Page 66 - Handbook of Plastics Technologies

THERMOPLASTICS

2.6 CHAPTER 2

TABLE 2.2 Selected Mechanical Properties of Cellulose Esters
Cellulose Cellulose Cellulose
acetate acetate butyrate acetate propionate

Composition, %
Acetyl 38–40 13–15 1.5–3.5
Butyrl – 36–38 –
Propionyl – – 43–47
Hydroxyl 3.5–4.5 1–2 2–3

Tensile strength at fracture, 23 °C, MPa 13.1–58.6 13.8–51.7 13.8–51.7
Ultimate elongation, % 6–50 38–74 35–60
Izod impact strength, J/m
Notched, 23°C 6.6–132.7 9.9–149.3 13.3–182.5
Notched, –40°C 1.9–14.3 6.6–23.8 1.9–19.0
Rockwell hardness, R scale 39–120 29–117 20–120

Percent moisture absorption at 24 hr 2.0–6.5 1.0–4.0 1.0–3.0
Formulation of cellulose esters is required to reduce charring and thermal discolora-
tion, and typically includes the addition of heat stabilizers, antioxidants, plasticizers, UV
44
stabilizers, and coloring agents. Cellulose molecules are rigid due to the strong intermo-
lecular hydrogen bonding that occurs. Cellulose itself is insoluble and reaches its decom-
position temperature prior to melting. The acetylation of the hydroxyl groups reduces
intermolecular bonding and increases free volume, depending on the level and chemical
45
nature of the alkylation. CAs are thus soluble in specific solvents but still require plasti-
cization for rheological properties appropriate to molding and extrusion processing condi-
tions. Blends of ethylene vinyl acetate (EVA) copolymers and CAB are available.
Cellulose acetates have also been graft-copolymerized with alkyl esters of acrylic and
methacrylic acid and then blended with EVA to form a clear, readily processable thermo-
plastic.
CA is cast into sheet form for blister packaging, window envelopes, and file tab appli-
cations. CA is injection molded into tool handles, toothbrushes, ophthalmic frames, and
appliance housings and is extruded into pens, pencils, knobs, packaging films, and indus-
trial pressure-sensitive tapes. CAB is molded into steering wheels, tool handles, camera
parts, safety goggles, and football noseguards. CAP is injection molded into steering
wheels, telephones, appliance housings, flashlight cases, and screw and bolt anchors and is
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extruded into pens, pencils, toothbrushes, packaging film, and pipe. Cellulose acetates
are well suited for applications that require machining and then solvent vapor polishing,
such as in the case of tool handles, where the consumer market values the clarity, tough-
ness, and smooth finish. CA and CAP are likewise suitable for ophthalmic sheeting and in-
jection molding applications that require many post-finishing steps. 47
Cellulose acetates are also commercially important in the coatings arena. In this syn-
thetic modification, cellulose is reacted with an alkyl halide, primarily methylchloride to
yield methylcellulose or sodium chloroacetate to yield sodium cellulose methylcellulose
(CMC). The structure of CMC is shown below in Fig. 2.3. CMC gums are water soluble
and are used in food contact and packaging applications. Its outstanding film-forming
properties are used in paper sizings and textiles, and its thickening properties are used in
starch adhesive formulations, paper coatings, toothpaste, and shampoo. Other cellulose es-

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