Novolak plastic

Phenolics. These plastics allow the preparation of both random prepolymers, such as Baekelands A stage and true structopendant prepolymers, commonly known under the term novolaks (Figure 6). Novolaks permit one to take advantage of the newer prepolymer technology. Monomers are phenol, cresols, and formaldehyde. Molecular weights of the novolaks are between 300 and 700. Novolaks are obtained through careful selection of reaction conditions and catalysis of the phenol-formaldehyde reaction. Molecular weight, as well as the ratio of 2,2 - and 2,4 -links, can be controlled. These structural factors, studied extensively by Wood (28), have an eflFect on the physical properties of the cured polymer network. 

Phenolic novolak resins have been also used as coreactants (hardeners) with epoxy resins to produce thermoset systems with high-quality "engineering plastic" properties. 

Neither alcohol-soluble spirit Novolaks nor plasticized phenolic resins dissolve in dry-hardening oils such as linseed oil, however. The first step in this direction was achieved by the combination of phenolic resin with ester gum, i.e., abietic acid (the main component of colophonium or rosin) esterified with glycerine. These modified phenol resins or oleoresinous phenolics dry better than copal-linseed oil resins. Even better elasticized phenolic resins were developed by introducing new groups into the basic components. When bisphenol A (see Chapter 26) is used as a phenolic component, both the solubility in dry-hardening oils and the resistance to yellowing are improved. 

Phenolic resins have been developed to a highly versatile degree as lacquer and paint bases. Pure Novolak is only soluble in polar solvents such as acetone, alcohol, low esters, etc. However, the market potential of alcohol-based lacquers is limited, and such lacquers are too brittle for many uses. For this reason, so-called plasticized and elasticized phenolic resins ( substituted phenolics ) were developed. Plastification is achieved either by partial etherification (e.g., with t-butyl alcohol), esterification (e.g., with fatty acids), or both etherification and esterification (e.g., with adipic acid and trimethylol propanol). These plasticized phenolic resins have increased elasticity, are soluble in aromatics, are compatible with polyvinyl compounds and fatty acids, and are suitable for use as stoving enamels. 

Packaging of the chip portion of the chip/lead combined unit into a plastic or ceramic container is a semi-automated process involving the use of epoxy resins, usually epoxy novolak resin formations [15]. These should be fully cured prior to use, but unreacted monomers may be present, which can sensitise, as may other diluent and flame retardant materials, such as halogenated derivatives of bisphenol A, which are added to the resin prior to use. The completed devices are then laser marked in an enclosed system and tested, again in an automated process, prior to delivery for sale. 

The use of straight resol and novolak resins derived from phenol in surface coatings is very limited, mainly because of the brittleness of the films. Further, vegetable oils cannot be used as plasticizers since they are incompatible with the resins. However, there are a number of ways in which oil-solubility can be achieved and large quantities of phenolic resins suitable for surface coatings are produced by such methods as the following ... 

---