Apple consumers are deeply attached to their cool-looking devices. Colour is an important part of this, so it’s not surprising that one or two recent filings by Apple, with the US patent office, have colour as their basis. Apple wrote in one application that it needed to make cost-effective and strong metallic surfaces that could be dark grey or black.

Apple Looks At Metal Bonding Adhesive Solutions For The Future

Cost-effectiveness a key constraint

The cost-effective point is a key one. Dark metals can be produced already in conventional ways by using zirconium or its alloys. But these materials are heavy, costly and difficult to work with during the manufacturing process. Titanium alloys, by contrast, are stronger and lower in weight.

There are various ways of finishing the surface of an object made from a titanium alloy. The surface can be anodized – an electrolytic process, which makes the surface more suitable for paint and glues than a bare metal. Alternatively, the surface can be treated with a coating such as a physical vapour deposition (PVD). This is a plating or coating which is applied to produce a colour. A PVD chromium carbide layer on a titanium or steel base layer will give dark colouring. However, no one has tried to use an oxide on a modified titanium surface.

Apple Looks At Metal Bonding Adhesive Solutions For The Future

Pores in the metal, to receive glue

At the same time, Apple wants to solve a plastic-to-metal bonding problem. The idea is that there will be pores or spaces in the metal and that these will contain a material that bonds better with the plastic – in technical terms, creates a mechanical or chemical bond that has more ‘bond energy’ than a simple bond of plastic with metal.

This infill material may even be an adhesive, for example, a metal bonding adhesive that is cured by heat. There is an increasing number of custom adhesives, an example is electronically clean adhesives – and these sophisticated adhesive products are now available online at a company such as

The pores in the metal could be made through a chemical etching process or by other methods. One advantage would be that this plastic-over-metal process will make the object waterproof so that it won’t corrode. In a note to their filing, Apple says it may make the object more impact resistant. Things the consumer will welcome.