Water sorption/solubility of dental adhesive resins

d e n t a l m a t e r i a l s x x x ( 2 0 0 6 ) xxx–xxx

avai lab le at www.sc iencedi rec t .com

journa l homepage: www. int l .e lsev ierhea l th .com/ journa ls /dema

Water sorption/solubility of dental adhesive resins

Juliana Malacarne, Ricardo M. Carvalho, Mario F. de Goes, Nadia Svizero,David H. Pashley, Franklin R. Tay, Cynthia K. Yiu, Marcela Rocha de Oliveira Carrilho ∗

Department of Restorative Dentistry, Dental Materials Division, Piracicaba School of Dentistry, University of Campinas,Av. Limeira, 901 Areiao - Materiais Dentarios, Piracicaba, SP, 13414-903, Brazil

a r t i c l e i n f o

Article history:

Received 31 May 2005

Accepted 1 November 2005

Keywords:

a b s t r a c t

Objectives. This study evaluated the water sorption, solubility and kinetics of water diffusion

in commercial and experimental resins that are formulated to be used as dentin and enamel

bonding agents.

Methods. Four commercial adhesives were selected along with their solvent–monomer com-

bination: the bonding resins were of Adper Scotchbond Multi-Purpose (MP) and Clearfil SE

Bond (SE) systems, and the “one-bottle” systems, Adper Single Bond (SB) and Excite (EX). Five

Dental resins

Water sorption

Solubility

Hydrophilicity

experimental methacrylate-based resins of known hydrophilicities (R1, R2, R3, R4 and R5)

were used as reference materials. Specimen disks were prepared by dispensing the uncured

resin into a mould (5.8 mm × 0.8 mm). After desiccation, the cured specimens were weighed

and then stored in distilled water for evaluation of the water diffusion kinetics over a 28-day

period.

Results. Resin composition and hydrophilicity (ranked by their Hoy’s solubility parameters)

influenced water sorption, solubility and water diffusion in both commercial and exper-

imental dental resins. The most hydrophilic experimental resin, R5, showed the highest

water sorption, solubility and water diffusion coefficient. Among the commercial adhesives,

the solvated systems, SB and EX, showed water sorption, solubility and water diffusion coef-

ficients significantly greater than those observed for the non-solvated systems, MP and SE

(p < 0.05). In general, the extent and rate of water sorption increased with the hydrophilicity

of the resin blends.

Significance. The extensive amount of water sorption in the current hydrophilic dental resins

is a cause of concern. This may affect the mechanical stability of these resins and favor the

rapid and catastrophic degradation of resin–dentin bonds.

© 2005 Published by Elsevier Ltd on behalf of Academy of Dental Materials. All rights reserved.

1. Introduction

Manufacturers have added hydrophilhydrophobic dimethacrylates in an atteffective bonding between hydrated denposites. Although such hydrophilic resinshigh immediate bond strength to dentin, mvitro studies have shown that resin–dentin

∗ Corresponding author. Tel.: +55 11 37650813E-mail address: [email protected] (M.R

much weaker over time [1–6]. The instability of resin–dentin

0109-5641/$ – see front matter © 2005 Publishdoi:10.1016/j.dental.2005.11.020

ic monomers toempt to promote

tin and resin com-are able to achieveany in vivo and ininterfaces become

; fax: +55 19 34125218..O. Carrilho).

bonds has been attributed to the porous nature of hybridlayer [7], which behaves as a permeable structure [8,9] thatseems to be susceptible to slow water hydrolysis [4,10]. Intheory, hydrolysis of resin–dentin bonds involves degradationof both components of hybridized dentin, resin and collagenfibrils. Nevertheless, in a recent in vitro study, the resincomponents of hybrid layers were rapidly lost [11] suggesting

ed by Elsevier Ltd on behalf of Academy of Dental Materials. All rights reserved.

DENTAL-854; No. of Pages 8

2 d e n t a l m a t e r i a l s x x x ( 2 0 0 6 ) xxx–xxx

that alterations in resin components may be the first step inthe degradation of such interfaces.

Ideally, polymer networks should be insoluble materialswith relatively high chemical and thermal stability. How-ever, most of the monomers used in dental resin materialscan absorb water and chemicals from the environment, andalso release components into the surrounding environment[12,13]. Moreover, it has been shown that the movement ofwater from hydrated dentin may cause the formation of water-filled channels within the polymer matrices of contemporaryhydrophilic dentin adhesives [14]. Accordingly, these water-filled channels may accelerate elution of unreacted monomersfrom polymerized resins [15], as well as promote weakening ofthe polymers by plasticization [7,8]. Thus, both water sorptionand solubility would lead to a variety of chemical and physicalprocesses that may result in deleterious effects on the struc-ture and function of dental polymers, including their retentivecapacity in adhesive dentistry.

Studies have mostly been focused on determining thewater sorption characteristics of epoxy-based polymers[16–19]. However, data are scanty on the kinetics of waterdiffusion in methacrylate-based resins that are employedas adhesives for bonding to hydrated dentin. It has beenspeculated that the permeability of polymerized resins maybe related to their polarity [20,21], and also that the incorpo-ration of more hydrophilic and ionic functional groups into apolymer network will lead to increased water absorption by

2. Materials and methods

2.1. Specimen preparation

Four commercially available dental adhesives, which werechosen according to their different solvent–monomer combi-nations, were tested: the solvent-free bonding resins of AdperScotchbond Multi-Purpose (MP) and Clearfil SE Bond (SE) sys-tems; and the “one-bottle” systems, Adper Single Bond (SB)and Excite (EX), both solvated with ethanol/water mixtures.Their composition and respective manufacturers are shownin Table 1.

Five non-solvated resin blends (R1, R2, R3, R4 and R5) ofpotential dentin and enamel adhesives were formulated byBisco Dental Products Co. (Schaumburg, USA). These resinswere ranked according to increasing hydrophilicity [21]. Theircomposition and Hoy’s solubility parameters are shown inTable 2. Hoy’s solubility parameters (ı) were calculated by sum-ming the molar attractive constants of each repeating func-tional group in the polymer using the method of Van Krevelen[23] and Barton [24]. These intermolecular attractive forces canbe categorized as polar forces (ıp), hydrogen forces (ıh), or dis-persive forces (ıd). The square root of the sum of these squaresforces yields the total cohesive energy density (ıt) that is equiv-alent to Hildebrand’s solubility parameter [25]. The ıt valuesare included in Table 2 for those who prefer the Hildebrand

s use

A, bisp-toluEMAEMA

c acidnd sta

e; CQed on

such polymers [20–22]. Nevertheless, there are no sufficientinformation from a single study that allow us to draw acorrelation between resin hydrophilicity of methacrylate-based resins and their corresponding water sorptioncharacteristics.

In order to assess the relationship between the hydrophilic-ity of methacrylate-based dental resins and the extent ofabsorbed water, this study evaluated the water sorptionand solubility in commercial dental adhesives as well asin a series of experimental resin blends of known compo-sition and hydrophilicity. These experimental dental adhe-sives were previously ranked for increasing hydrophilicity bytheir respective Hoy’s solubility parameters [21]. The kinet-ics of water diffusion through each group of dental adhe-sive (commercial and experimental) was also monitored uponageing in water. It was hypothesized that the degree ofhydrophilicity of methacrylate-based adhesives has a factualeffect on their water sorption, solubility and water diffusioncoefficient.

Table 1 – Composition of the commercial adhesive system

Adhesive and manufacturer

Clearfil SE Bond (SE); Kuraray Medical, Inc., Japan MDP, HEMdiethanol-

Adper Scotchbond Multi-Purpose (MP); 3M/ESPE, USA bis-GMA, HAdper Single Bond (SB); 3M/ESPE, USA bis-GMA, H

ethanolExcite (EX); Ivoclar Vivadent AG, Liechtenstein Phosphoni

catalysts a

Abbreviations: bis-GMA: bisphenol A diglycidyl ether dimethacrylatmethacryloyloxydecyl-dihydrogen phosphate. Basic composition bas

solubility parameter.Ten resin disks of each material were produced in a brass

mould (5.8 mm diameter, 0.8 mm thick). The liquid adhesivewas directly dispensed to completely fill the mould. The sur-face of the solvated, one-bottle systems (SB and EX), was gen-tly blown with an oil/water-free compressed air for 90 s to facil-itate solvent evaporation. All visible air bubbles trapped in theadhesives were carefully removed prior to photo-activation.A glass cover slip was placed on top of the adhesive, whichwas light-cured for 40 s using a quartz-tungsten-halogen lightsource operated at 650 mW/cm2 (Elipar TriLight, ESPE, Ger-many). After removing the specimen from the mould, photo-activation was repeated on its opposite surface for another40 s.

2.2. Water sorption and solubility

Water sorption and solubility were determined according tothe ISO specification 4049, except for specimens’ dimensions

d in the study

Components

-GMA, hydrophobic dimethacrylates, submicron silica fillers, N,N-idine, CQ, dimethacrylates, polyalkenoic acid copolymer and initiators, dimethacrylates, polyalkenoic acid copolymer, initiators, water and

acrylate, bis-GMA, HEMA, methacrylates, silicon dioxide, ethanol,bilizers

: camphorquinone; HEMA: 2-hydroxyethyl methacrylate; MDP: 10-manufacturers’ technical profiles.

d e n t a l m a t e r i a l s x x x ( 2 0 0 6 ) xxx–xxx 3

Table 2 – Composition and Hoy’s solubility parameter of the five experimental resins used in the present studya

Resin Composition wt% Hoy’s solubility parameters (J/cm3)1/2

ıd ıp ıh ıt

R1 bis-GMA-E 70.00 14.8 9.8 6.9 19.1TEGDMA 28.75CQ 0.25EDMAB 1.00

R2 bis-GMA 70.00 13.9 12.0 10.3 20.9TEGDMA 28.75CQ 0.25EDMAB 1.00

R3 bis-GMA 70.00 13.9 12.6 12.2 22.3HEMA 28.75CQ 0.25EDMAB 1.00

R4 bis-GMA 40.00 13.6 12.6 11.3 21.9HEMA 28.75TCDM 30.00CQ 0.25DMABA 1.00

R5 bis-GMA 40.00 13.9 12.9 12.9 23.0HEMA 28.752MP 30.00CQ 0.25EDMAB 1.00

Abbreviations: 2MP: bis[2-(methacryloyloxy)ethyl] phosphate; bis-GMA: bisphenol A diglycidyl ether dimethacrylate; bis-GMA-E: ethoxylatedbisphenol A diglycidyl ether dimethacrylate; CQ: camphorquinone; DMABA: dimethylaminobenzoic acid; EDMAB: ethyl N,N-dimethyl-4-aminobenzoate; HEMA: 2-hydroxyethyl methacrylate; TCDM: di(hydroxyethylmethacrylate) ester of 5-(2,5-dioxotetrahydrofurfuryl)-3-methyl3-cyclohexene-1,2dicarboxylic anhydride; TEGDMA: triethylene-glycol dimethacrylate; ıd: dispersion forces; ıp: polar forces; ıh: hydrogen bondingforces; ıt: total cohesive energy density.a According to Yiu et al. [21].

and period of water immersion that was extended up to 28days. Immediately after polymerization, the specimens wereplaced in a desiccator and transferred to a pre-conditioningoven at 37 ◦C. The specimens were repeatedly weighed after24 h intervals until a constant mass (m1) was obtained (i.e.,variation was less than 0.2 mg in any 24 h period). Thicknessand diameter of the specimens were measured using a digitalcalliper, rounded to the nearest 0.01 mm, and these measure-ments were used to calculate the volume (V) of each specimen(in mm3). They were then individually placed in sealed glassvials containing 10 mL of distilled water (pH 7.2) at 37 ◦C. Afterfixed time intervals of 1, 2, 3, 4, 5, 6, 7, 14 and 28 days of stor-age, the vials were removed from the oven and left at roomtemperature for 30 min. The specimens were washed in run-ning water, gently wiped with a soft absorbent paper, weighedin an analytical balance (m2) and returned to the vials con-taining 10 mL of fresh distilled water. Following the 28-daysof storage, the specimens were dried inside a desiccator con-taining fresh silica gel and weighed daily until a constant mass(m3) was obtained (as previously described). The initial massdetermined after the first desiccation process (m1) was usedto calculate the change in mass after each fixed time interval,during the 28 days of storage in water. Changes in mass wereplotted against the storage time in order to obtain the kineticsof water absorption during the entire period of water storage.Water sorption (WS) and solubility (SL) over the 28-days of

water storage were calculated using the following formulae:

WS = m2 − m3

V, SL = m1 − m3

V(1)

2.3. Diffusion coefficient

Five resin disks of each material were prepared from a brassmould as previously described. After a constant mass (m1) wasobtained the disks were immersed in distilled water at 37 ◦C.At fixed time intervals, they were removed from the vials,washed in running water, blot-dried, weighed and returnedto the water. Several readings were taken during the 1st day(every hour), and then at increasing intervals (every 12 h) untilequilibrium was attained. The diffusion coefficients of waterinto dental adhesives were determined by plotting the Mt/M∞ratios as a function of the square root of time (where Mt wasthe mass gain after time t and M∞ was the final mass gain).Since all plotted curves were linear when Mt/M∞ ≤ 0.5 (notshown), the diffusion coefficients of water (D) in the resinscould be calculated using the Stefan’s approximation [26]:

Mt

M∞= 4

L

(Dt

�

)1/2(2)

where L is the thickness of the specimen (in cm).


2.4. Statistics

Experimental resins and commercial adhesive systems wereseparately analyzed. The mean and standard deviation ofwater sorption and solubility were calculated for each materialgroup, and all data were analyzed by two individual one-wayANOVA (one for water sorption and the other for solubilitydata), using the materials as the main factor. Subsequently,changes in mass observed within each material as a functionof the increasing time of storage in water were also testedfor significance using a two-way ANOVA, using the materialand the storage time as the two factors. Finally, the diffu-sion coefficients of each group of material were analyzedusing a one-way ANOVA seeking for significant differencesamong the materials. Post hoc multiple comparisons wereperformed using Tukey’s test. Additionally, the relationshipbetween water sorption and Hoy’s solubility parameters of thefive experimental non-solvated resin blends were evaluatedby means of regression analysis. Statistical significance waspreset at ˛ = 0.05.

3. Results

3.1. Commercial adhesives

Results are summarized in Tables 3 and 5 and Fig. 1. Water

Fig. 1 – Changes in mass of commercial adhesives over28-days of water storage. Symbols represent mean values(n = 10). Since the standard deviations around all means aresmaller than the symbols, they have not been indicated.SB: Single Bond; EX: Excite; MP: Scotchbond Multi-Purposeadhesive; SE: Clearfil SE Bond adhesive.

non-solvated materials, MP and SE, were of the same order ofmagnitude and significantly lower than those observed for thesolvated materials, SB and EX (p < 0.05) (Table 5).

3.2. Experimental resins

Results are summarized in Tables 4 and 5 and Figs. 2–4.Water sorption and solubility varied significantly betweenmaterials. The least hydrophilic resin, R1, showed the low-est water sorption, followed by resins R2, R3, R4 and R5,with R1 < R2 < R3 = R4 < R5 (Table 4). The lowest solubility val-ues were obtained in R4 > R2 > R3 (p > 0.05), which were sig-nificantly different from the resins R1 and R5, with R1 < R5(p < 0.05) (Table 4). Mass variation curves during the 28-daysof immersion in water are presented in Fig. 2. Similar tothe commercial adhesives, all materials showed the great-est increase of mass within the 1st day of storage in water,except for R1 specimens, which did not present a significantmass variation along the entire period of water storage (Fig. 2).After the first 24-h of water storage, a continued increase ofmass was observed for the specimens of R2, R3 and R4 (with

Table 4 – Water sorption and solubility (�g/mm3) ofexperimental non-solvated resins after 28 days of waterstorage

Resins Water sorption Solubility

sorption and solubility varied significantly (p < 0.05) betweenmaterials. SE showed the lowest water sorption, followed byMP, EX and SB. All differences in water sorption behavior wereshown to be statistically significant (p < 0.05) (Table 3). Follow-ing the same trend, MP and SE showed lower solubilities whencompared to the solvated systems, SB and EX. The lowest sol-ubility value was observed for MP, followed by SE, EX and SB.All differences in solubility behavior were shown to be sta-tistically significant (p < 0.05) (Table 3). Mass variation curvesduring the 28-days of immersion in water are presented inFig. 1. Using m1 as a baseline mass, all adhesives showed thegreatest increase of mass within the 1st day of storage in water(Fig. 1). After that period, a continued and significant increaseof mass was observed for MP and SE until the equilibriumwas reached, which occurred between the 2nd and 3rd day.Conversely, SB and EX showed a constant decrease of massafter the 1st day of storage. For MP and SE specimens, a sig-nificant decrease of mass was observed only on the 28th dayof evaluation (Fig. 1). Water diffusion coefficients of the two

Table 3 – Water sorption and solubility (�g/mm3) ofcommercial adhesive systems after 28 days of waterstorage

Adhesives Water sorption Solubility

SE 59.32 (4.88)d −2.08 (2.20)c

MP 69.31 (3.07)c −9.56 (5.90)d

SB 207.39 (9.98)a 95.75 (6.68)a

EX 152.51 (6.45)b 87.96 (4.00)b

Analysis per column: Values are means (S.D.), n = 10 (per group).Same superscript letters (a–d) indicate no statistically significantdifference (p > 0.05).

R1 11.41 (2.25)d 29.84 (4.88)b

R2 35.42 (3.00)c −3.11 (3.01)c

R3 69.54 (3.91)b −3.60 (4.68)c

R4 65.91 (2.74)b −1.81 (2.34)c

R5 168.19 (3.45)a 113.28 (2.89)a

Analysis per column: Values are means (S.D.), n = 10 (per group).Same superscript letters (a–d) indicate no statistically significantdifference (p > 0.05).


Table 5 – Diffusion coefficients (×10−8 cm2 s−1) of theresins

Commercial adhesivesSE 7.00 (2.16)c

MP 6.74 (2.02)c

SB 69.16 (15.49)a

EX 40.95 (3.29)b

Experimental resinsR1 –*

R2 11.19 (3.19)b

R3 11.17 (2.74)b

R4 8.56 (1.71)b

R5 21.71 (2.63)a

Analysis per row: Values are means (S.D.), n = 05. Groups identifiedby different superscript letters (a–c) indicate no statistically signif-icant difference (p > 0.05).∗ Specimens of R1 did not show a significant mass gain over the

period of storage in water, thus it was not possible to calculate aprecise water diffusion coefficient for this resin blend.

Fig. 2 – Changes in mass of experimental resins over28-days of water storage. Symbols represent mean values(n = 10). Since the standard deviations around all means aresmaller than the symbols, they have not been indicated.

Fig. 3 – Regression analysis of the mean water sorption ofthe five experimental resins (R1–R5), plotted against Hoy’ssolubility parameter for total cohesive energy density.

Fig. 4 – Regression analysis of the mean water sorption ofthe five experimental resins (R1–R5), plotted against Hoy’ssolubility parameter for hydrogen bonding forces.

R2 > R3 = R4 and R2 = R4) until the equilibrium was reached,which occurred for all these resins between the 2nd and 3rdday of storage in water (Fig. 2). Conversely, after the first 24-hof water storage, a constant and significant decrease of masswas verified for R5 specimens (Fig. 2). The water diffusioncoefficients of the resins with intermediate hydrophilic fea-tures, R2, R3 and R4, were in the same order of magnitude, andwere significantly lower than the values presented by the mosthydrophilic experimental resin, R5 (p < 0.05) (Table 5). It wasnot possible to calculate a precise diffusion coefficient for resinR1, since it did not present a significant mass gain along theperiod of storage in water. In general, the extent of water sorp-tion increased with the hydrophilicity of the resin blends. Ahighly significant positive correlation was observed when themean water sorption was plotted against ıt (r = 0.99; p < 0.005)and ıh (r = 0.97; p < 0.005), respectively, Hoy’s solubility parame-ter for total cohesive energy density and for hydrogen bondingforces (Figs. 3 and 4). The correlations between water sorptionand ıp and ıd were not significant (not shown).

4. Discussion

The hydrophilic nature of a polymer is in large part a functionof the chemistry of its monomers and its polymerization link-ages. An examination of the most commonly used monomersin dental adhesive systems (HEMA, BPDM, MDP, bis-GMA)shows that they form polymers with carbon and oxygen back-
bones [13]. In addition, the structure of those polymers revealsthe presence of hydrolytically susceptible ester groups [12].The presence of hydroxyl, carboxyl and phosphate groups inmonomers and their resultant polymers make them morehydrophilic (Table 2) and, supposedly, more prone to watersorption [12]. The present results showed that both experi-mental and commercial dental adhesives absorbed most ofthe water within the 1st day of water storage. Water sorption,solubility and diffusion coefficients of the experimental andcommercial dental adhesives were all significantly dependenton material composition.
Recently, Yiu et al. [21] proposed the use of Hoy’s solubil-ity parameters as a tool for ranking the relative hydrophilicityof a series of experimental dental adhesives. In the present


study, a strong correlation between the mean water sorptionand the degree of hydrophilicity of the same experimentalnon-solvated resins was determined (Figs. 3 and 4). Positivecorrelations between the mean water sorption with Hoy’ssolubility parameters ıt and ıh of the experimental resinssuggest that the absorbed water seems more likely to formhydrogen bonding with the hydrophilic and ionic domains inthese methacrylate-based resins, which is in agreement withother studies [16,21]. It was not possible to calculate the sol-ubility parameters of the commercial adhesives employed inthis study because it would be necessary to know the exactamount of each component in each material, which is notprovided by the manufacturers. The highest values of watersorption and solubility (Table 3) as well as the highest rateof water diffusion (Table 5) were observed for SB and EXadhesives. Thus, by using the data obtained with the fivenon-solvated resin blends as a parameter for the relation-ship between water sorption and hydrophilicity, it is sugges-tive that SB and EX presented, indeed, a more hydrophilicbehavior than SE and MP. This is probably because of the sol-vents present in their composition. In view of these results,the anticipated hypothesis was confirmed.The results demon-strate that the kinetics of water uptake was also material-dependent. That is, the rate of water uptake was lower forthe less hydrophilic adhesives, MP and SE. In contrast, forthe more hydrophilic commercial adhesives systems, SB andEX, maximum water uptake occurred during the 1st day of

swell [31]. We speculate that the solvated commercial adhe-sives SB and EX probably formed polymer networks with poorpacking density or with higher free volume, as they exhib-ited the highest water diffusion coefficients, and they had thegreatest increase of mass during the 1st day of water storage.Additionally, it is likely that the residual solvent (water andethanol) entrapped within the polymerized structure of SB andEX may have evaporated during the pre-conditioning, dryingprotocol, thereby allowing more free-volume space for readi-aly water uptake [32]. In contrast, although the non-solvatedmaterials, MP and SE, also showed the greatest increase inmass during the 1st day of storage, they continued to absorbwater at a slower rate for 2 more days until equilibrium wasreached (Fig. 2). A slow and continued water uptake suggeststhat these adhesives formed denser polymer networks thatimposed a certain resistance to water penetration. Indeed, it isexpected that under clinical conditions (i.e. atmospheric pres-sure, presence of oxygen, at room temperature) solvated resinspolymerize in a sub-optimal way, as the presence of residualsolvent may prevent optimal monomer conversion [33–35] andinterfere with optimal polymer chain packing density, furtherenhancing plasticization of the cured polymer [36–38].

Nevertheless, the results obtained with the five experimen-tal resins lead us to believe that differences in the polarity ofeach material may play a preponderant role in determiningthe ability of polymers to absorb water. Although Hoy’s solu-bility parameter of commercial adhesives cannot be calculated

storage (Fig. 1). The extent and rate of water uptake intopolymer networks are predominantly controlled by two mainfactors: resin polarity, dictated by the concentration of polarsites available to form hydrogen bonds with water [16,20];and network topology, which is related to the cohesive energydensity of the polymer network [16,27–29]. While the poly-mer polarity (water affinity for hydrophilic polar groups in thepolymer) is a major determinant of water uptake into poly-mers [30], non-polar polymers with low Hoy’s cohesive energydensities permit water molecules to move freely throughnanovoids as “unbound” water. Because this unbound wateris filling free volume, it is not expected to cause dimen-sional changes of the polymer [17]. In contrast, the watermolecules that attach to the polymer chain via hydrogenbonding, referred to as “bound” molecules, disrupt the inter-chain hydrogen bonding, induce swelling and plasticize thepolymer [18,19].

According to the “free volume theory”, water sorption intoglassy polymers implies that “unbound” molecules of waterdiffuse through the nanovoids within the polymer withoutany inter-relationship with polar molecules in the material[30]. Thus, the macromolecular packing density of the polymerwould determine the hole-free effective volume for water dif-fusion [31]. Consequently, the quality of the network formedduring polymerization will also dictate, to some extent, whichmolecules can be taken up and how much swelling occurswhen a polymer is soaked in a solvent [13]. Thus, the struc-tural and topological features of polymers are fundamental indetermining the extent to which polymers will be affected byan aqueous environment. The presence of cross-links betweenpolymer chains generally results in a significant decrease inthe water permeability of the polymer, because they decreasethe hole-free volume and the ability of polymer chains to

because of the lack of information on their exact formulation,the water sorption values show that the most hydrophilic,non-solvated experimental resin, R5, presented a water sorp-tion and a water diffusion coefficient that were higher than thenon-solvated, less hydrophilic commercial materials, MP andSE. While we believe that “free volume theory” may explainwater sorption in R1 or in most of polymethyl methacry-lates [20], the “interaction theory” (i.e., water attached to polarsites of the polymer via hydrogen bonds) seems to providea more reliable interpretation of the water sorption data formore hydrophilic polar resins such as R3–R5. Polymer net-works (R3–R5) formed by polar groups such as hydroxyl, car-boxyl or phosphate have shown significant reductions in thestrength [21] and stiffness [15] after storage in water, andthis is probably because they are very prone to absorb wateras “bound water” [15,20,21]. Water molecules that are firmlybound to polar sites along the polymer networks exhibit highplasticizing effect, thus causing the reduction of polymer’smechanical properties by altering the mobility of their chainsegments.

At sufficiently high water sorption, macromolecularpolymer chains undergo a relaxation process as they swellto absorb the water. Initially, the presence of water soft-ens the polymer by swelling the network and reducing thefrictional forces between the polymer chains [38]. After therelaxation process, unreacted monomers trapped in thepolymer network are released to the surroundings in a ratethat is controlled by the swelling and relaxation capacitiesof the polymer. More hydrophilic polymer networks, whichhave a superior relaxation capacity, permit a faster releaseof unreacted monomers through nanovoids in the material[28,29], showing a decrease in mass within a short time ofwater immersion. This is somewhat dependent on the degree


of conversion and the quantity of pendant molecules existingwithin the network [31]. In addition, a high amount of poros-ity will facilitate fluid transport in and out of the network,leading to enhanced water uptake and elution [13]. Probablydue to these facts, the solvated commercial adhesives SB andEX presented the highest values of solubility and began toshow a significant decrease in mass from the 2nd day, whilefor MP and SE the decrease of mass was observed only at day28 (Fig. 1). Similarly, among the experimental resins the mosthydrophilic resin (R5) exhibited the greatest solubility. Curi-ously, the least hydrophilic resin R1 presented higher solubilitythan the most hydrophilic resin R5. This may be caused by areduced degree of conversion and/or reduced packing densityof R1. R1 is mostly composed of bis-GMA-E, a less flexible andmore viscous monomer, which, in high concentration, mayshow lower degree of conversion [31,39], poor cross-linkingdensity and a consistent solubility when soaked in water[31].

The measurement of mass changes as a parameter toestimate the capacity of polymers to absorb water demandscareful interpretation. In fact, some studies employed thesimple increase in mass of polymers stored in water as away of assessing their water sorption behavior [22,40]. How-ever, this approach seems to be consistent only when rel-atively hydrophobic resins are compared. In general, poly-mer networks prepared by free-radical polymerization ofhydrophilic dimethacrylates show a spatial heterogeneity,waamvmmawlvwlbowist

htiravrprroms

5. Conclusion

This study showed that water sorption, solubility and waterdiffusion coefficient of methacrylate-based resins, formulatedto be used as dentin/enamel bonding agents, are positivelydependent on adhesives’ composition and hydrophilicity.

Acknowledgements

This study was performed by Juliana Malacarne as partial ful-filment of her M.S. degree at the University of Campinas. Theauthors gratefully acknowledge the valuable assistance givenby Mr. Marcos Blanco Cangiani for the technical and laborato-rial support, and by Dr. Marcelo Alves of the School of Agron-omy Luiz de Queiroz of the University of Sao Paulo, who per-formed the statistical analysis. The experimental resins usedin this study were developed and donated by Bisco, Inc. Thisstudy was supported in part by Grants from CAPES/PRODOC(to PI Dr. Carrilho), 305300/04-0, 474226/03-4 from CNPq (toPI Dr. Carvalho), Brazil; by R01 grants DE 014911 and DE015306 (to PI Dr. Pashley) from the National Institute of Dentaland Craniofacial Research, Bethesda, MD, USA; and by grant10204604/07840/08004/324/01, Faculty of Dentistry, The Uni-versity of Hong Kong, China.

r
here some parts are densely cross-linked and some parts
re loosely cross-linked. Such spatial heterogeneity is presentlmost from the beginning of polymerization with unreactedonomers dispersed in a pool of microgel domains with a

ariable density of cross-links [41,42]. When this partially poly-erized structure is stored in water, part of it infiltrates theaterial, increasing its “weight”. At the same time, unre-

cted monomers and low molecular weight polymers thatere entrapped in the microgels and/or in the nanovoids can

each out, lowering the polymer’s mass [26,31]. Since the massariation is the net result of both the increase in mass due toater penetration, and the decrease in mass due to elution of

ow molecular weight material, it is impossible to conclude,y measuring only the increase in mass, the exact amountf absorbed water. For this reason, analysis of the kinetics ofater diffusion associated with water sorption and solubil-

ty measurements provides complementary information andhould be used in concert with assessing the behavior of den-al polymers in water.

Over the past 20 years, advances in adhesive dentistryave centered on improving resin–dentin bond strength, a goalhat has been ultimately achieved [43]. Apart from identify-ng the mechanisms responsible for the lack of durability ofesin–dentin bonds, the next decade should be devoted to cre-ting materials and techniques that can improve the preser-ation of these bonds or, at least, increase their lifetime. Theelatively poor stability of resin–dentin bonds [1–3,5,44] occursrobably due to the competing requirements for hydrophilicesins that can infiltrate moist dentin, in opposition to the pre-equisite that durable bonds should not absorb water. Basedn these concerns, we suggest that future attempt to createore stable resin–dentin bonds using more hydrophilic adhe-

ives should not be encouraged.

e f e r e n c e s

[1] Burrow MF, Satoh M, Tagami J. Dentin bond durabilityafter three years using a dentin bonding agent with andwithout priming. Dent Mater 1996;12:302–7.

[2] Hashimoto M, Ohno H, Kaga M, Endo K, Sano H, Oguchi H.In vivo degradation of resin–dentin bonds in humans over1 to 3 years. J Dent Res 2000;79:1385–91.

[3] Takahashi A, Inoue S, Kawamoto C, Ominato R, Tanaka T,Sato Y, et al. In vivo long-term durability of the bond todentin using two adhesive systems. J Adhes Dent2002;4:151–9.

[4] Hashimoto M, Ohno H, Sano H, Kaga M, Oguchi H. In vitrodegradation of resin–dentin bonds analysed bymicrotensile bond test, scanning and transmissionelectron microscopy. Biomaterials 2003;24:3795–803.

[5] De Munck J, Van Meerbeek B, Yoshida Y, Inoue S, Vargas M,Suzuki K, et al. Four-year water degradation of total-etchadhesives bonded to dentin. J Dent Res 2003;82:136–40.

[6] Carrilho MRO, Carvalho RM, Tay FR, Yiu CK, Pashley DH.Durability of resin–dentin bonds related to water and oilstorage. Am J Dent, in press.

[7] Wang Y, Spencer P. Hybridization efficiency of theadhesive/dentin interface with wet bonding. J Dent Res2003;82:141–5.

[8] Tay FR, Pashley DH, Suh BI, Carvalho RM, Itthagarun A.Single-step adhesives are permeable membranes. J Dent2002;30:371–82.

[9] Tay FR, Pashley DH, Peters MC. Adhesive permeabilityaffects composite coupling to dentin treated with aself-etching adhesive. Oper Dent 2003;28:610–21.

[10] Hashimoto M, Ohno H, Sano H, Kaga M, Oguchi H.Degradation patterns of different adhesives and bondingprocedures. J Biomed Mater Res 2003;66B:287–98.

[11] Carrilho MRO, Tay FR, Pashley DH, Tjaderhane L, CarvalhoRM. Mechanical degradation of resin dentin bondcomponents. Dent Mater 2005;21:232–41.


[12] Santerre JP, Shajii L, Leung BW. Relation of dentalcomposite formulations to their degradation and therelease of hydrolyzed polymeric-resin-derived products.Crit Rev Oral Biol Med 2001;12:136–51.

[13] Ferracane JL. Hygroscopic and hydrolytic effects inpolymer networks. In: Proceedings of conference onscientific insights into dental ceramics and photopolymernetworks, vol. 18. 2004. p. 118–28.

[14] Tay FR, Pashley DH, Garcia-Godoy F, Yiu CK. Single-step,self-etch adhesives behave as permeable membranes afterpolymerization. Part II. Silver tracer penetration evidence.Am J Dent 2004;17:315–22.

[15] Ito S, Hashimoto M, Wadgaonkar B, Svizero N, CarvalhoRM, Yiu C, et al. Effects of resin hydrophilicity on watersorption and changes in modulus of elasticity.Biomaterials 2005;26(33):6449–59.

[16] Soles CL, Yee AF. A discussion of the molecularmechanisms of moisture transport in epoxy resins. JPolym Sci 2000;38:792–802.

[17] Van Landingham MR, Eduljee RF, Gillespie JW. Moisturediffusion in epoxy systems. J Appl Polym Sci1999;71:787–98.

[18] Adamson MJ. Thermal expansion and swelling of curedepoxy resin used in graphite/epoxy composite materials. JMater Sci 1980;15:1736–45.

[19] Moy P, Karasz FE. Epoxy–water interactions. Polym Eng Sci1980;20:315–9.

[20] Unemori M, Matsuya Y, Matsuya S, Akashi A, Akamine A.Water absorption of poly(methyl methacrylate) containing4-methacryloxyethyl trimellitic anhydride. Biomaterials2003;24:1381–97.

[28] Brazel CS, Peppas NA. Mechanisms of solute and drugtransport in relaxing, swellable hyrophilic glassy polymers.Polymer 1999;40:3383–98.

[29] Brazel CS, Peppas NA. Dimensionless analysis of swellingof hydrophilic glassy polymers with subsequent drugrelease from relaxing structures. Biomaterials1999;20:721–32.

[30] Bellenger V, Verdu J. Structure-properties relationship fordensely crosslinked epoxy-amine systems based onepoxide or amine mixtures. J Mater Sci 1989;24:63–8.

[31] Sideridou I, Tserki V, Papanastasiou G. Study of watersorption, solubility and modulus of elasticity of light-cureddimethacrylate-based dental resins. Biomaterials2003;24:655–65.

[32] Yiu CK, Goracci C, Pashley EL, Hiraishi N, King MN, FerrariM, et al. Solvent and Water Retention in Dental AdhesiveFilms After Evaporation. J Dent Res.; 2005 [special issue;abstract 0511].

[33] Andrzejewska E. Photopolymerization kinetics ofmultifunctional monomers. Prog Polym Sci 2001;26:605–65.

[34] Dickens SH, Cho BH. Interpretation of bond failurethrough conversion and residual solvent measurementsand Weibull analysis of flexural and microtensile bondstrengths of bond agents. Dent Mater 2005;21:354–64.

[35] Jacobsen T, Soderholm K-J. Some effects of water ondentin bonding. Dent Mater 1995;11:132–6.

[36] Hotta M, Kondoh K, Kamemizu H. Effect of primers onbonding agent polymerization. J Oral Rehabil 1998;25:792–9.

[37] Paul SJ, Leach M, Rueggeberg FA, Pashley DH. Effect ofwater content on the physical properties of model dentin

[21] Yiu CKY, King NM, Pashley DH, Suh BI, Carvalho RM,Carrilho MRO, et al. Effect of resin hydrophilicity andwater storage on resin strength. Biomaterials2004;25:5789–96.

[22] Burrow MF, Inokoshi S, Tagami J. Water sorption of severalbonding resins. Am J Dent 1999;12:295–8.

[23] Van Krevelen DW. Properties of polymers. Their correlationwith chemical structure; their numerical estimation andprediction from additive group contributions. 3rd ed. NewYork: Elsevier; 1990.

[24] Barton AFM. Handbook of solubility parameters and othercohesive parameters. 2nd ed. Boca Raton: CRC Press; 1991.p. 69–156 [Chapter 5, Expanded Cohesion Parameters].

[25] Marom G. The role of water transport in compositematerials. In: Comyn J, editor. Polymer permeability. GreatBritain: Elsevier Applied Science; 1985. p. 341–74 [Chapter9].

[26] Sideridou I, Achilias DS, Spyroundi C, Karabela M. Watersorption characteristics of light-cured dental resins andcomposites based on bis-EMA/PCDMA. Biomaterials2004;25:367–76.

[27] Vrentas JS, Duda JL. A free volume interpretation of theinfluence of the glass transition on diffusion inamorphous polymers. J Appl Polym Sci 1978;22:2325–39.

primer and bonding resins. J Dent 1999;27:209–14.[38] Ferracane JL, Berge XH, Condon JR. In vitro aging of dental

composites in water—effect of degree of conversion, fillervolume, and filler/matrix coupling. J Biomed Mater Res1998;42:465–72.

[39] Sideridou I, Tserki V, Papanastasiou G. Effect of chemicalstructure on degree of conversion in light-cureddimethacrylate-based dental resins. Biomaterials2002;23:1819–29.

[40] Turner DT. Water sorption of poly(methylmethacrylate): 1.Effects of molecular weight. Polymer 1987;28:293–6.

[41] Kloosterboer JG, Van de Hei GMM, Boots HMJ.Inhomogenity during the photopolymerization ofdiacrylates: d.s.c. experiments and percolation theory.Polym Commun 1984;25:354–7.

[42] Allen PEM, Bennett DJ, Hagias S, Hounslow AM, Ross GS,Simon GP, et al. The radical polymerization ofdimethacrylate monomers—the use of NMR Spectrometryto follow the development of network structure. Eur PolymJ 1989;25:785–9.

[43] Tay FR, Pashley DH. Dental adhesives of the future. JAdhes Dent 2002;4:91–103.

[44] Gwinnett AJ, Yu S. Effect of long-term water storage ondentin bonding. Am J Dent 1995;8:109–11.

Water sorption/solubility of dental adhesive resins

Documents