Application of Tackifying Resins in Hot Melt Adhesive Formulations

rubber tackifying resins

Hot melt adhesive is an important category of adhesive, with 100% solid content, no solvent, environmental protection and non-toxic, fast curing characteristics, widely used in automated packaging, hygiene products, labeling tape, medical medicine, automotive interior, construction waterproof, home furniture, printing and binding, shoe making, clothing and other industries.

One of the most important performance indicators of hot melt adhesives is the adhesion to the substrate, and the first test to determine whether the hot melt adhesive is qualified is whether the adhesion of the hot melt adhesive is up to standard. Factors affecting the adhesion of hot melt adhesives include formulation design, bonding principle, substrate treatment, etc. In order to enhance the bonding strength, the most common method is to add tackifying resin to the formula to enhance the adhesive performance.

Tackifier resin is the most important raw material for hot melt adhesives, which can reduce the melt viscosity, improve the wetting and initial adhesion of the glued material, and adjust the heat resistance temperature and drying time of the adhesive, the amount is about half of the total amount of hot melt adhesives.

Hot melt adhesive requires good compatibility between the tackifier and the basic polymer, good adhesion to the bonded material, and good thermal stability at the melting temperature.

Classification of tackifying resins

Commonly used tackifying resins are mainly divided into 3 types: rosin resin, terpene resin and petroleum resin.

The main component of rosin is resin acid, which accounts for more than 90% of its total, and there are also a small amount of fatty acid and neutral substances. The rosin resin used for hot melt adhesive formulation is generally a low molecular polymer modified by esterification of natural rosin with glycerol ester or pentaerythritol ester.

Terpene resin is a polymer made from a-pinene or β-pinene polymerization. Most of the domestically produced terpene resins are dark in color and have a certain odor, which are not stable and not suitable for use in the sanitary products industry.

Petroleum resins can be roughly divided into C5 aliphatic petroleum resins, C9 aromatic petroleum resins, C5-C9 copolymer petroleum resins, and dicyclopentadiene (DCPD) alicyclic petroleum resins and hydrogenated modified petroleum resins in five categories.

Petroleum resin softening point is generally 80-140℃, relative density is 0.970-0.975, relative molecular mass is less than 2×103, insoluble in water, easily soluble in organic solvents, with low acid value, good adhesion, water resistance and chemical resistance, and has the characteristics of good viscosity regulation and thermal stability.

The appearance of petroleum resin solves the problems of limited source of natural rosin and terpene resin, poor thermal stability, expensive and easy aging, and the performance of rosin resin modified hot melt adhesive can be achieved by using petroleum resin with a mass fraction of 60%.

It is found that the hot melt adhesive performance is better when the relative molecular mass of tackifying resin is 300-2000 and the glass transition temperature is 0-70℃.

The most important factor affecting the cohesive strength and adhesive strength of hot melt adhesives is the compatibility between the tackifying resin and the base resin.

Hydrogenated alicyclic resins have good compatibility with the rubber phase, but poor compatibility with the plastic phase. Peel strength and cohesion will increase with its content, because hydrogenated alicyclic resins form aggregates in the rubber phase.

In SBS-based hot-melt adhesives, the best compatibility with the PB phase is achieved at a C9 hydrogenation amount of 0.7. At a low hydrogenation amount, the aromatic resin is compatible with PS, and although the high elastic modulus of SBS hot melt adhesive increases significantly, the adhesion is almost absent; as the hydrogenation amount increases, the SBS hot melt adhesive becomes more and more viscoelastic. In addition, when the amount of C5 is 50 parts, the compatibility with SBS blending system is the best, that is, the compatibility with PB phase is better.

One of the prerequisites for choosing a tackifying resin is to be fully compatible with other raw materials in the formulation system, especially elastomers. A tackifying resin with good overall performance should also have a narrow molecular weight distribution, be polar, have low odor, be light in color, have good aging resistance and be inexpensive.

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