silicone based impression materials

SILICONE BASED IMPRESSION MATERIALS

CONDENSATION SILICONE

AAMIR GODIL FIRST YEAR P.G.

DEPARTMENT OF PROSTHODONTICSM.A.R.D.C.

CONDENSATION SILICONE IMPRESSION MATERIAL

2

OUTLINE• Introduction • Chemistry of Condensation Silicone

Impression Material• Review of Physical and Chemical

Properties• Advantages and Disadvantages• Conclusion


3

INTRODUCTION

4

Elastic impression materials were developed from synthetic rubber by S.L. Pearson at the University of Liverpool in 1955.

The introduction of rubber-based, polysulfide impression materials was followed by silicone-based materials, both of which are still in use.

Following these, elastic polyether impression materials were introduced, followed in recent times by polyvinylsiloxane impression materials.

Hoffman-Axthelm, W. History of Dentistry. Chicago, Quintessence Put. CO., Inc., 1981. p.

284


5

• Condensation silicone was the first type of silicone impression material.

• Also known as conventional silicones.• Setting occurs at room temperature , and so are also

called RTV silicones.


6

CHEMISTRY OF CONDENSATION SILICONE IMPRESSION MATERIAL

(C- Silicone)


7


8

COMPOSITIONSupplied as two paste system:• Base paste:

• Hydroxyterminated polysiloxane polymer• Filler (cristobalite, talc, starch)

• Catalyst:– Liquid catalyst:

• Cross-linking agent (e.g tetraethoxy silane) +• Activator (dibutyl-tin dilaurate)

– Paste catalyst:• Cross-linking agent, activator, inert oil• Filler

9Institute of Medical Biochemistry, 1st Fac. Med., Charles

University, and Institute of Dental Research in Prague


10

• The instability of condensation-type silicone materials is attributed to the production of an alcohol byproduct during polymerization

• A two-step putty/wash technique was developed in which the use of a thin layer of wash material minimizes the amount of alcohol byproduct and thereby retains the dimensional stability within acceptable limits.

(Craig RG. A review of properties of rubber impression materials. J Mich Dent Assoc 1977;59:254-61.)

• To further overcome some of the problem associated with condensation silicones, another silicone material was introduced that polymerizes by an addition reaction in which no byproduct is produced.


11

REVIEW OF PHYSICAL AND CHEMICAL PROPERTIES


12

OVERVIEW


13

• The proposed definition of the working time of an elastomeric impression material suggested by ISO (1988) is "the period of time between the start of mixing and the commencement of the development of elasticity and the loss of plasticity"

• This would seem to be a very sensible definition, but most techniques used for the evaluation of working time simply monitor a change in viscosity or rigidity (Wilson, 1966; McCabe and Bowman, 1981; ISO, 1988)

WORKING AND SETTING TIME


14

At 230C3.3

minutesAt 370C

2.5 minutes

At 230C11.0

minutes

At 370C8.9

minutes

MEAN WORKING TIME MEAN SETTING TIME

Usually, working time is measured at room temperature and setting time at mouth temperature


15

HOW ARE THE WORKING AND SETTING TIME ASSESSED?

• Penetrometer tests are used• End of working time: The time when a blunt

needle of specified dimension fails to penetrate the volume of given material at a specified depth

• End of setting time: The time when a blunt needle of specified dimension fails to permanently indent the set material

• In British Standards Test, reciprocating rheometer is used. The property recorded is more close to viscosity and shear thinning properties compared to elasticity


16

WHAT ARE THE FACTORS INFLUENCING WORKING AND

SETTING TIME?Decrease in temperature

(Cooling of glass slab and storing materials at room temperature can help in prolonging the working and setting time)

Decrease in viscosity

Increase in temperature( Rise in temperature accelerates the curing rate )

Increase in viscosity(Changing the proportions of base/catalyst can alter the setting reaction)


17

DIMENSIONAL STABILITYWHAT ARE THE FACTORS AFFECTING

DIMENSIONAL STABILITY?

Polymerisation Shrinkage

By-Product Release

Thermal Contraction

ImbibitionIncomplete Recovery |

Viscoelsticity


18

Linear contraction of four elastomeric impression materials

SIGNIFICANCE-To attain maximum accuracy, polysulfide and C- silicone casts should be poured immediately.

Immediately is defined as the period within the first 30 minutes, even for putty wash technique. (Phillips’)

- Polyether and A-silicone will yield accurate casts even after 24 hours to one week.


19

DIMENSIONAL CHANGE AFTER SETTING

• The unrestrained dimensional change after setting has been reported by Craig (1986b), Mansfield and Wilson (1975), Eames et al (1979), Ohsawa and Jorgensen (1983), and Bell et al (1976).

• The values show some variation from product to product of the same type, but the decreasing order of dimensional stability is

ADDITION SILICONESPOLYETHERS

POLYSULFIDESCONDENSATION SILICONES

(After 24 hr in air of 50% relative humidity)


20

EFFECT OF HUMIDITY ON DIMENSIONAL STABILITY

Bell et al (1976)• The effect of low, medium, and high humidity on the dimensional

changes of stored rubber impressions of polysulfides, condensation silicones, and polyethers

• The dimensional change of all three types was affected. The standard polysulfide was affected the least, followed by the condensation silicone and polyether

• The optimum storage condition for polysulfides was high humidity; for condensation silicone, medium humidity; and for polyether, low humidity

BELL, J.W.; DAVIES, E.H.; and VON FRAUNHOFER, J.A. (1976): The Dimensional Changes of Elastomeric Impression Materials under Various Conditions of

Humidity, / Dent 4:73-82.


21

REPRODUCTION OF ORAL DETAIL

ACCURACY OF STONE MODELS• A variety of laboratory models has been used to

evaluate the accuracy of rubber impression materials.

(Reisbick and Matyas, 1975; Stackhouse, 1975; Stauffer et al., 1976; Marcinak et al., 1980; Lacy et al., 1981a,b; Augsburger et al., 1981;

Marcinak and Draughn, 1982; Finger and Ohsawa, 1983; Johnson and Craig, 1985)

PolyetherAddition Silicone

PolysulfideCondensation Silicone


22

RECOVERY FROM UNDERCUTSJohnson and Craig (1985)

• The addition and condensation silicones demonstrated the best recovery as a result of being removed from undercuts and the least change between models from an initial and a second pour


23

Tjan et al. (1986) and Linke et al. (1985) • Standard gold castings were used to evaluate the accuracy

of dies from the impressions of crowns, MOD's, and occlusal inlays.

• Agar was less accurate than were silicones and the polyether for making impressions of occlusal or MOD inlays.

• A delay in pouring condensation silicone impressions of MOD's and crowns resulted in a decrease in accuracy which was not observed for occlusal inlays.TJAN, A.H.L.; WHANG, S.B.; TJAN, A.H.; and SARKISSIAN, R. (1986): Clinically

Oriented Evaluation of the Accuracy of Commonly Used Impression Materials, / Prosthet Dent 56:4-8.LINKE, B.A.; NICHOLLS, J.I.; and FAUCHER, R.R. (1985): Distortion Analysis of Stone

Casts Made from Impression Materials,/ Prosthet Dent 54:794-802

ACCURACY


24

WETTING AND CONTACT ANGLE


25

COMPATIBILITY WITH DIE MATERIALS

• Lorren et al. (1976) measured the contact angles of mixes of dental stone on elastic impression materials and found values :

Condensation Silicone 95°

Polysulfide 70°

Polyether 50°

Addition Silicone (Hydrophillic) 40°

• They also found a direct relationship between the contact angle and the number of bubbles occurring on casts.LORREN, R.A.; SALTER, D.J.; and FAIRHURST, C.W. (1976): The Contact

Angles of Die Stone on Impression Materials, / Prosthet Dent 36:176-180.


26

• Norling and Reisbick (1979) measured the contact angles of saturated calcium sulfate dihydrate solutions to be:

– Polysulfide impressions = 80°– Condensation silicone impressions = 97°– polyether impressions = 15°

• When they incorporated non-ionic surfactants into polysulfide and silicone impression materials, the wettability of gypsum mixes increased and the number of surface bubbles decreased.

• The working time of the silicone was increased; however, the working time of the polysulfide was unchanged, as was the permanent deformation or dimensional change.NORLING, B.K. and REISBICK, M.H. (1979): The Effect of Nonionic Surfactants on Bubble Entrapment in Elastomeric Impression Materials, /

Prosthet Dent 42:342-347


27

DISINFECTION

• Recommended material for elastomeric impression materials: – Glutaraldehydes– Chlorine compounds– Iodophores– Phenolics

• Method: Immersion

• Disinfectant requiring more than 30 minutes exposure time are not recommended.


28

COMMERCIALLY AVAILABLE DISINFECTANTS


29

• An evaluation of polysulfides, poly ethers, and condensation and addition silicones with a variety of disinfectants was conducted.

• They found in some cases that some disinfectants adversely affected impression materials, and therefore care should be used in selection of a satisfactory combination.

• The smallest dimensional changes were observed for addition silicones with:

BERGMAN, M.; OLSSON, S.; and BERGMAN, B. (1980): Elastomeric Impression Materials. Dimensional Stability and Surface Detail Sharpness Following Treatment with Disinfection

Solutions, Siued Dent J 4:161-167.STORER, R. and McCABE, J.F. (1981): An Investigation of Methods Available for Sterilising

Impressions, Br Dent J 151:217-219.

Bergman et al (1980) Storer and McCabe (1981)

Cidex 2.4% activated glutaraldehydeTechno-sept PropanololHibitane 8g/L chlorhexidine gluconateK-644 Potassium bromideBenzalkon Benzalkonium chloride


30

RHEOLOGICAL PROPERTIES

• The data for condensation silicones could be fitted to kinetic equations, but the kinetics was not consistent with the stoichiometry.

• However, the dependence of reaction rate on catalyst-base ratio was qualitatively consistent with kinetic and network considerations.

COOK, W.D. (1982a): Rheological Studies of the Polymerization of Elastomeric Impression Materials. I. Network Structure of the Set State, / Biomed Mater Res 16:315-330.

COOK, W.D. (1982b): Rheological Studies of the Polymerization of Elastomeric Impression Materials. II. Viscosity Measurements, / Biomed Mater Res 16:331-344.

COOK, W.D. (1982c): Rheological Studies of the Polymerization of Elastomeric Impression Materials. III. Dynamic Stress Relaxation Modulus, / Biomed Mater Res 16:345-357.


31

Polysulfide

C-Silicone

A- Silicone

Polyether

Creep Compliance

The rate at which strain increases for a constant

applied stress


32

ELASTICITY• The material should be adequately elastic such

that it does not undergo permanent deformation on retrieval

• Setting time as mentioned by the manufacturer may not be adequate to provide sufficient elasticity under clinical conditions

A- Silicone

C-SiliconePolyether

Polysulfide

Amount of permanent deformation following strain in compression


33


34

• Refers to the resistance to fracture of an elastomeric material subjected to a tensile force acting perpendicular to surface flaw.

• Tear strength (N/m) (lb/m) = Maximum force needed to pull the specimen apart (N)

Thickness of the specimen (m)

TEAR STRENGTH


35

WHAT IS THE SIGNIFICANCE OF TEAR STRENGTH?

• Material can tear while recording impression of subgingival or interproximal areas

• If a material tears in such areas, a portion may remain embedded in the gingival sulcus or the interproximal area, which may be difficult to locate due to lack of radiopacity of these materials (except polysulfides)

Poly-sulfide

Polyether

Silicones

Tear

St

reng

th


36

BIOCOMPATIBILITY

Allergy Toxic Reactions

Hypersensitivity

Cell Cytotoxicity Analysis:

Polysulfides Silicones Polyether

+++Significance:

If evidence of tearing is detected during careful inspection of gingival sulcus, immediate retrieval of the remnant should be done


37

ADVANTAGES AND DISADVANTAGES


38

ADVANTAGES• Lower price• Good elastic recovery• Easily seen margins• Sufficient working time• Pleasant odor and taste

DISADVANTAGES• Low tear strength• Volatile by product• Shrinking of impression over time• Limited shelf life• Dimensional stability is affected due to by

product• Hydrophobic• Usually hand-mix version only


39

CONCLUSION


40

Thank You

silicone based impression materials

Health & Medicine