Filter paper
Encyclopedia
Filter paper is a semi-permeable paper
Paper
Paper is a thin material mainly used for writing upon, printing upon, drawing or for packaging. It is produced by pressing together moist fibers, typically cellulose pulp derived from wood, rags or grasses, and drying them into flexible sheets....

 barrier placed perpendicular
Perpendicular
In geometry, two lines or planes are considered perpendicular to each other if they form congruent adjacent angles . The term may be used as a noun or adjective...

 to a liquid or air flow. It is used to separate fine solids from liquids or air.

Properties

Filter paper comes in various porosities and grades depending on the applications it is meant for. The important parameters are wet strength
Wet strength
Wet strength of paper is a measure of how well the fibreweb is holding together upon a force of rupture. Wet strength is routinely expressed as the ratio of wet to dry tensile force at break.-Mechanism:...

, 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.

Manufacture

The raw materials are different paper pulps
Pulp (paper)
Pulp is a lignocellulosic fibrous material prepared by chemically or mechanically separating cellulose fibres from wood, fibre crops or waste paper. Wood pulp is the most common raw material in papermaking.-History:...

. The pulp may be from [softwood], [hardwood], [fiber crops], mineral fibers. For high quality filters, dissolving pulp
Dissolving pulp
Dissolving pulp is a bleached wood pulp that has a high cellulose content . It is produced chemically from the pulpwood, in a process that has a low yield...

 and mercerised pulp
Mercerised pulp
Mercerised pulp is a bleached pulp that is treated with hot dilluted alkali to get extra high bulk and high porosity. The pulp feels and looks similar to a cotton ball.-Manufacture:...

 are used. Most filter papers are made on small paper machines. For laboratory
Laboratory
A laboratory is a facility that provides controlled conditions in which scientific research, experiments, and measurement may be performed. The title of laboratory is also used for certain other facilities where the processes or equipment used are similar to those in scientific laboratories...

 filters the machines may be as small as 50 cm width. The paper is often crêped
Crêpe paper
Crêpe paper is tissue paper that has been coated with sizing and then creped to create gathers.-Production:Paper that is creped is produced on a paper machine that has a single large steam-heated drying cylinder fitted with a hot-air hood. The raw material is paper pulp. The Yankee cylinder is...

 to improve porosity. The filter papers may also be treated with reagents or impregnation to get the right properties.

Air filters

The main application for air filters are combustion air to engine
Engine
An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including internal combustion engines and external combustion engines burn a fuel to create heat which is then used to create motion...

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
Humidity is the amount of moisture the air can hold before it rains. Moisture refers to the presence of a liquid, especially water, often in trace amounts...

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

Coffee filter

Coffee filters of paper are made from about 100 g/cm2 crêped paper
Crêpe paper
Crêpe paper is tissue paper that has been coated with sizing and then creped to create gathers.-Production:Paper that is creped is produced on a paper machine that has a single large steam-heated drying cylinder fitted with a hot-air hood. The raw material is paper pulp. The Yankee cylinder is...

. The crêping allows the coffee to flow freely between the filter
Filter (chemistry)
In chemistry and common usage, a filter is a device that is designed to physically block certain objects or substances while letting others through. Filters are often used to remove solid substances suspended in fluids, for example to remove air pollution, to make water drinkable, and to prepare...

 and the filtration funnel
Funnel
A funnel is a pipe with a wide, often conical mouth and a narrow stem. It is used to channel liquid or fine-grained substances into containers with a small opening. Without a funnel, spillage would occur....

. The raw materials (pulp
Pulp (paper)
Pulp is a lignocellulosic fibrous material prepared by chemically or mechanically separating cellulose fibres from wood, fibre crops or waste paper. Wood pulp is the most common raw material in papermaking.-History:...

) for the filter paper are coarse long fiber
Softwood
The term softwood is used to describe wood from trees that are known as gymnosperms.Conifers are an example. It may also be used to describe trees, which tend to be evergreen, notable exceptions being bald cypress and the larches....

, often from fast growing trees. Both bleached
Bleaching of wood pulp
Bleaching of wood pulp is the chemical processing carried out on various types of wood pulp to decrease the color of the pulp, so that it becomes whiter. The main use of wood pulp is to make paper where whiteness is an important characteristic...

 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.

Fuel filters

The paper used for fuel filters is a crêped paper
Crêpe paper
Crêpe paper is tissue paper that has been coated with sizing and then creped to create gathers.-Production:Paper that is creped is produced on a paper machine that has a single large steam-heated drying cylinder fitted with a hot-air hood. The raw material is paper pulp. The Yankee cylinder is...

 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.

Laboratory filters

For laboratory use filter papers are made in a varieties of ways since specific applications require specific types of papers. The raw materials might be acid washed wooden fibers
Pulp (paper)
Pulp is a lignocellulosic fibrous material prepared by chemically or mechanically separating cellulose fibres from wood, fibre crops or waste paper. Wood pulp is the most common raw material in papermaking.-History:...

, carbon or quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

 fibers. Finishing and confectioning is the bulk of the production work.

In laboratories, filter paper is usually used with a filter funnel
Filter funnel
A filter funnel is a laboratory funnel used for separating solids from liquids via the laboratory process of filtering.In order to achieve this, a disk shaped piece of filter paper is usually folded into a cone and placed within the funnel. The suspension of solid and liquid is then poured through...

, Hirsch, or Buchner funnel
Büchner funnel
A Büchner funnel is a piece of laboratory equipment used in suction filtration. It is traditionally made of porcelain, but glass and plastic funnels are also available. On top of the funnel-shaped part there is a cylinder with a perforated plate separating it from the funnel. The Hirsch funnel has...

.

Ashless filter paper is mainly used for gravimetric methods
Gravimetric analysis
Gravimetric analysis describes a set of methods in analytical chemistry for the quantitative determination of an analyte based on the mass of a solid...

 in quantitative chemical analysis. It has a base weight of 80 g
Gram
The gram is a metric system unit of mass....

/m2.

These papers may be impregnated with various reagents for use in detection tests like pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

, pregnancy
Pregnancy
Pregnancy refers to the fertilization and development of one or more offspring, known as a fetus or embryo, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets...

 or diabetes.

Oil filters


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

Tea bags are made from abacá fibers
Abacá
Abacá, Musa textilis is a species of banana native to the Philippines, grown as a commercial crop in the Philippines, Ecuador, and Costa Rica. The plant is of great economic importance, being harvested for its fibre, once generally called Manila hemp, extracted from the trunk or pseudostem. On...

, a very thin and long fiber hemp. Often the paper is augmented with a minor portion of synthetic fiber
Synthetic fiber
Synthetic fibers are the result of extensive research by scientists to improve on naturally occurring animal and plant fibers. In general, synthetic fibers are created by forcing, usually through extrusion, fiber forming materials through holes into the air, forming a thread...

s. The bag paper is very porous and thin, 12 - 20 g/cm2, and has high wet strength
Wet strength
Wet strength of paper is a measure of how well the fibreweb is holding together upon a force of rupture. Wet strength is routinely expressed as the ratio of wet to dry tensile force at break.-Mechanism:...

.
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