PMMA and investment materials

Question 1

name 5 ideal properties of a denture base

Answer 1

high conductivity
high softening temperature
low density
non toxic/ irritant
unaffected by oral fluids
easy to repair
low thermal expansion

Question 2

free radical addition polymeristation

Answer 2

Chemical union of TWO molecules either the same or different to form a larger molecule WITHOUT the elimination of a smaller molecule

(via C=C bond)

Question 3

contents of heat cured acrylic powder

Answer 3

initiator
PMMA particles
plasticiser - allows quicker dissolving in liquid
pigments - give good colour
co - polymers - improve mechanical properties

Question 4

contents of heat cured acrylic liquid

Answer 4

methacrylate monomer
inhibitor - extends shelf life
co polymers - improve mechanical properties

Question 5

what are the 4 stages of acrylic polymerisation

Answer 5

1. activation - of initiator to provide free radicals
2. initiation - free radicals break C=C bonds in monomer
3. propagation - growing polymer chain
4. Termination - of polymerisation

Question 6

what initiator is used in both heat cured and self cured acrylic

Answer 6

benzoyl peroxide

Question 7

how is the initiator activated in self cure vs heat cure acrylic

Answer 7

heat cure - activated by heat
self cure - activated by a promoter (tertiary amine) present in the liquid

Question 8

compare heat and self cure acrylic
- chemical activation

Answer 8

self cure has less efficient chemical activation meaning less efficient polymerisation resulting in lower molecular weight and decreased mechanical properties

Question 9

compare heat and self cure acrylic
- unreacted monomer

Answer 9

self cure sees more unreacted monomer therefore an increased chance of irritation, a softer denture base and decreased transverse strength

Question 10

compare heat and self cure acrylic
- polymerisation contraction

Answer 10

self cure acrylic sees no contraction during polymerisation so sees a better initial fit. However, expands due to water absorption in mouth
(heat cure acrylic has this expansion compensated for by polymerisation contraction)

Question 11

what other denture base materials are available if a patient had an acrylic allergy

Answer 11

nylons (absorb water)
vinyl polymers (soften at low temperatures)

Question 12

what is an investment material

Answer 12

material that is suitable for forming a mold into which a molten metal or alloy can be cast to make a indirect restoration

Question 13

why do investment materials need to be able to expand

Answer 13

to compensate for the cooling shrinkage of alloys

Question 14

why do investment materials need to be porous

Answer 14

to allow trapped gasses to escape

Question 15

what is a sprue in relation to investment materials

Answer 15

a channel allowing the molten metal or alloy access to the mould of the indirect restoration

Question 16

process of creating and using an investment material (lost wax technique)

Answer 16

1. create wax pattern e.g of crown
2. investment material poured around wax pattern and allowed to set to create a mold
3. wax is then eliminated via boiling water or burning in oven
4. molten alloy is then forced into mold cavity now vacated by wax via sprues

Question 17

what are the two categories of components that make up an investment material powder

Answer 17

binder - forms coherent solid mass and determines what type of investment material it will be
refractory - withstands high temperatures and gives expansion

Question 18

binder component of investment materials

Answer 18

forms coherent solid mass and determines what type of investment material, there are 3 available:
gypsum , phosphate or silica

Question 19

refractory component of investment materials

Answer 19

withstand high temperatures and allow expansion
silica

Question 20

green strength

Answer 20

strength at room temperature

Question 21

what is investment material powder mixed with

Answer 21

water

Question 22

shrinkage of gold and cobalt chrome when cooled

Answer 22

gold - 1.4%
CoCr - 2.3%

Question 23

why can gypsum bonded investment material not be used for casting cobalt chrome

Answer 23

it is only capable of expanding 1.4% versus CoCr’s shrinkage of 2.3% - not compatible
can be used for gold

Question 24

properties of gypsum bonded investment material
- expansion
- particle size
- ease of use
- porosity
- strength

Answer 24

• 1.4% expansion , not suitable for CoCr but can be used for gold
• fine particles which produce a good smooth surface
• easy to use
• good porosity
• adequate strength if produced correctly

Question 25

why cant gypsum be heated above 1200 degrees celsius

Answer 25

sulphur trioxide is released which creates porosities
this means alloys that melt above 1200 degrees celsius cant be used with gypsum investment materials

Question 26

why is gypsum held at a high temperature for a long time during production

Answer 26

at around 700 degrees carbon monoxide and sulphur dioxide are produced
holding the material at a high temp allows these gases to escape

Question 27

properties of phosphate bonded investment
- strength
- ease of use
- porosity
- expansion

Answer 27

• good green strength
• easy to use
• good porosity
• 2% if use colloidal silica instead of water as liquid component

Question 28

why might colloidal silica solution be used instead of water for mixing phosphate bonded investment materials

Answer 28

increases strength
gives hygroscopic expansion - 2%

Question 29

why are silica bonded investment materials not used very often

Answer 29

they are complicated to use and non porous

Question 30

why do investment materials need to be strong

Answer 30

to withstand pressures during casting