BACKGRQUNP QF THE IHVfiWTIOB

The present invention relates to a fiber reinforced composite material and its method of manufacture. More particularly, the present invention provides a fiber reinforced composite material which exhibits superior high temperature stability and dielectric strength. The material is designed to retain its physical properties upon heat aging for extended periods of time and is, therefore, useful for high temperature electrical insulating applications and for structural heat barrier applications. A method for manufacturing such a composite material is also provided.

A number of materials have been developed in the past for high temperature electrical insulating and/or structural heat barrier applications. For example, U. S. Patent No. 4,571,357 discloses an electrical insulating laminate paper which is wound in a wet state on a conductive material and thereafter impregnated with oil. The laminate paper comprises an integrated assembly of a plastic film and fiber papers bonded to each other. The fiber paper is heterogeneously moistened with water to prevent the plastic film from swelling after being impregnated with the insulating oil.

Preparation of such a material requires a complex process. First, insulating papers composed of natural cellulose, such as kraft papers, or insulating papers composed of natural cellulose blended with synthetic fibers or pulps must be bonded to each other. The laminate may be formed by extruding onto the insulating paper an adhesive polyolefin

resin by means of an extruder. Alternatively, the laminate may be formed by bonding a polyolefin film to a natural cellulose paper in a body with heating and pressing. Once the laminate is formed, it must be moistened with a stream of small water drops having a broad particle size distribution of 10-1000 microns so as to result in a water content of 5-25% by weight. To form a stream of water drops having such a broad particle size distribution, ion exchanged water must be forced against a mesh wire netting under pressure through a nozzle.

In addition to the complex process required for making this material, the material has the further disadvantage of not being useful for structural heat barrier applications.

U. S. Patent No. 4,430,384 discloses an electrical insulating, heat resistant, flexible refractory tape useful for wrapping electrical wires and cables. To form the tape, a porous base fabric must be coated and impregnated with a mixture comprising a refractory material and a bonding agent. After coating and impregnating the base fabric with the refractory material, the fabric is preferably coated on both sides with a fire retardant polymeric coating.

The material suffers from several disadvantages. First, care must be taken when the refractory coating is applied to the porous fabric to ensure that the coating bonds to both the surface and the interstices of the porous base fabric. Second, to impart flame retardant properties to the material a separate coating must be applied to the refractory coating. Finally, the material cannot be manufactured on standard paper making equipment.

U. S. Patent No. 4,225,649 also discloses a fire protective coating for electrical cables and wires. The coating is composed of latex, clay, a low temperature fiber and a fire retardant material which provides a source of organically bound halogen. The coating has no electrical insulating properties of its own, and a separate layer of

insulating material must be applied to the wire or cable before the coating is applied. Moreover, the coating cannot be made by a conventional paper making process but is, instead, an aqueous emulsion applied to the wire or cable as a fluid by spraying, brushing, troweling, gunning, etc.

U. S. Patent No. 4,018,962 discloses an electrical insulating flame resistant tape used as a protective coating for electrical cables, wires, connectors, etc. The tape comprises a basic fabric upon which a heat liquified insulating material is deposited. The tape cannot be manufactured on standard paper making equipment. Instead, the liquid insulating material must be separately applied to the base fabric such as by knife coating. Moreover, to impart flame resistant characteristics to the tape, a fire retardant compound such as a halogenated plasticizer must be mixed into the insulating material before it is applied to the base fabric.

U. S. Patent No. 3,928,210 also discloses a fire protective coating for electrical cables or wires. The coating comprises an aqueous resinous emulsion, inorganic fillers, inorganic fibers, and organic fibers. To impart flame retardant characteristics to the material a source of organically bound halogen must be added. Moreover, the coating does not have any electrical insulating properties of its own.

U. S. Patent No. 3,666,615 discloses a composite layer of sheet material cut into a tape and used as an electrical insulating material. The sheet material comprises a fiber based member, a layer of thermosetting resin, a layer of hardening agent for hardening the resin and a contact preventive film layer between the resin layer and the layer of hardening agent. While the material has good insulating properties and can be formed as sheets, it does not appear to have any significant heat resistance and cannot be made by a paper making process.

Accordingly, it is the aim of the present invention to provide a fiber reinforced composite material which exhibits high temperature resistance, high dielectric strength, and retention of properties upon heat aging.

It is a further aim of the present invention to provide such a composite material which can be manufactured on standard paper aking equipment.

It is a further aim of the invention to provide a method of manufacturing such a composite material.

SUMMARY PF THE IWVENTIOH

The present invention provides a fiber reinforced composite material which exhibits high temperature resistance, high dielectric strength and superior retention of physical properties with heat aging. The material comprises 5-60% by weight of polyamide fibers, 0-40% by weight of polyester fibers, 15-40% by weight of inorganic particulate filler, 5- 40% by weight of elastomeric binder and 0-10% by weight of compounding agents.

The material is manufactured by forming an aqueous slurry containing the polyamide fibers, the polyester fibers, elastomeric binder, inorganic particulates and compounding agents. Using conventional papermaking equipment, the slurry is formed into a non-woven fabric or mat. The fabric is pressed, then heated to evaporate out the majority of the water. The fabric is then subjected to both heat and pressure to achieve the requisite fabric density.

In a preferred embodiment of the invention, a rheology modifier is dispersed in water, and the polyamide fibers are added to this dispersion to form an aqueous slurry. Once the polyamide fiber slurry is formed, the remaining furnish ingredients may be added and processing continues as outlined above.

The high temperature resistance and dielectric strength of the resulting material make it particularly useful in applications requiring electrical insulating properties at elevated temperatures such as electrical motors, transformers, composite structures and specialty film laminations.

DETAILED DESCRIPTION OF THE INVENTION

The fiber reinforced composite material of the present invention comprises 5-60% by weight, preferably 15-30% by weight, of polyamide fibers. The polyamide fibers form the basic structural network of the composite material. They exhibit superior heat aging, flexibility and dielectric properties and enhance the aesthetics of the final composite material.

Polyester fibers may be incorporated into the composite for those applications which require substantial flexibility of the final product. The composite may comprise 0-40% by weight, preferably 15-25% by weight, of polyester fibers depending on the nature of the application.

Any natural or synthetic rubber may be used as the elastomeric binder. Preferably, the elastomeric binder is selected from the group consisting of nitrile rubber, acrylic rubber, silicone rubber, neoprene rubber and mixtures thereof. The elastomeric binder functions as an adhesive for the fibrous network and enhances the physical performance of the final composite material, particularly in the high temperature environment for which the present invention is intended.

The inorganic filler may be any inorganic filler but is preferably selected from the group consisting of silica, calcium carbonate, clay and mixtures thereof. The inorganic fillers serve to increase the density, uniformity and performance of the final composite material.

The compounding agents used in the present invention are well-known and include retention aids, dispersion aids, formation aids and any other processing aids commonly employed by those skilled in the art.

Table 1 illustrates the acceptable and preferred quantities of the ingredients useful in the practice of the present invention expressed in percentages by dry weight in the final product.

Table I

Ingredients Acceptable Ranges Preferred Ranges Polyamide Fibers 5-60 15-30 Polyester Fibers 0-40 15-25 Elastomeric Binder 5-40 15-30 Inorganic Filler 15-40 20-40 Compounding Agents 0-10 3-7

The fiber reinforced composite material is manufactured on conventional paper aking equipment such as, for example, a cylinder, Fourdriner or rotoformer type papermaking machine. A standard volume of water is added to an appropriate papermaking dispersing system, such as a pulper, beater, chest, etc. The polyamide fibers are added to the system and dispersed to form a slurry. The fiber slurry is further processed through mechanical action, such as with a disk refiner or beater, to open up and fibrillate the individual fibers.

Following fibrillation of the polyamide fibers, the remaining ingredients may be added to the slurry. Caustic and alum are used in the process to cause deposition of the binder onto the fibers and to cause flocculation of the inorganic portion. The slurry is then processed into a non-woven fabric, and, after the non-woven fabric is formed, it is pressed and dried to evaporate out the majority of the water. The time and temperature required for drying is dependent on the composition and thickness of the non-woven fabric.

Forming the final product requires the consolidation of the non-woven fabric by the application of heat and pressure to achieve fabric densities of greater than or equal to 1 gram per cubic centimeter. Composite material densities of this magnitude are required to obtain the desired physical properties.

In a preferred embodiment of the invention, a rheology modifier is mixed with the polyamide fibers to counter the natural tendency of such fiber to knot or bundle during processing. Knots or bundles present in the final product can adversely affect its performance. The absence of fiber bundles is particularly important where a thin sheet of the composite is used in a high temperature environment. In such applications, even a small flaw in the material can create a weakness which causes the entire sheet to fail.

The present inventors have found that polyacrylamide polymers are suitable rheology modifiers. Prior to the addition of the polyamide fibers, a water/polyacrylamide polymer dispersion is formed in the dispersing system and continually agitated for a period of time sufficient to permit adequate molecular chain and molecular charged formation. The period of time required for proper chain and charge formation varies depending on the particular polyacrylamide polymer used.

After the required time period has elapsed, the polyamide fibers are added to the system and dispersed as described above. The polyacrylamide polymer raises the viscosity of the polyamide fiber slurry, and an ionic charge forms on the fibers through an ionic interaction of the fibers and the molecular chains of the polymer. As a result, the fibers are more easily and completely dispersed and an untangled, knot-free slurry is formed. Once the knot-free slurry of polyamide fibers is formed, the method of manufacturing the composite is identical to that described above.

The polyacrylamide polymer is used in an amount ranging from 0.025-0.25% by weight and preferably is present in an amount ranging from 0.05-0.1% by weight. Cationic or anionic polyacrylamide polymers may be used such as, for example, "SEPARAN" AP-273 manufactured and sold by Dow Chemical Co., Middland, MI.

The present invention is illustrated by the following examples.

Example i

Ingredients % of Total Composition

Aromatic Polyamide Fiber 40

Polyester Fiber 20

Inorganic Filler 20

Acrylated Silicone Binder 20

A non-woven fabric having a weight of approximately 1.75 ounces per square yard was prepared from the above ingredients. 0.075% of a polyacrylamide polymer was used to provide a knot-free slurry of the polyamide fibers. Upon drying of the non-woven fabric, it was consolidated by multiple passes at a rate of 40 feet per minute, a temperature of 200 _O F and a pressure of 2450 pounds per lineal inch.

The final product exhibits the following characteristics:

Table II

Parameter Results

Thickness (inches) .0022

Density (g/cc) 1.06

Tensile (lbs/in) 14.1

Mullen Burst (psi) 32

Dielectric Strength (v/mil) 396

The final product exhibits the following characteristics on heat aging: Table III

Parameter ______P-τ , 12 Hrs, 4øς»ϋf ' _ 24 HITS.

Basis Height lbs/2880 sq. ft. 106.7 100.0 oz./sq. yd. 5.33 5.00

6/ra ² 180.7 169.5

Caliper mils 8.25 7.8

mm 21 .20

Tensile Strength lbs/in MD 82 78

CD 58 55

N/cm MD 143.6 136.6

CD 101.5 96.3

Tear grams MD 208 188

CD 248 192

N MD 2.04 1.84

CD 2.43 1.88

Mullen

PSI 132 120

KPa 910 827

Pielectric Strength v/mils 225 247

Kv/mm 8.86 9.72

Example II

The following ingredients were formed into a non- woven fabric on conventional papermaking equipment. 0.075% by weight of a polyacrylamide polymer was used to provide a knot- free slurry of the polyamide fibers. The non-woven fabric was processed under conditions identical to those of Example II.

Ingredients % of Total Composition

Aromatic Polyamide Fiber 60

Inorganic Filler 20

Acrylated Silicone Binder 20

The final product exhibits the following characteristics:

Table IV

Parameter Results

Thickness (inches) .0019

Density (g/cc) 1.06

Tensile (lbs/in) 11.2

Mullen Burst (psi) 22.0

Dielectric Strength (v/mil) 441

The final product exhibits the following characteristics on heat aging:

Table V

Parameter 400G-F. 12 Hrs. 400 ^O -F. 24 Hrs.

Basis Height lbs/2880 sq. ft. 89.7 90.8 oz./εq. yd. 4.48 4.54

6/m ² 151.9 153.9

Caliper mils 5.5 5.7 mm .14 .14

Tensile Strength lbs/in MD 92 72

CD 75 72

N/cm MD 161.1 168.1

CD 131.3 126.1

Tear grams MD 128 124

CD 140 128

N MD 1.26 1.22

CD 1.37 1.26

Mullen

PSI 150 160

KPa 1,034 1,103

Pielectric Strength v/Mils 300 286

Kv/mm 11.81 11.26

While preferred embodiments have been shown and described, various modifications may be made thereto without departing from the spirit and scope of the invention. Accordingly, it must be understood that the present invention has been described by way of illustration and not limitation.