The most important optical properties of coating materials are clarity (transparency), gloss, refractive index and colour. High clarity requires that the refractive index is constant throughout the sample in the viewing line. The presence of interfaces between regions of different refractive index causes scattering of light and reduction in transparency. This can be seen in otherwise transparent coatings containing very fine air bubbles, or matting agents such as wax or silica. Amorphous polymers free from impurities are transparent unless chemical groups are present that absorb visible light. Crystalline polymers may or may not be transparent. Where crystalline structures are smaller than the wavelength of light then they do not interfere with the passage of light and the polymer is transparent. Where these structures are greater in diameter than the wavelength of light then light will be scattered, providing the crystal structures have a different refractive index (i.e. a different density) from that of amorphous regions.
Transparency may be defined as the state permitting perception of objects through or beyond the material (coating). It can be assessed as the fraction of normally incident light that is transmitted with less than 0.1° deviation from the direction of the primary beam. Some coatings, although transparent, may have a cloudy or milky appearance known as haze. This can be measured as the amount of light deviating by more than 2.5 from the transmitted beam direction and is often the result of surface imperfections.
When light falls on a material some is transmitted through it, some is reflected and some is absorbed. Transmittance is the ratio of light passing through to the light incident and reflectance is the ratio of reflected to incident light. The gloss of a film is a function of the reflectance and the surface characteristics of a material. A perfect mirror-like surface, known as a specular reflector, shows one extreme of behaviour. At the other extreme, a perfect diffuse reflector reflects light equally in all directions at all angles of incidence. Several different measures of gloss exist, each defining a distinct aspect of appearance. These include specular gloss, distinctness of image gloss, contrast gloss and sheen. Specular gloss refers to the reflection that occurs at the angle of reflection equal to the angle of incidence of a beam of light. Contrast gloss is the ratio of intensity of light that is reflected at two different angles relative to the surface. Distinctness of image gloss refers to the distinctness of patterns of light reflected from a surface. Sheen is a type of reflection, possessed by velvet, where a matt surface gives rise to pronounced specular reflection at a small angle to the surface.
There is a relationship between viscosity and gloss whereby, generally, it is easier to get a high gloss with a polymer of low degree of polymerization (and low molecular weight). This is because the viscosity (resistance to relative motion within the material) is lower at a given solids concentration. Viscosity is also affected by choice of solvent and by temperature. Viscosity grade provides a convenient index. High viscosity grade material becomes resistant to flow at an earlier stage in drying. Solvent coatings that form an immobile gel at a point when they still contain appreciable solvent tend to form a surface that follows the irregularities of the underlying substrate. If a coating dries with a rough surface its contrast gloss and distinctness of image gloss will be markedly affected. The higher molecular weight PVAC and acrylic polymers tend to give semi-matt finishes whereas dammar, mastic and AW2 tend to produce high gloss surfaces. The nature of the surface therefore depends partly on the ability of the film to level itself. This in turn depends on the nature of the solvents used to formulate the coating. Using solvents that evaporate slowly will reduce the rate at which viscosity rises and allow more time for levelling of the coating before drying. However, it will also allow more time for the coating to pick up dust. Considerable experience is needed for the proper selection of solvents for coatings, particularly when using materials of high viscosity grade. Selection of solvents for coatings may, however, be restricted by the potential effect of the solvent on the underlying paint (Tsang and Erhardt, 1990). A slow evaporating solvent will remain in contact with the paint for longer and there is therefore greater risk of solvent action on the paint. Provided that a polymer material used as a coating is capable of high gloss then it is possible to obtain a full range of effects using different solvents, application techniques and matting agents.
The ability of a coating to wet the surface is important in saturating the surface and allowing colours to be seen, particularly so with porous surfaces. Penetration into porous surfaces is more easily achieved with polymers of low viscosity grade. An appropriate degree of adhesion to the surface is also important. Failure in this respect leads to light being reflected by cracks and fissures. This is often noted with polyvinyl alcohols which generally have low adhesion on most kinds of paint. The interaction of a coating with the wood on a cellular level is difficult to ascertain. Microscopy may show coatings that sit on the top of the wood exhibiting no evidence of saturation. Other coatings may be found to penetrate many cells into the wood tissue. The adhesive properties of surface coatings are similar to those of the same polymers used as adhesives. Some components of a coating such as oils can be drawn into the wood structure selectively due to the heterogenous nature of wood and this can cause uneven ageing characteristics in the coating. Coatings that are water or alcohol based are capable of swelling the wood tissue and thereby penetrating through cell walls.
This is one reason why alcohol-based coatings, or 'spirit varnishes' are known for excellent mechanical bonding with wood.
The refractive index of a coating can affect the appearance of the underlying surface. Refractive index measures the extent that light is bent in travelling from one medium to another (strictly speaking, from a vacuum but air gives a close approximation). As the refractive index of a coating increases, more light is reflected from the top surface, less light escapes from the coating into the air again and less light is reflected at the coating/paint interface. The refractive index therefore affects the success with which the surface is revealed. However, in practice the effect of variations in refractive index is often less apparent than differences in gloss due to the relative smoothness of the upper surface of the coating and the ability of the coating to penetrate and wet out the surface. This point is demonstrated by polyvinyl alcohols, cellulose ethers, and soluble nylon which are, or have been, used as consolidants of loose dry pigment. They have a minimal darkening effect because of their poor adhesive and wetting properties.
Very few polymers absorb radiation in the visible spectrum that is roughly between 380 and 760 nm. Thus, most polymers are colourless. However, some thermosets (including phenol formaldehyde, epoxies, and polyurethanes) absorb slightly more strongly at the blue end of the spectrum and thus appear yellowish or brownish in transmitted or reflected light. These substances contain alternating double and single bonds or aromatic rings which act as chromophores, absorbing light at frequencies corresponding to the excitation energies of bonding electrons. Since the eye is not sensitive to wavelengths of less than about 400 nm it is not necessary for coatings to transmit light of shorter wavelength. Indeed, it is a definite advantage if they do not since this will protect the underlying surface against damage from UV. Fresh dammar resin films absorb nearly all radiation below 280 nm and some of that between 280 and 380 nm. When aged, films of dammar and mastic absorb more of the UV component and some of the shorter wavelength visible radiation. They therefore appear yellow and later even brown thus distorting the true colours of the surface under the coating.
The extent to which a coating is known to yellow is an important consideration in its selection. The methacrylate and vinyl acetate polymers possess good stability to light and do not yellow with age. However they are transparent to short wavelength radiation and have a sharp cut off of transmission in the region of 200 nm. Coatings that are opaque to short wavelengths offer useful protection to painted surfaces in situations where UV levels are not otherwise controlled. Ideally, such coatings should remove all light below 400 nm and none above. Several compounds exist which are designed to filter UV radiation and which are suitable for incorporation in coatings. One problem with their use, however, is that of incorporating sufficient absorber for efficacy in a very thin film without the absorber migrating out of the film. The presence of traces of solvent may provide protection against UV during the drying period.
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