Covering Sources of Toxic Vapors with Foam

Dr. Walter P. Aue and Fausto Guidetti
SPIEZ LABORATORY
CH-3700 Spiez, Switzerland

INTRODUCTION

In a case of chemical terrorism, first responders might well be confronted with an obvious source of toxic vapor which keeps spreading out its hazardous contents. With foam as an efficient and simple means, such a source could be covered up in seconds and its effects mitigated drastically. Once covered, the source could then wait for a longer time to be removed carefully and professionally by a decontamination team.

In order to find foams useful for covering up toxic vapor sources, a comprehensive set of measurements has been made in order to answer the three main questions:

Which foams could be used for this purpose?
How thick should the foam cover be?
For how long would such a foam cover be effective?

The basic idea behind the experiments was to measure the time to breakthrough of warfare agents and simulants as a function of layer thickness for a selection of fire fighter- and other foams.

MATERIALS AND METHODS

Method. In order to obtain consistent results for breakthrough times, the following laboratory model was developed:
Measurements were done in cylindrical glass beakers of 60mm i.d. and heights of 10, 20, 30, 50, 70, and 100mm. 0.5mL of agent was pipetted into small dishes of 37mm i.d. on the bottom of the beakers in order to avoid special effects between foam and beaker walls affecting the agent.

In order to simulate reality somewhat, foam was shot from a distance of 2m into a 60cm x 60cm x 20cm enamel coated metal tub mounted one corner pointing down and tilted, such that the foam would flow out of the tub. Then, it was transferred into the measuring beakers to cover the agent, depending on the fluidity either with a lab beaker or with an icing bag with a nozzle of 14mm diameter. With very stiff foam, gentle pressure had to be applied to the bag. After filling the beakers, excess foam was removed with a ruler to obtain the proper foam thickness.

The measuring beaker was then closed with a glass cover with an inner height of 12mm and four vents spaced equally on its circumference. Through 3 vents, 3 air streams of 80L/h in total were fed in order to clear the vapor space evenly through the 4th vent, to which a T-tube was attached. The one end of the tube was open; to the other end, a chemical monitoring device was attached. The air flow through the vapor space was in any case larger than the maximum suction of any of the monitors. By these means, the flow through the vapor space, and accordingly the dilution of the diffusing agent vapors, was kept constant irrespective of the suction of the monitor.

The breakthrough threshold concentration at the above flow was arbitrarily put at 100µg/m3. Finding a chemical agent monitor sensitive enough and not cross sensitive to the foams was quite a challenge. In the end, the monitors used, after appropriate calibration and with their respective evaluation softwares, were Proengin AP2C, Smiths Detection Swiss CAM, Bruker RAID M-100, and for the Jomos protein foam the Inficon Hapsite Smart II. On the latter the cycle time could be minimized to 10min.

In a given experiment, a series of foam thicknesses was measured simultaneously using the same batch of foam. To start with, the beaker cover was put onto the smallest beaker until breakthrough, then the next higher beaker was measured.

In order to monitor foam quality independently, dehydration of the foams was measured as a function of time in parallel with the breakthrough measurements in a 1L graded conical glass sedimentation vessel. This parallel monitoring proved to be a crucial tool for getting credible and consistent results. The characteristic parameter calculated was the dehydration time to 90%, i.e. D90.

All measurements were done twice at an ambient temperature of 20°C.

Foams. The selection of foams was a cross section of fire extinguisher foams available in Switzerland, together with the decontamination foam CASCAD introduced in the Swiss army for the NBC troops. In detail, they were:

- CASCAD: aqueous decontamination foam with active chlorine at pH = 10;
- 3M FC 602 ATC plus: AFFF lightwater foam at pH = 6;
- Jomos Schaumgeist: protein foam at pH = 6;
- Minimax MXOL 09 at pH = 4.5;
- Moussol: alcohol resistant AFFF;
- Primus LS TS: ATC air foam at pH = 8;
- Primus LW TN: ATC Light water foam;
- Solberg Arctic foam 600 ATC at pH = 5.5;
- Solberg Rehealing foam RF 3x6 at pH = 6.5.
In bold are the names of the foams as they will be used for the remainder of the paper.

CASCAD was prepared in the dedicated backpack set according to the company's procedures with a pressure of 6bar. MXOL and Primus foams were applied from regular fire extinguishers. FC 602 (6%), Jomos (3%), Moussol (3%), Arctic (6%) and Rehealing (6%) foams were prepared by respective dilutions of concentrates with deionized water and applied with a small foam generator at a pressure of 6bar, corresponding to the pressure found on fire fighter trucks.

Chemical warfare agents and simulants. So far, breakthrough times have been measured for several simulants and GB. At an early stage of the project, when breakthrough was assessed by nose, banana oil and methyl salicylate were the simulants of choice. Later on, with the monitors, DMMP was used. As for chemical warfare agents, GB has been measured. The purity of the simulants was technical grade; GB was better than 95% pure.

RESULTS

The essential results are assembled according to the three main topics, namely
- breakthrough times;
- correlation of breakthrough times vs. dehydration times; and
- correlation of breakthrough times of DMMP vs. GB.

Breakthrough times. Breakthrough times as a function of foam layer thickness are the key results of these experiments. They are shown in Fig. 1 for the 4 most long lived foams for DMMP and GB.

(Editor's Note: Because these very important and intricate Figures 1-3 are in color-we are not able to display them in the Newsletter; however, this complete Paper including Figures will be on the web, www.asanltr.com, by 18 July 2008.)

Fig.1: Breakthrough times as a function of foam layer thickness for the 4 most long lived foams for DMMP and GB. The dark colors are for GB, the light ones for DMMP. Two experiments have been performed per foam and agent under "identical" conditions. For technical reasons, no DMMP could be run for Jomos foam. GB did not break through CASCAD foam at 10cm thickness, i.e. this breakthrough time would be at infinity.

Four features are obvious from Fig.1:

- The breakthrough times fall in the foam sequence CASCAD - Jomos - FC 602 - Rehealing.
- For the same foam, the breakthrough times are clearly shorter for the highly volatile GB than for DMMP.
- According to the nature of experiments with foam, there is considerable scatter in the data.
- A10cm layer of CASCAD foam contains enough active chlorine to destroy 0.5mL of GB.

Breakthrough times vs. dehydration. In essence, the barrier effect of foam lasts as long as the foam contains some water. As an illustration of this fact, the relation of dehydration times D90 and breakthrough times at a 5cm layer of CASCAD, Jomos, Rehealing and FC 602 is illustrated for GB in Fig. 2.

Fig. 2: Correlation of dehydration times D90 and breakthrough times for GB for a choice of foams for a layer of 5cm.

From the clean correlation in Fig. 2, it seems obvious that assessing D90 from the dehydration process is an excellent means to monitor the quality of the different foams. This means that an erroneous experiment can always be detected at an early stage from an erroneous dehydration behavior. Furthermore, we can conclude that the easy to measure dehydration times D90 yield a good forecast for the breakthrough times.

Breakthrough times of DMMP vs. GB. The breakthrough times of DMMP and GB have been correlated in pairs for the same foam and the same layer. This should allow to investigate, whether the two agents are affected differently by the three foams, and therefore see the influence of the different chemistries of the three foams.

To this end, the correlation of breakthrough times through a series of layers of a choice of foams is shown in Fig. 3.

Fig. 3: Correlation of breakthrough times through a series of layers of CASCAD, FC 602 and Rehealing. E.g. a given data point correlates the breakthrough times of GB and DMMP through a 5cm layer of CASCAD foam. A 10cm layer of CASCAD has not been broken through and therefore can not be shown.

From Fig. 3, it becomes obvious, that the foams affect the agents DMMP and GB quite differently. For Rehealing, the breakthrough times for GB are approx. 40% of those for DMMP, whereas for FC 602, the ratio is approx. 20%. CASCAD lies in between. This means that the foams act not solely as physical barriers, but also have some chemical effects on the agents. For Rehealing and FC 602 with pH values of 6.5 and 6.0, respectively, this is quite surprising. Not so surprising is this aspect for CASCAD with a pH of 10 and a good load of active chlorine: Whereas GB breaks through a 7cm layer of it, it is chemically destroyed by 10cm. DMMP on the other hand is more stable and breaks through 10cm of CASCAD.

DISCUSSION

In this paper, we present our experimental approach to investigate the topic in some detail. The preliminary results represent six months of work and illustrate the fact that for making reproducible experiments foam is not an easy material. In spite of this, they clearly identify foams suitable or less suitable for the job.

The most suitable foam for this application appears to be CASCAD: With a layer of 7cm, it stops GB vapors for 200min; 10cm of foam on the other hand seem to contain enough active chlorine to destroy 0.5mL of GB and accordingly to prevent any breakthrough.

Of the conventional fire fighter foams, the best ones are Jomos, Rehealing and FC 602 with GB breakthrough times of 235min, 85min and 63min. Featuring pH values between 6.0 and 6.5 and consequently not having much chemical activity (hydrolysis), they act mainly as physical barriers, although they affect chemical agents to some extent.

For DMMP, the breakthrough times for CASCAD, Rehealing and FC 602 amount to 1095min, 211min and 324min, respectively, and are therefore approximately a factor of 2.5 to 5 longer than for GB, corresponding with the lower vapor pressure of DMMP. The Jomos/DMMP combination could not be measured, because all chemical agent monitors were strongly cross sensitive to the foam, and the Hapsite Smart II showed prohibitive memory effects for DMMP.

The remaining foams Arctic, LS TS, MXOL, LW TN and Moussol with D90s of 25min, 23min, 15min, 15min and 8min, respectively, were not investigated any further: Arctic and LS TS were expected to have breakthrough times similar to FC 602, based on the conclusions from Fig. 2. The others had even shorter lives and therefore were left aside.

As for the thickness of the foam layer, the rule "the thicker the better" applies. We limited our experiments to 10cm considering the use of portable fire extinguishers and the amount of foam needed to cover an agent spill with such a layer.

The next step in the project will be measurements with HD because of its different chemistry and higher stability. Later on, the experiments need to be validated in field trials; however some thinking might be required first, concerning different surface materials, application of the foam and measuring layer thickness and agent breakthrough in larger dimensions.

ACKNOWLEDGMENT

We thank Christian SchŠfer for his idea to monitor foam dehydration with sedimentation vessels and his miniature foam generator. Many thanks also to Christophe Curty, Benjamin Menzi and Roland Kurzo from the organic synthesis group who supplied the chemical warfare agents, Roland Liebi from personal NBC protection for lending us his AP2C measuring setup, and Silvia Sager for the icing bag.

Keywords: C-terrorism, first responder, foam, GB, mitigation, toxic vapor

FIGURES

Fig. 1: Breakthrough times as a function of foam layer thickness for the 4 most long lived foams for DMMP and GB. The dark colors are for GB, the light ones for DMMP. Two experiments have been performed per foam and agent under "identical" conditions. For technical reasons, no DMMP could be run for Jomos foam. GB did not break through CASCAD foam at 10cm thickness, i.e., this breakthrough time would be at infinity.