MAY 1986
In an industry as competitive as the geomembrane industry, companies have to be on top of quality control and assurance at all times. Many companies have gone to great lengths to assure their product is of the highest possible quality. This article outlines the quality control and assurance practices used in manufacturing PVC geomembranes. The article is a summary of a technical paper presented at the 1992 Geosynthetics Research Institute annual conference and is written with the permission of David C. Lauwers, Manager of Compounds and Film Technology for Occidental Chemical Corporation.
PVC Geomembrane sheeting is produced by the calender manufacturing process - a continuous extrusion between a pair of cylinders. Calenders in the United States consist mainly of four rolls, although they can range from three to six. This manufacturing method differs from an extrusion process, which is commonly used for polyethylene and related products, because a fixed die for each desired gauge is not necessary. There are a variety of advantages to using the calendering method. Calender rolls allow for flexibility to produce films over a wide range of gauges (from 1.5 to 100 mils), they provide the manufacturer with the ability to change a gauge without interrupting production and calendering the geomembrane allows the physical properties of the film to remain constant regardless of the width of the sheets. Calendered film also assures uniformity of the melt and development for optimum physical properties due to the high shear of the process.
The PVC geomembrane formulas are based upon PVC resin. However, other raw materials are added to provide flexibility, color and stability. Raw materials also improve processing. Formulas for PVC geomembranes may differ slightly from manufacturer to manufacturer; however, they all conform to NSF 54 standards for PVC or Oil resistant PVC. Occidental Chemical Corporation and Canadian General Tower also produce their products in accordance with the PVC Geomembrane Institute Quality Control Document. Manufacturers use similar quality control procedures in order to assure the quality of the finished product. Quality Control and Assurance procedures cover every aspect of the calendering process, from testing of raw materials to certification of the finished product. A good Quality Control and Assurance program includes testing of the following:
Incoming Raw Materials
In order to assure quality of the finished product, the quality of each raw material must be checked. Only approved suppliers are used. These suppliers have demonstrated an ability to provide a consistent product, within approved specifications. Suppliers also receive formal Quality Control and Assurance audits. Established procedures for each raw material are used to check incoming quality. Among these procedures are sampling methods, frequency of testing, test methods and acceptable value ranges. Results of this testing are plotted in order to track trends and identify any possible changes in raw material properties. The major raw materials used in PVC geomembranes are PVC resin, plasticizer and stabilizer. The following table outlines the various tests conducted on PVC geomembrane raw materials.
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Raw materials are not unloaded for use until the Certificates of Analysis are reviewed and/or raw material testing has been conducted. Any raw materials that fail to meet the specifications are returned to the supplier.
Blending Procedures
Individual raw materials used in PVC geomembranes are blended together using a variety of methods. Some typical blenders used include Ribbon and Henschel. Blending is done in order to obtain a uniform, free flowing mixture of raw materials prior to fusing. While the raw materials blend, the PVC resin is heated to the range of 160° F to l80° F, to open up the individual resin particles. Opening the resin particles enables the liquid components of the blend, including the stabilizer and plasticizer, to be absorbed. Specific blending procedures are established depending upon the formulation and equipment used. These procedures include order of raw material addition, timing between additions, loading instruction, along with safety and handling practices. Procedures such as these assure that a uniform dry blend is obtained without lumps or undispersed raw materials.
Blender Checks
Once blending is complete, a small sample of each blend is taken, identified and sent to the quality control laboratory for evaluation. The dry blend is inspected for uniformity, contamination and lumps. A sheet is made from the blend, using a laboratory two-roll mil and checked for color, undispersed material, processing and flexibility. This easy test identifies variations in resin, plasticizer, lubricant, stabilizers and pigments in the blend. After this testing is completed the blender is released to production.
Production Quality Control and Inspection
An "In Process" quality control specification is established for each type of PVC geomembrane calendered. These specifications include any inspection or physical testing that need to be conducted on the "fresh" geomembrane. Typical specifications include frequency of testing, physical property testing and visual inspection parameters. The specifications also set the requirements for color, packaging gauge and width. The types of tests conducted include modulus, tensile, elongation, color, surface and backside inspection. Gauge, profile, shrinkage and gloss tests are also used.
A three to five yard sample of the geomembrane is placed upon an inspection table with back and top lighting. Both sides of the geomembrane are also obtained at this time for morgue retains and certification testing. A calender condition sheet is maintained for each type, width and gauge of PVC. This sheet includes the temperature, speed, gap, differential and other physical settings required to produce that specific geomembrane. These settings are used to initially set up the calender train. Slight adjustments may be necessary to meet the finished product specifications.
Once the geomembrane is running on specification it continues to be tested and monitored throughout the order. Any deviations outside target ranges for properties are reported to production so that adjustments can be made at each calender. The gauge of the geomembrane is continuously monitored using a computer assisted gauge control device. The device measures the gauge across the sheet and automatically adjusts the gap between the calender rolls in order to maintain gauge and profile. Using this system, the gauge of the geomembrane can be controlled within 5 percent of the stated gauge. This tight gauge control offers reduced variation in physical properties as well as improved quality in comparison to other types of geomembranes.
Once this is completed the rolls are labeled and marked. Gauge, width, production date, weight, yardage and other important data is recorded on the label for each roll. This provides a method of tracking the rolls right to the fabricated panels and actual installation site.
Certifications
Certification samples are allowed to age at standard temperature and humidity conditions for a period of 24 to 48 hours. This allows the samples to obtain at least 95 percent of the full physical properties. A sample from each lot of PVC geomembrane representing approximately every 10,000 pounds is retained for testing. The retained samples are checked according to the National Sanitation Foundation Standard 54 for Flexible Membrane Liners. The following properties are checked for each sample:
Thickness, mils --- ASTM D1593, Para. 8.1.3
Specific Gravity --- ASTM D792, Method A
Breaking Factor --- ASTM D882
Elongation at Break, percent --- ASTM D882
Modulus at 100% --- ASTM D882
Tear Resistance, pounds --- ASTM D1004
Low Temperature, degrees C --- ASTM D1790
Dimensional Stability, percent --- AST1W D1204
Water Extraction, percent loss --- ASTM D3083
Volatile Loss, percent loss --- ASTM Dl203
Once all the information is gathered, it is reviewed by a quality control manager, who in turn, signs the certification. Copies of certification are supplied to customers with the shipment.
Feed Back
Any good Quality Control program involves addressing any quality issues that may arise in the field. Many companies provide a technical service that investigates any material in question. Quality control departments examine actual samples sent back from the field. Those samples undergo an investigation in order to determine cause of the defect or performance issue.
The exact methods used by each manufacturer of PVC geomembrane may vary slightly. However, every manufacturer takes steps to assure that proper quality control and assurance is used throughout the entire process.
It is the quality control and assurance programs that guarantee the quality needed for geosynthetic applications aimed at protecting the environment and the ultimate user of the PVC geomembrane.
Environmental Protection, Inc provides this PGI Technical Bulletin for your information.
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