PVC Optical Properties

The optical properties of a material can be defined in terms of refractive index, clarity or transparency/haze, gloss, and color. The refractive index (the amount of refraction that takes place, directly connected to the speed of light in a substance) of PVC is around 1.54. For comparison, that of PP is 1.49, that of water is 1.33, and that of glass is 1.51.

PVC, both rigid and ?exible, has good clarity when correctly formulated. This property is utilized in many different applications.

Flexible PVC ?lm (cling wrap) is extensively used for packaging of food products by allowing the customer to see the product but at the same time minimizing the passage of water vapor into or out of the package. It also permits naturally occurring gases to escape from the package. An industrial use, requiring excellent clarity for ?exible PVC, is hanging strip doors. Rigid PVC has uses in blister packs and blow moulded bottles.

It should also be noted that clear material can also have color. These can vary between blue clear, water clear, and neutral clear.

Gloss Level

Surface gloss is linked with the ability of a surface to re?ect more light in some directions than in other directions. Gloss is in?uenced by the refractive index, the angle of incident light, and the surface topography. Gloss meters are used, whereby gloss ratings are obtained by comparing the spectacular re?ectance from the sample to that from a highly polished black glass standard (with a refractive index of 1.567). A 60° angle (or an 85° angle for low and medium gloss) is normally used to measure gloss level. EN ISO 2813:2000 is the reference standard.

Gloss level can be in?uenced by the formulation ingredients but is most in?uenced by processing equipment conditions and techniques.

 Color

The ?nal color of a PVC article depends, obviously, on the pigments used but heat stabilizers can have an in?uence with color consistency and prevention of yellowing caused by degradation. Optical brighteners, which make materials whiter and brighter under ultraviolet (UV) light, are not generally used but may have speci?c uses.

The technical measurement of color is usually based on the CIE L*a*b* color sphere or color space to aid the numerical classi?cation of color differences, providing a standard scale for comparison of color values. The light source can be varied but, of course, must be standardized for a particular color measurement. A typical illuminate type would be D65 which relates to neutral daylight.

L* gives the level of color intensity. For opaque materials, L* = 0 (total black) and L* = 100 (white). For clear materials, L* = 0 (total black) and L* = 100 (colorless). a* is the red to green axis with positive a* giving the degree of redness and negative a* greenness. b* is the yellow to blue axis with positive b* giving the degree of yellowness and negative b* the blueness.

Thus, a typical shade of white could be L* = 92.70 a* = +1.60 b* = –5.10 and a pale grey (nearly white) could be L* = 83.70 a* = –0.50 b* = +0.50.

Delta (Δ) values of these ?gures can also be generated where ΔL*, Δa*, and Δb* indicate how much a standard and sample differ from one another in L*, a*, and b*.

The total color difference ΔE* summarises the differences of one color in comparison with a standard, considering their L*, a*, and b* values. ΔE* does not indicate which of these parameters are out of tolerance if ΔE* is out of tolerance. The color standard is established from the average of several readings over time.