Polyurethane (PUR) adhesives refer to a class of adhesives containing urethane groups (-NHCOO-) or isocyanate groups (-NCO-) in the molecular chain. Because -NCO in its molecular chain can react with a variety of functional groups containing active hydrogen to form interfacial chemical bonds, it has strong adhesion to a variety of materials, and has tough film, impact resistance, good flexibility, and peel strength. High, good ultra-low temperature resistance, oil resistance and wear resistance and other characteristics. Hot-melt adhesive (referred to as hot-melt adhesive) is a kind of solvent-free solid adhesive based on thermoplastic polymer. Hot melt adhesive has the advantages of rapid bonding, environmental protection and no pollution, energy saving and environmental protection, convenient storage, transportation and use, low cost, and wide application, mainly including EVA, PE, random PP, polyurethane, etc. Polyurethane hot melt adhesives can be divided into two categories: one is reactive hot melt adhesives, which can be further divided into terminal isocyanate group prepolymer type (moisture curing type) and blocked isocyanate group prepolymer type (latent curing type). ); the other is thermoplastic polyurethane elastomer hot melt adhesive, which is cross-linked 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 cross-link and solidify when exposed to moisture; while the latter's curing process mainly uses hydrogen bonding to produce physical cross-linking. When heated again, it will weaken the hydrogen bonding and become a viscous liquid, so it can be repeatedly heated - cooled. Cured, the product can be recycled or reused, and it is an environmentally friendly material.
There is a reactive NCO functional group at the end of the molecular chain of the PUR resin. Since the carbon-nitrogen double bond in this functional group is very active, it will react with air or moisture on the surface of the substrate to form an amine group, and at the same time release CO2, and the amine group will regenerate. Reacts with other NCO groups to form structures 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 to complete the so-called cross-linking reaction. Its reaction mechanism with moisture is as follows:
It can be seen from the above that the bonding and curing of PUR adhesives 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. Relatively speaking, chemical bonding will obtain higher strength adhesion than its bonding method. General EVA hot melt adhesives have bonding theories such as mechanical, adsorption, and diffusion; while PUR adhesives are more based on chemical bond theory bonding, which reacts with the substrate to form an "indivisible whole", so PUR can be cured after curing. resulting in excellent bond strength.
With the increasing awareness of people's environment and health, the society no longer only pays attention to the function and application of adhesives, and the requirements for the environmental protection of adhesives are becoming more and more strict. Green adhesives have become imperative. With the continuous improvement of people's living standards, especially in European and American countries, the printing products that people pursue 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 type of adhesive, PUR hot melt adhesive has the characteristics of solvent-free 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-plastic, paper-plastic lamination, and plastic box manufacturing.
PUR hot melt adhesive is a PUR prepolymer with active terminal groups. For printing 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 the prepolymer is wetted, it can penetrate into the paper well, and undergo cross-linking reaction 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 binding hot melt adhesives in book binding has achieved rapid development. EVA hot-melt adhesive has good adhesion and elasticity. At present, most of the book binding hot-melt adhesives on the domestic market are EVA-type hot-melt adhesives. However, EVA type 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. When the book is read, a groove is formed at the binding or the book is suddenly closed. superior. 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, can be repeatedly melted and used, has high product utilization rate, and has broad application prospects.