IB: Denture Acrylics

PMMA (polymethyl metacrylate)

• material of choice for full denture bases and “gumwork” for removable devices

• available commercially as heat-cure, cold-cure, or light-cure

heat-cure and cold-cure are most widely used in

denture fabrication

steps of addition polymerization

1) activation

2) initiation

3) propagation

4) termination

activation

• benzoyl peroxide and other compounds break down into free radicals when heated

• initiator starts the setting reaction

• inhibitors scavenge free radicals until inhibitor is depleted

initiation

• free radicals attack double bonds of monomers

• activated monomers initiate chain-growing reaction

propagation

• monomers continue to add to growing chain until most of monomer is consumed

• heat is given off at this stage

termination

• monomer is depleted

• two free radicals react and form stable molecule

• reaction stops

compared to heat-cure resins, self-cure resins have

• lower Mw

• higher porosity

• higher retained monomer

• lower strength

• require accelerator to cure at room temperature

accelerators/activators

• make a resin self-curing

• usually aromatic amines that make initiator unstable and form a free radical at ambient temps

• DMPT is most common

• must be mixed with initiator before polymerization will begin

• cause poorer color stability of self-cure resins

most widely used light-cure initiator system

combination of camphorquinone (photosensitizer) and amine (initiator)

commercial heat- and cold-cured denture base resins are usually supplied as

• two components

• POWDER and LIQUID mixed together in 3:1 ratio

• separates monomer from initiator

• for cold-cure, separates initiation from activator

components of powder

• acrylic polymer beads

• initiator

• pigments

• plasticizer

components of liquid

• monomer

• inhibitor

• accelerator/activator

• cross-linking agent

• plasticizer

if polymer/monomer ratio is too high

• insufficient monomer to wet the powder and fill voids → porous resin

• incomplete polymerization → granular resin

• insufficient plasticity to fill denture flask to form shape

if polymer/monomer ratio is too low

• excess monomer → excess polymerization shrinkage

• poor dimensional accuracy

stages in physical reaction during self-cure acrylic processing

sandy → stringy → doughy → rubbery → solid

doughy stage

• most of monomer has been absorbed into polymer beads

• mix loses its adhesiveness and separates cleanly

• material has sufficient body for adequate pressure in packing; still plastic and malleable enough to completely fill and take on shape of mold cavity

• “just right”

rubbery stage

• all the monomer is absorbed into polymer particles or evaporated

• swollen, plasticized polymer beads are completely coalesced → form rubbery mass

• “gone too far”

if monomer evaporates too quickly

• material reaches rubbery stage too soon

factors affecting dough-forming times

• particle size

• powder MW

• plasticizer

• temperature

• polymer/monomer ratio

• self-curing

particle size

• smaller polymer powder particles have greater SA/V → take up monomer rapidly and decrease dough-forming time

powder molecular weight

• absorption of monomer decreases as polymer MW in powder increases → increases dough-forming and working times

plasticizer

• decreases dough-forming time

• increases working time

temperature

• lower temperatures increase working times because polymerization is exothermic

polymer/monomer ratio

• the less monomer absorbed into polymer, the less dough-forming time

self-cure resins have shorter working times than

heat-cure resins

manufacturer controls

• particle size

• powder MW

• plasticizer

mold liners minimize

• monomer penetration into gypsum, which causes roughness and adhesion to mold

• diffusion of water from gypsum into dough, which causes blanching and crazing

packing is done during the

doughy stage → material has enough body to allow overfilling of mold

compression molding technique

• pressure squeezes dough into mold

• helps maintain denture’s proper dimensions

injection molding technique

• resin injected into mold under pressure

• improved clinical accuracy in fine detail

• much lower resin viscosity

fluid resin technique

• specially designed flask filled with reversible hydrocolloid investment medium

• uses low-viscosity, pourable chemical-cure resin

• pour resin into mold instead of packing dough

advantages of fluid resin technique

• improved adaptation to underlying soft tissues

• less damage to prosthetic teeth and denture bases during deflasking

• reduced material costs

• simplification of flasking, deflasking, and finishing procedures

disadvantages of fluid resin technique

• shifting of prosthetic teeth during processing

• air entrapment w/in denture base

• poor bonding between denture base and acrylic teeth

• technique sensitivity

if temperature is too low during heat-curing cycle

• incomplete polymerization

• large amount of residual monomer

• weak denture

if temperature is too high during heat-curing cycle

• monomer may boil → gas bubbles → porosity

with immersion of flasked denture case into a water bath, use

alternative heat-curing cycles

properties of cured resin

• molecular weight as high as 1,200,000

• 0.2-0.5% residual monomer

• 2% water at equilibrium

crazing

• tensile stresses in resin that may cause network of surface microcracks

• can eventually lead to cracks and fractures

craze resistance is markedly improved by

crosslinking

dimensional inaccuracy can arise from

• polymerization shrinkage

• mold cavity expansion

• thermal contraction