Early hot melt adhesives mainly used ethyl cellulose and animal glue or leather glue, and were soon replaced by synthetic resins such as polyamide and EVA copolymer. Thermoplastic rubber SBS and SIS are the fastest growing part of hot melt adhesive market at present because of their special molecular structure and properties.
SBS, SIS hot melt adhesive composition
SBS and SIS hot melt adhesives mainly include SBS, SIS thermoplastic rubber, tackifying resin, plasticizer and stabilizer. The surface is analyzed and discussed respectively.
SBS and SIS are one kind of block copolymers. Its rubbery middle block molecule is unsaturated rubber. It has the solubility and thermal plasticity of polystyrene, but at room temperature it has the toughness and elasticity of vulcanized natural rubber or vulcanized diene. Its characteristics come from its unique molecular structure.
SBS and SIS block copolymers have not only linear structure, but also Ang structure. All these block copolymers have two-phase composition. The dynamic mechanical test of SBS and SIS block copolymers shows that SBS and SIS block copolymers have two Tg peaks, which indicates that there are separated polystyrene phase and polyisoprene phase in these block copolymers. According to the different preparation processes, the morphology or shape of the two dispersed phases can be spherical, cylindrical or flaky.
2 tackifying resin
The influence of tackifying resin on the properties of adhesive depends on its distribution trend between two phases of block copolymer. Polyaromatic hydrocarbon, terpene resin, coumarone-indene or petroleum resin with softening point higher than 85℃ have good compatibility with the end block of polystyrene plastic, which is mainly used to adjust the melt viscosity and control the modulus of adhesive, so that the adhesive still has high cohesive strength at high temperature. The viscosity of low softening point resin is contrary to the above, which makes the adhesive sticky at low temperature and reduces the operating temperature of hot melt adhesive. Aliphatic olefin resin, rosin ester resin, polyterpene resin, terpene phenolic resin, C3 petroleum resin, etc. have good compatibility with the intermediate rubber block, which is mainly just to endow the adhesive with initial adhesive force and peel force, improve the adhesive strength of the adhesive to polar substrate, control the modulus of the adhesive, and soften the adhesive. This kind of resin reduces the low-temperature softness of adhesive, and can be improved by adding corresponding plasticizer. According to the performance requirements of the adhesive, one or more tackifying resins can be selected for use in order to obtain satisfactory results.
Plasticizer is used to improve the adhesive force, cold resistance, impact resistance and flexibility of adhesives, reduce the melt viscosity of adhesives and improve the operability of adhesives. Generally, SBS and SIS hot melt adhesives should choose welding oils which are incompatible with the middle rubber block of block copolymer and poor with the end polystyrene plastic, such as naphthenic oil, hydraulic oil, 40# engine oil, etc. If a plasticizer such as DOP with good compatibility with the terminal polystyrene plastic is used, the phase area of the terminal plastic will be softened, thus the adhesive strength of the adhesive will decrease rapidly. In actual use, according to the characteristics of each plasticizer, one or more plasticizers should be selected to meet various performance requirements.
As mentioned above, the rubbery middle block molecules of SBS and SIS block copolymers are unsaturated rubbers, which are vulnerable to chemical attack at carbon-carbon double bonds, and undergo oxidative degradation or molecular cross-linking to form refractory network structures, resulting in poor performance of adhesives. Therefore, it is necessary to add stabilizers such as antioxidants, anti-ozone punishment and ultraviolet stabilizers for protection. The choice of stabilizer depends on the selection of different ingredients and the selection of product stability index. Commonly used stabilizers are 1010, 168, 264, Butyl Zimate, etc. The research shows that a good protective effect can be obtained when a variety of stabilizers are mixed.
Hot melt adhesive with SBS and SIS as main components does not need vulcanization, has good creep resistance, good compatibility with various additives, and can bond various materials. In recent years, the application scope of this kind of hot melt adhesive has been expanding. However, it also has some disadvantages, such as poor heat resistance, ultraviolet radiation resistance and hydrocarbon solvent resistance. In the future, the important task of this kind of hot melt adhesive is to chemically modify SBS and SIS block copolymers and develop new tackifying resins to expand its application fields.