Phenol-formaldehyde resin durability

Phenol-formaldehyde resins are the oldest thermosetting polymers. They are produced by a condensation reaction between phenol and formaldehyde. Although many attempts were made to use the product and control the conditions for the acid-catalyzed reaction described by Bayer in 1872, there was no commercial production of the resin until the exhaustive work by Baekeland was published in 1909. In this paper, he describes the product as far superior to amber for pipe stem and similar articles, less flexible but more durable than celluloid, odorless, and fire-resistant. ° The reaction between phenol and formaldehyde is either base or acid catalyzed, and the polymers are termed resols (for the base catalyzed) and novalacs (for the acid catalyzed). 

Other well known condensation polymers include phenol-formaldehyde resins, the prototype of which is Bakelite (Figure 13.15 C). Such structures were known as early as the 1870s, and in the early 20th century these tough, durable thermosets were among the first synthetic polymers of commerical importance. More modern versions of this type of polymer are known as Novolac. This chemistry is also that which make.s calixarenes (Chapter 4), which are cyclic tetramers rather than linear polymers. 

A durable and popular composite, transite, was asbestos with up to 15% cement. In the form of boards and sheets, it was a substitute for wood, and in severe climates, was used for roofing, fence materials, and other structures because it weathers as extremely well. Transite with asbestos is no longer available. Its replacement, silica, has limited applications because of health concerns. Other composites are continuously being developed to serve the needs of industry. Other composites include linoleum (linseed oil and jute), Bakelite (phenol-formaldehyde resin and cellulose fiber), plywood, and vehicle tires. 

Strong durable bonds can be formed between metals and vinyl chloride polymers by using a polymeric adhesive which contains carboxyl groups and a phenol formaldehyde resin. 

General Properties. Phenolic resins generally are aqueous solutions of alkaline-catalyzed phenol-formaldehyde polymers. A typical resin would be about 40% solids, containing phenol, formaldehyde, and sodium hydroxide in molar ratios of about 1 2 0.75, and might average 10-50 phenol units linked together. These can be spray dried for application as a dried powder. Phenol-formaldehyde resins are cured with heat under pressure. The resultant bond is highly water resistant and heat resistant. The durability and weatherability of phenolic-bonded wood composites enables them to be rated for exterior use. 

Phenol—formaldehyde (PF) was the first of the synthetic adhesives developed. By combining phenol with formaldehyde, which has exceptional cross-linking abiHties with many chemicals and materials, and a small amount of sodium hydroxide, a resin was obtained. The first resins soHdified as they cooled, and it was discovered that if it was ground to a powder with a small amount of additional formaldehyde and the appHcation of more heat, the mixture would Hquify and then convert to a permanently hard material. Upon combination of the powdered resin mixture with a filler material such as wood flour, the result then being placed in a mold and pressed under heat and pressure, a hard, durable, black plastic material was found to result. For many years these resulting products were called BakeHte, the trade name of the inventor. BakeHte products are still produced today, but this use accounts for only a small portion of the PF resins used. 

Properties of the HDA boards prepared from the activated substrate may be compared to the phenol-formaldehyde (PF) boards prepared using 6 resin solids. Comparison may also be made to the property requirements from commercial standard CS 236-66, type 2 medium density (below 0.8 g/cc), Class 1 particleboard fabricated using durable and highly moisture resistant and heat resistant binders suitable for interior and certain exterior applications. Specifications of this class of particleboard, generally prepared from PF resins, are MOR, minimum = 2,500 psi M0E, minimum =... 

Phenol-formaldehyde type polymers had been the only exterior-durable adhesives for wood bonding, until the recent limited use of isocyanates. Both systems are petrochemical-based. Several researchers substituted carbohydrates for part of phenolic adhesives (1-4) > producing solid, fusible novolak resins. Recently, reaction of carbohydrate acid-degradation products with phenol and formaldehyde has produced liquid resols (5). Gibbons and Wondolowski (6,7) replaced a considerable amount of phenol with carbohydrate and urea to pro-... 

Acid Catalyzed Condensation Polymerizations. The strong protonic acids produced by the photolysis of onium salts I-III can also be employed to catalyze the condensation of phenolic, melamine, and urea formaldehyde resins. Very durable photoresists based on these inexpensive and readily available resins can be made. Such resists generally require a postbake prior to development to complete the condensation and to enhance image formation. 

Examples of synthetic resins include phenol-formaldehyde formulations, which withstand boiling water or slightly higher temperatures and are used in the chemical industry in the form of multiple coats, baked on, for resisting a variety of corrosive media. Silicone and polyimide resins are useful at stiU higher temperatures. Alkyd resins, because of favorable cost, fast-drying properties, and durability, have found wide application for protecting the metal surfaces of machinery and home appliances. 

The greatest volume of wood adhesives for structural applications are thermosetting phenol-formaldehyde (PF) or urea-formaldehyde (UF) polymers or their derivatives. PFs are the commodity for exterior applications, while UFs are the commodity for interior applications. Because each composite product and the associated process may require a somewhat different resin, many variations of PF and UF resins are available. Isocyanate crosslinking resins are used to a lesser extent than PFs or UFs in manufactured board. Although these resins impart a durable, water resistant bond suitable for exterior application, the cost per pound of adhesive is greater than for PFs or UFs. 

Urea-formaldehyde adhesives were patented in 1920 but were first eommercialized around 1937. During World War II, stareh was modified with urea resins to make both waterproof adhesives and impregnants for paper, which led in the 1940s to phenolic-impregnated paper for the first durable honeyeomb eore for lightweight rigid honeycomb panels. 

Formaldehyde additions overcome the effect of phenolic extractives in certain hardwood species, which prevent proper cure or adhesion of PF resins. Free formaldehyde appears to react rapidly with these phenolic extractives before they can interfere with the phenolic resin curing mechanism. With certain wood species that are rich in extractives, this technique has been used to increase bond durability from interior-grade to true exterior-grade performance. 

These synthetic thermosetting resins are condensation products of unsubstituted melamines and formaldehyde. They are equivalent in durability and water resistance to phenolics and resorcinols. Melamines are often combined with ureas to reduce cost. Melamines have higher service temperatures than those of ureas. 

These pure synthetic resins are made by reacting phenol with formaldehyde. There are two resin types one that is cooked into oil and used largely in trade sales and marine paints and another that is sold dissolved in a solvent and applied in that form and baked. The first resin type offers excellent water resistance and durability, making it ideal for exteriors, floors, porches, decks, and marine paints or varnishes. Since resin type 1 also offers fine chemical, alkali, and alcohol resistance, it can be used for furniture, bars, patios, and similar applications. In some instances, adhesion is rather poor. 

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