PAPER MANUFACTURING METHOD

Abstract: A paper manufacturing method capable of obtaining papier which meets the requirements for the smoothness and stiffness (rigidity) of the surface thereof. This method is characterized in that the paper having a bonedry water content of 1.8-7% is treated by a thermal calender under the conditions including a temperature of 150°-300°C and a linear pressure of not less than 40 kg/cm, to obtain paper suitably used as a support for photographs.

Description: This invention relates to a process for producing paper, more particularly to a process for producing paper which is satisfactory in both surface smoothness and rigidity (stiffness) and suited for use as a photographic support, although the use of the paper is not limited thereto. Among the means for improving the surface smoothness of paper are proper selection of pulp material, calendering of paper, increase of pressing force and increase of paper density. For enhancing the rigidity (stiffness) of paper, means are known such as properly selecting the pulp material, making the paper bulky, etc. However, the treatment for improving the surface smoothness of paper and the treatment for enhancing the rigidity (stiffness) of paper are incompatible with each other, and thus it has been difficult to satisfy both requirements at the same time. The object of this invention is to produce an improved paper, of excellent surface smoothness (flatness) and stiffness, suitable in particular for use as a photographic support. Another object of this invention is to provide an improved paper which hardly causes emulsion mottle and, preferably, has a small amount of liquid absorption when photographic emulsion is applied thereto. The present invention resides in a process for producing paper comprising: subjecting a paper which has been internally sized and strengthened and has an absolute dry moisture content (moisture content in absolute dry condition) of 1.8 - 7% to a heat calendering treatment at a temperature of 150 - 300°C under a linear pressure of at least 40 kg/cm; and after said heat calendering treatment, introducing an aqueous solution containing one or more surface strengthening agent(s)to adjust the final moisture content to 5% or more. Internal sizing is a term used in the art of paper making to describe the process of adding suitable chemicals to a papermaking stock slurry which precipitate on the fibres in order to control the penetration of liquids into paper made from it. US-A-3230867 describes a process for finishing paper by calendering to achieve a finer finish, using specified conditions as to moisture content, temperature and linear pressure. However, this document does not provide a process capable of simultaneously enhancing both the stiffness and the surface smoothness of the paper. US-A-2241554 describes a paper making process wherein moisture is introduced into paper before it is calendered. The paper used in this invention can be one which has been made by using whatever available type of pulp and which also may contain chemical additives such as sizing agent, fluorescent agent, etc., but it is preferred to use a paper obtained from, for example, the following method. That is, to a mixed pulp consisting of LBKP (bleached hardwood Kraft pulp), LBSP (bleached hardwood sulphite pulp) and NBSP (bleached softwood sulphite pulp) mixed in predetermined ratios are added additives such as alkyl ketene dimer, polyacrylamide, polyamide epichlorohydrin, starch, fluorescent agent, etc., to form a paper web of a predetermined basis weight. This paper web is dried and then heat-calendered. After this, surface sizing treatment is conducted using a modified polyvinyl alcohol as surface sizing agent, an inorganic electrolyte such as common salt and a fluorescent agent. The reason for defining the moisture content in absolute dry condition (hereinafter referred to as absolute dry moisture content) to 1.8 - 7% is based on the fact that if said moisture content is less than 1.8%, no satisfactory effect of the heat calendering treatment is provided resulting in a poor surface quality of the produced paper, while if said moisture content exceeds 7%, water tends to scatter in the heat calendering treatment to cause a poor paper surface quality. Such trouble will not occur when said moisture content is 1.8 - 7%. The reason for defining the heat calendering temperature to 150 - 300°C in the present invention is that if said temperature is below 150°C, the unevenness of the paper surface remains large as in the case of the conventional calendering treatment and the desired smooth surface can not be obtained, while if said temperature exceeds 300°C, not only the surface smoothness is worsened but there also arises the problem of paper parching. These problems will not occur and a smooth surface can be obtained if said temperature is in the range of 150 - 300°C. Heating can be accomplished by using such means as electric heating, electromagnetic induction and the like. The reason for defining the linear pressure to 40 kg/cm or above in the heat calendering treatment in the present invention is that if said linear pressure is less than 40 kg/cm, the desired surface smoothness can not be obtained. In the present invention, the heat calendering treatment must be carried out before the surface sizing treatment (conducted by using, for example, a modified polyvinyl alcohol solution). As apparent from the examples given later, an especially desirable result can be obtained when (i) the absolute dry moisture content of the paper to be subjected to the heat calendering treatment is 2.5 - 5%, (ii) the air permeability of the paper to be heat calendered is 300 sec or less, (iii) the heat calendering temperature is 160 - 270°c, and (iv) the linear pressure in the heat calendering treatment is 60 kg/cm or above. According to the present invention, water is added after the heat calendering to adjust the final moisture content, using an aqueous solution containing (a) surface strengthening agent(s) (such as polyvinyl alcohol, starch, casein, gelatin, SBR, NBR, polyacrylamide, etc), and optionally dye, fluorescent agent, antistatic agent, anti-fogging agent, etc, according to the object and purpose of use. By such addition of water, the final moisture content is adjusted to 5% or more, preferably 6% or more. Said supply of water to adjust the final moisture content and the addition of various additives to said aqueous solution make the paper obtained according to this invention very suitable as a photographic support described below. The paper obtained according to this invention is especially suited for use as a photographic support (paper support and resin-coated paper support) which must satisfy both required levels of rigidity (stiffness) and surface smoothness. For instance, the paper obtained according to this process can be used directly as a photographic paper support just like a white-and-black copying paper or DTR copying paper, and can be also used indirectly as a photographic paper support just like ordinary white-and-black printing paper, by coating it with an inorganic pigment such as baryta. Further, it can be used as a photographic resin-coated paper support by coating said paper with a polyolefin resin such as polyethylene by, for example, extrusion coating method like ordinary color printing paper. The copying paper and printing paper using the paper obtained according to this process can satisfy rigidity (stiffness) and also has fine smoothness. Regarding the rigidity, in case the paper obtained according to this invention is used as a photographic paper support, such support has a Taber rigidity (Trade Mark) of 11 g-cm or above when the thickness is 165 µm and 13 g-cm or above when the thickness is 175 µ. Also, the photographic resin-coated paper support made by providing a resin coating layer on both sides of the paper obtained according to this invention has a Taber rigidity of 16.5 g-cm or above when the base paper thickness is 165 µ and the total thickness is 220 µ and a Taber rigidity of 19 g-cm or above when the base paper thickness is 175 µ and the total thickness is 230 µ. The paper obtained according to this invention is small in surface unevenness and also has fine surface smoothness, so that when it is used as a photographic support and an emulsion is applied thereto, there hardly occurs the so-called Emulsion mottle (a partial disturbance of the layers in the multi-layer structure which occurs when a phototraphic emulsion is applied to the photographic support). Also, swelling of the emulsion layer after the wet photographic processing is minimized, and there seldom takes place scratching or exfoliation of the emulsion layer. Examples The present invention will hereinafter be described in further detail by way of examples thereof, but it should be understood that the present invention is not limited to these examples. Examples 1 - 5 and Comparative Examples 1 - 2 A pulp composed of 20 parts by weight of NBSP, 50 parts by weight of LBSP and 30 parts by weight of LBKP was beaten to a freeness of 300 ml. This pulp slurry was added with Aquapel (made by Dick Hercules Inc.) mainly composed of an alkyl ketene dimer as sizing agent, Stargum (made by Seiko Kagaku Company) mainly composed of polyacrylamide as strengthening agent and Epinox (made by Dick Hercules Inc.) mainly composed of polyamide epichlorohydrin in amounts of 0.5% by weight, 2% by weight and 0.5% by weight, respectively, based on the pulp and calculated as content in the paper web, to form a paper web having a basis weight of 180 g/m². This paper web was dried to obtain a paper having an absolute dry moisture content of 2.5%, an air permeability of 60 sec. and an internal bond strength of 2.3 kg-cm. The thus prepared paper samples were subjected to heat calendering at temperatures of 120°C, 145°C, 150°C, 160°C, 200°C, 270°C and 300°C, respectively, under a linear pressure of 150 kg/cm and then subjected to surface sizing by using a modified Poval solution as surface sizing agent, followed by drying and an additional calendering treatment at a temperature of 30°C under a linear pressure of 70 kg/cm to make the final moisture content 8.5%. The test results of the obtained papers are shown in Table 1. Heat calendering temperature Flatness (1) Comp. Example 1 120°C 4.5 Example 2 145 4.25 Example 1 150 4.0 Example 2 160 4.0 Example 3 200 3.5 Example 4 270 3.5 Example 5 300 4.0 Examples 6 - 9 and Comparative Examples 3 - 4 Papers were prepared with the same pulp and the same chemical blend as used in Examples 1 - 5 and Comparative Examples 1 - 2, with their absolute dry moisture content before heat calendering being adjusted to 1.5%, 1.8%, 2%, 2.5%, 5%, 7% and 9%, respectively. These papers were subjected to heat calendering at a temperature of 270°C under a linear pressure of 150 kg/cm before surface sizing and then subjected to the same treatment as in Examples 1 - 5 and Comparative Examples 1 - 2 to adjust the final moisture content to 8.5%. The test results of the obtained papers are shown in Table 2 along with the test results of Example 4. The air permeability and internal bond strength of said paper before heat calendering were 60 sec. and 2.3 kg/cm, respectively. Absolute dry moisture content before heat calendering Flatness Comp. Example 3 1.5% 4.5 Example 6 1.8 4.0 Example 7 2 4.0 Example 4 2.5 3.5 Example 8 2.5 3.5 Example 9 7 4.0 Comp. Example 4 9 4.75 As seen from Table 2, a smooth paper with small surface unevenness can be obtained when the absolute dry moisture content (moisture content in absolute dry condition) before heat calendering is 1.8 - 7%, especially 2.5 - 5%. Examples 10 - 12 and Comparative Example 5 Papers prepared by using the same pulp with the same chemical blend as in Examples 1 - 5 and Comparative Examples 1 - 2 and adjusted to have absolute dry moisture content of 5% before heat calendering, an air permeability of 60 sec. and an internal bond strength of 2.3 kg-cm were subjected to heat calendering at a temperature of 270°C under linear pressures of 10 kg/cm, 40 kg/cm, 60 kg/cm and 80 kg/cm, respectively, before conducing surface sizing and then subjected to the same treatment as in Examples 1 - 5 and Comparative Examples 1 - 2 to make the final moisture content 8.5%. The test results of the obtained papers are shown in Table 3. Linear pressure of heat calendering Flatness Comp. Example 5 10 kg/cm 5.0 Example 10 40 4.0 Example 11 60 3.75 Example 12 80 3.75 As seen from Table 3, a good result can be obtained when the linear pressure of heat calendering is 40 kg/cm or above, especially 60 kg/cm or above. Example 13 A pulp comprising 20 parts by weight of NBSP, 50 parts by weight of LBSP and 30 parts by weight of LBKP was beaten to a freeness of 300 ml. This pulp slurry was added with Aquapel (made by Dick Hercules Inc.) mainly composed of an alkyl ketene dimer as sizing agent, Stargum (made by Seiko Kagaku Company) mainly composed of polyacrylamide as strengthening agent and Epinox (made by Dick Hercules Inc.) mainly composed of polyamide epichlorohydrin in amounts of 0.5% by weight, 2% by weight and 0.5% by weight, respectively, based on the pulp and calculated as content in the paper web, to form a paper web having a basis weight of 180 g/m², and this paper web was dried to obtain a paper having an absolute dry moisture content of 2.5%, an air permeability of 350 sec. and an internal bond strength of 2.3 kg-cm. This paper was heat calendered at a temperature of 270°C under a linear pressure of 150 kg/cm and then subjected to surface sizing by using a modified Poval solution as surface sizing agent, followed by drying and additional calendering at a temperature of 30°C under a linear pressure of 70 kg/cm to adjust the final moisture content to 8.5%. The test results of the obtained paper are shown in Table 4 together with the test results of the paper of Examples 4 obtained in the same way as Examples 13 except that the air permeability of the paper was adjusted to 60 sec. Air permeability Flatness Example 4 60 sec 3.5 Example 13 350 4.0 Example 14 and Comparative Example 6 A pulp comprising 100 parts by weight of LBKP was beaten to a freeness of 300 ml, and this pulp slurry was added with Aquapel (made by Dick Hercules Inc.) mainly composed of an alkyl ketene dimer as sizing agent, Stargum (made by Seiko Kagaku Company) mainly composed of polyacrylamide as strengthening agent and Epinox (made by Dick Hercules Inc.) mainly composed of polyamide epichlorohydrin in amounts of 0.5% by weight, 2% by weight and 0.5% by weight, respectively, based on the pulp and calculated as content in the paper web, to form a paper web having a basis weight of 180 g/m². This paper web was dried to obtain a paper having an absolute dry moisture content of 2.5%, an air permeability of 45 sec. and an internal bond strength of 2.0 kg-cm. The thus prepared papers were heat calendered at temperatures of 120°C and 270°C, respectively, under a linear pressure of 150 kg/cm and then subjected to surface sizing by using a modified Poval solution as surface sizing agent, followed by drying and additional calendering at 30°C under a linear pressure of 70 kg/cm to produce papers having a final moisture content of 8.5%. Further, the back side of each of these papers was subjected to a corona discharge treatment and then coated with a mixture of a high-density polyethylene (density: 0.968, Melt Index (MI): 7) and a low-density polyethylene (density: 0.918, MI: 5) (mixing ratio = 1/1) to a thickness of 30 µ by using an extrusion melt coater. Then the opposite side of each paper was similarly subjected to a corona discharge treatment and coated with a low-density polyethylene containing 9% of anatase type titanium oxide (said polyethylene before addition of pigment having a density of 0.918 and a melt index of 5) to a thickness of 25 µ to produce a photographic support. The surface of this photographic support was further subjected to a corona discharge treatment and then coated with a blue-sensitive silver chlorobromide gelatin emulsion layer containing a yellow coupler, an intermediate layer, a green-sensitive silver chlorobromide gelatin emulsion layer containing a magenta coupler, an ultraviolet absorbing layer containing an ultraviolet absorber, a red-sensitive silver chlorobromide gelatin emulsion layer containing a cyan coupler and its protective layer in that order from the support by an extrusion system and dried to prepare a multi-layer silver halide color photographic printing paper. The coating speed in this process was 200 m/min, and the thickness of the emulsion layer after drying was 10 µ. The test results are shown in Table 5. Examples 15 - 16 A pulp prepared by blending 20 parts by weight of NBSP, 50 parts by weight of LBSP and 30 parts by weight of LBKP was beaten to a freeness of 300 ml, and this pulp slurry was added with Aquapel (made by Dick Hercules Inc.) mainly composed of an alkyl ketene dimer as sizing agent, Stargum (made by Seiko Kagaku Company) mainly composed of polyacrylamide as strengthening agent and Epinox (made by Dick Hercules Inc.) mainly composed of polyamide epichlorohydrin in amounts of 0.5% by weight, 2% by weight and 0.5% by weight, respectively, based on the pulp and calculated as content in the paper web, to form a paper web having a basis weight of 180 g/m². This paper web was dried to obtain a paper having an absolute dry moisture content of 2.5%, an air permeability of 45 sec. and an internal bond strength of 2.0 kg-cm. This paper was heat calendered at a temperature of 270°C under a linear pressure of 150 kg/cm and then surface sized by using a modified Poval solution as surface sizing agent, followed by drying and additional calendering at 30°C under a linear pressure of 70 kg/cm to produce papers having a final moisture content of 5% and 8%, respectively. (The moisture content was changed by dryer steam). Further, the back side of each of these papers was subjected to a corona discharge treatment and then coated with a mixture of a high-density polyethylene (density: 0.968, MI: 7) and a low density polyethylene (density: 0.918, MI: 5) mixing ratio = 1/1) to a thickness of 30 µ by using an extrusion melt coater. Then the opposite side of the paper was subjected to a corona discharge treatment and then coated with a low-density polyethylene containing 9% of anatase type titanium oxide (said polyethylene before addition of pigment having a density of 0.918 and a melt index of 5) to a thickness of 25 µ to produce a photographic support. The surface of this photographic support was further subjected to a corona discharge treatment and then coated with a blue-sensitive silver chlorobromide gelatin emulsion layer containing a yellow coupler, an intermediate layer, a green-sensitive silver chlorobromide gelatin emulsion layer containing a magenta coupler, an ultraviolet absorbing layer containing an ultraviolet absorber, a red-sensitive silver chlorobromide gelatin emulsion layer containing a cyan coupler and its protective layer in that order from the support by an extrusion system and dried to prepare a mulit-layer silver halide color photographic printing paper. Each of the thus obtained color printing paper samples was subjected to a heat treatment and then cut to a predetermined size. After measuring the weight of each sample, it was placed into a color processor and subjected to a color developing treatment for a period of 2 minutes and 30 seconds, a bleach-fixing treatment for a period of 3 minutes and water washing treatment for a period of one minute. Then the printing paper was drawn out in a wet state, the water droplets adhering to both sides of the paper were wiped out with a filter paper and the weight of the printing paper was measured quickly so that it remained wet to thereby determine the amount of liquid absorption into the emulsion. The results are shown in Table 6. Example 17 A pulp comprising a blend of 20 parts by weight of NBSP, 50 parts by weight of LBSP and 30 parts by weight of LBKP was beaten to a freeness of 300 ml, and this pulp slurry was added with Aquapel (made by Dick Hercules Inc.) mainly composed of an alkyl ketene dimer as sizing agent, Stargum (made by Seiko Kagaku Company) mainly composed of polyacrylamide as strengthening agent and Epinox (made by Dick Hercules Inc.) mainly composed of polyamide epichlorohydrin in amounts of 0.5% by weight, 2% by weight and 0.5% by weight, respectively, based on the pulp and claculated as content in the paper web, to form a paper web having a basis weight of 180 g/m² and dried to obtain a paper having an absolute dry moisture content of 2.5%, an air permeability of 60 sec. and an internal bond strength of 2.3 kg-cm. This paper was heat calendered at a temperature of 270°C under a linear pressure of 150 kg/cm and then subjected to surface sizing by using a modified Poval solution as surface sizing agent, followed by drying and additional calendering at 30°C with the linear pressure adjusted to 150 kg/cm so that the final thickness would become 165 µ, making the final moisture content 8.5%. The test results of the obtained paper are shown in Table 7. In Table 7 are also shown the test results of the paper of Example 4 obtained under the same conditions as in Example 17 except that the final calender linear pressure was adjusted to 70 kg/cm so that the final thickness would become 175 µ. Heat calendering temperature Final calender linear pressure Final thickness Taber rigidity Flatness Example 17 270°C 150 kg/cm 175 µ 11.2 g-cm 3.25 Example 4 270 70 175 13.5 3.5 Example 18 and Comparative Example 7 A pulp comprising 100 parts by weight of LBKP was beaten to a freeness of 300 ml, and this pulp slurry was added with Aquapel (made by Dick Hercules Inc.) mainly composed of an alkyl ketene dimer as sizing agent, Stargum (made by Seiko Kagaku Company) mainly composed of polyacrylamide as strengthening agent and Epinox (made by Dick Hercules Inc.) mainly composed of polyamide epichlorohydrin in amounts of 0.5% by weight, 2% by weight and 0.5% by weight, respectively, based on the pulp and calculated as content in the paper web, to form a paper web having a basis weight of 180 g/m². This paper web was dried to obtain papers having an absolute dry moisture content of 2.5%, an air permeability of 45 seconds and an internal bond strength of 2.0 kg-cm. These papers were heat calendered at temperatures of 120°C and 270°C, respectively, under a linear pressure of 150 kg/cm and then subjected to surface sizing by using a modified Poval solution as surface sizing agent, followed by drying and additional calendering at 30°C under a linear pressure of 150 kg/cm to produce papers having a final moisture content of 8.5%. Further, the back side of each of these papers was subjected to a corona discharge treatment and then coated with a mixture of a high-density polyethylene (density: 0.968, MI: 7) and a low-density polyethylene (density: 0.918, MI: 5) (mixing ratio = 1/1) to a thickness of 30 µ by using an extrusion melt coater. Next, the opposite side of each said paper was subjected to a corona discharge treatment and then coated with a low-density polyethylene containing 9% of anatase type titanium oxide (said polyethylene before addition of pigment having a density of 0.918 and a melt index of 5) to a thickness of 25 µ to produce a photographic support. The test results of the thus obtained photographic supports are shown in Table 8. Heat calendering temperature Flatness Comp. Example 7 120°C 4.5 Example 18 270 3.5 Example 19 A paper was produced with the same blend as used in Example 16, and this paper was calendered by adjusting the linear pressure of final calendering to 70 kg/cm so that the paper thickness would become 175 µ and both sides of the paper were coated similarly to Example 16 to produce a photographic support. The test results are shown in Table 9. Heat calendering temperature Final calender linear pressure Final thickness Taber rigidity (4) Flatness Example 19 270°C 70 kg/cm 175 µ 17.5 g-cm 4.0

Claims: A process for producing paper comprising: subjecting a paper which has been internally sized and strengthened and has an absolute dry moisture content (moisture content in absolute dry condition) of 1.8 - 7% to a heat calendering treatment at a temperature of 150 - 300°C under a linear pressure of at least 40 kg/cm; and after said heat calendering treatment, introducing an aqueous solution containing one or more surface strengthening agent(s) to adjust the final moisture content to 5% or more. The process according to claim 1, wherein the temperature of the heat calendering treatment is 160 - 270°C. The process according to claim 1, wherein the linear pressure of the heat calendering treatment is 60 kg/cm or above. The process according to claim 1, wherein the absolute dry moisture content of the paper being subjected to the heat calendering treatment is 2.5 - 5%. The process according to claim 1, wherein the air permeability of the paper being subjected to the heat calendering treatment is 300 seconds or less. The process according to any preceding claim wherein after the heat calendering treatment, the paper is treated with the said aqueous solution and then calendered to adjust the final moisture content. The process according to any preceding claim wherein the obtained heat-calendered paper is a photographic paper support. The process according to claim 7, wherein the photographic paper support has a thickness of 165 µ and a Taber rigidity of 11 g-cm or above. The process according to claim 8, wherein the photographic paper support has a thickness of 175 µ and a Taber rigidity of 13 g-cm or above. The process according to claim 7, 8 or 9 wherein the photographic paper support is provided with a resin coating layer on one or both sides thereof. The process according to claim 10, wherein the photographic resin-coated paper support has a thickness of 220 µ, made by providing a resin coating layer on both sides of 165 µ thick photographic paper support, and a Taber rigidity of 16.5 g-cm or above. The process according to claim 10, wherein the photographic resin-coated paper support has a thickness of 230 µ, made by providing a resin coating layer on both sides of a 175 µ thick photographic paper support, and a Taber rigidity of 19 g-cm or above. A process claimed in any preceding claim in which the final moisture content is adjusted to 6% or more. The process according to any preceding claim in which the said aqueous solution further contains one or more further additive for example dye, fluorescent agent, antistatic agent, anti-fogging agent.

Inventors: TASHIRO, NAOYA, UEHARA, HIROSHI
Applicants: MITSUBISHI PAPER MILLS, LTD.
Application year: 1986
Main classification: D21G1
IPC codes: D21G1