Rheology of acrylic denture-base polymers G. Mutlu* R. Huggett A. Harrison

Department of Prosthetic Dentistry and Dental Care of the Elderly University of Bristol Dental School Lower Maudlin Street Bristol BS1 2LY, United Kingdom J.W. Goodwin R, W. Hughes

Department of Physical Chemistry School of Chemistry University of Bristol Cantock's Close Bristol BS8 1TS, United Kingdom Received January 25, 1990 Accepted August 8, 1990

*For reprint requests Dent Mater 6:288-293, October, 1990

ost acrylic resin denture-base systems are supplied in a poly (methyl methacrylate) powder and methyl methacrylate liquid form. An initiator such as benzoyl peroxide is found in the powder compon e n t ( - 0 . 3 % ) . W h e n t h e s e two components are mixed, the mass passes through a series of stages, v/z.: (i) sandy, (ii) stringy, (iii) doughy, and (iv) rubbery. At the tin-st stage, the mixture has the appearance and consistency of slightly damp sand, i.e.., it is dilatant. After a few minutes, dependent on the temperature and the nature and composition of granules and liquid, the mixture begins to take on a new appearance. If the mixture is now touched with a spatula or finger, it will be found to be sticky. The stickiness gradually lessens, and the mix becomes workable in the fingers and may be handled like a dough. With time, the mixture becomes more elastic, and ritually it reaches the rubbery stage and cannot be worked easily between the fingers. So that ambiguity can be avoided, it is important for the terms to be clearly defined in discussions of the handling proper-

M

Abstract-The aim of this study was to investigate the changing rheological

behavior of a denture-base polymer from mixing to setting. In addition, monomer evaporation and exothermic behavior of the mix were evaluated. The results show that the material behaves as a pseudoplastic fluid. It is shown that the viscosity increases at different rates with respect to lapsed time, and increases with higher temperature. Also, it is shown that polymerization and monomer evaporation both play a part in dough formation.

ties of the mix. "Doughing time" is the time from mixing until the time the material is ready for manipulation. "Manipulation time" is the time from the end of the doughing time during which it is possible for the dough to be manipulated and packed. Ideally, the packing/manipulation stage should be as long as possible, and the rheology of the dough should exhibit good flow characteristics so that optimum packing plasticity will be achieved. The flow properties of the polymer/monomer mix used to pack the mould in denture-base construction is of importance, since flow characteristics of the dough mix can influence the thickness of flash and consequently the accuracy and quality of the moulded denture. There have been numerous reports on the properties of acrylic resin denture-base polymer since its introduction for use in dentistry (e.g., Stafford et al., 1980; Murphy et al., 1982; MacGregor et al., 1984; Huggett and Brooks, 1984; Huggett et al., 1984; Knott et al., 1988); however, few have concentrated on the rheology of the material. The stages through which mixes of the powder

G*,G',G" (kPa)

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Fig. 1.

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and loss moduli

288 MUTLU e$ aL/RHEOLOGY OF ACRYLIC DENTURE POLYMERS

of homopolymer

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Modulus Modulus

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strain).

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and liquid pass were described and the effect of temperature on these changes was first discussed by Tuckfield et al. (1943). No scientific quantitative rheological measurements were presented. Later, McCabe et al. (1975), in addition to studying the effect of temperature, also demonstrated that a short doughing time was dependent on the presence of very small beads of polymer in the powder component. The apparatus used was a reciprocating rheometer originally designed by Wilson (1966). Autopolymerizing aczTlic resin polymers are used during orthopedic surgery as bone cements, and there are reports of the rheological properties of these materials. For example, Schoenfeld (1974) measured the viscosity of bone cement (and dental acrylic resin custom-tray material) as a function of shear rate and the ratio of monomer to polymer. The viscosity of bone cements was also measured as a function of shear rate at single ambient temperature (Ferracane and Greener, 1981; Krause et al., 1982). The aim of this study was to investigate the changing rheological behavior of a heat-polymerizing denture-base polymer from mixing to setting. This is an important factor in successful utilization and handling of these materials, since flow characteristics can influence the accur a c y and quality of the moulded d e n t u r e . In a d d i t i o n , m o n o m e r evaporation and exothermic behavior of the mix are evaluated. MATERIALS AND METHODS The polymer powder used in this study is characterized in the Table. The liquid component used was m e t h y l m e t h a c r y l a t e containing 0.01% quinol. The doughing and manipulation time of the mix are also shown in the Table. For all test procedures, 10 mL of monomer were mixed with 32 mL of polymer and left to stand in the porcelain mixing vessel for 30 s. The mixes were then spatulated for a further 30 s. A stopwatch was started when the monomer was added to the polymer. All tests were carried out three times.

Measurement of Viscosity. -Oscillating deformation m e a s u r e m e n t s were

TABLE CHARACTERIZATIONOF POLY(METHYLMETHACRYLATE)POWDERUSED 405500 1506911 54.62 ~m 0.25-0.30% 22 rain 23 rain

Number averagemolecular weight Weight averagemolecular weight Powder particle size (mean diameter) Residual benzoyl peroxide Doughing time at 20°0 Manipulation time at 20°C Viscosity denture

of a c r y l i c r e s i n base polymer

Viscosity (kPas)

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Mean

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Fig. 2. Viscosityof homopolymer(a120°Cand 0.0026strain), Viscosity (kPas)

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V,isoosity at 10 °C

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Viscosity at 2 0 ° C

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Viscosity st 3 0 ° C

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Fig. 3. Viscosityof homopolymerat differenttemperature (0.0027 strain).

chosen as a means of monitoring the rheological changes of the mixes. This test procedure has several advantages. First, the measurement of

very ried tural both

small deformations can be carout, thus avoiding any strucdamage to the material. Second, the elastic and viscous proper-

Den~al Materials/October 1990

269

(A)

Viscosity of Homopolymer at different strains (10 °C)

The ratio of the peak stress to peak strain is the complex modulus G*, and this is the sum of the storage modulus G' and the loss modulus G". These are related to the complex modulus via the phase angle:

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(1)

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The loss term is a measurement of the viscous dissipation of energy and can be expressed as the dynamic viscosity ~' having the oscillation frequency ~:

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ties can be monitored. Third, it has some similarities with the subjective test performed by the rubbing of a material backward and forward between the fingers for assessment of the textm'e. The testing procedure was to CmTy out a forced sinusoidal deformation between parallel plates in a Bohlin VOR rheometer (Bohlin Reologi UK Ltd., Huntingdon, PE18 6EF, UK).

The i n s t r u m e n t controls the frequency of the deformation and the strain, and monitors the resulting stress. The two sine waves (representing stress and strain) will only be in phase for an elastic solid and will be 90° out of phase for a Newtonian fluid. A visco-elastic material has properties between these two limits and thus has a phase displacement ~ between the two sine waves.

2g{} MUTLU et al./RHEOLOGY OF ACRYLIC DENTURE POLYMERS

The measurement of viscosity as a function of time, at a fLxed frequency of 1 Hz, allowed a large number of measurements to be monitored over short time intelwals. The strain dependence of the material was also investigated. Since many visco-elastic materials show a marked dependence on the amplitude of strain, this parameter needs close control. The experimental procedure was as follows: After the material was handmixed for 30 s, it was placed between the parallel plates of the rheometer. A low-viscosity silicone fluid (DowCorning Corporation, Midland, MI) was placed around the edge of the specimen so that monomer evaporation would be prevented. The instrument was operated by sweeping through a programed linear increase in strain, with the tests being repeated every five rain from mixing to one h. The temperatures chosen for this study were 10°C, 20°C, and 30°C, and tests were canSed out three times at each temperature. From these, data plots were constructed of dynamic viscosity as a function of time at constant strain. Measurement of Weight Change in the Po/ymer/Monomer Mix. - For measurement of the weight loss due to evaporation of the monomer, an electronic top balance (Oertling RB 153, Warley, UK B66 2LP) was used. The weight of an empty container was compensated for and the weight display zeroed. After being hand-mixed for 30 s, the mix in its container was placed on the pan. The weight was recorded every five rain from mixing to one h.

Measurementof ResidualMonomer. -Gas liquid chromatography (GLC) was used for residual monomer estimations. Samples of 0.3 g were taken at five-minute, 30-minute, and 60minute intervals after the start of mixing and were refluxed with 5 mL of methanol in a round-bottomed flask for one h. This procedure ensured that all residual monomer in the samples was extracted into the solution (Huggett and Brooks, 1984). For the purpose of the subsequent GLC analysis, a standard 0.5 mL of 1% ethyl acetate in methanol was added to the solution at this stage. This added constituent provides a reference by which the quantity of methyl methacrylate monomer in the solution can be measured.

Measurementof TemperatureChangein the Polymer/Monomer M i x . - Temperature changes as a consequence of possible polymerization were measured by placement of two thermocouplesone into the mix and the other into the s u r r o u n d i n g c o n t a i n e r - a n d monitoring of the temperature differential as a function of time. The temperature in the surrounding container was constant at 24.3°C. RESULTS

in Fig. 5. During the dough formation, the weight of the mix reduced by about 3%.

(c)

evaporation of the monomer is shown

DISCUSSION A general representation of the viscosity of the sample is shown in Fig. 8. When the powder and monomer are mixed, a two-phase system is

Viscosity of Homopolymer at d i f f e r e n t strains (30°C)

Viscosity (kPas) 120 1 O0

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Viscosity. -(a) The complex, storage,

Weight Change. - W e i g h t loss due to

Temperature Change. - T h e result, which is displayed in Fig. 7, shows exothermic behavior. This is indicative of polymerization after the powder and the liquid were mixed.

Residual Monomer. -The monomer, when added to the polymer, represents - 3 1 % of the mix. This reduces as a consequence of evaporation and a slow rate of polymerization. The monomer remaining in the mix (residual monomer) was determined at five, 30, and 60 min after mixing. The results of the residual monomer determinations are shown in Fig. 6.

0

and loss moduli are shown in Fig. 1. With increasing time, the storage modulus increases more rapidly than the loss modulus. This indicates a transition toward a more elastic material. (b) The viscosity as a function of time is shown in Fig. 2. This was obtained by use of the measured loss modulus and Eq. (3). As expected, an increase in viscosity with respect to lapsed time is clearly demonstrated. (c) The viscosity as a function of temperature is shown in Fig. 3. The viscosity increases with increasing temperature. (d) Fig. 4 A, B, and C present the viscosity of the polymer as a function of strain at different temperatures. The viscosity decreases with increasing strain. This is an important factor in the packing. The polymer behaves as a nonNewtonian fluid (pseudoplastic or plastic materials).

The level reduced by 3% between the time intervals of five and 60 min.

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Fig. 5. Evaporation of residual monomer during dough formation (.% decrease in weight).

Dental Materials/October 1990

291

%

3O

CONCLUSIONS

The rheological characteristics of an acrylic resin denture-base polymer are presented in this study. The viscosity shows a decrease with inc r e a s i n g s t r a i n ( a m p l i t u d e of deformation) at a fixed frequency. As the amplitude increases at a given frequency, the average shear rate during oscillation increases. The material has shown that it behaves as a pseudoplastic f l u i d - t h a t is, the viscosity decreases as the shear rate increases. The measurements indicate that the viscosity increases at different rates with respect to increase of time. An increase of viscosity with increasing temperature has been demonsWated. As the temperature increased, the viscosity at a fixed time after mixing increased, indicating a faster or more rapid change in viscosity with time at elevated temperature. Further, it has been shown that polymerization plays a part in dough formation and that the formation of the dough is related to evaporation of the monomer. The rheological properties have been studied by oscillating deformation, and it has been shown that this method can be used to quantify the changes in material behavior. In future publications, we intend to measure and to model the response of denture-base polymers of different compositions.

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produced. It is composed of small granules of polyuner dispersed in a solvent formed by the monomer. The sample is dilatant. Two processes Occur:

(i) Swelling of the polymer granules and the subsequent entrapment of the monomer reduce the free volume, which leads to an increase in granule concentration and hence viscosity; and (ii) Polymerization of the monomer also increases the viscosity. As the chains of the swollen polymer on the periphe17 of the neighboring granules begin to entangle, a space-filling network is formed. At

this stage, the material behaves as a single phase, but it is not as yet homogeneous. This is because the monomer has not had enough time to diffuse to the centers of the grains. In this regime, the sample can be manipulated. If this region is long and fiat, the manipulation period is also long. The behavior at 20°C is shown (Fig. 2). As the temperature is increased, the viscosity rises much more rapidly, and the manipulation period is reduced. When cooled to 10°C, the sample shows an extended sigrnoidal shape, indicating that lowtemperature storage will increase the manipulation period.

292 MUTLU et aL/RHEOLOGY OF ACRYLIC DENTURE POLYMERS

ACKNOWLEDGMENTS

G. Mutlu is supported by a Turkish Government scholarship. Thanks are due to the Bristol and Weston Health Authority for financial support. REFERENCES FERRACANE, J.L. and GREENER, E.H.

(1981): Rheology of Acrylic Bone Cements, Biomat Med Dev Art Org 9:213224. HUGGETT, R. and BROOKS, S.C. (1984): Some Obselwations on Denture Base Materials, J Br Inst Surg Tech 1:7-14. HUGGETr, R.; BROOI~, S.C.; and BATES, J.F. (1984): The Effect of Different Curing Cycles on Levels of Residual Monomer in Acrylic Resin Denture Base Materials, Quint Dent Tech 8:365370. KNOTT, N.; RANDALL, D.; BELL, G.; SATGURUNATHAN,R.; BATES, J.F.; and

HUGGETT, R. (1988): Are P r e s e n t Denture Base Materials and Standards Satisfactory?, Br Dent J 165:198200. KRAUSE, W.R.; MILLER, J.; and NG, P. (1982): The Viscosity of Acrylic Bone Cements, J Biomed Mater Res 16:219243. MACGREGOR,A.R.; STAFFORD,G.D.; and HUGGETT, R. (1984): Recent Experience with Denture Polymers, J Dent 12:146-157. MCCASE, J.F.; SPENCE, D.; and WILSON, H.J. (1975): Doughing Time of Heatcured Dental Acrylic Resins and its Dependence on Polymer Partic]e Size, J Oral Rehabil 2:199-207. MURPHY, W.M.; HUGGETT, R.; and HANDLEY, R.W. (1982): A Laboratory and Clinical Study of Trevalon Denture Base Material, J Oral Rehabil 9:401--411. SCHOENFELD, C.M. (1974): The Rheology and Monomer Release of Unset Methyl Methacrylate for Bone Cements. PhD Thesis. Northwestern

University, Chicago. STAFFORD, G.D.; BATES, J.F.; HUGGETT, R.; and HANDLEY, R.W. (1980): A Review of the Properties of

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Some Denture Base Polymers, J Dent 8:292-306. TUCKFIELD, W.J.; WORNER, H.K.; and GUERIN, B.D. (1943): Acrylic Resins in Dentistry, Part II. Their Use for

Denture Construction, Aust Dent J 47:1-26. WILSON, H.J. (1966): Elastomeric Impression Materials, Br Dent J 121:277-283.

Dental Materials~October 1990 293