Vinyl acetate monomer (VAM) is a significant intermediate used in the production of a wide range of resins and polymers for paints & coatings, adhesives, glues & sealants, elastomers, textile finishes, paper coatings, binders, films, and a myriad of other industrial and consumer applications. It efficiently homo-polymerizes to polyvinyl acetate (PVA), and VAM can be used in numerous random copolymers and terpolymers such as ethylene-vinyl acetate copolymers, vinyl-acrylic resins, vinyl acetate-acrylic acid copolymers, and vinyl acetate-vinyl chloride copolymers. With the wide diversity of polymerization options, VAM has allowed the design of products with a wide spectrum of cost and performance profiles.
The largest end-use for VAM is in the production of polyvinyl acetate resins as a base for adhesives and coatings, as well as a feedstock for derivative resins like polyvinyl alcohol (PVOH). Polyvinyl acetate emulsions and resins are low in cost and convenient to use, and they have a wide application range. PVA is likely best known as the base component of household white glues used for bonding paper, fabrics, wood, and plastic.
Worldwide consumption of VAM measures over 4 million metric tons, with an annual growth rate of about 4.7 percent. PVA represents over half of the total usage of VAM. In addition to uses in paints, coatings, adhesives, and binders, PVAs are feedstocks for other large-volume systems such as polyvinyl alcohol (PVOH), polyvinyl butyral (PVB,) and polyvinyl formal (PVF). PVOH is the largest use for PVAs, followed by adhesives and paints & coatings.
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. However, flexibility, toughness and adhesive strength increase. At a level of 50 percent, EVAs are amorphous.
VAEs with more than 60 percent VAM content are used in coatings, adhesives, cements, and plasters. VAEs produce low-VOC emulsion systems, because the ethylene monomer serves as a plasticizing monomer and reduces the need for coalescing agents or plasticizers. Commercial VAE emulsions exhibit Tgs between -15 °C and +15 °C. Formulators may freeze-dry VAE emulsions to produce redispersible powders (RDPs), often referred to as “solid latexes.”
EVAs with less than 40 percent VAM content are thermoplastics used for elastomeric films, extrusion coating, and adhesives. EVAs are further employed in the production of ethylene vinyl alcohol (EVOH) copolymers with excellent gas barrier properties useful in multi-layer food packaging and agricultural films, beverage & cosmetic bottles, and barrier layers of plastic gasoline tanks.
A wide variety of vinyl acrylic copolymer options are available. Vinyl acrylic emulsions are economical products that find extensive usage in interior architectural paints & caulks, adhesives & sealants, paper & textile binders, engineered fabrics, and pigment dispersions. Acrylic monomers like ethyl, butyl and 2-ethylhexyl acrylates enhance the performance of copolymers by improving flexibility, water resistance, adhesion, and scrub & stain resistance. Ter-monomers are also used like ethylene and acrylic acid in these systems.
VAM Polymerization Considerations
VAM polymerization is easy in all processes, emulsion, bulk, solution, and suspension. Commercial VAM sources have purities greater than 99.9 percent, 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 viscosity of the final polymers. Formulators may polymerize hydroquinone-stabilized material without removing the HQ.
VAM polymerizations are exothermic, requiring provisions for cooling. In emulsion reactions, polymerization times are about 4 hours at temperatures of 70 °C. Formulators must maintain the pH at near neutral level during polymerization to minimize VAM hydrolysis. To ameliorate the hydrolysis reaction, they can use a buffer such as sodium acetate.
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 randomly copolymerize versus a preference for homopolymerization or block polymerization, is based on the measurement of so-called reactivity ratios. The reactivity ratio for a pair of monomers is the reaction rate constant for propagation of the first monomer in a growing polymer chain to itself versus addition to the second monomer. When the reactivity ratios of comonomer pairs are similar, random copolymerizations are observed. Ethylene with a reactivity ratio of r1=0.79 and VAM (r2=1.4) copolymerizes well, affording alternating copolymer compositions closely reflecting the compositions of the monomer feeds. When the reactivity ratios of two monomers differ significantly, compositions will tend towards that of a homopolymer of the more reactive co-monomer. As an example, random copolymerization of styrene (r1~50) with VAM (r2=0) cannot be achieved by conventional free-radical polymerizations.
For a thorough explanation of how monomer reactivity ratios can be used to predict copolymer compositions, and a tables of reactivity ratios for comonomer pairs, please see this lecture from Stanford University.
Another unique feature of VAM free radical polymerizations is their propensity to undergo chain transfer reactions that afford branched architectures. This dynamic is due to chain transfer reactions between the growing polymer chain free radical and abstraction of an acetate hydrogen position (predominant mode) or a tertiary hydrogen atom within the polymer chain. Branching process increases in significance as the conversion proceeds, with decreasing monomer concentrations. Branching results in PVAs with lower tensile properties, increased ductility, and lower solution and melt viscosities. The branches are both short and long chain (above the critical entanglement length), with the short chain branches responsible for the lower strength properties.
Safe Handling of Vinyl Acetate Monomer
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.
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:
- Keep VAM free of contamination.
- Use and maintain proper inhibitor levels, as inhibitors are consumed over time.
- HQ-inhibited VAM can be preferably stored under dry nitrogen.
- Avoid absorption of moisture, which can cause hydrolysis of VAM.
- Prevent contact with amines, strong acids, alkalis, silica, alumina, oxidizing agents and initiators, which can cause spontaneous polymerization.
- Avoid exposure to air, which can result in the formation of peroxides.
- Store VAM within recommended temperature limits, not exceeding 30 °C (86 °F).
- Use proper materials of construction and thoroughly clean tanks, reactors and piping before filling with VAM.
For further details on handling and health information, refer to the SDS for VAM.
VAM Sales Specification
Gantrade is one of the main global suppliers of high-purity vinyl acetate monomer. We maintain robust inventories in North America and Europe to meet global demand. Our package sizes range from 20 MT (44,080 lbs.) tank trucks to rail cars and steel drums.
Gantrade’s sales specifications for VAM are displayed 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|
Vinyl acetate monomer is a key intermediate used in the manufacture of homopolymers, copolymers, and derivatives for adhesives & sealants, paint & coatings, packaging films, construction materials, textile & paper finishes, and a myriad of other applications. VAM is a highly versatile monomer, creating copolymers such as ethylene-co-vinyl acetate, acrylic ester-co-vinyl acetate, vinyl chloride-co-vinyl acetate, etc. The broad copolymer options allow the design of numerous polymer systems to meet the economics and performance requirements of a multitude of industrial and consumer applications. The largest use for VAM is in polyvinyl acetate homopolymer, which is used in adhesives and paints, offering ease of use and good adhesion properties to paper, fabrics, wood, plastics and many other substrates at a low cost.
For more information on how VAM can work for you, contact Gantrade today.