I. Introduction
Polyurethane (PUR) adhesive refers to a type of adhesive containing urethane groups (-NHCOO-) or isocyanate groups (-NCO-) in the molecular chain. Because -NCO in its molecular chain can react with a variety of active hydrogen-containing functional groups to form interface chemical bonds, it has extremely strong adhesion to various materials, has a tough film, good impact resistance, flexibility, and peel strength High, good low temperature resistance, oil resistance and wear resistance. Hot-melt adhesives (referred to as hot-melt adhesives) are a type of solvent-free solid adhesives based on thermoplastic polymers that melt into a liquid when heated and are applied to the surface of the substrate in the form of a melt for bonding. Hot melt adhesive has the advantages of rapid bonding, non-toxic and non-pollution, energy saving and environmental protection, convenient storage and transportation, low cost, wide application, etc., mainly including EVA, PE, random PP, polyurethane and so on. Polyurethane hot melt adhesives can be divided into two categories: one is reactive hot melt adhesives, which can be divided into terminal isocyanate group prepolymer type (moisture curing type) and blocked isocyanate group prepolymer type (latent curing type) ); The other type is thermoplastic polyurethane elastomer hot melt adhesive, which uses the hydrogen bonding in the composition to crosslink, so that it has excellent elasticity and strength. The former melts when heated and reacts with moisture to cause crosslinking and solidification; while the latter solidifies mainly through the use of hydrogen bonding to physically crosslink. When heated again, it weakens the hydrogen bonding and becomes a viscous liquid, so it can be heated repeatedly-cooling It is solidified and the product can be recycled or reused. It is an environmentally friendly material.
2. Brief introduction of curing mechanism of polyurethane hot melt adhesive
The PUR resin molecular chain has a reactive NCO functional group at the end. Due to the very high carbon-nitrogen double bond activity in this functional group, it will react with air or moisture on the surface of the substrate to form an amine group, and at the same time release C02, the amine group will then Reacts with other NCO groups to form a structure with urea (urea) functional groups. Since there is still active hydrogen in the urea structure, it will further react with the unreacted NCO group to form a biuret structure, completing the so-called cross-linking reaction. Its reaction mechanism with moisture is as follows:
-NCO + H2O → NH2 + CO2 → -NH-CO-NH
-NH-CO-NH- + OCN- → NH-CO-N-CO-NH-
As can be seen from the above, PUR adhesive bonding and curing is a reactive curing method, which is essentially different from the curing of general EVA 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. Relatively speaking, chemical bonding will achieve higher strength than its bonding method. General EVA hot melt adhesives have mechanical, adsorption, diffusion and other bonding theories; while PUR adhesives are more chemical bond theoretical bonding, which reacts with the substrate to form an "inseparable whole", so PUR can be cured after curing Produces excellent bonding strength.
3. The application of hot melt adhesive in book binding and its advantages and disadvantages
1. Application in book binding
With the increasing environmental awareness and health awareness of people today, society no longer only focuses on the function and application of adhesives, the requirements for adhesives' environmental protection are more stringent, and the development of environmental protection that meets the three major requirements of "environmental protection, health and safety" Green adhesive is imperative. People's living standards continue to improve, especially in Europe and the United States, people's pursuit of printed products tend to be fashionable, novel, and personalized, which makes the production process more and more complicated, and the 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, fatigue resistance and so on. These meet the needs of current social development.
In the printing and packaging industry, PUR hot melt adhesives are used in book binding, plastic-plastic printing, paper-plastic lamination, and plastic box manufacturing. PUR hot melt adhesive is a PUR prepolymer with active terminal groups. For printed paper, when the active terminal groups are exposed to humid air or the paper reaches a certain humidity, a cross-linking reaction will occur. After being wetted, the prepolymer 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 wireless binding technology, the application of hot melt adhesives in book binding has achieved rapid development. EVA hot melt adhesive has good adhesiveness and elasticity. At present, most book binding hot melt adhesives on the domestic market are EVA type hot melt adhesives. However, EVA hot melt adhesive is expensive and has shortcomings such as memory, so it tends to close when you want to open the book, and tends to open when you want to close it, and when the book is read, a groove is formed in the binding place or the book suddenly closes on. The production process of polyurethane hot melt adhesive is simple and the cost is low, which can make up for the above defects. The adhesive has good elasticity and toughness, high bonding strength, the adhesive can be melted and used repeatedly, the product utilization rate is high, and it 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, lower sizing temperature, impact resistance and fatigue resistance, good flatness, durability, strong adhesion, Environmental protection and other advantages.
For book binding after printing, PUR hot melt adhesive can reach the strength of ordinary EVA hot melt adhesive binding after 6 hours, and the strongest bonding effect can be achieved after 48 hours. Paper coating and ink, varnish coating, UV curing layer, plastic film and other materials with large differences in surface properties will not greatly affect the binding properties of PUR hot melt adhesives. Compared with traditional EVA adhesives, The results of functional tests of finished products (such as book pull and pull pages test) will be more than 60% higher. The traditional EVA hot melt adhesive book binding sizing temperature is about 160-180 ℃, PUR hot melt adhesive sizing temperature is relatively low, only 120-150 ℃, is very suitable for high? 创 舾 å° â’ Make-up and noisy Scratch Ï„ é¤? The tip of the pinch is more and more confusing to send the xiong die? ⑶ Iridium a little bit and said Mou Tongdi stepped on? Bath 6 Miscellaneous ran to wash the steps of the fence? Car? VA hot melt glue needs to be processed on the book, The PUR hot melt adhesive can reduce this process, save costs and shorten the production lead time. The PUR hot melt adhesive layer is soft, which makes it easy to read and fold the book. As a network cross-linking compound, it exhibits excellent resistance to shock, vibration and fatigue. For some long-term books such as reference books, manuals, PUR hot-melt glue binding is undoubtedly the best choice for the flatness of books required for teaching books, cooking books, etc. (the degree to which the books are opened). Due to the chemical 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 preserved books such as photobooks, photo yearbooks, high-end cultural relics Calligraphy and painting, Bible books, etc., PUR hot melt adhesive can fully meet the requirements of these books. PUR hot-melt adhesive has excellent adhesion, and the amount of adhesive used is about 50% or less than that of traditional EVA hot-melt adhesive. EVA hot melt adhesive should reach a certain strength (the thickness of the back of the book according to the paper and the thickness of the book). The adhesive layer should be controlled between 0.6 and 2 mm. Under the same circumstances, the PUR adhesive layer can be used regardless of book thickness or paper size. Generally, it can reach the same strength as EVA hot melt adhesive at about 0.3 mm. The composition of PUR hot melt adhesive contains no solvent, and the curing process will not cause pollution of volatile solvents, which is the future development goal of environmentally friendly adhesives.
3. Main application disadvantages
Although the performance characteristics of polyurethane hot melt adhesives are very outstanding, there are still some technical problems that need to be solved, such as: reducing the melting temperature and melting viscosity; improving the initial adhesion; enhancing the flexibility, heat resistance and flame retardancy of the product. PUR hot-melt adhesives require higher operability and operation technology than traditional EVA adhesives. It is necessary to avoid contact between the adhesive and moisture in the air. Therefore, nitrogen should be used in the pre-heated adhesive box for protection. Because the amount of sizing of PUR hot melt adhesive nails can be lower than that of EVA hot melt adhesive, the operation stability of the machine is relatively high. When PUR hot melt adhesive is heated and used, a small amount of harmful stimulating gas (isocyanate) may be volatilized, which requires high temperature control of the melt adhesive and a high ventilation system. After the PUR hot melt adhesive melts and sizing and cools, crosslinking reaction solidifies, it can not be reheated and melted and used again, and EVA hot melt adhesive as a plastic resin can be repeatedly heated and used.
4. Development trend of polyurethane hot melt adhesive for book binding
Since the successful research of polyurethane hot melt adhesive in the 1930s, the average annual growth rate of polyurethane hot melt adhesive market demand reached more than 11% by 2007, becoming the third fastest growing adhesive after water-based adhesives and EVA hot melt adhesives.
The development of PUR hot melt adhesive for book binding is also very rapid. Since the late 1980s and early 1990s, the first generation of PUR hot-melt adhesives began to be used in book binding, but at that time, the best performance was achieved three to six days after the book was sizing, and then the book trimming, post-processing and transportation, etc., but Limited to the synthesis technology at the time, during the operation of the melt adhesive, a large amount of odor (MDI, TDI) residual isocyanate monomer volatilized, which stimulated the operator more. By the middle and late 1990s, the curing speed of the second-generation PUR hot melt adhesive was shortened to three days, and the residual monomers of isocyanate were effectively controlled. In the 21st century, the third generation of moisture-curable polyurethane PUR hot melt adhesive has been further improved. The book can be cut in 30 minutes after sizing and binding, and cured in 6-16 hours to achieve the best bonding effect.
Today, with high-speed development, we are pursuing higher production efficiency and safety. Obviously the third-generation PUR hot melt adhesive still fails to meet the production needs. Some large internationally renowned companies have made continuous efforts to shorten the curing time of PUR hot melt adhesives. The concept of "UV ultraviolet humidification" dual-cured PUR hot melt adhesive was proposed, and began to be used in actual production in batches. This hot melt adhesive is in the process of synthetic production, grafting some monomer components that can react with UV on the PUR molecular chain. When the book is sizing, the UV energy lamp installed on the binding equipment is used to initiate a cross-linking reaction of the UV monomer components in the PUR hot melt adhesive, so that the PUR hot melt adhesive is initially cured, and then further reacts with air moisture Until fully cured. This kind of PUR hot melt adhesive can be matched with the equipment for online production and book-cutting operation, and the curing degree can reach a satisfactory effect within 3 minutes, which is equivalent to the curing speed of ordinary EVA hot melt adhesive. Even this type of PUR hot melt adhesive can also be applied to The binding of the round-back hardcover book. At present, due to the relatively high price of such adhesives, it still has not been promoted and applied. But I believe that in the near future, technological progress and maturity of the process will definitely bring people newer PUR hot melt adhesives that are cheaper and of higher quality.
With the progress of society, people's pursuit of cultural enjoyment and taste is getting higher and higher, and books, reading materials, magazines, etc. tend to be more fashionable, personalized, durable, resistant to collection, flattening, and safer. So far, to meet people's needs, PUR hot-melt adhesive 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 keys for the promotion and use of PUR.
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