Contribution of postpolymerization conditioning and storage environments to the mechanical properties of three interim restorative materials

Contribution of postpolymerization conditioning and storage environments to the mechanical properties of three interim restorative materials Geoffrey A. Thompson, DDS, MS, BAa and Qing Luo, PhDb Marquette University School of Dentistry, Milwaukee, Wis; Carl Zeiss Microscopy LLC, Thornwood, NY Statement of problem. Because polymer-based interim restorative materials are weak, even well-made restorations sometimes fail before the definitive restoration is ready for insertion. Therefore, knowing which fabrication procedures and service conditions affect mechanical properties is important, particularly over an extended period. Purpose. The purpose of this study was to evaluate the effect of thermal treatment, surface sealing, thermocycling, storage media, storage temperature, and age on autopolymerizing poly(methylmethacrylate) and bis-acryl interim restorative materials. Outcome measures were flexural strength, Vickers surface microhardness, and impact strength. Material and methods. Flexural strength and microhardness of poly(methylmethacrylate) (Jet Acrylic) and 2 bis-acrylcomposite (Protemp 3 Garant and Integrity) interim restorative materials were evaluated as affected by storage media, storage temperature, storage time, thermocycling, postpolymerization thermal treatment, or application of a surface sealer. In total, 2880 beam specimens (25
2
2 mm) were fabricated. Mechanical property analyses were made at 10 days, 30 days, 6 months, and 1 year after specimen preparation. Flexural strength was determined by using a 3-point bending test in a universal testing machine with a 1 kN load cell at a crosshead speed of 5.0 mm min-1. Fracture specimens were recovered and used for determining Vickers microhardness. Measurements were made with a 0.1 N load and 15 second dwell time. Three microhardness measurements were made for each specimen, and the mean was used for reporting Vickers microhardness. Notched impact specimens (64
12.7
6.35 mm) were fabricated from Jet, Protemp 3 Garant, and Integrity interim restorative materials, yielding 288 impact specimens. Impact strengths were assessed at 10 days, 30 days, 6 months, and 1 year with a 2 J pendulum. The effects of the various experimental treatments were determined and rank ordered with analysis of variance, F ratios, and least square means differences Student’s t tests (a¼.05). Results. All experimental treatments investigated had significant effects on flexural strength, with material (P<.001) and thermocycling (P<.001) being dominant. Moreover, all experimental treatments investigated had a significant overall impact on Vickers microhardness with material (P<.001) and Palaseal glaze (P<.001) showing large effects. Material (P<.001) and age (P¼.010) had a significant effect on impact strength. Conclusions. Mechanical properties of some interim polymeric materials can be improved by postpolymerization heat treatments or surface glazing. This procedure may extend the useful lifetime of some bis-acryl interim restorations. (J Prosthet Dent 2014;-:---)

Clinical Implications Improvement of the mechanical properties of interim restorative materials may decrease the number of unscheduled repairs over the course of treatment. Interim dental prostheses are fabricated for restoring teeth when the definitive prosthesis is made indirectly. Although interim restorations are usually

a

in function for a few weeks or less, they must still satisfy certain biologic, esthetic, and mechanical requirements.1 Interim restorations should resist

removal, restore esthetics, protect the prepared tooth and periodontium, allow normal masticatory function, and maintain the relationship with the

Program director, Postgraduate Program in Prosthodontics, Marquette University School of Dentistry. Technical support engineer, Carl Zeiss Microscopy LLC, Thornwood, NY.

b

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Volume adjacent teeth and opposing dentition.2,3 Longer periods of use may become necessary when evaluating function or when replacing or stabilizing teeth during implant, periodontal, or maxillofacial procedures. No interim material possesses all the requirements necessary for every restorative situation.4-6 Because polymer-based interim restorative materials are weak (about one tenth to one half as strong as definitive restorations), even well-made restorations sometimes fail before the definitive restoration is ready for insertion.7 Therefore, observing how fabrication procedures can enhance mechanical properties is important, particularly when interim restorations are used for extended periods. Several investigations have reported that flexural strength, tensile strength, and hardness are improved when interim restorative materials receive a postpolymerization heat treatment.8-10 Energy in the form of heat may increase chain collisions, thereby generating enough energy to further the polymerization process.9,11,12 Ogawa et al13 found that immersing autopolymerizing acrylic resin in warm water during polymerization produced stronger restorations; however, the margins were not as accurate as the restorations immersed in room-temperature water. Although postpolymerization treatments can improve some properties, it may be at the expense of other properties. Residual monomer affects water sorption in acrylates and may exert a plasticizing effect by decreasing interchain forces and allowing increased deformation.14 Water sorption is a function of the pendant molecules on the polymer chains, specifically whether they are hydrophilic or hydrophobic.15 Highly cross-linked polymers and heatpolymerized PMMA have a lower diffusion coefficient for water than chemically polymerized PMMA.16 Studies have shown that water sorption detrimentally affects fatigue resistance, flexural strength, and toughness of urethane, bis-acryl, and acrylate interim

materials.17-20 Takahashi et al16 reported that immersing polymeric materials in water generally degraded strength over time; moreover, the length of time required by different polymers to reach their “equilibrium strength” varied. The equilibrium strength could be greater or lesser than the asfabricated strength. These findings are not only relevant to the results of shortterm in vitro studies but also to the effect long-term clinical use of interim restorative materials and the environment may have on properties. Few studies have evaluated the material properties of interim restorative materials over extended periods.21,22 The flexural strength of interim restorations has been correlated with load tolerance and resistance to removal.23 Although Hooke’s Law is not generally applicable to polymers, flexure testing may give a good approximation of strength.24 Measurement of flexural strength is important, but monotonic strength tests alone are not a good indicator of clinical performance.18 Fracture toughness (KIc) is a measure of resistance to crack propagation. KIc has been correlated to abrasion resistance and may be a more accurate predictor of clinical performance.20,25 Flexure tests are unable to characterize resistance to crack propagation, which is a factor of fracture toughness18 and is directly related to the ability to absorb impact energy.26 Hand-mixed resins are inherently porous, resulting in localized stress concentrations in response to an applied load. Therefore, it is beneficial to reduce porosity. One method for reducing or eliminating porosity is to add a surface glaze. Emmanouil et al27 applied glazes to acrylic baseplate resin and found that surface hardness was improved, but not strength. Hardness is an indicator of the degree of polymerization and may be used as a predictor of the wear resistance of dental materials.23,28,29 Diaz-Arnold et al30 measured the microhardness of 3 bis-acryl and 2 PMMA interim materials. The authors found that for 3 of the 5 materials surveyed, microhardness

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decreased over time, and bis-acryls were superior to methacrylates after storage in artificial saliva for 14 days. This is probably due to their rigid backbone and cross-linked chains, which should decrease susceptibility to solvents.28 Selection for a hardness test partially depends on the substrate being measured and its surface geometry. Generally, Knoop hardness tests are indicated for hard, brittle, and thin sections, while the Vickers hardness tester can be used for all types of materials.31 A Vickers microhardness test is less sensitive to surface conditions but more sensitive to measurement errors.32 Frequently, a lack of thermocycling is cited as a limitation of laboratory research. Balkenhol et al21 evaluated the effect of thermocycling on the mechanical properties of interim polymers and reported a 2-fold decrease in flexural strength after thermocycling. This study investigated the impact strength, flexural strength, and microhardness of a poly(methylmethacrylate) and 2 bis-acryl composite resin interim restorative materials as affected by storage medium, storage temperature, thermocycling, postpolymerization heat treatment, application of a surface sealer, and age. The 6 hypotheses considered were that storage temperature would not affect the microhardness, impact strength, or flexural strength of interim polymeric restorative materials; that the storage medium would not affect the microhardness, impact strength, or flexural strength of interim polymeric restorative materials; that thermocycling would not affect the microhardness, impact strength, or flexural strength of interim polymeric restorative materials; that the addition of a visible light-polymerized surface sealer would not affect the microhardness or flexural strength of interim polymeric materials; that postpolymerization heat treatment would not affect the microhardness or flexural strength of interim polymeric materials; and that age would not affect the microhardness, impact strength, or flexural strength of interim polymeric materials.

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Table I. Batch numbers and manufacturer’s information for interim restorative materials evaluated Material

Polymer Type

Batch

Integrity

Bis-acryl

040517, 042411, 010904, 030421

Dentsply Caulk

Protemp 3 Garant

Bis-acryl

129040, 151815, 159790, 169020

3M ESPE

Liquid: 1406, 1496; powder: 1430

Lang Dental Mfg Co

Jet Acrylic

Poly(methyl methacrylate)

Manufacturer

Table II. Characteristics of 10 specimens prepared for each group and for each interim restorative material undergoing flexural strength testing Group

Treatment

R

Store at 23 C in air

RW

Store at 23 C in water

B

Store at 37 C in air

BW

Store at 37 C in water

HR

Postpolymerization heat treatmentþstore at 23 C in air

HRW

Postpolymerization heat treatmentþstore at 23 C in water

HB

Postpolymerization heat treatmentþstore at 37 C in air

HBW

Postpolymerization heat treatmentþstore at 37 C in water

GR

Glazeþstore at 23 C in air

GRW

Glazeþstore at 23 C in water

GB

Glazeþstore at 37 C in air

GBW

Glazeþstore at 37 C in water

Matrix was repeated for 4 periods of storage (10 days, 30 days, 6 months, 1 year). Identical specimen set was prepared for those specimens undergoing thermocycling as treatment condition.

Three specimens were prepared for each group and for each interim restorative material undergoing impact testing

Table III.

Group

Treatment Store at 23 C in air

IR IRW

Store at 23 C in water Store at 37 C in air

IB IBW

Store at 37 C in water

Matrix was repeated for 4 periods of storage (10 days, 30 days, 6 months, 1 year). Identical specimen set was prepared for those specimens undergoing thermocycling as a treatment condition.

MATERIAL AND METHODS The materials and batch numbers are found in Table I, and a description of specimen preparation is shown in

Thompson and Luo

Tables II and III. Analyses were made at 10, 30, 180, and 365 days after specimen preparation. Flexure specimens were made in accordance with ISO standard 10477:2004(E) for polymerbased restorative materials.33 A stainless steel split block mold with a lower chase for holding the parts was manufactured for fabricating the specimens. Three identical molds were used simultaneously to hasten specimen preparation. One hundred twenty specimens (10 specimens
12 treatments) were prepared from each interim material. Specimens were manufactured for each of 4 aging periods and for both thermocycling and nonthermocycling tests. For each material and aging period, all 120 specimens were made and then randomly assigned to treatment groups. In total, 2880

flexure specimens and 288 impact specimens were prepared. PMMA specimens were fabricated in ambient conditions by mixing the polymer and monomer in a flexible dappen dish with a Teflon-coated spatula at the 2:1 powder-liquid ratio recommended by the manufacturer. Powder was weighed on an analytic balance (B203-S; MettlerToledo Inc), and the liquid was measured with a bulb pipette. After achieving a doughy consistency, the polymer was inserted into the split block mold, covered, and held with C clamps for 15 minutes before disassembling. The molds created specimens 25.0 (l)
2.0 (b)
2.0 (d) mm. The bis-acryl composite specimens were fabricated similarly, except that the material was extruded directly into the molds from a cartridge delivery system. The base and depth of each specimen was measured with a digital micrometer with accuracy to 0.001 mm (230M Series; The LS Starrett Co). Subsequently, specimens were subjected to different postpolymerization treatments and storage conditions before testing (Table II). Thermocycling (Thermo-cycling testing instrument; SABRI Dental Research Instruments Co) was always performed immediately before the date of impact or flexure testing. Water baths were filled with reverse osmosis (RO) filtered water (NANOpure Infinity Water Purification System; Barnstead International [now ThermoFisher Scientific]) kept at 5 C and 55 C. The dwell time was 25 seconds, and the transport time was 15 seconds. Five thousand cycles were completed before flexural or impact testing. Palaseal (Heraeus Kulzer-Jelenko) is a translucent, photo-activated, unfilled polymer resin surface sealer. It is dispensed and applied as a gel, then photopolymerized. For the postpolymerization heat treatment and polymerization of the surface glaze, a light polymerizing system (Triad 2000; Dentsply Trubyte) was used. Operating time for both postpolymerization heat treatment and Palaseal glaze was 90 seconds. Failure loads were determined by using a mechanical testing machine

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Table IV.

Overview of main effects on flexural strength, Hv, and impact strength

Condition

F

P

Maximum flexure strength 1621.80

<.001

Thermocycle (2 levels)

319.45

<.001

Temperature (2 levels)

Material (3 levels)

130.33

<.001

Storage medium (2 levels)

40.57

<.001

Age (4 levels)

25.12

<.001

Palaseal glaze (2 levels)

15.42

<.001

4.41

.04

Postpolymerization heat treatment (2 levels) Hv Material (3 levels)

457.69

<.001

Palaseal Glaze (2 levels)

566.99

<.001

Thermocycle (2 levels)

13.17

<.001

Storage Medium (2 levels)

12.96

<.001

Postpolymerization Heat Treatment (2 levels)

9.75

<.001

Age (4 levels)

9.65

<.001

Temperature (2 levels)

4.10

.04

25.96

<.001

Age (4 levels)

3.36

.010

Thermocycle (2 levels)

2.96

.087

Storage temperature (2 levels)

0.50

.479

Storage medium (2 levels)

0.17

.680

Impact strength Material (3 levels)

P<.05 denotes significant difference. F ratios were used to rank order magnitude of effect.

(Instron Model 5581; Instron Corp), 1 kN load cell (Instron Corp), and software (TestWorks 4; MTS Corp). All tests were performed in displacementcontrol mode and data acquired at 1 point s-1. The mechanical testing machine was calibrated annually. The 3-point bending apparatus (SATEC Systems) had a support span of 20.0 mm and 3.0 mm diameter hardened steel rollers. Specimens were centered over the supports, and a center load (3.0 mm diameter hardened steel roller) was applied at a crosshead speed of 5.0 mm min-1 until failure. Flexural strength (S) was determined by using the following equation:

S¼

3PL ; 2bd 2

where P is the load necessary to fracture the specimens, L the distance between the support spans, b the width

of the beam, and d the depth of the beam. Immediately after flexure testing, fracture pieces were recovered and used to determine Vickers microhardness. The pieces were measured at 3 locations on the tensile surface with a microhardness tester (MHT 200 and ConfiDent Software; Leco Corp) at
62.5 magnification, 0.1 N load, and 15 second dwell time. Measurements were averaged, and a mean Vickers hardness number (Hv) was determined. The microhardness tester was routinely calibrated by using hardness standards. Impact specimens were made in accordance with ISO standard D25697 to determine the Izod pendulum impact resistance of plastics.34 Specimens were 64.0 (l)
6.35 (b)
12.7 (d) mm. Specimens were prepared as previously described except for the use of a

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stainless steel split block mold for impact specimens. Because of the amount of material required and associated cost, 3 specimens were prepared for each group and for each interim restorative material undergoing impact testing (Table III). This matrix was repeated for 4 periods of aging (10 days, 30 days, 6 months, and 1 year). An identical specimen set was prepared for those specimens undergoing thermocycling. The base and depth of each specimen was measured with a digital micrometer with accuracy to 0.001 mm (230M Series; The LS Starrett Co). Before testing, notches were prepared in the face of the specimens to a depth of 2.50 mm with a motorized notching cutter (Model 22-05-03, Motorized Notching Cutter; Testing Machines Inc) at constant speed and rotation. Impact strength was determined with an Izod impact tester and a 2 J pendulum (Model 43-02, Monitor Impact Tester; Testing Machines Inc). The breaking energy was recorded in kJ/m2. The effects of the various experimental treatments on flexural strength, microhardness, and impact strength were determined by analysis of variance (ANOVA) and least square means differences Student t tests (JMP Statistical Software, v5.0.1; SAS Institute Inc) (a¼.05).

RESULTS Table IV shows the overall effects of the various experimental treatments on flexural strength, Hv, and impact strength. The effects are rank ordered from most to least impact with 1-way ANOVA F ratios for each measure. All experimental treatments investigated had significant effects on flexural strength, with material and thermocycling being dominant. Similarly, all experimental treatments investigated had significant overall impact on Hv, with material and Palaseal glaze showing large effects. Material and age were significant factors on impact strength. Table V refines the analysis shown in Table IV by examining the interactions

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Three-way ANOVA results for comparison of temperature, storage medium, and thermocycling on flexural strength and Hv

Table V. Source

Sum of Squares

df

Mean Square

F

P

Corrected model

148 003.90

7

21 143.40

25.86

<.001

Storage temperature

48 195.84

1

48 195.84

58.93

<.001

Storage medium

12 939.67

1

12 939.67

15.82

<.001

Thermocycle

57 027.17

1

57 027.1

69.72

<.001

2437.22

1

2437.22

2.98

.084

69.44

1

69.44

0.08

.771

20 440.72

1

20 440.72

24.99

<.001

2774.86

1

2774.86

3.39

.066

Error

2 340 914.60

2862

817.93

Corrected total

2 488 918.50

2869

Corrected model

968.82

7

138.40

1.81

.082

Storage temperature

212.85

1

212.85

2.78

.10

Storage medium

632.17

1

632.17

8.24

.004

Thermocycle

13.46

1

13.46

0.18

.68

Storage temperature
Storage medium

4.18

1

4.18

0.55

.82

Dependent variable: Maximum flexure stress, MPa

Storage temperature
Storage medium Storage temperature
Thermocycle Storage medium
Thermocycle Storage temperature
Storage medium
Thermocycle

Dependent variable: Hv

Storage temperature
Thermocycle

.24

1

0.24

0.003

.96

Storage medium
Thermocycle

2.70

1

2.70

0.04

.85

Storage temperature
Storage medium
Thermocycle

90.77

1

90.77

1.18

.28

Error

219 902.40

2868

76.67

Corrected total

22 871.23

2875

among storage temperature, storage medium, and thermocycling for flexural strength. A significant interaction was found between storage medium and thermocycling (P<.001). Figure 1 indicates that differences in flexural strength values between air and RO water were greater when the temperature was 37 C than when the temperature was 23 C. Similarly, the difference in flexural strength between air and RO water was greater when there was no thermocycling. No significant interactions were found among temperature, storage medium, and thermocycling for Hv or impact strength (Table V). Table VI shows the results of a 2way ANOVA with the significance levels for the interaction of Palaseal glaze and age on maximum flexural strength and Hv. For flexural strength,

Thompson and Luo

Palaseal glaze was significant (P¼.03), but the interaction between Palaseal glaze and age was not significant (P¼.50). The greatest difference between the Palaseal glaze versus the noPalaseal glaze treatment occurred at 10 days with another departure after 1 year (Fig. 2). For Hv, Palaseal glaze, age, and the interaction between Palaseal glaze and age were significant. Table VII highlights the interaction between postpolymerization heat treatment and type of material for flexural strength and Hv, showing the results of a 2-way ANOVA with the significant interaction between postpolymerization heat treatment and material. For flexural strength, material and the interaction between postpolymerization heat treatment and material were significant (P<.001). Protemp 3 Garant and Integrity

demonstrate slightly greater benefit compared with Jet PMMA. For Hv, the postpolymerization heat treatment, material, and the interaction between postpolymerization heat treatment and material were significant (P<.001), with the largest difference between the no-postpolymerization heat treatment and postpolymerization heat treatment occurring for Integrity and Protemp 3 Garant. Table VIII shows the mean flexural strength for the interim materials that received conditioning simulating the oral environment, that is, materials that were stored in 37 C water and thermocycled before flexure testing (BW, thermocycled). Integrity was significantly stronger than all other materials, with the exception of 10 day old Protemp 3 Garant. Jet PMMA was significantly weaker during all periods of

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Maximum Flexure Stress (MPa)

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quarters the surface microhardness of Jet PMMA.

85 37 deg. C

DISCUSSION

80

75

23 deg. C 70

65 RO Water

Air

Storage Medium 1 Plot showing interactions between effects of storage medium and temperature on maximum flexure stress. testing compared with Integrity and Protemp 3 Garant. Table IX shows the interaction between thermocycling and age on the impact strength for BW specimens. Integrity possessed significantly greater impact strength at 10 days and 6 months compared with all other materials and time periods. However, the impact strength of Integrity at 30 days

Table VI.

-

was not statistically greater than that of Protemp 3 Garant at 30 days. Moreover, 1 year Integrity was not statistically tougher than 1 year Protemp 3 Garant or 1 year Jet PMMA. Table X shows a similar interaction between thermocycling and age on Hv for BW specimens. Overall, the thermocycled BW bis-acryl resins used in this study exhibited one third to three

The hypothesis that storage temperature would not affect the microhardness, impact strength, or flexural strength of interim polymeric restorative materials was partially rejected, as storage temperature affected flexural strength but not microhardness or impact strength. The hypothesis that storage medium would not affect the microhardness, impact strength, or flexural strength of interim polymeric restorative materials was also partially rejected, as storage medium significantly affected flexural strength and microhardness but not impact strength. Thermocycling had a significantly detrimental effect on flexural strength but not on microhardness or impact strength; therefore, the hypothesis that thermocycling would not affect the microhardness, impact strength, or flexural strength of interim polymeric restorative materials was partially rejected. The hypothesis that the addition of a visible light-polymerized surface sealer would not affect the microhardness or flexural strength of interim polymeric materials was

Two-way ANOVA results for comparison of Palaseal glaze and age on maximum flexural strength and Hv

Source

Sum of Squares

df

Mean Square

F

P

Corrected model

39 915.6

7

5702.2

6.66

<.001

Palaseal glaze

4 106.39

1

4106.4

4.80

.03

Age

31 472.82

3

10 490.94

12.26

<.001

2033.25

3

677.75

0.79

.50

Error

2 449 002.9

286

855.70

Corrected total

2 488 918.5

22 869

Corrected model

35 776.17

7

5110.88

79.19

<.001

Palaseal glaze

31 287.75

1

31 287.75

484.89

<.001

Age

533.43

3

177.81

276

.041

Palaseal glaze
Age

2979.12

3

993.04

15.39

<.001

Error

185 095.06

2868

64.54

Corrected total

220 871.23

2875

Dependent variable: Maximum flexure stress, MPa

Palaseal glaze
Age

Dependent variable: Hv

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Vickers Microhardness (VHN)

-

20

Palaseal Glaze

No Palaseal Glaze 10

10 days

30 days

6 months

1 year

Age 2 Plot showing interactions between effects of Palaseal glaze, age, and Hv of all groups. rejected, as the addition of Palaseal surface glaze significantly affected microhardness and flexural strength. The hypothesis that postpolymerization heat treatment would not affect the microhardness or flexural strength of interim polymeric materials was partially rejected, as postpolymerization heat treatment increased only surface microhardness. The hypothesis that age would not affect the microhardness, impact strength, or flexural strength of interim polymeric materials was

rejected, as age had a significant effect on flexural strength, microhardness, and impact strength. Flexure beams stored in RO water demonstrated decreased flexural strength and elastic modulus compared with their counterparts stored in air (Fig. 1). However, specimens stored in 37 C water exhibited greater flexural strength and modulus compared with those stored in 23 C water (Fig. 1). Elevated storage temperature may have extended the polymerization reaction of

the interim materials, as evidenced by improvement in properties.8,12 Similar to Balkenhol et al,21 this study found autopolymerizing PMMA resin to have significantly lower fracture strength at all aging times compared with the bis-acryl materials (Table VIII). However, PMMA interim materials are linear molecules and therefore should exhibit lower strength and elastic modulus.30 The elastic modulus of the materials measured in this study differed greatly. Jet PMMA (1.93 GPa) exhibited an increased strain to failure and was approximately half as rigid as Integrity (3.43 GPa) and three quarters as rigid as Protemp 3 Garant (2.31 GPa). In addition, the inherent flaws and porosity produced by air entrapment when hand mixing interim polymer materials affect the strength, as ultimate failure will occur when the stress at any one flaw leads to unstable crack growth and extension. Automixed materials would be expected to have less porosity. Takahashi et al16 reported that interim materials took up to 30 days to achieve equilibrium flexure strength. However, in this study, equilibrium strength was sometimes not observed or was not stable, as evidenced by

Two-way ANOVA results for comparison of postpolymerization heat treatment and material on flexural strength and Hv

Table VII. Source

Sum of Squares

df

Mean Square

F

1 178 015.90

5

235 603

514.74

P

Dependent variable: Maximum flexure stress, MPa Corrected model Triad cure

<.001

12.40

1

12.40

0.027

.870

1 025 369.70

2

512 684.85

1120.09

<.001

7739.90

2

3869.95

8.45

<.001

Error

1 310 902.60

2864

457.72

Corrected total

2 488 918.47

2869

50 593.38

5

170.55

<.001

Material Material
Triad cure

Dependent variable: Hv Corrected model

10 118.70

3896.49

1

83 896.49

66.67

<.001

45 590.21

2

22 795.11

383.21

<.001

2121.54

2

1060.77

17.88

<.001

Error

170 277.84

2870

59.33

Corrected total

220 871.23

2875

Triad cure Material Material
Triad cure

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Table VIII.

Mean flexural strength values of BW specimens (thermocycled)

Flexural Strength, MPa

Material, Age Integrity, 30 d

A

108.62

Integrity, 6 mo

A

106.92

Integrity, 10 d

A

Protemp 3 Garant, 10 d

B

101.55

B

Integrity, 1 y

C

92.66

C

91.11

Protemp 3 Garant, 1 y

D

69.95

Protemp 3 Garant, 6 mo

D

69.87

Protemp 3 Garant, 30 d

D

69.21

Jet PMMA, 10 d

E

52.58

Jet PMMA, 1 y

E

F

50.79

Jet PMMA, 6 mo

E

F

44.42

F

42.62

Jet PMMA, 30 d Levels with different letters are significantly different (P<.05).

changing flexural strength (Figs. 3-5, Table VIII). Treatment groups BW, HBW, and GBW were the interim materials that received conditioning to simulate the oral environment; after preparation, they were stored at 37 C in RO water and thermocycled before testing. In addition, HBW received a postpolymerization heat treatment and GBW received a Palaseal glaze before aging. For Protemp 3 Garant flexure

Table IX.

specimens, Groups BW, HBW, and GBW possessed its greatest flexural strength at 10 days, then decreased markedly. The strength of Integrity specimens peaked at 10 days (GBW) or 30 days (BW, HBW) and was significantly weaker after 1 year compared with all other times (Figs. 3-5, Table VIII). BW and HBW Jet PMMA specimens decreased in strength and then increased in strength over time.

Mean impact strength values of IBW specimens (thermocycled)

Impact Strength, kJ/m2

Material, Age Integrity, 6 mo

A

2.34

Integrity, 10 d

A

2.26

Integrity, 30 d

A

Protemp 3 Garant, 30 d

B B

2.14 C

1.82

Integrity, 1 y

C

D

Protemp 3 Garant, 1 y

C

D

E

1.69

Protemp 3 Garant, 10 d

C

D

E

1.64

Protemp 3 Garant, 6 mo

C

D

E

1.63

PMMA, 1 y

C

D

E

1.49

PMMA, 6 mo

D

E

1.42

PMMA, 10 d

D

E

1.39

E

1.33

PMMA, 30 d Levels with different letters are significantly different (P<.05).

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For GBW Jet PMMA, the flexural strength peaked after 30 days (Figs. 3-5, Table VIII). Contrary to the results published by Akova et al28 and Diaz-Arnold et al,30 the BW bis-acryl resins used in this study exhibited statistically lower surface hardness than Jet PMMA (Table X). However, those studies used a Knoop indenter rather than a Vickers indenter, and thermocycling was not performed. In addition, Akova et al28 formed specimens against glass and Diaz-Arnold et al30 polished specimens before testing. The specimens in this study were formed in stainless steel split molds and were unpolished. For reasons previously explained, a Vickers indenter may be a better selection for polymeric materials. Considering microhardness and treatments BW, HBW, and GBW, the application of a Palaseal glaze did not significantly increase the Hv of Jet PMMA. However, comparing BW specimens with GBW, a Palaseal glaze approximately doubled the microhardness of Integrity (from 9.8 to 19.4 Hv) and almost tripled the microhardness of Protemp 3 Garant (from 6.25 to 16.6 Hv). For bis-acryl materials, a surface glaze may reduce the leaching of unpolymerized monomer or prevent absorption of water; both processes should increase microhardness. Considering all treatment groups and the interaction of age and Palaseal glaze, the application of a Palaseal glaze resulted in significantly greater surface microhardness (P<.001) compared with specimens receiving no glaze (Table VI, Fig. 2). Postpolymerization heat treatment significantly increased the surface microhardness of each interim material except Jet PMMA, but not to the magnitude achieved by Palaseal glaze (Table VII). Hardness may be an indicator for wear resistance. Conceivably, decreased wear would manifest itself as acceptable clinical performance, particularly as strength is related to cross-sectional area. The results indicate that Integrity and Protemp 3 Garant would benefit by postpolymerization heat treatments or

Thompson and Luo

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9

Table X.

Mean microhardness values of BW specimens (thermocycled)

Material, Age

VHN

Jet PMMA, 6 mo

A

Jet PMMA, 1 y

A

B

19.19

Jet PMMA, 30 d

A

B

18.14

Jet PMMA, 10 d

19.75

B

Integrity, 1 y

C

17.61

C

15.51

Integrity, 6 mo

D

12.89

Protemp 3 Garant, 6 mo

E

Integrity, 30 d

E

F

Integrity, 10 d

E

F

G

8.27

Protemp 3 Garant, 30 d

E

F

G

7.63

Protemp 3 Garant, 1 y

9.26

F

Protemp 3 Garant, 10 d

8.93

G

6.91

G

6.14

VHN, Vickers hardness number. Levels with different letters are significantly different (P<.05).

Palaseal glazing for increasing surface microhardness. Only aging and type of material had significant effects on impact strength (Table IV). With regard to interim materials that received conditioning simulating the oral environment (BW, thermocycled), 10 day, 30 day, and 6

month Integrity specimens possessed significantly greater impact strength than did Integrity specimens at 1 year. No significant differences were found between the impact strength of Protemp 3 Garant or Jet PMMA specimens at any time (Table IX). Similar to the results of flexural strength, 1 year

120

Maximum Flexure Stress (MPa)

110 100 Integrity 90 80 Protemp 3 Garant 70 60 Jet PMMA 50 40 30

10 days

30 days

6 months

1 year

Integrity possessed significantly less impact strength compared with 6 month specimens. Regarding individual materials, the mean impact strength for BW, thermocycled materials were Integrity (2.11 kJ/m2) > Protemp 3 Garant (1.70 kJ/m2) > Jet Acrylic (1.41 kJ/m2). Integrity was significantly tougher than Protemp 3 Garant, which was significantly tougher than Jet Acrylic. Aqueous solvents exert a plasticizing effect on materials that are not cross-linked28 and may increase toughness or impact strength. Although water sorption was not measured, longer periods of water storage may allow greater sorption and concomitantly greater toughness. Yet an increase in toughness over time was observed in Protemp 3 Garant and Jet PMMA but not Integrity, which may indicate a greater degree of polymerization and cross-linking. Some of the outcomes reported in this study contradict previous studies and are likely due to aging and conditioning treatments. Moreover, evaluation methods will affect the observed behavior of polymeric materials and consequently the characterization of properties. This study could not show a correlation between flexure strength, microhardness, and impact strength; therefore, extrapolation of performance from one test to another would be inappropriate at least for these materials. Although no interim restorative material exhibited the best properties under all conditions and at all times, clinical success likely relies on several of the measured parameters in this study, given that the evaluated materials have been successfully used by restorative dentists for years. A limitation of this study is that no assessment of dimensional change or marginal accuracy was made after thermocycling or aging.

CONCLUSIONS

Age 3 Plot showing interactions between effects of material and age and flexure strength of thermocycled BW specimens.

Thompson and Luo

1. A postpolymerization heat treatment or the application of a Palaseal

10

Volume 120

Maximum Flexure Stress (MPa)

110 100 Integrity

90 80

Protemp 3 Garant

70 60

Jet PMMA

40

10 days

30 days

6 months

1 year

Age 4 Plot showing interactions between effects of material and age and flexure strength of thermocycled HBW specimens. 120

Maximum Flexure Stress (MPa)

110 100 90

Integrity

80 70

Protemp 3 Garant

60 50 Jet PMMA

40 30

Issue

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Garant, but not microhardness or impact strength. 4. Storage medium significantly affected the flexural strength and microhardness of Jet PMMA, Integrity, and Protemp 3 Garant, but not impact strength. 5. Thermocycling had a significantly detrimental effect on flexural strength of Jet PMMA, Integrity, and Protemp 3 Garant, but not microhardness or impact strength.

REFERENCES

50

30

-

10 days

30 days

6 months

1 year

Age 5 Plot showing interactions between effects of material and age and flexure strength of thermocycled GBW specimens.

glaze significantly improves the microhardness of Integrity and Protemp 3 Garant interim restorative materials. 2. Aging has a significant effect on the flexural strength, impact strength,

and microhardness of Jet PMMA, Integrity, and Protemp 3 Garant. 3. Storage temperature significantly affects the flexural strength of Jet PMMA, Integrity, and Protemp 3

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30. Diaz-Arnold AM, Dunne JT, Jones AH. Microhardness of provisional fixed prosthodontic materials. J Prosthet Dent 1999;82: 525-8. 31. Meyers MA, Chawla KK. Mechanical behavior of materials. 2nd ed. Cambridge: Cambridge University Press; 2009. p. 220, 223, 507. 32. Souder W, Paffenbarger GC. Physical properties of dental materials. National Bureau of Standards Circular C433. Washington, DC: US Government Printing Office; 1942. p. 54. 33. International Organization for Standardization. Dentistry-polymer-based crown and bridge materials. ISO 10477. http://www. iso.ch/iso/en/prods-services/ISOstore/store. htm 34. International Organization for Standardization. Standard test methods for determining the Izod pendulum impact resistance of plastics. ISO D256e2010. http://www.iso. ch/iso/en/prods-services/ISOstore/store.htm Corresponding author: Dr Geoffrey A. Thompson Marquette University School of Dentistry PO Box 1881 Milwaukee, WI 53201-1881 E-mail: [email protected] Copyright ª 2014 by the Editorial Council for The Journal of Prosthetic Dentistry.