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biomaterials > Polymer Composites 2 > Flashcards

Polymer Composites 2 Flashcards

1
Q

Definition of a composite

A
  • Two or more dissimilar materials to achieve a new complex
  • These are chemically and physically different constituents with a distinct interface separating them
  • One or more discontinuous phase embedded in a continuous phase
  • The discontinuous phase is the reinforcing agent which is usually stiffer and stronger than continuous phase
  • The overall properties are different in type and magnitude than those of the separate constituents
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2
Q

Properties of polymeric continuous phase?

A

Matrix which is tough and ductile

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3
Q

Role of the polymeric matrix in composites?

A
  1. Transfer stresses to the load bearing fillers
  2. Protect the fillers from damage
  3. Maintaining the desired spacing of the fillers
  4. Reduce the tendency to transmit stress concentrations from broken fillers to intact fillers
  5. Allow for controlled interfacial bond failure between the polymer and filler (absorbing additional energy)
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4
Q

Factors affecting the properties of composites?

A
  1. Geometry of filler (e.g., particles, platelets, short fibers, long fibers)
  2. Distribution of the filler (e.g., homogeneity of dispersion)
    3, Properties of constituents (e.g., matrix and filler)
  3. Amount of constituents (e.g., volume fraction)
  4. Quality of interface (e.g., poor adhesion leads to void formation therefore lower density)
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5
Q

Overview of Dental Composites

A
  • Developed 1940s
  • Better aesthetics than dental amalgam
  • Cheap, insoluble, easy to manipulate
  • Mixtures of fillers and monomers are cured under visible light activation through polymerization
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6
Q

What are the current clinically used components of dental composites?

A
  1. Continuous phase matrix: organic resin
  2. Discontinuous phase: inorganic filler
  3. Adhesion promoters: silane coupling agents
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7
Q

What are the common monomers used as dental composite resins?

A
  1. Bis-GMA
  2. UDMA
  3. TEGDMA
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8
Q

What are some issues with the resin matrix Bis-GMA, and how can it be solved?

A

Because of its high molar mass, Bis-GMA has a high viscosity at room temperatures. Di-methacrylate is used as a diluent and allow for crosslinking to occur between chains, which increases resistance to solvent attack.

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9
Q

How is polymerization of dental composites carried out?

A

Free radical chain polymerization activated by light

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10
Q

What are the steps of polymerization of dental composites?

A
  1. The pre-polymer components can be kept in a single paste system
  2. A special photo-initiator and activator is used in a light activated polymer
  3. When exposed to light (468nm) polymerization takes place at increments of up to 2mm thick samples
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11
Q

Describe the change in modulus of composites as a function of curing time? (graph)

A

Modulus vs. Time

Paste starts with low modulus, curves up during polymerization, and ends at a high modulus when the composite is cured

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12
Q

applications of dental composites

A

Interface
Oral fluids
Restoration

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13
Q

Do dental composites experience shrinking? Which formula is used?

A

Yes

The polymerization volume shrinkage formula

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14
Q

What are the factors that affect shrinking? (same for dental polymers as regular PMMA bone cements)

A
  1. Number of C=C in the monomer relative to the molar mass
  2. Amount of filler
  3. Volume of composite
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15
Q

What are the classifications of dental composites and corresponding average filler particle sizes?

A
  1. Traditional composites, 8-12um
  2. Small particle-filled composite, 1-5um
  3. Hybrid composite, 0.6-1.0um
  4. Microfilled composite, 0.04-0.4um
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16
Q

How are fillers generally produced?

A

Grinding or milling of quartz or glasses to produce particles 0.1-100um
Difficult to grind down due to hardness of quartz, composites have less polished surfaces which can cause abrasions

17
Q

Describe the aspects of pyrogenic silica fillers discussed in class?

A
  • Silica micro-fillers of colloidal size (0.04um) can be obtained by pyrolytic process
  • Colloidal silica particles have a large surface area (50-300 m^2/g)
  • The large surface area increases the polar bonds with monomers, increasing the viscosity
18
Q

Rank the compressive strength for the different classes of dental composites?

A
  1. Small particle (350-400 MPa)
  2. Hybrid (300-350 MPa)
  3. Microfilled (250-350 MPa)
  4. Traditional (250-300 MPa)
  5. Unfilled acrylic (70 MPa)
19
Q

Rank the tensile strength for the different classes of dental composites?

A
  1. Small particle (75-90 MPa)
  2. Hybrid (70-90 MPa)
  3. Traditional (50-65 MPa)
  4. Microfilled (30-50 MPa)
  5. Unfilled acrylic (24 MPa)
20
Q

Rank the elastic modulus for the different classes of dental composites?

A
  1. Small particles (15-20 GPa)
  2. Traditional (8-15 GPa)
  3. Hybrid (7-12 GPa)
  4. Microfilled (3-6 GPa)
  5. Unfilled acrylic (2.4 GPa)
21
Q

What are adhesion promoters/coupling agents used for?

A
  • Adhesion promoters are used to wet the fillers and prevent agglomerates and void formation
  • The primary function of a coupling agent is to provide a strong chemical link between the oxide groups on the glass filler surface and the polymer
22
Q

How to adhesion promoters perform their function?

A
  • Reacts with both the polymer matrix and filler’s silanol groups by forming a siloxane (chemical) bond
  • Increase strength as stress is transferred from matrix to filler
  • When hydrated, silanol groups attach to the filler surface while the polymer matrix groups covalently attach to monomer
23
Q

silane coatings create which type of bond?

A

The silane coating leads to a strong water-resistant bond

24
Q

What happens to the glass-resin interface without a silane coupling agent?

A
  • The glass resin interface deteriorates rapidly in the presence of water which diffuse through the resin and attack through hydration
  • This impacts the interface between polymer/particles hydrolytic breakdown, loss of bond, high rate of wear
  • Causes swelling of the composite leading to cracking, leaching out of low molar mass components
25
Q

Describe the steps of silanisation of a filler particle surface

A

A) A H bond occurs btwn the hydrolyzed silanol and the OH group at the surface of the particle, another H bond occurs with the carbonyl group
B) A condensation reaction occurs between the silanol group and the surface releasing water and creating covalent bond
C) The other H bond orients the silane molecule parallel to the surface

26
Q

Requirements for monomers for dental composites

A

[LOOK AT TABLE IN SLIDES]

27
Q

Material requirement: Low volume polymerization shrinkage

A

Effect on Dental Practice:

Easier processing and no marginal gap

28
Q

Material requirement: Higher rate of polymerization

A

Effect on Dental Practice:

Shorter curing times

29
Q

Material Requirement: Crosslinking

A

Effect on Dental Practice:

Sufficient physical and mechanical properties

30
Q

Material Requirement: Greater resistance to oral conditions

A

Effect on Dental Practice:

Lower failure rate

31
Q

Material Requirement: Tg > 60 Deg C

A

Effect on Dental Practice: Longer term durability

32
Q

Material Requirement: High light and coloration stability of polymer

A

Effects on the Dental Practice: Longer term esthetics

33
Q

Material Requirement: Low toxicity

A

Effects on Dental Practice: Minimum toxicological risk for patient

biomaterials (18 decks)

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