Material sheet and process for its preparation

Abstract

Material sheets are provided which include at least one monolayer, wherein the at least one monolayer has a plurality of drawn unidirectional polymer fibers having a strength of greater than 1.2 GPa and a thickness of less than 100 μm, and wherein the material sheet includes a bonding agent of less than 13 wt % relative to the total weight of the material sheet.

Claims

1. A process for the production a material sheet comprising the step of: (a) unidirectionally aligning a plurality of drawn polymer fibers having a strength of greater than 1.2 GPa in abutting lengthwise contact with one another to thereby form a monolayer having a thickness of less than 100 μm and a bonding agent content of less than 13 wt % relative to the weight of the material sheet; and (b) compressing the material sheet to cause adjacent fibers to be mechanically fused to one another along the abutting lengthwise contact thereof.

2. The process according to claim 1, further comprising prior to step (b) the steps of: (a1) repeating step (a) thereby producing at least two monolayers, and (a2) stacking each monolayer such that the fiber direction in adjacent monolayers differs.

3. The process according to claim 1, wherein step (a) comprises aligning the drawn polymer fibers such that at least a portion of the fibers within at least one monolayer are in abutting contact along substantially an entire length of the fibers, and wherein step (b) comprises compressing the material sheet under at least 75 bar pressure such that the abutting fibers are mechanically fused together, thereby forming the material sheet.

4. The process according to claim 1, wherein the process comprises subjecting the material sheet to pressure and temperature conditions such that less than 5 wt % of the fibers within the material sheet have melted during the process as determined by DSC (10° C./min).

5. The process according to claim 1, wherein the process comprises subjecting the material sheet to pressure and temperature conditions such that there is no detectable melt bonding as determined by DSC (10° C./min).

6. The process according to claim 1, wherein each of the thickness of the monolayer is no more than 2 times a thickness of the unidirectionally aligned polymer fibers.

7. The process according to claim 1, wherein step (a) is practiced such that the monolayer comprises less than 10 wt % of the bonding agent.

8. The process according to claim 1, which further comprises providing the bonding agent as bonding strips or fibers to bond the fibers in the monolayer.

9. The process according to claim 8, which further comprises orienting the bonding strips or fibers in a different direction than the unidirectionally aligned polymer fibers within the monolayer.

10. The process according to claim 8, which comprises covering no more than 20% of a surface area of the monolayer surface area with the bonding strips or fibers.

11. The process according to claim 1, wherein step (b) is practiced such that the adjacent fibers are mechanically fused along at least 40% of an abutting length thereof.

12. The process according to claim 1, wherein step (b) is practiced such that the adjacent fibers are mechanically fused along at least 60% of an abutting length thereof.

13. The process according to claim 1, wherein step (b) is practiced such that the adjacent fibers are mechanically fused along at least 80% of an abutting length thereof.

14. The process according to claim 1, wherein step (b) is practiced such that the adjacent fibers are mechanically fused along at least 90% of an abutting length thereof.

15. The process according to claim 1, wherein step (b) is practiced such that the adjacent fibers are mechanically fused along at least 95% of an abutting length thereof.

Description

(1) FIG. 1 is a schematic drawing of process in accordance with the present invention of preparing at least one monolayer;

(2) FIG. 2 is a graph illustrating ballistic performance versus monolayer thickness;

(3) FIG. 3 is a Differential scanning calorimetry spectrograph illustrating the presence of an endothermic effect consistent with partial melt recrystallisation of the fibers;

(4) FIG. 4 is a Differential Scanning Calorimetry spectrogram illustrating the absence of an endothermic effect consistent with the absence of melt recrystallisation of the fibers.

(5) In a preferred embodiment, a material sheet is prepared through the unidirectional alignment of fibers to form a monolayer. This alignment of the fibers may be achieved through various standard techniques known in the art that is able to produce substantially straight rows of unidirectional fibers, such that adjacent fibers have substantially no gap between them. Preferably, the average thickness of such a monolayer is at 1.0, more preferably at least 1.3, 1.4 or 1.5 times the thickness of the individual fiber. This arrangement ensures that there is generally at least some overlap between adjacent fibers, such that the adjacent fibers may be mechanically fused under high pressure. Preferably, the maximum thickness of the monolayers is no more than 3, more preferably no more than 2.5, even more preferably no more than 2 and most preferably no more than 1.8 times the thickness of the individual fiber. Higher monolayer thickness tends to reduce anti-ballistic performance.

(6) The monolayer may suitably be formed by feeding a polymer fiber (1) from an unwinding station (2), under tension, through an alignment means, such as plurality of spreader bars (3) and onto a receiving device (4), such as plate as shown in FIG. 1.

(7) The tension of the fibers is preferably no more than 25%, more preferably no more than 10% of the tensile strength of the fibers, as higher tension increases the risk of breaking the fibers in the spreader bars; the need for heavy duty processing equipment and a potential reduction in winding speeds. Too low a tension and the alignment of the fibers through the spreader bar and onto the receiving device is difficult to control.

(8) The plate (4) rotates about a central axis (5) such that the fibers (1) working in cooperation with the spreader bars (3) create a monolayer of unidirectional fibers surrounding the circumference of the plate. In this case care should be taken that the alignment of the fibers is such that adjacent fibers are in longitudinal contact and are abutting each other.

(9) Preferably, one layer of fibers is wound around the receiving plate, so to minimise the areal density of each monolayer.

(10) The spreader bars are adjustable such that adjacent fibers are in close enough proximity for mechanical fusion of adjacent fibers to occur under high pressure once the required numbers of monolayers have been wound onto the receiving plate. Typically, the resultant mechanically fused adjacent fibers are fused along their substantial length (i.e at least 30%, 40%, 50%, 60%, 70% relative to the total length of the adjacent fibers).

(11) The radius of the tip of the spreader bar which contacts the polymer fiber is preferably at least 1 mm, as lower radii increase the risk of fiber breakage. Furthermore the radius of the tip of the spreader bar is preferably at most 20 mm, more preferably at most 10 mm.

(12) The number of spreader bars is preferably between 6 and 20, with a good balance between speed and control achieved over this range.

(13) After the completion of the first layer, the fiber end is fixated and the receiving plate may be rotated such that the winding of the second layer is at an angle to the preceding layer. The central axis preferably includes a clamp which may be removed and placed in alignment of the central axis of the second layer. Preferably, the receiving plate is of a rectangular configuration, such that the adjacent layers may be aligned at right angles to each other. Alternative configurations may also be used, such as various polygon configurations used depending upon the desired angles between adjacent monolayers of unidirectional fibers.

(14) The process of winding further monolayers is preferably repeated until the desired number of monolayers per material sheet is achieved. Preferably, there is at least 2, 4, 6, 8 or 10 monolayers of drawn unidirectional polymer fibers per material sheet. The obtained material sheets may be stacked to form a stack comprising preferably at least 20, more preferably at least 40, even more preferably at least 60 and most preferably at least 80 stacked material sheets. The maximum number of material sheets will depend upon the ballistic threat and may suitably be determined by routine experiments. The consolidation of the stacked material sheets may be performed in an analogue fashion to the consolidation of the monolayers to form the material sheet.

(15) The increasing number of monolayers favours hard anti-ballistic applications-whereby the stack of material sheets is further consolidated into a panel by pressing at a suitable temperature and pressure, while applications requiring flexibility, so called soft ballistics as e.g. a bullet resistant vest, use generally a lower number of material sheets.

(16) In embodiments, in which the polymer of the drawn unidirectional polymer fiber is UHMWPE, the areal density of each monolayer is preferably less than 0.10 kg/m.sup.2 and more preferably less than 0.08 kg/m.sup.2, 0.06 kg/m.sup.2, 0.05 kg/m.sup.2 0.045 kg/m.sup.2, 0.04 kg/m.sup.2 or 0.035 kg/m.sup.2. The lower the areal density the greater the number of interfacial sites between adjacent layers per given areal density.

(17) The receiving plate (4) in FIG. 1 is preferably loaded on both sides with at least one monolayer and is placed in a high pressure device and subjected to pressures of at least 100 bar The applied pressure is preferable applied in a one step process, with the pressure quickly (preferably within 30 seconds, more preferably within 20, 10 or 5 seconds) ramped up the target operating pressure to avoid melt bonding. As the time lag to change the temperature of the high pressure device is relatively long compared to the time lag to adjust pressure setting, the temperature is preferably maintained within the preferred temperature range (below the melting point of the fibers) for compression under high pressure.

(18) The time for mechanical fusing is dependant upon the combination areal density of the sheet, temperature and pressure, but is typically at least 30 seconds and up to several hours. The optimum time for consolidation generally ranges from 5 to 120 minutes, depending on conditions such as temperature, pressure and part thickness and can be verified through routine experimentation. Preferably, the compression time has a lower range of at least 5, 10 or 15 minutes and an upper range of no more than 2, 1.5 or 1 hours.

(19) After the completion of the mechanical fusing cycle, the product is cooled down to below 100° C., preferably while still maintaining the operating pressure. Pressure is preferably maintained at least until the temperature is sufficiently low to prevent relaxation, i.e. preferably below 80° C. Such temperature can be established by one skilled in the art. The plate is subsequently release from the high pressure device and the two consolidated stacks on either side of the plate obtained by cutting the fibers along the peripheral edge adjoining the two receiving plate surfaces.

(20) Test methods as referred to in the present application, are as follows: Intrinsic Viscosity (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 anti-oxidant in an amount of 2 g/I solution, by extrapolating the viscosity as measured at different concentrations to zero concentration; Tensile properties (measured at 25° C.): tensile strength (or strength), tensile modulus (or modulus) and elongation at break (or eab) are defined and determined on multifilament yarns as specified in ASTM D885M, using a nominal gauge length of the fiber of 500 mm, a crosshead speed of 50%/min. On the basis of the measured stress-strain curve the modulus is determined as the gradient between 0.3 and 1% strain. For calculation of the modulus and strength, the tensile forces measured are divided by the titre, as determined by weighing 10 metres of fiber; values in GPa are calculated assuming a density of 0.97 g/cm.sup.3. Tensile properties of thin films were measured in accordance with ISO 1184(H). DSC is measured using a power-compensation PerkinElmer DSC-7 instrument which is calibrated with indium and tin with a heating rate of 10° C./min. For calibration (two point temperature calibration) of the DSC-7 instrument about 5 mg of indium and about 5 mg of tin are used, both weighed in at least two decimal places. Indium is used for both temperature and heat flow calibration; tin is used for temperature calibration only. The furnace block of the DSC-7 is cooled with water, with a temperature of 4° C. This is done to provide a constant block temperature, resulting in more stable baselines and better sample temperature stability. The temperature of the furnace block should be stable for at least one hour before the start of the first analysis. The material sheet sample is taken such that a representative cross-sectional of adjoining peripheral fiber surfaces of adjacent fibers is achieved which may suitable be seen through light microscopy. The material sheet is cut into small pieces of 5 mm maximum and a sample size of at least about 1 mg (+/−0.1 mg) is taken. The represenative sample is put into an aluminum DSC sample pan (50 μl), which is covered with an aluminum lid (round side up) and then sealed. In the sample pan (or in the lid) a small hole must be perforated to avoid pressure build-up (leading to pan deformation and therefore worse thermal contact). This sample pan is placed in a calibrated DSC-7 instrument. In the reference furnace an empty sample pan (covered with lid and sealed) is placed. The following temperature program is run: 5 min. 40° C. (stabilization period) 40 up to 200° C. with 10° C./min. (first heating curve) 5 min. 200° C. 200 down to 40° C. (cooling curve) 5 min. 40° C. 40 up to 200° C. with 10° C./min. (second heating curve) The same temperature program is run with an empty pan in the sample side of the DSC furnace (empty pan measurement). Analysis of the first heating curve is used. The empty pan measurement is subtracted from the sample curve to correct for baseline curvature. Correction of the slope of the sample curve is performed by aligning the baseline at the flat part before and after the peaks (at 60 and 190° C. for UHMWPE). The peak height is the distance from the baseline to the top of the peak. Two endothermic peaks are expected for the first heating curve, in which case the peak heights of the two peaks are measured and the ratio of the peak heights determined. For the calculation of the enthalpy of an endothermic peak transition prior to the main melting peak, it is assumed that this endothermic effect is superimposed on the main melting peak. The sigmoidal baseline is chosen to follow the curve of the main melting peak, the baseline is calculated by the PerkinElmer Pyris™ software by drawing tangents from the left and right limits of the peak transition. The calculated enthalpy is the peak area between the small endothermic peak transition and the sigmoidal baseline. To correlate the enthalpy to a weight %, a calibration curve is used.

EXAMPLES

(21) Preparation of a Multilayered Material Sheet

(22) A square aluminium receiving plate with a thickness of one cm and length and width of 41 cm was used to wind gel-spun high strength polyethylene fibers with a tenacity of 35.3 cN/dTex and a filament thickness of about 19 micron). The receiving plate was clamped in a rotating device, such that the fiber could be unwound under tension from a spool or the like. The fiber was guided over ten spreader bars and one layer was wound around the aluminium receiving plate.

(23) The pitch of the windings may be set according to the desired areal density (AD)/thickness of the fiber layers. A minimum thickness of 30 μm was selected which corresponded to about 150% of the thickness of a single fiber to ensure adjacent fibers were is intimate contact (i.e they were generally at least partially overlapping or abutting).

(24) Three types of specimens, with different AD/thickness of the individual layers were made. The areal density of the total plate was also measured. After finishing the winding of one layer, the fiber end was fixated, the aluminium plate released from the clamps, rotated, and a new layer was wound with an angle of 90 degrees to the previous layer. This procedure was repeated, until the desired number of layers was reached, and the desired areal density of the stack of layers was obtained (Table 1).

(25) The multi-layered material sheet, still bound to the receiving plate, was then inserted into a hydraulic press and subjected to 300 bar pressure at 138° C. for 1 hour, before being cooled to 80° C. under pressure. The hydraulic press was then opened and the product demoulded from the receiving plate. The fibers were cut along the edge of the aluminium plate to obtain two plates per cycle, which were trimmed to a width and length of about 40 cm and subjected to ballistic performance testing.

(26) DSC analysis on the samples in the example detected no melt recrystallised fibers.

(27) Ballistic Performance

(28) Armoured plates produced from material sheets with cross plied monolayers of varying thickness (by stacking and pressing at 140° C., 300 bar during one hour, subsequently cooled under pressure to 80° C.) were subjected to shooting tests performed with 9 mm parabellum bullets full metal jacket round nose with nominal mass of 8 g (examples 1 to 5 and comparative experiment A). The first shot was fired at a projectile speed at which it is anticipated that 50% of the shots would be stopped (V.sub.50 value). The actual bullet speed was measured at a short distance before impact. If a stop was obtained, the next shot was fired at an anticipated speed being 10% higher than the previous speed. If a perforation occurred, the next shot was fired at a speed being 10% lower than the previous speed. The result for the experimentally obtained V.sub.50 value was the average of the two highest stops and the two lowest perforations. The kinetic energy of the bullet at V.sub.50 was divided by the total areal density of the plate, thus reaching the so-called E.sub.abs value. E.sub.abs is a good performance parameter for armour plates, because it reflects the stopping power, relative to the weight/thickness of the plate.

(29) TABLE-US-00001 TABLE 1 Ballistic performance versus areal density/thickness Example/Comp. thickness areal density of Experiment individual armoured plate (AD) E.sub.abs # monolayer [μm] [kg/m.sup.2] [J/(kg/m.sup.2)] 1 30 4 Na perforation at obtainable bullet speed 2 30 3 484 3 33 3 404 including 10% of bonding agent 4 60 4 357 5 60 2.9 302 including 10% bonding agent A 100 4 258 B 260 2.9 190 including 20% bonding agent

(30) Example 5 and Comparative experiment B were performed upon plates subjected to 20 bar for 5 minutes and then 165 bar pressure at 145° C. for 60 minutes.

(31) The results indicate that by increasing the number of monolayers in the armoured plate for a given areal density (i.e decreasing thickness/areal density per monolayer), anti-ballistic performance is significantly improved. As illustrated in FIG. 2 (a graphic representation of Table 1), the increase in anti-ballistic performance accelerates when the monolayer thickness drops below 100 μm and particularly below 60 m.

(32) Retention of Mechanical Strength

(33) The retention of mechanical strength in the longitudinal direction of the fibers was evaluated by subjecting individual fibers to high pressure conditions defined according to the present invention and comparing the tenacity of the fibers against the starting material. This was achieved through sandwiching test fibers within a multilayered construction.

(34) The winding procedure, as previously described, was performed to create five monolayers from the earlier mentioned UHMWPE fiber, with a filament thickness of about 19 microns. Individual test fibers were then unilaterally aligned, in between layers of silicon paper. A further five layers of UHMWPE fiber were wound over the test fibers. The thickness of each layer was about 30 μm.

(35) The tenacity of the fibers of 35.3 cN/dTex was measured prior to the mechanical fusing process.

(36) TABLE-US-00002 TABLE 2 Tenacity (cN/dtex) of fibers versus processing conditions Example/ Comparative Pressing Step Tenacity Experiment conditions cN/dtex 6 300 Bar, 131° C.  33.2 C 10 Bar, 131° C. 31.5 D 10 Bar, 144° C. 30.6

(37) As illustrated in Table 2, lower pressure and higher temperatures (Comparative experiment D) result in largest deterioration in tenacity (or tensile strength).

(38) DSC analysis indicated that no detectable melt recrystallised polymer fibers were observed for Example 6. Comparative example C did exhibit signs of a visible endothermic effect in the DSC curve between 130° C. and 140° C. (around 131° C.) consistent with the presence of a small amount (i.e <5 wt %) of partially melt recrystallised fibers (FIG. 3). In contrast this endotherm is absent in FIG. 4 for Example 6.

(39) The results from Table 2 confirm that increasing mechanical fusing temperature decreases the tenacity of the fibers. As indicated in comparative example D, the combination of low pressure and an elevated temperature close to the melting point of the fiber results in partial melting of the fibers which contributes to the reduction in tenacity of the fibers. The application of an initial low pressure step was generally detrimental to the tenacity of the fibers. Thus, for the UHMWPE tested, mechanical fusing in a one step process using a combination of high pressure (eg. at least 100 bar) and low temperature (e.g. less than 140° C.) achieved the optimal results.