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T E C H N I C A LN O T E S

RES IN QUAL ITY AND IMPACTON PERFORMANCE FOR HDPE GEOMEMBRANES

Covering the world.Protecting the Earth.

Rev.

05.1

5

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WHAT MAKES A GOOD GEOMEMBRANE

Geomembranes are liners or barriers of polymeric material used to control liquid or gaseous fluid migration in environmental, geotechnical, hydraulic, transportation and private development applications. Given their superior properties, HDPE geomembranes have been the ideal choice in various fields of applications, across several industries. These liners offer good containment options in several engineering applications thus preventing environmental hazards and limiting the anthropogenic impact on the environment.

What makes a high performance and reliable geomembrane are quality ingredients, a tested recipe and an unfailing process. On the field, performance of the liner system is derived largely from the quality of the raw material, polyethylene resin, as well as processing, installation and construction quality insurance.

In order to assess a liner’s quality; physical, mechanical and endurance properties are evaluated. Mechanical properties are a function of the resin. Manufacturers using the same resin should have the same mechanical properties. They can however differentiate themselves from one another through their recipe and process which determines physical as well as endurance properties.

Commercially available HDPE geomembranes use polyethylene resin in the range of 0.934 and 0.938 mg/l. The resin itself is actually in the medium-density range (MDPE). Only by adding carbon black and additives to the mixture is its formulated density is raised to 0.941 mg/l or slightly higher. Thus, what is called HDPE by the geomembrane industry is actually MDPE resin to producers and manufacturers (Koerner, 2012).

TABL

E 1

3

The Geosynthetics Research Institute (GRI) is the internationally respected technical reference for best practices and industry stan-dards. The GM-13 (HDPE) and GM-17 (LLDPE) Standard specifications constitute the world’s most stringent collection of polyethylene geomembrane minimum technical requirements. The following is a comparative analysis of two resin samples by different producers. The analysis serves to demonstrate how resin quality plays a major role in the properties of a geomembrane thus making it suitable or unfit across various fields of engineering applications.

The two samples in Table 1 demonstrate the basic differences between a resin that will lead to a reliable product (S3) in comparison to ones that will lead to liners with lesser properties and a shorter life-span requiring extensive and costly packaging with additives (S1 and S2). Resin samples were used to produce test pellets in order to evaluate mechanical and endurance properties.

Since numerous material properties are used for actual engineering design calculations and dimensioning, manufacturing processes become irrelevant as long as the raw materials denote acceptable testing values.

COMPARATIVE ANALYSIS OF RESIN SAMPLES

Table 1: Resin Samples – Excerpt from technical data sheets

Sample 1(S1*)

Sample 2(S2*)

Sample 3(S3*)

Density ASTM D792 g/cc 0.955 0.942 0.937

OIT ASTM D3895 Minutes 2.9 96.22 120

HP-OIT ASTM D5885 Minutes 118 305 1000

ESCR* ASTMD5397 hours 9 147 1500

Tensile Properties ASTM D6693

Tensile Strength at Yield for ESCR ASTM D5397 psi 4136 3457 2700

Elongation at Yield % 12.3 13.8 12

Elongation at break % 574 699 800

Puncture ASTMD4833 lbf 229.6 207.7 240

TABL

E 1

* For clarity purposes, the acronym S1, S2 and S3 are used to define the different resins and are used for the sake of text fluidity.

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CRISTALLINITY AND DENSITY FI

GURE

1

75 mil HDPE50

40

30

20

10

0 100 200

-26oC

-7oC+23oC

300 400

Strain (%)

Stre

ss (M

Pa)

500 600 700

Figure 1: Cold Temperature and Freeze-Thaw Cycling behavior of Geomembranes and their seams

Crystallinity refers to the degree of structural order in a solid. The degree of crystallinity has a big influence on hardness, density, transparency and diffusion. In both S1 and S2, crystallinity is a major point of distinction. According to conventional wisdom, the higher the density, the stronger are the intermolecular forces and tensile strength, but the more brittle is the material. Based on ASTM D792 testing method, S1 and S2 have a higher density than S3 meaning that, when packaged, liners made of these resins will be more compact and brittle and have a higher density than a liner made using S3.

The crystallinity of the resin used to manufacture a geomembrane is mostly responsible for the reaction of the latter to temperature. The stress-strain set of curves in figure 1 demonstrates the effect of temperature changes and freeze-thaw cycling on geomembranes and their seams. As temperature decreases, an HDPE geomembrane reaches its elastic limit more quickly. The elastic limit is defined as the limit at which a material will deform without permanent sequels and resume its original shape and size. At +23, -7 and -26 degrees, the stress required to reach the yield point is increasingly higher with a higher elastic modulus. This shows that a highly crystalline, high density HDPE geomembrane is harder to handle in cold temperatures than in a moderate climate but has also a higher risk of deformation and cracking. For engineers and installers, this aspect represents a major deal-breaker as it affects greatly the ease of installation and construction quality insurance.

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OXYDATIVE INDUCTION TIME (OIT)

FIGU

RE 2

AA =

B =

C =

B C

100

50

Aging Time (log scale)

Prop

erty

Cha

nge

(%)

depletion timeof antioxidants

induction time to onsetof polymer degradation

time to reach 50%degradation of a particular property

Figure 2: Stages of geomembrane service-life

Oxidative induction time (OIT) is the measure of antioxidants depletion rate. Antioxidants (AOs) are added to the resin and determine partially the endurance properties of a geomembrane. The service-life of a geomembrane is broken down into three stages: depletion time of antioxidants, induction time and degradation. There is a misconception around the fact that only additives determine the endurance properties of a geomembrane. But the reality is that both the quality of the resin and the additives determine the quality and endurance of a the geomembrane.

From a field perspective, exposed geomembranes are more prone to oxidation as they are in direct contact with air. Although oxidation for submerged geomembranes, under tailings or waste for example, happens at a slower rate due to the starved oxygen conditions, the expectations for their service-life are higher. Thus, minimum OIT and HP-OIT requirements as specified by the GRI-GM13 standards define the minimal acceptable standards. Based on ASTM 3895 and ASTM D5885 testing methods, S1 and S2 tested poorly for Oxidative Induction Time (OIT) compared to S3. The GRI specification for OIT requires geomembranes to have an OIT above 100 minutes for HDPE geomembranes. S1 and S2 fall clearly below specifications making them prone to generating liners that have shorter service-lives.

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In the last 15 years, high-density polyethylene (HDPE) materials have improved significantly and now meet more stringent performance standards; some performance evaluation tests that once took only days to run now take months given modern resins. One such performance criteria is Environmental Stress Crack Resistance (ESCR). Because standard test methods for measuring ESCR on plastics take such a long time (sometimes months) the quality of the material is judged acceptable if the failure time exceeds a certain limit. Many times, the test is terminated before an absolute fracture time is determined. Due to these long testing times, new tests and standards have been developed to differentiate these improved materials more easily. The major variables that affect ESCR in polyethylenes include Molecular Weight, Molecular Weight Distribution, Chain Branching (measured

The definition of stress cracking according to ASTM D883 is “an external or internal crack in a plastic caused by tensile stresses less than its short-term mechanical strength.” This type of cracking typically involves brittle cracking and poor ductility of the material. Slow crack growth is another term commonly used to describe stress cracking. ‘’Environmental stress cracking’’ or ESC is a behavior of polymeric material in harsh weather conditions or in the presence of active wetting agents. ESC manifests in microscopic imperfections and propagates through the crystalline regions of the polymer structure. The ability of a polymer to resist slow crack growth or environmental stress cracking is known as ESCR. Different polymers exhibit varying degrees of ESCR.

ENVIRONMENTAL STRESS CRACKING RESISTANCE

indirectly by density), and ESCR testing conditions (i.e., reagent concentration, temperature, stress). In general, resistance to

slow crack growth (ESCR) decreases as the amount of crystallinity increases in a material. Due to the higher crystallinity of S1 and S2, the ESCR test result is obtained as early as 147 hours whereas the minimum required value by GRI-GM 13 is 500 hours based on ASTM 5397 testing methods.

Many high density polyethylenes may crack when exposed to chemical environments or when submitted to tensile stresses. The standard polyethylene resins in North America are specially formulated to resist stress cracking. Therefore, stress cracking of polyethylene geomembranes should not be an issue with resin density below 0.937 g/cc and stress crack resistance is not required by GRI-GM17 for LLDPE geomem-branes (EPA Technical Guidance Document, 1993).

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Polyethylene geomembranes are there to serve a clear purpose: to be a reliable containment product and protect the environment. Soil contamination, among other environmental hazards, is a very common repercussion of anthropogenic activities. As resin producers and geomembrane manufacturers, we are responsible for ensuring good quality products and safe industry practices.

As manufacturers, it is utterly important to review the technical data sheet and test raw materials quality in order to ensure acceptable mechanical properties and produce superior quality products.

In several applications, the behavior of geomembranes in specific environmental and weather conditions will define its ease of installation, wrinkling and puncturing. The emergence of reflective white geomembranes contributed to solving several quality issues. However, highly crystalline and below minimum requirements raw materials will still generate low quality geomembranes with short service-lifes.

Choose your liners carefully, but even more so, beware of your raw materials quality and be a part of distinctive manufacturing practices.

OTHER MECHANICAL PROPERTIES

CONCLUSION

Several tests were developed to assess the mechanical properties and strength of polymeric sheet materials. Many have been adopted for use in evaluating geomembranes. Tensile tests are performed routinely for quality control and quality insurance and are indicative of material strength. Geomembranes are layed out on or backfilled with soil containing stones, sticks or hard debris. They are susceptible to puncture during and after loads are placed on them. Such puncture is important to consider as it occurs after the geomembrane is covered and cannot be detected until a leak from the completed liner system becomes obvious. Repair costs are then enormous and environmental hazards well underway. Numerous side slope failures have occurred. Often, cover soil slides over the geomembranes, but sometimes, the geomembrane fails and pulls out of the anchor trench moving on a lower friction surface beneath (Koerner, 2012). The three resin samples compared denote significant differences in tensile properties. On pallets, samples 1 and 2 demonstrated significantly weaker test values compared to sample 3.