Editing Liquid-crystal display (section)

==Chemicals used==
Several different families of liquid crystals are used in liquid crystal displays. The molecules used have to be anisotropic, and to exhibit mutual attraction. Polarizable rod-shaped molecules ([[biphenyl]]s, [[terphenyl]]s, etc.) are common. A common form is a pair of aromatic benzene rings, with a nonpolar moiety (pentyl, heptyl, octyl, or alkyl oxy group) on one end and polar (nitrile, halogen) on the other. Sometimes the benzene rings are separated with an acetylene group, ethylene, CH=N, CH=NO, N=N, N=NO, or ester group. In practice, [[eutectic]] mixtures of several chemicals are used, to achieve wider temperature operating range (−10..+60&nbsp;°C for low-end and −20..+100&nbsp;°C for high-performance displays). For example, the E7 mixture is composed of three biphenyls and one terphenyl: 39 wt.% of 4'-pentyl[1,1'-biphenyl]-4-carbonitrile (nematic range 24..35&nbsp;°C), 36 wt.% of 4'-heptyl[1,1'-biphenyl]-4-carbonitrile (nematic range 30..43&nbsp;°C), 16 wt.% of 4'-octoxy[1,1'-biphenyl]-4-carbonitrile (nematic range 54..80&nbsp;°C), and 9 wt.% of 4''-pentyl[1,1':4',1''-terphenyl]-4-carbonitrile (nematic range 131..240&nbsp;°C).<ref>{{cite book|url=https://books.google.com/books?id=zwmGAmsiTWoC&q=liquid+crystal+displays+molecules+biphenyl+fluorinated&pg=PA331|title=High-Performance Polymer...|first=Guy|last=Rabilloud|publisher=Editions OPHRYS|via=Google Books|isbn=9782710810957}}</ref>

===Environmental impact===
{{See also|Electronic waste}}

The production of LCD screens uses [[nitrogen trifluoride]] (NF<sub>3</sub>) as an etching fluid during the production of the thin-film components. NF<sub>3</sub> is a potent [[greenhouse gas]], and its relatively long [[half-life]] may make it a potentially harmful contributor to [[global warming]]. A report in ''Geophysical Research Letters'' suggested that its effects were theoretically much greater than better-known sources of greenhouse gasses like [[carbon dioxide]]. As NF<sub>3</sub> was not in widespread use at the time, it was not made part of the [[Kyoto Protocol]] and was deemed "the missing greenhouse gas".<ref name=nf3>{{Cite web|url=http://www.alternet.org/environment/95111/your_flat_screen_has_(greenhouse)_gas/|title=Your Flat Screen Has (Greenhouse) Gas|author-first1=Emily|author-last1=Udell|date=15 August 2008|website=AlterNet|access-date=May 3, 2019|archive-date=June 29, 2011|archive-url=https://web.archive.org/web/20110629164556/http://www.alternet.org/environment/95111/your_flat_screen_has_(greenhouse)_gas/|url-status=dead}}</ref> NF<sub>3</sub> was added to the Kyoto Protocol for the second compliance period during the Doha Round.<ref>{{cite web|url=https://unfccc.int/files/kyoto_protocol/application/pdf/kp_doha_amendment_english.pdf|title=Doha amendment to the Kyoto Protocol|website=Unfcc.int|access-date=2 March 2022|archive-date=December 24, 2022|archive-url=https://web.archive.org/web/20221224054705/http://unfccc.int/files/kyoto_protocol/application/pdf/kp_doha_amendment_english.pdf|url-status=live}}</ref>

Critics of the report point out that it assumes that all of the NF<sub>3</sub> produced would be released to the atmosphere. In reality, the vast majority of NF<sub>3</sub> is broken down during the cleaning processes; two earlier studies found that only 2 to 3% of the gas escapes destruction after its use.<ref name=natgas>{{cite journal | last=Hoag | first=Hannah | title=The missing greenhouse gas | journal=Nature Climate Change | volume=1 | issue=808 | date=2008 | issn=1758-678X | doi=10.1038/climate.2008.72 | pages=99–100| bibcode=2008NatCC...1...99H }}</ref> Furthermore, the report failed to compare NF<sub>3</sub>'s effects with what it replaced, [[perfluorocarbon]], another powerful greenhouse gas, of which anywhere from 30 to 70% escapes to the atmosphere in typical use.<ref name=natgas/>