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Filter Paper

Even in our time of high technology, classical filter papers are being used in many chemical, biological and research laboratories around the world. More often than not they are a piece of complex technology themselves, made of fibre configurations of different materials.

The most important goal of the filtration is to adequately select the type of filter according to its purpose. To do this, we will try, by way of standard, to reach the quickest possible velocity of filtration for the level of retention that we need. The filter papers are depth filters, that is, the retention of the particles take place not only on the surface of the filter but also within the paper by means of mechanisms that are, in some cases, rather complex. Johnson Test Papers offers an extensive range of qualitative, ashless, and technical filter papers that cover broad applications in laboratory and industrial applications. We also offer application specific filters and phase separator papers. The different grades of cellulose filters offer increasing degrees of purity, hardness, and chemical resistance. Some grades are also available in a prepleated format, enabling improved flow rate and increased loading capacity.

Johnson Test Papers’ cellulose filters are manufactured from the finest available raw materials on the market which includes high quality cotton linters which have been treated to achieve a minimum alpha cellulose content of 95% to maintain guaranteed quality, reproducibility, and uniformity. Cotton linters are short-fibred seed hairs from cotton seeds, which cannot be used for textile purposes, but which are highly suitable for the manufacture of soft and absorbent filter papers. In addition to cotton linters we use mainly pulp, which is obtained by chemical treatment of plant materials, e.g. coniferous or deciduous wood. Only the most experienced paper specialists select the raw materials in order to ensure the continuously high quality of our filter papers. The filters are tested for basic weight, thickness, air flow, and mechanical strength. They are mainly used for sample clarification or precipitate recovery before material identification or quantification in areas such as pharmaceutical, chemical, environmental testing, food and beverage, mining, cement and many other industries.

In the filtration process there are various factors that play a role and determine the retention, those factors are the following:

Retention on the surface: Particles larger than the holes formed by the network of cellulose fibers will remain on the surface of the filter. Likewise, as the surface becomes saturated the ability of retention increases due to the formation of a layer of particles until the fill-up level is reached. It is here when the process of filtration stops.

Retention depth: Refers to the mechanisms of retention produced inside the filter. Some of the most important processes are:

  • Electrostatic adsorption: according to the polarity of the filter fibres and of the particles that must go through it, in some cases, attraction occurs which makes these particles adhere to the fibre walls and particles smaller than the size indicated in the specifications of the filter are retained.
  • Effects of inertia: some particles will literally remain stuck inside the network of fibres due to the high kinetic energy with which they penetrate the pores of the frame.
  • Sedimentation: the particles can be captured by the filter network and deposited by gravity in somewhere of the interior space formed by the fibres.

In any case, the efficiency of retention of a filter paper is also determined by the other factors related to the liquid, which can be: the pH value, the viscosity and concentrations of the liquid to be filtered as well as the form and composition of the particles in suspension. Other causes or properties of the filter also affect the efficiency of retention: level of refinement of the cellulose fibres, resistance to the moist state of the filter, thickness, nature of the surface, etc.

So due to the extremely complex mechanisms on which filtration depends, it is sometimes impossible to theoretically determine the most adequate filter for filtration. It is at this moment when, in the case of difficult filtrations, it is essential to do some comparative testing of filtration between various samples of filter paper according to the parameters of retentions that we need. Johnson Test Papers are aware of the difficulty of some operations of filtration and because of this we are willing to help you resolve your problems of filtration for which we have our own laboratory for quality control ensuring consistency in performance.and development of new products.


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