「ネ ッ ト ワ ー ク ポ リ マ ー 」Vol .29 No.4(2008)

Original(Received October 8, 2008)

Novel Liquid Epoxy Resin and its Cured Products Providing

both of Excellent Flexibility and Outstanding Toughness.

Ichiro OGURA* and Nobuya NAKAMURA*

*DIC Corporation ,

(12, Yawata-kaigandori, Ichihara, Chiba, 290-8585, Japan)

Abstract

We succeeded in the development of a novel liquid epoxy resin providing both of excellent flexibility and

outstanding toughness to its cured resins. The main drawback of epoxy resins "Stiff but Brittle" was largely improved by

an epoxy resin based on bisphenol A and modified with a high concentration of a flexible skeleton.

We were able to synthesize the new type of epoxy resin by reaction of bisphenols with vinyl ethers, leading to

phenolic intermediate having a special acetal structure. This phenolic intermediate was epoxidized by epichlorohydrin. The

resulting epoxy resin exhibit both high flexibility and excellent toughness after curing. In view of this, it was expected

that such epoxy resins are able to solve the problems and future demands in difficult high-tech applications, such as printed

circuit board and the like.

The excellent performance is considered to be based on the synergy effect of the hard-segment (with the molecular

mobility reduced by cross-linking points located closely to that hard segment) providing much larger strength and the

soft-segment (mobility of which was larger because of the position far from cross-linking points) performing excellent

flexibility.

1. Introduction Epoxy resins provide excellent characteristics like water resistance, heat resistance, chemical resistance, high

adhesion, low curing shrinkage and electrical insulation properties, etc. However, epoxy resins have the essential problem

of "Stiff but Brittle" on the other hand.

Therefore, an improvement has been strongly requested from the application fields using epoxy resins in paints,

adhesives, structural materials and in electronic materials, etc. Moreover, the known fragility of cured epoxy resins causes

their destruction under the influence of kinetic load or strong impact. In particular, fatal defects occur in parts in fields of

adhesives and structural materials, etc. due to that effect.

A lot of researchers in the area of epoxy resins have made efforts to solve the problem, so far. As a result, the

compounding technologies using rubber and elastomers were developed, and some improvements were done 1-3). However,

a perfect improvement method had not been developed.

In particular, regarding the molecular design of the epoxy skeleton, revolutionary technology had been hardly made,

even though the attempts included methods of introducing an aliphatic skeleton and despite methods of introducing the

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rubber element etc.

For instance, the effect of a sufficient toughness improvement could not be confirmed by the modifying technology

using aliphatic dicarboxylic acid compound, as shown in Fig.1, despite an improvement of the elongation was observed.

Additionally, the viscosity increased greatly by the influence of the high polarity ester generated with the modifying

reaction, and the problem of flow ability etc. occurred. The alcoholic ether type epoxy resin etc. derived from ethylene

oxide modified BPA etc. could provide cured resins with high flexibility. However, because of their much lower toughness,

the cured resins cracked easily under strong load.

On the other hand, flexibility was hardly improved, though an improved effect against impact-resistance was

observed on the modifying technology using the terminal carboxylic acid rubber compounds (ex. CTBN). In addition, there

was a problem of brittlness due to the changing of properties of the rubber component after longer time at high temperatures

of 150•Ž or more.

A sufficient improvement was not yet obtained, even though flexibility and toughness improvement of epoxy resin

have been attempted in many manners for long time. Therefore, there is a high demand for the development of epoxy resins

having both of excellent flexibility and outstanding toughness, particularly for applications in the field of the electronic

industry materials, for automotive and for aircraft industries.

We had zealously researched the solution of such problems, and succeeded in the development of a novel liquid

epoxy resin that had very excellent flexibility and toughness at the same time ". In this paper, we report on molecular

design, synthesis method, and basic physical properties evaluation result in detail.

1.1 Hypothesis on molecular design and synthesis reaction

It was considered on the basis of our long-term studies that epoxy resins which satisfy the above-mentioned demand

characteristics must contain two following structure elements.

(1) A hard segment and a soft segment skeleton. (means to get both flexibility and toughness).

(2) The connecting parts between the hard segment and the soft segment skeleton must be ether bond (means to get both

flexibility and low viscosity).

1) Modified epoxy resins by aliphatic dicarboxylicacid

2) Modified epoxy resins based on dialcohol

Fig.1 Usual flexible epoxy resins that don' t accompany high toughness.

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Fig.2 shows the conceptual model of the molecular design that satisfies the above-mentioned two conditions.

First of all, it was considered that a bisphenol skeleton like bisphenol A is suitable for the hard segment, and a poly

alkylene skeleton like polyethylene glycol for the soft segment. This idea would allow achieving of the above-mentioned

(1), however it was still difficult to find the method for satisfying (2).

In order to fulfill this condition (2) we developed a unique synthetic route based on the addition of the phenolic

hydroxyl function to the vinyl ether group 7'). We expected that with the synthetic approach as shown in Scheme 1 we can

provide the requested properties.

Finally, these expectations were proven by the molecular design of the target epoxy resin described in Fig.3, its

successful synthesis, and evaluation of reaction products.

2. EXPERIMENTAL

2.1 Materials

All starting materials for the target epoxy resin: bisphenol A, epichlorohydrin (ECH), caustic soda (NaOHaq.),

triethyleneglycol divinyl ether (DVE-3) and toluene used for this synthesis, were industrial commodities without refining.

All amine compounds used as curing agents, triethylenetetramine (TETA) and polypropylene glycol diamine (D-400) were

also industrial commodities without refining.

Hard Segment (for example ; BPA)

Soft Segment (for example ; PEG)

Ether bond

(for example ; acetal)

1) Basic synthesis technology

2) Applied synthesis technology

Oligomer bisphenol modified by DVE

Fig.2 Concept model of a novel epoxy resin that can provide both of excellent flexibility and outstanding toughness.

Scheme 1 Basic and applied synthesis technology to make a novel epoxy resin with both of excellent flexibility and outstanding toughness

Fig.3 Target molecular structure of a novel epoxy resin with both of excellent flexibility and outstanding toughness

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BPA type epoxy resin (E-BPA; EPICLON 850S of DIC, epoxy equivalent of 188 g/eq.,viscosity of 13,000 mPa.s, 25 °C),

the most conventional one, and BPA-(six mole) ethylene oxide adduct type epoxy resin (E-EOBP; industrial product with

epoxy equivalent of 358 g/eq.) as the typical flexible epoxy resin, were used for the control experiments.

2.2 Methods of analysis and instrument for analysis of molecular structure "C-NMR (Made of Japan Electron Optics Laboratory Ltd., JNMFX-200, solvent: CDC13), FD-MS (made of

Japan Electron Optics Laboratory Ltd. and AX505H), and FT-IR spectrum device (Japanese spectrum Ltd., FT-IR-500,

KBr tablet method) were used for the structure analysis of the obtained compound. GPC (Made of TOSOH Ltd.,

HLC-8220 GPC, column: TSK-GEL2000Gx2, 3000G and 4000G) was used for the evaluation of the molecular weight

distribution. Perchloric acid-epoxy equivalent weight analysis method (JIS K-7236) was used for the determination of

the epoxy group content.

The viscous properties of the obtained various epoxy resins were evaluated with their viscosity measured by E type

viscosity meter.

2.3 Evaluation methods and instruments for investigations on cured resins

Epoxy resins were mixed with amine curing agent at room temperature, poured into the iron shells (65mm in the

diameter and 12mm in height), and heated for two hours at 125 °C and for two hours at 80 °C. Cured resin test pieces of

3mm thickness were obtained.

The discoid cured resin test pieces with thickness of 3mm, and diameter of 5cm were made and used for the toughness

evaluation under manual strong bent load. The test piece that flexuous of 180 degrees without cracking was given "pass" on

the evaluation of flexibility and toughness". The obtained cured resins were cut out to a prescribed dumbbell type test piece,

and their tensile strength was measured in accordance with JIS K-6301 (with the third dumbbell type).

For the evaluation of their adhesion, the epoxy resins were mixed with the amine curing agents at room temperature,

and coated on the cold-rolled steel (with 1.6mmx25mmx100mm, SPCC-SB, made by the test peace company, and

defatting with toluene). The test pieces were obtained by heating for two hours at 80 °C and for two hours at 125 °C and

for two hours at 150 °C. Their adhesion was evaluated by tensile shear test in accordance with JIS K6850. Moreover, the

same examination with an aluminum sheet (with 1.6mmx25mmx100mm, A1050P, made by the test piece company, and

deffating by toluene) was similarly evaluated.

2.4 Synthesis of DVE modified bisphenol compound (VEBP)

A target DVE modified bisphenol compound was synthesized in accordance with synthesis route shown in Scheme

2. Bisphenol A (BPA) 228 g (1.0 mole) and DVE-3 172 g (0.85 mole) were added to a 1 1 flask and stirred for six hours at

120 °C. As a result, DVE modified bisphenol compound (VEBP) with the appearance of transparent semi-solid could be

synthesized. The end point of the process is defined by the disappearance of the DVE-3 in GPC analysis.

2.5 Synthesis of DVE modified bisphenol type epoxy resin (E-VEBP)

The obtained VEBP400g (with 364 g/eq of the hydroxyl equivalent) was dissolved in 925 g of epichlorohydrin (ECH)

(10 moles) and 185 g of butanol. 122 g (1.5 moles) of aqueous sodium hydroxide solution (49 %) were added drop-wise at

65 °C within 5 hours. Then the mixture was allowed to stir for one additional hour. The generated salt was dissolved by adding

water, and the aqueous layer was dismissed by separation. Toluene (800 g) was added to the obtained crude resin after the

excess of ECH was removed (at 150 °C). The resulting organic phase was washed three times with 100 g of water, and the

inorganic salt was removed completely. Finally, toluene was removed and the target epoxy resin was obtained.

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1" step ; Synthesis of intermediate (VEBP)

2nd step ;Synthesis of epoxy resin (E-VEBP)

3. RESULTS AND DISCUSSION 3.1 Synthesis and analysis of VEBP

The reaction of BPA with DVE-3 was performed without any solvent or catalyst. The additional reaction of the

phenolic hydroxylic moietyand the vinyl ether group had been known to be reversible. The reverse-reaction (decomposition of the acetal group) was observed for this reaction, too, depending on process temperature. It was understood that the

decomposition of the acetal group became remarkable when the reaction temperature exceeded 120 °C. Therefore, the

range of 100-120 °C was chosen as the best temperature for the investigated process.

The chemical structure was identified with '3C-NMR, and it was confirmed to have the target compound described

in Fig.4. A feature spectrum distinctive for the acetal group was observed, as understood from the spectrum shown in

Fig.4. Moreover, the peak of M± = 658 that corresponds to n=1 and the one of M+ = 1088, corresponding to n=2 (based

on theoretical structural formula) were observed in the

mass spectrum. Finally, the hydroxyl equivalent weight was

measured to be 364 g/eq.

3.2 Synthesis and analysis of E-VEBP

The epoxy resin obtained according to above

described synthesis is a slightly yellow liquid. The epoxy

equivalent weight was determined to 462 g/eq. This is only

10% more than the theoretical value (420 g/eq.). The result

showed that the epoxidation reaction had progressed almost

perfectly. Interestingly, after the epoxidation reaction was

completed, the reverse-reaction of the acetal group (which

occurred at the stage of the phenol acetalization) was not

observed at all, even not at 200 °C.

That behavior of the system can be explained as

Scheme 2 Two synthesis steps to make the intermediate and the epoxy resin

Fig.4 Mass spectrum of VEBP derived from reaction of BPA

and DVE-3

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follows. As shown in Fig.5, the acetalization

reaction of bisphenol with vinyl ethers is a

reversible process. The corresponding reverse

reaction seems to occur easily under acidic

conditions in the presence of the phenolic

hydroxyl groups. However, it is considered that

this reverse-reaction was prevented completely,

because the acidic groups of the system were

fully consumed by the epoxydation of phenolic

hydroxyl moieties. Up to now, the industrial

value of the poly-phenol compound obtained by such modification reaction was low, due to its thermal instability. However,

the final epoxy resin prepared from that intermediate exhibits much higher thermal stability. It can be concluded that the

described conversion of the starting bisphenol via the acetalization step into the epoxy resin leads to novel products having

high value from the industrial point of view.

3.3 Physical properties of E-VEBP

Table 1 shows data that compares the resin properties of E-VEBP with general BPA type liquid and solid epoxy

resins (E-BPA). The viscosity was 13,000 mPa.s(25 °C) that was almost the same as that of BPA type liquid epoxy resin

(E-BPA liquid), the most typical epoxy resin. Therefore, it is understood that it is an industrially valuable resin that could

be treated and handled by the workability similar to E-BPA liquid.

Moreover, the observation that the viscosities of E-VEBP and E-BPA liquid are equal is interesting, because

E-VEBP shows larger molecular weight than E-BPA. It is considered that it is caused by the synergic effect of the large

content of flexible polyethylene glycol skeleton, which has the function of decreasing viscosity in E-DVEP, and of the very

few of the hydroxyl groups, which cause increasing viscosity due to strong hydrogen bonds.

There are some kinds of typical flexible epoxy resins on the market, which contain high content of ester bond

skeleton (e.g. containing the structure of dimmer-acid), however their workability is bad because of much higher viscosity

Table 1 Comparison between E-DVBP and conventional BPA type epoxy

resin on physical properties.

Fig.5 Reversible reaction of acetalization between phenol and vinyl ether and its stabilization by epoxidation

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caused by strong hydrogen bonds. Comparing with those, E-VEBP flexible skeleton of which is introduced through ether

group can provide both flexibility and low viscosity.

3.4 Physical properties of cured resins of E-VEBP

Its flexibility and toughness was evaluated by simple manual flexing of tests pieces prepared from cured epoxy

resins (thickness of the pieces about 3 mm).

For example, most of conventional epoxy resins, like BPA type, could not be bent because they behave very stiff

in this test. On the other hand, when conventional flexible epoxy resins were used, for instance, BPA-(six mole) ethylene

oxide adduct type epoxy resin (E-EOBP), the bend of 180-degree was possible, however the test piece was easily broken

like as being cut as shown in Fig.6. It means this experiment shows that its toughness is weak in spite of good flexibility.

On the other hand, in case of same examination with the cured resins of E-VEBP, the bend of 180-degree was possible by

weak load as shown in Fig.7, moreover, it was not broken at all. And it never cracked in course of a few tens of repeated bendings.

In addition, no change on the appearance (particularly neither blushing nor wrinkling) was observed on the test piece.

Such a big difference in toughness is very amazing, because both E-VEBP and E-EOBP are flexible epoxy resins,

which contain polyalkylene oxide (PEG) skeleton (as soft segment) as well as BPA skeleton (as hard segment).

The reason of E-VEBP's superexcellent performance is considered to be based on the synergy effect of the hard-

Fig.6 Conventional flexible epoxy resin (E-EOBP) that is easily broken

Fig.7 Cured resin of E-DVEP with both of excellent flexibility

and outstanding toughness

Fig.8 Difference between E-DVBP and E-EOBP on the cross-linking model

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segment (with the molecular mobility limited by cross-

linking points located closely to that hard segment) providing

much larger strength and the soft-segment (mobility of

which is larger because of the position far from cross-linking

point) performing excellent flexibility, shown in Fig.8. Poor

toughness of E-EOBP is considered to be due to the opposite

arrangement of the hard and soft segment positions.

The purpose of tensile shear tests was evaluating of "Flexibility and Toughness" from the viewpoint of adhesive

properties. As a result (shown in Table 2), it is understood

that E-VEBP can satisfy both high strength and large elongation.

On the other hand, the strength was much lower although the elongation of E-EOBP cured resin was similar to

E-VEBP. The result was in accordance with a bad result in the above flexing test.

4. CONCLUSION We succeeded in the development of a novel liquid epoxy resin (E-VEBP) that is able to improve the characteristic

"Stiff but brittle" drastically, So far, this property is considered as the most important weak point of current epoxy resins. The new epoxy resin was synthesized by the addition of bisphenol to vinyl ether, followed by the epoxidation of the acetal

generated on the first stage of the process. Moreover, this resin shows very low viscosity and exhibits excellent workability. In addition, it possesses very high adhesive strength. Such excellent performance was considered to be caused by the

position of the flexible skeleton based on triethylene glycol units and by a particular connection of the flexible skeleton through the special ether group (acetal).

In view of these results, it is expected that this epoxy resin can solve difficult problems in high-tech applications like

materials for printed circuit board etc.. Moreover, the possibility that various epoxy resins with versatile functions are able

to be synthesized by proper selection of other divinyl ethers and bisphenols is suggested.

REFERENCES AND NOTE 1) T.Kamon and K.Takii, Netsukoukaseijushi, 2, 140 (1981). 2) R.A.Pearson and A.F. Yee, J. Mater Sci.,24, 2571 (1989). 3) T. Ashida and T. Yamauchi, Nihonsecchakugakkaishi, 31, 171 (1995). 4) J.Heller, D.W.H.Penhale and R.F.Helwing, Journal of Polymer Science: Polymer Letters Edition, Vol.18, 293-297(1980). 5) S.Moon, K.Naitoh and T.Yamaoka, Chem. Mater., Vol.5, 1315-1320(1993). 6) H.Zhang and E.Ruckenstein, Journal of Polymer Science: Part A: Polymer Chemistry, Vol.38, 3751-3760(2000). 7) I.Ogura and N.Nobuya, JP 2004-156024. 8) I.Ogura and N.Nobuya, USP 7,087,702. 9) I.Ogura and N.Nobuya, EP 1411101.

Table 2 Comparison between E-DVBP and E-EOBP cured

resins on tensile properties

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