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Plastic containers


From a former plastics engineer: Most plastics are quite inert in
themselves, but often they are mixed with additives to enhance some
particular quality, or may come off the machine with some kind of residue:
Mold-release agents (tubs & boxes), slip-enhancing additives (plastic bags,
even the food-grade kind), PVC film's plasticizers, UV and IR-protection
additives for tubs & buckets rated for outdoor use ... and, don't forget the
dyes and pigments in non-transparent products.

As one case in point, last year we had heavy mortality and large proportion
of deformed critters in a shipment of shrimp naups (about 24 hrs
in-transit). Turned out the shipper had switched plastic-bag suppliers. Both
were certified food grade, but the new kind evidently had some extra or
different chemical, and these are very sensitive critters. Trials at the
shipper's plant conclusively pinpointed the new  bags as the culprits.
Another example: While testing a biofilter design trial, nitrifying bacteria
wouldn't "take" on the food-grade plastic beads we were trying. After
treatment with hot caustic plus thorough rinsing, the bugs settled in OK.

That same shrimp hatchery uses many kinds of plastic containers
uneventfully, for raising the naups all the way to 30-day old juveniles.
These include rigid and plasticized PVC, polyethylene, polystyrene and
fiberglass. Bottom line is that many plastics are quite safe, but it's very
hard to tell which. In commercial operations what they do is find out which
particular products are in regular, uneventful use by other people in the
business, and stick to those. If you don't have that choice, all you can do
is start with food-grade stuff if possible, and run trial batches.

Hope this helps more than it confuses

----- Original Message -----
> Date: Sun, 16 Jun 2002 18:31:15 +0300
> From: "Antonis" <muser at spark_net.gr>
> Subject: Moina containers
> I plan to start a Moina culture I have a plastic container where the Moina
will go in.But I have read an article
> which says always avoid plastics.Why would he say that I mean plastic is a
neutral material.If you have any experience on the subject please post.
>                                              Antonis
> P.S:Moina is similar to Daphnia that's why I made the connection with the