Processes for making release liners

Details Processes for making release liners Processes for making release liners

1999-01-28

2001-10-09

Pianalto, Bernard (Department: 1762)

Coating processes

Direct application of electrical, magnetic, wave, or...

Polymerization of coating utilizing direct application of...

C427S208000, C427S208400, C427S378000, C427S387000, C427S505000, C427S515000, C427S516000, C427S551000, C427S557000, C427S558000, C427S559000

Reexamination Certificate

active

06299945

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to sheet materials, and more particularly to release liner sheet materials, and to processes for making the same.
BACKGROUND OF THE INVENTION
Silicone containing materials are widely applied to film, paper and other substrates to impart release properties thereto. Historically, many of these silicone materials have been solvent based and catalyzed by metallic catalysts such as tin, rhodium or platinum. These release coatings have provided desirable levels of release for many applications. However, because of increased environmental concerns regarding the release of volatile organic compounds (VOCs) that can result from the use of such solvent based systems, the industry has been shifting its focus to solventless (100% solids) release systems.
Solventless systems include thermally cured silicone release compositions (e.g. platinum catalyzed systems) and radiation cured systems (cured by UV or EB radiation). Thermally cured silicone release systems can require large quantities of energy and extensive manufacturing floor space for the thermal cure equipment. Radiation cured systems require less manufacturing floor space and are more energy efficient. However, many release liner manufacturers have invested heavily in thermal cure equipment that is not readily displaced and/or converted to radiation curing lines.
Both solvent based and solventless silicone release systems can impart desirable release properties to the substrate to which it is applied. However, an increasing number of applications require a silicone release coating that minimizes the migration or transfer of unreacted silicone compounds in the release coating onto an adjacent surface such as an adhesive or the backside of the support substrate in addition to providing the desired release level.
For example, unreacted or non-crosslinked silicone compounds present in the cured silicone release layer can migrate onto an adjacent adhesive layer and reduce its adhesive properties. Silicone migration or volatilization is also problematic for silicone-sensitive end uses, such as in the microelectronics and automotive industries. Silicones can also transfer to the backside of the support substrate of a release liner stored in roll form resulting in a surface that is difficult or impossible to print. Furthermore, the silicone which transfers to the backside of the support substrate can re-transfer to another substrate which has been adhesively or otherwise laminated to the release liner. This re-transfer of silicone can also cause printing problems such as poor adhesion or non-wetting of the ink.
SUMMARY OF THE INVENTION
The present invention provides release liner sheets which include a substrate and a radiation cured silicone release coating on a surface thereof. The present invention also includes substrates wherein both surfaces are coated with radiation cured silicone release coatings. In this aspect of the invention, the release coatings may be the same or different.
The release coating exhibits desirable release properties as well as reduced amounts of undesirable components. For example, the release liners of the invention exhibit reduced total silicone extractables (unreacted silicone materials). Generally the release coatings include no more than about 1.5, preferably no more than about 0.9, and most preferably no more than about 0.2, micrograms per square centimeter total silicone extractables.
Further, the release liners can have reduced amounts of volatile silicone compounds, such as low molecular weight siloxanes typically found in radiation curable silicone compositions. Preferably the release liner of the invention has no more than about 10 parts per million (ppm), and more preferably less than about 2.0 ppm, of such compounds in the cured release layer.
As a result of significantly reduced amounts of total extractables and/or volatile silicone compounds in the cured release layer, the release liners of the invention can exhibit minimal or substantially no silicone transfer to adjacent surfaces, such as the backside of the support substrate, adjacent adhesive layers, and the like. This in turn provides several advantages, such as minimizing the reduction of adhesive properties of adjacent adhesive layers due to silicone transfer; minimizing the development of printing problems on face stock due to silicone contamination; minimizing or eliminating silicone migration in microelectronic applications; and the like.
The present invention also provides processes for making the release liners. In this embodiment of the invention, a composition comprising a radiation curable silicone release agent in an organic solvent is applied to a surface of a substrate. Any of the types of radiation curable silicone release materials can be used in accordance with the invention. A variety of organic solvents can also be employed, for example, aliphatic, aromatic, ketone and other solvents as well as mixtures of such solvents. Typically, the composition includes about 1 to about 90 weight percent, preferably about 4 to about 20, and more preferably about 6 to about 15, weight percent radiation curable silicone release agent, and about 99 to about 10, preferably about 96 to about 80, and more preferably about 94 to about 85, weight percent solvent, based on the total weight of the composition.
The coated substrate is then treated under conditions sufficient to remove substantially all of the solvent, for example by heat and/or high velocity air. The resultant coated substrate is then exposed to radiation, such as ultraviolet radiation or electron beam radiation, under conditions sufficient to substantially cure the radiation curable silicone release agent to form a radiation cured silicone release coating.
Unexpectedly, the inventors have found that application of radiation curable silicone release materials in a solvent forms a release liner having significantly reduced amounts of total extractables and/or volatile organic compounds, as discussed above. In addition, the process of the invention can provide many manufacturing advantages. For example, the process provides release liners having desired release and other properties using lower coat weights, as compared to conventional solventless radiation cured systems. Typically, radiation curable silicone release compositions are applied at relatively high coat weights, as compared to solvent or water based systems. Because radiation curable silicone compositions can be expensive, the ability to reduce coat weights without adversely affecting release properties can reduce raw material costs.
In addition, equipment currently used with thermal cure systems can be readily adapted for use in the process of the invention. Still further, productivity of thermal cure equipment can be increased. For example, the lines can be run at increased speeds because the dwell time required to remove solvent, as compared to the dwell time required to thermally cure traditional metal catalyzed silicone systems, is significantly less.


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