Impact modifiers thermosets

Key words polysulfone, membrane, thermoset, impact-modified,... 

Although the main use of impact modifiers is with thermoplastics, thermosets also benefit. The agent is added at the monomer stage. Thus epoxy polymers can be made less brittle by the addition of rubbers. Care has to be taken that the high temperature properties of the thermoset are not compromised. 

Core-shell emulsion polymers with a core or rubbery stage based on homopolymers or copolymers of butadiene are used as impact modifiers in matrix polymers, such as ABS, for styrene acrylonitrile copolymer methyl methacrylate (MMA) polymers, poly(vinyl chloride) (PVC), and in various engineering resins such as polycarbonate) (PC) poly(ester)s, or poly(styrene)s, further in thermosetting resins such as epoxies. 

Core-shell polymers were commercially introduced as impact modifiers for poly(vinyl chloride) PVC, in the 1960s. They are produced by a two-stage latex emulsion polymerization technique (Cruz-Ramos, 2000). The core is a graftable elastomeric material, usually crosslinked, that is insoluble in the thermoset precursors. Typical elastomers used for these purposes are crosslinked poly(butadiene), random copolymers of styrene and butadiene,... 

DMTA is a very interesting tool for characterizing heterogeneous materials in which domains of distinct Tg values coexist. The most interesting cases involve modified thermosets of different types (see Chapter 8). Examples are the use of rubbers (e.g., liquid polybutadiene and random copolymers), or thermoplastics (e.g., polyethersulphone or polyetherimide in epoxy matrices or poly(vinyl acetate) in unsaturated polyesters), as impact modifier (epoxies), or low-profile additives (polyesters). The modifier-rich phase may be characterized by the presence of a new a peak (Fig. 11.10). But on occasions there may be superposition of peaks and the presence of the modifier cannot be easily detected by these techniques. If part of the added polymer is soluble in the thermoset matrix, its eventual plasticizing effect can be determined from the corresponding matrix Tg depletion, and the... 

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. 

The polyester-type thermoset polyurethanes were commercialized in 1942, and the linear thermoplastic ones (TPU) ten years later. Polyester-type TPU, Texin resins for extrusion and injection molding, were introduced in 1961, whereas polyether-type, Roylar , in 1971. Owing to great diversity of the ingredients, the TPU performance can be readily modihed. Por this reason, as well as because of the cost, TPU s are seldom blended. Their use can be divided into three groups (i) blends with POM, (ii) blends in which TPU is used as a com-patibilizer and impact modifier, and (iii) others. 

Rubber toughened epoxy resins are the well known examples of impact modified thermosets utilizing reactive rubbery prepolymers. Epoxy resins can be toughened or flexibilized by any one of the following types of oligomeric reactive elastomers ... 

EPDM Rubber Sulfur-vulcanizable thermosetting elastomer produced from ethylene, propylene, and a small amount of nonconjugated diene such as hexadiene. Has good weatherability and chemical and heat resistance. Used as impact modifiers and for weather stripping, auto parts, cable insulation, conveyor belts, hoses, and tubing. Also called EPDM. 

A final word in this short summary of impact modification concerns the amount of imagination compounders and polymer scientists continue to invest in the development of rubber-modified compoimds. Efforts are being made to decrease their compounding costs, to enhance their paintability, and to formulate them to replace more competing materials such as ABS, thermoset rubber, and PVC—thereby allowing more recyclable "aU-olefin" automotive interiors, for example. Recent efforts have even been made to incorporate ground vehicle tire rubber or other "ultrafine rubber particles into PP to create vulcanized elastomeric thermoplastics that cost less than half as much as standard TPVs. Similar concepts will continue to push impact-modified POs into new applications [7-26, 7-27, 7-28]. 

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