a new class of highly reactive acrylic monomers, 2. light-induced copolymerization with difunctional...

Makromol. Chem. 192, 507-522 (1991) 507

A new class of highly reactive acrylic monomers, 2 a )

Light-induced copolymerization with difunctional oligomers

Christian Decker: Khalil Moussa

Laboratoire de Photochimie GCnerale (CNRS), Ecole Nationale SupCrieure de Chimie, 3, rue A. Werner, 68200 Mulhouse, France

(Date of receipt: May 7, 1990)

SUMMARY: Acrylic monomers containing a cyclic carbonate group in their structural unit have been shown

to copolymerize very rapidly and extensively with urethane and phenoxy oligomers having two acrylic functional end groups. With the most reactive resin, 90% conversion was reached within 20 ms of UV exposure in the presence of air. The kinetics of these ultrafast polymerizations was studied by infrared spectroscopy and their actual profiles were recorded in real time by RTIR spectroscopy. The rate of polymerization and the amount of residual unsaturation were determined for the three monomers studied, and compared to the performance of the mono-, di- and triacrylates commonly used as reactive diluents in UV-curable systems. Quantum yield measurements indicate a very efficient propagation reaction, with kinetic chain lengths in the order of lo4 mol per initiating radical, even in the presence of air. The crosslinked polymers obtained with these new monomers exhibit both a great hardness and a high flexibility, together with a good resistance to organic solvents, chemicals and UV radiation, which makes them well suited for applications as protective coatings.

Introduction

In a recent paper I ) , we have described the outstanding performance of a new class of acrylic monomers that contain a cyclic carbonate function in their structural unit. Upon UV irradiation, these monoacrylates were shown to polymerize within milliseconds, much more rapidly and extensively than the usual acrylic monomers, and even faster than triacrylates, known for their high reac t i~ i ty~ .~) . These new monomers also impart hardness and flexibility to the UV-cured polymer film, which was found to be strictly insoluble in organic solvents I ) , thus indicating that an efficient crosslinking process had occurred.

Mono- and multifunctional acrylates are now commonly used as reactive diluents in a large number of photocurable systems, such as fast-drying varnishes4, '1, printing inks 6 ) , adhesives '1, and as photoresists for microlithography ') and graphic arts6) applications. Besides reducing the resin viscosity, the role of the monomer is to increase both the rate of polymerization and the maximum cure extent, since the unreacted monomer and oligomer are known to adversely influence the long-term behavior of UV-cured polymerss). In addition, some of the final product properties, like the hardness, flexibility, heat and chemical resistance, will also be affected by the structure and functionality of the monomer employed 93 lo).

a) Part I : cf. I ) .

0 1991, Hiithig & Wepf Verlag, Base1 CCC 0025-1 16)</91/$03.00

508 C. Decker, K. Moussa

The purpose of this study was to determine whether the remarkable performance, already observed for the light-induced homopolymerization of cyclic carbonate monoacrylates, still remains valid when these monomers are used as reactive diluents in polyurethane-acrylate- or epoxy-acrylate-based UV-curable systems. Both the kinetics of such ultrafast photopolymerizations and the properties of the polymer film were investigated and compared with the results obtained previously with the usual mono-, di- or triacrylic diluents ' ' 9 12).

Experimental part

Materials

The photopolymerizable resin consisted of 3 main components: (i) a photoinitiator that cleaves readily upon UV exposure; (ii) an acrylate end-capped oligomer that will constitute the frame of the three-dimensional polymer network; (iii) a reactive diluent that participates in the polymerization and will thus be incorporated into the final material.

Photoinitiator: For most experiments, qa'-dimethoxydeoxybenzoin (Irgacure 65 I from Ciba- Geigy) was used as photoinitiator, owing to its high initiation efficiency 13, 14) . Since this compound was yet shown to yield slightly colored photoproducts, the polymer discoloration upon accelerated weathering was followed on film cured with 1-hydroxycyclohexyl benzoate (Irgacure 184 from Ciba Geigy), a photoinitiator known for its clear photoproducts 15).

Oligomer: Two types of difunctional oligomers were used in this study: (i) an aliphatic polyurethane-diacrylate (Actilane 20, trade name SNPE) that leads to soft and elastomeric coatings; (ii) a diacrylate derivative from the diglycidyl ether of bisphenol Aa) (Actilane 72, trade name SNPE), incorrectly called epoxy-acrylate, which yields hard and glassy polymer materials. Both of these oligomers are highly viscous at room temperature and need to be strongly diluted in order to make the viscosity decrease down into the 100 mPa . s range, which is required for pratical use in the coating industry.

Monomer: Three carbonate-acrylate monomers, recently synthetized by the SocietC Nationale des Poudres et Explosifs (SNPE) ' 6 ) , were used as reactive diluents: 2-0x0- 1,3-dioxolan-4-ylmethyl acrylate (l), 2-acryloyloxyethyl 2-0x0-I ,3-dioxolan-4-ylmethyl carbonate (2), and 2-0x0- -1,3-dioxolan-4-ylmethyl 3-acryloyloxypropionate (3).

For performance comparison, some acrylic monomers commonly used in UV-curable systems were tested under identical conditions: ethyl diethylene glycol acrylate b, (EDGA from Norsolor), tripropylene glycol diacrylate ') (TPGDA from Norsolor), hexanediol diacrylate d, (HDDA from Norsolor) and trimethylolpropane triacrylatee) (TMFTA from Norsolor).

Irradiation

Typical formulations contained 5 wt.-Vo of photoinitiator and equal parts of the acrylic oligomer and monomer. The resin was applied on an NaCl salt disc as a uniform layer of 25 pm thickness by means of a calibrated wire-wound applicator. Samples were exposed, in the presence of air, to the radiation of a 2 kW medium pressure mercury lamp, equipped with a semi-elliptical

Systematic name: 2,2-Bis[4-(2,3-epoxypropoxy)phenyl]propane.

Systematic name: 1,4,7-Trimethyl-3,6-dioxaoctamethylene diacrylate. b, Systematic name: 3,6-Dioxaoctyl acrylate.

dl Systematic name: Hexamethylene diacrylate. e , Systematic name: 2,2-Dimethylenebutyl triacrylate.

A new class of highly reactive acrylic monomers, 2 509

reflector, which had a power output comparable to that of the light sources used in industrial UV- curing lines (80 W per linear centimeter). The UV irradiance at the sample position was measured by actinometry and found to be 1,5 . 10 -6 einstein . s - ' or 500 mW . cm -2. A camera shutter was used to select precise exposure times in the range of 2 to 100 ms. For properties evaluation, the formulation was coated onto a glass plate and passed under the UV lamp at a belt speed of 10 m/min. The unsaturation content, the hardness and the light-fastness of the tack-free coating were determined.

Analysis

The extent of the polymerization reaction, after a given exposure, was evaluated quantitatively by IR spectroscopy, by monitoring the decrease of the sharp peak centered at 812 cm-' (twisting vibration of the acrylic CH2=CH bond). The rate of polymerization (R,) was determined from the maximum slope of the conversion versus irradiation time curve, which, in the presence of air, is usually reached when 25% of the double bonds have polymerized 1 7 ) . It should be emphasized that the R , value thus determined is not the actual polymerization rate, since it includes the dark polymerization which continues to develop during time lapse of a few seconds between the end of the exposure and the measure of the IR absorbance at 812 cm-'. This postcure was shown") to represent a substantial part (up to 80%) of the overall process, even for experiments carried out in the presence of air. The actual polymerization profiles have been recorded in situ by using the newly developed real-time infrared (RTIR) photospectroscopy '7). This very sensitive technique permits to determine in a single experiment, which lasts for a fraction of a second, the important kinetic parameters ( R , , quantum yield, kinetic chain length). IR spectroscopy was also used to evaluate the precise amount of unreacted acrylate groups in the tack-free UV-cured coating.

The hardness of the cured film was evaluated by monitoring the damping time of the oscillations of a pendulum (Persoz hardness), which is directly related to the softness of the sample. For a 50 pm thick UV-cured film, coated onto a glass plate, Persoz values typically range from 50 s for elastomeric materials to over 300 s for hard and glassy polymers.

The film flexibility was evaluated by bending UV-cured coatings 180" around mandrels of decreasing diameters and measuring the diameter at which cracks first appear. For a 3 mm diameter, the film undergoes an elongation of 30%. Highly flexible coatings were subjected to the most severe zero-T-bend test, where the coated substrate was bent onto itself, with no spacer between the two halves. Samples passing this test without film cracking or crazing were marked 0 in the flexibility scale.

The light-fastness of the UV-cured copolymers containing monomers 1, 2 or 3 was tested in a QUV-accelerated weatherometer equipped with UV-B-313 lamps and operated at 40 "C, 100% humidity and continuous illumination. At the sample position, the incident UV light intensity was measured to be 1,7 mW . cm-2. The extent of the degradation was evaluated from the variation of the yellow index and from the light-induced chemical modifications, which were monitored by IR spectroscopy.

Kinetic study of the photopolymerization

1. Polyurethane-acrylates

The photocuring of polyurethane-acrylate-based resins has already been thoroughly investigated 11, 12* owing to the various applications of these fast-drying systems, mainly in the coating industry for the protection of flexible materials. The functionality of the monomer used as reactive diluent was shown to have a drastic effect on the rate of polymerization and on the maximum conversion"). This is clearly illustrated by


.c I HDDA

L 4 A

60 Irradiation t ime in rns

Fig. 1. Light-induced copolymerization of a polyurethane-diacrylate (Actilane 20) with various acrylic monomers (for abbreviations see Exptl. part), in the presence of air. Light intensity: 500 mW . cm-2; film thickness: 25 pm; photoinitiator: 5 wt.-Yo of Irgacure 651

Fig. 1, which shows the conversion versus time curve obtained upon UV irradiation of a photosensitive resin made of equal amounts of a polyurethane-diacrylate oligomer and of a mono (EDGA), di (HDDA), or tri (TMPTA) acrylic monomer. The copolymerization kinetics follows a typical S-shape profile because of two major factors: (i) the initially low rates are due to the well-known inhibition effect of 0221); (ii) the rate slowing down observed above 30% conversion results from both the monomer consumption and the gelification which leads to the trapping of the reactive radicals2').

An increase in the monomer functionality leads to a much faster copolymerization, but the reaction stops earlier because of mobility restrictions of the reactive sites in the highly crosslinked polymer formed, so that the maximum conversion levels off at about 60% for the triacrylate TMPTA. The tack-free UV-cured film thus contains a certain amount of residual acrylic groups, depending on the type of diluent used. The unsaturation content varies from 2% of the original amount of acrylates, when the monofunctional EDGA was used as diluent, to 36% for the trifunctional TMPTA (Tab. 1). Compared to the light-induced homopolymerization of the same monomers '), the

7 -7 a -

z 1 A

- / - .- . * H3DA

, /

/ /

/

, I

30 10 Irradiation time in rns

Fig. 2. Light-induced copolymerization of a polyurethane-diacrylate (Actilane 20) with acrylic monomers containing a cyclic carbonate structure, in the presence of air. Light intensity: 500 mW. cm-'; film thickness: 25 pm; photoinitiator: 5 wt.-Yo of Irgacure 651

Tab.

1.

Phot

opol

ymer

izat

ion

of p

olyu

reth

ane-

acry

late

resi

ns in

the

pres

ence

of

air.

Act

ilane

20: 47,5 w

t.-%

; m

onom

er: 47,5 w

t.4'0;

Irg

acur

e 651: 5

wt.-

%.

Ligh

t int

ensi

ty: 500 m

W . c

m-*

. Fi

lm th

ickn

ess:

25

pm

d)

Rp/

['41O

R

ub)

h ')

Flex

ibili

ty

RP

a)

mol

. kg

. s - '

in '7

0 in

s

in m

m

RP

-

Mon

omer

[A

cryl

ate]

,

mol

. kg-

' m

ol.k

g-'.

sC'

5-1

Mon

oacr

ylat

e:

EDG

A

32

30

1 36

450

2 2,6

430

3 2,s

370

4 3,1

40

Dia

cryl

ate:

H

DD

A

5,o

100

TPG

DA

3,9

100

Act

ilane

20

1,4

40

Tria

cry la

te:

TMF'

TA

595

600

Ace

tate

: 5

0,7

120

a)

R, : m

axim

um r

ate

of p

olym

eriz

atio

n.

b,

RU

: re

sidu

al u

nsat

urat

ion

in th

e ta

ck-f

ree

film

. ')

Pers

oz h

ardn

ess

of t

he ta

ck-f

ree

film

. d,

R

ate

of t

he li

ght-i

nduc

ed h

omop

olym

eriz

atio

n (s

. ref

. I)).

10

2

30

0

5 125

4

260

0 220

165

6

100

0 750

130

5 190

0 150

13

5 30

0 7

20

16

130

2

25

28

10

120

1 30

40

28

10

-

__

110

36

270

5 100

-

-

170

2

W 8 Y

6'

N


reaction rate appears to be about 5 times higher for the copolymerization of the polyurethane-acrylate resin (Tab. I), probably because of a strong viscosity effect that favors propagation over termination 23).

Similar kinetic investigations have been carried out by taking as monomer the newly developed monoacrylates, which contain a cyclic carbonate group in their structural unit. The copolymerization was then found to proceed much faster than with the conventional mono- and diacrylates, and almost as rapidly as with the highly reactive triacrylate (Fig. 2). The most remarkable feature is that the polymerization develops more extensively with those monomers and reaches 90% conversion within 0,02 s of exposure, compared to 0,8 s for EDGA (with di- and triacrylic diluents, such high conversions are never reached). The UV-cured polymers contain therefore a low amount of residual unsaturation ( = ~ V O ) , with the expected benefit with regard to their long- term behavior. The monomer content of the crosslinked polymer can be further decreased by pursuing the UV exposure (Fig. 3), leading finally to a hard, scratch-free coating that contains, for monomer 1, less than 2% of unreacted acrylic functions.

CH,=CH-C-0-CH,-CH-CH,

0 /I b b \ /

C

0 1 II

CH,=CH-C-0-CH,-CH,-0-C-0-CH2-CH-CH2

0 I / b b II 0

\\ / C

0 2 I I

CH,=CH-C-O-CH,-CH,-C-O-CH,-CH-CH,

0 II b b ll 0

\ / C

0 3 I /

Fig. 3. Influence of the kind of monomer on the variation of the unsaturation content of a UV-cured polyurethane-acrylate copolymer upon further irradiation

Irradiation time in s


The three formulations containing monomers 1, 2 and 3 show comparable rates of copolymerization (Tab. l), whereas, in the light-induced homopolymerization, monomer 2, which contains in addition a linear carbonate group, was shown') to be markedly more reactive than monomers 1 and 3. It can be seen from Tab. 1 that the R, value decreases when the polyurethane-acrylate oligomer was added to monomer 2, just the opposite trend from what is usually observed in UV-curable systems, which clearly demonstrates the outstanding performance of this new monomer. Since the R , value depends on' the initial acrylate congentration, [A], , which varies amongst the different systems examined, the value of the ratio R,/[A], is also reported in Tab. 1 for a better comparison of the intrinsic reactivity of these acrylic monomers. For monomers 1, 2, and 3, R,/[A], values are significantly higher than for the triacrylate TMPTA and up to 10 times greater than for the usual mono- and diacrylates.

An interesting consequence of the large rate of polymerization observed with these new monomers is that oxygen inhibition, due to the diffusion of atmospheric 0, into the thin film, will be less pronounced, owing to the short exposure required to cure the sample. A similar effect was found previously by using powerful lasers as light sources in order to reduce the irradiation time24).

Another consequence of the remarkably high reactivity of monomer 2 is that, by lowering the photoinitiator concentration, relatively thick samples can be photopoly- merized at still acceptable rates. With 0,5 wt.-Yo of photoinitiator, a 500 pm thick film was deep-through cured within less than 1 s of UV exposure, in the presence of air. Actually the light-induced copolymerization of the polyurethane-diacrylate with monomer 2 was found to proceed even without any photoinitiator added: 15% conversion was reached after 2 s of irradiation in the presence of air, and after only 0,6 s in an inert atmosphere, a result in good agreement with our previous observations ') on the photopolymerization of pure monomer 2.

The kinetics of these ultrafast polymerizations has also been studied in real time by RTIR photospectroscopy 18,25), operated at an incident light intensity of 40 mW * cm-, at the sample position. The conversion versus time curves thus recorded (Fig. 4) confirm fully the high performance of the three cyclic carbonate-monoacrylates studied. The actual rate of polymerization can be determined at any time from the slope

Fig. 4. Polymerization profiles recorded by RTIR spectroscopy for a polyurethane-acrylate resin exposed to UV radiation in the presence of air. Dashed line: conversion after irradiation and 10 s storage in the dark at 25 "C. Light intensity: 40 mW . cm -2. Actilane 20: 47,5 wt.-%; monomer: 4 7 3 wt.-%; Irgacure 65 1 : 5 wt.-%

Irradiation time in

514 C. Decker. K. Moussa

of these kinetic curves. For monomer 2, the maximum rate was measured to be 10 mol . L - I . s-I, a value much lower than that reported previously in Tab. 1 , due to both the weaker light intensity used in the RTIR analysis and a substantial postpoly- merization which was included in the former evaluation. The latter effect may account for up to 70% of the total polymer formed, as shown by the dashed curve of Fig. 4, which represents the variation of UV exposure of the conversion reached after irradiation and 10 s of storage in the dark at room temperature.

The reason why these monomers are so reactive is still not clear, but it is most likely related to the presence of the cyclic carbonate group. Indeed, when this structure in the very reactive monomer 2 was replaced by an isopropyl group, the rate of copolymerization of monomer 4 with Actilane 20 was found to drop by a factor of 10 (Fig. 5 ) , with an R , value close to that observed by using EDGA as diluent.

CH,=CH-C-0-CH,-CH,-0-C-0-CH I1 I 0 CH3

II 0

4

CH,-C-0-CH,-CH-CH,

0 II b b \ /

C

0 II

5

In another approach to demonstrate the crucial role of the cyclic carbonate structure, an acetate group was substituted for the acrylate function in monomer 1, thus leading to a compound 5 which cannot be considered as a monomer anymore.

Nevertheless, a 1 / 1 mixture (by wt.) of this compound with the polyurethane-diacrylate (Actilane 20) was found to polymerize 3 times faster than Actilane 20 alone, despite the twice lower value of the acrylate concentration (Fig. 5). Actually this formulation

Fig. 5 . Influence of monomer 4 (linear carbonate- monoacrylate) and of compound 5 (cyclic carbonate- acetate) on the photopolymerization of a polyurethane-diacrylate (Actilane 20)

Irradiation time in ms


happens to have the highest intrinsic reactivity among the various systems studied so far, with an R,/[A], value of 170 s-I, compared to 28 s for the oligomer alone and 10 s -' for the Actilane 20 + EDGA resin. Recent experiments suggest that the acetate 5 acts rather as a catalyst. The fact that it can be totally extracted from the insoluble polymer formed, by simple treatment with chloroform 26), indeed indicates that it has not been incorporated into the polymer network. Further work on the mechanism of polymerization of this new class of monomers is in progress, in order to better understand the precise role of the cyclic carbonate structure.

2. Epoxy-acrylates

A similar kinetic study was carried out by using a polyphenoxy-diacrylate (Actilane 72) as functional oligomer and monomers 1, 2 or 3 as reactive diluents, in a 1 / 1 wt. ratio. All the UV-cured copolymers thus obtained were found to be very hard and glassy, which makes them well suited to protect rigid materials by scratch-free resistant coating^^^^^*). The conversion versus time curves are shown in Fig. 6, for the various systems exposed to UV radiation. The values of the maximum rate of polymerization and the amount of residual unsaturation in the tack-free film are reported in Tab. 2.

Here again, we observe the same trend as previously found in the polyurethane- acrylate photoresists. The R, value increased by a factor of 5 when the difunctional TPGDA was replaced by one of the monomers 1, 2 or 3, and by as much as a factor of 17 when compared to monofunctional EDGA-based formulation. With the most reactive system, a single pass under the lamp at a speed of 20 m/min proved to be sufficient to obtain the tack-free film. The polymerization quantum yield, @,, calculated from the ratio of R, to the absorbed light intensity, was found to be as high as 5000 acrylic groups polymerized per photon absorbed, despite the strong 0, inhibition effect on such a radically induced process. Assuming an initiation quantum yield of 0,4 for the photoinitiator usedt4), this would correspond to a kinetic chain length of 12500 mol per initiating radical.

The main difference between the kinetic profiles of polyurethane- and poiyphenoxy- based photoresists is that the polymerization stops at an earlier stage in the latter case, due to segmental mobility restrictions of the reactive sites which become trapped in the

.g 1001

Fig. 6. Light-induced copolymerization of an epoxy- diacrylate (Actilane 72) with various acrylic monomers, in the presence of air. Light intensity: 500 m w * cm -'; film thickness: 25 pm; photoinitiator: 5 wt.-Vo Irgacure 65 1

Irradiation time in ms

Tab.

2.

inte

nsity

: 500

mW

. cm

-'.

Film

thic

knes

s: 2

5 Vr

n Ph

otop

olym

eriz

atio

n of

epo

xy-a

cryl

ate

resi

ns in

the

pres

ence

of

air.

Act

ilane

72:

47,

5 w

t.-%

; M

onom

er: 4

7,5

wt.-

To; I

rgac

ure

651:

5 w

t.-%

. L

ight

Mon

omer

[A

cryl

ate]

, a)

R

,/[A

Io

Num

ber b

, RU~)

in T

o h

d,

Flex

ibili

ty e,

S-I

of

pas

ses

(20

m/m

in)

in s

in

mm

Mon

oacr

ylat

e:

ED

GA

4

5

1 43

9 2

339

3 4,1

Dia

cryl

ate:

H

DD

A

6,s

TPG

DA

5.3

35

500

500

600

130

120

8 6

100

1 13

0 2

145

2

20

3 23

3

a)

Max

imum

rat

e of

pol

ymer

izat

ion.

b,

N

umbe

r of

pas

ses

unde

r th

e UV la

mp

for

obta

inin

g a

tack

-fre

e fi

lm.

') RU

: re

sidu

al u

nsat

urat

ion

in t

he ta

ck-f

ree

film

. dl

Pe

rsoz

har

dnes

s of

the

tack

-fre

e fi

lm.

e,

Dia

met

er o

f m

andr

el a

t w

hich

cra

cks

firs

t app

ear.

5 19

18

15

23

20

60

0 36

0 1

320

1 34

0 1

300

12

260

7

0

U

n

"1 R 5 E 6


stiff and glassy polymer network29). It remains thus, in the UV-cured material, a relatively large amount of residual unsaturations ( = 20%). Pursuing the irradiation was yet shown to further reduce this level, down to 12% when the exposure time was extended to 2 s.

Properties of the UV-cured polymers

Both the polyurethane-acrylate and the epoxy-acrylate copolymers obtained with monomers 1,2 or 3 were found to be totally insoluble in the organic solvents, with little swelling, thus indicating that a highly crosslinked polymer network has been formed. They also exhibit a good resistance to moisture, strong acids and alcaline treatment. The optical properties of these materials make them well suited for applications as organic glasses or optical components, since they are perfectly transparent and quite resistant to UV radiation and laser beams 30). Their mechanical properties depend primarily on the functionality and chemical structure of the monomers and oligomers used4s6). In this study, the hardness, flexibility and light-fastness of the various copolymers have been investigated more thoroughly, in consideration of the potential applications of such materials in the coating industry.

Hardness and flexibility

Aliphatic polyurethane chains are known to impart elasticity and impact resistance to UV-cured polymers, while the polyphenoxy structure leads to hard materials with high tensile strength and low elongation at break4). One of the great advantages of the monomers containing a cyclic carbonate group is that the hardness and scratch-resistance of the polymer formed is markedly enhanced '1, compared to the usual monoacrylates. The same trend was observed when these monomers were copolymerized with either a polyurethane-diacrylate or an epoxy-diacrylate.

The values of the Persoz hardness (h) are reported in Tabs. 1 and 2 for the various systems examined. The hardness increase is particularly pronounced for monomer 1, since the polyurethane tack-free coating was found to be almost as hard (h = 260 s) as when using a triacrylic monomer like TMPTA (h = 270 s), and substantially harder than with a diacrylate like HDDA ( h = 130 s), or a monoacrylate like EDGA (h =

30 s). Upon further exposure to UV radiation, the polymer film continues to harden slightly (Fig. 7) due to the slow polymerization of the residual acrylic double bonds that are trapped in the three-dimensional network (Fig. 3). Close to 100% conversion can be reached by heating the UV-cured sample above 80°C, a treatment which relieves segmental mobility restrictions 29).

Very similar results were obtained by using the epoxy-acrylate oligomer (Fig. S), but the enhancement effect due to the cyclic carbonate monomers was less spectacular, since such polymers are already very hard. One can still notice that the hardness of the polymer obtained by using monomer 1 as reactive diluent ( h = 390 s) is almost as high as the hardness of glass (h = 420 s).

518

EDGA A-----:

I I I I

C. Decker, K. Moussa

C ._ in in a,

L

100

1

-------3 / 2

N 0 in I : 2 0 0 -

100

Fig. 7. Influence of the monomer on the hardness of a UV-cured polyurethane- -acrylate copolymer upon further exposure. Film thickness: 50 pm

Fig. 8. Influence of the monomer on the hardness of a UV-cured epoxy-acrylate copolymer upon further exposure. Film thickness: 50

EDGA -

I I I I w

For most polymers, an increase of the hardness is accompanied by a loss of the elongation at break, so that it is very difficult to produce materials that are hard and flexible a t the same time. The novel monomers studied here present the distinct advantage of providing both hardness and flexibility to the final product. All the monoacrylate/polyurethane-acrylate copolymers were found to pass the severe zero T-bend test, no cracks appearing when the polymer film was bent onto itself and a 10 N . cm-2 pressure applied (Tab. 1). By contrast, with di- and triacrylate monomers, cracks appeared upon bending the film around mandrels of diameter ( d ) of 2 and 5 mm, respectively. The outstanding performance of monomer 1 should be emphasized, since the UV-cured polyurethane copolymer proved to be very hard and


scratch-resistant, while it exhibits at the same time a high flexibility, with break elongation values above 100%. The same effect was observed for the epoxy-acrylate copolymers containing monomer 1, 2 or 3 (Tab. 2). The UV-cured films were found to be both harder and more flexible than the diacrylate-based systems, with a remarkably low d value of 1 mm, compared to 7 and 12 mm for TPGDA and HDDA coatings, respectively.

Light-fastness

Owing to their high crosslink density, UV-cured acrylic polymers resist quite well to photodegradation 3 i , 32), especially the aliphatic derivatives, which were shown to remain perfectly clear and transparent upon accelerated weathering 33). The effect of monomers 1,2 or 3 on the light-fastness of aliphatic polyurethane-acrylate copolymers was investigated by monitoring the discoloration and the chemical modifications occurring upon exposure in a QUV weatherometer.

Discoloration: A sharp but limited increase of the yellow index was observed upon the first 10 h of QUV aging (Fig. 9). This discoloration is directly related to the amount of residual photoinitiator in the UV-cured polymer, and can be substantially reduced by lowering its initial concentration from 5 to 1 wt.-%. Yellowing appears to be more pronounced with the novel monomers, probably because of some reaction of the initiator radicals with the cyclic carbonate structure. As shown by Fig. 9, a slow bleaching process occurs yet upon further exposure, so that heavily irradiated polymers are no more colored. After 2000 h of QUV aging, the yellow index value of a 25 pm thick film was less than 1 , for the various systems examined.

Chemical modifications: The main chemical changes occurring upon UV irradiation of such crosslinked polyurethanes are the destruction of the carbamate function and the formation of OH groups, which were both monitored by IR spectroscopy. When monomers 1, 2 or 3 were used as reactive diluents in polyurethane-based resins, the disappearance of the NH group, followed at 1 530 ern-', was found to be slower (Fig. 10) than in HDDA or TMFTA copolymers, and the oxidation process developed less

Fig. 9. Influence of the monomer on the discoloration of a UV-cured polyurethane-acrylate copolymer, upon QUV aging

"0 100 200 OUV exposure in h

520

cn I 5O- z 0

C 0

U

- ._ c

C. Decker, K. Moussa

TMPTA

p V Z K

1

Yellow indexa)

OH formation') in h Perzos hardness d, in s

NH consumption b, in h

Flexibilityd) in mm

Fig. 10. Influence of the monomer on the loss of the NH group upon QUV aging of a UV-cured pol yurethane-acrylate copolymer

O S 0,6 I 0,7 0,8 450 250 1 600 600 1 000 900 300 1300 850 750 130 270 260 100 190

2 5 0 0 0

Fig. 11. Influence of the monomer on the photo- oxidation of a UV-cured polyurethane-acrylate copolymer, upon QUV aging

QUV exposure in h

extensively (Fig. 11). The exposure time needed to destroy 20% of the carbamate functions was found to increase from 250 h for the TMPTA copolymer to 1600 h for the copolymer with monomer 1. Similarly, the hydroxyl absorbance at 3510 cm- ' increased by a value of 0,l after only 300 h of QUV exposure for TMFTA, compared to 1 300 h for monomer 1. Tab. 3 summarizes the results obtained with the various poly-

Tab. 3. Performance analysis of various UV-cured polyurethane-acrylate copolymers

Monomer I HDDA TMF'TA 1 2 3


urethane copolymers examined. It clearly shows the best overall performance of monomer 1, which leads to hard and flexible polymer materials that are quite resistant to photodegradation. It should be mentioned that the light-fastness of such UV-cured polymers can be greatly enhanced by addition of benzotriazole UV absorbers and hindered amine light-stabilizers (HALS), a more than tenfold drop in the degradation rate being observed with the most effective stabilizer^^^).

Conclusion

The high reactivity of acrylic monomers containing a cyclic carbonate structure, which was demonstrated in our previous study on the light-induced homopolymerization ’), was fully confirmed by the present investigation on the copolymerization with difunctional oligomers. The photopolymerization develops as fast as with triacrylates and as extensively as with monoacrylates, thus leading within milliseconds to highly crosslinked polymers with low monomer contents. A most remarkable feature is that this new class of very reactive monomers impart both hardness and flexibility to the UV-cured polymers, which in addition exhibit a good resistance to solvents, chemicals and accelerated weathering.

Such monomers are expected to find their main applications as reactive diluents in the coating industry where their outstanding performance should help solve some of today’s problems, like the cure speed, oxygen inhibition, tailor-made properties, deep- through cure, etc. The actual reason which makes these monomers so reactive is still unknown. Further work is in progress in order to elucidate the basic mechanism of this polymerization, and will be reported in a next paper.

The authors wish to thank SociPtP Nationale des Poudres et Explosifs (SNPE) for the grant of a graduate student fellowship and for the supply of monomer samples.

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a new class of highly reactive acrylic monomers, 2. light-induced copolymerization with difunctional...

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