J. BIOMED. MATER. RES.

VOL. 10, PP. 77-89 (1976)

Influence of Application Technique on

Microstructure and Strength of Acrylic Restorations* R. P. KUSY, J. R. RIAHAN, and D. T. TURNER, Dental Research Center, University of North Carolina at Chapel Hill, North Carolina ,27514

Summary An investigation was made of the influence of application techniques on the microstructure and properties of an acrylic tooth restorative. Mixtures of acrylic powder and monomer (“Sevriton Simplified”) were applied by the brush technique of Nealon ( J . Prosth. Dent., 2, 513, 1952) and by two bulk flow techniques. While similar porosities (about 4%) were observed, the brush technique resulted in a greater quantity of grains from the acrylic powder. Despite this, there was little difference in values of compression modulus, compressive yield stress, and diametral compressive strength. The mechanical strength of the materials studied was less than one-half that of high molecular weight poly(methy1 methacrylate) (PMMA). Crack propagation studies established that the interface between the grains and matrix was not a source of weakness. However, as the matrix was crosslinked this could not be checked by solution methods of polymer characterization.

INTRODUCTION Acrylic powder and liquid monomer are used to restore tooth structure. Such mixtures are designed to harden by polymerization within minutes. Facile application with good adaptation to the tooth cavity is of critical importance. It has been reported’ that this adaptation is best achieved by means of Nealon’s brush technique,2 but this is a complex question which probably depends on a variety of factors, including the shape and accessibility of the cavitll t o be filled. The more definitive objectives of the present study are, first, to investigate how the method of application influences the *Presented a t the Clemson University Seventh Annual International Biomaterials Symposium, April 1975. 77 @ 1976 by John Wiley & Sons, Inc.

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KUSY, MAHAN, AND TURNER

microstructure and mechanical strength of the acrylic restoration; and second, to check whether or not these materials are as weak as reported p r e v i ~ u s l y ~ and, * ~ if so, to investigate the cause. This second objective is important because clinical evaluations indicate that the major inherent fault of these materials is poor abrasion resistance5 which is related to mechanical strength. In order to meet the above objectives a commercially available acrylic product was chosen for study, since it has been evaluated in clinical tests. The methods of application studied were the currently favored nonpressure techniques. Two of the methods involved bulk flow of a mixture of powder and monomer. The third was Nealon's technique2 in which powder was carried to the cavity on a brush moistened with monomer.

EXPERIMENTAL Sevriton Simplified (Amalgamated Dental Trade Distributors Ltd., London, England) included an acrylic polymer powder with small amounts of pigment to simulate tooth shades. This was mixed a t room temperature (22°C) with an acrylic monomer which includes a crosslinking agent. These components contain a catalyst system based on a peroxide and derivatives of sulfinic acid which provides free radicals to initiate polymerization. The system is based primarily on methyl methacrylate. I n one bulk flow technique (I),recommended by the manufacturer, eight drops of monomer were dispersed in a mixing vessel, a dappen dish. Excess powder was added and the dish was tapped three times on a hard surface. The dish was then inverted and tapped once to remove excess powder from the caked mixture. More monomer (three drops) was added and the mixture stirred in one direction with a glass rod (15 sec). The mixture was carried in increments to the cavity with an instrument, a periodontal probe. A second bulk flow technique (11) was similar except that the mixture was poured into a plastic tube tapering to a capillary (a Jiffy tube). The mixture was squeezed through the capillary into the cavity. I n a third technique (111) a small brush moistened with monomer was used to pick up a small amount of powder and the mixture was transferred to the cavity. The brush was cleaned with monomer

ACRYLIC RESTORATIONS

79

and the procedure repeated until the cavity had been filled. This technique was first described by Nealon.2 Techniques I1 and 111 have been described in detail by Sockwell.6 Cavities were cut in extracted teeth by standard techniques used in operative dentistry. For microstructural studies cylindrical holes were drilled in a block of P3lMA. After filling, surfaces were polished t o 0.05 pm by standard metallurgical procedures. The surface was then etched by exposure to fumes of concentrated nitric acid (1 min).’,* Surfaces were examined by reflected light with a Zeiss Universal Microscope. Quantitative estimates of volume fractions of pores and grains were made by a standard technique in which a grid was superimposed on a photomicrograph and the number of intersections with particular features counted. Porosity intrinsic to the grains and fragmented grains were included in the grain count. As pores were counted on polished sections there is the possibility that some overestimates were obtained because of polishing artifacts. No significant differences were detected among samples made by three investigators and, therefore, all data are presented without this distinction being made. For crack propagation studies, a U-shaped filling was made in a sheet of PMMA 0.3 cni thick. After polishing, a wedge was driven in as indicated in Figure 1. After etching, the path cleaved by the crack through the surface was examined.1° The bulk fracture surfaces were also examined under the microscope, without etching.ll Samples for mechanical testing at room temperature (22°C) were formed in glass tubes (length 1.1 cm; diam 0.3 cm). These were removed and cut with a diamond saw into specimens 0.6 cm long two to seven days after formation. I n work on similar materials i t was found that mechanical properties changed up t o 24 hr after sample preparation.” With Teflon coated platens compressive yield stress tests, o - ~ ,were made on an Instron Rlachine a t a strain rate of 3 x 10-3 sec-1. Young’s modulus, E,, was estimated for the initially linear portion of this stress-strain curve. The yield stress was obtained by taking the maximum stress, which corresponded to an offset strain of ca. 0.02. A measure of tensile strength, C T T , was obtained from diametral compressive strength tests using the formula U T = 2P/?rDL; P = load a t fracture (kg), D = diameter of cylinder (mm), and L = length of cylinder (mrn).l2 The cross head extension rate was

80

KUSY, MAHAN, AND TURNER

lcm -

Fig. 1 . Schematic illustrating U-shaped restoration used for crack propagation studies: Top, surface profile before cleaving; bottom, after cleaving.

1 cm/min. I n this test both fracture and yielding were considered failures with no distinction made, since statistical techniques showed no significant difference. Most did fail by fracturing, however. Insoluble, “gel,” fractions were estimated gravimetrically on samples (length 1.1 em; diarri 0.3 cm) after immersion in chloroform for one week a t room temperature (eq. (1)). In addit,ion, equilibrium swelling values for the gel were estimated from dimensional changes (eq. ( 2 ) ) .

70 gel

=

% ’ volume increase

w3

100 --

‘101

=

100 ((Z2/ZJ3

- 1)

(2)

w 1 is the initial weight of the sample and w 3 is the weight of the extracted sample after the removal of chloroform. l 2 is a linear dimension of a swollen sample and Zl is the corresponding initial sample dimension.

RESULTS AND DISCUSSION An example of adaptation to a cavity in a tooth is shown by dark field illumination of a section cut with a diamond saw (Fig. 2a). The contrast between enamel (E), dentin (D), and microstructural details of thc filling are revealed more clearly by bright field illumina-

ACRYLIC RESTORATIONS

(c)

81

(4

Fig. 2 . Microstructure of Sevriton Simplified restorations. (a) Prepared in tooth, sectioned with diamond saw, unetched, darkfield. (b) Same as (a) except polished to 0.05 pm, brightfield. E = enamel, D = dentin, ap = artifact porosity. (c) Prepared in PMMA, polished to 16 pm, unetched, brightfield. (d) Same as (c) except polished to 0.05 pm.

tion of a polished surface (Fig. 2b). Similar results were obtained using blocks of PMMA in place of teeth (Figs. 2c, d). The granular microstructure, although visible in these materials even on the polished surface (Fig. 3a), is greatly enhanced by etching (Fig. 3b). The porosit,y within grains preexists in the powder as judged by separate microscopic examination. Occasionally very large pores are seen with irregular shapes which include portions of grains around their perimeters (Fig. 2b). This latter effect indicates

82

KUSY, MAHAN, AND TURNER

(a)

(b)

Fig. 3. General microstructure of Sevriton Simplified. (a) Polished to 0.05 pm, unetched, brightfield. (b) Same as (a) except etched with HNO, vapor (1 min). fg = Fragmented grain; ip = internal pores characteristic of powder.

a polishing artifact. Nevertheless with experience, i t was possible to minimize such artifacts and obtain photomicrographs similar to those shown in Figure 3a and b. Quantitative metallography techniques indicate that there is little difference among the application techniques with respect t o matrix porosity (Fig. 4). The most striking difference displayed in Figure 4 is that the brush technique (111) results in a greater volume fraction of grains, i.e., GOY, as compared to only about 40% for the two bulk flow techniques (I and 11). Presumably, this is due to the brush technique utilizing a higher powder to liquid ratio during application. This finding provides an interesting example of the use of microstructural analysis to provide information in a case where the powder to liquid ratio in the mixture is unknown. A further interesting difference, evident from Figure 4, is that the brush technique provides a more uniform granular product. When the brush technique is used to fill a number of preparations, 757, of these restorations yield powder fractions within 5y0 of the mean. However, since the standard deviation of the bulk flow techniques is greater, the likelihood of variation from one preparation t o another is more. Despite the differences in microstructure mentioned above, little differences were detected in the mechanical properties measured (Fig. 5). This insensitivity would appear consistent with earlier

ACRYLIC RESTORATIONS

83

P.38.l N37 SD* 6.6

0,401

N.37 p = 42

SD* 5.0 0.30.

I

020

0.10

0.10

a50

0.40-

(I[) E 4

030: 0.20.

-

-

N.67 P.423

so= 9 5

-

.

0.20

0.10.

0.10

0

0 0501

05 01 N.37 P.600 SD= 4.7

0.401

- 0.30.

.

4

.

020.

0.I 0.

N.37 p = 3.8 SD= 3.1

L 102030

0 % Particles. P

% Pprosity, p

Fig. 4. Quantitative evaluation of Sevriton Simplified microstructure as a function of technique: Left, probability distribution of powder particles, P ; right, probability distribution of matrix porosity, p . N = Sample population; = mean; SD = standard deviation.

x

observations by Smith that the strength of denture base materials, which arc also 2-phase acrylic polymers, is little influenced by the ratio of powder to liquid used in their preparation.’ The results in Figure 5 confirm previous results which show that these 2-phase acrylic materials, like the dental repair acrylics and

KUSY, MAHAN, AND TURNER

84

0501 040

g51.400 SD= 1.200

.~ ...... ....................................................... ~

~~.~ ...........................

050~ N=II

~

.

~.... ~ ........~ ......... . ............ ~ ........ ~

050

~

.

N=9

4= 4.300

0 20 010

~

..............................................

~.~~ ............~ . ~ . ~

N=IO SD: 220

Conpresswe Yield Stress

4 (p~1)x~o-3

Cmpession Modulus

E, ipso I10-5

50 60 Diarretrd Cwnpessive S h q t h

o,lpw)xlo-~

Fig. 5 . Mechanical properties of Sevriton Simplified as a function of technique: Left, probability distribution of compressive yield stress, u c ; center, probability distribution of compression modulus, E,; right, probability distribution of diametral compressive strength, U T .

orthopaedic bone cements, are much weaker than commercially prepared 1-phase samples of PRlRlA (Table I). Self-curing acrylic polymers of the type used for tooth restoration usually contain 2 to 3% residual monomer one day after p r e p a r a t i ~ n . ~ This would have a plasticizing effect which would account for the low modulus of elasticity relative to PAIMA. Residual monomer would also decrease tensile strength but reference t o Smith’s data’

~

ACRYLIC RESTORATIONS

85

TABLE I Comparison of Mechanical Propert,ies of 2-Phase Acrylics with PMMA Compressive Yield Stress or Compressive Strength (psi)

Diametral Compressive Strength or Tensile Strength (psi)

340,000 (2.4).

10,400 (1.9)

3,300 (3.0)

Sevriton (111), 279,000 (8.2) Present work

10,600 (6.6)

4,580 (4.8)

Material Sevriton3

Modulus of Elasticity (psi)

PMMA (M,%l X 106)b 444,000 (l.l)1314,200'3

10,400 (6.7)'s

Fracture Surface Energy (erg/cmz)

2.3 X lo4 (25) (matrix) 3.1 X 105 (2O)l.l

a Figures in parentheses indicate standard deviation as a percentage of mean value. M , = viscosity average molecular weight. 1 psi = 6.9 X 104 dyne/cmZ.

indicates that the values given in Table I are much too low to be explained by this effect alone. Moreover, the 2-phase materials are more brittle than PMRIA, notwithstanding any plasticization, as judged by their fracture behavior. Another indication of the brittleness of these materials is provided by the high ratio of compressive to tensile strength (2-3). In PMMA, this ratio generally results in a value of 1.3-1.4,15 as obtained from the data in Table I. Insight into the cause of brittle fracture was sought from crack propagation studies. These indicated that the interface between grains and matrix is not a source of weakness, as was found to be the case in other 2-phase acrylics formed a t high rates of polymerization.l0 Using the technique described in Figure 1, evidence for fracture solely by transgranular cleavage is shown in Figure 6a and b. This was confirmed by examination of the bulk fracture (Fig. 7a, b). The obvious difference in the fracture behavior is that extensive secondary cracking is limited to the bulk flow techniques. These secondary cracks either stem from the main crack or are close to it (Fig. 6a). They are also found on the fracture surfaces as black irregular features (Fig. 7a). There is some evidence that occasional grains act as crack stoppers.

KUSY, MAHAN, AND TURNER

86

(a)

(b)

Fig. 6. Surface profile of fractured Sevriton Simplified. (a) Bulk flow technique. c = secondary crack. (b) Brush technique. ap = artifact porosity. Both polished 0.05 pm, fractured (cf. Fig. l ) , etched with HNO, vapor (1 min), brightfield.

An indication that brittle behavior is due to the matrix and not the grains is provided by the appearance of the fracture patterns in the two phases. Some quantitative measure may be obtained by reference to fracture surface energy, y, which is a measure of the energy expended pcr unit area in cleaving a surface. I n the case of P A M A it has been shown that the lower this value of y, the more brittle the material. RIoreover, an empirical relationship has becn established between y and the periodicity of the ribs (Fig. 8).16 The spacing of the wave-like ribs in the matrices for Materials 1-111 is N 10 l m (Fig. 7a, b) which corresponds to a value of y N 2 X lo4 erg/cm2. This is much less than the value for a conventional sample of PRIlIA for which y = 3.1 x lo5 erg/cm2 (Table I).14 Like conventional PMPtlA, grains exhibit interference colors and parabolic markings indicative of this higher valuc of 7.'' It should be remarked that the correlation in Figure 8 is based on data obtained for linear samples of I'JIRIA which show that y decreases with decreasing molecular weight. It is also known that y is reduced by crosslinking,'* although a quantitative correlation is yet to be established. Therefore, i t is not known to what cxtent the brittleness of the matrix in the present 2-phase acrylic polymers is due to a low molecular weight, which might be expected from a

ACRYLIC RESTORATIONS

(a)

87

(b)

Fig. 7. Surface of fractured Sevriton Simplified. (a) Bulk flow technique. c = secondary crack. (b) Brush technique. It = ribs in matrix. Red and blue interference colors present on particulate grains (shading) with parabolas indicating fracture direction (arrow).

rapid rate of polymerization, or to crosslinking. Evidence for crosslinking, however, was obtained by standard techniques, characterizing gel content and equilibrium swelling values (Table 11). The higher gel content for the bulk flow techniques is consistent with the higher volume fraction of crosslinked matrix. Presumably these matrices are less tightly crosslinked, i.e., swell more, because less peroxide is carried in with the lower volumc fraction of powder. Further characterization of the crosslinkcd nctwork is in progress. TABLE I1 Characterization of Crosslinked Networks in Sevriton

____~___

_ _ _ _ . _ _ _ _ _ _ ~ ~ ~ _ _ _ _ ~ ~ _ _

yo Volume Increase (es. ( 2 ) ) Dimensional Change

% Gel Technique

I I1 I11

(eq. (1))

Length

Diameter

40.7 37.8 29.8

154

163 98

183 179 107

_ _ _ ~ ~

--

~

-

~-

-

-

KUSY, MAHAN, AND TURNER

88

2

I

0 log,,

Rib

Periodicity, r

3

(u)

Fig. 8. Influence of rib spacing on y for commercial PMMA (ref. 16).

CONCLUSIONS The brush technique results in materials with more grains, carried over from the powder, and having a more reproducible composition. Brush and bulk flow techniques yield materials with similar mechanical strength. All techniques result in materials which have less than one-half the strength of conventional PMAIA. Fractured sections indicate that mechanical weakness is not due to poor bonding between grains and matrix. The matrix is much more brittle than the granular phase.

ACRYLIC RESTORATIONS

89

Periodic rib spacings in the matrix phase indicate a fracture surface energy of E 2 X lo4 erg/cm2. The materials includc a crosslinked gel fraction. The authors are grateful to K. F. Leinfelder, D.D.S., M.S. for making and advising on cavity preparations. This investigation was supported by U.S. Public Health Service Research Grant No. I)E 02668 from the National Institute of Dental Research and, in part by General Research Support Grant No. ItR 5333 from the General Research Support Branch of the National Institutes of Health. Partial support is also acknowledged from the Materials Research Center, University of North Carolina, under Grant No. GH 33632 from the National Science Foundat ion.

References 1. T. Fusayama, M. Ishihashi, and T. Kitazaki, Bull. Tokyo Med. Den. U., 2, 235 (1956). 2. F. H. Nealon, J . Prosth. Dent., 2, 513 (1952). 3. W. T. Sweeney, W. D. Sheehan, and F:. L. Yost, J A I I A , 49, 513 (1954). 4. R. L. Bowen, J A D A , 66, 57 (1963). 5. K. F. Leinfelder, T. B. Sluder, C. L. Sockwell, W. 11. Strickland, and J. T. Wall, J . Prosth. Dent., in press. 6. C. L. Sockwell, The Art and Science of Operative Dentistry, ch. 16, C. M. Sturdevant, R . E. Barton, and J. C. Brauer, Eds., McGraw Hill, New York, 1968. 7. D. C. Smith, Brit. Dent. J., 111, 9 (1961). 8. R. P. Kusy and D. T. Turner, J . Dent. Res., 53, 948 (1974). 9. T. Lyman, H. E. Boyer, and W. J. Carnes, Eds., Metals Handbook 8: Metallography, Structures, and Phase Diagrams, American Society for Metals, Ohio, 1973. 10. R. P. Kusy and D. T. Turner, J . Biompd. Mater. Res., 8, 185 (19'74). 11. R. P. Kusy and 1). T. Turner, J . Dent. Res., 53, 520 (1974). 12. Guide to Dental Materials and Devices, 6th ed., A D A Spec. No. 1, American Dental Association, Illinois, 1972. 13. H. Moscowitz, Ph.D. Thesis, Drexel University, Philadelphia, Pennsylvania, 1972. 14. R. P. Kusy and D. T. Turner, Polymer, 15, 394 (1974). 15. P. B. Bowden and J. A. Jukes, J . Mater. Sci., 3, 183 (1968). 16. R. P. Kusy and D. T . Turner, unpublished work. 17. J. P. Berry, J . Appl. Physics, 33, 5, 1741 (1962). 18. L. J. Broutman and F. J. McGarry, J . A p p l . Poly. Sci., 9, 609 (1965).

Received March 5 , 1975 Revised April 21, 1975

Influence of application technique on microstructure and strength of acrylic restorations.

An investigation was made of the influence of application techniques on the microstructure and properties of an acrylic tooth restorative. Mixtures of...
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