Open time and set time are two key processing parameters for EVA-based hot melt adhesives (HMA) which will significantly affect the extent of instantaneous bonding and eventual fracture energy. It is not necessary to offer long open time and fast set time for any application. The optimum open time and set time are determined by the application system, actual bonding process, and bonded substrates.
- Open time is the time after adhesive is applied during which a serviceable bond can be made. Many factors affect open time, including temperature, substrate, adhesive, and amount of adhesive applied.
- Set time is the time it takes to form an acceptable bond when two or more substrates are combined with an adhesive.
It is easy to define open time and set time described above. In most HMA labs, however, the open time and set time are often determined by touch, which is a very fast but subjective test. Test methods are described as follows.
- Completely melt a HMA in a beaker at 325F.
- Dip a glass rod inside the molten adhesive until it reaches the equilibrium temperature.
- Remove the glass rod from the beaker and immediately apply a thin layer of adhesive on a piece of paper.
- Immediately press fingers on the adhesive film and count the time in seconds until finger print and separation force is diminished.
- Those finger print numbers are recorded as open time.
- Completely melt a HMA in a beaker or a jar at 325F.
- Tilt the beaker or jar and pour a long bead of HMA on a piece of Kraft paper.
- Laminate another piece of Kraft paper on the top of the coated one immediately.
- Slowly separate the two pieces of paper and count the time when one of the Kraft paper is torn. Record the time required to tear paper as set time.
Too many variables, such as adhesive temperature, adhesive amount, finger pressure, and the paper used, may affect the test results. For the same HMA, different people at different labs may obtain different results. As a result, most numbers reported on TDS for HMAs 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 HMAs. During the cooling of an HMA, 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, assuming 3 x 10E6 dyne/cm2. 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 adhesive’s energy and structure change at every single moment during the cooling stage. Based on this scientific information adhesive – 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.
For more information call or email Pierce Covert,
Glue Machinery Corporation