Filter paper is a semi-permeable

paper Paper is a thin sheet material produced by mechanically or chemically processing cellulose fibres derived from wood, rags, grasses or other vegetable sources in water, draining the water through fine mesh leaving the fibre evenly distribu ...

barrier placed

perpendicular In elementary geometry, two geometric objects are perpendicular if they intersect at a right angle (90 degrees or π/2 radians). The condition of perpendicularity may be represented graphically using the '' perpendicular symbol'', ⟂. It c ...

to a liquid or air flow. It is used to separate fine solid particles from liquids or gases. Filterpapier rund

The raw materials are different paper pulps. The pulp may be made from

softwood Scots Pine, a typical and well-known softwood Softwood is wood from gymnosperm trees such as conifers. The term is opposed to hardwood, which is the wood from angiosperm trees. The main differences between hardwoods and softwoods is that the s ...

hardwood Hardwood is wood from dicot trees. These are usually found in broad-leaved temperate and tropical forests. In temperate and boreal latitudes they are mostly deciduous, but in tropics and subtropics mostly evergreen. Hardwood (which comes from ...

, fiber crops,

mineral fibers In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ( ...

Properties

Filter paper has various properties. The important parameters are

wet strength The wet strength of paper and paperboard is a measure of how well the web of fibers holding the paper together can resist a force of rupture when the paper is wet. Wet strength is routinely expressed as the ratio of wet to dry tensile force at bre ...

, porosity, particle retention,

volumetric flow rate In physics and engineering, in particular fluid dynamics, the volumetric flow rate (also known as volume flow rate, or volume velocity) is the volume of fluid which passes per unit time; usually it is represented by the symbol (sometimes ). I ...

, 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 of the ability of a small piece of filter paper to absorb a significant volume of liquid.

Manufacture

The raw materials are different paper pulps. The pulp may be from

, fiber crops,

. For high quality filters,

dissolving pulp Dissolving pulp, also called dissolving cellulose, is bleached wood pulp or cotton linters that has a high cellulose content (> 90%). It has special properties including a high level of brightness and uniform molecular-weight distribution. This pul ...

and mercerised pulp are used. Most filter papers are made using small paper machines. For

laboratory A laboratory (; ; colloquially lab) is a facility that provides controlled conditions in which scientific or technological research, experiments, and measurement may be performed. Laboratory services are provided in a variety of settings: physi ...

filters, the machines may be as small as 50 cm in width. The paper is often crêped to improve porosity. The filter papers may also be treated with

reagents In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...

or impregnation to get the right properties.

Types

Air filters

The main application for air filters are combustion air to

engine An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy. Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power ...

s. 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 Moisture is the presence of a liquid, especially water, often in trace amounts. Small amounts of water may be found, for example, in the air (humidity), in foods, and in some commercial products. Moisture also refers to the amount of water vapo ...

. Some heavy duty qualities are made to be rinsed and thereby extend the life of the filter.

Coffee and tea

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 A funnel is a tube or pipe that is wide at the top and narrow at the bottom, used for guiding liquid or powder into a small opening. Funnels are usually made of stainless steel, aluminium, glass, or plastic. The material used in its construct ...

. 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. Tea bags also work as a kind of paper filter. They are made from abacá fibers, a very thin and long fiber manilla hemp. Often the paper is augmented with a minor portion of

synthetic fiber Synthetic fibers or synthetic fibres (in British English; see spelling differences) are fibers made by humans through chemical synthesis, as opposed to natural fibers that are directly derived from living organisms, such as plants (like cotton) ...

s. The bag paper is very porous and thin and has high

Fuel filters

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.

Horizontal plate filters

Horizontal plate filter paper is commonly utilized in industrial processing. Filter paper typically is designed to fit the manufacturers specifications. Absolute micron retention can range from 1–100 microns but

Diatomaceous earth Diatomaceous earth (), diatomite (), or kieselgur/kieselguhr is a naturally occurring, soft, siliceous sedimentary rock that can be crumbled into a fine white to off-white powder. It has a particle size ranging from more than 3 μm to l ...

is commonly used with filter paper to obtain sub-micron filtration.

Activated carbon Activated carbon, also called activated charcoal, is a form of carbon commonly used to filter contaminants from water and air, among many other uses. It is processed (activated) to have small, low-volume pores that increase the surface area availa ...

or other filter aids can be used with the filter paper to form a

filter cake A filter cake is formed by the substances that are retained on a filter. Filter aids, such as diatomaceous earth or activated carbon are usually used to form the filter cake. The purpose is to increase flow rate or achieve a smaller micron filtr ...

to achieve specific results. Filter paper can be impregnated with DE or activated carbon.

Oil filters

Engine oil Motor oil, engine oil, or engine lubricant is any one of various substances used for the lubrication of internal combustion engines. They typically consist of base oils enhanced with various additives, particularly antiwear additives, detergen ...

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.

Laboratory-grade paper filters

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. Historically, a type of soft, porous paper called ''charta emporetica'' was used in

pharmacy Pharmacy is the science and practice of discovering, producing, preparing, dispensing, reviewing and monitoring medications, aiming to ensure the safe, effective, and affordable use of medication, medicines. It is a miscellaneous science as it ...

as a filter and as packing paper.

Qualitative filter paper

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. Whatman filter paper

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

Quantitative filter paper, also called ash-free filter paper, is used for quantitative and gravimetric analysis. During the manufacturing, producers use acid to make the paper ash-less and achieve high purity.

Chromatography papers

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

Extraction thimbles are rod-shape filter paper often used in

soxhlet extractor A Soxhlet extractor is a piece of laboratory apparatus invented in 1879 by Franz von Soxhlet. It was originally designed for the extraction of a lipid from a solid material. Typically, Soxhlet extraction is used when the desired compound has a ' ...

s or atomized extractors. It is ideal for very sensitive detection, the performance depends on the thickness of inner diameter. Also, it is usually used in areas of food control and environmental monitoring.

Glass fiber filters

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

Quartz fiber filter paper has a high resistance to chemicals, does not absorb NO_x and SO_x dioxides, is unaffected by humidity and is easily sterilized. Thus, it is mostly used for air pollution analysis.

PTFE filter

Polytetrafluoroethylene Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. It is one of the best-known and widely applied PFAS. The commonly known brand name of PTFE-based composition is Teflon by Chemo ...

(PTFE) filter has wide operating temperature (−120 °C~260 °C) with high

air permeability Textile fibers, threads, yarns and fabrics are measured in a multiplicity of units. * A fiber, a single filament of natural material, such as cotton, linen or wool, or artificial material such as nylon, polyester, metal or mineral fiber, or man-m ...

. The resistance to high temperature makes PTFE filter paper suitable for use in autoclaves. It is often used to filter hot oils, strong solvents and collecting airborne particulates.

References

{{DEFAULTSORT:Filter Paper Paper Filters Analytical chemistry Laboratory equipment Liquid-solid separation Solid-gas separation