Studies on the aminolysis of RTV silicone rubber and modifications of degradation products

Abstract

The degradation of room temperature vulcanized (RTV) silicone rubbers, swelled in different solvents and followed by the aminolysis reaction using diethylamine as a nucleophilic reagent, was studied. The reaction was further facilitated with a potassium hydroxide catalyst dissolved in ethanol. When the silicone rubber swelled in a less polar solvent, toluene, the main product of the aminolysis reaction was a linear polydimethylsiloxane with hydroxyl and ethoxy end groups (EtO–PDMS–OH), and an average molecular weight of 15,000. On the other hand, when a polar solvent, tetrahydrofuran, was used, the degradation rate was increased, and significant amount of cyclic compounds were found in addition to the linear PDMS. Furthermore, if different alcohols were used to dissolve the KOH catalyst, the degradation rate and the product distribution were both changed. The dissolution time decreased in the following order: glycerol > ethylene glycol > 1,4-butanediol > ethanol. In this study, efforts have also been made to improve the reusability of the aminolysis products by changing their terminal groups.

Introduction

Silicone polymers or polysiloxanes possess several unique properties that are retained over a wide temperature range, such as low temperature flexibility, good dielectric strength and water repellency. Depending on the molecular weight and structures, they are produced in the form of fluids, resins, and elastomers. Silicone elastomers are either room temperature vulcanized (RTV) or heat-cured silicone rubbers, depending on whether crosslinking is carried out at ambient or elevated temperature. Though the mechanical strengths of RTV rubbers are generally lower than those of the corresponding heat-cured silicones, but for many applications, their convenience outweighs their reduced mechanical properties. To increase their strength or some specific properties, fillers like silicon oxide and titanium oxide are generally incorporated into the silicone rubbers [1], [2].

It is costly to recycle the RTV silicone rubbers, because they are thermosets. In addition, the reusability of the recycled silicone rubbers is limited. Most methods to recycle the RTV silicone rubbers are based on the degradation at high temperatures. However, Pappas and coworkers [3] have found that aliphatic primary and secondary amines completely dissolve the crosslinked silicone rubbers at room temperature. The dissolution is caused by the nucleophilic cleavage of Si–O bonds by amines, which occurs selectively at crosslinking sites. The selectivity is attributed to the enhanced silicone electrophilicity at crosslinking site, where the silicon atom is bonded to four oxygen atoms. In comparison, the other silicon atoms in the main chain are bonded to only two oxygen atoms. Their results show the way to recycle the RTV silicone rubbers at mild conditions. Later on in 1980, Pappas and Just [4] used various amines with different functionalities to degrade RTV silicone rubbers. They found that the dissolution time, the time needed to just dissolve the silicone rubber through the aminolysis degradation, increased with the number of carbon atoms in the aliphatic amines. They attributed this to the steric effect on the amine group. They also found that the dissolution is faster for the primary amines than the secondary amines.

Shimmel [5], [6], [7] found that the addition of potassium hydroxide (KOH) could accelerate the aminolysis rate of silicone rubbers by the secondary amines. Several conclusions were made on the steric effect and the need for a base as the catalyst in order to have an effective splitting reaction in the aminolysis process. However, the role of the KOH as a catalyst is not clarified. In order to further reduce the dissolution time, ethanol was employed to dissolve KOH in order to ensure a fixed homogeneous catalyst concentration [8]. In this study, the effects of different swelling solvents on the aminolysis reaction of RTV silicone rubbers were further examined. In addition, a reaction mechanism was proposed to elucidate the need of a base as the catalyst. Furthermore, in order to improve the functionality for possible applications, various chemical modifications of the degradation products have also been investigated.