Application and Market Status of Hot Melt Adhesives in Aluminum-Plastic Composite Pipes

Aluminum-plastic composite pipe is a new type of pipe that combines the advantages of both metal and plastic. Its typical structure consists of an inner and outer layer of polyethylene with a middle layer of aluminum alloy, with each layer bonded together by a specialized hot melt adhesive. This structure endows the composite pipe with the high strength of metal and the corrosion resistance of plastic, while also offering good flexibility and bend-without-springback characteristics. It has found wide applications in hot and cold water transport, floor radiant heating, and gas supply systems.

However, there is an inherent contradiction between aluminum and polyethylene: the aluminum surface carries polar groups, while polyethylene is a non-polar material. They are thermodynamically incompatible and cannot form a strong direct bond. This necessitates a specialized hot melt adhesive as an intermediate transition layer to firmly bond the inner and outer plastic layers to the aluminum core, thereby ensuring the full performance of the composite pipe.

I. Technical Approaches for Bonding Resins

The core technical route for aluminum-plastic pipe hot melt adhesives is maleic anhydride (MAH) graft-modified polyolefins.

Conventional polyolefins contain no polar groups and cannot effectively bond to metals. By introducing polar groups such as maleic anhydride onto the polyolefin molecular chain, the hot melt adhesive gains the ability to chemically bond with the metal surface. The MAH groups can chemically react with or form strong hydrogen bonds with the hydroxyl groups and oxide layers on the aluminum surface, achieving a firm chemical anchorage; meanwhile, the polyolefin backbone is fully compatible and entangles with the inner and outer polyethylene layers.

This dual function—anchoring to the aluminum layer on one end and compatibilizing with the plastic layer on the other—makes the specialty hot melt adhesive a molecular bridge connecting metal and non-metal. MAH-grafted polyethylene is suitable for hot-melt adhesive coatings in aluminum-plastic composite pipes, offering good adhesion properties and processability.

II. Key Performance Indicators

The quality of the hot melt adhesive directly affects the service life of the pipe. In practical applications, early issues with delamination and leakage in aluminum-plastic pipes were largely attributed to insufficient heat resistance and long-term durability of the bonding resin.

The industry has established clear requirements for key performance indicators of aluminum-plastic pipe hot melt adhesives. Peel strength is the core indicator for measuring bond strength, and is typically required to reach a high level. In addition, the Vicat softening point of the hot melt adhesive is generally required to exceed 100°C, and volatile content should be below 0.4% to reduce bubble formation and odor issues. Elongation at break must be no less than 400% to ensure that the adhesive layer does not crack during pipe bending and processing.

From a manufacturing perspective, the production of aluminum-plastic pipe hot melt adhesives involves multiple steps including dissolution, premixing, and reactive extrusion. The mixing temperature must remain below 60°C to prevent decomposition of the initiator, while the extrusion reaction temperature is controlled in the range of 160–250°C. This continuous automated process offers high efficiency and is essentially pollution-free.

III. Differentiated Requirements for Different Base Materials

Aluminum-plastic composite pipes have varying requirements for hot melt adhesives depending on the base material of the inner and outer layers.

PE-RT aluminum-plastic pipes are the most widely used type, with inner and outer layers made of heat-resistant polyethylene. The matching hot melt adhesive is mainly based on MAH-grafted polyethylene, with peel strength requirements typically above 120 N/25mm.

PP-R aluminum-plastic stabilized pipes represent a rapidly developing direction in recent years. PP-R pipes suffer from thermal expansion deformation when carrying hot water, which can be effectively overcome by combining with aluminum, offering a wide temperature and pressure range. Since PP-R is non-polar and contains no reactive groups, bonding to aluminum is more challenging and requires a PP-based hot melt adhesive. The hot melt adhesive for PP-R aluminum-plastic pipes is primarily composed of MAH-grafted atactic polypropylene, requiring not only good adhesion to aluminum but also excellent compatibility with the PP-R layer.

IV. Localization Progress and Market Status

Aluminum-plastic composite pipe technology was first proposed by British scholars in the late 1970s and achieved industrial production in Germany and the UK in the late 1980s. China began introducing aluminum-plastic pipe products in the early 1990s, but the specialty bonding resins long relied on imports, with annual imports reaching nearly 10,000 tons.

In recent years, with the maturation of domestic MAH grafting technology, locally produced aluminum-plastic pipe hot melt adhesives have gradually achieved import substitution. Domestic manufacturers use reactive extrusion grafting processes to melt-graft polyethylene with maleic anhydride, and the comprehensive performance of the produced bonding resins is progressively approaching that of international counterparts.

Shanghai Jiuju Polymer Materials Co., Ltd. has accumulated proprietary technical expertise in this field. Its bonding resin product series, through graft modification of polyolefins, retains the mechanical properties, low-temperature resistance, and chemical corrosion resistance of polyolefins while imparting excellent adhesion to aluminum. Product features include strong adhesion, durable bonding, and good processability, and they are widely applicable in aluminum-plastic composite pipes, aluminum-plastic panels, multi-layer co-extruded composite pipes, and other areas. The company's technical approach encompasses multiple mechanisms including chemical reactions (covalent bonds and hydrogen bonds), anchoring effects, and diffusion effects, ensuring reliable and long-lasting adhesion.

Conclusion

The reason why aluminum-plastic composite pipes cannot do without specialized hot melt adhesives lies in the contradiction between the non-polar polyethylene and polar aluminum within the "polyethylene-aluminum-polyethylene" structure. MAH-grafted polyolefin technology, by constructing a chemical architecture with "one end affinic to aluminum and the other end affinic to plastic" at the molecular level, enables the firm bonding of these two incompatible materials. With the continuous advancement of domestic grafting modification technologies, locally produced aluminum-plastic pipe hot melt adhesives have already met the performance requirements for high-end applications in terms of peel strength, heat resistance, and processability, providing a reliable material foundation for the steady development of the aluminum-plastic composite pipe industry.