Denture base resins plasticizer

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					Manipulation
4.       Processing: porosity, processing strains.
common mistakes can be happened while you are making a denture. The most common one is
     porosities, and the problem in these is that they are unstable and they cause weakness of
     the denture. And there are many types of porosities and each one has a specific reason.
             Porosity: caused by,
         1.      Polymerization shrinkage (contraction porosity)
         2.      Volatilization of monomer (gaseous porosity), which is caused by
                 the evaporation of the monomer, which happens when boil the water much
                 higher than 100°, because the evaporation temp. of the monomer is 100.3° .
         3.      And usually this happened in the thick portion of the denture, which is the
                 palatal portion. Since these areas are thick, the temp. raised quickly and cause
                 evaporation. So, during processing, temp. should be raised slowly.
         4.      Granular porosity, due to loss of monomer while resin mix
                 is left to stand until dough stage is reached. Also if the
                 resin mix is dry and it’s because of 2 reasons: 1- adding too much powder
                 when it’s mixing with the monomer, so the mix will be dry. 2- the mix it correct
                 but when we don’t cover it after we finish mixing, there’ll be evaporation and
                 it’ll be dry also.

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1.     Contraction porosity: occurs due to monomer
       contraction (shrinkage) by 20% during processing.
        Processing involves a raise in temperature to
         initiate polymerization at first as it’s put in a boiling water
         and then temperature raised due to the
         exothermic reaction
        During this, resin flows (under pressure) into
         spaces created by curing contraction and it’ll be less
         that what we want. SO, excess resin is important to
         maintain this pressure.

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    Once resin becomes rigid, thermal contraction
     may occur (change from curing temperature to
     room temperature). Curing temperature for cold
     cure resin is lower than heat cure resin.
    Insufficient amounts of resin packed in the flask
     may lead to voids or porosity. Also resin should be
     packed in the DOUGH stage. Prior to that the
     resin would flow too rapidly and pressure is lost




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Porosity

2.        Gaseous porosity:
     1.     Caused by a rise in the resin temperature
            during curing above 100°C (> boiling
            temperature of resin)
     2.     Gaseous monomer forms and causes gaseous
            porosity
     3.     This is avoided by allowing a slow and
            controlled rise in temperature



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       Processing strains:
           Internal strains occur during processing of resin,                             and
            that’s because the pressure and raising of the temp. that you cause while working.

           These stresses and strains shouldn’t be allowed to relax completely, because     If it’s
            allowed, warpage, distortion and crazing (tiny
            surface defects) occur.
           Some are relieved as the material flows but thermal
            contraction strains may remain. This can be minimized by:
               Slowly cooling flask (cooling and warming)
               Using acrylic rather than porcelain teeth to ensure
                compatible shrinking . Porcelain and acrylic resin has different
                coefficient of contraction and expansion. Now, this difference can cause stress
                inside the material if we use porcelain teeth. But if we use the acrylic one, the
                coefficient will be similar; so, there’ll be no stress created inside the material.


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Properties of resin

   Biocompatibility:
       High, however, allergy may occur due to
        leachable components mainly the monomer and
        benzoic acid.
       Allergy is mainly associated with cold cure resin
        due to high residual monomer
       As a replacement, denture bases maybe
        constructed from polycarbonate



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Properties continue,

   Dimensional stability and accuracy:
They are dimensionally stable if the patient takes care of them (putting them in a humid
    environment) and if our processing was correct according to the temp. and pressure.
    Otherwise, they’ll loose water and the shape might change.

       What is the difference between retention and
        stability?
       It is important for the denture to be retained
        intraorally. Why?
           Accurate fit to ensure good adhesion (large surface
            area) and cohesion (accurate fit)
           To ensure good peripheral seal (all of these things we take them
            next year)


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Properties continue,

   Mechanical properties: one of the properties of the resin material is low impact state,
    which mean, if the patient accidentally drops the denture, it’ll easily break.
     Creep is a problem, which is changes in shape and it’s susceptible to distortion. It’s
      minimized by cross-linking agents
     Dentures are prone to fracture
                                               Tensile strength 50 MPa
     Commonly, midline of upper
                                               Elastic modulus Low
    denture                                    Flexural                 2200-2500
                                               modulus                  MPa
     Mainly caused by:

           Trauma, leading to cracks then failure. So, if there are defects, pores, bubbles
            or tiny fractures inside the denture, the denture will be weaker.
           Poor quality processing: lack of bonding between resin and teeth
           Crazes



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Mechanical properties continue,
   Solution to patients who commonly fracture dentures:
       High impact resistant resin (contain rubber toughening
        agent), decrease crack, but the problem is that it may lower
        flexural modulus and lead to fatigue due to excessive
        flexure. And finally it might be broken.
       Incorporation of fibers to produce fiber reinforced resin:
           Carbon fibers: difficult to handle, poor esthetics
           Aramid fibers: lack of bonding with rein
           Ultra high molecular weight polyethylene fibers, UHMPE: low
            density, neutral color, biocompatible, bonds to resin but processing
            is time consuming
           Glass fibers: most promising, incorporated as short fibers or loose
            form.
        Sometimes, they add sheets of these fibers to make the denture stronger (specially the
           palatal area). The most common place that they put them is the midline (even upper
           or lower midlines), because the midline is easier to be broken.

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Physical properties
Thermal conductivity          Very low,       that’s why patients always
                              burn their mouths,   disadvantage:
                              Isolates tissue from
                              temperature sensation
Coefficient of thermal        High, if teeth are from
expansion (CTE)               porcelain, differential
                              expansion                loose teeth
Water sorption & solubility   Absorb water 1-2% wt.
                              slowly and we can lose it, so we should
                              put it in a humid environment.

                              Insoluble in oral fluids


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Denture lining material
After we make the denture and the patient used it; then after few years, it contacts telling that
    the denture is loosed or it’s not with the vertical dimension of the face. So, wrinkles will
    appear in the face and also the ability of eating will be affected as well.
Sometimes, there’s nothing wrong with the denture but still the patient is complaining from pain;
    then he/she just has to get used to it.

   Divided into:
       Permanent hard reline materials

       Semi-permanent soft liners

       Tissue conditioners/temporary liners


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Hard reline materials:

   Criteria for using it to reline dentures:
       Poor retention and stability
       Loss of vertical dimension
       Degradation of the denture base (destroyed for some reasons)
       For older patients for home getting use to a new
        denture base would be difficult (they can do relining to make it fit
        better)

       Lack of denture extension into mucobuccal fold
        areas (important for facial support)


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   Materials used:
   its composition is similar to the resin material. Sometimes it uses ethyl instead of methyl.
Ethyl is less irritant than methyl. So, they are different than each other; so, if the patient irritate
    from one of them, we should give him the other one.
       Heat cure resin, in the lab.
       Cold cure resin, chairside. (can be used in the clinic immediately)
        Disadvantages:
            Poor taste
            Poor color stability
            Exothermic reaction, it can cause irritation, so, we shouldn’t keep it in the
             patient’s mouth all the time; we place it for sometime and then we finish the setting
             out side.
            Lack of control over amount of denture removes & thickness of reline




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                  Two types of cold cure resin used

         Type I                         Type II (<irritant,
                                        < dimensional stability)
Powder   1.PMMA                         1.PEMA
         2.Benzoyl peroxide             2.Benzoyl peroxide
         3.Pigments                     3.Pigments

Liquid   MMA.                           Butyl.MA.
         Amine. Di-n-butylphalate       Amine




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Semi-permanent soft liners

   When is it used: In cases of discomfort and
    soreness from an otherwise satisfactory
    denture. Lasts for 6 months maximum.
   This discomfort is usually associated with the
    mandible due to small surface area,
    possibility of sharp, thin resorbed ridge
   Soft liner with absorb some of the masticatory
    forces


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   Desirable qualities in materials used:
       Rubbery
       Resilient
       Low elastic modulus
   Some materials (polymers) are naturally
    rubbery. Others can be modified by adding
    plasticisers
   Plasticisers: act as lubricants for polymer
    chains and make it easier for them to slide
    over one another, so material can deform
    easily. In other words, it adds some elasticity to the material. And if they leach out
    (lost), the material will be brittle.

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Semi-permanent reline materials:

   Silicon rubber: polydimethyl siloxane polymer+
    filler to achieve correct consistency
   The material solidifies by cross-linking rather
    than polymerization since its already a polymer
   An adhesive is needed to bond silicon to denture
    because they are from another material (not like the acrylic liners) . E.g.:

       Alkyl-silane coupling agent
       Silicon polymer dissolved in solvent
   Disadvantages: weak bond, encourage Candida
    albicans growth (susceptible for fungal infection)
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Semi-permanent liners
       Acrylic soft liners: can bind to denture base
    1. Leachable plasticizer systems, composition:
           Powder: mix of PMMA & PEMA
           Liquid: MMA with 25-50% plasticizer (dibutylphalate)
      Disadvantages and recommendations: plasticizer
       leaches out so I becomes stiff. Avoid using high
       temperature and strong bleaches
    2. Polymerisable plasticizer systems, advantage:
       resist dissolution. Hard at room temperature,
       softens in the mouth


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Relative merits of soft liners




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Tissue conditioners/temporary soft liners
If the patient feel a pain due to excess material or a hot one, we can use this type of liners to
        relief it. It has to be removed every 2 or 3 days because it has plasticizers.

         Usually needed in cases of tissue injury
          such as inflammation or ulceration.
1.        Tissue conditioners: soft material applied to
          fitting surface of denture to allow better
          stress distribution
     1.    Composition: PEMA+ ethyl alcohol solvent+
           plasticizer.
     2.    Needs to be replaced every few days due to
           leaching out of solvent and plasticizer


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   Disadvantages:
       Need for frequent replacement
       Prone to microorganism colonization
       Prone to damage by denture cleansers, so patient
        should be instructed to use plain soap and water




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    Acrylic teeth
                                          Construction considerations
   Advantages compared to                   Constructed in layers to
    ceramic teeth:                            simulate natural color
       Tough                                Gingival portion is
       Bond to denture base material         made from minimally
       Easy to grind during occlusal         cross-linked resin to
        adjustment                            ensure good bonding
       Do not wear natural, artificial       with denture base
        opposing teeth
       Easily repolished
   Disadvantages:
       Soft and easily wear
       Stain over time

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References

   Introduction to dental materials. Chapter 3.2
   Dental materials, clinical applications for
    dental assistants and dental hygienists.
    Chapter 13




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