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UV Protective Sol-Gel Coating

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fatih yalciin

on 3 June 2014

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Transcript of UV Protective Sol-Gel Coating

How we can solve this problem ?
UV absorbing
UV Protective Sol-Gel Coating
Malzeme Bİlimi ve Mühendisliği

534- Sol-Jel Teknolojisi
Doç.Dr. Özlem Yeşil Çeliktaş

Hazırlayan: Fatih Yalçın
91120021058
Ultraviolet
The solar radiation has wide wavelength ranges between
100
nm to
3000
nm.
consisting of :

ultraviolet (UV)
visible (VIS)
infrared (IR) radiation.

UV light brings the world interesting effects in different areas of human life, such as entertainment, medical applications, and scientific research.

Some examples of daily UV light usage are:
- television
- varieties of lamps
- photolithography
- sterilization
- fluorescence
- and even the sun tanning of human skin

Solar UV radiation varies strongly with

season
time of day
latitude and atmospheric conditions


UV light, natural or artificial, is responsible for

the decomposition and degradation of organic compounds
the discoloration of dyes and pigments
the loss of mechanical properties
gloss in polymers and plastics.




UV lights that reaches the Earth


Ultraviolet radiation


Ultraviolet radiation, sometimes called ultraviolet light, is invisible electromagnetic radiation of the same nature as visible light

Yet, having shorter wavelengths and higher energies.


In the electromagnetic spectrum, UV radiation extends between the
blue end of the visible spectrum
(400 nm) and
low-energy X-rays
, straddling the boundary between ionizing and non-ionizing radiation, which is conventionally set at 100 nm.


Radiation is conventionally classified into 3 bands in order of increasing energy.

This division was put forward by the Commission Interntionale de l’Eclairage (CIE), and corresponds broadly to the effects of UV radiation on biological tissue.


Very high energy: UV-C rays


(λ= 100-280 nm)


Highly damaging to human skin, but filtered by ozone layer, therefore not reaching the earth’s surface (18).



High-energy: UV -B rays

(λ= 280 nm - 320 nm)
Penetrating the skin to a depth of a few millimeters, induce the formation of stable pigments in the epidermis of the skin.

Melanoma or skin cancer is increased considerably due to a prolonged exposure to UV-B rays .

UVB organic absorbers (PABA derivatives, salicylates, cinnamates and camphor derivatives


Low energy: UV -A rays

(λ= 320 nm - 400 nm)
Penetrating the skin deeply , leading to premature aging.
Acute effects are only temporary and of short duration. UV-A penetrates deeper into the skin due to its longer wavelength and plays a role in skin photoaging.

UVA organic absorbers (benzophenones, anthranilates and dibenzoylmethanes)

UV-B radiation is strongly absorbed by stratospheric ozone and small changes in the protective ozone layer can produce large increases in UV-B radiation reaching the Earth’s surface.

Solar UV-C radiation does not reach the Earth’s surface because it is absorbed by ozone and other gases in the atmosphere. UVC can damage DNA and other molecules and is often used as a germicidal agent.


More frequent reports of skin cancer have made people increasingly aware of the danger of prolonged exposure to ultraviolet (UV) rays, which account for about 6% of the terrestrial sunlight and, under excessive doses, is proved to cause
Erythema
Certain skin cancers
Keratitis
Cataracts.

UV-B levels change rapidly with time of day while UV-A levels vary more slowly throughout the day.

Exposure to both, UV-B and UV-A leads to tanning and sunburn (erythema), and can affect the immune system.


Tanning lamps emit mostly UV-A radiation with a few percent content of UV-B.


Artificial uv light there are a variety of artificial sources of UV light. These sources are widely used in

some occupational
Medical
recreational settings


The most common artificial sources of UV radiation are arc-lamps, and fluorescent or incandescent lighting


Although the UV-B rays are only a small fraction (about 10 %) of total UV radiation, they are very dangerous since these are responsible for the acute and chronic illnesses, including



erythema (sunburn)
sun tanning
photo carcinogenesis
photoaging.


Arc lamps
emit in a wide range of wavelengths including a large amount of UV-A and UV-B and some UV-C light.


Short arc lamps
are usually used in movie theatre projectors, searchlights, specialized medical equipment with fibre-optic light delivery means, some stage spotlights ‘‘followspots’’), in some scientific equipment or when an extremely intense light source is required.



Mercury
, mercury–xenon, deuterium and metal halide short arc lamps emit large amounts of UV light and are the most common sources for artificial UV radiation.



Tungsten
halogen incandescent lamps emit a significant amount of UV light. Fluorescent and incandescent lighting are widely used but do not emit significant amounts of UV radiation under normal use.



Fluorescent
lamps produce UV radiation by ionizing low pressure mercury vapours. This radiation is absorbed by a phosphor coating on the inside of the tube that converts it to visible light.

Some UV radiation may leak through the phosphor coating, but the levels are generally low.



Common organic materials, such as most plastics, polymers, wood,etc., absorb ultraviolet radiation and undergo a
rapid photolytic
and
photo-oxidative
reaction that results in their
photodegradation
.


The damage to organic materials by UV light is a well known issue.

The energy of the photons in the ultraviolet region
(290–400 nm)
is sufficient to break chemical bonds in

Polymers
Wood
Paper
Other organic based materials
resulting in the formation of free radicals.

The loss of
strength
,
impact resistanc
e, and
mechanical integrity of polymers
exposed to UV light is well known.
These changes in mechanical properties reflect polymer chain scissionas a result of photodegradation.

At the same time, polymers progressively lose their
coloration
and acquire a
yellowish
tint after prolonged exposure to UV light.

Artwork in museums suffers an important photodegradation, as art pieces are exposed for large periods of time to natural and artificial illumination.

The dyes in paintings and photographs undergo a progressive fading, finally losing their
coloration
.

UV radiation is especially harmful to libraries and archival materials as it leads to the weakening and embrittlement of
cellulose fibres
and causes paper to bleach, yellow or darken, depending upon its constituents.

Effect of UV Light on Material
Protection of materials against UV radiation

There are different methods to protect matter from the harmful effects of UV radiation.

One of the most widely used methods in industry is the
dispersion of UV-absorbing
agents in the photodegradable material, reducing its degradation upon exposure to UV light.
There are
inorganic
and
organic
materials that can be used to protect against UV radiation.


Inorganic
materials are based mainly on mixed
oxide films
or particles, able to absorb or scatter light.

On the other hand, there are several
organic
molecules which efficiently absorb UV light, and can also be used for this purpose.

Why SOL-Gel ?

The inorganic coatings, however, were
limited
in application to heat resistant substrates, due to
the high curing temperature required
to obtain the protective coating.

On the other hand, dispersions of UV-absorbing molecules in polymer matrices exhibit low photostability upon prolonged irradiation with UV light, as the matrix itself is degraded during irradiation.

The sol–gel method allows the preparation of

Transparent
Solid
Porous inorganic matrices

at low temperatures, and the incorporation of organic photoactive molecules in its porosity.
The Sol–Gel method allows the preparation of
hybrid organic–inorganic matrices
, by using organically modified alkoxide precursors.

The non-hydrolysable organic groups of the alkoxide remain attached to the matrix porosity.

Alkoxides modified with non-hydrolysable phenyl groups were used for the preparation of phenyl modified
ormosil
matrices for the entrapment of the UV-absorber molecules.
Organic–inorganic hybrid sol-gel (HSG) materials, also called
‘‘Ormosil’s’’
.

Organically modified silicates or ‘
‘Ormocer’
’.

By combining the characteristics of sol-gel and polymer, the UVPHSG
(UV-patternable hybrid sol-gel)
glass provides many advantages over pure inorganic sol-gel coatings or organic polymer coatings

Protection of materials against UV radiation

There are different methods to protect matter from the harmful effects of UV radiation.


One of the most
widely
used methods in industry is the
dispersion of UV-absorbing agents in the photodegradable material
, reducing its degradation upon exposure to UV light.
There are inorganic and organic materials that can be used to protect against UV radiation.

Inorganic materials are based mainly on mixed
oxide films
or particles, able to
absorb
or
scatter
light.

On the other hand, there are several organic molecules which efficiently absorb UV light, and can also be used for this purpose.

Compared with organic polymer coatings, the hybrid sol-gel films also manifest advantages such as
-low optical propagation loss
-high chemical and mechanical stability

as well as good compatibility with different substrates to be coated.
The hybrid sol-gel films can be prepared by both
dipping
and
spin-on
techniques.

In the
spin coating
process, the film is deposited and dried
in a few seconds
which usually results in a highly uniform coating.

In the case of
dip coating
, the film is applied by dipping the substrates in a bath of coating media and removing them in a controlled manner at a rate of typically a
few centimeters per minute
.
Generally speaking,


Spin coating
is used for high uniformity coatings but is
not
suitable
for very thick layers
,



Dip coating
is suited for thick layers but usually with a
limited uniformity
, especially near the edge of the substrates.


Many research can get the coatings present

-very strong absorption about 270–400 nm range
-having a thickness of about 1 mm.

The protective film reduces the intensity of the UV light reaching the substrate
to less than 7%
of the incident radiation, providing an efficient protection against the UV radiation.
t

How to Analyse it ?
UV Protection Factor AS/NZS 4399


UV Protection Factor (UPF)
The UV protection factor states how long someone wearing UV protective clothing can stay out in the sun without suffering skin damage. 


Sun Protective Factor (SPF)

a time factor for the protection of skin compared to exposure without any protection.



Inorganic UV-absorbing systems
Many inorganic UV-absorbing systems based on particles or thin film coatings have been developed.
Inorganic semiconductor oxides
such as
TiO2, SiO2 CeO2 Al2O3, and ZnO
are the most widely used for protection against UV radiation.

They can applied;
-Directly
-Hybrid
-Embedding
-in a Capsule
The UV light is strongly absorbed by excitation of electrons from the valence band to the conduction band.

Compared with the
existing organic UV absorbers, the inorganic UV agents are more preferred
because of their unique features including, among others,
non toxicity
and
chemical stability
under both high temperature and UV-ray exposure.

For mechanism of
TiO2
some investigators believe that it provides good UV protection by
reflecting and/or scattering
most of the UV-rays through its high refractive index;


Others believe that it absorbs UV radiation because of its
semiconductive properties
.
Organic absorbers
However, the low stability of the extensively used polymer matrices to embed the absorbers, upon irradiation with UV light, limits their application seriously.

The attenuation of the UV radiation in these materials is accomplished by both,
bandgap

-absorption
-scattering of light.

The bandgap strongly
depends on
the
nanoparticle size
and the
presence of crystalline imperfections
.

Smaller size distributions tend to extend the absorption bands towards the visible range, which reduces the yield coating.
Light having a wavelength longer than the band gap wavelength will not be absorbed.

The intensity of scattered light is a function of particle size as well as the
refractive indices of the particles
and the media.
One the most important families of UV-absorber molecules include a phenolic group that plays an important role in the dissipation of the absorbed energy.
Phenolic-type UV absorbers involve compounds forming
O–H–O
bridges;
salicylates
2-hydroxybenzophenones
2,2-dihydroxybenzophenones
3-hydroxyflavones or xantones

Compounds forming O–H–N bridges

2-(2-hydroxyphenyl)benzotriazoles
2-(2-hydroxyphenyl)-1,3,5-triazines




Some precursor reflactive index at 598 nm


Rutile TiO2 n: 2,7
Brookite TiO2 n: 2,62
Anatase Ti n: 2,54
CeO2 n: 2,2
ZnO n: 2,02


Sun Protective Factor
Titanium
The titania films are normally applied by
dip coating
of an organic precursor followed by
heat treatment to remove the organic residue
and form the desired anatase phase.

Titanium dioxide and zinc oxide are wide bandgap semiconductors, which are transparent in the visible and absorb in the UV range.


E gap/TiO2= 3.0 eV 365nm


Transparent, self-cleaning titania coatings on glass are today widely used and sold under several trade names.

These coatings combine photocatalytic breakdown of absorbed organic dirt with light-induced superhydrophilic surface properties hat promote water to evenly wet and rinse away the decomposition products
Zinc Oxide
ZnO has bandgap energies corresponding to wavelengths 380 nm, respectively.


E gap/ZnO= 3.2 eV 380nm


Zinc oxide precursors are
less reactive than their titania counterparts
and the formation of the oxide nanoparticles typically proceeds through a series of intermediate compounds
zinc hydroxide species are
sensitive to the water content
and to ageing.
Transparent in visible region.

Zinc oxide nanoparticles show a
lower
photocatalytic performance than TiO2.
Cerium dioxide

cerium nitrate or cerium sulfate are type of it.

Cerium is also used as ZnO and TiO2 because of its bandgap energy.


for Cerium agglomeration unless surfactants are introduced during the precipitation process.


As a UV absorber and reinforced material
Tinuvin (213-360-1577)
Sema 20160

Cinnemate

ZnO, Al, Ce

Barium, Magnesium, Cobalt

Phenolic compound



-specialized band absorbing
-conservation coating
-film penetration rating
Full transcript