One of the modern-day innovations designed to make our life easier is the nanofilm spray product. That may not ring any bells, but most of us will have met NFPs in our time. They are available in industry and as consumer products in spray cans for applying non-stick or self-cleaning coatings to a range of domestic surfaces including bathroom tiles, windows, floors and textiles.

The coatings are based on siloxanes polymers that are prepared in situ by the sol-gel process after spraying. As the solvent evaporates, a series of hydrolysis and condensation reactions occurs to produce films having the desired properties, based on the initial components present. However, using the product might present a risk to the user.

During the spraying process and the subsequent film-forming reactions, it is possible that particles and volatile organic compounds (VOCs) are emitted into the air. This occurred with an early NFP which was responsible for acute respiratory problems in Germany.

Assessing the consumer risk is complicated by the fact that the components of the spray are not all listed in the manufacturers' material safety data sheets (MSDS), according to a group of researchers in Denmark. So, the team set out to identify the VOCs and analyse the particles that are produced in typical consumer products.

Peder Wolkoff and colleagues from The National Research Centre for the Working Environment in Copenhagen and the University of Copenhagen analysed four NFP products on sale in the UK, German and Nordic markets, selected to be representative of the different types of non-aqueous NFPs. The sprays were applied to a stainless steel plate in a closed container and the gases and particles were sampled through special ports.

The gases were trapped in sorbent tubes for analysis by GC-FID and GC/MS using a 14% cyanopropyl-phenyl, 86% dimethylpolysiloxane medium polarity column. The mass spectrometer was operated in electron ionisation mode for compound identification using the NIST 08 mass spectral library, or in chemical ionisation mode with isobutane as reagent gas when the spectra did not match.

The VOCs were quantified by GC-FID using calibration curves produced from standard compounds or by comparison of the FID response with those of similar compounds. In addition, the VOCs from the spray chamber were analysed directly by real-time membrane introduction mass spectrometry (MIMS) using a miniature portable instrument. Particle sizes were also measured.

All of the MIMS spectra showed peaks corresponding to the cyclic siloxanes from cyclotetra- to cycloheptasiloxane, which were confirmed by GC/MS. However, the MIMS and GC/MS spectra were different for each NFP, confirming the different product compositions.

Most of the airborne particles formed during spraying were below 100 nm in diameter for three of the NFPs and below 200 nm for the fourth, the size distributions differing from product to product. The researchers note that there is "no general understanding of the potential effects of nanoparticles exposure" but pointed out that particles of this size are known to have significant deposition efficiency in the respiratory tract and airways. There is a real possibility of airway damage.

The first NFP was intended for ceramic tiles and the MSDS stated that it contained an unspecified fluorosilane with isopropanol as solvent. However, many other compounds were detected after spraying, including diisopropoxymethane, 1-chloroacetone, 1,1- and 1,3-dichloroacetone and a volatile perfluorosilane. The presence of the chlorinated acetones was "surprising." They are toxic with similar effects to the tear gas bromoacetone and were not listed on the MSDS. Similarly, the non-listed perflorosilane accumulates in the body and is cytotoxic.

The team estimated that the total airborne concentration of the chloroacetones would be about 6700 µg/m³ in a standard room with no ventilation in a worst case scenario. This equates to twice the maximum level defined by the American Conference of Industrial Hygienists (ACGIH).

For a multipurpose NFP declared to contain propane and butane as propellant and kerosene, many C₄-C₁₄ hydrocarbons were emitted from a multipurpose NFP, including limonene. This monoterpene reacts with ozone to produce several oxygenated VOCs, including formaldehyde and secondary organic aerosols, which are known to cause health problems.

Of the remaining NFPs examined, one was produced for ceramic tiles and the MSDS listed ethanol, methanol and unspecified siloxanes. The fourth was intended for coating glass and contained photocatalytic titanium dioxide and ethanol. After spraying, low-molecular-weight ketones were found in both cases, plus low amounts of alkylcyclosiloxanes.

In all cases, VOCs and nanoparticles were emitted during application of the NFPs. A number of the VOCs were absent from the product MSDS and the research team suggested that "their presence may not be known to the manufacturer." Along with the nanoparticles, some of these are known to induce adverse health effects but it is very difficult at this stage to assess the associated risk.

They concluded that "further investigations of the particle phase and the toxicological effects of the complex NFP exposures are necessary for assessing the entire health risk associated with the use of these new product types."

Related Links:

• Environmental Science and Technology 2009, 43, 7824-7830: "Release of VOCs and particles during use of nanofilm spray products"

Article by Steve Down

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.