International Journal of Adhesion & Adhesives 31 (2011) 97–103

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International Journal of Adhesion & Adhesives

0143-74

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/ijadhadh

Characterization of polyurethane adhesives containing nanosilicas of differentparticle size

Mohammed A. Bahattab a,n, Jessica Donate-Robles b, Vanesa Garcıa-Pacios b,Jose Miguel Martın-Martınez b

a Petrochemical Research Institute, King Abdulaziz City for Science and Technology, 11442 Riyadh, Saudi Arabiab Adhesion & Adhesives Laboratory, University of Alicante, 03080 Alicante, Spain

a r t i c l e i n f o

Article history:

Accepted 17 October 2010Three nanosilicas with different particle sizes were added to a polyurethane adhesive (PU). The filled

adhesives were characterized by thermal gravimetric analysis (TGA), dynamic mechanical analysis

Available online 4 November 2010

Keywords:

Polyurethane

Stainless steel

Lap shear

96/$ - see front matter & 2010 Elsevier Ltd. A

016/j.ijadhadh.2010.11.001

esponding author. Tel.: +966 14813634; fax:

ail address: Bahattab@kacst.edu.sa (M.A. Baha

a b s t r a c t

(DMA), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and contact

angle measurements. Adhesive strength was evaluated from single lap shear test of solvent wiped

stainless steel/polyurethane adhesive joints.

Addition of nanosilica filler altered the degree of phase separation between the hard and soft segments

in the polyurethane, in different extent depending on the nanosilica particle size. Furthermore, upon

curing higher degree of crosslinking was obtained in the nanosilica filled polyurethane. The nanosilicas

agglomerated into the polyurethane matrix. On the other hand, the addition of nanosilica increased the

surface energy of the polyurethane to a greater extent by increasing the nanosilica particle size and

moderate increase in the single lap shear strength of stainless steel/polyurethane adhesive joints was

obtained.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Adhesive formulation properties contain several componentssuch as tackifiers [1], adhesion promoters [2] and fillers [3] amongother additives to produce an adequate performance in differentapplications. Thermoplastic polyurethanes are commonly used asadhesives and they may contain fillers to impart viscosity, improvemechanical properties and modify the rheological and surfaceproperties [4]. Thermoplastic polyurethanes are segmented poly-mers that exhibit a two-phase microstructure, which arises fromthe incompatibility between the soft and hard segments. The hardrigid segment segregates into a glassy or semi-crystalline matrices,and the polyol soft segments form amorphous or rubbery matricesin which the hard segments are dispersed [5]. Fillers are usuallyadded to polyurethane adhesives to impart improved mechanical,thermal and adhesion properties.

Polyurethanes can be tailored to meet the highly diversifieddemand of modern technologies such as coatings, adhesives, fibers,foams and thermoplastic elastomers. Several kinds of silicas havebeen added to improve different properties of polyurethanes. It hasbeen shown [6] that the density and mechanical properties of rigid

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+966 14813670.

ttab).

polyurethane foams filled with precipitated silica were decreasedwith increased filler loading due to cell damage. On the other hand,the thermal conductivity of the silica filled polyurethane foamincreased with filler loading.

TiO2–SiO2 nanoparticles of about 70 nm prepared by sol–gelprocess were incorporated as building blocks into polyurethanematrix [7]. The polyurethane/TiO2–SiO2 hybrid showed higherthermal stability as compared with that of the unfilled PU due tointeractions between the polymer chains and the inorganic parti-cles. On the other hand, polyurethane urea/silica-sol-modifiednano-calcium carbonate hybrids exhibited excellent thermal sta-bility, freezing resistance and mechanical stability [8]. Further-more, the tensile strength and hardness increased with increase inthe filler content. On the other hand, Tan et al. [9] preparednanosilica particles by hydrolysis of tetraethoxyorthosilicate inthe presence of methacryloxypropyltrimethoxy silane. The nano-silicas were dispersed evenly in PU matrix and imparted improvedUV–vis transmittance.

It has been shown [10] that the incorporation of nanosilicaparticles to conductive polyurethane/polyaniline materialincreased its thermal stability, Tg and extensibility due to a higherdegree of phase separation. Also, antimicrobial properties can beimparted to polyurethane by adding silica gel particles with aminecovalently bonded moieties [11]. Furthermore, Vuillequez et al.[12] prepared by UV-light and thermal cure an interpenetrating

M.A. Bahattab et al. / International Journal of Adhesion & Adhesives 31 (2011) 97–10398

polymer network composed of polyurethane methacrylate andsilica. The Tg decreased as the silica concentration increased, whichwas in accordance with the trend in surface hardness.

Jang et al. [13] prepared shape-memory polyurethane-silicananocomposites using fumed silica and hydrolyzed 3-amino-propyltriethoxysilane. Although aggregation of fumed silica parti-cles in the polyurethane matrix was found, the modulus and tensilestrength increased. On the other hand, a relatively narrow glasstransition region and a large difference between the modulus at theglassy and rubbery plateau regions were obtained. In a differentstudy, fumed silica (8–12 nm) was added to polyurethane in thepresence of 3-amino-propyltriethoxysilane coupling agent [14]. Ahigh connectivity between fumed silica and polyurethane wasfound and improved thermal, mechanical and dynamic mechanicalproperties were obtained. Finally, in a different study, nanosilicasof different particle size (150–200 nm) modified by a cycliccarbonate functional organoalkoxysilane were used to preparepolyurethane-silica nanocomposite [15]. Addition of the nanosili-cas increased the thermal decomposition as well as the abrasionresistance of the polyurethane.

To our knowledge no papers dealing with precipitated nanosi-lica filled polyurethane adhesives have been published and also acomplete study considering the structure–properties relationshiphas not been performed. Therefore, in this study, three nanosilicasof different particle sizes (20, 50 and 100 nm) were added topolyurethane adhesive formulations and the influence of thenanosilica particle size on the structure–properties relationshipof the filled polyurethane adhesives was studied.

Table 2Nomenclature of the polyurethane adhesives.

Filled polyurethane Nanosilica in the

polyurethane

Unfilled None

20 nm DP 5820

50 nm DP 5480

100 nm DP 5840

Table 3Some characteristics of the nanosilicas.

Property 20 nm 50 nm 100 nm

Nominal average primary particle size (nm) 20 50 100

Actual particle size in ethylene glycol solution

(nm)

8.1 10.1 25, 120

pH 3.0 3.0 3.0

Medium Ethylene

glycol

Ethylene

glycol

Ethylene

glycol

Viscosity at 25 %oC (mPa s) 60 60 85

SiO2 content (wt%) 30 30 30

2. Experimental

2.1. Polyurethane adhesive formulations

Three commercial precipitated nanosilicas (20, 50 and 100 nm,manufactured by Nyacol, Ashland, USA) were incorporated in thepolyurethane adhesive formulations. 160 g of hydroxyl polyacry-late polyol (Desmophen A-365, Bayer, Leverkusen, Germany) wasstirred for 5 min at 3000 rpm. 2 g of dispersing agent BYK110 (BYKadditives & instruments, Wesel, Germany) was added to the polyoland stirred for 5 min, followed by adding 10 g of leveling agent(EGA—ethylene glycol acetate) under stirring for 5 min more. Then10 g of colloidal nanosilica solution (prepared by mixing eachnanosilica with ethylene glycol) was added to the previous mixtureand stirred for 15 min, followed by adding 5 g methyl ethyl ketoneand 5 g xylene (Aldrich, Milwaukee, WI, USA) and additionalstirring for 5 min more. 55 g of the hardener (N75, Bayer, Leverku-sen, Germany) was added to the polyurethane solution by stirringfor 5 min. Table 1 shows the PU formulations in wt% and Table 2shows the nomenclature of the adhesives used in this study.

The solid filled polyurethanes films of about 1.5 mm thick wereprepared by placing about 50 cm3 solutions in a glass mould(10�10 cm2) allowing curing and a slow evaporation of theremaining trapped solvent.

Table 1Polyurethane adhesive formulations (in wt%).

PU nomenclature Polyol Disperser 1a Disperser 2b EGAc Silica

Unfilled 66.1 0.8 4.1

20 nm 64.8 0.8 4.1 4.1

50 nm 64.8 0.8 4.1

100 nm 64.8 0.8 4.1

a Disperser 1: BYK 110 (BYK additives & instruments, Wesel, Germany).b Disperser 2: BYK 331 (BYK additives & instruments, Wesel, Germany).c EGA: Ethylene glycol acetate.

2.2. Experimental techniques

Particle size distribution: The particle size distribution of thenanosilicas was obtained in a nanoparticle size analyzer Nano-ZS(Malvern Instruments Ltd., Malvern, U.K.). The nanosilica wasdispersed in ethylene glycol and the measurement was obtainedat room temperature. A square glass round bottom cell was used formeasurement.

Transmission electron microscopy (TEM): The topography andparticle size of the nanosilicas after ethylene glycol evaporationand the degree of dispersion of the nanosilicas in the polyurethanematrix were monitored by transmission electron microscopy (JEOLJEM-2010, Tokyo, Japan); an acceleration voltage of 100 kV wasused. The filled polyurethane films composites were cryogenicallymicrotomed and placed on a copper grid for analysis.

Thermal gravimetric analysis (TGA): TGA studies were carried out ina TA TGA Q500 instrument (TA Instruments, Newcastle, DE, USA),under nitrogen at a flow rate of 100 ml/min. Samples (10–15 mg)were heated from 25 1C up to 600 1C using a heating rate of 101 C/min.

Differential scanning calorimetry (DSC): DSC experiments werecarried out in a TA DSC Q100 V6.2 instrument (TA Instruments,Newcastle, DE, USA). Aluminium pans containing 10–15 mg samplewere heated from �70 1C to 300–400 1C under nitrogen atmosphere(gas flow: 50 ml/min). The heating rate was 5 1C/min. The first heatingrun was carried out to remove the thermal history of the samples.From the second heating run the glass transition temperature (Tg), thecrystallization temperature (Tc), and the melting temperature (Tm) ofthe polyurethanes were obtained.

20 nm Silica 50 nm Silica 100 nm MEK Xylene Hardener

4.1 2.1 22.7

2.0 2.0 22.3

4.1 2.0 2.0 22.3

4.1 2.0 2.0 22.3

M.A. Bahattab et al. / International Journal of Adhesion & Adhesives 31 (2011) 97–103 99

Dynamic mechanical analysis (DMA): The viscoelastic propertiesof the polyurethanes were measured in a TA DMA Q800 instrument(TA Instruments, Newcastle, DE, USA), using the shear sandwichmode. The experiments were carried out by heating the samplefrom �100 to 200 1C, using a heating rate of 51 C/min at a frequencyof 1 Hz and a strain of 0.5%.

Contact angle measurements: The wettability and the surfaceenergy of the polyurethane films surfaces were evaluated bycontact angle measurements using a Rame-Hart 100 goniometer(Rame-Hart Instrument Co., Mountain Lakes, NJ, USA). The poly-urethane films were placed into the hermetic and isothermal(25 1C) chamber of the goniometer previously saturated withstandard liquid. Two different standard liquids were used, bidis-tilled and deionized water and di-iodomethane. Drops (4 ml) ofbidistilled and deionized water or di-iodomethane were placed onthe polyurethane film surface using a microsyringe provided with aflat end needle, and static contact angle values were measured. Thecontact angle values were measured 5 min after drop deposition. Fivedrops placed on different places on the polyurethane film surfacewere measured and averaged. The experimental error was 721.

Adhesive joints formation: Adhesion was tested from single lapshear tests of stainless steel/filled polyurethane adhesive/stainlesssteel joints. Stainless steel test pieces were wiped with isopropanol.0.2–0.3 g adhesive solution containing 22.7 (unfilled polyurethane)or 22.3 (filled polyurethane) wt% hardener were mixed by hand for5 min and spread evenly on 3�3 cm2 area on one of the stainlesssteel test pieces. The other test sample was placed on the area to bebonded and a pressure of 0.8 MPa was applied for 10 s to achieve asuitable joint. Adhesive joints were left to cure for 72 h beforetesting in an Instron 4505 universal testing machine (Instron Ltd.,Norwood, USA) at a pulling rate of 100 mm/min. Four replicateswere carried out and the results were averaged.

Fig. 2. TEM micrographs of the nanosilicas (after ethylene glycol removal).

3. Results and discussion

3.1. Characterization of the nanosilicas

Table 3 shows some properties of the nanosilicas used in thisstudy. All nanosilicas have similar acidic nature. 30 wt% nanosilicaswere dispersed in ethylene glycol to facilitate the addition into thepolyurethane adhesive solutions. The actual particle size distribu-tion of the nanosilicas in the ethylene glycol solution was measured(Fig. 1) and the values are given in Table 3. The actual particle size ofthe nanosilica dispersions is smaller than that in the nanosilica. Allnanosilicas show monomodal particle size distribution always inthe nanometer range, except nanosilica 100 nm in which a bimodaldistribution of particles appears.

The morphology and particle size of nanosilica agglomeratesmay change once the solvent (ethylene glycol) is removed. Thisissue is of great importance to understand the distribution of the

Fig. 1. Particle size distribution of the nanosilicas in ethylene glycol solution.

M.A. Bahattab et al. / International Journal of Adhesion & Adhesives 31 (2011) 97–103100

nanosilicas in the polyurethane matrix once the filler is added tothe adhesive formulation. Thus, TEM was also used to determinethe morphology and degree of agglomeration of the nanosilicasonce the ethylene glycol is removed. Fig. 2 shows the TEM

Fig. 3. Derivative TGA thermograms of unfilled

Fig. 4. (a) TEM micrographs at low magnification of the nanosilica filled polyurethanes. (

(c) TEM micrographs at high magnification of the nanosilica filled polyurethanes.

micrographs of the nanosilicas. All nanosilica particles are sphericaland very regular in size, the distribution of particles is wider in the100 nm nanosilica. The evaporation of the solvent does not produceparticle agglomeration.

and 20 nm nanosilica filled polyurethane.

b) TEM micrographs at medium magnification of the nanosilica filled polyurethanes.

M.A. Bahattab et al. / International Journal of Adhesion & Adhesives 31 (2011) 97–103 101

3.2. Characterization of the polyurethane-nanosilica mixtures

Thermal gravimetric analysis is a useful technique to analyze thestructure of the polyurethanes [16]. The TGA thermogram of theunfilled polyurethane shows several decompositions (Fig. 3). Between300 and 400 1C, the hard segments in the polyurethane decompose,giving two separate decompositions due to urethane (at lowertemperature) and urea (at higher temperature). At 430 1C, thedecomposition of the polyol (soft segments) is found, and about500 1C the decomposition of the polymer is produced. Addition ofnanosilica filler does not produce new thermal decompositions butthe different decomposition steps are produced at different tempera-ture and/or weight loss. The polyurethane filled with 20 nm nano-silica shows the decomposition of the hard segment at lowertemperature and with higher weight loss than in the unfilledpolyurethane, indicating that nanosilica disrupts the interactionsbetween the polymer chains affecting the extent of phase separation.

The dispersion a degree of aggregation of the nanosilicas in thefilled polyurethanes was analyzed by TEM. Addition of nanosilicachanges the topography of the polyurethane matrix (Fig. 4a)creating bubbles and nanocracks on the surface. The higher thenanosilica particle size, the lower the bubble size and the higher thenumber of cracks. The bubbles can be created by segregation of thenanosilica and the cracks can be ascribed to stress concentrations inthe vicinity of the nanosilica agglomerates. Fig. 4(b) shows that allnanosilicas tend to agglomeration the polyurethane matrix andthe number of isolated nanosilica particles is quite reduced. Thesize of the nanosilica agglomerates is in the microns range (exceptin the 20 nm nanosilica filled polyurethane), the polydispersityof the agglomerates is more marked by increase in the nanosilicaparticle size. Fig. 4(c) shows the degree of agglomeration of thenanosilicas in more detail. The nanosilica particles do not interact

Table 4Some parameters obtained from the DSC curves of nanosilica filled polyurethanes. First

PU Tg (1C) Curing process I Curing process II

T (1C) DH (J/g) T (1C) D

Unfilled �46 47 17.7 – –

20 nm �49 44 1.3 80 2

50 nm �49 39 3.5 75 5

100 nm �57 37 5.8 81 4

100

20nm

50 nm

Fig. 5. DSC thermograms of the filled p

but they are compacted maintaining the individual entity ofthe particles. This is likely ascribed to the absence of adequatesurface polarity and the lack of affinity of the inorganic particlesand the organic polyurethane matrix. In summary, the polyur-ethane with 20 nm nanosilica shows the lowest degree ofagglomeration.

DSC is a sensitive technique to analyze the thermal stability andthe structure of polyurethanes. The first DSC heating run (notshown) of the unfilled polyurethane shows a glass transition at�46 1C due to the soft segments (Table 4); the addition ofnanosilica decreases the value of the glass transition to �49 or�57 1C due to higher degree of phase separation in the polyur-ethane induced by the nanosilica; the higher the nanosilica particlesize, the lower the value of the glass transition temperature. On theother hand, the first DSC heating runs of the polyurethanes showseveral exothermal peaks at about 40, 80 and 120 1C which maycorrespond to the complete curing of the polyurethane (Table 4),i.e. the starting samples were not fully cured. The enthalpy of curingof the polyurethane decreases upon addition of nanosilica indicat-ing the positive effect of the filler on the kinetics of cure of thepolymer. Furthermore, a melting process is always found at about250 1C, due to the collapse of the polymeric chains (Table 4).

The second DSC heating runs of the filled polyurethanes (Fig. 5)do not show any exothermal process indicating the full cure of thepolyurethanes. As a consequence of the cure, two glass transitiontemperatures appear, one located at around – 10 1C due to the softsegments and other at 65 1C likely due to the hard segments in thepolyurethane. In general, the nanosilica particle size does notgreatly influence the values of the glass transition temperature. Onthe other hand, the melting of the fully cured polyurethanes isproduced at 300–350 1C, a melting temperature higher than in theincompletely cured filled polyurethanes (Table 4).

DSC heating run.

Curing process III Tm (1C) DHm (J/g)

H (J/g) T (1C) DH (J/g)

118 1.2 257 4.8

.6 – – 245 0.4

.3 137 0.2 246 1.1

.9 – – – –

olyurethanes. Second heating run.

M.A. Bahattab et al. / International Journal of Adhesion & Adhesives 31 (2011) 97–103102

DMA experiment was used to determine the viscoelastic proper-ties of the filled polyurethanes. Fig. 6(a) shows the variation of thestorage modulus (G0) as a function of the temperature. All poly-urethanes exhibit the glassy region, the glass transition (noticed bya sudden decrease in storage modulus) and the rubbery plateau. Inthe glassy region the storage modulus does not vary with increasein temperature, because of the lack of movement of the polymerchains below the glass transition, and the addition of nanosilicadecreases the storage modulus due to the lack of miscibilitybetween the nanosilica particles and the polymer matrix. Afterglass transition is reached, the storage modulus in the rubberyplateau decreases in a greater extent in the filled polyurethanes.

The values of the glass transition temperature and degree ofcrosslinking in the filled polyurethanes can be better analyzed fromthe curves of tan delta vs temperature (Fig. 6b). The unfilledpolyurethane shows two glass transition temperature values at�15 1C (due to the soft segments) and 67 1C (the main glass

5

5.5

6

6.5

7

7.5

log

G'-P

a

Temper

unfilled

20 nm 100 nm

50 nm

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0-50-100

0-50-100

Tan

Del

ta

Temper

Fig. 6. Variation of (a) the storage modulus and (b) tan delta as a funct

transition likely due to the hard segments). The addition of thenanosilica filler removes the glass transition at lower temperaturedue to their influence on the degree of phase separation betweensoft and hard segments in the polyurethane matrix. Furthermore,only the addition of 20 nm nanosilica decreases the glass transitiontemperature of the polyurethane, whereas the addition of the othernanosilicas does not significantly change the glass transitiontemperature value of the polyurethane. On the other hand, theaddition of nanosilica decreases the degree of crosslinking of thepolyurethane, in a greater extent by adding 20 and 100 nmnanosilicas, because of an increase in the maximum value of thetan delta curve is noticed (Fig. 6b). This is in agreement with thelack of miscibility of the nanosilica particles in the polyurethanematrix. However, it should be considered that, according to the DSCexperiments the starting samples were not fully cured (exothermiccure between 40 and 120 1C—Table 4), hence additional cure mayalso take place during the DMA experiments.

ature (°°C)

20015010050

20015010050

ature (°C)

unfilled

20 nm

100 nm

50 nm

ion of temperature of unfilled and nanosilica filled polyurethanes.

Table 5Contact angle values (25 1C) on the nanosilica filled polyurethane surfaces.

PU H2O (deg.) CH2I2 (deg.) Surface energy (mJ/m2)

Unfilled 75 42 40.21

20 nm 67 36 44.87

50 nm 53 39 50.63

100 nm 57 40 48.22

Table 6Single shear adhesion strength values of stainless steel/nanosilica filled polyur-

ethane joints. 72 h after joint formation.

PU Shear strength (MPa)

Unfilled 14.270.1

20 nm 14.470.2

50 nm 14.770.1

100 nm 15.370.3

M.A. Bahattab et al. / International Journal of Adhesion & Adhesives 31 (2011) 97–103 103

Contact angle measurements were carried out on the surface ofthe filled polyurethanes to estimate the influence of addingnanosilica on their wettability and surface energy. The contactangle values were measured using a polar (H2O) and a nonpolar(CH2I2) liquid, and the values obtained are given in Table 5.Addition of nanosilica filler decreases the water contact anglevalues in a greater extent by increasing the nanosilica particle size;therefore, the addition of the filler increases the wettability of thepolyurethane. On the other hand, the di-iodomethane contactangle values practically do not change by adding nanosilica likelydue to the polar nature of the nanosilica.

The surface energy of the filled polyurethanes was calculatedfrom the water and di-iodomethane contact angle values on thepolyurethane films using the Owens and Wendt equation.

gLð1þcosYÞ ¼ 2ðgsdgldÞ1=2þ2ðgspglpÞ

1=2

in which gL is the surface tension of the liquid probe, Y the contactangle value, gsd the dispersive component of the surface energy ofthe solid, gld the dispersive component of the surface tension of theliquid, gsp the polar component of the surface energy of the solid,and glp the polar component of the surface tension of the liquid. Theaddition of filler increases the surface energy of the polyurethane(Table 5) this tends to increase with increase in the nanosilicaparticle size.

Finally, the adhesion properties of the filled polyurethanes wereobtained from single lap shear tests of stainless steel/polyurethaneadhesive joints (72 h after joint formation). The shear strengthslightly increases in the joints produced with the filled polyur-ethanes in a greater extent by increasing the nanosilica particle size(Table 6). The locus of failure in all joints was cohesive in theadhesive.

4. Conclusions

The nanosilicas aggregated into the polyurethane matrix due tolack of compatibility with the polyurethane matrix and the degreeof agglomeration increased with particle size. Furthermore, theaddition of nanosilica created bubbles and cracks on the filledpolyurethanes likely due to stress concentration between thenanosilica agglomerates and the polyurethane matrix. As a con-sequence, the viscoelastic properties of the filled polyurethaneswere inferior to that of the unfilled polyurethane. However, thesurface properties and adhesive strength of the stainless steel/polyurethane adhesive joints increased by adding nanosilicas in agreater extent by increasing the nanosilica particle size. Theincrease in shear strength in stainless steel/polyurethane adhesivejoints was small likely due to the lack of miscibility of the inorganicfiller into the organic polymer matrix, and the lack of dispersioninto separate particles.

Acknowledgments

M.A. Bahattab thanks King Abdulaziz City for Science andTechnology (Saudi Arabia) and University of Alicante (Spain) forfinancial support, and the assistance in some experiments ofMr. Mohammad Asif Alam.

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