Curing process epoxy resin

It is beyond the scope of this review to discuss in detail viscoelastic properties at and after gelation. The gel point is one of the important characteristics in the epoxy resin curing process from both kinetic and rheological aspects. The gelation transition has been widely studied for epoxy resin systems [39,44,133-138] and already discussed in some of this series, such as by Malkin and Kulichikhin [28], Williams et al. [139], and Winter and Mours [140]. Rheological techniques for the determination of the gel point have been summarized for thermosetting resins by Halley et al. [29,141]. 

Participation of radical products of thermal degradation of PMBs in the epoxy resin curing process at high temperatures. 

Aotlvatlon energy of flberlte epoxy, 141-43 Aerospace epoxy resins curing process, 127-29 density, 129,143 differential scanning calorimetry, 138-42... 

Although the simple rate expressions, Eqs. (2-6) and (2-9), may serve as first approximations they are inadequate for the complete description of the kinetics of many epoxy resin curing reactions. Complex parallel or sequential reactions requiring more than one rate constant may be involved. For example these reactions are often auto-catalytic in nature and the rate may become diffusion-controlled as the viscosity of the system increases. If processes of differing heat of reaction are involved, then the deconvolution of the DSC data is difficult and may require information from other analytical techniques. Some approaches to the interpretation of data using more complex kinetic models are discussed in Chapter 4. 

Since the epoxy resin cures primarily by a ring-opening mechanism, it exhibits a smaller degree of cure shrinkage than other thermosetting resins. In these reaction processes, the epoxy group may react in one of two different ways anionically and cationically. Both are of importance in epoxy resin chemistry. In the anionic mechanism, the epoxy group may be opened in various fashions to produce an anion, as shown in Fig. 2.10. 

Therefore, catalysts merely act as an initiator and promoter of epoxy resins curing reactions. The amounts of catalyst used with epoxy resins are usually determined empirically and are chosen to give the optimum balance of properties under the required processing conditions. Generally, only several parts per hundred of catalyst is used with an epoxy resin. Excess amounts of catalyst can result in poor physical properties and degraded resin. 

The fact that a viscosity increase after phase segregation (for t > tp) is connected with such mechanism is evidenced by the results of gel chromatographic (GPC) analysis of solfi action in the network formation process of low-molecular siloxane rubbers (Fig. 15). As the reaction proceeds the molecular mass of the sol fraction decreases and so does its viscosity. However, network formation of a number of epoxy resins cured with amines or other curing agents conform the homogeneous model without microgel formation [88 a]. 

It has been demonstrated that thick layers of fused silica filled epoxy resins with processable viscosities can be UV cured In... 

A variant of thermosetting has been described in that PPE resins are dissolved in epoxy resins. A variety of polymers can be dissolved in epoxy resins." In order to facilitate the processability of PPE, the PPE is dissolved in an epoxy resin as processing aid. After processing by kneading, the epoxy resin is cured. In contrast to other approaches where the thermoplastic polymer acts as a toughener for the epoxy matrix, the amount of epoxy resin added can be adjusted so that the PPE will form the continuous phase in the final state. 

Thermoplastics are sometimes added to epoxy resins. Thermoplastic-modified epoxy resins [43,44] based on tri- and difunctional epoxy resins cured with DDS and blended with polyethersulfone form the basis for the matrix material in a composite used for the Boeing 777 aircraft. The incorporation of the thermoplastic helps the processing characteristics and also improves the mechanical properties, notably the toughness. The thermoplastic is able to phase separate from the epoxy phase and acts as a reinforcement for the epoxy matrix, enhancing its high temperature properties. The maximum use temperatures of all these resins will typically be 30 to 50 degrees lower than the cited Eg, assuming the same cure schedule. 

Modification of epoxy resins by such modifiers is usually achieved in two ways by integrating the rubber into the epoxy pol5rmer or by copolymerizing the epoxy and rubber obgomers simultaneously with the process of epoxy resin curing at 393 or 433 K. 

The usual process of epoxy resin curing runs alongside the above reactions. 

Dynamic mechanical and differential scanning calorimetry experiments were used to analyze the BF catalyzed TGDDM-Novalac-DDS system. Dynamic mechanical experiments have been utilized in the past to characterize this epoxy resin (1,2,5,6,7). They have provided information concerning the viscoelastic response of the polymer as well as important morphological information. On the other hand. Differential Scanning Calorimetry (DSC) experiments provide a means for measuring extent of reaction, reaction kinetics and mechanisms of the epoxy system curing process (2). 

The choice of epoxy resin, curing agent, fillers, and other ancillary materials depends on factors such as cost, processing conditions, and the environment to which the insulated electrical or electronic component will be exposed. 

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