Step Addition Curing Resins

In the heater, the impregnated web is heated to boil away the solvent and to begin the polymerization of the thermosetting resin. This step is known as B-staging or prepreging. It is deshable to cure the resin enough so that the web is not tacky or dry and is handleable, but yet sufficiently uncured to permit an additional cure and flow at the lamination press. As the sheets exit the treater, they are sheared, stacked, and stored in temperature- and humidity-conholled rooms. 

Resoles. The advancement and cure of resole resins foUow reaction steps similar to those used for resin preparation the pH is 9 or higher and reaction temperature should not exceed 180°C. Methylol groups condense with other methylols to give dibenzyl ethers and react at the ortho and para positions on the phenol to give diphenyknethylenes. In addition, dibenzyl ethers eliminate formaldehyde to give diphenyknethanes. 

Hand Lay-Up and Spray-Up. In hand lay-up, fiber reinforcements in mat or woven form are placed on the mold surface and then saturated with a Hquid polymer, typicaHy a polyester resin, that has been chemically activated to polymerize (cure) without the addition of heat. Multiple pHes of reinforcement and multiple cure steps aHow very heavy waH thicknesses to be achieved. 

A typical cationic uv adhesive formulation contains an epoxy resin, a cure-accelerating resin, a diluent (which may or may not be reactive), and a photoinitiator. The initiation step results in the formation of a positively charged center through which an addition polymerization reaction occurs. There is no inherent termination, which may allow a significant postcure. Once the reaction is started, it continues until all the epoxy chemistry is consumed and complete cure of the resin has been achieved. Thus, these systems have been termed living polymers. 

The formation of networks by addition polymerization of multifunctional monomers as minor components included with the monofunctional vinyl or acrylic monomer is industrially important in applications as diverse as dental composites and UV-cured metal coatings. The chemorheology of these systems is therefore of industrial importance and the differences between these and the step-growth networks such as amine-cured epoxy resins (Section 1.2.2) need to be understood. One of the major differences recognized has been that addition polymerization results in the formation of microgel at very low extents of conversion (<10%) compared with stepwise polymerization of epoxy resins, for which the gel point occurs at a high extent of conversion (e.g. 60%) that is consistent with the... 

Several significant aspects of the RIM operation determine the moldabillty of a resin in a particular application. These are mixing, mold filling, and resin curing. In addition, the presence of fillers is of importance and can make the difference between a moldable and non-moldable system as the filler affects each of the above steps. 

Figure 22 illustrates typical DSC thermograms for UF and PF resins. The PF resin had larger exothermic heat of curing as compared to the UF resin. For illustration purposes, the reaction between phenol and formaldehyde in alkaline conditions to form PF adhesives is briefly discussed here. The first step is an exothermic addition reaction forming methylol derivatives at the ortho or para positions ... 

Epoxy-amine systems follow an addition step-growth polymerisation mechanism. The two principal reactions of primary and secondary amines with epoxy oligomers are shown in Reaction scheme 1 [30]. These reactions are catalysed by acids, phenols and alcohols (e.g. impurities in commercial epoxy resins). The presence of water causes a tremendous acceleration, but does not alter the network structure. The hydroxyl groups formed by the amine-epoxy addition steps are also active catalysts, so that the curing reaction usually shows an accelerating effect in its early stage (autocatalysis). 

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