Tuesday, January 6, 2015

Common misconceptions about PVC Geomembranes

Andre Rollin: The first misconception is that there exists only one type of PVC.  This is very wrong.  There are a multitude of formulations that result in an array of PVC geomembranes to fit most applications.

The second most common misconception is that PVC will not resist hydrocarbons, acids and bases; but, yes, PVC can resist many hydrocarbons (except strong solvents), acids and bases for a long period of time if the proper formulation is used.

Also, many seem to think that PVC will be punctured easily.  On the contrary, PVC is less easily punctured than a rigid geomembrane because PVC has a high elongation characteristic – flexibility is the most desirable property of a PVC geomembrane, offering no re-alignment of the molecular chains when elongated.  It conforms easily to sites, and in most applications does not require reinforcement.

A fourth misconception is that you should compare geomembrane thickness, even between disparate materials, when making a selection.  This is wrong.  Comparing geomembrane thickness can be irrelevant.  Performance is what matters.  For example, comparing the properties of a 60 mil rigid geomembrane to the properties of a 60 mil PVC geomembrane is like comparing nonwoven to woven geotextiles.  At like thicknesses, the results can be dramatically different and confuse the real objective of a design.  Consider that many agencies (such as New York’s Department of Transportation) recognize that though nonwoven geotextiles generally have lower strength characteristics than woven geotextiles, the behavior of nonwovens is acceptable, even desirable in many applications because of higher flexibility (elongation).  In many geomembrane designs, a lower thickness PVC geomembrane can offer functional performance similar to thicker, rigid geomembranes.

A fifth misconception with PVC geomembranes is that a chemically bonded seam is less resistant that a fusion seam.  This is not the case if proper bonding procedure are followed.  I and many others in the field have observed very strong, tensile-resistant, chemically bonded seams even after many years of their service life.

A sixth misconception is that the fragility temperature is too high.  In most applications the PVC geomembrane will be covered, and a liner temperature lower than 30˚ C (86˚ F) will not be attained.  Canadian and American users are well-aware that PVC membranes used as exterior pool liners can withstand very cold temperatures for many winters (more than 10 years) without cracking or suffering structural damages.

A seventh misconception is that a seam cannot be checked properly.  Yes, seams can be checked properly, which is to say they allow both non-destructive and destructive testing options.  For non-destructive measures, dual-fusion seams can be checked using the channel air pressure test similar to the one use for other types of geomembranes.  And chemical seams must be checked using an air lance and/or electrical leak detection techniques.  Destructive testing can be performed on both type of seams to measure the tensile and peel resistances.

Finally, there is a misconception that you cannot use PVC for exposed applications.  Well, yes and no – the general usage product’s blend (i.e., without UV inhibitors or absorbers) cannot resist long exposures; but, UV-resistant formulations will resist breakdown and perform very well for many years.

*Courtesy:  www.geomembrane.com

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