Epoxy resins rubbers, compatible

These findings suggest strongly that the composition of the elastomeric molecule and the nature of the functional groups affect its compatibility and rate of reaction with the epoxy resin, which in turn affect the molecular and morphological structure of the heterophase system. These data indicate the importance of the acrylonitrile comonomer and the carboxyl groups in controlling the polarity of the rubber, and subsequently its compatibility characteristics with the epoxy. We could also... 

Fig. 13 is a TTT cure diagram of three systems a neat epoxy resin and the same epoxy modified with two reactive rubbers at the same concentration level. The times to the cloud point, gelation and vitrification are shown for each system. The cloud point is the point of incipient phase separation, as detected by light transmission. The modified system with the longer times to the cloud point and gelation, and the greater depression of Tg, contains the more compatible of the two rubbers. The difference in compatibility could then be used to account for differences in the volume fractions of the phase separated rubber-rich domains and in the mechanical properties of the neat and the two rubber-modified systems. 

In the thermally initiated cure of rubber modified epoxy, the rubber may be present within in the epoxy matrix as distinct domains. The morphology of the cured resin has been shown to be dependent on (1) the cure temperature and accelerator concentration, since the extent of particle (domain) size growth appears to be limited by gelation and (2) the nature (percent acrylonitrile) of the rubber used, since mixture compatibility increases with the acrylonitrile content of the rubbers (1,2). 

All of the above solid rubber-modified epoxy resins visually displayed clearly biphasic morphological properties (i.e., discreet rubber domains in a continuous epoxy matrix). If there is a reaction between a rubber moiety and an epoxide, it would best be studied in a homogeneous reaction mixture. Lower molecular weight epoxy resins are more compatible with CTBN elastomers and will form homogeneous solutions at elevated temperatures. Reaction of an epoxide with a reactive moiety contained in the elastomer, R, will most likely obey the following rate law ... 

In studies of the ERC heterogeneous structure obtained in copolymerization between the epoxy and rubber oligomers that occurs along with the epoxy resin cure (i.e., hot cure compoimds), the average size of the rubber particles was found to be dependent on its polarity and on the level of compatibility with the epoxy resin as well as on the chemical composition of the modifier [132]. The author also showed that the size of the rubber inclusions decreases and the structure of the composition becomes more homogeneous with increase of the polarity of the elastomers and the curing temperature of the ERC. 

Let us consider the reasons for the improvement of the thermodynamic compatibility of the epoxy and rubber oligomers that is observed with increase of the polarity of the rubber. One of the factors that facilitate the compatibility of the compound components is the ability of their molecules to form various bonds between each other in particular, the molecules of the rubber and the epoxy resins under study have functional groups that can form hydrogen bonds (Table 3.12). 

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. ... 

Monomeric plasticizers such as dioctyl phthalate and tricresyl phosphate, commonly used as plasticizers for vinyl resins and rubber, have poor compatibility with epoxy resins and are seldom used. They tend to separate out during cure and after storage and the small amount which is retained is of little value for improvement in flexibility. 

As epoxy and silicone rubber are completely immiscible, the addition of a compatibilizer is necessary to obtain a satisfactory dispersion of the rubber in the resin. The main objective of Kasemura and coworkers [216] was to find an appropriate surface-active agent to reduce the interfacial tension between the resin and the rubber, in order to compatibilize the two components. These authors achieved adequate compatibility in the epoxy resin with the use of a polyester-modified silicone oil to disperse an RTV (room temperature vulcanizing) silicone rubber or silicone diamine. The results showed that the impact fracture energy of the resin was increased by the addition of the RTV silicone rubber, up to two times that of the unmodified resin, whereas the addition of silicone diamine had almost no effect, possibly because the molecular weight was too low. Moreover, T-peel strengths of aluminum plates bonded by epoxy resin filled with RTV silicone rubber and with silicone diamine effectively increased with the silicone content, showing a maximum at 10-20 pph. By scanning electron microscopy, many particles of silicone rubber, 1-20 xm, were observed across the whole of the fracture surface. 

More recently Crosbie and Philips [85,86] investigated the toughening effect of several reactive liquid rubbers (carboxyl terminated butadiene-acrylonitrile, vinyl terminated butadiene-acrylonitrile, hydroxyl terminated polyether, polyepichlorohydrin) and an unspecified experimental reactive liquid rubber developed by Scott Bader Ltd. on two different polyester resins a flexibilized isophthalic-neopentyl glycol polyester resin, PVC compatible and an epoxy modified polyester resin, which is preaccelerated. The results of these studies are summarized as follows ... 

---