Phenolic resins hardening

Typical transfer molding compositions for encapsulation of electronic devices are mixtures of an epoxy novolac resin, a phenolic resin hardener, a catalyst, large amounts of inorganic filler (e.g., Si02) flame-retardant ingredients, internal lubricants, carbon black, and sometimes other additives such as getters to trap ionic impurities (34,35), corrosion-protection materials, and stress-relief ingredients. 

Benzidine dihydrochloride 4,4 -Methylenebis (2-chloraniline) Phenolic resin hardener, steel Calcium cyanamide hardener, textiles Epoxy cresol novolac hardener, tissue Zinc PCA... 

Reaction of one mole of acetaldehyde and excess phenol in the presence of a mineral acid catalyst gives l,l-bis(p-hydroxyphenyl)ethane [2081-08-5], acid catalysts, acetaldehyde, and three moles or less of phenol yield soluble resins. Hardenable resins are difficult to produce by alkaline condensation of acetaldehyde and phenol because the acetaldehyde tends to undergo aldol condensation and self-resinification (see Phenolic resins). 

Prices of phenolic resins vary substantially depending on the appHcation. In 1995, the price of general-purpose and semisoHds was 1.50— 1.80/kg, whereas epoxy-hardener grades can exceed 2.20/kg. Because raw materials of phenoHc resins are derived from cmde oil and natural gas, the prices of phenohc resins depend on the prices of these resources. 

Phthalic anhydride (Figure 26.10 I) is an important example of the first class of hardener. It has a molecular weight of 148 and about 0.6-0.9 equivalent is used per epoxy group. For the lower molecular weight bis-phenol resins this works out at about 35-45 phr. The hardener is usually added at elevated temperature of about 120-140°C. It will precipitate out below 60°C but will again dissolve on reheating. 

In order to obtain cured products with higher heat distortion temperatures from bis-phenol epoxy resins, hardeners with higher functionality have been used, thus giving a higher degree of cross-linking. These include pyromellitic dianhydride IV, and trimellitic anhydride V. 

MUPF/PMUF-resins harden in the main under similar acid conditions to UFs and MUFs. Because MUF-resins harden in the acid pH range, but phenolic resins have a minimum reactivity under these conditions, there is the real danger that the phenolic portion of the resin might not really be incorporated into the aminoplastic... 

The addition of urea to a phenolic resin causes several effects (1) decrease of the content of free formaldehyde (2) decrease of the viscosity of the glue resin (3) acceleration of the hardening reaction via the higher possible degree of condensation of the resin at the same viscosity (4) reduction of the costs of the resin. 

Alkaline phenolic resin, ester hardened. The binder is a low viscosity, highly alkaline phenolic resole resin (1.2 to 1.7%). The hardener is a liquid organic ester (18 to 25%). Sand is mixed with hardener and resin, usually in a continuous mixer. The speed of setting is controlled by the type of ester used. Low sand temperature slows the cure rate, but special hardeners are available for cold and warm sand. In 24 h compression strength can reach 4000 kPa (600 psi). 

Ester-cured alkaline phenolic system. The resin is an alkaline phenolic resin (essentially the same as the self-hardening resins of this type). Sand is mixed with the resin and blown or manually packed into a core box. A vaporized ester, methyl formate, is passed through the sand, hardening the binder. The total resin and peroxide addition is 1.5%. Compression strengths of 5000 kPa (700 psi) are possible. 

PHENOL-FURFURAL RESIN, A phenolic resin that has a somewhat sharper transition than phenol-formaldehyde from the soft, thermoplastic stage to the cured, infusible state and can be fabricated by injection molding since it has little tendency to harden before curing conditions are reached. 

CA 42,7045(1948) (An expl consisting of NG, NC, NaNOg woodpulp is improved in its ability to fire under high liq press or after long immersion in w, by incorporating ca 3% cornmeal impregnated with 25% of a heat-hardenable urea -CHjO or phenolic resin)... 

Novolac and resol cold hardening oligomers habe been used 19-75-99). In the case of resol foams the process technology is not different from that used for epoxy foams. Glass, phenolic resins, carbon, polystyrene, polyacrylonitrile and poly(vinylidene chloride) microspheres have been used as fillers ... 

Finally, crosslinked structures of type VII may result from the hardening, for instance, of phenolic resins (503) through reaction of novolacs with hexamethylenetetramine, which is a preformed aminomethylating agent (see Table 31, Chap. III). 

Matsumoto and coworkers incorporated poly(A -(hydroxyphenyl)maleimides) into composite materials with phenol-formaldehyde resins. Poly(iV-(4-hydroxyphenyl)malei-mide) has been shown to form miscible blends with phenolic resins and, after crosslinking, produces composites with good thermal and chemical stability. The hardening or crosslinking agent most commonly used is hexamethylenetetramine (HMTA), to form an insolnble and infusible three-dimensional polymeric network (Scheme 29). 

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