Wholly aromatic polyamide fibers, a sheet comprising same and method of producing the sheet

Aromatic polyamide fibers which have a crystalline structure having (1) crystal size (A) in a (110) plane of 7.5 nm, (2) crystal size (B) in a (200) plane of 8.2 nm and (3) a product A.times.B of 61.50 to 630.00, and exhibit a thermal linear expansion coefficient of -1.0.times.10.sup.-6 /.degree. C. to -7.5.times.10.sup.-6 /.degree. C. and thus a high dimensional stability even upon moisture-absorbing and desorbing, are useful for forming a resin-reinforcing fiber sheet, a pre-preg containing the fiber sheet, and a laminate for, for example, an electric insulating material or electric circuit board, having an excellent cutting, shaving, perforating or laser processability and capable of forming a smooth cut, shaved or perforated face.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to wholly aromatic polyamide fibers, a sheet
 comprising same and a method of producing the sheet. More particularly,
 the present invention relates to wholly aromatic polyamide fibers having a
 high heat resistance and an excellent electric insulating property under
 high temperature and high humidity conditions, and a sheet comprising the
 heat resistant and electrically insulating wholly aromatic polyamide
 fibers and at least one member selected from binder resins and heat
 resistant organic polymer fibrids, and usable as an electric insulating
 material, and a method of producing the wholly aromatic polyamide fibers.
 2. Description of the Related Art
 It is well-known that wholly aromatic polyamide fibers have excellent heat
 resistance and mechanical strength and thus are useful for various uses.
 In the case where the wholly aromatic polyamide fibers are used as a
 resin-reinforcing material, for example, FRP material or parts of OA
 machines and devices which have to exhibit a very accurate dimensional
 stability, however, the dimensions of the aromatic polyimide fibers in the
 longitudinal direction thereof and in the transverse directions at right
 angles from the longitudinal direction change to a great extent in an
 atmosphere in which an increase and a decrease of the temperature are
 repeated, and the change in the dimensions due to absorption and
 desorption of moisture in the fibers is significant. Therefore, in a
 fiber-reinforced resin composite laminate plate, a strain is created in
 the interfaces between the reinforcing fibers and the resin, and the
 strain causes the laminate plate to exhibit a reduced durability to
 repeated load, and an interfacial separation occurs. Particularly, when
 used as an electric insulation material while an electric current is
 flowed therethrough for a long period, water is accumulated in a small
 amount in the interfaces between the resin and the reinforcing fibers, to
 promote movement of impurity ions contained in the resin or fibers, or to
 cause an interfacial separation due to a temperature-increase and
 water-evaporation. As an attempt for solving the above-mentioned problems,
 for example, Japanese Unexamined Patent Publication No. 5-83,096, No.
 62-218,425 and No. 62-225,539 discloses a method of improving interfacial
 adhesion of aromatic polyamide fibers to a resin by heat treatment.
 However, this method is disadvantageous in that the fiber-reinforced resin
 material has a high content of impurity ions and a high moisture content,
 and thus is unsuitable for electrically insulating uses. These problems
 have not yet been fully solved.
 The wholly aromatic polyamide fibers have a very high resistance to cutting
 and thus are used in various protective materials as disclosed in, for
 example, Japanese Unexamined Patent Publications No. 4-14,277, No.
 6-280,140. However, this very high cutting resistance causes such a
 problem in that when a resin composite laminate plate reinforced by the
 wholly aromatic polyamide fibers is subjected to a piercing work with a
 drill, the reinforcing fibers are not sharply cut, the reinforcing fibers
 are divided into fine fibrils or not completely cut, and thus the
 resultant hole has a rough inner face. Particularly, in recent piercing
 work, a very small size of hole is required to be formed with a high
 accuracy, and thus a new piercing method using a carbon dioxide gas laser
 has been attempted. Therefore, a solution of the above-mentioned problem
 is strongly demanded in industry.
 Also, when the fiber-reinforced composite resin laminate material is
 subjected to cutting work, the reinforcing fibers are not sharply cut and
 the resultant fine fibrils from the reinforcing fibers project from the
 cut face. This problem must be solved.
 SUMMARY OF THE INVENTION
 An object of the present invention is to provide holly aromatic polyamide
 fibers having a very high dimensional stability even when employed in an
 atmosphere in which the temperature repeatedly changes and in which the
 moisture is absorbed by or desorbed from the fibers, a sheet comprising
 the fibers and a method of producing the fibers.
 Another object of the present invention is to provide wholly aromatic
 polyamide fibers which, when used as reinforcing fibers for a
 fiber-reinforced resin composite laminate plate, and subjected to a
 piercing, cutting or laser treatment work, can be sharply cut, exhibit a
 high resistance to fibril-formation from the fibers, and thus cause the
 laminate plate to exhibit smooth and sharp cut faces, a sheet comprising
 the fibers, and a method of producing the sheet.
 The above-mentioned objects can be attained by the wholly aromatic
 polyamide fibers, the sheet comprising the fibers and the method of
 producing the sheet, of the present invention.
 The wholly aromatic polyamide fibers of the present invention are
 characterized by comprising wholly aromatic polyamide crystals having
 apparent crystal sizes determined from a wide angle X-ray diffraction
 intensity curve in accordance with the Scherrer's equation and satisfying
 the requirements (1), (2) and (3);
 (1) the crystal size (A) in a (110) plane is 7.5 nm (75 angstroms) or more,
 (2) the crystal size (B) in a (200) plane is 8.2 nm (82 angstroms) or more,
 and
 (3) the product (A.times.B) of the crystal size (A) and the crystal size
 (B) is 61.50 to 630.00, and having a thermal linear expansion coefficient
 within the range of from -1.0.times.10.sup.-6 /.degree. C. to
 -7.5.times.10.sup.-6 /.degree. C.
 In the wholly aromatic polyamide fibers of the present invention, when the
 wholly aromatic polyamide fibers in an amount of 2 g are cut into a short
 length of 1 mm or less and immersed in 50 ml of a distilled water, and are
 heat-treated in an autoclave at a temperature of 121.degree. C. under a
 pressure of 2 kg/cm.sup.2 G, to extract impurity ions from the fibers, and
 the extracted impurity ion-containing water is subjected to a ICP analysis
 and to a chromatographic analysis to determine the amount of the extracted
 impurity ions, namely sodium, potassium and chlorine ions, per liter of
 the extracted impurity ion-containing water, the extracted impurity ions
 preferably satisfy the requirements (a), (b) and (c):
 (a) the amount of the extracted sodium ions is not more than 40 mg/liter.
 (b) the amount of the extracted potassium ions is not more than 3.0
 mg/liter, and
 (c) the amount of the extracted chlorine ions is not more than 7.5
 mg/liter.
 The wholly aromatic polyamide fiber sheet of the present invention
 comprises, as principal components, 60 to 96% by weight of wholly aromatic
 polyamide staple fibers, and 4 to 40% by weight of at least one member
 selected from the group consisting of organic resin binders and
 heat-resistant organic polymer fibrids,
 said wholly aromatic polyamide staple fibers containing the specific wholly
 aromatic polyamide fibers of the present invention in the form of staple
 fibers.
 The wholly aromatic polyamide fiber sheet of the present invention
 preferably has a maximum change in the longitudinal dimension of 50 .mu.m
 or less per length of 20 mm, determined by such a test method for
 dimensional change of a sheet due to moisture-absorption and desorption of
 the sheet, that a sample of the sheet having a length of 20 mm and a width
 of 5 mm is left standing in the ambient atmosphere at room temperature at
 a relative humidity of 85% RH or more for 120 hours or more to allow the
 sheet to fully absorb moisture; the moisture-absorbed sheet sample is
 subjected to heating to a temperature of 280.degree. C. at a heating rate
 of 10.degree. C./minute, then to cooling to room temperature at a cooling
 rate of 10.degree. C./minute to dry the sheet sample, the above-mentioned
 heating and cooling procedures being repeated three times in total under
 the same conditions as mentioned above; and a maximum change in the
 longitudinal dimension of the sheet sample generated during the repeated
 heating and cooling procedures is measured per 20 mm in the longitudinal
 dimension.
 The method of the present invention for producing a wholly aromatic
 polyamide fiber sheet containing the specific wholly aromatic polyamide
 fibers of the present invention in the form of staple fiber, comprises;
 preparing an aqueous staple fiber slurry by dispersing aromatic polyamide
 staple fibers containing the above-mentioned specific wholly aromatic
 polyamide fibers in the form of staple fibers and heat resistant organic
 polymer fibrids in an aqueous medium; subjecting the aqueous staple fiber
 slurry to a paper-forming procedure; drying the resultant wet paper-like
 sheet; heat-pressing the dried sheet at a temperature of 210 to
 400.degree. C. under a linear pressure of 150 to 250 kg/cm, to partially
 soften, or melt or soften and melt the heat-resistant organic polymer
 fibrids in the dried sheet and thereby to causing the fibrids to be bonded
 to each other and to the aromatic polyamide staple fibers.
 The wholly aromatic polyamide fiber sheet of the present invention can be
 used to produce a pre-preg by impregnating the wholly aromatic polyamide
 fiber sheet with a thermosetting resin.
 The pre-preg is used to produce a shaped article, particularly a perforated
 shaped article, and a laminate plate, particularly a perforated laminate
 plate.
 The perforated shaped article or laminate plate can be produced by a laser
 perforation method.
 DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The inventors of the present invention have made an extensive study for
 attaining the above-mentioned objects and found that when a
 fiber-reinforced resin article is subjected to a laser perforation work,
 there is a difference in decomposition temperature between the reinforcing
 fibers and the resin and thus, to form a satisfactory hole, a portion of
 the reinforcing fibers must be melted to form a smooth cylindrical inner
 wall surface of the hole. Also, it was determined that the formation of
 the smooth inner wall surface is governed to a large extent by proportions
 of crystal portions and amorphous portions of the fibers and the apparent
 crystal size and thus, when the proportions and the apparent crystal size
 are not appropriate the smooth inner surface of the hole cannot be formed.
 Further, it was found that when a small amount of p-type aromatic
 polyamide fibers which can be easily softened and melted and have a high
 ultimate elongation are contained in the reinforcing fibers, the resultant
 hole has a smooth inner wall surface. The present invention has been
 completed on the basis of the above-mentioned findings.
 The smoothness and sharpness of the finished surface formed by a cutting,
 shaving or drill-perforating work are governed to a great extent by the
 apparent crystal size of the reinforcing fibers as mentioned above. When
 the reinforcing fibers include, as a portion thereof, a small amount of
 p-type polyamide fibers having a high ultimate elongation, and thus a high
 deformability, and are capable of being easily softened and melted, and
 the p-type polyamide fiber-containing resin composition is formed into a
 fiber-reinforced resin-based material, a shaped article or a laminate
 plate by a heat-pressing procedure, the p-type polyamide fibers are
 deformed, softened and partially melted during the heat-pressing
 procedure, to promote the intertwinement and adhesion with other
 reinforcing fibers and to increase the adhesion areas between the
 reinforcing fibers. Therefore, the reinforcing fibers in the shaped
 article or laminate plate are fixed with each other, and when the shaped
 article or laminate plate is subjected to a cutting, shaving or
 drill-perforating work, the fixed reinforcing fibers can be sharply cut by
 a cutter or drill without escaping from the cutter or drill, and thus the
 cut face of the shaped article or laminate plate is very smooth and sharp.
 The wholly aromatic polyamide fibers of the present invention comprises
 crystals of the wholly aromatic polyamide having specific apparent crystal
 sizes determined from a wide angle X-ray diffraction intensity curve
 thereof in accordance with Scherrer's equation. Namely, the wholly
 aromatic polyamide crystal sizes must satisfy the requirements (1), (2)
 and (3):
 (1) the crystal size (A) in a (110) plane is 7.5 nm (75 angstroms) or more,
 (2) the crystal size (B) in a (200) plane is 8.2 nm (82 angstroms) or more,
 and
 (3) the product (A.times.B) of the crystal size (A) in nm and the crystal
 size (B) in nm is 61.50 to 630.00.
 Also, the specific wholly aromatic polyamide fibers have a thermal linear
 expansion coefficient within the range of from -10.times.10.sup.-6
 /.degree. C. to -7.5.times.10.sup.-6 /.degree. C., preferably from
 -10.times.10.sup.-6 /.degree. C. to -6.5.times.10.sup.-6 /.degree. C.
 Preferably, the crystal size (A) in the (110) plane is 8.5 nm or more,
 more preferably by 10.0 nm or more, still more preferably 10.5 nm or more,
 further preferably 11.0 to 27.0 nm; the crystal size (B) in the (200)
 plane is 8.4 nm or more, more preferably 8.5 nm or more, still more
 preferably 9.0 nm or more, further preferably 9.5 to 20.0 nm; and the
 product (A.times.B) of the crystal size (A) in nm and the crystal size (B)
 in nm is 71.40 to 630.00, more preferably 85.00 to 630.00, still more
 preferably 94.50 to 575.00.
 When the crystal size (A) in the (110) plane is less than 7.5 nm, the
 crystal size (B) in the (200) plane is less than 8.2 nm, and/or the
 product A.times.B is less than 61.50, the apparent crystal sizes of the
 aromatic polyamide fibers in the (110) and (200) planes are too small, and
 when an aromatic polyamide fiber-reinforced shaped article or composite
 laminate plate is subjected to a drill-perforation work or a shaving work,
 the aromatic polyamide fibers located at the cut end faces or shaved faces
 exhibit a high resistance to fibrillation. However, the resultant aromatic
 polyamide fibers exhibit too low a rigidity and thus are easily deformed,
 and when the cutter or drill edge comes into contact with the fibers, the
 fibers can be easily deformed and can easily escape from the cutter or
 drill edge. Namely, the resultant aromatic polyamide fibers exhibit a
 decreased cutting or shaving processability.
 To enhance the processability of the reinforcing fibers, when a small
 amount of para-type aromatic polyamide fibers which can be easily softened
 or deformed or melted rather than the reinforcing aromatic polyamide
 fibers under the same temperature and pressure conditions, for example,
 specific p-type aromatic polyamide fibers having an ultimate elongation of
 6% or more, are contained in the reinforcing fibers, and the p-type
 aromatic polyamide fiber-containing reinforcing fibers are used for a
 production of a fiber-reinforced resin-based material or a resin composite
 laminate plate, the p-type aromatic polyamide fibers are easily elongated,
 softened, deformed, or partially melted so as to enhance the
 intertwinement and adhesion of the reinforcing fibers and to increase the
 adhesion area of the reinforcing fibers. Therefore, the reinforcing fibers
 are fixed to each other and are difficult to escape from the cutter or
 drill edge. Therefore, the resultant fiber-reinforced resin composite
 laminate plate exhibits an enhanced cutting, perforating and shaving
 processabilities. However, when the p-type aromatic polyamide fibers do
 not satisfy the requirements (1), (2) and/or (3) of the present invention,
 the enhancement in the cutting, perforating and shaving processabilities
 is unsatisfactory.
 For example, the inventors of the present invention have disclosed
 reinforcing fibers containing a small amount of p-type aromatic polyamide
 fibers. However, the disclosed p-type aromatic fibers did not satisfy the
 requirements (1), (2) and (3) of the present invention and should be
 distinguished from the wholly aromatic polyamide fibers of the present
 invention which can be easily elongated, deformed and melted at a
 relatively low temperature. Therefore, the disclosed p-type aromatic
 polyamide fibers are unsatisfactory in blendability and adhesion with
 other reinforcing fibers. Also, the disclosed p-type aromatic polyamide
 fibers exhibit higher softening, deforming and melting temperature than
 those of the specific wholly aromatic polyamide fibers of the present
 invention.
 Also, when the product (A.times.B) is more than 630.00, the resultant
 wholly aromatic polyamide fibers are advantageous in that the resultant
 aromatic polyamide fibers exhibit a high rigidity and enhanced cutting and
 shaving processabilities. However, this type of fibers is disadvantageous
 in that the cut faces of the fibers are easily divided along the
 longitudinal axes of the fibers and fine fibrils are formed. Therefore,
 even when the specific p-type aromatic polyamide fibers are mixed in a
 small amount in the reinforcing fibers, the resultant fiber-reinforced
 resin composite laminate plate cannot prevent the projection of finely
 divided fibers in the form of fibrils from the cut or shaved faces
 thereof, and thus exhibits degraded cutting, shaving and drill-perforating
 processabilities.
 Also, when the fiber-reinforced resin composite laminate plate is subjected
 to a perforation work by laser, for example, carbon dioxide gas laser, and
 when the apparent crystal sizes in the (110) and (200) planes of the
 aromatic polyamide fibers are too small, portions of the resin walls
 surrounding the holes are carbonized and colored in black color, and the
 electrical insulation properties of the laminate plate are degraded, and
 thus the laser-perforated laminate plate cannot be used as an electric
 insulating material.
 In this case, it is confirmed that when the specific wholly aromatic
 polyamide fibers of the present invention, which are easily softened,
 deformed or melted, are contained in a small amount in the reinforcing
 fibers, these specific fibers are melted by a small amount of energy
 within a short time to form smooth wall faces of the holes, and the
 black-coloration of the wall faces of the hole due to the carbonization of
 the resin is restricted. When the apparent crystal sizes are too large,
 the smooth and sharp faces are not formed on the hole inner surfaces, and
 thus the resultant inner surfaces of the holes are rough.
 The inventors of the present invention have confirmed, as a result of the
 inventor's extensive study, that the resin composite laminate plates
 reinforced by the specific aromatic polyamide fibers in which the crystal
 sizes of the aromatic polyamide crystals are controlled to the ranges as
 mentioned above, exhibits a very good laser-perforation, cutting and
 shaving processabilities.
 The specific wholly aromatic polyamide fibers of the present invention have
 a larger crystal size (A) in the (110) plane than that of the conventional
 aromatic polyamide fibers, and thus have an enhanced degree of
 crystallization and a decreased content of amorphous portion. Therefore,
 the specific wholly aromatic polyamide fibers of the present invention are
 characteristic in decreased dimensional changes both in the fiber axis
 direction and the transverse direction at right angles to the fiber axis
 direction, in the atmosphere in which a high temperature and a low
 temperature are repeatedly generated or in the atmosphere in which a
 moisture absorption and a moisture desorption of the fibers repeatedly
 occur.
 The specific wholly aromatic polyamide fibers of the present invention have
 a slightly broader range of the thermal linear expansion coefficient than
 that of the conventional aromatic polyamide fibers. However, when the
 specific wholly aromatic polyamide fibers of the present invention which
 are easily softened, deformed or melted at a relatively low temperature
 are mixed in a small amount in the reinforcing fibers, the intertwinement
 of the reinforcing fibers can be controlled. Also, when the specific
 aromatic polyamide fiber-containing reinforcing fibers are employed in
 combination with an appropriate resin, the thermal linear expansion
 coefficient of the resultant fiber-reinforced resin composite material
 along the surface plane of the material and at right angles to the surface
 plane can be unlimitedly decreased toward zero. Accordingly, the specific
 wholly aromatic polyamide fibers of the present invention are appropriate
 as reinforcing fibers for FRP materials which must have a highly accurate
 dimensional stability and are useful for aircraft and for resin composite
 laminate plate materials usable for parts of OA machines and devices.
 Particularly, the fiber-reinforced resin composite materials usable for
 the aircraft which are employed under conditions having a large
 temperature difference, are employed under high load in an atmosphere in
 which a high temperature and a low temperature are repeatedly applied to
 the materials. Therefore, generally, the repeated changes in temperature
 and in humidity cause changes in dimension of the material, and the
 dimensional changes cause a interfacial separation between the resin and
 the reinforcing fibers. This problem can be solved by using the specific
 wholly aromatic polyamide fibers of the present invention, and the
 resultant fiber-reinforced resin materials can exhibit an enchased
 durability in practice.
 The specific wholly aromatic polyamide fibers of the present invention
 satisfying the crystal size requirements (1), (2) and (3) surprisingly
 exhibit a low equilibrium moisture content of 3.2% or less, particularly
 2.7% or less, more particularly 2.0% or less at a specific crystal size.
 This equilibrium moisture content of 3.2% or less is definitely different
 from that of the conventional typical aromatic polyamide fibers of about
 4.5% or more. Therefore, when the composite laminate plate reinforced by
 the specific fibers of the present invention is used as an electric
 insulating material, the conventional problem that a small amount of water
 is accumulated in the interfaces between the resin and the reinforcing
 fibers, and the impurity ions contained in the accumulated water move
 through the interfaces while an electric current is applied to the
 material over a long period, can be solved even in the case where the
 accumulated water contains a large amount of the impurity ions. Also,
 another problem that the interfacial separation occurs due to
 heat-vaporization of the water, can be prevented.
 Among the conventional aromatic polyamide fibers, those having an
 equilibrium moisture content of about 1.5% or less are known. This type of
 the aromatic polyamide fibers have a outermost surface layers thereof
 having been heat-oxidized to a great extent, and thus having a decreased
 surface electric resistance, and thus cannot be used as an electric
 insulating material. The resistance of the aromatic polyamide fibers to
 cutting and shaving decreases with increase in moisture content of the
 fibers. Therefore, the wholly aromatic polyamide fibers of the present
 invention having above-mentioned low equilibrium moisture content are
 suitable for use in cloth having a high resistance to heat and/or cutting
 or shaving (for example, in protective suits).
 Also, paper sheet materials for electric insulating materials, particularly
 printed circuit boards, must be excellent in heat resistance, thermal
 dimensional stability, resistance to moisture absorption, wet dimensional
 stability, electric insulating property, light weight, and resistance to
 deformation during formation into a laminate material (for example,
 resistances to warping, distortion and corrugation). For these
 requirements, the aromatic polyamide staple fiber sheets of the present
 invention significantly contribute to enhancing the above mentioned
 properties in comparison with the conventional aromatic polyamide staple
 fiber sheets and other heat resistant organic polymer staple fiber sheets.
 Also, the aromatic polyamide staple fiber sheets of the present invention
 exhibit an excellent laser processability and cause the perforation and
 shaving works in a very small size which have been considered to be
 impossible when the conventional fiber sheet is used, to be realized.
 The aromatic polyamide fibers usable for the sheet of the present invention
 may be produced from an aromatic polyamide resin selected from
 homopolyamide having recurring units of the formula (I) and copolymers
 having at least 80 molar %, preferably at least 90 molar %, of recurring
 units of the formula (I).
 Formula (I)
EQU --NH--Ar.sub.1 --NHCO--Ar.sub.2 --CO-- (I)
 In the formula (I), Ar.sub.1 and Ar.sub.2 respectively and independently
 from each other represent a divalent aromatic group, preferably selected
 from those of the formulae:
 ##STR1##
 The above-indicated aromatic groups may be substituted by 1 to 4
 substituents selected from, for example, halogen atoms, lower alkyl groups
 preferably having 1 to 4 carbon atoms and a phenyl group.
 In the production of the aromatic polyamide fibers of the present invention
 having the specific crystal size properties (1), (2) and (3), the aromatic
 polyamide resin having the recurring units of the formula (I) is formed
 into fibers and the fibers are passed through a high temperature
 atmosphere or are brought into contact with a high temperature heater to
 impart a thermal history thereto. Alternatively, other aromatic polyamide
 fibers produced by a conventional fiber-producing process are passed
 through or stored in a high temperature atmosphere, or are brought into
 contact with a high temperature heater to impart a thermal history to the
 fibers in an after treatment step.
 The principle process for producing the above-mentioned aromatic polyamide
 fibers and the properties of the fibers are disclosed in, for example, UK
 Patent No. 1,501,948, U.S Pat. No. 3,733,964, U.S. Pat. No. 3,767,756, and
 U.S. Pat. No. 3,869,429, and Japanese Unexamined Patent Publications No.
 49-100,322, No. 47-10,863, No. 58-144,152, and No. 4-65,513. In the
 patents and publications, p-type aromatic polyamide fibers are disclosed
 as heat-resistant fibers. In these p-type aromatic polyamide fibers, 50
 molar % or more of the groups Ar.sub.1 and Ar.sub.2 are p-oriented
 aromatic groups. Particularly, the p-type aromatic polyamide fibers are
 poly (p-phenylene terephthalamide) staple fibers (available under the
 trademark of KEVLER, from Du Pont) and copoly
 (p-phenyllene/3,4'-oxydiphenylene terephthalamide) staple fibers
 (available under the trade mark of TECHNORA, made by TEIJIN). The latter
 aromatic copolyamide staple fibers have a low impurity ion content, thus
 exhibits an excellent electric insulation property and are preferably
 employed in practice.
 The copoly (p-phenylene/3,4'-oxydiphenylene terephthalamide) fibers having
 cation-exchanging and non-ion-absorbing solid inorganic compound particles
 fixed to the peripheral surfaces of the fibers, exhibit a high mixed
 varnish-penetration property and an enhanced adhesion to the mixed varnish
 through the inorganic compound particles. Thus in the production of
 electric insulating material and laminates for electric circuit boards,
 the copolyamide fibers contribute to decreasing the deformation of the
 electric insulating material or the electric circuit board and to
 enhancing the electric insulating property and the thermal dimensional
 stability of the material or board at a high humidity.
 The cation-exchanging, non-ion-absorbing inorganic compounds include
 inorganic compounds, for example, silica, alumina-magnesia, kaolin, acid
 clay, active clay, talc, bentonite and hydrated aluminum silicate (osmose)
 which are capable of exchanging cations and of absorbing non-ions.
 When these inorganic compounds are fixed in the state of a solid on the
 peripheral surfaces of the fibers, the adhesive properties of the fiber
 surfaces are enhanced. The solid particles of the inorganic compounds
 preferably have a particle size of about 0.01 to 5.00 .mu.m. To fix the
 solid inorganic compound particles to the fiber surfaces, for example, the
 particles are brought into contact with and pressed under pressure on the
 surfaces of the fibers which have been softened, to embed the solid
 particles into the surface portions of the softened fibers.
 The specific aromatic polyamide fibers of the present invention having the
 specific crystal size features can be produced by applying a severe heat
 treatment which is not taught or suggested by conventional heat treatments
 to the fibers during the fiber-producing procedures, or by applying the
 severe heat treatment, as an after-treatment, to the fibers, or by
 producing the aromatic polyamide from a low moisture-absorbing material
 partially substituted with substituents, for example, halogen atoms.
 When the aromatic polyamide fibers of the present invention are employed
 for an electric insulting material, the impurity ions such as extracted
 sodium ions, extracted potassium ions and extracted chlorine ions,
 contained in the inside of the fibers may cause a problem of a decreased
 insulation. Therefore, during the fiber-producing procedures or during the
 fiber-after treatment procedures, the fibers are fully washed to such an
 extent that preferably the following requirements (1), (2) and (3) are
 satisfied, more preferably the following requirements (4), (5) and (6) are
 satisfied.
 (1) Extracted sodium ion content .ltoreq.40 mg/liter
 (2) Extracted potassium ion content .ltoreq.3.0 mg/liter
 (3) Extracted chloride ion content .ltoreq.7.5 mg/liter
 (4) Extracted sodium ion content .ltoreq.35 mg/liter
 (5) Extracted potassium ion content .ltoreq.2.5 mg/liter
 (6) Extracted chloride ion content .ltoreq.6.5 mg/liter
 When the requirements (1), (2) and/or (3) are not satisfied, the resultant
 aromatic polyamide fibers may exhibit an unsatisfactory electric
 insulating property and thus may not be usable for an electric insulting
 material.
 The m-type aromatic polyamide staple fibers usable for the present
 invention include staple fibers comprising at least one member selected
 from homopolymers and copolymers comprising the recurring units of the
 formula (I) wherein 50 to 100 molar % of Ar.sub.1 and Ar.sub.2 are
 meta-type divalent aromatic groups. For example, in the m-type aromatic
 polyamides, the dicarboxylic acid component comprises at least one member
 selected from terephilalic acid and isophthalic acid and the diamine
 component comprises at least one member selected from m-phenylenediamine,
 4,4'-diaminophenylether, 4,4'-diaminodiphenylmethane, and xylylenediamine.
 Typical m-type aromatic polyamides include poly-m-phenyleneisophthalamide,
 poly-m-xylyleneterephthalamide and copolyamides of isophthalic acid
 chloride and/or terephthalic acid chloride with metaphenylenediamine.
 Generally, the m-type aromatic polyamide fibers exhibit a higher
 equilibrium moisture content (water content) and a higher impurity ion
 content than those of the p-type aromatic polyamide fibers and thus cause
 the fiber-reinforced resin materials to exhibit a reduced electric
 insulating property, particularly at a high humidity. Therefore, when the
 fiber-reinforced resin material is used for base materials of electric
 insulating materials or for laminate materials for electric circuit
 boards, the m-type aromatic polyamide fibers should be fully washed to
 reduce the impurity ion content or should have a more severe heat history
 than that of the p-type aromatic polyamide fibers.
 The p-type aromatic polyamide fibers usable for the present invention,
 having an ultimate elongation of 6% or more and capable of easily
 deforming, can be produced by controlling the fiber production conditions.
 For example, in the production procedure of
 copoly(p-phenylene/3,4'-oxydiphenylene terephthalamide) fibers, the draw
 ratio is controlled to a low level to prevent or restrict the promotion of
 crystallization, or the heat history is restricted. By these controls, the
 resultant p-type aromatic polyamide fibers of the present invention can
 exhibit unique properties definitely different from those of the
 conventional copoly(p-phenylene/3,4'-oxydiphenylene terephthalamide)
 fibers, namely a tensile strength of 10 g/d or less, an ultimate
 elongation of 6% or more and can be easily softened, deformed and melted
 at a relatively low temperature, although the chemical structure of the
 p-type aromatic polyamide fibers of the present invention is the same as
 that of the conventional p-type aromatic polyamide fibers. In this case,
 when the cation-exchanging and non-ion-absorbing inorganic compound is
 fixed in the state of a solid onto the peripheral surfaces of the fibers,
 the resultant inorganic compound particles exhibit an enhanced mixed
 varnish-penetration property, and the fibers can be firmly adhered to the
 mixed varnish through the fixed inorganic compound particles layer. Thus,
 in the production of the electric insulating materials or laminate
 materials for electric circuit boards, the deformation of the target
 materials can be reduced, and the electric insulating property and thermal
 dimensional stability of the target material can be enhanced.
 Further, the p-type aromatic polyamide fibers of the present invention,
 having the high elongating property, exhibit reduced impurity ion content
 and moisture content in comparison with those of the m-type aromatic
 polyamide fibers. Therefore, the p-type aromatic polyamide fibers of the
 present invention are useful as a material for base materials of electric
 insulating materials and laminate materials for electric circuit boards
 which need a high electric insulating property, particularly a high
 reliability in the electric insulating property at a high humidity.
 The organic resinous binder usable for the present invention preferably
 comprises at least one thermosetting resin selected from, for example,
 epoxy resins, phenol-formaldehyde resins, polyurethane resins and
 melamine-formaldehyde resins. Among these resins, the epoxy resins which
 have at least one functional epoxy group per molecule thereof and are
 dispersible in water have a high compatibility with the mixed varnish
 which is impregnated in the resin during the production of a pre-preg.
 The fibrids usable for the present invention and comprising an organic
 polymer material include all the fine and short fibers and flakes having
 an equilibrium moisture content of 8.0% or less, exhibiting a binding
 property for the sheet-forming fibers during a wet fiber-sheet forming
 procedure and in the form of thin flakes, fine scales or randomly
 fibrillated fine and short fibers. The fibrids are selected from, for
 example, fibrids produced, in accordance with the methods disclosed in
 Japanese Examined Patent Publications No. 35-11,851 and No. 37-5,732, by
 mixing a solution of an organic polymer with a precipitating agent for the
 organic polymer while applying a shearing force to the mixture system, and
 another fibrids produced, in accordance with the method as disclosed in
 Japanese Examined Patent Publication No. 59-603, by randomly fibrillating
 a shaped article produced from a polymer solution exhibiting an optical
 anisotropy and having a high molecular orientation, with a mechanical
 shearing force, for example, a beating force. Among these fibrids, the
 former fibrids are preferably employed for the present invention.
 The organic polymers usable for the binding fibrids are not limited the
 specific type of polymers as long as the polymers have a fiber or
 film-forming property and exhibit a thermal decomposition-starting
 temperature of 300.degree. C. or more.
 For example, the polymers for the binder fibrids may be selected from
 wholly aromatic polyesters having a liquid crystalline property when
 melted, and aromatic polymers having at least one heterocyclic structure.
 Among these polymers, copoly(p-phenylene/3,4'-oxydiphenylene
 terephthalamide (available under the trademark of TECHNORA, from TEIJIN)
 having a low impurity ion content, wholly aromatic polyesters (available
 under the trademark of VECTRAN, from KURARAY) which comprise, as acid
 components, p-hydroxybenzoic acid and 2,6-hydroxynaphthoic acid and
 exhibit a liquid crystalline property when melted, and a low equilibrium
 moisture content. When a high heat resistance is needed, poly-p-phenylene
 benzoxazole &lt;PBO, made by TOYOBO&gt; is advantageously used.
 In the heat resistant wholly aromatic polyamide fiber sheet of the present
 invention, the content of the binder comprising at least one member
 selected from the organic resinous binders and the organic polymer fibrids
 is preferably 4 to 40% by weight, more preferably 5 to 30% by weight,
 based on the total weight of the sheet. When the content of the binder
 fibrids is controlled to a relatively low level, for example, the fibrids
 produced by the fibrid-producing method disclosed in Japanese Examined
 Patent Publications No. 35-11,851 and No. 37-5,732, are advantageously
 employed. Also, where the content of the binder fibrids is adjusted to a
 relatively high level, the fibrids produced by the method disclosed in,
 for example, Japanese Patent Publication No. 59-603 are advantageously
 employed. The fibrids produced in accordance with the methods of the
 above-mentioned Japanese publications may be employed together.
 When the content of the binder fibrids is less than 4% by weight, the
 resultant aromatic polyamide fiber sheet produced by a wet paper-forming
 method may not exhibit a tensile strength sufficient to forming the fiber
 sheet. Also, when the content of the binder fibrids is more than 40% by
 weight, the resultant aromatic polyamide fiber sheet may exhibit too high
 a bulk density and may hinder the penetration of a mixed varnish into the
 sheet. The organic polymer fibrids include fibrids which shrink when
 moisture is removed therefrom, and other fibrids which elongate when
 moisture is removed therefrom.
 When both the shrinking fibrids and the elongating fibrids are employed in
 combination with each other in an appropriate combining ratio, the
 resultant heat-resistant aromatic polyamide fiber sheet exhibits an
 enhanced dimensional stability, even when water-washing and drying are
 repeatedly applied to the sheet, a high thermal dimensional stability and
 a high wet dimensional stability. Therefore, two different types of
 fibrids may be used for the aromatic polyamide fiber sheet.
 The aromatic polyamide fibers of the present invention have a large crystal
 sizes in the (110) and (200) planes. Namely, in the fibers,
 crystallization of the aromatic polyamide is proceeded to a large extent
 and the content of amorphous portions is reduced. Thus, even in the
 atmosphere in which a high temperature and a low temperature are
 repeatedly generated, the aromatic polyamide fibers of the present
 invention exhibit a small dimensional change both in the fiber axis
 direction and the transverse direction at right angles to the fiber axis
 direction. Also, the dimensional change of the aromatic polyamide fibers
 of the present invention due to the repeated moisture-absorption and
 desorption in the fiber axis and transverse directions is very small.
 Also, the aromatic polyamide fibers of the present invention satisfying the
 crystal size requirements (1), (2) and (3) exhibit a specifically low
 thermal linear expansion coefficient within the range of minus values,
 namely from -1.0.times.10.sup.-6 /.degree. C. to -7.5.times.10.sup.-6
 /.degree. C., and thus are appropriate as reinforcing fibers for resins
 having a positive thermal linear expansion coefficient and enable the
 thermal linear expansion coefficient of the fiber reinforced resin
 material to unlimitedly approach zero.
 The aromatic polyamide fibers of the present invention satisfying the
 crystal size requirements (1), (2) and (3) exhibit a low equilibrium
 moisture content of 3.2% or less, particularly, when the crystal sizes (A)
 and (B) are within specific ranges 2.7% or less, more particularly 2.0% or
 less, and thus are definitely distinguished from the conventional aromatic
 polyamide fibers which generally have an equilibrium moisture content of
 4.5% or more. Therefore, no peeling occurs at interfaces between the resin
 and the reinforcing aromatic polyamide fibers of the present invention.
 The aromatic polyamide fiber sheet formed from the aromatic polyamide
 fibers of the present invention exhibit, even when an organic resinous
 binder having a relatively high impurity ion content is employed, an
 extracted sodium ion content of 75 mg/liter or less, an extracted
 potassium ion content of 35 mg/liter or less and an extracted chlorine ion
 content of 95 mg/liter. When the crystal sizes (A) and (B) are optimized
 and the fiber sheet are fully washed, the impurity ion contents of the
 aromatic polyamide fiber sheet can be decreased to the levels of: 60
 mg/liter or less of the extracted sodium ion content, 20 mg/liter or less
 of the extracted potassium ion content and 80 ml/liter or less of the
 extracted chlorine ion content.
 Further, when the p-type aromatic polyamide fibers of the present invention
 having a high ultimate elongation and capable of being easily deformed are
 used under optimum conditions, the extracted impurity ion contents can be
 decreased to the levels of: 50 mg/liter or less of the extracted sodium
 ion content, 17 mg/liter or less of the extracted potassium ion content
 and 0.5 mg/liter or less of the extracted chlorine ion content. Also, the
 aromatic polyamide fiber sheet of the present invention has a low moisture
 absorption and thus even when an electric current is applied to the sheet
 at a high humidity, no movement of the impurity ions occurs.
 When the aromatic polyamide fibers are cut, the occurrence of the
 fibril-formation at the cut face and the unsatisfactory cutting is greatly
 influenced by the apparent crystal sizes (A) and (B) of the aromatic
 polyamide fibers. Namely, when the apparent crystal sizes (A) and (B) are
 too small, while the fibril-formation at the cut faces is restricted, the
 resultant fibers exhibit a decreased rigidity and can be easily deformed.
 Therefore, when a cutting force is applied with a cutting edge to the
 fibers, the fibers can easily escape from the cutting edge, and thus the
 cutting property of the fibers decreases. Also, when the crystal sizes (A)
 and (B) are too large, the cutting property of the resultant aromatic
 polyamide fibers increases. However, the fibers exhibit a reduced
 resistance to finely dividing of the cut face portions along the
 longitudinal axes of the fibers, and thus form fine fibrils projecting
 from the cut faces of the fibers. Therefore, the fibers have a reduced
 shaving property and perforating property.
 In the aromatic polyamide fibers of the present invention, the apparent
 crystal sizes (A) and (B) are controlled to appropriate ranges. Therefore,
 when a resin composite laminate plate reinforced by the aromatic polyamide
 fibers of the present invention is subjected to a drill perforation work,
 the reinforcing fibers can be smoothly cut without fibril-formation on the
 cut faces and without occurrence of unsatisfactory cutting. Therefore, the
 inside wall faces of the resultant holes are smooth and not rough. Also,
 the carbon dioxide gas laser-perforation work can be advantageously
 applied to the resinous materials reinforced by the aromatic polyamide
 fibers of the present invention to obtain smooth holes.

EXAMPLES
 The present invention will be further illustrated by the following
 examples.
 In the examples and comparative examples, the following tests were applied
 to the resultant products.
 (1) Apparent crystal size
 Crystal size (A) in a (110) plane and crystal size (B) in a (200) plane of
 the aromatic polyamide fibers were determined by preparing a wide angle
 X-ray diffraction intensity curve of the fibers; then separating Ganssian
 distribution combined with Cancy distribution shown in the X-ray
 diffraction intensity curve from each other, in accordance with the method
 of Hindeleh et al. (A.M.H in deleh and D. J. Johns, Polymer, 19, 27
 (1978); and from the obtained half width, calculating the apparent crystal
 sizes (A) and (B) in the (110) and (200) planes in accordance with the
 Sherrer's equation shown below.
 Apparent crystal size=K.lambda./(.beta. cos .theta.) wherein K represents a
 constant numeral of 0.94, .lambda. represents a wavelength of x-rays of
 0.154 nm (1.54 angstroms), .beta. represents the half value width in
 radian units of the reflection profile, calculated from the equation:
 .beta..sup.2 =.beta..sub.M.sup.2 -.beta..sub.B.sup.2
 wherein .beta..sub.M represents as measured value and .beta..sub.E
 represents a constant for the measurement apparatus used, and .theta.
 represents a bragg angle.
 (2) Thermal linear expansion coefficient
 The fibers having an initial fiber length of 200 mm between a pair of
 fiber-holding clips were repeatedly heated and cooled between room
 temperature and 250.degree. C. at a heating and cooling rate of 10.degree.
 C./minute, by using a thermal analysis apparatus (TMA, thermoflex type,
 made by RIGAKU DENKI K.K.) and the thermal linear expansion coefficient of
 the fibers was calculated from dimensional changes of the fibers at a
 temperature of 100 to 200.degree. C.
 (3) Specific gravity
 The specific gravity of the fibers was determined by a density gradient
 tube method in n-heptane/tetrachloromethane at a temperature of 25.degree.
 C.
 (4) Equilibrium moisture content
 The equilibrium moisture contents of the fibers and the fiber sheet were
 determined in accordance with JIS L 1013, by the following methods.
 (A) Equilibrium moisture content of fibers
 Aromatic polyamide fibers were absolutely dried in the ambient air
 atmosphere at a temperature of 120.degree. C., left to stand at a
 temperature of 20.degree. C. at a relative humidity (RH) of 65% for 72
 hours to moisture-condition the fibers, and then subjected to a
 measurement of moisture content of the fibers. The moisture content of the
 fibers was indicated by percentage (%) of the weight of water contained in
 the moisture-conditioned fibers based on the absolute dry weight of the
 fibers.
 When the fibers subjected to the moisture content test consisted of a
 mixture of two or more different types of fibers, the moisture contents
 (%) of the individual types of fibers were separately determined, and a
 weight average moisture content of the mixed fibers was calculated from
 the moisture contents of the individual types of fibers and the weight
 proportions of the individual types of fibers based on the total weight of
 the mixed fibers, and indicated in %.
 (B) Equilibrium moisture content of fiber sheet
 A fiber sheet produced by a paper-forming method was absolutely dried at a
 temperature of 120.degree. C. and moisture-conditioned at a temperature of
 20.degree. C. at a relative humidity (RH) of 65% for 72 hours. Then, the
 moisture-conditioned fiber sheet was subjected to a moisture content
 measurement, and the moisture content of the fiber sheet was indicated by
 percentage (%) of the weight of water contained in the
 moisture-conditioned fiber sheet based on the absolute dry weight of the
 fiber sheet.
 (5) Surface electric resistivity of fibers
 With reference to the surface electric resistivity test method in
 accordance with Japanese Industrial Standard (JIS) C 6480, electrodes were
 formed by using a silver paste on a fiber with intervals of 20 mm in the
 longitudinal direction of the fiber, and an electric resistivity between
 the electrodes was measured. The surface electric resistivity of the fiber
 was represented by the measured electric resistivity between the
 electrodes.
 Before the test, the fiber was moisture-conditioned at a temperature of
 20.degree. C. at a relative humidity (RH) of 40% for 24 hours and the
 surface electric resistivity measurement was carried out at a temperature
 of 20.degree. C. at a relative humidity (RH) of 40%.
 (6) Amounts of extracted impurity ions
 Fibers were cut short, immersed in 50 ml of distilled water and
 heat-treated in an autoclave at a temperature of 121.degree. C. under a
 pressure of 2 kg/cm.sup.2 G for 24 hours. The amounts of the extracted
 individual types of impurity ions in the distilled water were determined
 by an ICP analysis and an ion-chromatographic analysis. The amounts of the
 individual types of impurity ions were indicated in mg per liter of the
 extracting distilled water.
 (7) Bulk density of fiber sheet
 The bulk density of each staple fiber sheet was determined in accordance
 with Japanese Industrial Standard (JIS) C 2111, Section 6.1.
 (8) Tensile strength of fiber sheet
 (A) Tensile strength and ultimate elongation of fibers
 The tensile strength and ultimate elongation of fibers were tested in
 accordance with JIS L 1013, Section 7 by using a constant stretching rate
 type tensile tester.
 (B) Tensile strength of fiber sheet
 The tensile strength of a fiber sheet was tested in accordance with JIS C
 2111, Section 7, by using a constant stretching rate type tensile tester.
 (9) Interlaminar peeling strength of fiber sheet
 In the determination of the interlaminar peeling strength of a fiber sheet,
 a sample of the fiber sheet having a length of 200 mm and a width of 15 mm
 was peel-divided at the middle layer portion thereof at a peeling angle of
 180 degrees into two pieces, by a T-peeling method using a constant
 stretching rate type tensile tester. A stress created on the sample was
 measured.
 (10) Thermal dimensional change of fiber sheet
 A fiber sheet having a length of 300 mm and a width of 50 mm was subjected
 to a measurement of the longitudinal length of the sheet before and after
 a heat treatment at a temperature of 280.degree. C. for 5 minutes, by
 using a high accuracy two dimensional coordinate-measurement tester (made
 by Muto Kogyo K.K.). The change (%) in the longitudinal length was
 calculated in accordance with the following equation:
 Dn(%)=(Lb-La)/La.times.100
 wherein Dn represents a thermal dimensional change in percent in the
 longitudinal length of the fiber sheet between before and after the heat
 treatment, Lb represents a longitudinal length of the fiber sheet after
 the heat treatment and La represents a longitudinal length of the sheet
 before the heat treatment.
 (11) Dimensional change of fiber sheet due to moisture absorption and
 desorption thereof
 A sample of a fiber sheet was left to stand in an atmosphere at room
 temperature at a relative humidity (RH) of 85% or more for 120 hours or
 more to allow the sample to fully absorb moisture therein. The sample
 having a width of 5 mm was held at two end portions thereof by a pair of
 holders of a thermal analyzer (trademark: Thermoflex type thermal analyzer
 TMA, made by Rigaku Denki K.K.). The initial distance between the holder
 was 200 mm. The sample was heated in the thermal analyzer from room
 temperature to 280.degree. C. at a heating rate of 10.degree. C./minute
 and then cooled to room temperature at a cooling rate of 10.degree.
 C./minute. The heating and cooling procedures were repeated further twice
 or more (three times or more in total), while recording a dimensional
 change of the length of the sample on a chart. The initial length of the
 sample after the moisture-conditioning step and before the first heating
 step was compared with the length of the sample after the second or later
 cooling step was completed. The maximum difference between the length of
 the sample after the moisture-conditioning step and before the first
 heating step and the length of the sample after the second or later
 cooling step was calculated, and then the percentage of the maximum length
 difference (maximum elongation or maximum shrinkage), based on the sample
 length after the moisture-conditioning step and before the first heating
 step was calculated. The lower the length change, the higher the thermal
 dimensional stability and deformation resistance of the sample in the
 temperature- and moisture content-varying conditions.
 (12) Production of laminate for electric circuit board and evaluation of
 cutting processability thereof
 A varnish composition was prepared by preparing a mixture of a high-purity
 brominated bisphenol A-type epoxy resin with an o-cresol novolak type
 epoxy resin, a curing agent consisting of dicyandiamide and a
 curing-promoting agent consisting of 2-ethyl-4-methyl-imidazole and
 dissolving the resultant mixture in a mixed solvent consisting of
 methylethylketone and methyl cellosolve in a mixing ratio of 50/50 by
 weight. A sample of a fiber sheet was impregnated with the varnish
 composition, and then the impregnated varnish composition was dried at a
 temperature of 100 to 120.degree. C. for 5 to 10 minutes, to provide a B
 stage pre-preg sheet having a varnish resin content of 55% by volume.
 Two pieces of the pre-preg sheet were laminated on both the front and back
 surfaces of a first copper foil having a thickness of 35 .mu.m, and then,
 both the front and back surfaces of the resultant laminate was further
 laminated with two pieces of a second copper foil having the same
 thickness as that of the first copper foil. The resultant laminate was
 heat-pressed in a reduced pressure atmosphere at a temperature of
 170.degree. C. under a pressure of 40 kg/cm.sup.2 for 50 minutes, and then
 cured in a hot air dryer at a temperature of 200.degree. C. for about 20
 minuets. A cured laminate for an electric circuit board was obtained.
 The electric circuit board laminate was perforated at five points thereof
 by a high speed-revolving drill having an outer diameter of 0.5 mm at a
 revolving rate of 70,000 turns/minute, and an electron microscopic
 photograph of upper portions of the inside wall surfaces of the resultant
 five cylindrical holes was taken. The cut surface conditions of the holes
 were observed and evaluated into the following three classes.

Class Cutting processability
 3 Best cutting processability
 The inner wall surface of the hole
 has no fine fibrils and is quite
 smooth.
 2 Satisfactory cutting processability
 The inner wall surface of the hole
 has a small number of fine fibrils
 which do not cause any problem in
 practice.
 1 Bad
 The inner wall surface of the hole
 has a large number of fine fibrils
 and is not smooth.
 (13) Deformation of a laminate for electric circuit board
 The same pre-preg sheets as that used in test (12) was laminated on both
 the front and back surfaces of a first copper foil having a thickness of
 18 .mu.m and then second copper foils having a thickness of 35 .mu.m were
 laminated on the pre-preg sheet layers. The resultant laminate was
 heat-pressed by a hot press. The resultant laminate was cut into square
 pieces having length and width of 15 mm; and then central square portions
 of the second copper foils of the square pieces having a length and width
 of 110 mm were removed, so as to retain edge portions of the second copper
 foils in a square frame form having a width of 20 mm, by means of etching,
 to prepare specimens for the test. The resultant partially etched laminate
 pieces were heat-treated at a temperature of 260.degree. C. for 10
 minutes. When the specimens were deformed, due to the heat treatment, the
 maximum deformation of the specimens were measured as a difference in the
 level between the deformed portions and the central portions of the
 specimens. The deformation include warping, distortion, torsion, and
 corrugation.
 (14) Insulation resistance at a high humidity
 The same laminate for electric circuit board before drilling as that in the
 test (12) was used. One surface of the laminate was partially etched to
 form comb-shaped electrodes at intervals of 0.15 mm and consisting of the
 second copper foil. The resultant specimen was stored for 1,000 hours
 while applying a direct voltage of 35 V to the electrodes at a temperature
 of 60.degree. C. at a relative humidity (RH) of 95%, and then in the
 atmosphere at a temperature of 20.degree. C. at a relative humidity (RH)
 of 6% for one hour. After the storage, a direct voltage of 35 to 90 volts
 was applied to the electrodes for 60 seconds, to measure the insulation
 resistance of the specimen
 Example 1
 Poly-p-phenylene terephthalamide fibers having an individual fiber
 thickness of 1.67 d tex (1.50 denier), a specific gravity of 1.442, an
 equilibrium moisture content of about 7.0%, an apparent crystal size at a
 (110) plane of 5.5 nm (55 angstroms) and an apparent crystal size at a
 (200) plane of 6.2 nm (62 angstroms) were suspended in flowing distilled
 water and stirred at a low-stirring rate for about one day to fully wash
 the fibers, and to decrease the impurity ion content of the fibers. Then
 the washed fibers were dried and successively heat-treated in a high
 temperate atmosphere at a temperature of 280.degree. C. for one minute.
 The resultant aromatic polyamide fibers were subjected to a measurement of
 the apparent crystal sizes (A) and (B) in the (110) plane and the (200)
 plane. In the results of the measurement, the apparent crystal size (A) in
 the (110) plane of the fibers was 8.0 nm (80 angstroms), crystal size (B)
 in the (200) plane of the fibers was 8.3 nm (83 angstroms), the product
 (A.times.B) was 66.40. The properties of the fibers tested by the
 above-mentioned test methods are shown in Table 1.
 Comparative Example 1
 The same procedures as in Example 1 were carried out, except that the
 washing in the flowing water and heat-treatment of the aromatic polyamide
 fibers were omitted.
 The resultant aromatic polyamide fibers had an apparent crystal size (A) in
 the (110) plane of 3.5 nm (55 angstroms) and an apparent crystal size (B)
 in the (200) plane of 6.2 nm (62 angstroms) and a product (A.times.B) was
 34.10. The properties of the fibers are shown in Table 1.
 Example 2
 The same procedure as in Example 1 were carried out, except that the
 heat-treatment was applied at a temperature of 325.degree. C. to the
 fibers.
 The resultant aromatic polyamide fibers had an apparent crystal size (A) in
 the (110) plane of 10.2 nm (102 angstroms) and an apparent crystal size
 (B) in the (200) plane of 8.6 nm (86 angstroms) and a product (A.times.B)
 was 87.72. The properties of the fibers are shown in Table 1.
 Example 3
 Poly-p-phenylene terephthalamide fibers having an individual fiber
 thickness of 1.58 d tex (1.42 denier), a specific gravity of 1.455, an
 equilibrium moisture content of about 4.5%, an apparent crystal size at a
 (110) plane of 7.2 nm (72 angstroms) and an apparent crystal size at a
 (200 plane) of 8.5 nm (85 angstroms) were washed in the same manner as in
 Example 1 to fully wash the fibers, and to decrease the impurity ion
 content of the fibers. Then the washed fibers were dried and successively
 heat-treated in a high temperature atmosphere at a temperature of
 400.degree. C. for one minute.
 The resultant aromatic polyamide fibers were subjected to a measurement of
 the crystal sizes (A) and (B) in the (110) plane and the (200) plane. In
 the results of the measurement, the apparent crystal size (A) in the (110)
 plane of the fibers was 15.4 nm (154 angstroms), the apparent crystal size
 (B) in the (200) plane of the fibers was 12.2 nm (122 angstroms), the
 product (A.times.B) was 187.88. The properties of the fibers tested by the
 above-mentioned test methods are shown in Table 1.
 Example 4
 The same procedures as in Example 3 were carried out, except that the
 heat-treatment was applied at a temperature of 450.degree. C. to the
 fibers.
 The resultant aromatic polyamide fibers had an apparent crystal size (A) in
 the (110) plane of 17.0 nm (170 angstroms) and an apparent crystal size
 (B) in the (200) plane of 14.5 nm (145 angstroms) and a product
 (A.times.B) was 246.50. The properties of the fibers are shown in Table 1.
 Comparative Example 2
 The same procedure as in Example 3 were carried out, except that the
 heat-treatment was applied at a temperature of 550.degree. C. to the
 fibers.
 The resultant aromatic polyamide fibers had an apparent crystal size (A) in
 the (110) plane of 28.5 nm (285 angstroms) and an apparent crystal size
 (B) in the (200) plane of 22.2 nm (222 angstroms) and a product
 (A.times.B) was 632.70. The properties of the fibers are shown in Table 1.
 The resultant fibers were thermally oxidized on the surface portions
 thereof, colored into a brown color, exhibited a low surface electric
 resistivity of 3.times.10.sup.-8 .OMEGA./cm and thus were useless as an
 electric insulating material in practice.
 Example 5
 The same procedures as in Example 3 were carried out, except that after the
 impurity ion content of the fibers was decreased by washing the fibers
 with an industrial flowing water while immersing the fibers in the water,
 the washed fibers were brought into contact with a super high temperature
 heater at a temperature of 550.degree. C. under tension for 0.6 second,
 while forwarding on the heater to heat-treat the fibers.
 The resultant aromatic polyamide fibers had an apparent crystal size (A) in
 the (110) plane of 10.7 nm (107 angstroms) and an apparent crystal size
 (B) in the (200) plane of 9.2 nm (92 angstroms) and a product (A.times.B)
 was 98.44. The properties of the fibers are shown in Table 1.
 Example 6
 The same procedures as in Example 3 were carried out, except that after the
 impurity ion content of the fibers were decreased by immersing the fibers
 in an industrial flowing water and washing the fibers with the flowing
 water, the washed fibers were brought into contact with a super high
 temperature of 550.degree. C. under tension for 0.5 second while
 forwarding the fibers along the heater, to heat-treat the fibers.
 The resultant aromatic polyamide fibers had an apparent crystal size (A) in
 the (110) plane of 28.0 nm (280 angstroms), an apparent crystal size (B)
 in the (200) plane of 22.4 nm (224 angstroms) and a product (A.times.B)
 was 627.20.
 The physical properties of the aromatic polyamide fibers are shown in Table
 1.
 Example 7
 The same procedures as in Example 6 were carried out, except that the
 temperature of the heater was changed to 600.degree. C., the contact time
 of the fibers with the heater was changed to 0.6 second.
 The resultant aromatic polyamide fibers had an apparent crystal size (A) in
 the (110) plane of 27.5 nm (275 angstroms), an apparent crystal size (B)
 in the (200) plane of 20.8 nm (208 angstroms) and a product (A.times.B) of
 572.00.
 The physical properties of the aromatic polyamide fibers are shown in Table
 1.
 Comparative Example 3
 The same procedures as in Example 6 were carried out, except that the
 temperature of the heater was changed to 650.degree. C. and the contact
 time of the fibers with the heater was changed to 0.9 second.
 The resultant aromatic polyamide fibers had an apparent crystal size (A) in
 the (110) plane of 28.2 nm (282 angstroms), an apparent crystal size (B)
 in the (200) plane of 22.5 nm (225 angstroms) and a product (A.times.B)
 was 634.50.
 The physical properties of the aromatic polyamide fibers are shown in Table
 1. In the resultant fibers, the surface portions of the fibers were
 heat-oxidized and colored brown. The resultant fibers had a low surface
 electric resistivity of 2.times.10.sup.-9 .OMEGA./cm and thus are suitable
 as an electric insulating material in practice.
 TABLE 1
 Principal properties of fibers
 Thermal
 Crystal size linear Equilibrium Surface
 Impurity ion content
 (110) (200) expansion moisture resis-
 Sodium Potassium Chloride
 Item plane plane Product Specific coefficient content tivity
 ions ions ions
 Example No. (nm) (nm) A .times. B gravity (.times.10.sup.-6 /.degree.
 C.) (%) (.OMEGA./cm) (mg/l) (mg/l) (mg/l)
 Example 1 8.0 8.3 66.40 1.449 -4.1 3.1
 10.sup.12 15.2 1.2 2.2
 2 10.2 8.6 87.72 1.462 -3.9 2.6
 10.sup.13 14.5 0.9 1.9
 3 15.4 12.2 187.88 1.464 -4.1 1.7
 10.sup.14 10.0 0.4 2.7
 4 17.0 14.5 246.50 1.466 -3.7 1.6
 10.sup.14 10.3 0.4 2.5
 5 10.7 9.2 98.44 1.468 -4.0 1.9
 10.sup.14 20.3 1.3 2.4
 6 28.0 22.4 627.20 1.477 -2.5 1.4
 10.sup.12 16.6 0.6 4.4
 7 27.5 20.8 572.00 1.472 -2.8 1.6
 10.sup.13 17.5 0.6 4.9
 Comparative 1 5.5 6.2 34.10 1.422 -4.3 6.9
 10.sup.11 52.1 1.5 4.8
 Example 2 28.5 22.2 632.70 1.475 -2.4 1.4
 10.sup.8 9.8 0.4 4.3
 3 28.2 22.5 634.50 1.478 -2.2 1.4
 10.sup.9 17.5 0.7 6.1
 Example 8
 A staple fiber blend comprising 94% by weight of the same aromatic
 polyamide (poly-p-phenylene terephthalamide) fibers as in Example 4 having
 a fiber length of 3 mm, an equilibrium moisture content of 1.6% and 6% by
 weight of fibrids comprising a copoly(p-phenylene/3,4'-oxydiphenylene
 terephthalamide), made by TEIJIN and an equilibrium moisture content of
 4.1% were suspended in water by using a pulper and mixed with 0.02% by
 weight of a dispersing agent (Trademark: YM-80, made by MATSUMOTO YUSHI
 K.K., to provide an aqueous slurry having a staple fiber-fibrid total
 content of 0.15% by weight, for a sheet-forming procedure.
 The aqueous staple fiber slurry was subjected to a paper-forming procedure
 using a TAPPI hand paper-forming machine, the resultant wet staple fiber
 web was lightly dewatered under pressure, the dewatered wet staple fiber
 web was dried in a hot air drier at a temperature of 150.degree. C. for 15
 minutes. A heat-resistant fiber sheet was obtained. Then the dried staple
 fiber web was heat-tested by using a calender machine with a pair of metal
 rolls having hardened surfaces and a diameter of about 350 mm, at a
 temperature of 200.degree. C. under a linear pressure of 160 kg/cm, then
 further heat pressed by using a high temperature high calendering machine
 with a pair of metal rolls having hardened surfaces and a diameter of
 about 400 mm, at a temperature of 320.degree. C. under a linear pressure
 of 200 kg/cm, to soften and/or melt the
 copoly(p-phenylene/3,4'-oxydiphenylene terephthalamide) fibrids and to
 firmly bond the poly-p-phenylene terephthalamide staple fibers to each
 other through the softened and/or molten fibrids. An aromatic polyamide
 fiber sheet having a basis weight of 72 g/m.sup.2 was obtained.
 The resultant aromatic polyamide fiber sheet had an equilibrium moisture
 content of 1.73%. The sheet-production conditions of the fiber sheet are
 shown in Table 2 and the properties of the fiber sheet determined by the
 above-mentioned tests are shown in Table 3.
 Also, a pre-preg was produced from the fiber sheet and a laminate for an
 electric circuit board was produced from the pre-preg, by the method as
 shown in the test (12)
 The laminated was tested by the tests 12, 13 and 14. The test results are
 shown in Table 3.
 Comparative Example 4
 An aromatic polyamide fiber sheet was produced by the same procedures as in
 Example 8, except that the fiber blend comprised 98% by weight of the
 aromatic polyamide staple fibers and 2% by weight of
 copoly(p-phenylene/3,4'-oxydiphenylene terephthalate) fibrids.
 The production conditions of the fiber sheet is shown in Table 2, and the
 properties of the fiber sheet tested by the above-mentioned tests are
 shown in Table 3. The properties of the laminate for electric circuit
 boards produced from the fiber sheet was tested by the tests (12), (13)
 and (14). The test results are shown in Table 3.
 Examples 9 to 13 and Comparative Examples 5 and 6
 In each of Examples 9 to 13 and Comparative Examples 5 and 6, an aromatic
 polyamide fiber sheet was produced by the same procedures as in Example 8,
 except that the fiber sheet was prepared from a staple fiber blend
 comprising (1) the same aromatic polyamide staple fibers as in Example 4,
 having a fiber length of 3 mm and an equilibrium moisture content of 1.6%;
 (2) aromatic polyamide staple fibers comprising a
 copoly(p-phenylene/3,4'-oxydiphenylene terephthalamide, surface-coated
 with 0.5% by weight of talc and 0.1% by weight of osmose and having an
 individual fiber thickness of 1.67 d tex (1.50 denier), a fiber length of
 3 mm and an equilibrium moisture content of 1.8% which fibers are
 available under a trademark of TECHNORA from TEIJIN); and (3)
 heat-resistant organic polymer fibrids (made by TEIJIN) comprising a
 copoly(p-phenylene/3,4'-oxydiphenylene terephthalamide) having an
 equilibrium moisture content of 4.1%, in the proportions as shown in Table
 2. The staple fiber blend was suspended in water, and converted to a fiber
 sheet in the same method as in Example 8.
 The production conditions of the fiber sheet are shown in Table 2 and the
 test results of the fiber sheet are shown in Table 3.
 Also, a laminate for electric circuit boards was prepared from a pre-preg
 produced from the fiber sheet impregnated with the varnish composition.
 The results of the tests (12), (13) and (14) are shown in Table 3.
 Comparative Example 7
 An aromatic polyamide fiber sheet was produced by the same procedures as in
 Example 8, except that the same aromatic polyamide staple fibers as in
 Example 8 were used in an amount of 98% by weight, and the
 copoly(p-phenylene/3,4'-oxydiphenylene terephthamide) fibrids were
 replaced by 2% by weight of an epoxy resin binder which was employed in
 the state of an aqueous solution having a dry solid content of the binder
 of 10% by weight and applied by a spray method during the fiber
 sheet-forming procedure.
 The production conditions of the fiber sheet are shown in Table 2 and the
 test results of the fiber sheet are shown in Table 3.
 Also, the fiber sheet was converted to a pre-preg and then further to a
 laminate for electric circuit boards by the method shown in the test (12).
 The results of the tests (12), (13) and (14) applied to the laminate are
 shown in Table 3.
 Examples 14 to 15 and Comparative Examples 8 and 9
 An aromatic polyamide fiber sheet was produced by the same procedures as in
 Example 10, except that the same aromatic polyamide staple fibers as in
 Example 4, and copoly(p-phenylene/3,4'-oxydiphenylene terephthalamide
 staple fibers (trademark: TECHNORA, made by TEIJIN) having an individual
 fiber thickness of 1.67 d tex (1.50 denier), a fiber length of 3 mm, and
 an equilibrium moisture content of 1.8% and surface-content with 0.5% by
 weight of talc and 0.1% by weight of osmose, were blended in the blend
 proportions shown in Table 2.
 Example 16
 An aromatic polyamide fiber sheet was produced by the same procedures as in
 Example 10, except that the same aromatic polyamide staple fibers as in
 Example 2, having a fiber length of 3 mm and an equilibrium moisture
 content of 2.6% and copoly(p-phenylene/3,4'-oxydiphenylene terephthalamide
 staple fibers (trademark: TECHNORA, made by TEIJIN) having an individual
 fiber thickness of 1.67 d tex (1.50 denier), a fiber length of 3 mm, and
 an equilibrium moisture content of 1.8% and surface-coated with 0.5% by
 weight of talc and 0.1% by weight of osmose, were used.
 The production conditions of the fiber sheet are shown in Table 2 and the
 test results of the fiber sheet are shown in Table 3.
 The contents of the impurity ions contained in the fiber sheet were
 measured by the above-mentioned test (6). In the test results, the
 extracted sodium ion content was 38 mg/liter, the extracted potassium ion
 content was 3.8 mg/liter and the extracted chlorine ion content was 7.5
 mg/liter. These impurity ion contents were satisfactory.
 Also, the fiber sheet was converted to a pre-preg impregnated with the
 varnish composition and then further to a laminate for electric circuit
 boards by the method shown in the test (12). The results of the tests
 (12), (13) and (14) applied to the laminate are shown in Table 3.
 Examples 17 to 23
 In each of Examples 17 to 23, an aromatic polyamide fiber sheet was
 produced by the same procedures as in Example 10, except that the same
 aromatic polyamide staple fibers as in Example 4, having a fiber length of
 3 mm and an equilibrium moisture content of 1.6%,
 copoly(p-phenylene/3,4'-oxydiphenylene terephthalamide staple fibers
 (trademark: TECHNORA, made by TEIJIN) having an individual fiber thickness
 of 1.67 d tex (1.50 denier), a fiber length of 3 mm and an equilibrium
 moisture content of 1.8% and surface-coated with 0.5% by weight of talc
 and 0.1% by weight of osmose, poly-m-phenylene isophthalamide staple
 fibers (trademark: CORNEX, made by TEIJIN) having an individual fiber
 thickness of 3.33 d tex (3.0 denier), and a fiber length of 5 mm, and
 copoly(p-phenylene/3,4'-oxydiphenylene terephthalamide fibrids (made by
 TEIJIN), which has a high heat resistance and an equilibrium moisture
 content of 4.1%, were mixed with each other in mixing proportions as shown
 in Table 2, and the mixed staple fibers and fibrids were suspended in
 water.
 The production conditions of the fiber sheet are shown in Table 2 and the
 test results of the fiber sheet are shown in Table 3.
 Also, the fiber sheet was converted to a pre-preg and then further to a
 laminate for electric circuit boards by the method shown in the test (12).
 The results of the tests (12), (13) and (14) applied to the laminate are
 shown in Table 3.
 Examples 24 to 29
 In each of Examples 24 to 29, an aromatic polyamide fiber sheet was
 produced by the same procedures as in Example 19, except that, the
 poly-m-phenylene isophthalamide staple fibers of Example 19 were replaced
 in Example 24 by poly-p-phenylene benzo-bisthiazole staple fibers; in
 Example 25 by wholly aromatic polyester staple fibers exhibiting a liquid
 crystalline property when melted; in Example 26 by polyphenylene sulfide
 staple/fibers; in Example 27 by polyetherimide staple fibers; in Example
 28 by polyetheretherketone staple fibers; and in Example 29 by
 polytetrafluoroethylene staple fibers.
 The production conditions of the fiber sheet are shown in Table 2 and the
 test results of the fiber sheet are shown in Table 3.
 Also, the fiber sheet was converted to a pre-preg and then further to a
 laminate for electric circuit boards by the method shown in the test (12).
 The results of the tests (12), (13) and (14) applied to the laminate are
 shown in Table 3.
 TABLE 2
 Mixing of component
 fibers (wt %)
 Binder
 Staple Staple component High calendering
 fibers fibers fibrid/ conditions
 A/B C/D/E resin (C. .degree.) (kg/cm)
 Comparative 4 98/0 0/0/0 2/0 320 200
 Example 5 6/92 0/0/0 2/0 320 200
 Example 8 94/0 0/0/0 6/0 320 200
 9 6/88 0/0/0 6/0 320 200
 10 15/75 0/0/0 10/0 320 200
 11 20/60 0/0/0 20/0 320 200
 12 20/45 0/0/0 35/0 320 200
 13 45/20 0/0/0 35/0 320 200
 Comparative 6 15/35 0/0/0 50/0 320 200
 Example 7 98/0 0/0/0 0/2 320 200
 8 6/92 0/0/0 0/2 320 200
 Example 14 15/75 0/0/0 0/10 320 200
 15 20/45 0/0/0 0/35 320 200
 Comparative 9 45/20 0/0/0 0/50 320 200
 Example
 Example 16 15/75 0/0/0 10/0 320 200
 17 15/73 2/0/0 10/0 320 200
 18 15/70 5/0/0 10/0 320 200
 19 15/60 15/0/0 10/0 320 200
 20 5/55 20/0/0 10/0 320 200
 21 5/70 15/0/0 10/0 320 200
 22 30/45 15/0/0 10/0 320 200
 23 55/20 15/0/0 10/0 320 200
 24 15/60 0/0/15 10/0 320 200
 25 15/60 0/0/15 10/0 320 200
 26 15/60 0/0/15 10/0 260 200
 27 15/60 0/0/15 10/0 260 200
 28 15/60 0/0/15 10/0 300 200
 29 15/60 0/0/15 10/0 300 200
 [Note] Staple fiber A: Poly-p-phenylene terephthalamide staple fibers
 Staple fiber B: Copoly(p-phenylene/3,4'-oxydiphenylene terephthalate staple
 fibers
 Staple fiber C: poly-m-phenylene isophthalamide staple fibers
 Staple fiber D: Copoly(p-phenylene/3,4'-oxydiphenylene terephthalamide
 staple fibers having an ultimate elongation of 8% or more
 Staple fiber E: Heat-resistant organic polymer staple fibers other than the
 above-mentioned staple fibers
 TABLE 3
 Properties of heat resistant fiber sheet
 Dimensional
 change
 due to
 Inter- Thermal Equili- moisture-
 Tensile laminar dimen- brium absorption
 Electric circuit board laminate
 Bulk stren- peeling sional moisture and
 Deform- Cutting Surface
 Item density gth strength change content desorption
 ation process- resistivity
 Example No. (g/cm.sup.3) (kg/15 mm) (g/15 mm) (%) (%)
 (.mu.m) (mm) ability (.OMEGA./cm)
 Comparative 4 0.29 1.1 9.6 0.09 1.68 16
 1.2 3-2 10.sup.9
 Example 5 0.35 1.3 10.2 0.12 1.85 39
 1.9 3-2 10.sup.9
 Example 8 0.66 3.4 29.5 0.02 1.73 12
 0.8 2-3 10.sup.11
 9 0.68 3.8 43.2 0.02 1.91 14
 0.9 2-3 10.sup.12
 10 0.70 5.8 44.8 0.01 1.94 9
 0.6 3 10.sup.13
 11 0.77 6.9 56.3 0.04 2.06 21
 1.0 3 10.sup.13
 12 0.82 8.1 62.4 0.06 2.24 25
 1.4 3 10.sup.12
 13 0.80 6.2 57.8 0.03 2.19 16
 0.9 2-3 10.sup.11
 Comparative 6 0.91 10.2 81.1 0.18 2.51 55
 2.2 2-3 10.sup.8
 Example 7 0.21 0.8 7.5 0.24 1.71 21
 2.6 1-2 10.sup.9
 8 0.29 1.1 9.2 0.36 1.89 44
 3.5 1-2 10.sup.9
 Example 14 0.42 2.2 17.8 0.15 2.22 43
 1.8 2-3 10.sup.13
 15 0.67 5.2 64.6 0.22 2.68 35
 2.6 2-3 10.sup.12
 Comparative 9 0.77 6.7 66.8 0.29 3.21 31
 2.8 2-3 10.sup.8
 Example
 Example 16 0.64 4.1 38.8 0.03 2.10 22
 0.9 2-3 10.sup.11
 17 0.72 6.1 45.7 0.02 2.02 10
 0.8 2-3 10.sup.13
 18 0.73 6.3 46.2 0.03 2.14 7
 0.9 3 10.sup.13
 19 0.75 6.6 47.1 0.05 2.45 5
 0.7 3 10.sup.12
 20 0.78 7.1 50.2 0.08 2.62 8
 1.4 3 10.sup.11
 21 0.67 4.5 31.7 0.02 2.46 16
 0.7 3 10.sup.12
 22 0.57 4.3 33.6 0.01 2.50 10
 0.6 3 10.sup.12
 23 0.49 4.7 41.2 0.00 2.35 3
 0.4 3 10.sup.11
 24 0.69 4.8 32.9 0.02 1.66 4
 0.9 3 10.sup.12
 25 0.78 7.2 62.1 0.07 1.67 20
 1.2 2-3 10.sup.12
 26 0.76 6.6 42.4 0.37 1.65 46
 3.4 2-3 10.sup.10
 27 0.75 6.3 42.1 0.28 1.71 42
 2.8 2-3 10.sup.11
 28 0.68 5.7 39.1 0.04 1.71 15
 0.9 3 10.sup.13
 29 0.59 4.5 37.6 0.06 1.64 18
 1.1 2 10.sup.12
 Example 30
 An aromatic polyamide fiber sheet was produced by the same procedures as in
 Example 10 with the following exceptions.
 The fiber sheet was prepared from:
 (1) 15% by weight of the same aromatic polyamide staple fibers (A) as in
 Example 4, having a fiber length of 3 mm and an equilibrium moisture
 content of 1.6%;
 (2) 70% by weight of copoly(p-phenylene/3,4'-oxydiphenylene terephthalamide
 staple fibers (B) (trademark: TECHNORA, made by TEIJIN) having an
 individual fiber thickness of 1.67 d tex (1.50 denier), a fiber length of
 3 mm and an equilibrium moisture content of 1.8% and surface-coated with
 0.5% by weight of talc and 0.1% by weight of osmose;
 (3) 7.5% by weight of copoly(p-phenylene/3,4'-oxydiphenylene
 terephthalamide) fibrids having an equilibrium moisture-content of 4.1%,
 as heat-resistant organic polymer fibrids; and
 (4) 7.5% by weight of a water-dispersible epoxy resin binder which was
 applied in the state of an aqueous dispersion thereof having a solid
 content of 1% by weight by a spray method to a precursory fiber sheet
 formed from the above-mentioned fibers (1) and (2) and fibrids (3), during
 the fiber sheet-forming procedure.
 The principal sheet-forming conditions of the fiber sheet is shown in Table
 4, and the properties of the resultant fiber sheet evaluated by the
 above-mentioned tests are shown in Table 5.
 A pre-preg was produced by impregnating the fiber sheet with the varnish
 composition by the procedures as shown in Test (12), and a laminate for an
 electric circuit board was produced from the pre-preg and subjected to the
 tests (12), (13) and (14). The test results are shown in Table 5.
 Example 31
 An aromatic polyamide fiber sheet was produced by the same procedures as in
 Example 30 with the following exceptions.
 A half amount by solid weight of the water-dispersible epoxy resin binder
 applied by the spray method was replaced by a water-dispersible phenol
 resin binder in the state of an aqueous dispersion having a solid content
 of 20% by weight (made by SHOWA KOBUNSHI).
 The principal sheet-forming conditions of the fiber sheet is shown in Table
 4, and the properties of the resultant fiber sheet evaluated by the
 above-mentioned tests are shown in Table 5.
 A pre-preg was produced by impregnating the fiber sheet with the varnish
 composition by the procedures as shown in Test (12), and a laminate for an
 electric circuit board was produced from the pre-preg and subjected to the
 tests (12), (13) and (14). The test results are shown in Table 5.
 Example 32
 An aromatic polyamide fiber sheet was produced by the same procedures as in
 Example 10 with the following exceptions.
 The copoly(p-phenylene/3,4'-oxydiphenylene terephthalamide fibrids were
 replaced by fibrids comprising a wholly aromatic polyester fibrids (made
 by KURARAY) having a liquid crystalline property when method.
 The principal sheet-forming conditions of the fiber sheet is shown in Table
 4, and the properties of the resultant fiber sheet evaluated by the
 above-mentioned tests are shown in Table 5.
 A pre-preg was produced by impregnating the fiber sheet with the varnish
 composition by the procedures as shown in Test (12), and a laminate for an
 electric circuit board was produced from the pre-preg and subjected to the
 tests (12), (13) and (14). The test results are shown in Table 5.
 Example 33
 An aromatic polyamide fiber sheet was produced by the same procedures as in
 Example 10 with the following exceptions.
 The copoly(p-phenylene/3,4'-oxydiphenylene terephthalamide fibrids were
 replaced by poly-p-phenylene benzobis-thiazole fibrids.
 The principal sheet-forming conditions of the fiber sheet is shown in Table
 4, and the properties of the resultant fiber sheet evaluated by the
 above-mentioned tests are shown in Table 5.
 A pre-preg was produced by impregnating the fiber sheet with the varnish
 composition by the procedures as shown in Test (12), and a laminate for an
 electric circuit board was produced from the pre-preg and subjected to the
 tests (12), (13) and (14). The test results are shown in Table 5.
 Examples 34 to 39 and Comparative Examples 10 to 13
 In each of Examples 34 to 39 and Comparative Examples 10 to 13, an aromatic
 polyamide fiber sheet was produced by the same procedures as in Example 10
 with the following exceptions.
 The high calendering temperature and pressure of Example 10 were changed to
 as shown in Table 4.
 The principal sheet-forming conditions of the fiber sheet is shown in Table
 4, and the properties of the resultant fiber sheet evaluated by the
 above-mentioned tests are shown in Table 5.
 A pre-preg was produced by impregnating the fiber sheet with the varnish
 composition by the procedures as shown in Test (12), and a laminate for an
 electric circuit board was produced from the pre-preg and subjected to the
 tests (12), (13) and (14). The test results are shown in Table 5.
 Comparative Examples 14 to 16
 In each of Comparative Examples 14 to 16, an aromatic polyamide fiber sheet
 was produced by the same procedures as in Example 10 with the following
 exceptions.
 The aromatic polyamide staple fibers used in Example 10 were replaced by
 the same aromatic polyamide fibers in Comparative Example 14 as in
 Comparative Example 1, in Comparative Example 15 as in Comparative Example
 2, and in Comparative Example 16 as in Comparative Example 3.
 The principal sheet-forming conditions of the fiber sheet is shown in Table
 4, and the properties of the resultant fiber sheet evaluated by the
 above-mentioned tests are shown in Table 5.
 The fiber sheet of Comparative Example 14 was subjected to the impurity ion
 content test (6) and the equilibrium moisture content Test (4)-B.
 In the test results, the extracted sodium ion content was 76.9 mg/liter,
 the extracted potassium ion content was 5.3 mg/liter, the extracted
 chlorine ion content was 11.9 mg/liter and the equilibrium moisture
 content was 2.72%. Thus the fiber sheet was unsuitable for electric
 insulating materials.
 A pre-preg was produced by impregnating the fiber sheet with the varnish
 composition by the procedures as shown in Test (12), and a laminate for an
 electric circuit board was produced from the pre-preg and subjected to the
 tests (12), (13) and (14). The test results are shown in Table 5.
 Examples 40 and 41
 In each of Examples 40 and 41, an aromatic polyamide fiber sheet was
 produced by the same procedures as in Example 10 with the following
 exceptions.
 The fiber sheet was prepared from:
 (1) the same aromatic polyamide staple fibers (A) as in Example 1, having a
 fiber length of 3 mm and an equilibrium moisture content of 3.1%;
 (2) copoly(p-phenylene/3,4'-oxydiphenylene terephthalamide staple fibers
 (B) (trademark: TECHNORA, made by TEIJIN) having an individual fiber
 thickness of 1.67 d tex (1.50 denier), a fiber length of 3 mm and an
 equilibrium moisture content of 1.8% and surface-coated with 0.5% by
 weight of talc and 0.1% by weight of osmose;
 (3) copoly(p-phenylene/3,4'-oxydiphenylene terephthalamide) staple fibers
 (D) (TECHNORA, made by TEIJIN) having an ultimate elongation of 65.7%, an
 individual fiber thickness of 3.33 d tex (3.00 denier) and a fiber length
 of 5 mm;
 (4) as heat-resistant organic polymer fibrids,
 copoly(p-phenylene/3,4'-oxydiphenylene terephthalamide) fibrids (made by
 TEIJIN) having an equilibrium moisture content of 4.1%, in the mixing
 proportions shown in Table 4.
 The principal sheet-forming conditions of the fiber sheet is shown in Table
 4, and the properties of the resultant fiber sheet evaluated by the
 above-mentioned tests are shown in Table 5.
 A pre-preg was produced by impregnating the fiber sheet with the varnish
 composition by the procedures as shown in Test (12), and a laminate for an
 electric circuit board was produced from the pre-preg and subjected to the
 tests (12), (13) and (14). The test results are shown in Table 5.
 Example 42
 An aromatic polyamide fiber sheet was produced by the same procedures as in
 Example 41 with the following exceptions.
 As the staple fibers (D), copoly(p-phenylene/3,4'-oxydiphenylene
 terephthalamide) staple fibers (D) (trademark: TECHNORA, made by TEIJIN)
 having an individual fiber thickness of 2.44 d tex (2.2 denier), a fiber
 length of 5 mm, and an ultimate elongation of 32.3% were employed.
 The principal sheet-forming conditions of the fiber sheet is shown in Table
 4, and the properties of the resultant fiber sheet evaluated by the
 above-mentioned tests are shown in Table 5.
 A pre-preg was produced by impregnating the fiber sheet with the varnish
 composition by the procedures as shown in Test (12), and a laminate for an
 electric circuit board was produced from the pre-preg and subjected to the
 tests (12), (13) and (14). The test results are shown in Table 5.
 Example 43
 An aromatic polyamide fiber sheet was produced by the same procedures as in
 Example 41 with the following exceptions.
 As the staple fibers (D), copoly(p-phenylene/3,4'-oxydiphenylene
 terephthalamide staple fibers (trademark: TECHNORA, made by TEIJIN) having
 an individual fiber thickness of 3.33 d tex (3.0 denier) a fiber length of
 5 mm and an ultimate elongation of 13.5% were used.
 The principal sheet-forming conditions of the fiber sheet is shown in Table
 4, and the properties of the resultant fiber sheet evaluated by the
 above-mentioned tests are shown in Table 5.
 A pre-preg was produced by impregnating the fiber sheet with the varnish
 composition by the procedures as shown in Test (12), and a laminate for an
 electric circuit board was produced from the pre-preg and subjected to the
 tests (12), (13) and (14). The test results are shown in Table 5.
 Example 44
 An aromatic polyamide fiber sheet was produced by the same procedures as in
 Example 41 with the following exceptions.
 As the heat resistant organic polymer fibrids,
 copoly(p-phenylene/3,4'-oxydiphenylene terephthalamide fibers (made by
 TEIJIN) having an equilibrium moisture content of 4.1% were used in an
 amount of 7.5% by weight, and a water-dispersible epoxy resin binder was
 applied in a solid amount of 7.5% by weight in the state of an aqueous
 dispersion having a solid content of 10% by weight to precursory fiber
 sheet comprising the staple fibers (1), (2) and (3) and the
 above-mentioned fibrids.
 The principal sheet-forming conditions of the fiber sheet is shown in Table
 4, and the properties of the resultant fiber sheet evaluated by the
 above-mentioned tests are shown in Table 5.
 A pre-preg was produced by impregnating the fiber sheet with the varnish
 composition by the procedures as shown in Test (12), and a laminate for an
 electric circuit board was produced from the pre-preg and subjected to the
 tests (12), (13) and (14). The test results are shown in Table 5.
 Comparative Example 17
 An aromatic polyamide fiber sheet was produced by the same procedures as in
 Example 41 with the following exceptions.
 In place of the aromatic polyamide staple fibers (D) used in Example 41,
 the same aromatic polyamide staple fibers as in Comparative Example 1
 having a fiber length of 3 mm and an equilibrium moisture content of 6.9%
 were employed.
 The principal sheet-forming conditions of the fiber sheet is shown in Table
 4, and the properties of the resultant fiber sheet evaluated by the
 above-mentioned tests are shown in Table 5.
 A pre-preg was produced by impregnating the fiber sheet with the varnish
 composition by the procedures as shown in Test (12), and a laminate for an
 electric circuit board was produced from the pre-preg and subjected to the
 tests (12), (13) and (14). The test results are shown in Table 5.
 Comparative Example 18
 An aromatic polyamide fiber sheet was produced by the same procedures as in
 Example 42 with the following exceptions.
 In place of the aromatic polyamide staple fibers (A) used in Example 42,
 the same aromatic polyamide staple fibers as in Comparative Example 1
 having a fiber length of 3 mm and an equilibrium moisture content of 6.9%
 were employed, and in place of the staple fibers (B) used in Example 42,
 copoly(p-phenylene/3,4'-oxydiphenylene terephthalamide staple fibers
 (trademark: TECHNORA, made by TEIJIN) having an ultimate elongation of
 13.5%, an individual fiber thickness of 3.33 d tex (3.0 denier) and a
 fiber length of 5 mm were employed.
 The principal sheet-forming conditions of the fiber sheet is shown in Table
 4, and the properties of the resultant fiber sheet evaluated by the
 above-mentioned tests are shown in Table 5.
 A pre-preg was produced by impregnating the fiber sheet with the varnish
 composition by the procedures as shown in Test (12), and a laminate for an
 electric circuit board was produced from the pre-preg and subjected to the
 tests (12), (13) and (14). The test results are shown in Table 5.
 TABLE 4
 Mixing proportions of
 component staple fibers High calendering
 and binder conditions
 Staple Staple Binder Linear
 Item fibers fibers Fibrids Temperature pressure
 Example No. A/B C/D/E resin (.degree. C.) (kg/cm)
 Example 30 15/70 0/0/0 7.5/7.5 320 200
 31 15/70 0/0/0 7.5/7.5 320 200
 32 15/75 0/0/0 10/0 320 200
 33 15/75 0/0/0 10/0 320 200
 34 15/75 0/0/0 10/0 240 200
 35 15/75 0/0/0 10/0 280 200
 36 15/75 0/0/0 10/0 350 200
 37 15/75 0/0/0 10/0 380 200
 38 15/75 0/0/0 10/0 320 170
 39 15/75 0/0/0 10/0 320 240
 Comparative 10 15/75 0/0/0 10/0 180 200
 Example 11 15/75 0/0/0 10/0 430 200
 12 15/75 0/0/0 10/0 320 120
 13 15/75 0/0/0 10/0 320 300
 14 15/75 0/0/0 10/0 320 200
 15 15/75 0/0/0 10/0 320 200
 16 15/75 0/0/0 10/0 320 200
 Example 40 5/70 0/15/0 10/0 320 200
 41 30/45 0/15/0 10/0 320 200
 42 55/20 0/15/0 10/0 320 200
 43 30/45 0/15/0 10/0 320 200
 44 30/45 0/15/0 7.5/7.5 320 200
 Comparative 17 30/45 0/15/0 10/0 320 200
 Example 18 55/20 0/15/0 10/0 320 200
 [Note]
 Staple fibers A: Poly-p-phenylene terephthalamide staple fibers
 Staple fibers B: Copoly(p-phenylene/3,4'-oxydiphenylene terephthalamide
 staple fibers
 Staple fibers C: Poly-m-phenylene isophthalamide staple fibers
 Staple fibers D: Copoly(p-phenylene/3,4'-oxydiphenylene terephthalamide
 staple fibers having an ultimate elongation of 8% or more
 Staple fibers E: Heat resistant organic polymer staple fibers different
 from the staple fibers A, B, C and D
 TABLE 5
 Properties of heat resistant fiber sheet

Dimensional