Setting Reactions Flashcards
What happens during the gelation stage of a GIC setting reaction?
Initial set is due to Ca ions crosslinking with the polyacid by chelation with carboxyl groups. Ca ions are bivalent so can react with two molecules. This gelation equates to the initial set of the material and takes several mins depending on the particular material. Initial set is caused by formation of calcium polyacrylate. Material will appear hard in mouth following this reaction.
What happens during the dissolution stage of a GIC setting reaction?
Acid dissolves into solution. H+ ions attack glass surface. Ca, Al, Na and F ions are released. Leaves a silica gel around unreacted glass.
What happens during the hardening phase of a GIC setting reaction?
Trivalent Al ions ensure good crosslinking with an increase in strength. Aluminium polyacrylate formation takes a long time. This process does not start for at least 30mins and can take a week or longer to be complete. The aluminium reaction ensures a much higher degree of crosslinking. This process greatly improves the mechanical properties of the material (gets stronger and more translucent).
What is the setting reaction for conventional amalgam?
Reaction of mercury (liquid) and silver, tin, copper (powder).
Ag3Sn + Hg => Ag3Sn + Ag2Hg3 Sn7Hg9
Y + Hg => Y + y1 + y2
What are the properties of each product of the amalgam setting reaction?
Y = good strength and good corrosion resistance ( TS=170MPa
Y1 = good corrosion resistance but poor strength (TS=30MPa)
Y2 = weak and poor corrosion resistance (TS=20MPa)
Amalgam TS=60MPa
Describe the setting reaction of gypsum.
Hemihydrate dissolves and dihydrate forms. Dihydrate solubility is low so a supersaturated solution forms. Dihydrate crystals precipitate on impurities as crystals. More hemihydrate dissolves and continues until it is all dissolved.
Initial set - dihydrate crystals come into contact ie. push apart - expansion starts and material has properties of a weak solid and will not flow - can be carved
Final set - strong and hard enough to be worked (strength continues to develop)
During setting, excess water is trapped in the powder mass. On completion of setting, excess water evaporates and voids are produced (porosity)
How does Camphorquinone contribute to the setting reaction of composites?
It is activated by blue light. Produces radical molecules which initiate free radical addition polymerisation of resin (Bis-GMA). This leads to changes in the resin properties (I.e. increased molecular weight, increased viscosity, strength). Causes 35-80% degree of conversion of resin.
Describe the setting reaction for alginate.
Reaction is sodium alginate and calcium sulphate changes to sodium sulphate and calcium alginate gel.
2NaAlg + CaSO4 => Na2SO4 + CaAlg
Long cross-linked fibril entangling undissolved particles.
What is the setting reaction for dispersion modified amalgam?
Y + Hg => Y + y1 + y2
Y2 + AgCu => Cu6Sn5 + y1
Describe the acrylic polymerisation.
Acrylic undergoes free radical additional polymerisation - four stages:
Activation - of initiator to provide free radicals
Initiation - free radicals break C=C bond in monomer and transfer free radical
Propagation - growing polymer chain
Termination - of polymerisation
What is the initiator in polymerisation of acrylic?
Benzoyl peroxide (symmetrical molecule C6H5COO-OOCH5C6)
How is acrylic activated during polymerisation?
Heat >72C
Or
Self cured
Why does GI need protecting following placement? How can it be protected?
It is vulnerable to acid attack therefore must be protected by using a resin (dentine/enamel bonding agent, unfilled Bis-GMS resins), varnish (copal ether, acetate) or gel (vaseline)
What is the setting reaction for poly ethers?
Three stages: activation, initiation, propagation
Poly ethers base paste is an imine terminated pre polymer with an inert filler which catalyses by sulphuric acid ester which initiates cross linking in prepolymers
What is the setting reaction for CaOH?
chelation reaction between the zinc oxide and the butylene glycol disalicylate
