Dental Materials - Polymers

What are polymers used for in dentistry?

dentures, composites, acrylic, partial dentures, nightguards, temporary crowns, bridges, post and core, incisal guide table

What are requirements for an acceptable polymer material?

- adequate strength + durability

- satisfactory thermal properties

- processing accuracy and dimensional stability

- good chemical stability

- moderate cost

- absence of taste and color

- biocompatibility

- natural appearance

- color stability

- adhesion to plastics, metals, and porcelain

- easy to fabricate and repair

- insolubility and low sorption of oral fluids

What were historical materials used in dentistry?

wood, ivory/bone, ceramics, metals and metal alloys, polymers

What were pros and cons of using wood in dentistry?

Pros - readily available, inexpensive, carvable

Cons - degraded over time

What were pros and cons of using ivory/bone in dentistry?

Pros - carvable, stable in oral environment, esthetic/hygienic

Cons - expensive

What were pros and cons of ceramics in dentistry?

Pros - additive + subtractive, intimate tissue contact, stable in oral cavity, characterized

Cons - brittle, heavy

What are examples of natural polymers?

celluloid, vulcanite

What are examples of synthetic polymers?

PMMA, urethane dimethacrylate

What are characteristics of vulcanite?

- introduced in 1885

- produced by heating natural rubber in presence of sulfur

- hard, reddish-brown

- questionable esthetics

- demanding fabrication process

What are characteristics of celluloid?

- composed of 65% nitrocellulose and 35% camphor

- late 1860s (sub for ivory)

- first commercial use --> dentures

- cons: camphor taste, thermoplastic, highly flammable

What are synthetic resins used in dentistry today?

- poly methyl methacrylate (PMMA)

- urethane methacrylate

- nylons

- polyolefins

- aryl ketone polymers

What are polymers?

many units of mers bonded together

What are type I denture base polymers?

heat-processed polymers

What are type II denture base polymers?

auto-polymerized polymers (no external heat)

What are the two types of polymerization mechanisms?

condensation and addition polymerization

How is condensation polymerization characterized?

produces H2O, acids, ammonia as byproducts

How is addition polymerization characterized?

- no by products

- need an unsaturated group (double bond) and free radical (activator/initiator)

What are the steps of addition polymerization?

1) induction (initiation and activation)

2) propagation (chain growth)

3) termination

4) chain transfer (may or may not happen)

What are reasons why polymerization inhibition may occur other than complete exhaustion of the monomer?

impurities, presence of O2

How does crosslinking control mechanical properties of materials?

- provides sufficient number of bridges btw. linear molecules

- pros: increases strength, decreases solubility and water sorption

What is a crosslinking agent?

A) benzoyl peroxide

B) dibutyl phthalate

C) glycol dimethacrylate

D) tertiary amine

C) glycol dimethacrylate

How do co-polymers control mechanical properties of materials?

- increase impact resistance

- allows for elasticity/flexibilit

What is an example of a co-polymer?

A) benzoyl peroxide

B) glycol dimethacrylate

C) butadiene-styrene rubber

D) hydroquinone

C) butadiene-styrene rubber

What is an example of a plasticizer?

A) benzoyl peroxide

B) glycol dimethacrylate

C) dibutyl phthalate

D) hydroquinone

C) dibutyl phthalate

In general, what are physical properties of denture base resins?

polymerization shrinkage, porosity, water absorption, processing stresses, strength, creep

Describe characteristics of polymerization shrinkage

- density change

- volumetric shrinkage (6-7%)

- linear shrinkage = 0.7%

- type II resins have less shrinkage than type I

Describe characteristics of porosity

- results from vaporization of unreacted monomers + low molecular weight polymers

- can be caused by inadequate mixing, inadequate pressure, insufficient material, air inclusion

Describe characteristics of water absorption and how it affects physical properties of denture base resins

- H2O molecules penetrate PMMA mass + occupy positions btw. polymer chains

- affected polymer chains forced apart and cause slight expansion

- interferes w/ crosslinking

Describe processing stresses and how they affect physical properties of denture base resins

- friction btw. mold walls + soft resin may inhibit normal shrinkage of polymer chains

- result: polymer chains stretched + cause tensile stresses

- release of stress = cumulative dimension changes

Describe characteristics of strength

- compressive strength = 11,000 PSI

- depends on: composition of resin, processing technique (heat > chemical), conditions of oral environment, degree of polymerization (cold cured > heat cured)

Describe characteristics of creep

- deformation over time

- polymers have measurable creep due to viscoelastic behavior

What are physical properties of denture bases?

- low in strength

- fairly flexible

- low thermal conductivity

- high volumetric shrinkage

- high water sorption

- good color stability

Describe characteristics of heat activated (type I) PMMA

- free radicals obtained by heating benzoyl peroxide (initiator)

- free radicals from benzoyl peroxide initiate polymerization of PMMA

What are the components of the powder for heat-activated PMMA?

- prepolymerized PMMA spheres

- benzoyl peroxide (initiator)

- dibutyl phthalate (plasticizer)

- pigments, opacifiers (CdSu, TiO, C)

- synthetic fibers (acrylic or nylon)

What are components of the liquid of heat-activated PMMA?

- unpolymerized methyl methacrylate

- hydroquinone (inhibitor)

- dibutyl phthalate (plasticizer)

- glycol dimethacrylate (crosslinker)

What are characteristics of chemical activated PMMA?

- consists of 2 separate reactants that undergo chemical reactions to generate free radicals

- activator = tertiary amine

- initiator - benzoyl peroxide

- tertiary amine catalyzes split of benzoyl peroxide into free radicals which initiates PMMA polymerization

What is the composition of the powder of chemical activated PMMA

- pre-polymerized PMMA spheres

- benzoyl peroxide (initiator)

What is the composition of the liquid of chemical activated PMMA?

- unpolymerized methyl methacrylate

- glycol dimethacrylate (crosslinker)

- tertiary amine activator (N, N-dimethyl-p-toluidiene)

- hydroquinone (inhibitor)

What is the ideal polymer:monomer ratio?

A) 1:2

B) 2:1

C) 3:1

D) 1:3

C) 3:1

What happens if the P:M ratio is too high?

cured resin becomes porous, granular, and weaker (insufficient monomer to wet all powder)

What happens if the P:M ratio is too low?

excess monomer results in excess polymerization shrinkage (poor dimensional stability)

What are disadvantages of PMMA?

hydrophobic, porous, brittle

Describe characteristics of urethane methacrylate

- light activates initiator to generate free radicals

- photoinitiator = camphoroquinone (CQ)

- activator - tertiary amine

- light exposure produces interaction of tertiary amine w/ photoinitiator to form free radicals that initiate polymerization

What is the activator in a light activated polymer?

A) heat

B) tertiary amine

C) glycol dimethacrylate

D) benzoyl peroxide

B) tertiary amine