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Progress in Organic Coatings 67 (2010) 264–268

Contents lists available at ScienceDirect

Progress in Organic Coatings

journa l homepage: www.e lsev ier .com/ locate /porgcoat

ynthesis and properties of novel polyurethane acrylate containing-(2-hydroxyethyl) isocyanurate segment

ong He ∗, Mengbo Zhou, Bo Wu, Zhanglin Jiang, Jun Nieollege of Material Science and Engineering, Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education,eijing University of Chemical Technology, Beijing 100029, PR China

r t i c l e i n f o

rticle history:eceived 25 August 2009eceived in revised form7 November 2009

a b s t r a c t

Novel UV curable multi-functional polyurethane acrylate (PUA) containing 3-(2-hydroxyethyl) isocya-nurate (THEIC) segment was synthesized through three step reactions, the ring-opened reaction of�-caprolactone with THEIC under the catalysis of tetrabutyl titanate (TBT), the polyaddition reactionbetween formed hydroxyl compounds and isophorone diisocyanate (IPDI) and condensation reaction

ccepted 21 November 2009

eywords:ulti-functional polyurethane acrylate

-Caprolactone-(2-Hydroxyethyl) isocyanurate

between the product of the second step and pentaerythritol triacrylate (PETA). The chemical structureof PUA oligomer and some influence factors in synthesis process were characterized with GPC, 1H NMRand FTIR. And its photopolymerization process and properties of cured films were also investigated.

© 2009 Elsevier B.V. All rights reserved.

V-curingigh hardness

. Introduction

UV-curable coatings have been widely used through manyelds since 1960s, which consist of oligomer, monomer and pho-oinitiator [1–4]. The coating film properties, such as hardness,brasive resistance, flexibility and weatherability, mainly accountn the oligomer structure and its concentration in the formula-ion. Polyurethane acrylate (PUA) is a kind of most widely andiversely applied oligomer, which synthesized from the polyol, iso-yanate and hydroxyalkyl-acrylate or -methacrylate. So looking forore new structure and special property PUA would play the key

ole in the development of UV curable chemistry. For example, theigh-hardness PUA would find its more and more utility in optics,lectric, inks and decoration [5–7].

The hardness of the film is influenced by its crosslinking density,hain flexibility and monomer rigidity. THEIC, a kind of aliphatic tri-ydroxyl compound containing isocyanuric ring, and its derivatesave been used in magnet wire enamels, electrical insulating var-

ishes, and rigid urethane foams with excellence hardness and heatesistance [15,16]. It would be an interesting candidate polyol torepare PUA and denote the good rigid and optical property toligomer. And at the same time it is also convenient to synthesishe high functionality PUA due to its three hydroxyl groups in the

∗ Corresponding author. Fax: +86 1064421310.E-mail address: heyong@mail.buct.edu.cn (Y. He).

300-9440/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.porgcoat.2009.11.005

ring, thus increase the crosslinking density with fast curing rate ofthe oligomer.

It is well known that �-caprolactone can react with hydroxylcompounds through ring opening polymerization (ROP) and poly-caprolactone is a kind of biomaterial for its biocompatibility andbiodegradability [8]. In general, tin (II) salts especially, tin (II)octoate [9,10] and rare earth compounds, such as yttrium iso-propoxide [11], lanthanide alkoxides [12,13] had been used asinitiators for the ring-opening polymerization of lactones. And thetensile strength modulus and the hardness of ROP products woulddecrease with the content of �-caprolactone unit increased, whichwas consistent with the trend of crystallinity [14].

Thus this article describes an attempt to synthesis a new kindof high-hardness PUA by introducing THEIC as the hard segmentand polycaprolactone as the soft segment and increasing the dou-ble bonds density. THEIC reacted as the hydroxyl compound to�-caprolactone through ring open reaction with tetrabutyl titanate(TBT) as catalyst, in which the formed polyol holds both the hardsegment and soft segment whose crystallinity and solubility is dif-ferent from those of THEIC. Their molecular weights are analyzedand the influencing factors, such as reaction temperature, concen-tration of catalyst and molar ratio of raw materials are studied.The product of ROP contains three hydroxyl groups and could be

used to prepare PUA as polyol while IPDI and PETA were chosen asthe diisocyanate and hydroxyalkyl acrylate. So the final PUA wouldhave nine double bonds theoretically, while the common PUAs havetwo, three or at most six double bonds. And the UV-curing processand properties of cured films were measured respectively.

Y. He et al. / Progress in Organic Coatings 67 (2010) 264–268 265

ture o

2

2

(((cSCDfw

2

ncavp

2

ftod3N

ttrwfiN(

2

tw

Scheme 1. Struc

. Methods

.1. Materials

�-Caprolactone was distilled under reduced pressure, tris-2-hydroxyethyl) isocyanurate (THEIC), isophorone diisocyanateIPDI), 2-hydroxyethylacrylate (HEA), pentaerythritol triacrylatePETA, Eternal Co, Taiwan), tetrabutyl titanate (TBT, Beijing Chemi-al Co.), 2-hydroxy-2-methyl-phenyl-propan-1-one (Irg1173, Ciba,weden), dibutyltindilaurate (DBTDL, Tianjing Fucheng Chem. Co.hina), CN373 and CN9006 (Sartomer Co., USA), BP (Benzophenone,alian Xueyuan Specialty Chem. Ltd., China) were used without

urther purification. The chemical structure of some raw materialsere showed in Scheme 1.

.2. Synthesis procedure of triol compound

THEIC and tetrabutyl titanate were added into a four-ecked flask equipped with mechanic stirrer, thermometer, refluxondenser and dropping funnel. After reaching the setting temper-ture, �-caprolactone was slowly dropped into the above reactionessel within 30 min and kept reacted with predetermined tem-erature and time.

.3. Synthesis procedure of PUA

IPDI (MOH:MNCO = 1:1) and DBTDL (500 ppm) were added into aour-necked flask equipped with a mechanic stirrer, thermome-er, reflux condenser and dropping funnel. The triol compoundbtained in the first step was diluted with ethyl acetate and slowlyropped into the reaction vessel keeping temperature no more than0 ◦C. The progress of reaction was determined by measuring theCO value using di-n-butylamine back titration method [4].

Thus, after the theoretical NCO value was achieved at the end ofhe above procedure, PETA or HEA was dropped immediately intohe flask, then the temperature was heated up to 70 ◦C slowly. Theatio of residual NCO groups to the hydroxyl groups in the acrylatesas set at 1:1.1, so as to eliminate all the residual NCO groups. Thenish of reaction was determined by the total disappearance of theCO peak (around 2267 cm−1) in the Fourier transform infrared

FTIR) spectrum.

.4. Measurements

1H NMR spectra were recorded on a Bruker AV600 unity spec-rometer operated at 600 MHz using TMS as an internal reference,ith CDCl3 as the solvent.

f raw materials.

Fourier transform infrared spectra (FTIR) were recorded on aNicolet 5700 instrument (Nicolet Instrument, Thermo Company,USA). Real-time infrared spectra (RTIR) were used to determine theconversions of double bond. The mixture of monomer and initia-tor was applied between two KBr crystals and irradiated with theUV radiation with UV spot light source (Rolence-100 UV, Taiwan,China) at room temperature. The light intensity on the surface ofsamples was 80 mW/cm2 detected by radiometry (UV-A, BeijingNormal University, China).

The molecular weights and molecular weight distributionswere determined by gel permeation chromatography (GPC) witha Waters 515-2410 GPC System equipped with a set of Styragel(HT3 HT5 HT6E). THF was used as diluents at a flow rate of1.0 mL/min at 30 ◦C.

The formula was applied on tin-plated steel strips polished withsandpaper by a coater with a clearance of 20 �m, with the exceptionof films for pendulum hardness test coated with a 40 �m coateron the glass plates. The coating films were exposed with UV lamp(12 mW/cm2, LUV Special Lamps Co., China) in the air to totallycure.

Pendulum hardness, pencil hardness and film glossy were testedaccording to China national standards GB/T1730-93, GB/T6739-1996 and GB1743-79.

3. Results and discussion

3.1. Synthesis of triol compound

3.1.1. Structure characterizationThe mechanism of ring-opening reaction of �-caprolactone has

been studied for many years [19]. 1H NMR data could be used toanalyze the structure. The symbol and assign of different H atom oftriol compound are shown below (Scheme 2).

Theoretically, the number of H atoms on the ring-opening prod-uct obeyed the following rules:

c = d = j = k,

e = f = g = n × d,

h = i = (n − 1) × d,

f + h + j = 2 × g

The integration of 1H NMR peak values of all H-atoms in the threering-opening products in Table 1 was in accordance well with the

266 Y. He et al. / Progress in Organic Coatings 67 (2010) 264–268

Scheme 2. 1H NMR spectrum and symbol of different H atom in mono-substitution triol compound.

astrriTe

3

ltwmtaowpdc

TI

Fig. 1. Molecular weight as a function of time.

bove rules. The result showed that the triol had been synthesizeduccessfully. The values of “a, b” varied as the ring-opening reac-ion proceeded on different sides of THEIC. When the ring-openingeaction propagates on the one, two or three sides in THEIC, theatio of a/d were 2, 0.5 or 0, respectively. Therefore in Sample 2n Table 1, the substitution nearly propagated on only one side ofHEIC. The other two samples were the mixtures of one, two andven three substitutions of ring-opening products.

.1.2. Influence factorsThe ring-opening reaction products were non-crystalline, from

iquid to semi liquid, and more soluble in ethyl acetate at room-emperature, while THEIC was a white crystalline powder. So,ith the increase of reaction time, reactant became more andore clear. And to determine a proper reaction time, the reac-

ion of THEIC and 3 times mass �-caprolactone were performedt 100 ◦C for 11 h. As seen in Fig. 1, molecule weight of the ring-

pening reaction product reached maximum value after 3 h andas in nearly linear relationship with time in this stage. In thisaper, the term of peak molecular weight (Mp.) was used toescribe the tendency of molecular weight with changed reactiononditions.

able 1ntegration values in 1H NMR of the three ring-opening products.

Samples (THEIC: �-caprolactone) Integration values of H proton peaks

a b c d

1 (1:1) 1.60 1.61 0.98 1.002 (1:2) 2.06 1.96 1.00 1.023 (1:3) 1.40 1.35 1.00 1.02

Fig. 2. Molecular weight as a function of time for different temperature.

To investigate the influence of reaction conditions on the molec-ular weight, parallel reactions were carried out with differentreaction temperature, reactant molar ratio and initiator concen-tration respectively. From Fig. 2, it can be seen that the molecularweights increased with the increase of temperature, which at170 ◦C is about 1.5 times larger than that of at 120 ◦C. And thereaction times to reach the maximum molecular weight to threetemperature condition are around 1 h.

Fig. 3 shows the tendency of molecule weights at different molarratios of THEIC to �-caprolactone. The molecular weight increasedas the �-caprolactone monomers increases. When the ratio is 1:1,little ring opening polymerization occurred.

When 0.1 wt% initiator exists in the reaction system, molecu-lar weight of product is bigger than that with 0.5 wt% and 1 wt%systems (Fig. 4). Too much chain transfer to polymer as well asto monomer would be promoted while more initiator was added,which will lead to more termination and bigger molecular weight.The low range of molecular weight can be achieved by using more

amount of initiator [17].

Typical molecular weights range of the polyols in polyurethaneacrylate locates from 1000 to 5000 [18]. But differing from otherresearches, high molecular weight polycaprolactone is not the aimof us and what expected is obtaining a kind of polyol which can

e f + h g i j k

1.80 2.64 1.83 0.77 1.06 1.002.25 3.43 2.25 1.16 1.08 1.022.73 4.37 2.72 1.68 1.05 1.02

Y. He et al. / Progress in Organic C

Fig. 3. Molecular weight as a function of time for different molar ratios.

F

rwfwh

temperature is an effective way to obtain the desired molecules

ig. 4. Molecular weight as a function of time for different amount of catalyst.

epresent THEIC’s property. Too much �-caprolactone contents

ould weaken the effect of THEIC, thus make the product not dif-

erent with the commonly used polycaprolactone diol compound,hich could reduce the mechanic properties and thus reduce theardness of the coating films.

Fig. 5. IR spectrum o

oatings 67 (2010) 264–268 267

Therefore the molecular weights would be controlled withthe amount of �-caprolactone monomers and the ratio ofmonomer and initiator. But with the lower molar ratio ofTHEIC to �-caprolactone, for example 1:1 and 1:0.5, most ofTHEIC remain unreacted. Thus, 1 wt% initiator concentration,140 ◦C and 1:3 or 1:2 THEIC/�-caprolactone molar ratios is theoptimum condition in all experiments that have been carriedout.

3.2. Synthesis and structure characterization of PUA

In the former reports and industrial production methods, poly-ols were dropped by the isocyanates. But in this paper, the oppositereaction path was used, in which the above polyols were dilutedwith ethyl acetate. By doing this, the concentration of isocyanate(NCO) groups was always more than the concentration of OHgroups. Because that IPDI has two NCO groups with different reac-tivities, it was expected that only one NCO group of IPDI moleculeparticipates in the first step reaction, and another remains undis-turbed and reacts with 2-hydroxyl acrylate in the second step. So,through the way of dropping by the triols rather than by the iso-cyanates, the minimum molecular weight product of ROP can beobtained.

The reaction rate constant of two NCO groups and speed ofPUA synthesis reaction are determined by temperature. The reac-tion rate constant also depend on the amount of catalyst, whichaccelerates the speed of the reaction and separates the selectivityof the two NCO groups. However, the amount of catalyst is sen-sitive in the reaction, which easily causes high viscosity or evenimplosion as it exceeds. In the second step, hydroxyl acrylate wasadded into the flask at one time. The temperature and the con-centration of NCO group in the reaction system were comparablylow. Nearly most of isocyanates contain only one NCO group sothat crosslink reaction is unlikely to happen. Heating up after theaddition hydroxylethyl acrylate was at in interval of 10 ◦C in orderto accelerate the rate of reaction. Therefore, the tight control of

(Table 2).From the peaks around 810 cm−1 (C C), 1720 cm−1 (C O) and

3372 cm−1 (N–H) in FTIR spectrum, it is confirmed that PUA hasbeen obtained (Fig. 5).

PUA oligomer.

268 Y. He et al. / Progress in Organic C

Table 2Raw materials for PUA oligomer.

Example Hydroxyl value of triol THEIC/caprolactone Hydroxyl acrylate

PUA-A 280 1:3 PETAPUA-B 340 1:2 PETAPUA-C 450 1:1 PETAPUA-D 340 1:2 HEAPUA-E 450 1:1 HEA

Fig. 6. Double bond conversion of PUA oligomer.

Table 3Properties of cured films.

Example Curing time (s) Pencil hardness Pendulum hardness Glossy

PUA-A 15 3H 0.89 148

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3

rhoa

[[[

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PUA-B 15 3H 0.91 146PUA-C 15 3H 0.90 140PUA-D 30 2H 0.88 117PUA-E 30 2B 0.08 135

.3. Real-time infrared

Real-time FTIR was carried out to monitor the process of theeaction. It can be seen from Fig. 6, double bonds conversion reachts maximum (around 80%) at a very short time, which is about no

ore than half minute. Comparing with CN9006, a kind of famousix functional PUA, it was found that both of them had a fast andlmost similar curing time, but the ultimate conversion of CN9006as 10% less than that of our PUA, which maybe the result of dif-

erence of their molecular moving ability. And high double bondsensity can be explained for their fast curing rate.

.4. Properties of cured films

Two-functional PUA, commonly adduct of HEA, HEMA, are infe-ior to multi-functional ones in the curing time as well as inardness, abrasive assistance, etc. In the UV-curing step, adductf HEA needs to be irradiated under the UV light for 30 s, whiledducts of PETA need just 15 s for total cure.

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oatings 67 (2010) 264–268

Multi-functional PUA coatings possess high hardness and highglossy mainly due to their high crosslink density, while lost theflexibility, adhesion compared with the two-functional ones. Thehardness of the cured films was both measured by pendulum hard-ness on the glass plates and by pencil hardness on the polished tinplates. In Table 1, three �-caprolactone substituted products withdifferent molar ratios have close hardness, because their hard seg-ment contents are close to each other. Because that no reactivediluents or other additives were used to show the films’ originalproperty (Table 3).

4. Conclusions

A series of UV curable polyurethane acrylates, containing THEICand �-caprolactone segments were synthesized. Different molec-ular weights triols, ranged from 400 to 1000, were obtained bycontrolling reaction temperature, reactant molar ratio and initiatorconcentration. The possible structures of the final products wereanalyzed that one product proceeded the ring-opening reactiononly on one side of THEIC, while others proceeded on all sides ofTHEIC. And further synthesis of polyurethane with above triols, IPDIand PETA or HEA was accomplished. The RTIR and the cured filmproperties tests showed that the multi-functional polyurethaneacrylates exhibited very fast-curing and high hardness comparedwith the commercial six functional PUA. In the further work, theflexibility of the coating films and a better formulation are stillneeded to be improved.

Acknowledgements

The authors would like to thank the National Natural Sci-ence Foundation of China (50703003) and the Program forChangjiang Scholars and Innovative Research Team in University(No. 50473024) for their financial support.

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