Morphology and Elastic Properties of PP/EVA Polymer Blends

Polymer International 41 (1996) 227-236

Morphology and Elastic Properties of PP/EVA Polymer Blends

A. Maciel," A. Del-Real,b M. V. Garcia-Garduiio,b*c E. Oliva,b 0. Manero," & V. M. Castaiiob**

a Instituto de Investigaciones en Materiales, UNAM, Apartado postal 70-360, Mexico, DF 04510 Instituto de Fisica, UNAM, Apartado postal 20-364, Mexico, DF 01000

Division de Posgrado e Jnvestigacion, Facultad de Odontologia, UNAM, Mexico, DF 04510

(Received 4 December 1995; revised version received 21 February 1996; accepted 27 March 1996)

Abstract: Several polypropylene-ethylene vinyl acetate (PP/EVA) copolymers with compositions ranging from 90/10 to 10/90 PP/EVA were prepared and characterized in terms of their morphology by transmission and scanning electron microscopy, and their mechanical properties were also studied. The results show a wide range of spatial structures which correlate well to the corresponding measurements of elastic modulus of the blends.

Key words : blends, morphology, mechanical properties, electron microscopy, polypropylene, ethylene vinyl acetate, TEM, SEM

I NTRO D U CTI 0 N reports on the blending of polymers, copolymers, elasto- mers, interpenetrating polymer networks, e t ~ . ~ - ~ In par-

The mechanical behaviour of binary polymer blends ticular, there has been some interest in the binary depends critically on the morphology and connectivity blending of polypropylene (PP) and ethylene vinyl of the distinct phases. Almost all two-component poly- acetate (EVA), due to the wide engineering use of PP meric combinations form immiscible phases. Polymer and the ability of EVA to act as an impact m~d i f i e r .~ miscibility is not only important in the case of simple Accordingly, in this article, we present our findings on polymer mixtures, but also determines the physical the morphology-property relationships of a number of nature of block and graft copolymers, interpenetrating PP/EVA blends, prepared by different methods and networks, and thermosetting networks of polymer mix- with different compositions. tures. The degree of compatibility has often been used In processing operations of polymer blends the to describe good adhesion between the constituents, and sample morphology may be substantially modified to predict average mechanical properties, the behaviour depending on the type of process (extrusion, mixing, of two-phase block or graft copolymers, and capabil- moulding, etc.) to which the sample is subjected. In this ities for blending. regard, the resulting morphology of the polymer blend

The term miscibility, in most polymer blends, does depends strongly on the method of preparation of the not necessarily imply ideal molecular mixing but it samples and previous processing conditions. Attention rather suggests that the level of molecular mixing is ade- in this present work is given to the morphology of quate to yield macroscopic properties expected in a samples that were processed under different conditions single-phase material.' Therefore, any effort towards the and prepared for microscopy studies following different understanding of the very basic scientific aspects of methods. Examination of the resulting micrographs polymer blending is more than welcome in the com- provides evidence of the influence of these factors on the munity interested in developing novel materials with resulting morphological features. improved properties. In this regard, there exist several Previous studies performed on PP/EVA (vinyl acetate

content 45%) blends,' show a maximum in impact * To whom all correspondence should be addressed. strength at 70/30 PP/EVA apparently due to the

227 Polymer International 0959-8103/96/$09.00 0 1996 SCI. Printed in Great Britain

228 A. Maciel et al.

uniform distribution of 1.75 ,um size elastic type particles. The decrease of impact strength for larger EVA contents is caused by larger rubber particle sizes. On the other hand, when the EVA content is larger, a peak in the elongation at break is observed at 30/70 PP/EVA. This has been attributed to the presence of interpenetrating co-continuous rubber and plastic phases formed at this content.

In another study using smaller vinyl acetate contents (9, 12 and 19%), PP/EVA blends form a system with increased pseudoplasticity due to the dispersed EVA domains.' These domains are smaller (particle diameter is approximately 0.45,um) in the blend with 80/20 PP/EVA composition (12% vinyl acetate content). In this particular study, melt viscosity and elasticity decrease and the morphology shows a uniqueness at this composition. The average particle diameter is close to the limits stated for optimum impact toughening of PP by an ela~tomer.~ A shear-induced droplet break-up is observed as the stress is increased, and coalescence is observed at lower stresses. At 10-20% EVA, droplet size is higher and they can deform more easily, whereas with 20-30% EVA, droplet size decreases with a conse- quent increase in viscosity. Interestingly, the smaller droplet size is found at 5 and 30% EVA composition.

Different procedures have been used for processing the blend before morphological studies, and also different routes have been used for sample preparation. In one case' preparation of samples for impact and mechanical tests was made after they were mixed in a mixing chamber and press-moulded in an hydraulic press. In a previous study,' samples were prepared in a single- screw extruder and the extrudate was freeze-fractured and etched with toluene to dissolve out the EVA particles. Hence, it is difficult to attribute the difference in size of EVA particles only to the difference in vinyl acetate content (45% in Ref. 1 and 12% in Ref. 2, since the processing of the blends was also different.

180°C for lOmin, then the blend was pelletized and extruded at 225°C.

Mechanical testing

Tensile strength tests were performed in a Instron machine, model 1125, at a deformation rate of 100mm min-' according to ASTM procedure 1708.

Morphology characterization

Two preparation techniques were employed. Technique 1 : ultramicrotomy. The extruded speci-

mens were immersed in a 2% aqueous solution of OsO, for 48h to stabilize the sample, washed with distilled water, dried and cut by ultramicrotomy in a microtome model MT 6000-XL (RMC). The cuts were vacuum- coated with carbon for examination by transmission electron microscopy (TEM). The pyramids were vacuum-coated with gold in order to observe the surface by scanning electron microscopy (SEM).

Technique 2 : cryofracture. Two sections were cut from the extruded specimen: one parallel and the other perpendicular to the extrusion direction. The samples were frozen in liquid N, for 1 to 2min to make them brittle enough for fracture. The fractured polymers were vacuum-coated with gold for observation by SEM, to analyse the anisotropy in the resulting morphology of the PP/EVA blends and to evaluate the deformation of particles.

Another method of preparation used follows a procedure described by Thomas.' The extrudate was freeze- fractured and etched with toluene. In this case, samples with 20 and 40% EVA content were used.

The morphology observations were carried out in a transmission electron microscope (JEOL-100CX) at lOOkeV and a scanning electron microscope (JEOL JSM-T20) in secondary electron mode.

RESULTS AND DISCUSSION EXPER I M E NTAL

Blend preparation

Polypropylene (Himont) with molecular weight of 150000 and melting temperature of 173"C, was blended with ethylene-vinyl acetate (28%; Atochem), with molecular weight and melting temperature of 117 000 and 46°C respectively. The blends were prepared by two methods.

Method A: double extrusion. The components were extruded in a Haake R-4000 mixer with a screw L/D of 25 : 1 and length 71 cm at 225°C. They were pelletized and then extruded once more at the same conditions.

Method B: blending before extrusion. The components were processed in a blender (Rheomix 254) at

Figure 1 contains the plot corresponding to the elastic modulus of the blends prepared by the two methods described above. It can be observed that the samples subjected to the double extrusion process (method A) present, in general, a higher modulus than the samples prepared by method B.

The morphology of the blends with higher PP content and prepared by method A is less homogeneous than that of the blends prepared by method B. This indicates that, in general, samples prepared according to the first method show poor phase dispersion, as can be seen in Figs 2 and 3, which show the micrographs of PP/EVA samples with concentrations of 90/10. As observed there, the specimens prepared by method A (Fig. 2), present a more irregular microstructure, with

POLYMER INTERNATIONAL VOL. 41, NO. 3, 1996

Morphology and elastic properties of P P / E V A blends 229

Elastic Modulus EVA - PP

. . - 0.w 1

0 20 40 60 ao 100

Weight (X) Polypropylene

Fig. 1. Elastic modulus versus weight (%) PP. Blends prepared by (-) method A, (A method B.

some voids, whereas those prepared by method B have a smoother morphology with some elongated features, of the order of a few micrometres, which correspond to the other phase (Fig. 3). The roughness of the former is

probably due, according to the contrast in the micrographs, to the presence of EVA regions segregated on top of the PP matrix.

In 80/20 PP/EVA blends prepared by method A (double extrusion), the elastic modulus reaches a very high value, that is, a maximum in the mechanical properties was found. The morphology (Fig. 4) shows a continuous PP phase with ‘tongues’ of EVA, as com- pared to Fig. 5, which corresponds to the same composition but prepared by method B, showing the same continuous phase with the EVA. Perhaps these ‘tongues’ provide the blend with higher modulus, acting as a sort of in-situ reinforcing agent. It is possible that at this particular concentration, the method of mixing is not severe enough to adequately disperse the EVA in the PP, and this produces the EVA segregation. The modulus of this blend is closer to that of pure PP.

For those samples with low PP content the corresponding moduli and morphology are very similar, regardless of the mixing method, as shown in Figs 6 and 7.

Figures 8 and 9 show micrographs of 80/20 PP/EVA blends prepared by method A (double extrusion) using technique 2 (freeze-fracture). The samples were fractured parallel to the extrusion direction (Fig. 8), where super- imposed plates with a few holes and some patches, corresponding to EVA, are observed. The particle size is similar to that found in Ref. 1 when the vinyl acetate content is 45% and the EVA content is 30% (1.75 pm).

Fig. 2. Method A (double extrusion) 90110 PP/EVA blend; SEM micrograph technique 1 (ultramicrotomy).



Fig. 3. Method B (blending before extrusion) 90/10 PP/EVA blend; SEM micrograph technique 1 (ultramicrotomy).

The system with 80/20 PP/EVA composition, with low vinyl acetate content (12%), was shown to present high processability and lower viscosity and elasticity.’ On the other hand, the vinyl acetate content of the EVA

copolymer considered in this work is relatively high (28%), and consequently the particle size is also larger, similar to those observed in Ref. 1 (where the vinyl acetate content is 45%). Peaks in the impact strength

Fig. 4. Method A (double extrusion) 80/20 PP/EVA blend; SEM micrograph technique 1 (ultramicrotomy).


Morphology and elastic properties of PPIEVA blends 23 1


and elongation at break in Ref. 1 are observed at an Figure 9 shows the same sample, having been EVA concentration of 30%, whereas here a peak in the immersed in toluene for 48 h, dried and prepared for elastic modulus is observed at 80/20 PP/EVA composi- SEM observation. The EVA phase is shown as voids tion (Fig. 1). and is distributed as nearly spherical domains in the

Fig. 6. Method A (double extrusion) 20/80 PP/EVA blend; SEM micrograph technique 1 (ultramicrotomy).




continuous PP matrix. According to our experience, this dissolution technique affects the original morphology and can lead to mistaken conclusions. A compara- tive study on the influence of the preparation technique is being prepared and will be discussed separately.

As the proportion of the EVA phase increases to 40%, the size of the EVA particles increases (Fig. lo), in agreement with the results of Ref. 1. The size of the elongated structures in samples fractured parallel to the direction of extrusion is quite large. The elongation and

Fig. 8. Method A (double extrusion) 80/20 PP/EVA blend; SEM micrograph technique 2 (freeze-fracture along the extrusion direction).


Morphology and elastic properties of PPIE V A blends 233

Fig. 9. Method A (double extrusion) 80120 PP/EVA blend; SEM micrograph of the same sample as in Fig. 8, but with toluene dissolution.

Fig. 10. Method A (double extrusion) 60140 PP/EVA blend; SEM micrography technique 2 (freeze-fracture along the extrusion direction).



Fig. 11. Method A (double extrusion) 30/70 PP/EVA blend; SEM micrograph technique 2 (freeze-fracture along the extrusion direction).

coalescence of EVA particles is clearly observed parallel In Ref. 1, at this composition, impact properties to the direction of extrusion, where the presence of the decrease due to larger particles size. elongated domains contrasts with those reported in pre- As the EVA composition increases more, to 70%, a vious studies,' where such elongation is not observed. phase inversion is observed where the PP particles are

Fig. 12. Method A (double extrusion) 30/70 PP/EVA blend; SEM micrograph technique 2 (freeze-fracture perpendicular to the extrusion direction).


Morphology and elastic properties of PPlEVA blends 235

Fig. 13. Method A (double extrusion) 70/30 PP/EVA blend; TEM micrograph technique 1 (ultramicrotomy).

embedded in a continuous EVA phase. Figures 11 and 12 show the resulting morphology parallel (Fig. 11) and perpendicular (Fig. 12) to the extrusion direction. small with a mean particle diameter of 2 pm (Fig. 12). Figure 11 shows high polydispersity of particle sizes, where the larger particles are substantially deformed.

Sizes vary from 1 to about 7pm. Deformation of particles in the plane normal to the extrusion direction is

TEM micrographs taken at 30% EVA composition following methods A (double extrusion) (Fig. 13) and

Fig. 14. Method B (blending before extrusion) 70/30 PP/EVA blend; TEM micrograph technique 1 (ultramicrotomy).



method B (blending before extrusion) (Fig. 14) also show interesting features: a much lower degree of particle coalescence is observed in Fig. 13, where particle dispersion is better. These observations are in agreement with those reported in Ref. 1 in the 70/30 PP/EVA composition. When the blends were produced by double extrusion, a major coalescence was generated (Fig. 13). These observations are quite similar to those shown in Ref. -2. It is apparent that the processing operations prior to sample preparation influence drastically the resulting morphology in these systems.

CONCLUSIONS

The results show that for a system of two immiscible polymers, as in the case of the PP/EVA blends studied here, the specific preparation method used has a clear influence on the morphology. The detailed thermodyna- mic reasons for this effect are rather complicated and beyond the scope of this article. However, some specific conclusions can be drawn. For example, at low concentrations of the EVA phase, the blend morphology is largely controlled by the continuous PP phase. As the concentration increases, this phase gradually forms itself into a network that eventually dominates the elastic modulus of the blends.

For 90/10 PP/EVA blends, a clear microstructural difference is observed between samples prepared by the two preparation methods reported here, and this differ-

ence is also reflected in the mechanical properties. The modulus for samples prepared by method B increases monotonically, whereas that for samples prepared by method A shows discontinuities, giving a clue to the homogeneity achieved in each case. Modelling of the above microstructure-morphology relationships, by using a percolation approach, is currently underway and will be reported separately.

ACKNOWLEDGEMENTS

The technical support of M. en C. Jaqueline Caiietas, Mr Juan Caire, Mr Pedro Mexia, Mr JosC Guman, Ms Trinidad Reynosor, Mr Gustavo Arango and Mr Ernest0 Sanchez is gratefully acknowledged.

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Morphology and Elastic Properties of PP/EVA Polymer Blends