Wettability of NonaqueousElastomeric Impression

Materials

lohnny Y. Chai, BDS, MSAssistant ProfessorDirector, Advanced Education ProsthodonticsNorthtvestern University Dental SchoolChicago, Illinois

Tze-Cheung Yeung, BDS, MSAltending StaffDental DepartmentVeterans General HospitalTaipei, Republic of China

The wettability of eight nonaqueous elastomeric impressionmaterials was studied by comparing their contact angles.The materials included three polyethers (one of which waslight activated), three hydrophilic poly(vinyl siloxanes), oneconventional poly(vinyl siloxane), and one poly(vinylsiloxane) putty. Extracted teeth were prepared toapproximate the roughness of a tooth preparation. Contactangles were measured at different time intervals after thestart of mixing but were not shown to be significant. Thenonhydrophilic poly(vinyl siloxane) materials and thepoly(vinyl siloxane) putty v /ere found to be significantly lesswettable. Int j Prosthodont 1991,-4:555-560.

T he entrapment of air bubbles when introducingstone into an impression can result in substan-

tial defects in the resulting cast. The better the wet-ting of the adherent (impression material) by theadhesive (dental stone), the less likely air is to beentrapped. Also, the viscosity of the adhesive hasto be considered.' When a bubble is entrappedaround a loose particle, eg, a piece of temporarycement in the impression, the bubble diminishesat a rate dependent on the viscosity and the surfacetension of the adhesive and the abilities of theadhesive to wet the adherent and the particles.-While the viscosity and the surface tension of thedental stone cannot be altered without compro-mising other important physical properties, differ-ent impression materials exhibi t differentwettabilifies. The purposes of this article were (1]to review the literature on adhesion and wettingand (2) to report the findings on the wettability ofseveral newer "hydrophilic" impression materials.

Review of the Literature

An atom in the lattice of a solid is attractedequally by its neighhoring atoms. However, those

Reprint requests: Johnny Y. Chai, BDS, MS, Department ofProsthodontics, Northwestern University Dental School, 240E Huron Street, Chicago, Illinois 60611.

at the surface are attracted to the subsurface atoms,thus creating a force imbalance. The attraction ofsurface atoms to each other is stronger than theattraction between the surface and subsurfaceatoms, thus creating a phenomenon known as sur-face tension, which is commonly measured indynes per centimeter.' Surface energy, measuredin ergs per square centimeter, is numerically iden-tical to surface tension. The attraction of unlikeatoms to the surface of a material is termed adhe-sion. Long-range attractive interactions aremediated through electrostatic forces and van derWaals dispersion forces. Short-range attractiveinteractions are effective only at the immediateadhesive area. Examples are various types of chem-ical bonds and intermediate bonds (such as hydro-gen bonds].'

It has been shown that the reversible work orforce of adhesion between a liquid and a solid (Wa)is governed by Dupré's equation"";

Wa = âSV + ÖLV - ÖSL (î)

where âSV, âLV, and ôSL are respectively the freesurface energies or surface tensions of the solid-vapor, liquid-vapor, and solid-liquid interfaces atequilibrium.

Another important relationship between the freesurface energies or surface tensions of the inter-faces was proposed by Young^;

dSV - (2)

S.--^-—4, Number 6, 1991 555 The International Jojrnal of Pro^thodontics

queous Elaslomeric Impresi Cli,ii/Yeung

where i is tbe contact angle formed when thereis a balance between tbe free surface energies atall tbe interfaces.

Good wetting of a liquid on tbe surface of a solidoccurs wben the contact angle approacbes zero.In such situations, the force of adbesion betweenthe liquid and solid molecules approaches that ofcohesion between tbe liquid molecules. To theother extreme is nonwetting between a liquid anda solid when tbe contact angle approacbes 180°.A contact angle of 90° has been arbitrarily selectedto distinguisb tbe wetting from the nonwettingphenomena.*

Altbougb one may rank the wettabilities of liq-uids on a solid by the magnitude of the contactangles, quantitative assessment of their wettabili-ties remains difficult. For example, a liquid with a35° degree contact angle may not indicate tbat tbeliquid has twice tbe wetting ability of another liquidhaving a 70° contact angle. The reversible work orforce of adhesion between a liquid and a solid (Wa)is a better indication of the adhesiveness betweena liquid and a solid.

When equation 1 is combined with equation 2:

Wa = äLV + 3LV cos4> = dlV (1 (3)

Hence, by knowing the surface tension of the liq-uid and its contact angle with the solid, tbe revers-ible work of force of adbesion between the liquidand tbe solid can be determined. Tbe surface ten-sion of a liquid and the contact angle can be esti-mated witb a tensiometer and by geometricmeasurement, respectively. In addition, wbencomparing the wettabilities of different surfaces,the cosine of tbe contact angles is a better param-eter tban are tbe contact angles.

While surface tensions of liquids are less tban100 dynes/cm, tbe free surface energies of bardsolids are between 500 and 5000 ergs/cm^ Thoseof organic and polymeric solids are comparable toliquids. The free surface energies of solids seem toincrease with their hardness and melting points.'Increasing tbe free surface energy of a solid ordecreasing tbe surface tension of a liquid improvesthe adhesion.

The direct measuremenl of the free surface ener-gies of solids is difficult, hence a more easily obtain-able and more usable parameter, called the criticalsuTÍace tension, is used. A review of tbe conceptof critical surface tension is found elsewbere.'

Some factors that influence surface energies andsurface tensions warrant discussion. The presenceof a surface-active agent (surfactant), eg, soap,reduces tbe surface tension of water, and thus pro-motes its wetting ability. Tbe presence of humidity

reduces tbe surface energy of a bigb-energy solid.Tbis pbenomenon is more obvious if the surfacemolecules of the solid have an affinity to water. Amonolayer of water, wbich has lower surface ten-sion, adsorbed onto tbe surface of tbe solid is suf-ficient to iower its surface energy.

Surface tensions of liquids are reduced as tem-perature is raised.« The rate of wetting of a solidsurface by a liquid depends on tbe force of adbe-sion as well as tbe viscosity of tbe liquid. In otherwords, tbe contact angle of a liquid varies witbtime. More time elapses for a viscous liquid toattain equilibrium at the interfaces, bence to bavea constant contact angle.^^

Tbe contact angles differ depending on wbetbera liquid bas advanced onto a solid surface or aliquid recedes from a previously wetted surface.These are termed advancing contact angle andreceding (retreating) contact angle, respectively.'Tbe observation of a smaller receding contact anglewas tbougbt to be tbe fill-in of surface irregularitieson tbe solid surface by tbe liquid, thus facilitatingwetting. Altbougb it bad been claimed tbat therewas no difference between advancing and reced-ing contact angles if tbeir determination bad beencarefully done,' tbe introduction or removal of liq-uid from tbe center of a tbermodynamically stabledrop would change its contact angle. Increasing thevolume of liquid inside tbe drop increases its pres-sure, tbus generating an increasing sbear force attbe contact line {liquid-solid-air interface). Tbecontact line remains stationary as the pressure inthe drop increases. Tbe contact angle of the dropincreases simultaneously to a maximum value(advancing contact angle) until tbe drop spreads.Tbe drop only spreads wben the sbear force at tbecontact line exceeds a critical limit. Likewise, witb-drawal of liquid from tbe drop decreases its contactangle to a minimum (receding contact angle]before tbe contact line retreats.'"

Tbe relationship between surface rougbness andcontact angle is defined by Wenzel's equation":

R = R7R° =

wbere R is tbe rougbness factor, R' is the true sur-face area of a rougb solid, R° is tbe surface area ofan ideally plane solid, ip' is the contact angle of aliquid on a rough solid, and ^° is the contact angleof a liquid on an ideally plane solid. Hence, tberougber a surface, tbe larger tbe rougbness factor,wbicb is always greater than or equal to one:

R = cos(í>7cosií° > 1

Increasing roughness increases cos0', since cos<l>°is constant. Tbe contact angle (0') decreases wben

The Internalional iournal of Prosthodontics 5 5 6 Volume 4, N

CSai/Veung Nonaqueous Eiasiomeric Impn

Table 1 Impression Materials Evaiuated

Impression materials Brands

Batch numbers

Catalyst

Polyethar

HyOrophilicpoly{vinyl siloxane)

Nonhydrophilicpoly (vinyl siloxane)Poly(vinvl siloxane] putty

Impregum-F- 473 130Parmadyne low uiscosity' 28 75Genesis low viscosityt 11588 11588

Mirror-3 low viscosity^ 81284 81284Express low vrscosity*§ 6FT2XJ1 6FT2XJ1Imprint medium viscosityg eAE1F8 8AE1F8

Reprosil medium viscosityt 122185A 122185A

Mirror-3 very high viscosityt 81351 81351

'ESPE-Premigr, Norrisfown, Pa.fLD Caulk Div/Dentsply, Miltord. Delà.tKerr Manufacturing Co, Romulus. Mich.§3M Dantal ProOucts D;v, St Paul, Minn.

the value of cosii'/cos0° increases for 0' between0° and 90°. However, the contact angle increasesfor an increasing value of cos^'/costíf" for 4,between 90° and 180°. Therefore, rougheningincreases the wettability of a solid for liquids withcontact angles below 90°. The reverse is true forliquids with contact angles above 90°,'

Earlier studies estimated the wettabilities ofimpression materials by measuring the contactangles formed by standard mixes of die stone.'^-^Some felt that this method produced mushroom-shaped drops and made contact angles difficult tomeasure. In addition, the procedure was subjectedto variables such as the duration and degree ofvibration.'" Instead of die stone, a saturated solu-tion of calcium sulfate dihydrate or water had beenused."" ' Another method of measuring wettabili-ties of impression materials was counting the num-ber of air bubbles on the surface of stone casts ofimpressions of tooth preparations'^-^" or standarddies.'""The order of increasing wettability of elas-tomeric impression materials has been shown tobe as follows: condensation silicone, conventionalpoly(vinyl siloxane), polysulfide, polyether, revers-ible hydrocolloid.'^''''^'"' The addition of nonionicsurfactants of nonylphenoxypoly(ethyleneoxy)ethanol homologs of specific ethyieneoxy chainlength to polysulfide and silicone was found toincrease their wettabilities. Not only was the effectof these homologs specific to the type of impres-sion material, their effects could be different forimpression materials of the same type.''' Althoughsome proprietary surfactants improved the wetta-bility of polyether and hydrocolloid, none of theseven products investigated in one study showedany efïect on polysulfide or silicone." However,another study showed that two surfactants were

effective in increasing the wettabilities of five kindsof impression materials.'"

Materials and Methods

Eight elastomeric impression materials weretested (Table 1), Five of the materials werepoly(vinyl siloxanes); three were marketed ashydrophilic type, one did not claim to be hydro-philic, and one was a putty. Two of the materialswere polyethers and one was a light-activatedpolyether ureihane dimethacrylate.

Since tooth preparations were never entirelysmooth and surface roughness had been known toaffect the contact angle, impression materials wereallowed to polymerize against prepared tooth sur-faces. A flat surface was prepared on five molarteeth, embedded in autopolymerizing acrylic resin(Orthodontic Resin, LD Caulk Div/Dentsply, Mil-ford, Delà), using a diamond instrument (round endtaper 8850-014, Brasseler, Savannah, Ga), Fivenylon molds, 6.5 mm in thickness with a circularopening of 12.5 mm in diameter, were used to holdthe impression materials.

Each impression material was mixed accordingto the manufacturers' instructions to a streak-freemix. They were loaded into a syringe and injectedinto the opening of each of the molds, which wereplaced over a polyvinylchloride sheet. The pre-pared teeth were randomly placed over the circularopening in each nylon mold and the impressionmaterial was allowed to polymerize at room tem-perature for 15 minutes. Five samples of each ofthe impression materials were prepared from singlemixes. Samples of the light-activated impressionmaterial were prepared by polymerizing the mate-rial for 3 minutes through a clear plastic plate cov-

-Î 4, Number 6, 1991 5 5 7 The International Iournai of Prostliodontics

onaq eous Eläitomeric Imp

Table 2 Contact Angle Measurements

m près si on materials

Genesismpregum=ermadynaExpressmprintMirror-3 (low viscosity)Reprosil\flirror-3 (putty)

20 min

.553 (.063)

.486 (.086)

.474 (.018)

.377 (.057)

.288 (.115)

.347 (.112)

.031 (.061)-,046 (.047)

es5ion Materials

Cosine of contact Í

50 min

.454 (.075)

.468 (.076)

.375 (.086)

.358 (.038)329 (.093).217 (.072).059 (.088)

-.094 (.065}

J Chai/Yeung

ngles

80 min

.455 (.102)

.423 (.093)

.344 (.091)

.436 (.056)

.340 (.142)

.251 (.033)-.027 (.060)-,052 (.056)

Ove ral

60.7 (5-87)62.5 (5.35)66.5 (5.49)67.0 (3.6B)71.2(6.80)74.1 (5.59)88.9 (4.47)94.0 (3.25)

mean

cos í

.487 (.090)

.459 (.083)

-.064 (.057)

Standard deviations m parenthèses.tia signiticant öitterence (P > .OtI beWeen materials connected by vertical bar.

eringthe nontesting surface ofthe samples (Prisma-Lite, LD Caulk Div/Dent5ply),

All samples were disinfected in accordance withthe guidelines of the American Dental Associa-t ion." The polylvinyl siloxane) materials wereimmersed in 3.2% glutaraldehyde solution (Cidex-Plus, Johnson and Johnson, East Windsor, N]¡ for10 minutes, according to the manufacturer'sinstructions. The polyether materials were sprayedwith the same disinfectant and placed in a sealedplastic bag for 10 minutes. All samples were rinsedwith water and dried with an air syringe beforetesting.

Contact angle measurements of each samplewere made 20, 50, and 80 minutes after the startof mixing. A calibrated pipette was used to placea drop of saturated calcium sulfate dihydrate (0.05

mL) onto each sample. A photograph was made ofeach drop 30 seconds after its placement using a2:1 magnification. The 30-second duration wasselected based on a pilot study which indicatedthat the magnitude of the contact angles at thisinterval was very close to those at thermodynamicequilibrium and that the setting die stone wouldstill be workable in this timeframe. A level rule(Skandor SKY 272-0464, Korea) was used to alignthe camera and the samples to ensure they werehorizontal, A 35-mm camera (Nikon FM 2)equipped with a 105-mm macro lens (Micro-Nik-kor) and two extension tubes (Nikon PN-11, PF-13) recorded a 2X magnified image. No attemptwas made to align the direction of tooth prepara-tion on the sample to the optical axis of the lens.Hence, the effect of the direction of tooth prep-

G^fiç^iï Iiripregurr |mo,i„| M¡,,oi-3 Heprasil Mi.,or.3 pull»

Fig 1 Cosines of contact angles of impression materials.

The imerrational Journal of Prosthodontics 5 5 8 Volume 4, Niimhipt f,

: Imptesîion Material;

aration on the contact angle was ranidom. The con-tact angles were measured on 3 X 5-inchphotographs of the samples. The average of tworeadings from each (drop was calculated. The tem-perature and relative humidity recorded during theexperiment were 23.5°C ± 0.5°Cand55% ± 1%,respectively. Two investigators independentlymeasured the contact angles of all samples. Sincea good correlation was found between the inves-tigators (Pearson correlation, r = .9277), one setof the measurements was used. A two-way analysisof variance (ANOVA) and Newman-Keuls muhiplecomparison test were used to study the effect ofmaterials and time on the cosine of contact angles.

Results

The cosines uf contact angles of all impressionmaterials tested at different intervals are presentedin Table 2 and Fig 1. The two-way ANOVA showedsome significant differences among the impressionmaterials (P < .01) but failed to show any signifi-cant difference between the time intervals {P >.01). Hence, the time intervals for eath impressionmaterial were pooled. A one-way ANOVArevealed significant differences among the impres-sion materials. Newman-Keuls multiple compari-son test showed that there were no significantdifferences (P > .01) among Genesis, Impregum,Permadyne, and Express as a group; Permadyne,Express, and Imprint as a group; and Imprint andMirror-3 (low viscosity) as a group. Any other com-bination was significantly different (P < -01). Thenonhydrophil ic poly(vinyl siioxane) and thepoly(vinyl siioxane) putty were found to have sig-nificantly lower wettabilities than the hydrophilicpoly(viny! siioxane) and the polyether materials.

Discussion

The time of contact angle measurement aftermixing did not affect the wettability of the impres-sion materials tested. The poiyether materials werethe most hydrophilic materials tested, which is con-sistent with previous studies.'^-^^'« The polyetherand poly(vinyl siioxane) impression materials weresignificantly more hydrophilic than the conven-tional poly(vinyl siioxane) material. Although notall hydrophilic poly(vinyl siioxane) materialsapproached the wettahility of the polyether mate-rials, they were significantly more hydrophilic thanthe conventional poly(vinyl siioxane). Thus, themanufacturers' claim of increased wettability is jus-tified. This finding is consistent with that of somerecent studies."''"''' It should be noted that at the

time of writing this paper, the nonhydrophilicpoly(vinyl siioxane) was replaced with its hydro-philic formulation. Although the poly(vinyl siiox-ane) putty (Mirror-.î) was marketed together withits low-viscosity material as hydrophilic, it wasunlikely that the manufacturer had intended toclaim the putty to be hydrophilic. The putty mate-rial exhibited a significantly lower wettability thanthe conventional poly(vinyl siioxane). However, aputty material might not need to be hydrophilic,since the material would seldom contact the toothpreparation.

Counting air bubbles on stone dies from animpression of standard dies or tooth preparationwas not performed in this experiment because ear-lier studies had confirmed the correlation of thenumbers of air bubbles on stone dies with the wett-abilities of impression materials.''•"•'""^' It shouldbe noted that all impression materials tested weredisinfected according to the manufacturers' pro-tocol. However, it is not known whether this dis-infectant affected the wettabil i t ies of theimpression materials, nor is it known if the variationin disinfection method (spray vs immersion) intro-duced a variable. One study demonstrated adecrease in the wettability of a hydrophilic impres-sion material affer immersion in any of five disin-fectants.'^

Conclusions

1. Varying the time between 20 and 80 minutesafter the start of mixing did not affect the wet-tability of the impression materials tested.

2. The light-activated polyether urethane dime-thacr>'late impression material showed thebest overall wettability, which was not signif-icantly different from the other polyethermaterials tested.

3. The nonhydrophilic poly(vinyl siioxane) mate-rial exhibited significantly lower wettabilitythan the hydrophilic poly(vinyi siioxane)materials, the polyether materials, and thelight-activated polyether.

4. The poly(vinyl siioxane) putty demonstratedthe lowest wettability, which was significantiylower than all other materials tested.

References

1. Phillips RW: Skinner's Science of Dnntai Materials, ed 8Philadelphia, WB Saunders, 1982, pp 20-27.

2. von Fraunhofer JA: Scientific Aspects of Dental Materials.London, Butterworthî, 1975, pp 59-62.

3. GIsntz PO: Wettability and adhesiveness. A study ofenamel, dentine, some roçtorative dental iiiateiials, and

4, Number 5, 1991 559 Tiie International Journal oí Prosthodontics

Chai/Yeung

dental plaque. Odontol Rev 1969;12(suppl 17):8-16.4. Duprê A: Theorie Mechanique de ¡a Chaleur. Paris, Gau-

Ihier-Villsis, 1869, p 369.5. Young T: An essay on the cohesion of fluids. Philoi Trans

R Soc Lond ÍBio!) 1805;65, 95.6. Greener EH, Harcourl IK, Lautenschlager EP: Materiais

Science in Dentistry. Baltimore, Williams and Wilkins,1972, p 40.

7. Glanti PO: Wettability and adhesiveness. A study ofenamei, denline, îome restorative dental malerials. anddéniai plaque. Odonto! Rev 1969;12(5Uppl 17]:19-31.

8. Craig RG: Restorative Dental Materials. St Louis, CVMosby Co, 1989, PP 31-32.

9. Koran A; Impression material for recording the denlurebearing mucosa. Dent Clin North Am 1980,24:97-111.

10. Darvell BW, Murray MD. Ladizesky NH: Contact angles:A note. ) Denn987;1 5:82-84.

11. Wenzel R Resistanceofsoiid surfaces to wetting by water.Ind Eng Chem 1936;2e:98e.

12. Lorren RA, Salter D|, Fairhurst CW: The contact angios ofdie slone on impression maleiials. ; Prosthel Dent1976;36:176-t80.

13. Lacy A, Treieaven S, lendresen M: The effect of selectedsurfactants on the wetting behavior of gypsum die stoneon impression malerials. CDA J 1977;Nov:36-40.

14. Noriing BK, Reisbick MH: The effect of nonionic surfac-(anl; on bubble enlrapmeni in elastomeric impressionmaterials. / Frosthet Dent 1979:42:342-347.

15. Peulïfeldt A, Asmussen E: Impression materials: Lííecl ofhydrophiiicity and viscosity on ability to displace waterfrom dentin surfaces. Scand I Dent Res 1988;96:25i-259.

16. Prallen DH, Craig RG: Weltability oía hydrophiliC addi-tion silicone impression material. / Prosthet Dent1989:61:197-202.

17. GerrowlD, Roda MR, West D: Comparison of advancingcontact angles of stone on impression materials. J DentKes1989;68:l007, IADR abslrno. 1126.

18. McCormick |, Shiliingburg H, Antony S, Duncanson M:Wettabilily of elastomeric impression materials: Effect ofselected surfactants. I Dent Res 1988;67:375, IADR abstrno, 2101.

19. Council of Denial Malerials, instruments, and Equipment;Council on Dental Practice; Council on Dental Therapeu-tics: Infection control recommendations for tiie dentaloffice and the dental laboralory. / Am Dent Assocl988;116:24l-248.

20. Cullen D, Mikeseil |, Sandnk |: Contact angle and voidsin gypsum casts from elastomeric impressions. ; Dent Res19e9;68:397, IADR abslr no. 1723.

21. Noding BK, Conn L|: Wetting properties of hydrophilicand conventional poiylvinylsiloxanes). / Dent ResI988;67:282, IADR abstr no. 1353.

22. Pratten D, Covey D. Sheats R: Effect of disinfectants onthe wettabilily of impression materials. / Denl Res1989;68:397, IADR abstr no. 1727.

Literature Abstract.

Radiation Absorbed From Dental Implant Radiography:A Comparison of Linear Tomography, CT Scan, andPanoramic and Intra-oral Techniques

The radiation dose delivered to five critical tissues during simulated dental implant radiographieprocedures was measured. A humanoid tissue-equivalent x-ray phantom was used for simulation of adental implant patient. One hundred ribbon lithium fiuonde thermoluminescent dosimeters were usedfor dose measurements. They were placed within and on the phantom at anatomic locationscorresponding to bone marrow, salivary gland, upper thyroid gland, eyes, and skin entrance areas.The CT examination delivered the greatest dose, while linear tomography delivered the lowest.Reported panoramic and intraoral doses were similar to those of linear tomography.

Clark DE, Dnnlorth RA, Barnes RW, Burlch ML. Scand J Dent Res 1991:99|3):236-24D. References: 22. Reprints: DrDennis E Clark, Associate Professor of Orai Radiology, School ot Dentistry. Loma Linda, Caiifornia 92354.—Stet'enP. Haug, DDS, Indiana University School oi Dentistry

The International lournai of Prosthodontics Volume 4, Number 6, 1991