Advances in cellulose ester performance and application

Abstract

In this review, we examine those areas of cellulose ester application in which there has been the greatest recent activity, and we choose to define the word ‘recent’ to include roughly the last 20 years. We focus on work that increases understanding of cellulose ester structure-property relationships, as well as how they relate to performance in specific applications. Our focus is on the performance of cellulose esters in modern coatings, controlled release of actives, plastics (with particular focus on biodegradable plastics), composites and laminates, optical films, and membranes and related separation media. We think that the review will prove useful to those who work with cellulose esters in these and related applications, as well as those who may wish to use this background to help them imagine new ones.

Introduction

Our laboratories have for many years had great interest in the relationships between structure, properties, and performance of cellulose esters. Cellulose is converted to its organic esters, in the laboratory and commercially, for two reasons. First, cellulose is poorly soluble in common solvents and is not melt-processible because it decomposes before it undergoes melt flow. Conversion of cellulose to its esters affords materials that are processible into various useful forms, such as three dimensional objects, fibers, and solutions to be used for coating or casting (of films or membranes, for example). Secondly, the physical properties of cellulose (for example, solubility parameter) can be greatly modified by derivatization. The modified properties of these cellulose derivatives give entry into a range of applications greatly expanded from those available to the parent polysaccharide. It is important to gain a fundamental understanding of how structural changes are effected by cellulose esterification, the ability to predict how those changes will impact properties, and an understanding of how those property changes translate into performance vs. application requirements, in some cases in cooperation with other materials or ingredients.

As will be evident from this review, cellulose esters have been commercially important polymers for nearly a century, and have been known for the better part of two centuries. In spite of their long history, we shall see that there is substantial research activity around novel cellulose esters, new ways to increase their performance, and creative ways to exploit them in commercial applications. It may seem contradictory to describe a family of polymers as having both great longevity and great currency. This seeming contradiction may be simply explained by the fact that cellulose esters are based on, and prepared from, the renewable polysaccharide cellulose. Our society faces the probability of increasing price and dwindling supply of petroleum, and petroleum-based products. At the same time, the environmental impact of the polymers that we use in our daily lives is subject to increasing scrutiny. Society is developing to a larger extent the expectation that the polymeric materials we use will be renewable-based, biodegradable, recyclable, or some combination thereof [1]. Certainly the health impacts of polymeric materials are attracting considerable interest, with endocrine disruption being a particular societal concern [2]. The more careful scrutiny of the polymeric materials of commerce that results from these societal concerns has greatly increased interest in non-toxic, renewable-based materials such as cellulose esters. It is fortunate that esterification of cellulose affords access to a range of properties that make cellulose esters workhorse materials in many applications.

In this review, we shall examine those areas of cellulose ester application in which there has been the greatest recent activity, and we choose to define the word recent to include roughly the last 20 years. We feel that this area has not previously been subject to a review of the scope of this one, and that this time period covers significant developments in the understanding of cellulose ester structure-property relationships, as well as how they relate to performance in specific applications. It is our hope that the following review will prove useful to those who work with cellulose esters in these and related applications, as well as those who may wish to use this background to help them imagine new ones.