Mona K. Marei, BDS, CAGS, MSC.D, Ahmed El-Sabrooty, BDS,
Ahmed Y. Ragab, BDS, Mohamed A. El-Osairy, BDS
 Alexandria University, Alexandria, Egypt.

 
This study was performed to evaluate the effect of adding tin or aluminum powder with average particle size of 10/u.m to heat curing acrylic resin in a concentration of 5% by volume. Four physical and mechanical properties were tested for the purpose of this evaluation: thermal conductivity, impact strength, compressive strength, and warpage. The addition of 5% by volume of either metal powder to polymethylmethacrylate (PMMA) improved the four tested properties. However, tin powder was superior to aluminum powder in improving the impact and compressive strength of PMMA, while aluminum powder was superior to tin powder in improving thermal conductivity of PMMA and decreasing its warpage. Both metal powders caused undesirable discoloration to the heat curing acrylic resin. The use of metal-filled resin is therefore recommended in the areas where it is not displayed to get the benefit of improved physical and mechanical properties of PMMA.

 
Polymethylmethacrylate has proved to be the most satisfactory denture base material currently available. Almost all dentures are being fabricated with this type of polymer. Although the properties of this material are not ideal in every respect, it has several desirable features that account for its popularity. Acrylic resins have excellent aesthetic properties, adequate strength, low water sorption, low solubility, and freedom of toxicity. They can reproduce surface details accurately and can be easily repaired. However, few but important disadvantages are inherent in this resin such as low thermal conductivity, high coefficient of thermal expansion that causes internal stresses to be released during the process resulting in dimensional inaccuracy and relatively low modulus of elasticity which causes its rapid deformation at low stresses.1
A number of investigators have shown that polymethylmethacrylate can be reinforced with various types of fibers and fillers to overcome its negative properties.2'3 Carbon fibers were added to denture resin to improve its fatigue behavior and impact strength.4 Furthermore, polyethylene and sapphire fibers were claimed to enhance the physical properties of acrylic resin.5'6 Addition of metal fibers improved some physical and mechanical properties of acrylic resin, while the incorporation of silver, copper, and/or aluminum in the form of powder to the resin was found to improve its thermal conductivity, curing shrinkage and water sorption.7 Recently, the autopolymerizing polymethyl methacrylate resin was reinforced by silane treated glass fibers and increased the strength of resin.8
The present study was conducted to evaluate the effect of the addition of 5%, by volume, of tin or aluminum to PMMA on the thermal conductivity, impact strength, compressive strength and warpage.

 
The materials used in this study were polymethyl- methacryiate (no fillers added to act as a control)*,polymethylmethacrylate plus 5% by volume of tin powder** and polymethylmethacrylate plus 5%, by volume, of aluminum powder**.7 The particle size of both metal filler powders was 10 /mm.7 The metal fillers were added to' polymethylmethacrylate powder in a mortar and mixed until a homogeneous color was attained.
The physical and mechanical properties of the metal-filled acrylic resin studied in the present investigation were (1) thermal conductivity, (2) impact strength, (3) compressive strength and (4) warpage. For each test, 15 specimens were prepared which were divided into three groups with five specimens each: Group ! represents unmodified PMMA which was used as control; Group II represents PMMA with 5% by volume of tin powder; and Group III represents PMMA with 5% by volume of aluminum powder. In all tests, specimens were flasked in stone model of certain dimensions, specific for each test, in the usual manner of flasking a denture.
Specimens for all the tests, except for warpage, were produced from machined pieces of copper having the required dimensions according to British Dental Association Specification No. 771.9

Thermal Conductivity Test:

Specimens were in the form of discs, 2.5 mm thick and 2.5 cm in diameter,as shown in Fig. 1.9 The special apparatus***, shown in Fig. 2, was designed specially for this purpose, and is schematically represented in Fig. 3. The apparatus consisted of a healer (2) made of a 0.2 mm diameter tungsten wire insulated with mica and rests on asbestos base (1). The heater is inserted into a copper block 2.5 cm in diameter and 8 mm high (3).

Between the two specimens [the control specimen (4) and the metal-filled specimen (6)], there was an intermediate copper plate 2 mm thick and 2.5 mm in diameter (5). A 2.5 cm diameter and 3 cm high cylinder made of copper served as a heating sink to radiate heat conducted by the two specimens.
Three copper constant thermocouples (8) were soldered in three holes drilled in the heater block, intermediate plate and the heat sink, respectively. The vacuum chamber consisted of a thick steel base plate filled with eight connecting copper rods insulated with vacuum tight teflon.9,10 The apparatus rested on copper O-ring and was covered by a bell jar.11,12

Electric Circuit:

The electric circuit [Fig. 41 consisted of a 220-volt stabilizer (7) connected with low voltage DC power supply (6) to feed the heater (3). Variable resistance (4) was used to maintain the current passing through the heater wire at 0.5 ampere during the test. The three thermocouples from thermal conductivity apparatus connected through a connecting box (1) to a digital thermometer (2) recorded the temperature with an accuracy of 0.10C.

Experiment Procedure:

The two parallel surfaces of the disc specimens prepared for thermal conductivity measurement were polished by the use of a very fine emery paper on a flat glass surface. The thickness of each specimen was measured by a micrometer with an accuracy of (± 0.01 mm). A thin layer of silicon grease was applied between the specimens and the contact surface of the apparatus to allow thermal resistance. The acrylic resin specimen was placed between the heater surface and the intermediate plate, while the metal filled specimen was placed between the intermediate plate and the heat sink. The vacuum chamber was closed and the rotary pump was switched on. The current passing through the heater was adjusted at 0.5 ampere and maintained constant during the work by the use of a variable resistance. Four hours later, stability was achieved and the attained temperatures T,, T2, T3 of heater surface, intermediate plate and surface of the heat sink, respectively, were recorded using the connecting box and the digital thermometer. The heat lost from the lateral surface of the specimen and intermediate plate was neglected with respect to the heat conducted to the specimen. Relative thermal conductivity was calculated from the following equation:

K2      =     d2(T,-T2)

K1             d,(T2-T3)

Where:

K1: Thermal conductivity of PMMA specimen.

K2: Thermal conductivity of metal-filled specimen.
d2: Thickness of control specimen.
d2: Thickness of metal-filled specimen.
T1: Temperature of the heater surface.
T2: Temperature of the intermediate plate.
T3:   Temperature of the surface of heat sink.
(T, - T2) = (D-q1): Temperature difference

across the acrylic resin specimen.

(T2 - T3) = (D-q2): Temperature difference

across the metal-filled resin specimen.

(D  q)      = Temperature gradient.

     d

Impact Strength Test:

This test aims at subjecting the denture to sudden load. Charpy's pendulum impact testing machine* [Fig. 5] was used to determine the relative impact resistance of acrylic resin and metal-filled acrylic resin. The machine is a pendulum-type with a disc­shaped hammer carrying a knife edge.

Rectangular specimens were prepared (7.5 cm long, 1 cm in both thickness and width), each having a 2-mm deep standard notch with 60° angle in the middle of the bar [Fig. 6]. The specimen was simply supported horizontally at both ends and not clamped.
A striking energy of 10 kg at a velocity of 5.5 meter/second was used. The specimen was struck by the hammer at the mid span while the notch in the specimen was at the opposite surface of striking position. The impact strength of a specimen was recorded as the number of kg.cm of energy absorbed in breaking the specimen.1

Compressive Strength Test:

All specimens for this test were in the form of cubes having an edge of 1 cm [Fig. 7], Cubes were subjected to compressive load until failure, using a universal testing machine*. Each specimen was placed between two parallel flat surfaces and the compressive load was gradually increased until sample failure occurred. Great care was required to insure proper transfer of load from testing machine to specimen. The load was made axially and applied uniformly over the end of the specimen. The maximum load of the testing machine selected for our purpose was five tons applied through hydraulic compressive piston. The compressive strength calculated as stress in MN/ m'2.

Warpage Test:

This test aimed at determining the areas of strain (deformation) that lead to separation of the resin base from the model subsequent to processing of a denture.
An edentulous dentiform model** was duplicated to produce 15 edentulous models. These models were used to construct the denture bases that were utilized to test for warpage. The dentures were divided equally into three groups. The first group of five denture bases had no filler. The second group of denture bases had their palatal portion made of acrylic with 5% tin by volume and the third group of denture bases had their palatal portions (same size) made of acrylic but with 5% aluminum. On the posterior surface of each model, 5 points (A, 8, C, D, E) were located [Fig. 8]:

-      Point A: the crest of the ridge (left side).
-      Point E: the crest of the ridge (right side).
-      Point C: the center of the palate (half the distance between points A and E).
-      Point B: midway between points A and C.
-      Point D: midway between points E and C.

The amount of separation between acrylic bases and the model at these selected points after flasking, was measured using a special magnifying micrometer microscope before removal of acrylic bases from the models. The measurements were made to the nearest 0.01 mm. To compare the three groups, arithmetic means and standard deviations were calculated. A student's f-test was used to determine significance of differences among means.

Thermal conductivity

Table 1 represents the ratio of thermal conductivity of tin-filied acrylic resin specimens (KTin) and acrylic resin specimens with no fillers (K,). Table 2 shows the ratio between KAluminum and Kj. The higher the ratio, the greater the ability of the substance to transmit energy and vice-versa. The mean ratio of the thermal conductivity of acrylic resin filled with tin relative to that with no filler was 1.312 ± 0.47 watt/mK. While the mean ratio of thermal conductivity for the acrylic resin filled with aluminum relative to acrylic resin with no filler was 1.439 ± 0.046 watt/mK. Thermal conductivity of acrylic filled with aluminum (KAl) was significantly (P = < 0.05) higher than the thermal conductivity of acrylic filled with tin.

Impact strength

The impact strength for both experimental groups was significantly higher (P = < 0.05) than that for the control group.

Compressive strength

Table 4 presents the compressive strength values. The mean compressive strengths of acrylic filled with tin (111.82 MN/m2) and acrylic filled with aluminum (105.74 MN/m2) were significantly (< .05) higher than that of acrylic resin with no filler (92.2 MN/m2).

Warpage

The acrylic resin group with no filler showed more warpage at the five points (A, B, C, D, E) as evidenced by greater values of separation than that which occurred with acrylic filled with lin or aluminum as shown in Table 5. However, the acrylic filled with aluminum had a lesser warpage than that filled with tin. The addition of 5% by volume of either powder tin or aluminum caused discoloration of the acrylic resin. In the case of tin powder the discoloration was more severe than in the case of aluminum powder

 
Several metals can be added in the form of powder to PMMA. Tin and aluminum were selected here because of their good thermal conductivities, low cost and ability to attain high polish. Marked gradual decrease in the tensile strength was noticed as the filler ratio increased which, in turn, limits the addition of fillers greater than 5% by volume.7
The particle size is another factor, larger particles decreased the tensile strength because they settle when mixed with monomer. Average particle size was 10 mm to match the particle size of resin powder which permits the use of a conventional method in finishing the specimens.7
In the present study, the addition of metal fillers to PMMA increases the relative thermal conductivity by 31% in case of tin powder and 44% in case of aluminum powder. The increase in thermal conductivity of resin upon the addition of metal powder has been ascribed to successful distribution of metal particles in the resin matrix even where in small percentage (5% by volume). Good filler distribution renders the resin more conductive and stronger. It seems that the distribution of metal particles permitted an overlap sufficient enough to bridge the insulating effect of polymethylmethacrylate.7,10-12
In the present study, the impact strength of metal-filled resin specimens was higher than that of the control (10% increase in case of tin and 3% increase in case of aluminum) as shown in Table 3. Also, the compressive strength of these metal-filled resin specimens had a marked improvement over the control (21.2% increase in case of tin and 13.8% in case of aluminum over the control specimens) (Table 4).
While prior studies showed an appreciable increase in the compressive strength of the resin as the filler reached 20 to 25% by volume,1,7 in the present study, an increase in both properties was recorded by the addition of 5% metai powder by volume. A recent study recorded 2.5% increase in the compressive strength of acrylic when filled with aluminum (5% by volume),7
The greatest mean warpage of all the tested denture bases occurred at the central position of the posterior border (Table 5) which is in agreement with previous studies.14-18 The addition of metal filler reduced the warpage at C-point from 0.62 mm in control specimens to 0.28 mm when tin was added (Table 5) and 0.26 mm in case of aluminum addition. Significant decrease in warpage is likely to increase the retention of upper dentures, in addition to its good thermal conductivity. Reduction in warpage was noticed in all the measured locations of the metal-filled bases.
The addition of metal powder to PMMA powder described in this study has advantages over other techniques,17 in that it was much easier and did not involve a lengthy procedure.
The unpleasant discoloration that occurred even with the inclusion of a small percentage of metal indicated that the use of metal-filled PMMA should be in areas where it is not seen. The strength of these composite might allow their use in the posterior occlusal regions to withstand chewing stresses.13 Even with this apparent discoloration, which restricts the use of metal-filled resin to the palatal portion of upper and/or lingual flanges of lower dentures, the metal reinforcement is, nevertheless, likely to reduce the fracture incidence of acrylic denture.

 
This study was conducted to evaluate the effect of adding tin or aluminum powder (5% by volume) to PMMA on four properties of heat-curing acrylic resin. The four tested properties were thermal conductivity, impact strength, compressive strength and warpage.
Results showed that the addition of either metal powder significantly improved the properties as compared to PMMA with no filler. The addition of aluminum improved the thermal conductivity and decreased the warpage of PMMA more effectively than tin. On the other hand, the addition of tin powder had superior effect regarding impact and compressive strength. The undesirable discoloration of the resin that occurred, due to metal incorporation, limits the use of metal-filled resin to areas where it is not displayed.