Filter paper comes in various porosities and grades depending on the applications it is meant for. The important parameters are wet strength, porosity, particle retention, flow rate, compatibility, efficiency and capacity.
There are two mechanisms of filtration with paper; volume and surface. By volume filtration the particles are caught in the bulk of the filter paper. By surface filtration the particles are caught on the paper surface. Filter paper is mostly used because even a small piece of filter paper will absorb a significant volume of liquid
The raw materials are different paper pulps. The pulp may be from softwood, hardwood, fiber crops, mineral fibers. For high quality filters, dissolving pulp and mercerised pulp are used. Most filter papers are made on small paper machines. For laboratory filters the machines may be as small as 50 cm width. The paper is often crêped to improve porosity. The filter papers may also be treated with reagents or impregnation to get the right properties.
The main application for air filters are combustion air to engines. The filter papers are transformed into filter cartridges, which then is fitted to a holder. The construction of the cartridges mostly requires that the paper is stiff enough to be self-supporting. A paper for air filters needs to be very porous and have a weight of 100 - 200 g/m2. Normally particularly long fibrous pulp that is mercerised is used to get these properties. The paper is normally impregnated to improve the resistance to moisture. Some heavy duty qualities are made to be rinsed and thereby extend the life of the filter.
Coffee filters of paper are made from about 100 g/m2 crêped paper. The crêping allows the coffee to flow freely between the filter and the filtration funnel. The raw materials (pulp) for the filter paper are coarse long fiber, often from fast growing trees. Both bleached and unbleached qualities are made. Coffee filters are made in different shapes and sizes to fit into different holders. Important parameters are strength, compatibility, efficiency and capacity.
The paper used for fuel filters is a crêped paper with controlled porosity, which is pleated and wound to cartridges. The raw material for filter paper used in fuel filters are made of a mixture of hardwood and softwood fibres. The basis weight of the paper is 50 - 80 g/m2.
For laboratory use filter papers are made in a variety of ways since specific applications require specific types of papers. The raw materials might be acid washed wooden fibers, carbon or quartz fibers. Finishing and confectioning is the bulk of the production work.
Tea bags are made from abacá fibers, a very thin and long fiber hemp. Often the paper is augmented with a minor portion of synthetic fibers. The bag paper is very porous and thin, 12 - 20 g/cm2, and has high wet strength.
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