Flashcards in Biomaterials 3 (Gypsum Products) Deck (47):
Gypsum is a white to milky yellowish mineral that is mined from the earth in the _______ form. The form used in dentistry is the _______, which is produced by heating the dihydrate.
-dihydrate form (CaSO4•2H2O)
All forms of gypsum including alabaster, plaster, dental stones and die stones are chemically the same; they only differ by ______, _______, and _______.
additives, particle size and shape, and density
The hemihydrates are mixed with _____ to undergo an exothermic reaction to revert back to the dihydrate before use.
-properties can be easily modified
-reproduce medium-fine detail
-have good dimensional accuracy and stability
-have good contrast with colored waxes
-inexpensive and easy to use
-adequate strength for most uses.
why are gypsum products one of the most widely used materials?
-poor abrasion resistance, which is important in a die material
The only major deficiency of gypsum products
-chemistry of the setting reaction
-factors affecting dimensional accuracy and porosity
Important concepts to understand for dental applications include...
• Plaster models
• Dental stone - form die to duplicate oral anatomy when poured into any type of impression
• Binder - for silica, gold,
and low-melting nickel- chromium casting investments
• Soldering investment
• Mold material -
processing complete dentures
The conformation and
density of the hemihydrate
particle determine the
amount of water necessary to produce a workable mix. That water/powder ratio ultimately determines the relative _____ of the set product, with less water producing the _____ product.
• Made by heating the dihydrate to form the hemihydrate in open kettles at 110 ̊-120 ̊C (calcined)
• Produced as beta particles
• Has poor crystallinity. As the dihydrate is calcined, the water
boils away creating a lack of medium for the ions to diffuse and
form crystals; hence, the plaster has poor crystallinity.
• Powder is irregular shaped and porous
• Weak: 9 MPa after 1 hour
• Most setting expansion: 0.3%
• Optimum water/powder ratio: 50 ml/100 g
Type II - Model Plaster
• Prepared by dehydrating gypsum under pressure in the presence of water vapor at 125 ̊C
• Powder particles are uniform in shape and more dense
• Produced as alpha particles
• Stronger: 21 MPa after 1 hour
• Less expansion: 0.2%
• Optimum water/powder ratio: 28-30 ml/100 g
Type III - Dental Stone
• Made by dehydrating in boiling 30% calcium chloride and washing away chlorides with hot water (100 ̊C)
• Alpha particle: more regular
• Densest powder
• Strongest: 35 MPa after one hour
• Lowest expansion: 0.10%
• Optimum water/powder ratio: 22-24 ml/100 g
• Contains extra salts to reduce setting expansion
Type IV - Improved/Die Stone
• Made in the same way as Type IV
• Similar properties as Type IV but stronger (48 MPa)
• Does not contain the extra salts that reduce setting expansion
• Has higher expansion (0.3%) than Type IV
• Optimum water/powder ratio: 18-22 ml/100 g
Type V - High Strength/High Expansion Dental Stone
During the preparation of stones, the boiling point of the liquid is _____ substantially either due to the higher pressure or the presence of calcium chloride (or other salts). This allows more liquid water to be present as the dihydrate decomposes to the hemihydrate permitting the formation of well-formed, regular, non-porous crystals. These particles pack better than plaster particles and allow for the use of smaller volumes of water during processing for clinical use.
Figure 3 shows the change in the composition of gypsum to plaster of Paris (the hemihydrate form) to an anhydrite with two
crystal forms depending on the temperature it is heated to. The orthorhombic form reacts very slowly with water and does not form a useful setting mixture with water. Because the water molecules in the dihydrate are found in alternate double layers with the calcium and sulfate ions, its removal results in the destruction of that crystal. Also, the need for heat (energy) for this process shows that the dihydrate form is a more stable form than the hemihydrate.
Dental gypsum products rely on the ______ of the hemihydrate to produce a less soluble dihydrate. Here the hemihydrate dissolves easily in water, the solution soon becomes supersaturated with the dihydrate, and with appropriate nucleation, begins to precipitate from the solution by forming needle-like crystals. The precipitation prevents the solution from being saturated with the hemihydrate and allows any remaining hemihydrate to continue to dissolve. This process continues until all the hemihydrate or water has reacted or until the remaining hemihydrate has been occluded by the precipitant.
This setting reaction is ______. When the mixing water becomes saturated with the hemihydrate upon mixing, the precipitation of the dihydrate does not happen immediately because of the ________ required to form new crystals (nucleation). This delay is called the ______.
- induction period
It takes less energy to add new ions to the surface of existing crystals and so once precipitation has been initiated, crystal growth is only limited by the ______ to the surfaces of the crystals and so progresses quickly. This crystal growth generates heat and so the greatest rate of temperature rise coincides with the greatest rate of crystal growth, which is some time after the start of mixing when sufficient nucleation has taken place. This change in temperature can be used to monitor the progress of the reaction.
diffusion of free ions
The first few ions that precipitate must go into the correct ______ or crystal growth cannot occur. These ions may precipitate onto random sites on other types of solid materials or small crystals of the same material. Nucleation that involves another solid material is called _______ and that which is formed spontaneously in solution is called _______. Homogeneous nucleation occurs much more slowly. Other factors affecting the setting reaction are discussed below.
The final setting time is defined as the time required for the reaction to be _______, i.e. completely set and can be separated from the impression without distortion or fracture. However, this is difficult to define since no material exhibits an abrupt end to the reaction, and setting reactions may continue for hours after physically being able to be separated from an impression.
Measurement of setting time is useful since if the rate of reaction is too fast, then the mixed mass may harden before the operator can manipulate it. To further aid this assessment, an initial setting time is defined as the time required for the mass to reach a certain arbitrary state of firmness and it can be measured using a ________, a loss of gloss from the surface of the stone due to the incorporation of water into the dihydrate crystals, or the cooling of the temperature.
Two penetration tests
-Gilmore needle test
Basically, the mass is penetrated as it is setting with a needle or a thin rod at a certain force. When a 2.12 mm diameter Gilmore needle weighing 113.4 g can no longer penetrate the gypsum, it is said to be in ______. When a 1.06 mm diameter Gilmore needle weighing 453.6 g can
no longer penetrate the gypsum, it is said to be in ______.
The setting reaction of gypsum products is driven by the higher solubility of the ______ (CaSO4•1/2H2O) compared to the dihydrate (CaSO4•2H2O). At 20 ̊C the hemihydrate is 4 times as soluble as the dihydrate. At 100 ̊C the solubilities are equal, and no setting reaction will occur. Thus, the dissolved calcium sulfate precipitate as calcium sulfate dihydrate
This setting reaction can be controlled by controlling the solubility of the reactant and product, the ____ of nucleation of new product crystals, the crystal ______, the crystal rate of _____, and the amounts of reactants.
increase the dissolution rate of the hemihydrate, thus increasing the ratio of the solubility of the hemihydrate to the dihydrate and decreasing the setting time (increasing the setting rate). These salts and ions are added by the manufacturer to control setting time, and is the most reliable method. Figure 7 shows the effect of different salts on the setting rate. Thus, distilled water is recommended over tap water for use in the setting reaction since tap water may contain various ions that affect setting rate.
Accelerators (K2SO4, NaCl)
inhibit crystal growth. Borax induces the precipitation of a layer of calcium tetraborate on the hemihydrate particles, reducing their dissolution rate and setting rate.
Retarders (Borax, saliva)
accelerates dissolution below 40 ̊C, mainly reducing the induction period, but is a retarder above 50 ̊C. The solubility of the hemihydrate decreases with increasing temperature while the solubility of the dihydrate increases until 40 ̊C before it decreases. Heat also affects the ion mobility. As the temperature increases, ion mobility increases, accelerating the setting reaction. Thus, the ratio of solubility of the hemihydrate to the dihydrate and the ion mobility both play a part in the setting time. Generally, as temperature is increased to 40 ̊C the ratio decreases but the effect of the increase in ion mobility is greater and so the reaction accelerates slightly. When the temperature increases above 40 ̊C, the decrease in ratio outweighs the increase in ion mobility and the reaction is retarded.
Water addition (water/powder ratio = W/P) - decreased W/P --> saturation is reached sooner --> setting time shortened. Less water = _________
shorter setting time
a. The manufacturer includes some dihydrate for nucleation
b. Rapid and/or prolonged mixing creates more nucleation sites and decreases setting time.
c. "Slurry" water contains nucleating dihydrate particles.
d. Damp storage allows the dihydrate to form - thus producing unwanted nucleation sites and a
setting reaction that is more rapid than expected. However, if sufficient dihydrate is formed on the hemihydrate powder, it can retard the dissolution of the hemihydrate and retard the reaction
Addition of nucleation sites (to accelerate setting time)
Different types of gypsum products differ in the _____ and ____ of the calcium sulfate hemihydrate crystals. Some are irregular and porous (model plaster Figure 1) and some are regular and dense (dental stone Figure 2). This difference makes it possible to obtain the same consistency for mixing with less amount of water for dental stones, resulting in increased strength.
shape and form
The theoretical (stoichiometric) ratio for complete conversion of hemi- to dihydrate is _____ hemihydrate. More than this, however, is required to make a mix of usable consistency that will flow into the details of an impression.
18.6 g water/100 g
This excess water is distributed as free water in the set mass without taking part in the chemical reaction and is necessary to wet the powder particles during mixing. If not enough water is present, the mixed mass is _____, more difficult to handle, and traps ______ easily when poured into the mold.
The water in excess of 18.6 g has no load bearing capacity and just increases the volume of void space in the set product, thus lowering strength. At the typical ratios used, _______ is strongest, stone is intermediate and plaster is weakest. If mixed at identical water/powder ratios, the strengths of the three products are identical because the chemistry of the material is the same. Thus, _______, which requires more water to achieve a mixable consistency, is more porous, less dense and has a lower compressive strength.
_____ of the excess water approximately doubles the compressive strengths of the products because the presence of excess water induces porosity
of the final product, and porosity will reduce the relative strengths of the material since neither water nor air has any strength.
Drying at normal room temperature and humidity requires about _____. Elevated temperatures speed drying and consequently attainment of maximum strength. However, above about 60 ̊C, water of crystallization (the two water molecules in Ca2SO4•2H2O) may be driven off along with excess water, reconverting part of dihydrate to hemihydrate, and weakening the cast.
If not enough water is present, i.e. volume fraction of excess water is below 0, then _______ are introduced while mixing, decreasing the relative strength. If too much water is introduced, _____ due to liquid-filled pores decrease the relative strength.
At the theoretical stoichiometric point, the relative strength should be theoretically the maximum. However, because of the difficulty in mixing at this ratio, the actual maximum is reached substantially _____ the theoretical maximum strength and also slightly above the 0 excess volume fraction. Note that increase in compressive strength corresponds to the increase in hardness of the material.
Theoretically, the hemihydrate should contract about 7% volumetrically and 2% linearly during setting, but in reality, there is actually a slight linear expansion of _____. This is because dihydrate crystals grow on the nuclei of crystallization in the form of radiating long needle-like crystals called spherulites
After a period of growth, the crystals growing from adjacent nuclei impinge on one another, thrusting the particles apart and causing the setting expansion. Since there really is both a volumetric contraction caused by the chemistry of the reaction and an expansion caused by the thrusting of the crystals, porosity is induced in the set material. Typically, ____ of the expansion occurs in the first hour of setting.
Setting expansion may be controlled by changing the interaction of the growing crystallites. It can be: ______ by moving the nuclei further apart – increasing the water/powder ratio of the mix.
Setting expansion may be controlled by changing the interaction of the growing crystallites. It can be: ______ by changing the habit of the growing crystals to a fatter, more readily compacted form – e.g.,
the manufacturer can include additives such as K2SO4.
Setting expansion may be controlled by changing the interaction of the growing crystallites. It can be: _______ by causing fewer crystals to grow by taking on a more needle-like, hence longer, shape – e.g.,
the manufacturer can include additives such as calcium acetate.
Setting expansion may be controlled by changing the interaction of the growing crystallites. It can be: ______ by water immersion to obtain "hygroscopic expansion." As the mixing water is used up, the continuous water phase is replaced by a surface film that coats each particle. Surface tension of this film tends to retard the growth of the crystallites. When water is added to the setting mass (powder + correct volume of H2O for mixing), the surface tension is relieved and additional crystal growth occurs. This additional expansion is called hygroscopic expansion. This effect is used in metal casting.
Setting expansion may be controlled by changing the interaction of the growing crystallites. It can be: ______ by increasing spatulation due to the creation of more nucleation sites.
In the absence of additives that the manufacturer adds, ______ would be the main determinate of setting expansion.
water to powder ratio
To retain surface detail, casts must not be subjected to operations that will abrade or dissolve the surface. _____ of wax patterns on dies is almost an unavoidable source of abrasion. Attempts to increase abrasion resistance by soaking in glycerin and oils have resulted in some improvement.