Rubber matrix hardeners

In recent years PETN sheet explosive, consisting of PETN in a rubber-like elastic matrix, has found considerable use in metal-forming, metalcladding and metal-hardening. Physical expl characteristics of rubber-bonded sheet expl are described by W. Kegler R. Schall (Ref 45, p 496), by Kegler (Ref 59), and in Refs 30c,... 

Formulations have been developed where small rubber domains of a definite size and shape are formed in situ during cure of the epoxy matrix. The domains cease growing at gelation. After cure is complete, the adhesive consists of an epoxy matrix with embedded rubber particles. The formation of a fully dispersed phase depends on a delicate balance between the miscibility of the elastomer, or its adduct with the resin, with the resin-hardener mixture and appropriate precipitation during the crosslinking reaction. 

Because of its elastic texture and tendency to harden, marshmallow was assumed to exist in a rubber state. Molecules in the rubbery state of marshmallow can be mobile and lead to sugar crystallization, loss of moisture from the matrix, cross-linking of proteins in gelatin, and collapse of foam. In order to stabilize the product, it is necessary to identify the factors limiting the shelf life of the marshmallow under a specific storage condition. 

This is a simplification of the process occurring in a curing resin-hardener system and a detailed discussion may be found in Pascault et al (2002), Williams et al (1997) and Inoue (1995). The main parameter that it is important to control in the reactive phase separation is the diameter of the elastomer particle. This is because the toughness of the resulting network is controlled by the energy-absorbing mechanisms such as particle cavitation and rubber bridging of cracks. Also of importance is the limitation of the effect of the rubber dispersed phase on the critical properties of the cured epoxy resin such as the stiffness and Tg. This will be affected by the extent to which the rubber dissolves in the matrix-rich phase. 

Figure 15.7a shows that the two phases are with irregular domain sizes and shapes. This indicates that the NR/EPDM blends were completely immiscible, large EPDM domains being dispersed in the NR matrix. The average domain size of the dispersed phase was 4.1 pm. The compatibility of the NR/EPDM system was improved by the addition of a compatibilizer, as can be seen in Fig. 15.7b-g the treatment resulted in noticeable surface hardening, and the physical changes in the surface were expected to influence physically both the deformation and adhesion of the two mbbers, that is, the compatibilizers improved both the morphology and compatibility of the blends because of the reduction in the interfacial tension between EPDM and NR rubbers. The size of the dispersed phase (EPDM) domain decreased with the addition of compatibilizers, and no gross phase separation was present in the blends (Fig. 15.7). For NR/BR/EPDM, the domain size was approximately 3.8-1.26 pm NR/PVC/EPDM, 2.7-0.75 pm NR/chlorosulfonated PE/EPDM, 2-0.75 pm NR/p-radiation/EPDM 4-1.5 pm and NR/MAH/EPDM. 1-0.25 pm. These results are in agreement with the observations of Anastasiadas and Koberstein (58) and Meier (59), who reported that compatibilizers reduced the phase domain size.

Examples of the uses of ceramic particles in a matrix (rather than as a powder) include the use of glass beads for hardening rubber for tires and adding kaolin to paper to make it smoother and easier to print on. The properties depend not only on the particle but also on the PB that encloses it. Particles are used to seed crystallization and other phase transformations. Hematite particles can be used to seed the growth of a-Al203 at temperatures lower than the usual phase transformation so that the grain size can be kept small. 

In aU cases, a premature rupture of the rubber will restrict extensive deformation of the matrix as it leads to the formation of voids larger than critical, resulting in fast crack initiation and propagation. When rubber rupmre occurs later, after some advance of matrix deformation, the orientation-induced hardening of the matrix can alleviate to some extent the effect of such flaws and the material is allowed to deform further. 

The flexibilizer markedly modifies the relaxation behaviour of the epoxy resin systems shown in Figs. 20 and 21. Its incorporation results in crosslinked two-phase systems. But the two phases are compatible and therefore not clearly separate. The Tg of the resin-hardener matrix (a relaxation) is smoothly passing into the Tg of the incorporated oligoester (a2 relaxation). The maximum of the relaxation shifts to lower temperatures as the flexibilizer content of the system is increased. The QL2 relaxation always occurs at nearly the same characteristic temperature, as is evident from the modulus decay at about 240 K in Figs. 20 and 21. The reduction in modulus observed in the rubber-elastic state shows the decrease in crosslinking density caused by an increase in flexibilizer content. ... 

The effect of orientation hardening of matrix polymers on the toughness of polymer blends was examined by Ishikawa et.al. (Ishikawa et al., 1996). Both PMMA and PVC with different characteristics of orientation hardening were used as matrix polymers and a silicon/acrylic composite rubber graft copolymer was used as the... 

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