Page 65 - Handbook of Plastics Technologies

THERMOPLASTICS

THERMOPLASTICS 2.5

does affect molecular weight, molecular weight distribution, degrees of orientation, and
morphological structure. Material described commonly as “cellulose” can actually contain
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hemicelluloses and lignin. Wood is the largest source of cellulose, is processed as fibers
to supply the paper industry, and is widely used in housing and industrial buildings. Cot-
ton-derived cellulose is the largest source of textile and industrial fibers, with the com-
bined result being that cellulose is the primary polymer serving the housing and clothing
industries. Crystalline modifications result in celluloses of differing mechanical proper-
ties, and Table 2.1 compares the tensile strengths and ultimate elongations of some com-
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mon celluloses.

TABLE 2.1 Selected Mechanical Properties of Common Celluloses

Tensile strength, MPa Ultimate elongation, %
Form Dry Wet Dry Wet
Ramie 900 1060 2.3 2.4
Cotton 200–800 200–800 12–16 6–13
Flax 824 863 1.8 2.2
Viscose rayon 200–400 100–200 8–26 13–43
Cellulose acetate 150–200 100–120 21–30 29–30

Cellulose, whose repeat structure features three hydroxyl groups, reacts with organic
acids, anhydrides, and acid chlorides to form esters. Plastics from these cellulose esters are
extruded into film and sheet and are injection molded to form a wide variety of parts. Cel-
lulose esters can also be compression molded and cast from solution to form a coating.
The three most industrially important cellulose ester plastics are cellulose acetate (CA),
cellulose acetate butyrate (CAB), and cellulose acetate propionate (CAP), with structures
as shown in Fig. 2.2.
These cellulose acetates are noted for their toughness, gloss, and transparency. CA is
well suited for applications requiring hardness and stiffness, as long as the temperature
and humidity conditions don’t cause the CA to be too dimensionally unstable. CAB has
the best environmental stress cracking resistance, low temperature impact strength, and di-
mensional stability. CAP has the highest tensile strength and hardness. A comparison of
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typical compositions and properties for a range of formulations is given in Table 2.2.
Properties can be tailored by formulating with different types and loadings of plasticizers.

FIGURE 2.2 Structures of cellulose acetate, cellulose acetate butyrate, and cellulose acetate propi-
onate.

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