Blanket substrate and blanket

Abstract: Provided are a blanket substrate having excellent properties of good dimension stability, uniform thickness, resistance of cyclic compression and good adhesiveness to rubber, and a blanket comprising the substrate. The substrate comprises spun yarn of polyvinyl alcohol based fibers, in which the fibers have primary ridged streaks as formed on their surface in the direction of the fiber axis with finer secondary ridged steaks formed in the primary ridged streaks, and have a cross section circularity of at least 80 %.

Description: The present invention relates to a blanket substrate and a blanket comprising it for offset printing, etc. A blanket of a laminate composed of 3 or 4 fabric layers and a rubber layer, of which the surface to be contacted with ink is of a smooth rubber layer, has heretofore been widely used. In order to obtain blankets with good printing characteristics, high-quality blanket substrates must be used. Concretely, blanket substrates are required to have high-level properties of (1) good dimension stability with little "elongation", (2) good adhesiveness to rubber layers, and (3) uniform thickness. Blanket substrates with poor dimension stability will be elongated by the load of machine driving, and the elongation lowers the printing accuracy. In order to ensure the intended printing, the "elongated" part must be wound up, which, however, interferes with efficient printing operation. On the other hand, those with low adhesiveness to rubber layers and those of which the thickness is not uniform give uneven prints, and the printing accuracy with them is unsatisfactory. As being uniform in thickness and having good adhesiveness to other layers, substrates made of high-quality yarn of Egyptian cotton are widely used, which, however, are problematic in their production that requires particular treatment with wet heat so as to enhance the dimension stability of the substrates. In addition, even after the treatment, the dimension stability of the substrates is lowered when they are again wetted. As a result, in repeated offset printing, the blankets comprising the substrates are elongated while being pressed against rolls and wetted. If the degree of "elongation" is too large, the blankets must be re-tightened, for which the printing operation must be stopped. In addition, the "elongation" changes the thickness of the blankets, whereby the blankets become not uniform in thickness. The maintenance of the blankets requires much labor. Blankets, if not maintained well, will have poor printing characteristics. In that situation, polyvinyl alcohol (PVA) fibers having good dimension stability and having high affinity for rubber have been proposed for blanket substrates (see JP-A 47-32908, 62-282986). However, since ordinary PVA based fibers have a cocoon-like cross section, blanket substrates comprising them are problematic in that they are often not uniform in thickness, and, in addition, when used for a long period of time, they often lose "resistance to cyclic compression" and their capabilities become unsatisfactory. Specifically, JP-A 47-32908 proposes using spun yarn of PVA based fibers for preventing the elongation of blankets. However, the fibers constituting the spun yarn shall have "interfiber slips" when a mechanical load is applied thereto. After all, therefore, even though high-strength fibers having a high modulus of elasticity are used for substrates, the "elongation" of the substrates comprising them is inevitable. On the other hand, JP-A 62-282986 proposes using high-strength, low-elongation PVA filament yarn for blanket substrates. According to the proposed method, the substrates produced will be elongated little, but their adhesiveness to other layers is low as the surface of the filament yarn has no nap. Therefore, the substrates could not ensure satisfactory printing accuracy. In order to solve the problems noted above, using core yarn prepared by applying short fibers onto the surface of synthetic filaments or long staple fibers having a length of from 10 to 30 cm has been proposed for blanket substrates (see JP-A 63-249696, 6-297877). They say that the core of the core yarn prevents the "elongation" of the blanket substrates comprising the core yarn, and that the short fibers on the surface of the core improve the adhesiveness of the substrates to other layers. In practice, however, it is difficult to produce homogeneous, high-quality core yarn. Therefore, the substrates comprising core yarn are still problematic in that their thickness will be not uniform. In addition, since the short fibers existing on the surface of the core will have "interfiber slips", the dimension stability of the substrates could not be improved to a satisfactory degree. In consideration of the problems noted above, the object of the present invention is to provide a blanket substrate having excellent properties of good dimension stability, uniform thickness, resistance to cyclic compression and good adhesiveness to rubber, and also a blanket comprising the substrate. Specifically, the invention provides the following: (1) A blanket substrate comprising spun yarn of polyvinyl alcohol based fibers, in which the fibers have primary ridged streaks formed on their surface in the direction of the fiber axis with finer secondary ridged steaks formed in the primary ridged streaks, and have a cross section circularity of at least 80 %. (2) The blanket substrate of (1), wherein the width of the primary ridged streaks on the surface of the polyvinyl alcohol based fibers falls between 0.1 and 2 µm, the depth thereof falls between 0.05 and 0.4 µm, and the length thereof is at least 5 µm. (3) The blanket substrate of (1) or (2), wherein the width of the secondary ridged streaks on the surface of the polyvinyl alcohol based fibers falls between 0.01 and 0.05 µm, and the depth thereof falls between 0.01 and 0.05 µm. (4) The blanket substrate of any one of (1) to (3), which has a tensile strength at break in the warp direction of at least 4 g/d, and has a degree of stress of at least 1 g/d when 2 % elongated in the warp direction. (5) The blanket substrate of any one of (1) to (4), which has a degree of thermal shrinkage at 150°C in the warp direction of at most 2 %. (6) The blanket substrate of any one of (1) to (5), wherein the strength of the polyvinyl alcohol based fibers is at least 8 g/d, and the Young's modulus thereof is at least 180 g/d. (7) A blanket comprising at least the blanket substrate of any one of (1) to (6). Fig. 1 is an electromicroscopic picture (x 10,000) showing the surface structure of one embodiment of the fibers to be used in the invention. The invention is characterized in that spun yarn of specific PVA based fibers is used for constructing a blanket substrate. In general, substrates comprising spun yarn could have high adhesiveness to rubber layers, but are problematic in that the short fibers constituting the spun yarn have interfiber slips to lower the dimension stability of the substrates. If blankets are elongated while being used for actual printing operation, they must be re-tightened, for which the printing operation must be stopped. In addition, the "elongation" changes the thickness of the blankets, whereby the blankets become not uniform in thickness, and the printing accuracy with them is lowered. The maintenance of the blankets requires much labor. Moreover, if blankets are deformed while used in printing operation, good prints could not be obtained. The invention is based on the finding that spun yarn of fibers having a specific surface structure is almost free from interfiber slips and that a substrate comprising the spun yarn has good dimension stability. Concretely, in the invention, used are PVA based fibers having primary ridged streaks formed on their surface in the direction of the fiber axis and having finer secondary ridged steaks formed in the primary ridged streaks (see Fig. 1). As having such specific ridged streaks on their surface, the PVA based fibers are effectively prevented from having interfiber slips, and the dimension stability of the substrate comprising the fibers is remarkably improved. In general, it is said that fibers having a high cross section circularity have poor adhesiveness to other layers. However, having the specific ridged structure on their surface, the adhesiveness of the PVA based fibers for use in the invention to rubber layers is remarkably improved even though the fibers have a high cross section circularity. In view of the dimension stability, the uniform thickness, the ability not to lose resistance to cyclic compression, and the adhesiveness of the blanket substrate, it is desirable that the primary ridged streaks on the surface of the PVA based fibers constituting the substrate have a width of from 0.1 to 2 µm, a depth (height) of from 0.05 to 0.4 µm and a length of at least 5 µm, more preferably have a width of from 0.1 to 1 µm, a depth (height) of from 0.07 to 0.3 µm and a length of from 10 to 300 µm. For the same reasons as above, it is also desirable that the secondary ridged streaks on the surface of the PVA based fibers have a width of from 0.01 to 0.05 µm, a depth (height) of from 0.01 to 0.05 µm and a length of at least 0.01 µm. From the viewpoint of the uniform thickness and the ability not to lose resistance of cyclic compression of the substrate, the PVA based fibers constituting the substrate must have a cross section circularity of at least 80 %, but preferably from 90 % to 100 %. The substrates comprising the fibers having such a high cross section circularity are ready to have a uniform thickness, and, in particular, it is easy to apply uniform pressure thereto. Therefore, even when a blanket comprising the substrate is used in printing for a long period of time, it may well keep its resistance to cyclic compression, and its printing characteristics and even durability are good. Fibers having a small cross section circularity are unfavorable, since blanket substrates comprising them are often not uniform in thickness even though their adhesiveness to other layers could be high. It is desirable that the cross section profile of the PVA based fibers is circular, more preferably, substantially completely round, since the substrate comprising the fibers could be uniform in thickness and since uniform pressure could be applied to the substrate blanket to ensure good printing capabilities of the blanket with ease. In general, blankets comprising fibers with a higher cross section circularity are often problematic in that their adhesiveness to other layers is poor. However, in the invention, the fibers to be used have fine ridged streaks on their surface, and therefore, the substrate comprising the fibers has good adhesiveness to rubber. The cross section circularity of fibers as referred to herein indicates a value of B/A x 100, in which A means the area of the minimum circumscribed circle around the cross section of the fiber, and B means the area of the cross section of the fiber. The single fiber denier of the PVA based fibers for use in the invention is not specifically defined, but preferably falls between 0.1 and 20 d. In view of the spinning step for preparing the fibers and of the uniform thickness and the adhesiveness to other layers of the blanket substrate comprising the fibers, it is more desirable that the single fiber has a fineness of from 0.5 to 3 d. In view of the durability and the dimension stability of the blanket substrate, it is also desirable that the single fiber strength is at least 8 g/d, more preferably at least 10 g/d, even more preferably at least 12 g/d, and that the Young's modulus of the fibers is at least 180 g/d, more preferably at least 200 g/d, even more preferably at least 250 g/d. The uppermost limit of the fiber strength and that of the Young's modulus are not specifically defined. In general, however, the fiber strength may be at most 30 g/d, and the Young's modulus may be at most 500 g/d. For the same reasons as above, it is still desirable that the elongation at break of the fibers falls between 2 and 8 %. The method for producing the PVA based fibers for use in the invention is not specifically defined. One preferred method comprises wet-jetting a spinning solution as prepared by adding PVA to an organic solvent, into a coagulation bath. One preferred embodiment of the method is mentioned below. It is desirable that PVA to be used has a mean degree of polymerization, as obtained according to a viscosity method in an aqueous solution at 30°C, of at least 500. PVA of that type is ready to give PVA based fibers having a high strength and a high modulus of elasticity. Especially preferred is PVA having a viscosity-average degree of polymerization of at least 1000, more preferably at least 1500, as being more ready to give high-strength PVA based fibers. In view of the cost, it is preferably 5,000 or less. The saponification degree of PVA to be used is not also specifically defined. However, in view of the heat resistance and the mechanical properties of the PVA based fibers to be produced, it is desirable that PVA has a saponification degree of at least 98.5 mol%, more preferably from 99.0 mol% to 100 mol%. The PVA based fibers produced could have good durability and good dimension stability even under severe conditions. The vinyl alcohol-based polymers to be used may be copolymerized with any other monomers. However, in order not to interfere with the properties of PVA, the copolymerization rate is preferably at most 10 mol%, more preferably at most 2 mol%. The PVA based fibers may contain any other components (polymers, etc.) except vinyl alcohol-based polymers, so far as the additional components do not interfere with the effect of the invention. The solvent to be used for the fiber production is not specifically defined, and any and every organic solvent capable of dissolving PVA may be used. It includes, for example, polar solvents such as dimethylsulfoxide (DMSO), dimethylformamide, dimethylimidazolidine, etc., and polyhydric alcohols such as glycerin, ethylene glycol, etc. Mixtures of two or more of those solvents and even mixtures of the solvent with water may also be used. Of many such solvents, DMSO is preferred, as being able to dissolve PVA at relatively low temperatures without thermally deteriorating and coloring the resulting PVA solution. The PVA concentration in the spinning solution varies, depending on the degree of polymerization of PVA and the type of the solvent used. In general, it may fall between 2 and 30 % by weight, but preferably between 3 and 20 % by weight. The spinning solution to be used in the invention may contain various additives, in addition to PVA and the solvent. The additives include, for example, surfactants, antioxidants, pH-controlling agents such as acids, gelation promoters such as boric acid, etc. A predetermined amount of any of those additives may be added to the spinning solution. Where DMSO or the like having a relatively high freezing point is used as the solvent, methanol or the like having a coagulating ability could be added to the spinning solution within the range not coagulating PVA in the solution. Adding methanol or the like to the spinning solution within that range is preferred, as the solution is protected from being frozen owing to freezing point-depressing effect of methanol or the like added thereto, even when the temperature of the coagulation bath used for spinning the solution is lower than the freezing point of the solvent. The spinning solution may be jetted out into the coagulation bath through nozzles having a desired diameter. The coagulation bath comprises an organic solvent having the ability to coagulate PVA. The solvent is not specifically defined, and any and every solvent having the ability to coagulate PVA is employable. It includes, for example, alcohols such as methanol, ethanol, etc., and ketones such as acetone, methyl ethyl ketone, etc. Of those, preferred is methanol, as it is inexpensive and its coagulating ability is relatively mild enough to easily form uniform and fine crystal structures. The organic solvent may be combined with an inorganic salt such as calcium chloride, sodium rhodanide, etc. However, in view of the mechanical properties of the fibers to be produced, it is desirable that a solvent of spinning solution is incorporated into the coagulation bath. The solvent of spinning solution content of the coagulation bath varies, depending on the solvent having coagulation capabilities, but is preferably from 5 to 70 % by weight. The bath gives a uniform gel through mild coagulation therein. More preferably, the solvent content of the bath is from 10 to 50 % by weight, even more preferably from 14 to 45 % by weight. For producing fibers having a high strength and a high modulus of elasticity, it is desirable that the temperature of the coagulation bath is not higher than 20°C, more preferably not higher than 15°C, even more preferably from 0°C to 10°C. The spinning method for producing the PVA based fibers for use in the invention must be a wet-spinning method in which the nozzle is kept in direct contact with the coagulation bath. Any other dry/wet-spinning method or gel-spinning method in which the nozzle is spaced from the coagulation bath via an air gap layer therebetween is not employable herein, since the surface of the fibers produced therein could not have the desired ridged structure. Specifically, in such a dry/wet-spinning method or gel-spinning method, secondary ridged streaks could be formed on the surface of the fibers produced, but primary ridged streaks having a larger structure could not be formed thereon. The fibers not having primary ridged streaks will often have interfiber slips, and substrates comprising them could not have good adhesiveness to other layers and, therefore, their properties including durability are poor. The reason why the structure of the fiber surface varies, depending on the spinning method employed, is not as yet clear. At least at present, it is believed that, in the wet-spinning method, the spinning solution having been jetted out through the nozzle into the coagulation bath is immediately solidified, and, as a result, the viscoelastic condition of the spinning solution just before being jetted out through the nozzle could be directly transferred to the surface of the solidified fibers to thereby make the fibers have specific ridged streaks on their surface. On the other hand, it is believed that, in the dry/wet-spinning method and the gel-spinning method, the spinning solution is jetted out through the nozzle into the air gap layer between the nozzle and the coagulation bath, in which the solidification rate of the jetted solution is small, and, as a result, the solution is solidified after the viscoelastic condition of the solution has been attenuated in some degree, and therefore, the solidified fibers could not have specific ridged streaks on their surface. Specifically, it is believed that the reason for the significant difference in the surface structure between the fibers as produced in the wet-spinning method and those as produced in the dry/wet-spinning method (or in the gel-spinning method) will be that, in the wet-spinning method, the fibers produced are relaxed after the solidification of their surface since the solidification rate of the surface of the polymer flow just after having been jetted out through the nozzle is extremely high, while, in the dry/wet-spinning method, the fibers produced are first relaxed and then solidified. Next, the fibers having been solidified in the coagulation bath are taken out, and it is desirable to remove the solvent and others from the solidified fibers through extraction washing. As the extraction bath for the removal, preferably used is an organic solvent having coagulation capabilities. Next, a desired oil agent is applied to the thus-washed fibers, which are then dried. In order to prevent the fibers from being stick together, it is desirable that the fibers are wet-drawn in one or more stages in any desired step before the drying step. Preferably, the wet-drawing magnification falls between 2.5 and 5.5. It is desirable that the thus-obtained fibers, which are to be spun into spun yarn, are drawn under heat at high temperatures for orientation and crystallization to thereby make them have a high strength and a high modulus of elasticity. The thus-processed fibers are not stuck together, and therefore have high thermal drawability. Accordingly, they may be drawn with ease to a high degree of magnification into high-strength, high-modulus fibers. The thermal drawing system employable herein is not specifically defined, in which is used any of non-contact or contact heaters, hot air furnaces, oil bathes, high-temperature water vapors, etc. The thermal drawing may be effected in two or more stages, for which the temperature is controlled in plural stages. Preferably, the drawing temperature is not lower than 210°C, and, more preferably, it falls between 220 and 250°C. Also preferably, the total drawing magnification falls between 8 and 26, more preferably between 10 and 24. After having been thermally drawn, the fibers may be optionally processed with an oil agent. If desired, they may be further processed for crosslinking the hydroxyl groups therein. Observing the surface of the fibers obtained in the manner as above, in a replica method that will be mentioned hereinunder, the fibers are seen to have, on their surface, a microscopic double-ridged structure that comprises relatively large primary ridged streaks running continuously in the direction of the fiber axis, and secondary ridged streaks definitely smaller than the primary streaks. In. the invention, the PVA based fibers must be spun into spun yarn. In place of spun yarn, if filament yarn or core yarn of fibers is used as the essential component for producing blanket substrates, the resulting substrates could not have good adhesiveness to other layers and are not uniform in thickness, and, therefore, they could not attain the object of the invention. Needless-to-say, the spun yarn may be combined with any other yarn (filament, core yarn, etc.) within the range not interfering with the effect of the invention, but it is more desirable that the substrate of the invention is substantially composed of spun yarn only. In the invention, the spun yarn for the substrate is of specific PVA based fibers as above. Therefore, the substrates composed of the spun yarn could have good adhesiveness to other layers, and, in addition, they have good dimension stability and are uniform in the thickness. In particular, according to the spinning method noted above, in which a spinning solution prepared by dissolving PVA in a solvent is used, the fibers produced are hardly glued together. Therefore, the fibers can be efficiently spun into spun yarn of high quality, and the blanket substrate composed of the spun yarn has extremely excellent properties of good adhesiveness to other layers, good mechanical characteristics and good dimension stability. Specifically, in order to obtain spun yarn of high quality, the fibers must be homogeneously carded in the carding step in the spinning process, and it is important that the short fibers which are in aggregate are not stuck together. PVA based fibers produced in a conventional wet-spinning method in which, for example, an aqueous solution of PVA is jetted out into Glauber's salt or the like are lightly stuck together in the step of drying them, and therefore, could not be spun into spun yarn of high quality. The spun yarn of such conventional PVA based fibers could not attain the effect of the invention. The method of spinning the specific PVA based fibers into spun yarn for use in the invention is not specifically defined. One preferred example of the spinning method is as follows : The fibers are previously crimped in a crimping step, then cut into pieces having a length of from 10 to 80 mm, and the resulting fiber aggregates that are ready to be spun are spun in a spinning system like that for spinning cotton, into the intended spun yarn. In this process of producing the spun yarn, any other fibers except the specific PVA based fibers may be combined with the specific PVA based fibers within the range not interfering with the effect of the invention. In order to efficiently attain the effect of the invention, it is desirable that the proportion of the specific PVA based fibers in the spun yarn is at least 50 % by weight, more preferably at least 80 % by weight, even more preferably at least 90 % by weight. Most preferably, the spun yarn is of 100 % by weight of the specific PVA based fibers. The thickness of the spun yarn may be suitably determined. For example, the spun yarn may have a yarn count number of from #10 to #80. For example, #20 spun yarn of the PVA based fibers of the invention may have a yarn quality (this is represented by U %) of 9 %, and this is comparable to high-quality, Egyptian cotton spun yarn. The PVA based fibers of the invention may be spun into #50 or higher spun yarn, although spinning conventional PVA based fibers into it is difficult. The blanket substrate that comprises at least partly the spun yarn of that type is excellent, as having much better dimension stability and adhesiveness to other layers, and as being much more uniform in thickness. It is not always necessary that the fabric for the substrate is exclusively composed of spun yarn having the same yarn count. In view of the uniformity of thickness, however, it is desirable that the weft or the warp of the fabric is of spun yarn having substantially the same yarn count (within the range of the ratio, largest yarn count/smallest yarn count ≤ 1.1). For the purpose of reducing the weaving shrinkage of the warp, it is desirable that the yarn count of the spun yarn for the warp is larger than that of the spun yarn for the weft. Concretely, for this, it is desirable to satisfy the condition of (yarn count of the spun yarn for the weft) x 3 ≥ (yarn count of the spun yarn for the warp) ≥ (yarn count of the spun yarn for the weft). If desired, twist yarn composed of from 2 to 10 spun yarns may be woven into the fabric for the substrate. In view of the dimension stability and the durability of the blanket substrate, it is desirable that the strength of the spun yarn is at least 4 g/d, and it is further desirable that the degree of elongation of the spun yarn falls between 5 and 12 % or so. The uppermost limit of the strength of the spun yarn is not specifically defined, but may be generally at most 20 g/d. From the same viewpoints as above, it is desirable that the "U %" of the spun yarn is at most 15 %, more preferably at most 12 %, even more preferably from 0 % to 10 %. The spun yarn mentioned above is formed into fabric for the blanket substrate of the invention. For the substrate, woven fabric of the spun yarn is suitable, as having better mechanical properties, especially having much better mechanical properties and dimension stability as selectively improved in one direction. Above all, more preferred are plain weaves, in view of the easiness of its production and of the mechanical properties of the fabric. In the invention, it is not always necessary that the blanket substrate is composed of only the specific spun yarn comprising the specific PVA based fibers mentioned above. Without interfering with the effect of the invention, any other yarn (spun yarn, filament yarn, etc.) may be combined with the spun yarn comprising the specific PVA based fibers to construct the blanket substrate of the invention. The fibers usable for the additional yarn include PVA based fibers except the specific PVA based fibers noted above, polyester fibers, rayon fibers, cotton fibers, etc. If desired, even twisted yarn composed of the spun yarn comprising the specific PVA based fibers and other spun yarn may also be used in the invention. In order to fully ensure the effect of the invention, it is desirable that the warp for the substrate fabric is partly or entirely of the spun yarn comprising the specific PVA based fibers. It is more desirable that at least 80 % by weight of the warp of the fabric, but more preferably, substantially the whole of the warp thereof is composed of the spun yarn comprising the specific PVA based fibers defined herein. The weft of the fabric is not required to have so high quality, as compared with the warp thereof. Therefore, for the weft, the spun yarn of only the specific spun yarn comprising the specific PVA based fibers as defined herein may not be used to obtain the intended, high-quality blanket substrate. However, in order to more surely attain the effect of the invention, it is desirable that at least 80 % by weight, but more preferably, substantially the whole of the weft is composed of the spun yarn comprising the specific PVA based fibers defined herein. The method for producing the substrate of the invention is not specifically defined. The substrate may be produced with any known method. In view of the dimension stability, the resistance to cyclic compression and the printing utility of the substrate, it is desirable that the thickness of the substrate just before being laminated with other layers into blankets falls between 0.1 and 0.5 mm , and that the unit weight thereof falls between 100 and 300 g/m 2 . From the viewpoints of the dimension stability and the adhesiveness to other layers of the substrate, it is also desirable that the total denier per 1 cm in width in the warp direction falls between 5000 and 15000 d/cm, and that the density of the warp falls between 30 and 130/in. In order to enhance the dimension stability of the substrate, it is desirable that the substrate is subjected to thermal fixation treatment. One preferred method for the thermal fixation comprises stretching the substrate to a degree of at least 5 % in the warp direction followed by thermally fixing it at a temperature of 130°C or higher. The substrate thus having been subjected to thermal fixation in that manner could have much better dimension stability at high temperatures and at ordinary temperature. The stretching step is to remove the structural relaxation of the spun yarn in the fabric, in which the undulations of the yarn oriented in the warp direction are removed so as to straighten the yarn. The most preferred stretching degree varies, depending on the structure of the fabric. It is desirable that the stretching degree is higher than the degree of weaving shrinkage of the fabric in the warp direction. More preferably, the stretching degree is at least 5 %, even more preferably at least 10 %. However, if the stretching degree is too large above a certain value, the fibers constituting the fabric will have internal strain to shrink. If so, the fibers thus having shrunk will be subjected to the thermal fixation in the next step. In that case, the increase in the shrinking degree will be no more effective. For these reasons, it is desirable that the stretching degree is at most 20 %, more preferably at most 15 %. The degree of weaving shrinkage as referred to herein is one to be measured according to the ordinary woven fabric test method of method B in JIS-L-1096. Where the "warp direction" of woven fabric could not be identified, the direction in which the fabric has the highest tensile strength is recognized as the warp direction of the fabric. The concrete stretching method is not specifically defined. For example, the woven fabric to be stretched is held by rubber rollers, and this is stretched between them while the rotating rate of the plural rollers applied to the fabric is separately controlled. As the case may be, the rubber rolls will slid on the raw fabric to be stretched therewith so that the fabric could not be stretched to the desired stretching degree. In that case, the raw fabric may be previously marked in some points, and the stretching degree may be confirmed from the marked points, and may be controlled, if necessary. Through the stretching treatment, the density of the weft of the stretched fabric decreases. Therefore, the decrease in the density of the weft could be the index of the stretching degree. PVA based fibers have a high modulus, and therefore fabric comprising them requires large force for stretching it. For example, for stretching fabric of PVA based fibers to a degree of 5 %, needed is force of at least 9.8066 kN/m (1 tonf/m). Therefore, when the fabric is stretched to a high degree, it is desirable that the fabric is treated under dry heat so as to soften the PVA based fibers constituting it, and thereafter the thus heat-treated fabric is stretched. The fabric having been subjected to such dry heat treatment could be efficiently stretched, and the stretching treatment does not have any significant influence on the structure of the fibers constituting the fabric. Therefore, the dry heat treatment prior to the stretching treatment is preferred for the fiber properties and for the resistance to cyclic compression of the blanket substrate. In particular, the dry heat treatment much increases the degree of stress of the stretched fabric in 2 % elongation. The stress in 2 % elongation varies, depending on the yarn count of the spun yarn constituting the fabric and on the constitution of the fabric. Where fabric having the same constitution is subjected to the same stretching treatment, its properties could be improved by the dry heat treatment. For facilitating the stretching treatment, it is desirable that the temperature for the dry heat treatment is 100°C or higher, more preferably 150°C or higher, even more preferably 180°C or higher. However, in order not to deteriorate the properties of the woven fabric, the temperature for the dry heat treatment is preferably not higher than 230°C. Stretching may be effected after the dry heat treatment, or may be effected simultaneously with it. Stretching may be effected under wet heat (for example, at 100°C or higher). However, since PVA based fibers are softened little with hot water, the latent heat for vaporization of water will be the energy loss. Therefore, wet heat stretching is not efficient, but will rather cause interfiber sticking of PVA based fibers by which the fabric for the substrate will lose its flexibility. For these reasons, wet heat stretching is unfavorable. In order to reduce the shrinkage stress at high temperatures, it is still desirable that the thermal fixation treatment is effected at a temperature of not lower than 140°C, more preferably not lower than 160°C. Where the dry heat treatment is followed by the stretching treatment and further by the thermal fixation treatment in series, it is desirable that the thermal fixation treatment is effected at a temperature lower by at least 10°C, more preferably by at least 20° C, than the temperature for the dry heat treatment, in view of the structural stability and the dimension stability of the substrate. From the viewpoint of the properties of the substrate, it is desirable that the temperature for the dry heat treatment is not higher than 230°C, more preferably not higher than 200°C. The thermal fixation treatment may be effected in a constant length condition. However, from the viewpoint of the dimension stability, it is desirable that the thermal fixation treatment is effected in a relaxed condition in some degree. Through the stretching treatment and the thermal fixation treatment, the blanket substrate obtained could have much improved dimension stability, and its thickness could be more uniform. From the viewpoint of the dimension stability of the blanket, it is desirable that the tensile strength at break of the substrate in the warp direction is at least 4 g/d, more preferably at least 5 g/d, even more preferably at least 6 g/d, and that the degree of stress of the substrate in 2 % elongation in the warp direction is at least 1 g/d, more preferably at lest 1.1 g/d, even more preferably at least 1.2 g/d. The uppermost limit is not specifically defined. In general, however, the tensile strength at break of the substrate in the warp direction may be at most 20 g/d, and the degree of stress thereof in 2 % elongation in the warp direction may be at most 10 g/d. In order to prevent the dimension change (shrinkage) in vulcanization to be effected at high temperatures after the lamination of the blanket substrate with a rubber layer, it is desirable that the degree of thermal shrinkage of the substrate at 150°C in the warp direction is at most 2 %, more preferably at most 1 %, even more preferably at most 0.7 %, still more preferably from 0 % to 0.5 %. The blanket that comprises the substrate having such a small degree of shrinkage could have much more excellent properties. In its practical use, the blanket is not elongated, and its thickness does not vary. The method for producing the blanket of the invention is not specifically defined, so far as the blanket comprises at least the substrate of the invention. The blanket is generally composed of a plurality of substrate layers and a surface rubber layer, for which the blanket substrate of the invention may be combined with any other substrates, or the plural substrate layers are all of the substrate of the invention. In view of the printing characteristics, it is desirable that the blanket comprises four substrate layers. Especially in view of the uniformity in quality of the blanket, it is desirable that the two interlayers are of substrates having substantially the same constitution. In addition, in order to enhance the dimension stability of the blanket, it is desirable that one outer layer (layer X) to which a rubber layer is adhered is of a substrate made of spun yarn having substantially the same constitution as that of the spun yarn constituting the substrate for the interlayers, while the density of the warp and the weft of the substrate for the layer X is larger than that of the substrate for the interlayers, that the other outer layer (layer Y) opposite to the layer X is of a substrate made of spun yarn having a smaller yarn count than that of the spun yarn constituting the substrate for the interlayers, and that those substrate layers are laminated in the defined order. More concretely, it is desirable that, in the substrates for the interlayers and the layer X, the warp is of spun yarn of from #10 to #30, and the weft is of spun yarn of from #50 to #70, and that, in the substrate for the layer Y, the warp and the weft are both of spun yarn of from #10 to #30. In general, in blankets, the dimension change in the substrate layers remoter from the surface rubber layer is larger. The blanket substrate of the invention has especially excellent dimension stability. Therefore, it is desirable that, in the blanket of the invention, at least the outermost layer (this is the most remotest from the surface rubber layer) is of the substrate of the invention. Needless-to-say, the blanket of the invention may have any additional layers except the substrate layers and the surface rubber layer. For example, it may have a compressible layer of sponge or the like. It is desirable that the interlayer adhesive is of a liquid substance of nitrile rubber, chloroprene rubber or the like. The substrate may be processed for enhancing its adhesiveness to other layers. For the surface rubber layer to be laminated on the substrate, for example, employable are natural rubber, chloroprene rubber, nitrile rubber, vulcanized rubber, polyurethane rubber, fluorine rubber, acrylic rubber, hydrin rubber, etc. In view of the printing characteristics of the blanket, especially preferred is nitrile rubber. If desired, additives of a vulcanizing agent, a vulcanization promoter and the like may be added to the rubber for the rubber layer. The method of laminating the rubber layer to the substrate layers is not specifically defined. For example, a calender roll may be used for the lamination. As the case may be a solution of rubber may be applied onto the laminate of substrate layers. In that case, the rubber solution may be applied onto the laminate of substrate layers by the use of a knife coater, a roll coater or the like, thereby forming the surface rubber layer on the laminate of substrate layers. It is desirable that the rubber layer thus laminated has a unit weight of from 100 to 1000 g/m 2 . After having been laminated in that manner, the rubber layer may be vulcanized to complete the blanket. The blanket of the invention is applicable to any and every type of printing, but is preferably used in offset printing. The invention is described in more detail with reference to the following Examples, which, however, are not intended to restrict the scope of the invention. Degree of polymerization of PVA According to JIS K6726, the limiting viscosity [η] of an aqueous solution of PVA at 30°C is measured. From the value measured, obtained is the degree of polymerization of PVA, as log P = 1.63 log([η] x 10 4 /8.29) in which P is the mean degree of polymerization of PVA. Tensile strength (g/d), elongation at break (%), and Young's modulus (g/d) of fibers According to JIS L1013, a 20-cm sample of fibers having been previously conditioned for the moisture content is tested at a deformation rate of 50 %/min under an initial load of 0.1 g/dr for its physical properties of tensile strength, elongation at break and Young's modulus. Width, depth and length of primary ridged streaks, and width, depth and length of secondary ridged streaks on the surface of fibers Using a sheet film of polyethyl methacrylate, formed is a one-stage molding replica of the surface of fibers under the condition of 120° C/0.8 kg/cm 2 . This is shadowed through vacuum vapor deposition with a platinum-palladium alloy, at an angle of tan  = 0.7 in the direction perpendicular to the fiber axis. The shadowed replica is reinforced by vacuum depositing carbon thereon in the direction perpendicular to the fiber axis and the film surface and then the polyethyl methacrylate Carbon is deposited thereover at the top of the replica also through vacuum vapor deposition, and the carbon is reinforced. The sheet film of polyethyl methacrylate film is dissolved off. The 2-stage replica thus prepared is held on a sheet mesh and photographed with a transmission-type electron photomicrographer at magnification of 5,000. Measurement of the fine, ridged streaks on the surface of the fibers is made on the reversed print (x 30000) of the picture. The depth of the streaks is obtained, based on the angle for the shadowing. Cross section circularity, % In the microphotographic picture presenting the cross section of fibers, the cross section circularity of the fibers, B/A x 100, is obtained, in which A indicates the area of the minimum circumscribed circle around the cross section of the fiber, and B indicates the area of the cross section of the fiber. U % U % indicates the percentage of mean unevenness deviation of yarn, and this is obtained according to method A for fiber unevenness in JIS-L-1095 (test method for ordinary spun yarn). Constitution of woven fabrics A: This is a plain weave, in which the warp is of twisted yarn of two #20 spun yarns and its density is 50/in, and the weft is of single #20 spun yarn and its density is 50/in. B: This is a plain weave, in which the warp is of twisted yarn of four #60 spun yarns and its density is 65/in, and the weft is of single #30 spun yarn and its density is 65/in. C: This is a plain weave, in which the warp is of twisted yarn of two #60 spun yarns and its density is 110/in, and the weft is of single #30 spun yarn and its density is 75/in. Tensile strength at break of fabric in the warp direction, g/d The tensile tenacity at break of fabric (g/cm) in the warp direction is divided by the fiber denier (d/cm) that corresponds to the total thickness of the warp existing in 1 cm-width in the warp direction of the fabric to obtain the tensile strength at break of the fabric in the warp direction (g/d). The tensile tenacity at break of fabric is obtained according to JIS-L-1096 for the test method of ordinary woven fabric. Stress of fabric in 2 % elongation in the warp direction, g/d The stress per 1 cm in width of fabric in 2 % elongation, which is obtained from the tension-load curve of the fabric, is divided by the fiber denier that corresponds to the total thickness of the warp existing in 1 cm-width in the warp direction of the fabric to obtain the stress of the fabric in 2 % elongation in the warp direction (g/d). The tension-load curve of fabric is obtained according to JIS-L-1096 for the test method of ordinary woven fabric. Degree of thermal shrinkage at 150°C of fabric in the warp direction, % A fabric is left in a hot air oven at 150°C under no tension for 15 minutes, and the length of the dimension shrinkage in the warp direction is measured. The length of shrinkage is divided by the original length of the non-treated fabric to obtain the percentage of thermal shrinkage (%) of the fabric. Dimension stability A blanket is set in an offset printer, which is run to give about 100 test prints. Then, the printer is continuously run under the same condition as that for the test prints. The continuous printing gives 1000 prints. The prints are checked for image gaps, if any, therein, on the basis of which the blanket is evaluated. If the blanket used is elongated, the prints shall have image gaps. The dimension stability of blankets tested is represented by A (for good blankets that gave prints all with no image gaps) or C (for bad blankets that gave some prints with image gaps). Uniformity in thickness A blanket is set in an offset printer, which is continuously run under the same condition as that for the test prints as above. The continuous printing gives 1000 prints. The prints are checked with the naked eye and through a loupe for the details, as to the condition of the dots and as to the presence or absence of any defective parts where the density of the images is not uniform. If the blanket used has some swollen defects, the density of the images shall be partly increased and the dots are enlarged. The printing is effected on A1-size paper. The uniformity in thickness of blankets tested is represented by A (for good blankets that gave prints all substantially having no defective parts), B (for average blankets that gave some prints having from 1 to 3 defective parts), or C (for bad blankets that gave some prints having 4 or more defective parts). Resistance to cyclic compression A roll with a piece of embossed paper (size 1 cm x 1 cm, thickness 0.1 mm) attached on its surface is pressed 100 times against the surface of a blanket, and the blanket is tested in continuous printing under the same condition as that for the test prints above. The condition of the prints is checked for its change, if any. The prints obtained by the use of the blanket are compared with those obtained by the use of a comparative blanket having a cotton substrate, on the basis of which the resistance to cyclic compression of the blanket tested is evaluated. If the blanket used has lost its resistance to cyclic compression in some parts, the density of the images printed shall be partly thin and the images are partly whitened. The resistance to cyclic compression of blankets tested is represented by A (for good blankets of which the resistance to cyclic compression <image whitening resistance> is better than that of the comparative cotton blanket), B (for average blankets of which the resistance to cyclic compression is comparable to that of the comparative cotton blanket), or C (for bad blankets of which the resistance to cyclic compression is worse than that of the comparative cotton blanket). The comparative cotton blanket used is one as prepared in Comparative Example 8. Adhesiveness to rubber (peeling-resistant tenacity), kg/in Blankets were produced with the same method as Example 7, and subjected to the T-type peeling test of JIS K6323 for "rubber-laminated fabrics". In the test, the peeling-resistant tenacity between the fabric layer and the rubber layer is measured. Reference Examples 1 to 3 PVA having a viscosity-average degree of polymerization of 1700 and a saponification degree of 99.8 mol% was added to DMSO to be 10 % by weight, and dissolved therein at 90°C for 8 hours in a nitrogen atmosphere, and the resulting solution was wet-spun into a coagulation bath of methanol/DMSO = 70/30 by weight at 5°C through a circular nozzle with 1000 orifices each having a diameter of 0.08 mm. The resulting, solidified fibers were drawn to a total drawing ratio of 4 times in a wet-drawing bath of methanol/DMSO = 95/5 by weight at 40° C, then contacted with a countercurrent flow of methanol to remove DMSO therefrom through extraction, then dried in a hot air drier, and then drawn under heat in a hot air furnace at 240°C. The total drawing ratio was 17 times. An oil agent was applied to the fibers, which were then dried. Thus was obtained fiber tow. The fibers obtained herein had a single fiber denier of 1.0 d, a strength of 14.5 g/dr, a degree of elongation at break of 5.1 %, a Young's modulus of 298 g/d, and a cross section circularity of 100 %. The cross section profile of the fibers was substantially completely round. The surface of the fibers was observed with an electronic microscope according to a replica method. The primary ridged streaks found on their surface had a width of from 0.2 to 0.9 µm, a depth of from 0.1 to 0.2 µm and a length of at least 50 µm. The width and the depth of the secondary ridged streaks also found thereon were both from 0.02 to 0.3 µm, and the length thereof was at least 0.05 µm. The fiber tow was crimped under heat, and then cut into fiber pieces having a length of 38 mm. These are to be spun into spun yarn. The fibers had high quality with no interfiber sticking found therein. The fibers were spun according to a cotton-spinning method into #20 spun yarn (Reference Example 1, having a mean tenacity of 1354 gf, a mean strength of 5.1 g/d, a mean elongation of 9.2%, and U % of 9.2), #30 spun yarn (Reference Example 2, having a mean tenacity of 886 gf, a mean strength of 5.0 g/d, a mean elongation of 8.2 %, and U % of 11.1), and #60 spun yarn (Reference Example 3, having a mean tenacity of 435 gf, a mean strength of 4.9 g/d, a mean elongation of 7.0%, and U % of 12.1). Reference Example 4 The same process as in Reference Example 1 was repeated, except that the spinning solution of aqueous PVA was jetted into a bath of Glauber's salt to prepare PVA based fibers (Kuraray's "1005C20/1"). The fibers obtained herein had a single fiber denier of 1.0 d, a strength of 7 g/dr, a degree of elongation at break of 13.5 %, a Young's modulus of 180 g/d, and a cross section circularity of 30 %. The cross section of the fibers had a cocoon-like profile. The surface of the fibers was observed with an electronic microscope according to a replica method. Neither primary ridged streaks nor secondary ridged streaks were found. The fibers were spun according to the same cotton-spinning method as in Reference Example 1, into #20 spun yarn. Regarding its properties, the resulting #20 spun yarn had a mean tenacity of 850 gf, a mean strength of 3.2 g/d, a mean elongation of 16.0 %, and U % of 16.0. The fibers were partly stuck together, and their quality was poor. Reference Example 5, Reference Example 6 The same process as in Reference Example 1 was repeated, except that PVA based fibers of Kuraray's "1006C20/1" were used herein. The fibers used herein had a single fiber denier of 1.0 d, a strength of 9.8 g/dr, a degree of elongation at break of 11 %, a Young's modulus of 130 g/d, and a cross section circularity of 30 %. The cross section of the fibers had a cocoon-like profile. The surface of the fibers was observed with an electronic microscope according to a replica method. Neither primary ridged streaks nor secondary ridged streaks were found. The fibers were spun according to the same cotton-spinning method as in Reference Example 1, into #30 spun yarn (Reference Example 5) and #60 spun yarn (Reference Example 6). Regarding their properties, the #30 spun yarn had a mean tenacity of 1400 gf, a mean strength of 5.6 g/d, a mean elongation of 10.0 %, and U % of 11, and the #60 spun yarn had a mean tenacity of 720 gf, a mean strength of 5.2 g/d, a mean elongation of 9.5 %, and U % of 12.3. Reference Examples 7, 8, 9 In the same manner as in Reference Example 1 except that Egyptian cotton was used in place of the PVA based fibers, prepared were #20, #30 and #60 spun yarns. Regarding their properties, the #20 spun yarn had a mean tenacity of 770 gf, a mean strength of 3.0 g/d, a mean elongation of 9 %, and U % of 9.0, the #30 spun yarn had a mean tenacity of 570 gf, a mean strength of 2.9 g/d, a mean elongation of 8.3 %, and U % of 9.8, and the #60 spun yarn had a mean tenacity of 290 gf, a mean strength of 2.7 g/d, a mean elongation of 7.6 %, and U % of 10.5. Examples 1 to 6, Comparative Examples 1 to 6 Using the spun yarns obtained hereinabove, plain weaves were prepared as in Table 1 below. Next, the fabrics were subjected to thermal fixation treatment under the condition shown in Table 1 to produce blanket substrates. The dry heat treatment and the stretching treatment both at 210°C were effected by passing the blanket held between two rubber rolls under tension through a hot air furnace over a period of 2 minutes. The thermal fixation treatment was effected under mild tension. The test data are shown in Table 1. Example 7 Two blanket substrates as produced in Example 2 were bonded with a nitrile rubber-type adhesive, and vulcanized under heat at 150°C to obtain a laminate. One blanket substrate as produced in Example 3 was laminated on one surface of the laminate (over this substrate, a surface rubber layer is to be laminated), and vulcanized under heat in the same manner as above. Next, one blanket as produced in Example 1 was laminated on the other surface of the laminate (this surface is opposite to that to be laminated with a surface rubber layer), and vulcanized under heat also in the same manner as above. Thus was obtained a substrate layer of a laminate of four blanket substrates. Next, a nitrile rubber solution was repeatedly applied onto the surface of the substrate layer, and then vulcanized under heat at 150°C to form a surface rubber layer thereon. Thus was finished the production of a blanket. The blanket produced herein was on the grade A with respect to all the dimension stability, the resistance to cyclic compression and the uniformity in thickness. In addition, the adhesiveness of the substrate layer to the rubber layer was 6.0 kg/cm and was high. It is known that the properties of the blanket are extremely good. After the dimension stability test, the blanket was subjected to a continuous printing test. In the continuous printing test, the dimension stability of the blanket of this Example was much better than that of the cotton blanket of Comparative Example 8 to be mentioned below. The prints obtained by the use of the blanket of this Example all had no image gaps. Example 8 A blanket was produced in the same manner as in Example 7, except that the substrate of Example 4 was used in place of that of Example 1, the substrate of Example 5 was used in place of that of Example 2, and the substrate of Example 6 was used in place of that of Example 3. The blanket produced herein was on the grade A with respect to both the dimension stability and the uniformity in thickness, and on the grade B with respect to the resistance to cyclic compression. In addition, the adhesiveness of the substrate layer to the rubber layer was 6.0 kg/cm and was high. It is known that the properties of the blanket are extremely good. Comparative Example 7 A blanket was produced in the same manner as in Example 7, except that the substrate of Comparative Example 1 was used in place of that of Example 1, the substrate of Comparative Example 2 was used in place of that of Example 2, and the substrate of Comparative Example 3 was used in place of that of Example 3. The dimension stability, the uniformity in thickness and the resistance to cyclic compression of the blanket produced herein were all not good, as being all on the grade C. In addition, the adhesiveness of the substrate layer to the rubber layer was 4.5 kg/cm and was lower than that in the Examples. Comparative Example 8 A blanket was produced in the same manner as in Example 7, except that the substrate of Comparative Example 4 was used in place of that of Example 1, the substrate of Comparative Example 5 was used in place of that of Example 2, and the substrate of Comparative Example 6 was used in place of that of Example 3. The dimension stability of the blanket produced herein was on the level A, but the uniformity in thickness and the resistance to cyclic compression thereof were both on the level B. In addition, the adhesiveness of the substrate layer to the rubber layer was 4.5 kg/cm and was lower than that in the Examples. While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

Claims: A blanket substrate comprising spun yarn of polyvinyl alcohol based fibers, in which the fibers have primary ridged streaks as formed on their surface in the direction of the fiber axis with finer secondary ridged steaks formed in said primary ridged streaks, and have a cross section circularity of at least 80 %. The blanket substrate as claimed in claim 1, wherein the width of the primary ridged streaks on the surface of the polyvinyl alcohol based fibers falls between 0.1 and 2 µm, the depth thereof falls between 0.05 and 0.4 µm, and the length thereof is at least 5 µm. The blanket substrate as claimed in claim 1 or 2, wherein the width of the secondary ridged streaks on the surface of the polyvinyl alcohol based fibers falls between 0.01 and 0.05 µm, and the depth thereof falls between 0.01 and 0.05 µm. The blanket substrate as claimed in any one of claims 1 to 3, which has a tensile strength at break in the warp direction of at least 4 g/d, and has a degree of stress of at least 1 g/d when 2 % elongated in the warp direction. The blanket substrate as claimed in any one of claims 1 to 4, which has a degree of thermal shrinkage at 150°C in the warp direction of at most 2 %. The blanket substrate as claimed in any one of claims 1 to 5, wherein the strength of the polyvinyl alcohol based fibers is at least 8 g/d, and the Young's modulus thereof is at least 180 g/d. A blanket comprising at least the blanket substrate of any one of claims 1 to 6.

Inventors: HAMADA TOSHIHIRO, ISE TOMOKAZU, SHIRAKI KUNIHIRO, TAKAI NOBUYOSHI, HAMADA, TOSHIHIRO, ISE, TOMOKAZU, SHIRAKI, KUNIHIRO, TAKAI, NOBUYOSHI
Applicants: KURARAY CO, KURARAY CO., LTD.
Application year: 1999
Main classification: B41N10
IPC codes: D01F6, B41N10