Acrylic Resins
On this page 11 sections
One Liners:
- For impression of obturator = Black GP
- Desired polymer structure = Non cyrstilliane
- Polystyrene = used in injection mold technique
- Hydroquinone is = 0.006%
Notes
Volumetric polymerization shrinkage = 21%
6–7% volumetric shrinkage after using the 3:1 mixture, and ~0.5% linear shrinkage
| Material | Residual monomer | Linear shrinkage |
|---|---|---|
| Chemically activated | 3–5% | ~0.3% |
| Heat activated | 0.2–0.5% | ~0.5% |
Curing cycles
74°C for 8 hours
74°C for 7 hours → 100°C for 3 hours
Important temperatures
Benzoyl peroxide
- Decomposes above 65°C
- Minimum porosity occurs around 71–74°C
MMA boiling point
- Approximately 100°C
- One slide: 100.33°C
- Earlier slide: 100.8°C
Composition
| Component | Phase | Function / significance |
|---|---|---|
| Poly(methyl methacrylate) | Powder | Main prepolymer |
| Benzoyl peroxide | Powder | Initiator |
| Pigments / organic dyes | Powder | Tissue-like colour |
| Nylon/acrylic fibres | Powder | Simulate blood vessels/capillaries |
| Methyl methacrylate | Liquid | Monomer |
| Tertiary amine — N,N-dimethyl-p-toluidine | Liquid of self-cure resin | Activates benzoyl peroxide |
| Glycol dimethacrylate | Liquid | Cross-linking agent |
| Plasticizer | Liquid | Produces softer/more resilient resin |
| Hydroquinone | Liquid | Inhibitor; prevents premature polymerization |
Heat-activated liquid composition
Hydroquinone
- Inhibitor
- Prevents undesirable polymerization during storage
- Liquid is kept in dark bottles to limit polymerization from UV light.
Cross-linking agent = Glycol dimethacrylate
Effects stated:
- ↓ solubility
- ↓ water sorption
- ↑ resistance to crazing
Plasticizers
External plasticizer
- Dibutyl phthalate
- Can leach into oral fluids
Internal plasticizer
- Butyl or octyl methacrylate
- Does not leach out
Used to make resin softer/more resilient.
POLYMER–MONOMER RATIO
Standard heat-cure ratio:
3 : 1 polymer : monomer
Why?
- Adequate wetting of powder
- Affects workability
- Affects dimensional changes during setting.
Very high P/L ratio
Causes:
- Dry, unmanageable mix
- Poor flow under pressure
- Insufficient monomer to bind all polymer beads
- Granular porosity
TYPES OF POROSITY
| Type of porosity | Main cause / mechanism | Important points | Prevention |
|---|---|---|---|
| Gaseous porosity | Monomer evaporates/boils when temperature becomes too high during polymerization | Polymerization is exothermic; resin + stone are poor thermal conductors. More common in thick central areas; peripheral thin areas have less porosity. MMA boiling point ≈ 100.33–100.8°C | Properly controlled curing cycle; avoid excessive temperature rise |
| Contraction porosity | Contraction of monomer during polymerization | Poor/inadequate mixing → some regions contain excess monomer → these regions shrink more and form voids | Correct P/L ratio, proper mixing, maintain pressure, bench curing, injection molding |
| Granular porosity | Monomer evaporation + failure of polymer particles to unite | Can occur during mixing; after mixing if container is uncovered; monomer may also be absorbed into plaster during packing | Cover mixing container, use proper P/L ratio, apply separating medium to plaster surfaces |