I believe that the manufacturers of hot melt pressure sensitive adhesive know that the most commonly used term to describe the anti-aging properties of hot melt pressure sensitive adhesive is: thermal stability.
Thermal stability refers to the ability of hot melt pressure-sensitive adhesive to maintain no obvious changes in color, viscosity or adhesive properties during a long time of melting.
The thermal stability of hot-melt pressure-sensitive adhesive will be affected by the chemical structure and composition of the raw materials used, the production process of the hot-melt pressure-sensitive adhesive manufacturer, the setting temperature of the melting machine and the use environment of the end user.
Most polymers and adhesives possess specific molecular structures and physical properties prior to aging. In order to ensure the best thermal stability of hot-melt pressure-sensitive adhesive, manufacturers need to pay special attention to the quality of adhesive raw materials and the overall production process conditions.
Usually, hot melt pressure sensitive adhesives must be melted into a fluid at 170°C before being coated. The duration of hot melt pressure-sensitive adhesive at a particular high temperature is mainly determined by the size of the melt groove and the speed of entering and leaving the adhesive.
The adhesive can show better heat resistance and maintain stable performance for a long time when the adhesive is fused in the melt groove and applied by the slit die extrusion or spray adhesive system using the closed or nitrogen-injected melt adhesive system.
Under high temperature environment, most adhesives do not contact with air, only a very small amount of adhesives on the melt surface will oxidize with air. However, through the roller coating machine adhesive, because the hot melt pressure sensitive adhesive has been in contact with the air, will show poor thermal stability.
In addition to the operating environment, we also have to consider the molecular structure of the material.
Materials that are saturated (or hydrogenated) or of low polarity usually exhibit better heat resistance. Because the single bond (σ-bond) and the hydrogenated double bond (π-bond) require very high temperatures to open.
On the other hand, unsaturated double bonds of polymer and viscosifier are easier to open and can generate free radicals (R?) under high temperature and shear above 170°C.
Once formed, free radicals are spontaneously oxidized by oxygen in the surrounding air, forming unstable peroxide radicals (ROO?). These peroxide radicals immediately react with unreacted hydrocarbons, stripping protons to form hydroperoxides (ROOH). ROOH further splits into two unstable reactive species, RO? and HO?.
These two free radicals will react with two unreacted hydrocarbons, stripping protons to form stable water (H2O) and alcohol (ROH), but at the same time regenerate into two new free radicals (R?) to continue a new cycle of oxidation.
In short, the "aging resistance" or "stability" of hot-melt pressure-sensitive adhesive depends on the following factors: the quality of the material used by the adhesive manufacturer, the mixing conditions and methods used in the production of hot-melt pressure-sensitive adhesive, the coating system and the setting of the melting and coating temperature, and the storage time of the product by the end user
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