Innovation in clear labels and co-extrusion surface-protection films.
Editor’s Note: Please visit www.adhesivesmag.com to view all of the figures for this article.
KRATON® styrenic block copolymers (SBCs) are thermoplastic and elastomeric materials widely used in pressure-sensitive adhesive applications such as tapes, labels, and construction adhesives. This family of polymers presents flexibility in design that allows the development of various architectures to meet differentiated end-use requirements (see Figure 1).
This article focuses on hydrogenated styrenic block copolymers (HSBCs) in 1.) clear, hot-melt pressure-sensitive adhesives (HMPSAs) for labels, due to their excellent stability; and 2.) co-extrusion of HSBCs to polyolefin backings for use in surface-protection films.
It is possible to produce adhesives that can either be soft and tacky, hard and cohesive, and UV-resistant or not, depending on the rubber type, the polystyrene content, the diblock/triblock ratio, and the molecular weight of the polymer. Hence, their unique makeup provides formulating latitude to meet the end-use demands in labels and co-extrusion films.
The most commonly used SBCs for pressure-sensitive adhesives are tri-blocks 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 polymers1 (see Figure 2).
To improve 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 demonstrate excellent weatherability, thermal stability and high tensile strength, as well as the characteristics of a non-polar olefinic mid-block. When mixed with stable/hydrogenated formulating materials, adhesives can be developed with high stability, transparency and clarity - all of which provide great aesthetic and performance value to clear-label and film-adhesive formulations.
HSBCs in Hot-Melt Adhesives
Hot-melt adhesives are cost-effective and environmentally friendly to manufacture. Upfront benefits include elimination of the drying step associated with processing solvent or aqueous adhesive coatings, reduced handling/emissions of volatile organic compounds (VOCs) in process, and eliminating the need for expensive ovens and their energy requirements in manufacturing.
KRATON HSBCs can be used for clear-label hot-melt pressure-sensitive adhesives. They can be designed to withstand degradation from sunlight and outdoor conditions while demonstrating good adhesion properties. When properly formulated with stable resins and plasticizers (and with a suitable stabilizer package2 ), they exhibit the required stability for clear-label pressure-sensitive adhesives. Table 1 shows two polymers that are particularly well-suited for this application. KRATON G1657 has a relatively low polystyrene content (13%) and contains 30% diblock; commercialized KRATON G1642 has a modified midblock to make it softer with a lower viscosity. It also has a medium polystyrene content.
Some formulation examples can be found in Table 2. The tackifying resin is a partially hydrogenated aromatic hydrocarbon resin. The plasticizer is paraffinic white oil.
The adhesive formulation based on KRATON G1657 shows good tack properties and moderate temperature resistance. KRATON G1642 shows adequate tack (RBT <10 cm) and excellent cohesion at 70°C. The label die-cutting performance3 of these formulations was qualitatively assessed and exhibited very good results. Therefore, the suggested adhesives could be excellent starting formulations for clear labels.
SEBS polymers can be formulated in pressure-sensitive adhesives and demonstrate wide formulation flexibility. They provide excellent moisture resistance (due to the hydrophobic nature of SBCs), good adhesion to a range of substrates (including polyolefins) and good UV stability.
Hot-melt pressure-sensitive adhesives (HMPSA) of this type are formulated to have a relatively low melt viscosity. This is required so 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 speeds.
HSBCs in Adhesives for Co-Extruded Films
It is also possible to formulate adhesives based on the SEBS polymers suitable for making adhesive films by co-extrusion. Co-extrusion (cast or blown films) is a process in which the adhesive layer and polyolefin film are simultaneously extruded into sheet form. These polymers have viscosities much higher than those used in HMPSAs. They are particularly attractive where high elasticity, good clarity and tailored adhesion/cohesion balance is required. As polyolefin films become increasingly popular as backing substrates, the use of these formulated adhesives in surface-protection films will broaden.
Co-extrusion technology provides the following advantages over adhesive coating technology.
- Manufacturing costs are reduced as the coating step is eliminated - the backing and the adhesive films are produced simultaneously.
- Adhesives with higher viscosities can be handled. This allows production of adhesives with lower plasticizer content and differentiated properties, such as lower migration problems and higher temperature resistance.
- Very thin adhesive layers can be achieved.
The adhesive should be available in free-flowing pellets and be able to withstand high processing temperatures to ensure good melt strength and optimal film quality by avoiding the formation of gels.
HSBC Products for Co-Extrusion
When only very low levels of adhesion are required, a neat HSBC polymer may be used as the adhesive layer on films. Unformulated polymers are usually soft polymers with good flow properties and good compatibility with the co-extruded polyolefin layer. The most useful polymers are listed in Table 3.
KRATON G1657 has a high melt-flow viscosity and shows excellent adhesion to polyolefins. KRATON G1643 has a modified rubber midblock for lower viscosity, higher softness and excellent adhesion to polypropylene. Developmental KRATON MD6945 combines low styrene content and a modified rubber. It is also the softest hydrogenated SBC and demonstrates excellent adhesion to polypropylene. Finally, KRATON FG1924 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.
It is critical that the adhesive compound be available in the form of free-flowing pellets. Extensive studies have led to the development of free-flowing adhesive compounds that produce clear adhesive films (see Table 4).
These compounds can be co-extruded (on either cast or blown film equipment) with polyolefins to produce films with adhesion values from very low to semi-pressure sensitive. MD6718 and MD6649 are particularly suited for removable films, and demonstrate very low adhesion values. MD6718 is a very soft compound with low Tg. MD6666 and MD6700 have higher Tg, and they target semi-PSA protective films. MD6700 has a lower MFR (4 g/10 min) and is better suited for large cast film lines.
Although the adhesive films can meet several application requirements, their adhesion level is particularly desirable for removable films in temporary surface protection.
Surface-protection films are used in a range of metal- and plastic-fabricating industries. They are commonly used during the manufacturing process of the product, as well as during shipping and storage. These films protect fragile and highly finished surfaces, such as polished chrome, stainless steel, acrylic sheeting, or furniture; the nature of the surface to be protected and its finish govern the formulation of the film’s adhesive mass. The adhesive layer thickness varies according to technical requirements, and is usually 4-14 µm. The film also needs to be easily removed without leaving any traces of residue on the surface. Main applications are protection of plastics (like PMMA and PC), automobiles during transport, pre-installed appliances, and window profiles (see Figure 3).
In general, the films used for these types of applications predominantly include polyethylene, polystyrene and PVC films. The bi-phasic nature of HSBC (i.e., polystyrene domains in an ethylene-butylene continuous elastomeric phase) allows for co-extrusion with a variety of polymers. Only contact with flexible PVC is to be avoided, because the polar plasticizer has a tendency to migrate into the polystyrene domains and destroy the cohesion of the HSBC.
The adhesion level (see Table 5) in surface-protection films can be tailored by adjusting the adhesive layer thickness or dry blending with polyolefins, allowing film converters to cover the widest possible range attainable with co-extruded films.
Table 6 shows that the adhesion level can be tailored by varying the ratio of compounds F/I. The adhesive layer and the LDPE layer are 40 and 50 µm thick, respectively.
New, soft HSBCs are promising polymers for the development of clear, hot-melt coated PSA. Target applications include clear labels, UV-resistant masking tapes and durable adhesives for outdoor applications. The superior thermal and UV stability of the hydrogenated polymers also extend the potential towards new processing techniques, such as co-extrusion on standard cast or blown-film lines.
The co-extrusion process provides economic and performance advantages over adhesive coating of surface protection films. Manufacturing costs are reduced as the coating step is eliminated and thin layers can be obtained with highly viscous adhesives. In addition, the use of HSBCs rather than solvent or aqueous adhesives eliminates the drying step (VOC removal) and space required for processing ovens.
A range of polymers is available, each providing specific features. KRATON HSBCs, which contain different polystyrene contents, molecular weights, di-block-to-tri-block ratios and rubber structures, provide formulators with the flexibility to modify process and film properties.
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 the temporary protection of surfaces. They are highly elastic and conformable for thermoforming, not to mention free of the drawbacks typical of adhesive-coated films such as residue transfer. Compound F has indeed been specially developed to avoid residue transfer, even on high-gloss surfaces.
Martin Dupont, Geert Vermunicht and Wayne Higgins are gratefully acknowledged for providing much of the data that appears in this article.
For more information, visit www.kraton.com.
We believe the information set forth above to be true and accurate, but any recommendations or suggestions that may be made in the foregoing text are without any warranty or guarantee whatsoever, and shall establish no legal duty or responsibility on the part of the authors or their employer. Furthermore, nothing set forth above shall be construed as a recommendation to use any product in conflict with any existing patent rights. Kraton Polymers expressly disclaims any and all liability for any damages or injuries arising out of any activities relating in any way to this publication.