October 4, 1991
PVC Calendering FML Materials
Sheet Integrity - Pinholes
An understanding of sheet integrity must begin with an understanding of the process, formulation, and raw materials.
In calendering, the raw materials are fed to a simple mixer, gradually heated and homogenized by passage through an internal mixer/and or a roller mill, then the hot mass is fed to the calender where it is compressed between heated rollers to form a film of desired thickness. The hot film is peeled from the calender, cooled and woundup.
Contemporary PVC calendars have four (4) rolls and are usually in an inverted L configuration. Sheeting 84" wide and higher are capable of being produced in a thickness range of .003" to .050".
Calendering offers the following advantages:
- Very high outputs. Output is controlled by mixing capacity.
- Excellent thickness control. ± 5% of nominal.
- Low strain in the membrane.
- Attractive appearance.
- A four (4) roll plastic calender, reforms the sheet three times ensuring exclusion of air in the membrane.
Attached for further consideration and perusal are the following articles:
- Calendering for high volume PVC sheet - contains schematics of the process.
- A table comparing methods of PVC manufacture.
- A monograph on vinyl chemistry basic compounding.
- An article on critical properties of raw materials.
The calendering process then produces membrane of homogenous composition, uniform physical dimensions ensuring membrane integrity within the parameters of its design. In the range of thicknesses used in the geomembrane industry (.010" - .060") pinholes are a non event.
To ensure the manufacture of pinhole free membrane, two procedures are followed:
1. A backlit vertical viewing station - full width, 2 yards long - is installed in the calender train, prior to the windup. This allows for continuous monitoring.
2. Periodic sampling - full width, two yards long - every third roll. The sample is placed over a viewing table, which is backlit, hand tension applied and the membrane examined.
The following are some examples of applications where PVC films or sheeting are used and where sheet integrity is of prime importance.
- geomembranes (.010" - .060")
- swimming Pool liners (.015" - .040")
- roofing membranes
- shower pan liners
- medical collection bags for liquids and solids
- vapour barriers (.004")
- tarpaulins
- inflatable aquatic designs (.008 - .010")
- medical enteral feeding and plasma bags
- baby pants
- shower curtains
Some examples of product or application specifications are shown here:
a) ASTM D3083 - PVC sheeting for Pond Canal Reservoir Lining
1 pinhole allowed per 10 sq. yards (backlit viewing)
b) ASTM D4551 - PVC Plastic Flexible Concealed Water Containment Membrane
For .030 and .040 sheeting
No pinholes allowed (backlit viewing)
3. Federal Specification L-P-375C Types I and II Plastic Film, Flexible, Vinyl Chloride.
2 pinholes/sq. yd. for film thickness .004" - .008"
NO pinholes allowed for sheeting thickness .010" - .020"
4. CGT specification for Inground Swimming Pool Vinyl
No pinholes allowed. 20 mil membrane (backlit viewing).
Conclusion
Process, formulation, raw materials, quality control testing, all combine to produce a defect free, pinhole free geomembrane.
PAUL LUSSIER, Canadian General-Tower, Ltd.
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