I56 Australian Dental Journal, June, 1983

Volume 28, No. 3

Polymerization shrinkage of resin-based restorative mate rials

Michael Goldman

Australian Dental Standards Laboratory

ABSTRACT-The polymerization shrinkage of a range of chemical and photopolymerized resin-based restorative materials available in Australia was investigated using a previously reported volumetric shrinkage measuring method.

The method proved easy to use, reproducible, independent of specimen size, and capable of use in both chemically and photo-initiated systems. The values obtained for the polymerization shrinkages ranged from 1.67 to 5.68 per cent with most materials being in the 2 to 3 per cent range and the overall average result being 2.81 per cent. It was observed that powder-liquid systems seemed to have the highest shrinkage and light-activated materials the least with the paste-paste materials having intermediate results. An experiment carried out to examine the effect of voids incorporated upon mixing showed that increased void content caused a greater shrinkage of material per unit mass.

This, together with other factors such as amount and size of reacting monomer, is seen to partially explain the observed order of polymerization shrinkages.

(Received for publication November 1982.)

Introduction Factors of importance in the marginal adaptation of

resin based restorative materials to dental cavity walls include polymerization shrinkage, long-term water sorption characteristics, placement viscosity and polymerization time of the material, bonding between the material and the walls, finishing technique of the dentist and the thermal coefficient of expansion of the material.'-4

Studies over the past decade have revealed that the marginal gap caused by initial polymerization shrinkage is often not eliminated by acid etching',' or volumetric expansion due to water Internal stresses

' Asmussen E, Jorgensen KD. A microscopic investigation of the adaptation of some plastic filling materials to dental cavity walls. Acta Odont Scand 1972;30:3-21.

' Jacobsen PH. Clinical aspects of composite restorative materials. Brit Dent J 1975;139:276-80.

' Asmussen E. Marginal adaptation of restorative resins in acid etched cavities. Acta Odont Scand 1977;35: 125-34.

' Asmussen E. Composite restorative resins-Composition versus wall-to-wall polymerization contraction. Acta Odont Scand 1975;33:337-44.

generated by the polymerization shrinkage have also been associated with fatigue type failure and wear in dental and analogous composites.6-9

Thus although the relationship between all the above factors and the long term marginal adaptation of a restoration is not completely clear, the elimination or reduction of polymerization shrinkage is considered of major importanceb.8 and work towards this end has been reported.b

To enable this aim to be achieved, a standardized and reproducible method of determining polymerization

Brauer GM, Dulik DM, Hughes HN, Dermann K, Rupp NW. Marginal adaptation of BIS-GMA based composites con- taining various diluents. J Dent Res 1981;60:1966-71.

' Thompson VP, Williams EF, Bailey WJ. Dental resins with reduced shrinkage during hardening. J Dent Res 1979;58: 1522-32.

' Boenig HV, Walker N. Shrinkage of glass reinforced polyesters. Modern Plastics Feb 1961;38:123.

Dickson G. Physical and chemical properties and wear. J Dent Res 1979;58:1535-43.

' Broutman LJ, Krock RH, eds. Composite materials. Vol. 5, 6; New York: Academic Press, 1974.

Australian Dental Journal, June, 1983

shrinkage is required. A test method based on the drop of water level in a capillary tube attached to a pycnometer containing a material exhibiting polymerization shrinkage has recently been described.'O The purpose of the present work was to be used this method to tabulate the volumetric polymerization shrinkages of a wide range of resin based restorative materials, including microfine and photopolymerized materials, available on the Australian market and to consider some of the factors important in determining the extent of shrinkage.

Materials The materials used in the study were a range of those

commercially available to the dental profession in Australia at the time of the study. They are listed in Table I with the manufacturer's name and the batch numbers. To facilitate analysis, the results have been presented in three groups according to dispensing method:

(a) Chemically curing powder-liquid materials; (b) Chemically curing paste-paste materials; (c) Photopolymerized materials.

Each material was mixed or photo-initiated according to the manufacturer's instructions.

I57

begun at 70 * 20 s (depending on the ease of manipula- tion and mixing time of the material) and a base reading was made at 90 s. Polymerization shrinkage was then measured from 90 s to 60 minutes.

Methods The method is based o n that reported by

Bandyopadhyay.'' The test apparatus consists of a dilatometer tube of length approximately 250 mm and graduated in divisions of millimetres, containing a capillary of uniform diameter attached by means of a ground glass fit to a density bottle of volume 0.025 L. Five different dilatometer tubes were used and the volumes of the capillaries per mm division were determined using a mercury thread method." They ranged between 0.143 and 0.152 mm'/division.

The volumetric shrinkage was determined in a ther- mostatically controlled waterbath operating at 25 * 0.1 "C. Two testing procedures were used.

(a) Chemically curing materials Volumes of from 80-200 mm' of the materials were mix-

ed according to the manufacturer's instructions at 23 k2"C and 50 per cent relative humidity. The mixed material was quickly manipulated into an approximately cylindrical shape and dropped into a density bottle containing water at 25 * 0.1 "C (the density bottles had undergone conditioning in the water bath for several hours before testing). The dilatometer tube was then placed tightly into the neck of the bottle, displacing excess water through the capillary. The excess water was wiped off and the time from the start of mixing was recorded against the drop of meniscus in the capillary tube. Readings were

"' Bandyopadhyay S. A study of the volumetric setting shrinkage of some dental materials. J Biomed Mater Res 1982; 16: 135.44.

" Bekkedahl N. Volume dilatometry. J Res 1949;43:145-56.

(b) Photopolymerized materials The materials tested in this category were all supplied

in syringe dispensers. A volume of between 50 and 200 mm' was extruded from the dispenser straight into a density bottle hand warmed to a temperature just above 25°C. The dilatometer tube was placed into the neck of the bottle, excess water wiped off and the apparatus placed in the water bath. As the temperature of the apparatus and specimen approached that of the bath the meniscus dropped. The apparatus was conditioned in the bath for about ten minutes and then meniscus readings were taken every two minutes. When three meniscus readings were within 0.1 mm, the apparatus was considered to be conditioned. The apparatus was then removed from the bath, wiped dry and the material activated uniformly through the glass for 40 seconds using the apvopriate light source. (Preliminary investigations had shown that the glass density bottles did not decrease the intensity of the light in the wavelengths of interest.) The apparatus was replaced in the bath. After twenty- five milbutes a reading was taken and thereafter every two minutes. When three consecutive readings were within 0.1 mm (usually after 30-35 minutes) this was considered to be the final volume.

A supplementary experiment was carried out where, in order to incorporate air, light activated specimens were well spatulated on a mixing pad before dropping into the density bottles. The testing then proceeded as usual.

After the polymerization was essentially complete, the specimen was removed from the density bottle, air blown dry, weighed and the volume of the polymerized specimen (V) determined pycnometrically.

The volume shrinkage was determined from: AV

where S =per cent shrinkage

s=- V + AV 'Ooq0

V = final volume of the specimen AV =change in volume due to polymerization

(a) from 90 s to 60 min for chemically cured materials;

(b) between when readings were constant for light ac-

At least four specimens were tested for each material.

shrinkage.

and

tivated composites.

Results 1. Preliminary investigations

These had shown that: (a) Water uptake by the setting materials was negligi-

ble in the hour during which readings were taken and this

158 Australian Dental Journal, June, 1983

TABLE I Materials investigated and their polymerization shrinkages

Volumetric Coefficient of Material Manufacturer Batch No. shrinkage variation

per cent (specimens tested)

Po wder-liquid C e r v i d e n t Powderlite Rest o d e n t Isocap N* Enamelite Isocap S* Compocap*

Paste-pasle Lee Microfill Profile Ultrafill Clearfil FII Miradapt Silar Cosmic PS Finesse Isopast Adaptic P10 Adaptic KO Prestige Plus Concise Concise KO Vytol

Light aclivatedt

Shade) Visiodispers (Standard

Prismafil (Shade G. B.) Visiofil (Standard Shade) Heliosit (Shade 35)

S. S. White-Penwalt S. S. White-Penwalt Lee Pharm. Vivadent Lee Pharm. Vivadent Vivadent

Lee Pharm. S. S. White-Penwalt Southern Dent Ind Kuraray Co. Johnson & Johnson 3M A. D. Inter. L. D. Caulk Co. Vivadent Johnson & Johnson 3M Johnson & Johnson Lee Pharm. 3M 3M L. D. Caulk Co.

Espe L. D. Caulk Co. Espe Vivadent

0976088/247812 174561'17452 1027 & 1126 1282 I l l 1 & 9347 1182 1181

1065 368109/69 31480611 lo002 04 2381 860 1 AG 9281108 061 08 I /052981 620581/471280 8M026 052781 9H 108 1 l04E 04308 I I01580 03 1681/031781

HO 808C 072581 & 090981 HI 7655 640581

5.68 3.95 3.31 3.27 3.24 3.02 2.52

5.17 2.94 2.77 2.74 2.69 2.69 2.67 2.57 2.52 2.49 2.46 2.32 2.29 2.14 2.03 1.90

2.58 2.38 1.67 1.67

3.3 (4) 5.8 (6) 3.5 (4) 4.1 (6) 2.5 (4) 9.1 (11) 3.0 (10)

1.6 (6) 2.1 (4) 3.3 (6) 2.4 (7) 2.1 (6) 4.0 (6) 4.5 (7) 3.0 (6) 4.8 (10) 4.4 (7) 2.1 (7) 4.7 (5) 2.6 (5) 4.0 (6) 2.6 (5) 5.7 (9)

4.6 (6) 3.4 (5) 2.6 (4) 1.7 (6)

* Capsules. t llsing recommended light sources.

was confirmed for the mixed, photopolymerized materials in the supplementary experiment.

(b) Evaporation and leakage into and out of the ap- paratus was negligible within the time of the experiment.

(c) Polymerization of the light activated materials due t o the artificial laboratory light was not noticeable.

(d) Although the materials set more rapidly at 3 7 T , the extent of polymerization shrinkage after one hour a t 2S"C closely approached that of materials tested at 37°C.'' The tests were consequently performed at 25°C to minimize thermal effects due to specimen and capillary tube being initially a t room temperature (23 k2"C).

(e) Varying specimen size from about half to three- times that normally used gave results within the experimental error of the normal size specimens. Thus the test was independent of specimen size for the volume range in interest.

2. Polymerization shrinkages Results for the polymerization shrinkages of the

materials tested with coefficients of variation and number of specimens tested are given in Table 1.

The results can be seen to range from 1.67 to 5.68 volume per cent. The highest results overall tend t o be for the powder-liquid materials, photopolymerized materials appear to shrink the least with paste-paste materials being intermediate.

As can be seen from the coefficients o f variation, the results are reproducible with only Isocap S having a coefficient of variation greater than 6 per cent and most coefficients being less than 5 per cent. The high value for the coefficient of variation of Isocap S can perhaps be attributed to variations in the powder-liquid ratios between capsules and different shades.

3. Effect of air incorporation The results of the supplementary experiment to examine

the effect o f incorporation of air upon mixing of light activated materials are given in Table 2.

Comparison of the densities, polymerization shrinkages and changes in volume per unit mass of material (a value calculated t o enable comparisons of material shrinkage independently of volume changes due to density differences alone) show that after mixing the density was significantly (P<0.05) lowered in each material and that

Australian Dental Journal, June, 1983 159

TABLE 2 D!/./eretice.s in dentistry, volume change per unit mass and shrinkage between mixed and unmixed light activated materials

Unmixed Mixed

Density AV/m Shrinkage Density AV/m Shrinkage Material x I0’kg/m’ (mm’/g) (per cent) x IO’kg/m’ (mm’/g) (per cent)

Visiodi\per\ (standard) I .61 16.60 2.58 I .55 19.53 2.94 Prismafil (Shade GB) 2.07 1 I .68’ 2.38’ I .96 12.21’ 2.33’ Visiol’il (ttandard) 2.09 8.14 I .67 2.00 10.66 2.08 Heliosit (Shade 20) I .39 13.34 1.82 I .35 14.80 I .95

* Value$ not significantly different at the P =0.05 student t test confidence level.

TAHI.E 3 Densitieh, inorganic and organic filler contents. estimated reacting monomer content and polymerization shrinkage

Material

Inorganic Organic Estimated

(mass Yo) (mass TO) (vol. Vo)

Density filler filler reacting Polmerization x IO’kg/m’ content content monomer shrinkage (vol. Vo)

Cervident l’owderlite Restodent Isocap N Enamelite Isocap S Compocap Lee Microfill Profile Ultrafill Clearfil FII Miradapt Silar Cosmic Finesse I+opa\t Adaptic PI0 Adaptic RO Prestige Plus Concise Concise RO Vytol Visiodispers Pritmafil Visiofil Heliosit

1.69 1.90 1.88 1.34 1.75 1.33 2.01 1.52 2.18 2.03 I .95 2.17 I .50 2.16 I .38 I .35 I .97 2.07 2.03 2.03 I .99 2.22 2.09 I .60 2.08 2 . m I .36

61 75 65.8 30.3 64.5 31.2 79.4 52.6 80.2 76.0 75.3 81.1 50.9 79.4 33.7 37.2 77.2 83.6 77.5 78.1 78.7 81.5 79.6 61.8 75.2 83 38.8

36.3

38.3

4.0

24.3

28.0 32.7

-

29.2

61 46 53 38 57 36 40 59 34 42 45 34 33 35 45 38 43 35 41 40 41 32 37 55 41 35 41

5.68 3.95 3.31 3.21 3.24 3.02 2.52 5.17 2.94 2.77 2.74 2.69 2.69 2.67 2.57 2.52 2.49 2.46 2.32 2.29 2. I4 2.03 1.90 2.58 2.38 I .67 I .67

there was a significant (P<0.05) increase of both volumetric shrinkage and shrinkage per unit mass in three materials out of four. I t should be noted that the results from unmixed Heliosit a re slightly different from those in Table I as a different material shade was used.

4. Reacting monomer content An attempt was made to determine the amount of

reacting monomer in each material. Inorganic filler

’ ’ Ruyter IE, Sjovik I J . Composition of dental resin and com- posite materials. Acta Odont Scand 1981;39: 133-46.

‘ I Degussa Technical Bulletin Pigments. N o I I Basic Characteristics. Frankfurt: Degussa Pigments Division, 1973.

’‘ Mabie CP, Menis DL. Microporous glassy fillers for dental composite% J Biomed Mat Res 1978;12:435-72.

content was determined according to I S 0 Sfondurd 4049 1978. The amount of pre-polymerized material present as ‘organic filler’ in the microfine materials was deter- mined following the method of Ruyter and Sjovik’* using ethanol as the solvent. Densities of the set specimens were determined by dividing the mass of the dry specimens by their pycnometrically determined volumes.

An estimate of the volume per cent of reacting monomer in each material was then made by using the values determined above and the approximations that:

(a) The density of the large particle filler is 2.65 x 103kg/m’.”.14

(b) The density of microfine filler is 2.2 x IO’kg/m’.” (c) The density of pre-polymerized ‘organic filler’ is

1.13 x lO’kg/m’.

E

160

Results for the mass percentage of inorganic filler con- tent, density of the set materials, mass percentage organic filler content a n d an estimate of the volume per cent of reacting monomer are given in Table 3 together with the determined values for polymerization shrinkage. From the Table it can be seen that the estimates of percentage reacting monomer vary from 32 to 61 volume per cent with more than three-quarters of the materials having less than 46 per cent estimated reacting monomer.

Australian Dental Journal, June, 1983

resin density is only a n estimate and, hence general conclusions are difficult to draw. It will be noted, however, that of the six materials having the highest estimated monomer content ( 2 4 6 per cent), five have large polymerization shrinkages of more than three per cent while the sixth, Visiodispers, has the largest shrinkage of the photopolymerized materials.

The size of reactive monomer in a system has also been ascribed much importance in determining the level of both in vivo and in vitro ~hr inkage .~ .”” It is generally believed that the greater the amount of small reactive monomers in a system the larger will be the shrinkage. This can be observed in a comparison of the polymerization shrinkage of methyl methacrylate (21 per cent) and the large aromatic dimethacrylates (eight to nine per cent).”

Thus products with high mass percentages of ethylene glycol dimethacrylate (M. W = 198) or triethylene glycol dimethacrylate (M.W = 286) should exhibit higher shrinkage than materials consisting mainly of BIS-GMA (M.W =512) or the urethane dimethacrylates. Quan- titative analysis of the effects of different reactive monomer types is impossible because of lack of certain- ty of the compositions of each material, but two specific examples are noteworthy. It can be seen that Profile, even with a high level of filler content and consequent low level of reactive monomer has the highest shrinkage of the highly filled paste-paste systems. This is probably due to the high level (30 per cent by mass) of ethylene glycol dimethacrylate in that reactive monomer.” Likewise it is probable that Silar, even with its very low estimated reactive monomer content shrinks to quite a high degree because its reactive phase contains a high percentage (50 per cent)I2 of triethylene glycol dimethacrylate. Most materials contain 15-30 per cent of triethylene glycol dimethacrylate as diluent.”

The present research did not cover the effect of degree of cure on polymerization shrinkage, however, reports in the literature indicate that:

(a) Materials containing a high proportion of low M.W reacting monomer have higher degrees of conversion of unsaturated methacrylate groups than materials consisting mainly of the larger aromatic d imetha~ryla tes . ’~ . ’~ Thus polymerization shrinkage should be higher for the high diluent monomer content materials.

(b) Although conversion rates vary widely between materials, those found for photopolymerized materials

Discussion Taking into account differences in materials, times and

temperatures of measurement as well as methods, the results for polymerization shrinkage of from 1.7 to 5.7 per cent found in the present research agree well with those found by other worker^.'^-'' The low spread of results, the observed independence of per cent shrinkage on specimen size in the volume range of interest and the possibility of determining post gelation shrinkage (often considered of more interest in the clinical situation than total shrinkage)6.’5 are all encouraging aspects of the test method used. T h e main problems faced are the need to maintain very accurate temperature control and the verification for each material that water absorption is not responsible for a part of the measured drop in meniscus level.

Four factors are believed to be of major importance in causing the observed shrinkages.

( I ) The amount of reacting monomer. (2) The type of reacting monomer. (3) The degree of cure. (4) The amount of air incorporated in the material. The effect of decreasing the amount of reacting

monomer in a system by incorporating inert filler is to decrease the polymerization shrinkage. Lee and Orlowski” and Bowen” have shown that the introduction of about 60 volume per cent of filler into dimethacrylate systems decreases the shrinkage from about nine to three per cent. The filler does not have to be inorganic. Thus prepolymerized resin, acting as a n organic filler in the microfine materials and dibutyl phthalate of which the Isopast catalyst was largely made up” will also reduce the amount of monomer available to react and hence the polymerization shrinkage.

Unfortunately the exact ingredients of most of the commercial materials tested a re not known. Therefore the figure calculated for the amount of reacting monomer and based o n approximations of filler and prepolymerized

’’ Lee H, Orlowski JA. Adhesive dental composite restoratives. Los Angeles: Lee Pharmaceuticals, 1974.

I 6 De Gee AJ, Davidson CL, Smith A. A modified dilatometer for continuous recording of volumetric polymerization shrinkage of composite restorative materials. J Dent I98 1 ;9:36-42.

I’ Bowen RL. Properties of a silica-reinforced polymer for dental restorations. J Am Dent Assoc 1963;66:57-64.

‘ I Ruyter IE, Svendsen SA. Remaining methacrylate groups in composite restorative materials. Acta Odont Scand 1978; 36: 75-82.

’’ Vankerckhoven H. Lambrechts P, Van Beylen M , Davidson CL, Vanherle G. Unreacted methacrylate groups on the surface of composite resins. J Dent Res 1982;61:791-5.

*‘ Stupp SI, Weertman J . Characteristics of monomer to polymer conversions in dental composites. J Dent Res 1979;58: IADR Abstr 949:329.

I ’ Ruyter IE, Oysaed H. Conversion in different depths of ultraviolet and visible light activated composite materials. Acta Odont Scand 1982;40:179-92.

Australian Dental Journal, June, 1983

are around the midpoint of those found for the majority of chemically cured Thus the low polymerization shrinkage of the light cured materials cannot be explained by a markedly lower degree of cure.

The role of voids in the polymerization shrinkage of resin based dental restoratives has not yet been studied although suggestions have been made that incorporated voids should increase the per cent shrinkage.” It is likely that powder-liquid materials should incorporate more air upon mixing than paste-paste materials and that unmixed light activated materials should contain the least voids. An examination of the general order of shrinkage which shows that six out of the top seven highest shrinking materials are powder-liquid materials and that all the light activated materials are in the lower half of the shrinkage table adds credibility to this suggestion but as there are many other variables involved in the shrinkage, these results alone do not constitute proof.

The experiment for which the results are given in Table 2 was carried out in an attempt to isolate this variable. In all cases the density of the material was seen to decrease which meant that air had been incorporated. In three cases out of four the shrinkage and change in volume per unit mass was found to be significantly higher. (Water uptake was again tested and found to be negligible.) This is a strong indication that the voids are in fact playing a noticeable part in increasing the polymerization shrinkage. Thus, with all other factors being equal, a system with a lot of incorporated air should shrink more than one with fewer voids.

Of the materials tested, three (Cervident, Enamelite, and Restodent) are labelled for use in specific restora- tions which may require high fluidity and involve large free surface areas from which shrinkage can take place without unduly affecting the integrity of the margin.

161

Hence the large shrinkages found for these materials are not neaessarily contra-indications for their use in these restorations.

Of the materials intended for general use (Class 111, IV and V restorations with some materials also claiming suitability for posterior use) the great majority were found to have shrinkages of between two and three per cent. Only Llee Microfill and Powderlite shrank much more than thfee per cent while only the unmixed light activated Heliosit and Visiofil shrank much less than two per cent and these exhibited shrinkage of around two per cent when they were mixed.

Conclusion As mentioned in the introduction, many factors besides

polymerization shrinkage are involved in determining the product to give the best marginal adaptation and of course in the general choice of material. With all else being equal however, it would seem that the chances of obtaining the best marginal integrity and of inducing the least internal strain into a material would be improved by selecting a material with a low polymerization shrinkage and being careful to avoid entrapping excess air during mixing.

Acknowledgements The author is indebted to all the staff of the Australian

Dental Standards Laboratory for their willing help, especially the Director, Dr D. R. Beech, Dr W. D. Cook, and Mr M. Chong.

Address for reprints: Australian Dental Standards Laboratory,

240 Langridge Street, Abbotsford, Vic., 3067.