what are impression materials used for?
register or reproduce the form and relationship of the teeth, dental arches and oral tissues
impression materials behave as ?
viscoelastic materials
what is strain rate dependance?
mechanical response of dental materials depending on how fast they are loaded
what is elastic recovery?
ability to recover to the original shape after stretch, relevant for removal of impression materials from the mouth
what is tear strength?
the ability to resist tearing in thin sections
what is dimensional stability?
ability to retain its absolute dimensional size over time
what is surface resolution?
- hydrophilic meaning <90 degrees has a high degree of surface detail meaning surface gets more “wet”
- hydrophobic meaning >90 degrees has a low degree of surface detail and liquid doesn’t wet sufficiently
what are aliginate hydrocolloids?
- a dissolving solid is dispersed into liquids and depending on the quality of water depends on the different state i.e. gel or sol
- colloid particles are hydrophilic
what is the chain of reactions for alginate?
- Na-alginate mixed with water (sol) where Ca2+ is released from dissolution of CaSO4
- Ca2+ ions replace the Na ions and cross link the alginate chain to form a gel
what is working time?
the time allowed for mixing the alginate, loading the tray and positioning the tray into the patients mouth
what is setting time?
the time required for the reaction to be completed and the material solidified
what are the modifiers of setting and working time?
- increased temp shorts time
- proportions of water to powder where thinner increases time
- under or over mixing
ANSI-ADA requires what percent elastic recovery?
95% after compressed 20% for 5 seconds
when does permanent deformation increase?
- time before testing decreases
- during removal
- when time is held under compression increases
- thinner mixes are used
a limitation of alginate is?
loss of accuracy with increased time of storage
what are the advantages of alginate?
-ease of mixing
- minimum equipment needed
- flexibility of set impression
- accuracy
- low cost
what are the disadvantages of alginate?
- low tear strength
- limited elasticity
- limited surface detail
- storage stability
elastomeric impression materials are ???
silicone rubbers that are flexible cross linked polymers when set
compared to alginates, elastomeric have greater ?
- accuracy
- dimensional storage stability with time
- excellent record of detail
- tear strength
addition silicone impression materials are supplied as a what system?
two paste system: base + catalyst
- paste 1: silicone, vinyl, fillers and acid catalyst
- paste 2: silicone with hydrogen groups and fillers
addition silicone impression materials do not generate?
volatile. by-products
what are the types of consistencies of addition silicone impression materials?
low viscosity
medium viscosity
high viscosity
what are some characteristics of addition silicone impression materials?
- short working time
- mid to low tear strength
- elastic recovery is highest among all impression materials
- relies on shear thinning behavior for dispensing
polyether impression materials have properties similar to?
addition silicones
