EVA Hot Melt Adhesive Tips Fully Revealed!

Hot melt adhesive is a solid adhesive that uses thermoplastic resin as the base material, adds various tackifiers, antioxidants, and other auxiliary materials, and is melted and mixed. It has the advantages of solvent free, convenient synthesis, short curing time, wide bonding range, convenient storage and transportation, and green environmental protection, thus achieving rapid development.

The hot melt adhesive industry in China started in 1985, and between 2004, the industry began to develop rapidly, with a sharp increase in production and vigorous development in various industries. Among them, EVA (ethylene vinyl acetate copolymer) hot melt adhesive has become a rapidly developing, widely used, and widely used type of hot melt adhesive due to its excellent bonding performance, good adhesion to all materials, and good adhesive layer toughness, weather resistance, heat resistance, low melt viscosity, and low price.

EVA hot melt adhesive is usually composed of EVA resin, tackifying resin, viscosity regulator, antioxidant, filler, etc.

EVA Resin

EVA resin is an essential matrix resin in the composition of EVA hot melt adhesive, which determines the cohesive strength, flexibility, and adhesion strength to the substrate of the hot melt adhesive. EVA (ethylene vinyl acetate copolymer) is made by bulk polymerization of ethylene and vinyl acetate copolymer at a certain temperature and pressure (structural formula shown in Figure 1). Due to the introduction of vinyl acetate monomer in the molecular chain, the branching degree of the polymer is increased, resulting in a decrease in crystallinity, improved flexibility, impact resistance, filler compatibility, and thermal sealing. The resin has good softness, impact strength, environmental stress cracking resistance, and good optical properties over a wide temperature range, as well as low-temperature and non-toxic properties.

The proportion and content of ethylene and vinyl acetate (VA) monomers in EVA resin have a significant impact on the performance of the resin. Typically, the VA content of EVA resin used in EVA hot melt adhesives ranges from 18% to 40%, with an increase in VA content. The toughness, impact resistance, softness, stress cracking resistance, viscosity, thermal sealing, and repeated bending of the resin at low temperatures increase, resulting in an increase in peel strength of the adhesive and rubber elasticity, but strength, hardness The melting point and thermal deformation temperature also decrease accordingly. Therefore, according to the different needs of hot melt adhesive, EVA resin with different VA content is usually selected for use alone or in combination.

Tackifying Resin

Viscose increasing resin refers to a type of small molecule polymer that can increase the viscosity of rubber materials. Its relative molecular weight is usually below 10000, its softening point is between 60 ℃ and 150 ℃, and its glass transition temperature is higher than room temperature. It has a certain degree of thermoplastic properties but extremely low mechanical properties. Tackifier resins are usually not used alone and are mainly used as low molecular weight modification additives in fields such as hot melt adhesives, polymer coatings, and rubber. Among them, tackifier resins are the most widely used in the field of hot melt adhesives, and the global consumption of tackifier resins for hot melt adhesives accounts for more than 60% of its total production every year.

The commonly used tackifying resins for EVA hot melt adhesives are mainly divided into three types: rosin resin, terpene resin, and petroleum resin. The main component of rosin is rosin acid, which accounts for over 90% of its total amount. In addition, there are small amounts of fatty acids and neutral substances. The structure of rosin acid contains double bonds and carboxyl groups, which have strong reactivity and are unstable under light, heat, and oxygen conditions. It exhibits poor aging resistance, poor weather resistance, and is prone to discoloration. Therefore, various derivatives such as hydrogenated rosin, disproportionated rosin, polymerized rosin, esterified rosin, and maleic acid rosin have been derived on the basis of rosin. These derivatives not only improve the instability of rosin, but also endow rosin with superior properties and are widely used in tackifying resins.

Terpene resin, also known as pinene resin. It mainly utilizes a-pinene from turpentine oil or β- Pinene is a series of linear polymers obtained through cationic polymerization under the action of a Fork's catalyst, ranging from liquid to solid. And it has good solubility in various synthetic substances. Easily soluble in aromatic hydrocarbons and vegetable oils.

Petroleum resin is a compound obtained by cationic polymerization of fatty or aromatic hydrocarbons, a byproduct of cracked petroleum. Petroleum resins include pure C5 petroleum resin, C9 petroleum resin, and C5/C9 copolymer petroleum resin. The comprehensive performance of petroleum resin is slightly inferior to other resins, but the price is relatively suitable. The modified C5 petroleum resin not only improves adhesion and softening point, but also greatly enhances its polarity due to the introduction of polar groups.

Viscosity Modifier

A viscosity regulator, as the name suggests, is an additive used to regulate the viscosity of polymers. It is often used in EVA hot melt adhesives to reduce melt viscosity, improve colloid fluidity, shorten drying time, and reduce wire drawing phenomena. The dosage in hot melt adhesive generally does not exceed 25%.

The commonly used viscosity regulator is usually a wax type substance, and there are many types of waxes, but not all waxes are suitable for the field of hot melt adhesives. The esters and unsaturated bonds present in animal and plant waxes can affect the aging resistance and thermal stability of hot melt adhesives. Therefore, the main waxes used for preparing hot melt adhesives are mineral waxes and synthetic waxes, such as microcrystalline waxes, paraffin waxes, polyethylene waxes, and Fischer's wax.

The carbon atom distribution of paraffin in mineral wax is relatively narrow and almost exists in a straight chain form. It has good stability and low melting viscosity, but the softening point and melting point of paraffin are too low, which directly affects the thermal properties of hot melt adhesives; Compared with paraffin wax, microcrystalline wax has a wider distribution of carbon atoms and a higher molecular weight. In addition to the straight chain structure, microcrystalline wax also has branched and circular structures in its molecular chain. Its molecular crystallinity is low and there is an amorphous zone. microcrystalline wax has better low-temperature resistance and has a significant impact on the glass transition temperature of hot melt adhesives, making it suitable for the preparation of low temperature resistant hot melt adhesives.

Polyethylene wax and Fischer Tropsch wax in synthetic wax belong to high crystalline wax, which contains a large amount of n-alkane structure and has characteristics such as high hardness, high melting viscosity, low temperature resistance, high melting point and softening point. They are generally suitable for the preparation of high-temperature resistant and rapid curing hot melt adhesives.

Antioxidants

Antioxidants are a type of chemical additives commonly used in polymers, which can delay and prevent polymer oxidation. A small amount of antioxidant is generally added to hot melt adhesives to prevent deterioration and decrease in bonding strength caused by oxidative decomposition during high-temperature preparation and construction. Commonly used antioxidants include antioxidants BHH264, 1010, 1135, 168, etc.

Filler

Adding fillers can reduce the cost of hot melt adhesive and improve some of its properties. Adding fillers to EVA hot melt adhesive can improve the heat resistance of the gel, reduce the shrinkage of the adhesive, and control the fluidity of the adhesive. Commonly used fillers include clay, white carbon black talc powder, titanium dioxide powder, barium sulfate, etc.

At present, hot melt adhesives are mainly used in industries such as packaging, wood, book binding, clothing, automobiles, and construction. In wood processing, it is often used for furniture edge sealing, and in the packaging industry, it can be used in areas such as cardboard and cardboard boxes, cement bags, labeling, food packaging, protective packaging, and packaging container sealing. In the clothing industry, it is often used as an adhesive for hot melt adhesive interlining. In addition, it can also be used in the manufacturing of televisions, air conditioners, refrigerators and other electrical appliances, as well as in the shoemaking industry, the connection between PET blow molded bottles and polyolefin cups, SO non-woven composite cement packaging bags, carpet backing, cigarette filter crimping, outer packaging tearing, cable joint sealing, oil pipeline anti-corrosion and other fields.

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