Types of polyurethane adhesives for automobiles

Types of polyurethane adhesives for automobiles
China’s automobile industry is flourishing. In 2011 alone, the national automobile production reached 18.7 million vehicles, with a rapid growth rate of nearly 10% per year. The market for adhesives used in the automotive industry is very broad.
Automotive adhesives/sealants can be classified into five categories based on their application areas: automotive body adhesives, automotive interior adhesives, automotive engine chassis adhesives, automotive component adhesives, and automotive manufacturing process adhesives. In 2007, the annual consumption of automotive adhesive in China was about 120000 tons, of which the annual consumption of single component moisture cured polyurethane windshield adhesive was about 5500 tons. In addition, there are still water-based polyurethane adhesives for car roof linings that have not been counted yet. In recent years, the annual demand for polyurethane adhesives in China has increased at an average rate of 30%.

1. The advantages of PU adhesive for automobiles

(1) Good adhesion performance within -40~+100 ℃;

(2) Due to its good toughness, especially at low temperatures, it has excellent impact and vibration resistance, and a long service life;

(3) Due to the presence of a large number of polar and reactive groups, various substrates used in automobiles, such as glass, plastic, metal and other surface smooth materials, as well as various interior and exterior materials and fabrics, have excellent adhesive properties;

(4) By adjusting the formula of PU adhesive, different hardness and elongation adhesives can be made. The adhesive layer can be adjusted from flexible to rigid to meet the bonding needs of different materials;

(5) PU adhesive can be cured by heating or at room temperature, with a simple bonding process and good operational performance. There are no side reactions during the curing process, so the adhesive layer is less prone to defects.

(6) PU adhesive has good properties such as wear resistance, oil resistance, solvent resistance, chemical resistance, ozone resistance, and bacterial resistance.

The development of automotive industry technology requires the lightweight of automotive components, therefore a large number of plastic components are used, especially high-strength FRP (glass fiber reinforced plastic) and SMC (sheet molded composite material), which require the use of PU structural adhesive and sealant for bonding and assembly. With the continuous increase of modern car speed, the safety requirements for car windows and windshields have been raised. The direct bonding process for windshields has been adopted, and a large amount of single component moisture cured PU adhesive has been used. In order to improve the environmental performance of PU adhesive, water-based PU adhesive, solvent-free PU structural adhesive, and reactive PU hot melt adhesive have been developed.

2. Common varieties and applications of polyurethane adhesives for automobiles

(1) Single component moisture cured polyurethane adhesive

The process of directly bonding car windshields to the vehicle body began in the 1960s in the United States. The single component moisture cured PU adhesive was first developed by ESSEX Chemical Company in the early 1970s and successfully applied to General Motors in the United States. In 1976, Audi also applied it to the Audi C2 model. Subsequently, Japanese and other European car manufacturers successively adopted the direct bonding process of windshield. Due to its simple construction and the use of mechanical adhesive, over 95% of the world’s windshield and side window glass are currently bonded using this adhesive.

Single component moisture cured PU adhesive, due to the presence of active – NCO groups, can react with trace amounts of moisture on the adhered surface or in the air to solidify. Single component moisture cured PU windshield adhesive requires sensitivity to moisture, fast curing speed, excellent elasticity after curing, and single packaging with good storage stability. It is a product with high technological content in automotive adhesive, and it is also the most widely used type of PU adhesive in China’s automotive industry.

By using this glass bonding and sealing process, the windshield and vehicle body can be tightly integrated, increasing the rigidity and anti twisting ability of the vehicle body while ensuring the sealing effect. According to Article 212 of the Federal Motor Vehicle Safety Standards (FMVSS) in the United States, when a car collides with a concrete wall at a speed of 50km/h, the adhesion integrity of the windshield must be above 75%. At present, the United States, Japan, Germany, France, and China almost all use this process in the installation of car windshields, and at the same time, adhesive methods are mostly used for the windshields and side windows of passenger cars.

Other application examples of single component PU adhesive: For example, the original design of FAW’s J5, J6 and other series trucks was a flat top steel plate top cover, which was changed to a high top to meet the needs of long-distance transportation with sleeper installation, and the top cover was changed to SMC material, using single component PU adhesive for bonding. In addition, the roofs of FAW’s Xiaohongqi sedan and FAW Volkswagen’s “Bora” sedan are both made of PU adhesive, which integrates the roof and cover. This not only enhances the strength of the roof, but also achieves the purpose of shock absorption and noise reduction.

Single component moisture cured PU adhesive is suitable for bonding porous surfaces. It is usually used in combination with glass activator, glass primer, and paint primer for non porous surfaces such as glass and metal. To ensure reliable adhesion performance between windshield and vehicle body, it is necessary to apply primer on the glass surface, which can improve the bonding strength of PU adhesive on the glass layer.

In order to improve the speed and quality of adhesive application, major automotive companies currently use robotic arms to automatically apply single component moisture cured PU adhesives. The automatic gluing system consists of a gluing pump, a measuring device, a robotic arm, a gluing gun, and a workbench. To ensure the stability of the adhesive application, a pressure plate and adhesive conveying pipeline with a heating system are usually used. When using an automatic glue application system, it is necessary to ensure that air is discharged every time the glue bucket is replaced, prevent bubbles, and ensure the cleanliness of the gun nozzle to ensure accurate glue application.

At present, single component PU adhesive has been localized and partially applied in the automotive industry. However, due to the need for glass activator, paint surface, and glass primer to ensure sufficient adhesive strength when using this single component PU adhesive, and the slow curing speed and cumbersome assembly process, there is a need to apply activator, primer, and instantaneous positioning windshield adhesive. If a heating coating device is equipped during the assembly of windshield glass, the slightly hot adhesive material immediately produces a high initial adhesive strength on the adhered surface, ensuring the positioning of the window glass and gradually achieving complete curing, thereby solving the slow curing speed of PU adhesive and the inconvenience of requiring fixtures for assembly.

The varieties of single component moisture cured PU windshield adhesive currently being applied and developed include:

① High modulus single component PU glass adhesive;

② High contact viscosity single component PU glass adhesive;

③ Pre coating single component PU adhesive and its process;

④ Silane modified PU adhesive sealant;

⑤ Low conductivity single component PU adhesive is used to prevent corrosion of aluminum body and adverse effects on antenna reception.

(2) WPU adhesive for automotive interior parts

WPU adhesive refers to the adhesive formed by dissolving or dispersing polyurethane in water, which has the advantages of non flammability, low odor, no environmental pollution, energy conservation, and convenient operation and processing. In the 1980s, it was first applied in leather finishing, fabric finishing, and coatings abroad, and it was not until the 1990s that it gradually gained application in the bonding of automotive interior components. The solid content of water-based PU adhesive can be adjusted according to user needs, and the viscosity can also be adjusted freely. It can be cured at room temperature or by heat. Nowadays, water-based PU adhesive has been used for the bonding of some automotive components, such as PVC artificial leather for car interiors, instrument panels, mudguards, door panels, floor felt, and roof lining. Another example is the soft roof with PVC film compounded with polyurethane foam or fabric compounded with polyurethane foam.

In order to accelerate the application process of WPU adhesive, FAW Changchun Fuao Johnson Control Automotive Decoration System Co., Ltd. has invested a huge amount of money to introduce a complete set of water-based polyurethane car roof lining adhesive and car roof manufacturing equipment and molds from Italy. It now has the ability to independently produce high-quality and environmentally friendly car roofs and the conditions to match luxury interior parts for the automotive industry.

Our company is the first in China to launch LP-7002A and LP-7002B water-based polyurethane adhesives, which have the characteristics of strong initial adhesion, fast drying speed, high bonding fastness, and strong water resistance. They respond to the requirements of reducing VOC emissions in passenger cars and promoting health and environmental protection both domestically and internationally. We are actively cooperating with major manufacturers to promote them.

In addition, foreign countries are now committed to research on reducing costs and improving performance of WPU adhesive. Leung Pak T Company and others in the United States have developed WPU adhesive suitable for hot forming lamination processes. Its adhesive strength and heat resistance are much higher than commonly used solvent based or other water-based adhesives. The outer PVC film products bonded with it can be used to manufacture automotive instrument panels, door panels, and other interior components. Japan has also addressed the issue of low initial adhesion of WPU adhesive by introducing epoxy resin into WPU adhesive, successfully developing products with good initial adhesion performance.

(3) Two component PU structural adhesive

Structural adhesive refers to an adhesive used for bonding load-bearing structural components, which can withstand large dynamic and static loads, and can be used for a long time. PU adhesives used for automotive structural components are generally solvent-free two-component PU adhesives.

Modern cars widely use plastic and polyurethane materials, and PU adhesive is one of the best adhesives for bonding plastic and polyurethane materials. In 1967, Goodyear was the first American company to successfully use two-component PU adhesive for the bonding of SMC engine covers on heavy-duty vehicles. Subsequently, General Motors and Ford Motor Company in the United States successively used PU adhesive to bond SMC components of large heavy-duty trucks, and then promoted the bonding of FRP (glass fiber reinforced plastic) components. At the same time, it has been successfully used for bonding components such as the roof, doors, water tank brackets, driver’s cab roof, and door panels on trucks. In 1977, the United States adopted a series of polyurethane adhesives for the bonding of car engine covers, driver’s cab covers, door panels and other components. Subsequently, countries such as Japan also adopted this type of technology.

(4) Reactive PU hot melt adhesive

Reactive PU hot melt adhesive is a type of adhesive that, under the condition of suppressing chemical reactions, is heated and melted into a fluid for easy application. After cooling, it solidifies and plays a bonding role. Then, with the help of moisture present in the air or on the surface of the adhesive, it reacts and expands chains to generate high cohesive polymer, further improving adhesion and heat resistance. This reactive PU hot melt adhesive is a thermoplastic resin adhesive containing reactive groups. Its main component is a terminal NCO prepolymer, which is made by adding non reactive thermoplastic resins and thickening resins with isocyanate groups, as well as antioxidants, catalysts, plasticizers, flame retardants, mold inhibitors, fillers, etc.

Reactive PU hot melt adhesive is mainly used in car roofs, instrument panels, trunk lids, side decorative strips, and headlights. In addition, it can also be used for door composite decoration, PP, ABS, polycarbonate car lampshade bonding and fixation, cargo box panel bonding, interior decoration bonding, etc. In recent years, there have been reports on the use of reactive PU hot melt adhesive for direct bonding of automotive windshields, but there is still much research work to be done to achieve the performance of single component moisture cured PU windshields.

(5) Polyurethane flocking adhesive

In recent years, China has established multiple electrostatic flocking production lines. Flocking adhesive is mainly used for flocking on various substrate surfaces in automobiles, such as EPDM, PVC, EPDM sealing strips, ABS, PC, PP, metal and other materials. Currently, there are many types of acrylic adhesive used in the market for electrostatic flocking, but the adhesion strength is poor. Generally, the dry and wet rubbing strength is around 2000-4000 times, while the dry/wet rubbing strength of polyurethane electrostatic flocking adhesive is generally above 10000 times. PU flocking adhesive can be divided into solvent type and solvent-free type, single component and two-component types.

Our company can provide a full range of LP-4001 and LP-4002 automotive sealing strip flocking adhesives, as well as LP-6518 and LP-6519 high solid and water content polyurethane electrostatic flocking adhesives, which have high bonding strength, excellent wear resistance, water resistance, solvent resistance, and independent intellectual property rights.

3. Conclusion

(1) A single component moisture cured PU windshield adhesive urgently needs to be developed to replace imported polyurethane windshield adhesives with fast curing speed, UV curability, and humidity insensitivity.

(2) The domestic automotive industry has not yet established performance standards and specialized testing methods for automotive adhesives/sealants, which not only affects the application process of automotive adhesives/sealants in the automotive industry, but also restricts the standardization and development of automotive adhesive research and production units, making it difficult to establish a complete quality assurance system.

(3) Due to the lack of strict environmental regulations for adhesives used in the automotive industry in China, coupled with cost considerations by automotive manufacturers, the development and application of environmentally friendly automotive adhesives have not received high attention, and solvent based adhesives with environmental pollution are still being used in the automotive industry.

(4) There are many universal automotive adhesive products that meet general usage requirements, while there are relatively few high-tech and high-performance products used for special needs. The independent innovation ability is weak, and the research and development ability needs to be further improved.

Transferred from Chinese adhesives and adhesive tapes
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