Many characterization techniques are used to measure the bond strength of hot-melt pressure-sensitive adhesives (HMPSAs). These calculations are usually performed by measuring the force required to debond the adhesive holding two substrates together. Measurement procedures vary by substrate, contact time, area and force used to make the bond; debonding procedure variables include speed and the debonding angle.
PSAs are typically soft and tacky at temperatures higher than their glass-transition temperature (Tg); conversely, they are firmer at lower-than-ambient temperatures. This allows tack to be measured at standard evaluation conditions (23°C and 50% relative humidity).
Unlike HMPSAs, hot-melt non-pressure-sensitive-adhesives (HMNPSAs) do not develop tack at room temperature. Instead, they become tacky when they are applied at high temperatures; after cooling to room temperature, tack is lost. Therefore, procedures commonly used to measure tack cannot be used to evaluate these materials.
In some packaging processes, such as carton box forming and closure using HMAs, optimal process productivity depends on the right application temperature for optimal cohesion. Long and costly trial-and-error tests are usually used to find optimal application temperature for NPSAs for those applications.
This article discusses probe-tack measurements and rheological results at high temperatures for HMNPSAs formulated with Dynasol styrene-butadiene-styrene (SBS) copolymers.
ExperimentalAdhesives were formulated using SBCs with different structures, molecular weights and compositions (see Table 1). Toluene solution viscosity can be used as reference of the polymer’s molecular weight.
Adhesives were formulated and evaluated under controlled temperature and humidity conditions (23°C and 50% humidity). Formulated adhesives were applied to 0.002-inch PET film. Texture analyzer TA-XT Plus was used to measure the probe tack at different temperatures. The flat, stainless-steel probe is 8 mm in diameter and was put in contact with a NPSA film at a fixed load (100 g), then removed at 0.05 mm/sec.
For the dynamic mechanical measurements, parallel-plate systems with 8 mm plates (low temperature) and 25 mm plates (high temperature) were used in conjunction with the liquid nitrogen chamber to control the temperature from the glassy to the terminal region of each adhesive. The samples were evaluated at a frequency of 10 rad/s and a strain of 0.3-3%; the temperature was increased at 3°C/min from 0°C to 150°C.
Results and DiscussionSBCs can easily be formulated as hot-melt pressure-sensitive or non-pressure-sensitive adhesives. Figure 2 shows comparative characterization results for the adhesives proposed in Table 2, which are based on three linear SBS/SB polymers. F-01, based on an SBS/SB polymer with high molecular weight and 10% of diblock (S-4301), developed higher viscosity and a higher softening point.
F-02 has an SB/SBS polymer with 80% of diblock and lower molecular weight; hence, the viscosity is reduced. An SBS triblock polymer with styrene content of 40% provides an excellent cohesive strength for adhesives, as demonstrated by F-03.
Several factors enable the production of high tack, including fluidity (to allow good surface wetting) or a certain viscosity (to permit flow over the substrate’s surface to make good contact). Therefore, it’s important to measure the probe tack as a function of temperature to optimize productivity and performance.
In addition, the tack phenomenon requires the adhesive to act as a viscous liquid. In this case, the materials reached the maximum value as the temperature increased and the viscosity was reduced.
Rheological measurements can be used to describe the characteristics of adhesives as well as to understand the phenomenon of tack. Dynamic mechanical analysis (DMA) has been an excellent method for determining thermal transitions.
The length of the rubbery plateau is a function of the molecular structure. As shown in Figure 4, F-01 exhibits a higher elastic modulus (G’) and larger rubbery plateau, indicating that the adhesive has a higher cohesive strength. However, when the modulus becomes too large, the ability to wet the substrate and the degree of tack is reduced.
Hot-melt adhesives are expected to wet quickly and develop a high degree of tack; however, the cohesive strength is crucial, with the balance of properties the ultimate goal.
ConclusionThis article shows that NPSAs develop optimal performance at temperatures higher than those specified for standard adhesion evaluations. In addition, probe tack tests, when used as a function of temperature and rheological measurements, can provide useful information to optimize processing, performance, and application of adhesives.
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