Discussion on the application of the hottest polyu

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Discussion on the application of polyurethane hot melt adhesive in book binding

I. Introduction

polyurethane (PUR) adhesive refers to a kind of adhesive containing carbamate group (-nhcoo-) or isocyanate group (-nco-) in the molecular chain. Because -nco in its molecular chain can react with a variety of functional groups containing active hydrogen in the electronic universal testing machine for thermal insulation materials to form an interface chemical bond, it has extremely strong adhesion to a variety of materials, and has the characteristics of tough adhesive film, good impact resistance and flexibility, high peel strength, good ultra-low temperature resistance, oil resistance and wear resistance. Hot melt adhesive (hot melt adhesive for short) is a kind of solvent-free solid adhesive based on thermoplastic polymer. It melts into liquid state after heating, and is applied to the surface of substrate in the form of melt for bonding. Hot melt adhesive has the advantages of rapid bonding, non-toxic and pollution-free, energy conservation and environmental protection, convenient storage, transportation and use, low cost, wide range of applications, mainly including EVA, PE, random PP, polyurethane, etc. Polyurethane hot melt adhesives can be divided into two types: one is reactive hot melt adhesives, which can be divided into isocyanate terminated prepolymer type (wet curing type) and isocyanate containing prepolymer type (latent curing type); The other is thermoplastic polyurethane elastomer hot melt adhesive, which is crosslinked by hydrogen bonding in the composition, so that it has excellent elasticity and strength. After the former is heated and melted, it will react and crosslink and solidify when wet; The latter curing process mainly uses hydrogen bonding to produce physical cross-linking. Heating again will weaken the hydrogen bonding and turn into viscous liquid. Therefore, it can be heated repeatedly - cooled and solidified. The product can be recycled or reused. It is an environmentally friendly material

II. Introduction to the curing reaction mechanism of polyurethane hot melt adhesive

there is a reactive NCO functional group at the end of the molecular chain of PUR resin. Because the carbon nitrogen double bond activity in this functional group is very high, it will react with air or water vapor on the surface of the substrate to form an amino group, and release C02 at the same time. The amino group reacts with other NCO groups to form a structure with urea functional group. Because there is still active hydrogen in the urea structure, it will further react with the unreacted NCO group to form a biuret structure and complete the so-called cross-linking reaction. The 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 the bonding and curing of PUR adhesive is a reactive curing method, which is essentially different from the curing of general EVA hot melt adhesive and water-based adhesive. According to the bonding theory, it is considered that there are mechanical theory, adsorption theory, diffusion theory, electrostatic theory, chemical bond theory and other bonding mechanisms. Relatively speaking, chemical bond bonding will obtain higher strength than its bonding method. The general EVA hot melt adhesive has the bonding theory of mechanism, adsorption, diffusion and so on; While pur adhesive is more about chemical bond theory bonding, which reacts with the substrate to form an "indivisible whole", so it can produce excellent bonding strength after pur curing

III. application of hot melt adhesive in book binding and its advantages and disadvantages

1 Application in book binding

with people's increasing awareness of environment and health, society no longer only pays attention to the function and application of adhesives, and the requirements for environmental protection of adhesives are becoming more and more strict. It is imperative to develop environmental friendly green adhesives that meet the three requirements of "environmental protection, health and safety". With the continuous improvement of people's living standards, especially in European and American countries, the printing products pursued by people tend to be fashionable, novel and personalized, which makes the production process more and more complicated and the product quality requirements higher and higher. As a new kind of adhesive, PUR hot melt adhesive has the characteristics of solvent-free 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 adhesive is applied to the end of Book Packaging experiments, plastic plastic and paper plastic lamination of printed materials, plastic box manufacturing, etc

pur hot melt adhesive is a pur prepolymer with active terminal group. For printing paper, when the active terminal group is exposed to humid air or the paper reaches a certain humidity, cross-linking reaction will occur. After being wetted, the prepolymer can penetrate into the paper well and cross-linked 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 adhesive in book binding has developed rapidly. EVA hot-melt adhesive has good adhesion and elasticity. At present, most of the hot-melt adhesives for book binding in the domestic market are EVA hot-melt adhesives. However, EVA hot melt adhesive is expensive and has memory and other shortcomings, so it tends to close when you want to open the book, and open when you want to close it. When the book is read, it forms a groove in the binding or the book suddenly closes. Polyurethane hot melt adhesive has simple production process and low cost, which can make up for the above defects. The adhesive has good elasticity, toughness and high bonding strength. The adhesive can be melted and used repeatedly, and the product utilization rate is high. It has a broad application prospect

2. Main application advantages

in the application of book binding, compared with the traditional EVA hot melt adhesive, PUR hot melt adhesive has the advantages of rapid curing, simple use, low sizing temperature, impact resistance, fatigue resistance, good flattening, durability, strong adhesion, environmental protection and so on

for post press book binding, PUR hot melt adhesive can reach the nailing strength of ordinary EVA hot melt adhesive after 6 hours, and the strongest bonding effect can be achieved after 48 hours. The coating of paper and materials with large differences in surface properties such as ink, varnish coating, UV curing layer and plastic film will not have a great impact on the binding properties of PUR hot melt adhesive. Compared with traditional EVA adhesive, the functional test results of finished products (such as book page pulling and opening test) will be higher than 60%. The traditional EVA hot melt adhesive book binding sizing temperature is about ℃, and the pur hot melt adhesive sizing temperature is low, only need ℃, which is very suitable for the application of high temperature sensitive and variable materials, which can not only save energy consumption, but also increase the operation safety of workers. For paperback books with large thickness, the traditional EVA hot-melt adhesive needs to lock the books, while pur hot-melt adhesive can reduce this process, save costs and shorten the production lead time. Pur hot melt adhesive layer is soft, which makes it very convenient to read and fold books. As a cross-linked compound, it shows excellent performance in shock vibration resistance and fatigue resistance. For some long-term books such as reference books, manuals, teaching books, cookbooks, etc., PUR hot melt adhesive binding is undoubtedly the best choice for the flattening of books (the degree of opening Books). Due to the chemical crosslinking reaction of PUR hot melt adhesive, it has excellent adhesion properties such as high heat resistance, low temperature resistance, solvent resistance, water resistance, etc. for some long-term preserved books such as photo albums, picture yearbooks, high-end cultural relics calligraphy and painting, Bible books, 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 less than that of traditional EVA hot melt adhesive. EVA hot melt adhesive should reach a certain strength (the thickness of sizing on the back of the book depends on the condition of the paper and the thickness of the book). The adhesive layer should be controlled between 0.6 ~ 2mm. In the same case, the pur adhesive layer can reach the same strength as EVA hot melt adhesive at about 0.3mm, regardless of the thickness of books and papers. The composition of PUR hot melt adhesive does not contain solvents, and the curing process will not cause the pollution of volatile solvents, which is the future development goal of environmental friendly adhesives

3. Main application shortcomings

although the performance characteristics of polyurethane hot melt adhesive are very prominent, there are still some technical problems to be solved, such as: reducing the melting temperature and melting viscosity; Improve the initial adhesion; Enhance the flexibility, heat resistance and flame retardancy of products. The operability and operation process of PUR hot melt adhesive are higher than those of traditional EVA adhesive. It is necessary to try to avoid the contact between the adhesive and moisture in the air. Therefore, nitrogen should be used for protection in the preheating glue box. Due to the fact that the application amount of PUR hot-melt adhesive nailing can be less than that of EVA hot-melt adhesive, Jinan tension machine has high requirements for the stability of machine operation and the guarantee service quality. During the heating and use of PUR hot melt adhesive, a small amount of harmful irritant gas (isocyanate) may volatilize, which requires high temperature control and ventilation system of the melt adhesive

when pur hot melt adhesive melts and cools, crosslinking reaction occurs and solidifies, it cannot be reheated and melted for reuse, while EVA hot melt adhesive can be heated and used repeatedly as a plastic resin

IV. development trend of polyurethane hot melt adhesive for book binding

since the successful research in the 1930s, the average annual growth rate of polyurethane hot melt adhesive market demand has reached more than 11% in 2007, becoming the third fastest growing adhesive after water-based adhesive and EVA hot melt adhesive

pur hot melt adhesive for book binding is also developing rapidly. Since the late 1980s and early 1990s, the first generation of PUR hot-melt adhesive has been used in book binding, but at that time, the best performance was achieved three to six days after book sizing before Book trimming, post-processing and transportation. However, it was limited to the synthesis technology at that time. In the process of glue melting, a large amount of peculiar smell (MDI, TDI) remained isocyanate monomer volatilization, which greatly stimulated the operators. In the late 1990s, the curing speed of the second generation pur hot melt adhesive was shortened to three days, and the residual isocyanate monomer was effectively controlled. Entering the 21st century, the third generation of moisture cured polyurethane pur hot melt adhesive has been further improved. The book edge can be cut 30 minutes after book sizing and binding, and cured within hours to achieve the best bonding effect

today, with the rapid development, we pursue higher production efficiency and safety. Obviously, the third generation of PUR hot melt adhesive still fails to meet the needs of production. Some internationally renowned large companies have made continuous efforts to shorten the curing time of PUR hot melt adhesive. The concept of "UV plus moisture" double curing pur hot melt adhesive was put forward and began to be used in batch production. This hot melt adhesive is grafted with some monomer components that can react with UV on the pur molecular chain in the process of synthesis and production. After book sizing, the UV energy lamp installed on the binding equipment will cause a cross reaction of UV monomer components in pur hot melt adhesive, but the prospect is not optimistic, so that pur hot melt adhesive can be initially cured, and then further react with air moisture until it is completely cured. This kind of PUR hot melt adhesive can cooperate with the online production of equipment and the operation of cutting book edges, and the curing degree can achieve satisfactory results 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 round spine hardcover books. At present, due to the relatively expensive price of this kind of adhesive, it is still not popularized and applied. But I believe that in the near future, the progress of technology and the maturity of technology will bring people cheaper and higher quality new pur hot melt adhesive

with the progress of society, people are pursuing higher and higher cultural enjoyment and taste. Books, books, magazines, etc. tend to be of a higher level of fashion, personalization, durability, collectability, flattening and safety. So far, PUR hot melt adhesive binding technology is the best choice to meet people's needs. It can be predicted that reducing the price of PUR hot melt adhesive, improving the ease of use performance of PUR, widening the applicability to the hardcover binding process is pur's push

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