You might have noticed how the air quality around us is changing constantly. Do you remember the last time that you have filled your lungs with fresh and clean air? Every day we are exposed to pollutants in the air we breathe - chemicals as well as fine particles - whether we are staying outdoors or indoors. This problem not only affects the people in developing countries, but the majority of the population on Earth.
You might think that the title sounds a bit far-fetched but let’s take below a closer look to the problem and see what could be the role of cellulose fibrils.
It is estimated that 6.5 million people die annually as a consequence of air pollution, being the fourth leading cause of premature death in 2013. The portion of outdoor pollution has been steadily increasing since the 1990, reaching the level of indoor pollution. Air pollution consists of several substances from which the most harmful ones are nitrogen oxides (NOx), sulphur oxides (SOx), ozone and particulate matter (PM). A special concern is related to the fine fraction of the particulate matter consisting of particles below 2.5 and 10 µm (PM2.5 / PM10) which are cable to penetrate deep into the lungs and blood streams. Although there is a worldwide effort to decrease the air pollution, in the meanwhile, effective personal protection and air filters offer relevant alternatives.
We have earlier discussed about the role of cellulose fibrils in water purification.
Typically, air filters are made of fibrous materials such as cellulose, cotton, glass or synthetic fibers. The filtering efficiency is mainly controlled by varying the thickness of the fibers as well as the density of the filter, in other words, by controlling the surface area of the filter. One of the common filter types, used for air purification are the so called HEPA filters, capable to removing particles above 0.3 µm. HEPA filters are typically used in clean rooms but also in vacuum cleaners and household air purifiers.
HEPA filters are typically made of borosilicate glass fibers or plastic fibers (e.g., polypropylene) bound together with up to 5% binder (acrylic, urethane, silanes or epoxy) by a wet method. The binder usually sets the maximum temperature where the filter can be used as well as the level of possible leachables.
Challenges related to the HEPA filters are related to the airflow and filtering efficiency. To increase the efficiency, the diameter of glass fiber is normally decreased in order to increase the surface area. This naturally weakens the structure. Also, in certain cases, the binders may cause problems, for example, trace amounts of siloxanes contained in silicone resin are known to disturb the semiconductor production.
Role of cellulose fibrils in filters
When we think about the properties of cellulose fibrils: film forming properties, strong adhesion on glass, high tensile strength, large surface area, freeness of volatile compounds or other impurities, chemical stability, pH stability and temperature stability (can be sterilized by autoclaving), we can see immediate match for filters.
On HEPA filters, cellulose fibrils could act as a strengthening material, enabling the production of filters with thinner glass fibers, leading to more efficient filters. This would allow to produce even more efficient micro- and ultrafilters for filtering liquid and air. Nemoto et al. reported lately a method producing high performance air filters by forming aerogels from TEMPO oxidized nanocellulose by freeze drying, using commercial HEPA filter as a support. The increased efficiency was a results of increased specific surface area formed by the spider-web-like networks of nanocellulose within the filter.
Cellulose and cotton has been used for decades for filtering air and liquids. The efficiency is naturally restricted by the fiber dimension of these natural fibers. There will definitely be still many steps to be taken before we will see a filter made of completely out of cellulose fibrils. But if we consider all the activities around cellulose fibrils and nanocellulose, the day when cellulose fibrils is part of a filter may be even closer than we believe.
Otto Soidinsalo works as a technical application manager at Borregaard. He has a Ph.D. in organic synthesis from the University of Helsinki and his working experience ranges from organic synthesis, cellulose ethers and its applications to nanocrystalline cellulose and microfibrillated cellulose.
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