Production process of single-side-sizing ultra-low-gram-weight high-strength corrugated paper and ultra-low-gram-weight high-strength corrugated paper

Abstract

The invention discloses a production process of single-side-sized ultra-low-gram-weight high-strength corrugated paper and the ultra-low-gram-weight high-strength corrugated paper, which are used for solving the technical problem of non-uniform sizing on the surface of raw paper caused by frequent abrasion of a sizing roller for gluing the bottom surface of the raw paper in the existing corrugated paper production. The method comprises the following steps: preparing raw materials; the method comprises the following steps: treating raw materials into slurry through a pulping process, and adding a chemical additive into the slurry; the pulp is conveyed to a paper machine through a drifting system, the pulp becomes raw paper after passing through a net part, a pressing part and a pre-drying system of the paper machine, a sizing device of the paper machine conducts sizing treatment on the surface of the raw paper, and a post-drying system of the paper machine conducts secondary drying on the raw paper to obtain a corrugated paper finished product. The sizing device comprises a first roller and a second roller, a gap allowing body paper to penetrate through is formed between the first roller and the second roller, the first roller is a sizing roller, grooves are formed in the surface of the first roller, the second roller is a non-sizing roller, and the surface of the second roller is a smooth surface.

Description

Technical Field

[0001] The invention relates to the technical field of papermaking, and in particular to a production process for ultra-low grammage and high-strength corrugated paper with single-sided gluing and ultra-low grammage and high-strength corrugated paper. Background Art

[0002] In the existing corrugated paper production process, after the paper machine integrates the pulp into base paper, it needs to pass through a sizing device to achieve sizing treatment on the surface of the base paper. At present, the sizing device generally uses two relatively arranged sizing rollers to respectively apply sizing to the upper surface and the lower surface of the base paper. Because, in the actual production process, the sizing rollers used to apply sizing to the lower surface of the base paper are frequently worn on both sides, and the wear position is not fixed, resulting in uneven gluing on the paper surface, base paper folding and other quality problems, and also causing paper breaks on both sides of the gluing, which undoubtedly has a serious impact on the production of corrugated paper.

[0003] Therefore, finding a technical solution that can solve the above technical problems has become an important topic studied by technical personnel in this field. Summary of the invention

[0004] The embodiment of the present invention discloses a single-sided gluing ultra-low grammage high-strength corrugated paper production process and ultra-low grammage high-strength corrugated paper, which are used to solve the technical problem that the gluing roller used for gluing the bottom surface of the base paper in the existing corrugated paper production process is often worn, resulting in uneven gluing on the surface of the base paper and folding of the base paper.

[0005] The present invention provides a single-sided gluing ultra-low grammage high-strength corrugated paper production process, which is used to produce corrugated paper with a grammage of less than 60 grams per square meter, comprising the following steps:

[0006] S1. preparing raw materials, wherein the raw materials include high-end waste paper and low-end waste paper, wherein the high-end waste paper is grade A domestic waste or grade B domestic waste, and the low-end waste paper is grade D domestic waste;

[0007] S2, processing the raw materials into slurry through a pulping process, and adding chemical additives into the slurry;

[0008] S3, the pulp is transported to the paper machine through the flow conveying system, the wire part of the paper machine dehydrates and integrates the pulp to form a wet paper web, the press part of the paper machine presses and dehydrates the wet paper web, then the pre-drying system of the paper machine dries the wet paper web to obtain base paper, the sizing device of the paper machine applies sizing to the surface of the base paper, and finally, the post-drying system of the paper machine performs secondary drying on the base paper to obtain a finished corrugated paper product;

[0009] The gluing device comprises a first roller and a second roller arranged opposite to each other, wherein a gap is formed between the first roller and the second roller for the base paper to pass through, the first roller is a gluing roller and the surface of the first roller has grooves, and the second roller is a non-gluing roller and the surface of the second roller is a smooth surface.

[0010] Optionally, in step S1, the raw materials specifically include 40-50% of high-end waste paper and 50-60% of low-end waste paper.

[0011] Optionally, in step S2, the pulping process specifically includes:

[0012] a. Pulping the raw materials and conveying the pulped raw materials to a high-concentration cleaner;

[0013] b. The slurry after being treated by the high-concentration cleaner is subjected to coarse screening and graded screening in sequence, and the graded screened slurry is transported to the low-concentration cleaner;

[0014] c. The pulp treated by the low-concentration cleaner is finely screened, and the finely screened pulp is subjected to disc grinding treatment. The pulp treated by the grinding disc is transported to the feeding system of the paper machine.

[0015] Optionally, in the step S2, the pulp obtained through the pulping process has a beating degree of 33-39° SR.

[0016] Optionally, in step S2, the chemical additives include retention aids, filter aids, dry strength agents, sizing agents in the slurry, surface sizing agents, bentonite and polyaluminium chloride;

[0017] Among them, the dosage of the retention agent is 0.3-0.5kg / t, the dosage of the filter aid is 0.4-0.6kg / t, the dosage of the dry strength agent is 5-10kg / t, the sizing agent in the slurry is AKD emulsion with cationic charge, and its dosage is 10-15kg / t, the dosage of the surface sizing agent is 0.8-1.5kg / t, the dosage of the bentonite is 2-3kg / t, and the dosage of the polyaluminium chloride is 5-9kg / t.

[0018] Optionally, in step S3, specific operating parameters of the wire section of the paper machine are:

[0019] The lip opening of the mesh is 8-10%;

[0020] The flow rate of the headbox of the wire section is 60000-80000l / min;

[0021] The speed difference between the headbox of the wire section and the forming wire of the wire section is 50-60m / min;

[0022] Internet concentration 1.0-1.5%;

[0023] The net retention rate is 68-75%;

[0024] The dryness of the wet paper web leaving the wire is 19-22%;

[0025] High vacuum of couch roller 49-53%.

[0026] Optionally, in step S3, specific operating parameters of the press section of the paper machine are:

[0027] The pressure of the first pressing in the press section is 75-85 kN / m, the pressure of the second pressing in the press section is 120-130 kN / m, the pressure of the third pressing in the press section is 1300-1350 kN / m, and the dryness of the wet paper web leaving the press section is 50-55%.

[0028] Optionally, in step S3, the steam pressure of the pre-drying system of the paper machine is 200-300 kPa, and the steam pressure of the post-drying system of the paper machine is 30-60 kPa, wherein the temperature of the pre-drying system is 85-95°C; and the moisture content of the wet paper web after passing through the pre-drying system is 7-8%.

[0029] Optionally, in step S3, the glue flow rate of the glue used by the glue application device of the paper machine is 1000-1100 l / min, the starch consumption in the glue is 15-22 kg / t, the viscosity of the glue is 20-40, and the solid content of the glue is 7.5-8.5%;

[0030] The pressure of the first roller is 100-160 kPa; the rubber transfer rate of the first roller is 23-25 ​​l / min, the mid-height value of the first roller and the second roller is 0.292 mm, and the gap between the first roller and the second roller is 1.8-2.3 mm;

[0031] The linear pressure of the glue application device is 28-33 kN / m, and the width of the pressing zone is 26-30 mm.

[0032] The present invention provides an ultra-low grammage and high-strength corrugated paper, which is produced according to the above-mentioned single-sided gluing ultra-low grammage and high-strength corrugated paper production process, wherein the grammage of the ultra-low grammage and high-strength corrugated paper is less than 60 grams per square meter.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] The single-sided gluing ultra-low grammage high-strength corrugated paper production process of the present embodiment is used to produce corrugated paper with a grammage of less than 60 grams per square meter. In the above process, when gluing is applied to the surface of the base paper, only the first roller in the gluing device applies glue to the surface of the base paper, and the second roller in the gluing device does not apply glue to the other surface of the base paper. By adopting the process of gluing a single surface of the base paper to produce ultra-low grammage high-strength corrugated paper, the current problem of frequent wear of the gluing roller is completely solved, and the quality problems such as uneven gluing on the surface of the base paper and folding of the base paper are solved, so that the operation of the paper machine is improved, and the paper breaks on both sides of the gluing are reduced by about 80-90% compared with the existing technical solution, thereby ensuring the quality of the corrugated paper products. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0036] Figure 1 A schematic diagram of a process for producing ultra-low grammage and high-strength corrugated paper with single-sided gluing provided by the present invention;

[0037] Figure 2 A structural schematic diagram of a gluing device for a single-sided gluing ultra-low grammage high-strength corrugated paper production process provided by the present invention;

[0038] Figure 3 A schematic structural diagram of a second roller in a gluing device in a single-sided gluing ultra-low grammage high-strength corrugated paper production process provided by the present invention;

[0039] Figure 4 A schematic diagram of the end face of a second roller in a gluing device in a single-sided gluing ultra-low grammage high-strength corrugated paper production process provided by the present invention;

[0040] Illustration: second roller 1; first roller 2; base paper 3. DETAILED DESCRIPTION

[0041] The invention discloses a single-sided gluing ultra-low grammage high-strength corrugated paper production process and ultra-low grammage high-strength corrugated paper, which are used to solve the technical problem that the gluing roller used for gluing the bottom surface of base paper in the existing corrugated paper production process is often worn, resulting in uneven gluing on the surface of the base paper and folding of the base paper.

[0042] In order to enable those skilled in the art to better understand the scheme of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and specific implementation methods. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0043] See also Figures 1 to 4 The embodiment of the present invention provides a single-sided sizing ultra-low weight high-strength corrugated paper production process, which is used to produce corrugated paper with a weight of less than 60 grams per square meter, wherein the process includes the following steps:

[0044] S1. preparing raw materials, wherein the raw materials include high-end waste paper and low-end waste paper, wherein the high-end waste paper is grade A domestic waste or grade B domestic waste, and the low-end waste paper is grade D domestic waste;

[0045] S2, processing the raw materials into slurry through a pulping process, and adding chemical additives into the slurry;

[0046] S3, the pulp is transported to the paper machine through the flow conveying system, the wire part of the paper machine dehydrates and integrates the pulp to form a wet paper web, the press part of the paper machine presses and dehydrates the wet paper web, then the pre-drying system of the paper machine dries the wet paper web to obtain base paper, the sizing device of the paper machine applies sizing to the surface of the base paper, and finally, the post-drying system of the paper machine performs secondary drying on the base paper to obtain a finished corrugated paper product;

[0047] The gluing device comprises a first roller and a second roller arranged opposite to each other, wherein a gap is formed between the first roller and the second roller for the base paper to pass through, the first roller is a gluing roller and the surface of the first roller has grooves, and the second roller is a non-gluing roller and the surface of the second roller is a smooth surface.

[0048] In the production process of ultra-low grammage and high-strength corrugated paper with single-sided gluing in the present embodiment, when gluing is applied to the surface of the base paper, only the first roller in the gluing device applies glue to the surface of the base paper, and the second roller in the gluing device does not apply glue to the other surface of the base paper. By adopting the process of gluing a single surface of the base paper to produce ultra-low grammage and high-strength corrugated paper, the current problem of frequent wear of the gluing roller is completely solved, and quality problems such as uneven gluing on the surface of the base paper and folding of the base paper are solved, so that the operation of the paper machine is improved, and the paper breaks on both sides of the gluing are reduced by about 80-90% compared with the existing technical solution, thereby ensuring the quality of the corrugated paper products.

[0049] In addition, due to the single-sided gluing process, the wear degree of the second roller can be greatly reduced, the service life of the second roller is effectively extended, the replacement frequency of the second roller is reduced, and the production cost is effectively saved.

[0050] Furthermore, in step S1 of this embodiment, the raw materials specifically include 40-50% of high-end waste paper and 50-60% of low-end waste paper.

[0051] Specifically, in the present embodiment, grade A domestic waste or grade B domestic waste accounts for 40-50% of the raw materials, and grade D domestic waste accounts for 50-60% of the raw materials.

[0052] It should be noted that the Grade A domestic waste, Grade B domestic waste and Grade D domestic waste involved in this embodiment are all waste paper raw materials that can be purchased on the market. Among them, Grade A, Grade B and Grade D are distinguished according to their quality, among which Grade A is a better grade of waste paper, and Grade D is a worse grade of waste paper.

[0053] Furthermore, in step S2 of this embodiment, the pulping process specifically includes:

[0054] a. Pulping the raw materials and conveying the pulped raw materials to a high-concentration cleaner;

[0055] b. The slurry after being treated by the high-concentration cleaner is subjected to coarse screening and graded screening in sequence, and the graded screened slurry is transported to the low-concentration cleaner;

[0056] c. The pulp treated by the low-concentration cleaner is finely screened, and the finely screened pulp is subjected to disc grinding treatment. The pulp treated by the grinding disc is transported to the feeding system of the paper machine.

[0057] It should be noted that in the above pulping process, a high-concentration cleaner and a low-concentration cleaner are used to remove heavy impurities in the pulp to avoid affecting the quality of the subsequent finished product;

[0058] In the above pulping process, coarse screening is used to remove light residues and larger impurities, and fine screening is used to remove some adhesives. Among them, the coarse screening is carried out by outflow, and the pulp passes through the screen under the work of the high-speed rotating rotor, and large pieces of plastic impurities are squeezed out of the pulp pipe at the end of the barrel. KADANT's UV500 series screen frame is used for coarse screening, and the hole screen and the slot screen are selected to be used in combination to ensure the throughput of the pulp while also having a more reasonable average service life; KADANT's UV500 series screen frame is used for grading screen and fine screening, and the screen slot is selected to be 0.15mm-0.25mm, which can separate the pulp into good pulp and long, medium and short fibers.

[0059] Furthermore, the pulp obtained through the pulping process has a beating degree of 33-39°SR.

[0060] Furthermore, in step S2 of this embodiment, the chemical additives include retention aids, filter aids, dry strength agents, sizing agents in the slurry, surface sizing agents, bentonite and polyaluminium chloride;

[0061] Among them, the dosage of the retention agent is 0.3-0.5kg / t, the dosage of the filter aid is 0.4-0.6kg / t, the dosage of the dry strength agent is 5-10kg / t, the sizing agent in the slurry is AKD emulsion with cationic charge, and its dosage is 10-15kg / t, the dosage of the surface sizing agent is 0.8-1.5kg / t, the dosage of the bentonite is 2-3kg / t, and the dosage of the polyaluminium chloride is 5-9kg / t.

[0062] It should be noted that the retention aid in this embodiment connects the fibers together to form a network or agglomerate through the polymer chain bridging principle, thereby wrapping fine fibers and fillers to improve the retention effect. The polymer is selected as the retention aid.

[0063] The functional groups of the sizing agent in the present embodiment react with the hydroxyl groups on the fibers to form covalent bonds, thereby being adsorbed on the fiber surface to prevent the fibers from absorbing water and moisture too quickly, thereby improving the water resistance.

[0064] In addition, the filter aid in this embodiment is specifically a substance that can improve the filtration efficiency of the filtrate. In order to prevent the filter residue from being too densely accumulated and to make the filtration proceed smoothly, insoluble inert materials with different degrees of fineness are used.

[0065] The dry strength agent in this embodiment is an important chemical used in the papermaking industry to increase the strength of paper. Many water-soluble polymers that can form hydrogen bonds with fibers can become dry strength agents. Dry strength agents are usually used to compensate for the decrease in paper strength caused by adding fillers or low-grade fibers (such as recycled fibers).

[0066] Furthermore, in step S3 of this embodiment, the specific operating parameters of the wire section of the paper machine are:

[0067] The lip opening of the mesh is 8-10%;

[0068] The flow rate of the headbox of the wire section is 60000-80000l / min;

[0069] The speed difference between the headbox of the wire section and the forming wire of the wire section is 50-60m / min;

[0070] Internet concentration 1.0-1.5%;

[0071] The net retention rate is 68-75%;

[0072] The dryness of the wet paper web leaving the wire is 19-22%;

[0073] High vacuum of couch roller 49-53%.

[0074] Furthermore, in step S3 of this embodiment, the specific operating parameters of the press section of the paper machine are:

[0075] The pressure of the first pressing in the press section is 75-85 kN / m, the pressure of the second pressing in the press section is 120-130 kN / m, the pressure of the third pressing in the press section is 1300-1350 kN / m, and the dryness of the wet paper web leaving the press section is 50-55%.

[0076] It should be noted that, through the above design, the moisture on the wet paper web can be squeezed as much as possible, effectively reducing the drying time of the wet paper web by the pre-drying system, and further saving the energy required in the production process.

[0077] Further, in step S3 of this embodiment, the steam pressure of the pre-drying system of the paper machine is 200-300 kPa, and the steam pressure of the post-drying system of the paper machine is 30-60 kPa, wherein the temperature of the pre-drying system is 85-95°C;

[0078] It should be noted that the moisture content of the wet paper web after passing through the pre-drying system is 7-8%.

[0079] Furthermore, in step S3 of this embodiment, the glue flow rate of the glue used by the glue application device of the paper machine is 1000-1100 l / min, the starch unit consumption in the glue is 15-22 kg / t, the viscosity of the glue is 20-40, and the solid content of the glue is 7.5-8.5%;

[0080] The pressure of the first roller is 100-160 kPa; the rubber transfer rate of the first roller is 23-25 ​​l / min, the mid-height value of the first roller and the second roller is 0.292 mm, and the gap between the first roller and the second roller is 1.8-2.3 mm;

[0081] The linear pressure of the glue application device is 28-33 kN / m, and the width of the pressing zone is 26-30 mm.

[0082] It should be noted that during the operation of the paper machine, the lubricating water of the first roller needs to be used normally and the rubber cleaning component is opened normally to ensure the cleanliness of the rubber and the uniformity of the film formation on the roller surface.

[0083] Furthermore, in step S3 of this embodiment, both the first roller and the second roller are made of PU material.

[0084] It should be noted that this embodiment does not limit the manufacturing materials of the first roller and the second roller. In other specific implementations, the first roller and the second roller may also be made of metal or alloy materials.

[0085] See also Figures 1 to 4An embodiment of the present invention provides an ultra-low weight high-strength corrugated paper, wherein the ultra-low weight high-strength corrugated paper is produced by the above-mentioned ultra-low weight high-strength corrugated paper production process with single-sided gluing, wherein the ultra-low weight high-strength corrugated paper has a grammage of less than 60 grams per square meter.

[0086] It should be noted that the performance parameters of the corrugated paper produced by the above process are as follows: the paper index is normal, the moisture content of the paper is 7.8-8.5%, the water absorption value is 30-40g / ㎡, and the tear length is 4.0-5.0km.

[0087] If the grammage of the corrugated paper exceeds 60 g / m2, the various performance parameters of the corrugated paper produced by the above process will show a gradual downward trend, which is not conducive to the corrugated paper entering the market for use.

[0088] The above is a detailed introduction to the production process of ultra-low grammage and high-strength corrugated paper with single-sided gluing and the ultra-low grammage and high-strength corrugated paper provided by the present invention. For those skilled in the art, according to the ideas of the embodiments of the present invention, there will be changes in the specific implementation methods and application scopes. In summary, the content of this specification should not be understood as a limitation on the present invention.

Claims

1. A single-sided sizing process for producing ultra-low-weight high-strength corrugated paper, used to produce corrugated paper with a weight of less than 60 g / m2, characterized in that: The following steps are involved: S1. preparing raw materials, wherein the raw materials include high-end waste paper and low-end waste paper, wherein the high-end waste paper is grade A domestic waste or grade B domestic waste, and the low-end waste paper is grade D domestic waste; S2, processing the raw materials into slurry through a pulping process, and adding chemical additives into the slurry; S3, the pulp is transported to the paper machine through the flow conveying system, the wire part of the paper machine dehydrates and integrates the pulp to form a wet paper web, the press part of the paper machine presses and dehydrates the wet paper web, then the pre-drying system of the paper machine dries the wet paper web to obtain base paper, the sizing device of the paper machine applies sizing to the surface of the base paper, and finally, the post-drying system of the paper machine performs secondary drying on the base paper to obtain a finished corrugated paper product; The gluing device comprises a first roller and a second roller arranged opposite to each other, wherein a gap is formed between the first roller and the second roller for the base paper to pass through, the first roller is a gluing roller and the surface of the first roller has grooves, and the second roller is a non-gluing roller and the surface of the second roller is a smooth surface.

2. The process for producing ultra-low weight high-strength corrugated paper with single-sided gluing according to claim 1, characterized in that: In the step S1, the raw materials specifically include 40-50% of high-end waste paper and 50-60% of low-end waste paper.

3. The process for producing ultra-low weight high-strength corrugated paper with single-sided gluing according to claim 1, characterized in that: In step S2, the pulping process specifically includes: a. Pulping the raw materials and conveying the pulped raw materials to a high-concentration cleaner; b. The slurry after being treated by the high-concentration cleaner is subjected to coarse screening and graded screening in sequence, and the graded screened slurry is transported to the low-concentration cleaner; c. The pulp treated by the low-concentration cleaner is finely screened, and the finely screened pulp is subjected to disc grinding treatment. The pulp treated by the grinding disc is transported to the feeding system of the paper machine.

4. The process for producing ultra-low weight high-strength corrugated paper with single-sided gluing according to claim 3, characterized in that: In the step S2, the pulp obtained through the pulping process has a beating degree of 33-39° SR.

5. The process for producing ultra-low weight high-strength corrugated paper with single-sided gluing according to claim 1, characterized in that: In step S2, the chemical additives include retention aids, filter aids, dry strength agents, sizing agents in the slurry, surface sizing agents, bentonite and polyaluminium chloride; Among them, the dosage of the retention agent is 0.3-0.5kg / t, the dosage of the filter aid is 0.4-0.6kg / t, the dosage of the dry strength agent is 5-10kg / t, the sizing agent in the slurry is AKD emulsion with cationic charge, and its dosage is 10-15kg / t, the dosage of the surface sizing agent is 0.8-1.5kg / t, the dosage of the bentonite is 2-3kg / t, and the dosage of the polyaluminium chloride is 5-9kg / t.

6. The process for producing ultra-low weight high-strength corrugated paper with single-sided gluing according to claim 1, characterized in that: In step S3, the specific operating parameters of the wire section of the paper machine are: The lip opening of the mesh is 8-10%; The flow rate of the headbox of the wire section is 60000-80000l / min; The speed difference between the headbox of the wire section and the forming wire of the wire section is 50-60m / min; Internet concentration 1.0-1.5%; The net retention rate is 68-75%; The dryness of the wet paper web leaving the wire is 19-22%; High vacuum of couch roller 49-53%.

7. The process for producing ultra-low weight high-strength corrugated paper with single-sided gluing according to claim 1, characterized in that: In step S3, the specific operating parameters of the press section of the paper machine are: The pressure of the first pressing in the press section is 75-85 kN / m, the pressure of the second pressing in the press section is 120-130 kN / m, the pressure of the third pressing in the press section is 1300-1350 kN / m, and the dryness of the wet paper web leaving the press section is 50-55%.

8. The process for producing ultra-low weight high-strength corrugated paper with single-sided gluing according to claim 1, characterized in that: In step S3, the steam pressure of the pre-drying system of the paper machine is 200-300 kPa, and the steam pressure of the post-drying system of the paper machine is 30-60 kPa, wherein the temperature of the pre-drying system is 85-95° C.; the moisture content of the wet paper web after passing through the pre-drying system is 7-8%.

9. The process for producing ultra-low weight high-strength corrugated paper with single-sided gluing according to claim 1, characterized in that: In the step S3, the glue flow rate of the glue used by the glue application device of the paper machine is 1000-1100 l / min, the starch consumption in the glue is 15-22 kg / t, the viscosity of the glue is 20-40, and the solid content of the glue is 7.5-8.5%; The pressure of the first roller is 100-160 kPa; the rubber transfer rate of the first roller is 23-25 ​​l / min, the mid-height value of the first roller and the second roller is 0.292 mm, and the gap between the first roller and the second roller is 1.8-2.3 mm; The linear pressure of the glue application device is 28-33 kN / m, and the width of the pressing zone is 26-30 mm.

10. An ultra-low weight high-strength corrugated paper, characterized in that: The ultra-low weight high-strength corrugated paper is produced according to the ultra-low weight high-strength corrugated paper production process with single-sided gluing as described in any one of claims 1 to 9, wherein the ultra-low weight high-strength corrugated paper has a weight of less than 60 g / m2.

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