Procedures conditioning the absorption/desorption behavior of cold-cured epoxy resins

Procedures Conditioning the Absorption/DesorptionBehavior of Cold-Cured Epoxy Resins

MARIAENRICA FRIGIONE,1 MARIATERESA LETTIERI2

1Department of Innovation Engineering, University of Salento, Via per Arnesano, 73100 Lecce, Italy

2Istituto per i Beni Archeologici e Monumentali, IBAM-CNR, Via Monteroni, 73100 Lecce, Italy

Received 19 December 2007; accepted 4 April 2008DOI: 10.1002/polb.21466Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: The absorption/desorption behavior of a commercial cold-cured bisphe-nolic epoxy resin, subjected to different treatments prior to exposure to water, wasanalyzed. The epoxy system has been already used as both matrix and adhesive forthe manufacture and application, respectively, of fiber reinforced polymers compositesemployed for rehabilitation procedures. The effects of different curing, conditioning,and storing conditions on the water absorption/desorption process taking place in thecured resin were evaluated. The different conditioning procedures used to dry thespecimens before their exposure to water caused a different extent of physical agingand of curing on each system, influencing the amount and the rate of diffusion of thewater molecules inside the specimens. Moreover, if the specimens are subjected tothermohygrometric cycles prior to immersion in water, the rate of diffusion and theamount of water also depends on the presence of water molecules inside the curedresins not easy to remove by any drying treatment. During all the hygrometric treat-ments performed, a deaging process took place. The kinetic of this deaging processfor the not-fully cured systems depends on the additional crosslinking taking place inthe samples. The different procedures used to condition the specimens also affect thevariations in glass transition temperature (Tg) of the cured systems during and afterimmersion in water. Finally, the different drying procedures employed proved to benot equally appropriate for cold-cured epoxy resins. VVC 2008 Wiley Periodicals, Inc. J Polym

Sci Part B: Polym Phys 46: 1320–1336, 2008

Keywords: adhesives; ageing; diffusion; thermosets

INTRODUCTION

The use of thermosetting resins, in particularepoxy based, in different applications involvedin the rehabilitation or renewal of civil infra-structures and cultural heritage has gaineda continuous increase in the last years, dueto their improved quality, easiness of applica-tion along with economic advantages. A moreextensive utilization of the thermosetting resins

in the mentioned applications, however, hasbeen hindered by the lack of a detailed knowl-edge of their long term performance that forceddesign engineers to use factors higher to ensuresatisfactory performance in service.

The epoxy resins used in civil engineeringrepairing operations and in cultural heritagerestoration are ‘‘cold-curing’’ resins, that is, theyare based on bisphenolic epoxy resins that, forpractical and economical reasons, are cured atambient temperatures.1 Aliphatic amines areused as curing agents, since they are able toinduce the crosslinking of the resin at relativelylow curing temperatures, leading to hardenedsystems with acceptable mechanical properties,

Correspondence to: M. Frigione (E-mail: [email protected])

Journal of Polymer Science: Part B: Polymer Physics, Vol. 46, 1320–1336 (2008)VVC 2008 Wiley Periodicals, Inc.

1320

with the further advantage of possessing lowcosts.

The two main consequences of a curing pro-cess carried out at ambient temperatures are asfollows: long curing times (in the order of weeks)are necessary to achieve sufficient mechanicalproperties of the resins; and a moderate glasstransition temperature (Tg) is attainable bythese systems, that is, in the range 50–65 8C.2

The crosslinking reactions taking place in epoxyresins at ambient temperatures are often notcompleted, due to kinetic restrains, even whenlong curing times are employed.

The effects produced by environmental agentson cold-curing resins has received a limitedattention in the literature only in the last fewyears. It can be assumed that the adverse condi-tions that may affect the long-term performanceof a thermosetting resin will be: aqueous envi-ronments; high and low temperatures; solvents;cleaning agents and chemicals; biological agents.

The previous experience in other fields provedthat the good properties of thermosetting, inparticular epoxy resins, undergo a considerabledecay after a long period of immersion in water.3

The effect of exposure to moisture is to alter thephysical, mechanical, and chemical characteris-tics of the polymers. Water may enter a resineither by diffusion or by capillary action throughcracks and crazes. Once inside, the water maymodify the properties of the polymer either in areversible manner, for example by plasticization,or in an irreversible manner, for example byhydrolysis.

The first factor affecting the diffusion processis essentially a geometrical one, that is, theamount of a liquid that can diffuse into a poly-mer depends on the size of the diffusing mole-cule relative to the free volume available withinthe polymer.4 The second factor concerns thechemical nature of the penetrant versus that ofthe polymer. This factor determines the affinityof the diffusing molecule to the specific func-tional groups present in the polymer.5 Epoxyadhesives are prone to water absorption becausethey possess polar sites that are able to attractwater molecules.6 Moreover, in crosslinked ep-oxy resins the packing of macromolecules is rel-atively low and the access for a small penetrantmolecule, such as water, is easy.

The mechanism of water absorption in differ-ent types of epoxy and, more important, thestate of aggregation of the water moleculeswithin the bulk of the material, with the related

effects on the polymer’s properties, are very im-portant issues that have been widely investi-gated in the last 20 years giving rise to a plentyof theories, often contradictory.7

One of the most generally recognized theoriesstates that water entering an epoxy resin can beclassified in ‘‘free’’ water and ‘‘bound’’ water.8–10

The former fills the microcavities of the networkwithout interacting with any unit contained inthe crosslinked network. It is characterized,therefore, by an elevated mobility. On the otherhand, bound water molecules interact throughH-bonding to different types of specific sites uni-formly distributed in the resin, which is bindingthe polar groups of the resin. While free wateris relatively easy to be removed on desorption, itwould not be easy to remove the bound watercompletely.11,12

Many scientists agree that bound water inthe polymer matrix rather than the totalamount of water uptake is responsible for thelowering of the Tg of the polymer. Water mole-cules tightly bound to the specific sites along theresin exhibit a negligible mobility, due to thehigh energy barrier to be overcome, and areable to disrupt the interchain Van der Waalsforces inside the network producing an increaseof segmental mobility.13–15 On the other hand,the species of water with lowest interactionswith epoxy, restricted in the free volume of theresin, are characterized by a high mobility alongthe network and are expected to exhibit areduced plasticizating effect.

The water molecules that act as a plasticizercause severe reductions in the Tg values of anepoxy resin, along with reduction in modulusand strength with a consequent marked unsuit-able decrease of load-bearing capacity.16 Sincethe multiplicity of states of the penetrant mole-cules within the matrix generates differenteffects on the properties of the cured epoxyresin, the overall water up-take cannot be con-sidered as a reliable measure of the degree ofplasticization. As a consequence, the traditionalrule of the thumb for most thermosetting heat-cured resin systems, that is, the Tg is reducedby 10–20 8C for every 1% of absorbed water ormoisture,17,18 should be applied with a limitedconfidence.

In the case of cold-curing epoxy resins, theimmersion in water for prolonged times provedto be less severe, in terms of reductions of Tg,than in the case of heat cured epoxy adhe-sives.19 The modification in Tg reflects not only

ABSORPTION/DESORPTION BEHAVIOR OF COLD-CURED EPOXY RESINS 1321

Journal of Polymer Science: Part B: Polymer PhysicsDOI 10.1002/polb

the degree of resin plasticization but also thatwater/resin interactions occur in the material.Relatively short time of exposure to water leadto more or less reversible plasticization, produc-ing a lowering of the Tg, as confirmed by dif-ferent studies.19–21 In the case of cold-curingepoxies, as the resin would not be fully cured atroom temperature, an excessive reduction in theTg of the wet resin can promote additional cross-linking.22 Actually, the lowering in Tg uponmoisture ingress allows the polymer chains tobecome mobile; this increases the probability ofa contact between the groups that have notreacted, thus promoting additional crosslinking(i.e., postcuring).23 This assumption can justifythe new increase in the glass transition temper-ature observed at longer immersion times, whenthe increase in Tg due to the additional cross-linking prevails on its reduction due to plastici-zation.19 On the other hand, some water mole-cules are likely not to contribute significantly tothe plasticization of the resin but they can formmultisite interconnective bond complexes whichcreate bridging between chain segments. Thisresults in secondary crosslinking (pseudo cross-linking) and contributes to an increase in Tg inwater saturated epoxy resins.14

Depending on the number of H-bonds thatwater molecules are able to form with polarsites, they can plasticize the resin, that is, form-ing only one H-bond, or, conversely, create bridg-ing between chain segments through two H-bonds.14

Whatever was the phenomenon occurred incold-curing epoxy adhesives immersed for pro-longed periods in water, it was found that insaturation condition the adhesives showed adecrease in Tg never exceeding 10 8C; in somecases, moreover, the adhesives almost recoverthe initial Tg value.19 In conclusion, waterabsorption will generally produce a deteriorationin the already poor high temperature load-bearing capacity of epoxy resins cured at roomtemperature.

The value of maximum moisture content atinfinite time, that is, M?, is strongly dependenton the polarity of the cured resin structure: themore polar, the higher the equilibrium moistureconcentration. Heat-cured epoxies are oftenbased on aromatic amines, which are less reac-tive than aliphatic amines but possess muchmore rigid molecules. This rigidity reduces theirlevel of molecular motion and hinders water dif-fusion. With either type of curing agent, com-

plete cure leads to a large number of crosslink-ing and this is expected to reduce the watertransport.16 However, this is unlike to be thecase of cold-curing epoxy resins, that, as previ-ous stated, often do not reach the completion ofcrosslinking reactions.

As already stated, the water transport in athermosetting resin is related to the availabilityof molecular-sized holes in the polymer struc-ture and to the polymer-water affinity. In aseries of comprehensive publications, the roleof polarity proved to be predominant withrespect to topology in establishing the maximumamount of water absorbed by an epoxy/aminesystem, that is, highly polar resins, possessingnumerous hydroxyl and amine groups, absorbmore water that less polar ones.24,25 Only withinresins of comparable polarity the free volumecorrelates with the equilibrium moisture uptake.Moreover, it was also found that, by increasingthe crosslinking density, the free volume fractionis increased along with the presence of polargroups. Steric hindrances at the crosslink junc-tions, in fact, introduce unoccupied volume ele-ments coincident with the polar hydroxyls andamines. This opens the matrix and facilitatesthe access to and the association of water mole-cules with the polar groups, thus increasing thewater uptake.

Specific water-epoxy interactions increasewater uptake but retard the transport kinetics,that is, the diffusion coefficient. On the otherhand, it was reported that the free-volume con-tent affects to a minor extent the transportkinetics.25 Sorbed water associates with the po-lar hydroxyl and amine groups. These polarsites, in fact, placed adjacent to the crosslinkjunctions, provide a sort of ‘‘trapping’’ sites for awater molecule diffusing through the epoxy,causing the water molecules to stop and inter-act, thus hindering diffusion. Therefore, ahigher resin polarity accounts for a large num-ber of trapping sites, a lower coefficient of diffu-sion and more water molecules in the boundstate. As emphasized, the coincidence of the po-lar sites and unoccupied volume elements makesthese epoxy-water interactions favorable fromboth an energetic and steric point of view.

Finally, another process can take place incold-cured thermosetting resins modifying theirproperties during time, that is, the ‘‘PhysicalAging.’’ The physical aging is a universal phe-nomenon that occurs in all the amorphous poly-mers below their Tg and that leads to a reduc-

1322 FRIGIONE AND LETTIERI


tion in the polymer’s free volume over time, thatis, in a ‘‘densification.’’26 Even though all large-scale molecular motion stops below the Tg, thepolymer glasses, not being in a thermodynamicequilibrium, continue to proceed toward theequilibrium through small-scale conformationalchanges. Structural relaxation in the glassystate is a very slow process, but it becomesquicker in the proximity of the glass transitiontemperature.27,28 The main effect of physicalaging resides in the modification of all the tem-perature-dependent properties.27,29–33 The struc-tural changes produced by physical agingdepends on time and temperature of aging;the behavior of a polymer at temperatures belowthe glass transition are normally the result ofa slow relaxation of the glass from its initialnonequilibrium state toward its final thermody-namic equilibrium state at that temperature.The physical aging, finally, is a thermorevers-ible phenomenon that can be erased by heat-ing the polymer above its glass transitiontemperature.26,27

The occurrence of physical aging in a polymerisothermally aged can be measured in adynamic DSC experiment as an endothermicpeak in the glass-transition region, whose posi-tion and intensity depends on temperature andtime of aging.34–36 The area of this peak isrelated to the relaxation enthalpy (DHrel) andincreases with the aging time, becoming con-stant when the thermodynamic equilibrium isachieved.

The process of densification of the resin, dueto the physical aging, is able to impede thetransport of water, thus retarding the moistureuptake. Referring to a semirigid epoxy aminesystem (Tg ¼ 118 8C for the dry system), it isreported that, at short times, the aged resinpicks up less moisture than the deaged one.However, as time progresses, the differencesbetween the aged and deaged systems arediminished, until they appear to absorb thesame amount of water at saturation condition.24

The deaged resin, therefore, reaches the equilib-rium more rapidly than the aged material.

Ingress of water in an aged resin is capableto rearrange the network so as to erase thephysical aging, particularly in epoxy systemspossessing a more flexible network (for example:not fully cured low Tg systems). On the otherhand, in more rigid network, that is, a 100%crosslinked high Tg system, water is less able tosignificantly alter the effects of physical aging.

It can be concluded, therefore, that an increaseof the water content in aged specimens has asimilar effect than an increase in temperatureon the same systems, both leading to the ‘‘reju-venation’’ of the systems. Quantitatively, how-ever, the kinetics of the structural recovery dueto the ingress of water molecules is much slowerthan that due to a thermal treatment above theTg of the system.37

The difficulty to assess the long-term per-formance of the cold-curing thermosetting res-ins, mainly arising on their low Tg and resultingin an intrinsic instability of the cured systems,is even amplified by the lack of appropriatestandards for durability tests. Most of theexperiments involving durability aspects, infact, are conducted following standard proce-dures not appropriate for the particular systems.In particular, the standard frequently employedfor water absorption tests38 recommends a con-ditioning procedure, able to assure the completedryness to samples, to perform on the specimensbefore their immersion in distilled water, as fol-lows: each sample is dried in vacuum oven for24 h at 50 8C and then cooled in a desiccator.The described conditioning procedure performedon not fully crosslinked cold-cured resins can beregarded as a thermal treatment, that is, a post-cure, which will influence the properties of theresins. It must be considered, therefore, notappropriate for the analysis of cold-cured resins.

In this article the absorption/desorptionbehavior of a commercial epoxy resin, specifi-cally designed for civil engineering applications,subjected to different treatments prior to expo-sure to water, was analyzed. The effects of dif-ferent curing, conditioning, and storing condi-tions employed on the water absorption/desorp-tion process taking place in the resin wereevaluated.

EXPERIMENTAL

Material

A commercial two-component epoxy adhesivewas analyzed (trade mark: Epojet, supplied byMAPEI S.p.A., Milano, Italy). It is based ondiglycidylether of bisphenol-A cured with a mix-ture of polyethylenamine–xylenediamine–nonyl-phenol. This system is produced and purchasedas both matrix and adhesive for the manufac-ture and application, respectively, of fiber rein-



forced polymers composites employed for reha-bilitation procedures.

As indicated by suppliers, the resin and thehardener were carefully mixed using the recom-mended ratio, that is, resin:hardener ¼ 4:1 byweight. Samples of standard rectangular form(dimensions: 90 mm 3 10 mm 3 4 mm) wereobtained by pouring the mix into Teflon molds.The samples were left to cure at a temperatureof 23 8C 6 2 8C and a relative humidity (R.H.)of 50% 6 5%. They were removed from themolds after 26 h and they were, then, main-tained under controlled conditions of tem-perature and humidity, as reported in the nextparagraph.

Treatments

Tests of water and moisture absorption wereperformed on specimens of the epoxy adhesivecured at ambient temperature or previously sub-jected to thermohygrometric treatments. To thisaim, the prescriptions of the proper code38 werefollowed, with the exception of the conditioningprocedure, as detailed next.

A thermohygrometric treatment was per-formed on some samples cured in the laboratoryambient for 10 days. It consisted in a cyclicalvariation in both temperature and relative hu-midity, carried out in an environmental chamber(Mod. UY 600, ACS Angelantoni Climatic Sys-tems, Massa Martana, Perugia, Italy). Each cyclewas performed in accordance with the followingplan: 2 h at 23 8C and 50% R.H.; 1 h to changethe conditions; 22 h at 45 8C and 90% R.H.; 1 hto restore the initial conditions. The samples

were divided in two sets removed from thechamber after 10 and 30 cycles, respectively. Af-ter both the thermohygrometric treatments, thesamples were left in the laboratory ambient for8 months.

The untreated samples as well as those previ-ously subjected to the thermohygrometric treat-ments were completely dried employing differentprocedures before the exposure to liquid orvapor water. The first procedure consisted instoring the specimens in a desiccator with silicagel (supplied by Carlo Erba Reagenti, Rodano,Milano, Italy) at room temperature until con-stant weight was achieved. Weight measure-ments were registered using an analytical bal-ance with an accuracy of 60.1 mg (Sartorius,Mod. BP 2215, Sartorius Stedim Italy, S.p.A.,Bagno a Ripoli, Firenze, Italy). Some of theuntreated specimens were dried employing thisprocedure up to 3 months. In a second proce-dure, the specimens were kept in an oven(Selecta, Mod. Conterm, Swiss Vacuum Technol-ogies SA, Neuchatel, Switzerland) at 50 8C for7 days. This procedure was used on the remain-ing untreated samples, earlier allowed to cureat ambient temperature for 5 months, and onall the specimens previously subjected to thethermohygrometric cycles and subsequentlyleft in the laboratory ambient for 8 months.In Table 1, the curing and conditioning proce-dures performed on all the samples before thetests of water and moisture absorption, weresummarized.

The samples were, then, exposed to saturatedwater vapor (100% R.H.) or immersed in dis-tilled water, at a constant temperature (23 8C 6

Table 1. Curing, Drying, and Conditioning Procedures Performed on Each System Analyzed Before the Tests ofWater Absorption and Exposure to Moisture

SystemCure

Conditions CyclesProcedure BeforeExposure to Water Exposure to Water

H1 Ambient for 10 days No Silica gel for 3 months 100% R.H. up tosaturation

W1 Ambient for 10 days No Silica gel for 3 months Immersion up tosaturation

W2 Ambient for 5 months No Oven at 50 8C for 7 days Immersion up tosaturation

C10 Ambient for 10 days 10 thermohygrometriccycles

Ambient for 8months

Oven at 50 8Cfor 7 days

Immersion up tosaturation

C30 Ambient for 10 days 30 thermohygrometriccycles

Ambient for 8months

Oven at 50 8Cfor 7 days

Immersion up tosaturation

Control Silica gel for 4 years



2 8C). In both cases, the samples were periodi-cally taken from the storage box, wiped with adry cloth and weighed to calculate the percent-age of water absorption. After the measure-ments the samples were replaced in the con-tainer. From the weight variations of all thesamples subjected to these treatments, theamount of water absorbed during any exposurewas assessed. This procedure continued untilthe decrease in weight for three subsequentperiods, as shown by two consecutive weight-ings, was less than 1% of the original weightof the sample; at this point, the sampleswere considered to have reached the saturationcondition.

All the saturated samples were, then, placedin a desiccator with silica gel at room tempera-ture and periodically weighed to calculate thepercentage of water desorption. When the de-crease in weight for three subsequent periods,as shown by two consecutive weightings, wasless than 1% of the original weight of the sam-ple, the percentage of water totally desorbedfrom any sample was calculated.

For comparison purposes, control specimenswere stored in silica gel for the whole period ofthe analysis, that is, 4 years.

Tests

The thermal properties of the untreated samplesas well as of specimens subjected to the differenttreatments performed were determined usingdifferential scanning calorimetry. A DifferentialScanning Calorimeter (DSC 822, Mettler Toledo,Novate Milanese, Milano, Italy) was employed,performing calorimetric dynamic scans between5 and 200 8C with a heating rate of 10 8C/min,under nitrogen atmosphere at a flow rate of 80mL/min. The thermal properties (glass transi-tion temperature, relaxation enthalpy, and re-sidual heat of crosslinking reactions) were calcu-

lated as the average of at least three measure-ments.

SEM observations were carried out on thefractured surface of samples immersed for about1 min in liquid nitrogen and cracked, to producea brittle fracture. A Environmental ScanningElectron Microscope (ESEM, Mod. XL 30, FEICompany, Eindhoven, Netherlands) was used,performing the examinations in low vacuummode (80 Pa), with a voltage of 25 kV and a spotsize 5.1. The secondary electron imaging (GSE)was recorded at several magnifications. Themorphology of control samples (cured for 4 yearsin a controlled environment), of specimensimmersed in water up to saturation and aftertheir complete evaporation (W1 system), wasanalyzed.

RESULTS AND DISCUSSION

Tests of Absorption of Water

In Table 2, the results obtained after the immer-sion tests and those after the exposure to mois-ture are listed. In particular, the diffusion coeffi-cient, the content of water in correspondence ofthe saturation condition, and the time requiredto reach the saturation condition are reportedfor each system analyzed. In Figure 1, thecurves of water absorption for the same systemsare reported as a function of the square root oftime of immersion or of exposure to moisture.Saturation conditions were reached for all thesamples analyzed, even though the saturationtimes were different, mainly depending on theconditioning procedures used, as discussed next.

The diffusion coefficient (D) was calculated asfollows:

D ¼ p16

L

M1

8>:

9>;

2 Mt2 �Mt1ffiffiffiffit2

p � ffiffiffiffit1

p8>>:

9>>;

2

ð1Þ

Table 2. Results From Tests of Water Absorption/Desorption

System

DiffusionCoefficient(cm2/s)

Water Contentat Saturation

(wt %)

Time toSaturation(weeks)

Diffusion Coefficienton Evaporation

(cm2/s)

H1 1.98 3 10�9 1.818 60 1.50 3 10�9

W1 1.19 3 10�9 1.880 60 1.89 3 10�9

W2 2.52 3 10�9 1.762 27 6.52 3 10�8

C10 3.34 3 10�9 1.572 27 6.81 3 10�8

C30 3.38 3 10�9 1.779 27 7.11 3 10�8



where: Mt1 and Mt2 represent the mass of pene-trant absorbed at time t1 and t2, respectively,M? the absorbed water in correspondence tosaturation condition, and L is the sample thick-

ness. Therefore, by plotting Mt/M? versus(time)0.5/L and measuring the initial slope of thecurve (usually for Mt/M? ratios less than 0.6),the coefficient of diffusion D was estimated.

In Figure 2(a,b) the glass transition tempera-tures calculated on all the systems analyzed arereported as function of time of immersion/expo-sure to water/moisture and of water content,respectively. As expected, the ingress of watermolecules in the cured systems determines clearreductions in Tg as consequence of the plastici-zation process.

In Figure 3(a), the relaxation enthalpy, DHrel,measured on all the systems at different timeof immersion/exposure to water/moisture, is re-ported. In Figure 3(b), this property is reportedas a function of the percentage of water content.The numerical values of DHrel measured on eachsystem before the exposure to water are alsolisted in Tables 3 and 4. As expected, a generaldecrease of the relaxation enthalpy is observedby increasing the immersion time and the watercontent, with the only exception of W2 system.

Figure 1. Water absorption curves versus squareroot of immersion time or of exposure to moisturetime.

Figure 2. Glass transition temperature (Tg) meas-ured on different systems versus (a) time of immer-sion in water or of exposure to moisture; (b) watercontent.

Figure 3. Relaxation enthalpy measured on differ-ent systems versus (a) time of immersion in water orof exposure to moisture; (b) water content.



The segmental mobility in thermosetting resins,induced by water ingress, may reflect themotions of segments of unreacted molecules,which can be present in the cured system, aswell as the motions of chain segment and endschain in crosslinked regions of network;36 hence,it can also produce the erasure of physicalaging. In a previous research, similar resultswere found, confirming that, during a hygromet-ric treatment, a deaging process took place.20

Samples Kept in a Dry Atmosphere

The removal of water in samples of H1 and W1systems before their exposure to water was per-formed by storing them in a dry ambient until aconstant weight was reached. In Figure 4 it canbe observed that the reduction in weight ofspecimens stored in silica gel is continuous upto about 1 month, with an overall decrease inweight of about 0.2% with respect to the initialweight. These specimens kept in a dry environ-ment for 3 months showed a Tg of about 57 8Cand a residual heat of reaction of about 9 J/g, asreported in Table 3. The presence of a residualheat of reaction indicates that, even after thislong time, the curing reactions carried out atambient temperature did not reach the comple-

tion. The enthalpy of relaxation measured at theend of the 3-month period is 8.7 J/g, which canbe regarded as the limiting value reached by thesystem due to its asymptotic approach towardthe metastable equilibrium state.

By comparing the results of immersion/expo-sure to water reported in Table 2 and the curvesshown in Figure 1, similar values for diffusioncoefficient, content of water at saturation andtime for saturation, were found for H1 and W1systems: that is, their behavior is comparableirrespective to the fact that water was in liquidor in vapor state.7

The immersion/exposure to water producedreductions in the Tg of H1 and W1 systems, asshown in Figure 2(a,b). In particular, theabsorption of water in the system H1 leads tothe lowering of Tg, that follows a linear decreasewith the percentage of water uptake, at a rateof about 13 8C for 1% of water absorbed at leastup to 1-month exposure time, which is in agree-ment with the mentioned empirical rule. W1system, on the other hand, shows a more rapidreduction in its Tg up to a value measured aftera 2-week immersion similar to that measured onH1 after a 4-week exposure to moisture. Forlonger immersion times, the Tg of W1 increasesagain. This can be attributed to the reactivation

Table 3. Thermal Properties Measured on H1 and W1 Systems Before and After Exposure to Water

System

Before exposure to water After evaporation

Tg (8C)ResidualDH (J/g)

RelaxationDH (J/g) Tg (8C)

ResidualDH (J/g)

RelaxationDH (J/g)

H157.1 6 0.3 9.2 6 0.4 8.7 6 0.3

n.m.W1 66.2 6 0.3 1.9 6 0.2 5.0 6 1.0Control Tg ¼ 64.1 8C 6 1.5 8C

n.m. ¼ not measured.

Table 4. Thermal Properties Measured on W2, C10, and C30 Systems Before and After theThermal Treatments Performed

System

Before the thermal treatment Before exposure to water

Tg (8C)RelaxationDH (J/g) Tg (8C)

ResidualDH (J/g)

RelaxationDH (J/g)

W2 55.1 6 0.1 6.5 6 0.9 63.4 6 0.3 0 4.5 6 0.8

C10

End ofcycles

After 8months

End ofcycles

After 8months

63.0 6 0.7 0 4.3 6 0.958.1 6 3.0 59.4 6 0.5 3.6 6 0.3 7.3 6 0.5C30 51.7 6 0.7 60.1 6 0.5 2.0 6 0.4 5.6 6 0.6 62.5 6 1.8 0 4.1 6 1.1



of the incomplete curing reactions with the con-sequent new increase in the Tg, as alreadystated. The latter would have been occurred inthe first 4 weeks of immersion in the case ofW1, whose Tg was severely reduced during thisperiod. Unfortunately, the measurements of Tg

were performed on H1 and W1 systems onlyduring the first period of immersion/exposure(up to 4 weeks) and it was not possible to con-firm the same behavior for H1 system at longerexposure times.

The values of relaxation enthalpy measuredon H1 and W1 systems at different times ofimmersion/exposure and at different levels ofwater content, were found reasonably similar, asshown in Figure 3(a,b).

Firstly, the initial values of their relaxationenthalpy were the highest with respect to theother systems, that is, about 9 J/g, since theywere not subjected to any thermal treatmentand the physical aging proceeded entirely onthese systems. This last observation explainsalso the lowest diffusion coefficients and thehighest saturation time found for these systemswith respect to the others. It is reported, in fact,that the physical aging produces a decrease ofpermeability and diffusivity of small moleculesof penetrant, with a consequent reduction in thesorption kinetics and equilibrium sorption level,due to the reduction of free volume of the ‘‘den-sified’’ resin.8,24,39,40 The value of water contentat saturation for W1 was found comparable withthat of specimens on which the physical agingwas erased by the thermal treatment, that is,W2 system. This suggests that the decelerationof the diffusion process in W1 samples did notproduce the expected reduction in the equilib-rium sorption level. An explanation of such dis-

crepancy can be the presence of a large numberof unreacted polar sites in the samples of W1,not completely cured before immersion. It isreported, in fact, that specific water-epoxy inter-actions can increase the equilibrium of wateruptake in an epoxy resin.25,41 On the otherhand, the presence of such unreacted polar sitesseems to slow down the diffusion kinetics,due to the frequent formation of trappingsites limiting the movement of the water mole-cules. This last aspect, however, needs furtherinvestigations.

Secondly, the reductions in DHrel found forH1 and W1 systems with increasing the contentof absorbed water were more marked than forany other system. The decrease in relaxationenthalpy was continuous for H1 system. In thecase of W1, after a large reduction in DHrel dur-ing the first 2 weeks of immersion, the decreasein relaxation enthalpy became less prominent atlonger immersion times. The last result can beexplained by the reactivation of curing reactionstaking place in W1 system in correspondence ofthe same immersion period, as envisaged by theincrease in its Tg. The additional crosslinking,reducing the segmental mobility of unreactedmolecules, would have been able to slow downthe deaging process.

Finally, the presented results found for H1and W1 systems are in a good agreement withthose obtained in a previous research perform-ing the tests of water absorption in similar con-ditions, with few differences ascribed to a cer-tain scattering of results.20

Samples Subjected to a Thermal TreatmentAfter a Permanence in Atmosphere

In Table 4, the data relative to the thermalproperties measured on W2 system before andafter the thermal treatment are reported. TheTg measured on W2 kept in laboratory environ-ment for 5 months before the thermal treat-ment, that is, 55.1 8C, is only slightly lowerthan that measured on W1 system after a 3-month storage in silica gel (about 57 8C). Thissuggests that the presence of a small amount ofwater in W2 system before the thermal treat-ment has had a limited effect on its Tg. On theother hand, the thermal treatment performed todry up the specimens of W2 system before theirimmersion in water produced the postcure of thesamples: the crosslinking reactions, not com-pleted during the ‘‘cold-cure’’ performed at ambi-

Figure 4. Weight decrease measured on H1 and W1systems, stored in desiccator with silica gel, versusstoring time.



ent temperature for 5 months, started anew andcompleted at a temperature (50 8C) close to theinitial Tg (about 55 8C). As a consequence, anincrease in Tg of about 8 8C (from 55 to 63 8C)and the absence of any residual heat of reactionwere measured for this system.

The relaxation enthalpy measured on W2 sys-tem after the ambient-cure performed for 5months (i.e., 6.5 J/g) is relatively close to thatcalculated on W1 system after the 3-month stor-age in silica gel (i.e., 8.7 J/g), confirming in bothsystems the achievement of the limiting equilib-rium value. On the other hand, the thermaltreatment subsequently performed on W2 at50 8C, that is, a temperature not much lowerthan its Tg, caused the partial ‘‘deage’’ of thesystem, reducing its relaxation enthalpy (downto 4.5 J/g).

The thermal treatment performed at 50 8C onW2 also affects its behavior toward the immer-sion in water, as can be inferred by comparingthe data reported in Table 2 and shown in Fig-ure 1 for W1 and W2 systems both immersed inpure water. In particular, the time to reach thesaturation condition was much higher for thesamples not subjected to any thermal treatmentbefore immersion, that is, W1. Accordingly, thediffusion coefficient for this latter system waslower than that found for W2, indicating a lowerrate for the diffusion process of water inside thespecimens. In W2 system, in fact, the effects ofphysical aging were partly removed by heatingthe specimens at a temperature close to their Tg

before immersion.The immersion in water produced in W2 sys-

tem a continuous decrease in Tg within the first3 months of immersion, with a maximum reduc-tion in Tg of about 10 8C. The reduction in Tg

follows a linear relationship with the percentageof water uptake, at a rate of about 7 8C for 1%of water absorbed. Then, a new small increasein Tg is observed, that cannot be ascribed to anadditional postcuring of the resin, since it wascompletely cured before the immersion. It wasattributed to the pseudo crosslinking broughtabout by the water molecules forming bridg-ing between chain segments, as previously de-scribed. The same behavior is also reported in astudy performed on a bisfenolic epoxy resin(DGEBA) fully cured with an amine-terminatedpoly(propylene oxide) at 100 8C to obtain a Tg of72 8C.37 The authors stated that, even thoughthe system was relatively stable in the dry state,when the system was subjected to water sorp-

tion-desorption cycles, its Tg steadily increasedup to 76 8C.

Referring to the relaxation enthalpy meas-ured on W2 system with increasing immersiontime and water content, Figure 3(a,b), after theexpected initial decrease of DHrel (i.e., in thefirst 2 weeks), it remained fairly constant for a3-month immersion period and, then, it in-creases again. The new increase of DHrel beganin correspondence of the minimum value mea-sured for its Tg. It appears, therefore, thatthe significant reduction of the Tg yields thereactivation of the physical aging process. Thephysical-thermodynamic explanation of this as-pect needs further investigations, once verifiedthat the differences of DHrel values measured onW2 systems during the immersion in water aresignificant.

Samples Subjected to ThermohygrometricTreatments

Specimens subjected to repeated thermohygro-metric cycles, that is, contemporary exposed to atemperature of 45 8C and to 90% R.H., are likelyto experience an increase in Tg, due to the com-pletion of crosslinking reactions, along with areduction in Tg due to the high level of humid-ity.20 The neat increase in the Tg was lowerthan that expected if the effects of the two treat-ments, that is, thermal and hygrometric, wereadditive.

The results reported in Table 4 shows thatthe Tg measured immediately after the end ofthe thermohygrometric treatments was slightlyhigher, for C10 system, or slightly lower, for C30system, than that measured on W2 system leftin ambient for 5 months. The increase in Tg

found for C10 system confirmed the occurrenceof an additional cure on the system. On theother hand, the lower Tg measured after the 30cycles can be attributed to a larger amount ofwater absorbed after the longer exposure timeemployed in the 30-cycle treatment and, hence,to a more marked effect of plasticization. How-ever, by repeating the measurements on thesame systems left in laboratory ambient for 8months, the Tg of both systems experienced anadditional increase, more marked in the case ofC30 system (DTg % 18 8C). The new increase inTg suggests that the water absorbed during thecycles, particularly by specimens of C30 system,was able to evaporate, at least partially, from



the samples stored in a laboratory ambient for aprolonged time.

A noticeable reduction in relaxation enthalpywas measured on both systems at the end of thecycles, indicating that the thermohygrometrictreatments also produced a partial erasure ofthe physical aging, brought about by the con-temporary ingress of water into the specimensand, mostly, the increase of temperature. Then,the physical aging started again, as proved bythe increase in DHrel measured after the 8-month storage, since the specimens were kept atlower humidity and temperature levels. The pre-sented results are again in a good agreementwith those reported in our previous study, eventhough, in that case, the specimens have beensubjected to a different conditioning procedure.20

The results of both works prove that the effectsof limited variations in temperature and mois-ture are to some extent reversible.

Before immersion in water, samples of C10and C30 were subjected to a thermal treatmentat 50 8C for 7 days. Both the thermohygrometricand the subsequent thermal treatments con-curred to complete the crosslinking reactions inthese systems, as confirmed by the measure-ments of the residual heat of reaction found forboth systems equal to 0. Moreover, the values ofTg and DHrel measured at the end of the ther-mal treatment on C10 and C30 systems werefound comparable to those calculated on W2 sys-tem at the end of the same treatment (Table 3).It can be concluded, therefore, that the thermaltreatment performed was able to erase theeffects of a previous hydrothermal history onthe thermal properties that can be evaluated bya DSC.

The results of the immersion tests performedon samples of C10 and C30 are reported inTable 2 and in Figures 2(a,b) and 3(a,b) and canbe compared with those found for W2 system.The behavior of both systems subjected to thethermohygrometric cycles is reasonably similarto that observed for W2 system, at least if com-pared to the behavior displayed by H1 and W1systems. The saturation time was the same forthe three systems. The water content at satura-tion found for C30 is close to that measured onW2 system, while it was slightly lower in thecase of C10 system. On the other hand, the sam-ples of C10 and C30 show a certain increase ofthe diffusion rate of water with respect to W2that cannot be ascribed to physical aging effects,since the values of DHrel found for C10 and C30

systems before immersion were comparable tothat calculated on W2 system.

As a first explanation, a possible cause of thehigher diffusion coefficients of the sorption pro-cess for C10 and C30 samples with respect toW2 could be due to the presence of ‘‘bound’’water in the specimens previously subjected tothe thermohygrometric treatments. This water,not completely removed by the thermal treat-ment, would be more strictly linked to the resinwith two hydrogen-bonds. This would agreewith the similar values of Tg found for C10,C30, and W2 systems before the immersion inwater, since the bound water does not produceany plasticization in epoxy.14 Therefore, uponimmersion in water, the water molecules diffus-ing inside the systems C10 and C30 cannot bindthe polar sites of the polymer, already occupiedby the bound water molecules, and they wouldhave an easy ingress path. On the other hand,bound water, forming interactions with specificsites distributed on the polymer network, isreported to slow down the diffusion process.10

This justification, therefore, would be not suita-ble to explain the differences in diffusion coeffi-cients, at least in the whole period of immersionanalyzed.

Referring to the very beginning of the sorp-tion stage (up to 1–2 days), a slower diffusionprocess is observed for C10 and C30 systemswith respect to W2 system (as showed for C30in Figure 5(a). After this period, the diffusionprocess of W2 seems to slow down, displaying astep-like behavior up to saturation point. In cor-respondence to the same initial period, W2 dis-

Figure 5. (a) Water weight gain and (b) Tg versussquare root of immersion time for W2 and C30 sys-tems.



plays a significant decrease in Tg up to 1 monthof immersion, as shown in Figure 5(b). A highdiffusion coefficient along with a remarkabledecrease in Tg would suggest the access of plas-ticizing water inside the specimens of W2. Forlonger immersion times, the decrease of diffu-sion coefficient found along with a smallerreduction in Tg values [Fig. 2(a)] suggest thatthe entering water starts to form double hydro-gen-bonds.

Comparing the water weight gain and the Tg

values for C30 system for the very beginning ofthe sorption stage as function of square root ofimmersion time, Figure 5(a,b), respectively, theinitial small increase in Tg can be ascribed tothe pseudo crosslinking produced by the watermolecules absorbed in this initial immersion pe-riod, leading to a reduced diffusion coefficientrespect to W2 system. Then, the diffusion pro-cess continues at an almost constant rate withlimited reductions in Tg (not exceeding 6–7 8C).

The relaxation enthalpy measured on C10and C30 systems continuously decreased withimmersion time and water content as a conse-quence of the ingress of water, [Fig. 3(a,b)]. Thereductions in DHrel occurred slowly than that forH1 and W1 systems; the initial values of relaxa-tion enthalpy measured for C10 and C30 sys-tems, however, were lower than those calculatedfor H1 and W1 and smaller reductions wereexpected. C10 and C30 systems were, in fact,completely crosslinked before their immersion inwater. Thus, the deaging process upon wateringress for these systems was slower than forH1 and W1 due to a reduction of the segmentalmobility inside the polymeric molecule. Thereactivation of the physical aging process in thiscase did not take place.

Tests of Desorption of Water

Once the saturation condition was achieved, allthe systems analyzed were removed from thestorage box and allowed to dry placing them ina desiccator with silica gel. Weight changes dur-ing the drying period was monitored and theresults are shown in Figure 6.

Samples Kept in a Dry Atmosphere

The decrease in weight observed for H1 and W1systems appears to be slower than any othersystem. This would confirm the hindrance to dif-fusion of water molecules in absorption stage

brought about by a higher extent of physicalaging taking place in H1 and W1 specimens,able to slow down the motion of water moleculesin these systems. The comparison of diffusioncoefficients found for desorption process for allthe systems analyzed, as reported in Table 2,confirms this statement. The diffusion coefficientduring desorption process was calculated usingeq 1, already employed for the calculation of thediffusion coefficient in absorption stage.

Small differences between the diffusion coeffi-cients measured during absorption and desorp-tion processes were found for H1 and W1 speci-mens. This would be again explained taking intoaccount the larger amount of physical agingoccurred in the latter systems. Once again, wecannot exclude that the presence of unreactedsites inside the structure of H1 and W1 systemscan also affect the rate of diffusion during de-sorption; however, this aspect needs furtheranalysis.

Water uptake seems to be only partially re-versible upon drying in H1 and W1. After dryingto an asymptotic equilibrium value, in fact, asmall net mass increase was observed, if com-pared with the initial weight of unexposed speci-mens, suggesting also in these systems the pres-ence of some ‘‘bound’’ water not removable evenby using silica gel.

Finally, in Figure 7 the comparison of the lat-ter stage of the desorption process taking placein W1 system, previously immersed in water upto saturation, with the desorption of W1 and H1during the drying process performed before itsimmersion in water, is presented. The evapora-tion of the few water molecules during the ini-tial drying process occurred at the same rate(slope ¼ �6.7 3 10�3) that the evaporation of

Figure 6. Water desorption curves versus squareroot of time.



the latter water molecules absorbed during thecomplete immersion in liquid water (final part ofthe curve enclosed in the box: slope ¼ �7.0 310�3).

Samples Subjected to a Thermal Treatment After aPermanence in Atmosphere

In the case of W2 system, the water absorbedduring the sorption stage is almost completelyremoved during the subsequent desorption pro-cess. A very limited amount of ‘‘bound’’ water,however, is still measured also in this case.

Moreover, by comparing the diffusion coeffi-cients on absorption and desorption processesfor W2, a large increase in diffusivity is observedduring the evaporation of water with respect tothe sorption stage. This phenomenon has al-ready been observed by different authors andexplained in terms of ‘‘free’’ more mobile waterthat is the first to leave the specimens duringdesorption stage.12,25

Samples Subjected to ThermohygrometricTreatments

In the case of C10 and C30 specimens, the equi-librium weight after evaporation is even lowerthan the initial one, that is, the weight meas-ured before their immersion in water. Before ex-posure to liquid water, C10 and C30 systemswere subjected to hydrothermal cycles, left for aprolonged time in laboratory atmosphere andkept in oven at 50 8C for a week. As previouslydiscussed, this treatment was probably not able

to remove completely the water previously ab-sorbed by the samples during the cycles. There-fore, it can be hypothesized that, when thesamples were dried using silica gel, a slightlylarger part of water was removed. It can be con-cluded, therefore, that the permanence for a pro-longed time in silica gel is much more efficientto remove moisture from these crosslinked sys-tems than a thermal treatment performed at awarm temperature. As previously observed forthe other systems, however, even using thistreatment, it is not possible to remove com-pletely the ‘‘bound’’ water.

The diffusion coefficients calculated for C10and C30 during the desorption process wereagain much higher than those calculated duringabsorption stage.

Thermal Properties

Finally, the calorimetric properties measured onthe different specimens after the different treat-ments performed were compared to those foundon un-treated specimens. These control speci-mens were stored in silica gel for 4 years. At theend of this period, they were analyzed by DSCand the following results were found: Tg ¼64.1 8C (6 1.5); residual heat of reaction ¼ 6.5J/g (6 0.5); DHrel ¼ 6.8 J/g (6 0.2). This provedthat, after curing the system in a dry atmos-phere at ambient temperature for prolongedtimes (several years), the crosslinking reactionswere still not completed. However, a reasonablyhigh Tg was reached by samples kept for thisvery long period in an extremely dry atmos-phere (R.H. about 10%).

These properties are first compared withthose measured on W1 system at the end of theevaporation procedure (reported in Table 3). Inthis condition, W1 showed a Tg slightly higherthan that measured on control samples, that is,about 66 8C, and a much lower residual heat ofreaction, about 2 J/g. These results confirmedthat, during the immersion in water, the reacti-vation of the curing reactions occurred in W1specimens, leading to a higher Tg and a lowerresidual heat of reaction with respect to controlspecimens never exposed to water. We cannotexclude, however, that the presence of a smallamount of plasticizing water (i.e., that forming asingle hydrogen-bond with polar sites) still con-tained in the specimens, as concluded by theweight measurements, is able to plasticize to alimited extent W1 system, producing a very

Figure 7. Comparison between the desorption pro-cess taking place in W1 system, previously immersedin water up to saturation, and the desorption of W1and H1 during the drying process performed beforeits immersion in water.



small decrease in its Tg. The clear difference inthe residual heat of reaction measured on W1and control systems, in fact, does not seem toreflect the small difference in their Tg values.

Referring to W2, the calorimetric data of thissystem at the end of the desorption process arenot available. The comparison is made, there-fore, with the properties calculated on W2 sys-tem before its immersion (Table 4). The thermaltreatment performed on this system at 50 8Cbefore immersion in water led to the completepostcure of the system: a Tg of about 63 8C andthe absence of any residual heat of reactionwere found. This Tg is comparable to that mea-sured on control specimens that, on the otherhand, did still show a residual heat of reaction.This contradictory result could be possiblyexplained in terms of the temperature used topostcure W2 system, higher than the ambienttemperature at which the cure of control speci-mens takes place. It is known, in fact, that thereaction of cure of a monomer/curing agent isvery temperature dependent, that is, highertemperatures shorten the curing time, but it hasalso an important influence on the properties ofcured products. The development of good me-chanical properties requires the attainment ofa sufficiently high degree of crosslinking. Thiscondition is typically obtained by curing or post-curing at high temperatures, even though net-works obtained at temperatures higher thanthat required for the specific curing agent showa progressive reduction of the glass transitiontemperature.2 Explanations of such phenomenahave been given in terms of the formation of aless dense network at high curing temperaturespossibly due to volatilization and/or oxidation ofthe hardener at these temperatures. Referringto cold-curing epoxy resins, an attempt toincrease the Tg of similar commercial bisphe-nolic epoxy resins by curing them at a slightlyhigher temperature (60 8C) resulted ineffective,leading, on the contrary, to a slight reduction ofthe glass transition temperature.

The relaxation enthalpies calculated on con-trol specimens (6.8 J/g) and on W1 and W2 sys-tems before the exposure to water and the ther-mal treatment, respectively, are reasonablyclose, indicating in all cases the achievement ofthe limiting equilibrium value. A slightly lowervalue of DHrel was found on W1 after evapora-tion, confirming that the occurrence of addi-tional crosslinking during immersion wouldhave reduced the physical aging in W1 system.

Morphology from SEM Observations

In Figures 8–10, the internal morphology of thedifferent systems evaluated by SEM microscopyis shown. The observation of these micrographscan provide a macroscopic qualitative view ofthe effect of sorbed water inside the systems.

A surface with distinct fracture lines charac-teristic of brittle resins is evidenced in the con-trol sample, that is, that cured for 4 years in acontrolled environment, as well as in a W1 sam-ple analyzed after the complete evaporation ofwater (Figs. 8 and 10, respectively). In bothcases, the substantial absence of water is con-firmed by the absence of zones displaying ductilefracture features. On the other hand, the inter-nal morphology of a specimen of W1 system justafter the achievement of saturation conditionsevidences two distinct zones [Fig. 9(a)]: an exter-nal region, that is, close to the edge of the sam-ple, that appears to have fractured in a ductilemanner [Fig. 9(b)] and an internal zone, that is,the ‘‘core’’ of the specimen, where a brittle frac-ture is observed [Fig. 9(c)]. The different fea-tures observed in the last system proved the ex-istence of an advancement front of water insidethe specimen. Therefore, the fracture occurs in amore ductile way in the external zone, due tothe presence in this region of a higher amountof water able to plasticize the resin.

CONCLUSIONS

The absorption/desorption behavior of a cold-cured epoxy resin was investigated in relation tothe different curing, conditioning, and storing

Figure 8. SEM micrograph of the central part ofa control sample cured for 4 years in a controlledenvironment.



treatments performed before the exposure towater. It was found that both the amount ofequilibrium water uptake and the rate of waterabsorption depend on the different processescarried out on the systems.

Diffusion of water, in liquid or vapor state, isfound to be sensitive to the physical aging

effects. The effects of physical aging, on theother hand, are to some extent reduced by theingress of water molecules: thus the effect of ahygrometric treatment is similar to that of athermal treatment, both being able to ‘‘deage’’an aged system. The rate of a hygrometric deag-ing procedure, however, is markedly reducedwith respect to a thermal deaging one. Theoccurrence of additional crosslinking in the notfully cured systems reduces further the rate ofthe deaging process.

Polar affinity existing between specific sitesinside the cured resin and the water moleculesaffects the diffusion kinetic and, to a lowerextent, the amount of water absorbed at satura-tion. This polar affinity is related to the pres-ence of unreacted groups (for the systems notcompletely cured before exposure to water), or tohydroxyl groups (formed during crosslinkingreactions).

The rate of diffusion and the amount of wateralso depends on the presence of ‘‘bound’’ waterinside the cured resins, that is, water formingstrong interactions with the polar segments ofthe polymer and not easy to remove by any dry-ing treatment.

The ingress of ‘‘plasticizing’’ water moleculesin the cured systems determines clear reduc-tions in their Tg. It is confirmed, however, thatin cold-curing epoxy resins the decrease in Tg

never exceeds 10 8C with respect to the initialTg measured on dry specimens. In addition, anew increase in Tg was observed in some cases,attributed to the reactivation of the incompletecuring reactions for the not fully cured samples,or to the formation of pseudo crosslinkingbrought about by water molecules forming

Figure 10. SEM micrograph of the central part of asample of W1 system after its complete evaporation.

Figure 9. SEM micrograph of a sample of W1 sys-tem immersed in water up to saturation: (a) generalview (lower magnification); (b) internal part close tothe edge of the specimen; (c) internal part placed inthe center of the specimen.



bridging between chain segments for the speci-mens completely cured before exposure to water.

Finally, it was proved that the permanence ina very dry atmosphere (silica gel) for a pro-longed time is much more efficient to removemoisture from cold-cured epoxy systems than athermal treatment performed at a warm temper-ature. A much higher temperature, higher than100 8C, would probably be more efficient in dry-ing samples, but it is not practicable in suchcases. Any thermal treatment, in fact, is likelyto act as a postcuring process, thus altering thecharacteristics of the system under analysis.However, even by using a silica gel treatment, itis not possible to remove completely boundwater from the samples previously exposed towater.

The results presented in this article confirmthat the cold-cured epoxies employed in rehabili-tation or renewal of civil infrastructures andcultural heritage are in an unstable thermody-namic equilibrium, mainly attributed to theincomplete curing reactions taking place in theresins at ambient temperature. The externalenvironmental agents can easily perturb thisequilibrium, leading to soft or severe modifica-tions in the structure of the cured resins and, inturn, to permanent changes in their long-termperformance. These agents are, in fact, able toinduce different chemical or thermodynamicprocesses in the systems, whose effects are noteven easy to separate and analyze.

The authors thank M. Masieri (of Italian CNR-IBAM,Lecce) for his technical support in performing ESEManalysis.

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Procedures conditioning the absorption/desorption behavior of cold-cured epoxy resins

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