Factors of polymer weathering
Encyclopedia
The aging of natural and artificial polymeric materials is a natural phenomenon in metal
s, glass
, mineral
s and other inorganic materials. The main environmental parameters influencing the degradation of polymer
ic materials is daylight combined with the effects of temperature
, moisture
and oxygen
. These act as the main parameters of stress for outdoor weathering
.
, atmospheric temperature
and moisture in its various forms. This, combined with the effects of wind and atmospheric gases and pollutant
s. Although the ultraviolet
(UV) portion of solar radiation is mainly responsible for initiating weathering effects, the visible and near-infrared portions can also contribute to the weathering processes. Colored materials are susceptible to visible radiation, and near-infrared radiation can accelerate chemical reaction
s by raising the material temperature. The other factors act synergistically with solar radiation to significantly influence the weathering processes. All weather factors, including the quality and quantity of sunlight
, vary with geographic location, time of day and year, and climatalogical conditions. In order to fully understand and predict the effect of weather on materials, data is required on each factor that may contribute to degradation.
breaking reactions caused by the absorbed light, either through direct or indirect processes. Chemical bond breaking is a prerequisite to any chemical reaction, and chemical reactions are a prerequisite to observable or measurable physical changes. Other weather factors mainly promote weathering through their influence on the secondary reactions which follow the breaking of bonds. Degradation of most materials exposed to outdoor conditions is caused mainly by the ultraviolet portion of solar energy, with the shortest wavelength
s often having the greatest effect. Therefore, variations in both the quantity and quality of ultraviolet in both the direct solar beam and diffuse sky radiation
are important factors in the design and evaluation of weathering tests.
s have greater mobility. Therefore, the rate of oxygen diffusion increases and free radical fragments formed in primary photochemical processes are more readily separated. Thus, the chance of recombination
is reduced and secondary reactions are promoted. Reactions may take place at higher temperatures that occur at a very low rate or not at all at lower temperatures.
In the presence of sunlight the surface temperature of an object is usually considerably higher than the temperature of the air. Solar absorptivity is closely related to color, varying from about 20% for white materials to 90% for black materials; thus samples of different colors will reach different on-exposure temperatures. Because the thermal conductivity
and heat capacity
of polymeric materials are generally low, much higher temperatures can be obtained on the surface than in the bulk of the material. Therefore, both the surface temperatures of the samples, produced largely by infrared radiation absorption which varies by material color, and ambient air temperature and its fluctuations during exposure do play a role.
Diurnal and seasonal variations occur in solar radiation. Temperature cycling
can cause mechanical stress, particularly in composite systems consisting of materials with widely differing temperature coefficient
s of expansion. Temperature and its cycles are also closely linked with water in all of its forms. Drops in temperature can cause water to condense on the material as dew, a rise in temperature causes evaporation
, and sudden rainfall can cause thermal stress.
, dew
, rain
, snow
, frost
or hail
, depending on the ambient temperature. Moisture, in combination with solar radiation, contributes significantly to the weathering of many materials. This is due both to the mechanical stresses imposed when moisture is absorbed or desorbed and to the chemical participation of moisture in the chemical evolution
(and in some instances physical effects such as impact). The span of time over which the precipitation occurs and the frequency of wetness are more important in the weathering of materials than the total amount of precipitation. The mechanical stresses induced by freeze/thaw cycling can cause structural failures in some systems, or accelerate degradation
already initiated.
Moisture participates both physically and chemically in degradation. Water absorption
by synthetic materials and coatings from humidity and direct wetness is a diffusion controlled process. This hydration of the surface layers produces a volume expansion which places mechanical stress on the dry subsurface layers. A following drying out period signifies a desorption
of water. The drying out of the surface layers would lead to a volume contraction; the hydrated inner layers resist this contraction, leading to surface stress cracking. This oscillation between hydrated and dehydrated states may result in stress fracture
s. Because of diffusion
rates in organic materials, it may takes weeks or months to reach a moisture equilibrium.
The chemical effects of moisture can be seen in the chalking of titanium dioxide
(TiO2) pigmented coatings and polymers; the anatase
form is particularly sensitive to wavelengths below about 405 nm while the rutile
forms absorb energy above that wavelength. Chalking results from the degradation of the binding material resulting in a release of the TiO2 pigment particles. These particles form a dull layer on the surface which may be wiped off. Experience shows that chalking is strongest where more water is available on the surface; little to no chalking occurs in dry atmospheres. TiO2 is a semiconductor
where electron transitions from the valence band
to the conduction band
result from the absorption of light at wavelengths in the near UV range, below 400 nm. Ultraviolet radiation causes electron hole pairs to be created in the TiO2 lattice. These react with the hydroxide groups on the surface and the Ti4+ ions. Hydroxyl
and perhydroxyl radicals are formed through the conversion of oxygen and a water molecule whereby the TiO2 surface again resumes the initial form and acts as a catalyst for continued activity, thus repeating the chalking cycle. The hydroxide and perhydroxyl radical then cause oxidative decomposition of the binder with the subsequent release of TiO2 particles.
In addition to the reactions of oxygen in its normal ground state, some reactions of oxygen are due to the excited singlet state, a highly reactive form of the molecule. Singlet oxygen
is responsible for the rapid deterioration of materials, particularly those with conjugated unsaturation such as natural rubber and synthetic elastomers. It is formed when triplet oxygen
, the normal ground state, reacts with sensitizers, such as certain dye
s and ketone
s, excited by radiation to their triplet states. Oxygen also increases the amount of solar radiation absorbed by conjugated unsaturated hydrocarbon
s through formation of a complex with these materials.
The extent of photochemical reactions involving oxygen differ in the inner and outer layers of both aromatic and aliphatic polymers due to their dependence on the diffusion of oxygen through the polymer. Photooxidation is significantly reduced at depths beyond which oxygen penetrates. Studies of the degradation profiles of low density polyethylene
(LDPE), polymethylmethacrylate (PMMA) and polyvinylchloride (PVC) show that photooxidation was higher at the front and back surfaces than in the interior bulk of the material. Because ultraviolet radiation is not strongly absorbed by these materials, a considerable amount of the radiation incident on the front surface is transmitted to the back surface where it initiates photooxidation.
is produced by short wavelength (110 nm - 220 nm) UV photolysis of oxygen in the upper atmosphere. The photochemical reaction of nitrogen oxide
s and hydrocarbon
s from automobile exhausts is another source. Ozone plays a dual role in weathering. The concentrated layer in the upper atmosphere absorbs the short wavelength (≤300 nm) ultraviolet radiation emitted by the sun and thus plays a critical role in protecting terrestrial objects from this actinic radiation. Ozone is also a powerful oxidant and reported to react rapidly with elastomer
s and other unsaturated polymers. Ozonolysis
typically results in stiffening and cracking, particularly under mechanical stress. However, the contribution of ozonolysis reactions to the overall photooxidation process is still subject to controversy.
Atmospheric pollutants (e.g., sulfur dioxide
, nitrogen oxides, hydrocarbons, etc.), in combination with solar radiation, can also be responsible for severe damage. Acid-base induced chemical changes may also be responsible for much pollution-caused damage. Unsaturated alkyl and aromatic compounds may act as catalysts in the photooxidation of polymers. In the presence of sulfur dioxide and oxygen, ultraviolet radiation causes crosslinking of polyethylene
and polypropylene
and is responsible for the rapid loss of color in pigmented coatings.
Moisture, in combination with temperature, may also promote microbial growth. Mold
, mildew
and other microbiological and botanical agents may play a significant role in material degradation, particularly in tropical and subtropical climates, although they may not be generally thought of as weathering factors.
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
s, glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
, mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s and other inorganic materials. The main environmental parameters influencing the degradation of polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
ic materials is daylight combined with the effects of temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
, 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...
and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
. These act as the main parameters of stress for outdoor weathering
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...
.
Introduction
The components of the weather cycles responsible for the deterioration of most materials are non-ionizing radiationNon-ionizing radiation
Non-ionizing radiation refers to any type of electromagnetic radiation that does not carry enough energy per quantum to ionize atoms or molecules—that is, to completely remove an electron from an atom or molecule...
, atmospheric temperature
Atmospheric temperature
Atmospheric temperature is a measure of temperature at different levels of the Earth's atmosphere. It is governed by many factors, including incoming solar radiation, humidity and altitude...
and moisture in its various forms. This, combined with the effects of wind and atmospheric gases and pollutant
Pollutant
A pollutant is a waste material that pollutes air, water or soil, and is the cause of pollution.Three factors determine the severity of a pollutant: its chemical nature, its concentration and its persistence. Some pollutants are biodegradable and therefore will not persist in the environment in the...
s. Although the ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
(UV) portion of solar radiation is mainly responsible for initiating weathering effects, the visible and near-infrared portions can also contribute to the weathering processes. Colored materials are susceptible to visible radiation, and near-infrared radiation can accelerate chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
s by raising the material temperature. The other factors act synergistically with solar radiation to significantly influence the weathering processes. All weather factors, including the quality and quantity of sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...
, vary with geographic location, time of day and year, and climatalogical conditions. In order to fully understand and predict the effect of weather on materials, data is required on each factor that may contribute to degradation.
Solar Radiation
Physical changes resulting from exposure to the environment are initiated by chemical bondChemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...
breaking reactions caused by the absorbed light, either through direct or indirect processes. Chemical bond breaking is a prerequisite to any chemical reaction, and chemical reactions are a prerequisite to observable or measurable physical changes. Other weather factors mainly promote weathering through their influence on the secondary reactions which follow the breaking of bonds. Degradation of most materials exposed to outdoor conditions is caused mainly by the ultraviolet portion of solar energy, with the shortest wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s often having the greatest effect. Therefore, variations in both the quantity and quality of ultraviolet in both the direct solar beam and diffuse sky radiation
Diffuse sky radiation
Diffuse sky radiation is solar radiation reaching the Earth's surface after having been scattered from the direct solar beam by molecules or suspensoids in the atmosphere. It is also called skylight, diffuse skylight, or sky radiation and is the reason for changes in the colour of the sky...
are important factors in the design and evaluation of weathering tests.
Temperature
The temperature of materials exposed to solar radiation has a significant influence on the effect of the radiation. The destructive effects of light are usually accelerated at elevated temperatures as a result of the increased rate of secondary reactions, with reaction rates about doubling with each 10°C rise; this may not be true of all materials but is often found with polymers. At high temperatures moleculeMolecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
s have greater mobility. Therefore, the rate of oxygen diffusion increases and free radical fragments formed in primary photochemical processes are more readily separated. Thus, the chance of recombination
Recombination
Recombination may refer to:* Recombination , the process by which genetic material is broken and joined to other genetic material* Recombination , in semiconductors, the elimination of mobile charge carriers...
is reduced and secondary reactions are promoted. Reactions may take place at higher temperatures that occur at a very low rate or not at all at lower temperatures.
In the presence of sunlight the surface temperature of an object is usually considerably higher than the temperature of the air. Solar absorptivity is closely related to color, varying from about 20% for white materials to 90% for black materials; thus samples of different colors will reach different on-exposure temperatures. Because the thermal conductivity
Thermal conductivity
In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....
and heat capacity
Heat capacity
Heat capacity , or thermal capacity, is the measurable physical quantity that characterizes the amount of heat required to change a substance's temperature by a given amount...
of polymeric materials are generally low, much higher temperatures can be obtained on the surface than in the bulk of the material. Therefore, both the surface temperatures of the samples, produced largely by infrared radiation absorption which varies by material color, and ambient air temperature and its fluctuations during exposure do play a role.
Diurnal and seasonal variations occur in solar radiation. Temperature cycling
Temperature cycling
Temperature cycling is the process of cycling through two temperature extremes, typically at relatively high rates of change. It is an environmental stress test used in evaluating product reliability as well as in manufacturing to catch early-term, latent defects by inducing failure through...
can cause mechanical stress, particularly in composite systems consisting of materials with widely differing temperature coefficient
Temperature coefficient
The temperature coefficient is the relative change of a physical property when the temperature is changed by 1 K.In the following formula, let R be the physical property to be measured and T be the temperature at which the property is measured. T0 is the reference temperature, and ΔT is the...
s of expansion. Temperature and its cycles are also closely linked with water in all of its forms. Drops in temperature can cause water to condense on the material as dew, a rise in temperature causes evaporation
Evaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
, and sudden rainfall can cause thermal stress.
Moisture
Moisture can take the form of humidityHumidity
Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...
, dew
Dew
[Image:Dew on a flower.jpg|right|220px|thumb|Some dew on an iris in Sequoia National Park]]Dew is water in the form of droplets that appears on thin, exposed objects in the morning or evening...
, rain
Rain
Rain is liquid precipitation, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet. Rain requires the presence of a thick layer of the atmosphere to have temperatures above the melting point of water near and above the Earth's surface...
, snow
Snow
Snow is a form of precipitation within the Earth's atmosphere in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by...
, frost
Frost
Frost is the solid deposition of water vapor from saturated air. It is formed when solid surfaces are cooled to below the dew point of the adjacent air as well as below the freezing point of water. Frost crystals' size differ depending on time and water vapour available. Frost is also usually...
or hail
Hail
Hail is a form of solid precipitation. It consists of balls or irregular lumps of ice, each of which is referred to as a hail stone. Hail stones on Earth consist mostly of water ice and measure between and in diameter, with the larger stones coming from severe thunderstorms...
, depending on the ambient temperature. Moisture, in combination with solar radiation, contributes significantly to the weathering of many materials. This is due both to the mechanical stresses imposed when moisture is absorbed or desorbed and to the chemical participation of moisture in the chemical evolution
Chemical evolution
Chemical evolution may refer to:*Nucleosynthesis, the creation of chemical elements in the universe either through the Big Bang, or supernovae*Abiogenesis, the transition from nonliving elements to living systems...
(and in some instances physical effects such as impact). The span of time over which the precipitation occurs and the frequency of wetness are more important in the weathering of materials than the total amount of precipitation. The mechanical stresses induced by freeze/thaw cycling can cause structural failures in some systems, or accelerate degradation
Degradation
Degradation may refer to;* Biodegradation, the processes by which organic substances are broken down by living organisms* Cashiering or degradation ceremony, a ritual performed when cleric is deprived of office or a knight is stripped of the honour...
already initiated.
Moisture participates both physically and chemically in degradation. Water absorption
Water absorption
During the transmission of electromagnetic radiation through a medium containing water molecules, portions of the electromagnetic spectrum are absorbed by water molecules...
by synthetic materials and coatings from humidity and direct wetness is a diffusion controlled process. This hydration of the surface layers produces a volume expansion which places mechanical stress on the dry subsurface layers. A following drying out period signifies a desorption
Desorption
Desorption is a phenomenon whereby a substance is released from or through a surface. The process is the opposite of sorption . This occurs in a system being in the state of sorption equilibrium between bulk phase and an adsorbing surface...
of water. The drying out of the surface layers would lead to a volume contraction; the hydrated inner layers resist this contraction, leading to surface stress cracking. This oscillation between hydrated and dehydrated states may result in stress fracture
Stress fracture
A stress fracture is one type of incomplete fracture in bones. It is caused by "unusual or repeated stress" and also heavy continuous weight on the ankle or leg...
s. Because of diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
rates in organic materials, it may takes weeks or months to reach a moisture equilibrium.
The chemical effects of moisture can be seen in the chalking of titanium dioxide
Titanium dioxide
Titanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
(TiO2) pigmented coatings and polymers; the anatase
Anatase
Anatase is one of the three mineral forms of titanium dioxide, the other two being brookite and rutile. It is always found as small, isolated and sharply developed crystals, and like rutile, a more commonly occurring modification of titanium dioxide, it crystallizes in the tetragonal system; but,...
form is particularly sensitive to wavelengths below about 405 nm while the rutile
Rutile
Rutile is a mineral composed primarily of titanium dioxide, TiO2.Rutile is the most common natural form of TiO2. Two rarer polymorphs of TiO2 are known:...
forms absorb energy above that wavelength. Chalking results from the degradation of the binding material resulting in a release of the TiO2 pigment particles. These particles form a dull layer on the surface which may be wiped off. Experience shows that chalking is strongest where more water is available on the surface; little to no chalking occurs in dry atmospheres. TiO2 is a semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
where electron transitions from the valence band
Valence band
In solids, the valence band is the highest range of electron energies in which electrons are normally present at absolute zero temperature....
to the conduction band
Conduction band
In the solid-state physics field of semiconductors and insulators, the conduction band is the range of electron energies, higher than that of the valence band, sufficient to free an electron from binding with its individual atom and allow it to move freely within the atomic lattice of the material...
result from the absorption of light at wavelengths in the near UV range, below 400 nm. Ultraviolet radiation causes electron hole pairs to be created in the TiO2 lattice. These react with the hydroxide groups on the surface and the Ti4+ ions. Hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...
and perhydroxyl radicals are formed through the conversion of oxygen and a water molecule whereby the TiO2 surface again resumes the initial form and acts as a catalyst for continued activity, thus repeating the chalking cycle. The hydroxide and perhydroxyl radical then cause oxidative decomposition of the binder with the subsequent release of TiO2 particles.
Atmospheric Oxygen
Photooxidation accounts for most polymer failures that occur during outdoor exposure. It results from the effects of solar radiation in combination with oxygen. Oxygen can promote degradation in several ways. Free radicals, formed as a result of the cleavage of chemical bonds by solar radiation, react with oxygen to form peroxy radicals that initiate a series of radical chain reactions. The destructive effect of the radiation is multiplied manifold by propagation of bond breakage and the formation of hydroperoxides that further absorb solar ultraviolet radiation. This cascade effect results in an auto-acceleration of the weathering process, and may partially account for the general non-linearity of the weathering response to radiant exposure.In addition to the reactions of oxygen in its normal ground state, some reactions of oxygen are due to the excited singlet state, a highly reactive form of the molecule. Singlet oxygen
Singlet oxygen
Singlet oxygen is the common name used for the diamagnetic form of molecular oxygen , which is less stable than the normal triplet oxygen. Because of its unusual properties, singlet oxygen can persist for over an hour at room temperature, depending on the environment...
is responsible for the rapid deterioration of materials, particularly those with conjugated unsaturation such as natural rubber and synthetic elastomers. It is formed when triplet oxygen
Triplet oxygen
Triplet oxygen is the ground state of the oxygen molecule. The electron configuration of the molecule has two unpaired electrons occupying two degenerate molecular orbitals...
, the normal ground state, reacts with sensitizers, such as certain dye
Dye
A dye is a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and requires a mordant to improve the fastness of the dye on the fiber....
s and ketone
Ketone
In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...
s, excited by radiation to their triplet states. Oxygen also increases the amount of solar radiation absorbed by conjugated unsaturated hydrocarbon
Unsaturated hydrocarbon
Unsaturated hydrocarbons are hydrocarbons that have double or triple covalent bonds between adjacent carbon atoms. Those with at least one double bond are called alkenes and those with at least one triple bond are called alkynes...
s through formation of a complex with these materials.
The extent of photochemical reactions involving oxygen differ in the inner and outer layers of both aromatic and aliphatic polymers due to their dependence on the diffusion of oxygen through the polymer. Photooxidation is significantly reduced at depths beyond which oxygen penetrates. Studies of the degradation profiles of low density polyethylene
Low density polyethylene
Low-density polyethylene is a thermoplastic made from petroleum. It was the first grade of polyethylene, produced in 1933 by Imperial Chemical Industries using a high pressure process via free radical polymerization. Its manufacture employs the same method today. LDPE is commonly recycled and has...
(LDPE), polymethylmethacrylate (PMMA) and polyvinylchloride (PVC) show that photooxidation was higher at the front and back surfaces than in the interior bulk of the material. Because ultraviolet radiation is not strongly absorbed by these materials, a considerable amount of the radiation incident on the front surface is transmitted to the back surface where it initiates photooxidation.
Secondary factors of weather
OzoneOzone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
is produced by short wavelength (110 nm - 220 nm) UV photolysis of oxygen in the upper atmosphere. The photochemical reaction of nitrogen oxide
Nitrogen oxide
Nitrogen oxide can refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds:* Nitric oxide, also known as nitrogen monoxide, , nitrogen oxide* Nitrogen dioxide , nitrogen oxide...
s and hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
s from automobile exhausts is another source. Ozone plays a dual role in weathering. The concentrated layer in the upper atmosphere absorbs the short wavelength (≤300 nm) ultraviolet radiation emitted by the sun and thus plays a critical role in protecting terrestrial objects from this actinic radiation. Ozone is also a powerful oxidant and reported to react rapidly with elastomer
Elastomer
An elastomer is a polymer with the property of viscoelasticity , generally having notably low Young's modulus and high yield strain compared with other materials. The term, which is derived from elastic polymer, is often used interchangeably with the term rubber, although the latter is preferred...
s and other unsaturated polymers. Ozonolysis
Ozonolysis
Ozonolysis is the cleavage of an alkene or alkyne with ozone to form organic compounds in which the multiple carbon–carbon bond has been replaced by a double bond to oxygen...
typically results in stiffening and cracking, particularly under mechanical stress. However, the contribution of ozonolysis reactions to the overall photooxidation process is still subject to controversy.
Atmospheric pollutants (e.g., sulfur dioxide
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...
, nitrogen oxides, hydrocarbons, etc.), in combination with solar radiation, can also be responsible for severe damage. Acid-base induced chemical changes may also be responsible for much pollution-caused damage. Unsaturated alkyl and aromatic compounds may act as catalysts in the photooxidation of polymers. In the presence of sulfur dioxide and oxygen, ultraviolet radiation causes crosslinking of polyethylene
Polyethylene
Polyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons...
and polypropylene
Polypropylene
Polypropylene , also known as polypropene, is a thermoplastic polymer used in a wide variety of applications including packaging, textiles , stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and polymer banknotes...
and is responsible for the rapid loss of color in pigmented coatings.
Moisture, in combination with temperature, may also promote microbial growth. Mold
Mold
Molds are fungi that grow in the form of multicellular filaments called hyphae. Molds are not considered to be microbes but microscopic fungi that grow as single cells called yeasts...
, mildew
Mildew
Mildew refers to certain kinds of molds or fungi.In Old English, it meant honeydew , and later came to mean mildew in the modern sense of mold or fungus....
and other microbiological and botanical agents may play a significant role in material degradation, particularly in tropical and subtropical climates, although they may not be generally thought of as weathering factors.