Kraft paper is usually used as packaging material. It is tawny. The semi-bleached or fully bleached kraft pulp is light brown, cream or white. The crack length is generally above 6000m. Tear strength, breaking work and dynamic strength of kraft paper are high. Most of kraft paper are web and flat paper. Sulfate softwood pulp is common raw material, and it is beaten and made paper on a long net paper machine. Apply for cement bag paper, envelope paper, rubber seal paper, asphalt paper, cable protective paper, insulation paper, and other kinds of papermaking.

Fine kraft paper is a new generation of ordinary kraft paper, widely used in laminating packaging, medical packaging, portfolios, envelopes, melon bags, hand bags, composite bags, tapes, valve pockets, paper-plastic composites and other fields. The fine kraft paper produced from micro-bleached sulfate pulp, which is a popular raw material for packaging industry due to its light color, smoothness, and strength, it can meet the packaging and printing requirements of packaging materials. In the large range of ordinary color kraft paper, the market demand is growing rapidly.

Kraft paper classification

Kraft paper usually maintains its virginal color of yellow-brown and is suitable for making bags and wrapping paper. Kraft paper has a variety of uses depending on nature and use. Kraft paper is a general term for a piece of paper. There is no specific specification. It is generally classified according to its nature and use.

• By the color, it can be divided into original color kraft paper, red kraft paper, white kraft paper, flat kraft paper, single-light kraft paper, two-color kraft paper, and the like.
• By the different uses, it can be divided into packaging kraft paper, waterproof kraft paper, pitched kraft paper, anti-rust kraft paper, pattern kraft paper, insulated kraft paperboard, kraft stickers and so on.
• By the different materials, it can be divided into recycled kraft paper, kraft core paper, kraft base paper, cowhide wax paper, wood pulp kraft paper, composite kraft paper and so on.

Kraft paper and pulp making

Due to the wide application of fine kraft paper, we take it as an example. In addition to high strength requirements in terms of breakage resistance, tearing and tensile strength, fine kraft paper has high requirements in terms of color, smoothness, uniformity and appearance quality, and usually requires a lower Tightness and high air permeability. Generally, it is prepared by using kraft pulp wood pulp. In order to meet the requirements of color and appearance quality, it is necessary to bleach the pulp, so that the whiteness of the pulp reaches 24%-34%, and keep the yellow and red values of the pulp relatively stable, that is, to maintain the stability of the hue of the bleached pulp.

As for the technical solution of fine kraft paper, for the quality requirements and use characteristics of fine kraft paper, we focus on the selection of fiber raw materials, pulp cooking, pulp bleaching, pulp beating, net forming, pressing, drying, paper finishing and other production processes and process control.

Materials preparation

The distinguishing feature of fine kraft paper includes good physical strength, uniformity, and smoothness. In terms of paper properties, paper made from kraft softwood pulp has higher physical strength, while paper made from hardwood wood pulp is better than softwood pulp in terms of uniformity and flatness. The kraft paper made from long-fiber softwood pulp is difficult to achieve the desired effect in the formation but has a higher physical strength index. According to the quality technical requirements of fine kraft paper, we choose the process of softwood and hardwood mixing pulp. The proportion of hardwood pulp accounts for about 30%. Based on this raw material ratio, the physical strength of the paper does not decrease significantly, but the uniformity, smoothness and other indexes are obviously improved. At the same time, it has a good effect in developing new sources of fiber raw materials, reducing the pressure on the purchase of masson pine and reducing production costs.

Pulp cooking and bleaching

The pulp of fine kraft paper is required to have less coarse fiber bundles and stable color, and it has high-quality requirements for cooking and bleaching processes. It is well known that there is a big difference in the performance of cooking and bleaching between softwood and hardwood. If the pulp production line can separate pulping of softwood and hardwood, the cooking and bleaching of softwood and hardwood alone can be preferred. Under the production conditions without separate pulping, we use the mixed cooking of softwood and hardwood, mixed bleaching after cooking, and the cooking quality, Non-uniform, coarse fiber bundles, pulp hue instability and other quality problems are easy to appear in the production process. For this reason, we strictly control the stability of the fiber material ratio in the material preparation, improve the chip yield, use DCS control operation technology and computer programs to control cooking.

As for the pulp cooking equipment, because the steaming ball adopts the direct steaming mode, it will appear some problems like low alkali concentration, poor cooking uniformity and low pulp quality in the late cooking. Comparatively speaking, the pulp cooked in the vertical pulp digester has better quality. The paper and pulp company generally adopts DDS replacement cooking process for cooking. Because of the large liquid ratio during the cooking cycle, the degree of automation is high, the temperature difference between different parts is small, and the cold blow technology can be used to obtain paper with uniform cooking quality, the small change of Kappa number and high strength. Besides, the pulp quality is better than the pulp produced by the traditional batch cooking process, especially suitable for the production of fine kraft paper. It also saves energy consumption, compared with the traditional batch cooking process, saving Cooking heat consumes more than 50%. In the process of pulp screening, the introduction of foreign sewing screen can not only greatly increase the output, but also reduce the bark, coarse fiber bundle and other impurities in the pulp, and improve the quality of the pulp. Reduce the dusty substances and coarse fiber bundles in the paper to make the paper more beautiful.

In order to meet the printing performance and appearance quality requirements of fine kraft paper, the whiteness is required at 24%-34%. If adopts pulp with lower hardness, although the whiteness can meet its requirements, it is difficult to achieve its quality requirements in terms of yellowness, tightness, and physical strength. While sulfate wood pulp with higher kappa number, its whiteness is generally less than 20%, so, the method of single-stage bleaching of hypochlorite can be used in the production.

Beating

To improve the paper strength of fine kraft paper, optimizing the pulping process is an important measure. In general, to improve the physical strength of the paper, the uniformity, and flatness of the paper, it is necessary to appropriately increase the beating degree of the pulp, while maintaining its good air permeability and low tightness, the degree of beating should not be too high. Because the low tightness and high air permeability requirements of fine kraft paper are contradictory to the improvement of physical strength, uniformity, and flatness, it is especially important to find the best balance point in production.

Kraft papermaking

The fine kraft paper has higher quality requirements in the evenness, the quantitative error of the vertical and horizontal differences. To this end, we improve the quality by using appropriate pulp-to-web ratios, using mesh shakes and net shapers, etc.

The breathability, tightness, and smoothness of the paper are related to the press production process. The pressing reduces the porosity of the paper, thereby reducing the paper permeability and suction height, and improving the tightness; the air permeability and tightness of the paper are rapidly increasing with the increase of the pressing pressure and the pressing time. The physical strength of the paper can be improved.

The paper sheet not only evaporates moisture, increases the mechanical strength during the drying process, but also affects the properties of paper tightness, absorbency, air permeability, smoothness, and sizing. These changes are closely related to the drying method and process. It is generally believed that high temperature enhanced drying with rapid heating will increase the softness, porosity, absorbency and air permeability of the paper, and reduce the tightness, transparency and mechanical strength of the paper.