Blending Resins For SIS-Based Nonwoven, Hot Melt Adhesives

This article investigates the effect on adhesive performance of blending a fully hydrogenated hydrocarbon resin with different grades and amounts of pure monomer aromatic hydrocarbon resins. It also reports the results of blending these resins to reproduce the effect of a chemical-resin modification (partial C₉-resin hydrogenation) on SIS-based (styrene-isoprene-styrene) hot melt adhesive performance.

The results show that blends of aromatic and aliphatic hydrocarbon resins can be used to modify polymer and resin compatibility in SIS-based nonwoven, hot melt adhesives. In fact, they can effectively replace and can be a competitive alternative to partially hydrogenated C₉ resins in these systems.

One of the basic hypotheses of the blending concept is that resins seem to act like nonvolatile solvents for most polymers, in which case their polymer solubility or compatibility can be adjusted by mixing them with each other.

Strengthening SIS Hot Melt Adhesives with Aromatic Resins

A series of tests compared the performance of an SIS-based hot melt adhesive system containing 250 parts of a hydrogenated hydrocarbon against formulations containing increasing amounts of pure monomer aromatic hydrocarbon resins blended with the other resin. This approach determined the influence of the softening point (glass transition temperature or Tg) and/or the molecular weight distribution of the various pure monomer resins at a constant formulation rate.

Table 1 compares the performance of a control formulation with formulations that substituted 25 parts, or 10 percent, of the hydrogenated hydrocarbon resin with a variety of pure monomer resins. Formulations 1, 2 and 3 contain substituted resins having softening points equal to, or lower than, 100°C. Formulation 4 includes a substituted resin with a softening point greater than 100°C at 152°C.

Room-temperature adhesion results, which mainly indicate differences in SIS midblock compatibility, show hardly any difference between formulations containing the various pure monomer resins with regard to peel and loop tack. At first, the effect of substituting only 10 percent of the hydrogenated hydrocarbon with a pure monomer resin seems quite limited. However, comparing cohesive performance (shear-adhesion properties) at various temperatures produces a totally different picture.

Cohesive performance of the hot melt adhesive at 23°C improves considerably by substituting the hydrogenated hydrocarbon resin with pure monomer aromatic resin, even with low-softening-point grades. This may be caused by the fact that these aromatic resins swell the polystyrene domains. At 40°C, cohesion is improved only when the pure monomer resin used has at least a 100°C softening point. Lower-softening-point grades plasticize the polystyrene endblock at this elevated temperature.

At 70°C, the polymer/resin interaction changes again, as shown in Table 1. Pure monomer resins with softening points of less than 120°C appear to decrease shear value. The lower the softening point of the aromatic resin, the more pronounced the decrease. This is caused by an increasing solubility toward the styrene endblock phase of the polymer, which delays the order-disorder of the styrene domains. A similar behavior occurs when changing the degree of hydrogenation of mixed-aromatic hydrocarbon resins.

Only the pure monomer resin having the 152°C softening point does not follow the trend. Because it is an endblock-reinforcing resin, it accelerates the phase-separation process between polymer midblocks and endblocks, thus resulting in higher shear values.

Doubling the amount of pure monomer resin substituted for the hydrogenated mixed-aromatic resin to 50 parts, or 20 percent of the resin constituency in the formulation, produces basically the same trends, even at high temperatures. Table 2 (page 29) shows the results.

Incorporation of a Liquid, Pure Monomer Resin

In another set of tests illustrated by Table 3 (page 29), a liquid, pure monomer resin substituted for part of the pure monomer resin can bring loop-tack adhesion back to the level of the standard formulation without any aromatic resin, while also improving shear values below 70°C. These data show that blending resins can produce quite interesting results in SIS-based hot melt adhesive systems.

The effect on formulation performance of steadily increasing the amounts of a pure monomer resin appears in Table 4. The resin was selected for having a softening point identical to that of the hydrogenated mixed-aromatic hydrocarbon in the standard formulation to eliminate any differences in the resin phase. Formulations 1 through 4 substitute 25, 50, 75 and 100 parts respectively of a pure monomer resin for the original resin. Results indicate that cohesive strength increases with higher levels of the pure monomer resin. At substitution levels of 50 phr or less, adhesion to steel, measured by peel and loop-tack tests, is retained. Above this level of pure monomer resin, adhesion decreases.

Influence of Degree of Resin Hydrogenation

A similar study on the effect of aromatic-resin substitution also was conducted by using partially hydrogenated resins in place of fully hydrogenated Regalite® R101 hydrocarbon resin (Table 5). The softening point of the resins used was kept constant at 100°C. In this case, aromatic structures were introduced chemically into the adhesive system, rather than by physical blending.

It is apparent that the data in the two tables show similarities, but there are also differences. In general, peel and loop-tack properties decrease, and cohesive properties at temperatures below 70°C increase. One difference is tan delta peak temperatures and tan delta peak values. Based on experience from other tests, decreasing the degree of hydrogenation produces higher tan delta peak temperatures and lower tan delta peak values.

Experience also suggests that the 100 parts of the pure monomer resin blended with 150 parts of the hydrogenated resin shown in Formula 4 in Table 4 would interact only with the styrenic endblocks, thus significantly decreasing tan delta temperature of the continuous phase because there is less resin available for midblock modification. But that did not happen. In general, the tan delta peak temperature does not change, but the tan delta peak values do decrease. Since the glass transition of the continuous phase does not change when substituting increasing amounts of pure monomer resin for the hydrogenated resin, it appears the pure monomer resin is pulled into the midblocks. Figure 1 shows this dynamic clearly.

Another difference in the test results shown in Tables 4 and 5 is that the effect of blending with the pure monomer resin on adhesion-cohesion balance at ambient temperatures seems stronger than by changing the degree of hydrogenation of the mixed-aromatic hydrocarbon resins. Comparing loop-tack and shear performance of the formulas in the two tables clearly shows this. At equivalent loop-tack values, cohesive strength is much higher when aromaticity is introduced by blending. The comparatively broader molecular weight distribution of the pure monomer resin to the hydrogenated resin is the probable reason for this.

Other interesting differences between degrees of hydrogenation and resin blending become apparent when comparing viscosity trends. The lower the degree of hydrogenation, the lower the viscosity, especially at 120°C. Blending with the pure monomer resin seems to follow the same trend at first, but viscosities increase again as the percentage of pure monomer resin increases.

One probable reason for this is that although the solubility toward the styrene domains will increase with a higher amount of pure monomer resin, compatibility with the isoprene midblock will decrease. This outcome seems stronger with blending than with partial hydrogenation. Since only about six percent of the formulation consists of styrene endblocks and about 19 percent is the isoprene-midblock phase, there will be a point in the cooling curve that the decreasing midblock solubility will be the dominating factor for adhesive viscosity. It seems that this point occurs earlier with blending. Nevertheless, using a resin with a lower degree of hydrogenation than MBG 275 in Table 5 should increase the viscosity, as well.

A second reason for the correlation between increasing viscosities and increasing percentages of pure monomer resin in a formulation is the broader molecular weight distribution of the pure monomer resin compared with the hydrogenated resin.

Summary

This article addressed the question, "Can the effect of a chemical-resin modification on hot melt adhesive performance be reproduced in styrenic block copolymer-based hot melts by blending two different resins?"

To find the answer, blends of hydrogenated mixed-aromatic hydrocarbon resins and varying grades and amounts of pure monomer aromatic resins in an SIS-based nonwoven hot melt standard formulation were evaluated. The performance of blends with adhesive-system formulas containing partially hydrogenated mixed-aromatic hydrocarbon resin that differ in degree of hydrogenation also were compared.

Results of this work answer the question with a confident, "Yes." Such blends can effectively modify polymer/resin compatibility in SIS-based nonwoven hot melt adhesives.

Additional information on Kristalex®, Endex® and Regalite® hydrocarbon resins is available from Hercules Resins, Hercules Plaza, 1313 N. Market St., Wilmington, DE 19894-0001; call 800-235-0543.