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Waterborne Epoxy-Acrylic Dispersions Modified by Siloxane

Kai ZhangSchool of Chemical and Energy Engineering, South China University of Technology, Guangzhou, P. R. China and School of

Chemistry and Applied Chemistry, Huanggang Normal University, Huangzhou, P. R. China

He-qing Fu, Hong Huang, and Huan-qin ChenSchool of Chemistry and Applied Chemistry, Huanggang Normal University, Huangzhou, P. R. China

Waterborne epoxy dispersions have been an active area of research since the early 1980sbecause of legislative restrictions on the use of organic solvents in conventional solvent-based products and also because they exhibit almost the same high performance levels assolvent-borne epoxy. In the present study, first the epoxy-acrylic graft copolymers (EAG)were synthesized through grafting acrylic monomers onto the epoxy resin, and then EAGwere modified by amine-siloxane through the incorporation of the latter into the former viaring-opening reaction. The graft copolymers act as an effective compatibilizer between thestyrene-acrylic resin phase and the epoxy matrix, which make the waterborne epoxy disper-

sions stable. The effects of methacrylic acid (MAA) content, siloxane content, neutralizationdegree on the particle size and the apparent viscosity of the dispersions were investigated.The structure of EAG, and siloxane-modified EAG copolymers (EAGS) was confirmed withFourier transform infrared (FTIR) spectroscopy analyses. The physical properties of the dis-persions and the physicochemical properties of the films show the dramatic improvement inproperties of waterborne epoxy dispersions due to epoxy-acrylic-graft-copolymer modifiedby siloxane.

Keywords Waterborne coatings, epoxy-acrylic-graft-copolymers, siloxane, epoxy dispersion,physicochemical properties

1 INTRODUCTION

Epoxy resins have been widely used as coatings, adhesives,and sealants because of their combined properties of toughness,

flexibility, adhesion, and chemical resistance.[1,2] However, in

order to make them tractable, it is necessary to dilute the resins

with organic solvents. Since the 1960s a substantial research

effort has been made into the development of waterborne

epoxy systems because of the limit of volatile organic com-

pounds, and some interesting methods have been proposed.[36]

One is dependent on the external emulsifier to make the resin

dispersed in water. The other is by way of chemical modifi-

cation to introduce polar groups which confer water dispersi-

bility to the resin. Woo and Toman[6,7] synthesized the

water-reducible epoxy-acrylic composite copolymers via

grafting copolymerization, later many researchers used

similar methods to prepare the waterborne epoxy dispersions,but the storage stability of the dispersions and the chemical

resistance of the films is inconsistent.[8,9] Amine-siloxanes

are known for high reactivity, unusual flexibility, high

thermal stability, excellent chemical resistance, and high cor-

rosion protective efficiency.[1014]

The conventional methods used to introduce siloxanes into

polymers involve a common blending process.[15,16] Blending

may increase the viscosity of the resin and a larger amount of

the solvent can be consumed in the preparation of the coatings;

the evaporation of the excess solvent, during and after curing,

may cause shrinkage and produce internal stresses in the

coatings, causing leaching after a certain time. In the case of

epoxy-siloxane blends, phase separation and bleeding of thesilicon component occur.[15,17] The phase separation and low

surface energy of silicone-polymer-modified epoxies hamper

their use in surface coatings.[17] Thus, to solve this problem, it

is mandatory to incorporate a siloxane moiety into the polymer

backbone through some chemical reaction.[15,18] The incorpor-

ation of siloxane into epoxy through chemical reaction

improves the aforementioned drawbacks.[15,18]

This work was financially supported by Science and TechnologyResearch Program (07BQ010), Hubei Provincial Department ofEducation.

Received 18 October 2006; Accepted 29 October 2006.

Address correspondence to Kai Zhang, School of Chemical andEnergy Engineering, South China University of Technology, Guangz-hou 510610, P. R. China. E-mail: kaizhangchem@yahoo.com.cn

Journal of Dispersion Science and Technology, 28:12091217, 2007

Copyright# Taylor & Francis Group, LLC

ISSN: 0193-2691 print/1532-2351 online

DOI: 10.1080/01932690701527896

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In this study, epoxy-acrylic graft copolymers (EAG) were

modified by amine-siloxane through the incorporation of the

latter into the former via ring-opening reaction. The graft

copolymers act as an effective compatibilizer between the

styrene-acrylic resin phase and the epoxy matrix, which

make the waterborne epoxy dispersion stable. The factors influ-

encing the particle size and the apparent viscosity of epoxy

resin dispersions were discussed in detail. The physical proper-

ties of the dispersions were evaluated. The structure of EAG

and siloxane-modified EAG (EAGS) was confirmed with

Fourier transform infrared (FTIR) spectroscopy analyses. The

physicochemical analysis of EAGS was carried out by such

instrumental methods as scanning electron microscope

(SEM), thermogravimetric analysis (TGA). The chemical

resistance of the coated samples also was investigated. The

modification of EAG through siloxanes had a synergistic

effect on the properties of the EAGS paints.

2 EXPERIMENTAL

2.1 MaterialsCommercially grade liquid epoxy resin diglycidyl ether of

bisphenol-A (E-44, Guangzhou Dongfeng Chemicals Ltd., Co.,

China) was used as basic material. Methacrylic acid, methyl

methacrylate, and styrene were used as grafted monomers.

Benzoyl peroxide was used as initiator. A blend of 2-butoxy-

ethanol and n-butanol was used as solvent. Monomers and

solvents used were of analytical grade and purchased from

Guangzhou Chemicals Factory, China. g-aminopropyl methyl

diethoxysilane (A-2100, Commercially grade, Guangzhou

Shuangjian Trade Ltd., Co.) was used as modifier. Trimethyl-

ethanolamine (A.R., Guangzhou Chemicals Factory, China)

was used as neutralization agent. Deionized water also was used.

2.2 Synthesis of Epoxy-Acrylic Graft Copolymers (EAG)

The graft copolymers were synthesized by the copolymeri-

zation of epoxy resin with grafted monomers. The copolymer-

ization was carried out at 1101158C for 6 hours using benzoyl

peroxide and a blend of 2-butoxyethanol and n-butanol as

initiator and solvent, respectively, under a nitrogen atmos-

phere. After taking out the solvent, epoxy-acrylic graft copoly-

mers were obtained.[7]

2.3 Preparation of Siloxane-Modified Epoxy-Acrylic-Graft-Copolymer Dispersions

The grafted copolymers mixture was then fed into an

agitated reducing vessel with trimethyl-ethanolamine for 15

minutes at room temperature. Then calculatedg-aminopropyl

methyl diethoxysilane was added dropwise through an

additional funnel. The system was reacted for 70 minutes at

508C. Siloxane-modified epoxy-acrylic-graft-copolymer dis-

persions with a solid content of 30 wt% were obtained by

adding calculated deionized water to above system, and the

dispersions were agitated for 1 hour. All the process was

agitated with stirrer.

2.4 Film Formation

Films were prepared with a dry thickness of about 0.5 mm.

After casting the dispersions onto glass plates (10 cm 5 cm),

the films were allowed to dry for 6 hours at 1108C in the oven.

2.5 Characterization

Formation of EAG and siloxane-modified EAG was ascer-

tained from FTIR spectra. The FTIR spectra were recorded

on the Perkin-Elmer 1730 (Perkin-Elmer Co. Ltd., USA).

The fracture surface morphology of the samples was

observed using SEM (HITACHA S-510, Japan). Prior to the

examination, the surface was coated with a thin layer of gold

in order to improve the conductivity and prevent charging.

The thermal stability of the films was assessed by TGA using

TGA-7 (Perkin-Elmer Co. Ltd., USA) at a heating rate of

108C/minutes in an inert atmosphere. The particle size of thedispersions was measured by using dynamic light scattering

(DLS, MALVERN Autosizer Lo-C, USA). The 633.7 nm

of laser wavelength and the 908 scattering angle were used.

The transparence of films was studied by using optical micro-

scope. Circumvolved Viscometer (BROOKFIELD DV-II

Viscometer Engineering /ABS. INC. MIDDLEBORO, MA

02346, USA) was also employed to measure the viscosity of

the dispersions.

3 RESULTS AND DISCUSSION

3.1 Evaluations of Physical Properties of the EAGSDispersions

3.1.1 Particle SizeThe stability of dispersions is related to the size of the

particle closely, the smaller the particle size is, the steadier

the dispersion is. Research has been conducted on the factors

influencing the particle size of the EAGS dispersions. In

summary, the particle size was mainly related to MAA

content and A-2100 content. The results were illustrated in

Tables 1 and 2.

Particle size of dispersions primary depends on hydrophili-

city which is mainly governed by the ionic group content. The

hydrophilicity of epoxy resin was greatly enhanced after

grafting because the carboxylic acid groups in MAA were

attached on epoxy resin chains. With the amount of MAA

increasing, the hydrophilicity of the modified resin was

improved, the modified resin displayed different morphology

in water, ranging from opaque cream to white-milky dispersion

and the particle size of the dispersions corresponded from large

to small. This may be due to the higher proportion of MAA,

which imparts better dispersibility of the particles of EAG

copolymer. When the amount of MAA was increased from

18 to 22.2%, the average particle size of dispersions was

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decreased from 263.9 nm to 91.7 nm. Further increase of MAA

content in modified resin didnt result in further decrease of the

average particle size. Whereas, which resulted in a decrease of

the ability of chemical resistance of the films. Therefore the

optimal amount of MAA in our system is 22.2%.The main reason for bad stability of EAG dispersions is the

opening-ring reaction of oxirane groups with carboxylic acid

groups occurs in the presence of trimethyl-ethanolamine as cat-

alyzer during the stored process. In our experiment, the amino

groups of A-2100 reacted with oxirane groups of the EAG

copolymers, which can avoid opening-ring reaction occurring

during the stored process, and also can form new crosslinking

structure. However, the average particle size of EAGS disper-

sions was decreased in a nonlinear manner with an increase of

the A-2100 content. When more A-2100 was introduced, the

particle size of the dispersions increased, and which even

made the modified resin cant be dispersed by water. This

could be due to the possibility that certain A-2100 may

undergo hydrolysis association, resulting in entanglements

and clustering of the polymer molecules. Only when the

A-2100 content was in the range of 1 to 2.5%, the neighboring

group of EAGS copolymers can shield from A-2100 hydrolyz-

ing effectively, and the EAGS dispersions have good stability.

3.1.2 Apparent Viscosity

The rheologic properties of the dispersions not only haveeffects on the construction property, but also on storage stab-

ility and films property. It is important to study the rheologic

properties of the dispersions for valuing the qualities and mas-

tering the application of the dispersions.[19]

Figure 1 described the curve of the apparent viscosity of dis-

persions via the MAA content at the same rate of shear. As can

be seen from Figure 1, the apparent viscosity of the dispersions

was improved greatly with the amount of MAA increasing.

From the viewpoint of thermodynamics, the total Gibbs free

energy of the polymers in the dispersions is constant.

The hydrophilicity of the modified resin was enhanced with

the amount of MAA increasing, which inevitably causes the

weakness of the boundary tension between modified resin and

water and the enlargement of the surface area of the modified

resin. The number of the absorbed hydrous layer was increased

with the increasing of the surface area of the modified resin,

TABLE 1

Effect of MAA content on the physical properties of EAGS copolymers dispersionsa

Samples

MAA content

(mass%)

Siloxane content

(mass%)

Mean diameter

(nm)

Compatibility

with water

Shelf life at

508C 1 month

Zk137 18.8 2 263.9 PC, milky PS

Zk131 22.2 2 91.7 C, White

translucent

E

Zk135 24.4 2 95.2 C, White

translucent

E

Zk136 26.6 2 71.1 C, Yellow

translucent

E

aC compatible; CS complete separation; E excellent; IC incompatible; NS No separation; PC partially compatible; PS

partial separation.

TABLE 2

Effect of siloxane content on the physical properties of EAGS copolymers dispersionsa

Samples

Siloxane content

(mass%)

MAA content

(mass%)

Mean diameter

(nm)

Compatibility

with water

Shelf life at 508C

1 month

Zk141 0.5 22.2 Cant test IC CS

Zk139 1 22.2 257.4 PC, milky PSZk131 2 22.2 91.7 C, White

translucent

E

Zk140 2.5 22.2 147.2 C, White

translucent

NS

Zk142 3 22.2 Cant test IC CS

a

C compatible; CS complete separation; E excellent; IC incompatible; NS No separation; PC partially compatible; PS

partial separation.

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which was equal to enlarge the volume of the disperse phase.

According the Moony theory, the larger the volume of thedisperse phase is, the greater the apparent viscosity of the disper-

sions is. In addition, the electroviscosity occurred when the dis-

persions was flowing, which required external force to overcome

the interaction between the surface charge of micelle and the ion

of the inner electric double layer, then also induced the increas-

ing of the apparent viscosity.

The chain of EAGS copolymers has many carboxylic

groups, which can be neutralized with an amine such as

trimethyl-ethanolamine and diluted with water to obtain water-

borne systems. The reaction degree of carboxylic groups with

amine groups is simply called neutralization degree. Figure 2

described the curve of the apparent viscosity of dispersions

via neutralization degree at the same rate of shear. It wasshown that the dispersions were flocculent and unstable when

the neutralization degree was below 80%, and the apparent vis-

cosity of the dispersions was almost kept unchanged when the

neutralization degree was in the range of 90% to 110%, while

the apparent viscosity of the dispersions was sharply increased

when the neutralization degree was above 110%. Such phenom-

ena indicated that the neutralization degree has a buffer effect on

the apparent viscosity of the dispersions. The reason for this is

that the dispersions of the waterborne epoxy are a heterogeneous

system. The pH value of the dispersive medium (water phase)

was changed with the neutralization degree, however, as the

surface of polymer micelle structured with electric double

layer, the hydrous layer of the surface of polymer micelle withwater was formed through hydrogen bond and Coulomb force.

There existed equilibriums (shown in Figure 3) for ions of the

surface of polymer micelle, and then buffer solution occurred

in hydrous layer, so the pH value of the hydrous layer

was kept little changed. Once the pH value in the water phase

was changed greatly, the buffer effect of the hydrous layer

was damaged. When the pH value was much little, ionization

of -COOH was prevented, the hydrophilicity of the modified

resin was so poor that the dispersions were flocculated, whilewhen the pH value was much high, the hydrophilicity group

of the modified resin was almost ionized, and the hydrous

layer was thickened, then it was easy to generate particle

network, so the apparent viscosity of the dispersions was

increased greatly.

The effect of A-2100 content on the apparent viscosity of

the dispersions also was examined (shown in Figure 4). From

Figure 4 it can be observed that the apparent viscosity of the

dispersions was reduced greatly with the amount of A-2110

increasing when the content of A-2100 was low than 2%.

The main reason is the difference of structure and polarity

between A-2100 and the EAG copolymers. As we all know

that the solvation and hydrogen bond action make the increas-ing of the dispersion viscosity. For nonpolar unit of A-2100, the

solvation and hydrogen bond action is poor. However, when

the content of A-2100 was above 2%, the hydrolysis effect of

A-2100 increased gradually, which made the increasing of

the apparent viscosity.

3.1.3 Compatibility with Water and Shelf Life

The shelf life of waterborne dispersions is a very important

characteristic, which determines their safe storage period. The

experimental sets of epoxy dispersions were kept at 508C for

one month in an incubator and any kind of phase separation

FIG. 1. Effect of the MAA content on apparent viscosity of dispersions. FIG. 2. Effect of neutralization degree on apparent viscosity ofdispersions.

FIG. 3. Equilibrium equation for ions of the surface of polymer micelle.

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was noticed. The compatibility of EAG dispersions contained

MMA content above 18.8% was quite satisfactory, but theshelf life was less than 1 week at room temperature. When

EAG copolymers were modified by siloxane, the shelf life of

the EAGS dispersions was improved greatly when the

A-2100 content was in the range of 1 to 2.5%. The results

reported in Tables 1 and 2 also revealed that the dispersions

with small particle size showed very good performance in

terms of shelf life.

3.2 Films Characteristics

3.2.1 FTIR Analysis

The formation of EAGS proceeds in two steps and is ascer-

tained from FTIR spectra. In the first step, the grafting ofacrylic monomers onto epoxy resin occurred in the presence

of free radical initiator. The formation of and

of epoxy at 1731 1607 cm21 and 1582 cm21, respectively,

are used to ascertain the completion of the reaction

(Figure 5).[7] The second step involves reaction between

oxirane group of EAG copolymers and amino group of

A-2100, which is confirmed by the disappearance of epoxy at

916 cm21

, increaseing in the intensity of hydroxyl group of

epoxy at 3420 cm21, and formation of Si-OCH2CH3 at

1100 cm21 and Si-CH3at 1256 cm21, but these peaks are not

obvious because of little A-2100 amount (Figure 6).

3.2.2 Optical Microscopy Observation

The miscibility of bisphenol-A epoxy resin with Styrene-

Acrylic copolymers (SA) is poor. Even when they were

mixed with the help of some solvent, the films made from

which, however, were whitely opaque and easily peeled off.

It showed that the blends separated into two phases after

solvent evaporated at room temperature. As the components

in our system have the similar degrees of affinity to solvent,

it is difficult to separate pure EAG copolymers from the

system with common method of precipitating separation. It

was found that the films are transparency and smooth, and

they are very difficult to be peeled off from the plates. Such

kind of phenomena indicated that the EAG copolymers inter-

connected bisphenol-A epoxy resin with Styrene-Acrylic copo-

lymer. The graft copolymers acted as an effective

compatibilizer between the styrene-acrylic resin phase and

the epoxy matrix, which leaded to the high miscibility of

FIG. 4. Effect of the siloxane content on apparent viscosity of dispersions.

FIG. 5. FTIR of EAG copolymers.

FIG. 6. FTIR of EAGS copolymers.

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network. The incorporation of siloxane into EAG system

improves the thermal stability and enhances the degradation

temperature to an appreciable extent. The delay in degradation

caused by siloxane moiety may be attributed to its ablative

behavior imparted by partial ionic nature, high bond energy

and thermal stability of -Si-O-Si- linkage.[22] Although the

oxirane groups of EAG copolymers were reacted with amino

groups of siloxane, the thermal stability of EAGS copolymerswas not decreased comparing with that of EAG copolymers.

From these observations, it was concluded that films of

EAGS copolymers showed high thermal resistivity.

3.2.5 Chemical Resistance

Films also were tested for chemical resistance by soaking of

the coated specimens in 5.0 wt% vitriol (H2SO4), 5.0 wt%

sodium hydroxide (NaOH) solutions, ethanol, ethyl acetate,

gasoline, and distilled water at room temperature (Table 4).

Table 4 showed that the chemical resistance of SA copolymers

is poor, which was quickly peeled off from the plates in

ethanol, water or 5% H2SO4, and in ethyl acetate, gasoline or5% NaOH, it also only remained for 24 hours. The chemical

resistance of EAG copolymers is better than that of SA copo-

lymers, which is attributed to the strong adhesive force to the

glass of epoxy. When the EAG copolymers were blended

with siloxane, chemical bond on the glass surface was

formed by siloxane, which improved the adhesive force

between EAG copolymers and glass. This force would

become greater when siloxane was introduced by way of

chemical reaction, so EAGS copolymers have a good chemical

resistance after curing. But the films contained silicon was

easily damaged under basic condition comparing with other

mediums, the reason might be that -Si-O- linkage is quite easyto be attacked under basic condition.

3.2.6 Water Absorption

Dried films (30 mm 30 mm; original weight designated

as W0) were immersed in water for 24 hours at 258C. After

the residual water was wiped from the films using filter

paper, the weight (W1) was measured immediately.[23]

It was calculated as follows: water absorption, R (%)

(W12W0)/W0 100 (%)Figure 9 showed that the water absorption of EAGS copoly-

mers decreased with the increasing of siloxane weight fraction,

which is attributed to the excellent hydrophobicity of siloxaneand the cross-linking characteristics of silanol, the hydrolysate

of siloxane. The water absorption of films decreases, which can

cause the size of hydrophilic microdomain turn small after

swelling, not enough to the half-wavelength of visual rays, at

TABLE 3

The results of TG analysis

SampleDiscompose temperature at different weight loss percent (8C)

number 5% 10% 20% 30% 40% 50% 60% 70% 80%

1 290.86 326.29 345.96 360.6 371.82 381.45 392.23 404.65 422.31

2 138.99 186.58 228.68 272.76 299.37 318.14 335.01 360.32 400.383 193.57 212.35 301.65 335.14 356.43 374.05 388.9 402.89 418.56

4 181.26 220.32 304.34 343.68 371.88 396.66 415.43 429.88 452.9

Note: 1: E-44; 2: SA copolymers; 3: EAG copolymers; 4: EAGS copolymers (2 wt% siloxane).

TABLE 4

The chemical resistance of different samplesa

SA

copolymers

EAG

copolymers

EAGS

copolymers

(by blending)

EAGS

copolymers

(by grafting)

Ethanol A C D EEthyl acetate B C D E

Gasoline B D D E

Distilled water A C D E

5% H2SO4 A D D E

5% NaOH B C C D

Note: A: films damaged after 3 hours; B: films damaged after 24 hours; C: films damaged after 48 hours.aD: unchanged after 24 hours, bubbly after 48 hours; E: unchanged after 48 hours.

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the level where the films could remain transparent. This charac-

teristic[24] is very important for coating materials. However,

when the weight fraction of siloxane exceeded 2.5%, thestorage stability of waterborne EAGS dispersions turned bad.

4 CONCLUSIONS

A waterborne epoxy-acrylic-graft-copolymer dispersion

modified by siloxane was synthesized. The physical properties

of the dispersions and the physicochemical properties of the

films of the EAGS copolymers proved to be very interesting.

The following conclusions were reached:

1. The EAGS copolymers act as an effective compatibilizer

between the styrene-acrylic resin phase and the epoxy

matrix, which make the waterborne epoxy dispersions

stable.2. When the MAA content was 22.2% and the siloxane

content was 2%, the waterborne EAGS copolymers disper-

sions were stable and the particle size was minimal.

The apparent viscosity of the dispersions increased

with the increasing of the MAA content, and decreased

with the increasing of the A-2100 content, whereas the

neutralization degree has a buffer effect on the apparent

viscosity of the dispersions.

3. The physicochemical properties of the films showed that the

EAGS copolymers had comparatively thermal stability to

EAG copolymers, and the mass loss was 5% at 1818C.

The films of EAGS copolymers dispersions had good

chemical and water resistance.

LIST OF ABBREVIATIONS

EAG epoxy-acrylic graft copolymers

MAA methacrylic acid

EAGS siloxane-modified EAG copolymers

FTIR Fourier transform infrared spectrometer

SEM scanning electron microscope

TGA thermogravimetric analysis

E-44 diglycidyl ether of bisphenol-A

A-2100 g-aminopropyl methyl diethoxysilone

DLS dynamic light scattering

SA Styrene-Acrylic copolymers

H2SO4 vitriol

NaOH sodium hydroxide

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