45 A m i n o Resins 1
L. L. WILLIAMS, I. H. UPDEGRAFF2, and J. C. PETROPOULOS1,3 Stamford Research Laboratories, Polymer Products Division, American Cyanamid Company, 1
Stamford, CT 06904-0060 407 Den Road, Stamford, CT 06903
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Chemistry Manufacturing Applications Trends in Amino Resins Future Trends Summary The term "amino resins" is normally applied to condensation products of formaldehyde and polyfunctional amides and amidines such as urea and melamine. The bulk of amino resins in use today are based on these two compounds. These highly functional, reactive materials find a broad range of applications. In the plastics industry, these include molding compounds, coating resins, and adhesives; other traditional markets are as treating agents for textiles and paper. The objective of this paper is to provide a broad outline of the uses and chemistry of amino resins with major emphasis on the coatings area and a lesser amount on molding compounds. There are a number of leading references on the chemistry and applications of amino resins (1-4). Although the chemistry of the reaction of formaldehyde with urea and other amino compounds was investigated much earlier, the first useful product did not come on the market until the 1920s. The first commercial application for amino resins was in molding compounds and utilized a resin made with an equimolar combination of urea and thiourea, the invention of Edmond C. Rossiter (5). The Beetle brand name was applied to the new molding compound and has remained prominent in amino resins ever since. It is of special interest that the very first commercial product based on amino resins should be a complex formulation such as a molding compound. Deceased 3
0097-6156/85/0285-1101 $06.00/0 © 1985 American Chemical Society
In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
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APPLIED POLYMER SCIENCE
Adhesive resins based on aminos are much simpler and now are far larger volume products than molding compounds. The reason that the more complicated product came first is that the amino molding compound was unique for its time. In contrast to the phenolics, the amino resin could be supplied in pastel and transluscent colors. It had no objectionable phenolic odor yet had a hard, lustrous surface that was not easily stained. Melamine resins did not appear until a few years before World War II, first in Europe and later in America (6, 7). At that time melamine was prepared from dicyandiamide (dicy), but in recent years a process utilizing urea has been shown to be more economical. The various urea-based processes (high and low pressure) have completely supplanted the process using dicy. Once introduced, melamines moved rapidly into applications already established by urea, and corresponding formulations for molding, laminating, coating, gluing, and textile finishing were soon available from a number of manufacturers. The remarkable stability of the symmetrical triazine ring made these products resistant to chemical change once the resin had been cured to the cross-linked, insoluble state. Melamine has naturally remained more expensive than its precursor urea and so finds use in those applications demanding superior performance. Molded plastic dinnerware takes advantage of the exceptional hardness and water-resistance offered by the triazine resin. Melamine cross-linkers for coatings provide hardness, waterresistance, and freedom from yellowing. The only other triazine resins of commercial importance are based on benzoguanamine, 2-phenyl-4,6-diamino-l,3,5-s-triazine. Its use is small relative to that of urea and melamine, representing only a few million pounds per year. Benzoguanamines are used in surface coatings because they provide better adhesion than melamine; however, their poor resistance to ultraviolet light compared to that of melamine has always limited their use. In recent years the sharply increased price of benzoguanamine relative to that of melamine has further reduced its importance as an item of commerce. Prior to the rapid rise of thermoplastics following World War II, aminoplastics served a broad, diversified market. They continue to show a moderate growth today, but this is a result of good growth in some areas and contraction in others. Growth of overall amino resin volume was at a rate of 7% from 1969 to 1979 (8), reaching a total volume of 1.6 billion pounds in the latter year. However, the major component of this growth has been the urea-formaldehyde resins used as adhesives in various wood products, such as chip and f lakeboard. More than 60% of the total volume of aminos goes to this end use. In the higher technology areas, molding compounds and protective coatings, growth has been much less. Amino molding compounds particularly show l i t t l e or no growth, while use of aminos in protective coatings is s t i l l slowly expanding. The major reason for the growth in aminos in coatings has been that baking finishes are incorporating higher percentages of cross-linkers, rather than any significant growth in coatings volumes. While coatings employ the largest number of different aminos, the volume in the United States s t i l l does not exceed 100 million pounds on a real basis. In spite of their modest volume, amino coating resins are the segment of this class of materials showing the greatest technological change (see Table I).
In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
Downloaded by SUNY STONY BROOK on October 25, 2014 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch045
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W I L L I A M S ET AL.
Table I.
Amino Resins
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Pattern of Consumption for Urea and Melamine Resins Thousands of Metric Tons Market
Adhesives for fibrous and granulated wood Adhesives for laminating Adhesives for plywood Molding compounds Paper treatment and coating resins Protective coatings Textile treatment and coating resins Export Other
1982
1983
345 12 27 28 30 29 27 8 6 512
415 17 20 34 33 35 31 10 7 602
Note: As this table shows, the largest outlet for amino resins by far i s their use as adhesives or binders for reconstituted wood products made from sawdust and wood chips. Urea-formaldehyde resin is most commonly used. Melamine-formaldehyde resin can provide improved water resistance and may be combined with the urea resin to provide an improved product. Molding compounds are about the next most important outlet for amino resins. It is approximately evenly divided between urea and melamine. The primary use for urea moldings is in the electrical f i e l d , while the most important area for molded melamine plastic is dinnerware.
In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
Downloaded by SUNY STONY BROOK on October 25, 2014 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch045
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A P P L I E D P O L Y M E R SCIENCE
A factor that may affect the growth of amino resins i s the question of formaldehyde t o x i c i t y . Like many other chemicals, aminoformaldehyde resins have encountered questions of product safety in recent years. This has been largely due to reports that formaldehyde causes nasal tumors in rats, although epidemiological studies of workers exposed to formaldehyde have shown no confirmation of this. The controversy surrounding formaldehyde is complicated by the fact that i t s odor i s obnoxious to humans i n very s m a l l concentrations. Thus, most humans w i l l not tolerate formaldehyde at a concentration of 1 ppm, far below the 15 ppm that was shown to cause tumors in rats. Thus far, the evidence has not led to any reduction in the already f a i r l y low 3-ppm OSHA limit on formaldehyde exposure. As noted, a lower exposure l e v e l i s self-enforcing because of the highly unpleasant odor of formaldehyde. The one area of amino resin usage that has been severely affected by formaldehyde toxicity i s the use of foamed ureaformaldehyde as insulation. The material has many good qualities for this application: i t can be injected readily into the walls of existing housing and cured in place, providing a good "R" factor. However, in practice there were too many cases where enough formaldehyde was given off by the cured foam to make houses u n l i v a b l e . Had more technical sophistication been used in the industry this might well have been avoided, but at this point use of urea-formaldehyde foam has been outlawed in some states and i s widely rejected by consumers. No early r e v i v a l of this use of aminos appears possible. The impact of formaldehyde on the use of amino resins in other areas has been much less. There is pressure to reduce the l e v e l of free formaldehyde in the urea-formaldehyde-bonded construction materials. However, i t appears l i k e l y that this w i l l be resolved technically with either better resins or a higher degree of cure. This use is sensitive to the free formaldehyde issue because i t can affect the end user, the people l i v i n g in the homes built with the material. Other uses of aminof ormaldehyde resins are less l i k e l y to be affected. Molded articles, laminates, and surface coatings are generally highly cured materials where any release of formaldehyde occurs in the factory where the resins are cured. In such environments control of formaldehyde l e v e l s i s a straightforward problem of providing adequate v e n t i l a t i o n in the work areas. Although formaldehyde w i l l probably have a further dampening effect on growth of amino resins, i t does not seem likely to greatly reduce the use of this type of chemical. Given that aminoformaldehyde resins w i l l probably continue as a major item of commerce, a brief review of their chemistry i s appropriate. Chemistry Amino resin chemistry goes back to the 1880s. The first mention of a urea-formaldehyde reaction product was by Toi lens (9) in 1884 in a report of work done by an associate named Holzer. Very few references r e l a t i n g to amino resins can be found in the chemical l i t e r a t u r e prior to 1900. Ludy (10) in 1889 and Carl Goldschmidt (11) in 1896 mentioned products that were similar to the material
In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
Downloaded by SUNY STONY BROOK on October 25, 2014 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch045
45.
Amino Resins
W I L L I A M S ET AL.
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prepared by Holzer. The next significant contribution was the isolation of mono- and dimethylolurea by Einhorn and Hamburger (12) in 1908. Ten years l a t e r , Hans John (13), an Austrian c i t i z e n , was granted a patent for an adhesive made from urea and formaldehyde. During the early 1920s Hans Goldschmidt and Oskar Neuss (14) were experimenting with urea resins in Germany, while Kurt Ripper and F r i t z Pollak (15, 16) were making amino resins i n Austria. As mentioned above, Rossiter (5} introduced the f i r s t commercial product, his Beetle molding compound, in England about this time (1926). Two main reactions are involved in the formation and cure of amino resins. The f i r s t i s a simple addition reaction of formaldehyde to the amino compound, as i l l u s t r a t e d in Equation 1, that shows the formation of a typical methylol intermediate. R-NH2*HCH0
