Vinyl acetate monomer (VAM) is a significant intermediate used in the production of a wide range of resins and polymers for paints & coatings, adhesives & sealants, elastomers and a myriad of other industrial and consumer applications. VAM efficiently homo-polymerizes to polyvinyl acetate (PVA) and is used in numerous random co- and terpolymers. A wide spectrum of cost and performance profiles have been produced using VAM.
The much lower cost of VAM versus other monomers from the (meth)acrylate series or styrene is a significant driver of preferences for this monomer. Outdoor weathering of VAM-based polymers is not as good as found with (meth)acrylic monomers, thus most VAM based paint & coating uses are limited in interior applications. The glass transition temperature of VAM is + 30 C, so it is often copolymerized with a “soft monomer” like ethylene and butyl acrylate to reduce the system Tg and improve low temperature performance. Significant branching occurs in the free radical polymerization of VAM through chain transfer reactions. This results in lower viscosities associated with polymers and good adhesive characteristics.
The facile ability to hydrolyze the acetate group to a hydroxy moiety on a polyvinyl acetate chain accounts for a very large proportion of VAM’s usage, in polyvinyl alcohol (PVOH) and ethylene vinyl alcohol copolymer (EVOH) production.
A fast-growing use of VAM is the manufacture of vinyl acetate-ethylene (VAE) and ethylene-vinyl acetate (EVA) copolymers. As the VAM content increases in an ethylene-vinyl acetate copolymer, crystallinity decreases and tensile properties decrease, but flexibility, toughness and adhesive strength increase. At a level of 50 percent, EVAs are amorphous.
EVAs with less than 40 percent VAM content are thermoplastics used for films and molded parts, extrusion coating and hot melt adhesives.
VAEs with more than 60 percent VAM content are used in coatings, adhesives, cements, grouts and plasters. Commercial VAE emulsions exhibit Tg’s between -15 °C and +15 °C. VAE emulsions can be freeze dried to afford redispersible powders (RDPs). These RDPs contain emulsifiers in the final product and are not thermoplastics in nature.
All acrylic latex products produce high performance paints, now on a par with the quality of solvent based paint formulations. VAM-based paints also show good qualities in architectural paints, but are deficient in outdoor weathering, water resistance and flexibility. VAM is a lower cost monomer vs. acrylates, so paint formulators balance the performance attributes of both to keep costs down. Copolymerizing acrylic monomers with vinyl acetate enhances the performance of the PVA by improving flexibility without using plasticizers, increasing water resistance of the final product and improving durability. Vinyl acrylic paints, caulks and sealants are mostly used in interior architectural applications.
VAM polymerization is facile in all processes, emulsion, bulk, solution and suspension. Commercial VAM sources have purities > 99.9% with trace levels of water, acetic acid and acetaldehyde. Water and acetic acid do not have much of an effect on polymerizations, but acetaldehyde acts as a chain transfer agent and has a significant effect on the molecular weight and properties of the final polymers. Hydroquinone-stabilized VAM can be polymerized without removal of the HQ.
VAM polymerizations are exothermic, requiring provisions for cooling. In emulsion reactions, polymerization times are about 4 hours at temperatures of 70 °C. The pH must be maintained near neutral during polymerization to minimize VAM hydrolysis, which produces acetaldehyde.
Free radical VAM copolymerizations occur easily with some monomers and with difficulty with other comonomers. The method for measuring the tendency of monomer pairs to copolymerize is based on the measurement of “reactivity ratios”. Ethylene with a reactivity ratio of r1=0.79 and VAM (r2=1.4) copolymerize well, affording near alternating copolymer compositions. Random copolymerization with styrene (r1~50) and VAM (r2=0) cannot be achieved by conventional free-radical polymerizations.
Polymerization of VAM is highly exothermic and uncontrolled polymerizations can result in explosions. The following principles need to be applied in handling and polymerization of VAM.
The Vinyl Acetate Council published a Vinyl Acetate Safe Handling Guide to provide procedures and best practices for the safe handling, storage and transport of vinyl acetate. http://www.vinylacetate.org/library.php
Gantrade’s sales specifications for polymerization-grade high purity VAM are outlined in the chart below.
|Vinyl Acetate, wt. %||99.9 min.||GC|
|Acetaldehyde, ppm||150 max.||GC|
|Methyl Acetate, ppm||200 max.||GC|
|Ethyl Acetate, ppm||300 max.||GC|
|Acetic Acid, ppm||50 max.||ASTM D2086|
|Water, ppm||400 max.||ASTM D1364|
|Specific Gravity 20°/20°C||0.9334-0.9345||ASTM D4052|
|Platinum-Cobalt Color||5 max.||ASTM D1209|
|Hydroquinone, ppm||3-9 Typically||ASTM D2193|
|Appearance||Free & Clear, no Suspended Matter||Visual|
Package sizes are 20 MT (44,080 lbs.) tank trucks, rail cars and steel drums.
For more information on how high purity vinyl acetate monomer can work for you, contact Gantrade today. Our team can offer the right technical advice for virtually any market application.