This document discusses reversible hydrocolloids, which are polysaccharide colloidal suspensions used for dental impressions. Reversible hydrocolloids exist as either a sol or gel, transitioning between the two states during the impression process. They are extracted from seaweeds and set via cooling and physical bonding. Examples include agar, which is supplied in tubes or syringes and sets in the mouth. Reversible hydrocolloids are dimensionally unstable due to water absorption and evaporation, but provide accurate impressions and are easy to use.

1. Reversible
Hydrocolloids
ASST PROFESSOR
Dr Mumtaz ul Islam
B.Sc. BDS. MHR. M.Phil.

2. What are hydrocollides
• Polysaccharides colloidal suspensions in water
• Not solution not suspension
• No solid particles present not a solution
• Particles bears charges which repels each other
• Medium is water so hydro

3. Types
• Two sol and gel
• Sol low viscosity fluid random chains
• Gel viscosity increases plastic chains are aligned
• Fibrils in the colloidal phase produces a jelly like consistency
• More the fibrils structure in gel stronger jelly will form
• Strength depends on concentration of polysaccharides

4. Setting
• Conversion from sol to gel
• Gel formation commences in two ways reversible and irreversible
• Cooling produces alignment
• Van der waals forces
• Hydrogen bonding
• Reversible process reversible hydrocollides

5. Reversible hydrocollides (agar)
• Polysachrides extracted from sea weeds
• Primarily 15% colloidal suspension of agar in water
• High molecular weight more free hydroxyl groups
• Presentation flexible tubes, syringes
• Borex provides bulk retards setting of gypsum model and die materials
• Potassium sulphate activates the retarding effect of ----------------

6. Manipulation
• Prepared (conditioned) in conditioning bath before use
• Three compartments having water at different temperature
• Tube or syringe first immersed in 100°C bath → sol
• Then in 65°C till required for use mixing by squeezing
• Few minutes before impression tube transferred to 45°C bath
• Impression by conventional method

7. Application
• Different consistencies are available
• Crown and bridge work impressions
• High viscosity sol tray
• Low syringed over prepared tooth or teeth

8. Temperature hysteresis effect
• Present at sol to gel transition and vice versa
• Transition from sol to gel is slow at mouth temperature
• To accelerate it cold water spray or water-cooled trays used
• Material near the water tubes sets rapidly
• Near tooth can flow and record even the tray moved slightly
• Removal through snatching

9. Syneresis and imbibition
• Syneresis (squeezing) expulsion of water on surface
• Imbibition swelling of material through water uptake
• Agar is very much susceptible for both
• Impression should be rinsed and kept covered in a damp gauze
• Model must be poured within 30 minutes
• Not possible to make metal coated or epoxy resin dies

10. Clinical advantage
• Its water absorption capacity
• Impression of a provisional ill fitting crown difficult
• Gingival inflammation
• Increased crevicular flow
• First impression takes up moisture 2nd one would be accurate
• Can be reused cross infection is a limitation along with others

11. Types & Applications
• Type 1 high consistency for complete and partial dentures
• Alone or in combination with type 2 or type 3 type 1 in tray
• Type 2 medium consistency multi purpose
• Itself used in extruded technique (combination technique)
• Type 3 low consistency multi purpose
• only syringe with type 2 or type 3 type

12. Properties
• Fluid in sol form for recording fine details
• Mucostatic low viscosity
• Reproducibility ISO 1564 copy 0.02mm line in a metallic block
• Compatibility with gypsum products 0.05mm line cast production
• Flexible for undercuts cylinder 20mm long and 12.5mm diameter
• Measuring load 125g then 1.25kg strain minor 4% and major 15%

13. Properties contd,
• Viscoelastic elastic recovery can be optimized
• Amount of permanent deformation undercut severity stressed time
• If stressed time reduced elastic recovery enhanced
• Quick Snatch recovery (snatching)
• Cylindrical specimen 20mm high and 12.5mm diameter compressed 4mm
• Recovery after one second load application must be 96%

14. Properties contd,
• Mechanically poor Tear strength is low
• Interproximal and sub gingival areas very difficult to record
• Tear resistance Ts (tear strength)
• Type 1material 0.75 N/mm
• Type 2 and type 3 materials 0.5 N/mm

15. Properties contd,
• Often small drops of water are visible on surface
• High water content evaporation causes shrinkage
• Imbibition produces swelling
• Dimensionally unstable
• Not always possible nor advised make more than one cast
• Duplicating material can be reused (low cost)

16. Duplicating
• Cast or model on a glass slab surrounded by duplicating flask
• Duplicating material is thinner heated at 50°C
• Poured in to fill the flask temperature kept as low as possible
• Contraction prevented also by cooling the flask from base
• Cast removed immediately after setting and poured
• Retarding effect avoided by treating the surface by accelerator