Wn

SKN

a

ARRAA

KWcCHH

1

rrOistht

hlolsihtamci

0h

Progress in Organic Coatings 76 (2013) 729– 735

Contents lists available at SciVerse ScienceDirect

Progress in Organic Coatings

jou rn al h om epage: www.elsev ier .com/ locate /porgcoat

aterborne polyurethane-acrylic copolymers crosslinked core–shellanoparticles for humidity-sensitive coatings

hao-Fei Zhang, Rong-Min Wang ∗, Yu-Feng He, Peng-Fei Song, Zhan-Min Wuey Laboratory of Eco-Environment-Related Polymer Materials of Ministry of Education, Key Laboratory of Polymer Materials of Gansu Province, Institute of Polymer, Northwestormal University, Lanzhou 730070, China

r t i c l e i n f o

rticle history:eceived 25 October 2012eceived in revised form 2 January 2013ccepted 3 January 2013vailable online 24 January 2013

a b s t r a c t

A novel crosslinked core–shell emulsion of waterborne polyurethane-acrylic copolymers (PUA) was suc-cessfully synthesized by emulsion polymerization. The average particle size of the PUA particle wasapproximately 130 nm and its core–shell morphology was proved with transmission electron microscopy(TEM) and dynamic light scattering (DLS), whose structure was also confirmed by FT-IR and TGA. PUA wasapplied to prepare the humidity controlling coatings (PUA-C) by compositing with pigments and porous

eywords:aterborne polyurethane-acrylic

opolymersrosslinked core–shell structureumidity sensitivity

fillers. The structure and properties of humidity controlling coatings were investigated, with particularattention to the effects of the humidity controlling. The surface morphology of the PUA-C was observedby scanning electron microscope (SEM). The humidity controlling coatings showed excellent propertiesof humidity sensitivity and humidity retention.

© 2013 Elsevier B.V. All rights reserved.

umidity controlling coatings

. Introduction

Indoor humidity is an important influence on our lives. Too highelative humidity (RH > 60%) could cause the metal surface cor-osion, electrical insulation fall, material deformation and so on.n the contrary, when the moisture content is too low (RH < 40%),

t causes skin chapping, respiratory system resistance decreased,tatic electricity, etc [1,2]. Thus, it is known that the humidity con-rolling becomes particularly important [3,4]. In recent years, theumidity controlling materials have been paid more attention forheir moisture absorption and desorption properties [5,6].

Generally, the humidity controlling materials include inorganicumidity controlling materials, organic polymer humidity control-

ing materials and composite humidity controlling materials. Mostf inorganic humidity controlling materials are internal porous,arge specific surface area and good adsorption ability, but it is nottable at normal temperature, extremely easy to produce the salt-ng out and then pollutes the environment [7,8]. Organic polymerumidity controlling materials have high absorption ability, butheir response to water is slow and their desorption ability is poors well [9]. The polymer/inorganic composite humidity controlling

aterials show high water absorbing capacity and water retention

apacity. The humidity controlling coating was one of compos-te humidity controlling materials [10,11]. We had synthesized

∗ Corresponding author. Tel.: +86 931 7970358; fax: +86 931 7970358.E-mail address: wangrm@nwnu.edu.cn (R.-M. Wang).

300-9440/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.porgcoat.2013.01.003

acrylate-based copolymers emulsion (PA) and applied to preparehumidity controlling coating [12]. PA coating showed good waterresistance and excellent humidity controlling activities. However,there were some disadvantages, i.e., the water resistance and scrubresistance should be increased. As waterborne polyurethane emul-sion (PU) shows good performances, like high water absorption,low temperature resistance, good flexibility, excellent abrasionresistance and high gloss [13]. We considered that combiningpolyacrylic emulsion with waterborne polyurethane emulsioncould overcome the shortcomings of each other. The water basedresin, which can be prepared with low cost, has good abrasionresistance, excellent flexibility and high tensile strength. In thisarticle, waterborne polyurethane-acrylic copolymers crosslinkedcore–shell emulsion (PUA) was synthesized and applied to preparethe humidity controlling interior wall coatings (PUA-C). After mea-suring the basic properties and surface morphology of the PUA-C,its properties of humidity controlling and humidity retention werealso investigated.

2. Experimental

2.1. Materials

Butyl acrylate (BA), methyl methacrylate (MMA) (Shang-

dong, Qilu Petrochemical Kaitai Industry Co.) and hydroxyethylmethacrylate (HEMA) (Wuxi Yangshi Sanlian Chemical Co.) werewashed with 10% sodium hydroxide solution to remove inhibitorsand then washed with deionized water to remove the base residual.

7 rgani

Tpdt((swbCaCw

2c

(

(

(

2(

paafiw

2

gs(

S

30 S.-F. Zhang et al. / Progress in O

he washed monomers were dried over calcium chloride. All theurified monomers were stored at −2 ◦C until used. Isophoroneiisocyanate (IPDI), polypropylene glycol (PPG) (Shanghai Crys-al Pure Industrial Co.), 2,2-dimethylolpropionic acid (DMPA)Shanghai Biological Engineering Co.), dibutyltin dilaurate (DBTDL)Changzhou Cary Technology Co.), potassium persulfate (KPS) andodium hydrogen carbonate (NaHCO3) were received analyticallyithout further purification. And the talc, montmorillonite, kaolin,

entonite, diatomite (CD02), diatomite (CD05) (Guangzhou Tiantaihemical & Light Industry Co.), natural crystalline calcium carbon-te and precipitated calcium carbonate (Lanzhou Shuangshuanghemical Industry Co.) were all industrial grade. Distilled wateras used throughout the experiments.

.2. Preparation of polyurethane-acrylic copolymers (PUA)rosslinked core–shell emulsion

1) Preparation of NCO-terminated polyurethane (NCO-PU) prepoly-mer. A mixture of core monomers solution consisting of IPDI,PPG, DMPA in acetone was added to a four-neck flask (250 mL)equipped with a thermometer, a reflux condenser, droppingfunnels, a mechanical stirrer and nitrogen gas inlet. After addingthe catalyst (DBTDL), the system was dept over a period of 3.5 hat 80 ◦C. Then, TEA, as a neutralization agent, was added andstirred for 30 min at 60 ◦C to react with carboxyl group in theside chain of prepolymer, which afforded the core solution ofNCO-terminated polyurethane (NCO-PU) prepolymer.

2) Grafting of HEMA to the NCO of PU. Firstly, an amount of HEMAwas added to the core solution of NCO-PU prepolymer. Then,the mixture was stirred at 60 ◦C for 30 min, during which thehydroxyl groups of HEMA could react with NCO group of PUcore. Finally, high speed shearing was used to emulsify thesolution after adding an amount of distilled water into the reac-tion system. HEMA-terminated polyurethane was obtained bygrafting of HEMA to the NCO.

3) Preparation of PUA. HEMA-terminated polyurethane (HEMA-PU), SDS and Tween-20 aqueous solution were added intothe four-neck flask, while NaHCO3 was used to maintain pHvalue. A mixture of shell monomers consisting of MMA andBA was placed in a dropping funnel, and KPS aqueous solu-tion was placed in another dropping funnel, simultaneously.The KPS solution and shell monomers were added into theabove-mentioned system dropwise over a period of 3 h. Themixture was stirred at 80 ◦C for a further 2 h to ensure completemonomer conversion. Finally, the PUA was obtained.

.3. Preparation of the polyurethane-acrylic copolymers coatingPUA-C)

The humidity controlling coating (PUA-C) was prepared by PUA,igments, porous fillers and distilled water. Firstly, the pigmentsnd fillers were dispersed in water with stirring. Then, the PUA wasdded. The mixture was ground for a period of 1 h with an attritorlled with glass balls of 0.3 mm in diameter at room temperature,hich afforded the humidity controlling coating (PUA-C).

.4. Measurements of the PUA emulsion and coating (PUA-C)

The solid content and the final conversion were measured by theravimetric method. The solid content (Solid %) and overall conver-ion (X), respectively, were calculated by the following formulas

Eqs. (1) and (2)):

olid% = Wde

We× 100 (1)

c Coatings 76 (2013) 729– 735

X = (solid%/100) − (W1 + W2 + W3 + W4)Wm

(2)

where solid% is the solid content, Wde is the weight of dry emulsion,We is the weight of emulsion; and W1, W2, W3, W4, and Wm are theweight fractions of KPS, NaHCO3, compound emulsifier, dibutyltindilaurate, and initial monomer respectively, in the feed for eachsample.

The filterable solids obtained from each run were dried. Thecoagulum contents were then calculated according to Eq. (3):

Coagulation% = Mf

(solid% × Ms + Mf)× 100 (3)

where Mf and Ms are the weights of dried filterable solids and thefiltered emulsion, respectively.

The average particle size and particle size distribution of theemulsion particles were measured by dynamic light scattering(DLS) (Nano series, Malvern Instruments Ltd., UK) at 25 ◦C, and thesamples were highly diluted (<0.01%) to prevent multiple scatteringbefore tested. The micrographs of the PUA were observed by JEM-1230 transmission electron microscope (TEM) from JEOL, at 200 kV.For physical and mechanical characterization, coatings with a wetthickness of 100 �m were cast on a clean glass and galvanized ironplate at ambient temperature (25 ◦C) for 48 h. The morphology ofthe glass samples were observed with a scanning electron micro-scope (SEM), type JSM-5600LV after coated with gold. The films ofthe PUA were prepared by drying its resin at room temperature.The FT-IR spectra were detected by Digital FTS3000 spectrometer.The TGA was performed with a Pyris Diamond (Perkin Elmer) underthe nitrogen atmosphere at a heating rate of 10 ◦C/min from 20 to750 ◦C.

3. Results and discussion

3.1. Preparation of the crosslinked core–shell polymer particles(PUA)

The blending of polymer particles is a common approach tocreate waterborne nanocomposites [14]. However, in most cases,incompatibility between the polymers causes their phase sepa-ration, which hindered the desired synergy during application ofthe obtained product. In past decades, the polyurethane-acrylicsystem has been paid attention with the formation of inter-penetrating networks [15,16] and core–shell structure [17,11].Crosslinked polyurethane dispersions based on hydroxylatedpolyesters were prepared and their film properties were studied[18], but the dispersion of it was bad. Recently, the crosslinkedcore–shell structure between polyurethane and 3-aminopropyltriethoxysilane, pyromellitic dianhydride, N-hydroxyphthalimidewas reported [19]. It was also reported that the crosslinkedcore–shell structure between polyurethane and polyacrylate, fluo-rinated acrylic could improve the surface tension of latexes, thesurface free energy and thermal property of films [20,21]. How-ever, most of reports emphasized the preparation and propertiesof film. There were few reports about the application of hybridparticles.

Here, we focus on the application of crosslinked core–shellnanoparticles by preparing functional copolymers. We used HEMA,MMA and BA as functional monomers because they were crosslink-ing agent, hard monomer and soft monomer respectively. Theacrylic copolymer shell was grafted on polyurethane core, whichformed stable hybrid particles. With adjusting the proportion of

MMA and BA, we got the optimized PUA emulsion, which could beused to prepare the smooth and transparent PUA film. The obtainedPUA emulsion was applied to prepare the humidity controllingcoatings by compositing with pigments and porous fillers. The

S.-F. Zhang et al. / Progress in Organic Coatings 76 (2013) 729– 735 731

d core–shell (PU/PA) polymer particles.

sptr(p(mmnTwsetlo

sccw

3

(aoepwcir

TTo

0

50

100

150

200

250

0 50 100 150 200

wat

er a

bsor

ptio

n/%

PA:PU=1PA:PU=2PA:PU=3PA:PU=4PA:PU=5

Fig. 1. Formation of the crosslinke

uggested mechanism of forming the crosslinked core–shellolymer particles (PUA) was described in Fig. 1. Firstly, A NCO-erminated polyurethane (PU) prepolymer was synthesized witheaction of isocyano group ( NCO) in IPDI and hydroxy group

OH) in PEG and DMPA. Secondly, vinyl group was grafted to PUrepolymer by surface isocyano group reacting with hydroxy groupOH) in hydroxyethyl methacrylate (HEMA). Finally, the acryliconomers (MMA, BA) were copolymerized with hydroxyethylethacrylate on the surface to form PA shell. Waterborne PU acted

ot only as macromonomers but also as macromolecular emulsifier.he aqueous solution of KPS was dropped into the vessel at 80 ◦Cithin 2–3 h. During the radical polymerization, the crosslinked

tructure was formed between the core and shell by the copolym-rization of vinyl groups at the end of PU molecular chains andhe vinyl monomers. After completing copolymerization, hybridatexes of PUA possessing crosslinked core–shell structure werebtained.

In order to get the optimum reaction conditions, effect ofhell/core rate, effect of MMA/BA rate, Tween-20/SDS rate, andontent of HEMA had been investigated. PUA and its coating wereharacterized by TEM, SEM, DLS, FT-IR and TG/DTA. Their propertiesere also measured.

.1.1. Effect of shell/core rateIn order to investigate the influence of acrylic copolymer

PA)/PU rate on properties of emulsion and film, the particle sizend coagulum content of PUA emulsion, and water resistancef PUA film were measured. A small amount of obtained PA/PUmulsion was brushed on a clean glass, and placed at ambient tem-erature for 2 days to determine water resistance (Table 1) and

ater absorption (Fig. 2) of PA/PU films. It showed that the parti-

le size and coagulum content of PUA emulsion increased with thencreasing PA/PU rate. In Table 1, it also indicated that the wateresistance of PA/PU film was normal if PA/PU was more than 3.

able 1he influence of PA/PU rate for particle size, coagulum contents and water resistancef PUA.

PA/PU Particle size (nm) Coagulum (%) Water resistance (24 h)

1 105 0.3 Yellow2 112 0.5 Light yellow3 126 0.7 Normal4 258 4 Normal5 465 11 Normal

Time/min

Fig. 2. Effect of PA/PU rate on water absorption.

The water absorption of PA/PU film was also measured (Fig. 2). Itshowed that the water absorption of PA/PU film decreased withincreasing PA/PU rate. The best rate of PA/PU was 3/1 and the bestwater absorption of PA/PU film was 156%.

3.1.2. Effect of MMA/BA rateHard monomer (MMA) and soft monomer (BA) have great influ-

ence on gloss and hardness of PUA films. Fig. 3 and Table 2 showedthat the appearance of PUA films. It indicated that the best ofMMA/BA rate for 0.65/1.

3.1.3. Effect of HEMA content

Chemical incorporation of HEMA in the PU prepolymer is an

important reaction step as the amount of HEMA further deter-mines the number of grafting points between core and shell.Therefore, it is essential to control the chemical reaction between

Table 2The influence of MMA/BA rate for appearance of PUA films.

MMA/BA Appearance of PUA films

1/1 Crack0.85/1 Transparency poor0.65/1 Bright and transparent0.55/1 Soft0.45/1 Very sticky

732 S.-F. Zhang et al. / Progress in Organic Coatings 76 (2013) 729– 735

ate on

Hwsbm

3

tacfiewttr

Fig. 3. Effect of MMA/BA r

EMA and PU. Fig. 4 shows that the particle size and conversionere influenced by the amount of HEMA. The emulsion particle

ize and conversion were the minimum, and emulsion was sta-le especially when the content of HEMA was 8.3% (wt) of shellonomers.

.1.4. Effect of Tween-20/SDS rateThe choice of emulsifier was not only directly related to freeze-

haw stability, coating rheology and water sensitivity, but alsoffected the cost of emulsions. Here, an anionic emulsifier and aationic emulsifier were mixed with two typical non-ionic emulsi-ers to obtain compound emulsifiers. The effects of the compoundmulsifier on the particle size and conversion rate of the emulsionere measured and presented in Fig. 5. It could be seen that when

he compound emulsifier of SDS/Tween-20 rate was 3/1, it reachedhe best system as it exhibited the highest monomer conversionate (93.5%) and the smallest particle size (126 nm).

0

500

1000

1500

2000

2500

0 5 10 15 20

HEMA/%

Parti

cle

size

/nm

60

65

70

75

80

85

90

95

100

Con

vers

ion/

%

The particle size of PUA

The conversion of PUA

Fig. 4. Effect of HEMA content on particle size and conversion.

appearance of PUA films.

3.2. Characterization and properties of PUA emulsion and coating

3.2.1. Emulsion particle morphology and sizeThe morphology of PUA emulsion particles was observed by TEM

(transmission electron microscopy) and showed in Fig. 6. A clearcore phase and shell phase structures of the emulsion particles wasobserved due to the difference of electron penetrability to the corephase and the shell phase. The light and the dark regions in theparticles correspond to polyurethane core phase and polyacrylateshell phase, respectively. Furthermore, the observed emulsion sizewas about 130 nm. Fig. 7 showed the average particle size and theparticle size distribution of PUA hybrid latexes. As showed in Fig. 7,the particle size distribution was narrow and the average particlesize was 126 nm. This result was consistent with the particle sizemeasured by TEM.

3.2.2. FT-IR analysisThe FT-IR spectra of prepolymers, such as NCO-terminated

polyurethane (NCO-PU) (a), HEMA-terminated polyurethane

0

100

200

300

400

500

600

700

800

900

0 1 2 3 4 5Tween-20/SDS

Parti

cle

size

/nm

0

10

20

30

40

50

60

70

80

90

100C

onve

rsio

n/%

The particle size of PUA

The conversion of PUA

Fig. 5. Effect of Tween-20/SDS rate on particle size and conversion.

S.-F. Zhang et al. / Progress in Organic Coatings 76 (2013) 729– 735 733

(I3NoamcpaHwwc(gIc

3

p(bfiatcp

ing 1250 times). That meant the PUA-C would be widely employed

Fig. 6. The particle morphologies of PUA emulsion.

HEMA-PU) (b) and the product, PUA (c) were showed in Fig. 8.n PUA and its all prepolymers, sharp bands appearing between300 and 3500 cm−1 were due to the presence of hydrogen-bonded

H group [22]. In NCO-PU, the characteristic absorption bandf isocyanate group [23] appeared at 2270 cm−1, and the bandst 1715 cm−1 (C O) and 1107 cm−1 (C O C) confirmed the for-ation of urethane group. Compared with the NCO-PU (a), the

haracteristic absorption band in HEMA-PU (b) changed. The disap-eared band at 2265 cm−1 and the appearance of the characteristicbsorption band at 1637 cm−1 (� C C) and 811 cm−1 (ı HC C) inEMA-PU (b) confirmed that NCO groups (NCO-PU) had reactedith the OH groups in HEMA. That meant the CH CH2 groupsere grafted to the polyurethane. Compared with HEMA-PU, the

haracteristic absorption band of C C at 1637 cm−1 and 811 cm−1

� HC C) disappeared in PUA (c). It indicated that the CH CH2roups copolymerized with the vinyl groups of acrylate monomers.t revealed that the crosslinked structure was formed between theore and shell.

.2.3. Thermo gravimetric analysis (TGA)Here, the TG of crosslinked core–shell waterborne

olyurethane-acrylic copolymers (PUA) and polyurethane filmPA) were measured (Fig. 9). In order to compare with covalentond bonding and physical blended composite, the compositelm of PU and PA physical blended (PUPA) was also measured

nd analyzed. It showed that the PUA was more stable thanhe PUPA and PU, and it also proved that the existence of thehemical bonds between PU and PA. Improving the thermalroperty of polyurethane has many methods, such as make

Fig. 7. The particle size distribution of PUA emulsion.

Fig. 8. FT-IR spectra of NCO-PU (a), HEMA-PU (b) and PUA (c).

use of SiO2 as a crosslinker to improve the thermal property ofpolyurethane [24]. Compared with it, the preparation of crosslinkedcore–shell waterborne polyurethane-acrylic copolymers is moreeasier.

3.2.4. The basic property of PUA films and coatingHumidity controlling coating (PUA-C) is mainly used in interior

wall decoration, and plays an important role in regulating indoorhumidity. Therefore, as common architectural indoor coatings,the basic properties of the humidity controlling coatings weremeasured, including state in container, application property,appearance, freezing–thawing resistance, drying time, alkali resis-tance, scrub resistance, water resistance, and adhesion. The resultswere showed in Table 3. It indicated that the basic properties ofthe humidity controlling coatings met the standard for interior wallcoatings, and particularly the scrub resistance was very high (reach-

in indoor coatings.

01020304050

60708090

100

0 100 200 300 400 500 600 700

Temp/Cel

TG/%

PUPUAPUPA

Fig. 9. The thermo gravimetric of PU, PUPA and PUA films.

734 S.-F. Zhang et al. / Progress in Organic Coatings 76 (2013) 729– 735

Table 3Basic properties of the PUA film and coating.

Film properties Results Coatings properties(GB/T 9756-2009a) Results

Appearance Smooth and transparent Low temperature stability NormalSurface dry time (<h) 1.5 Alkali resistance (24 h) NormalHard dry time (<h) 5 Water resistance (48 h) NormalHardness 2H Surface dry time (<h) 2Adhesion/grade 1 Scrub resistance (times) 1250

a

3

bPemmtTmbP

g

Water absorption (%) 156

The indoor coating standard of synthetic resin emulsion.

.2.5. The surface morphology of the PUA-CThe PUA coatings were brushed on a clean glass and an asbestos

oard respectively. As showed in Fig. 10a and b, the surface ofUA-C was smooth without any cracks in the coatings, and thexternal structure was compact. The surface and cross profileorphologies of PUA-C were also observed by scanning electronicroscope (SEM) and presented in Fig. 10c and d. It indicated

hat the humidity-sensitive coatings had micro-structure like coral.here are two methods to modified inorganic porous materials, oneethod is modified by the surfactant [25] and another is modified

y polymers [26]. In our study, the PUA-C was obtained by blendingUA and inorganic porous fillers, which is simple synthetic process.

In summary, the basic properties and surface morpholo-ies of the PUA-C were investigated. It revealed excellent

Fig. 10. The surface morph

Water absorption (%) 185

properties of humidity controlling and humidity retention becausefillers of porous structure were combined with waterbornepolyurethane-acrylic copolymers crosslinked core–shell emulsion.Therefore, it could be used in indoor coatings for controllinghumidity.

3.3. The humidity controlling properties of PUA-C

On one hand, the water absorption abilities of PA-C and PUA-Cwere measured, and they were 106% and 186% respectively. It indi-

cated that the PUA coating has strong water absorption and goodwater retention, which means faster response to water. The PUAhad a lot of hydrophilic groups, such as carboxyl group ( COOH,

COOR) and hydroxyl group ( OH), porous and multi-layer fillers

ology of the PUA-C.

S.-F. Zhang et al. / Progress in Organi

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10Time/hrs

RH

/%

PUA-I

PUA-D

B-I

B-D

Fig. 11. The humidity increasing and dehumidifying properties of the humiditycontrolling coatings (PUA-I: humidity increasing with moist coating; B-I: blankhumidity increasing without coating; PUA-D: humidity decreasing with dry coating;B

wmfiocc

picTsarfe

scrm0tlw[ih

ethsedimfpsr

[

[[

[[[[

[[

[

[[[

[[

[

[

[71 (2011) 369–375.

[28] A.K. Mishra, R. Narayan, T.M. Aminabhavi, S.K. Pradhan, K.V.S.N. Raju, J. Appl.

-D: blank humidity decreasing without coating).

hich belonged to water-absorbent materials. Furthermore, poly-er chains of the PUA could stretch along the pore and surface of

llers, form three dimensional network structure so that store a lotf water. The PUA-C with strong permeability and gas permeabilityontained a large amount of void, which was suitable for humidityontrolling coatings of the interior wall.

On the other hand, the increasing humidity and dehumidifyroperties of PUA-C were measured and showed in Fig. 11. The

ncreasing humidity property of PUA-C was measured in artificiallimate box (42 L) using saturated water coated plates with PUA-C.he controlling experiment was measured by replacing the coatedheet with a dish-like glass vessel filled with water, whose surfacerea was equal with the former coated sheets. It indicated that theelative humidity (RH) in artificial climate box could be increasedrom 30% to 85% in 0.5 h. However, it needed 9 h in the controllingxperiment.

In order to investigate the dehumidify property, similarly, theample plates were coated with PUA-C and put into the artificiallimate box, then the changing values of RH at different time wereecorded. The decreasing humidity property of PUA-C was alsoeasured (Fig. 11). The PUA-C decreased RH from 90% to 60% in

.5 h, whereas the process of decreasing humidity needed 3 h whenhere were no coated sheets. It revealed that the humidity control-ing coatings had an excellent property of dehumidify. Compared

ith the acrylate-based copolymer humidity controlling coatings27], the PUA-C showed excellent humidity controlling and humid-ty retention, it could be used in indoor coatings for controllingumidity.

Compared with traditional humidity controlling materials orquipments, such as humidity controlling textiles, humidity con-rolling porous ceramics or air humidifier, the advantages ofumidity controlling coatings not only include no energy con-umption, low cost and no pollution to environment, but highffectiveness of using the large surface of interior wall as well. Beingifferent from the conventional composite coatings [28], PUA coat-

ng showed strong absorption and good water retention, whicheans faster response to water. It could be coated on surface to

orm a protective, decorative and humidity-sensitive coating. Com-ared with acrylate-based copolymers coatings [29], PUA coatings

howed excellent water resistance, weather resistance and scrubesistance.

[

c Coatings 76 (2013) 729– 735 735

4. Conclusions

A novel waterborne polyurethane-acrylic copolymerscrosslinked core–shell emulsion (PUA) was successfully pre-pared and the formation mechanism of crosslinked core–shellstructure was suggested. The different proportions and thecontents of HEMA, PA/PU rate, MMA/BA rate and Tween-20/SDSrate on the stability of emulsion were investigated. The particlesize and core–shell morphology of the particle were provedwith TEM and DLS, which indicated that the distribution ofemulsion particle size was narrow and uniform. The formationof crosslinked structure between PU and PA was confirmed byFT-IR and TGA. In addition, the humidity controlling coating(PUA-C) was synthesized by PUA, pigments and porous fillers. Thesurface morphology of the PUA-C was observed by SEM. Throughmeasuring the basic properties, PUA-C coating met the standardfor interior wall coatings, and particularly the scrub resistance wasvery high (reaching 1250 times). PUA-C coating showed excellentproperties of humidity sensitivity and humidity retention, andthey could be widely used in indoor coatings for controllinghumidity.

Acknowledgement

The authors are grateful to the NSFC (21263024, 21244003,20964002), PCSIRT (IRT1177), the Gansu Sci & Techn SupportProject (1011GKCA017), Funda Res Funds Gansu Univ (2010-176) and Lanzhou Sci Techn Bureau (2009-1-14) for financialsupport.

References

[1] H. Huang, S. Kato, R. Hu, Y. Ishida, Build. Environ. 46 (2011) 1817–1826.[2] H. Zhang, H. Yoshino, Build. Environ. 45 (2010) 2132–2140.[3] R.J. Wu, Y.L. Sun, C.C. Lin, H.W. Chen, M. Chavali, Sens. Actuators B 115 (2006)

198–204.[4] M. Sadakiyo, H. Okawa, A. Shigematsu, M. Ohba, T. Yamada, H. Kitagawa, J. Am.

Chem. Soc. 134 (2012) 5472–5475.[5] R. Demir, S. Okur, M. Sekar, Ind. Eng. Chem. Res. 51 (2012) 3309–3313.[6] D.H. Vu, K.S. Wang, B.H. Bac, Mater. Lett. 65 (2011) 940–943.[7] W. Lv, R.M. Wang, Y. He, H. Zhang, Prog. Chem. 20 (2008) 351–361.[8] Y. Tomita, R. Takahashi, S. Sato, J. Ceram. Soc. Jpn. 112 (2004) 491–503.[9] Y. Moriya, N. Ishii, JSME Int. J. 39 (1996) 653–661.10] V. García-Pacios, V. Costa, M. Colera, M. Martín-Martínez, Prog. Org. Coat. 71

(2011) 136–146.11] Y. Lu, Y. Xia, R.C. Larock, Prog. Org. Coat. 71 (2011) 336–342.12] R. Wang, J. Wang, W. Lv, J. Guo, Y. He, M. Jiang, J. Appl. Polym. Sci. 120 (2011)

3109–3117.13] Z. Zhou, W. Xu, J. Fan, F. Ren, C. Xu, Prog. Org. Coat. 62 (2008) 179–182.14] Y. Nakayama, Prog. Org. Coat. 33 (1998) 108–116.15] D.L. Merlin, B. Sivasankar, Eur. Polym. 45 (2009) 165–170.16] M. Sultana, H.N. Bhattia, M. Zuberb, I.A. Bhattia, M.A. Sheikha, Carbohydr.

Polym. 86 (2011) 928–935.17] M. Hirose, J. Zhou, K. Nagai, Prog. Org. Coat. 38 (2000) 27–34.18] R. Narayan, D.K. Chattopadhyay, B. Sreedhar, K.V.S.N. Raju, N.N.

Mallikarjuna, T.M. Aminabhavi, J. Appl. Polym. Sci. 99 (2006)368–380.

19] A.K. Mishra, R. Narayan, K.V.S.N. Raju, T.M. Aminabhavi, Prog. Org. Coat. 74(2012) 134–141.

20] S.L. Chai, M.M. Jin, H.M. Tan, Eur. Polym. 44 (2008) 3306–3313.21] H. Xin, Y. Shen, X. Li, Colloids Surf. A 384 (2011) 205–211.22] A.V. Raghu, G.S. Gadaginamath, N.T. Mathew, S.B. Halligudi, T.M. Aminabhavi,

React. Funct. Polym. 67 (2007) 503–514.23] A.V. Raghu, H.M. Jeong, J. Appl. Polym. Sci. 107 (2008) 3401–3407.24] K.K. Jena, S. Sahoo, R. Narayan, T.M. Aminabhavi, K.V.S.N. Raju, Polym. Int. 60

(2011) 1504–1513.25] A.K. Mishra, S. Allauddin, R. Narayan, T.M. Aminabhavi, K.V.S.N. Raju, Ceram.

Int. 38 (2012) 929–934.26] L. Zampori, G. Dotelli, P.G. Stampino, C. Cristiani, F. Zorzi, E. Finocchio, Appl.

Clay Sci. 60 (2012) 140–147.27] R.M. Wang, J.F. Wang, X.W. Wang, Y.F. He, Y.F. Zhu, M.L. Jiang, Prog. Org. Coat.

Polym. Sci. 125 (2012) E67–E75.29] T. Nguyen, J. Martin, E. Byrd, N. Embree, Polym. Degrad. Stab. 77 (2002)

1–16.