Sealing projects in groundwater protection, civil engineering, structural sealing and corrosion protection place different kinds of requirements on the sealing membrane.  The material properties, structure and surface design, and thickness of HDPE sheets are adapted to the specific requirements of a project.
The following criteria are especially important for raw material selection:

• Dimensional stability
• Behavior under mechanical stress
• Relaxation Behavior
• Chemical resistance
• Resistance to biological attack
• Stress-crack resistance
• Ageing resistance (behavior)
• UV resistance
• Impermeability
• Panel joints as strong as the parent sheet

 State-of-the-art manufacturing, on-site welding, constructive design and speed of installation are other very important factors

• Stress Factors
  Sealing membranes are generally exposed to stresses that are cause mainly by the following factors:
  • Ageing
  • Physical Stress, e.g. tensile strain
  • Biological attack, e.g. microorganisms The demand of state-of-the-art technology for large area sealing properties require the
  • widest range of properties to effectively resist these stresses over the long term.

Years of experience have proven that High Density Polyethylene (HDPE) meets all these requirements. This material has a high crystalline content combined with   excellent mechanical strength, flexibility, and deformability. HDPE is according to present scientific and state-of-the- art technology, an ideal combination for environmental protection application.

Physical :

• The liner must be free of holes, pores, voids and inclusions
• The liner must have a minimum density within the tolerance limits .
• The liner strength and deformation properties must be greater than the required minimum value .
• The anticipated low and high temperatures must not alter its dimensions or physical and mechanical properties
• The permeability of water or other stored media must be negligible
• The liner must have good stress crack resistance.
• The liner must have good UV resistance .

Chemical :

• The liner must resist the concentration of the contained media for the planned project's lifetime .
• The liner must resist diluted liquids and gasses that it may come into contact with during the operation of the project .

Biological :

• To sustain its physical properties and ensure a long working lifetime the lining must be resistant to biological attacks
• An essential element must be the biological resistance to soil contaminants and soil microorganisms .
• As a ground barrier the liner s mechanical properties must resist attacks from small mammals, roots, and insects .

General :

• Sound site joining capability .
• Ease of joint quality assurance testing methods .
• Ruggedness under site conditions .
• Ease of handling and processing .

HDPE Properties

• HDPE membrane liner is produced from virgin polyethylene resin , entirely free of plasticizers or other filler materials. HDPE is enhanced with carbon black to provide the best protection against the effect of ultraviolet radiation

• HDPE membrane liner has unique physical, chemical, biological, and mechanical properties. Within a certain range, properties may be varied to suit desired specifications for specific liners. The desired properties are usually determined after careful consideration of prevailing conditions under which the liner will be installed and operated. Of prime importance are the following criteria:
  • Strength characteristics.
  • Elastic and plastic deformability.
  • Relaxation characteristics.
  • Chemical rankness.
  • Stress cracking resistance.
  • Ageing resistance.
  • Process ability.
  • Thickness.

1. Dimensions
   • HDPE membrane liner is manufactured in up to 7.3 m wide rolls, up to 200m long, depending on thickness. Standard rolls are of a length providing a roll weight of about 1600kg. In the past, 10 m wide rolls were produced; however, experience has shown that 7.3 m wide rolls provide optimum results considering ease of site handling, installation, welding, quality assurance and cost factors such as production and transportation.
   • HDPE membrane liners are produced from 0 .75 mm to 6.0 mm in thickness.
   • The large dimensions of the HDPE membrane rolls result in a minimum of site welds. Accordingly, the combination of optimum unit sizes and minimum site joints ensures a most cost effective installation.
     
     
2. Melt Flow index
   The melt flow index measures the process ability of the polyethylene. This property determines the possibility of extruding the polyethylene both for membrane manufacture and also for welding during installation on site. Melt index is not a membrane performance property. The test method often used for geomembranes polymers is ASTM D 1238. A given amount of the polymer is heated in a furnace until it melts. A constant load pushes it through an orifice and out of the bottom of the test device. The melt index value is the weight of extruded material in grams for l0 min. Duration, the higher the value of melt flow index, the lower the density of the polymer, all other things being equal. This in turn suggests a lower molecular weight.
   
   
3. Water Absorption
   Water absorption indicates the capability of absorption and diffusion of the sealing membrane with regards to polar liquid media. The relatively low residual water absorption of polyethylene can be attributed to the hygroscopic properties of the carbon black that is added to the material. From a technical point of view, there is on water absorption of the polyethylene itself.
   
   
4. Water tightness - Permeability
   Flexible liner membranes based on polyethylene are, when correctly manufactured, water tight with respect to liquid media. The material absorbs very little water. Contrary to mineral liners, permeation is not influenced by differential pressure. FML's are impermeable barriers for all inorganic pollutants, and the permeation of hydrocarbons that contaminate groundwater is very low, especially in comparison to other liners. The very small permeation that does occur is not a flow process, but is based on sorption and diffusion processes. This depends on the type and combination of permeates and difference in concentration between the upper and lower side of the liner layers. The crystalline of the density, the construction of the lining system and the thickness are HDPE sheet properties affecting permeation that can be adapted to the Specific requirements of individual projects.

WATER VAPOR TRANSMISSION VALUES AFTER HAXO {2}

• Indentation Hardness
  Liners will usually be subjected to gravel and sand under high liquid weights or pressures and are exposed to indentation action. Membranes must have relatively high surface hardness to resist these pressures and puncturing. HDPE combines high surface hardness together with other mechanical properties that enable it to move smoothly when exposed to sand and gravel.
  
  
• Stress - Strain Behavior of HDPE
  
  1. HDPE under stress may undergo elastic (reversible) or plastic (irreversible) deformation. The limit of the elastic performance is typically reached for HDPE at a deformation of 15-20% of the original length (elongation at yield). However, HDPE will continue to deform under higher stresses up to deformation of 500% to 1000% (elongation at break).
     
     
  2. The HDPE stress strain behavior is characterized by four main Parameters which are usually determined by the standard tensile test
     -Tensile stress at yield
     -Tensile stress at break
     -Elongation at yield
     -Elongation at break
     

     Stress-Deformation Behavior
     (Tensile Load)
     

  3. There are a number of tensile tests Performed on a geomembrane samples. The response for several of these same geomembranes is given in Figure.
     

     
     Index tensile results of commonly used geomembranes

     Quantitative data gained from these curves are focused around the following.
     • Minimum stress (at ultimate for PVC and VLDPE, at scrim break for CSPE-R and at yield for HOPE).
     • Minimum strain (as noted earlier). o Modules (the slope of the initial Portion of the stress – strain Curve).
     • Ultimate stress (at Complete Failure). o Ultimate strain (at Complete Failure).
       
       
     For the three materials shown in figure. These Values are given in the first column of table. While all of the listed values of strength are significant, attention is often focused on the stress at Particular allowable strain for materials Like PVC the scrim-breaking stress for reinforced materials Like CSPE-R and the yield stress for HDPE materials. It must be recognized, however, that polymers are viscelastic materials and strain invariably plays an important role.

  Tensile Behavior Properties of30-Mil PVC, 36-MiLCSPE and 30-Mil HDPE

  Property	Dumbble shape			Narrow-width (1.0-in {25 mml}) shape			Three-dimensional shape
  	PVC	CSPE-R	HDPE	PVC	CSPE -R	HDPE	PVC	CSPE-R	HDPE
  maximum stress* Ib./in.2 (megapascals)	3400	5700	3200	2900	5100	3000	1200	3300	2300
  	23	39	22	20	35	21	8.3	23	16
  Maximum strain* (%)	300	17	11	300	35	13	120 +	100	47
  modulus (Ib./in.2) (megapascals)	9000	33,000	94	9000	15,000	40000	4000	5,000	25000
  	62	227	648	62	103	275	28	34	172
  ultimate stress(lb./in.2)(megapascals)	3400	1300	≈4000	2700	1200	≈3500	d.n.f.	3300	2300
  	23	9	28	19	8.3	24		23	16
  Ultimate strain (%)	300	100	≈700	300	58	≈600	d.n.f.	100	47

  Notes:
  PVC   : values are at ultimate       CSPE-R: values are at scrim break
  HDPE: values are at yield            d.n.f. = did not fail
  
  
• Impact Resistance
  This property describes the material behavior under very low temperature and/or rapid deformation - causing loads. Falling objects, including cover materials, can penetrate geomembranes, causing leaks themselves or acting as initiating points for tear propagation to proceed from thus an assessment of geomembrane impact resistance is necessary. The thicker geomembranes have greater impact resistance than do the thinner ones. The effect of scrim reinforcement is not significantly different from that of nonrein forced. There is a significant difference in impact resistance between different types of geomembranes. For geomembranes with greater impact resistance, or for geotextiles underlying and/or overlying the geomembrane, significantly higher impact resistance will result.
  
  
• Long Term Behavior
  In practice, membrane liners can be subjected to sustained deformation and stress. Sustained deformation is of particular importance. A high quality HDPE membrane should have an optimum combination of creep and relaxation properties. With constant long term deformation, the material relaxes and with correct dimensioning can reach an almost stress-free state. It must be noted, however, that higher stress results in higher deformation and higher temperatures result in more rapid creep.
  
  
• Stress - Crack Resistance
  FML5 are exposed to combined chemical and mechanical attacks. Some polymer materials will undergo stress - crack corrosion and ultimate failure when exposed to these kinds of attacks. Polyethylene that has a density in the range of 0.93 to 0.94g /cm3 is especially stress – crack resistant.

  Stress Cracking (Bent Strip):

  In ASTM D 1693, small test specimens of 1.5 by 0.5 in. (38 by 13mm) are prepared With a Controlled imperfection on one surface, which is a notch about one half of the thickness running centrally along the long dimension. The specimens are bent into a U shape and placed within the flanges of a channel holder. This assembly is then immersed in a surface wetting agent at an elevated temperature, usually 50°c. Since stress - records the proportion of the total number of specimens that crack in a given time. Usually geomembrane specifications call for a zero number of stress-cracked specimens within: 1500 hr. The test is not very challenging to geomembranes due to the ability of the test specimens to stress relax during the test. A newer and more challenging test for stress crack resistance of plaques made from resin pellets, geomembrane sheets and geomembrane seams is described stress-cracking (constant load).

• Surface Design " Geo-mechanical Behavior
  The geo-mechanical behavior describes the interaction between the FML and the ground layers. Deformability and liner strain under loading are affected by the geo-mechanical properties: friction and adhesion. With regard to the FML's behavior when exposed to settlements and differential settling, the smooth surface of HDPE sheet ensures that locally initiated loading is distributed over large, areas which leads to low deformation, over loading is thus avoided. Liner systems on inclined surfaces, e.g. capping or base liners for hillside landfills in mountainous areas, may cause instability and result in sliding problems. By enhancing the friction coefficient on one or both sides of the HDPE sheet DRS, exact adaptation to these situations can be obtained. The geo-mechanical properties of HDPE sheet DRS; ensure that the joining of sheet/earth will have the same shearing resistance as the ground itself. The surface structure of HDPE sheet DRS does not change its characteristics tensile deformation behavior, and overloading does not occur because forces from the ground are distributed over the whole liner.

1. Density
   -Density indicates the crystalline of a polyethylene, which, together with the molecular structure of the material, greatly influences its stress - strain behavior. Polyethylene, with a high degree of crystalline, will offer high strength but low deformation. Middle to high density polyethylene offers a very good combination of strength and deformability, and has very high stress-crack resistance at a relatively low degree of crystalline .
   
   -In current practice, the term "high density polyethylene (HDPE)" is used to describe geomembranes whose base resin may actually be medium density polyethylene (MDPE). There are several resins of different densities currently used in the manufacture of polyethylene geomembranes, see Table 3.1.
   
   -The density ranges on Table 3.1 are for the basic polymer, i.e. there’s in, before addition of carbon black and other additives to both increase performance and durability or assist in production. This document will utilize the ASTM designation HDPE to reflect the material in use today. Very low density polyethylene (VLDPE) resin falls into the density range below 0.910 g/cc and is not yet designated by ASTM

Table 3.1 Polyethylene Types.*

Acronym	Type	Nominal Density Range	ASTM D 1248 Type
HDPE	High Density Polyethylene	>=0.960 g/cc	IV
HDPE	High Density Polyethylene	0.941 to 0.959 g/cc	III
MDPE	Medium Density Polyethylene	0.926 to 0.940 g/cc	II
LPDE	Low Density Polyethylene	0.910 to 0.925 g/cc	I
VLDPE	Very Low Density Polyethylene	<0.910 g/cc	0

Uncolored, unfilled materia
2. Coefficient of thermal Expansion
   

   The coefficient of expansion is an important material and sheet property which describes its behavior under thermal changes. HDPE like most plastic products has a higher coefficient of thermal expansion than some common construction materials such as wood and concrete. The coefficient of thermal expansion for HDPE is not constant, but is of the order of 0.012% per degree centigrade change in temperature. This property must be accounted for in lining design and installation. There are a number of procedures that can be used to determine the coefficient of thermal contraction or expansion of a material for example, ASTM D2102 and D2259 for contraction, and D 1042 for expansion and dimensional changes. All of them subject the test specimen to a constant source of cold (or heat) and carefully measure the separation distance between two given initial locations. Some typical data are presented in Table. An example using this data in adding slack during the installation of a Geo.-membrane is given below.

   Example: Calculate the amount of slack to be added during the installation of a HDPE liner for a surface impoundment anticipating a 100°f temperature change. Base the calculations on a 100 ft. side slope distance.

   Solution: Minimum: 8 x 10'5 (100) (100) (12) = 9.6 in.
   Maximum: 12 x 10-5 (100) (100) (12) = 14.4 in.
   Easily seen is that the calculated amounts are very significant and the importance of adding slack for temperature compensation cannot be overemphasized.
   
   

   COEFFICENTS OF LINEAR THERMAL EZPANSION
   (VARIOUS REFERNCES)
   

   Polymer type	Thermal linear expansively x 10'5
   	Per1°F	Per1°C
   Polyethylene
   high density	8-12	15-22
   medium density	6-8	11-15
   low density	5-7	9-13
   Polypropylene	3-5	5-9
   Polyvinyl chloride
   unplasticized	3-10	5-18
   35 % Plasticizer	4-14	7-25
   Polyamide
   nylon 6	3-4	5-7
   nylon 66	4-5	7-9
   Polystyrene	3-4	5-7
   Polyester	3-4	5-7

3. Effect of temperature
   -The membrane may be subjected to high temperatures either due to the contained material or expos
   
   - Increased temperatures have two opposing effects, on one hand they reduce the membrane internal tension through stress relief due to the increased flexibility under elevated temperatures, on the other hand, the increased temperature reduces the material strength. However, even at temperatures as high as 80° C, the HDPE retains a strength of about 6 N/mm2; other thermoplastics used for membrane liners would be severely softened and weakened before reaching these temperatures. High quality HDPE membrane can perform satisfactorily at temperatures ranging from - 40° C to +80° C, with even short shock loading temperatures exceeding even those values
   
   
4. Chemical Properties
   -The chemical resistance of a geomembrane vis-a-vis the substance (S) it is meant to contain is always important, and often it is the most critical a aspect of the design process, for example, in domestic waste or hazardous waste containment, the pollutant will interface directly with the geomembrane. Thus the geomembrane resistance must be assured for the life of the facility.
   
   - A material is chemically resistant when its mechanical properties are not altered irreversibly or negatively by the media to which it is exposed. HDPE has an extraordinary resistance to ambient conditions of organic or inorganic solvent attacks. The paraffin structure of HDPE accounts for its extremely high chemical resistance, therefore, use of HDPE lining in acids and alkaline solutions is highly recommended. HDPE lining systems tested long term (more than 10,000 hours (SLT, 1982)) in various waste disposal sites showed no change in the physical properties even under high temperatures (70°C) These tests were applied on the liner joints as well as the lining material itself.
   

   General chemical resistance guidelines of commonly used geomembranes*

   Geomembrane Type

   Chlorinated polyethylene (CPE)

   Chloro- sulfonated polyethylene  (CSPE)

   Ethylene propylene diene monomer  (EPDM)

   polychloro prene  (neoprene)

   polyethylene

   polyvinyi chloride  (PVC)

   Chemical

   100°f

   158°f

   100°f

   158°f

   100°f

   158°f

   100°f

   158°f

   100°f

   158°f

   100°f

   158°f

   General:

   Allphatic

   hydrocarbons

   x

   x

   x

   x

   x

   x

   Aromatic

   hydrocarbons

   x

   x

   x

   x

   Chlorinated

   solvents

   x

   x

   x

   x

   Oxygenated

   solvents

   x

   x

   x

   x

   x

   x

   Crude

   petroleum

   solvents

   x

   x

   x

   x

   Alcohols

   x

   x

   x

   x

   x

   x

   x

   x

   x

   x

   x

   x

   Acids:

   Organic

   inorganic

   Bases:

   Organic

   x

   x

   x

   x

   x

   x

   x

   x

   x

   inorganic

   x

   x

   x

   x

   x

   x

   x

   x

   x

   x

   x

   Heavy

   x

   x

   x

   x

   x

   x

   x

   x

   x

   x

   x

   Metals Salts

   x

   x

   x

   x

   x

   x

   x

   x

   x

   x

   x

   *x = generally good resistance. Source: After Vandervoort [21].
   
   
5. Biological Properties
   

   There are a tremendous number of living organisms in the soil.

   • A major concern for soil-buried geomembranes is the burrowing of animals through them. Fungi include yeasts, molds, and mushrooms, they depend on organic matter for carbon, nitrogen, and other elements, their numbers can be very large, as much as 10 to 20 million per gram, of dry soil.

    

   • Bacteria are single-cell organisms, among the simplest and smallest known forms of life. They rarely, exceed 5mm in
   • Length and are usually round, rod like. Or spiral in shape their numbers are enormous: more than 1 billion per gram of soil. As with fungi the greatest concern of bacteria regarding geomembranes is not polymeric degradation, but fouling and clogging of the drainage systems often constructed in conjunction with the geomembrane itself.
     
     
6. Weathering Resistance
   Increased temperatures, ozone and UV light are weathering effects than can lead to irreversible damage in the material structure generally classified as aging. By adding an exact amount of carbon black to the HDPE sheet material, excellent weathering stability can be achieved. The thermal- oxidative and photochemical influence on sheets has been tested in artificial time compression tests; these tests have shown that HDPE sheet offers long-term weathering resistance which has been confirmed by numerous projects carried out in various climatic zones in the past 15 years. HDPE sheet stabilized with carbon black can be exposed to almost all climate zones for more than 2 years without loss of essential properties.

• Geo-textile
  Geo-textiles consist of synthetic fibers rather than natural ones such as cotton, wool. or silk. Thus biodegradation is not a problem. These synthetic fibers are made into flexible, porous fabric by standard weaving machinery or are matted together in a random or non woven, manner. some are also knit. The major point is that they are porous to water flow across their manufactured plane and also within their plane, but to a widely varying degree. The majority of geotextiles are made from polypropylene or polyester polymers formed into fibers or yams and finally into a woven or non- woven fabric, which when placed in the ground is a Geotextile. In general, the words fabric and Geotextile will be used interchangeably. The choices of fabric styles are as follows:
  
  • Woven monofilament
  • Woven multifilament
  • Woven slit-film monofilament
  • Woven slit-film multifilament
  • Non woven continues filament heat bonded
  • Non woven continues filament needle punched
  • Non woven staple needle punched
  • Non woven resin-bonded
  • Other woven or non woven combinations
    
    
• Geotextile Functions & Mechanism
  

  There are at least 80 specific application areas for geotextiles that have been developed, however the fabric always performs at least one of five discrete functions :

  1. Separation
  2. Reinforcement
  3. Filtration
  4. Drainage
     Liquid barrier (When impregnated)