Most hot melt pressure sensitive adhesives (HMPSAs) are designed or developed for use at room temperature or in an air conditioned environment. A HMPSA exhibits excellent room temperature pressure sensitive adhesion, such as peel and tack, but may show insufficient or non-acceptable performance at sub-ambient or low temperatures. If good adhesion performances at both room and low temperatures are required, can HMPSA formulators develop optimum products to satisfy this requirement?
In the North American and western European countries, even though the weather and temperature may vary dramatically at different geographic zones and seasons, most HMPSAs are designed for use inside an air conditioned room. Some low temperature grade HMPSAs may be designed particularly for use inside a refrigerator and/or freezer, but are not for use at room temperature. However, this situation is not the case in other countries, such as India and China. Air conditioning is not always available in a plant, an office, and a room disregarding the weather and seasons. HMPSAs designed specifically for use at room temperature (70-75F), may fail in the winter time. An HMPSA possessing a wider temperature servicing range is therefore needed.
Is it possible to develop a formulation for use at both room temperature and low temperatures? To answer this question, we first need to understand why an adhesive can provide pressure sensitive adhesion at room temperature? Although there are so many hypotheses explaining the cause of pressure sensitivity of a material at room temperature, recently, most adhesion researchers believe the major cause of this unique behavior is primarily related to the glass transition temperature (Tg) of the adhesive formulation. Based on rheological evaluations, Tg is designated by the temperature at the peak (maximum) value of the bell-shape Tan delta curve at low temperature zone. Polymer molecular chains show the largest free volume or the greatest flow character at the vicinity of Tg.
The Tg of most general purposed, room temperature use HMPSAs ranges from 0-10, based on a rheological testing with an oscillatory frequency of 10 radian/sec (about 1.59 Hz). Within this temperature range, tack and peel force normally lower with the decrease of Tg. To improve the low temperature flexibility or adhesion performances, the Tg of HMPSAs must be tailored to low temperature regions. Theoretically, when the entire Tan delta curve is shifted horizontally to the low temperature end, Tg is decreased. In other words, a greater flow/wetting; better flexibility, and larger energy dissipation upon separation of HMPSAs will occur at lower temperatures. Therefore, low temperature adhesion performances are improved. Unfortunately, when the Tan delta curve is shifted to the low temperature direction, the Tan delta value at room temperature area is lowered. This means the flow/wetting character of the HMPSA at room temperature is reduced and pressure sensitive adhesion performances are also lowered.
According to the above description, theoretically, it is difficult to obtain excellent adhesion performances at both room and low temperatures simultaneously. Technically, if formulators are able to raise and broaden the bell shape Tan delta curve, the HMPSA will offer a broader range of PSA performances. Two formulating tips are as follows. First, to raise the value of Tan delta peak and entire curve, an SIS with higher percentage of Isoprene mid-block and SI di-block are recommended. Secondly, to broaden the Tan delta peak area, molecular weight distribution of Isoprene mid-block should be wider; Isoprene mid-block and selected tackifiers should be less compatible.