Ask Dr. Dave: We noticed that there are a lot of brown particle in the dispenser we use for hot-melt adhesives. What is it?
Examining char in hot-melt adhesives.
Question: In the dispenser we use for hot-melt adhesives, we noticed that there are a lot of brown particles in the adhesive when it is received in the head. What is happening? Can the formulation burn? The adhesive bond’s heat resistance is good.
Answer: First, make sure that your adhesive tank is thoroughly cleaned. However, my experience over the years with hot-melt adhesives is that this can occur with badly formulated adhesives, particularly with ethylene-vinyl acetate (EVA) resins. When such an adhesive is heated at high temperatures for long periods of time without being dispensed, it tends to oxidize. In addition, thickening of the adhesive (increased viscosity) is often observed and charring (i.e., brown or black discoloration) occurs. This often happens when hot-melt machines are left with the heat on over the weekend or during vacation periods.
Most likely, you have a poorly formulated product. Modern adhesives from reputable suppliers have the correct heat stabilizers incorporated in the adhesive to prevent this from happening.
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