PAPER OR PLASTIC? NEW DEVELOPMENTS IN SILICONIZING FILMIC SUBSTRATES

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1 PAPER OR PLASTIC? NEW DEVELOPMENTS IN SILICONIZING FILMIC SUBSTRATES Peter T. Vert, Applications Engineer, Dow Corning Corporation, Midland, MI Dr. Stephen Cray, Development Group Leader, Dow Corning Corporation, Midland, MI Introduction Often paired with film face stocks, filmic release liners are used in many pressure sensitive applications today, ranging from no-label-look beverage labels to diaper closures, medical applications and labels for health and beauty aid products. The market for filmic release liners is growing at a rate of 10 percent per year, fueled in part by the emergence of premium applications that require the level of optical clarity, water resistance, flexibility or fiber-free performance films can provide. Figure 1. The no-label look Key Factors and Trends Impacting Technology Selection Many factors influence the selection of materials and coating technologies for filmic laminates, such as end-user requirements; converting, laminating, die-cutting and printing requirements; adhesive and substrate characteristics; environmental concerns; locally available material choices and silicone release coating technology. A self-adhesive laminate is a complex structure. The face stock, adhesive, silicone release coating and backing substrate all have an impact on each other. The silicone release coating plays a critical, enabling role that allows the entire release construction to function. Compatibility and robustness are key, but the silicone technology used can also drive process decisions and other trade-offs.

2 Desirable End-Use Properties Films offer a degree of surface smoothness and added value functionality, such as flexibility, water resistance and perhaps even stain resistance that positively impact label performance and appearance. Because of the surface smoothness of films, there is virtually no pattern transfer from the liner to the adhesive. This is essential for clear-on-clear applications where surface roughness can translate into a cloudy or hazy appearance in the end-use application. The flexibility and texture of films are benefits in applications like diaper closures. The water resistance of films makes them ideal for labeling health and beauty aid products exposed to moist environments. Filmic release liners are also ideal for building materials which require strong liners that are capable of withstanding harsh environments. Also, some medical applications require the use of fiber-free materials, e.g., films. While on the rise, the use of films in label applications is relatively low today, creating an excellent opportunity for future growth. Cost Reduction Trends The global pressure sensitive marketplace is driven by the need to produce increasingly more labels, faster and at lower cost. Consequently, there is a growing need for release coating systems that can be applied at high line speeds and cure quickly. Low-temperature cure is desirable, as well, because it reduces energy costs and supports the coating of temperature-sensitive filmic substrates. Another cost-reduction effort that impacts material selection is the trend toward reducing substrate caliper. Lower-caliper materials cost less. However, in the case of paper, lowering caliper lowers strength and increases the potential for tearing. Films potentially have several advantages over paper. Their surface smoothness facilitates even coverage during the coating process and their superior hold-out reduces the amount of silicone coating required. Additionally, films are relatively incompressible, and can have the required stiffness to provide sharp die-cutting. Their inherent strength, even at lower calipers, offers the potential for higher conversion speeds and better throughput while minimizing concerns over costly web breaks. To balance this however, films must be carefully selected to withstand the conditions in the siliconizing and conversion processes. Films are less able to withstand elevated temperatures compared to paper. This is particularly true of polyolefin films. Polyester, films are somewhat more robust physically, and are less difficult to handle as a web. It is advisable to measure shrinkage and tensile strength at the conversion temperature to ensure that the substrate can be processed at the required tension levels without an unacceptable level of shrinkage or permanent film stretching. It also must be realized that the paper manufacturers are making large strides to meet the growing demands of evolving end-user applications. For example, some grades of paper have excellent smoothness and hold-out performance and can cost effectively compete with films in some of these value-added segments. One less direct cost-reduction strategy that is beginning to find favor today is that of involving silicone release coating and other material suppliers early in the laminate development process. By working together, suppliers are often able to help laminate makers take advantage of material synergies that optimize the cost effectiveness and end-use performance of the entire structure.

3 Technology Trends Due to environmental concerns and regulatory pressures, many release coating operations, especially in Europe and the Americas, have decreased their reliance on solvent-based coatings and moved toward solventless, emulsion-based and UV-cure technologies. On the adhesives side, water-based acrylics dominate the pressure-sensitive label market. However, the use of hot melt and UV-cure adhesives is growing. This has important release coating implications, because the coating s primary function is to provide the appropriate release profile against the chosen pressure sensitive adhesive. The market is seeing changes in substrates as well. The rise in the use of polyester (PET), oriented polypropylene (OPP) and polyethylene (PE) films, both as face stocks and liners, has led to the development of UV- and low-temperature-cure release coatings designed especially to meet the needs of temperature-sensitive materials. A Comparison of Filmic Substrate Properties and Applications Filmic substrates differ considerably in their filmic properties (see Table 1). Selecting the best one for any given application often involves making trade-offs. Polyester offers the best across-the-board performance, but often requires priming (which increases its cost) to achieve good release coating anchorage. However, recent advances in silicone release coating technology now enable coating of lesscostly unprimed polyester films. Table 1. Key substrate properties Some of the Key Filmic Properties Ease of Thermal Cure Initial Silicone rub-off Delayed Rub-off Caliper control Cost Treated PET Untreated PET LDPE HDPE BOPP MOPP Difficult Fair Good Very Good

4 For polyolefin liners, thermal cure particularly with LDPE is challenging, although rub-off issues tend to be less severe compared to general purpose grade PET. Polypropylene can have quite different properties depending on the method of manufacture. BOPP films can compete with PET in liner applications. Thermal cure and delayed rub-off can be quite challenging and the silicone system must be carefully selected to get the desired balance of properties. MOPP can generally withstand higher cure temperatures than BOPP. Table 2 shows the pressure sensitive applications where PE, PET and OPP films are most frequently used today. The highlighted boxes indicate where their use is expected to grow in the future. Table 2. Pressure sensitive applications for filmic substrates today and tomorrow Applications PE PET OPP Labelstock Graphic Arts Tapes Hygiene/Health Care Building Covering Films Decorative Vinyl Silicone Release Coating Technologies There are two basic ways to cure silicone release coatings onto films via heat (thermally) or via UV or e-beam radiation. Thermally curable coatings employ either solvent-based, emulsion, or solventless (100 percent silicone solids) delivery systems. Solventless systems may be based on either vinyl or hexenyl polymers. Solvents are frequently added to solventless coatings to improve spreading and to aid in processing. Table 3 compares the performance of solvent-based, emulsion, UV-curable and today s new solventless release coating technologies for films.

5 Table 3. A comparison of current release coating technologies for films Properties Solvent Emulsion UV Solventless New technologies Cure Speed Slow Medium Very fast Medium- Fast Cure Temperature >85 C (185 F) >110 C (130 F) Ambient >90 C (194 F) Anchorage Good Good Good Coatweight (lb/ream) Range of Release Broad Broad Limited Broad Release Stability Moderate Environment Poor Material Cost Low-Medium Low-Medium Very High Medium Solvent-Based Coatings Solvent-based coatings offer the advantages of low coat weight, excellent anchorage to films, very good wetting and optical clarity. However, they cure more slowly than solventless coatings and have a more limiting health, safety and environmental profile. Emulsion Coatings Emulsion coatings offer the advantages of low coat weight, good anchorage to films, very good wetting and optical clarity. In addition, they do not have the health, safety, and environmental concerns that solvent-based coatings do. However, like solvent-based coatings, they do not cure as quickly as other available technologies. UV-Curable Solventless Coatings UV-curable solventless release coatings can be used on a broad range of thermally sensitive substrates. They have excellent bath life, low misting potential and cure quickly. The capital investment in curing equipment for a dedicated UV line is significantly less than for a thermal line. However, raw material costs are high. Catalyst inhibition can be a problem on clay-coated papers and nitrogen inerting may be required. More importantly, only a limited range of release profiles can be created. Additionally, UVcurable coatings may become unstable against hot melt adhesives and some aggressive acrylics. Thermally Curable Solventless Coatings Thermally curable solventless release coatings are compatible with a broad range of adhesive types. They offer a wide range of stable release forces and can be applied using conventional coating equipment. Although not suitable for very heat-sensitive substrates, some of today s new thermally curable technologies can be cured at temperatures as low as 90 C (194 F). Coat weights are limited to current solventless coating techniques (gravure). Process Considerations Process considerations that affect the outcome of siliconizing films with thermally curable coatings include: Web tension control Good web tension control is essential when using thin calipers of some substrates. Temperature control Some films are more temperature-sensitive than others, LDPE in particular.

6 Coat weight Reducing coat weight can reduce costs; but reducing it too much can negatively impact coverage and consequently release performance. Filmic substrates generally offer the lowest coat weight potential. Film variability Different additives, such as anti-oxidant, anti-fungal and anti-blocking agents, can affect anchorage and inhibit cure. Process conditions Corona versus non-corona. Catalyst Tin catalysts are less prone than platinum catalysts to catalyst inhibition. Key Challenges for Coating Films Key challenges for coating unprimed polyester are primarily anchorage related. However, it is possible to achieve excellent immediate and aged anchorage by: Changing process conditions (e.g., using corona treatment). Using anchorage additives. Adopting one of the new silicone coatings designed especially for thermally curing on films; these innovative coatings have unique architectures that improve anchorage performance even on unprimed polyester. Key challenges for coating polypropylene or polyethylene are related to the temperature sensitivity of these substrates and the need for low-temperature cure. Improvements in low-temperature cure performance can be made by Increasing the concentration of platinum catalyst in the release coating formulation. Increasing the SiH:Vi ratio. Using more reactive polymers and crosslinkers. Selecting filmic substrates that are non-inhibiting. Changing process conditions corona versus non-corona. Summary The use of filmic release liners is growing, fueled by high-value no-label look applications. In addition to their ability to deliver optical clarity, films possess beneficial properties, such as strength, hold-out and surface smoothness that are likely to increase market interest and consequently application growth. Different films present different challenges, including anchorage, low-temperature cure requirements and the need for release modification. However, major technology breakthroughs occurring in thermal solventless release coatings are making it easier to overcome those challenges. UV-curable and especially the new low-temperature-cure solventless silicone systems designed especially for films are likely to out-pace solvent-based systems as the preferred vehicles for siliconizing film. Acknowledgements The authors would like to thank Dave Rich, Lori Jones, & Norm Kanar for their contributions to this paper.