Page 185 - Engineering Plastics Handbook

158 Engineering Plastics

O O

O O H 2 N O NH 2

O O
Biphenyl dianhydride (BPDA) Oxy dianiline (ODA)

O O
OH
HO
NH NH O
O O
Polyamide acid

O O
−H O
2
N N O
O O
Polyimide
Equation 8.3 Biphenyl dianhydride ODA polyimide.

The solution-based polyamide-acid process limits the type and speed at
which articles can be made from these polyimides. This limitation is true
of many polyimides that have very rigid backbones. These polyimides,
which have very little flexibility in the polymer chain and a high T , are
g
very hard to melt process into complex shapes, especially on a commer-
cial scale. Such intractable polymers are processed into articles in the
more flexible polyamide-acid form, often with solvent present. The solvent
needs to be removed and recovered to form the final polyimide article. This
is true of polyimides based on PMDA and BPDA with ODA, as discussed
previously. They have only one flexible link in the polymer chain—the
ether group of the ODA—and can only be melt-formed in the amide-acid
stage or by machining from a solid block of resin.
While such rigid polyimides have enjoyed tremendous commercial suc-
cess based on good physical properties, the difficulties in processing
them have limited their applications. It was, therefore, desirable to find
a method to produce polymers that combined the excellent physical prop-
erties of traditional polyimides with the simple melt processability of
other thermoplastics, such as polycarbonates, polyesters, or polyamides.
The key to developing a truly melt-processable polyimide was to build
more flexible linkages into the polymer backbone. This had to be done
in such a way that the polyimide still retained other desirable features
(e.g., heat resistance, strength, high use temperature, and low flam-
mability). Scientists at the General Electric Corporate Research and