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Effect of dynamic vulcanization on the microstructure and performance of polyethylene terephthalate/elastomer blends.


Polymer blending is a widely practiced method of modifying the properties of polymers. In general, the dispersion of a rubber inside a thermoplastic crystalline polymer improves its toughness. Significant improvements over the simple blends can be achieved by dynamic vulcanization of the elastomeric phase. This process generates thermoplastic vulcanizate (TPV) blends, which have the processing characteristics of a plastic, yet perform with the flexibility and durability of a thermoset rubber. Under dynamic shear, the elastomer is preferentially crosslinked to generate fine particles in a melt processable thermoplastic matrix. Improvements in properties include higher tensile strength and impact resistance, better elastic recovery, improved property retention at high temperatures, greater resistance to chemical attack and swelling by solvents, improved fatigue resistance, greater stability of morphology and more consistent processability (1-3).

Processing Conditions

The basic parameters controlling the properties of polymer blends are the morphology of the blend, the composition, the mixing conditions, the properties of individual phases, the interfacial tension, the viscosity ratio and the elasticity ratio (4). In the case of TPVs, the curing system and the degree of crosslinking have also an effect on its properties (5-8). Since the emergence of TPV blends in the field of polymer science in the 1980s, over 500 patents and publications have appeared (2). Most of these publications are based on works done with an internal mixer and few have studied the processing-morphology-property relationships of TPV blends produced with a twin-screw extruder. Cai and Isayev studied the dynamic vulcanization of copolyester/nitrile rubber blends by using various mixing techniques such as internal mixing, open mill mixing and twin-screw extrusion (9). They showed that better performance characteristics are obtained with the latter technique. Moffett and Dekkers worked on the compatibiiz ation and dynamic vulcanization of polybutylene terephthalate (PBT) and an epoxy grafted ethylene-propylene-diene (EPDM) terpolymer (10). During melt mixing in the twin-screw extruder, the epoxy groups react with PBT endgroups to form a graft copolymer while the EPDM is crosslinked by a peroxide or a diamine. Blends obtained with the diamine vulcanization gave the same level of elastic recovery as the peroxide vulcanization and was proposed to be an alternative vulcanization method that eliminates degradation of the thermoplastic induced by free radical generation. In a recent work by Verbois et al. (11), a TPV blend based on ethylene-vinyl acetate and polypropylene was produced by extrusion using a silane as the crosslinking agent. The evolution of the morphology was also followed during blending and crosslinking in an internal mixer. In both processes, the phase inversion region was found to be at similar gel content.

Phase Inversion

In TPV blends, the most common morphology is a dispersed elastomeric phase in a thermoplastic matrix. This morphology is usually made of droplet particles and when the rubber is present as the major component, large globular domains dispersed in the thermoplastic are observed (6, 12). At the initial stages of dynamic vulcanization, a co-continuous structure can be generated, and, as the degree of crosslinking progresses to completion during mixing, a dispersed phase is always obtained (12). Karger-Kocsis recently claimed the presence of an interpenetrating network (IPN) from a dynamically cured LDPE/ SBR blend, but it is restricted to only a microscale level (3). Nowadays, polymer blends exhibiting a co-continuous structure are of special interest because the properties of both components can be fully exploited. The development of continuity is well described by many authors (13-17). At low concentration, a two-phase blend exhibits a dispersed phase/matrix type morphology. The further addition of minor phase will lead to a percolation point, and at higher concentrations, phase inversion occurs. At that point, the two immiscible phases commingle in such a way that each phase remains continuously connected throughout the bulk of the blend. Most studies on co-continuity have been carried out on immiscible binary blends and in particular with respect to the role of the viscosity ratio and elasticity ratio in determining the position of phase inversion. Recently, Li et al. went even further to investigate the role of the blend interface type on co-continuous morphologies (18). Some models have been developed to predict the phase inversion point based on the phase deformability or on a thermodynamic approach. The first model based on the phase deformability was proposed by Paul and Barlow (19). It is based on observations made by Avgeropoulos (20) and was generalized by Miles and Zurek (21) as:

[[theta].sub.1]/[[theta].sub.2] = [[eta].sub.1]/[[eta].sub.2] = [lambda] (1)

where [[theta].sub.i] and [[eta].sub.i] are the volume fraction at phase inversion and the viscosity of phase i at the shear rate of blending respectively. Another model has been proposed by Metelkin and Blekht and is based on the theory of capillary instabilities (22):

[[theta].sub.2] = 1/1 + [lambda]F([lambda]) (2)

where F([lambda]) = 1.25 log [lambda] + 1.81 [(log [lambda]).sup.2] and [lambda] = [[eta].sub.1]/[[eta].sub.2]

According to these equations, these models predict that the less viscous phase has the greatest tendency to be the continuous phase. Hence, the more easily deformed phase will tend to encapsulate the least deformed one. From a thermodynamic point of view, Van Oene has demonstrated that the interfacial tension under dynamic conditions will be lower for a system with a more elastic matrix and a less elastic dispersed phase (23). Bourry and Favis (13) observed a correlation between the elasticity ratio and the phase inversion point. They postulated the following equations:

[[theta].sub.1]/[[theta].sub.2] = [G'.sub.2]/[G'.sub.1] (3)

and by using the tan [[delta].sub.i] = [G.sup.n.sub.i]/[G'.sub.i]

[[theta].sub.1]/[[theta].sub.2] = tan [[delta].sub.1]/tan [[delta].sub.2] (4)

Therefore, it would be expected that the more elastic phase would tend to encapsulate the less elastic one (24). Hence, it is a challenge to elaborate the effect of dynamic vulcanization on the phase inversion region in TPV blends since no such work has been rigorously conducted to date.

In this study, the objective is to elaborate a new type of TPV blend based on a polyethylene terephthalate (PET) and ethylene-ethyl acrylate-maleic anhydride (EEA-MAH) system dynamically crosslinked by the reaction of a diamine with MAH moieties and prepared in a twin-screw extruder. The composition range of phase inversion will be defined. The influence of viscosity and elasticity on this phenomenon will be considered. The effect of dynamic crosslinking and blend composition on the microstructure and mechanical properties of a PET/E-EA-MAH system will be investigated



The polyethylene terephthalate (Selar PT 7086) supplied by DuPont is a homopolymer having an intrinsic viscosity of 1.0 dL/g, a melting point of 254[degrees]C and a crystalline density of 1.40 g/[cm.sup.3]. The E-EA-MAH (Lotader 4700) obtained from Atofina is a random terpolymer with a content of 29.6% by weight of ethyl acrylate and 1.5% of maleic anhydride. It has a melting point of 65[degrees]C and a density of 0.94 g/[cm.sup.3]. at 25[degrees]C. The densities of PET and E-EA-MAH at 270[degrees]C were measured by a PVT-Gnomix apparatus and are found to be 1.17 g/[cm.sup.3]. and 0.77 g/[cm.sup.3]. respectively. The crosslinking agent used is a polyoxypropylenediamine (Jeffamine D-230) supplied by Huntsman. It is a difunctional primary amine having an average molecular weight of approximately 230 g/mol. The crosslinking reaction is illustrated in Fig. 1.

Curing Kinetics in the Brabender

A Brabender Plasticorder PL2000 was used to blend and cure a sample of E-EA-MAH using the diamine as the crosslinking agent at a ratio of [NH.sub.2]/MAH = 0.5. With the temperature of the mixing chamber initially set at 270[degrees]C and the rotors turning at 60 RPM, a volume of 55 ml of E-EA-MAH was first fed into the chamber and was allowed to mix for 5.5 mm under a flow of dry nitrogen. Next, the diamine was introduced and the mixture was blended for another 5.5 mm. The melt was rapidly quenched in liquid nitrogen to freezein the morphology. The cured elastomer was then treated with toluene to evaluate its gel content.

Blend Preparation by One-Step Extrusion

Prior to mixing, the PET was dried overnight at 130[degrees]C to minimize hydrolytic degradation. The blends were all prepared in a one-step extrusion process in a Leistritz 34 mm co-rotating twin-screw extruder. The screw configuration is given in Fig. 2. The resins were first dry blended and fed at 10 kg/h into the hopper under nitrogen. The extruder was operated at a screw speed of 200 RPM and a barrel temperature of 270[degrees]C. In the case of dynamically vulcanized blends, the diamine was directly fed into the extruder at 1/3 of its length using a HPLC pump and the unreacted diamine was removed through a devolatilization port. The ratio of [NH.sub.2]/MAH was maintained at 0.75. At the die exit, all the blends were quenched in cold water and then pelletized. The concentrations given in the text are based on the volume fraction.


Rheological characterization was carried out using an ARES (Advanced Rheometric Expansion System) at 270[degrees]C. Measurements were performed under a blanket of dry nitrogen using a parallel-plate tooling geometry setup. A 15% strain was used for all dynamic tests. This is well within the linear viscoelastic region as determined by a strain sweep experiment. Pellets were loaded between the parallel plates and the disk-shaped specimen was prepared in the rheometer oven chamber. This procedure allows avoidance of morphological changes that can result from an additional sample preparation step, minimizes thermal degradation, and ensures that all the specimens experienced the same thermal history. After melting, the gap was set and the excess material was trimmed. A 3-minute soak time was allowed prior to the beginning of the test to allow for the thermal equilibrium of both the instrument and the specimen. The rheological curves of the pure components were obtained by this method and are shown in Fig. 3. The P ET used in this study is a high molecular weight polymer and has a higher viscosity than the elastomer over the whole frequency range investigated. In order to verify the stability of the dynamically vulcanized blends after extrusion, a time sweep test was performed.

Rheological measurements were performed on samples of E-EA-MAH containing diamine in a ratio of [NH.sub.2]/MAH = 0.75. Blends were first prepared in a Brabender operating at 60 RPM and 50[degrees]C. This temperature was chosen in order to reduce crosslinking in the mixing chamber. Initially, E-EA-MAH was added in the chamber and was allowed to melt mix for 8 min. The diamine was then incorporated and further mixing was maintained for another 5 min. The blend was removed to be compression molded into disks 25 mm in diameter at 100[degrees]C for 2 minutes with a load of 6 tons.

Morphology Observations, Extent of Continuity and Gel Content

The morphology of the blends was studied with scanning electron microscopy (SEM) by examining the cryofractured surface and the microtomed surface (transverse and axial direction) of extruded strands sputtered previously with gold-palladium. Micrographs were taken using a JEOL JSM 840 scanning electron microscope at 5 and 10 kV.

For the uncrosslinked blends, the extent of continuity was quantified using a gravimetric method. The extraction of the dispersed E-EA-MAH was performed on extruded strands in a Soxhlet extraction apparatus using toluene for 24 hrs. The continuity of the E-EA-MAH is defined as the weight of E-EA-MAH present initially minus the calculated weight of the residual E-EA-MAH after extraction all divided by the weight of E-EA-MAH present initially.

The gel content of dynamically vulcanized blends was determined by extraction of powdered samples in a Soxhlet apparatus. This was achieved in two steps. First, it is necessary to remove the PET phase with pure trifluoroacetic acid for 24 hours since no solvent can extract both the PET and the uncrosslinked elastomer phase. Then, the uncrosslinked E-EA-MAH phase was extracted with fresh toluene for 72 hours. The gel content is then calculated.

Mechanical Properties

The blends were dried at 100[degrees]C for 12 hours prior to injection molding into ASTM type IV dumbbells and Izod bars. Tensile tests were performed at room temperature according to ASTM D638 at an elongation rate of 20 mm/min. This speed of testing was selected in order to produce rupture of all the specimens within 0.5 to 5 minutes. The Izod impact strength was measured using notched specimens according to ASTM D256 on a Custom Scientific Instruments model CS-137C impact tester. Notched impact testing eliminates the effect of the skin morphology.


Some works have reported a degradation reaction that causes chain scission of PET fibers by aminolysis with some amines leading to a reduction of desired properties (25-28). This reaction was not found to occur in our system as confirmed by infrared spectroscopy analysis of extruded PET/diamine blends. The IR spectra did not show any peak at 1630 [cm.sup.-1] attributed to the presence of amide moieties, the expected product of PET aminolysis.

Prior to conducting studies in the extruder, E-EA-MAH and diamine were blended in the Brabender in order to estimate the curing kinetics. As depicted in Fig. 4, the crosslinking can be easily followed by monitoring the mixing torque. Initially, E-EA-MAH was added and was allowed to mix until equilibrium. A drastic increase of the torque with time was registered, once the curing agent was added, as a result of the crosslinking of the elastomer. The maximum torque was reached within 1 min. indicating completion of the reaction. This fast reaction rate with maleic anhydride groups is provided by the strong nucleophilic character of amine end-groups. Such time-scale is consistent with the residence time in the twin-screw extruder. However, a part of the curing agent evaporated after addition owing to the high temperature in the Brabender. A lubricating effect of this viscous fluid acting on the walls and rotors of the mixing chamber was also observed. As a result, the elastomer is less crosslinked than expected a nd an inhomogeneous system is obtained. Melt mixing with the twinscrew extruder enables the pumping of the diamine directly in the pressurized polymer melt and also provides high shear rates required to break up the particles of elastomer into a fine and homogeneous dispersion.

Uncrosslinked Blends

Morphology of uncrosslinked blends at 20%, 30%, 40% and 80% E-EA-MAH are depicted in Fig. 5. At 20% E-EA-MAH. the elastomer is dispersed as droplets in the PET matrix. With an increase in the concentration to 30% of E-EA-MAH, an almost co-continuous structure is observed with 96% of E-EA-MAH continuity after extraction. This is well illustrated on the corresponding micrograph. Extraction of the blend with toluene at 40% E-EA-MAH disintegrated the structure, indicating the occurrence of phase inversion between 30% and 40%. The micrographs of cryofractured blends from 40% to 80% E-EA-MAH clearly illustrate the evolution from mainly droplets and elongated fibers of PET dispersed in the elastomer matrix to a fully nodular morphology.

The predicted phase inversion point for this system was calculated from Eqs 1 to 4 using viscosities, storage moduli, and tan [delta] at shear rates of 1, 10 and 100 rad/s. Deformation rates greater than 100 rad/s are typical in the twin-screw extruder. Results are reported in Table 1. At these shearing rates, the models from the phase deformability approach underestimate the phase inversion point, whereas the empirical equations from the thermodynamic approach overestimate it. Although these models and equations failed to predict the phase inversion point, it is found that models based on the phase deformability correctly predict the encapsulating matrix. At 270[degrees]C and 100 rad/s. E-EA-MAH is 20 times less viscous than PET. Based on our observations, the less viscous and more easily deformed phase dominates and has the greatest tendency to become the matrix for the uncrosslinked system.

Dynamically Vulcanized Blends

One has to be careful when interpreting SEM and TEM micrographs of complex or co-continuous morphologies since these do not provide an accurate representation of the three-dimensional structure and hence can be misleading. One way to characterize a co-continuous morphology is through gravimetric measurements in which one phase is completely dissolved by a selective solvent without collapsing the structure. Unfortunately, this technique cannot be applied to TPV blends because of the insoluble crosslinked elastomer and its swelling behavior. Instead, a judicious examination of the axial and transverse directions was performed on microtomed samples, allowing for the range of co-continuity to be established. Figure 6 depicts the morphologies of blends at 40%, 70% and 80% E-EA-MAH. The high contrast obtained in these SEM micrographs is sufficient to distinguish the two distinct phases. The dark phase on the micrographs corresponds to the crosslinked E-EA-MAH phase. This contrast effect compensates to a degree for the lack of a solvent extraction step. Micrographs of the TPV blend at 40% and 70% of elastomer in Figs. 6a and b clearly depict a dispersion of elongated fibers of E-EA-MAH in the PET matrix. E-EA-MAH is the dispersed phase at concentrations up to 70%. With a further increase in elastomer content to 80%, the crosslinked E-EA-MAH becomes the continuous phase with PET dispersed as elongated fibers. These observations are clearly reflected in Figs. 6b and c and indicate that phase inversion has occurred at an elastomer concentration between 70% and 80%. By comparing the uncrosslinked and crosslinked systems, dynamic vulcanization shifts the midpoint of phase inversion from approximately 35% to 75% of elastomer content. This shift is associated to the increase of viscosity and elasticity of the elastomer phase due to the dynamic crosslinking and is expected from phase inversion models based on the phase deformability.

The degree of crosslinking is obtained by measuring the gel content (i.e. unextractable crosslinked phase) after the extraction of the PET phase and the uncrosslinked E-EA-MAH phase. Figure 7 shows the gel fraction based on E-EA-MAH as a function of the composition of E-E-MAH in TPV blends. For the same [NH.sub.2]/MAH concentration ratio, the degree of crosslinking is almost negligible at 20% of elastomer content because of a weak network formed at low concentration. As the elastomer concentration is increased, the gel content increases until a plateau is reached at 50% of E-EA-MAH. The maximum gel content obtained is around 33%.


The crosslinking reaction was monitored by curing the elastomer with diamine in the rheometer in order to evaluate the viscous and elastic properties as the material crosslinks. The dynamic viscosity and modulus at 270[degrees]C of an E-EA-MAH/diamine blend as a function of time at 1 rad/s are shown in Fig. 8. Note the absence of the crossover modulus, indicating that the gel point is already exceeded. The crossover between G' and G" is often defined as the gel point (29). At all times, the elastic behavior of the rubbery network formed (G' > G") dominates. This is not the case for neat E-EA-MAH, which demonstrates the opposite behavior (see Fig. 3). The kinetics of crosslinking is slower in the rheometer than in the Brabender because most of the crosslinking reaction occurred during preparation, and this cannot be avoided. After 10 min of curing in the rheometer, the viscosity of the elastomer increases by a factor of 12, while its storage modulus shows a 600-fold increase when compared to neat E-EA-MAH. Aft er one hour, the viscosity has not reached an equilibrium value. E-EA-MAH becomes less deformable after crosslinking, and this explains the direction of the shift of the region of dualphase continuity.

Figure 9 shows the rheological curves of the 40/60 PET/E-EA-MAH blends before and after crosslinking, Stability measurements made on the crosslinked blend did not show any increase of the viscosity with time, thus evidencing that no further reaction occurred once the material is extruded. Adding 40% PET in uncrosslinked E-EA-MAH slightly increases the viscosity and dynamic moduli over those of neat E-EA-MAI-I. In contrast, the crosslinked blend has a much higher viscosity and moduli than the uncrosslinked material, especially in the low frequency range. The rheology curves are reflecting the phase inversion phenomenon evidenced by the morphology analysis. The uncrosslinked blend is composed of an E-EA-MAH matrix containing viscous PET domains. It then behaves like neat E-EA-MAH. The reactively modified blend is made of a viscous PET matrix filled with partially crosslinked E-EA-MAH domains. As a result, the material has a lower viscosity than the neat PET and shows a severe shear-sensitivity over the whole fr equency range investigated.

Mechanical Properties

Figure 10 shows the tensile properties of blends with and without diamine crosslinking agent as a function of composition. In general, tensile strength and modulus are reduced as the elastomer content is increased for both cases. These reductions should be expected as a result of the rubbery nature of E-EA-MAH. However, dynamic crosslinking slightly increases the tensile strength and modulus, but decreases the elongation at break probably because of the stiffening of the elastomer phase after crosslinking. Similar results are also reported for other systems (6, 30). Tensile properties of blends with less than 30% E-EA-MAH are not affected by the diamine crosslinking agent because of the low degree of crosslinking obtained. In the uncrosslinked blends, the elongation at break curve displays a minimum in the phase inversion region at 30%-40% of E-EA-MAH. This phenomenon is also observed for the crosslinked system with the transition located in the 70%-80% range where phase inversion occurs. Both uncrosslinked a nd crosslinked systems display a negative deviation in the elongation at break, which is symptomatic of poor interfacial adhesion. This PET/E-EA-MAH system has a high interfacial tension probably similar to a PET/HDPE system. For a PET/HDPE system, Ihm et al. determined a value of 9.7 mN/m at 270[degrees]C using the breaking thread method (31).

Polymers can break in distinct manners under impact. The type of fracture can be characterized from the amount of plastic deformation at the crack tip and the stability of crack propagation. Many observations can help to distinguish whether a fracture occurred in a ductile or fragile manner. When a fracture is brittle, the crack grows in an unstable manner with a speed greater than the impact speed. This leads to the phenomenon of shattering of the part in practice and to rough surfaces often showing a branching effect. When the sample breaks in a ductile manner, the two halves remain attached by a thin ligament and are pushed away by the hammer at a much lower velocity. The surface exhibits a whitening effect or could become bright, reflecting light due to craze formation. Materials exhibiting a ductile mode of fracture perform better in terms of impact resistance than a brittle material. Figure 11 shows the variation of the Izod impact strength of notched samples of uncrosslinked and dynamically crosslinked blends with E-EA-MAH content. In the first system, the impact strength increases gradually with E-EA-MAH content until 30% and then reaches a plateau. As the E-EA-MAH content is increased from 0% to 30%, a nearly 4-fold increase in the impact strength is observed. At 20% E-EA-MAH, the sample demonstrates a fragile fracture behavior whereas blends with E-EA-MAH contents above 30% exhibit a ductile mode of fracture. Fractured surfaces of these blends are shown in Figs. 12a and b. The fragile-ductile transition appears to be related to the morphological change from dispersed droplets to a co-continuous structure. Factors governing this will be studied in a future paper. In all the range of studied compositions, dynamically vulcanized blends have a higher impact strength than the corresponding uncrosslinked blends. At 40% E-EA-MAH, dynamic vulcanization enhances the impact strength of the blend by a factor of 5. Compared to the neat PET, it is a 17-fold increase. In these dynamically vulcanized blends, the impac t strength increases with the elastomer content up to 20% E-EA-MAH and rises significantly between 20% and 40% E-EA-MAH. In this range of compositions, the fracture behavior changes from a fragile to a ductile mode. The fractured surfaces at 40% E-EA-MAH is depicted in Fig. 12c. At 60% of elastomer content, the samples did not break under the conditions used in this study.

From the mechanical performance, it is shown that dynamic vulcanization dramatically improves the impact strength while decreasing the elongation at break. In general, these two properties should follow the same trend but it appears that dynamically vulcanized blends demonstrate anomalous behavior. It appears that the impact strength in dynamically vulcanized blends is not as sensitive to the poor interfacial adhesion as the elongation at break. Future work leading to the understanding of the fracture mechanisms of these blends will be conducted. This can also be an important route to establish novel polymer blends in the absence of an interfacial modifier.

Future work will be carried out in order to study the effect of compatibilization, process conditions, and the concentration of the crosslinking agent on the structure-property relationship. Systems with various viscosity ratios and elasticity ratios will also be examined in future studies.


Blends of PET/E-EA-MAH were dynamically crosslinked in a one-step extrusion process by a bi-functional amine-terminated glycol. The phase inversion was shifted from the 30%-40% range to the 70%-80% range of elastomer content after dynamic vulcanization. The phase inversion point of the uncrosslinked blends is not well predicted using models and empirical equations based on the thermodynamic approach or on the phase deformability. However, models based on the phase deformability correctly predict the matrix. The low viscosity of pure E-EA-MAH is found to predominate and to preferentially encapsulate the highly viscous PET. Dynamic vulcanization slightly improves the tensile strength and modulus while decreasing the elongation at break due to a weak interfacial adhesion. A significant increase of 17-fold in the impact strength is obtained over the neat PET.







able 1

Prediction of Phase Inversion Point for the Uncrosslinked System

Shear rate [[eta].sup.*.sub.1] /
(rad/s) [[eta].sup.*.sub.2] [G'.sub.2]/[G'.sub.1]

 1 0.013 369.8
 10 0.027 88.6
 100 0.047 32.9


 Predicted E-EA-MAH volume
 at phase inversion point
Shear rate tan [[delta].sub.1] / Paul & Metelkin
(rad/s) tan [[delta].sub.2] Barlow & Blekht

 1 5.50 0.013 0.040
 10 3.07 0.026 0.049
 100 1.97 0.045 0.054

Observed 0.35

 Predicted E-EA-MAH
 volume fraction
 at phase inversion
Shear rate
(rad/s) Eq 3 Eq 4

 1 0.997 0.846
 10 0.989 0.754
 100 0.971 0.663



2: PET


The authors would like to express their appreciation to Mr. Robert Lemieux for his help in carrying out the blend preparation, to Mr. Pierre Sammut for providing the rheological data and to Mr. Michel Carmel for preparing the injection molded samples. Appreciation is also extended to Novoplas, the National Research Council of Canada, and the National Science and Engineering Research Council of Canada for funding of this project.


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Basil D. Favis *

* To whom correspondence should be addressed.
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Author:Ma, Pei Lian; Favis, Basil D.; Champagne, Michel F.; Huneault, Michel A.; Tofan, Florin
Publication:Polymer Engineering and Science
Date:Oct 1, 2002
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