Mixed
Metals PVC Stabilisers Risk Characterisation
Mixed Metal
Compounds used as PVC Stabilisers
Mixed Metal
PVC stabilisers contain zinc, magnesium, barium or calcium stearates,
oleates, octoates or p-tert-butyl benzoates singly or in combination,
together with co-stabilisers and lubricants.
The co-stabilisers
can be divided in several product families:
Depending
on the physical state and the concentration of the co-stabilisers
and the presence or not of solvent, the stabiliser packages
can be liquid or solid.
Lubricants
such as paraffins, polyethylene waxes or ester waxes can be
used when required by processing conditions (window frames).
Classification
and labelling of Mixed Metal PVC Stabilisers
Zinc, magnesium
and calcium are essential to life at low doses. The recommended
daily intake for zinc is about 15 mg for adults. At high doses,
toxic effects are observable. As with other chemical substances,
exposure to humans and to the environment must be understood
at each step of the life cycle of mixed metal compounds.
Zinc stearate
is currently under review under the Existing Chemicals Directive.
An extensive Environmental Health Criteria for Zinc has been
developed by IPCS . An article in the Environment section of
Zincworld
gives comments about the status of the risk assessment.
Soluble
baryum salts are acutely toxic by ingestion and insoluble salts
such as baryum sulphate can be toxic by inhalation. The oxide
and hydroxide are irritant to skin.
Inorganic
magnesium and calcium compounds are considered to have low toxicity.
For example, there are no regulatory limits to the concentration
of magnesium or calcium in drinking water. The recommended average
daily intake for humans is about 1 g for calcium and about 0.2
g for magnesium.
Depending
on the end-use and on performance requirements different co-stabilisers
can be used in varying concentrations. For example, the substitution
of cadmium or baryum by calcium and/or zinc results in a stability
loss which must be compensated by an increase in the concentration
of certain co-stabilisers: twice the amount of phosphite is
required when switching from Ba/Zn to Ca/Zn systems.
Co-stabilisers
have low acute toxicity (LD50 <1,600 mg/kg BW).
Manufacturing
and Use of Mixed Metal PVC Stabilisers
The workplace
exposure limit for soluble baryum compounds is 0.5 mg/m3 (as
Ba), and 10 mg/m3 for insoluble baryum compounds (sources: Directive
80/1107/EEC and US ACGIH).
The only
real occupational hazard known for zinc compounds is the exposure
to "fumes" (zinc oxide). The workplace occupational
TLV are 10 mg/m3 for zinc stearate (5 mg/m3 in the UK) and for
zinc oxide dust. The TLV drops to 5 mg/m3 for zinc oxide "fumes".
The TLVs
for dusty calcium or magnesium salts is 6 mg/m3 in Germany.
At high
temperatures (>180°C) emissions of the volatile components
of the stabiliser formulation can occur. Some chemical transformation
can also result in the formation of traces of ZnCl2, BaCl2 and
CaCl2.
Manufacturing
of the PVC Article Containing Mixed Metal Stabilisers
There are
no unusual occupational risks associated with these systems
during manufacture.
Use
of the PVC Article Containing Mixed Metal Stabilisers
As with
other PVC stabilisers, Ba/Zn and Ca/Zn systems are strongly
bound to the PVC matrix.
Calcium/Zinc
stabilised PVC pipes were tested by KIWA (Dutch authority responsible
for approving pipes for potable water). The results obtained
were:
| Stabiliser |
First
washing |
Second
Rinse |
| Zinc |
<0.6
mg/dm2 |
- |
| Calcium |
278
mg/dm2 |
<20
mg/dm2 |
Industry
tests on similarly stabilised pipes show values for Zn of <0.02
mg/l using the standard test procedure. Similar values were
obtained on Ca/Zn stabilised PVC window frames.
All calcium,
magnesium and zinc salts used in PVC stabilisers are listed
for indirect food contact.
