Page 184 - Engineering Plastics Handbook

Thermoplastic Polyetherimide (PEI) 157

solution, or a solution that is only partially imidized, is used to make a film,
laminate, or other shapes. The polyamide acid is then heated during (or
after) the forming process to remove solvent and water to form the final poly-
imide. This complicates the forming process, which requires a chemical reac-
tion, the elimination of water, in addition to removal of solvent. These
additional requirements can be accommodated in simple operations, e.g.,
casting thin film, where a high surface area is present to aid in solvent and
water removal. However, making more complex shapes is difficult.
The well-known polyimides based on pyromellitic dianhydride (PMDA)
®
and oxy dianiline (ODA)—Eq. (8.2) (Kapton from DuPont Co.) [6–8]—and
the biphenyl dianhydride (BPDA) polymer with oxy dianiline—Eq. (8.3)
®
(Upilex from Ube Co.) [9]—are produced by using variations of the amide-
acid process. The Kapton-type polyimides have thermal transitions
between 360 and 410°C (680 and 770°F). The Upilex R grade has a glass
transition temperature T of about 285°C (545°F). Upilex S resin is a crys-
g
talline polyimide made from BPDAand para-phenylene diamine (PPD). It
has a very high modulus with a T of greater than 500°C (932°F). While
g
these rigid polyimides have good properties, once imidized, they are not
easily melted or re-formed. Simple stock shapes can be made by high-
pressure sintering of polyimide powders. Polyimide parts can be machined
®
from slabs and rod stock formed from the PMDA-ODA (Vespel from
®
DuPont) and BPDA-ODA (Upimol from Ube, Inc.) polyimide powders.
However, these methods are not attractive when large numbers of parts
must be made.
O O

O O H N O NH 2
2
O O
Pyromellitic dianhydride (PMDA) Oxy dianiline (ODA)

O O
OH
HO
NH NH O
O O
Polyamide acid
O O
−H O N N O
2
O O
Polyimide
Equation 8.2 PMDA-ODA polyimide.