How to Choose Hardcoats?

Every lens surface reflects light &#; an inevitable fact of life that can often prove to be very irritating. But a remedy is at hand. A high-quality AR coating not only ensures that you look better by enhancing the visibility of your eyes, but it also eliminates most distracting reflections. AR coatings considerably facilitate night driving or reading. And anyone looking at you also benefits from an AR coating: as there are hardly any reflections on the front surface of the lens, they have an unhindered view of your eyes. A high-quality AR coating consists of various layers that interact to provide the optimum effect. In this way, reflections caused by light of various wavelengths and from different directions are minimized and reduce eye fatigue.

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There is a wide range of transparent coatings available commercially to provide plastics with some level of protection against damage due to abrasion. The market for protective coatings is thought to be worth at least $4bn, and there are a number of generic systems, including:

• Polymer coatings
• Silicones
• Vacuum deposited ceramics
• Nanocomposite/hybrids

The largest class of coatings in terms of volume and value are the polymer systems, frequently based on urethane, acrylic, epoxy, ester compounds or some combination of these. In general terms, however, these systems tend to serve the lower added value sector of the market and are consequently relatively low cost whilst providing a moderate level of protection.

Superior protection to the underlying substrate is provided by silicone hardcoats. (The term hardcoat is applied to any system that gives a level of abrasion protection beyond a threshold value. This value is an increase in haze (loss of transparency) of less than 15% after 500 cycles of Taber abrasion). Silicones are the benchmark for commercially available high performance coatings, since they can provide very high abrasion resistance and also protect the substrate from weathering.

The choice of which system to select is determined by both economic and technical considerations. The technical criteria for selection are determined by considering the operational environment of the coated component, and the predictable excursions beyond normal conditions. More critical components have a wider range of criteria that must be passed. Some of these criteria may nominally seem mutually exclusive, for example, it is desirable to have a coating that is both flexible and abrasion resistant. In practice, thresholds are set that are component specific and so the required performance characteristics are representative of the real-life conditions to be faced.

As the end-user demands more from their components, there is a need to further develop protective coatings to keep pace. There are two generic approaches to this issue; the refinement and improvement of existing coatings, or the development of new materials. For example, the silicone hardcoats are the most widely used hardcoats for polycarbonate and acrylic. These coatings have been in use for many years and there are many variations available for specific applications. Significant research work has been undertaken to try to improve the performance of these coatings. The properties of interest include:

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• Abrasion resistance
• Adhesion
• Chemical resistance
• Visible transparency
• Ultra-violet stability/opacity
• Flexibility

The basic chemistry of silicones is well known and closely defined. Modification to the properties to achieve future enhancements will be made not by changing this chemistry, but by incorporating additives that have a functional role, or by changing the evolution of the network during the coating process.

The development of new materials with superior properties, has been one of the challenges taken up by many research groups, including TWI. A common approach is to fabricate organic-inorganic hybrid materials with the aim being to combine the constituents to allow them to behave as an homogeneous whole (and retain properties such as optical transparency), whilst each constituent can exert its influence to modify specific properties, for example increasing the inorganic content to improve abrasion resistance.

TWI has developed, Vitolane TM , a technology platform that allows nanostructured organic-inorganic hybrids for use in coating formulations to improve abrasion and scratch resistance.

Further information

FAQ: Why do plastics need hardcoats?

FAQ: Why replace glass with plastics?

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