CLOSURE COATING

Abstract: Elastomeric closures, such as pharmaceutical vial stoppers are coated by plasma polymerization of a lower alkane to reduce the coefficient of friction of the closure and thereby facilitate handling of the closures. Application of the coating at a low polymerization rate limits adverse effects of the coating on leak-resistance of the stopper. Stoppers (80) are subjected to a lower alkane gas in a rotatable drum (10) which may also be rocked in an arc. Helical coil (58) produces sufficient rf energy to form a low plasma gas from the lower alkane which polymerizes on the stoppers as the drum is rotated.

Description: DESCRIPTION CLOSURECOATING TECHNICAL FIELD The present invention relates to the field of coated elastomeric closures and methods of making the same. BACKGROUNDART Elastomeric elements such as plugs, stoppers, "o"-rings, gaskets and others are used to provide fluid- tight seals. Elastomeric elements which can be used to provide fluid-tight seals are referred to herein as "closures." One application for elastomeric closures is in sealing pharmaceutical vials. An elastomeric closure for a pharmaceutical vial typically is in the form of a stopper which includes a relatively thick-walled rubber ring with flat top and bottom surfaces encircling a relatively thin septum formed integrally with the ring. Such a stopper may also include a hollow, cylindrical collar projecting from the bottom surface of the ring and surrounding the septum. In use, the stopper is placed on the mouth of a pharmaceutical vial so that the collar enters into the mouth of the vial and maintains the septum in alignment with the mouth. The ring overlies the rim of the vial surrounding the mouth. A metallic element is clamped over the rubber ring and the vial rim so that the metallic element holds the bottom surface of the ring against the vial rim. The bottom surface of the ring serves as a sealing surface. This surface on the stopper conforms to minor irregularities in the vial surface under the pressure applied by the metallic crimp element. Many other elastomeric closures are used in the pharmaceutical industry and in other industries. In each case, however, a sealing surface of the closure conforms to a mating surface of a container or other object to make the fluid-tight seal. Coatings have been applied on elastomeric closures such as pharmaceutical vial stoppers to reduce the coefficient of friction of the stoppers. This facilitates handling and feeding of the stoppers in automated equipment and facilitates engagement of the stoppers with the vials. Coatings may also provide a barrier against extraction of materials from the closure by the product stored in the container. One type of coating includes a liquid lubricant such as silicone oil, glycerine or the like disposed on the surface of the closure but not bonded to the closure itself. Such coatings are unsatisfactory in many applications due to contamination of the product in the vial by the coating material, and by materials extracted from the stopper. Also, liquid coatings can diffuse into the elastomer itself, thereby removing the coating from the surface. This may cause inconsistent results in processing. Other attempts have been made to provide stoppers coated with polymeric materials such as polytetrafluoroethylene, polyethylene, tetrafluoro- ethylene, polypropylene and polyparaxylylene. These materials generally reduce the coefficient of friction of the stopper and generally reduce contamination of the product in the vial. However, these coatings tend to impair conformance of the closure to imperfections in the vial, resulting in leakage problems. This effect is described in the article "Quantative and Mechanistic Measurements of Parenteral Vial Container/Closure Integrity - Leakage Quantitation, by Morton et al, J. Parenteral Science and Technology, 43:88 - 97 (1989) . Thus, there is a need for a closure coating which is chemically inert and which does not contaminate the product; which provides a substantial reduction in coefficient of friction; and which does not substantially impair the ability of the elastomeric closure to seal. Preferably, the coating should also retard extraction of materials from the elastomer by the product or at least should not itself contribute extractables. All of these requirements, taken together, present a formidable engineering task. Despite considerable effort expended by those in the art, there has remained a significant, unmet need for improved coated closures and for improved methods of coating closures. SUMMARYOFTHEINVENTION The present invention addresses these needs. One aspect of the present invention provides a method of making coated elastomeric closures characterized by the steps of providing uncoated elastomeric closures and plasma polymerizing a lower alkene to form a coating on the uncoated closures to reduce the coefficient of friction of the closures. The coefficient of friction of closures is commonly expressed in terms of the "slip angle", i.e., the angle to the horizontal at which a plane or channel bearing the closures must be positioned before the closures will slide down of their own accord. Most preferably, the plasma polymerizing step is conducted in a gradual manner, so that the slip angle of the closures decreases from the value for the uncoated closures to about 45° over a period of at least about 15 minutes. Typically, the uncoated closures have a slip angle of at least about 60°. Although the rate of decrease of the slip angle during the process will vary during the process, the average rate of decrease over the entire process desirably does not exceed 2 degrees per minute, and more desirably is about 1.5 degrees per minute or less. The lower alkene desirably is selected from the group consisting of ethylene, propylene and combinations thereof, most preferably propylene. The plasma polymerization step may include the steps of providing a gaseous mixture of the lower alkene with inert gas and forming a low temperature plasma of the gaseous mixture in contact with the closures. This aspect of the present invention includes the realization that the rate and conditions of plasma polymerization have a substantial influence on the properties of the coated closure. A coating applied gradually, in a slow polymerization process requiring considerable time to reach a given slip angle, normally will have lower leakage than a closure coated to the same slip angle but using a rapid polymerization process. Although the present invention is not limited by any theory of operation, it is believed that this difference is related to the extent of cross-linking induced by rapid and slow processes. That is, it is believed that slower processes tend to yield coatings with less cross-linked polymeric structures whereas rapid processes tend to yield more cross-linked coatings. Regardless of the reason, however, the closures produced by the slow process with a relatively slow rate of polymerization and with slow rate of decrease in the slip angle of the closures have better leak resistance than equivalent closures produced with a rapid polymerization process. Typically, the gas mixture is converted to a plasma by applying electrical energy, such as RF or microwave energy so as to produce a glow discharge in the plasma adjacent the closures. The closures may be in a pile within a reaction vessel maintained under subatmospheric pressure so that there are interstices between the closures. The plasma may be formed within these interstices. Typically, the glow discharge fills about one-fourth to about three-fourth of the interstices in the pile. The pile may be agitated during the plasma-forming step, as by rotating the reaction vessel around a horizontally-extensive rotation axis so as to tumble the pile. In a particularly preferred arrangement, the reaction vessel is also pivoted around a horizontally-extensive pivot axis transverse to the rotation axis so as to further tumble the pile, and cause the closures to circulate in directions parallel to the rotation axis. The gas mixture flows through the reaction vessel generally codirectionally with the rotation axis. Because the closures circulate, no single closure remains in the gas inlet region where the gas mixture is rich in alkenes, or in the outlet region where the gas mixture is alkene- depleted. Further aspects of the present invention provide coated elastomeric closures. Most desirably, the coated closures are made in accordance with processes as aforesaid. The coated closures desirably include an elastomeric body, such as a body consisting essentially of rubber and defining a sealing surface, at least the sealing surface being covered by a plasma polymerized lower alkene such as plasma polymerized ethylene or plasma polymerized propylene. Most preferably, closures according to this aspect of the present invention have a slip angle of less than about 30° and desirably about 28° or less. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a diagrammatic view of apparatus in accordance with one embodiment of the invention. Figure 2 is a sectional view of a closure together with part of a test fixture. Figure 3 is an elevational view of a test figure. BEST MODES OF CARRYING OUT THE INVENTION Apparatus in accordance with one embodiment of the present invention is illustrated in FIG. 1. The apparatus includes a hollow, tubular cylindrical reaction vessel 10 formed from glass, the reaction vessel having a central axis 12. The inlet and outlet ends 14 and 16 of the reaction vessel are closed by end panels 18 and 20 respectively. The end panels are arranged so that the same can be readily detached from the reaction vessel and reattached thereto to permit access to the interior of the reaction vessel. The end panels of the reaction vessel are equipped with conventional seals (not shown) . Reaction vessel 10 is mounted in circular journals 22. Journals 22 rest on rollers 24, which in turn are supported for rotation on a frame 26. Rollers 24 are connected to a rotation drive motor 28 so that motor 28 can drive the rollers and rotate vessel 10 around the central axis 12. Frame 26 in turn is pivotally mounted to a subframe 27 by a pivoted joint, schematically indicated at 30. A reciprocating linear actuator 32 is connected between the subframe 27 and a point on frame 26 remote from the pivot joint 30 so that the reciprocating actuator will cause the frame 26 to rock, relative to subframe 27, around a pivot axis passing through pivot joint 30. The pivot axis is transverse to the axis 12 of the reaction vessel; as seen in Fig. 1, the pivot axis extends into and out of the plane of the drawing through pivot joint 30. Gas supply apparatus 36 includes a plurality of gas sources 38 each connected by a valve 40 to a manifold 42. The gas sources themselves may include conventional elements such as storage tanks containing the desired gases, pressure regulators, flow meters, safety valves, purge valves and the like. As further discussed below, some or all of the gas sources may be actuated together so as to supply a gas mixture containing the required reactants. Manifold 42 is connected through a flexible bellows 44 and a rotary joint 46 to an inlet port 48 in end panel 20, at the inlet end of vessel 10. An outlet port 51 in end panel 18 at the outlet end 14 of the vessel is connected through a similar rotary joint 48 and flexible bellows 50 to an exhaust system 52 which includes a vacuum pump and conventional auxiliary equipment. The gas supply systems and exhaust systems also include conventional control elements such as pressure, temperature and flow rate sensors, programmable controllers and the like. These elements are also linked to conventional control elements (not shown) arranged to control the operation of motor 28 and linear actuator 32. A blower 54 is connected to a cooling air manifold 56 disposed adjacent the exterior of vessel 10. The cooling air manifold may, for example, be mounted on frame 26. A helical metallic coil electrode 58 encircles vessel 10, the coil being spaced slightly outside of the exterior of the vessel. The coil is fixed to frame 26, and does not rotate with the vessel. The ends 60 of the coil are electrically connected to ground potential, whereas a center tap 62 at the middle of the coil is electrically connected to the output of an impedance matching network 64. The input of network 64 is connected to a conventional radio frequency generator 66. The impedance matching network may be of conventional construction and may include elements such as variable capacitors and/or inductors. As is conventional in the RF plasma art, the impedance matching network is adjusted for efficient power transfer between the RF generator and the coil, and between the coil and the plasma as discussed below. The RF generator 66 can be arranged to operate at any suitable frequency, typically between about 100 KHz and about 300 MHz. However, the generator preferably is set to operate at a so-called "ISM" or industrial-scientific-medical frequency as required by radio communications authorities. 13.56 MHz is a particularly preferred ISM frequency. Vessel 10 and coil 58 are surrounded by a grounded metallic shield 68, only partially illustrated in FIG. 1. The shield may be provided with appropriate openable access panels (not shown) for access to the end panels 18 and 20 on the vessel. In a method according to one embodiment of the invention, uncoated elastomeric closures are disposed within the interior of vessel 10 by opening end panel 16 and resealing the end panel. The closures may be closures of any shape which can be used to provide fluid tight seal, but most preferably are closures of the type used for sealing pharmaceutical vials and other containers. Closures intended for sealing mouth openings on common pharmaceutical vials are referred to herein as "stoppers". As illustrated in FIG. 2, a typical stopper includes a relatively thick ring portion 80 defining a top surface 82 and a bottom surface 84, a hollow cylindrical protrusion 86 extending from the bottom surface and a relatively thin puncturable diaphragm or septum 88 aligned with the interior of the hollow cylindrical protrusion. Closures such as stoppers typically are formed from rubber compositions including polymers such as butyl rubber, natural and synthetic polyisoprene, silicones and combinations of these together with vulcanizing or cross-linking agents, catalysts, retarders, pigments and the like. The rubber composition may also include particulate fillers such as carbon black and others. Alternatively, the closures may be formed from non-rubber elastomers such as thermoplastic and thermosetting polyurethanes, and other synthetic polymers having elastomer properties. These materials may also be blended with additives and fillers. Accordingly, as used herein, the term "elastomer" refers to any composition having elastomeric properties, regardless of whether the same includes rubbers or other polymers. In the coating process, the closures 80 are placed within the interior of vessel 10 so that the closures form a pile 90 in the bottom of the vessel. The closures in the pile will define interstices 92 therebetween. The shapes and sizes of these interstices will depend upon the shapes and sizes of the closures and upon the random arrangement of the closures in the pile. After sealing the chamber, exhaust system 52 is actuated to bring the chamber to a sub-atmospheric pressure, preferably between about 0.01 and 10 Torr, and more preferably between about 0.1 and 1 Torr. Gas supply unit 36 is actuated to provide a gas mixture including at least one lower alkene. As used herein, "lower alkene" refers to unsaturated hydrocarbons and hydrocarbon derivatives including between 2 and 8 carbon atoms, preferably between 2 and 5 carbon atoms and most preferably 3 carbon atoms. Propylene is particularly preferred. Mixtures of alkenes may also be employed. The gas mixture also includes a monoatomic, Group VIII gas, commonly referred to as an inert gas. Helium and argon are preferred monoatomic gases, helium being especially preferred. Typically, the molar ratio of alkene to inert gas is about 5:1 to about 10:1, more preferably about 7:1 to about 9:1. The gas mixture enters the chamber through inlet 48 at the inlet end 16 and passes downstream, in directions generally parallel to axis 12 to the outlet 51. As exhaust system 52 and gas supply device 36 continue to operate, the space within vessel 10, including the interstices 92 in the pile of closures is gradually purged of air and filled entirely with the gas mixture at the aforementioned subatmospheric pressure. The flow rate of the gas mixture desirably is between about 0.2 to about 0.8 standard cubic centimeters per minute for each liter of volume in the interior of chamber 10, (including the volume occupied by closures) . Drive motor 28 is actuated to rotate vessel 10 about axis 12, causing the pile 90 to continually tumble and rearrange itself, thereby also causing new interstices 92 to form within the pile. At the same time linear actuator 32 rocks frame 27, and hence vessel 10, about pivot axis 30. This causes the closures 80 to continually move back and forth between the ends of the chamber, in upstream and downstream directions parallel to the axis of the chamber. Thus, each closure will continually travel between an upstream region adjacent inlet end 16 and a downstream region adjacent outlet end 14. The optimum rotational speed, rocking movement rate and degree of rocking movement will vary with the diameter of the vessel, the number of closures, the type of closures and the degree of circulating movement desired. However, for typical conditions a rotation rate of between about 1 and about 5 revolutions per minute; a rocking motion of frame 26 encompassing about 20 degrees of arc and a rocking rate of about 1 to about 3 cycles per minute are satisfactory. Power supply 66 is actuated to apply radio frequency excitation to coil 58 through impedance matching network 64. This in turn applies electric fields through the wall of vessel 10 to the gas mixture within the vessel. The applied electric field creates a glow discharge and forms a low-temperature plasma within the vessel. As used herein, the term "low-temperature plasma" refers to a plasma in which the temperature of the atoms and positively charged ions is relatively low, and substantially below the electron temperature of the plasma. Preferably, the temperature of the atoms and ions in the plasma is less than about 100°C and more desirably less than about 40°C and most preferably at about room temperature, i.e., at about 20-25°C. The temperature of the closures desirably is in the same range. The energy applied through coil 58 tends to heat the vessel and its contents. Cooling air supplied by fan 54 blows over the outside of the vessel and carries off this heat. The glow discharge occurs principally within the interstices 92 in the pile of closures. As further discussed hereinbelow, it is desirable to conduct the process at a moderate rate. One useful indicator of the process rate is the degree to which the glow discharge fills the interstices. Under normal conditions, when the process is being conducted at the desired, moderate rate, the glow discharge fills about one-fourth to about three-fourths of the interstices, and most desirably about one-half of the interstices as measured by visual observation. The plasma formed from the gas mixture in turn forms polymeric coatings on the surfaces of the closures. The polymers consist essentially of hydrocarbons. The polymeric coating reduces the coefficient of friction of the closures. Coefficient of friction can be measured and expressed in terms of a "slip angle." As referred to herein, the slip angle is measured by placing a closure in a generally u-shaped track 96, seen in end view in Fig. 2, dimensioned so that the track will guide the closures but will not bind the closures. The track 96 is mounted on a test fixture 98 (Fig. 3) arranged to pivot the track gradually upwardly from a horizontal position so that the angle A of the track with the horizontal increases slowly. The angle A at which the closure 80, moves 2.5 cm along the track, is referred to herein as the slip angle. The track 96 ordinarily is fabricated especially for each type of closure. The fixture 98 may be a coefficient of friction testing machine of the type sold under the designation 32-25-00 by Testing Machines Incorporated of Amityville, New York. Typically, the uncoated closures prior to processing have slip angles of more than about 45°, and most typically more than about 60°. The coated closures should have a slip angle of less than 45°. The precise slip angle desired varies with the application. For many typical closures installed by automatic machinery, slip angles of less than 35° may be used. The slip angle decreases progressively as the process continues. The rate of such decrease indicates the rate of plasma polymerization. This rate is principally controlled by the power applied by the power supply 66 and is also influenced by the flow rates and pressures in the system. Most preferably, the process is operated at a relatively slow rate so that the slip angle decreases gradually, at an average rate over the entire process of about 2° per minute or less, and most preferably about 1.5° per minute or less. The slip angle of the closure desirably does not decrease below about 45° until at least about 15 minutes of treatment have elapsed. Preferably, the slip angle of the closures decreases from its initial value to about 35° over a period of about 25 minutes or more; up to about 100 minutes or more are preferred. Surprisingly, by operating the process in this gradual manner, the leakage resistance of the closures is markedly enhanced, vis-a-vis similar plasma coated closures made using a more rapid polymerization process. Although the present invention is not limited by any theory of operation, it is believed that the gradual polymerization process forms the polymeric coating with fewer cross-links between polymer chains, and which is more flexible and more conformable. The following non-limiting examples illustrate certain features of the invention: EXAMPLE 1: A batch of 5,000 standard 20 mm size elastomeric pharmaceutical closures of the type commonly referred to as vial stoppers are loaded into a reactor vessel as illustrated in FIG. 1 having an interior diameter of about 30 cm and an axial length of about 1 meter. The vessel is rotated about its axis at about 2 rpm, but is not rocked about a pivot axis. A gas mixture of propylene and helium is passed through the vessel at a flow rate of 23 standard cubic centimeters per minute propylene and 3 standard cubic centimeters per minutes helium. RF excitation at 13.56 MHz is applied. Different rates of power application are used on different runs. A fixed 30 minute coating time is employed for runs 1-4. A 100 minute coating time is used for run 5. The results, as shown below in Table 1, indicate that, in runs 1-4, leakage rate increases markedly between about 500 and 600 watts excitation. Notably, the coated samples prepared in run 5 have a slip angle lower than that of the coated samples from the other runs, and have lower leakage as well. TABLE I RF EXCITATION LEAKAGE RUN WATTS) SLIP ANGLE RELATIVE SCORE 1 400 30 2 500 29 3 600 102 44 770000 127 5 260 30 Uncoated ——_ 65 10 Numerous variations and combinations of the features discussed above can be used without departing from the present invention as defined by the claims. Merely by way of example, the plasma can be excited by energy other than radio frequency energy. Microwave excitation may be employed, using known forms of microwave energy applicators. Also, although the tumbling and pivoting reaction vessel discussed above is particularly advantageous, the reaction can be conducted in static vessels. For example, closures can be mounted in racks for fixtures so as to maintain each closure in a preselected orientation and assure that the coating is applied to particular surfaces of the closure. This approach is the most practical in the case of very large closures. Also, the process may be performed in a continuous fashion along with continuous input and output of closures from the reaction vessel where the vessel is appropriately equipped. As these and other variations and combinations of the features discussed above can be utilized without departing from the present invention as defined by the claims, the foregoing description of the preferred embodiment should be taken by way of illustration rather than by way of limitation of the claimed invention. INDUSTRIALAPPLICABILITY The present invention is applicable to processing and use of elastomeric closures.

Claims: CLAIMS: 1. A method of making coated elastomeric closures characterized by the steps of providing uncoated elastomeric closures and plasma polymerizing a lower alkene to form a coating on said uncoated closures and thereby reduce the slip angle of the closures, said plasma polymerizing step being conducted gradually so that the slip angle of the closures decreases to 30 degrees over a period of at least about 15 minutes. 2. A method as claimed in claim 1 further characterized in that said uncoated closures have a slip angle of at least about 60 degrees. 3. A method as claimed in claim 1 further characterized in that said plasma polymerizing step is performed so that the slip angle of the closures decreases to 30 degrees over a period of at least about 20 minutes. 4. A method as claimed in claim 1 further characterized in that said lower alkene is selected from the group consisting of ethylene, propylene and combinations thereof. 5. A method as claimed in claim 1 further characterized in that said plasma polymerization step includes the steps of providing a gaseous mixture of said lower alkene and helium and forming a low- temperature plasma of said mixture in contact with said closures. 6. A method as claimed in claim 1 further characterized in that said plasma polymerization step includes the steps of providing a pile (90) of the closures (80) within a reaction vessel (10) so that there are interstices between the closures in the pile and forming a low temperature plasma in said interstices. 7. A method as claimed in claim 6 further characterized in that said step of forming a low temperature plasma in said interstices is performed so that a glow discharge is present in at least some of said interstices. 8. A method as claimed in claim 7 further characterized by the step of agitating said pile of closures during said plasma-forming step. 9. A method as claimed in claim 8 further characterized in that said step of agitating said pile includes the step of rotating said reaction vessel about a generally horizontal rotation axis (12) to tumble said pile, and pivoting said reaction vessel about a generally horizontal pivot axis (30) transverse to said rotation axis to further tumble said pile. 10. A method as claimed in claim 9 further characterized in that said step of forming a plasma includes the step of introducing a gaseous mixture including said lower alkene into said reaction vessel adjacent one end of said rotation axis and withdrawing said mixture adjacent the other end of said rotation axis, and further characterized in that said tumbling causes said closures in said pile to move along said rotation axis. 11. A closure made by a process as claimed in any one of the preceding claims. 12. A closure as claimed in claim 11 including an elastomeric body, said body defining a sealing surface, said sealing surface being covered only by the coating applied by said plasma polymerization step and being devoid of other coatings. 13. A closure as claimed in claim 12 further characterized in that said elastomeric body consists essentially of butyl rubber. 14. A closure characterized by an elastomeric body having a sealing surface and a coating of a plasma polymerized lower alkene overlying at least said sealing surface of said body. 15. A closure as claimed in claim 14 further characterized in that said elastomeric body consists essentially of butyl rubber. 16. A closure as claimed in claim 15 further characterized in that said sealing surface of said body is devoid of coatings other than said plasma polymerized lower alkene. 17. Plasma treatment apparatus characterized by: (a) a reaction vessel for holding articles to be treated, the vessel (10) having an inlet end (16) , an outlet end (14) and a rotation axis (12) extending between said inlet end and said outlet end; (b) vessel support means (26, 27, 28, 30, 24) for supporting the vessel, rotating the vessel about said rotation axis and pivoting the vessel about a pivot axis transverse to the rotation axis so that articles contained in the vessel tumble about the rotation axis and also circulate in directions parallel to the rotation axis, towards the inlet end and towards the outlet end; (c) gas handling means (42, 40, 38, 44, 46) for admitting a gas stream to the vessel adjacent the inlet end and withdrawing the gas stream from the vessel adjacent the outlet end; and (d) energy application means (60, 58, 64, 66) for supplying energy to the vessel so as to convert the gas in the stream to a plasma, whereby the articles in the vessel will be treated uniformly by the plasma. 18. Apparatus as claimed in claim 17 further characterized in that said pivot axis is substantially horizontal. 19. Apparatus as claimed in claim 17 or claim 18 further characterized in that the vessel support means is operative to perform said rotation and pivoting continuously during operation of the apparatus, and further characterized in that the gas handling means is operative to admit and withdraw the gas stream continuously during operation of the apparatus.

Inventors: BABACZ ROBERT J, BABACZ, ROBERT, J.
Applicants: POLAR MATERIALS INC, POLAR MATERIALS INCORPORATED
Application year: 1995
Main classification: B05C13
IPC codes: A61J1, B05D1, B05D3, B05D7, C08J7