1. Hot melt adhesive
Hot melt adhesive generally refers to a solid at room temperature, which is heated and melted into a liquid. After coating, wetting the adherend, pressurizing, and cooling, the adhesive completes the bonding within a few seconds. Hot melt adhesive is a multi-component mixture made of thermoplastic resin or rubber as the main material, together with other auxiliary materials. In most cases, hot melt adhesives contain no water or solvents and are 100% solid adhesives.
The main advantages of hot melt adhesives are:
a. The lamination speed is fast, which is convenient for continuous automatic high-speed operation;
b. No solvent hazards, no burning, environmentally friendly products;
c. No drying process is required, the lamination process is simple, and the product quality is easy to control;
d. The product itself is sturdy and easy to package, transport and store;
e. Better bond strength and flexibility.
Among them, EVA (ethylene vinyl acetate) type hot melt adhesives are widely used in Europe, but due to their high temperature resistance, the use temperature is higher (170 ° C to 200 ° C), so the shape of plastic doors and windows. Can not be used, European scientists have finally developed a polyurethane hot-melt adhesive with better bonding reaction after years of testing.
2. Reactive polyurethane hot melt adhesive
To introduce reactive polyurethane hot melt adhesives, we first need to introduce reactive hot melt adhesives. Hot melt adhesive is a 100% solids one component adhesive. Flowable after heating, easy to apply and wet on the adherend. Once cooled, it solidifies immediately and creates cohesion that holds the two adherends together. Their low contamination, high initial tack and fast tack are well received by the modern automated assembly industry and are developing rapidly. However, due to the thermoplasticity of the main resin itself, it is prone to creep due to heat and has limited heat resistance; cold brittleness affects mechanical strength and performance; high melting temperature makes it unsuitable for heat-sensitive adherends, which limits its application in modern industry. .
Inspired by the molecular structure of reactive binder resins, scientists introduced reactive reactive groups into thermoplastic resin molecules. The hot melt adhesive formed from the adhesive is applied to the adherend and reacted by reactive groups. It cures and forms a thermoset resin; this improves bond strength, heat resistance, solvent resistance, chemical resistance and creep resistance. This adhesive, which combines the properties of hot melt and reactive adhesives, is called reactive hot melt adhesive (RHMA).
There are two types of reactive polyurethane hot melt adhesives (PUR): hot melt moisture curing and hot melt heat reaction. The hot-melt heating reaction type is also a closed plastic polyurethane. The hot-melt moisture-curable polyurethane adhesive is mainly used here. There are many kinds of it, and the main component is an isocyanate-terminated polyurethane prepolymer. The characteristics of this reactive polyurethane hotmelt adhesive (PUR) are as follows:
(1) Solvent free, one liquid type. Unlike solvent-based adhesives that require a drying process, there are no problems of environmental pollution and poisoning caused by solvents. The bonding process is simple, and it can be applied by scraping or spraying, and is suitable for various automated assembly lines.
(2) It can be painted at low temperature. The melting temperature of polyurethane reactive hot-melt adhesive is lower than that of ordinary hot-melt adhesive (170-200℃), and it can be applied at low temperature of 100-150℃. It not only saves energy but also reduces the corrosiveness of the sizing device. Especially suitable for the bonding of heat-sensitive materials such as plastics.
(3) Good operability. The two adherends can be fixed in a short time, so the assembly can be quickly transferred to the next processing step, improving work efficiency.
(4) The exposure time is long. In recent years, researchers in various countries have been working to extend the research on reactive hot melt adhesives and their exposure times (adjustable from 5 seconds to 60 minutes) to the assembly of large-area complex parts.
(5) Long pot life: The pot life of 150°C at 90°C can reach 16 hours and 3 days.
(6) Excellent heat resistance, cold resistance, water vapor resistance, chemical resistance and solvent resistance. The properties of the listed hot melt adhesives are greatly improved compared to the original hot melt adhesives.
