Hot melt adhesive is a very rapidly developing adhesive product with a wide range of applications. The formula of hot melt adhesive is formulated with a mixture of basic polymers, tackifying resins (tackifiers), waxes and antioxidants, etc. Each component substance has its role. For example, a certain amount of plasticizer, filler, and other polymers can be added as appropriate to improve the adhesion, fluidity, heat resistance, cold resistance, and toughness.
The basic polymer is the adhesive of the hot melt adhesive, and its role is to make the adhesive have the necessary bonding strength and cohesive strength.
The basic polymers of hot melt adhesives are thermoplastic resins. More basic polymers are polyolefins and their copolymers, such as ethylene-vinyl acetate copolymer resin (EVA), low molecular weight polyethylene (PE), ethylene - ethyl acrylate copolymer resin (EEA), random polypropylene (APP), ethylene-acrylic copolymer (EAA); thermoplastic elastomers, such as butyl rubber (BR), styrene-butadiene block copolymer (SBS), styrene - isoprene block copolymer (SBS), styrene - isoprene block copolymer (SBS), styrene - isoprene block copolymer (SBS), styrene - isoprene block copolymer (SBS). Styrene-isoprene block copolymer (SIS); in addition to cellulose derivatives, polyamide resins (PA), polyester resins (PES), polyurethane resins (PU).
If a single basic polymer cannot meet the performance requirements, two or more basic polymers with elastic properties can be mixed. Suppose you want to improve the hot melt adhesive's cold resistance, flexibility, impact resistance, and creep resistance. In that case, you can add a small amount of synthetic rubber such as isobutyl rubber or styrene-butadiene block copolymer. You can also add random polypropylene or asphalt to reduce the cost of rubber.
As the viscosity of the basic polymer is generally quite high, called the interface of the wettability and initial viscosity is not good, so it should not be used alone, often adding good compatibility with the viscosity enhancers mixed-use.
The main role of viscosity enhancers is to reduce the melt viscosity of the hot melt adhesive, improve its wettability and initial viscosity of the glued surface to improve the strength of the glue, improve operational performance and reduce the cost of the purpose. In addition, it can also be used to adjust the heat-resistant temperature and drying time of the adhesive.
The requirements of the tackifier are: must have good compatibility with the basic polymer; good adhesion to the glued material; good thermal stability at the melting temperature of the hot melt adhesive. The amount of tackifier is 20%-150% of the basic polymer.
Commonly used adhesion enhancers are as follows.
(1) rosin and its derivatives are the most used in hot melt adhesives as a tackifier. Usually can be divided into three categories.
Rosin: fat rosin, wood rosin, rosin floating oil, etc... Its main component is rosin acid, containing polar carboxyl groups, small molecular weight, EVA resin, and another polar basic polymer compatibility. But its softening point is not high (70-85 ℃), and because the molecule has a conjugated double bond, easy to be oxidized, so the thermal stability and oxidation resistance are poor.
Modified rosin: hydrogenated rosin, disproportionated rosin, polymerized rosin, etc. The softening point of rosin increases after modification, and there is no conjugated double bond, so the thermal stability and oxidation resistance are better.
Rosin resin: rosin glycerides, hydrogenated rosin pentaerythritol esters, polymerized rosin glycerides, etc... The comprehensive performance is better.
(2) terpene resin and its modified resin: terpene resin is obtained by the polymerization of terpene compounds contained in turpentine. It is stable, does not change color when exposed to light and heat, is resistant to alkali and acid, and has good electrical properties.
Modified terpene resin is obtained by the polymerization of phenol or maleic anhydride. The polymerization product with phenol can be called terpene resin, which has a high softening point, good resistance to oxidation, acid, and alkali.
(3) Petroleum resin is a polymer of unsaturated hydrocarbon fractions from petroleum pyrolysis by-products. It can be divided into aliphatic, aromatic, and alicyclic, according to the carbon chain structure of unsaturated hydrocarbons.
Commonly used C5 and C9 petroleum resin, such as m-pentadiene petroleum resin, is obtained from carbon five separated products m-pentadiene by cationic catalytic polymerization, solid aliphatic petroleum resin, with pure color and odorless, good thermal stability, strong adhesive force, and other characteristics.
In addition to the above three types of tackifiers, thermoplastic phenolic resin, low molecular weight polystyrene, and Gummaron resin are also available as a tackifier.
The main role of waxes is to reduce the melting point and melt viscosity of hot melt adhesives, improve the fluidity and wettability of the glue, improve the adhesive strength, prevent hot melt adhesives from caking and reduce stress costs. In addition to polyester, polyamide hot melt adhesive basic wax, most hot melt adhesives add a certain amount of wax. But the amount is too much, the shrinkage of the adhesive becomes larger, but the bonding strength is reduced, so in need to achieve greater bonding strength due to control the amount of wax. Commonly used waxes are alkane paraffin wax, microcrystalline paraffin wax, synthetic wax. The amount of wax generally does not exceed 30% of the weight of the basic polymer.
Microcrystalline paraffin wax improves the flexibility of hot melt adhesive, bonding strength, thermal stability, and cold resistance are due to alkane paraffin wax. Still, the ability to prevent caking is lower than alkane paraffin wax, and the price is higher. Synthetic waxes have good compatibility with basic polymers and good chemical stability, thermal stability, and electrical properties and are better than the first two types of paraffin waxes in use.
The role of filler is to reduce the shrinkage of hot melt adhesive textile over-penetration on the surface of porous glue joint, improve the cold resistance and heat capacity of hot melt adhesive, extend the operable time and reduce the cost. But the amount of filler can not be too much. Otherwise, it will make the melt viscosity of the hot melt adhesive rise, the wettability, and initial viscosity becomes poor, thus reducing the strength of the glue.
The role of plasticizers is to accelerate the melting speed, reduce the melt viscosity, improve the wettability of the glued material, improve the flexibility and cold resistance of the hot melt adhesive. However, if the amount of plasticizer is too much, the cohesive strength of the adhesive layer will be reduced. At the same time, the migration and volatilization of plasticizers will also reduce the strength of the glue and the heat resistance of the glue layer. Therefore, the general hot melt adhesive only adds fewer goods without plasticizers.
Commonly used plasticizers are dibutyl benzodicarboxylate (DBP), dioctyl benzodicarboxylate (DOP), butyl benzyl benzodicarboxylate (BBP), etc.
The role of antioxidants is to prevent the oxidation and decomposition of hot melt adhesives at high melting temperatures for a long time.
Generally speaking, if the hot melt adhesive is heated at 180℃-230℃ for more than 10h or if the component used has poor thermal stability, it is necessary to add an antioxidant, such as the use of heat-resistant components, and not at high temperatures for a long time, but not good antioxidants.
Of course, not all hot melt adhesive products contain all these components, and some hot melt adhesives may contain only a few or one of these components. However, the basic polymer must be included because it is the main component of the hot melt adhesive.