STYRENIC BLOCK COPOLYMERS IN ADHESIVES FOR CO-EXTRUDED FILMS

Transcription

1 STYRENIC BLOCK COPOLYMERS IN ADHESIVES FOR CO-EXTRUDED FILMS M. Dupont, Research Chemist, Kraton Polymers Belgium, Mont St Guibert, Belgium D. St. Clair, Research Chemist, Kraton Polymers U.S. LLC, Houston, TX Abstract Styrenic block copolymers are widely used in pressure sensitive adhesives which can be applied by hot melt coating. SBCs can also be used in compounds which have higher melt viscosity that are suitable for use in a co-extrusion process. This paper will show that compounds can be formulated which, when coextruded with polyolefins, give films with adhesion values that can be controlled over a wide range, from very low to semi-pressure sensitive. The compounds are based on the hydrogenated SBCs and so they have good UV and thermal stability. Films can be co-extruded on either cast or blown film equipment, thus possibly reducing production costs by eliminating a separate adhesive coating step. These films are particularly well suited for use as surface protective films. 1. Introduction Styrenic block copolymers are used in a wide variety of adhesive applications such as tapes, labels, construction adhesives, etc. (Figure 1). This family of polymers presents a flexibility of design, which allows the development of various architectures to meet the differentiated end-use requirements. Depending on the rubber type, the polystyrene content, the diblock / triblock ratio and the molecular weight, it is possible to produce an adhesive that can either be soft and tacky, or hard and cohesive, UV resistant or not. PS polyisoprene PS 0-55% diblock polystyrene-polybutadiene-polystyrene UV stable Polystyrene-polyethylene/butylene-Polystyrene Figure 1. Styrenic block copolymers In this presentation we will focus on the hydrogenated styrenic block copolymers (HSBC) in mainly two types of applications. First, they are very promising base polymers for clear hot melt pressure sensitive 215

2 adhesive (HMPSA) applications, due to their excellent stability. Then, their benefits are even more obvious with new production technologies such as adhesive co-extrusion with a polyolefin backing. We will show that they can be formulated into compounds which, when co-extruded with polyolefins, give films with adhesion values that can be controlled over a wide range, from very low to semi-pressure sensitive, in combination with excellent UV and thermal stability. Several applications will be described, with particular emphasis on protective films. 2. Styrenic Block Copolymers Styrenic block copolymers (SBC) are thermoplastic and elastomeric materials. The most commonly used for pressure sensitive adhesives are triblocks containing an elastomeric midblock and polystyrene endblocks. The rubber midblock is typically a polydiene, either polybutadiene or polyisoprene, resulting in the well-known families of SBS and SIS polymers 1 (Figure 2). Butadiene Isoprene Unhydrogenated SBC + H2 Hydrogenated SBC improved stability (-CH 2 -CH=CH-CH 2 -)-(CH 2 -CH-) n CH m CH 2 (-CH 2 -CH 2 -)-(CH 2 -CH-) 2n m CH 2 (-CH 2 -CH=C-CH 2 -) CH 3 + H2 + H2 Ethylene-Butylene = Ethylene-Propylene (-CH 2 -CH 2 -CH-CH 2 -) CH 3 CH 3 Figure 2. Styrenic block copolymers To improve the thermo-oxidative and UV stability, the polybutadiene rubber block of an SBS polymer can be hydrogenated to form a styrene-ethylene-butylene-styrene block copolymer (SEBS). The SEBS polymers show excellent weatherability, thermal stability, high tensile strength, and also the characteristics of a non-polar olefinic mid-block. When mixed with stable, hydrogenated formulating ingredients, adhesives can be made which have high stability, transparency and clarity. The elasticity of SBCs is due to the thermodynamic incompatibility between the polystyrene endblocks and the elastomer midblock, creating a two-phase structure consisting of polystyrene domains enveloped in an elastomeric matrix. The exact morphology of the two-phase structure depends on the molecular structure of both phases and in particular on their molecular weights. For the thermoplastic elastomers commonly used in adhesive applications, the endblock / midblock ratio is relatively low and so the polystyrene domains are spherical. These polystyrene domains give cohesion to the system and act not only as physical crosslinks in the three-dimensional network, but also as reinforcing filler for the elastomeric matrix (Figure 3). 216

3 200Å PS! Rubber! Figure 3. Domain formation The hydrogenation of the rubber double bonds leads to some changes in elastomeric behaviour. The elastomer becomes less flexible, as the molecular weight between entanglements decreases. The ethylene-butylene backbone also has a lower solubility parameter than an unsaturated midblock and therefore the incompatibility of the rubber matrix and the polystyrene domains is increased. This makes the phase transition between the rubber and the polystyrene sharper and produces material with higher modulus and improved cohesion but also higher viscosity. 40 S-EB-S S-B-S 30 S-I-S Stress, MPa Elongation, % Figure 4. Stress strain curves of various SBCs Figure 4 clearly shows that the hydrogenation step increases the tensile strength at break and reduces the elongation power of the styrenic block copolymers. SEBS polymers are usually stiffer molecules, with higher elastic modulus compared to unhydrogenated SBCs. SBCs as such do not exhibit any strong adhesive character and therefore need to be compounded with lower molecular weight ingredients to develop the required properties. These properties may vary depending on the final application. Finally, SBCs being thermoplastic materials are ideal candidates for hot melt adhesive formulations. 217

4 3. HSBC in Hot-Melt Adhesives Hot-melt manufacturing of adhesives is a cost-effective, environmentally friendly process. Drying is not required, thus eliminating space occupied by expensive ovens needed for processing of solvent or aqueous adhesive coatings. Also, the energy requirements are much lower. HSBC can be used for clear pressure sensitive adhesives that need to withstand degradation from the sunlight and outdoor conditions, while showing good adhesion properties. They indeed exhibit the required stability for these demanding applications, when properly formulated with stable resins and plasticizers and with a suitable stabilizer package 2. Two polymers that are particularly well suited for this application are shown in Table 1. Polymer A has a relatively low polystyrene content (13%) and contains 30% diblock. Recently commercialized polymer B has a modified midblock to make it softer and to lower its viscosity. It also has a medium polystyrene content. Table 1. Polymer characteristics Polymer Grades Type % Polystyrene Diblocks Polymer A SEBS % Polymer B SEBS 21 0% Some formulation examples can be found in Error! Reference source not found.. The tackifying resin is a partially hydrogenated aromatic hydrocarbon resins. The plasticizer is paraffinic white oil. The adhesive formulation based on polymer A shows good tack properties and moderate temperature resistance. Polymer B shows adequate tack (RBT <10 cm) and excellent cohesion at 70 C. The label die-cutting performance 3 of these formulations was qualitatively assessed and showed very good results. The suggested adhesives could therefore be excellent starting formulations for clear labels. Table 2. SEBS based formulations and adhesive properties Polymer A, wt. % 32 Polymer B, wt. % 31 hydrogenated hydrocarbon resin, wt.% white oil, wt. % mixing T, C 180 C 180 C adhesive properties PA, N/25 mm p.t 9 LT, N/25 mm 8 12 RBT, cm 2 7 HP 70 C, 500g, h die-cutting behaviour Very good Very good p.t.: paper tear 218

5 SEBS polymers can be formulated in good pressure sensitive adhesives and show wide formulation flexibility. They additionally combine an excellent moisture resistance as SBCs are very hydrophobic, a good adhesion on a wide range of substrates, including polyolefins, and a good UV stability. Hot melt pressure sensitive adhesives (HMPSA) of this type are formulated to have relatively low melt viscosity. This is required in order that they can be pumped to a coating head of a conventional hot melt coater where the PSA is applied in a very thin layer onto a plastic film at very high speed 4. HSBCs in Adhesives for Co-Extruded Films It is also possible to formulate adhesives based on the SEBS polymers which have much higher viscosity in order that they are suitable for making adhesive films by co-extrusion, like their use in co-extrusion cast and blown adhesive films. They are particularly attractive where high elasticity, good clarity and tailored adhesion / cohesion balance are required. Their compatibility with polyolefins even increases film application possibilities as polyolefin film substrates become more and more popular as the backing. The co-extrusion technology brings significant advantages over the coating technology: - Manufacturing costs are reduced as the coating step is eliminated: the backing and the adhesive films are produced at the same time; - Adhesives with higher viscosities can be handled: this allows the production of adhesives with lower plasticizer content and hence differentiated properties such as lower migration problems and higher temperature resistance. - Very thin adhesive layers can be achieved However the adhesive should be available in free flowing pellets and withstand the high processing temperatures to ensure good melt strength and optimal film quality by avoiding the formation of gels. 4.1 HSBC Products for Co-Extrusion When only very low levels of adhesion are required, a neat HSBC polymer can be used as the adhesive layer. Unformulated polymers are usually soft polymers with good flow properties and good compatibility with the coextruded polyolefin layer. The most useful polymers are listed below (Error! Reference source not found.). The polymer A, already discussed in paragraph 3, has high melt flow and shows excellent adhesion to polyolefins. Polymer C has a modified rubber midblock for lower viscosity, higher softness and excellent adhesion to polypropylene. The developmental polymer D combines low styrene content and a modified rubber. It is the softest hydrogenated SBC and shows excellent adhesion to polypropylene. Finally, polymer E is a low styrene polymer to which about 1% maleic anhydride has been grafted. The grafting of maleic anhydride onto the rubber backbone increases the adhesion of this polymer to polar substrates. 219

6 Table 3. Characteristics of SEBS polymers for adhesive coextruded films polymer A polymer C polymer D polymer E Structure SEBS SEBS SEBS SEBS-MA Styrene Content %w Diblock Content %w Hardness Shore A Melt Flow g/10min 200 C / 5 kg C / 2.16 kg C / 5 kg Tg C These neat polymers might not develop the adhesive properties required for diverse applications such as co-extruded tie-layers, fusible films, laminating films, protective films, etc. So for some applications, it is therefore necessary to compound them with lower molecular weight ingredients to tailor the adhesive properties. It is critical that the adhesive compound be available in the form of free-flowing pellets. Extensive studies have lead to the development of adhesive compounds which are free-flowing and produce clear adhesive films. They can be found in Error! Reference source not found.. Table 4. Compounds for adhesive coextruded films Compound F G H I MFR (190 C/2.16Kg) g/10min Specific Gravity Hardness Shore A C MPa Calc DSC Tg C These compounds can be co-extruded with polyolefins, to give films with adhesion values that can be controlled over a wide range, from very low to semi-pressure sensitive. Compounds F and G are particularly suited for removable films, and show very low adhesion values. Compound F is a very soft compound with low Tg. Compounds H and I have higher Tg and target semi-psa protective films. Compound I has a lower MFR (4 g/10 min) and should be better suited for large cast film lines. Although the adhesive films can meet several film applications requirements, their adhesion level is particularly well suited for removable films for surface temporary protection. This application will therefore be further investigated. 4.2 Protective Films Protective films are used in a broad range of metal and plastic fabricating industries, both during the manufacturing process of the product, and during shipping and storage. This is to protect fragile and highly finished surfaces, such as polished chrome, stainless steel, acrylic sheeting, furniture, etc. The nature and the surface finishes to be protected govern the formulation of the film's adhesive mass. The adhesive layer thickness varies according to technical requirements and usually ranges between 4 and

7 "m. The film also needs to be easily removed, leaving no trace of residue on the surface. Main applications are protection of plastics like PMMA and PC, protection of cars during transport, protection of appliances, and protection of window profiles (Figure 5). Figure 5. Protective film Generally, the films used for this type of applications predominantly include polyethylene, polystyrene and PVC films. The biphasic nature of HSBC, i.e. polystyrene domains in an ethylene-butylene continuous elastomeric phase, allows for co-extrusion with a wide variety of polymers. Only contact with flexible PVC is to be avoided because the polar plasticizer has a tendency to migrate into the PS domains and destroy the cohesion of the HSBC. Experimental Because of the elastomeric nature of these HSBC polymers and compounds, extruding them is different than extruding a conventional thermoplastic resin. Here are some hints concerning the processing conditions to be used. The processing temperatures should ideally be selected between 170 C and 230 C, although temperatures up to 260 C can be used without significantly altering the polymer. Generally, viscosity of the neat HSBC polymers is about a factor of 10 higher than the viscosity of the adhesive compounds. Therefore, processing temperatures for the neat polymers will be near the upper end of this range and for the compounds near the lower end of this range. The melt viscosity is very responsive to shear and relatively insensitive to temperature (Figure 6). Thus, optimum flow during processing of these polymers is obtained at high shear rates. 221

8 Apparent Shear Rate, 1/sec Figure 6. Capillary viscosity of compounds G and H Long extruders are preferred, with a screw with a mixing section or mixing pins to improve melt uniformity. As the adhesive pellets have a tendency to stickiness, the feed zone should not be warmer than 80 C and the feeder screw should be cooled down. The adhesive compounds were coextruded with a LDPE polymer on a lab-size co extrusion cast film line. The parameters were set as follows (Error! Reference source not found.): Table 5. Extruder Settings Single screw extruders Diameter, mm 25 Length / diameter 24/1 Compression ratio 3/1 Temperature profile, C Zone Zone Zone Zone Die 180 The adhesive compounds were coextruded at a 50"m thickness with a 75 "m thick LDPE layer. The 180 peel adhesion on stainless steel was measured after one day at room temperature or after one day ageing at 60 C. Results can be seen in Error! Reference source not found.. Although this lab co-extrusion line does not allow for very thin coatings, results give an indication of the range of properties achievable. The adhesion level can be further tailored by adjusting the adhesive layer thickness or dry blending with polyolefins; allowing film converters to cover the widest possible range attainable with coextruded films. 222

9 Table 6. Peel adhesion of adhesive coextruded films Compound F G H I LDPE / adhesive film 180 Peel on Steel N/25 mm after 1 25 C after 1 60 C In the case of shiny surface protection, it is very important there is no residue after removal of the film. This is the reason why a special compound was developed for this application. The newly developed compound F exhibits significantly lower tendency to leave residues on the surface than coated systems and other compounds. It exhibits a very low peel force, but this can be adjusted if needed by blending with compound I, which shows significantly higher adhesion level. The MFR of the blend and the peel adhesion on stainless steel were measured. Table 7 shows that the adhesion level can be tailored by varying the ratio of compounds F / I. The adhesive layer and the LDPE layer are respectively 40 and 50 "m thick. Table 7. Peel adhesion 180 of compounds F/I blends Compound Compound F Compound I MFR (190 C/2.16 kg) LDPE / adhesive film 180 Peel on Steel, N/25 mm after 1 25 C Conclusion New, soft HSBC are promising polymers for the development of clear hot-melt coated PSA. Target applications are clear labels, UV resistant masking tapes, outdoor adhesives. The superior thermal and UV stability of the hydrogenated polymers also extends the potential towards new processing techniques such as coextrusion on standard cast or blown film lines. The co-extrusion process brings economic and performance advantages over adhesive coating. The manufacturing costs are reduced as the coating step is eliminated and thin layers can be obtained with highly viscous adhesives. An adhesive with fewer low molecular weight components such as plasticizers often exhibits better ageing behavior (lower migration problems) and higher temperature resistance. A range of polymers is available, each providing specific features. HSBC with different polystyrene contents, molecular weights, diblock to triblock ratios and rubber structures give the formulator flexibility in modifying processing and film properties. 223

10 However semi-pressure sensitive properties can only be obtained by formulating the block copolymer into compounds. It was shown that HSBC free flowing compounds achieve high adhesion levels previously only attainable through adhesive coating. The properties can further be tailored by dry blending with other polyolefins or other compounds. Films based on these HSBC compounds are particularly well suited for temporary protection of surfaces. They are highly elastic and conformable for thermoforming, and free from the drawbacks typical of adhesive coated films, such as residue transfer. Compound F has indeed been specially developed to avoid residue transfer, even on highly glossy surfaces. On-going developments target to widen the range of compounds available, increasing the adhesion levels achievable to fulfill more stringent adhesive requirements of other PSA applications. 6. Acknowledgments Geert Vermunicht and Wayne Higgins are gratefully acknowledged for providing much of the data which appeared in this paper. 7. References 1. Kraton Polymers : Drivers for Innovation J.G. Southwick M.A. Masse J.K.L. Schneider FEICA September 2000, Barcelona Spain 2. New hydrogenated styrenic block copolymer for pressure sensitive adhesives N. De Keyzer X. Muyldermans AFERA April 2002, Brussels - Belgium 3. Styrenic block copolymers for labels- M. Dupont 27 th Munich Finishing and Adhesive Symposium October 2002, Munich - Germany 224

11 TECH 31 Technical Seminar Speaker Styrenic Block Copolymers in Adhesives for Co-Extruded Films Richard Schmidt, Kraton Polymers LLC Richard Schmidt is applications and market development engineer at Kraton Polymers LLC, a large producer of styrenic block co-polymers (SBCs). Prior to joining Kraton, Schmidt worked in the area of polymers and packaging for 13 years with companies including Quantum Chemical, Chevron Phillips Chemical and A. Schulman Inc. His responsibilities have included manufacturing, technical service, applications development, sales, business and marketing management. He has four patents in the field of oxygen scavenging polymers. Schmidt received his B.S. degree in chemical engineering from the University of Texas at Austin and completed an online MBA course with Tulane University. 213