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E12 Long term performance of adhesives in stainless steel plumbing
Background
Millions of metres of austenitic stainless steel water tubing are used each year for carrying hot and cold water, and occasionally gases, in both domestic and commercial installations. Stainless steel is frequently chosen in preference to traditional materials because of its attractive and hygienic appearance, easy cleaning and high corrosion resistance. The most common methods of joining have involved the use of conventional fittings or capillary fittings with soft solder and a phosphoric acid based flux. However, these fittings are made of copper, or copper alloys, and for aesthetic and corrosion reasons a wholly stainless system is desirable. Stainless steel compression fittings are more bulky and expensive than stainless steel capillary fittings, which are readily available at a reasonable price. Unfortunately, stainless steel is very difficult to soft solder because of its protective oxide film making fluxing difficult. One answer is adhesive bonding, developed by British Steel.
Lancashire Fittings have used an anaerobic, acrylic adhesive to bond their stainless steel fittings for the past 24 years. These fittings have to withstand mains water pressure and stresses due to freezing. In certain circumstances they must withstand temperatures of up to 80° C for long periods in hot water supply and heating applications. They must also have sufficient strength to withstand stresses imposed during installation and minor knocks in service.
What was Done
Long term tests were undertaken on stainless steel adhesive pipe joints simulating a domestic central heating using adhesives based on polyethylene and methacrylic acid These fittings were operated in an environment with water at 80ēC for over 10 years. Tensile tests were undertaken on representative samples of these joints compared to compression fittings and soldered copper joints. Flexibility without leakage was assessed as well as burst strength.
Joint Details
Adherends
Austenitic stainless steel
Adhesive
Anaerobic acrylic
Pre Treatment
Degrease / surface abrade / apply activator
Bonding
To ensure a good joint, the stainless steel surfaces are degreased and then abraded, both operations that can be carried out on a building site. The adhesive is cured at room temperature using an activator so that handling strength is achieved in around 60 seconds.
Cure Cycle
Room temperature cure
Location
The bonding was carried out under factory conditions
Results
Long term tests simulating a domestic central heating system showed that adhesives based on polyethylene and methacrylic acid perform satisfactorily. These fitting have been operating in an environment with water at 80ēC for over 10 years without any leakage. Tensile tests on representative samples of these joints have shown that they are stronger than both compression fittings and soldered copper joints, and can accommodate a reasonable degree of flexing without leakage as well as having exceedingly good burst strength.
Discussion
This case study demonstrates that adhesive joints can perform well in hot wet environments
Lessons learnt
Anaerobic acrylic adhesive joints on stainless steel can perform well in hot wet environments
Source
MTSADBD2 Dissemination Guides ADH3 SSPIPE
NPL/ Lancashire Fittings Ltd., The Science Village, Claro Road,Harrogate, North Yorkshire, HG1 4AF www.lancs-fittings.co.uk
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