Water sorption and mechanical properties of dental composites C.Bastioli and G. IstmdoG. Donegani,

Roman0

Novara,

&a/y

C. Migliwesi Department

of Engineering,

(Received

8 November

The physical scanning tests.

University

1988:

properties

calorimetry,

disappeared linking

sorption

scanning

indicated

Italy

1989;

accepted

dental

and desorption

calorimetry

20

February

composites

in water,

of the material

1989)

were

measurements

curves of samples

that the small residual monomer

after immersion

reaction

Trento,

14 February

of four commercial water

The differential

ageing in water

of Trento,

revised

investigated and flexural

as prepared

reactivity,

monomer

desorption.

differential properties

and after different

present

which probably acts as a plasticizer

and the residual

through mechanical

times

of

in the as prepared samples,

and facilitates Consistently,

a further crosswater

causes

the

embrittlement of the material, as detected from the flexural mechanical properties. Water sorption and desorption kinetics were measured at different temperatures, the water diffusion coefficients were calculated

and the activation

energies

fracture surfaces and the decrease filler/matrix adhesion. Keywords:

Dental

Bis-phenolglycidyl have gained materials

composites,

methacrylate

increasingly

in dentistry’.

mechanical

(bis-GMA)

larger Many

of the diffusion

properties,

as restorative

of the commercial

of various

impart

types,

high long-term

ization shrinkage3, molecules4

geometry dimensional

reduced

stability’,

finishing

and

chosen

to

low polymer-

release of low molecular

and good surface

weight

to the

high viscosity

dimethacrylate,

methyl

the same

improve

resins’. resin

time

However, and

favour

increase

release

themselves

composites,

about

(MMA),

30%

degree

the water

of

low

included.

In

by weight

fillers of different

monomer

etc., which

the final conversion

diluents the

basic

uptake

molecular most

type, geometry,

at

of the of the weight

commercial

of diluents

are

used.

size and content

ventional

main classes ones, which

silica and silicates ___--___ Correspondence 1990

Butterworth

of dental

contain

composites

70-85%

having a dimension

are: (1) con-

by weight

of about 20 pm, and

and lower mechanical In this

by weight

(< 1 pm).

properties,

the latter shows

properties,

i.e.

high polymerization

properties

paper,

the

properties

compared

effect and

composites

and desorption

MATERIALS

high

shrinkage

with conventional

diffusion

of water

the

were

type

studied

coefficients

sorption

on the

of fracture and water

of four sorption

determined.

AND METHODS

Materials In Table 7, the compositions are reported thin

with

contain

bis-GMA

the class of poly(ethylene The nature, content, of the

inorganic

of the tested dental composites

commercial

layer chromatography

sedigraph 12/90/0302

finishing

size

behaviour-dependent

fillers

means of emission

Ltd. 0142-96

up to 50%

composites.

of quartz,

to Dr C Mlgliaresl. Et Co (Publishers)

of the

of a good

containing

water sorption (about 2 wt%*)

materials

are utilized. The

excellent

conventional

are added to the basic resin, dimethacrylates), bis-phenol

methacrylate

substances, Moreover,

of the

composites,

resin

mechanical

(> 10 P so’) different diluents such as poly(ethylene glycol

the existence

silica of very fine particle

Despite greater

and colour stability5

to the material. Owing

(2) microfine of pyrolytic

products

diluents

and content

The SEM analysis

indicated

sorption

based composites

applications

consist of the same type of base resin, different fillers

process were determined.

of the water uptake on the temperature

methods.

names analysis,

and producers. it was

found

resin, with diluents

From that

all

belonging

to

dimethacrylates)‘. surface were

spectrometry,

area and size distribution

determined

respectively

ash analysis,

Each commercial

product

by

BET tests and is applied

in

19-05 Biomatenals

1990,

Vol

1 1 April

219

Properties of dental composites: C. Bastioli et al.

Table 1 Composition composites

and other characteristics

of the tested

dental

Product name

Adaptic

Miradapt

Profile

Silar

Manufacturer

Johnson Et Johnson Quartz

Johnson Et Johnson Barium silicate 8 1.o 0.37 17.0

S.S. White

3M

Strontium silicate 81.7 0.97 15.8

Pyrolitic silica 50.0 -

40 20 16

26 18 14

Filler type

Filler content (wt%) 77.4 Surface area (m2/g) 0.92 Average diameter (pm) 17.9 Filler size’ s95 40 S60 25 SlO 6

b

*Expressed as wt% of filler with size higher than that indicated in the first column.

components: catalyst and activator. The catalyst contains matrix, filler and benzoyl peroxide; the activator contains matrix, filler and an aromatic amine (NJ/-diidrossi ethyl paratoluidine). Samples were prepared from a catalystactivator of 1 : 1 ratio. two

Methods Differential scanning calorimetry (DSC) measurements were performed by using a differential scanning calorimeter Mettler model DSC30 with a scanning rate of 20”C/min. Water sorption and desorption kinetics were determined on flat samples 0.045 X 3 X 0.5 cm3, using a protocol where the polymerization of the samples was conducted in a mylar mould for 15 min at r.t. After polymerization, all samples were immersed in distilled water at 60°C and then dehydrated under vacuum at 6O”C, until constant weight was attained. Sorption and desorption kinetics were measured by weighing at different times the previously conditioned samples after immersion in water (sorption) and then suspended in a closed bottle on silica gel (desorption), at 37, 48 and 60°C. The fracture surfaces of the samples were observed with a scanning electron microscope (SEM) (Cambridge Stereoscan model 604, Cambridge Instruments, Cambridge, UK). The flexural mechanical properties were measured in air at r.t. (25”C, 50% r.h.) using an lnstron machine on samples previously aged in water for 3, 15 and 60 d, at a crosshead speed of 0.1 cm/min.

RESULTS AND DISCUSSION DSC diagrams of Miradapt, after different times of immersion in water at 37 “C, are shown in Figure 1. The curves obtained with the other materials are very similar. A wide exotherm peak can be observed for the samples aged for 10 min. being associated with the residual curing of the low molecular weight unreacted components. After 24 h. the exothermic peak is no longer evident (in some samples and in other measurements it still appears) and no more residual reactivity can be detected for all samples aged for 60 d in water. The water acts as a plasticizer and favours both the reaction and the desorption of low molecular weight molecules. Generally an increase of the resin glass transition temperature (T,) occurs, which is particularly relevant for the longer time aged samples, indicating the lowering of the Tg due to the presence of water is less important than the effect, due to unreacted species initially present in the composite". Sorption and desorption experiments were performed in order to evaluate the water diffusion coefficients at 37,48

220

Biomaterials

1990, Vol 11 April

I -20

I

I

I

I

I

I

I

0

20

40

60

80

100

120

T,Oc Figure 1 DSC thermograms for Miradapt after (a) 10 min. (b) 24 h and (c) 60 d of immersion in water at 37°C.

and 60°C and the activation energies of the diffusion process. The curves reported in Figures 2 and 3 refer to samples which have undergone a previous sorption/ desorption cycle at 60°C. in order to remove any unreacted monomer. The initial linear dependence of M,, the water content of the sample at the time, t, on the square root of time divided by the sample thickness (t o.5I-’ ), suggests a Fickian behaviour of the transport phenomenon”. The diffusion coefficients can be calculated from the initial slope of the sorption and desorption curve through Equation 1 “: M,/M

= 1 - exp [-7.3

(Df/p2)o.75]

(1)

where D is the diffusion coefficient and M the equilibrium water content. The water diffusion coefficients calculated from the sorption and desorption experiments at 37, 48 and 60°C. are reported in Table 2. In the same table, the percent weight loss after the first conditioning cycle at 60°C. M, and the equilibrium water uptakes in the following sorption cycle, referred to the composite total weight, Mg,,c, and to the amount of resin in the composite, M, r, are indicated. At each temperature, the diffusion coefficients for sorption are lower than the ones for desorption and the difference increases with increasing T. The activation energies of water transport in all materials were calculated by assuming an Arrhenius dependence of the diffusion coefficients on the temperature as shown in Figure 3. where the logarithm of D is plotted versus 1/T. The slopes of the straight lines which correlate D and 1/T, divided by the gas constant, R, give the activation energies, which appear to be higher for sorption (Table 3). The result is frequently found for glassy polymers and can be explained in terms of overall plasticizing effects due to the incoming water. No deviations from the ideal were observed in the sorption kinetic curves (Figure 2) as a consequence of filler/ matrix debonding. In fact, this phenomenon would be characterized by an increase of water uptake with temperature.

Properrim

&

4

Ii

Yt/l Figure 2

at Rj

Table 2 Sorption /O,) and dasffrptkx? fDJ diffusion coefficients (cm2/si: water uptakes after sorption referred to the composite weight lMII. J and to rhe resin weight f M, ,I; wt% decrease IMJ of the dry samples after the firsi conditioning cycle in water at 60°C M,

Adaptic

0.31

37°C

-

Miradapt

0.25

Profile

0.47

Silar

0.88

D, 0, M %C M 9.r D, D, M 9.c M 9.’ D, 0, M 9.c Y?. r 0, D, M Q.C &l9,’

4.97 1.10 4.87 7.11 1.58 0.953 5.01 4.40 8.27 0.788 4.31 8.75 1.11 2.89 5.78

x 10-g

x 10-g x 10-E

x 1O-9 x 10-9

x lo-’ x lo-.8

48 “C

60°C

1.11 1.16 1.04 4.60 1.02 2.30 1.07 5.63 9.89 1.19 0.732 4.00 1.74 2.00 2.44 4.88

2.06 2.57 1.04 4.60 2.55 3.20 1.00 5.26 2.94 3.29 0.690 3.77 3.25 3.52 2.46 4.92

x 10-g x 10-8

x 10-B x 1O-8

x lo-’ x 1O-’

x 1C?-8 x 10-8

Miradapt Profite Silar Adaptic

48.75 65.83 47.48 51.66

26.20 51.29 43.47

37°C. fA/ 48°C and #)

1’0

72

Matertal

immersion time (d) __________ ,--...

X 1O-8 X lo-*

x ?O-8 x IO-’

15

-.. 60

E (GPa)

x 10-a x 10-a

x lo-* x lo-’

60°C as indicated

Table 4 Herural modulus (E] after different ageing times in water 8? 3 7 “C. and weight loss &I,/ and gain @A,) after 60 d of immersion in water

3

Activation energies for water sorption /E,J and the waterdesorption IEd) processes

Table 3

I3

- I raea, secClrGm

Sorption (a/ and desorption lb) curves of Miradapt and War

Material

composites: C. Bastioli et al.

a

silar

0

of den&

Adaptic Miradapt Profile SilW

8.4 6.2 13.5 4.3

9.5 8.6 9.3 5.4

10.4 12.1 9.9 5.9

0.76 0.60 1.40 2.35

0.85 1.07 1.09 2.97

On the contrary, the water uptake decreases with temperature (Table 2). Furthermore, the SEM of.the fracture surface of composites after the water ageing cycle at 60°C (Figure 4), do not show any debonding between filler and matrix. Flexurai mechanical properties were determined in samples aged in water at 37°C for 3, 15 and 60 d. Data for the modulus are reported in Table 4, together with the percent weight loss, M,, and weight gain, M,, determined after 60d. These quantities differ from those reported in Table 2, being measured at 37 “C and on thicker samples not previously conditioned. The difference can be explained, considering that at 60 “C, the polymerization proceeds faster rhan at 37 “C, causing a decrease of the low molecular weight substances that can diffuse out from the material, even if the diffusion coefficients increase. The trend, however, is the same and an approximately similar ratio among the weight losses of different composites was

Biomaterials

1990, Vol 1 1 April

221

Properties of dental composites: C. Bastioli et al.

-17

-12

-1s

-17

0 c -18

-1s

b -17

-12

-IQ

a

310

.

.

3.1

3.2

Figure 3 Arrhenius plot for (0) water sorption and (0) desorption processes of (a} Silar. (b) Profile and (c) Miradapt.

observed. As for the data of Table 2, the water loss and gain of Silar, which contains less filler, is higher. All composites display an elastic modulus which is higher than that of the Silar composite, consistent with their higher filler content. Moreover, whilst, for Adaptic, Miradapt and Silar, an increase of the elastic modulus with the ageing time is observed, a large initial reduction is caused by the aging phenomenon in the flexural modulus of the Profile cement, indicating a different effect of the overall plasticization associated with the incoming water and the outcoming monomers’“. In conclusion, water sorption and desorption experiments demonstrated that the Fick’s law can be used to model the sorption and the desorption behaviour of the tested composites, with diffusion coefficients depending on the temperature following an Arrhenius-type equation. Immersion in water ages the materials and the mechanism appears to be associated with an overall plasticization effect, caused by the release of the unreacted species and the sorption of water. An increase of the elastic flexural modulus for Miradapt, Silar and Adaptic was observed, whilst, for Profile, the modulus decreases after immersion in water. The

222

Biomaterials

1990, Vol 11 April

Figure 4 SEM of fracture surfaces of Profile (a, c) and Silar (6. d) samples before (a, b) and after (c, d) the water sorption at 60°C.

Properties

difference can be explained by a different role played on plasticization by the outcoming species and the incoming water. No debonding occurs for any material between resin and filler, as observed by SEM and demonstrated from the water sorption curves.

5

6

7

REFERENCES 8 Bowen. R.L., Properties of a silica-remforced polymer for dental restoratlons, J. Am. Dent Assoc. 1963, 66, 57-64 Hirasawa, T.. Hlerano, S., Hirabayashi, S., Harashima. I. and Aizawa. M.. Initial dimensional change of composites in dry and wet conditions, J. Dent. Res. 1983, 62, 28-33 Antonucci, J.M. and Toth, E.E., Extent of polymerization of dental resins by differential scanning calorimetry. J. Dent Res. 1983, 62, 121-126 Fletcher,A.M., Purnaveja,S..Amin.W.M., Rltchie.G.M., M0radians.S. and Dodd, A.W.,The level of residual monomer in self-curing denture base materials, J. Dent Res. 1983, 62, 1 18-l 20

9

10

11 12

of denfal compostfes:

C. Basfioli ef al

Powers, J.M., Demlnson. J.8. and Koran, A., Color stability of restorative resins under accelerated aging, J. Den?. Res. 1978, 57, 964-968 Bowen, R.L., Barton, J.A. and Mullineax, A.L., Composite restorative materials, In Dental Marerials Research. National Bureau of Standards Special Publication 354, (Eds G.R. DIckson and J.M. Cassel). US Government Printing Dfflce, Washington DC, 1972, p 379 St. Germain, H., Swartz. M.L., Phillips, R.W., Moore, 8.K. and Roberts, T.A., PropertIes of microfilled composite resins as Influenced by filler content, J. Dent Res. 1985, 64, 155-l 60 Braden, M. and Clarke, R.L., Water absorption characteristics of dental microfmecomposltefilling matenals,Biomarena/s 1984,5,369-372 Cowperthwaite, G.F., Fey, J.J. and Malloy, M.A., The nature of the crosslinking matrix found in dental composite fillmg materials and sealants, In Biomedical and DenralApplicarions of Polymers (Eds C.G. Gebelein and F.F. Koblitz). Plenum Press, New York, 1981, p 379 Apicella. A., Migliaresi. C., Nlcodemo. L., Nvzolals, L.. laccanno. L. and Roccotelli. S., Water sorption and mechanical propertIes of a glass reinforced polyester resin, Composites 1982, 13, 406-410 Barrie, J.A.. in Diffusion in Polymers (Eds J. Crank and G.S. Park), Academic Press, London, 1968, p 259 Crank, J.. MarhemaricsofDiffusion, Clarendon Press, Oxford, 1975

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