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Progress in Organic Coatings 57 (2006) 44–49

Siloxane additive as modifier in cationic UV curable coatings

M. Sangermano ∗, R. Bongiovanni, G. Malucelli, I. Roppolo, A. PriolaDipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, C.so Duca degli Abruzzi 24, 10129 Torino, Italy

Received 16 January 2006; accepted 1 June 2006

bstract

A polysiloxane hydroxyl terminated oligomer (SIOH) was used as chain transfer additive in cationic UV curing of a dicycloaliphatic epoxy

esin (CE). An increase on curing rate and final epoxy conversion was achieved together with a flexibilization of the network and a consequentoughness improvement. Also an important increase on surface hydrophobicity was induced already in the presence of very low amount of thedditive.

2006 Elsevier B.V. All rights reserved.

eywords: Polysiloxane additive; Cationic photopolymerization; Surface properties; Toughness

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. Introduction

The ultraviolet (UV) curing process is becoming of increasingmportance in different industrial applications: the formulationsre solvent-free, the production rate is high and the energyequired is much less than in thermal curing [1,2]. These advan-ages have led to the rapid growth of the technique in differentelds, mainly in the production of films, inks and coatings onvariety of substrate, including paper, metal and wood. More-ver, a variety of high-tech and electronic applications, such ashe coating of optical fibers and the fabrication of printed circuitoards, have been developed [3].

From a chemical point of view the curing process is a poly-erization of multifunctional oligomers and monomers giving

ise to tridimensional networks.Both radical and cationic species can be generated by the

nteraction of UV light with a suitable photoinitiator; thereforehe reaction mechanism is either radical or cationic.

Cationic photopolymerization is receiving renewed interestue to its interesting characteristics that cannot be achievedy using a radical photopolymerization process. They mainly

nclude the absence of oxygen inhibition; moreover the

onomers employed show low shrinkage, and good adhesionroperties and are generally characterized by being less toxic and

∗ Corresponding author.E-mail address: marco.sangermano@polito.it (M. Sangermano).

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300-9440/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.porgcoat.2006.06.001

rritant compared with acrylic and methacrylic resins employedn radical processes [4].

The initiation of cationic UV curing polymerization is a mul-istep process involving first, the photoexcitation of diaryliodo-ium or triarylsulfonium salts, and then the decay of the resultingxcited singlet state with both, heterolytic and homolytic cleav-ges: cations and aryl-cations generated are very reactive withonomers to give the formation of a Bronsted acid, which is

he actual initiator of cationic polymerization [5,6]. Initiationf polymerization takes place by protonation of the monomer,ollowed by the addition of further monomer molecules, thusesulting in chain growth reaction [7], as shown for an epoxyonomer in Scheme 1.In addition to the usual mechanism, shown in Scheme 1 for the

ing opening polymerization of epoxides, Penczek and Kubisaescribed a new mechanism for the polymerization of theseonomers called activated monomer mechanism that takes placehen the cationic polymerization of epoxides is carried out in

he presence of alcohols [8–10]. This mechanism is reported incheme 2.

During the polymerization, the growing ionic chain endndergoes a nucleophilic attack by the alcohol to give a pro-onated ether. Deprotonation of this latter species by the epoxy

onomers results in the termination of the growing chain and

he proton transfer to the monomer can start a new chain. Theolymer has now an alcohol fragment as an end group.

The effect of the chemical structure of the alcohol on theccurrence of this chain transfer reaction, during the UV cur-

M. Sangermano et al. / Progress in Organic Coatings 57 (2006) 44–49 45

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ng process, has been deeply reported and discussed in theiterature [11–15]. We have recently reported the use fluori-ated alcohols as surface modifying agents through a chainransfer mechanism in cationic photopolymerization of epoxyystem [16]; a surface modification of the photocured filmsas achieved in the presence of very low amount of the fluoro

dditive.By a selective migration of the fluoroalcohols towards the

uter-layer of the coating and their specific self-aggregation,igh hydrophobicity and oleophobicity were achieved. At theame time the substrate interface was kept unmodified. Theseeatures are interesting for practical applications: the epoxy coat-ngs have the same adhesion as the unmodified one, at the sameime the outer-layer, having a much lower surface tension, canuarantee low friction, antifouling, antigraffiti and antistatingroperties, as reported in a recent patent [17].

A study on the surface modification of an epoxy curableystem, obtained by using silicon epoxides [18] was investi-ated; silicon monomers posses some unique properties suchs low glass transition temperature, low surface energy, non-ammability and thermal stability [19,20]. Therefore, besidesuorinated oligomers, silicon materials have also been investi-ated for their interfacial properties such as low wetting, spread-

ng and adhesion and as non-toxic fouling release properties.

In this paper the use of a hydroxylated terminated silox-ne oligomer was investigated as bulk and surface modifier inationic UV curing of a typical epoxy system.

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Scheme 2

.

. Experimental

.1. Materials

3,4-Epoxycyclohexylmethyl-3′,4′-epoxycyclohexyl car-oxylate, UVR 6110 from Dow-Union Carbide (CE) wasmployed as a reference epoxy resin. A polydimethylsiloxaneydroxyl terminated (PS340 Mn = 600) was purchased fromluorochem, England (SIOH).

The photoinitiator was a commercially available mixture ofntimonate sulfonium salts (UVI 6976) supplied from Dow: its given as a solution in propylene carbonate (50%, wt./wt.) andt was added to the curable mixtures at a concentration equal to% (wt./wt.). The structure of the monomer together with theiloxane additive are reported in Table 1.

.2. Samples preparation

The photocurable formulations were prepared by mixingifferent amount of the siloxane additive to the epoxy resinnd adding 2 wt.% of the cationic photoinitiator. The formu-ations were coated to a glass or to a polystyrene substratesing a wire-wound applicator. The wet film was exposed to

V irradiation by using a Fusion lamp (H bulb), with radiation

ntensity on the surface of the sample of 280 mW/cm2 and aelt speed of 6 m/min. Fully cured films of about 100 �m werebtained.

.

46 M. Sangermano et al. / Progress in Organic Coatings 57 (2006) 44–49

Table 1chemical structure of the monomer and of the siloxane additive

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sspecies increases, so that the polymerization can be almost com-pleted and the epoxy group conversion increases from 75% inthe pure CE resin up to 90% in the presence of 7 wt.% of theSIOH additive.

.3. Characterization techniques

The kinetics of the photopolymerization were determined byeal-Time FT-IR spectroscopy, employing a Thermo-Nicolet700 instrument. The liquid formulations were coated ontosilicon wafer. The sample was exposed simultaneously to

he UV beam, which induces the polymerization, and to theR beam, which analyses in situ the extent of the reaction.ecause the IR absorbance is proportional to the monomer con-entration, conversion versus irradiation time profiles can bebtained. Epoxy group conversion was followed by monitor-ng the decrease in the absorbance of the epoxy groups in theegion 760–780 cm−1 and normalized it to the ester band at670 cm−1. A medium pressure mercury lamp equipped withn optical waveguide was used to induce the photopolymer-zation (light intensity on the surface of the sample of about0 mW/cm2).

The gel content was determined on the cured films by mea-uring the weight loss after 24 h extraction with chloroform atoom temperature, according to the standard test method ASTM2765-84.Dynamic mechanical thermal analyses (DMTA) were per-

ormed with a Rheometric Scientific MKIII (UK) instrument, atfrequency of 1 Hz in the tensile configuration. TGA analysesere performed using a LECO TGA-601 instrument in the rangeetween 30 and 700 ◦C, with a heating temperature of 10 ◦C/minn air.

Impact resistance measurements were performed with anTS FAAR Charpy pendulum, according to ASTM D256ethod; samples were prepared by coating and UV curing the

ifferent formulations onto polystyrene specimens. The PS spec-mens were 0.5 cm thick and 8 cm long, the cured formulationsere about 100 �m thick. The pendulum break the coated sur-

aces.Contact angle measurements were performed with a Kruss

SA10 instrument, equipped with a video camera. Analyses

ere made at room temperature by means of the sessile drop

echnique. Three to five measurements were performed on everyample and the values were averaged. The measuring liquid wasouble distilled water (γ = 72.1 mN/m).

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. Results and discussion

.1. Kinetic investigation

The photopolymerization kinetics were monitored by real-ime FT-IR. The epoxy conversion curves as a function of therradiation time for the pure CE monomer and in the presencef increasing amount of siloxane SIOH additive are reportedn Fig. 1. While the plateau value gives the final epoxy grouponversion, the slope of the curve gives an indication of theolymerization rate.

From the curve reported in Fig. 1, the increase of the epoxyroup conversion by increasing the SIOH content in the pho-ocurable formulation is evident.

These results are explained on the basis of the chain transfereaction involving the SiOH groups: the scheme of the interac-ion of the polysiloxane additive with the epoxy group of theycloaliphatic epoxy resin is reported in Scheme 3 in which thehain transfer mechanism is evidenced.

As a consequence of the chain transfer reaction, the networktructure has greater flexibility and the mobility of the reactive

ig. 1. Real-time FT-IR conversion curves for CE monomer and in the presencef growing amount of SIOH.

M. Sangermano et al. / Progress in Organic Coatings 57 (2006) 44–49 47

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.2. Thermal properties

Thermo-dynamic-mechanical characterization (DMTA) waserformed on UV-cured films; this analysis allows the evaluationf the elastic and viscous components of the modulus (E′ and′′) of the material in a large temperature range. From the E′/E′′

atio, a third parameter named tan δ is obtained, whose maximumorrespond to the Tg of the material. Therefore, this techniqueives a complete characterization of the thermal and viscoelasticroperties of the polymer network.

In Table 2 the Tg values, obtained as the maximum of tan δ

eak from DMTA thermogram, are reported for the samplesnvestigated, together with the gel content after 24 h extractionn chloroform.

A decrease of Tg values by increasing the amount of SIOHdditive in the photocurable formulation is evident. This flexibi-ization effect is a consequence of the chain transfer mechanism

hich gives rise to a decrease of crosslinking density and intro-uce siloxane flexible structure in the network. These data aren agreement with the previously discussed kinetic results.

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able 2hermal properties and gel content of Cured network

ample Tg (◦C, DMTA) Gel content (%)

E 243 100E + 3 wt.% SIOH 235 98E + 5 wt.% SIOH 230 98E + 7 wt.% SIOH 200 99

ig. 2. TGA curves for CE monomer and in the presence of growing amount ofIOH.

The high gel content values (Table 2) confirm that the siloxanedditive interact with the growing chain and is tightly crosslinkedo the polymeric network.

The thermal stability of the cured films was investigated byGA analysis. The TGA curves of pure epoxy resin and in theresence of increasing amount of SIOH are shown in Fig. 2. Thealues of the 10% and 50% weight loss and of the non-volatileart (or char content) are reported in Table 2.

These data clearly indicate that by increasing the amount ofhe siloxane additive in the photocurable formulation the thermaltability of the epoxy network is significantly enhanced. A slightncrease on char content in the presence of SIOH additive is alsovidenced. This thermal stability increase can be attributed tohe insertion of siloxane moieties in the photocured network,s well as to the silica formation during thermal degradation,s demonstrated by the char content increase, that induces anxidation process retardation and an insulation effect [21].

.3. Resilience behavior

The photocurable formulations, containing increasingmount of SIOH in the range between 0 and 7 wt.%, were coatednto polystyrene specimens and cured by UV light. Resilienceehavior was investigated by Charpy measurements and the data

esistance by increasing the amount of SIOH in the photocur-ble resin. This toughness effect can be related to the decreasef Tg values and can be attributed to the plasticization effect

10% weight loss 50% weight loss Char content (%)

337 407 0– – –

494 572 4.0524 580 3.0

48 M. Sangermano et al. / Progress in Org

Table 3Resilience properties PS coated with 100 �m cured films

Sample Resilience (J/cm2)

CE 1.21CE + 3 wt.% SIOH 1.24CC

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nduced by the chain transfer reaction with SIOH, as previouslyiscussed.

This resilience increasing is particularly important for epoxyoatings because they are characterized by good mechanicalroperties, but low impact resistance.

.4. Surface properties

Contact angle measurements with water were performed onV-cured films coated on glass substrate, obtained from pho-

ocurable formulations containing increasing amount of SIOHontent; the data are reported in Fig. 3.

These data show an important increase on hydrophobicitylready at very low siloxane concentration. At the plateau, theettability of the films overcomes 90◦, which is considered the

hreshold value for hydrophobicity.Small pieces of the coatings were detached from the glass

ubstrates with some difficulties, because of their high adhesion,nd water wettability measured.

The side of the coating in contact with glass did not reportny modification in the presence of SIOH, showing an advancingontact angle of 70◦, as the pure CE resin (see Fig. 3).

In this way it is possible to obtain highly hydrophobic surfacesithout affecting the good adhesion properties, typical for epoxy

esins: the modified coatings containing SIOH showed a 100%ross-cut adhesion on polar substrate (glass) as well as the pureE cured epoxy resin.

As is well known that silicon coatings show poor adhesionecause of their very low surface tension, the high adhesion val-es measured confirm a selective surface enrichment of the lower

urface energy additive, SIOH, toward the less polar interfaceair) during the curing process.

The surface modification is permanent: the wettability washecked after many weeks from the film preparation and the

ig. 3. Advancing contact angle measurements with water on cured films.

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anic Coatings 57 (2006) 44–49

ontact angle values were unchanged. This is due to the facthat the siloxane additive is covalently linked to the network ast takes part into the polymerization process through the chainransfer mechanism as shown above.

. Conclusions

A polysiloxane hydroxyl terminated oligomer (SIOH) wassed as chain transfer additive in cationic UV curing of a dicy-loaliphatic epoxy resin (CE).

An increase of the epoxy group conversion was achievedy increasing the SIOH content in the photocurable formu-ations. This result was explained on the basis of the chainransfer reaction involving the Si–OH groups. A networkexibilization was induced by increasing the amount of theolysiloxane additive, with a Tg decrease and a consequentesilience increase. An important increase on thermal stabilitys obtained for the modified systems, because of the pecu-iar properties of silicones. Also a deep surface modifications induced in the presence of even low amount of SIOH1 wt.%). It was demonstrated that the polysiloxane oligomer,aving a low polarity, selectively migrates to the film sur-ace exposed to air; the top layer of the films was thereforeade highly hydrophobic while the opposite side, in contactith glass substrate, kept the same properties of the reference

esin.In conclusion, in the presence of a polysiloxane hydroxyl

erminated additive it is possible to induce a deep modificationf bulk and surface properties of an epoxy network, improvingignificantly the final properties of the coatings.

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