Novolac phenolic resins applications

The most widely used epoxy resins are reaction products of either bisphenol A or a novolac phenolic resin with epichlorhydrin. When used to manufacture corrosion-resistant structures for use in the chemical process industry, epoxy resins are generally hardened with either aromatic or cycloaliphatic amines. The hardeners for epoxy resins are, with few exceptions, added at levels varying from 20phr (parts per hundred resin) to lOOphr. This means that the hardener is actually quite a high proportion of the matrix resin and has quite a profound effect on the mechanical and corrosion properties of the cured resin. Thus the selection of the most suitable hardener is critical to the eventual success of the application. Epoxy resins have viscosities of several thousand mPas at room temperature, which makes it much more difficult to wet out glass fibre efficiently with them than with polyesters. Wet-out therefore involves heating the resin formulation to between 40°C and 60°C to reduce the viscosity to less than 1000 mPas. 

Cured phenolics are universally brittle in nature. This is true of both resoles and novolacs and does not depend much on the source of methylene used to promote cure. Consequently, the fillers used in molded articles are highly important to the design of the manufactured product. With resoles, the fiber or filler are usually the primary component of the final composite, with the resole acting as a binder or impregnating agent. With novolacs the resin may be the major component in the molded part. Poly-silanes and other organic polymers are also added in some applications to promote impact resistance and toughness [192]. 

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While "conventional positive photoresists" are sensitive, high-resolution materials, they are essentially opaque to radiation below 300 nm. This has led researchers to examine alternate chemistry for deep-UV applications. Examples of deep-UV sensitive dissolution inhibitors include aliphatic diazoketones (61-64) and nitrobenzyl esters (65). Certain onium salts have also recently been shown to be effective inhibitors for phenolic resins (66). A novel e-beam sensitive dissolution inhibition resist was designed by Bowden, et al a (67) based on the use of a novolac resin with a poly(olefin sulfone) dissolution inhibitor. The aqueous, base-soluble novolac is rendered less soluble via addition of -10 wt % poly(2-methyl pentene-1 sulfone)(PMPS). Irradiation causes main chain scission of PMPS followed by depolymerization to volatile monomers (68). The dissolution inhibitor is thus effectively "vaporized", restoring solubility in aqueous base to the irradiated portions of the resist. Alternate resist systems based on this chemistry have also been reported (69,70). 

Both resole and novolac types of phenolic resin are used in aerospace applications. The resoles are generally made by using alkaline catalysts and an excess of formaldehyde. The resole structure is highly complex and involves both methylene and dimethyl ether bridges between the phenolic moieties. During cure both water and formaldehyde are evolved. 

Dow developed the phenol novolac epoxy resins, Shell introduced polyglycidyl ethers of tetrafunctional phenols, and Union Carbide developed a triglycidyl p-aminophenol resin. These products continue to find uses today in highly demanding applications such as semiconductor encapsulants and aerospace composites where their performance justifies their higher costs relative to bisphenol A based epoxies. 

Conversely, novolac PF resins are produced by the reaction of excess phenol with formaldehyde (P F molar ratio 1 0.8 to 1 1) in the presence of an acid catalyst. Unlike resole resins, novolac PF resins have a more linear structure (Fig. 4) and do not self-cure because they lack the residual reactive methylol groups of resoles. Therefore, an external curing agent such as hexamethylenetetraamine must be added to novolac resins to yield a cross-linked structure. Novalac PF resins cured under acidic conditions are not recommended for wood composites for long-term structural applications. 

Phenolic resins, which are formed as condensation products of formaldehyde and different phenols, form the basis of a unique class of rigid, cross-linked polymers. From a structural point of view, these resins are categorized into two types, one-step (resole) resins and two-step (Novolac) resins. One-step (resole) phenolic resins are formed from phenol and an excess of formaldehyde, in an aqueous catalytic medium. The reaction is terminated before the completion to produce a resin material that may be cross-linked in its final application. Both liquid and solid forms of these resins are available, and are formed by removing water or retaining water from the reaction. Phenolic resole resins are a mixture of the following compounds ... 

HMT is commonly used by the plastics industry as the crosslinking agent for novolac phenol formaldehyde thermosetting resins. These molding resins are used in many commercial applications. 

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