Additive migration pipes

Health and safety concerns over PVC are mostly due to the possible leaching out of the residual monomer in addition to certain additives and plasticisers used. In this context, there has been a growing concern over PVC toys (pacifiers and teething rings in particular), that these additives can be released into a child s mouth when the toy is chewed. Old vinyl intravenous (IV) bags, as used in hospitals, have also been shown to leach some DEHP plasticiser. It has been shown that there is also a possibility of the residual monomer and other additives migrating into drinking-water from PVC bottles [13], or from PVC water pipes [14, 15]. 

Hydrocarbon sensors (qv) placed directiy below the tank bottoms can be effective. However, old contamination or contamination from other tanks or piping can yield misleading results. In addition, the low permeabUity of some areas in the soil can prevent the migration of vapors to the sensing ports under the tank bottom. 

Some additives can contain certain heavy metals or their ions, for different purposes, (e.g., stabilisers used in PVC window profiles and pipes, which are mostly lead-hased or barium/cadmium/zinc compounds, in addition to a number of organonickel compounds used for UV stabilisation). These can pose health hazards if they migrate out of the system. 

Another concern underlying some of the newer products has been to minimise the loss of stabiliser (and hence loss of durability under outdoor conditions), whether caused by extraction by solvents in pipes carrying fluids, or by chemical decomposition or volatility. One new Cytec UV light absorber, Cyasorb THT, combines a HALS additive with triazine chemistry. The combination was tried partly to reduce loss of additive by migration, but it was foimd to maintain the strength of polyethylene for more than twice as long as some of its competitors. 

In addition to plasticisers, there are a number of different additives used for different purposes, i.e., stabilisers used in PVC window profiles and pipes are mostly lead-based, or they can be either barium/cadmium/or zinc compounds. All of these can pose a health hazard if they migrate out of the system above certain concentrations. 

It is well known from pipe pressure tests that immersion in water enhances antioxidant migration in comparison to air, as was observed in our experiments. Smith et al. attributed this to the influence of carbon black (Smith et al. 1992). Antioxidants are adsorbed at the earbon black surface, whereby the migration process is retarded considerably. During immersion in water some moisture is also present in the HDPE bulk material and this is preferentially adsorbed by the carbon black aggregates. The adsorption of water thereby supersedes that of the antioxidants and decreases the retardation coefficient and increases the apparent diffusion coefficient of the latter. In addition, it was suggested that the antioxidant forms loosely bonded clusters within the dry PE bulk material which may disperse slowly in contact with diffusing water thus enhancing the antioxidant diffusion coefficient (Le Poidevin 1977). 

Physical losses occur because of volatilization, poor solubility, diffusion, leachability, blooming, etc. Efficiency means inherent activity and physically retained by polymers. There are still less information about chemical nature and migration of transformation products of additives. The antioxidants diffuse in PE hot water pipes leading to their consumption and pipe deterioration [71]. 

The surface areas of test coupons are typically much smaller than the surface area of the rest of the loop, and piping materials typically have corrosion rates in SCW similar to those of the materials being tested. Therefore, the test fluid will contain impurities introduced from loop surfaces in addition to those released by corrosion of the test coupons [34]. Dissolved species released into the water can deposit onto test specimens by precipitation or incorporation into the growing corrosion film [34]. Chromium released from the autoclave by corrosion was shown to migrate to test specimen surfaces, leading to improved corrosion resistance of the test alloy [39,40]. 

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