Poly-Flex Introduces Easy Welding Textured Sheet
by Jim Green

A new textured sheet product with smooth edges was introduced by Poly-Flex, Inc. in April of 1996. The new blown-film product contains a smooth edge of approximately 6" in width on both sides of the sheet. This patent-pending innovation was the result of a number of years of research and development work.
The new textured sheet provides the monolithic textured surface preferred for slope applications, but with the welding installation characteristics of smooth sheet. Installers will benefit from the increased ease of installation and the wider window of weldability. In addition to increased weldability, the strength of the joint between the layers should be comparable to smooth geomembrane welds. Available in all standard HD and LL thicknesses, this sheet is now the standard Poly-Flex textured product for all applications.
Initial response from the geotechnical market as well as technical organizations such as GFR has been extremely positive. Let us know how this product works for you.

Does Thickness Control Mean Better Quality?
by Mark Cadwallader

Thickness specification and control is an important consideration for geomembranes. Flat-die geomembrane manufacturing delivers greater thickness and gauge control than blown-film, and has sought to create the impression of improved material quality because of it. Standard thickness specifications are usually thought of as the following:

Nominal
Nominal thickness is typically maintained to a ± 10% tolerance. For example, 60 mil nominal material would be acceptable at any measured thickness between 54 and 66 mils, although upper thickness limits are usually not enforced.

Minimum Average Roll Value
The ± 10% tolerance is still applied to the nominal thickness, however, the average of all readings must be the nominal thickness or greater. For example, 60 mil minimum average roll value (M.A.R.V.) material could have individual readings as low as 54 mil. but the average of all readings must be 60 mil or greater. To manufacture this material, the target manufacturing thickness is set above 60 mils to assure the average is at least 60 mil. This requires more raw material to be used at a higher cost per square foot.

Minimum
No individual reading may fall below the nominal thickness. For example, at ± 10% manufacturing tolerance, 60 mil minimum material could have individual readings between 60 and 72 mils. To manufacture this material the target manufacturing thickness would be set at 66 mils to assure that no reading falls below 60 mil. In other words, even though the specification says 60 mil, 66 mil is what must be supplied at an even higher cost per square foot.

Obviously, thickness control provides a slight economic advantage for flat-die production, by not giving away excess resin to meet minimum specifications. Yet this is only one factor in the competitive mixture of production economics. Furthermore, blown film provides certain advantages of polymer orientation not present in flat-die produced material.
Due to weight of the vertical column of film, the blown film process tends to introduce a degree of balanced orientation to the liner. Polymer orientation has been purposely incorporated into the production of HDPE pipe and geogrid because it improves creep resistance, stress crack resistance, chemical resistance, and overall durability of the material. When compared with unoriented HDPE, oriented pipe has exhibited enhanced mechanical and physical properties (1). Thus for the same resin, and all else being equal, blown film liner can arguably be said to have better quality for the purposes of long term performance as a geomembrane than flat-die liner.
Thickness control has very little to do with the performance properties engineers are concerned about. Thickness control has been for the most part a means of creating an "impression" for flat-die technology and of introducing a slight economic advantage. Practical contractors and engineers want a little more substance. Thickness variation does not influence material quality nor project quality, especially with the welding and installation methods of today.
The gap between gauge control of flat die produced geomembrane and blown film produced geomembrane is currently being narrowed by advances in gauge control for the blown film production. The thickness control gap will probably never be completely bridged due to the inherent difficulties of thickness uniformity in concentric-circle die lips. But considerable progress is being made.
Poly-Flex, Inc. has introduced a number of process improvements that have resulted in decreased thickness variation. Today in most products, the actual tolerance is better than ± 10% allowing a more aggressive pricing strategy on M.A.R.V. and minimum thickness specifications. Process improvement research and development will continue in an effort to further close the gap.

(1) Ward, I.M., et al, The Production and Properties of Die-Drawn PE Pipe, Plastic Pipes VI, York, England, 1985.