Thermoplastic elastomer SIS has attracted great attention since it came out in 1963. It is a triblock copolymer composed of styrene and isoprene.
Middle is a soft rubber segment isolated from each other, and the two sides are hard plastic segments, which have properties of vulcanized rubber at room temperature, exhibit plasticity at high temperatures. It has advantages of good elasticity and bonding strength, low temperature resistance, good solubility resistance, low solution viscosity, fast curing, etc., so it is usually used to prepare adhesives with SBS or other materials. It is mainly used as hot-melt adhesive and pressure-sensitive adhesive, used in medical treatment, electrical insulation, packaging, protection and masking, marking, bonding and fixing, and interlayer bonding of composite bags.
However, SIS has low polarity, poor oil resistance and solvent resistance, which greatly limits its application range. For example, when used as an adhesive at high temperature or used to bond polar substrates such as shoes and wood, there are many disadvantages: low bonding strength with polar materials, poor heat resistance and weather resistance, especially when used as hot melt pressure sensitive, its softening point is low.

 

Current research on modification of SIS hot-melt pressure-sensitive adhesives mainly focuses on three aspects: The first is to modify SIS elastomer by introducing polar groups or segments on elastomer to change polarity of molecule, including : Epoxidation, graft modification.
The second is to modify SIS pressure-sensitive adhesive, mainly by adding other types of adhesives or additives to change surface tension and polarity of adhesive, thereby improving adhesion between adhesive and adhered material ; Secondly, it is modified by changing components of pressure-sensitive adhesive;
The third is use of electron beam or ultraviolet light to break double bond of SIS elastomer to generate free radicals, then carry out intra-molecular, intermolecular, polymerization, grafting, and cross-linking processes with other polymers. .

Since polystyrene and polyisoprene are non-polar substances, their miscibility with polar substances and indirectness of adhesive formed are restricted. Modification of SIS is mainly to introduce polar groups on it. Performance of pressure-sensitive adhesive prepared by ESIS modified by epoxidation according to the best formula was compared with pressure sensitive adhesive prepared by unmodified SIS according to the best formula. Results show that peel strength, stickiness and aging resistance of ESIS pressure-sensitive adhesive are better than those of unmodified SIS pressure-sensitive adhesive.

There are double bonds in SIS elastomer, which can increase initial tack, heat resistance and adhesion of adhesive through grafting. Use monomers similar to polyolefin structure and surface properties to graft SIS.

At present, domestic research on graft modification of SIS and acrylic esters generally adopts method of solution grafting, with methyl methacrylate (MMA), butyl methacrylate (BMA), acrylonitrile (AN), acrylic acid ( MAH) is a monomer to modify SIS. Experimental results show that under action of benzoyl peroxide (BPO), MMA, BMA and their mixture can effectively graft SIS, enhance its polarity and flexibility, improve adhesion performance shown by polar materials.

Because polyisoprene segment in SIS is easily broken due to action of oxygen, heat, light, etc., it is degraded and cross-linked, which affects performance of pressure-sensitive adhesive. Low chemical unsaturation of butyl rubber, coupled with inactivity of polyisobutylene chain, makes heat and oxidation resistance of butyl rubber far superior to other general-purpose rubbers.

SIS elastomer itself has no initial tack. When it is formulated into a pressure-sensitive adhesive, binding resin, softener, anti-aging agent and other additives must be added.
There are two main ways to modify SIS pressure-sensitive adhesive. One is to change composition or content of pressure-sensitive adhesive, select different tackifying resins, solvents and additives or content of each component according to requirements of adhesive.; The second is blending modification, by adding other types of adhesives or additives to increase polarity of system, so that adhesive force of mixed binder and polar fabrics increase, which is currently method often used to produce adhesives with special properties.
Modification of SIS pressure-sensitive adhesive can significantly improve adhesion to polar materials, heat resistance and weather resistance will also be improved. This modification is relatively simple and requires simpler equipment. Compared with elastomer modification, raw material and energy consumption are lower, which is suitable for company to produce binders that meet different performance requirements.

Ultraviolet light or electron beam modification is to break double bond of SIS elastomer to generate free radicals through short electron beam or ultraviolet irradiation after hot melt pressure-sensitive adhesive is coated, then proceed with polymerization reaction. Generally, after coating, if elastomer is cooled to below Tg of SIS segment, SIS will only be physically cross-linked, then irradiated with electron beam or ultraviolet rays, part of chemical cross-linking will be performed, which can make up for lack of physical cross-linking. It can greatly improve temperature resistance and solvent resistance of adhesive without affecting viscosity.
UV and electron beam modification can improve peel strength and stickiness, is suitable for applications in some special fields. It has low environmental pollution, very few raw materials and energy consumption, is a promising technology.