Toughening effect epoxy resin

Jijima and coworkers [194-197] have used N-phenylmaleimide-styrene copolymers (PMS), N-phenylmaleimide-styrene-p-hydroxystyrene (PMSH), and polyCaryl eter ketone)s as hybrid modifiers to improve the toughness of bisphenol A diglycidyl ether epoxy resin cured with p,p -diamino-diphenyl sulfone. These hybrid modifiers were effective in toughening the epoxy resin. When using the modifier composed of 10% PMS (M = 313,000) and 2-5% PMSH (M = 316,000), the fracture toughness for the modified resins increased 100% with no deteriora-... 

Sohi. M.M., Hahn, H.T. and Williams, J.G. (1987). The effect of resin toughness and modulus on compressive failure modes of quasi-isotropic graphite/epoxy laminates. In Toughened Composites. ASTM STP 937 (N.J. Johnston cd.). ASTM. Philadelphia, PA. pp 37 60. 

The catalyst does not make up part of the final epoxy network structure or have a significant effect on the final properties of the cured resin. Thus, the final cured properties of the epoxy system are primarily due to the nature of the epoxy resin alone. Homopolymerization normally provides better heat and environmental resistance than polyaddition reactions. However, it also provides a more rigidly cured system, so that toughening agents or flexibilizers must often be used. In adhesive systems, homopolymerization reactions are generally utilized for heat cured, one-component formulations. 

In general, the elastomer must be prereacted (adducted) with the epoxy for the toughening effect to take place. Adducts reduce the likelihood of early phase separation and maintain the solubility of the elastomer in the uncured resin system. For CTBN the reaction is carried out at high temperatures (150 to 160°C) and usually in the presence of a catalyst, such as tris-dimethylamino phenol or piperidine. The resulting epoxy-CTBN adducts are available from several suppliers, and they can be easily formulated into epoxy adhesives. 

Table II shows that the chemical bonding between the epoxy matrix and the rubbery phase is important. The terminal reactive groups are more effective than the pendant groups in toughening epoxy resins.

Pearson and Lee (1991) examined the effects of particle-size and particle-distribution effects on rubber-toughened epoxy resins. They examined a variety of CTBN liquid rubbers and a methacrylated butadiene styrene core-shell particle in a DGEBA-piperidine system. They found that the toughening mechanism for small particles was internal cavitation of the... 

It is postulated that the role of the HBP in toughening will be to act similarily to a coreshell particle that is, the core of the HBP will act to cavitate and promote shear yielding, and the shell will be able to be tailored to control aggregation and interactivity with the epoxy-resin matrix. Increases in core should promote cavitation, and shell-chemistry functionalization should increase dissolution and reactivity with the epoxy resin. However, unlike with the core-shell particles, the inherently greater number of shell sites and low viscosity of the HBP will enable the toughening to occur without deleterious effects on other properties. 

The effects of ceramic particles and filler content on the thermal shock behavior of toughened epoxy resins have been studied. Resins filled with stiff and strong particles, such as silicon nitride and silicon carbide, show high thermal shock resistance, and the effect of filler content is remarkable. At higher volume fractions (Vf > 40%), the thermal shock resistance of these composites reaches 140 K, whereas that of neat resin is about 90 K. The highest thermal shock resistance is obtained with silicon nitride. The thermal shock resistance of silica-filled composites also increases with increasing filler content, but above 30% of volume fraction it comes close to a certain value. On the contrary, in alumina-filled resin, the thermal shock resistance shows a decrease with increasing filler content. 

This study demonstrated that the final destination of the added core-shell rubber particles, in PC, PA, or both, in the PC-PA binary blend can be controlled by properly selecting the chemical structure of the shell in the core-shell rubber. The unreactive MBS rubber tends to reside in the PC phase and near the vicinity of the PC-PA interface. The reactive MBS-MA rubber can have a chemical reaction with PA end groups and can therefore be retained within the PA phase. High-molecular-weight bisphenol A epoxy resin has proved to be an efficient compatibilizer for PC-PA blends. Rubber-toughening of the PC-PA blend in which PC is the matrix is much more effective than with blends in which PA is the matrix. 

In view of the functionality thus created, it is interesting to consider possible applications for the epoxldlzed oils mentioned as epoxy monomers per se. Indeed, some epoxldlzed oils are commonly used as reactive diluents for other epoxy prepolymers in order to reduce cost or Improve processability (10,11) examples claimed in reference 11 Include epoxldlzed linseed, butylated linseed, soybean, and tall oils. However, although some fundfimental studies of the effects of monofunctional reactive diluents on the viscoelastic and other properties of epoxy resins have been published (see, for example, reference 12), little or no analogous Information on the effects of multifunctional reactive diluents appears to exist. At the same time, some reactive additives such as polyols (13), poly(ether esters) (14) and carboxy-terminated elastomers (15) have been used to provide an elastomeric toughening phase for epoxies. 

Consequent to documentation surrounding methods of employing reactive nitrile elastomers to modify epoxy resins is a growing body of literature which serves to characterize and elucidate these systems. Such topics as morphology in the cured and uncured state, transitions from toughening to flexlbilization, viscoelastic effects, equilibrium physical properties and phase structure are available to the investigator (12-17). 

When the epoxy resin is modified with eavitating rubbery particles or debonding glass spheres with a combined volume fraction of/, additional toughening results, arising primarily from an effective reduction of the plastic resistance, Y. The effect of this is obtained by formally replacing Jo with Jo l —f) and Y with F(1 —/) in eq. (13.56) to give... 

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