Modified thermosets

R. Siebert, "Rubber-Modified Thermoset Resins," in C. K. Riew and J. K. GiUham, eds., ACS Advances in Chemistry Series 208, American Chemical Society, Washington, D.C., 1983, p. 179 W. D. Bascom and D. L. Hunston, "Rubber Toughened Plastic," Adv. Chem. Ser. No. 222, American Chemical Society, Washington, D.C., 1989. 

Riew, C.K. and Gillham, J.K. Rubber Modified Thermoset Resins. Advances in Chemistry, 208, American Chemical Society, Washington, DC, 1984. 

Williams, R.J.., Rozenberg, B. A. and Pascault, J.-P. Reaction Induced Phase Separation in Modified Thermosetting Polymers. Vol. 128, pp. 95-156. 

Solvent-modified thermosets display enhanced toughness due to the incorporation of a second phase material. A brittle-tough transition has been observed which cannot be attributed to changes in the interparticle distance. The chemically induced phase separation technique offers new routes and strategies to prepare such materials and enter new areas of applications. Hence, engineered porosity is demonstrated as a research concept developed into a toolbox for material scientists. 

Fiber reinforced prepregs containing a MMBS modified thermoset resin are particularly useful in the fabrication of electrical and office equipment enclosures (24). 

A composition is prepared by dissolving the MMBS modified thermoset resin in acetone. To the resulting solution, additives may be added, for example, pigments, flame retardants, lubricants or cure accelerators, e.g., hexamethylenetetramine. 

The effect (or lack of effect) of crosslinks on basic physical properties of thermosetting polymers is discussed in Chapter 10, while the effect on elastic and viscoelastic properties is analyzed in Chapter 11. Yielding and fracture of neat and modified thermosetting polymers are discussed in Chapters 12 and 13. Finally, the very important problem of the durability of polymer networks is presented in Chapter 14. 

There are two main procedures used to generate a second phase in a modified thermoset ... 

Figure 8.4 Temperature vs. composition transformation diagram for a modified thermoset with an upper critical solution temperature (UCST) behavior (Crit = critical point for a and p see text).

Another procedure for the preparation of modified thermosets consists of introducing preformed particles in the initial formulation. This technique is also well documented for modified thermoplastics (Paul and Bucknall, 2000). In Chapter 7 different macromolecular architectures such block copolymers, crosslinked microparticles, hyperbranched polymers, and den-drimers, were presented (Fig. 7.11). All these compact molecules can be used as thermoset modifiers. Thermoplastic powders and core-shell polymers are the more accessible preformed molecules. Some examples are given below. 

Depending on applications, both procedures used to generate two-phase morphologies in modified-thermosets (a) reaction-induced phase separation (RIPS) and (b) the use of preformed particles - have their advantages... 

Riew CK, Gillham JK (eds), Rubber-Modified Thermoset Resins, Adv. Chem. Sci., 208, ACS, Washington DC, 1984. 

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... 

The fracture modeling of rubber-modified thermosets was developed by Huang and Kinloch (1992a), Kinloch and Guild (1996), Huang et al. (1993b), and Yee et al. (2000). 

Following the requests to increase toughness by keeping a high Tg, for several applications (the aerospace industry in particular), high-Tg or semicrystalline thermoplastics (TP) can be used instead of rubbers to modify thermosetting polymers (Hedrick et al., 1985 Pearson, 1993 Hodgkin et al., 1998 Pascault and Williams, 2000). 

Different TPs have been used to modify thermosets, such as poly(ether sulfone) (PES), polysulfone (PSF), poly(ether ketone) (PEK), polyether imide (PEI), poly(phenylene oxide) (PPO), linear polyimides, polyhydan-toin, etc. (Stenzenberger et al., 1988 Pascal et al., 1990, 1995 Pascault and Williams, 2000). 

When TP-modified thermosets are used as matrix for composites, the resulting morphologies can be strongly affected by the presence of glass or carbon fibers (Varley and Hodgkin, 1997). 

Core-shell rubber (CSR) particles are prepared by emulsion polymerization, and typically exhibit two or more alternating rubbery and glassy spherical layers (Lovell 1996 Chapter 8). These core-shell particles are widely used in thermoplastics, especially in acrylic materials (Lovell, 1996), and have also been used to modify thermosets, such as epoxies, cyanates, vinyl ester resins, etc. (Becu et al., 1995). 

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