Development of a Cationic UV Curable Inkjet Ink – Formulation

Effect on Curing Behavior

Atsushi Tomotake, Toshiyuki Takabayashi, Nobumasa Sasa, Atsushi Nakajima, and Shuji Kida; 1st R&D Group, R&D Division,

Konica Minolta IJ Technologies, Inc.; #1 Sakura-machi, Hino-shi, Tokyo 191-8511, Japan

Abstract

Two alternative curing mechanisms may be used; free radical

polymerization and cationic polymerization. Free radical

polymerization currently dominates because of its low cost and the

ease of design afforded by a wide selection of usable monomers.

Cationic polymerization is a new process but has become to get

many attracts because of its unique advantages. One of the

challenges of the cationic polymerization is the tendency to be

affected by moisture. Studying about the polymerization under

several moisture levels, we found the reactivity of monomers

controlled the effect of moisture. We will report results of

mechanistic study of the effect as well as ink performances at high

humidity environment.

Introduction

UV curable inkjet systems have proliferated because they can

be used with non-absorbing media and because they fix quickly.

Two alternative curing mechanisms may be used, free radical

polymerization or cationic polymerization. Free radical

polymerization currently dominates because of its low cost and the

ease of design afforded by a wide selection of usable monomers.

However, free radical polymerization has an inherent

disadvantage: oxygen inhibits their polymerization. Inkjet inks

have lower viscosity than conventional off-set inks, and

atmospheric oxygen can diffuse into inks with lower viscosity.

Consequently, inhibition takes place more easily in inkjet inks than

in off-set inks.[1]

Cationic polymerization, the alternative curing mechanism,[2]

avoids these difficulties by adopting a different polymerization

mechanism that is free of oxygen inhibition. Key component for

the polymerization is photo-generated acid. The acid protonates

cyclic ether oxygen. It can be cleaved to carbonium cation (a).

Unprotonated cyclic ether reacts with the cation to form alkyl

oxonium cation (b). The oxonium cation can also be formed by

the reaction of the cation (a) with unprotonated ether in SN2 type

reaction. The cation (b) reacts with the ether sequentially to form

polyethers.

One of the issues of the cationic polymerization is the

tendency to be affected by moisture. It was said that water

molecule can react with the cation intermediate to terminate the

polymerization because of its higher reactivity.[3]

In this paper, we present our results with a focus on effects of

water on curing performance as well as mechanistic study.

Figure 1 Cationic polymerization mechanism

Experimental

Epoxide (2) was synthesized by a conventional method.[4]

Other monomers and photo-initiator used in this study are

commercially available. Monomers and the photo-initiator were

mixed with under yellow light and used for the study. Curing

speeds were measured with FT-IR spectrophotometer.

Results and Discussions

Effect of water on ring-opening reaction of

epoxides

The effect of water on ring-opening reaction of epoxides was

evaluated by using real time FT-IR spectroscopy at high and low

humidity environments.

We mixed epoxide monomers, (1), (2), and (3) with photo-

initiator (4) separately. The mixtures were irradiated with high

pressure mercury lamp and a decay of peak intensity at 775 cm-1,

which was assigned to the absorption of epoxide ring, was

monitored to evaluate the reaction rate. The measurements were

carried out at 20%RH and 80%RH environments and the results

were summarized in figure 2.

The curves showed the rate of ring-opening reaction of

epoxide. Epoxide (1) and (2) showed higher rates while epoxide

(3) showed slower rate at 20%RH. At high humidity environment,

80%RH, all the ring-opening rates were retarded greatly. In

particular, epoxides (2) and (3) showed a big retardation.

OCPIHh.HHOCHOC(a)

HOOCOCPOLYMER(b)

 10

Figure 2. Ring-opening reaction of epoxides at 20%RH (solid lines) and

80%RH (dashed lines)

Effect of water on curing behavior of oxetane-

epoxide mixtures

It was reported that mixture of oxetane and epoxide showed

good curing behavior for practical use of cationic polymerization

as UV curable ink.[2] Although the epoxide used in the literature

showed higher reaction rate alone at initial stage of polymerization,

its conversion ratio stayed low. Although the oxetane used

showed higher conversion ratio alone, its reaction rate was low at

the initial stage. It was found the mixture of the epoxide and the

oxetane showed a synergistic effect on curing behavior; the

epoxide gave higher reaction rate and oxetane gave higher

conversion rate.

To evaluate the effect of water on practical usage, we then

measured the effect of water on curing sensitivity with coatings of

the mixture of oxetane (5) and above mentioned epoxides. We

mixed the oxetane with epoxides and the photo-initiator to form

model inks. The ratio was 67/28/5 in weight. The mixtures were

coated on PET with wire bar, and the thickness of the coating was

adjusted to be 3 µm. Then they were irradiated with high pressure

mercury lamp at 3 humidity levels, 20%RH, 50%RH, and 80%RH.

To compare the curing sensitivity at every humidity level, we

measured irradiated energy that was necessary to become durable

against scratch. The results were summarized in figure 3.

In figure 3, we observed similar effect of water to that of on

ring-opening reaction. Efficiency of polymerization was

decreased as humidity level was increased. Much energy was

necessary at higher humidity levels. Combining with the result in

figure 2, it seemed that the ring-opening reaction rate affected the

curing sensitivity. Epoxide (1), which showed the highest rates at

both high and low humidity alone, showed higher polymerization

efficiency with the oxetane. Epoxide (3), which showed the

slowest rates, showed lower polymerization efficiency.

Figure 3 Humidity dependence of curing sensitivity.

Reaction mechanism of cationic polymerization

Although the carbonium cation derived from epoxide was

reported to be a key intermediate and its formation step is rate-

determining of the polymerization, the effect of water did not

reported.[2] To clarify the effect, we studied the reaction

mechanism of the polymerization of oxetane-epoxide system in the

presence of water.

OOOOSSPF6(1)

(4)

OOOOOO(2)

C4H9O(CH2)7OOOC5H11(3)

80.0%

90.0%

OOO(5)

01002003004005000%20%40%60%80%100%

Relative humidityCuring energy (mJ/cm2)

(1)+(5)

(2)+(5)

(3)+(5)

We evaluated tendency of protonation by photo-generated

acid of the materials, oxetanes, water, and epoxides, by comparing

their basicity, pKb values.

The basicity of oxygen atoms of oxetane and epoxide were

reported as shown below. Oxetane-oxygen has the biggest pKb

value; it means that oxetane has the highest basicity of the three.

The photo-generated proton will be added on the oxetane-oxygen

in higher ratio. The oxonium cation derived from the oxetane,

however, was reported to be much stable than the oxonium cation

derived from epoxide; the polymerization will be promoted only

from the epoxide-derived oxonium cation.

 oxetane water epoxide

pKb 3.1 7.0 7.4

When the polymerization will be carried out at higher

humidity environment, the ink can absorb certain amount of water.

Water molecule is less basic than oxetane but much basic than

epoxide. Water molecule can trap the photo-generated proton and

retard to form the epoxide-derived oxonium cation (B). The

concentration of the oxonium cation will be decreased, and the

polymerization will be retarded accordingly. The mechanism was

proposed in figure 4.

If the ring-opening reaction of the epoxide-derived oxonium

cation (B) is fast, much amount of epoxide (A) will be protonated

through the equilibrium EQ. Consequently, efficiency of the

polymerization with such epoxide will be improved as shown in

figure 3.

Figure 4. Polymerization mechanism in the presence of water

Conclusion

We evaluated the effect of water on ring-opening reaction of

epoxides by using real time FT-IR spectroscopy. The ring-

opening reactions were retarded greatly at high humidity

environment. We also evaluated the effect of water on curing

behavior of oxetane-epoxide mixture system. The curing

sensitivity was also affected by moisture and much amount of

energy was necessary at high humidity environment to be cured.

Comparing pKb values of materials, epoxide was found to be

less basic than oxetane and water. The generation of the key

intermediate, oxonium cation (B), will be affected by water

accordingly. The concentration of the cation will be decreased by

water and the polymerization will be retarded accordingly.

From the mechanism, it was considered that accelerating of

the ring-opening reaction will suppress the effect of water on the

curing. Epoxides with higher ring-opening rate showed good

curing behavior even at high humidity environment.

References

[1] N. Caiger and S. Herlihy: Oxgen Inhibition Effects in UV-Curing

Inkjet Inks, IS&T’s NIP 15(1990) International Conference on Digital

Printing Technologies, 116-119

[2] H. Sasaki, TOAGOSEI TREND, 2(1999)

[3] “UV curable materials”, Johokiko co.,ltd, 11( 2006)

[4] Japanese Patens, No.3770274

Author Biography

Atsushi Tomotake received his Ph.D. degree in Synthetic Organic

Chemistry in University of Tsukuba in 1987. He joined Konica

Corporation in the same year. He was working to develop new materials

for color silver halide photographic paper for many years. Now his

interest is focused on the materials for color hard copy, such as thermal

dye transfer printing and ink jets.

OHO+OO+HOOH3OH2OH2OHOHOOOPOLYMER(A)

OEQ(B)

(C)