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Laminate Construction Chopped Strand Mat Chopped strand mat is widely used in the fabrication of corrosion -resistant structures to obtain consistent resin/ glass lamination ratios. Many types of glass mat are available, and the importance of proper mat selection should not be overlooked. Mats are available with a variety of sizings and binders, and even the glass itself can vary between manufacturers. These differences manifest themselves in the ease of laminate wet -out, corrosion resistance, physical properties, and the tendency of the laminate to jackstraw. Manufacturers of glass mat can provide assistance in selecting the most suitable mat for specific and end-use applications. Woven Roving Woven continuous fiberglass roving at 24 oz/ sq.yd. may be used to improve the structural performance of FRP laminates. If more than one ply of woven roving is used, it should be laminated with alternating layers of glass mat separating each ply, otherwise, separation under stress can occur. Due to the wicking action of continuous glass filaments, woven roving should not be used in any surface layer directly in contact with the chemical environment. Continuous Filament Roving Continuous roving may be used for chopper -gun lamination and in filament winding. Filament winding is widely employed for cylindrical products used in the chemical equipment market and is the predominant manufacturing process for chemical storage tanks and reactor vessels. Glass contents of up to 70% can be achieved using filament winding, which provides uniform, high-strength structural laminates. Because the capillary action of continuous rovings can carry chemical penetration deep into the composite structure, a well constructed, intact corrosion barrier is essential for filament -wound structures. Topcoats are often used for filament -wound products intended for outdoor exposure to protect the glass fibers from UV attack. Resin Curing Systems One of the most important factors governing the corrosion resistance of composites is the degree of cure that the resin attains. For general service, it is recommended that the laminate reach a minimum of 90% of the clear cast Barcol hardness value listed by the resin manufacturer. For highly aggressive conditions, it may be necessary to use extraordinary measures to attain the highest degree of cure possible. One effective way to do this is to post -cure the laminate shortly after it has gelled and completed its exotherm. Some laboratory studies have suggested that the combination of benzyl peroxide (BPO) and dimethylaniline (DMA) may provide a more complete cure before post -curing than the standard cobalt DMA/ MEKP system. In some instances, resins have demon- strated a permanent undercure for reasons that are not fully understood. One theory is that undercure is related to initiator dispersion. Typically BPO is used in paste form, which is prepared by grinding solid BPO particles in an inert carrier. Dispersion and dissolution of BPO paste is clearly a more challenging procedure than blending in low -viscosity MEKP liquid, especially in cold conditions. Another advantage of MEKP systems is a more positive response to post -curing. Vinyl ester resin promoted with cobalt/ DMA tends to foam when MEKP initiator is added. This increases the difficulty of eliminating entrapped gases from the laminate. Foaming can be reduced in a number of ways. BPO/ DMA reduces foaming, as does the use of an MEKP/ cumene hydroperoxide blend or straight CHP. Using a resin that does not foam, such as DION® 9800 urethane - modified vinyl ester resin or a bisphenol fumarate resin, is another alternative. High-quality composite products can be fabricated using either of the promoter/ initiator combinations described above. For end-users, it is suggested that the preferences of the fabricator involved be taken into account when specifying initiator systems.