What are appropriate temperature settings for hot melt tanks, hoses, and guns? These are the most frequently asked questions by hot melt users.
Many hot melt applicator and adhesive providers suggest users set the initial temperature at 325° for every zone of the hot melt system. Users frequently ask if they can set a temperature higher than 325° when the viscosity of a selected hot melt adhesive is very high and is hard to apply.
With higher temperatures the double bond (or π-bond) of a polymeric molecule tends to open, creating two free radicals (when the temperature exceeds 325°). Those free radicals are very unstable or reactive; and will spontaneously react with the surrounding oxygen from the air. As a result, hot melt adhesives are oxidized and aged upon heating in the melting tank.
It is noticeable that most hot melt adhesives basically do not form free radicals at temperatures lower than 325°. When hot melts in the melt reservoir are protected with a nitrogen blanket or under vacuum, they will not be oxidized even though free radicals exist in the adhesives.
To insure precise application and limit adhesive degradation with high viscosity adhesives, set the tank at a maximum of 325° and raise the temperature of the hose and gun first. Since adhesives inside the hose and guns are not exposed to air or oxygen; they will have no chance to be oxidized. Actually, users can raise the temperature of melting tank above 325F but should always maintain the adhesive level at 80-90% of the total tank depth and insure that the adhesive is used regularly.
Most melting tanks are heated with heating pads, cartridge heaters or cast in heaters. These are typically focused on the lower portion or the bottom of the melting tank. If users constantly charge ambient hot melt adhesive into melting tank to maintain an adhesive level, the solid room temperature adhesive will also lower the temperature of the molten adhesive particularly at the surface level In fact, the temperature of those adhesives on the top layer, which are exposed to the air, will not exceed the tank set temperature. Consequently, adhesive users may increase the setting temperature for the melting tank as well without worrying too much about overheating of adhesives. To prevent oxidation, always keep in mind that only those adhesives exposed to air (surface level) should be maintained at a temperature below 325°.
Melt On Demand tanks are extremely effective at keeping adhesive in the tank at lower temperatures prior to use and should be considered.
In summary, we recommend that adhesive in a melter reservoir does not exceed 325° if it exposes to air. If hot melt adhesive users can ensure the temperature of the adhesive at the top or surface of the tank is lower than 325° through consistent filling/use, then the tank can be operated at higher temperature levels than 325°.
For more information call or email Pierce Covert,
Glue Machinery Corporation
1(888)202-2468 info@gluemachinery.com
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.
For more information call or email Pierce Covert,
Glue Machinery Corporation
1(888)202-2468 info@gluemachinery.com