edited hv
George
products of chemistry
B. Kauitrnan
California Slate Univesily, Fresno
Fresno, CA93740
Chemical Magic Polymers from a Nonexistent Monomer Raymond B. ~ e ~ m o u r '
University of Southern Mississippi, Hattiesburg, MS 39406-10076 George B. Kauffman
California State University, Fresno, California 93740 As is well known, attempts to produce vinyl alcohol (HzC = CHOH) by the hydrolysis of acetylene have been unsuc-
cessful. Vinyl alcohol is an enol, which is in equilibrium with acetaldehyde (H3CCHO),thepreferred keto structure of the two tautomers: HBCCHO+ H2C=CHOH Keto farm
Nevertheless, poly(viny1 alcohol) (PVA) is a commercial, water-soluble product that is used a s a viscosity control additive and a s a reactant for the nroduction of uolv(vinv1 .. " butyral, (I'VR,, which is used as an inner component of the elass-olastics sandwich in automobile safetv elass. The s p t h i s i s of WA is a tribute to the ingenui6 i f polymer chemists who recognized that commercial poly(viny1 acetate) (PVAC, (CHzCHOOCCH&) was an ester of the en01 tautomer of acetaldehyde. Hence, it was logical to hydrolyze this polyester in order to obtain PVA ((CH2CHOH)J. Klatte vatented the synthesis of PVAC in 1912 ( I ) . Hermann ;nd Haehnel o f i Farbenindustrie, ~ which was a recipwnt ofa patent on PVAC, also descrihcd thc production o f ~ ~1924 ~ i(2). n PVAC is a low-melting, amorphous solid produced by the emulsion polymerization of vinyl acetate monomer. Its ~ r i n c i ~uses a l are a s a waterborne ~ a i n tadhesive. . textile Lizini paper coating, and source of h\. ~ommerc&lPVA, which is sold under the trade or generic names of Gelvatol, Elvanol, Resistoflex, or Vinyon,-is available in molecula~ weights ranging from 2,000 to 125,000 and with a controlled extent of hydrolysis ranging from 73 to 100%.The tensile strength of PVA is greater than that of PVAC and increases from 1,000 psi to 15,000 psi a s the extent of hydrolysis increases. Vinyl acetate is used as a flexibilizing and solubilizing copolymer with vinyl chloride. The copolymer of vinyl chloride (87%)and vinyl acetate (Vinylite, VYHH), which was patented by Reid in 1928 (31, is widely used as a solventbased coating. Copolymers of vinyl acetate and methyl acrylate (-C=CHCOOC-1 a n d of ethyl acrylate (HC2=CHCOOC2-)and ethylene (C-=C-) are commercially available. Copolymers of ethylene and vinyl acetate (EVA) are produced a t an annual rate in excess of 1.1 billion pounds in the United States under the trade names of Elvax, Escrene, and Ultrathene. More than 40% of the EVA pro-
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duced is used as meat wrapping, bag and box containers, stretch film, ice bags, and extrusion coatings. Films with excellent barrier properties are produced by the hvdrolvsis of these crvstalline , . of EVA. The uro~erties , EVAcopolymers vary with composiriun. .More than 40 million oounds of thrse FDA-aooro\,cd films are used in the ~ n i t k dStates. Because of the strong intermolecular hydrogen bonds between hydrogen and oxygen atoms on adjacent chains, PVA is a strong polymer that can be used a s a water-soluble coating as well as a strong fiber. The utility of WAfiber is demonstrated by its use in the Kurlon fiber-reinforced polyester ties that are used in place of wood or reinforced concrete to hold the rails together on the Japanese bullet railway system. These fibers form a bond with the resin that is superior to the bond between glass fibers and resins. Polyvinyl alcohol is a nonionic surfactant and acts as a protective colloid in aqueous systems. The specific gravity of PVA is 1.20-1.27, and its heat-sealing temperature is 110-150 "C, depending on the extent of hydrolysis of the precursor PVAC. When solubility in water is undesirable, the surface of the WAfilms or fibers are formalized by immersion in 37% aqueous formaldehyde (formalin). Polyvinyl formal (PVF) (Formvar, Mowital) is produced when all the available hydroxyl groups in PVA have been reacted with formaldehyde. Polyvinyl acetals were patented by Hopf in 1934 (4). The properties of PVF are dependent on the extent of hydrolysis of the PVAC precursor and the availability of dual hydroxyl groups on alternating carbon atoms in the polymer chain. The typical commercial PVF (Formex) has a degree of polymerization (DP) (the number of repeating units) of about 500, with 5% unreacted VAunits and 10% unreaded VAC units. These amorphous polymers are soluble in many organic solvents such as ethylene chloride, methyl isobutyl ketone, and mixtures oftoluene (60%)and ethanol (40%) as well as in glacial acetic acid. A typical PVF has a specific gravity of 1.2, a softening point of 190 "C, and a tensile strength of 20,000 psi. PVF was produced by Wacker Chemie, I.G. Farbenindustrie (now BASF), Du Pont, Union Carbide, and Shawinigan Chemicals (now Monsanto) during the mid 1930's. Blends of PVF and phenolic resins are used a s wire coatings and structural adhesives (Redux). Polyvinyl acetal can be produced by the condensation of acetaldehyde and PVA but is not available commercially Since this acetal is a condensation product of acetaldehyde
and the polymer of the nonexistent en01 of acetaldehyde, polymer science is utilized for this "sleight of hand" trick. The condensation product of PVA and butyraldehyde (PVB, Butvar, Butacite, Saflex),which was used in 1936by H. F. Robinson (5)to pmduce a glass-resin-glasslaminate, is used worldwide as safety glass for automobile windshields. Because about 40% of the unreacted groups are present, PVB has extra adhesion to glass. PVB is also used as a film between acrylic resin sheets in aircraft. The structures for PVB and PVF are:
Aqueous dispersions of PVB are used as textile sizings, and solutions of PVB are used as adhesives, coatings, and printing inks. The versatility of PVB is demonstrated by its use in "wash primers," which serve as metal conditioners for metal-treated steel surfaces. The two-package wash primer formulation consists of a solution of PVB and zinc tetraoxychromate and a mixture of isopmpyl alcohol and toluene ~ l u as solution of ~ h o s ~ h o racid i c in aoueous isopropy~aicohol. In 1845 the Enelish biblical scholar John William Burgon (1813-1888),Tn his poem, Tetra," stated: A
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By labour wrought as wavering fancy plans, but from the raek as if by magic grown.
This phrase may aptly be applied to PVA, which has grown from a nonexistent monomer as if by chemical magic to a water-soluble polymer with myriad applications. Liierature Cited Poly(viny1 formal)(WF)
General References Seymour, R. 8.;Camsher. C. E. Giant Moleo&s; mLey: NewYmk, 1990. Seymous R. B; Camsher C. E. Polymer Chemistry: An Inimduelion. 3rd ed.; Dekker: New York, 1992. Seymour, R.B: Mark. H. FAppiievlians afP0iymm:Plenum: New York, 1988.
Volume 71 Number 7 July 1994
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