Formulation of hot-melt pressure-sensitive adhesive

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Pressure-sensitive adhesives mainly fall into two categories: resin-based and rubber-based. In recent years, hot-melt pressure-sensitive adhesives have been developed; the main component of these adhesives is thermoplastic elastomers. Thermoplastic elastomer-based pressure-sensitive adhesives are composed of SBS, SIS, tackifying resins, softeners, antioxidants, colorants, etc. First, SBS and SIS: SBS is a styrene-butadiene terpolymer block copolymer, while SIS is a styrene-isoprene-styrene terpolymer block copolymer. Both possess both rubbery and plastic-like properties, are readily available at moderate prices, and are therefore very suitable as elastic components in pressure-sensitive adhesives. Based on their structure, SBS can be divided into linear and star types. Linear structures have a lower molecular weight and good solubility, but their cohesion strength is insufficient ； The star-shaped structure has a relatively high molecular weight and high cohesion strength, but a high melting temperature. Therefore, SBS with a linear structure should be selected for manufacturing pressure-sensitive adhesives. The ratio of the relative contents of styrene (St) to butadiene (Bd) has a significant impact on the properties: a higher St/Bd ratio results in lower viscosity and greater adhesion, but poorer elasticity and cold resistance ； As St/Bd decreases, viscosity increases and elasticity rises, but adhesion strength and heat resistance decrease. For use as a pressure-sensitive adhesive, SBS generally has a St/Bd ratio of 30/70. SIS is a incompatible two-phase structure, with PS dispersed in the polyisoprene continuous phase, serving to effect \"vulcanization\" and reinforcement. The structure contains polyisoprene segments with multiple methyl side chains, which confer strong adhesion; therefore, it is more suitable for producing pressure-sensitive adhesives, especially hot-melt pressure-sensitive adhesives, than SBS. The glass transition temperatures of SIS are Tg1 at 550°C and Tg2 at 10,000°C; it has high elasticity but is not resistant to aging, yet it is resistant to water, alcohols, weak acids, and weak bases. Esters, analogs, aromatic compounds, and hydrocarbon compounds can dissolve or swell SIS. II. Viscosity-enhancing resins: Thermoplastic elastomer SIS does not possess initial adhesion on its own; viscosity-enhancing resins must be added to it in order to achieve pressure-sensitive properties. The key to the performance of pressure-sensitive adhesives lies in the viscoelasticity of the adhesive; the role of tackifiers is primarily to endow the pressure-sensitive adhesive with the necessary viscosity. Since thermoplastic elastomers have a two-phase structured morphology, compatibility between the tackifying resin and the two phases of the elastomer must be taken into consideration when selecting such resins. Viscosity-enhancing resins compatible with the rubber phases (PB, PI) in thermoplastic elastomers include rosin and rosin esters, terpene resins, C5 petroleum resins, etc., which impart viscosity to the pressure-sensitive adhesive. Viscosity-enhancing resins compatible with the plastic phase (PS) include guayule resin, aromatic petroleum resins, PS resin, etc., which can improve the cohesiveness of the pressure-sensitive adhesive. There are also some viscosity-increasing resins that are compatible with both phases, such as terpene phenol resins with a high softening point and aromatic petroleum resins with a low softening point. Phenolic resin is incompatible with both phases of thermoplastic elastomers. The amount of tackifying resin used is generally equal to or slightly more than that of the thermoplastic elastomer. It is true that as the amount of tackifying resin increases, the peeling strength of the pressure-sensitive adhesive improves; however, once the peeling strength reaches its peak, further increase in the amount of tackifying resin will instead cause a sharp decline in peeling strength. III. Softeners In thermoplastic elastomer pressure-sensitive adhesives, in addition to tackifying resins, it is necessary to add softeners as well ； A certain amount of softener (also known as plasticizer) must also be added; its role is to improve the properties of the pressure-sensitive adhesive, such as increasing the initial adhesion force, reducing the viscosity of the adhesive solution or melt, and improving the coating process ； Costs can also be appropriately reduced. IV. Antioxidant SDS: Thermoplastic elastomers contain unsaturated bonds, and they undergo aging as a result of environmental factors such as oxygen, light, and heat. This aging is particularly severe in the case of hot-melt pressure-sensitive adhesives, which are prepared and applied by melting at high temperatures. Therefore, anti-aging agents with antioxidants as the main component must be added to the pressure-sensitive adhesive formula. If no anti-aging agent is added during the preparation of the hot-melt pressure-sensitive adhesive, yellow coking appears on the bottle wall at the gas-liquid interface. As the temperature rises, the color deepens, affecting the appearance and performance of the product. Actual tests have shown that the peeling strength of pressure-sensitive adhesives with antioxidants added is about 30% higher than that of those without antioxidants. V. Crosslinking agents: The cohesive strength and adhesion of thermoplastic elastomer pressure-sensitive adhesives decline rapidly at high temperatures. To improve their performance at such temperatures, certain crosslinking agents need to be added to the formula. VI. Colorants: By including pigments and fillers in the formulation of thermoplastic elastic pressure-sensitive adhesives, colored adhesives can be produced; for example, adding an appropriate amount of titanium dioxide can result in a white pressure-sensitive adhesive. VII. Other additives: To reduce costs or increase the viscosity of certain solvent-based pressure-sensitive adhesives, natural rubber or synthetic rubber can be used to replace part of the thermoplastic elastomers; however, attention must be paid to their compatibility with each other. Natural rubber and isoprene rubber can blend well with SIS, while styrene-butadiene rubber and cis-butadiene rubber can be used in SBS pressure-sensitive adhesive systems. VIII. Guidelines for Formulating Thermoplastic Elastomer Pressure Sensitive Adhesives When designing formulations, it is essential to be familiar with the effects of various components on thermoplastic elastomer pressure sensitive adhesives. The following points summarize these aspects: (1) The proportion of tackifying resins; (2) Peel strength; (3) Tackifying resins that are compatible with plastics can make the pressure sensitive adhesive harder, increasing its elastic modulus and cohesive strength ； And tackifying resins and softeners compatible with rubber. On the contrary. (4) Factors determining the maximum operating temperature of pressure-sensitive adhesives. (5) Factors determining the minimum operating temperature of pressure-sensitive adhesives. (6) Issues related to the use of viscosity-enhancing resins and softeners that are compatible with the rubber phase. (7) The proportion of antioxidant used. (8) The performance characteristics of inorganic pigments and fillers. (9) The addition of appropriate cross-linking agents can improve the high-temperature resistance of pressure-sensitive adhesives. (10) Natural rubber or synthetic rubber can be used as the main component of some elastomers. The above is an introduction to the formulations of hot-melt pressure-sensitive adhesives