Acrylic Microwave

8/6/2019 Acrylic Microwave

http://slidepdf.com/reader/full/acrylic-microwave 1/4

Cl in i ca l Ma te r i a l s 12 (1993 ) 11 7 -120

The Use of Microwave Energy to Cure DentureAcrylic Resins

Randa IDiwan, E. C. Com be & A. A. GrantDepartment of Restorative Dentistry, University Dental Hospital of Manchester, Higher Cambridge Street,Manchester M 15 6FH, UK

(Received 2 December 1991; sent for revision 9 March 1992; accepted 10 April 1992)

Abstract: Experiments have been carried out on the curing of poly(methy1methacrylate) denture base material in a microwave oven, using conventionaldental flasks and flask clamps. It has been shown that it is important (a) tomaintain sufficient pressure on the materials, (b) to avoid gaseous porosity by notheating too rapidly initially, and (c) to ensure that all metal is protected fromexposure to microwaves. When the appropriate conditions of polymerisation wereused, microwave-cured samples had satisfactory physical and mechanicalproperties according to American D ental Association specification number 12.

INTRODUCTION

Since the introduction of acrylic resin into thedental field for the construction of denture bases,several techniques have been employed for itsfabrication. One option is to use injection mould-ing,l though the most widely used technique iscompression moulding of an acrylic resin doughwhich consists of a mixture of poly(methy1 meth-acryiate) together with monomeric methyl meth-acrylate and an initiator such as benzoyl peroxide.This dough is polymerised in a two-part metallicflask, either using a water bath or dry heat oven attypically 72 “C for up to 14 h. Use of such a methodto obtain a denture base that is free of porosity andhas satisfactory physical and mechanical propertiesconsumes much time and energy. More rapid heatcuring cycles have been explored by Jerolimovet cd2

can be used for curing denture acrylic resins.4s5 A

technique using microwave energy has been re-ported in which a conventional denture base resinwas cured in specially designed non-metallic flasks,for example, made of glass-fibre reinforced poly-ester resin flasks with polycarbonate bolts.5-11Porosity of the resin was a potential problem,particularly for thicker sections of material.

This work was undertaken to explore microwavetechniques of curing, to see to wh,at extent thiswould be feasible using conventional dental flasksand materials, and to determine the necessaryconditions for the production of porosity-freespecimens with suitable mechanical properties.

MATERIALS AND METHODS

The chemically activated (self-curing) materials,containing a tertiary amine activator, require lessenergy but the resultant resin has a relatively highresidual monomer content, high creep values, lowerstrength and stiffness, poorer colour stability, and is

not usually free of porosity.3

This work was carried out with a hea&-cured resin;this was an unpigmented materiatl, to enableassessment of porosity to be undertaken readily. Apolymer/monomer ratio of 2.5/l by weight wasemployed. The moulds were prepared from KaffirD dental stone, mixed at a water/powder ratio of

0.3.It has been suggested that microwave energy The microwave oven was an AEG Micromat

117C i i n i c a l M a t e r i d s 0 2 6 7 -6 6 0 5 / 9 3 / $ 0 6 . 00 01993 Elsevier Science Publishers Ltd, England 10-2



11 8 R. Diwan, E. C. Combe, A. A. &YXFT~

which operates at a frequency of 2450 MHz. Themaximum power was 650 W. The apparatusenabled different power settings to be selected: inthis work these ranged from 10 % to 100 % of theavailable power, thus enabling different rates of

polymerisation to be studied.In initial work, the moulds were designed to give

acrylic specimens of rectangular cross-section, ofdimensions 65 mm x 40 mm x 7 mm. Preliminaryexperiments were carried out under different con-ditions without using a conventional dental flask,because of the inadvisability of exposing metallicobjects to microwaves. Techniques were thenevolved, as detailed below, which permitted the useof dental flasks. Subsequently, specimens wereprepared under a wide range of polymerisationconditions, and examined visually for porosity.When optimum curing conditions had been found,some properties of the cured resin were determinedby the methods of American ental Association(ADA) specification number 12. These includedtransverse deflection, water sorption and solubility,and colour stability. Finally, dentures were made inthe unpigmented resin, to assess the practicalviability of the suggested curing technique

RESULTS AND DISCUSSION

Preliminary experiments

The first test was carried out at 100% power for10 min in moulds without any means of applyingpressure to the material, to determine if gypsumcould transmit microwaves to the resin. It wasobserved that the acrylic mass expanded after about7 min. At the end of the test, the gypsum was

Fig. 1. Porous acrylic specimens produced in a gypsum mouldwith pressure applied by G-clamps.

10 min then 50 %100 % power for

from the vo~at~~~satio~ of ~~~o~~~---.-~~ uncuring, leading to excessive residual ~~r~~~~~~r n

ower with visual ~bse~vati~~ of tspecimens after I 60 min it -was

hat this power setting was insucient ~~ly~e~~sati5~. At 2.0 %

findings were essentially similar. Thirty and 40 %power for 60 min gave ised bnt porous

was that cracking ofn observed. Attempts

essnre to gypsum mogauze were also often ~~s~~~essf~~~ n sc9me experi-

ments, a V-shaped pattern of porosity wasobserved owing ts uneven ~~~~~~~t~~~. f pressure

place.Following this, an ex

with the stonekept in its metal flask; wmetal discs removed, onmicrowaves could pass thrto reach the acrylic dough.the flask clamp waspressure could be a



Microwave energy to cure denture resins 11 9

Table 1 . Relation between curing cycle and porosity

Cur ing cyc let i m e a n d p o w er

(% I

Poros i ty oft h i n s p e ci m e n s

Poros i ty oft h i c k s p e ci m e n s

1 0 min at 10%60 min at 40 % Clear, non-porous10 min at 100%10 min at 1 0%90 min at 40 % Clear, non-porous

5 min at ‘100%10 min at 1 0%90 min at 40 % Clear, non-porous15 min at 100%

Clear with a singletiny central pore

Clear, non-porous

Clear, non-porous

10min at 10%100 min at 40 % 1

Clear, non-porous

20 min at 110% Incomplete40 min at 50 % polymerisation20min at IO%

1Won-porous, quite

50 min at 50 % transparent

Incompletepolymerisation

Slight porosity incentre of specimen

A few pores incentre of specimen

Slight porosity incentre of specimen

Did not polymerise

20 min at 1.0%60 min at 5 0 % 1 Clear, non-porous

20min at 10%90 min at 50% }

Clear, non-porous

20min at 10%30 min at 60% !-

Did not polymerise

20min at 10%30 min at 60% Clear, non-porous

Porosity in centre

5 min at 1 00%with branch shape

20min at 10%40 min at 60 %

IClear, non-porous Porosity in centre

20min at 10%45 min at 60% ?

Clear, non-porous Porosity in centre

Clear, non-porous

processed at 20 % power for 10 min, 50 % powerfor 10 min and full power for 10 min. Porousspecimens resulted, possibly because the gypsumwas not ex.erting sufficient pressure on the flask withthe acrylic dough. Also, the odour of monomercould be detected, suggesting that polymerisationwas incomplete.

In the next experiment, two flasks were used inthe clamp, and a longer curing time was adopted.The upper flask contained gypsum only, and the

Fig. 2. Dentures produced in an unpigmented resin by amicrowave technique.

lower one contained the dough. As before, this flaskwas used without its upper or lower metal discs.Curing was carried out at 20 %, 50 % and 100 %power consecutively, at 20 min for each powersetting. The specimens contained a small amount ofporosity.

On the assumption that this porosity was of thegaseous type, the above experiment was repeated,using a longer curing time, namely, 10 % power for20 min, 40 % for 40 min, 60 % for 10 min, then100 % for 10 min. This yielded apparently sat-isfactory specimens.

Table 2. Physical and mechanical properties of microwave-cured acrylics

Effect of curing cycle

Following this, many specimens were cured underdifferent conditions, and examined for porosity.Table 1 shows the observations for thick specimens(7 mm as above) and thin specimens (3 mm).

S a m p l e Deject ion (mm ) , for loads”1 .5-3 .5 kg 1 .5-H kg

Wa t e rsorp t ion’(m glcm’)

Wa t e rso lubi l i tyb(mglcm ”)

Col’ours tab i l i ty

Microwave cured :10 min at 10%90 min at 40% >15 min at 100%ADA specification

1.48 ( f 0.28) 3.24 ( f 0.44) 0.75 0.005

max. 2.5 min. 2.0max. 5.5

max. 0.8 max. 0.04 Slight change

Slight changein colour

a Mean of five determinations as required by specification.b Mean of two determinations as required by specification.



12 0 R . Diwan, E. G. Corn&, A. A. Grant

Properties of cured specimens

Table 2 shows data for microwave cured specimens,tested according to the ADA specification number12. It appears that satisfactory acrylic specimens

can be produced by this technique.Full upper and lower dentures were prepared in

the microwave oven using an unpigmented resin toenable observation of porosity, if any, to be carriedout. Figure 2 shows dentures cured at 10% powerfor 10 min, followed by 40 % power for 90 min,then 100 % power for 15 min. It can be seen that thetechnique appears to have been satisfactory.

CONCLUSIONS

Acrylic denture materials can be cured usingmicrowave energy. Conventional dental equipmentcan be used, provided care is taken to preventexposure of metal to microwaves. The techniqueoffers the potential of ensuring adequate curingwith minimal consumption of energy. Because ofthe potential of this system, further studies arebeing conducted on the effect of curing conditionson residual monomer and mechanical properties.

1.

2.

3.

4.

5.

6.

7.

8.

9.

10 .

81 .

Psyor, W. J., h jec t i on moulding of plastics for dentures. J,Am . Dent. Assoc. 29 (1942) 1408-8.

rooks, S. C., Huggetc, R. Ba’ies, J. F.,denturekase materials. Dent.

0 7 ~ De n l d Mme i i a b j ! i t h e d n ) .

Edinburgh, 1986, p. 264.thods of polymerisation of dental

niversity of Manchester, 198 3F., On the microwave poly-

merisation method by the developed flask. Quint. Dem.Teechnol., 9 (1984) 961-74.

denture acrylic resinsperties. Quin r. hr.,Clerck, I. P ) Mic ve ~~~y~~~~~~~~~~~ of ac ryk

IZS used in dental pro stheses. J. Prosthef. Dent. 59(198-i) 6X--8.Sanders. .I. I,., Levin, B. & Reitz, P. V., Porosity in dentuse

acrylic resins cured by microwave energy. Quint.Invn:_ 1,

ori, D., Ifmggett, is., Bates, 1 , F. & Brooks; S. CT k.mm of denture base acrylic resins polymerised by

microwave irradiation and by conventional water bathcuring systems. Dent. M uteu., 4 (1988 ) 25--X.-hong, V.-T. & Thomasz; F. G. V., Comparison ofdenture acrylic resins cured by bshg water and mi-crowave energy. Amt. Denl. 3.: 37 (1988 ). 201-204.h&i, B., Sanders, J &. & Reitz, 37 ii., The use cfmicrowave energy for processing acr$ic resim. J, Brosthe;.

Acrylic Microwave

Documents