THERMOSTABLE COMPOSITE AND PREPARATION METHOD THEREOF
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application claims benefit and priority to Indian Patent Application
No. 201641029690, filed on August 31, 2016. The disclosure of that India application is incorporated herein by reference for all purposes. Provisional application
FIELD OF THE INVENTION
[0002] The disclosure relates generally to a thermostable composite material and
preparation method thereof and in particular to high performance carbon fiber reinforced thermoplastic composites.
DESCRIPTION OF THE RELATED ART
[0003] There is a huge demand for high performance polymeric composites, which
are light with improved specific properties and functionalities, in aviation and space applications. However, most of the polymeric composites employed for aviation and space applications are thermosetting polymers, which are thermally unstable. Thermosetting polymer based composites attain limited tensile strength and are not very stable at high temperatures. Therefore, there is a need for development of high temperature resistant thermoplastic polymer based composites which further are recyclable.
[0004] The US patent US6998434B2 discloses carbon fiber reinforcement and a
polymeric resin selected to form the matrix material of the composite. The European patent EP2508552A1 relates to carbon fiber reinforced composite materials wherein poly olefin resin is used. The Chinese patent CN1837285A discloses a method for interface modification of continuous fiber reinforced polyaryl ether resin-base composite materials containing diazacyclo group that includes plasma treating the reinforcement material.

[0005] The PCT patent application WO2005030836 Al discloses production of
thin walled components; or components with highly filled compositions from poly ether ketone as polymer material and granular fiber fillers. The US patent US5683757A relates to a method of modifying the surface of polymers and carbon-based materials by ion implantation and oxidative conversion techniques, which includes implantation of the substrate with energetic ions, including ions of at least one metal or semi-metal element able to form a stable, non-volatile oxide.
[0006] However, there remains a need to develop a high temperature resistant
thermoplastic polymer composite suitable for hyper velocity impact resistance with enhanced mechanical and physical properties. Therefore, attention is given to developing an excellent high temperature resistant thermoplastic polymeric composite.

SUMMARY OF THE INVENTION
[0007] The disclosure relates to thermostable composite and in particular to a
thermostable composite for high temperature applications with high impact resistance.
[0008] In various embodiments a thermostable composite is disclosed. The
thermostable composite includes a fabric layer sandwiched between a top and a bottom matrix layer. The fabric layer comprises carbon fiber of 0.2-0.5 mm thickness and the matrix layer comprises poly ether ketone (PEK) of 0.05-0.15 mm thickness. The composite is cross-linked by surface plasma treatment and electron beam irradiation, and comprises up to 80 wt.% carbon fiber.
[0009] In some embodiments a thermostable composite includes a plurality of
fabric layers. Each fabric layer comprises carbon fiber of 0.2-0.5 mm thickness and a plurality of matrix layers alternately stacked between the fabric layers, wherein each matrix layer comprises poly ether ketone (PEK) of 0.05-0.15 mm thickness. The composite comprises a top and a bottom matrix layer cross-linked by surface plasma treatment and electron beam irradiation, and up to 80 wt. % carbon fiber.
[0010] In various embodiments the composite includes 2-10 fabric layers and 3-11
matrix layers. In some embodiments the matrix layer of the composite further comprises 0 - 2 wt. % cemented carbide nanoparticles. In various embodiments the cemented carbide is selected from tungsten carbide (WC), titanium carbide (TiC), tantalum carbide (TaC), or mixtures thereof.
[0011] In some embodiments melting temperature of the composite is at least
380°C. In various embodiments glass transition temperature of the composite is at least 143°C. In some embodiments continuous operating temperature for the composite is up to 2800C. In various embodiments tensile strength of the composite is at least 1200 MPa. In some embodiments the composite withstands hyper velocity impact of up to 3400 m/sec.

[0012] In various embodiments reduction of impact velocity, energy absorption,
specific energy absorbed and absorbed velocity of the composite are in the range of 600-700 m/sec, 100-300J, 150-250 J/(g/cm2) and 650-750 m/sec, respectively.
[0013] In some embodiments a stack comprising a plurality of the composite
further includes one or more foam layers of 2-4 mm thickness. In various embodiments the composite is a thermoplastic composite.
[0014] In various embodiments a method of preparing a thermostable composite
includes the steps of providing a plurality of fabric layers and resin layers, wherein the fabric layer comprises carbon fiber and the resin layer comprises poly ether ketone (PEK). Arranging the fabric layers and the resin layers alternately, thereby forming a stack with a top and bottom resin layer. Curing the stacked material in an autoclave to obtain a composite. Modifying the surface of the composite by plasma treatment and electron beam irradiation, thereby enhancing cross-linking between the resin layer and fabric layer.
[0015] In some embodiments the resin layer further comprises a cemented carbide
of 0-2 wt. % added to the resin as a nanoparticle dispersion. In various embodiments the cemented carbide is selected from a group consisting of tungsten carbide (WC), titanium carbide (TiC), tantalum carbide (TaC), or mixtures thereof.
[0016] In various embodiments the plasma treatment induces activation sites in the
composite. In some embodiments plasma treatment increases tensile strength of the composite. In various embodiments at least 500 to 2000 nm of the surface is modified. In some embodiments absorbed dose from electron beam irradiation by the composite is 280-320 kGy. In various embodiments depth to which the composite is irradiated is at least 10 mm.

BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The invention has other advantages and features which will be more readily
apparent from the following detailed description of the invention and the appended claims, when taken in conjunction with the accompanying drawings, in which:
[0018] FIG. 1 A. illustrates thermostable composite showing surface modified layer
[0019] FIG. IB depicts a thermostable composite showing layered structure.
[0020] FIG. 1C shows a pair of thermostable composite stack sandwiching a foam
layer.
[0021] FIG. 2 depicts a method of fabricating a thermostable composite.
[0022] FIG. 3 depicts the thermo-stability of PEK-Carbon fabric and epoxy carbon
fabric composite after high temperature autoclave processing.
[0023] FIG. 4 depicts the Influence of Tungsten Carbide Nano Powder on
mechanical properties of PEK.
[0024] FIG. 5 depicts the tensile strength of PEK (20%) -carbon fiber (80 %)
composite and 2% tungsten carbide dispersed PEK (20%)-carbon fiber (80%) composite.
[0025] FIG. 6 depicts comparison of tensile strength of PEK (20%) - carbon fiber
(80%) composite, 2% tungsten carbide dispersed PEK (20%) - carbon fiber (80%) composite, aluminium and steel.
[0026] FIG. 7 depicts specific strength of PEK (20%) - Carbon Fiber (80%)
composite, 2% Tungsten Carbide dispersed PEK (20%) - Carbon Fiber (80%) composite, aluminium and steel

[0027] FIG. 8 depicts Rockwell hardness C (HRC) of PEK (20%) - Carbon fiber
(80%) composite, 2% Tungsten Carbide dispersed PEK (20%) - Carbon fiber (80%) composite, aluminium and steel.
[0028] FIG. 9A depicts hyper velocity impact on PEK-Carbon fabric composite
before impact
[0029] FIG. 9B depicts hyper velocity impact on PEK-Carbon fabric composite
entry side
[0030] FIG. 9C depicts hyper velocity impact on PEK-Carbon fabric composite
exit side.

DETAILED DESCRIPTION
[0032] While the invention has been disclosed with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from its scope.
[0033] Throughout the specification and claims, the following terms take the
meanings explicitly associated herein unless the context clearly dictates otherwise. The meaning of "a", "an", and "the" include plural references. The meaning of "in" includes "in" and "on." Referring to the drawings, like numbers indicate like parts throughout the views. Additionally, a reference to the singular includes a reference to the plural unless otherwise stated or inconsistent with the disclosure herein.
[0034] The invention in its various embodiments discloses a thermostable
composite suitable for hyper velocity impact resistance applications that includes a carbon fiber fabric material reinforcement with poly ether ketone (PEK) polymeric resin matrix. Further a method of fabrication of the thermostable composite is disclosed. The disclosed thermostable composites are suitable for various aerospace applications yield up to 70% weight reduction, lower density and higher specific strength over composites of metal and metal alloys.
[0035] In various embodiments, provided herein is a thermostable composite 100
that includes a carbon fiber fabric layer 101 sandwiched between top and bottom matrix layers of poly ether ketone (PEK) 103 as illustrated in FIG. 1A.
[0036] In some embodiments, the thermostable composite 100 includes a plurality
of fabric layers 101 stacked with a plurality of matrix layers 103 in an alternating manner, with a top and a bottom matrix layer 103, as shown in as shown in FIG. IB. In some embodiments, the thermostable composite 100 includes 2-10 fabric layers 101 and

3-11 matrix layers 103. In one embodiment, the composite consists of 2 fabric layers and 3 matrix layers. In another embodiment, the composite consists of 3 fabric layers and 4 matrix layers.
[0037] In some embodiments, the thickness of the fabric layer 101 is 0.2-0.5 mm.
In some embodiments, the thickness of the matrix layer 103 is 0.05-0.15 mm. In some embodiments, the carbon fiber is 50-80% by weight of the thermostable composite 100. In other embodiments, the PEK is 15-50% by weight of the thermostable composite 100. In one embodiment the ratio of carbon fiber to PEK is 80:20.
[0038] In some embodiments, the matrix layer 103 of the composite material 100
further includes a cemented carbide 105 as shown in FIG. 1A. The composite 100 may include 0-2 wt. % of the cemented carbide 105. The cemented carbide 105 is selected from tungsten carbide, titanium carbide, tantalum carbide or any other hard carbides, or mixtures thereof. In various embodiments, the cemented carbide 105 strengthens the composite 100 by providing friction against movement of the PEK polymer fibers during deformation.
[0039] In various embodiments, the thermostable composite 100 includes a matrix-
impregnated fabric layer 107 with enhanced surface activity and crosslinking, as shown in FIG. 1A. In some embodiments, the matrix-impregnated fabric layer 107 is obtained by surface modification to further crosslink the composite 100. In some embodiments, the surface modification includes atmospheric pressure plasma treatment and electron beam irradiation. In some embodiments, the surface modified composite 100 is characterized by surface energy and X-ray Photo Electron Spectroscopy (XPS). In some embodiments, the composite 100 demonstrate about 8 times the shear strength as compared to the corresponding unmodified composite of carbon fiber and PEK in lap shear test. In some embodiments, the surface modified region 109 is at least 0.5-1 um, as shown in FIG. 1A and IB.

[0040] In some embodiments melting temperature of the thermostable composite
100 is at least 380 C. In some embodiments the glass transition temperature is at least 143 C. In various embodiments the composite 100 shows thermal stability at 280 C for continuous operating temperature and a thermal stability up to 570°C for shorter durations, as shown in FIG. 3.
[0041] In various embodiments density of the composite 100 is 1.5 gm/cm3. Said
density is 1/2- l/5th of the density of steel and aluminium.
[0042] In various embodiments tensile strength of the composite 100 is in the
range of 1200 MPa - 1400 MPa. The tensile strength of the composite 100 is at least 5 times greater than aluminium and is at least 10 times greater than steel.
[0043] In various embodiments a hyper impact velocity resistant thermostable
composite 110 is formed by stacking at least two thermostable composite 100 with one or more foam layers 113 sandwiched there between, as shown in FIG. 1C. In some embodiments, the foam layer 113 is made of silicone foam. In some embodiments, the composite 110 includes up to 60 fabric 101 and matrix layers 103 and a silicone foam layer 113 for developing a bullet proof vest. In other embodiments, up to 60 fabric and matrix layers and a silicone foam.
[0044] In various embodiments the hyper velocity impact resistance of the
composite 100 is observed at a velocity in the range of 1300-1500 m/sec. Reduction in impact velocity is in the range of 600-700 m/sec. Energy absorption of impact energy is in the range of 100-300J. Specific energy absorbed is in the range of 150 -250 J/(g/cm2) and absorbed velocity is in the range of 650- 750 m/sec. In various embodiments the composite 100 is resistant to an impact with velocity of up to 3400 m/s.
[0045] In various embodiments, a process 200 for the fabrication of a thermostable
composite is disclosed herein, as illustrated in FIG. 2. In step 201, a plurality of carbon

fiber fabric layers and poly ether ketone (PEK) resin layers are provided. In step 203, the plurality of fabric layers and resin layers are arranged in an alternate manner to form a stack. In step 205, the stacked material is cured at a temperature of 400 °C and under a pressure of 8 bar to obtain a composite. In step 207, the surface of the composite is modified by plasma treatment. In one embodiment, atmospheric pressure plasma treatment is used. In step 209, the composite is irradiated using electron beam to obtain a thermostable composite.
[0046] In some embodiments, a nanoparticle dispersion of cemented carbide is
added to obtain the PEK resin layer. The dispersion may include cemented carbide in the range of 0- 2 wt % to the resin. The cemented carbide is selected from a group of tungsten carbide, titanium carbide, tantalum carbide or any other hard carbides or mixture thereof. In some embodiments the dispersion used is nanoparticle tungsten carbide (WC) dispersion.
[0047] In various embodiments, the composite is surface modified by atmospheric
pressure plasma treatment. The plasma treatment induces activation sites on the surface of the fabric layer 101 and the matrix layer 103, thereby enhancing the surface energy of the composite 100. The depth to which the composite 100 may be modified is between 500 - 2000 nm. Also, the plasma treatment results in better impregnation of the PEK matrix 103 within the fabric material 101. The surface modified region 109 includes a matrix-impregnated fabric layer 107 with enhanced surface activity and crosslinking. In some embodiments, the composite 100 shows increased mechanical properties.
[0048] In various embodiments the composite 100 is irradiated. Irradiation is
carried out with concentrated, highly charged stream of electrons. The electrons alter various chemical and molecular bonds present within the polymer matrix. Further depending on the absorbed dose rate a radiation induced cross linking in polymer matrix chain is induced thereby resulting in significant improvement of thermo-mechanical

properties. The composite 100 is irradiated in the range of 280-320 kGy. The depth to which the composite 100 is irradiated is at least 10 mm.
[0049] In various embodiments the composite 100 shows low smoke and toxic gas
emission. Inherent flame retardancy and the very low toxicity of the resulting combustion gases during a fire accident, makes composite 100 an ideal candidate in the aerospace industry.
[0050] While the invention has been disclosed with reference to certain
embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material the teachings of the invention without departing from its scope. Further, the examples to follow are not to be construed as limiting the scope of the invention which will be as delineated in the claims appended hereto.
EXAMPLES
[0051] Example 1 - Characterization of thermostable composite
[0052] High temperature thermoplastic - carbon fiber composite was made (i) first
sample: 55% carbon fiber with 45% PEK, (ii) second sample: 80% Carbon Fiber with 20%) PEK. Surface modification of the composite was carried out by atmospheric pressure plasma treatment in order to impart activation sites over the surface of the polymer matrix and the fabric. The composite is then irradiated to 300 kGy by electron beam irradiation. The thermal stability of the composites was found to be up to 570 deg Celsius (for short duration) and up to 280 deg Celsius (for long duration).The composite also demonstrated shore hardness: 96, density: 1.5 g/cm3 (l/5th of steel and 1/2 of aluminium) and was therefore, higher in specific strength than steel and aluminium. It was also tested under aggressive chemical (acidic pH 1 and alkaline pH 13) and high

energy radiation (Gamma radiation 5000 kGy) and there was no deterioration of mechanical properties. The impregnation of 80% carbon fiber (weight ratio) with 20% PEK (weight ratio) demonstrated the highest impregnation achieved and is highly promising for various strategic applications.
[0053] The composite was characterized by surface energy and X-ray photo
electron spectroscopy (XPS). The cross linking was characterized by Fourier transform infrared spectroscopy (FTIR) and subsequent test of mechanical properties under tensile test. FIG. 3 demonstrates result of thermo gravimetric analysis (TGA) of PEK-CF composite to determine the stability over a short duration. Lap shear tests conducted for the surface modified composite demonstrated 8 times the shear strength as compared to the corresponding unmodified composite.
[0054] Example 2 - Analysis of the physical properties of dispersion strengthened thermostable composite
[0055] Rockwell Hardness C (HRC) for the exemplary second sample PEK-
Carbon fiber fabric composite was 67 as against that of mild steel (< 55) and hardened steel (55 to 65). HRC was carried out according to ASTM standard ASTM E-18. Dispersion of tungsten carbide nano powder (2% wt ratio) in PEK resin followed by PEK film impregnated with carbon fiber fabric with 80% weight ratio, lead to further improvement of Rockwell Hardness C (HRC) of PEK-Carbon fiber fabric composite to 69.
[0056] Additionally, toughness of PEK-tungsten carbide nano powder film and
toughness of this material essentially absorbs the energy. A comparative study on influence of dispersion of tungsten carbide nano powder in PEK on toughness is shown in FIG. 4.

[0057] The tensile strength of 20% PEK and 80 % Carbon Fiber is 1200 MPa,
whereas tensile strength of 2% tungsten carbide nano powder dispersed PEK (20%)-Carbon fiber (80%) composite is 1400 MPa shown in FIG. 5. 61 layers of resin and 60 layers of carbon fibers were impregnated. Consequently tensile strength is higher than aluminium and steel as shown in FIG. 6 and specific strength is significantly higher than aluminium and steel shown in FIG. 7. Rockwell Hardness C (HRC) also shows higher than aluminium and steel shown in FIG. 8.
[0058] Example 3 Analysis of thermostable composite as a hypervelocity impact resistant material
[0059] A light gas gun has been used to accelerate a projectile to hypervelocity
speed. A 5.56cp mm aluminium sphere made with A12017-T4 was used as a projectile. The projectile is fired using a two stage firing system. The magnetic sensor and a laser sensor with an intervalometer to sense the velocity of the projectile using time difference for a known distance before and after the impact with the laminate. The maximum speed attained was up to 3400 m/s.
[0060] Hyper velocity impact on high temperature thermostable composite of 5
layers (3 layers of PEI with 2 layers of Carbon Fabric) was carried out with a velocity of 1380 m/sec. It is to be noted that velocity of projectile after impact was reduced to 680 m/sec with significant energy absorption up to 200 J and specific energy absorbed was 199.59 J/(g/cm2). Due to significant absorption of impact energy, the absorbed velocity was 700 m/sec. Further, there was no delamination from polymer matrix to carbon fabric and insignificant cracks on polymer matrix as shown in FIGs. 9A, 9B, 9C.
[0061] This composite material is at par with the material operable at high
temperatures and resistant to high impact velocity. Therefore, finds a potential to be used as composite where high impact resistance and temperature resistance is required.

We claim:
1. A thermostable composite comprising: a fabric layer sandwiched between a top
and a bottom matrix layer,
wherein the fabric layer comprises carbon fiber of 0.2-0.5 mm thickness and the matrix layer comprises poly ether ketone (PEK) of 0.05-0.15 mm thickness,
wherein the composite is cross-linked by surface plasma treatment and electron beam irradiation, and comprises up to 80 wt.% carbon fiber
2. A thermostable composite comprising:
a plurality of fabric layers, wherein each fabric layer comprises carbon fiber of 0.2-0.5 mm thickness; and
a plurality of matrix layers alternately stacked between the fabric layers, wherein each matrix layer comprises poly ether ketone (PEK) of 0.05-0.15 mm thickness,
wherein the composite comprises a top and a bottom matrix layer cross-linked by surface plasma treatment and electron beam irradiation, and up to 80 wt.% carbon fiber.
3. The composite of claim 2, comprising 2-10 fabric layers and 3-11 matrix layers.
4. The composite of claim 1 or 2, wherein the matrix layer further comprises 0-2 wt.% cemented carbide nanoparticles.
5. The composite of claim 4, wherein the cemented carbide is selected from tungsten carbide (WC), titanium carbide (TiC), tantalum carbide (TaC), or mixtures thereof.
6. The composite of claim 1 or 2, wherein melting temperature of the composite is atleast380°C.

7. The composite of claim 1 or 2, wherein glass transition temperature of the composite is at least 143°C.
8. The composite of claim 1 or 2, wherein continuous operating temperature for the composite is up to 280°C.
9. The composite of claim 1 or 2, wherein tensile strength of the composite is at least 1200 MPa.
10. The composite of claim 1, wherein the composite withstands hyper velocity impact of up to 3400 m/sec.
11. The composite of claim 9, wherein reduction of impact velocity, energy absorption, specific energy absorbed and absorbed velocity are in the range of 600-700 m/sec, 100-300J, 150-250 J/(g/cm2) and 650-750 m/sec, respectively.
12. A stack comprising a plurality of the composite of claim 1 or 2, further comprising one or more foam layers of 2-4 mm thickness.
13. The composite of claim 1 or 2, wherein the composite is a thermoplastic composite.
14. A method of preparing a thermostable composite comprising the steps of:

a) providing a plurality of fabric layers and resin layers, wherein the fabric layer comprises carbon fiber and the resin layer comprises poly ether ketone (PEK);
b) arranging the fabric layers and the resin layers alternately, thereby forming a stack with a top and bottom resin layer;

c) curing the stacked material in an autoclave to obtain a composite;
d) modifying the surface of the composite by plasma treatment and electron beam irradiation, thereby enhancing cross-linking between the resin layer and fabric layer.

14. The method of claim 13, wherein the resin layer further comprises a cemented carbide of 0-2 wt.% added to the resin as a nanoparticle dispersion.
15. The method of claim 14, wherein the cemented carbide is selected from a group consisting of tungsten carbide (WC), titanium carbide (TiC), tantalum carbide (TaC), or mixtures thereof.
16. The method of claim 13, wherein plasma treatment induces activation sites in the composite.
17. The method of claim 13, wherein plasma treatment increases tensile strength of the composite.
18. The method of claim 13, wherein at least 500 to 2000 nm of the surface is modified.
19. The method of claim 13, wherein the absorbed dose from electron beam irradiation is 280-320 kGy.
20. The method of claim 19, wherein depth to which the composite is irradiated is at least 10 mm.