Solidification and properties of dental gypsum

The most important process in the manufacture of orthopedic products from dental plaster is its solidification. 

It can occur in two ways: normal and hygroscopic. During the hardening process, needle-shaped crystals are formed in the gypsum mass. 

As they harden, the crystals grow and expand, creating an increase in pressure in the mass, which increases its volume. The ability of gypsum to expand slightly when solidified is key for many dental applications, as orthopedic models and dies are made slightly larger than the minimum required size, and the expansion compensates for the shrinkage of the metal casting during cooling. However, uncontrolled expansion is unacceptable, as the resulting volume can easily be excessive.

To solve these problems, there are modifying additives that allow fine adjustment and precisely achieve the required level of expansion of the material. The usual adjustment ranges are 0.2-0.3% (by volume of material) for simple gypsum and 0.05-0.1% for high-strength gypsum. It is possible to significantly increase the degree of expansion of the material by immersing it in water during the hardening process, since in the absence of the surface tension factor of water (as when solidifying in air), gypsum crystals grow more freely, providing a larger volume of material. This method of hardening gypsum materials is called hygroscopic.

After complete solidification, gypsum materials in dentistry also differ in a number of properties. First of all, this includes the mechanical properties of the material. Compressive strength is mainly determined by the initial liquid / powder ratio: the less water, the higher the compressive strength.

In practice, reducing the amount of water in the mixture leads to a too thick mixture, from which it is difficult to remove air bubbles, which in the future will significantly reduce the strength of the product, increasing its porosity. Also, a small amount of water may not be enough for the entire gypsum mass to react. Given the above, adjusting the strength using the ratio of water and gypsum is used, as a rule, with great care.

Another parameter is the tensile strength. Due to its porosity, gypsum initially has a low value of this indicator, which leads to rapid damage to models made of ordinary gypsum. High-strength gypsum largely solves this problem — the tensile strength of products made from it is approximately twice as high, so orthopedic products, stamps and bridges are made from it. Due to the low surface hardness of gypsum, the material is easily worn out, so with high service life requirements, gypsum can be replaced with various plastics, although they have a characteristic polymerization shrinkage. 

An important property of gypsum is dimensional stability: after hardening, the dimensions of the product do not change.