curing and photostabilization of thermoset and photoset acrylate polymers

Macromol. Mater. Eng. 2001, 286, 5–16 5

Curing and Photostabilization of Thermoset and

Photoset Acrylate Polymers

Christian Decker,* 1 Khalid Zahouily,1 Andreas Valet 2

1 Departement de Photochimie Generale (UMR-CNRS N8 7525), Ecole Nationale Superieure de Chimie de Mulhouse– Universite de Haute-Alsace, 3, rue Werner – 68200 Mulhouse, France

2 Ciba Specialty Chemicals Inc, Additives Division, BU Imaging and Coating Additives, CH-4002 Basle, Switzerland

Introduction

Protective coatings are commonly used to improve the

surface properties of a large variety of materials, and their

resistance to weathering in exterior applications as well.

Such coatings must fulfil a number of requirements with

respect to their chemical resistance (organic solvent,

moisture, UV-radiation, oxygen), their mechanical prop-

erties (resistance to abrasion, scratching and impact) and

their optical properties (gloss, transparency). They are

usually made of highly crosslinked polymers which show

superior performance than their linear counterparts, with

respect to their physico-chemical properties and their

exterior durability, because of the reduced mobility of the

reactive species and the tight interbonding of the polymer

chains. For automotive finishes, most of today’s top coats

consist of thermosetting acrylate polymers which contain

light stabilizers to protect the painted basecoat and avoid

photobleaching and delamination upon outdoor expo-

sure.[1, 2]

A promising alternative to these solvent-based coat-

ings, which are typically cured at 1308C for 30 min, has

recently appeared with the development of UV-cured

acrylate clearcoats which, in spite of the presence of the

needed photoinitiator, resist surprisingly well to photode-

gradation in the presence of adequate light stabilizers.[3–7]

By using UV-radiation to initiate the crosslinking poly-

merization of multifunctional monomers, one can trans-

form almost instantly a liquid resin into a solid polymer

material. Therefore, such solvent-free formulations can

be cured at ambient temperature with minimum energy

consumption. If such UV-cured coatings are to be used as

automotive finishes, their mechanical properties, as well

as their exterior durability, need to be at least equal to

that of the thermoset polymers currently utilized as top-

Full Paper: The light-stability of thermosetting acrylateand UV-cured acrylate polymers has been tested in anaccelerated QUV weatherometer. Infrared spectroscopywas used to monitor in situ the chemical changes occur-ring upon both curing and photoageing of 35 lm thickclearcoats. The curing was shown to proceed more exten-sively in photoset than in thermoset acrylate polymerswhich contain 6 and 20% unreacted functional groups,respectively. During photoageing, the loss of the binderstructural groups is accompanied by the production of oxi-dation products (carbonyl and hydroxyl groups). The UV-cured polyurethane-acrylate coating was shown to bemore resistant to accelerated weathering than the mela-mine/acrylate thermoset currently used as automotivefinishes. The addition of a hydroxyphenyltriazine UV-absorber and a hindered amine light stabilizer (HALS)radical scavenger was found to increase substantially thelight stability of acrylate clearcoats.

Macromol. Mater. Eng. 2001, 286, No. 1 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2001 1438-7492/2001/0101–0005$17.50+.50/0

Influence of HALS and UVA stabilizers on the photodegrada-tion of TMA, TUA and PUA coatings exposed to wet cycleQUV-B ageing. HALS: [Tinuvin 123] = 1 wt.-%; UVA: [Tinu-vin 400] = 2 wt.-%.

6 C. Decker, K. Zahouily, A. Valet

coats. The scratch and abrasion resistance of some UV-

cured polyurethane-acrylate clearcoats was already

shown to be as high as that of 2-pack polyurethane/acry-

late thermoset clearcoats.[7] The objective of the present

study was to compare the performance of thermoset and

photoset acrylate polymers, with respect to both the cur-

ing process and the weathering resistance of well-stabi-

lized clearcoats.

Experimental Part

Materials

The solvent-based thermoset resins used in this study con-sisted of a mixture of an acrylate polyol copolymer and acrosslinking agent which was either a methoxylated mela-mine (TMA coatings) or a trifunctional isocyanate (TUAcoatings). The TMA formulation was a mixture of Joncryl510 (from S.C. Johnson and Sons) and Cymel 303 (fromAmerican Cyanamid Corp.). The TUA coatings were madefrom a 2-pack formulation consisting of Macrymal SM 510N (from Hoechst) and Desmodur N75 (from Bayer). Uponheating at 1308C of the liquid formulation the solvent israpidly removed and crosslinking occurs by transetherifica-tion (TMA) or polycondensation (TUA) to yield a hard andinsoluble polymeric material. In dual-cure systems, thecrosslinking agent was an isocyanate functionalized acrylateoligomer (Roskydalm UAVP LS 2337 from Bayer).

The solvent-free UV-curable resin (PUA coating) con-tained 3 components: an aliphatic urethane-acrylate teleche-lic oligomer (Ebecryl 284 from UCB Chemicals), hexanedioldiacrylate (HDDA from UCB Chemicals) as reactive diluent(30 wt.-%), and a photoinitiating system (3 wt.-%) consistingof a 1/3 mixture of a bisacylphosphine oxide (Irgacure 819)and a hydroxyphenyl ketone (Darocur 1173), both from CibaSpecialty Chemicals.

Some experiments have been performed with a dual-curesystem based on urethane-acrylate chemistry. It consisted ofa 2-pack formulation which was prepared just before use bymixing the 2 following components in a 86/14 weight ratio:part A was a mixture of an acrylate polyol and an acrylatefunctionalized urethane, and part B was a disocyanate-mono-acrylate crosslinker.

Two types of light stabilizers were dissolved in the liquidformulations, thus ensuring an homogeneous distributionwithin the solid polymer after curing: a hydroxyphenyl-s-triazine (Tinuvin 400) acting as UV absorber, and an N-alkoxy-substituted piperidine (Tinuvin 123) acting as aHALS-type radical scavenger, both stabilizers from CibaSpecialty Chemicals.

Curing

The liquid formulation was applied by means of a wire-wound bar onto a barium fluoride crystal as a uniform layer,to get after curing a 35 lm thick dry film. The thermosetresins were typically cured by a 30 min cure at 1308C. Thephotoset resin was cured by a 3 s exposure to the UV radia-tion of a medium pressure mercury lamp (IST-80 W/cm), at

ambient temperature. For the dual-cure system, the cross-linked polymer was obtained either by a 10 min cure at 808Cfollowed by a UV-irradiation, or by a 2 s UV exposure fol-lowed by a 10 min cure at 808C. For the 3 systems investi-gated, the cure extent was evaluated by infrared spectro-scopy, monitoring disappearance of the reactive functionsupon heating or UV-irradiation.

Weathering

To evaluate the resistance to photodegradation, the varioussamples were placed in an accelerated QUV weatherometerequipped with UVB-313 or UVA-340 fluorescent lamps. Theacceleration factor is greater with UVA-313 lamps becauseof the emission of UV radiation in the 280–300 nm range,which is lacking in UVA-340 lamps, like in sunlight. For thisreason, the slower but more reliable UVA-340 ageing hasbeen used in most of our accelerated weathering experi-ments. The QUV weatherometer was operated under the fol-lowing wet cycle conditions: 8 h UV irradiation at 708C, fol-lowed by 4 h in the dark at 508C, with water condensation.The thermoset and photoset coatings were exposed for up to3000 h and taken out at regular time intervals to check theextent of the photodegradation.

Analysis

Infrared spectroscopy was used to monitor both the curingreaction and the photoageing process. The crosslinking ofthe thermosetting TMA coating was followed through thedecrease of the IR band at 1072 cm–1 of the methoxy groupundergoing transetherification with the hydroxyl groups ofthe polyol-acrylate. For the 2-pack thermosetting TUA coat-ing, the polycondensation reaction was followed through thedecrease of the isocyanate IR band at 2272 cm–1. The light-induced crosslinking polymerization of the photosettingPUA coating was followed through the decrease of the acry-late double bond which absorbs at 812 cm–1.

The chemical modifications occurring upon acceleratedQUV weathering were monitored quantitatively through thedecrease of the binder structural bands and the buildup of theoxidation products, mainly carbonyl groups and hydroxylgroups. The following IR bands were found to be the mostaffected by the photodegradation process:

– the ether band of TMA coatings at 1072 cm–1

– the carbamate band of TUA and PUA coatings at 1521cm–1

– the CH band of the 3 polymers in the 2930–2960 cm–1

region

The relative amount of remaining functional groups wasdetermined by means of the ratio of the IR absorbance at thecorresponding wave number after a given QUV exposure tothe IR absorbance of the unexposed sample.

The cure extent was also followed through pendulum hard-ness measurements (Persoz), which are correlated with theelastic modulus of the crosslinked polymer. The Persozmethod consists of monitoring the damping time of the oscil-lations of a pendulum placed onto the polymer sample coated

Curing and Photostabilization of Thermoset and Photoset Acrylate Polymers 7

on a glass plate. The physical significance of pendulum hard-ness has been reviewed by Sato.[8] According to the Voigtmodel of retarded elasticity, the theory predicts that the pen-dulum hardness value is proportional to the reciprocal of thedamping capacity or mechanical loss. Therefore, the validityof this method is restricted to homologous materials, but itmay be reasonably used for measuring the viscoelastic prop-erties of coating films.[8] Persoz values typically rangebetween 50 s for soft elastomeric materials and 350 s forhard and glassy polymers, to reach a maximum value of 400s for mineral glass. Hardness is mainly a function of the elas-tic modulus and its dependence on the glass transition tem-perature was recently studied by Schwalm et al. for UV-cured acrylate coatings.[9]

Curing of Acrylate Coatings

The main difference between the thermoset and photoset

acrylate polymers examined in this study lies not only in

the type of energy used to promote the crosslinking pro-

cess (heat or radiation) but also in the type of reaction

used to generate the crosslinks between the polymer

chains:

– a transetherification or a condensation reaction

between the hydroxyl groups of a polyol-acrylate and

the functional group of a crosslinker for thermoset

coatings

– a radical-induced polymerization of a difunctional

acrylate monomer or oligomer for the photoset coating

As a result, the crosslink density of the network is sub-

stantially higher in photosets, where the branching point

concentration reaches a value of 5 mol N kg–1, than in ther-

mosets where this concentration reaches typically a value

of 1.5 mol N kg–1. For both types of polymers we have

studied the cure kinetics upon heating or UV-irradiation.

Thermal Curing of TMA and TUA Coatings

The type of polymer network formed upon heating of a

polyol-acrylate in the presence of a methoxylated mela-

mine crosslinker is shown in Scheme 1. The cure extent

can be evaluated quantitatively from the decrease of the

melamine methoxy group which is easily monitored by

infrared spectroscopy. Figure 1 shows how the methoxy

band of TMA disappears with cure time. It should be noted

that the IR band at 1072 cm–1 does not disappear comple-

tely upon heating, because of the concomitant formation

of the ether crosslink between the polyacrylate chain and

the melamine crosslinker (Scheme 1). We can still evalu-

ate fairly well the cure extent by considering that the trans-

etherification is completed after a 3 h cure at 1308C, at

which time the IR band at 1072 cm–1 is leveling off.

A similar polymer network is obtained by using a tri-iso-

cyanate crosslinker, as shown in Scheme 2. Here, the con-

densation reaction can be easily followed through the dis-

appearance of the distinct and intense IR band of the iso-

cyanate group at 2272 cm–1 (Figure 1). Upon heating at

1308C, the crosslinking reaction proceeds significantly

faster in urethane/acrylate than in melamine/acrylate ther-

mosets, as shown in Figure 2. The condensation reaction

can be conducted already at 808C, and it even proceeds

slowly at ambient temperature. The pot lifetime of the for-

mulation is only of a few hours, which explains why the

urethane/acrylate thermoset is made of a 2-pack system.

Both TMA and TUA coatings were found to reach their

maximum hardness, a Persoz value of 310 s, after a

15 min bake, when about half of the functional groups

have reacted (Figure 3). Additional crosslinks formation

has apparently no effect on the physical properties of the

cured polymer. It should be noted that the hardening of

the TMA coating did occur somewhat faster than that of

the TUA coating, even though it is slightly less reactive.

Scheme 1. Polymer network formed upon thermal curing of amelamine/acrylate coating.

Scheme 2. Polymer network formed upon thermal curing of a2-pack urethane/acrylate coating.


The molecular mobility restrictions resulting from the

hardening are mainly responsible for the cure slowing

down observed upon further heating. After 30 min, the

usual cure time for thermoset clearcoats, the two cross-

linked polymers still contain about 20% unreacted isocya-

nate or methoxy groups. To get completely cured poly-

mers, the thermal treatment at 1308C had to be extended

for 2 h, a too long cure time for most industrial applica-

tions. Thermoset acrylate clearcoats currently used, in

particular as automotive finishes, therefore contain a sig-

nificant amount of unreacted functional groups of the

crosslinker.

Figure 1. Infrared spectra TMA and TUA coatings before (1) and after thermal treatment at 1308C for 30 min (2)and 180 min (3).


Photocuring of PUA Coatings

The three-dimensional polymer network formed by

photopolymerization of difunctional monomers and oli-

gomers has a quite different structure than the thermoset

polymers, as shown in Scheme 3. The polymer chain,

generated by polymerization of the acrylate double bond,

is branched every two carbon atoms, so that the crosslink

density (5 mol N kg–1) is about 3 times as large as in ther-

moset polymers.

The photoinitiated crosslinking polymerization of the

PUA urethane coating proceeds extensively within sec-

onds upon intense illumination at ambient temperature.

Figure 4 shows the polymerization profile obtained by

monitoring the disappearance of the acrylate double

bond. In UV-cured polymers, the final conversion

depends on a number of factors, such as the rigidity of the

oligomer chain, the crosslink density, the Tg of the cured

polymer and the temperature reached by the sample. With

the PUA coating used here, as many as 96% of the acry-

late double bonds had reacted within a 3 s UV exposure.

As expected, the addition of a UV absorber (UVA)

([Tinuvin 400] = 2 wt.-%) was found to slow down the

polymerization rate (Figure 4). Indeed, this compound

reduces the rate of initiation by interfering with the

photoinitiator for the scavenging of the incident photons,

as shown in Scheme 4. Among the various UVAs and

photoinitiators tested, the less pronounced inner radiation

filter effect was obtained with a combination of a bisacyl-

phosphine oxide photoinitiator and a hydroxyphenyltria-

zine UV-absorber. It was thus possible to keep the resi-

dual unsaturation content to a low level (L 6%), as shown

by the IR spectra represented in Figure 5. The unreacted

acrylate double bonds present in the UV-cured polymer

were actually found to disappear rapidly upon accelerated

QUV weathering, due to the enhanced photopolymeriza-

Figure 2. Loss of methoxy and isocyanate groups upon curingof TMA and TUA thermoset acrylate coatings.

Figure 3. Dependence of the hardness of thermoset acrylatecoatings on the amount of functional groups consumed.

Scheme 3. Polymer network formed upon UV-curing of anurethane/acrylate telechelic oligomer.

Figure 4. Influence of the UV-absorber ([Tinuvin 400] =2 wt.-%) on the polymerization kinetics of a UV-curableurethane-acrylate coating.


tion at higher temperature (708C). The same trend was

observed for the residual photoinitiator which was essen-

tially gone after a 30 min QUV exposure (Figure 6). This

is an important result with respect to the weathering resis-

tance of UV-cured coatings which would be affected by

the presence of such a photosensitive compound.

Dual-Cure of TUA-PUA Coatings

For automotive applications of these acrylate clearcoats,

dual-cure systems combining thermal and photochemical

induced crosslinking have been recently developed to

address the issue of curing in shadow areas on 3D sub-

strates (10). The basic idea was to use, as crosslinking

agent, a UV-curable urethane-acrylate oligomer bearing

isocyanate groups. This compound was combined with a

polyol-polyacrylate in such proportions to have an NCO/

OH ratio of 1. Because the polycondensation reaction

proceeds slowly already at ambient temperature, this type

of dual-cure system will be a 2-pack formulation. A 1-

pack formulation could be obtained by using a partially

acrylated methoxylated melamine as crosslinker. Scheme

5 shows a schematic representation of the polymer net-

work formed after dual curing of a TUA-PUA coating.

The 2-step curing process can be started either by the

UV-irradiation or by the thermal treatment. In the first

case, the solvent had to be removed by a short heating,

followed by the UV irradiation and then by the 30 min

cure at 1308C. The polymerization of the acrylate double

bonds was essentially completed after a 2 s UV exposure,

while the thermal crosslinking was proceeding like in the

TUA coating (80% conversion of the isocyanate). Similar

results were obtained by starting with the thermal treat-

ment and performing the UV-irradiation on the hot sam-

ple. By letting the thermally cured sample cool off before

UV exposure, the acrylate polymerization proceeded less

extensively and leveled off at 80% conversion.

Scheme 4. Competition between the photoinitiator and the UV absorber forthe incident photons, in UV-curing of a photostabilized acrylate resin.

Figure 5. Infrared spectra of a stabilized PUA coating, beforeand after UV-irradiation for 4 s.

Figure 6. UV absorption of a stabilized PUA coating before (1)and after UV-curing (2), followed by 30 min QUV exposure (3).


In some particular applications (car repairing), the cure

temperature should not exceed 808C while the cure time

should be reduced to 10 min. It can be seen from the IR

spectra shown in Figure 7 that, under those conditions,

the condensation reaction is developing less extensively,

the isocyanate conversion reaching hardly a value of

25%, while the UV-curing reaction still proceeds effi-

ciently, as expected. Figures 8 and 9 show some typical

kinetic profiles of the 2 types of functional groups (iso-

cyanate and acrylate double bonds) for a TUA-PUA coat-

ing undergoing dual curing, by starting either with the

thermal treatment (Figure 8) or with the UV-irradiation

(Figure 9). During UV-curing, the temperature of the

sample can rise up to 808C, depending of the film thick-

ness and the type of support, because of the heat rapidly

evolved by the exothermal polymerization. During this

short period of time, thermal curing of the isocyanate

groups occurs to a small extent, as shown in Figure 9.

The glass temperature of the sample, which is initially on

the order of –208C, rises up to 608C after cure.

In both cases, the cured polymer contains about 75% of

the original amount of isocyanate functions. These groups

were found to disappear slowly upon storage in air, but

significantly faster in a 100% humid atmosphere, as

shown in Figure 10. After 5d, the polymer contained only

15% residual isocyanate groups. This process is due to

the hydrolysis of the NCO functions into amines, which

further react with isocyanates to yield urea-type cross-

links,[10] according to Scheme 6:

It should be noted that, while the hydroxyl IR band was

decreasing during the thermal treatment due to the

reaction with NCO groups, it remained essentially

unchanged during the moisture curing, as expected. This

reaction is beneficial as it leads ultimately to a polymeric

material which contains no unreacted functionalities any-

more.

Scheme 5. Schematic representation of a polymer network formed bydual-curing of a TUA-PUA formulation made of a polyol-polyacrylate andan isocyanate functionalized urethane-acrylate telechelic oligomer.

Figure 7. Infrared spectra of a dual-cure TUA-PUA coating, before andafter curing (10 min cure at 80 8C followed by a 4 s UV-irradiation).


Compared to thermally cured polyurethane-acrylate

coatings, dual-cure systems show additional benefits with

respect to their scratch resistance and their chemical

resistance.[11] As in the case of the currently used clear-

coat/basecoat systems, however, the combination of a

basecoat and a UV-curable clearcoat must be specifically

designed in order to achieve the correct overall mechani-

cal properties required for the given application.

Photostabilization of Acrylate Coatings

To ensure a long-lasting protection of a polymeric mate-

rial used in exterior applications, the coating applied to

its surface must not only be very resistant to photodegra-

dation, but it must also prevent efficiently the penetration

of the harmful UV radiation of sunlight into the substrate.

The UV absorber introduced in the coating formulation

for this purpose has therefore to be quite photostable and

have the longest possible lifetime in outdoor exposure.

The light-stability depends not only on the UVA chemical

structure[12] but also on the type of coating used.[13]

Photodegradation of Acrylate Clearcoats

FTIR spectroscopy was used to monitor the photodegra-

dation of thermoset and photoset acrylate polymers,

because this technique gives quantitative data on the

chemical modifications which have occurred at an early

stage of the photodegradation process, well before physi-

cal and optical changes can be observed.[3, 13] Already

Figure 8. Kinetic analysis of the dual-curing of a TUA-PUAcoating (10 min cure at 80 8C followed by a 4 s UV-irradiation).- - -: isocyanate thermal curing at 80 8C for 60 min.

Figure 9. Kinetic analysis of the dual-curing of a TUA-PUAcoating (UV-irradiation for 4 s, followed by a 10 min cure at80 8C).

Figure 10. Influence of humidity on the isocyanate consump-tion upon storage of a dual-cured TUA-PUA coating.

Scheme 6. Moisture curing of isocyanate groups.


after 200 h QUV exposure distinct changes in the infrared

spectrum can be observed, in particular a decrease of the

structural bands of the various coatings, namely:

– the ether band at 1072 cm–1 of TMA

– the urethane C1N band at 1521 cm–1 of TUA and

PUA

– the C1H band at 2930 cm–1 of TMA, TUA and PUA.

Figure 11 shows some typical IR spectra of these bands

before and after QUV exposure. The ether crosslink of the

melamine/acrylate thermoset appears to be more sensitive

to photodegradation than the carbamate bond in the

urethane/acrylate thermoset and photoset. The decrease

of these functional groups upon wet cycle QUV-A ageing

of the 3 unstabilized coatings is shown in Figure 12. At

the same time, oxidation products are being formed as

revealed by the increase of the IR absorbance in the

3000–3500 cm–1 region (hydroperoxides and alcohols)

and in the 1650–1750 cm–1 region (carbonyl com-

pounds).

The photodegradation of the polymer film is accompa-

nied by a loss of the gloss, which yet becomes significant

only after 500 h, once the polymer has already suffered

substantial chemical modifications. Infrared spectroscopy

proved therefore to be a better analytical tool to make an

early evaluation of the weathering resistance of a given

polymer.

Photostabilization of Acrylate Clearcoats

The addition of a UV-absorber and a HALS radical sca-

venger has a pronounced stabilizing effect on the photo-

degradation of the 3 types of coatings studied. A concen-

tration of 1 wt.-% of HALS (Tinuvin 123) proved already

sufficient to get an optimum effect, while for the UV

absorber, the radiation filtering effect is increasing expo-

nentially with the UVA concentration according to the

Lambert-Beer law:

It = I0 exp(–2.3 e N l N c)

where It is the light transmitted by a sample of thickness l

exposed to an incident light I0 and containing a UV absor-

ber at a concentration c, e being the UVA extinction coef-

ficient. At a Tinuvin 400 concentration of 2 wt.-%, 99%

of the a350 nm UV radiation are absorbed by a 40 lm

thick coating.

The stabilizing effect of Tinuvin 123 and Tinuvin 400

on the photodegradation of TMA, TUA and PUA coatings

is illustrated in Figure 13. It can be seen that a combination

of these additives leads to a 10-fold increase in the light

stability of these polymers. For an easy performance ana-

lysis, we have represented in Figure 14 the differential IR

Figure 11. Infrared spectra of unstabilized thermoset and photoset acrylate coatings, before and afterwet cycle QUV-A ageing.

Figure 12. Loss of the TMA ether groups and the TUA andPUA urethane groups upon wet cycle QUV-B ageing of unstabi-lized coatings.


spectra obtained for the well-stabilized TMA and PUA

clearcoats, by subtracting the IR spectrum of the original

sample from the IR spectrum of the sample exposed to

QUV-B ageing for 1400 h and 2100 h, respectively. In this

representation, the positive values of the IR absorbance

correspond to the buildup of the oxidation products, while

the negative values correspond to the loss of the binder

functional groups. It clearly appears that the urethane acry-

late photoset has suffered less chemical modification upon

QUV ageing than the melamine/acrylate thermoset cur-

rently used as automotive topcoat. This is also true for the

urethane/acrylate thermoset which shows a weathering

resistance similar to that of the PUA coating[15] and which

is generally considered as the state of the art for automo-

tive finishes. It is yet not widely used in today’s applica-

tions because of the inconvenience of a 2-pack formulation

and its higher cost. This is not the case for the equally per-

forming urethane-acrylate photoset which, in addition, has

the advantage of being a solvent-free formulation readily

curable at ambient temperature.

To compare the efficiency of the HALS and UVA light

stabilizers in the three coatings, we have represented in

Figure 15 the relative loss of CH groups after 1400 h wet

cycle QUV-B ageing. The superior performance of the

UVA+HALS combination in urethane-acrylate polymers

is clearly apparent. Similar results were obtained with the

dual-cure urethane acrylate system which was yet found

to be slightly less resistant to weathering than TUA and

PUA coatings. This could be due to the fact that the ther-

mal curing was performed at 808C during only 10 min.

The urea and amine groups formed subsequently by

moisture curing of the 75% unreacted isocyanate groups

might be less resistant to photodegradation than the car-

bamate groups of TUA and PUA polymers.

With respect to the optical properties, the UV-cured

coating showed similar performance upon accelerated

weathering as the thermoset TUA and TMA coatings. The

well stabilized PUA clearcoat remained non-colored,

transparent and glossy for up to 3000 h of wet cycle QUV-

A ageing.[5] The outstanding weathering resistance of UV-

cured urethane-acrylate coatings has been confirmed by

outdoor exposure tests performed in Florida.[7] Based on

gloss retention data, their light fastness was found to be

superior to that of TMA automotive topcoats, and quite

comparable to that of 2-pack TUA thermosets stabilized

Figure 13. Influence of HALS and UVA stabilizers on thephotodegradation of TMA, TUA and PUA coatings exposed towet cycle QUV-B ageing. HALS: [Tinuvin 123] = 1 wt.-%;UVA: [Tinuvin 400] = 2 wt.-%.

Figure 14. IR differential spectra of stabilized thermoset(TMA) and photoset acrylate (PUA) clearcoats before and afterQUV-B ageing.

Figure 15. Influence of HALS and UVA stabilizers on thephotodegradation of TMA, TUA and PUA coatings exposed towet cycle QUV-A ageing. HALS: [Tinuvin 123] = 1 wt.-%;UVA: [Tinuvin 400] = 2 wt.-%.


with the same UVA+HALS combination. Figure 16 shows

some typical gloss retention profiles for a stabilized PUA

clearcoat exposed to natural weathering in Florida. It can

be seen that, for both types of basecoats used (dark blue

metallic and red solid shade), the 208 gloss value has

decreased by only 20% after a 7 year exposure.

When thermoset or photoset coatings are used to

improve the weathering resistance of polymeric materi-

als, a crucial factor is the permanence of the UV absorber

during outdoor exposure. Indeed, the UVA has to stay as

long as possible to fulfil its role as a sunlight screen. The

UVA lifetime can be easily evaluated by monitoring its

slow disappearance upon photoageing, by means of UV

spectroscopy.[3] Its value was found to be directly related

to the binder weathering resistance. The longest lifetime

was measured in a fluorinated thermoset acrylate polymer

(Lumiflon) which is known for its outstanding light stabi-

lity. The chemical structure of the UV absorber has also a

strong effect on the UVA permanence, the longest life-

times being observed with hydroxybenzotriazoles and

hydroxyphenyl-s-triazines.

In photoset acrylate coatings, a substantial increase in

the UVA lifetime was obtained by the addition of a

HALS radical scavenger.[12] This light stabilizer is there-

fore essential because it not only increases the weathering

resistance of the coating, but it also helps the UV absor-

ber stay longer in the protective film, thus ensuring a

long-lasting radiation filtering effect. The efficiency of

such stabilized photoset urethane-acrylate clearcoats to

protect organic materials against photodegradation has

been tested on painted metallic panels exposed to QUV-A

ageing for up to 5000 h. Their weathering resistance was

improved 10-fold by means of a 50 lm thick UV-cured

PUA topcoat which remained clear and glossy and did

not delaminate.[5]

It should be emphasized that the stabilization efficiency

of a UV absorber is much more pronounced when this

additive is introduced in a topcoat, rather than in the bulk

of the polymer material to be stabilized. This follows

from the classical light absorbance equations where AP

and AS designate the absorbance of the polymer and of

the stabilizer, respectively.

Intensity of the light absorbed by the polymer in:

– the neat polymer: IP = I0 [1 – exp(–2.3 AP)]

– the UVA stabilized polymer: IPS = I0 [1 – exp(–2.3

(AP+AS))] N (AP/AS)

–- the UVA coated polymer: IPC = I0 [1 – exp(–2.3

AP)] N exp(–2.3 AS)

The stabilization efficiencies of the two systems can

then be calculated from the following ratios:

Sbulk ¼IP

IPS

¼ ½1ÿ expðÿ2:3APÞ�AS

½1ÿ expðÿ2:3ðAP ÿ ASÞÞ�AP

Scoated = IP/IPC = exp(2.3 AS)

An illustration of the superior radiation filtering effect

achieved by using a coating can be found by considering

a typical system where the polymer is 90% transparent to

UV light (AP = 0.046) and the UVA absorbs 99% of the

incident light (AS = 2). From the relative amount of

photons absorbed by the polymer, the stabilization effi-

ciency was evaluated for the bulk polymer and the coated

polymer:

IP/I0 = 10% Sbulk = 4.3IPS/I0 = 2.2%

)gggs

IPC/I0 = 0.1% Scoated = 100

The same amount of UV absorber appears to be 23

times as effective in screening the UV radiation when it is

introduced in a topcoat, rather than in the bulk of the

polymer. This result explains the increasing use of protec-

tive coatings to improve the weathering resistance of

organic materials.

When the UVA concentration has been cut by half

upon photoageing (AS = 1), the S values are dropping to

Sbulk = 2.4 and Scoated = 10, so that the superiority of the

coating process greatly diminished (a factor of 4 only).

These data show how crucial it is to use a UV absorber

having a long lifetime.

A performance analysis of the thermoset and photoset

acrylate polymers, based on the curing and weathering

data obtained in this study, is given in Table 1. The main

advantage of the UV-technology, besides the much faster

and more complete cure, is that the coating is obtained

from a 1-pack solvent-free formulation cured at ambient

temperature.

Figure 16. Gloss retention of a stabilized UV-cured clearcoatafter Florida exposure. HALS: [Tinuvin 292] = 1 wt.-%; UVA:[Tinuvin 400] = 1.5 wt.-%. Film thickness: 40 lm.


Conclusion

An effective method to improve the weathering resistance

of polymeric material is simply by protecting their sur-

face with a highly crosslinked polymer film containing

adequate light-stabilizers. These coatings can be cured by

either a thermal treatment or by UV-irradiation. The latter

process is particularly attractive because the photopoly-

merization occurs within seconds at ambient temperature

to yield a tightly crosslinked polymer network. So-called

“dual-cure” coatings might be an alternative for sub-

strates where curing in shaded areas might be an issue.

The light-fastness of UV-cured polyurethane-acrylate

coatings was shown to be superior to that of the mela-

mine/acrylate thermosets currently used as automotive

finishes, and similar to that of 2-pack urethane/acrylate

thermosets. The addition of a hydroxyphenyl-s-triazine

UV absorber and a HALS radical scavenger increases

substantially the weathering resistance of both types of

coatings. The stabilized urethane-based clearcoats

remained essentially unchanged after 2000 h wet cycle

QUV-A ageing and proved very effective to enhance the

weathering resistance of painted metals. This technology

can be applied to a large variety of substrates used in

exterior applications and requiring therefore an outstand-

ing and long-lasting outdoor durability.

Acknowledgement: This work was supported by a researchgrant from Ciba Specialty Chemicals Additives Division (Basle,Switzerland).

Received: July 24, 2000Revised: September 25, 2000

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Table 1. Performance analysis of thermoset and photoset acry-late clearcoats stabilized with UVA (2 wt.-%) + HALS (1 wt.-5).

Acrylate clearcoat QUV-A Thermoset Photoset——

hTMA TUA PUA

Cure time 30 min at 130 8C 2 sResidual functions/% 20 20 6Crosslink density/(mol N kg–1) 1.5 1.5 5Persoz hardness/s 310 300 270Loss of CH group/% 1 400 20 4 3Loss of ether or urethane/% 1 400 48 8 10Gloss retention/% 3 000 70 95 92

curing and photostabilization of thermoset and photoset acrylate polymers

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