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International Journal of Adhesion & Adhesives 30 (2010) 448–455

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International Journal of Adhesion & Adhesives

0143-74

doi:10.1

n Corr

E-m

m.p.ans1 Fa2 Fa

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

Effect of nano- and micro-particle additions on moisture absorption inthixotropic room temperature cure epoxy-based adhesives for bonded-intimber connections

Zakiah Ahmad a,n, Martin P. Ansell b,1, Dave Smedley c,2

a Faculty of Civil Engineering, University Technology Mara Malaysia, 40450 Shah Alam, Selangor, Malaysiab Materials Research Centre, Department of Mechanical Engineering, University of Bath, BA2 7AY, UKc Rotafix (Northern) Limited, Rotafix House, Abercraf, Swansea SA9 1UR, UK

a r t i c l e i n f o

Article history:

Accepted 9 March 2010The in-situ bonding of pultruded fibre-reinforced plastic rods into timber structural members is a

commonly used technique for making timber-to-timber connections and for the strengthening and

Available online 8 April 2010

Keywords:

Novel adhesive

Epoxy

Microscopy

Durability

Fickian and non-Fickian diffusion

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

016/j.ijadhadh.2010.04.001

esponding author. Tel.: +6 03 55435236; fax

ail addresses: zakiahah@hotmail.com (Z. Ahm

ell@bath.ac.uk (M.P. Ansell), daverotafix@aol

x: +44 01225 386928.

x: +44 01639 730858.

a b s t r a c t

repair of timber structures. Ideally, the adhesive should be thixotropic, shear thinning, room

temperature cure, environmentally stable, solvent-free and applied without pressure. This study

investigates the moisture absorption characteristics of three adhesives, specially formulated for

bonded-in timber connections, where two of the adhesives are modified with nano- or micro-particles.

The three adhesives are denoted as CB10TSS (standard adhesive), Albipox (standard adhesive with

CTBN rubber additions) and Timberset (standard adhesive filled with ceramic particles). The aim of the

additions is to improve the environmental stability of the standard adhesive as well as enhancing

mechanical properties and raising the glass transition temperature. The effect of high temperatures and

high humidity on the properties of the three adhesives was determined following conditioning at

different combinations of temperature and relative humidity (20, 30 and 50 1C/95% RH) and soaking in

water at 20 1C. In all cases the moisture uptake for the rubber-modified adhesive was less than for the

standard adhesive, but the ceramic particle-filled adhesive exhibited the lowest moisture uptake

overall. Exposure to humid environments at temperatures lower than Tg resulted in water uptake

characterized as Fickian, which had only a modest effect on properties. However, exposure to humid

environments at temperatures higher than Tg resulted in non-Fickian uptake of water and significant

changes to the diffusion and permeability coefficients.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Moisture absorbed into adhesives can act as a plasticizer,solvent or hydrolysis agent [1]. Some effects, such as plasticiza-tion and swelling, are reversible. Other effects, such as micro-cracking and hydrolysis, are irreversible processes that contributeto the degradation of the adhesive properties (thermal, mechan-ical, physical and chemical). Moisture can reduce Tg and decreasethermal stability [2]. Brewis et al. [3] analyzed the Tg of a range ofepoxies following immersion in water and found that the Tg

initially decreased, but after prolonged exposure the Tg of eachadhesive increased, in some cases above the Tg of the dryadhesive. This was explained by the formation of additional

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cross-links in the wet adhesive. Moisture can also cause adecrease in yield strength and other mechanical properties anda change in deformation mechanism [4–7].

Moisture can cause permanent chemical and physical changes.Antoon and Koenig [8] investigated the effects of moisture onanhydride-cross-linked epoxy resin films by means of Fouriertransform infrared spectroscopy (FTIR) measurements. Theydemonstrated that hydrolytic attack of water takes place on theepoxy films and the hydrolysis effects are accelerated undertensile stress in alkaline water. They also reported that slowoxidation processes occur at 100% relative humidity. Moisture canalso cause structural damage by inducing micro-cavities or crazesin polymeric materials [9] and the formation of such structuraldamage can further accelerate moisture diffusion [3]. Water mayhave a role in modifying the behaviour of the crack tip duringfracture.

Moisture can cause swelling of an adhesive, which can weakenadhesive joints. Depending on the exposure environment andtype of adhesive, the volumetric expansion can reach 10%.

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PVC frame

Glass plate

3 mm

500mm

500 mm

Fig. 1. Mould for plates of adhesive.

Z. Ahmad et al. / International Journal of Adhesion & Adhesives 30 (2010) 448–455 449

Adamson [10] recorded about 4% and 5% swelling of an epoxywhen exposed to water at 40 and 74 1C, respectively. El-Saad et al.[11] recorded about 1% and 10% swelling of cured rubber filledepoxy adhesive when exposed to 23 and 60 1C, respectively, inwater. Chang et al. [12] found that the swelling was linearlyproportional to the relative humidity and the swelling generallyincreased with moisture content and exposure temperature. Theswelling of the adhesive has an effect on Tg and the mechanicalproperties of the adhesive. Fernandez-Garcia and Chiang [13]studied the effect of hygrothermal ageing on the swelling and Tg

of a particle-filled epoxy-based adhesive. They found thatmoisture absorption, swelling and Tg are related to the apparentfree volume of the adhesive because the Tg value obtainedfollowing saturation was almost the same as that found followingdrying and re-saturation, although the specimens had differentsaturation levels.

Most epoxy-based commercial adhesives are formulated withfillers, which are employed to improve properties and reducecosts. The water resistance of adhesive joints depends on theamount and type of filler, particle size, the wettability of the fillerby the adhesive and other factors. Some research has shown thatabsorbed water can attack the matrix/filler interface and causedebonding at the interface [14]. Bowditch [15] investigated theeffect of water on mechanical properties of particle-filled(aluminium) adhesives and the progressive loss in strengthassociated with increase in exposure to water.

A major aim of the research was to improve environmentalstability of the CB10TSS adhesive and timber joints adhesivelybonded with CB10TSS. Under ambient conditions, the addition ofnano- and micro-sized particles into CB10TSS significantlyimproves the glass transition temperature and mechanicalproperties of bulk adhesives and increases joint strength[16–18]. It is desirable that these adhesives should maintainthese properties in high humidity and high temperature environ-ments. This paper is concerned with determining the environ-mental stability of thixotropic, room temperature cure epoxyadhesives used for bonding fibre-reinforced plastic (FRP) rods intotimber for the purpose of making structural connections or forreinforcing or repairing the timber [19].

Three adhesive formulations are investigated based on astandard adhesive formulation (CB10TSS), which is thixotropicand shear thinning to allow application into overhead locationswithout loss of adhesive. The adhesive is designed to cure at roomtemperature, so the glass transition temperature Tg is quite low inthe range 30–40 1C. The second and third adhesives weremodified versions of the standard adhesive formulated by theaddition of nano-rubber (designated Albipox) or ceramic micro-particles (designated Timberset) to raise Tg and to improveenvironmental stability at elevated temperatures and relativehumidities. Previous papers by the authors [16–18] havedescribed the mechanical properties of the three adhesives. Thetensile and flexural strengths, fracture toughness, shear strengthand pull-out strength of the CB10TSS, Albipox and Timberset werecompared and the addition of nano-rubber or micro-ceramicparticles significantly enhanced the mechanical properties. Theglass transition temperatures of the modified adhesives were alsoincreased. This study compares moisture uptake in these threeadhesive formulations in hot, moist environments and determinesthe nature of the diffusion process in each case.

2. Materials, specimen preparation and experimentalprocedures

The standard two-part CB10TSS epoxy adhesive comprises ofpart A: a mixture of diglycidyl ether of bisphenol-A (DGEBA), a

monofunctional glycidyl ether reactive diluent, silica fumeparticles (to control rheology) and a thixotropic agent and partB: the hardener, a mixture of polyetheramines and a thixotropicagent. The two parts were carefully mixed in equal volumes tominimize air entrapment. The second adhesive (Albipox) alsocontained a carboxyl terminated butadiene nitrile (CTBN) nano-rubber supplied in an epoxy base, which was intended to toughenthe standard adhesive, control moisture uptake and raise Tg. Thethird adhesive (Timberset) was a high elastic modulus formula-tion containing ceramic micro-filler particles including bentonite,quartz and mica and the effect of filler on moisture uptake wasalso under consideration.

The two-part adhesive was mixed using rotary motor with aconstant speed for 5 min. This technique was found to be moreeffective than manual mixing in terms of rate of energy suppliedto the adhesives. After mixing, the adhesive was placed in avacuum chamber for about 2 min to release air bubbles.

Plates of adhesive, 3 mm thick, 500 mm wide and 500 mmlong were prepared in a mould from the CB10TSS, Timberset andAlbipox adhesives as shown in Fig. 1. Each adhesive was left in themould for 10 days to cure. After demoulding, the adhesive platewas cut into 76.2 mm�25 mm�3 mm using a diamond cutter toprepare the moisture uptake test specimens in accordance withBS EN 2243-5:1992: Standard test methods for structuraladhesives—Part 5: Ageing tests.

The adhesive specimens were exposed to three high humidityenvironments, namely 20 1C/95% RH, 30 1C/95% RH and 50 1C/95%RH in a humidity chamber and were also immersed in distilledwater at room temperature. In the humidity chamber water wasintroduced into the air in a very fine dispersion to prevent directcondensation on the specimens at 95% RH. Moisture uptake wasmeasured by recording the weight change before and afterexposure as a function of time using

Mt ¼mt�mo

mo� 100% ð1Þ

where Mt is moisture uptake at any time t, mt is the mass of thespecimens at any time t during ageing and mo is the oven drymass of the specimen. These measurements were used todetermine the moisture diffusion coefficient, activation energyand permeability coefficient values. Subsequently, scanningelectron microscopy (SEM) was performed on the adhesivesamples.

3. Results and discussion

3.1. Experimental results

Results from the absorption measurements were plotted asmoisture uptake, Mt versus the square root of time (t1/2). Figs. 2–5show the absorption curves for CB10TSS, Albipox and Timberset

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Linear graph at Mt / Ms = 0.6

Fig. 2. Moisture absorption curves for adhesives aged at 20 1C/95% RH with line

fitted to curve CB10TSS to check for linearity at Mt/Msr0.6.

Fig. 3. Moisture absorption curves for adhesives aged at 30 1C/95% RH.

Linear graph at Mt / Ms < 0.6

Fig. 4. Moisture absorption curves for adhesives conditioned at 50 1C/95% RH

showing Fickian and non-Fickian behaviour.

Fig. 5. Moisture absorption curves for adhesives soaked in water.

Z. Ahmad et al. / International Journal of Adhesion & Adhesives 30 (2010) 448–455450

after exposure to the three environments. Each point on thecurves represents the average of three specimens. The time ofexposure to each environment varies between 5 and 7 months.

The experiments were stopped when the specimens reached theequilibrium state. The absorption of moisture was notaccompanied by any visible damage to the material exceptcolour changes (yellowish) in the case of CB10TSS aged at 30and 50 1C/95% RH.

3.2. Calculation of diffusion parameters

The absorption behaviour of the adhesive can be quantified bydiffusion coefficient D. The diffusion coefficient is a measure ofthe ability of the penetrant molecule, in this case the watermolecule, to move among the polymer segments.

For one-dimensional diffusion through an infinite plate ofthickness, h, Fick’s second law reduces to

@c

@t¼D

@2c

@x2ð2Þ

where c is the concentration of water, t is time and D is thediffusion coefficient or diffusivity through the thickness of thematerial.

Simplified solutions of Eq. (3) by Crank [20] showed that forthe initial stage of the sorption process where Mt/Mso0.6, thefollowing relationship can be applied:

Mt

Ms¼

4ffiffiffiffiffiffiphp

ffiffiffiffiffiffiDtp

ð3Þ

Rearranging the equation above gives the relationship for D as

D¼ p h

4Ms

� �2 Mtffiffitp

� �2

ð4Þ

where Ms is the equilibrium moisture uptake, Mt is the moistureuptake at any time t, D is the diffusion coefficient and h is thethickness of the specimen. Mt=

ffiffitp

is the initial slope of the graphof Mt versus

ffiffitp

.The diffusion behaviour is also quantified by another para-

meter, which is permeability P. Permeability is a measure of theability of a material to transmit fluids. The permeabilitycoefficient, P [21], expresses the combined effect of both D andMs as

P¼Ms � D ð5Þ

The values of D, Ms and P for all adhesives systems at eachenvironmental condition are summarized in Table 1.

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Table 1Values of diffusion coefficient D, maximum moisture uptake Ms and permeability

coefficient P for all adhesives.

D�10�13 m2/s Ms (%) P�10�13 m2/s

CB10TSS

20 1C/95% RH 2.38 1.54 3.66

30 1C/95% RH 4.70 1.57 7.38

50 1C/95% RH 22.50 1.03 30.83

Soaked in water 2.45 2.78 6.81

Albipox

20 1C/95% RH 2.31 1.23 2.84

30 1C/95% RH 2.80 1.37 3.84

50 1C/95% RH 10.90 1.32 15.15

Soaked in water 1.92 1.92 3.69

Timberset

20 1C/95% RH 0.14 0.44 0.06

30 1C/95% RH 2.10 0.54 1.13

50 1C/95% RH 3.70 0.49 1.81

Soaked in water 1.60 1.01 1.62

Z. Ahmad et al. / International Journal of Adhesion & Adhesives 30 (2010) 448–455 451

3.3. The effect of environment (temperature and humidity) on the

diffusion coefficient

Table 1 displays the variation in diffusion coefficient andpermeability of moisture in the adhesives corresponding todifferent temperatures of conditioning at 95% RH. It can be seenthat CB10TSS, Albipox and Timberset show the same trends indiffusion behaviour where the diffusivity and permeability of theadhesives increase with temperature. At lower temperatures, thedifferences in the diffusion parameters between CB10TSS andAlbipox do not appear to be highly significant but at 50 1C, therate of moisture diffusion into CB10TSS increases significantly.Timberset has the lowest value for all the diffusion parameters.Therefore it can be said that exposure to high humidity at lowtemperatures results in less moisture absorption but as thetemperature increases more moisture is absorbed.

3.4. The effect of environment (temperature and humidity) on the

moisture uptake of the adhesives

Figs. 2–4 indicate that the moisture diffusion rate is differentfor each adhesive for all of the environmental conditions.

The relationship between the time to reach the equilibriumvalue and the temperature appears to depend on the glasstransition temperature of the adhesive (the values of Tg for unagedCB10TSS, Albipox and Timberset are 31.7, 42.8 and 53.8 1C,respectively [17]). Comparing Figs. 2 and 3 the moisture uptakeincreases as the temperature increases from 20 to 30 1C, below theglass transition temperature of the three adhesives. However at50 1C/95% RH, CB10TSS and Albipox show very different absorptioncharacteristics compared to Timberset. In Figs. 2 and 3, Mt increasesrapidly in proportion to

ffiffitp

and then the rate increases slowly untilsaturation, which seems to follow Fickian behaviour. HoweverFig. 4 shows evidence of a multistage absorption process forCB10TSS and Albipox. The first moisture uptake stage occurs veryrapidly at a rate approximately proportional to

ffiffitp

and then relaxesat a temporary saturation point before the second stage ofabsorption begins and the moisture uptake very slowly increases.

To determine whether the sorption behaviour of theseadhesives follows Fickian or non-Fickian behaviour, a detailedanalysis of the moisture diffusion characteristics of the adhesivesis presented below.

Alfrey et al. [22] introduced the following classification for thediffusion process in glassy polymers:

Fig. 6. Log Mt/Ms versus log t plotted for CB10TSS at 20 1C/95% RH including

� regression line where R2 is the coefficient of correlation, n¼0.5 and k¼�1.63.

Case I or Fickian diffusion: the rate of diffusion is much smallerthan the rate of the relaxation process,

�

Case II diffusion: the rate of diffusion is much faster than therate of the relaxation process and � Non-Fickian or anomalous diffusion: the rate of diffusion is

associated with finite rates at which the polymer structurereacts to the penetrant molecule.

These different cases are distinguished by the shape ofabsorption curves of Mt/Ms versus t according to the followingexpression [21]:

Mt

Ms¼ ktn ð6Þ

where Mt represents the moisture uptake at time t and Ms is themoisture uptake at equilibrium saturation. k is a constant thatdepends on the structural characteristics of the polymer and itsinteraction with moisture and the value n determines the mode oftransport.

Equate (3) and (6)

4ffiffiffiffipp

hffiffiffiffiffiffiDtp ¼ ktn

where D in Eq. (4) can also be written as

D¼ðkhÞ2tp

16ð7Þ

For Fickian diffusion, n¼1/2 when the diffusion obeys Fick’slaw. Case II diffusion is when nZ1. For non-Fickian system, nA(1/2, 1) or changes sigmoidally from one diffusion mode to theother. In addition, non-Fickian behaviour needs two or moreparameters to describe the inherent interacting diffusion andrelaxation effects. The n and k values are obtained from the linearplot of log Mt/Ms versus log t. Figs. 6 and 7 show the examples ofthe graph of log Mt/Ms versus log t for CB10TSS aged at 20 1C/95%RH and Albipox aged at 50 1C/95% RH, respectively. A linearregression line is fitted though the data. Similar graphs forCB10TSS, Albipox and Timberset aged at 20, 30 and 50 1C/95% RHand soaked in water at room temperature are not shown here butthe values of k, n and R2 values are presented in Table 2.

The magnitude of n for most test conditions is close to 0.5 withthe major exceptions being CB10TSS and Albipox aged at 50 1C/95% RH. The values of n close to and less than 0.5 down to 0.38 forCB10TSS at 30 1C/95% RH suggest diffusion of the moisturethrough the adhesives is either normal Fickian behaviour orslightly anomalous Fickian behaviour. However the n¼0.22 and0.33 values for CB10TSS and Albipox aged at 501C/95% RH were

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Z. Ahmad et al. / International Journal of Adhesion & Adhesives 30 (2010) 448–455452

not taken as anomalous as they fit the non-Fickian criterionrelated to the ratio of Mt/Ms, that is, the graph is linear at Mt lessthan 0.6Ms (Fig. 4). Besides the values of n to determine theFickian or non-Fickian behaviour, it also must satisfy thecondition that the curve is linear upto Mt¼0.6 Ms as in the caseof CB10TSS exposed to 20 1C/95% RH (Fig. 2). Therefore thesecriterions were used as the justification for the Fickian and non-Fickian behaviour. From Table 2, it is seen that CB10TSS, Albipoxand Timberset follow Fickian diffusion behaviour under all ageingconditions except for CB10TSS and Albipox when aged at50 1C/95% RH.

The effect of temperature on Ms does not appear to besignificant for samples exhibiting Fickian absorption. However,temperature increases the rate of diffusion as seen by the value ofD in Table 1 and might change the relative rates of diffusionthrough low density and high density phases, thus causing morepronounced non-Fickian anomalies at 50 1C than at 20 1C. At 50 1Cand 95% RH, the polymer chains for CB10TSS and Albipox aremuch more mobile in the rubbery state at a temperature higherthan the Tg for CB10TSS and Albipox. Big voids are formedinducing stresses at the silica fume or nano-rubber to matrixinterface or interphase. The high loading of ceramic micro-particles ensures that Timberset takes up less moisture comparedto CB10TSS and Albipox and the non-Fickian behaviour of thelatter two adhesives at 50 1C/95% RH may be the result of

Fig. 7. Fitted curve to check for goodness-of-fit of diffusion behaviour for Albipox

at 50 1C/95% RH where Y¼ log Mt/Ms, x¼ log t, R2 is the coefficient of correlation,

n¼0.33 and k¼�1.11.

Table 2Values of E, n, k, R2 and assessment of Fickian diffusion behaviour based on n and rati

Parameters from log grap

n k

CB10TSS E¼72.7 kJ/mol 20 1C/95% RH 0.50 �1.63

30 1C/95% RH 0.38 �1.28

501C/95% RH 0.22 �0.67

Soaked in water 0.48 �1.67

Albipox E¼46.8 kJ/mol 20 1C/95% RH 0.48 �1.66

30 1C/95% RH 0.44 �1.43

50 1C/95% RH 0.33 �1.11

Soaked in water 0.50 �1.63

Timberset E¼110 kJ/mol 20 1C/95% RH 0.44 �1.53

30 1C/95% RH 0.45 �1.30

50 1C/95% RH 0.46 �1.18

Soaked in water 0.46 �1.60

exposure to a temperature greater than Tg. In general, CB10TSSabsorbs more moisture in all environments than Albipox, andTimberset absorbs the least amount of water. Therefore theaddition of CTBN in Albipox and ceramic particles in Timbersethelp in reducing moisture uptake. Shen and Springer [23]mentioned that the water saturation value attained in theadhesive they studied depends significantly on the relativehumidity of its environment, but it is insensitive to thetemperature. Conversely, the maximum moisture content mea-sured for the adhesives studied here increases with increase intemperature. This increase has also been observed in neat epoxy-based adhesives [24] and it could be a result of polymer structuralrelaxation caused by a shift in the glass transition temperature orwater interacting with fillers in the adhesives.

Fickian and non-Fickian diffusion behaviours have beendetermined by using the parameters shown in Table 2. Adhesivesageing at lower temperatures exhibit Fickian behaviour but whenconditioned at 50 1C/95% RH, they behave in a non-Fickian fashionexcept for Timberset. To further verify that CB10TSS and Albipoxexhibit non-Fickian behaviour at 50 1C/95% RH, the Fickian curvesare generated using the least-squares fit of the solution of Fick’slaws as described in BS EN ISO 62:1992 as shown in Eq. (8) withmeasured diffusivities D, sample thickness h and saturation levelMs for CB10TSS and Albipox as inputs:

Mt ¼Ms�Ms8

p2

X20

k ¼ 1

1

ð2k�1Þ2exp �

ð2k�1Þ2Dp2

ht

" #ð8Þ

where,

k¼ 1,2,3,. . .,20:

The fitted Fickian curves for CB10TSS, Albipox and Timbersetwhen conditioned at 50 1C and 95% RH are shown in Fig. 4. FromFig. 4, it is seen that Timberset exhibits Fickian absorption upto astable saturation level that is reached after about 1600 h.However, significant deviations from Fickian behaviour areobserved for CB10TSS and Albipox after about 100 h of exposureto the 50 1C/95% RH environment. There are numerous physicaland chemical mechanisms that could be considered responsiblefor non-Fickian behaviour. The rapid first stage of moisture uptakemay be mainly attributed to the diffusion of water molecules intothe pre-existing free volume in the materials. On the other hand,the second stage of moisture uptake may be a consequence of arelaxation process in the materials or chemical interactionbetween the polymer and the absorbed molecules [25]. Theconditioning temperature of 50 1C may further cross-link CB10TSSand Albipox evidenced by the increase in Tg (Table 3 shows thevarious Tg after exposed to different environment from authors’

o of Mt/Ms.

h Mt/Ms¼ktn Mt/Ms¼0.6 Fickian or non-Fickian

R2

0.99 Yes Fickian

0.99 Yes Fickian

0.84 No Non-Fickian

0.99 Yes Fickian

0.99 Yes Fickian

0.99 Yes Fickian

0.93 No Non-Fickian

0.99 Yes Fickian

0.99 Yes Fickian

0.99 Yes Fickian

0.98 Yes Fickian

0.99 Yes Fickian

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Z. Ahmad et al. / International Journal of Adhesion & Adhesives 30 (2010) 448–455 453

previous work [17]). With the increase in the degree of curing, theepoxy groups react to form cross-links and the polar groupsattract water molecules that are chemically bound, therebycontributing to increase in water absorption. Another reasoncould be the dissolution of the polymer causing mass loss inconjunction with moisture gain.

The moisture-transport rate D in epoxy adhesives is expectedto increase with temperature as observed in this study. Theactivation energy E is needed for transport processes since thewater molecules have to overcome the energy barriers set up bythe interaction potential from intra-molecular forces [7]. When thediffusion is Fickian and D is a function of temperature, thediffusivity is related to temperature by the Arrhenius relationship:

D¼D0exp �E

RT

� �ð7Þ

where D0 is a constant pre-exponential coefficient, R is thegas constant that is equal to 8.314 J/K mol, E is the activationenergy for diffusion and T is the absolute temperature in Kelvin.The E values for CB10TSS, Albipox and Timberset are presented inTable 2.

Timberset has the highest activation energy followed byCB10TSS, and Albipox has the lowest activation energy. In theprevious work on mechanical testing and thermal analysis of theadhesives [17], Timberset was found to have a higher cross-linkdensity. Thus the superior activation energy can be attributed tothe higher cross-link density of the network, which minimizes theavailability of molecular-sized holes in the polymer structure.

Based on the looseness of the polymer network the activationenergy of Albipox is expected to be higher than the activationenergy of CB10TSS. However this is not the case. The intermediateactivation energy for Albipox may be due to the interactionbetween the moisture and the rubbery particles.

Table 3Summary of the glass transition temperature values for the three adhesives before

and after exposure to different environmental conditions [17].

Conditions Tg (1C)

CB10TSS Albipox Timberset

Control 31.7 42.8 53.8

20 1C/95% RH 26.7 40.0 51.6

30 1C/95% RH 22.0 38.5 61.4

50 1C/95% RH 26.9 48.7 77.8

50 1Ca 33.8 66.7 87.9

Soaked in water at 20 1C 19.2 44.9 59.8

a Placed in the oven at constant temperature 50 1C to show all the adhesives

further post-curing at high temperature.

A

Fig. 8. SEM micrographs of the fracture surface of CB10TSS: (a) as-received at �1500, (

plasticization, �1500.

The results of weight gain measurements for CB10TSS, Albipoxand Timberset immersed in water at approximately 20 1C areshown in Fig. 5. The moisture diffusion curves for these adhesivesshow Fickian diffusion behaviour (see Table 2). For Albipox andTimberset, after immersing in water for about 5625 h the wateruptake reaches maximum values. However, for CB10TSS wateruptake is still increasing at 7921 h and at this time CB10TSS hasthe highest water uptake (2.83%) followed by Albipox (1.92%) andTimberset has the lowest water uptake (0.97%).

The general behaviour of moisture uptake for full immersionat 20 1C is similar to the exposure at 20 1C and 95% RH asthey fit Fickian behaviour. From Table 1, comparing full immer-sion with 95%RH, Ms and P are higher in all cases. The D values areabout the same for CB10TSS and Albipox but the D value forTimberset is much higher under full immersion. This may bedue to capillary action associated with the micro-particles inTimberset.

3.5. Fracture surface morphology of the adhesives

Fracture micrographs for CB10TSS, Albipox and Timbersetspecimens in the as-received state and aged specimens afterabout 70 days exposure (soaked in water and 50 1C/95% RH) ispresented in Figs. 8–10,respectively. Other micrographs are notshown here due to limited space.

For CB10TSS there are distinct micro-structural differences.The rougher cleavage pattern for as-received CB10TSS (Fig. 8a)loses it roughness (Fig. 8b) when soaked in water, with noyielding of the adhesive and the development of numerous micro-cracks marked A commensurate with the high degree of moistureuptake in CB10TSS. When aged at 50 1C/95% RH, the micrographsreveals plasticization of the adhesive due to high temperature andhigh moisture uptake.

In the fracture micrographs of Albipox after soaking in water(Fig. 9b) the fracture surface is rather smooth with radiatingsoft flex-ridges indicating plastic deformation compared to thecontrol (Fig. 9a), which shows cleaves in a petal-like form.Micro-voids indicating loss of rubbery particles can also beseen. When exposed to 50 1C/95% RH (Fig. 9c), the fracturesurface becomes smooth and the petal shapes and ridgesdisappear.

The increase in diffusion rate in Timberset is likely to be due tothe degradation of the interfaces between the matrix and thefillers, which allows for the increase in moisture uptake. Thepresence of moisture plasticizes the adhesive matrix (Fig. 10b)and debonding of the fillers but the presence of high temperature(Fig. 10c) induces more debonding and increases plasticity.However moisture uptake for Timberset is still very much lowerthan for CB10TSS and Albiox.

b) after soaking in water for 90 days, �1200 and (c) aged at 50 1C/95% RH showing

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Fig. 9. SEM micrographs of fracture surface of Albipox: (a) as-received at �1600, (b) after soaking in water, �1200 and (c) aged at 50 1C/95% RH showing plasticization, �1600.

Fig. 10. SEM micrographs of the fracture surface of Timberset: (a) as-received at �1400, (b) after soaking in water for 90 days, showing debonding of micro-particles and

plasticization �1200 and (c) aged at 50 1C/95% RH, �1 400.

Z. Ahmad et al. / International Journal of Adhesion & Adhesives 30 (2010) 448–455454

4. Conclusions

The moisture absorption characteristics of CB10TSS, Albipoxand Timberset at room temperature cure epoxy adhesives wereinvestigated after conditioning at 20, 30, 50 1C/95% RH andsoaking in water at room temperature. Diffusion models werefitted to the experimental data and the following results wereobtained.

�

The diffusion of moisture into the CB10TSS, Albipox andTimberset follows a Fickian process when aged at 20 and 30 1C/95% RH and soaked in water at room temperature. However,moisture uptake by diffusion for CB10TSS and Albipox deviatesfrom Fickian diffusion behaviour when aged at 50 1C/95% RH.Between CB10TSS and Albipox, at 50 1C, Albipox is only mildlydeviant from Fickian behaviour. � Increasing the temperature of exposure has the effect of

increasing the rate of water absorption in epoxy basedadhesives even with the addition of fillers. However the baseadhesive CB10TSS absorbs more moisture under all conditionsthan Albipox, which contains CTBN rubber, and Timberset,which contains ceramic micro-particles and absorbs the leastamount of water.

� The addition of CTBN (Albipox) and ceramic particles

(Timberset) reduces moisture uptake but the results of fullwater immersion at 20 1C suggest that the mechanism is quitedifferent for Timberset and may involve capillary action.

� The rate of diffusion increases with temperature and obeys the

Arrhenius equation, consistent with the higher temperatureincreasing the extent of reaction between hydrogen and watermolecules leading to the formation of micro-voids in thesystem, thus increasing moisture uptake.

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