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{{Short description|Semi-permeable paper barrier}}
{{Use dmy dates|date=June 2023|cs1-dates=y}}
{{Use list-defined references|date=June 2023}}
'''Filter paper''' is a semi-permeable [[paper]] barrier placed [[perpendicular]] to a liquid or air flow. It is used to separate fine solid particles from liquids or gases.
[[File:Filterpapier rund.jpg|thumb|Filter paper: used for separating solid from liquid.]]
The raw materials are typically different [[pulp (paper)|paper pulp]]s. The pulp may be made from [[softwood]], [[hardwood]], [[fiber crop]]s, or [[mineral fiber]]s.

==Properties==
Filter paper has various properties. The important parameters are [[wet strength]], [[porosity]], particle [[Retention rate|retention]], [[volumetric 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 of the ability of a small piece of filter paper to absorb a significant volume of liquid.

==Manufacture==
{{Main|Paper machine}}
The raw materials are different [[pulp (paper)|paper pulps]]. The pulp may be from [[softwood]], [[hardwood]], [[fiber crops]], [[mineral fibres|mineral fibers]]. For high quality filters,  [[dissolving pulp]] and [[mercerised pulp]] are used. Most filter papers are made using small [[paper machine]]s. For [[laboratory]] filters, the machines may be as small as 50&nbsp;cm in width. The paper is often [[Crêpe paper|crêped]] to improve porosity. The filter papers may also be treated with [[reagents]] or impregnation to get the right properties.

==Types==
===Air filters===
[[File:Air filter, opel astra(2).JPG|thumb|Air filter paper in an auto engine]]
{{Main|Air filter}}
The main application for air filters are [[combustion air]] to [[engine]]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/m<sup>2</sup>. 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]].<ref name="Paulapuro_2000"/>{{rp|page=113}} Some heavy duty qualities are made to be rinsed and thereby extend the life of the filter.

===Coffee and tea===
{{main|Coffee filter|Tea bag}}
Historically, [[blotting paper]] or [[cloth filter coffee|cloth]] were used to extract filter coffee.

Modern coffee filters of paper are made from about 100 g/m<sup>2</sup> [[crêpe paper|crêped paper]]. The crêping allows the coffee to flow freely between the [[Filtration|filter]] and the filtration [[funnel]].<ref name="Gagné_2020"/> The raw materials ([[pulp (paper)|pulp]]) for the filter paper are coarse [[softwood|long fiber]], often from fast growing trees. For example, [[Melitta]] uses up to 60% of [[bambus]] in their filters since 1998.<ref name="Melitta_1"/> Both [[Bleaching of wood pulp|bleached]] and unbleached qualities are made.<ref name="Paulapuro_2000"/>{{rp|page=114}} Coffee filters are made in different shapes and sizes to fit into different holders.
Most notable are the (paper) [[coffee filter system]]s introduced by [[Melitta filter system|Melitta]] (1908, 1932, 1936, 1965), [[Chemex filter system|Chemex]] (1941) and [[Hario filter system|Hario]] (2004<!-- some sources state 2005 -->). 

Important parameters are strength, compatibility, efficiency and capacity.

Tea bags also work as a kind of paper filter. They are made from [[Abacá|abacá fibers]], a very thin and long fiber manilla hemp. Often the paper is augmented with a minor portion of [[synthetic fiber]]s. The bag paper is very porous and thin and has high [[wet strength]].<ref name="Paulapuro_2000"/>{{rp|page=114}}

===Fuel filters===
{{Main|Fuel filter}}
The paper used for fuel filters is a [[crêpe paper|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&nbsp;g/m<sup>2</sup>.<ref name="Paulapuro_2000"/>{{rp|page=114}}

===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]] is commonly used with filter paper to obtain sub-micron filtration. [[Activated carbon]] or other filter aids can be used with the filter paper to form a [[filter cake]] to achieve specific results. Filter paper can be impregnated with DE or activated carbon.

=== Oil filters ===
[[File:Engine oil filter cutaway.JPG|thumb|Oil filter cut-away with paper visible inside]]
{{Main|Oil filter}}

[[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.<ref name="Paulapuro_2000"/>{{rp|page=113}}

==Laboratory-grade paper filters==
{{See also|Filtration}}
[[File:Filter paper 840 3x3 copy.jpg#file|thumb|left|A scan of Whatman Filter Paper 4 Qualitative taken at 840 magnifications under a [[scanning electron microscope]].]]

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.<ref name="UCI_2013"/>

Historically, a type of soft, porous paper called ''charta emporetica'' was used in [[pharmacy]] as a filter and as packing paper.<ref name="Cyclopaedia"/>

=== 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&nbsp;4; the smallest pore size is grade 602&nbsp;h; the most commonly used grades are grade&nbsp;1 to grade 4.<ref name="qualitative"/>
[[File:Whatman filter paper.jpg|thumb|150mm qualitative filter paper]]
Grade&nbsp;1 qualitative filter paper has the pore size of 11&nbsp;μm. This grade of filter paper is widely used for many different fields in agricultural analysis, [[air pollution]] monitoring and other similar experiments.<ref name="qualitative"/>

Grade&nbsp;2 qualitative filter paper has the pore size of 8&nbsp;μm. This grade of filter paper requires more filtration time than Grade&nbsp;1 filter paper. This filter paper is used for monitoring specific contaminants in the atmosphere and soil testing.<ref name="qualitative"/>

Grade&nbsp;3 qualitative filter paper has the pore size of 6&nbsp;μm. This grade of filter paper is very suitable for carrying samples after filtration.<ref name="qualitative"/>

Grade&nbsp;4 qualitative filter paper has the pore size of 20~25&nbsp;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.<ref name="qualitative"/>

Grade 602&nbsp;h qualitative filter paper has the pore size of 2&nbsp;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.<ref name="qualitative"/>

=== 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.<ref name="Fisher"/><ref name="Papers"/>
[[File:Chromatography paper.jpg|thumb|filter paper uses for paper chromatography]]

=== 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&nbsp;cm to 18&nbsp;cm and the thickness is from 0.17&nbsp;mm from 0.93&nbsp;mm.<ref name="Labtek"/>

=== Extraction thimbles ===
Extraction thimbles are rod-shape filter paper often used in [[soxhlet extractor]]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.<ref name="Thimbles"/>

=== Glass fiber filters ===
Glass fiber filter has the pore size of 1&nbsp;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.<ref name="Pall"/> 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<sub>x</sub> and SO<sub>x</sub> dioxides, is unaffected by humidity and is easily sterilized. Thus, it is mostly used for air pollution analysis.<ref name="quartz"/>

=== PTFE filter ===
[[Polytetrafluoroethylene]] (PTFE) filter has wide operating temperature (−120&nbsp;°C&nbsp;~ 260&nbsp;°C) with high [[air permeability]]. 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.<ref name="quartz"/>

==See also==
{{Commons category|Paper filters}}
* [[Air purifier]]
* [[Filtration|Filter]]
* [[Particulates]], e.g., PM10
* [[Respirator]]
* {{ill|Ulaxfilter|sv}}

==References==
{{Reflist|refs=
<ref name="Paulapuro_2000">{{cite book |author-last=Paulapuro |author-first=Hannu Viljami |author-link=:fi:Hannu Paulapuro |title=Paper and Board grades |series=Papermaking Science and Technology |volume=18 |date=2000 |publisher=Fapet Oy |location=Finland |isbn=952-5216-18-7 |pages=113–114 |chapter=5}}</ref>
<ref name="Gagné_2020">{{cite book |title=The Physics of Filter Coffee |chapter= |author-first=Jonathan |author-last=Gagné |author-link=:d:Q56561291 |editor-first=Jean |editor-last=Zimmer |publisher=Scott Rao |date=2020<!-- 2020-10-23? --> |edition=1 |isbn=978-0-578-24608-6 |pages=}} [https://web.archive.org/web/20230227202755/https://www.scottrao.com/the-physics-of-filter-coffee] (xvi+249+3 pages)</ref>
<ref name="Melitta_1">{{cite web |title=111 Jahre Melitta Zeitstrahl Allgemein |language=de |trans-title= |publisher=[[Melitta]] |date= |url=https://www.melitta-group.com/datei.php?src=download/111-Jahre-Melitta_Zeitstrahl-Allgemein.pdf |access-date=2023-06-08 |url-status=live |archive-url=https://web.archive.org/web/20230310205124/https://www.melitta-group.com/datei.php?src=download/111-Jahre-Melitta_Zeitstrahl-Allgemein.pdf |archive-date=2023-03-10}}</ref>
<ref name="UCI_2013">{{cite web |title=Filtration Techniques |url=http://faculty.sites.uci.edu/chem1l/files/2013/11/RDGfiltration.pdf |website=Filtration Techniques |access-date=2015-11-21 |url-status=live |archive-url=https://web.archive.org/web/20150501090643/http://faculty.sites.uci.edu/chem1l/files/2013/11/RDGfiltration.pdf |archive-date=2015-05-01}}</ref>
<ref name="Cyclopaedia">{{Cite Cyclopaedia 1728 |wstitle=Charta_emporetica}}</ref>
<ref name="qualitative">{{cite web |title=Filter Papers and Membranes |url=http://www.chimica.unipd.it/nicola.tiso/pubblica/_private/Utile/catalogo%20whatman.pdf |website=Filter Papers and Membranes |access-date=2015-11-21 |url-status=live |archive-url=https://web.archive.org/web/20151122045201/http://www.chimica.unipd.it/nicola.tiso/pubblica/_private/Utile/catalogo%20whatman.pdf |archive-date=2015-11-22}}</ref>
<ref name="Fisher">{{cite web |title=Filter Papers for the Laboratory and Industry |url=https://static.fishersci.com/cmsassets/downloads/segment/Scientific/pdf/filter_papers.pdf |website=Filter Papers for the Laboratory and Industry |publisher=Sartorius Stedim |access-date=2015-11-21 |url-status=live |archive-url=https://web.archive.org/web/20151122011251/https://static.fishersci.com/cmsassets/downloads/segment/Scientific/pdf/filter_papers.pdf |archive-date=2015-11-22}}</ref>
<ref name="Papers">{{cite web |title=Lab Filtration Papers |url=https://labfiltrationpapers.com/collections/quantitative-filter-papers |website=Lab Filtration Papers}}</ref>
<ref name="Labtek">{{cite web |title=Laboratory Filter Paper |url=https://www.labtek.com.au/SuppliersData/Advantec/Laboratory%20Filter%20Paper.pdf |website=Laboratory Filter Paper |access-date=2015-11-21 |url-status=live |archive-url=https://web.archive.org/web/20160309211418/http://www.labtek.com.au/SuppliersData/Advantec/Laboratory%20Filter%20Paper.pdf |archive-date=2016-03-09}}</ref>
<ref name="Thimbles">{{cite web |title=Cellulose extraction thimbles |url=http://www.hahnemuehle.com/en/filtration/lab-filtration/extraction-thimbles/cellulose-extraction-thimbles.html |website=Hahnemuehle |access-date=2015-11-21 |url-status=live |archive-url=https://web.archive.org/web/20151122083336/http://www.hahnemuehle.com/en/filtration/lab-filtration/extraction-thimbles/cellulose-extraction-thimbles.html |archive-date=2015-11-22}}</ref>
<ref name="Pall">{{cite web |title=Glass Fiber Filters |url=http://www.pall.com/main/laboratory/product.page?id=20027 |website=Glass Fiber Filters |publisher=Pall Corporation |access-date=2015-11-21 |url-status=live |archive-url=https://web.archive.org/web/20151122000545/http://www.pall.com/main/laboratory/product.page?id=20027 |archive-date=2015-11-22}}</ref>
<ref name="quartz">{{cite web |title=Laboratory Filter Paper |url=https://www.labtek.com.au/SuppliersData/Advantec/Laboratory%20Filter%20Paper.pdf |website=Laboratory Filter Paper |access-date=2015-11-21 |url-status=live |archive-url=https://web.archive.org/web/20160309211418/http://www.labtek.com.au/SuppliersData/Advantec/Laboratory%20Filter%20Paper.pdf |archive-date=2016-03-09}}</ref>
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[[Category:Paper]]
[[Category:Filters]]
[[Category:Analytical chemistry]]
[[Category:Laboratory equipment]]
[[Category:Liquid-solid separation]]
[[Category:Solid-gas separation]]