What is the paper bulk?

Bulk is an important indicator of paper and cardboard, it is the ratio of quantitative to thickness and can be expressed in cubic centimeters per gram. And it indicates the degree of paper thinning, that is, the size of the paper porosity.

Why improve the paper bulk?

Bulk is a very important property of paper and paperboard, high bulk of paper and paperboard can maintain the necessary stiffness, can reduce fiber consumption, saving pulp costs.

High bulk means that paperboard, light paper and other types of paper requiring thickness can reduce paper weight, for copy paper, cardboard and other requirements of the stiffness of the paper species can improve the opacity of the paper, stiffness performance.

Therefore, the loose thickness is of great significance to the cost control, product performance and added value of paper and paperboard products.

How to improve the paper bulk?

In this article 7 ways to help improve paper bulk will be concluded.

• Fiber Pull Properties
• Pulp type
• Pulping
• Drying
• Press section operation
• Operation of calender
• Fillers

1 Fiber Pull Properties

The fiber form of paper fiber raw material has an important influence on the loose thickness of paper.

The porosity of the coarser fiber paper is higher, and the loose thickness of the paper is also higher, and vice versa.

Generally speaking, the fiber has high coarseness and loose thickness.

However, the bulk thickness is not only related to the coarseness of the fiber, but also has a very important relationship with the collapse of the fiber in the paper forming process. The bulk thickness of the paper ultimately depends on the collapse and deformation of the fiber.

Therefore, the fibers with small diameter and thick wall are more stiff, not easy to collapse, and easy to form paper with high bulk thickness.

2.Pulp type

Pulp type has a significant effect on paper bulk.

The high bulk thickness of high yield pulp itself is incomparable with other pulp types, so in high-grade paper, high yield pulp partially replaces bleached kraft hardwood pulp and is widely used; In paperboard, high yield pulp as the core layer of paperboard can greatly improve the loose thickness and stiffness of paperboard.

High yield pulp has similar physical strength and brightness to bleached kraft hardwood pulp. Compared with bleached kraft hardwood pulp, in addition to low pulping cost and high yield, high yield pulp also has some good other characteristics, such as high bulk thickness, high opacity and high fine fiber composition.

Because most of the lignin is retained in the high yield pulp, and the high yield pulp fiber is generally quite hard and not easy to be crushed in the papermaking process.

3.Pulping

The higher the pulping degree of the pulp, the greater the tightness of the copied paper sheet, so in the pulping stage, the operation of pulping should be adjusted according to the changes in the incoming material. Especially for waste paper pulp, the source is complex, the nature is very different, some waste paper after the hydraulic pulper is about 50 ° SR, such waste paper can not be added knife pulping, only let it sparring through. For the waste paper that does not reach 45°SR after passing through the hydraulic pulper, the knife can be added slightly to ensure the stability of the pulping degree, and not to overpulping.

The pulping of the surface pulp should not be excessive, the long fiber pulp can be beaten above 40°SR to facilitate the uniform formation of the net, while the short fiber pulp is not necessary to beat too fine, because the short fiber pulp is short, too much cutting will affect its strength, a large number of small fibers, in addition to reduce its thickness, short fiber pulp pulping to about 35°SR is appropriate.

4.Drying

In general, the bulk thickness of paper and board decreases as the drying time increases, and decreasing the drying time can increase the bulk thickness. Some studies indicate that paper bulk thickness and maximum drying shrinkage rate are roughly inversely related, because as drying proceeds, the inter-fiber and intra-fiber bond areas are contracted, resulting in an increase in tightness and a decrease in bulk thickness. Paper bulk increases as the maximum drying shrinkage rate decreases because the reduction in wet shrinkage induces an increase in paper bulk, which increases the paper bulk.

5.Press section operation

The main purpose of the press is dewatering, but other properties of the sheet must also be taken into account. Too much pressure increases the tightness of the sheet and decreases its bulkness. If the thickness does not meet the requirements, the pressure should be reduced to the maximum extent possible to ensure the final paper quality. Of course, if the conditions allow, choose to use a wide nip press, shoe press, can better solve the contradiction between dewatering and bulk.

6.Operation of calender

Calendering includes hard calendering and soft calendering. Its function is to improve the smoothness of the paper surface. When the pressure is increased, the smoothness of the paper surface will be improved, but its loose thickness will be affected. To solve this contradiction, appropriate adjustments should also be made according to the requirements of product quality. If the loose thickness is good, the pressure should be appropriately increased to achieve better smoothness. If the smoothness is far beyond the standard, it is unnecessary to apply too much pressure to obtain the best bulk thickness.

7.Fillers

The effect of fillers on the paper bulk is negative. Because the density of fillers is higher than that of fibers, when the paper quanitative is fixed, increasing the amount of filler will reduce the paper bulk.