Application of Polyurethane Hot Melt Adhesive in Book Binding
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First, Introduction
Polyurethane (PUR) adhesive refers to a type of adhesive containing a urethane group (-NHCOO-) or an isocyanate group (-NCO-) in the molecular chain. Because the -NCO in the molecular chain can react with a variety of active hydrogen-containing functional groups, it forms an interface chemical bond, which has strong adhesion to a variety of materials, has a tough film, good impact resistance, flexibility, and peel strength. High, good resistance to ultra-low temperature, oil and abrasion resistance. Hot melt adhesive (referred to as hot melt adhesive) is a kind of solvent-free solid polymer based on thermoplastic polymer. It is melted into a liquid state by heat and applied to the surface of the substrate as a melt for bonding. Hot melt adhesive has the advantages of quick bonding, non-toxic and non-polluting, energy saving and environmental protection, convenient storage and transportation, low cost and wide application, including EVA, PE, random PP and polyurethane. Polyurethane hot melt adhesives can be divided into two categories: one is reactive hot melt adhesive, which can be further divided into terminal isocyanate based prepolymer type (moist curing type) and closed isocyanate based prepolymer type (latent curing type). The other type is a thermoplastic polyurethane elastomer hot melt adhesive which is crosslinked by hydrogen bonding in the composition to give it excellent elasticity and strength. After the former is heated and melted, it reacts and cures in the case of moisture; while the latter solidification process mainly uses physical interaction of hydrogen bonds, and the heat is weakened again, which weakens the hydrogen bond and becomes a viscous liquid, so it can be repeatedly heated-cooled Cured, product recyclable or secondary, environmentally friendly materials.
Second, polyurethane hot melt adhesive curing reaction mechanism
The PUR resin has a reactive NCO functional group at the end of the molecular chain. Since the carbon-nitrogen double bond activity in the functional group is very high, it reacts with air or water vapor on the surface of the substrate to form an amine group, and at the same time releases CO 2 and amine groups. Reacts with other NCO groups to form a structure having a urea (urea) functional group. Since the active hydrogen is still present in the urea structure, it is further reacted with the unreacted NCO group to form a biuret structure, and the so-called crosslinking reaction is completed. Its reaction mechanism with moisture is as follows:
-NCO+H2O→NH2+CO2→-NH-CO-NH
-NH-CO-NH-+OCN-→NH-CO-N-CO-NH-
It can be seen from the above that PUR adhesive bonding curing is a reactive curing method, which is essentially different from the curing of general EVA type hot melt adhesives and water-based adhesives. According to the theory of bonding theory, it is believed that there are bonding mechanisms such as mechanical theory, adsorption theory, diffusion theory, electrostatic theory, and chemical bond theory. In contrast, chemical bonding gives a higher strength of adhesion than bonding. In general, EVA hot melt adhesives have mechanical, adsorption, diffusion and other bonding theories; while PUR adhesives have more chemical bond bonding, which reacts with the substrate to form an "indivisible whole", so PUR can be cured. Excellent bond strength produced.
Third, the application of hot melt adhesive in book binding and its advantages and disadvantages
1. Application in bookbinding
With the increasing awareness of environmental awareness and health, people no longer only pay attention to the function and application of adhesives, and the requirements for environmental protection of adhesives are becoming more stringent, and environmentally friendly products that meet the three requirements of “environmental protection, health and safety” are developed. Green adhesives are imperative. People's living standards continue to improve, especially in Europe and the United States, people's pursuit of printing products tends to be fashionable, novel, and personalized, which makes the production process more and more complex, product quality requirements are getting higher and higher. As a new type of adhesive, PUR hot melt adhesive has the characteristics of no solvent volatility, heat and cold resistance, solvent resistance and fatigue resistance. These are in line with the needs of current social development.
In the printing and packaging industry, PUR hot melt adhesives are used in bookbinding, plastic printing, plastic-plastic lamination, and plastic box manufacturing. PUR hot melt adhesives are PUR prepolymers with active terminal groups. For printed paper, cross-linking occurs when the active terminal is exposed to moist air or the paper reaches a certain humidity. After the prepolymer is wetted, it can penetrate into the paper well and cross-link with the fiber molecules in the paper to achieve higher viscosity and higher paper tension. In the past 20 years, thanks to the development of the wireless binding process, the application of hot melt adhesives in bookbinding has been rapidly developed. EVA hot melt adhesive has good adhesion and elasticity. At present, most of the bookbinding hot melt adhesives on the domestic market are EVA hot melt adhesives. However, EVA type hot melt adhesive is expensive and has the disadvantages of memory, so it tends to close when you want to open the book, and tends to open when you want to close it, or form a groove or book at the binding when the book is read. on. The polyurethane hot melt adhesive has simple production process and low cost, which can make up for the above defects. The adhesive has good elasticity and toughness, high bonding strength, and the adhesive can be repeatedly melted and used, and the product utilization rate is high, and has broad application prospects.
2. Main application advantages
In book binding applications, compared with traditional EVA hot melt adhesives, PUR hot melt adhesives have fast curing, easy to use, low sizing temperature, impact and fatigue resistance, good flattening, durability, and strong adhesion. Advantages such as environmental protection.
For post-print bookbinding, PUR hot melt adhesive can reach the strength of ordinary EVA hot melt adhesive after 6 hours, and the strongest bonding effect can be achieved after 48 hours. Paper coating and materials with different surface properties such as ink, varnish coating, UV-cured layer, and plastic film do not have a great influence on the binding properties of PUR hot melt adhesive. Compared with traditional EVA adhesives, Finished product functional testing (such as book pull pages, uncovering pages) results will be more than 60% higher. The traditional EVA hot melt adhesive bookbinding sizing temperature is about 160-180 ° C, PUR hot melt adhesive sizing temperature is low, only 120-150 ° C, very suitable for high temperature sensitive, variable material applications, can It saves energy and increases the operational safety of workers. For thicker paperback books, traditional EVA hot melt adhesives require lock processing for books, while PUR hot melt adhesives can reduce this process, save costs, and shorten the production lead time. PUR hot melt adhesive layer is soft, which makes the book easy to read and fold. As a network cross-linking compound, it has excellent performance against shock vibration and fatigue resistance. For some long-term books such as reference books and manuals, The flattening of books required for textbooks, cookbooks, etc. (the degree to which books are opened), PUR hot melt binding is undoubtedly the best choice. Due to the high cross-linking reaction of PUR hot melt adhesive, it has high heat resistance, low temperature resistance, solvent resistance, water resistance and other excellent adhesion properties. For some long-term preservation books such as photo albums, photo yearbooks, and high-end cultural relics. Words and calligraphy, Bible books, etc., PUR hot melt adhesive can fully meet the requirements of these books. PUR hot melt adhesives have excellent adhesion and are about 50% less or less than conventional EVA hot melt adhesives. EVA hot melt adhesive should reach a certain strength (the thickness of the back of the book is based on the paper condition and the thickness of the book). The glue layer is controlled between 0.6 and 2 mm. Under the same circumstances, the PUR adhesive layer can be used in the same thickness as the EVA hot melt adhesive, generally at a thickness of about 0.3 mm. PUR hot melt adhesive composition does not contain solvents, and the curing process does not cause pollution of volatile solvents, which is the future development goal of environmentally friendly adhesives.
3. Main application shortcomings
Although the performance characteristics of polyurethane hot melt adhesives are very prominent, there are still some technical problems that need to be solved, such as: lowering the melting temperature and melt viscosity; improving the initial tack; enhancing the flexibility, heat resistance and flame retardancy of the product. The operation and operation process of PUR hot melt adhesive is higher than that of traditional EVA adhesive. It is necessary to avoid contact between the adhesive and the air in the air. Therefore, nitrogen should be protected in the preheated melt tank. Since the amount of glue applied to the PUR hot melt adhesive can be less than that of the EVA hot melt adhesive, the running stability of the machine is relatively high. During the heating process of PUR hot melt adhesive, a small amount of harmful irritating gas (isocyanate) may be volatilized, which requires high temperature control and ventilation system. When the PUR hot melt adhesive melts and cures and the cross-linking reaction is solidified, it cannot be reheated and melted and reused, and the EVA hot melt adhesive can be repeatedly used as a plastic resin.
Fourth, the development trend of polyurethane hot melt adhesive for bookbinding
Since the successful research of polyurethane hot melt adhesives in the 1930s, the annual average growth rate of polyurethane hot melt adhesives in the market reached more than 11% in 2007, becoming the third fastest growing adhesive for waterborne adhesives and EVA hot melt adhesives.
The development of PUR hot melt adhesives for bookbinding is also very rapid. Since the late 1980s and early 1990s, the first generation of PUR hot melt adhesives began to be used for bookbinding, but at the time the books were glued to perfect performance three to six days before the book was cut, processed and transported. Limited to the synthetic technology at the time, during the melt operation, a large amount of residual odor (MDI, TDI) residual isocyanate monomer volatilization, which is relatively irritating to the operator. In the mid-to-late 1990s, the curing speed of the second-generation PUR hot melt adhesive was shortened to three days, and the isocyanate residual monomer was effectively controlled. Stepping into the 21st century, the third generation of moisture-curing polyurethane PUR hot melt adhesive has been further improved. After 30 minutes of bookbinding, the book can be cut and cured within 6-16 hours to achieve the best bonding effect.
Today, with rapid development, we are pursuing higher production efficiency and safety. It is clear that the third generation of PUR hot melt adhesives still failed to meet the production needs. Some internationally renowned large companies have made continuous efforts to shorten the curing time of PUR hot melt adhesives. The concept of "UV UV humidification" dual-cured PUR hot melt adhesive was proposed and started to be used in actual production. This hot melt adhesive is a monomer component that reacts with UV on the PUR molecular chain during the synthetic production process. After the book is glued, the UV energy lamp installed on the binding equipment is used to initiate the crosslinking reaction of the UV monomer component in the PUR hot melt adhesive, so that the PUR hot melt adhesive is initially solidified and then further reacted with the air moisture. Until fully cured. This kind of PUR hot melt adhesive can be combined with equipment for on-line production and cutting edge operation. It can achieve satisfactory results in 3 minutes, which is equivalent to the curing speed of ordinary EVA hot melt adhesive. Even this type of PUR hot melt adhesive can be applied to The binding of the round ridge hardcover book. At present, due to the relatively expensive price of such adhesives, it has not been promoted and applied. But I believe that in the near future, advances in technology and maturity of the process will surely bring new PUR hot melt adhesives that are cheaper and of higher quality.
With the progress of society, people are pursuing higher and higher cultural enjoyment and taste. Books, reading materials, magazines, etc. tend to be fashionable, personalized, durable, resistant to collection, leveling, and safer. So far, to meet the needs of people, PUR hot melt binding technology is the best choice. It can be predicted that reducing the price of PUR hot melt adhesive, improving the ease of use of PUR, and broadening the applicability to the hardcover binding process are the most important key points for the promotion of PUR.