A Study of the Effect of PP-g-MA and SEBS-g-MA on the Mechanical and Morphological Properties of Polypropylene/Nylon 6 Blends.
RICHARD L. EINSPORN [+]
The mechanical and morphological properties of polypropylene/nylon 6 blends compatibilized with PP grafted with maleic anhydride (PP-g-MA) and styrene/ethene-co-butene/styrene grafted with maleic anhydride (SEBS-g-MA) are studied using a special version of a factorial design known as extreme vertices. Properties examined include yield stress, modulus, elongation, toughness, impact strength and morphology. Comparisons are made between various treatment combinations (i.e. a variety of blends) and polypropylene homopolymer using various statistical methods including analysis of variance (ANOVA), Scheffe's Test and Duncan's Multiple Range Test. Significant differences were found for yield stress, modulus, elongation, toughness and impact strength for specific treatment combinations versus PP as well as on average. Ternary diagrams are used to plot response surfaces of the measured data illustrating the main effects and interactions involved, while allowing correlations to be made with blend morphology. Indicatio ns from test results and analysis of response surfaces show a strong relationship between nylon/compatibilizer ratio and mechanical properties.
Owing to the high cost of homopolymer development and exhaustion of new homopolymer possibilities, the development of polymer blends has received much attention in recent years with 4500 blend patents and 50,000 articles published annually. Homopolymer development costs an average of $10 million (1990) with an additional $100 million for pilot plant costs in the development and commercialization steps. By contrast, polymer blend development usually costs less than a few million dollars . Blends of polyolefins such as polypropylene are of particular interest because of their low cost, high processability and the numerous applications for which they are well suited. However, most polymer blends are not compatible without the addition of a third component known as a compatibilizer [2-6], and polypropylene/polyamide blends are no exception. The use of maleic anhydride or acrylic acid grafted to polypropylene, ethylene-propylene rubber or styrenic block copolymers have all effectively improved mechanical proper ties and dispersion over uncompatibilized polypropylene/polyamide blends [7-19]. The presence of commercially available compatibilzing agents such as Uniroyal Chemical's Poylbond[R] and Shell Chemical's Kraton[R] allow these materials to be blended with a variety of polymers enhancing properties and/or reducing cost thereby eliminating the need to develop entirely new compatibilzing agents.
Polypropylene is frequently toughened by the addition of ethylene/propylene rubber (EPR), SEBS based materials [20-33] or by copolymerization with ethylene near the end of the polymerization process [34, 35]. The toughening of nylon is often accomplished using EPR or SEBS grafted with maleic anhydride or acrylic acid (36-43). The pendant graft functionalities are capable of reacting to form amide linkages with the terminal amine of polymides creating a macromolecule that is composed of nylon and rubber modifier. This macromolecule reduces the polyamide/rubber modifier interfacial energies thus creating a finer dispersion improving stress transfer, which in turn increases toughness. The use of the same materials is common in the compatibilization of polypropylene/polyamide blends owing to the relative affinity of rubber modifiers for polypropylene even though all rubber modifiers are not miscible with polypropylene. This creates a ternary mixture with improved strength and toughness versus polypropylene homopolymer but oftentimes reduces tensile strength (13). The use of polypropylene grafted with either acrylic acid or maleic anhydride in polypropylene/polyamide blends creates a blend with improved impact properti es versus polypropylene homopolymer while maintaining or improving strength (13, 14, 16).
Previous studies show the varying effect each compatiblizer has on PP/nylon blends. PP-g-MA, being essentially polypropylene in nature, is quite miscible with polypropylene in relatively small amounts allowing for a finer dispersion of nylon in polypropylene upon blending. SEBS-g-MA, on the other hand, is not miscible in either polypropylene or nylon but does have an affinity for each (16). SEBS-g-MA creates a core-shell morphology with nylon whereby nylon is encapsulated in SEBS-g-MA, both of which constitute the non-dispersed phase in polypropylene (13).
The properties of the system are dependent on a number of variables, including molecular weight, graft level and the ratios of the various components. It is proposed that the ratio of polyamide/compatibilizer plays a major role for each property of interest. This paper uses statistical modeling of experimental results to create response surfaces illustrating the feasibility of using such techniques to predict the performance of these materials over a range of compositions while allowing better visualization and interpretation of experimental results.
The experimental design used in this study is a highly efficient variation of factorial design specifically used for mixtures known as extreme vertices (44, 45). The efficiency of the design is due to the mutual dependence of the component levels. Specification of two of three components in a ternary system specifies the level of the third component since the sum of the component fractions must equal one. In this design, the model equation intercept is eliminated algebraically upon formulation of the model equation. With the elimination of the intercept, the addition of an another model term is made possible improving the power of the model. In the present study, polypropylene/nylon 6 blends compatiblized with PP-g-MA and SEBS-g-MA are studied for the purpose of understanding the interactions involved in this system.
Polypropylene used in this study was obtained from Exxon Chemical. It is a 34g/ 10 mm 230[degrees]C MFI material known as Escorene[R] 3155. Nycoa[R] 2169 from Nyltech is a low molecular weight nylon 6 (Mw = 10,000) with a melt flow of 50g/ 10 min at 230[degrees]C. SEBS-.g-MA was obtained from Shell Chemical as Kraton[R] FG1901X, a material with a 22g/ 10 mm MEl at 230[degrees]C and a graft level of 1.84%. The PP-g-MA used is Polybond[R] 3150 obtained from Uniroyal Chemical having a 0.7 wt% graft level and a MEI of 50g/l0 mm at 190[degrees]C.
Polymer processing equipment includes a Brabender EX-200 single screw extruder with a 20:1 L/D and a 3/4" screw equipped with a mixing section (Brabender 05-0-052) to aid in the blending process. Injection molding made use of a Boy 15S equipped with molds designed to conform to the dimensions specified by ASTM Test D638 type M-1 and ASTM D 256. Tensile properties were tested using an Instron model 4204 at 50 mm/min with a 50 kN load cell while Izod impact tests used a model 43-1 impact machine from Testing Machines Inc. with a 2-lb weight. Electron microscopy consisted of first sputter coating the samples with 25 nm of gold followed by analysis at 20kV in an Amray 1600 SEM.
Prior to blending, nylon and the compatibilizing agents were dried for 8 hours under vacuum at 80[degrees]C to remove water absorbed during shipping and handling. Materials were blended immediately upon removal from the oven to avoid absorption of atmospheric moisture. Melt blending of the polymers occurred under gravimetric feed conditions at 32 rpm and 225[degrees]C. Two passes through the extruder were used to help facilitate sufficient blending with each pass being followed by quenching in an ice-bath, granulation, and drying for 8 hours under vacuum at 80[degrees]C. Injection-molding of blends used an operating temperature of 275[degrees]C with the mold temperature set at ambient conditions.
Statistical Design and Testing
The range of component levels is based on the need for a design encompassing a wide range of levels for each factor within reasonable limits based on knowledge obtained from literature and cost considerations. The factor levels are based on the desire to test polypropylene/nylon blends containing between 5% and 20% nylon 6 by weight. The range of nylon amounts is a balance between a discernible change in test results versus polypropylene homopolymer and cost considerations as nylon is much more expensive than polypropylene. The amount of compatiblizer is limited to between 1% and 5% by weight based on preliminary research and a literature review (11-16). The polypropylene fraction makes up the remaining portion of the mixture varying between 75% and 94% by weight. A plot of the experimental design is given in Fig. 1 in the form of a ternary plot. The treatment combinations selected for run points correspond to the corners of the design and two center runs so that an estimate of variance is established. Each mechanical test uses five samples with the exception of impact tests, which use eight samples.
The treatment combinations correspond to the blend compositions given in Tale 1. The model used to predict the response contains all main effects and two of the three two-way interactions as shown in Eq 1.
Y = a[X.sub.1] + b[X.sub.2] + c[X.sub.3] + ab[X.sub.1][X.sub.2] + bc[X.sub.2][X.sub.3] (1)
The letter X represents the fraction of material used while a, b and c represent model coefficients, which are simply the parameter estimates of the analysis. Two independent experimental designs are used for each compatiblizer type, one for PP-g-MA blends and one for SEBS-g-MA blends.
The use of analysis of variance (ANOVA] tests blends for model significance, main effects and two-way interactions. The statistical software used, called JMP, was developed by SAS Institute. This particular software works especially well for analyzing and plotting mixture models. Other analyses make use of Scheffe's Test to compare the average effect of each family of blends versus polypropylene homopolymer and Duncan's Multiple Range Test to compare all individual treatment means. Scheffe's test is a form of orthogonal contrasting while Duncan's Test is a variation oft-tests that helps suppress the inflation of Type I error while remaining powerful enough to detect significant differences (46].
RESULTS AND DISCUSSION
Table 2 contains a summary of the mechanical properties for all blends tested. Table 3 contains the results of statistical testing comparing the overall effect of each compatibilizer on the blends for all treatment combinations and the level of significance. Scheffe's test is used at significance levels of 95% and 99% to compare the effect of each family of blends versus PP homopolymer. Yield stress results reveal opposing effects for each type of compatibilizer. This is due to the means by which each type of compatibiizer reduces interfacial energies between polypropylene and nylon 6 as mentioned previously. The incorporation of nylon into the polypropylene matrix for PP-g-MA compatibilized blends allows nylon to contribute strength much more effectively than in SEBS-g-MA compatibiized blends where nylon is essentially encapsulated. Increases of as much as 12.6% are observed for that blend containing 20% nylon and 5% compatibilizer although the blend containing a similar nylon/compatibilizer ratio using 12. 5% nylon and 3% compatibilizer shows an increase of 12.0%. The effect of SEBS-g-MA on yield stress reveals that increases in SEBS-g-MA reduce yield stress, possibly owing to the negative contribution to yield stress inherent in SEBS materials versus polypropylene and the absence of a contribution from the encapsulated nylon. Although the model for SEBS-g-MA blends is not significant with a P-value of 0.1627, PP-g-MA blends are significant with a P-value of 0.0429. Figure 2 shows the response surface of maleated polypropylene blends. The shape of the surface reveals a maximum In yield stress along a ridge between blends containing 12.5% nylon, 3% PP-g-MA and 20% nylon. 5% PP-g-MA. This corresponds to a nylon/PP-g-MA ratio of 4.2 and 4.0 respectively, whose close proximity indicates the importance of this ratio in strength optimization.
Modulus results show that PP-g-MA blends have a positive impact on modulus while SEBS-g-MA blends fail to show a significant effect. Increases of 23% on average are seen using PP-g-MA, with individual treatments showing increases as high as 34% for the blend containing 20% nylon and 1% compatibilizer. Once again an interactive effect is observed upon examination of the response surface given in Fig. 3. The presence of vertical lines in the surface corresponds to an interaction between nylon and compatibilizer. The trend observed not only shows interaction, but also that very little compatibilizer is needed to provide sufficient compatibiization to allow the stiffness of nylon to contribute to modulus. Additional compatibilizer has a negative effect on modulus as nylon becomes better dispersed In the polypropylene matrix. This effect may be related to the formation of a third phase (excess PP-g-MA), or represents an optimum dispersion of nylon 6 in the matrix polypropylene. The negative effect of additional c ompatibilizer beyond that needed on yield stress has been observed in data published by Duvall et al. in yield stress (11). Modulus data coupled with yield stress data shows that although stiffness is increased with a small amount of PP-g-MA, additional elongation requires more PP-g-MA in order for nylon to properly contribute strength although a maximum level is observed that is related to the nylon/compatibilizer ratio.
Elongational characteristics show striking differences between PP-g-MA blends, SEBS-g-MA blends and PP homopolymer. Before continuing the analysis, however, the significant reduction in elongation of the PP control is worth noting. The PP homopolymer control has been subjected to the same processing condition as all blends. This includes two passes through the extruder and injection molding. A significant negative effect is observed when the processed PP is compared to PP, which has been injection molded only. The PP subjected to the injection-molding step only shows a significant increase in elongation of 2700% versus processed PP. Nevertheless, the use of SEBS-g-MA allows elongational characteristics of PP/nylon blends to maintain significantly higher elongation than the processed PP by an average of 685% with increases as high as 1660% for a family of blends containing between 12.5% nylon, 3% SEBS-g-MA and 5% nylon, 1% SEBS-g-MA. Once again an interaction between SEBS-g-MA and nylon is present, evidenced b y the response surface and the ridge of maximum elongation corresponding to a nylon/SEBS-g-MA ratio of between 4:1 and 5:1 shown in Fig. 4. Although the use of SEBS-g-MA results in significantly enhanced elongation, it also presents more erratic behavior when compared to the blends containing PP-g-MA. This can only be attributed to experimental irregularities within the blended material. The use of PP-g-MA has no effect on elongation for all treatment combinations tested. It appears that SEBS contributes a heat stabilizing effect for PP although further tests are needed to validate this claim.
Toughness, the area under the stress/strain curve, is thus closely related to elongation for this study. The use of PP-g-MA has an average increase of 30% versus PP homopolymer while SEBS-g-MA containing blends increase toughness by 71%. Figure 5 shows that an interaction is present for SEBS-g-MA blends with optimum toughness values corresponding to a nylon/SEBS-g-MA ratio of between 4:1 and 5:1. All treatment combinations except one (20% nylon. 1% PP-g-MA] showed increased toughness versus polypropylene homopolymer for both families of blends.
Impact strength shows significant differences for compatibilizer type. On average, the effect of PP-g-MA has no significant effect on impact strength while SEBS-g-MA containing blends show an increase of 88% on average versus PP homopolymer. Individual treatments of PP-g-MA blends show increases of as much as 41% for the two blends containing 12.5% nylon, 3% PP-g-MA and 39% for the blend containing 20% nylon and 5% PP-g-MA. This once again corresponds to between a 4:1 and 5:1 nylon/PP-g-MA compatibiizer ratio. The use of SEBS-g-MA significantly increases impact strength for all blends tested with individual increases as high as 158% for that blend containing 20% nylon and 5% SEBS-g-MA. Analysis of the response surface given in Fig. 6 shows the strong dependence of impact strength on SEBS-g-MA level with only a slight contribution from nylon being present. This is due to the strong ability of SEBS to dissipate energy versus PP or nylon 6.
Figures 7 through 11 show the morphology of PP-g-MA blends while Figs. 12 through 14 show SEBS-g-MA blends respectively at 5000X magnification. Compatibilization effects are most clearly indicated by comparing the fracture surfaces of blends containing 5% nylon. Figures 7 and 11 show blends containing 1% compatibilizer with relatively large particles protruding from the matrix. In contrast, Figs. 8 and 14 show blends containing 5% compatibilizer in which the particles are embedded within the matrix. Increasing the percentage of compatibilizer has significantly increased the dispersion and particle size distribution of the nylon within the polypropylene.
For blends containing 20% nylon and only 1% compatibilizer, the SEBS-g-MA provides better wetting of the nylon than PP-g-MA. leading to improved adhesion to the polypropylene matrix. There is no clear distinction between the two compatibilizers for the other levels tested. However, trends are visible when nylon/compatibilizer ratio are considered. Another interesting observation is that the non-dispersed particle size appears indistinguishable for blends compatibilized with PP-g-MA and SEBS-g-MA in ratios of 4:1 to 5:1. This trend remains true even though total nylon amount for these blends ranges between 5% and 20% of the total blend composition. This is another strong indication that the nylon/compatibiizer ratio is critical to optimization of blend properties and correlates to response surfaces showing a relative independence of properties along the ridge of optimum properties.
The following conclusions are drawn from this statistical investigation of polypropylene/nylon 6 blends compatibilized with PP-g-MA and SEBS-g-MA. The use of PP-g-MA enhances the properties of yield stress, modulus, toughness and impact strength while the use of SEBS-g-MA increases elongational properties, toughness and impact strength while decreasing yield stress. The effect of nylon/compatibilizer ratio is critical to the optimization of PP/nylon compatibilized blends irrespective of nylon amount over the range of compositions studied. The effect is observed in response surfaces of yield stress and modulus for PP-g-MA compatibilized blends, and toughness for SEBS-g-MA compatibilized blends. The model also predicts an optimum level for these properties, beyond which there is a negative effect.
The experimental design employed, extreme vertices, is able to effectively use experimental data to model mechanical properties even when very few treatment combinations are used. The quantification of main effects and interactions allows a better understanding of the complex behavior of ternary polymer blends. The extreme vertice design is ideally suited to mixtures and is superior to conventional factorial designs and variations thereof as it more efficiently and more effectively determines main effects and interactions. This allows a more complete understanding of complex phenomena associated with ternary blends. This type of analysis is ideally suited for the optimization of properties especially when coupled with multivariate analysis and the path of steepest ascent method of determining optima.
MA Maleic Anhydride
PP-g-MA PP grafted with maleic anhydride
SEBS-g-MA SEBS grafted with maleic anhydride
ANOVA Analysis of Variance
EPR Ethylene-propylene rubber
(*.) Corresponding author: Dr Sunggyu Lee C. W. LaPierre Professor and Chair Department of Chemical Engineering University of Missouri-Columbia W2030 Engineering Building East Columbia, MO 65211 Email: firstname.lastname@example.org
(*.) Department of Chemical Engineering The University of Missouri-Columbia Columbia, MO 65211
(+.) Department of Mathematical Sciences The University of Akron Akron, OH 44325
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Blend Compositions. Treatment Component Percentages Combination Polypropylene Nylon 6 PP-g-MA SEBS-g-MA 1 90 5 5 0 2 94 5 1 0 3 84.5 12.5 3 0 4 84.5 12.5 3 0 5 75 20 5 0 6 79 20 1 0 7 90 5 0 5 8 94 5 0 1 9 84.5 12.5 0 3 10 84.5 12.5 0 3 11 75 20 0 5 12 79 20 0 1 Mechanical Properties. Treatment Yield Stress Modulus Elongation Toughness Combination (MPa) (MPa) (%) (MPa) PP Control 33.4 (0.239) 505 (88.6) 24.3 (2.48) 576 (7.33) 1 36.0 (0.341) 574 (40.8) 32.6 (3.69) 746 (75.6) 2 35.2 (0.159) 576 (32.5) 35.9 (4.29) 710 (61.4) 3 37.1 (0.066) 624 (43.1) 30.7 (4.61) 778 (109) 4 37.0 (0.403) 622 (10.9) 27.9 (4.58) 724 (29.7) 5 37.4 (0.345) 647 (88.1) 34.5 (6.92) 820 (53.7) 6 34.7 (0.147) 675 (4.47) 21.5 (1.26) 606 (5.71) 7 29.2 (0.384) 482 (14.4) 199 (51.5) 985 (90.3) 8 32.3 (0.507) 530 (25.3) 286 (111) 1100 (66.8) 9 29.8 (0.371) 503 (20.1) 206 (60.63) 956 (113) 10 30.4 (0.406) 472 (57.1) 296 (121) 1110 (120) 11 29.3 (0.493) 456 (111) 117 (47.8) 954 (125) 12 33.2 (0.341) 518 (83.2) 39.5 (2.32) 669 (25.5) Treatment Impact Strength Combination (ft-[lb.sub.t]/in) PP Control 0.62 (0.13) 1 0.72 (0.14) 2 0.64 (0.08) 3 0.90 (0.13) 4 0.85 (0.16) 5 0.86 (0.067) 6 0.59 (0.044) 7 1.2 (0.20) 8 0.86 (0.10) 9 1.2 (0.22) 10 1.3 (0.15) 11 1.6 (0.057) 12 0.99 (0.072) Note: Numbers in parenthesis represent the standard deviation Summary of Scheffe's Test. The Overall Effect of Compatibilizer Type. Response Variable PP-g-MA Blends Deviation Significance Yield Stress +8.6% 99% Modulus +23 95% Elongation Not Significant Toughness +30% 99% Impact Strength Not Significant Response Variable SEBS-g-MA Blends Deviation Significance Yield Stress -8.0% 99% Modulus +685% Not Significant Elongation +71% 99% Toughness +88% 99% Impact Strength 99%
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|Author:||TUCKER, J. DAVID; LEE, SUNGGYU; EINSPORN, RICHARD L.|
|Publication:||Polymer Engineering and Science|
|Article Type:||Statistical Data Included|
|Date:||Dec 1, 2000|
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