Mineral Oil Adhesives

Mineral Oil Adhesives in Metallocene Polyolefin Hot Melts In recent years, metallocene polyolefin elastomers (mPO) have gained significant market share and continue to expand their presence across various industrial segments. Strong market penetration can be observed in hot melt adhesives (HMA), where styrene block copolymer (SBC) remains the dominant polymer. Owing to their unique properties, versatile applicability and supply advantages, polyolefin-based alternatives are increasingly preferred and have begun to replace SBC in certain HMA applications. Global demand for SBC and polyolefins is expected to reach comparable levels by 2020. Mineral oil is used as a plasticizer in various hot melt applications (such as tapes, labels, floor coverings, hygiene products, etc.). The function of the plasticizer is to reduce melt viscosity, adjust glass transition temperature and final use properties. Similar to SBC, mPO are high molecular weight compounds that require plasticizers to meet certain performance requirements. Therefore, measuring the compatibility and impact of mineral oils on the performance of mPO-based HMA is of great importance. The current study aims to investigate the effect of different mineral oils on peel strength, shear adhesion failure temperature (SAFT) characteristics in mPO-based HMA formulations. Three polymers were tested: an olefin block copolymer (OBC), an ethylene-octene copolymer (EO) and a polypropylene-based (PP) elastomer. To monitor the effect of different mineral oils, naphthenic and paraffinic products were compared. Since the main sales channel for these adhesives is the hygiene industry, and more specifically nonwoven applications such as baby diapers and feminine care products, formulations were based on general construction adhesive recipes. To limit the effect of different components, formulations were kept simple and created with only the most basic ingredients (Table 1). All samples were coated via spraying onto PE nonwoven substrates and the spray pattern was optimized. Three different add-on levels of 2, 3 and 4 grams were used to evaluate the possibility of adhesive weight reduction.

Comparison of naphthenic and paraffinic plasticizer performance in PE and PP-based formulations:

The melt viscosity results of EO and PP-based construction formulations are shown in Figure 1. Measurements were performed at 165°C. In the EO-based formulation, no difference was observed between naphthenic and paraffinic plasticizers. In PP-based samples, naphthenic viscosity was recorded slightly higher. This is due to the higher polarity of naphthenic molecules. Another oil property affecting melt viscosity is the viscosity index (VI), which is a measure of viscosity decrease as a function of temperature. Typically, VI values of naphthenic molecules are lower compared to paraffins. This means that viscosity decrease is greater for naphthenics as temperature increases. Therefore, in the current study, if temperature is raised to 180°C, both naphthenic and paraffinic viscosity would be equal or naphthenic viscosity would be slightly lower. Melt viscosity is particularly important in terms of applying the adhesive layer onto the substrate. Selection of a mineral oil with low VI (high viscosity decrease) would allow application temperature to be reduced. Lower temperatures would result in less energy consumption, increasing efficiency in production and processing stages. When considering baby diaper adhesives or other nonwoven material adhesives that come into contact with the human body or are used at elevated temperatures, performance in the working temperature range is of particular importance. Typically, peel properties such as peel strength decrease with increasing temperature. Figure 2 shows peel strength values at 37°C for both EO and PP-based samples, including different add-on levels. It is clearly evident that in both polymers containing naphthenic oil, peel values are much higher at high add-on levels compared to paraffins. When the weight of adhesive on the substrate is reduced, peel strength tends to decrease. This is expected since the adhesive layer between substrates becomes thinner. When peel values of naphthenic and paraffinic formulations are compared as a function of decreasing adhesive add-on levels, the difference between the two mineral oil plasticizer types narrows. However, even at the lowest add-on level (2 grams), naphthenic plasticizers provide significantly higher peel performance. Shear performance is of great importance for tapes and labels, but is certainly also important for nonwoven product adhesives. When considering PP-based formulations, the expected difference between naphthenic and paraffinic products was not observed (Figure 3). This property is primarily dependent on the polymer and adhesive. However, it was found that naphthenic oils can contribute to shear performance in EO adhesives. This resulted in recording much higher SAFT values compared to the paraffinic sample. The viscoelastic behavior and rheology of the adhesive fundamentally determine overall performance, production and processing conditions. Therefore, in-depth analysis of rheology curves can be a very suitable tool for rapid formulation development and adaptation. Many different properties can be predicted from rheology data (e.g. peel strength, friction, open time, viscosity, glass transition, etc.). Significant differences were observed when comparing rheology curves of formulations containing naphthenic and paraffinic plasticizers (Figures 4-5). Since naphthenic oils contribute higher storage and loss moduli, higher peel values can be expected. This correlates with peel adhesion results. There is also a significant difference at the tan δ peak compared to paraffinic formulations. Glass transition temperature (Tg) shifts approximately 5-10°C. This is due to the difference between naphthenic and paraffinic oils. Paraffins typically have 5-10°C lower Tg, which accounts for this difference. Using naphthenics may be more beneficial since the higher Tg allows more oil to be added to the formulation and thus resin content can also be reduced. This would lower the formulation cost of the adhesive. In addition to Tg, the characteristic of the tan δ curve may also indicate other properties. The higher the tan δ at Tg, the better the flow and wetting properties of the adhesive. This directly affects the processing and application of the hot melt. Naphthenics are typically higher and this was observed in both EO and PP formulations. The difference in tan δ curve characteristic between naphthenic and paraffinic oils is more pronounced in PP samples. The shape of the Tg peak corresponds to compatibility. Based on this, naphthenic and paraffinic oils are expected to have similar compatibility with EO, but for naphthenics in PP the Tg peak is much narrower and sharper, showing a higher degree of compatibility. No significant difference was observed in other predicted properties. Regardless of mineral oil grade, friction resistance, open time and melting point are expected to be similar. Effect of oil loading on OBC-based hot melt adhesives Oil content is of high importance in hot melt formulation since it affects the properties and cost of the final product. Therefore, to investigate how different oil levels contribute to HMA performance, general nonwoven product formulations with increasing oil and add-on levels were prepared (Table 2) and the performance of each formulation was evaluated. Melt viscosity of each formulation was measured at different temperatures. With increasing oil content, viscosity decrease was expected as the plasticizing effect became more pronounced (Figure 6). At 140°C, very little difference was observed between samples containing 15% and 20% oil. As temperature increased, the difference showed a slight upward trend, but the viscosity of these two formulations was still recorded as quite similar at certain temperatures. An increase in plasticizing effect was only seen at the highest oil loading (25%). In this case, viscosity at 140°C was half that of the other two samples and showed little change when temperature was raised to 160°C. This indicates that high oil content can have a significant effect on hot melt processing. With higher oil content, application temperature can be reduced. This can provide benefits in terms of energy efficiency in the processing stage. In general, increased plasticizing effect is expected to have a negative impact on peel properties. Figure 7 shows peel strength of adhesives at 37°C as a function of oil content. At 4 and 3 gram add-on levels, peel strength shows a downward trend as a function of decreasing add-on level. At 15% oil content, peel strength could not be measured due to substrate failure. This indicates that the peel strength of the sample in question was higher than the others. At 2 gram add-on level, oil level shows very little effect on peel values. Regardless of oil amount, peel strength was recorded at around 50 g/25 mm. This means that at low adhesive add-on levels, a hot melt with higher oil content will perform as well as a formulation with lower oil content. Similar results were obtained in shear adhesion failure temperature results. Although primarily dependent on polymer and resin, some difference in SAFT value would be expected. However, since oil content had no effect on SAFT, such a difference was not recorded in this study. The effect of oil content on rheology can be predicted in parallel with plasticization theories. Typically, as plasticizer increases, the material becomes softer and therefore storage and loss moduli decrease (Figure 8). As oil content increases, Tg decreases. If the objective is to modify the resin and Tg is a key factor, resin selection plays a critical role. If the tackifier resin is replaced with another resin with higher Tg, Tg can be kept constant by increasing oil content. Tan δ curves also show that increasing oil content has no effect on compatibility and wetting behavior, and therefore melting point and open time decrease slightly with higher oil content.

Conclusion

The study demonstrates that naphthenic oils are highly compatible with mPO-based hot melt adhesives. In both EO and PP formulations, naphthenic oils outperform paraffins. Mineral oils have minimal effect on shear adhesion failure temperature, while this property is primarily determined by the type of polymer and tackifier resin. In rheology curves, higher moduli were observed in formulations containing naphthenic oil, parallel to peel adhesion results. Additionally, higher wetting and better compatibility were recorded with naphthenic plasticizers. It was demonstrated that increasing oil content has no effect on peel strength at low add-on levels and has a positive effect on processing stage properties (e.g. application temperature). This shows that while oil content can be increased to 20-25%, tackifier resin amount can be reduced. This would lower costs in both the processing stage and formulation. Dr Peter Kaali is Technical Adviser for the chemicals industry division of Nynas AB. He holds a doctorate from the Royal Institute of Technology (KTH) in Stockholm, Sweden in polymer technology. Peter has approximately 10 years of experience in the polymer industry. Author: Dr. Peter Kaali Technical Adviser Nynas AB