Optical Properties
The optical properties of a plastic part generally involve the way in which the plastic materials interact with light. These interactions can be grouped into three different types: (1) how the light passes through or is diffused by the plastic material, (2) the way light is reflected off the surface of the material, and (3) the color of the plastic material. These optical properties are important in most applications and are critically important in some.
In many plastics, optical properties in combination with other properties, such as toughness, flexibility, and moldability, are the crucial combination that gives real value. Some applications in which optical properties are especially important are outdoor signs, optical fibers, automotive tail lights, safety glasses, window glazing, merchandise display cases, instrument panels, contact lenses, prisms, low-cost camera lenses, magnifiers, and numerous boxes, packages, and coatings where clarity is desired.
5.7.1. Light Transmission
Plastics differ greatly in their ability to transmit light. Some plastics allow light to pass through them with little change. Images are distinct when viewed through these materials. These materials are called transparent. Good examples of transparent materials (even though they are often colored) are plastic lenses for eyeglasses, bulletproof glass, and automobile backup lenses. Window glass and clear water would, of course, be other non-plastic transparent materials. The plastic materials most often associated with high transparency are acrylics (Plexiglass™, Acrylite™, and Lucite™), polycarbonate (Lexan™ and Sparlux™), and polystyrene (Dylene™, Opticite™, and Santoclear™).
Other plastic materials do not allow any light to pass through them. These opaque polymeric materials can be used for car tires, football helmets, and computer housings. They are often polymers to which fillers or pigments (inorganic colorants) have been added. A few polymers can be naturally opaque.
Some plastic materials have light transmission properties that are intermediate between transparency and opacity. These materials are semi-transparent materials, sometimes called translucent. They allow light to pass through them but the materials appear to be cloudy or shadowy. Images are detected but are not distinct. Some common translucent materials are nylon gears, plastic milk bottles, and covers for fluorescent lights that have been surface roughened to diffuse the light. Plastic materials can also be semi-opaque and allow a small amount of light to pass but do not permit the detection of images through the plastic material.
The boundaries between the various categories of light transmission are not well defined. Hence, some convenient rules of thumb in discussing the various degrees of light transmission are useful. If light will pass through a plastic part such that a newspaper can be easily read through the plastic, then the material can generally be considered to be transparent. If a newspaper cannot be easily read through the part but general shapes can be perceived, such as holding your hand on the plastic part and detecting the outline of the hand, the material is translucent. Semi-opaque materials allow only vague shadows to be perceived through the part. Nothing is perceived through an opaque plastic material.
Even in highly transparent materials, some light is absorbed by the plastic. This absorption causes minor heating of the material and, in the case of the high-energy ultraviolet light, can cause degradation and the plastic part will become less transparent, often with an accompanying yellowing of its color. (The yellow color is caused by a preferential absorption of blue light by many degraded plastics.) The transmission can be measured by comparing the amount of light that passes through the plastic part with the amount of light passing through clean air or a vacuum. In many plastics these light transmission changes are gradual and so a plastic material can be transparent with little perceived change for long periods of time. Polymethylmethacrylate (PMMA) has been found to lose less than 20% of its light transmission over 20 years if properly formulated. This is the best plastic material for retention of optical clarity.
The standard test for measuring light transmittance and haze is ASTM D1003. Luminous transmittance is defined as the ratio of transmitted light to the incident light. The value is generally reported as a percentage of the transmitted light. Haze is the cloudy appearance of an otherwise transparent specimen caused by light scattered from within the specimen or from its surface. Both luminous transmittance and haze are measured by using a hazemeter.
Transmission depends upon the relatively un-obstructed passage of light through the plastic part. If the light is scattered by inhomogeneities in the part, the amount of light transmitted is reduced. Relatively small reductions or minor scattering cause the material to become translucent. A typical cause of this phenomenon is the scattering of light by the crystal structure in a crystalline polymer such as polyethylene. Low-density polyethylene that has few crystals is transparent. However, as the number of crystals increases, the transparency decreases until, in high-density polyethylene, the material is translucent (as with plastic milk bottles). This scattering by the polymer crystal structure occurs because the size of the crystal is approximately the same as the wavelength of the visible light. If the crystals were significantly smaller or larger, little scattering would occur. In general, polymer crystals are about the right size to cause scattering, and so a useful rule of thumb is that transparent polymers are noncrystalline and translucent polymers are crystalline. However, some crystalline polymers, such as PET, are transparent because the crystal size is not in the wavelength range of visible light.
The effect of additives can significantly alter this rule. Additives will almost always decrease the light transmission capability of the plastic material. This is especially true for many fillers that not only scatter light but absorb light.
Just as fillers tend to make plastics opaque, so also do additives that create separate phases. For instance, ABS is a mixture of three types of plastic materials (polyacrylonitrile, polybutadiene, and polystyrene). The mixtures of these materials will often have small, separate phases where one or two of the materials are dispersed through the other. Light will reflect off the phase boundaries where the phases touch and be scattered or absorbed, thus resulting in an opaque material.
5.7.2. Colorants
The color of the plastic part is affected by the way the light is absorbed or diffracted by either the polymer itself or by additives in the plastic material. Additives which cause specific light to be absorbed are called colorants. These colorants can be either semitransparent or opaque; they can have either a small effect on the transmission of light or a major effect. One class of colorants will dissolve in either the resin or some other component of the plastic mix. These colorants are called dyes and are often organic. The other major class of colorants will not dissolve in the resin or any of the components and is, therefore, more like a particulate additive. These colorants are called pigments and are frequently, but not always, inorganic. So, a plastic part that is clear was likely colored by a dye, whereas a plastic part that is dark and opaque was likely colored by a pigment. The most common pigment used to color plastics is carbon black, which also acts as an ultraviolet light absorber and therefore provides additional weathering protection.
Materials with highly delocalized electrons tend to absorb light because the frequency of light is often the same as or close to the frequency needed to excite these electrons. This absorption property is readily apparent when the molecular structure of dyes is examined. Almost all dyes are aromatic molecules with added carbon-carbon double bonds to increase the delocalization of the electrons. Hence, they absorb certain light frequencies and let others pass through. The absorption of part of the visual light spectrum results in a colored material. Conductive and semiconductive polymers have highly delocalized electrons and are almost always highly colored. Graphite, for instance, which is conductive, is black, indicating that light of almost all visible frequencies is absorbed.
Color matching is an important property in some plastic applications. This matching can be done analytically by using the light reflectance or transmittance spectrum of the plastic material in comparison with the desired color match partner. The amount of pigment or dye is carefully adjusted to give the same reflectance or transmittance between the two materials to achieve the proper match. Care should be taken to monitor this color match after the dye or pigment has been thoroughly mixed into the plastic material, as additive dispersion has a major effect on the color.
These color matches are obtained visually by using specific light sources to view the color and then comparing the color with standard color samples. The test method for this procedure is ASTM D 1729. Automated color matching machines indicate when a color match has been achieved (with a selected variance percentage) and, when the color match is not correct, suggest combinations of standard pigments that can be added to the mix to obtain the match.
5.7.3. Surface Reflectance
The reflection of light off the surface of a plastic part determines the amount of gloss on the surface. The reflectance is dependent upon a property of materials called the index of refraction, which is a measure of the change in direction (angle) of an incident ray of light as it passes through a surface boundary. If the index of refraction of the plastic part is near the index of air, light will pass through the boundary without significant change in direction. If, on the other hand, the index of refraction between the air and the plastic material is large, the ray of light will significantly change direction, causing some of the light to be reflected back toward its source. This reflection emphasizes the presence of the surface. For instance, glass is a material with nearly the same index of refraction as air, therefore glass window surfaces are not readily apparent. However, if the glass is coated with a shiny metal, as is done in a mirror, the surface becomes very visible. (A mirror will reflect the light because the silver surface on the back of the mirror has a very high index of refraction compared to air or glass.)
The refractive index is measured by ASTM D 542 and is actually defined as the ratio of the sine of the angle of the incident light to the sine of the angle of refracted light.
Refractive index values are important to designers of lenses for optical instruments such as microscopes and binoculars. The optical index of most transparent plastics is near that of glass. The test is conducted using a refractometer or a microscope to compare the actual thickness of a specimen with the apparent thickness when viewed through the material. The result is a number (index) without units.
Surface reflectance is also affected by geometric features of the boundary that may cause the light to change directions. For instance, roughing the surface or coating the surface with irregular materials will cause the light to change direction and therefore cause surface reflectance. In some cases, surface reflectance is desired. For instance, the cover over a light fixture is often roughened or lightly pigmented so that the light is scattered, thus diffusing the light over a wider area and decreasing direct shadows. The glare of the light, which is a measure of the amount of light that is moving directly to the viewer, is reduced because of the diffraction caused by the change in surface reflectance. For good transparency, a matching of the indices of the plastic part and of air is generally desired. Some of the plastics that give this good match are PMMA, polycarbonate, and polystyrene.
A convenient test for assessing the amount of light reflected off the surface is the specular gloss test, ASTM D 523. Specular gloss is defined as the relative luminous reflectance factor of a specimen in the specular (observed) direction. In other words, the specular gloss is the amount of shininess exhibited by a surface. Specular gloss usually is measured by a glossmeter, which shines a light on a sample, measures the reflected light at a particular angle, and compares this reflected light with the amount that would be reflected from a shiny black surface.
A light analysis technique called birefringence, or double refraction, has proven to be very useful in detecting internal stresses and molecular orientation in transparent plastics. In some cases, the molecular orientation will enhance mechanical properties and is desired. An example is the biaxial orientation of PET films for improved strength and formability. In other cases, the residual stresses are not desired, as they may indicate places where premature failure could occur.
The most common methods of measuring birefringence depend upon the property of photoelasticity (as defined using Brewster's law, which states that the index of refraction in a strained material becomes directional and that the change in direction is proportional to stress). When molecules are oriented, as, for instance, from plastics processing, the effects of the orientation can be seen using birefringence techniques. The measurement of birefringence is made by placing the sample between a plate that polarizes light and a plate that analyzes the polarization. When light is shined through the polarizer, then the sample, and then the analyzer, the stress pattern or the molecular orientation becomes visible. The pattern can be analyzed qualitatively (visually) or quantitatively by instrumentation. Because these results are nearly instantaneous, birefringence can even be used for online quality control as well as part inspection.
Standard polarizing optical microscopy is a valuable tool for investigating the nature of the crystalline structure of polymers. Spherulites can be seen using this technique and, therefore, the amount of crystallinity and the size of the crystals can be studied. The standard microscope can also be useful in examining surfaces for defects and areas of high strain as detected by crazing or blushing. The scanning electron microscope (SEM) is a tool that gives surface information at much higher magnifications than are available with optical microscopes. SEM spectra can give valuable information about phenomena such as fractures and fracture surfaces, crack initiations, surface contamination, adhesive bonding, and surface treatments. Some SEM spectrometers allow the use of a technique called electron dispersive spectroscopy (EDS) which can detect the presence and concentration of atomic species on the surface of the polymer. For instance, the amount of oxidation of a surface can be measured by comparing the EDS spectra of the surface before and after oxidation. The ratio of the oxygen to carbon peaks will reveal the amount of oxidation that has occurred.