Fazal Ghani1 , Rafique Moosa2
How to CITE:
Moosa R, Ghani F. Effect of CuringMethods and Temperature on Porosity inAcrylic Resin Denture Bases. J Pak DentAssoc. 2012;21(03) : 127-135
To investigate the influence of the curing method and curing temperature as variables of porosity in poly-methyl-methacrylate (PMMA) denture bases.
Two hundred standardized square-shaped acrylic specimens (2cm2) with thickness of 1 mm were fabricated by using the fabrication techniques of wax pattern, investing, de-waxing, packing and curing. Forty specimens of these (GroupA)were fabricated in self-curing resin.The remaining specimens in GroupB,C,D, and E with 40 specimens in each group were cured by using thermostatically and time controlled water-curing equipment. Specimens in Group B were cured by subjecting them to a curing cycle involving immersion in water at room temperature and raising the temperature to reach boiling and thenmaintaining it for 45 minutes. Specimens in Group C were cured with 70 C & 7 hours curing regimen, Specimens in Group D were processed by the 70 C and 9 hours cycle. Specimens in Group Ewere cured by immersing them in room temperature water and raising the temperature slowly to reach 100 Candmaintaining it for another one hour.After finishing and polishing the specimens, porosity was assessed using scanning electronmicroscopy.The data collected were analyzed using SPSS version 14.0.
Specimens in all the four groups displayed varying porosity percentages with both the within and between group variations. The range of the mean percent porosity in the water-cured specimens was 2 to 7 as compared to specimens in Group A (self-curing resin specimens ) exhibiting significantly higher mean porosity percentage of 63.Themean porosity percentwas lowest (2%) in the specimens cured by the 70 Cand 9 hours cycle.
Use of lower curing temperature and prolonged curing time had a beneficial effect on reducing the occurrence of porosity in the acrylic denture bases.
Denture base materials, Polymerization methods, Polymerization cycles Polymerization times Porosity in acrylic denture bases.
Poly-methyl-methacrylate (PMMA) commonly known as acrylic resin is the most popular dental material used in the construction of contemporary removable prostheses. Since its invention in 1936, nearly all conventional removable partial and complete dentures zremade in thismaterial. Despite thewide use of acrylic resin in removable denture work over a period of 76 years, the search for the understanding and improvement of its behavior continues. Upon mixing the conventional powder (polymer) and liquid (monomer) system, polymerization is achieved by a number ofmethods, such as heat activation (water bath or microwave- energy), chemical activation and visible light application. Currently, heat polymerization in a water-bath is the most widely usedmethod4.
During polymerization of acrylic resin, pores are formed in its mass leading to porosity. Porosity occurs due to the air trapped during mixing, monomer contraction and evaporation of the monomer during
curing. Upon polymerization of acrylic resin, small pores of almost uniformshape and size are acceptable; however, extensive and deeper porosity can weaken a denture base resin, promote staining, render denture unesthetic due to colour change, harbor organisms such as Candida albicans, and can cause bond failures between the artificial tooth and denture base resin. The compressive strength of acrylic resin is inversely proportional to the total porosity and that a decrease in porosity could increase the compressive strength. A porous denture base is also very difficult if not impossible to finish and polish and thus is an unwanted aspect of denture fabrication. Porosity or void fraction is a measure of the void (empty) spaces in a unit volume of a material. Despite, the importance of making denture bases that are void-free and non-porous, our knowledge of void removal strategies is very limited. Experience has shown that it is not easy to routinely ensure the making of porosity free dentures. An abundant research effort has been made till now to overcome this problem and studies of assessing and minimizing the porosity and voids have remained a matter of focus.
Porosity can bemeasured bymanymethods including conservative (non-invasive) and invasive (destructive)
methods. Compagnoni et al1 calculated porosity in acrylic resin specimens by measurement of the pecimen olume before and after its immersion in water. Among the other methods used in studies of porosity assessment are included; laser-opto-acoustic method, optical (conventional and electro-photographic and microscopic methods, surface painting and printing, radiography including micro-computed tomography, the Archimedes method of assessing the volume changes after diffusion of a liquid medium. Each of these methods has its own indications, advantages and disadvantages. For example the Archimedes method of measuring porosity is the easiest and non invasive method of porosity assessment.
On the other hand the methods involving image analyses such as micro-computed tomography give additional information about the internal structure of the specimen and thus are well-suited for qualitative evaluation of the specimens. The interpretation of the SEMmicrographs is difficult and time-consuming but is the most reliable and precise method. Furthermore, they give relatively poor
information. Laser opto-acoustic method is based on measurements of phase velocities of the rmo-optically excited longitudinal ultrasonic waves in specimens. In the direct methods the bulk volume of the specimen is determined and then it is compared with the volume of the skeletal material with no pores using the equation;Pore volume = Total volume of specimen with pores Original material volume with no pore.
In the optical methods, the area of the material is determined versus the area of the pores visible under the microscope. In electro-photography pore portions are traced by a tracing film and processing a painted out image. Computed tomography method uses CT scanning by creating a 3D image rendering of external and internal geometry, including voids and the defect analysis utilizing computer software. mbibition methods involveimmersion of the porous sample, under vacuum, in a fluid
that preferentially wets the pores. In the water saturation method, the pore volume is determined by subtracting the volume of water left after soaking the specimen in it from the total volume of water used for soaking the specimen. In the water evaporation method, the pore volume is determined as the weight of saturated sample minus weight of dried sample divided by the density ofwater.
Several variables influence porosity in acrylic resin. Mathew et al1 investigated the effect of the curing mode on the type and location and distribution of pores and found that that pores gathered near the surface of cooler moulds and near the center in warmer molds for all their test resins. Pressure in the initial stages of polymerization was considered to minimize the material’s porosity.20 A post packing pressure could reduce voids by 70%. Maintaining the applied pressure also affects the morphometry of voids including their shape, size, and spatial distribution.
Water sorption in acrylic resin, partly may be due to presence of deeper porosity and surface flaws. The thicker samples have been shown to exhibit greater water sorption. Similarly, fiber reinforced acrylic materials exhibited less changes because of reduced water sorption as compared to plain acrylic resin. Pero et al state that mean percent porosity was related to the absolute density of the acrylic resin and the weight of the specimen. Also another variable is the proportion of themonomer used for mixing. It has been shown that lowliquid percentage in the ix accompanied without a temperature rise above 100 °C, excluded porosity Compagnoni et al studied rectangular resin specimens (65×40×5 mm) Al-Doori et al15 suggested that porosity free material could only be guaranteed in sections not thicker than 3 mm. Truong and Thomas31 found that porosity was observed in thick specimens with a cross-section of 14×10mm. However, they suggested that the microwave programme could be optimized to prevent porosity without prolonging the curing time or sacrificing the physical properties of the resins by starting the curing process at low wattage. Lai et al in a comparative study involving 10mm thick acrylic specimens cured by water-bath and various levels of microwave energy found reduced porosity in the waterbath-
cured specimens as compared to that in the microwave-cured specimens.
An adequate solution for storing specimens must be used tomeasure porosity by water absorption. It has also been found thatmicrowave polymerization cycles and the specimen thickness of acrylic resin could influence porosity. Yau et al found that the highest temperature reached by heating of resin during processing was well below the elevated boiling point of monomer. They concluded thatmonomer does not boil in clamped denture flasks under sufficient pressure. They suggested that adequate clamp pressure would prevent gaseous porosity irrespective of curing cycle used.While emphasizing the beneficial effect of increased curing pressures on decreased porosity in acrylic resins, they were able to demonstrate elimination of the most of larger pores by using curing pressures of approximately 0.68 to 3.44MPa.
Compagnoni et al found no significant differences in mean porosity among the acrylic specimens cured by
heating in water-bath or microwave energy. They concluded that porosity in the test resin specifically
designed for microwave polymerization was not affected by different polymerization cycles. Porosity was similar to the conventional heat-polymerized denture base resin tested. Kusy mentioned about porosity in self-curing acrylic bone cements using standard quantitative metallography and pointed out to the fact that porosity could range from 1 to 8%. Bafileetal compared porosity of denture resin cured by microwave energy to denture resin cured by the conventional heat method and found no significant differences in the mean porosity between the control group (cured in a curing tank at 165° F for 9 hours) and the four groups of microwave-processed samples that used micro liquid monomer. The two groups of microwave-processed samples of methyl methacrylate monomer showed a significantly higher mean porosity.
Microscopic photography as method of investigating the effect of thickness (3mm and 6mm) of the specimens and differentmicrowave energy cycles on the porosity of 2 heat-activated denture base resins has also been used.16 The area of each pore was measured with a digital planimeter, and the total area of pores per surface was calculated in percentage form. The total number of pores on each surface and the topographical distribution of the pores also were recorded. They observed that some specimens exhibited no pores and in the thicker specimens there were giant pores (3.69 mm2) with small, gaseous pores of almost uniform shape and size. Of the observed surfaces, 75.3%were free of pores and 24.7% contained at least one pore. The thicker specimens exhibited the greatest amount of porosity (P<.0001); and that polymerization cycle had no effect on porosity (P=.19). The 3 other factors (material, specimen thickness, and surface) and all possible interactions among them were significant (P<.05). They concluded that minor porosity could occur in thin and more severe porosity in thicker areas of conventional resin specimens that underwent microwave polymerization. The rein designed specifically for microwave polymerization exhibited no clinically significant porosity Mattie et al investigated the influence of an experimental radiopaque additive, triphenyl bismuth (TPB), on polymethyl methacrylate resins formulated for dental use.They found a tendency to entrain air bubbles, because of the hydrophobicity ofTPB, resulting in increased susceptibility to brittle failure at the higherTPBlevels. Pero et al An investigation of porosity, Pero et al focussed on the presence of porosity at the interface of one artificial tooth acrylic resin and three denture base resins including one microwave-polymerized and two heatpolymerized. Ten specimens of each denture base resin with artificial toothwere processed.After polymerization, specimens were polished and observed under a microscope at 80x magnification. The area of each pore present between artificial tooth and denture base resinwas measured using computer software, and the total area of pores per surface was calculated inmm . Porosity analysis was directed to record the average number of pores, porosity area range and range for pore diameter. They found varying but not significantly different values for these among the test samples. In a study, Ganzarolli et al25 evaluated the porosity of conventional heat-polymerized, microwave-polymerized and injection-molded resins. Porosity was evaluated by weighing each specimen in air and in water using an analytical scale balance. The injection-molded resin showed no relevant improvement of porosity, transverse and impact strength. Jerolimov et al26 studied the influence of a rapid-cure polymerization cycle, including a terminal boil, with respect to transparency, residual monomer, mechanical properties, and generation of gaseous porosity. They suggested that porosity-free resin could be produced during rapid curing if a low concentration ( 0.26%) of benzoyl peroxide initiator in the powder component was ensured. Similarly, a very small concentration ( 0.025%) of the chemical activator dimethyl-p-toluidine in themonomer componentwas also considered as an advantage. Mechanical properties were only influenced where substantial porosity existed. They emphasized on the importance of formulating a material “tailor-made” for rapid curing de Oliveira et al evaluated the effect of the number and position of flasks on porosity of a microwave-cured acrylic resin. Samples were made in microwave resin processed at 500 W. For porosity assessment, the specimens were polished and immersed in permanent ink and counting the pores were counted under a stereo light microscope. They found no statistically significant differences in porosity among the groups. In a study, Gay and Gordon28 found that all samples cured by the heat platen press for 9 hours at 75 °C had no porosity. Marie observed that untreated glass fibers increased the transverse strength by 11% but produced some porosity in the polymericmatrix. Kuhar&Funduk using contact profilometer compared the effects of 4 chairside polishing kits and 2 conventional laboratory polishing techniques in 3 different acrylic denture base resin specimens (15x303mm). In the results, the highest means every surface roughness (Ra=2.86+0.8 um to 3.99+1.31 um) was measured surfaces finished with a tungsten carbide bur. The lowest surface roughness values (Ra =0.02+0.01 um) were determined for acrylic resin specimens polished with a lathe and polishing paste. The Ra values of resin specimens after polishing with chairside silicone polishing kits ranged from 0.05 ± 0.0 um to 0.35+0.05 am) Mean average Ra values of specimens polished with a polishing cream alone (Ra= 1.01+0.17 um to 1.68+0.47 um) were significantly higher (P<.05) than those obtained with other polishing systems tested. Significant differences in mean average surface roughnesswere found between auto polymerizing and injected heat-polymerizing resin specimens. In addition, scanning electron microscopy revealed increased porosity of auto polymerizing resin specimens. They concluded that conventional laboratory polishing was found to produce the smoothest surface of denture base acrylic resin. Chairside silicone polishing kits produced a significantly smoother surface of acrylic resin than specimens polished with a tungsten carbide bur. The presence of large pores was characteristic for the auto polymerizing resinmaterial. The purpose of this study is to evaluate the relative influence of some variable of denture processing on the extent of porosity in acrylic complete denture bases.
Two hundred specimens (2cm2 and 1mm thick) to represent test acrylic denture bases were fabricated using either self-curing or conventional heat-cured polymer and monomer based acrylic resin systems of the same manufacturer (Dentsply, Pacific Link Tower,Wong Chuk Hang, Hong Kong). The wax patterns for the acrylic specimens were laid down using 1 mm thick sheet modeling wax and sealing them on plaster slabs. They were then invested in plaster using two piece brass dental flask.Dewaxingwas carried out by placing flasks inwater bath (Electrical ThermostaticWater Baths, 2L4H, Anjum MC 028110202Made in China.) heating for 30 minutes at 600C. The flasks were taken out from the water-bath and opened. Hot water was slowly flown on plaster mould surfaces in the flask to eliminate wax traces. Flasks were allowed to cool before applying a coat of single layer of separating medium and allowed to dry. Subsequently, 40 moulds in flasks were packed with a mix of self-curing resin to constitute Group A specimens. For the heat-cure specimens, an acrylic resin mix was prepared in a mixing jar with lid following the polymer / monomer ratios recommended by the manufacturer. Subsequently, upon reaching the dough stage, the resin mix was packed to fill the moulds in the remaining 160 flasks. The flasks were kept bench pressed following the recommended packing pressures for each consecutive trial packing until acrylic flash no longer came out. Samples were kept bench pressed for 30 minutes. The flasks packed with the heatcuring resin were divided into four groups (with 40 specimens in each group) and designated as group ‘B’ ‘C’ ‘D’ and ‘E’. Subsequently, all the specimens inside the tightly clamped packed flasks were polymerized chemically or by immersing in room temperature water and using varying curing temperature and curing timings.
The specimens in group ‘A’ were cured chemically using self-curing PMMA resin. During curing these samples, the clamped flasks remained immersed in warm water. Specimens in Group B were cured by keeping immersed the flasks in the water in the curing equipment having a thermostat and time controller. The cycle started with immersion of flasks in water at ambient temperature and then raising water temperature gradually to reach boiling and maintained at this for 45 minutes. The specimens in group C’ were cured by immersing the flasks inwater bath at ambient temperature and raising the water temperature gradually to 70 °Candmaintained at this for 7 hours. The specimens in group D were cured by immersing the flasks in the water bath at ambient temperature and gradually raising the water temperature to reach 70 ºCand thenmaintaining at this temperature for 9 hours. The specimens in group E were cured by immersing the flasks in water at ambient and raising the temperature of water to reach 100 0C and maintained for an hour. After curing, flasks were bench cooled before recovering the specimens. Specimenswere finished using acrylic trimmer and sandpaper. Polishing was accomplished using a dental polishing lathe with pumice slurry and brush and finally with polishing cake and cloth buff.
Assessment of Porosity in Specimens:
Porosity in specimens in each group was assessed by themethod of scanning electronmicroscopy (SEM) using the scanning electron microscope (Olympus Bx51 Research Microscope with Olympus DP 12 Digital Microscope Camera). In the materials testing laborato y, specimens were prepared for porosity assessment by first fine polishing using water slurry of fine pumice powder and bristle brush in lathe for about 2minutes at slow speed, Finally, each acrylic resin specimen was placed under the polisher and freelymoved by the operator.After polishing, specimens were cleaned, dried and stored individually in sterile containers. The routine and standard technique was used for coating the specimens with thin gold-coat for SEM examination. Surface porosity and morphology of the specimens in each group was investigated by SEM using (FEI / EO, A type Quanta Holland) at accelerating voltage of 10KV.
There were wide variations in the percent porosity values of the specimens both within and between the groups. Inspection of the data in Table 1, show very high porosity and cracks percent values among specimens in Group A (chemically-cured) as compared to the specimens in the other groups. It can also be seen in this table that Group B samples had a mean porosity and cracks percent value of 5, with such values of 4.2, 2 and 7 for the specimens in Group C, D & E respectively. However, the differences in the mean porosity percent values for the specimens in Group B and C were statistically insignificant. Group D specimens (watercuring at 70 oC for 9 hours) had the lowest mean porosity and cracks percent values of 2.Group Esamples exhibited relatively higher mean porosity and crack percent value (7%) as compared to samples in Groups B, C & D. The ranges and mean values of percent porosities in the various groups of specimens are shown inTable 1.
bath is a relatively superior curing method in terms of controlling the porosity. The samples in the self-curing group not only revealed increased number of porosity but also there weremore pores and some were larger than 100 um diameter. Figure 1 displays porosities observed in specimens fromthe various groups.9
Curing acrylic resin in the water bath proved to be a superior curing method in terms of controlling the porosity. The samples in the self-curing group not only revealed increased number of pores but also and some were more pores that were larger than 100 um diameter showing incomplete polymerized acrylic resin powder particles which had not come into contact with the monomer liquid during mixing. Our this finding is supported by the results of Kuhar and Funduk.30 This much larger possibly could be an indication of incomplete polymerized acrylic resin powder particles which had not come into contactwith themonomer liquid duringmixing. Figure 1 displays porosities observed in some specimens fromthe various groups.With the use of themethod in this study, less than 5% porosity in the heat-cured acrylic resin specimens were found (Table 1 & Figure 1). In contrast, the specimens of the auto-ploymerized resin showed extensive and larger pores that would encourage food, bacteria and Candida lodgment and would thus lead to inflammation of the tissues underneath the denture in case oral and denture hygienewas suboptimal.
The total porosity of a porous specimen is the ratio of the pore(s) volume to the total volume of the specimen. It is expressed either as a fraction or as a percentage. Generally porosity is affected by some micro-structural parameters including the size, shape and packing and distribution of particles in the specimens. A processed material like acryilc denture base can be characterized in terms of the mean porosity, pore size diameter, and total pore surface. Similarly pores can be categorized as; through pores, blind pores, closed (internal) pores. Thus the selection of the method of assessment of porosity ill e influenced by the type of information required about porosity in amaterial.
Several variables influencing porosity in acrylic resin samples have been identified. Of these important one include; specimen thickness, curing method, curing time and curing temperature. In our studywe noted the effect of time and temperature which is supported by other studies. Matthew et al investigated the effect of the curing mode (varying curing temperature) on the type and location and distribution of pores. They found that pores gathered near the surface of cooler molds and near the enter in warmer molds for all the test resin specimens. The differing ranges and mean values of porosity seen in our study from those of the other studies could be due to several reasons and factors. Mathew et al pointed out to the varying proportion of the liquid recommended for the various brands of resin as a reason. The varying sized specimens used by different investigators could be another reason. Compagnoni et al studied rectangular resin specimens (65×40×5mm). In our study, the specimens have been thin enough to be examined under microscope for porosity analysis. Al-Doori et al15 suggested that porosity free material could only be guaranteed in sections not thicker than 3 mm. Truong and Thomas found that porosity was observed in thick specimens with a cross-section of 14×10mm. Lai et al in a comparative study involving 10mm thick acrylic specimens cured by water-bath and various levels of microwave energy found reduced porosity in the water-bath-cured specimens as compared to the that in the microwave-cured specimens. Pero et al suggested that an adequate solution for storing specimens must be used tomeasure porosity bywater absorption,Variations in the porosity percentages and pore sizes and areas have also been commonly seen in studies. The total area of the pores was found as 6.5 % and the mean diameter of the pores as 97 μm by Compagnoni et al. They found varying but much lower mean values of the percent porosity as compared to the values seen in our study Kusy assessed porosity in elf-curing acrylic bone cements using standard quantitative metallography and found that porosity ranged from 1 to 8%. Bafile et al compared porosity of denture resin cured by microwave energy to denture resin cured by the conventional heatz method and found no significant differences in mean porosity between the control group (cured in a curing tank at 165° F for 9 hours) and the four groups of microwaveprocessed samples that usedmicro liquidmonomer.
The effect of thickness (3mm and 6mm) of the specimens and different microwave energy cycles on theporosity of 2 heat-activated denture base resins has been determined by micrography. It was found that the resin designed specifically for microwave polymerization exhibited no clinically significant porosity Kuhar & Funduk compared the effects of 4 chairside polishing kits and 2 conventional laboratory techniques used for polishing 3 different acrylic denture base resins. They found that conventional laboratory polishing produced the smoothest surface of denture base acrylic resin and chairside silicone polishing kits produced significantly smoother surface of acrylic resin than specimens polished with a tungsten carbide bur. They also noted that the presence of large pores was characteristic for the autopolymerizing resinmaterial.
1. Reduced porositywas seen in specimens processed by curing in hotwater.
2. Specimens cured by the 70 oC and 7 or 9 hours curing cycles exhibited significantly lower values of ean percent porosity.
3. The very high porosity level of the selfcuring resin would not only adversely affect the strength and aesthetic properties of the processed resin but as per clinical experience and evidence the highly porous nature of the resin base will encourage the harbouring and growth of Candida in these dentures causing inflammation and soreness of the soft tissues underneath the denture.
4. The use of self-curing resins for the making of removable prostheses is not advisable.
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