Camouflage materials adapting to improved detection
As the means of detection improve, so too must camouflage technology, as Simon Frost reports.
Military camouflage once had a simple purpose – to hide personnel and equipment from the enemy’s sight. But advances in detection technologies have added obstacles to the pursuit of invisibility, and materials scientists are now beginning to discuss new technologies, including adaptive and heat-concealing materials, in the public domain. In March, the Advances in Camouflage Science and Engineering conference, jointly chaired by UK and Swedish members of the IOM3 Defence, Safety and Security Committee, offered a platform for discussion between international researchers.
Hans Kariis of the Swedish Defence Research Agency summarised the wavelengths that camouflaging materials must conceal now and in the near future. Those that can be detected by reflected radiation include the visible (VIS), near infrared (NIR), radar and, increasingly, ultra violet (UV). Emitted radiation – from body heat, for example – can be widely detected on the thermal infrared (TIR), while detection in the short wave infrared (SWIR) is expected to become more common in the next 10–15 years.
‘New detection technologies are making their way onto the battlefield and becoming more widespread because they’re getting cheaper and easier to use,’ Dr Eoin O’Keefe, Senior Technical Consultant at QinetiQ, UK, told Materials World. ‘Key among those at the moment is the thermal infrared.’
While the danger to military vehicles of heat-seeking missiles has been present for many years, dismounted soldiers now increasingly face the threat of manned portable weapons equipped with TIR detection, making the foot soldier’s thermal signature just as important as the visible.
Gilda Santos, Technical Project Manager for the Portuguese Technological Centre for Textile and Clothing Industries, discussed a project being carried out in collaboration with a consortium that includes the Portuguese Army to determine methods for reducing the thermal emissivity of textiles to match the thermal pattern of the environment.
Santos explained that cenospheres – inert, hollow spheres of alumina silicate – or porous bioceramic membranes can mask the wearer’s thermal signature by reflecting radiation, while providing thermal insulation. But physical vapour deposition of metal films is interesting the researchers most, as it can fulfil the brief of reducing the thermal signature by mitigating emissivity more flexibly, and inexpensively, without any significant evaporative resistance and consequent lack of breathability for the wearer.
From dusk till dawn
Key in new camouflage technologies is self-adaptation – materials that respond to their environment to provide the concealment required in sunlight and darkness alike. Environmentally stimulated changes discussed by Dr Peter Hobson, of QinetiQ, included spectral absorption and reflection, emissivity, diffuse or specular properties and thermal and electrical conductivity.
As a means of sidestepping discussion of sensitive, real-world military applications, Hobson presented examples of inspiration from nature, such as the horn-eyed ghost crab (Ocypode ceratophthalmus), the juvenile of which can modify its reflectance and colouration depending on the time of day, and the giant Australian cuttlefish (Sepia apama), which ceases sexual signalling as night falls and hides from its predators by rapidly shifting between countless variations of uniform, mottled and disruptive colouration patterns, depending on its surroundings.
‘Although it has been going on for quite a while, it’s only recently that we have been able to talk about adaptive camouflage,’ his colleague, O’Keefe told Materials World. How long? I asked. ‘That, I can’t tell you. I can tell you that it’s been going on for quite a while.’
Hobson did detail, however, QinetiQ’s work in negatively photochromic pigments – the same premise for technology that turns glasses into sunglasses in sunlight, but reversed so that pigments switch with rather than against the external stimuli.
In collaboration with the University of Huddersfield, UK, the ongoing work identified an intense blue nickel complex (nPC) dye as the system for examination in 2011. The researchers synthesised various coloured nPCs, but incorporating these into textile coatings is a work in progress. They require transformation into a powdered pigment form to be suitable for textile coating.
One method under investigation is microencapsulation – isolating droplets of the liquid dye within a continuous polymeric shell on the sub-micron scale. Two processes have been attempted – in situ polymerisation with melamine/formaldehyde, and complex coacervation, which is widely used in the pharmaceutical industry and involves phase separation of two oppositely charged polymers, such as gelatine and acacia.
Glint and glare
For large equipment and vehicles, the reduction of glint – the transient, specular reflection of a direct light source such as that seen on a car windscreen for a brief moment – and glare – the glow caused by reflections of an extended, ambient light source, such as sunlight spread by cloud cover – is essential.
Simon Childs, of Malvern Optical Ltd, UK, presented the company’s research into hydrophobic matt paints for the reduction of glint. Hydrophobicity, he explained, is desirable in a matt coating, because the surface slopes and rough texture used to create a matt effect typically hold water and remain wet for longer than smooth surfaces, meaning that they become reflective in wet conditions.
Funded by the Centre for Defence Enterprise, UK, Malvern studied a combination of binders and commercial off-the-shelf paints, with variously sized particulates and surfactants. The team measured the hydrophobicity of different combinations using a contact angle goniometer to determine and evaluate water roll-off angles, in addition to gloss meter and ellipsometer tests to examine the gloss and low-angle reflectivity. Finally, BAE Systems, which also supplied the gel-coated composite and rubber substrates for the tests, was enlisted to assess the adhesion of the coatings before and after water immersion and salt spraying.
They found that abrasion and treatment with ethyl acetate and trichloroisocyanuric acid improved the adhesion properties, while the most promising coating achieved a water contact angle greater than 150° and a gloss unit of 0.4GU, reducing the reflectivity of the rubber substrate by a factor of around 100.
O’Keefe explains, ‘The way a surface appears depends on where the light is from and where you are seeing the reflected light from, and it changes with angle, which can be the illumination angle or the receiver angle. How it varies by those two angles is the bidirectional reflectance distribution function. You can see it, for example, on a desk surface – when you look straight down at it, it might look brown, but when you move lower and start to look at it from what is known as the grazing angle, you start to see things reflected in it – it becomes more specular.’
QinetiQ, as O’Keefe discussed at the conference, has been developing matt paints of its own using sharp, angular granules at the surface of the paint. I asked O’Keefe what materials those granules were made from. That, he couldn’t say.