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 )
The correct polymer to monomer ratio for denture base fabrication is (……..) or (……)
The correct polymer to monomer ratio for denture base fabrication is ( 3:1 by volume ) or ( 2:1 by weight )
Type IV and V die stone are produced when a alpha hemihydrate are mixed with ( ……..) or ( ………)
Type IV and V die stone are produced when a alpha hemihydrate are mixed with ( 30% CaCl2 / MgCl2 ) or ( sodium succinate)
How can the setting time of gypsum products be controlled
1) WP ratio
- higher WP ratio —> slower set due to less nuclei of crystallisation per unit volume
2) Spatulation time
- Longer spatulation time —> faster set as more crystal particles are broken down —> more nuclei of crystallisation formed per unit volume , particles more exposed to water & nuclei of crystallisation more evenly dispersed throughout the mix
3) Temperature
- cold water —> prolong setting time
- hot water —> shortens setting time
4) Addition of chemicals
- accelerator : K2SO4 (>2% concentration ) , sodium sulfate , sodium chloride
- retarders : borax
How can the setting expansion of gypsum products be controlled
1) WP ratio
- Higher WP ratio —> more water —> less nuclei of crystallisation per unit volume —> less interaction b/w crystals —> less outward thrust —> lower setting expansion
2) Mixing time
- Longer mixing time —> more crushing of particles —>. More nuclei of crystallisation formed —>. More interaction b/w crystals —> more outward thrust —> more setting expansion
3) Addition of chemicals by manufacturer
What are the factors affecting strength of gypsum product?
1) WP ratio - more water -> more porosities formed -> lower strength
2) Spatulation time - strength increase with spatulation time up to certain point (~ 1 min ) , past which the nuclei of crystallisation are broken up & the growth of crystal is affected
3) Water content - dry strengthen of gypsum ( when all excess water is evaporated ) is 2-3x more than wet strength of gypsum
4) Additives - retarders & accele both lower strength
5) Temperature - GP stable at < 40 C ; > 100 C loss of crystallisation occurs
6) Particle size - properly ground , decreased size —> increase strength due to more nuclei of cryst
State the composition of alginate
1) Potassium / sodium / triethanolamine alginate - reacts with Ca in calcium sulfate to form Ca - alginate
2) Calcium sulfate - reacts with potassium / sodium alginate , provides Ca ions that cross link the alginate sol .
3) Potassium titanium fluoride - gypsum hardener , counters the effect of formation of Na2SO4 which is a gypsum retarder
- ensure that the cast that is poured has a hard surface
4) ZnO - filler particles
5) Diatomaceous earth - filler particles
6) Sodium phosphate ( Na3PO4)
- retarder , prevents instant gelation of alginate during setting reaction
7) Coloring agents
State the chemical reaction of setting of alginate
Na/ K- alginate + CaSO4 —> Ca-alginate + Na2 /K 2So4
Na3PO4 ( retarder ) slow down the gelation reaction by reacting with CaSO4 so,that the setting of alginate is not instantaneous
State the working time & setting time of normal set and fast set alginate
Normal set -
WT : 3-4.5 min
ST 2-4.5
Fast set - Wt & St : 1-2min
State the advantages and disadvantages of alginate
Describe the compatibility of gypsum with agar & alginate
Setting of alginate produced Na2SO4 ( accelerator at low concentration but retarder at high concentration )—> gypsum retarder , hence potassium titanium fluoride is added as a gypsum hardener . Agar contain borax —> gypsum retarder , hence the agar impression can be immersed in 2% K2SO4 solutioprior to pouring of gypsum , K2SO4 solution prior to pouring of the gypsum. K2SO4 is a gypsum accelerator ?.Without gypsum hargpderners , the surface of gypsum which is poured will be very soft & inadequate to make casts and models
Denture bleaching can occur if soaked in water overnight at too (………….. ) temperature , exposure of denture to solvent like (……………. )
Denture bleaching can occur if soaked in water overnight at too ( high ) temperature , exposure of denture to solvent like ( acetone )
What are the ideal requirements of impression materials ?
1) Non- irritant , no foul odor , non toxic
2) Acceptable Color & appearance
3) Dimensionally stable after setting
4) Able to accurately capture surface details
5) Easy to manipulate with minimal equipment
6 ) Viscous enough to be contained in the impression tray but fluid enough to adapt to the oral tissue
7) Cost effective
8) Resisitant to distortion or tearing upon removal from the mouth.
9 ) Compatible with cast and die materials
Describe the composition of polyether impression material?
3 Pastes :
I) reactor paste - Imine terminated polyether m undergoes cross-linking to form rubber - colloidal silica - filler - glycol ether phthallate : plasticizer
II) base plate - ester derivative of aromatic sulfonic acid : cross linking agents - colloidal silica : filler - glycol ether pthallete : plasticizer
III) body modifier paste - octal phtallate ( thinner ) ;reduce viscosity of the material - 5% methyl cellulose : acts as a retarder
Elastomers are formulated in various consistencies : extra low , low medium, heavy and putty in increasing order of filler content . Described the consistencies of condensation silicones , polyethers & polysulfides
Addition silicone : extra light , light , regular , heavy & putty
Condensation silicone : extra light , light , regular , putty ( no heavy)
Polyether : light , regular & heavy
Polysulfide : only light & heavy body
State the advantages & disadvantages of polyether impression materials
Advantages : I)highly accurate II) dimensionally stable III) no byproduct formed ( like polysulfide ) IV) Long shelf -life V) Good pseudoplastic properties I) multiple cast pouring is possible
Disadvantages
I)sulfonic acid reactant may be irritating to tissue
II) high stiffness
III) low tear strength
IV) not available in complete range of viscosities ( only light , regular & heavy )
V) Expensive compare to other elastomer
VI) hypersensitivity in some patients due to presence of ethylene imine rings in base paste
Describe the composition of addition silicone /PVS
1) Base paste
I) Polymethyllhydrosiloxane : undergoes cross-linking
II) other siloxane and polymers
III) silica : fillers
2) Reactor paste
I) Divinylpolysiloxane : cross linking agent
II) platinum salt ( chloroplatinic acid ) : catalyst
III)palladium : absorb H2 gas ( sometimes H2 gas released during reaction )
IV) retarders
V) Fillers
State the advantages & disadvantages of addition silicone
Advantage : I) excellent reproduction of surface details ( best) II) high dimensionally stability III) Pleasant odor & Color IV) if) short setting time ( 4-7mins ) V) automix available VI) high tear strength
Disadvantages
I) Hydrophobic, hence must have a dry working field
II) expensive
III) sometimes hydrogen gas can be released
