Stretch modified elastomeric netting

Extruded netting having at least some elastomeric strands, the properties of which have been modified by stretch conditioning.

DETAILED DESCRIPTION OF THE INVENTION Stretch modified extruded elastomeric netting may be used in numerous ways. It may be used as a component in fabrics and other materials or used alone to create stretch and recovery in any direction required. It “bounces back” under vigorous use. It retains its elasticity to conform to a wide variety of shapes and sizes. For example, a preferred use of the netting at present is in mattress pads where it provides a tight fitting pad which does not come off at the comers. In such a pad, the structure is a layered or laminated structure of a “sandwich” type including the net interiorly. Hysterisis stress-strain curves illustrate the elastomers modified according to the invention. By making use of the invention, i.e. stretch modification, resins and resin blends making up elastomeric netting or elastomeric/orientable netting hybrids can be custom engineered for different hysterisis requirements and product configuration. 
 Design Principles Critical end use performance criteria (load force &commat; a specified elongation, unload force &commat; a specified elongation, set, stress relaxation, creep, strand count, etc.) must be known for custom engineering. Once these are established for a particular application, the following parameters must be taken into consideration. 
 Primary Design Parameters Raw material: Thermoplastic elastomer (blend) selection 1 Netting weight Processing: Stretch modification ratio (i.e., stretch ratio) Stretch modification temperature 
 Secondary Design Parameters 2 Processing: Stretch rate Holding time &commat; maximum elongation MD line web tension Line speed Roll wind tension Roll TD width when wound The secondary design parameters affect the final netting properties, but to a much lesser extent than the primary design parameters do, and need not be considered further herein in any detail as they will be apparent to those familiar with this art. 
 Design Process The selection of a thermoplastic elastomer resin forms the basis for the netting's performance characteristics. A library of hysterisis curves for various thermoplastic elastomer resins and resin blends is useful for product design but not necessary. Ideally, such hysterisis curves would be available at various stretch ratios and stretch temperatures. The netting weight may have to be adjusted to reach a specified force at a specified elongation (either on the load or unload cycle, or both). The stretching temperature can be increased to yield a greater first cycle set, improved dimensional stability and greater product width. The stretch ratio should generally exceed the stretch range expected in the netting's end use application. Specifically, the stretch ratio should be adjusted so that subsequent hysterisis cycles (at lower stretch rate for the end use application) meets specified predetermined performance targets. Often (but not always), the second load cycle follows closely the first cycle unload curve, this may serve as a first approximation in product design. Initial design targets may be established by simulating actual processing and end use conditions using a tensile tester capable of performing hysterisis testing. After initial product has been made and tested, the process will typically need to be fine-tuned. This can usually be done by a slight modification of the primary or secondary design parameters. Referring now to FIG. 1 , this graph contains stress-strain curves for the transverse direction of a netting made with Hytrel 4056, a polyether-ester thermoplastic elastomer, a block copolymer. Curve &num; 1 is the load curve to break. The two downward spikes are caused by strand slippage in the jaws of the testing machine. Curve &num; 2 shows the first hysterisis cycle to 100% elongation for the non-stretch modified netting. Curve &num; 3 shows the first hysterisis cycle to 375% elongation for the non-stretch modified netting. This cycle is similar to what the netting experiences in the TD stretch modification process. The (return) set is 180%. Curve &num; 4 shows a second hysterisis cycle to 100% elongation, that follows the first cycle to 375% (Curve &num; 3 ). Curve &num; 4 has been repositioned, so that it starts &commat; 0% strain, rather than &commat; 180% strain. Curve &num; 4 simulates the first hysterisis cycle in the end use application of the stretch modified product. When comparing the stretched product (curve &num; 4 ) to the non-stretched product (curve &num; 2 ), it can be seen that in this case the stretched product has: lower set (18% vs 35%) higher modulus (in this example, this only applies to the 20-100% elongation range) lower energy loss higher force (in this example, this only applies to the 75-100% elongation range) than the non-stretched product. It is also a slightly wider, lower weight product. Note that if curve &num; 4 would have started at 180% strain, it would have extended out to 360% strain (for 100% elongation when starting the second cycle at 0% strain). Also note that with this medium hardness resin, the first cycle's load curve shows a definite yield point. This yield point is eliminated in the second cycle, and therefore also for a stretch modified elastomeric netting. Referring now to FIGS. 2A and 2B , these two graphs show two ( 2 A) and four ( 2 B) cycle hysterisis curves of a netting made with a blend of styrene-butadiene-styrene (SBS) resins. Compared to the netting in FIG. 1 , this netting is relatively soft (low hardness), has a low force, low modulus, low set, and low energy loss. FIGS. 2A and 2B show that subsequent hysterisis cycles approximately follow the first cycle unload curve. The additional set (after the first cycle) is minimal. Most of the energy loss takes place in the first hysterisis cycle. When this product is stretched, very little additional width is gained, but set and energy loss are reduced and close to zero. Referring now to FIG. 2 A, it includes several curves A, B, C and D. Curve A represents a first loadcycle applied to an all elastomeric net in the MD direction in which stretch modification occurs. Curve B represents a first unload cycle in which the load applied in Curve A is relaxed and released. Curve C represents a second load cycle which is representative of load in actual use of the product. Curve D represents a second unload cycle in which the load of Curve C is relaxed and released. It can be readily seen from the graph that curves A and B, the first cycle, show some “set” but it is low compared to that obtained with orientable polymer nets. In subsequent cycles (C-D) there is low set which does not significantly increase the original “set” or there is low additional set. Also, it can be seen that the energy loss takes place primarily in the first cycle (A-B) with very little loss occurring in subsequent cycles. The combination of low energy loss and low set makes this a product that performs very close to a true elastomer. Referring now to FIG. 2 B, it also includes several curves 1 - 4 similar to those in FIG. 2A . Curve 1 shows first hysterisis cycle (both load and unload). Curve 2 shows a second hysterisis cycle (both load and unload). Curve 3 shows a third hysterisis cycle (both load and unload). Curve 4 shows a fourth hysterisis cycle (both load and unload). All hysterisis cycles include an upper curve (load cycle) and a lower curve (unload cycle). What is demonstrated by the curves of FIG. 2B is that subsequent cycles after the first cycle do not result in significant changes in the set and energy loss properties. 
 Comparison Of Low And Medium Hardness Stretch Modified Elastomeric Netting The performance difference between a netting made from Hytrel 4056 (a medium hardness thermoplastic elastomer, FIG. 1 ), and the netting made from the SBS blend (a low hardness thermoplastic elastomer, FIGS. 2A and 2B ) show a typical performance range for resins used for stretch modified elastomeric netting. Within the same resin class: The harder the resin, the higher the set, and the greater the width gained with the stretch modified elastomeric netting. The harder the resin, the higher the modulus and force. A stretch modified elastomeric netting will increase the modulus and force (at equivalent elongation) further yet. The softer the resin, the lower the energy loss. A stretch modified elastomeric netting will reduce the energy loss further yet when not exceeding the original stretch modification or first cycle elongation. 
 EXAMPLES 
 EXAMPLE &num;1 The starting product is an extruded, square, all-elastomeric netting made from Hytrel 4056. Hytrel 4056 is a polyether-ester resin made by Du Pont. The netting is biaxially stretched using equipment similar to that described in U.S. Pat. No. 4,152,479 (Larsen). The product is stretched in the machine direction (“drafted”) at a temperature of 128° F., and an applied MD stretch ratio of 2.52. The resulting effective MD stretch ratio (after full product relaxation) is 1.79. The product is then stretched in the cross direction (“tentered”) at a temperature of 150° F., and an applied CD stretch ratio of 4.75. The resulting effective CD stretch ratio is 2.38. Cf. Example 1 in Table 1 below. Table 1 below includes data for Example 1 and for additional sets of stretch modification trials, containing information for additional Examples 2 and 3. 3 TABLE 1 POLYETHER-ESTER COPOLYMER Ex. &num;1 Biaxially Ex. &num;2 Ex. &num;3 MD stretched Biaxially Stretched-only Product Description product product product. Resin Hytrel 4056 Hytrel 3078 Hytrel 4056 Relaxed strandcount, 2.8 × 2.8 2.7 × 3.6 6.8 × 2.6 MD × TD (per inch) (on roll: 2.5 × 2.8) Calculated relaxed 6.7 6.6 16.3 weight (PMSF) (on roll: 5.8) Relaxed width, excluding 30.4 31.5 32.9 edgetrim (in) (on roll: 35.0) Applied/Effective draft 2.52/1.79 3.35/1.68 3.21/1.92 ratio Applied Effective tenter 4.75/2.38 4.83/2.10 — ratio (on roll: 2.74) MD stretch temperature 128 70 90 (° F.) TD stretch temperature 150 146 — (° F.) MD set &commat; 35% 5.5 1.7 (est.) 4.0 elongation (%) MD force &commat; 35% 1,600 210 (est.) 4,220 elongation (g/3 in) MD force &commat; break 10,300 28,600 (g/3 in) TD force &commat; break 3,030 2,500 (g/3 in) MD elongation &commat; 450 440 break (%) TD elongation &commat; 220 720 break (%) 
 EXAMPLE &num;5 This all-elastomeric extruded “square” netting is made from Hytrel 3078. Hytrel 3078 is a polyether-ester resin made by Du Pont Company. The netting is first MD stretch modified in-line with the extrusion process, and subsequently CD stretch modified using part of a biaxial orientor (as in Example &num;1). The product is stretched in the machine direction at room temperature (70° F.), and an applied MD stretch ratio of 3.61. The resulting effective MD stretch ratio after full product relaxation is 1.67. The product is stretch modified in the CD at room temperature (70° F.), and an applied CD stretch ratio of 5.29. The resulting Effective CD stretch ratio is 1.93. Cf. Example 5 in Table 2 below. Table 2 not only includes data for Example &num;5 but for additional stretch modification trials regarding Examples 4-9. 4 TABLE 2 POLYETHER-ESTER COPOLYMER Ex. &num;5- Ex. &num;6-CD Ex. &num;7- Ex. &num;8 Ex. &num;9- Ex. &num;4-CD Biaxially stretch- CD stretch- Biaxially Biaxially strech-only stretch only only stretched stretched Product description product product.(1) product product product.(2) product(2) Resin Hytrel 3078 Hytrel 3078 Hytrel 3078 Hytrel 3078 Hytrel 3078 Hytrel 3078 Relaxed strandcount, 3.0 × 6.2 2.9 × 3.7 2.5 × 6.1 2.6 × 5.1 2.6 × 3.1 2.7 × 3.6 MD × TD (per inch) Calculated relaxed 12.6 7.2 10.1 9.0 5.5 6.6 weight (PMSF) Relaxed width, (in) 26.3 27.0 32.0 32.4 32.5 31.5 Applied/Effective 1.00/0.98 3.61/1.67 1.00/1.00 1.00/1.19 3.20/1.96 3.35/1.68 MD stretch ratio Applied/Effective CD 5.29/1.88 5.29/1.93 5.29/2.29 4.83/2.16 4.83/2.17 4.83/2.10 stretch ratio MD Stretch — 70 — — 70 70 Temperature (° F.) CD stretch 70 70 153 150 147 146 temperature (° F.) MD set &commat; 25%-50%- 0.7-3.1-6.0 0.9-3.1-5.2- 0.9-2.9-4.8- 75%-100% 8.7 7.0 6.7 elongation (%) TD set &commat; 25%-50%- 0.6-2.7 0.5-2.6-5.0- 0.9-2.9-5.3- 75%-100% 5.2-7.8 7.4 7.8 elongation (%) MD elastic recovery 1.0-1.2-1.4 1.0-1.2-1.3- 1.0-1.1-1.2- ration for 25%-50%- 1.5 1.4 1.3 75%-100% elongation TD elastic recovery 1.0-1.1-1.3- 1.0-1.1-1.3- 1.0-1.1-1.3- ratio for 25%-50%- 1.4 1.4 1.4 75% 100% elongation MD force &commat; 25%- 190-330 120-210- 110-190- 50%-75%-l00% 440-550 300-480 290-240- elongation (g/2 in) TD force &commat; 25%- 280-490- 210-360 170-290- 50%-75%-100% 670-830 480-630 400-510 elongation (g · 2 in) MD stress relaxation, 15.1 5 min &commat; 50% elongation (%) TD stress relaxation, 15.5 5 min. &commat; 50% elongation (%) 
 EXAMPLE &num;12 This all-elsatomeric extruded “square” netting is made from Exact 4041, a VLDPE copolymer resin made by Exxon. The netting is MD stretch modified in-line with the extrusion process, and subsequently CD stretch modified using part of a biaxial orientor (as in Example &num;1). The product is stretched in the MD at room temperature and an applied MD stretch ratio of 2.11. The resulting effective MD stretch ratio after full product relaxation is 1.58. The product is stretch modified in the CD at 130° F., and an applied CD stretch ratio of 3.49. The resulting effective CD stretch ratio is 2.37. Cf Example &num;12 in Table 3 below. Table 3 not only includes data for Example &num;12 but for additional stretch modification trials regarding Examples 10-15. 5 TABLE 3 VLDPE COPOLYMER Ex. &num;10- CD Ex.&num;11- Ex. &num;12- Ex. &num;13- Ex. &num;14- stretch- Biaxially Biaxially Biaxially Biaxially Ex. &num;15 Product only stretch stretch stretch stretched Tenter- description product product. product. product. product. ed-only Resin Exact 4041 Exact 4041 Exact 4041 Exact 4041 Exact 4041 Exact 4041 Relaxed 3.7 × 3.4 3.5 × 3.6 3.6 × 3.4 5.3 × 4.2 4.9 × 4.3 4.7 × 5.1 strandcount, MD × TD (per inch) Calculated relaxed 5.7 6.0 5.7 5.6 5.3 6.0 weight (PMSF) Relaxed width, (in) 88.6 91.3 92.5 88.3 93.4 97.8 Finished slit roll 102 95 95 95 95 width (in) Applied/Effective 1.00/ 2.11 2.11/ 2.14 2.14/ 1.00/ MD stretch ratio 1.23 1.47 1.58 2.03 1.68 1.02 Applied/Effective 3.53/ 3.58/ 3.49/ 3.49/ 3.56/ 3.57/ CD stretch ratio 2.30 2.40 2.37 2.26 2.44 2.57 Tenter temperature, 121 127 130 131 134 135 (° F.) MD set &commat; 25%- 1.3-4.6- 1.8-5.7-11- 1.7-5.3-11- 1.7-5.3-11- 1.6-5.1- 1.3-5.0- 50%-75%-100% 9.6-17 19 18 19 9.8-18 11-21 elongation (%) TD set &commat; 25%- 2.1-6.5- 2.8-7.2-12- 2.2-6.2-11- 2.0-5.8-11- 2.1-6.2-11- 1.7-5.1- 50%-75%-100% 11-18 19 18 17 19 9.7-17 elongation (%) MD elastic 1.1-1.3- 1.1-1.4-1.8- 1.1-1.3-1.7- 1.1-1.4-1.7- 1.1-1.4- 1.0-1.1- recovery ration for 1.5-1.8 2.0 1.9 2.0 1.7-1.9 1.2-1.3 25%-50%-75%- 100% elongation TD elastic recovery 1.1-1.4- 1.2-1.4-1.6- 1.1-1.4-1.6- 1.1-1.4-1.6- 1.1-1.4- 1.1-1.4- ratio for 25%-50%- 1.6-1.8 1.8 1.9 1.8 1.6-1.9 1.6-1.9 75%-100% elongation MD force &commat; 25%- 290-390- 350-460- 410-570- 460-630- 410-560- 270-340- 50%-75%-100% 450-500 510-570 660-730 730-820 650-730 390-420 elongation (g/2 in) TD force &commat; 25%- 190-290- 150-240- 190-310- 140-220- 160-250- 170-280- 50%-75%-100% 410-550 340-450 440-750 320-440 370-490 400-510 elongation (g · 2 in) The invention finds a preferred use in composite materials comprised of multi-layers such as that shown in FIG. 3 . Two layer composites of net and another material may be used but composites with more than two layers such as shown in the FIG. 3 are most likely. That composite comprises outer covering layers 60 and 62 , the stretch conditioned elastomer net being indicated at 64 . All such composites are referred to herein generally as composites comprised of a plurality of layers including net. They may be manufactured in the known manner as by subjecting them, when assembled, to heat and pressure. Adhesives may be included. The above Examples and disclosure are intended to be illustrative and not exhaustive. These examples and description will suggest many variations and alternatives to one of ordinary skill in this art. All these alternatives and variations are intended to be included within the scope of the attached claims. Those familiar with the art may recognize other equivalents to the specific embodiments described herein which equivalents are also intended to be encompassed by the claims attached hereto.