Hot melt adhesive is a solid adhesive based on thermoplastic resin, adding various viscosifier, antioxidant and other auxiliary materials, after melting and mixing. It has the advantages of no solvent, easy synthesis, short curing time, wide bonding range, convenient storage and transportation, and green environmental protection, so it has achieved rapid development.

Chinese hot melt adhesive industry started in 1985, during 2004, the hot melt adhesive industry began to develop rapidly, the output also increased sharply, and the application in various industries also appeared vigorous development. Among them, EVA (ethylene - vinyl acetate copolymer) hot melt adhesive due to excellent bonding performance, has good adhesion to all materials, and has good adhesive toughness, weather resistance, heat resistance, low melt viscosity, low price and other advantages, so it has become a hot melt adhesive in the rapid development, wide application, large amount of hot melt adhesive.

EVA hot melt adhesive is usually composed of EVA resin, viscosifying resin, viscosity regulator, antioxidant, filler and so on.

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

The proportion and content of vinyl and vinyl acetate (VA) monomers of EVA resin have a great impact on the performance of the resin. Usually, the VA content of EVA resin used in EVA hot melt adhesive ranges from 18-40%. As the VA content increases, the toughness, impact resistance and softness of the resin at low temperature will be improved. The stress cracking resistance, viscosity, thermal sealing and repeated bending increased, the stripping strength of the adhesive increased, the rubber elasticity increased, but the strength, hardness, melting point and thermal deformation temperature also decreased. Therefore, EVA resins with different VA contents are usually used separately or in combination according to different needs of hot melt adhesives.

Viscosifying resin is a kind of small molecule polymer that can increase the viscosity of rubber materials. The relative molecular weight is usually below 10000, the softening point is between 60℃ and 150℃, the glass conversion temperature is higher than room temperature, and it has a certain thermoplastic but very low mechanical properties. Viscosifier resins are usually not used alone. They are mainly used as low molecular modification agents in hot melt adhesives, polymer coatings and rubber. Among them, viscosifying resin is the most widely used in the field of hot melt adhesive. The global annual consumption of viscosifying resin for hot melt adhesive accounts for more than 60% of its total production.

EVA hot melt adhesive commonly used adhesive resin is mainly divided into 3 types: rosin resin, terpene resin and petroleum resin. The main component of rosin is rosin acid (structure shown in Figure 2), accounting for more than 90% of its total, in addition to a small amount of fatty acids and neutral substances. The structure of rosin acid contains double bonds and carboxyl groups, with strong reactivity, unstable under light, heat and oxygen conditions, showing poor aging resistance, poor weather resistance, easy to produce discoloration and other phenomena. Therefore, hydrogenated rosin, disproportionated rosin, polymerized rosin, esterified rosin and maleic rosin were derived on the basis of rosin. These derivatives can not only improve the instability of rosin, but also give rosin more excellent properties, and have been widely used in viscosifying resins.

Terpene resin, also known as pinene resin. It is a series of linear polymers from liquid to solid produced by cationic polymerization of A-pinene or β-pinene of turpentine under the action of Furk's catalyst. And has a good compatibility with various synthetic substances. Soluble in aromatic hydrocarbon and vegetable oil.

Petroleum resin is a compound obtained by cationic polymerization of aliphatic hydrocarbons or aromatic hydrocarbons, by-products 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 worse than that of other resins, but the price is more suitable. The modified C5 petroleum resin not only improves the adhesion and softening point, but also greatly enhances its polarity due to the introduction of polar groups.

Viscosity regulator, as the name suggests, is a kind of additives to adjust the viscosity of polymer, in EVA hot melt adhesive is often used to reduce melt viscosity, improve colloidal fluidity, but also can shorten the drying time, reduce the phenomenon of wire drawing. The amount is generally not more than 25% in hot melt adhesive.

There are many kinds of wax, but not all wax are applicable to the field of hot melt adhesive. The ester and unsaturated bonds in the wax of animals and plants will affect the aging resistance and thermal stability of hot melt adhesive. Therefore, wax for the preparation of hot melt adhesive is mainly mineral wax and synthetic wax, such as: microcrystalline wax, paraffin wax, polyethylene wax and Fischer-Tropsch wax.

The distribution of carbon atoms in paraffin wax is narrow and almost exists in the form of straight chain. It has good stability and low melting viscosity, but the softening point and melting point of paraffin wax are too low, which will directly affect the thermal properties of hot melt adhesive. Compared with paraffin wax, microcrystalline wax has a wider distribution of carbon atoms and a higher molecular weight. Besides the straight chain structure, there are also branched chain and ring structure in the molecular chain of microcrystalline wax. The molecular crystallinity of microcrystalline wax is low and there are amorphous regions.

Polyethylene wax and Fischer-Tropsch wax in synthetic wax belong to high crystalline wax, which contains a lot of n-paraffin structure, and has the characteristics of high hardness, high melting viscosity, low temperature resistance, high melting point and softening point, etc., which is generally suitable for the preparation of hot melt adhesive with high temperature resistance and fast curing.

Antioxidant is a kind of chemical additives commonly used in polymers, which can delay and prevent the oxidation of polymers. A small amount of antioxidants are generally added to hot melt adhesives, mainly to prevent deterioration and bond strength reduction caused by oxidation decomposition of hot melt adhesives during high temperature preparation and construction. Commonly used antioxidants include antioxidants BHH264, 1010, 1135, 168, etc.

Adding filler can reduce the cost of hot melt adhesive and improve some properties of hot melt adhesive. Adding fillers in EVA hot melt adhesive can improve the heat resistance of colloid, reduce the shrinkage of rubber, control the fluidity of rubber. Commonly used fillers are clay, white carbon black talc, titanium dioxide, barium sulfate and so on.

At present, hot melt adhesive is mainly used in packaging, wood, book binding, clothing, automobile, construction and other industries. In wood processing is used for furniture edge sealing, in the packaging industry can be used for cardboard and cardboard boxes, cement bags, labeling, food packaging, protective packaging, packaging container sealing and other fields. In the clothing industry is often used as a hot melt bonding interlining adhesive, in addition to the manufacturing of television, air conditioner, refrigerator and other electrical appliances, as well as shoe industry, PET blow bottle and polyolefin based cup, SO non-woven composite cement packaging bag, carpet backing connection, cigarette filter roll, outer packaging strip, cable joint sealing, oil pipeline anticorrotion and other fields.