Trapping VOCs in paint to improve indoor air quality

Indoor Air Pollution

With huge plumes of dark gas arising from factories and smoggy hazes being seen surrounding big cities, outside air pollution is often easier to visualise as a problem (Figure 1). Indoor air pollution is generally not as visible, and sources aren’t as obvious, however, it is thought that concentrations of pollutants are generally 2-5 times higher indoors than outdoors.

Figure 1: Factory chimneys give off a lot of gas which is quite an obvious source of outdoor air pollution. Indoor air pollution is not so obvious.

Examples of indoor air pollution

The World Health Organisation (WHO) has published documentation on indoor air quality focusing on compounds that are of particular note indoors (Figure 2). Many of these have in common that they are classified as volatile Organic Compounds (VOCs). VOCs have high vapour pressures and as such exist as invisible gasses. They also often have strong odours and are generally responsible for the “new” smell on many products.

Sources of indoor air pollution

The popular “new car” or “new book” smell can be attributed to VOCs but the main factor in most indoor environments is decorating materials. Paint and carpet often cover most of a room. As will be discussed in more detail, paint has historically contained VOCs and carpets often contain them in the backing and adhesives. Although a lot of the VOCs are emitted soon after application, they can continue to emit low levels for years afterwards in a process called off-gassing. Therefore, even after the smell has gone away, the risk has not. Furnishings and common office equipment, such as copiers, are also sources, so the air quality in work environments can be of particularly high concern.

Figure 2: Some of the key indoor air pollutant compounds highlighted by the WHO in the clouds along with the most prominent sources in the orange labels. 

Effects of indoor air pollution

The WHO does recognize indoor air quality as a health risk. Some of the associated concerns of indoor air pollution and VOCs, in particular, are eye, nose and throat irritation, headaches and some VOCs are suspected or known to be carcinogenic.

These do not apply to all VOCs and some only from long-term, prolonged exposure but the need to reduce concentrations is clear.

Indoor air quality standards

Despite this, the WHO do not set guidelines for VOC concentrations indoors. Some countries have brought in their own, particularly with regard to paint. France (Afsset) and Germany (AgBB) have regulations that consider VOC concentrations after 3 days from the application of the product and again after 28 days. Anything above 10 mg m^-3 after 3 days or 1 mg m^-3 after 28 days is deemed unsafe. The UK differs slightly but does still have limits on VOC concentration in the product at the ready-to-use stage in g l^-1 (Table 1) where a VOC is defined as “any organic compound having an initial boiling point less than or equal to 250 °C at a standard pressure of 101.3 kPa”. The VOC content allowed differs in each kind of product and also between the type of product (water-based or solvent-based). Concentration limits range from 30 g l^-1 for interior matt paints and water-based primers to 750 g l^-1 for solvent-based binding primers.

Table 1: The maximum VOC content allowed in coatings, both solvent and water-based, as specified by the UK legislation.

Solutions to indoor air pollution

Most manufacturers, despite governmental restrictions, have adjusted their products to reduce the VOC levels below the limits set above because consumers are becoming more aware of the issue and looking for low VOC alternatives. This, however, does not mean the problem is solved. Older materials can still be giving off VOCs for years afterwards and other sources, particularly in an office, remain prominent.  

VOC in Paint and Coatings

VOCs are used in solvent-based coatings to coalesce the polymer and lower the minimum film formation temperature (MFFT) to below room temperature. Their high vapour pressure means that they will evaporate from the product quickly leaving behind just the paint that is then harder and more durable thanks to the coalescent (Figure 3). This is more prominent in solvent-based coatings because, in water-based coatings, water can provide a similar role and evaporate off. The VOCs, therefore, allowed reduced drying times, which was particularly useful in DIY household jobs. While they prove to be an effective addition to paint, for indoor areas with little to no ventilation or DIY projects where the room could not be left to dry and off-gas for a few days, the health risks outweigh the benefits and alternatives are now available.

Figure 3: Schematic of the film formation of the polymers in paint upon evaporation of the water or VOCs depending on whether the paint is water or solvent based.

VOC free paint

Low VOC or sometimes even VOC free paints are now readily available for a whole range of uses. Due to the role VOCs played in previous paints, the challenge was to overcome thick, difficult to apply paint that had long drying times. While some still need more applications than regular paints to achieve the same colour, options are improving (Figure 4). This has mainly been achieved through improved polymer chemistry that allows a lower MFFT. Blends of hard and soft monomer units within the polymer can be arranged such that all desired application and durability properties are achieved.

Compounds with boiling points above the specified 250 °C, can also be used in coatings to produce similar results as VOCs but with fewer health implications.

Figure 4: Low or no VOC paints can need more applications than solvent-based paint with VOCs.

How to Reduce Indoor Air Pollution

Despite these alternatives, indoor air pollution is still a concern. Newer technologies utilise chemical sieves, zeolites, in paint to trap VOCs so that they are not only non-contributing to indoor air pollution, but they are actively reducing it.

Zeolites

What are Zeolites?

Zeolites are the name for a broad range of compounds based around an aluminium silicate framework. Their cage-like structure means that other molecules can be accommodated and adsorbed into the central pore (Figure 5). Synthetic production means zeolites can be tweaked to selectively accommodate desired molecules. Selectivity is normally achieved due to the size of the molecule, but it can also be done based on polarity.

Figure 5: The crystal structure of a zeolite based on an aluminium silicate framework. 

ZEOflair

ZEOCHEM offer several additives that can adsorb VOCs called ZEOflair. ZEOflair traps VOCs irrespective of size due to the mixture of pore sizes. They also offer additives for a range of VOC hydrophobicities, so optimal adsorption can be achieved (Figure 6). By trapping the VOC within the framework, it prevents the compounds being breathed in meaning they eliminate odours as well as their harmful effects.

For use in flooring and carpets, ZEOflair can be added to PVC. Measurements taken using dynamic olfactometry and GC-FID found that as little as 1% ZEOflair added to PVC produced a 60% reduction in odour concentration and 23% reduction in VOC concentration.

Within paints, 1.5% ZEOflair (by weight) in the formulation offered an 80% reduction in formaldehyde concentrations for over 5 years before reaching saturation. This was calculated based on a daily inlet of 50 µg m^-3 into a room of 4 x 5 x 2.5 m and the assumption the air would recycle 5 times per day. The zeolite pores allow for the adsorption of formaldehyde up to 15 % of the initial weight of ZEOflair added. In the room size given above, it's estimated about 9 kgs of paint would be used on the wall, of which 135 g would be ZEOflair and so 15 % of that weight, 20.25 g of formaldehyde can be adsorbed before saturation is reached.

Figure 6: Different ZEOflair additives are better for different VOCs. If the hydrophobicity of the VOC is known, then the optimal additive can be chosen.

As a free-flowing powder with high thermal conductivity, this technology can be used in a wide range of materials and applications without altering the physical, chemical or mechanical properties. Alongside paints and coatings ZEOflair can be used in in automotive parts, packaging, textiles and personal care products. This technology offers the opportunity to improve indoor air quality in a range of different environments and reduce the risk of adverse health effects.

For further insight and for recommendations for your formulation, contact your account manager or call us to discuss your requirements.