Crosslinking phenolic

The route to crosslinked phenol-formaldehyde resins via resoles corresponds to that used by Baekeland in his original commercial technique. They now tend to be used for adhesives, binders, and laminates. The resole... 

Alkenyloxystyrene monomers such as 4-allyloxystyrene are useful components of photocured cationically polymerizable compositions. Used alone or in combination with divinyl ethers they provide low viscosity formulations, which are excellent solvents for commercial onium salt photoinitiators. Photocuring rates are comparable to vinyl ether monomers and the initially photocured alkenyloxystyrene polymers may be further heat processed to yield crosslinked phenolic type resins having outstanding thermal resistance properties. The new materials have good adhesive properties and are potentially useful where a combination of ease of processability and high performance is required. 

The advanced applications for nitrocellulose plastisol propellants require that they be integrally bonded to the motor case. Successful case bonding for the multiyear storage life of a rocket calls for special adhesives and liners which are completely compatible with these highly plasticized propellants. Best results have been obtained with a combination of an impervious rubber liner and a crosslinked adhesive system with a limited affinity for the plasticizers used in the propellants. Examples of effective liners are silica-filled butyl rubber and chlorinated synthetic rubber. Epoxy polyamides, isocyanate-crosslinked cellulose esters, and combinations of crosslinked phenol-formaldehyde and polyvinyl formal varnishes have proved to be effective adhesives between propellant and impervious liners. Pressure curing of the propellants helps... 

A pyrogram for a crosslinked phenol-formaldehyde resin (made with a basic catalyst) was done similarly to that for other polymers exemplified in this book, at 600° C in He and with the separation on a Carbowax column and mass spectral detection (see Table 4.2.2). The peak identification for the chromatogram shown in Figure 8.3.1 is given in Table 8.3.1. 

Figure 8.3.1. Pyrogram of a crosslinked phenol-formaldehyde resin sample. Pyrolysis done on 0.4 mg material at 60(f C in He, with the separation on a Carbowax type column.

Hexamethylene tetramine is an important chemical. Its most important application is as a source of formaldehyde for crosslinking phenolic molding powders, shell molding resins, and two-step curing resins for chip board. Quatemization of HMTA with an alkyl chloride (R-Cl) gives a family of bactericides (developed by Dow) for use in latex paints and as a dermatitis preventative in water-soluble cutting oil. Hexamine is used in pharmaceutical formulations to combat urinary tract infections and also as an intermediate in the production of chloramphenicol. 

Showa High Polymer [127] demonstrated the use of onium salts for crosslinking phenol resins in 1986. 

Acid-catalyzed condensation has been the primary and dominant foundation for aqueous base developable negative resist systems [354-363]. The first commercial chemical amplification resist was built on this mechanism. The condensation resists are typically three-component systems comprising a base soluble binder resin bearing reaction sites for crosslinking (phenolic resin), a radiation-sensitive acid generator, and an acid-sensitive latent electrophile... 

The route to crosslinked phenol-formaldehyde resins via resoles corre.sponds to that used by Baekeland in his original commercial technique, They now tend to be used for adhesives, binders, and laminates. The resole prepolymers are made typically in batch processes, using a trace of ammonia (about 2% on phenol) as the alkaline catalyst. Care has to be taken with this process since, despite the molar excess of formaldehyde, there is sufficient of each component present in the prepolymer to permit the formation of a highly crosslinked product. Indeed, such a product will form if the resole is heated excessively, but the problem can be avoided by careful attention to the conditions of reaction and by ensuring that polymerisation is not allowed to proceed for too long. 

Lignin, one of the major polymeric constituents of wood, is a highly crosslinked phenolic polymer derived from the free-radical coupling of substituted phenylpropa-noids (Figure 5.9). It reacts with aqueous chlorine to form oxidized and chlorinated species. This is a key reaction in the bleaching of wood pulp by the kraft process... 

Thermoset, those that cannot be resoftened, including polyethylene (crosslinked), phenolics, and alkyds like the polyesters (polymers based on maleic, terephthalic, or other carboxylic acids and glycol, propylene glycol or similar alcohols). 

Adhesives are formed from phenolic resirts and thermoplastic materials such as polyvinylformal, polyvinylacetals polyvinylbutyral, and nitrilbutyl mbber. Chemical reactions occur during Mend formation. However, the specific morphology of the polymer Mend formed is the main cause for toughness. The cured material contains a well dispersed discontinuous thermoplastic phase in the crosslinked phenolic matrix. 

Schematic Cut through a Crosslinked, Fiber-Reinforced Phenolic Material Rectangles Connected by Short Lines Represent the Crosslinked Phenolic Phase and the Broad Lines Represent the Fibers. Some Molecules of the Crosslinked, Phenolic Phase are Covalently Bonded to the Fiber.

Fillers and other additives are used to make molded objects from phenolic resins to reduce shrinkage during cure, improve strength, and provide flow control. Representative materials added are mineral flour, wood flour, nutshell flour, pressed olive stones, lignite, and natural rubber. Crosslinked phenolics containing inorganic or organic fillers are schematically illustrated in Scheme 35. 

Thermomechanical analysis (TMA) is the measurement of the deformation of the phenolic material imder a constant strain as a function of temperature or time. TM A invesfigalion of crosslinked, modified phenolics gives the linear expansion coefficient and the glass tiansifion temperature of this material [33]. The linear expansion coefficient ( ) is measured by thermal expansion in one dimension. It is defined by equation 62 where dL/dT is the slope of the jdot of the longest sample dimension versus sample temperature and L is the original length of the crosslinked phenolic sample. 

The heating rate strongly influences the shape of a TGA curve, as does the atmosphere, the gas flow rate, and the sample mass [243]. Heating of a crosslinked phenolic up to its ignition temperature leads to formation of char. Char yields are an important parameter in flammability studies. The oxygen... 

Which methods are applicable to obtain information about temperature behavior of crosslinked phenolics ... 

Crosslinked phenolics combine high strength, excellent thermal and mechanical stress, very good electrical and thermal insulation, fire resistance, veiy low smoke generation with a good compatibility with fillers and fibers. Phenolics are not expensive. 

The temperature behavior of crosslinked phenolics can be described by using difierent thermal analytical methods. DTA, DSC and DMA can be applied for analysis of transitions in crosslinked phenolics. TMA investigation of crosslinked phenolics gives the linear expansion coefficient and the glass transition temperature. TGA is used to study the thermal stability of the material and its decomposition. 

Crosslinked phenolic resins exhibit both high hardness as well as good resistance to heat, oxidation, and solvents. Thus they are the material of choice as reinforcing materials for rubber mixtures. Vulcanized materials soon reach their limits of hardness and stiffness therefore hexamethylene tetramine-cured phenol-novolacs and active fillers gained importance as reinforcement resins as early as the late f 940s. We now know that an inherent condensation network is formed that permeates the vulcanized rubber. Phenolic reinforcement resins can be used alone or directly in combination with fillers. The resin system results in an improvement in rubber-specific properties as well as in increased heat resistance and good solvent and oil resistance. 

Resols are intermediates in the formation of highly crosslinked phenol formaldehyde polymers, and they usually have molecular weights in the 300-700 range. A typical resol structure is shown in Fig. 4,... 

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