Page 72 - Handbook of Plastics Technologies

THERMOPLASTICS

2.12 CHAPTER 2

FIGURE 2.9 Synthesis of nylon.

changes in part dimensions and reduce the properties, particularly at elevated tempera-
96
tures. As a result, the material should be dried before any processing operations. In the
absence of moisture, nylons are fairly good insulators but, as the level of moisture or the
97
temperature increases, the nylons are less insulating.
The strength and stiffness will be increased as the number of carbon atoms between
amide linkages is decreased, because there are more polar groups per unit length along the
98
polymer backbone. The degree of moisture absorption is also strongly influenced by the
number of polar groups along the backbone of the chain. Nylon grades with fewer carbon
atoms between the amide linkages will absorb more moisture than grades with more car-
bon atoms between the amide linkages (nylon 6 will absorb more moisture than nylon 12).
Furthermore, nylon types with an even number of carbon atoms between the amide groups
have higher melting points than those with an odd number of carbon atoms. For example,
99
the melting point of nylon 6,6 is greater than either nylon 5,6 or nylon 7,6. Ring opened
nylons behave similarly. This is due to the ability of the nylons with the even number of
carbon atoms to pack better in the crystalline state. 100
Nylon properties are affected by the amount of crystallinity. This can be controlled to a
great extent in nylon polymers by the processing conditions. A slowly cooled part will
have significantly greater crystallinity (50 to 60 percent) than a rapidly cooled, thin part
101
(perhaps as low as 10 percent). Not only can the degree of crystallinity be controlled,
but also the size of the crystallites. In a slowly cooled material, the crystal size will be
larger than for a rapidly cooled material. In injection molded parts where the surface is
rapidly cooled, the crystal size may vary from the surface to internal sections. 102 Nucleat-
ing agents can be utilized to create smaller spherulites in some applications. This creates
materials with higher tensile yield strength and hardness, but lower elongation and im-
pact. 103 The degree of crystallinity will also affect the moisture absorption, with less crys-
104
talline polyamides being more prone to moisture pickup.
The glass transition temperature of aliphatic polyamides is of secondary importance to
the crystalline melting behavior. Dried polymers have T values near 50°C, while those
g
with absorbed moisture may have T s in the range of 0°C. 105 The glass transition temper-
g
ature can influence the crystallization behavior of nylons. For example, nylon 6,6 may be
above its T at room temperature, causing crystallization at room temperature to occur
g
slowly leading to post mold shrinkage. This is less significant for nylon 6. 106
Nylons are processed by extrusion, injection molding, blow molding, and rotational
molding, among other methods. Nylon has a very sharp melting point and low melt viscos-
ity, which is advantageous in injection molding but causes difficulty in extrusion and blow
molding. In extrusion applications a wide MWD is preferred, along with a reduced tem-
107
perature at the exit to increase melt viscosity.

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