The importance of composite-water interaction has been acknowledged in the ISO standard 4049 which states the maximum values for water sorption and concurrent solubility for resin-based materials (composites and cements). In order to comply with this ISO standard, resin-based materials must have water sorption and solubility values equal or lower than 40 micrograms per cubic milimetre (sorption) and 7.5 micrograms per cubic milimetre (solubility) for specimens 15 mm in diameter and 2 mm thick.
Water sorption and solubility values are based on mass changes of the samples before (m1) and after immersion (m2) in water and after dessication (m3) until constant mass is achieved. Mass changes m2-m3 are divided by sample volume to calculate water sorption and m1-m3 divided by sample volume give solubility values.
Recent papers* published in Dental Materials investigated water sorption and solubility and hygroscopic dimensional changes of several resin-based composites:
low-shrinkage Filtek Silorane
universal Gradia Kalore
micro-hybrid Gradia Direct Anterior and Posterior andself-adhering flowable Vertise Flow.
After 150 days of storage in de-ionized water, the lowest sorption of about 13 μg/mm³ was found for Filtek Silorane and the greatest of about 72 μg/mm³ was found for Vertise Flow. Vertise Flow also showed the greatest solubility of about 16 μg/mm³ whereas other materials showed either negative values (Filtek Silorane and Gradia Kalore) or values below 4 μg/mm³. The authors suggested that the negative solubility values for Filtek Silorane and Gradia Kalore meant that the dessication was not sufficient or that some water was irreversibly bound to the resin matrix.
Hygroscopic dimensional expansion as the result of water sorption over the 150-day period was lowest for Filtek Silorane (about 0.7%) and highest for Vertise Flow (about 4.8%) whereas the values for Gradia composites were between 1.5 and 2%. Hygroscopic expansion may compensate to a certain extent polymerization shrinkage which was found to be 0.99% for Filtek Silorane, 1.7-2.4% for Gradia composites and 4.4% for Vertise Flow. However, this expansion occurs over a much slower time scale than shrinkage and its effect on the clinical performance of resin-based composite is yet to be determined.
The greatest stability in the aqueous environment found for Filtek Silorane may be explained by the hydrophobic siloxane and low-shrinkage ring-opening oxirane units of the silorane monomer. Furthermore, cationic polymerization is relatively oxigen-insensitive with the potential of reaching higher degree of conversion than methacrylate-based composites.
On the other hand, aqueous instability of Vertise Flow was attributed to the hydrophilic monomer, GPDM, which is responsible for the self-adhesive property of Vertise Flow but also seems to attract more water uptake by the resin matrix compared to other resin-based composites.
* Wei YJ, Silikas N, Zhang ZT, Watts DC. Hygroscopic dimensional changes of self-adhering and new resin-matrix composites during water sorption/desorption cycles. Dent Mater. 2011 Mar;27(3):259-66.
[Reprints of the cited papers may be obtained from the corresponding authors]
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