What Is Ethylene-Vinyl Acetate Copolymer?
Ethylene-vinyl acetate copolymer (EVA) is a thermoplastic made by copolymerising ethylene with vinyl acetate (VA, CAS 108-05-4) in a high-pressure process. EVA is a polymer family rather than a single substance: it is identified by two numbers, the VA content and the melt flow index (MFI), and those two values decide everything about how a grade performs. Ethylene units give stiffness, strength and low cost; VA units are bulky and polar, so they disrupt crystallinity and bring flexibility, clarity, polarity, adhesion and compatibility with other polymers and fillers.
| Property | Typical range |
|---|---|
| Comonomer | Vinyl acetate, CAS 108-05-4 |
| VA content | 5-40 percent by weight |
| Density | About 0.92-0.95 g/cm3 at 23 degrees C |
| Melt flow index | 0.3-400 g/10 min depending on grade |
| Appearance | Colourless to slightly yellowish pellets |
| Softening behaviour | Decreases as VA content rises; no sharp crystalline melting point at high VA |
| Solubility | Insoluble in water; soluble in warm aromatic and chlorinated solvents |
Because polarity increases with VA content, high-VA grades bond strongly to glass, metal foils and polar substrates, while low-VA grades behave much like a soft polyethylene with better toughness and stress-crack resistance.
How VA Content Controls Properties
Choosing an EVA grade is essentially a question of matching VA content to the end use. The table below summarises the customary mapping from VA content to application.
| VA content | Character | Typical applications |
|---|---|---|
| 5-9 percent | Stiff, tough, PE-like, good stress-crack resistance | Stretch and shrink film, heavy-duty bags, cable compounds, moulded caps |
| 9-18 percent | Flexible, clear, good low-temperature sealing | Packaging film, agricultural film, extrusion coating, lamination tie layers |
| 18-28 percent | Soft, rubbery, high polarity, excellent adhesion | Hot melt adhesives, coating binders, wax blends, polymer modification |
| 28-40 percent | Very soft, elastomeric, highly polar, transparent | Photovoltaic encapsulant film, foam, blends, speciality coatings |
MFI works in the opposite direction to molecular weight: a high MFI gives easy flow for extrusion coating and hot melt adhesives, while a low MFI gives melt strength and toughness for blown film and foam. Two grades with the same VA content but different MFI are not interchangeable on a given line.
Packaging Films, Lamination and Adhesives
EVA films are chosen where a combination of toughness, translucency and low-temperature sealing is required. Shrink wrap for pallets and consumer goods, liners, ice bags and agricultural film all exploit the polymer's impact resistance and resistance to environmental stress cracking. In flexible packaging, EVA layers improve puncture resistance of freezer film and provide good hot-tack at seal temperatures lower than those needed for a straight polyethylene seal.
Extrusion coating: a thin EVA layer anchors a barrier or print layer to board, foil or paper
Lamination: EVA-based adhesive layers bond paper, foil and plastic films into laminates used for food and industrial packaging
Hot melt adhesives: EVA with tackifying resin and wax gives fast-setting bonds for cartons, bookbinding, labels and non-woven hygiene products, with viscosity measured by ASTM D3236
Binder and coating: EVA dispersions and solutions act as binders for paper coatings, primers and sealants
Because VA groups are polar, EVA adheres to substrates that polyethylene will not wet, which is the main technical reason it appears in laminate and coating structures rather than only in plain film.
Footwear, Foam, Automotive and Cable Applications
Crosslinked and chemically blown EVA foam is one of the polymer's best-known uses. Foamed sheet at densities of roughly 0.15-0.35 g/cm3 provides cushioning at very low weight, so it appears in shoe midsoles and insoles, flip-flops, sandals, gym and yoga mats, puzzle mats and protective padding for sport and leisure goods.
In transport and electrical applications, low-VA EVA grades are moulded into interior trim, door panels, headliners, sound-deadening layers and weatherstripping, where flexibility, UV resistance and sealing behaviour matter. Cable compounds use EVA for wire and cable insulation and jacketing because the polymer accepts high filler loadings - flame retardants, carbon black and inorganic fillers - while keeping flexibility and electrical resistance. Low-density medical tubing and drug delivery components use medical-grade EVA for its flexibility and low extractables.
Photovoltaic Encapsulation and Quality Control
Solar modules are the most demanding EVA application. An encapsulant film of high VA content, typically 28-33 percent, is laminated between the front glass, the solar cells and the backsheet. During lamination the film melts, wets the cells and cures with a peroxide crosslinking system, forming a transparent, electrically insulating and moisture-barrier layer that mechanically holds the module together for decades. Typical film thickness is 0.4-0.6 mm, and crosslinking is controlled through gel content measurement to confirm the cure level.
Encapsulant films are characterised by the test methods in IEC 62788-1-2, while the finished module is type-approved against the design qualification and safety standards IEC 61215 and IEC 61730; film products for this market are commonly specified against GB/T 29848 for photovoltaic encapsulation film. Incoming inspection covers VA content by Fourier transform infrared spectroscopy per ASTM D5594, melt flow index per ASTM D1238, thickness, transmittance, peel strength after lamination and gel content after curing. EVA pellets should be stored in sealed bags in a cool, dry place away from direct sunlight, since the material is moisture-tolerant but should still be dried to the resin supplier's recommendation before extrusion coating or film blowing.
Frequently Asked Questions
Q: What is ethylene-vinyl acetate copolymer used for?
A: Main uses are packaging and agricultural film, extrusion coating and lamination, hot melt adhesives, foamed footwear and sports products, wire and cable compounds, and photovoltaic encapsulant film.
Q: What does VA content mean in EVA?
A: VA content is the weight percentage of vinyl acetate comonomer in the polymer; higher VA content gives a softer, clearer, more polar material with better adhesion, while lower VA content is stiffer and more polyethylene-like.
Q: Why does EVA dominate solar module encapsulation?
A: High-VA EVA offers optical clarity, electrical insulation, low moisture transmission and strong adhesion to glass and cells, and it can be crosslinked during lamination to lock the module assembly together.
Q: How is EVA characterised in quality control?
A: VA content is measured by infrared spectroscopy per ASTM D5594, flow behaviour by melt flow index per ASTM D1238, and cure level of encapsulant film by gel content after lamination.
Q: Can EVA be used as a hot melt adhesive base polymer?
A: Yes. EVA with a tackifying resin and a wax produces a fast-setting adhesive for cartons, bookbinding and non-woven products, with viscosity routinely tested by ASTM D3236.
Q: How should EVA resin be stored?
A: Keep pellets in sealed bags in a cool, dry warehouse away from direct sunlight, use the recommended drying profile before processing, and observe first-in-first-out stock rotation to avoid moisture uptake and ageing.