JP7655877B2 - Kraft Paper - Google Patents

Description

The present invention relates to kraft paper that can be used as a material for packaging, envelopes, and paper bags.

Kraft paper is a strong paper made from pulp produced by the Kraft process. Kraft paper is used, for example, for packaging paper, envelopes, paper bags, corrugated cardboard materials, and adhesive tape. For paper bags, kraft paper is known that has a longitudinal breaking elongation of 2.2% or more, a transverse breaking elongation of 4.0% or more, and a longitudinal/lateral breaking elongation ratio (longitudinal/lateral) of 0.50 or more as specified in JIS P8113:2006 (see, for example, Patent Document 1). Such kraft paper has excellent elongation and strength properties so that it is less likely to cause bag breakage when processed into bags.

JP 2014-055372 A

Kraft paper is often used as a material for packaging, envelopes, and paper bags. Kraft paper used for packaging, envelopes, and paper bags naturally has the strength required for these applications, and more specific qualities are required in terms of production and use. In terms of production, it is resistance to corner cracking when forming packages, envelopes, and bags. In terms of use, it is ease of gripping and opening envelopes, and durability of string holes when paper bags have string handles.
SUMMARY OF THE PRESENT EMBODIMENTS In view of the above, an object of the present invention is to provide a kraft paper having the more specific qualities as mentioned above.

The object of the present invention can be achieved by the following kraft paper:
[1] A kraft paper containing at least unbleached softwood kraft pulp, a sizing agent, a paper strength agent, and aluminum sulfate, wherein the kraft paper has a tensile strength index calculated by the following formula of 0.060 (kN·m/g) or more in the machine direction and 0.040 (kN·m/g) or more in the cross direction, and an A value calculated by the following formula of 36.0 to 46.0 in the machine direction and 141.0 to 184.0 in the cross direction.
Specific tensile strength (kN·m/g)=(tensile strength (kN/m) ÷ basis weight (g/m ² ))
A value = elongation rate of kraft paper (%) ÷ specific tensile strength (kN m/g)

When used for wrapping paper, envelopes, and paper bags, the kraft paper of the present invention has resistance to corner cracking when forming the wrapping, envelopes, and bags, ease of gripping and opening the envelopes, and durability of string holes when the paper bag has a string handle.

The kraft paper of the present invention is described in detail below.

Kraft paper contains at least kraft pulp, a sizing agent, a strength agent, and aluminum sulfate. The sizing agent, strength agent, and aluminum sulfate are contained in order to obtain strength of the kraft paper.
Furthermore, the kraft paper may contain additives other than those mentioned above that are conventionally known in the papermaking field, such as fillers, fixing agents, retention agents, water resistance agents, dispersants, thickeners, flow improvers, defoamers, foam suppressors, release agents, foaming agents, penetrating agents, colorants, fluorescent whitening agents, ultraviolet absorbing agents, antioxidants, preservatives, and mold inhibitors.

In some embodiments, the kraft paper is substantially free of fillers. Generally, fillers have the effect of reducing the strength of the paper. Here, "substantially" means that the fillers are not contained in an amount that would reduce the strength of the paper. The kraft paper may contain fillers in such a small amount that the strength of the paper is not reduced. In at least one embodiment, the kraft paper is free of fillers.

The filler is a white inorganic pigment conventionally known in the papermaking field. Examples of fillers include kaolin, clay, light calcium carbonate, heavy calcium carbonate, talc, titanium dioxide, satin white, aluminum silicate, diatomaceous earth, calcium silicate, synthetic amorphous silica, colloidal silica, aluminum hydroxide, and alumina.

The filler content in kraft paper can be estimated by its ash content, which depends on the content of the filler and the metal components of inorganic salts such as aluminum sulfate contained in the kraft paper.
In some embodiments, the kraft paper has an ash content of 2% by weight or less as determined in accordance with ISO 1762:2001 "Paper, board and pulps - Determination of residue (ash) on ignition at 525 degrees C." In at least one embodiment, the kraft paper has an ash content of 0.3% by weight or more and 2% by weight or less, depending on the aluminum sulfate.

Kraft paper can be obtained by mixing a slurry of kraft pulp dispersed in water with a sizing agent, strength agent, aluminum sulfate, and other additives as necessary, and then using a conventional papermaking machine such as a Fourdrinier papermaking machine, a cylinder papermaking machine, or a twin-wire papermaking machine to make paper stock. The resulting paper can then be subjected to a size press treatment using a size press liquid containing a surface sizing agent such as starch, polyvinyl alcohol, styrene acrylic resin, or olefin maleic acid resin, and/or calendaring using a machine calendar, soft nip calendar, super calendar, multi-stage calendar, or multi-nip calendar, as necessary.

The size press process is carried out using a size press device conventionally known in the papermaking field. Examples of size press devices include an inclined size press, a horizontal size press, and a rod metering size press, a roll metering size press, and a blade metering size press as film transfer methods, a shim sizer, an opti sizer, a speed sizer, and a film press as rod metering size presses, and a gate roll coater as a roll metering size press. Further examples include a bill blade coater, a twin blade coater, a velbapa coater, a tab size press, and a calendar size press.

Kraft pulp is a type of chemical pulp produced by the kraft process, which is conventionally known in the papermaking field. Bleached kraft pulp is kraft pulp that has been bleached by a bleaching process. Unbleached kraft pulp is kraft pulp that has not been subjected to the bleaching process.
Pulp raw materials are conventionally known wood or non-wood, such as coniferous and broad-leaved wood, and non-wood such as hemp, linter, straw, bamboo, sugarcane, reed, fruit, paper mulberry, Mitsumata, and gampi. Bleached kraft pulp is called coniferous bleached kraft pulp if the pulp raw material is coniferous, and unbleached kraft pulp is called coniferous unbleached kraft pulp if the pulp raw material is coniferous.

Kraft paper contains unbleached softwood kraft pulp as the kraft pulp.
The kraft paper may contain various pulps known in the art other than unbleached softwood kraft pulp. Examples of various pulps known in the art other than unbleached softwood kraft pulp include bleached softwood kraft pulp, unbleached hardwood kraft pulp, bleached hardwood kraft pulp, semi-bleached softwood kraft pulp, semi-bleached hardwood kraft pulp, mechanical pulp, and deinked waste paper pulp. In some embodiments, the kraft paper contains 65% or more of unbleached softwood kraft pulp by mass based on the total pulp contained in the kraft paper. In some further embodiments, the kraft paper contains 80% or more of unbleached softwood kraft pulp by mass based on the total pulp contained in the kraft paper. In at least one embodiment, the kraft paper contains 100% of unbleached softwood kraft pulp by mass based on the total pulp contained in the kraft paper. The reason for this is that the strength of the kraft paper is easily obtained, and the above-mentioned corner crack resistance and string hole durability can be obtained.

In some embodiments, the kraft paper contains bleached softwood kraft pulp along with unbleached softwood kraft pulp. In some embodiments, the kraft paper contains 35% or less by mass of bleached softwood kraft pulp in the total pulp contained in the kraft paper. In at least one embodiment, the kraft paper contains 35% or less by mass and 10% or more by mass of bleached softwood kraft pulp in the total pulp contained in the kraft paper. The reason for this is that the ease of opening of the kraft paper is improved.

The kraft paper has a specific tensile strength, calculated by the following formula, of 0.060 (kN·m/g) or more in the machine direction and 0.040 (kN·m/g) or more in the cross direction.
Specific tensile strength (kN·m/g)=(tensile strength (kN/m) ÷ basis weight (g/m ² ))
Furthermore, the kraft paper has an A value calculated by the following formula of 36.0 or more and 46.0 or less in the machine direction of the kraft paper, and 141.0 or more and 184.0 or less in the cross direction of the kraft paper.
A value = elongation rate of kraft paper (%) ÷ specific tensile strength (kN m/g)
By satisfying these values, the kraft paper can have corner crack resistance when forming packages, envelopes, and bags, ease of gripping and opening the envelope, and string hole durability when the paper bag has a string handle. If the kraft paper does not satisfy these values, it cannot have at least one of the corner crack resistance when forming packages, envelopes, and bags, ease of gripping and opening the envelope, and string hole durability when the paper bag has a string handle. There is no particular upper limit for the specific tensile strength. This is because the upper limit for the specific tensile strength is inevitably determined because the elongation and tensile strength of the kraft paper are in conflict with each other and there is a limit to the strength of the paper.

The specific tensile strength and elongation of kraft paper can be determined in accordance with ISO1924-2:1994 "Paper and board-Determination of tensile properties-Part 2: Constant rate of elongation method". These measurements can be performed using a measuring device that follows the method in accordance with ISO1924-2:1994. An example of a measuring device is the Tensilon universal material testing machine manufactured by Orientec Co., Ltd. The specific tensile strength and elongation are conventionally known in the papermaking field, and can be adjusted, for example, by adjusting the ash content, density, paper strength, pulp type and freeness of the base paper, as well as the pulp jet speed/fourdrinier wire speed (J/W ratio), the jetted pulp concentration and the dewatering speed of the wire part during papermaking.

In some embodiments, the kraft paper contains unbleached softwood kraft pulp with a freeness of 500 mlcsf or more. In some embodiments, the kraft paper contains bleached softwood kraft pulp together with unbleached softwood kraft pulp, and the freeness of the unbleached softwood kraft pulp is 500 mlcsf or more and the freeness of the bleached softwood kraft pulp is 400 mlcsf or more and 500 mlcsf or less. The reason for this is that the kraft paper is more likely to satisfy the above numerical ranges, and as a result, the kraft paper is more likely to obtain any one of corner crack resistance when forming packages, envelopes, and bags, ease of gripping and opening an envelope, and string hole durability when the handle of a paper bag is a string.
The freeness is a value determined in accordance with ISO5267-2:2001 "Pulps-Determination of drainability-Part 2: Canadian Standard Freeness method". The freeness can be adjusted by beating and/or refining the pulp. The value of the freeness decreases as the beating and/or refining of the pulp proceeds. The upper limit of the freeness of softwood unbleached kraft pulp, which has a freeness in the range of 500 mlcsf or more, is the freeness of pulp that is not subjected to beating and/or refining. The upper limit of the freeness of general softwood unbleached kraft pulp is in the range of 680 mlcsf to 740 mlcsf.

The sizing agent is an internal sizing agent used in the papermaking process to impart sizing properties. Due to the action mechanism of the sizing agent, the sizing agent also has a positive effect on the strength of the paper. Sizing agents are conventionally known in the papermaking field and are not particularly limited. Examples of sizing agents include rosin-based sizing agents for acidic paper, alkenyl succinic anhydride-based sizing agents, alkyl ketene dimer-based sizing agents, neutral rosin-based sizing agents, fatty acid-based sizing agents, and cationic styrene acrylic resin-based sizing agents for neutral paper. In some embodiments, the sizing agent is a rosin-based sizing agent. This is because the sizing properties of the kraft paper made by papermaking a paper material that does not substantially contain a filler can be stably expressed.
The content of the sizing agent in the kraft paper may be any content that is conventionally known in the papermaking field, for example, from 1 part by mass to 20 parts by mass per 1000 parts by mass of pulp contained in the kraft paper.

Paper strength agents are also called paper strength enhancers, and are used in the papermaking process to give strength to paper. Paper strength agents are conventionally known in the papermaking field, and are not particularly limited. Examples of paper strength agents include starch, modified starches such as dialdehyde starch, oxidized starch, and cationized starch, vegetable gums such as guar gum, locust bean gum, and tragacanth gum, celluloses such as methylcellulose, carboxymethylcellulose, hydroxymethylcellulose, and hydroxyethylcellulose, and synthetic polymers such as polyurethane resins, polyethyleneimine resins, urea resins, polyethylene oxide resins, and polyvinyl alcohol resins.
In some embodiments, the strength agent is starch and modified starches such as dialdehyde starch, oxidized starch, and cationic starch, for safety reasons when the kraft paper is used for food packaging.
The content of the paper strength agent in the kraft paper may be any content that is conventionally known in the papermaking field. The content of the paper strength agent is, for example, 5 parts by mass or more and 30 parts by mass or less per 1000 parts by mass of pulp contained in the kraft paper.

Aluminum sulfate is effective in fixing the sizing agent in the paper. It is particularly effective when the sizing agent is a rosin-based sizing agent. The aluminum ions in aluminum sulfate can bond with the pulp fibers, thereby increasing the tensile strength of the kraft paper. However, the inclusion of aluminum sulfate makes the paper acidic and prone to deterioration. For this reason, the amount of aluminum sulfate that can be contained is limited.

In some embodiments, the kraft paper contains retention aids conventionally known in the papermaking art because they improve the corner crack resistance and string hole resistance discussed above.
The retention agent is, for example, a cationic resin or an anionic resin. The cationic resin used as the retention agent is a resin containing a primary to tertiary amine or a quaternary ammonium salt to which protons are easily coordinated and which dissociates when dissolved in water to exhibit cationicity. Examples of the cationic resin include polyethyleneimine, polyvinylpyridine, polyamine sulfone, polydialkylaminoethyl methacrylate, polydialkylaminoethyl acrylate, polydialkylaminoethyl methacrylamide, polydialkylaminoethyl acrylamide, polyepoxyamine, polyamidoamine, dicyandiamide-formaldehyde condensate, dicyandiamide polyalkyl-polyalkylene polyamine condensate, polyvinylamine, polyallylamine, and other compounds and salts thereof, as well as diallylamine acrylamide copolymers, polydiallylmethylamine hydrochloride, poly(diallyldimethylammonium chloride), poly(diallyldiethylammonium chloride), copolymers of diallyldimethylammonium chloride and acrylamide, copolymers of diallyldimethylammonium chloride and methacrylamide, copolymers of diallyldiethylammonium chloride and acrylamide, copolymers of diallyldiethylammonium chloride and methacrylamide, copolymers of dimethylamine ammonia epichlorohydrin, and diethylamine epichlorohydrin ammonia copolymers.
The content of the retention agent in the kraft paper may be any content that is conventionally known in the papermaking field, for example, from 0.05 to 1 part by mass per 1,000 parts by mass of pulp contained in the kraft paper.

A kraft paper containing at least unbleached softwood kraft pulp, a sizing agent, a paper strength agent, and aluminum sulfate, wherein the freeness of the unbleached softwood kraft pulp is 500 mlcsf or more, and the kraft paper has a tensile strength index calculated by the following formula of 0.060 (kN·m/g) or more in the longitudinal direction of the kraft paper and 0.040 (kN·m/g) or more in the transverse direction of the kraft paper, and an A value calculated by the following formula of 36.0 to 46.0 in the longitudinal direction of the kraft paper and 141.0 to 184.0 in the transverse direction of the kraft paper.
Specific tensile strength (kN·m/g)=(tensile strength (kN/m) ÷ basis weight (g/m ² ))
A value = elongation rate of kraft paper (%) ÷ specific tensile strength (kN m/g)

The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to the following examples as long as it does not deviate from the gist of the invention. Furthermore, the mass percentages and parts by mass shown in the examples indicate values of dry solids or substantial components. The amount of surface sizing agent applied indicates the value of dry solids.

<Kraft paper>
A slurry of unbleached kraft pulp and/or bleached kraft pulp adjusted to a predetermined freeness was dispersed in water in a predetermined amount of parts by mass totaling 1000 parts by mass, and 4 parts by mass of a rosin sizing agent (Seiko PMC, CC-1404), 15 parts by mass of a cationized starch (Grain Processing Corporation, CHARGEMASTER R462) as a paper strength agent, 10 parts by mass of aluminum sulfate, and 0.1 parts by mass of an acrylamide resin (Himoc (registered trademark) ND300, Himo Co., Ltd.) as a retention agent were mixed into the slurry to prepare a paper stock. The paper stock was made using a Fourdrinier paper machine while controlling the pulp concentration, J/W, and drying process to obtain a paper stock. Both sides of the paper were subjected to a size press treatment with a size press liquid containing oxidized starch (Oji Cornstarch Co., Ltd., Oji Ace A) and styrene-acrylic resin (Arakawa Chemical Industries Co., Ltd., Polymeron (registered trademark) 1308S) as surface sizing agents to obtain kraft paper with a basis weight of 70 g/ ^m2 and specific tensile strength and elongation shown in Table 1. The amounts of surface sizing agents applied were 0.3 g/ ^m2 for oxidized starch and 0.07 g/ ^m2 for styrene-acrylic resin per side. The ash content was 1.5% by mass.

The parts by weight and freeness (mlcsf) of unbleached kraft pulp and bleached kraft pulp are shown in Table 1.
In Table 1, unbleached softwood kraft pulp is abbreviated as NUKP, and bleached softwood kraft pulp is abbreviated as NBKP.

Each kraft paper was evaluated for the following qualities. The evaluation results for each kraft paper are shown in Table 1.

<Corner crack resistance>
For the test paper bags, 100 square bottom bags measuring 360 mm long x 280 mm wide x 80 mm gusset were made from kraft paper using an automatic square bottom bag making machine. The lengthwise direction of the kraft paper was aligned with the lengthwise direction of the paper bag. The state of corner cracking at the fold of the obtained square bottom bag was observed and evaluated according to the following criteria. In the present invention, kraft paper is considered to have corner crack resistance if it is rated A, B, or C.
A: The number of pieces with a corner crack in one place is 1 or less out of 100 pieces.
B: The number of pieces with corner cracks in one place is 2 to 3 out of 100 pieces.
C: The number of parts having a corner crack in one place is 4 to 5 parts out of 100 parts.
D: The number of parts with corner cracks in one place is 6 or more out of 100 parts,
Or in the case of a corner-cracked corner-bottom bag, corner cracks are found in multiple places.

<Easy to grip and open>
An A4 size envelope was made from craft paper. The length of the craft paper was aligned with the long side of the envelope. Ten monitors were asked to seal and open the envelope. The sealing was performed by inserting three A4 size clear files into the envelope and sealing the mouth of the envelope with a stick glue. The opening was performed by forcing a thumb into the gap at the end of the sealed envelope mouth and tearing the flap part of the envelope mouth while moving the thumb along the envelope mouth. Each monitor performed 20 tasks. The monitors performed these tasks and performed a sensory evaluation based on the following criteria from the viewpoint of the ease of gripping and opening the craft paper. The final evaluation result was calculated by the average value of the 10 monitors and rounded off to the nearest whole number. In the present invention, if the craft paper is rated A, B, or C, it is considered to have ease of gripping and opening.
A: 5 points (very good)
B: 4 points (good)
C: 3 points (generally good)
D: 2 points (normal)
E: 1 point (bad)

<Leg hole durability>
A square-bottom bag was prepared in the same manner as in the evaluation of <corner crack resistance> above. A handle was attached to the square-bottom bag. The part where the acrylic string for the handle (diameter 2.5 mm/length 400 mm) was attached was made by folding the craft paper twice. The width of the fold was 100 mm. On the front and back of the paper bag, holes (string holes) were opened symmetrically at positions 50 mm from the mouth of the paper bag and 50 mm from the center in the width direction, and the string was passed through the holes to make a knot and attach a handle with a horizontal hole fastening. The string hole durability of the paper bag with handle prepared in this manner was evaluated. The string hole durability was evaluated by placing a commercially available unopened 500 ml PET bottle in a paper bag placed on the floor, lifting it up, and visually observing the state of the string hole, and the observation results were evaluated according to the following criteria. In the present invention, if the craft paper is rated A, B, or C, it is considered to have string hole durability.
A: With five or more PET bottles, no tears were observed in the string holes.
B: With four or fewer PET bottles, no tears were observed at the string holes.
C: With three or fewer PET bottles, no tears were observed at the string holes.
D: Two or less plastic bottles had tears in the string holes.

From Table 1, it can be seen that Examples 1 to 10, which satisfy the configuration of the present invention, are kraft papers that can be used as materials for packaging, envelopes, and paper bags, and have resistance to corner cracking when forming packaging, envelopes, and bags, ease of gripping and opening envelopes, and string hole durability when the paper bag has a string handle. On the other hand, it can be seen that Comparative Examples 1 to 5, which do not satisfy the configuration of the present invention, are unable to obtain any of the above effects.

Claims (1)

A kraft paper containing at least unbleached softwood kraft pulp, a sizing agent, a paper strength agent, and aluminum sulfate, wherein the kraft paper has a specific tensile strength calculated by the following formula of 0.060 (kN·m/g) or more in the longitudinal direction of the kraft paper and 0.040 (kN·m/g) or more in the transverse direction of the kraft paper, and an A value calculated by the following formula of 36.0 to 46.0 in the longitudinal direction of the kraft paper and 141.0 to 184.0 in the transverse direction of the kraft paper.
Specific tensile strength (kN·m/g)=(tensile strength (kN/m) ÷ basis weight (g/m ² ))
A value = elongation rate of kraft paper (%) ÷ specific tensile strength (kN m/g)