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