Polyamide compositions

The high temperature aging of polyamides is substantially improved by incorporating therein an antioxidant and an oxide of a metal of Group II or III of the Periodic Table, preferably a crystalline aluminosilicate such as a synthetic zeolite. The amount of antioxidant is preferably 1 to 5% and the amount of metal oxide is preferably 0.5 to 5%. Polyamides which can be stabilized in this way include Nylon-6, Nylon-11 and Nylon-12.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to polyamide compositions containing stabilizers 
which retard embrittlement of the polyamide when exposed to high 
temperatures. 
2. Introduction of the Invention 
When exposed to elevated temperatures, polyamides become brittle. Many 
polyamides are commercially available in the form of stabilized 
compositions which contain limited amounts, less than 0.5% by weight, of 
antioxidants. However, even these stabilized compositions fall far short 
of practical requirements. 
SUMMARY OF THE INVENTION 
I have now discovered that the embrittlement of polyamides can be retarded 
by incorporating therein an antioxidant and an oxide of a metal of Group 
II or III of the Periodic Table. 
In one aspect, the present invention provides a composition which comprises 
(a) an organic polymer component which comprises a polyamide: 
(b) distributed in said polymer component, at least 0.5% by weight, based 
on the weight of the polyamide, of an antioxidant; and 
(c) dispersed in said polymer component, a compound of a metal of Group II 
or III of the Periodic Table which can function as an acid scavenger, 
preferably an oxide, chloride or sulfate. 
DETAILED DESCRIPTION OF THE INVENTION 
The organic polymer component generally comprises at least 70%, preferably 
at least 80%, particularly at least 90%, especially essentially 100%, of 
at least one polyamide. [Percentages are by weight throughout this 
specification.] The polyamide generally comprises at least 25%, preferably 
at least 75%, particularly essentially 100%, of units having the formula 
EQU --NH--(CH.sub.2).sub.p --CO-- 
where p is an integer, preferably 6, 9, 10, 11 or 12. Preferred polyamides 
are Nylon-6 (polycaprolactam), Nylon-6,6 (polyhexamethylene adipamide), 
Nylon-6,9 (polyhexamethylene azelaamide), Nylon-6,10 (polyhexamethylene 
sebacamide), Nylon-6,12 (polyhexamethylene dodecanoamide), Nylon-11 
(polyundecanoamide) and Nylon-12 (polydodecanoamide). The polymer 
component can (but preferably does not) contain one or more other polymers 
which are not polyamides, e.g. an olefin homopolymer or copolymer. 
The metal compound which is used (in conjunction with an antioxidant) as a 
stabilizer in this invention is a compound of a metal of Group II or III 
of the Periodic Table, preferably Mg, Ca, Zn, Mn or Al. Oxides are the 
preferred metal compounds. The oxide may be present as a discrete 
compound, e.g. zinc oxide or alumina, or as part of a mixed oxide, e.g. a 
crystalline aluminosilicate or other mineral. Two or more oxides can be 
used together. Particularly good results have been obtained using a 
zeolitic molecular sieve, preferably a synthetic zeolite. Suitable 
zeolites are disclosed in the Encyclopaedia of Chemical Technology 
(Kirk-Othmer), 3rd Edition, Volume 15, pages 638-669, the disclosure of 
which is incorporated herein by reference. The amount of oxide used is at 
least 0.5%, e.g. 0.5 to 5%, especially at least 1%, e.g. 1 to 3%, by 
weight of the polyamide. The oxide is preferably free from any coupling 
agent or other additive which substantially reduces its efficacy as an 
acid scavenger. 
The amount of antioxidant used in this invention is at least 0.5%, e.g. 0.5 
to 7.5%, preferably 1 to 5%, based on the weight of the polyamide. Any 
type of antioxidant can be used in this invention, but particularly good 
results have been obtained using the following classes of antioxidant: 
(1) aromatic compounds containing amine groups, preferably a reaction 
product of an aromatic polyamine and an aliphatic ketone, e.g. 
di-naphthyl-p-phenylene diamine or a reaction product of diphenylamine and 
acetone; 
(2) aromatic mercapto compounds and salts thereof, e.g. a metal salt of 
2-mercapto benzothiazole; and 
(3) compounds containing at least one, preferably two or more, hindered 
phenol moieties, e.g. a 3,5-di-t-butyl-4 hydroxy phenyl group, for example 
N,N'-hexamethylene bis(3,5-di-t-butyl-4-hydroxy hydrocinnamide) or 
tetrakis[methylene3-3',5'-di-t-butyl-4-hydroxy phenylpropionate]methane. 
The antioxidant will usually be a discrete compound which is dispersed in 
the polymer but the invention includes antioxidants which are chemically 
linked to the polymer. 
The compositions of the invention can be in the form of compositions 
suitable for melt-shaping, e.g. by extrusion or molding, or in the form of 
shaped articles, which can optionally be cross-linked, e.g. by 
irradiation. Particularly useful are cross-linked articles which have been 
rendered heat-recoverable. In this connection, reference should be made to 
copending and commonly assigned Application Ser. No. 324,222, filed Nov. 
23, 1981 by Richards et al., which describes the radiation cross-linking 
of polyamides comprising substantial amounts of Nylon-11 and Nylon-12 
units and the disclosure of which is incorporated herein by reference. 
The compositions can contain other ingredients, in addition to the polymer, 
antioxidant and metal oxide, e.g. organic and/or inorganic flame 
retardants, fillers and processing aids. The amount of such additives may 
be, for example, 10 to 40% by weight of the composition. Preferred flame 
retardants are disclosed in Ser. No. 324,222 incorporated by reference 
herein. The compositions preferably contain sufficient pigment to mask the 
yellowing of the polyamide which often takes place on exposure to elevated 
temperatures.