6. ACRYLATE RESINS
A new development of widespread importance in the synthetic resin industry is the commercial production of the polymers of certain derivatives of acrylic acid. The commercial exploitation of the acrylates is another example of the belated realization of the value of substances known for many years. Acrylic acid has been known for about a hundred years, and the polymer of methyl acrylate was first described in 1880. It was not until 1927, however, that a suitable method for their commercial production was developed. The study of the many derivatives of acrylic and methacrylic acids leads to the conclusion that those of greatest practical application in the resin field are the lower esters, such as methyl and ethyl, polymerized separately or together.
Colorless transparency, stability against aging, thermoplasticity, and chemical resistance to many reagents are the general characteristics of the acrylate resins. In consistency they range from soft, sticky, semiliquids to hard, tough, thermoplastic solids. Since these widely varying properties are obtained by control of manufacturing conditions, rather than by the use of plasticizers, the resins retain their initial properties indefinitely. Aging and weathering have no effect as they are stable under exposure to heat, light, and oxidizing agents. The methacrylates are harder and tougher but less elastic than the acrylates.
The acrylate resins are marketed in a number of forms, such as solutions in organic solvents, dispersions in water, solid cast sheets, rods and tubes, and molding powders. All of these are distinguishable from many other resins by their colorless transparency, adhesivequalities, great elasticity, and chemical resistance. The brilliant water-white color makes it possible to secure masses having a high degree of light transmission and great optical clarity.
The earliest commercial use of the acrylate resins was in laminated safety glass marketed as Plexigum in the United States and as Luglas and Sigla in Europe. The extensibility and elasticity of the resin film gives the laminated glass a flexible or yielding type of break when subjected to a hard impact. Having excellent adhesion to glass there is no need of an auxiliary cement to bond the resin to the glass, nor is it necessary to seal the edges since the resin has good resistance to moisture. The acrylate resin used for this purpose is in the form of a viscous solution in an organic solvent. A film is applied to each sheet of glass, the solvent removed by drying, and the sheets are pressed together.
The harder acrylic resins are used in the form of solid thermoplastics. Methyl methacrylate is of special interest. As the monomer is a mobile liquid it can be cast-polymerized to a solid of any desired shape in predesigned molds or produced in finely divided form for use as molding powder. The cast resin is marketed in this country as Crystalite, Plexiglas and Lucite, and in England as Diakon.
The solid acrylate resins are clearer than cast phenolic resins, not as brittle as the polystyrene resins, and not as tough as cellulose acetate or nitrocellulose plastics. Their transparency and resistance to aging and weather permit their use in applications not previously considered for synthetic resins. Sheets of this resin may be formed or molded into many useful shapes. The aircraft industry has found them suitable for windshields and cockpit enclosures to effect streamlining and thus greatly reduce wind resistance.
Methyl methacrylate is probably the nearest approach to organic glass thus far developed. Its optical properties make it suitable for spectacle lenses, camera lenses, magnifying glasses, and protective goggles. Spectacle lenses are now being made to prescription by molding. It is estimated that 900 molds will supply the requirements of about 98 percent of the prescriptions. The excellent light transmitting quality of methyl methacrylate permits its use in edge lighting, advertising displays, and instrument dials. It is also used in inspection windows in various types of machinery where curved sections are necessary and where glass might be broken.
A synthetic resin combining the properties mentioned, together with high tensile and impact strength, good dielectric properties, ultraviolet transmission, and resistance to water, oil, acids, and alkalies is an important contribution. The acrylates may be colored or have fillers added to give any desired translucency or opaqueness. They can be sawed, cut, blanked, turned, drilled, ground, polished, and sanded much the same as are nitrocellulose plastics.
Airplane Cockpit Enclosures of Cast Acrylate Resin.Source: Rohm & Haas Company, 222 W. Washington Square, Philadelphia, Pa.
Airplane Cockpit Enclosures of Cast Acrylate Resin.
Source: Rohm & Haas Company, 222 W. Washington Square, Philadelphia, Pa.
Spectacle Lenses Molded To Optical Prescription From Acrylate Resin.Source: Rohm & Haas Company, 222 W. Washington Square, Philadelphia, Pa.
Spectacle Lenses Molded To Optical Prescription From Acrylate Resin.
Source: Rohm & Haas Company, 222 W. Washington Square, Philadelphia, Pa.
A new and interesting application of the acrylate resins is as molded reflectors in a system of indirect highway lighting. The reflectors are pressed from colorless, transparent methyl methacrylate resin and are 1⅝ inches in diameter. They are assembled in a pressed metal housing to form a double facing marker which is snap-locked to the top of an angle iron post. The posts are so located that the reflectors are accurately aligned 3 feet above the pavement edge. An installation has been made on U. S. Highway No. 16 between Detroit and Lansing, Mich., at a cost of about $340 per mile.The motorist provides his own light from his headlights which strikes the reflectors and is returned as a narrow beam of brilliant illumination. The chief of the United States Bureau of Public Roads states that this is a definite contribution to the safety and utility of the highways at night. The reflector is a group of tiny cube corners, over 300 in each disk. Each cube corner is a complete retrodirective optical system; a light ray entering the front surface is reflected from surface to surface of the cube and after the third reflection is directed back toward the headlight regardless of the entrance angle. If the cubes are made with a high degree of dimensional accuracy, the reflected light has a high candlepower, strong enough to be seen for a mile.
Other uses for these resins are in sound recording records, dentures, telephone and radio transmitter diaphragms, novelties, and lighting fixtures.
The monomer (unpolymerized methyl methacrylate) may be used to impregnate wood, cloth, wallboard, cork, paper, electrical coils, tile, or stone, and then polymerized to form the resin. Paper and cloth so treated have many uses, such as in the electrical and food-packaging industries. Laminated sheets find wide possibilities for use in the aircraft field, and for lamp shades. Wood may be impregnated with as much as 60 percent of the monomer. Solutions of these resins in organic solvents, such as ethylene dichloride, ethyl acetate, and toluol, are used in surface coatings, undercoats on difficult adhesion jobs, to impregnate paper and textiles, and in insulation. These coating solutions are marketed in the United States under the trade name Acryloids and in Europe under the trade names, Borron, Plexigum, and Acronol. They may be brushed, sprayed, dipped, and baked. Baking is recommended to give a higher gloss, better adhesion, and a harder film. The dried film has an elasticity of 1,000 percent at ordinary room temperature and the light transmission of clear films is intermediate between ordinary window glass and quartz.
Acrysol is an adhesive consisting of a dispersion of the resin in water and is recommended for use where adhesion is difficult, as on rubber or rubberized surfaces.
Commercial production of acrylate resins in the United States was started in 1931 by Rohm and Haas, Philadelphia, Pa., under United States Patents Nos. 1,388,016 of August 16, 1921, and 1,829,208 of October 27, 1931.
Commercial production of methyl methacrylate resins was started in 1937 by E. I. du Pont de Nemours & Co. This development is under United States Patent No. 1,980,483, issued in 1934. The liquid monomer is produced at Belle, W. Va., and shipped to Arlington, N. J., where it is polymerized by heat to the solid resin.
The output of acrylate resins was hardly more than experimental in 1935 but increased somewhat in 1936 and very appreciably in 1937. Although statistics of production are not publishable, it can be stated that in 1937 the output approached that of other synthetic resins made in commercial quantities. The properties of these resins indicate very large commercial production in the near future. Pricesof the several types are still high as compared with other resins but should eventually be somewhat lower than those of cellulose acetate and nitrocellulose plastics and slightly higher than those of cast phenolic resins.
There have been no recorded imports of acrylate resins. The two domestic producers have agreements, licenses, or affiliations with the principal foreign makers of these products, one in England and one in Germany. Such arrangements would account for the absence of imports, except for sample or experimental lots, and might also limit export markets.