Epoxy resins crack resistance

Most organic solvents penetrate very easily into epoxy resins. Epoxy resins are resistant to hydrocarbons and aliphatic alcohols. In ketones and esters they exhibit only limited resistance, in particular at elevated temperatures. Chlorinated hydrocarbons cause swelling. Some organic solvents, such as acetone or carbon disulfide, can also lead to severe softening and crack formation. 

Polymerized epoxy adhesives are amorphous and highly crosslinked materials. This microstructure results in many useful properties such as high modulus and failure strength, low creep, and good chemical and heat resistance. However, the structure of epoxy resins also leads to one undesirable property—they are relatively brittle materials. As such, epoxy adhesives tend to have poor resistance to crack initiation and growth, which results in poor impact and peel properties. In sealant formulations, epoxy resins do not often provide the degree of elongation or movement that is required for many applications. 

Ichirou, I. Epoxy Resin Composition Having Improved Adhesiveness and Cracking Resistance. JP 56-120,726, Sep 22, 2004 Toshiba Chem. 

The achievement of enhanced toughness in a brittle polymer such as an epoxy resin is vital in applications such as adhesives, for which peel strength and resistance to interfacial debonding through crack propagation are important criteria for performance. This may be achieved by incorporation of elastomeric or other phase-separated particles into the network. These may be present before reaction or they may form during the process of... 

Chapters 8 and 9 consider the mechanical properties of rubber- and ceramic-particle toughened-epoxy materials. The importance of rubber cavitation is highlighted in Chapter 8. It is well known that this mechanism can relieve the high degree of triaxiality at a crack tip in the material and enable subsequent plastic hole growth of the epoxy resin, which is a major toughening mechanism. We return to rigid particles in Chapter 9, which examines their use to increase the thermal shock resistance of epoxy resins. 

The thin inner shell of the core-shell polymers contained less than 20% rubber by weight, with 60% by weight of a plastic core in the particles. It is unusual to enhance the toughness of the normally brittle epoxy-resin system at such a low level of rubber content. We believe that the unique design, the use of a multilayer toughener with a thin, elastomeric inner layer, and the large plastic core together produced behavior like that of solid rubber particles and, hence, enhanced crack resistance. 

Monoethanolamine-containing formulations can be used to make polycarbonate molds with good dyeabihty and crack resistance (186). Monoethanolamine, diethanolamine, and triethanolamine have been cited in formulations for epoxy resins compounds (187—192). 

The B. F. Goodrich Chemical Company produces monopolymers and copolymers with acrylonitrile. These are known as the Hylar series. Acrylonitrile increases the viscosity and imparts oil resistance, adhesion, and compatibility with epoxy resins. Carboxy-terminated butadiene-acrylonitrile copol)uner (CTBN) improves impact strength, low-temperature shear strength, and crack resistance in epoxy formulations. 

Analogously to thermoplastic injection molding, metallic inserts should be preheated to 120-150 C before being placed in the mold for duroplastics as well. This applies less to small threaded sockets and more to larger magnetic coils encased in epoxy resin. A positive factor in this connection is the high level of crack resistance of EP resins. 

The polymer-cement concrete reveal, apart from the higher compressive strength— better adhesion to aggregate and lower porosity of the interfacial transition zone in comparison with traditional concrete. The propa tion of cracks in concrete is thus changed [61,62]. The tightness of polymer-cement concrete and resistance to the attack of corrosive environment, including the acidic one, are generally improved. In the latter case the epoxy resin is reconunended. In the two papers by Czamecki (one with Lukowski) [5,61] the recent achievements in the area of polymer-cement concrete are presented. 

Among the most effective modifiers allowing an increase in the impact viscosity and cracking resistance of epoxy polymers are the oligobutadiene acrylonitrile rubbers with reactive end groups. The structures of the reactive rubbers that are commonly used to increase the impact viscosity of the epoxy resins are as follows. CTBN is a... 

Let us consider the effect of the EEC phase state upon its crack resistance. The maximum modifying effect from addition of the rubber into the epoxy resin is achieved for rubber particles in the form of the individual phase being available in the epoxy polymer matrix. As was shown for EEC obtained by the PEE method, this condition is observed when using SKD-KTEA as a modifier. 

Thus, from the above data one could conclude that to achieve the maximal values of Kjc the method of modification with PER is most effective. Additionally, this method is more technologicsLlly apphcable because it can be used in formation of cold-cure compounds. Special attention must be paid to the correlation of the Kjc characteristic with X2,3 foi mixtures such as epoxy resin—modifier and curing agent-modifier. The presented characteristics of the ERG cracking resistance indicate the strong dependence of Kic on the structure of these materials, which enables the use of this characteristic in optimizing the compositions of new and already known epoxy rubber materials. 

Figure 3.20 Effect of the method of ED-20 epoxy resin modification on the cracking resistance Kic. (1) mechanical mixture with 20% SKN-30 KTR (2) modification with 20% SKN-30 KTR (3) modification with 20% SKD-KTRA with preformed PER (4) heat-treated mixture with 20% SKD-KTRA (5) heat-treated resin (6) mechanical mixture with 20% SKD-KTRA (7) cured resin without heat treatment.

Gravity fill is also used when cracking is random or patterned. The material normally used is gravity polymer covered with hard and durable sand. Epoxy resin-based materials instead of elastomer/plastomer-based materials are preferred when a longer-lasting surface skid resistance is required. However, this method of repair can be very expensive. 

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