High performance filter media typically comprise a number of separate component materials selected to give a balance of performance characteristics including: structural integrity, dimensional stability, filtration efficiency, pressure drop, chemical resistance and downstream processing properties.
Fibres are a key component of many filter media. These may be selected for their raw materials origin and chemical properties, their dimensions, or their electrostatic function. Modified fibres that interact with ionic species to give selective removal of materials can also be used. In addition to functional requirements, fibres are also selected based on the manufacturing process used to produce the filter media; for example wet-laying and resin bonding, dry-laying and needle-bonding, extrusion and spunbonding, meltblowing or electrospinning.
Cellulose is probably the longest established raw material for filter manufacture. Different types are available from many plant sources as well as man-made processes and microbial based sources. Cellulosic fibres may also be modified mechanically or chemically to tailor their performance.
Man-made cellulose fibres
Viscose fibre, the most common form of man-made cellulose suffered a gradual decline in production volume during the final decades of the 20th Century. However, limitations on cotton availability and the impact of high oil prices on synthetic fibre costs have given new impetus to the manufacture of fibres from renewable resources and viscose production is now increasing, driven by strong growth in China. In contrast to natural cellulose, man-made cellulosic fibres exhibit consistent properties and controlled fibre dimensions.
Viscose fibres optimised for filtration applications during manufacture may exhibit specific fibre titres and cut lengths, tailored cross-sections, modified cellulose chemistry or incorporate performance enhancing additives within the fibre structure.
Tencel fibres are a newer class of man-made cellulose fibres, produced on commercial scale and marketed by Lenzing Fibers. The Tencel production process has a low environmental impact and an almost 100% conversion efficiency from woodpulp to the final fibre. In common with other cellulosics, the fibres are fully biodegradable after use, thereby providing a low environmental burden disposal method. Tencel fibres are produced in different titres and cut lengths and can also feature modified chemistries. Mechanical modification, by means of refining, to achieve fibrillation is an important means of affecting and controlling filtration performance. The fibres at the point of manufacture feature a smooth surface and round or ovoid cross section. This is dramatically changed after refining.
Refining
The main objectives of refining are:
to generate small diameter or even sub-micron fibrils to enhance filtration efficiency and/or sheet mechanical performance
(possibly) to reduce fibre length to improve sheet formation,
or conversely, to minimise fibre length reduction in order to maximise sheet performance.
Woodpulp is the most commonly refined fibre. Refining yields relatively fine fibrils that exhibit the same flat, ribbon-like structure as the original fibres. The sheet structure is relatively dense and sheet permeability is compromised.
Refined Tencel fibres produce a distribution of round fibrils emanating from the surface of the primary fibres. This effect is well illustrated in this micrograph, in which the fibrillar structure of the main filament and the surface separation of sub-micron diameter microfibrils are clearly shown.
Analysing the diameter distribution of fibrillated Tencel shows that a broad distribution is obtained ranging from around 100nm up to the initial diameter of the original filaments, which is approximately 11μm. The refining conditions affect the peak diameter and the detailed shape of the distribution but typically the peak fibril diameter is in the range 300-500nm.
The refiner types suitable for processing Tencel fibres range from lab scale valley beaters and PFI mills to laboratory blenders or even domestic food blenders. At production scale, Hollander beaters, disc refiners and conical refiners are used. It is critical to choose refining conditions that maximise fibril formation without excessive shortening of the fibres and fibrils to optimise the performance of the refined Tencel. The geometry of the refiner fillings can be selected to give optimum refining of the chosen fibre type being processed.
Filter media tests
Micro-glass can be considered as a benchmark fibre with regards to fine particle filtration characteristics. However, micro-glass media can release abrasive glass fragments during use and glass fibres are neither biodegradable nor suitable for incineration and so can present difficulties for disposal after use. Therefore, alternative filter media that overcome these deficiencies, whilst maintaining excellent fine particle filtration efficiency, are required.
A grade of micro-glass was selected that gave a broadly similar fibre diameter distribution to that of refined Tencel fibres. The micro-glass in this case had a range of fibre diameters between 0.2μm and 1μm and exhibited a Schopper Reigler value of 43°.
A series of test sheets was prepared comprising a mixture of woodpulp types and synthetic fibres. Into each stock, a proportion of refined Tencel or micro-glass fibre was added to give an even distribution of “high performance fibre” throughout the test media. Comparing the relationship between air permeability and blend ratio, we see that the Tencel containing media gave comparable or slightly higher permeability values than the micro-glass containing media.
The average pore size within these four sets of filter sheets followed similar trends to the air permeability values. Comparing the flow data for the two sets of media, the Tencel containing media gives a lower pressure drop than the 43°SR micro-glass media at a given blend ratio.
The dust capacity of the Tencel containing media is at least as good as that of the micro-glass containing sheets.
Interestingly, looking at the mechanical properties of the filter sheets, the burst strength of the micro-glass containing media is adversely affected by the glass fibre content, whereas incorporating refined Tencel actually improves the strength compared to the woodpulp base formulations.
Benefits
Lenzing suggests a series of benefits of using refined Tencel in filter media could include:
Sheet porosity can be tailored by controlling the incorporation level and degree of refining for Tencel fibre and fine pore structures can be attained that yield high filtration efficiency.
Incorporation of refined Tencel can increase the mechanical performance of filter media.
In contrast to some nano-materials, there are no ill-health concerns with the use of refined Tencel and there is no risk of the release of hard, abrasive particles from the filter media, which can be of concern when using micro-glass.
The raw material for Tencel production (woodpulp) is sustainable and the fibre manufacturing process has a low environmental impact. After use Tencel fibres are biodegradable or can be disposed by incineration.
Modern filter media typically comprise a blend of components selected to impart specific performance attributes. Cellulosic fibres of all forms remain major raw materials for filter manufacture.
The incorporation of refined Tencel within a filter media formulation can yield similar performance attributes to the use of micro-glass, combined with improved mechanical properties and elimination of the potential release of abrasive glass fibre particles.