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6

Afnan & Lina Hasan

…

Rahaf Al-Jafari

Nesreen Salim

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Acrylic resins: prosthesis materials that we use in patient’s mouth

for different uses.

What is the complete denture?

A removable dental prosthesis that replaces the

entire dentition and associated structures of the

maxilla and the mandible.

**The teeth (white part) and the denture base (pink part) are made of

acrylic resins.

**We take the patient’s impression by plastic tray, then pour it into gypsum

product to give a cast, which gives an exact copy of the patient’s arch ,after that

by using the acrylic material we make a special tray on the cast and one more

time we take an impression using the special tray, pour it in gypsum product ,after

that we start setting the teeth (which are also made of acrylic material ) in wax

above the cast, finally we replace the wax with acrylic material .. this is a simple

review about how we made the acrylic denture…

Acrylic resins

You should

imagine the uses

of the materials

to make it easier

to understand

Use acrylic resin to replace

missing tissue “as in completely

edentulous paitent”

Acrylic denture

األطقم المتحركة

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The material of choice?

Name: Acrylic resin (polymethyl methacrylate).

Why do we use this material? Because of its desirable

qualities:

1-Good esthetics: give acceptable pink color like gingiva.

2-Cheap material. 3-Easy to process: doesn’t need expensive equipment.

Impression using Metal tray Cast

Special tray

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Dental uses (applications):

1-Denture base material: the pink part in the denture (gingiva)

2-Denture repair: once patient broke his denture there is no need

to make a new denture, just repair the broken part by acrylic resin.

3-Denture teeth: although they are different in color “denture

teeth & base” but they are also made of acrylic resin.

4-Denture liners: overtime the denture may looses (واسع), so

instead of changing the whole denture, we only reline it تبطين with a

layer of acrylic resin to remake it good fit on patient.

5-Construction of customized trays (special trays)

6-Soft tissue replication on cast metal framework

Types of acrylic resin: *are named according to the procedures and their

activation methods*

1-Heat cured 2-Cold cured =chemical cured= self-cured 3-Microwave cured resins 4-Light cured ________________________

Monomer or MER: single organic molecule capable

of uniting. -The monomer in acrylic resin is

Methyl methacrylate (MMA). -It’s synthetic resin commonly used in dentistry.

*Keep in mind* When we have different applications for the same material, we

should have different types. Ex: Acrylic resin uses= (denture base, trays, liners, teeth,

repair..)

Acrylic resin (liquid & powder)

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-It has a weak double bond which can be broken down after

activating it, making the molecule active (free radical),

so it will bind with another monomer making it free radical,

again binding with another monomer and so on, until we

reach the final polymer.

Polymer: macromolecule formed by small repeating units called monomers -Polymethyl methacrylate its large number of monomers units, linked together (PMMA).

Copolymer: is formed by joining two or more different types of

monomers together.

monomer + monomer= polymer ➔ methyl methacrylate + methyl methacrylate=polymethyl methacrylate

monomer

monomer

monomer

monomer monomer monomer

methyl methacrylate

methyl methacrylate

methyl methacrylate methyl methacrylate

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Acrylic resin: Are resilient plastics formed by joining multiple methyl methacrylate molecules or (mers). The reaction is an addition polymerization reaction to convert MMA (methyl methacrylate) to PMMA (polymethyl methacrylate).

-Addition polymerization (adding monomers) ➔ NO by-product

-Condensation polymerization ➔ Produce by-product (water,

hydrogen).

Cross linked polymers: adjacent polymers that bond together by side chains (bonds between the linear polymers): 1-Weak bond between side chains leads to easily manipulated and stretched polymer (resilience) 2-If bond is highly charged, resulting polymer is stiffer (strong)

Linear chains

Cross linked

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Polymerization: (how reaction occur)

a-Addition polymerization:

1. Activation: by producing free radicals. (breaking the double

bond in the initiator, transforming it into free radical), activators:

1. Tertiary amine to activate (chemically cured).

2. Heat to activate (heat cured).

3. Light to activate (light cured).

2. Initiation: when an initiator such as benzoyl peroxide is

decomposed, and free radicals are produced (external free

radical that breaks down the double bond in the first monomer and

the polymerization reaction starts).

Past papers: which of the following is the initiator? Benzoyl peroxide

(because it has week double bond).

3. Propagation: occurs when free radicals break the bond

between carbon atoms in monomer, and adjacent

monomers bond together (adding the monomers through the

process).

4. Termination: occurs when monomer units are used up,

or free radical is tied up by a reaction

Briefly... free radical comes from the initiator benzyl peroxide (formed in whatever the method of activation)➔ breakage of the double bond in monomer (methyl methacrylate)➔monomer becomes free radical➔so it will bind

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with the adjacent monomer after breaking the double bond in it & so on …Until we reach the polymer.

b-Condensation polymerization:

Usually more than one type of monomers are used. The

reaction produces by-products such as water, hydrogen or

alcohol.

1)Heat cured resin: 1-Powder: Pre-polymerized beads of PMMA (They are the same material of acrylic resins but have already undergone addition polymerization and become rigid, then they got grinded and added to the powder of the material in order to participate in the production of a new acrylic resin material).

Initiator: benzoyl peroxide

Pigments: organic dyes, e.g.: ferric oxide, cadmium sulfide

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Opacifiers: titanium/zinc oxide. Also, barium or radiopaque

glass fillers (up to 20%) which may decrease strength and

change appearance. (opaque= not transparent)

Plasticiser: dibutyl phthalate (give the material flexibility &

resiliency)

Synthetic fibers: nylon/acrylic (strength).

2-Liquid: Non-polymerized methyl methacrylate monomer, boiling

point of 100.8°C. (volatile, very strong smell)

Crosslinking agent: glycol dimethacrylate (two double

bonds). This component improves mechanical properties by

joining polymer chains.

Inhibitor: hydroquinone, to prolong shelf life of monomer.

It reacts with free radicals and stabilizes them.

-Store in a dark(brown) container to prevent occurrence of

free radical.

Plasticizers: used to reduce the brittleness of cross linked polymers.(giving more resiliency), generally do not enter reaction, severe as interference for the cross-linking, or lubricant between chains.

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2)Cold cured resin: *needs activator* Composition: identical to heat cure resin except

polymerization is activated by tertiary amine (e.g.

sulfonic acid or dimethyl-p- toluidine) which is an accelerator.

• Working time is less than heat cured acrylic, and viscosity rapidly increases, so more danger of increased vertical dimension. *faster than hot cure due to the chemical reaction so the working

time is less, thus the viscosity increases rapidly. • Higher residual monomer → less biocompatibility,

inferior mechanical properties.

*What are the residual monomers? They are the monomers that

remain after the end of the reaction and stay entrapped between the

chains without getting polymerized (not 100% consumption of

monomer).

*What are the side effects of these residual monomers?

1) it will leach in the patient’s oral cavity, and some people are

allergic or sensitive to such these materials.

2) it may affect the mechanical properties of the material (resin).

*Residual monomers in cold cured resins are higher than in heat

cured resins, WHY?

Because in cold cured resins the reaction is not organized (the

polymerization reaction occurs quickly), thus not gradually

consumption of monomers.

Accordingly, mechanical properties are inferior (less) in cold cured

resins.

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Residual monomers are higher→because the reaction is faster →so the mechanical properties are inferior.

Inferior color stability →oxidation of amine accelerator. The only difference between heat and cold cured resins from

chemical composition side is the tertiary amine in cold cured resins,

and that’s what makes the color stability lesser than in heat cured

resins, due to the oxidation of amine accelerator, thus the denture

becomes more yellowish over time.

Cold VS heat cure: • Smaller polymer beads are used in cold cure resin to ease dissolution in monomer. (thus, accelerate the reaction).

• Cold curing method results in a polymer with a lower molecular weight leading to:

➢ Lower strength ➢ Residual uncured monomer, so, higher solubility ➢ Lower color stability, more prone to yellowing

• Allowed to cure at room temperature or in warm water.

• Start to polymerize soon after mixing (2.5 hours).

• Packing consistency is reached in 5 minutes

• Chemically activated resins display less shrinkage and more dimensional stability→ due to reduction in residual stresses during the processing cycle.

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3) Microwave cured resin:*not common*

Using glass fiber-reinforced plastic flask. we use flask (metal container) when we want to transform the wax

to acrylic in dentures making, then we expose the flask with wax to

*heat* in order to remove the wax and replace it with acrylic, this

process occurs in both heat and cold cure , but here in microwave

cure we use special flasks not metal (glass fiber- reinforced plastic

flasks).

Very short curing cycle (3 min). advantage

while in heat cured → overnight.

cold cured →2.5 hours.

Cost, high equipment expenses. Like the special flasks,

that’s why it is not a very common method.

Cleaner, more time efficient.

**In general, the properties of this type are good and better than the

cold cured, and nearly at the same level with heat cured or a little

bit less.

**before we start with the fourth type we have to know

something→if we know the properties of each type we also have to

know the appropriate uses for each one (base,repair,teeth,liner

dentures or customized trays).

for example:

1) Denture base material: in dentures that the patient uses for

years→ heat cure because it is the most superior in mechanical

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properties ,lower residual monomer ,healthy, biocompatible for

patients ,very strong ,durable and so on.

2) Special trays (customized)→cold cure or light cure.

4) Light cured resin:

used as denture hard relining material, repair

material, construction of trays (special or customized

trays),

**we can also use cold cured for these uses.

Composition: حفظ

Urethane dimethacrylate (UDMA)

Colloidal silica to control flow

Photoinitiator: camphorquinone

Activator: visible light

High molecular weight acrylic resin monomers

**the same composition as composite, the composite is a light

cured material that is exposed to a special light; which it is used

as activator.

-Inferior bond strength to resin denture teeth,

bonding agents.

-No methacrylate monomer & could be used in

monomer sensitive patients.

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-Lower elastic moduli & flexural strength.

**the best material for denture base material (pink) is heat cure as

we said before because it is strong and has less residual monomer,

another reason that the denture teeth (white) are made by heat cure,

so when the two materials (base and teeth) are the

same(matching)→the bonds between them are very strong.

while in case of light cure resin for the base, and the teeth are made

by heat cure resin → the bonds are inferior(weaker).

**The question that present itself here; why we didn’t use the heat

cure for the base from the beginning? Because there are some people

who are allergic or sensitive to the monomer (methyl methacrylate),

so the priority here is the patient’s health.

؟light cureليه ما نعملهم من ال heat cure من ال teethسأل الدكتورة مهي الا حد

-.- light cureال من teethانهم يعملوا ال بكلف ريحكت كتير صعب وكت

Other types of cured resins

Pour and cure resin (fluid resin): same

composition, smaller bead size. Mixed and

poured into a mould of hydrocolloid (low

viscosity resin). Excellent reproduction of details

but inferior in other aspects (strength, residual

MMA, distortion) → so beads are smaller, higher fluidity,

more details but less strength. -.-اله أي استخدام ألنه مش قوي ام

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High impact strength acrylics: reinforced with

butadiene-styrene rubber, bonds to resin matrix.

Powder and liquid form and processed as heat-

cured acrylic.

Heat cure + (butadiene- styrene rubber)→high impact strength acrylic

وطبعاً ،heat curedا عأسنانهم وما بزبط معهم ال بضغطوهاد للناس اللي كتير

هاد أغلى.

Rapid-heat polymerized acrylics: contain heat

activated and chemical activated initiators.

Processed in boiling water immediately. ساعتين مثالً ، بنستخدم هاد النوع.لو واحد بده يسافر بعد

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Mixing and curing of heat cured resin Powder liquid ratio affects the workability of the

mix and dimensional stability (3:1)

3 for the Powder and 1 for the liquid.

Liquid is poured into a container then powder is added gradually

Too little monomer (powder ↑( leads to incomplete wetting of powder and streaked (lines)or blanched appearance in denture (white in color).

Too much monomer (liquid↑) leads to more polymerization shrinkage and porosity.

Setting stages:

بنالحظ عليه تغيرات من حيث تدريجياً،بعد ما جبنا السائل وحطينا عليه الباودر

القوام .....

1. Sandy stage: immediately after mixing متل الرمل

2. Sticky or stringy stage

. stickyبكون زي اللي فيه خيوط و مثال لما نمسكه بمعلقة

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3. Dough stage: time to pack in the mould the best

stage of the material that we can use and put it in a flask.

(flasking stage or packing in the mould).

▪ Dough stage period is the working time.

▪ Time taken to reach dough stage is dough forming time

4. Rubbery stage.لما نشدها بترجع نفس شكلها عكس المرحلة اللي قبل

5. Stiff

Flasking and Heat curing

1. Flasking

2. Dewaxing

3. Application of separating medium (water soluble

alginate solution)

4. Placing acrylic dough

5. Packing and removing excess using cellophane paper

6. Final packing: metal to metal contact

7. Heat curing under pressure

** when the dough is ready, we apply it to the flask after

melting the wax and we close both parts of the flask together.

Then we put them under pressure so excess material will come

out through the sides of the flask. After we remove the excess

material we put the flask with pressure exerted on it in water

bath in case of heat cure.

**اذا ما فهمتوا كتيرأو ما تخيلتوا شو بصير بالزبط كملوا ما عليكم حتفهموا اكتر بالصور

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**Why pressure?

Ensure flow to all parts (to ensure that the dough spreads

everywhere in the mould).

Prevents raised bite (to prevent increasing the thickness of

the denture thus increasing the vertical dimension of the patient’s

face, so it must be thin).

Reduce porosity (there are no spaces between the particles).

**Curing cycle example: Heating process

Long cycle: 8 hours or longer in 74°C water. (Overnight).

Short cycle: heat at 74°C for 1.5 hours then boil (100

°C) for an additional hour

-SO the total is 2.5 hours.

**the long cycle is way better than the short cycle, but in

some cases (time limitation or your patient wants to travel

after 3 hours for example) we use short cycle.

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1) This is the flask with wax denture inside it (1 and 2), we put it

under temperature to remove the wax outside the flask ,the result

as shown in pic3.

2) After that we make the acrylic mixture and start packing it in

the mould (In dough stage), after we finish packing, we close the

flask and put it under pressure, remove the excess dough, finally

in water bath at 74°C (long cycle).

Suggested video: https://www.youtube.com/watch?v=vowbYucpJgQ&t=525s

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At 74°C most of the polymerization occurs.

Temperature then increases due to exothermic

reaction. Addition polymerization reaction is exothermic reaction, that’s why

we don’t put the flask in 100°C water bath, because there are two

sources of heat: from the water bath (74°C) and from the reaction

itself, the total heat can reach 100°C, but not 100.8°C (which is the

boiling temperature of the monomer).

we shouldn’t reach 100.8°C *boiling point* in order to prevent

evaporating of the monomer and making porosity, so the most

suitable temperature with the exothermic reaction is 74°C

(74+exothermic reaction<100.8°C).

In thick section, temperature is higher, curing is

better, but porosity is more likely to happen.

If pressure is lost during processing, porosity

results which is uniformly distributed.

It’s important to allow slow cooling to room

temperature to minimize internal stress formation.

(Gradual cooling).

Residual monomer: the use of 1-hour terminal boil

(raise the temp. higher than 74°C) help to minimize

residual monomer, but do not raise temperature

too high until most polymerization occurred.

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**Why there is no risk in raising the temperature at the end?

Because the reaction is nearly over.

Polymerization shrinkage:

Polymerization shrinkage: is reduced due to the

presence of pre-polymerized grains

or beads (7%) rather than the monomer itself

(21%). ممه **With the pre-polymerized beads (already undergone addition

polymerization reaction) the percentage of polymerization

shrinkage decreases from 21% to 7%.

Heat generation occurs due to the polymerization

reaction. This is reduced due to the presence of the

already polymerized resin.

MMA monomer:

▪ Volatile

▪ Flammable

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Precautions:

▪ Kept in a sealed container.

▪ Dark container away from light source or heat

source. Activation عشان ما يصيرلها

▪ No contact should be allowed with the polymer

beads (carry benzoyl peroxide initiator).