Dicyclopentadiene (DCPD)-based hydrocarbon resins are widely used as tackifiers in many applications. In particular, hydrogenated DCPD-based hydrocarbon resins are frequently used in premium hot-melt adhesives (HMAs) and hot-melt pressure-sensitive adhesives (HMPSAs) because of their water-white color and excellent stability to light and heat.
Such resins are characterized by their softening point, degree of modification with aromatic olefins and degree of hydrogenation. When the tackifiers are formulated with an elastomeric polymer, these characteristics have a bearing on the adhesive performance.
Hydrogenated DCPD ResinsThere are two main types of DCPD-based hydrocarbon resins: DCPD resins, which predominantly contain DCPD; and DCPD-C9 resins, which contain substantial amounts of aromatic unsaturated components such as styrene, a-methylstyrene, vinyltoluene, or aromatic fractions. The hydrogenation of hydrocarbon resins affords resins that are very light or water-white in color and that possess remarkable stability to light and heat.
In this study, various hydrogenated DCPD resins were used as tackifiers to afford styrene-isoprene-styrene (SIS)- and styrene-butadiene-styrene (SBS)-based HMPSAs. Details about the characteristics of the tackifiers are listed in Table 1.
A TA instruments AR 2000 was used to measure the rheological properties of the HMPSAs. Specimens with a thickness and diameter of 2 and 8 mm, respectively, were prepared and measured at a shear rate of 1 Hz. Testing temperatures ranged from -50 to 120°C, and the heating rate was 7°C/min.
Results and DiscussionEvery tackifier used in this study can easily tackify SIS and be used to formulate HMPSAs. Figure 1 shows comparative test results of the adhesive performance of SIS-based HMPSAs containing different tackifiers.
From the viewpoint of being PSAs, all adhesives have sufficient peel strength adhesion and tackiness; however, some trends were identified. Among tackifiers having the same softening point, those with higher aromatic content had higher peel strength and loop tack compared to those with low or no aromatic content. For example, SU-400, SU-100, SU-100S, and SU-500 have the same softening point; however, SU-400, which has the highest aromatic content, has relatively higher peel strength and loop tack. In addition, tackifiers having a high softening point increase the peel strength and loop tack of SIS-based HMPSAs.
Rheological measurements can be used to describe the characteristics of adhesives and to clarify the tack and high-temperature performance. The glass-transition temperature (Tg) indicates the point at which a polymer changes from a glassy to a rubbery material. The Tgis considered to be the point at which the tan d curve is maximum; hence, the adhesives have different glass-transition temperatures depending on the tackifiers used in their formulation.
Rheological analysis such as a temperature sweep test also provides information about the adhesive’s holding power at high temperatures. Generally, the magnitude of the crossover temperature (where G’ = G” and tan d = 1) of PSAs correlates very well to the actual experimental shear adhesion failure temperature (SAFT).
The softening point of the tackifier affected the low-temperature properties (e.g., Tgof PSAs) and high-temperature properties (e.g., crossover temperature and softening point of PSAs). The molecular weight of the tackifier mainly affected the low-temperature properties. The aromatic H wt% affected only high-temperature properties.
Figure 2 shows the temperature ranges from Tgto the crossover temperature, as calculated from the rheological analysis of SIS-based HMPSAs. Depending on the characteristics of the constituent tackifiers, the formulated adhesives have different workable temperatures as PSAs. SU-500, a tackifier with no aromatic content, has wider range of workable temperatures than SU-400, a tackifier with high aromatic H wt%, even though both have the same softening point. In tackifiers with higher softening points, the workable temperature range of the PSA shifts to a higher temperature.
Figure 3 shows comparative test results of the adhesive performance of SBS-based HMPSAs using different tackifiers. In these results, only some of the tackifiers exhibit tackiness at room temperature. Tackifiers having an aromatic H wt% higher than 3% and/or a softening point lower than 110°C have significant loop tack strength at room temperature; SU-490, SU-400, H-2300 and SU-90 are examples of such tackifiers.
ConclusionThis article shows the correlation between the characteristics of tackifiers and the adhesive performance of styrene-block-copolymer-based HMPSAs. The higher the softening point of the tackifier, the higher the Tg, softening point and crossover temperature of the PSAs. High aromatic H wt% content reduces the high-temperature resistance of PSAs, as suggested by the decrease in the crossover temperature and softening point of the PSAs. Only some tackifiers that have high aromatic H wt% content and/or low softening point exhibit tackiness at room temperature when they are used to formulate SBS-based HMPSAs.
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