Aniline-formaldehyde resins

Although occasionally in demand because of their good electrical insulation properties, aniline-formaldehyde resins are today only rarely encountered. They may be employed in two ways, either as an unfilled moulding material or in the manufacture of laminates. 

To produce a moulding composition, aniline is first treated with hydrochloric acid to produce water-soluble aniline hydrochloride. The aniline hydrochloride solution is then run into a large wooden vat and formaldehyde solution is run in at a slow but uniform rate, the whole mix being subject to continuous agitation. Reaction occurs immediately to give a deep orange-red product. The resin is still a water-soluble material and so it is fed into a 10% caustic soda solution to react with the hydrochloride, thus releasing the resin as a creamy yellow slurry. The slurry is washed with a counter-current of fresh water, dried and ball-milled. 

Because of the lack of solubility in the usual solvents, aniline-formaldehyde laminates are made by a pre-mix method. In this process the aniline hydrochloride-formaldehyde product is run into a bath of paper pulp rather than of caustic soda. Soda is then added to precipitate the resin on to the paper fibres. The pulp is then passed through a paper-making machine to give a paper with a 50% resin content. 

Aniline-formaldehyde resin has very poor flow properties and may be moulded only with difficulty, and mouldings are confined to simple shapes. The resin is essentially thermoplastic and does not cross-link with the evolution of volatiles during pressing. Long pressing times, about 90 minutes for a in thick sheet, are required to achieve a suitable product. 

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Aniline—formaldehyde resins were once quite important because of their excellent electrical properties, but their markets have been taken over by newer thermoplastic materials. Nevertheless, some aniline resins are stiU. used as modifiers for other resins. Acrylamide (qv) occupies a unique position in the amino resins field since it not only contains a formaldehyde reactive site, but also a polymerizable double bond. Thus it forms a bridge between the formaldehyde condensation polymers and the versatile vinyl polymers and copolymers. 

The term aminoplastics has been coined to cover a range of resinous polymers produced by interaction of amines or amides with aldehydes. Of the various polymers of this type that have been produced there are two of current commercial importance in the field of plastics, the urea-formaldehyde and the melamine-formaldehyde resins. There has in the past also been some commercial interest in aniline-formaldehyde resins and in systems containing thiourea but today these are of little or no importance. Melamine-phenol-formaldehyde resins have also been introduced for use in moulding powders, and benzoguanamine-based resins are used for surface coating applications. 

A large proportion of the amino resin produced is used as compression-molded plastics, with or without carrier materials such as cellulose. Amino resins are less colored and less light sensitive then phenolic resins, but they are more sensitive to humidity and temperature. Urea formaldehyde resins can be employed up to temperatures of 90° C and melamine formaldehyde resins up to 150°C. The urea formaldehyde resins are particularly suitable for rapidsetting compression molding materials. Of course, aniline formaldehyde resins must be used as pre-cross-linked products, since no aromatic nucleus condensation occurs in the absence of acid catalysts and a postcuring with... 

Mustata F., Bicu 1., Epoxy aniline formaldehyde resins modified with resin adds, Polimery, 46(7-8), 2001, 534-539. 

Thus, urea formaldehyde resins are particularly suitable as rapid-setting (curing, hardening) molding materials. Aniline formaldehyde resins are hardened with compounds such as paraformaldehyde, hexamethylene tetramine, or furfurol, since as no polycondensation reactions occur in the absence of acid catalysts, pre-cross-linked products have to be used for this purpose. Aniline resins therefore cannot be used as rapid-setting molding materials (cf. Section 12.2.2). 

Aniline a-n l-on [Gr aniliny fr. Anil indigo, fr. FP, fr. Arabic an-nil the indigo plant, fr. Sanskrit fr. mil indigo, fr. feminine of ntla dark blue] (1850) (phenylamine, amino-benzene) n. C6H5NH2. A colorless, oily liquid made by the reduction of nitrobenzene with iron chips and an acid catalyst. It is used in the production of aniline-formaldehyde resins and certain catalysts and antioxidants. 

Aniline-formaldehyde resin n. An amino-plastic that is made by condensing formaldehyde and aniline in an acid solution. The resins are thermoplastic and are used in making molded and laminated insulating materials with high dielectric strength and good chemical resistance. 

MA copolymer reaction, 283, 430 polyester Michael reaction, 490 Aniline-formaldehyde resins, MA condensations, 513, 516... 

Fig. 46. (a) Dielectric constant and (b) dissipation factor at 20°C versus frequency for undried (solid line) and dried (broken line) aniline-formaldehyde resin. 

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