In 1989, PUR binding adhesives first began trials in North America, when PUR was used only for normal paperback binding. The printing and bookbinding factory that originally used PUR hot melt adhesives found that PUR has a high value in applications such as hardcover books, annual reports, catalogs, yellow pages, and magazines.
Today, PUR is rapidly applied in North America. In 1995, only 28 manufacturers reported using PUR in North America. By the end of 2002, this number has 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 adhesive strength. After opening the book, the appearance is smooth and easy to read.
What is PUR glue
PUR is a macromolecular compound having a urethane bond (NH-CO-O), and the terminal of the molecular chain is an isocyanate group. The basic composition of the PUR is the addition polymer of an active hydrogen-terminated polyol (polyester, polyether, etc.) and a low-molecular-weight glycol (chain extender) and isocyanate. The diol reacts with isocyanate to form a rigid molecular segment, while the polyol and isocyanate react to form a flexible molecular segment, so PUR is a rigid and flexible hot melt adhesive. In addition, the isocyanate base encounters irreversible cross-linking reactions in the air, forming a strong adhesive layer, which is characteristic of PUR hot melt adhesives.
PUR background
With the diversification of printing products, the printing industry uses a variety of different grammage printing papers, plastic films, coating oils, and inks. When the latest PUR adhesives are applied to the bonding of these materials, there will be no great difficulty. . PUR hot melt adhesive can firmly bond the coated paper, its adhesive strength is 40-60% higher than the traditional adhesive. In ordinary wireless bonding, the printing ink on the paper migrates to the spine, and PUR hot melt does not have this problem at all.
What makes PUR unique is its ability to adhere to a variety of materials, including clearcoats, UV-cured layers, plastic films, and coated paper.
The development of PUR
Although the first-generation PUR hot melt adhesives can achieve linked production, it takes up to 24 hours after book binding to achieve the maximum bond strength, in order to withstand external forces during transportation.
The second-generation PUR hot melt adhesive was introduced in 1994 and achieved joint 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, a sufficient adhesive strength can be obtained. Despite this, the second-generation PUR has its disadvantages: in the cold winter, the relative humidity of the air drops to 20%, and cross-linking of the PUR glue is a problem.
The third-generation PUR can be fully dried and fixed in just 1 hour, and it requires only the moisture in the paper to complete the moisture-curing cross-linking reaction. The book can be directly off-line for plastic sealing. However, the improvement of the third-generation PUR hot melt adhesive has also had a negative impact. Since PUR can achieve a preliminary adhesive strength in a relatively short period of time, some manufacturers found that the PUR soon became sticky in the plastic pot during use. This is especially evident in wet areas and those who mainly book thin books, because PUR has to stay in the applicator for a long time when thin books are bound.
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.
Advantages of PUR Hot Melt Adhesives
1. The book is very smooth
According to the recommended brushing thickness, the thickness of the book block brush is about 0.025cm. At this time, the PUR adhesive layer is much softer than the EVA hot melt adhesive. Therefore, the book is opened very flatly, and the book does not have to be pressed by hand when reading, and the book flatness improves. The reader's reading comfort. Therefore, PUR is favored by some large companies. For example, Microsoft and Texas Instruments use PUR hot melt adhesives to bind product user manuals.
2. High temperature and low temperature resistance
Put a book bound with PUR hot melt and a book bound with EVA hot melt in the refrigerator for one night. Then take the two books from the refrigerator the next day and open the book. You hear the cracking sound of the EVA hot melt binding book, and the PUR hot melt bookbinding book is intact.
The refrigerator test simply compares the environmental conditions that the book may face in the winter. To test the durability of the PUR binding book under summer environmental conditions, the book can be placed in the back seat of the car during the day. At this time, the ambient temperature can reach 43°C. At this temperature, the EVA hot melt bookbinding book begins to spread. The page came off and the PUR book was still intact.
PUR binding books will not be affected even under high or low temperature conditions, even at 94°C or minus 40 degrees without problems.
3. Spine formation
When ordinary hot melt adhesives are used to bind irregularly textured art paper book blocks, a large amount of wrinkles are generated at the grooves. Because PUR hot melt adhesives are cold glue adhesives, the working temperature is low, so the moisture of the paper at the spine is not lost. At the same time, the PUR adhesive layer has a certain degree of ductility during the drying process, allowing the paper fibers to recover.
In addition, due to the small amount of PUR hot melt adhesive used in the binding process, very little adhesive is squeezed out on the encapsulating table, and the two sides of the splint firmly wrap the cover on the book block to form a stiff spine.
4. The smoothness of the book block cut is trapezoidal
Since the PUR hot melt adhesive has only half the thickness of other hot melt adhesives, the incision is smooth without trapezoidal shape and the edge of the book cover is also very smooth.
5. Solvent resistance
Some bookbinding experts believe that PUR is the only binding material that is both solvent and grease resistant. The dry-fixed PUR adhesive layer is immersed in solvents and greases used in the printing industry, such as methyl ethyl ketone and alcohol, and these chemicals have no effect on the PUR layer. On the other hand, some of the solvents used in the printing industry will completely soften or even dissolve frequently used hot-melt adhesives, such as EVA.
6. For hardcover cost savings
For many years, the book block of the hard-back book has been used for sewing, and before brushing, a tough adhesive is applied to the book block. According to the use of some users, put the lock book line book block flat, and then brush the PUR hot melt adhesive, this binding method can save about 30-40% of the cost.
7. Good rounding performance and arc deformation
Many hardcover books use round spine backs to enhance the beauty and durability of the book. Brush the PUR adhesive on the book block, and then round and ridge to get a more beautiful round spine. Since the PUR coating and the book block are dried in a circular state, the durability of the round book back is maximized. In the case of ordinary hot melt adhesives, when the book block is bound, the glue is generally first brushed and then rounded, so the durability of the circle cannot reach the effect of PUR adhesion.
PUR usage notes
Some precautions must be taken when using the PUR to prevent the PUR from being cross-linked enough to cause the adhesive layer to come off easily. In order to meet the needs of PUR gluing, there are already open plastisol and closed pressure nozzle systems for PUR gluing.
The PUR binding device using roller coating requires a sealed pre-melter that is not affected by the humidity of the air. The coating device, which is easy to clean at the end of the job, must remove the PUR residue after the gluing is completed. The coating roller can be easily detached and adhered. The glue is removed.
Because the PUR hot melt will react and solidify when exposed to air, an inert gas (such as nitrogen) must be used to form a protective layer. In addition, the fumes generated by the hot melt adhesive nozzle and the upper space of the plastic tank must also be promptly discharged.
When PUR is overheated (above 140°C), isocyanate gas is released, causing an impact on the human body, such as irritating the mucous membranes of the eyes, nose, and throat, and causing headaches and other symptoms when large amounts of smoke are inhaled. According to the experiment, the usual operating temperature cannot exceed 120°C. From a safety point of view, it is advisable to set a temperature control device and a local exhaust device for the heating of the glue to promote gas diffusion. When binding a thick book, it is necessary to configure a conveyor belt that is long enough to prevent the spine from deforming and to prevent the adhesive from adhering to the cutting edge.
Looking to the future
Annual reports and business magazine binding are one of the main application markets for PUR because these businesses need to bind advanced coated paper and textured art paper. Future PUR will continue to exert its advantages in this market.
With the popularity of PUR, some publishers have already felt the advantages of PUR. They demand the use of PUR binding books to ensure the quality of binding and the durability of books. Books with PUR binding will be more and more.
Hardcover books are the next area of PUR application. PUR adhesives are cheaper than lock stitches. Fourth-generation PUR hot melt adhesives can be interlocked and many hardcover bookbinding manufacturers have begun investing in PUR binding. Due to the cost advantages and high quality of PUR's hardcover, more and more hardcover manufacturers use PUR hot melt adhesives.
