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A comparison of denture base acrylic resins polymerised by microwave irradiation and by conventional water bath curing systems
A comparison of denture base acrylic resins polymerised by microwave irradiation and by conventional water bath curing systems
D. AI Doori, R. Huggett, J. F. Bates, S. C. Brooks Department of Restorative Dentistry, University
of Wales College of Medicine,Heath Park, Cardiff, Wales
Al Doori D, Huggett R, Bates JF, Brooks SC. A comparison of denture base acrylic resins polymerised by microwave irradiation and by conventional water bath curing systems. Dent Mater 1988: 4: 25-32. Abstract - The application of microwave energy to the polymerisation of denture base acrylic resin is discussed. Comparisons with conventional water bath curing systems were made in respect of molecular weight, conversion of monomer and porosity of four acrylic materials. Molecular weight values of materials cured using microwave energy and the water bath system were essentially the same. Conversion using microwave energy was substantial, but minimal residual monomer levels attainable with the water bath system were not achieved. Microwave curing at 70W for 25 min minimized porosity problems associated with rapid heating of the dough, but porosity free material could only be guaranteed in sections not thicker than 3 mm. A t the present stage in this assessment it appears that microwave curing offers no advantage in time saving over rapid water curing systems.
The most popular denture base acrylic resin in use is heat cured poly(methylmethacrylate) (PMMA). Virtually all complete dentures are constructed from this material using the conventional polymer/monomer dough moulding technique and cured using a water bath system. Since the introduction, over 4 decades ago, of the acrylic resins for denture construction there has been a continual search for modified procedures to process the resin. In this study a comparison is made between conventional water bath curing and polymerisation undertaken by microwave irradiation. Polymerisation of PMMA, whether by microwave or conventional means, may be affected by a variety of time and temperature parameters. Principles of microwave heating
A microwave is an electromagnetic wave having a wavelength in the microwave region. Although this region is not bound by definition, it is commonly regarded as extending from 300,000
megacycles to 100 megacycles per second (MHz). This corresponds to wavelengths between 1 mm and 30 cm longer wavelengths (thus lower energy) than infrared rays, but shorter than those of radio and TV. Microwave ovens for general cooking purposes produce microwaves of 2450 Hz. This means that an electrostatic field is generated which changes direction 2450 times a second. The microwaves produced have several typical characteristics. They may be absorbed, reflected or transmitted depending on the materials present. The oven cavity is made with metallic walls which reflect the microwaves. Any material which can be heated by microwaves has polarized molecules. This means that one end of each molecule has a slight positive charge, while the other has a slight negative charge. In an electrostatic field which rapidly changes direction, polarized molecules are flipped over rapidly and generate heat due to molecular friction (Fig. 1). Although the microwave frequency itself is constant at 2450 Hz, the power output (wattage) can be adjusted by a
Key words: dental, PMMA, curing cycle, molecular weight, residual monomer, porosity, - microwave energy. R. Huggett, Department of RestorativeDentistry, University of Wales College of Medicine,Heath Park, Cardiff, CF4 4XY, Wales.
ReceivedNovember 18, 1986; accepted April 21, 1987.
control regulating the frequency of emission of microwaves. Microwave heating, therefore, does not depend on thermal conductivity. The major advantages over conventional heating for cooking food are that temperature rise occurs within the food and not in the oven and that there is an extremely short period of time required to cook food. It is this time-saving element which has encouraged workers to extend the principle of microwave heating to the polymerisation of denture base materials. Literature review
There is a voluminous literature in respect of the polymerisation of PMMA's. In the following review, a limited number of pertinent papers are discussed in its development and use as a denture base material. Taylor (1) (1941) warned that porosity resulted when an initial processing temperature greater than 74~ was used due to a temperature rise within the specimens to above the boiling point of monomer. Incomplete packing
26
A l Doori et al. ao
b. (-
+
)
~
~
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§
C,
-
.
9
!~-1
q
'.I
U-
','.
!?
'11
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Fig. 1. Generation of heat in microwave oven. a_).Random arrangement of polar molecules in absence of the electrostatic field; b) Alignment of polar molecules in the electrostatic field; c) Flipping over of polar molecules as the direction of the electrostatic field is reversed rapidly, generating heat.
of the mould and inadequate compression on the flask were also reported by Taylor as a cause of porosity. Sweeney, Paffenbarger and Beall (2) (1942) demonstrated that, in addition to those causes cited by Taylor, insufficient mixing of monomer and polymer would also cause porosity. Tuckfield, Warner and Guerini (3) (1943) showed that the temperature rise within the resin was greater in the thicker portions, thus giving more complete polymerisation. This was confirmed by Harman (4) (1949) who also demonstrated that the stiffness and the strength developed by the resins after processing for 9 h at 71~ was equal to that obtained by processing for one hour at 71~ and then boiling for 30 min. Vernonite work bench (5) (1946) published a method of curing acrylic resins using a water vapour technique. They compared the results obtained by vapour bath curing with conventional water bath curing and they concluded that the vapour bath was a satisfactory
method of curing and that the physical properties of the cured resins are as good, but not superior to those obtained in the water bath. They reported that for practical denture construction; strength and moulding, the result will be the same for either vapour or water bath provided the temperature cycles are similar. Matthews and Tyldesley (6) (1950) reported on the effect of different curing methods on the mean molecular weights and tensile strength developed in cured specimens of the resins. They concluded that boiling for longer than one hour neither raised the molecular weight nor the strength enough to warrant the extra time. They added that good physical properties and chemical inertness can be obtained by either the vulcanizer cure 8 minutes at 45 lb pressure or by the boiling method. Peyton (7) (1950) reviewed other methods of curing other than using the water bath. These included the use of dry heat developed by two electrically heated platens applied to the flask under eom-
pression, a dry air oven, infra red light bulbs as a heat source and induction heating by an electronic generator. He concluded that when all factors were considered it was doubtful that any of these methods have any real advantage over the water bath method and it was the careful control of the temperature during processing that remained the important factor for consideration. Smith (8) (1958) realised that the amount of residual monomer was one of the principal factors affecting the properties of denture base materials produced under different curing cycles. He stated that the time required for substantially complete conversion of monomer to polymer would be impractically long at temperatures much below 100~ and that, except for curing schedules involving longer than one hour at 100~ all P M M A dentures would contain appreciable amounts of residual monomer dependent on the ambient conditions and the efficiency of heat transfer. Austin a n d Basker (9) (1982), after examining a survey of processing procedures in UK dental laboratories, warned that inadvertent shortening of the curing time and/or failure to adhere to the recommended curing temperature could result in the production of resins containing very high levels of residual monomer. As a result, mechanical properties would suffer and the patient may be at a higher risk of mucosal irritation. In order to avoid this, it is essential to ensure complete polymerisation so that residual monomer is kept as low as possible. The first reported use of microwave energy to polymerise denture base materials was by Masamishinishi (10) in 1968. He realised that there was a problem of reflection of microwaves by metallic flasks. To counteract this he used flasks which contained holes at various positions. In most cases either undercuring, due to lack of microwave penetration, or severe porosity, due to rapid heating of the resins, occurred. However, he was able to produce porosity-free acrylic resin specimens of similar physical and mechanical properties to water bath cured specimens by the use of a holed denture flask and by placing a b e a k e r containing 76 ml of water in the oven at the same time. The effect of the water was to absorb much of the microwave energy, thus, in effect reducing the wattage. The actual wattage is not given in the paper, but it seems that a continuous emission of mi-
M i c r o w a v e irradiation a n d water-bath cures
crowaves occurred with the microwave oven used. Kimura (11) et al. in 1983 also sought to apply microwave energy to the dough forming and curing of acrylic resins. They solved the problem of reflection by metal flasks thus: After the mould was packed with dough and the halves of the flask pressed together, the mould was meticulously removed from the flask, bound tightly with a rubber tube and irradiated in t h e microwave oven. The wattages quoted are 500 W and 200 W. Their conclusions were that dough time could be reduced, clasp wires could be incorporated, adaptability was improved and that porosity free specimens of 3 mm thickness or less could be produced with an irradiation time of 3 min. Neither the degree of cure nor examination of physical and mechanical properties were assessed. Further research by Kimura and Teraoka (12) (1984) saw the introduction of a polymeric flask suitable for use in a microwave oven. A more recent publication by Reitz, Sanders and Levin (13) (1985) also described the use of this type of flask and went on to compare some physical properties of identical strips of resin, some of which were cured by microwaves and some by conventional water bath heating. They found that for specimens 2.5 mm thick processing for 289 minutes on each side at 400 W produced specimens that were not statistically different in respect of porosity, hardness or transverse strength from specimens cured in a water bath at 74~ for 8 h. But in thicker sections (up to 10 ram) reducing the wattage (to 90 W) and increasing the exposure time (to 689 minutes each side) was not sufficient to eliminate porosity. In this pilot study it seemed appropriate to determine the optimum conditions for the microwave curing of acrylic denture base materials. It was decided to evaluate the molecular weight, the residual monomer levels and extent of porosity and to compare with the same materials cured by the conventional water bath system. It was considered that by these means a good initial characterization could be achieved. Materials
The 4 materials examined in this study are listed in Table 1, with their sources. The Trevalon material used in this
27
Table 1. Materials examined in study. Material
Number
Trevalon
1
Lucitone 199
2
TSl195 Homopolymer & methyl methacrylate monomer Experimental material TSl195 homopolymer and methylmethacrylate monomer containing 0.025% DMPT & 5% EGDM.
study is a poly(methyl-methacrlylate) (PMMA) denture base polymer marketed as a powder containing mainly P M M A and a small percentage of poly (butyl-methacrylate) copolymer and a methyl-methacrylate (MMA) monomer containing 7% ethylene glycol dimethacrylate cross linking agent. Trevalon is a standard heat-cured doughmoulded denture base material. Lucitone 199 is an impact resistant material. It is a modified P M M A denture base polymer marketed as a powder containing 95-98% PMMA and 2-5% rubber which is probably formed in a similar way to that described in US Patent No. 3427 214. In this patent the M M A and the butadiene styrene are described as being copolymerised in an emulsion with a second coating of M M A covering the beads. These polymer beads are then mixed with monomer in the usual way. The liquid monomer contains 8% E G D M . This material is also dough moulded, T h e third material used was a P M M A homopolymer; clear powder (TSl195). The monomer used with this powder was pure methylmethacrylate. The fourth material, an experimental material similar to Rapid curing materials (QC20 and Acron Rapid), used the same powder as in the third material, but the monomer was modified by the addition of 0.025% dimethyl-p-toluidine (DMPT) as an initiation activator and 5% of E G D M as crosslinking agent. The modifications enable this material to be cured using a 20 min period in boiling water and there are in effect 2 initiators, the DMPT and heat to activate the benzoyl peroxide. It is thus a hybrid material of cold and heat cure.
Manufacturer De Trey Division, Dentsply Ltd, Weybridge, Surrey, England. Caulk Co Delaware, USA. Dentsply York Division, Dentsply International Inc. York, PA 17445. Cole Polymers Ltd, Mitcham Road, Croydon, England. British Drug House, Poole, Dorset, England. As above,
All the powders contain benzoyl peroxide initiator and in the case of Trevalon and Lucitone 199 an inorganic pigment. All the monomers contain small amounts ( ~ 0.006%) of hydroquinone which acts as an inhibitor preventing unwanted polymerisation during storage. A mixing ratio of 3.2 : 1 v.v. was selected for all materials.
Methods Microwave curing
The microwave oven used in this study was a Panasonic model NE-651 (Matsushita Electric Trading Co Ltd, Osaka, Japan) (Fig. 2). A fan-like stirrer blade and a rotating turntable ensured uniform absorption of microwaves during operation. The output power was controlled by adjustment of the variable power controller unit. The maximum output of the oven was 600 W. When this setting was selected, microwaves were produced by the magnetron and fed into the oven cavity through the wave guide continuously during the 30 s cycle of the power motor. When lower wattage settings were selected, then microwaves were only produced for the appropriate time during the 30 s cycle (Fig. 3). For example, when the lowest setting of 70 W was selected the microwaves were only produced for 5 s of the 30-s cycle. For production of specimens using the microwave technique, the special dental FRP flask of H. Kimura (12) was used. The flask is made of glass fibrereinforced polyester resin and is used with polycarbonate bolts. Moulds were prepared by investing master pattern
28
A/Doori
et al.
Waveguide Stirrer \ blade
/
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~ F.R.P.
I
\ttllltl
III
I []
_
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Magnetron
I \ r
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Flask "
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9
chromatography. The technique is described fully by Huggett, Brooks and Bates (14) (1984) and can be summarized as follows: Drill cuttings were taken from a specimen using a low speed twist drill. Precisely 0.300 g of these cuttings were refluxed with 3 ml of methanol solvent for 3 hours to ensure complete extraction of the residual monomer present. On completion, 0.5 ml of 1% ethyl acetate in methanol solution was added to serve as an internal standard during chromatographic analysis. The Poropak Q column selected enabled separation of methanol solvent, ethyl acetate internal standard and methyl methacrylate monomer using the Pye Unicam 104 equipment. A typical chromatograph is shown in Figure 5. Monomer concentration was calculated relative to the internal standard and expressed as a weight percentage of the sample.
J
J
Oven
cavity
Turntable
Fig. 2. Diagrammatic representation of microwave oven.
blanks in gypsum. Mould separation (alginate solution), packing and clamping procedures followed standard practise. Flasks were allowed to bench cool prior to devesting. It was found that although curing could be achieved at high wattage in short times (around 3 min at 600 W for 3 mm thick specimen) there was increased likelihood of porosity which became inevitable with thicker specimens. It was, therefore, decided that the use of the lowest wattage (70 W) would probably be most suitable for curing of acrylic denture base materials particularly when fairly thick sections are involved. The next part of the investigation was to follow the polymerisation by microwaves by monitoring the exotherm temperature by the incorporation of thermocouples* into the dough in the flask. Typical exotherm plots of the materials examined are shown in Fig. 4. It was found that the exotherm was always completed after 24 min and so the microwave curing procedure of 24 rain at 70 W was selected for all materials. For the water bath cure specimens, mould separation packing and clamping procedures were the same as for the microwave group except that conventional metal dental flasks were used. Previous research (14) indicated that 7 h at 70~ followed by 3 h at *PTFE insulated fast response type Ni-Cr/ Ni-N1. Comark Electronics Ltd, Littlehampton, Sussex, England.
100~ achieved minimal levels of residual monomer and so this curing procedure was selected for 3 of the materials in this group. The fourth material was cured using the popular 20-min cure in boiling water. In terms of processing time, this cure is competive with the microwave method. Residual monomer determinations
The residual monomer content of t h e materials was determined using gas
Control Setting High
Output Power 600W
(1oo%) 480W
Medium
( 80 %) 360W
Molecular weight determinations
The molecular weights of the powder components of the materials examined and the two-phase cured product were determined by Rubber and Plastics Research Association, Shawbury, Salop, England using gel permeation chromatography (GPC). In the GPC analysis, tetrahydrofuran was used as the solvent and the system was calibrated with standard samples of polystyrene of known molecular weight. As the mo-
On, O f f Time on Variable power switch ON
I
1 30S
ON
Low
( 60 %)
Defrost
240W (40%)
113s
180W
ON I 'Os
( 30 %)
Warm
70W (12%)
L
1 5s
ON
Ires
Simmer
OFF
12ss
OFF
]
ON
ON
Fig. 3. Variable out )ut of microwave energy.
l l
lls
OFF
izs
OFF 20s OFF 25S
Microwave irradiation and water-bath cures
29
sumption is made that the materials tested are PMMA homopolymers. The molecular weight figures, therefore, are only correct for those polymers that are PMMA homopolymers and reasonably correct for those that are not.
140
120
I0C
Results and discussion g_
Residual monomer 60
40
20
0
i
i
i
i
2
4
6
8
10
l
i
l
12
14
16
i 18
20
22
24
Time ( m i n u t e s )
Fig. 4. Typical exotherm developed during microwave curing, where: 9 = Material 1; 9 = Material 2 ; 0 = Material 3; 9 = Material 4.
ucts of the materials that contain a cross-linking agent, the presence of the cross-linked agent prevents the product from going into complete solution. The product separates into a sol and a gel by filtering. These are separated and the sol is used for the molecular weight determinations and the values presented in this study. One further problem in respect of the molecular weight determinations is the constituents of the materials. In the determinations the as-
lecular weight relationship of polystyrene and PMMA in tetrahydrofuran is known, then the calibration can be used for PMMA determinations. The molecular weight distribution obtained from GPC analysis allows for the derivation of a variety of averages. In this study the number and weight averages are presented, together with the percentage fraction of polymer with molecular weight below 105. It should be noted that with the two-phase prod-
A
-- M e t h a n o l
B
= Ethyl
C
= Methyl
(large
Acetate
Methacrylate
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u
A
C
B
I
I
1
2 Time
Fig. 5. Typical GLC chromatogram.
(mins)
excess) (Internal
i
i
3
4
standard)
(Residual
monomer)
Previous research (14) indicated that there exists a minimal obtainable level of residual monomer using conventional dental laboratory curing technique and that this level is obtained only when a terminal boil of at least one hour is employed. Levels around 0.3% were considered minimal (Table
2). It can be seen that these minimal levels of residual monomer were achieved with 3 of the materials in the waterbath cure group. The fourth material of course was cured using the 20-min cure and, thus, showed a slightly higher level of residual monomer. For the microwave group, a high degree of conversion was achieved, but higher levels of residual monomer than the minimal were present. Thicker specimens also had higher levels of residual monomer (Table 3). The effect of using the modified monomer component (Material 4), however, was more marked in that approximately 2.5 times more residual monomer was present. The reason for this is not altogether clear, although a comparison of the mechanisms of free radical production may give some clues. When benzoyl peroxide is broken down by heat alone the process is fairly straightforward as shown in Fig. 6. A n initiating radical is then able to react with a monomer molecule to start polymerisation. When an activator such as the tertiary amine dimethyl-p-toluidine is used to break down the benzoyl peroxide then an electron transfer mechanism is favoured (15) as shown in Fig. 7. The aminium radical cation can undergo a series of complex secondary reactions leading to a variety of products. Conversion of monomer proceeds when initiating radicals react with monomer molecules in exactly the same way as when benzoyl peroxide is decomposed by heat. After initiation and a period of conversion there is a rapid rise in the rate of polymerisation, as the viscosity increases. The resulting exotherm increases the rate of polymerisation
30
A I Doori et al.
Table 2. Residual monomer levels for microwave curing and water-bath curing. Material
version of the traces of monomer present to the limiting value may be hindered due to the following reasons:
Percentage residual monomer Microwave cure
Water-bath cure
Mean
s.d.
C of V (%)
Mean
s.d.
C of V (%)
0.55 0.45 0.51 1.25
0.11 0.18 0.11 0.16
20.0 40.0 21.6 12.8
0.3l 0.24 0.31 0.68
0.06 0.07 0.08 0.21
19.4 29.2 25.8 30.9
1 2 3 4
Table 3. Dependence of residual monomer on thickness for material 3 PMMA homopolymer/methyl methacrylate monomer. Curing procedure
Thickness % Residual (ram) monomer
Microwave 24 min at 70W Microwave 24 min at 70W Water bath 7h 70~ + 3h 100~ Water bath 7h 70~ + 3h 100~
C.c.o I
O
0
2.5
0.51___0.11
7.0
0.79___0.05
2.5
0.3l_+0.04
7.0
0.31_+0.04
~
Heat>
Oe
I
which continues until substantial conversion of monomer is achieved. The rate of polymerisation then falls rapidly in the final stages due to lack of monomer. The concentration of initiator radicals is also a factor since they must begin the growth of each new polymer chain. Smith (8, 16) concluded that residual monomer tended towards a small approximately constant value at a particular temperature as a monomer/polymer equilibrium is approached. When a tertiary amine is present in the monomer component the final con-
C.c.o I
Oe I
I
Initiating Radical
0 e
Initiating Radical
Benzoyl Peroxide
Fig. 6. Decomposition of benzoyl peroxide by heat.
~\C
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CH3
~
.0
I
~\ CH3
CH3
~ --
J
/C'o
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/C -Oe
9
Initiating
Initiating
Radical
Radical
Fig. 7. Decomposition of benzoyl peroxide by reaction with tertiary amine.
Molecular weight
The GPC determinations, Tables 4-6, show that all of the specimens examined had molecular weight values greater than 105. In general, polymers vary with molecular weight in a relationship such as shown in Fig. 8. Materials of high molecular weight have a greater degree of molecular attraction and entanglement; they have greater rigidity and strength and higher glass transition and melting temperatures. Above a molecular weight of 105, the polymer behaves as a typical long chain polymer with optimum physical and mechanical properties. Increases in molecular weight into the million range give no appreciable improvement in many properties (17). At the other end of the scale, the presence of very low molecular weight polymers will have a plasticising effect. Because the material borders on the liquid state, it will lubricate the other large chain molecules. With the materials examined in this study, it
CD I
0 Transfer C H 3 < ~ - NIl"[+] []?"
/c~o
1) There may be much less benzoyl peroxide remaining at this stage if its decomposition by tertiary amine in the initial stages of polymerisation was very efficient. 2) The initiating radicals may have been converted to less efficient initiating species due to reactions with aminium radical cations or products from secondary reactions associated with the initiator-accelerator reactions.
--CH:~
[-] 0 ~'0 +
CH 3
Aminium
ion
M i c r o w a v e irradiation a n d water-bath cures
Table 4. Molecular weight values of the powder components of the materials examined. Material (powder) 1 2 3
IVln x 105
IVIw x 105
4.68 2.19 4.34
13.20 5.79 14.50
/Vlw/1Vln
Percentage of polymer < 105
2.82 2.65 3.36
2.1 9.9 2.7
3
t
2
c~
0 a.
I
!
10 3
!
10 4
I
10 5
10 6
Molecular Weight Fig. 8. Variation in polymer properties with molecular weight.
can be seen that there were only very small percentages of polymer with molecular weight less than 105. The molecular weight distribution plots, and the 2 examples given in Fig. 9, show that the low molecular weight tail is short and that there is no fraction of polymer below 104. Thus, in all these materials it could be expected that the molecular weight values would give optimum physical and mechanical properties. It should, however, be noted that the low levels of residual monomer (Klw = 100) which are present in these specimens are not recorded under these particular conditions of the GPC system. In comparing the molecular weight values of the materials polymerised by conventional means and microwave energy, account must be taken of the reproducibility of GPC molecular weight determinations. In an earlier study (18) it has been shown that GPC determinations produce a high level of reproducibility. Standard deviation values of 0.65 • l0 s 1Vlw are obtained. It will be appreciated that in this study, and in general, only one GPC determination is made on test specimens. However, by using the reproducibility standard deviation value x 2, i.e.
.....
Conventional cure Microwave cure
E
c"
3
..D L
/ /
c-
"6 to
I
i
IL
I
Porosity
I
When dentures are polymerised using rapid curing systems the possibility of porosity in thick sections is always a major consideration. With commercially available systems such as Acron Rapid* or QC20**, which are essentially similar to Material 4, used in this study, the manufacturers add caution-
7= I I
/ i
i
lO 4
i
10 5
!
10 6
0.65 x l0 s • 2 = 1.3 x 105,
it is possible to establish whether there are significant differences between results. Thus, with the materials examined in this study (Tables 5, 6), it can be reasoned that with each material, whether it is polymerised by conventional means or microwave energy, that there is no significant difference in the molecular weight values. This similarity in molecular weight values of a material polymerised by the two curing systems is clearly demonstrated in the molecular weight distribution plots of Material 3 shown in Fig. 9.
-5
.o
31
i
10 7
Molecular Weight (log scale) Fig. 9. Molecular weight distribution plots of material 3.
!
1o 8
* Austenal Dental Products Ltd, Harrow, UK. **Dentsply International, Weybridge, UK.
32
A I Doori et al.
Table 5. Molecular weight values of the materials polymerised by microwave energy. Material
1 2 3 4
]VIn x 105
/vlw x 105
3.50 1.95 4.47 3.22
9.14 4.15 13.00 9.33
b/Iw/19In
2.61 2.13 2.91 2.90
Percentage of polymer < 105 3.0 11.0 2.5 4.7
Table 6. Molecular weight values of the materials polymerised in the water bath. Material
1 2 3 4
1Vln x 105
1Vlw x 105
3.20 1.78 3.87 2.77
8.77 3.62 13.90 8.22
ary notes in their instructions with respect to thick dentures. With QC20, the instructions recommend the following curing cycle for average dentures: Immerse in boiling water, continue heating to regain boiling temperature and boil for 20 rain. For abnormally thick cases, the instructions recommended are modified to: Immerse in boiling water, turn Off the heat for 20 min, then reheat and boil for 20 min. With the instructions for Acron Rapid there is just one recommended curing cycle for all dentures including bulky dentures viz: Immerse in boiling water, increase heat until boiling point is regained, then boil for 20 rain. However, with the Acron Rapid system the instructions advise that with extra thick dentures the dough should be packed at a 'snap dough' consistency. It will be appreciated that with rapid curing systems that contain D M P T as an initiation activator, that polymerisation begins at the time of mixing the polymer and monomer, thus the cautionary notes listed by the manufacturers for thick cases effectively reduce the curing temperature in the early stages. The manufacturers instructions do not quantify 'thick' dentures. However, in this study it was found, using Material 4, and water bath curing for 20 min, packing at the dough stage and without
/Vlw/Mn
2.74 2.04 3.59 2.97
Percentage of polymer < 105 4.7 12.3 3.9 6.1
any curing delay, that specimens 60 x 45 x 7 mm thick could be produced without porosity (specimens were examined using a microscope s for the presence of porosity). However, using the same material and time with microwave curing, porous free specimens could only be obtained up to 3 mm thick.
Summary This study has shown that microwave energy can efficiently polymerise denture base polymers, but in thick sections there is a risk of porosity. Further, at the present state of our studies, apart from the advantage that a microwave oven offers in respect of cleaner processing-equipment handling, there is no advantage in time saving over rapid water-curing methods. However, further studies are currently progressing to examine the potentiality of microwave curing.
References 1. Taylor PB. Acrylic resins: their manipulation. J A m Dent Assoc 1941: 28: 373. Microscope: Carl Zeiss, 63095, FRG.
2. Sweeney WT, Paffenbarger GC, Beall JR. Acrylic resins for dentures. J A m Dent Assoc 1942: 29: 7-33. 3. Tuckfield WJ, Warner HK, Guerini BD. Acrylic resins in dentistry. Aust J Dent 1943: 27: 172. 4. Harman IM. Effects of time and temperature on polymerisation of a methacrylate resin denture base. J A m Dent Assoc 1949: 38: 188-203. 5. The Vernonite work bench. 1946: 5. 6. Matthews E, Tyldesley WR. The polymerisation of acrylic denture base materials. Br Dent J 1950: 89: 148--150. 7. Peyton FA. Packing and processing denture base resins. J A m DentAssoc 1950: 40: 521-526. 8. Smith DC. The acrylic denture base. Br Dent J 1958: 105: 86-91. 9. Austin AT, Basker RM. Residual monomer levels in denture bases. Br Dent J 1982: 153: 424-426. 10. Masamichi Nishii. Studies on the curing of denture base resins with microwave irradiation: with particular reference to heat-curing resins. J Osaka Dental University 1968: 2: 23-40. 11. Kimura H, Teraoka F, Ohnishi H, Saito T, Yato M. Applications of microwave for dental technique (Part 1). Doughforming and curing of acrylic resins. J Osaka Dent Sch 1983: 23: 43-49. 12. Kimura H, Teraoka F. On the microwave polymerisation method by the developed flask. Quint Dent Tech (Japan) 1984: 9: 967-974. 13. Reitz PV, Sanders JL, Levin B. The curing of denture acrylic resins by microwave energy. Phys Prop Quint Int 1985: 8: 547-551. 14. Huggett R, Brooks SC, Bates JE The effect of different curing cycles on levels of residual monomer in acrylic resin denture base materials. Quint Dent Technol 1984: 8: 365-371. 15. Brauer GM, Davenport RM, Hanson WC. Accelerating effect of amines on polymerisation of methyl methacrylate. Mod Plastics 1956: 34: 153. 16. Smith DC. The acrylic denture base. The peroxide concentration in dental polymers. Br Dent J 1959: 107." 62-67. 17. Beech DR. Molecular weight distribution of denture base acrylic. J Dent 1975: 3: 19-24. 18. Huggett R. Some structure and fracture property relationships in heat cured polymethylmethacrylate denture base materials. MSc Thesis, University of Bath, UK (1982).
D. AI Doori, R. Huggett, J. F. Bates, S. C. Brooks Department of Restorative Dentistry, University
of Wales College of Medicine,Heath Park, Cardiff, Wales
Al Doori D, Huggett R, Bates JF, Brooks SC. A comparison of denture base acrylic resins polymerised by microwave irradiation and by conventional water bath curing systems. Dent Mater 1988: 4: 25-32. Abstract - The application of microwave energy to the polymerisation of denture base acrylic resin is discussed. Comparisons with conventional water bath curing systems were made in respect of molecular weight, conversion of monomer and porosity of four acrylic materials. Molecular weight values of materials cured using microwave energy and the water bath system were essentially the same. Conversion using microwave energy was substantial, but minimal residual monomer levels attainable with the water bath system were not achieved. Microwave curing at 70W for 25 min minimized porosity problems associated with rapid heating of the dough, but porosity free material could only be guaranteed in sections not thicker than 3 mm. A t the present stage in this assessment it appears that microwave curing offers no advantage in time saving over rapid water curing systems.
The most popular denture base acrylic resin in use is heat cured poly(methylmethacrylate) (PMMA). Virtually all complete dentures are constructed from this material using the conventional polymer/monomer dough moulding technique and cured using a water bath system. Since the introduction, over 4 decades ago, of the acrylic resins for denture construction there has been a continual search for modified procedures to process the resin. In this study a comparison is made between conventional water bath curing and polymerisation undertaken by microwave irradiation. Polymerisation of PMMA, whether by microwave or conventional means, may be affected by a variety of time and temperature parameters. Principles of microwave heating
A microwave is an electromagnetic wave having a wavelength in the microwave region. Although this region is not bound by definition, it is commonly regarded as extending from 300,000
megacycles to 100 megacycles per second (MHz). This corresponds to wavelengths between 1 mm and 30 cm longer wavelengths (thus lower energy) than infrared rays, but shorter than those of radio and TV. Microwave ovens for general cooking purposes produce microwaves of 2450 Hz. This means that an electrostatic field is generated which changes direction 2450 times a second. The microwaves produced have several typical characteristics. They may be absorbed, reflected or transmitted depending on the materials present. The oven cavity is made with metallic walls which reflect the microwaves. Any material which can be heated by microwaves has polarized molecules. This means that one end of each molecule has a slight positive charge, while the other has a slight negative charge. In an electrostatic field which rapidly changes direction, polarized molecules are flipped over rapidly and generate heat due to molecular friction (Fig. 1). Although the microwave frequency itself is constant at 2450 Hz, the power output (wattage) can be adjusted by a
Key words: dental, PMMA, curing cycle, molecular weight, residual monomer, porosity, - microwave energy. R. Huggett, Department of RestorativeDentistry, University of Wales College of Medicine,Heath Park, Cardiff, CF4 4XY, Wales.
ReceivedNovember 18, 1986; accepted April 21, 1987.
control regulating the frequency of emission of microwaves. Microwave heating, therefore, does not depend on thermal conductivity. The major advantages over conventional heating for cooking food are that temperature rise occurs within the food and not in the oven and that there is an extremely short period of time required to cook food. It is this time-saving element which has encouraged workers to extend the principle of microwave heating to the polymerisation of denture base materials. Literature review
There is a voluminous literature in respect of the polymerisation of PMMA's. In the following review, a limited number of pertinent papers are discussed in its development and use as a denture base material. Taylor (1) (1941) warned that porosity resulted when an initial processing temperature greater than 74~ was used due to a temperature rise within the specimens to above the boiling point of monomer. Incomplete packing
26
A l Doori et al. ao
b. (-
+
)
~
~
§
§
C,
-
.
9
!~-1
q
'.I
U-
','.
!?
'11
',',
§
~+',.
-:;
Fig. 1. Generation of heat in microwave oven. a_).Random arrangement of polar molecules in absence of the electrostatic field; b) Alignment of polar molecules in the electrostatic field; c) Flipping over of polar molecules as the direction of the electrostatic field is reversed rapidly, generating heat.
of the mould and inadequate compression on the flask were also reported by Taylor as a cause of porosity. Sweeney, Paffenbarger and Beall (2) (1942) demonstrated that, in addition to those causes cited by Taylor, insufficient mixing of monomer and polymer would also cause porosity. Tuckfield, Warner and Guerini (3) (1943) showed that the temperature rise within the resin was greater in the thicker portions, thus giving more complete polymerisation. This was confirmed by Harman (4) (1949) who also demonstrated that the stiffness and the strength developed by the resins after processing for 9 h at 71~ was equal to that obtained by processing for one hour at 71~ and then boiling for 30 min. Vernonite work bench (5) (1946) published a method of curing acrylic resins using a water vapour technique. They compared the results obtained by vapour bath curing with conventional water bath curing and they concluded that the vapour bath was a satisfactory
method of curing and that the physical properties of the cured resins are as good, but not superior to those obtained in the water bath. They reported that for practical denture construction; strength and moulding, the result will be the same for either vapour or water bath provided the temperature cycles are similar. Matthews and Tyldesley (6) (1950) reported on the effect of different curing methods on the mean molecular weights and tensile strength developed in cured specimens of the resins. They concluded that boiling for longer than one hour neither raised the molecular weight nor the strength enough to warrant the extra time. They added that good physical properties and chemical inertness can be obtained by either the vulcanizer cure 8 minutes at 45 lb pressure or by the boiling method. Peyton (7) (1950) reviewed other methods of curing other than using the water bath. These included the use of dry heat developed by two electrically heated platens applied to the flask under eom-
pression, a dry air oven, infra red light bulbs as a heat source and induction heating by an electronic generator. He concluded that when all factors were considered it was doubtful that any of these methods have any real advantage over the water bath method and it was the careful control of the temperature during processing that remained the important factor for consideration. Smith (8) (1958) realised that the amount of residual monomer was one of the principal factors affecting the properties of denture base materials produced under different curing cycles. He stated that the time required for substantially complete conversion of monomer to polymer would be impractically long at temperatures much below 100~ and that, except for curing schedules involving longer than one hour at 100~ all P M M A dentures would contain appreciable amounts of residual monomer dependent on the ambient conditions and the efficiency of heat transfer. Austin a n d Basker (9) (1982), after examining a survey of processing procedures in UK dental laboratories, warned that inadvertent shortening of the curing time and/or failure to adhere to the recommended curing temperature could result in the production of resins containing very high levels of residual monomer. As a result, mechanical properties would suffer and the patient may be at a higher risk of mucosal irritation. In order to avoid this, it is essential to ensure complete polymerisation so that residual monomer is kept as low as possible. The first reported use of microwave energy to polymerise denture base materials was by Masamishinishi (10) in 1968. He realised that there was a problem of reflection of microwaves by metallic flasks. To counteract this he used flasks which contained holes at various positions. In most cases either undercuring, due to lack of microwave penetration, or severe porosity, due to rapid heating of the resins, occurred. However, he was able to produce porosity-free acrylic resin specimens of similar physical and mechanical properties to water bath cured specimens by the use of a holed denture flask and by placing a b e a k e r containing 76 ml of water in the oven at the same time. The effect of the water was to absorb much of the microwave energy, thus, in effect reducing the wattage. The actual wattage is not given in the paper, but it seems that a continuous emission of mi-
M i c r o w a v e irradiation a n d water-bath cures
crowaves occurred with the microwave oven used. Kimura (11) et al. in 1983 also sought to apply microwave energy to the dough forming and curing of acrylic resins. They solved the problem of reflection by metal flasks thus: After the mould was packed with dough and the halves of the flask pressed together, the mould was meticulously removed from the flask, bound tightly with a rubber tube and irradiated in t h e microwave oven. The wattages quoted are 500 W and 200 W. Their conclusions were that dough time could be reduced, clasp wires could be incorporated, adaptability was improved and that porosity free specimens of 3 mm thickness or less could be produced with an irradiation time of 3 min. Neither the degree of cure nor examination of physical and mechanical properties were assessed. Further research by Kimura and Teraoka (12) (1984) saw the introduction of a polymeric flask suitable for use in a microwave oven. A more recent publication by Reitz, Sanders and Levin (13) (1985) also described the use of this type of flask and went on to compare some physical properties of identical strips of resin, some of which were cured by microwaves and some by conventional water bath heating. They found that for specimens 2.5 mm thick processing for 289 minutes on each side at 400 W produced specimens that were not statistically different in respect of porosity, hardness or transverse strength from specimens cured in a water bath at 74~ for 8 h. But in thicker sections (up to 10 ram) reducing the wattage (to 90 W) and increasing the exposure time (to 689 minutes each side) was not sufficient to eliminate porosity. In this pilot study it seemed appropriate to determine the optimum conditions for the microwave curing of acrylic denture base materials. It was decided to evaluate the molecular weight, the residual monomer levels and extent of porosity and to compare with the same materials cured by the conventional water bath system. It was considered that by these means a good initial characterization could be achieved. Materials
The 4 materials examined in this study are listed in Table 1, with their sources. The Trevalon material used in this
27
Table 1. Materials examined in study. Material
Number
Trevalon
1
Lucitone 199
2
TSl195 Homopolymer & methyl methacrylate monomer Experimental material TSl195 homopolymer and methylmethacrylate monomer containing 0.025% DMPT & 5% EGDM.
study is a poly(methyl-methacrlylate) (PMMA) denture base polymer marketed as a powder containing mainly P M M A and a small percentage of poly (butyl-methacrylate) copolymer and a methyl-methacrylate (MMA) monomer containing 7% ethylene glycol dimethacrylate cross linking agent. Trevalon is a standard heat-cured doughmoulded denture base material. Lucitone 199 is an impact resistant material. It is a modified P M M A denture base polymer marketed as a powder containing 95-98% PMMA and 2-5% rubber which is probably formed in a similar way to that described in US Patent No. 3427 214. In this patent the M M A and the butadiene styrene are described as being copolymerised in an emulsion with a second coating of M M A covering the beads. These polymer beads are then mixed with monomer in the usual way. The liquid monomer contains 8% E G D M . This material is also dough moulded, T h e third material used was a P M M A homopolymer; clear powder (TSl195). The monomer used with this powder was pure methylmethacrylate. The fourth material, an experimental material similar to Rapid curing materials (QC20 and Acron Rapid), used the same powder as in the third material, but the monomer was modified by the addition of 0.025% dimethyl-p-toluidine (DMPT) as an initiation activator and 5% of E G D M as crosslinking agent. The modifications enable this material to be cured using a 20 min period in boiling water and there are in effect 2 initiators, the DMPT and heat to activate the benzoyl peroxide. It is thus a hybrid material of cold and heat cure.
Manufacturer De Trey Division, Dentsply Ltd, Weybridge, Surrey, England. Caulk Co Delaware, USA. Dentsply York Division, Dentsply International Inc. York, PA 17445. Cole Polymers Ltd, Mitcham Road, Croydon, England. British Drug House, Poole, Dorset, England. As above,
All the powders contain benzoyl peroxide initiator and in the case of Trevalon and Lucitone 199 an inorganic pigment. All the monomers contain small amounts ( ~ 0.006%) of hydroquinone which acts as an inhibitor preventing unwanted polymerisation during storage. A mixing ratio of 3.2 : 1 v.v. was selected for all materials.
Methods Microwave curing
The microwave oven used in this study was a Panasonic model NE-651 (Matsushita Electric Trading Co Ltd, Osaka, Japan) (Fig. 2). A fan-like stirrer blade and a rotating turntable ensured uniform absorption of microwaves during operation. The output power was controlled by adjustment of the variable power controller unit. The maximum output of the oven was 600 W. When this setting was selected, microwaves were produced by the magnetron and fed into the oven cavity through the wave guide continuously during the 30 s cycle of the power motor. When lower wattage settings were selected, then microwaves were only produced for the appropriate time during the 30 s cycle (Fig. 3). For example, when the lowest setting of 70 W was selected the microwaves were only produced for 5 s of the 30-s cycle. For production of specimens using the microwave technique, the special dental FRP flask of H. Kimura (12) was used. The flask is made of glass fibrereinforced polyester resin and is used with polycarbonate bolts. Moulds were prepared by investing master pattern
28
A/Doori
et al.
Waveguide Stirrer \ blade
/
@,,\\\
~ F.R.P.
I
\ttllltl
III
I []
_
///
Magnetron
I \ r
\
Flask "
'[11; \
9
chromatography. The technique is described fully by Huggett, Brooks and Bates (14) (1984) and can be summarized as follows: Drill cuttings were taken from a specimen using a low speed twist drill. Precisely 0.300 g of these cuttings were refluxed with 3 ml of methanol solvent for 3 hours to ensure complete extraction of the residual monomer present. On completion, 0.5 ml of 1% ethyl acetate in methanol solution was added to serve as an internal standard during chromatographic analysis. The Poropak Q column selected enabled separation of methanol solvent, ethyl acetate internal standard and methyl methacrylate monomer using the Pye Unicam 104 equipment. A typical chromatograph is shown in Figure 5. Monomer concentration was calculated relative to the internal standard and expressed as a weight percentage of the sample.
J
J
Oven
cavity
Turntable
Fig. 2. Diagrammatic representation of microwave oven.
blanks in gypsum. Mould separation (alginate solution), packing and clamping procedures followed standard practise. Flasks were allowed to bench cool prior to devesting. It was found that although curing could be achieved at high wattage in short times (around 3 min at 600 W for 3 mm thick specimen) there was increased likelihood of porosity which became inevitable with thicker specimens. It was, therefore, decided that the use of the lowest wattage (70 W) would probably be most suitable for curing of acrylic denture base materials particularly when fairly thick sections are involved. The next part of the investigation was to follow the polymerisation by microwaves by monitoring the exotherm temperature by the incorporation of thermocouples* into the dough in the flask. Typical exotherm plots of the materials examined are shown in Fig. 4. It was found that the exotherm was always completed after 24 min and so the microwave curing procedure of 24 rain at 70 W was selected for all materials. For the water bath cure specimens, mould separation packing and clamping procedures were the same as for the microwave group except that conventional metal dental flasks were used. Previous research (14) indicated that 7 h at 70~ followed by 3 h at *PTFE insulated fast response type Ni-Cr/ Ni-N1. Comark Electronics Ltd, Littlehampton, Sussex, England.
100~ achieved minimal levels of residual monomer and so this curing procedure was selected for 3 of the materials in this group. The fourth material was cured using the popular 20-min cure in boiling water. In terms of processing time, this cure is competive with the microwave method. Residual monomer determinations
The residual monomer content of t h e materials was determined using gas
Control Setting High
Output Power 600W
(1oo%) 480W
Medium
( 80 %) 360W
Molecular weight determinations
The molecular weights of the powder components of the materials examined and the two-phase cured product were determined by Rubber and Plastics Research Association, Shawbury, Salop, England using gel permeation chromatography (GPC). In the GPC analysis, tetrahydrofuran was used as the solvent and the system was calibrated with standard samples of polystyrene of known molecular weight. As the mo-
On, O f f Time on Variable power switch ON
I
1 30S
ON
Low
( 60 %)
Defrost
240W (40%)
113s
180W
ON I 'Os
( 30 %)
Warm
70W (12%)
L
1 5s
ON
Ires
Simmer
OFF
12ss
OFF
]
ON
ON
Fig. 3. Variable out )ut of microwave energy.
l l
lls
OFF
izs
OFF 20s OFF 25S
Microwave irradiation and water-bath cures
29
sumption is made that the materials tested are PMMA homopolymers. The molecular weight figures, therefore, are only correct for those polymers that are PMMA homopolymers and reasonably correct for those that are not.
140
120
I0C
Results and discussion g_
Residual monomer 60
40
20
0
i
i
i
i
2
4
6
8
10
l
i
l
12
14
16
i 18
20
22
24
Time ( m i n u t e s )
Fig. 4. Typical exotherm developed during microwave curing, where: 9 = Material 1; 9 = Material 2 ; 0 = Material 3; 9 = Material 4.
ucts of the materials that contain a cross-linking agent, the presence of the cross-linked agent prevents the product from going into complete solution. The product separates into a sol and a gel by filtering. These are separated and the sol is used for the molecular weight determinations and the values presented in this study. One further problem in respect of the molecular weight determinations is the constituents of the materials. In the determinations the as-
lecular weight relationship of polystyrene and PMMA in tetrahydrofuran is known, then the calibration can be used for PMMA determinations. The molecular weight distribution obtained from GPC analysis allows for the derivation of a variety of averages. In this study the number and weight averages are presented, together with the percentage fraction of polymer with molecular weight below 105. It should be noted that with the two-phase prod-
A
-- M e t h a n o l
B
= Ethyl
C
= Methyl
(large
Acetate
Methacrylate
tO Q. er
u
A
C
B
I
I
1
2 Time
Fig. 5. Typical GLC chromatogram.
(mins)
excess) (Internal
i
i
3
4
standard)
(Residual
monomer)
Previous research (14) indicated that there exists a minimal obtainable level of residual monomer using conventional dental laboratory curing technique and that this level is obtained only when a terminal boil of at least one hour is employed. Levels around 0.3% were considered minimal (Table
2). It can be seen that these minimal levels of residual monomer were achieved with 3 of the materials in the waterbath cure group. The fourth material of course was cured using the 20-min cure and, thus, showed a slightly higher level of residual monomer. For the microwave group, a high degree of conversion was achieved, but higher levels of residual monomer than the minimal were present. Thicker specimens also had higher levels of residual monomer (Table 3). The effect of using the modified monomer component (Material 4), however, was more marked in that approximately 2.5 times more residual monomer was present. The reason for this is not altogether clear, although a comparison of the mechanisms of free radical production may give some clues. When benzoyl peroxide is broken down by heat alone the process is fairly straightforward as shown in Fig. 6. A n initiating radical is then able to react with a monomer molecule to start polymerisation. When an activator such as the tertiary amine dimethyl-p-toluidine is used to break down the benzoyl peroxide then an electron transfer mechanism is favoured (15) as shown in Fig. 7. The aminium radical cation can undergo a series of complex secondary reactions leading to a variety of products. Conversion of monomer proceeds when initiating radicals react with monomer molecules in exactly the same way as when benzoyl peroxide is decomposed by heat. After initiation and a period of conversion there is a rapid rise in the rate of polymerisation, as the viscosity increases. The resulting exotherm increases the rate of polymerisation
30
A I Doori et al.
Table 2. Residual monomer levels for microwave curing and water-bath curing. Material
version of the traces of monomer present to the limiting value may be hindered due to the following reasons:
Percentage residual monomer Microwave cure
Water-bath cure
Mean
s.d.
C of V (%)
Mean
s.d.
C of V (%)
0.55 0.45 0.51 1.25
0.11 0.18 0.11 0.16
20.0 40.0 21.6 12.8
0.3l 0.24 0.31 0.68
0.06 0.07 0.08 0.21
19.4 29.2 25.8 30.9
1 2 3 4
Table 3. Dependence of residual monomer on thickness for material 3 PMMA homopolymer/methyl methacrylate monomer. Curing procedure
Thickness % Residual (ram) monomer
Microwave 24 min at 70W Microwave 24 min at 70W Water bath 7h 70~ + 3h 100~ Water bath 7h 70~ + 3h 100~
C.c.o I
O
0
2.5
0.51___0.11
7.0
0.79___0.05
2.5
0.3l_+0.04
7.0
0.31_+0.04
~
Heat>
Oe
I
which continues until substantial conversion of monomer is achieved. The rate of polymerisation then falls rapidly in the final stages due to lack of monomer. The concentration of initiator radicals is also a factor since they must begin the growth of each new polymer chain. Smith (8, 16) concluded that residual monomer tended towards a small approximately constant value at a particular temperature as a monomer/polymer equilibrium is approached. When a tertiary amine is present in the monomer component the final con-
C.c.o I
Oe I
I
Initiating Radical
0 e
Initiating Radical
Benzoyl Peroxide
Fig. 6. Decomposition of benzoyl peroxide by heat.
~\C
CH:~<~ ~H?~V6 Electron DMPT
CH3
~
.0
I
~\ CH3
CH3
~ --
J
/C'o
Intermediate 0
/C -Oe
9
Initiating
Initiating
Radical
Radical
Fig. 7. Decomposition of benzoyl peroxide by reaction with tertiary amine.
Molecular weight
The GPC determinations, Tables 4-6, show that all of the specimens examined had molecular weight values greater than 105. In general, polymers vary with molecular weight in a relationship such as shown in Fig. 8. Materials of high molecular weight have a greater degree of molecular attraction and entanglement; they have greater rigidity and strength and higher glass transition and melting temperatures. Above a molecular weight of 105, the polymer behaves as a typical long chain polymer with optimum physical and mechanical properties. Increases in molecular weight into the million range give no appreciable improvement in many properties (17). At the other end of the scale, the presence of very low molecular weight polymers will have a plasticising effect. Because the material borders on the liquid state, it will lubricate the other large chain molecules. With the materials examined in this study, it
CD I
0 Transfer C H 3 < ~ - NIl"[+] []?"
/c~o
1) There may be much less benzoyl peroxide remaining at this stage if its decomposition by tertiary amine in the initial stages of polymerisation was very efficient. 2) The initiating radicals may have been converted to less efficient initiating species due to reactions with aminium radical cations or products from secondary reactions associated with the initiator-accelerator reactions.
--CH:~
[-] 0 ~'0 +
CH 3
Aminium
ion
M i c r o w a v e irradiation a n d water-bath cures
Table 4. Molecular weight values of the powder components of the materials examined. Material (powder) 1 2 3
IVln x 105
IVIw x 105
4.68 2.19 4.34
13.20 5.79 14.50
/Vlw/1Vln
Percentage of polymer < 105
2.82 2.65 3.36
2.1 9.9 2.7
3
t
2
c~
0 a.
I
!
10 3
!
10 4
I
10 5
10 6
Molecular Weight Fig. 8. Variation in polymer properties with molecular weight.
can be seen that there were only very small percentages of polymer with molecular weight less than 105. The molecular weight distribution plots, and the 2 examples given in Fig. 9, show that the low molecular weight tail is short and that there is no fraction of polymer below 104. Thus, in all these materials it could be expected that the molecular weight values would give optimum physical and mechanical properties. It should, however, be noted that the low levels of residual monomer (Klw = 100) which are present in these specimens are not recorded under these particular conditions of the GPC system. In comparing the molecular weight values of the materials polymerised by conventional means and microwave energy, account must be taken of the reproducibility of GPC molecular weight determinations. In an earlier study (18) it has been shown that GPC determinations produce a high level of reproducibility. Standard deviation values of 0.65 • l0 s 1Vlw are obtained. It will be appreciated that in this study, and in general, only one GPC determination is made on test specimens. However, by using the reproducibility standard deviation value x 2, i.e.
.....
Conventional cure Microwave cure
E
c"
3
..D L
/ /
c-
"6 to
I
i
IL
I
Porosity
I
When dentures are polymerised using rapid curing systems the possibility of porosity in thick sections is always a major consideration. With commercially available systems such as Acron Rapid* or QC20**, which are essentially similar to Material 4, used in this study, the manufacturers add caution-
7= I I
/ i
i
lO 4
i
10 5
!
10 6
0.65 x l0 s • 2 = 1.3 x 105,
it is possible to establish whether there are significant differences between results. Thus, with the materials examined in this study (Tables 5, 6), it can be reasoned that with each material, whether it is polymerised by conventional means or microwave energy, that there is no significant difference in the molecular weight values. This similarity in molecular weight values of a material polymerised by the two curing systems is clearly demonstrated in the molecular weight distribution plots of Material 3 shown in Fig. 9.
-5
.o
31
i
10 7
Molecular Weight (log scale) Fig. 9. Molecular weight distribution plots of material 3.
!
1o 8
* Austenal Dental Products Ltd, Harrow, UK. **Dentsply International, Weybridge, UK.
32
A I Doori et al.
Table 5. Molecular weight values of the materials polymerised by microwave energy. Material
1 2 3 4
]VIn x 105
/vlw x 105
3.50 1.95 4.47 3.22
9.14 4.15 13.00 9.33
b/Iw/19In
2.61 2.13 2.91 2.90
Percentage of polymer < 105 3.0 11.0 2.5 4.7
Table 6. Molecular weight values of the materials polymerised in the water bath. Material
1 2 3 4
1Vln x 105
1Vlw x 105
3.20 1.78 3.87 2.77
8.77 3.62 13.90 8.22
ary notes in their instructions with respect to thick dentures. With QC20, the instructions recommend the following curing cycle for average dentures: Immerse in boiling water, continue heating to regain boiling temperature and boil for 20 rain. For abnormally thick cases, the instructions recommended are modified to: Immerse in boiling water, turn Off the heat for 20 min, then reheat and boil for 20 min. With the instructions for Acron Rapid there is just one recommended curing cycle for all dentures including bulky dentures viz: Immerse in boiling water, increase heat until boiling point is regained, then boil for 20 rain. However, with the Acron Rapid system the instructions advise that with extra thick dentures the dough should be packed at a 'snap dough' consistency. It will be appreciated that with rapid curing systems that contain D M P T as an initiation activator, that polymerisation begins at the time of mixing the polymer and monomer, thus the cautionary notes listed by the manufacturers for thick cases effectively reduce the curing temperature in the early stages. The manufacturers instructions do not quantify 'thick' dentures. However, in this study it was found, using Material 4, and water bath curing for 20 min, packing at the dough stage and without
/Vlw/Mn
2.74 2.04 3.59 2.97
Percentage of polymer < 105 4.7 12.3 3.9 6.1
any curing delay, that specimens 60 x 45 x 7 mm thick could be produced without porosity (specimens were examined using a microscope s for the presence of porosity). However, using the same material and time with microwave curing, porous free specimens could only be obtained up to 3 mm thick.
Summary This study has shown that microwave energy can efficiently polymerise denture base polymers, but in thick sections there is a risk of porosity. Further, at the present state of our studies, apart from the advantage that a microwave oven offers in respect of cleaner processing-equipment handling, there is no advantage in time saving over rapid water-curing methods. However, further studies are currently progressing to examine the potentiality of microwave curing.
References 1. Taylor PB. Acrylic resins: their manipulation. J A m Dent Assoc 1941: 28: 373. Microscope: Carl Zeiss, 63095, FRG.
2. Sweeney WT, Paffenbarger GC, Beall JR. Acrylic resins for dentures. J A m Dent Assoc 1942: 29: 7-33. 3. Tuckfield WJ, Warner HK, Guerini BD. Acrylic resins in dentistry. Aust J Dent 1943: 27: 172. 4. Harman IM. Effects of time and temperature on polymerisation of a methacrylate resin denture base. J A m Dent Assoc 1949: 38: 188-203. 5. The Vernonite work bench. 1946: 5. 6. Matthews E, Tyldesley WR. The polymerisation of acrylic denture base materials. Br Dent J 1950: 89: 148--150. 7. Peyton FA. Packing and processing denture base resins. J A m DentAssoc 1950: 40: 521-526. 8. Smith DC. The acrylic denture base. Br Dent J 1958: 105: 86-91. 9. Austin AT, Basker RM. Residual monomer levels in denture bases. Br Dent J 1982: 153: 424-426. 10. Masamichi Nishii. Studies on the curing of denture base resins with microwave irradiation: with particular reference to heat-curing resins. J Osaka Dental University 1968: 2: 23-40. 11. Kimura H, Teraoka F, Ohnishi H, Saito T, Yato M. Applications of microwave for dental technique (Part 1). Doughforming and curing of acrylic resins. J Osaka Dent Sch 1983: 23: 43-49. 12. Kimura H, Teraoka F. On the microwave polymerisation method by the developed flask. Quint Dent Tech (Japan) 1984: 9: 967-974. 13. Reitz PV, Sanders JL, Levin B. The curing of denture acrylic resins by microwave energy. Phys Prop Quint Int 1985: 8: 547-551. 14. Huggett R, Brooks SC, Bates JE The effect of different curing cycles on levels of residual monomer in acrylic resin denture base materials. Quint Dent Technol 1984: 8: 365-371. 15. Brauer GM, Davenport RM, Hanson WC. Accelerating effect of amines on polymerisation of methyl methacrylate. Mod Plastics 1956: 34: 153. 16. Smith DC. The acrylic denture base. The peroxide concentration in dental polymers. Br Dent J 1959: 107." 62-67. 17. Beech DR. Molecular weight distribution of denture base acrylic. J Dent 1975: 3: 19-24. 18. Huggett R. Some structure and fracture property relationships in heat cured polymethylmethacrylate denture base materials. MSc Thesis, University of Bath, UK (1982).