The Influence of Water Absorption of Dental PMMA on Performance.

The influence of water absorption of dental PMMA on its performance is mainly reflected in the following aspects:

Volume and size changes

Water absorption and swelling: PMMA is a polar molecule. After being soaked in water, the denture base made of it can absorb a certain amount of water. The absorbed water will affect the performance of the resin base. After absorbing water, the base resin undergoes a slight volume expansion. It is calculated that for every 1% of the mass fraction of water absorbed by PMMA, the material will undergo a linear expansion of 0.23%. Both in the laboratory and in clinical practice, it has been found that the expansion of the base after absorbing water can partially compensate for the volume shrinkage caused by the polymerization and cooling processes, and improve the suitability of the denture base.

Dimensional stability: As the water absorption of the base resin affects the mechanical properties and dimensional stability of the material, there are strict regulations on the water absorption of the base resin. The current ISO standard stipulates that the water absorption value of the base resin after being immersed in distilled water at 37℃ for 7 days should not exceed 32μg/mm². If the denture loses water and dries out, it will cause the denture base to deform.

Changes in mechanical properties

Stress release and deformation: The moisture absorbed by the resin material can interfere with the interaction between the polymer chains, acting as a plasticizer and affecting the physical and mechanical properties of the base. As the interaction between the molecules of the polymer chain weakens, the stress generated during polymerization is released. During the stress release process, the resin base may deform. However, since this change is relatively small, it will not have a significant impact on the fit of the denture base.

Impact and flexural strength: PMMA absorbs moisture, which may damage its physical and mechanical properties during use and make it prone to failure under cyclic loading, resulting in insufficient impact and flexural strength.

Changes in chemical properties

Solubility: PMMA has a very low solubility in water and the liquids that the mouth usually comes into contact with. However, its solubility may be affected to some extent after absorbing moisture. The current ISO standard stipulates that when the base resin is immersed in distilled water at 37℃ for 7 days, the solubility of the self-curing resin shall not exceed 8.0g/mm³, and that of other types shall not exceed 1.6g/mm³.

Aging performance: Compared with other plastics, PMMA has better aging resistance. However, as time goes by, the impact strength of PMMA slightly increases, while its tensile strength and light transmittance slightly decrease. Its resistance to silver streaks and molecular weight significantly decline, and its color gradually turns yellow. Water absorption may accelerate this aging process.

Changes in biological properties

Allergic reaction: Theoretically, as long as the human body comes into contact with the base resin, residual monomers, benzoyl peroxide, pigments or the products of the reaction between the base resin components and the usage environment, an allergic reaction may occur. Among them, residual monomers are considered to be the main cause of allergic reactions. In vitro studies have shown that acrylic base resin has certain cytotoxicity, and the level of cytotoxicity is related to the powder-liquid ratio of the resin, the base polymerization method, and whether the denture is soaked in water after production. Soaking dentures in water after they are made helps reduce the cytotoxicity of the resin base.