09.03 Coatings and films

Categories: Advanced Glazings

Introduction
Films and coatings are discussed thoroughly in the book by Johnson [1991], and are also discussed in some depth in the article by Valdes [1988].  The following notes summarise the information provided by Johnson and Valdes and also raise some additional issues.

Coatings with different optical or thermal properties may be applied to the surface of the glass as chemical coatings during manufacture (either as part of the on-line manufacturing process or as a second-stage operation) or may be added in the form of adhesive-backed sheets during refurbishment.

It should be noted here that most coatings and films consist of several layers of different materials, and a full discussion of each possible combination is beyond the scope of FACETS.  Furthermore there is a wide variability in the properties of some types of coating, and two coatings which share one property may vary considerably in others.  Therefore this section only considers coatings in general terms.
 

Directly applied mirror (optically reflective) coating
A mirror coating (light reflective) may be used to create glazing which reflects a substantial part of the incident light (note that a coating may be selective, so that a coating which reflects light need not necessarily reflect heat, and vice-versa).  Mirror coatings may be decorative (thin gold-plating for example) or functional (to prevent a view in under day-light conditions) but they should allow a view out, although the view may be coloured and will appear darker.  Note that when back-lit (at night for example) light-reflective coatings may allow a view in (in some cases this has been used to dramatically enhance the architectural effect).

Reducing the amount of light that enters through the glass helps to reduce the heat generation within a building, at the expense of slightly lower natural light levels.

Mirror coatings may be produced as an on-line part of the float glass manufacturing process, or may be added to the glass after manufacture.

Some typical performance values for reflecting glasses are:
 

T = fraction transmitted, R = fraction reflected, A = fraction absorbed
From Button and Pye (1993)

The light reflection of these glasses is not particularly high.  However appearance depends on the relative amounts of transmitted and reflected light.  A 6 mm reflective glass only reflects three to four times as much light as a clear float glass.  However, a 6 mm clear float glass transmits 87% of the building light outwards, and this is significant when compared to the 8% of daylight light that is reflected.  With the reflective coating the reflection of daylight (at least 19% for the 6 mm glasses above) is much greater than the light transmitted from inside the building (10%)

The proportion of energy that is absorbed by these coated glasses is similar to that of body-tinted glasses - an optically-reflective coating does not necessarily reflect heat.  These glasses can therefore reach high surface temperatures, and the same precautions are required as for coloured glass - shield the building occupants from radiated heat and toughen the glass.

Coatings generally have to be protected from damage - although single panes of coated glass are possible they are rarely used, and when they are used the coating is usually on the room-side of the glazing.  Mirror coatings should also be applied to flat glass surfaces or distorted reflections will occur.  The deflection of glass due to thermal expansion or contraction should always be considered, as this will also distort the reflected image, and unevenness in the framing system will also be apparent for large expanses of reflective glass.

The colour shift of transmitted light experienced with many reflective coatings cannot be avoided, nor can the fact that the coating is still present in winter and may prevent useful winter solar gains.

The reflective nature of these coatings does extend into the infra-red region (although not necessarily uniformly), and optically-reflective coatings can help to reduce the U-value of glazing units.
 

Mirror (optically reflective) coating applied as part of an adhesive film
A reflective coating may be applied in the form of a pre-coated adhesive film (this is the only option for retrofit, but it can also be used on new-build).  However, if the glass has a film applied during or after installation then the glass is unlikely to be toughened, and so may be more likely to fail through thermal stresses.  Furthermore the film is thicker than an on-line coating (there is a substrate and an adhesive layer involved) and this will generally let less light through and absorb more solar radiation.

As with any product that can be used in retrofit there is a tendency for the installer to use the product without fully understanding its true implications in terms of absorbed solar energy.  The addition of a film to the room-side of a double-glazed unit is particularly undesirable, as the higher thermal resistance of the glazing air-space prevents the loss of heat from the warmer pane of glass.

Putting a film on the external surface of the glazing will reduce problems of energy absorption, but the film must be resilient, and the user must allow for the accumulation of dirt on the film and possible damage to the film by the cleaning process.

Another issue with adhesive films is the durability of the adhesive.  If the film is applied after the glass is installed then the film is only likely to be fitted up to the edge of the glazing gaskets, leaving a path for cleaning fluids and water to reach the adhesive layer and the edge of the film.  Delamination of the film and separation of the film from the glass could then occur.  The effect of fire should also be considered - if a film has a plastic substrate then it should be of a material that does not give off poisonous fumes or burn freely in the event of a fire.

Films can also be sold on the basis that they improve the security of the glazing and hold the glass together in the event of an explosion.  Both of these issues require that the film is applied carefully and extends to the very edge of the glazing (the glazing must be removed).  If the film is only installed up to the glazing gaskets then in the event of an explosion the glass can be punched out of its frame as a single piece.
 

Low-emissivity (thermally reflective) coating applied on-line during the manufacturing process (hard coatings)
Un-corroded metal surfaces generally have a low radiation emissivity and a high radiation reflectivity; some metal oxides also have this property.  If a metal-based coating is applied to the surface of the glass then infrared radiation can be reflected, and heat transfer is reduced.  The metal coating may only be a few tens of atoms thick, and so it appears transparent when viewed from either side (the coating is thermally-reflective but not optically-reflective).  A typical coating will reduce the amount of infra-red that the surface emits by 80%.

Coatings can be applied during the float glass manufacturing process.  On-line coating has obvious cost advantages over the alternative of having to transport the glass to a coating plant, and higher rates of production are possible.  Coatings which are applied on-line are referred to as pyrolitic coatings, and they form an integral part of the glass surface, being baked onto the glass during production.  These coatings are often referred to simply as ‘hard’ coatings, because they are difficult to remove and can be bonded to directly (this type of coating does not have to be removed when the glass is fabricated into a multiple glazing unit).

Some work has been performed on the etching of hard coatings to produce a grid (spacing on the scale of mm).  This modification of the coating increases transmittance and reduces reflectance.  Lampert and Ma [1992 pp56] suggest that a typical grid etched onto an indium tin oxide (ITO) film saw the total solar energy transmittance increased from 0.8 to 0.9, whilst the reflectance reduced from 0.91 to 0.83.  However, this extra step in the manufacturing process increases the cost of the coating.

The coating material is already an oxide (usually tin oxide or indium tin oxide) and so cannot corrode when exposed to the atmosphere.  These coatings can therefore be used on the exposed surfaces of glass, but this raises issues related to cleaning.  If an exposed low-e coating becomes wet or dirty it quickly becomes a high-e coating because water and dirt have a high radiation emissivity - cleaning may be difficult because hard low-e coatings have a rough surface, and repeated cleaning may result in damage and erosion of the coating.  Erosion of exposed coatings on the outer surface of the glazing will be more rapid due to wind-borne dust and debris.  Hard low-e coatings are therefore usually used within multiple glazing units.

For the exclusion of solar energy low-e coatings in double glazing units are best used on surface 2 (the convention is to count from the outside, so that surface 2 is on the back of the outer pane of glass).  However, small variations in the thickness of the coating may lead to a slightly oily appearance (iridescence), and this effect is usually hidden by putting the coating on surface 3.  The difference in thermal performance is that extra solar radiation is absorbed at the glass pane which has the low-e coating, and if this pane is on the room-side of the glazing unit then more of the extra heat is radiated into the room.  It is suggested however that the iridescence of hard low-e coatings is less of a problem now that multi-layered hard coatings have been developed (Lampert and Ma [1992 pp55]).

A key gain in performance from using a low-e coating is the increase in the room-side glazing temperature.  The reduction of condensation on such glazings can actually be a better selling point than the reduction of energy usage because condensation is a visible problem and energy usage is not.
 

Low-emissivity (thermally reflective) coating applied after the manufacturing process (soft coatings)
Low-e coatings may also be applied after the glass has been manufactured, by the process of sputtering (also referred to as vacuum deposition).  In this process the cooled glass is placed into a vacuum chamber and a pure metal target is bombarded with a beam of ionised gas to produce a stream of metal atoms which then condense onto the surface of the glass.  The resulting coating has a very low emissivity (about 0.04, which is as low as is practically possible - any further reduction in emissivity would not give a significant reduction in heat transfer).  This type of coating can also be applied to plastics materials, because it is produced at a low temperature.

The sputtering process occurs at a much lower temperature than pyrolysis and so the coating does not bond itself to the surface of the glass.   As a result the soft coating must be removed from the edge of the glass (it is burnt off) before assembly into a multiple glazing unit.  Applying the coating onto hot glass would not work because the coating metal would corrode during deposition, and its effectiveness would be significantly reduced.

At one time using a soft coating on an exposed surface was not possible, because the coating corroded (tarnished) when exposed to atmospheric moisture.  Soft coatings therefore had to be protected by being hermetically sealed into a dry environment (such as the cavity of a multiple glazing unit).  Now, however, multi-layer soft coatings are possible, in which the low-e layer is protected between layers of a metal oxide (a typical layer arrangement is bismuth oxide-silver-bismuth oxide).  The metal oxide is selected to be transparent to infrared (otherwise the low-e properties of the silver coating would be lost) and cannot corrode (because it is already an oxide).

Soft coatings may be more expensive than hard coatings, because the glass must be transported to the coater and because the coating must be removed from the edge of the glass before it can be used.  The glass may also have to be cut to size before coating.  A useful refinement would be to improve the bond between the soft coating and the glass, although the ease of removing the coating may make this unnecessary.  Advances in processing speed will also reduce the cost difference between hard and soft low-e coatings.
 

Low-emissivity (thermally reflective) coating applied as part of an adhesive film
Low-e coatings can be applied first to a plastic film, and then fitted to the glazing.  In this case the low-e coating will probably be protected with a clear coating that transmits infra-red (otherwise the properties of the low-e coating will be lost).

Adhesive films may be applied to single glass in order to cut down solar heat transmission or to improve privacy.  However, these films do increase the amount of energy that is absorbed by the glass, which is often annealed and more likely to break due to thermal fracture.  These films should not be applied to ordinary double glazed units or the risk of failure is very high.
 

Diffractive coating (holographic)
A diffraction grating comprises a series of parallel lines with uniform spaces between them.  The lines are of some material that does not transmit radiation.  When solar radiation strikes the diffraction grating radiation which passes close to the edge of a line is bent.  If ‘waves’ of radiation are considered, as shown in this image, then it is apparent that one of the diffracted waves must travel an extra distance x in order to reach the target surface - if the distance x is equal to one-half of the wavelength of the light then the two waves will cancel each other out (destructive interference) and there will be no illumination of the target surface at that particular wavelength.  Similarly if the distance x is equal to the wavelength then the two rays will add together (constructive interference) and there will be bright illumination of the target surface.

Clearly the angle a is important, and the distance x need not be only one-half or one whole wavelength - reinforcement occurs at any whole number multiple of the wavelength and interference occurs at any whole number multiple of the wavelength plus one-half wavelength.  The light passing through the diffraction grating therefore appears as a series of bands of light and dark.  It should be noted that this effect is only visible if the spacing of the parallel lines is small - typically around the wavelength of the radiation - and that different wavelengths of radiation behave slightly differently.  This can cause the transmitted visible light to be separated into a colour spectrum as each colour favours a slightly different viewing angle to its neighbours.

Holographic coatings work on the basis of a diffraction grating - a 3-dimensional network of parallel lines is created which have a uniform spacing similar to the wavelength of solar radiation.  Only light of the same wavelength as the spacing of the grid is allowed straight through - nearby wavelengths are diffracted whilst distant wavelengths are reflected.

By varying the spacing of the holographic grid it is possible to refract incoming solar heat and light through different angles, so that one part of the spectrum is transmitted and another is diverted.  Holographic films can thus be used to create shading devices, or to redirect light to where it is best used.

Holographic films may also be produced which can have several layers, so that light can be selectively used when the sun is at several positions in the sky.

An interesting use of holographic films is described by Müller [1994] in which holographs on sliding glass panels are placed over a glazing system with clear glass bands delineated with strips of photovoltaic cells - most of the direct incident light is focused onto the photovoltaic cells and about half of the diffuse light passes between the cells to provide natural light within the building.

Holographic coatings work well with direct light, but do not work properly if the light is diffuse.  There may also be a rainbow effect when the glazing is viewed from some angles (UCD-OPET [1994]).
 

Plastic glazing sheet with a low-emissivity (thermally reflective) coating applied during manufacture
Low-e coatings can also be applied to plastics.  The application of a soft-coating to a film which is then suspended in the gas-space of a multiple glazing unit is described in the Section 09.01, but it is also possible to produce thick plastic sheets with an integral low-e coating.

‘Plexiglas X Heatstop’ (Endres and Benz [1994]) is a plastic sheet which is co-extruded with an infrared-reflective coating.  The active ingredient of the coating is mica platelets enclosed in a metal oxide layer, the thickness of which is controlled to preferentially reflect infra-red radiation.  This coated product can be used in single sheets - the coating is an integral part of the sheet, and should not corrode.

It is reported by Endres & Benz [1994] that ‘Plexiglas X Heatstop’ has a reddish-blue ‘shimmer’ in incident light, and that transmitted light has a green tinge.  As with other low-e coatings the daylight is also slightly reduced.  However, the coated plastic can be curved and bent after manufacture, to create components such as skylight domes.  The product typically has 51% light transmittance and 38% total solar energy transmittance, which compares to 92% light and 85% total solar energy transmittance for the clear un-coated product.
 

Other issues relating to coatings and films
Coatings are far more complex than has been described above.  Multi-layer coatings are being produced in more and more forms, and development of new coatings is ongoing.  Many of the problems associated with coatings will probably be solved, and in some countries glasses with coatings form a major part of the new-glazing market.

A subject that has not been described above is the development of coatings specifically for the purpose of excluding ultraviolet radiation.  Such coatings are particularly useful because of the detrimental effects of ultraviolet on some materials and pigments.

Some films are applied to glass for the purpose of holding the glass together should it break (particularly as the result of an explosion).  Such films will also modify the thermal performance of the glass, even if the film is transparent.  Patterned films might be used to create privacy, and this too can modify thermal performance.  Generally the application of any adhesive film to a window will result in more heat being absorbed by the glass, and care should be taken to check that the glazing system can tolerate any additional thermal expansion that occurs as a result.