A rapid method for investigating the absorption of formaldehyde from air by wool
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- Curling, S.F., Loxton, C. & Ormondroyd, G.A. J Mater Sci (2012) 47: 3248. doi:10.1007/s10853-011-6163-7
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Formaldehyde emitted from household products, such as furniture produced with medium density fibreboards, has been reported as causing health concerns in both domestic and business environments, these concerns being generally known as ‘sick building syndrome’. A number of differing approaches to removing formaldehyde from the atmosphere have been investigated. It is known that formaldehyde binds to wool fibres when the formaldehyde is in the liquid phase. However, few investigations into the sorption potential of wool for vapour phase formaldehyde have been made. This article details a rapid, novel method to directly measure the uptake of formaldehyde by wool and by inference, other materials. The data detailed in this article also demonstrates the significant ability of wool to sorb formaldehyde in the vapour state.
In recent years, there have been an increasing number of health concerns about the effect of airborne contamination by volatile organic compounds (VOCs) in work and domestic environments—frequently termed ‘sick building syndrome’ [1, 2, 3, 4, 5]. One such VOC that has been the focus of many investigations is formaldehyde (CH2O), which occurs naturally at various concentrations in a variety of natural materials, e.g. meat (2–20 mg/kg), fruit and vegetables (6.3–35 mg/kg) , wood (0.04 mg/kg) . It is also a product of human metabolism and can be detected in human breath at levels ranging from 1.2 to 72 ppb . Current guidelines stipulate limits of 0.1 mg/m3 in interior air to avoid adverse health effects .
Formaldehyde has been used in a large number of industrial products ranging from textiles to disinfectants, but historically the most of the concern over formaldehyde has been concentrated on emissions from urea formaldehyde foam insulation (UFFI) (particularly in mobile homes) and from wood products, particularly particleboard, OSB and medium density fibreboard (MDF), bound with formaldehyde containing resins, e.g. urea formaldehyde (UF), melamine-urea formaldehyde (MUF) and phenol formaldehyde (PF) [1, 10].
In contrast, few studies into the effectiveness of wool to sorb gaseous state formaldehyde can be found in the scientific literature. Some chamber studies, using methods to measure the reduction of formaldehyde levels in the air, have shown that wool-based furnishings can reduce the levels of formaldehyde . However, these methods do not directly measure the formaldehyde sorption and also require substantive facilities to perform the tests. This study details a relatively rapid method utilising Dynamic Vapour Sorption (DVS) techniques to directly measure the uptake of formaldehyde by wool using small scale methods. This method will allow the assessment of vapour sorption of formaldehyde not only for wool fibres but possibly for a wide range of natural fibres. As there is considerable commercial interest in the ability of fibres to sorb formaldehyde, this method could act as a screening method for fibre properties.
The DVS system is designed to measure accurately a change in sample mass, as it sorbs precisely controlled concentrations of water vapour in a nitrogen carrier gas. The relative humidity (RH) is varied by passing the nitrogen stream through water in varying proportions depending on the RH required. The sample mass readings from the microbalance are then used to reveal the vapour sorption/desorption behaviour of the sample.
Theoretically, if a dilute aqueous solution of formaldehyde is used in place of pure water, then the amount of formaldehyde sorbed by the wool fibres can be calculated, from the difference in mass of the fibre pre- and post-exposure. In a standard water isotherm, the change in mass should return to zero at 0% RH; however, if formaldehyde has been sorbed, then there should be a difference in mass at the 0% RH point. Based on this difference, the sorption of formaldehyde by the fibres can be calculated based on the unit weight of the fibre.
Sorption analyses were performed using DVS system (Surface Measurement Systems, London, UK).
Sheep wool fibre was used as the sample material. A small amount (approximately 10 mg) was teased out and placed onto a pre-cleaned sample pan which was carefully hooked onto the hang-down wire connected to the microbalance. The water reservoir in the DVS system was filled with deionised water, and the sample chamber was then sealed. Nitrogen was passed over the sample at a flow rate of 200 cm3/s at a temperature of 21 ± 0.2 °C. The schedule for the DVS was set to steps of 0%, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% and 90% RH for both the sorption and desorption steps. Data on mass change were acquired every 20 s. At each RH, an algorithm was set to ensure that equilibrium had been reached when the ratio of change in mass in relation to change in time was less than 0.002%/min for at least 10 min. Integral humidity and temperature probes located close to the sample allowed direct monitoring of the actual humidity and temperature, with these data being used in the analysis of the results, rather than the set values. It was noted during the experimental study that RH readings when using the formaldehyde solution were lower than that with water, probably because of differences in vapour pressure, highlighting the need to use the recorded data rather than the set targets.
After the determination of the water isotherm, the water reservoir in the DVS system was filled with a 9.25% solution of formaldehyde in water (diluted with deionised water from a 37% solution (Sigma-Aldrich, UK)), and the wool/formaldehyde isotherm was determined in the same way as above.
As noted above, wool sorbs formaldehyde by both chemisorption and physisorption. Whilst chemisorption should be irreversible, the physisorption component of the total sorption may not be. To determine whether the sorption of formaldehyde was reversible, samples that had been exposed to formaldehyde were subsequently retested using water vapour only (after cleaning and purging of system for removing formaldehyde).
As it was thought possible that a single sorption/desorption cycle may not give a true maximum value of formaldehyde sorption, fibre samples were subjected to repeated sorption/desorption cycles. As the determination of the maximum sorption levels would not require a full isotherm and to obtain more rapid determination of the equilibrium, the cycles consisted of repeated 0%, 90%, 0% steps rather than the full range of steps.
Results and Discussion
An important consideration is that the sorption values given above will correspond to maximum values obtained at the given temperature of 21 °C and when using high formaldehyde concentrations. In practical use of woollen products, it is possible that environmental variables and various treatments and finishes of the product could affect the sorption properties of the fibres. This method could be used to test the significance of these variables and treatments.
The investigation shows that the technique proposed in this article is a novel and valid method for directly measuring the sorption of formaldehyde by wool fibres. The method is rapid (4–5 days duration) and utilises small sample sizes. This method will allow rapid testing of wool fibres derived from different sources and under different environmental conditions and could be adaptable to other fibre types and other vapour state compounds
The data provided also show that wool fibre is capable of sorbing significant amounts of vapour phase formaldehyde from air, although some may be re-released under certain conditions. These data imply that wool used in textiles and insulation may be a realistic method of reducing airborne formaldehyde contamination.