Epoxide Novolac crosslinking

Void-free phenolic networks can be prepared by crosslinking novolacs with epoxies instead of HMTA. A variety of difunctional and multifunctional epoxy reagents can be used to generate networks with excellent dielectric properties.2 One example of epoxy reagents used in diis manner is the epoxidized novolac (Fig. 7.34) derived from the reaction of novolac oligomers with an excess of epichlorohydrin. 

The epoxy novolacs synthesized by reaction of novolacs with chloromethyloxiiane may contain traces of hydrolyzable and nonhydrolyzable organically boimd chlorine as impurities, which reduce not only the functionality of the epoxidized novolac but negatively affects the properties of the crosslinked products. Furthermore, traces of chloride and alkali metal ions also need to be carefully removed. In contrast to this, epoxy novolacs obtained by epoxidation of allylphenol are fiee of chloride and alkali metal ions. 

For some highly crosslinked thermosetting polymers with rigid backbones, topological restrictions limit the maximum attainable conversion. For the reaction between epoxidized novolacs and cresol-based novolacs, a maximum conversion, Xmax= 0.80 was reported (Hale et al., 1991). A similar value of x ,ax was reported for the cure of an epoxidized novolac with 4,4 -diaminodiphenylsulfone (DDS) (Oyanguren and WilKams, 1993a). In these cases, partially reacted networks may be modeled as a random solution of reacted functionalities with a concentration equal to x/x ax... 

The diglycidylether of Bisphenol A (DGEBA) is where n = 0. With increasing n, the viscosity of the epoxy resin increased imtil it becomes a solid epoxy. At high molecular weight, the structure is basically the polyhydroxyether of Bisphenol A termed Phenoxy (PHE). Epoxidized novolacs (phenol-formaldehyde resins) are also employed in epoxy thermosetting systems. Epoxies are typically crosslinked with di- (or higher) amines (either aliphatic or aromatic). 

Novolacs are thermoplastic materials, that need to be transformed by crosslinking reactions to get insoluble, nonmelting, thermally stable thermosets. Different crosslinking agents have been used for thermal curing, for instance l,3,5,7-tetiaazatricyclo[3.3.3. P Jdecane (hexamethylenetetramine), oxiranes (epoxides), diisocyanates, urea, melamine, urea methanal resins, melamine metfaanal resins, and imide precursors. Furthermore, photoreactive compounds crosslink novolacs with methacrylate and methacryloylchloride resins, respeetively by itradialion [1]. 

Resole/epoxide networks possess a higher crosslinking density and show a higher chemical resistance than novolacs crosslinfced with epoxides. Resole/epoxide networks have been applied as coatings. 

Crosslinked epoxy novolacs have improved thermal and chemical resistance after curing when compared to networks made from 2,2 -[(l-methyle1hyiidene)-bis(4,l-phenylene-oxymethylene)bis-oxirane], an epoxidized monomer. However, similar structures can be made by using phenol or a novolac to crosslink conventional epoxy resin to form an insoluble, nonmelting product (see subchapter C.2.). 

Influence of the Functionality of Epoxidized 4,4 -(l-Methylethylidene) bisphenol Novolac Prepolymers on the Time of Gelation during Crosslinking with 4,4 -Diaminodiphenyl-methane at Different Temperatures (epoxy groups amino hydrogen=l) (data from ref. [91 ])... 

Novolacs can be transformed into insoluble, nomnelting, thermally stable fheimosets by crosslinking reaction with 1,3,5,7-tetraazatricyclo-[3.3.3. H ]-decane (hexamethylenetetramine), oxiiane (epoxides), diisocyanates, urea. 

The viscosity of the epoxide prepolymers in many instances is reduced by additions of thin liquid epoxy compounds fliat may include cycloaliphatic epoxides or lactones. Various epoxidized vegetable oils, like epoxidized linseed, sunflower, soybean, and others are also used in some formulations. Table 3.4. lists come cycloaliphatic epoxy monomers used for such purposes. Vinyl ethers polymerize at a faster rate than do cycloaliphatic epoxides. They are, therefore, included in some formulations. Other materials fliat are included for such purposes or to control viscosity can be epoxy novolacs, glycidyl ethers of long chain alcohols. To control viscosity, polyols might also be used, because it is assumed that hydroxy group bearing compounds crosslink with the epoxy resins. 

Curing by the reaction of epoxide with phenolic OH is usually done using a phenolic novolac. These are high-temperature cures and produce very highly crosslinked polymers due to the frequency of OH groups on the novolac. 

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