irreversable hydrocolloids and dental gypsum products (complete) Flashcards
what is the common name for irreversable hydrocolloids
alginate
when is irreversable hydrocolloid used (alginate)
diagnostic impressions
facial moulages
removable prosthesis
used in wound healing to form non-adhesive bandages
What does irreversable mean (irreversable hydrocolloids)
once the impression is made, it can’t be resoftened and used again (one way gelation)
what do the two parts of hydro-colloid mean
hydro = water colloid = glue
are there reversable hydrocolloids
yes
when are reversable hydrocolloids used
impressions of teeth for crowns and bridges
what are the problems with reversable hydrocolloids
need special equipment
harder to do
fairly fragile
what are the benefits of reversable hydrocolloids
they are accurate
can be heated up, melted, then reused again
where does the agar component of alginate come from
giant kelp
what kind of reaction takes the alginate from the sol state to the gel state
a chemical reaction
why is alginate so widely used
because it is easy to use
what are the two things in alginate, the ratio of which determines the strength and elasticity of the alginate
mannuronate
guluronate
what is the alginate like if the ratio of mannuronate to guluronate is high
they are weaker and more elastic
what is the alginate like if the ratio guluronate to mannuronate is high
they are stronger and more brittle
What is the most abundent material in alginate
diatomaceous earth
what is diatomaceous earth
fossilized diatoms (hard shelled algae)
what is the purpose of diatomaceous earth
a filler material that is used to kill bugs
what are the two main chemical reactors in alginate
potassium alginate and calcium sulfate
are gypsum products versitile or not
very versatile
what are gypsum products used for in dentistry
making edentulous impressions making casts from impressions mounting casts on articulators investments for casting and soldering indexes for dental repairs
where do gypsum products come from
a rock that is formed from the evaporation of sea water, has calcium and sulfate in it. it is mined in open pit mines or quarries
what are the steps in getting from calcium sulfate dihydrate to gypsum(hardened calcium sulfate dihydrate)
- heat up calcium sulfate dihydrate
- gives up water and becomes calcium sulfate hemihydrate
- mix that with water and the reaction reverses, gives off heat, and hardens into gypsum (hardened calcium sulfate dihydrate)
is the reaction of calcium sodium hemihydrate with water that forms gypsum (calcium sodium dihydrate) endo or exothermic
exothermic
how do you know when your gypsum has sufficiently hardened
when the gypsum is no longer warm
What are the different types of dental stones
Type 2 = plaster (pourous plaster)
Type 3 = low to moderate strength
Type 4 = high strength low expansion
Type 5 = high strength high expansion
what are the characteristics of plaster
irregular crystal structure
porous
soft
less accurate
what are the characteristics of dental stone
regular crystal structure less porous harder greater compressive strength more accurate
what are the important parameters to evaluate gypsum
- fluidity/viscosity at pouring
- setting time
- setting expansion
- compressive strength
- reproduction of detail
- surface hardness
- abrasion resistance
what is the viscosity of gypsum
how runny it is,
the lower the viscosity the more runny
what is the setting time of gypsum
the amount of time it takes the gypsum to harden after mixing
why is setting time one of the most important characteristics of gypusm
because too short of a setting time doesn’t allow it to get set into the impression, and too long of a setting time means it takes too long
what is setting expansion of gypsum
the amount of volumetric expansion that occurs as the stone hardens
what would happen to a crown made on a stone that has high setting expansion
the crown would be too large
what is the compressive strength of the stone
how resistant to failure the gypsum is to compressive strength
compressive strength is highly dependent on what
the water to powder ratio (more water = less compressive strength)
what is the reproduction of detail of dental stones
how detailed of impressions the stone will be
which have higher reproduction of detail type 1 and 2 dental stone or type 3,4 and 5 dental stone
type 3, 4, and 5 have higher reproduction of detail
what is surface hardness and resistance to abraision for gypsum
how hard the surface is, and how hard to scratch it it is
what things affect the parameters of gypsum products
ratio of powder to water spatulation humidity temperature additives pH
what does increasing the ratio of water to powder do to gypsum
- decreases compressive strength
- decreases surface hardness and abrasion resistance
- increases the porosity of the gypsum
what does increased spatulation do to the gypsum
- accelerates setting time (causes more solidifying nuclei to form)
What does humidity do to gypsum
humidity below 70% = faster setting time
humidity above 70% = slower setting time
what is the easiest way to make sure that humidity doesn’t affect the setting time of your gypsum
keep the container sealed
what affect does temperature have on gypsum
higher temp = shorter setting time
lower temp = longer setting time
which is the physical property has the most effect on gypsum
temperature
what are the two common additives to gypsum
potassium sulfate
NaCl
what does potassium sulfate do to gypsum
accelerates setting time
improves surface quality
what does NaCl do to gypsum
increases setting expansion
shortens setting time
how does pH affect gypsum
low pH increases setting time (saliva is low pH)
high pH decreases setting time
