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The protection of flexible plastics from fungal attack

Akcros Chemicals GmbH : 25 February, 2002  (New Product)
This report - A Collaborative Study of the Plastics Protection Working Group (PPG) of the International Biodeterioration Research Group (IBRG)* - represents an abstract of work aimed towards the development of a method for assessing the activity of fungicides incorporated into plasticised plastics.
Members of the PPG, representing approximately 40 organisations from industry, academic institutes, government and independent testing laboratories, used the so-called Nutrient Salts Agar (NSA) test method (2) for carrying out a collaborative study programme.

The method involves placing thin plastic test samples onto a layer of non-nutrient agar and overlaying it with a thin film of similar agar inoculated with a fungal spore suspension. Such a technique overcomes the often hydrophobic character of many plastic surfaces and provides a very humid environment, encouraging the germination of fungal spores.

Samples tested in this way are visually rated for growth using the scale 0 = no growth, 1 = moderate and 2 = heavy. Any zone of inhibition formed around the test piece can be measured and noted but such data is not part of the overall assessment.

Collaborative studies carried out over six years were split into several phases. Firstly, it was necessary to familiarise the participants with the basic principles of the test and to determine the most important factors to obtain reproducible results. The second and third phases, carried out with plastic films (foils) prepared both in the laboratory and under manufacturing conditions, were concerned with reproducibility and repeatability of the technique.

Phase 1

In the first, induction, phase plastic foils, incorporating 370 and 500 ppm of the biocide 10,10´-oxybisphenoxyarsine (OBPA) were compared with biocide-free control samples. The work clearly showed complete fungal growth in the case of the controls whilst the foils treated with 500ppm of OBPA were free from any observable infection. Difficulties were encountered with assessing test pieces treated with the borderline concentration of 370 ppm OBPA. However, this problem was overcome and, in a two-year study, good reproducibility and repeatability were achieved.

During the first phase it was evident that this type of specialist testing needs be carried out by trained microbiologists working in laboratories appropriately equipped and being familiar with interpretation of results generated by such studies.

Phase 2

In the second phase of the work the spectrum of biocides investigated was broadened to include 4,5-dichloro-2-(n-octyl)-4-isothiazolin-3-one (DCOIT) and Zinc-bis-(2-pyridinethiol-1-oxide) (ZPT). These biocides were incorporated at concentrations of 2,000 and 2,500 ppm respectively.

All laboratories involved in the study obtained similar results for the PVC foils protected with DCOIT but variable results were found with ZPT. Additional studies demonstrated that such variability could be overcome by incorporating a buffering system into the agar-agar medium. Subsequent studies then resulted in better reproducibility, comparable to those found in the first phase of the work.

Phase 3

The third collaborative study was carried out using PVC foils prepared under manufacturing conditions. The test pieces contained the biocides already mentioned but incorporated a number of additional ones. Results from the third programme confirmed the conclusion of the second phase of the work, that the NSA method can be used as a standard test for determining the efficacy of biocides added to flexible plastics to prevent fungal attack.

Summary

During the six years of the collaborative programme a large number of parameters necessary for development of a standardised test method were investigated. It was found to be important to control the following variables:

* Preparation of the fungal spore suspensions. For example, it was shown that washing of the spore suspension had no appreciable influence on the final results. Therefore the NSA test does not include this step in the preparation of the inoculum.
* The control of spore viability. Although the polycaprolactone esterase test was investigated as a possible viability control, difficulties in carrying out such a procedure and interpretation of results from it led to its exclusion. Thereafter the viability control consisted of an unprotected PVC foil, included in the test and treated in the same way as the biocided samples.
* Preparation and application of the Nutrient Salts Agar. The work to investigate the amount of medium in the test vessels and the quantity required to cover the surface of the test pieces was carried out and quantities determined. The amount of covering agar was thereafter specified as 15 cm3 with a maximum of 5 hours elapsing between placing the test sample on the agar surface and applying the agar overlay. During this work the effect of pH on reproducibility of results was noted and further studies resulted in this being buffered at pH 5.5.

These developments, improving both the methodology and interpretation of results, were incorporated into the test method.

Conclusion

By performing the collaborative studies and approaching the question of testing flexible plastics in a stepwise manner the Plastics Protection Group of the IBRG has shown that the NSA method can be used as a repeatable and reproducible method for determining the efficacy of fungicidal additives.

Eugène Bessems, Akzo Nobel Chemicals, Chairman IBRG Plastics Protection Group

Reference
1. Bessems, E., 2000. The Protection of Flexible Plastics against Fungal Attack, Journal of Industrial Textiles, 30 (3), 185- 200.
2. Borgmann-Strahsen, R., Bessems, E., 1994. Evaluating Microbiological Susceptibility of Plasticised PVC Films, Kunstoffe Plast. Europe, 84 (2), 158 – 162
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