The trick is to make it stick

Packaging tape sticks better when made with refined naphthenic oil. Comparing the impact of different oils, Nynas found a way to finetune the adhesive properties.

Packaging tape sticks better when made with refined naphthenic oil. Comparing the impact of different oils, Nynas found a way to finetune the adhesive properties.

When optimizing the performance of an adhesive – in this case a styrenic block copolymer pressure sensitive adhesive – attention is often given to the polymer and resin system but not to the oil used as plasticizer. However, since many adhesive formulations contain significant amounts of plasticizer, we decided it was worthwhile to examine what impact different types of oil have on specific adhesive systems.

Although it is difficult to predict the performance of a given oil in an adhesive, our study has provided us with some conclusions that can facilitate the adhesive formulation process. Mineral oil consists of a mixture of different hydrocarbon molecules with different chemical characteristics that influence the performance of the oil in an adhesive. In our study, we took into consideration both the origin of the oil and the refining technique. However, as the results show that the latter has less impact, that part of the study will not be discussed here.

The study was made on an adhesive system designed for use as packaging tape. In order to minimize the amount of variables, the formulation was made as simple as possible, with only polymer, resin, oil and antioxidants. As base polymer we used Kraton® D1160 E, which is a linear SIS triblock copolymer with bound styrene of 18.5% mass. The resin used was Piccotac 1094-E from Eastman, an aliphatic hydrocarbon resin of low molecular weight. The formulation consisted of 44% base polymer, 46% resin, 9% plasticizing oil and 1% of the antioxidant Irganox 1010.

Our objective was to compare two oils of equal refining degree and refining technique but of different crude origin, naphthenic and paraffinic. Both oils were severely hydrotreated and classified as technical white oil.

The main differences between naphthenic and paraffinic oil can be gleaned from the following characteristics: density, flash point, pour point and solubility (see Table 1). Naphthenic oil displays higher density due to a higher ratio of ring structures. However, it has a lower viscosity index, which means that its viscosity decreases faster as the temperature increases.

The flash point for naphthenic oil is generally a little lower, but more significantly, and due to its lack of waxes, naphthenic oil also displays a lower pour point, resulting in better lowtemperature properties. Finally, in the case of the highly refined oils we tested, the naphthenic oil had a better solubility profile, represented by a higher viscosity gravity constant (VGC).

We used dynamic mechanical analysis (DMA) to evaluate changes to the viscoelastic properties when using different types of oil in the same adhesive formulation. DMA provides nuanced information of the viscoelastic properties of a polymeric material. In brief, the DMA highlighted several positive results for the naphthenic oil, for instance higher shear resistance and improved miscibility (see Fig. 1).

However, in order to better assess the adhesive performance we also tested the peel adhesion, probe tack and static shear on stainless steel (see Fig. 2). In these tests, the adhesive prepared with naphthenic oil showed stronger peel adhesion. In addition, the naphthenic oil adhesive showed slightly better probe tack. This is in line with the DMA measurements, where the naphthenic oil displayed a slightly higher glass transition peak, which represents the material’s ability to lose energy through deformation, thus achieving better wet-out.

The static shear, which measures the inherent strength of the adhesive, also called the cohesion, was significantly better for the adhesive prepared with naphthenic oil. Again, the DMA results corresponded very well to these measurements. At the rubber plateau region (Fig. 1), the adhesive is at rest, and there are no phase transitions. A low Tan delta value in this region indicates that the material has the ability to store energy at service temperature – in other words, it has good shear resistance.

Improved cohesion means that the packaging tape has a better chance to withstand tough transport conditions. A premium packaging tape should, of course, stay on the package throughout the entire transport chain.

The results from this study demonstrate the impact that different highly refined plasticizing oils have on SIS-based packaging adhesives and that it is possible to fine-tune the adhesive properties with the choice of oil. Although the formulation studied had a rather low concentration of plasticizing oil, just 9%, the effects of the naphthenic and paraffinic oils were evident.

We found that highly refined naphthenic oil gives close interaction with midblock and enhances phase compatibility of the triblock copolymer without softening the styrene endblock. This gives a unique combination of excellent adhesion and wet-out properties as well as maintained or improved cohesive strength. Furthermore, the higher density of naphthenic oil might give rise to slightly lower free volume between the flexible isoprene chains, resulting in higher intermolecular forces – thus even stronger adhesion.

Table 1

Characteristics ASTM Method Nyflex 222B N-TWO P-TWO
Density at 15°C (kg/dm3) D4052 0.893 0.874
Vicosity at 40°C (cSt) D445 100 75
Vicosity at 100°C (cSt) D445 9 9
Tg (°C) DSC -67 -77
Flash Point (°C) D92 227 235
Pour Point (°C) D97 -27 -9
VGC D2501 0.825 0.805
Hydrocarbon type (%) D2140    
CA   1 1
CP   57 65
CN   42 34
FDA status   § 178.3620 (b) § 178.3620 (b)

Table 1: Characteristics of the tested oils; Naphthenic Technical White Oil (N-TWO) and Paraffinic Technical White Oil (P-TWO)

Figure 1



Fig. 1. DMA, Viscoelastic properties as a function of temperature (8 mm top plate, 25 mm bottom plate, 10 rad/s, 5 °C/min). G’ illustrates the material’s ability to store energy. G’’ illustrates the material’s ability to flow. Tan Delta is the ratio between G’’ and G’.

Figure 2

Fig. 2. Results from tests of peel adhesion, probe tack and static shear on stainless steel show overall better performance for the adhesive with naphthenic oil, N-TWO.

Author: Anna Eriksson

Title: MSc Chemical Engineer (Technical Service Engineer)
Nynas Naphthenics
Technical Market Support

OILS FOR ADHESIVESTECHNICAL ARTICLE

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