Hygroscopic and hydrolytic effects in dental polymer networks

Summary

Objectives

The objective of this manuscript is to outline the factors associated with hygroscopic and hydrolytic effects in dental polymer networks, and to review the literature generated over the past thirty years or more in this area.

Methods

Information was gathered from nearly 90 published articles or abstracts appearing in the dental and polymer literature. Studies were predominantly identified through a search of the PubMED database.

Results

Studies were included that provided direct evidence for the uptake of solvent by a polymer network and its subsequent physical or chemical effect, or the loss of molecular species into solvents. An attempt was made to select articles that spanned the timeframe from approximately 1970 to today to ensure that most of the classic literature as well as the latest information was included.

Conclusions

Dental polymer networks have been shown to be susceptible to hygroscopic and hydrolytic effects to varying extents dependent upon their chemistry and structure. The importance of these effects on the clinical performance of polymer restoratives is largely unknown, though numerous investigators have alluded to the potential for reduced service lives.

Significance

While the physical and mechanical properties of these materials may be significantly altered by the effects of solvent uptake and component elution, what may constitute the greatest concern is the short-term release of unreacted components and the long-term elution of degradation products in the oral cavity, both of which should be strongly considered during restorative material development.

Introduction

Many polymer networks are considered to be largely insoluble structures with relatively high chemical and thermal stability. However, these networks may absorb water and chemicals from the environment. In turn, the network may release components to its surroundings. The phenomena of sorption and solubility may serve as precursors to a variety of chemical and physical processes that create biological concerns as well as producing deleterious effects on the structure and function of the polymeric material. These effects may include volumetric changes such as swelling, physical changes such as plasticization and softening, and chemical changes such as oxidation and hydrolysis (Fig. 1). The properties of the network may be permanently altered by these events, and the performance of the polymer may be compromised. It is generally accepted that all polymers degrade to some extent, and in this light, it is appropriate to suggest that the greatest difference between degradable and non-degradable polymers may be temporal. There are many applications in which the degradation of polymers is acceptable and even required. In any case, this manuscript will only be concerned with non-degradable polymers, defined as those requiring a much longer time to degrade than their expected time of service [1].

There is concern that the effects of solvent uptake and hydrolytic degradation may lead to a shortened service life of dental restorations. But an even greater concern is the possibility of biological effects elicited by the species evolved from polymeric dental restoratives, thus requiring the effects of hygroscopic and hydrolytic phenomena to be investigated and explained. There are a variety of polymers used in dentistry, but the concerns over sorption and solubility mainly are limited to restorative and prosthetic materials residing semi-permanently within the oral cavity. While sorption and solubility do occur with other polymers, such as impression materials, these will not be addressed here. The objective of this manuscript is to outline the factors associated with hygroscopic and hydrolytic effects in dental polymer networks, and to review the literature generated over the past thirty years or more in this area.