Chemical permeation

These pages give depth to the information in the services area.
Dr Bromwich has been actively involved with permeation research since 1992.
These pages describe the special permeation cells, instrumentation and techniques he has developed
To help you get acquainted, a Permeation Tutorial is provided on chemical permeation concepts. More detailed workshop notes are also available to download as a PDF file.

Permeation Research

How Gloves Fail

Dr Bromwich is continuing work started at Griffith University to investigate How and Why chemical protective clothing fails. A local safety supply company Industrial Decontamination Services (IDS) has been developing systems for laundering chemical protective clothing for ten years.
By laundering better quality gloves, it is often economic for companies to reuse a glove a number of times before it is visually degraded. Work done by NIOSH in the US has demonstrated that gloves can be thermally decontaminated and reused many times without significant degradation.

Gao, P., N. El-Ayouby and J. T. Wassell (2005). “Change in permeation parameters and the decontamination efficacy of three chemical protective gloves after repeated exposures to solvents and thermal decontaminations.” Am. J. Ind. Med. 47: 131-143.
Gao, P. and B. Tomasovic (2005). “Change in Tensile Properties of Neoprene and Nitrile Gloves After Repeated Exposures to Acetone and Thermal Decontamination.” Journal of Occupational and Environmental Hygiene 2(11): 543 – 552.

At present most gloves are bought on price rather than performance and discarded without knowledge of whether their protective life has been exceeded. In many cases, chemicals invisibly permeate the glove and the glove acts like a poultice, preventing the chemical from escaping and enhancing skin permeation. In other cases, almost new gloves are discarded.
When gloves are sorted, a more elaborate management system can be constructed and the gloves can be intercepted after laundering to determine How Gloves Fail.
To classify the way the gloves failed, a batch of 1000 gloves was examined and sorted into 30 categories of failure. For more details on this work and directions of the research, download the file below.

Clues, L., Bromwich, D. W. and B. Jones (2007). “How Gloves Fail.” Journal of Occupational Health and Safety – Australia and New Zealand 23(3):251-258 PDF

Griffith Permeation cells

Griffith Cell

Initially the ASTM F739 cell was redisgned to make the cell suitable for teaching chemical permeation in undergarduate laboratory session in an intrductory occupational hygeine subject. The cells had to be rugged (not glass), fast to assemble and easy to decontaminate between trials – and affordable. The Mk1 Griffith cell was borne, using Drager tubes as an integrating detector, and a personal sampling pump to suck or blow air through the cell and stain tube.

Automated intermittent exposure cell

The US standard ASTM F1383 prescibes an octopus-like glass permeation cell for measuring chemical permeation through chemical protective clothing under conditions of intermittent exposure. However the cell exacerbated the inherent problems in the ASTM F739 cell and it had to be manually creating additional problems. An automated intermittent cell was developed and the permeation data compared with published data in ASTM F1383. The data was also modelled, giving insights into intermittent exposure, including how exposures on one day can affect the performabce of a glove or suit the next day.

Pressurised FTIR-ATR Cell

Most chemical perfromance data for gloves is for pure, volatile liquids because they are easier to measure. There is little data on non volatile chemicals, complex mixtures and formulations and poorly water soluble chemicals. This challenge to be able to measure the “hard” chemicals lead to a pressurised FTIR-ATR cell. Fourier Transform InfraRed – Attenuated Total Reflectance is a powerful analytical chemical technique that has the potential to measure the permeation of most “difficult” chemicals including solids.

Evaporation Studies

Sometimes simple experiments can give intersting results. Standardard atmospheres are often generated in Tedlar (polyvinyl fluoride) bags. The PVF is very polar, but the polymer chains are very tight, making permeation through Tedlar quite slow. It was not known whether the injection of solvent onto the same spot for sucessive calibrations could affect the calibration, as the bag may retain a “memory” of the last claibration, particulatly for polar solvents. A 1 cm circle of Tedlar was placed on a microbalance and weighed as smaples of n-hexane (non-polar) amd MEK (polar) evaporated from the sample. A control experiment was conduted using a smaple of aluminium foil. The solvent initially began to simultaneosly evaporate and diffuse into the Tedlar, so two stages of weight loss were observed – an initial fast loss from evaporation and a much slower weight loss as the solvent diffused out of the Tedlar. Similar experiments could be conducted on gloves, chemcial suits and skin to simulate the effect of droplets of chemcials landing on the surface. As permeation is generally condentration dependent (see Crank “The Matematics of Diffusion”, very different chemcial expsores would be expeted from this type of chemcial exposure compared to continuous expure to a chemical.

Permeation and Penetration

Most people think that if a glove looks good, it will protect continue to protect the wearer. Even with an intact glove, the wearer will be exposed to chemicals though the process of permeation. It is not a matter of whether they are exposed, but how long before the exposure is toxic. It applied to gloves, chemical suits, boots, aprons, exhaust valves on respirators and even the breathing hoses on airline respirators. Have you ever had a funny taste in your mouth when using an airline respirator?

Permeation is a molecular process, driven by solubility and diffusion and highly dependent on temperature. The process is invisible and immediate, though the rate increases with time.
Penetration is a hydraulic process and is driven by pressure. We talk about penetration though holes, seams and zips.

Permeation through gloves

The permeation process can be seen as three stages –

The chemical dissolving in the glove polymer
The chemical diffusing though the glove, driven by the chemical gradient across the glove
The chemical coming in contact with the skin on the inside of the glove. This if often with evaporation, but the chemical can also dissolve in sweat and skin oils.

Once in contact with the skin, the health effects may be immediate with local irritation or other local effects, or can be systemic, with a similar permeation process transporting the chemical to blood vessels and the rest of the body.
This is of course, simplified as the immune system may be involved, or the chemical may be biotransformed in the skin. Also real-world formulations may have thousands of components (most fuels) that may act synergistically.

Consulting Occupational Hygienists