Resin system, choice

Substituted heat-reactive resins are most widely used in contact-adhesive appHcations and, to a lesser extent, in coatings (77,78) -butylphenol, cresol, and nonylphenol are most frequendy used. The alkyl group increases compatibiHty with oleoresinous varnishes and alkyds. In combination with these resins, phenoHcs reduce water sensitivity. Common appHcations include baked-on and electrical insulation varnishes, and as modifiers for baking alkyds, rosin, and ester gum systems. Substituted heat-reactive resins are not used for air-dry coatings because of theh soft, tacky nature in the uncured state substituted nonheat-reactive phenoHcs are the modifying resin of choice in this case. 

Color Concentrates. Color concentrates have become the method of choice to incorporate colorants into resins. Color concentrates have high ratios of colorant to a compatible vehicle. The colorant may be added at 70% colorant to 30% vehicle in a titanium dioxide mixture whereas the ratio may be 15% colorant to 85% vehicle in a carbon black mixture. The amount of colorant that can be added is dependent on the surface area and the oil absorption of the colorant and the wetting abiHty of the vehicle. The normal goal is to get as much colorant in the concentrate as possible to obtain the greatest money value for the product. Furthermore, less added vehicle minimizes the effect on the physical or chemical properties of the resin system. 

Glass-reinforced polyester is the most widely used reinforced-resin system. A wide choice of polyester resins is available. The bisphenol resins resist strong acids as well as alkahne solutions. The size range is 2 through 12 in the temperature range is shown in Table 10-17. Diameters are not standardized. Adhesive-cemented socket joints and hand-lay-up reinforced butt joints are used. For the latter, reinforcement consists of layers of glass cloth saturated with adhesive cement. 

Gel permeation chromatography Is the method of choice for analysis of thermoplastic resin systems. Corrected for imperfect resolution, chromatogram interpretation yields accurate molecular descriptions, including theoretical, kinetic distributions (, ) The current research is designed to extend the utility of this analytical tool to the analysis of thermoset resins. 

Table II summarizes the overall results and indicates which solvents were the best choices for further evaluation. Fortunately, the generally recommended co-solvent for each of the three resin systems was 2-bu-toxyethanol. Comparisons thus could be made using this solvent as a common base. The data are not quantitative but are expressed on a better-than or poorer-than basis, with +3 being superior and —3 being inferior.

It is evident from these results, as well as from results cited later in this discussion, that the choice of solvent is of critical importance in achieving the best film properties from a resin system. The cheapest solvent combination may not and probably will not give best film properties. It is also evident that in the process of evaluating the merits of resins, conclusions could be erroneous unless the optimum solvent blend is chosen for each resin. Extrapolating these considerations further, it can be expected that the same resin could yield entirely different properties if it is applied to a surface in different forms, for example, as a solution, a dispersion in solvents, a dispersion in water, or as a powder. 

Choice of an appropiate surface treatment and a suitable primer are important because adhesion to the substrates presents difficulties [11.37]. Primers based on modified alkyd resins or two-pack epoxy-resins for derusted ferrous metals mainly contain zinc phosphate and zinc OKide as corrosion protection pigments. Nonferrous metals are first washed with an ammoniacal wetting agent before applying the primer that contains a binder based on synthetic resins (e.g., PVC copolymers, chlorinated rubber) which ensure good adhesion to the substrate. The same primer must be used on zinc or galvanized surfaces because the use of alkyd resins causes embrittlement [11.38] The primed surfaces are largely topcoated with alkyd resin systems. 

For pipe construction, steel mandrels are generally used. However, where cylinders or spheres are to be made, an alternative material has to be used so that it can be removed once the resin system has cured. In these cases the mandrel can be made from wax, a low melting metal alloy, or an inert plaster held together with a water soluble binder. Clearly, in these cases the mandrel can only be used once. Material choice for the mandrel will depend on the cure cycle needed for the resin system. 

By the correct choice of resin system, sheeting can be manufactured which will not yellow to any extent after exposure to tropical weather conditions for several years. However, to ensure that this is the case the resin system must be chosen with care and must be fully cured. Also the release film must be removed before installation and the laminate should contain not less than 75% by weight of resin. In other cases the resin content of the laminate may fall between 65 and 75% by weight. 

Adhesion and stability of bonding at the fibre surface is important against resistance to creep, particularly in the presence of moisture, and selection of fibre finishes should ensure compatibility with choice of the resin system to optimise against interface degradation. 

To assist in the choice of the correct cure conditions, it is advantageous to know the gel time of a resin system. 

In controlling z-axis expansion, the key factors to consider are the choice of resin system, the resin system Tg, and the resin content of the base materials. Fillers in the resin system, in addition to the fiberglass cloth, can also be used to lower the CTE of the material. Table 8.1 compares the thermal expansion of several commercially available base materials. These values can vary significantly based on the exact resin content of the material or PCB tested. In multilayer PCBs, the amount of copper in the sample will also have a significant impact as the z-axis expansion of copper is very low compared to the resin system. 

---