COMPOSITE MATERIAL

Abstract

A composite material comprising an elastomer having ceramic platelets dispersed therein, and applications thereof including an armour system. The ceramic platelets each have a first plate surface and a second plate surface, the first and second plate surfaces being separated by a height H. The ceramic platelets each having a maximum diameter D measured in the first and second plate surfaces. The ceramic platelets have a mean height H.sub.m and a mean maximum diameter D.sub.m. The mean height H.sub.m is 0.1 to 1 μm and the ratio D.sub.m:Hm is 20 or more.

Claims

1. A composition comprising an elastomer having ceramic platelets dispersed therein, the ceramic platelets each having a first plate surface and a second plate surface, the first and second plate surfaces being separated by a height H; the ceramic platelets each having a maximum diameter D measured in the first and second plate surfaces; and the ceramic platelets having a mean height Hm and a mean maximum diameter Dm; wherein Hm is 0.1 to 1 μm and the ratio Dm:Hm is 20 or more.

2. The composition of claim 1, wherein the ratio Dm:Hm is 30 or more.

3. The composition of claim 1, wherein the mean height Hm of the ceramic platelets is 300 nm or more and/or the mean maximum diameter Dm is 10 μm or more.

4. The composition of claim 1, wherein the elastomer comprises butyl rubber, polyisobutylene (PIB), natural rubber (polyisoprene), polybutadiene rubber (BR), styrene-butadiene rubber (SBR), nitrile rubber (NBR), chloroprene rubber (CR), ethylene propylene rubber (EPM), ethylene propylene diene rubber (EPDM), and/or silicone.

5. The composition of claim 1, wherein the elastomer comprises butyl rubber, polyisobutylene (PIB), natural rubber (polyisoprene), polybutadiene rubber (BR), styrene-butadiene rubber (SBR), nitrile rubber (NBR), chloroprene rubber (CR), ethylene propylene rubber (EPM), and/or ethylene propylene diene rubber (EPDM).

6. The composition of claim 1, wherein the elastomer comprises one or more saturated rubbers, optionally blended with one or more unsaturated rubbers.

7. The composition of claim 1, wherein the elastomer comprises butyl rubber, optionally blended with polyisobutylene.

8. The composition of claim 1, wherein the ceramic platelets are metal oxide, metal nitride and/or a metal carbide platelets.

9. The composition of claim 1, wherein the ceramic platelets are alumina platelets.

10. The composition of claim 1, wherein the ceramic platelets are silane treated ceramic platelets.

11. The composition of claim 1, consisting of the elastomer having the ceramic platelets dispersed therein.

12. The composition of claim 1, wherein the ceramic platelets constitute no more than 50 vol % of the composition.

13. The composition of claim 12, wherein the ceramic platelets constitute 20 to 30 vol % of the composition.

14. A method for the preparation of the composition of claim 1, the method comprising dispersing ceramic platelets in an elastomer, the ceramic platelets each having a first plate surface and a second plate surface, the first and second plate surfaces being separated by a height H; the ceramic platelets each having a maximum diameter D measured in the first and second plate surfaces; and the ceramic platelets having a mean height Hm and mean maximum diameter Dm; wherein Hm is 0.1 to 1 μm and the ratio of Dm:Hm is 20 or more.

15. The method of claim 14, which is a melt mixing method.

16. The method of claim 14, additionally comprising hot-pressing, extruding or bi-axial stretching to increase alignment of the platelets within the elastomer.

17. (canceled)

18. An armour system comprising a rigid substrate and the composition of claim 1.

19. (canceled)

20. The composition of claim 1, wherein the ratio D.sub.m:H.sub.m is 30 to 50.

Description

[0073] The invention is further described, in a non-limiting manner, with reference to the following figures:

[0074] FIG. 1 is a schematic diagram (of a ceramic platelet for use in embodiments of the invention;

[0075] FIG. 2 shows SEM images of Alusion® alumina sub-micro platelets (FIG. 2A, scale 20 μm) and PWA 20 micro platelets (FIG. 2B, scale 10 m);

[0076] FIG. 3 is a schematic diagram demonstrating hot-pressing;

[0077] FIG. 4 shows an FIB section of Ex. 7 with a scale of 10 m;

[0078] FIG. 5 is graph showing true stress for a composition in accordance with the invention (upper line) and comparative examples (lower lines);

[0079] FIG. 6 shows graphs of G′ (storage modulus), G″ (loss modulus) and tan δ (phase lag between stress and strain), all measured at 1 Hz;

[0080] FIG. 7 shows graphs of true stress and strain energy;

[0081] FIG. 8 shows the results of ballistics tests;

[0082] FIG. 9 shows schematic diagrams (not to scale) of armour systems in accordance with embodiments of the invention; and

[0083] FIG. 10 shows schematic diagrams (not to scale) of vibration mounts in accordance with embodiments of the invention.

[0084] FIG. 1 is a schematic diagram (not to scale) of a ceramic platelet 10, shown from above (upper image) and a perspective view from the side (lower image). The platelet 10 has a first (upper) plate surface 12 and a second (lower) plate surface 14, the plate surfaces 12, 14 being separated by a height H. The first plate surface 12 has a maximum diameter D. The second plate surface 14 is identical to the first surface so the maximum diameter of the second plate surface 14 is equal to the maximum diameter of the first plate surface 12. If the plate surfaces were different, then the longer maximum diameter would be considered.

[0085] Each of the ceramic platelets within the elastomer will have a maximum diameter D and a height H, and there is likely to be dispersion. However a mean diameter D.sub.m and mean height H.sub.m can be determined. The ceramic platelets of the invention have a ratio of D.sub.m to H.sub.m of at least 20.

[0086] The platelet 10 is shown as having an oval cross-section for simplicity, but the cross-section is likely to be irregular in practice. For example, platelets may be prepared from a single crystal having a uniform height which breaks to provide platelets of various sizes, but identical heights.

Summary of Examples

[0087]

TABLE-US-00001 Vol Example Manufacture Elastomer Ceramic particle % Comp. Melt mix PIB Alumina powder 30 Ex. 1 Comp. Melt mix PIB PWA 20 micro 30 Ex. 2 platelets Ex. 1 Melt mix PIB Alusion ® sub-micro 30 platelets Ex. 2 Melt mix, rubber Butyl Alusion ® sub-micro 30 compounding rubber platelets without sulphur Ex. 3 Melt mix, rubber Butyl Alusion ® sub-micro 30 compounding rubber platelets with sulphur Ex. 4 Solvent cast PIB Alusion ® sub-micro 10 platelets Ex. 5 Solvent cast PIB Alusion ® sub-micro 20 platelets Ex. 6 Solvent cast PIB Alusion ® sub-micro 30 platelets Ex. 7 Solvent cast PIB Alusion ® sub-micro 40 platelets Ex. 8 Solvent cast PIB Alusion ® sub-micro 50 platelets

[0088] Materials and Methods

[0089] Composites were manufactured with a matrix polymer of polyisobutylene (Oppanol® N80, BASF, UK) MW 1,100,000, or butyl rubber (Butyl 402, LANXESS) with an unsaturation of 2.25% mol and a Mooney viscosity of 33 MU. The platelet dimensions were determined from SEM imaging (FIG. 2) and are consistent with the literature values shown in the table below.

TABLE-US-00002 Mean maximum Mean diameter height Ceramic particle D.sub.m (μm) H.sub.m (μm) D.sub.m:H.sub.m Alumina powder 10 10 1 (Sigma Aldrich, 99.9% pure) PWA 20 micro platelets 19.52 (SD 4.98) 3.12 (0.79) 6.3 (Fujimi Corporation) Alusion ® sub-micro 12.66 (SD 2.28) 0.36 (SD 0.08) 35.2 platelets (Antaria Ltd, Australia)

[0090] Melt Mixing

[0091] Composites with PIB matrices were melt mixed using a HAAKE® Rheomix600 mixing rheometer with Banbury rotors, resulting in a chamber volume of 78 cm.sup.3. For all compounds a chamber fill ratio of 0.7 was applied. If a surface modifier (e.g. silane treatment) is desired, it can be applied to platelets through a pre-treatment process.

[0092] PIB composites were melt mixed at 140° C. at 50 rpm for 10 minutes, reaching a maximum of no more than 175° C. to avoid degradation. The resulting mixture was air cooled before being hot pressed (160° C. for 4 minutes with a force of 250 kN). This was repeated once more to make a 1 mm thick sheet for DMA samples, remaining material was then repressed into a 4 mm thick sheet for SHPB samples. DMA strips of 12×30 mm were punched from the 1 mm sheet. Cylindrical SHPB sample discs of 8 mm diameter were cut from the 4 mm thick sheet using a biopsy punch.

TABLE-US-00003 Start End thickness thickness Temperature Load Pre-heat Press Cool Stage Presses (mm) (mm) (° C.) (kN) (minutes) (minutes) (minutes) 1 2 4 1 160 250 2 2 1 2 1 6 2 160 250 2 2 1 3 1 8 4 160 250 4 2.5 2

[0093] Hot pressing was used as a method of increasing alignment. To explore the influence of alignment, samples were taken after one pressing (P1) and three pressings (P3), as illustrated in FIG. 3. The melt mixed composites exhibited a significant increase in both modulus (330 to 390 MPa) and strength (4.7 to 5.3 MPa) with repeated pressings (P1 to P3). This could be a result of increasing dispersion or alignment with repeat pressings.

[0094] A basic curing system was employed for the butyl rubber examples, using a sulphur crosslinking mechanism with TMTD (tetramethylthiuram disulphide), as an accelerator. Zinc oxide was also added to the compound as an activator. The compounding formulation used in the manufacture of the composites is shown below:

TABLE-US-00004 Parts per hundred Weight percent Volume fraction (pph) (% wt) (V) Sulphur 2 0.70 0.006 Zinc oxide 3 1.05 0.004 TMTD 5 2.09 0.026 Butyl rubber 100 34.84 0.675 Alumina 177 61.67 0.289

[0095] Butyl rubber composites were compounded in two stages, a high temperature melt mixing of the platelets, and a reduced temperature mixing of curing agents. Both stages were performed with the use of a HAAKE® Rheomix600 with Banbury rotors, resulting in a chamber volume of 78 cm.sup.3. For all compounds a chamber fill ratio of 0.7 was applied.

[0096] Solvent-Gel Casting

[0097] A solvent cast and hot-pressing method was developed based on Bonderer et al (J. Mater. Res. 24, 2741-2754 (2009). A suspension of alumina platelets in toluene was created and PIB was added to achieve the desired polymer solution.

[0098] The resulting polymer-ceramic-solvent mixtures were cast into large thin films, ≈500 m thick when dry, and dried at 60° C. for 24 hours. The dry films were cut into 20×20 mm sheets, stacked and hot pressed at 160° C. for 4 minutes with a force of 250 kN, then rapidly cooled, this was repeated once more to make a 1 mm thick sheet for DMA samples, remaining material was then repressed into a 4 mm thick sheet for SHPB samples. DMA strips of 12×24 mm were punched from the 1 mm sheet. Cylindrical SHPB sample discs of 8 mm diameter were cut from the 4 mm thick sheet using a biopsy punch (see table above for hot-press parameters).

[0099] Results

[0100] Structure

[0101] A combination of cryofracture face and FIB section views were used to characterise the structure. Imaging cryofractures allowed the alignment of platelets in the composites to be measured and gave an indication of dispersion. FIB (focussed ion beam) sections allowed dispersion to be studied in more accuracy but alignment cannot be assessed due to the small section area.

[0102] Both cryofractures and FIB sections showed good platelet dispersion over all contents. Within the small area of FIB sections platelets appeared evenly dispersed. The majority of platelets are singularly dispersed and even at 0.4 V.sub.p (40 vol %) when platelets are forced into close proximity, a layer of elastomer is present between adjacent platelets. For example, FIG. 4 shows a gallium ion FIB section of Ex. 7, which is a PIB composite comprising 40 vol % sub-micro alumina platelets.

[0103] Dynamic Materials Analysis (DMA) and Split Hopkinson Pressure Bar (SHPB)

[0104] DMA was used to study viscoelastic properties across the Tg, and SHPB tests were employed to determine high strain rate and high deformation properties.

[0105] DMA was performed using the TA Instruments Q800 with a single cantilever configuration. Tests were performed over a temperature range of −115 to 30° C. at a ramp rate of 3° min.sup.−1, in order to traverse the Tg. All samples were analysed at frequencies of 1, 10 and 100 Hz.

[0106] High strain rate, high deformation, compression properties were determined using a SHPB. Aluminium alloy 6082 T6 bars with a diameter of 12.7 mm were used. The system uses a rapid release of a vacuum chamber to accelerate a striker bar within a sabot at speeds of up to 20 ms.sup.−1. Stress waves within the bars were measured using pairs of strain gauges with a resistance of 120Ω.

[0107] FIG. 5 demonstrates that the type of ceramic platelet affects the properties of the resulting composite: Comp Ex 0.1 employs alumina powder (D.sub.m:T.sub.m=1), Comp Ex. 2 employs alumina micro platelets (D.sub.m:T.sub.m=6.3) and Ex. 1 employs sub-micro alumina platelets (D.sub.m:T.sub.m=35.2).

[0108] The modulus and strength properties are shown in the table below.

TABLE-US-00005 SHPB 20° C. SHPB −35° C. Modulus UCS Modulus UCS (MPa) (MPa) (GPa) (MPa) Ex. 1 394 53.3 1.7 112 Comp Ex. 1 148 21.0 1.2 74 Comp. Ex. 2 198 24.2 — —

[0109] Both ambient and low temperature SHPB compressive tests show a large reduction in both modulus and strength compared to that of the sub-micro platelets. Comp. Ex. 1 (alumina powder) exhibiting a 61% and 34% reduction in strength at 20° C. and −35° C. respectively when compared to Ex. 1 (sub-micro platelets).

[0110] It is clear from this that the increase in modulus, strength and strain energy observed from the addition of sub-micro alumina platelets are far greater than would normally be achieved by simple particle reinforcement. This being direct evidence that the size and aspect ratio of these platelets is highly beneficial in reinforcement both above and below the T.sub.g.

[0111] FIGS. 6A, 6B and 6C show that the use of ceramic platelets in accordance with the invention provided huge increases in storage and loss modulus and a broadening of tan δ but a reduction in peak height. The reinforcing effect of the platelets changes with strain rate and proportion of ceramic platelets.

[0112] When considering platelet volume, a steady increase in G′ was observed with increasing V.sub.p across the full temperature range. There was a noticeable increase in G′ from 0.3 to 0.4 V.sub.p above the Tg. G″ peaks increased proportionally, up to 0.4 V.sub.p. At 0.5 V.sub.p the composites exhibited a G″ peak lower than the 0.4 V.sub.p and similar to the 0.3 V.sub.p. There were also large increases in G″ both above and below the Tg with increasing platelet content up to a content of 0.4 V.sub.p.

[0113] FIG. 7 shows the SHPB (Split Hopkinson Pressure Bar) properties of composites depending on the vol % of ceramic platelets (examples 4 to 8). 40 vol % showed early failure with reduced strain energy absorption. 30 vol % showed the same ultimate strength but increased toughness with only 15% plastic strain on release. Compression was determined at −35° C. to simulate ballistic performance.

[0114] Ballistic Performance

[0115] A 5 mm diameter 1.1 g steel fragment was employed to simulate a projectile, at a projectile velocity of 320 ms.sup.−1 (716 miles per hour). The composite was applied as a 2 mm layer to a 5 mm Armox® 440 steel plate. There was a visible reduction in dent severity when 30 vol % sub-micro alumina platelets were employed, as shown in FIG. 8A, and illustrated in the bar chart FIG. 8B.

[0116] Damping or relaxation mechanisms are thought to have a significant contribution to strike face ballistic performance. As such, how the relaxation mechanisms are affected by the addition of platelets could therefore be critical in determining the ballistic performance of the composite. The relaxation mechanisms were investigated across the Tg using DMA, with detailed tan δ peaks being obtained at 1 Hz.

[0117] It was observed that the peak form changed significantly with the introduction of alumina platelets. PIB has two identifiable relaxation peaks within the tan δ T.sub.g peak. These peaks being identified as P1 and P2, corresponding to the sub-Rouse and Rouse relaxation modes respectively.

[0118] Armour System

[0119] The composition of the invention is particularly useful for ballistics applications, such as armour systems.

[0120] FIG. 9A shows a cross-section of a blast panel 20 in accordance with an embodiment of the invention. The blast panel 20 contains a layer of composite material 22, which is sandwiched between two rigid plates 24 (e.g. steel or aluminium alloy). It can be observed that composite material 22 contains aligned ceramic platelets, the alignment being in parallel with the rigid plates 24.

[0121] FIG. 9B shows a cross-section of an armour system 30 for protection against small arms. The armour system 30 has an environmental cover 32 on its front face and the composite material 22 is located as a layer between the environmental cover and a rigid backing 34 (e.g. steel, aluminium or a another ballistic resistant composite, such as Kevlar®). A spall liner 36 can be applied to the opposite side of the rigid backing 34.

[0122] FIG. 9C shows an armour system 40 with ceramic tiles for protection against armour piercing rounds. The armour system is similar to that shown in FIG. 9B except that there is an additional layer of ceramic tiles 42 located between the layer of composite material 22 and the rigid backing 34.

[0123] Vibration Damping

[0124] FIG. 10 shows anti-vibration mounting systems comprising a composition in accordance with embodiments of the invention.

[0125] FIGS. 10A and 10B show vibration mounts 50 and 60, each of which comprise the composition 22 in accordance with an embodiment of the invention. The composition is “sandwiched” between two rigid (e.g. steel) discs 52. In system 50 the ceramic platelets are aligned parallel to the discs 52 whereas in system 60 the ceramic platelets are aligned perpendicular to the discs 52.

[0126] FIG. 10C shows a cylindrical anti-vibration mount 70 comprising a composition 72 in accordance with an embodiment of the invention. The composition 72 is formed into a tubular shape “sandwiched” between an outer rigid collar 74 and an inner cylindrical core 76. The ceramic platelets within the composition are aligned parallel with the rigid outer collar 76.