J. Dent. 1992;
Effect of the curing cycle on residual monomer levels of acrylic resin denture base polymers A. Harrison and R. Huggett Department
and Dental Care of the Elderly, Dental School, Bristol, UK
ABSTRACT It has been shown previously that high levels of residual monomer have a deleterious effect on the properties of denture base polymers. Levels of residual monomer were determined on a homopolymer and a copolymer using gas-liquid chromatography. A wide range of recommended and ‘short cut’ curing cycles were then investigated which produced values ranging from 0.56 to 18.46%. From the ranges examined an optimum cycle of 7 h at 70°C and 1 h at 100°C was established which was used to polymerize 23 currently available synthetic denture base polymers. Only small differences were found between the materials tested with a range from 0.54 to 1.08% of residual monomer. KEY WORDS: J. Dent. 1992; 1992)
Denture base materials, 20:
Curing cycles reviewed
Correspondence should be addressed to: Professor A. Harrison, Department of Prosthetic Dentistry Et Dental Care of the Elderly, Dental School, Lower Maudlin Street, Bristol BSI 2LY. UK.
INTRODUCTION High levels of residual monomer have a deleterious effect on the mechanical properties of denture base polymers (Harrison et al., 1977; Lamb et al., 1983). Also, there have been several reports in the literature regarding levels of residual monomer in denture base polymers which, it has been postulated, are responsible for mucosal irritations (Bradford, 1948; Nyquist, 19.52; Fisher, 1954,1956; Giunta and Zablotsky, 1976; Basker et al., 1978; Ali et al., 1986). However, there has only been one reported paper concerning residual monomer levels in acrylic resin teeth (Huggett et al., 1989). The commonest denture base material in use is heatcured poly(methy1 methacrylate). Virtually all complete dentures are made from this material. It is not worthy that there is no recommended residual monomer level stipulated in the British Standard Specification for Denture Base Polymers (BS 2487 : 1989; IS0 1567 : 1988). Only the British Standard Specification for Orthodontic Resins (BS 6747 : 1987) specifies an upper limit, viz. not more than 3.5% by mass of methyl methacrylate monomer. Levels of residual monomer < 1% can be obtained using @ 1992 Butterworth-Heinemann 0300-5712/92/060370-05
conventional laboratory polymerizing procedures (Huggett et al., 1984). Considerably higher levels, up to 20%, have been demonstrated by taking short cuts when using manufacturers’ recommended short polymerizing procedures (Austin and Basker, 1982). Gas-liquid chromatography (GLC) has been used with success for residual monomer determinations by many workers (e.g. Smith, 1958; Douglas and Bates, 1978; Austin and Basker, 1980; Huggett et al., 1984). The object of this study was to establish an optimum curing cycle for a wide range of acrylic resins currently available in the UK (Synthetic Denture Base Polymers Approved List, 1990; Table I). It is pertinent to comment that in addition to obtaining minimal levels of unpolymerized monomer, dentures must be porosity free and therefore an appropriate curing cycle is essential to prevent high exothermic reactions. The rate of heat application and the thickness of the methyl methacrylate mass will obviously be influencing factors in the exotherm. The wide variation in denture thickness is largely an unknown variable and therefore the correct choice of curing cycle is the predominant factor.
Table 1. Synthetic denture-base polymers tested (Dental Practice Board 1990 Approved list)
Table II. Characterization of Trevalon* and methyl
65.0 ym wt% 0.92
2.74 x IO6 0.28%
B. Monomer component Methyl methacrylate Hydroquinone Ethylene glycol dimethacrylate Tertiary amine
93% 0.006 % 6% 99% 0.006%
*Bonar Polymers Ltd, Newton Aycliffe, Co. Durham, UK. tBritish Drug House, Poole, Dorset, UK.
In this study tests were initially undertaken on a representative denture base copolymer and a homopolymer.
The materials investigated are listed in Table I. Characterization of the representative denture base copolymer and homopolymer are presented in Tables ZZand ZZZ.
Specimens were produced following carefully standardized methods of preparation. The moulds were prepared by investing master pattern blanks 65 X 40 X 5 mm in gypsum (South Western Industrial Plasters, Chippenham.
UK) using conventional denture flasking techniques. A powder : liquid ratio of 3.2 : 1 v/v was used to form the doughs. Mould separation. packing and clamping procedures followed standard practice. Except for method 7 (Table IV) all materials were polymerized in a thermostatically controlled water-bath. The curing cycles used are detailed in Table IV. After devesting, the specimens were conditioned by placing them in distilled water in sealed polythene bags held in a water-bath maintained at 37°C for 23 h. The specimens were removed from their containers, dried of visible water and stored in air at 23 + 2°C for 1 h prior to taking drill-cuttings.
GLC analysis The technique enables mixtures of volatile liquids to be separated, each component appearing as a separate peak
Tab/e K Reproducibility of residual monomer determinations
(3) Ten separate samples (7) One sample run 7 0 times iw
Mean s.d. s.e. Range C.V.
(2) Ten separate samples taken from one plate (56)
0.74 0.77 0.75 0.74 0.73 0.69 0.73 0.75 0.73 0.74
0.65 0.57 0.61 0.61 0.62 0.61 0.59 0.61 0.63 0.56
0.65 0.76 0.69 0.76 0.70 0.68 0.63 0.67 0.65 0.62
0.74 0.020 0.006 0.08 2.65
0.61 0.025 0.008 0.09 4.19
0.68 0.046 0.015 0.14 6.28
of area proportional to its volume present. The apparatus used for the analysis was a Philips PU 4500 Gas Chromatograph (Pye Unicam Ltd, Cambridge, UK) linked to a Hewlett Packard Integrator, fitted with a 80100 mesh, 1.5 m long Porapak Q column (Jones Chromatography Ltd, Llanbradach, UK) and operated at an oven temperature of 230°C with an inert gas as the carrier at 60 ml mini flow rate. The samples analysed contained methanol (in large excess), methyl methactylate monomer and ethyl acetate for an internal standard. The calibration procedure consisted of measuring peak height ratios of various methyl methacrylate-ethyl acetate solutions in standardized concentrates of methanol, thus enabling the methyl methacrylate proportion of a sample to be determined. This amount was then expressed as a percentage of residual monomer in the specimen. The procedure used in this study was essentially that laid down in British Standard Specification 6447 : 1987 for Orthodontic Resins. However, an internal standard was included which we have found from previous studies enhances the precision of measurement and reduces the possibility of error because of operator technique variability. Basically, preparation involved taking a series of drill-cuttings (> 0.4 g), distributed across the surface of the processed plate to make up the specimen and then extracting all the residual monomer from the specimen by refluxing with methanol. The solutions were then analysed by GLC and the results expressed as percentages of residual monomer (by weight) of the specimens.
Reproducibility determinations It is of particular
of residual relevance
taken from IO separate plates 1%)
evaluating the reproducibility values presented for these residual monomer tests for a distinction to be made between the accuracy of the measuring techniques and various measures of its precision upon one prepared sample for GLC determination (Table V (I)), as distinct from the residual monomer variation that might be expected from 10 samples prepared from an individual processed plate using a specific curing cycle (Table V (2)), and 10 separate samples taken from 10 separate processed plates using the same specific curing cycle (Table V (3)). From Table V it can be seen that a high level of reproducibility is obtained. As would be expected the largest standard deviation and standard error of the mean are demonstrated in the 10 samples taken from 10 separate plates processed in identical fashion. It will be appreciated that as usually only one GLC test specimen is prepared from a single processed plate, the value 0.092% (s.d. X 2) can be used with reasonable confidence as a guide to significant differences. Thus differences between means of < 0.092% are probably not significant. Many of the materials tested provided essentially the same result and it is reasonable to assume that they are the same material marketed (retailed) under different tradenames.
of curing cycle
On completion of the tests on the 10 water-bath curing cycles and the single dry heat curing system using both Trevalon and homopolymer, it was shown that cycles 3,4 and 11 were superior. There was no significant difference between the levels of residual monomer achieved with these three cycles. Since cycle 3 saves 7 h compared to cycle 4 and should be available in all dental laboratories, and also is suitable for all foreseeable denture applications, it was chosen as the optimum cycle for the
levels in denture
Table VI. Residual monomer content of Trevalon polymerized
Table VII. Residual monomer content polymerized by 1 1 different cycles
Curing cycle (Table IV)
Curing cycle (Table IV)
by 1 1 different cycles
1 2 3 4 5 6 7 : 10 11
Residual monomer (%I 2.91 2.14 0.79 0.82 1.32 3.97 0.91 15.65 1.39 2.01 17.72
subsequent tests. Although the dry heat system achieved comparable results it is not generally available in laboratories and has the disadvantage that the upper temperature limit may be more difficult to control than a water-bath which cannot be heated above the boiling point of water.
RESULTS AND DISCUSSION The results of the residual monomer content estimation of Trevalon polymerized by the 10 water-bath curing cycles and one dry heat system listed in Table IV are presented in Table VI. The curing cycles fall into four main groupings. In the first group, overnight cures (l)-(4), it can be seen that where a terminal boil is used the levels of residual monomer are markedly decreased. This result is in agreement with previous work (e.g. Austin and Basker, 1980; Huggett et al., 1984; Jerolimov et al., 1985). The overnight cures, without a terminal boil, show levels of residual monomer approximately three times that of those where a terminal boil is used. It can be expected that a period of 7 h at 70°C plus a terminal boil would produce a porosity-free denture irrespective of thickness. Therefore, either of these overnight cures, with a terminal boil, will assure an optimum result. In the second group of curing cycles, short and reverse cures ((5) and (6)) undertaken following the manufacturers’ recommendations, cycle (5) produces a high degree of polymerization but the levels of residual monomer are raised in comparison to the overnight cycles plus terminal boil ((3) and (4)). The shorter of the two cures (cycle (6)) gives a residual monomer level of almost 4%. In both of these cycles there is a risk of porosity in thick sections of denture bases due to possible high exothermic reaction. The risk will obviously be greater in cycle (6) than cycle (5). In those curing cycles (‘short cut’) where planned errors have been included the results hold no surprises. Cycle (S), 7 h at 6O”C, produces levels of residual monomer approximately five times higher than the recommended
2 3 4 5 6 7 : 10 11
Residual monomer 6) 3.90 2.79 0.69 0.60 1.21 3.69 0.56 16.03 1.48 3.23 18.46
cure. Almost double the level of residual monomer is recorded with cycle (9) compared to cycle (3). Comparisons between the short and reverse cures using recommended and ‘short cut’ cycles again give predictable results, with cycle (11) giving levels of residual monomer of almost 18%. Again, with both of these cycles there is a serious risk of porosity in thick sections. In those cycles where planned errors were used similar high levels of residual monomer were demonstrated by Austin and Basker (1982). The results of the residual monomer content estimation of the homopolymer polymerized by the 10 water-bath curing cycles and one dry heat system listed in Table IV are presented in Table VZI. A similar pattern of results to those obtained with Trevalon was shown. The homopolymer has no cross-linking agent or tertiary amine in the monomer component, compared to Trevalon which has < 1% tertiary amine and 6% ethylene glycol dimethacrylate as a cross-linking agent. Whilst it is unlikely that the absence of cross-linking agents would affect the degree of polymerization, it is possible that the tertiary amine may do so. Its presence as a chemical activator causes a complex reaction with the benzoyl peroxide (Brauer et al., 1956) which results in a variety of secondary products, some of which may consume benzoyl peroxide, thereby reducing its availability so that polymerization of the final traces of monomer is less efficient. It might also be reasoned that absence of the chemical activator would delay the onset of polymerization until the heat activation begins and therefore a less controlled and higher exothermic reaction may be expected. The higher exotherm may result in lower levels of unpolymerized monomer as a consequence of the heat evolved. The results shown in Tables VZ and VII do not appear to demonstrate any pattern in relation to the presence or absence of chemical activator. The result of the residual monomer determinations using the optimum curing cycle, 7 h at 70°C + 1 h at lOO”C, for all the synthetic denture base polymers tested are shown in Table VIII. In view of the reproducibility values obtained with this test method (Table V) it can be reasoned that some of the materials may be grouped 7 h at 70°C
J. Dent. 1992;
20: No. 6
Tab/e VIII. Residual monomer content using the optimum curing cycle
Denture base polymer
% Residual monomer
Acron Rapid Acron Standard Betacryl I I Croform Exten Doric Fast-cure LP-22 Meadway-super-cure Meliodent Metrocryl LWT Metrocryl original Metrocryl Rapid Cure Metrocryl Universal Minacryl Universal Plastex QC 20 Redilon Stellon 100 Ex Stellon 100s Trevalon Trevalon C Trevalon Hi WHW
0.80 0.54 0.91 0.47 0.77 0.78 0.85 0.77 1.08 0.59 0.94 1.02 0.82 0.94 0.76 0.94 0.91 0.74 0.99 0.79 0.82 0.58 0.74
together as similar results are obtained, and it is quite possible that these are the same basic material marketed under different tradenames. Only small differences are shown between the 23 materials tested with a range from 0.54 to 1.08% of residual monomer. These differences are probably a consequence of variations in the amount of benzoyl peroxide in the powder and of the chemical activator in the liquid. Jerolimov et al. (1989) have demonstrated the influence of these two components on the degree of polymerization and also the optimum proportions in relation to speed of onset of polymerization. It is possible that variation in the amounts of inhibitor and cross-linking agents of different types and amounts in the monomer component may also be expected to influence polymerization. However, it is unlikely that there are any consequential differences in the amounts used by the different manufacturers. This study has shown that ‘short cut’ curing cycles should not be used. Further, whilst rapid curing may be advantageous in saving time it will increase the possibility of generating gaseous porosity thereby reducing important mechanical properties. The cycle of 7 h at 70°C and 1 h at 100°C will give an optimum level of polymerization and avoid the risk of porosity even in thick sections.
CONCLUSIONS 1. A curing cycle of 7 h at 70°C followed by a terminal boil is the optimum curing cycle resulting in maximum monomer conversion. 2. Short-cut curing cycles are undesirable and result in significantly raised levels of residual monomer.
3. The residual monomer levels of 23 currently available denture base polymers cured using the optimum cycle ranged from 0.54 to 1.08%.
Acknowledgements The authors gratefully acknowledge the financial assistance provided by the Department of Health and by the Bristol and Weston Health Authority Medical Research Committee.
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