Title
Author(s)
Citation
Issued Date
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The pressure and temperature changes in heat-cured acrylic
resin during processing
Yau, Wai-fung, Elizabeth.; 邱慧鳳.
Yau, W. E. [邱慧鳳]. (1999). The pressure and temperature
changes in heat-cured acrylic resin during processing. (Thesis).
University of Hong Kong, Pokfulam, Hong Kong SAR. Retrieved
from http://dx.doi.org/10.5353/th_b3195410.
1999
http://hdl.handle.net/10722/40677
The author retains all proprietary rights, (such as patent rights)
and the right to use in future works.
THE PRESSURE AND TEMPERATURE CHANGES IN
HEAT-CURED ACRYLIC RESIN DURING
PROCESSING
YAU WAI FUNG ELIZABETH
M. D. S. DISSERTATION
THE UNIVERSITY OF HONG KONG
I 999
ACKNOWLEDGEMENTS
My sincere gratitude is expressed to the following:
Dr. T.W. Chow, my supervisor of the M.D.S. course, for his invaluable
support, guidance and the treasured experiences shared throughout my course of
study.
Mr. Leo Y.Y. Cheng, my supervisor of the research, for his guidance,
generous help and full technical support.
Professor R.K.F. Clark, consultant of United Medical and Dental School, for
his invaluable advice on the use of the transducer.
Dr. C.W. Kwan, Assistant Research Officer, for his generous assistance with
the data analysis and the computer work.
Miss S.W. Cheng, Technician, Oral Rehabilitation, for her sincere help and
great technical assistance.
Mr. S.W. Ho, Mr. K.D. Lee and Mr. D.B. Yuen, Staff in Dental Materials
Sciences, for their generous support on the instrumentation.
- vi -
THE UNIVERSITY OF HONG KONG
LIBRARY
Thesis Collection
Deposited by the Author
THE PRESSURE AND TEMPERATURE CHANGES IN
HEAT-CURED ACRYLIC RESIN DURING
PROCESSING
YAU WAI FLING ELIZABETh, 13.DÒS.
This thesis is presented in partial fuHilment
ofthe requirements for the degree of
Master of Dental Surgery
Faculty of Dentistry
The University of Hong Kong
July 1999
\
:
'
I
1p
Abstract
Acrylic resìn is commonly used for denture construction. It is generally
believed that the gaseous porosity sometimes found in acrylic resin after processing is
a result of the temperature having reached the boìling point of monomer durìng heat
curing. The important factors involved are temperature and pressure. The aims of this
study are to measure the pressure arid temperature changes of acrylic resin during
processing, to record the highest temperature achieved when curing in boiling water
and to determine the elevated boiling point ofmonomer under high pressure.
A submìniature pressure transducer (temperature compensated to 94°C) and a
thermocouple were placed on the palate of a standardized maxillary complete denture
base. A heat-cured resin (Trevalon C) was polyrnerized by a long heating cycle (72°C
for 6.5h and 92°C for 1.5h). Recordings ofpressure and temperature (n=6) were made
at initial clamping of denture flasks and throughout the heating and cooling cycles of
resin processing. The temperature ofthe acrylic resin was also monitored during a fast
cycle, which was accomplished by placIng the flask directly into boiling water for
40mm.
The pressure of acrylic dough inside the clamped flask was initìafly i i .S3atrn
(SD=323) and reached a peak of 22.Olatm (SD3.45) during the long heating cycle.
The elevated boiling point of monomer at increased pressure was calculated to be
about 193.33°C (at ll.53atm) and 227.86°C (at 22.Olatm). These elevated boiling
points are higher than the maximum temperature 13 1.21°C (SD=6.56) reached during
the fast curing cycle. No porosity was observed iii the denture bases heat cured by the
fast cycle.
In conclusion, the highest temperature reached by heating of resin during
processing ìs well below the elevated boiling point of monomer. Monomer therefore
does not boil in clamped denture flasks under sufficient pressure. Thus adequate
clamp pressure prevents gaseous porosity irrespective ofcuring cycle used.
CONTENTS
Contents
i
Index of Figures
iv
Index of Tables
y
Acknowledgements
vi
1. INTRODUCTION
I
2. LITERATURE SURVEY
3
2. 1 .
History of Denture Base Materials
.
3
Early Natural Materials
2.1.2 Vulcanite
2.1.3 Cellulose Nitrate
2.1 .4 Phenol-formaldehyde Resin
2.1.5 VinylPolymers
2.1.6 EpoxyResin
2.1.7 Polystyrenes
2.1.8 Nylon
2.1.9 Polycarbonate
3
3
2.1.10 Poly(methyl methacrylate)
9
2. 1 I
.
4
5
6
7
8
8
8
2.2. Types of Acrylic Resins
i
Heat-cured Acrylic Resin
2.2.2 Cold-cured Acrylic Resin
2.2.3 Microwave-cured Acrylic Resin
2.2.4 Light-cured Acrylic Resin
2.2. 1
5
17
18
19
21
23
2.3. Processing Methods
2.3.1 Compressionmou.1ding Tecbnìque
2.3.2 Jnjection-moulthng Technique
2.3.3 Pour Resin Technique
2.3.4 Mìcrowave Curing Cycle
2.3.5 LightCuring
23
26
26
28
29
30
2.4. Porosìty
2.4.1 GaseousPorosity
2.4.2 Contraction Porosity
2.4.3 Porosity Caused by In&lequate Homogenicity
ofthe Dough
31
32
32
2.5. Temperature Control during Processing
33
2.6. Pressure Control during Processing
37
-1-
2.7. Elevation of Boìlìng Point
40
3. OBJECTJVESOFTRE STUDY
43
4. MATERIALS AND METHODS
44
4.1.Materials
4.1 I Pressure Transducer
.
Thermocouple
4.1.3 Dynamic Strainnieter
4. 1 .4 Standard Maxillary Casts and Patterns
4. 1 .5 Modified Hanau Denture Flasks
4.1.6 AcrylicResin
4.1.7 CuringTank
4.1.8 ChartRecorder and Computer
4_ i .2
44
46
48
49
52
52
54
54
55
4.2. Methods
55
57
62
4.2.1 Preparations
4.2.2 Packing the Resin Dough
4.2.3 Curing Cycles
4.3. Observation ofPorosity
62
4.4. Elevation ofBoìling Point
63
5. RESULTS
5. 1 .
64
64
Pressure Change during Packing
5 .2. Temperature and Pressure Changes in Long Curing Cycle
5 .3 .
69
Temperature Change in Fast Cycle
5.4. Elevation ofBoíling Point during Processing
6. DISCUSSION
6. 1 .
67
71
73
73
Packing Pressure
6.2. Temperature and Pressure Changes during Processing
75
75
77
6.2.1. Temperature Changes
6.2.2. Pressure Changes
6.3. Temperature Change in Fast Cycle
83
6.4. Elevation ofBoiling Point and Gaseous Porosity
84
6.5. DiseussionofMaterials andMethods
86
88
7. CONCLUSIONS
-11 -
8. REFERINCES
89
9. APPENDICES
109
9.1. Calibration of Thermometer
109
9.2. Calibration of Strainmeter for Measuring Temperature
110
10. CONFERENCE PRESENTATION
111
-Ill -
LIST OF FIGURES
SECTION 4
Fig. 4.1
Subminiature Pressure Transducer
47
Fìg. 4.2
Transducer Placed Insìde Protective Sleeve
47
Fig. 4.3
Thermocouple
Fig. 4.4
Strainmeters
50
Fig. 4.5
RTV Silicone Moulds of Maxillary Edentulous Arch
51
Fìg. 4.6
Standardized Wax Pattern of Complete Denture Base
51
and Thermometer
50
Formed on Duplicate Cast
Fig. 4.7
Modìfied Hanau Denture Flasks
53
Fig. 4.8
Chart Recorder
53
Fig. 4.9
Flaskìng ofthe Cast
59
Fìg. 4.10
Assembly ofthe Flask
59
Fig. 4.11
Cross-section ofthe Set-up ofTransducer and
60
Thermocouple on the Denture Base
Fig. 4.12
Trial Pack under Hydraulic Press
61
Fig. 4.13
Torque Applied
61
Fig. 5.1
Typical Pressure Change during Packing and Clamping
66
Fig. 5.2
Typical Pressure and Temperature Changes during
68
Section 5
Long Curing
Fig. 5.3
Typical Temperature Change ofAcrylic Resin during
70
Fast Curing
Section 6
Fig. 61
Pressure and Temperature Changes during the First
81
12 Hours
Fig. 6.2
Pressure Change in Clamped Flask with No Heat Applied 82
-IV -
LIST OF TABLES
Section 4
Table 4.1
Range and Gain of the Strainmeter
48
Section 5
Table S . i
Pressure of Acrylic Resin at the First and the Second Thai 65
Pack
Table 5.2
Pressure ofAcrylic Resìn at the Final Closure and at the
65
Steady State
Table 5.3
Peak Temperature ofAcrylic Resìn durIng Fast Curing
69
Table 5.4
Pressure ofAcrylìc Resin in Clamped Denture Flask at
71
Ambient Temperature
Table 5.5
Peak Pressure and the Elevated Boiling Point
71
Table 5.6
Lowest Pressure ofthe Acrylic Resin
72
Table 5.7
Pressure at the End ofthe Cycle
72
Table 9. 1
Calibration of Thermometer
i 09
Table 9.2
Calibration of Strairimeter for Temperature
i 10
Section 9
-V-
INTRODUCTION
1. Introduction
Acrylic resins were first introduced to dentistry in 1937 (Peyton, 1975). They
are now the most commonly used materials in constructing partial and complete
dentures. In addition to its excellent physical, colour arid optical properties, the
advantage of acrylic resins is the ease with which it can be mould.ed and processed for
the construction of dentures.
Polymerization of acrylic resins can be either heat activated,
chemical
activated or light activated. Heat-cured acrylic resin is always the choice for
constructing denture
bases
because heat curing provides a higher degree of
polymerization, which in turn gives the denture bases superior strength and colour
stability (Winkler, 1984; Al-Mulla et al.. 1988; Combe, 1992; Phillips, 1996).
In heat-cured acrylic resins, polymerization is started by free radicals from the
beazoyl peroxide, which is activated by heat. As the polymerization proceeds, a
certain amount of heat is liberated. It is generally believed that excessìve heat
generated may boil the monomer before the process is complete. Tuckfield et al.
(1943) suggested that the vapour trapped in the rapidly polymerizìng resin would give
rise to internal porosity. Thus the rate and degree of temperature rise must be well
controlled by reducing both the rate of heating and the temperature to whIch the flask
is subjected during the curing cycle. Yeung et al. (1995) has shown that a temperature
i
INTRODUCTION
spike was absent if a slow heating cycle (6.5b at 72°C and then 3Omìn at 100°C) was
used.
Faraj and Ellis (1 979) suggested that porosity would form in the cured resin
only ifthe vapour pressure in the monomer (methyl methacrylate) at above 100°C was
above atmospheric pressure; and that such pressure might exceed the denture flask
clamping pressure. The clamping pressure is originally designed for adapting the
denture base to the master cast during
the
thermal contraction
in the
cooling process.
The boiling point of the methyl methacrylate, however, will be raised if it is under
pressure. Whether the monomer will boil inside the flask then becomes questionable.
The pressure mentioned in most studies in the literature unfortunately is the
external pressure applied on the flask during trial closure; the main crIterion for such
pressure is to obtain metal-to-metal contact between the two halves of the flask in a
hydraulic press (Peyton, 1950; Faraj and Ellis, 1979; SeIg, 1982; Morrow et al.,
1986). The flasks subsequently were fransferred to the spring clamp, which was hand-
tightened without recording the magnìtude of the pressure applied (Sowter and
Barton, 1986; Phillips, 1996). Also, no attempt has been made to record the internal
pressure of the acrylic resin during curing cycle. In the present study, internal pressure
arid the temperature of the resin during packing and processing of heat-cured acrylic
denture base were investigated.
2
LITERATURE SURVEY
2. Literature Survey
21 History of Fenture Base Materials
Replacement of missing teeth is one of the main problems dentists face. In
order to provide effective dental services to patients dentists require ideal materials.
The ideal material is one which is biologically compatible, readily available,
reasonably ìnexpensive, and sìmple to manipulate with a readily controlled technical
procedure to develop a prosthesis that is functionally effective and pleasing in
appearance (Peyton, 1975).
2.1.1 Early Natural Materials
It is interesting to note that as recently as i 840 dentistry depended to a large
degree on naturally occurring materials for replacement of missing teeth. Structures
resembling dentures, fashioned from hardwood, ivory or bone and fastened to natural
teeth by screws, gold wire or other means, were reported to be common before i 800
(Peyton, 1975).
2.1.2 Vulcanite
The art of vulcanizing rubber was developed in i 839 by Charles Goodyear
(Winkler, 1984). In about 1855 vulcanized hard rubber was developed and introduced
as a denture base material, so-called ebonite or vulcanite (Gibson, 1922). The
3
LITERA TURE SUR VEY
combination of porcelain teeth mounted in vulcanite greatly improved comfort and
function of dentures for edentulous patients.
Porcelain teeth mounted in vulcanite was the standard in denture making for
the next 75 years, but its principal disadvantage was poor esthetic quality. The natural
colour of vulcanite is a dark olive browm Colouring it meant the addition of
considerable amounts of dyes and pigments, whìch decreased ìts slrength and.
durability (Winkler and Vernon, i 978). Early colouring attempts used ester-soluble
dyes, but these were either easily degraded or deslioyed by the catalyst or the heat, or
turned out to be ìnhibitors. Water-soluble dyes were tried but their colour stability was
poor. Also, because rubber ìs opaque only surface colour was visible.
Moreover, vulcanite was easily contaminated by the absorption of saliva and
food flavors and odours. Attempts to address this problem were made by subjecting
the materìal to various processing conditions. However, results were less than ideal.
2.1.3 Cellulose Nitrate
In I 868 John Wesley Hyatt developed a. thermoplastic polymer popularly
known as "celluloid" by combining cellulose nitrate and camphor while searching for
a substìtute for ivory for the manufacture of billìard balls (Purrias, 1942; Sweeney
1958; Brydson, 1982). Celluloid was produced in pink colours (Woodforde. 1968;
Greener et aL, 1972), which were more pleasing than the dark colours of vulcanite.
4
LITERA TLJRE SUR VEF
Celluloid, however, discoloured and stained
easily
because of its porous
structure. Also, the camphor - added in maximum quantity of 30% as plasticizer to
facilitate moulding - made the taste and odour of the finished dentures unpleasant to
patients (Woodforde, 1968; Greener et al., 1972; Combe and Grant, 1973;
Paffenbarger and Rupp, 1973; Combe, 1992). In addition, because cellulose nitrate
lacked stability, the dentures had a tendency to warp and distort in service (Peyton,
1950). Furthermore, the dentures were difficult to repair if fractured, and the material
was ìnflanirnable.
Other cellulose compounds, such as acetate, acetate-butyrate, and ethyl
cellulose, were produced but they had warpage and distortion characteristics similar to
the nitrate compound and thus were not well accepted.
2.1.4 Phenol-formaldehyde Resin
Dr. Leo Bakeland invented phenol-formaldehyde resin - known as
"Bakelite" -ìn 1907 (Peyton, 1975) but phenol-formaldehyde dentures were not
prepared until 1924 by Dr. Stryker (Paffenbarger and Rupp, 1973). The main
disadvantages of these dentures were their lack of uniformity and the inability to
control their physical qualities, which depended to a large degree on the processing
conditions. Undercuring the resin resulted in dimensional instability, while overcuring
reduced strength and colour quality.
LITERA TURE SURVEY
When processed, it had an excellent appearance but the material was very
brittle (Barber, 1934; Kimball, 1936) due to extensive cross-linking (Combe, 1992); it
was also very stiff and difficult to mould, probably owing to vaporization of phenol
durìng preparation (Kìmball. 1936; Gieler and Skinner, 1939; Combe and Grant,
1973; Combe, 1992). It was ìmpossible to repair, discoloured in service and retained
the taste of phenol. Because of the organic properties of phenol and formaldehyde,
polymerization is affected by water, thus the investment needed to be completely
dried before processing (Greener et al., 1972).
2.1.5 Vinyl Polymers
Mixtures of polynierized vinyl chloride and vinyl acetate were available for
dentures after 1932 (Paffenbarger and Rupp, 1973). Their colour was pleasing but the
processing method was difficult to follow without residual stresses developing. These
stresses frequently caused slow warpage (Greener et al., 1972) arid fracture in service.
Modifications of vinyl polymers, for example,
Luxene 44,
were introduced in
1939. They contained the original proportions of vinyl chloride to vinyl acetate
copolymer, plasticìsed with methyl methacrylate monomer (Skinner and Cooper,
1943; Smith, 1957). The materials showed lìttle water absorption (Woelfel et al.,
1963; Brauer, 1966; Stafford arid Smith, 1968b), high fatigue resistance (Stafford and
Smith, 1970; Combe and Grant, 1973). and hìgh ìmpact resistance.
LITERATURE SURVEY
Nevertheless, they had a low elastic modulus (Brauer, 1966; Braden and
Stafford, 1968) and a low transitìonal temperature of 80°C (Braden and Stafford,
I 968), which could lead to distortion when cleaning or polishing at high temperatures.
Furthermore, polyvinyl acrylics were supplied in gel form and required special
injectìori-moiilding equipment for processing (Winkler, 1984). The injection
moulding technique involved feeding compressed air into a cylinder that drove a
piston, forcing the resin into the mould. Such a processing method was extraordinary
long and complìcated.
2.1.6 Epoxy Resin
Epoxy resins were first synthesized in 1 937 by Pierre Casten (Brauer, i 97Th).
Their advantages are toughness (Causton, 1978), strength, hardness. low curing
shrinkage and good adhesion to metals (Kydd and Wykhuis, 1958; Smith, 1962).
Their shortcomings are suspected toxicity of some of the polyaniine curing agents
(Combe and Grant, 1973; Brauer, 1977a; Combe, 1992), discolouration (Chevitarese
et al., 1962; Smith, 1962; Causton, 1978), high water sorption (Greener et al., 1972;
Paffenbarger and Rupp, i 973) and poor adhesion to vinyl polymers, resulting in poor
bonding with the synthetic teeth (Sinìth, 1962).
7
LITERATURE SURVEY
2.1.7 Polystyrenes
High-impact polystyrene, Jectron, was introduced in 1951 for denture-base
construction. It showed lower water sorptìon than other denture base-materials
(Woelfel
et ciL, 1963; Brauer,
1966) but numerous upper midline fractures were
reported (Woelfel, 1971 ; Smith, 1973). Also Jectron does not reproduce the
impression surface as faithfully as does poly(methyl methacrylate) and this makes it
less useful.
2.1.8 Nylon
Nylon was first used in the construction of denture bases in the 1950s
(Matthews and Smìth, 1955; Watt, 1955). Nylon displayed considerable strength
(Anderson et al., 1974) but was unsatisfactory for denture bases because of its high
moulding shrinkage, high water absorption and discolouration (Smith, 1962; Combe
and Grant, 1973; Combe, 1992), high flexibility and poor chemical union with the
teeth. The surface cannot be as highly polished as acrylic resin, and roughening leads
to plaque accumulation (Smith, 1957; Greener et al., 1972).
2.1.9 Polycarbonate
Polycarbonates can be synthesized from bisphenol A and diphenyl carbonate
(Brauer. 1 977a). Polycarbonates have higher impact resistance than poly(methyl
methacìylate) (Smith, 1962; Stafford and Smith, 1968b; Bikales, 1971; Combe and
LITERATURE SURVEY
Grant, 1973; McCabe and Wilson, 1974; Brauer, 1977a; Williams and Cunningham
1979) and good dimensional stability due to their low water sorption (Brauer, 1966;
Bates, 1 973 ; Williams and Cunningham, I 979). They also have high tensile strength,
high elastic modulus, high abrasion resistance and good colour stability (Wennstrom.
1965; Brauer, 1966; Stafford and
SIIIÌth,,
1967; Bikales, 1971).
Polycarbonates, however, were not widely used in dentistry because their use
requires the installation of complex instruments and because of the high incidence of
dentures cracking, owing to the stresses induced by the differential thermal shrinkage
rates of polymer and gypsum mould. Also, the use of polycarbonate is limited to
porcelain teeth because ofits required high moulding temperature (Brauer, 1966).
2.1.10 Poly(methyl methacrylate)
A more satisfactory plastic denture base material emerged in 1937, when Dr.
Walter Wright made known the results of his clinical evaluation of poly(methyl
methacrylate), known as Vernonite (Peyton, 1975).
PoIy(methyl methacrylate) is popular as denture-base material because ìt has
excellent esthetic properties, adequate strength, low water sorption, and low solubility
(Brauer, 1977a). It is bard, with a Knoop hardness number 18 to 20 (Phillips, 1996).
Its tensile strength is about 59MPa and its specific gravity is 1.19. The modulus of
LITEk4 TURF SUR VEY
elasticity is about 2.4GPa. The polymer starts to soften at 125°C and depolynierize at
200°C. At 450°C 90% of the polymer will have depolymerized to the monomer.
In addition, poly(methyl methacrylate) dentures are free from toxicity. can be
easily repaired, have the ability to reproduce accurately, and retain indefinitely the
details and dimensions of a pattern. Poly(methyl methacrylate) can be used with a
simple moulding and processing technique (Leong and Harcourt, I 974; Brauer,
1977a) for the construction of denture base. Poly(methyl methacrylate) is colourless
in its pure state and can be easily pigmented and characterized (Winkler, 1984).
The main deficiency of poly(methyl methacrylate) is its low tensile strength,
particularly under fatigue and impact condìtions, together with its high notch
sensitivity (Stafford and Smìth, 1968a; Hargreaves, 1971), high coefficient of thermal
expansion (Leong and Harcourt, I 974) and poor resistance to abrasion (Hargreaves,
197!).
Moreover the dimensìonal accuracy of a poly(methyl methacrylate) denture ìs
greatly affected by processing shrinkage and water sorption. Reported linear
shrinkage ranges between 0.26% and i .20% (Grunewald et al., 1952; Matthews
1952; Chen
aL, 1992;
et al.,
et aL,
1988; DaBreo and Herman, 1991; Dixon et aL, 1992; Huggett et
Cheng et al., 1993; Yeung et al., 1995; Sadamori et al., 1997) It has been
reported that typical dental acrylic resins show a water sorption of about O.5mass%
(Peyton arid Mann, 1942; Cheng et aL, 1993; Wong et
aL, 1999).
lo
LiTERATURE SURVEY
While many resins were introduced and subsequently withdrawn in the past 60
years, poly(methyl methacrylate) remains the most popular denture base material.
Attempts to improve its clinical performance have been made by modì1.'ing the resin.
The incorporation of low-molecular-mass styrene-butadiene rubber was found
to increase the impact strength and fatigue resistance of poly(methyl methacrylate)
while maintaining the elastic modulus (Stafford and Smith, I 970; Stafford and Lewis,
1980; Braden and Causton, 1979; Manley and Stonebanks, 1980; Winkler, 1984;
Rodford, 1990; Craig, 1991). The polybutadiene rubber has a molecular mass of about
30,000 and forms a copolyrner with methyl methacrylate in bead form (Cornell,
i 969). The beads form an interpenetrating network with poly(methyl methacrylate)
during polymerization (Stafford et ai, 1980). This increases the impact strength up to
0.133 and the elastic modulus to 2.3GPa, compared with about OE025J and 2-OGPa,
respectively, for traditional materials. The major disadvantage of polybutadiene
rubbers is their cost. They also have a higher diffusion coefficìent (Stafford and
Huggett, 1973; Stafford et al., 1983) and are technique sensitive.
The addition of fibres was also used to reinforce the acrylic resins. Carbon
fibres were found to be associated with increases in transverse strength, impact
resistance, tensìle strength and fatigue resìstance (Schreìber, 197 1 and 1974;
Wylegala, 1973; Manley et al, 1979; Manley and Stonebanks, 1980; Kimura, 1982).
The main disadvantage ofthe fibres was their unsìghtly appearance withìn the denture
and also the potential risk ofbìocompatibility ifexposed (Bowman et al., 1974). Glass
11
LITERATURE SUR VEY
fibres (Smith, 1957; Vallittu, 1996; Waltimo et al., 1999) and aramid fibres (Berrong
et al., 1990) were also tried but the same problems were encountered. Furthermore, a
reduced transverse strength was reported if the fibres were poorly prepared (Braden,
1966; Kobiltz et al., 1974; Stafford and Handley, 1975; Ilirabayashi et al., 1984;
Vallittu, 1997).
Polyethylene fibres were developed and incorporated into the denture base
resins in 1988 Polyethylene fibres were biocompatible (Rubin, 1983) and were
reported to increase in Young's modulus and strength in axial direction when drawn
at a temperature below their melting point (Capaccio and Ward, 1973). These fibres
were added into the dentures, which then showed an increase in toughness and
stiffliess and a reduction in water sorption, without deterioratìon in other properties
(Braden et aL, 1988; Ladizeskyetal., 1990 and 1994; Chowetal., 1992 and 1993).
In the past, swallowing of fragments of fractured dentures was occasionally
encountered. To be able to locate inspirated or swallowed fragments in patients' upper
respiratory or digestive tracts, researchers attempted to incorporate the resins with
radiopaque substances such as silver alloy, lead acetate, finely ground gold, gold leaf,
magnesium oxide, bismuth subcarbonate, barium sulphate, poiy(barium acrylate), and
barium fluoride (Chandler et al., 1971; Combe, 1971 and 1972; Stafford and.
MacCulloch, 1971; McCabe and Wilson, 1976; Winkler, 1984). Ten to 15% bismuth
or uranyl salts (BiCl3, BiBr3, UO2(NO3)2) or 35% Zirconyl dimethylerylate were
found to produce a satisfactory radiopacity (Rawls et al., 1990) but they were not
12
LITERATURE SURVEY
used, especially in cold-cured acrylic resins because amìne in the monomer would
precipitate the metal ions present (Craìg i 991).
Esthetics is always an important factor in determining the success of a
prosthesìs. Colouring of the denture base was done by adding inorganic pigments:
cadmium suiphide, cadmium sulpho-seleiiide, iron oxide, zinc chromate and mercuric
suiphide. Titanium dioxide was most frequently used as ari opacifier (Winkler and
Vernon, 1978). The pìgments were added in very small proportions in the powder and
did not affect the mechanical properties of the resins. However, the resultant colour
varied because of variations in both the preparations and the types of resin. In the late
1950s organic pìgments aimed at overcoming the problems of inorganic pigments
began to appear on the market. Although they could be used in smaller amounts and
in simplìfied procedures, they were unstable under ultraviolet light and affected by the
temperatures necessary for polymerization. hitemally coloured polymers were then
tried during polymerìzation but these failed because polymerizing monomer rejects
inorganic pigments.
Different fibres are also used to increase denture characterization. For
example, veined acrylìc resin contains short (3mm) fibres to simulate blood vessels of
the mucosa. Acetate rayon fibres were firstly used because they were completely
insoluble in monomer. However, they were capable of absorbing a considerable
amount ofmonomer, leading to changes in diameter and length.
13
LITERATURE SURVEY
When rayon fibres were exposed durìng services they absorbed the oral fluids
or were broken off leaving voids and trapping matters that discoloured. Therefore, the
characterization was lost within a short period of time and the capillary effect was
replaced by discolouration. Acrylic resin fibres showed negligible amount of swelling
and were compatible with the denture base resin. Nylon fibres were also used to
simulate the minute blood vessels of the oral mucosa (Quinhivan, 1975; Winkler arid
Vernon, 1978).
14
LITERATURE SURVEY
22 Types of Acrylic Resins
Different types of acrylic resins are developed for dìfferent clinical and
laboratory applications. They are usually supplìed as powder-liquid systems. The
powder (polymer) is mixed with the lìquìd (monomer). Acrylic dough is formed and
packed into the mould prior to polymerization of monomer (Winkler, i 984; Phillips,
i 996).
The liquid contains unpolymerized methyl methaciylate. It ìs clear and
transparent. It has a melting point of -48°C. boiling point of 1OO.3C (Ellis and
Carleton, 1936; Taylor, 1941), density of O.945g/mL at 20°C and the heat of
polymerization of 54.OkJ/mol. It exhibits a light vapour pressure and is ari excellent
organic solvent (O'Brien and Ryge, 1978).
Cross-linkage agents such as ethylene and butylene glycol dimethacrylate are
used to improve the craze resistance, solvent resistance (Mark, i 942; Jagger and
Huggett, 1990), heat resistance, hardness, and fatigue limit ofthe final poiymer (Fujii,
1989). These cross-linkage agents are chemically and structurally similar to methyl
methacrylate and therefore believed to be incorporated into growing chains.
However, it was reported that abrasion resistance and impact strength were not
significantly improved (Harrison et al., 1978). This was because the powder provided
by manufacturers was pre-fomied polymer, and only a small amount of cross-linkings
15
LITERATURE SURVEY
fanned during polymerization; thus most of the cross linkings could not incorporate
into the original polymer chain.
If pure methyl methacrylate is left standing it may polymerize very slowly
without the aid of initìators and activators, as free radicals and chance of collisions are
inevitably present. An inhibitor, such as hydroquinone or monomethyl ether of
hydroquinone is added to prevent premature polymerization, but some inhibitors may
affect the colour of the final product. Plasticizers such as dibutyl ptbalate are usually
added to the liquid to increase the solubility ofthe polymer.
The powder is composed mainly of poly(methyl methacrylate) beads that may
have been modified with small amounts of ethyl methacrylate or ethyl acrylate to
produce a softer ñnal product. Benzayl peroxide is added for the initiation of
polymerization. The thy, finely ground pigments are mechanically blended with the
polymer particles, or the polymers are internally coloured during their production in
the reactor kettle. Characteristic fibres are present to ìmprove the esthetics; acrylic
fibres are found to be more serviceable than rayon fibres (Winider and Vernon, 1978).
Also supplied is gel or plastic cake, in which monomer and polymer are
premixed and packed as soft, rubbery, one-unit cakes ready for immediate use.
Though their respective proportions are accurate and their mixing is thorough
(Winkler, 1964) their use is limited owing to their short shelf life, even when stored
under refrigeration (Winkler, 1984).
LITERA TUI SUR VEY
Conventional acrylic resin is polymerized by heat, and advances in technology
have seen the development of self-cured, lìght-cured and microwave-cured acrylic
resins.
2.2.1 Heat-cured Acrylic Resin
In heat-cured acrylic resin, benzoyl peroxide ìs commonly used to initiate
reaction because of its low cost. Molecules of benzoyl peroxide decompose.
producing pairs of free radicals. when heated above 60°C. Each radical rapidly attacks
one carbon atom of the double bond ìn methyl methacrylate, forming further free
radicals, inìtiaIing chain reaction.
CH3
R+CH2=C
r
R- CH -C
COOCH3
COOCH3
The chains propagate rapidly and terminate either by the coupling of two radicals
directly or by the Iransfer of a single hydrogen ion from one chain to another.
Benzoyl peroxide can also dissociate at room temperature but at a slow rate;
heat is usually required to achieve a sufficiently high rate of polymerization. Thermal
energy is supplied either by immersing a denture flask and flask carrier in a hot water
tank (compression mould technique and injection mould technique) or by microwaves
(Nishìì, 1968; Hashìmoto etal, 1968).
17
LITERATURE SURVEY
2.2.2 Cold-cured Acrylic Resin
In i 947, chemical activators that initiated polymerization at room temperature
were developed. Such acrylic resins are described as self-cured, cold-cured, or
autopolymerized. Tertiaiy amine such as NN-dimethy1-para-to1uidine is added to the
monomer and decomposes benzoyl peroxide to fonii sufficient free radicals on
mixing. The use of an Initiator system based on barbìturic acid is quite popular among
European manufacturers (Schaefer, 1970; Gross, 1976).
Cold-cured resins and heat-activated resins are quite sìniilar but the degree of
polymerization achieved in cold-cured resins is usually less. Therefore less
polymerization shrinkage is usually observed (Mowery et aL, 1958). The linear
shrinkage reported ranged from 0.01% to 0.43% (Skinner and Jones, 1955; Mowery et
al., 1958). Thus cold-cured acrylic resìns were commonly reported as having less
dimensional change and better adaptation to the cast than heat-cured systems
(McCracken, 1952; Woelfel
et
aL, 1960; Anthony and Peyton, 1962; Peyton and
Anthony, 1963). It is also possible that the lower polymerization temperature
introduces less strain in the dentures.
Cold-cured acrylic resin ìs usually associated with a higher level of residual
monomers. The unreacted monomers may decrease the transverse strength of the
denture base (Arab
et al.,
1989). They also lower the tensile strength, hardness,
stiffness and fatigue resistance (Smith, 1962; Honorez et aL, 1989), possibly due to
the plasticizing effect ofthe monomers.
18
LITERATURE SURVEY
Several reports in the literature stated that a hìgh level of resìdual monomers
was responsible for mucosal irritation (Bradford, 1948; Nyquist, 1952; Fisher, 1954
and 1956; Turrell, 1966; McCabe and Basker, 1976; Giunta and Zablotsky, 1976;
Austin and Basker, 1980; asker et al., 1978; Ali er al., 1986; Dogan
et aL 1995).
McCabe and Basker (1976) reported the residual monomer levels causing mucosal
irritation ranged between 0.186% and 0.233%. In Austin and Basker studies (1980),
the level ranged between 1.7% and 3.2%. On the other hand, Smith and Bains (1955)
and Woelfel (1 977) argued that residual monomers were unlikely to be the cause of
tissue irritation because most of them would be exiracted by water or salìva within the
first few hours of a denture's service.
It was also noted that the average expansion of autopolymerized dentures
during service owing to water sorption was slightly greater than that of heat-cured
dentures (Mowery et al. , i 958; Mirza, I 961).
Moreover, the presence of tertiary amìnes makes their colour stability inferior
to that of heat-actIvated acrylic resins, owing to the subsequent oxidation forming
coloured end products.
2.2.3 Microwave-cured Acrylic Resin
Microwaves are electromagnetic waves produced by a generator called a
magnetron (Rohren and Bulard, i 985). Methyl methacrylate molecules orient
19
LITERA TURE SUR VEY
themselves in the electromagnetic field of the microwaves - 2,45OIvIBz, as produced
in domestic ovens
and change their direction nearly 5 billion times a second. Heat
released during the resulting collisions ofthe molecules is adequate to break down the
benzoyl peroxide
molecules into
free radicals, initiating chain reactions (De Clerck,
1987).
The reported advantages of this method are shorter curing time (Reìtz et al.,
1985; Hayden, 1986; Levin
et aL, 1989), ease,
cleanliness, and substitution of the
heavy brass flask and water-bath curing tank by the lighter plastic flask and
mìcrowave oven (Nìshii, I 968, Hasbimoto et al., i 968). The physical properties of
microwave-cured resins were reported to be as good as those of heat-cured resins
(Nishii.
1968).
Lower
(Sanders
residual
et al.,
monomer levels (De Clerck, 1987) and reduced porosity
1987) were also shown. Some authors, however, continued to
encounter the problem of porosity when the sections were 2.5-3.0mm thick (Reitz
et
aL, 1985;Al-Doorietal., 1988).
In 1991, Bafíle et al. claimed to have reduced the mean porosity of
microwave-cured resin by processing with a special liquid (Micro Liquìd, H.D. Justi.,
Oxnard, Calif.). The liquid's formula has never been revealed by the manufacturer but
the authors have suggested that it may contain eìther a triethylene or tetraethylene
glycol, which are dìmethacrylates having
a reaetìve group on each end.
20
LITERATURE SURVEY
Dimethacrylates have low vapour pressure, which would allow processing at elevated
temperatures without porosity occurring.
Kimura et aL (1984) suggested that improvement in acbptatìon was the result
of the homogerious heating of the investing plaster and resin by the microwaves
causing fewer internal stresses to be introduced into the processed denture. On the
other hand, Shlosberg (1 989) concluded that the density, dimensional accuracy,
transverse strength, hardness, and residual-monomer levels of denture base resin
polymerized by microwave were not different from those of denture base resIn
polymerized by hot water.
Although most of the physical properties of microwave-cured resins are
comparable with those of heat-cured resins, microwave processing requires a
microwave oven, a special resin and unique non-metal flasks, which deteriorate more
rapidly than metal flasks (Rudd, 1996). The procedure is also technique sensitive to
porosity or insufficient curing of resin (Baille et al., 1991). Hence, the use of
microwave energy to cure denture resins is not readily accepted.
2.2.4 Light-cured Acrylic Resin
In 1985, Dentsply International Company developed visible-light-cured resin
denture base material, which it marketed under the trade naine Triad'. It is composed
of a matrix of urethane dimethacrylate, and small amounts of rnicrofine silica to
21
LITERATURE SURVEY
control rheology. The filler consìsts of varying sizes of acrylic-resin beads, which
become part of an interpenetrating polymer network structure when cured. It is
supplied in theet and rope forms, reducìng the risk of air bubbles being trapped during
mixing, and is packed in light-proofpouches to prevent premature polymerization.
Visible light, usually the shorter blue 400 to 500 nanometer wavelength, is the
activator. and eamphorquinone is the initiator. Although visible-light-cured resin had
comparable transverse strength, hardness, rigidity, nontoxicity with heat-cured acrylic
resins, its susceptibility to staining and brittleness and its low-impact resistance
limited its use (Ogle
et
aL, 1986; Khan et al., 1987; Al-Mulla
et al.,
1988). Light-
cured dimethacrylate denture material was reported to absorb water at a much slower
rate than heat-cured acrylic resìn, but had a higher percentage of water uptake (AlMulla et al., 1989).
22
LITERATURE SURVEY
23 Processing Methods
Along with the continual evolution of denture base materials, different
processing methods were developed, not only to cope with the changes in materials
but also to improve the efficiency and properties ofthe final dentures.
2.3.1 Compression-moulding Technique
Traditionally, acrylic resin dentures are processed in brass denture flasks by
compression moulding the acrylic resin in its dough state. The flasks are placed in a
temperature-controlled water bath for a specified time to permit resin polymerization.
Physical properties are generally believed to be related to the curing time and
temperature provided and possibly the stresses accumulated (Stanford et al., 1955;
Rupp et aL, 1957; Lorton and Phillìps, 1979; Jerolimov et al., 1991). The rate of
cooling also has a significant effect on dìmensìonal accuracy. Warpage and larger
shrinkage were reported when the dentures were quenched in water (Chen et al.,
1988; YeungetaL, 1995).
2.3.1.1 Long Curing Cycle without Terminal Boil
A long curing cycle of 7 to 14h at around 70°C ìs used to cure the acrylic
(Harrison and Huggett, 1992; Vallittu et al., 1998). It ìs believed that the relatively
low curing temperature prevents boiling of the monomer during the exothermic
23
LITERA TUPE SUR VEY
reaction. The reaction is slow, because fewer radicals are produced and therefore the
process must be prolonged to achieve a high degree of polymerization.
Unfortunately, a relatively high content of residual monomer was found ìn
dentures cured at 70°C for 9h (Vallittu et aL , i 998). The author suggested that this
was due to the relatively low polymerization temperature. The low temperature,
which is below the glass transition temperature of the polymer, not only produced
fewer radicals but also caused lower molecular chain motions for polymerization. The
rate of free-radicals formation and the rate of polymerization were higher at 100°C
than at 70°C.
When the terminal boil was omitted in the curing cycles a lower glass
transition temperature of the denture was reported (Jerolimov et al., 1991) and
reduced transverse bend strength and surface hardness were found (Arab et al. , i 989).
It is probable that the plasticizing effect of the high levels of residual monomers
effectively reduced interchain forces, leading to greater deformation under loading.
2.3.1.2 Long Curing Cycle with Terminal Boil
A two-stage curing cycle of 7h at 70°C followed by a terminal boil for 1h at
100CC was suggested and found to produce the best indentation strength and tensile
strength (Jagger, i 978). This cycle was also shown to
have an optimum level of
polymerization and to avoid the risk ofporosity even in thick sections (Hathson. and
24
LITERATURE SURVEY
Huggett, i 992). This cycle allows near-absolute polymerization at 70°C and removal
ofmost ofthe residual monomer at 100°C. Using this cycle, residual monomer levels
ranged between 0.54 and 1.08% among different brands ofactylic resins.
The dimensional accuracy of the denture base was reported to be adversely
affected by the terminal boil but the effect was too small to be clinically significant
(Huggett et al., 1984).
2.3.1.3 Short Cycle
Curing the acrylic resin by boiling the dough for 30mm to 2k Is sometimes
recommended by the manufacturers and commonly done in commercial laboratories
owing to time and cost savings. However, consìderably higher levels of residual
monomer, up to 20%, were demonstrated (Austin and Basker, 1982; Harrison and
Huggett, 1992; Vallittu et aL, 1998). It was also claimed that, in short cycles
there
was a. potential risk of porosity in the thick sectìons of denture bases, owing to
possible high exothermic reaction (Jerolìmov et al. 1989a; Harrison and Huggett,
I 992).
Moreover, McCartney (1984) showed a 25% increase in the amount of palatal
base distortion space and a 50% increase m induced malocclusion. Quenching the
flask immediately after processing is contraindicated owing to the large stress and
distortion introduced. A bench cool is usually recommended.
25
LITERA TUILE SURVEY
2.3.2 Inj ection-moulding Technique
A specially designed flask is used. It can be placed into a carrier that maintains
pressure on the assembly during resin introduction and processing. Thermoplastic
resin is softened by heat and, while still hot, injected ìnto the mould. The resin is
allowed to cool and solidify. In the case of powder-Iìquid resin, the components are
mixed and injected into the mould at room temperature. The flask is then placed in a
water bath for polymerization. Powder-lìquid resin can also be supplied mechanically
premixed and packaged in capsule form. This premixing is an attempt to produce a
more evenly mixed, homogenous denture base (Trage, 1980).
The injection-moulding technique can be used either for beat-cured or selfcured acrylic resins and was first introduced in 1942 by Pryor, who claimed that it had
the advantages of eliminating flash and the consequent "raised bite" improving
dimensional form and reducing porosity. The injection moulding technique was
demonstrated to cause statistically sinìficant less contraction than the compression
moulding method (Huggett et ai, 1992). Furthennore it reduced production time and
cost. Reduced skin contact and monomer-vapour inhalation are other suggested
advantages.
2.3.3 Pour Resin Technique
Pour resin technique was first developed in
1955, and
generally accomplished
using cold-cured acrylic resins and hydrocolloid moulds. The basic principle of pour
26
LITERATURE SURVEY
resin is similar to that of the ìnjection-moulding technique. The wax pattern of the
final denture ìs placed in the specially designed flask, which is then filled with
hydrocolloid. Cold-cured acrylic is preferred for this technique because the
hydrocolloid is easily destroyed by high temperature.
The main advantages of the pour resin technique are the elimìnation of stress
and increase in aeclusal vertical dimensìon commonly found when using the
compression-moulding technique, and time saving.
A reduction in the occlusa! vertical dimension was sometimes found because
of shrinkage in the resin during polyrnerizatìon. Antonopoulos (1978) found that fluid
resin underwent more curing shrinkage (0.617%) than did heat-cured resin (0.377%).
Tooth movement occasionally occurs when the flasks vibrate during assembly
(Winkler, 1984). Voids or porosity in the resin can occur owing to ìncorrect spruce
positioning or ifthe pouring resin is too viscous.
In a four-year follow up examination of patients with complete dentures
processed ìn either pour resin or heat-activated acrylic
resine
the pour resin denture
bases were poorer in mechanical properties, leading to fractures and increased wear
(Trudso et aL , I 980). Dislodgment of teeth was also encountered because of the heavy
anterior tooth contact resulting from curing shrinkage ofthe resin (Winkler, 1984).
27
LITERATURE SU1VEY
2.3.4 Microwave Curing Cycle
The microwave oven used is usually in 550W power and 2,450MHz with
12cm wavelength (Ilbay eÍ
al.,
1994). A teflon flask (polytetrafluoroethylene) is used
instead of metal because microwaves are reflected by metal surfaces (IDe Clerck,
1987; Kimura et al., 1983; Reìtz et al., 1985). Continuous pressure is applied by
tightening the teflon screws on the flask.
In respect of the quality of cured resin, in terms of colour, texture and
porosity, Kimura
et al.
(1983) reported that the best result was obtained in 3mm of
curing at 550W. This was supported by Ilbay studies (1994), which found that the
Vickers hardness of microwave-cuied resin (22.46VKN)
is comparable with
that of
heat-cured resin (21VKN) (Wilson et al., 1987).
Hayden (1 986) stated that curing occurred in 4mm and that no curing occurred
at low power. On
the other
hand, Reitz
et al.
(1985) showed that curing took 5mm at
high power and 13mm at low power, 90W only. The time was reduced to 10mm if the
power was
raised to 113W (ilbay et al., 1994). De Clerck (1987) reported that
acceptable curing was obtained in 8mm at high power.
No specific porosìty was observed in mìcrowave-cured resin (Kimura et al.,
1983; Reitz et al., 1985; Ilbay et al., 1994). It has been stated that choice ofa suitable
acrylic resin, current power and polymerization time is important in order to reduce
porosìty to aminimurn level (Levin et ai, 1989; Sanders et al., 1987).
28
LITERATURE SURVEY
2.3.5 Light Curing
The curing unit for light-cured resin emits high-intensity collimated shielded
light from quartz halogen lamps. The light is concenirated in the shorter blue 400 to
500nm wavelength spectrum ofvisible light (Triad VLC system). High-intensity light
results in deep polymerization of the material to a depth of 5 to 6mm. The unit
contains a rotating platform that is elevated to a chamber where the soft uncured resin
ìs polymerìzed (Ogle et al., 1986). Applying an air-bather coating before fInal
polymerization can prevent air inhibition ofthe surface layer.
LITERATURE SURVEY
2.4 Porosity
Porosity is one ofthe undesirable characteristics ofpoly(methyl methacrylate).
Its presence in denture base is often attributed to the presence of residual monomer
(Beech, 1975; Jagger and Huggett, 1975). Porosity wìIl result in high internal stress
and make the dentures vulnerable to distortion and warpage (Craig, 1980).
The mechanical properties are adversely affected by porosity (Firtell and
Harman, 1983; Chee et al, 1988; Honorez et al. , 1 989). The tensile strength of porous
acrylic resin was reported to be one sixth to one eighth that of dense poly(niethyl
methacrylate) (Gettleman et
aL, 1977).
Harbouring of organisms, retention of fluids, staining and calculus deposits are
common problems caused by surface porosity (Tuckfield
et al.,
1943; Davenport,
1970, 1972). The accumulation offood debris and plaque ìn the voids Inevitably leads
to foul odour (Firtell and Harman, 1983).
Porosity can be classified as gaseous porosity, contraction porosity arid
porosity caused by inadequate homogenicity of the dough (Osborne, 1942; Thckfield
et al., 1943; Phillips, 1996)
30
LITERATURE SURVEY
2.4.1 Gaseous Porosity
Gaseous porosity results from the vaporization of unreacted monomer and
low-molecule-mass polymers. when the temperatnre of a resin reaches or surpasses its
boiling point (1003°C) (Taylor, 1941; Tucklield eÍ al., 1943; Peyton, 1950; Atkinson
and Dennis, 1959; Atkinson and Grant, 1966; Faraj and Ellis, 1979). Gaseous porosity
is usually nearer the centre of the mass, whereas the non-porous section is in the
periphery, as heat ìs conducted away (Phillips, 1996; Greene et aL, 1972). Gaseous
porosity usually appears in regular shape (Osborne and Wìlson, 1970; Phillips, 1996)
and the affected area is smallest in comparison with the other two types of porosity
(Wolfaardt et al., 1986). Bubble size ranges between 1.92 x 102 to 4.23 x 102
_;
average diameter is 2.9 x lOmm (Faraj and Ellis, 1979).
It is generally believed that
the
exothermie heat during polymerization is the
major factor causing gaseous porosity (Peyton, 1950; Faraj and Ellis, 1979). The
maximum exothermic temperature reached during curing of acrylic denthre base
depends on the composition of the powder and the liquid, the powder/liquid ratio
used, the size and geometry of the test piece or denture, the thermal conductivìty of
the mould and the heat-transfer medium in wbìch the mould is placed (Paraj and Ellis,
1979; Gay and King, 1979; Firtell and Harman, 1983; Wolfaardt et al., 1986;
Jerolimov etal., 1989a and 1989b).
31
LITERATURE SURVEY
2.4.2 Contraction Porosity
Contraction porosity is produced as a result of inadequate pressure or
insufficient material in the mould during polymerization (Phillips, I 996). Packing the
resins at the sandy or stringy stages also produces contraction porosity. The large
amount of monomer present between the polymer particles the resin will flow too
readily out of the mould, which then becomes under-packed (Mutlu et al., i 992).
The voids of contraction porosity may occur locally or may be dispersed
throughout the denture and. unlìke gaseous porosity, they can occur in thin sections
(Tuckfleld et al., 1943; Phillips, 1996). Contraction porosìty is usually irregularly
shaped and the affected area is of intermediate size, compared with the other two
types ofporosìty (Wolfaardt et aL, 1986).
2.4.3 Porosity Caused by Inadequate Homogenicity of the Dough
Inadequate mixing of powder and liquid, or air inclusion, will lead to some
regions having more monomer than others and thus shrink more. Such localized
shrinkage tends to produce voids. This kind of porosity occurs either on surface or
subsurface locations. It is irregular in shape and usually in larger
size than
the other
two types of porosity (Wolfaardt et al., 1986). Occurrences of this type of porosity
can be minimized by ensuring proper polymer-to-monomer ratio, well-controlled
mixing procedures and delaying packing until the dough stage is reached (Phillips,
1996).
32
LITERATURE SURVEY
2.5 Temperature Control during Processing
In order to improve the working effcìency and mechanìcal properties of
denture base without boiling the monomer during processing, nmerous studies were
conducted to investigate the optimal temperature for the curIng cycle (Harnian and
Pittsburgh, 1949; Atkinson and Dennis, 1959; Jagger, 1978; Harrison and Huggett,
1992;
VallittuetaL,
1998).
The polymerization reactìon of poly(methyl methacrylate) is an exothennie
reaction. The temperature ñse was first demonstrated by Tuckfield et al. in 1943.
They found a peak of temperature occurring in a one-inch cube specimen when the
temperature in the centre of the acrylic cube reached approximately 73°C. The
maximum temperature recorded was almost 140°C, which may be sufficient to cause
boiling of free monomer and thus lead to the formation of porosity. A temperatare
peak during processing was also demonstrated by other researchers (Hannan and
Pittsburgh 1949; Aticinson and Grant,
1966; Faraj and Ellis,
1979).
It has been determined that methyl methacrylate polymerizes readily at a
temperature of 70 to 75°C (Tylman and Peyton, 1946) when a common catalyst,
benzoyl peroxide, is used. The justification. for using low temperature with a longer
period (7 to 14h) was based on the critical polymerization temperature together with
the exothermie heat liberated during the polymerization process. Peyton (1950) stated
that a greater tendency toward porosity was found when the acrylic resin was cured
above 75CC.
33
LITERA TU1tE SURVEY
This cycle was supported by Hai-man and Pittsburgh (1949). They reported
that using a 3mm thick, 60 x 65mm specimen plate under extended constant cure at
71°C caused a continuous change in the mechanical properties of the resin produced.
Both stiffliess and breaking load increased in the same order as the increase in curing
time when 2.5, 5.5, 9, 18, 24 48 and 72h were used. Those cured for 9h or more
showed acceptable mechanical properties according to ADA specification No.12.
Those cured for 48h and 72h demonstrated a lack of flexìbility, which made the
dentures brittle. Thus Harman and Pittsburgh suggested that 9h at 71ÓC was the best
cycle.
Allowing polymerization to take place at a slower rate without causing large
exothermic heat is an effective way ofminimizing porosity (Jerolimov et al., 1989a).
FIrtell et aL (1981), however, noted that although low processing temperature could
reduce porosity, it produced a weakened material because of the formation of shorter
molecular chains of the resìn, and a higher level ofresidual monomer.
In 1958 Smith studied the residual monomer concentratìon in different curing
cycles. The residual monomer was found to lower the transverse strength, tensile
strength, hardness, stiffness and fatigue resistance of the denture (Smith, 1962;
Honorez et al. 1989; Arab et al., 1989). Smith found that 80 to lOOh were needed for
the residual monomer concentrations to reach a limiting value at 70°C. This was
impractìcal and complete polymerization was considered to be impossible. He then
suggested a curing cycle of 16 to 24h at 7OC followed by bench cool (Smith, 1959).
34
LITERA TITRE SUR VEY
A two-stage curìng cycle was developed to overcome the problems of residual
monomer without causing porosity. The resin is cured at a lower temperature until
most of the
monomers convert to polymer, and followed by a terminal boil to remove
the unreacted monomer. Tuckfield et al. (1943) found a satisfactory curing cycle was
65.5°C for l.Sh followed by raising to the boil at IOOÖC for O.5h.
Harman and Pittsburgh (1949) attempted to find a shorter cycle. They found
that a curing cycle of 71°C for l.25h and boiling for another half-hour was a good
cycle to preclude bubbles in thick sections, and provide the necessary strength in thin
portions.
Jagger (1978) suggested that 7h at 70°C and lii at 100°C could be used. He
explained that the initial low temperature of 70°C ensured that porosìty was avoided
and hìgh molecular mass was obtained. The residual monomer was then removed
quickly in the terminal boil. A curing cycle of 7h at 70°C plus a terminal boil
producing a porosity-free denture irrespective of thickness was also shown (Hathson
and Huggett, I 992).
In addition Clarke et al. (1992) showed that the temperature changes inside a
denture base with embedded thermocouples followed closely the cuÑig cycle of the
water bath; the variation is not more than 2°C. Yeung
et aL (1995)
had also shown
that a temperature spìke was absent if a slow heating cycle (6.5h at 72°C and then
LITERATURE SURVEY
30mm
at 100°C) was used. Therefore monomer is uriJikely to boil in the long curing
cycle with terminal boil.
Apart from temperature control, it was also noted that the maximum
temperature reached in the processing cycle is also affected by the geometry of the
denture base and the type ofpoly(methyl methzcrylate) used. The peak temperature of
polymerization was reduced by decreasing the thickness of the denture base and by
the substitution of higher-molecular-mass polymer of methacrylates for poly(methyl
methacrylate) (Brauer et al. , 1986; Vallittu, i 996).
LITERATURE SURVEY
2.6 Pressure Control during Processing
According to Faraj and Ellis (1979) porosity was not found in acrylic discs
(50mm in diameter and 8mm thick) heat-cured at 7OC provided that sufficient
clamping pressure was used and the flask was fully packed. They suggested avoiding
porosity either by processing initially at a maximum temperature of 70°C or
increasing the clampìng pressure. This was suppotied by other authors (Atkinson and
Dennis,
1959;
Keller et al., 1978; Keng et al., 1979; Berg and Gjerdet, 1985; Chee et
aL, 1988).
Pressure is generally believed to be necessary for resin to flow into, fill and
conform to the shape of the mould (O'Brien arid Ryge, 1978; Faraj and Ellis, 1979;
Mutlu et al., 1992 and 1994). However, few studies have been done to investigate the
optimal pressure required and pressure changes during processing.
In 1941, Taylor studied pressure caused by the thennal expansion of acrylic
during processing and believed that pressure applied could overcome a portion of the
polymerization shrinkage. The experiment was done by confining the material in a
cylìnder and following its dimensional changes by the movement of a piston upon
which was maintained a given load. As the resin contracted and expanded,, the
platform of the testing machine moved up and down, and this movement was
recorded. The resin dough. was then packed into the mould at room temperature with a
pressure of 1 3øpsi (9. 1 atm) and then heated, the pressure required to compress the
resin back to its original volume during the processing was measured. It was
37
LITERA T1JRE SUR VEY
expressed in pounds per cubic inch. The maximum pressure found was close to 6OOO
pounds per cubic inch (29.4atm). However, this method was criticized because of the
fact that the resin tended to stick to the sìdes of the mould and a fin of the resin was
forced up between the piston and the cylinder, making the whole setup less sensitive
to pressure change.
Taylor then suggested that a compressive stress of 6,000 pounds per cubìc
inch (29.4atm) employed at the time that polymerization commenced could
compensate part of the shrinkage of the resin. High pressure is objected because the
teeth are prone to displacement.
Woelfel (1977) stated that no more than 1,500psi (lOSatm) should be used for
trial closures and only 3,500ps1 (245atm) should be used for final closure. These
pressures were adopted (Shlosberg et al. , I 989; Nelson et al. , i 99 1). Becker et al.
(1977) used higher pressures - 2,500 to 3,SOOpsi (175 to 245atm) for trial closures
and 4,SOOpsi (3 1 5atm) for final closures.
Xia
et al. (1996)
showed that the acrylic resìn cured at 120°C in a pressure
cooker with 6kg'cm2 (5 .8atm) did not have any porosity. They suggested that the
boiling point ofmonomer under this pressure was raised to 168.9°C and that therefore
it would not boil. On the other hand, Keng et ai. (1979) encountered porosity when
boiling acrylic resin of thicknesses of 25mm and 5mm, respectively, for i 5mm. The
IITERATUPE SURVEY
pressure they
applied on the flasks was 50kg/cm2
(48.5atm) in the final closure and
the flasks were transferred to sprìng clamp during curing.
Since there ìs no standard pressure suggested, the main criterion used by most
authors is to obtain metal-to-metal contact between the two halves of the flask under
hydraulic
pressure during final closure (Peyton, 1950; Faraj and ElIìs, 1979; Morrow
et al., 1986). To maintain the pressure during processing, the flasks arc transferred to
the spring clamp, which ìs then only hand tightened (Becker et al., 1977; Sowter and
Barton, 1986; Rudd, 1996; Phillips, 1996).
All the above-discussed pressures are the external
pressures applied on the
flasks; a higher pressure in the acrylic dough is expected during processing. This is
because pressure builds up due to thermal expansìon as the acrylic is completely
confined inside the mould (Taylor, 1941). To date the amount of pressure developed
in the acrylic dough is still unknown.
LITERATURE SUR VEY
2.7 Elevation of Boiling Point
The boiling temperature of a substance is the temperature at which ìts vapour
pressure is equal to the ambient pressure (Moore, 1972). A relation between the
changes in pressure and temperature for vapour and liquid phases that remain in
equilibrium when either variable is modified can be expressed by the Clapeyron
equation. This equation was first proposed by the French engineer Clapeyron in 1834
(Williams and Williams, 1973). The equation contains three variables, pressure (p),
volume (V), and temperature (T)
dpidT=
where
tr
Si
V
S is the entropy change and t
V is the volume change for the
phase transition.
Clausius niodìfied this equation about 30 years later. The Clausius-Clapeyron
equation can be regarded as an equation for the temperature variation of the vapour
pressure of the liquid. That ìs because the equation contains only two variables,
pressure and temperature provided that the heat of vaporization is assumed constant
over the temperature and pressure range of interest. This equation can be transformed
to
ptpeX
wheH/R(1/T'-1/T)
40
LITERATURE SURVEY
In this expression,
vap
H is the heat of vaporization, R is the gas constant, p is
the vapour pressure at a temperature T, and p' is the vapour pressure at T.
According to Clapeyron-Clausius equation, it is logical to expect that the
boiling point of the monomer will be elevated under pressure. Thus, the elevated
boiling point of monomer in the acrylic dough, if the pressure is measured, can be
calculated by the following equation:
-H/R(1!T2- l/T)
1nP2TP1
where
2
S
the pressure measured during processing,
P1
is the atmospheric
pressure, H is the heat of vaporization of methyl methacrylate (37.5kJ/mol), R is the
gas constant(8.314J/Kmol),
T2
is the elevated boiling point at P2 and T1 is the boiling
point at Pi (atmospheric pressure).
Unfortunately, the pressure applied on the resIn is still not clearly defined in
the literature. Therefore the level to which the boiling point is elevated and whether
the monomer will boil are still questionable.
To summarize, denture base materials have been under constant development
in order to improve their strength, ease of manipulation, esthetic qualìty arid patients'
comfort. Acrylic resin remains the most popular material used for consiructing
41
LITERATURE SURVEY
dentures 60 years after its introduction to
the dental profession.
performance, however, is sensìtìve to manipulation techniques.
compression moulding and heat curing techniques is
a widely
Its
clinical
Combining
accepted processing
method because a superior material can be obtained. Processing defects such as
porosity are encountered occasionally and usually require careful attention to control
temperature. On the other hand, the boiling of monomer is believed to be affected by
the pressure applied. There is insufficient data on pressure during processìng and this
issue requires further investigation.
OBJECTI VES
3. Objectives of the Study
Because the gaseous porosìty caused when the monomer boils durIng
processìng has detrimental effects on the mechanical properties and esthetics of the
final dentures and encourages plaque accumulation during service, temperature during
processing cycles has been well studied and controlled, to prevent boiling of the
monomer. In the compression-moulding technique the boiling point of monomer will
be raìsed when the resin is under pressure. Whether the monomer will actually
vaporize insIde the flask then becomes questionable. The pressure mentioned in most
studies in the literature, as discussed earlier, does not ìndicate the true pressure inside
the acrylic dough. Therefore there is no data on the internal pressure of the acrylic
resin during processing.
This study aims:
i)
to measure the changes in pressure and temperature of acrylic resin dunng
processing;
iì)
to record the highest temperature reached by the acrylic resin when fast
cured in boiling water;
iii)
to calculate the elevated boiling point ofmonomer under high pressure.
43
VÍATERJALS AND METHODS
4. Materials and Methods
In thìs study, a subminiature pressure lransducer and a thermocouple were
placed into the palate of an acrylic denture base. The pressure and the temperature
changes during the packing and curing cycle were recorded simultaneously. Two
curing cycles were investigated: a long cycle with a terminal boiJ, and a fast cycle. A
thermocouple was placed into the water bath to monitor the temperature of the curing
cycle.
4,1 Materials
4.1.1 Pressure Transducer
A subminiature pressure transducer (model S-1000 psig, Sensotec, Ohio,
USA) (Fig. 4.1) was placed in the centre of the palate to measure pressure. It
consisted of a diaphragm at one end and temperature-sensitive electronic components
at the other end, at which the cable is connected. The main advantage is the
transduce?s small size (16.5 1mm in diameter and 19.05mm in length). It can be fitted
into the denture flask and occupy only a small volume ofthe acrylic dough.
The diaphragm is made of stainless steel and is 9.53mm in diameter. It ìs
unitized with the sidewalls, that is, both are made from a single piece of stainless
steel. Although the diaphragm is rugged ìt can be made thin enough to measure low
pressure. The flush diaphragm detects pressure changes in the environment, fluids,
44
MATEJUALSAND METIIOI)S
gases and semi-solids, and gives a voltage output. The overload safety is up to 50%
over its capacity. Compared with the semiconductor type oftransducer, this foil gauge
transducer is less affected by thermal changes and provides high, long-term stability.
The transducer is temperature compensated by the temperature-sensitive
electronic components located inside the end portion. According to the manufacturer,
the temperature compensation ranges from 2OO to 2OOF (-2&89° to 93.33°C).
Nevertheless the dìaphragm can operate under a wider temperature range of -65° to
3OO'F (54Ó to 150°C) without damage, although it is not temperature compensated.
The 3m-long submersible cable
attached to
the transducer terminated in 4
twisted leads.
The transducer was calibrated by the manufacturer as follows: capacity of
l000psi (68.O5atm), maximum excitation voltage of SV, calibration ratio of
2.438OmV/V
Special
care was required to prevent damaging the thin diaphragm,
overloading the transducer and overheating the electronic components. Care was also
needed to prevent damage to the cable through scratching and tearing ofthe coating.
A brass sleeve (Fig. 4.2) was made for the transducer to protect it from
damage by the stone during flasking, processing and deflasking. Notches were made
45
MA TEPJALS AND METHODS
on ìts outer surface to increase interlocking with the stone. In order to prevent acrylic
resin from getting ìnto the screw space between the head of the transducer and the
internal threads of the sleeve during trIal packs and processing. the space was sealed
with putty (Lasticomp, Kettenbach Germany). To facilitate removal after processing
no catalyst was added to the putty. The head ofthe sleeve together with the diaphragm
was then wrapped and sealed with microwave wrap (Glad microwave wrap, First
Brands Corporation, Danbury, CT, USA). The microwave wrap is about 0.005mm
thick and capable oftolerating temperatures of-60° to about 140°C. It was used to
prevent the acrylic resin from adhering to the diaphragm and to facilìtate deflasking.
4.1.2 Thermocouple
Two K-type thermocouples (Fig. 4.3) each 0.25mm in diameter and sheathed
with Kapton (TC Limìted, Uxbridge) Middlesex, UK) were used. Each thermocouple
was w&ded with a Vulcan 3a Welder (Orthomax Dental Ltd., Bradford, West
Yorkshire, UK). One thermocouple was placed in the centre of the acrylic dough just
below the area of dìaphragm. It was connected to a thermometer (KM 2013, KaneMay Instrumentation Ltd., Hertfordshire, UK), which converted the temperature
input
into voltage output (Appendix 1) for recordìng. The second thermocouple was placed
in the curing tank to monitor the water temperature.
46
MA TERL4LS AND METHODS
Fig.4. i Subminiature pressure transducer.
Fig.4.2 Transducer placed inside protective sleeve.
47
MATERIALS AND METHODS
4.1.3 Dynamic Strainmeter
Two dynamic strainmeters (DC-92D, Tokyo Sokki Kenkyajo, Tokyo, Japan)
(Fig. 4.4) were used as amplífíers, by which small voltage inputs were amplified for
improved recording. The strainmeters have a maximimi measuring range of 5OmV
and a voltage output of± 10.5V. The gain ofthe strainmeter is listed in Table. 4.1.
Table.4.l Range and gain of the strainmeter
Range
Gain
lOOxlOE6
Approx. 10,000 times
200xl0
5,000
500x106
2,000
lKx10&
1,000
2Kx10
500
5Kx106
200
The maximum voltage output ofthe subminiature pressure transducer is:
5Vx 2.4380mV1V
12.l9mV
As the maximum voltage output ofthe strainmeter is ±10.5V, the amplìfication ratio
is:
10.5V I 12.l9mV = 814
The closest amplification ratio provided by the strairimeter is i ,000.
MATERIALS AND METHODS
The voltae output of thermometer is l5O.2mV at lOO.2C (Appendix
9.1), the
amplification is:
10.5V / 150.2mV =70 times
Since the minimum gain of the strainmeter i 200 times, a custom-made resistance
circuit was constructed to give a resultant amplìfication of 50 times. With this
amplification, a temperature range ofü° to 140ÒC (Appendix 9.2) could be measured.
4.1 .4 Standard Maxillary Casts and Patterns
A room-temperature vulcanizing (RTV) silicone mould (Silastic E, Dow
Cornìng, Michigan, USA) (Fig. 4.5) was formed from a master edentulous maxillary
cast. Twelve maxillary casts were recovered in artificial stone (Kaffir D, British
Gypsum, Newark, Nottinghamshire, UK); powder-water ratio was 100g per 30ml
distilled water.
A wax pattern of a denture base was made on the master cast. The thickness of
the wax denture base in the palate was 2.5mm. Two wax spruces were attached to the
tuberosity regions. Another silicone mould was made over the wax base. A wax base
could be formed by pouring molten wax into the mould through the spruce holes.
The casts and wax-patterns (Fìg. 4.6) recovered from the moulds were of
standard size, shape and thickness and had no undercut.
49
MA TEPJALS AND
METHODS
Fig.4.3 Thermocouple and thermomeLer.
Fig.4.4 Strainmeters.
50
MATERIALS AND METHODS
Fig.4.5 RTV silicone inoulds of maxillary edentuous arch.
Fig4.6 Standardized wax pattern of coniplete denim e
base formed on duplicate cast.
Dl
MATERIALS AND METHODS
4.1.5 Modified Hanau Denture Flasks
Hanau denture flasks (Teledyne Hanau, Buffalo, NY, USA) (Fig. 4.7) were
used. The cast was flasked into the bottom halfofthe lower flask, which is shallower
than the top half. This arrangement has the advantage of having more room for the
placement of the transducer. A 3cm-diameter hole was drilled in the centre of the lid
of the ower flask and a i 5 x 8mm rectangular hole was drilled at the edge of the lid
of the upper flask to accommodate
the transducer with its
protective
sleeve, its cable
and the thermocouple.
4.1.6 Acrylic Resin
Maxillary acrylic denture bases were made of clear heat-cured acrylic resin
(Trevalon C, Dentsply Limited, De Trey Division, Weybridge, Surrey, UK). The
powder contains copolyrner ofbutyl (6%) and methyl methacrylate (94%), and 0.28%
benzoyl peroxide. The liquid contains 93% methyl methacrylate, 6% ethylene glycol
dirnethacrylate and 0.006% hydroquinone.
The curing cycles suggested by the manufacturer ìnclude short cycles of
boiling the resìn for 20mm and any type oflong curing cycles. A 10mm bench cool
before water cool is recommended for the short cycles.
52
MATERIALS AND METHODS
Fig4.7 Modified Hanau denture flasks.
Fig.4.8 Chart recorder.
53
MATERIALS AND METHODS
4.1.7 Curing Tank
A thermostatically controlled curing tank (Reco Dental, Wiesbaden, Germany)
was used and was programmed to cure the acrylic resin at 72°C for 6.5h and 92CC for
I .5h for the long cycle. lt was adjusted to keep the water boiling for 40mm during the
fast cycle.
4.1.8 Chart Recorder and Computer
A 12-channel hybrid chart recorder (AH series AHS2O-GNN, Chino, Japan)
(Fig. 4.8) was used. It is capable of recording temperature and pressure every 30s.
Owing to the limitations ofits speed, a computerized data acquisition program (Visual
Basic 4.0, Microsoft Corporation, Redmond, Washington, USA) was custom-made to
record the pressure and temperature of the resin at the high speed of once every
second. Thus the pressure and the temperature ofthe acrylic dough were monitored by
both the chart recorder and the computer in order to detect any rapid changes. The
second thermocouple, which had been placed in the curing tank to monitor the heat
supply, was connected to the chart recorder only.
54
MATERIALS AND METHODS
4.2 Methods
4.2.1 Preparations
4.2.1.1 Calibration of Strain Amplifiers
For measuring pressure, a stramnmeter was calibrated by
DC voltage
calibrator (model 2003, Time Electronìcs Ltd., Tonbridge, Kent, UK) to amplify the
subminiature pressure transducer output 1OOO times for improved recording.
The thermocouple in the acrylic dough was connected to a thermometer that
converted the temperatures measured into voltage signals. The second strainmeter in
conjunction with a custom-made resistance circuit, was calibrated to gìve a resultant
amplification ofthermometer signais by 50 times.
The thermostatically-controlled curing tank was calibrated to ensure that the
temperature did not go above 94°C in the long cycle.
4.2.1.2 Making the Maxillary Cast andthe Wax Pattern
The silicone moulds were cleaned with detergent and dried. A thin layer of
Aurofiim (Aurofiim, BEGO, Germany) was applied to the mould for the stone cast. It
was used as a wetting agent to obtain a smooth stone surface. 100g artificial stone was
vacuum (Model D, Whip-Mix CorporatìOn Louisville, Kentucky, USA) mixed with
30ml distilled water and then poured into the mould. When the stone was set, the cast
was transferred into the second mould for making the wax pattern. Molten rtiodeling
MATERJALSANI METHODS
wax was poured into the
mould
through
the spruce holes. The wax baseplate and the
cast were recovered after cooling of the modeling wax. The spi-aces were cut and the
surface was smoothed by rubbing with a tissue paper. A total of twelve samples were
prepared.
4.2.1.3 Flasking the Wax Pattern
The bases of maxillary casts were trimmed in order to give more room for the
transducer inside the flask. The thickness of the cast at
the palate was about i .5cm.
The cast was then flaskeci in the bottom halfofthe lower flask (Fìg. 4.9).
A thermocouple was embedded in middle of the palate and just beneath the
oral surface of the wax. The diaphragm of the transducer was fixed immediately over
the position ofthe thermocouple.
The top half of the flask was then filled with stone in 2 stages because it could
facilitate the
removal of the
transducer aÍer the experiment. Only half of the top
mould was filled in each stage. After the stone had set, the pattern was carefully de-
waxed with boiling water and detergent, without giving excessive heat to the
electronic components near the end of transducer.
MATERIALS AND METHODS
4.2.L4 Connection to the Chart Recorder and Computer
An empty upper flask was placed on the top of the lower flask (Fig. 4.10). The
end portion and the transducer cable together with the thermocouple wire were fed
through the base hole of the flask. The cable and the wIre were connected to their
respective amplification complexes, which were already connected to both the
computer and the chart recorder. The setup can be seen in Fig. 4. 1 i.
4.2.1.5 Preparations for the Fast Curing Cycle
A similar set up
cycle. The transducer
of the
acrylic
but without the transducer was prepared for the fast curing
was
omìtted because the fast cycle involved continuous heating
dough for 40mm
at
100°C, which is above the temperature
cornpensatable by the transducer.
4,2.2 Packing the Resin Dough
Two thin layers of sodium alginate solution (Separating fluid, Ivoclar AG,
Schuan, Liechtenstein) were applied to the stone with a soft brush and allowed to dry
completely.
20mg of heat-cured Trevalon C polymer powder was mixed with 9ml of
monomer liquid according to the manufacturer's instructions. The acrylic resin was
left undisturbed in a covered jar for i 5mm until the dough stage was achìeved. The
57
MA TEJUALS AN» METHODS
actylic dough was kneaded and then adapted to the mould. A small amount of resin
was put around the thermocouple. A sheet of polyethylene (Dentsply, DeTrey GmbH,
Weybridge, Surrey, UK) was placed between the upper and the lower halves of the
flask for trial packs.
Two trial packs were performed in a hydraulic press (Hydrofix, BEGO Bremer
Goldschlägerei With, Herbst Gmbh and Co., Bremen, Germany) (Fig. 4.12). The
force was gradually increased to 2OkN over a period of approximately
maintained at this force for
5mm
hmm
and
before removal of flash. The pressure and the
temperature changes were recorded.
The flasks were then transferred to the spring clamp. A 36Nm torque was
applied to the spring clamp by a torque bar (Fig. 4.13) in order to maintain the
pressure achieved by the hydraulic press at the final closure.
58
MATERL4LS AND METHODS
Fig.4.9 Flasking of the cast.
Fig.41O Assembly of the flask.
59
MATERIALS AND METHODS
To strain meter
Lead wire with temperature
sensitive components for temperature compensation
-=--
rL
---
Brass sleeve for
______ holding and protecting
the pressure transducer
Top half of mould
Screw for fixation
of the pressure
transducer
Pressure
¡a ph ragm
Bottom half
of mould
I.J';i,.J.J I
Position of
thermocouple
Fig.4J i Cross section of the set-up of transducer and thermocoup'e on
the denture base.
MATERIALS AND METHODS
Fig.4. 12 Trial pack under hydraulic press.
Fig.4.13 Torque applied.
61
MA TEPJALS AND METHODS
4.2.3 Curing Cycles
The clamped flasks were placed into the thermostatically controlled curing
tank. Two curing cycles were used: a long curing cycle arid a fast cycle. Six
samples
were used for each cycle.
4.2.3.1 Long Curing Cycle
A long curing cycle (72°C for 6.5h and 92°C for i .5h) was performed. The
flasks were left undisturbed in the curing tank for at least 36h to allow the acrylic
resin to cool slowly to the ambient temperature (24° to 28°C). Pressure and
temperature were recorded for the entire duration until
the flask
was opened.
4.2.3.2 Fast Curing Cycle
The clamped flasks were placed into the curing tank with boiling water
(100°C) for 40mm. They were then bench-cooled for 10mm and then water-cooled in
a water tank of22° - 24°C for a further 20mm.
4.3 Observation of Porosity
All the denture bases from both curing cycles were observed for gaseous
porosìty by visual inspection (International Organization Por Standardization 1567,
1988).
62
MATERJALÇ4ND METHODS
4.4 Elevation of Boiling Point
The ekvation of boiling point of methyl methacrylate was calculated by the
following equation:
InP2ÍP1 =-H/R(l/T2-l/T1)
where
P2 =
pressure measured,
P1
= atmospheric pressure, H - heat of vaporization
(37.5kJImolforMMA), R= gas constant (8.314J/Kmol), T2 =boilingpoint at P2 and
Ti = boiling poInt at P.
63
RESULTS
5. Results
5.1 Pressure Change during Packing
The pattern of pressure change during packing of the acrylic dough was
reproducible in all six samples (Table 5.1 and 52). A typical tracing is shown in
Fig.5J.
When the hydraulic pressure was increased, the pressure of the acrylic dough
increased from I atm. It reached a peak (25.2latm) when the hydraulic force was
increased up to 2OkN. The pressure then dropped rapidly. Ructuations of pressure
were recorded when the hydraulìc press was reactivated in order to maintain 2OkN.
The pressure returned to the atmospheric pressure when the hydraulic pressure was
released.
A similar pattern of pressure change was found ìn the second trial pack.
However, there was typically a higher peak (29.l7atm) arid the pressure dropped at a
slower rate than the first trial pack.
In the final closure, the pressure of the dough was lower than that of the trial
packs and the pressure was aimost constant throughout the five minutes. The average
maximum pressure was about l5atm. Only a small drop from the peak was recorded
and the pressure then became almost steady (12.38atm). When the flasks were spring
clamped to a torque of 36Nm, the pressure was about llatm, which was close to the
RESULTS
pressure achieved in the final closure.
Table3.1 Pressure of acrylic resin at the first and the second trial pack
Sample
First trial pack /attn
Second trial pack /alm
i
22.70
27.63
2
23.12
27.27
3
22.87
25.10
4
25.11
30.82
5
27.86
29.64
6
29.61
34.54
Mean
25.21(2.91)
29.17(3.34)
( ) = standard deviation
Table.5.2 Pressure of acrylic resin at the final closure and at the steady state
Sample
Peak pressure /atm
Steady State latin
I
16.77
13.82
2
13.63
8.89
3
18.40
15.73
4
17.78
15.60
5
12.44
8.89
6
12.84
11.35
Mean
15.32(2.64)
12.38(3.13)
( ) = standard deviation
65
40
35
30
25
20
15
10
o
0
5
10
15
20
25
Time I min
Fig.5.1 Typical pressura change during packing and clamping.
30
35
40
RESULTS
5.2 Temperature and Pressure Changes in Long Curing Cycle
Similar patterns of temperature and pressure changes of acrylic resin during
the long curing cycle were found in the six samples. A typical tracing is shown in Fig.
5.2. The recorded temperature of the acrylic dough during the processing cycles
followed closely the heat supplied and. no temperature spike was observed. The
temperature difference between the acrylic dough and the curing tank was about 1° to
2°C only.
The pressure of the acrylic dough in the clamped denture flask ranged from
7.07 to 14.78atm. When the curing cycle started the temperature and the pressure of
the acrylic dough increased continuously for 30mm. The pressure reached a peak
(17.35 to 27.l3atm) when the temperature was raised to 72°C. The temperature was
maintained at 72°C the pressure dropped gradually and was close to the atmospheric
pressure, I to I .2 latin, at its lowest pressure. The pressure of the resin remained low
until the terminal boil.
A tiny drop in pressure was recorded when
the temperature was raised from
72°C to 92°C. The pressure was almost steady when the temperature was kept at
92°C. The heater was turned off automatically after l.5h, the temperature of the
acrylic resin started to drop gradually, closely following that of the water. On the
contrary, the pressure increased slowly until a plateau ranging from 7.70 to i i .56atrn
was reached at ambient temperature (26° ± 2°C).
67
100
25
90
80
20
70
P60
15
E
&
10
40
30
20
10
o
O
10
20
30
40
50
60
70
Time I h
Fig5.2 Typlc& pressure and temperature changes during long curing.
80
RESULTS
5.3 Temperature Change in Fast Cycle
A typical tracing of the temperature of the acrylic resin is shown in Fig. 5.3.
When the flasks were placed into the curing tank with boìling water at I 00°C,
the temperature of the resin Increased rapidly from the ambient temperature (20° to
23°C). A temperature spike occurred around 25-30mm and reached a peak (1 19.17° to
136.34°C). The peak temperature recorded in the six samples are shown in Table 5.3.
Table.5.3 Peak temperature of acrylic resin during fast curing
Sample
Temperaturc/C
I
136.34
2
131.37
3
14.88
4
129.18
5
136.34
6
119.17
Mean
131.21(6.56)
( ) = standard deviation
The temperature then dropped to and stayed at 100°C. After boiling for 40mìn
the flasks were allowed to bench cool and the temperature dropped to 85°C. The
flasks were then cooled under water (22°C to 24°C) and a faster drop in temperature
was recorded (about 25°C in 10mm).
Pk 1i0C
140
120
100
C.)
8C
60
40
20
0
20
40
60
80
Time 1mm
Fig. 5.3 Typical temperature change of acrylic resin during fast curing.
Q
100
120
RESULTS
54 Elevation of Boiling Point during Processing
According to the pressures measured, the elevated boiling point of methyl
methacrylate was calculated. The results are shown in Table 5.4 to 5.6.
Table.5.4 Pressure of acrylic resin in damped denture flask at ambient temperature
(
)
Sample
Pressurelatm
Corresponding boiling pointl°C
1
13.60
202.89
2
7.07
172.22
3
14.78
207.01
4
14.31
205.45
5
8.47
180.29
6
10.92
192.14
Mean
11.53(3.23)
193.33(14.44)
standard deviation
Table.5.5 Peak pressure and the elevated boiling point
Sample
Pressure/attn
Corresponding boiling pointi°C
Temperature of resiri?C
I
20.90
225.44
68
2
17.35
215.42
70
3
22.67
229.95
70
4
27.13
240.21
69
5
24.21
233.73
72
6
19.77
222.40
72
Mean
22.01(3.45)
227.86(8.74)
70.17(1.60)
( ) - standard deviation
71
RESULTS
Table.5.6 Lowest pressure of the acrylic resin
Sample
Pressure/atm
Conesponding boiling pointl°C
I
LOO
100.3
2
1.00
100.3
3
1.00
100.3
4
1.21
106.27
5
1.04
101.51
6
1.00
100.3
Mean
1.04(0.08)
101.50(2.39)
( ) = standard deviation
The recorded pressure of the denture base at the end of the processing cycle is
shown in Table 5.7. Since the denture was hardened, the boiling point of methyl
methacrylate was not elevated.
Table.5.7 Pressure at the end of cycle
( ) - standard
Sample
Pressure/atm
i
7.74
2
11.40
3
11.56
4
10.03
5
9.57
6
7.70
Mean
9.67(1.69)
deviation, ambient temperature
= 24° to 28°C.
72
DISSCUSION
6. Dis cussion
6.1 Packing Pressure
The purpose of trial closure is to remove the excess resin and conform the
acrylic dough into the shape of mould (O'Brien and Ryge, 1978; Faraj and Ellis,
I 979, Mutlu
et aL , I
994). Pressure inside the acrylic dough depends on the magnitude
and the rate of pressure applìed, the amount of acrylic resin placed and the
consistency ofthe acrylic dough when packed.
In this experiment, a peak pressure ranging from 22.70 to 29.6latm (mean
25.2latm) was recorded in the frst
trial
pack under 2OkN hydraulic press but it
dropped rapidly because the excess resin was extruded out of the flask quickly.
When the monomer molecules had penetrated to the centre of the polymer
beads, entanglements were formed between the polymer chains. This would make the
acrylic dough viscous and stiff. A higher resistance force against the removal of
excess resin was developed inside the acrylic dough (O'Brien and Ryge, 1978;
Williams and Cunningham, 1979; Phillips, 1996). Therefore, a typically higher
pressure (29. i 7atm) was found in the second trial pack and the flow of resin out of the
flask was slower.
Most of the excess resin had been removed ìn the first two trial packs,
therefore in the final closure of the flask the pressure became almost steady. Since the
73
DISSCUSION
cross-sectional area of the denture flask is approximately 80cm2, under 2OkN
hydraulic press, the pressure actìng on the acrylic dough is calculated to be about
25atm. However, the recorded pressure of the aerylìc dough in the final closure at the
steady state (12.38atm) was well below this calculated pressure. Lt was probably due
to a relatively soft dough and a rigid gypsum mould in the metal flask. Thus the force
acting on the flask indicated by the hydraulic press might not be sufficient to reflect
the actual pressure of the acrylic dough.
In daily laboratory practice, the acrylic dough is usually cured under clamped
denture flask because ofthree suggested reasons (O'Brien and Ryge, 1978): to reduce
the effect of thermal expansion. to compensate for polymerization shrinkage and to
reduce vaporization of the monomer. The spring clamp is often hand-tightened with
or without the help of a hammer (Peyton, 1950; Faraj and Ellis, 1979; Seig, 1982;
Morrow ei al., 1986). Thus the actual amount of torque given to the spring is
subjected to individual differences and is not clearly known (Sowter and Barton,
1986; Phillips, 1996).
In this experiment, 36Nm torque applied on the spring clamp was shown to
give a pressure comparable with that of the final closure. Too little pressure might
lead to contraction voids and excessive pressure would lead to mould distortion and
teeth dìsplacement (Taylor, 1941; O'Brien and Ryge, 1978).
74
DISSCUSION
6.2 Temperature and Pressure Changes during Processing
6.2.1 Temperature
Changes
Polymerization of poly(methyl metliacrylate) is an exothermic reaction.
Temperature control has been well studied to prevent porosity formation and to
improve efficiency (Harman and Pìttsburgh, 1949; Atkìnson and Dennìs 1959;
Jagger, 1978; Harrison and Huggett, 1992; Vallittu
et aL, 1998). lt
has been suggested
that the peak temperature is affected by the volume of polymer, powder-to-liquìd
ratio. amount of fillers, composition of the resins, geometry of the denture base,
thermal conductivity of the stone mould and the heat transfer medium in which the
mould ìs placed (Faraj and Ellis, 1979; Vallittu, 1996).
Atkìnson and Dennis (1959) showed that a difference of4cC in the processing
temperature could make the difference of 60öC within the acrylic resin. A temperathre
spike was also demonstrated in a long curing cycle, as shown in sorne dental material
science textbooks (Philips, 1996) and publìcations (Tuckfield et al., 1943; Harman
and Pittsburgh, 1949; Faraj and Ellis, 1979).
Tuekfield et aL (1943) demonstrated that a temperature spike (about 90°C)
occurred in an acrylic cube of O.Sin (12.7mm) when the water temperature was 73°C
only. Harman and Pittsburgh (1949) recorded the temperature changes in an archshaped acrylic sample of 60 X 65 x 3mm at thìn sectIon and 60 x 65 x 12.7mm at thick
section under constant cure at 71°C. The peak temperatures were 83°C and I 16°C
respectively. Faraj and Ellis (1979) recorded a temperature peak of 88°C in 95mm
75
DISSCUS!ON
when the acrylic resin was cured at 70°C. The samples they used were acrylic discs
8mm thick and 50mm in diameter.
Although all the discs showed a temperature spike in the
1011g
curing cycle, a
criticism was that the acrylic samples were unrealistically thick. The amount of
exothermic heat is related to the rate of reaction, which in turn is related to the
processing temperature and thickness of the resin (Woelfel et aL, 1960). The peak
temperature correlating to the thickness of the acrylic resin was also due to a
reduction in surface area to volume ratio, affecting heat dissipation (Faraj and Ellis,
1979; Vallittu, 1996).
In this experiment, the temperature of the acrylic
resin
followed that of the
curing tank closely and a temperature spike was absent ìn the long curing cycle. These
factors could be confirmed by previous studies (Taylor, 1941; Honorez et al., 1989,
Clarke et aL, 1992; Yeung et al., 1995). The temperature difference between the water
and the acrylìc dough (2° - 3°C on heating up), and the time lag (about 3mm) were
small. This could be explained by the slow heating of the acrylic resin and conduction
of heat by the gypsum mould.
The heat conducted away from the thermocouple was probably negligible
because its
diameter is
only 0.25mm and its outer surface is insulated.
DISSCUSION
6.2.2 Pressure Changes
Pressure and temperature changes in the curing cycle are shown ¡ri Fig. 6.1.
When the temperature ofthe curing tank was increased, both the flasks and the acrylic
dough would undergo thermal expansion (Komiyama and Kawara, 1998). The
expansion of the flasks
(20x106
I °C for brass) and the stone (1 1x106 I °C) would
result in a larger mould and lead to a drop in pressure. However, the acrylic resin has
a greater coefficient of thermal expansion
(81x1OE'6
I °C) and expands to a larger
extent than that of the brass flasks and the stone. As the acrylic dough was confined
inside the mould, thennal expansion in this confined space would exert a higher
pressure inside the body ofthe acrylic dough. Therefore the pressure increased rapidly
from the clamped pressure i I .53atm.
The peak pressure (22.Olatm) was found when the temperature reached 72°C.
Peaking was probably due to the fact that the thermal expansion of the acrylic dough
and the flasks stopped as the temperature reached 72°C and did not go higher. The
pressure then started to drop because of the polymerization shrinkage of poly(inethyl
methacrylate).
In fact, polymerization took place almost as soon as the polymer powder and
the liquid were mixed. Polymerization was due to the collisions of molecules, which
are ìnevitably present. In some heat-cured acrylic resins (Acron Rapid, DeTrey QC
20, Donc, President Quick Cure, Stellon i 00. Trevalon and some others), the
manufacturers add a small amount of tertiary amine (<1%) in order to increase the
77
IiISSCUSION
rate of reaction. This will cause some polymerizatìon to occur at a low rate alter
mixing so that a large exothermic heat associated with sudden polymerization is
avoided (Jerolimov et al., 1989a; Harrison and Huggett, 1992).
However, the rate of such polymerization is very slow. The rate will increase
when benzoyl peroxide is heated and breaks down at 60°C. Nevertheless, the amount
of free radicals produced is small and the rate of reaction is impractical at 60°C.
Therefore the curing temperature is raised to 70° - 72°C (Harman and Pittsburgh,
1949; Jagger, 1978; Harrison and Huggett, 1992).
When the polymerization proceeded, the volume was expected to reduce by
21% as the density of the monomer changed from 0.94 to
1.19mg/cm3
(Phillips,
1996). Since two-thirds of the monomer was replaced by the polymer beads in the
acrylic dough, polymerization shrinkage would reduce the volume by 7%. The acrylic
resin shrinking away from the transducer and the surroundings would reduce the
pressure.
The effect ofpolymerization shrinkage on the pressure was shown (Eig.6.2) in
a pilot study in which acrylic dough was placed in a spring clamped flask and the
flask left at room temperature for more than 40h. As the pressure and the temperature
of the flasks were kept steady, the pressure of the acrylic resin dropped gradually,
probably owing to the polymerization shrinkage.
78
DISSCUSION
The pressure then dropped to its lowest point of about latm. The acrylic resin
was probably at an advanced stage of polymerization and had shrunk away from the
diaphragm ofthe transducer. This occurred around 15h after starting the curing cycle
and small variations (3.4 to 18h) were found. Since then no more fluctuation of
pressure was observed because the polynierized resin became hardened.
On heating up to 92°C, which simulates the terminal boil ìn practice, a rapid
drop in pressure was caused by rapid expansion ofthe moulds. In this stage the acrylic
resin was just about to expand because of the small time lag in heat conduction. When
the temperature reached 92°C, the mould stopped expandìng and became stable, and
the polymerized acrylic resin
expanded but
only slìghtly as it was already hardened.
Therefore there was a small increase of pressure. When the temperature was kept
steady at 92°C both the mould and the acryLic resin remained stable arid the pressure
was steady until the cooling process started.
On cooling, the thermal shrinkage (Komiyarna and Kawara 1998) of the
acrylic resin was expected to cause a drop ìn pressure. However, an increase in
pressure was recorded. This might be explained by both the spring clamp and the
contraction of the moulds compressing the resin and trying to get close adaptation of
the denture base (Taylor, 1941; Orien and Ryge, 1978), resulting in a net increase
of pressure. When the curing tank reached the ambient temperature, the pressure
became steady because both the flasks and the acrylic resin were dimensionally
stable.
79
DISSCUSION
On deflasking, the pressure dropped to i atm, that is, atmospheric pressure. In
some cases a small gap was visually detected between the cured denture base arid the
gypsum after processing. This was also reported by
other
investigators (Pickett and
Appleby, 1970; McCartney, 1984; Lechner and Lautenschlager, 1984; Al-Hanbali et
al.
1991). Pickett
and Appleby (1970) studied the posterior palatal seal of denture
bases cured under pressure in long cycles at 71 °C, with or wìthout terminal boil. They
reported that the denture bases were closely adapted to the models on the buccal
portion, the crest, and the crestal third of the lingual portion of the ridges, but not at
the palatal border, and that there was a space ranging from 0.24 to 0.28mm at the
posterior edge.
loo
25
90
80
70
60
E
50
E
40
10 Q.
30
20
5
10
o
o
2
4
6
8
10
Time I h
Fig.6.1 Pressure and temperature changes during the first 12 hours.
12
14
18
16
14
12
10
4
2
0
5
10
15
20
25
Time ¡ h
FIg.6.2 Pressure change in clamped flask with no heat applied.
30
35
DISSCUSIOÍV
6.3 Temperature Change in Fast Cycle
Continuous heating of the acrylic dough at i OOC dissociated a large number
of benzoyl peroxide molecules and created many free molecules to initiate the
polymerization. In addition, the heat energy supplìed would increase the vibrations
and movements of the molecules, by which reaction takes place more rapìdly. The
large amount of exotherniic heat created a temperature spike of about 131°C in the
fast cycle.
A temperature spike was also shown in previous studies (Faraj and Ellis, 1979;
Vallittu, 1996). Faraj and Ellis (1979) showed that a temperature peak of 136°C
occurred 25rnjn after placing the denture flask in a water bath at 100°C. Vallittu
(1996) showed that different temperature peaks occurred in different volumes of
acrylìc bar cured at 98°C: 137°C for 750mm3, 189°C for 1500mm3 and 199°C for
3000mm3.
The peak exothermic reaction represents a maximum rate of polymerization
taking place. In this experiment, the peak occurred around 25 to 30mm. A curing time
of 20mm, however, is recommended by the manufacturer. If those instructions are
followed during denture construction, a certain amount of monomer may remain
unpolymerized, with reaction still taking place when the flask starts to cool in the air.
The sudden reduction of the surrounding temperature will cause the denture flasks to
contract and stress may be introduced.
83
DISSCUSION
6.4 Elevation of Boiling Point and Gaseous Porosity
According to the Academy of Denture Prosthetics (1968), a denture is
considered to be hygìenically acceptable when it is non-porous, because porosity will
detrimentally affect the resistance of the material to staìning. calculus deposition and
adherent substances. Furthermore, porosity in acrylic denture bases also results in
reduced tensile strength, foul odours, high internal stress and vulnerability to
distortion and warpage (Gettleman et al., 1977; Craig, 1980; Firtell and Haiman.
1983).
To prevent porosity formation, it was suggested that thick dentures should be
avoided or that lower curing temperatures should be used (Gay and King, 1979).
Since the thickness of a denture ìs deterniìned by the anatomy of alveolar ridge,
curing temperature was well studied to obtaìn an optimal cycle (Harman and
Pittsburgh, 1949; Atkinson and Dennis, 1959; Jagger, 1978; Harrison and Huggett,
1992; Vallittu et aL, 1998). Harman and Pittsburgh (1949) were able to produce
porosity-free resin in acrylic blocks of 12.7mm thick at a curing temperature of 70°C.
Long curing cycle either with or without a terminal boíl was a well-adopted practice
to reduce the risk of porosity.
Besìdes temperature control, pressure was also applied in attempt to reduce
porosity (Smith et aL, 1967). Keller et al. (1978) demonstrated a reduction in porosity
in a two-stage process at curing pressure of about 0.68 to 3.44MPa (671 to
3395atm).
DISSCUSION
The
elevation of the boiling point of monomer under pressure was noted by
Taylor in 1941. However, lack of sufficient instrumentation arid data dId not permit
him further discussion.
Xia et aL (1996)
cooker at
6kgf/cm2
cured 10 acrylic samples of 6 x 50 x 60mm in a pressure
(5.8atm). Water temperature was raised, reaching 120ÖC. The
temperature and pressure were held for 10mm before the power was turned off. No
porosity was found in any of the samples. It was explained that the boiling point of
the monomer was elevated to 168.9°C under
6kgflcm2
and that it was unlikely that the
monomer had boiled. However, the pressure suggested was the air pressure on the
cooker and not the pressure on the water nor on the acrylic resins. Doubt was raised
on the actual elevated boiling point of monomer.
As the pressure of the acrylic resins was measured in this experiment, the
elevated boiling point of methyl methacrylate could be calculated. It was elevated to
193.46°C when spring clamped at 36Nm (ll.53atm). The boiling poìnt was elevated
to 227.97°C when the acrylic dough was at 22.Olatm at 72°C. The maximum
temperature achìevcd among the six samples in the fast cycle (136°C) was well below
these elevated boìling points.
Porosity of denture bases was detected by the suggested criteria: 'there shall
be no bubbles or voids when viewed without magriificatìon' (American Dental
Associations' specification, No.12, 1975; ISO 1567, 1988). No porosity was observed
DISSCUSION
by direct visìon in the denture bases cured in the fast cycle. Thìs confirmed that under
sufficient pressure monomer would not boìl.
6.5 Discussion ofMaterials and Methods
No studies have previously been carried out to investigate pressure during
processing. This is mainly because of instrument limitations. However, a
subminiature pressure transducer, as used in this study, solved the instrumentation
problem. Although the transducer is only temperature compensated to 94°C because
of the electronic components incorporated in its end portion, this does not affect the
validity of the experiment for the Jong curing cycle. This is because the acrylic resin
has already polymerized before the terminal boil.
However, the transducer could not be used in the fast curing cycle, because the
high temperature could have damaged it. Thus the actual pressure was not known and
the elevated boiling point of the monomer could not be calculated. The pressure was
assumed, following the Iogìcal explanation in the long curing cycle. An even higher
elevated boiling point of the monomer might be expected because of the greater
thermal expansion of the acrylic dough at the higher temperature achieved before
polymerization. Further studies may reveal the higher pressure.
Because of the concavity of the palate the thickness of the acrylic resin
beneath the transducer was about 4mm and the base had to be waxed up to the flat
DISSCUSION
diaphragm of the transducer. Although this was thicker than is usual in dentures, a
4iiim thick denture base or even thicker was commonly encountered in certain parts of
dentures replacing the tissues of severely resorbed alveolar ridge. As thick areas and
the palate were always believed to be at highest risk of porosity, testing on a thick
palate could be particularly useful.
In this experiment, a torque bar was used to indicate and standardize the
amount of torque given to the spring clamp. In laboratory practice the spring clamp is
usually tightened by hand or with a hammer and the force applied is purely subjective.
Individual variations among technicians, however, subject the spring clamp to
different pressures and may lead to porosity.
This study was based on heat curing the acrylic denture base in a spring
clamped denture flask, which is a commonly employed laboratory technique. The
pressure changes in other processìng methods, such as the injection moulding
technique or use of flask with a nonspring clamp, is still unknown. Further
investigations are needed.
CONCLUSIONS
7. Conclusions
I.
The temperature of the acrylic resin followed closely the curing temperature and
no temperature spike was observed in the long curIng cycle.
2. A temperature spike (131.21°C) occurred at about 25 to 30mm when the acrylic
resin was processed at 100°C in a fast curing cycle.
3.
The maximum pressure of acrylic resin durìng processìng was 22.Olatm, at which
the boiling point ofmonomer was elevated to 227.86°C.
4. The hìghest temperature reached by the acrylìc resìn during processing was well
below the elevated boiling point of monomer.
5.
Monomer will not boil in clamped denture flasks under sufficient pressure.
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APPENDICES
9. Appendices
9.1 Calibration of Thermometer
Thermometer: Kane-May Instrumentation
Thermocouple: K-type
Ambient Temperature: 21.5°C
Table 9.1 Calibration of Thermometer
Temperature I °C
Meter reading I mV
24.8
36.5
50.0
74.5
75.1
112.4
100.2
150.2
125.3
189.0
150.4
227.0
199.0
302.0
300.0
454.0
400.0
604.0
500.0
756.0
601.0
908.0
700.0
1060.0
800.0
1210.0
899.0
1361.0
i 09
4PPENLJ!CES
9.2 Calibration of Strainmeter for Measuring Temperature
Dynamic strainmeters: DC-92D, Tokyo Sokki Kenkyujo, Tokyo, Japan
Thermometer: Kane-May Irstrumeritation
Thcniocouplc: K-type
Amplification factor: 50
Th!e 9.2 Calibration ofStrainmeter for Temperature
Temperature I °C
Meter reading / V
30.6
2.208
40.5
2.929
50.2
3.656
60.1
4.374
70.0
5.113
72.0
5.258
so:i
5.847
90.0
6.585
94.1
6.880
100.1
7.323
hO
CONFERENCE PRESENTATION
10.
Conference Presentation
A poster presentation was done in the
British Society
46th
Annual Conference of
for the Study ofProsthetic Dentìstry in March 1999, Liverpool.
Ill
CONFERENCE PRESENTATION
Monomer Never Boils!
Yau WFE*, Cheng YY, Chow TW and Clark RKF
The University of Hong Kong and GKT Dental Institute
It n gen,raft bheaerl thai tire gneorrn porool
nomebines found n acryhc room after processeig
ta a resub of the lofrrfterahoe hooted rtaachod tite
bodhrg pomi of monomer durIng heOt Conng
Cousu__ci
I To measure tiro changos In pressura 800
temperature cit cicnyIic tasio during prote soìnJ
2 To record tir. lodheol IatflpItraIUrn rsothmt
by aa-pirro rosin wtten laOt c1od ei boiling
Annambty cil (lairs. and (rl) (laStro
sptlrrg clamped lo 3611m torque
(C)
water-
3 To calculada filin Utevated bailing point
IflOtlOfiter tardai high Pressura
of
Long cycle
The acrylIc rosin dough wss cured by a orig
s
j_,_
_*l. b
fr; I
software,
pressure
damped
priai cyc,.
A loader let-up of tite ceinte doctas brano,
willi ftrennocouple only. was processed by a
teal curing C)lCle )100C fctr IOni(o)
Equation
mn of m*idtwy
a
Cuflng cydo (lic for 8 bIt and 94t for i irti)
The citenpos (ti pressano cand lemp.ralure
emra recorded Willi conryirter data icqulattlonI
(aimed one
lnPIJpt
'u
(tilodal S-tOdd.
irtIP
Wgfti.ttipt0n UK) voll
acmpwreaban ta alt . and
tIfi tosida PrOectIOS
__._
.-- -
rOechoti e ire.tl cli 22Oefm during Irre (OrIg
curing cycte (Tabt t ) The alevatod boning
point of monomer ai Iii. inr.raesod pressura
wan oalculated to 1w about 105C at In harm)
rand about 228C at 22081m) Tiras. cilaullad
boiling points ere higher than tite rnnaxtmum
temperatura (131t) macbent durIng Inc fasi
CuiiniO cycle )F1g 2). Thus lire motnomar did
not resell It, bolting p0(01 end no gaseous
pettily wits observad In (tre curad denture
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