Today we learn about paper strength characteristics: stiffness
Stiffness refers to the ability of paper and cardboard to resist bending. It represents the softness or stiffness of paper, namely rigidity or flexibility. Stiffness is especially an important performance index of cardboard. After cardboard is made into cartons or other appliances, it must have sufficient stiffness to withstand external pressure without deformation or damage.
The stiffness of paper should not be confused with stiffness and apparent hardness. The stiffness and apparent stiffness are based on the sound, tactile stiffness and compressibility when turning the paper, while the stiffness is only based on the ability to resist bending.
If the stiffness of the paper is low, it will bring difficulties to paper feeding and delivery during the printing process, and the printing speed cannot be increased, which will easily cause printing wrinkles.
The factors that affect the stiffness of paper are:
① The stiffness of the paper increases with the third power of the thickness, and the thickness is the most important factor affecting the stiffness.
②The raw materials and pulping methods are different, and the stiffness of the paper is different, in order: unbleached softwood sulfite pulp>bleached softwood sulfite pulp>bleached softwood kraft pulp>kraft pulp>groundwood Pulp.
③The stiffness of paper made from high-beaten pulp is higher than that of lightly beaten paper.
④The stiffness of paper with high hemicellulose content in the pulp is higher than that of paper with low hemicellulose content, and the stiffness of paper made of short fiber pulp is usually higher than that of long fiber pulp.
⑤ Apply starch or other additives in the slurry to increase the stiffness.
⑥Humidity has a great influence on the stiffness of paper. After being wet, the stiffness of paper and cardboard will decrease linearly.