Phenolic Casting resin

Table 3.4 Typical properties of phenolic cast resins (reference 3.5).

Lactic acid (1790) (milk acid, a-hydroxypropionic acid) n. CH3CHOH COOH. A colorless or yellowish liquid with several applications in plastics. Reacted with glycerine, it forms an alkyd resin. It is a catalyst for vinyl polymerizations, and an additive for phenolic casting resins. It has a bp of 122°G/15mmHg, mp of 18°C, and Sp gr of 1.2 Also known as alpha-hydroxypropionic acid and milk acid. 

Cast phenolics. Phenolic casting resins are available as syrupy liquids produced in huge kettles by the condensation of formaldehyde and phenol at high temperature in the presence of a catalyst and the removal of excess moisture by vacuum distillation. These resins, when blended with a chemically active hardener, can be cast and cured solid in molds constructed from various materials and of a variety of mold designs. They will exhibit a broad-based property profile as described in Table 2.14. 

Phenolic casting resins were at one time important materials, being used for umbrella handles, artificial jewellery, knobs and such objects where decorative effects were required. They are now little used in such applications, having been displaced by more readily processed plastics. However, phenolic casting resins still find some use for the production of various metal-forming tools. 

The thermal stabiUty of epoxy phenol—novolak resins is useful in adhesives, stmctural and electrical laminates, coatings, castings, and encapsulations for elevated temperature service (Table 3). Filament-wound pipe and storage tanks, liners for pumps and other chemical process equipment, and corrosion-resistant coatings are typical appHcations using the chemically resistant properties of epoxy novolak resins. 

In addition to electrical uses, epoxy casting resins are utilized in the manufacture of tools, ie, contact and match molds, stretch blocks, vacuum-forrning tools, and foundry patterns, as weU as bench tops and kitchen sinks. Systems consist of a gel-coat formulation designed to form a thin coating over the pattern which provides a perfect reproduction of the pattern detail. This is backed by a heavily filled epoxy system which also incorporates fiber reinforcements to give the tool its strength. For moderate temperature service, a Hquid bisphenol A epoxy resin with an aHphatic amine is used. For higher temperature service, a modified system based on an epoxy phenol novolak and an aromatic diamine hardener may be used. 

It is likely that the quinone methide and related structures formed at these temperatures account for the dark colour of phenolic compression mouldings. It is to be noted that cast phenol-formaldehyde resins, which are hardened at much... 

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. 

Cured phenolic resins have outstanding heat resistance, resistance to cold flow, good electric (insulation) properties, and good dimensional stability. Phenolic resins have good adhesive properties and are employed in the production of sandpaper, abrasive wheels, and brake linings. These resins are also used as casting resins. 

Superior toughness and mechanical properties The strength of properly formulated epoxy resins usually surpasses other types of casting resins. The cured epoxy resins [Structure (4.35)] are seven times tougher than the cured phenolic resins [Structure (4.36)] as is evident from Figure 4.15. The relative toughness is attributed to the distance between the crosslinking sites and presence of internal aliphatic chains. 

Processing of phenol-aldehyde oligomers into various articles is based on a polycondensation reaction which leads to solidification of the material at temperatures below 200°C and pressures exceeding 10 MPa. The process is accompanied by volatile product formation. However, phenol-formaldehyde resins of the resol type can be cast without additional pressure and heat. The raw molding reactants contain different organic and mineral fillers and other additives in addition to the basic resin. 

One of the earliest commercial plastics was Bakelite , formed by the reaction of phenol with a little more than one equivalent of formaldehyde under acidic or basic conditions. Baeyer first discovered this reaction in 1872, and practical methods for casting and molding Bakelite were developed around 1909. Phenol-formaldehyde plastics and resins (also called phenolics) are highly cross-linked because each phenol ring has three sites (two ortho and one para) that can be linked by condensation with formaldehyde. Suggest a general structure for a phenol-formaldehyde resin, and propose a mechanism for its formation under acidic conditions. (Hint Condensation of phenol with formaldehyde resembles the condensation of phenol with acetone, used in Problem 26-17, to make bisphenol A.)... 

The potential applications for conducting polymers are enormous and this has stimulated a large amount of research into this area. Not surprisingly, solid-state NMR spectroscopy has been applied to study these amorphous, insoluble and in many cases crosslinked materials [24]. Looking at the CP/MAS spectra of a series conducing polyacenic polymers, some of which were doped with iodine, it was possible to see the effect of the halogen upon conductivity. These resins were prepared by a conventional procedure for the preparation a Novolak-type phenol-formaldehyde resin. After synthesis, the phenol-formaldehyde resin were dissolved and solutions were cast as a film and heat treated to between 590-670°C in a N2 atmosphere to form the polyacenic film. The electrical conductivity of the films was shown to increase... 

Chem. Descrip. Aluminum hydroxide CAS 21645-51-2 EINECS/ELINCS 244-492-7 Uses Filler, flame retardant for thermoset plastics, polyester (concrete, artificial marble), epoxy casting resins, crosslinked elastomer cables, carpetbacking latexes, urea/phenolic chipboard, paints, varnishes... 

Chem. Descrip. Aluminum trihydrate CAS 21645-51-2 EINECS/ELINCS 244 92-7 Uses Flame retardant, filler for polyester, acrylic, phenolic, and epoxy resins, polyurethanes (casting resins), powd. coatings Properties Powd. = 8 pm median particle size bulk dens. = 600 kg/m oil absorp. = 17 cmVl 00 g conduct. < 120 pS/cm < 0.3% moisture Martinal ON-921 [Martinswerk GmbH Lonza]... 

---