Impregnant compositions for porous articles

Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...

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526 90, 526212, C08F 2026

Patent

active

050176692

DESCRIPTION:

BRIEF SUMMARY
The invention relates to the sealing of small pores in porous rigid articles by impregnation with a liquid synthetic resin impregnant and subsequent curing of resin in the pores. The articles include particularly metal castings and sintered metal parts but also include materials such as wood or inorganic materials such as brick, stone or concrete.
The impregnation of metal castings with resins to seal porosity is a process which is well established. Porosity in castings is invariably inconsistent in size and shape, thus rendering the components unusable due to leakage. This is especially true where they are subjected to pressure. Apart from this well known cause for rejection of castings used in the motor industry there are many other problems caused by porosity including plating failures, "blow out" during stove enamelling, and sites for corrosion, entrapment of organic material and possible bacteria growth.
Generally, porosity can be divided into three types: through porosity, blind porosity and enclosed porosity. Through porosity causes leakage and is the type with which founders are primarily concerned. Blind porosity, having one entrance to the surface only, will not produce a leakage but can cause surface finishing problems through absorption of treatment fluids. Enclosed porosity causes no problems unless present in excess where it can cause structure failure. Other similar defects often encountered in the foundry include cold laps, cracks, blow holes, and inclusions all of which are often referred to, incorrectly, as forms of porosity. Frequently examination of a casting rejected for leakage and marked by the inspector as porous, reveals that it actually has a cold lap, crack or a blow hole.
Various methods of sealing porosity employed over the years include plugging, coating with epoxy resin, and welding. All of these techniques are highly labour intensive and therefore expensive, and there is no guarantee that the treatment will be successful.
One preferred impregnation technique for sealing porous articles comprises immersing the article in liquid impregnant under vacuum conditions in an autoclave, allowing the pressure to rise so forcing the impregnant into the pores, removing the impregnated article, washing the impregnant from the surface and curing the impregnant which remains in the pores. The article is preferably subjected to vacuum conditions before immersion in the liquid impregnant but it is also possible to immerse the article in the impregnant before drawing the vacuum in the autoclave. The washing step is desirably carried out using water and in many cases it will be desirable to include a surfactant either in the water or in the impregnant. The impregnants normally used are based on one or more esters of (meth)acrylic acid, at least some of esters being derived from polyfunctional alcohols in order to provide cross linking on curing. The impregnant will normally contain a free radical-producing curing agent such as a peroxide or azo compound and an inhibitor such as a sterically hindered phenolic compound. It is also desirable to include a small amount of metal chelating agent. Compositions catalysed by peroxy compounds and azo compounds can be heat-cured e.g. by immersing the article in hot water at say 90.degree. C. Anaerobically curing compositions on the other hand require peroxy catalysts.
The impregnant compositions should have a sufficiently low viscosity to permit them to be drawn into the pores of the article (it should be noted in this connection that metal castings tend to have smaller pores than sintered metal articles). On the other hand, too low a viscosity encourages unwanted leaching out of impregnant from the pores during the washing step. Also, of course, water solubility has to be taken into account if an aqueous wash liquid is used. Furthermore, impregnants of low volatility clearly have to be used in order to avoid boiling off under vacuum conditions. A low surface tension liquid impregnant is also desirable since it permits wetting of the metal in the pores an

REFERENCES:
patent: Re32240 (1986-09-01), De Marco
patent: 4069378 (1978-01-01), De Marco

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