Additional Composite Properties

Question 1

Describe the water sorption & solubility characteristics of composites in general (5)

Answer 1

1. Polymer matrix- absorb water–> swelling–> doesn’t counteract polymerisation shrinkage
2. Water uptake–> reduction in surface hardness, wear resistance & colour stability (inc. stain retention)
3. Microfills have higher water sorption values–> high volume fraction of matrix, weak bonding btwn pre-polymerised particles & matrix
4. Major leachable components: residual monomer & surface treatment of filler (lvl of leaching indicated by lvl of silicon in saliva)
5. Most water sorption occurs in 1st few hours following restoration placement

Question 2

What is leaching?

Answer 2

Removal of soluble constituents from a material by percolating action of a fluid e.g. saliva

Question 3

List 3 general mechanical properties of composites

Answer 3

1. Increased compressive strength of fine particle & hybrid composites
2. More highly filled composites have compressive strengths similar to/higher than enamel
3. Young’s Modulus- relates to stiffness of material; composites tend to have Y.M. that are a fraction of that of enamel, fine particle materials have Y.M. equivalent to dentine, deflection of restoration under high load strains tooth-composite bond, deflection also inc. tensile stress on adjacent cusps

Question 4

List 4 wear characteristics of composites

Answer 4

1. Increased filler volume= decreased wear
2. Highly filled fine particle composites have decreased rate of loss compared to microfilled composites
3. Constant volume fraction wear–> resistance to wear increased by decrease size of filler particle
4. Soft filler particles w/ hardness characteristics similar to enamel gives decreased wear as it absorbs energy of mastication; less transmitted to matrix

Question 5

List 2 thermal properties of composites

Answer 5

1. Thermal conductivity of composites closely match those of dentine & enamel, and is much lower than that of amalgam
2. Weak bonds hold polymer molecules/chains together; tend to expand/contract with temp changes. Stress @ composite-tooth interface due to thermal cycling–> marginal break down. (Effect greater in resin-rich microfill resins than in fine particle materials or hybrids)