New Microbial Risks For Modern Water Borne Coatings
The adverse effect of low VOC coatings
Extract of a paper presented at the first Trans Tasman
Surface Coatings Conference, Surfers Paradise, Australia, August 1994
INTRODUCTION
Increasing concern is being expressed world wide about
uncontrolled emission of volatile organic compounds (VOC's) into the atmosphere.
The interaction of these compounds with other atmospheric gases in the presence
of ultraviolet light leads to an increase in low level ozone, a significant
factor in photochemical smog. This is not to be confused with the effect of
chlorine containing compounds on stratospheric ozone. (1) It should also be
remembered that the automobile is the biggest contributor to this problem.
Although solvent based systems are primary culprits in
this scenario, water borne coatings also contain VOC's and may make a
contribution to the atmospheric levels. The modern trend is therefore to
eliminate VOC's from coatings.
The major sources of VOC's in water borne coatings are:
1. Coalescing Solvents.
2. Free Monomer from Polymer Binders.
3. Other Additives (including biocides).
We shall therefore look at the effects of reducing the
levels of these various classes of material, on the microbial preservation of
the coatings.
COALESCING SOLVENTS
Coalescing solvents are the most significant contributor
to VOC's in water based coatings as they are present at levels of 1-2%. They are
added to the paint to assist in the process of film formation.
In Germany in 1990 approximately 460,000 tons of flat wall
and house paint and 120,000 tons of dispersion bound plasters were used.
Assuming 1-2% coalescent in these products then 10,000 tons of VOC were released
to atmosphere (2) Similar levels probably apply to other industrialised
countries.
Work carried out at the Paint Research Association has
shown that some coalescing agents may make a significant contribution to the
preservation of the paint.
The following results are produced with the permission of
R. Springle of the Paint Research Association
The conclusions drawn from this were that the removal of
these materials could dramatically alter the susceptibility of the paint system
to microbial spoilage requiring much higher levels of in can preservative to
provide protection.
There exists a class of so called conformal solvents to
replace the above but as yet no one has examined them to assess their
microbiological effect.
The PRA are currently setting up a project to examine the
effects of lowering VOC's on preservation of Paint.
It is perhaps worthwhile mentioning at this stage that the
initial tests were carried out with freshly prepared paint and that there is no
indication as to the long term effectiveness of these solvent as preservatives.
Such effects as phase partioning and hydrolysis may render them less effective
long term.
FREE MONOMERS
Free monomers are the unreacted residues left at the end
of the polymer latex production. They are present at lower levels than the
coalescent agents but due to their higher volatility they contribute
significantly to the VOC during the application stage (2). They also affect the
aesthetic properties of the product, in as much as they often have relatively
high odours. They also are more toxic then coalescing agents, and it is possible
that in an unventilated room exposure limits for monomers could be exceeded.
Polymer manufactures have for a number of years been
attempting to reduce the level of free monomer to the parts per million level.
This has led in some cases to an increase in microbiological problems. Unlike
coalescents, free monomers at the levels typical before the attempts to reduce
VOC began, do not significantly contribute to the preservation of the latex.
This runs contrary to the folklore in the polymer latex industry that free
monomer is preservative. This might have been true post war when levels of vinyl
acetate were 0.5-1% but is no longer true.
To understand how problems occur it is necessary to
understand a little about the method of latex production. Latex polymerisation
is carried out by means of free radical polymerisation of various monomers using
peroxy compounds such as persulphate. This drives the reaction to 95-99%
completion and in order to lower the free monomer level the resultant latex is
either stripped or subjected to a post treatment. Stripping involves the removal
of free monomers by evaporation usually involving heating and applying vacuum.
Post treatment usually involves the addition of a redox couple to generate free
radicals at low temperature to drive the polymerisation reaction. A redox couple
consists of a reducing agent and an oxidising agent.
It is in the post treated latex systems that contamination
problems have been occurring. This is usually due to deactivation of the biocide
active ingredients. This is caused by unreacted residues of the reducing or
oxidising agent used in post treatment Consequently systems that have been
produced happily for a number of years suddenly become subject to
microbiological problems.
This first came to our notice in 1991 when a long standing
customer using a1,2-Benzisothiazolin-3-one (BIT) containing biocide, suddenly
had spoilage complaints. It was found that the post treatment system had been
changed from an ascorbic/acid hydrogen peroxide system to a system containing
sodium hydrosulphite, an extremely powerful reducing agent acting by liberating
sulphite ions.
In a simple experiment BIT solution was incubated at
various temperatures for 6 hours in the presence of 500 ppm of various reagents.
The level of BIT was determined by HPLC. The results are shown in Figure 1.
Figure 1 Early experiments with BIT in solution
As a number of polymer manufactures were experience
similar problems, it became necessary to develop a reliable method to
determining the actives of interest at in use concentration. The actives were
BIT and the mixture of 2-methyl-4-isothianzolin-3-one and
5-chloro-2-methyl-4-isothiazolin-3-one (MI and CMI)
After some consideration the technique of dialysis was
found to be most universally applicable. The HPLC analysis was carried out using
an instrument fitted with a Diode Array detector. This equipment is capable of
carrying out determinations at two wavelengths simultaneously and also collects
a spectrum of the analyte. In Figure 2 is a typical chromatogram. The MI elutes
at 3.3 min and is interfered with by other early eluting analytes. The CMI
elutes at 5.3 minutes and the BIT at 6.2 minutes. In some cases the BIT was
interfered with by monomer and analysis was done at the secondary wavelength
maximum of 320. See Figure 3 for the spectra of the analytes.
Figure 2 Typical HPLC chromatogram of a polymer
dialysate
Figure3 Spectra of the BIT and CMI
As the methyl isothiazoline is less microbiologically
active than the chloromethyl this was not determined.
Using the technique of dialysis followed by HPLC then it
was possible to work closely with polymer latex manufactures to examine what was
happening at the critical period following the post treatment of the latex. As
can be seen in Figure 4 the longer the delay between adding the post treatment
and adding the biocide the more CMI remains.
Figure 4 The effect of delaying the addition of biocide
to an acrylic adhesive emulsion polymer.
Due to the complex nature of the situation following redox
post treatment, it was not always as clear cut, and a set of experiments was set
up under controlled conditions. In these experiment the isothiazolinone actives
were exposed to the redox reagents in a fixed molar ratio both alone and in
combination. This was done in phosphate buffers at pH 4, 7 and 9 approximately.
Some of the information is represented graphically in Figures 5, 6 and 7.
Figure 5 The effect of individual Redox reagents
(0.1mM) on CMI (0.2mM) at different pH's
Figure 6 The effect of combinations of Redox reagents
(both at 0.1nM) on CMI (0.2mM) at different pH's
Figure6 The effect of individual Redox reagents (1mM)
on BIT (3mM) at various pH's
We gained considerable useful information from this
exercise and it allowed us to predict the behaviour of products after certain
post treatment regimes. And we have successfully assisted some of the major UK
polymer manufactures to reduce free monomer levels without increasing the
microbial risks (3, 4).
OTHER ADDITIVES
As manufactures of biocides, my remarks will be confined
to this area. Manufactures of other paint additives will face similar problems.
Volatile organic compounds are present in biocide
formulations as:
1. Active Ingredients.
2. Solvent or Cosolvent
for Active Ingredients.
3. Antifreeze etc
In the case of active ingredients one of the most popular
class of actives is the formaldehyde release Biocides. These have been out of
favour in some areas for some time and the trend will be almost certainly to
reduce their usage. This will restrict the available number of raw materials.
The removal of formaldehyde releases will rule out a very popular formulation
type, that of formaldehyde releaser and CMI, MI combinations
CONCLUSION
The move to low VOC coatings will inevitably lead to an
increase in microbial spoilage problems. This is something all coatings
manufacturers should be aware of.
L. Conquer
REFERENCES
(1) E Pratt, Surface Coatings International, 1994 (4)
pp132-137
(2) H Zeh et al, Surface Coatings International, 1994
(4) pp144-151
(3) L Conquer, Polymer Paint Colour Journal, Sept 1993
pp421-423
(4) L Conquer, European Coatings Journal,
Sept 1993 pp592-597
