Effect of temperature and duration of post-cure on selected mechanical properties of resin composites containing carboxylic anhydrides

Anne Peutzfeldt and Erik Asmussen Department of Dental Materials and Technology, Royal Dental College, Copenhagen, Denmark

Peutzjeldt A, Astnusseti E: Effect of temperature attd duration of post-cure on selected mechanical properties of resin composites containing carboxylie anhydrides. Scattd J Dettt Res 1992; 100: 296-8.

Resin composites containing carboxylic anhydrides have recently been formulated for use as inlay materials. To optimize the post-cure, the present study measured the effect of post-cure temperature and duration on four mechanical properties of anhydride-containing resin composites. Temperatures were varied between 37°C and 225"C, and durations were varied between 0 and 24 h. An optimum in strength and stiffness was found when post-eure temperatures of 125-150°C were used. Strength and stiffness were found to increase with increasing duration of post-cure. Due to discoloration of the resin composites when post-curing for 6 or 24 h, a post-cure period of 1 h was preferred.

Because of inadequate resistance to abrasion in areas of occlusal contact, resin composite is not yet the material of choice for restoring large occlusal cavities (1, 2). In an attempt to improve abrasion resistance, resin composites containing carboxylic anhydrides have been formulated (3). For the crosslinking reactions of the anhydride resins to occur, heating of the resin composites is required, enabling the resins to be used as inlay materials only. Addition of anhydride to resin composites was found to improve mechanical properties significantly (3). In developing the anhydride-containing resin composites, light-cured resin composites were postcured at 150"C for 1 h. Working with resin composites without crosslinking additives, we had earlier found 150°C to be the optimum post-curing temperature (4). To optimize the post-curing conditions of anhydride-containing resin composite, the present study measured four mechanical properties of resin composites post-cured at different temperatures or for different periods of time. i\/iateriai and methods The study was divided into two parts: in part A the effect on mechanical properties of post-cure

Key words: dental materials; diametral tensile strength; flexural strength; resilience; restorative resins; stiffness Anne Peutzfeldt, Department of Dental Materials and Technology, Norre Alle 20, DK-2200 Copenhagen N, Denmark Accepted for publication 11 October 1991

temperature was measured, and in part B the effect of post-cure duration. The effect of post-cure conditions was tested on four mechanical properties: 1) diametral tensile strength (T), 2) flexural strength (S), 3) modulus of elasticity (E), and 4) modulus of resilience (R). Two resin composites, the compositions of which are given in Table 2, were prepared from the cornpounds listed in Table 1. To make the resin composites light-curing, 0.2% w/w CQ and 0.2% w/w CEMA were dissolved in the resins. Silanized glass filler was then added to a load of 80% w/w. A. Fffect of temperature - Unpolymerized material was applied in respective molds and irradiated with a Luxor unit (ICI, Macclesfield, England) for 40 s on each side. The resin composite specimens were then separated from the molds and post-cured for 1 h at one of the temperatures given in Table 3. The specimens were placed in water at 37°C for 1 wk prior to testing. B. Effect of duration - Following the initial cure, the composite specimens were post-cured at 150°C for one of the periods of time listed in Table 4, and then stored in water for 1 wk. After water storage, the specimens were ground flat and the dimension of the specimens measured. The specimens were subjected to testing in a Universal Testing Machine (Instron Ltd., High Wy-

Anhydrides in composites: post-curing Table 1

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297

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Materials used for preparation of resin composites Initials Supplier Compound BISGMA Rohm GmbH Chem. Fabrik, Darmstadt, Germany Bisphenol-A-glycidyl dimethacrylate TEGDMA Aldrich-Chemie, Steinheim, Germany ^ • Triethyleneglycol dimethacrylate UEDMA Ivoclar AG, Schaan, Liechtenstein Urethane dimethacrylate* HEMA Merck-Schuchardt, Munich, Germany 2-Hydroxyethyl methacrylate CQ EGA-Chemie, Albuch, Germany Camphor quinone CEMA Ivoclar AG, Schaan, Liechtenstein . . _ . N,N-cyanoethyl rnethylaniline MAA Fluka Chemie AG, Buchs, Switzerland [ Methacrylic acid MAAM Merck-Schuchardt, Munich, Germany ' Methacrylamide MAN Merck-Schuchardt, Munich, Germany Maleic anhydride SUCAN Aldrich-Chemie, Steinheim, Germany Succinic anhydride GM 31685 SchoU-Schleiffer AG Feldbach, Switzerland Silanized glass filler * RUYTER (7).

combe, England) and the four mechanical properties calculated as earlier described in detail (3). Eight specimens were used in each testing mode for each of the post-cure conditions. The statistical treatment of the results involved one-way analyses of variance, regression analyses (5), and NewmanKeuls' Multiple range tests (6) with P = 0.05 as the level of significance.

Table 2 Composition of resin cotnposites (in mol-%) used jbr measuring ttie effect of tetnperature (A) and of duration (B) of post-cure A B Compound Mol-% Mol-% BISGMA 20 TEGDMA 20 45 ; UEDMA 20 15 HEMA 5 MAA 5 25 15 MAAM 20

MAN

SUCAN

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,

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10

Al

C 37 75

A3 A4 A5 A6 A7 A8

100 125 150 175 200 225

. - : , : . : -

:• .

The mean values' and pooled standard deviations of T, S, E, and R are presented in Table 3 for part A and in Table 4 for part B. A. Effect of temperature - Results of the statistical test are summarized in Table 3. Compared to the control series (Al), T increased as a result of post-cure at elevated temperatures. Only when the temperature was very high, did T decrease (A8). The highest values of T were found when postcuring at 100°-175°C. With respect to S, a postcure of 75°C caused S to decrease (A2) while 125°C, 150°C and 200°C post-curings meant an increase in S. Post-curing at 125°C or 150°C also improved E. As with S, a 75''C post-cure gave a reduction in R. B. Effect of duration - Table 4 shows the statistical results. T was enhanced as a result of post-cure. One h, 6 h and 24 h post-cures gave highest and similar T values. As in the case of T, S increased as a result of post-cure compared to the control value (Bl). Six hour and 24 h post-cures resulted in highest S values. Only post-curing for 6 or 24 h improved E compared to the control series (Bl).

Table 3

Diametrat tensite strength (T). ftexurat strettgth (S), tnodutus of elasticity (E), and modulus of resilience (R) of resin composite A, post-cured for I h at temperatures listed. Pooled statidard deviations were: Sr = 4.3 MPa, Ss = 17 MPa. S!: = 1.0 GPa, and SR = 0.35 MJ/m\ Values with same lettering were not significantly dijferent Results Temperature Code

Results

T (MPa) S (MPa) E (GPa) R (MJ/m') 63" 182" 9.3"" 1.8" 71' 148" 8 5" 1.3" 1.9" 9.4"" 190" 12.7" 1.8" 213'" 76'" 2.2" 11.5' 224" 76'" 2.2" 9.3"" 200"' 73'" 2.3" 10.2" 217'" 64" 2.0" 192" 9.1"" 43"

Table 4 Diametral tensite strength (T), flexural strength (S). modulus of elasticity (E), atid tnodutus of resitience (R) of resin cotnposite B. post-cured at 150" for different periods of time. Pooted standard deviations were: Sj = 3.4 MPa. s^=13 MPa, s,. = ].O GPa, and .SR = 0.34 MJ/tn'. Values witti same ettertng 1 were trot signtficantty different Results Duration S (MPa) E (GPa) R (MJ/m') (MPa) h T Code 6.7" 1.6" 148" 58" 0 Bl B2 1/6 63" 164" 6.7" 2.0" 65"' 175"' 7.5"" 2.0" 1/2 B3 7 y;,bc 1 68'" 170" 1.9" B4 6 71" 187'" 8.7'" 2.0" B5 24 70" 193" 8.9' 2.1" B6

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Feutzfeldt and Asmussen

With respect to R, no difference was found between the six R values. Significant correlations were found between duration of post-cure and T, S, and E, respectively (/>< 0.001). Discussion An optimum in tensile strength, flexural strength and modulus of elasticity was formed when postcuring took place at 125°-150°C. This is in agreement with what was earlier found with four resin composites with no carboxylic anhydrides or other crosslinking additives (4). Besides poorer mechanical properties, post-curing at higher temperatures (175°-225°C) caused discoloration of the resin composite specimens, disqualifying the use of temperatures higher than 15O'C. Tensile strength, flexural strength, and modulus of elasticity were found to increase with increasing duration of the post-cure, which means that a 24 h post-cure seemed the curing time of choice. However, a brown discoloration of the resin composite was observed when post-curing took place for 6 h or for 24 h. The 1 h post-cure resulted in values of tensile strength, stiffness, and resilience of similar magnitude to those achieved with the 6

and 24 h post-cures. Thus, in spite of a small decrease in flexural strength when post-cure duration was reduced to 1 h, the 1 h post-cure is preferable. Also, from the point of view of the dental technician manufacturing the resin composite inlays, a 1 h post-cure is more practicable than a 6 or 24 h post-cure. -







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References 1. LEINFELDER K F , TAYLOR D F , BARKMEIER WW, GOLDBERG

2. 3. 4. 5. 6. 7.

AJ. Quantitative wear measurements of posterior composite resins. Dent Mater 1986; 2: 198-201. LuTZ F, IMFELDT T, PiiiLLiPS RW. P-10 - Its potential as a posterior composite. Dent Mater 1985; 1: 61-5. PEUTZFELDT A, ASMUSSEN E. Influence of carboxylic anhydrides on selected mechanical properties of heat-cured resin composites. J Dent Res: 1991; 70: 1537-41. ASMUSSEN E, PEUTZFELDT A. Mechanical properties of heat treated restorative resins for use in the inlay/onlay technique. Scatid J Dent Res 1990: 98: 564-7. HALD A . Statistical theory with engineering application. New York; John Wiley & Sons, 1952. BRUNING JL, KiNTZ BL. Cotnputatiotml hatidbook of statistics. Glenview, IL; Scott, Foresman & Co., 1977. RuYTER IE. Monomer systems and polymerization. In: VANHERLE G, SMITH DC, eds. Intertiatiottal sytnposiutn on

posterior composite resin dental restorative tttaterials. St. Paul, MN: Minnesota Mining and MFG. Co., 1985; 109-35.

Effect of temperature and duration of post-cure on selected mechanical properties of resin composites containing carboxylic anhydrides.

Resin composites containing carboxylic anhydrides have recently been formulated for use as inlay materials. To optimize the post-cure, the present stu...
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