Page 291 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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Encyclopedia of Physical Science and Technology En012c-604 July 26, 2001 16:2

800 Polymers, Thermally Stable

CO CO
sol n or melt
N R N + H 2 N R′ NH 2 peroxide catalyst O R 1 O R 1
CO CO N ∆ N
R + R
O
O
CO CO
(LI) R 1 ∆
N R N O O N R 1
NH R′ NH N
CO CO (XLIX) R 1 O
n O O
R N
O
∆ pressure
The resulting modified materials have exhibited en-
cross-linked hanced mechanical and environmental (hot/wet) prop-
resin
erties over the base BMI resin system. For example,
2
compared with a fracture toughness (G IC = 50 J/m )
for base resin, the optimized cured BMI/allylphenyl
BMIs have attractive processing characteristics, sim-
comonomer blend exhibited significantly enhanced tough-
ilar to the epoxy resins but with the bonus of higher 2
ness (G IC = 500–600 J/m ). Properties of the carbon fiber-
temperature performance. Prepregs are produced by im-
reinforced laminates have reflected the improved prop-
pregnating fiber matrix from the melt or from a solu-
erties of the unreinforced resins. As discussed earlier
tion of the monomers. BMI resins are available in a
(Section 2), the blending of BMIs with cyanate ester (CE)
wide range of commercial resin systems. Although sin-
resins has enhanced the toughness characteristics of the
gle monomers can be used, they invariably produce rather R
BMIs, resulting in a range of (Skyflex ) BT resins.
brittle products and more frequently eutectic mixtures of
The reactivity of the norbornylene (endomethylene
monomers are supplied in order to lower the viscosity of
tetrahydrobenzene) group to “pyrolytic polymerization”
themeltandimprovethewetting/flowbehavioronthefiber
above 300 C forms the basis of the volatileless ther-
◦
matrix.
mal cure of bisarylnadimides (LII) to yield cross-linked
Other techniques have been used to improve the tough-
resins.
ness characteristics of the BMIs. The introduction of flex-
ible groups, e.g., urethane or epoxy segments in the poly- CO CO CO CO >300°C
N R N A N R N Cross-linked
mer chain, while improving toughness, limits the thermal CO CO CO n=2 CO resin
(LII)
stability of the BMI resin. Addition of liquid rubber during
cure has produced a two-phase system that contributes to R = CH 2 , , O
a much tougher resin. Unfortunately, the presence of the e.g.,
CO C(CF 3 ) 2
rubber modifier also increases susceptibility to hot/wet A = ,
degradation.
Improvementstotheprocessingandmechanicalproper- In the thermal polymerization, the rate-determining
ties of BMI resins have also been achieved by melt blend- steps involve reverse Diels–Alder reactions to produce
ing base monomers, including close-to-eutectic mixtures, N-arylmaleimide moieties, which then provide the key
with propenyl (L) or allyl (LI) substituted comonomers. monomers for the final polymerization to the cross-
Proposed reaction schemes that postulate Diels–Alder cy- linked resin. Overall fabrication problems associated with
cloaddition mechanisms are shown below. this process led to the development of the PMR poly-
imides. These are formed by the in situ polymerization of
monomeric reactants on the fiber matrix according to the
O
CH 3 scheme shown below.
CH 3
∆
R + N R 1 R Processing of fiber composites (prepregs) involves con-
O
version of the initially formed low-molecular-weight pre-
O N
(L) O polymer into the required component by compression or
R 1
∆ autoclave moulding.
O
R 1 CO
N O PMR-15 (LIII, R = CH 2 A = ), a key matrix
resin for high temperature applications, is the most widely
N R 1
O
O commercially accepted of the norbornene-terminated
CH 3
R range of addition PIs. Compared with other high-
O
temperature materials, it is relatively easy to process, and
N
O carbon fiber reinforced composites can be fabricated by a
R 1
variety of techniques. Uses of these materials have been