This invention relates to desiccant stoppers for a wide range of containers, and concerns in particular plastic stoppers, comprising a breathable plastic membrane to absorb any free moisture (or odours and the like) in the container. 
Desiccant stoppers are used to control the moisture or odour vapour levels of air, within a sealed container, such as a bottle, jar, bag or box, and to control the closed atmosphere to the benefit of sensitive products such as pharmaceuticals packaged within.  They are produced in a number of sizes and types relevant to the size and nature of the container and the content to be protected.  They must be non-toxic, resistant to water, strong, sterile, and able to provide a microbial barrier.  They must also be dust-free, and dust-proof. 
The present invention relates to the type of desiccant stopper described in WO 02/038465 which comprise a suitably-sized capsule, rather like a small pot or jar, as the desiccant holder, and after this has been filled with the chosen desiccant it is capped with either a porous-type material wad (such as a thin disc of cardboard) crimped into place, or capped with a moulded plastic lid with cast-in perforations. 
Dependent upon their end use, desiccant stoppers can be filled with a wide variety of desiccant-material content.  In the event that they are required to control moisture, suitable absorbent materials are silica-gel, or molecular sieve, while for the control of odours, granulated carbon is used.  Some devices can be used to control the level of a specific gas within a sealed container where the gas might have an ill effect upon the contents.  One such gas is oxygen; oxygenation can often lead to serious content deterioration.  In some instances, a mixture of several different materials, and types of material, will be formulated and used, and there are a number of proprietary brands of admixtures on the market. 
Regardless of their content, these stoppers are generally referred to as “desiccant stoppers”; their use is extensive within Pharmaceutical, Veterinary, Foodstuffs, and Electronics goods packaging. 
WO 02/038465 discloses a method for the production of such a stopper, in which method a fibrous fabric sheet is held in place between the two relevant parts—the stopper body and the lid—and these two are then fused together and to the sheet so that on cooling and solidifying they form a solid, integral plastic supporting frame around the sheet fused thereinto.  The term “fused” as used herein means that the materials of the body and lid and the materials of the sheet have become one integral body, as though the entire stopper had been cast in a single piece.  Thus, the body and lid materials are not merely attached to either side of the fibrous sheet, and do not even merely extend integrally through the sheet, but are instead actually integral with—have become one with—the sheet. 
The fibrous fabric sheet is that spun-bonded breathable plastic (Low Density Polyethylene, or LDPE) material known as TYVEK HBD 1059, and manufactured by Dupont
Spun-bonded materials made from LDPE or the like—referred to hereinafter for convenience simply as “spun-bonded plastics”—are immensely strong, and will not tear.  They will bond with other plastic parts of similar plastic specification providing temperature profiles are strictly observed (being extremely thin, typically as little as 0.15 mm thick, they are easily performance-impaired, even destroyed, by the application of too much heat). 
Spun-bonded plastics, and particularly the TYVEK types of material, are most suitable to act as a permeable membrane for desiccant stoppers due to the superb transfer through them of moisture and odour vapours.  The microporosity of the material controls dust emission, and the high tear strength and puncture resistance is perfect for ensuring the integrity of the final finished article.  Another example of such a spun-bonded plastics is that material available under the name TEIJIN, and manufactured by Unisel (now amalgamated with Dupont). 
The method might be said to be characterised by the manner in which the membrane and body/lid parts are disposed to enable the required sonic fusing to take place.  A high density pot/lid plastic can most reliably be fused with a low density membrane plastic provided that the pot/lid make physical contact with each other, to transmit the vibrations and make them melt, with the membrane retained in a recess between the two such that it, too, melts—to become fused integrally with the pot/lid—but only at its very outermost edge.