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Applications and Limits of Polycarbonate and Acrylic Lenses

Polycarbonate and acrylic plastics have achieved very wide usage in the design and manufacture of luminaire lenses.
Their advantages over glass are many, including impact resistance, weight and viscosity of flow characteristics that allow them to be injection molded into optical designs of extremely high precision.

Yet, for all their similarities, there are significant differences in performance in the areas of optical stability and impact resistance. These differences make the occasional claims touting one or the other as the superior lens material difficult to evaluate. In fact, comparative analysis shows that neither material meets all design requirements better than the other. Combinations of factors may make acrylic the choice for one application; change those needs slightly and polycarbonate is the best selection.

Time and again, specifiers and owners of a project have been disappointed in the performance and longevity of luminaires that are being overdriven or misapplied. Purchasing a fixture that uses lamps too hot for the lens material will lead to premature failure. For instance, a 10" square by 7" deep fixture with a polycarbonate lens and a given optical configuration can easily support a 70 HPS source and provide long life and performance, A 100 HPS lamp in the same fixture may well cause yellowing of the lens and loss of performance and aesthetic characteristics. A larger fixture must be used if the higher source is the design choice. This paper will describe the materials and then explore the two critical performance areas of impact resistance and optical stability with emphasis on thermal characteristics and effects. Further, primary application criteria will be established. The goal is to enable the specifier, through evaluation of his specific project, to select the most appropriate lens material.

Description:
Both acrylic and polycarbonate are thermoplastics. This means the material can be re-melted from solid state for use in injection or casting process. Acrylic is derived from methyl methacrylate. This plastic has excellent optical qualities, good chemical resistance and thermal and electrical properties and has moderate strength. This strength can be enhanced by use of an additive in the formative chemical reaction, yielding high impact acrylic. The additive, however, reduces clarity, weatherability and flexural modulus. (1) Polycarbonate results from the linking of dihydric or polyhedric phenols through carbonate groups. The material has very high impact resistance, is easily processed by all thermoplastic methods and has high temperature performance. (2) Additives will make it resistant to ultraviolet radiation which can cause long term degradation of the plastic.

From these descriptions alone, guidelines for use begin to take shape. The following specific analyses will make these guidelines clear.

Impact Resistance:
Two frequently used methods to evaluate strength of these materials are the notched Izod test and falling dart impact. Results are measured in footpounds. The notched Izod test evaluates shear stress while the falling dart measures resistance to direct, penetrative impact. Comparison of polycarbonate and two types of acrylic shows:
 
Notched Izod
Falling Dart
Polycarbonate (3)
16
125
High Impact Acrylic (4)
1.2
8.0
Standard Acrylic (4)
0.4
1.0

In refractor design, the falling dart test is the most appropriate because concern is focused on the likelihood of vandalism or the impact of an object on the lens.

Factors come into play that can radically affect each material's resistance to impact. Temperature change can reduce material strength. High impact acrylic shows straight line correlation between temperature and strength. The 8.0 ft/lb value at 60°F in the dart test decreases to 4 ft/lbs at 9°F. Conversely, its impacts strength will increase beyond 8 ft/lbs as temperatures exceed 60°F. (5) Polycarbonate also exhibits some loss of strength in low temperatures but over the same 0°-60°F range, it loses only 15% as opposed to acrylic's 50% loss. (6) Polycarbonate demonstrates exceptional low temperature tolerance.

Another factor altering impact resistance is lens design itself. With respect to high impact acrylic, W.C. Burkhardt notes that there is ". . . difference in the impact strength of lighting parts depending on whether the prismatic or non-prismatic side was impacted. Unfortunately, impacting the smooth side usually resulted in reduced impact strength."(7) The variations in impact strength appear to be functions of part thickness, size and shape of the optical element and even the molding process used to create the optical element. Burkhardt further notes "…whenever prisms or other light obscuring or controlling elements are to be used', if possible they are best designed on the outside of a lens from an impact strength stand-point."(8).

This is an application drawback. In terms of dirt depreciation and maintenance, it is far preferable to keep the optical elements on the inside of the lens and present a flat, smooth surface to the exterior. Dirt will not adhere readily to such a surface and it is easily cleaned.

Polycarbonate exhibits some stressing characteristics that are typical of curved surfaces. Sharp corners and radii are high stress points, concentrating loads in very localized areas. Lenses designed with softer corners and larger radii would show correspondingly less stress. However, the larger radius solution leads to some small loss of light control capability.

When it comes to selecting a lens material for strength, the, consider these factors: basic strength, ambient temperature and design. Will the luminaire be located in an area where damage through vandalism is likely or would the only lens contact be occasional and light? Will it be exposed to temperatures with wide shifts in range? Will maintenance be a problem? Is the environment dusty or dirty? In the following chart boxes with an "X" indicate an appropriate design choice.
 
Impact:
Heavy
Light

Ambient Temperature:
Cold
Moderate
Hot

Environment:
Clean
Dusty
Polycarbonate

X
X



X
X
X


X
X
Acrylic


X




X
X


X

Optical Performance:
The second major attribute of these plastics is their light transmitting abilities. Both acrylic and polycarbonate are excellent choices for lens material as they have high transmissivity ratings and show very little hazing.
 
Transmittance:
Haze:
Refractive Index:
Polycarbonate
86-89%
1-3%
1.586
High Impact Acrylic
89%
4%
1.49
Standard Acrylic
92%
2%
1.49

Of the three, standard acrylic performs best as an optical medium. However, because of strength considerations, polycarbonate and high impact acrylic are the most common choices for outdoor use. The critical difference between these two in the long run is the fashion in which each reacts to the environmental stresses of ultraviolet radiation and heat.

In lighting applications UV is a well known stressing agent of plastics - all transparent plastics will yellow under UV - but it is in many ways the most controllable. Polycarbonate without a UV inhibiting additive will show strong yellowing upon exposure to natural and artificial sources of ultraviolet (such as sunlight and HID lamps). High impact acrylic also yellows, though not to the same degree, and standard acrylic shows little UV induced yellowing. The use of a UV inhibitor in polycarbonate formulation reduces yellowing significantly.

Aside from the nature of the plastic itself, lamp selection can affect the yellowing process. Figure 1 shows typical energy emissions of the three major HID lamps.

Lighting

Note that the mercury vapor and metal halide lamps both emit ultraviolet radiation while the high pressure sodium does not. Naturally, selection of a given lamp type would in itself indicate the degree to which UV would affect the lens. However, between LTV stabilization in polycarbonate, the general acrylic characteristics and judicious lamp selection, LTV is a minor problem. A more pressing concern for both materials is heat.

Refer again to Figure 1. Note that all lamps emit a relatively large portion of their total output in convective/conductive energy. This is heat; it has a remarkably adverse effect on acrylic and polycarbonate.

Figures 2, 3 and 4 clearly illustrate the close correlation between heat and yellowing in thermoplastics exposed to HID sources. (Note that the time scale for the acrylic test differs from the polycarbonate scale.)

Lighting    Lighting Lighting

(Charts 2 and 3 are black and white reproductions of colored photos demonstrating relative Yellowness Index over time These charts are available on request.)

Aside from lamp, other factors that influence the rate of heat induced yellowing include ambient temperature, size of luminaire enclosure, distance from lens to lamp center as well as the distance from lens to reflector and the shape of the reflector itself. The charts therefore represent specific and controlled tests and should be considered a guide to possible material responses in the most general sense.

Does yellowing present more than an aesthetically displeasing effect? Strangely, the data are split. Standard falling ball impact tests on polycarbonate indicate that there is no loss of material strength. Furthermore, an ASTM Yellowness Index Rating of 25 for polycarbonate results in a loss in transmissivity of only 5%.

On the other hand, yellowing is a sign of degradation of the plastic molecule. Heat and ultraviolet act to break the molecules. This surrenders the intrinsic strength of the material as the molecular structure no longer consists of long intertwined chains but fractured segments. This may be reflected in reduced strength of parts with formed surfaces as these surfaces tend to localize stresses.

From a specification point of view, one must be concerned with the temperature of the lens. Polycarbonate has a viable working temperature of around 90°C (approximately 195'F). Working temperature is that maximum allowable temperature for a material that will not result in a loss of physical characteristics. For acrylic, ". . . the designer concerned with retention of of optical properties would certainly want to design for plastic temperatures in the 150°-160° F range."(14)

Unless the lens/housing enclosure is large, these temperature limitations mean the use of lamps with high amounts of conductive/convective radiation is not recommended. This includes the mercury vapor and metal halide lamps. High pressure sodium is a good choice as its higher efficiency makes the use of a lower wattage lamp with less emitted heat feasible. At any rate, in specifying relatively small fixtures with plastic lenses, it is advisable to request a tested internal lens temperature. This can allow the specifier to easily determine if the source/wattage combination he desires is compatible with long lens life.

Unfortunately, there are other causes of yellowing that are extremely difficult to anticipate. Stresses in the manufacturing process can cause premature yellowing even if the other environmental stresses are within the limitations described. Some of these fabrication stresses include the amount of regrind or reprocessed material in the lens, excessive molded in stresses (a function of the lens design itself), processing temperature and improperly or insufficiently dried resin. These factors are not perceivable in a finished lens and the manufacturer must be constantly aware of the possibilities of molding stress and must test samples for quality.

Another phenomenon of the thermoplastic lens is haze. The percentage amount of haze (see Optical Performance chart) indicates how much light is scattered outside a small conical angle by the material. The larger the amount of haze, the more diffuse the source will appear to be. In some applications, this is desirable but if acrylic or polycarbonate is selected because of transmissivity or clarity, haze is a definite shortcoming.

Little has been tested as to the causes in haze generation. There appears to be a link between the causes of yellowing and an increased amount of haze but the percentage of light improperly diffused rarely increases by more than a few percentage points.

Selection of a lens with an emphasis on retention of optical characteristics requires an analysis based on heat. This is where the tested temperature comes in handy. If a high UV emitting source is preferred, also keep in mind that these lamps are quite hot.

Conclusion:
As pointed out earlier, the superiority of a given plastic lens material depends not on intrinsic attributes but is determined by the application itself. An overall comparison shows that polycarbonate is more applicable and fits more situations. Acrylic is a good performer when chances of surface impact are small and when continued aesthetic appearance is important with the use of high UV sources. The single most destructive element, though, is heat. In specifying any fixture with a thermoplastic lens this must be considered carefully.

Footnotes:
(1) Redfoot, H.I., "Acrylic", Modern Plastics Encyclopedia, 1985- 1986
(2) Carhart, R. 0., et al, "Polycarbonate", Modern Plastics Encyclopedia, 1985-1986
(3) General Electric Plastics Division, "Designing with LEXAN Resin', G.E. publication CDC-536B 4182
(4) Burkhardt,,W.C., 'High impact acrylic for lenses and diffusers', Lighting Design and Application, April 1977 (5) Ibid.
(6) General Electric Plastics Division, 'Designing with LEXAN Resin", G.E. publication CDC-536B 4182
(7) Burkhardt, W.C., 'High impact acrylic for lenses and diffusers', Lighting Design and Application, April 1977 (8) Ibid.
(9) General Electric Plastics Division, 'Designing with LEXAN Resin', G.E. publication CDC-536B 4/82
(10) Burkhardt, W.C., 'High impact acrylic for lenses and diffusers', Lighting Design and Application, April 1977
(11) Fisher, W.S., and Weinstein, S., 'Heat Transfer with High Intensity Discharge Lamps and Luminaires", Illuminating Engineering, Vol. 65, No. 4, April 1970
(12) Reprinted with permission of General Electric Plastics Division
(13) Burkhardt, W.C., 'High impact acrylic for lenses and diffusers", Lighting Design and Application, April 1977
(14) Ibid.

Article reproduced courtesy of DEVINE Lighting
Hubbell Canada | Manufacturer of Lighting, Wiring & Electrical Products