I know nothing about this. Just what I found on the Web:
Water Soluble Purge Material
There are water soluble papers and plastics which can be used for purging and are ideal for the purging of pipe welds where there will be no access after welding to remove an inflatable purge system or disc assembly. Most pipe systems can be washed out prior to use and the water soluble PURGE DAM will dissolve and vanish.
The paper material has been shown to leave large fibres after dissolving which can block filters in sensitive systems and paper is becoming an ecologically undesirable material as trees need to be cut down to make paper.
The plastic will dissolve away to molecular size leaving no traces.
Both the paper and the plastic will dissolve in cold water but they both dissolve better in hot water or hot water with steam
[Sorry, lost the source]




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Note added at 7 hrs 20 mins (2004-12-19 21:36:15 GMT)
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ASW The purge dam material that simply flushes out of pipe with water.

In the past, purge chambers for gas-tungsten arc (Tig) welding have been made of such materials as cardboard or heavy paper. But these often were costly in time and effort to remove and could leave dangerous residue in the line. The introduction of ASW paper and tape, greatly simplified and literally revolutionized purge damming. Because ASW is water soluble, it can be dissolved away with water when the weld is completed, and flushed completely out of the system.
ASW Is soluble in hot or cold water or steam. ASW paper and tape are made of a CELLULOSE POLYMER. Flushing the pipe with water or steam after welding removes not only the ASW soluble paper dam but also the tape which is a combination of ASW paper and a tackified water soluble resin. And ASW leaves no undesirable residue or trace elements in the line.
ASW Is strong Enough to maintain the important oxygen purge. Although ASW materials disperse in water when no longer needed, they are strong enough to dam ARGON, helium and other inert gases. They are simple to use, require no special tools or training. They can be formed to fit all size pipe and tube.
ASW Provides multiple advantages over other purge dam material. Fits all sizes and shapes of piping and tubing. ASW use is not limited to small diameter pipes as cones are. No special skills or tools required. Easy to remove. ASW dams flush away automatically and completely. They can be used even where the pipe interior is inaccessible after welding. It is clean, leaves no harmful residue when flushed away. It is not subject to risk of bursting inside the pipe like bladders. Economical, Non-disposable dams such as bladders and cones are expensive to replace when damaged, lost or stolen. ASW paper and tape are inexpensive and disposable. They also save time and purging gas. Easier to handle and transport. A small amount of ASW paper will take care of a welders needs for an entire day.
ASW has been proven effective in thousands of applications in stainless and other steel alloy pipes. ASW has become the preferred purge dam material of the welding industry. ASW is today being used successfully in NUCLEAR and fossil fuel plants, breweries, processing and chemical plants, offshore drilling rigs, tankers, pipeline, pulp mills, and similar applications.
[http://www.junctiontools.com.au/Pta_Site/Cms1.htm]