Resin Based Materials

Question 1

Why are poly methyl methacrylate granules used in the power to form heat cure acrylic resins

Answer 1

Lower polymerisation shrinkage from 20% (pure monomer) to 5-8%

Question 2

What is present in polymer powder of heat cure acrylic resin

Answer 2

Poly methyl methacrylate granules

Benzoyl peroxide (initiator)

Pigments and opacifiers

Synthetic fibres

Question 3

What is the nature of the liquid component in heat cure acrylic resins

Answer 3

Highly flammable and volatile

Question 4

What is the inhibitor present in heat cure acrylic resin

Answer 4

Hydroquinone

Question 5

What is function of plasticiser in heat cure acrylic resin liquid

Answer 5

Make polymer softer

Make polymer more resilient as plasticiser helps to absorb shock by breaking some existing bonds, making material less brittle

Question 6

What happens if too little liquid is used in mixture toform heat cure acrylic rein

Answer 6

Too little monomer, lessened mechanical properties

Question 7

What happens if too little powder is used in mixture to form heat cure acrylic resin

Answer 7

Too much residual monomer, cause tissue irritation

Question 8

What are some good properties of heat cure acrylic resin

Answer 8

Good aesthetics, good colour stability
In expensive
Easy to process
Low thermal conductivity to insulate tissues from thermal sensation
High biocompatibility as few patients allergic, if problem is due to residual monomer causing irritation
Good dimensional stability during function
Virtually insoluble in water (though soluble in organic solvents)

Question 9

What are the mechanical properties of heat cure acrylic resin

Answer 9

Moderate mechanical properties

Low impact strength, will fracture when dropped, but adequate to resist fracture in normal use

Rigid, low proportional limit and high elastic modulus

Question 10

What does knoops test measure. What is its value and what does it indicate in heat cure acrylic resin

Answer 10

Measure of hardness of material.

20KHN in acrylic resin ie low hardness. Easy to polish but low abrasion resistance and is susceptible toa ttrition

Question 11

DIsadvantages of heat cure acrylic resin

Answer 11

Susceptible to distortion during processing
High CLT
No anti microbial properties and can get colonised by C. Albicans

Question 12

What may cause denture whitening

Answer 12

Soaking it in hot water

Contact with organic solvents (alcohol degrade polymer network, loss of colour pigments)

Question 13

What is curing temperature of acrylic resin to be maintained at

Answer 13

74 degrees celsius

Exothermic reaction will add heat to raise material to POLYMERISATION temperature

Question 14

What are the stages of polymerisation of acrylic resin

Answer 14

1. Sandy
   Fluid incoherent mass
2. Stringy
   Monomer penetrate polymer, cannot be manipulated
3. Dough
   >5min working time, undissolved polymer suspended in matrix of monomer and dissolved polymer. Plastic and homogenous
4. Rubbery
   Monomer disappears by further penetration or evaporation. cannot be molded
5. Stiff

Question 15

What is the purpose of flasking acrylic resin

Answer 15

Remove excess material

Question 16

Advantages of long curing cycle acrylic resin

Answer 16

Increased polymerisation time —> less residual monomers

Less distortion and warpage as more heating time for molecules to find ideal position

Fewer bubbles and pores

Question 17

Describe slow cure cycle

Answer 17

Heat flask at 74 degrees celsius for 90 min then increase temperature to boiling for 1 hour

Question 18

Should you place acrylic dough in hot water to cure it

Answer 18

No. Place in cold water and increase temperature slowly. Rapid initial heating will produce internal stresses and warpage after deflasking, cracking and crazing

Question 19

What causes internal porosities in acrylic resin

Answer 19

Vaporisation of unreacted monomer (low molecular weight polymer, temperature surpass boiling temperature)

Question 20

What causes external porosities in acrylic resin

Answer 20

Lack of homogenity of dough

Lack of adequate pressure

Question 21

What causes cracking and crazing of acrylic resin

Answer 21

Increased mechanical stress eg during flasking

Incorporation of water

Rapid cooling

Question 22

How is benzoyl peroxide activated in autopolymerised acrylic resins

Answer 22

Tertiary amine in liquid activates it

Question 23

Degree of conversion of autopolymerised acrylic resins

Answer 23

At least 5% still monomers

Question 24

Properties of autopolymerised acrylic resin

Answer 24

Less colour stability

Less processing shrinkage since lower peak temperature

Greater porosity

Initial strength not as high but similar over time

Question 25

What is contraction porosity

Answer 25

Inadequate pressure during polymerisation

Question 26

Disadvantage of using polymethyl methacrylate to make dentures

Answer 26

Susceptible to denture wear due to low hardness

Question 27

What is granular porosity

Answer 27

Localised porosities

Too much powder, unwet resin that does not completely mix. Tends to happen at thinner parts of denture as volatilisation takes place more readily there

Creates stress in denture making it prone to fracture

Can fix using autopolymerised resin