In the course of continuous improvement and development, PUR hot melt adhesives have become one of the important varieties in the binding material after printing due to its excellent adhesive strength, temperature resistance, chemical resistance, and aging resistance. Correct understanding of the history and problems of PUR hot melt adhesives is a prerequisite for better application.
PUR hot melt adhesives have strong adhesion, cold and heat resistance, and have high resistance to oil components in printing inks or toners. However, the recent trend shows that this adhesive is not consistent with the concept of "absolute quality and reliability", what is the truth, and listens carefully to this article.
Development comes from demand
The first-generation reactive adhesives entered the market in 1984. Due to the limited raw materials at the time, the adhesives had a low initial strength and required a long period of chemical reaction to reach the final adhesive strength. In general, stapled products require up to 6 days of reaction time to proceed to the next step. Especially for web offset products, since the tape is not rewetted and contains only a small amount of moisture, a reaction time of several weeks is required, greatly affecting the delivery time.
In the late 1990s, the second-generation PUR hot melt adhesive, which significantly shortened the reaction time, was launched in the market. The main characteristics of this generation of PUR adhesives are: the complete hardening time is reduced to 3 days; the process is more stable and reliable than before. However, in the cold winter, when the relative humidity of the air drops to 20%, cross-linking of the PUR hot melt adhesive is still a problem.
Into the 21st century, the third-generation PUR hot melt adhesive debuted. This high-response adhesive requires only a few hours of reaction time. In addition, the initial strength of the adhesive is greatly improved, which facilitates the operation of the processing equipment (trihedral knife and double-cutting saw), and the production rate can exceed 10,000 per hour. The third-generation hot melt adhesives have low viscosity and good adhesion stability in the production process, especially in completely closed systems. For example, the glue binding machine nozzle coating is more suitable for the use of this highly reactive adhesive. Agent.
At this time, PUR hot melt adhesive users have further satisfied the high-quality and quick glue binding wishes, but the cover side adhesive is still not satisfactory. At the time, PUR Adhesive was still using traditional EVA hot melt adhesives for side coating, which often had a bad influence on the overall image of the product, especially for the printing of magazines and magazines, because ink was often found on the cover. Thick offset images and solids exist, so there is always an interaction between the ink and the side glue that uses the EVA adhesive. Although PUR hot melt adhesives can maintain the style of the bound product, there will be grease on the first and last pages of the cover and booklet. This problem was not solved until after the manufacturers supplied the PUR side glue in 2005. This type of side glue has a continuously high viscosity at the beginning and does not exhibit the properties of the PUR hot melt after cross-linking.
The high quality achieved by the use of PUR hot melt adhesives in booklet binding has also attracted the enthusiasm of traditional book producers. This type of adhesive, which is still referred to as the "slow" cross-linking reaction time, greatly hinders the on-line production of bookbinding, stitch bookbinding and bookbinding. In the "transition phase" of the adhesive, the internal strength (cohesive force) of the film cannot participate in elastic or plastic deformation, and no film fracture occurs. Therefore, in order to speed up the cross-linking, it is necessary to use an adhesive that greatly accelerates cross-linking immediately after the UV light reaction and exposure. It was during this development that the fourth-generation PUR hot melt adhesive appeared. This product, which is known for its double-cured polyurethane hot melt adhesive, is also used for rubber binding books. Just as the name of the glue is expressed, accelerated cross-linking with UV irradiation helps to increase the cohesive strength of the glue, and the final strength of the glue is then generated by wet cross-linking of the isocyanate.
The future responsibility is significant
Since the relevant association has classified a certain concentration of free isocyanate contained in the reactive adhesive system as a class of "harmful preparations", the PUR hot melt adhesive for bookbinding must be marked with hazardous materials to produce the Xn mark. Occupational safety and health information (BGI 790-015) states that “The isocyanate harmful to health includes uncombined free isocyanates (individual diphenylmethane diisocyanate MDI, polymeric MDI) in the form of inhalable mists and aerosols. And MDI polyurethane prepolymers with free NCO isocyanate groups. Currently, 0.1% to 4% of uncomplexed isocyanates are contained in PUR hot melt adhesives. When the temperature exceeds 100°C, isocyanates will be in air at harmful concentrations. In this regard, the plastics industry responded quickly and started to develop products that ensure compliance with the occupational exposure to isocyanate extremes in all circumstances. This is when the fifth-generation PUR hot melt adhesive was born to meet the industry. The environmental requirements for low emissions of isocyanates meet the growing safety requirements of the plant.
In order to achieve the goal of safety and environmental protection, many reactive hot melt adhesive manufacturers today also reduce emissions by reducing the melting and processing temperatures of hot melt adhesives to the 100oC limit. The requirement for processing adhesives in this low temperature range is that the common properties of the adhesive, such as the cross-linking mechanism, high 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, unstable viscosity properties in the production can have adverse consequences on difficult-to-stick paper. In other words, in the case of unstable viscosity, there is an adverse effect on the firmness of the binding.
Other styling businesses have begun to develop so-called hybrid adhesive systems that replace traditional polyurethane-based materials with traditional hot-melt adhesives. However, in view of many people, this kind of development is not necessarily beneficial to the binding factory and post-press processing. A comparison of the quality of a glued product made with a hybrid adhesive system to a product made with a PUR adhesive will reveal that the firmness of the product bound with a hybrid adhesive is significantly reduced. Of course, 30% to 40% reduction in firmness is also related to paper quality. When using offset paper and partially matte coated paper, even if the percentage of reduced binding firmness is high, the use of the product is not greatly affected. The use of hard-to-bond glossy coating papers is completely different, and the pull test results of these papers soon fell into the category of “low binding firmness”. Another problem with this hybrid adhesive system is the altered resistance to mineral oil distillation and bookbinding resistance in extreme temperature ranges. Because the uncross-linked components of the adhesive react with the ink, ie, interact with each other, as was the case with EVA hot melt adhesives in the past few years, this has caused the binder to worry. Since the introduction of PUR hot melt adhesives, binder companies no longer need to pay attention to this interaction problem. Temperature tolerance is also similar. PUR hot melt adhesives can withstand temperatures from -30oC to +200oC.
The fifth-generation PUR hot melt adhesive (low emission) is also not completely accepted in terms of process reliability and binding quality. From the figure below, it can be seen that the reaction and bonding firmness of the PUR hot melt adhesive and its application interval are moving in what direction. For all low-emission PUR hot melt adhesives, although the cross-linking process does not fully meet the needs of the market, a 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 adhesion (see figure below: yellow curve). The third-generation PUR hot melt adhesive (high-response system) achieves 75% final firmness in the same amount of time (see figure below: red curve). The reaction process of the fifth-generation PUR hot melt adhesive (low-emission system) has risen almost linearly (see the figure below: green curve), and its bond strength is limited to a 50% increase. However, in the current complaints, we were surprised to find that the reaction process of the PUR hot melt adhesive system is not responsible for guarantees. Application technical defects, paper problems, and low indoor air humidity have been ignored as the cause of the problem. As far as the current situation is concerned, the adhesive reaction time is a difficult problem to solve.
Given the ever shorter time interval between printing, post-press processing and delivery, the importance of adhesive reactions cannot be included as a secondary requirement for product requirements. For short-term delivery catalog printing plants, bookbinding plants for double production, and digital printing users, the reaction speed and stability of different batches of adhesives are important quality standards.
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