Chemical Stages of Polymerization of Acrylic Resin

DENTURE BASE MATERIALS

DENTURE BASE MATERIALS

VULCANITE (1855)– Rubber with 32 % Sulphur & Metallic oxides.

 Advantages – It is nono-toxic, non-irritant,

has excellent Mechanical properties 

material is sufficient hard to polish.

Limitation – Absorbs saliva – becomes unhygiene, leads to bacterial growth & unpleasant odour

Poor esthetics

Dimensional changes

Thermal expansion

Contraction of 2-4% by volume during addition of sulphur  to the rubber

DENTURE BASE MATERIALS

NITROCELLULOSE– Dimensionally unstable

Contains unpleasant tasting plasticizers

Excessive warpage

High water absorption

Poor colour stability

DENTURE BASE MATERIALS

PHENOL FORMALDEHYDE

Becomes discoloured & unesthetic

It is thermoset type – so it is difficult to repair

PORCELAIN – This material is tolerated well,

But it is difficult to fabricate

Can be easily broken

CLASSIFICATION OF DENTURE BASE MATERIALS

METALLIC

Cobalt – Chromium

Gold Alloys

Aluminium

Stainless Steel

Titanium

TEMPORARY

Self-cure Acrylic resin

Shellac Base Plate

Hard Base Plate Wax

NON-METALLIC

Acrylic Resin

Vinyl Resin

PERMANENT

Heat-cure Acrylic resin (1937)

METALLIC

CHEMISTRY OF
SYNTHETIC RESINS

CLASSIFICATION

Synthetic Resins are often called as PLASTICS

A substance that although dimensionally stable in normal use was plastic at some stage of manufacture

Thermoplastic – they soften again when reheated (above GTT)

Thermosetting – they are resistant to change after further application of heat

Third Group – ELASTOMERS Founded on Naturally occurring LATEX isolated from Hevea brasiliensis tree

Thermoplastic Resins

Are fusible, soluble in organic solvents

Better  flexural & impact properties

Most plastics in Dentistry belong to this group

PMMA, Polyvinyl, Polystyrene

Thermosetting Resins

These become permanently hard when heated above critical temp. & they do not soften again on heating

Usually cross-linked in state

These are insoluble, infusible

Crosslinked PMMA, Silicones.

Superior  abrasion resistance

Superior Dimensional Stability

IDEAL REQUIREMENTS

Tasteless, odourless, non-toxic, non-irrtant

Esthetically satisfactory – transparent, translucent, easily  pigmented, colour should be permanent

Dimensionally stable, should not expand during Processing & subsequent use by the patient

Should have adequate  Strength, Resilience,    Abrasion Resistance

IDEAL REQUIREMENTS

Insoluble & Impermeable to oral fluids

Low specific gravity

Softening temp. should be well above temp. of any hot food

Easy to fabricate

Good thermal conductivity

Radioopaque

USES

Preparation of Dentures

Artificial teeth

Tooth restorations

Orthodontic space maintainance

Crown & Bridge facings, Temporary Crowns

Maxillofacial prosthesis, Athletic Mouth Protector

Inlay patterns

Implants

Dies, Impression trays

Endodontic filling materials

Basic Nature of Polymers

1. Polymer – Molecule that is made up of may parts

Chemical possessing a molecular weight of more than 5000

Monomer – Molecule from which polymer is constructed

Molecular Wt. of various  mers  X  no. of mers

(determines its physical properties)

Degree of Polymerization --- total no. of mers in polymers .

Strength increases with increase in Deg. Of Poly.

STUCTURE OF POLYMERS (SPATIAL STRUCTURE)

1 LINEAR – linear homopolymer, random copolymer, block

2 BRANCHED – branched homopolymer, random copolymer, graft copolymer

3 CROSS-LINKED- homopolymer crosslinked with single crosslinking agent

Polymerization – series of chain reaction by which a macromolecule or polymer is formed from a single molecule

Condensation

Slow method

Repeated Elimination of small molecules

By- products – NH3, H2O, halogen acids

Functional groups are repeated (Amide, Urethane, Ester or Sulfide)

Here by-product formation is not necessary

Addition

In Dental procedures

No change in chemical composition & no by-products

Giant molecules (unlimited size)

Simple, but not easy to control

CHEMICAL STAGES OF POLYMERIZATION

INDUCTION (INITIATION)

Is the time during which the molecules of the initiator becomes energized or activated & start to transfer the energy to the monomer.

Impurity --- increases length of this period

Increase temp. --- shorter is length of Induction period

Initiation energy is 16000 to 29000 cal/mol.

CHEMICAL STAGES OF POLYMERIZATION

3 INDUCTION SYSTEMS

HEAT ACTIVATION –free radicals are liberated by heating Benzoyl peroxide

CHEMICAL ACTIVATION – atleast 2 reactants --- chem. Reaction--- liberate free radicals              Benzoyl peroxide + Aromatic Amine(dimetyl-p-toluidine)

LIGHT ACTIVATION –  photons of light energy activate the initiator – free radicals . Under visible light Camphoroquinone & an amine --- free radical

INHIBITION OF POLYMERIZATION

Occurs when there is

Complete exhaustion of monomer  Or

Formation of High Molecular Weight  polymer

Inhibited by :

IMPURITIES (react with Activated Initiator / Nucleus)

Hydroquinone (0.006%) is in Monomer for storage

OXYGEN retards polymerization

Influence the length of Initiation period & degree of polymerization

COPOLYMERIZATION

Is required to improve physical properties

Two or more chemically different monomers polymerize to form COPOLYMER

TYPES

Random

Graft

Block

Applications of copolymerization

ETHYL ACRYLATE+ PMMA = FLEXIBILITY

BLOCK & GRAFT Polymers = Improves IMPACT STRENGTH (good adhesive properties + surface characteristics)

CROSS-LINKING (chemical bond between linear polymers)

Applications – Improves strength,                         reduces solubility & water sorption

Highly Cross-linked Material provides - increased resistances ------ to solvents, crazing & surface stresses

Plasticizers

Increases solubility of polymers in monomer

Reduces brittleness

But it also reduces Strength & Hardness & Softening point

EXTERNAL – penetrates macromolecules & neutralizes secondary bond. It Evaporates / Leaches out

INTERNAL - Copolymer

Types of resins

Acrylic resin

Vinyl resin

Polystyrene

Epoxy resins

OTHER RESIN SYSTEMS

Polycarbonates

Polyurethanes

Cyanoacrylates

ACRYLIC RESINS

Are Derivatives of Ethylene & contain a vinyl group in their structural formula

Acrylic resins used in dentistry are esters of

1 Acrylic acid

2 Methacrylic acid

Available as Methyl methacrylate [liquid] &             Poly (Methyl methacrylate) [powder]

Poly (Methyl Methacrylate) Resins

Widely used --- easy to process

It is Thermoplastic resin

Liquid [monomer] Methyl Methacrylate is mixed with Powder [polymer ]

Monomer plasticizes the polymer to dough-like consistency which can be easily moulded

Types ---- based on method used for its activation

Heat activated resins

Chemically activated resins

Light activated resins

HEAT – ACTIVATED DENTURE BASE RESINS

AVAILABLE AS Powder+ Liquid & Gels – Sheets &cakes

COMPOSITION

Liquid

Methyl Methacrylate

Dibutyl pthalate ---  plasticizer

Gylcol dimethylacrylate [1-2%] ---- cross-linking agent

Hydroquinone ---- inhibitor

Stored in tightly sealed Amber coloured bottle – to prevent evaporation , premature poymerization [by light or U.V radiation]

COMPOSITION

Powder

Poly (Methyl Methacrylate)

Other copolymers (5%)

Benzoyl Peroxide ---- Initiator

Compounds of Mercuric sulphide,

               Cadmium sulphide ---- Dyes

Zinc / Titanium oxides --- Opacifiers

Dibutyl pthalate --- plasticizer

Dyed organic filler

Inorganic particles like glass fibers / beads

High mol. Wt. polymers dissolves slowly in monomer

So,  to increase in solubility

Additive – (Ethyl acrylate copolymer)

Plasticizer – Dibutyl phthalate

Adding low mol. Wt. PMMA

POLYMERIZATION REACTION

 Powder (Poly)+ Liquid (mono) +heat = polymer + heat

Technical consideration

COMPRESSION MOULDING TECHNIQUE

Prep of wax pattern [waxed dentures]

Prep of Split mould [Investing & Dewaxing]

Applictn of Separating Media

Mixing of powder & liquid

Packing

Curing

Cooling

Deflasking

Finishing & polishing

COMPRESSION MOULDING TECHNIQUE
Prep of wax pattern [waxed dentures]

Prosthetic teeth are selected & arranged – esthetic & functional requirements

Impression making, cast generation, record bases

Articulator mounting, teeth arrangement, wax contouring

Waxed dentures are sealed to master casts – removed from articulator

Prep of Split mould [Investing ]

Master cast is coated with thin layer of separator

Base flasking

Counter flasking – dental stone in intimate contact with all external surfaces,

Incisal & Occlusal surfaces are slightly exposed – to facilitate deflasking

Third Pour – to fill remaining flask

Lid is gently placed & stone is allowed to set

Prep of Split mould [Dewaxing]

On complete setting – record base & wax has to be removed

Flask is immersed in boiling water for 4 mins

Base flask & counter flask segments are separated

Residual wax is removed by wax solvents

Mold cavity is cleaned with mild detergent solution & rinsed with boiling water

Application of Separating Media

To prevent water from the mould to enter into Acrylic resin [affects rate of polymerization & colour of resin]

To prevent Monomer penetrating into the mould [plaster to adhere to the acrylic resin & produce rough surface]

Can lead to compromises in Physical & Esthetic properties

TYPES

Tinfoil

Tinfoil substitutes - Cellulose lacquers, Solution of Alginate compounds, Evaporated milk, Soap, Sodium silicate, Starches

Mixing of powder & liquid
Polymer:Monomer ratio

Accepted ratio – 3:1 by volume   or    2:1 by weight

If more Monomer [lower polymer/monomer ratio]

Greater poly. Shrinkage

Additional time is reqd. to reach the packing consistency

Tendency for porosity

If less Monomer [lower polymer/monomer ratio]

Less wetting – Granular acrylic

Dough will be difficult to manage – not fuse into continous unit of plastic

Physical stages of Polymerization

Stage 1 – Sandy / Wet sand stage – polymer gradually settles in monomer, forms a fluid, incoherent mass. Also described as ‘coarse or grainy’

Stage 2 – Stringy / Sticky stage – monomer enters into polymer, if the mixture is touched --- it forms cobweb like structure

Stage 3 – Dough-like / Gel stage : mass becomes more saturated, smooth & dough like. It does not adhere to container or spatula. Mass is plastic & homogenous at this stage. Time reqd – 10 mins

Physical stages of Polymerization

Stage 3 – Rubbery / Elastic Stage: Monomer disappears by penetration into the polymer & evaporation. Mass is cohesive, rubber-like, non-plastic & cannot be moulded as it rebounds when compressed or stretched, it does not flow freely

Stage 5 – Stiff – due to evaporation of free monomer. Mix appears very dry & is resistant to mechanical deformation

Dough-forming Time

Time reqd for the resin mixture to reach a dough-like stage

ADA specification no. 12 – in less than 40 mins

Clinically – most resins reach a doughlike consistency in less than 10 mins

Depends on

Controlled by manufacturer

1 Deg. Of polyn. – higher the polyn, lower the Dough-forming Time

2 Particle size – Smaller the particle size, shorter the Dough-forming Time

Controlled by operator

3 Polymer:Monomer ratio : If this is high (less monomer), there is shorter dough forming time

4 Temperature – Higher the temp., shorter dough forming time

5 Plasticizer – reduces the dough forming time

Working time

May be defined as the time that a denture base material remains in dough like stage

At least 5 mins

Affected by temp., extended via refigeration (moisture can degrade properties)

Can be avoided by storing in air tight container

PACKING

Introduction of denture base resin into mould cavity

Overpacking: excessive thickness, malpositioning of teeth

Underpacking : noticeable denture base porosity

Rope-like form ----- packed into flask

Polyethylene sheet is placed ---- flask is assembled

Application of pressure --- resin dough flows evenly into mold space

Flask portions are separated – sheet is removed with a rapid, continuous tug

Excess resin – flash

Second trial closure

Final closure – no polyethylene sheet

Polymerization Procedure / Curing

DB Resins – Benzoyl Peroxide{Initiator} – when heated above  60*C – decompose to form Free Radicals  - reacts with Monomer to initiate chain-growth polymerization

Heat is termed as Activator

After Final closure – flasks are kept at Room temp. for 30 to 60 mins – Bench Curing

Longer flow period – equalization of pressure in Mold

Allows time for more uniform dispersion of monomer

Longer exposer of resin teeth to monomer- better bond

Curing cycle / Heating process

1. Processing denture base resin in Constant temp water bath at 74*C (165*F) for 8 hrs or longer, with no terminal boiling

2. Processing the resin at 74*C for approx. 2 hrs & then increasing the temp. of water bath to 100*C & processing for 1 hour more

Other Methods of supplying heat for activation

Steam, Dry air Oven, Dry heat (electrical), Infrared heating, Induction/Dielectric heating, Microwave radiation [Specially formulated resin & Non-metallic Flask: Speedy process]

Internal Porosity

Resin & Dental stone – Poor thermal conductors, heat of reaction cannot be dissipated, so temp. of resin rises above 

that of stone & surrounding water

Temp. exceeds the boiling pt. of Monomer (100.8*C)

Porosity – not seen on surface, as heat is dissipated

Centrally, heat generated in thick portions cannot be dissipated --- boiling of unreacted monomer ---- porosity

External Porosity

1. Lack of Homogenity – Portions containing more monomer will shrink more than the adjacent areas, results in voids & resin appears white.(proper powder:liquid, homogenous mix – pack in dough stage)

2. Lack of adequate pressure – Lack of dough during final closure (Flash indicates adequate material)

OTHER PROBLEMS : Crazing[Cracks] & production of Internal Stresses

Cooling

After Curing – Denture flasks should be cooled slowly to room temp.

Rapid Cooling – warpage of denture base because of differences in thermal contraction of resin & stone

Slow Cooling – Minimizes potential difficulties

So, Bench-Cooling for 30 mins, then flask should be immersed in cool tap water for 15 mins

Cooling overnight is ideal

Deflasking, Finishing & Polishing

Deflasking – has to be done with care to avoid flexing & breaking of Acrylic denture

Finishing – Metal Trimmer, Acrylic/Alpine Stone, Dry & Wet Sand paper

Polishing – suspension of finely ground pumice in water

Injection molding Technique

Mold space can be filled by injecting resin under pressure in specially designed flasks

Sprue hole / Vent hole are formed in stone mold

Soft resin (dough stage) is contained in injector & is forced into mold

Resin under pressure until it has hardened

Polystyrene resin – polymer is first softened under heat & injected while hot, then it solidifies in mold upon cooling

No trial closures are required

Injection molding Technique

Advantages

Dimensional accuracy

Low free monomer content

Good impact strength

Disadvantages

High capital costs

Difficult mold design problems

Less craze resistance

Less creep resistance

Special flask is required

Polymerization by Microwave energy

It is cleaner & faster than polymerization with  conventional technique

Fit of denture is comparable or superior

Acrylic resins are less prone to porosity

Advantages : good appearance, high glass transition temp, ease of fabrication, low capital cost & good surface finish

Disadvantages : Radiolucency, Free monomer content/formaldehyde may cause sensitization, fatigue life too short & low impact strength

Chemically Activated Denture Base Resin (Autopolymer, Self-cure, Cold-cure)

Composition

Liquid

Methyl Methacrylate

Dimethyl-p-toluidine ---- Activator

Dibutyl phthalate----------- Plasticizer

Glycol dimethacrylate ----- Cross linking agent

Hydroquinone ------------- Inhibitor

Chemically Activated Denture Base Resin (Autopolymer, Self-cure, Cold-cure)

Composition

Powder

Poly (Methyl Methacrylate)

Other copolymers  -  5%

Benzoyl Peroxide         ---------------- Initiator

Compounds of mercuric sulphide,                           Cadmium sulfide   -------------- Dyes

Zinc / Titanium Oxide   ------------- Opacifiers

Dibutyl phthalate                ---------- Plasticizer

Dyed organic fillers

Inorganic particles like glass fibers / beads

Uses of Autopolymerizing Resin

With fillers (pumice), for construction of custom trays

For denture repair, relining & rebasing

For making removable orthodontic appliances

For adding a post-dam to adjust upper denture

Advantages

Better initial fit

Less thermal contraction

For repairing dentures, as it avoids warpage due to    re-curing

Disadvantages

Colour stability is inferior, due to subsequent oxidation of the tertiary amine

Lesser degree of polymerization, so these have slightly inferior physical properties

Manipulation

1. Sprinkle – On technique

2. Adapting technique

3. Fluid resin technique

4. Compression moulding technique

5. Injection moulding technique

Fluid resin technique (pour-type acrylic resin)

These have high molecular wt powder that are smaller in size & when they are mixed with monomer, the mix is very fluid

They are used with lower powder-liquid ratio – 2 : 1 -2.5 : 1

This aids to prevent undue increase in viscosity during mixing & pouring stages

This technique commonly involves use of Agar Hydrocolloid for the mould preparation

Fluid mix is poured in the mould quickly & allowed to polymerize under pressure at 0.14 Mpa (20 psi).               

Advantages

Better  tissue fit

Fewer open bites.

Less fracture of porcelain teeth during deflasking procedure

Reduced material cost

Simplification of lab procedure for flasking (no trial closure),deflasking & finishing of denture.

Disadvantages

Air occlusion(bubbles)

Shifting of teeth during processing

Infraocclusion (closed bites)

Occlusal imbalance due to shifting of teeth

Incomplete flow of denture base material over neck of anterior teeth

Formation of films of  denture base material over cervical portions of plastic teeth that had not been previously covered with wax

Poor bonding to plastic teeth..

Technique sensitivity.

Autopolymerizing

Heat is not necessary for polymerisation

Porosity is greater.

Have lower average molecular weight.

Higher residual monomer content.

Material is not strong.(coz of their lower molecular weight mols.)

Poor  color stability.

Easy to deflask.

Rheological properties:

A) Show greater distortion.

B) More initial deformation.

C) Increased creep & slow recovery.

Heat cured

Heat is necessary for polymerisation

Porosity is less

Higher average molecular weight (5 lakhs-10 lakhs)

Lower residual monomer content

Material is strong

Good color stability

Difficult to deflask

A) Show lesser distortion

B) Less initial deformation

C) Less creep & quicker recovery

Light activated denture base resin

Composition

Urethane dimethacrylate matrix

Acrylic copolymer

Microfine silica fillers

Photoinitiator system – Camphoroquinone amine

Supplied in pre-mixed sheets having clay-like consistency

Provided in opaque light packages – to avoid premature polymerization

Adapted to cast when in plastic form

Polymerized in light chamber with light of 400-500 nm from high intensity quartz halogen bulbs

Properties of Denture Base Resin

Methyl Methacrylate Monomer: Clear, transparent, volatile, has sweetish odour.

Melting pt:  -48*C

Boiling pt:  100.8*C

Heat of polymerization:  12.9Kcal/mol

Volume shrinkage during polymerization:  21%

Poly (Methyl Methacrylate )

Tasteless, odourless, clear transparent, has adequate compressive & tensile strength, has low hardness-can be easily scratched & abraded

Shrinkage ---- thermal shrinkage on cooling & polymerization                                                  shrinkage

Volume shrinkage is 8% & Linear Shrinkage is 0.53%

Resin Teeth – PMMA copolymerized with a cross linking agent

Resin teeth

High fracture toughness

Crazing, if not crosslinked

Clinically significant wear

Easily ground & polished

Silent on contact

Dimensional change (water sorpn)

Cold flow under stress

Loss of Vertical dimension

Self adjusting

Chemical bond to denture

Minimal abrasion of opposing

Porcelain teeth

Brittle

Crazing by thermal shock

Insignificant wear

Grinding is difficult (glaze)

Sharp impact sound

Dimensionally stable

No permanent deformation

Stable VDO

Difficult to fit in diminished interarch space

Mechanical retention necessary

Abrades opposing teeth

Recent Advances

High Impact strength materials: butadenestyrene rubber-reinforced PMMA

Rapid heat polymerized resins: hybrid acrylics that are polymerized in boiling water immediately after packing (place in boiling water & then full boil for 20 mins)

Denture Reliners

Heat cure --- compression molding & low curing temp.

There is a tendency for previously cured material to warp

Self cure --- directly in mouth, but fades, smells

SOFT/RESILIENT LINERS

Purpose is to Absorb some of the energy produced by masticatory impact

Used – irritation in mucosa, area of severe undercut, congenital/acquired defects of palate

Plasticized acrylic resin- PEMA/PMMA/Copolymer + aromatic ester-ethanol liquid containing 60-80% plasticizer (dibutyl phthalate)

Vinyl resin, Silicone rubbers – RTV, heat cured silicones, Polyurethanes

Problems: inadequate bonding, loose stiffness as plasticizer is leached out, loss of denture base strength, trimming, polishing is difficult, disagreeable taste & odour, cannot be cleaned easily (fungal growth)

Materials in Maxillofacial prosthesis

To correct facial defects resulting from cancer surgery, accidents / congenital deformities

Ideally – easy, inexpensive to fabricate, biocompatible, strong, stable, skinlike in appearance, soft & must be colour stable, easy to clean

PMMA

Latexes – but they are weak, degenerates rapidly with age

Plasticized polyvinylchloride – has got plasticizers, crosslinking agent & UV stabilizers

Silicone rubber --- RTV & heat-vulcanized

Polyurethane polymers