Blends of polyamide (PA) and acrylonitrile-butadiene-styrene (ABS) copolymers yield polymeric materials that are highly solvent resistant, easy to process and have high impact strengths over a wide temperature range. These properties make these blends interesting materials for various applications such as automotive applications. However, due to the immiscibility of PA and ABS a compatibilizer is necessary to obtain well dispersed blends and good interfacial adhesion between both blend phases. A fine dispersion and good adhesion between the blend phases is needed to obtain tough polymer blends. Up to now, a significant amount of research was focused on the factors that influence the compatibilization of PA/ABS blends. In these investigations it was shown that styrene-acrylonitrile-maleic anhydride (SAN-MAh) terpolymers are effective reactive compatibilizers for PA/ABS blends. During melt blending the amine endgroup of polyamide molecules reacts with MAh groups of the terpolymer, yielding SAN-g-PA copolymers at the blend interface. An optimum in the terpolymer MAh content was reported at which maximum blend impact strength was obtained. The objective of the research described in this thesis was to gain more insight into the compatibilization mechanism of reactive SAN-MAh compatibilizers in PA6/ABS blends. Well-defined SAN-MAh terpolymers were prepared to investigate the simultaneous influence of MAh content and compatibilizer molar mass on the impact strength of PA/ABS blends. In addition, the influence of the chemical composition distribution of the SAN-MAh terpolymers on the blend toughness was studied. To produce well-defined SAN-MAh terpolymers in a continuously operated stirred tank reactor, a model for the terpolymerization reaction of styrene, acrylonitrile and MAh was developed. Chapter 2 describes the development of the model which is based on the penultimate unit approach. The model accurately predicts the copolymer composition and molar mass in the product stream at steady state for the production of SAN-MAh terpolymers in a CSTR, as reported in Chapter 4. 180 Summary The techniques developed for the characterization of the SAN-MAh terpolymers are described in Chapter 3. A SEC method was developed which enables accurate determination of the MAh content in SAN-MAh polymers, even at low MAh contents, i.e. <0.5 wt%. Next to the determination of the overall MAh content, a method was developed to measure the average weight fraction of MAh as function of the molar mass distribution (MMD). The analysis of the average copolymer composition was carried out with Fourier Transform InfraRed (FTIR) spectroscopy. Chapter 5 describes the simultaneous influence of MAh content and molar mass of reactive SAN-MAh compatibilizers on the blend impact strength of PA/ABS blends. Contrary to literature, our results indicate that no clear optimum in the relation between MAh content and impact strength exists. Our results demonstrate that at least two MAh units per SAN-MAh molecule are needed to create effective reactive compatibilizers for blends of PA and ABS. Increasing the compatibilizer MAh content, as well as changes in molar mass of the reactive compatibilizer, did not have a significant influence on the blend toughness. Moreover, orientation in the material turned out to have a large influence on the blend impact strength. A four times higher impact strength was measured perpendicular to the orientation compared to measurements parallel to the orientation. A possible explanation for the influence of orientation on the blend impact strength is that craze growth in the PA phase perpendicular to the orientation is terminated by the rubber particles in the neighboring ABS phases, while parallel to the orientation crazes can develop throughout the length of the PA domains, leading to fatal cracks. Next to the blend impact strength, the Melt Volume-flow Rate (MVR) of the blends was investigated. A relation between the MVR of the blend and the MAh content of the compatibilizer was found. Transmission Electron Microscopy (TEM) revealed that a significant number of micelle-like structures were present in the PA phase of the blend. These micelles are formed by compatibilizer molecules which are pulled away from the interface during extrusion. More and smaller micelles were formed with increasing compatibilizer MAh content. These micelle-like structures lead to an increase in the viscosity of the blend and hence reduce the blend MVR. Summary 181 Chapter 6 deals with the influence of compatibilizer structure on the impact strength of PA/ABS blends. The feasibility of in-situ production of SAN-b-PA diblock copolymers during extrusion of the blend was investigated. It was expected that these block copolymers remain at the blend interface during extrusion, as the SAN block of the copolymer was expected to form more entanglements with the SAN phase of the ABS compared to the relative short SAN blocks between the reacted MAh units in SAN-g-PA graft copolymers. Reversible Addition-Fragmentation chain Transfer (RAFT) polymerization was used for the production of MAh end-functional SAN copolymers in a single CSTR. The MAh end-functional SAN copolymers were used as reactive compatibilizers in blends of PA and ABS. However, the in-situ made SAN-b-PA block copolymers did not result in well dispersed PA/ABS blends and, hence, no tough blends were obtained. The low amount of MAh present in the blends and the non-optimized conditions for the RAFT polymerization are most likely the reason for the low degree of dispersion and the low blend impact strengths. However, the RAFT technique proved to be a useful route to prepare end-functional polymers. Optimization of the RAFT polymerization conditions is required to further investigate the influence of in-situ formed SAN-b-PA block copolymer compatibilizers on the impact strength of PA/ABS blends.
|Qualification||Doctor of Philosophy|
|Award date||15 Dec 2005|
|Place of Publication||Eindhoven|
|Publication status||Published - 2005|