Polymers

Question 1

Fabrication

Answer 1

combination of spherical POLYMER BEADS
•liquid MONOMER.
•the plasticized polymer can be shaped, as required, before the monomer present is polymerised to yield a relatively rigid and permanent result.

Question 2

POLYMERISATION two types of

Answer 2

1. By heating to 70ºC (HEAT-CURED PMMA)
   •Injection Moulding
2. By chemical activation at room temperature (COLD-
   CURING, or AUTO-POLYMERISING PMMA)
3. Light Activation

Question 3

PLASTICISATION OF PMMA BY METHYLMETHACRYLATE.

Answer 3

PMMA is non-crystalline,
•Appropriate solvents will be absorbed by, and swell, the polymer.
•The effect is purely physical as a chemical reaction is not involved.

Question 4

Polymer with liquid results in

Answer 4

1. WET SAND STAGE
   Very little attack on the polymer by the monomer.
2. STRINGY STAGE
   Some polymer from the outer surface of the polymer
   beads has dissolved in the monomer to give a very
   viscous solution of polymer in monomer.

Question 5

Then final stage of polymer with liquid

Answer 5

GEL (FULL DOUGH) STAGE
•flow properties of the combination are maximised and it is at this stage that the material is shaped, before the monomer is polymerised.
•Factors affecting doughing time
•temperature of the mix
•size of the polymer beads
•molecular weight of the polymer
•presence of plasticiser.

Question 6

ELASTOMERIC STAGE

Answer 6

If the mixture is allowed to stand further, in the open
laboratory, evaporation of monomer will take place
from the gel to yield a rubber-like consistency.

Question 7

HEAT-CURED PMMA - PROCEDURE

Answer 7

Polymerisation of methylmethacrylate in the
MA/PMMA dough
•activated by the unstable organic peroxide, DIBENZOYL PEROXIDE (BPO,white crystalline solid,),
•which decomposes to form phenyl free-radicals at a temperature of 70ºC.
•dry heat, or via a heated water bath.
important that an excessive temp increase doesn’t occur in the dough (exothermic).
• If temperature exceeds the bp of the monomer
(100.3 ºC under 1 atm Pressure) the monomer will
evaporate

Question 8

COMPOSITION OF HEAT CURED POWDER.

Answer 8

●Emulsion-polymerised beads of PMMA.
●Plasticiser (eg dibutyl phthalate) The polymer beads are plasticized in the manufacturing process to allow facile absorption of the monomer.
●Initiator (dibenzoyl peroxide): 1%
●Pigment Inorganic, or organic, pigments to colour the denture base.

Question 9

COMPOSITION OF HEAT CURED LIQUID.

Answer 9

●Monomer (methylmethacrylate)
●Cross-linking agent (eg glycoldimethacrylate): 1-2%. Increases the surface hardness of the polymer and reduces the incidence of surface crazing.
●Inhibitor (eg hydroquinone): 0.1% Hydroquinone (colourless crystalline solid, soluble in MMA)
●donate H atoms readily to free-radical species forming monomer radicals (through thermal effects, or the action of light)

Question 10

CONTRACTION POROSITY.

Answer 10

If not enough acrylic dough is packed into the mould then contraction porosity will occur.
- contraction of the denture base of 7%.
- avoided by packing an excess of dough into the mould to allow for the contraction.
The excess is accommodated through the elastic deformation of the mould material (investment, usually dental plaster, or stone), where the halves of the mould are closed under spring pressure.

Question 11

Surface crazing

Answer 11

If not enough acrylic dough is packed into the mould then contraction porosity will occur.
- contraction of the denture base of 7%.
- avoided by packing an excess of dough into the mould to allow for the contraction.
The excess is accommodated through the elastic deformation of the mould material (investment, usually dental plaster, or stone), where the halves of the mould are closed under spring pressure.

Question 12

WATER ABSORPTION

Answer 12

PMMA is slightly hydrophilic.
this results in a slight swelling of the material.
PMMA dentures may require periodic chemical sterilisation to reduce bacterial content.

SURFACE POLISHING
acrylic denture should be highly polished. Removes surface cracks

Question 13

BONDING OF HEAT-CURED PMMA WITH ACRYLIC TEETH

Answer 13

Chemical bonding (adhesion) occurs through the absorption of monomer by the surface layers of the teeth.
•This monomer is subsequently copolymerised with the denture base.

Question 14

HIGH-IMPACT ACRYLIC POLYMERS

Answer 14

Failure
- accidental dropping (impact failure), through fatigue failure, or by stress-relief within the material.
Acrylic denture base materials with increased impact strengths are available
- combinations of rubber & PMMA.

Question 15

TECHNIQUES FOR INCREASING THE IMPACT STRENGTH

Answer 15

•Materials based on rubber-PMMA copolymers

•Inclusion of carbon fibres

•Inclusion of polythene fibres, or mats

Answer 16

While the inclusion of rubber particles toughens the material but are more flexible than PMMA.
- styrene-butadiene random copolymer (SBR rubber) are grafted onto the PMMA chains in the fabrication of the polymer particles.
•The rubber particles in the processed denture yields a composite effect.
•Cracks travelling through the PMMA matrix are interrupted at the interfaces with the rubber particles
•large amounts of energy can be absorbed through the deformation of the rubber. Such materials have higher impact strengths than PMMA.

Question 17

Cold curing pmma

Answer 17

Methyl methacrylate may be polymerised through the induced chemical decomposition of dibenzoyl peroxide by a suitable tertiary amine, at room temperature.
•Such materials are called COLD-CURING or AUTO-POLYMERISING acrylic.

Question 18

RAPID CONVERSION OF MONOMER TO POLYMER IS DELAYED DUE TO THE FOLLOWING EFFECTS:

Answer 18

Neutralisation of inhibitor (consumes radicals)

•Inhibiting effect of air (O2 dissolved in monomer)

•Early termination of short & mobile polymer chains

Significant conversion to polymer only occurs after the system has achieved the dough state. This conversion is accompanied by an evolution of heat which may result in gaseous porosity in the final article, as with the heat-polymerised material.

Question 19

MAIN COMPONENTS OF COLD-CURING PMMA POWDER

Answer 19

Emulsion-polymerised beads of PMMA.

•plasticiser: the polymer beads are more heavily plasticized than the heat-curing materials.

•Initiator (dibenzoyl peroxide): 1%

•Pigment

Question 20

MAIN COMPONENTS OF COLD-CURING PMMA LIQUID

Answer 20

Monomer (methylmethacrylate)

•Cross-linking agent (eg glycoldimethacrylate)

•Inhibitor (eg hydroquinone)

•Tertiary amine.

Question 21

COMPARISONS WITH HEAT-CURED MATERIAL

Answer 21

The transverse strength of cold-cured PMMA is approximately 80% of the heat-cured material.
•The residual monomer content may be as high as 5%.
•The colour stability of the peroxide/DMPT activation system is poor in that coloured by-products are formed in the setting reaction.

Question 22

SOFT-LINING MATERIALS FOR DENTURES

Answer 22

Sometimes an elastomeric lining may be required
on the fitting surface of denture.

The following types of materials maybe used:

1. Systems with leachable plasticiser
2. Systems with polymerisable plasticiser.
3. Systems without plasticiser.
4. Systems without monomer

Question 23

SOFT-LINING MATERIALS FOR DENTURES

Answer 23

Many soft lining materials for dentures may become hard and foul (bacteria/fungi) in the oral cavity, due to :
-The loss of plasticiser
-absorption of saliva, in the mouth
-so caution must be exercised with the use of a denture cleanser on any denture which is fitted with a soft lining.

Question 24

SYSTEMS WITH LEACHABLE PLASTICISER

Answer 24

adequate chemical bonding due to material similarities.
•gradual hardening due to loss of plasticiser and the absorption of saliva
•This soft-lining may be processed with denture manufacture, or by processing onto PMMA.

Question 25

SYSTEMS WITH POLYMERISABLE PLASTICISER

Answer 25

•Plasticiser which also undergoes a polymerisation reaction which therefore should be more difficult to leach out of the base material.
•The polymerised plasticiser lubricate the polymer chains.
•Such systems are not widely available commercially.

Question 26

SYSTEMS WITHOUT PLASTICISER

Answer 26

Silicone polymer - SILICONE SOFTLINING MATERIALS - - -
•heat-cured an cold-curing silicone
- usually based on poly(dimethylsiloxane)
- setting chemistry may relate closely to the condensation-curing silicone impression materials
- mechanical properties of the cold-curing types appear
to be particularly poor, on long-term water immersion.

Question 27

SYSTEMS WITHOUT MONOMER

Answer 27

The material does not contain free monomer, so these materials are frequently used at the chair-side as a temporary soft lining
•The heat-cured materials,
• loss of plasticiser (& ethanol) occurs in the mouth with a hardening of the material. However, bonding toPMMA is relatively poor, and the system is only suitable for temporary use.