Aminoplastics urea-formaldehyde resins

By the mid-1990s world production of aminoplastics was estimated at about 6 000 000 t.p.a. of which more than 5 000 000 t.p.a. were urea-formaldehyde resins. The bulk of the rest were melamine-formaldehyde. Such bald statistics, however, disguise the fact that a considerable amount of aminoplastics used are actually co-condensates of urea, melamine and formaldehyde. 

Aminoplastics In this group, melamine-formaldehyde resins with their good heat resistance, scratch resistance and stain resistance, are usually preferred to urea-formaldehyde resins where chemical resistance is important. Unlike the phenolics these materials are not restricted to dark colours. 

In far too many instances trade-name polymer nomenclature conveys very little meaning regarding the structure of a polymer. Many condensation polymers, in fact, seem not to have names. Thus the polymer obtained by the step polymerization of formaldehyde and phenol is variously referred to a phenol-formaldehyde polymer, phenol-formaldehyde resin, phenolic, phenolic resin, and phenoplast. Polymers of formaldehyde or other aldehydes with urea or melamine are generally referred to as amino resins or aminoplasts without any more specific names. It is often extremely difficult to determine which aldehyde and which amino monomers have been used to synthesize a particular polymer being referred to as an amino resin. More specific nomenclature, if it can be called that, is afforded by indicating the two reactants as in names such as urea-formaldehyde resin or melamine-formaldehyde resin. 

W. Blakey, The history of aminoplastics The sixth Chance Memorial Lecture of the Society of Chemical Industry, Chemistry and Industry, (25 July 1964), 1349-1357, on p. 1352. Blakey received a D.Phil. in organic chemistry from Kings College, Cambridge, in 1928, and shortly after joined British Cyanides to undertake research into aminoplastic moulding compounds. Blakey was appointed chairman of British Industrial Plastics in 1962, one year after the firm was merged with Turner Newall Ltd. This lecture is an extremely useful source of information on aminoplastic developments. See also C.P. Vale and W.G.K. iy oi Aminoplastics (London Iliffe Books, 1964), which includes a historical introduction, and a portrait of Hanns John who in 1918 patented the first industrial use of urea-formaldehyde resins. 

Aminoplastics (urea/formaldehyde and melamine/formaldehyde resins) are even better in flammability properties than phenolics. They can be further improved by admixing 7 to 20 per cent of halogenated phosphoric esters. 

Urea-formaldehyde resins and similar aminoplast precondensates form the greatest proportion of all the resins used as additives. Mono-methylated and dimethylated ureas are used, as are the analogous condensation products of formaldehyde with melamine. The monomeric compounds penetrate into the intermicellar space in the cellulose in aqueous solution, and there harden with heat to form insoluble resins (cf. also Section 28.2). Since the formation of mono- and dimethylated urea is reversible, CH2O occurs in equilibrium. Formaldehyde can form methylene cross-link bridges between the individual chains. In addition, longer cross-linking... 

Blais, J. F. (1959) Amino Resins, Reinhold Publishing Corporation, New York. Vale, C. P. and Taylor, W. G. K. (1964) Aminoplastics, Iliffe Books Ltd, London. Meyer, B. (1979) Urea-Formaldehyde Resins, Addison-Wesley, Reading. 

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. 

The crosslinkers examined in this study were aminoplast resins 1-4 selected from melamine-formaldehyde, urea-formaldehyde, benzoguanamine-formaldehyde, and glycoluril-formaldehyde resins, all of which undergo the crosslinking sequence shown in Scheme 1. The response of these crosslinkers to acid catalysis in thin films is compared on a relative basis to the well studied methylated melamine, 1 19-11). 

Formaldehyde resins with better water- and temperature-stabilities are obtained if the urea is partly or wholly replaced by melamine (aminoplasts). These condensations are likewise carried out mainly in alkaline medium, again yielding soluble pre-condensates consisting essentially of N-[tris- and hexakis-(hy-droxymethyl)] compounds of melamine. 

Soils stabilized with urea-formaldehyde have strengths comparable to the phenoplasts and like those materials are less sensitive to testing strain rate than other chemical grouts. (For optimum mechanical properties and to keep free formaldehyde levels low, one molecule of urea should be provided with three molecules of formaldehyde.) Little data are available, but it is probable that aminoplasts break down comparatively quickly under cyclic wet iry and freeze thaw conditions. The creep endurance limit is probably a relatively high percentage of the UC. Except as noted above, the resins have good stability and are considered permanent. 

U.S. Pat. Nos. 3,407,154 [16] and 3,407,155 [17] describe thermosetting urea-formaldehyde and aminoplast resinous molding composition comprising fusible reactive urea-formaldehyde and aminotriazine-formaldehyde resin, respectively, and purihed a-cellulose hbers (14-25% by weight) as a hller. 

Formaldehyde is employed in the production of aminoplasts and phenoplasts, which are two different but related classes of thermoset polymers. Aminoplasts are products of the condensation reaction between either urea (urea-formaldehyde or UF resins) or melamine (melamine-formaldeliyde or MF resins) with formaldehyde. Phenoplasts or phenolic (phenol-formaldehyde or PF) resins are prepared from the condensation products of phenol or resorcinol and formaldehyde. 

The principal feature that distinguishes thermosets and conventional elastomers from thermoplastics is the presence of a cross-linked network structure. As we have seen from the above discussion, in the case of elastomers the network structure may be formed by a limited number of covalent bonds (cross-linked rubbers) or may be due to physical links resulting in a domain structure (thermoplastic elastomers). For elastomers, the presence of these cross-links prevents gross mobility of molecules, but local molecular mobility is still possible. Thermosets, on the other hand, have a network structure formed exclusively by covalent bonds. Thermosets have a high density of cross-links and are consequently infusible, insoluble, thermally stable, and dimensionally stable under load. The major commercial thermosets include epoxies, polyesters, and polymers based on formaldehyde. Formaldehyde-based resins, which are the most widely used thermosets, consist essentially of two classes of thermosets. These are the condensation products of formaldehyde with phenol (or resorcinol) (phenoplasts or phenolic resins) or with urea or melamine (aminoplastics or amino resins). 

Aminoresins or aminoplastics cover a range of resinous polymers produced by reaction of amines or amides with aldehydes [14,46,47]. Two such polymers of commercial importance in the field of plastics are the urea-formaldehyde and melamine-formaldehyde resins. Formaldehyde reacts with the amino groups to form aminomethylol derivatives which undergo further condensation to form resinous products. In contras to phenolic resins, products derived from urea and melamine are colorless. 

The characteristics of PF resins and the reactive chemical groups they present render them particularly suitable for the preparation of binders by coreaction with other resins. This is still a relatively young field, and the most interesting and relevant co-resins that are being used or explored in this respect are the aminoplastic resins, in particular urea-formaldehyde (UF) and melamine-formaldehyde (MF) (the copolymerization with the latter being a somewhat older use), and the diisocyanates. 

Results obtained by a series of techniques for the curing of several resin systems [18,20-24] have indicated, however, that posttreatment and hotstacking (postcuring) conditions capable of improving the mechanical performance of aminoplastic resin-bonded particleboard without any degradation should instead exist. This is of some importance, firstly because the performance of UF- and melamine-urea-formaldehyde (MUF)-bonded particleboard could be improved with very little process change from the present industrial conditions to yield better board performance (or the same performance at lower adhesive content levels), and secondly because at parity of board performance such an approach may well lead to the use of even shorter industrial press cycles than today, even for aminoplastic resins. 

When the pendant groups are epoxides, like glycidyl esters, cross-linking can be carried out with dianhydrides or with compounds containing two or more carboxylic acid groups [241]. Aminoplast resins (urea-formaldehyde or melamine-formaldehyde and similar ones) are also very effective [242]. 

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