In 1989, PUR binding adhesives were first tried in North America. At that time, PUR was only used for general paperback binding. Now, the printing and binding factory that originally used PUR hot melt adhesive found that PUR has high use value in hardcover books, annual reports, catalogs, yellow pages and magazines.
Today, PUR has been rapidly promoted and applied in North America. In 1995, it was reported that only 28 manufacturers used PUR in North America. By the end of 2002, this number had increased to 50. PUR has many advantages over commonly used hot-melt adhesives (EVA hot-melt adhesives, PVA latex). The most important point is that PUR has a high bonding strength, and the appearance of the book after opening is flat and easy to read.
What is PUR
PUR is a polymer compound with an urethane bond (NH-CO-O), and the molecular smelting terminal is an isocyanate group. The basic composition of PUR is a polyhydric alcohol (polyester, polyether, etc.) with active hydrogen at the end and an addition polymer of low molecular weight diol (elongation agent) and isocyanate. Glycol and isocyanate react to form a rigid molecular refining section, while polyol and isocyanate react to form a flexible molecular refining section, so PUR is a rigid and flexible hot melt adhesive. In addition, the isocyanate group encounters moisture in the air and produces an irreversible cross-linking reaction, forming a strong adhesive layer, which is the characteristic of PUR hot melt adhesive.
The background of PUR
With the diversification of printed products, the printing industry uses a variety of different weights of printing paper, plastic films, varnishes and inks. The latest PUR adhesives will not be difficult when applied to the bonding of these materials. . PUR hot melt adhesive can firmly bond coated paper, and its bonding strength is 40-60% higher than traditional adhesive. In ordinary wireless binding, the printing ink on the paper will migrate to the spine, and PUR hot melt adhesive will not have this problem at all.
PUR is unique in that it can bond a variety of materials, including varnish coatings, UV curing layers, plastic films and coated paper.
The development process of PUR
Although the first-generation PUR hot-melt adhesive can achieve linked production, it takes 24 hours after book binding to reach the maximum bonding strength, and then it can withstand the external forces in transportation.
The second generation of PUR hot-melt adhesive was launched in 1994, and realized linkage cutting. If there is a certain amount of moisture in the paper, the binding area maintains a high air humidity, and after 4 hours of glue application, sufficient adhesion strength can be obtained. Nevertheless, the second-generation PUR still has deficiencies: in the cold winter, the relative humidity of the air drops to 20%, and the cross-linking of PUR glue is a problem.
The third-generation PUR can be completely dried and fixed in only 1 hour, and only requires the moisture in the paper to complete the moisture curing cross-linking reaction, and the books can be directly taken offline for plastic sealing. However, the improvement of the third-generation PUR hot-melt adhesive has also brought negative effects. Because PUR can achieve initial bonding strength in a short time, some manufacturers found that PUR soon became sticky in the glue pot during use. Especially in wet areas and those manufacturers that mainly bind thin booklets, because PUR stays in the glue applicator for a long time when binding thin booklets.
The fourth-generation PUR is already on the market. It has better fluidity than the third-generation PUR hot-melt adhesive and can achieve the expected initial tack strength, while extending the residence time of PUR in the glue pot. The fourth-generation PUR has replaced most of the second- and third-generation PUR products, and is especially successful in new nozzle systems. At the same time, books bound by the fourth-generation PUR hot-melt adhesive can be shipped to all places on the same day. Due to the fast drying and fixing speed, the productivity of books and periodicals is improved, and at the same time, the production flexibility is also improved.
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