937

Novel acrylic resins for dental applications

R. Labella, M. Braden and K.W.M. Davy Dental School of The London Hospital Medical College, University of London, Turner Street, London El 2AD. UK

A heat-cured resin based on a copolymer of bis-phenol-A glycidyl methacrylate (bis-GMA) and tetrahydrofurfuryl methacrylate (THFM) was investigated. Workable pastes were made by adding 90110 w/w bis-GMA/THFM copolymer powder to a 70130 w/w monomer of the same composition. The organic filler content was 60-64% w/w. Young’s modulus, flexural strength, impact strength, hardness, water absorption and desorption, linear thermal expansion, polymerization shrinkage and glass transition temperature were determined. The materials studied showed high elastic modulus, hardness and glass transition temperature. A relatively low linear thermal expansion was obtained but poor impact strength and low flexural strength, indicating brittleness. Acceptable values were obtained for water absorption.

Keywords: Dental materials, acrylic resins, glycidyl methacrylate, tetrahydrofurturyl methacrylate

Received 16 December 1991; revised 10 March 1992; accepted 22 April 1992

The achievements introduced to restorative dentistry by the composite technology and the use of dimethacrylate monomers (e.g. bis-GMA) are considerable and include increased elastic modulus and hardness and reduced polymerization shrinkage” ‘. More recently, composites polymerized under heat and pressure have been intro- duced as inlays and onlays.

The present investigation originated from preliminary studies on the viscoelastic and elastic properties of bis- phenol-A derived resins carried out by Clarke3v4 and Davy and Braden’ using tetrahydrofurfuryl methacrylate (THFM) as a diluent for bis-GMA. Surprising elevations in Young’s modulus and glass transition temperature suggested further work on its potential for dental applications.

This work attempted to exploit such resins by making a composite of the pulverized resin in its own monomer, a technique first used in the manufacture of microfine composites6-s.

MATERIALS AND METHODS

2,2-bis(p[2-hydroxy-3-methacryloyloxy-propoxy]phenyl) propane (bis-GMA) was obtained as Nupol” from Freeman Chemicals Ltd, UK, and THFM was obtained from Rohm GmbH, Chemische Fabrik, Germany. Benzoyl peroxide was used as an initiator in the form of Lucidol’” CH50 (Azo Chemicals, Gillingham, Kent, UK). This is a 50/50 master batch of benzoyl peroxide with dicyclo- hexylphthalate. A monomer system of 90% bis-GMA, 10% THFM and 1% benzoyl peroxide (2% Lucidol) was

Correspondence to Dr R. Labella.

heat-cured overnight at 8O”C, fragmented and then ball- milled to give a powder. Figure 1 shows two main particle sizes of -2 and -10 pm. The liquid was prepared from a 70130 w/w ratio of bis-GMA/THFM and 1% benzoyl peroxide (2% Lucidol). Workable pastes ready to be packed in the mould were obtained using two powder/ liquid ratios of 80140 and 63137 for water absorption and Vickers hardness: of 60140, 63137 and 84136 for Young’s modulus and flexural strength; of 63/37 for thermal expansion, impact strength and glass transition tem- perature. The pastes were placed in dental stone moulds and in conventional dental flasks. The moulds were prepared by investing a rigid pattern of the required shape in dental stone. Tin foil was used as the separating medium between the mould and the resin. The flasks were closed and pressure gradually applied by a hand screw press. The specimens were then dry heat-cured in an oven set 7 h delay followed by 3 h ‘boiling’ temperature of approx. 9O’C. Finally, cylindrical samples were prepared for polymerization shrinkage and glass transition temperature tests. Hollow Teflon@ cylinders were used as moulds. The resin was dry heat-cured overnight at 8O’C. The Newman-Keuls test was used for significance of variance of results obtained from Young’s modulus and flexural strength tests, each test providing four sets of data. The Student’s t test was performed on results from impact strength, Vickers hardness and water uptake tests, each presenting two group sets of data.

Modulus of elasticity (Young’s modulus)

Young’s modulus was measured using a three-point bending test on a tensile testing machine manufactured by J.J. Lloyd Instruments (Model No. M30K), South- ampton, UK.

0 1992 Butterworth-Heinemann Ltd Biomaterials 1992, Vol. 13 No. 13 0142-9612/92/130937-07

938 Dental acrylic resins: R. Label/a et al.

Figure 1 Scanning electron micrograph of ball-milled 70/30 w/w bis-GMA/THFM copolymer.

Eighteen rectangular specimens were used, 10 mm X 80 mm X 2 mm. For each specimen the test was run three times to check the results. The supporting bars of the lower part of the rig were set 30 mm apart, which was considered the effective length of the samples.

A 500 N load cell and a 5 mm/min speed were selected for the cross-head. The results were used to calculate the Young’s modulus using:

13c E=LL

4bd3y

where E is Young’s modulus; 1 is length of specimen: F is load applied: b is width of specimen: d is thickness of specimen: and y is deflection.

Flexural strength

The flexural strength was obtained in a three-point bending test measuring the load applied at break. The same equipment and samples for the Young’s modulus test were used. The full-scale load was programmed for 100 N. The following formula was used to calculate the flexural strength [omax):

3Fl o =-

max 2bd2

Impact strength

The method used to evaluate impact strength was of the Charpy type, which uses supported but unclamped notched cylindrical specimens struck centrally by a pendulum. The instrument used was a Hounsfield Plastics Impact Tester manufactured by the then Tensometer Ltd of Croydon, UK.

Twenty specimens were tested. They were in the form of circular rods turned on a lathe to a diameter of 7.6 mm and cut to a length of 44 mm. Each rod was subsequently notched to a depth of l/8 inch using a Hounsfield impact specimen notching machine.

The specimens were placed on horizontal supports, in the path of the pendulum and facing the striker with the unnotched surface. The pendulum was then released from its preset starting position and its swing amplitude reduced by fracturing the specimen, This reduction was indicated by a pointer which allowed the impact strength

to be read directly, in foot pounds (ft lb), which was converted to joules (J),

Hardness

A Vickers Armstrong Ltd (Crayford, UK) tester was used. A load of 1 kg was applied for 10 s to a penetrating diamond point in the shape of a square-based pyramid (angle between its faces 136’). The indentation area on the surface of six samples was then observed on a microscope (objective 2/3”) and the length of the diagonal measured. A table was used to obtain the Vickers Hardness Number (VHN) corresponding to each measured value. For each sample the test was run ten times and an average number given. The room temperature was nominally 20%

Water absorption and desorption

Four rectangular samples (3 cm X 6 cm) were prepared. The thickness was only 0.5 mm so that it could be assumed that diffusion occurs only through the major surfaces, and to reach a quick equilibrium. Samples were stored in a desiccator for 72 h and then weighed to 0.0001 g and returned to the water bath. Weighing intervals were 5, 15, 30, 60, 120, 180, 240, 320, 360 and 420 min during the first day, twice during the second day: daily from the third to the seventh day: twice during the second week and then once a week until a constant weight was reached and maintained for 2 wk or more. After saturation the samples were transferred to a 37°C oven containing calcium sulphate as desiccant and the weighing procedure followed at the same timed intervals as during absorption, until a new equilibrium was attained after desorption. Two absorptionldesorption cycles were completed for each sample.

The diffusion coefficients (0) of absorption and desorption can be calculated from the following equation:

M,IM, = 2(Dtln12)“2 (3)

where M, is the mass absorbed (or lost) at equilibrium, Mt the mass absorbed (or lost) at time t and 1 is the sample half thickness. The value of D is readily calculated from the slope(s) of M,/M, versus t”’ using the equation:

D(cm’ s-‘) = s2n12/240 (4)

The water uptake at equilibrium is here expressed in weight %,

Linear thermal expansion

A 63% organic filler content sample was prepared in a cylindrical shape, 53 mm long and 5 mm diameter. It was placed deep in the bottom of a quartz tube and upon it a quartz rod placed vertically as a movable piston. The quartz rod made contact with the sample top at one end and with the extension rod of a dial gauge at the other.

The dial gauge was calibrated in graduations of 0.001 mm. The tube was then immersed in a bath containing a non-freezing liquid. The temperature was set at the starting value of -2O’C. Every 20 min the temperature was raised by 3°C and the expansion read, until a final temperature of +45%. Contraction was then registered at the same intervals from -20 to 45%. Two expansion/contraction cycles were performed.

Biomaterials 1992. Vol. 13 No. 13

Dental acrvlic resins: R. Label/a et al. 939

As the coefficient of linear expansion of fused quartz is 0.5 X 10m6 per “C, and that of the materials studied transpired to be >l.O X 10e5, the error in neglecting the expansion of the quartz is ~5%.

Polymerization shrinkage

Polymerization shrinkage was determined by measuring the density of the monomer (70/30 w/w bis-GMA) and the corresponding polymer. The density of the monomer was measured using a 25 ml density bottle. This was weighed empty to 0.0001 g, filled with the monomer and weighed again. The same bottle was then emptied, thoroughly washed and dried. It was weighed again empty, filled with distilled water and weighed again. The measure- ments were carried out at nominally 2O’C. The density of the monomer was then calculated using the following formula:

density of monomer = weight of monomer

weight of water (5)

Since the density of water is lower than unity at the temperature used, the corrected density of the monomer was then calculated as:

corrected density = density of monomer X p(T) (6)

where p(T) = 1 - a(T - 4) - /3(T - 4)‘. a and p are constants: a = 1.1458 X 10m4 “C-‘; p = 6.1979 X 10m6

?Z2. The density of the polymer was determined using the

hydrostatic weighing technique. A polymerized sample of the starting composition was suspended from a thread with negligible weight and weighed in air and in water. The density of the polymer was then calculated as:

density of polymer = weight in air

weight in air - weight in water

(7)

The volume shrinkage was calculated as:

% shrinkage

density of polymer - density of monomer =

density of polymer x 100 (8)

The volume shrinkage of the 60% and 63% filled polymers was finally calculated.

Glass transition temperature

The glass transition temperature was determined by differential scanning calorimetry using a Perkin-Elmer model DSC7 (Norwalk, Connecticut, USA]. The tempera- ture range was -25 to +375”C, at scan speeds 5,10,15, 20,25,30,35,4O”C per min consecutively. The specimens were prepared by ball-milling the 63% filler polymerized material and the unfilled polymer to a thin powder. The weight ranged from 16 to 24.6 mg. The apparent T8 was plotted against scan speed and the actual Ts obtained by extrapolating to zero rate.

RESULTS

The results for Young’s modulus, flexural strength, impact strength and Vickers hardness are displayed in

Table 2, The water uptake and polymerization shrinkage results are shown in Tables 2 and 3 respectively. Figure 2 is a typical plot of M,/M, water uptake results versus t1/2

Table 1 Mechanical properties

Test Group Mean n SD Significance

Young’s modulus (GPa)

Flexural strength (MPa)

Impact strength (J) Vickers hardness number

1 25.1 3 +1.9 2 24.5 3 +0.4 NS

4.42 3.59 3.77 3.44

z 6 6

kO.69 +0.40 kO.31 +0.31 NS

73.0 59.5 85.3 76.7

: 6 6

k12.5 +6.0 +16.0 k13.6 NS

0.018 8 kO.005 0.013 20 k0.003 NS

Table 2 Water uptake

Group Mean n SD Significance

Diffusion coefficients (lOmg cm’/s) Absorption 1

2 Desorption 1

2

Equilibrium uptake (% w/w) I Cycle 1

2 II Cycle 1

2

4.85 2 kO.05 4.84 2 +0.15 NS 3.28 2 f0.87 8.44 2 fo.39 NS

3.02 2 kO.03 2.87 2 kO.02 NS 2.98 2 to.03 2.84 2 kO.05 NS

Table 3 Density and polymerization shrinkage

Density of monomer 1.1263 g/ml Density of polymer 1.2277 g/ml Volume shrinkage (Av/v X 100) 8.25% Calculated shrinkage of 60% filled system 3.30% Calculated shrinkage of 63% filled system 3.05%

1.0

0.5

0 40 80 120 160

tf(minJf

Figure 2 Plot of uptake and desorption data as a function of t”*. Absorption cycles: A, 1st; 0, 2nd. Desorption cycles: A, 1st; 0, 2nd.

Biomaterials 1992, Vol. 13 No. 13

940 Dental acrylic resins: R. Label/a et a/.

The thermal expansion of the polymer is not linear with temperature but shows a gradually increasing slope (Figure 3). The same behaviour has been recorded for contraction, with a gradually decreasing slope with decreasing temperature. By plotting expansion values of AL/T against T, a straight line is obtained (Figure 4). This gives:

L, = L, + 293pc + pc2 (91

where L is length of sample, with L, the length at C degrees Celsius and L, at O”C, C is temperature in degree Celsius, jI is slope of graph in Figure 4, equal to 1.77 X

10e8 mm K-‘. Figure 5 shows a typical curve obtained from DSC test.

The 63% filler material shows two transition temperatures at any of the scan rates used. By plotting the apparent TB

values against scan speed and extrapolating to zero rate, the actual T, temperatures of 255’C and 70°C are obtained (Figure 61.

The unfilled material shows two main transition temperatures at any of the scan rates and two extra transitions at scan rates from 20 to 50”C/min. The plot of the two main TB temperatures against scan rates was extrapolated to give 245% and 60°C (Figure 7).

DISCUSSION

Young’s modulus

The tested materials showed moduli ranging from 3.4 to 4.4 GPa. These are high values, considering that: 11) heat- cured Nupol alone has a modulus about 3.6 GPa (Ref. 5); (2) THFM homopolymer has a modulus of about 2 GPa (Ref. 9); (3) the final product obtained from polymerization of workable pastes usually shows reduced modulus compared with bulk polymerized materials of the same composition.

It can be inferred that THFM as a diluent does not reduce the modulus of bis-GMA as might be expected but has increased its rigidity. Davy and Braden’ speculated that THFM could increase the degree of conversion of bis-GMA, which polymerizes incompletely when used alone (about 60% conversion”). Davy and Braden’ obtained an elastic modulus of 4.6 GPa for heat-cured copolymer of 95/5 of NupoVTHFM and a value still

-20 -10 0 10 20 30 40 50

T(‘Cl

Figure 3 Plot of linear thermal expansion (AL) as a function of T.

x 10-4(cm/~C)

7

6

5 I

L 41

0 1 I I I I I I ,

253 259 265 271 277 283 289 295 301 307 313 317 323

UK1

Figure 4 Plot of AL/T ratio as a function of T.

150

I /

112.5

OC I

-10 30 70 110 150 190 230 270 310

Temperature (“C)

Figure5 Diagram obtained from DSC test at scan rate of 10Wmin. ---, 1st derivative.

400 t

350 t

300 -

255. 250 = d

I transition

0 d a

II transition

loo 70 I . .

50 A

I

0 5 10 15 20 25 30 35 40 45 50

Scan rate (OClmin)

Figure 6 Plot of apparent T, values obtained from DSC as a function of scan speed and extrapolation to zero rate. Filled material. A, I transition: A, II transition.

equal to that of the parent Nupol with a 20% addition of THFM. Clarke4 has shown that the addition of THFM to Nupol produced polymers with higher TB values and a higher activation energy. This would explain the higher modulus values.

Finally, the values obtained are also greater than the data obtained for the elastic modulus from currently available crown and bridge resins and denture base

- Biomaterials 1992, Vol. 13 No. 13

Dental acrylic resins: R. Label/a et al. 941

50 60

0 5 10 15 20 25 30 35 40 45 50

Scan rate (°C/min)

Figure 7 Plot of apparent TB values obtained from DSC as a function of scan speed and extrapolation to zero rate. Unfilled material. A, A, Cl, W are I, II, Ill, IV transitions respectively.

PMMA [see Tables 1 and 4). Some caution is necessary in such a comparison because different test conditions were used. Nevertheless, the differences are substantial.

Flexural strength

The values obtained ranged from 55.5 to 85.3 MPa. These values are higher than those for denture base PMMA but lower than data available from conventional crown and bridge resins and microfilled heat-cured composites (Tables 3 and 4).

Impact strength

The resin investigated gave impact strength values 0.013-0.016 J, low compared with PMMA denture base resins. For example Trevalon (De Trey Dentsply] showed an impact strength of 0.028 J (Ref. 17). This is typically observed in highly cross-linked polymers because of the extremely reduced mobility not only of the main chains

but also of short segments of the main chains and of side chains. However, with such a high modulus, there is potential for rubber toughening.

Vickers hardness

Vickers hardness numbers obtained ranged from 24.5 to 25.1. The tested polymers showed a hardness similar to that of Isosit, which is a microfilled heat-cured composite but a much higher hardness value than that of conven- tional crown and bridge veneering resins and denture base PMMA resin (Tables 3 and 4).

Water absorption and desorption

Plots of M,/M, gave linear dependence on t”’ at early values of time and departure from linearity at the later stage (Figure z), as predicted by the diffusion theory. The higher slope, hence the higher diffusion coefficient during desorption is a well-known phenomenon with the uptake of water by polymers**. This is due to the coefficient being dependent on concentration and decreasing with increasing concentration. The materials tested showed reversible behaviour during two sorption- desorption cycles.

Diffusion coefficient values from 4.81 to 4.95 X 10mg cm’/s for absorption and from 8.17 to 9.90 X 10mg cm’/s for desorption were obtained. The equilibrium uptake ranged from 2.81 to 3.04% w/w.

These data are quite consistent with the behaviour of bis-GMA based resins recorded in previous studies. Braden and DavyI found that bis-GMA homopolymer has a diffusion coefficient of 6.27 X lo-’ cm’/s for desorption. Equilibrium uptake was 2.8% w/w.

Pate1 and Braden” tested samples of THFM homo- polymer, which did not equilibrate in 2.75 yr. THFM by itself absorbs >30% water, without equilibrating. The materials studied in this work when compared with PMMA and conventional crown and bridge materials,

Table 4 Physical characteristics of PMMA and polymeric materials used in crown and bridge construction11-16

Young’s modulus @Pa)

Thermal coefficient of expansion (lO+C’)

Hardness (kg/m2)

Water sorption: Equilibrium uptake (% w/w)

Diffusion coefficient (lo-* cm2 s-l)

Absorption Desorption

Polymerization (% v/v)

Flexural strength (MPa)

Denture base PMMA

2.75

8.0-9.0

17

2.25

1.59 3.5

5.2-6

50-60

Crown and bridge veneering resin

1.55-2.12

8.3

17-18

2.04-2.24

1.64 2.6

84-89

lsosit microfilled heat-cured composite

2.89-3.08

7.2

27

1.7

0.32 0.54

1.4 (filling composite resin)

103

Polyvinyl acrylics

2.8

7.1

16

1.99 3.6

6

Crown and bridge microfilled light-cured composites

7.6-8.5

27.2-30.0

Biomaterials 1992, Vol. 13 No. 13

942 Dental acrylic resins: R. Labella et al.

absorb water more slowly but in larger quantities, Isosit shows similar diffusion coefficients (Tables 2 and 4).

Linear therm81 expansion

The expansion data were not linear with temperature, hence a unique coefficient of expansion cannot be ascribed. The expansion appears to be parabolic (Equation 9).

Conventionally, the coefficient of linear expansion is defined as:

I aL, a=LdC WI

(111

2938 + 2/X

a = L, + 293pc f @I? I121

~q~a~jon 12 could be used to calculate an average value at a chosen temperature. For example, when C = 37, a = 1.22 x 10-5”c-‘.

Polymerization shrinkage

Density measurements led to 8.25% v/v polymerization shrinkage of the polymer matrix. A value of 8.25% confirms the high degree of conversion for bis-GMA, which showed a pol~e~zation shrinkage of 5.04% v/v when used alone by Pate1 et af.‘l, who also gave a value for THFM polymerization shrinkage of 14.3% v/v.

Glsss transition temper8ture

The extrapolated Ts values show for the unfilled polymer two values at high temperatures (245°C and 207’C) and two at lower. This appearance of doublets, albeit at different temperatures, has been noted by Clarke3 using differentia1 thermal mechanical analysis, and this author has discussed their significance. However, for the filled system the behaviour is simpler with values at 255°C and 70% respectively. The former very high value is presumably the main transition (Tsa] and the lower is a @ transition. These values are much higher than those found by Clarke3* *. However, the two methods, differential scanning calorimetry and differential thermal mechanical analysis, are presumably measuring different properties as the former measures discontinuities in heat capacities (C,) and hence entropy.

CP (13)

Obviously these materials are thermally very stable.

CONCLUSIONS

The polymers evaluated in this study showed the following characteristics, compared with conventional PMMA-based resins:

(1) High Young’s modulus. (2) High hardness, approaching that of microfilled

composite resins. (3) Low thermal expansion, although expansion was

not linear. (4) Very high glass transition temperatures. (5) Low impact strength. (6) Low flexural strength. (7) Low polymerization shrinkage.

With regard to the water absorption, it can be concluded that the polymer slowly absorbs a moderate amount of water, higher than other available acrylics but still acceptable.

It is postulated that the use of THFM as a diluent for bis-GMA produces a high cross-linked resin, with a high degree of conversion. This would explain the rigid, hard, thermally stable polymer absorbing reasonably little water.

It would also explain the poor impact strength and the low flexural strength.

REFERENCES

1

2

3

4

5

6 7 8

9

10

II

12

13

14

15

16

17

Bowen, R.L., Dental filling material comprising vinyl silane treated fused silica and a binder consisting of the reaction product of bis-phenol and glycydyl acrylate, US Patent No. 3,066,112, 1962 Bowen, R.L., Properties of a silica reinforced polymer for dental restorations, 1. Am. Dent. Assoc. 1962, 68, 57 Clarke, R-L., The visco-elastic nature of polymers related to dentistry, PhD Thesis, University of London, 1988, pp 273-285 Clarke, R.L., Dynamic mechanical thermal analysis of dental polymers. II, B&-phenol A related resins, Bio- materials 1989, 10, 549-552 Davy, K.W.M. and Braden, M., Study of polymeric systems based on 2,2-bis-4(2-hydroxy-3-methacryloyl- oxypropoxy)phenylpropane, Eiomaterials 1991, 12, 406-410 German Patent Application No. 2,403,111, 1975 German Patent Application No. i&405,578, 1975 Nemcek, J., Arwell, R.T. and Raymond, SF. (ICI], Dispersion of siliceous solids in liquid organic media, European Patent Application 0,013,491 AT, 1979 Patei, M.P., Braden, M. and Davy, K.W.M., Polymer- isation shrinkage of methac~late esters, Bjoma~e~~~s 1987, 8, 53-56 Patel, M.P. and Braden, M., Heterocyclic methacrylates for clinical applications. I. Mechanical properties, Biomaterjals 1991, 12, 645-648 Stafford, G.D. and Braden, M,, Water absorption of some denture base polymers, J Deat. Res, 3968, 47, 341 von Fraunhofer, J.A., The surface hardness of polymeric restorative materials, Er. Dent. I. 1971, 130, 243-245 Michl, R.J., Isosit - A new dental material, Quintessence International 1978, 3, 29-33 Wilson, G.S., Physical and chemical studies of heat cured polymers used for crown and bridge construction, PhD Thesis, University of London, 1982, pp 525-668 Craig, R.G., Restorative Dental Materials, 8th edn. CV Mosby, St Louis, USA 1989, pp 514-520 Greener, E. and Duke, S., Physical properties of two new crown and bridge veneering resins, J. Oral Redskin. 1989, 16, 203409 Rodford, R., The development of high impact strength denture base materials, J. Dent. 1986, 14, 214-217

BiomateriaIs 1992, Vol. 13 No. 13

Dental acrylic resins: R. Label/a et al.

16 Barrie, J.A., Water in polymers, in Diffusion in Polymers (Eds J. Crank and G.S. Park), Academic Press, NY, USA, 1966, pp 259-313

19 Braden, M. and Davy, K.W.M., Water absorption characteristics of some unfilled resins, Biometerials 1966, 7, 474-475

943

20 Patel, M.P. and Braden, M., Heterocyclic methacrylate for clinical applications. III. Water absorption character- istics, Biomaterials 1991, 12, 653-656

21 Patel, M.P., Braden, M. and Davy, K.W.M., Polymer- isation shrinkage of methacrylate esters, Biomaterials 1967, 8, 53-56

Biomaterials 1992, Vol. 13 No. 13