INTEGRAL POLYETHYLENE TEREPHTHALATE GRIDS, THE METHOD OF MANUFACTURE, AND USES THEREOF

Abstract

An integral polymer grid with a plurality of interconnected, oriented polyethylene terephthalate strands and an array of openings therein is made from a polyethylene terephthalate sheet-like starting material having perforations or indentations therein that form the openings when the sheet-like material is uniaxially or biaxially stretched. The grid has a higher tensile strength to weight ratio and a higher creep reduced strength to weight ratio than corresponding ratios associated with a grid made from a non-polyethylene terephthalate starting material.

Claims

1. An integral polymer grid comprising a plurality of interconnected, oriented polyethylene terephthalate strands having an array of openings therein.

2. The integral polymer grid according to claim 1, wherein the oriented polyethylene terephthalate strands have been uniaxially or biaxially stretched.

3. The integral polymer grid according to claim 1, wherein the polyethylene terephthalate is a homopolymer or a copolymer.

4. The integral polymer grid according to claim 1, wherein the polyethylene terephthalate is selected from the group consisting of amorphous polyethylene terephthalate, crystalline polyethylene terephthalate, and polyethylene terephthalate glycol.

5. The integral polymer grid according to claim 1, wherein the grid has a higher tensile strength to weight ratio and a higher creep reduced strength to weight ratio than corresponding ratios associated with a grid made from a non-polyethylene terephthalate starting material.

6. The integral polymer grid according to claim 1, wherein the plurality of interconnected, oriented polyethylene terephthalate strands includes transverse strands interconnected by substantially longitudinally oriented strands.

7. The integral polymer grid according to claim 1, wherein the grid is a geogrid configured for structural or construction reinforcement purposes.

8. A starting material for making an integral polymer grid comprising a polyethylene terephthalate sheet-like material having perforations or indentations therein that provide openings when the sheet-like material is uniaxially or biaxially stretched.

9. The starting material according to claim 8, wherein the polyethylene terephthalate is crystalline polyethylene terephthalate.

10. The starting material according to claim 8, wherein the sheet-like material has an initial thickness of at least 3 mm.

11. The starting material according to claim 8, wherein the sheet-like material has an initial thickness of at least 1.4 mm.

12. The starting material according to claim 10, wherein the stretched sheet-like material exhibits a substantially linear relationship between a stretch ratio and a specific tensile strength.

13. A geogrid construction comprising a mass of particulate material strengthened by embedding therein an integral polymer grid as claimed in claim 1.

14. A method of strengthening a mass of particulate material, comprising embedding in the mass of particulate material the integral polymer grid as claimed in claim 1.

15. A method of making an integral polymer grid, comprising orienting a polyethylene terephthalate sheet-like starting material having perforations or indentations therein to provide a plurality of interconnected, oriented polyethylene terephthalate strands and to configure the perforations or indentations as grid openings.

16. The method according to claim 15, wherein the polyethylene terephthalate sheet-like starting material is oriented by uniaxial or biaxial stretching.

17. The method according to claim 15, wherein the polyethylene terephthalate is crystalline polyethylene terephthalate.

18. The method according to claim 17, wherein the sheet-like starting material has an initial thickness of at least 3 mm.

19. The method according to claim 15, further comprising a step of manipulating a variable associated with the polyethylene terephthalate to improve resistance of the integral grid to hydrolysis, the variable being selected from the group consisting of carboxyl end group, molecular weight, crystallinity, orientation, surface area, temperature, pH, and cation presence.

20. A method of providing a geogrid construction, comprising: uniaxially or biaxially stretching a crystalline polyethylene terephthalate sheet-like starting material having perforations or indentations therein to provide an integral geogrid having a plurality of oriented polyethylene terephthalate strands and a plurality of grid openings; and embedding the integral geogrid in a mass of particulate material.

21. An integral geogrid comprising a plurality of interconnected, oriented polyethylene terephthalate strands having an array of openings therein that is made from a crystalline polyethylene terephthalate sheet-like starting material having perforations or indentations therein that form the openings when the sheet-like material is uniaxially or biaxially stretched.

22. The integral geogrid according to claim 21, wherein the polyethylene terephthalate strands include substantially transversely oriented strands interconnected by substantially longitudinally oriented strands.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] FIG. 1 illustrates a sample of a crystalline polyethylene terephthalate (CPET) grid according to one embodiment of the present invention in which the starting sheet was uniaxially stretched to a stretch ratio of 3.1:1.

[0028] FIG. 2 is a creep test chart showing a creep curve for a CPET sample, initially 3 mm thick, which is uniaxially stretched to a stretch ratio of 5.3:1 and suspended under 60% loading.

[0029] FIG. 3 is a creep test chart showing a creep curve for a CPET sample, also initially 3 mm thick, which is uniaxially stretched to a stretch ratio of 5.3:1, but suspended under 70% loading.

[0030] FIG. 4 is a graph illustrating comparative creep curves for various polymeric geotextile materials under loading of 40% ultimate strength.

[0031] FIG. 5 is a graph illustrating representative creep curves for filaments of various polymers.

[0032] FIG. 6 is a table summarizing the tensile test results of the various polymeric samples tested.

[0033] FIG. 7 is a chart illustrating the specific strength of the samples shown in FIG. 6.

[0034] FIG. 8 is a table summarizing the specific strength of certain representative samples shown in FIG. 7.

[0035] FIG. 9 is a chart summarizing the specific strength of the representative samples shown in FIG. 8.

[0036] FIG. 10 is a table summarizing the effect of the stretch ratio on the specific strength of a PET sample.

[0037] FIG. 11 is a graph depicting the effect of the stretch ratio on the specific strength summarized in FIG. 10.

[0038] FIG. 12 is a creep test chart showing a creep curve for a CPET sample, initially 1.4 mm thick, which is stretched to a stretch ratio of 4.25:1 and suspended under 60% loading.

[0039] FIG. 13 is a table summarizing basic properties of various polymers typically used to produce staple fibers, continuous filaments, and oriented tapes.

[0040] FIG. 14 is a table summarizing the relative chemical resistance of various fiber-forming polymers used in geotextiles.

[0041] FIG. 15 is a table summarizing the effect of pH on the tensile strength of various geotextile polymers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0042] Although only preferred embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its scope to the details of construction and arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or carried out in various ways.

[0043] Also, in describing the preferred embodiments, terminology will be resorted to for the sake of clarity. It is intended that each term contemplates its broadest meaning as understood by those skilled in the art, and includes all technical equivalents which operate in a similar manner to accomplish a similar purpose.

[0044] In order to understand the behavior and properties of the PET material, and to establish the parameters of using PET to make integral grids, instead of HDPE, standard commercially available extruded sheets of PET were procured. Three types of PET sheets were used: amorphous PET (“APET”), crystalline PET (“CPET”), and PET glycol (“PETG”).

[0045] Punched samples for each of the three types of PET sheet were prepared with standard sheet punches such as those used to manufacture HDPE UX products sold by the Tensar International Corporation, Inc. (hereinafter “Tensar”) (Atlanta, Ga.), the assignee of the instant provisional application for patent. For example, FIG. 1 shows a CPET starting sheet sample prepared using a punch (1.53″×0.375″) and uniaxially stretched to a stretch ratio of 3.1:1.

[0046] The punched sheet uniaxial stretching was performed on a Tensar laboratory stretcher. Tensar carried out initial laboratory work in accordance with the present invention by stretching narrow strips, i.e., from 2 mm to 4 mm wide, of PET and HDPE to establish the temperature and stretch ratio conditions under which the testing would be conducted. The temperature range was established to be between 100° C. and 240° C., and the stretch ratio range to be between 2:1 and 10:1. These temperature and stretch ratio conditions were then used during stretching of the approximately 8″×10″ punched samples.

[0047] For high temperature stretching, i.e., above 160° C., an Instron “Hot-Box” was installed on an Instron Model 1125 tensile testing machine. Standard “dog bone”-shaped samples of CPET, having an initial thickness of 3 mm, were heated to, and then conditioned at, 180° C. for 15 minutes. The heat-conditioned samples were then uniaxially stretched in the lab stretcher in accordance with standard stretching protocols used by Tensar. The samples were stretched to the maximum stretch ratio that was allowed by the size of the Hot-Box, i.e., a ratio of 5.3:1. These 5.3:1 samples were then tested for both tensile and creep properties.

[0048] The unstretched and stretched strips and ribs from the 8″×10″ samples were tested for tensile properties on an Instron Model 1125 tensile testing machine using serrated pneumatic grips. The testing grips were padded with cardboard and/or sandpaper to prevent slippage and edge break. The tensile data was normalized for the difference in sheet thickness between the HDPE and the PET by dividing the tensile results by the initial cross-sectional area of the test specimen (i.e., the resultant tensile data has the units of N/mm.sup.2). This normalization enabled a material-to-material comparison without the need to standardize the physical dimensions of the test specimens.

[0049] Finally, a strength to basis weight comparison was made between standard Tensar HDPE UX products and the 8″×10″ PET punched and stretched samples.

[0050] For creep testing, the 5.3:1 stretch ratio samples were suspended under a load at room temperature in a quality control laboratory. One sample was suspended under a load corresponding to 60% of ultimate tensile strength (FIG. 2), and a second sample was suspended under a load corresponding to 70% of ultimate tensile strength (FIG. 3).

[0051] Tensile strength test data associated with the aforementioned 5.3:1 stretch ratio samples is presented in FIGS. 6 and 7. A summary of representative data from FIGS. 6 and 7 is presented in FIGS. 8 and 9. FIG. 8 presents representative specific strength values for each type of polymer at unstretched and maximum stretch ratio conditions.

[0052] Since the 3 mm thick CPET starting sheet provided the maximum specific strength among the samples summarized in FIGS. 6-8, samples of the 3 mm CPET starting sheet were uniaxially stretched at various ratios of from 3.1:1 to 5.3:1. These results are shown in FIGS. 10 and 11. From this data it is evident that for the 3 mm CPET starting sheet there exists a substantially linear relationship between stretch ratio and specific tensile strength (see FIG. 11).

[0053] As is evident from the results presented herein, CPET is a good candidate for the manufacture of extruded and uniaxially stretched integral grids because of a higher specific strength and better creep characteristics. At comparable stretch ratios, CPET exhibits almost double the specific strength of HDPE. That is, as is evident from FIG. 8, sample HDPE 2 (initial thickness of 2.9 mm) has a specific strength of 163 N/mm.sup.2, while sample CPET 4 (initial thickness of 3 mm) has a specific strength of 373 N/mm.sup.2.

[0054] Since APET starting sheets crystallize during the heated stretching operation, APET starting sheets can achieve a specific strength similar to that of HDPE. But, the required crystallization associated with the heated stretching of APET starting sheets makes the process slow and more expensive. Hence, CPET starting sheets are clearly preferred for the present invention.

[0055] The PETG starting sheets did not show any significant difference between unstretched and stretched samples even at a stretch ratio of 8.5:1.

[0056] As is evident from FIGS. 2 and 3, the first 5600 hrs of creep data show that there is minimal strain associated with the CPET samples. That is, the strain is only 0.78% at 60% loading, and 1.34% at 70% loading.

[0057] Another test demonstrated the ability to prepare a grid from a CPET sheet having an initial thickness of 1.4 mm. The sample was first punched, then stretched to a stretch ratio of 4.25:1, and finally placed under a loading of 60% of ultimate tensile strength. As is evident from FIG. 12, creep data for the 1.4 mm sample for the first 2000 hours shows a strain of only about 2.4%.

[0058] While the integral PET grid according to the present invention exhibits the above-described advantageous characteristics, PET in general can be susceptible to hydrolysis during wet processing and in end use. PET is hydrolyzed by certain acids and by all strong bases, including some organic bases. The factors that can affect this hydrolysis include carboxyl end group (“CEG”), molecular weight, crystallinity, orientation, surface area, temperature, pH level, and the presence of cations. See FIG. 14, which summarizes the relative chemical resistance of various fiber-forming polymers used in geotextiles, and FIG. 15, which summarizes the effect of pH on the tensile strength of various geotextile polymers.

[0059] For example, one literature source has proposed that PET hydrolysis is proportional to the square root of the CEG concentration. I. M. Ward, Mechanical Properties of Solid Polymers, Wiley Interscience, New York, 1971. Molecular weight inversely affects the CEG concentration. Hence, a higher molecular weight PET will be less susceptible to hydrolysis. Orientation can also lessen the hydrolytic effect in that it reduces the diffusion rate of the penetrant.

[0060] Thus, while there are no specific additives that can retard hydrolysis in PET materials, one or more of the aforementioned variables of CEG, molecular weight, crystallinity, orientation, surface area, temperature, pH, and the presence of cations can be manipulated to improve resistance to hydrolysis.

[0061] The foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes may readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation described and shown.