Evaluation of analytical techniques for measurement of denture-base acrylic resin glass-transition temperature R. Huggett 1" S.C. Brooks2 A.M. Campbell 3 R. Satguranathan 3
G.A. BelF 1Department of Prosthetic Dentistry Dental School, University of Bristol Lower Maudlin Street Bristol BS1 2L4, United Kingdom 2Department of Basic Dental Science University of Wales College of Medicine, Cardiff, Wales
United Kingdom 3Courtaulds Research Coventry, United Kingdom 4Courtaulds Coatings Felling, Tyne & Wear United Kingdom Received January 16, 1989 Accepted November 16, 1989 *Corresponding author Dent Mater 6:17-19, January, 1990
Abstract-The glass-transition temperature of a range of acrylic resin materials used in prosthetic dentistry was determined. The techniques used to make the measurement included: thermal mechanical analysis, dynamic mechanical thermal analysis, and differential scanning calorimetry. It was found that the measuring techniques used yielded very similar results, and as a consequence it was concluded that: familiarity and easy availability of thermal mechanical analysis lead to the recommendation that this technique should be employed as the standard glass-transition evaluation technique for denture-base acrylic resins.
lass-transition temperature (Tg) is recognized as one of the most important parameters of amorphous polymeric materials. It is an inherent property of the material which exhibits profound consequences on the potential application of the polymer. Tg is commonly defined as the temperature at which the polymer undergoes a transition from glassy to rubber-like behavior. During this transition, dramatic changes in properties such as roodulus, elasticity, heat capacity, and refractive index are observed. These changes are reversible and are a function of the molecular motion of the polymer chains. In the context of denture-base polymers, the determination of the Tg is a very important a s p e c t - i n particular, the relationship of Tg to moulding temperature. McCabe and Wilson (1980), using differential scanning calorimetry, reported on the Tg of 14 denture-base materials. Among their conclusions were the facts that: (i) in order to avoid dimensional changes due to creep, a high value of glass-transition temperature is advantageous, and (ii) a low value of glass-transition temperature results in the production of relatively small internal stresses within heat-curing materials but has no such effect on autopolymerizing materials. Barsby and Braden (1981), in studies on a number of pour-type denture-base resins, found that these materials displayed low glass-transition temperatures, indicating a propensity to thermal distortion. Furthermore, they found that these low values reflected high residual monomer contents. Eisenberg (1984), in a relatively recent review, has highlighted that powder Tg is the key for true injection moulding where a "molten" polymer system is required. The powder Tg is not so imp o r t a n t for dough moulding techniques (including reaction injee-
tion moulding); however, the dough Tg is crucial. In dough moulding, the dough Tg is initially significantly below the moulding temperature, generally 70100°C for heat-cured systems. However, as polymerization proceeds, the dough Tg increases toward that of the final fully-polymerized material. This second Tg commonly exceeds the moulding temperature. Once this situation is achieved, the potential for flow is curtailed. A numerical expression for this behavior is given by the Williams, Landel, and Ferry equation (Eisenberg, 1984). As an aid to characterization of acrylic resin denture-base systems, Huggett et aL (1984) used the method of thermomechanical analysis to determine the effects of cross-linking, molecular weight, higher methacrylates, and stereo chemistry on Tg. Finally, the Tg of the ultimate material will exert a significant influence over the mechanical properties of the denture. There are many methods available to determine the Tg of polymers, among which Differential Scanning Calorimetry (DSC), Thermal Mechanical Analysis (TMA), and Dynamic Mechanical Thermal Analysis (DMTA) are commonly used. This communication describes the use of the above techniques in relation to measuring the Tg's of denture-base polymers. A range of common materials, including auto-cure polymerizing resins, was employed for this study. Differentia/Scanning Calorimetry (DSC). Differential Scanning Calorimetry is essentially a calorimetric technique commonly employed to determine thm~al transitions of polymeric materials. It measures the amount of heat required to increase the sample temperature by a pre-determined incremental value T over that required to heat a reference sample by
Dental Materials/January 1990 17
the same T. The DSC thermogram is recorded as the heat differential between the reference material samples vs. temperature. Since polymeric materials exhibit a change in heat capacity when passing through a phase transition, an inflection in the DSC thermogram is indicative of the polymer Tg. A Perkin-Elmer DSCII was used for this purpose. Derma/Mechanical Analysis (TMA). Thermal mechanical analysis is a relatively simple technique for measuring the glass-transition temperature of polymers. A Stanton Redcroft TMA was used in this study. The instrument utilizes a sensing element which is essentially composed of a movable-core differential transformer. A quartz probe in contact with the sample is attached to the movable core. The probe measurements are detected by a change in transformer output. The instrument e m p l o y s two o p e r a t i n g modes, namely, penetration and expansion. In this study, the measurements were performed by use of the penetration mode. The weighted probe is allowed to 'set' upon a uniform square sample. The temperature of the sample is then raised by a heating control unit. As the sample begins to s o f t e n at a t r a n s i t i o n temperature, probe displacement is noted and recorded on a chart recorder. Dynamic Mechanical Derma/Analysis (DMTA). - D y n a m i c Mechanical Thermal Analysis is one of the most sensitive t e c h n i q u e s available for characterizing and interpreting the mechanical behavior of materials with r e s p e c t to t e m p e r a t u r e and frequency. It is designed to study the behavior of polymeric materials through a wide range of transitional phase states. Dynamic Mechanical Thermal Analysis is based on observing the viscoelastic response of a material subjected to a small oscillatory strain, while undergoing a t e m p e r a t u r e program. This technique separates the viscoelastic behavior into two c o m p o n e n t s of modulus: a real part, the elastic or storage modulus (E'); and an imaginary part, the damping or loss modulus (E"). The loss tangent is the
quotient of the loss and storage moduli, as shown below tan g = E"/E' Plots of tan 5E' and E" vs. temperature essentially yield all motional and structural transitions within a polymer as they occur over a wide temperature sweep. The glass-transition temperature (Tg) of a polymer is commonly defined as the temperature corresponding to the maximum displacement in the tan ~ vs. temperature plot. Two readily available commercial thermal analyzers were used in this study: DuPont 983 Dynamic Mechanical Analyzer (DMA) and Polymer Laboratories Dynamic Mechanical Thermal Analyzer (PLDMTA). EXPERIMENTAL
Materials. - T h e dental polymers are formed by the mixing of a fine polymer powder which contains a polymerization initiator with a liquid monomer to form a dough. Polymerization is initiated either by heat or, in the case of autopolymerized materials, by a chemical activator present in the monomer component. Manufacturers' recommended curing conditions were followed for each of the materials studied. The material TSl195 is a heatcured poly(methylmethacrylate) homopolymer denture-base material. Lucitone 199 is also a heat-cured denture-base material but contains minute rubber particles which give the material a high impact strength. DeTrey SOS, DeTrey Orthoresin, and Peripheral Seal are autopolymerized materials, but w h e r e a s DeTrey SOS is poly(methylmethacrylate), DeTrey Orthoresin is a copolymer of poly(methylmethacrylate) and poly(ethylmethacrylate). The rigid reline material, Peripheral Seal, uses poly(ethylmethacrylate) as the polymer powder component, and butylmethacrylate is used as the mono m e r c o m p o n e n t in place of m e t h y l m e t h a c r y l a t e . The use of higher ester methacrylates makes the resulting polymer softer and more flexible. Perspex is an industrially produced clear sheet of bulk-polymerized poly(methyl methacrylate).
DSC. - A Perkin Elmer DSC II was used for the Differential Scanning Calorimetric measurement. The instrument was equipped with a scanning autozero to correct for baseline irregularities. Thermograms over the range of 25 to 150°C were recorded for the polymer samples at a heating rate of 4°C p e r min. The samples were p r e p a r e d by the crushing of the moulded polymer strips and weighed approximately 15 mg. The crushed polymer was sealed within aluminium pans. Tg was measured by the Tg onset technique. TMA. -Measurements were carried out with a Stanton Redcroft Thermomechanical Analyzer model 691 linked to a conventional X-Y recorder. The probe of 1 mm 2 surface area rested on a specimen measuring 4 x 4 x 6 mm. The probe was connected to a transducer which allowed vertical movement of the probe to be monitored on the recorder. A removable heating cuff and thermos surrounded the probe and specimen. A thermocouple was located close to the specimen, and the temperature was monitored on the recorder. A heating rate of 10°C p e r min was used and a nominal 25-g load applied to the probe to increase penetration. Plots of probe response against temperature were recorded and the glasstransition temperature measured for five specimens of each material. DMAJPL-DMTA. - Both the DMA and DMTA were operated in flexure mode with a fLxed sinusoidal frequency of 1 Hz. The temperature was varied from 0°C to 150°C, with a temperature ramp of 4°C p e r rain. Material sample sizes of 60 x 11 x 2 mm were used for both techniques. Plots of tan ~ vs. temperature were r e c o r d e d , and the t e m p e r a t u r e corresponding to the maximum in tan value was taken as the Tg. Five samples of each material were evaluated. RESULTS The experimental results are recorded in Tables 1 and 2. DISCUSSION In general there is a good correlation between the Tg values obtained from each of the techniques. The
HUGGETT et aL/GLASS-TRANSITION T E M P E R A T U R E M E A S U R E M E N T S OF A C R Y L I C R E S I N S
TABLE 1 MEASURED GLASS-TRANSITIONTEMPERATURESOF COMMERCIALDENTURE-BASEMATERIALS BY USE OF THE VARIOUS THERMAL ANALYZERS
Tg°C Sample Code DSC TMA PI-DMTA DMA-983 Perspex 1 117 127 (1.1) 129 (0.5) 130 (0.4) DeTrey SOS 2 118 100 (2.5) 106 (0.8) 108 (0.4) Lucitone 199 3 120 125 (1.0) 129 (0.4) 128 (0.8) TS1195b 4 127 t25 (2.5) I31 (0.3) 137 (0.8) TSl195c 5 109 109 (0.9) d d DeTrey Orthoresin 6 NO 83 (2.7) 92 (0.6) 112 (1.7) Peripheral Seal 7 75 67 (1.1) 86 (0.3) 97 (1.3) Numbers quoted for TMA, PI-DMTA, and DMA are mean values (n = 5). The numbers in parenthesis are standard deviations. (a) Only one sample per material was evaluated, due to difficult sample preparation. (b) Seven hours at 70°0 with three hours of terminal-boil curing cycle employed. (c) Fourteen hours at 70°0 curing cycle employed. (d) Two transitions observed; results given in Table 2. NO, Not Obtainable.
strongest correlations are between TMA and the other groups. The results of this investigation with respect to the relationship between TMA and DSC show that the strength of the relationship is r = 0.910, r 2 = 0.828 obtained from linear regression analysis, so that approximately 83% of the variation is directly related. The tr value obtained was P