Hydrocolloids (Alginate) PDF

Hydrocolloids (Alginate) Hydrocolloids (Alginate) Dr. Nesma Abdel Fattah Amer  Mixing a solute (dispersed phase) with a solvent (dispersion medium) may result in 1-Solution 2-Colloid 3-Suspension Homoge...

Hydrocolloids (Alginate) Hydrocolloids (Alginate) Dr. Nesma Abdel Fattah Amer  Mixing a solute (dispersed phase) with a solvent (dispersion medium) may result in 1-Solution 2-Colloid 3-Suspension Homogeneous Heterogeneous Heterogeneous Single Phase system Two Phase system Two Phase system. Solute exists as small the solute particle size is Solute particle size is molecules or ions in the smaller than that of the largest insoluble molecules solvent suspension and larger like sands like sugar in water. than solution.  N.B: Mixing 2 Immiscible Liquids Emulsion As suspension, but the dispersed phase is liquid instead of solid. Ex: like oil in water.  Colloids are called hydrocolloids when the dispersion medium is water. They are used for dental impressions. They exist in the sol and gel state. They are inserted in the mouth in sol state (sufficiently fluid to record details) Then, they are removed after the gel is formed and exhibit elastic properties  Sol-gel transformation occur through agglomeration of molecules of dispersed phase to form connected fibrils or chains in a network pattern.  Hydrocolloids may be: Reversible (2ry Irreversible (1ry bonds) bonds) Agar Alginate heating Chemical reaction Sol gel Sol gel cooling  It is a viscoelastic impression material which changes from a sol to a gel state through a chemical reaction, so it is irreversible. It is indicated as a primary impression for the dentulous and edentulous patients Forms of supply: A- bulk form in container with a scoop for measuring powder and plastic measuring cylinders for liquid. B- Small sealed packets/ individually sealed pouches B A Composition: 1- Potassium (or sodium) alginate 15 % ○ Soluble salt of Alginic Acid (comes from marine plants) ○ Main Rx ingredient because it forms sol with water. 2- Calcium sulfate dihydrate (CaSO4.2H2O)16% It is the reactor of the material, it reacts with the soluble alginate form to produce insoluble form. Composition: 3- Sodium phosphate Na (Po ) 3 4 2 It acts as a retarder, as it allows longer working time (where Ca reacts with PO4 before the cross linking occurs). Its conc. determines whether it is regular or fast set material 4- Potassium sulfate It is accelerator for the setting time of the gypsum products used in pouring models 5- Fillers As zinc oxide or diatomaceous earth which strengthens the gel. 6-Flavoring, coloring and disinfecting agents (chlorhexidine) 7-Chemical indicator (present in some products) Changes color with PH change, to indicate different stages in manipulation (chromatic alginate) Setting reaction: The reaction of alginate is called gelation chemical reaction. 1. When the powder is mixed with water & Then , the calcium reacts with phosphate to form insoluble calcium phosphate. (First: Retardation reaction) 2- This process continues until the whole amount of phosphate is depleted. 3-Then the Calcium ions start to react with the soluble alginate (sodium or potassium alginate) to form insoluble alginate (calcium alginate). Ca + Sodium alginate → Calcium alginate (final elastic gel) (Second: Setting reaction) Where monovalent Na is replaced by divalent Ca achieving crosslinking of chains Manipulation Equipments Alginate powder Mixing rubber bowel Wide bladed Metal spatula Perforated or rim locked stock tray (allowing imp. thickness not less than 4 mm) 1- Dispensing and mixing Fluff the alginate container (to evenly distribute the constituents). W/ P ratio is measured acc. To manufacturer instruction and placed in a clean mixing rubber bowl with a stirring motion then vigorous mixing with a metal spatula against rubber bowl wall squeezing the material until a creamy and smooth mix is obtained Mixing time is 45 s to 1 min. 2- Tray loading and impression taking A thin layer of alginate adhesive is applied and allowed to dry before loading the alginate (Optional) 3- Impression removal Use the alginate remaining in the bowl as an indicator for Setting Time. If the alginate in the bowl is set, then the material in the mouth is also set. The impression should be removed with a snap movement as this increases the tear strength and elastic recovery. 4- Disinfection The disinfection should be relatively rapid to prevent dimensional changes using iodophors, sodium hypochlorite and glutaraldehyde, done by: Spraying technique The impression is rinsed in water, The disinfectant is sprayed on the surface. The impression is wrapped in a disinfectant soaked paper towel and placed in a sealed plastic bag for 10 minutes. The impression is unwrapped, rinsed, and shaken to remove excess water then pouring. 5- Pouring impression: Without a separating medium  Affect the setting and surface properties of the gypsum materials. The water content of the impressions inhibits the setting of the gypsum at the surface and results in a soft surface. The hydrocolloids are hydrophilic and so absorb water from the gypsum mix which results in chalky appearance of gypsum models. 1. Immersion of the impression in 2 % K2SO4 before pouring (gypsum accelerator). 2.Incorporation of the gypsum accelerator in the alginate impression material itself, such as potassium titanium fluoride. 3. Separate cast from imp. as soon as its setting occur.  Dimensional unstable on storage Causes: 1- Water loss It occurs either by Syneresis (exuding of fluids onto the surface) or evaporation. It results in shrinkage. 2- Water uptake It occurs by Imbibition. It results in swelling and expansion. 1. Pouring the impression immediately (within 10 min). 2. If the pouring must be delayed, the impression must be stored for 30-60 min maximum in various storage media such as 2 % potassium SO4 3. The impression is wrapped in a surgical paper towel saturated with water, and placed in a sealed plastic bag or a humidor (100% relative humidity).  Advantages Disadvantages 1. Inexpensive 1. Dimensionally unstable 2. Easy to use 2. Low tear strength 3. Hydrophilic so good wetting to oral 3. Difficult to disinfect and electroplate tissues. 4. Immediate pour and Single cast 4. Stock trays with no need for special 5. Lower detail reproduction trays. So used for primary impression  5 Modified forms 2- Dust free alginate: 3- Siliconized alginate: Adding dust-reducing agent as Silicone polymers are added to glycerin or glycol to reduce allergies strengthen the material. 1- Two pastes system: One paste contains the Na/K alginate and the other paste contains the calcium sulfate reactor. 5- Chromatic alginate Chemical 4- Alginat containing indicators are added to the alginate disinfectants powder to indicate different stages in manipulation QACs or CHX are added to powder