Expansion of phosphate-bonded investments: Part II—Thermal expansion PDF

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E x p a n s i o n of p h o s p h a t e - b o n d e d i n v e s t m e n t s : Part II T h e r m a l e x p a n s i o n J . E. H u t t o n , DDS, M S , M P H , a a n d G. W . M a r s h a l l , DDS, MPH, PhD b The University of California, School of Dentistry, Department of Restorative Dentistry, San Fr...

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E x p a n s i o n of p h o s p h a t e - b o n d e d i n v e s t m e n t s : Part II T h e r m a l e x p a n s i o n J . E. H u t t o n ,

DDS,

M S , M P H , a a n d G. W . M a r s h a l l ,

DDS,

MPH,

PhD b

The University of California, School of Dentistry, Department of Restorative Dentistry, San Francisco, Calif. This study investigated the thermal expansion of phosphate-bonded investment m a t e r i a l s . T h r e e i n v e s t m e n t s w e r e m i x e d w i t h e i t h e r d i s t i l l e d w a t e r or t h e i r s p e c i a l l i q u i d s a n d a l l o w e d a s e t t i n g t i m e o f 1 or 2 4 h o u r s . H y d r a t i o n a n d t h e r m a l expansions were measured with a vertical dilatometer and were analyzed for i n t e r a c t i v e a n d m a i n e f f e c t s . T h e a m o u n t o f e x p a n s i o n a t t r i b u t e d to h y d r a t i o n w a s minimal, whereas thermal expansions varied and were of greater magnitudes. M a t e r i a l I e x h i b i t e d t h e g r e a t e s t t h e r m a l e x p a n s i o n . M a t e r i a l s II a n d III r e c o r d e d smaller thermal expansions that increased when the materials were mixed with t h e i r s p e c i a l l i q u i d s . S e t t i n g t i m e did n o t s u b s t a n t i a l l y i n f l u e n c e h y d r a t i o n e x p a n s i o n s b u t did i n f l u e n c e t h e r m a l e x p a n s i o n s , a n d h e a t i n g t h e m a t e r i a l s to 8 0 0 ~ C i n s t e a d o f 7 0 0 ~ C did n o t d r a m a t i c a l l y e l e v a t e e x p a n s i o n s . (J PROSTHET DENT 1995;73:127-31.)

T-~ r h o s p h a t e - b o n d e d investment materials have become more popular for casting high melting noble-metal (precious 1 and semiprecious 2) and base-metal (nonprecious 3) alloys in fixed prosthodontics. Considerations for these investments include (1) adequacy of investment expansion to compensate for casting shrinkage, (2) ease of retrieving the casting from the fired high-strength investment, and (3) ability to obtain complete, nonporous castings while reducing casting defects. 4, 5 Stamenkovic 6 demonstrated that conditions during refractory mold preparation can affect the casting quality, but there is limited additional information on the characteristics of the phosphate-bonded investment materials. 7 Part I of this study confirmed that the most important variable responsible for differences in setting expansion was the mixing liquid that consisted of a special liquid or distilled water, s Furthermore, setting expansion was believed complete within 1 hour and did not increase if setting time was extended to 24 hours. In this continuation of the investigation of phosphatebonded investment materials, parameters were measured to determine individual and interactive contributions to the thermal expansion characteristics of these investment systems. MATERIAL

AND METHODS

The three phosphate-bonded investments, coded materials I, II, and III, were evaluated for thermal expansion

aAssociate Professor, Division of Prosthodontics. bprofessor, Division of Biomaterials. Copyright 9 1995 by The Editorial Council of THE JOURNALOF PROSTHETICDENTISTRY. 0022-3913/95/$3.00 + 0 1 0 / 1 / 5 9 5 3 6

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after setting for 1 or 24 hours. The liquid/powder proportions investigated in this study are listed in Table I with the manufacturers' reported values for thermal expansion. Each material was mixed with either room-temperature distilled water or the manufacturer's room-temperature special liquid. The special liquid was designed for selective dilution with water to adjust mold expansion, 9 but dilutions were not used in this study. Procedures for mixing the components, including mixing rates, preparation of the samples, and measurement of the setting expansions, have been previously reported, s All samples were allowed to set at room temperature in air for 1 or 24 hours. At the conclusion of the dilatometer setting expansion measurements, the samples were removed from their molds and soaked in room-temperature distilled water. This soak was recommended by the manufacturers to reduce the occurrence of mold cracking during heating because these cracks would create defective castings. If the setting of the materials was incomplete at the time of soaking, hygroscopic expansion could occur. 1~Any additional setting expansion is referred to in this study as the "hydration effect" to distinguish it from the traditional hygroscopic expansion that occurs if the mold is deliberately exposed to water during setting. 11 During the soaking period the samples were monitored with use of the dilatometer system so that dimensional change as a result of the hydration effect could be measured. On setting, each sample was approximately 3.9 cm in height and 2.0 cm in diameter. Each sample included a small conical (8 mm deep • 8 mm diameter) indentation in the base that accommodated a thermocouple during subsequent thermal expansion measurements. The samples were positioned in the dilatometer and heated at a rate of 4 ~ C per minute to an ultimate temperature near 900 ~ C. Each sample was measured twice for thermal expansion

VOLUME 73

NUMBER 2

HUTTON AND MARSHALL

during the heating cycle, once at 700 ~ C and once at 800 ~ C. Each experimental condition was replicated three times, which resulted in a total of 36 samples (3 samples x 3 materials x 2 setting times x 2 mixing liquids) and 72 observations (36 samples x 2 temperatures) for this phase of the study. Linear expansions caused by the hydration effect and thermal effect were monitored with a vertical dilatometer. The dilatometer used a fused-quartz holding chamber and push-rod attached to the core of a linear variable differential transducer (Hewlett Packard Corporation, Waltham, Mass.) previously described, s The transducer output was automatically recorded with a voltmeter and digital printer (Hickok model DMS 3200P voltmeter and model PR4900 printer, Hickok Electronic Instruments, Cleveland, Ohio) during the 5-minute soaking period and heating cycle. Expansion curves were determined by conversion of the transducer output to distance, on the basis of calibration curves for the transducer. Analysis of variance (ANOVA) considering the effects of material, setting time, and liquid on hydration expansion was done to evaluate possible singular and interactive effects of these basic parameters. Because the thermal expansions of the 32 samples were measured at both 700 ~ C and 800 ~ C, repeated-measures ANOVA considering the effects of material, setting time, liquid, and temperature on expansions was done to evaluate possible main and interactive effects of these basic parameters. RESULTS

Hydration effect An ANOVA was computed to determine the effects of main and interactive variables associated with the hydration expansion (Table II). The materials, setting time, mixing liquid, and two-way and three-way interactions of these variables were of interest. The three-way interaction (material, liquid, setting time) was not significant (probability p 0.09). The only significant two-way interaction was between material and liquid. The expansions attributed to this interaction were nearly zero for materials I and III but greater for material II (Table III). The main effect of setting time (Table II) was not significant (p 0.83), so allowing samples to bench-set either 1 or 24 hours before they were soaked in distilled water did not affect subsequent hydration expansion. The greatest mean hydration expansion was recorded for material II (0.42 % ) if the material was mixed with special liquid (Table III), but when the material was mixed with distilled water the effect was less (0.12 % ). The least effect from hydration was recorded for material III, which expanded an average of 0.01% when mixed with water. The maximal effect for material III when it was mixed with special liquid was only 0.04 % compared with the effect for material II of 0.42 %. Material I recorded hydration effects similar to those of material III for both mixing liquids.

FEBRUARY 1995

THE JOURNAL OF PROSTHETIC DENTISTRY

Table I. Manufacturers' reported properties at various liquid-to-powder ratios

L i q u i d - t o - p o w d e r ratio

Percent thermal expansion (at 7 0 0 ~ C)t

11 ml water/60 gm 11 ml special liquid/60 gm 11 ml water/65 gm 11 ml special liquid/65 gm 9.5 ml water/60 gm 9.5 ml special liquid/60 gm

1.20 1.40 1.55 1.75 Not reported? Not reported?

Investment

material*

I II III

*Material I is Biovest, Dentsply Int., York, Pa., which is presently available only as a soldering investment; material II is Ceramvest, Kerr Sybron Corp., Romulus, Mich., which is experimental and not commercially available; material III is Ceramigold, Whip Mix Corp., Louisville, Ky., which is commercially available. teach manufacturer reported that intermediate thermal expansions could be achieved by using a combination of special liquid and water.

Tukey's Student range test revealed that material II when mixed with special liquid was significantly different from all other material liquid combinations. The others comprised a single homogeneous group. Thermal

effect

The three investment materials when mixed with their special liquid or distilled water had thermal expansions unique in magnitude and minimal or no additional expansion occurred at temperatures higher than 700 ~ C. Regardless of mixing liquid, material I had the greatest thermal expansions, which ranged from 1.64% to 1.76% (Table IV). Materials II and III had similar intermediate thermal expansions, except when material III was mixed with distilled water. In that instance, it recorded the least mean thermal expansions (0.80% to 0.88%). The repeated-measures ANOVA (Table V) revealed that the four-way interaction of material, liquid, setting time, and temperature was not significant (p 0.95). However, the three-way interaction of material, setting time, and temperature was significant (p 0.0002). The other threeway interactions (material, liquid, and setting time; material, liquid, and temperature; and liquid, setting time, and temperature) were not significant (p 0.74, p 0.31, and p 0.13, respectively). Therefore thermal expansion, like setting expansion, s could not be evaluated directly, in this case because of the material, setting time, and temperature interaction. Because these singular effects were not additive, comparisons were made between material, setting time, and temperature combinations. The data were restructured combining the distilled water and special liquid groups (Table VI) to reveal relationships between material, setting time, and temperature. The greatest mean thermal expansion was obtained for material I (1.72 %) if the material set for 24 hours and was heated to 800 ~ C. The smallest thermal expansion was recorded for material III. When material III set for 24 hours and was

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Table

HUTTON AND MARSHALL

II. ANOVA for hydration effect (percent expansion) as dependent variable Source

Sum o f squares

Material Liquid Setting time Material and liquid Material and setting time Liquid and setting time Material, liquid, and setting time

0.4879 0.1188 0.0006 0.1661 0.0020 0.0414 0.0625

Model Error (residual Totals

0.8792 0.2820 1.1612

D e g r e e s of freedom

Mean square

F value

p value

2 1 1 2 2 1 2

0.2439 0.1188 0.0006 0.0830 0.0010 0.0414 0.0313

20.76 10.11 0.05 7.07 0.08 3.52 2.66

0.00 0.00 0.83 0.00 0.92 0.07 0.09

11 24 35

0.0799 0.0117

6.80

0.00

Mean hydrationexpanmon0.10% _+0.11%.

Table

III. T r e a t m e n t effect of mixing liquid M e a n h y d r a t i o n effect (percent expansion)

Material

Distilled water

Special liquid

I II III

0.02 -+ 0.04 0.12 -+ 0.17 0.01 -+ 0.00

0.03 -* 0.03 0.42 -+ 0.23* 0.04 _+ 0.03

*Material II when mixed with its special liquid was significantlydifferent (p ~ 0.05) from all others. The other material and liquid combinations constituteda singlehomogeneoussubset. Eachmeanrepresentssix samples (three sampleswere allowedto set for 1 hour and three for 24 hours).

heated to 700 ~ C it expanded only 1.01%. Material II had thermal expansions intermediate to the extreme values of materials I and III. Tukey's S t u d e n t range test revealed that material and setting time combinations generally had significantly greater expansions (p _< 0.05) when the materials were heated to 800 ~ C than when they were heated to 700 ~ C. However, there were exceptions: (1) materials I and II when allowed to set for 24 hours were not different at 700 ~ and 800 ~ C and (2) material II when set for 1 hour expanded significantly less at 800 ~ C t h a n at 700 ~ C. All thermal effect differences, whether statistically significant or not, were minute (Table VI). The main effect of mixing liquid was significantly (p 0.0001) related to thermal expansion (Table V) and therefore could be measured directly. The materials had a mean thermal expansion of 1.22 % when mixed with distilled water. The expansion increased to 1.43% when the materials were mixed with their special liquids (Table VII). However, analyzing the material and liquid interactive effect was more revealing. Materials II and III recorded significant increases in thermal expansion when they were mixed with their special liquids. Material I expanded less but the change was not significant. 128

DISCUSSION Hydration

effect

The three phosphate-bonded investment materials investigated in this study had small expansions as a result of hydration. A significant (p 0.004) material and liquid twoway interaction was detected (Table II). Therefore the variations in expansion for each material and liquid combination were considered individually, rather than as a direct comparison of the hydration effects of the three materials. The effect was independent of setting time (p < 0.83), so the increase in setting time from i to 24 hours did not significantly alter the expansion. This finding was consistent with prior setting expansion results s and suggested that all three materials had essentially completed their setting reactions within the first hour. The expansion values indicated that although expansion caused by hydration for material II when the material was mixed with special liquid was higher than those values observed for the other two materials (Table III), all these expansions were small. The fact that a small but detectable effect was observed suggested that these three materials would expand hygroscopically if exposed to water before their setting reactions were complete. This was generally in accord with concepts of the hygroscopic expansion of phosphate-bonded systems. 1~Material II had eradicable reaction to the water soak. This could be attributed to a slower setting reaction for material II or could indicate inconsistency in the data, so additional samples should be investigated. Conversely, none of the materials possessed large hydration effects when mixed with distilled water. Because the materials in this group were completely set before soaking, they did not experience additional hygroscopic expansion. Consequently, the recommended water soak, suggested to reduce thermal cracking, should not promote additional expansion caused by a hygroscopic effect. Although there was a statistically significant two-way interaction between material and liquid, from a clinical perspective this small expansion (Table III, 0.01% to 0.42 %) is probably not of practical importance except for that in material II. VOLUME 73 NUMBER 2

HUTTON AND MARSHALL

Table

THE JOURNAL OF PROSTHETIC DENTISTRY

IV. Mean thermal effect (percent expansion) 700 ~ C Distilled water

Special liquid

Distilled water

Special liquid

I II III

1.67 _+ 0.17 1.06 _+ 0.01 0.86 _+ 0.07

1.64 _+ 0.09 1.36 + 0.05 1.30 _+ 0.09

1.69 _+ 0.18 1.03 _+ 0.02 0.88 _+ 0.07

1.67 _+ 0.09 1.33 _+ 0.05 1.32 _+ 0.09

I II III

1.74 + 0.12 1.17 +_ 0.09 0.80 _+ 0.06

1.69 _+ 0.07 1.35 _+ 0.05 1.20 _+ 0.05

1.76 _+ 0.12 1.18 +_ 0.09 0.83 _+ 0.05

1.69 _+ 0.08 1.35 _+ 0.04 1.22 _+ 0.04

Setting time

1 Hour Material Material Material 24 Hours Material Material Material

800 ~ C

Each mean represents three samples. All samples were measured at both 700 ~ and 800 ~ C.

T a b l e V. Repeated-measures ANOVA* for thermal effect (percent expansion) at 700 ~ and 800 ~ C as dependent variables Source

Between-subject effects Material Liquid Setting time Material and liquid Material and setting time Liquid and setting time Material, liquid, and setting time Error (residual) Within-subject effects Temperature Material and temperature Liquid and temperature Setting time and temperature Material, liquid, and temperature Material, setting time, and temperature Liquid, setting time, and temperature Material, liquid, setting time, and temperature Error (residual)

T y p e III s u m of squares

Degrees of freedom

Mean square

F value

p Value

5.2763 0.7403 0.0034 0.6324 0.0749 0.0237 0.0091 0.3634

2 1 1 2 2 1 2 24

2.6382 0.7403 0.0034 0.3162 0.0374 0.0237 0.0046 0.0151

174.25 48.90 0.23 20.88 2.47 1.56 0.30

0.0001 0.0001 0.6375 0.0001 0.1056 0.2231 0.7428

0.0011 0.0030 0.0003 0.0002 0.0002 0.0017 0.0002 0.0000 0.0016

1 2 1 1 2 2 1 2 24

0.0011 0.0015 0.0003 0.0002 0.0001 0.0008 0.0002 0.0000 0.0001

17.74 23.28 3.97 2.81 1.22 12.91 2.45 0.05

0.0003 0.0001 0.0578 0.1066 0.3140 0.0002 0.1307 0.9479

*Thirty-six samples with two measurements made on each sample. Mean thermal expansion 1.32% • 0.01%.

Thermal

effect

T he three phosphate-bonded investment materials had large thermal expansions at both 700 ~ C and 800 ~ C. All three materials had small differences in thermal expansion if heated to 800 ~ C instead of 700 ~ C. T h e expansion values at these two temperatures were similar for each material, setting time, and liquid combination (Table IV) except for t h a t in material II when it set for I hour. T he four-way interaction of material, liquid, setting time, and temperature was not significant (p 0.95), so interactions between three of the variables were examined (Table V). Th e only significant three-way interaction was that for material, setting time, and temperature (p 0.0002). Therefore the variations in thermal expansion for each FEBRUARY 1995

material and setting time combination were examined individually, rather than as a direct comparison of the thermal effects of the three materials. The interactions of material, setting time, and t em p er at u r e revealed that four of the combinations increased in expansion at 800 ~ C, two recorded no change, and one (material II with 1 hour of setting) shrank (Table VI). Material I displayed the greatest mean thermal expansion of all materials and it expanded most when allowed 24 hours of setting time (1.71% to 1.72%). Under these conditions, material III recorded the least amount of thermal expansion (1.01% to 1.02 %) and material II recorded intermediate expansions (1.26% for both setting times). Heating these samples to 800 ~ C instead of 700 ~ C did not increase expansion. Thes...