CHAP. 2 PHOTOPOLYMERIZATION

CHAP. 2 PHOTOPOLYMERIZATION

2.1 Description of photopolymerization; applications 1-5

The transformation of a reactive liquid into a solid, by UV-radiation, leading to

polymerization and cross-linking is termed photopolymerization or UV-curing.

UV-curing is defined as:

FAST TRANSFORMATION OF 100% REACTIVE, SPECIALLY FORMULATED,

LIQUIDS INTO SOLIDS BY UV PHOTONS.

Photons generated by UV-light are absorbed by the chromophoric site of a molecule

in a single event; this molecule generates radicals or protons, the initiating species that

promote the fast transformation (time range 10-2-1 s) from the liquid into the solid. As a

result of the curing process, a solid polymer network, totally insoluble in the organic

solvents and very resistant to heat and mechanical treatments, is formed from a 100%

reactive liquid.

The entire process is schematized in Fig. 2.1.


Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion

17

Fig. 2.1: Schematic representation of photocuring process.

As shown in Fig. 2.1, a UV-curable formulation is made of three basic components:

1. photoinitiator, which absorbs the incident light and readily generates reactive

radicals or ions;

2. functionalized oligomer, which, by polymerizing, will constitute the back-bone of

the three-dimensional polymer network formed;

3. a mono- or multifunctional monomer, which acts as a reactive diluent and will be

incorporated into the network.

The photoinitiator is the key of all process, because it determines both the rate of

initiation and the penetration of the incident light into the sample, governing in this case

also the depth of cure.

Depending on the photoinitiator used, the reactive species generated can be radicals

or ions, so the process can be named radical or cationic photopolymerization. As

described in the following paragraphs, radical and cationic photopolymerizations are

very different not only for the active species that start the reaction, but also for the types

of monomers used and for the cure mechanism and experimental conditions in which

the process is performed. In Fig. 2.2, the differences in the reactive species generated

and the initiation step for the two processes are schematically illustrated.

During the initial part of the reaction, polymerization rate depends on the reactivity

and concentration of the functional group as well as on the viscosity of the matrix

medium. Other important parameters are chemical micro-structure and functionality of

monomers and/or oligomers: they will determine the final degree of polymerization,

physical, and chemical characteristics of the final polymer.

Photoinitiator

UV radiation

Reactive species(radicals or ions)

Multifunctional monomer

Crosslinked polymer



18

Fig. 2.2: Initiation step for radical (I) and cationic (II) photopolymerization.

APPLICATIONS

Nowadays, UV-curing technology is well established in many industrial fields and in

particular applications, it offers new possibilities of development. The principal

industrial use of UV-curing technology is in the coating industry for the surface

protection of all kind of materials, due to high speed process and good energy yield. A

typical industrial line UV processor is made of two parts: the coating machine, where

the UV-curable resin is applied on the substrate, and the UV oven, where the liquid

resin is dried within a fraction of a second by passing under a powerful lamp.

In Fig. 2.3 an industrial processor for the UV-curing of organic coatings is

schematically presented.

Fig. 2.3: UV-curing industrial processor for coatings.

R CH2 CHR' R CH2 CHR'

(I)H+ CH2 CHR CH3 CHR+

(II)



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Acrylate resins, cured with radical photopolymerization, are the most widely used

UV-systems, with a total annual production of approximately 60000 tons, while

cationic-type resins, cured with cationic photopolymerization, represent a minor part,

i.e. about 2000 tons, but in continuous growth (10-12% per year).

Here are reported the main industrial fields in which UV-curing technology is

employed1,2.

Graphic arts/Coatings

Adhesives

Electronics

Stereolithography

Dental composite materials

Graphic Arts

UV-curing is used both in the pre-press part to produce the printing plate as well as

in the printing process itself, thanks to the development of fast-drying UV-curable inks.

The printing process consists of the rapid transfer through an ink of a given image

from a printing plate to the substrate (usually paper), thus allowing a fast production of

prints.

The main printing processes in which UV-curing is involved are: letterpress, gravure,

flexography, screen printing and lithography. On the other side new UV inks have been

developed. They present a number of advantages over conventional solvent-based inks:

the higher viscosity allows several colours to be applied successively;

their solvent-free formulations lead to a better print definition and high gloss

images;

the UV process is more economic because it requires less energy and

achieves a higher productivity; the entire process is performed at ambient temperature,

without any solvent emission, which makes it a environmental friendly procedure.

Finally, for some specific applications, it is necessary to further improve surface

properties of printed material (ex. gloss, smoothness, and abrasion and scratch

resistances, weathering resistance). This can be achieved by applying a thin layer of a

UV-curable varnish, which is known to give high gloss and smooth surface.



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Coatings

Coatings are applied to a surface. They can be divided into:

Functional coatings, improving the surface by:

protecting it from abrasion, scratch, mar, chemicals;

providing different properties such as release, slip, adhesion, electrical

conductivity or insulation, antifogging, flame retardance;

acting as a barrier to various liquids or gasses.

Decorative coatings are applied to:

change appearance ( colour, gloss or mat finish, texture);

hide surface (imperfections, electrical circuitry, etc.).

Usually coatings are classified according to the substrate they are applied to:

paper and paperboard

wood

plastics

metal

glass and ceramic

miscellaneous.

This type of employ of UV-curable varnishes is increasingly used to obtain highly

resistant coatings to protect any substrate: wood, plastic, metal, glass, optical fibres,

paper, leather, fabrics, etc.; the film thickness is of the order of 20-100 µm to assure a

long-lasting protection.

Adhesives

Radiation curing has two main areas of application in the field of adhesion:

1. to bond together two parts of a laminate, acting as a quick-setting glue. In this

case the use is limited by the UV-transparency of one of the two parts of the laminate.

The whole process is divided in three steps: applying of the adhesive in the liquid state;

assemblage of the two parts; exposure of the assembly to UV-light.

UV-cured laminates show a great potential because they are produced by a process

that is faster, cheaper and easier to work out than the usual thermal cure carried out for

hours under high pressure.



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2. To produce pressure-sensitive adhesives and release coatings. It consists in a

rapid photoinitiated crosslinking producing a viscoelastic system with predetermined

properties.

Electronics

Here UV-curable systems have found applications as photoresists in the imaging

step, fast drying adhesive and conformal coatings.

Stereolithography

This new technology is based mainly on the capability of UV-curable systems to give

three-dimensional solid objects, by scanning the surface of a resin with a laser to form a

thin solid pattern, and building up the model step-by-step by adding one layer on top of

another. Complex parts can be obtained faster, with great precision, and more flexible

processing than with conventional modelling techniques. Besides it allows the direct use

of digital design information to guide the formation of a model that closely represents

the original design.

Dental Composite Materials

Adding mineral fillers such as glass or silica particles to UV formulations is possible

to obtain extremely hard and abrasion-resistant composite materials. These types of

resins present a number of advantages over conventional systems: immediate readiness

for use, extended working time, higher polymerization rate, and short setting time,

better adhesion of the filler particles to the matrix.

The curing of these systems has to be performed at visible light and it is necessary to

take into account that inert filler can be up to 60% in volume, so the penetration of light

in these composite resins is limited therefore it has to be carried a multiple step process.

PRINCIPAL ADVANTAGES/DISADVANTAGES1,2

The main advantages of UV-curing technique are better understood if compared with

the traditional thermal-curing polymerization (Tab. 2.1).



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Tab. 2.1: Comparison of UV and thermal curing1.

Parameter UV Thermal

Commercial

Capital cost + -

Operational cost + -

Formulation cost - +

Floor space + -

Cure speed + -

Skill level required 0 +

Environmental

No solvent release + -

Energy consumption + -

Technical

Chemical resistance + -

Formulation variety 0 +

Curing of pigmented films - +

No substrate damage + 0

Low cure temperature 0 -

Sensitivity to oxygen + +

Health & safety

Fire hazard + -

Radiation hazard 0 +

Irritant raw materials - +

+ = advantage - = disadvantage 0 = intermediate



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Arguments in favour of the replacement of thermal curing by UV-curing are mainly

lower capital and running costs, lower floor space requirements, higher running speeds,

less substrate heating, the high quality of the cured coating or ink, no solvent release

during curing and the development of new curable formulations having less or no skin

irritant raw materials.

There are also economic and ecological factors that encourage the continuous growth

of radiation curing technology such as:

Raw materials containing a low amount of volatiles, less or no skin irritant, have

been developed and increase the range of formulation variety.

Low-viscosity monomer-free oligomers and water reducible oligomers can be

used in spray coating applications.

New applications in metal and glass coatings are possible thanks to oligomers

that adhere well to critical substrates.

Weather resistant products are available for outdoor applications.

More reactive photoinitiators allow lower concentrations in formulations or less

powerful UV sources to be used.

Photoinitiator-free UV-curable systems appear on the market.

New monochromatic UV-sources were introduced.

On the other side thermal curing still holds a strong position due to the advantage in

formulation costs and variety, the avoidance of radiation and the lower skill level

required. Moreover the thickness of the sample that can be photocured is normally very

thin if compared to a thermal cured one. Mainly for this reason UV-cure technology is

still not widespread in the composites industry.

2.2 Radical photopolymerization1,2,4

The radical polymerization mechanism can be schematically represented in Fig. 2.4:

Fig. 2.4: Radical polymerization mechanism. R CH2 CHR' R CH2 CHR'



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R is the active specie generated by photodecomposition of the initiator.

It should be pointed out that it is only the initiation step, radical formation from the

photoinitiator, which is different from thermal polymerization.

Radical photoinitiators can be divided into two groups according to the way the

active species are generated2:

1. by photocleavage, if radicals are generated by a intramolecular scission ;

2. by hydrogen abstraction, if radicals are generated by the abstraction of an atom of

hydrogen from a donor molecule.

In Fig. 2.5 are illustrated the two ways of radicals’ generation.

A-B* → A• + B• A* + RH → AH• + R• homolitic cleavage hydrogen abstraction

Fig. 2.5: Mechanism of radicals’ generation in radical photopolymerization.

1. Photocleavage: in this class we found aromatic carbonyl compounds that

undergo to homolytic C-C bond scission upon UV exposure, with the formation of two

radical fragments; the benzoyl radical was shown to be the major initiating species.

The process is schematized in Fig. 2.6; examples of photoinitiators belong to this

class are: benzoin ethers derivatives, benzilketals, hydroxyalkylphenones, α-amino

ketones, and acylphosphine oxides.

Fig. 2.6: Radical formation reaction for aromatic carbonyl compounds.

2. Hydrogen abstraction: this is a typical reaction of some aromatic ketones, like

benzophenone, thioxanthone, or camphorquinone. Under UV irradiation, they do not

undergo fragmentation, but abstract a hydrogen atom from an H-donor molecule to

generate a ketyl radical and the donor radical.

C

O

C Xhv

O

C XC



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The process is schematized in Fig. 2.7:

O

Cvh

C

O

*

RHC

OH

R

Fig. 2.7: Radical formation reaction for aromatic ketones.

In this case initiation of polymerization occurs through the H-donor radical. The

most frequently used H-donor molecules are tertiary amines, because of the high

reactivity of the α-amino alkyl radical towards the double bond, as shown in Fig. 2.8:

Fig. 2.8: Radical formation reaction in case of tertiary amine used as co-initiator.

This latter class of photoinitiators have also the advantage of reducing the inhibition

effect of oxygen because they promote a peroxidation mechanism that consumes the

oxygen present in the monomer.

In Fig. 2.9 are listed the principal classes of radical photoinitiators.

C

O

C

OR

R'

benzoin derivatives

benzil ketals C

O

C

OR

OR'

hydroxyalkylphenone C

O

C

R

OH

R'

acylphosphine oxides C

O

P

O

benzophenone derivatives C

O

thioxanthone derivativesC

S

O

Fig. 2.9: Radical photoinitiators commonly used.

Ar2C O N CH2hv CHNAr2C OH

CH2 CHN CH CH2



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The main classes of resins that can be cured with radical system are: acrylate and

methacrylate monomers, thiol-ene systems, and unsatured polyester resins.

Acrylate and methacrylate monomers are by far the most used in industry because

they are very reactive and can be used to create a large variety of crosslinked polymers

with tailor-made properties. Their polymerization is very fast at the beginning, but

progressively slows down when gelification and vitrification occur; for this reason there

are always some residual unreacted insaturations trapped in the polymer network.

They can be divided into:

functionalized oligomers

mono- or poly-functional monomers

The most important types of functionalized oligomers are:

epoxy acrylic resins

urethane acrylic resins

polyalkylene glycol diacrylates

polyester diacrylates

The most important monomers are:

diethylene glycol diacrylate

hexanediol diacrylate

trimethylolpropane triacrylate.

In Fig. 2.10 is presented the typical reaction scheme for this class of monomers.

Epoxy acrylates are highly reactive and produce hard and chemically resistant

coatings, so they are used in wood finishing applications, varnishes for paper, and

cardboard as well as for hard coatings2,4.

Polyesters acrylates are often applied in wood coatings, varnishes, lithographic and

screen inks.

Methacrylates monomers have similar reactivity to acrylates monomers, but with a

lower propagation rate2,4.



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C

O

CH2 CH C

O

O C

O

CH2 CH C

O

O

Propagation

C

O

CH2 CH C

O

Omonomer

C

O

CH2 CH

C

CH2 CH CH2 CH

CO

C O

CHCH2 CH2

O

C O

CH CH2CH CH2CH

Termination

Pn Pm PnPm

Pn vitrification

Initiation

Fig. 2.10: Polymerization reaction of acrylates.

The most important advantages of acrylate formulations are high reactivity and

adjustable viscosity. Rapid cure speed and low viscosity combined with brittleness and

poor adhesion are obtained when acrylate monomers are used; acrylate oligomers have

higher viscosity and lower reactivity than monomers, but they guarantee a broad range

of coating property requirements. Therefore radiation curable formulations usually

consist of monomers as reactive thinners and oligomers as binders.

Thiol-ene systems are used in many applications such as coatings, adhesives,

sealants, etc.

Their polymerization reaction can be represented as follows (Fig. 2.11):

Fig. 2.11: Polymerization reaction of thiol-ene systems.

Using multifunctional monomers it is possible to obtain a three-dimensional network

in which connecting chains are made of alternating copolymer. It should be noticed that

OAr2C RSHvh Ar2C OH RS

RS CH2 CH R' RS CH2 CH R'

R'RS CH2 CH RSH RS CH2 CH2 R' RS



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thiol-ene systems are less sensitive to air inhibition that other radical systems because

peroxy radicals are also capable to extract H from the thiol (Fig. 2.12) forming the thiil

radicals which continue the polymerisation process.

Fig. 2.12: Hydrogen abstraction from thiol molecule.

Unsatured polyester resins are mainly employed in the wood finishing industry; the

radical-initiated crosslinking occurs by direct addition copolymerization of the vinyl

monomer with the unsaturations at the polyester backbone, as shown in Fig. 2.13:

Fig. 2.13: Polymerization of unsatured polyesters.

In Fig. 2.14 the principal classes of radical monomers are listed.

polyester/styrene C

O

CH CH C

O

CH CH2

thiol/ene C(R SH)4 CH2 CH R' CH CH2

acrylates

(CH2 CH C

O

O CH2)3 CH2 CH2 CH3

CH2 CH C

O

O R O C

O

CH CH2

R = polyester, polyether, polyurethane, polysiloxane

Fig. 2.14: Radical monomers commonly used.

RSCHCH2

P PO2RSH

PO2H RS

R O C

O

CH CH C

O

O CH CH2 crosslinked polymer



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2.3 Cationic photopolymerization6-9

The cationic polymerization mechanism is schematically represented in Fig. 2.15:

Fig. 2.15: Cationic polymerization mechanism.

H+ is the active specie generated by photodecomposition of the initiator.

Photoinitiators for cationic photopolymerization can be divided into three groups:

Aryl diazonium salts

Ferrocenium salts

Diaryliodonium/triarylsulfonium salts.

The latter are named “onium salts” and are nowadays the photoinitiator class most

used in cationic polymerization. They are stable crystalline compounds, readily soluble

in a wide variety of common polar solvents and cationically polymerizable monomers

and absorb strongly in the UV region. In Fig. 2.16, their structure is represented.

Fig. 2.16: General structure of “onium salts”:

diaryliodonium (I) and tryarylsulfonium (II) salt.

Under UV light, they are subjected to photolysis through a quite complex

mechanism. In the case of diaryliodonium salts, one can have photoexcitation of the salt

and after the decay of the resulting excited singlet with heterolytic and homolytic

H+ CH2 CHR CH3 CHR+

I

MtXn

S

MtXn

(I) (II)



30

cleavages of carbon-iodine bond. Free-radicals, cationic and cation-radical fragments

are produced according to the scheme reported in Fig. 2.17.

*

HMtXnArArI MtXnAr2I MtXnhv MtXnAr2I ZH ArI Z

Fig. 2.17: Photolysis of diaryliodonium salt under UV light.

Protonic acids, denoted as HMtXn, derive from the reaction between the aryl cations

and aryliodine cation radicals with solvents, monomers, or impurities. HMtXn is the real

initiator of cationic polymerization, as shown in Fig. 2.18.

nM

MHMtXn H M+ MtXn

H M+ MtXn H (M)nM+ MtXn Fig. 2.18: Initiation mechanism for cationic polymerization.

For triarylsulfonium salts the photolysis is similar, but the heterolytic cleavage is

dominant on homolytic cleavage.

The anion generally indicated as MtX-n must have non-nucleophilic characteristics

because any cationic species generated during photolysis or by addition to a monomer

would give combination with a nucleophilic anion and, as result, retardation or

complete suppression of polymerization reaction. According to their non-

nucleophilicity, the most useful anions are: PF-6, AsF-

6, and SbF-6.

The type of anion determines also the strength of the Brønsted acid generated via

photolysis: bigger anions generate stronger acids, so the reactivity order is:

SbF-6 > AsF-

6 > PF-6 > BF-

4.

In Fig. 2.19 are shown the differences observed changing the anion on the kinetics of

photopolymerization of cyclohexene oxide.



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Fig. 2.19: Photopolymerization of cyclohexene oxide using 0.02% mol of (C6H6)3S+X- salts6.

The onium salts show a very high degree of thermal stabilty due to their cation part

which is stabilized by the resonance of benzenic rings and by the d-orbital of central

atom. As a result of this stability, they undergo thermal decomposition at very high

temperatures, as shown in Fig. 2.20.

Fig. 2.20: TGA analysis of (C6H6)3S+ AsF-

6 in nitrogen and air during an heating

ramp of a rate of 10 C/min6.

In Fig. 2.21 are summarized the various critical functions that can be assigned to the

cation and anion portion of an onium salt.



32

Fig. 2.21: “Anatomy” of an onium salt photoinitiator.

Studies made on reactive systems using photo-calorimetric technique6, i.e. photo-

DSC, have revealed that there are other parameters controlling the reaction:

Concentration of photoinitiator, for each of them is possible to observe that there is a

specific concentration for which is obtained an optimum cure rate. Further increase in

photoinitiator level does not produce a corresponding increase in the cure rate, possibly

due to the light screening effects by the triarylsulfonium salt itself or its photolysis

products.

UV-light intensity, because the system is limited by the absorption of the

photoinitiator, so it is useless to have very high light intensities. At very low intensities

there appears to be some type of inhibition effect.

Temperature effect, it has been observed that in all cationic systems cure at the

highest temperature the substrate give the highest cure rate, of course this is not always

possible.

CATION

DETERMINES PHOTOCHEMISTRY

λmax

molar absorption coefficient

quantum yield

photosensitization

thermal stability

ANION

DETERMINES POLYMER CHEMISTRY

acid strength

nucleophilicity

anion stability

initiation efficiency

propagation rate constants

MtXnI



33

Water effect, because the presence of water (or other hydroxyl containing impurities)

can change both the rate and the extent of polymerization of epoxy monomers.

Two other classes of cationic photoinitiators have been mentioned above:

Aryldiazonium salts

Ferrocenium salts.

Aryldiazonium salts were the first class of cationic photoinitiators developed in the

1970s. They can be used in the ring opening polymerization of epoxides through the

reaction scheme represented in Fig. 2.22.

Fig. 2.22: Photolysis mechanism of diaryldiazonium salt and cationic polymerization

of an epoxy monomer.

This class of cationic photoinitiators had no success essentially for two reasons:

1. the thermal instability of aryldiazonium salt leads to poor latency so that the

systems spontaneously gelled in few hours even in absence of light.

2. The generation of nitrogen gas as photolysis product leads to film defects.

Ferrocenium salts are a very different class of cationic photoinitiators. They undergo

photolysis to generate an iron-based Lewis acid with the loss of the arene ligand. This

species coordinates to an epoxy monomer to give ring-opening polymerization as shown

in Fig. 2.23.

Ar N2 BF4hv Ar F BF3 N2

O BF3

H2O

O

n



34

R1

Fe X hv XFe

R1

O

R1Fe

O

R1

( )3

X- R1

nO

R1

polymer Fig. 2.23: Photolysis mechanism of ferrocenium salt and cationic polymerization of

an epoxy monomer.

The use of this class of photoinitiator is limited to the monomers that can bond

effectively with the photogenerated coordinatively unsatured ion center.

Cationic photopolymerization is used to cure monomers that are reactive towards

cationic species. In Fig. 2.24, the most important monomers that can be UV-cured in the

cationic way are scheduled. Among all the monomers presented, the most interesting

classes for cationic photopolymerization are multifunctional vinyl ethers and epoxides

because they are very reactive and commonly available.

Fig. 2.24: Polymerizable monomers with cationic photoinitiators.

Cationic Photoinitiators

hv

nCH

R

CH2 O

O

R

nCH2 CH2 S

S

nCH

OR

CH2OR

nN

C O

R

CH2 CH2

N

OR

n(CH2)4 O

O

n(CH2)5 O C

O

O

O

nCH

R

CH2

R

nCH2O CH2O CH2O

O

O O



35

The epoxy monomers can be UV-cured through the opening of the epoxy ring,

catalyzed by the acid species generated by photolysis of the initiator. The reaction

mechanism is presented in Fig. 2.25.

X OCH

CH

R

R'X O

CH

CH

R

R'oxonium ion

monomer

X (O CH

R

CH

R'

)n OCH

CH

R

R'

Fig. 2.25: Polymerization scheme for an epoxy monomer.

In presence of difunctional epoxides UV-curing leads to a crosslinked polymer.

The reactivity of this class of monomers is quite broad, for example monomers

containing the epoxycyclohexane group are much more reactive than glycidyl ethers or

glycidyl esters, due to steric and electronic factors.

Two examples of epoxy monomers commonly used are cyclohexane dimethanol

diglycidylether, denoted DGE and 3,4-epoxycyclohexyl-3’,4’-

epoxycyclohexanecarboxilate, denoted CE. Their structures are given in Fig. 2.26.

Fig. 2.26: Chemical structures of DGE and CE.

O

O

OO

CE

CH2

CH2

O CH2

O CH2

O

O

DGE



36

Vinyl-ethers monomers are the most reactive towards cationic photopolymerization,

giving a three-dimensional polymer network with a low number of residual

insaturations. The high reactivity of these monomers is due to the presence of the

double bond C=C that, with the oxygen atom, stabilizes the cation through the chain

growth (Fig. 2.27).

R CH2 CH OR CH2 CH OCH2 CH OR+

Fig. 2.27: Growth of the polymer chain and its stabilization by resonance.

Even if vinyl ethers are ideally suited for cationic photopolymerization, their use in

industry is limited by their high cost and the hazards of using acetylene under high

pressure during their synthesis.

PRINCIPAL ADVANTAGES/DISADVANTAGES

Effect of oxygen

One of the main advantages of cationic-initiated polymerization, if compared to the

radical induced process, is that the former is not sensitive to oxygen, thus allowing

coatings to be cured rapidly even in the presence of air.

Influence of film thickness

In thin films the photopolymerization develops at the same rate, but as the film

thickness is increased, the propagation rate value, Rp, drops, due to the UV filter effect

of the top layer (Fig. 2.28). Film thickness has a pronounced effect also on the

maximum conversion level. Moreover atmospheric oxygen will diffuse less rapidly in

thick coatings.



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Fig. 2.28: Influence of the film thickness on the photopolymerization of a

cycloaliphatic diepoxy7.

Post-polymerization

One of the distinct features of cationic photopolymerization, compared with radical-

induced process, is the post-cure phenomena: it consists in a further and not negligible

polymerization taking place once the light has been switched off. Such an important

post polymerization is due to the fact that two cations cannot interact to undergo

coupling or disproportionation, so that the living polymer chain continues to grow in the

dark, until termination occurs by transfer reaction or bimolecular interaction with

another species present in the polymerization mixture (as water, bases, or another

portion of polymer chain).

Fig. 2.29 shows some typical conversion vs. time curves recorded after exposure,

compared to continuous irradiation: post-polymerization is relatively more important in

the early stages of the reaction, but a significant increase of the degree of conversion

could be noticed even after 20 minutes of storage in the dark.



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Fig. 2.29: Polymerization profiles recorded after UV exposure of various durations for a cycloaliphatic diepoxy7.

2.4 Why using cationic photopolymerization?

The main differences between radical and cationic photopolymerization has been

described and it becomes evident that the cationic UV-curing process offers many

important advantages that are summarized here:

the initiating species is a stable compound only consumed by anions or

nucleophiles;

after UV exposure, cationic polymerization continues for a long time;

since no radicals are involved, cationic photopolymerization is not sensitive to

oxygen;

films made from cationic formulations show low shrinkage and good adhesion.

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CHAP. 2 PHOTOPOLYMERIZATION

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