Basic knowledge of plastics: what is EVA?

What is EVA?

EVA is an ethylene-vinyl acetate (vinyl acetate) copolymer, which is made by copolymerizing ethylene (E) and vinyl acetate (VA). The English name is: Ethylene Vinyl Acetate, abbreviated as EVA, or E/VAC. Generally, the content of vinyl acetate (VA) is between 5% and 40%. Compared with polyethylene, EVA introduces vinyl acetate monomer into the molecular chain, thereby reducing crystallinity, improving flexibility, impact resistance, filler compatibility and heat sealing performance. Generally speaking, the performance of EVA resin mainly depends on the content of vinyl acetate in the molecular chain. Due to the adjustable ratio of constituent components to meet the needs of different applications, the higher the content of vinyl acetate (VA content), the higher its transparency, softness and toughness will be relatively improved.

The polymerization method uses high-pressure bulk polymerization (for plastics), solution polymerization (PVC processing aid), emulsion polymerization (adhesive), and suspension polymerization. Emulsion polymerization is used when the content of vinyl acetate (VA) is higher than 30%, and high-pressure bulk polymerization is used when the content of vinyl acetate is low.

1. Features

EVA resin is characterized by good softness, rubber-like elasticity, good flexibility at -50°C, good transparency and surface gloss, good chemical stability, anti-aging and ozone resistance strength Good, non-toxic. Good blending with fillers, good coloring and molding processability.

It has a great relationship with vinyl acetate content, molecular weight and melt index. When the melt index (MI) is constant and the content of vinyl acetate (VAC) increases, its elasticity, flexibility, compatibility, and transparency will also increase. When the VAC content is reduced, the performance is close to that of polyethylene, the rigidity is increased, and the wear resistance and electrical insulation are improved. If the VAC content is constant and the melt index increases, the softening point will decrease, and the processability and surface gloss will improve but the strength will decrease. Otherwise, with the decrease of MI, the molecular weight will increase, and the impact performance and environmental stress crack resistance will increase. The polarity of acetate increases elasticity and viscosity, decreases crystallinity and electrical properties, and dissolves in hydrocarbon solvents and oils.

The characteristics of EVA and PEVA are:

1. Biodegradable: it will not harm the environment when discarded or burned.

2. The price is similar to that of PVC: EVA is more expensive than toxic PVC, but relatively cheaper than phthalate-free PVC.

3. Lighter weight: The density of EVA ranges from 0.91 to 0.93, while that of PVC is 1.32.

4. Odor-free: EVA does not contain ammonia or other organic odors.

5. Does not contain heavy metals: in line with the relevant international toy regulations (EN-71 Part 3 and ASTM-F963).

6. No phthalates: suitable for children’s toys and will not cause the risk of plasticizer release.

7. High transparency, softness and toughness: the application range is very wide.

8. Super low temperature resistance (-70C): suitable for freezing environment.

9. Resistant to water, salt and other substances: it can maintain stability in most applications.

10. High heat adhesion: It can be firmly attached to nylon, polyester, canvas and other cloths.

11. Low bonding temperature: can speed up the production speed.

12. Can be silk-screened and offset-printed: it can be used for multi-pattern products (but EVA inks must be used).

2. Purpose

EVA resin is very versatile. Under normal circumstances, the main products of EVA with vinyl acetate content of less than 5% are films, wires and cables, LDPE modifiers, adhesives, etc.; EVA products with vinyl acetate content of 5% to 10% are elastic films, etc.; vinyl acetate EVA with a content of 20-28% is mainly used for hot melt adhesives and coating products; vinyl acetate content is 5%-45%, and the main products are films (including agricultural films) and sheets, injection molding and molding products , Foam products, hot melt adhesives, etc. like:

(1) Films, sheets and laminated products: with sealing properties, adhesion, flexibility, toughness, shrinkage, suitable for elastic packaging films, heat shrinkable films, agricultural films, food packaging films, laminated films, can be used Used as the middle layer of polyolefin laminated film. (2) General supplies: It has the advantages of flexibility, resistance to environmental stress cracking, and good weather resistance. It is suitable for industrial materials such as power wire insulation bags, household appliances accessories, window sealing materials, etc.

(3) Daily groceries include sporting goods, toys, cushions, straps, airtight container covers, EVA rubber footballs, etc. (4) Auto parts include shock absorbers, fenders, interior and exterior decorative accessories, etc.

(5) Foamed products: foam slippers, sandals, building materials, etc. are pressurized and foamed. Injection foaming has various industrial parts, women’s shoe soles, hot melt adhesives, etc.

3. Molding and processing of ethylene-vinyl acetate copolymer

EVA can be injection molding, extrusion, blow molding, calendering, rotational vacuum thermoforming, foaming, coating, heat sealing, welding and other molding processing.

Fourth, the consumption of EVA materials in my country

In 2001, the market consumption of EVA resin in my country was about 290kt. With the development of my country’s shoe industry and the increase in the amount of functional greenhouse film, my country’s demand for EVA resin will also increase year by year.

(1) Foam shoe material

Shoe material is the most important application field of EVA resin in my country. In the EVA resin used in shoe materials, the vinyl acetate content is generally 15% to 22%. Because EVA resin blended foam products have soft, good elasticity, chemical resistance and other properties, they are widely used in the soles and interior materials of middle and high-end tourist shoes, hiking shoes, slippers, and sandals. In addition, this material is also used in the field of sound insulation boards, gymnastics mats and sealing materials. Shunde and Zhongshan in Guangdong, Jinjiang and Quanzhou in Fujian and Wenzhou in Zhejiang are the main production bases of my country’s footwear industry, which consume a large amount of EVA resin products every year. In 1999, my country consumed about 150kt of EVA resin in foamed shoe materials and shock absorption.

(2) Film

The main purpose of EVA film is to produce functional greenhouse film. Functional greenhouse film has high weather resistance, anti-fogging and heat preservation performance. Because polyethylene has no polarity, even if a certain amount of anti-fogging agent is added, its anti-fogging performance can only be maintained for about two months; and The greenhouse film made by adding a certain amount of EVA resin not only has a higher light transmittance, but also has a greater improvement in anti-fogging performance, which generally lasts more than four months. In addition, EVA can also be used to produce packaging films, medical films, laminated films, casting films, etc.

In 1999, there were more than 50 companies using EVA resin to process films in my country. In 1999, my country produced a total of 800kt of shed film, of which 220kt of functional shed film, consuming about 30kt of EVA resin, plus packaging film, medical film and other fields, my country consumed 37kt of EVA resin in the field of film.

(3) Wire and cable

With the continuous development of computer and network engineering, people are increasingly using halogen-free flame-retardant cables and silane cross-linked cables for the safety of computer rooms. Because EVA resin has good filler tolerance and cross-linkability, it is used more in halogen-free flame-retardant cables, semiconductor shielded cables and two-step silane cross-linked cables. In addition, EVA resin is also used to make sheaths for some special cables. In the EVA resin used in wire and cable, the vinyl acetate content is generally 12% to 24%. In 1999, the wire and cable industry consumed about 6000t of EVA.

(4) Toys

EVA resin also has many applications in toys, such as children’s wheels and seat cushions. In recent years, my country’s toy processing industry has developed rapidly, and production is mostly concentrated in coastal areas such as Dongguan, Shenzhen, and Shantou, mainly for export and foreign processing. According to analysis, these manufacturers consume about 5000t of EVA resin each year, and the grades used are basically the same as the materials used for shoes.

(5) Hot melt adhesive

The hot melt adhesive with EVA resin as the main component, because it does not contain solvents, does not pollute the environment and has high safety, it is very suitable for automated assembly line production, so it is widely used in book binding, furniture edge banding, automobiles and household appliances Assembly, shoemaking, carpet coating and metal anti-corrosion coating.

Hot melt adhesives mainly use varieties with a vinyl acetate content of 25% to 40%. Although there are products of this brand in China, production has not been arranged for a long time, so they are all occupied by imported materials. In 1999, my country’s hot melt adhesive field was estimated to consume 17kt of EVA resin.

(6) Other

EVA resin is also widely used in the fields of inks, bags, wine bottle cushions, etc. It is estimated that the consumption of EVA resin in these areas is no less than 15kt.

Post time: Nov-19-2021