Protective Shield for Screens
The touchscreens of smartphones and tablets are crucial human-machine interfaces, functioning as both displays and control elements. Their surfaces are shielded against damage by covering them with protective films that incorporate self-adhesive silicones from WACKER.
Natural color expression and retina displays along with low energy consumption: manufacturers don’t spare the superlatives when extolling the screens of their smartphones, tablets and the like. The screen surfaces are the all-important human-machine interfaces, functioning as both displays and control elements. Phoning, taking high-res snaps, or gaming – everything you do pivots around the touchscreen. The screen’s quality therefore plays a key role in the purchasing decision.
“It is becoming increasingly important to protect high-quality smartphone screens against scratches and knocks, as these devices are often very expensive.”
Dr. Timo Hagemeister
“Protecting high-quality screens against scratches and impacts is becoming ever more important in view of the high price of many smartphones,” explains Dr. Timo Hagemeister, until recently silicones development manager at WACKER in Shanghai (China) and now head of the Group’s Consumer Care business team. “The equipment manufacturers protect them with thin films, which are permanently applied to the surfaces during production.” At the same time, consumers can do their touchscreens a favor by applying an additional self-adhesive protective film, which guards the display against minor accidents and extends its lifetime. Hardened plastic films or glass protectors are also commercially available. They can be made extremely thin, as well as robust and flexible and even defend the display against hot cigarette butts or violent shocks when the device is dropped, for example.
Release Force Is Key
Source: Gartner, May 2017
Whether the protective films are applied by manufacturers or consumers, or films are employed permanently or as temporary protection during transportation, it is essential to bond them effectively on the surface. For this purpose, the films are treated with pressure sensitive adhesives (PSA). These self-adhesive coatings only need to be pressed on gently. To enable the films to be easily peeled off completely as required, the release force of the PSA film is adjusted precisely. The release force indicates how much force is required to peel a film off its backing.
The manufacturers decide whether it has to be easy or difficult to peel off, depending on the specific requirements. The best systems should be capable of being individually tailored to the application. With our new DEHESIVE® PSA 765 silicone, that is exactly what we have achieved,” explains Ki-Eon Kim of the Group’s applications technology center in Seongnam-si, near Seoul in South Korea. Like his colleague Timo Hagemeister, the WACKER expert works in the beating heart of the electronics industry. East Asia is not only where many smartphone manufacturers have their production sites, but is also the home of their subcontractors – including protective-film manufacturers.
The exact properties of the self-adhesive films are determined by their structure and chemical composition. The protective films have a multilayer sandwich structure. A key role is played by the PSA layer, “It may be made of polyacrylic, polyurethane or rubber – or silicones, as in this case,” explains Hagemeister. The PSA layer is usually between 15 and 25 micrometers thick, and has to perform several different functions, such as anchoring firmly to the glass surface of the display. At the same time, the layer must remain bonded to the backing material, usually a polyethylene terephthalate (PET) film. The adhesive film is thus the connecting element between the PET layer of the film and the glass surface of the smartphone.
“However, the materials have very different chemical and physical properties,” says Hagemeister. “That is why it is a challenge to formulate the silicone PSAs such that they remain anchored to both materials – but with different adhesion strengths.” It should be possible to strip the adhesive film off the glass surface, though not off the PET layer. Manufacturers of protective film need a higher adhesion strength here.
This is the balancing act that the new product DEHESIVE® PSA 765 is required to perform. Its properties have convinced a South Korean protective-film manufacturer – which now uses WACKER’s silicone PSAs.
Market share of different PSA grades (in the markets of Greater China and South Korea) – WACKER estimate
“Silicones are more expensive on the whole. That is why they only command some 8% of the market. Our customers therefore only use silicones where they offer a performance advantage,” says Kim, explaining the challenges facing developers. “In practical terms, our silicone PSAs show ideal behavior in the production process. To ensure rapid processing workflows, the viscosity and curing behavior must be precisely matched,” adds Hagemeister.
Other important criteria that the protective-film manufacturers use to choose their adhesive suppliers include storage stability, abrasion resistance and surface adhesion. In addition, the handling also plays an important role. “The adhesive layer must ensure that the protective film conforms to the display surface without troublesome air bubbles,” says Kim. The silicones achieve all that – as well as another advantage: they do not yellow and they retain their properties both at high and low temperatures.
The chemical and physical properties of the release coating can be tested at WACKER’s technical center in South Korea.
Silicone PSAs are multicomponent systems, including high-adhesion and less high-adhesion silicones, various additives and a platinum catalyst. This catalyst triggers the crosslinking of the silicones to form a three-dimensional polymer structure. This step only takes place during manufacture of the protective films, after the silicone PSA has been applied to the backing material. “The platinum catalyst is thus sold as part of the product – and makes up a significant share of the total price. The less precious metal that is needed for the curing reaction, the better,” explains Hagemeister. “We managed to reduce the platinum content of DEHESIVE® PSA 765 to the point where we have a competitive product that performs very well on various PET films, and which protective-film manufacturers can tailor to their requirements,” he adds.
We exploit the special talents of silicones here, since, by artfully functionalizing them – that is to say introducing special chemical groups and combining different silicone grades – we can produce complementary properties, with low or high adhesion, and flexible or stiff characteristics. The reason lies in their chemical structure. Whereas silicone fluids consist of linear molecular chains, silicone resin chains are highly branched, and introduce a high degree of crosslinking into the adhesive layer. And that has an effect on the release force: where the degree of curing is higher, the film remains strongly bonded. Thus, the release force can be precisely adjusted by adding silicone resins. This opens up a wide range of applications for DEHESIVE® PSA 765 coatings on protective films: in display production as a whole, various protective films are used – for example to protect the components against damage during manufacture. “Our goal is to develop a product family that supplies the entire value chain with solutions for self-adhesive films,” explains Kim, summarizing his future development ambitions.