The adhesive strength of PUR cannot reach the specified strength at once after coating, and it cannot be judged that the effect is good or bad. Therefore, it is necessary to fully understand this feature and adjust the binding machine. In particular, it should be understood that when the pressure value of the coating roller is different from the standard value, the glue will not reach its maximum adhesion strength value, and the production should be conducted in strict accordance with the binding operation standards.
As for the PUR coating method, companies have introduced various wrapping machines based on different coating methods, and only general roll coating is discussed here. In addition to roll coating, a part of the PUR wrapper in use sprays the PUR through a nozzle mounted on the cover feeder.
1) Stapling reaction time (hardening time) characteristics
When using PUR, it should be noted that it takes a certain amount of time to reach its maximum strength, and the grasp of the variation of its strength is very important in product management. Depending on the type of binding and the paper used, the intensity and time required for the PUR to reach are different, but in any case the book is opened one hour after the book is wrapped, or the book is distorted. It takes about half a day to reach the maximum strength of the glue. Therefore, during the processing operations such as shipping books and book covers, attention should be paid to inspection to ensure that sufficient time for reinforcement is maintained.
2) Effect of coating roller pressure on adhesive strength
When the roller pressure is insufficient, the intensity of the PUR reaction does not meet the requirement to generate a page fall. Conversely, when the pressure is too high, the glue penetrates between the pages of the book and affects the leveling of the book. Therefore, the pressure adjustment of the roller must be taken seriously.
3) Effect of coating amount on adhesive strength
Compared with EVA, the coating amount of PUR is preferably less. Although the adhesive strength is high, it affects the flatness of the book. If you can reduce the amount of glue coating, in order not to affect the strength of the bond, you can make a good leveling books.
4) The effect of the depth of notch on the bond strength
Compared to when using EVA, the effect of book back depth on the binding quality seems to be smaller, but it cannot be ignored. If the groove is opened too deep and the amount of glue that penetrates is too much, the leveling of the book will be deteriorated. Therefore, it is desirable that the groove be shallower than when using EVA. In addition, the shape of the groove is preferably two shapes such as "" and "□". In the future, it is necessary to find out what is the best slot shape related to adhesive strength, coating amount, and leveling property.
5) The ink solvent causes the bond strength to decrease over time
In the presence of petroleum-based ink solvents, EVA strength is affected. In the sealed storage state, the EVA of the site-dominated page bonding site softens, lowers the adhesive strength, and generates page breaks. In contrast, the PUR intensity is not affected by the solvent and can avoid softening or page breaks of the glue. Two types of glue-bonded full-size field books have been tested under high-temperature sealed pressure conditions. After 2 weeks of testing, the bond strength of EVA decreased and page breaks occurred, while the bond strength of PUR was completely unaffected.
4 PUR binding equipment
Roll coating PUR binding equipment includes:
1) Closed pre-melting device for moisture isolation
Several companies have introduced pre-melting devices that do not require nitrogen gas. The pre-melting box is divided into a single-page box and a large box, specifications are 10kg, 20kg, 200kg and so on.
2) Coating device that facilitates completion of job cleaning work
Roll coating uses "One-Shot-2Roll" as the mainstream coating equipment. Use this equipment to clean up the PUR residue after reaction at the end of the job. In order to easily remove the coating roller and the like and to easily remove the adhered glue, the surface of each component of the coating apparatus is coated with a Teflon layer. In addition, in order to reduce the amount of plastic that cannot be completely discarded after the end of the job, measures must be taken to reduce the tank volume.
3) Local exhaust device
When the PUR is heated too high (above 140°C), isocyanic gas is produced. The gas is said to cause irritation to the eyes, nose, and throat of the human's mucous membranes. Excessive inhalation can cause headaches and other symptoms. From the test results, harmful gases generated under normal 120°C operating conditions did not exceed the allowable values. However, from a safety point of view, the paste heating should be equipped with a temperature control device and a local exhaust device to accelerate the gas diffusion, and promote the reaction of harmful gases with the water in the air and become non-toxic. In addition, a conveyor belt must be provided to ensure that the PUR glue dries, in order to prevent deformation of the back of the thick book or sticking to the blade of the book.
Concluding remarks
PUR has excellent performance in terms of adhesive strength, durability, leveling, recycling, etc. From the point of view of binding production, bookbinding quality can be improved. Japan does not use PUR for a long time, PUR costs more than EVA, so Japan does not use much. The use of PUR in a bookbinding factory requires the initial investment of the equipment, a long reaction time for fixing the PUR glue, and cleaning after the completion of the work.
First of all, there is a general understanding of PUR glue, and the price has fallen to be similar to that of EVA. This may be the key to popularizing PUR. In addition, improving the ease of use of PUR adhesives and lowering the price of special packaging machines are important conditions for the promotion of PUR.
