Coatings of many colours

C&I Issue 8, 2011

Following the economic downturn, the global optical coatings market is expected to resume steady growth in coming years, rising from estimated sales of $6.35bn in 2010 at a compounded annual rate (CAGR) of 0.8% to reach $6.77bn by the year 2015, according to Global Industry Analysts (GIA). The US is the largest market with sales of $2.77bn in 2010, while Europe is poised to register the fastest CAGR of 2.2% over the period 2007-2015 (Table 1).

A key growth driver is the replacement of incandescent bulbs with other lighting alternatives, together with demand from flat screen displays, the improving construction industry and green windows and solar energy as well as military and defence systems.

Transmissive or antireflective coatings are the largest segment, with estimated sales of $2.78bn in 2010. However, coatings used in flat screens and advance cameras provide the highest margins. Filters, including electrochromic and infrared coatings, are expected to register the fastest growth, with demand increasing at a CAGR of over 4.0%.

During the global recession, smaller coatings producers struggled. As a result, this sector is currently undergoing consolidation with numerous M&As, according to GIA. Coatings houses with an innovative edge are expected to be most successful. ‘Thin film coatings can be used to manage the full electromagnetic spectrum of light from ultraviolet to infrared by selectively adding or deleting portions of the spectrum and thus tailoring the light from a given source to shift the colour temperature, chromaticity, light or thermal properties,’ observes Robert Naum, chief technology officer at Applied Coatings. ‘Companies that can develop thin film designs and processes to provide unique engineered thin film coatings to solve specific application requirements will definitely have an advantage in the marketplace.’

coloured thingsHighly sophisticated coatings take other factors into consideration, including the shape and position of the light source and reflector in the lamp, the type of bulb and the properties of the filament and its location in the bulb. The morphology of the coating itself can affect the fundamental properties of light, according to Naum. A highly crystalline film could have extensive internal reflections or absorption, while an amorphous system may typically exhibit very different reflective properties. Dislocations and physical discontinuities in the coating will affect the absorption, reflection and transmission characteristics.

Surface properties of the substrate are important as well, and affect the nucleation, formation and bonding of thin film coatings. Substrate geometry, the position of light sources, the angle of incidence of light beams to the thin film surface and whether a surface is smooth, faceted or stippled surface will have an impact on management of the spectrum and be dictated by the end use application.

Thin film coatings typically are composed of up to 40–50 different metal species, mostly oxides, nitrides, borides or silicides. Common metals include tantalum, niobium, silicon, titanium, halfnium, zirconium and germanium. Oxides are used in ranges of stoichiometries, such as Al2O3, Al1.8O2.2, etc. For highly engineered coatings, several layers of different materials are deposited on a substrate. Materials are chosen based on their refractive indices, and the thickness of each layer is also important, according to Markus Bilger, a product line manager at JDS Uniphase (JDSU), based in Milpitas, California, US.

Antireflective coatings are found on eye glasses and computer screens to reduce glare and also are widely used in solar applications to increase the amount of light captured by solar cells. Reflective coatings, which are largely based on aluminum and silver, on the other hand, manage the heat load of a light source. ‘Hot mirrors transmit visible light but reflect the rest of the solar spectrum (IR and UV) so that heat is reflected away, while cold mirrors only reflect the visible region,’ Bilger explains. Reflective coatings find use in all types of instrumentation, optics and medical lighting applications.

Applied Coatings, located in Rochester, New York, US, has developed coatings for reflectors used on emergency vehicles that provide more intense blue and red colours, making them more visible. Spectral Metal from Deposition Sciences (DSI), based in Santa Rosa, California, US, are coated stainless steel sheets that can be cut and formed to specific sizes and shapes after the coating is applied, reducing the complexity and cost of assembly, according to engineering supervisor David Gray.

Hot mirrors are also being developed for use with conventional incandescent light bulbs to improve their energy efficiency. ‘Placing a hot mirror around the filament in a bulb causes the heat given off by the filament to be reflected back onto the filament, enabling the same level of light output at lower energy consumption,’ says Sternbergh. The company’s EcoWhite Silver thin film coating provides up to 14% improved light output but has a silver content about 50% less than typical protected silver reflectors, so reducing costs.

Colour correcting films are often used to make artificial light more like natural sunlight. ‘Artificial light is often more yellowish than natural light, and in certain applications, it is desirable to have indoor light be more like natural daylight,’ says Gray. Optical films can make the correction by preferentially allowing more blue light to pass through to compensate for the additional red generated by the light source. Thin film coatings can also convert the ‘cold’ light of fluorescent bulbs to be like the ‘warm’ light observed with incandescent lights sources and modify the colour of LED lights.

In medical applications, particularly operating rooms or diagnostic instruments, in addition to colour temperature shifting, optical coatings must also be able to provide a highly consistent light source, which can be a challenge when different bulbs are used within a lamp. ‘In such applications, several bulbs are often included in order to have replacements if one bulb burns out,’ Naum says. ‘However, the location of the point source is thus variable, but the coatings must still provide the exact same properties of the light regardless.’

These types of coatings are examples of band pass filters that allow certain wavelengths of light to pass through; these filters can be very narrow, focused on a specific wavelength, or very broad, covering say the visual region of the spectrum. Genetic sequencers and gesture recognition technology use filters to filter out ambient light so that either fluorescence or specific reflected light can be detected. Optical filters in satellites enable analysis of gas plumes on Mars and protect external solar cells from excess heat when facing the sun. Optivex films from Applied Coatings also prevent UV degradation of artwork in museums while maintaining the desired colour attenuation of the lens.

A goal at JDSU is to develop optical thin film coatings that mimic human light perception. ‘The human eye response curve is well understood and our goal is to develop optical coatings that make it possible for devices to provide output that matches that curve,’ Bilger remarks. The company recently developed new microlithography or patterning processes that allow products to sense and adjust, according to ambient light conditions. End products include smart phones with lighted screens that fade out when pressed to a caller’s ear or adjust the lighting level and colour balance and intensity displays, according to external conditions.

At Applied Coatings, much work has focused on optical films that also serve other functions. ‘These films must transmit colour or reflect portions of the spectrum while simultaneously being responsive to other properties, such as the electrical, mechanical or thermal characteristics of a system,’ Naum notes. ‘This type of application is an example of highly specialised, multifunctional optical coating.’

Other research is focused on improving the stress resistance, durability, density, colour and other properties of thin film coatings with new starting materials and advance processing techniques. ‘Thin films have been beneficial ever since artificial lighting has been used. They make it possible to customise light for individual applications, and therefore there will always be demand for optical coatings,’ Gray concludes.

Fibre providers

The first optical fibres with attenuation less than 20dB (decibels)/km, and the process to produce them, were developed by glass and ceramics firm Corning in 1970 and they were being commercially produced and protected by DeSolite UV curable optical fibre coatings by 1977. The DeSolite technology is now owned by DSM. Demand for optical fibre has been increasing since 2001 and, with the volume of data transmitted over broadband networks expected to increase more than 10-fold by 2015, the market will remain robust. To date, according to Steven Schmid, an application development director with DSM Desotech, worldwide deployment of optical fibre has already exceeded more than 1.5bn km. China has become one of the fastest growing regional producers of optical fibre; however, manufacturing does still take place in the US and Europe as well as in India, Brazil and Russia.

25 optical tableOptical fibres are approximately 125μm in diameter — roughly the size of a human hair — with a core diameter of 8-62.5μm, depending on the product and application. Inner core and outer cladding glass layers have refractive indices (RI) that differ by about 0.05%. The inner region has higher RI due to doping of the silica with germania, which keeps the light inside the core and allows it to be transmitted through the fibre, according to Ian Davis, product line manager for high data rate products for Corning. The fibres are drawn from a glass rod with inner and outer regions that reflect the composition and refractive indices of the core and cladding of the finished fibres.

Coating application is critical to coating performance, notes Davis. The glass must be properly centred. The thickness and uniformity of the coating layers are also important. Corning uses its proprietary production methods and UV curable CPC protective coatings, which are custom formulated and supplied solely to Corning by DSM.

Once the coating process is complete, the fibres are wound onto a spool for storage. When ready for use, the fibres are then unwound from the spool and typically coloured with UV curable inks before being placed into 2-3mm O.D. plastic tubes, each containing up to 12 fibres, and each of a different colour. The colours serve as an identification system. Multiple tubes are then fabricated together into a cable. Alternatively, fibres can be bonded in a planar array, commonly referred to as a ribbon, using a UV curable matrix material. Multiple ribbons can then be inserted into a cable.

speakersCoatings for optical fibres generally contain monomers, oligomers, at least one photoinitiator and certain additives, such as adhesion promoters, and are based on urethane acrylate chemistry, which affords both toughness and elasticity. The oligomers are typically prepared from hydroxy functionalised acrylate terminated polyurethanes, which in turn are synthesised from polyols and diisocyanates. The choice of acrylate, isocyanates, repeating units in the polyol and overall molecular weight determine the modulus, viscosity, cure speed, water and oil resistance and other desirable properties of the coating. The liquid coatings are converted to solid films through free radical polymerisation upon exposure to UV irradiation.

When very high temperature (>300OC) resistant coatings are required, polyimide-based coatings are used. These must be thermally cured, which is a slower process, and are thus limited to specialty applications, such as in oil well logging. Fibre lasers for use in various industrial and medical applications require optical fibres coated with low refractive index fluoropolymers, which are often UV curable.

For most optical fibres, the primary coating is a rubbery-like material, with a low modulus, low glass transition temperature and high elongation, and is formulated to adhere to the glass under a range of environmental conditions. The outer optical fibre coating protects the soft inner coating and is a much stiffer, highly crosslinked material. It also provides mechanical protection and creates a slick handling surface.

The coatings are designed to cushion the fibre and decouple it from outside forces that cause microbending attenuation. Microbending, or localised deformations in the fibre core of tenths of a millimeter or less, occurs as a result of non-uniform external forces such as may be encountered in fibre production, cabling, installation or changes in temperature. If microbending occurs, the light can be deflected from its path, leading to loss of signal.

Both layers of coating must also be easily stripped and removed for manipulation of the fibres during processing, but otherwise maintain adhesion to the glass. And they must be compatible with the gels often used inside cables to protect the fibre against moisture.

‘Overall,’ stresses Davis, ‘the coating must protect the fibre attributes of bend and attenuation and protect the glass during further processing, installation and while it is in-service. While the coatings are very important for maintaining the performance of the optical fibres, it is the glass fibre itself that provides the characteristics necessary for high speed voice, data and video communications, and the coating is there to ensure that the fibre maintains those key properties.’

Cynthia Challener is a freelance science writer based in Hardwick, Vermont, US.

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