synthesis and characterization of multiple cross-linking uv-curable waterborne polyurethane...

ORIGINAL PAPER

Synthesis and characterization of multiple cross-linkingUV-curable waterborne polyurethane dispersions

Zong Niu • Fengling Bian

Received: 20 August 2011 / Accepted: 2 January 2012 / Published online: 2 March 2012

� Iran Polymer and Petrochemical Institute 2012

Abstract A series of multiple cross-linking ultraviolet

(UV) curable waterborne polyurethane dispersions (UV-

PUDs) were synthesized by modification of diglycidyl

ether of bisphenol-A-based epoxy resin (E51) through ring-

opening by 3-aminpropyltriethoxysilane (APTES). Ini-

tially, APTES-E51 was synthesized using APTES to open

the epoxy groups of E51. Then, APTES-E51 was incor-

porated into the chains of polyurethane, and multiple cross-

linking UV-PUDs were produced. The chemical structures

were confirmed by the Fourier-transform infrared spec-

troscopy (FTIR) and the effect of the APTES-E51 content

on the UV-PUDs properties was investigated. The average

particle size of UV-PUDs was determined by dynamic light

scattering (DLS). The result showed that the average par-

ticle size increased with increasing APTES-E51 content

and the stability of the UV-PUD storage diminished when

the content of APTES-E51 was 10.0%. After modification

by APTES-E51, the water absorption of the UV-cured

films decreased and the water contact angle (CA) increased

significantly. Thermogravimetry analysis (TGA) of the

UV-cured films illustrated that APTES-E51 modified UV-

curable waterborne polyurethane could exhibit good ther-

mal stability. In addition, mechanical property of the cured

films showed that the incorporation of APTES-E51 also

improved tensile strength of the cured films. We can obtain

good storage stability, satisfied water resistance, and high

thermal stability and tensile strength when the APTES-E51

content of the UV-PUD was 9.1%.

Keywords Waterborne polyurethane � Multiple cross-

linking � UV-curable � 3-Aminpropyltriethoxysilane �Epoxy resin

Introduction

In recent years, owing to fast curing speed and environ-

mental-friendliness, UV-curable waterborne technology

has been gained more and more attention and developed

fast [1–3]. It combines the advantages of waterborne

technology and UV technology. At the same time, it

overcomes some disadvantages such as poor film properties

for waterborne technology and potential skin irritation for

UV technology. Therefore, UV-curable waterborne mate-

rials are expected to substitute the solvent-based products.

As a unique kind of polymer, UV-curable waterborne

polyurethane can obtain various properties and enhance

performance by the selection of different components. And

it can be satisfactorily applied in coatings for wood, elec-

tronic materials, biologic materials, printing inks, etc.

Therefore, the UV-curable waterborne polyurethane has

become one of the most favorable materials [4–6].

Despite all the advantages, their use has been limited.

The conventional preparation of the UV-curable water-

borne polyurethane is usually through capping the linear

polyurethane (PU) prepolymer with the single-hydroxyl

acrylate, and the obtained resins cannot possess high

molecular weight. Therefore, the films are soft with un-

ideal mechanical properties and water resistance compared

with solvent-based products [7, 8].

Silicone-modified resins exhibit excellent properties

such as high-temperature stability for the excellent bond

strength of Si–O–Si, good hydrophobic behavior, scratch

resistance, etc. For many years, silicone intermediates are

Z. Niu � F. Bian (&)

College of Chemistry and Chemical Engineering, Lanzhou

University, Lanzhou 730000, People’s Republic of China

e-mail: [email protected]

Iran Polymer andPetrochemical Institute

123

Iran Polym J (2012) 21:221–228

DOI 10.1007/s13726-012-0021-6

widely used to modify polyurethane to increase its prop-

erties such as heat resistance and water resistance [9–11].

Lee et al. [12] prepared UV-curable waterborne polyure-

thane/silica nanocomposites with silica functionalized by

allyl isocyanate. With the addition of silica, swelling, ini-

tial and rubbery moduli, strengths and thermal stability of

the film have been improved.

Epoxy resin is well known for its unique properties,

including high mechanical strength, heat resistance, tough-

ness, etc. [13, 14]. Chen et al. [15] synthesized polyurethane/

polyacrylate/epoxy resin latex interpenetrating networks

which possessed high water resistance and tensile strength

when the epoxy resin content was 10%. Therefore, the per-

formance of polyurethane can be improved by the employ-

ment of silicone intermediate and epoxy resin.

The properties of polyurethane film are affected not only

by the components but also by the molecular weight and

structure of the polyurethane [16]. For example, high cross-

linking density has a positive effect on the thermal stability

and mechanical property of PU film [17].

In this work, we used 3-aminpropyltriethoxysilane

(APTES) to open the epoxy ring of bisphenol-A-based epoxy

resin (APTES-E51) and incorporated it into the chains of

polyurethane to synthesize a series of multiple cross-linking

UV-curable waterborne polyurethane dispersions (UV-PUDs).

Because the APTES-E51 is a kind of multifunctional

oligomer, it can act as a chain extender during the preparation

of UV-PUDs. It can increase the nonlinear structure of dis-

persions. In addition, APTES can hydrolyze into silanol in

aqueous environment and form Si–O–Si linkages by self-

condensation reaction of silanol. Therefore, the high cross-

linking density films can be obtained by the incorporation of

APTES-E51. Moreover, it can also combine the advantages

of APTES and epoxy resin. It is expected that such material

can possess better water resistance, high thermal stability and

mechanical property.

Experimental

Materials

Isophorone diisocyanate (IPDI, Shanghai Rongrong Chem-

ical Co. Ltd., China) and poly (ethylene glycol) (PEG,

Mn = 800, Shanghai Yuanbang Resin Industries Co. Ltd.,

China) were dried and degassed at 110 �C under vacuum for

2 h before using. Dimethylol propionic acid (DMPA;

Chengdu Xiya Chemical Co. Ltd., China) was dried at 50 �C

for 48 h. Bisphenol-A-based epoxy resin (E51, the average

epoxy value of 0.51 mol/100 g, Shenzhen Jiadida Chemi-

cal Co. Ltd., China), pentatry thritol triacrylate (PETA,

Shanghai Meida Chemical Reagent Co. Ltd., China),

3-Aminopropyltriethoxysilane (APTES, Nanjing Xiangqian

Chemical Co. Ltd., China) were used as they received.

2-Hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-

propanone (Irgacure 2959, Ciba Specialty Chemicals Inc.)

was used as a photo-initiator. Dibutyltin dilaurate (Shanghai

Rongrong Chemical Co. Ltd., China) was used as a catalyst.

p-Methoxyphenol (Luoyang Reagent Co., China) was used

as an inhibitor. Triethylamine (TEA, Tianjing Chemical

Reagent Factory, China) and acetone (Tianjin Chemical

Reagent Factory, China) were used as they received.

Synthesis of UV-PUDs

The UV-PUD was synthesized as follows: first of all, E51

and APTES were added into a round-bottom four-necked

flask equipped with a mechanical stirrer, a thermometer and a

condenser with drying tube, and reacted at room temperature

for 3 h under N2 atmosphere. APTES-E51 was obtained at

this step. Then, IPDI and PEG were added and reacted at

85 �C until the NCO content reached a theoretical value A.

Subsequently, DMPA was added and reacted at 70 �C until

the NCO content reached a theoretical value B. Acetone was

added to reduce the viscosity during the process. PETA,

DBTDL and p-methoxyphenol were added and reacted at

60 �C for 3–4 h. As neutralization agent, TEA was added to

neutralize carboxylic groups at 50 �C and the mixture was

stirred for 1 h. Finally, distilled water was added at high

speed shearing for 0.5 h to disperse the polymer.

Then, UV-PUD was obtained after removal of the ace-

tone from the dispersion by rotary vacuum evaporation.

The pH value and solid content of the dispersions were 7–8

and 35%, respectively. A series of UV-PUDs with different

APTES-E51 content were synthesized. The preparation

procedure and basic formulation based on mole ratio are

given in Scheme 1 and Table 1, respectively.

The theoretical value of the NCO content was calculated

according to the following formula [18]:

NCO% ¼ ðMNCO �MOHÞ � 42

Wall

� 100% ð1Þ

where, MNCO is the mole number of NCO group, MOH is

the mole number of OH group, Wall is the weight of all the

compositions and 42 is the molecular weight of NCO

group. The real NCO content is measured by the standard

di-n-butylamine titration method.

Preparation of UV-PUDs films

An amount of 3 wt% of photo-initiator Irgacure 2959 with

respect to the solid content of the dispersion was added into

the dispersion. Then, the formulated dispersion was cast onto

a glass plate at room temperature and dried at 60 �C in an

oven to reach a constant weight. Subsequently, the film was

cured under mercury lamp (the UV dose is 125 mw/cm2).

222 Iran Polym J (2012) 21:221–228


123

Structure characteristics of UV-PUDs

The structure of dispersions was characterized on an

Avater-360 Fourier-transform infrared spectrometer (Nicolet

Instrument Corporation, USA).

Acceleration test of storage stability

Acceleration test was carried out as follows: the disper-

sions were put in an oven at 60 �C for 72 h and then

transferred into a refrigerator at 0 �C for 72 h, then again to

the oven at 60 �C for another 72 h. The above steps were

repeated three times.

Measurement of the average particle size

The average particle size of UV-PUDs was determined by

dynamic light scattering (DLS) equipment (Coulter-LS

230). The dispersions were diluted by distilled water, and

then were homogenized by ultrasonication.

Scheme 1 Preparation process

of UV-PUDs

Table 1 Compositions

of UV-PUDs

The mole ratio of TEA/DMPA

was fixed at 1.1/1 to secure

100% neutralization

Samples APTES-E51 (wt%) IPDI/PEG/APTES-E51 DMPA PETA TEA

UV-PUD0 0 1/0.38/0 0.34 0.23 0.37

UV-PUD1 3.6 1/0.34/0.04 0.34 0.23 0.37

UV-PUD2 6.2 1/0.31/0.07 0.34 0.23 0.37

UV-PUD3 9.1 1/0.28/0.10 0.34 0.23 0.37

UV-PUD4 10.0 1/0.27/0.11 0.34 0.23 0.37

Iran Polym J (2012) 21:221–228 223


123

Measurement of water resistance

The water resistance property was measured by immersing

the UV cured films in distilled water at room temperature

and the absorbed water content (W) was calculated by the

following equation:

Wð%Þ ¼ mt � md

md

� 100% ð2Þ

where, md is the weight of the dried film, mt is the weight of

the film at various immersion times and is determined after

wiping off the surface water. When swelling reached

equilibrium, the W(%) value reached a maximum known as

maximum water absorption (Wm). The results were the

mean values of five measurements.

Measurement of contact angle

The contact angle of water on the film was measured at

room temperature by a contact angle goniometer and the

final results are the mean values of five measurements.

Measurement of thermal stability

The thermal analysis was measured by TG/DTA 7300

(Seiko Electronic Nano Science and Technology Co. Ltd.,

Japan) from room temperature to 700 �C and 10 �C/min

heating rate in N2 atmosphere.

Measurement of tensile property

Tensile property was measured using a tensile tester

(RHD-2 A, Shanghai Ruihong Mechanical and Electrical

Technology Co. Ltd., China) at a crosshead of 500 mm/

min at room temperature. All results are the average of five

runs at least.

Results and discussion

Analysis of chemical structure

The preparation procedure of the UV-PUDs is given in

Scheme 1 and their chemical structures are confirmed by

FTIR. Figure 1a shows the FTIR spectrum of APTES-E51.

The typical absorption peaks at 3,360 and 1,364 cm-1 are

assigned to the N–H and C–N stretching vibration

absorption peaks, respectively. The absorption peaks at

1,079, 957 and 830 cm-1 correspond to the Si–O,

–OCH2CH3 and Si–C stretching vibration absorption peaks

in APTES, respectively. And the absence of absorption

peak at 914 cm-1 illustrates that epoxy groups had been

completely opened by APTES.

As an example, the FTIR spectrum of UV-PUD3 sample

is shown in Fig. 1b. The typical absorption peaks at 3,330,

2,873–2,951, 1,715, 1,637 and 1,359 cm-1 correspond to

the N–H, C–H (CH2 and CH3), C=O, C=C and C–N

stretching vibration bands, respectively, and the peak at

1,536 cm-1 correspond to the N–H bending vibration band.

The absorption peaks at 838 and 1,102 cm-1 correspond to

the Si–C, Si–O–Si stretching vibration bands, respectively.

The absorption peak at 954 cm-1 is much weaker than that

in Fig. 1a which is due to hydrolysis of ethoxy groups

of APTES and condensation of silanol groups forming

Si–O–Si linkage. Absence of absorption peak at 2,270 cm-1

implies that NCO groups had been reacted completely [19].

It could be said that the UV-PUDs has been synthesized

successfully.

Characteristics of UV-PUDs

Effect of APTES-E51 content on the particle size of UV-

PUDs is shown in Fig. 2 and the characteristics of UV-

PUDs are shown in Table 2. It can be included that when

the content of APTES-E51 increased from 0 to 10%, the

average particle size of the dispersions increased from 82.9

to 224.3 nm, correspondingly. The first reason is that when

the APTES-E51 is incorporated into polyurethane chains,

the branched structures of dispersions increase due to its

polyfunctionality. In addition, APTES can hydrolyze into

silanol in aqueous environment and form Si–O–Si linkage

by polycondensation reaction of silanol which would lead

to the cross-linking of particles. It may also be ascribed to

the hydrophobicity of Si–O–Si linkage [19, 20]. Therefore,

the average particle size increased with increasing APTES-

E51 content. However, it was found that the transparency

decreased with increasing the APTES-E51 content and

the dispersion was layered after acceleration test, when

the content of APTES-E51 was 10.0%. Therefore, the

Fig. 1 FTIR spectra of a APTES-E51 and b UV-PUD3

224 Iran Polym J (2012) 21:221–228


123

Fig. 2 Effect of APTES-E51 content on particle size of UV-PUD samples: a UV-PUD0, b UV-PUD1, c UV-PUD2, d UV-PUD3, and e UV-PUD4

Table 2 Characteristics

of UV-PUDsSamples APTES-E51

(wt%)

Average particle size

(nm)

Appearance

Before test After acceleration test

UV-PUD0 0 82.9 Transparent Transparent

UV-PUD1 3.6 103.7 Transparent Transparent

UV-PUD2 6.2 122.6 Sub-transparent Sub-transparent

UV-PUD3 9.1 164.6 Sub-transparent Sub-transparent

UV-PUD4 10.0 224.3 Milky Layered

Iran Polym J (2012) 21:221–228 225


123

maximum content of APTES-E51 was restricted to 9.1 wt%

to obtain good storage stability.

Water resistance of the cured films

Compared with the solvent-based products, the poor water

resistance of the cured films of UV-PUDs is one of the

major disadvantages. Improvement of water resistance of

the cured films has attracted extensive attention. Correla-

tion between the water absorption and the soaking time of

the cured films was investigated as shown in Fig. 3. It can

be seen that the water absorption of the cured films

increases with the soaking time and then levels off. In

addition, the water absorption decreases with increasing the

APTES-E51 content. It may be ascribed to the increase in

cross-linking density of the cured films and hydrophobic

Si–O–Si linkages with increasing the APTES-E51 content.

The values of the maximum water absorption (Wm) are

obtained from Fig. 3 and the results are shown in Table 3.

Table 3 illustrates the contact angle and the maximum

water absorption (Wm) of the cured films, as well. It was

found that the maximum water absorption decreased with

increasing the APTES-E51 content, but the contact angle

increased. As Si–O–Si linkages have a much lower surface

energy than polyurethane, the former may migrate to the

surface layer during the film-forming and UV-curing which

result in a film with low surface energy [21]. Therefore,

the water contact angle of the films increased with the

raise of APTES-E51 content. As mentioned above, the

incorporation of APTES-E51 has a positive effect on the

water resistance.

Thermal analysis of the cured films

Thermal stability of the cured films is evaluated by TGA

and the results are shown in Fig. 4. It was found that

incorporation of APTES-E51 enhanced the decomposition

temperature and improved the thermal stability. As is

known, thermal decomposition of polyurethane chains has

at least two stages. The initial stage is mainly the decom-

position of hard segment which involves the urea and

urethane groups. Second stage is the decomposition of soft

segment which involves the polyester and polyether [22].

Generally speaking, the decomposition temperature for

normal polyurethane is at about 240 �C [23] but for the

UV-PUD films, the initial decomposition temperature is

higher than 270 �C. It can be seen in Fig. 4 that the initial

decomposition temperature of the samples is in the order

of UV-PUD0 \ UV-PUD1 \ UV-PUD2 \ UV-PUD3. The

UV-PUD3 degrades at 295 �C while the UV-PUD0

degrades at 271 �C.

Final decomposition temperature and final residue mass

are also different. For the UV-PUD0 sample, the decom-

position terminates at 442 �C and the final residue is 3.5%.

For UV-PUD3, the decomposition ends at 487 �C and the

final residue is 9.2%. The higher APTES-E51 content, the

better is the thermal stability of the film. This difference

may be attributed to the increased cross-linking density due

Fig. 3 Plots of water absorption versus soaking time

Table 3 Performance of the

cured films

SD Standard deviation

Samples APTES-E51 (wt%) Contact angle Maximum water absorption (Wm)

Degree SD % SD

UV-PUD0 0 62.2 0.16 11.6 0.18

UV-PUD1 3.6 66.6 0.21 9.2 0.26

UV-PUD2 6.2 73.3 0.14 6.5 0.30

UV-PUD3 9.1 80.8 0.12 4.7 0.22

Fig. 4 Thermogravimetric curves of the cured films

226 Iran Polym J (2012) 21:221–228


123

to the increased nonlinear structures of the dispersions and

formation of Si–O–Si linkages.

Mechanical property of the cured films

As is known that the epoxy resin has excellent mechanical

strength owing to plenty of rigid benzene rings, the

incorporation of APTES-E51 into the chains of polyure-

thane is likely to increase the tensile strength of the cured

films. Figure 5 shows the correlation between the tensile

property and APTES-E51 content. It can be seen that

tensile strength of the cured films increases with increasing

the APTES-E51 content but the extensibility decreases. In

addition, the high cross-linking density due to the incor-

poration of APTES-E51 increases the tensile strength.

Conclusion

A series of multiple cross-linking UV-PUDs modified by

E51 were synthesized through ring-opening by APTES. On

the one hand, the high cross-linking density can be

obtained with the incorporation of APTES-E51. On the

other hand, such material also can combine the advantages

of APTES and epoxy resin. Consequently, the performance

of films with regard to water resistance, thermal stability

and mechanical property can be improved after modifica-

tion with APTES-E51. The average particle size of the

dispersions increased with increasing APTES-E51 content.

Maximum content of APTES-E51 was controlled at 9.1%

to obtain good storage stability. After modification, water

contact angle increased and water absorption of the cured

films decreased significantly. Investigation of thermal sta-

bility of the cured films indicated that it was raised with the

increase of APTES-E51 content. Study of mechanical

property illustrated that the incorporation of APTES-E51

had a positive effect on the tensile strength of the cured

films. Therefore, we can obtain good storage stability,

satisfied water resistance, high thermal stability and tensile

strength when the APTES-E51 content was 9.1%. It is

hopeful that this work will provide a fundamental basis for

the industrial applications.

Acknowledgments This research is supported by the Fundamental

Research Funds for the Central Universities (lzujbky-2010-32).

References

1. Otts DB, Heidenreich E, Urban MW (2005) Novel waterborne

UV-crosslinkable thiol–ene polyurethane dispersions: synthesis

and film formation. Polymer 46:8162–8168

2. Van den Berg KJ, Van der Ven LGJ, Van den Haak HJW (2008)

Development of waterborne UV-A curable clear coat for car

refinishes. Prog Org Coat 61:110–118

3. Chen S, Sui J, Chen L, Pojman JA (2005) Polyurethane–nano-

silica hybrid nanocomposites synthesized by frontal polymeri-

zation. J Polym Sci Pol Chem 43:1670–1680

4. Yang ZL, Wicks DA, Hoyle CE, Pu HT, Yuan JJ, Wan DC, Liu

YS (2009) Newly UV-curable polyurethane coatings prepared by

multifunctionathiol- and ene-terminated polyurethane aqueous

dispersions mixtures: preparation and characterization. Polymer

50:1717–1722

5. Lee SK, Kim BK (2009) High solid and high stability waterborne

polyurethanes via ionic groups in soft segments and chain ter-

mini. J Colloid Interface Sci 336:208–214

6. Lee HT, Wu SY, Jeng RJ (2006) Effects of sulfonated polyol on

the properties of the resultant aqueous polyurethane dispersions.

Colloids Surface A 276:176–185

7. Pichavant L, Coqueret X (2008) Optimization of a UV-curable

acrylate-based protective coating by experimental design. Prog

Org Coat 63:55–62

8. Kim YB, Kim HK, Yoo JK, Hong JW (2002) UV-curable poly-

urethane dispersion for cationic electrodeposition coating. Surf

Coat Technol 157:40–46

9. Pathak SS, Khanna AS (2009) Investigation of anti-corrosion

behavior of waterborne organosilane–polyester coatings for

AA6011 aluminum alloy. Prog Org Coat 65:288–294

10. Easton T, Poultney S (2007) Waterborne silicone-organic hybrid

coatings for exterior applications. J Coat Technol Res 4:187–190

11. Dhoke SK, Bhandari R, Khanna AS (2009) Effect of nano-ZnO

addition on the silicone-modified alkyd-based waterborne coat-

ings on its mechanical and heat-resistance properties. Prog Org

Coat 64:39–46

12. Lee SK, Yoon SH, Chung I, Hartwig A, Kim BK (2011)

Waterborne polyurethane nanocomposites having shape memory

effects. J Polym Sci Pol Chem 49:634–641

13. Li Y, Mao S (1996) Study on the properties and application of

epoxy resin/polyurethane semi-interpenetrating polymer net-

works. J Appl Polym Sci 61:2059–2063

14. Vabrik R, Czajlik I, Tury G, Rusznak I, Ille A, Vig A (1998) A

study of epoxy resin–acrylated polyurethane semi-interpenetrat-

ing polymer networks. J Appl Polym Sci 68:111–119

15. Chen L, Chen S (2004) Latex interpenetrating networks based on

polyurethane, polyacrylate and epoxy resin. Prog Org Coat

49:252–258

16. Krol P (2007) Synthesis methods, chemical structures and phase

structures of linear polyurethanes. Properties and applications of

linear polyurethanes in polyurethane elastomers, copolymers and

ionomers. Prog Mater Sci 52:915–1015

Fig. 5 Mechanical properties of the cured films versus APTES-E51

content

Iran Polym J (2012) 21:221–228 227


123

17. Bai CY, Zhang XY, Dai JB, Li WH (2006) A new UV curable

waterborne polyurethane: effect of C=C on the film properties.

Prog Org Coat 55:291–295

18. Stagg HE (1946) A method for the determination of isocyanates.

Analyst 71:557–559

19. Lai XJ, Li XR, Wang L, Shen YD (2011) Synthesis and char-

acterizations of waterborne polyurethane modified with 3-ami-

nopropyltriethoxysilane. Polym Bull 65:45–57

20. Jeon HT, Jang MK, Kim BK, Kim KH (2007) Synthesis and

characterizations of waterborne polyurethane–silica hybrids using

sol–gel process. Colloids Surface A 302:559–567

21. Bai CY, Zhang XY, Dai JB (2007) Synthesis and characterization

of PDMS modified UV-curable waterborne polyurethane disper-

sions for soft tact layers. Prog Org Coat 60:63–68

22. Lin M-F, Shu Y-C, Tsen W-C, Chuang F-S (1999) Synthesis of

polyurethane-imide (PU-imide) copolymers with different dian-

hydrides and their properties. Polym Int 48:433–445

23. Bai CY, Zhang XY, Dai JB, Wang JH (2008) Synthesis of UV

crosslinkable waterborne siloxane–polyurethane dispersion

PDMS-PEDA-PU and the properties of the films. J Coat Technol

Res 5:251–257

228 Iran Polym J (2012) 21:221–228


123

synthesis and characterization of multiple cross-linking uv-curable waterborne polyurethane...

Documents