Looking at the plastic film industry from the point of view of an inspection supplier can be quite narrow. This article focuses on measurement requirements often ignored.
The following is a rundown of two ways to measure transparency in plastics (and other materials) -The Refractive Index and Optical Clarity.
The Refractive Index
The refractive index is a measure of how much light is bent (or refracted) as it passes through a substance. It is defined by: n = sin i /sin r, where i and r are the angles of incidence and refraction respectively. The refractive index is also the ratio of the speed of light in a vacuum to the speed in the transparent material. The refractive index will vary slightly with the wavelength of the light used to measure the refractive index. If ‘white’ light (a mixture of various wavelengths) is used as the incident beam, then the variation in the refractive index for the various wavelengths will lead to splitting of light into the colors of the spectrum, a process known as dispersion. When light enters a dense material from a less dense material then the refracted ray is bent towards the normal. When light enters a less dense material from a dense material the refracted ray is bent away from the normal. When light passes through a transparent material with parallel sides, the refractions ‘cancel out’ and the path of the light is displaced due to the presence of the transparent material.
Measuring optical clarity
The boundaries between ‘transparent’ or ‘clear’ and ‘translucent’ or ‘opaque’ are often highly subjective. What is acceptable for one observer is possibly not acceptable for another observer. It is possible to measure the degree of light transmission using ASTM D-1003 (Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics). This test method is used to evaluate light transmission and scattering of transparent plastics for a defined specimen thickness. As a general rule, light transmission percentages over 85 are considered to be ‘transparent’. The perceived transparency or optical clarity is dependent on the thickness of the sample used for assessment, and the optical clarity will decrease with increasing thickness. Standard glass can be relatively optically clear in thin sections but will show a green tint (due to iron in the glass) as the thickness increases. Optical clarity can only be achieved when the refractive index is constant through the material in the viewing direction. Any areas of opaque material (such as colorants) or areas of different refractive index, will result in a loss of optical clarity due to refraction and scattering.
Optical clarity is also dependent on surface reflections from the sample. The surface reflections at the air/plastic interface create significant transmission losses. For example, PMMA’s transmission loss is around 93%, and PS's is around 88%. These surface reflections can come from two basic causes: specular reflection, which is the normal reflection from a smooth surface, and diffuse reflection, which is dependent on the surface flatness of the sample. The transmission loss as a result of surface roughness or embedded particles is more often termed ‘haze’, and this is generally a production concern and not a property of the material. In producing blown film, haze can be caused either by melt fracture at the surface or by interfacial instability between the layers of the film.
An object's transparency is measured by its total transmittance. Total transmittance is the ratio of transmitted light to the incident light. There are two influencing factors; reflection and absorption.
Incident light = 100% - (Absorption = -1% + Reflection = -5%) = Total Transmittance = 94%
ASTM International (formerly known as the American Society for Testing and Materials) is the main body which works within the industry and develops standards for various tests/instruments. They dictate that the industry standard for the clarity meter entails the following;
- Reference beam, self-diagnosis, and enclosed optics
- Built-in statistics with average, standard deviation, coefficient of variance, and min/max
- Large storage capacity and data transfer to a PC.
The attribute of clarity of a sheet, measured by its ability to transmit image-forming light, correlates with its regular transmittance. Sensitivity to differences improves with decreasing incident beam- and receptor-angle. If the angular width of the incident beam and of the receptor aperture (as seen from the specimen position) are of the order of 0.1° or less, sheeting of commercial interest have a range of transparency of about 10 to 90 % as measured by this test. Results obtained by the use of this test method are greatly influenced by the design parameters of the instruments; for example, the resolution is largely determined by the angular width of the receptor aperture. Caution should therefore be exercised in comparing results obtained from different instruments, especially for samples with low regular transmittance.
Property testing is important for a number of reasons. These may include:
- Meet customer standards and specifications.
- Provide quality control to verify the manufacturing process.
- Establish a history for new materials.
Testing often includes material evaluation such as density, and mechanical property evaluation such as tensile strength. The methods of testing often vary depending on the capabilities of the manufacturer. For example, testing the tensile properties of bags may entail filling the bag with a weight rather than using the traditional tensile testing machine per ASTM specifications.
- Density. A materials characteristic, the specific density of the overall blend has an effect on end product properties. Lower density blends often have better mechanical properties in film applications. Density is determined by ASTM specification D792 or D1505.
- Melt Index. A materials characteristic, the melt index of the blend may affect the processing and melt mixing characteristics of the blend. Blends with lower melt indices may decrease throughput and require increased mixing in order to obtain consistent mechanical properties.
- Gel Count. A materials characteristic, gels are materials composed of oxidized or high molecular weight materials. The presence of gels in plastic films is objectionable due to appearance and to the problems associated with printing on the films. Gel count is determined by ASTM specification D-335 1-74.
- Tensile Strength. A mechanical property, tensile strength is a measure of the maximum stress a material will withstand when subjected to a load in tension. ASTM specifications for plastic films include D 88291. A simple method is to load the film or bag with a weight.
- Tear/Shear Strength. A mechanical property, tear strength is a measure of the maximum stress a material will withstand when subjected to a load in shear. ASTM specification for plastic films is D 1004-90.
- Dart Drop Impact Strength. A mechanical property, dart drop impact measures the toughness of the material by introducing a polyaxial load. ASTM specification for plastic films is D 1709-91.
- Haze. An optical characteristic, haze is a measure of the clarity or transparency of the material. ASTM specifications for plastic films is D 1746-92 and ASTM D l003.
- Gloss. An optical characteristic, gloss is a measure of the reflectance of the material. ASTM specification for plastic films is D 2457-90.
In addition, there may be a number of other material or mechanical properties that are important to the manufacturer. These will vary depending on the performance requirements and customer specifications. Other characteristics that may be tested include: burst/seal strength, odour, freeze resistance, print quality, and modulus.
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