PUR hot melt adhesive development track and application bottleneck

In the continuous improvement and development, PUR hot melt adhesives have become one of the important varieties of post-press binding materials due to their excellent bonding strength, temperature resistance, chemical resistance and aging resistance. Correctly understanding the development history and existing problems of PUR hot melt adhesives is the premise of better application of them.

PUR hot melt adhesive has strong adhesion, cold resistance and heat resistance, and has high resistance to oil components in printing ink or toner. However, recent trends have shown that this adhesive does not conform to the concept of "absolute quality and reliability", what is the truth, and listen to this article carefully.

Development stems from demand

The first generation of reactive adhesives entered the market in 1984. Due to the limitations of the raw materials at that time, this adhesive had a low initial strength and required a long time of chemical reaction to reach the final adhesive strength. Under normal circumstances, the bound product requires a reaction time of up to 6 days before proceeding to the next step. Especially for web offset products, because the paper tape is not rewet and contains only a small amount of water, it takes a few weeks of reaction time, which greatly affects the delivery time.

At the end of the 1990s, the second generation of PUR hot-melt adhesive, which significantly reduced the reaction time, was launched on the market. The main characteristics of this generation of PUR adhesives are: the complete hardening time is shortened to 3 days; the processing process is more stable and reliable than before. But in the cold winter, when the relative humidity of the air drops to 20%, the cross-linking of PUR hot melt adhesive is still a problem.

Entering the 21st century, the third generation of PUR hot melt adhesives debut, this high-response adhesive only requires a few hours of reaction time. In addition, the initial strength of the adhesive is greatly improved, which is conducive to the operation of processing equipment (three-sided knife and double-joint cutting saw), and the production speed can exceed 10,000 copies per hour. The third-generation hot-melt adhesive has a lower viscosity and good adhesion stability during the production process, especially in a completely closed system, such as the glue coating of the nozzle of the perfect binding machine is more suitable for using this high-reaction adhesive Agent.

At this time, PUR hot melt adhesive users' desire for high-quality and fast binding is further satisfied, but the side glue of the cover is not satisfactory. At that time, PUR binding was still using traditional EVA hot-melt adhesive for side coating, which often had a bad influence on the overall image of the product, especially for the printing of graphic magazines and magazines. Because the ink is often next to the cover Thick offset images and solids exist, so there is always an interaction between the ink and the side glue using EVA adhesive. Although PUR hot melt adhesive can maintain the style of the bound product, oil stains will appear on the first and last pages of the cover and booklet. This problem could not be solved until the manufacturers of PUR side glue were supplied in 2005. This type of side glue has a continuous high viscosity at the beginning, and the performance of PUR hot melt adhesive will be shown after crosslinking.

The high quality achieved by using PUR hot melt adhesives for booklet binding has also attracted the covetment of traditional book producers. This kind of adhesive, which is still called "slow" cross-linking reaction time, greatly hinders the on-line production of glue binding, thread-locking book cores and round rounds. In this kind of adhesive, in the "transition stage", the inherent strength (cohesion) of the adhesive film cannot participate in elastic or plastic deformation, nor will the adhesive film break. Therefore, in order to accelerate the cross-linking speed, it is necessary to use an adhesive that greatly accelerates the cross-linking immediately after UV light reaction and exposure. It was during this development that the fourth generation of PUR hot melt adhesives appeared. This product, known as dual-curing polyurethane hot-melt adhesive, is also used for bookbinding. Just as the name of this kind of glue says, it is helpful to improve the cohesive performance of the glue when UV irradiation starts to accelerate the crosslinking, and then the final strength of the glue is produced by the wet crosslinking of isocyanate.

Great responsibility in the future

Since the relevant association has classified certain concentrations of free isocyanate contained in reactive adhesive systems as "harmful preparations", PUR hot-melt adhesives for binding must be marked with the dangerous goods Xn mark. Occupational safety and health information (BGI790-015) states that "isocyanates are harmful to health including unsynthesized free isocyanates (monomer diphenylmethane diisocyanate MDI, polymerized MDI in the form of inhalable mists and aerosols) And MDI polyurethane prepolymers with free NCO isocyanate groups.) At present, PUR hot melt adhesives contain 0.1% to 4% uncombined isocyanate. When the temperature exceeds 100oC, the isocyanate will be in the air at a harmful concentration in the air Zhong Yi Chu. ”To this end, the rubber industry quickly responded and began to develop products that ensure compliance with the occupational exposure isocyanate limit value under any circumstances. At this time, the fifth generation of PUR hot melt adhesive was born to meet the industry The environmental protection requirements for low emissions of isocyanates meet the growing safety requirements of the factory.

In order to achieve the goal of safety and environmental protection, many reactive hot melt adhesive manufacturers now also reduce emissions by reducing the melting and processing temperature of the hot melt adhesive to the limit of 100oC. The requirements for processing adhesives in this low temperature range are: the common properties of the adhesive, such as the cross-linking mechanism, the higher initial strength and the stability of the viscosity value should be maintained at normal levels. Due to the lack of moisture on the edge of the paper, the unstable viscosity performance in the production will have undesirable consequences on difficult-to-stick paper. In other words, in the case of unstable viscosity, the firmness of the binding will be adversely affected.

Other glue-making businesses have begun to develop so-called hybrid adhesive systems, which use traditional hot-melt adhesive materials instead of polyurethane-based materials. But in the eyes of many people, this development is not necessarily beneficial to bookbinding factories and post-press processing. Compare the quality of the products made with the mixed adhesive system with the products made with the PUR adhesive. You will find that the firmness of the products with the mixed adhesive is significantly reduced. Of course, the 30% to 40% reduction in firmness is also related to the quality of the paper. When using offset paper and partially matte coated paper, even though the percentage of binding firmness reduction is high, it has little effect on the usability of the product. The situation of using hard-coated gloss coated paper is completely different, and the tensile test results of this paper are quickly reduced to the range of "less binding firmness". Another problem with this mixed adhesive system is the changed resistance to mineral oil distillation and the binding resistance in extreme temperature ranges. Because the uncrosslinked component of the adhesive will react with the ink, that is, interact, just like the use of EVA hot melt adhesives in the past few years, this makes the binding factory worried. Since the introduction of PUR hot melt adhesives, bookbinding factories no longer need to pay attention to this interaction problem. The temperature resistance is similar. The temperature resistance of PUR hot melt adhesive can range from -30oC to + 200oC.

From the perspective of process reliability and binding quality, the fifth-generation PUR hot melt adhesive (low emission) is not completely accepted. It can be seen from the figure below, in which direction the reaction of PUR hot melt adhesive and the process of adhesive firmness and its application interval are developing. For all low-emission PUR hot-melt adhesives, although the cross-linking process does not fully meet market demand, a development trend can be seen. The "normal reaction system" of the second-generation PUR hot-melt adhesive reached 20% of its final firmness 8 hours after bonding (see figure below: yellow curve). The third-generation PUR hot-melt adhesive (high reaction system) reached 75% of the final firmness in the same time (see the following figure: red curve). The reaction process of the fifth-generation PUR hot-melt adhesive (low-emission system) rises almost linearly (see the figure below: green curve), and its bonding strength is limited to a 50% increase. However, in the existing complaint, we were surprised to find that the reaction process of the PUR hot melt adhesive system is not responsible for guarantees. The defects in application technology, paper problems and low indoor air humidity have been ignored as the cause of the problem, so From the current situation, the reaction time of the adhesive is a difficult problem to deal with.

In view of the shorter and shorter time intervals between printing, post-press processing and delivery, the importance of adhesive reaction can no longer be regarded as a secondary position for product requirements. For the short-term delivery of catalogue printing plants, double-book production bookbinding plants and digital printing users, the reaction speed and stability of different batches of adhesives are quite important quality standards.