WO2001028768A1

WO2001028768A1 - Silica fiber/carbon matrix composites for semi-structural applications

Info

Publication number: WO2001028768A1
Application number: PCT/US2000/026904
Authority: WO
Inventors: Doug Wilson
Original assignee: Hitco Carbon Composites, Inc.
Priority date: 1999-10-15
Filing date: 2000-09-29
Publication date: 2001-04-26
Also published as: AU7738800A

Abstract

A semi-structural composite comprising a carbon matrix reinforced with a plurality of substantially pure silica fibers, wherein said part is substantially carbonized, and wherein said part has a flexural strength of at least about 5 Ksi, a cross-ply strength of at least about 0.1 Ksi, a beam shear strength of at least about 0.5 Ksi, and a Izod impact strength of at least about 1 ft-lb./in. A process for the production of semi-structural chemical process apparatus parts comprising the steps of providing a plurality of substantially pure microporous silica fibers; impregnating said plurality of substantially pure microporous silica fibers with a carbon matrix precursor to form a prepreg; laying-up and curing said prepreg to form a part; and carbonizing said part in an inert atmosphere is also described.

Description

SILICA FIBER/CARBON MATRIX COMPOSITES FOR SEMI-STRUCTURAL APPLICATIONS

TECHNICAL FIELD OF THE INVENTION

The present invention is directed to fiber reinforced matrix composites, and more particularly directed to silica fiber-reinforced carbon matrix composites and articles manufactured from the composites. The composites of the present invention are particularly useful in the manufacture of "lightly-loaded" apparatus and reactor parts for use in industrial applications, such as in the chemical process industry, and in energy storage applications.

BACKGROUND OF THE INVENTION

There are many industrial applications that require the use of high temperature resistant and corrosion resistant material components and parts. For example, the chemical process industry requires high temperature and corrosion resistant components, such as pump bodies, heat exchanger tubes, pipes, flanges and the like. Traditionally, the high temperature resistant and corrosion resistant parts have been manufactured from graphite materials.

A fundamental disadvantage of the use of chemical process apparatus components manufactured from graphite is their limited mechanical properties and durability. Although chemical processing parts and components manufactured from graphite exhibit excellent corrosion resistance to aggressive chemicals, such as strong acids and alkalis, they are brittle and have a very low impact resistance. This low impact resistance often leads to cracked or broken components, which require replacement at frequent intervals leading to undesirable equipment downtime with serious economic consequences to those in the chemical process industry. Other common disadvantages of graphite parts include generally poor mechanical properties, low strength, and poor durability brought about by its highly brittle nature, and its tendency to microcrack when exposed to repeated temperature cycles.

Carbon fiber-reinforced carbon matrix composites or carbon/carbon composites have been proposed as a possible replacement for graphite components in many industrial applications, especially in the chemical process industry, which require high temperature resistance, corrosion resistance, high mechanical stability and durability. Such composites offer similar corrosion resistance properties to graphite components and exhibit much superior mechanical properties, namely improved strength, dimensional stability, and impact and thermal shock resistance, in part due to the incorporation of reinforcing carbon fibers within the composite structure. However, the principle disadvantage of chemical process apparatus parts manufactured from carbon/carbon composites is the very high cost of production, which may be attributed to the high cost of the carbon fibers used, and the long manufacturing cycles required for the repeated densification of the carbon/carbon composites.

United States Patent No. 5,834,114 to Economy et al, discloses a method of making a fiber material for adsorption of contaminants comprising the steps of coating a fiber substrate with a resin; cross-linking the resin with a cross-linking agent; heating the coated fiber substrate to carbonize the resin; and exposing the coated fiber substrate to an etchant to activate the resin. The fiber substrate is selected from glass or mineral fibers. Exposing the coated fibers to the etchant produces a chemically active fiber having a desired surface porosity for filtering out toxic and noxious contaminants from air and water.

It, therefore, remains desirable to develop low cost composite materials and parts manufactured from the composite material, which are high temperature resistant, corrosion resistant, and that offer improved durability as compared to conventional parts used in the chemical process industry manufactured from graphite materials. SUMMARY OF INVENTION

It is therefore an object of the present invention to provide a composite material and parts manufactured from the composite material that are high temperature resistant.

It is another object of the present invention to provide a composite material and parts manufactured from the composite material that are corrosion resistant.

It is another object of the present invention to provide a composite material and parts manufactured from the composite material that exhibit improved mechanical properties, such as high impact resistance, as compared to component parts made from graphite materials.

It is another object of the present invention to provide a composite material and component parts manufactured from the composite material that have a lower cost of production as compared to component parts manufactured from carbon/carbon composite materials.

These and other objects, together with the advantages thereof over composite materials of the existing art, which shall become apparent from the specification which follows, are accomplished by the invention as hereinafter described and claimed.

The present invention therefore provides a semi-structural composite comprising a carbon matrix reinforced with a plurality of substantially pure microporous silica fibers, wherein said part has a flexural strength of at least 5 Ksi, a cross-ply strength of at least 0.1 Ksi, a beam shear strength of at least 0.5 Ksi, and an Izod impact strength of at least 1 Ksi.

The present invention further provides a process for the production of a semi- structural composite part comprising providing a plurality of substantially pure microporous silica fibers; impregnating the plurality of said substantially pure silica fibers with a carbon matrix precursor to form a prepreg; laying-up and curing said prepreg to form a net shape; and carbonizing said net shape in an inert atmosphere to form a part.

The present invention further provides a semi-structural composite part produced by the steps comprising providing a plurality of substantially pure microporous silica fibers; impregnating said plurality of substantially pure silica fibers with a carbon matrix precursor to form a pregreg; laying-up and curing said prepreg to form a net shape; and carbonizing said net shape in an inert atmosphere to form a part.

DETAILED DESCRIPTION OF THE INVENTION

It has now been found that silica fiber reinforced carbon matrix composites can offer improved mechanical properties, durability, high temperature resistance, and high corrosion resistance to aggressive chemicals relative to graphite materials, together with a reduced manufacturing cost as compared to carbon fiber reinforced carbon matrix composites or carbon/carbon composites. The silica fiber-reinforced carbon matrix composite structures have particular application in the manufacture of "lightly-loaded" semi- structural parts and components for use in industry, such as the chemical process industry. The term lightly-loaded, as used herein, refers to the ability of a semi-structural part of the present invention to support a particular load without undue deformation or failure of the part.

The semi-structural properties of the chemical process apparatus parts of the present invention may be achieved by the use of substantially pure non-porous silica fibers or silica fibers having a surface microporosity. Preferably, the semi-structural parts of the present invention include silica fibers having a surface microporosity. The term "substantially pure", as used in the specification, refers to fibers having a silica content of at least 90 weight percent, and wherein the silica content may be as high as 99.9 weight percent. The term "microporous", as used in the specification, refers to that characteristic of silica fibers having a surface porosity. Specifically, the microporous silica fibers have pore sizes of at least about 10 angstroms (A), and pore sizes preferably in the range of about 10 angstroms to about 30 angstroms in diameter, and a fiber surface area that is at least about 20 m²/g. The microporous silica fiber reinforced carbon matrix composites of the present invention are prepared by providing a plurality of substantially pure microporous silica fibers, preferably those produced from the leaching of glass fibers. The leaching of glass fibers is the process by which there is a gradual removal of soluble constituents from the glass. The leaching process results in the production of a substantially pure microporous silica fiber, having increased thermal and strength characteristics and high abrasion resistance. The method of leaching glass fibers to produce substantially pure microporous silica fibers is described in United States Patent No. 2,624,658, which is incorporated herein by reference. A particularly useful silica fiber is commercially available from Hitco Carbon Composites, Inc., under the trademark Refrasil ®.

The silica fibers useful in the silica fiber reinforced carbon matrix composite of the present invention may take the form of short discontinuous fiber, continuous fiber, chopped fiber, yarn, chopped yarn, tape, cloth, chopped cloth, needled fabrics, non-woven fabric, non-woven felt, woven fabric and woven felt. The silica fibers are preferably provided in the form of a woven fabric.

In a preferred embodiment, the substantially pure silica fibers are subsequently impregnated with a carbon matrix precursor, such as an organic resin, and are heated to a temperature of at least 600°C in the absence of oxygen to produce a carbon char on the surface of the silica fibers. Impregnation is the process by which carbon matrix precursor is deposited on the surface of the silica fibers, or is otherwise infiltrated into the micropores of the substantially pure silica fibers, and in the dimensional structure defined by the fibers, to form a "green" silica fiber/carbon matrix composite. Impregnation of the microporous silica fibers with the carbon matrix precursor can be achieved by any known means of impregnation including, but not limited to immersing the substantially pure silica fibers in a "bath", dipping the fibers, spraying the fibers, and the like.

After the substantially pure silica fibers have been impregnated with a liquid carbon matrix precursor, lay-up of the desired structure and curing, using conventional curing conditions, is performed.

The matrix precursors of high purity carbon which may be used to impregnate the porous silica fiber to form the silica fiber/carbon matrix composite of the present invention include liquid sources of carbon, such as organic resins and pitch. Suitable organic resins include thermosetting resins, such as phenolic epoxy, vinyl ester resins, polyester resins, and thermoplastic resins, such as polyether sulfone (PES), polyether ether ketone (PEEK), polyether imides (PEI) and polyacrylonitrile (PAN).

In a preferred embodiment, the carbon matrix precursor is a phenolic resin such as liquid novolak and resole resins. Examples of suitable phenolic resins include, but are not limited to those phenolic resins that are commercially available from Ashland chemical under the trade designation SCI 008, and those commercially available from Borden Chemical under the trade designations USP 39 and 91LD.

The silica fiber-reinforced carbon matrix composites of the present invention can be fabricated into a variety of desired shapes, by a variety of techniques. As discussed above, the silica fiber-reinforcement may take the form of a fabric. A plurality of resin impregnated silica fiber fabrics are placed or stacked (commonly known as laying-up) on a tool or die and are autoclave- or press-molded into the desired shape. The impregnated fibers can be molded in a hydraulic press or in an autoclave by conventional procedures for curing phenolic resins.

In a preferred embodiment, a woven fabric or other fiber structure comprising a plurality of silica fibers impregnated with a carbon matrix precursor is referred to as a prepreg. The prepregs are typically stacked or laid-up and cured at a curing temperature in the range of about 125°C to about 175°C for a period of time effective to cure the carbon matrix precursor, by way of example but not limitation, such as a period of about 2 hours. The curing process forms a net shape that comprises the precarbonized silica fiber/carbon matrix composite. The cured net shape is then carbonized by heat treatment, which results from the pyrolysis of the carbon compound.

The cured prepreg is subjected to heat treatment at temperatures in the range of about 600°C to about 1200°C in an inert atmosphere such as nitrogen for a period of time effective to carbonize the desired component or part. The carbonization of the prepreg generally takes place for about 9 hours to about 48 hours. This heat treatment produces a sufficient amount of carbon char from the pyrolysis of the phenolic resin, and serves to reinforce the bond between the carbon matrix and the silica fibers. The term carbon char, as used throughout the specification, refers to the carbonaceous residue on the surface of the silica fibers that results from the pyrolysis of the carbon matrix precursor.

Although the carbonized composite part may be a final product, further machine finishing may be performed if desired using diamond tools, carbide tip tools, and the like.

Those having ordinary skill in the art would expect that a silica fiber reinforced carbon matrix composite would not maintain structural integrity, or otherwise fall apart due to the expected poor coverage of the silica fiber surface by the carbon matrix layer. However, optical and electron microscopy has confirmed that the carbon layer uniformly covers the entire surface of the silica fiber.

The silica fibers of the present invention are microporous, typically as a result of their manufacture from the leaching of the soluble constituents from glass fibers.

The porosity of the silica fibers, as measured by nitrogen adsorption, ranges from about 20 to about 500 m²/g, depending on the glass precursor and the leaching conditions. It has been determined that the bond between the silica fiber and the carbon matrix is much stronger than expected, and the bond strength may be attributed to the excellent "wet-out" or coverage of the microporous silica fiber surface by the carbon matrix.

A silica fiber-reinforced carbon matrix composite was prepared according to the present invention. Several properties indicative of mechanical strength and durability of composite materials, such as bulk density (g/cc), flexural strength (Ksi), cross-ply strength (Ksi), beam shear strength (Ksi) and Izod impact strength (ft-lb./in) were determined for the silica fiber/carbon matrix composite. A comparison of the properties of the silica fiber/carbon matrix composite of the present invention and a carbon/carbon composite, prepared by methods known in the art, is shown in Table I below.

TABLE I

Comparison of the Properties for Carbon Fiber-Reinforced and

Silica Fiber Reinforced Carbon Matrix Composites.

As shown in Table I, above, the silica fiber-reinforced carbon matrix composite of the present invention exhibited a flexural strength of about 9.8 Ksi, a cross-ply strength of about 0.2 Ksi, a beam shear strength of about 0.92 Ksi and an Izod impact strength of about 2.4 ft-lb/in. The properties of the silica fiber-reinforced carbon matrix composites of the present invention are surprisingly and unexpectedly superior to comparative carbon fiber reinforced carbon matrix composites.

The bulk density was determined by ASTM test number C559, which is a method for determining the bulk density of manufactured carbon and graphite articles having a volume of at least 500 millimeters (mm). The flexural strength and beam shear strength were determined by ASTM test number D790, which is a test method for determining the flexural properties of composite materials. The test requires the composite material to be in the form a rectangular bar molded directly or cut from sheets, plates or molded shapes. In addition, the cross-ply tensile was determined by ASTM test number D952, which is a test method for determining the bond strength or ply adhesion.

The Izod impact strength was determined by ASTM test number D256, which is a test method for measuring the impact resistance of composite materials. The impact resistance is measured as the resistance of a specimen to breakage by flexural shock. The flexural shock is produced by a standardized pendulum-type hammer mounted in a machine. The results of the test are reported in terms of the amount of energy absorbed per unit width of the specimen.

As shown in Table I, the bulk density, flexural strength, cross-ply strength, beam shear strength and Izod impact strength are all greater for the silica fiber- reinforced carbon matrix composite of the present invention as compared to the carbon fiber-reinforced carbon composite material. According to one embodiment of the present invention, the silica fiber reinforced carbon matrix composite can be produced to exhibit a bulk density of about 1.2 to about 1.7 g/cc, a flexural strength of about 5 to about 20 Ksi, a cross-ply strength of about 0.1 to about 0.5 Ksi, a beam shear strength of about 0.5 to about 5 Ksi and an Izod impact strength of about 1 to about 5 ft-lb/in. These properties are key for applications that require both corrosion resistance and durability of a chemical process apparatus component or part.

The tensile strength (Ksi), compressive strength (Ksi), flexural strength (Ksi), Izod impact resistance (ft-lb./in), and bulk density (g/cc) of a further silica fiber/carbon matrix composite prepared according to the present invention were determined. A comparison of these properties of the silica fiber/carbon matrix composite of the present invention with those of comparative carbon-carbon (C/C) composite and traditional graphite parts is shown in Table II below. TABLE II

Comparison of Silica/carbon matrix composite, Carbon/carbon composite and Graphite.

As shown in Table II, the tensile strength, flexural strength and Izod impact strength of the silica fiber-reinforced carbon matrix composites of the present invention are superior to components made from graphite. The compressive strength of the silica fiber-reinforced carbon matrix compositions of the present invention is similar to that of composites made from graphite.

Table II also shows that the Izod impact strength and flexural strength properties of the silica fiber reinforced carbon matrix composites of the present invention are superior to graphite materials, but are somewhat inferior to the much more expensive carbon/carbon composites. Furthermore, properties which are "fiber dominated", such as tensile strength and compressive strength are, as expected, much superior for the carbon fiber based composite materials as compared to the silica fiber based composite materials of the present invention. The term "fiber-dominated", as used in the specification, refers to characteristics or properties of the silica fiber reinforced carbon matrix composites of the present invention that are largely influenced by the silica fibers.

However, in many of the intended "lightly loaded" applications, the nature of the bond between the fiber and the matrix of the silica fiber/carbon composite of the present invention are adequate to provide the mechanical properties needed for the intended chemical process applications. The mechanical properties, such as composite toughness, impact resistance, fatigue resistance, interlaminar shear strength and flexural strength, that dominated by the silica-carbon bond are surprisingly high.

According to the present invention, the silica fiber/carbon matrix composites of the present invention can be used to manufacture "lightly loaded" or semi-structural components and parts for use in chemical process reactors and apparatus such as distillation column packings (both structural and loose packings), heat exchanger tubes, pump bodies, shields, thermocouple tubes, distillation column trays and spray nozzles.

The silica fiber/carbon matrix composite of the present invention is also suitable for the manufacture of electrochemical apparatus or fuel cell components, such as bipolar plates and gas diffusion electrodes, and wet frictional coupling agents in torque converters.

Based on the foregoing disclosure, it is therefore demonstrated that the objects of the present invention are accomplished by the production and use of the silica fiber reinforced carbon matrix composites and chemical process apparatus components or parts manufactured from the composite material. It is further demonstrated that the present invention provides a composite material that has improved corrosion resistance and mechanical durability as compared to conventional materials made from graphite, and a lower cost of production as compared to carbon/ carbon composites. It should be understood that the selection of specific silica fibers and carbon matrix precursors, as well as processing time and conditions for the various steps of manufacture, can be determined by one having ordinary skill in the art without departing from the spirit of the invention herein disclosed and described. It should therefore be appreciated that the present invention is not limited to the specific embodiments described above, but includes variations, modifications and equivalent embodiments defined by the following claims.

Claims

I CLAIM:

1. A semi-structural composite comprising a carbon matrix reinforced with a plurality of substantially pure microporous silica fibers, wherein said matrix is substantially carbonized, and wherein said part has a flexural strength of at least about 5 Ksi, a cross-ply strength of at least about 0.1 Ksi, a beam shear strength of at least about 0.5 Ksi, and an Izod impact strength of at least about 1 ft-lb./in.

2. The semi-structural composite of claim 1, wherein the carbon matrix is derived from an organic resin precursor.

3. The semi-structural composite of claim 2, wherein the organic resin is a phenolic resin selected from novolak and resole resins.

4. The semi-structural composite of claim 1, wherein the plurality of substantially pure microporous silica fibers are in a form selected from at least one of a continuous fiber, chopped fiber, yarn, chopped yarn, tape, cloth, chopped cloth, non- woven fabric, non-woven felt, woven fabric and woven felt.

5. The semi-structural composite of claim 5, wherein the plurality of substantially pure microporous silica fibers are in the form of a woven fabric.

6. The semi-structural composite of claim 1, wherein said substantially pure microporous fibers have a surface area of at least 20 m /g.

7. The semi-structural composite of claim 1, wherein said substantially pure microporous fibers have a pore size of at least 10 A.

8. The semi-structural composite of claim 1, further characterized by at least one of the following: i) wherein the part has a bulk density of at least about 1.2 g/cc; ii) wherein said part has a flexural strength of about 5 to about 20 Ksi; ii) wherein said part has a cross-ply strength from about 0.1 to about 0.5 Ksi; iv) wherein said part has a beam shear strength from about 0.5 to about 5 Ksi; v) wherein said part has an Izod impact strength from about 1 to about 5 ft- lb./in.; and vi) wherein the part has a tensile strength of at least about 2.5 Ksi.

9. A chemical process apparatus part comprising the semi-structural composite of claim 1 , wherein the chemical apparatus part is selected from the group consisting of pump bodies, heat exchanger tubes, distillation column packings, shields, distillation column trays, thermocouple bodies, stirrers, spray nozzles and baffles.

10. An electrochemical cell component comprising the semi-structural composite of claim 1, wherein said component is selected from bipolar plates and gas diffusion electrodes.

11. A torque converter frictional coupling agent comprising the semi-structural composite of claim 1.

12. A semi-structural composite part produced by the process comprising: providing a plurality of substantially pure microporous silica fibers; impregnating said plurality of substantially pure silica fibers with a carbon matrix precursor to form a prepreg; laying-up and curing said prepreg to form a part; and carbonizing said part in an inert atmosphere.

13. The semi-structural composite part as in claim 12, wherein said silica fibers are produced by the leaching of glass fibers.

14. The semi-structural composite part as in claim 12, wherein the carbon matrix precursor is an organic resin, wherein the organic resin is a phenolic resin selected from novolak and resole resins.

15. The semi-structural composite part as in claim 12, wherein the plurality of substantially pure microporous silica fibers are in a form selected from at least one of a continuous fiber, chopped fiber, yarn, chopped yarn, tape, cloth, chopped cloth, non- woven fabric, non-woven felt, woven fabric and woven felt.

16. The semi-structural composite part as in claim 12, wherein impregnating the silica fibers is accomplished by one selected from the group consisting of dipping or immersing the fibers in a carbon matrix precursor and spraying the fibers with a carbon matrix precursor.

17. The semi-structural composite part as in claim 12, wherein curing is effected at a temperature in the range of about 125°C to about 175°C for a period of time effective to cure the precursor.

18. The semi-structural composite part as in claim 12, wherein the cured part is carbonized in the temperature range of about 600°C to about 1200°C for a period of time effective to substantially carbonize the precursor material.

19. The semi-structural composite part as in claim 12, wherein the part is further characterized by at least one of the following: i) wherein the part has a bulk density of at least about 1.2 g/cc; ii) wherein the part has a flexural strength of at least about 5 Ksi; iii) wherein the part has a cross-ply strength of at least about 0.1 Ksi; iv) wherein the part has a beam shear strength of at least about 0.5 Ksi; v) wherein the part has an Izod impact strength of at least about 1 ft-lb./in; and vi) wherein the part has a tensile strength of at least about 2.5 Ksi.

20. A process for the production of a semi-structural composite part comprising: providing a plurality of substantially pure microporous silica fibers; impregnating said plurality of substantially pure silica fibers with a carbon matrix precursor to form a prepreg; laying-up and curing said prepreg to form a part; and carbonizing said part in an inert atmosphere.

21. The process of claim 20, wherein said substantially pure microporous silica fibers are produced by the leaching of glass fibers.

22. The process of claim 20, wherein impregnating the silica fibers is accomplished by a technique selected from the group consisting of dipping or immersing the fibers in a carbon matrix precursor or spraying the fibers with a carbon matrix precursor.

23. The process of claim 20, wherein curing is effected at a temperature in the range of about 125°C to about 175°C for a period of time effective to cure the precursor.

24. The process of claim 20, wherein carbonizing is effected at a temperature in the range of about 600°C to about 1200°C for a period of time effective to substantially carbonize the precursor material.