Sack kraft paper is a type of paper that combines two major requirements – toughness and porosity. This type of paper is specifically designed to be used for manufacturing sacks, which are often handled roughly during transportation and storage.

One of the primary requirements for sack kraft paper is toughness. It needs to be able to resist tough handling, such as being unloaded from a lorry and thrown to the ground. To meet this requirement, the paper must be able to absorb shocks or have a high energy absorption capacity, also known as “work to burst.” Work to burst is determined by the elongation of the paper multiplied by the straining force. In addition, tear strength is essential to prevent minor damages from spreading further.

The second major requirement for sack kraft paper is that the sacks must be easily filled. This means that the paper must be porous enough to prevent a build-up of counterpressure when the sack is filled. This porosity allows for efficient filling and prevents the sack from bursting due to pressure.

To meet these requirements, sack kraft paper is manufactured using medium to low refined long fiber kraft pulp. The refining process is done at high consistency, while the forming process occurs at a low consistency. This manufacturing strategy ensures good fiber-to-fiber bonding with high porosity. Both bleached and unbleached fiber can be used in the production of sack kraft paper.

After the paper is formed, it is dried with loose draws, which allows it to shrink in the dryer section of the paper machine. This process generates microcreping, which gives the paper a high elongation before bursting. Typical values of elongation for sack kraft paper are approximately 10% in the cross machine direction and 4% in the machine direction.

To further improve the elongation in the machine direction, the clupac method is used. This method involves causing the paper to microcrepe between a pair of very elastic cylinders. This additional step enhances the overall performance of the sack kraft paper.

Sack kraft paper is typically produced on large, high-efficiency paper machines, with production scales of 100,000 tons per year or more. The paper sack itself is made up of multiple layers, with the number of layers determined by the required toughness.

In conclusion, sack kraft paper is a specialized type of paper that combines toughness and porosity. It is designed to withstand rough handling while also allowing for easy filling of sacks. The manufacturing process involves using medium to low refined long fiber kraft pulp and generating microcreping to achieve high elongation before bursting. Sack kraft paper is an essential material for the production of durable and efficient sacks.