Epoxy resins (EP) are an important class of thermosetting resins. They are a variety of thermosetting resins that are used in large amounts and are widely used. However, as a thermosetting material with a high degree of crosslinking, epoxy resin has a typical crack brittleness growth. After curing, it has shortcomings such as insufficient toughness, poor impact resistance, and easy cracking, which limits its application. EP toughening modification has always been a research hotspot of Chinese and foreign researchers. At present, in various epoxy resin toughening technologies, the use of epoxy tougheners is a convenient technology in industrial production, which can save costs and improve efficiency for enterprise production.
Toughening with epoxy toughener
Epoxy toughener is an amine-activated blocked isocyanate group modified ether-containing branched chain polymer. Polyamine usually mixed with epoxy resin curing agent is the ideal curing agent. Use with cycloaliphatic diamines (such as Laromin® C 260, Ancamin® XT, TCD-diamine, xylene diamine, etc.) to obtain good results. When mixed with epoxy resin, it acts as a reactive toughening agent. At present, the main domestic manufacturer of epoxy tougheners is Shandong Huacheng Hi-tech. The product models are epoxy tougheners PA662 and PA660, which are mainly used for the softening of epoxy resins and the preparation of elastic coatings and sealants.
Rubber-based elastomer toughening EP is an epoxy resin toughening method that started earlier. The toughening mechanism is mainly the "silver-nail anchor" mechanism and the "silver-shear band" mechanism. The toughening effect depends not only on the strong strength of the connection between rubber and epoxy resin, but also on the compatibility and dispersion of the two and the curing process of EP.
The rubber currently used to toughen EP is generally a liquid rubber with active end groups. When toughening EP, the active end groups carried by this type of rubber are combined with the active groups in the EP molecular chain under the action of a curing agent. (Such as epoxy, hydroxyl, etc.) reaction. This not only enhances the strength and compatibility of the combination of rubber and EP, but also introduces the soft rubber segment of the flexible chain structure into the epoxy resin crosslinking network, thereby improving the impact toughness of EP. Su Hang and others studied different kinds of active end-group rubbers as tougheners to toughen EP. The results show that the modified EP's impact resistance, bending resistance and tensile shear properties have been significantly improved. The research on rubber toughened EP has been relatively mature, but due to the low strength and modulus of rubber and poor heat resistance, the effective toughening of EP tends to weaken the strength, modulus and heat resistance of the material.
Toughening of thermoplastic resin
Thermoplastic resin toughened EP generally adopts shear yield theory or particle tearing to absorb energy and disperse phase particles to initiate crack riveting mechanism to explain. Thermoplastic resins are used to modify epoxy systems in the form of high molecular weight or low molecular functional oligomers. Because high-performance thermoplastic polymers have good toughness, high modulus and good heat resistance, they are Using them to modify EP can not only improve the toughness of EP, but also not reduce the rigidity and heat resistance of EP. Hu Bing et al. modified toughened EP with polyetheretherketone. While the toughness of the material was improved, the compression strength and Martin heat resistance were not reduced. Judging from the shape of the fracture surface, it belongs to ductile fracture. When the addition amount of polyetheretherketone is 6%, the toughness is good, reaching 19.1kJ/m2, which is an increase of 107.6% compared with pure EP.
The disadvantage of thermoplastic resin toughening EP is that the thermoplastic resin used to modify EP is not easy to dissolve in common solvents (ethanol, acetone, etc.), and the processing and curing conditions are demanding.
Toughening of silicone resin
There are two methods of silicone resin toughening EP: blending and copolymerization. Simple blending and curing has the problems of excessive interfacial tension of two phases, poor modification effect, and poor compatibility. Therefore, copolymerization modification is generally used. Methods.
Toughened by rigid particles
Rigid particles in plastic deformation, tensile stress can effectively suppress the expansion of the matrix resin cracks, while absorbing part of the energy, thereby playing a toughening role. Appropriate addition of rigid silica, kaolin, glass beads and calcium carbonate particles can improve the toughness of EP, the degree of improvement depends on the size and shape of the particles and the volume fraction.
Toughening of nanoparticles
Due to the small particle size of nanoparticles, high surface atomic occupancy rate, and the special structure of unsaturated bonds or dangling bonds on the surface, nanoparticles have very large surface activity, and there is a high possibility of physical or chemical bonding with polymers. At the interface, it forms a force far greater than the van der Waals force with the epoxy group, forming a very ideal interface, thereby playing a better role in initiating micro-cracks and absorbing energy.
Other toughening methods
In addition to the above commonly used toughening methods, EP toughening methods also include interpenetrating polymer network toughening, dendritic molecular toughening, and ionomer toughening EP. Interpenetrating polymer network toughening (IPN) is a polymer mixture formed by two or more interpenetrating polymers interpenetrating and intertwined. It is characterized by the random penetration of one material into another. Go, play the role of "forced tolerance". This structure can not only greatly improve the synergistic effect of material performance, but also the network can undergo large deformation under the action of external forces, absorb external energy, increase stress transmission, increase the effective number of stress concentration chains, and have a significant toughening effect on EP . Synchronous interpenetration can limit phase separation to a great extent, so its toughening effect is good.