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Impression materials (2)

Last time we stopped at "Properties of polysulfide /Slide #54"

Properties of clinical interest of Polysulfide:

Disadvantages:

Needs a long setting time (10 min or more).

Can be irritant to the oral mucosa.

Messy (we use orange solvent to remove stains).

Very unpleasant taste and odor.

Advantages :

Higher tear strength than hydrocolloids/agar.

Accuracy improves if impression is poured within 30 minutes.

Moderately hydrophilic.

Viscoelastic with slow but incomplete recovery.

For these disadvantages, Polysulfide has been largely replaced

by other rubber/elastic materials.

It is supplied in different viscosities and these viscosities are

controlled by the amount of filler, HOW?

The more filler you have, the more viscous the material will be as it will

be more dimensionally stable but the accuracy will be less because of the

viscosity

This is the 1st part of script 6 which will be included in the exam and the 2nd part is

for the quiz next week

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light body(blue)

these pictures represents using 2 viscosities together ,the blue one express the details so it is accurate but

has less stability as it can change the shape after removing it, that’s why we use it with a more viscous

/heavy material

**More viscosity: -heavier body

-Good dimensional stability, WHY?

Because they have less reactive groups, which means less ability of polymerization, shrinkage or subfraction after setting. -BUT LESS Accuracy.

Silicon rubber (condensation silicon)

Composition and setting reaction which are 2 in different viscosities according to

the amount of filler: * Base: Hydroxyl-terminated polydimethylsiloxane (liquid silicone prepolymer) + silica

filler.

*Catalyst: Alkyl silicate such as tetraethylsilicate +Tin compound such as dibutyl tin

dilaurate. All of these when mixed together will give the condensation reaction

if we don’t remove the impression product-ethyl alcohol byThe reaction produces -

in the right time, this by-product may evaporate causing changes in the dimensional

stability of the impression.

-They are both hydrophobic the manufactures make modifications to produce a less

when fielddry , it would be better having a but generally speaking hydrophobic version

taking an impression

**We use this material for Crown, bridge work,

complete and partial dentures.

Heavy body (orange)

Light Body (Blue)

The other

type is called

addition

silicon

sillicon

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*some of them contain the putty (more viscous) and other contain the wash (florable

material), these are used for teaching, but the other type (addition) is used in clinics

because it is better even though it is more expensive.

Properties: Hydrophobic

Compared to polysulphides:

◦ Shorter setting time (less than 8).

◦ Become elastic quicker.

The putty version has good dimensional stability.

Dimensional changes due to evaporation of by product.

Adequate tear strength.

Non toxic and no bad odor.

Liquid catalyst can be irritant if not mixed well, as each one can be

toxic, but the final product after mixing well is non toxic.

Silicon rubbers (addition curing) 2 paste systems

◦ Base: silicon pre-polymer with vinyl groups +filler

◦ Catalyst: silicon pre-polymer with hydrogen terminal groups + filler +

Chloroplatinic cid

Setting reaction:

-It is slightly quicker than the condensation (5min in the pt mouth)

-Addition polymerization reaction with no byproducts.

-In some products, hydrogen gas can be released.

Solution the manufacturer can modify the product to allow the

absorption of the ions and prevent its release and affect the stability Or

sending it to the lab quickly before the release of H.

SO it depends on:

1- Manufacturer.

2- Management.

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*there are

- wash material

Heavy and medium body-

-putty

((More viscous more stability less accuracy less reactive body))

*Usually when we take impressions we use the wash and the putty, or the medium one

without anything.

*There are machines for mixing, which are better,

because they minimize human errors; as hand mixing can

cause bubbles to appear, which in turn causes errors. Auto mixing gun

NOTE: we don’t mix wearing gloves, hand should be washed and dried,

because the latex found in gloves can interfere with the setting or

the polymerization, so we use bare hands, special machines or free

latex gloves in mixing.

Properties: Reasonable setting time.

Good tear strength.

High elasticity.

Excellent dimensional stability.

Hydrophobic.

Solution (dry field or using surfactant spray which make the surface of the

hydrophobic material hydrophilic to become more combatable to the impression

)

Less hydrophobic products are available

Disinfected as condensation silicon.

Clinical applications

- Similar uses to condensation silicon

* Problems associated with the addition silicones:

◦ Hydrophobicity

◦ Improper setting, due to contact with latex rubber, glove powder, freshly placed

methacrylate based materials such as composites or RM-GICs.

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Polyethers * Composition

- Base: polyether pre-polymer + filler +plasticizer

- Catalyst: aromatic sulphonic acid + filler +plasticizer

*Setting reaction

-Cationic Ring opening addition polymerization reaction.

-Ionized form of sulphonic acid provides the initial source of cations.

- As the reaction proceeds, more are produces.

-No by-product is formed

Properties and clinical considerations

- Adequate tear resistance.

- More rigid than other silicones once set (an area with severe undercuts

,this material will not be a good choice ,you should use a flexible one)

- Hydrophilic.

- Good dimensional stability under low humidity.

- Used as monophase (available as regular and putty).

- Can be used in taking impression for implants when they are placed in a

non-paralllel relationship intraorally.

- May cause allergic reactions because of the catalyst component.

- Disinfected for ten minutes as other silicones (when setting it for a long

time, there will be water absorption (swelling), which leads to changes in

the dimension).

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Comparison of properties of elastomers *There are differences between these silicone products (setting

time/accuracy/working time /rigidity)

*The BEST material in term of elastic behavior /dimensional stability

Polyethers and additional silicons.

SLIDE #67:

Working time

Setting time

Transition from plastic to elastic: polysulphides set longer, Polyethers set

faster with a distinct transition.

Accuracy (recording fine details and dimensional stability)

Polymerization and thermal contractions (viscosity and CTE have an effect)

Rigidity (Polyethers are the most rigid polysulphides are the least stiff) an

indication of how easy it is to remove impression from patients mouth.

SLIDE #68:

The graphs in this slide are not imp and we don’t have to know the details, they

are tests used in measuring the elasticity

There is an available standard to call an material as excellent elasticity and so

on.

There are differences between the elasticity in materials so if the

manufacturer introduces an material, he needs to use these standards and test

it, so he can compare it and specify it ,this is how they rank the materials from

high to low stability.

Tear resistance: (by measuring tensile strength or elongation by pulling

it in several directions) or more accurately, the force required to

propagate a tear from a notch tip in the material (v. good for addition

silicon).

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Jaw (bite) registration materials

* We use this method to find out the occlusal relationship between the 2

jaws in centric occlusion.

* We use: Addition silicones and Polyethers.

Also, we can use wax because it is cheaper but it is less accurate.

* How to apply it: Gun-mix format and of sufficient viscosity so they

won't drip off the teeth when applied, we apply it for ~2 min.

*One clinical problem (disadvantage) is: will record too much surface detail from the occlusal areas of the molar teeth, which is not recorded and/or cannot be re-engaged by the registration when placed onto the working mode.

DONE BY : Laila Malhas