Лако-красочные материалы — производство

Hardened PF resin have a specific gravity of approximately 1.2 to 1.3, a refractive index of 1.6, and a specific heat of 0.5. They are typically brown in color, and novolaks are lighter in color than resols. Resols are dark yellow, orange reddish, or brownish even when made with pure raw materials. However, if the alkali is neutralized resols become almost color­less. The best results were obtained with citric, lactic, and phosphoric acids. Pale-colored, hardened resins can be prepared with them [46]. Phenolic resins are relatively stable up to about 200 to 250°C, although oxidative degradation takes the form of attack at the methylene bridges to produce substituted, dihydroxy benzophenones [47]. Above this temperature, they begin to char slowly, and at higher temperatures charring is more rapid. At about 400°C decomposition is rapid, yielding phenols and aldehydes, and leav­ing a cokelike residue.

In the A stage, simple PF resins are readily soluble in alcohol, esters, ketones, phenols, and some ethers, and insoluble in hydrocarbons and oils. As a class, resols tend to be more soluble in alcohols and water, and novolaks tend to be more soluble in hydrocarbons. In the early stages of condensation, resols are often soluble in water, owing to the presence of methylolphenols, especially polyalcohol. This is more pronounced with resols that are derived from phenol. Cresilic resols are less soluble, and xylenolic resols are almost insoluble in water. The solubility of A-stage resins in dilute aqueous sodium hydroxide or in mixtures of water and alcohols follows the same trend.

Solubility in alcohols and insolubility in hydrocarbons appear to go together. The alcohol and water solubility can be reduced only by using acetaldehyde or other aldehydes in the place of formaldehyde, and by introducing hydrocarbon chains, particularly in the ortho or para positions in the aromatic ring. B-stage resins are soluble in only a few solvents, such as boiling phenols, acetone, aqueous sodium hydroxide, and deca — and tetrahydronaphthalenes. Resins in the hardened or C stage are very resistant to most chemical reagents. They are unaffected by all ordinary organic solvents and water, although a few percent of water may be absorbed in filled material, mainly by the filler, thus causing slight swelling. The C-stage resins dissolve slowly in boiling phenols such as naphthols. Resins from the simplest phenols can also be broken down and dissolved by hot, strong alkali solutions.

Simple PF resins are readily attacked by sodium hydroxide. However, cresol — formaldehyde, and especially xylenol-formaldehyde resins, are much less susceptible to attack. Resins are often more resistant to strong alkaline solutions (i. e., 15 to 20%) than to dilute solutions (i. e., 5%). The filler has a considerable influence on the chemical resistance of the resins. Inert mineral fillers have a better resistance than cellulosic fillers. C-stage resins are resistant to most acids, except sulfuric acid stronger than 50%, formic acid, and oxidizing acids such as nitric and chromic acids. The insolubility of hardened resins in acetone is used to test the degree of cure of the resin. The curing temperature influences the
amount of matter that is insoluble in acetone after prolonged heating [48]. The higher the hardening temperature, the lower the amount of acetone extractives. The mechanical properties of hardened PF resins are greatly influenced by the moisture content. This applies even more to resins containing fillers, plasticizers, and other ingredients. The rate of water absorption decreases with time, but thick samples may not reach an equili­brium even after several months in water. Therefore, in measuring the mechanical proper­ties of resins, it is necessary to condition the test pieces under carefully controlled temperature and humidity prior to making the tests. In many cases, the mechanical prop­erties of hardened resins are largely dependent on the type and orientation of the filler. This applies particularly to water absorption, tensile strength, and impact strength. It also applies to shear strength, with the condition that in the plane of the laminations the shear strength depends on the adhesion between the laminae of sheet material. The properties of the resin are more important than those of the filler in determining the compression strength.