There have been reports of problems with staining on swimming pool vinyl liners. The following information is from Union Carbide Corporation of A&E SAC, a producer of liners for swimming pools.
Generalised dark stains have occasionally been observed on the surface of vinyl swimming pool liners, particularly in-ground installations. The reported frequency of this condition is very low, yet the discoloration is rapid, unsightly and stubborn enough to be of considerable concern to dealers and fabricators, as well as owners. Thus, the Vinyl Fabrics Department of Union Carbide Corporation initiated an investigation into the problem.
These stains appear typically as mottled grey, cloud-like formation against the blue or green background colour of the liner. A significant aspect is that the discoloration is apparent only on that portion of the liner in contact with sand or soil; it does not continue up into the part of the liner supported by metal or wooden sidewalls. Removal of an affected liner discloses that the vinyl sheeting is stained all the way through. The grey spotty nature of the discoloration can be reduced substantially by scrubbing, but not removed entirely. These observations eliminate from consideration two well known causes of liner staining: overchlorination and migration of asphaltic wood preservatives. The former condition results in yellow bleached areas, while the latter produces dark yellow-brown stains only where the liner contacts an asphalt treated wooden sidewall.
The presumption that the grey mottled discoloration results from microbiological attack was confirmed by a series of analytical tests. Portions of stained vinyl liners together with samples of sand that was in contact with the vinyl were evaluated by two analytical laboratories specializing in microbiological examinations. Both reported that high populations of fungal organisms were present in the stained liner and in the sand. The fungi isolated were largely of the Penicillium, Aspergillus and Trichoderma species, all of which can produce greenish-grey discoloration. High bacteria counts were also found on vinyl and sand, although these organisms are a less likely cause of staining.
The conclusion is that the mottled grey stains of the type described were caused by fungi present in the sand and thence attacking the vinyl liner. The high count of fungal organisms plus the fact that severe staining has taken place within one pool season strongly suggests that the sand used in such cases was highly contaminated with organic matter or pollutants. One pool dealer, several of whose installations suffered fungus staining, reported his opinion that the sand used came from river bottoms.
The mechanism by which the liner staining takes place thus becomes clear. The proper environment for most fungus growth is always present beneath a pool liner: dampness, absence of light and "food for the organisms in the form of plasticizers and lubricants in the vinyl formulation. The factor that results in fungus staining in the few cases reported and not in the vast majority of vinyl pool installations is the presence of high fungus and bacteria populations introduced by use of contaminated sand bases.
Microbiological staining of vinyl swimming pool liners can be effectively prevented by the following measures:
1. Dealers should endeavour to obtain clean sand, free of contamination by organic matter.
2. Vinyl sheeting may be ordered with a fungicide/biocide incorporated into the formulation. Such liner material will resist microbiological attack from normal populations of organisms, but will likely only delay, not prevent, staining from the abnormal conditions described previously.
3. If there is a reason to believe that the sand is contaminated, or as an added precaution, the sand base should be sprayed with a 5 - 10% solution of sodium hypochlorite. This chemical is readily available as household bleach or as a pool water sanitiser. Application may be conveniently made by means of a pressure-type garden sprayer. The solution must be allowed to dry thoroughly before installation of the liner.
The following information is from Union Carbide Corporation:
Enumerated below are our plant comments on pink staining of white pool sheeting. This staining is most frequently seen as pink blotches above the water line, and at lesser severity on pool bottoms. The culprit is a combination of bacteria/fungus called Streptomyces Rubrireticuli. This organism is present in the soil, is both aerobic and anaerobic, and produces a red pigment. It grows on various substrates (wood, cloth, etc.), but not on vinyl. However, the red pigment is absorbed by plasticiser and migrates through and becomes visible on the face of the vinyl sheeting. Discoloration is not limited to white vinyl, but takes the form of less noticeable grey or tan on green or blue liners. The best hope of prevention is incorporation of a microbiocide in the sheeting, thereby setting up a zone of inhibition to limit growth of the organism on the adjacent pool wall or sand base. A premium compound would result as the cost of the microbiocide is substantial.
Finally, our pool sheeting customers should be informed of the best means of reducing existing pink stains. This is by application of a strong caustic solution to the affected areas, since the organism is very sensitive to a high pH environment. Solutions of sodium carbonate (used to raise water pH) or household ammonia with or without trisodium phosphate should be effective.
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