Plastic and nonmetallic article shaping or treating: processes – Direct application of electrical or wave energy to work – Conveying or aligning particulate material
Reexamination Certificate
1998-11-16
2003-11-18
Staicovici, Stefan (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Direct application of electrical or wave energy to work
Conveying or aligning particulate material
C264S255000, C264S309000, C264S484000, C425S17480E
Reexamination Certificate
active
06649116
ABSTRACT:
This invention relates to a process and apparatus for making thin-walled elastomeric articles.
Conventional processes for making thin-walled rubber articles, such as gloves, involve dipping a shaped former into a rubber latex, or other suitable dipping medium, to coat the former with the latex, or other medium, and curing or drying the rubber latex on the former to produce a thin-walled article, then stripping the finished article from the former. This so-called dipping process has become highly developed so that it is now possible reliably to produce very thin-walled articles such as condoms and surgeons' gloves of extremely high quality. There have been proposals to make thin-walled elastomeric articles by other methods, such as by spraying the rubber latex (or the like) on to the former (instead of dipping) but it has not proved possible to produce such high quality products reliably in these ways and these processes are rarely used.
It is known to form a protective coating on a surface by electrostatic deposition of a coating composition thereon, and this process has been extremely successful, for example, in painting irregularly shaped objects. In U.S. Pat. No. 3,698,847, the electrostatic deposition process is unusually used to form a generally non-elastomeric hollow article by electrostatically depositing a synthetic thermoplastic resin on a collapsible former, and then withdrawing the collapsed former through a hole in the wall of the self-supporting hollow article so formed. The process is described for use for making Christmas tree ornaments.
Certain elastomeric thin-walled articles such as (but not exclusively) condoms and surgeons' gloves, have to be of extremely high quality in order to be acceptable. For example, they must be essentially pore-free so as to provide reliable protection to the user. They must also have an adequate strength so as not to tear or otherwise disintegrate in use. They must also be thin-walled, and often of a wall thickness of less than 200 &mgr;m, or even below 100 &mgr;m. As stated above, the dipping process has been considerably refined over a long period of use to overcome or minimise various problems, so that such products can now be reliably and consistently produced economically and in an automated fashion, in this way.
We have now found, however, that it is possible to make thin-walled elastomeric articles reliably and without certain of the disadvantages of the well-established dipping process, by another process. In particular, we have found that such articles can be made by a process involving electrostatic deposition.
According to the present invention, there is provided a process for forming a thin-walled article, which process comprises electrostatically spraying charged particles of an elastomeric composition into a chamber containing a rigid shaped former having an earthed conductive surface whereby the charged particles of the composition are attracted to the conductive surface of the former to form a coating of the composition thereon; consolidating the coating to produce the thin-walled article on the former; and stripping the article from the shaped former.
The invention also includes apparatus for forming a thin-walled article, comprising a chamber containing a rigid shaped former having an earthed conductive surface, means for electrostatically spraying charged particles of an elastomer composition within the chamber wherein the particles can be attracted to the conductive surface and can form a coating thereon, and stripping means for stripping the thin-walled article from the shaped former after the coating has been consolidated on the former.
The provision of a satisfactory alternative to the dipping process is a surprising achievement, as is the possibility of making highly satisfactory elastomeric condoms and gloves, for example, by this technique having regard to the severity of the requirements for the production of commercially acceptable products.
In the process of the invention, the consolidation step is important. The degree of consolidation will be sufficient to ensure that the thin walled article can be stripped from the former as an integral article, without damaging the structure of the article. Further details are described hereinafter.
The elastomeric composition may be any composition that is capable of being electrostatically charged when sprayed in particulate form; it may be a liquid or it may be a solid. When the sprayed particles are in the form of a liquid, then the consolidation will generally involve some kind of solidification; when the sprayed particles are in the form of a solid, then the consolidation will generally involve some kind of amalgamation of the particles into an integral mass. The particle coating may be consolidated by a variety of different techniques. For example, the composition that is sprayed into the chamber may be curable, so that the coating can be consolidated by curing. However, it is preferred that the coating can be consolidated by evaporation: for example, the composition that is sprayed may include a solvent that can be evaporated to leave a solid coating on the former.
When the composition is a solid, the coating will normally comprise a plurality of solid particles electrostatically adhered to the surface of the former. These particles may be amalgamated to form the thin-walled article by, for example, heating them to cause them to soften or melt and coalesce, and then cooling the coalesced coating.
The consolidation may occur naturally, without any specific treatment. However, it is usually desirable or essential to speed up the consolidation process with some form of treatment. We prefer to carry out the consolidation using heat treatment. Although the heat treatment may be carried out in an oven, it is especially preferred that the heat treatment is carried out by inducing electrical currents in the former.
When the composition is a liquid, it can be atomised to form a plurality of small liquid particles, which can be electrostatically charged. The liquid form of the composition may include an organic or aqueous solvent, or it may comprise solid particles dispersed or dissolved in a liquid. Suitable liquid compositions comprise film-forming elastomeric materials, and include: polyurethanes (solvent and water based solutions and emulsions); fluoroelastomers; triblock copolymer rubbers (SBS, SIS, SEBS), styrene-butadiene rubber latex (SBR); acrylonitrile-butadiene rubber latex (NBR); carboxylated-SBR, carboxylated-NBR, and other carboxylated rubber latices; styrene-vinylpyridine-butadiene terpolymer latex; natural rubber latex (NR); polychloroprene rubber latex (CR); and butyl rubber latex (isobutene-isoprene rubber).
The preferred elastomeric compositions are those containing thermoplastic elastomeric polyurethanes, more preferably aqueous dispersions of thermoplastic elastomeric polyurethanes, and most preferably aqueous dispersions of thermoplastic elastomeric polyurethanes containing a cosolvent such as methyl isobutyl ketone or the like.
When the composition is a solid, it is almost invariably provided in the form of solid powder.
The particles of the composition can be sprayed using a conventional electrostatic spray gun. This can impart an electrostatic charge to the particles of the composition. When the composition is a liquid, the spray gun may be adapted to atomize the liquid to produce a plurality of small particles. When the composition is a solid, the spray gun is usually provided with the solid particles in powder form. Typically, the particle size of the sprayed composition is dependent upon the parameters of the spraying apparatus which, in turn, will be dependent upon the composition being sprayed.
The former may be rotated or otherwise moved while it is being coated, in order to help provide a uniform coating.
In a preferred aspect of the invention, the elastomeric composition, in the form of a solution or dispersion in water or organic solvent, is electrostatically sprayed by means of a rotary atomiser. This has the advantage of
Stephenson Peter John
Tickner Timothy Martin
Rankin, Hill Porter & Clark LLP
Staicovici Stefan
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