Manufacturing of Filter Tow
Cellulose Acetate Production
THE RAW MATERIAL – CELLULOSE
The basic raw material used to manufacture cellulose acetate polymer is plant-derived cellulose. Cellulose, the structural component of the primary cell wall of green plants, makes up 40 to 50 percent of a tree’s composition. Today, the most common source of cellulose is pulp derived from trees, grown in responsibly managed forests. In many cases, these are managed under guidelines such as the Sustainable Forestry Initiative® Inc.1 or the Forest Stewardship Council (FSC)2.
The natural polymer cellulose is modified by combining it with acetic acid to form cellulose acetate. Acetic acid, one of the simplest organic acids and a main component of vinegar, may be produced by either natural or synthetic chemical processes. Practically, this acetylation of cellulose is not done directly with acetic acid, but via acetic anhydride as reactant, which in turn is obtained from acetic acid by elimination of water.
The industrial manufacturing process of cellulose acetate polymer can be divided into four principal steps:
STEP 1: The Activation of Cellulose
The cellulosic raw material (e.g. wood pulp) is mixed with acetic acid and a sulfuric acid catalyst under mechanical stress, resulting in a homogenous slurry. Thereby, the cellulose fibers are being made equally accessible for the reaction, and the even distribution of the reagents is facilitated and ensured.
STEP 2: The Acetylation
Acetic anhydride is added to the mixture from the activation step and reacts, catalyzed by sulfuric acid, with cellulose to form fully acetylated cellulose (cellulose triacetate) and acetic acid. The cellulose fibers are only completely dissolved at the end of the acetylation step, therefore the reaction cannot be stopped at a lower degree of acetylation.
STEP 3: The Hydrolysis
For many applications, fully acetylated cellulose does not offer the desired properties (e.g. with respect to solubility). Therefore, water is added to the mixture from the acetylation step to remove a certain number of acetate groups from the cellulose acetate polymer molecules. In chemical terms, this reaction is called a hydrolysis. Based on production quantity, in most cases around 20% of the acetate groups are removed, resulting in so-called cellulose diacetate (actually a cellulose-2.5-acetate), which is soluble in acetone and used for filter tow and textile fibers production. Finally, the sulfuric acid is neutralized.
STEP 4: Precipitation and processing
Cellulose acetate is precipitated in the form of flakes by addition of larger amounts of water. Subsequently, the flakes are washed with water to remove remaining acid. To form the final product, the flakes are dried.
Manufacturing Filter Tow from Flakes
As with cellulose-2.5-acetate in textile applications, the core process in the manufacturing of cellulose acetate filter tow is dry spinning using acetone as a solvent.
The production of filter tow can be described in four principal steps.
STEP 1: Preparation of the spinning solution
Cellulose acetate flakes and acetone are mixed together intensively. A small amount of titanium dioxide is added as a delustering agent for the later product. To achieve a spinning solution free of particles able to block the spinnerets, the mixing is followed by an intensive filtration.
STEP 2: Spinning
After passing the spinneret of a spinning cell the acetone starts to evaporate from the spinning solution. The evolving filaments solidify and are brought together while the acetone is almost completely removed and recovered.
STEP 3: Crimping
The resulting filaments from numerous spinnerets are combined to form a cable of up to several tens of thousands of filaments. These cables are subsequently crimped in a stuffer box to obtain the typical filter tow structure.
STEP 4: Drying and processing
The fresh crimped structure is fixed by drying, and the filter tow is compressed in bale presses to form filter tow bales.