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Application of thermally activated water-based polyurethane adhesive in automotive trim

Application of thermally activated water-based polyurethane adhesive in automotive trim

  • 2023-03-28

Basic components of thermally activated water-based polyurethane adhesive.

Some people refer to thermally activated water-based polyurethane as water-based polyurethane hot melt adhesive, which has some rationale in terms of adhesion mechanism. Hot melt adhesive is solid at room temperature, and when heated to a certain degree of melting, it can be applied to the surface to be bonded and cooled. On the other hand, thermally activated water-based polyurethane is first applied to the surface to be bonded, then dried in a heated oven or at room temperature to become a solvent-free solid. It is then heated for activation, bonded, and then insulated or cooled. Unlike hot melt adhesive, during the activation process, the soft segments of the polyurethane chain transition from a crystalline state to an amorphous state, while the physical crosslinking of the hard segments remains intact and the polyurethane molecule chains do not undergo relative slippage. In simple terms, the non-adhesive glue becomes adhesive after being heated, and can then be bonded to the material. The temperature at which this transition from non-adhesive to adhesive occurs is the theoretical minimum activation temperature.

Thermally activated water-based polyurethane is mainly composed of high molecular weight polyols, isocyanates, hydrophilic chain extenders, and a small amount of organic solvents.

High molecular weight polyols usually use polyester polyols, which are also one of the core raw materials and relatively complex.

Isocyanates include IPDI, HDI, HMDI, TDI, and MDI, and for thermally activated water-based polyurethane, IPDI and HDI are generally used in combination. If only HDI is used, the initial bonding strength of the product is appropriate, but the pre-polymer viscosity is high, the heat resistance of the product is poor, and the final bonding strength is weak.

Hydrophilic chain extenders such as DMPA or DMBA can meet the product requirements when the solid content is less than 45%, but if the solid content needs to be further increased, sulfonic acid salt-type hydrophilic chain extenders need to be used.

Organic solvents: In the synthesis of prepolymers, a small amount of high-boiling point solvents are generally added to reduce the viscosity of the prepolymers, such as N-methylpyrrolidone and N,N-dimethylacetamide. In the acetone synthesis method, a relatively large amount of acetone is added, and after emulsification and dispersion, it is removed by vacuum distillation. The recovered acetone can be reused, which can reduce costs and emissions.

Thermally activated water-based polyurethane is widely used in the field of automotive interior trim adhesives due to its environmental friendliness. When used, it is generally combined with water-dispersible isocyanate curing agents, with a mixing ratio of 100/4.5-6.5. After mixing, it is evenly sprayed on the surface to be bonded (single-side/double-side spraying), and the amount of adhesive is generally controlled at 80-100g/cm3. After drying and activation at 50-70°C, it is then bonded under certain pressure and temperature conditions. Finally, it is stored at room temperature or slightly higher temperature for at least 24 hours, allowing the curing agent, resin, and substrate to fully cure and crosslink.

The performance indicators and representative products of thermally activated water-based polyurethane are determined by the molecular structure adjustment of the products. The selection of a suitable thermally activated water-based polyurethane adhesive product depends on the specific application. For leather, PVC and other substrates with poor heat resistance, products with low activation temperatures should be considered first. For high-temperature applications, heat resistance is more important. For bonding of special-shaped profiles, initial bonding strength and other factors should be considered.

The lowest activation temperature and heat resistance are important parameters of thermally activated water-based polyurethane. The crystallinity and crystallization rate also have a significant impact on the bonding strength and other performance of the product. The molecular structure of the product can be analyzed from a structural perspective, which can guide the selection of thermally activated water-based polyurethane.

© Copyright: Anhui Dowell Huatai New Materials Co., Ltd All Rights Reserved.

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