Water Sorption and Adhesion

8/6/2019 Water Sorption and Adhesion

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d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) 617626

a v a i l a bl e a t w w w . s c i en c e d i r e c t .c o m

j o u r n a l h o m e p a g e : w w w . i n t l . e l s e v i e r h e a l t h . c o m / j o u r n a l s / d e m a

Water sorption/solubility of self-etching dentin

bonding agents

Shuichi Ito a,, Tomohiro Hoshino a, Masahiro Iijima b, Naohiro Tsukamoto a,David H. Pashley c, Takashi Saito a

a Health Sciences University of Hokkaido, Division of Cariology and Endodontology, Department of Oral Rehabilitation, School of

Dentistry, 1757 Tobetsu, Hokkaido 0610293, Japanb Health Sciences University of Hokkaido, Division of Orthodontics and Dentofacial Orthopedics, Department of Oral Growth and

Development, Tobetsu, Hokkaido, Japanc Medical College of Georgia, Department of Oral Biology, Augusta, GA, USA

a r t i c l e i n f o

Article history:

Received 12 September 2008

Received in revised form

13 November 2009

Accepted 5 March 2010

Keywords:Water sorption

Water solubility

Dentin bonding

Percent conversion

a b s t r a c t

Objectives. The purpose of this study was to compare the water sorption/solubility, percent

conversion and microtensile bond strength of three single-step self-etching adhesives with

those of a two-step self-etching primer adhesive system.

Methods. Solvent evaporation from the adhesives was determined gravimetrically. After

removal of volatile solvents, theresins were cast into disks andpolymerized. One-half of the

disks were incubated in water while the other half were incubated in hexadecane. Repeated

measurements of water sorption were made for 10 days followed by drying for 2.5 days to a

constant weight. Percent conversion was done using FTIR spectroscopy. Microtensile bondstrengths were measured 24h after bonding.

Results. All of the adhesives lost 2030% of their weight after 4 min of forced air except for

Fluorobond II which lost no weight. All resins stored in water exhibited a time-dependent

increase in water sorption and solubility. The resins stored in hexadecane showed very low

sorption and solubility. Water sorption was highest for Absolute 2 (20.7%), intermediate for

Fluorobond Shake One (10.2%) and lowest for Clearfil 3S (8.9%) and Fluorobond II (7.5%). Per-

cent conversions ranged from a low of 68.3% for Absolute 2 to a high of 87.4% for Clearfil3S. The two-step self-etching primer adhesive (Fluorobond II) gave the lowest water sorp-

tion and lowest solubility of any of the tested adhesives. SEM observations of resin disks

incubated in hexadecane looked similar to unincubated controls. Incubating resin disks in

artificial saliva covered the surfaces of the resins with mineral crystallites.

Significance.Single bottle self-etching adhesives showhigher watersorption/solubilities than

two-step self-etching adhesives. The former products would not be expected to function aswell as the latter products.

2010 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Corresponding author. Tel.: +81 133 23 1423; fax: +81 133 23 1423.E-mail address: [email protected] (S. Ito).

0109-5641/$ see front matter 2010 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

doi:10.1016/j.dental.2010.03.001
mailto:[email protected]://dx.doi.org/10.1016/j.dental.2010.03.001http://dx.doi.org/10.1016/j.dental.2010.03.001mailto:[email protected]


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1. Introduction

In 2003, Tay and Pashley [1] asked the question have dentin

adhesives become too hydrophilic? It has become clear that

water left on acid-etched dentin surfaces in the wet bonding

technique is necessary to keep collagen fibrils collapsing [2],

but can water also causes phase changes when dimethacry-lates encounter too much water [37]. Even in self-etching

adhesives that are applied to dry smear layer-covered dentin,

1030wt% [810] water is added to the hydrophilic formula-

tions to ionize the carboxylic or phosphate methacrylates and

to solubilize calcium and phosphate ions liberated from the

action of self-etching adhesives with dentin. If this water is

not evaporated, it will dilute the comonomers, and may inter-

fere with bonding [11,12] and lower the mechanical properties

of the resin [6,13,14]. Manufacturers seem to concentrate on

how rapidly their materials can create a bond rather than

determining the optimum application time and evaporation

time for maximum bond strength. It has been reported that

reduction in mechanical strength [2] and modulus of elas-ticity [3] are associated with increasing hydrophilicity of the

copolymer blends after water storage. This could explain why

resindentin bonds made with hydrophilic resin monomers

significantly degrade over time in vitro [1520] and in vivo

[2124].

Water sorption within polymer matrices created by

contemporary hydrophilic dentin adhesives is not always uni-

form. Using ammoniacal silver nitrate to trace the distribution

of absorbed water, Tay et al. have shown both uniform and

non-uniform water uptake into commercial adhesive resins

[2527]. Uniform absorption was seen as isolated individual

silver grains, while the non-uniform water uptake resulted in

the formation of liner, branched, water-filled channels [26].When dentin bonded with adhesive resins was storedin water

for 12 months, the distribution of absorbedwater changeddra-

matically [27]. Clearly, water sorption by dental resins and the

hybrid layer is more complex than expected [1].

All-in-one adhesives and self-etching primers are intrin-

sically hydrophilic owing to the presence of acidic, highly

polar functional groups substituted on methacrylates, and the

presence of water and ethanol solvents. They rapidly absorb

water, which results in polymer swelling, plasticizing [2830]

andweakening of the polymer network [2830]. Water absorp-

tion into polymers is assumed to be directly related to the

hydrophilicity of the polymers [3134].

The water sorption and solubility of newly developed adhe-sives needs to be fully investigated. The nullhypothesis tested

was that none of the adhesive resins tested will have different

water sorption/solubility values.

2. Materials and methods

2.1. Evaporation of dentin bonding resin components

by storage in room temperature and by air-drying

Forty microliters of unpolymerized adhesive were placed on a

tared Teflon slide. For two-step adhesive, FB II, equal volumes

of primer and adhesive were rapidly (5 s) mixed together and

placed on a tared slide. The Teflon slide was weighted before

and immediately after depositing the drop of bonding resin,

enabling the weight of the bonding resin (A) from the differ-

ence of the two weights to be calculated (0-min storage time).

To allow the volatile dentin bonding components to sponta-

neously evaporate, the plastic-plate with the unpolymerized

adhesive mixture was stored in the dark for 60 min at room

temperature (25 C). When an air-stream was used to evapo-rate solvents, a 3-way air-water syringe was clamped 15 cm

from the adhesive and the control button depressed half-way

to produce an air flow of 20 L/min for 30 min in the dark. Five

specimens were prepared per adhesive. The weight of plastic-

plate with the bonding resin was successively remeasured

every minute for the first 5min, every 5 min from 10 to 30min

and every 10 min for the rest of the time. The weight of resid-

ual adhesives at each time (B) was determined by subtracting

the weight of the plastic-plate.

The degree of spontaneous evaporation of volatile solvents

(Ep in wt%) at room temperature was followed for 60 min. The

rate of evaporation of volatile solvents in response to an air-

stream was calculated as follows:

Ep =(A B) 100

A

with A the initial weight of the adhesive mixture and B the

weight of the mixture after each storage time.

2.2. Water sorption/solubility

After the time required for spontaneous or air-stream-

induced solvent evaporation was known, the commercial

comonomers/solvent mixtures were placed in a circular well

made in Teflon to form disks 10.00.1 mm in diameter and

1.00.02mm thick. While in the dark, the Teflon well con-taining the solvated comonomers was exposed to the same

air-stream used to evaporate the solvents in the previous sec-

tion, for 30min to insure that all volatile solvents had been

removed prior to light curing. The surface of the comonomers

was covered with a glass cover slip to exclude atmospheric

oxygen, forming a flat surface, and the resin was light-cured

for 30 s using a dental curing light (Morita, Kyoto, Japan)

operated at 600 mW/cm2, with the tip held 1 mm from cover

slip. After removing the disk from the mold, a similar light

exposure was applied to the lower disk surface. Specimen

dimensions to the nearest 0.01mm were measured using a

digital micrometer. Ten resin disks were made for each of the

commercial resins. They were dry polished to a thickness of0.500.02mm.

Water sorption was measured following the method out-

lined in ISO 4049 (12-1998). However, disk diameters were

10 mm and 0.5 mm in thickness, to match the dimensions

of specimen molds. Immediately after polymerization, the

specimens were placed in desiccators and transferred to a pre-

conditioning oven at 37 C. The specimens were repeatedly

weighed after 24h intervals until a constant mass (m1) was

obtained (i.e.variation waslessthan 0.2mg in any 24h period).

Thickness and diameter of the specimens were measured

using a digital caliper, rounded to the nearest 0.01mm, and

these measurements were used to calculate the volume (V) of

each specimen (in mm3). The resin disks were then individu-


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Table 1 Composition of the adhesive used.

Material (Manufacture) Composition Resin application

Single-step Absolute 2 (AB2) (Dentsply

Sankin, Tokyo, Japan

Acetone, pyro-EMA, PEM-F, 4-MET, CQ,

nanofillers, UMDA

Apply to tooth for 5 s with Agitation.

Repeat this Procedure 2 times during

15 s. Gentle air-blast for 5s

Single-step Clearfil Tri-S Bond (3S)

(Kuraray Medical, Osaka, Japan)

MDP, Bis-GMA, HEMA, microfiller,

initiator, water ethanol

Apply to tooth for 20 s. Strong air-blast

for 5 s

Single-step Fluoro Bond Shake One

(SO) (Shofu, Kyoto, Japan)

Bottle A: Glass filler, acetone water;

Bottle B: HEMA, 4-AET, Bis-GMA,

initiator

Shake bottles, dispense equal

volumes of liquid from bottle A and

bottle B. Leave 20s. Gentle air-blast

Two-step Fluoro Bond II (FB II) (Shofu,

Kyoto, Japan)

Primer: Carboxylic acid Monomer,

Phosphonic acid Monomer, Water,

Solvent, Initiator; Bond: S-PRG filler,

UDMA, TEGDMA, HEMA, Initiator

Apply primer to tooth for 10s. Gentle

air-blast. Apply bonding agent to

tooth. Leave 10 s

Abbreviations : Pyro-EMA = tetramethacryloyloxyethyl pyrophosphate; PEM-F= pentamethacryloyloxyethyl cyclohexaphosphazene monoflu-

oride; 4-MET= 4-methacryloxyethyltrimellitic acid; UMDA= urethane dimethacrylate; HEMA = 2-hydroxyethyl methacrylate; MDP = 10-

methacryloyloxydecyldihydrogen phosphate; Bis-GMA = bisphenyl A diglycidylmethacrylate; TEGDMA= triethyleneglycol dimethacrylate;

CQ = camphorquinone; 4-AET = 4-acryloxyethyltrimellitic acid.

ally placed in sealed glass vials containing 10 mL of distilled

water(pH 7.2) at 37

C.Afterfixedtimeintervalsof1,2,3,4,5,6,7 and10 days of storage,the vials were removedfrom the oven

and left at room temperature for 30 min. The specimens were

washed in running water, gently wiped with a soft absorbent

paper, weighed in an analytical balance (m2) and returned to

the vials containing 10 mL of fresh distilled water. Following

the 10 days of storage, the specimens were dried inside a des-

iccator containing fresh silica gel and weighed daily for 2.5

days until a constant mass (m3) was obtained (as previously

described). The initial mass determined after the first desic-

cation process (m1) was used to calculate the change in mass

after each fixed time interval, during the 10 days of storage in

water calculated as the difference in dry mass before immer-

sion and after reaching the water sorption plateau, followingdrying in a sealed chamber filled with anhydrous calcium sul-

fate [35]. Disk volume was determined by measuring diameter

and thickness before and after water exposure. Dry weight

measurements were followed daily for 10 days. The values

(%) for water sorption (WS) and solubility (SL) were calculated

asWS =M2 M3/V,

SL =M1 M3/V,where M1 is the initial dry constant mass

(mg) before water immersion; M2 is the mass (mg) after water

immersion; M3 is the mass (mg) after drying specimens that

hadreached their maximum water sorption and V is the spec-

imens volume in mm3. Netwater uptakewas calculated as the

sum of water sorption and solubility [36].

Resin disks were also immersed in hexadecane as anexample of a water-free pure oil with a viscosity similar to

water. At each time period, resin disks were removed from

the hexadecane and blotted dry of excess hexadecane before

weighing. These specimens were run in parallelwith the water

immersed specimens as controls.

2.3. Monomer conversion

To ascertain that the increase in water sorption of the more

hydrophilic resin blends was not caused by their lower extent

of cure, the monomer conversion of the commercial resins

was measured using infrared spectroscopy, according to the

method of Rueggeberg et al. [37]. Briefly, 25L of solvated

blends were placed in a small tared vessel on an analyti-

cal balance. After obtaining an initial weight, the vessel wasremoved from the balance and evaporated with an air-stream

(0.5L/s) for 10 min or until there was no further weight loss.

Then 5L of the solvent-free blend was placed in a 6-mm

hole was punched in a piece of single side Scotchtape, that

was placed directly over the 22mm diamond crystal of

a horizontal attenuated total reflectance unit (Golden Gate-

SPECAC, Inc., Woodstock, GA). As the tape was501m thick,

the hole provided a convenient well into which was placed

5L of each evaporated resin blends. The fluid was covered

with a thin Mylar film which, in turn, was covered by a glass

slide. The resin was cured as described above, using a 120-

s exposure. Multiple scans were made before and after light

exposure at 2 cm

1 resolution between 1680 and 1550 cm

1at a rate of one scan/s for 305 s, using a Fourier Transform

Infrared spectrophotometer (IR Prestige-21, Shimazu, Tokyo,

Japan). The degree of conversion was calculatedusing changes

in the molar ratios (represented as peak absorbance height)

of aliphatic (1636cm1)/aromatic (1608 cm1) carbon double

bonds on the cured (C) and uncured (U) states. Conversion was

calculated by using the following equation:

% Conversion =

1C

U

100

2.4. Microtensile bond test

Sixteen extracted noncarious human third molars were used

in this study. The teeth were collected after obtaining the

patients informed consent under a protocol approved by the

Health Sciences University of Hokkaido Institutional Review

Board. Flat dentin surfaceswere created in mid-coronal dentin

perpendicular to the tooths longitudinal axis using a slow-

speed diamond saw (Isomet, Buehler, Lake Bluf, IL, USA) to

remove occlusal enamel and superficial dentin. Each surface

of mid-coronal dentin was ground with320-grit silicon carbide

paper under running water for 30 s just prior to bonding.

One two-step and three one-step self-etching adhesives

were used in this study (Table 1). The commercial adhesives

were Fluoro Bond II (FB II, Shofu, Kyoto, Japan), Clearfil Tri-S


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Bond (3S, Kuraray Medical Inc., Tokyo, Japan), Absolute 2 (AB,

Dentsply-Sankin, Tokyo, Japan), Fluoro Bond Shake One (SO,

Shofu, Kyoto, Japan).

Each bonding resin was applied on the dentin surface

according to the manufactures instruction. That means their

solvents were not completely removed as they were in

the water sorption/solubility specimens. All adhesives were

subsequently light-cured for 10s with the light-curing unit(Morita, Kyoto, Japan) operated at 600 mW/cm2. Following

adhesive treatment, five 1-mm increasements of a resin

composite (Beautifil, Shofu, Kyoto, Japan) were built up and

individually light-activated for 60 s. After the bonded speci-

mens had been stored in water at 37 C for 24h, the built-up

teeth were sectioned perpendicular to the adhesive interface

to remove the peripheral 3 mm from each side of the tooth,

yielding a cube-like specimen that represents the center of the

tooth. This central cube was cut into 0.9-mm-thick slabs. Each

slab, in turn, was sectioned into 1.0-mm-wide beams (adhe-

sive area: approximately 0.9 mm2) with a diamond saw under

water cooling/lubrication. Although 9 beams were produced,

only the 6 with the greatest dentin thickness were selectedfor testing. Four teeth were used for each group. Beams were

attached to a testing apparatus with a cyanoacrylate adhe-

sive (Model repair 2, Dentsply-Sankin, Tokyo, Japan). A tensile

load was applied with a material tester (EZ test, Shimazu,

Kyoto, Japan) at a crosshead speed of 1.0 mm/min until fail-

ure. The load (N) at failure divided by the cross-sectional area

of each failed beam measured with a digital micrometer per-

mitted calculation of the microtensile bond strength that was

expressed in MPa. Four teeth were used per bonding mate-

rial. As each tooth yielded 6 beams, there were 24 beams per

material. Themean microtensile bond strengths of each tooth

were calculated separately and then averaged. There were no

pretesting failures.

2.5. SEM examination

Resin disks were stored in an artificial saliva for 10 days. The

composition of the buffer solution (mmol/L) was: NaCl (130),

CaCl2 (0.7), MgCl2 (0.2), KH2PO4 (4.0), KCl (30), NaN3 (3), 4-

Fig. 1 Spontaneous evaporation of solvated adhesives of

dentin bonding agents over time during storage in the dark

at room temperature. The mass of the solvated adhesives

(SO, 3S and AB2) gradually decreased 1933% over 60min.

The weight of FB II did not change over 60 min.

Fig. 2 Evaporation of solvated adhesives using a

continuous gentle air-stream over time. The solvated

adhesives of AB, SO, and 3S rapidly lost weight during the

first 5 min. The weight of FB II did not change during the

30 min of exposure to a continuous air-stream.

(2-hydroxyethl) piperazine-1-ethanesulfonic acid (HEPES, 20),

pH = 7.4. After storage, resin discs were immediately fixed in

2.5% glutaraldehyde buffered in 0.1 M cacodylate titrated to

pH 7.2, for 72h. The specimens were then rinsedseveral times

with 0.1 M sodium cacodylate buffer. The resins disks were

dehydrated in increasing concentrations of ethanol (40, 50, 60,

70, 80 and 90%) for 30 min each and in 100% ethanol for 24 h.

Final chemical drying was conducted according to the proto-

col of Perdigo et al. [38] using hexamethyldisilazane (Kyowa

chemicals, Tokyo, Japan). The dry resin disks were sputter-

coated with gold for 200 s and then examined with a scanning

electron microscope (SSX-550, Shimazu, Tokyo, Japan).

3. Results

3.1. Evaporation of volatile adhesive components by

storage at room temperature for 60 min and by an

air-stream for 30 min

The degree of spontaneous evaporation of the volatile solvents

of the solvated adhesives was measured at room tempera-

ture for 60min (Fig. 1). The rate of evaporation induced by a

stream of air over 30 min is graphically presented in Fig. 2.

The weights of adhesives SO, 3S and AB2 adhesive mixtures

gradually decreased 1933% during dark storage at room tem-perature. In contrast, the weight of FB II showed no change

during the 60-min storage period (Fig. 1).

Fig. 2 shows similar evaporation of volatile adhesive com-

ponents induced by an air-stream. The weight loss of AB2, SO

and 3S reached a plateau in 35 min. However, there was no

weight loss in FB II specimens over the 30 min of a directed

air-stream.

3.2. Water sorption and solubility changes over time

When mass gain (i.e. water sorption) and mass loss (i.e. sol-

ubility) of disks made from the bonding resin were plotted

against time, the lowest water sorption (72.5g/mm3) was


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Table 2 Percent conversion of the dentin bondingresins.

Bonding resins Conversion (%)

Absolute 2 68.3 (0.9)a

Clearfil 3S Bond 87.4 (0.5)b

Fluoro Bond Shake One 86.1 (0.8)b

FluoroBond II 75.0 (0.9)c

Means values (standard deviation, n = 5) percent conversion. Resin

composition is presented in Table 1. Groups identified by different

superscript letters are significantly different (p


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Fig. 4 SEM photograph of SO control surface (a)

immediately after polishing but before immersion in

storage media. The surface was relatively rough; (b) similar

resin disks showed rougher surfaces after 10 days

immersed in artificial saliva; (c) SO resin disks immersed in

hexadecane for 10 days showed a surface roughness that

was similar to control surfaces (a) (magnification 2400).

Fig. 5 SEM photographs of3S disks of (a) control surfacenot immersed in storage media showing relatively rough

surface; (b) surface covered by small crystals found on disks

immersed in artificial saliva after 10 days; (c) relatively

smooth resin surface of disks immersed in hexadecane for

10 days (magnification 2400).


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Fig. 6 SEM photograph of AB2 control surface (a)

immediately after polymerization against glass. The

surface was relatively smooth; (b) similar resin disks

showed a rougher surface after 10 days immersed inartificial saliva; (c) AB2 resin disk immersed in hexadecane

for 10 days showed a surface roughness that was similar to

control surfaces (a) (ac: magnification 2400).

The SEM appearance of 3S control surface is shown in

Fig. 5a. When 3S specimens were immersed in artificial saliva,

the surface became covered with crystalline material (Fig. 5b).

Hexadecane-immersed specimen surfaces were not different

from controls (Fig. 5c).

When AB2 polymerized against glass, they gave a typical

smooth SEM appearance (Fig. 6a). When they were immersed

in artificial saliva, they become covered with crystalline

material (Fig. 6b). When AB2 specimens were immersed in

hexadecane for 10 days, they looked very similar to controls

(Fig. 6d).

4. Discussion

The results of this study clearly show that water sorption ofthe resins tested varied significantly from each other. Thus,

the test null hypothesis that there are no differences in water

sorption/solubility of single-step self-etching adhesives must

be rejected. The increased water sorption of AB2 may have

been due, in part, to the fact that this resin had the low-

est percent conversion (Table 2). However, the lowest water

sorption/solubility (Table 3) was seen in FB II even though its

percent conversion was intermediate rather than the highest.

Water sorption varies widely among adhesives depending on

their composition [2830].

One of the most hydrophobic commercial dentin adhe-

sives Superbond C&B (Sun Medical Co.) contains 5 vol%

4-methacryloxyethyltrimellitic anhydride (4-META) in 95 vol%methyl methacrylate. Unemori et al. [36] reported that this

material had an extremely low water sorption of 0.4 wt% at

37 C. Burrow et al. [39] reported that adhesive resins com-

monly usedin 1999in the products Universal Bond(L.D.Caulk),

All Bond 2 (Bisco) and Clearfil Liner Bond II (Kuraray Medi-

cal) had maximum water sorption values of 2.0, 3.9, 4.8 and

5.5%, respectively. These values are somewhat lower than the

water sorption values observed in SO, 3S, in the current study.

Conversely, AB2 exhibited a water sorption value that was

much higher (ca. 21%, Table 3), and thus should be regarded

as very hydrophilic. This value is similar the water sorption

of Xeno III reported by Fabre et al. [40], of 26.8%. Reis et al.

[41] reported water sorption of two-step self-etching primeradhesives (Clearfil SE Bond and Protect Bond) of 7 and 8%,

respectively. These values were similar to FB II in the current

work (7.2%, Table 3).

A previous study [28] showed that extensive water sorp-

tion and solubility was coupled with large reductions in resin

stiffness after water sorption for the most hydrophilic resin

blend. This is cause for concern, as the concentration of acidic

resin monomers utilized in the most acidic resin blends in

that study were similar to those employed in contemporary

aggressive self-etching dentin adhesive.

Figs. 1 and 2 showed that the optimal drying time for dif-

ferent solvated adhesives is quite different. For example, the

manufacture of 3S recommends a 5-s air-drying after 20 s ofactive agitation of the solvated adhesive. In the current study,

evaporation of volatile solvent from the solvated 3S adhe-

sive after 1 min with an air syringe was only removed about

one-fourth of the total weight loss achieved after 5 min of an

air-blast (Fig. 2). Thus, even 1min of evaporation, that is far

in excess of manufacturers recommendations, would leave

three-fourths of the solvents in the adhesive.

This study revealed that the evaporation volatile solvent

from self-etching of bonding resins depends largely on the

amount of solvent added to the blend and the chemical com-

position of the adhesive blends. Complete evaporation of

solvents is difficult to achieve, even by what appears to be

thorough air-drying [4245].


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Table 3 Means of water sorption and solubility of dentin bonding resin.

Resin Water sorption(g/mm3) (%A)

Water solubility(g/mm3) (%)

Hexadecane sorption(g/mm3)

Hexadecane solution(g/mm3)

Net wateruptake (%)B

Absolute 2 207.7 (4.5)a (20.7) 115.6 (5.8)a (11.6) 17.6 (1.8)a 41.2 (3.4)a 32.3

Fluoro Bond

Shake One

101.9 (3.8)b (10.2) 143.1 (3.8)b (14.3) 7.8 (1.5)b 21.6 (2.3)b 24.5

Clearfil 3S 89.2 (3.6)c (8.9) 127.4 (2.4)c (12.7) 5.9 (1.3)b 19.6 (1.5)b 21.6

Fluorobond II 72.5 (2.3)d (7.3) 2.0 (0.5)d (0.2) 13.7 (2.1)c 3.9 (0.8)c 7.5

Values are mean (standard deviation, n = 5)g/mm3. Same superscript letters (ad) in the columns and rows indicates no statistical difference.

Water sorption is given in absolute terms (g/mm3) and in relative terms (%) to provide comparisons to literature values which include both

expressions.A 207.7g/mm3 = 0.2077 mg/mm3 100=20.7mg/100mm3 = 20.7%.B Net water uptake is the sum of water sorption and solubility (%).

Table 4 Microtensile bond strengths for self-etching adhesives.

Adhesive AB2 3S SO FBII

Tensile bond strength 24.8 (6.9)a 57.9 (11.5)b 40.6(9.7)b 51.5(5.4)b

Values are means (standard deviation, n = 24) MPa. Superscript letters designate statistically significant differences (p < 0.05).

Formulation compromises are made when creating self-

etching adhesives. The inclusion of relatively high concentra-

tions of acidic monomers and water, to permit ionization of

those monomersand solubilization of calciumand phosphate,

makes these polymers very hydrophilic. The advantages that

these systems provide during bonding may be compromised

by relatively large subsequent water sorption behavior that

lowers the stiffness of adhesivelayerwhich couples resin com-

posites to dentin. This change may result in poor load transfer

across the bonded interfaceduring function over time, leading

to catastrophic joint failure.

The lack of solvent evaporation from FB II was unex-pected. The only other product that has been reported not to

lose weight during attempts to evaporate its solvents was a

solvent-free experimental resin and the solvent-free adhesive

of Clearfil SE Bond [31]. In that same study, One-Up Bond F

only lost 5.4% of its weight during exposure to an air-stream

for 10 min. We speculate that the water used to ionize the

acidic monomers and solubilize released calcium and phos-

phate immediately reacts with the prereacted surface of glass

filler particlesin FB II in an acidbase reactionthat binds water

to the silica gel layer that forms in the glass fillers. In addi-

tion, that water likely hydrogen bonds to the polycarboxylate

matrix. The net result may be no water evaporation. Since

water evaporation from FB II is not possible clinically, themanufacturers directions to use gentle air-blast to evaporate

volatile solvent seems to be superfluous.

There were no significant differences in the microten-

sile bond strengths between adhesives that included the

S-PRG filler (i.e. FB II) or not (Table 4). However, in the SEM

observation, a crystalline structure was observed on the sur-

face of all of the resins after incubation in artificial saliva

(Figs. 3b, 4b and 5b) but not when they were incubated in hex-

adecane or in control resin disks polymerized on glass. These

crystals were much smaller than those reportedby Hashimoto

et al. [46] in an artificial 40m gap between the resins and

dentin disks over a 1000-day period. In the current study, the

smaller crystals formed on resin within 10 days from a solu-

tion of artificial saliva. We speculate that water trees in the

resin become saturated with the inorganic salts in the arti-

ficial saliva and interact with acidic polymers in the resin to

form microscopic crystals of calcium phosphates.

The somewhat higher bond strength of FB II compared to

SO made by the same company may be due to the fact that

the hydrophilic primer of FB II is covered by a solvent-free

mixture of UDMA and TEGDMA that may be more hydropho-

bic than the ingredients of SO. This might also be responsible

for the significantly lower water sorption of FB II compared

to SO and the extraordinary low water solubility of FB II (ca.

2%). The low water sorption probably plasticitized FB II muchless than those resins showing higher water sorption (Table 3).

The extremely low hexadecane absorption/solubility is prob-

ably due to its hydrophobicity andits relatively large size (MW

226) making it relatively impermeable to these resins.

Clearly, long-term studies on the durability of resindentin

bonds are needed to provide a more realistic evaluation of the

longevity of bonds made with new, more hydrophilic self-etch

adhesives.

This study demonstrated that the unfilled phosphoric acid

esters of methacrylate(Absolute 2) had the highest water sorp-

tion. Single-step self-etching dentin adhesives gave higher

water sorptions and solubilities compared with the two-step

self-etching primer adhesive system (FluoroBond II).

Acknowledgements

The materials (Absolute; Dentsply Sankin, 3S Bond; Kuraray,

Shake One and FL Bond II; Shofu) used in this study were gen-

erouslysupplied by the respective manufacturers. The authors

are grateful to Michelle Barnes for secretarial support.

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Water Sorption and Adhesion

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