Biomaterials 2 (Polymers and Waxes)

Question 1

List the four categories of natural waxes.

Answer 1

• Mineral
• Plant
• Insect
• Animal

Question 2

Examples of mineral wax

Answer 2

Paraffin, microcrystalline, Barnsdahl, Ozokerite, Ceresin, Montan

Question 3

Examples of plant wax

Answer 3

Carnauba, Ouricary, Candelilla

Question 4

Examples of insect wax

Answer 4

Beeswax

Question 5

Examples of animal wax

Answer 5

Spermaceti

Question 6

List the three classifications of dental waxes

Answer 6

• Pattern Waxes
• Processing Waxes
• Impression Waxes

Question 7

Distinguish Pattern Waxes

Answer 7

• Used to form general predetermined size and contour of an artificial dental restoration
• Problems: Thermal change in dimension and tendency to warp on standing
• Inlay Wax
• Casting Wax
• Baseplate Wax

Question 8

Distinguish Processing Waxes

Answer 8

• Used as auxiliary aids in constructing a variety of restorations and appliances
• Boxing Wax
• Utility Wax
• Sticky Wax
• Blockout Wax

Question 9

Distinguish Impression Waxes

Answer 9

• Used to correct small imperfections in other impressions
• Corrective Wax
• Bite Registration Wax

Question 10

List the three major categories of additives used in dental waxes.

Answer 10

• Gums
• Fats
• Resins

Question 11

Function of gums

Answer 11

Viscous and amorphous and harden when exposed to air

Question 12

Function of fats

Answer 12

• Used to modify the melting range and hardness of waxes

- Have a pronounced effect on the properties of paraffin by softening the wax and allowing for ease of polishing

Question 13

Function of resins

Answer 13

• Used to modify the melting range, hardness, adhesiveness, flake-resistance, gloss, and toughness of dental waxes
• Also used to make films

Question 14

Identify the primary components in most dental waxes.

Answer 14

• Waxes
• Gums
• Fats
• Resins
• Paraffin

Question 15

Explain why waxes have a melting range instead of a melting point.

Answer 15

Since natural waxes consist of a mixture of many compounds, there is no single melting point, but rather there is a melting range.

Question 16

Advantage of waxes for use in dentistry

Answer 16

• Ability to be carved to a smooth surface and molded without cracking
• Can be formulated to yield products with specific and markedly different physical properties
• Soften when heated and solidify when cooled
• Can be readily molded to a desired form
• Are easily polished or flamed to give a high surface gloss
• Are nontoxic

Question 17

Disadvantages of waxes for use in dentistry

Answer 17

• Thermal expansion is very large and yield points are low since they are typically moderately large hydrocarbons with weak intermolecular forces
• Chain lengths are too long to permit good crystallization, but not long enough to allow entanglements like polymers
• Dimensional stability since waxes are viscoelastic and will continue to deform under applied stress when cooled and over time due to recovery from stresses frozen in

Question 18

Functions of Inlay Wax

Answer 18

Used to form direct or indirect patters for the casting of metal or hot pressing of ceramic inlays, crowns and bridge units using the lost wax technique

Question 19

Functions of Casting Wax

Answer 19

Used to fabricate patterns for the metallic framework of removal partial dentures and other similar structures

Question 20

Functions of Baseplate Wax

Answer 20

• Used to establish the initial arch form in the construction of complete dentures
• Bite rim to restore occlusal relationship, registering arrangement of teeth, and checking the denture inside the patient’s mouth.

Question 21

Functions of Boxing Wax

Answer 21

Used to form a dam around an impression to confine the stone poured into the impression to form a model of the edentulous arch and other types of impressions

Question 22

Functions of Utility Wax

Answer 22

• Beading impression trays
• Added to the borders of the tray to extend its length or height
• Placed around the tray periphery to protect and cushion soft tissues
• Placed on the posterior region of the maxillary tray to prevent alginate from flowing down the throat

Question 23

Functions of Sticky Wax

Answer 23

Used where rigidity and adhesion are important, e.g., for temporarily holding broken dentures together during repair, to obtain adhesion between casting waxes and stone models

Question 24

Functions of Blockout Wax

Answer 24

Used for filling the undercut area on the cast during processing of the Cr-Co framework

Question 25

Functions of Corrective Wax

Answer 25

Used in the corrective impression technique in partial and complete denture prosthesis, to restore regions in the edentulous patients, and to reproduce the details of mucous membrane.

Question 26

Functions of Bite Registration Wax

Answer 26

Used to record the relationship between the upper & lower teeth in dentulous patients

Question 27

Characterize the magnitude of solidification shrinkage of waxes and explain the effect of this property on the selection of acceptable manipulative techniques for dental waxes.

Answer 27

• Unusually large, about 11% to 15% for paraffin
• Accurate patterns require that either the wax be molded in bulk, in the plastic state (i.e., softened but not melted), or that steps are taken to assure that solidification shrinkage occurs only in areas and in directions that are not critical (e.g., by adding thin increments of melted wax to build up to the desired shape - incremental build-up)

Question 28

Characterize the magnitude of the thermal expansion of dental waxes and explain why most of the thermal expansion curves are not linear.

Answer 28

• Largest coefficients of thermal expansion of all dental materials
• Most curves are not linear because thermal expansion is not linear due to weak secondary valence forces

Question 29

Relate thermal expansion and conductivity of dental waxes to manipulation techniques.

Answer 29

• Large thermal expansion coefficients may contribute to dimensional inaccuracies, so waxes are formulated to undergo transformation to a plastic (moldable) state at temperatures only slightly above the temperatures at which they must be rigid. Heating above the temperature required to reach a moldable condition results in excessive shrinkage on cooling.
• Low thermal conductivity means that heat moves through wax very slowly, and heating must be gentle and over a long enough time to ensure that the entire mass is made homogeneously plastic (soft) while not melting.

Question 30

Define solid-solid transformation as a function of temperature in dental waxes and explain its relation to proper manipulation technique.

Answer 30

Waxes can have multiple solid-solid transformation temperature. Waxes may be manipulated above the solid-solid transformation temperature. If the upper limit of the solid-solid transformation range is exceeded, the melting range will be entered and some components of the wax will begin to melt. If there is melting, high contraction and distortion will occur upon cooling and solidification. Thus, great care should be taken not to exceed the solid-solid transformation temperature range when, for example, using the bulk-molding method for making wax patterns in the direct technique.

Question 31

Define flow (creep) and relate the property to the expected outcome of a described wax pattern fabrication technique.

Answer 31

-Flow (also known as creep) is a time dependent response to stress that can result in large plastic deformations over a long period of time in response to stresses that are so low as to produce negligible immediate elastic or plastic deformation. It exists due to the slippage of molecules over each other due to the low intermolecular valance forces.
-Flow is especially important
in direct inlay waxes because it must have relatively high
flow a few degrees above mouth temperature to allow it to be molded, but must not be uncomfortably warm to the patient.
-At mouth temperature, it
must be below the
transformation temperature and the flow must be low to
minimize distortion during removal. However, flow will
never be negligible, and application of even low stresses over prolonged periods to already-formed patterns must be avoided.

Question 32

Describe the relationship between wax flow and solid-solid transformation temperature.

Answer 32

If the upper limit of the solid-solid transformation range is exceeded, the melting range will be entered and some components of the wax will begin to melt. If there is melting, high contraction and distortion will occur upon cooling and solidification.

Question 33

Define ductility and describe its relationship with melting point and melting range.

Answer 33

• Ability to deform under tensile stress
• Ductility of waxes increases with temperature; waxes with lower melting points or melting ranges that begin at lower temperatures typically have greater ductility at a given temperature; waxes and blends of waxes with wider melting ranges generally have greater ductility.

Question 34

Define stress relaxation and explain the relationship between this property and proper or improper wax pattern fabrication technique.

Answer 34

-After molding is complete and the pattern or impression is finished, slow deformation will also occur in the absence of external stresses or elevated temperatures due to the relaxation of stresses that were frozen into the wax pattern.
-Heating and shaping the wax quickly creates
residual stress, and after time, this stress relaxes
creating distortions. Consequently internal stresses
must be avoided by assuring completely uniform
heating and plasticity when using the bulk-carve technique, or by using the wax additive technique in which each increment anneals the previous increment.

Question 35

Define Crosslinked Polymers

Answer 35

• Crosslinks are permanent connections between chains.
• Theoretically, a highly crosslinked polymer can be one giant molecule. Crosslinking happens either because there was a “crosslinker” that formed bonds between the chains during polymerization, or the monomers had 2 or more reactive sites allowing the formation of branches able to connect with neighboring chains.

Question 36

Define Copolymers

Answer 36

• Polymers formed by using more than one type of monomer.
• Usually the order of the different monomers is random (random copolymer), but they can be formulated where a large number of one mer type is connected to another mer type (block copolymer)

Question 37

Define Tacticity

Answer 37

Describes the relative position of the side groups or branches of the polymer from one unit to the next

Question 38

Define Isotactic Polymer

Answer 38

This is when the relative position of the side groups or branches of the polymer is always the same.

Question 39

Define Syndiotactic Polymer

Answer 39

This is when the sequence of the relative position of the side groups or branches of the polymer is alternating.

Question 40

Define Atactic Polymer

Answer 40

This is when the substitutes are randomly arranged.

Question 41

Define Molecular Organization

Answer 41

• Molecular organization can also affect material and mechanical properties.
• In some polymers the chains can align themselves to form a highly ordered (i.e., crystalline) structure.
• In others the chains are randomly coiled and entangled into an “amorphous” structure.
• Many polymeric materials combine these two forms of organization.

Question 42

Define Elastic Recovery

Answer 42

This is when the material is deformed but rapidly returns to the original shape like a rubber ball. It occurs in the amorphous regions of polymers where the randomly coiled chains straighten and then recoil and return to their original size and location in the material.

Question 43

Define Plastic Deformation

Answer 43

This is when the material is deformed and molded into a new, permanent shape like stretching silly putty. It occurs because polymer chains slide past one another and become relocated within the material, resulting in permanent deformation.

Question 44

Distinguish the effects of chain length, chain branching and crosslinking on the mechanical properties of a polymer.

Answer 44

• The longer the polymer chain the greater the number of entanglements (temporary connections) among chains. Thus, the longer the chains the more difficult it is to distort the polymeric material; consequently, rigidity, strength and melting temperature increase (there is a strong analogy with spaghetti).
• Branched and crosslinked polymers have stronger material/mechanical properties as compared to linear polymers.

Question 45

Define Thermoplastics

Answer 45

These are materials that can soften (flow or melt) when heated and can be molded. The process is reversible since no new chemical bonds are formed.

Question 46

Define Thermosets

Answer 46

These are materials whereby heat causes polymerization and crosslinking or just crosslinking to make new bonds that lock the material into a new shape. The process is non-reversible (why?). Applying heat speeds the solidifying reaction but once solidified, the material will not melt.

Question 47

Define Curing

Answer 47

This is the chemical reaction in which low molecular weight materials (monomers or small polymers) are converted into higher molecule weight materials (polymers) in order to attain desired properties. It is also called polymerization. This process may also involve crosslinking.

Question 48

Define Setting

Answer 48

This is the extent to which curing has progressed.

Question 49

Define Viscoelastic Behavior

Answer 49

When a tensile load is applied, the random coils in the amorphous regions are straightened and chains become oriented. In effect, this material becomes more crystalline, making it stiffer in tension. With further drawing, chain slippage occurs and the deformation becomes plastic.

Question 50

Define Final Set

Answer 50

This is when the reaction (polymerization) is complete.

Question 51

Define Accelerator

Answer 51

This is a catalyst that makes the initiator unstable at ambient (i.e., normal) temperatures. Therefore, additional heat is not needed. In dental materials, they are usually aromatic amines. They are always found in “self”-curing (also called “cold”-curing or “chemical” curing) polymer resin systems. They must be mixed with the initiator (“catalyst”) before polymerization will begin. Thus, two part systems are used. One part has the initiator, the other the accelerator.

Question 52

Define Glass Transition Temperature

Answer 52

• At a certain temperature, called the glass transition temperature (Tg), there is a change from being glassy and brittle to ductile or rubbery. This is not due to the polymer melting. The melting point of the polymer is much higher.
• Can also be defined as the temperature where there is a sudden alteration in the rate change of specific volume

Question 53

Distinguish the effects of the three types of polymer tacticity on polymer molecular orientation, crystallinity, mechanical properties, and optical properties.

Answer 53

• Due to the repeating structure of isotactic and syndiotactic polymers, they can readily form regular arrays and crystallize.
• Crystalline regions are highly ordered, hard and rigid, high melting point, and are transparent or translucent
• Amorphous regions are random coils, soft and flexible or glassy, low glass transition temperature, and transparent
• Combination are translucent or opqaue

Question 54

Distinguish the effects of polymer crystallinity on polymer mechanical and optical properties.

Answer 54

Crystalline regions are highly ordered, hard and rigid, high melting point, and are transparent or translucent

Question 55

Describe the 4 steps of addition polymerization.

Answer 55

• Step 1: Activation - Application of energy (heat or light) to cause an initiator (“catalyst”) to chemically decompose and form free radicals (very reactive molecular fragments).
• Step 2: Initiation - Breaking of monomer double bonds causing them to begin reacting with other monomers.
• Step 3: Propagation - Continuation of the addition of monomer to the activated, growing chain.
• Step 4: Termination - Two free radicals can react, a stable molecule then forms and the reaction stops.

Question 56

Compare and contrast advantages and disadvantages of addition and condensation polymerization.

Answer 56

• Addition Polymerization is when monomers are added sequentially to another in a chain-growth reaction. Polymers produced by this method are simple multiples of the monomer.
• Condensation Polymerization is also called step growth polymerization, where all monomers react simultaneously. A catalyst is used to activate the monomer and small molecules (e.g. water and HCl) are usually eliminated from the polymer chain (“condensed out”). Thus, the unit parts of the polymer have a slightly different chemical formula than the original monomer. Examples include some elastomeric impression materials.

Question 57

Describe how ‘spherulites’ form when a crystalline polymer forms during polymerization or when a crystalline polymer cools from a melt.

Answer 57

When a crystalline polymer is polymerized, the crystalline regions begin forming at the center of the spherulite. This phenomenon can also be observed when a polymer is cooled from a melt.

Question 58

Relate how the rate (fast or slow) at which a force is applied to a viscoelastic polymeric material affects its tendency to be deformed or recover.

Answer 58

• Fast rate (impact) – Viscoelastic substance will tend to exhibit elastic recovery after an impact force has been applied. If substance is very brittle, an impact force could cause fracture.
• Slow rate (squeezing) – Applying the same amount of force as seen in the impact force, but over a longer duration, may cause the substance to deform plastically, as longer-term stress may cause crystals to slide past each other and amorphous regions to either clump or stretch.

Question 59

Pick out the three factors that affect swelling in polymeric materials.

Answer 59

Polymers tend to absorb a solvent, swell and soften rather than dissolve, but as chain length decreases, they will dissolve. The longer the chains, the slower a polymer dissolves and crosslinking prevents complete chain separation and inhibits dissolution. Thus, highly crosslinked polymers cannot be dissolved. Amorphous polymers swell more than crystalline polymers, and elastomers swell more than plastics (Find out the definition of elastomer and why it swells more.) Even a small amount of swelling in dental devices can have undesirable results.

Question 60

Distinguish material properties, such as specific volume and mechanical properties, and molecular motion above and below the glass transition temperature (Tg).

Answer 60

• Specific Volume – As the temperature placed on the glassy material reaches the Tg, the once steady change in specific volume will change to a much higher rate of volume increase.
• Mechanical Properties – The transition will begin with the hard glass steadily changing into a soft glass, but once the Tg is reached, the soft glass will rapidly become leathery, then rubbery with a slower transition to rubbery semi-solid flow, and finally transform into a liquid (fusion)
• Molecular Motion – As the material transitions from glassy to soft glass, the first changes will be from bond stretching and bending, followed by movement of side-groups. Once the Tg is reached, chains will rapidly begin to coil and uncoil until a rubbery stage is reached. Once enough heat is applied, chain slippage will occur from the rubber, allowing the material to flow. Thus, at low temperature, the stiffness of primary valance bonds dominates the behavior, and at high temperatures, chain slippage gives rise to a very low modulus. Note that these properties are independent of chain length but rather dependent on the ability of the chain segments to rotate due to thermal energy.

Question 61

Define Initiator.

Answer 61

In the activation step, the initiator (“catalyst”) will chemically decompose and form free radicals (very reactive molecular fragments).

Question 62

Define an elastomer and why it swells more than plastics.

Answer 62

An elastic polymer that exhibits viscoelasticity and has very weak intermolecular forces, low stiffness, and high failure strain. Elastomer are made out of monomers with a very low melting temperature which might suggest very weak interactions between the monomers and in consequence in the elastomer molecule itself. Given the fact that the elastomer molecules are crosslinked, they will behave in a rubbery way instead of flowing in a plastic nature. Crosslinking of elastomers contributes to its increased swelling capabilities