Denture base resins by wanghonghx

VIEWS: 80 PAGES: 28

									Denture base resins




                      1
Denture construction




               Main use is




                             2
Old materials used to construct
dentures
 Material              Disadvantage

 Cellulose products    Warpage, bad taste, blisters
                       staining (leaking camphor),
                       and loss of color
 Phenol formaldehyde   Loss of color, difficult to
                       process
 Vinyl resins          Low fracture resistance

 Vulcanite             Inferior esthetics

                                                      3
The material of choice?
   Acrylic resin (polymethyl methacrylate) PMMA
   Why?
   Desirable qualities –reasons why it is used nowadays- :
       Good esthetics
       Cheap
       Easy to process & use
   Disadvantages:
       Not strong enough especially impact strength (denture fall accidentally
        and it is broken easily)
       Susceptible to distortion with time
       Low thermal conductivity leads to the pt frequently burning the
        mucosa as they not actually feeling the heat immediately.
       Radiolucent if by a chance, part of it broken or pt is wearing a small
        partial denture & swallows it, you cant see it in radiographs, so you don’t
        know if it went to the lung or to the stomach, if it went to the stomach it is
        fine, its gonna come out but in the lung it is a problem.
       so they include opcifiers to the composition to make it opaic (radio-opaic)4
Dental uses

   Denture base material
   Denture repair
   Denture teeth
   Soft liners
   Construction of customized trays
   Soft tissue replication on cast metal
    framework


                                            5
                                              Material is supplied as powder &
                                              liquid



                                                         Acrylic resin




        Acrylic denture
The end product with acrylic resin teeth or
porcelain.




                                                                                 6
Composition
Different types, sometimes it is classified according to the way it sets.


 Heat cured set by heat
 Cold cures it doesn’t need heat only chemical rxn cause them
    to become hard.

 Light cured harden as subjected to light at certain wavelength.
 Pour and cure resins very fluid & poured into the
    model.




                                                                            7
Continue,

   The reaction is an addition polymerization
    reaction to convert MMA (methyl
    methacrylate) by cross linking to PMMA (polymethyl
    methacrylate).
   Mono= single
   Poly= many
No by product not like condensation polymerization reaction.




                                                               8
Polymerization & crosslinking
   Polymer: , large long chain molecules formed by
    chemically joining smaller molecules called
    monomers
   Copolymer: is formed when two or more different
    types of monomers join together
   Cross-linked polymers: adjacent polymers that bond
    together by side chains:
       Weak bond not highly charged between side chains leads to a
        easily manipulated and stretched polymer so the end result is
        soft polymer.
       If bond or cross-linked or side chains is highly charged, resulting
        polymer is stiffer rigid

                                                                              9
Polymerization
1. Addition polymerization:
  1. Activation: by producing free radicals very active component & cause
     the connection between monomers. Activators; needed for activation:
     1. Tertiary amine
     2. Heat
  2. Initiation: when an initiator such as benzoyl peroxide source of
     free radicals is decomposed and free radicals are produced.
  3. Propagation: occurs when free radicals break the bond
     between carbon atoms in monomer and adjacent monomers
     bond together, keep going until either there is no more free radicals or no
      more monomers.
  4. Termination: occurs when monomer units are used up, or
     free radical is tied up by a reaction

                                                                             10
Polymerization

2. Condensation polymerization: usually more
   than one type of monomer is used. The
   reaction produces by-products not like addition
   polymerization such as water, hydrogen or alcohol.




                                                    11
Heat cured resins

      Powder:
        Beads or granules of PMMA so they are already
           polymerized not monomers.

          Initiator: benzoyl peroxide
          Pigments
          Opacifiers: titanium/zinc oxide to make it radio-opac
          Plasticiser: dibutyl phthalate make the material softer,
           added flexibility.

          Synthetic fibers: nylon/acrylic to look like blood vessels –
           small blood vessels- to give the gingiva a natural appearance.


                                                                            12
I.       Heat cured resins

        Liquid:
          Methyl methacrylate monomer

          Cross-linking to make polymers at the end agent:
           ethylene glycol dimethacrylate. This
           component improves mechanical properties add
           strength.




                                                        13
14
    Flasking and Heat curing
   Flasking                                            Processing at 74°C for 8
   Dewaxing                                             hours or longer
   Application of separating medium                    Processing at 74°C for 8
    prevent the 2 parts of the flask from sticking       hours then increasing
    to each other   (water soluble alginate              temperature to 100°C for 1
    solution                                             hour
   Placing acrylic dough                               Processing at 74°C for 2
   Packing closing the flask together and               hours and raise temperature
    removing excess                                      to 100 for 1 hour.
   Heat curing under pressure so the                   Cooling should be slow to
    2 flasks are placed on top of each other &           avoid warpage so it will not fit in pt’s
    pressed. Pressure is important to let the            mouth.
    dough to flow all over & cover all the              Most important is heating gradually &
    necessary components. If pressure was lost           cooling gradually, it should not be over
    from the flask or you pack the flask before          heated & if you cool it down quickly there
    dough stage (sandy or sticky stage) the              will be distortion heating should be
    material will not flow & there will be defect.       controlled & cooling should be slow.
                                                                                             15
Heat curing, continue

   In addition to previously mentioned curing
    methods (heating of acrylic resin is in water path to raise the temperature),
    curing may be done in microwave, where the
    flasks used should be non metallic. But in that case
    we can’t place it in the metal flask, the flask is a device that contain the model which
    has the denture in it, so sometimes heat cure resin instead of using warm water to

    heat it, it can be heated in microwave.




                                                                                               16
                 This is the flask, the metal device, this flask contain the model
FLASKING STEPS   & the teeth set on the gypsum model in the wax.




                                2 flasks –one for upper arch & one for lower arch-,
                                it is placed in this water path & then we raise the
                                temperature, after melting the wax, we re-open the
                                flask, the acrylic resin as a dough is placed where
                                the wax used to be, then the flask is closed back
                                again & they place it in the hot water to harden the
                                resin.
                                                                                 17
Setting stages
When the powder & liquid mixed, you can’t immediately use that mix to make
  the denture, you have to wait for sometime for the reaction to occur. So
  there are stages for the setting of the material or after you mix the material.

   Sandy stage: immediately after mixing sandy
    texture (no use)

   Sticky or stringy stage
   Dough stage: mix is cohesive and less tacky,
    easy to manipulate (time to pack in the
    moulds) , use the material & place it in the flask.
       Dough stage period is the working time.
       Time taken to reach dough stage is doughing time
   Rubbery stage -wait too long-
So if you try to place it where it should as it is elastic, it will not remain in its
   position, it will attend to come back & rebound, so deficiency occur, so you
   should never wait, it should be used during the dough stage.
There is a fifth stage called stiff stage.                                            18
Continue,

   The previous composition allows for:
       Processing by the dough technique
       Minimal polymerization shrinkage
       Heat generation is minimal because in the powder we have
       already PMMA, so you don’t need to polymerize it, it is already
       polymerized.
    As the polymerization rxn of acrylic resin is exothermic but because we are
       placing pre-polymerized beads, they are already polymerized so no
       excess is necessary so this will minimize heat generation & also
       shrinkage will be reduced.




                                                                                  19
 Continue,
   Dough technique: The powder-liquid mix is
    used, when it reaches the dough stage, it is
    used to pack the flask containing the teeth and
    the set plaster which will then be closed under
    pressure. Manipulation of the mix in the dough
    stage is easier and produces better results.
If you stretch it, it will stay stretched & not re-bound & go back. So it is placed on
    the model, excess is removed & at the end the flask is re-putted again, always
    put excess material. Excess is necessary to compensate for any shrinkage of
    defect that might occur during setting, so this is what we do after de-waxing.




                                                                                    20
Continue,

   Polymerization shrinkage: is reduced due to
    the presence of pre-polymerized grains or
    beads rather than the monomer itself.
   Heat generation occurs due to the
    polymerization reaction (80 kJ/mol). This is
    reduced due to the presence of the already
    polymerized resin.



                                                   21
Continue,
   MMA monomer:
       Volatile
       Flammable & also irritant for the skin or the mucosa.
   Precautions:
       Kept in a sealed container
       Dark container away from light source or heat
        source or flame
       No contact should be allowed with the polymer
        beads (carry benzoyl peroxide initiator)
    Sometimes if the processing is incorrect, there will be residual monomer
      >>> processing should be accurate, otherwise if there is residual
      monomer remaining un reactive, it will cause irritant to the skin & the
      mucosa inside the pt’s mouth.

                                                                                22
II.         Cold cure resin

   Composition: identical to heat cure resin
    except polymerization is initiated by tertiary
    amine, it will react with benzoyl peroxide & produce free radicals (e.g.
    sulfinic acid or dimethyl-p- toluidine). They
    contain an inhibitor (hydroquinone) that
    destroys free radicals to prolong working time
    to give us time to work with the dough
   So no heat but chemical –tertiary amine- produce the free radicals & there is also
    inhibitors.




                                                                                         23
Cold cure resin used for making special
trays




                                          24
 Cold VS heat cure
Cold cure powder is a little bit different from heat cure powder

   Smaller polymer beads are used in cold cure
    resin to ease dissolution (easy to mix) in monomer there
    is no heating & we need it to mix very well with the liquid& the end result the denture will
    have a lower Molecular Weight, so it have less strength sometimes there is residual

                                        .
    monomer because there is no heat to evaporate residual monomers

   Cold curing method results in a polymer with a
    lower molecular weight leading to:
       Lower strength
       Residual uncured monomer
       Lower color stability color might change with time, more prone to
        yellowing
       More prone to creep and therefore distortion so more
                                                                                              25
        susceptible to distortion, so the fit will get worse.
III.       Pour and cure resin: not used much. Mixed and poured
           into a mould of hydrocolloid elastic material. Excellent
           reproduction of details as it is more soluble but inferior to cold
           and heat cure resin in other aspects but it is not very strong as it is
           very flowable.

IV.        Light cure resin (blue light): used as denture hard
           relining material, repair material, construction of trays.
           Different components so that they are sensitive to light so when they are subjected
           to light they become harden & no need of heat. We cover it by a translucence
           material to allow the light to get in, usually the light cure resin come in form of sheets
           as one component so we don’t mix the powder & the liquid. You adapt the sheet to
           the cast (model), you cut the excess, set the teeth & then cure it. & usually in clinics
           or labs we use this type of resin to make special trays (in our center we don’t use it
           to make dentures but to make special trays.)

 Composition:
              UDMA
              Colloidal silica to control flow
              Filler of acrylic beads                                                          26
 Manipulation
1.       Powder: liquid, THE GOLDEN RULE:
         always use powder: liquid ratio recommended
         by manufacturer. (2.0/1.0 wt %, 1.6/1.0 vol.%)
         to keep polymerization shrinkage at 5-8%
          Too much powder the mix will be too dry, there will not be good mixing
           or good wetting of the powder by monomer

          Too much monomer lead to irritation at the end to the pt & if it is too
           much it evaporates while the material is still setting & cause porosity like bubbles
           inside the denture.
          Porosities inside the denture cause it to become weaker & if you can see it, it is
           unaesthetic.
          You don’t need a dry mix or the mix to be too wet but you want a proper
           proportioning.


                                                                                             27
Manipulation
2.   Control of color: pigments responsible for
     the color are incorporated into the polymer,
     sometimes on the beads surface and can
     be washed away if incorporated into
     monomer too quickly
3.   Mould lining: to prevent penetration of resin
     into plaster, separating medium used e.g.
     sodium alginate, tin foil, vasline.
4.   If the resin go inside the pours of plaster & set & become hard, it will be
     difficult to separate the 2 flasks from each other, as the flask is containing
     the model & the model has teeth set on top so between the cast & the
     flask there is space & this space is filled with plaster which has pours.

                                                                                 28

								
To top