A NEW GENERATION OF HOT MELT TAPE FORMULATIONS, USING BLENDS OF SIBS AND SBS BLOCK COPOLYMERS AND NEW HYDROCARBON TACKIFYING RESINS.

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1 A NEW GENERATION OF HOT MELT TAPE FORMULATIONS, USING BLENDS OF SIBS AND SBS BLOCK COPOLYMERS AND NEW HYDROCARBON TACKIFYING RESINS. Chrétien Donker, Product Application Manager, Eastman Chemical Middelburg BV, Middelburg, The Netherlands. Introduction Adhesive formulators are looking for alternative polymer systems due to the impact of isoprene availability on the supply of SIS (styrene- isoprene- styrene) block copolymers. These polymers are an essential part of most hot melt pressure sensitive adhesive (HMPSA) formulations. KRATON Polymers have developed a new type of block copolymer with an SIBS (styrene- isoprenebutadiene- styrene) structure. The midblock of this block copolymer consists of a blend of isoprene and butadiene monomers and needs less isoprene and hence is less dependant on isoprene monomer availability. Eastman Chemical Company has developed a series of new hydrocarbon resins that are suitable to tackify this new generation of block copolymers. Another type of block copolymer is SBS (styrene- butadiene- styrene). This polymer does not contain isoprene at all, but is difficult to formulate to provide the right balance of adhesive properties and coatability for a number of applications. This paper will discuss the possibility of using a blend of SIBS and SBS block copolymers in HMPSA formulations, thus minimizing the amount of isoprene monomer needed. Such a blend needs specially developed hydrocarbon resins to formulate adhesives for HMPSA applications. SIBS Block Copolymer HMPSA Formulations Traditionally HMPSA formulations are based on SIS block copolymers with a styrene amount of 15-20%, with or without diblocks. These block copolymers offer a wide range of application possibilities, when used with appropriate tackifying resins. They give a good balance of adhesion and cohesion with a good ageing performance. These types of formulations are specifically popular in hot melt packaging tapes, but are also used in specialty tapes and label applications. Typical tackifying resins used to tackify SIS are full aliphatic C5 hydrocarbon resins, or slightly aromatic modified C5 resins. KRATON Polymers have developed a new thermoplastic elastomer (TPE). This is an SIBS block copolymer, specifically designed for the packaging tape industry. This polymer cannot be tackified with the same C5 resins used for SIS, as they do not have the optimum compatibility with the isoprene butadiene midblock of this block copolymer. Eastman Chemical Company has developed two new tackifiers that are designed to have the correct compatibility with SIBS to provide the performance expected from SIS based HMPSA. These new tackifiers require a relatively high amount of aromatic modification. Resin A hydrocarbon resin as well as Resin B hydrocarbon resin have been developed to tackify SIBS. Resin B is designed to be used in packaging tape formulations, whereas Resin A finds application in specialty tape applications. Initial customer testing demonstrated that these resins provide

2 good overall tape performance, giving a good balance between tack, adhesion and cohesion. Formulations using KRATON MD-6455 with either Resin A or Resin B could be coated at sufficiently low temperatures and also performed well after ageing. Blends of Block Copolymers Historically people have tried to use blends of block copolymers, more specifically blends of SIS and SBS, in various applications. As SBS is typically less expensive than SIS, this could provide an economic advantage. It is known, that blends of these copolymers can be difficult to process and the final adhesive properties not acceptable for some applications. Acceptable formulations could be made in the laboratory by using resins which are compatible in both polymers and by limiting the amount of SBS in the formulation to less than 20%. In practice, the difficult processability of the SBS polymer forced people to avoid using these blends in most tape applications. With the newly developed SIBS block copolymer, more blending options are now feasible. On the one hand, blending of SIS and SIBS is possible, requiring different properties of tackifying resins, depending on the chosen ratio of the two polymers. The more SIBS is used in the formulation, the higher should be the aromatic modification of the resin. Another alternative is to use blends of SIBS and SBS. Both polymers are compatible, which should provide opportunities for blending with higher amounts of SBS in the formulation. The objective of this paper is to explore the feasibility of combining SIBS and SBS block copolymers in HMPSA tape formulations. Experimental Section Two sets of experiments were carried out, following a full factorial design, using the computer program "Design-Expert ". In each set, a number of hotmelt formulations were prepared in a Z-blade mixer at 170 C. The visco-elastic properties of the formulations were evaluated by dynamic mechanical analysis (DMA), using a Rheometrics Ares Reometer. The formulations were coated onto 23 micron polyester (Mylar) film with a coating weight of approx 20 g/m 2 and the resulting tapes were tested for a number of specific tape properties, as shown in the table below. A final set of experiments was carried out with an optimized fixed formulation. Peel adhesion Afera 5001/PSTC 101 Rolling ball tack PSTC 6 Loop tack PSTC 16 Shear adhesion to steel at 23 C, 2.5 kg Afera 5012/PSTC 107 Shear adhesion to steel at 40 C, 2.5 kg Afera 5012/PSTC 107 Shear adhesion to steel at 70 C, 0.5 kg Afera 5012/PSTC 107 SAFT, 0.5 kg Eastman W-126

3 Discussion of the Results Screening Design Two polymers, one resin and plasticizing oil were selected: the newly developed SIBS block copolymer and a commercially available SBS block copolymer (SBS 1), Resin A Hydrocarbon Resin as the tackifying resin and naphthenic oil. Initially an eleven point screening design was carried out, varying the SIBS/SBS ratio to a maximum of 50 phr (parts per hundred rubber) of SBS, resin content from 115 to 175 phr and oil content from 10 to 40 phr. An optimum tape performance window was defined by the following parameters: Peel adhesion > 10 N/25mm Rolling ball tack < 5 cm Loop tack > 15 N/25 mm Shear adhesion to steel at 23 C, 2.5 kg > minutes Shear adhesion to steel at 40 C, 2.5 kg > 300 minutes Shear adhesion to steel at 70 C, 0.5 kg > 300 minutes SAFT, 0.5 kg > 80 C DMA characteristics indicated that at a high resin loading (175 phr) and low oil content (10 phr), the tan delta peak temperature starts to shift outside the acceptable use range at the high end. When compensated with high oil loading (40 phr), the melt flow temperature starts to suffer. On the other end of the design a combination of low resin (115 phr) and high oil (40 phr) is below the usable tan delta temperature range for normal HMPSA. Results of the tape testing against the defined norm are represented in figure 1. Especially high resin loading gave inferior tape performance. The design provided three (out of eleven) formulations with acceptable tape performance. This is not sufficient enough to optimize any of the components varied, but was used as a starting point for a second design. Optimized Design The results of the screening design suggested a second design, even richer in SBS, at lower resin and oil contents. In this second eleven point statistical design the SBS content was varied between 25 and 100 % of the total polymer content; the resin content varied between 100 and 145 phr and the oil content between 0 and 35 phr. The design contained three center points at the following composition: SIBS/SBS/Resin/Oil = 37/63/122.5/17.5. All formulations were characterized by DMA and by measuring tape properties. The optimum tape performance window was defined as described above. Again formulations with high resin and low oil or low resin and high oil loading were found to have an undesirable high or low tan δ maximum temperature in the DMA diagram. To obtain a better understanding of the general impact of the components varied, the data of the first and second design were combined. Analysis of the data confirmed that the visco-elastic properties of these HMPSA formulations, as determined by DMA, showed only minor variations with changing SIBS/SBS ratio, while the effect of the polymer/resin/oil ratio was significant.

4 Review of the adhesive data suggests that compositions around the center point should give good PSA performance. In contrast to the analysis by DMA, there was a clear effect of the SIBS/SBS content noticeable on tape performance, as demonstrated in figure 2. The contour plots demonstrate that the window to formulate a pressure sensitive adhesive with good overall adhesive properties gets smaller with increasing SBS content. This is especially true for the loss of loop tack at high SBS levels. From the analysis of the statistical design and the DMA contour diagrams, a favorable formulation was selected for further investigations containing 125 phr of resin and 25 phr of oil. Optimization of the SIBS/SBS Blend Ratio and Components In the last set of experiments, the hotmelt formulation was set at 100 phr of TPE, 125 phr of resin, 25 phr of oil and also the usual 2 phr of antioxidant was added. SIBS/SBS ratio was varied between 0 and 100 %. The performance of the SIBS/SBS blends improves on the one hand with higher SBS content, notably for shear adhesion and SAFT. On the other hand, SIBS has the tendency to provide better tack and adhesion. These findings are consistent with the results of the second design. Some results are available to suggest that at lower oil contents, the performance for shear adhesion of SIBS will improve relative to SBS and the tack of SBS will decrease further relative to SIBS. The desired adhesive application properties will be the decisive factor to decide on the final tape formulation. It was then decided to continue with three different resins, the newly developed Resin A Hydrocarbon Resin, Resin B Hydrocarbon Resin and Resin C hydrocarbon Resin (see table below for their characteristics), as well as another commercial SBS copolymer (SBS 2). Name Softening point *1) Molecular weight *2) MMAP *3) DACP *4) ( C) Mn Mw Mz ( C) ( C) Resin A Resin B Resin C *1) R&B softening point according to ASTM E-28 *2) Molecular weight according to Eastman test method 53/0/W228 *3) MMAP according to ASTM D-611 *4) DACP according to Eastman test method 53/0/W216 The formulations were characterized again with DMA and adhesives testing. An overview of the tape properties of all formulations prepared for this section is given in Table 1 (see appendix). When reviewing the performance of the different tackifying resins, formulations containing Resin C Hydrocarbon Resin failed at elevated temperatures, both for shear adhesion, as well as for SAFT. This is a direct result of the styrene endblock weakening caused by the relatively high aromatic modification of Resin C. Overall Resin A Hydrocarbon Resin performed best; particularly better shear adhesion performance at elevated temperatures. All formulations were aged for 14 days at 40 C and evaluated again. No significant differences with the initial results were observed, which is a clear indication of the good compatibility of all ingredients in these HMPSA formulations.

5 Extruder Mixing Experiments All formulations discussed above were blended in a Z-blade mixer. In practice many people use a twinscrew extruder to mix the adhesives. Some additional experiments were therefore executed using a laboratory scale extruder to blend the adhesives. Resin A and Resin B were the resins of choice. They were mixed with SIBS and SBS in different ratios and also aged for 14 days at 40 C. Clearly Resin A gave the best overall results in all tested SIBS/SBS ratios, also after ageing. The adhesive results were similar to the adhesive performance when blended in a Z-blade mixer. Resin B however gave different results. Especially when the amount of SBS in the formulations was more than 30 phr, the rolling ball tack started to deteriorate, going to unacceptable levels of more than 25 cm initially and more than 30 cm after ageing. This was most likely caused by the higher molecular weight (more specifically the Mz value) of Resin B. Again, the extruder experiments confirmed that Resin A is the superior resin for formulations using SIBS/SBS block copolymer blends. Conclusions Blends of SIBS and SBS block copolymers can be prepared with up to 50% of SBS, provided the right type of tackifying resin is being used. Aromatically modified C5 hydrocarbon resins are the best choice, in particular the newly developed Resin A Hydrocarbon Resin. If the aromatic modification is too high, as in Resin C, the cohesion and high temperature performance of the tape are becoming inferior. When blended in an extruder, also Resin B gave signs of incompatibility at higher than 30% addition of SBS, resulting in unacceptable high rolling ball tack values. A good starting formulation for a 50/50 blend of SIBS and SBS block copolymer contains 125 phr of resin and 25 phr of oil. The formulation can be further optimized according to the desired adhesive performance. In the experimental set-up, using Z-blade mixers and relatively short residence times, both SIBS and SBS were easily processed. Also in a laboratory scale twin-screw extruder, the adhesives were easy to mix and gave no signs of gelling or other processing problems. It is recognized, that in commercial practice there are sometimes issues in processability of, in particular, SBS block copolymers, which are not reflected in this work. Acknowledgement I would like to thank my colleagues, Dr. Wim Stevels and Jos Ooms for their help in getting the data, figures and tables. A special thanks goes to Dr. Michaela Hofbauer for all her help and support. Registered trademark of Stat-Ease Inc.

6 Figure 1: Overlay plot of the first screening design. B: Resin A: Oil Figure 2: Overlay plots of the second design, showing adhesive performance differences in SIBS/SBS ratio 25 % SBS 50 % SBS 75 % SBS

7 Table 1: Results of the SIBS/SBS = 50/50 formulations, blended in a Z-blade mixer (initial and after ageing) Initial aged Initial aged Initial aged Initial aged Initial aged Initial aged AHA SIBS SBS SBS Resin A Resin C Resin B Naphtenic oil Irganox Coating weight (g/m²) Shear 23 C, 2.5 Kg, Steel (min) Avg >10k >10k >10k >10k >10k >10k >10k >10k >10k >10k >10k >10k Std Shear 40 C, 2.5 Kg, Steel (min) Avg Std Shear 70 C, 0.5 Kg, Steel (min) Avg Std Loop tack to steel (N/25mm) Avg Std Peel adhesion to steel (N/25mm) Avg Std Rolling Ball Tack (cm) Avg Std SAFT, 0.5 Kg, Steel ( C) Avg Std

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