R&D StoryAl-Plastic direct-bonding
Showa Denko's Al-plastic direct-bonding technique shows infinite possibilities to bond different types of materials such as metal and resin without using traditional adhesive agents or screws. The technique overcomes the problem that traditional adhesive agents take time to cure, and it can successfully bond a metal to polycarbonate (PC) resin, which was considered difficult to commercialize.
- What prompted the development?
Most products used today, such as automobiles, smartphones, and electrical appliances, include different types of materials. Conventionally, these materials are joined with screws or adhesive agents. But recently, with growing needs for lighter and smaller parts, attention has been focused on the technique of directly bonding materials together.
Currently, bonding is mainly done using adhesive agents, but curing at room temperature takes a long time, sometimes several days. However, if the temperature is increased to shorten the curing time, the length of the material is changed as rails lengthen in summer, and when the temperature falls, the material returns to its original length. However, some products using multiple materials with different linear expansion coefficients may become deformed and the strength or reliability of the adhesive may deteriorate.
Adhesive bonding has several problems: one is the adhesive agents have limited pot life, another is some of them must be stored at low temperature. So, there are many restrictions. Although these restrictions can be overcome by time and effort, doing so increases the product cost. To deliver products more quickly without raising the cost, Showa Denko decided to pursue the direct-bonding technique.
- What direction did you set for the development?
The popular direct-bonding technique creates minute dimples on the metal surface before making an injection molding of heated resin for physical bonding. However, the high-viscosity-material does not fully get into them, so that the method results in lower bond strength.
PC resin, which is well known and used for automobile headlamps, smartphones, and many other products, is a high-viscosity-resin and it has never achieved a high bond strength with direct-bonding. However, because the material is used so widely in daily life by so many people, there must be huge potential needs for direct-bonding with PC resin. So, we decided to focus the technical development to make a huge impact in the world.
- How did you carry out the development?
For other project, we had been developing a resin-coating-technique to improve the corrosion resistance of aluminum products which was on the way of commercialization. It is usually difficult to bond aluminum to resin, but we thought that the technique, which achieves a high bond strength through chemical bonding, would lead direct-bonding with different materials. Using the technique for a primer to achieve direct-bonding, we first tried bonding thermosetting resin with aluminum and obtained good results. This gave us confidence, so we next tried to bond PC resin to aluminum with this kind of primer. However, we could never achieve it. When we opened the injection mold, the material dropped out without bonded … we were stunned. We eventually tried 70 patterns of primer compositions, but all were ended in vain.
We have studied thermosetting resins for a long time and tried to apply the theory about them to other resins. But we failed again and again, and so finally decided to choose a completely different primer structure. We thought that by devising a special structure of a primer to chemically bond the primer with aluminum, and using molecular diffusion to bond the primer with PC resin, we could establish a strong layer.
This method worked well. After several trials, we found the solution which was very excited that no one else in the world had ever been able to do.
- What is a key to solve the problem?
Many companies have been researching metal-PC direct-bonding, and most of them focused on studying the surface treatment of metals. We have both business, aluminum and resin, so that we can discuss on how to make adequate structures of metal and resin bonding.
Furthermore, our researchers are constantly acquiring knowledge in various fields to assist development. Molecular diffusion was not on our primary research field, but our researchers are always gathering knowledge in various fields through reports and lectures, and finally came up with good idea. This is the main reason why we could find a solution from a different viewpoint.
- What are the advantages of the developed Al-plastic direct-bonding?
In this technique, a primer which is optimized for the resin to be bonded provides sufficient bond strength. Therefore, PC resin, which is high-viscosity-resin, and PBT or nylon, which is a crystalline resin with low viscosity, can be bonded. Polypropylene, which is a non-polar material that is difficult to bond even with adhesive, can also be bonded with a practical level of bond strength.
The technique will greatly help us replace adhesive agents which take much care for stocks. We should be able to simplify manufacturing processes and reduce costs.
- What will you develop in the future?
Currently, we are trying to directly bond various resins that are thought to be difficult to bond. Our development team with a wide range of technologies and applications, such as organic and inorganic materials and aluminum, are systematically cooperating with each other. We will work on new developments while listening to customer needs.
- Koji Maekawa
Project Manager of Direct Bonding Project
- Hidehiko Higashimura
Director of Corporate Marketing