Abstract:
A peroxide treated LLPDE material that can used as a geomembrane liner in land areas storing toxic waste, municipal landfills, leachate or slurry ponds, and other such land containment applications as well as applications in which a strong, non-biodegradable material which is impermeable to air and moisture is needed.

Description:
FIELD OF THE INVENTION 
     The invention relates to a linear low density polyethylene material that can be formed into thick sheets for use in geomembrane and other applications. 
     BACKGROUND OF THE INVENTION 
     It is known that linear low density polyethylene (LLDPE) when treated with peroxide has an improved transparency for use in making thin polyethylene films. Furthermore, peroxide treated LLDPE can be used to make finished objects such as film under operating conditions used for the conversion of high pressure polyethylene while still maintaining the excellent mechanical properties of LLDPE such as: greater rigidity at equal density; superior hot mechanical strength; improved resistance to cracking under tension; less jamming when tubular film is being produced; and better stretchability and improved resistance to perforation in film form. 
     Specifically, a process is known for improving melt strength of LLDPE by pretreating the polymer in melt form at a temperature of at least 230° C. in the presence of an organic peroxide for a period of time equal to at least three times the half-life of the organic peroxide at the melt temperature. Such a treatment improves bubble stability in blown film extrusion. 
     It is also known that processing characteristics of LLDPE may be improved by adding a chemically modified ethylene polymer to a base resin. The use of the modified polymer appears to improve the melt strength of the overall blend, providing greater bubble stability when blowing films from such blends. 
     As stated above, peroxide treated LLDPE is known to be effective when used for the fabrication of thin films. Such thin films are used to manufacture various products, especially products used in conventional food wrapping and packaging, such as grocery sacks, plastic bags, bread bags and trash bags. In all of these applications, the material manufactured from the peroxide treated LLDPE is a very thin, flimsy material. 
     Polyethylene is also used to fabricate thicker materials which have quite different applications than food or trash packaging. One polyethylene application which has assumed increased importance in recent years is its use for geomembranes. These are the sheets of material used to line areas of land where toxic waste is stored, leachate ponds, municipal landfills, and other such types of land containment areas. Conventionally, the bottoms and sides of these land areas are lined with high density polyethylene (HDPE) sheets having a thickness of 40-100 milliinches (mils). The material produced is strong, non-biodegradable, impermeable to air and moisture, and is effective as a barrier against leakage of the toxic waste, household chemicals and other such liquids into the soil and drinking water. However, when such material is used to cover the top of such an area, it is too rigid and lacks the capacity to stretch and form a leak-proof barrier around the top of the often irregularly shaped material underneath. 
     Pure LLDPE has not been used as a material for geomembrane applications because previous attempts to fabricate materials of the thickness necessary for such applications (i.e., in the range of 30-100 mils) have been unsuccessful. Attempts to extrude either blown or cast (flat) LLDPE sheet material at such a thickness range have been unsuccessful because the material simply will not hold together during processing. 
     A material has been needed for geomembrane applications which could be formed into thick sheets which would be strong, non-biodegradable, impermeable to air and moisture, but which would also have the capacity to stretch and change shape in order to provide an effective covering for a toxic waste storage area, leachate ponds, municipal landfills, and other such land containment areas. 
     The present invention provides such a material made from peroxide-treated LLDPE material formed to the specifications needed to provide cover and liners for geomembrane lined land areas where which toxic waste and household chemicals are dumped, slurry ponds, and other land containment applications. The present inventors has also successfully overcome previous failures to extrude in either cast or blown sheet from LLDPE materials having a thickness in the range of 30 to 100 mils. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide a peroxide modified LLDPE material having a thickness of from 0.030 to 0.100 inches. 
     Another object of the present invention is to provide a peroxide modified LLDPE material having an axi-symmetric strain at failure of from 50 to 130%. 
     Still another object of the present invention is to provide a peroxide modified LLDPE material having a stress at rupture (break) of from 600 to 5000 psi. 
     Another object of the present invention is to provide a peroxide modified LLDPE material having a percent elongation at rupture (break) of from 30 to 1000%. 
     Yet another object of the present invention is to provide a peroxide-treated LLDPE material made by a method comprising the steps of: 
     (a) mechanically compounding linear low density resin with a solid or liquid form of peroxide in a co-rotating twin screw extruder to make a peroxidetreated resin; and 
     (b) extruding the peroxide-treated resin as a blown sheet either smooth or textured. 
     Still another method of the present invention is to provide a method of using linear low density polyethylene resin to make a geomembrane liner comprising the steps of: 
     (a) mechanically compounding linear low density resin with a solid or liquid form of peroxide in a co-rotating twin screw extruder to make a peroxide-treated resin; and 
     (b) extruding the peroxide-treated resin as a cast or flat sheet either smooth or textured. 
     Yet another object of the invention is to provide a method of treating linear low density polyethylene (LLDPE) resin with peroxide for use as a geomembrane film comprising mechanically compounding linear low density resin with a solid or liquid form of peroxide in a co-rotating twin screw extruder to make a peroxide treated resin. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph plotting the stress (psi) against the elongation (%) for a smooth, non-impled, geomembrane sheet sample. 
     FIG. 2 is a graph plotting the shear viscosity (poise) of 7105, 7108 and ER2257 against the shear rate (1/s) at 230° C. 
     FIG. 3 is a graph plotting the shear viscosity (poise) of 7105, 7108 and ER2257 against the shear rate (1/s) at 190° C. 
     FIG. 4 is a graph plotting the shear viscosity (poise) of ER3212 against the shear rate (1/s) at 190° C. 
     FIG. 5 is a graph plotting the shear viscosity (poise) of ER3212 against the shear rate (1/s) at 230° C. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention relates to a material comprising peroxide treated LLDPE. When peroxide treated LLDPE is extruded in the form of a sheet having a thickness in the range of from 30 to 100 milliinches, a material results which has qualities of flexibility, strength and non-biodegradability which are ideal for use as a geomembrane liner material for lining and covering land areas of toxic waste sites, municipal landfills, leachate ponds, and other such land containment areas. The following non-limiting examples describe the conditions under which several embodiments of peroxide treated LLDPE resin and geomembrane materials were made. 
     EXAMPLES 
     Example 1 
     13,923 pounds of peroxide treated LLDPE type pellets are obtained by placing 50-pound batches of conmix (polyethylene pellets) (7109Z) in a blender/extruder (Extruder No. 58MM) with 284 grams Zinc Oxide, 711 grams Irgafos 168, 142 grams Lupersol 101 (peroxide), and 711 grams Irganox 1010. Mixing time in the blender is 30 minutes. Temperatures in the extruder are kept within the range of from 400 to 500° F. Extruder speed is 318 rpms at a production rate of 300 lb/hr. Gear pump suction is 300 psi. Melt index for the peroxide treated pellets is measured intermittently throughout the mixing process to assure that the melt index is maintained in the range of from 0.25 to 0.55 gm/10 min. The peroxide treated LLPDE pellets obtained have a melt index in the range of from 0.39 to 0.41 gm/10 min., an HLMI in the range of from 20-21 gm/10 min., a melt flow ratio in the range of from 50 to 54, a density of 0.921 gm/cc, and an oxidative induction time in the range of from 87 to 120 minutes. 
     Example 2 
     The peroxide treated LLDPE pellets (ER2257) of Example 1 are used to manufacture, by conventional sheet extrusion methods, sample sheets of geomembrane material having a thickness in the range from 0.040 to 0.043 inches. Thickness of the test specimens was measured in accordance with ASTM D5199. Axi-symmetric tensile testing was performed using a 24″ diameter pressure vessel. Pressure was ramped at 1 psi per minute. System pressure was monitored via a pressure transducer and self adjusted every 5 seconds to maintain the pressure ramp. Center region deformation was monitored using a sonar transducer. Data was acquired via a computer monitoring system that collected both pressure and displacement data every 1 second. Samples were of a smooth (non-dimpled) variety. Axi-symmetric tensile properties (ASTM D 5617) of various of these samples were tested. Stress at yield was in a range of from 1119 to 1208 psi. Percent elongation at yield was in a range of from 12.5 to 27.4%. Stress at rupture was in a range of from 676 to 1017 psi. Percent elongation at rupture was from 35.7 to 116%. For each sample, the ruptures in the material as a result of the testing were multi-directional tears. Furthermore, the ruptures were sufficiently far enough away from the edge of the device to assume that the device itself did not lead to the rupture in the material. FIG. 1 plots the stress (psi) against the elongation (%) for a smooth (non-dimpled) geomembrane sheet sample (864-8001). 
     Example 3 
     Two shipments (7108 and 7105) of non-peroxide-treated LLDPE resin are prepared under slightly different conditions than the shipment (ER2257) described in Examples 1 and 2. The LLDPE resin pellets were prepared with at least one of the following criteria differing from ER2257. The differing resin criteria include: Melt Index (Ml) (gm/10 min, ASTM D1238 Condition E), High Load Melt Index (HLMI)(gm/10 min., ASTM D1238 Condition F), and Melt Flow Ratio(MFR). Table 1 tabulates the comparative qualities of the peroxide treated LLPDE resin of the three shipments as well as the comparative qualities of geomembrane materials manufactured from the three resin shipments. 
     
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Property 
                 7108 
                 7105 
                 ER2257 
               
               
                   
               
             
             
               
                 Density, gm/cc 
                 0.92 
                 0.92 
                 0.921 
               
               
                 Melt Index, gm/10 min. 
                 0.8 
                 0.5 
                 0.4 
               
               
                 HLMI, gm/10 min. 
                 26 
                 14 
                 20 
               
               
                 MFR 
                 33 
                 28 
                 50 
               
               
                 Ash Content, ppm 
                 770 
                 450 
                 700 
               
               
                 Oxidative Induction Time 
                 &gt;100 
                 &gt;100 
                  87-115 
               
               
                 (OIT), min. 
               
               
                 Environmental Stress Crack 
                 &gt;6050 
                 &gt;6050 
                   
               
               
                 Resistance (ESCR) FO hours, 
               
               
                 10% Igepal, 50° C. 
               
               
                 Tensile Properties - Resin 
               
               
                 Tensile at Yield, psi 
                 2,840 
                 2,850 
                 2,800 
               
               
                 Tensile at Break, psi 
                 1,260 
                 4,760 
                 5,250 
               
               
                 Elongation, % 
                 800 
                 790 
                 810 
               
               
                 Tensile Properties-Sheet 
               
               
                 Thickness 
                 50 mils 
                   
                 40 mils 
               
               
                 Test Rate 
                   
                   
                 2′/min 
               
               
                 Elongation at Break, % 
                 790 
                   
                 685 
               
               
                 Tear, lbs 
                 31 
                   
                 25-27 
               
               
                 Puncture, lbs 
                   
                   
                 89 
               
               
                 Multi-Axial % Elongation 
                   
                   
                  72-116 
               
               
                 Avg. Tensile Force at Break, 
                 1.7 
                 1.8 
                 2.1 
               
               
                 cN 
               
               
                 Avg. Elongation Ratio 
                 17.3 
                 6.6 
                 6.3 
               
               
                 Molecular Weight Data 
               
               
                 Mn 
                 41,270 
                 43,260 
                 40,300 
               
               
                 Mw 
                 124,680 
                 134,160 
                 122,080 
               
               
                 Mz 
                 336,380 
                 352,350 
                 312,700 
               
               
                 Mw/Mn 
                 3.02 
                 3.08 
                 3.03 
               
               
                   
               
             
          
         
       
     
     FIG. 2 is a graph plotting the shear viscosity (poise) of 7105, 7108 and ER2257 against the shear rate (1/s) at 230° C. FIG. 3 is a graph plotting the shear viscosity (poise) of 7105, 7108 and ER2257 against the shear rate (1/s) at 190° C. 
     Example 4 
     ER3212 was produced on commercial scale production equipment under conditions similar to ER 2257 as described in Example 1. 
     Table 2 contains resin physical property data for ER3212, a peroxide treated LLDPE resin similar to ER2257. 
     
       
         
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 ER 3212 Resin Data 
               
             
          
           
               
                 Property 
                 G078367 
                 G078369 
                 G078370 
                 G078374 
                 G078378 
                 G078382 
                 G078385 
                 G078389 
                 G078396 
                 G048720 
                 G048726 
               
               
                   
               
               
                 Density, gm/cc 
                 0.9200 
                 0.9205 
                 0.9207 
                 0.9208 
                 0.9207 
                 0.9207 
                 0.9205 
                 0.9203 
                 0.9194 
                 0.9198 
                 0.9201 
               
               
                 Melt Index, 
                 0.36 
                 0.35 
                 0.36 
                 0.38 
                 0.38 
                 0.40 
                 0.43 
                 0.45 
                 0.45 
                 0.33 
                 0.34 
               
               
                 gm/10 min. 
               
               
                 HLMI, gm/10 
                 15.5 
                 15.1 
                 15.5 
                 15.8 
                 18.7 
                 15.8 
                 17.6 
                 17.9 
                 18.7 
                 12.9 
                 13.7 
               
               
                 min. 
               
               
                 MFR 
                 43.1 
                 43.1 
                 43.1 
                 41.6 
                 49.2 
                 39.5 
                 40.9 
                 39.8 
                 41.6 
                 39.1 
                 40.3 
               
               
                 Initial Tension @ 
                 5.0 
                 4.3 
                 5.0 
                 5.3 
                 4.4 
                 4.5 
                 4.5 
                 3.4 
                 3.5 
                 6.1 
                 5.8 
               
               
                 190° C. 
               
               
                 Initial Tension @ 
                 3.4 
                 2.8 
                 3.5 
                 3.5 
                 3.5 
                 3.0 
                 2.7 
                 3.4 
                 2.4 
                 3.6 
                 3.4 
               
               
                 230° C. 
               
               
                 Vicat Softening 
                 105 
                 104 
                 106 
                 107 
                   
                 105 
                 105 
                 104 
                 106 
               
               
                 Temp, ° C. 
               
               
                 C-13 NMR Butyl 
                 15.5 
                 16.4 
                 16.2 
                 13.8 
                 13.6 
                 13.1 
                 14.0 
                 16.0 
                 18.7 
               
               
                 Branches 
               
               
                 Additive AB, 
                 625 
                 555 
                 580 
                 580 
                 580 
                 585 
                 600 
                 575 
                 600 
                 590 
                 595 
               
               
                 ppm 
               
               
                 Additive VIII, 
                 1305 
                 1290 
                 1395 
                 1320 
                 1290 
                 1470 
                 1450 
                 1390 
                 1400 
                 1180 
                 1255 
               
               
                 ppm 
               
               
                 Additive XVI, 
                 1610 
                 1500 
                 1545 
                 1505 
                 1585 
                 1455 
                 1620 
                 1555 
                 1665 
                 1615 
                 1535 
               
               
                 ppm 
               
               
                 GPC Data 
               
               
                 Mn 
                 39,400 
                 39,500 
                 40,300 
                 39.900 
                 39,300 
                 39,400 
                 39,300 
                 38,900 
                 38,600 
                 43,100 
                 43,060 
               
               
                 Mw 
                 133,170 
                 131.820 
                 133,990 
                 131,950 
                 130.780 
                 130.760 
                 129,560 
                 128,030 
                 127,420 
                 137,100 
                 134,050 
               
               
                 Mz 
                 362,840 
                 354,290 
                 363,180 
                 353,030 
                 348,710 
                 347,200 
                 344,510 
                 343,000 
                 341,140 
                 353,480 
                 342,710 
               
               
                 Mw/Mn 
                 3.38 
                 3.33 
                 3.33 
                 3.31 
                 3.32 
                 3.32 
                 3.3 
                 3.29 
                 3.3 
                 3.18 
                 3.11 
               
               
                 Flexural 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 48,650 
                 48,065 
               
               
                 Modulus, psi 
               
             
          
           
               
                 Tensile Properties, 
               
               
                 (20 in/min testing) 
               
             
          
           
               
                 Strength at Yield, 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 1,560 
                 1,700 
               
               
                 psi 
               
               
                 Strength at Break, 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 4,400 
                 4,530 
               
               
                 psi 
               
               
                 Elongation, % 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 740 
                 750 
               
             
          
           
               
                 Tensile Properties, 
               
               
                 (2 in/min testing) 
               
             
          
           
               
                 Strength at Yield, 
                 1620 
                 1640 
                 1620 
                 1690 
                 1650 
                 1650 
                 1620 
                 1620 
                 1590 
                   
                   
               
               
                 psi 
               
               
                 Strength at Break, 
                 4390 
                 4600 
                 4330 
                 4480 
                 4590 
                 4480 
                 4620 
                 4220 
                 4610 
               
               
                 psi 
               
               
                 Elongation, % 
                 770 
                 780 
                 790 
                 780 
                 770 
                 790 
                 780 
                 750 
                 780 
               
               
                 OIT, minutes 
                 130 
                 125 
                 125 
                 120 
                 125 
                 140 
                   
                 130 
                 145 
                 145 
                 130 
               
               
                 NCTL, hrs. 
                   
                   
                   
                   
                   
                 &gt;530 
                   
                   
                   
                 &gt;1483 
                 &gt;1460 
               
               
                   
               
             
          
         
       
     
     Example 5 
     The ER3212 resin was used to produce geomembrane sheets under conditions similar to those described in Example 2. 
     Table 3 contains sheet data for geomembrane produced from ER3212. Both smooth sheet and textured sheet data are shown. Axi-symmetric data for strain at failure ranged from 55 to 125%. The sheet thicknesses ranged from 40 to 69 mils. Tensile strength at break ranged from 2869 to 4918 psi. Elongation at break ranged from 495 to 950%. 
     FIGS. 4 and 5 are graphs plotting shear viscosity (poise) of ER3212 against the shear rate (1/s) at 190° C. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 ER 3212 Sheet Data 
               
             
          
           
               
                   
                 ER 3212 
                 ER 3212 
                 ER 3212 
                 ER 3212 
               
               
                 Property 
                 Smooth 
                 Textured 
                 Smooth 
                 Textured 
               
               
                   
               
               
                 Process 
                 Cast 
                 Blown 
                 Blown 
                 Blown 
               
               
                 Density, gm/cc 
                   
                 0.929 
                 0.930 
                 0.930 
               
               
                 Carbon Black Content, % 
                   
                 2.3 
               
               
                 Axi-Symmetric Data 
               
               
                 Strain at Failure, % 
                 125 
                 55 
                 130 
                 75 
               
               
                 Puncture Resistance, psi 
                 2310 
                 1385 
                 1963 
                 1705 
               
               
                 MD 
               
               
                 Tensile @ Yield, psi 
                 1510 
                 1660 
               
               
                 Elong @ Yield, % 
                 25 
                 19 
               
               
                 Tensile @ Break, psi 
                 4590 
                 3660 
                 4918 
                 3379 
               
               
                 Elong @ Break, % 
                 630 
                 535 
                 947 
                 655 
               
               
                 Sample Thickness, mils 
                 49 
                 69 
                 40 
                 40 
               
               
                 Tear Resistance, psi 
                 610 
                 665 
                 616 
                 629 
               
               
                 TD 
               
               
                 Tensile @ Yield, psi 
                 1540 
                 1660 
               
               
                 Elong @ Yield, % 
                 21 
                 19 
               
               
                 Tensile @ Break, psi 
                 4530 
                 3280 
                 4902 
                 2869 
               
               
                 Elong @ Break, % 
                 915 
                 495 
                 950 
                 568 
               
               
                 Sample Thickness, mils 
                 50 
                 69 
                 40 
                 40 
               
               
                 Tear Resistance, psi 
                 590 
                 660 
                 566 
                 617 
               
               
                   
               
             
          
         
       
     
     Although a few embodiments of the invention have been described in detail above, it will be appreciated by those skilled in the art that various modifications and alterations can be made to the particular embodiments shown without materially departing from the novel teachings and advantages of the invention. Accordingly, it is to be understood that all such modifications and alterations are included within the spirit and scope of the invention as defined by the following claims.