CA1045743A - Synthetic fiber comprising acrylonitrile-polyvinylalcohol graft copolymer and styrene-acrylonitrile copolymer and process therefore - Google Patents

Abstract

Abstract of the Disclosure A synthetic fiber substantially comprising:
(A) 5-40% by weight of a copolymer consisting of (a) 20-80% by weight of polyvinyl alcohol, and (b) 80-20% by weight of acrylonitrile, and (B) 60-95% by weight of a copolymer consisting of (c) 55-95% by weight of styrene, and (d) 5-45% by weight of acrylonitrile.
This fiber is used for producing paper and has a potentially fibrillar texture. When beaten, it forms synthetic pulp having stalk fibrils and micro fibrils extending from the stalk fibrils.
High-tenacity synthetic paper can be made because this synthetic pulp is self-adhesive.

Description

-The present invention relates to a synthetic fiber :Eor making paper, and to a method for producing the same.
Heretofore, many proposals have been made for the manufacture of paper from synthetic fibers rather than from natural pulp~ thereby utilizing the physically or chemically superior properties of synthetic fibers. Accord-ingly, various proposals have been made with reference to a method for pro-; ducing synthetic pulp from synthetic fibers. For example, with respect to polymers of the acrylic series it is known to produce a porous gel fiber from the polymers and then to beat the fiber, or to treat the polymers with 1~ a liquid agent such as a strong mineral acid, a swelling agent, or a deterio-rating agent, or to promote fibrillation by polymer blending. ~urther, with respect to polymers of the olefin series, it is known to split a film to bring about fibrillation.
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r~ -2-' ' . ', ', : ' ~S743 However, the resulting pulp by such known methods does not have hydrophilic properties which are necessary for satisfactory dispersion in water at the time of wet paper making, and for satisfactory adhesion of fibrils after the sheet is formed.
On the other hand, from the viewpoint of performance of the paper, synthetic pulps have not been known which are capable of producing a paper having a high degree of opacity and having high wet dimensional stability.
As materials for improving such characteristics of wood pulp, various inor-ganic fillers or powdered synthetic polymers have been used. However9 they have no paper-forming capability and can be used only as additives.
Japanese Patent Application Publication Nc. 10655/1964 discloses fibrillating synthetic fibers in a swelling agent to obtain so-called hooked fibers having short, fine hooks. In such hooked fibers, however~ the so-called hooks necessary for intertwining are short and are not self-adhesive, and therefore intertwining of fibers is not sufficient for the purpose of producing paper of high quality. The paper made from such fibers tends to be non-uniform.
It is disclosed in Japanese Patent Application Publication No.
20757/1961 that gel-like, non-collapsed, wet spun acrylic fibers tend to become fibrillated. 1' ~' . . .
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S7~3 ~lowever, fibrillation is carried out, as reported in said publication using gel-like, non-collapsed fibers. In order to convert fibrillated fibers into a sheet-like form, it is preferable that they have hydrophilic properties, and self-adhesion properties. If fibrillated fibers should fail to exhibit self-adhesion, a sheet-like material having high tensile strength cannot be obtained; a special adhesive is required in order to obtain high tensile strength~ As described in the publication, a sheet obtained according to the disclosed method has very low tensile strength, which is a clear indicat-ion of the drawback that the fibrillated material fails to have the property of self-adhesion.
Japanese Patent Application Publication No. 11851/1960 discloses a synthetic pulp fibril having feeler-like protrusions which are said to be capable of intertwining. As distinguished from the product obtained by ordinary spinning processes, and because this fibril essentially lacks any so-called stalk fibers, it has the drawback, when synthetic paper is made therefrom, that the paper product has poor physical properties, especially tenacity. This fibril is effectively utilized as an adhesive together with other fibrous materials. However, good paper cannot be obtained from this fibril alone.

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, ~4 ' : . ' ~ 57~3 We, the present inventors, have previously proposed fibers for paper and synthetic pulp consisting of (A) 15-90% by weight of a fiber-forming hydrophobic polymer and ~B) 10-85% by weight of a copolymer in which a hydro-philic portion and a hydrophobic portion are bonded chemically, in which the hydrophilic portion is dispersed and oriented in the direction of the fiber axis.
Since then, we have conducted strenuous studies with a view to producing a fiber which is capable of yielding a pulp having even more excellent opacity and wet dimensional stability in respect of paper perform-ance, high level dispersing properties in water and fibril adhering strength after forming the sheet. The present invention is a result of these studies.
Thus the present invention seeks to provide a fiber for a syn~hetic paper which is capable of yielding paper having high opacity, degree of whiteness and wet dimensional stability not attainable by paper made of conventional wood pulp.
In a second aspect the present invention seeks to provide a fiber for a synthetic paper imparted withperformances capable of producing pulp and making paper by exactly the method normally used for wood pulp. This is done by imparting hydrophilic properties to pulp -- a point which has been little considered in the case of conventional synthetic pulp, improving the dispersion properties in water when made into an aqueous slurry, and imparting adhesion strength between the fibrils.
Alternatively the present invention seeks to provide a method Eor making a synthetic fiber which is capable of obtaining a beaten fibril (pulp) having excellent intertwinement and adhesion among the fibrils, which can be made into paper in a wet system, using ordinary beating means of the ~;
type normally used with natural pulp, and capable of using the resulting ` beaten Eibril in making paper consisting of 100% of said beaten fibrils, or making paper consisting of beaten fibril and natural pulp in desired ratios.
In a first embodiment this invention seeks to provide a ~ - 5 -.: .

~5~3 synthetic fiber capable o~ being fibrillatcd for forming paper, consisting essentially of: (A) about 5-40% by weight of a graft copolymer consisting of (a) about 20-80% by weight of polyvinyl alcohol, and (b) about 80-20% by weight of acrylonitrile; and (B) about 60-95% by weight of a copolymer consisting of (c) about 55-95% by weight of styrene, and (d) about 5-45% by weight of acrylonitrile.
In a second embodiment this invention seeks to provide a method for producing a synthetic fiber capable of being fibrillated for forming paper, which comprises dissolving a composition consisting essentially of:
(A) about 5-40% by weight of a graft copolymer consisting of (a) about 20-80%
by weight of polyvinyl alcohol, and (b) about 80-20% by weight of acrylo-nitrile, and (B) about 60-95% by weight of a copolymer consisting of (c) about 55-95% by weight of styrene, and (d) about 5-45% by weight of acrylonitrile in a solvent selected from the group consisting of dimethyl sulEoxide and dimethyl acetamide, and wet spinning the resulting solution into an aqueous coagulating bath.
In a third embodiment this invention seeks to provide synthetic pulp which comprises: (A) about 5-40% by weight of a graft copolymer consist-ing of (a) about 20-80% by weight of polyvinyl alcohol, and (b) about 80-20%
by weight of acrylonitrile, and (B) about 60-95% by weight of a copolymer consisting of (c) about 55-95% by weight of styrene~ and (d) about 5-45% by weight of acrylonitrile.
As a preferred embodiment of the present invention, there may be provided, in addltion to components ~A) and ~B), unreacted polyvinyl alcohol and acrylonitrile produced as by-products in the process of graft copolymerization. Further, polymers of the polyvinyl alcohol series and the acrylic series may be added separately.

-~LO~Si7~3 In the practice of the present invention, as a further preferred embodiment, the fiber may comprise a mixed polymer system containing (A) about 5-~0% by weight of a copolymer in which polyvinyl alcohol and acrylo-nitrile bond chemically, containing about 2 -80% by weight of polyvinyl alcohol, and (B) about 60-95% by weight of an acrylonitrile - styrene copoly-mer containing about 5-~5% by weight of acrylonitrile, in which system poly-vinyl alcohol is present in an amount of about 2-55% by weight based on the weight of the entire mixed polymer system.
A method which is desirable for obtaining a fiber having the characteristics of the present invention involves dissolving such polymer composition in dimethyl sulfoxide or dimethyl acetamide and wet-spinning the resulting solution into an aqueous coagulating bath. A method involving further drawing or heat-treating the spun fiber while it is a water-contain-ing gel, as occasion demands, is desirable.
Because of the application of such heat-treatment, the fiber may contract by more than ~5%.
The present invention has succeeded in producing a paper which has remarkably high opacity and wet dimensional stability by using a graft copoly-mer consisting of polyvinyl alcohol (hereinafter referred to as PVA) having hydrophilic properties and acrylonitrile (hereinafter referred to as AN) to impart hydrophilic characteristics to the pulp. At the same time, it disperses the copolymer into a copolymer consisting of styrene (hereinafter referred to as St) and AN.

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~5i743 It is an indispensable requirement that the copolymer ~A) should be a copolymer in which an AN component and a PVA component are chemically bonded to each otherJ so that the PVA component as a hydrophilic component and the AN component as a hydrophobic component exist in the form of, for example, a graft or block copolymer, and two different types of copolymer (A) and copolymer (B) are caused to exist in a mixed system, in the afore-mentioned ratios.
In the reaction for obtaining copolymer (A), a small amount of an AN polymer is produced which is not bonded to the hydrophilic component or the unreacted hydrophilic component, and is not bonded to AN. However, when the copolymers ~A) and ~B) exist within the aforesaid range, then without fail, ~he exis~ence of such AN polymer or unreacted PVA does not become an obstacle in the practice of the present invention, insofar as its amount is within the aforesaid range. Therefore, the AN polymer need not be intention-ally removed. What is important is that the AN component and the PVA com-ponent should chemically bond to each other within the aforesaid range and exist in chemically bonded combination, by which it is possible to impart excellent hydrophilic properties, dispersing properties in water and self-adhesion to the beaten fibrils ~synthetic pulp) which are obtained by beating the fiber. When the AN component and the PVA component are simply mixed and exist, it is not possible to impart such characteristics to the beaten fibril.

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i7~3 When an AN - PVA copolymer used in the practice of the present invention is a graft copolymer, it is possible to produce the sam~ by aqueous non-uniform polymerization or solution uniform polymerization. Referring to the degree of polymerization of the PVA~ it is preferable that the average degree of polymerization be within the range of about 500-3400, preferably within the range of 600-1800.
It is possible to carry out polymerization by dissolving such PVA
in a polymerization solvent, for example, dimethyl sulfoxide (hereinafter referred to as DMSO), mixing with and dissolving in the resultant solution, 80-400% by weight (based on PVA) of AN or a vinyl monomer consisting mainly of AN, and polymerizing the resulting mixed solution using as a catalyst, for example, a persulfate at a relatively low temperature of from room temp-era~ure to about 70DC. By using such a method, a PVA/AN graft copolymer, unreacted PV~, polyacrylonitrile or a polymer of the AN series are produced.
It is also possible to add the same AN to an aqueous solution of PVA and carry out the polymerization reaction. The obtained PVA/AN graft copolymer can be isolated by re-precipitation and filtration.
It is necessary that the PVA content in the graft copolymer should be about 20-80% by weight, preferably about 35-65% by weight. In case said conten~ is less than abou~ 20% by weight, the molecular weight of the poly-acrylonitrile component in the graft copolymer is too high, seriously harm-ing processability and impeding the development of hydrophilic property. On the other hand, when said ~ _g_ .

~457gL3 content exceeds about 80% by weight, when the graft copolymer is made into an aqueous slurry as fiber and pulp, the graft copolymer flows out in water and the objects of the present invention cannot be achieved.
A graft copolymer containing PVA having an average degree of poly-merization of less than about 500 drops too much in water resistance and swelling strength, decreasing the water resistance of the paper obtained from the fiber, and not imparting preferred qualities to such paper.
; On the other hand, when said average degree of polymerization exceeds about 3400, hydrophilic property decreases, fibrillation is not carried out smoothly and performance characteristics necessary for use as paper are not developed. Upon preparing the graft copolymer, besides AN, less than about 40 mol % of a vinyl monomer other than AN, but which is co-polymerizable with AN, (for example, vinyl acetate, methyl acrylate, styrene and vinyl chloride) may be copolymerized.
A St/AN copolymer, (component B) of the present invention may be prepared by oridinary methods of random copolymerization and known methods of block copolymerization such as aqueous non-uniform polymerization and bulk polymerization, for example.
In order to achieve high opacity and a high degree of whiteness, it is not preferable that the St/AN copolymer be compatible with the PVA/AN
graft copolymer and the polymer of acrylic series. As one criterion for judging whether the former is compatible with the latter two, one may check the transparency of a solution obtained by dissolving the Eormer and the latter two in a common solvent. For example, when a polymer of acrylic 5~3 series and a PVA/AN gra~t copolymer are dissolved in DMSO, a somewhat trans-parent solution is formed. We have found that by adding an St/AN copolymer which is incompatible with both the PVA/AN graft copolymer and the polymer of acrylic series, paper having remarkably high opacity and degree of white-ness is obtained. To that end, it is necessary that the styrene content in the St/AN copolymer should be within the range of about 55-95% by weight, preferably about 60-85% by weight.
When the styrene content is less than about 55% by weight, however, a large amount of the St/AN copolymer may be added, and the compatibility with the other constitutional element increases. Therefore, the improvement of opacity and degree of whiteness is not satisfactory.
On the other hand, when the styrene content exceeds about 95% by weight, the solubility in DMSO and DMAc (dimethyl acetamide), which functions as solvents, is reduced and a satisfactory product cannot be obtained.
The composition of the present invention consists of about 5-40%
by weight of such a PVA/AN graft copolymer and about 60-95% by weight of such an St/AN copolymer. When the amount of said graft copolymer is less than about 5% by weight, the fiber produced by the method of the present invention is not fibrillated by beating, and has essentially no hydrophilic property. When the amount of the graft copolymer exceeds about 40% by weight, the water resistance of the resulting paper grows worse. In addition, the opacity of the resulting paper is harmed. When the amount of the St/AN
copolymer is less than about 60% by weight, the desired high opacity cannot be achieved.

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~4S7~3 The composition of the present invention is not limited to those consisting of said two copolymers only, but it may contain the unreacted PVA
and a polymer of the AN series produced as by-products in the process of graft copolymerization, and may include an intentionally added polymer of acrylic series.
Homopolyvinyl alochol has the property that a greater part thereof falls off when it is formed into an aqueous slurry in the process of making fibers and pulp, which is not essential for achieving the object of the pres~
ent invention. However, referring to the amount of PVA, this should not exceed about 23% by weight. When said amount exceeds about 23% by weight, the opacity of the paper is harmed. Further, this is not desirable in respect of foaming and contamination of the aqueous slurry.
The amount of the polymer of acrylic series should not exceed about 35% by weight. When higher, the desired high opacity cannot be achieved.
With respect to the polymer of acrylic series, a separately poly-merized linear polymer may be used. One having a molecular weight of 20000-100000 is preferable. A monomer of the vinyl series in an amount within the range not exceeding about 40 mol % which may be used as a copolymerization component for graft copolymerization, may be used as well.

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Substitution for the St/A~ copolymer by adding less than about 35%
by weight of a polymer of acrylic series results in somewhat reduced opacity.
However, by reinforcing the toughness of the beaten fibrils, it has the function of increasing the tenacity of paper. However, when it is substi-tuted for the PVA/AN copolymerJ there is no such effect.
The high opacity is measured by a method of measuring to be ment-ioned later, and should be in excess of at least about 80%. When less, there is no measurable effect of improvement with respect to a value of about 70% obtained by paper made from wood pulp.
As mentioned above~ existence of the unreacted PVA has no essential significance in the present invention. However, when the amount of such unreacted PVA exceeds about 23% by weight, foaming takes place in the aqueous slurry due to bleed-out of the PVA at the time of beating, or the degree of opacity of the resulting paper is sharply lowered.
It is further preferable that in fibers constituted by these mixed polymers~ PVA be contained in an amount of about 2-55% by weight based on the total amount of the polymers. Existence of PVA in this amount~ including PVA which is chemically bonded, gives favorable dispersing properties in water. The shape of the fibrils obtained by beating the fiber is very effect-ive for promoting intertwinement of fibrils and adhesion of paper made from such fibrils. But when this amount is less than about 2% by weight of PVA, development of such properties is insufficient. Using an amount exceeding about 55% by weight of PVA, the water resistance, opacity and degree of white-ness of the resulting paper are reduced.

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A wet spinning method using a solvent - water coagulating bath is especially suitable for producing a syn-thetic fiber of the p:resent invention. As solvents, dimethyl sulfoxide (DMSO) and dimethyl acetamide (D~Ac) are suitable. When the solvent is used and a solvent -water coagulating bath is used in producing a Eiber> the product is very suitable for beating and making paper.
The spun undrawn yarn is drawn to a predetermined draw ratio by ordinary methods in steam, hot water or a solvent -water bath, and thereby becomes capable of attaining the tenacity and shape of fibrils that are suitable for form-ing paper. .

As mentioned above, the composition of the present invention may be dissolved in DMSO or DMAc. Further, 15 this solution may be wet spun by ordinary means into an ., aqueous spinning bath, for example, a.n aqueous solution of ~ -DMSO or Dl~Ac containing up to the maximum of about 80% by weight of DMSO or DMAc to product an undrawn water-contain-ing gel yarn. Further, such undrawn yarn may be drawn in a hot water bath, or in a steam atmosphere as occasion demands.

One speci~ic method accorclirlg to the practice o:E
the present invention is dissolving such polymer composition in tne aforesaid solvent at a concentration within the range of about 8-40% by weight based on the weight of said solvent.

In case the concentration is less than about 8% by weight, when the wet spun yarn is beaten~ strong fibrils are unlikely -to be present and the paper tends to be very brittle.

7~3 On the other hand, when the concentration exceeds about 40% by weight, the viscosity of the spinning solution is increased too much, harming spinnability. Spinning into an aqueous spinning bath increases the tendency ; of the fibrils to disperse in water when an aqueous slurry is formed in a later step. This is indispensable for forming a fiber having a structure which is suitable for producing a pulp that is capable of developing high opacity, which is one of the objects of the present invention. Both in-compatibility of the graft copolymer (A) with the copolymer (B) and the for-mation of a coagulated composition upon being contacted with an aqueous medium derived from the hydrophobic property of the copolymer ~B) work very favorably, and fibers of the present invention develop porous structures including micro-voids which are roughly uniformly dispersed throughout the entire fiber structure. This is one reason why the beaten pulp has very high opacity.
Only the micro-voids have the effect of increasing the opacity of the fibers. It is preferable that fibers of the present invention contain micro-voids, the average diameter of a greater part thereof not exceeding 5 microns, and that they be uniformly dispersed througllout the entire structure of the fibers.
For example, it is possible to make artifically an annular hollow yarn using a composition of the present invention. I-lowever, hollows having diameters of more than S microns brought about at such time do not contribute to opacity. However, when the origanization of a substantial part of the structure is in accordance with the present invention, it is possible to achieve excellent results.
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~45~743 One of the factors which relate to the porous structures of fibers of the present invention is ~he apparent specific gravity (pa), which can be calculated from the average cross-sectional area o-f the substantial part of the fiber (S), and the average denier of the fiber (d).
Further, by heating fibers of the present invention to a tempera-ture higher than the softening points of the polymers, it is possible to pro-duce a transparent yarn having no voids in which the porous structure collap-ses. By measuring the specific gravity at this time~ it is possible to obtain a specific gravity pd of the product in the densely compac~ed state.
It is preferable that the structure of a fiber of the present invention be such that:
pa/pd < 0.8 When the ratio is greater than 0.8, increase of opacity cannot be expected.
Further, undrawn water-containing gel yarns obtained by wet spinning into an aqueous spinning bath are subsequently heat-treated at a wet tempera-ture of 80-180C, or preferably drawn at such wet temperature.
We have found that when the composition of the present invention is drawn, a remarkable effect is observed. When each of a solvent solution of the graft copolymer ~A) of the present invention alone and a solvent solut-ion of the copolymer (B) of the present invention alone is drawn after being spun into a solvent/water coagulating bath, the best that can be obtained is continuous drawing of at most 2 times ~in the case o:E the graft ~t,~ -16-~ ~S ~ ~ 3 1131~7~

copolymer (A), drawing is cornpletely impossible) is possible.
~lowever, when a solvent solution of a composition obtained by mixing the graft copolymer (A) and the copolymer (B) at a predetermined ratio in accordance wi-th the present invention, and the resulting composition is draw~ by the operations de-scribed in the present invention, it is possible to draw this composition to a ratio of at least 9, between draw rollers.

Drawing is carried out with wet heating, for example, in hot water containing the solvent, or in a steam atmosphere.
So~called known wet heat drawing is applicable.

In this case, it i5 necessary ~or the tempera~ure to be at least 80C. Especially, when the amount o~ a component which is amorphous and hydrophobic (such as styrene) is large, smooth drawing cannot be carried out at a temperature lower than 80C. On the other hand, when the temperature exceeds 180C/ the St/AN copolymer melts and the monofilaments stic~
to each other. As a result, various obstacles are encountered when the fibrils separate and disperse by heating. The draw ratio is selectecl in connection wi-th the desired beating con-ditions.

Further, the resulting fiber has a denier of about0.1-30, and is cut into a length of about 1~50 mm, preferably

2.5-25 mm. In the resulting synthetic flber, the copolymer (A) is dispersed as a plurality of particles in the copolymer (B) and arranged as independent phases in the direction of the fiber axis.

5~3 A fiber obtained by this production method is easily fibrillated by beating and may be made into pulp which has excellent dispersing properties in water. It is possible to make such fibers into an aqueous dispersion having a concentration of about 1-20% by weight and to beat the same by use of commercially available beating devices such as, for example, ball mills, beaters, PFI mills, and refiner. When, for example, a PFI mill is used, it is possible to use a linear pressure of 3.4 kg/cm2, a clearance of 0.1-0.3 mm and a cut fiber concentration of 3-12% by weight and by varying the total number of rotation of the rolls, it is possible to obtain a slurry having a freeness of about 40-750 cc.
Normal wet paper making methods are applicable without change, and it is possible to mix this synthetic pulp with wood pulp in desired proport-ions and to make paper from the mixed pulp -- especially from wood pulp blended with about 10-35% of the synthetic pulp, and the effect of the pres-ent invention is remarkable.
Fibers obtained according to the present invention are easily fibrillated, forming a pulp having excellent dispersing properties in water.
With reference to paper making, known wet paper making methods are applicable.
As a result, it is possible to make paper having high levels of opacity, a high degree of whiteness and an excellent wet dimensional stability not attainable by conventional paper made from wood pulp.

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It is preferable that a high draw ratio ~iber of the present invention be subjected to wet heat-treatment in boiling water, or in a steam atmosphere. It is preferable that the heat-treating temperature be about 80-180CJ preferably about 90-12QC. The time is preferably within the range of about 30 seconds to about 8 minutes, and it is to be expected that the final fiber will contract by at least about 45% of the initial length.

When a fiber whose shrinkage is less than about ~5%, or a fibril obtained by subjecting a fiber to beating opera-tions witnout prior treatment with wet heat, e~cessive ~ibrils are someti~es brought about from the fiber stalk, and the ~ibrils tend to cohere to form flocks. Therefore, the dispersing properties in water deteriorate and, as a result, the te~ture of paper tends to become inferior.
The wet heat-treatment can be carried out, before the fiber is cut, as a continuous yarn, and after the fiber contracts, it may be cut into a length of about 1-15 mm, preferably about 7-15 mm, and then the cut fiber may be beaten.

A low draw ratio yarn of the present invention (such a~ a draw ratio between 1.0 and 2.5, ~or example) ma~ be beaten wi~hout being subjected to such wet heat-treatment to produce a pulp having excellent dispersing properties.

A fiber of the present invention need not have unifo~
denier throughout the direction of the fiber length. In spinning according to the present invention, the solution of said compo-sition may be jetted at a high speed into the coagulating bath and may be discharged into a coagulating bath ~lowing at high speed.

The composition of a PVA/AN graft copolymer of the present invent-ion may be determined in a given case by the following means. From the polymer solution after graft copolymerization, it is possible to extract the polymer as a solid component by known methods, such as re-precipitation oper-ations and filtering operations. After the solid component is dried, it is subjected to hot water extraction for 48 hours using a Soxhlet extractor.
One component extracted by such operations is the unreacted PVA polymer.
Subsequently, after the solid component is dried again, it is extracted with dimethyl formamide (hereinafter referred to as DMF) at 100C for 5 hours, and the extracted component is a polymer of the AN series.
The remaining component is a PVA/AN graft copolymer. From the amount charged, the amount of the re-precipitated polymer, the amount of product extracted with hot water, and the amount of product extracted with DMF, the compositions of the graft PVA and graft PAN in the graEt copolymer are determined.
With reference to an AN/St copolymer, by measuring the amount of nitrogen (N) by elemental analysis, the amount of the AN component is deter-mined, and from the remaining amount, the amount of the ST component is deter-mined.
The degree of opacity and the degree of whiteness of paper referred to, are measured as follows. Using an integral sphere-type ~R meter, manu-factured by Nihon Seimitsu Kogaku Co., Ltd. and using a green filter, when the reflexibility obtained when a standard white plate is placed at the back of a ~k~¦ ~

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sheet o~ paper having a basis weight of 40 g/m2 is made 100, the degree of opacity is expressed by a reflexibility (%) when a black plate is placed at the back of said paper; and using a blue filter, in case a re~lexibility of a standard MgO plate :is made 100, the reflexibility (%) when at least 6 sheets of the sample paper are accumulated is read and made a degree of whiteness.
In order to discriminate dispersing properties in water, when a very small amount of pulp is put into a test tube together with water and shaken, said properties are thereafter easily determined by observing whether or not flocks due to coherence of the fibrils are produced. HoweveT, when extreme fibrils are produced, this may have a bad influence upon the spinning, remark- -ably prolonging the dehydrating time when the diluted slurry is dehydrated from above the mesh. Therefore, discrimination is possible in this respect.
As a most simple method, paper may be made and the uniformity of the texture of the paper may be determined.
A beaten fibril ta synthetic pulp) obtained by beating the resulting fiber may be formed into paper consisting of substantially 100% of said beaten fibril and into paper consisting of a mixture of said beaten fibril and wood pulp at an optional mixing ratio. The resulting paper is characteri~ed in that the degree of whiteness, degree of opacity, wet dimensional stability, per-2Q meation resistance, surface picking strength and printability are simultan-eously attainable at balanced high levels which have not been realizable by conventional natural pulp. The paper can develop excellent aptitute in many uses for paper such as paper for reprography like coated ~ 21 ;7~3 paper, photographic paper, India bible paper, thin paper, tracing paper, electrostatic recording paper, electrophotographic paper, magnetic recording paper and copying paper; and paper used in combinat;on with these papers for reprography or independently like pressure sensitive copying paper, card for a statistical machine, punch tape, business form, optical mark recognition paper and optical character recognition paper as well as magnetic ink charac-ter recognition paper, release paper, paperboard and wrapping paper.
Hereinbelow, specific examples of the present invention are pres-ented in order to clarify the disclosure of the present inventlon. The examples shown hereinbelow are illustrated in conjunction with drawings for clarifying the fundamental requirements for showing the constitution of the present invention and these examples are not intended to limit the scope of the present invention.
Of the Drawings:
Figure 1 is a graph showing the relationship between pulp freeness and degree of beating, using a yarn prepared in accordance with Example 1 herein, and Figure 2 is a graph showing a similar relationship for the pulp obtained in the procedure of Example 2 hereof.

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, Examp 1 e l kg of PVA having a degree of polymerization of 1400 (NM~
manufactured by Nihon Gosei Kakagu Co., Ltd.) was dissolved in 19 liters of DMS0 at a temperature of 50-60C for about 2 hours with stirring to obtain a uniform solution. To this solution were added ammonium persulfate (herein-after referred to as APS) in an amount of 0.1 mol % based on AN as a catalyst '` and dodecyl mercaptan (hereinafter referred to as DM) in an amount of 2% by ; weight based on AN as a molecular weight control agent, and the APS and DM
were uniformly dissolved in said solution with sufficient stirring. A mix-ture of 3 kg of AN and 0.2 kg of DMSO was added dropwise to this solution which was kept at a temperature of 52C. This solu~ion was polymeri~ed with stirring for about 2 hours to obtain a polymer solution having a viscosity of 200 poise.
l~hen this condition was attained, hydroquinone in an amount of 0.1 mol % based on AN was added to the polymer solution, and the mixture was stirred sufficiently to stop the polymeri~ation reaction. In the polymer, the AN conversion was 61%, the extraction ratio with hot water was 16%, the PVA content was 35% and the extraction ratio with dimethyl formamide (DMF) was 7.5%. From these values, i~ was determined that PVA bonded to AN to form a graft copolymer. In the polymer obtained, the component which was soluble in hot water was PVA not bonded to AN and the component soluble in DMF was polyacrylonitrile (hereinaEter referred to as PAN) not bonded to PVA. ~lowever, these components did not have to be purified and removed, but could be used per se as a polymer -^ ~ s -23-.

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; 1131-7~ 7 ~ 3 solution. And ~hen it was necessary for any reason, it was possible to take out the graft copolymer by precipitation and dissolving and extracting operations, and -~o use ~he graft copolyIner. The PVA used herein was not particularly limited, however, when a judgment was made from the viewpoint of the mechanical properties of the obtained fiber, the adhesion and dispersing properties in water of the fibril after being beaten, a degree of polymerization within the ra.nge of 500-3400 was found to be preferable.

10 In a separately prepared polymerization vessel, 20 parts of an AN/St (24/76) mixture were added to 150 parts of water. To the resulting mixture were added, with vigorous stirring, tertiary dodecyl mercaptan (TDM) in an amount of 0.2%
based on the monomer, a catalyst of the azobis series in an amo~mt of 0.3% based on the monomer and a small amount of sulfuric acid, and polymerization was carried out at a tempera-ture of 80-100C for about 3 hours to obtain a beads-like AS
(acrylonitrile - styrene) copolymer (AN/St = 24/76). The de-gree of polymerization of this copolymer expressed as intrlnsic viscosi~y (n) measured in met'hyle-thyl ketone (hereinafter re ferred to as MEI~) at 30C was 0.5. The resulting 'beads-li'ke AS copolymer was dried by a ~lush dryer to remove the moisture completely. Next, this copolymer was dissolved in D~IS0 at 70C wi.th stirring to obtain a uniform polymer solution having a concentration of 27%.

~ .

-2~-7~3 Next, proper amounts of a DMSQ solution of the afore-said PVA - AN graft copolymer and a DMS0 solution of the AS
copolymer were taken, respectively. Further, a necessary amount of DMSO was added to prepare a mixed polymer solution having a concentration of 13% containing 12% of the PVA component.

This mixed polymer solution was stirred by a spiral-type stirring blade for 3 hours to obtain a mechanically uniform mixed solution, which was spun as a spinning solution through spinning nozzles each having a diameter o~ 0.1 mm into a Dl~O/
water (70/30~ coagulating bath, and the resulting undrawn yarn was continuously drawn. After drawing, the drawn yarn was washed with water sufficiently to remove the solvent. The denier of the resulting yarn was 5.

This yarn had the following values:
pa = 0.~1 I pd = 1.11 and pa/pd = 0.37.

The PV~ portion of this yarn was dyed with OsO~
(osmic acid) and when an ul.tra thin (cross-sectional) cut piece was taken and observed under an electron microscope, it was ob-served that a plurality of voids having an average diameter of less than 5 ~icrons were uniformly dispersed in the cross-secti.on ancl the copolymer (A) was dispersed in the copolymer (B) as the dispersed phase. By the same toke~, by observa-tion of a longi-tudinal section of the fiber also, it was observed that the com-ponent (A) was dispersed ln the component (B) QS StriQe.

~25-This yarn was cut into a fiber length of 3 mm and the cut pieces were beated in a PFI mill manufactured by Kumagaya Riki Co., Ltd., Japan (clearance 0.2 mm, weight 3.4 kg and pulp concentration 5%) to carry out fibrillation. Figure 1 of the drawings shows the effect of such beating on pulp freeness, by showing the relation between the number of revolutions of the PFI mill and the freeness. It is clearly observed from Figure 1 that a fiber based on the present invention forms paper-forming fibrils by beating.
Beaten fibrils whose freeness values were 200 cc, 320 cc, and 390 cc were made into aqueous solutions having concentrations of 0.02%, and were introduced into a manual paper-making sheet machine (using 80 mesh metal screen) manufactured by Kumagaya Riki Co., Ltd., Japan to obtain three different manually made papers (wet papers). The basis weights of these wet papers were 40.1 g/m . The wet papers were dried by an FC dryer manu-factured by FC Seisakusho Co., Ltd. at 105C for 2 minutes.
In Table 1, measured results of various characteristics of the three kinds of papers (samples A, B and C) as well as paper made from wood pulp are shown. Of these characteristics, as conspicuous characteristics, it is apparent that a high degree of whiteness, opacity, wet strength and excellent degree of air permeation, which we have been unable to attain with conventional wood pulp, are developed despite the absence of any addition of any filler or additive in the case of the three papers prepared according to the present invention.

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, ~4S7~3 The degree of opacity was expressed as reflexibility when a black plate was placed at the back of sample paper. The standard was the reflexi-bility obtained by placing a standard white plate at the back of the sample paper, using a green filter, and was designated as 100.
The degree of whiteness was expressed as reflexibility when at least 6 samples were accùmulated, when as a standard the reflexibility of a standard MgO plate using a blue filter was designated as 100.
Example 2 (1) 61 g of PVA having a degree of polymerization of 1800 were dissolved in 550 g of DMSO at 50C for 2 hours with stirring to prepare a uniform solution. To this solution was added 0.61 g of DM and the mixture was stirred. To the resulting solution, a mixed solution of 61 g of AN
and 61 g of DMSO was added dropwise. When necessary, the degree of acidity of the system was controlled. Next, to the solution, 0.5 g of APS was added dropwise and polymerization was carried out at 50C for 6 hours.
Thereafter, 0.57 g of hydroxylamine sulfate was added as a polymerization stopper together with a small amount of DMSO to the system to complete the polymerization. The polymer solution so obtained, having a graft copolymer, had 100 grams of the entire polymer, 80 grams of the graft copolymer of ZO 80 g, a solution concentration of 12.4~ and a viscosity of 250 poise.

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(2) By suspension polymerization the same as in Example 1, an AS (AN/St = 25/75) copolymer having an intrinsic viscosity (n) measured in MEK at 30C of 0.65 was prepared, which was dissolved in DMS0 at 50G to prepare a ~miform poly-mer solution having a concentration of 25%.

(3) The polymer solution obtained in (1) and the polymer solu-tion obtained in (2) and a measured necessary amount of DMS0 were mixed to prepare a polymer solu~ion hav-ing a concentration of the entire polymers of 19% and a PVA
content of 13%, and the resulting polymer solution was stirred at 50C for 2 hours to prepare a uniformly dispersed mixed polymer solution, which was made a spinning solution.

This spinning solution was spun from spinning nozzles each having a diameter of 0.15 mm into a DMS0/water =
(40/60) coagulating bath at 30C, and the undrawn yarn was drawn to 4.5 times in hot water and washed with water to re-move the remaining solvent.

The denier of the resulting yarn was 4. This yarn had values of:
pa = 0,40, pd - 1.10, and pa/pcl = 0.36.

This yarn was cut into fiber lengths of 3 mm, and the cut pieces were beaten in a PFI mil.l manufactured by Kumagaya Riki Co., Ltd., Japan (clearance 0.2 mm, weight 3.4 kg, pulp concentration 5%) to carry out fibrillation. Fig.2 shows various stages of thoroughness of beating as variation of degree of freeness (C.S.F.) as well as the total number of revolutions of the PFI mill. From Fig. 2, it is apparent ~hat beating of the fiber proceeds and fibrils are formed by the beating treatment.

1~L5743 This fiber was made into 3 different beaten fibrils having freeness values of 400 cc, 300 cc and 200 cc by the PFI mill. Each of these beaten fibrils was further beaten by a home mixer ~SM-225, pulp concentration 2%, manufactured by Sanyo Denki Co., Ltd., Japan) into pulp having a freeness of 305 cc (pulp A), 215 cc (pulp B), and 95 cc (pulp C).
These pulps A, B and C were caused to have pulp concentrations of 0.2% and were made manually into papers using a manual paper-making sheet machine (using 80 mesh metal screen) manufactured by Kumagaya Riki Co., Ltd., Japan. The basis weights of these papers were 50 g/m2 and 35 g/m . The wet papers were dried in an FC dryer (at 105C for 2 minutes). In Table 2, the measured results of various characteristics of these papers are shown. As conspicuous, remarkable characteristics, a remarkably high degree of white-ness, a degree of opacity and a wet strength which we had been unable to realize with conventional wood pulp were obtained. The shrinkage at the time of drying the wet papers was low to about the same degree as paper made from wood pulp, and their air permeating degree was high.
Next, an examination was made of the characteristics of paper made from mixtures of said synthetic pulps of the present invention and wood pulp.
As such wood pulp, needle-leaved tree-refined kraft pulp (NBKP) and broad-leaved tree-refined kraft pulp (LBKP) were beaten by a PFI mill (clearance 0.2 m~, weight 3.4 kg, concentration 5%) to obtain beaten pulps having a freeness of 310 cc. This NBKP and LBKP were mixed at a ratio of N/L = 4/6 and the obtained mixture was made base pulp of wood pulp. This mixed base pulp and each of said beaten pulp .

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: ' , ' 9L 57 9L3 r A and pulp B were mixed, the resultant 2 kinds of mixed pulps were caused to have concentrations of 0.02%, from which papers were manually made. The wet papers were dried at 105C for 2 minutes by an FC dryer. Table 3 shows the results, from which it is understood that papers obtained from mixtures obtained by mixing lower amounts of pulps obtained by beating a fiber shown in the present invention (beaten fibrils) with said base pulp of wood pulp exhibit a high degree of whiteness, degree of opacity, wet strength, break-ing length and surface strength which have heretofore been una~tainable with paper made from 100% wood pulp. It was able to develop well balanced paper 10 characteristics, exceeding the properties of natural pulp.

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7~L3 Example 3 A PVA - AN graft copolymer as in Example 2 and an AS copolymer ([~] = 0.65, AN/St = 28/72) were dissolved in DMSO and spinning solutions having concentrations of 15% by weight containing PVA's in a~ounts of 10%
by weight and 20% by weight based on the entire amounts of the polymers were prepared.
On the other hand, PVA (NM-14, manufactured by Nihon Gosei Kagaku Co., Ltd.), PAN (molecular weight 63000) and an AS copolymer ([n] = 0.65, AN/St = 28/72) were so prepared that they became the same as the PVA, PAN
and AS components of the aforesaid graft copolymer and AS copolymer.
These four different spinning solutions were spun from spinning nozzles each having a diameter of 0.1 mm into a solvent - water coagulating bath (DMSO/water = ~5/55) and continuously drawn to 3.5 times in a hot water bath. After drawing, the remaining solvent was removed by washing with water.
The denier of the resulting yarns was 3.5. These yarns were cut into lengths of 3 mm, and the cut pieces were beaten by a PFI mill the same as in Example 2. The degrees of beating are shown in Table ~. In the cases of fibers in which the PVA - AN graft copolymers were used, beating proceeded smoothly and forming of fibrils was carried out. In contrast, fibers consist-ing of simple mixtures of the respective components were very difficult to beat and breakage of fiber and bleed-out of PVA took place at the time of beating and beating as such was not desirable.

57~3 Next, these beaten fibrils were made into four different we-t papers by a manual paper making instrument manufactured by Kumagaya Riki Co., Ltd., Japan, dried at 105C for 2 minutes by an FC dryer and the characteristics of the dry papers were mèasured. The results are shown in Table 5. The fibers consisting of simple mixtures of the respective components cannot obtain required paper strength and bulk density as paper and did not become paper that was capable oE withstanding actual use. In contrast, the fibers consisting of the graft copolymers and the AS copoly-mers developed excellent paper-forming performances as will readily be understood from Table 5.

Table 4 Amenability to Beating 15 Sample PVA Beating ~ -content ........... . ~
Fiber from PVA/AN
graft copolymer 10 Beatin~ was smooth, formation and AS copolymer of :Eibrils proceeded.

.. 20 Beating was very smooth, ; formation of fibrils proceeded.

Fiber from simple mixture of PVA) 10 Very difficult -to beat, many 25 P~l and AS copoly- fibers remained unbeaten, mer breakage occurred and PVA
bled out.
- 20 _ -3~

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Example 4 By the same method as in Example 2, a PVA - AN graft copolymer was obtained by DMS0 solution polymerization. The solution was precipitated using methanol as a precipitating agent to obtain a solid PVA - AN graft copolymer, which was treated by vacuum drying for 24 hours to remove the methanol and a very small amount of the solven~. Next, this copolymer ;~ was dissolved in DMAc at 60C to prepare a ~miform solution having a concentration of 15V/o~ Separately, an AS copolymer .
10 (~n) = 0.51, ~NISt = 23/77) was dissolved in DMAc at 80C
to prepare a uniform solution having a concentration of 15%.
: The aforesaid two kinds of solutions were mixed to prepare spinning solutions containing 10, 25, 40, 60% of PVA. Each of these spinning solutions was spun from a spinning nozzle 15 each having a diameter of 0.11 mm into a solvent - water (DMAc/water = 60l40) coagulating bath, and continuously washed with steam. The denier of the resulting yarn was 4.
I'his yarn was cut in~o :Eiber lengths of 3 mm, the cut pieces were beaten in a PFI mill and :Eurther beaten by a home mixer to obtain a beaten fibril hav.Lng a :Ereene~,s (C.S.F.) o:E 250 cc, which was made into an aqueous dispersed liquid having a con-centration of 0.02%, from which manually made wet paper having a basis weight of 50 g/m2 was made, which was dried at 102C
for 15 minutes by a drum dryer to obtain paper. rne results of measuring the characteristics of these four kinds of paper are shown in Table 6.
, ~ 3~- I

57~3 ll31-74 It is confirmed that each of these four kinds of paper was excellent in degree of whiteness, degree of opacity, air permeating degree and tenacity and well balanced as a whole, w'nich was not true of paper made from wood pulp. And S it should be understood that when the PVA content reached 60%, the balance of paper quality was lost.

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~.,___ . ___ __ __ .~ -yl_ ~57~3 Example 5 1.125 kg of PVA (NM-14, manufactured by Nihon Gosei Kagaku Co., Ltd.) was dissolved little by little at 50C in 10 kg of DMS0. Next, 9.68 g of APS, 11.25 g of DM and a small amount of sul:Euric acid were added to the resulting DMS0 solution, the ph of the mixture was adjusted ~o 4.5, and to this mixture, a solution obtained by dissolving 1.125 kg of AN in 2.75 kg of DMS0 was added dropwise for 50 minutes. The mixed solution was stirred for 7 hours while the temperature was kept at 50C. Thereafter, 14 g of hydroxylamine sulfate and 4 g of H2S04 were added to stop the reaction. As a result, a solution having a viscosity at 50C of about 300 poises and a polymer concentration of 12.4% by weight were obtained. The polymer obtained by re-precipitating this solution in methanol consisted of 11.7% by weight ,~ of PVA, 3.8% by weight of PAN and 84.5% by weight of PVA/AN graft copolymer containing 60.1% by weight of PVA. This solution was designated (A).
Next, 150 parts of water were placed in a separate polymerization vessel and while the temperature was kept at 85C, DM in an amount of 0.3%
by weight based on a monomer and N,N'-azobisisobutyronitrile in an amount of 0.35% based on the monomer were added. Next, 30 parts of an AN/St = 24/76 (by weight monomer and a small amount of a surface active agent were added ~0 to the mixture, and the solution was stirred for 2 hours to obtain a beads-like resin (~) having an intrinsic viscosity (~) measured in ~lEK at 30C of 0.5 and a degree of polymerization of 98.5%.

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~31 4~7~3 121 parts of the solution (A), 85 parts of the resin (B) and 183 parts of DMS0 were mixed at 60C for 3 hours to obtain an opaque polymer solution having a polymer concentration of 25.0%. This solution was spun into an aqueous solution at 30C containing 57% by weight of DMS0 and the resulting undrawn yarn was drawn in hot water at 95C. The draw ratios and deniers per filament of the resulting yarns are shown in Table 7. These yarns were cut into lengths of 3-10 mm and introduced into boiling water for wet heat-treatment for predetermined periods. After this, the heat-treated fibers were made into aqueous slurries having concentrations of 5% by weight, and these slurries were beaten in a PFI mill (load 3.4 kg/cm2, clearance 0.2 mm) after 30000 revolutions of the rotor. Thereafter, the concentrations of the slurries were adjusted to 1% (by weight), 800 cc of each of the beaten fibers were thrown into a home mixer, stirred and disaggregated to obtain pulp slurries. The results appear in Table 7.

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~ ~0~743 Example 6 ; : ' The pulp slurries obtained in Example.S were formed into papers by a square-type manual papermaking sheet machine manufactured by Kumagaya P~iki Co., Ltd., Japan, while observ-ing the standard operations described in JIS P-820.9.

Pulp slurries having papermaking concentrations of ~ 0.02% by weight were used, 12 liters of the liquids were .~ formed into papers by a 80 mesh metal screen for making paper to make papers having basis weights of 40 glm .

The wood pulp used was NBKP/LBKP = S0l50 ~weight rat.io) having a freeness of 413 cc. The papermaking characteristics and paper properties are shown in Table 8.

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; 1131-7~ 45~43 Example 7 By the same method as in Example 5, various composi-tions of PVA/~ graft copolymers and AN/St copolymers were pre-pared as shown in Table 9. These copolymers were prepared by the same method as in Example l, formed into spinning solutions and wet spun.

; The draw ratio was 4.5, the denier was 3 and the boil-ing water treatment time was 1.5 minutes, the. beating conditions ` were made the same as in Example 1 and the papermaking conditions .~ !
.~ 10 were the same as in Example 6. The results are shown in Table 9.

, From Table 9, it is clear that only the compositions accordin~ to the present invention gave Sood results.
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Example 8 AN was solution polymerized in DMS0 using N,N'-azobisisobutyronitrile as a catalyst to prepare a DMS0 solution containing 15.4% by weight of PAN having a molecular weight of 67000.

Composi~ions having various compositions were prepared from the resulting polymer solution and a P~A/AN
graft copolymer prepared by the same method as in Example 5 to carry out the similar determinations to those of Example 7. The results are shown in Table 10. In Table lO, it is shown that only the compositions according to the present invention gave ~ood results.

.

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5~3 Example 9 18 g of PVh (~-1-26, manufactured by Nihon Gosei Kagaku Co., Ltd.) were dissolved in 300 ml of water at 50C. To the resulting aqueous solution were added 0.45 g of APS and 0.49 g of sodium thiosulfate, both dissolved in 5 ml of water. To the resulting solution, 80 g of AN was added dropwise and tha mixture was polymerized for 1.5 hours. After the polymerization, a sat-urated aqueous solution of sodium sulfate was added to the polymer solution, and the resulting mixed solution was heated to 100C to aggregate the poly-mer particles, which was cooled and filtered. Next, the filtered polymer was washed with hot water and thereafter washed with methanol and dried to obtain 97 g of the polymer. The composition of this polymer consisted of 43 g of PAN and 57% of a PVA/AN graft copolymer containing 28% by weight of PVA.
To 100 parts of this polymer was added 250 parts of an St/AN
copolymer containing 70% of St separately polymerized by the same method as in Example 5. The resulting mixture was dissolved in D~c to prepare a solution having a concentration of 20% by weight, which was spun into a DMAc/water = 50/50 solution at 40C and the resulting undrawn yarn was drawn 4.2 times in hot water at 95C to obtain a 2.8 d continuous yarn. This yarn was cut into a length of 6 mm. Thereafter, sample cut pieces were put on a belt conveyer and passed through a steam treating machine capable of blowing off steam from the lower nozzles for a length of 2 m from the entrance through the exit at a speed of 1 m/min. The temperature inside said machine was 95C.

,' f, ~

74l;3 This wet heat-treatment caused the yarn to contract by 71%.
The resulting yarn was subjected to beating in a PFI mill using the method shown in Example 6. The total number of the roll revolution was varied and various beaten pulps were prepared. As a reference, yarn not treated with steam was similarly processed.
; By the method shown in Example 7, the dehydration time of mixed (synthetic pulp/wood pulp = 20/80) pulps was measured and the results are shown in Table 11.

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-l~S743 Example 10 ibers according to the method of the present invention obtained in examples up to Example 8 were beaten, and the resulting beaten fibrils (synthetic pulps) were mixed with wood pulp. From the resulting mixtures paper products were made. The wet strength and dimensional stability of these papers were measured and the results are shown in Table 12. The wet dimensional stability was shown by change of dimension (%) when the relative humidity (RH) was varied from 65% to 95% at 20C using a TAPPI paper elasticity tester.
Wet strength was shown by the ratio (%) of the tensile strength of a sample immersed in water at 20C for 20 minutes obtained by a tensile ~ test carried out immediately thereafter to the tensile strength of the same - sample which was dry. The values of paper made from 100% wood pulp are also shown in Table 12.
Table 12 Used Pulp No. Synthetic pulp/ Wet Strength Wet dimensional wood pulp (%) stability 2 20/80 18 0.57 :
3 35/65 21 0.~1 11 20/80 14 0.52

6 0/100 1.4 1.51 . .
. . .

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l~S7~3 Example 11 10 kg of PVA (NM-14, manufactured by Nihon Gosei Kagaku Co., Ltd.) was dissolved little by little in 10 kg of DMS0 at 50C. To the DMSO solut-ion, 9.1 g of ASP, 2.1 g of DM and a small amount of sulfuric acid were added, the pH of the solution was adjusted to 4.5, and a solution obtained by dissolving 2.1 kg of AN in 2.5 kg of DMS0 was added dropwise to the same solution for 50 minutes. While ~he temperature was kept at 50C, the solut-ion was stirred for 8.5 hours, and then 6.4 g of hydroxylamine sulfate and '~ 7 g of H2S04 were added thereto to stop the reaction. As a resul~, a poly-mer solution having a viscosity of about 175 poises at 50C and a polymer concentration of 14.5% by weight was obtained. A polymer obtained by re-precipitating this polymer solution in methanol consisted of 11.9% by weight of unreacted PVA, 8.7% by weight of polyacrylonitrile and 79.4% by weight of a PVA/AN graft copolymer containing 50.1% by weight of PVA. This polymer solution was designated (A).
Next, 150 parts of water were introduced into a diEferent polymer-ization vessel, and while the tamperature was kept at 80C, DM in an amount of 0.3% by weight based on a monomer and N,N'-azobisisobutyronitrile in an amount of 0.35% based on the monomer were added to said water. Next, 30 parts of an AN/S~ = 24/76 (weight ratio) monomer and a small amount of a surface active agent were added ~o the resulting aqueous solution, and by continuing stirring of the solution for 2 hours, a beads-like resin ~B) ; having a degree of polymerization of 98.5% and an intrinsic viscosity [n]
msasursd in ME~ at 30C Eor 0.5 was obtained :' ,'' .

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. . .

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In still another polymerization vessel, by using N,N'-azobisisobut-yronitrile as a catalyst in DMS0, an AN/methyl acrylate = 93/7 (molar ratio) mixture was polymerized to obtain a DMSO solution (C) having a polymer con-centration of 20.8% by weight. The molecular weight of the acrylic polymer was 67000.
207 parts of the solution (A), 60 parts-of the resin (B) and 48 parts of the solution (C) were mixed together with constant amounts of DMS0 to prepare solutions having different polymer concentrations.
Each of these solutions was spun into an aqueous solution contain-ing 52% by weight of DMS0, the resulting undrawn yarn was drawn under various drawing conditions and washed with water, the results being shown in Table : 13.
Table 13 shows the results obtained by making papers from beaten pulps obtained by cutting the wet drawn yarns into a length of 6 mm adding 400 g of such cut pieces in a state of aqueous slurry having a concentration of 1% by weight into a home mixer (SM-226, 1200 cc, manufactured by Sanyo Denki Co., Ltd., Japan) and rotating such aqueous slurry at about 10,000 r.p.m. for 30 minutes.
The papers were made using a square-type sheet machine manufactured by Kumagaya Riki Co., Ltd., Japan, while observing the standard operations described in JIS P-8209. The basis weights of the papers were 50 g/m2. As a reference, paper from wood pulp ~NBKP/LBKP = 50/50 (weight ratio) was made from a slurry having a freeness of 413 cc under the same con~itions. The results appear in Table 13.

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415~ 3 Example 12 - By the same methods as in Example 11, various PVA/AN graft copoly-mers, St/AN copolymers and polymers of the AN series were prepared and vari-ous fibers of compositions shown in Table 14 were spun.
DMS0 solutions having polymer concentrations of 25% by weight were spun into an aqueous solution containing 70% by weight of DMS0 at 30C, and the resulting undrawn yarns were drawn 4.0 times in hot water at 98C to obtain 3.0 d yarns.
The wet drawn yarns were cut into lengths of 3 mm, the cut pieces as aqueous slurries having csncentrations of 5% by weight were beaten using a PFI mill manufactured by Kumagaya Riki Co., Ltd., Japan at linear pressure of 3.4 kg/cm , clearance of 0.2 mm and rotor revolution number of 30,000.
Thereafter, they were treated with a home mixer by the method of Bxample ll for 8 minutes and made into paper sheets.
The paper sheets were made in the same way as in Example ll to obtain samples made from 100% synthetic pulps having basis weights of 50 g/m . The results are shown in Table 14. Table 14 shows that only the compositions of the present invention gave good opacity.

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.;: ~ 1. ~5~ 1~ 57q3 . 1 ; -'T - .. - - ' - .. .-, . ............... .. .. _,_.. _._ L,~ ' . ' S~3 Example 13 18 g of PVA (NH-18, manufactured by Nihon Gosei Kagaku Co., Ltd.) were dissolved in 300 ml of water at 50C. To the resulting aqueous solut-ion, 0.45 g of APS and 0.49 g of sodium thiosulfate, both dissolved in 5 ml of water, were added. To the resulting solution, 65 g of AN were added drop-wise and the solution was polymerized for 2.0 hours. A saturated aqueous solution of sodium sulfate was added to the polymer solution, the entirety was heated to 100C to aggregate the polymer particles, and the polymer was cooled and flltered. Next9 the polymer was washed with hot water and then - 10 with methanol and cooled to obtain 87 g of the polymer. The composition of this polymer consisted of 35% by weight of PAN and 65% by weight of a PVA/AN
graft copolymer containing 26.5% by weight of PVA.
To 100 parts of this polymer, 270 parts of an St/AN copolymer con-taining 76% of styrene copolymerized by the same method as in Example 11 dissolved in DMAc was added, and the mixture was formed into a solution hav-ing a concentration of 20% by weight, which was spun into a mixed DMActwater = 50/50 solution at 60C, the resulting undrawn yarn was drawn 4.0 times in , hot water at 95C to obtain a 2 denier drawn yarn. The pa/pd of this yarn was 0.39.
This yarn was cut into a length of 4 mm, beaten and formed into paper by the same means and method as in Example 11 to obtain paper having a basis weight of 50 g/m2, a length at break of 3.9 km, a degree of whiteness of 94% and a degree of opacity of 97%.
.' , , ~ ~ -60-: , ,; ' ' ~S~43 Example 14 :' Th0 wet strength and dimensional stability of the papers made from the beaten fibers according to the method of the present invention obtained in examples up to Example 13 were measured and the results are shown in Table 15.
The wet dimensional stability is shown as the change of dimension : ~%) when the relative humidity (RH) was varied from 65% to 95% at 20C using a TAPPI paper elasticity tester.
The wet strength is shown as the ratio of the tensile strength of : 10 a sample immersed in water at 20C for 20 minutes and is obtained by compari-son of a tensile test carried out immediately thereafter to the tensile ,. strength of the same sample when it was dry. The same measured results on paper made from 100% wood pulp are shown in Table 15.
Table 15 ,.,~ :
Used Pulp Synthetic paper Wet strength Wet dimensional stability . No. or wood paper (%) (%) .
, _ _ ; 23 Synthetic paper 50 0.3 ,, .

,, 20 32 Synthetic paper 57 0.1 . 34 Synthetic paper 12 1.4 ., _ . 35 Synthetic paper 48 0.2 ~
. .
Wood pulp paper 1.4 1.5 ;' .
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Accordingly, it will be app~eciated that -the synthetic fibers according to this invention "consist essentially of"
(A) about 5-40% by weight of a graft copolymer consisting of . (a) about 20-8Q% by weight of polyvinyl . ` alcohol, and (b) about 80-20% by weight of acrylo-.~ nitrile, and (B) about 60-95% by weight o a copolymer consist-ing of (c) about 55-95% by weight of styrene, , and i (d) about 5-45% by weight of acrylo-;¦ nitrile.

, 15 It will be appreciated from the foregoing disclosure that the use of this expression does not imply that these com-:: i ponents must be free of any other additives. Indeed, natural pulp, homo polyvinyl alcohol, acrylic polymers, and many other additives may also be present, as disc].osed herein, without interfering with the advantages and beneficial properties that are attained) and without departing from the spirit an~l scope of the invention as defined in the appended claims ~ Ga~
.~ . .

: ' ~

.

Claims (12)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A synthetic fiber capable of being fibrillated for forming paper, consisting essentially of:
(A) about 5-40% by weight of a graft copolymer consisting of (a) about 20-80% by weight of polyvinyl alcohol, and (b) about 80-20% by weight of acrylonitrile, and (B) about 60-95% by weight of a copolymer consisting of (c) about 55-95% by weight of styrene, and (d) about 5-45% by weight of acrylonitrile.

2. A synthetic fiber according to claim 1, wherein said graft copoly-mer (A) is dispersed as a plurality of particles in said copolymer (B) and oriented as an independent phase along the direction of the fiber axis.

3. A synthetic fiber according to claim 1, wherein the average degree of polymerization of said polyvinyl alcohol is about 600-3400.

4. A synthetic fiber according to claim 1 further containing, in a range from trace amounts to about 23% by weight, homopolyvinyl alcohol.

5. A synthetic fiber according to claim 4, wherein said polyvinyl alcohol and homopolyvinyl alcohol in said graft copolymer are present in an amount of about 2-55% by weight based on the entire weight of the fiber.

6. A synthetic fiber according to claim 1 further containing, in a range from trace amounts to about 35% by weight, a polymer of the acrylic series.

7. A synthetic fiber according to claim 6, wherein said polymer of the acrylic series is polyacrylonitrile.

8. A method for producing a synthetic fiber capable of being fibrill-ated for forming paper, which comprises dissolving a composition consisting essentially of:

(A) about 5-40% by weight of a graft copolymer consisting of (a) about 20-80% by weight of polyvinyl alcohol, and (b) about 80-20% by weight of acrylonitrile, and (B) about 60-95% by weight of a copolymer consisting of (c) about 55-95% by weight of styrene, and (d) about 5-45% by weight of acrylonitrile in a solvent selected from the group consisting of dimethyl sulfoxide and dimethyl acet-amide, and wet spinning the resulting solution into an aqueous coagulating bath.

9. A method according to claim 8 wherein the spinning is conducted in a manner to produce an undrawn yarn and the resulting undrawn yarn is contract-ed by subjecting it to wet heat-treatment.

10. Synthetic pulp which comprises (A) about 5-40% by weight of a graft copolymer consisting of (a) about 20-80% by weight of polyvinyl alcohol, and (b) about 20-80% by weight of acrylonitrile, and (B) about 60-95% by weight of a copolymer consisting of (c) about 55-95% by weight of styrene, and (d) about 5-45% by weight of acrylonitrile.

11. Synthetic paper made from the synthetic pulp of claim 10.

12. Synthetic paper made from the synthetic pulp of claim 10, combined with wood pulp.