Phenol/formaldehyde curing

CF = cardanol-formaldehyde resin (cured) MCPAF = monocar-danyl phosphoric acid-formaldehyde resin (cured) BrCF = bromo derivative of CF BrMCPAF = bromoderivative of MCPAF PF = phenol-formaldehyde (cured) MPPAF = monophenyl phosphoric acid-formaldehyde (cured), BrPF = bromo derivative of PF BrMPPAF = bromoderivative of MPPAF PPF = phenol-formaldehyde resin phosphorylated (cured). 

Polyethylene, polypropylene, polyvinyl chloride, polyamide Phenol-formaldehyde-cured rubber styrenated polyester Polyimide (ladder molecules) Polyethylene terephthalate Terylene, cellulose acetate Chloroprene rubber, polyisoprene Heat-resistant polymers... 

Methylol Terminated, p-Alkyl Substituted, Phenol-Formaldehyde Curing Resin... 

Methylol-terminated para-alkyl-substituted phenol formaldehyde curing resin is a condensation product from a reaction of the proper proportions of para-alkyl phenol with formaldehyde under controlled conditions see Figure 10.67. 

Figure 10.67 Reaction of para-octyl phenol with formaldehyde to produce methylol-terminated para-octyl-substituted phenol formaldehyde curing resin...

Methylol-terminated p-alkyl-substituted phenol formaldehyde curing resin... 

A very small amount of the total formaldehyde is used to make methylol-terminated phenol formaldehyde curing resins, usually used in rubber bladders for tire curing. 

In practice, synthetic polymers are sometimes divided into two classes, thermosetting and thermo-plMtic. Those polymers which in their original condition will fiow and can be moulded by heat and pressime, but which in their finished or cured state cannot be re softened or moulded are known as thermo setting (examples phenol formaldehyde or urea formaldehyde polymer). Thermoplastic polymers can be resoftened and remoulded by heat (examples ethylene polymers and polymers of acrylic esters). 

Quinone dioximes, alkylphenol disulfides, and phenol—formaldehyde reaction products are used to cross-link halobutyl mbbers. In some cases, nonhalogenated butyl mbber can be cross-linked by these materials if there is some other source of halogen in the formulation. Alkylphenol disulfides are used in halobutyl innerliners for tires. Methylol phenol—formaldehyde resins are used for heat resistance in tire curing bladders. Bisphenols, accelerated by phosphonium salts, are used to cross-link fluorocarbon mbbers. 

Most processors of fiber-reinforced composites choose a phenol formaldehyde (phenoHc) resin because these resins are inherently fire retardant, are highly heat resistant, and are very low in cost. When exposed to flames they give off very Htde smoke and that smoke is of low immediate toxicity. PhenoHc resins (qv) are often not chosen, however, because the resole types have limited shelf stabiHty, both resole and novolac types release volatiles during their condensation cure, formaldehyde [50-00-0] emissions are possible during both handling and cure, and the polymers formed are brittle compared with other thermosetting resins. 

Other modifications of the polyamines include limited addition of alkylene oxide to yield the corresponding hydroxyalkyl derivatives (225) and cyanoethylation of DETA or TETA, usuaHy by reaction with acrylonitrile [107-13-1/, to give derivatives providing longer pot Hfe and better wetting of glass (226). Also included are ketimines, made by the reaction of EDA with acetone for example. These derivatives can also be hydrogenated, as in the case of the equimolar adducts of DETA and methyl isobutyl ketone [108-10-1] or methyl isoamyl ketone [110-12-3] (221 or used as is to provide moisture cure performance. Mannich bases prepared from a phenol, formaldehyde and a polyamine are also used, such as the hardener prepared from cresol, DETA, and formaldehyde (228). Other modifications of polyamines for use as epoxy hardeners include reaction with aldehydes (229), epoxidized fatty nitriles (230), aromatic monoisocyanates (231), or propylene sulfide [1072-43-1] (232). 

The importance of the nature of the catalyst on the hardening reaction must also be stressed. Strong acids will sufficiently catalyse a resol to cure thin films at room temperature, but as the pH rises there will be a reduction in activity which passes through a minimum at about pH 7. Under alkaline conditions the rate of reaction is related to the type of catalyst and to its concentration. The effect of pH value on the gelling time of a casting resin (phenol-formaldehyde ratio 1 2.25) is shown in Figure 23.15. 

Phenol-formaldehyde resins. These arc used to cure butyl rubber forming thermally stable carbon-carbon cross-links. 

Nitrile rubber is compatible with phenol-formaldehyde resins, resorcinol-formaldehyde resins, vinyl chloride resins, alkyd resins, coumarone-indene resins, chlorinated rubber, epoxies and other resins, forming compositions which can be cured providing excellent adhesives of high strength, high oil resistance and high resilience. On the other hand, NBR adhesives are compatible with polar adherends such as fibres, textiles, paper and wood. Specific formulations of NBR adhesives can be found in [12]. 

An additional activating hydroxyl group on the phenolic ring allows resorcinol to react rapidly widi formaldehyde even in die absence of catalysts.8 Hiis provides a method for room temperature cure of resorcinol-formaldehyde resins or mixed phenol-formaldehyde/resorcinol-formaldehyde resins. Trihydric phenols have not achieved commercial importance, probably due to tiieir higher costs. 

The most common crosslinking agent for novolac resins is HMTA which provides a source of formaldehyde. Novolac resins prepared from a phenol-formaldehyde (F/P) ratio of 1/0.8 can be cured with 8-15 wt % HMTA, although it has been reported that 9-10 wt % results in networks with the best overall performance.3... 

Cured phenol-formaldehydes are resistant to attack by most chemicals. Organic solvents and water have no effect on them, though they will swell in boiling phenols. Simple resins are readily attacked by sodium hydroxide solutions, but resins based on phenol derivatives, such as cresol, tend to be less affected by such solutions. Simple phenol-formaldehyde polymers are resistant to most acids, though formic and nitric acids will tend to attack them. Again, cresol-based polymers have resistance to such attack. 

Sulfur cross-links have limited stability at elevated temperatures and can rearrange to form new cross-links. These results in poor permanent set and creep for vulcanizates when exposed for long periods of time at high temperatures. Resin cure systems provide C-C cross-links and heat stability. Alkyl phenol-formaldehyde derivatives are usually employed for tire bladder application. Typical vulcanization system is shown in Table 14.24. The properties are summarized in Tables 14.25 and 14.26. 

Diene rubbers can be vulcanized by the action of phenolic compounds like phenol-formaldehyde resin (5-10 phr). Resin-cured NR offers good set properties and low hysteresis [54]. 

Curing of Urea-Formaldehyde and Phenol-Formaldehyde. Urea-formaldehyde resins and phenol-formaldehyde resins can be cured by various mechanisms. 

Acid Curing. Urea-formaldehyde resins and resol-phenol-formaldehyde resins can be acid-cured by wastes from the production of maleic anhydride [1902]. The waste from the production of maleic anhydride contains up to 50% maleic anhydride, in addition to phthalic anhydride, citraconic anhydride, benzoic acid, o-tolulic acid, and phthalide. The plugging solution is prepared by mixing a urea-formaldehyde resin with a phenol-formaldehyde resin, adding the waste from production of maleic anhydride, and mixing thoroughly. 

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