Impact Modifiers Tougheners

Impact modifiers for PET are generally elastomeric compounds that increase impact strength and elongation while usually decreasing modulus. An effective way to enliaiice the impact strength and to induce a brittle/ductile transition of the fracture mode, is by the chspersion of a rubber phase witliin the PET matrix. The 

The basic mechanism of toughening is one of void formation and shear band formation (cavitation) when stress is applied. 

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The well-known examples of blends are impact modified, toughened polymers, where polymers with different glass transition temperatures are blended, such as a rubber with a thermoplastic. Many other blends are known, such as barrier polymers for packaging, where specific polar or nonpolar polymers improve the properties of polymer blends, combined to increase the resistance against transport of water and a certain gas (oxygen, carbon dioxide, etc.), such as PA (barrier to oxygen) with a polyolefin (barrier to water vapor). 

Schlagzkhmacher polym impact modifier, toughening agent Schlamm/... 

Mechanical Properties. Properties of typical grades of PBT, either as unfiUed neat resin, glass-fiber fiUed, and FR-grades, are set out in Table 8. This table also includes impact-modified grades which incorporate dispersions of elastomeric particles inside the semicrystalHne polyester matrix. These dispersions act as effective toughening agents which greatly improve impact properties. The mechanisms are not fiiUy understood in all cases. The subject has been discussed in detail (171) and the particular case of impact-modified polyesters such as PBT has also been discussed (172,173). 

Improve both impact strength and rigidity of thermoplastics by using up the energy of crack propagation. Elastomers are prototypical toughening additives. Examples of high-polymeric impact modifier/thermoplastic matrix systems are EVA, CPE and MBS in PVC, EP(D)M and SBS in PA, and acrylic rubbers in polyesters. 

Reactive impact modifiers are preferred for toughening of PET since these form a stable dispersed phase by grafting to the PET matrix. Non-reactive elastomers can be dispersed into PET by intensive compounding but may coalesce downstream in the compounder. Reactive impact modifiers have functionalized end groups. Functionalization serves two purposes - first, to bond the impact modifier to the polymer matrix, and secondly to modify the interfacial energy between the polymer matrix and the impact modifier for enhanced dispersion. Some examples of commercially available reactive impact modifiers for PET are shown in Table 14.3. An example of a non-reactive elastomer that can be used in combination with reactive impact modifiers is ethylene methyl acrylate (EMA), such as the Optema EMA range of ethylene methyl acrylates manufactured by the Exxon-Mobil Chemical Company (see Section 4.2). 

Most non-reactive (unfunctionalized) elastomeric impact modifiers such as general purpose rubbers, are not highly effective at toughening polyesters because they are unable to adequately interact with the polyester matrix so as to achieve... 

Pecorini and Calvert [28] attribute the role of small particles and a small interparticle distance to inducing high toughness in PET by promoting massive shear yielding in the matrix. Their study showed that the non-reactive impact modifier gives a system in which the rubber phase is not well dispersed. It was shown that this is not effective in toughening PET at levels of either 10 or 20%. The... 

A twin-screw extruder is generally preferred for producing rubber-toughened, glass-filled PET compounds for injection moulding applications. The PET and impact modifier are added at the throat while the glass reinforcement is added downstream. The size of the rubber domains will depend on the amount of energy and the capability of the equipment used for dispersion. 

Impact Modifiers. Notched impact strength and ductility can be improved with the incorporation of impact modifiers, which also can lower the britde— ductile transition temperature and give much improved low temperature toughness. Impact modifiers are rubbers (often olefin copolymers) that are either modified or contain functional groups to make them more compatible with the nylon matrix. Dispersion of the mbber into small (micrometer size) particles is important in order to obtain effective toughening (29). Impact modifiers can be combined with other additives, such as glass fiber and minerals, in order to obtain a particular balance of stiffness and toughness. 

It would appear that these rubbers have a similar toughening effect as the S-EB-S block copolymers. However, a synergistic effect is present when a mixture of both impact modifiers is employed [27]. 

For most applications, in particular in the automotive industry, the toughness of nylon is insufficient. The most successful method developed for modifying brittle polymers is rubber-toughening. By incorporation of a minor amount of a dispersed rubber phase (Impact modifier) the fracture resistance is improved significantly and the impact strength can be increased severalfold. There is an unavoidable decrease in flexural modulus and tensile strength, but the balance of properties of the rubber modified nylon is much better than that of unmodified nylon. Several kinds of morphology which... 

Dr. Riew has presented more than 50 technical papers and holds more than 25 patents on emulsion polymers, hydrophilic polymers, synthesis and application of telechelic polymers, and toughened plastics for adhesives and composites. His latest research is in the synthesis, characterization, and performance evaluation of impact modifiers for thermosets and engineering thermoplastics. His research interests include correlating polymer chemistry and physics, morphology, engineering, and static and dynamic thermomechanical properties to the failure mechanisms of toughened plastics. 

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