The Effect of Injection Speed on Morphology and Mechanical ... - Core

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ScienceDirect Energy Procedia 56 (2014) 57 – 64

11th Eco-Energy and Materials Science and Engineering (11th EMSES)

The effect of injection speed on morphology and mechanical properties of Polyoxymethylene/Poly(lactic acid) blends Suchalinee Mathurosemontria *, Putinun Auwongsuwana, Satoshi Nagaib, Hiroyuki Hamadaa F

b

a Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto, 606-8585, Japan Mitsubishi Engineering Plastic Corporation, Hiratsuka, _kanagawa, 254-0016, Japan

Abstract Polyoxymethylene (POM)/Poly(lactic acid)(PLA) blends were injection molded at three different injection speeds as 50 mm/s, 100 mm/s and 300 mm/s for investigation the effect of injection speed on morphology and mechanical properties of this blends. The tensile test was carried out for examination of mechanical properties. Morphology was observed by polarized optical microscope and polarizing micrographs that was supported by Raman spectra. At injection speed of 50 mm/s, mechanical properties of POM/PLA blends increased with decreasing POM content but when 20 %wt of POM was added into PLA, the results dropped obviously due to a poor distribution of both POM and PLA phases. That can be confirmed by polarizing micrographs and Raman spectra. It was found that the POM-rich phase mostly combined in the core area. The dispersion of POM-rich phase expanded to a wide area when the injection speed increased to 100 mm/s and 300 mm/s, respectively. The improvement of distribution of POM-rich phase has an influence on the enhancement of mechanical properties. Tensile modulus of POM 20 wt.% increased from 2.8 GPa to 3.0 GPa while its tensile strength increased from 50.5 MPa to 55 MPa. Thus, the high injection speed can enhanced the phase distribution and mechanical properties at low POM content but has not a significant effect when POM content over 40 %wt. © 2014 2014Elsevier The Authors. Published byaccess Elsevier Ltd. © Ltd. This is an open article under the CC BY-NC-ND license Energy System, Rajamangala University of Technology Thanyaburi Peer-review under responsibility of COE of Sustainalble (http://creativecommons.org/licenses/by-nc-nd/3.0/). (RMUTT). under responsibility of COE of Sustainalble Energy System, Rajamangala University of Technology Thanyaburi (RMUTT) Peer-review Keywords: Polyoxymethylene; Poly(lactic acid); Injection molding; Injection speed; Polymer blend

* Corresponding author. Tel.:+81-906-552-085-1. E-mail address: zucha_k_t_89@hotmail.com

1876-6102 © 2014 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of COE of Sustainalble Energy System, Rajamangala University of Technology Thanyaburi (RMUTT) doi:10.1016/j.egypro.2014.07.131

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Suchalinee Mathurosemontri et al. / Energy Procedia 56 (2014) 57 – 64

1. Introduction Polyoxymethylene (POM) is an important engineering thermoplastic that was derived from formaldehyde synthesis, which has been found an application in the fields of vehicles, electronics applications and precision machinery. POM has good-balance mechanical properties, excellent creep resistance, and excellent thermal stability during molding, low friction coefficient as well as excellent anti-wear properties. However, the disadvantage as its low toughness and difficult decorated surface are a limitation of application [1-3]. To improve the mechanical properties of POM, the polymer blending is an efficient and convenient technique to overcome the disadvantages of polymer. Sometime the blending technique does not always improve the mechanical properties. They may be intermediate between those of the individual components. In some case, they show either negative or positive deviation from intermediate feature. In other that the mechanical of polymer blends are depend on the miscibility of polymers. There are too many factors that affect in miscibility and immiscibility of polymer blend such as polarity, specific group attraction, molecular weight and ratio as well as crystallinity [4]. However, several literatures have been reported on POM blending with polymer such as Poly (ethylene-glycolco-cyclohexane-1, 4-demethanol terephathalate) (PETG), which is an amorphous copolyester that can improved the ductility and toughness of POM [5]. POM has been blended with poly(ethylene oxide) (PEO) and exhibited a desirable notched impact strength [6]. In 2011, there was reported on miscibility of POM and poly (lactic acid) (PLA) [7], which is thermoplastic that can be biodegradation due to it derived from renewable resources such as corn or sugarcane. Nowadays, PLA has gained in replacing petroleum-based plastic in consumer good applications, with primary uses limited to biomedical applications such as sutures and developed to packaging, fibers, paper coating and film. The mechanical properties of PLA were high stiffness and strength. However, the disadvantage of the PLA was extremely brittle and low toughness [8]. That report still showed the extremely high elongation at break of the POM/PLA blends although the tensile strength reduced when POM content increased. Morphology of POM/PLA blend shown an uneven fracture surface but it did not show a phase separation. In other word, POM/PLA blend is a miscible blend, which may be both POM and PLA is a semi-crystalline polymer and there are the similar melting temperature and melt flow rate. With the related characteristics of them, it is easy to manage the viscosity and to obtain optimum dispersion [9]. According the difference and the similarity of both polymers as mentioned before, it is interesting to study the mechanical properties of this blend system at the various contents. However, the mechanical properties of polymer associated with the morphology. Thus, the effect of processing parameters on morphology and mechanical properties were studied in this research. 2. Experimental 2.1. Materials The commercial grades of POM (copolymer) used in this study was obtained by Mitsubishi Engineering Plastic Co, Ltd., namely Lupital V20-HE, which is a good wear resistance property. PLA grade TERRAMAC TE-2000 was obtained by Unitika Ltd. for blending with POM. The properties of POM and PLA were listed in Table 1. Table 1. Material properties Material

Density (g/cm3)

MFR (g/10min)

Tm (qC)

@ 190 qC, 2.16 kg POM (V20-HE)

1.41

10

155

PLA (TE-2000)

1.25

10

165

2.2. Specimen preparation POM and PLA pellets were dried in an oven under the temperature of 80˚C for 16 hours. After that they were mixed together by a dry blending technique. The composition of POM/PLA blends were listed in Table 2. The


blends were injection molded into dumbbell shaped specimen by using the single screw injection molding (Po Yuen UM50). The barrel temperatures were set at 140, 170, 180, 190 and 190˚C from hopper to die while mold temperature and cooling time were 30˚C and 20sec, respectively. The injection speeds were varied by 50, 100 and 300 mm/s. Table 2. Composition and specimen codes Code

POM content (wt.%)

PLA content (wt.%)

POM0

0

100

POM20

20

80

POM40

40

60

POM60

60

40

POM80

80

20

POM100

100

0

2.3. Mechanical testing Tensile test was carried out by an Instron universal testing machine (Instron model 4206) with testing speed 10 mm/min and grip distance of 115 mm. The tensile modulus of elasticity was measured by using strain gauge extensometer at room temperature. 2.4. Polarized optical microscope (POM) The cross section of POM/PLA blends specimens were cut on the center position and delicately polished to a mirror finish. The rough surface was be smoothed by using the sandpaper index from 200, 400, 800, 1200 and 2000, respectively. The alumina suspension in water was used in the hindmost process. The damage from heat and flushed away the debris between the process can avoided by allowing water flow over the specimen and sandpapers. The final thickness of specimen was around 30 μm. The specimens were observed by polarized optical microscope with x10 magnification. 2.5. Raman analysis The specimens were polished similar to the polish process that was mentioned before but the final thickness was around 10 mm and placed under x10 magnification in the Raman spectroscope, which made by Renishaw (Nagoya, Japan). The laser excitation source was 633 nm wavelength, spectral range was from 400 – 4000 cm-1. Fig.1 shows the cross section size and measuring position along X axis and Y axis. The distance between each position was 20 μm.

Fig. 1. Cross section size and measuring position.

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3. Results and discussions 3.1. Morphology of polymer blends The final products by injection molding of POM/PLA blends indicated the visible non-uniform characteristic of dumbbell specimens as show in Fig.2. Cross section of POM/PLA blends specimens were observed by polarize optical microscope to investigate the non-uniform characteristic of POM and PLA blends.

(a)

(b) (a)

(c)

(d)

(e)

(f)

Fig. 2. Dumbbell specimens by injection molding of POM/PLA blends neat PLA; (b) POM 20; (c) POM 40; (d) POM 60; (e) POM 80; (f) neat POM.

Generally the specimens molded by injection molding with the processing concept of imparts high stress and rapid cooling led to a clear multiphase structure of skin layer and a core layer. The polished cross section is observed with a polarized optical microscope. The relative thickness of the skin and core vary depending on the material rheological and process conditions. The skin regions occur due to the fountain flow at regions adjacent to the mold is quenched by the cold mold wall rapidly. The region in the frozen layer will be highly elongated, which is called the shear layer while the cooling rate at the core region is considerably lower than skin layer [10-13]. Polarizing micrographs of POM, PLA and POM/PLA blends shown in Fig.3. It can be investigated that the polarizing micrograph of the neat PLA showed the more transparency than that of the neat POM. This is expected because the crystallinity of PLA is normally lower than that of POM. This explanation was supported by the previous study [7]. As mentioned above, there was reported on miscibility of POM and poly (lactic acid) (PLA). However, regarding the effect of POM/PLA blends, the polarizing micrographs of the POM/PLA blends at blending ratio between 40 – 80 wt% of POM showed distribution of translucent crystalline phase of POM throughout the cross-section of the specimens. While agglomeration of the POM rich phase at the center of the specimen was appeared for the blend at 20 wt% of POM, indicating the poor distribution of the minor phase (POM) in the POM/PLA blend. Moreover, these polarizing micrographs indicate the partial miscibility behavior between POM and PLA blend system.

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 3. Polarizing micrographs of cross section of POM/PLA blends (a) neat PLA; (b) POM20; (c) POM 40; (d) POM 60; (e) POM 80; (f) neat POM.

The results by Raman analysis were employed to substantiate the skin-core layer of the blend. Two sets of Raman shift peaks were selected for intensity analysis of the blend compositions, the 872 (PLA) and 917 (POM) shifts that


917

872

are representative of the backbone chemical structure in the respective polymers as shown in Figure 4 [14, 15]. It was found that an increase in the POM content resulted in monotonic increase and decrease of the lines peaked at 872 and 917 cm-1, respectively. The ratio of I917/(I872+I917) which is the intensity of POM content was calculated for identification phase of POM illustrated in Fig.4. From the results in Fig.5 found that the ratio of I917/(I872+I917) along X axis and Y axis did not smooth and indicated a high fluctuation at the translucent area of specimen. This results confirmed that the translucent phase in the POM/PLA blends at blending 20 wt% of POM was a POM-rich phase.

Raman intensity

POM 0 POM 20 POM 40 POM 60 POM 80 POM 100 Raman Shift / cm

-1

Fig. 4. Raman shift of POM/PLA blends.

X axis

Y axis

Fig. 5. Raman shift of POM/PLA blends with 20 wt% POM.

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3.2. Mechanical properties of polymer blends Tensile modulus of POM, PLA and their blends were shown in Fig.6(a). It was found that POM/PLA blends system at 40, 60 and 80 wt.% of POM indicated the intermediate values between neat POM and neat PLA, which has been called additive behavior. However, at 20 wt.% of POM shown the high negative results from rule of mixture. Tensile strength of POM/PLA blends revealed the unsynergistic behavior as illustrated in Fig.6(b). The tensile strength exhibited a negative deviation from the rule of mixture. This was because the partial miscible behavior of POM and PLA as previous discussed.

(a)

3.5 3 2.5 2 1.5 0

20

40

60

80

Tensile strength (MPa)

Tensile modulus (GPa)

4

75 70 65 60 55 50 45 40 35

(b)

0

100

POM contents (wt.%)

20

40

60

80

100

POM contents (wt%)

Fig. 6. (a) Tensile modulus; (b) Tensile strength of POM/PLA blends at injection speed of 50 mm/s.

3.3. Mechanical properties of polymer blends at the different injection speeds It was found that tensile modulus still indicated a similar tendency to the results of POM/PLA blends at injection speed 50 mm/s. The results of 40, 60 and 80 wt% of POM did not obviously show the deviation when the injection speed was adjusted to 100 and 300 mm/s, respectively. Those were similar to tensile strength. However, the adjustment of injection speed to 20 wt% of POM exhibited the increasing of mechanical properties as presented in Fig.7 (a) and (b). Tensile modulus increased and got close the rule of mixture line. While its tensile strength increased over the result of 40 wt% of POM.

Tensile Modulus (GPa)

(a)

3.5 3.0 2.5 2.0

Tensile Strength (MPa)

75

4.0

(b)

70 65 60 55 50 45 40 35

1.5 0

20

40

60

80

POM content (%wt) 50 mm/s

100 mm/s

100

0

20

40

60

80

100

POM content (%wt)

300 mm/s

Fig. 7. (a) Tensile modulus; (b) Tensile strength of POM/PLA blends at injection speed of 50, 100 and 300 mm/s.

The increasing of mechanical properties at 20 wt% of POM can be described by the polarizing micrographs as shown in Fig.8. It was found that the increasing of injection speed led to a better distribution of minor phase as POM. Even a polarizing micrographs at injection speed 100 mm/s and 300 mm/s did not show the complete distribution, the distribution of POM rich phase on the core broaden out obviously. There was cause from the


difference of shear viscosity. In general polymer which is a low shear viscosity can easy to bleed out from core to outside.

(a)

(b)

(c)

Fig. 11 Polarizing micrographs of cross section of POM/PLA blends at 20 wt% of POM (a) 50 mm/s; (b) 100 mm/s; (c) 300 mm/s.

4. Conclusion The effect of injection parameter on morphology and mechanical properties of POM/PLA blends were investigated. The following conclusions can be drawn. At injection speed 50 mm/s : x Morphology of 20 wt% POM specimen exhibited a non-uniform of phase distribution between POM and PLA obviously whereas another contents shown a normal uniform. x Mechanical properties of POM/PLA blends indicated the intermediate values between POM and PLA whereas at 20 wt% POM was quite low. After increased injection speed : x Morphology of 20 wt% POM specimen exhibited a better phase distribution between POM and PLA obviously. x Mechanical properties of 20 wt% POM increased with the increasing of injection speed. References [1] Xiaodong W, Hangquan LI, Riguang J, Study on the miscibility and phase behavior of polyoxymethylene with novolak, J Master Sci Technol Commun 2000;16:427-430. [2] Xiaoling G, Cheng Q, Qiang F, Toughening mechanism in polyoxymethylene/thermoplastic polyurethane blends, Polym Int Commun 2004;53:1666-71. [3] Pecorini T. J, Hertzberg R. W, Manson J. A, Structure-property relations in an injection-moulded, rubber-toughened, semicrystalline polyoxymethylene, J Mater Sci Commun 1990;25:3385-95. [4] Ramiro J, Eguiazabal J. I, Nazabal J, Synergistic mechanical behaviour and improved processability of poly (etherimide) by blending with poly (trimethylene terephthalate), Polym Adv Technol Commun 2003;14:129-136. [5] Lam K. L, Abu Bakar A, Mohd Ishak Z. A, Amorphous copolyester/polyoxymethylene blends: thermal, mechanical and morphological properties, Kgk Kautsch Gummi Kunstst Commun 2004;57:570-578. [6] Shibing B, Qi W, Effect of injection speed on phase morphology, crystallization behavior, and mechanical properties of polyoxymethylene/ poly(ethylene oxide) crystalline/crystalline blend in injection processing, Polym Eng Sci Commun 2012;52:1938-44. [7] Satoshi N, Akira I, Patcharat W, Kazushi Y, YEW L, Hiroyuki H. Miscibility of polyoxymethylene and poly(lactic acid), ANTEC proceeding Commun 2011. [8] Ray E. D, Patrick R. G, David E. H, Polylactic acid technology, Adv Mater Commun 2000;23:1841-46. [9] Technology Focus Report : Blends of PLA with other Thermoplastics. NatureWorks 2007;2. [10] Fritch L, Injection-molding ABS – mold in quality with the machine, Plast Eng Commun 1979;35:68-72.

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