Testing and Analysis for Quality Assurance
5/11/2016 7:15:42 PM
Materials testing is a vital component throughout the PVC industry. Testing ensures the PVC supplier that the compound performs as intended and will be consistent across production runs. Designers base their selection of materials for new products on the results of standard tests. Plastics manufacturers use test results to help establish process parameters and quality control personnel use standard tests as a benchmark to be sure products are meeting customers' requirements.
An understanding of the commonly used tests can help give you a competitive edge and ensure a quality end product.
Testing and Evaluation Methods for PVC Compounds
The most commonly used tests for flexible pvc can be broken into the following categories.
- Physical Property Testing
- Thermal Property Testing
- Electrical Property Testing
- Flammability Testing
- Rheology Testing
- Color Testing
- Accelerated Weathering Testing
Several national and international agencies establish and publish testing specifications for industrial materials. The most commonly used standards in the United States come from ASTM International.
A major international organization similar to ASTM is the International Organization for Standardization (ISO). Both agencies have a common goal of developing standards in hopes of facilitating the exchange of quality goods and services and ensuring their safe use.
Physical Property Testing
Physical property tests measure the molecular structure of the material which can often effect the physical properties. The most important properties for measurement include:
- Specific Gravity (ASTM D792) - The ratio of the density of a liquid or solid mass to the density of water at a specific temperature.
- Hardness (ASTM D2240) - The resistance of a plastic material to indentation. It is measured on a Durometer machine which led to the industry to call hardness "Duro." Normal specification is plus or minus 3 and three scales are used: Shore A for flexible, Shore C for semi rigid and Shore D for rigid. Usually a delayed reading of 10 or 15 seconds is used.
- Tensile Strength (ASTM D638) - The maximum nominal stress sustained by a test specimen being pulled from both ends, at a specific temperature and a specific rate of stretching. Specification is a minimum amount in PSI.
- Elongation (ASTM D638) - The amount of increased length of a material until breakage. Specification is a minimum percentage.
- Tear Strength (ASTM D1004) - The force required to tear a plastic material, usually expressed in pounds per inch. Specification is a minimum number.
Thermal Property Testing
Thermal analysis provides material characteristics data in relation to temperature and time.
- Melt Temperature - The temperature required for the material to process.
- Maximum Continuous Operating Temperature (ASTM D794) - Accelerated tests essentially used for the wire and cable industry to determine the resistance of the compound to elevated use temperatures. The ratings are based on the maximum continuous operating temperatures temperatures that wire insulation can withstand without degrading enough to fail in service.
- Brittleness Temperature (ASTM D746) - The temperature at which 50% of the tested specimens exhibit brittle failure at specified impact conditions. The lower the brittleness point, the better the low temperature properties.
Electrical Property Testing
Electrical tests, in general, are measurements of the resistance, conductivity or charge storage either on the surface or throughout the PVC material.
- Volume Resistivity - (ASTM D257)- The resistance to leakage current through the body of an insulating material.
- Surface Resistivity (ASTM D257)- The resistance to leakage current along the surface of an insulating material.
- Insulation Resistivity (ASTM D257) - The resistance between two conductors of a circuit or between a conductor and the ground when they are separated by an insulator. Expressed in ohm-centimeters.
- Dielectric strength (ASTM D149)- The measurement of electrical voltage required to break down or arc through a plastics material. The units are reported as volts per millimeter of thickness.
- Dissipation (power) factor (ASTM D150) - The measure of the power (watts) lost in the plastics insulator. Measurements are made at one million hertz.
Rheology is the study of flow and deformation of materials under applied forces. Put simply, it indicates how a material will process. A rheometer is typically used for accurate measurement.
Melt flow rate (ASTM D1238 or D3364) - The number one indication of the materials viscosity in the melt phase. It is defined as the mass of polymer in grams flowing per 10 minutes through a capillary of specific diameter and length by a pressure applied via a range of standard weights at specified temperature.
Color's basic structure (visual light waves) can be measured with a Spectrophotometer which assigns numerical values which act as names or identities for specific colors. This device uses three different sources of light including daylight, incandescent and fluorescent light. The resulting set of color values is then compared to stored color standards.
Flammability, also called flame resistance measures the ability of a material to support combustion. A term used with flammability testing is "self-extinguishing" which indicates the material will not continue to burn once a flame has been removed.
There are numerous flammability specifications that a material can meet. It is not always convenient to conduct the actual flame test on every production run. There is a test however called Oxygen Index that can be done to show the consistency of lot to lot flammability properties.
Oxygen Index (ASTM D2863) - The minimum concentration of oxygen in a flowing mixture of oxygen and nitrogen that will just support flaming combustion of a plastic specimen.
Accelerated Weathering Testing
Accelerated Weathering Testing is necessary to ensure that materials and products meet their expected functionality and lifetime. An array of variables exist that ensure that the effects of aging and weathering are hard to predict and as such can cause problems in the future which may reflect back upon the product and your business in a negative way.
Weathering tests include the following:
Ultraviolet Resistance (ASTM D4329, ASTM D 4587, ISO 4892) - Measures resistance to the effects of direct sunlight or artificial weathering devices. UV radiation when combined with water and other environmental factors may cause material degradation.
Permeability (ASTM D1434) - The volume or mass of gas or vapor penetrating an area of the material in 24 hours.
Biochemical Resistance (ASTM G21) - PVC is inherently resistant to bacteria and fungi but depending on the application, antimicrobial additives may be required.
Chemical Resistance (ASTM D543) - Tests resistance to chemical reagents by simulating performance in potential end use environments.
Our experienced technical team can perform these as well as other quality assurance evaluations. If you would like more information about flexible PVC material testing methods, please contact John Schlitzer at email@example.com or 717.336.2823 ext 104.