Page 73 - Handbook of Plastics Technologies

THERMOPLASTICS

THERMOPLASTICS 2.13

When used in injection molding applications, nylons have a tendency to drool, due to
their low melt viscosity. Special nozzles have been designed for use with nylons to reduce
this problem. 108 Nylons show high mold shrinkage as a result of their crystallinity. Aver-
age values are about 0.018 cm/cm for nylon 6,6. Water absorption should also be consid-
ered for parts with tight dimensional tolerances. Water will act to plasticize the nylon,
relieving some of the molding stresses and causing dimensional changes. In extrusion, a
screw with a short compression zone is used, with cooling initiated as soon as the extru-
date exits the die. 109
A variety of commercial nylons are available, including nylon 6, nylon 11, nylon 12,
nylon 6,6, nylon 6,10, and nylon 6,12. The most widely used nylons are nylon 6,6 and ny-
lon 6. 110 Specialty grades with improved impact resistance, improved wear, or other prop-
erties are also available. Polyamides are used most often in the form of fibers, primarily
nylon 6,6 and nylon 6, although engineering applications are also of importance. 111
Nylon 6,6 is prepared from the polymerization of adipic acid and hexamethylenedi-
amine. The need to control a 1:1 stoichiometric balance between the two monomers can be
improved by the fact that adipic acid and hexamethylenediamine form a 1:1 salt that can
be isolated. Nylon 6,6 is known for high strength, toughness, and abrasion resistance. It
112
has a melting point of 265°C and can maintain properties up to 150°C. Nylon 6,6 is
used extensively in nylon fibers that are used in carpets, hose and belt reinforcements, and
tire cord. Nylon 6,6 is used as an engineering resin in a variety of molding applications
such as gears, bearings, rollers, and door latches because of its good abrasion resistance
and self-lubricating tendencies. 113
Nylon 6 is prepared from caprolactam. It has properties similar to those of nylon 6,6,
but a lower melting point (255°C). One of the major applications is in tire cord. Nylon
6,10 has a melting point of 215°C and lower moisture absorption than nylon 6,6. 114 Nylon
11 and nylon 12 have lower moisture absorption and also lower melting points than nylon
6,6. Nylon 11 has found applications in packaging films. Nylon 4,6 has found applications
in a variety of automotive applications due to its ability to withstand high mechanical and
thermal stresses. It is used in gears, gearboxes, and clutch areas. 115 Other applications for
nylons include brush bristles, fishing line, and packaging films.
Additives such as glass or carbon fibers can be incorporated to improve the strength
and stiffness of the nylon. Mineral fillers are also used. A variety of stabilizers can be
added to nylon to improve the heat and hydrolysis resistance. Light stabilizers are often
added as well. Some common heat stabilizers include copper salts, phosphoric acid esters,
and phenyl-β-naphthylamine. In bearing applications, self-lubricating grades are available
that may incorporate graphite fillers. Although nylons are generally impact resistant, rub-
ber is sometimes incorporated to improve the failure properties. 116 Nylon fibers do have a
tendency to pick up a static charge, so antistatic agents are often added for carpeting and
other applications. 117
2.2.5.1 Aromatic Polyamides (Polyarylamides). A related polyamide is prepared when
aromatic groups are present along the backbone. This imparts a great deal of stiffness to
the polymer chain. One difficulty encountered in this class of materials is their tendency to
decompose before melting. 118 However, certain aromatic polyamides have gained com-
mercial importance. The aromatic polyamides can be classified into three groups: amor-
phous copolymers with a high T , crystalline polymers that can be used as a thermoplastic,
g
and crystalline polymers used as fibers.
The copolymers are noncrystalline and clear. The rigid aromatic chain structure gives
the materials a high T . One of the oldest types is poly(trimethylhexamethylene terephtha-
g
®
latamide) (Trogamid T ). This material has an irregular chain structure, restricting the ma-
119 ®
terial from crystallizing, but a T near 150°C. Grilamid TR55 is another polyamide
g

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