Hot Melt Adhesive - Open Vs Set Time II
Too many variables, such as adhesive temperature, Hot Melt Adhesive amount, finger pressure, and the paper used, may affect the test results. For the same hot melt adhesive, different people at different labs may obtain different results. As a result, most numbers reported for hot melt adhesives are simply for reference only.
Sophisticated equipment, called the Olinger glue bond tester, is available in the market for determining open time and set time. The tester is able to control temperature, the size of glue bead, and preset the time required for bonding and separation process. Based on this microprocessor controlled machine, lab people will be able to more precisely determine the open time and set time resulting from various substrates.
Rheological determination offers another scientific insight into understanding and comparing open time and set time for different hot melt adhesives. During the cooling of an hot melt adhesive, those crystalline segments (PE backbone) can set immediately without offering any open time for bonding purpose. Those amorphous segments (VA branches), on the other hand, can retard the solidification of the adhesive matrix and offer different degrees of open time and set time depending upon the compatibility of selected EVAs and tackifying resins.
According to a “time sweep” rheological measurement from high temperatures for a molten adhesive, during cooling, the G’ (elastic modulus, cohesion) will first increase steeply to a certain value which is resulted from the crystalline segment, and level off at a lower plateau region for a certain period of time, and then continue to increase to another higher plateau region. The adhesive will eventually set at this higher plateau with a minimal slope. The open time can be defined as the range (time) that G’ is below a certain value. Beyond this value, the adhesive no longer has the capability of wetting those bonded substrates. If the adhesive is bonded onto substrates before the assumed G’ value within the open time range, then, the set time is defined as the G’ value that reaches the higher plateau region.
Although the rheological measurement dose not provide the actual open time and set time in seconds as those determined by human finger or a bond tester, this scientific evaluation can precisely reveal the hot melt adhesive’s energy and structure change at every single moment during the cooling stage. Based on this scientific information, scientists can easily compare the rheological property difference of each formulation during cooling process and apply it to distinguish the length of open time and set time.
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