JP6722596B2 - Flash-spun plexifilamentary strands and sheets - Google Patents

Description

The present invention relates to flash spun plexifilamentary sheets, fabrics or fibrous webs suitable for protective apparel, air filtration, and other end uses.

Protective clothing is intended to protect the wearer from being exposed to something that is in the wearer's environment, or to protect the wearer's environment from being contaminated by the wearer. Includes any coveralls, gowns, smocks and other clothing. Examples of protective clothing include suits worn in clean rooms for microelectronics manufacturing, medical suits and gowns, coveralls for dirty work, and suits worn for protection against liquids or particulate matter. The particular application for which the protective garment is suitable depends on the composition of the cloth or sheet material used to make the garment and how some cloth or sheet materials are held together in the garment. For example, one type of cloth or sheet material may be very expensive or uncomfortable to use in medical garments, but may be superior for use in hazardous chemical protective clothing. .. Another material is lightweight and breathable enough for use in clean room suits, but not durable enough for dirty work applications.

The physical properties of the fabric or sheet material determine the application of protective clothing to which the material is suitable. It has been found that it is desirable for a wide variety of protective garment applications that the materials used in making the protective garment provide good barrier protection against liquids such as body fluids, paints or sprays. It is also desirable that the material used in making the protective garment prevent the passage of fine dust, dust and fiber particles. Another group of desirable properties of the cloth or sheet material used in protective clothing is that the material has sufficient strength and tear resistance that garments made using the sheet material are It is not to lose its integrity. Also, the cloth and sheet materials used in protective clothing transfer and dissipate both moisture and heat, allowing the wearer to physically work while wearing the garment without excessive heat and sweat. It is also important to be able to do. Finally, most armor materials must have a resilience that allows them to regain their shape when crushed or otherwise deformed. Post-crush recovery is a frequently used metric for resilience. In the context of the present invention, resilience encompasses both elastic and plastic deformation as long as the material substantially recovers its original shape and essential properties after the stress gradients responsible for crushing have been removed.

Adhering fabrics to form garments may require fusing the fabric material with other materials. Such fusion is even easier with materials having a reduced crystal structure. Therefore, what is needed is a fabric with high resilience and lower crystallinity than previously available to enhance adhesion with other layers, as evidenced by the crush value. is there. There are several conditions in which deformation can occur during placement or use of the material of interest. In that case, it is desirable for the material to regain its original shape and essential properties.

In one embodiment, the present invention relates to flash spun plexifilamentary fiber strands having a total crystallinity of 55% or less.

In a further embodiment, the fiber strands have a BET surface area of less than 12 m ² /g, a crush value of 0.9 mm/g or more. In yet another embodiment, the fiber strands predominantly include fibers formed from polyethylene.

In yet another embodiment, the fiber strands comprise predominantly fibers formed from polyethylene, said fibers having a total crystallinity of less than 52%.

The fiber strands of the present invention may further have monoclinic and orthorhombic structures as measured by X-ray analysis as described herein, wherein the monoclinic structure has a crystallinity of 1% or 1. % greater than.

In further embodiments, any of the embodiments disclosed herein of plexifilamentary fiber strands may be consolidated into a sheet structure. The sheet structure may then optionally be bonded thermally or mechanically.

In yet another embodiment, the present invention relates to a multi-layer structure that includes two or more consolidated sheets.

The multilayer structure of the present invention may further comprise two or more sheets, wherein at least one sheet is a polyethylene sheet comprising the plexifilamentary structure of any of the embodiments described herein. is there. For example, the plexifilamentary structure may be a consolidated sheet made from fiber strands according to any of the embodiments described herein. In a further embodiment, the plexifilamentary sheet may be thermally solidified.

Figure 3 shows a non-scaled cross-sectional schematic diagram of a spinning cell illustrating a method for making a flash spun plexifilamentary sheet. 3 is an example of X-ray signals from polyethylene with monoclinic and orthorhombic crystal structures. 1 shows a plot of total crystallinity versus BET for inventive and comparative examples. 3 shows a plot of crush versus total crystallinity for inventive and comparative examples.

Applicants specifically incorporate the entire contents of all cited references in this disclosure. Further, when an amount, concentration, or other value or parameter is given as either a range, a preferred range, or a list of preferred values on the higher side and preferred values on the lower side, this is not a disclosure of the ranges separately. It is to be understood as specifically disclosing all ranges, whether or not present, formed by any pair of any range upper limit or preferred value with any range lower limit or preferred value. Where ranges of numerical values are recited herein, unless otherwise stated, ranges are meant to include their endpoints and all integers and fractions within the range. It is not intended to limit the scope of the invention to the particular values recited when defining the range.

As used herein, the term "polymer" generally includes, but is not limited to, homopolymers, copolymers (such as block, graft, random and alternating copolymers, etc.), terpolymers, and the like, and blends and modifications thereof. Including things. Furthermore, unless otherwise specifically limited, the term “polymer” is intended to include all possible geometrical arrangements of materials. These configurations include, but are not limited to, isotactic, syndiotactic, random symmetry, and the like.

As used herein, the term "polyethylene" includes not only homopolymers of ethylene, but also copolymers in which at least 85% of the repeating units are ethylene units, such as copolymers of ethylene and alpha-olefins. Intended.

As used herein, the term "nonwoven, sheet or web" is arranged in an irregular manner to form a planar material without discernible patterns, such as patterns, as found in knitted fabrics. Means a single fiber or thread structure. Single fibers that are locally organized in some preferential manner or orientation are also considered to be arranged in an irregular manner due to this definition.

As used herein, "machine direction" is the longitudinal direction within the plane of the sheet, ie, the direction in which the sheet is manufactured. "Lateral" is the direction in the plane of the sheet that is perpendicular to the machine direction.

As used herein, the term "plexifilamentary" means a plurality of thin, ribbon-like, three-dimensional monolithic networks of film fibril elements of irregular length and median fibril width of less than about 25 micrometers. Means In plexifilamentary structures, the film fibril elements are generally aligned co-extensive with the axis of the structure and are discontinuous with irregular spacing at various locations throughout the length, width and thickness of the structure. To form a continuous three-dimensional network structure.

The term "spinning fluid" refers to the total composition that is spun using the spinning equipment described herein. The spinning fluid contains a polymer and a spinning agent.

The term "spinning agent" relates to the solvent or mixture of solvents and any additives, solubilizers and blends used to initially dissolve the polymer to form the spinning fluid.

A "multilayer structure" is a composite structure containing layers of different materials that may be layered and adhered in a face-to-face arrangement on at least a portion of their faces. In one embodiment, the multilayer structure of the present invention relates to two or more sheets, wherein at least one sheet is a polyethylene sheet comprising any of the plexifilamentary structures described herein.

In one embodiment, the present invention relates to flash spun plexifilamentary fiber strands having a total crystallinity of 55% or less.

In a further embodiment, the fiber strands have a BET surface area of less than 12 m ² /g, a crush value of 0.9 mm/g or more. In yet another embodiment, the fiber strands predominantly include fibers formed from polyethylene.

In yet another embodiment, the fiber strands primarily comprise fibers formed from polyethylene, said fibers having a total crystallinity of less than 52%.

The fiber strands of the present invention may further have monoclinic and orthorhombic structures as measured by X-ray analysis as described herein, wherein the monoclinic crystallinity is 1% or 1 Greater than % .

In further embodiments, any of the embodiments disclosed herein of plexifilamentary fiber strands may be consolidated into a sheet structure. The sheet structure may then optionally be bonded thermally or mechanically.

In yet another embodiment, the present invention relates to a multi-layer structure including two or more solidified sheets.

The multilayer structure of the present invention may further comprise two or more sheets, wherein at least one sheet is a polyethylene sheet comprising the plexifilamentary structure of any of the embodiments described herein. is there. For example, the plexifilamentary structure may be a consolidated sheet made from fiber strands according to any of the embodiments described herein. In a further embodiment, the plexifilamentary sheet may be thermally solidified.

A method for producing flash-spun plexifilamentary sheets, specifically Tyvek® spunbonded olefin sheet material, is described in US Pat. No. 3,081,519 to Blades et al. (issued to DuPont). First mentioned. U.S. Pat. No. 3,081,519 discloses a liquid spin agent that is below the normal boiling point of the liquid spin agent and above the normal boiling point of the liquid spin agent and below the normal boiling point of the liquid spin agent and is not a solvent for the polymer. Of fiber forming polymers are spun into regions of lower temperature and substantially lower pressure to form plexifilamentary film-fibril strands. As disclosed in U.S. Pat. No. 3,227,794 to Anderson et al. (issued to DuPont), plexifilamentary film-fibril strands are prepared by reducing the pressure of the polymer and spinner solution immediately before flash spinning in a vacuum chamber. It is best obtained using the method disclosed in Blades et al. when slightly lowered in (letdown chamber).

A general flash spinning device selected for the purposes of describing the present invention is the flash spinning device disclosed in US Pat. No. 3,860,369 to Brethauer et al. (incorporated herein by reference). Similar The apparatus and method for flash spinning a spinnable polymer is fully described in US Pat. No. 3,860,369 and is shown in FIG. Flash-spinning processes are usually carried out in a chamber 10, sometimes referred to as a spinning cell, which has spin-agent removal holes 11 and openings 12 through which the nonwoven sheet material produced in the process is removed. A spinning fluid containing a mixture of polymer and spin agent is provided to a spinning orifice 14 through a pressurized feed conduit 13. The spinning fluid passes from the supply conduit 13 to the chamber 16 through the chamber opening 15. In certain spinning applications, chamber 16 may act as a pressure decompression chamber, where the reduction in pressure is the phase of the spinning fluid as disclosed in US Pat. No. 3,227,794 to Anderson et al. Cause separation. A pressure sensor 22 may be provided to monitor the pressure in chamber 16.

The spinning fluid in chamber 16 then passes through spinning orifice 14. It is believed that the pressurized polymer and spin agent pass from the chamber 16 to the spinning orifice to create an extensional flow near the entrance of the orifice, which helps align the polymer. As the polymer and spin agent flow out of the orifice, the spin agent expands rapidly as a gas, leaving behind fibrillated plexifilamentary film-fibrils. Gas exits chamber 10 through holes 11. Preferably, the gaseous spin agent is concentrated for reuse in the spinning fluid.

The polymer strands 20 emitted from the spinning orifice 14 are conventionally guided towards a rotating deflector baffle 26. The rotating baffle 26 spreads the strands 20 into a more planar structure 24 in which the baffle guides alternating left and right. As the spread fiber strands descend from the baffle, the fiber strands are electrostatically charged, holding the fiber strands in the spread open configuration until the fiber strands 24 reach the moving belt 32. Fiber strands 24 are deposited on belt 32 to form batt 34. Ground the belt to help ensure proper anchoring of the charged fiber strands 24 on the belt. The fiber batt 34 may be fed under the roller 31 thereby compressing the batt into a lightly solidified sheet 35 formed of plexifilamentary film-fibril meshes arranged in overlapping multi-directional arrangements. The sheet 35 exits the spinning chamber 10 through the outlet 12 and is collected on the sheet collecting roll 29.

A "heat-set" or "heat-bonded" sheet is a sheet made by heat setting the web of the present invention. Some examples of thermal bonding methods are by gas bonding, steam entanglement, ultrasonic bonding, stretch bonding, hot calendering, hot roll embossing, hot surface bonding.

Thermal surface bonding can be performed by the method described in US Pat. No. 3,532,589 to David to obtain a hard bonded surface. In this method, the plexifilamentary sheet is then passed over a heated drum-cooling drum-heating drum-cooling drum to thermally bond both sides of the material. The heated drum is maintained at a temperature that results in partial melting of the plexifilamentary structure, including sheet bonding. The cooling drum is intended to reduce the temperature to a value that does not cause the sheet to shrink or deform when it cannot be suppressed. During the bonding process the sheet is slightly compressed by the flexible belt and there is a controlled shrinkage.

Alternatively, the plexifilamentary sheet may be adhered by an embossing roll and a rubber coated backup roll to adhere one or two sides of the sheet. The embossing rolls are smooth or, for example, the patterns shown in the following documents: dot patterns (US Pat. No. 3,478,141, US Pat. No. 6,610,390, US patent application). Publication 2004/241399A1), rib patterns (US 2003/0032355A1), random patterns (US 7,744,989) or different patterns (US 5). , 964, 742) and can have different patterns. The sheet may pass through one or more parts of the embossing roll as well as the rubber coated backup roll. Further, the sheet may be in contact with a preheat or chill roll as described in US Pat. No. 5,972,147 before and after the embossing roll and backup roll pair. Finally, the material may be softened by an adhesive process, such as the button breaking device described in US Pat. No. 3,427,376 by Dempsey.

Test Methods The following test methods were used in the description, examples, and claims to quantify various reported properties and properties.

The surface area of the plexifilamentary fiber strand product is a measure of the degree and fineness of fibrillation of the flash spun product. The surface area is S. Brunauer, P.; H. Emmett and E.I. Teller, J.; Am. Chem. Soc. , V. Measured by the BET nitrogen absorption method of 60 p 309-319 (1938) and reported as square meters/gram (m ² /g).

Crush value describes the ability of a fiber strand to recover its initial shape after being compressed. They were quantified using the following procedure: Three plexifilamentary fiber strands of different size were drawn from a Reemay® sheet. The three samples weighed about 1, 2 and 3 grams. The reported crush value is the average of the values measured on three samples. Minimal pressure was applied to form each sample plexifilamentary strand into a ball shape to avoid crushing, then the sample was weighed in grams. The crush value of each sample was measured using a crush tester consisting of an acrylic sample holder and a crusher. The sample holder included a cylindrical portion having an inner diameter of 2.22 inches (5.64 cm) and an outer diameter of 2.72 inches (6.91 cm). The center of the cylinder was located at the geometric center of a square base measuring 6.00 inches by 6.00 inches (15.24 cm by 15.24 cm). The crusher is a disk having a first disk-shaped surface (0.25 inch (0.64 cm) thick and 2.20 inch (5.59 cm) diameter) located at one end of the plunger rod. ) And a second disc on the plunger rod spaced 1.50 inches (3.81 cm) behind the first disc (diameter=0.75 inches (1.91 cm)). Inclusive. The second disk also had a thickness of 0.25 inch (0.64 cm) and a diameter of 2.20 inch (5.59 cm). The disc was sized slightly smaller than the inner diameter of the cylindrical sample holder to allow air to escape from the sample during crushing. The plexifilamentary samples were placed one at a time in the sample holder, and a thin piece of paper approximately 2.2 inches (5.59 cm) in diameter was placed on the plexifilamentary sample prior to crushing. The plunger rod was then inserted into the cylindrical sample holder so that the first disc-shaped surface was in contact with the piece of paper. The second disk served to hold the axis of the plunger rod in line with the axis of the cylindrical sample holder. Each plexifilamentary strand sample was crushed by placing a 2 lb (0.91 kg) weight on the plunger rod. The crush height (mm) was obtained by measuring the height of the sample from the bottom of the cylindrical sample holder to the bottom of the crusher. The plunger and weight were removed from the sample after about 2 minutes, leaving a piece of paper in place to facilitate measurement of the recovered height of the sample. Each sample was allowed to recover for approximately 2 minutes and the height of the paper from the center of each of the four sides of the sample holder was measured and the measurements averaged to obtain the recovered height (mm) of the sample. To calculate the crush value (mm/g), the average crush height is subtracted from the average recovered height and divided by the average sample weight. The crush value is a measure of how much the sample will recover to its original size after being crushed, with higher values indicating greater recovery of the sample's original height.

Crystallinity index
The crystallinity of polyethylene was measured using X-ray analysis according to the following procedure.

A reflection θ-θ Bragg-Brentano type diffractometer was equipped with a Cu-K _α X-ray tube source having a wavelength of 1.54 Å and a one-dimensional detector. The sample was placed horizontally on the flat holder in the center of the diffractometer and perpendicular to the scattering vector. The sample was rotated on this plane during the measurement.

The method used for the quantification of crystallinity is described by S. L. Aggarwal, G.; P. Tilley, Determination of crystallinity in polyethylene by X-Ray diffractometer, Journal of Polymer Science, Vol. 18, pp. 17-26, 1955 based on the ratio of scattered intensity in the crystalline region to total intensity. Only the analysis reported in this document considers the case where a tetragonal phase is present. Furthermore, monoclinic phases may also be present, in which case the procedure described below is used.

1. The local linear background, subtracted from the scattering angle 2θ=13±1±29±1°, was subtracted.

2. The scattering signal was applied to four different peaks: one for amorphous diffuse scattering (2θ=21.8°, peak width (full width at half maximum, FWHH) about 4.5-5°, and integrated intensity, I _amorphous ). The two peaks are mapped to the 110 (2θ=21.59°, I ₁₁₀ ) and 200 (2θ=24.03°, I ₂₀₀ ) reflections of the polyethylene orthorhombic crystal form, the last peak being polyethylene. Corresponds to the 100 (2θ=19.47°, I ₁₀₀ ) reflection of the monoclinic crystal form. The quoted angular position was slightly changed to account for the expected 2θ shift. The crystal peak had a width (FWHH) of about 1°. The Gaussian peak shape well explained the observed intensity, but the Pearson VII peak shape was also used with good results. Grams AI peak fitting software was used.

3. Total crystallinity was calculated from the ratio of crystal scattering to total scattering. Crystal scattering was defined as the sum of integrated intensities from crystal peaks (monoclinic and orthorhombic). Total scattering was defined as the sum of integrated intensities of crystalline and amorphous peaks:

Therefore, the partial crystallinities CI _orthorhombic and CI _monoclinic were calculated from these equations, respectively.

FIG. 2 is a graphic representation of the X-ray signals obtained using X-ray analysis of polyethylene with monoclinic and orthorhombic crystal structures. Different crystalline phases are identified by peak profiles and associated vertex positions and heights.

Experimental Flash-Spun Plexifilamentary webs are produced on a 1 gallon flash-spinning experimental apparatus. The 1 gallon capacity flash spinning apparatus used here is a modification of the 50 cc unit described in U.S. Pat. No. 5,147,586 with a larger size. The device consisted of two high pressure cylindrical chambers, each with a piston adapted to exert pressure on the contents of the chamber by a hydraulic pump. Each cylinder had an internal volume of 1 gallon. The cylinders were connected to each other at one end by a channel with a static mixer. The piston was driven by high pressure oil supplied by a hydraulic system. The output of one of the cylinders was coupled to a chamber with a spinneret device at the other end. The two cylinders are heated to a temperature similar to the desired spinning temperature. The polymer is placed in one cylinder. Then a vacuum is drawn on the cylinder. To provide the desired polymer concentration. Spin agent is added by high pressure pump. The polymer and spin agent were then heated to the desired mixing temperature as measured by a Type J thermocouple and typically held at that temperature for 60-120 minutes. During heating, a piston was used to create a differential pressure between the two cylinders instead. This procedure repeatedly pressed the polymer and spin agent through one mixing channel from one cylinder into the other, resulting in mixing and forming a spinning fluid. After mixing and just before spinning, the other piston was moved over that cylinder in order to place the contents completely in one cylinder. The valve is then opened to direct the spinning fluid to the chamber opening of the spinneret. The flash spun plexifilamentary web is guided by baffles onto a moving Reemay® coated belt in a nitrogen purged stainless steel housing to collect the plexifilamentary web.

Description of Materials Dichloromethane is commercially available from Brentag Northeast, 81 W.C. Industrial grade purity from Huller Lane, Reading, PA 19605, United states, used as received. Dichloromethane is CAS Nr. 75-09-2. Dichloromethane is also known as methylene chloride.

2,3-dihydrodecafluoropentane is commercially available from E.I. I. DuPont de Nemours and Company, 1007 Market Street, Wilmington DE, United States, obtained from CAS Nr. Hydrofluorocarbon having 138495-42-8, used as received.

Trans-1,2-dichloroethylene is commercially available from Diversified CPC International Inc. 24338 W.I. Durkee Rd. Purchased from Channahon IL 60410-9719, United States and used as received. Trans-1,2-dichloroethylene is also known as trans-1,2-dichloroethene, and is described in CAS Nr. 156-60-5.

HFE7100 is manufactured by 3M Center, Building 224-3N-11, St. A commercial grade hydrofluoroether known under the tradename Novec® 7100 by 3M®, purchased from Paul MN 55144-1000. Novec® 7100 is a 20-80 wt% contribution of methyl nonafluoroisobutyl ether, CAS Nr. 163702-08-7 and 20-80 wt% methyl nonafluorobutyl ether, CAS Nr. It is a mixture with 163702-07-6. Novec® 7100 has a purity of 99.5% and is used as received.

1H-perfluorohexane (1,1,1,2,2,3,3,4,4,5,5,6,6-tridecafluorohexane) was prepared according to CAS Nr. It is a hydrofluorocarbon having 355-37-3. 1H-perfluorohexane is commercially available from Fluoryx Inc. , 1933 Davis St. Ste. 294, San Leandro, CA 94577, purchased from United States. 1H-perfluorohexane has a purity of greater than 98% and is used as received.

1H,6H-perfluorohexane (1,1,2,2,3,3,4,4,5,5,6,6-dodecafluorohexane) was prepared according to CAS Nr. It is a hydrofluorocarbon having 336-07-2. 1H,6H-perfluorohexane is commercially available from Exfluor Research Corporation, 2350 Double Creek Dr. , Round Rock, TX, 78664, United States. 1H,6H-perfluorohexane has a purity level of about 95% and is used as received.

Examples 1 and 2 and the comparative example are 2.35 g/10 min (measured according to EN ISO 1133 at 190° C. and 5 kg load), and 24.5 g/10 min (190° C. and 21.6 kg load). Was spun from high density polyethylene having a melt index of 0.96 g/cm ³ (measured according to EN ISO 1133), a density of 0.96 g/cm ³ (measured according to EN ISO 1183). Examples 3-6 were 0.74 g/10 min (measured according to ASTM D 1238 at 190° C. and 2.16 kg load) and 29.6 g/10 min (ASTM D 1238 at 190° C. and 21.6 kg load). Spun from high density polyethylene having a melt index (measured in accordance with) and a density of 0.95 g/cm ³ .

Sheets of the present invention were obtained by flash spinning performed from an upstream vacuum chamber of at least 15 cm ³ and a discharge pressure of 70 bar gauge minimum, resulting in 200-400 denier fibers.

Results Examples Table 1 describes the spinning conditions of the examples and Table 2 describes the properties obtained for the examples.

Table 1

Table 2

Table 3 describes the spinning conditions for the comparative examples and Table 4 describes the properties obtained for the comparative examples.

Table 3

Table 4

The BET surface area of 12 m ² /g or less, the crushing value of 0.9 mm/g or more, and the crystallinity of 55% or less, which are a series of properties that serve the purpose of the present invention, are the spinning conditions and the It can only be achieved using the composition. None of the comparative examples meet the desirable set of properties of the present invention.

The plot of _total crystallinity (CI _total ) versus BET in FIG. 3 for Comparative Examples A-C made using a hydrocarbon spin agent is shown by the black diamonds in the figure, showing a decrease in CI _total with BET. .. For Cases 1-8, shown as open circles, for other spinner systems, Figure 3 shows the trend of increasing CI _total with BET.

FIG. 4 shows the collapse value as a function of total crystallinity. For the comparative examples Cases A-C, shown as black diamonds, the crush increases with increasing total crystallinity, whereas for Examples 1-8, the white diamonds, 0.9 mm/gram. A crush value of more than corresponds to a crystallinity of less than 55%.
Next, an aspect of the present invention will be described.
1. Flash-spun plexifilamentary fiber strands containing fibers having a total crystallinity of 55% or less.
2. The flash-spun plexifilamentary fiber strand according to 1 above, which has a BET surface area of 12 m ² /g or less and a crush value of 0.9 mm/g or more.
3. The flash spun plexifilamentary fiber strand according to 2 above, which comprises mainly fibers formed from polyethylene.
4. The fiber strand according to 3 above, wherein the fibers have a total crystallinity of 52% or less.
5. The fiber strand according to 3 above, wherein the fibers have a monoclinic structure and a orthorhombic structure as measured by X-ray characterization, and the crystallinity of the monoclinic structure is 1% or more.
6. A solidified sheet produced from the fiber strand according to 1 above.
7. A multilayer structure comprising two or more sheets or strands, wherein at least one sheet is the sheet described in 6 above.
8. A thermally or mechanically solidified sheet produced from the fiber strand according to 1 above.
9. A multilayer structure comprising two or more sheets or strands, wherein at least one sheet is the sheet described in 8 above.

Claims (3)

A thermally or mechanically solidified sheet containing flash spun plexifilamentary fiber strands comprising fibers having a total crystallinity of 55% or less, said flash spun plexifilamentary fiber strands having a surface area of 12 m ² / has a BET surface area of g or less and a crush value of 0.9 mm/g or more,
Crystallization of the monoclinic structure, the fiber strands predominantly comprising fibers formed from a homopolymer of ethylene, and having a monoclinic and orthorhombic structure as measured by X-ray characterization. Said sheet having a degree of greater than 1%. The sheet of claim 1, wherein the fibers have a total crystallinity of 52% or less. A multilayer structure comprising two or more sheets, wherein at least one sheet is the sheet of claim 1.