The Cloud Point of Hot Melt Adhesive
“Cloud Point” is the temperature at which a hot melt adhesive becomes cloudy on cooling and is a measure of compatibility of the formula components. Hot melt adhesives (HMA) with a lower cloud point normally impart longer open time and better hot tack. However, a hot melt adhesive possessing a very low cloud point may present a certain degree of undesired surface tack.
Following is the simplest method to determine the cloud point of hot melt adhesive.
1. Heat up the hot melt adhesive sample to a clear (transparent) molten state, typically at 325℉-350℉, in a beaker.
2. Dip the mercury end of a glass thermometer into the molten adhesive mass.
3. Remove the thermometer from the molten adhesive mass, immediately rotate the thermometer downward slowly, let extra amount of adhesive flow back to the adhesive mass.
4. Watch the cloudiness formation at the tip of the thermometer.
5. Record the temperature at the thermometer when the adhesive turn cloudy.
Although this is a cost free, very simple and subjective test, the repeatability is good and relatively accurate with a careful observation.
If the VA (vinyl acetate) domain of EVA exhibits better compatibility with the incorporated tackifying resins, the blend remains clear for a longer time during cooling and results in a lower value of cloud point. A blend of EVA with high VA content and rosin esters may form transparent hot melt adhesives, even though they are fully set at room temperature. Normally, a more polar tackifying resin blended with an EVA will present a lower cloud point.
When waxes are included in the formulation, their softening points become the primary factor affecting cloud point. The cloud point of a micro-crystalline wax is lower than that of poly-ethylene wax and synthetic paraffin wax, due to its more cyclic (polar) molecular structure.
If an EVA-based hot melt adhesive is pigmented with fillers, it is impossible to perform the same test as described above. An alternative way is to perform a temperature scan from molten to solid state with a rheometer. Below is a suggested test method.
1. Hot melt adhesive is melted first in between a pair of stainless steel parallel plates.
2. Step cooling the tested sample with an oscillatory frequency under a large strain.
3. Observe the change of G” (viscous modulus) or η’(elastic viscosity).
4. When the slope of the G” curve suddenly changes, upward, upon cooling, the temperature at the intersection of two slopes reveals the point where the incompatible species in the formulation abruptly phase out from the adhesive matrix. This is similar to the concept of determining the cloud point by our bare eyes.
Based on the rheological measurement, disregarding the color, clarity, and any ingredients used in hot melt formulations, the temperature where G” showing an abrupt change can be considered as a good indicator to describe the compatibility of EVA and incorporated tackifiers.
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