What is the “aging” performance of hot melt adhesives? The most common terminology to describe “aging” performance is “heat stability” and some refer to this as “Pot Life”.
Heat stability is the ability of a hot melt adhesive to be held in a molten state for extended periods of time without changing in color, viscosity or bond characteristics. Common testing methods call for the evaluation of color, viscosity, and appearance changes after 96 hours (4 days) at 350°F. Polymers and tackifiers have their own specific molecular structures. These hot melt polymers and tackifiers show very different heat stability after heating at 350°F for 96 hours.
Heat stability is affected most by the chemical composition of the hot melt, the hot melt manufacturers’ production methods and the end users consumption over time and temperatures of the hot melt prior to use.
To insure maximum heat stability users of hot melt need to be most concerned with the quality of the adhesive and the production methods. Identical hot melt formulation produced in different mixing systems may result in dissimilar heated stability, adhesion performance and heat resistance. For instance, when a formulation is produced under a nitrogen blanket or vacuum, very minimal adhesives are oxidized. As a result, it may show very good heat stability. On the other hand, if the same formulation is made in an open system without a nitrogen blanket and/or vacuum, the resulted properties may be poor and varied between batches.
Equally as important are the end users’ temperature settings of their own hot melt systems as well as usage rates and exposure to oxygen. Hot melt adhesives prior to application must be melted at elevated temperatures of 325F +. The amount of time a hot melt is kept at a specific temperature is typically based on the size of the tank, usage and filling rates. To simulate the worst aging condition heat stability tests are performed at 350°F for 96 hours without any usage or agitation.
Using a closed hot melt system where the adhesive is melted in a tank and applied with a slot die, extrusion or spray system; adhesive may exhibit better heat resistance and maintain consistent performances and heated stability for a longer time. This is because adhesive is not exposed to the air at elevated temperatures during process. Minimal adhesive oxidation occurs. Hot melts applied by a roll coater will show less heat stability due to constant exposure to air.
When we compare the heat stability of various hot melt adhesives; we actually are dealing with materials molecular structure. A saturated (hydrogenated) or less polar material generally exhibit better heat resistance. Those double bonds (π-bonds) of hydrocarbon polymers and tackifiers tend to open and create free radicals upon heating at a temperature beyond 325°F. However, the single bond (σ-bond) and those double bonds being hydrogenated are very strong and require very high temperature to break them apart.
At a typical working temperature, they normally do not form free radicals and therefore are not oxidized. Natural resins like rosin, rosin esters, and most rosin derivatives contain certain ratio of free acid functional group (denoted by acid number) and unsaturated double bonds. Therefore, these natural tackifiers are very unstable and tend to hydrolyze and oxidize when they are exposed to oxygen and air, even though they are stored at room temperature.
In conclusion, hot melt “Aging”, “Heat Stability” and “Pot Life” are determined by the quality of materials used by the adhesive manufacturer, the method of mixing the components that make up the hot melt, the actual temperature of the components used to process the hot melt at the end user’s application, and the time the hot melt is held at that temperature at the end user’s factory.