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.
Filter papers are widely used in laboratory experiments across many different fields, from biology to chemistry. The type of filter used will differ according to the purpose of the procedure and the chemicals involved. Generally, filter papers are used with laboratory techniques such as gravity or vacuum filtration.
Qualitative filter paper is used in qualitative analytical techniques to determine materials. There are different grades of qualitative filter paper according to different pore size. There are total 13 different grades of qualitative filter paper. The largest pore size is grade 4; the smallest pore size is grade 602 h; the most commonly used grades are grade 1 to grade 4.
Grade 1 qualitative filter paper has the pore size of 11 µm. This grade of filter paper is widely used for many different fields in agricultural analysis, air pollution monitoring and other similar experiments.
Grade 2 qualitative filter paper has the pore size of 8 µm. This grade of filter paper requires more filtration time than Grade 1 filter paper. This filter paper is used for monitoring specific contaminants in the atmosphere and soil testing.
Grade 3 qualitative filter paper has the pore size of 6 µm. This grade of filter paper is very suitable for carrying samples after filtration.
Grade 4 qualitative filter paper has the pore size of 20~25 µm. This grade of filter paper has the largest pore size among all standard qualitative filter papers. It is very useful as rapid filter for cleanup of geological fluids or organic extracts during experiment.
Grade 602 h qualitative filter paper has the pore size of 2 µm. This grade of filter paper has the smallest pore size among all standard qualitative filter papers. It is used for collecting or removing fine particles.
Quantitative filter paper, also called ash-free filter paper, is used for quantitative and gravimetric analysis. During the manufacturing, producer use acid to make the paper ash-less and achieve high purity.
Chromatography is a method chemists use to separate compounds. This type of filter paper has specific water flow rate and absorption speed to maximize the result of paper chromatography. The absorption speed of this type of filter paper is from 6 cm to 18 cm and the thickness is from 0.17 mm from 0.93 mm.
Extraction thimbles are rod-shape filter paper often used in Soxhlet extractors or atomized extractors. It is ideal for very sensitive detection, the performance it depends on the thickness of inner diameter. Also, it is usually used in areas of food control and environmental monitoring.
Glass fiber filter has the pore size of 1 µm, it is useful for filtering highly contaminated solutions or difficult-to-filter solution. Also, glass fiber filter has extends filter life, wide range of particulate loads and can prevent sample contamination. In addition, different types of glass fiber filter are suitable for different filtration situation. There are 7 different types of glass fiber filters and the major difference is thickness.
Quartz fiber filter paper is high resistance chemical, does not absorb NOx and SOx dioxides, unaffected by humidity and easily sterilized. Thus, it is mostly use for air pollution analysis.
PTFE filter has wide operating temperature (-120 °C~260 °C) with high air permeability. The resistance of high temperature makes PTFE filter paper autoclavable. It is often use for filter hot oils, strong solvents and collecting airborne particulates.
Engine oil is filtered to remove impurities. Filtration of oil is normally done with volume filtration. Filter papers for lubrication oils are impregnated to resist high temperatures.
Tea bags are made from abacá fibers, a very thin and long fiber manilla hemp. Often the paper is augmented with a minor portion of synthetic fibers. The bag paper is very porous and thin and has high wet strength.