uv curing
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UV CURABLE COATINGS: Mechnism of UV CuringTRANSCRIPT
PowerPoint Presentation
UV Coating Technology
UV curing has now been established as an alternative curing mechanism to thermal hardening,contrary to the past, where it was only considered for the curing on temperature sensitive substrates, like wood, paper and plastics.
UV Curable coating used for
decorative appearance,(colour, gloss)
protective barrier. (against mechanical, biological and chemical attack etc. )
Major End Applications of UV Coatings
Basic principle of UV Curing
UV curing technology uses the energy of photons of radiation sources in the short wavelength region of the electromagnetic spectrum in order to form reactive species, which trigger a fast chain growth curing reaction.
electromagnetic spectrum Range
UV-A, UV-B, and UV-C radiation, is mainly used for this technology.
Energy calculation in UV Curing
The energy content of a photon is defined by the equation
E = h = hc/,
where is the frequency and is the wavelength (nm).
This equation tells us, that the shorter the wavelength, the higher the energy of a photon.
Interaction of UV process design parameters
Components of UV Coatings
Reactive resins containing a plurality of polymerizable double bonds, which govern mainly the desired properties of the final coating;
Copolymerizable, monomeric diluents, which are responsible for the reduction or adjustment of the viscosity of the formulation, a function taken by the solvent in conventional formulations;
Photo initiators or a photo initiating system containing photoinitiator and photosensibilizer or coinitiators;
Additives, like surface active additives, slip additives,
fillers, pigments, light stabiliser, etc.
Types of UV Curing
Chemistry of Radical UV Coatings
The chemistry involved in the radical initiated UV induced crosslinking can be divided into the three steps,
Initiation
Propagation
Termination
Importance of Photo-Initiator
Although the UV energy applied in photo curing may cleave CC and CH bonds, the commonly used monomers do not produce sufficient amounts of initiating species, which is due to low absorbance and poor cleavage efficiency. Thus, photoinitiator is required to initiate photo curing reaction of Coating .
What happens , when UV Light hit PI
Absorption of a photon by the photoinitiator Molecule
Excitation of an electron into higher singlet states
By intersystem crossing (ISC) an electron spin inversion leads to the excited triplet state.
Molecule posses two unpaired electrons, rather than from the singlet state.
From the triplet state two main reactions can lead to initiating species, the intramolecular scission of an -bond, or the intermolecular abstraction of a hydrogen atom.
The quantum yield of initiation
The number of growing chains per photon
absorbed reflects the importance of the processes leading to initiation over all the indicated processes of deactivation.
The efficiency of the photo-initiation
The efficiency of the photo-initiation is a function of different quantum yields, since several side reactions can occur in every step. Thus, the overall yield of initiation is a complex function of different quantum yields, represented exemplarily in Slide .
Jablonsky-type diagram for photoinduced radical photoinitiation.
Light is absorbed by a molecule what happens?
The electronic configuration changes.
This leads to an initial geometry of the excited state which is usually not the energy minimum.
During excitation the electron spin remains un-changed.
Spin inversion during excitation is forbidden by quantum mechanics and therefor unlikely.
Note: The Franck-Condon principle says that the heavy atom nuclei do not change their positions.
Right after the excitation several things may happen.
1) Vibronic relaxation brings the molecule quickly into the new energy minimum structure for the excited state. Energy is released into the solvent.
2) Intersystem crossing leads to triplet states by spin inversion.
3) Emission of light and return to the ground state (luminescence, fluorescence, phosphorescence).
4) Quenching of the exited state: Energy is transferred to another molecule.
5) Radiation less deactivation. Molecule goes back to ground state by vibrational (thermal) deactivation (no light emission). The energy goes to the solvent/environment of molecule.
Science behind Radial UV Curing
First law of Photo-Chemistry
Second Law of Photo-Chemistry
Excitation state and Radiation Energy
Role of Photon into Radical Formation
The first law of photochemistry, the Grotthuss-Draper law:
The law was first proposed in 1817 byTheodor Grotthussand in 1842, independently, byJohn William Draper.
According to this law, light must be absorbed by a compound in order for a photochemical reaction to take place.
The second law of photochemistry, the Stark-Einstein law:
Each photon of light absorbed by a chemical system, only one molecule is activated for subsequent reaction.
Named after German-born physicistsJohannes StarkandAlbert Einstein
The photon is a quantum of radiation, or one unit of radiation. Therefore, this is a single unit of EM radiation that is equal to Planck's constant (h) times the frequency of light. This quantity is symbolized by , h, or .
The photochemical equivalence law is also restated as follows: for everymoleof a substance that reacts, an equivalent mole of quanta of light are absorbed.
The formula is:
where NAisAvogadro's number.
Radiation energy must match energy difference of ground and excited state
Formation of Radicals
Cont..
The ejected electrons, known asphotoelectrons, carry information about their pre-ionized states.
For example, a single electron can have akinetic energyequal to the energy of the incident photon minus theelectron binding energyof the state it left. Photons with energies less than the electron binding energy may be absorbed orscatteredbut will not photo ionize the atom or ion.
Photo-Generation :Alpha -Cleavage
Photo-Generation : H-Abstraction
TheNorrish reactioninorganic chemistrydescribes thephotochemical reactionstaking place withketonesandaldehydes.
The reaction is named afterRonald George Wreyford Norrish
Propagation
Propagation is the key step to very efficient curing, since it is a chain reaction where for instance one produced radical can add more than 1000 monomer units within a fraction of a second.
Propagation and Transfer
Termination
Risks in UV Coating Curing
Irradiation. Low-pressure mercury lamps have their main output at 254 nm. This light severely damages cells, eyes and skin. Shield reactors; turn lamps off before checking the reaction. Never look into the beam of a high power LED; the lights very high intensity damage your eyes.
Ozone generation: Short wavelength light may generate ozone from oxygen. Perform reactions always in a well ventilated fume hood.
Lamps: Most lamps operate at high temperature and at high vapor pressure. Never move or touch lamps during operation. Never switch of the cooling right after switching of the lamp!
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Thank you