What are EVA Sheets?
EVA Foam Sheet or EVA Sheet is a substance that entraps on the inside, well-dispersed bubbles/cells. Polymer foams are composed of two phases, in which a blowing agent is used to generate stabilized
bubble structures inside the polymer matrix.
Figure: Ethylene Vinyl Acetate EVA Molecule
Cross-Linked EVA Foam Sheet
Polyolefin foams (EVA Sheets, LD Sheets) are fourth ranked, after polyurethane (PU),
polystyrene (PS), and poly (vinyl chloride) (PVC) foams. The increasing demand for cross-linked
EVA Foam Sheets makes them have one of the highest growth rates Polyolefin foam is one of the most important categories within polymer foams. It had first been marketed inthe early 1960s and the areas of application include footwear, packaging, sports and leisure, toys, insulation,automotive, buoyancy, cushioning and others .
In terms of final foam properties, the wide range of polyolefin foams can be divided into
hard foams, which are obtained using polypropylene (PP) or other high strength basic polyolefin, and softer foams, which are obtained using co-polymer, such as ethylene vinyl acetate or EVA Soft polyolefin foam from EVA comes with the property of elasticity, which is due to the ability of the long chains to reshape them back to the original configuration after the applied stress is released. Soft EVA Foam Sheets made of polyolefin have a wide application, such as cushioned packagingmaterials, floatation materials, padding in various sports equipment, shock absorbers, and sportsshoe soles/slippers/sandals.
Key properties of EVA Sheets:
- Resiliency/ Cushioning
- Sound & Energy Absorption
- Buoyancy
- Resistance to chemicals
- Low Thermal Conductivity & Insulation
- Low Water Vapor Transmission
- Thermoformability
- Ease of Fabrication
Microcellular plastics, otherwise known as microcellular foam, is a form of manufactured plastic, specially fabricated to contain billions of tiny bubbles less than 50 microns in size (typically from 0.1 to 100 micrometers).
This type of plastic is formed by dissolving gas under high pressure into various polymers, relying on "thermodynamic instability phenomena" to cause the uniform arrangement of the gas bubbles, otherwise known as nucleation.
Its main purpose was to reduce material usage while maintaining valuable mechanical properties. The main room for variance in these foams is the gas used to create them; the density of the finished product is determined by the gas used.
Depending on the gas used, the density of the foam can lie between 5% and 99% that of the pre-processed plastic. Design parameters, focused more on the final form of the foam and the molding process afterward, include the type of die or mold to be used, as well as the dimensions of the bubbles, or cells, that classify this material as a foam.
Since the size of cells is close to the wavelength of light, to the casual observer this foam retains the appearance of a solid light colored plastic.
Cross-Linking or Vulcanization Process of EVA Sheets
Cross-linking and/or vulcanization are defined as a process for converting a thermoplastic material like EVA or elastomer into a thermoset or vulcanizate.
This process converts unbound polymer molecular chains into a single network which retains many desirable physical and chemical properties of the base polymer.
The two major chemical processes by which cross-linking occurs are peroxide and sulfur cure systems. Peroxide systems are more versatile since they can be used to cross-link both saturated and unsaturated polymers, thereby providing a wider selection of elastomers, and more opportunities for cost savings.
Cross-linking technology is applied to EVA Sheet foaming. Cross-linking is a chemical
bond between adjacent polymer chains, which can stabilize bubbles during foam expansion, enhance the resistance of the cellular product to thermal collapse, and also improve the mechanical properties (such as anti-creep ability, weather-ability, impact absorption, etc) of the final foamed products. Soft EVA Foams Sheets are usually cross-linked during the manufacturing process, and other related polymer resins, which include LDPE, TPE, RUBBER and other blends.
Figure: Polymer Chains and Cross-Linking
There are several representative processes which are used to manufacture cross-linked
EVA Foam Sheets: irradiation cross-linked foam process, chemically cross-linked polyolefin foam process, chemically cross-linked polyolefin foamed BUN process, grafted resin cross-linked polyolefin foam process, injection molded foam process, and nitrogen autoclave process.
In the irradiation cross-linked foam process, an irradiation-unit, which generates
electron beams, is used to crosslink foamable solid matrix, after the completion of the extrusion.
In the chemically cross-linked polyolefin foam process, the cross-linking agent is used to
strengthen polymer resin under a lower heat, that is between the end of an extruder and a hot air oven. In the chemically cross-linked polyolefin foamed BUN or compression moulding process, a press, other than an extrusion system, is used to make foam with a chemical cross-linking agent. In the grafted resincross-linked polyolefin foam process, a grafted polyolefin resin is used for cross-linking under the
condition of heat and moisture. In the injection molded foam process, polyolefin with high melt fluidity is cured inside the press, and its foam densities are in the range of 100 – 300 kg/m3.
Figure: Microscopic View of EVA Sheets
All of the above cross-linked foaming process can be composed of three major steps:
mixture formation, crosslinking, and foaming (See Figure).
Figure: Schematic of the foaming technology of the cross-linked EVA Sheet.
Figure: Production Flow Chart EVA Sheet Manufacturing (I Stage), Compression Moulding
Figure: Production Flow Chart EVA Sheet Manufacturing (II Stage), Compression Moulding
Materials used in Manufacturing EVA Sheets
Ethylene Vinyl Acetate or EVA
Ethylene-vinyl acetate (EVA), also known as poly (ethylene-vinyl acetate) (PEVA), is the copolymer of ethylene and vinyl acetate. The weight percent vinyl acetate usually varies from 10 to 40%, with the remainder being ethylene.
Broadly speaking, there are three different types of EVA copolymer, which differ in the vinyl acetate (VA) content and the way the materials are used.
The EVA copolymer which is based on a low proportion of VA (approximately up to 4%) may be referred to as vinyl acetate modified polyethylene. It is a copolymer and is processed as a thermoplastics material – just like low density polyethylene. It has some of the properties of a low density polyethylene but increased gloss (useful for film), softness and flexibility. The material is generally considered as non-toxic.
The EVA copolymer which is based on a medium proportion of VA (approximately 4 to 30%) is referred to as thermoplastic ethylene-vinyl acetate copolymer and is a thermoplastic elastomer or TPE material.
It is not vulcanized but has some of the properties of a rubber or of plasticized polyvinyl chloride particularly at the higher end of the range. Both filled and unfilled EVA materials have good low temperature properties and are tough. The materials with approximately 11% VA are used as hot melt adhesives.
The EVA copolymer which is based on a high proportion of VA (greater than 40%) is referred to as ethylene-vinyl acetate rubber or EVA Rubber.
EVA is an elastomeric polymer that produces materials which are "rubber-like" in softness and flexibility. The material has good clarity and gloss, low-temperature toughness, stress-crack resistance, hot-melt adhesive waterproof properties, and resistance to UV radiation.
Low Density Polyethylene or LDPE
LDPE is defined by a density range of 0.910–0.940 g/cm3. LDPE has a high degree of short- and long-chain branching, which means that the chains do not pack into the crystal structure as well. It has, therefore, less strong intermolecular forces as the instantaneous-dipole induced-dipole attraction is less. This results in a lower tensile strength and increased ductility. LDPE is created by free-radical polymerization. The high degree of branching with long chains gives molten LDPE unique and desirable flow properties. LDPE is used for both rigid containers and plastic film applications such as plastic bags and film wrap. In 2013, the global LDPE market had a volume of almost US$33 billion. LDPE is normally blended with EVA for strength and stiffness modification. LDPE based foam sheets have better thermo formability.
Rubber
Rubber is a material which can stretch and shrink. It is a polymer. It can be produced from natural sources (eg. natural rubber) or can be synthesised on a industrial scale. Many things are made from rubber, like gloves, tires, plugs, and masks. A few things can be made only from rubber.
Sometimes the word means only natural rubber (latex rubber). Natural rubber is made from the white sap of some trees such as the Heveabrasiliensis(Euphorbiaceae). Other elastomers, called synthetic rubbers, are made by chemical processes.
The flexibility of rubber is appealing in hoses, tires and rollers for devices ranging from domestic clothes wringers to printing presses; its elasticity makes it suitable for various kinds of shock absorbers and for specialized machinery mountings designed to reduce vibration. Its relative gas impermeability makes it useful in the manufacture of articles such as air hoses, balloons, balls and cushions.
The resistance of rubber to water and to the action of most fluid chemicals has led to its use in rainwear, diving gear, and chemical and medicinal tubing, and as a lining for storage tanks, processing equipment and railroad tank cars.
Because of their electrical resistance, soft rubber goods are used as insulation and for protective gloves, shoes and blankets; hard rubber is used for articles such as telephone housings, parts for radio sets, meters and other electrical instruments.
The coefficient of friction of rubber, which is high on dry surfaces and low on wet surfaces, leads to its use for power-transmission belting and for water-lubricated bearings in deep-well pumps. Indian rubber balls or lacrosse balls are made of rubber.
Rubber is often blended with EVA foam sheets for improving various performance properties. Rubber increases low temperature flexibility and resilience properties of EVA Sheets.
DCP or Di-Cumyl Peroxide
Organic peroxide is a molecule containing at least two oxygen atoms, connected by a single bond to organic chemical groups, as shown below. Depending on the groups attached, this oxygen-oxygen bond is designed to break on heating, leaving one unpaired electron on each oxygen, called a “free radical”. These free radicals are able to promote certain chemical reactions, such as:
• Polymerization of one or more monomers
• Curing of thermosetting resins (polymer + monomer)
• Cross-linking of EVA Sheets and LDPE Sheets
Organic peroxides that are thermally decomposed generate free radicals that consequently create an active site on a polymer backbone. The reaction between two active sites creates a strong link between the polymer chains, forming a polymer network exhibiting very desirable mechanical properties, particularly excellent heat resistance and compression set.
Another advantage of using a peroxide cure instead of sulfur vulcanization is the wide range of polymers that can be cross-linked (unsaturated polymers like EVA and Rubber as well as saturated polymers like LDPE). Due to the nature of the strong carbon-carbon crosslink bond created by the use of organic peroxides, it is possible to use the full engineering capabilities of these peroxide cross-linkable EVA.
ADC or Azodicarbonamide
The principal end use of azodicarbonamide is as a blowing agent in the rubber and plastics industries. It is used in the expansion or foaming of a wide range of polymers, including polyvinyl chloride, EVA, and natural and synthetic rubbers. The blowing action occurs when the azodicarbonamide decomposes on heating (process temperatures ~190–230 °C) to yield gases (nitrogen, carbon monoxide, carbon dioxide, and ammonia), solid residues, and sublimated substances. Decomposition accelerators, in the form of metal salts and oxides, may also be added to bring about decomposition at lower temperatures.