KRAFT LINER BOARD AND METHOD OF PRODUCING KRAFT LINER BOARD FROM BLEACHED OR UNBLEACHED KRAFT PULP, TMP PULP, SCMP OR SULFITE PULP

Abstract: The invention relates to a method of improving the wet strength of paper products produced from bleached or unbleached kraft pulp, thermomechanical pulp, SCMP or sulfite pulp. The method comprises the steps of subjecting paper to high pressure densification during its production and heating the paper so as to raise its internal temperature to at least 216°C for a period of time sufficient to increase the wet strength thereof.

Description: This invention relates to the art of papermaking, particularly to treating paper products with pressure and heat to improve their wet strength while preserving their folding endurance. This invention, more specifically, relates to kraft paper products, bleached or unbleached paper products produced from SCMP and sulfite pulps, TMP paperboard. TMP paperboard means thermomechanical pulp paperboard. The term. "SCMP" process means semichemical mechanical pulping process. The kraft process is a method of preparation of an aqueous slurry of fibers by treatment of a suitable renewable raw material. In most pulping process, a considerable portion of the natural lignin in wood, grass or other vegetative matter is rendered soluble by chemical reaction with one or more nucleophilic reagents. In the kraft process, the nucleophilic reagents are sulfide and hydroxide ions, which are used under highly alkaline conditions. Variations of the kraft process include the earlier practiced soda process, using hydroxyl ions derived from metals in Group IA of the periodic table, namely lithium, sodium, potassium, rubidinium and cesium. A second variation involves the use of anthraquinone (AQ) or substituted anthraquinones as additional nucleophiles. Anthraquinone can be used in the soda process, in which case the process is known as the soda-AQ process, or in the kraft process which is then known as the kraft-AQ process. Such variations in the kraft process are well known in the industry and pulps prepared by any of these variations can be used in practicing the present invention. If desired, the soda-AQ, kraft and kraft AQ pulps can be rendered white by application of suitable bleaching agents. Such agents are usually electrophilic in nature and may include chlorine, chlorine dioxide, sodium hypochlorite, hydrogen peroxide, sodium chlorite, oxygen and ozone. Use is often in sequential stages and a suitable nucleophilic agent, customarily hydroxyl ion, may be used in intermediate stages. "Kraft paner" is paper made from pulp produced by the kraft process. Bleached kraft paper, because of its low lignin content, has low wet strength; hence it is desirable to develop this quality of bleached kraft products. Linerboard is a medium-weight paper product used as the facing material in corrugated carton construction. "Kraft linerboard" is linerboard made from pulp produced by the kraft process. The thermomechanical pulping process is a method of producing an aqueous slurry of fibers by mechanical treatment of a suitable renewable raw material. In most pulping processes, a considerable portion of the natural lignin in wood, grass or other vegetative matter is rendered soluble by chemical reaction with one or more nucleophilic reagents. Such nucleophilic reagents are not used in thermo­ mechanical pulp production. Instead, fibers are liberated from a thermally softened source material, usually wood , chips, by passage through a suitable shredder or disc refiner operating under ambient steam pressure at a temperature of 120-150°C or more. Such fibers are usually brittle and sheets prepared from them are brittle and have little or no wet strength. As used herein, "TMP pulp" is pulp produced as described above, by a thermomechanical process. Despite the fiber brittleness, TMP pulp has the advantage of avoiding the odor, disposal problems, and other drawbacks of chemical pulp production. "TMP paperboard" is a medium-weight paper product made from TMP pulp. It is notoriously weak when wet, and inflexible, compared with products identically made from more flexible pulps such as kraft pulps. Hence, a process for improving both the wet strength and flexibility of TMP paper products is desirable. In the art of making kraft paper products, TMP paperboard and SCMP paperboard, it is conventional to subject felted fibers to wet pressing to unite the fibers into a coherent sheet. Pressure is typically applied to a continuous running web of paper by a series of nip rolls which, by compressing the sheet, both increases its volumetric density and reduce its water content. The accompanying Fig. 1 shows in simplified diagramatic form a typical papermaking machine, including a web former and three representative pairs of wet press rolls. Also shown are drying rolls whose purpose is to dry the paper to a desired final moisture content, and a calender stack to produce a smooth finish. At least some of the rolls are ordinarily heated to hasten drying. (The drawing is simplified - there are many more drying rolls in actual practice.) The semichemical mechanical pulping process is a method of production of an aqueous slurry of fibers by treatment of a suitable renewable raw material. In most pulping processes, a considerable portion of the natural natural lignin in wood, grass or other vegetative matter is rendered soluble by chemical reaction with one or more nucleophilic reagents. Minimization of the lignin portion solubilized and removed whilst so altering the lignin as to permit recovery of fibers by the mechanical action of a disk or other refiner or shredder in a condition of little damage is the goal of the SCMP process. It is also the goal of certain related processes known as the chemimechanical process (CMP), the chemithermomechanical process (CT-MP) and the neutral sulfite semichemical (NSSC) processes. Such pulps are normally considered to be more brittle and of inferior strength when compared to lower lignin pulps produced by the kraft, sulfite, kraft-anthraquinone (AQ), soda-AQ or alkaline sulfite AQ processes. However, properties are adequate for many end-uses, including corrugated medium and even as a linerboard component. In the sulfite process, sulfite or bisulfite ion is the nucleophilic agent. The sulfite or bisulfite ions cause the lignin molecules to break into small fragments. During this chemical reaction, the sulfite or bisulfite ions become chemically bonded to the lignin fragments thereby providing water solubility. A variation of the sulfite process involves the use of anthraquinone (AQ) or substituted anthraquinones as a second nucleophile. AQ is reduced in situ during the earliest stages of the cook to anthrahydro­ quinone (AHQ). As AQ is insoluble and only the salt from of AHQ is soluble, alkali presence is necessary for solution formation and uniform penetration of AQ into the wood chip, grass stem or any other fiber-containing vegetative matter. Such a cooking process is known as an alkaline sulfite-AQ process. Both the sulfite process and the alkaline sulfite-AQ variation of the sulfite process are well known to the industry and pulps thus prepared can be used to give the benefit of our invention. There is currently considerable interest in treatments involving heat and pressure, or heat alone, during or after the production process, to improve various qualities of paper products. Quantifiable paper qualities include dry tensile strength, wet tensile strength, reverse folding endurance, compressive strength and stiffness, among others. Which qualities should desirably be enhanced depends upon the intended application of the products. For paper and TMP paperboard to be used in humid or wet environments two qualities of particular interest are wet strength and folding endurance, both of which can be measured by well-know standard tests. As used herein, then, "wet strength" means wet tensile strength as measured by American Society for Testing and Materials (ASTM) Standard D829-48. "Folding endurance" is defined as the number of times a board can be folded in two directions without breaking, under conditions specified in Standard D2176-69. "Basic weight" is the weight per unit area of the dried end product. For linerboard to be used in manufacturing corrugated cartons for use in humid or wet environments, three qualities of particular interest are wet strength, folding endurance and high humidity compression strength. "Compression strength" is edgewise linear compression strength as measured by a standard STFI (Swedish Forest Research Institute) Tester. Prior workers in this field have recognized that high-temperature treatment of linerboard can improve its wet strength. See, for example E. Rack, "Wet stiffness by heat treatment of the running web", Pulp & Paper Canada , vol. 77, No. 12, pp. 97-106 (Dec. 1976). This increase has been attributed to the development and cross-linking of naturally occurring polysaccharides and other polymers, which phenomenon may be sufficient to preserve product wet strength even where conventional synthetic formaldehyde resins or other binders are entirely omitted. It is important to note that wet strength improvement by heat curing has previously been thought attainable only at the price of increased brittleness (i.e., reduced folding endurance). Therefore, most prior high-temperature treatments have been performed on particle board, wallboard, and other products not to be subjected to flexure. The known processes, if applied to bleached kraft paper, TMP paperboard and SCMP paperboard, would produce a brittle product. Embrittled paper and paperboard are not acceptable for many applications involv­ ing subsequent deformation such as the converting operation on a corrugating machine to make corrugated boxes out of linerboard, and therefore heat treatment alone, to develop wet strength of linerboard, has not gained widespread acceptance, and is not viable for TMP paperboard. As Dr. Back has pointed out in the article cited above, "The heat treatment conditions must be selected to balance the desirable increase in wet stiffness against the simultaneous embrittlement in dry climates." Significantly, in U. S. Patent 3,875,680, Dr. Back has disclosed a process for heat treating already manufactured corrugated board to set previously placed resins, the specific purpose being to avoid running embrittled material through a corrugator. It is plain that added wet strength and improved folding endurance were previously thought incompatible results. It is therefore an object of the invention to produce paper products and TMP paperboard and linerboard having both greatly improved wet strength and good folding endurance. Another goal is to achieve that objective without resorting to synthetic resins or other added binders and wet strength agents. These objects are achieved by the method and paper products according to the claims. With a view of the foregoing, a process has been developped which dramatically and unexpectedly increases not only the wet strength of paper products, TMP Paperboard, SCMP paper­ board and lineboard, but also preserves its folding endurance. In its broadest sense, the method according to the invention comprise steps of 1) subjecting paper or linerboard, produced from bleached or unbleached kraft pulp or paperboard produced from thermo­ mechanically produced pulp or SCMP paper pulp, to high pressure densification to achieve a density of at least 600 kg/m³ and 2) heating the product to an internal temperature of at least 420°F (216°C) for a period of time sufficient to increase the wet strength of the product. This method produces a product having folding endurance greatly exceeding that of similar products whose wet strength has been increased by heat alone. This is clearly shown by our tests exemplified below. While the tests set out in the Examples have carried out the invention in a static press, it is preferred that the heat and pressure be applied to continuously running paper and board by hot pressure rolls, inasmuch as much higher production rates can be attained. In the case of linerboard from unbleached kraft, we prefer to raise the internal temperature of the board to at least 550°F (289°C), as greater wet strength is then achieved. This may be because at higher temperatures, shorter step duration is necessary to develop bonding, and there is consequently less time for fiber degradation to occur. Also, shorter duration enables one to achieve higher production speeds. In the case of other paper products from kraft pulp, and TMP boards, we prefer to raise the internal temperature of the paper to at least 465°F (240°C), as greater wet strength is then achieved. In the case of SCMP paperboard, it is preferred to raise the internal temperature of the board to at least 450°F (232°C). It should be noted that the heating rate, and thus the required heating duration at a particular temperature, depends on method of heat transfer chosen. Furthermore, it is desirable to raise the web temperature as rapidly as possible to the chosen treating temperature. Improved heating rates can be achieved by using high roll temperatures and/or by applying high nip forces to the press roll against the sheet on the hot rolls. That high pressure dramatically improves heat transfer rates has previously been disclosed. One worker has attributed this to the prevention of vapor formation at the web-roll interface. While the invention may be practiced over a range of temperatures, pressures and durations, these factors are interrelated. For example, the use of higher temperatures requires a heating step of shorter duration, and vice-versa. At 550° F (288°C), in the case of linerboard, a duration of 2 seconds has been found sufficient to obtain the desired improvements, while at 420°F, (216°C), considerably longer time is required. AT 465°F (240°C), a duration of 60 seconds has been found sufficient to obtain the desired improvements, while at 420°F (216°C), considerably longer time is required. In the case of SCMP board, at 450°F (232°C), a duration of 5 seconds has been found sufficient to obtain substantial improvement. It is presently preferred that, for safety reasons, the roll temperature be not greater than the web ignition temperature (572°F, 300°C); however, even higher roll tempe­ ratures may be used if suitable precautions, such as the provision of an inert atmosphere, or rapid removal of paper from the hot environment, are taken. The invention is described in detail in connection with the drawings in which Figure 1 shows, in greatly simplified diagrammatic form, a conventional apparatus for producing linerboard; Figure 2 shows, in like diagrammatic form, an apparatus for practicing the present invention. Figure 2 illustrates a preferred apparatus for carrying out the inventive process, although it should be understood that other devices, such as platen presses, can be used and in fact the data below was obtained from platen press tests. In the machine depicted, bleached or unbleached kraft paper fibers or unbleached TMP pulp fibers or SCMP or sulfite pulp fibers in aqueous suspension are deposited on a web former screen 10, producing a wet mat or fibers. The mat is then passed through a series of wet press nip rolls 12, 13, 14, 15, 16 and 17 which develop a consolidated web. Suitable wet presses known today include long nip presses and shoe-type presses capable of developing high unit press pressures on the wet fiber web. This step is known as "high pressure wet pressing". The web is then passed over pre-drying rolls 18, 19 to remove water from the wet web. Once the moisture content of the web has been reduced to less than 70% by weight, steps of the high pressure densification and high temperature treatment are applied according to the invention. To densify the web, a series of drying rolls 20, 21, 22, 23 are provided with respective pressure rollers 25, 26, 27, 28 which are loaded sufficiently to be able to produce a web density of at least 700 kg/m³. We define this step as "press drying" or "high pressure densification". In the preferred embodiment, the high pressure densification step of the invention is carried out both at normal drying temperatures (substantially below 400°F (204°C) in the press drying section, and also in the high temperature heat treatment section described below. It should be understood, however, that the two steps may be performed sequentially or simultaneously. In the heat treatment section, one or more drying rolls (e.g. 30, 31, 32, 33) are heated to or slightly above the desired maximum internal web temperature. Pressure rolls 35, 36, 37, 38 are used to improve heat transfer between the drying rolls and the web, and preferably, these pressure rolls are also highly loaded to continue the high pressure densification step during heat treatment. The drying roll temperature necessary to achieve target web temperature is a function of several factors including web thickness, web moisture, web entering temperature, web speed, nip pressure, and roll diameter; its calculation is within the skill of the art. It is presently believed optimum in the case of linerboard from unbleached kraft pulp to achieve an internal web temperature of 550°F (289°C) and to maintain such temperature for two seconds or 465°F (240°C) in the case of paperboard from bleached kraft pulp or TMP board and to maintain this temperature for 60 seconds or 450°F (232°C) in the case of paperboard from SCMP and other sulfite pulp and to maintain this temperature for 5 seconds. In any event, the roll temperature must be at least 420°F (216°C) which is well in excess of the temperature of normal drying rolls. The heat treatment rollers are contained within an enveloppe 40, and air caps 41, 42, 43, 44 may be used to heat the web further as it passes over the rolls. An inert gas, steam or superheated steam atmosphere may be used for this purpose and to prevent oxidation or combustion at high temperatures. Following heat treatment, the web may be passed over final rolls 50, 51 having air caps 60, 61 to condition the web, which is then calendered and reeled in a conventional manner. The combined effect of high pressure densification and high temperature produce an unexpected combination of good wet strength and good folding endurance in the finished product. The invention has been practiced as described in the following examples. The improvement in board quality will be apparent from an examination of the test results listed in the tables below. EXAMPLE 1 Pine wood chips from the southeastern United states were cooked by the kraft process to an extent typical of pulp used in linerboard production. The cooked chips were converted to a pulp by passage through a disk refiner. The pulp was thoroughly washed with water to remove residual black liquor and was stored in the wet state at 38-42°F (3°-6°C) in a refrigerator until sheets were prepared. The cooked, washed pulp had a kappa number of 98, indicating presence of 15% residual lignin and had a freeness of 720 ml by the Canadian Standard Freeness test, which values are typical of a pine linerboard pulp prior to beating. A dispersion of the pulp in distilled water was converted to handsheets using a TAPPI sheet mold. The quantity of fiber in the dispersion was adjusted to give a TAPPI sheet weight of 3.6 g in the oven dried state, said weight being close to that of an air dried, 42 lb/1000 ft² (205 g/m²) commercial linerboard sheet. The sheets were wet pressed with blotters at 60 psi (415 kPa) prior to drying. Three sets of sheets were prepared. Sheets from the first set were dried on TAPPI rings at room temperature according to TAPPI standard T205 om-81. This is a conventional (C) drying procedure. Sheets from the second set were also dried by the conventional procedure but this procedure was followed by a heat treatment (HT). The paper sheet was placed between two 150 mesh stainless steel screens, which assembly was placed in the platen press. Heat treatment was in accordance with the conditions found optimum for this invention, namely 2 seconds at 550°F (289°C) sheet internal temperature. To do this, single sheets were placed in a 550°F (289°C) Carver platen press for 4 seconds with 15 psi (105 kPa) as applied pressure. Previous experiments using a thermocouple buried in the sheet had shown that the sheet required 2 seconds to reach the target 550°F (289°C) temperature. Individual sheets from the third set were inserted in the wet state in a different platen press at 280°F (138°C). A pressure of 15 psi (105 kPa) was maintained for 5 seconds to dry surface fibers, after which the pressure was increased to 790 psi (5450 kPa) for 20 seconds. On completion of this press densification process (PD) sheet moisture was about 10%. Each sheet was removed from the PD press and immediately placed in the other, HT press for 4 seconds at 550°F (289°C). All three sets of sheets were conditioned at 73°F (23°C) and 50% humidity for at least 24 hours before testing. Fold, wet and conditioned tensile strength and conditioned compressive strength were the tests that were carried out. Wet tensile tests were carried out immediately after excess water was blotted from test sheets which had been removed after 4 hours immersion in distilled water. Otherwise, this test was the same as the ASTM standard wet tensile test. The results summarized in Table I show superior folding endurance and wet strength for the densified and heat treated sheets. EXAMPLE 2 Hardwood chips from the southeastern United States were cooked by the kraft process to yield, after disk refining and washing, a 98 kappa pulp of 618 ml Canadian Standard Freeness. This pulp was mixed with the softwood of example 1 to give a mixture containing 60% softwood and 40% hardwoodfiber. Sheets were prepared and tested following the procedure in Example 1. The superior fold and strength properties that were obtained are given in Table II. EXAMPLE 2A Pine wood chips were processed into a pulp as in Example 1, first paragraph. A dispersion of the pulp in distilled water was converted to handsheets using a Noble & Wood sheet mold. The quantity of fiber in the dispersion was adjusted to give a sheet weight of 7.9 g in the oven dried state. The sheets were wet pressed with blotters at 50 psi (346 kPa) prior to drying. Three sets of sheets were prepared. Sheets from the first set were dried on a rotary drum dryer in a conventional (C) manner. Sheets from the second set were heat treated (HT) as in Example 1, and sheets from the third set were densified and then heat treated (PD & HT) as in Example 1. One sample from each set was conditioned at 73°F (23°C), 50% relative humidity ("dry"); another sample was conditionned at 90°F (32°C), 90% relative humidity ("moist"). Folding endurance, wet tensile strength and compressive strength tests were then carried out as in Example 1. The results, summarized below, show a marked improvement in both folding endurance and in tensile and compressive strength in high moisture conditions. EXAMPLE 3 The pine pulp used in Example 1 was subjected to three levels of beating by multiple passes through an Escher Wyss refiner to decrease the freeness of the pulp. Sheets were prepared and tested at each process level following the procedure in Example 1. The results in Table 3 again clearly demonstrate the lack of brittleness of the PD+HT sheets in comparison with sheets treated by the C+HT procedure. These values may be compared to those shown in Table I, for unbeaten pulp (720 Canadian Standard Freeness). EXAMPLE 4 On a conventional linerboard machine, three hard covered 12" diameter press nip rolls were located on drier cans # 43, 45 and 47. Furnish of 100% softwood kraft pulp was run on the machine and a 42 1b/1000 ft² (205g/m²) basis weight linerboard was obtained at a speed of 1550 ft/min. (473 m/min.). No nip pressure was applied to the nips rolls mentioned during the first stage of the trial and with conventional drying temperature, properties outlined below in Table IV were obtained. In the table, "MD" denotes testing along the machine length; "CD" denotes testing across the machine width. When this board was subject to high temperature treatment of 464°F for 30 seconds, properties shown in Table V were obtained. The increase in wet strength, coupled with the very great reduction in folding endurance, conform to prior art experience. To test the effect of densification, the press nip rolls were then activated. A force of 230 pli (41 kg/cm) gave a nip pressure of 1225 psi (8445 kPa) and when three pressure nips were applied, the densified board gave test results as follows: The densified board was then heat treated at 464°F for 20 seconds. The following results were obtained. The unexpected lack of brittleness (as measured by the folding endurance test) of the densified and heat treated product (Table VII) when compared with the other high wet strength paperboard (Table V) can be identified as a direct result of the sequence of densification and high temperature treatmeant. EXAMPLE 5 To illustrate the effect of densification prior to conventional or dynamic press drying, handsheets were prepared from a 60% softwood, 40% hardwood high yield pulp blend of the linerboard type. The sheets were divided into two main groups. The first group of sheets were wet pressed at an intensity level approximating that in a conven­ tionally equipped production machine wet press (CWP). The second group were pressed at an intensity level approximating that of a modern production machine equipped with a shoe press (SP). Each group of sheets was further subdivided into individual sheets which were retained for testing after drying on a steam-heated rotating drum, or press drying by passage through the nip between a press roll and the rotating drum, or by static press drying between 150 mesh stainless steem screens at 465°F (240°C) for 30 seconds with 15 psi (104 kPa) pressure applied by means of a suitable press. Heat treated control sheets which had been subjected to conventional wet pressing (CWP) and drying on the rotating drum had high caliper. Such thick sheets have minimal fiber-fiber contacting points. As adhesive forces develop at such points during drying, minimal contacting points result in poor folding endurance and wet tensile strength properties after heat treatment. Densification by use of the shoe press gave lower caliper and improved contact between fibers, and wet strength also increased. Dynamic press drying gave somewhat more efficient densification and provided a further improvement in wet tensile strength. The combination of shoe wet pressing and dynamic press drying provided further improvements after heat treatment. The final data in the table show what can be obtained by application of static press drying followed by heat treatment of sheets which had been subjected to the shoe pressing procedure. EXAMPLE 6 Pine wood chips from the southeastern United States were cooked by the kraft process to an extent typical of pulp used in linerboard production. The cooked chips were converted to a pulp by passage through a disk refiner. The pulp was bleached and washed with water to remove residual black liquor and was stored in the wet state at 38-42°F (3°-6°C) in a refrigerator until sheets were prepared. The cooked, bleached pulp contained substantially no lignin and had a freeness of 720 ml by the Canadian Standard Freeness test, which values are typical of a bleached pine pulp prior to beating. A dispersion of the pulp in distilled water was converted to handsheets using a TAPPI sheet mold. The quantity of fiber in the dispersion was adjusted to give a TAPPI sheet weight of 3.6 g in the oven dried state, said weight being close to that of an air dried, 42 lb/1000 ft² (205 g/m²) commercial sheet. The sheets were wet pressed with blotters at 60 psi (415 KPa) prior to drying Three sets of sheets were prepared. Sheets from the first set were dried on TAPPI rings at room temperature according to TAPPI standard T205 om-81. This is a conventional (C) drying procedure. Sheets from the second set were also dried by the conventional procedure but this procedure was followed by a heat treatment (HT). The paper sheet was placed between two 150 mesh stainless steel screens, which assembly was placed in the platen press. Heat treatment was in accordance with the conditions found optimum for this invention, namely 60 seconds at 465°F (240°C) sheet internal temperature. To do this, single sheets were placed in a 465°F (240°C) Carver platen press for 60 seconds with 15 psi (105 kPa) as applied pressure. Individual sheets from the third set were inserted in the wet state in a different platen press at 280°F (138°C). A pressure of 15 psi (105 kPa) was maintained for 5 seconds to dry surface fibers, after which the pressure was increased to 790 psi (5450 kPa) for 20 seconds. On completion of this press densification process (PD) sheet moisture was about 10%. Each sheet was removed from the PD press and immediately placed in the other, HT press for 4 seconds at 465°F (240°C). All three sets of sheets were conditioned at 73°F (23°C) and 50% humidity for at least 24 hours before testing. Folding endurance and wet tensile strength were the tests that were carried out. Wet tensile tests were carried out immediately after excess water was blotted from test sheets which had been removed after 4 hours immersion in distilled water. Otherwise, this test was the same as the ASTM standard wet tensile test. The results summarized in Table IX show superior folding endurance and wet strength for the density and heat treated sheets. EXAMPLE 7 A southern hardwood bleached kraft pulp in the never-dried state was processed in accordance with the procedure in Example 6. The test results illustrate the lack of wet pulp strength and the somewhat brittle nature of conventionally dried hardwood pulp sheets. Heat treatment of the conventionally dried sheets produced rather mediocre wet strength accompanied by increased brittleness. However, sheets processed in accordance with this invention gave fold values improved by a factor of almost four, thereby demonstrating a pronounced lowering of brittleness in the sheets, which also had significantly improved wet strength. Example 8 illustrates the process of this invention when applied to TMP board. EXAMPLE 8 Pine wood chips from the southeastern United States were converted to a pulp by passage through a 250°F (121°C) disk refiner. The pulp was stored in the wet state at 38-42°F (3°-6°C) in a refrigerator until sheets were prepared. A dispersion of the pulp in distilled water was converted to handsheets using a TAPPI sheet-mold. The quantity of fiber in the dispersion was adjusted to give a TAPPI sheet weight of 3.6 g in the oven dried state, said weight being close to that of an air dried, 42 lb/1000 ft² (205 g/m²) commercial linerboard sheet. The sheets were wet pressed with blotters at 60 psi (415 kPa) prior to drying. Three sets of sheets were prepared. Sheets from the first set were dried on TAPPI rings at room temperature according to TAPPI standard T205 om-81. This is a conventional (C) drying procedure. Sheets from the second set were also dried by the conventional procedure but this procedure was followed by a heat treatment (HT). The paper sheet was placed in a 465°F (240°C) Carver platen press for 60 seconds with 15 psi (105 kPa) as applied pressure. Individual sheets from the third set were inserted in the wet state in a different platen press at 280°F (138°C). A pressure of 15 psi (105 kPa) was maintained for 5 seconds to dry surface fibers, after which the pressure was increased to 790 psi (5450 kPa) for 20 seconds. On completion of this press densification process (PD) sheet moisture was about 10%. Each sheet was removed from the PD press and immediately placed in the other, HT press for 60 seconds at 465°F (240°C). All three sets of sheets were conditioned at 73°F (23°C) and 50% humidity for at least 24 hours before testing. Folding endurance and wet tensile strength were the tests that were carried out. Wet tensile tests were carried out immediately after excess water was blotted from test sheets which had been removed after 4 hours immersion in distilled water, Otherwise, this test was the same as the ASTM standard wet tensile test. The results summarized in Table X show superior folding endurance and wet strength for the densified and heat treated sheets. surface fibers, after which the pressure was increased to 790 psi (5450 kPa) for 20 seconds. On completion of this press densification process (PD) sheet moisture was about 10%. Each sheet was removed from the PD press and immediately placed in the other, HT press for 60 seconds at 465°F (240°C). All three sets of sheets were conditioned at 73°F (23°C) and 50% humidity for at least 24 hours before testing. Folding endurance and wet tensile strength were the tests that were carried out. Wet tensile tests were carried out immediately after excess water was blotted from test sheets which had been removed after 4 hours immersion in distilled water. Otherwise, this test was the same as the ASTM standard wet tensile test. The results summarized in Table X show superior folding endurance and wet strength for the densified and heat treated sheets. Example 9 A mixture of spruce and fir wood chips was cooked by the SCMP process to a yield of 92% by weight of dry chips. The cooked chips were converted to a pulp by passage through a disk refiner. The pulp was washed with water to remove residual cooking chemical and solubilized material. Latency removal was accomplished by stirring at 4% consistency for 20 minutes at 85-90°C. Pulp freeness was 705 ml by the Canadian Standard Freeness Test. A dispersion of the pulp in distilled water was converted to handsheets using a TAPPI sheet mold. The quantity of fiber in the slurry fed to the mold was adjusted to give a basis weight of 42 lb/1000 ft² (205 g/m²) in the oven dried state. Two sets of sheets were prepared. Sheets from the first set were dried on TAPPI rings at room temperature after wet pressing. Wet pressing and drying were in accordance with the procedure in TAPPI T-205 om-81. Sheets from the second set were placed between two 150 mesh stainless steel screens and pressed in a platen press at 300 psi (2067 kPa) and 450°F (232°C) platen temperature for different times between 5 and 60 seconds. This drying procedure effectively combines the densification and heat treatment stages and is known as high temperature press drying (HTPD). All sheets were conditioned at 73°F (22,5°C) and 50% humidity for at least 48 hours before testing. Folding endurance, wet tensile and conditioned tensile strengths were the tests that were carried out. Wet tensile tests were run immediately after excess water was blotted from test sheets which had been removed after four hours immersion in distilled water. Otherwise, this test was the same as the ASTM standard tensile test for a conditioned sheet. The improved tensile properties, both wet and conditioned, and the lowered brittleness as illustrated by the increased number of double folds, are in accordance with the invention. Example 10 A sample of a commercial low yield sulfite pulp in the never dried state, prepared from northern softwood chips, was obtained and converted to handsheets using a TAPPI mold. The quantity of fiber in the slurry fed to the mold was adjusted to give a basis weight of 42 lbs/1000ft² (2.05,5 kg/M²) in the oven dried state. Four sets of sheets were prepared and wet pressed as specified in accordance with the procedure in TAPPI T-205 om-81. Two of the four sets of sheet were dried on rings as required by the procedure. These sheets were considered to be dried by a conventional (C) method. One of the two sets of dry sheets was then subjected to heat treatment. For heat treatment, each sheet was placed between two 150 mesh stainless steel screens and inserted between the platens of a preheated platen press. Press temperatures of 392, 428, 454°F (200, 220 and 240°C) were studied. The platens were immediately closed and 15 psi (103.4 kPa) pressure was applied for 5 seconds. Sheets were immediately removed from screens and allowed to cool after pressing. Preliminary experiments using a thermocouple wire buried in the sheet showed the sheet internal temperature after 2 seconds is only 1-2°C lower than the platens temperature. The third and fourth sets of sheets were placed between the 150 mesh screens and densified by a press densi­ fication (PD) procedure during the process of drying. To carry out the PD procedure, the wet sheets and the screens were placed between the platens of a second press and subjected to 15 psi (103.4 kPa) pressure at 138°C for 5 seconds to dry surface fibers, after which the pressure was increased to 790 psi (5443 kPa) for 20 seconds. On completion of this PD process, sheet moisture was about 10%. One set of sheets was retained for testing. Each individual sheet and screens from the second set were removed from the PD press and immediately placed in the other, HT press for 5 seconds. HT press temperatures of 200, 220 and 240°C were studied. HT pressure was 15 psi (103.4 kPa). All sheets were conditioned at 22.5°C for at least 48 hours before testing. Fold and wet tensile strengths were determined as specified in Example 8. The results show that wet strength improves as heat treatment temperature is increased. Fold decreases as heat treatment temperature is increased, but the decrease is much less pronounced for the densified, heat treated sheets. This shows that the densified sheets are much less brittle than the conventional sheets, even after beat treatment to yield enhancement of wet strength.

Claims: 1. A method of producing paper products with improved wet strength from bleached or unbleached kraft pulp, thermomechanical pulp, SCMP or sulfite pulp, while preserving its folding endurance, comprising steps of subjecting paper to high pressure densification during its production to achieve a density of at least 600 kg/m³, and heating the paper so as to raise its internal temperature to at least 420°F (216°C) for a period of time sufficient to increase the wet strength thereof. 2. The method of claim 1, wherein said densi­ fication and heating steps are carried out simultanecosly. 3. The method of claim 1, wherein said densi­ fication step precedes said heating step. 4. The method of claims 1 to 3. wherein said internal temperature is in the range of 420°F (216°C) to 572°F (300°C). 5. The method of claims 1 to 4, wherein the kraft pulp is unbleached and the internal temperature is about 550°F(289°C), and when the pulp is bleached, or when the pulp is thermomechanical pulp, the internal temperature is about 465°F (240°C) and when the pulp is SCMP or sulfite pulp, the internal temperature is about 450°F (232°C). 6. The method of claims 1 to 5, wherein in said densification step the paperboard is compressed to a density of 600 to 1200 kg/m³. 7. The method of claims 1 and 3 to 6, wherein said densification includes applying sufficient pressure to the paper to produce density in range of 700-900 kg/m³ prior to said heating step. 8. The method of claims 1 to 7, wherein said paper, prior to said densification step, has a moisture content in the range of 10% to 70% by weight. 9. The method of claims 1 to 8, wherein said paper product is linerboard or paperboard. 10. The method of claim 9, wherein said paper product is linerboard and has a basis weight in the range of 125 to 464 g/m². 11. The method of claims 1 to 9, wherein said paper product is paperboard and has a basis weight in the range of 30 to 464 g/m² and when the paperboard is produced form thermomechanical pulp, the basis weight is 60-464 g/m². 12. A linerboard having a wet strength of at least 2,63 kN/m (15 lb/in), and satisfying a folding endurance test of at least 300 cycles, preferably at least 1000 cycles. 13. A linerboard form SCMP or sulfite pulp, having a wet strength of at least 1,05 kN/m (6 lb/in), and satisfying a folding endurance test of at least 10 cycles. 14. A bleached kraft paperboard having a wet strength of at least 0.88kN/m (5 lb/in), and satisfying a folding endurance test of at least 50 cycles. 15. A bleached kraft paperboard according to claim 14, having a wet strength of at least 2.63 kN/m (15 lb/in), and satisfying a folding endurance test of at least 300 cycles. 16. A paperboard having a wet strength of at least 1.75 kN/m (10 lb/in), and satisfying a folding endurance test of at least one cycle.

Inventors: FIELD, JASPER H., HALL, F. KEITH, MACDONALD, DONALD M., RING, MICHAEL, SWENSON, ROY S.
Applicants: INTERNATIONAL PAPER COMPANY
Application year: 1986
Main classification: D21F11
IPC codes: D21F11