*1.2.1.1 Composition*

The components of the agar gels are 12–15% agar, 1% potassium sulphate to ensure a proper set of the gypsum material poured in the impression, 0.2% borax as a strengthener for the gel, 0.1% alkyl benzoate as an antifungal during storage, and 85% water (**Table 1**). Borax and agar retard the set of gypsum products, so potassium sulphate is added to cancel out their effect [13].

The composition described in **Table 1** may differ slightly depending on the dispersing medium for the gel; which could be either loading on an impression tray or a syringe. When fine details of preparation are needed, a less concentrated gel type is used in a syringed. A more concentrated gel is used to in water-cooled tray to form the bulk of the impression. Agar possesses relatively good elastic recovery, reproduction of details, pleasant tasting and easy to clean up. But it cannot be used to produce electroplated pies due to its dissolution inside the electrolytic bath [14]. Agar is a technique sensitive impression material due to its low tear strength of 27.6 KPa. Agar is dimensionally unstable due to the loss of water from the agar gels even when stored at 100% humidity. The consequence of this is an inaccurate model if left for a while before the cast is poured [9]. Agar hydrocolloids are supplied as sticks or gel and require specific equipment for its manipulation before the impression making process. Thereby making the process cumbersome but can be reused once the setup is done. Although agar hydrocolloid is an inexpensive


**Table 1.** *Composition of agar gels.* impression material with very good accuracy, its use has declined over the years due to the inability to pour impressions immediately, low dimensional stability, ease of manipulation and water-cooled impression trays and the inability to produce electroplated dies [9].

#### *1.2.2 Alginate*

Alginate is an irreversible hydrocolloid largely used in dentistry [15]. It is mainly used for diagnostic and planning in the rehabilitation of oral, orthodontics and maxillofacial prostheses [16–18]. The advantage of alginate materials is that it is easy to manipulate, cheap and provides a good level of comfort for patients without the need for specialized instruments and equipment [17, 19].

#### *1.2.2.1 Composition and setting reactions*

Discovered in 1945, as a substitute for agar whose importation was hampered by the outbreak of the Second World War. Alginates are salts of alginic acid, a polysaccharide extracted from the cell walls of brown algae (washed, ground and chemically treated, especially the pulp) belonging to the Phaeophyceae family, widespread especially in America [20]. Like agar, alginic acid, chemically known as anhydro-B-D-mannuronic acid has a high molecular weight (30,000 to 200,000) linear polymer [9].

The extracted alginic acid is then converted into a salt (alginate) of sodium, calcium, potassium or magnesium. Although alginate is insoluble in water, its alkaline salts are water-soluble. The production process of sodium alginate from brown algae can be done in two ways; using the calcium alginate method or the alginic acid method [21]. To extract alginic acid, the algae are placed in a sodium carbonate bath, exploiting the solubility of alkaline alginates in water. The alginic acid is recovered from the obtained solution by precipitation with hydrochloric acid or sulfuric acid [21]. The difficulty of the processes lies in the required physical separations; such as in the filtration of muddy residues from viscous solutions or in the separation of gelatinous precipitates that retain a large amount of liquid in their structure, resisting filtration and centrifugation [2].

The alginate impression materials for dental use contain several additives such as sodium alginate, calcium sulphate, trisodium phosphate, diatomaceous earth, zinc oxide, and potassium titanium fluoride, all in the form of a powder [2]. They are irreversible hydrocolloids because the picking reaction is a chemical reaction of irreversible precipitation therefore they cannot return in sol form using physical means, such as temperature, as with reversible hydrocolloids.

The chemical reaction occurs two times: a first phase called 'slowing' and a second phase called 'setting'. Initially, the powder is mixed with water, a sol is formed and the sodium or potassium salts of alginic salts react with the calcium sulphate [2] to allowing crosslinking of the alginic salts [9, 22, 23]. After the sodium phosphate has reacted, the remaining calcium sulphate reacts with sodium alginate to form insoluble calcium alginate that forms a gel with water which acts as a catalyst. There are many commercial variations of alginate that vary in consistency, setting time, elasticity, strength, and dimensional stability; manufacturers also add fillers, which impact on its properties, application, setting time, and pouring time [22]. The standard composition of alginate is as described in **Table 2**.

The alginates available on the market can be of two types: fast setting (hardening time of 1–2 min) or normal setting (setting time between 2 and 5 min). The setting time depends on the composition (water/powder ratio, where increasing the powder accelerates the hardening reaction) and the temperature at which mixing

**151**

*Hydrocolloids in Dentistry: A Review*

of alginic acid in water.

• Organic glycols • Flavoring agents • Coloring agents • Disinfectants

**Table 2.**

Calcium Sulphate, where:

(Normal set) setting time [20].

chemical reaction [2].

**1.3 Summary**

*DOI: http://dx.doi.org/10.5772/intechopen.94987*

*Composition and properties of alginate use in dentistry.*

**Material Percentage** 

takes place [2]. The reaction that causes the alginate impression materials to form makes use of the different solubilities of the sodium, potassium, ammonium and calcium H2O % Na & alginate % CaSO4 ÿ (paste) Ca & alginate % Na2SO4 (gel) salts

Other additives: chemical indicators Very small quantities Colour change

**(Approximate)**

Sodium or potassium alginate 15–20% Colloidal particles as basis of the gel Calcium sulphate dihydrate 14–20% Creates irreversible gel with alginate Potassium sulphate 10% Ensures set of gypsum materials Trisodium phosphate 2% Retarder to control setting time Diatomaceous earth 55–60% Filler to increase thickness and strength

**Purpose**

Reduce dust when powder is handled Improve taste of material Provide pleasant colors Cause antibacterial action

The setting reaction is a chemical reaction between Sodium Alginate and

This reaction (1) can be retarded with Calcium Phosphate, which acts as a retarder, thereby increasing the setting time and obtain a type I (fast set) or type II

The irreversible hydrocolloids, which are the most commonly used, are a mixture of manual or mechanized techniques through the union of powder and water [2]. Alginate impression materials are easy to use and manipulate without specialized equipment but can be mixed manually or mechanically, it is less expensive and has more rapid setting times. The reaction time and the setting time can be controlled with the temperature of the water used. They are slightly flavoured and in recent formulations, have colour indicators according to the phases of the

What started as a trial in the 19th century gradually became a benchmark in the history of dentistry and has today found its way into different aspects of medical, pharmaceutical and food industries with more studies on how to improve its effectiveness for optimal use. We are currently in a technologically advanced era which is gradually employing the use of CAD/CAM technologies for the diagnosis and treatment of patients which is still very expensive due to the cost of the equipment and specialized training required for the operators to interface with it. Amid all these, studies are still being carried out on convectional irreversible hydrocolloids to

Recent studies have shown that the dimensional stability of hydrocolloids has been improved upon with the materials which have extended cast pouring times [25]. The incorporation of disinfectant gels into the powder which when mixed with

improve their physical, mechanical and biological properties [24].

2Na PO 3CaSO Ca PO 2 3Na SO 34 4 3 4 24 +→ + ( ) (1)


#### **Table 2.**

*Colloids - Types, Preparation and Applications*

*1.2.2.1 Composition and setting reactions*

electroplated dies [9].

linear polymer [9].

*1.2.2 Alginate*

impression material with very good accuracy, its use has declined over the years due to the inability to pour impressions immediately, low dimensional stability, ease of manipulation and water-cooled impression trays and the inability to produce

Alginate is an irreversible hydrocolloid largely used in dentistry [15]. It is mainly

Discovered in 1945, as a substitute for agar whose importation was hampered by the outbreak of the Second World War. Alginates are salts of alginic acid, a polysaccharide extracted from the cell walls of brown algae (washed, ground and chemically treated, especially the pulp) belonging to the Phaeophyceae family, widespread especially in America [20]. Like agar, alginic acid, chemically known as anhydro-B-D-mannuronic acid has a high molecular weight (30,000 to 200,000)

The extracted alginic acid is then converted into a salt (alginate) of sodium, calcium, potassium or magnesium. Although alginate is insoluble in water, its alkaline salts are water-soluble. The production process of sodium alginate from brown algae can be done in two ways; using the calcium alginate method or the alginic acid method [21]. To extract alginic acid, the algae are placed in a sodium carbonate bath, exploiting the solubility of alkaline alginates in water. The alginic acid is recovered from the obtained solution by precipitation with hydrochloric acid or sulfuric acid [21]. The difficulty of the processes lies in the required physical separations; such as in the filtration of muddy residues from viscous solutions or in the separation of gelatinous precipitates that retain a large amount of liquid in their

The alginate impression materials for dental use contain several additives such as sodium alginate, calcium sulphate, trisodium phosphate, diatomaceous earth, zinc oxide, and potassium titanium fluoride, all in the form of a powder [2]. They are irreversible hydrocolloids because the picking reaction is a chemical reaction of irreversible precipitation therefore they cannot return in sol form using physical

The chemical reaction occurs two times: a first phase called 'slowing' and a second phase called 'setting'. Initially, the powder is mixed with water, a sol is formed and the sodium or potassium salts of alginic salts react with the calcium sulphate [2] to allowing crosslinking of the alginic salts [9, 22, 23]. After the sodium phosphate has reacted, the remaining calcium sulphate reacts with sodium alginate to form insoluble calcium alginate that forms a gel with water which acts as a catalyst. There are many commercial variations of alginate that vary in consistency, setting time, elasticity, strength, and dimensional stability; manufacturers also add fillers, which impact on its properties, application, setting time, and pouring time [22]. The

The alginates available on the market can be of two types: fast setting (hardening time of 1–2 min) or normal setting (setting time between 2 and 5 min). The setting time depends on the composition (water/powder ratio, where increasing the powder accelerates the hardening reaction) and the temperature at which mixing

used for diagnostic and planning in the rehabilitation of oral, orthodontics and maxillofacial prostheses [16–18]. The advantage of alginate materials is that it is easy to manipulate, cheap and provides a good level of comfort for patients without

the need for specialized instruments and equipment [17, 19].

structure, resisting filtration and centrifugation [2].

means, such as temperature, as with reversible hydrocolloids.

standard composition of alginate is as described in **Table 2**.

**150**

*Composition and properties of alginate use in dentistry.*

takes place [2]. The reaction that causes the alginate impression materials to form makes use of the different solubilities of the sodium, potassium, ammonium and calcium H2O % Na & alginate % CaSO4 ÿ (paste) Ca & alginate % Na2SO4 (gel) salts of alginic acid in water.

The setting reaction is a chemical reaction between Sodium Alginate and Calcium Sulphate, where:

$$\text{2Na}\_3\text{PO}\_4 + \text{3CaSO}\_4 \rightarrow \text{Ca}\_3(\text{PO}\_4)2 + \text{3Na}\_2\text{SO}\_4 \tag{1}$$

This reaction (1) can be retarded with Calcium Phosphate, which acts as a retarder, thereby increasing the setting time and obtain a type I (fast set) or type II (Normal set) setting time [20].

The irreversible hydrocolloids, which are the most commonly used, are a mixture of manual or mechanized techniques through the union of powder and water [2]. Alginate impression materials are easy to use and manipulate without specialized equipment but can be mixed manually or mechanically, it is less expensive and has more rapid setting times. The reaction time and the setting time can be controlled with the temperature of the water used. They are slightly flavoured and in recent formulations, have colour indicators according to the phases of the chemical reaction [2].

#### **1.3 Summary**

What started as a trial in the 19th century gradually became a benchmark in the history of dentistry and has today found its way into different aspects of medical, pharmaceutical and food industries with more studies on how to improve its effectiveness for optimal use. We are currently in a technologically advanced era which is gradually employing the use of CAD/CAM technologies for the diagnosis and treatment of patients which is still very expensive due to the cost of the equipment and specialized training required for the operators to interface with it. Amid all these, studies are still being carried out on convectional irreversible hydrocolloids to improve their physical, mechanical and biological properties [24].

Recent studies have shown that the dimensional stability of hydrocolloids has been improved upon with the materials which have extended cast pouring times [25]. The incorporation of disinfectant gels into the powder which when mixed with water will dissolve thereby preventing surface inaccuracies when soaked/sprayed with disinfectants [25]. Dust-free particles with the use of glycerine making the powder denser and the two-sol system like elastomeric materials to reduce inaccuracies due to annual mixing [25]. Chromatic products to indicate the different chemical reactions within the sol [25].

An important advancement is the use of agar-alginate laminate for making impressions which give better accuracy, thereby, eliminating the water-cooled trays for agar impressions. Certainly improving the quality and definition of these materials would be possible to expand their use with benefits for patients. Also, the reduced setting time and their single-footprint technique will provide added benefits for dentists in terms of time available for manipulation. The prospect is that these materials will continue to evolve as has happened since the 40s, thus producing high-performing impression materials [2].

#### **1.4 Conclusion and recommendation**

Despite the advancement in science and technology, hydrocolloids have remained relevant in dentistry, particularly as an impression material. The resilient properties of hydrocolloids coupled with the simplicity of use and biocompatibility with the oral tissues had endeared it in dental practices. While hydrocolloids had some inherent disadvantage in its properties, the advancement in material science and modification of hydrocolloids with other additivities has improved their properties and usefulness in the oral care practice.

## **Conflict of interest**

The authors declare no conflict of interest.
