Coating processes – Medical or dental purpose product; parts; subcombinations;... – Flexible web – sheet – film – or filament base
Reexamination Certificate
2001-09-21
2004-10-26
Beck, Shrive P. (Department: 1762)
Coating processes
Medical or dental purpose product; parts; subcombinations;...
Flexible web, sheet, film, or filament base
C427S002100, C427S243000, C427S372200, C427S374200, C427S384000, C427S386000, C427S391000, C427S402000, C427S407100, C427S411000, C427S412500
Reexamination Certificate
active
06808739
ABSTRACT:
BACKGROUND OF THE INVENTION
Many products are formed by coating a solvent-containing composition onto a suitable substrate. Such compositions generally include enough solvent so that the composition has a viscosity that allows the composition to be coated onto the substrate at a desired thickness using a desired coating technique. After coating the composition onto the substrate, it is often desirable to remove the solvent to dry the coating.
One drying technique involves heating the coated substrate in an oven to remove the solvent(s) by evaporation. Oven drying typically involves conveying large volumes of heated gas (e.g., air, an inert atmosphere, or the like) through the oven in order to heat and evaporate the solvent(s). As the stream of heated gas is conveyed through the oven, nearly all of the volatile components of the coated composition having a measurable vapor pressure at the drying temperature being used will tend to evaporate from the coating into the heated gas.
Evaporation of any particular volatile component occurs until the partial pressure of that component in the heated gas equals that component's vapor pressure at the surface of the liquid coating, meaning that an equilibrium with respect to that component has been established. However, because of the large volume of heated gas in the chamber at any one time and the fact that fresh heated gas is continuously supplied to the oven during drying, an equilibrium is rarely reached between the heated gas and any volatile component of the coating. As a general consequence, all of the volatile components of the coating tend to continuously evaporate and be carried away in the stream of heated gas throughout the entire drying process.
Of course, when drying is controlled by gas phase mass transfer more volatile components of the coating will evaporate at a faster rate than less volatile components. Nonetheless, evaporation of all the volatile components still occurs even if the drying temperature is well below the boiling point of one or more of the volatile components. In other words, the amount of non-solvent, volatile ingredients in the resultant dried coating will be different, and substantially so in some instances, than the amount of non-solvent volatile ingredients in the initial wet coating. Thus, oven drying offers poor control over the component drying process, because more than just the solvent(s) may be removed from the coating.
The impact of oven drying upon the composition of a dried coating can be illustrated in connection with the manufacture of a transdermal drug delivery device, also known as a transdermal patch. A conventional “peel and place” transdermal patch generally includes a drug-in-adhesive layer sandwiched between an impermeable backing and a release liner. At the time of use, the release liner is removed so that the patch can be attached to a patient, adhesive side down. Over time, the drug in the adhesive layer penetrates into the patient, or is topically active, in furtherance of the desired therapeutic treatment. Optionally, the drug-in-adhesive formulation may include one or more compounds known as penetration enhancers that increase the permeability of the patient's tissue to the drug.
Transdermal patches may be manufactured by coating a suitable substrate (e.g., the release liner, the impermeable backing material, or adhesive coated web, as the case may be) with a coating composition that includes one or more pharmacologically active agents (the drug or drugs, as the case may be), a pressure sensitive adhesive, optionally one or more penetration enhancers, and optionally one or more other excipients. Typically, one or more solvents are also included in the coating composition to facilitate forming a homogeneous coating composition having a suitable coating viscosity. For purposes of illustration, the solvent and the penetration enhancer will be deemed to be the only volatile components of the coating composition, wherein the solvent is substantially more volatile than the penetration enhancer. After such a composition is coated, the solvent is removed to dry the coating. However, because both the solvent and penetration enhancer are volatile components, both the solvent and the penetration enhancer will evaporate upon drying. Thus, a portion of the penetration enhancer in the original formulation is lost during drying. This loss of some of the penetration enhancer during drying is particularly problematic since the drug in adhesive layer of a transdermal patch must often meet tight composition and performance specifications.
Although this discussion of the drug-in-adhesive layer has assumed that only the solvent and the penetration enhancer are volatile components of the coating composition, this might not always be the case. In actual practice, for instance, at least the solvent and any one or more other ingredients of the coating composition may be volatile components.
Knowing that drying often will remove more than just the solvent from a coating being dried, an original coating formulation can include extra amounts of any one or more volatile, non-solvent ingredients in an attempt to compensate for losses that might occur during drying. For example, if a specification requires five weight percent (on a solids basis) of a penetration enhancer in a transdermal patch, but it is known that approximately two weight percent (on a solids basis) of the penetration enhancer might be lost during drying, then the original coating formulation can be formulated with about seven weight percent (on a solids basis) of the penetration enhancer in order to compensate for drying losses. This technique of incorporating extra amounts of ingredients into a coating formulation to compensate for drying losses is often referred to as “over formulation”. Over formulation can result in significant cost increase in producing the final product due to the amount of excess materials lost during conventional drying practices.
Accordingly, what is needed is a more accurate approach for drying coatings so that the composition of the resultant dried coating meets precise specifications.
SUMMARY OF THE INVENTION
The present invention relates to a method of selectively removing volatile components from a coated composition. It has now been discovered that the gap drying technique can be used in order to selectively cause some volatile components (i.e., “nonresident” volatile components) to be removed from a coating during drying without removing significant amounts of other volatile components (i.e., “resident” volatile components). In many instances, nonresident ingredients will be the solvents incorporated into a coatable composition. Resident ingredients may be any other ingredients other than the solvent(s).
Gap drying generally involves positioning a coated substrate between a condensing surface and a heating surface. The coated surface of the substrate faces the condensing surface and is separated therefrom by a relatively small gap. The heating surface is in thermal contact with the other side of the coated substrate. The energy from the heating surface is transferred through the substrate to the coating and causes certain components to evaporate from the coating. The resultant vapor travels across the gap above the coating and condenses on the condensing surface. The condensate is collected and removed. Advantageously, the vapor may be condensed, collected, and removed continuously so that the partial pressure of the evaporated component(s) never reaches the corresponding vapor pressure that would be exhibited at steady state equilibrium. As a consequence, the component to be removed from the coating can be continuously evaporated until substantially none of the component remains in the coating.
It has now been discovered that the gap drying process can be used to selectively remove one or more specific volatile components from a coated composition while substantially all of one or more other volatile components remain in the composition. The present invention is able to adapt the gap drying ap
Benson Peter T.
Cooklock Kathleen M.
Huelsman Gary L.
Jain Nirmal K.
Kolb William Blake
Beck Shrive P.
Kolb Michener Jennifer
Szymanski Brian E.
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