311
J. Dent. 1992; 20: 31 l-31 7
Determination of doughing and manipulation times of acrylic resin denture base material and a proposal for a specification for a packing plasticity test G. Mutlu, R. Huggett and A. Harrison Department UK
of Prosthetic
Dentistry
8 Dental Care of the Elderly,
University
of Bristol
Dental School,
Bristol,
ABSTRACT The doughing and manipulation times of a range of acrylic resin denture base polymers are presented in this study. The methods evaluated were: (i) probing following American Society for testing materials specification, F451-76; (ii) hole penetration following British Standard, 2487 : 1989 and International Standard Organisation, 1567 : 1988; (iii) probing using a penetrometer employing a 20 g load and 1 mm diameter needle; (iv) oscillating rheometer apparatus used at a fixed frequency of 1 Hz by sweeping through a programmed linear increase in strain. The results indicate that each of the methods is capable of evaluating the doughing and manipulation times of the unprocessed polymers. It is concluded that the Bohlin VOR Oscillating Rheometer is excellent for investigating the flow properties of denture base polymers but it cannot be recommended for a standards specification test because of the high cost. The penetrometer method has been shown to be superior to the current standards specification test and would be ideal for a packing plasticity standards test. KEY WORDS: plasticity J. Dent. 1992; 1992)
Acrylic
resins,
Denture
base
materials,
20: 3 1 1-3 17 (Received 2 1 November
Doughing
and
manipulation
199 1; reviewed 16 January 1992;
times,
Packing
accepted 27 April
Correspondence should be addressed to: Dr G. Mutlu, Department of Prosthetic Dentistry Et Dental Care of the Elderly, University of Bristol Dental School, Lower Maudlin Street, Bristol BSl 2LY. UK.
INTRODUCTION Over the centuries a variety of materials have been used for denture construction. The historical development of these materials from the early dentures carved from ivory, bone and wood up to the latest polymers have been discussed (Mutlu et al., 1989). Today most dentures are constructed using acrylic resin employing a relatively simple, low cost dough moulding press/pack technique resulting in dentures with good aesthetics. Studies on the rheological properties of acrylic resin denture base materials (Mutlu et al.. 1990, 1991) have shown that when the polymer and monomer components of denture base material are mixed the resultant material is at its lowest viscosity during the early time period and the viscosity increases gradually with respect to increase in time. When a 1992 Butterworth-Heinemann 0300-5712/92/050311-07
Ltd.
these two components are mixed the mass passes through a series of stages: (i) sandy, (ii) stringy, (iii) doughy and (iv) rubbery. The time from mixing until the time the material is ready for manipulation, or until the doughy stage, is called doughing time. The time from the end of the doughing time until the material becomes rubbery, or until the setting time, is called manipulation time during which it is possible to manipulate and pack the dough. The material should be packed during the manipulation stage since manipulating a material too early or beyond this time results in either breakdown of the material structure or damage to the mould and consequently influences the accuracy and quality of the moulded denture (Phillips, 1982; Craig, 1985; Jerilimov et al., 1985, 1989). If it is packed at the sandy or stringy stages too much
312
J. Dent.
1992;
20:
No. 5
Table I. The denture base polymers tested Constituents *
Manufacturer
Curing type Conventional heat cure
MMA
ED5
Bonar polymers, Newton Aycliffe, UK Bonar polymers
Conventional heat cure
ED6
Bonar polymers
Conventional heat cure
MMA (95%), (5%) MMA (90%), (10%)
Material D80
EMA BMA
*MMA. methyl methacrylate; EMA. ethyl methacrylate; BMA, butyl methacrylate.
monomer will be present between the polymer particles and the material will not have enough body to pack well and will flow too readily out of the mould. Packing too early may therefore lead to lack of pressure in the mould resulting in contraction porosity in the final denture base. If packed at the rubbery stage, the material will be too stiff to flow under the available pressure of the flask press and metal to metal contact between the flask halves will not be obtained. Delayed packing may result in loss of detail in the denture, movement of the teeth and a distortion of the vertical relation of the denture. In addition, because of the limited flow of the material at this stage it would be necessary to apply high pressure in order to till the mould and this could cause fracture of the mould. Packing the material at the correct stage facilitates more complete mould closure and gives better adaptation of the denture to the cast resulting in better retention and stability of the final denture. In spite of its importance, standard methods for the determination of the doughing and manipulation time of denture base materials do not exist. The British Standard (BS 2487 : 1989) and International Standard (IS0 1567 : 1988) for denture base polymers specify the method of determining the end of the manipulation stage (setting time). This method is termed a packing plasticity specitication test. It does not give the full length of the handling and manipulation time interval. The handling properties of many other dental materials have been widely studied: impression materials (e.g. Wilson, 1966; Jamani et al., 1989); dental cements (e.g. Houston and Miller, 1968; Eames et al., 1977); root canal cements (e.g. Grossman, 1976; Harcourt et al., 1981); composite resins (e.g. Ferracane et al., 1981; de Gee et al., 1989) and denture soft lining materials (e.g. Jones et al., 1982). However, this property of denture base polymers has received little attention. McCabe et al. (1975) demonstrated the effect of temperature and size of polymer beads of the powder component on the doughing time. The apparatus used was a reciprocating rheometer originally designed by Wilson (1966). The aim of this study is to develop a quick and accurate method of measuring the doughing and manipulation times of the denture base polymer as an alternative to the British Standard and International Standard test method and to propose a specification test.
MATERIALS
AND METHODS
The polymer powders used in this study are characterized in Table 1. The liquid component used was methyl methacrylate containing 0.01% hydroquinone. For all test procedures 10 ml of monomer were mixed with 32 ml of polymer and left to stand in the porcelain mixing vessel for 30 s. This mixing ratio is specified by the manufacturer. The mixes were then spatulated for a further 30 s. A stopwatch was started when the monomer was added to the polymer. Four different methods were used to evaluate the rheological properties of a range of denture base polymers: 1. Probing following ASTM specification F451-76. 2. Hole penetration following BS 2487 : 1989 and IS0 1567 : 1988. 3. Probing using a needle penetrometer. 4. Viscosity measurements using an oscillating rheometer. All tests were carried out at 20” If: 1 “C in a temperaturecontrolled environment. Although BS 2487 stipulates only one specimen and the ASTM F451-76 stipulates two separate determinations, in the present study each test was repeated three times.
Probing and hole penetration In the first stage, the determinations by probing following ASTM F451-76 for acrylic bone cements were used for measuring the doughing time of the material. Approximately 2 min after the onset of mixing, the mixture was probed gently with a gloved finger. Visual notice was taken as to the formation of fibres between the surface of the mix and the finger as it leaves the surface. This process of probing was repeated every minute until the gloved finger separated cleanly from the surface. The time at which this was first observed was denoted as the doughing time. In the second stage, the determinations by hole penetration following BS 2487 (1989): IS0 1567 (1988) for denture base polymers were used for measuring the manipulation time of the material. The packing plasticity specification test apparatus is shown in Fig 1. Immediately on completion of the doughing time a 10 g sample of the mix,
Multu
et al.: Determination
shaped to a thickness of approximately 5 mm, was placed on the upper surface of the perforated brass die (with 16 perforations each having a diameter of 0.75 mm) and covered with a sheet of polythene film. The glass plate (60 X 60 X 5 mm) and 5 kg weight were placed on top. After 10 min the weight was removed. When the material was firm, the depth of penetration was recorded by measuring from the lower surface of the brass die to the intruded polymer and subtracting this from the thickness of the brass die. The test was repeated every 5 min after doughing time until the mix was capable of being intruded into at least two holes of the die to a depth of not less than 0.5 mm.
of doughing
penetrometer
A microprocessor-controlled penetrometer (Model PNRlO, Sommer and Runge, Sur, Berlin) was used (Fig. 2). The control unit regulates the depth and time of penetration. A 20 g plunger load and 1 mm diameter needle penetrator was used in this study with a penetration time of 5 s. The mix was placed in a plastic bowl on the table of the penetrometer and the illuminator positioned so that the tip of the penetrator threw a clear shadow on the surface of the material to be measured. The penetrator was lowered slowly towards the surface of the material with the fine adjustment so that the tip of the penetrator touched the material but did not penetrate it. This was done by adjusting until the light gap between the tip of the penetrator and its shadow disappeared. A magnifying glass was used for better observation. The interlock for the plunger and penetrator was released for 5 s. The penetration depth in millimetres was displayed in the indicator section. The plunger and penetrator interlock position were reset and the needle wiped clean. The test was repeated every 5 min from mixing to setting. The sample was covered with a sheet of polythene film between the test intervals to prevent surface drying.
fig. 7. Packing plasticity test apparatus.
times
313
Since this method is later proposed as a standards testing procedure, a reproducibility study was undertaken. Mixes were prepared, in identical fashion, on 10 separate occasions and penetration measurements were made at the sandy, stringy, doughy and rubbery stages. Determinations were made in a controlled environment at 20°C f 1 “C over a 48-h period. Between each determination the apparatus was turned off, so that each test was discrete. The results are shown in Table II: a high level of reproducibility was obtained at the sandy, stringy and doughy stages; at the rubbery, unworkable stage, a greater variation in penetration depth was recorded.
Oscillating Needle
and manipulation
rheometer
The viscosity changes during dough formation of the mixes were measured using a Bohlin VOR Rheometer (Bohlin Reologi UK Ltd, Huntingdon, UK) (Fig. 3). A mix was placed between the parallel plates of the rheometer. A low viscosity silicone fluid (Dow-Corning Corp., Midland, MI, USA) was placed around the edge of the specimen to prevent evaporation of monomer. The testing procedure was to carry out a forced sinusoidal deformation at a fixed frequency of 1 Hz. The instrument was operated by sweeping through a programmed linear increase in strain (Strain Sweep program), with the test being repeated every 5 min from mixing to setting. From these, data plots were constructed for dynamic viscosity as a function of time at constant strain (0.001) since these materials have been shown to be strain dependent (pseudoplastic) (Mutlu et al., 1990).
Fig. 2. Microprocessor controlled penetrometer.
J. Dent.
314
1992; 20: No. 5
Table II. Reproducibility of needle penetration tests: depth of penetration (mm) time intervals on 10 separate mixes determined on 10 different occasions Specimen mix/ test occasion
S/Sandy
Time (min)/stage 7 5Atringy
at four
of determination 40/Doughy 50/Rubbery
1 2 3 4 5 6
14.2 14.4 14.5 14.1 14.4 13.9
13.8 14.4 14.3 14.4 14.2 14.2
4.12 4.06 4.05 4.47 4.11 3.99
1.17 1.44 1.88 1.40 1.50 1.20
: 9 10
14.3 13.7 13.9 14.4
13.9 14.2 13.8 13.7
4.49 3.86 3.95 4.25
1.84 1.40 1.43 1.66
14.18 0.27 1.90 0.08
14.09 0.26 1.87 0.08
4.13 0.20 4.89 0.06
1.49 0.23 15.90 0.07
Mean s.d. C.V. s.e. mean
RESULTS The doughing and the manipulation times of denture base polymers obtained by probing (ASTM F451-76) and the packing plasticity test method (BS 2487 : 1989; IS0 1567 : 1988) are shown in Fig. 4. The lower part of the bar represents the doughing time and the upper part represents
the manipulation time of the material. The needle penetration of the material as a function of time from mixing to setting is shown in Fig. 5. The viscosity of the materials as a function of time is shown in Fig. 6. As each test is repeated three times, the graph represents the median and range of the three measurements.
60 -z ._
50
E
40
.: c-
30 20 10 Ok
ED6
ED5
DEOFC
Fig. 4. Doughing and manipulation time of acrylic resin , gjj, denture base materials. Cl, Doughing time; 0, manipulation time.
Fig. 3. Bohlin VOR Rheometer.
5
.
E E 5 3 c
Yl : a
10 8 6
5
80
z .t
60
:: 40
4
5
2 0
100
5
10
15
20
25
30
35
Time
40
45
50
55
60
65
70
(min)
fig. 5. Needle penetration of acrylic resin denture base materials. X, D80 FC; 9, ED5; A, ED6.
20 0
5
10
15
20
25
30
35
Time
40
45
50
55
60
65
70
(min)
Fig. 6. Viscosity of acrylic resin denture base materials. -X- , 080 FC;+, ED5; +, ED6.
Multu et a/.: Determination
DISCUSSION This study has shown that the various stages of the polymer/monomer mix are directly influenced by the flow of the resin. In general, the penetration of the acrylic resin has decreased with increasing time during dough formation. The results are in good agreement with the viscosity curves of the same materials obtained from the rheometer. The experiments have provided and clearly demonstrated the advantages of early resin packing during the manipulation stage, because the depth of penetration at the early stage was greater than that for later penetration in both methods. These results suggest that from a rheological stand-point, acrylic denture base resin should be used as soon as possible after reaching the dough stage. This practice would take advantage of the early low viscosity to enhance the flow of the resin into the mould. Knowledge of the flow properties and handling times of the polymer provides a quick and convenient reference for the user and results in a more precise moulding process. The results of the present investigation indicate that the doughing and manipulation times of acrylic resin denture base material can be determined by different methods. Although these methods can be used to quantify the rheological changes in material behaviour, each method has its advantages and disadvantages.
-
Time consuming. Not practical. Unable to measure handling characteristics
Needle
the appropriate of the material.
range
of
penetrometer
Since rheological behaviour of acrylic denture base materials changes from mixing to setting, the flow properties of these materials should be examined
315
carefully. This study demonstrates that the present specification (BS 2487 : 1989; IS0 1567 : 1988) needs to be modified. It should cover all the handling characteristics of acrylic resin including the doughing, manipulation, setting times and plasticity measurements. It is clearly shown that between the three different techniques used in this study, the needle penetrometer test is considered to be practically more relevant than the two other methods. The needle penetrometer indicates flow curves, doughing time and manipulation time. When the polymer and monomer are mixed, the needle penetration of the mix decreases very slowly during the early stage. When the material reaches the dough stage, the penetration decreases sharply. This point is the end of the doughing time and relates closely with the time obtained from the probing method (ASTM F45 l-76). The penetration decreases with increasing time during the manipulation stage. When the acrylic resin dough was not workable according to the standard specifications (BS 2487 : 1989; IS0 1567 : 1988) a penetration of less than 3 mm was achieved with a 1 mm needle and a 20 g plunger load. This study supports the contention that the needle penetrometer is an ideal method for the standard packing plasticity test for denture base polymers. It has many advantages: - It is not time consuming. - It is practical and simple. - Doughing and manipulation times can be obtained
Probing and hole penetration Although probing by finger is a very easy method, it is purely subjective and there is the possibility of error in finding the real doughing time due to hand temperature. As mentioned earlier, the standard specification hole pentration test method does not give the full length of the handling time intervals but only evaluates the end of the manipulation stage which is the rubbery/setting time of the material. It is worth noting that the standard specification packing plasticity test (BS 2487 : 1989; IS0 1567 : 1988) is criticized by polymer manufacturers because it is not practical. Placing the dial gauge probe into the holes of the brass mould is extremely difficult and may indent the polymer. Leaving the material until it is harder (leathery) and withdrawing it and then measuring with a feeler gauge can lead to error of measurement from elongation (by stretching) of the material that has penetrated the hole. If the resin is left until rigid it may prove impossible to extract from the holes. In this study it was also found that the test was:
of doughing and manipulation times
-
efficiently. Plasticity of the material can be measured. It is readily available commercially and inexpensive.
Oscillating
relatively
rheometer
The Bohlin VOR Rheometer gives an excellent characterization of rheological measurements. It can be used to measure an extremely wide range of fluids and semifluids from low viscosity to very high viscosity which is achieved by selecting different combinations of measuring geometry and torque bar rating. The measurements can be followed visually on the screen as the tests are performed. It allows the user to construct a programme of user-defined tests which are automatically performed by the rheometer. Each set of test parameters is loaded according to a userdefined sequence. These tests are performed and the data stored on hard disk for review whenever desired. The instrument is supplied with a temperature control unit which is controlled from the operation programme. The circulation unit feeds a water-bath surrounding the lower measuring geometry and a platinum resistance thermometer which continually monitors the temperature close to the surface of the measuring geometry. With this waterbath system a temperature range of 5-90°C is accessible. Although the Bohlin VOR rheometer has many advantages, it cannot be recommended for standard specification tests because of: -
High cost of apparatus. Use requires tuition and experience.
316
J. Dent. 1992;
20: No. 5
CONCLUSIONS The determination of the doughing and manipulation times of three acrylic resin denture base polymers are presented in this study. From the results, the following conclusions can be drawn: 1. When the polymer and monomer are mixed the viscosity increases and the penetration decreases at different rates with respect to increases in time. 2. The Bohlin VOR Rheometer is excellent for investigating the rheological behaviour of denture base polymers, but the cost of the apparatus is high. 3. A needle penetrometer has been demonstrated as being capable of quantifying the changes in material behaviour. This technique is recommended for the standard flow properties evaluation technique for denture base polymers and has been shown to be superior to the current standards specification test. 4. The study has highlighted the need for updating the current standards specification for packing plasticity of denture base polymers. An initial outline proposal is presented which could serve as a basis for the development of a new standards test specification.
time from mixing until the time the material is ready to manipulate and pack). Manipulation time: the manipulation time is the period between doughing and setting time in which it is possible to manipulate and pack the dough. Setting time: the setting time is defined as the time from mixing until the material becomes rubbery and unworkable (at the completion of the manipulation time).
Requirements
Apparatus
It will be appreciated that any standard test which can only be performed in highly specialized laboratories, with expensive equipment, greatly limits the usefulness of a specification. Any specification test should fulfil the following by being:
-
Practical. Reproducible. Undertaken using inexpensive equipment. As simple and efficient as possible. Suitable for all currently available materials. Able to present practical information needed application.
for its
The proposed specification outlined below fulfils the requirements listed above and covers the necessary handling and packing characteristics of acrylic resin denture base materials. It includes the doughing, manipulation and setting times and packing plasticity of the materials which are related to penetration or flow of the resin.
Definitions So that ambiguity described.
-
A microprocessor-controlled plunger (20 g) and a standard and 2.5 g). Plastic container (6 cm volume).
penetrometer with a needle (1 mm diameter diameter
and
60 ml
Test procedure A sample of resin with a mass of 30-35 g is prepared in accordance with the manufacturer’s instructions and placed in the plastic container on the table of the penetrometer. Immediately prior to the recommended initial packing time, the penetrator is slowly lowered towards the surface of the material with the fine adjustment so that the tip of the penetrator touches the material but does not penetrate. The interlock of the plunger and penetrator is released for 5 s and the penetration depth, which will be displayed in the indicator section, is recorded. The plunger and penetrator is reset to the interlock position and the test repeated every 5 min from mixing to the maximum manipulation time recommended by the manufacturer. The sample is covered with a sheet of polyethylene film between the test intervals.
Acknowledgements can be avoided
Doughing time: the doughing when the material
resin, packing
When the resins are tested in accordance with the packing plasticity test using the needle penetration system, at the initial and final packing times (at the manipulation time) recommended by the manufacturer, they shall be capable of being penetrated not more than 10 mm and not less than 3 mm. The time at which less than 10 mm penetration is first observed is denoted as doughing time. The time at which less than 3 mm penetration is first observed is denoted as setting time. The initial and final packing times recommended by the manufacturer must be the same as the doughing and setting times.
PROPOSAL FOR A SPECIFICATION FOR PACKING PLASTICITY OF DENTURE BASE POLYMERS UTILIZING A NEEDLE PENETROMETER
-
for unprocessed
plasticity
reaches
the following
terms are
time is defined as the time the dough stage (which is the
The authors are grateful to Dr J. W. Goodwin and Dr R. W. Hughes for their helpful advice when using the Bohlin rheometer. G. Mutlu was supported by a Turkish Government Scholarship.
Multu et al.: Determination
of doughing and manipulation times
317
References American Society for Testing Materials. Standard Specification for Acrylic Bone Cements. ASTM Designation F451-76. British Standard. Specification for Denture Base Polymers. BS 2487 : 1989, London. Craig R. G. (1985) Restorative Dental Materials, 7th edn. St Louis: C. V. Mosby. de Gee A. J., ten Harke H. C. and Davidson C. L. (1989) The influence of mixing ratio on the working time, strength and wear of composites. Aust Dent. J. 34,466-469. Eames W. B., Monroe S. D., Roan J. D. et al. (1977) Proportioning and mixing of cements: a comparison of working times. Oper. Dent. 2,97-104. Ferracane J. L., Moser J. B. and Greener E. H. (1981) Rheology of composite restoratives. J. Dent. Res. 60, 1678-1685. Grossman L. I. (1976) Physical properties of root canal cements. J. Endodont. 2, 166-175. Harcourt J. K., Moser J. B. and Young A. R. (1981) Working and setting times of root canal sealants determined by the oscillating rheometer. Aust. Dent. J. 26,292-294. Houston W. J. B. and Miller M. W. (1968) Cements for orthodontic use. Dent. Practit. 19, 104-109. International Organization for Standardization. International Standard for Denture Base Polymers. IS0 1567 : 1988. Jamani K. D., Harrington E. and Wilson H. J. (1989) Consistency, working time and setting time of elastomeric impression materials. J. Oral Rehabil. 16, 353-366.
Forthcoming
Jerolimov V., Huggett R., Brooks S. C. et al. (1985) The effect of variations in the polymer/monomer mixing ratios on residual monomer levels and flexural properties of denture base materials. Quintessence Dent. Technol. 9, 431-434. Jerolimov V., Brooks S. C., Huggett R. et al. (1989) Some effects of varying denture base resin polymer/monomer ratios. Int. J. Prosthodont. 2, 56-60. Jones D. W., Sutow B. S., Graham B. S. et al. (1982) Setting characteristics and flow of denture soft lining materials. J. Dent. Res. 61, 285 (abstr. 962). McCabe J. F., Spence D. and Wilson H. J. (1975) Doughing time of heat cured dental acrylic resins and its dependence on polymer particles. J. Oral Rehabil. 2, 199-207. Mutlu G., Harrison k and Huggett R. (1989) History of denture base materials. Quintessence Dent. Technol.
Yearbook 13, 145-151. Mutlu G., Huggett R., Harrison A. et al. (1990) Rheology acrylic resin denture base polymers. Dent. Mater. 6, 288-293.
Mutlu G., Harrison A and Huggett R. (1991) The effect of powder particle size on the rheology of acrylic resin denture base materials. Med. Sci. Res. 19, 425-426. Phillips R. W. (1982) Skinner’s Science of Dental Materials, 8th edn. London: W. B. Saunders. Wilson H. J. (1966) Elastomeric impression materials. Part IThe setting material. Br. Dent. J. 121, 277-283.
Articles
Review Tooth
wear
and
facial
morphology
A. J. R. Crothers
Intraligamentary J. G. Meechan
Original
anaesthesia:
Research
a review of the literature
Papers
The surface pH of resin-modified glass polyalkenoate M. J Woolford and R. G. Chadwick
(ionomer)
cements
Effect of a resin lining and rebonding on the marginal leakage of amalgam restorations F. Bradley Dutton, J, B. Summitt, D. C. N. Chan and F. Garcia-Godoy Titanium nitrate coatings in clinical P. R. Mezger and N. H. J. Creugers Selection of dental procedures endocarditis S. Y. Tan and G. Gill
dentistry
for antibiotic
Microleakage of three generically different W. H. Douglas and J. W. Fundingsland
prophylaxis
fluoride-releasing
of
against
infective
liner/bases
In vivo confocal microscopy in clinical dental research: an initial T. F. Watson, W. M. Petroll, H. D. Cavanagh and J V. Jester
approach
J. Dent. 1992; 20: 31 l-31 7
Determination of doughing and manipulation times of acrylic resin denture base material and a proposal for a specification for a packing plasticity test G. Mutlu, R. Huggett and A. Harrison Department UK
of Prosthetic
Dentistry
8 Dental Care of the Elderly,
University
of Bristol
Dental School,
Bristol,
ABSTRACT The doughing and manipulation times of a range of acrylic resin denture base polymers are presented in this study. The methods evaluated were: (i) probing following American Society for testing materials specification, F451-76; (ii) hole penetration following British Standard, 2487 : 1989 and International Standard Organisation, 1567 : 1988; (iii) probing using a penetrometer employing a 20 g load and 1 mm diameter needle; (iv) oscillating rheometer apparatus used at a fixed frequency of 1 Hz by sweeping through a programmed linear increase in strain. The results indicate that each of the methods is capable of evaluating the doughing and manipulation times of the unprocessed polymers. It is concluded that the Bohlin VOR Oscillating Rheometer is excellent for investigating the flow properties of denture base polymers but it cannot be recommended for a standards specification test because of the high cost. The penetrometer method has been shown to be superior to the current standards specification test and would be ideal for a packing plasticity standards test. KEY WORDS: plasticity J. Dent. 1992; 1992)
Acrylic
resins,
Denture
base
materials,
20: 3 1 1-3 17 (Received 2 1 November
Doughing
and
manipulation
199 1; reviewed 16 January 1992;
times,
Packing
accepted 27 April
Correspondence should be addressed to: Dr G. Mutlu, Department of Prosthetic Dentistry Et Dental Care of the Elderly, University of Bristol Dental School, Lower Maudlin Street, Bristol BSl 2LY. UK.
INTRODUCTION Over the centuries a variety of materials have been used for denture construction. The historical development of these materials from the early dentures carved from ivory, bone and wood up to the latest polymers have been discussed (Mutlu et al., 1989). Today most dentures are constructed using acrylic resin employing a relatively simple, low cost dough moulding press/pack technique resulting in dentures with good aesthetics. Studies on the rheological properties of acrylic resin denture base materials (Mutlu et al.. 1990, 1991) have shown that when the polymer and monomer components of denture base material are mixed the resultant material is at its lowest viscosity during the early time period and the viscosity increases gradually with respect to increase in time. When a 1992 Butterworth-Heinemann 0300-5712/92/050311-07
Ltd.
these two components are mixed the mass passes through a series of stages: (i) sandy, (ii) stringy, (iii) doughy and (iv) rubbery. The time from mixing until the time the material is ready for manipulation, or until the doughy stage, is called doughing time. The time from the end of the doughing time until the material becomes rubbery, or until the setting time, is called manipulation time during which it is possible to manipulate and pack the dough. The material should be packed during the manipulation stage since manipulating a material too early or beyond this time results in either breakdown of the material structure or damage to the mould and consequently influences the accuracy and quality of the moulded denture (Phillips, 1982; Craig, 1985; Jerilimov et al., 1985, 1989). If it is packed at the sandy or stringy stages too much
312
J. Dent.
1992;
20:
No. 5
Table I. The denture base polymers tested Constituents *
Manufacturer
Curing type Conventional heat cure
MMA
ED5
Bonar polymers, Newton Aycliffe, UK Bonar polymers
Conventional heat cure
ED6
Bonar polymers
Conventional heat cure
MMA (95%), (5%) MMA (90%), (10%)
Material D80
EMA BMA
*MMA. methyl methacrylate; EMA. ethyl methacrylate; BMA, butyl methacrylate.
monomer will be present between the polymer particles and the material will not have enough body to pack well and will flow too readily out of the mould. Packing too early may therefore lead to lack of pressure in the mould resulting in contraction porosity in the final denture base. If packed at the rubbery stage, the material will be too stiff to flow under the available pressure of the flask press and metal to metal contact between the flask halves will not be obtained. Delayed packing may result in loss of detail in the denture, movement of the teeth and a distortion of the vertical relation of the denture. In addition, because of the limited flow of the material at this stage it would be necessary to apply high pressure in order to till the mould and this could cause fracture of the mould. Packing the material at the correct stage facilitates more complete mould closure and gives better adaptation of the denture to the cast resulting in better retention and stability of the final denture. In spite of its importance, standard methods for the determination of the doughing and manipulation time of denture base materials do not exist. The British Standard (BS 2487 : 1989) and International Standard (IS0 1567 : 1988) for denture base polymers specify the method of determining the end of the manipulation stage (setting time). This method is termed a packing plasticity specitication test. It does not give the full length of the handling and manipulation time interval. The handling properties of many other dental materials have been widely studied: impression materials (e.g. Wilson, 1966; Jamani et al., 1989); dental cements (e.g. Houston and Miller, 1968; Eames et al., 1977); root canal cements (e.g. Grossman, 1976; Harcourt et al., 1981); composite resins (e.g. Ferracane et al., 1981; de Gee et al., 1989) and denture soft lining materials (e.g. Jones et al., 1982). However, this property of denture base polymers has received little attention. McCabe et al. (1975) demonstrated the effect of temperature and size of polymer beads of the powder component on the doughing time. The apparatus used was a reciprocating rheometer originally designed by Wilson (1966). The aim of this study is to develop a quick and accurate method of measuring the doughing and manipulation times of the denture base polymer as an alternative to the British Standard and International Standard test method and to propose a specification test.
MATERIALS
AND METHODS
The polymer powders used in this study are characterized in Table 1. The liquid component used was methyl methacrylate containing 0.01% hydroquinone. For all test procedures 10 ml of monomer were mixed with 32 ml of polymer and left to stand in the porcelain mixing vessel for 30 s. This mixing ratio is specified by the manufacturer. The mixes were then spatulated for a further 30 s. A stopwatch was started when the monomer was added to the polymer. Four different methods were used to evaluate the rheological properties of a range of denture base polymers: 1. Probing following ASTM specification F451-76. 2. Hole penetration following BS 2487 : 1989 and IS0 1567 : 1988. 3. Probing using a needle penetrometer. 4. Viscosity measurements using an oscillating rheometer. All tests were carried out at 20” If: 1 “C in a temperaturecontrolled environment. Although BS 2487 stipulates only one specimen and the ASTM F451-76 stipulates two separate determinations, in the present study each test was repeated three times.
Probing and hole penetration In the first stage, the determinations by probing following ASTM F451-76 for acrylic bone cements were used for measuring the doughing time of the material. Approximately 2 min after the onset of mixing, the mixture was probed gently with a gloved finger. Visual notice was taken as to the formation of fibres between the surface of the mix and the finger as it leaves the surface. This process of probing was repeated every minute until the gloved finger separated cleanly from the surface. The time at which this was first observed was denoted as the doughing time. In the second stage, the determinations by hole penetration following BS 2487 (1989): IS0 1567 (1988) for denture base polymers were used for measuring the manipulation time of the material. The packing plasticity specification test apparatus is shown in Fig 1. Immediately on completion of the doughing time a 10 g sample of the mix,
Multu
et al.: Determination
shaped to a thickness of approximately 5 mm, was placed on the upper surface of the perforated brass die (with 16 perforations each having a diameter of 0.75 mm) and covered with a sheet of polythene film. The glass plate (60 X 60 X 5 mm) and 5 kg weight were placed on top. After 10 min the weight was removed. When the material was firm, the depth of penetration was recorded by measuring from the lower surface of the brass die to the intruded polymer and subtracting this from the thickness of the brass die. The test was repeated every 5 min after doughing time until the mix was capable of being intruded into at least two holes of the die to a depth of not less than 0.5 mm.
of doughing
penetrometer
A microprocessor-controlled penetrometer (Model PNRlO, Sommer and Runge, Sur, Berlin) was used (Fig. 2). The control unit regulates the depth and time of penetration. A 20 g plunger load and 1 mm diameter needle penetrator was used in this study with a penetration time of 5 s. The mix was placed in a plastic bowl on the table of the penetrometer and the illuminator positioned so that the tip of the penetrator threw a clear shadow on the surface of the material to be measured. The penetrator was lowered slowly towards the surface of the material with the fine adjustment so that the tip of the penetrator touched the material but did not penetrate it. This was done by adjusting until the light gap between the tip of the penetrator and its shadow disappeared. A magnifying glass was used for better observation. The interlock for the plunger and penetrator was released for 5 s. The penetration depth in millimetres was displayed in the indicator section. The plunger and penetrator interlock position were reset and the needle wiped clean. The test was repeated every 5 min from mixing to setting. The sample was covered with a sheet of polythene film between the test intervals to prevent surface drying.
fig. 7. Packing plasticity test apparatus.
times
313
Since this method is later proposed as a standards testing procedure, a reproducibility study was undertaken. Mixes were prepared, in identical fashion, on 10 separate occasions and penetration measurements were made at the sandy, stringy, doughy and rubbery stages. Determinations were made in a controlled environment at 20°C f 1 “C over a 48-h period. Between each determination the apparatus was turned off, so that each test was discrete. The results are shown in Table II: a high level of reproducibility was obtained at the sandy, stringy and doughy stages; at the rubbery, unworkable stage, a greater variation in penetration depth was recorded.
Oscillating Needle
and manipulation
rheometer
The viscosity changes during dough formation of the mixes were measured using a Bohlin VOR Rheometer (Bohlin Reologi UK Ltd, Huntingdon, UK) (Fig. 3). A mix was placed between the parallel plates of the rheometer. A low viscosity silicone fluid (Dow-Corning Corp., Midland, MI, USA) was placed around the edge of the specimen to prevent evaporation of monomer. The testing procedure was to carry out a forced sinusoidal deformation at a fixed frequency of 1 Hz. The instrument was operated by sweeping through a programmed linear increase in strain (Strain Sweep program), with the test being repeated every 5 min from mixing to setting. From these, data plots were constructed for dynamic viscosity as a function of time at constant strain (0.001) since these materials have been shown to be strain dependent (pseudoplastic) (Mutlu et al., 1990).
Fig. 2. Microprocessor controlled penetrometer.
J. Dent.
314
1992; 20: No. 5
Table II. Reproducibility of needle penetration tests: depth of penetration (mm) time intervals on 10 separate mixes determined on 10 different occasions Specimen mix/ test occasion
S/Sandy
Time (min)/stage 7 5Atringy
at four
of determination 40/Doughy 50/Rubbery
1 2 3 4 5 6
14.2 14.4 14.5 14.1 14.4 13.9
13.8 14.4 14.3 14.4 14.2 14.2
4.12 4.06 4.05 4.47 4.11 3.99
1.17 1.44 1.88 1.40 1.50 1.20
: 9 10
14.3 13.7 13.9 14.4
13.9 14.2 13.8 13.7
4.49 3.86 3.95 4.25
1.84 1.40 1.43 1.66
14.18 0.27 1.90 0.08
14.09 0.26 1.87 0.08
4.13 0.20 4.89 0.06
1.49 0.23 15.90 0.07
Mean s.d. C.V. s.e. mean
RESULTS The doughing and the manipulation times of denture base polymers obtained by probing (ASTM F451-76) and the packing plasticity test method (BS 2487 : 1989; IS0 1567 : 1988) are shown in Fig. 4. The lower part of the bar represents the doughing time and the upper part represents
the manipulation time of the material. The needle penetration of the material as a function of time from mixing to setting is shown in Fig. 5. The viscosity of the materials as a function of time is shown in Fig. 6. As each test is repeated three times, the graph represents the median and range of the three measurements.
60 -z ._
50
E
40
.: c-
30 20 10 Ok
ED6
ED5
DEOFC
Fig. 4. Doughing and manipulation time of acrylic resin , gjj, denture base materials. Cl, Doughing time; 0, manipulation time.
Fig. 3. Bohlin VOR Rheometer.
5
.
E E 5 3 c
Yl : a
10 8 6
5
80
z .t
60
:: 40
4
5
2 0
100
5
10
15
20
25
30
35
Time
40
45
50
55
60
65
70
(min)
fig. 5. Needle penetration of acrylic resin denture base materials. X, D80 FC; 9, ED5; A, ED6.
20 0
5
10
15
20
25
30
35
Time
40
45
50
55
60
65
70
(min)
Fig. 6. Viscosity of acrylic resin denture base materials. -X- , 080 FC;+, ED5; +, ED6.
Multu et a/.: Determination
DISCUSSION This study has shown that the various stages of the polymer/monomer mix are directly influenced by the flow of the resin. In general, the penetration of the acrylic resin has decreased with increasing time during dough formation. The results are in good agreement with the viscosity curves of the same materials obtained from the rheometer. The experiments have provided and clearly demonstrated the advantages of early resin packing during the manipulation stage, because the depth of penetration at the early stage was greater than that for later penetration in both methods. These results suggest that from a rheological stand-point, acrylic denture base resin should be used as soon as possible after reaching the dough stage. This practice would take advantage of the early low viscosity to enhance the flow of the resin into the mould. Knowledge of the flow properties and handling times of the polymer provides a quick and convenient reference for the user and results in a more precise moulding process. The results of the present investigation indicate that the doughing and manipulation times of acrylic resin denture base material can be determined by different methods. Although these methods can be used to quantify the rheological changes in material behaviour, each method has its advantages and disadvantages.
-
Time consuming. Not practical. Unable to measure handling characteristics
Needle
the appropriate of the material.
range
of
penetrometer
Since rheological behaviour of acrylic denture base materials changes from mixing to setting, the flow properties of these materials should be examined
315
carefully. This study demonstrates that the present specification (BS 2487 : 1989; IS0 1567 : 1988) needs to be modified. It should cover all the handling characteristics of acrylic resin including the doughing, manipulation, setting times and plasticity measurements. It is clearly shown that between the three different techniques used in this study, the needle penetrometer test is considered to be practically more relevant than the two other methods. The needle penetrometer indicates flow curves, doughing time and manipulation time. When the polymer and monomer are mixed, the needle penetration of the mix decreases very slowly during the early stage. When the material reaches the dough stage, the penetration decreases sharply. This point is the end of the doughing time and relates closely with the time obtained from the probing method (ASTM F45 l-76). The penetration decreases with increasing time during the manipulation stage. When the acrylic resin dough was not workable according to the standard specifications (BS 2487 : 1989; IS0 1567 : 1988) a penetration of less than 3 mm was achieved with a 1 mm needle and a 20 g plunger load. This study supports the contention that the needle penetrometer is an ideal method for the standard packing plasticity test for denture base polymers. It has many advantages: - It is not time consuming. - It is practical and simple. - Doughing and manipulation times can be obtained
Probing and hole penetration Although probing by finger is a very easy method, it is purely subjective and there is the possibility of error in finding the real doughing time due to hand temperature. As mentioned earlier, the standard specification hole pentration test method does not give the full length of the handling time intervals but only evaluates the end of the manipulation stage which is the rubbery/setting time of the material. It is worth noting that the standard specification packing plasticity test (BS 2487 : 1989; IS0 1567 : 1988) is criticized by polymer manufacturers because it is not practical. Placing the dial gauge probe into the holes of the brass mould is extremely difficult and may indent the polymer. Leaving the material until it is harder (leathery) and withdrawing it and then measuring with a feeler gauge can lead to error of measurement from elongation (by stretching) of the material that has penetrated the hole. If the resin is left until rigid it may prove impossible to extract from the holes. In this study it was also found that the test was:
of doughing and manipulation times
-
efficiently. Plasticity of the material can be measured. It is readily available commercially and inexpensive.
Oscillating
relatively
rheometer
The Bohlin VOR Rheometer gives an excellent characterization of rheological measurements. It can be used to measure an extremely wide range of fluids and semifluids from low viscosity to very high viscosity which is achieved by selecting different combinations of measuring geometry and torque bar rating. The measurements can be followed visually on the screen as the tests are performed. It allows the user to construct a programme of user-defined tests which are automatically performed by the rheometer. Each set of test parameters is loaded according to a userdefined sequence. These tests are performed and the data stored on hard disk for review whenever desired. The instrument is supplied with a temperature control unit which is controlled from the operation programme. The circulation unit feeds a water-bath surrounding the lower measuring geometry and a platinum resistance thermometer which continually monitors the temperature close to the surface of the measuring geometry. With this waterbath system a temperature range of 5-90°C is accessible. Although the Bohlin VOR rheometer has many advantages, it cannot be recommended for standard specification tests because of: -
High cost of apparatus. Use requires tuition and experience.
316
J. Dent. 1992;
20: No. 5
CONCLUSIONS The determination of the doughing and manipulation times of three acrylic resin denture base polymers are presented in this study. From the results, the following conclusions can be drawn: 1. When the polymer and monomer are mixed the viscosity increases and the penetration decreases at different rates with respect to increases in time. 2. The Bohlin VOR Rheometer is excellent for investigating the rheological behaviour of denture base polymers, but the cost of the apparatus is high. 3. A needle penetrometer has been demonstrated as being capable of quantifying the changes in material behaviour. This technique is recommended for the standard flow properties evaluation technique for denture base polymers and has been shown to be superior to the current standards specification test. 4. The study has highlighted the need for updating the current standards specification for packing plasticity of denture base polymers. An initial outline proposal is presented which could serve as a basis for the development of a new standards test specification.
time from mixing until the time the material is ready to manipulate and pack). Manipulation time: the manipulation time is the period between doughing and setting time in which it is possible to manipulate and pack the dough. Setting time: the setting time is defined as the time from mixing until the material becomes rubbery and unworkable (at the completion of the manipulation time).
Requirements
Apparatus
It will be appreciated that any standard test which can only be performed in highly specialized laboratories, with expensive equipment, greatly limits the usefulness of a specification. Any specification test should fulfil the following by being:
-
Practical. Reproducible. Undertaken using inexpensive equipment. As simple and efficient as possible. Suitable for all currently available materials. Able to present practical information needed application.
for its
The proposed specification outlined below fulfils the requirements listed above and covers the necessary handling and packing characteristics of acrylic resin denture base materials. It includes the doughing, manipulation and setting times and packing plasticity of the materials which are related to penetration or flow of the resin.
Definitions So that ambiguity described.
-
A microprocessor-controlled plunger (20 g) and a standard and 2.5 g). Plastic container (6 cm volume).
penetrometer with a needle (1 mm diameter diameter
and
60 ml
Test procedure A sample of resin with a mass of 30-35 g is prepared in accordance with the manufacturer’s instructions and placed in the plastic container on the table of the penetrometer. Immediately prior to the recommended initial packing time, the penetrator is slowly lowered towards the surface of the material with the fine adjustment so that the tip of the penetrator touches the material but does not penetrate. The interlock of the plunger and penetrator is released for 5 s and the penetration depth, which will be displayed in the indicator section, is recorded. The plunger and penetrator is reset to the interlock position and the test repeated every 5 min from mixing to the maximum manipulation time recommended by the manufacturer. The sample is covered with a sheet of polyethylene film between the test intervals.
Acknowledgements can be avoided
Doughing time: the doughing when the material
resin, packing
When the resins are tested in accordance with the packing plasticity test using the needle penetration system, at the initial and final packing times (at the manipulation time) recommended by the manufacturer, they shall be capable of being penetrated not more than 10 mm and not less than 3 mm. The time at which less than 10 mm penetration is first observed is denoted as doughing time. The time at which less than 3 mm penetration is first observed is denoted as setting time. The initial and final packing times recommended by the manufacturer must be the same as the doughing and setting times.
PROPOSAL FOR A SPECIFICATION FOR PACKING PLASTICITY OF DENTURE BASE POLYMERS UTILIZING A NEEDLE PENETROMETER
-
for unprocessed
plasticity
reaches
the following
terms are
time is defined as the time the dough stage (which is the
The authors are grateful to Dr J. W. Goodwin and Dr R. W. Hughes for their helpful advice when using the Bohlin rheometer. G. Mutlu was supported by a Turkish Government Scholarship.
Multu et al.: Determination
of doughing and manipulation times
317
References American Society for Testing Materials. Standard Specification for Acrylic Bone Cements. ASTM Designation F451-76. British Standard. Specification for Denture Base Polymers. BS 2487 : 1989, London. Craig R. G. (1985) Restorative Dental Materials, 7th edn. St Louis: C. V. Mosby. de Gee A. J., ten Harke H. C. and Davidson C. L. (1989) The influence of mixing ratio on the working time, strength and wear of composites. Aust Dent. J. 34,466-469. Eames W. B., Monroe S. D., Roan J. D. et al. (1977) Proportioning and mixing of cements: a comparison of working times. Oper. Dent. 2,97-104. Ferracane J. L., Moser J. B. and Greener E. H. (1981) Rheology of composite restoratives. J. Dent. Res. 60, 1678-1685. Grossman L. I. (1976) Physical properties of root canal cements. J. Endodont. 2, 166-175. Harcourt J. K., Moser J. B. and Young A. R. (1981) Working and setting times of root canal sealants determined by the oscillating rheometer. Aust. Dent. J. 26,292-294. Houston W. J. B. and Miller M. W. (1968) Cements for orthodontic use. Dent. Practit. 19, 104-109. International Organization for Standardization. International Standard for Denture Base Polymers. IS0 1567 : 1988. Jamani K. D., Harrington E. and Wilson H. J. (1989) Consistency, working time and setting time of elastomeric impression materials. J. Oral Rehabil. 16, 353-366.
Forthcoming
Jerolimov V., Huggett R., Brooks S. C. et al. (1985) The effect of variations in the polymer/monomer mixing ratios on residual monomer levels and flexural properties of denture base materials. Quintessence Dent. Technol. 9, 431-434. Jerolimov V., Brooks S. C., Huggett R. et al. (1989) Some effects of varying denture base resin polymer/monomer ratios. Int. J. Prosthodont. 2, 56-60. Jones D. W., Sutow B. S., Graham B. S. et al. (1982) Setting characteristics and flow of denture soft lining materials. J. Dent. Res. 61, 285 (abstr. 962). McCabe J. F., Spence D. and Wilson H. J. (1975) Doughing time of heat cured dental acrylic resins and its dependence on polymer particles. J. Oral Rehabil. 2, 199-207. Mutlu G., Harrison k and Huggett R. (1989) History of denture base materials. Quintessence Dent. Technol.
Yearbook 13, 145-151. Mutlu G., Huggett R., Harrison A. et al. (1990) Rheology acrylic resin denture base polymers. Dent. Mater. 6, 288-293.
Mutlu G., Harrison A and Huggett R. (1991) The effect of powder particle size on the rheology of acrylic resin denture base materials. Med. Sci. Res. 19, 425-426. Phillips R. W. (1982) Skinner’s Science of Dental Materials, 8th edn. London: W. B. Saunders. Wilson H. J. (1966) Elastomeric impression materials. Part IThe setting material. Br. Dent. J. 121, 277-283.
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