Composites Journal of Reinforced Plastics and

Journal of Reinforced Plastics and Composites http://jrp.sagepub.com

Effect of Organoclay and Ethylene-Octene Copolymer Inclusion on the Morphology and Mechanical Properties of Polyamide/Polypropylene Blends M. U. Wahit, A. Hassan, A. R. Rahmat, J. W. Lim and Z. A. Mohd Ishak Journal of Reinforced Plastics and Composites 2006; 25; 933 DOI: 10.1177/0731684406063529 The online version of this article can be found at: http://jrp.sagepub.com/cgi/content/abstract/25/9/933

Published by: http://www.sagepublications.com

Additional services and information for Journal of Reinforced Plastics and Composites can be found at: Email Alerts: http://jrp.sagepub.com/cgi/alerts Subscriptions: http://jrp.sagepub.com/subscriptions Reprints: http://www.sagepub.com/journalsReprints.nav Permissions: http://www.sagepub.com/journalsPermissions.nav

Downloaded from http://jrp.sagepub.com by Mat Uzir Wahit on November 21, 2007 © 2006 SAGE Publications. All rights reserved. Not for commercial use or unauthorized distribution.

Effect of Organoclay and Ethylene–Octene Copolymer Inclusion on the Morphology and Mechanical Properties of Polyamide/Polypropylene Blends M. U. WAHIT, A. HASSAN,* A. R. RAHMAT AND J. W. LIM Faculty of Chemical and Natural Resources Engineering Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia Z. A. MOHD ISHAK School of Materials and Mineral Resources Engineering Universiti Sains Malaysia, Engineering Campus 14300 Nibong Tebal, Penang, Malaysia ABSTRACT: A series of compatibilized polyamide 6/polypropylene (PA6/PP) blends, of composition 70/30, 50/50, and 30/70 have been prepared in a twin screw extruder followed by injection molding. The four types of PA6/PP blends involved are the neat PA6/PP blends, PA6/PP/ ethylene–octene copolymer (polyolefin elastomer, POE), PA6/PP/organoclay, and PA6/PP/POE/ organoclay. Tensile, flexural, and Izod impact test are performed on these blends. The morphology of the blends is characterized by scanning electron microscopy (SEM). The mechanical properties of the blends are found to be strongly dependent on the PA6/PP blend ratio. For any blend system, with the increase in PP concentration, the strength and stiffness decreases, while the toughness increases. The incorporation of 4 wt% organoclay into PA6/PP blends significantly increases the modulus and strength but with corresponding reductions in impact strength. Conversely, the incorporation of POE increases the toughness, while the strength and stiffness decrease. However, PA6/PP blends containing both organoclay and POE elastomer are shown to have the potential to be more rigid and tougher than the neat PA6/PP itself. Blend ratio and presence of organoclay are found to influence the morphology (e.g., POE particle size and distribution) of POE toughened nanocomposites systems. A finer particle size and better distribution of POE elastomer has been observed in high PP concentration PA6/PP blends and organoclay filled PA6/PP blends. KEY WORDS: nanocomposites, polyamide 6/polypropylene blends, morphology, strength, stiffness, toughness.

INTRODUCTION N RECENT YEARS, there has been increasing interest in the development of polymer blends based on polyamide 6 (PA6) and polypropylene (PP) [1–4]. The reason for blending PA6 with PP is to bridge the property gap between the two resins. The PP is

I

*Author to whom correspondence should be addressed. E-mail: [email protected]; [email protected]

Journal of REINFORCED PLASTICS

AND

COMPOSITES, Vol. 25, No. 9/2006

0731-6844/06/09 0933–23 $10.00/0 DOI: 10.1177/0731684406063529 ß 2006 SAGE Publications Downloaded from http://jrp.sagepub.com by Mat Uzir Wahit on November 21, 2007 © 2006 SAGE Publications. All rights reserved. Not for commercial use or unauthorized distribution.

933

934

M.U. WAHIT

ET AL.

widely employed because of its low cost, high barrier properties to moisture, and its ease of processing, but its high permeability to oxygen and many organic solvents limits its potential use. On the other hand, PA6 is a good barrier material for oxygen and organic compounds but it is relatively expensive, hygroscopic and thus a poor barrier for water. The PA6/PP blends offer a wide range of desirable characteristics such as good chemical resistance to oxygen permeability and organic solvents, low water absorption, and reduced cost [5–8]. Much effort has been devoted to the compatibilization of incompatibility in PA6/PP blends owing to different polarities and crystalline structure using PP functionalized with maleic anhydride (PPgMAH) [9–12], acrylic acid or other modified polar thermoplastic [13–15]. However, low notched impact strength is a common feature of these blends [1–6]. Therefore, other efforts concentrated mainly on how to increase the impact toughness by adding an elastomer into the blends. For neat PA6/PP blends, some results on incorporation of maleated POE into PA6/PP blends were published [9]. The usage of organoclay, particularly montmorillonite (MMT) as a reinforcing filler for thermoplastic to produce polymer nanocomposites has attracted great interest. These nanocomposites often exhibit remarkable improvement in materials properties when compared with virgin polymer or conventional micro- and macro-composites [16–19]. These improvements can include high moduli, increased strength and heat resistance, decreased gas permeability and flammability, and increased biodegradability of biodegradable polymers. Numerous researchers described polymer–clay nanocomposites on the basis of single-polymer matrix, including PA [20–22], PP [23,24], polystyrene [25,26], epoxy [27], and others [28–32]. However, incorporation of organoclay usually resulted in severe embrittlement manifested in a drop of impact strength and tensile elongation at break. Blending polymer with an elastomeric modifier may provide the way to improve the impact resistance of the base resin. In order to overcome the low impact resistance of PA/PP blend and its nanocomposites, polyethylene–octene copolymer (polyolefin elastomer, POE) was used as a toughening agent to form rubber-toughened PA6/PP nanocomposites in this study. The POE is a novel polyolefin elastomer which was developed using metallocene catalyst by Dow Chemical Co. This elastomer offers a controlled level of chain branching along the polymer backbone. The narrow composition and molecular weight distribution result in improved rheological properties. Compared with conventional polyolefin elastomer ethylene propylene diene monomer (EPDM), POE typically exhibits faster mixing and better dispersion when blended with PP [33]. The aim of the research is to study the effectiveness of organoclay as reinforcing filler and POE as impact modifier for PA6/PP blends, comprising of five blend ratios i.e., 100/0, 70/30, 50/50, 30/70, and 0/100. The microstructure and mechanical properties of the formed nanocomposites and the effect of blend ratio will be investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and mechanical analysis. EXPERIMENTAL Materials and Sample Preparation The blends used in this work are described in Table 1. The PP copolymer (SM-240) was supplied by Titan, Malaysia. The melt flow index (MFI) and density are 25 g/10min (at 230 C and 2.16 kgf load) and 0.9 g/cm3, respectively. The PA6 (Amilan CM 1017 with


935

Effect of Organoclay and Ethylene–Octene Copolymer Inclusion Table 1. PA6/PP systems used in the study. System

Designation

Neat

N BC5(70/30) BC5(50/50) BC5(30/70) P NFN4 BC5FN4(70/30) BC5FN4(50/50) BC5FN4(30/70) PFP4 NEP10 BC5EN10(70/30) BC5EN10(50/50) BC5EP10(30/70) PEP10 NFN4EP10 BC5FN4EP10(70/30) BC5FN4EP10(50/50) BC5FP4EP10(30/70) PC5FP4EP10

Organoclay reinforced

POE toughened

Organoclay/POE

PA6 (wt%)

PP (wt%)

PPgMAH (wt%)

Organoclay (wt%)

POE (wt%)

100 66.5 47.5 28.5 0 96 63.7 45.5 27.3 0 90 59.5 41.5 25.5 0 86 56.7 40.5 24.3 0

0 28.5 47.5 66.5 100 0 27.3 45.5 63.7 91 0 25.5 41.5 59.5 90 0 24.3 40.5 56.7 86

0 5 5 5 0 0 5 5 5 5 0 5 5 5 0 0 5 5 5 5

0 0 0 0 0 4 4 4 4 4 0 0 0 0 0 4 4 4 4 4

0 0 0 0 0 0 0 0 0 0 10 10 10 10 10 10 10 10 10 10

a density of 1.13 g/cm3) was supplied by Toray, Malaysia. The maleated PP used is Orevac CA 100 with 1 wt% of maleic anhydride (MA) produced by ATOFINA, France. Two different grades of organoclay are used in this study. Both grades of organoclay are commercial products from Nanocor Inc. USA. They are as follows. NANOMER 1.30TC This organoclay is a white powder containing MMT (70%) intercalated by octadecylamine (30%) suitable for use in polyamide resin. NANOMER 1.30P This organoclay is surface modified MMT minerals intercalated by stearyl ammonium chloride suitable for use in polyolefin resins. The cation exchange capacity is 110 meq/100 g. Following the pre-designed composition ratios given in Table 1; PA6, PP, PPgMAH, POE, and organoclay were dry blended in a tumbler mixer, prior to being compounded in a Berstoff co-rotating twin-screw extruder. The barrel temperatures were maintained at 200, 220, 230, and 240 C and the rotating screw was fixed at 50 rpm. The pelletized materials were dried and injection molded into standard ISO specimens for mechanical tests. Mechanical Testing Tensile and flexural tests were carried out according to ASTM D638 and ASTM D790 respectively, using an Instron 5567 Universal Testing Machine under ambient conditions.


936

M.U. WAHIT

ET AL.

Crosshead speeds of 50 and 3 mm/min were used for a tensile and flexural test, respectively. The Izod impact test was carried out on notched impact specimens using a Toyoseiki Impact Testing Machine under ambient conditions. Five specimens of each formulation were tested and the average values were reported. X-ray Diffraction XRD was performed with the Siemens XRD. The XRD were recorded with a step size of 0.02 from 2 ¼ 1.5–10 . The interlayer spacing of organoclay was derived from the peak position (d001-reflection) in XRD diffractograms according to the Bragg equation. Microscopy Examination The morphology of the blends was examined using a Philips SEM. Samples were cryogenically fractured in liquid nitrogen and etched in heptane at 50 C for 3 h to extract the elastomeric POE phase. Samples were coated with gold prior to examination under the electron beam. An operating voltage of 10 kV and a magnification of 2000 were used. The particles sizes were calculated from the diameter of the particles which were approximated to spheres. The POE dispersed phase size (number average diameter, Dn) was measured using image analysis software package (Zeiss KS 300 Imaging System Release 3.0 software). Typically over 50 particles and several fields of view were analyzed. RESULTS AND DISCUSSION Mechanical Properties Figures 1–4 show the tensile strength, flexural strength, elastic modulus (E-modulus), and flexural modulus for compatibilized PA6/PP blends, PA6/PP/organoclay, PA6/PP/ POE, and PA6/PP/POE/organoclay as a function of wt% ratio of PA6 and PP. It can be seen that tensile strength and modulus of all blend systems lie between the neat PA6 and PP and declined monotonically as the weight fraction of PP increased. Similar trends were observed for flexural strength and modulus. This trend is expected because the strength and stiffness of PP were lower than that of PA6. On the contrary, a higher impact strength value was observed for PA6/PP blends at any blend ratio as compared to neat PA6 (Figure 5). According to Tseng et al. [20] this was attributed to the presence of the PP copolymer dispersed phase in PA6 matrix, which caused higher impact strength as compared to PA6. Furthermore, the incorporation of PPgMAH caused the formation of PP6 grafted PP (PA6gPP) copolymer which strengthened the interface between the PA6 and PP phases. The reaction of the graft copolymer formation was previously proposed by Duvall and co-workers [31]. For PA6/ PP/PPgMAH blends, the succinic anhydride groups on PPgMAH were able to react with PA6 amine terminal group competitively to form PPgMAH-co-PA6 copolymer during melt processing (Figure 6). The grafted copolymers preferentially resided at the interface and improved interfacial adhesion through the chemical linkage across the interfaces [16,32]. As shown in Figures 2 and 4, the E-modulus and flexural modulus of the neat PA6, PP and PA6/PP blends increased with the incorporation of 4 wt% organoclay. A significant improvement was also evident in the tensile and flexural strength


937

Effect of Organoclay and Ethylene–Octene Copolymer Inclusion 60

Neat Organoclay POE Organoclay/POE

Tensile strength (MPa)

55 50 45 40 35 30 25 20 15 10 PA

PA6/PP (70/30)

PA6/PP (50/50)

PA6/PP (30/70)

PP

Figure 1. Effect of PA6/PP blend ratio on the tensile strength.

3000


Elastic modulus (MPa)

2500 2000 1500 1000 500 0

PA

PA6/PP (70/30)

PA6/PP (50/50)

PA6/PP (30/70)

PP

Figure 2. Effect of PA6/PP blend ratio on the E-modulus.

(Figures 1 and 3). Similar improvements in mechanical properties were also reported by previous researchers [7,19,23,34]. The reinforcement effects depend on four factors with respect to organoclay: rigidity, aspect ratio, the affinity with the matrix polymer, and degree of exfoliation [35]. According to Liu et al. [24] and Wang et al. [35], generally the enhancing effect of the organoclay on stiffness and strength was due to the higher modulus of organoclay as compared to polymer. The stiffness of organoclay contributes to the presence of the immobilized or partially immobilized polymer phase [36]. These improvements are also related to the degree of exfoliation of layered silicate in the polymer matrix and high aspect ratio of platelet structure possibly because it increases the interaction between silicate layers and polymer.


938

M.U. WAHIT 90


80

Flexural strength (MPa)

ET AL.

70 60 50 40 30 20 10 0 PA

PA6/PP (70/30)

PA6/PP (50/50)

PA6/PP (30/70)

PP

Figure 3. Effect of PA6/PP blend ratio on the flexural strength.


Flexural modulus (MPa)

2500

2000

1500

1000

500

0 PA

PA6/PP (70/30)

PA6/PP (50/50)

PA6/PP (30/70)

PP

Figure 4. Effect of PA6/PP blend ratio on the flexural modulus.

Besides that, there was a strong interaction between the PA6 matrix and silicate layers. It is believed that hydrogen bonding could form between the amide group of the PA6gPP copolymer and octadecylamine group of the organoclay intercalant (Figure 7). According to Chow et al. [36], this amide–amine reaction could happen when organoclay was exfoliated in the PA6/PP matrix. Subsequently, octadecylamine (intercalant) was capable of forming a chemical linkage with PA6gPP copolymer. Furthermore, Usuki et al. [37] suggested that organoclay interacted strongly with PA6 by ionic interaction. The ammonium NHþ 3 end group in PA6 interacted with a negative charge of the silicate layer. It was also observed that there were notable differences in level of strength and modulus for PA6, PP, and PA6/PP blends. The enhancement on tensile strength and modulus


939

Effect of Organoclay and Ethylene–Octene Copolymer Inclusion 14

Impact strength (kJ/m²)

12


10 8 6 4 2 0 PA

PA6/PP (70/30)

PA6/PP (50/50)

PA6/PP (30/70)

PP

Figure 5. Effect of PA6/PP blend ratio on the impact strength.

became less significant as the PP content increased. For example, increases in tensile strength and E-modulus for the PA6/PP (70/30) nanocomposites system, as compared to neat PA6/PP blends were about 24 and 33%, respectively. Whereas, for PA6/PP (30/70) they were about 9 and 5%, respectively. Flexural properties also showed a similar trend (Figures 3 and 4). For example, flexural strength of PA6/PP(70/30)/organoclay improved about 32% relative to neat PA6/PP blends, whereas for PA6/PP(30/70)/organoclay the improvement was about 9%. The reason for these observations is explained as follows. The degree of exfoliation of organoclay is dependent on compatibility between polymer and organoclay, processing method and its condition [22,29,37]. Silicate clay layers have hydroxyl groups and are compatible only with polymer containing polar functional groups [7,17]. In the case of PA6/organoclay as in this work, it is believed that either full or partially delaminated clay formation was favored by the compatibility between PA6 and organoclay and the shear forces during extrusion compounding and injection molding. The evidence for organoclay delamination is described later. On the other hand, PP has low polarity in its backbone. As a result, it is difficult to get exfoliated and homogenous dispersion of the silicates layer in PP. Fornes et al. [38] have pointed out that shear stress alone could not achieve exfoliation when the matrix polymer did not have good compatibility with organoclay. In this study, only 5 wt% of PPgMAH was used to mediate the polarity between the clay surface and PP. In principle, the desired nanoscale dispersion of organoclay in the PP matrix is achieved with PPgMAH by strong hydrogen bonding between hydroxyl groups of the silicates and maleic anhydride group [7]. Hasegawa et al. [26] have confirmed that a mixture of roughly up to 3 : 1 by mass of PPgMAH to organoclay was found to be effective in forming hybrid composites. It is also noted that the addition of 4 wt% organoclay has deteriorated the Izod impact strength of the nanocomposites. In the case of PA6 and PA6/PP (70/30), Izod impact strength was reduced from 4.6 to 2.9 kJ/m2 and 8.6 to 5.3 kJ/m2, respectively. Wang et al. [29] have previously reported that the impact strength decreased from 115 J/m for neat PA6/PP to 17 J/m for PA6/PP filled with 5 wt% organoclay. According to Stevenson [39], there are few reasons why fillers have detrimental effects on impact performance. One important reason is that a significant volume fraction of


940

M.U. WAHIT PP

PPgMAH

ET AL.

PA6

O CH3

H

H

CH2 C

CH2

CH

+

CH2

C

O

n

CH3

N

N

+

C O

H C

PA end group O Anhydride

O CH3

CH2 C

CH2

CH2

CH n

H NH

N

C CH3

C O

C

OH

O −H2O O CH3

CH2

H

C CH2

CH2

CH n

C CH3

N C O

N

C O

PA6gPP copolymer

Figure 6. Reaction between a PA6, PP, and PPgMAH i.e., PA6gPP formation.

polymer, which can dissipate stress through the shear yielding or crazing mechanism, is replaced by the filler, which generally cannot deform and dissipate the stress easily. The total ability of the material to dissipate the stress is therefore decreased. However, this is particularly true at a high concentration of filler. A second reason is that filler may hinder the local chain motions of polymer molecules that enable them to shear yield. The hindering process can sharply decrease the impact resistance of the materials. The third reason is that nanocomposites inherently contain incomplete dispersion of nanoparticles, which form aggregates, that cause premature crack formation which in turn lead to embrittlement or both [40]. However, this drawback is believed to be overcome by the incorporation of the rubber phase.


941

Effect of Organoclay and Ethylene–Octene Copolymer Inclusion Physical entanglement between PP and PA6gPP

PA6gPP copolymer O

PP CH3

CH3

H

C CH2

CH2

CH

C

N

N

C

n

CH2

C

O

POE CH2CH2CHCH

O

Hydrogen bonding

CH2(CH2)6CH3 H Physical entanglement between POE and PP / PA6gPP

H

N

CH2(CH2)16CH3

Octadecylamine group in the organoclay Figure 7. Interaction between PA6gPP copolymer and organoclay, between PA6gPP and PP, and between POE and PP.

The strength and stiffness of POE toughened PA6, PP, and PA6/PP blends are shown in Figures 1–4. The strength and stiffness of all systems dropped with the incorporation of POE. This is due to the softening or diluting effect of POE [41,42]. The influence of POE on toughness i.e., notched Izod impact strength are shown in Figure 5. The Izod impact strength increased with higher PP loading. The result showed that the impact strength of PP/POE increased most significantly, reaching the value of 12.8 kJ/m2, which was around 40% higher than that of neat PP, while, the incorporation of POE into PA6 led to a limited increment in impact strength, which was around 8% as compared to neat PA6. Referring to Figure 5, for the PA/PP (70/ 30)/POE blend, the impact strength only improved about 26%, but, for the PA6/PP (30/ 70)/POE blend, the impact strength was drastically improved by about 40%. It can be concluded that as the proportion of PP in the blends increased, the degree of impact strength enhancement increased. This was due to the compatibility between POE and PP which was much better than POE and PA6 alone. According to Yu et al. [42], POE exhibited faster mixing and better dispersion when blending with PP due to structure similarity. The scanning electron micrographs that will be discussed later also show that POE particles dispersed better and finer in PP as compared to the PA6 matrix. The results and discussion from previous sections have shown that the incorporation of organoclay into polymer has increased the strength and stiffness relative to neat polymer. However, it causes a significant reduction in the toughness property. On


942

M.U. WAHIT

ET AL.

Table 2. Impact strength performance of different systems. System

Impact strength performance

PA PA6/PP (70/30) PA6/PP (50/50) PA6/PP (30/70) PP

With With With With With

organoclay