Classifications

• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
  • C08F10/00—Homopolymers and copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond
  • C08F10/02—Ethene
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C55/00—Shaping by stretching, e.g. drawing through a die; Apparatus therefor

Definitions

• a process for preparing a disentangled ultra-high molecular weight polyethylene product is provided herein.
• the product has density not less than lg/cc.
• the present disclosure further relates to the use of articles obtained from the polyethylene product for ballistic applications.
• disentangled ultra-high molecular weight polyethylene used in the context of the present disclosure refers to a homo-polymer or copolymer of ethylene having molar mass in the range of 2M to 15M, wherein the polyethylene chains have low entanglement or are completely disentangled.
• Ultra-High Molecular Weight Polyethylene is a linear homopolymer produced by the polymerization of ethylene with a Ziegler-Natta catalyst. This class of linear polyethylene is termed as "ultra-high” because its average molecular weight is 10-100 times greater than the regular polyethylene especially high density polyethylene (HDPE).
• Ultra-high molecular weight polyethylene (UHMWPE) in the form of fiber, solution or powder is used to prepare ballistic resistant articles because of its high resistance to impact.
• UHMWPE has a melt viscosity in the region of 10 10 Pa.s and therefore cannot be processed by shear methods, as shear methods have limitation of processing polymers having melt viscosity in the range of 10 4 to 10 6 Pa.s.
• UHMWPE powder is heated above its melting temperature i.e., 180-220° C and fused by compacting at high pressure i.e., 3-5 MPa to obtain a densely packed structure.
• DCM direct compression molding
• UHMWPE is dissolved in a suitable solvent such as decalin or paraffin in order to disentangle the molecular network followed by removing the solvent to obtain UHMWPE in the form of a film or a tape.
• a suitable solvent such as decalin or paraffin
• This method is difficult to execute and is expensive as it requires capital expenditure and high amount of solvent. Further, the solvent content in the resulting UHMWPE product cannot be reduced below 0.1 %.
• the blocks obtained by pressing UHMWPE at a temperature of 100° C to 130° C and at a pressure of 20 kg/cm 2 are skived to obtain film or tape.
• the film or tape thus obtained is drawn at different temperatures varying from 136° C to 153° C in four stages.
• the polyethylene product obtained by this process exhibit density value less than 1 g/cc, making them less suitable choice for ballistic applications.
• a process for preparing polyethylene films having density not less than 1 g/cc is disclosed in one of the prior arts.
• the polyethylene obtained by this method is unsuitable for ballistic applications as it degrades in water or sunlight due to the presence of various fillers.
• the inventors of the present invention envisage a simple and cost effective in-situ process for preparing a polyethylene product from disentangled ultrahigh molecular weight polyethylene powder which on subjecting to low flow and high compacting process gives a product having density not less than 1 g/cc and exhibit enhanced crystallinity and thermo-mechanical properties for ballistic applications.
• An object of the present disclosure is to provide a process for the preparation of a disentangled ultra-high molecular weight polyethylene product having density not less than 1 g/cc.
• Another object of the present disclosure is to provide a process for the preparation of a polyethylene product having enhanced crystallinity and thermo-mechanical properties.
• Still another object of the present disclosure is to provide a disentangled ultra-high molecular weight polyethylene product.
• Yet another object of the present disclosure is to prepare articles from a disentangled ultra-high molecular weight polyethylene product which are suitable for ballistic applications.
• a process for preparing a disentangled ultra-high molecular weight polyethylene product is provided.
• This product has a density not less than 1 g/cc and enhanced crystallinity and thermo-mechanical properties.
• a positive mold of a compression molding machine is first preheated, followed by introducing disentangled ultra-high molecular weight polyethylene material into the preheated mold.
• the preheated material in contact with the preheated mold is then brought to room temperature.
• the material is then maintained at a predetermined temperature and then pressure is increased in a stepwise manner on the mold for different intervals of time to compact and fuse the polymeric material.
• the product, obtained at a temperature above room temperature is allowed to be cooled to obtain a compacted and fused product which is in the form of a block or a sheet, having a density not less than 1 g/cc.
• the polyethylene product obtained in the form of sheet may further be stretched below its melt temperature in an environmental chamber, into highly oriented tapes, films and fibers having high thermal and mechanical properties.
• the disentangled UHMWPE material used in the process can be a homopolymer or a co-polymer, with additives or without any additives.
• the disentangled UHMWPE product was characterized by measuring their physical properties (density, thickness), thermal properties (melting and crystallization temperatures, degradation temperatures) and mechanical properties (tensile strength and modulus).
• the product in the form of sheet, block, film, tape or fibers may be used for ballistic or any other suitable application.
• Figure 1 illustrates DSC thermograms showing multiple melting peaks of stretched fibers obtained by stretching polyethylene product obtained in Example 4.
• a process for preparing a disentangled ultra-high molecular weight polyethylene product is provided.
• This product has a density not less than 1 g/cc and enhanced crystallinity and thermo-mechanical properties.
• a positive mold having a cavity and a top plate is heated inside a compression molding machine at a temperature ranging from 80° C to 120° C, particularly at 100° C.
• the cavity of the heated mold is then filled with disentangled UHMWPE.
• the preheated material in contact with the preheated mold is then brought back to room temperature.
• the compaction of disentangled UHMWPE material is carried out below the melt temperature in a solid state at a temperature ranging from 23° C to below the melting range of the material, preferably below 130° C and at a pressure ranging from 10 kg/cm 2 to 400 kg/cm 2 in a compressing molding machine for a time period ranging from 0.5 hour to 10 hours to obtain a compacted and a highly fused disentangled UHMWPE product without grain boundaries.
• the product if obtained at a temperature above room temperature, is allowed to be cooled for a time period in the range of 10 min to 12 hours, to obtain a compacted and fused product which is in the form of a block or a sheet, having a density not less than 1 g/cc.
• the mold is then depressurized to recover the product.
• the pressure applied for compacting disentangled UHMWPE may be gradually increased step wise at fixed temperature for different intervals of time.
• the method of compacting may be carried out at a temperature of 125° C and applying a pressure in the following manner:
• the total time period of the compaction or application of pressure is in the range of 0.5 hours to 10 hours.
• the mold with compacted disentangled UHMWPE is then cooled to 23° C to obtain a polyethylene product which is in the form of a block or a sheet, having a density not less than 1 g/cc.
• the cooling is carried out by circulating water or air or chilled water. To achieve the desired quality of the resulting polyethylene product the cooling is carried out for a time period ranging from 10 minutes to 12 hours.
• the cooling step includes gradually cooling up to 80° C followed by sudden cooling to a temperature in the range of 20 to 30° C. After cooling the compacted material, the mold is then depressurized to recover the product.
• the disentangled UHMWPE product obtained in form of the sheet may further be converted into a shape selected from the group consisting of tape, film and fiber by way of stretching.
• the temperatures Ti and T 2 are equal to 125° C.
• the temperatures ⁇ and T 2 are 125°C and 137°C respectively.
• the stretching may be varied depending on the end application of the resulting tape, film or sheet or fiber.
• the molecular weight of the disentangled UHMWPE used for the preparation of the polyethylene product may be varied between 2 M and 15M, preferably in the range of 2M and 13M and more preferably in the range of 2M and 12.3 M.
• the average particle size of the disentangled UHMWPE may range from 25 to 800 microns, preferably from 50 to 600 microns, more preferably from 200 to 450 microns.
• the disentangled UHMWPE used for the purpose has very low bulk density and high crystallinity.
• the disentangled UHMWPE used is a homopolymer; however, a co-polymer or a combination can also be used.
• the disentangled UHMWPE is a copolymer of ethylene with an alpha-olefin of C 3 to C 8 .
• disentangled UHMWPE is essentially devoid of any additives
• primary and/or secondary antioxidants may be uniformly mixed with the disentangled UHMWPE before processing to prevent thermo-oxidative degradation.
• other additives such as curing agents, plasticizers, modifiers, stabilizers, surfactants, binders and the like may also be uniformly mixed with disentangled UHMWPE before subjecting it to processing.
• the disentangled UHMWPE can be in any form which includes but is not limited to a granule and a powder.
• the density of polyethylene product is dependent on the processing parameters, catalytic residue in the polymer powder and is independent of other factors such as molecular weight, average particle size and bulk density of the disentangled UHMWPE.
• the density of the polyethylene product obtained by using disentangled UHMWPE having molecular weight of 3.53M is substantially equal to that of the density of the polyethylene product obtained by using disentangled UHMWPE having molecular weight of 9.4M.
• the polyethylene product obtained by the process of the present disclosure is characterized for physical properties such as density and thickness, thermal properties such as melting temperature, crystallization temperature and degradation temperatures and mechanical properties such as tensile strength and tensile modulus.
• the characterization observed was that the melting temperature of the stretched products is higher than that of the powder form of the disentangled UHMWPE.
• the melting temperature for powder form of the disentangled UHMWPE is 140°C whereas the melting temperature of the resulting sheet, film and fiber obtained from the powder form of disentangled UHMWPE is 143° C, 144° C and 160°C respectively.
• the stretched products like tape, films and fibres showed efficient orientation as indicated by multiple splitting of melting endotherm (146° C, 154° C and 159° C) ( Figure 1) with very high melting temperature of 160° C, crystallization temperature of 121°C to 140°C, high crystallinity and tensile modulus of 155 GPa.
• the orientation of the molecules in the stretched product is dependent on the number of stretching, type of stretching and temperature applied for stretching.
• polyethylene product characterized by at least one of the following properties:
• - degradation temperature (@ 50% weight loss) is in the range of 471° C to 478° C;
• the polyethylene product may be used for preparing articles for ballistic or any other suitable application.
• Example 1 Processing of Disentangled UHMWPE having molecular weight of 3.53M
• a positive mold was preheated to 100° C for 10 min by keeping it inside a compression molding machine. After heating, the mold was taken out of the compression molding machine and allowed to be cooled to room temperature. The mold cavity was then filled with 10 g of disentangled UHMWPE powder having molecular weight of 3.53M, bulk density of 0.054 g/cc and average particle size of 227 microns. The mold was then closed by placing top plate and placed in the compression molding machine which was maintained at a temperature of 125° C. Compression molding was carried out using the following molding conditions: Pressure 10kg/cm 2 for 5 min, 50 kg/cm 2 for 5 min, 100 kg/cm 2 for 5 min, 150 kg/cm 2 for 5 min, 180 kg/cm 2 for 70 min.
• the temperature of the mold was brought down to 80° C by natural cooling followed by quenching the mold in water kept at room temperature.
• the mold was depressurized to obtain a molded disentangled ultra-high molecular weight polyethylene product.
• the molded polyethylene product (DPE Sheet - 1) showed a density of 1.00170 g/cc and was seen sinking in water.
• the physical and thermal properties are depicted in Tables 1, 2 and 4.
• Example 2 Processing of Disentangled UHMWPE having molecular weight of 4.9M
• Example 3 Solid state processing of disentangled UHMWPE having molecular weight of 4.9M was carried out in a similar manner as Example 1, The temperature, however, was maintained at 128° C and the pressure at the last stage was maintained at 175 kg/cm 2 and the time period for which this pressure was maintained was 90 min. The mold was cooled to room temperature by circulating water at room temperature. The polyethylene product (DPE Sheet - 2) obtained showed a density of 1.02778 g/cc. The sample was seen sinking in water. The physical and thermal properties are depicted in Tables 1 - 4.
• Example 3 Processing of Disentangled UHMWPE having molecular weight of 12.1M
• Solid state processing of disentangled UHMWPE having molecular weight of 12.1M was carried out in a similar manner as Example 1.
• the pressure of 180 kg/cm 2 at the last step was, however, maintained for a time period of 110 minutes.
• the mold was cooled to room temperature by circulating water at room temperature and depressurized.
• the polyethylene product (DPE Sheet-3) obtained showed a density of 1.03035 g/cc and was seen sinking in water.
• the physical and thermal properties are depicted in Tables 1, 2 and 4.
• Solid state processing of disentangled UHMWPE having molecular weight of 12.3M was carried out in a similar manner as Example 1 at a temperature of 125° C.
• the pressure of 175 kg/cm 2 at the last step was, however, maintained for a time period of 100 minutes.
• the mold was cooled by natural cooling by air for 12 hours followed by depressurizing the mold.
• the polyethylene product (DPE Sheet-4) obtained showed a density of 1.00210 g/cc and was seen sinking in water.
• the physical and thermal properties are depicted in Tables 1- 4 and Figure 1.
• Example 5 Processing of Disentangled UHMWPE having molecular weight of 5.4M at high pressure and room temperature
• the stretching was carried out using strips cut from the round molded sheets obtained by compression molding as in Example 1.
• the stretching speed was kept as 20mm/min.
• the samples were stretched up to 150 times.
• the mechanical and thermal properties are given in Tables 2 and 4.
• Comparative Example 1 Processing of entangled UHMWPE having molecular weight of 3.5M
• Density of disentangled ultra-high molecular weight polyethylene sheets, stretched tape, films / block was measured as per ASTM D-792 using Mettler Toledo unit @ 23° C in butyl acetate.
• Disentangled UHMWPE powder, polyethylene products and stretched products of varying molecular weights were subjected to nucleating efficiency test using Differential Scanning Calorimeter (Q2000 MDSC from M/s. TA instruments).
• the polymers were heated from ambient to 200° C in N 2 atmosphere with a heating rate of 10° C / min, held at 200° C and cooled to room temperature at the same rate.
• Melting temperature (T m ), Crystallization temperature (T c ), Heat of fusion during melting (AHTm) and cooling (AHTc) were recorded.
• the efficiency of various polyethylene products was evaluated by measuring their Tm, Tc and ⁇ and provided in Table 2.
• TGA Thermo-gravimetric analysis

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• Chemical & Material Sciences (AREA)
• Health & Medical Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Medicinal Chemistry (AREA)
• Polymers & Plastics (AREA)
• Organic Chemistry (AREA)
• Casting Or Compression Moulding Of Plastics Or The Like (AREA)
• Manufacture Of Macromolecular Shaped Articles (AREA)

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Cited By (5)

Citations (1)

• 2015
  • 2015-03-13 IN IN873MU2014 patent/IN2014MU00873A/en unknown
  • 2015-03-13 WO PCT/IB2015/051845 patent/WO2015140681A1/en active Application Filing

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