Aminoplastics

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. 

During the next 15 years the urea resins were also developed for use as adhesives, as textile finishing agents and in the production of surface coatings and wet-strength paper. Since World War II the development of chipboard has resulted in a large new outlet for urea-based resins which have also found other uses, such as in firelighters and foams. 

In 1935 Henkel S patented the production of resins based on melamine. Today these resins are important in the manufacture of decorative laminates and in tableware. 

The development of the aminoplastics has been described in two interesting reviews.  

These thermosetting polymers are synthetic resins containing the amine group (-NHj) in their macromolecules. The major commercial resins in this group are  

Urea-formaldehyde first appeared in 1929 in the USA as a substitute of glass for windows while melamine-formaldehyde was first commercialized by the American Cyanamid Co. in 

Urea-formaldehyde. This thermoset is obtained by the condensation of urea [(NHj)jCO] or derivatives such as hydroxymethylurea and formaldehyde (HCHO) in the presence of a proper catalyst either basic or acid. The general equation of the first reaction is given below  

Melamine-formaldehyde. The monomer used for preparing melamine formaldehyde is formed by reacting melamine with formaldehyde to yield hexamethylolmelamine. The monomer can further condense in the presence of an acid catalyst ether linkages can also form. A wide variety of resins can be obtained by careful selection of pH, reaction temperature, reactant ratio, amino monomer, and extend of condensation. Liquid coating resins are prepared by reacting methanol or butanol with the initial methylolated products. These can be used to produce extreme surface hardness, discoloration and solvent-resistant coatings by heating with a variety of hydroxy, carboxyl, and amide functional polymers to produce a cross-linked film. 

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Eyrol 51 is a water-soluble Hquid containing about 21% phosphoms. It is made by a multistep process from dimethyl methylphosphonate, phosphoms pentoxide, and ethylene oxide. The end groups are principally primary hydroxyl and the compound can thus be incorporated chemically into aminoplasts, phenoHc resins, and polyurethanes. Eyrol 51, or 58 if diluted with a small amount of isopropanol, is used along with amino resins to produce a flame-retardant resin finish on paper used for automotive air filters, or for backcoating of upholstery fabric to pass the British or California flammabiHty standards. 

Figure 4d represents in situ encapsulation processes (17,18), an example of which is presented in more detail in Figure 6 (18). The first step is to disperse a water-immiscible Hquid or soHd core material in an aqueous phase that contains urea, melamine, water-soluble urea—formaldehyde condensate, or water-soluble urea—melamine condensate. In many cases, the aqueous phase also contains a system modifier that enhances deposition of the aminoplast capsule sheU (18). This is an anionic polymer or copolymer (Fig. 6). SheU formation occurs once formaldehyde is added and the aqueous phase acidified, eg, pH 2—4.5. The system is heated for several hours at 40—60°C.

The term amino resin is usually appHed to the broad class of materials regardless of appHcation, whereas the term aminoplast or sometimes amino plastic is more commonly appHed to thermosetting molding compounds based on amino resins. Amino plastics and resins have been in use since the 1920s. Compared to other segments of the plastics industry, they are mature products, and their growth rate is only about half of that of the plastics industry as a whole. They account for about 3% of the United States plastics and resins production. 

Miscellaneous. In ore flotation, sodium sulfite functions as a selective depressant. In textile processing, sodium sulfite is used as a bleach for wood (qv) and polyamide fibers and as an antichlor after the use of chlorine bleach. Synthetic appHcations of sodium sulfite include production of sodium thiosulfite by addition of sulfur and the introduction of sulfonate groups into dyestuffs and other organic products. Sodium sulfite is useful as a scavenger for formaldehyde in aminoplast—wood compositions, and as a buffer in chrome tanning of leather. 

Cross-linking with aminoplasts and phenoplasts constitutes an important class of hardeners for high molecular-weight epoxy resins that require elevated temperature cures (see Amino resins). 

NB Daia for the three important ihermosetting materials (phenolics, aminoplastics and epoxide resins) were not covered in the 1998 review on which the 1997 data was based. The 1987 figures for these materials do include a substantial percentage of use in adhesive, surface coating and laminate applications. 

The tonnage of plasticisers consumed each year exceeds the annual tonnage consumption of most plastics materials. Only PVC, the polyolefins, the styrene polymers, the aminoplastics and, possibly, the phenolics are used in large quantity. 

There are now commercially available a large range of laminated plastics materials. Resins used include the phenolics, the aminoplastics, polyesters, epoxies, silicones and the furane resins, whilst reinforcements may be of paper, cotton fibre, other organic fibres, asbestos, carbon fibre or glass fibre. Of these the phenolics were the first to achieve commercial significance and they are still of considerable importance. 

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. 

The method for producing formaldehyde was described in Chapter 19. In aminoplastics manufacture it is used in the form of formalin (36-37% w/w CH2O). As in the case of phenolic resin production, formalin with both high and low methanol content is used according to the needs of the manufacturer. The low methanol content formalin is more reactive but is also less stable and must be used soon after its preparation. For this reason some resin manufacturers prefer to use formalin with a high 7-10%) methanol content. 

As with the other aminoplastics, the chemistry of resin formation is incompletely understood. It is, however, believed that under acid conditions at aniline-formaldehyde ratios of about 1 1.2, which are similar to those used in practice, the reaction proceeds via p-aminobenzyl alcohol with subsequent condensation between amino and hydroxyl groups (Figure 24.10). 

VALE, c. p, Aminoplastics, Cleaver-Hume Press, London (1950)... 

Because of their favourable price, polyesters are preferred to epoxide and furane resins for general purpose laminates and account for at least 95% of the low-pressure laminates produced. The epoxide resins find specialised uses for chemical, electrical and heat-resistant applications and for optimum mechanical properties. The furane resins have a limited use in chemical plant. The use of high-pressure laminates from phenolic, aminoplastic and silicone resins is discussed elsewhere in this book. 

Laminates have been prepared for the manufacture of chemical plant. They have better heat and chemical resistance than the polyester- epoxide- phenolic- or aminoplastic-based laminates but because of the low viscosity of the resins were not easy to handle. Because they were also somewhat brittle, furan-based laminates have been limited in their applications. 

Reactivity and hardening reactions of aminoplastic adhesive resins... 

MUPF/PMUF-resins harden in the main under similar acid conditions to UFs and MUFs. Because MUF-resins harden in the acid pH range, but phenolic resins have a minimum reactivity under these conditions, there is the real danger that the phenolic portion of the resin might not really be incorporated into the aminoplastic... 

Correlations between the composition of aminoplastic resin and the properties of wood-based panels... 

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