SECTION EDITORS

William Lefkowitz William F. Malone J&n E. Rhoads Robert S. StatYanou Ronald D. Woody

Adhesive Part II

properties

of several

Johnny Y. Chai, BDS, MS,B Lee M. Jameson, and Rich.ard A. Hesby, DDS, MSDd Northwestern

University,

impression DDS,b

John

B. Moser,

material

systems:

PhD,C

Dental School, Chicago, Ill.

The effect of saliva contamination on the tensile adhesive bond strengths of four impression adhesive systems, (1) polysulfide, (2) polyether, (3) polyvinylsiloxane, and (4) condensation silicone impression adhesive, was studied. Standard acrylic resin tray samples of l-inch square surface were made. Tray surfaces were contaminated with salziva either before or after the application of tray adhesives. Tensile tests were performed with a Universal Load testing machine. The tensile adhesive b’ond strengths of contaminated tray surface and contaminated adhesive were compared with a group of control. Contamination of all impression adhesives resulted in significantl:y lower bond strength. Except for the polyvinylsiloxane and polysulfide impression adhesive systems, saliva contamination of tray material signilicant:ly decreased adhesive bond strengths. Impression trays should be tried in the mouth, rinsed, and dried before adhesive is applied. (J PROSTHET DENT 1991;66:267-92.)

T

he use of impression tray adhesive in retaining elastomeric impression material has definite benefits. Ciesco et al.’ found the immediate accuracy and dimensional stability of five impression materials studied over a l-week period were improved when a tray/adhesive system was used. This was attributed to the ability of a tray/adhesive -Presented at the American cago, Ill.

Prosthodontic

Society meeting, Chi-

BAssistantProfessor,Division of Yixed Prosthodontics, Department of %ofessor CF’rofessor, Science. dProfessor

Restorative Dentistry. and Director, Advanced Education Prosthodontics. Division of Biological Materials, Department of Basic and Director.

Division

of Fixed Prosthodontics,

De-

partment 01’Restorative Dentistry. 10/l/26188

system to restrain the polymerization shrinkage of impression material. Tjan and Wbang2 compared the accuracy of dies cast from impressions made with nonperforated trays coated with adhesive, perforated trays without adhesive, and perforated trays coated with adhesive. They concluded that impression adhesives should be used if repeat pours were required. For the desirable properties of an impression adhesive/ tray system to be effective, care should be taken not to contaminate the tray’s surface. When a custom tray is constructed, a wax spacer is usually made. Asbestos has also been used as spacer material. 3,4 To prevent contamination of the tray by wax4 and to facilitate removal,5 tin or aluminum foil should be burnished over the wax. The surface preparation of tray resin affects the bond strength of a polysulfide impression material. The lowest bond strength

Table I. Batch numbers of impression materials and adhesives Batch numbers Impressic~n materials

-

Brands

Base

Catalyst 130 71268 :30387

Rdhesive 0036/2 12688 71387

Polyether Polyvinylsiloxane regular bot-ly Polysulfide reE:ular body

Impregum-F* Mirror-3t Permlastict

473 71268 30:X37 1012

1012

Condensation

:silicr,ne

Denture

12388 1334

12388 1334

111887

putty

Mirror-3t

71268

71268

12688

Polyvinqlsiloxane ----

*ESPE-Premier, tKerr Manufactl

Xorrisl.own Penn. ring Co., Rr mulus, Mic h.

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Elasticont

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CHAI

Fig.

1. Mean adhesive bond strengths of all impression materials tested.

Table II. Adhesive bond strengths of all impression materials under different conditions Adhesive

bond strengths

SD PS Tray material Adhesive Control PE Tray material Adhesive Control PVS Tray material Adhesive Control cs Tray material Adhesive Control

(psi) Significance

(p > .06)

53.86 14.68 58.90

8.3 6.6 2.6

*

51.02 52.32 72.62

8.4 8.1 9.3

* *

59.04 13.78 66.72

7.8 3.6 14.8

*

36.92 9.80 60.48

10.8 3.1 5.7

*

*

PS, Polysulfide; PE, polyether; PVS, polyvinylsiloxane; CS, condensation silicone. *A pair of asterisks for each impression material indicates no significant difference @ > .05) between treatments according to Scheffe multiple comparison test.

was found for samples cured against wax following immersion in boiling water for wax removal. Those cured against asbestos and those roughened with a No. 60 pesh silicone carbide paper showed intermediate values. The highest

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bond strength recorded were for samples cured over tinfoil samples.6 In fixed prosthodontics, impression trays are often checked intraorally before the impression procedure. Contamination by saliva, as a result, is likely to affect the ability of tray surface or adhesive film to retain the impression material. Saliva contamination is even more unavoidable when a custom tray for removable prosthodontics has to be border-molded. This has not been reported in the prosthodontic literature. Thus the aim of this study was to investigate the effect of saliva contamination of impression tray and impression adhesive on the tensile adhesive bond strength of four impression materials.

METHODS

AND MATERIAL

Four impression material systems were studiedpolysulfide, polyether, medium-viscosity (regular) polyvinylsiloxane, and condensation silicone impression material (Table I). Acrylic resin blocks with a l-inch square testing surface were prepared from an aluminum mold. Details of sample preparation and testing were outlined in a previous article.7 All samples were prepared at least 24 hours before experimentation and were selected randomly for each part of the experiment. Fresh human saliva was applied (with a cotton tip) onto the testing surface of each autopolymerizing acrylic resin block, The contaminated surface was dried with a handheld air syringe 1 cm from the sample surface for 15 seconds, and one layer of impression adhesive was painted and allowed to air dry for 15 minutes. The corresponding impression material was prepared to a streak-free mixture

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Fig. 2. Polysulfide samples. Top row, contaminated adhesive samples; middle row, con trol; bottom row, contaminated tray samples

according to the manufacturer’s instruction. A second spatula wa:sused to place the impression material between a metal mesh and the testing surface. The metal mesh was used to retain th.e impression material. The samples were allowed to polymerize in a thermostat-controlled water bath (Lab-line Imperial III Water Bath, Lab-line instruments, Inc., Melrose Park, Ill.) at 37’ C for 15 minutes be:fore testing. Tensile testing was performed on an Universal Load testing machine (Instron Corporation, Canton, Mass.) with tensile loads at 5 inches per minute. The tensile adhesive bond strength of a sample was estimated by the maximum load needed to separate an impression material from the acrylic resin testing surface as recorded on graph paper. Five samples were tested for each impression material. The second part of the experiment studied the effect of saliva contamination of impression adhesives. Acrylic resin blocks were contaminated with saliva 15 minutes after the impression adhes’ive had been applied and then dried with compressed.air for 15 seconds lcm from the sample surface. The mode of failure of all samples were also observed. Uncontaminated samples tested under the same conditions served as controls in this experiment. One-way analysis of variance (ANOVA) and Scheffe multiple comparison were used to analyze the data.

RESULTS Table II and Fig. 1 depict the means and standard deviations of tensile adhesive bond strengths of the control and experimental groups. For the polysulfide impression adhesive system, the tensile adhesive bond strength of contaminated tray samples did not differ significantly from control samples (p > 0.05).

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Contaminated adhesive samples exhibited tensile adhesive bond strength significantly lower than either the control samples or the contaminated tray samples (p < 0.01). When the testing surfaces were inspected after the experimental procedure, approximately half to seven eighths of the total surface area of the control samples still had adhesive attached whereas less than one tenth of the impression material remained. Approximately one quarter to seven eighths of the surface area of the contaminated tray samples were covered with adhesive whereas one tenth of the impression material remained attached. For contaminated adhesive samples, all of the adhesive remained on the acrylic resin block after testing (Fig. 2). No significant difference was found in adhesive bond strength between contaminated tray and adhesive samples for the polyether impression adhesive system (p > 0.05). However, both experimental groups demonstrated significantly lower adhesive bond strength than the control samples (p < 0.05). The modes of failure of contaminated tray and control samples were similar, with less than one tenth of the adhesive and none of the impression material remaining after testing. Contaminated adhesive samples had three quarters to seven eighths of their surface areas covered with impression adhesive and no impression material remaining (Fig. 3). Contaminated tray samples of polyvinylsiloxane adhesive system did not show significantly different adhesive bond strength from those of the control samples (p > 0.05), whereas contaminated adhesive samples demonstrated significantly lower values than either contaminated tray or control samples (p < 0.01). The contaminated tray and control samples had more than three quarters of their adhesive remaining on the testing surface with less than one

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CHAl

ET AL

Fig. 3. Polyether samples. Top row, contaminated adhesive samples; middle row, control; bottom row, contaminated tray samples.

Fig. 4. Polyvinylsiloxane samples. Top row, contaminated adhesive samples; middle row, control; bottom row, contaminated tray samples.

tenth of the impression material remaining. Contaminated adhesive samples had all surfaces covered with adhesive after testing (Fig. 4). Contaminated condensation silicone adhesive samples had lower adhesive bond strength than contaminated tray samples, which in turn had lower bond strength than control samples. There were significant differences among control and experimental groups @ < 0.01). One eighth to one third of the surface areas of control samples was covered with impression material and adhesive remained on all samples. Contaminated tray samples had less than one tenth their surfaces covered with impression material and its adhesive. All adhesive remained on contaminated adhesive samples, with no impression material present (Fig. 5). 290

DISCUSSION Different types of impression adhesives are not interchangeable because their chemistry differs. Adhesives for polysulfide impression materials consist of butyl rubber or styrene and acrylonitrile dissolved in a volatile solvent such as chloroform or a ketone. The impression adhesives used for silicone impression materials contain polydimethylsiloxane or a similar reactive silicone, and ethyl silicate. Polydimethylsiloxane adheres to the silicone impression material whereas ethyl silicate forms a hydrated silica that bonds to the impression tray material physically.* Retention of an impression adhesive on a tray material thus depends on the ability of the solvent in the adhesive to dissolve the tray material. Saliva contamination of tray material before the appliSEPTEMBER

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IFig. 5. Condensation silicone samples. 7’op row, contaminated row, control; bottom row, contaminated tray samples.

c.ation of polysulfide or polyv .nylsiloxane impression adhesive did not decrease its adhesive bond strength significantly. However the contamination of impression adhesives decreased the adhesive bclnd strength to less than one third and approximat.ely one fifth of the original values for polysulfide and polyvinylsiloxane impression adhesives respectively. Contamination of either the condensation silicone adhesive or its tray surf’ace decreased its adhesive bond strength to an unacceptable value. Although contamination of polyether adhesive ,zid its tray surface significantly decreased its adhesion, the mean adhesive bond st.rengths were greater than 3.2 psi. The effect of saliva contamination on the mode of failure di.d not differ too much from what had been expected. Contamination of adhesives prevented adhesion of impressi’on materials to them; therefore, failure occurred at the adhesive-impression material interface. Contamination of tray samples decreased the adhesion of adhesives to the tray material. The amount of adhesive and impression material left on tray surfaces was less than or equal to that of co’ntrol samples. As mentioned, the amount of materials left on tray surface is not a good indication of adhesive bond st:rength.7 Although similar bond strengths were demonstrated by contaminated tray and control samples for polysulfide and polyvinylsiloxane impression adhesives, more adhesive and impression material was left on the contaminated tray samples after testing.

CLINICAL

IMPLICATIONS

‘The formulation of a clinically acceptable minimal value for sufficient adhesive bond strength is difficult. No standards have been established for bond strengths of impressiosn adhesives, probably because there would have to be difrerent minimal ,standards for each type of impression adbesive system. Because the ri,gidity of impression materials varies for the same thickness of impression material TH.E

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adhesive samples; middlr>

and the same amount of undercut that two different impression materials have to overcome, the more rigid impression material would require stronger adhesion to the tray to prevent its separation. Since the standards are presently lacking, it is recommended that every attempt should be made to maintain the maximum adhesion that an impression adhesive can provide. Contamination of any adhesive in this study significantly decreased the adhesive bond strength. Applying this observation clinically, impression trays should be tried in the mouth and the necessary adjustment made before adhesive is applied. If impression adhesive has been applied before try-in, rinsing with tap water and drying with compressed air followed by application of a second layer of adhesive should be performed.”

SUMMARY

AND CONCLUSIONS

The effect of saliva contamination on the tensile adhesive bond strengths of four impression adhesive systems were studied-polysulfide, polyether, polyvinylsiloxane, and condensation silicone impression adhesive systems. The tensile adhesive bond strengths of contaminated tray surface and contaminated adhesive were compared with a control group. Contamination of all impression adhesives resulted in significantly lower bond strength. Except for the polyvinylsiloxane and polysulfide impression adhesive systems, saliva contamination of tray material significantly decreased adhesive bond strengths. Clinical implications of the results were discussed and recommendations were made. REFERENCES

291

CHAIETAL

3. Ellam AH, Smith DC. The relative effectiveness of adhesives for polysulphide impression materials. Br Dent J. 1966;120: 135-8. 4. Johnston FF, Philips RW, Dykema RW. Modern practice in crown and bridge prosthodontics. 3rd ed. Philadelphia: WB Saunders, 1971;181. 5. Shillingburg HT, Hobo S, Whitsett LD. Fundamentals of fixed prosthodontics. 2nd ed. Chicago: Quintessence Publishing Co Inc, 1981; 229. 6. Davis GB, Moser JB, Brinsden GI. The bonding properties of elastomer tray adhesives. J PROSTHET DENT 1976;36:278-85.

Effects of strain impression

7. Chai J, Jameson LM, Moser JB, Hesby RA. Adhesive properties of several impression material systems. Part I. J PROSTHET DENT 1991;66: 201-g.

8. Phillips RW. Skinner’s science of dental materials. 8th ed. Chicago: WB Saunders, 1982:150.

Reprintrequeststo: DR. JOHNNY Y. CHAI DENTAL SCHOOL NORTHWESTERN UNIVERSITY 240 E. HURON ST. CHICAGO, IL 60611

rate on the behavior

of elastomeric

Judson Klooster, DDS,* G. Irving Logan, DDS, MS,b and Anthony H. L. Tjan, DrDent, DDSC Loma Linda University, School of Dentistry, Loma Linda, Calif. The effects of strain rate on the ultimate tensile strength, ultimate elongation at fracture, and elastic recovery of five commonly used elastomeric dental impression materials were studied. Each specimen was prepared by standard mixing techniques, placed for setting in a standard ASTM mold, removed, trimmed, and mounted in an Instron testing unit. Tensile load was applied and plotted on a load versus displacement curve as the specimen was stretched axially until rupture. Five specimens of each material were tested, respectively, at strain rates of 100, 200, and 500 mm/min crosshead speed. Specimens were then removed, reassembled, and measured with an electronic caliper to assess the elastic recovery; measurements were repeated after 1 minute, 30 minutes, 1 hour, and 24 hours. Findings of this study indicated that (1) polysul5de materials exhibited the greatest amount of deformation, and the lowest tensile strengths of the materials studied, (2) higher tensile strengths were observed with higher strain rates; and (3) polysulfide materials showed the greatest amount of elongation at rupture, with the highest values occurring at the higher strain rates. (J PROSTHET DENT 1991;66:292-8.)

G ood dental

impressions require accurate and detailed replication of teeth and the contiguous oral integument. Numerous sources of inaccuracy can develop during impression making; among these are the selection of type, size, and rigidity of tray; the application of tray adhesive; impression technique; and manipulation of the impression material.1-g In addition, dentists must understand the important physical properties and behavioral characteristics of the impression material they are using, in order to optimize the result and reduce possible failures. For example, since elastomeric impression materials are viscoelastic,1°.12

Presented at the Pacific Coast Society of Prosthodontists meeting, Sun River, Ore. BDean and Professor, Department of Restorative Dentistry. bChairman and Professor, Department of Restorative Dentistry. cProfessor and Director of Biomaterials Research, Department of Restorative Dentistry. 10/l/27303

292

the strain is dependent on the length of time that a load is applied. Therefore it is suggested that an impression made with a viscoelastic material must be removed from the mouth with a snap, in order to minimize permanent deformation.13y l4 This study evaluated and compared the effect of strain rate on the ultimate tensile strength, ultimate elongation or elongation at break, and elastic recovery of five types of elastomeric impression materials.

MATERIAL

AND METHODS

For this experiment, “strain rate” was defined as crosshead speed (in millimeters per minute) times the gauge length of each specimen. Since the gauge length of each specimen in this study was made constant (79 mm), the strain rate was solely dependent on the variation of the crosshead speed (for example, the speed of pulling). Elastomeric impression materials are categorized into three major classes: (1) polysulfide, (2) silicone or polysiloxane, and (3) polyether.15 Two types of polysulfides are

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