ー 論文ー●Oxetanes: Curing Properties in Photo-Cationic Polymerization
Hiroshi Sasaki
Oxetane ProjectAbstract
Novel oxetane monomers are evaluated in a photo-cationic polymerization system. Monomer with p-methoxy-phenylgroup on the second position of oxetane ring exhibited high reactivity. In the formulation of oxetanes withcycloaliphatic diepoxide monomer, the viscosity of the formulation was reduced effectively while keeping high surface
cure rate as epoxide alone. Difunctional oxetanes exhibited improved solvent resistance.

1 Introduction

measurements, the oxetane monomer was proved to possess

In photoinitiated cationic polymerization, many different types

polymerization characters different from those of epoxides towardof monomers and oligomers have been examined and used [1]. In

photoinitiated cationic polymerization. Although rather longparticular, the photopolymerization of epoxides gives coatings ainduction period compared with epoxide was seen in the earlyhigh thermal capability, excellent adhesion and good chemical

stage of oxetane polymerization, after that stage, polymerization

resistance. Although photo-polymerized epoxy coatings are known

underwent smoothly until high conversion of monomer. In thefor their high performance, commercially available epoxides, suchformulation with epoxy monomers, oxetane exhibited fairly fastas bis-phenol-A-diglycidyl ether, undergo photoinitiated cationicpolymerization. Computational studies explained the acceleration

polymerization at rather slow rates.

mechanism of oxetane polymerization to be the fast initiation ofThree major factors contribute to the reactivity of cyclic ethers

epoxide through SN1 ring opening of oxirane ring.

during cationic ring-opening polymerization [2]. These are

To enhance the reactivity of oxetane in the initiation stage,

basicity, ring strain and steric factors. The basicity and ring strain

introduction of electron donating group in the second position ofenergy of cyclic ethers are listed in scheme 1. The ring strain ofoxetane ring, which results in the formation of stable secondary

ethylene oxide has been calculated to be 114 kJ/mol while that forcarbocation intermediate, might be effective for the SN1 ringoxetane is 107 kJ/mol [3]. At the same time, the pKa for these twoopening. Based on the above investigation, 3,3-dimethy-2-(p-ethers is 3.7 and 2.02, respectively [4-5]. Thus, while both cyclic

methoxy-phenyl)-oxetane(MPO) was selected and the reactivity ofethers have similar steric factors and ring strains, the basicity ofthe monomer was studied. This paper details the evaluation ofoxetane is considerably greater than that of ethylene oxide. This

MPO toward photo-cationic polymerization using photo-DSC.

latter parameter should, therefore, dominate by making oxetanes

As shown so far, the high reactivity of oxetane monomers seems

more reactive than epoxides during photoinitiated cationic ring-attractive. In this paper, five oxetane monomers (scheme 2) wereopening polymerization.

selected and the performance of their formulation with cycloaliphatic diepoxide monomer in the UV cationic cure wasPreviously, the curing property of oxetane monomer in

evaluated.

photoinitiated cationic polymerization was investigated incomparison with epoxy monomer [6]. Through real time FT-IR

Scheme 1 : Ring-strains and Basicities of Cyclic Ethers

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Scheme 2 : Monomers used in this study

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2 Experimental

can be explained by the hydrogen bonding of hydroxy methyl2.1. Raw Materials

groups in the molecule. The simplest di-functional monomer,The oxetane monomers used throughout this study wereDOX, exhibited rather low viscosity and specific gravity comparedsynthesized from OXA (DOX, XDO) or 3-ethyl-3-chloromethyl-with XDO with xylene moiety in the molecule. Each of theoxetane (EHOX, POX, DOX) through phase-transfer reaction.

monomers was found to be completely miscible with UVR-6110 at

MPO was synthesized from 1-(4-methoxyphenyl)-2, 2-dimethyl-1,any ratio. All the monomers listed were AMES Test negative.

3-propanediol according to the procedures of Xianming andEHOX exhibited low surface tension caused by the alkyl side

Kellogg [7]. Cycloaliphatic diepoxide monomer (UVR-6110) andchains. In the formulation with UVR-6110, the surface tension ofsulfonium salt cationic photoinitiator (UVI-6990) used in thethe formulation was reduced effectively (Figure 1).
The heat of polymerization of XDO and UVR-6110 wasformulation were obtained from Union Carbide Co.

Table 1 Typical Properties of Oxetane Monomers
2.2. Sample formulations
The formulations were prepared by mixing the monomers with
UVI-6990 in amber vials. The viscosity of monomers and theirformulations was measured at 25oC using Brookfield viscometer.
The surface tension of EHOX and the formulations with UVR6110 were measured at 20oC.
2.3. Photo-DSC measurement
Heat of polymerization during photo-cationic polymerization
was measured by DSC220C (Seiko Instruments Inc.) equipped witha UV-1 lighting unit (200W Hg-Xe lamp, Seiko Instruments Inc.).
Typical values for light intensity were 11 to 90 mW/cm2 at 365nm.
2.4. Curing Test Methods
A uniform coating of desired coating thickness was applied to atest panel using a #3 bar applicator. The liquid coating was cured
with a 80 W/cm of high pressure Hg lamp.
Surface Cure Rate (S.C.R.) is defined as maximum conveyor speed
which can be used to produce a coating that becomes dry to theFigure 1. Surface Tension values versus EHOX contentin intouch immediately after UV exposure.

UVR-6110 formulations（at 20℃）

Samples for the evaluation of adhesion, flexibility and acetone

measured by Photo-DSC and the results are shown in Figure 2. In

resistance were cured using a UV irradiator at 10m/min. of

the photo-cationic polymerization of XDO, higher amount of exo-conveyor speed. Polycarbonate (PC), tin free steel (TFS),

therm was measured compared with that of UVR-6110 in the earlypolymethylmethacrylate (PMMA) and glass were used as teststage of irradiation (10 to 30 sec.) and decreased smoothly after thepanels for adhesion and TFS for flexibility. Measurement ofpeak (30 to 80 sec.). This smooth decrease could be explained byadhesion and flexibility of cured samples was performed according

the fast propagation rate because of the high basicity of oxetane

to the procedures of JIS K-5400. The acetone resistance of thering. Small amount of heat was seen for UVR-6110 and the heatcoatings was defined as the maximum number of rubs with a cotton

generation lasted for long during the irradiation (until 10min.). Thisball impregnated with acetone until the coating starts to belong heat flow could be explained by the slow polymerization ofdestroyed.

oxirane ring. With addition of 30 wt% of UVR-6110 to XDO, theinduction period of polymerization was shortened showing fast

3 Results

initiation reaction. The heat flow for the formulation lasted long

Typical properties of oxetane monomers are listed in Table 1.

(after 30 sec.) compared with that for XDO alone, showing theEHOX, possessing long alkyl side chain, showed the lowest

polymerization of remaining epoxide.

viscosity and specific gravity. The slightly high viscosity of OXA

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The properties ofOXA

Table 2 Formulations of OXA withUVR-61101)

formulations

with UVR-6110 wereshown in Table 2.
The viscosity of theformulation exhibiteda decrease as theamount of monomer
was increased and,with addition of 15wt.% of OXA, it
reached almost half.
The adhesion to PC
Figure 2. Photo-DSC exo-therm curves for XDO and UVR6110 fromulations(I=20.0mW/cm2 at 365nm, 3parts of UVI-6990, 40℃)

and glass was good until 10 wt% of OXA. With higher content ofThe heat of polymerization of MPO and POX with 1wt% ofadding 20 wt.% of OXA. On the other hand, the exo-therm curve

sulfonium salt photoinitiator (UVI-6990) were measured by Photo-became sharper and bigger by adding larger amount of OXA,DSC and the results are shown in Figure 3. In the photo-cationicshowing the higher rate of ring opening reaction in the formulation

polymerization of MPO, sharper exotherm curve was measured(Figure 4).

OXA, the flexibility of the coating was improved. The surface curerate, measured on a conveyor type UV irradiator, was decreased bycompared with that of POX. This higher reactivity of MPO toward
cationic polymerization might be explained by faster ring-opening
during the initiation stage, which is caused by stable benzyl-typecarbocation intermediate. With addition of 10 or 20 wt% of MPO
to POX, exotherm curves changed to narrower uni-modal shapes. If
the polymerization of MPO and POX proceeded separately, theexotherm curve should be bi-modal. These uni-modal curvesindicate that the polymerization of these two monomers underwent
co-polymerization and the sharper curves exhibit accelerated
polymerization of oxetane rings in POX.

Figure 4. Photo-DSC exo-therm curves for OXA and UVR6110 fromulations(I=20.0mW/cm2 at 365nm, 3parts of UVI-6990, 40℃)
In cationic ring opening polymerization, hydroxy-functionalmaterials such as alcohol is known to work as a chain transfer
agent. OXA possesses not only oxetane ring, which undergoes
cationic polymerization, but also hydroxy methyl group. With
lower concentration of OXA in the formulation, the hydroxy groupshould work as chain transfer site and accelerate cure rate. The
higher content of hydroxy group in the formulation should lead toshorter polymer chain length, which results in a weaker surface ofFigure 3. Photo-DSC for POX, MPO and their fromulations

the coating, and might decrease the cure rate. Another possibility(I=11.2mW/cm2 at 365nm) with 1wt% of UVI-6990

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group in the coating, which should increase the tackiness of theThe formulations of di-functional oxetanes (XDO and DOX)
were listed in Table 4. While the formulation with XDO exhibitedsurface.
The Photo-DSC curves for the 30 wt% of oxetanes (EHOX,rather small decrease in viscosity, DOX reduced viscosity

POX, DOX and XDO) formulations were shown in Figure 5. The

effectively. In the combination of XDO with OXA, the viscosity ofheat generation curves for all the oxetane formulations werethe formulation was also low. Each formulation showed very highnarrower compared with that for UVR-6110 alone, exhibiting highsurface cure rate. In the formulation of OXA and XDO, even withreactivity of the formulations.

20 wt.% of OXA, the cure rate over 50 m/min was achieved. Sameas the mono-functional oxetanes, the adhesion of the formulations
were poor. This low adhesion might be explained by stressconcentration between the surface of substrate and the cured
coating caused by fast polymerization of oxetane ring.
Combination with OXA, slightly improved the flexibility of thecoatings.
Table 4 Formulations of XDO, DOX and OXA with UVR-61101)

Figure 5. Photo-DSC exo-therm curves for POX, EHOX,DOX and XDO with UVR-6110 fromulations(I=20.0mW/cm2 at 365nm, 3parts of UVI-6990, 40℃)
The formulations of EHOX and POX are shown in Table 3.
EHOX with long alkyl side chains gave low viscosity formulations.
Although the oxetane monomers are mono-functional, surface curerates were higher than that of epoxide alone. The adhesion of theformulations for PC and glass was poor compared with the UVR6110 alone. The flexibility of the coatings were improved by theFor the cationic curing system using epoxy monomers, it is wellformulation of either EHOX and POX.

known that post cure is required for complete cure of the coating.

Table 3 Formulations of EHOX, POX and OXA with UVR-61101)

Coating of UVR-6110 alone exhibited very poor acetone resistance
until 24 hours after the irradiation. As shown in figure 6, addition ofoxetane monomers improved the acetone resistance even 10 minutes.
Higher amount of oxetane content increased the acetone resistance.
The improvement effect could be explained by the fast generation of
the cross-linked network of oxetanes accelerated by epoxide. TheFigure 6. Acetonc Resistance vs. Time東亞合成研究年報

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References

higher acetone resistance of DOX formulation can be explained by1) J. V. Crivello, J. L. Lee and D. A. Conlon, J. Radiat. Curing,lower molecular weight per oxetane group of DOX, which resulted in10(1), 6, 1983

shorter length of cross-links i.e. a stronger network.

2) S. Penczek, P. Kubisa, and K. Matyjaszewski, Adv. in Polym.

4 Conclusion

Sci., 37, 5, 1980
3) A. S. Pell and G. Pilcher, Trans. Faraday Soc., 61, 71, 1965

Oxetanes are reactive monomers effective for photo-cationiccuring systems. All the monomers were completely miscible with4) E. M. Arnett, in "Progress in Physical Organic Chemistry",
Interscience, New York, 7, 243, 1967

UVR-6110 and were AMES Test negative.

5) S. Searles, M. Tamres and E. R. Lippincott, J. Am. Chem. Soc.,

Formulating with cyclo-aliphatic diepoxide, accelerated the75, 2775, 1953

polymerization rate of oxetanes. MPO, possessing electron6) H. Sasaki, J. M. Rudzinski, T. Kakuchi, J. Polym. Sci. Part A,

donating p-methoxyphenyl group in the 2nd position of oxetane

33,11,1807, 1995

ring, was proved to work as a good accelerator for other oxetane

7) H. Xianming, R.M. Kellogg, Synth., 533, 1995

monomers without such group.
In the formulation with monofunctional oxetanes with epoxide,

本報告は、Radtech North America 2000にて発表を行った予

the viscosity of the formulations was reduced effectively. HOX

稿集に加筆、修正したものである。

with long alkyl side chains exhibited the lowest viscosity while
keeping the surface cure rate high.
In the difunctional oxetanes formulation, DOX alone and thecombination of XDO with OXA gave low viscosity formulations
with high surface cure rate. Addition of difunctional oxetane
monomers improved acetone resistance. Higher amount of oxetane
content increased acetone resistance.
As investigated in this study, mono- or di-functional oxetane
monomers provide cationic UV-curing formulations of epoxide withhigh cure rates and low viscosity. This viscosity reduction could beadvantageous for some applications such as coating industry.

●オキセタン化合物：光カチオン重合における硬化特製
オキセタンプロジェクト

佐々木 裕

四員環の環状エーテルであるオキセタンは光カチオン重合においてエポキサイドとは異なる重合特性を示す事が明らかになっている。本報告においては、重合性官能基としてオキセタニル基を有するモノマーを用いて、フォト−DSCを用いた光カチオン重合挙動の評価および市販の脂環式エポキシであるUVR-6110(ユニオン・カーバイド社製)との配合物の特性評価を行い、オキセタン系材料の光硬化型材料への応用を検討した。
フォト−DSCによる重合時の発熱量の測定から、オキセタン環の2位にp-メトキシフェニル基を導入したMPOは単独重合のみならず、無置換のオキセタンモノマーであるPOXとの共重合においても重合を促進する事が明らかになった。
工業化が予定されている5種類のモノマーは、一般的な脂環式エポキシであるUVR-6110(ユニオン・カーバイド社製)との相溶性は良好であり、比較的分子量が低いにもかかわらずＡｍｅｓ試験が陰性であった。また、側鎖に２−エチルヘキシル基を有するEHOXは希釈性および表面張力低下能に優れていた。
UVR-6110との配合により光硬化型材料への応用の検討を行い、単官能オキセタン化合物の添加により配合物の粘度を大きく低減する事ができる事、二官能オキセタン化合物の配合により脂環式エポキシモノマー単独の場合と比較して光照射後の耐溶剤性が大きく向上できる事が明らかになった。

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