What is PUR hot melt adhesive made?
Macromolecular polyol is the soft segment of PU structure, which will directly affect the flexibility and low temperature resistance of PUR hot melt adhesive. Polyester polyols contain a large number of ester groups and have high cohesive energy. The prepared PUR hot melt adhesive has excellent adhesion, good heat resistance and oil resistance, but poor water resistance. Polyether polyols contain a large number of ether linkages and are not easily hydrolyzed. The prepared PUR hot melt adhesive has excellent hydrolysis resistance, good low temperature resistance and high flexibility of the molecular chain, but due to its low internal polymerization energy, its adhesion is not good. Usually polyester/polyether polyol compounding is used to adjust the content of ester and ether groups, soft and hard segments, and then adjust
PUR hot melt adhesiveIt is mainly composed of macromolecular polyols, isocyanates, tackifying resins, small amounts of additives and fillers.
1. High molecular polyol
Macromolecular polyol is the soft segment of PU structure, which will directly affect the flexibility and low temperature resistance of PUR hot melt adhesive. Polyester polyols contain a large number of ester groups and have high cohesive energy. The prepared PUR hot melt adhesive has excellent adhesion, good heat resistance and oil resistance, but poor water resistance. Polyether polyols contain a large number of ether linkages and are not easily hydrolyzed. The prepared PUR hot melt adhesive has excellent hydrolysis resistance, good low temperature resistance and high flexibility of the molecular chain, but due to its low internal polymerization energy, its adhesion is not good. Polyester/polyether polyol is usually used to adjust the content of ester group and ether group, soft segment and hard segment, and then adjust the flexibility and adhesion of PUR hot melt adhesive. Although a single polyester or polyether has some defects in performance, PUR hot melt adhesives can also be controlled by crystallization kinetics. Therefore, some crystalline polyols can often be selected as starting materials in experiments.
2. Isocyanate
Common isocyanate types are aromatic isocyanates and aliphatic isocyanates. In PUR hot melt adhesives, MDI is commonly used. MDI has high reactivity and relatively little toxicity, so it can be used as a fast-reacting polyurethane adhesive. In addition, MDI has a symmetrical molecular structure and can be designed into a microphase structure in which the molecular chain is regularly ordered, so the elasticity, adhesive strength and wear resistance of the adhesive layer are excellent.
3. Tackifying resin
PUR hot melt adhesives can be added to tackifying resins by grafting or blending to rapidly increase viscosity, heat resistance and initial viscosity. The tackifying resin can change the viscoelasticity of the main resin, and different types of tackifying resins have different effects on the performance of the adhesive. First of all, the tackifying resin should be well dissolved in the PUR prepolymer without stratification and precipitation. Viscose resins are generally small molecule monomers or macromolecular polymers that can be reacted with PUR prepolymers to form polymers. Commonly used tackifying resins include acrylic resins, ethylene-vinyl acetate copolymers, and TPU.
4. Additives
Commonly used PU catalysts include organotin and tertiary amines. In actual operation and application, the combination of various catalysts can play a catalytic synergistic effect. For PUR hotmelts, one or more different catalysts are usually required to catalyze the different stages of the reaction. Some catalysts have a significant effect on the-NCO/-OH reaction, and some catalysts have a significant effect on the-NCO/H2O reaction. The selection and amount of catalyst also have an effect on the performance of PUR hot melt adhesives.
In addition to the additives described above, additives compatible with the prepolymer may also be added to the single component PUR hot melt adhesive depending on various factors such as application and composition. Examples thereof include a plasticizer, a thixotropic agent, a silane coupling agent, a latent curing agent, and an ultraviolet absorber.
5. Filler
It can thicken, harden, toughen, compatibilize, strengthen, flame-retardant, coloring and anti-corrosion in the filling glue. It can also increase the wear resistance, thixotropy, thermal conductivity and electrical conductivity of the material, and can also improve the resistance of the product. Aging performance. In addition, it can also reduce production costs and curing shrinkage, extend the pot life of the product, and broaden the application range of the product. Commonly used PUR adhesive fillers are titanium dioxide, carbon black, talc, calcium carbonate, fumed silica, etc. The additives and fillers have to be dried before being added to the PUR prepolymer in order to avoid the-NCO content falling below the theoretical design value.
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