US20110233342A1

US20110233342A1 - Load bearing sheet comprising reinforcing tapes

Info

Publication number: US20110233342A1
Application number: US13/121,490
Authority: US
Inventors: Roelof R. Marissen; Daniela D. Ribezzo
Original assignee: DSM IP Assets BV
Current assignee: DSM IP Assets BV
Priority date: 2008-10-07
Filing date: 2009-10-05
Publication date: 2011-09-29
Also published as: EP2334854A2; US20110173874A1; TW201022493A; JP2012505316A; EP2334493A1; WO2010040714A1; JP5545500B2; CN102177284B; WO2010040711A2; AU2009301171A1; US8578693B2; CN102177284A; WO2010040711A3

Abstract

A load bearing sheet comprises a flexible layer reinforced with a plurality of reinforcing elements arranged on one or both sides of the flexible layer. The plurality of reinforcing elements comprises at least one HMwPE or UHMwPE tape. The invention also relates to a process for making the sheet. The invention further relates to applications of the sheet.

Description

TECHNICAL FIELD OF THE INVENTION

The invention relates to a sheet comprising a flexible layer reinforced with a plurality of reinforcing elements, and a method for producing thereof. The invention also relates to applications of such a sheet.

BACKGROUND OF THE INVENTION

Sheets that interact with wind or air are reinforced for the purpose of bearing the load exerted thereon during use. A typical example of such a sheet is a sail, which catches wind to generate thrust. The function of the reinforcing elements is to prevent the sheet from being ripped or torn, and they therefore should have a high stretch resistance and a high tensile modulus. One example of a sail with reinforcing elements is disclosed in U.S. Pat. No. 7,383,783 B2. This publication discloses a sail comprising a plurality of sailcolths made of light and flexible material, joined together, each of which comprises two outer layers, between which a plurality of reinforcing elements are positioned. As the material for the reinforcing elements, polymer materials, metal or metal alloys, and ceramic material are disclosed. Various polymer materials are disclosed, including high module polyethylene such as Spectra®, Dyneema® and Cetran®. It is taught that the reinforcing elements can be subjected to treatments which improve their mechanical characteristics, their characteristics of adhesion etc. by means of e.g. covering with thermoplastic adhesives. However, there is still a need for improvements on various properties of the sail, especially the reinforcing elements in the sail.

OBJECT OF THE INVENTION

Accordingly, it is an object of the present invention to provide an improved sheet with reinforcing elements.

DISCLOSURE OF THE INVENTION

This object is achieved according to the invention by a sheet according to claim 1. According to the invention a load bearing sheet is provided comprising a flexible layer reinforced with a plurality of reinforcing elements arranged on one or both sides of the flexible layer, wherein the plurality of reinforcing elements comprises at least one high molecular weight polyethylene (HMwPE) or ultrahigh molecular weight polyethylene (UHMwPE) tape.
According to the present invention, a larger area of the sheet is reinforced by the tape compared to the sheet reinforced with traditional fibers with a circular cross section of the same weight. The sheet is also more evenly reinforced by the tape according to the present invention. Furthermore, the distance between the reinforcing elements will be smaller according to the present invention, reducing the non-reinforced “weak spots” of the sheet.
By a load bearing sheet, it is meant herein as a sheet which bears a stretching load during use, e.g. from air or wind. In other words, the sheet is subjected to and functions to withstand a changing or a constant force for stretching the sheet during use.
The flexible layer may be made of various materials, and can be selected by the skilled person depending on the need. Suitable materials for the flexible layer include polymer materials such as polyethylene, polypropylene, polyester, polyethylene terephthalate, polyethylene naphthalene, polyamide.
A reinforcing element is herein understood to mean an element which assists an item in maintaining its shape and in preventing it from being ripped or torn.
A tape is herein understood to mean a flat elongated body the length dimension of which is much greater than its cross section dimension, e.g. the length dimension is at least 5 times larger than the width. Typically, the length dimension is at least 10 times larger, or even at least 50 times larger, than the width dimension. The cross section may have various anisotropic shapes, such as rectangular or elliptical. The longer axis of the cross section is referred as width and the shorter axis of the same, perpendicular to the width direction, is referred as thickness.
By high molecular weight polyethylene (HMwPE), it is herein meant a polyethylene with a molecular weight from 50,000 to 400,000. Ultrahigh molecular weight polyethylene (UHMwPE) is defined herein as a polyethylene with a molecular weight of at least 400,000. UHMwPE may have a molecular weight of up to several millions.
Intrinsic viscosity may be used for determining the molecular weight. Intrinsic viscosity is a measure for molar mass (also called molecular weight) that can more easily be determined than actual molar mass parameters such as Mn and Mw. The IV is determined according to method PTC-179 (Hercules Inc. Rev. Apr. 29, 1982) at 135° C. in decalin, the dissolution time being 16 hours, with DBPC as the anti-oxidant in an amount of 2 g/l solution, and the viscosity at different concentrations is extrapolated to zero concentration. There are several empirical relations between IV and Mw, but such relation is highly dependent on molar mass distribution. Based on the equation Mw=5.37*10⁴[IV]^1.37(see EP 0504954 A1) an IV of 4.5 dl/g would be equivalent to a Mw of about 422 kg/mol.
Because of their long molecule chains, stretched UHMwPE fibers with an IV of more than 5 dl/g have very good mechanical properties, such as a high tensile strength, modulus, and energy absorption at break. More preferably, a polyethylene with an IV of more than 10 dl/g is chosen. This is because a yarn made by gel-spinning such UHMwPE yarn offers a combination of high strength, low relative density, good hydrolysis resistance, and excellent wear properties. Suitable UHMwPE has an intrinsic viscosity of typically above 5 dl/g, preferably between about 8 and 40 dl/g, more preferably between 10 and 30, or 12 and 28, or between 15 and 25 dl/g.
Preferably, the HMwPE and UHMwPE of the present invention are a linear polyethylene, i.e. a polyethylene with less than one side chain or branch per 100 carbon atoms, and preferably less than one side chain per 300 carbon atoms, a branch generally containing at least 10 carbon atoms. Preferably, only polyethylene is present, but alternatively the polyethylene may further contain up to 5 mol % of alkenes that may or may not be copolymerized with it, such as propylene, butene, pentene, 4-methylpentene or octene. The polyethylene may further contain additives that are customary for such fibres, such as anti-oxidants, thermal stabilizers, colorants, etc., up to 15 weight %, preferably 1-10 weight %.
The load bearing sheet may be made by various ways of attaching the tape and one or both sides of a flexible layer to each other. The reinforcing elements may be stitched to the flexible layer. Alternatively, the reinforcing elements may be adhered to the flexible layer. The adhering of the reinforcing elements to the flexible layer may be done in various ways known to the skilled person and is not described here in detail. Suitable adhesives may be applied to the reinforcing elements and the reinforcing elements may then be applied to the flexible layer. It is also possible to adhere the reinforcing elements to the flexible layer by means of heat and pressure. For example, the arrangement of the flexible layer and the reinforcing layer may be heated so that the flexible layer and the arranged reinforcing elements are fused at the interface. Preferably, the flexible layer has a melting temperature lower than that of the reinforcing elements. This will prevent the degradation of the mechanical properties of the reinforcing elements. The reinforcing elements may be arranged in various patterns suitable for respective applications. Suitable patterns are known to the skilled person.
It is also possible to first arrange the reinforcing elements in a desired pattern, and then apply a resin or a polymer in a fluid state onto the pattern. Thereafter the resin or the polymer may be solidified by processes known to the skilled person, e.g. curing, to form the flexible layer of the load bearing sheet. An advantage of this method is that no adhesives are required and that there is more freedom in the process of making the load bearing sheet.
The peeling strength between the flexible layer and the reinforcing elements of the sheet according to the present invention improves significantly. The larger contact area between the tape and the flexible layer, resulting from the flat shape of the tape, leads to this improved peeling strength. In contrast, traditional fibers having a generally circular cross section have a small contact area with the sheet.
A sail or a kite comprising the sheet according to the present invention has a further advantage that the sail or the kite has a smoother surface because of the flat shape of the tape, which is aerodynamically advantageous. Furthermore, the smoother surface means that the tapes do not protrude from the sheet as much as traditional fibers with a generally circular cross section. Less protrusion results in less chance of the reinforcing elements being subjected to forces from outside to detach the reinforcing elements from the sheet. Accordingly, chance of the peeling is considerably reduced, without the need of special treatments to improve adhesion.
The tape preferably has a width of at least 0.3 cm, more preferably at least 0.6 cm, even at least 1.5 cm. A tape with a large width is advantageous in terms of the manufacturing process, in that fewer steps are required for arranging and adhering the tape. It is to be noted that a fiber with a circular cross section has a limitation in its cross section, since a fiber with a large circular cross section will result in a larger protrusion and a less smooth surface of the sheet, which are undesirable for the reasons explained above. However, such limitation is not present for a reinforcing element which is a tape.
A sail comprising tapes is disclosed in U.S. Pat. No. 5,097,783. In this publication, stress-bearing structural members are formed of multiple filament strands, ribbons or strips of a stretch resistant polymer, advantageously an aramid such as a Kevlar. It is disclosed that the members, in the form of tape comprising one or a plurality of strands adhered to a backing tape such as polyethylene film, are attached to the skin by adhesive after being laid on the skin by hand in a predetermined pattern. Such a member, i.e. a PE backing tape which in itself is reinforced by aramid fibers, is very different from a tape made of HMWPE or UHMWPE in that it has much lower mechanical properties than a HMWPE or UHMWPE tape.
The superior mechanical properties of the HMwPE or UHMwPE tape in combination with the high adhesion make the HMwPE or UHMwPE tape especially suitable for use in a sail or a kite.
Preferably, at least 50 wt % of the reinforcing elements are HMwPE or UHMwPE tapes, UHMwPE tapes being more preferred. More preferably, at least 75 wt %, or even 100 wt % of the reinforcing elements are HMwPE or UHMwPE tapes, UHMwPE tapes being more preferred.
According to a preferred embodiment, the sheet further comprises a second flexible layer and at least some of the plurality of reinforcing elements are arranged between the two flexible layers.
This arrangement increases the strength of the sheet. In this embodiment, the advantage of the improved peeling strength is further enhanced. In such a laminated structure, the reinforcing elements between the two flexible layers also have the function of holding the two layers of the sheet together. Peeling of the reinforcing elements from the sail results in a delamination of the layers of the sheet from each other, therefore the prevention of the peeling is even more important.
In this embodiment, an open space is present between the two layers and the reinforcing element, where none of these layers and the reinforcing element contacts each other. Delamination of the sail cloths from each other tends to start from this open space. This open space is much smaller when the reinforcing element is a tape, compared to a fiber with a generally circular cross section.
The load bearing sheet according to the present invention may be shaped into a hollow object for use in products which bear loads during use. Examples of the hollow objects include a bag and a container which support the weight of items inside, and an air balloon and an air ship which support gas pressure, e.g. air or helium pressure. With a hollow object, it is meant as an object that can surround or enclose in its hole a solid material or fluid such as air or water. The hollow object may be e.g. ball-shaped with or without one or more holes, cylinder shaped with or without one or more holes, or bent or straight tube-shaped. The sheet is especially advantageous for use in an air balloon and an air ship, since being lightweight is especially important in these products. With the same amount or weight of reinforcing elements, the reinforcing elements in the form of tape can cover more area compared to the reinforcing elements in the form of a traditional fiber with a generally circular cross section. In other words, the thickness of the reinforcing elements can be made smaller for reinforcing the same area, thus reducing the area density of the sheet. The overall weight of the balloon of an air balloon or an air ship is thereby reduced with the same strength according to the present invention.
The hollow object may also be made by first shaping a sheet to be reinforced into a desired hollow shape, and then providing the tape thereon. This may be done e.g. by a filament winding process. The sheet may also be used as a reinforcement of the hollow object. The structure of the hollow object may be made first with a material which is not sufficiently strong, and then the load bearing sheet according to the present invention may then be provided on the surface thereof.
The tape used in the load bearing sheet according to the present invention is preferrably provided by a process comprising the steps of:

- a) extruding a fluid composition comprising HMwPE or UHMwPE from one spinhole having an anisotropic shape;
- b) cooling the fluid tape so that the composition solidifies and
- c) drawing the solidified tape in longitudinal direction in at least one drawing step to obtain a monofilament tape.

According to a preferred embodiment, the tape is a gel-spun monofilament UHMwPE tape. This gel spinning process generally comprises the steps of a) spinning at least one filament from a solution of ultra high molecular weight polyethylene in a solvent; b) cooling the filament obtained to form a gel filament; c) removing at least partly the solvent from the gel filament; and d) drawing the filament in at least one drawing step before, during or after removing solvent. Gel spinning of UHMwPE has been described in various publications, including EP 0205960 A, EP 0213208A1, U.S. Pat. No. 4,413,110, WO 01/73173 A1, and Advanced Fiber Spinning Technology, Ed. T. Nakajima, Woodhead Publ. Ltd (1994), ISBN 1-855-73182-7, and references cited therein. These publications are incorporated herein by reference.
Therefore, according to one aspect of the present invention, the tape is provided by extruding a solution of UHMwPE in a solvent from one spinhole having an anisotropic shape; cooling the fluid tape so that the composition solidifies to form a gel filament tape; at least partly removing the solvent; and drawing the gel filament tape in longitudinal direction in at least one drawing step before, during or after removing solvent.
A monofilament is herein understood to mean a filament obtainable from a single spin hole. It will be appreciated that the monofilament used herein does not need to have a circular cross section. It is noted that a monofilament which has been reformed by e.g. folding a single monofilament and fusing different parts of the monofilament is still called a monofilament, but a yarn which is made from a plurality of fibers that have been fused together is not called a monofilament.
In the process, any of the known solvents for gel spinning of UHMwPE can be used. Suitable examples of spinning solvents include aliphatic and alicyclic hydrocarbons, e.g. octane, nonane, decane and paraffins, including isomers thereof; petroleum fractions; mineral oil; kerosene; aromatic hydrocarbons, e.g. toluene, xylene, and naphthalene, including hydrogenated derivatives thereof, e.g. decalin and tetralin; halogenated hydrocarbons, e.g. monochlorobenzene; and cycloalkanes or cycloalkenes, e.g. careen, fluorine, camphene, menthane, dipentene, naphthalene, acenaphtalene, methylcyclopentandien, tricyclodecane, 1,2,4,5-tetramethyl-1,4-cyclohexadiene, fluorenone, naphtindane, tetramethyl-p-benzodiquinone, ethylfuorene, fluoranthene and naphthenone. Also combinations of the above-enumerated spinning solvents may be used for gel spinning of UHMWPE, the combination of solvents being also referred to for simplicity as spinning solvent. In one embodiment, the spinning solvent of choice has a low vapor pressure at room temperature, e.g. paraffin oil. It was also found that the process of the invention is especially advantageous for relatively volatile spinning solvents at room temperature, as for example decalin, tetralin and kerosene grades. Most preferably, the spinning solvent is decalin.
A gel-spun monofilament UHMwPE tape was found to have a very high tenacity, which is very advantageous as a reinforcing element. Preferably, the UHMwPE tape has a tenacity of at least 20 cN/dtex, preferably at least 25 cN/dtex, even more preferably at least 30 cN/dtex, most preferably at least 35 cN/dtex. Such a high tenacity is obtainable due to the fact that the tape is a drawn UHMwPE tape.
Furthermore, the gel-spun monofilament UHMwPE tape was found to have a very high modulus. Preferably, the tape has an modulus of at least 600 cN/dtex, more preferably at least 900 cN/dtex, even more preferably at least 1300 cN/dtex. The high modulus is especially advantageous in a sail application, since the high modulus helps the sail to maintain its shape optimized for catching the wind for maximum speed.
According to a further embodiment, the UHMwPE tape is formed of a plurality of gel-spun UHMwPE filaments which have been at least partly fused. The method for making such a tape comprises exposing a plurality of gel-spun UHMwPE filaments to a temperature within the melting point range of the polyethylene for a time sufficient to at least partly fuse adjacent filaments under tension. This process is explained in detail in WO2006074823, which is incorporated herein by reference. Such a UHMwPE tape has a tenacity and modulus comparable to a gel-spun monofilament UHMwPE tape.
The UHMwPE tape may also be made by feeding a UHMWPE powder between a combination of endless belts, compression-moulding the UHMWPE powder at a temperature below the melting point thereof and rolling the resultant compression-moulded polymer thereby forming a tape, and subsequently drawing of that tape.
According to a further embodiment, the tape is a melt-spun monofilament HMwPE tape or a melt-spun monofilament UHMwPE tape wherein the UHMwPE has a molecular weight of up to 800,000. The melt-spinning process is widely known in the art, and involves heating a PE composition to form a PE melt, extruding the PE melt, cooling the extruded melt so that the melt solidifies, and drawing the solidified PE at least once. The process is mentioned e.g. in EP0344860A1, WO03/037590A1 and EP1743659A1, which are incorporated herein by reference.
Therefore, according to one aspect of the present invention, the tape is provided by extruding a HMwPE melt or a melt of UHMwPE having a molecular weight of up to 800,000 from one spinhole having an anisotropic shape; cooling the melt so that the composition solidifies and drawing the solidified tape in longitudinal direction in at least one drawing step.
In this embodiment, PE is chosen in view of the processibility. HMwPE can be melt-spun without difficulty, and UHMwPE with a molecular weight of up to 800,000 can also be melt-spun. A higher molecular weight provides a tape with more desirable mechanical properties, but processibility is decreased, and especially extruding becomes more difficult. Preferably the melt spun tape has a tenacity of at least 13 cN/dtex, preferably at least 16 cN/dtex, even more preferably at least 20 cN/dtex.
In a preferred embodiment, the tape is formed of longitudinally folded and fused layers. One way of making this tape comprises extruding a fluid composition comprising HMwPE or UHMwPE from one spin hole having an anisotropic shape; cooling the fluid tape so that the composition solidifies; folding the solidified tape in the longitudinal direction; and exposing the solidified tape to a temperature within the melting point range of the polyethylene for a time sufficient to at least partly fuse adjacent layers under tension. Optionally, the solidified tape may be drawn one or more times before the folding step.
It is advantageous to fold the solidified tape before the tape is dried, since the layers formed by the folding adhere to each other and no special equipment is required for holding the layers in place. In one embodiment, after the step of solidifying, the solidified tape is wound by a winder which has a slot with a width smaller than the width of solidified tape. This results in the solidified tape being folded in the longitudinal direction.
This process is especially advantageous for obtaining tapes of different thickness with a single die. The folding may be performed in various ways and various cross sectional shape of the formed tape may be made. The thickness of the tape can be controlled by the number of folds in the tape.
The tape may be folded such that a homogeneous thickness is obtained over the width of the tape. In this case, the cross section is substantially rectangular.
The number of folds may alternatively be varied depending on the location of the tape. This results in a tape with a cross section with different thickness depending on the location. For example, it is possible to fold the tape such that the thickness at the edge is higher than at the center. However, in a preferred embodiment, the tape is folded in such a way that the thickness in the center would be higher than the edge. In this case, the resulting cross section may have an elliptical-like shape.
The cross section having a higher thickness in the center is highly advantageous in terms of peeling strength. The curved peripheral of the tape fits to the sheet. This advantage is especially emphasized in an embodiment where the sheet comprises two layers. The open space present between the two layers and the reinforcing element, which is the starting point of the delamination, is minimized.
It is also possible to obtain the tape formed of longitudinally folded and fused layers by first making a non-folded dried tape and thereafter performing the steps of folding and fusing. The non-folded dried tape may be a monofilament tape made by the gel-spinning or the melt-spinning process described earlier. The non-folded dried tape may also be made from a plurality of UHMwPE filaments by fusing. In such embodiment, process for making the tape to be folded comprises the steps of arranging UHMwPE fibers in the form of a tape, and exposing the UHMwPE fibers to a temperature within the melting point range of the polyethylene for a time sufficient to at least partly fuse adjacent fibers under tension.

The invention will be described referring to the drawings in which:

FIG. 1 diagrammatically illustrates a cross section of a sail comprising a sail sheet comprising two flexible layers and a reinforcing element having a generally circular cross section; and

FIG. 2 diagrammatically illustrates a cross section of a sail comprising a sail sheet comprising two flexible layers and a reinforcing element having a generally elliptical cross section.

FIG. 1 diagramatically shows a cross section of a conventional sail. A reinforcing element 31 is sandwiched between a first layer 11 and a second layer 21 of a sail sheet. The reinforcing element 31 has a circular cross section. A large open space 41 is present between the first layer 11, the second layer 21 and the reinforcing element 31. The space 41 acts as a starting point for delamination of the two layers 11 and 12. The large size of the space 41 leads to a larger chance of the delamination.
FIG. 2 shows a cross section of a sail according to the present invention. Similar to the sail described in FIG. 1, a reinforcing element 32 is sandwiched between a first layer 12 and a second layer 22 of a sail sheet. In this example, the reinforcing element 32 has a elliptical cross section. The open space 42 is much smaller compared to the example in FIG. 1, reducing the risk of delamination.
The invention will be explained more fully below with reference to the following examples.

Methods:

- IV: the Intrinsic Viscosity was determined according to method PTC-179 (Hercules Inc. Rev. Apr. 29, 1982) at 135° C. in decalin, the dissolution time being 16 hours, with DBPC as anti-oxidant in an amount of 2 g/l solution, by extrapolating the viscosity as measured at different concentrations to zero concentration.
- Dtex: fibers' linear density (dtex, g/10 km) was measured by weighing a piece of fiber of 10 m length (about 18 cm). The measured weight in mg is the dtex.
- Tensile properties: tensile tests were carried out on an Instron Z010 tensile tester equipped with a 1 kN load cell and Instron parabolic fiber grips, in accordance with ASTM D885M, using a nominal gauge length of the fibre of 500 mm. Tensile strength was determined from the force at break and the linear density measured on each individual sample. Tensile modulus was determined as the chord modulus between 0.3 and 1.0% strain. Elongation at break and strain were determined by using a gauge length of 100 mm with a tension of 0.08 N at zero strain. The gauge length incorporated the full fiber length on the parabolic grip sections until the beginning of the flat pneumatical grip sections. Strain rate during tensile testing was 50 mm/min.

EXAMPLE 1

A UHMwPE monofilament tape to be used for the load bearing sheet was made via a gel spinning process. A solution of 8 wt % of UHMwPE of IV 20 dl/g in decalin was spun at about 160° C. through a spin plate having one spin hole into a solution monofilament tape. The solution monofilament tape was issued from a slot of 72 mm by 0.8 mm to an air-gap of 5 mm and entered a water bath. The solution monofilament tape was cooled in the water bath kept at about 30° C. to obtain a gel tape, and taken-up at such rate that a draw ratio of 1.8 was applied in the air-gap. Spin velocity was kept constant at 2.8 m/min. The gel tape was subsequently further drawn with a draw ratio of 20 at an average temperature of 125° C. to obtain a partially drawn tape. The partially drawn tape was wound into a bobbin using a winder which has a slot with a smaller width than the width of partially drawn tape. This caused longitudinal folding of the partially drawn tape. Spin finish was not applied. This folded partially drawn tape was subsequently drawn with a draw ratio of 3.5 at an average temperature of 151° C., to obtain a fully drawn tape.
The flat section and cross section of the tape were observed by SEM. It was observed that the folded sections of the tape have been fused during the final drawing. The thickness of the tape varied depending on the number of layers of the tape on top of each other, ranging from about 10 to 40 μm. The width of the tape was about 0.85 mm.
The tensile properties of the tape were measured according to the methods described hereinabove and the results are presented in Table 1.

EXAMPLE 2-5

Experiment was carried out analogously to example 1, but the partially drawn tape was not further drawn for example 2, and the draw ratio of the second drawing step to obtain a fully drawn tape was 2.5, 3.0 and 4.0, respectively for examples 3-5. The tensile properties of the yarn were measured according to the methods described hereinabove and the results are presented in Table 1.

TABLE 1

Draw ratio
in second				Elongation
drawing	Titer	Tenacity	E-modulus	at break
step	dtex	cN/dtex	cN/dtex	%

Ex. 1	3	334	30.4	1174.1	2.98
Ex. 2	1 (PDT*)	996	18.2	372.1	5.81
Ex. 3	2.5	411	28.3	1069.5	3.18
Ex. 4	3.5	292	30.3	1440.2	2.56
Ex. 5	4	252	32.7	1522.3	2.66

*PDT: Partially drawn tape; a tape obtained without the second drawing step

EXAMPLE 6

Experiment was carried out analogously to example 1, but the partially drawn tape was wound using a winder with a slot wider than the tape width. Thus, a tape was obtained without longitudinal folding.
The width and the thickness of the tape were determined by SEM to be 4000 μm and 10 μm, respectively. This high ratio between the width and the thickness is surprising, since it is much higher than that of the spin hole of the spin plate.
The invention has been explained above in relation to a load bearing sheet reinforced with a tape. However, it is noted that the tape described in relation to some examples have specific advantages when used in other applications. For example, the UHMwPE tape is suitable as a dental tape. Furthermore, the UHMwPE tape which is a gel-spun monofilament is especially advantageous for use in medical products requiring a high tenacity. Examples of suitable medical products include a surgical suture, a surgical mesh, a medical cable and a medical balloon. Especially, the tape formed of longitudinally folded layers of a gel-spun monofilament is advantageous in view of the strict requirement of the residual solvent content. According to the process for making this type of tape, a thick tape is obtained without special consideration for solvent removal. Since the solvent is removed in this process while the tape is formed of one thin layer, the solvent is easily removed. The thickness of the tape that can be made is thus virtually unlimited.

Claims

1. A load bearing sheet comprising a flexible layer reinforced with a plurality of reinforcing elements arranged on one or both sides of the flexible layer, wherein the plurality of reinforcing elements comprises at least one HMwPE or UHMwPE tape.

2. A load bearing sheet according to claim 1, wherein the sheet further comprises a second flexible layer and at least some of the plurality of reinforcing elements are arranged between the two flexible layers.

3. The load bearing sheet according to claim 1, wherein the tape is a gel-spun monofilament UHMwPE tape.

4. The load bearing sheet according to claim 1, wherein the tape is a UHMwPE tape formed of a plurality of gel-spun UHMwPE filaments which have been at least partially fused.

5. The load bearing sheet according to claim 1, wherein the tape has a tenacity of at least 20 cN/dtex, preferably at least 25 cN/dtex, even more preferably at least 30 cN/dtex, most preferably at least 35 cN/dtex.

6. The load bearing sheet according to claim 1, wherein the tape has modulus higher than 600 cN/dtex.

7. The load bearing sheet according to claim 1, wherein the tape is a melt-spun monofilament HMwPE or UHMwPE tape, the UHMwPE having a molecular weight of up to 800,000.

8. The load bearing sheet according to claim 1, wherein the tape is formed of longitudinally folded and fused layers.

9. A sail or a kite comprising the load bearing sheet according to claim 1.

10. A hollow object comprising the load bearing sheet according to claim 1.

11. A process for making a load bearing sheet, comprising the steps of providing a HMwPE or UHMwPE tape and attaching the tape and one or both sides of a flexible layer to each other.

12. The process according to claim 11, wherein the step of providing the tape comprises the steps of:

a) extruding a fluid composition comprising HMwPE or UHMwPE from one spinhole having an anisotropic shape.

b) cooling the fluid tape so that the composition solidifies;

c) drawing the solidified tape in longitudinal direction in at least one drawing step to obtain a monofilament tape.

13. The process according to claim 12 wherein the fluid composition is a solution of UHMwPE in a solvent and the step of providing the tape further comprises the step of at least partly removing the solvent.

14. The process according to claim 12, wherein the fluid composition is a melt of a HMwPE or a UHMwPE having a molecular weight of up to 800,000.

15. The process according to claim 12, 13 or 14, wherein the step of providing the tape further comprises the steps of:

a) folding the solidified tape in the longitudinal direction; and

exposing the solidified tape to a temperature within the melting point range of the polyethylene for a time sufficient to at least partly fuse adjacent layers under tension.