PUR bookbinding hot melt adhesive concept and development

What is PUR What is Polyurethane

PUR is a polymer compound having a urethane bond (NH-CO-O), and the end of the molecular chain is an isocyanate group. The basic composition of PUR is the addition polymer of active hydrogen terminated polyol (polyester, polyether, etc.) with low molecular weight diol (chain extender) and isocyanate. Diols react with isocyanates to form rigid molecular segments, while polyols react with isocyanates to form flexible molecular segments, so PUR is a rigid-flexible hot melt adhesive. In addition, the isocyanate group encounters an irreversible cross-linking reaction in the air to form a strong adhesive layer, which is the characteristic of PUR hot melt adhesives.

Polyurethane Background

With the diversification of printing products, the printing industry uses a variety of printing papers, plastic films, coating oils and inks of different grammage. When the latest PUR adhesives are applied to bond these materials, there is no great difficulty. . PUR hot melt adhesive can firmly bond coated paper, and its bond strength is 40-60% higher than traditional adhesives. In normal wireless bonding, the printing ink on the paper migrates to the spine, while PUR hotmelts do not have this problem at all.

PUR is unique in its ability to adhere to a variety of materials, including varnishes, UV-cured layers, plastic films and coated paper.

Development of PUR glue

Although the first-generation PUR hot-melt adhesive can achieve linkage production, in order to withstand external forces during transportation, it takes up to 24 hours after the book is bound to reach the maximum bond strength.

In 1994, the second generation of PUR hot melt adhesive was launched to realize seam 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 brushing, sufficient adhesive strength can be obtained. Nevertheless, the second-generation PUR also has disadvantages: in the cold winter, the relative humidity of the air drops to 20%, and the cross-linking of the PUR glue is a problem.

The third-generation PUR can be completely dried and shaped in only 1 hour, and only the moisture in the paper can complete the moisture-curing cross-linking reaction. Books can be plastic-sealed directly offline. However, the improvements in the third generation of PUR hot melt adhesives also brought negative effects. Because PUR can reach initial bond strength in a relatively short period of time, some manufacturers have found that PUR quickly becomes sticky in plastic pans during application. This is especially noticeable in wet areas and people who primarily book thin books, as the PUR has to stay in the applicator for a long time when thin books are bound.

The fourth generation of PUR is already on the market. It flows much better than third-generation PUR hotmelts, achieving the desired initial tack and extending the PUR residence time in the plastic tank. Fourth generation PUR has replaced most second and third generation PUR products, especially in new nozzle systems. At the same time, the fourth-generation PUR hot-melt binder can be shipped around the same day. Due to the fast fixing speed, the production efficiency of books is improved, and the flexibility of production is also increased.
The fourth-generation PUR is already available. It is much more fluid than the third-generation PUR hot melt adhesive, achieving the desired initial tack strength and extending the residence time of the PUR in the plastic pot. The fourth-generation PUR has replaced most of the second- and third-generation PUR products, especially in new nozzle systems. At the same time, the fourth-generation PUR hot melt binding book can be sent to and shipped around the same day. Due to the fast speed of fixation, the productivity of books is increased, and at the same time production flexibility is also increased.