polymers Article
Influence of the Processing Parameters on the Fiber-Matrix-Interphase in Short Glass Fiber-Reinforced Thermoplastics Anna Katharina Sambale *, Marc Schöneich and Markus Stommel Chair of Plastics Technology, TU Dortmund University, Leonard-Euler Str. 5, D-44227 Dortmund, Germany;
[email protected] (M.Sc.);
[email protected] (M.St.) * Correspondence:
[email protected]; Tel.: +49-231-755-5753 Academic Editor: Naozumi Teramoto Received: 12 April 2017; Accepted: 8 June 2017; Published: 13 June 2017
Abstract: The interphase in short fiber thermoplastic composites is defined as a three-dimensional, several hundred nanometers-wide boundary region at the interface of fibers and the polymer matrix, exhibiting altered mechanical properties. This region is of key importance in the context of fiber-matrix adhesion and the associated mechanical strength of the composite material. An interphase formation is caused by morphological, as well as thermomechanical processes during cooling of the plastic melt close to the glass fibers. In this study, significant injection molding processing parameters are varied in order to investigate the influence on the formation of an interphase and the resulting mechanical properties of the composite. The geometry of the interphase is determined using nano-tribological techniques. In addition, the influence of the glass fiber sizing on the geometry of the interphase is examined. Tensile tests are used in order to determine the resulting mechanical properties of the produced short fiber composites. It is shown that the interphase width depends on the processing conditions and can be linked to the mechanical properties of the short fiber composite. Keywords: polymer-matrix composites; interphase; nano-scratch; mechanical testing; injection molding; short glass fiber composites
1. Introduction Plastics are versatile materials that have gained an increasingly important role as a structural and functional material in recent decades. A significant increase in the strength and stiffness of plastics is achieved by introducing reinforcing particles into the polymer matrix [1]. Especially short glass fiber-reinforced thermoplastics are widely used in several industrial applications to create high performance, lightweight parts and devices. The combination of stiff glass fibers in a thermoplastic matrix yields an anisotropic composite material with improved mechanical properties especially in the direction of the fiber orientation [1]. The mechanical properties of the composite are largely dependent on the local microstructure and the interaction between the different material phases. Particularly the adhesion between the fibers and the matrix is crucial to ensure an optimum load transfer from the matrix phase into the glass fibers in order to improve the macroscopic composite performance [2,3]. At the microscale, recent literature demonstrates the presence of a third phase between short glass fibers and the polymer matrix [4]. Generally, this interphase region is defined as a zone at the interface of the matrix and the fiber exhibiting modified polymer properties [5]. Thus, this region represents an additional, three-dimensional material phase [6,7] whereupon the differing mechanical properties compared to the bulk material are induced by morphological and thermomechanical effects [5,8,9]. In semi-crystalline composites, the fiber-matrix interphase varies from several hundred nanometers to a few microns in width around the glass fiber [5,10]. Particularly, the fiber-matrix Polymers 2017, 9, 221; doi:10.3390/polym9060221
www.mdpi.com/journal/polymers
Polymers 2017, 9, 221
2 of 23
adhesion and thus the interphase are affected by the choice of glass fiber sizing and the conditions of crystallization during the cooling of the melt [11–13]. While cooling down, the fibers are acting as nucleation agents leading to a high density of spherulitic structures around the fiber, which is one explanation for the existence of an interphase. If the crystalline structures are growing mainly perpendicular to the fiber surface, the created morphology is described as trans-crystallinity [14]. The formation of columnar structures on the fiber surface, for example row-nucleated cylindritic structures, is possible due to melt shearing. The appearance and the difference between trans-crystallinity and shear-induced crystalline structures are studied by Varga and Karger-Kocsis [15]. Further studies regarding the effects of trans-crystallization and their impact on the interphase region are given by Quan et al. and Bergeret et al. [16,17]. Since crystalline structures are linked to the presence of an interphase, the conditions for the crystallization process greatly influence the formation of the interphase region. These process conditions include the rate of cooling, the crystallization temperature and the prevailing pressure during crystallization [16]. During the injection molding process for the manufacturing of short fiber plastic parts, different process parameter settings determine the cooling of the melt. The resulting cooling conditions alter the crystallization process and thus the morphology of the polymer matrix linked to the mechanical composite properties [11]. The formation of an interphase region is also dependent on the surface texture, the roughness and the surface energy of the glass fibers and the corresponding sizing [17,18]. The relation between the fiber sizing and the interphase is studied by Mäder [19] for semi-crystalline thermoplastics. The glass fiber sizing represents a thin layer applied to the fibers during the manufacturing process, which protects the fibers from mechanical surface damage [20]. In addition, the sizing contributes strongly to the fiber-matrix adhesion by maximizing the shear strength at the interface between the glass fiber and the polymer matrix by means of a chemical affinity. To ensure a good fiber-matrix adhesion, an organosilicon compound is typically used as a component of the sizing [3]. These silane compounds are acting as adhesion promoters and are constituted by the chemical structure R − SiX3 whereby X is a hydrolyzable group and R refers to an organic radical [3]. The silane adhesion promoter is bound to the glass fiber surface through hydrogen bonding and covalent siloxane bonds, whereas the polymer matrix chains are linked to the reactive radical of the silane [21]. A further prerequisite for good fiber-matrix adhesion is a complete wetting of the sizing on the glass fibers [22]. In order to improve the adhesion, the sizing not only has to be adapted to the glass fibers, but also to the surrounding polymer matrix. However, the composition of an adhesive promoter providing an optimized adhesion to the polymer matrix is still a challenging issue [18]. In contrast to semi-crystalline plastics, amorphous thermoplastics do not contain molecular chains forming crystalline structures. However, an interphase can still be expected based on the interdiffusion of macromolecules between the matrix material and the glass fiber sizing controlled by thermodynamic forces [23]. The exact mechanisms in terms of the interdiffusion and the resulting adhesion of the fibers with the matrix have still not been fully understood [9,10,13,24]. In the literature, different experimental methods for the geometrical mapping and mechanical characterization of the interphase are discussed. Friedrich et al. provide transmission electron microscopy (TEM) images of the crystalline regions around fibers. Here, trans-crystalline structures of thin layers in microfibrillar reinforced composites are studied by TEM. Thomason et al. show transgranular zones due to isothermal crystallization around glass fibers by means of polarized light microscopy. Varga et al. identify different shear-induced crystalline structures by pulling embedded fibers in the quiescent melt on a hot stage, imaging the crystalline structures by using a polarizing optical microscope [15]. The mechanical properties of the interphase are determined by Cech et al. [5] using atomic force microscopy (AFM). For this purpose, the surface of the glass fiber-reinforced samples is scanned in the tapping mode of the AFM to determine the surface topography, as well as the phase shift. However, the measurements using AFM only provide near-surface information about the interphase, which is highly dependent on the sample preparation [25]. For a mechanical
Polymers 2017, 9, 221
3 of 23
characterization of the interphase region, AFM and nano-indentation are applied in order to examine local changes regarding contact stiffness and microhardness [8,9,11]. The local interphase width between the fiber and the matrix phase can be measured by means of nano-scratching. For this purpose, the sample is placed in contact with the nano-indenter tip and is moved at a constant speed. Consequently, the tip scratches successively across each phase (polymer matrix, interphase, glass fiber). Following a measurement method developed by Schöneich et al. [4], the normal force is determined in order to maintain a constant scratch depth between the matrix and the glass fiber. By analyzing the measured normal force over the scratch path, the interphase geometry within the cutting plane of the sample can be identified [4]. The present work is focused on the influences concerning the formation of a fiber-matrix interphase in short fiber-reinforced thermoplastics. In this study, the thermoplastic polybutylene terephthalate (PBT) is used as the matrix material. The glass fiber sizing and the process parameters of the specimen production are considered to have an impact on the formation of an interphase. Therefore, a systematic study is conducted. The measurement of the interphase is performed by means of nano-scratches in accordance with the method of Schöneich et al. [4]. Four hypotheses on the formation of the interphase are proposed as a part of this study: Hypothesis 1. The fiber sizing affects the fiber-matrix adhesion and the fracture properties due to the formation of an interphase: The influence of the glass fiber sizing on the formation of an interphase is investigated by means of two compounded composite granulates. Coated and uncoated glass fibers are compounded in a PBT matrix using a twin screw extruder. Tensile specimens, consisting of the compounded granulates, are produced via the injection molding process. The processing parameters for the granulate with sized fibers and for the granulate with unsized fibers are equal and adjusted following the product data sheets. For the characterization of the macroscopic composite properties, tensile tests are carried out. For the measurement of the interphase width, nano-scratches are performed using parts taken from the tensile specimen. Subsequently, the resulting mechanical properties are correlated with the available microscale interphase information. Hypothesis 2. The interphase formation and its width are influenced by the processing parameters of the injection molding process through crystallization kinetics: Regarding the processing effects on the interphase, the present study investigates different injection molding parameters using the statistical tool “design of experiments (DoE)”. For this purpose, the parameters “melt temperature”, “mold temperature” and “holding pressure” are varied during the processing of different samples. The parameters “melt temperature” and “mold temperature” set the kinetics of the solidification and are related to the crystallization behavior of the polymer matrix [26]. The holding-pressure is considered, since the injection molding process causes a volume contraction during solidification of the melt resulting in a shrinkage of the final part. In order to compensate component shrinkage during the holding pressure phase, an additional melt cushion is used after the complete filling of the cavity. The involved compression affects the crystallization behavior during the cooling phase, since the kinetics of crystallization from the melt change under different pressure conditions [16]. Consequently, the adjustment of the holding pressure, i.e., the amount of compression of the melt, is suspected to influence the interphase formation. Hypothesis 3. The interphase is not only a crystallization phenomenon for semi-crystalline composites: In order to exclude the effects of semi-crystalline thermoplastics on the fiber-matrix interphase, additional samples are made from amorphous thermoplastics. In this case, acrylonitrile butadiene
Polymers 2017, 9, 221
4 of 23
styrene (ABS) specimens are fabricated to study the existence of the interphase in comparison to previous semi-crystalline samples. Hypothesis 4. The width of the interphase correlates with the mechanical behavior of thermoplastics: In addition to the measurement of the width of the interphase by nano-scratches, the mechanical properties of the different injection molded samples are determined via tensile tests with load directions parallel to the fiber orientation, as well as perpendicular to the fiber orientation. The width of the interphase and the mechanical properties’ tensile strength, the stiffness and the elongation at break are compared in order to determine a possible correlation. 2. Materials A polybutylene terephthalate reinforced with 20 wt % of short glass fibers (PBT GF20) is used. The applied “Pocan® B 3225” is a commercial-grade material, which is provided by the company Lanxess (Cologne, Germany). The PBT matrix of the composite is a semi-crystalline thermoplastic and the composite is used in a wide range of technical applications [27]. The relevant material properties are listed in Table 1 [28]. Detailed information about the glass fiber sizing is not provided by the manufacturer. Table 1. Material properties of the investigated polybutylene terephthalate reinforced with 20 wt % of short glass fibers (PBT GF20) “Pocan B 3225/Lanxess” [28]. Property
Value
Unit
Density Tensile modulus Tensile stress at break Tensile strain at break Melting temperature
1460 7100 120 3.4 225
kg/m3 MPa MPa % ◦C
In addition to the semi-crystalline PBT-GF20, an amorphous acrylonitrile butadiene styrene (ABS) reinforced with 20 wt % glass fibers (“Polyman FABS-20-GF”) from the company A. Schulman (Akron, OH, USA) is investigated. The material properties of Polyman FABS-20-GF are illustrated in Table 2 [29]. There are no specifications given about the used glass fibers and the glass fiber sizing in the composite. Table 2. Material properties of the investigated ABS-GF20 “Polyman FABS-20-GF/A. Schulman” [29]. Property
Value
Unit
Density Tensile modulus Tensile stress at break Tensile strain at break Melting temperature
1200 5500 65 2 105
kg/m3 MPa MPa % ◦C
Furthermore, sized and unsized glass fibers are compounded in a PBT matrix material using a twin screw extruder Thermo Fisher Scientific, Waltham, MA, USA). The PBT matrix material used for this purpose is provided by the company Lanxess (Cologne, Germany). The material properties of the PBT matrix material (“Pocan® B 1305”) can be found in Table 3 [30].
Polymers 2017, 9, 221
5 of 23
Table 3. Material properties of the investigated PBT matrix material “Pocan® B 1305/Lanxess” [30]. Property
Value
Unit
Density Tensile modulus Tensile stress at break Tensile strain at break Melting temperature
1310 2800 60 9.0 225
kg/m3 MPa MPa % ◦C
A composite granulate with sized glass fibers “GF CS 7968” (chopped strands) and a composite granulate with unsized glass fibers “GF MF 7980” (milled fibers) from Lanxess are compounded. The material properties of the two glass fiber types are shown in Table 4 [31,32]. Table 4. Material properties of the investigated glass fibers (unsized [31]/sized [32]). Property
Value
Unit
Sized fibers (GF CS 7968/Lanxess) Glass type Fiber sizing Sizing content Mean fiber length Mean fiber diameter
E-glass Silane-based polymer-coating approximately 0.9 4.5 11
wt % mm µm
Unsized fibers (GF MF 7980/Lanxess) Glass type Fiber sizing Sizing content Mean fiber length Mean fiber diameter
E-glass 190 14
wt % µm µm
3. Experimental Methods Different specimen types are manufactured in order to determine the manifold influences on the interphase formation. The procedure is subdivided into in three steps: the composite production, the processing of the specimen and an experimental analysis. The specimen processing and sample analysis distinguish between the following influences on the interphase:
• Influence of the glass fiber sizing: To produce a short glass fiber-reinforced composite, sized glass fibers (GF CS 7968) and unsized glass fibers (GF MF 7980) are added to a pure PBT matrix material (Pocan B 1305 from Lanxess) via compounding using a twin screw extruder. The compounded granulate types are injection molded involving the same processing parameters from a reference specimen type produced with an industrial-grade PBT-GF20 material. The influence of the glass fiber sizing on the formation of an interphase is determined by a comparison to the reference specimens.
• Influence of the processing parameters of the injection molding: By using design of experiments (DoE), the influences of different injection molding parameters “melt temperature”, “mold temperature” and “holding pressure” on the interphase formation are determined. A reference specimen type is processed according to the product data sheet. Based on these reference injection molding parameters, various specimen types are produced by varying the three factors according to an experimental design. The used granulate type for the specimens of the experimental design is a PBT-GF20 Pocan B 3225 of the company Lanxess. The effects of the injection molding factors are identified via mechanical testing and nano-scratches.
Polymers 2017, 9, 221
6 of 23
• Influence of the matrix morphology: To identify the influence of the semi-crystalline matrix material PBT, an amorphous thermoplastic matrix (ABS) reinforced with 20 wt % of glass fibers (ABS-GF20 from A. Schulman) is also used to fabricate specimens. The injection molding parameters are chosen according to product data sheets. A determination of the interphase is carried out to identify the interphase thickness. 3.1. Composite Production Short glass fiber-reinforced thermoplastics can be processed continuously by extrusion or discontinuously by injection molding in the melting phase. Both processing operations are used in this study to produce specimens from the required materials. At first, the compounding of the two different composite types, consisting of PBT matrix material and sized fibers for the composite, as well as unsized glass fibers are considered. These composites are used to identify the influence of a glass fiber sizing on the interphase. The compounding of the two granulate types is processed by means of a co-rotating twin screw extruder. The used Thermo Fisher Scientific “Haake OS Rheomex PTW16” twin screw extruder is composed of two modular constructed screws in the inside of a barrel, parted into ten heating zones. Two different granulates are produced by compounding. At first, the PBT-matrix material and sized glass fibers (BC) are compounded, and the output composite strand is cut to granulate pieces by using a fly cutter. A second granulate type is manufactured the same way by compounding unsized glass fibers combined with the PBT matrix material (UC). The fiber content of the composite is set to a weight ratio of 8:2 to compare the compounded granulates with the industrial processed ones. The process parameters are kept constant during the compounding of the two granulate types. The compounding parameters are listed in Table 5. Table 5. Applied parameters of the compounding process. Parameter
Setting
Unit
Temperature of the heating zones (1–10) Drive Output feeder matrix Output feeder fibers Pellet length
280 in all heating zones 100 1 0.25 1.5
◦C
1/min kg/h kg/h mm
3.2. Specimen Processing The different specimen types and the utilized materials for the specimen production are listed in Table 6. The different specimen are fabricated with the help of the injection molding machine “Arburg Allrounder 370 S 700-290 U” (Arburg, Loßburg, Germany). Table 6. Listing of the utilized composites for the specimen processing to examine the influencing factors on the formation of a fiber-matrix interphase. Influencing Factor
Material
Trading Name
Specimen Shortcut
Reference Process of injection molding With glass fiber sizing Without glass fiber sizing Morphology of the matrix
PBT-GF20 PBT-GF20 PBT-GF20 PBT-GF20 ABS-GF20
Pocan® B 3225 Pocan® B 3225 Pocan® B1305 + GF CS 7968 Pocan® B1305 + GF MF 7980 POLYMAN FABSABS 20 GF
Reference DoE 1–4 BC UC ABS
In pilot tests, the optimal parameter settings are determined with the help of the utilized granulate, the given specimen geometry and their dimensional accuracy, as well as the product data sheets of the materials. The listed processing parameters of the injection molding in Table 7 are kept constant for all injection molded specimens.
Polymers 2017, 9, 221
7 of 23
Table 7. Constant adjusted processing parameters to process the specimen in the injection molding process. Parameter
Setting
Unit
Dosage
76.5
(cm3 )
Circumferential speed
5
(m/min)
Injection pressure
1600
(bar)
Volume flow
30
(cm3 /s)
Switchover volume
2.3
(cm3 )
Holding pressure time
2
(s)
36
(s)
Residual cooling time Desiccation of the granulate before processing Residual moisture of the granulates Barrel temperature
Mold temperature
Holding pressure
120/2 (PBT-GF20) 80/3 (ABS-GF20) 0.001 270 (PBT-GF20) 250 (ABS-GF20) 100 (PBT-GF20) 60 (ABS-GF20) 600 (PBT-GF20) 600 (ABS-GF20)
(◦ C/h) (%) (◦ C) (◦ C)
(bar)
These determined parameters are defined to be the reference parameters. In further steps, the barrel temperature, the holding pressure level, as well as the mold temperature are varied in order to investigate the influence of the processing parameters regarding the interphase formation. In Table 7, the represented parameters are varied in the context of an experimental design based on the parameters for the PBT-GF20 reference specimens, whereas the processing parameters to manufacture ABS-GF20 specimens are kept constant. 3.3. Design of Experiments The influence of the injection molding parameters on the interphase formation in short glass fiber-reinforced thermoplastics is studied by applying a statistical experimental design. Fractional factorial experimental designs are applied to reduce time-consuming pretests. For this study, a screening design with a resolution step of III is chosen. This design determines the influences of three factors in four tests. The parameters “melt temperature”, “holding pressure” and “mold temperature” are considered to be the crucial influencing factors on the crystallization. In pilot tests, specimens are produced according to the product data sheet, and the determined injection molding parameters are designated as reference settings. Based on the reference settings, low (“−1”) and high (“+1”) levels for the three varying factors “mass temperature”, “level of holding pressure” and “mold temperature” are chosen at a constant distance from the reference setting. The remaining injection molding parameters are kept constant during the entire test duration. The chosen levels are at equal distance from the reference parameters. Additional pilot tests verify the system to be executable between the chosen levels. Table 8 shows the high, the low and the reference level settings for the three factors, respectively. Table 8. Level settings for the low, the reference and the high level setting. Level Setting
Melt Temperature (◦ C)
Holding Pressure (bar)
Mold Temperature (◦ C)
−1 Reference Setting +1
255 270 285
200 600 1000
80 100 120
Polymers 2017, 9, 221
8 of 23
By using the software MiniTab 17 (MiniTab Inc., State College, PA, USA), a statistical program for the evaluation of experiments, a fractional factorial design involving the level settings from Table 8 is generated and listed in Table 9. Table 9. Applied fractional factorial experimental design. Polymers 2017, 9, 221
8 of 23
Melt Temperature (◦ C)
Level Setting 1 2Level Setting 1 3 2 4 3 Reference Setting 4 Reference Setting
Holding Pressure (bar)
Mold Temperature (◦ C)
200
120
Table 9. Applied fractional factorial experimental design.
255
Melt Temperature (°C) 285 255 255 285 285 255 270 285 270
Holding Pressure 200(bar) 200 1000 200 1000 1000 600 1000 600
Mold Temperature (°C) 80 120 80 80 120 80 100 120 100
The first column of Table 9 defines the succession of each test. The following columns specify the The first column of Table 9 defines the succession of◦ each test. The following columns specify level settings of the three factors “melt temperature in ( C)”, “holding pressure in (bar)” and “mold the level settings of the three factors “melt temperature in (°C)”, “holding pressure in (bar)” and ◦ temperature in (temperature C)”. Figure 1 shows the experimental spacespace for the applied factorial design. “mold in (°C)”. Figure 1 shows the experimental for the appliedfractional fractional factorial The darkerdesign. spheres in the experimental space mark the performed factor settings of the design. The darker spheres in the experimental space mark the performed factor settings of the design.
Figure 1. Applied settings of the experimental space for the fractional factorial design after [33].
Figure 1. Applied settings of the experimental space for the fractional factorial design after [33]. After a resetting of the injection molding parameters, the first three processed specimens are for every setting in ordermolding to obtain constant conditions processing. Additionally, Afterrejected a resetting of the injection parameters, the while first three processed specimens are attention was paid to a low residence time of the melt within the heated plasticizing unit to avoid rejected for every setting in order to obtain constant conditions while processing. Additionally, thermal degradation of the melt [34]. attention was paid to a low residence time of the melt within the heated plasticizing unit to avoid thermal degradation of the melt 3.4. Specimen Preparation and[34]. Testing A specimen preparation is performed for the analysis after the processing procedure. To prepare
3.4. Specimen Preparation the specimens forand the Testing different testing methods, they are detached from their sprue system. To
determine the width of the interphase, the examination area is extracted by cutting the specimen. The
A specimen preparation is performed for the analysis after the processing procedure. To prepare extracted sample is ground and polished in serval steps. the specimensThefor the different they detached fromThis their sprue determination of the testing interphasemethods, is performed by a are nano-scratch method. method is system. To determine width of interphase, the examination area is extracted cutting the specimen. usedthe to measure thethe width of the interphase in the composite material. Table 10by lists the applied parameters foris theground nano-scratch The extracted sample and tests. polished in serval steps. The determination of theTable interphase is performed by a nano-scratch method. This method is used 10. Applied parameters of the nano-scratching tests. to measure the width of the interphase in the composite material. Table 10 lists the applied parameters Parameter Setting Unit for the nano-scratch tests. Indenter tip Cube corner Scratches per sample 5 on different glass fibers Table 10. Applied parameters of the nano-scratching tests. Scratch depth pre scratches 100 nm Scratch depth main scratch 150 nm Parameter Setting Unit Scratch velocity 0.1 µm/s Path length per 10 corner µm Indenter tip scratch Cube -
Scratches per sample Scratch depth pre scratches Scratch depth main scratch Scratch velocity Path length per scratch
5 on different glass fibers 100 150 0.1 10
nm nm µm/s µm
Polymers 2017, 9, 221
9 of 23
Polymers 2017,characterization 9, 221 9 of 23are For the of the stress-strain behavior of the respective material, all tensile tests performed at a constant strain rate of 1 mm/min. The internal pressure tests perpendicular to the fiber For the characterization of the stress-strain behavior the of respective material, all to tensile testsrate orientation in the specimen proceed, including with 2 cm3of /min water, which refers a strain are performed at a constant strain rate of 1 mm/min. The internal pressure tests perpendicular to the of 1.3%/min in the peripheral direction. In order to obtain reliable average properties, five repetitions fiber orientation in the specimen proceed, including with 2 cm3/min of water, which refers to a strain of the tests are performed. A digital image correlation (DIC) camera system of the company LIMESS rate of 1.3%/min in the peripheral direction. In order to obtain reliable average properties, five and the analysis program ISTRA 4D (Limess, Krefeld, Germany) are used to measure the strain during repetitions of the tests are performed. A digital image correlation (DIC) camera system of the the performed tests. company LIMESS and the analysis program ISTRA 4D (Limess, Krefeld, Germany) are used to The wall thicknesses of the tube specimens are measured by a magnetostatic sensor called MiniTest measure the strain during the performed tests. 7200 FH 4 of the company Elektrophysik (Cologne, Germany). The applied parameters of the tensile The wall thicknesses of the tube specimens are measured by a magnetostatic sensor called tests are shown 11. company Elektrophysik (Cologne, Germany). The applied parameters of MiniTest 7200 in FHTable 4 of the
the tensile tests are shown in Table 11.
Table 11. Applied parameters of the tensile tests and the internal pressure test. Table 11. Applied parameters of the tensile tests and the internal pressure test. Parameter Setting Unit
Parameter
Strain Rate Tensile Testing Strain Rate Tensile Testing Flow Rate for Internal Pressure Test Flow Rate for Internal Pressure Test Corresponding Strain Rate for Internal Pressure Test
Corresponding Strain Rate for Internal Pressure Test
Setting 1 2 1.3
1 2 1.3
Unit mm/min mm/min 3 /min cm cm3/min %/min %/min
4. 4. Determination Determinationofofthe theInterphase Interphase The local bymeans meansofofnano-scratching, nano-scratching, shown in Figure The localinterphase interphasewidth widthcan can be be measured measured by asas shown in Figure 2. 2. In In this case, the sample is moved at a constant speed in contact with the nano-indenter tip such this case, the sample is moved at a constant speed in contact with the nano-indenter tip such that that scratches from matrix over interphase to the fiber. present study, local thethe tiptip scratches from thethe matrix over thethe interphase to the fiber. In In thethe present study, thethe local ® ® interphase withaaHysitron Hysitron 900 TriboIndenter interphasewidth widthisismeasured measuredby by means means of of nano-scratching nano-scratching with TITI 900 TriboIndenter (Bruker Hysitron, (Bruker Hysitron,Minneapolis, Minneapolis,USA). USA). Themethodology methodologydeveloped developed by by Schöneich Schöneich et thethe The et al. al. [4] [4] is is applied. applied.During Duringthe thenano-scratch, nano-scratch, sampleis ismoved movedatata aconstant constantspeed speed of of 0.1 0.1 µm/s µm/s while tiptip is is sample whilethe thecontact contactwith withthe thenano-indenter nano-indenter maintained.As Asshown shownin inFigure Figure 2a, 2a, the the penetration penetration depth byby thethe normal maintained. depthofofthe thetip tipisiskept keptconstant constant normal force being controlledduring duringthe theprocess. process. force being controlled
Figure 2. 2. Nano-scratch with constant scratch depth following [35]: (a)(a) representation ofof thethe scratch path Figure Nano-scratch with constant scratch depth following [35]: representation scratch at path constant depth from the from matrixthe to the fiber;to(b) of the PBT-GF20 sample after nano-scratching; at constant depth matrix theimaging fiber; (b) imaging of the PBT-GF20 sample after (c)nano-scratching; 3D-imaging of (b). (c) 3D-imaging of (b).
Polymers 2017, 9, 221
10 of 23
In order to avoid artefacts from the sample surface topography, a pre-scratch is first performed. For that purpose, the tip is initially immersed 100 nm into the sample. Thereafter, the main Polymers 2017, 9, 221 10 of 23 scratch proceeds along an identical scratch path with 150 nm of penetration depth to the surface. In order to avoid artefacts from the sample surface topography, a pre-scratch is first performed. The uniformity of the material pile-up displays the advantage of this approach. In addition, For that purpose, the tip is initially immersed 100 nm into the sample. Thereafter, the main scratch the nano-indenter tip is continuously located at the same depth in the material. Thus, the engaged proceeds along an identical scratch path with 150 nm of penetration depth to the surface. The contactuniformity area of theofindenter tip and the sample constant, whereby correction factor is required to the material pile-up displaysisthe advantage of thisno approach. In addition, the calculate the self-imaging effect of the tip. at the same depth in the material. Thus, the engaged contact nano-indenter tip is continuously located area of thedisplays indenter the tip and the samplepaths is constant, whereby no described correction factor is requiredIn toorder to Figure 2b,c nano-scratch according to the methodology. calculate the self-imaging effect of the tip. determine the interphase width, different slopes are identified through the measured normal force Figure 2b,c displays the nano-scratch paths according to the described methodology. In order to progression during nano-scratching. determine the interphase width, different slopes are identified through the measured normal force Figure 3 shows a representative progression during nano-scratching.normal force profile of a nano-scratch coming from the matrix to the glassFigure fiber.3 A nearly constant normal is measured over thecoming scratch path the area of shows a representative normal force force profile of a nano-scratch from the in matrix to matrix the glassand fiber.fiber A nearly constantDue normal force is measured over the scratch path in the area the the pure material. to the presence of different slopes between theofmatrix and pure matrix and fiber material. Due to the presence of different slopes between the matrix and fiber fiber phases, the width of the interphase area can be identified. The onset of the interphase starts phases, the width of the interphase area can be identified. The onset of the interphase starts with the with the deviation from the linear gradient from the matrix material. The subsequent linear increase deviation from the linear gradient from the matrix material. The subsequent linear increase in the in the normal force refers to the interphase. This interphase area ends with a further increase of the normal force refers to the interphase. This interphase area ends with a further increase of the normal normalforce. force.The The renewed ofnormal the normal is referred to the initial contact of the indenter tip renewed rise rise of the force isforce referred to the initial contact of the indenter tip with with the pure fiber material. Further information about the measurement of the interphase width by the pure fiber material. Further information about the measurement of the interphase width by nano-scratches can be taken from the studies of Schöneich et al. [4]. nano-scratches can be taken from the studies of Schöneich et al. [4].
3. Measured normal force over scratchpath, path,identification identification ofofthe width using Figure Figure 3. Measured normal force over thethe scratch theinterphase interphase width using the the slope method. slope method.
To determine the mean interphase width for each selected composite material, the
Tonano-scratches determine theare mean interphase width for each selected composite the nano-scratches performed on different fibers within the cutting planematerial, of the corresponding sample. Subsequent to fibers the nano-scratch, AFM image taken the sample surface as presented are performed on different within theancutting planeis of the of corresponding sample. Subsequent to in Figure 2b,c. Table 10 lists the parameters used for the nano-scratch tests. The results of theTable 10 the nano-scratch, an AFM image is taken of the sample surface as presented in Figure 2b,c. nano-scratches for different sample types are presented in Figures 4 and 5. lists the parameters used for the nano-scratch tests. The results of the nano-scratches for different Figure 4 shows the results of the nano-scratch tests of the various samples to investigate the sampleinfluence types areofpresented in Figures 4 and 5. formation. At least five scratches are performed on the fiber sizing on the interphase Figure 4 shows the for results of thetype, nano-scratch of theonvarious tothe investigate the different glass fibers each sample respectively.tests Depending the glasssamples fiber sizing, width of the interphase varies on widely. The specimens containingAt unsized fibers “UC” show the influence of the fiber sizing the interphase formation. least glass five scratches are performed on smallest interphase area and standard deviation with a value of 353 ± 4 nm. The interphase width for different glass fibers for each sample type, respectively. Depending on the glass fiber sizing, the width the sized glass fiber samples hasspecimens a wider interphase with the mean values of “UC” 417 ± 39 nm. the Thesmallest of the interphase varies widely. The containing unsized glass fibers show reference specimens show an interphase width of 526 ± 42 nm. interphase area and standard deviation with a value of 353 ± 4 nm. The interphase width for the It is noticeable that the values for the standard deviations vary significantly concerning the sized glass fiber samples a wider interphase the mean values of has 417 an ± 39 nm. The reference measured interphasehas width. The sample type with containing unsized fibers extremely low specimens show an interphase width of 526 ± 42 nm. It is noticeable that the values for the standard deviations vary significantly concerning the measured interphase width. The sample type containing unsized fibers has an extremely low standard
Polymers 2017,2017, 9, 221 221 Polymers Polymers 2017, 9, 9, 221
11 of2323 1111 ofof 23
standarddeviation, deviation,whereas whereasthe thespecimen specimentype typewith withsized sizedfibers, fibers,asaswell wellasasthe thereference referencesamples samples standard deviation, whereas the specimen type with sized fibers, as well as the reference samples show higher showhigher higher standard deviations. show standard deviations. standardAdeviations. possiblereason reasonrepresents representsthe theapplied appliedsizing sizingononthe theglass glassfibers, fibers,which whichisisalso alsomeasured measuredininthe the A possible A possible reason represents the applied sizing on the glass fibers, which is also measured in nano-scratch test and included to the interphase width. For this purpose, the quantity of the applied nano-scratch test and included to the interphase width. For this purpose, the quantity of the appliedthe nano-scratch test and included toaccording the interphase width. For this purpose, the quantity of thethickness applied sizing sizingofof0.95 0.95mass mass percentaccording theproduct product data sheet converted sizing thickness sizing percent totothe data sheet isisconverted totoa asizing ofof of 66.4 0.95 mass percent according to the product data sheet is converted to a sizing thickness of 66.4 nm. 66.4nm. nm.On Onthis thisoccasion, occasion,a aconstant constantwidth widthofofthe theapplied appliedsizing sizingaround aroundthe thefiber fiberisisassumed. assumed.The The calculated sizing thickness corresponds good approximation the difference the meanwidth width Oncalculated this occasion, a constant width of the applied sizing around the fiber is difference assumed. The calculated sizing sizing thickness corresponds inina agood approximation totothe ofofthe mean of the interphase for a comparison of the unsized sample type (353 nm) and the sized sample type thickness corresponds in a good approximation to the difference of the mean width of the interphase of the interphase for a comparison of the unsized sample type (353 nm) and the sized sample type (417 nm). for(417 a comparison of the unsized sample type (353 nm) and the sized sample type (417 nm). nm). The wider interphase area thereference reference sample can explained unknown additives The wider interphase area of the reference sample can can be explained by unknown additives added The wider interphase area ofofthe sample bebeexplained bybyunknown additives added theindustrially-manufactured industrially-manufactured thereference reference specimens. Theseadditives additives usually added totothe ofofthe specimens. These to the industrially-manufactured materialmaterial ofmaterial the reference specimens. These additives usuallyusually improve improvethe theflowability flowability andthe thedispersibility. dispersibility. addition, theadhesion adhesion between glass fibersizing sizing and InInaddition, the glass fiber theimprove flowability and the dispersibility. In addition, the adhesion betweenbetween glass fiber sizing and the and the matrix material can be optimized by additives. and the matrixcan material can be optimized by additives. matrix material be optimized by additives.
Figure4.4.Interphase Interphasewidth widthfor forthe theinfluence influence ofthe the fibersizing sizing measuredwith with theintroduced introduced Figure Figure 4. Interphase width for the influence of of the fiber fiber sizingmeasured measured withthe the introduced nano-scratch method with sized fibers (SC) and unsized fibers (UC). nano-scratch method with sized fibers (SC) and unsized fibers (UC). nano-scratch method with sized fibers (SC) and unsized fibers (UC).
Theresults resultsofofthe thenano-scratching nano-scratchingtests testsfor forthe thespecimens specimensinfluenced influencedbybythe theprocessing processing The The results of the nano-scratching tests for the specimens influenced by the processing parameters parameters of the injection molding are shown in Figure 5. The interphase widths are measured for parameters of the injection molding are shown in Figure 5. The interphase widths are measured for every specimen type. At least five scratches are performed on different glass fibers for each sample of the injection molding are shown in Figure 5. The interphase widths are measured for every specimen every specimen type. At least five scratches are performed on different glass fibers for each sample type, respectively. type. Atrespectively. least five scratches are performed on different glass fibers for each sample type, respectively. type,
Figure Interphase width for theinfluence influence ofthe theprocessing processing parameters measured withthe the Figure 5. Interphase width for the influence of the of processing parameters measured with the with introduced Figure 5.5.Interphase width for the parameters measured introduced nano-scratchmethod. method. nano-scratch method. introduced nano-scratch
Polymers 2017, 2017, 9, 9, 221 221 Polymers
12 of of 23 23 12
The reference specimens show the widest interphase area of 526 ± 42 nm. Specimen Numbers 2 The reference specimens show the widest interphase area of 526 ± 42 nm. Specimen Numbers and 3 are closely matched at a width of 432 ± 32 nm and 436 ± 22 nm. The smallest width can be 2 and 3 are closely matched at a width of 432 ± 32 nm and 436 ± 22 nm. The smallest width can measured at Specimen Number 4 with 423 ± 41 nm. Differences of approximately 100 nm in the be measured at Specimen Number 4 with 423 ± 41 nm. Differences of approximately 100 nm in the interphase widths are measured for the same material. To identify the injection molding parameter interphase widths are measured for the same material. To identify the injection molding parameter with the highest influence on the interphase formation, the experimental design has to be evaluated. with the highest influence on the interphase formation, the experimental design has to be evaluated. The main effects plot for the influences on the interphase is shown in Figures 14–16. The main effects plot for the influences on the interphase is shown in Figures 14–16. The investigations in the present study mentioned above are performed with a semi-crystalline The investigations in the present study mentioned above are performed with a semi-crystalline thermoplastic matrix material. In a semi-crystalline thermoplastic material, a crystallization occurs in thermoplastic matrix material. In a semi-crystalline thermoplastic material, a crystallization occurs a privileged place at the interfaces of the fibers, and the degree of crystallization depends on the in a privileged place at the interfaces of the fibers, and the degree of crystallization depends on the process conditions of the crystallization. Amorphous thermoplastics are not able to crystallize process conditions of the crystallization. Amorphous thermoplastics are not able to crystallize because because of their macromolecular structure, and correspondingly, it is not possible to form a of their macromolecular structure, and correspondingly, it is not possible to form a long-range order long-range order like a semi-crystalline structure. In amorphous thermoplastics, there is only a like a semi-crystalline structure. In amorphous thermoplastics, there is only a near-range order. On this near-range order. On this basis, it is obvious to study the possibility of the formation and the detection basis, it is obvious to study the possibility of the formation and the detection of an interphase in of an interphase in amorphous fiber-reinforced thermoplastics. amorphous fiber-reinforced thermoplastics. Figure 6 lists the values for the width of the interphases in amorphous ABS-GF20 samples. It can Figure 6 lists the values for the width of the interphases in amorphous ABS-GF20 samples. It can be seen that an interphase is detectable by using the applied nano-scratch method. The interphase be seen that an interphase is detectable by using the applied nano-scratch method. The interphase width amounts 395 ± 35 nm for the amorphous material. width amounts 395 ± 35 nm for the amorphous material.
Figure 6. 6. Interphase Interphase width width for for the the influence influence of of the theamorphous amorphous material material measured measured with with the the introduced introduced Figure nano-scratch method. nano-scratch method.
Despite the theamorphous amorphousstructure structure of the matrix, the formation of an interphase occurs of the ABSABS matrix, the formation of an interphase occurs between between the matrix and fiber the glass fiber the or rather the glass fiber Thevalues absolute values of this the matrix and the glass or rather glass fiber sizing. Thesizing. absolute of this interphase interphase width deviate compared to the of the semi-crystalline Since the structure width deviate compared to the values of values the semi-crystalline samples.samples. Since the structure of the of the macromolecules from ABS from differs the macromolecular chain structure of material, the PBT macromolecules from the ABSthe differs thefrom macromolecular chain structure of the PBT material, a comparison of the overall width of the interphase is not appropriate. The of formation of the a comparison of the overall width of the interphase is not appropriate. The formation the interphase interphase in an amorphous thermoplastic shows that the is not only a of phenomenon of in an amorphous thermoplastic shows that the interphase is interphase not only a phenomenon crystallization. crystallization. Similar to thermoplastics, semi-crystalline the thermoplastics, the amorphous thermoplastics form a Similar to semi-crystalline amorphous thermoplastics form a three-dimensional three-dimensional due to a thermodynamically-controlled interdiffusion and of the interphase due to ainterphase thermodynamically-controlled interdiffusion of the macromolecules macromolecules and the molecules of the glass fiber sizing [8]. molecules of the glass fiber sizing [8]. 5. Determination Determination of the Mechanical Properties In the present present contribution, the characterization characterization of the macroscopic macroscopic mechanical properties of the fiber-reinforced thermoplastics is performed by means tensileoftests. Therefore, presented short shortglass glass fiber-reinforced thermoplastics is performed by of means tensile tests. tube specimens introducedintroduced by Kaiser et [36] are used. 7 shows tube specimen geometry Therefore, tube specimens byal.Kaiser et al. [36]Figure are used. Figurethe 7 shows the tube specimen
geometry with a 3D representation of the specimen in (a) and the connecting dimensions in (b).
Polymers 2017, 9, 221
13 of 23
Polymers 2017, 9, 221
13 of 23
with a 3D representation of the specimen in (a) and the connecting dimensions in (b). Besides the Besides the possible application of multi-axial loading cases, the main feature of the specimen is a possible application of multi-axial loading cases, the main feature of the specimen is a unidirectional unidirectional fiber orientation in the central measurement section. The fiber orientation in the fiber orientation in the central measurement section. The fiber orientation in the measurement section measurement section is determined by Kaiser with µCT measurements and micro sections [36]. The is determined by Kaiser with µCT measurements and micro sections [36]. The mechanical testing of mechanical testing of the tube specimens is performed by uniaxial tensile tests to determine the the tube specimens is performed by uniaxial tensile tests to determine the mechanical properties in the mechanical properties in the fiber direction of the specimens and by internal pressure tests to fiber direction of the specimens and by internal pressure tests to determine the mechanical properties determine the mechanical properties perpendicular to the fiber direction. perpendicular to the fiber direction.
Figure Figure 7. Tube Tube specimen specimen geometry geometry for for experimental experimental investigations: investigations: (a) (a) 3D 3D representation; representation; (b) (b) connecting connecting dimensions dimensions [36]. [36].
To To convert convert the the measured measured loads loads of of the the tensile tensile tests tests into intothe therequired required true true strains strainsfor foraastress-strain stress-strain diagram, diagram,the thefollowing followingformula formulaisisused used[37]. [37].InInFormula Formula(1), (1),the thetrue truestrain strainσσtrue in in MPa MPa is is calculated calculated 2 of 2 with the load in N applied to the specimen and the effective cross-sectional area in mm the with the load F in N applied to the specimen and the effective cross-sectional area A a in mm of specimen. the specimen. F σ σtrue = A a (1)(1) While the load is applied to the specimen, the wall thickness of the tube specimen is reduced due While the load is applied to the specimen, the wall thickness of the tube specimen is reduced to the necking of the specimen, and therefore, the effective cross-sectional area decreases. To measure due to the necking of the specimen, and therefore, the effective cross-sectional area decreases. To the wall thickness during the tests, a magnetostatic sensor is used. The effective cross-sectional area is measure the wall thickness during the tests, a magnetostatic sensor is used. The effective calculated using Formulae (2)–(7). cross-sectional area is calculated using Formulaeπ (2)–(7). 2 2 A a = ·( D − d ) (2) 4 ̅ ∙ − (2) 4 with: d = D − 2s (3) with: and:
and: and: and: ⇔
⇔
• • •
̅
−2 ̅ D = D0 ·(1 + εt )
(3) (4)
∙ 1 ε s = s 0 ( 1 + εr )
(4) (5)
̅ ε =̅ s1 −ε1 r s0
(5)(6)
A a = π·( D0 ·s0 ·(1 + εt )·(1̅ + εr ) − s0 2 ·(1 + εr )2 ) ε −1 In Formulae (2)–(7), the following symbols are used:
(7) (6)
D0 : average outer diameter − ̅ the π ∙ of the ∙ ̅ tube ∙ 1 specimen ε ∙ 1 εbefore ∙ test 1 εstarted measured four times (7) around the tube specimen with a caliper in mm; In Formulae (2)–(7), the following symbols are used: D: current outer diameter during the test in mm; : average outer diameter of the tube specimen before the test started measured four times around the tube specimen with a caliper in mm;
Polymers 2017, 2017, 9, 9, 221 221 Polymers Polymers 2017, 9, 221
14 of of 23 23 14 14 of 23
•• ••• • •• • •• • •• •• •
current outer outer diameter diameter during during the the test test in in mm; mm; :: current d:: current current internal internal diameter diameter during during the the test testin inmm; mm; d current internal diameter during the test in mm; d: ̅̅ :: wall wall thickness thickness of of aa representative representative measurement measurement point point before before the the test test started started in in mm; mm; s0 : wall thickness of a representative measurement point before the test started in mm; ̅: current current wall wall thickness thickness during during the the test test in in mm; mm; s:̅ : current wall thickness during the test in mm; εε :: tangential tangential strain strain ((--); ); εt : tangential strain (-); radial strain strain ((--). ). εε :: radial εr : radial strain (-). The symbols symbols used used in in Formulae Formulae (2)–(7) (2)–(7) are are displayed displayed in in Figure Figure 8. 8. The The mechanical mechanical material material The The symbols used in Formulae (2)–(7) are displayed in Figure 8. The mechanical material properties perpendicular perpendicular to to the the fiber fiber orientation orientation are are measured measured by by means means of of internal internal pressure pressure tests. tests. properties properties perpendicular to athe fiber orientation are into measured by means of test internal pressure tests. This load case is realized by constant flow of a fluid the sealed, parallel area of the tube test This load case is realized by a constant flow of a fluid into the sealed, parallel test area of the tube test This load case isflow realized by a constant of a fluid into the sealed, parallel test area of the tube test 3flow specimen. The rate is set to 2 cm /min, which refers to a strain rate of 1.3%/min. Analogous to specimen. The flow rate is set to 2 cm3 3/min, which refers to a strain rate of 1.3%/min. Analogous to specimen. The flow rate is set to 2 cm /min, which refers to a strain rate of 1.3%/min. Analogous to the calculation calculation of of the the true true stresses stresses of of the the tensile tensile tests, tests, the the true true stresses stresses of of the the internal internal pressure pressure tests tests the the calculation of the Formulae true stresses of the(9). tensile tests, the true stresses of the internal pressure tests are are calculated using (8) and The pressure is measured in bar. are calculated using Formulae (8) and (9). The pressure is measured in bar. calculated using Formulae (8) and (9). The pressure p is measured in bar. σ , (8) σ (8) , F σtrue,internal = (8) Aa −2 2 ∙∙ ̅ ̅ ∙∙ 1 1 1 εε ∙∙ ∙∙ 1 ̅̅ − (9) σ , (9) σ , p·( D0 ·(1 2 2+∙∙ εt̅ ̅ ) ∙∙−1 12·s0εε·(1 + εr )) (9) σtrue,internal = 2·s0 ·(1 + εr )
Figure 8.8.Geometric Geometric variables totocalculate calculate the effective cross-sectional area ofofthe the tube specimens. Figure8. Geometricvariables variablesto calculatethe theeffective effectivecross-sectional cross-sectionalarea areaof thetube tubespecimens. specimens. Figure
Figure 999 represents represents the the performed performed tensile tensile and and internal internal pressure pressure load load cases cases in in and and perpendicular perpendicular Figure represents the performed tensile and internal pressure load cases in and perpendicular Figure to the fiber orientation of the tube samples. to the the fiber fiber orientation orientation of of the the tube tube samples. samples. to
◦ ◦ Figure Figure 9. 9. Representation Representation of of the the performed performed tensile tensile and and internal internal pressure pressureload loadcases casesin in00° 0° and and 90 90° fiber fiber Figure 9. Representation of the performed tensile and internal pressure load cases in and 90° fiber orientation of the tube specimen. orientation of the tube specimen. orientation of the tube specimen.
Polymers 2017, 9, 221
15 of 23
It is well known that the mechanical properties, especially the stiffness, of a composite are influenced by its fiber content. To verify the glass fiber content of the compounded granulate types in Polymers 2017, 9,incinerations 221 15 of 23 the study, matrix according to DIN EN ISO 1172 and DIN EN ISO 3451 are implemented. The results of the matrix incinerations are given in Table 12. The fiber weight fractions for the two It is well known that the mechanical properties, especially the stiffness, of a composite are granulate types are averaged for three measurements in each case. For the incineration, a temperature influenced by its fiber content. To verify the glass fiber content of the compounded granulate types of 625 ◦ C for at least 3 h is applied to incinerate all organic substances. in the study, matrix incinerations according to DIN EN ISO 1172 and DIN EN ISO 3451 are implemented. The results of the matrix incinerations are given in Table 12. The fiber weight fractions Table 12. Results of the incineration of the matrix material according to DIN EN ISO 1172 and DIN EN for the two granulate types are averaged for three measurements in each case. For the incineration, a ISO 3451. temperature of 625 °C for at least 3 h is applied to incinerate all organic substances. Granulate Type of the matrixAveraged Glass Fiber Content (wt %)and DIN Table 12. Results of the incineration material according to DIN EN ISO 1172 EN ISO 3451. SC 16.49 ± 0.52 UC
17.92 ± 1.32
Granulate Type
Averaged Glass Fiber Content (wt %)
SC
16.49 ± 0.52
The average glass fiber contents slightly from the aspired 20 weight UC of the granulate types 17.92 ±deviate 1.32 percent of the reference sample material. The influence of the glass fiber sizing on the mechanical averagebyglass fiberten contents the granulate types deviate slightly aspired type. propertiesThe is tested at least tensileoftests and ten internal pressure testsfrom per the specimen 20 weightparameters percent of the sample material. The influence the glass sizing the in The applied of reference the tensile testing are given in Tableof11. The fiber results are on shown is tested by at least ten11. tensile tests and ten internal pressure tests per specimen Tablesmechanical 13 and 14,properties as well as in Figures 10 and type. The applied parameters of the tensile testing are given in Table 11. The results are shown in Tables13. 13Averaged and 14, astensile well asstrengths in Figures 10elongations and 11. Table and at break of the tensile tests of samples with (SC) and without (UC) glass fiber sizing and reference samples. Table 13. Averaged tensile strengths and elongations at break of the tensile tests of samples with (SC) and without (UC) glass fiber sizing and reference samples.
Specimen Name
Specimen Name UC UC SC SC Reference Reference
Tensile Strength (MPa)
Tensile Strength (MPa) 39.7 ± 2.9 39.7 ± 2.9 87.8 ± 1.9 87.8 ± 1.9 87.4 ± 0.6 87.4 ± 0.6
Elongation at Break (-)
Elongation at Break (-) 0.010 ± 0.002 0.010 ± 0.002 0.021 ± 0.003 0.021 ± 0.003 0.014 ± 0.001 0.014 ± 0.001
The results of the tensile testsare aregiven givenby by Figure Figure 10 that that the the The results of the tensile tests 10 and andTable Table13.13.It is It recognizable is recognizable stress-strain behavior is influenced by the presence of a glass fiber sizing. In additional, differences stress-strain behavior is influenced by the presence of a glass fiber sizing. In additional, differences in in the stiffness of the sample are determined specimentypes. types. the stiffness of the sample typestypes are determined for for thethe specimen
Figure 10. Average stress-strain diagramfor forthe the tensile tensile tests with (SC) andand without (UC)(UC) Figure 10. Average stress-strain diagram testsofofsamples samples with (SC) without sizing reference samples (R). glass glass fiber fiber sizing andand reference samples (R).
The tensile tests with loads in the fiber direction of the tube specimens show that the tensile
The tensile with loads in the fiber direction of the tube specimens show the tensile strength andtests the stiffness of the specimens with unsized glass fibers are lower thanthat the tensile strength and the of the thesamples specimens glass fibers are than the tensile strength strength andstiffness stiffness of withwith sizedunsized glass fibers, although the lower fiber content of the samples
Polymers 2017, 9, 221
16 of 23
andPolymers stiffness of the samples with sized glass fibers, although the fiber content of the samples with 2017, 9, 221 16 of 23 unsized glass fibers is higher. In comparison, the stress-strain plot of the tensile tests from the reference specimens and the samples with the sized glass fibers similar plot tensile strengths with unsized glass fibers is higher. In comparison, theshow stress-strain of the tensile and tests stiffnesses, from the butreference the elongation at break higher for the with sized glass fibers. Considering these results, specimens and isthe samples withsamples the sized glass fibers show similar tensile strengths and it isstiffnesses, evident that the glass fiber sizing has an enormous impact on the fiber-matrix adhesion, because but the elongation at break is higher for the samples with sized glass fibers. Considering results, is evident thebreak glass at fiber sizing haslower an enormous impact on the be fiber-matrix the these samples with itunsized glassthat fibers significantly loads. The load cannot transferred adhesion, because the samples with unsized glass fibers break at significantly lower load through the fibers because of a low adhesion between the matrix material and the glassloads. fibersThe resulting cannot be transferred through the fibers because of a low adhesion between the matrix material and from the absent sizing. the glass fibers resulting from the absent sizing. The results of the internal pressure tests for the samples influenced by the glass fiber sizing, as Thereference results ofspecimens the internalare pressure tests for 14 theand samples influenced by the glass fiber sizing, as well as the listed in Table Figure 11. well as the reference specimens are listed in Table 14 and Figure 11. Table 14. Average tensile strengths and elongations at break of the internal pressure tests of samples Table elongations at break of the internal pressure tests of samples with (SC) 14. andAverage withouttensile (UC) strengths glass fiberand sizing and reference samples. with (SC) and without (UC) glass fiber sizing and reference samples. Specimen Name Name Specimen UC UC SC SC Reference Reference
Tensile Strength (MPa)Elongation Elongation at Break (-) Tensile Strength (MPa) at Break (-) 38.8 ± 2.8 0.007 ± 0.001 38.8 ± 2.8 0.007 ± 0.001 55.7 ± 7.4 0.011 ± 0.002 55.7 ± 7.4 0.011 ± 0.002 64.6 ± 1.3 0.014 ± 0.002 64.6 ± 1.3 0.014 ± 0.002
Figure 11. 11. Average stress-strain diagram for the the samples samplesinfluenced influencedbyby Figure Average stress-strain diagramofofthe theinternal internalpressure pressure tests for the the injection molding parameters. injection molding parameters.
determined stress-strain diagramsofofthe theinternal internal pressure pressure tests TheThe determined stress-strain diagrams tests with withloads loadsperpendicular perpendiculartoto the fiber direction of the manufactured tube specimens show similar stiffnesses for every sample the fiber direction of the manufactured tube specimens show similar stiffnesses for every sample type. type. The similarity of the stiffnesses results due to the reduced influence of the fibers on the The similarity of the stiffnesses results due to the reduced influence of the fibers on the mechanical mechanical composite properties when loads are applied perpendicular to the fiber orientation. In composite properties when loads are applied perpendicular to the fiber orientation. In the internal the internal pressure test, the fibers are oriented perpendicular to the applied load, and the load pressure test, the fibers are oriented perpendicular to the applied load, and the load cannot be cannot be transferred through the fibers. In this load case, the fibers only act as inactive fillers, and transferred thematrix fibers.stiffness In this load case, the fibers only act sample as inactive fillers, and therefore, therefore,through only the is measured. The different types show significant only the matrix is measured. differences instiffness the breaking strengths.The different sample types show significant differences in the breakingTable strengths. 15 shows the results of the mechanical testing in the 0° direction to the fiber orientation of Table shows the results of testingparameters in the 0◦ direction to the fiberdesign. orientation the tube15 specimens influenced bythe themechanical injection molding in the experimental The of the tube specimens influenced injection molding parameters in the experimental design. parameters for the applied testsby arethe listed in Table 11. Connected to the values of Table 15, the Thecorresponding parameters for the applied tests are listedininFigure Table12. 11. Connected to the values of Table 15, the stress-strain diagram follows corresponding follows in Figure 12.elongations at break of the specimens differ It can bestress-strain determineddiagram that tensile strengths and the significantly. The Specimen and 3 exhibit tensile strengths higher elongations at It can be determined that Types tensile1strengths and higher the elongations at breakatof the specimens differ break thanThe theSpecimen other specimens. The3mechanical properties can be compared the reference significantly. Types 1 and exhibit higher tensile strengths at higher with elongations at break specimens. Specimen Numbers 2 and 4 fail at lower tensile strengths and smaller strains. However,
Polymers 2017, 9, 221
17 of 23
than the other specimens. The mechanical properties can be compared with the reference specimens. Polymers 2017, 9, 221 2 and 4 fail at lower tensile strengths and smaller strains. However, it can 17 of also 23 Specimen Numbers be recognized that there are only small differences in the stiffnesses of the specimen types. Obviously, the it can also be recognized that there are only small differences in the stiffnesses of the specimen types. stiffness of the samples is not influenced by the injection molding parameters. Obviously, the stiffness of the samples is not influenced by the injection molding parameters.
Table 15. Average tensile strength and elongation at break of the stress-strain behavior from the tensile Table 15. Average tensile strength and elongation at break of the stress-strain behavior from the tests tensile for thetests samples by the injection molding parameters. for theinfluenced samples influenced by the injection molding parameters. Specimen Specimen Name Name 1 1 2 2 3 3 4 4 Reference Reference
TensileStrength Strength(MPa) (MPa) Tensile 95.5 ± 5.3 95.5 ± 5.3 75.9 ± 0.6 75.9 ± 0.6 93.1±± 2.5 2.5 93.1 54.6±± 1.0 1.0 54.6 87.4±± 0.6 0.6 87.4
Elongation at Break Elongation at (-) Break (-) 0.016 ± 0.003 0.016 ± 0.003 0.010 ± 0.002 0.010 ± 0.002 0.016 ±0.016 0.002± 0.002 0.007 ±0.007 0.003± 0.003 0.014 ±0.014 0.001± 0.001
Figure 12. Average stress-strain diagram testsfor forthe the samples influenced byinjection the injection Figure 12. Average stress-strain diagramofofthe the tensile tensile tests samples influenced by the molding parameters. molding parameters.
Table 16 lists the average tensile strengths and elongations at break of the stress-strain behavior
Table 16 lists the average tensile strengths and elongations at break of the stress-strain behavior from the internal pressure tests conducted for the design of experiments. The corresponding from the internal pressure tests conducted for the design of experiments. The corresponding stress-strain curves are shown in Figure 13. stress-strain curves areTable shown in Figure Figure 13 and 16 indicate that13. Specimen Type 3 shows the highest average tensile strength Figure 13±and Tablefor16the indicate Specimen 3 showsspecimens the highest average tensile strength with 66.7 3.5 MPa internalthat pressure tests. Type The reference have an average tensile with strength 66.7 ± 3.5 MPa± 1.3 forMPa. the internal pressure The reference specimens have anwhereas averagethe tensile of 64.5 The elongations at tests. break of Specimen Type 3 amount to 0.016, elongation of the references amounts to 0.013. Specimen Types 2 and 4 show significantly lower strength of 64.5 ± 1.3 MPa. The elongations at break of Specimen Type 3 amount to 0.016, whereas tensile strengths elongations at break theSpecimen residual specimen stiffnesses oflower the elongation of theand references amounts tothan 0.013. Types 2types. and 4While showthe significantly Specimen 2 and 3 and the reference sample are the comparable, 4 and 1 show lower stiffnesses. tensile strengths and elongations at break than residualSamples specimen types. While the stiffnesses of Hence, it can be concluded that there are significant differences in the fracture behavior of the Specimen 2 and 3 and the reference sample are comparable, Samples 4 and 1 show lowervarious stiffnesses. specimen types. The injection molding parameters influence the tensile strengths and the elongations Hence, it can be concluded that there are significant differences in the fracture behavior of the various at break of the samples appreciably, while the elastic moduli of the samples remain comparable. specimen types. The injection molding parameters influence the tensile strengths and the elongations at break of the samples appreciably, while theelongation elastic moduli remain comparable. Table 16. Averaged tensile strength and at breakof ofthe the samples stress-strain behavior of the internal pressure tests for the samples influenced by the injection molding parameters.
Table 16. Averaged tensile strength and elongation at break of the stress-strain behavior of the internal Specimen Name Tensile Strength (MPa) Elongation at Break (-) pressure tests for the samples influenced by the injection molding parameters. 1 2 Specimen Name 3 4 1 2 Reference 3 4 Reference
52.0 ± 1.2 35.4 ± 2.6 Tensile Strength (MPa) 66.7 ± 3.5 21.6 ± 5.7 52.0 ± 1.2 35.4 ± 2.6 64.6 ± 1.3 66.7 ± 3.5 21.6 ± 5.7 64.6 ± 1.3
0.016 ± 0.002 0.007 ± 0.003 Elongation 0.016 ± 0.006at Break (-) 0.0050.016 ± 0.002 ± 0.002 ± 0.003 0.0140.007 ± 0.005 0.016 ± 0.006 0.005 ± 0.002 0.014 ± 0.005
Polymers 2017, 9, 221 Polymers 2017, 9, 221 Polymers 2017, 9, 221
18 of 23 18 of 23 18 of 23
Figure Average stress-straindiagram diagramof ofthe the internal internal pressure byby Figure 13.13. Average stress-strain pressure tests testsfor forthe thesamples samplesinfluenced influenced Figure 13. Average stress-strain diagram of the internal pressure tests for the samples influenced by injection molding parameters. thethe injection molding parameters. the injection molding parameters.
6. Correlation of the InterphaseProperties Properties with the the Mechanical Mechanical Behavior 6. Correlation ofof the Behavior 6. Correlation theInterphase Interphase Properties with with the Mechanical Behavior To correlate the formation of the interphase with the mechanical properties, an evaluation of the ToTo correlate the formation the mechanical mechanicalproperties, properties,ananevaluation evaluation correlate the formationofofthe theinterphase interphase with with the of of thethe experimental design is implemented by MiniTab 17, a statistical evaluation program. The correlation experimental design is implemented by MiniTab 17, a statistical evaluation program. The correlation experimental design is implemented by MiniTab 17, a statistical evaluation program. The correlation of of the measured widths of the interphases and the tensile strength of the load in 0° to the fiber ◦ to the theofmeasured widths of the of interphases and theand tensile strengthstrength of the load in 0load fiber orientation the measured widths the interphases the tensile of the in 0° to the fiber orientation (tensile◦ tests) and 90° to the fiber orientation (internal pressure tests) are shown in three (tensile tests) (tensile and 90 tests) to theand fiber pressure tests) are shown in three main effects orientation 90°orientation to the fiber(internal orientation (internal pressure tests) are shown in three main effects plots in Figures 14–16. Main effects plots quantify the impacts of the respective factors plots in effects Figuresplots 14–16. Main effects thequantify impactsthe of the respective factors to factors a quality main in Figures 14–16. plots Main quantify effects plots impacts of the respective to a quality feature (here: “tensile strengths” in 0° to the fiber orientation for the tensile tests, “tensile to a quality feature (here: “tensile in 0°orientation to the fiberfor orientation fortests, the tensile tests, “tensile in feature (here: in “tensile strengths” in strengths” 0◦ to the tensile “tensile strengths” strengths” 90° to the fiber orientation forfiber the internal pressurethe tests and “width of the interphase” ◦ in 90° to the fiber for the internal pressure tests and “width of the interphase” 90 strengths” thehigh fiber orientation fororientation the internal tests and the “width of the in thethe high intothe and the low level setting). In apressure main effects plot, gradient ofinterphase” the line describes inthe thelow highlevel and setting). the low level setting). In aplot, main effects plot,ofthe gradient of the line describes of thethe and In a main effects the gradient the line describes the influence influence of the factor on the experimental results. The larger the gradient of the line, the higher the influence the factor on the experimental results. The larger the gradient of the line, the higher the factor on theof experimental results. influence on the considered factor.The larger the gradient of the line, the higher the influence on the influence on the considered factor. considered factor.
Figure 14. Main effects plot of tensile tests for the influencing factors “mass temperature”, “holding Figure Main effectsplot plotofoftensile tensiletests tests for the the influencing influencing factors “mass “holding Figure 14.14. Main factors “masstemperature”, temperature”, “holding pressure” and effects “mold temperature” on the for tensile load in the fiber orientation. pressure” and “mold temperature” on the tensile load in the fiber orientation. pressure” and “mold temperature” on the tensile load in the fiber orientation.
Figure 14 shows the main effects plot for the influencing factors temperature, holding pressure Figure 14 shows the main effects plot for the influencing factors temperature, holding pressure and mold14 temperature the effects tensile load in 0°the to influencing the fiber orientation. The mass temperature of the Figure shows theon main plot for factors temperature, holding pressure and mold temperature on the tensile load in 0° to the fiber orientation. The mass temperature of the ◦ melt has the highest effect on the tensile strength, while the factors holding pressure and mold and mold on theontensile load in 0 to the fiber The pressure mass temperature melt hastemperature the highest effect the tensile strength, while theorientation. factors holding and mold of temperature show less impact. Additionally, the lower level settings are connected with the higher thetemperature melt has the highest effect onAdditionally, the tensile strength, the factors pressure mold show less impact. the lowerwhile level settings are holding connected with theand higher tensile strengths. temperature show less impact. Additionally, the lower level settings are connected with the higher tensile strengths. tensile strengths.
Polymers 2017, 9, 221 Polymers Polymers 2017, 2017, 9, 9, 221 221
19 of 23 19 19 of of 23 23
Figure 15. 15. Main Main effects effects plot plot of of tensile tensile tests tests for for the the influencing influencing factors factors “mass “mass temperature”, temperature”, “holding “holding Figure pressure” and “mold temperature” on the tensile load perpendicular to the fiber orientation. pressure” and “mold temperature” on the tensile load perpendicular to the fiber orientation.
Figure 15 15 shows shows the the main main effect effect plot plot for for the the tensile tensile load load perpendicular perpendicular to to the the fiber fiber orientation orientation Figure impacted by the internal pressure tests. Once again, the mass temperature of the melt shows the the impacted by the internal pressure tests. Once again, the mass temperature of the melt shows biggest impact on the mechanical properties of the specimens, whereas the effects of the holding biggest impact on the mechanical properties of the specimens, whereas the effects of the holding pressure and and the the mold mold temperature temperature are are comparatively comparatively low. low. The The respectively respectively lower lower level level settings settings for for pressure the factors relate to the higher tensile strengths. the factors relate to the higher tensile strengths.
Figure 16. 16. Main Main effects effects plot plot for for the the width width of of the the overall overall interphase interphase for for the the influencing influencing factors factors “mass “mass Figure temperature”, “holding pressure” and “mold temperature”. temperature”, “holding pressure” and “mold temperature”.
The main effects plot pictured in Figure 16 shows that the mass temperature has the biggest The main effects plot pictured in Figure 16 shows that the mass temperature has the biggest effect on the width of the interphase. Additional, a major impact of the mass temperature can be seen. effect on the width of the interphase. Additional, a major impact of the mass temperature can be seen. For both factors, the widest interphase is measured for the low level settings. The mold temperature For both factors, the widest interphase is measured for the low level settings. The mold temperature can be detected as the smallest influence on the system. Concerning Figure 16, the parameter settings can be detected as the smallest influence on the system. Concerning Figure 16, the parameter settings of the mass temperature and the holding pressure both have an impact on the interphase. At the of the mass temperature and the holding pressure both have an impact on the interphase. At the lower lower mass temperature or rather the lower holding pressure, a wider interphase is expected. mass temperature or rather the lower holding pressure, a wider interphase is expected. Conversely, Conversely, a smaller interphase is formed by a higher mold temperature. This effect can be a smaller interphase is formed by a higher mold temperature. This effect can be explained by a higher explained by a higher cooling rate of the melt [16]. The three factors “mass temperature”, “holding cooling rate of the melt [16]. The three factors “mass temperature”, “holding pressure” and “mold pressure” and “mold temperature” influence the width of the interphase by affecting the temperature” influence the width of the interphase by affecting the crystallization kinetics of the matrix crystallization kinetics of the matrix material. The highest impact on the mechanical properties and also on the interphase is given by the mass temperature. The experimental design shows a clear
Polymers 2017, 9, 221
20 of 23
material. The highest impact on the mechanical properties and also on the interphase is given by the mass temperature. The experimental design shows a clear dependency between the tested factors and the resulting effects in the main effects plots. However, the interaction between the factors themselves cannot be dissolved because of the experimental design type. Further tests to complete the fractional factorial design to a full factorial design have to be implemented to recognize the interactions between the factors. Regarding the stress-strain curves of the mechanical testing and the determination of the interphase width, a correlation between interphase width and material strength can be stated. The study to determine the influence of the injection molding settings shows that the settings influence the formation of the fiber-matrix-interphase and therefore have an impact on the tensile strength, as well as the elongation at break. The width of the interphase does not affect the stiffness, but the fracture behavior, including tensile strength and elongation at break. Comparing the results of the nano-scratch tests in Figure 5 and the tensile testing in Figure 12, the tensile strength and elongations at fracture correlate with the width of the interphase. The widest interphases and breaking strengths are measured for the reference specimen and Specimen Type 1, and furthermore, the smallest interphase and the lowest breaking strengths are detected for Specimen Types 2 and 4. 7. Summary and Conclusions The present study is focused on the influence of the processing parameters on the interphase formation in short fiber-reinforced thermoplastics. For this purpose, different experimental methods are applied to determine the mechanical properties of the composites regarding the existence of a fiber-matrix interphase. To characterize the macroscopic mechanical properties, tensile tests are carried out in and perpendicular to the fiber orientation of the injection molded test specimen. The microstructural study of the interphase is done by means of a nano-tribological indentation technique according to the methodology of Schöneich et al. [4]. In this method, a cube corner indenter tip of a nano-indenter is impressed into the thermoplastic matrix and moved up to the glass fiber at a constant depth. The progression of the controlled normal force to maintain the constant scratch-depth is referred to an interphase between the matrix and the fiber. The knowledge gained from the study is summarized below: The fiber sizing affects the fiber-matrix adhesion and the fracture properties due to the interphase formation. (Hypothesis 1) Sized and unsized glass fibers are compounded into a matrix material to determine the influence of the glass fiber sizing on the fiber-matrix adhesion and samples that are injection molded. Additionally, equivalent industrial-grade composite samples are injection molded using the same injection molding parameters. The samples are tested in fiber orientation direction (tensile tests) and perpendicular to the fiber orientation direction (internal pressure tests), and the width of the interphase is examined by means of nano-scratching. It can be shown that samples with unsized glass fibers have lower tensile strengths and significantly smaller interphases than the samples with coated glass fibers. Furthermore, the nano-scratch results display an additional thickness around 60 nm, which is induced by the sizing of glass fibers. The results show that the fiber sizing has a huge impact on the fiber-matrix adhesion and therefore on the breaking strength. The formation and the width of the interphase are influenced by the processing parameters of the injection molding process by crystallization kinetics. (Hypothesis 2) The evaluation of the experimental results in the DoE study reveals that the melt temperature has the highest influence on the tensile strength in the 0◦ and 90◦ fiber orientation, as well as on the interphase width. As a consequence, the highest tensile strengths and widest interphases are identified
Polymers 2017, 9, 221
21 of 23
at a lower melt temperature. The holding pressure and the mold temperature have a smaller effect on the tensile strength and the interphase width. Regarding the holding pressure, higher strengths and interphase widths are measured at the lower factor level. At a higher mold temperature, a wider interphase is measured, but a slight reduction of the tensile strengths of the investigated samples is also detected. However, the performed DoE study does not allow further conclusions about interactions and commingling of the individual process parameters, since they are not explicitly resolved in screening test plan. The interphase is not only a crystallization phenomenon for semi-crystalline thermoplastics. (Hypothesis 3) In semi-crystalline thermoplastic composites, the presence of an interphase can be explained by concentrated nucleation sites and increased spherulitic formation in the vicinity of the fiber material. Amorphous thermoplastics, however, do not contain crystalline structures due to their near order polymer chain configuration. In this work, several samples of short glass fiber-reinforced amorphous thermoplastics (ABS-GF20) are produced and tested for their interphase via nano-scratching. As a result of the present contribution, fiber-matrix interphases can be identified in amorphous polymer composites, as well. Therefore, the interphase cannot solely be described as a crystallization phenomenon. In the formation process of the interphase, additional thermodynamic aspects, such as the interdiffusion of the matrix macromolecules, as well as the interaction with the fiber coating, are of challenging importance and should be studied further. The width of the interphase correlates with the mechanical behavior of thermoplastics. (Hypothesis 4) The results of the performed tensile tests, as well as the nano-scratches produced with different injection molding parameters show that the width of the interphase can be influenced by the parameters “melt temperature”, “holding temperature” and “mold temperature”. The resulting interphase widths and tensile strengths, as well as the elongations at break for the processed samples vary significantly while the stiffnesses remain comparable in the mechanical tests. A correlation of the interphase thickness with the breaking strength and the elongation at break can be seen. The interphase is an important area for the adhesion between the matrix material and the fibers. In further studies, the influence of the interphase on the mechanical long time behavior will be studied in fatigue tests. Acknowledgments: The authors thank Christian Motz and Mohammad Zamanzade for the possibility to use the TriboIndenter associated with the chair “Materials Science and Methods” at Saarland University, Saarbrücken, Germany. This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors. Author Contributions: Anna Katharina Sambale, Marc Schöneich and Markus Stommel conceived and designed the experiments; Anna Katharina Sambale performed the experiments; Anna Katharina Sambale, Marc Schöneich and Markus Stommel analyzed the data; Anna Katharina Sambale wrote the paper with support from Marc Schöneich. Conflicts of Interest: The authors declare no conflict of interest.
References 1. 2. 3. 4. 5.
Bonnet, M. Kunststoffe in der Ingenieuranwendung. Verstehen und Zuverlässig Auswählen; Vieweg + Teubner: Wiesbaden, Germany, 2009. Witten, E. Handbuch Faserverbundkunststoffe/Composites. Grundlagen, Verarbeitung, Anwendungen; Springer: Wiesbaden, Germany, 2014. Ehrenstein, G.W. Faserverbund-Kunststoffe. Werkstoffe-Verarbeitung-Eigenschaften; Hanser: München, Germany, 2006. Schöneich, M.; Zamanzade, M.; Stommel, M. Fiber-matrix interphase in applied short glass fiber composites determined by a nano-scratch method. Compos. Sci. Technol. 2015, 119, 100–107. [CrossRef] Cech, V.; Palesch, E.; Lukes, J. The glass fiber–polymer matrix interface/interphase characterized by nanoscale imaging techniques. Compos. Sci. Technol. 2013, 83, 22–26. [CrossRef]
Polymers 2017, 9, 221
6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
21. 22. 23. 24.
25. 26. 27. 28. 29. 30. 31. 32. 33.
22 of 23
Drzal, L.T.; Rich, M.J.; Lloyd, P.F. Adhesion of Graphite Fibers to Epoxy Matrices. I. The Role of Fiber Surface Treatment. J. Adhes. 2006, 16, 1–30. [CrossRef] Papanicolaou, G.C.; Theocaris, P.S.; Spathis, G.D. Adhesion efficiency between phases in fibre-reinforced polymers by means of the concept of boundary interphase. Colloid Polym. Sci. 1980, 258, 1231–1237. [CrossRef] Hodzic, A.; Stachurski, Z.; Kim, J. Nano-indentation of polymer–glass interfaces Part I. Experimental and mechanical analysis. Polymer 2000, 41, 6895–6905. [CrossRef] Munz, M. Evidence for a three-zone interphase with complex elastic–plastic behaviour: Nanoindentation study of an epoxy/thermoplastic composite. J. Phys. D Appl. Phys. 2006, 39, 4044–4058. [CrossRef] Kim, J.-K.; Sham, M.-L.; Wu, J. Nanoscale characterisation of interphase in silane treated glass fibre composites. Compos. Part A Appl. Sci. Manuf. 2001, 32, 607–618. [CrossRef] Gao, S.-L.; Mäder, E. Characterisation of interphase nanoscale property variations in glass fibre reinforced polypropylene and epoxy resin composites. Compos. Part A Appl. Sci. Manuf. 2002, 33, 559–576. [CrossRef] He, T.; Porter, R.S. Melt transcrystallization of polyethylene on high modulus polyethylene fibers. J. Appl. Polym. Sci. 1988, 35, 1945–1953. [CrossRef] Pompe, G.; Mäder, E. Experimental detection of a transcrystalline interphase in glass-fibre/polypropylene composites. Compos. Sci. Technol. 2000, 60, 2159–2167. [CrossRef] Thomason, J.L.; van Rooyen, A.A. Transcrystallized interphase in thermoplastic composites. J. Mater. Sci. 1992, 27, 897–907. [CrossRef] Varga, J.; Karger-Kocsis, J. The difference between transcrystallization and shear-induced cylindritic crystallization in fibre-reinforced polypropylene composites. J. Mater. Sci. Lett. 1994, 13, 1069–1071. Moneke, M. Die Kristallisation von verStärkten Thermoplasten Während der Schnellen Abkühlung und Unter Druck; Technische Universität Darmstadt: Darmstadt, Germany, 2001. Quan, H.; Li, Z.-M.; Yang, M.-B.; Huang, R. On transcrystallinity in semi-crystalline polymer composites. Compos. Sci. Technol. 2005, 65, 999–1021. [CrossRef] Thomason, J.; Adzima, L. Sizing up the interphase. An insider’s guide to the science of sizing. Compos. Part A Appl. Sci. Manuf. 2001, 32, 313–321. [CrossRef] Mäder, E. Study of fibre surface treatments for control of interphase properties in composites. Compos. Sci. Technol. 1997, 57, 1077–1088. [CrossRef] Zorn, H. Alkaliresistente Glasfasern—Von der Herstellung bis zur Anwendung. Textile Reinforced Structures. In Proceedings of the 2nd Colloqium on Textile Reinforced Structures (CTRS2), Dresden, Germany, 29 September–1 October 2003; p. 528. Teschner, R. Glasfasern; Springer: Berlin/Heidelberg, Germany, 2013. Pristavok, J. Mikromechanische Untersuchungen an Epoxidharz-Glasfaser-Verbundwerkstoffen unter Zyklischer Wechselbelastung; Technische Universität Darmstadt: Darmstadt, Germany, 2006. Bergeret, A.; Bozec, M.P.; Quantin, J.-C.; Crespy, A.; Gasca, J.-P.; Arpin, M. Study of interphase in glass fiber-reinforced poly(butylene terephthalate) composites. Polym. Compos. 2004, 25, 12–25. [CrossRef] Voleppe, Q.; Pardoen, T.; Bailly, C. Interdiffusion and phase separation upon curing in thermoset-thermoplastic interphases unravelled by the characterization of partially cured systems. Polymer 2016, 106, 120–127. [CrossRef] Cheng, X.; Putz, K.W.; Wood, C.D.; Brinson, L.C. Characterization of local elastic modulus in confined polymer films via AFM indentation. Macromol. Rapid Commun. 2015, 36, 391–397. [CrossRef] [PubMed] Park, C.-S.; Lee, K.-J.; Nam, J.-D.; Kim, S.-W. Crystallization kinetics of glass fiber reinforced PBT composites. J. Appl. Polym. Sci. 2000, 78, 576–585. [CrossRef] Chisholm, B.J.; Fong, P.M.; Zimmer, J.G.; Hendrix, R. Properties of glass-filled thermoplastic polyesters. J. Appl. Polym. Sci. 1999, 74, 889–899. [CrossRef] Lanxess. Datenblatt. Pocan B 3225 000000; Lanxess: Cologne, Germany, 2016. A. Schulman. Datenblatt. Polyman FABS 20 GF; A. Schulman GmbH: Akron, OH, USA, 2016. Lanxess. Datenblatt. Pocan B 1305 000000; Lanxess: Cologne, Germany, 2016. Lanxess. Datenblatt. Lanxess-Glasfaser MF7980; Lanxess: Cologne, Germany, 2015. Lanxess. Datenblatt. Lanxess-Glasfaser CS 7968 4,5MM; Lanxess: Cologne, Germany, 2015. Siebertz, K.; van Bebber, D.; Hochkirchen, T. Statistische Versuchsplanung. Design of Experiments (DoE); Springer: Heidelberg, Germany, 2010.
Polymers 2017, 9, 221
34. 35.
36.
37.
23 of 23
Ferrano, F.; Lipka, A.; Stommel, M. Der Einfluss von Prozessparametern auf das mechanische Verhalten kurzfaserverstärkter Kunststoffbauteile in Lenksystemen. Kunstst. J. Plast. Technol. 2015, 1, 271–304. [CrossRef] Sambale, A.-K. Einfluss der Verarbeitungsparameter auf die Faser-Matrix Interphase in kurzfaserverstärkten Thermoplasten. Master’s Thesis, Universität des Saarlandes/Technische Universität Dortmund, Dortmund, Germany, 2016. Kaiser, J.-M. Beitrag zur Mikromechanischen Berechnung Kurzfaserverstärkter Kunststoffe—Deformation und Versagen. Available online: http://scidok.sulb.uni-saarland.de/volltexte/2013/5560/pdf/Beitrag_ zur_mikromechanischen_Berechnung_kurzfaserverstArkter_Kunststoffe_Deformation_und_Versagen_ Kaiser.pdf (accessed on 12 April 2017). Welsch, F. Entwicklung eines Prüfverfahrens zur Bestimmung biaxialer Werkstoffkennwerte von Kunststoffen. Master’s Thesis, Saarland University, Saarbrücken, Germany, December 2014. © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
