New High Performance Styrenic Block Copolymer for Hot-Melt Adhesives

Transcription

1 New High Performance Styrenic Block Copolymer for Hot-Melt Adhesives Donn A. Dubois, KRATON Polymers U.S. LLC Westhollow Technology Center, Houston, Texas 1. - Introduction KRATON Polymers have been leading innovative developments in the field of styrenic block copolymers 1 (SBC) since their first commercialization in These polymers are constructed using anionic chemistry which allows a high degree of precision in engineering molecular architectures. As a result new structures are continually being developed which show unique properties in adhesives. Until now, two families of non-hydrogenated SBC have been widely used in Hot Melt Adhesives (HMA): - styrene isoprene styrene block copolymers, called SIS and - styrene butadiene styrene block copolymers, called SBS. Both families have specific properties and are used in large volumes in their respective application fields. Further, with variable successes, SIS and SBS have also been combined in a single formulation for several reasons. For instance: to make economically attractive packaging tape and disposable non-woven adhesives; to improve the hot-melt stability of adhesives; or to improve label converting (matrix stripping without adhesive threads). KRATON Polymers now have developed a new styrenic block copolymer with a hybrid elastomer composition that combines both isoprene and butadiene in the mid-block. This polymer is a completely new concept that can broaden the application field of SBC s in hot-melts. In this presentation, I will highlight the beneficial and sometimes unexpected properties that such novel chemistry brings to the Pressure Sensitive Adhesive (PSA) Market Properties of the new SIBS block copolymer 2.1 Polymer Characteristics The first member of this new family of poly (styrene- block isoprene/butadiene block styrene) called SIBS - has been designed to serve primarily the tape and label markets. The polymer, SIBS-19, is briefly described in Table 1 and compared to two SIS polymers. It has a low polystyrene content, a low molecular weight for low hot-melt viscosity, tailor-made di-block content and an optimized

2 isoprene/butadiene structure. It has been designed to approach the adhesive properties of SIS in PSA formulations. Structure SIS-15 SIS-19 SIBS-19 Midblock polyisoprene polyisoprene poly(isoprene/butadiene) Molecular weight medium medium medium Diblock content yes no yes PSC % MFR g/10 min * Solution viscosity Pas ** *200 C/5kg **25 % mass in toluene at 25 C Table 1: Basic characteristics of SIS and SIBS The hybrid elastomer composition of this SBC has a significant influence on intrinsic physico - chemical properties of the polymer. Amongst those of particular importance for adhesives are the rubber glass transition temperature Tg, the plateau modulus G 0 n and the solubility parameter. 2.2 Dynamic Mechanical Analysis From the DMA curves in Figure 1, the following differences between SIBS and SIS are observed: The plateau modulus G 0 n of SIBS is higher than that of an SIS, but low enough to make PSA. The plateau modulus G 0 n is inversely proportional to the molecular weight between entanglements M e ; as M e of IB is smaller than that of I, G 0 n of SIBS is higher. The glass/rubber transition phase shows two effects: lower glass transition temperature and a smaller transition. The Tan Delta curve of SIBS shows a mid-block Tan Delta Peak value at about 62 C, some 12 degrees Celsius lower than that of the isoprene mid-block of an SIS whose value is at around 50 C. The lower glass transition temperature is an attractive feature to develop low temperature adhesives. The Tan Delta Peak height is lower for the SIBS mid-block than for the SIS mid-block.

3 C 1.0E E C E+07 Tan Delta 1.0 SIBS-19 SIS E+06 G' Pa 1.0E E E Temperature C Figure 1: Tan Delta and G -curves for SIBS-19 and SIS-19 The (Hildebrand) solubility parameters of poly-isoprene and polybutadiene are different, respectively δ = 8.1 and δ = 8.4 (cal/cm 3 ) ½. As both poly-isoprene and poly-butadiene are present in the mid-block, its solubility parameter has a value in between these two extreme values. This feature will have an important influence on the tackifying resin selection Compatibility with Tackifying Resins The performance characteristics of a resin in a polymer system, such as an adhesive, are directly related to the compatibility of the resin with the polymer. Compatibility windows and charts for resins with SIS and SBS are for instance defined with different tools such as Hildebrand Solubility Parameters (HSP), the MMAP/DACP solvent cloud point 2 or Refractive Index methods 3. As shown in Figure 2, SIBS-19 has a solubility parameter between that of SIS and SBS and therefore exhibits its own compatibility window towards mid-block tackifying resins. SIBS-19 is compatible with most of the resins available on the

4 market except with solid aliphatic C5 hydrocarbon resins (if they are used alone). These C5 resins have HSP values slightly too low to be in the compatibility window of SISB-19. Hildebrand Solubility Parameter 7 Hydrogenated resins Aliphatic C5 Mixed C5/C9 Partially hydrogenated C9 resins Rosin esters C9 aromatic resins 8 9 Polyethylene/propylene 7.8 Polyethylene/butylene 7.9 Polyisoprene 8.1 SIBS Polybutadiene 8.4 Polystyrene Key element for tack Figure 2: Resin Compatibility Map for SBC s SISB-19 has good compatibility with mixed aliphatic/aromatic hydrocarbon resins, fully or partially hydrogenated resins. There is a wide range of such resins available from several suppliers. Consequently, the formulator can select the most appropriate resin for his application. The resins presented in this study are: Resin A (SP 94 C) and Resin B (SP 86 C), Resin C, Resin D and Resin E (All three SP 94 C), Resin F (SP 94 C) and Resin G (liquid resin) from, Resin H (SP 89 C) and Resin K (SP 93 C) Hot Melt Formulation Technology SIBS are formulated in hot melt adhesives with similar hydrocarbon resins, plasticizers, antioxidants and additives conventionally used with respectively SIS and SBS block copolymers.

5 However, the formulation of SIBS allows additional flexibility compared to SIS as shown in Figure 3 that displays the well known PSA windows concept. To formulate a PSA similar to a PSA based on SIS, more resin must be added to the SIBS formulation to obtain the same formulation Tg. This indicates the possibility to develop slightly more diluted formulations compared to SIS. It is also clearly visible that low temperature PSAs can more easily be made because of the lower elastomer Tg. G' at 25 C (Pa) 1E+07 1E+06 1E+05 1E+04. SBS SIBS oil. SIS NR SEBS + oil Low Peel Label Freezer Label + mid - block resin + C5/C9 resin Cold Temp Label by J.D Carper, Hercules Inc. PSA tapes Dahlquist criteria : G (10 rad/s ) < Pa High Peel Label Disposables Tan Delta Peak Temperature or Tg ( C) Formulation Modulus = F( Me rubber, PSC, ingredients) Formulation Tg = F(Tg rubber, Tg ingredients, compatibility) Figure 3: PSA windows related to G and Tg SIBS formulations are compounded with conventional high shear mixers like Z- blade mixers and/or twin screw extruders. As with other non-hydrogenated SBC s, the setting temperatures should not exceed 180 C to avoid undesirable thermal degradation. Thanks to the good stability of the new polymer, no special antioxidant is required and conventional primary (phenolic) AO types can be used or a combination of primary and secondary antioxidants. Inerting by Nitrogen or CO 2 during Z-blade mixing is also recommended for SIBS as for SIS to avoid the thermo-oxidation of the polymer.

6 4. - Packaging Tape formulations The new technology polymer SIBS has been designed to provide adhesive formulations close in properties to equivalent SIS based formulations. However, SIBS are not similar to SIS and therefore might not be considered as a direct replacement polymer for SIS. SIBS have obviously their own set of characteristics that the formulator has to take in consideration. The following examples highlight the performance of this novel polymer Influence of the Resin Aromatic Content on PSA properties SIBS-19 has been solvent compounded in three packaging tape formulations consisting of polymer/resin/oil/a.o. 100/110/15/3 phr. Three resins were selected from the same supplier; they have similar R&B Softening points, 94 C, a DSC Tg of around 50 C but they differ in their respective aromatic content. Adhesive layer of 22 g/m 2 was applied on 36 microns PET. Results in Table 2 indicate that the increase in resin aromatic content improves the tack properties, such as Rolling Ball Tack and Loop Tack, reduces the cohesion as for the Flap Kraft and Holding Power tests and decreases the service temperature (SAFT). These effects are attributed in part to the interaction (solubility parameter) of the resins with the polymer end-blocks and mid-block. SIBS-19 Resin Resin C Resin D Resin E % aromatics* Packaging Properties RBT 23 C cm Flap Kraft 0.5 kg min Flap Kraft 1 kg min PSA properties Loop tack N/25 mm Peel Adhesion 180 N/25 mm HP ss 2 kg hours > SAFT 0.5 kg C * determined by KRATON Polymers Table 2: Influence of the resin aromatic content on PSA properties Reduced Polymer Content Formulations Packaging tape for sealing cardboard boxes is probably the largest volume application for PSA. Bi-oriented Polypropylene (BOPP) film is the backing of choice for that HMPSA application. There have been several recent tentative

7 proposals to develop formulations with wider service temperature comparable to acrylic water based system or to offer economically attractive systems. Amongst those, it has been suggested to use a blend of SIS and SBS, together with blends of two hydrocarbon resins and a plasticizer 4. This five component system did not meet the market formulation flexibility. Our SIBS concept brings to the market a new opportunity for a conventional three components system on existing mixing/coating equipment. Table 3 shows the results obtained in our laboratory with SIBS-19 in packaging tape formulations in combination with different hydrocarbon resins from different suppliers and at different polymer content. The adhesives have been prepared in a lab-scale Z-blade mixer and coated at 22 g/m 2 on PET out of solvent. Ingredients Units F-1 F-2 F-3 F-4 SIS SIBS Resin A Resin F phr Naphthenic Oil Antioxidant Rubber Content % G 0 n calculated 10 4 Pa Packaging Properties RBT cm Flap Kraft 0.5 kg hours Flap Kraft 1 kg hours HP Kraft - 1 kg/40 C min PSA Properties Loop tack N/25 mm Peel adhesion 180 N/25 mm HP ss - 2 kg hours > 500 > 500 >500 >500 SAFT 0.5 kg C HMV 177 C Pa.s Table 3: Adhesive properties of packaging tape formulations Figure 4 shows the Tan Delta and G moduli curves of both adhesives F-1 and F- 2 based respectively on SIS-19 and SIBS-19 with Resin A as given in Table 3. The shape of the Tan Delta curve confirms the good compatibility of this resin with the new polymer. The plateau modulus for the SIBS-19 formulation is higher than that of the SIS explaining the slightly lower tack properties. The Tan Delta Peak Value of the SIBS-19 formulation is lower than the value of the SIS-19 formulation due to the higher G of the SIBS polymer.

8 E C + 4 C SIS E E+08 Tan Delta 1.0 SIBS E+07 G' Pa 1.0E E E Temperature C Figure 4: Tan Delta and G -curves of adhesives F1 and F2 Results in table 3 also show that comparable performance to SIS formulations can be reached with this new polymer by adding more resin into the formulation, F1 compared F3 and F4. The tack properties are better and the cohesion remains almost unaffected. Figure 5 shows that addition of resin, decreases the formulation plateau modulus G 0 n to values closer to those of the SIS formulation. This opens up opportunities to develop economically more attractive formulations.

9 C 1E SIS-19 / Resin A / Oil 100 / 110 / 15 1E SIBS-19 / Resin F / Oil 100 / 120 / C 1E+08 1E+07 Tan Delta 1.0 1E+06 G' Pa 0.5 1E E Temperature C Figure 5: Tan Delta and G -curves of adhesives F1 and F3 Industrial trials to produce box closing tapes based on SIBS-19 have been carried out on a commercial scale; a HMPSA formulation was compounded with a high shear mixer and a 28 µm BOPP backing was coated on a high speed coating line with a 19 µm adhesive layer. Selected tackifying resin for this production trial was Resin F which was added in the amount of 110phr, 15phr of oil was added as plasticizer. Results in Table 4 indicate that equivalent tape performances can be achieved with this new hybrid elastomer SBC as with standard SIS for packaging tapes. Commercial SIS SIBS phr resin BOPP backing µm Coating thickness µm RBT cm 6 3 Flap kraft 0.5kg min HP kraft 40 C min Loop tack N/25mm Peel adhesion N/25mm HP ss 2kg hours SAFT 0.5kg C

10 Box closing 40 C 5 days All closed All closed Table 4: Adhesive properties of industrially produced tapes Hot Melt Viscosity Stability SBS is sometimes added to SIS to improve the adhesive hot melt stability, namely to prevent viscosity decrease and PSA properties changes due to SIS polymer degradation. However SBS are also prone to generating gels, particularly if the working temperature is above 170 C. Figure 6 shows for various formulations consisting of SBC/resin/oil/AO with respective ratios 100/110/15/3, the % of hot melt viscosity retention at 177 C in function of the test period, namely 24 hours. The % of viscosity retention is expressed as the % ratio of the viscosity at a time t i divided by the viscosity at the time t 0. Results indicate that SIBS-19, in combination with different aromatic modified hydrocarbon resins, provides to the formulation a stable hot melt viscosity during the 24 hours test period. The comparative SIS based formulations with Resin A and Resin K decrease in viscosity in function of time, as is well known for polyisoprene based SBCs. The HM viscosity values depend also on the type of resin used. This is related to their intrinsic chemical compositions and molecular weight parameters. It is worth mentioning that the formulations have been protected only with a conventional primary phenolic antioxidant, and no gel was observed at the end of the test.

11 SIBS-19 / Resin A SIBS-19 / Resin H SIBS-19 / Resin F SIBS-19 / Resin C % Viscosity Retention SIS-19 / Resin A SIS-19 / Resin K Time in hours Figure 6: Hot-Melt viscosity stability of SIS-19 and SIBS-19 adhesives 5. Adhesive with Low Temperature Tack The lower elastomer mid-block Tg (- 62 C) of SIBS-19 compared to SIS (- 50 C) opens the door for two opportunities: To develop HMPSA with good tack at low temperature, even for packaging tape applications. To achieve low formulation Tg with less plasticizer compared to SIS. This can be important because it means less plasticizer migration, less strike through in the front material (i.e. paper) and less edge oozing on tape rolls. To develop the optimal low temperature properties, suitable hydrocarbon resins having low glass transition Tg and low molecular weight, such as Resin B, have to be chosen. Figure 7 shows the Rolling Ball Tack in function of decreasing temperature for SIS and SIBS HMPSA packaging tape formulations. The formulation consists of polymer/resin/oil/a.o. 100/110/15/3 phr.

12 30 25 SIS-19 /Resin B Rolling Ball Tack - cm SIBS-19 /Resin B Temperature C Figure 7: Rolling Ball Tack = f (T) for Resin B based SIS-19 and SIBS-19 adhesives The curves indicate that, with the same formulation composition, the new SIBS polymer enables the PSA to maintain good tack at temperatures as low as approximately 10 C, which is about 15 C lower than the SIS formulation without the extra addition of plasticizer General Purpose PSA SISB-19 polymer based adhesives can be produced with adhesive properties similar to adhesive properties of SIS based adhesives. This is shown in the results of Table 5, where the polymer has been compared to SIS-15, a polymer with 15% PSC and slightly higher molecular weight. Two examples are given in this Table : Example 1: Formulations F-5 and F-6 have similar adhesive properties but SIBS-19 gives better RBT values, especially at lower temperatures, than SIS-15. To improve the RBT of the SIS-15 formulation, more oil should be added with the consequence of possible more oil bleeding or strike through the front material.

13 Example 2: Formulations F-7 and F-8 have been compounded with a liquid C5 hydrocarbon resin in combination with an aliphatic/aromatic hydrocarbon resin. Results indicate that a good performance adhesive can be made with a better balance of properties than the corresponding SIS formulation. Ingredients Units F-5 F-6 F-7 F-8 SIBS SIS Resin B phr Resin G Oil Antioxidant RBT at 23 C RBT at 15 C cm RBT at 10 C 3 9 RBT at 5 C 26 >>30 Loop tack N/25 mm Peel Adhesion 180 N/25 mm HP ss 2 kg Hours SAFT 0.5 kg C Table 5: Comparison of PSA properties of SIBS-19 and SIS-15 based adhesives 7. - Conclusions It has been demonstrated that the newly developed hybrid elastomer SBC SIBS- 19 polymer offers attractive formulating opportunities and advantages for the production of HMPSAs in several tape and label applications. General purpose PSA and box closing tapes matching SIS based tapes with readily available aromatic modified hydrocarbon resins. The hybrid elastomer chemistry results in more stable Hot Melt Viscosity compared to SIS or SBS based HMA. The lower elastomer Tg allows the production of lower temperature PSA and packaging tapes with good Rolling Ball Tack values at temperatures 15 C lower compared to SIS based HMPSA. The lower elastomer Tg and higher plateau modulus allow production of more economical formulations as more resin can be added to bring both formulation Tg and plateau modulus closer to those of SIS HMA.

14 References 1.- KRATON Polymers Drivers for Innovation J. G. Southwick, M.A. Masse, J K. Schneider - FEICA Hercules Technical Information E2 and E Best Performance - J.A. Schlademan European Adhesives & Sealants September 2003 p SIS/SBS based HMPSA for packaging tapes L. Jacobs PSTC May 2001 p235 Orlando

15 TECH XXVII Global Conference Speaker NEW HIGH PERFORMANCE SYTRENIC BLOCK COPOLYMER FOR HOT-MELT ADHESIVES Donn Dubois Donn DuBois is a staff research chemist at KRATON Polymers, Westhollow Technology Center in Houston, Texas. He is responsible for product development and polymer design to support markets in the adhesives, sealants and coatings industry. DuBois holds a Ph.D. in chemistry from the University of New Mexico and a B.S. in chemistry from the University of California, San Diego. He joined Shell Chemical Company in 1987 and has worked extensively with styrenic block copolymers, liquid polyketones, monomer synthesis and catalytic reactions to modify polymers.