Observations on the Elastic Behavior of a Synthetic Orthodontic Elastomer E.F. HUGET1, K.S. PATRICK2'3, and L.J. NUNEZ'2 'College of Dentistry and 'College of Pharmacy, University of Tennessee, 894 Union Avenue, Memphis, Tennessee 38163; and3 College of Phannacy, Medical University of South Carolina, 171 Ashley Avenue, Charleston, South Carolina 29425 The study focused on changes in elasticity that accompany water storage of a synthetic orthodontic elastomer. We plotted loading and unloading curves to permit the direct measurement of instantaneous elastic recovery (IER) and permanent set (PS) and the calculation of delayed recovery (DR). We obtained baseline data by testing dry asreceived material. Comparable tests were performed on material that had been stored in water at 37'C for one, seven, 14, 42, and 70 days. Gas chromatography-mass spectrometry was used for analysis of organic substances leached from the elastomer by water. A two-way ANOVA revealed that extension distance and water storage duration affected load requirement, IER, PS, and DR. The presence of leachable organic substances in 14-, 42-, and 70-day storage water was evidence of time-dependent matrix decomposition. Findings from tests of elastic performance and analysis of specimen storage water indicate that exposure of the elastomer to water leads first to weakening of noncovalent forces and subsequently to degradation. J Dent Res 69(2):496-501, February, 1990
Introduction. Synthetic elastic closure devices enjoy extensive use in canine retraction, diastema reduction, rotational correction, and general space closure. Presently, the use of these materials for intra-arch tooth movement appears to exceed that of natural latex elastics (De Genova et al., 1985). Although knowledge of the exact compositional features of synthetic orthodontic elastomers remains proprietary, it is likely that most are polyurethanes. Industrial polyurethanes are not inert materials. They decompose under prolonged contact with enzymes (Phua et al., 1987), water (Schollenberger and Stewart, 1971), and moist heat (Magnus et al., 1966). Moreover, in the dental context, the clinical behavior of synthetic elastic polymers is highly problematic. When stretched beyond their elastic limit and maintained under constant strain (extension), the elastomers exhibit an undesirable feature known as load relaxation. Clinically, first-day load losses can lead to a 50 to 75% reduction of the initial movement force applied to a malpositioned tooth (Brantley et al., 1979). The orthodontic literature is replete with reports on the load relaxation of synthetic closure devices (Andreasen and Bishara, 1970; Bishara and Andreasen, 1970; Hershey and Reynolds, 1975; Kovatch et al., 1976; Brantley et al., 1979; Young and Sandrik, 1979; De Genova et al., 1985). Lacking, however, is definition of the mechanisms that contribute to timedependent load decay. Such mechanisms may include physical breakdown, chemical decomposition, or combinations thereof. Also lacking are data that characterize the components of conjoint viscous and elastic behavior. We initiated the present research to (1) study an elastomer's behavior under finite conditions of loading, unloading, and specimen storage; (2) detect the presence of degradation products leached from the elastomer upon prolonged exposure to Received for publication May 25, 1989 Accepted for publication November 2, 1989
496
water; and (3) correlate observed changes in the test material's elastic behavior with structural and compositional changes resulting from water storage.
Materials and methods. One hundred and eight 6.3-cm segments were cut from a roll of the test material, Ormco® Grey Power Chain® II (Ormco Corporation, Glendora, CA). Eighteen segments were reserved for characterization of baseline elastic behavior. Each remaining piece was placed in a separate glass vial that contained 20 mL of de-ionized water. The vials were capped, transferred to an oven maintained at 370C ± 20C, and stored for periods of one, seven, 14, 42, and 70 days. At the end of each storage period, 18 vials were removed from the oven and distributed randomly so that three six-member test groups would result. This process yielded equal specimen populations for study of the test material's recovery from three different extension distances. The test pieces were removed from their respective storage vials (one at a time) and positioned between the vertically aligned, SC Elastomeric Grips of a constant strain-rate testing machine (Instron Universal Testing Machine, Instron Corp., Canton, MA). The provisionally positioned specimens were manipulated manually for placement of a 3.8-cm length of test material between the spring-loaded roller and contact plate of each grip. This arrangement yielded a linear, unstrained, vertically-oriented test-strip consisting of a continuous train of 11 circular modules and 10 intermodule couplers in the following configuration: 0-0-0-0. Specimen testing entailed the generation of loading and unloading curves. Initial loading profiles were established for extensions of 50% (1.9 cm), 100% (3.8 cm), and 200% (7.6 cm). When the desired extension was reached, the direction of crosshead movement was reversed for initiation of unloading. Completion of unloading was marked by the crosshead's return to the 3.8-cm working distance between specimen grips. After a wait of a prescribed time (1.5 min) for recovery to occur, the specimen was subjected to a second loading. Machine crosshead speeds for loading and unloading were 1.27 cm/min and 5.08 cm/min, respectively. Chart speed for loading and unloading was 2.54 cm/min. The use of loading and unloading curves for assessment of elastic behavior is illustrated in Fig. 1. For the depicted situation, specimen length was 3.8 cm. Total extension, as indicated by crosshead travel, was 3.8 cm (100%). Load (4.7 N) at total extension was coincident with the highest point of the loading curve (A). Measurement of instantaneous elastic recovery (IER) was made possible by a perpendicular line being drawn from the point of maximum extension to the common x-axis of the loading and unloading curves, and extension of the linear portion of the unloading curve to the x-axis (A). The 0.2-cm distance between the x-axis intercepts of the perpendicular line (7.6 cm) and the extended linear portion of the unloading curve
Vol. 69 No. 2
ELASTIC BEHAVIOR OFAN ORTHODONTIC ELASTOMER
(7.8 cm) reflects chart travel time affiliated with "instantaneous recovery". Time (t) is calculated by: CT t = C-s Cs where: CT = chart travel in cm, CS = chart speed in cm/ min, and t = time in min. During the time the chart traveled 0.2 cm, the machine's crosshead, moving at an unloading speed of 5.08 cm/min, traversed a distance of 0.4 cm. Instantaneous recovery crosshead travel (ICHT) is calculated by: ICHT = UCHS x t where: UCHS unloading crosshead speed in cm/min, t = time in min, and ICHT = instantaneous recovery crosshead travel in cm. Thus, IER is defined as the first component of elastic behavior to manifest upon rapid unloading of the visco-elastic elastomer. Instantaneous elastic recovery is calculated by: ICHT IER= T x 100
497
CROSSHEAD TRAVEL = 3.8 cm
r 5
z cl
4
0 -j
=
0
where: ICHT = instantaneous recovery crosshead travel in cm, TE = total extension in cm, and IER = instantaneous elastic recovery in percent. For measurement of permanent set (PS), the moving chart was marked at 11.8 cm so that termination of the 1.5-minute recovery period and the concurrent re-activation of the crosshead (B) would be denoted (chart distance at onset of recovery period = 8 cm; chart travel = 2.54 cm/min x 1.5 min = 3.8 cm; 8 cm + 3.8 cm = 11.8 cm). The axis-segment between this mark and the point from which re-loading began (12.6 cm) denotes the 0.8-cm distance the chart traveled during the take-up of specimen slack. Permanent set (PS) is calculated by: CT LCHS PS=-X ~~XlOO TB Cs where: CT = chart travel in cm, LCHS = loading crosshead speed in cm/min, TE = total extension in cm, CS = chart speed, and PS = permanent set in percent. Delayed recovery (DR) is calculated from the Eq.: DR = TE - IER - PS where: TE = total extension in percent, IER = instantaneous elastic recovery in percent, PS = permanent set in percent, and DR = delayed recovery in percent. Through the use of a statistical package (Analysis System Version 5, SAS Institute, Inc., Raleigh, NC), data pertaining to elastic behavior were stored and analyzed on a computer (VAX 8600, Digital Equipment Corp., Maynard, MA). The experiment's two-factor design facilitated a two-way ANOVA for extension effect, storage duration effect, and the effect of extension and storage duration interaction. For each specimen storage period, storage water affiliated with the six randomly selected test pieces was decanted into a beaker and evaporated to dryness in an oven at 370C ± 20C. A control beaker containing 120 mL of de-ionized water was treated similarly. Then the beakers were covered and kept in the oven until analysis of their residual contents could be accomplished.
2
4
6
8
10
18
20
CHART DISTANCE, cm
CROSSHEAD TRAVEL, cm
I 0.4
2.4
B
z
0~ot -J
21
FNERYM
1
10
12
14
16
CHART DISTANCE, cm
Fig. 1-(A) Loading and unloading curves for a synthetic orthodontic elastomer. Specimen length = 3.8 cm; specimen extension at peak load (4.7 N) = 3.8 cm (100%); chart speed = 2.54 cm/min; crosshead speeds for loading and unloading = 1.27 and 5.08 cm/min, respectively; recovery = chart travel interval (8.0 to 11.8 cm) affiliated with a 1.5-minute pause between unloading and subsequent re-loading of the specimen. (B) Reloading curve (continuation of A); vertical lines at chart travel distance of 11.8- and 12.6-cm mark termination of the recovery period and the onset of re-loading, respectively; PS = permanent set; chart and crosshead speeds for re-loading = 2.54 and 1.27 cm/min, respectively. [Note: criteria and formulae for calculation of instantaneous elastic recovery (IER), permanent set (PS), and delayed recovery (DR) are presented in "Materials and methods".]
HUGET et a.
498 1 0-
z
J Dent Res 20
* EXT. 1.9 cm * EXT. 3.8 cm 0 EXT 7.6 cm
w I-. w
8-
Tr
6-
T z z 4c
4-
4
as0
-j
EXT. 1.9 cm EXT. 3.8 cm EXT. 7.6 cm
15-
-j
4 cx 0
-
U U
2F
0 z z
February 1990
04
0.0
wL
IJ
2-
1.0
10-
7.0 14.0 42.0 70.0 TIME, DAYS Fig. 2-Effect of extension distance and water storage duration on load requirement. Standard deviations are denoted by error bars.
0.0
1.0
7.0
14.0
42.0
70 0
TIME, DAYS Fig. 4-Effect of extension distance and water storage duration on permanent set. Standard deviations are denoted by error bars.
20 * EXT. 1.9 cm * EXT. 3.8 cm * EXT. 7.6 cm
100 W
* EXT. 1.9 cm * EXT. 3.8 cm O EXT. 76cm
15-
80
0 0 w 0 Ch -J
w 0
10-
w
60
0
(n
m
0
40 4c
5
-j
wL0
z en I--
20
CA)
z
0-
0.0
1.0
7.0
14.0
42.0
70.0
TIME, DAYS Fig. 3-Effect of extension distance and water storage duration on instantaneous elastic recovery. Standard deviations are denoted by error bars.
Analysis of organic substances leached from the elastomer was performed on a 9610 gas chromatograph-4000 mass spectrometer (Finnigan Corp., San Jose, CA). The instrumentation was calibrated with perfluorotributylamine. Approximately 60 AtL of methanol was added to the control beaker and to each of the five beakers that had contained pooled-specimen storage water. Approximately 1 ,uL of solute-free methanol or a resultant methanol solution of residual organic matter was withdrawn from each beaker and injected onto a 30 m x 0.32 mm DB-1 dimethylsilicone fused silica column (J and W Scientific, Folsom, CA). The injector port and column were operated at 270'C, and the helium carrier velocity was 50 cm/s. Fragmentation of eluting components was accomplished by electron impact at an ionizing voltage and current of 70 V and 250 ALA, respectively. The data system (Teknivent, St. Louis, MO) scanned and acquired masses ranging from 50 to 700 atomic mass units (A). Scans were made every 2.4 s, with a sweep width of 0.2 p integrating each acquisition for 1 ms.
04 0.0
1.0
7.0 14.0 TIME, DAYS
42.0
70.0
Fig. 5-Effect of extension distance and water storage duration on delayed recovery. Standard deviations are denoted by error bars.
Results. Data relating to the elastic behavior of the polymeric material are presented in Figs. 2-5. Mean baseline values for "dry" control specimens (water storage duration = 0.0 days) were as follows: Loads required for achievement of total extensions of 1.9 cm, 3.8 cm, and 7.6 cm were 3.38 N, 4.62 N, and 7.38 N, respectively. Instantaneous recovery from respective deformations of 1.9, 3.8, and 7.6 cm was 0.20 cm (IER = 10.5%), 0.40 cm (IER = 10.5%), and 0.80 cm (IER = 10.5%). After elongation to 50, 100, and 200% of their initial length, control specimens exhibited unrecovered deformation of 0.16 cm (PS = 8.4%), 0.39 cm (PS = 10.4%), and 0.80 cm (PS = 10.5%), respectively. During the 1.5minute pause between unloading and re-loading, the material's recovery from respective elongations of 50, 100, and 200% was 1.54 cm (DR = 81.1%), 3.01 cm (DR = 79.0%), and 6.02 cm (DR = 79.1%).
Vol. 69 No. 2
ELASTIC BEHAVIOR OF AN ORTHODONTIC ELASTOMER
Loads required for production of the desired extension of specimens stored in water for one day declined sharply from baseline levels (Fig. 2). Deceleration of the load requirement's decline occurred between the first and seventh days of water storage. Load requirements for 14-day specimens were remarkably higher than those of specimens exposed to water for shorter durations. This transient change was followed by a renewed decline in load requirement for 42- and 70-day test pieces. The respective loads required for production of specified extensions of one-, seven-, and 70-day specimens were strikingly similar. Instantaneous elastic recovery of the test material from extensions of 50% and 100% was not affected by water storage (Fig. 3). After storage for seven days, the mean IER value for specimens extended to 200% (7.6 cm) fell to 10.1%. The mean IER value for 42- and 70-day specimens extended to 200% (7.6 cm) was 8.7%. Permanent set peaked on the 14th day of water storage (Fig. 4). Mean values for PS associated with extension of 50% (1.9 cm), 100% (3.8 cm), and 200% (7.6 cm) were 9.4, 11.0, and 12.3%, respectively. Recovery that occurred during the 1.5-minute pause between unloading and re-loading accounted for about 88 to 90% of the elastomer's total recovery. Mean values for DR (Fig. 5) ranged from 77.2% (TE = 200% and storage time = 14 days) to 81.8% (TE = 100% and storage time = 42 days). A two-way ANOVA revealed that extension distance and water storage duration affected load requirement, IER, PS, and DR. Interactions of extension distance and storage duration affected load requirement, IER, and PS, but not DR (p > 0.05). The statistical significance of all findings relating to the test material's elastic behavior is summarized in the Table. Gas chromatography failed to establish the presence of organic material in beakers that had contained one-day and sevenday storage water. Also devoid of residual organic matter was the control beaker. On the other hand, chromatographic analysis of material recovered from 14-, 42-, and 70-day storage water confirmed the presence of organic substances. Based on column retention times and mass spectra, these samples contained common degradative components. The chromatogram pertaining to the 14th day of specimen storage is shown in Fig. 6. Mass spectra of compounds that gave rise to peaks at retention times of 3.9 min (compound 1) and 5.9 min (compound 2) are presented in Figs. 7 and 8, respectively. Base peak ions for compounds 1 and 2 were at m/z 99 and m/z 127, respectively. Prominent high mass ions appeared at m/z 173 for compound 1 and m/z 201 for compound 2.
499
I.cn
z LU
z
uJ
LU
4
2
T
Inject
6
RETENTION
TIME,
8
MINUTES
Fig. 6-Gas chromatographic-mass spectrometric total ion chromatogram of substances leached from a synthetic elastomer by water. 1
0
75
Ur z
z
50
25
0 50
70
90
110
130
150
170
nitz Fig.
7-Mass spectrum of
compound 1.
Discussion. Water was selected as a specimen-storage medium over synthetic saliva and other aqueous solutions so that degradation of the synthetic elastomer would occur by the mildest possible means. Inordinately long specimen-storage periods of 42 and 70 days were used solely for comparison purposes. Clinically, orthodontic elastomers are used for relatively short periods of time and are replaced at seven- to 14-day intervals. Values for PS are relative, and their magnitude and experimental reproducibility are influenced significantly by the length of the recovery period. A preliminary experiment that used various recovery-period durations revealed that a 1.5-minute pause between the unloading and re-loading of the synthetic elastomer yielded the most reproducible values for PS. On this
TABLE SUMMARY OF TWO-WAY ANALYSIS OF VARIANCE Significance of Compared Variances Experimental Extension Storage Duration Extension-Storage Measurement Effect Effect Duration Interaction Load IER ** ** ** ** ** * PS DR ** ** N.S. [ER = Instantaneous elastic recovery, PS = permanent set, and DR = delayed recovery. * = p
Introduction. Synthetic elastic closure devices enjoy extensive use in canine retraction, diastema reduction, rotational correction, and general space closure. Presently, the use of these materials for intra-arch tooth movement appears to exceed that of natural latex elastics (De Genova et al., 1985). Although knowledge of the exact compositional features of synthetic orthodontic elastomers remains proprietary, it is likely that most are polyurethanes. Industrial polyurethanes are not inert materials. They decompose under prolonged contact with enzymes (Phua et al., 1987), water (Schollenberger and Stewart, 1971), and moist heat (Magnus et al., 1966). Moreover, in the dental context, the clinical behavior of synthetic elastic polymers is highly problematic. When stretched beyond their elastic limit and maintained under constant strain (extension), the elastomers exhibit an undesirable feature known as load relaxation. Clinically, first-day load losses can lead to a 50 to 75% reduction of the initial movement force applied to a malpositioned tooth (Brantley et al., 1979). The orthodontic literature is replete with reports on the load relaxation of synthetic closure devices (Andreasen and Bishara, 1970; Bishara and Andreasen, 1970; Hershey and Reynolds, 1975; Kovatch et al., 1976; Brantley et al., 1979; Young and Sandrik, 1979; De Genova et al., 1985). Lacking, however, is definition of the mechanisms that contribute to timedependent load decay. Such mechanisms may include physical breakdown, chemical decomposition, or combinations thereof. Also lacking are data that characterize the components of conjoint viscous and elastic behavior. We initiated the present research to (1) study an elastomer's behavior under finite conditions of loading, unloading, and specimen storage; (2) detect the presence of degradation products leached from the elastomer upon prolonged exposure to Received for publication May 25, 1989 Accepted for publication November 2, 1989
496
water; and (3) correlate observed changes in the test material's elastic behavior with structural and compositional changes resulting from water storage.
Materials and methods. One hundred and eight 6.3-cm segments were cut from a roll of the test material, Ormco® Grey Power Chain® II (Ormco Corporation, Glendora, CA). Eighteen segments were reserved for characterization of baseline elastic behavior. Each remaining piece was placed in a separate glass vial that contained 20 mL of de-ionized water. The vials were capped, transferred to an oven maintained at 370C ± 20C, and stored for periods of one, seven, 14, 42, and 70 days. At the end of each storage period, 18 vials were removed from the oven and distributed randomly so that three six-member test groups would result. This process yielded equal specimen populations for study of the test material's recovery from three different extension distances. The test pieces were removed from their respective storage vials (one at a time) and positioned between the vertically aligned, SC Elastomeric Grips of a constant strain-rate testing machine (Instron Universal Testing Machine, Instron Corp., Canton, MA). The provisionally positioned specimens were manipulated manually for placement of a 3.8-cm length of test material between the spring-loaded roller and contact plate of each grip. This arrangement yielded a linear, unstrained, vertically-oriented test-strip consisting of a continuous train of 11 circular modules and 10 intermodule couplers in the following configuration: 0-0-0-0. Specimen testing entailed the generation of loading and unloading curves. Initial loading profiles were established for extensions of 50% (1.9 cm), 100% (3.8 cm), and 200% (7.6 cm). When the desired extension was reached, the direction of crosshead movement was reversed for initiation of unloading. Completion of unloading was marked by the crosshead's return to the 3.8-cm working distance between specimen grips. After a wait of a prescribed time (1.5 min) for recovery to occur, the specimen was subjected to a second loading. Machine crosshead speeds for loading and unloading were 1.27 cm/min and 5.08 cm/min, respectively. Chart speed for loading and unloading was 2.54 cm/min. The use of loading and unloading curves for assessment of elastic behavior is illustrated in Fig. 1. For the depicted situation, specimen length was 3.8 cm. Total extension, as indicated by crosshead travel, was 3.8 cm (100%). Load (4.7 N) at total extension was coincident with the highest point of the loading curve (A). Measurement of instantaneous elastic recovery (IER) was made possible by a perpendicular line being drawn from the point of maximum extension to the common x-axis of the loading and unloading curves, and extension of the linear portion of the unloading curve to the x-axis (A). The 0.2-cm distance between the x-axis intercepts of the perpendicular line (7.6 cm) and the extended linear portion of the unloading curve
Vol. 69 No. 2
ELASTIC BEHAVIOR OFAN ORTHODONTIC ELASTOMER
(7.8 cm) reflects chart travel time affiliated with "instantaneous recovery". Time (t) is calculated by: CT t = C-s Cs where: CT = chart travel in cm, CS = chart speed in cm/ min, and t = time in min. During the time the chart traveled 0.2 cm, the machine's crosshead, moving at an unloading speed of 5.08 cm/min, traversed a distance of 0.4 cm. Instantaneous recovery crosshead travel (ICHT) is calculated by: ICHT = UCHS x t where: UCHS unloading crosshead speed in cm/min, t = time in min, and ICHT = instantaneous recovery crosshead travel in cm. Thus, IER is defined as the first component of elastic behavior to manifest upon rapid unloading of the visco-elastic elastomer. Instantaneous elastic recovery is calculated by: ICHT IER= T x 100
497
CROSSHEAD TRAVEL = 3.8 cm
r 5
z cl
4
0 -j
=
0
where: ICHT = instantaneous recovery crosshead travel in cm, TE = total extension in cm, and IER = instantaneous elastic recovery in percent. For measurement of permanent set (PS), the moving chart was marked at 11.8 cm so that termination of the 1.5-minute recovery period and the concurrent re-activation of the crosshead (B) would be denoted (chart distance at onset of recovery period = 8 cm; chart travel = 2.54 cm/min x 1.5 min = 3.8 cm; 8 cm + 3.8 cm = 11.8 cm). The axis-segment between this mark and the point from which re-loading began (12.6 cm) denotes the 0.8-cm distance the chart traveled during the take-up of specimen slack. Permanent set (PS) is calculated by: CT LCHS PS=-X ~~XlOO TB Cs where: CT = chart travel in cm, LCHS = loading crosshead speed in cm/min, TE = total extension in cm, CS = chart speed, and PS = permanent set in percent. Delayed recovery (DR) is calculated from the Eq.: DR = TE - IER - PS where: TE = total extension in percent, IER = instantaneous elastic recovery in percent, PS = permanent set in percent, and DR = delayed recovery in percent. Through the use of a statistical package (Analysis System Version 5, SAS Institute, Inc., Raleigh, NC), data pertaining to elastic behavior were stored and analyzed on a computer (VAX 8600, Digital Equipment Corp., Maynard, MA). The experiment's two-factor design facilitated a two-way ANOVA for extension effect, storage duration effect, and the effect of extension and storage duration interaction. For each specimen storage period, storage water affiliated with the six randomly selected test pieces was decanted into a beaker and evaporated to dryness in an oven at 370C ± 20C. A control beaker containing 120 mL of de-ionized water was treated similarly. Then the beakers were covered and kept in the oven until analysis of their residual contents could be accomplished.
2
4
6
8
10
18
20
CHART DISTANCE, cm
CROSSHEAD TRAVEL, cm
I 0.4
2.4
B
z
0~ot -J
21
FNERYM
1
10
12
14
16
CHART DISTANCE, cm
Fig. 1-(A) Loading and unloading curves for a synthetic orthodontic elastomer. Specimen length = 3.8 cm; specimen extension at peak load (4.7 N) = 3.8 cm (100%); chart speed = 2.54 cm/min; crosshead speeds for loading and unloading = 1.27 and 5.08 cm/min, respectively; recovery = chart travel interval (8.0 to 11.8 cm) affiliated with a 1.5-minute pause between unloading and subsequent re-loading of the specimen. (B) Reloading curve (continuation of A); vertical lines at chart travel distance of 11.8- and 12.6-cm mark termination of the recovery period and the onset of re-loading, respectively; PS = permanent set; chart and crosshead speeds for re-loading = 2.54 and 1.27 cm/min, respectively. [Note: criteria and formulae for calculation of instantaneous elastic recovery (IER), permanent set (PS), and delayed recovery (DR) are presented in "Materials and methods".]
HUGET et a.
498 1 0-
z
J Dent Res 20
* EXT. 1.9 cm * EXT. 3.8 cm 0 EXT 7.6 cm
w I-. w
8-
Tr
6-
T z z 4c
4-
4
as0
-j
EXT. 1.9 cm EXT. 3.8 cm EXT. 7.6 cm
15-
-j
4 cx 0
-
U U
2F
0 z z
February 1990
04
0.0
wL
IJ
2-
1.0
10-
7.0 14.0 42.0 70.0 TIME, DAYS Fig. 2-Effect of extension distance and water storage duration on load requirement. Standard deviations are denoted by error bars.
0.0
1.0
7.0
14.0
42.0
70 0
TIME, DAYS Fig. 4-Effect of extension distance and water storage duration on permanent set. Standard deviations are denoted by error bars.
20 * EXT. 1.9 cm * EXT. 3.8 cm * EXT. 7.6 cm
100 W
* EXT. 1.9 cm * EXT. 3.8 cm O EXT. 76cm
15-
80
0 0 w 0 Ch -J
w 0
10-
w
60
0
(n
m
0
40 4c
5
-j
wL0
z en I--
20
CA)
z
0-
0.0
1.0
7.0
14.0
42.0
70.0
TIME, DAYS Fig. 3-Effect of extension distance and water storage duration on instantaneous elastic recovery. Standard deviations are denoted by error bars.
Analysis of organic substances leached from the elastomer was performed on a 9610 gas chromatograph-4000 mass spectrometer (Finnigan Corp., San Jose, CA). The instrumentation was calibrated with perfluorotributylamine. Approximately 60 AtL of methanol was added to the control beaker and to each of the five beakers that had contained pooled-specimen storage water. Approximately 1 ,uL of solute-free methanol or a resultant methanol solution of residual organic matter was withdrawn from each beaker and injected onto a 30 m x 0.32 mm DB-1 dimethylsilicone fused silica column (J and W Scientific, Folsom, CA). The injector port and column were operated at 270'C, and the helium carrier velocity was 50 cm/s. Fragmentation of eluting components was accomplished by electron impact at an ionizing voltage and current of 70 V and 250 ALA, respectively. The data system (Teknivent, St. Louis, MO) scanned and acquired masses ranging from 50 to 700 atomic mass units (A). Scans were made every 2.4 s, with a sweep width of 0.2 p integrating each acquisition for 1 ms.
04 0.0
1.0
7.0 14.0 TIME, DAYS
42.0
70.0
Fig. 5-Effect of extension distance and water storage duration on delayed recovery. Standard deviations are denoted by error bars.
Results. Data relating to the elastic behavior of the polymeric material are presented in Figs. 2-5. Mean baseline values for "dry" control specimens (water storage duration = 0.0 days) were as follows: Loads required for achievement of total extensions of 1.9 cm, 3.8 cm, and 7.6 cm were 3.38 N, 4.62 N, and 7.38 N, respectively. Instantaneous recovery from respective deformations of 1.9, 3.8, and 7.6 cm was 0.20 cm (IER = 10.5%), 0.40 cm (IER = 10.5%), and 0.80 cm (IER = 10.5%). After elongation to 50, 100, and 200% of their initial length, control specimens exhibited unrecovered deformation of 0.16 cm (PS = 8.4%), 0.39 cm (PS = 10.4%), and 0.80 cm (PS = 10.5%), respectively. During the 1.5minute pause between unloading and re-loading, the material's recovery from respective elongations of 50, 100, and 200% was 1.54 cm (DR = 81.1%), 3.01 cm (DR = 79.0%), and 6.02 cm (DR = 79.1%).
Vol. 69 No. 2
ELASTIC BEHAVIOR OF AN ORTHODONTIC ELASTOMER
Loads required for production of the desired extension of specimens stored in water for one day declined sharply from baseline levels (Fig. 2). Deceleration of the load requirement's decline occurred between the first and seventh days of water storage. Load requirements for 14-day specimens were remarkably higher than those of specimens exposed to water for shorter durations. This transient change was followed by a renewed decline in load requirement for 42- and 70-day test pieces. The respective loads required for production of specified extensions of one-, seven-, and 70-day specimens were strikingly similar. Instantaneous elastic recovery of the test material from extensions of 50% and 100% was not affected by water storage (Fig. 3). After storage for seven days, the mean IER value for specimens extended to 200% (7.6 cm) fell to 10.1%. The mean IER value for 42- and 70-day specimens extended to 200% (7.6 cm) was 8.7%. Permanent set peaked on the 14th day of water storage (Fig. 4). Mean values for PS associated with extension of 50% (1.9 cm), 100% (3.8 cm), and 200% (7.6 cm) were 9.4, 11.0, and 12.3%, respectively. Recovery that occurred during the 1.5-minute pause between unloading and re-loading accounted for about 88 to 90% of the elastomer's total recovery. Mean values for DR (Fig. 5) ranged from 77.2% (TE = 200% and storage time = 14 days) to 81.8% (TE = 100% and storage time = 42 days). A two-way ANOVA revealed that extension distance and water storage duration affected load requirement, IER, PS, and DR. Interactions of extension distance and storage duration affected load requirement, IER, and PS, but not DR (p > 0.05). The statistical significance of all findings relating to the test material's elastic behavior is summarized in the Table. Gas chromatography failed to establish the presence of organic material in beakers that had contained one-day and sevenday storage water. Also devoid of residual organic matter was the control beaker. On the other hand, chromatographic analysis of material recovered from 14-, 42-, and 70-day storage water confirmed the presence of organic substances. Based on column retention times and mass spectra, these samples contained common degradative components. The chromatogram pertaining to the 14th day of specimen storage is shown in Fig. 6. Mass spectra of compounds that gave rise to peaks at retention times of 3.9 min (compound 1) and 5.9 min (compound 2) are presented in Figs. 7 and 8, respectively. Base peak ions for compounds 1 and 2 were at m/z 99 and m/z 127, respectively. Prominent high mass ions appeared at m/z 173 for compound 1 and m/z 201 for compound 2.
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Fig. 6-Gas chromatographic-mass spectrometric total ion chromatogram of substances leached from a synthetic elastomer by water. 1
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Discussion. Water was selected as a specimen-storage medium over synthetic saliva and other aqueous solutions so that degradation of the synthetic elastomer would occur by the mildest possible means. Inordinately long specimen-storage periods of 42 and 70 days were used solely for comparison purposes. Clinically, orthodontic elastomers are used for relatively short periods of time and are replaced at seven- to 14-day intervals. Values for PS are relative, and their magnitude and experimental reproducibility are influenced significantly by the length of the recovery period. A preliminary experiment that used various recovery-period durations revealed that a 1.5-minute pause between the unloading and re-loading of the synthetic elastomer yielded the most reproducible values for PS. On this
TABLE SUMMARY OF TWO-WAY ANALYSIS OF VARIANCE Significance of Compared Variances Experimental Extension Storage Duration Extension-Storage Measurement Effect Effect Duration Interaction Load IER ** ** ** ** ** * PS DR ** ** N.S. [ER = Instantaneous elastic recovery, PS = permanent set, and DR = delayed recovery. * = p
