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BDS2 CLINICAL Dental Materials > Resin Composites > Flashcards

Flashcards in Resin Composites Deck (44)
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what are the 3 components of resin based composites

• Resin forms the matrix of the composite material, binding the individual filler particles together though the coupling agent


what is the resin matrix

initially a fluid monomer but is converted to a rigid polymer by a radical addition reaction


what is the most common monomer



what is bis-GMA derived from

reaction of bisphenol-A and glycidylmethacrylate


what is the advantage of using bis-GMA over MMA

○ Higher molecular weight than MMA which helps to reduce the polymerization shrinkage


what is another common resin that is often used instead of Bis GMA

urethane dimethacrylate resin


why are bis-GMA and UDMA highly viscous fluids

because of their high molecular weights


what does UDMA and Bis-GMA being highly viscous mean

the addition of even a small amount of filler would produce a composite with a stiffness that is excessive for clinical use


how is the problem of over viscosity in UDMA/BisGMA composites overcome

low viscosity monomers known as viscosity controllers are used


what are common examples of low viscosity monomers



what ensures an adequate shelf life

nhibitor is also used to prevent premature polymerisation


what does the resin matrix contain that achieves the cure

The resin matrix also contains the activator/initiator systems for achieving the cure - components depend on which kind of reaction is employed, may be either chemical curing or visible-light activated curing


what can be done to reduce PM shrinkage

To reduce polymerisation shrinkage, can incorporate large amounts of glass filler particles as filler does not take part in polymerisation process


why is a coupling agent required

For composite to have acceptable mechanical properties, it is of the utmost importance that the filler and the resin are strongly bonded to each other


what happens if there is a breakdown in the interface between resin and GF

the stresses of the load will not be effectively distributed throughout the material and this interface will act as a primary for fracture leading to the subsequent disintegration of the composite


what are coupling agents



what is the most common coupling agents



what happens if there is not a good bond between resin and glass

stress transfer between resin and glass will be inefficient and most of the stress will be will have to be carried by the resin matrix -this will result in excessive creep and eventually fracture and wear of the restoration


what is the fundamental problem with resin and glass

resins are hydrophobic whereas silica-based glasses are hydrophilic due to a surface layer of hydroxyl groups bound to the silica hence the resin does not have a natural affinity to bond to the glass surface - the solution to the problem lies in a suitable coupling agent


how do coupling agents work

• The silane coupling agent has been so chosen as to have hydroxyl groups on one end which are attracted to the hydroxyl groups on the glass surface and the other end consists of a methacrylate group that is able to bond to the resin via the carbon double bond


what is the oxygen inhibition surface

When there is an air interface with resin, the resin will not cure and a sticky surface is readily discernible - this is of benefit when carrying out an incremental procedures as it ensures that layers of composite will be well bonded together


why is a high degree of conversion of C=C desirable

to achieve the optimum mechanical properties and this relates to the curing time and the power of the light unit


what does a lack of cure result in

• Any lack of cure provides a poor foundation for the restoration and may leaed to fracture - this is due to a lack of support at the cervical margin


what can affect the depth of cure

type of composite
quality of light source
method used


how can type of composite affect the depth of cure

as light hits the composite, it is reflected, scatters and absorbed and this limits the amount of penetration that is achieved. There is particular concern for darker shades - importance of incremental technique and curing time!


how can quality of light source affect depth of cure

light source should be designed so as to produce its maximum light output at approximately 460-480 nm where the maximum of the camphorquinone absorption coefficient is located


how does the method used affect the depth of cure

tip should be placed as close as possible to the restoration


what is the limitation of composite

One limitation of composites is that there will be a marginal gap as the composite shrinks away from the cavity wall on setting - PM SHRINKAGE


why is composite more prone to secondary caries than amalgam and GIC

• Composites do not have any intrinsic defence mechanisms against caries attack unlike GICs and amalgam and hence once a gap is formed, microleakage will occur which can quickly lead to the spread of recurrent caries


what does a higher degree of conversion also lead to

greater polyermisation shrinkage