Application of Polyurethane Hot Melt Adhesive in Bookbinding

I. Introduction

Polyurethane (PUR) adhesives refer to a class of adhesives that contain urethane groups (-NHCOO-) or isocyanate groups (-NCO-) in their molecular chains. Because of its molecular chain -NCO can react with a variety of active hydrogen-containing functional groups to form the interface chemical bond, has a strong adhesion to a variety of materials, has a tough film, impact resistance, flexibility, peel strength High, good resistance to ultra-low temperature, oil and abrasion resistance. Hot-melt adhesives (referred to as hot-melt adhesives) are a class of solvent-free solid adhesives based on thermoplastic polymers that are melted to a liquid state and applied to the surface of a substrate in the form of a melt for bonding. Hot-melt adhesives have the advantages of rapid bonding, non-toxic and non-polluting, energy-saving and environmental protection, convenient storage and transportation, low cost, and wide application. They mainly include 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 divided into isocyanate-based prepolymer type (moisture-curing type) and blocked isocyanate-based prepolymer type (precursor type Another type is a thermoplastic polyurethane elastomer hot melt adhesive, which utilizes hydrogen bonding in the composition to crosslink, thereby giving it excellent elasticity and strength. After the former is melted by heat, it reacts and crosslinks to cure in the presence of moisture; while the latter is cured by the use of hydrogen bonding to physically crosslink, and when it is heated again, it weakens the hydrogen bond and becomes a viscous liquid, so it can be repeatedly heated—cooled. Cured, the product can be recycled or used twice, it is environmentally friendly material.

Second, polyurethane curing adhesive curing mechanism

PUR resin has reactive NCO functional groups at the end of the molecular chain. Due to the very high carbon-nitrogen double bond activity in this functional group, it reacts with air or water on the surface of the substrate to form an amine group, and at the same time releases CO 2 and amine groups. Reaction with other NCO groups forms a structure with urea (urea) functional groups. Since there is still active hydrogen in the urea structure, it is further reacted 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-

It can be seen from the above that the adhesive curing of the PUR adhesive is a reactive curing method, which is essentially different from the curing of a general EVA hot melt adhesive or a water-based adhesive. According to the bonding theory, it is believed that there are adhesion mechanisms such as mechanical theory, adsorption theory, diffusion theory, electrostatic theory, and chemical bond theory. Relatively speaking, chemical bond bonding will obtain higher strength tack than its bonding method. General EVA hot melt adhesives have the bonding theory of mechanics, adsorption, diffusion, etc.; while PUR adhesives have more chemical bond bonding theory, which reacts with the substrate to form an “indivisible whole”, so the PUR can be cured The resulting excellent bond strength.

Third, the application of hot melt adhesive book binding and its advantages and disadvantages

1. Application in Bookbinding

With the increasing awareness of people's environmental awareness and health awareness today, the society is no longer focused on the function and application of adhesives. The environmental protection requirements of adhesives are increasingly stringent, and the development of three types of environment-friendly products that meet the requirements of “environmental protection, health, and safety” Green adhesives are imperative. People's living standards are continuously improving, especially in Europe and the United States. The printing products pursued by people tend to be fashionable, novel, and personalized, thus making the production processes more and more complicated and the product quality requirements higher and higher. As a new type of adhesive, PUR hot melt adhesive has the characteristics of solvent-free volatility, heat and cold resistance, solvent resistance and fatigue resistance. These meet the needs of current social development.

In the printing and packaging industry, PUR hot melt adhesives are used in bookbinding, plastics and plastics, paper and plastic lamination, and plastic box manufacturing. PUR hot melt adhesives are PUR prepolymers with active terminal groups. For printed paper, cross-linking reactions occur when the active terminal group is exposed to moist air or the paper reaches a certain humidity. After the prepolymer is wetted, it penetrates well into the paper and cross-links with the fiber molecules in the paper to achieve a 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 seen rapid development. EVA hot melt adhesive has good adhesiveness and elasticity. At present, the majority of book binding hot melt adhesives on the domestic market are EVA hot melt adhesives. However, EVA type hot melt adhesives are expensive and have disadvantages such as memory. Therefore, when they want to open a book, they tend to close. When they want to close, they tend to open. When a book is read, a groove is formed at the binding, or the book suddenly collapses. on. Polyurethane hot melt adhesive production process is simple, low cost, can make up for the above defects. The adhesive has the advantages of elasticity, good toughness, high bonding strength, adhesive can be repeatedly used for melting, high product utilization rate and wide application prospect.

2. The main application advantages

In book binding applications, PUR hot melt adhesives have a faster cure, are easier to use, have a lower sizing temperature, are more resistant to impact and fatigue, have better flatness, are durable, and have stronger bonding power than traditional EVA hot melt adhesives. Environmental protection and other advantages.

For post-press bookbinding, PUR hot melt adhesives can reach the strength of ordinary EVA hot melt adhesives 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 curing layer, and plastic film will not greatly affect the binding properties of PUR hot melt adhesives. Compared to traditional EVA adhesives, Finished functional tests (such as book pull, page break test) result will be higher than 60%. The traditional EVA hot melt glue book glue sizing temperature is about 160-180 °C, PUR hot melt glue sizing temperature is low, only 120-150 °C, very suitable for high temperature sensitive, volatile material applications, either Save energy and increase worker safety. For thicker paperback books, traditional EVA hot melt adhesives require stitching for books, while PUR hot melt adhesives can reduce this process, save costs, and shorten production lead times. PUR hot-melt adhesive layer is soft and can be used for reading and folding of books. As a cross-linking compound, it has excellent performance in impact vibration and fatigue resistance. For some long-term use books such as reference books and manuals. The leveling of books required for textbooks, cookbooks, etc. (to the extent that books are opened), PUR hot melt binding is undoubtedly the best choice. Due to the chemical crosslinking reaction of PUR hot melt adhesive, it has high heat resistance, low temperature resistance, solvent resistance, water resistance and other excellent adhesive properties. For some long-term preserved books such as photobooks, photo yearbooks, and high-grade cultural relics PUR hot melt adhesives can fully meet the requirements of these books. PUR hot melt adhesives have excellent adhesion with about 50% less or less glue than conventional EVA hot melt adhesives. EVA hot melt adhesive to achieve a certain degree of strength (back sizing thickness according to the paper and the book's different thickness) adhesive layer controlled between 0.6 ~ 2mm. In the same situation, the PUR adhesive layer can reach the same strength as the EVA hot melt adhesive, regardless of the thickness and thinness of the paper, generally about 0.3 mm. PUR hot melt adhesive composition does not contain solvent, curing process will not cause pollution of volatile solvents, environmental protection adhesive for the future development goals.

3 major application disadvantages

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: lower melting temperature and melt viscosity; increase the initial adhesion; enhance the flexibility, heat resistance and flame retardancy of the product. PUR hot melt adhesives have higher operability and operation requirements than conventional EVA adhesives. It is important to avoid contact between the adhesive and the moisture in the air. Therefore, nitrogen should be protected in the preheated adhesive tank. Because the PUR hot melt glue sizing can be less than the EVA hot melt glue, the stability of the machine is relatively high. PUR hot melt adhesive heating process may generate a small amount of harmful gas (isocyanate) volatilization, the melt temperature control, ventilation system requirements. When PUR hot melt adhesive melt sizing cooling cross-linking reaction curing, can not be re-heated and used again for melting, and EVA hot melt adhesive can be heated repeatedly as a plastic resin.

Fourth, book binding polyurethane hot melt adhesive development trend

Since the polyurethane hot melt adhesive was successfully researched in the 1930s, the average annual growth rate of the polyurethane hot melt adhesive market demand in 2007 reached over 11%, becoming the third fastest growing adhesive for waterborne adhesives and EVA hot melt adhesives.

The development of PUR hot melt adhesives for bookbinding has also been rapid. Since the late 1980s and early 1990s, the first generation of PUR hot melt adhesives began to be used in book binding, but the best performance was only achieved three to six days after the book was slashed. Book trimming, post-processing and transportation can be performed. Confined to the synthesis technology at that time, a large amount of odor (MDI, TDI) residual isocyanate monomer was volatilized during the operation of the melted rubber, which greatly stimulated 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. In the 21st century, the third generation of moisture-curing polyurethane PUR hot melt adhesives is further improved. Book binding can be cut in 30 minutes after bookbinding and cured in 6-16 hours to achieve the best bonding effect.

Today's rapid development, the pursuit of higher productivity, safety. It is clear that the third-generation PUR hot melt adhesive still fails to meet production needs. Some internationally renowned companies have made continuous efforts to shorten the curing time of PUR hot melt adhesives. The concept of "UV UV Moisture" dual-cured PUR hot melt adhesive was proposed and started to be used in batches for actual production. This hot melt adhesive is a process in which a monomer component capable of reacting with UV is grafted onto the PUR molecular chain. After the sizing of the book, the UV energy lamp installed on the binding device is used to initiate the crosslinking reaction of the UV monomer components in the PUR hot melt adhesive, so that the PUR hot melt adhesive is initially cured and then reacts further with air moisture. Until completely cured. This PUR hot melt adhesive can be used in conjunction with equipment production, cutting edge operation, curing in 3 minutes to achieve satisfactory results, and the curing speed of ordinary EVA hot melt adhesive is equivalent, even this type of PUR hot melt adhesive can also be applied to Bound hardback book binding. At present, due to the relatively expensive price of such adhesives, it still cannot be applied. However, I believe that in the near future, technological advances and the maturity of the process will surely bring people new PUR hot melt adhesives that are cheaper and of higher quality.

With the progress of society, people are increasingly pursuing cultural enjoyment and taste. Books, books, magazines and other trends tend to be fashionable, personalized, durable, resistant to collection, leveling, and safer. So far, to meet people's needs, PUR hot melt binding technology is a best choice. It can be predicted that reducing the price of PUR hot melt adhesives, improving the ease of use of PUR, and widening applicability to the hardcover binding process are the most important keys for PUR promotion.