Effect of Absorbed Moisture on the Properties of Commercial Thermoplastics and Model Epoxies
Authors: Colmenero M.A. , Letton A. 
Published: October 2008

In order to better understand the effects of moisture on the properties of polymers studies are conducted on commercial grade polycarbonate and polyestercarbonate as well as model epoxy resins based on Diglycidel ether of Bisphenol A (DGEBA). Owing to a greater number of hydrogen bonding sites the resins exhibit a greater moisture affinity, 1-470, as compared to the thermoplastics, .35?40. Moisture induced plasticization is seen to result in as much as a 50 degree drop in the glass transition for the resins while the thermoplastics are essentially unaltered. Correspondingly, there is seen to be a marked increase in the fracture toughness of thermosetting resins where again the thermoplastics are not affected.

INTRODUCTION

Several studies have indicated that there are large deposits of oil and natural gas that have yet to be recovered at depths far exceeding those of present day exploration. Recovery at these depths, in excess of six thousand feet in some cases, is feasible but as is the case all too often there are many difficulties. The use of steels and other conventional materials to explore at these far reaching distances would prove to be structurally difficult and expensive.

As an alternative, the use of polymers, or neat resins, and fiber reinforced polymers, composites, have been considered a solution. There are, however, many problems that are encountered when using polymers in an underwater application.

The nature of polymers is such that they absorb moisture from the surrounding environment. It is widely known that in the presence of moisture the material properties are altered. There has been a lack of effort to characterize material properties in terms of fatigue, fracture toughness, and viscoelastic deformation in a moisture saturated state. Studies have been conducted relating moisture content and the resulting properties to the degree of cure, i.e. crosslink density. In these studies the crosslink density has been varied by incomplete reaction. The structure can be characterized as dangling chain ends extending into segmental network structures. By varying the crosslink density in this manner material properties such as fracture toughness, glass transition, etc. are altered. These are the same properties that are affected by the presence of moisture therefore the results are misleading.

To more directly ascertain the variations of molecular structure as related to moisture uptake and the resulting moisture induced material properties several thermosetting resins based on the chemistry of Diglycidyl ether of Bisphenol A (DGEBA) are formulated. Crosslink density is varied through extension of the main chain backbone prior to crosslinking. This allows for the cure reaction to proceed to completion resulting in a complete network structure. In addition, representative commercial thermoplastics based on polycarbonate are investigated as to the impact of moisture on material properties.

EXPERIMENTAL

Sample selection includes commercial grade polycarbonate (PC) and polyestercarbonate (PEC) as well as two thermosetting resins of DGEBA crosslinked with Diaminodiphenylsulfone (DDS). The thermosetting structures consist of a tightly crosslinked network (EP1) and loosely crosslinked network (EP2) with molecular weight between crosslink, MC, of 340 and 6300, respectively.