- April 30, 2004, Brian D. Chapman, Dow Corning Corp.
Successful use of film liners calls for selecting the right release coating.
Demand for premium performance is fueling the growing popularity of film liners and substrates. End-users are attracted to film's optical clarity, flexibility, and fiber-free nature. Converters like film's relative strength advantages. As costs go down, film is gaining further ground. Reduced caliper means there are more feet per lineal roll in addition to lower costs.
Many factors must be considered in selecting a liner and the right release coating to use with it. They include the laminate's end use, the converting environment, and cost in use. Other considerations, such as environmental issues and global availability, are not related to performance but still might be deciding factors.
In the end, the decision comes down to end-use performance — the compatibility and stability of all the materials combined. Silicone release coatings with different cure mechanisms and physical properties are available for various applications.
Market share for silicone release coatings that cure on exposure to ultraviolet (UV) radiation is small but stable. While UV-cure coatings do not have wide application, they are essential in the niches they serve, particularly temperature-sensitive films, where the benefits outweigh their high cost.
UV-cure silicones can be used on thermally sensitive substrates. Because the curing mechanism does not depend on heating the web, radiation-curable silicones are ideal for siliconizing extremely temperature-sensitive films, particularly at high speeds. Finally, capital investment required for a dedicated UV line is relatively low.
The limitations that continue to prevent wider acceptance are high raw material costs, nitrogen inertion, and poor stability against many important adhesives, particularly aggressive acrylics. Most importantly, the release properties of UV-cure silicone coatings make them less versatile than other silicone release coatings. They have a limited range of release profile forces at different speeds and limited release differentials. They also can be difficult to process and achieve robust and repeatable release performance.
Solvent-Based Thermal Cure
Use of solvent-based coatings is not growing because there are more environmentally friendly alternatives, but they continue to be popular in areas in which users are experienced in handling the solvents. They offer the advantages of good wetting, low coat weight, good anchorage, and low coefficient of friction. In particular, they offer good release stability and lower temperature cure.
However, solvent-based silicones cure more slowly than newer solventless systems and sometimes create an undesirable appearance that is particularly noticeable in film laminates. As regulatory pressures mount, the main limitation remains the health, safety, and environmental profile of solvent-borne silicone systems.
Solventless Thermal Cure
Demand for faster, more environmentally neutral materials that are compatible with a very wide range of adhesives has pushed development of vinyl and hexenyl solventless silicone release coatings. These versatile materials offer the best range of release differentials and stable release forces. They can be used with a variety of adhesives and with conventional coating equipment. They are suitable for substrates that can operate at temperatures above 90°C (194°F) but not for very heat-sensitive substrates.
This table compares the performance of UV-cure, solvent-based, and solventless silicone release coating systems.
|Properties||Solvent||UV||Solventless New Technologies|
|Cure Speed||Slow||Very Fast||Medium-Fast|
|Cure Temperature||>85°C (185°F)||Ambient||>90°C (194°F)|
|Coat Weight (lb/ream)||0.4||0.8-1.3||0.8-1.3|
|Range of Release||Broad||Limited||Broad|
|Material Cost||Low-Medium||Very High||Medium|
|Source: Dow Corning Corp.|
New Technologies for Films
The most significant recent advance is in thermal solventless release coatings for polyester (PET) films. Of all films used as release liners today, PET has the best heat stability, making it ideal for use with thermal-cure silicones. PET has seen wide application in labeling, tape, and industrial markets.
Traditionally, only PET film that has been primed with adhesion promoters has succeeded, driving costs higher. Next-generation materials are being introduced now for unprimed (natural) PET. For a coating to have broad market application on unprimed PET, it must have exceptional anchorage performance while retaining silicone extractables below 5%, easy manipulation of delamination force at various speeds, stable release against a variety of adhesives, and of course, maintain the valuable clear appearance.
Anchorage has been the greatest hurdle for unprimed PET, but new silicone systems not only resist rub-off immediately after coating but also after two weeks at 90% relative humidity in contact with cast acrylic adhesive, and after six months at ambient temperatures and 50% relative humidity.
Thermal solventless silicone coatings for unprimed PET require some trade-off between dwell time and temperature to achieve anchorage. They will develop good anchorage at lower temperatures if the dwell time is longer. Higher temperatures can increase line speeds, but this can cause stress marks and wrinkles as the lines stop or start. Currently, PET generally is processed at slower speeds than paper to eliminate this problem.
New silicone materials now available offer premium performance without added corona treating. They offer the opportunity for wide release differentials and release stability unmatched by UV systems.
The use of filmic release liners is growing, with PET leading the way because of its versatility, strength, and stability. Films offer an exciting complement to existing paper substrates for coaters and converters alike. Advances in thermal solventless silicone technology are keeping pace. The best way to optimize the processing and performance of liner, adhesive, and release coating together is to work closely with a coating supplier early in the planning process.
Brian Chapman is market manager for the Americas area of Dow Corning's pressure-sensitive release coatings business. At Dow Corning, he held a variety of positions in technical service/application development for the pressure-sensitive industry and in supply chain and process development for the company's resins and polymers finishing unit. Chapman has a bachelor of science degree in chemical engineering from Michigan Technological Univ. and a master's in business administration from the Univ. of Michigan. He can be reached at firstname.lastname@example.org.
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