What is PUR hot melt adhesive?
PUR is the abbreviation of Polyurethane Reactive. It belongs to the polyurethane family. Different PUR types have different chemical properties. Polyurethane hot-melt adhesives have two categories: one is thermoplastic, and the other is reactive.
The former one is heated and then hardened by cooling, to form a bond. The latter is heated and then cross-linked and solidified, by the reaction of cooling and moisture, to produce a bond. We call thermoplastic type as polyurethane hot melt adhesive (TPU).
The reactive polyurethane hot-melt adhesive (PUR) has two types: moisture-curing and closed-type polyurethane hot-melt adhesive. What we often mention about PUR type, is a moisture-curing reactive polyurethane hot melt glue.
PUR has excellent bonding strength, temperature, chemical and aging resistance. It has extensive usage in packaging, wood processing, automotive, textile, elector-mechanical, aerospace industries.
Features of PUR hotmelts
PUR contains chemically active urethane groups (-NHCOO-) or isocyanate groups (-NCO). It also has materials containing active hydrogen. It has excellent adhesion to porous materials such as wood, leather, fabric, paper, ceramics, as well as smooth surface materials such as plastic, metal, glass, rubber, etc.
During bonding process, application machine heats PUR hot melt adhesive into a fluid. Its chemical reaction is inhibited for easy coating. The two adherends are laminated, and then cooled and has bonding power. The moisture in the air reacts with PUR, and extends chemical chain to generate a high cohesive polymer. This significantly improves the adhesion, high and low temperature resistance. Its high reactivity promote excellent adhesion to a variety of materials.
PUR has different shapes, like film, tape, powder. The material surface solution volatilizes and becomes a hot melt adhesive film shape. The PUR tape is backed by a reinforcing material or carrier. The film structure is supported by adhesive itself.
Difference to common-used hot melt glue
Most hot melt adhesives use resins such as ethylene vinyl acetate (EVA), polyester, and polyamide. EVA and polyester hot melt adhesives has poor strength and elasticity. Therefore, they cannot withstand too much external force.
Polyamide hot melt adhesives have high melting point and hardness. They have limitation in application. The urethane PUR contains, is unstable at application temperature. Polyurethane hot melt adhesives types are not many, and the application is limited. Polyurethane has not achieved much success as a hot melt adhesive.
In recent years, the research on polyurethane hot-melt adhesives has become active again in various countries. They have made new developments, in aspects of varieties and properties improvement.
Advantages of PUR hot melt glue:
- Solvent free. Solvent-based adhesives require a drying process. PUR does not need this. There is no environmental pollution or poisoning problems caused by solvents. It meets the requirements of environmental protection.
- Easy to operate. Bonding time is short. The assembly can go to the next processing quickly. It improves work efficiency.
- Excellent resistance to heat, cold, vapor, chemicals and solvents. The cross-linked structure greatly improves the chemical properties as well as the bond strength.
PUR curing mechanism
Moisture-curing polyurethane hot melt adhesive has base resin of terminal-NCO-based prepolymer. It also consists of thermoplastic resin, tackifying resin, antioxidant, catalyst, filler. It also contains other additives that do not react with isocyanate group. After cooling and bonding, moisture in air reacts with NCO groups, and compounds containing active hydrogen. This generates cross-linked polymers with high cohesion. It further enhances adhesion.
PUR hot melt applications
- Automotive structures and components such as windshield seals and light assembly;
- Textile industry and footwear industry, such as fabric seam bonding and sole bonding. It has great water resistance, softness and comfort.
- Book Binding. Due to the flexibility, the flatness of the book can be high when opened. It can prevent the formation of grooves in the binding. Or it help reduce the sudden closing of the book during reading. It is more superior than EVA binding.
- Food packaging industry. It can withstand high and low temperature, during disinfection treatment. It can pass food hygiene regulations.
- Wood processing and furniture industry. The adhesive layer has good water resistance and aging resistance. It does not pollute nor corrode wood.
- Electronic and electrical industry. Quickly cured, no glue required. Curing shrinkage rate is small. It is especially suitable for the bonding of precision components.
Future Development of PUR hotmelts glue
Although PUR performance is very outstanding, there are still some technical problems that need to be solved:
- Reduce melting temperature and melt viscosity;
- Increase initial tack force, shorten positioning time;
- Enhance product flexibility, heat resistance and flame retardancy;
- Shortage and high cost of PUR applicators.
The next step should focus on improving the creep resistance flexibility, reducing melt viscosity, and raise heat resistance. In the meanwhile, improve preparation process and gluing equipment.