B1 Flashcards
Describe the addition polymerisation
- The self addition of monomer molecules to each other , very rapidly through a chain reaction without simultaneously production of byproducts
- 3 major steps : initiation , propagation & termination
Describe the condensation polymerisation
- brought about by monomers containing 2 or more reactive functional groups such as -OH , - COOH , and -NH2
- byproduct formed : water
Example :polysulfide
Basically functional groups of monomers react together to form polymer
Describe the stages of free radical addition polymerisation
1) Initiation
- Activators ( Heat , N-N dimethyl P-toludine , microwave , visible light ) react with initiators (benzoyl peroxide , benzoin methyl ether , camphoroquinone ) to form free radicals
- Free radicals reacts with monomer molecule to form free radical monomer complex
- R’ + Monomer = R-M’
2 ) Propagation
- free radical monomer complex acts as a new free radical centre –> reacts with another monomer to form dimmer ( which is also become free radicals )
- R-M’ +M —> R-M-M ‘
- continues until the chain growth is stopped by the free radicals being killed by some impurities or all the monomer has been reacted.
3) Termination
- any further addition of the monomer units to the growing chain is stopped & growth of the polymer chain is arrested
- may be terminated by direct coupling of 2 free radical chain ends or by the exchange of H2 atom from one growing chain to the other to form double bonds ( disproportionation)
4) Chain transfer
- in this process the free radicals molecule of a growing chain is transferred onto another molecule ( example : inactive monomer or already terminated polymer chain )
- causes termination of original chain & activation of the chain in which the free radical is transferred onto
** although chain transfer can result in termination , addition polymerisation reactions are usually terminated via direct coupling or exchange of H atom from one chain to the other
Describe the copolymers and its types
- Polymer which is composed of more than one types of repeating units
- monomers from which a copolymer is made are known as comonomers
Types :
I) alternating : nA + nB = A-B-A-B-A-B
II) Random : nA+ nB : A-A-B-B-A-A-B-B
III) Block : nA +nB m A-A-A-A-A-B-B-B-B-B
IV) Graft copolymer : A forms backbone , B forms branches
Describe cross linking
- it is formation of chemical bonds between linear polymer molecules
- forms non linear , branched or cross linked polymer
- cross linking agents : ethylene glycol dimethacrylate
Describe plasticizers
- Substances added to resin to decrease their softening or melting / fusion temperature — less heat required for the material to become soft
- It increase the solubility of the polymer in monomer
- It decrease the brittleness ( harder to crack , more flexible ) but it decrease the strength and hardness
- used to form soft , cushioning liners for dentures
Types :
I) External plasticizers :
- penetrates the macromolecule and partially neutralise the intermolecular attraction which normally prevent the resin chains from slipping past one another ( ex: undergoing plastic flow ) when the material is stress — hence it helps make the resin flow more easily when stresses as bonds are neutralised
- insoluble , high boiling compound
- not widely used as it may evaporate or leach out during normal use of resin — loss of plasticity
II) internal plasticizers
- accomplished by copolymerisation with suitable comonomer
- plasticizers become part of the polymer
- ex m butyl methylacrylate is added to methylacrylate
State the classification of denture base resins & its ADA specification number
ADA specification number 12
Type 1 -Heat polymerisable polymer
I) Class 1 - in powder or liquid form
II) Class 2 - in cake form
Type 2 - Autopolymerizable polymers / cold cured
Type 3 - Thermoplastic blank or powder
Type 4 - Light activated materials
Type 5 - Microwave - cured materials
List some requirements of denture base resins
1) Dimensionally stable
2) Easily manipulate
3) high strength , stiffness , hardness , & toughness
4) ease of repair
5) natural appearance
6) resistant to absorption of oral fluids
7) absence of odor , taste , taste product
8) able to accurately reproduce surface details
Describe the 5 stages of polymer-monomer interaction
Polymer-to-monomer ratio —> 3:1 by volume — limits volumetric shrinkage to approx 7%
1) Sandy stage
- Little or no interaction occurs on molecular level
- polymer beads remain unaltered
- consistency of the mix —> coarse / grainy
2 ) Stringy stage
- Monomer attackers the surface of individual polymer beads & is absorbed into the beads
- polymer chains uncoil , increasing the viscosity of the mix
- characterised by stringiness when the materials is touched
3) Dough stage
- an increased number of polymer chains enter the solution — monomer & dissolved polymer are formed
- mass behaves as pliable dough , no longer tacky and does not adhere to the surface of the mixing vessel or spatula.
- ideal for compression moulding
4) Elastic / rubbery stage
- monomer is dissipated by evaporation & further penetration into remaining polymer beads
- mass rebounds when compressed or stretched
- no longer flows freely & cannot be modded by conventional compression technique
5) Stiff stage
- due to continued evaporation of unreacted monomer
- mixture is very dry & resistant to mechanical deformation
Describe the composition of heat cured & autopolymerizing denture resins
Powder component
*I) prepolymerized spheres of polymethyl methacrylate
*II) Benzoyl peroxide ( initiator — forms free radicals
III) pigments ( salts of cadmium , mercury , iron or organic dyes )
IV) radioopacifiers ( titanium / zinc dioxides )
V) dyed organic fibers for natural appearance of tissue ( nylon , acrylic )
VI) dibutyl phtalate ( plasticisers)
Liquid component
I) methyl methacrylate monomer
II) hydroquinone ( inhibitor )
III) glycol dimethyacrylate at 1-2% by volume ( cross linking agent )
IV) ** N N’ - dimethyl-p-toludine ( activator , only in autopolymerizing
-Glycol dimethyacrylate —> 2 C=C bonds per molecule while MMA —>1 C=C bond per molecule , hence 1 molecule of glycol dimethyacrylate can participate in the polymerisation of 2 separate polymer chains that unites 2 polymer chains , if sufficient glycol dimethyacrylate —> form extensive interconnection —> increase resistance to deformities
Described light activated / light cured denture base resins ( composition , activator , initiator )
Composition
I) Urethane dimethacrylate ( UDMA)
II) microfine silica
III) high molecular-weight acrylic resin monomer —> acts as organic fillers
Activator
- high intensity visible blue light
Initiator
- camphorquinone ( photosensitizing agent)
- supplied in sheet & rope forms in lightproof package ( prevent accidental polymerisation ) —> denture base mold Ed on cast & teeth positioned —> exposed to blue light —> polymerisation —> denture is finished & polished
Compare heat cured acrylic resins with autopolymerized ( cold cured ) acrylic resin ( classification , activator , degree of polymerisation , dimensional stability , colour stability )
Heat cure denture base resin
- Type 1
- Thermal heat >60 ( water bath , microwave oven)
- Benzoyl peroxide
- More complete ( Hence stronger impact strength , hardness , lower creep )
- less dimensional stability
- more stable in colour stability & aesthetics
Auto polymerized denture base resin
- Type 2
- N-N’ dimethy-P-toluidine
- Benzoyl peroxide
- Less complete ( greater amt of unreacted monomer present —> acts as a plasticizer & decrease transverse strength of denture base resin , also act as potential tissue irritant
- Fractures & abrades more easily
- More ( Less shrinkage displayed )
- Less stable ( Due to N-N’ dimethyl-P- toluidine —> susceptible to oxidation —> Color change ) (bleaching)
State the reason why porosities may appear in a denture base resin
1) Inadequte mixing of liquid & powder component
2) Inhomogeneity of resin mass
3) Inadequate pressure or insufficient material
4) Air inclusions incorporated during mixing & pouring procedures
5) Excess monomer used
Define relining
Process of replacing the tissue-contacting surface of an existing denture w/ a new material
Define rebasing
- Process of replacing the entire denture base of an existing complete or partial denture
- Only the original teeth & their arrangement remain
Distinguish btw hard liner , soft liner & tissue conditioners
Hard liners
- A layer of plastic is removed from the denture interior surface & then filled with putty like material
- done every 2 years
Soft liners
- A layer of soft , pliable material that is fitted onto the impression surface of the denture
- It absorb shock btw the hard base of the denture and gums
Tissue conditioner
- a short term soft-liner ( up to 14D)
- applied to tissue surface of the denture
- mainly elastomers used to treat irritated tissue , once it loses their elasticity it should be replaced
Examples : PMMA ( heat & cold cured ) , butyl methacrylate , silicone materials
State the indications for relining and rebasing
1) Alveolar ridges resorption
2) loss of retention & stability due to loss of correct relationship to the supporting tissue
3) decreased occlusal vertical dimension
4) In immediate dentures
5) socioeconomic constrains
- does not have money to replace denture
6) physical / mental state of the patient
State the contraindications of relining and rebasing
1) unresolved TMJ dysfunction & myofacial pain
2) abused , inflammed & pathologic basal seat mucosa
3) malpositioning of artificial teeth
4) Multiple fractured & severely worn artificial teeth
5) An unfavourable occlusal plane that produces a poor appearance
6) vertical dimensional occlusion that must be increased > 3-5mm
State the calcination & setting reaction of gypsum product
Calcination ( dihydrate —> hemihydrate )
CaSO4.2H2O —> CaSO4. 1/2 H20
Dry calcination : open kettle @ 110C , produce beta-hemihydrates (type I& II)
Wet calcination : steam pressure , produces alpha hemihydrates ( type III stone )
Wet calcination + 30% MgCl2 / CaCl2 or sodium succinct : produces alpha hemihydrates ( type IV & V die stone )
Setting ( hemihydrate —> dihydrates )
( CaSO4 ) 2 . H2O + 3H2O —> 2 CaSO4.2 H2O + heat
Compare & contrast between alpha & beta hemihydrates
Alpha hemihydrates
- Uniform and fine crystal
- Smooth and smaller SA
- Higher strength
- Lower W:P ratio
- Type III , IV, V
Beta-hemihydrates Irregular and porous crystals Rough and SA Lower strength Higher W:P ratio Type I and II
Describe the dissolution-precipitation theory of gypsum product
1) Hemihydrates dissolve in water and form a suspension
2) Hemihydrates dissolve until forming saturated solution of Ca2+ and (SO4)2-
3) Solution is supersaturated with respect to the solubility of dihydrate
4) Precipitation of dihydrate occurs
5) Hemihydrates continue to dissolve while dihydrates are precipitated , until all hemihydrates —> dihydrates
Explain normal & hygroscopic setting expansion of gypsum products
Normal setting expansion
- When setting occurs in normal condition
-Stage I : hemihydrate + H2O cause dihydrate to form on nuclei of crystallisation
- Stage II : Water is reduced by hydration & the particles are drawn closely together because surface tension of the water keeps the water surface area at a minimum.
- Stage III : dihydrate crystals grow & contact each other & H2O around particles decreases
- Stage IV : Further dihydrate growth consumes more water and should draw the crystals closer together , but the outward thrust of the growing crystals opposes this contraction
Stage V : eventually the crystals become intermeshed & entangled
Hygroscopic setting expansion
- When setting occurs while the gypsum is immersed in water
- Stage I : Initial Mix : hemihydrate+ H2O causes dihydrates to form on nuclei of crystallisation
- Stage II : Initial Crystal Growth : since Hemihydrates are under H2O , water consumed by hydration is immediately replenished & distance between particles remains the same
- Stage III : Solid phase Contact : Dihydrates crystals continue to grow & contact each other with no reduction in distance between the crystals
- Stage IV : Expansion : crystal will grow more freely before intermeshing
Stage V :Termination : intermeshing prevents futher expansion
How long should the curing cycle be and at what temperature in denture fabrication?
7 hours at 70 C followed by 3 hours at 100C
Final 3 hours ensure almost complete conversion of monomer in thinner areas of the denture bases where the effect of exothermic heat of the reaction is less pronounced
I) 74 C for 8 hours or longer with no terminal boiling point
II) 74C for approx 8 hrs then increasing to 100C for 1 hr
III) 74C for approx 2 hrs then 100C at 1 hr
Denture bleaching can occur if soaked in water overnight at too (…..) temperature ; exposure of dentures to solvent like (……)
Denture bleaching can occur if soaked in water overnight at too (high) temperature ; exposure of dentures to solvent like ( acetone )
