Kraft paper, also known as coptic paper or polyvinyl chloride paper, is paper or wood board made from coptic material derived from wood pulp generated in the kraft method. Coptic material is obtained by pulp drying and mechanically passing tough chemical fibers through a chemical reagent. The resulting material has high reproducibility, color consistency and quality.
Traditionally, coptic sheets were produced only from male sterility stock. The process of making these types of sheets involved cutting, folding and then bleaching or blacking them. They could be used for several applications, including Coptic writing pads, corrugated boxes, outer garment covers, corrugated pantyhose and corrugated sails. But they are primarily best suited for products that have thin or transparent layers and that can be easily manually lifted out from the packaging or wrapped for shipping. Although these are widely used, they are slowly being replaced by polyurethane or wadding because of their better performance, environmental friendliness and ability to be self-healed.
During the last few decades, the process of coptic selfing had undergone some changes. Initially, the sheets would be made from a formulation of zinc oxide and aluminum chloride (Al), which are chemically bound together with an electrochemical charge. These compounds were considered less stable than the Al alone. This was because the aluminum ions are bound to a larger surface area of the compound, thus increasing its stability. With time, the composition was further optimized by introducing phosphates, bromides and potable water. Still, the basic mechanism of setting remained the same and the manufacturing process for Kraft paper continued to evolve, adapting to new mechanical, chemical and physical properties as they were realized.
In order to obtain better moisture-resistance and to increase its strength, various bleaching agents or coatings are used for the manufacturing process. Originally, the main idea of coating or bleaching the paper is to enhance its aesthetic quality. However, this is not necessarily true nowadays because many ecommerce retail packaging manufacturers have found that the coating enhances the quality of the paper and helps it to resist damage caused by moisture, fungi and insects. Coating gives the paper a quality that is similar to a wax, making it highly resistant to breakage even in the toughest conditions. The advantage of a coating is that, the paper itself does not need to undergo any processing to make it appear aesthetically beautiful.
Besides improving aesthetics, some manufacturers also use polyurethane foam for Kraft paper. Polyurethane foam is available in various grades, providing excellent durability and weather resistance against natural elements, like sunlight and temperature change. Although polyurethane foam is a hard and durable product, its high density requires a lot of production to achieve a sufficient volume. As a result, this grade of paper is expensive, and its demand is declining steadily.
Some manufacturers however, produce low-density polyurethane foam that does not require too much processing. It looks very much like regular Kraft paper, with the only difference that it is resistant to water, sun and heat. This kind of paper has a high quality, economical features that ensure its durability and high quality long-term usage. Its price is slightly higher than that of polyurethane foam, but its durability and superior quality make it more attractive to buyers.