Development of Guidelines for the Formulation of Microbial Resistant Coatings with Optimal Levels of Added Biocide [DEFOG] - Project Updates
Note: The following summaries are provided for dissemination purposes to those who may have an interest in the work but not be directly involved in the projects. They may however contain references to other project reports containing full experimental details, access to which is restricted to Industrial Advisory Group Members.
Overall Summary to the end of the Fourth Quarter, Project DME 4.3
Structural materials are frequently coated with organic
surface coatings for aesthetic purposes and to afford protection
from degradation in the environment. The organic components of
typical surface coatings make them intrinsically susceptible to
infestation and spoilage by micro-organisms. In order to provide
an acceptable service life, biocidal additives are usually
incorporated into the coating formulation but, by their mode of
action, such additives are potentially toxic so that their use is
subject to increasingly stringent regulation and restriction.
It is suggested that many of the raw materials that are
commonly employed in coating formulę have intrinsic microbial
effects. Some of the ingredients may support and promote
microbial spoilage of coatings while others may be inhibitory in
their action. Currently these factors are not usually taken as a
primary consideration when designing a coating formulation with
antimicrobial protection.
The research contract awarded to the PRA will permit
development of screening methods for evaluating the influence of
raw materials on microbial activity. A database of the behaviour
for individual materials is being compiled. The information will
be used for formulation assistance when designing the optimal
concentration of biocide additive required for effectiveness.
General formulation guidelines are to be developed.
In the first quarter of the Project, procurement of a wide
range of raw materials for study had taken place. These had been
selected to provide representative samples from each of the
generic groups of the formulation ingredients. The materials were
catalogued and a list circulated to the members of the Industry
Advisory Group (IAG) for comments and advice.
Preliminary studies were carried out to find effective
microbial activity screening tests for the individual materials.
An important element of the investigation is the quantification
of microbial effects for each of the various materials in the
presence of other typical formulation components. PRA's initial
approaches to activity testing methods were questioned by the IAG
who felt the techniques did not adequately take account of the
influences from the other components. As a consequence a revised
experimental design was sought which would examine the behaviour
of each material in association with a selection of other
ingredients.
The range of raw materials used in coating formulations is
extensive. Restricting the level of each component to that
typically used in a coating, while employing simple mixtures,
still generates a requirement for a very large number of
experiments. Various techniques for handling the scale of the
programme have been examined. The team concluded the so termed
"Latin Square" experiment design was an efficient and
effective means for undertaking the evaluation task.
Work undertaken with a small range of materials in a (4 x 4)
Latin Square design, reported in the First Quarterly Report,
showed that the method was capable of generating an index value
for activity of a material with regard to its effect on growth
response for a yeast and for a mould.
To improve the efficiency of the screening programme, larger (9
x 9) Latin Square designs have subsequently been employed. In two
series of 9 x 9 Latin Square experiments 51 of the raw materials
have been assessed for response to mould and yeast in model
combinations comprised from a Resin, a Pigment and an Additive.
The results of these experiments were presented in the Second
Quarterly Report.
Evaluation of the raw materials will be an ongoing exercise
and in the Fourth Quarter Report results for a further 9 x 9
Latin Square are provided, covering in all 75 materials.
Statistical methods have been applied in order to link the
results derived from the individual Latin Squares experiments.
The design for a fourth 9 x 9 screening experiment is introduced.
Within the experimental sets a numerical response rating for
each of the tested materials is generated. In most cases a
distinction can be drawn for each material, relative to its peers,
in terms of its tendency to support or to inhibit micro-organism
growth.
When considering any individual rating it must be appreciated
that in the simple screening experiments only restricted
combinations are being examined and also that, for the Latin
square design, interaction effects are not probed.
In the 3rd Quarter, effort had been applied to completing the
preliminary screening and to finding the most suitable way to
represent the results in the material database. Concerns had been
expressed over the possible commercial implication of assigning a
specific rank to any raw material (IAG, 1st Meeting Minutes).
This is still the subject of much discussion by the IAG members.
Categorising the activity of a material towards supporting or
inhibiting microbial growth into broad bands, for example, very
good, good, neutral or poor could provide an acceptable
compromise, but it is difficult to provide agreeable boundaries.
The approach currently taken is to report the material rating
with a standard error.
It was noted that the level of assessed microbial activity
varies with time as well as with the type of organism used in the
challenge tests. The importance of the time of assessment and its
significance in determining a response index for a material is
still receiving consideration.
In response to the IAG's observation that some raw materials
may themselves contain undisclosed quantities of biocidal
additives, experiments where the test films were leached with
water prior to inoculation have been carried out. It is proposed
that this treatment should remove a substantial amount of any
added biocide. The results of these experiments, which were
presented in the Third Quarter Report, do show instances of
variation between the leached and non-leached systems.
It is necessary to test the predictive power of the material's
activity indices in actual coating formulations. During the 3rd
Quarter consideration had been given to this aspect of the
project, and formulation proposals were prepared for review by
the IAG. Subsequently, a Plackett Burman experimental design was
selected to explore the effects of material when combined in a
"realistic" paint formulation and in the Fourth Quarter
Report results of the laboratory microbial challenge tests on the
full formulations are reported.
While in most instances, each formulation ingredient shows an
effect which could be predicted from its behaviour in the
screening tests, a number of materials show anomalous behaviour,
for which explanations must be sought.
The full formulation paints are also undergoing field exposure
trials, and two months of assessment are reported. Although only
a short exposure period has occurred, the results so far confirm
that the component behaviour, predicted from the Latin Squares
screening tests, is in general being validated.
With the Fourth Quarter Report a pilot version of the
materials' database is being released (on CD-rom), to the IAG
members.
Attention is now being turned to the construction of the
Formulation Guidelines. With this in mind the Fourth Quarter
Report concludes with an outline of the experimental programme
which will probe the effects of other relevant factors on
microbial behaviour, eg. PVC, biocide concentration and leaching.
Also to be examined are the influences of secondary factors such
as gloss, surface energy, and water rub resistance.
Dr R Holman
Project Manager
15 November 1999
Note: The Funding Programmes "Degradation
of Materials in Aggressive Environments" (DME) and "Materials
Degradation in Aggressive Environments" (MDE) form parts of
the UK Government's, Department of Industry (DTI) support for
manufacturing industries.
