1、 Concept of PUR hot melt adhesive
Pur (polyurethane reactive), the full Chinese name is moisture curing reactive polyurethane hot melt adhesive. The main component is isocyanate terminated polyurethane prepolymer. The adhesion and toughness (elasticity) of PUR are adjustable, and has excellent bonding strength, temperature resistance, chemical corrosion resistance and aging resistance In recent years, it has become one of the important varieties of adhesive industry. Now it is widely used in packaging, wood processing, automobile, textile, electromechanical, aerospace and other national economic fields.
1.2 properties of polyurethane hot melt adhesive
Reactive hot melt adhesive is heated and melted into fluid under the condition of inhibiting chemical reaction, so as to facilitate coating; After the two adherends are bonded and cooled, the adhesive layer agglomerates and plays a bonding role; Then, with the help of the moisture existing in the air and the moisture attached to the surface of the adherend, it reacts and expands the chain to form a high polymer with high cohesion, which significantly improves the adhesion, heat resistance and low temperature resistance. Because of its high reactivity, it shows excellent adhesion to a variety of materials. It is widely used in the bonding of laundry top cover, disinfection cabinet top cover, book binding, car lights, furniture edge sealing, shoe making, etc.
Pur adhesive is an urethane group (- nhcoo -) or isocyanate group (- NCO) containing polarity and chemical activity in its molecular structure. It has excellent adhesion with materials containing active hydrogen, such as porous materials such as wood, leather, fabric, paper and ceramics, and surface cleaning materials such as plastics, metals, glass and rubber.
1.3 type of polyurethane hot melt adhesive
According to chemical properties, polyurethane hot melt adhesives can be divided into two categories: one is thermoplastic polyurethane hot melt adhesive, and the other is reactive polyurethane hot melt adhesive. The former is cooled and solidified after heating and liquefaction, and the latter is crosslinked and solidified through cooling and moisture reaction after heating and liquefaction. Thermoplastic polyurethane hot melt adhesive is also known as hot melt polyurethane hot melt adhesive (TPU). Reactive polyurethane hot melt adhesive (PUR) can be divided into wet curing polyurethane hot melt adhesive and closed polyurethane hot melt adhesive.
There are other classification standards for polyurethane hot melt adhesive. For example, according to shape, it can be divided into three types: adhesive film, adhesive tape and powder. The solution volatilizes on the surface of the material to be bonded and becomes a hot-melt adhesive film. The adhesive tape is backed by reinforced material or carrier, and the adhesive film is supported by the adhesive itself.
2、 Differences and relations between PUR and similar products
In the past, most hot-melt adhesives were prepared from ethylene vinyl acetate (EVA), polyester, polyamide and other hot-melt resins. Due to the poor strength and elasticity of EVA and polyester hot melt adhesive, it can not bear too much external force; Due to the high melting point and hardness of polyamide hot melt adhesive, it is limited in use. As a hot-melt adhesive, polychlorinated ester has always been considered to be less successful. Because its carbamate bond is unstable at application temperature, there are few varieties of polyurethane hot-melt adhesive and its application field is relatively narrow. In recent years, the research on polyurethane hot melt adhesive is active again, which makes the hot melt adhesive have new development in variety and performance.
Compared with EVA hot melt adhesive and aqueous / solvent adhesive, PUR has the following advantages:
1. Solvent free, one liquid type. Unlike solvent based adhesives, which do not need a drying process, there is no environmental pollution and poisoning caused by solvents, and meet the requirements of environmental protection. The bonding process is simple and can be applied by roller coating or spraying
2. Good operability. The two adherends can be fixed in a short time, so the assembly can be quickly transferred to the next processing process to improve work efficiency.
3. Excellent heat resistance, cold resistance, water vapor resistance, chemical resistance and solvent resistance. Compared with ordinary hot melt adhesive, the listed properties and bonding strength are greatly improved due to the cross-linking structure of reactive hot melt adhesive.
3、 Curing mechanism of polyurethane hot melt adhesive
3.1 thermoplastic polyurethane hot melt adhesive
More polyester polyols are used for thermoplastic polychlorinated ester hot melt adhesive, and the physical properties are better than those made of polyether polyols. MDL and HDI (non discoloration) are used for polyisocyanates. Thermoplastic polyoxyester is mixed with additives such as colorant, plasticizer and stabilizer to make adhesive film, tape or powder. In the past, this kind of glue was mostly used for fabric lamination and laminated bonding of composite films. After modification, it can be used to bond a variety of porous, smooth or rough materials, such as glass, metal, fabric and leather. It can also be used to install the liquid crystal box on the indicating part of electronic instruments. The peel strength of bonded textiles is 39 ~ 49kN / m, and it has good water washing and dry cleaning resistance. It can be used for the bonding of textiles and glass.
3.2 reactive polyurethane hot melt adhesive
In 1984, reactive polyurethane hot melt adhesive appeared in the American market, marking the new development of polyurethane hot melt adhesive technology.
3.2.1 wet curing polyurethane hot melt adhesive
The wet curing polyurethane hot melt adhesive is made of NCO terminated prepolymer as base material, thermoplastic resin, tackifying resin, antioxidant, catalyst, filler and other additives that do not react with isocyanate group. The adhesive is heated and melted into a fluid, coated on the surface of the adhered substrate, and the two adherends are bonded. After cooling, the adhesive is formed. Then, the moisture in the air or the trace moisture in the adhered substrate and other compounds containing active hydrogen react and expand the chain with NCO groups to form a polymer with cross-linked network structure and high cohesion, so as to further enhance the adhesive force. The - NCO group in Isocyanates can easily react with compounds containing active hydrogen.
3.2.2 closed polyurethane hot melt adhesive
A closed prepolymer is formed by sealing the end NCO group in the polyurethane prepolymer with a sealing agent under certain conditions. In fact, the free NCO group is protected so that it has no reaction activity at room temperature, which increases the storage stability of the glue: when heated to a certain temperature, the sealing agent dissociates, and the active NCO group regenerates, which can be cross-linked by chemical reaction with active hydrogen containing compounds such as polyols, amines, water, etc. Common sealing agents include oximes, phenols, alcohols, sodium bisulfite, pyrrolidone, etc.
4、 Application of reactive polyurethane hot melt adhesive
Since the dissociation temperature of the sealing agent is mostly above 100 ℃, it often causes bubbles in the adhesive layer, so the closed polyurethane hot-melt adhesive is only used for maintenance treatment. Although the wet curing reactive hot melt adhesive has excellent properties, its popularization is limited because it must strictly isolate moisture during manufacturing, storage, transportation and sizing. In recent years, due to the breakthrough progress of technology and equipment, these problems have been gradually solved, and the application is gradually increasing.
In general, the reasons for the application of hot-melt polychlorinated ester are: strong reliability, good uniformity, simple bonding process and less waste (the unused glue can be saved and reused later). In addition, because the hot melt adhesive does not use organic solvents and will not pollute the environment, it is welcomed by users.
Polyurethane hot melt adhesive can be designed for different grades and specifications. Its initial bonding time can be adjusted from a few seconds to tens of minutes. The yield strength of the adhesive film can reach 13.7mpa and the elongation can reach 600%. This product can adapt to the changes from large plate coating to automatic production line coating, and has been used in a variety of industries in developed countries.
Reactive Pu hot melt adhesive is the most widely used. Reactive polyurethane hot melt adhesive can not only bond porous materials, such as foamed plastics, ceramics, wood, fabrics, etc., but also bond materials with smooth surfaces, such as steel, aluminum, stainless steel, metal foil, glass, plastic, leather, rubber, etc. It also has high cohesive strength. The ratio of raw materials can be adjusted according to needs to obtain a series of adhesives from flexible to rigid.
Reactive polyoxyester hot melt adhesive can be cured under environmental conditions, which is lower than the application temperature of traditional hot melt adhesive. In short, it is a hot melt adhesive with good comprehensive performance and has been applied in the following industries:
(1) Automobile structure and parts, such as windshield seal and lamp assembly:
(2) Textile industry and footwear industry, such as fabric seam bonding and sole bonding, have the advantages of water resistance, softness and comfort:
(3) The book is bound wirelessly. Due to the flexibility of the adhesive layer, it can make the flatness of the book high when turning over, and prevent the formation of grooves at the binding place or the sudden closing of the book during reading, which is superior to EVA binding;
(4) The food packaging industry can withstand the high and low temperature disinfection treatment specified by food hygiene I
(5) In wood processing and furniture industry, the adhesive layer has good water resistance and aging resistance, and does not pollute and corrode wood:
(6) In the electronic and electrical industry, the adhesive layer solidifies rapidly without glue, and the curing shrinkage is small. It is especially suitable for the bonding of precision components in the electronic and electrical industry.
5、 Problems and Prospect of reactive polyurethane hot melt adhesive
5.1 existing problems
Although the performance characteristics of reactive polyurethane hot melt adhesive are very prominent, there are still some technical problems to be solved, such as reducing melting temperature and melting viscosity; Improve the initial adhesion and shorten the positioning time; Enhance the flexibility, heat resistance and flame retardancy of products. In addition, the preparation process and equipment and gluing equipment are still lacking and imperfect, and the cost is relatively high.
As a new generation of adhesive with excellent properties, reactive polyurethane hot melt adhesive has broad application fields and development prospects. Based on the current research results, the next step should focus on improving its comprehensive properties, improving the creep resistance of hot melt adhesive, improving flexibility, reducing melt viscosity and improving heat resistance and flame retardancy. At the same time, the preparation process and gluing equipment should be improved. This kind of high-performance environment-friendly green glue not only has great economic benefits and social value, but also has positive significance for promoting the green consumption of the whole people.