Phenolic resins branching

Much interest has centred on the branch of cyclophanes known as calixarenes. They are polyphenol systems that can act as hosts in the formation of inclusion compounds, where a small guest molecule resides completely in a cavity within a single host they are cone-shaped cavitands . Several accounts have appeared of their history. The discovery by Baeyer of a formaldehyde/phenol resin led to Bakelite and to the work of A. Zincke and E. Ziegler, who gave to the first oligomer a tetrameric structure of a calix[4]arene. Later syntheses by Gutsche (1978) led to calixarenes with 4, 6 or 8 phenol residues.107-109... 

In other systems, such as in some Friedel-Crafts alkylations, ortho-substitution is desirable. For example, extensive alkylation at both the ortho-and para-positions of phenol with formaldehyde in the presence of an acid catalyst yields highly branched novolac phenolic resin prepolymers [Eq. (26)]. 

It is clear that in a certain sense a branched PRF will behave as a more advanced, almost precured phenolic resin. While the first effect described is a definite advance on the road to better engineered PRF resins, the second effect can also be obtained with more advanced (reactionwise) linear resins. The contribution of the second effect to the decrease in resorcinol is not less marked than that of the first effect. It is, however, the second effect that accounts for the difference in behavior between branched and linear PRF adhesives. 

The hardening of a phenolic resin can be seen as the transformation of molecules of different sizes via chains lengthening, branching, and crosslinking to a three-dimensional network with theoretically an endlessly high molar mass. The hardening rate depends on various parameters, such as molar mass, molecular structure of the resin, the portions of various structural elements as well as possible catalysts and additives. 

With higher proportions of formaldehyde and the para-position vacant, branching is possible. An alkaline catalyst is used to get a slower rate of reaction, thus making it easier to stop the reaction before cross-linking occurs. This type of phenolic resin is called a resole. 

Other blends formed from phenolic prepolymeis and an ethylene-vinyl acetate (EVA) copolymer show similar effects. Blending ofphenolic resins and EVA copolymers results a single-phase system. However, reactions of phenolic resins lead to a molecular weight increase, to branching, and to crosslinking of the phenolic prepolymerthat cause a phase separation [183]. 

Products of polymerized vegetable oil acids and polyamines, which are highly branched liquid polyamides, are mixed with phenolic resins to produce a thermosetting composition with a wide range of properties. Cure usually is accomplished in 60 min at 300°F. The reaction involves the condensation between methylol groups and amine groups. 

Plastic is a material that can be plasticized into certain shapes under certain conditions (temperature, pressure, etc.) and can keep its shape unchanged at room temperature and normal atmosphere pressure. According to their performance after heat treatment, plastics can be divided into thermoplastic and thermosetting plastics. A thermoplastic plastic is generally a linear or branched polymer. It melts when heated and solidifies when cooled, and this kind of behavior can be repeated, so the plastic can be used multiple times. The main varieties are polyethylene, polypropylene, polyvinyl chloride, polystyrene, and acrylonitrile-butadiene-styrene terpolymer. Thermosetting plastic is a space network polymer, which is formed by direct polymerization of monomers or by cross-linking of linear prepolymers. Once the solidification is finished, the polymer cannot be heated back to the plasticizing state. The main varieties are phenolic resin, epoxy resin, amino resin, and unsaturated polyester. 

In 1927, Schiebler et ai, [14] found that in acid solution the methylolureas are converted to insoluble substances similar to Goldschmidt s compound. Today the polymerization mechanisms involved are similar to those discussed for other methylol compounds such as phenolic resins (Chapter 2) or melamine resins (see Section 3 of this chapter). It is interesting to note that because urea has four active hydrogens and three sites for polymerization, linear, branched, and cyclic structures are possible. In fact, Kadowaki [15] has isolated several low-molecular-weight condensation products of urea-formaldehyde and has described their properties. The cyclic structures commonly called urones, such as dimethylolurone (iV,N -dimethyloltetra-hydro-4//-l,3,5-oxidiazin-4-one), have also been prepared by Kadowski. 

Phenolic resins used in lamination are primarily the one-step resoles, although novolacs may be added to improve resistance to water. Further improvements in water resistance and a reduction in brittleness are achieved by using para-cresol in place of phenol. This inhibits branching since the para position is blocked by the methyl group. 

Amino resins are based on reactions of compounds containing amino, imino, or amide groups with aldehydes [3]. As in phenolic resins, the most widely used aldehyde is formaldehyde. In an aqueous environment a condensation reaction between formalin and the amino, imino, or amide compound leads to highly branched, low molecular weight polymers. Introducing an acid catalyst is usually necessary to initiate further condensation reactions that lead to curing. 

Phenolic resins are obtained by polymerizing phenol with formaldehyde. When polymerized at low pH (i.e., acidic reaction medium), the resultant material is a straight-chain polymer, normally called novolac. However, under basic conditions, a higher-branched polymer called resole is formed. To cure novolac, a cross-linking agent, hexamethylenetetramine, is required, which has the following chemical formula ... 

Stopping the polymer at this point requires the ratio of formaldehyde to phenol to be less than unity. Both methylene and ether bridges are known to be present. The reaction is either acid or base catalyzed, and branching is uncommon at this stage. The products are variously known as A stage resins, novolacs, or resole prepolymers. 

Paint and varnish manufacturing Resin manufacturing closed reaction vessel Varnish cooldng-open or closed vessels Solvent thinning Acrolein, other aldehydes and fatty acids (odors), phthalic anhydride (sublimed) Ketones, fatty acids, formic acids, acetic acid, glycerine, acrolein, other aldehydes, phenols and terpenes from tall oils, hydrogen sulfide, alkyl sulfide, butyl mercaptan, and thiofen (odors) Olefins, branched-chain aromatics and ketones (odors), solvents Exhaust systems with scrubbers and fume burners Exhaust system with scrubbers and fume burners close-fitting hoods required for open kettles Exhaust system with fume burners... 

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