Cellulose acetate is a substance that, like a number of other materials used in human industry, owes its existence to cellulose, a naturally occurring polysaccharide found in plants. (A polysaccharide is a carbohydrate molecule made up of a great many repeating sugar units; glycogen, a storage form of glucose in humans and other animals, is another polysaccharide.) First developed in the 1860s, cellulose acetate eventually changed the motion-picture industry by making it possible to store images on a substance that didn't have a tendency to burst into flames, as did the celluloid-based cousins of the material that predated cellulose acetate in the film world.
While cellulose acetate was eventually replaced by polyester in the making of film, it turned out to be an exceedingly versatile substance. It is strongly associated with the modification of cotton and rightly so, but it has found a home in a number of other applications as well.
What Is Cellulose?
Cellulose is a polymer of glucose molecules. In turn, glucose – which is the primary source of energy for living cells whether it is ingested (as in animals) or synthesized (as in plants) – is a six-carbon molecule that includes a hexagonal ring. One of the six carbons lies above the ring and is attached to an -OH, or hydroxyl, group; two of the carbons within the ring itself are also attached to a hydroxyl group. These three -OH groups can readily react with other molecules to form hydrogen bonds.
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Other polymers of glucose exist, but in cellulose, which is made by a variety of plants, the individual glucose monomers are the most extended, or stretched out. Also, individual cellulose chains line up next to one another in parallel, which encourages hydrogen bonds between adjacent chains and strengthens the entire cellulose structure. In the cotton type of cellulose, the chains are so tightly bound and aligned that it is difficult to dissolve them using conventional non-aggressive methods, such as merely getting them wet.
History of Cellulose Derivatives
In the early days of motion pictures, in the beginning of the 20th century, the film run through projectors consisted of nitrocellulose, which went by the trade name Celluloid. Like a lot of nitrogen-rich compounds, nitrocellulose is highly combustible, and in fact can catch on fire spontaneously under the right conditions. Because of the heat generated by projectors and the obvious need to keep the film dry, this set the stage, so to speak, for fiery mishaps at precisely the least opportune times.
Back in 1865, a French chemist, Paul Schützenberger, discovered that if he mixed wood pulp, which is rich in cellulose, with a compound called acetic anhydride, the latter substance was able to worm its way between the hydrogen-bonded cellulose chains and attach itself to the many hydroxyl groups available there. Initially, this newfound substance, cellulose acetate, was not put to any use. But 15 years later, the Swiss brothers Camille and Henri Dreyfus discovered that cellulose acetate could be dissolved in the strong solvent acetone and then re-formed into a variety of different compounds. For example, when it is assembled into thin solid sheets, it can be used as film.
Cellulose Acetate Structure
Recall that glucose molecules include three hydroxyl groups, one of them attached to the carbon exterior to the hexagonal rings and two others projecting from the ring itself. The hydrogen atom of the hydroxyl group, which is attached to the oxygen that is also attached to carbon on the other side, can be readily displaced by certain molecules that then take that hydrogen's spot in the parent glucose construct. One of these molecules is acetate.
Acetate, the form of acetic acid that has lost its acidic hydrogen, is a two-carbon compound often written CH3COO-. This implies that acetate has a methyl (CH3-) group at one end and a carboxyl group at the other end. A carboxyl group has a double bond with one oxygen and a single bond with the other. Since oxygen can form two bonds and carries a negative charge when it only has one bond, it is at this oxygen that the acetate becomes bound to the glucose molecule where a hydroxyl group previously sat intact.
Cellulose acetate as the term is commonly used actually refers to cellulose diacetate, in which two of the three available hydroxyl groups in each glucose monomer have been replaced by acetate. If enough acetate is made available, the remaining hydroxyl groups also begin to be replaced by acetate groups, forming cellulose triacetate.
Acetic acid, by the way, is the active ingredient in vinegar. In addition, an acetic acid derivative called acetyl coenzyme A, or acetyl CoA, is a key molecule in the tricarboxylic acid (TCA) cycle in aerobic cellular respiration.
Uses of Cellulose Acetate
As noted, cellulose acetate has been largely replaced by a form of polyester in the making of film, but both are largely passe now that digital photography and filmography have rapidly become the standard of the times. Cellulose acetate is also a major component of cigarette filters.
When aircraft came on the scene in the early 1900s, chemists soon found that cellulose acetate could be layered into the material used to form the bodies and wings of airplanes and thereby make them sturdier without adding a great deal of extra weight.
Acetate fabrics, as they are called, are everywhere in the clothing world. Cotton shirts are one popular product that includes acetate material. (When you see "acetate" on a clothes label, what is actually being listed is cellulose acetate.) But in the earliest uses of cellulose acetate in the garment industry, it was actually used in conjunction with silk, a more expensive treat, than as the basis for mass-produced, inexpensive attire. Here, it was used to help maintain the intricate patterns often seen in silk materials.
In the 1940s, when it was possible to make transparent forms of the material, cellulose acetate found a home in the U.S. Department of Defense, which used it to make aircraft windows and the eye-covering portions of gas masks. Today it is used in various plastics and remains a common alternative to glass windows, though it has been largely supplanted by acrylic in this regard.
Cellulose Acetate and the Environment
Cellulose acetate products are by definition made to resist degradation of all types, and in particular chemical degradation. This means that when you think of a list of "biodegradable" products, anything made with cellulose acetate should sit at the bottom of your mental list, because these products persist in the environment for long periods in which they become litter. (Consider the number of cigarette butts you probably saw the last time you took a stroll along a typical roadway. Unfortunately, these are not quite large enough, a la bottles and cans, to be spotted and picked up by litter crews, but they are ubiquitous enough to present as a collective eyesore.)
When cellulose acetate products sit in the sun for long enough, the light energy that strikes them can begin to dissolve the cellulose acetate. This allows molecules in the environment, mostly esterases, to attack the bonds in cellulose acetate in earnest. This combination "attack" is known as photochemodegradation.