FLAME-PROOF CORRUGATED PAPERBOARD AND METHOD FOR PRODUCING SAME

Abstract

Provided are a flame-proof linerboard and a flame-proof corrugated paperboard in which with a small coating amount, a flame-proof layer is formed on the surface of a general linerboard so as to reduce penetration of the flame-proofing agent, and which are thus low in cost and exhibit high flame-proof performance. A flame-proof corrugated paperboard (21) is produced which includes, as a linerboard (15), a flame-proof paper including a paper layer (11), a flame-proof layer (12) containing a flame-proofing agent and a polymer having a molecular weight of 8,000 or more and 10,000,000 or less, and an overcoat layer (13) for protecting the flame-proof layer (12), the overcoat layer (13) being disposed closer to the surface of the corrugated paperboard than the flame-proof layer (12) is.

Claims

1. A flame-proof corrugated paperboard comprising, as a flame-proof linerboard, a flame-proof paper including: a paper layer; a flame-proof layer containing a flame-proofing agent and a polymer having a molecular weight of 8,000 or more and 10,000,000 or less; and an overcoat layer for protecting the flame-proof layer, the overcoat layer being disposed closer to a surface of the flame-proof corrugated paperboard than the flame-proof layer is.

2. The flame-proof corrugated paperboard according to claim 1, wherein the polymer is a water-soluble polymer.

3. The flame-proof corrugated paperboard according to claim 2, wherein the flame-proofing agent is a water-soluble salt.

4. A production method for producing a flame-proof linerboard, the method comprising: applying to one surface of a paper layer of the flame-proof linerboard a coating liquid containing a flame-proofing agent and a polymer having a molecular weight of 8,000 or more and 10,000,000 or less, thereby forming a flame-proof layer; and applying an overcoat agent to a surface of the flame-proof layer on a side of the flame-proof layer opposite from the paper layer, thereby forming an overcoat layer.

5. The production method according to claim 4, wherein the coating liquid has a viscosity of 20 mPa.Math.s or more and 1,900 mPa.Math.s or less.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG. 1 is a sectional view of the flame-proof linerboard used in a flame-proof corrugated paperboard embodying the present invention.

[0020] FIG. 2 is a sectional view of the flame-proof corrugated paperboard embodying the present invention.

[0021] FIG. 3A is a photograph showing a result of a flame-proofing test in Example 1.

[0022] FIG. 3B is a photograph showing a result of a flame-proofing test in Example 2.

[0023] FIG. 3C is a photograph showing a result of a flame-proofing test in Comparative Example 2.

[0024] FIG. 3D is a photograph showing a result of a flame-proof test in Example 14.

BEST MODE FOR CARRYING OUT THE INVENTION

[0025] The present invention will be described below in detail with reference to embodiments. The present invention is directed to a corrugated paperboard having flame-proof performance. FIG. 1 shows a sectional view of a linerboard 15 used in a corrugated paperboard embodying the present invention, and FIG. 2 shows a sectional view of the corrugated paperboard.

[0026] The linerboard 15 includes a paper layer 11, a flame-proof layer 12 containing a flame-proofing agent and a polymer having a molecular weight of 8,000 or more and 10,000,000 or less, and an overcoat layer 13 for protecting the flame-proof layer 12. Basically, the flame-proof layer 12 is disposed on the side of a surface of the corrugated paperboard (hereinafter simply referred to as the surface side) so as to be kept in contact with the paper layer 11, and the overcoat layer 13 is disposed on the surface side so as to be kept in contact with the flame-proof layer 12. The present invention is achievable even if a different layer is disposed between these layers. However, since the closer the flame-proof layer 12 is located to a flame or a heat source from the surface side, the more likely the flame-proof layer 12 is to exhibit its flame-proof performance, it is desirable that a different layer is not disposed between the flame-proof layer 12 and the overcoat layer 13.

[0027] As the paper layer 11, a general linerboard can be used. While a linerboard having an inorganic layer on its surface can also be used, since, in the present invention, the flame-proof layer 12 reduces penetration into the paper layer 11, even a general linerboard is suitably usable which is especially not subjected to surface treatment for, e.g., providing an inorganic layer. Such a general linerboard is desirable in terms of easy availability and cost, too.

[0028] The flame-proof layer 12 contains the flame-proofing agent and the polymer, and uniformly covers the entire surface of the paper layer 11 on one side thereof so as to exhibit its flame-proof performance. As the flame-proofing agent contained in the flame-proof layer 12, for example, a phosphorus compound, a halogen compound or a metal hydroxide can be used. Among them, use of especially a water-soluble salt is preferable because a water-soluble salt is easily mixed as an aqueous solution of the flame-proofing agent when mixed with a water-soluble polymer described later to prepare a coating liquid. Examples of the flame-proofing agent which is a water-soluble salt include, e.g., phosphorus-nitrogen compounds such as ammonium polyphosphate and polyphosphoric acid amide, guanidine salts such as guanidine sulfamate and guanidine phosphate, halogen compounds such as ammonium bromide and ammonium chloride, boric acid compounds such as borax and sodium borate, and inorganic salts such as ammonium sulfate. The flame-proofing agent having water solubility refers to an agent in which solubility with respect to 100 g of water is 1 g or more, and preferably 10 g or more.

[0029] As the polymer contained in the flame-proof layer 12, a polymer is usable which can be dispersed or dissolved in water, and can form a film on the surface of the paper layer 11 by drying. Among them, use of especially a water-soluble polymer that can be dissolved in water is preferable because even when the water-soluble polymer is mixed with the flame-proofing agent, the water-soluble polymer can be applied while being uniformly dissolved without being aggregated by the flame-proofing agent, and this facilitates formation of the flame-proof layer 12 having high uniformity. The water-soluble polymer, which has water solubility, refers to a polymer in which the solubility with respect to 100 g of water is 0.2 g or more, and preferably 6 g or more. The water-soluble polymer is preferably a polymer having a relatively large oxygen index (OI) of 20 or more, and may be, e.g., a homopolymer such as polyvinyl alcohol (OI=about 22), polyacrylamide (OI=about 27) or polyvinylpyrrolidone (OI=21); or a copolymer containing monomers forming such homopolymers.

[0030] The polymer has, as a weight average molecular weight, a molecular weight of preferably 8,000 or more, and more preferably 30,000 or more. If a polymer having a molecular weight of less than 8,000 is used, it is difficult to achieve an appropriate concentration that is easy to coat, and the penetration of the flame-proofing agent into the paper layer 11 cannot be ignored, so that it is difficult to form the flame-proof layer 12. On the other hand, the polymer has a weight average molecular weight of preferably 10,000,000 or less, and more preferably 8,000,000 or less. If it exceeds 10,000,000, the solubility in water is significantly reduced, and the polymer concentration in the coating liquid cannot be increased. As a result, it is difficult to prevent the penetration into the paper layer 11, and thus the flame-proof performance deteriorates. The number average molecular weight generally shows a value equal to or less than the weight average molecular weight. Therefore, when the number average molecular weight is 8,000 or more, the above lower limit condition is satisfied. On the other hand, the viscosity average molecular weight shows an intermediate value between the number average molecular weight and the weight average molecular weight, though this depends on the molecular weight distribution of the polymer. Therefore, when the number average molecular weight or the viscosity average molecular weight is 10,000,000 or less, the above upper limit condition is substantially satisfied.

[0031] With respect to the flame-proofing agent and the polymer that are contained in the flame-proof layer 12, the mixing ratio of the flame-proofing agent to the polymer is preferably 0.5 times or more and 10 times or less. If the mixing ratio is less than 0.5 times, it is difficult to exhibit the flame-proof performance. On the other hand, if the mixing ratio exceeds 10 times, the strength of the coating film is reduced, so that the flame-proofing agent could fall off.

[0032] The flame-proof layer 12 is formed by applying to one surface of the paper layer 11 the coating liquid containing the flame-proofing agent and the polymer. The viscosity of the coating liquid is adjusted by the concentration in the coating liquid according to the molecular weight of the polymer used, and is preferably 20 mPa.Math.s or more, and more preferably 30 mPa.Math.s or more as a value measured at 20 C. by a B-type viscometer manufactured by Toki Sangyo Co., Ltd. If the viscosity is less than 20 mPa.Math.s, the viscosity is too low, and the amount of the flame-proofing agent penetrating into the paper layer 11 during coating is too large, so that a larger amount of the coating liquid is required for making a sufficient amount of the flame-proofing agent remain on the surface. On the other hand, the viscosity of the coating liquid is preferably 1,900 mPa.Math.s or less, and more preferably 1,800 mPa's or less. If the viscosity exceeds 1,900 mPa.Math.s, the viscosity is too high, and it is difficult to form a uniformly coated layer.

[0033] Examples of the method of applying the coating liquid onto the surface of the paper layer 11 to form the flame-proof layer 12 include, e.g., a gravure coater, a roll coater, a bar coater and a die coater. Among them, a direct gravure coater or a reverse gravure coater is preferable as a gravure coater. Even if the coating liquid is directly applied to the paper layer 11, since the molecular weight of the polymer and the viscosity of the coating liquid are adjusted to the above ranges, a layer of the flame-proofing agent is easily formed on the surface of the paper layer 11 while reducing the penetration into the paper layer 11, so that sufficient flame-proof performance can be obtained. Coating with especially a gravure coater is also suitable for production in a small lot using a gravure printing machine.

[0034] Since the flame-proof layer 12 is formed as a film containing the polymer, it is possible to reduce separation of the layer 12 during coating. In addition to this, since the overcoat layer 13 is disposed on the flame-proof layer 12, it is possible to reduce separation of the flame-proofing agent from the surface while forming the linerboard on the corrugated paperboard. Therefore, it is possible to improve the heat and wear resistance.

[0035] The applied amount of the flame-proofing agent contained in the flame-proof layer 12 is preferably 0.2 g/m.sup.2 or more, and more preferably 0.7 g/m.sup.2 or more in solid content. If the applied amount is less than 0.2 g/m.sup.2, the amount of the flame-proofing agent is too small, so that the flame-proof layer 12 could fail to exhibit sufficient flame-proof performance. On the other hand, the applied amount is preferably 20.0 g/m.sup.2 or less, and more preferably 5.0 g/m.sup.2 or less. Even if the applied amount exceeding 20.0 g/m.sup.2 is used, improvement in the flame-proof performance cannot be expected as much as the added amount, and it is impossible to make the most of it.

[0036] The linerboard 15, which partially constitutes the flame-proof corrugated paperboard 21 of the present invention, includes the overcoat layer 13 on the surface of the flame-proof layer 12 on its side (upper side in FIG. 1) opposite from (i.e., remoter from) the paper layer 11. The overcoat layer 13 prevents separation and wear of the flame-proof layer 12, and protects the flame-proof layer 12 so that the flame-proof layer 12 continuously exhibits its flame-proof performance even after, e.g., heating while machining the corrugated paperboard. The overcoat layer 13 needs to uniformly cover the entire flame-proof layer 12 so as to protect the flame-proof layer 12. The overcoat layer 13 is formed by forming the flame-proof layer 12, and then applying an overcoat agent thereon. As the overcoat agent, a varnish, or the combination of a varnish and an additive can be used. The type of varnish used may be, for example, a nitrocellulose-based or acrylic-based one, but is not particularly limited, because the flame-proof layer 12, which has excellent flame-proof performance, is formed on the paper layer 11, and thus the flame-proof performance is not affected. That is, any varnish may be used if the varnish exhibits wear resistance to such an extent that the varnish is not easily peeled off in a general corrugated paperboard production process and use process.

[0037] The applied amount of the overcoat agent is preferably 0.4 g/m.sup.2 or more and 18.0 g/m.sup.2 or less in solid content. If the applied amount is 0.4 g/m.sup.2 or less, it is difficult to sufficiently protect the flame-proof layer 12. On the other hand, if the applied amount is 18.0 g/m.sup.2 or more, the flame-proof performance could deteriorate, coating is difficult, and there is a problem in terms of cost, too.

[0038] The overcoat layer 13 may contain an inorganic substance as the above-mentioned additive in addition to the above-mentioned varnish. As compared to the overcoat layer 13 formed of only the varnish, the overcoat layer 13 containing the inorganic substance exhibits even higher flame-proof performance. Examples of the inorganic substance contained include oxides and metal compounds such as calcium carbonate, kaolin, silicon dioxide and titanium dioxide.

[0039] The content of the inorganic substance in the overcoat layer 13 is preferably 3% by mass or more. If the content is less than 3% by mass, the effect of improving the flame-proof performance is small, and even if the inorganic substance is added, the flame-proof performance is not improved. On the other hand, the content of the inorganic substance is preferably 35% by mass or less. If the content exceeds 35% by mass, the inorganic substance is likely to be precipitated in the coating liquid, so that a uniform overcoat layer may not be formed, or the inorganic substance may fall off from the overcoat layer.

[0040] The linerboard 15, which includes, on the paper layer 11, the above-described flame-proof layer 12 and overcoat layer 13 in this order, is a flame-proof linerboard that sufficiently exhibits its flame-proof performance against a flame or a heat source from the side of the overcoat layer 13. By using the linerboard 15 as at least one of a front linerboard and a rear linerboard, the at least one linerboard exhibits its flame-proof performance against a flame or a heat source from the direction in which the at least one linerboard is used. Especially if linerboards 15 as described above are used as the front linerboard and the rear linerboard, respectively, a flame-proof corrugated paperboard is obtainable which exhibits flame-proof performance equivalent to the flame-proof standard acceptance (45 Meckel burner method, such as a disaster partition) defined by the Japan Fire Retardant Association.

[0041] Further, the flame-proof corrugated paperboard 21 in which linerboards 15 as described above are used, respectively, as the front linerboard and the rear linerboard exhibits sufficient flame-proof performance, and thus a special flame-proof treatment to its corrugated medium 17 is not required. Also, since each linerboard 15 includes the overcoat layer 13, the flame-proof layer 12 is also protected from a load such as heat or friction applied in a normal production process of corrugated paperboard, and thus, the flame-proof performance is continuously exhibited. For this reason, the flame-proof corrugated paperboard according to the present invention can be produced by a method similar to that of general corrugated paperboard, but exhibits sufficient flame-proof performance.

[0042] The flame-proof corrugated paperboard according to the present invention can be used not only for applications such as building material panels, display panels, and partitions, but also for transport boxes and storage boxes for specific articles that are required to have flame-proof performance by taking advantage of its sufficient flame-proof performance.

EXAMPLES

[0043] Next, the flame-proof corrugated paperboard according to the present invention will be described more specifically with reference to examples in which the present invention is actually used. First, e.g., materials used will be enumerated.

<Flame-Proofing Agent>

[0044] Phosphorus-nitrogen flame-proofing agent: NONNEN R061-3 (aqueous solution: active ingredient=40%), produced by Marubishi Oil Chemical Co., Ltd. [0045] Guanidine flame-proofing agent: Plasafety SC-1000 (solid: guanidine sulfamate content90%), produced by Manac Incorporated. [0046] Halogen flame-proofing agent: ammonium bromide: special grade reagent, produced by FUJIFILM Wako Pure Chemical Corporation

<Water-Soluble Polymer>

[0047] Polyacrylamide 1: product code 555-77731, viscosity average molecular weight 5,000,000 to 6,000,000, produced by FUJIFILM Wako Pure Chemical Corporation [0048] Polyacrylamide 2: product code 738743, number average molecular weight 40,000, produced by Sigma-Aldrich Japan [0049] Polyacrylamide 3: Helios Floc HA-825, viscosity average molecular weight 11,000,000, produced by Helios K.K. [0050] Polyvinyl alcohol 1: JF-02, average polymerization degree 200 (viscosity average molecular weight 9,000), produced by JAPAN VAM & POVAL CO., LTD. [0051] Polyvinyl alcohol 2: JF-10, average polymerization degree 1,000 (viscosity average molecular weight 44,000), produced by JAPAN VAM & POVAL CO., LTD.

<Overcoat Agent>

[0052] Varnish 1: Gratone PCN (nitrocellulose-based), produced by SAKATA INX CORPORATION [0053] Varnish 2: New FK MR OP varnish N-2000 (acrylic-based), produced by SAKATA INX CORPORATION

<Inorganic Substance for Overcoat Agent>

[0054] Silicon dioxide: special grade reagent, produced by FUJIFILM Wako Pure Chemical Corporation [0055] Calcium carbonate: special grade reagent, produced by FUJIFILM Wako Pure Chemical Corporation [0056] Titanium dioxide: titanium oxide production only, produced by FUJIFILM Wako Pure Chemical Corporation

<Linerboard>

[0057] General linerboard: RKA170, produced by Rengo Co., Ltd. [0058] Inorganic layer-containing linerboard: CRC230, produced by Rengo Co., Ltd.

<Verification of Coatability by Viscosity>

Example 1

[0059] A coating liquid as a flame-proofing agent aqueous solution was prepared by mixing together a phosphorus-nitrogen flame-proofing agent, polyacrylamide 1 and water such that the phosphorus-nitrogen flame-proofing agent is 28% by mass, the polyacrylamide 1 is 6% by mass, and the water is 66% by mass. The coating liquid had a viscosity of 30 mPa.Math.s. By applying the coating liquid to a general linerboard surface constituting the front surface of a corrugated paperboard with a bar coater (Select-Roller manufactured by OSG SYSTEM PRODUCTS CO., LTD.) such that the applied amount would be 3.4 g/m.sup.2, a flame proof-layer was formed. After drying the flame-proof layer, by applying varnish 1 as an overcoat agent onto the flame-proof layer with the bar coater such that the applied amount would be 2.0 g/m.sup.2, an overcoat layer was formed, so that a flame-proof linerboard was produced. The overcoat layer formed was a uniform and even layer in appearance. Using thus-formed flame-proof linerboards, respectively, as front and rear linerboards, a general corrugated medium (manufactured by Rengo Co., Ltd.: S120) formed into a wave shape with a test fluter (manufactured by Nihon T.M.C. Corporation) was bonded with starch adhesive to form a A-flute corrugated paperboard, and the flame-proof performance was evaluated. The flame-proof performance was tested in accordance with the 45 Meckel burner method by the Japan Fire Retardant Association, and determined according to the following criteria.

[0060] means passed residual flame time (10 seconds or less) and afterglow time (30 seconds or less), means passed either one of residual flame time and afterglow time, and x means failed both of residual flame time and afterglow time.

[0061] According to the above determination criteria, Example 1 was determined as passed (). FIG. 3A shows a surface photograph after the flame-proof test in which the flame-proof performance was measured.

TABLE-US-00001 TABLE 1 Example/Comparative Example Number Comparative Comparative Example 3 Example 2 Example 1 Example 2 Type of Low High Flame- flame- Example 1 molecular molecular proofing proofing Content Reference weight PAM weight PAM agent only agent Linerboard RKA170 Flame- Flame- Phosphorus-nitrogen 28% by mass proofing proofing compound agent agent Guanidine compound 28% by mass Halogen compound Water- PAM1 (medium 6% by mass 6% by mass soluble molecular weight) polymer PAM2 (low molecular 6% by mass weight) PAM3 (high molecular 0.2% by mass weight) Water 66% by mass 71.8% by mass 72% by mass 66% by mass Viscosity of coating liquid 30 25 7 30 (mPa .Math. s) Coating method Bar coater Applied amount 3.4 2.82 2.8 (solid content g/m.sup.2) (Flame- (Flame- proofing proofing agent = 2.8, agent = 2.8, polymer = 0.6) polymer = 0.02) Overcoat Varnish Varnish 1 100% by mass agent (nitrocellulose-based) Applied amount 2.0 (solid content: g/m.sup.2) Flame-proof Without heat friction X X performance Example/Comparative Example Number Example 4 Type of Example 5 Example 6 Example 7 flame- Applied amount of Applied amount of Applied amount of proofing flame-proofing flame-proofing flame-proofing Content agent agent agent agent Linerboard Flame- Flame- Phosphorus-nitrogen 28% by mass proofing proofing compound agent agent Guanidine compound Halogen compound 28% by mass Water- PAM1 (medium soluble molecular weight) polymer PAM2 (low molecular weight) PAM3 (high molecular weight) Water Viscosity of coating liquid 30 30 (mPa .Math. s) Coating method Applied amount 0.34 0.85 24.1 (solid content g/m.sup.2) (Flame- (Flame- (Flame- proofing proofing proofing agent = 0.28, agent = 0.7, agent = 19.8, polymer = 0.06) polymer = 0.15) polymer = 4.3) Overcoat Varnish Varnish 1 agent (nitrocellulose-based) Applied amount (solid content: g/m.sup.2) Flame-proof Without heat friction performance

<Verification of Molecular Weight of Water-Soluble Polymer>

Example 2

[0062] A flame-proof corrugated paperboard was prepared in the same manner as in Example 1 except that the viscosity of the coating liquid was 25 mPa.Math.s by changing the water-soluble polymer used to polyacrylamide 2, which has a different molecular weight. As a result, the determination was . FIG. 3B shows a surface photograph after the flame-proof test in which the flame-proof performance was measured.

Comparative Example 1

[0063] A flame-proof corrugated paperboard was prepared in the same manner as in Example 1 except that by changing the water-soluble polymer used to polyacrylamide 3, which has a different molecular weight, the solubility in water was remarkably small and the concentration of the water-soluble polymer in the flame-proofing agent aqueous solution was 0.2% by mass. As a result, the flame-proofing agent aqueous solution infiltrated into the paper layer, and a sufficient flame-proof layer was not obtained.

Comparative Example 2

[0064] A flame-proof corrugated paperboard was prepared in the same manner as in Example 1 except that polyacrylamide was excluded from the flame-proofing agent aqueous solution to adjust the viscosity to 7 mPa.Math.s. As a result, the flame-proof performance was x, and most of the flame-proof corrugated paperboard was burned. A surface photograph of Comparative Example 2 is shown in FIG. 3C. It is considered that this occurred because the water-soluble polymer was not added to the flame-proofing agent aqueous solution, the viscosity decreased, most of the flame-proofing agent aqueous solution was absorbed into the linerboard, and a flame-proof layer was not formed on the surface.

<Verification of Type of Flame-Proofing Agent>

Examples 3 and 4

[0065] In each of Examples 3 and 4, a flame-proof corrugated paperboard was prepared in the same manner as in Example 1 except that the flame-proofing agent was changed to a guanidine flame-proofing agent (Example 3), or a halogen flame-proofing agent (Example 4), and the viscosity of the coating liquid was 30 mPa.Math.s by adding water for dissolution such that the concentration of the agent in the flame-proofing agent aqueous solution would be the same as that of Example 1 since the agent was provided in the form of a solid or a powder. The flame-proof performance in each of Examples 3 and 4 was determined as .

<Adjustment of Applied Amount>

Examples 5 to 7

[0066] In each of Examples 5 to 7, a flame-proof corrugated paperboard was prepared in the same manner as in Example 1 except that the applied amount of the flame-proofing agent was changed to 0.28 g/m.sup.2 (Example 5); 0.7 g/m.sup.2 (Example 6) or 19.8 g/m.sup.2 (Example 7). As a result of determining the flame-proof performance of each corrugated paperboard prepared, Example 5, in which the applied amount of the flame-proofing agent was small, was , but Examples 6 and 7 were . This shows that as the applied amount of the flame-proofing agent increase, the flame-proof performance improves.

<Type of Water-Soluble Polymer>

Examples 8 and 9

[0067] In each of Examples 8 and 9, a flame-proof corrugated paperboard was prepared in the same manner as in Example 1 except that the viscosity of the coating liquid was 30 mPa.Math.s (Example 8) or 80 mPa.Math.s (Example 9) by changing the polymer used from polyacrylamide 1 to polyvinyl alcohol 1 (Example 8) or polyvinyl alcohol 2 (Example 9). The flame-proof performance of each corrugated paperboard prepared was determined as . It was confirmed that even if the type of the polymer used is changed under the condition that it is a water-soluble polymer, the flame-proof corrugated paperboard according to the present invention can be produced.

TABLE-US-00002 TABLE 2 Example/Comparative Example Number Comparative Example 9 Example 10 Example 3 Example 8 Type of Mixing ratio of Mixing ratio of Example 11 Content Type of polymer polymer polymer polymer Type of vamish Linerboard RKA170 Flame- Flame- Phosphorus-nitrogen 28% by mass proofing proofing compound agent agent Water- PAM1 (medium 15% by mass 17% by mass 6% by mass soluble molecular weight) polymer PVA1 (low molecular 6% by mass weight) PVA2 (high molecular 6% by mass weight) Water 66% by mass 57% by mass 55% by mass 66% by mass Viscosity of coating liquid 30 80 1,800 2,000 30 Coating method Bar coater Gravure printing Bar coater machine Applied amount 3.4 4.3 4.5 3.4 (solid content g/m.sup.2) (Flame-proofing (Flame-proofing (Flame-proofing (Flame-proofing agent = 2.8, agent = 2.8, agent = 2.8, agent = 2.8, polymer = 0.6) polymer = 1.5) polymer = 1.7) polymer = 0.6) Overcoat Varnish Vamish 1 100% by mass agent (nitrocellulose-based) Varnish 2 (acrylic) 100% by mass Applied amount 2.0 (solid content g/m.sup.2) Flame-proof Without heat friction performance (Flame-proof layer was non-uniform and could not be evaluated) With heat friction Example/Comparative Example Number Comparative Example 5 Example 12 Example 13 Comparative Excessive Applied amount Applied amount Example 4 applied amount Content of varnish of varnish No varnish of varnish Linerboard Flame- Flame- Phosphorus-nitrogen proofing proofing compound agent agent Water- PAM1 (medium soluble molecular weight) polymer PVA1 (low molecular weight) PVA2 (high molecular weight) Water Viscosity of coating liquid Coating method Applied amount (solid content g/m.sup.2) Overcoat Varnish Vamish 1 100% by mass 100% by mass agent (nitrocellulose-based) Varnish 2 (acrylic) Applied amount 0.5 15.0 20.0 (solid content g/m.sup.2) Flame-proof Without heat friction performance With heat friction

<Adjustment of Mixing Ratio of Flame-Proofing Agent and Water-Soluble Polymer>

Example 10

[0068] When the mixing ratio of polyacrylamide 1 was changed to 15% by mass in Example 1, the viscosity of the flame-proofing agent aqueous solution was 1,800 mPa.Math.s, but a uniform flame-proof layer was obtained by coating at a coating speed of 80 m/min using a gravure printing machine. The flame-proof performance of a flame-proof corrugated paperboard prepared using this linerboard was determined as .

Comparative Example 3

[0069] When the mixing ratio of polyacrylamide 1 was changed to 17% by mass in Example 10, the viscosity of the flame-proofing agent aqueous solution was 2,000 mPa.Math.s, and a uniform flame-proof layer was not obtained. Therefore, it was not possible to prepare a flame-proof corrugated paperboard, and determine the flame-proof performance.

<Type of Varnish of Overcoat Layer>

Example 11

[0070] A flame-proof corrugated paperboard was prepared in the same manner as in Example 1 except that instead of varnish 1 (nitrocellulose-based), varnish 2 (acrylic-based) was used to form an overcoat layer. The flame-proof performance was determined as . It was confirmed that even if the type of varnish of the overcoat layer is changed, the flame-proof corrugated paperboard according to the present invention can be produced.

<Adjustment of Amount Applied to Overcoat Layer>

Examples 12 and 13

[0071] In each of Examples 12 and 13, a flame-proof corrugated paperboard was prepared in the same manner as in Example 1 except that the applied amount of varnish 1 for forming an overcoat layer was changed to 0.5 g/m.sup.2 (Example 12) or 15.0 g/m.sup.2 (Example 13). The flame-proof performance of each corrugated paperboard prepared was , and the flame-proof performance was not lost in the production stage of the corrugated paperboard.

Comparative Example 4

[0072] A flame-proof linerboard was prepared in the same manner as in Example 1 except that no overcoat layer was formed. This linerboard was placed in a color fastness rubbing tester (AB-301 manufactured by TESTER SANGYO CO., LTD.), and rubbed for 30 minutes under the conditions of a heating plate temperature of 180 C., a load of 500 gf, and a reciprocating speed of 30 cpm. Thereafter, a corrugated paperboard of A flute was prepared in the same manner, and the flame-proof performance was evaluated. As a result, the flame-proof performance of a portion of the corrugated paperboard that was not subjected to friction was determined as , but the flame-proof performance of a portion of the corrugated paperboard that was subjected to friction was determined as .

Comparative Example 5

[0073] A flame-proof linerboard was prepared in the same manner as in Example 1 except that the applied amount of varnish 1 for forming an overcoat layer was changed to 20.0 g/m.sup.2. After conducting a wear test on this linerboard in the same manner as in Comparative Example 4, a flame-proof corrugated paperboard was prepared using the linerboard, and the flame-proof performance was evaluated. As a result, the flame-proof performance of both a portion of the corrugated paperboard that was not subjected to friction and a portion thereof that was subjected to friction was determined as .

<Adjustment of Amount of Inorganic Substance in Overcoat Layer>

Examples 14 and 15, Comparative Example 6

[0074] In each of Examples 14 and 15 and Comparative Example 6, a flame-proof corrugated paperboard was prepared in the same manner as in Example 1 except that the overcoat agent contained silicon dioxide in an amount of 5% by mass (Example 14), 25% by mass (Example 15), or 30% by mass (Comparative Example 6). In each of Examples 14 and 15, both the residual flame time and the afterglow time were shortened although in the determination criterion, and flame-proof performance higher than that of the corrugated paperboard in Example 1 was exhibited. However, as a result of confirming dispersion stability by allowing the overcoat agent containing the inorganic substance to stand at room temperature for 1 day, no particular change was observed in Examples 14 and 15, but most of the inorganic substance was precipitated in Comparative Example 6. FIG. 3D shows a surface photograph of Example 14 after the flame-proof test.

TABLE-US-00003 TABLE 3 Example/Comparative Example Number Comparative Example 6 Example 14 Example 15 Excessive Blending Blending blending Example 16 Example 17 amount amount amount of Type of Type of Example 18 of inorganic of inorganic inorganic inorganic inorganic Type of Content substance substance substance substance substance linerboard Linerboard RKA170 CRC230 Flame- Flame- Phosphorus-nitrogen 28% by mass proofing proofing compound agent agent Water- PAM1 (medium 6% by mass soluble molecular weight) polymer Water 66% by mass Viscosity of coating liquid 30 (mPa .Math. s) Coating method Bar coater Applied amount 3.4 (solid content: g/m.sup.2) (Flame-proofing agent = 2.8, polymer = 0.6) Overcoat Varnish Varnish 1 95% by mass 75% by mass 70% by mass 95% by mass 100% by mass agent (nitrocellulose-based) Inorganic Silicon dioxide 5% by mass 25% by mass 30% by mass substance Calcium carbonate 5% by mass Titanium dioxide 5% by mass Dispersion stability of inorganic X substance Applied amount 2.0 (solid content: g/m.sup.2) Flame-proof Without heat friction performance (Overcoat layer could not be formed, and was unevaluated)

<Adjustment of Type of Inorganic Substance in Overcoat Layer>

Examples 16 and 17

[0075] In each of Examples 16 and 17, a flame-proof corrugated paperboard was prepared in the same manner as in Example 14 except that the type of the inorganic substance used was changed from silicon dioxide to calcium carbonate (Example 16) or titanium dioxide (Example 17). In each of Examples 16 and 17, there was no problem in the dispersion stability of the inorganic substance, and the result of the flame-proof test was also , and good flame-proof performance was exhibited.

<Change of Linerboard>

Example 18

[0076] A flame-proof corrugated paperboard was prepared in the same manner as in Example 1 except that instead of the general linerboard, an inorganic layer-containing linerboard was used. Such a different linerboard was used, but the result of the flame-proof test was , and good flame-proof performance was exhibited.

DESCRIPTION OF REFERENCE NUMERALS

[0077] 11: Paper layer [0078] 12: Flame-proof layer [0079] 13: Overcoat layer [0080] 15: Flame-proof linerboard [0081] 17: Corrugated medium [0082] 21: Flame-proof corrugated paperboard