However, viscose fibres are manufactured from natural wood pulp in a chemical process, in which natural variations of the raw material are levelled out. This article demonstrates that viscose speciality fibres are tailor-made cellulosic fibres for filter media manufacture and looks at potential future applications.
Figure 1. Viscose fibres of different cross-sections, relative surface areas
of the fibres. (Images: Kelheim Fibres GmbH, Kelheim, Germany)
Fibres are a commonly used material in filtration. They are used either as filter auxiliaries or as raw material to manufacture filter media. The most common natural fibres are cellulose fibres, used to manufacture for example paper and nonwoven filter media. They are non-allergenic and physiologically neutral. Processing of these fibres into filter media is safe as they do not release harmful substances and do not irritate the skin. Cellulosic fibres have a high moisture and humidity absorbency and are fully biodegradable.
Due to their natural origin, they can be incinerated CO2 neutral. Even composting of filters from viscose fibres is possible as long as the residues in the filter are also compostable. The disadvantage of natural cellulose fibres, however, is a certain variation of their properties, which is common for most natural substances. Especially in cases where filter media manufacturing requires a very narrow property profile these variations may be too big and hence may have a negative impact on the final product as processing of the fibres cannot equilibrate all variations.
Just like natural cellulosic fibres, viscose fibres consist of cellulose and do have the same advantageous health, security and environmental properties. Unless the natural fibres, however, viscose fibres are manufactured from natural wood pulp in a chemical process, in which natural variations of the raw material are levelled out.
Figure 2. Selection of viscose fibres in different geometries and cut-lengths for sedimentation experiments.
During the viscose fibre manufacturing process the cellulose is derivatised and dissolved in lye. The cellulosic solution is then extruded into fibres of definite cross-section and length. This allows designing the fibres to the specific needs of the user independent from the origin of the cellulosic raw material used for fibre manufacture. The modification of the physical and chemical fibre properties is as well possible as the modification of the fibre geometry (Figure 1).
The fibre geometry is important for processing the fibres into filter media as many processing technologies have size restrictions. A common application for viscose fibres in filter media are filter papers. Paper manufacturing requires short fibres of cut-lengths of 10 mm or less to avoid braid formation in the process, which lead to inhomogeneous paper formation and may cause process interruptions. Hence, the viscose fibres are manufactured as short cut in the desired cut-length with a very narrow length distribution. Viscose fibres consist of cellulose like pulp from wood or fibrous plants and hence are highly compatible with pulp for processing.
Figure 3. Sedimentation of round viscose fibres of different titers and cut-lengths.
During paper manufacture, natural wood pulp is blended with viscose fibres to adjust paper properties and to compensate variations from the natural pulp. Round fibres with rather big diameters will strongly increase paper bulk and porosity and hence are suitable for pressure drop reduction in filter papers. Fibres with a smaller diameter have less impact on the pressure drop, but they have a positive influence on paper strength. Trilobal fibres have a similar influence on paper porosity than round fibres.
Flat fibres, however, in most cases will lower the porosity of the papers, especially when their thickness-to-width ratio is low, as the flat fibres tend to orientate parallel to the paper surface and reduce the flow through the paper. Using flat fibres will strongly increase paper strength as the paper bulk decreases, which increases the number of binding points between the different fibres.
The fibre geometry is not only important for filter paper manufacture or for the manufacture of filter media using carding techniques. If viscose fibres are used as a filter auxiliary, for example as a substituent for other auxiliaries like diatomite, the adaptation of the fibre geometry is a basic requirement to be able to use the existing process equipment and to obtain comparable filtering results.
Figure 4. Pores-to-solids volume ratio of different viscose fibre sediments.
The viscose fibre manufacturing process allows the intrinsic functionalisation of the fibres. The functionalisation of natural cellulosic fibres is only possible by post-treatment, whereas the chemical and physical functionalisation of viscose fibres is also possible during the manufacturing process. The fibre modification during fibre manufacture can be performed similar to a post-treatment on never-dried fibres, which are particularly accessible for chemical reactions, or it can even be done before fibre extrusion by incorporation of functional additives into the spinning dope.
Particulate additives, which are incorporated during the spinning process, are locked up inside the fibre matrix and therefore cannot be separated from it anymore. Other additives, for example water soluble polymers, bind to the viscose fibre matrix by forming strong hydrogen bonds or build up a polymeric network inside the fibres after extrusion and therefore also cannot be separated from the viscose fibre matrix anymore. In all cases, the functionalisation of the viscose fibres is intrinsic as the function becomes part of the fibres themselves and is not just attached to them. Trying to separate function and fibres in most cases will destroy the fibres.
An example of such intrinsic functionalisation are ion exchange fibres. They can either be obtained by using the ion exchange functionality of cationic or anionic polyelectrolytes, which are bound to the polymer network of the viscose fibres by hydrogen bonds or by incorporation of ion exchange resin particles, which are locked up inside the fibre matrix. Other examples for functionalisation are flame retardant fibres composed of a silicate network entangled with the cellulose network, electrically conductive fibres containing carbon black or PCM fibres loaded with microencapsulated wax particles as heat accumulators.
The use of viscose fibres is not limited to one single application and to one single material construction. Even though textiles and nonwovens made from viscose fibres are generally very common for filtration the most common filter media containing viscose fibres are filter papers.
In technical applications, filter papers containing with viscose fibres are used for example for fluid filtration of operational fluids like lubricants or fuels. They are also used to manufacture pleated filter media for air filtration.
Especially in technical filter papers where the modification of the fibre geometry is already used to adapt filtration properties like porosity and pressure-drop using functionalised fibres is bringing an additional benefit as within the particle filtration step also non-particulate components like malodours or corrosive substances can be removed with the help of functional viscose fibres.
Food and beverage filtration
One of the most common applications for viscose fibres in filtration are filter papers for food and beverage filtration like coffee pad and teabag papers or plug wrap papers for cigarettes. As viscose fibres are physiologically and taste neutral, non-allergenic and they produced in a controlled hygienic environment, they are perfectly suitable for sensitive applications like hygiene or food and beverage. A completely new application area for viscose speciality fibres in food and beverage filtration, however, is the pre-coat filtration. Beverages like beer are filtered by pre-coat filtration to remove turbidity and tanning substances, which may alter the taste and reduce shelf life. The most common filter auxiliary for pre-coat beverage filtration is diatomite (diatomaceous earth), a fossil mineral with limited availability. The most important problem associated with diatomite is the dust generation of the dry material. As diatomite dust is considered harmful, filter cakes from diatomite pre-coat filtration need to be disposed to landfills.
A tightening of waste legislation may lead to a new classification of diatomite filter cakes, which then may be disposed as hazardous waste, which will significantly increase disposal costs. Even though membrane filtration as an alternative to pre-coat filtration gives good filtration results, most users want to continue using pre-coat filtration, not only because they have the filtration equipment but also because the pre-coat filtration process is more flexible and less energy and process water consuming than membrane filtration.
As viscose fibres consist of fully biodegradable cellulose, and during beverage filtration additional valuable and fully biodegradable organic material is accumulated in the filter cakes, it is possible to sell the filter cakes as fertiliser for agriculture instead of paying for their disposal. Another advantage of viscose fibres for pre-coat filtration is the possibility of functionalisation. Before pre-coat filtration, PVPP is added to beer to remove tanning substances by chemical interaction. During pre-coat filtration PVPP with the bound tanning substances is removed together with the yeast particles. As the functionalisation of viscose fibres to have comparable interactions with tanning substances than PVPP seems possible tanning substances may be removed together with yeast within the pre-coat viscose filtration step.
Figure 5. Cake build-up on filter candles of pilot filter line before and after process and fibre optimisation. (Images: Krones AG, Neutraubling, Germany)
In a research project viscose fibres, which are specially adapted to pre-coat filtration as a renewable substitute for diatomite, are developed. As the fibre chemistry can be adapted independent from the fibre geometry on already geometrically optimised fibres, the adaptation of the fibre geometry to the requirements of pre-coat filtration was taken as starting point (Figure 2).
After analysing the different geometrical fibre parameters, sedimentation and cake build-up of viscose fibres in function of their fibre geometry were studied with the aim of developing a predictive model for the cake build-up of viscose fibres. It became obvious that the orientation of the fibres during sedimentation only had little influence on sedimentation speed and cake build-up, whereas especially the fibre cut-length and the fibre denier as well as the fibre geometry had a considerable impact on cake formation. Generally, shorter and thicker fibres sediment quicker and form more compact sediments than longer and thinner fibres (Figure 3).
The comparison to diatomite sediments revealed that most sediments from viscose fibres were significantly more porous than sediments from diatomite particles. The sediments of fibres with a high denier and a short cut-length, however, had similar porosities to the sediments of diatomite (Figure 4). During the first observations on cake build-up, no liquid flow was applied. The liquid flow through the cake during pre-coat filtration, however, as well as yeast particles filtered off during filtration may have an influence on cake porosity. Unless diatomite filter cakes, which are nearly incompressible and hence rather insensitive to liquid flow, viscose fibre filter cakes underwent a compression. As expected, filter cakes from longer fibres, which had a higher porosity, were more compressible. Furthermore, compressibility increased with the liquid flow rate through the filter. For viscose fibres in pre-coat filtration, not only the cake build-up and the influence of yeast particles on the filter cake but also the compressibility of the filter cake under flow conditions needed to be taken into account.
After the first lab tests cake build-up was studied on a pilot line and the process and the fibres were optimised to obtain a homogeneous filter cake on the filter candles (Figure 5). In the meantime, filtration was also tested with yeast suspensions and artificial beer. The test results confirm the potential of viscose fibres as a replacement for diatomite. During the first filtration tests, the turbidity values of the filtrate were close to the limit values imposed by the European Brewers Convention (EBC). In the meantime, it was even possible to reduce them below these limits. As fibre optimisation is still in process, a further improvement of the measured values is expected.
It was shown that viscose speciality fibres are tailor-made cellulosic fibres for filter media manufacture. They are physiologically and taste neutral, non-allergenic and therefore highly suitable for food and beverage applications. By intrinsic functionalisation, the integration of additional functions into the fibres is possible without altering their processing or application properties. Common applications for viscose fibres are filter papers, where they are used for example to regulate the paper porosity.
A new application for viscose speciality fibres is the pre-coat filtration where renewable and fully biodegradable fibres may replace potentially hazardous fossil filter auxiliaries like diatomite in the future. Due to the high versatility of viscose speciality fibres, it can be expected that there are many other application areas of these fibres in filtration to be filled in the future.