Ultralow-gram-weight environment-friendly composite kraft paper and preparation method thereof

Abstract

The invention discloses ultralow-gram-weight environment-friendly composite kraft paper and a preparation method thereof, and the preparation method comprises the following steps: (1) raw material preparation: in parts by weight, wood pulp accounts for 40%-45% of the raw materials, and kraft paper regenerated pulp accounts for 55%-60% of the raw materials; (2) slurry treatment: adding a chemical additive into the mixed slurry obtained in the step (1), and uniformly stirring to obtain slurry to be put on a net; (3) forming a net part: feeding the to-be-meshed pulp into the net part of a double-net paper machine; (4) squeezing: feeding the wet paper web formed by the net part into a squeezing part, and sequentially squeezing the wet paper web through a first squeezing roller, a second squeezing roller and a third squeezing roller; (5) drying; (6) surface sizing treatment; and (7) coiling and forming to obtain the environment-friendly composite kraft paper with ultralow gram weight. According to the technical scheme provided by the invention, the ratio of the ultra-low gram weight kraft paper pulp is greatly reduced, but the strength still meets the packaging scene use requirements of chemical fertilizers, cement and other industrial woven bags, so that the purposes of environmental protection, cost reduction and efficiency improvement are achieved.

Description

Technical Field

[0001] This invention relates to the field of papermaking technology, and in particular to an ultra-low basis weight environmentally friendly composite kraft paper and its preparation method. Background Technology

[0002] Kraft paper, due to its excellent properties such as high strength, tear resistance, and folding resistance, is widely used in heavy-duty packaging applications for fertilizers, cement, and chemical raw materials. With the packaging industry's increasing demand for "lightweight, environmentally friendly, and low-cost" products, ultra-low basis weight kraft paper (basis weight ≤ 70g / m³) is being developed. 2 This is becoming a trend. However, existing technologies for preparing ultra-low basis weight kraft paper still have the following shortcomings: (1) The contradiction between strength and basis weight: Traditional kraft paper relies on a high proportion of virgin wood pulp (≥70%) to ensure strength. If the basis weight is reduced and the proportion of recycled pulp is increased (especially when the proportion of recycled pulp exceeds 50%), the tensile strength and bursting index of the paper will drop significantly. (2) Poor production stability: Ultra-low grammage kraft paper has a thin paper width and low wet strength. It is prone to problems such as poor uniformity and screen penetration during the wire forming stage. In the pressing stage, it is prone to crushing due to improper line pressure control. During winding, it is prone to paper breakage due to tension imbalance. The production qualification rate is low.

[0003] In view of the above technical problems, this invention achieves high strength and production stability of ultra-low basis weight kraft paper by optimizing the raw material ratio, refining the recycled pulp, and controlling the process parameters, based on 55%~60% recycled kraft pulp and 40%~45% wood pulp. Summary of the Invention

[0004] In view of the shortcomings of the prior art, the purpose of this invention is to provide an ultra-low basis weight environmentally friendly composite kraft paper and its preparation method, aiming to solve the technical problems in the prior art where the ultra-low basis weight kraft paper needs to maintain a high wood pulp ratio to ensure strength, which leads to limited increase in the proportion of recycled pulp and difficulty in balancing strength and environmental protection and lightweight requirements.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing ultra-low basis weight environmentally friendly composite kraft paper includes the following steps: (1) Raw material preparation: Take wood pulp produced by the wood pulp line and kraft paper recycled pulp produced by the waste paper line and mix them together; by weight, the wood pulp accounts for 40%~45% of the raw materials and the kraft paper recycled pulp accounts for 55%~60% of the raw materials; the beating degree of the wood pulp is 20~25SR; the beating degree of the mixture of long fibers and short fibers in the kraft paper recycled pulp is 30~40SR; (2) Slurry treatment: Add chemical additives to the mixed slurry obtained in step (1) and stir evenly to obtain the slurry to be fed into the web; the chemical additives include at least 0.3~0.5 kg / t of retention aid, 0.4~0.6 kg / t of filter aid and 3~5 kg / t of dry strength agent; (3) Wire forming: The pulp to be wired is fed into the wire section of the wire paper machine. The lip plate opening is controlled at 9%~9.8%, the headbox flow rate is 65000~70000 l / min, and the pulp-wire speed difference is 35~40m / min. After forming, a wet paper web is obtained. The dryness of the wet paper web exiting the wire section is 22%~24%. (4) Pressing: The wet paper web formed by the wire section is fed into the pressing section and pressed in sequence through the first press roll, the second press roll, and the third press roll. The linear pressure of the first press roll is controlled to be 75~85kN / m, the linear pressure of the second press roll is 120~130kN / m, and the linear pressure of the third press roll is 1300~1350kN / m. After pressing, the dryness of the paper web leaving the pressing section is 50%~55%. (5) Drying: The pressed paper web is fed into the dryer, and the steam pressure of the pre-drying is controlled at 250~350kPa and the steam pressure of the post-drying is controlled at 20~50kPa; (6) Surface sizing treatment: The dried paper web is sizing treated, and the glue flow rate of the sizing machine is controlled at 900~1000 l / min and the scraper pressure of the sizing machine is 100~160kPa; (7) Rolling and forming: The paper web after surface sizing is rolled up to obtain ultra-low grammage environmentally friendly composite kraft paper.

[0006] Further, in step (1), the wood pulp is softwood pulp with a fiber length of 1.5~2.0 mm.

[0007] Furthermore, in step (1), the recycled kraft pulp is obtained by deinking and purifying recycled industrial kraft paper.

[0008] Further, in step (2), the stirring speed of the slurry treatment is 300~500 r / min, and the stirring time is 20~30 min.

[0009] Furthermore, in step (2), the chemical additives also include 3~5 kg / t of AKD in the slurry, 3~5 kg / t of polyaluminum chloride and 0.8~1.0 kg / t of starch in the slurry.

[0010] Furthermore, in step (3), during the mesh forming process, the vacuum degree of the tumbler roller is controlled to be 56~58kPa.

[0011] Furthermore, in step (5), during the drying process, the pre-drying temperature is controlled at 85~95℃ and the moisture content of the pre-dried paper web is controlled at 7.5~8.5%.

[0012] Furthermore, in step (7), the winding tension of the winding process is 600N, the winding speed is 500~600m / min, and steam is not introduced into the post-drying section during winding.

[0013] Accordingly, the present invention also provides an ultra-low basis weight environmentally friendly composite kraft paper, which, unlike the prior art, is prepared by the preparation method of ultra-low basis weight environmentally friendly composite kraft paper described above.

[0014] Beneficial effects: This invention provides an ultra-low basis weight environmentally friendly composite kraft paper and its preparation method, which has at least the following advantages compared with the prior art: (1) By optimizing the raw material ratio (40%~45% wood pulp, 55%~60% recycled kraft paper pulp) and refining the recycled pulp, the utilization rate of recycled pulp is greatly improved, while the tensile strength of the paper in the longitudinal direction can reach 4.5~5.2kN / m and the bursting index can reach 2.1~2.5kPa・m. 2 / g, the packaging needs of industrial woven bags for fertilizers, cement and other products.

[0015] (2) The paper web is pressed in three stages and the linear pressure parameters of the three press rolls are set in a gradient. After pressing, the dryness of the paper web at the press section meets the requirements and there is no crushing or bubbling.

[0016] (3) The application of high proportion (55%~60%) recycled pulp significantly reduces the dependence on virgin wood pulp, and the raw material cost is reduced by about 15%~20%, achieving the goals of environmental protection, cost reduction and efficiency improvement. Detailed Implementation

[0017] To make the objectives, technical solutions, and effects of this invention clearer and more explicit, the invention is further described in detail below. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of the invention.

[0018] The preparation method of the ultra-low basis weight environmentally friendly composite kraft paper includes the following steps: (1) Raw material preparation: Wood pulp produced by the wood pulp line and recycled kraft paper pulp produced by the waste paper line are mixed; by weight, the wood pulp accounts for 40%~45% of the raw materials, and the recycled kraft paper pulp accounts for 55%~60% of the raw materials. The beating degree of the wood pulp is 20~25SR, which can ensure that the fiber surface is appropriately fracturized to enhance the bonding force, and avoid over-beating that causes fiber cutting. The beating degree of the long fibers (length ≥1.0mm) and short fibers (length <1.0mm) in the recycled kraft paper pulp is 30~40SR, which ensures that the long fibers provide skeletal support in the paper web, while the short fibers fill the gaps, thereby improving the uniformity of the paper web. In practical applications, the wood pulp and recycled kraft paper pulp can be added to the mixing tank, water can be added to adjust the pulp concentration, the stirring device can be turned on, and the mixture can be stirred for a period of time (e.g., 20~30min) to make the wood pulp and recycled pulp evenly mixed.

[0019] (2) Pulp treatment: The mixed pulp obtained in step (1) is added to chemical additives and stirred evenly to obtain the pulp to be fed into the web; the chemical additives include at least 0.3~0.5 kg / t of retention aid, 0.4~0.6 kg / t of filter aid, and 3~5 kg / t of dry strength agent. The retention aid is preferably cationic polyacrylamide, which can improve the retention rate of fibers and fillers and reduce web loss; the filter aid is anionic polyacrylamide, which accelerates pulp dewatering and can improve web forming efficiency; the dry strength agent is preferably a modified starch-based dry strength agent, which improves the dry strength of paper by enhancing the hydrogen bonding force between fibers. The unit kg / t refers to the number of kilograms of additives added per ton of oven-dry pulp processed.

[0020] (3) Wire forming: The pulp to be fed into the wire section of the wire-fed paper machine is fed in, and the lip plate opening is controlled at 9%~9.8%. The opening is dynamically adjusted according to the pulp flow rate to avoid excessive opening causing the paper web to be too thick, and insufficient opening causing insufficient pulp supply. The headbox flow rate is 65,000~70,000 l / min to ensure uniform basis weight of the paper web (basis weight deviation ≤ ±2g / m). 2 The pulp-to-wire speed difference is 35-40 m / min, which ensures the pulp is evenly spread on the wire and reduces differences in fiber orientation. After forming, a wet paper web is obtained, with a wet paper web dryness of 22%-24% exiting the wire.

[0021] (4) Pressing: The wet paper web formed by the wire section is fed into the press section and pressed sequentially through the first press roll, the second press roll, and the third press roll. The linear pressure of the first press roll is 75~85kN / m, which plays a preliminary role in dewatering. The linear pressure of the second press roll is 120~130kN / m, which further removes free water in the fiber gaps and further improves the dryness. The linear pressure of the third press roll is 1300~1350kN / m, which achieves tight fiber bonding under high pressure and improves the smoothness of the paper web surface. After pressing, the dryness of the paper web exiting the press section reaches 50%~55%, without crushing or bubbling.

[0022] (5) Drying: The pressed paper web is fed into the drying cylinder, and the pre-drying steam pressure is controlled at 250~350 kPa and the post-drying steam pressure is controlled at 20~50 kPa. The "pre-drying steam pressure" refers to the saturated steam pressure introduced into the drying cylinder in the pre-drying section of the drying process; the "post-drying steam pressure" refers to the saturated steam pressure introduced into the drying cylinder in the post-drying section of the drying process. When the steam pressure is controlled at 250~350 kPa, the corresponding surface temperature of the drying cylinder (i.e., the "pre-drying temperature") is 85~95℃. This temperature range can quickly break the binding force between the fiber and the moisture to achieve efficient dehydration, and can also avoid the problem of uneven paper web shrinkage, wrinkling or embrittlement caused by excessively high temperature leading to rapid evaporation of surface moisture and failure of internal moisture to migrate in time. When the post-drying steam pressure is controlled at 20~50 kPa, the corresponding surface temperature of the drying cylinder is 45~55℃. This temperature range can slowly release the thermal stress generated inside the paper web due to rapid drying, reduce the brittleness caused by the directional alignment of fibers, and at the same time make the moisture content of the paper web edges and center more consistent.

[0023] (6) Surface sizing treatment: The dried paper web is then surface sizing treated. The sizing flow rate of the sizing machine is controlled at 900~1000 l / min to ensure that the sizing liquid evenly covers the surface of the paper web. The pressure of the scraper of the sizing machine is 100~160 kPa, which can be finely adjusted according to the basis weight of the paper web. After sizing, the paper web can be dried a second time in a hot air dryer (preferably at a temperature of 70~80℃ and a wind speed of 2~3 m / s) to form a uniform sizing film on the surface of the paper web, thereby improving the bursting strength index and surface water resistance.

[0024] (7) Rolling and forming: The paper web after surface sizing is rolled up to obtain ultra-low grammage environmentally friendly composite kraft paper.

[0025] Furthermore, in step (1), the wood pulp is softwood pulp with a fiber length of 1.5~2.0 mm. Fibers in this length range can form a dense fiber interwoven network at a lower basis weight, resulting in superior tensile strength compared to short fiber wood pulp.

[0026] Further, in step (1), the recycled kraft paper pulp is obtained by deinking and screening of recycled industrial kraft paper. First, deinking is performed, resulting in a pulp whiteness ≥75% ISO. Then, screening and purification are carried out, sequentially passing the pulp through a 1.2mm mesh sieve and a 0.8mm mesh sieve to remove large particles, followed by fine screening using a pressure sieve (preferably with a 0.15mm mesh) and a slag remover to remove fine sand particles, metal fragments, and other impurities.

[0027] Further, in step (2), the stirring speed of the slurry treatment is 300~500 r / min, and the stirring time is 20~30 min.

[0028] Furthermore, in step (2), the chemical additives also include 3~5 kg / t of AKD, 3~5 kg / t of polyaluminum chloride, and 0.8~1.0 kg / t of starch. AKD can improve the water resistance of paper; polyaluminum chloride acts as a synergist for the retention and filtration system, promoting the aggregation of colloidal particles. The significance of adding starch is that, through the hydrogen bonding and gelatinization of starch molecules with pulp fibers, the bonding strength between fibers is enhanced, solving the problem of insufficient folding endurance of paper caused by the weak bonding force of traditional pulp fibers.

[0029] Furthermore, in step (3), during the forming process of the wire section, the vacuum degree of the folding roller is controlled to be 56~58kPa to ensure that the wet paper web is stably peeled from the wire section, and there is no tearing or wrinkling of the wet paper web during peeling.

[0030] Furthermore, in step (5), during the drying process, the pre-drying temperature is controlled at 85~95℃ and the pre-drying paper web moisture content is 7.5~8.5%. This setting ensures dehydration efficiency to support production speed and avoids defects such as uneven sizing and paper web embrittlement caused by excessively high or low moisture content.

[0031] Furthermore, in step (7), the winding tension of the winding process is 600N, the winding speed is 500~600m / min, and no steam is introduced into the drying section during winding. This can avoid paper web stretching and deformation or paper breakage, and also prevent the paper web from getting too hot and sticking together after winding, thus ensuring the quality and stability of ultra-low basis weight paper roll forming.

[0032] Accordingly, the present invention provides an ultra-low basis weight environmentally friendly composite kraft paper, which, unlike the prior art, is prepared by the preparation method of ultra-low basis weight environmentally friendly composite kraft paper described above.

[0033] To facilitate understanding, the following examples are provided for further illustration.

[0034] Example 1 (1) Raw material preparation: Take wood pulp produced by the wood pulp line and kraft paper recycled pulp produced by the waste paper line and mix them together; by weight, the wood pulp accounts for 40% of the raw materials and the kraft paper recycled pulp accounts for 60% of the raw materials; the beating degree of the wood pulp is 20SR; the beating degree of the mixture of long fibers and short fibers in the kraft paper recycled pulp is 30SR. (2) Slurry treatment: Add chemical additives to the mixed slurry obtained in step (1) and stir evenly to obtain the slurry to be fed into the web; the chemical additives include 0.3 kg / t retention aid, 0.4 kg / t filter aid, 3 kg / t dry strength agent, 3 kg / t AKD in the slurry, 3 kg / t polyaluminum chloride and 0.8 kg / t starch in the slurry; (3) Wire forming: The pulp to be wired is fed into the wire section of the wire paper machine. The lip plate opening is controlled at 9%, the headbox flow rate is 65,000 l / min, and the pulp-wire speed difference is 35 m / min. After forming, a wet paper web is obtained. The dryness of the wet paper web exiting the wire section is 22%. (4) Pressing: The wet paper web formed by the wire section is fed into the pressing section and pressed in sequence through the first press roll, the second press roll, and the third press roll. The linear pressure of the first press roll is controlled to be 75 kN / m, the linear pressure of the second press roll is 120 kN / m, and the linear pressure of the third press roll is 1300 kN / m. The dryness of the paper web leaving the pressing section after pressing is 50%. (5) Drying: The pressed paper web is fed into the dryer, and the steam pressure of the pre-drying is controlled at 250 kPa and the steam pressure of the post-drying is controlled at 20 kPa; (6) Surface sizing treatment: The dried paper web is sizing treated by controlling the sizing flow rate of the sizing machine to be 900 l / min and the scraper pressure of the sizing machine to be 100 kPa. (7) Rolling and forming: The paper web after surface sizing is rolled up to obtain ultra-low grammage environmentally friendly composite kraft paper.

[0035] Example 2 (1) Raw material preparation: Take wood pulp produced by the wood pulp line and kraft paper recycled pulp produced by the waste paper line and mix them together; by weight, the wood pulp accounts for 42% of the raw materials and the kraft paper recycled pulp accounts for 58% of the raw materials; the beating degree of the wood pulp is 22SR; the beating degree of the mixture of long fibers and short fibers in the kraft paper recycled pulp is 33SR. (2) Slurry treatment: Add chemical additives to the mixed slurry obtained in step (1) and stir evenly to obtain the slurry to be fed into the web; the chemical additives include 0.35 kg / t retention aid, 0.45 kg / t filter aid, 3.5 kg / t dry strength agent, 3.5 kg / t AKD in slurry, 3.5 kg / t polyaluminum chloride and 0.85 kg / t starch in slurry; (3) Wire forming: The pulp to be wired is fed into the wire section of the wire paper machine. The lip plate opening is controlled at 9.2%, the headbox flow rate is 66,500 l / min, and the pulp-wire speed difference is 37 m / min. After forming, a wet paper web is obtained, and the dryness of the wet paper web exiting the wire section is 23%. (4) Pressing: The wet paper web formed by the wire section is fed into the pressing section and pressed in sequence through the first press roll, the second press roll, and the third press roll. The linear pressure of the first press roll is controlled to be 78 kN / m, the linear pressure of the second press roll is 123 kN / m, and the linear pressure of the third press roll is 1320 kN / m. The dryness of the paper web after pressing is 52%. (5) Drying: The pressed paper web is fed into the dryer, and the steam pressure of the pre-drying is controlled at 280 kPa and the steam pressure of the post-drying is controlled at 30 kPa; (6) Surface sizing treatment: The dried paper web is surface sizing treated, and the glue flow rate of the sizing machine is controlled at 930 l / min and the scraper pressure of the sizing machine is 120 kPa. (7) Rolling and forming: The paper web after surface sizing is rolled up to obtain ultra-low grammage environmentally friendly composite kraft paper.

[0036] Example 3 (1) Raw material preparation: Take wood pulp produced by the wood pulp line and kraft paper recycled pulp produced by the waste paper line and mix them together; by weight, the wood pulp accounts for 43% of the raw materials and the kraft paper recycled pulp accounts for 57% of the raw materials; the beating degree of the wood pulp is 23SR; the beating degree of the mixture of long fibers and short fibers in the kraft paper recycled pulp is 35SR. (2) Slurry treatment: Add chemical additives to the mixed slurry obtained in step (1) and stir evenly to obtain the slurry to be fed into the web; the chemical additives include 0.4 kg / t retention aid, 0.5 kg / t filter aid, 4 kg / t dry strength agent, 4 kg / t AKD in the slurry, 4 kg / t polyaluminum chloride and 0.9 kg / t starch in the slurry; (3) Wire forming: The pulp to be wired is fed into the wire section of the wire paper machine. The lip plate opening is controlled at 9.4%, the headbox flow rate is 67,500 l / min, and the pulp-wire speed difference is 38 m / min. After forming, a wet paper web is obtained. The dryness of the wet paper web exiting the wire section is 23%. (4) Pressing: The wet paper web formed by the wire section is fed into the pressing section and pressed in sequence through the first press roll, the second press roll, and the third press roll. The linear pressure of the first press roll is controlled to be 80 kN / m, the linear pressure of the second press roll is 125 kN / m, and the linear pressure of the third press roll is 1325 kN / m. The dryness of the paper web leaving the pressing section after pressing is 53%. (5) Drying: The pressed paper web is fed into the dryer, and the steam pressure of the pre-drying is controlled at 300 kPa and the steam pressure of the post-drying is controlled at 35 kPa; (6) Surface sizing treatment: The dried paper web is surface sizing treated, and the glue flow rate of the sizing machine is controlled at 950 l / min and the scraper pressure of the sizing machine is 130 kPa. (7) Rolling and forming: The paper web after surface sizing is rolled up to obtain ultra-low grammage environmentally friendly composite kraft paper.

[0037] Example 4 (1) Raw material preparation: Take wood pulp produced by the wood pulp line and kraft paper recycled pulp produced by the waste paper line and mix them together; by weight, the wood pulp accounts for 44% of the raw materials and the kraft paper recycled pulp accounts for 56% of the raw materials; the beating degree of the wood pulp is 24SR; the beating degree of the mixture of long fibers and short fibers in the kraft paper recycled pulp is 38SR. (2) Slurry treatment: Add chemical additives to the mixed slurry obtained in step (1) and stir evenly to obtain the slurry to be fed into the web; the chemical additives include 0.45 kg / t retention aid, 0.55 kg / t filter aid, 4.5 kg / t dry strength agent, 4.5 kg / t AKD in the slurry, 4.5 kg / t polyaluminum chloride and 0.95 kg / t starch in the slurry; (3) Wire forming: The pulp to be wired is fed into the wire section of the wire paper machine. The lip plate opening is controlled at 9.6%, the headbox flow rate is 68,500 l / min, and the pulp-wire speed difference is 39 m / min. After forming, a wet paper web is obtained. The dryness of the wet paper web exiting the wire section is 24%. (4) Pressing: The wet paper web formed by the wire section is fed into the pressing section and pressed in sequence through the first press roll, the second press roll, and the third press roll. The linear pressure of the first press roll is controlled to be 83 kN / m, the linear pressure of the second press roll is 128 kN / m, and the linear pressure of the third press roll is 1340 kN / m. The dryness of the paper web leaving the pressing section after pressing is 54%. (5) Drying: The pressed paper web is fed into the dryer, and the steam pressure of the pre-drying is controlled at 330 kPa and the steam pressure of the post-drying is controlled at 45 kPa; (6) Surface sizing treatment: The dried paper web is surface sizing treated, and the glue flow rate of the sizing machine is controlled at 980 l / min and the scraper pressure of the sizing machine is 150 kPa. (7) Rolling and forming: The paper web after surface sizing is rolled up to obtain ultra-low grammage environmentally friendly composite kraft paper.

[0038] Example 5 (1) Raw material preparation: Take wood pulp produced by the wood pulp line and kraft paper recycled pulp produced by the waste paper line and mix them together; by weight, the wood pulp accounts for 45% of the raw materials and the kraft paper recycled pulp accounts for 55% of the raw materials; the beating degree of the wood pulp is 25SR; the beating degree of the mixture of long fibers and short fibers in the kraft paper recycled pulp is 40SR. (2) Slurry treatment: Add chemical additives to the mixed slurry obtained in step (1) and stir evenly to obtain the slurry to be fed into the web; the chemical additives include 0.5 kg / t retention aid, 0.6 kg / t filter aid, 5 kg / t dry strength agent, 5 kg / t AKD in the slurry, 5 kg / t polyaluminum chloride and 1.0 kg / t starch in the slurry; (3) Wire forming: The pulp to be wired is fed into the wire section of the wire paper machine. The lip plate opening is controlled at 9.8%, the headbox flow rate is 70,000 l / min, and the pulp-wire speed difference is 40 m / min. After forming, a wet paper web is obtained. The dryness of the wet paper web exiting the wire section is 24%. (4) Pressing: The wet paper web formed by the wire section is fed into the pressing section and pressed in sequence through the first press roll, the second press roll, and the third press roll. The linear pressure of the first press roll is controlled to be 85 kN / m, the linear pressure of the second press roll is 130 kN / m, and the linear pressure of the third press roll is 1350 kN / m. The dryness of the paper web leaving the pressing section after pressing is 55%. (5) Drying: The pressed paper web is fed into the dryer, and the steam pressure of the pre-drying is controlled at 350 kPa and the steam pressure of the post-drying is controlled at 50 kPa. (6) Surface sizing treatment: The dried paper web is surface sizing treatment, and the glue flow rate of the sizing machine is controlled at 1000 l / min and the scraper pressure of the sizing machine is 160 kPa. (7) Rolling and forming: The paper web after surface sizing is rolled up to obtain ultra-low grammage environmentally friendly composite kraft paper.

[0039] The ultra-low basis weight environmentally friendly composite kraft paper obtained in Examples 1 to 5 was tested, and the test results are shown in the table below.

[0040] Test Project Example 1 Example 2 Example 3 Example 4 Example 5 Quantitative (g / m 2 )]> 68 68.5 69 69.5 70 Moisture (%) 8.2 8.0 8.1 7.9 7.8 <![CDATA[Bulk density (cm 3 / g)]]> 1.62 1.63 1.64 1.65 1.66 <![CDATA[Bursting strength index (kPa·m 2 / g)]]> 2.1 2.2 2.3 2.4 2.5 Tensile strength (kN / m) - Longitudinal 4.5 4.7 4.8 5.0 5.2 Tensile strength (kN / m) - transverse 1.9 2.0 2.1 2.2 2.3 <![CDATA[Water absorption (positive / negative, g / m 2 )]]> 52 / 51 49 / 48 47 / 46 45 / 44 43 / 42 Smoothness (front view, s) 3.5 3.2 3.1 3.0 2.9 Breathability 720 700 680 670 660 Whiteness (% ISO) 26.3 26.5 26.6 26.7 26.8 Color value - L value 73.1 73.0 72.9 72.8 72.7 Color value - a value 2.6 2.5 2.55 2.5 2.4 Color value - b value 24.8 24.7 24.6 24.5 24.4 Whether or not it is crushed none none none none none Based on the above embodiments and test results, it can be concluded that: (1) The basis weight of the ultra-low basis weight environmentally friendly composite kraft paper prepared in each embodiment is stably controlled at 68~70 g / m³. 2 It falls within the ultra-low weight range; its tensile strength is 4.5~5.2 kN / m in the longitudinal direction and its bursting index is 2.1~2.5 kPa·m. 2 / g, which can meet the heavy packaging needs of industrial woven bags such as fertilizer and cement; the product performance has reached the preset technical target, and there are no key indicators that fail to meet the standard.

[0041] (2) As the proportion of wood pulp increased from 40% to 45% and the dry strength agent increased from 3 kg / t to 5 kg / t, the bursting index increased from 2.1 kPa·m 2 / g increased to 2.5 kPa・m 2 / g, the longitudinal tensile strength increased from 4.5kN / m to 5.2kN / m, indicating that the parameters of raw materials and chemical additives can be flexibly adjusted according to the actual strength requirements in industrial production.

[0042] (3) No crushing occurred during the pressing process in each embodiment, indicating that the parameter settings of the three-stage pressing of the present invention are reasonable and effectively avoid the risk of crushing of ultra-low basis weight paper web.

[0043] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any way. It is understood that those skilled in the art can make equivalent substitutions or modifications based on the technical solution and inventive concept of the present invention, and all such modifications or substitutions should fall within the protection scope of the present invention.

Claims

1. A method for preparing an ultra-low grammage environment-friendly composite kraft paper, characterized in that, The method comprises the following steps: (1) raw material preparation: mixing wood pulp produced by wood pulp line and kraft paper recycled pulp produced by waste paper line; the wood pulp accounts for 40-45% of the raw material by weight, and the kraft paper recycled pulp accounts for 55-60% of the raw material by weight; the beating degree of the wood pulp is 20-25SR; the mixed beating degree of long fibers and short fibers in the kraft paper recycled pulp is 30-40SR; (2) pulp treatment: adding chemical additives to the mixed pulp obtained in step (1) and stirring uniformly to obtain pulp to be laid on the wire; the chemical additives at least include 0.3-0.5 kg / t of retention aid, 0.4-0.6 kg / t of filter aid and 3-5 kg / t of dry strength agent; (3) wire section forming: sending the pulp to be laid on the wire into the wire section of the gap former, controlling the lip opening degree to be 9-9.8%, the flow of the headbox to be 65,000-70,000 l / min and the speed difference between the pulp and the wire to be 35-40 m / min, and obtaining a wet paper web after forming, and the dryness of the wet paper web out of the wire section is 22-24%; (4) pressing: sending the wet paper web formed in the wire section into the pressing section, and sequentially passing through the first pressing roll, the second pressing roll and the third pressing roll for pressing, and respectively controlling the linear pressure of the first pressing roll to be 75-85 kN / m, the linear pressure of the second pressing roll to be 120-130 kN / m and the linear pressure of the third pressing roll to be 1,300-1,350 kN / m, and the dryness of the paper web out of the pressing section after pressing is 50-55%; (5) drying: sending the paper web after pressing into the drying section, and controlling the front drying steam pressure to be 250-350 kPa and the back drying steam pressure to be 20-50 kPa; (6) surface sizing treatment: performing surface sizing treatment on the dried paper web, and controlling the sizing flow of the sizing machine to be 900-1,000 l / min and the doctor bar pressure of the sizing machine to be 100-160 kPa; (7) winding and forming: winding the paper web after completing the surface sizing treatment to obtain the ultra-low basis weight environment-friendly composite kraft paper.

2. The process for the preparation of ultra-low grammage eco-friendly laminated kraft paper as claimed in claim 1, wherein, In step (1), the wood pulp is coniferous wood pulp, and the fiber length is 1.5-2.0 mm.

3. The process for the preparation of ultra-low grammage eco-friendly laminated kraft paper as claimed in claim 1, wherein, In step (1), the kraft paper recycled pulp is obtained by deinking and screening purification of recycled industrial kraft paper.

4. The process for the production of ultra-low grammage eco-friendly laminated kraft paper as claimed in claim 1 wherein, In step (2), the stirring speed of the pulp treatment is 300-500 r / min, and the stirring time is 20-30 min.

5. The process for the production of ultra-low grammage eco-friendly laminated kraft paper as claimed in claim 1 wherein, In step (2), the chemical additives further include 3-5 kg / t of AKD in pulp, 3-5 kg / t of polyaluminum chloride and 0.8-1.0 kg / t of starch in pulp.

6. The process for the production of ultra-low grammage eco-friendly laminated kraft paper as claimed in claim 1 wherein, In step (3), during the wire section forming, the couch vacuum degree is controlled to be 56-58 kPa.

7. The process for the production of ultra-low grammage eco-friendly laminated kraft paper as claimed in claim 1 wherein, In step (5), during the drying, the front drying temperature is controlled to be 85-95℃, and the moisture of the paper web in the front drying is controlled to be 7.5-8.5%.

8. The process for the production of ultra-low grammage eco-friendly laminated kraft paper as claimed in claim 1 wherein, In step (7), the winding tension of the winding process is 600 N, the winding speed is 500-600 m / min, and no steam is supplied to the back drying section during winding.

9. An ultra-low grammage environment-friendly composite kraft paper, characterized in that, The ultra-low basis weight environment-friendly composite kraft paper is prepared by the preparation method in any one of claims 1-8.