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
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