Process for the preparation of a thermoplastically processable aromatic polyamide with phosphorus acid catalyst

A thermoplastically processable aromatic polyamide is prepared by polycondensing diacid monomer A with diamine monomer B: (A) HOOC--Ar--COOH;(B) H.sub.2 N--Ar'--NH.sub.2 ; wherein Ar is 1,3-- or 1,4-phenylene; 1,4--, 1,5--, 2,6-- or 2,7-naphthylene, ##STR1## and Ar' is ##STR2## wherein X is --SO.sub.2 -- or --CO--; Y is --O-- or --S--; Z is --O--, --S--, --SO.sub.2 --, --CO-- or ##STR3## wherein R and R' each is --H or C.sub.1 -- to C.sub.4 -alkyl and n is 0 or 1 in the melt at a temperature in the range of from 200.degree. to 400.degree. C. in the presence of a catalyst selected from the group consisting of alkyl- or aryl-phosphonic acids, -phosphonous acids, -phosphinic acids, esters thereof, halides thereof and mixtures thereof, the catalyst content being 0.01 to 2.0 mol % relative to the total content of components A and B.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a process for the preparation of a 
thermoplastically processable aromatic polyamide. 
2. Description of the Background 
In principle, the preparation of thermoplastically processable aromatic 
polyamides are known (see, for example, DE-A-3,609, 011; Brode et al., 
Polym. Prepr., Am. Chem. Soc., Div. Polym. Chem. 15, 761 (1974); Adv. 
Chem. Ser. (1975), 142; CA 84, 5530 sf). However, these aromatic 
polyamides have a high melt viscosity. Consequently, very high 
temperatures are necessary during their preparation and processing, 
usually at least 350.degree. C. At these temperatures, damage to the 
product is often observed, recognizable from discolorations or an 
impairment of the mechanical properties. 
Aromatic polyamides are used as high temperature resistant materials in the 
aerospace, automobile, electrical and electronics industry, where they are 
exposed to temperatures in the region of 200.degree. C. and above in the 
presence of atmospheric oxygen. Under these conditions, discoloration of 
the product readily occurs with simultaneous diminution of the mechanical 
properties. 
There have already been many attempts to redress these deficiencies. In 
particular, it has been proposed to carry out the polycondensation of the 
aromatic dicarboxylic acids and aromatic diamines in the presence of 
triphenyl phosphite or of an acid which has been derived from phosphorus 
having the formula:H.sub.3 PO.sub.m, where m is 2, 3 or 4 as catalyst and 
of a dialkylaminopyridine as cocatalyst (DE-A-3,539,846, 3,601,011, 
3,609,011, 3,731,185, and German Patent Application number P 39 35 466.0). 
However, the resulting polyamides cannot always completely satisfy the 
prescribed requirements. A need therefore continues to exist for aromatic 
polyamides of improved properties. 
SUMMARY OF THE INVENTION 
Accordingly, one object of the present invention is to provide molding 
compositions based on aromatic polyamides which exhibit improved heat 
stability. 
Briefly, this object and other objects of the present invention as 
hereinafter will become more readily apparent can be attained in a process 
for preparing a polyamide by polycondensing the monomers: 
##STR4## 
wherein Ar is 1,3- or 1,4-phenylene; 1,4-, 1,5-, 2,6- or 2,7-naphthylene, 
##STR5## 
wherein: X is --SO.sub.2 -- or --CO--; 
Y is --O-- or --S--; 
Z is --O--, --S--, --SO.sub.2 --, --CO-- or 
##STR6## 
wherein R and R' each is --H or C.sub.1 -- to C.sub.4 -alkyl; and n is 0 
or 1, 
in the melt at a temperature in the range of from 200.degree. to 
400.degree. C. in the presence of a catalyst of an alkyl- or 
aryl-phosphonic acid, -phosphonous acid, -phosphinic acid, ester thereof, 
halide thereof or combinations thereof in an amount of 0.01 to 2.0 mole % 
relative to the content of monomers A and B. 
Suitable aromatic dicarboxylic acids (component A) include isophthalic 
acid, terephthalic acid, 1,4-, 1,5-, 2,6-and 2,7-naphthalenedicarboxylic 
acid, 4,4'-diphenyl ether dicarboxylic acid or 
4,4'-benzophenonedicarboxylic acid, 4,4'-diphenyl sulfone dicarboxylic 
acid, 4,4'-biphenyl-dicarboxylic acid and mixtures thereof. Isophthalic 
acid and terephthalic acid may be substituted on the phenylene group by 
alkoxy, aryloxy or alkyl groups. An example of a suitable acid of this 
type is 2-phenoxyterephthalic acid. 
Examples of suitable aromatic diamines (component B) include 
4,4'-bis(p-aminophenoxy)diphenyl sulfone, 4,4'-bis(m-aminopheloxy)diphenyl 
sulfone, 4,4'-bis(p-aminophenoxy)-benzophenone, 
4,4'-bis(m-aminophenoxy)benzophenone, 
4,4'-bis(p-aminophenylmercapto)benzophenone, 
4,4'-bis(p-aminophenylmercapto)diphenyl sulfone and mixtures thereof. 
The components A and B are usually reacted together in approximately 
equimolar quantities. 
The molecular weight of the polyamide product can be regulated by adding, 
as component C, aromatic monocarboxylic acids (see German Patent 
Application P 39 35 468.7) or aromatic carboxamides of the formula given 
below to the reaction medium. These additions are generally 0.01 to 10, 
preferably 0.05 to 5 mol %, relative to the total of components A and B. 
##STR7## 
In the above formula, Ar and Ar' are optionally substituted phenyl 
radicals. Examples of suitable molecular weight regulators are 
benzanilide, 4-chlorobenzoic acid anilide, tolylanilide, 
4-(p-benzoylamidophenoxy)-4'-phenoxy-diphenyl sulfone and 
2-naphthalenecarboxylic acid anilide. 
The molecular weight of the polyamide can also be regulated by using 
aromatic carboxylic acids such as, for example, benzoic acid, 
naphthalenecarboxylic acid, chlorobenzoic acid and/or aliphatic carboxylic 
acids having 1 to 20 carbon atoms, on the one hand mixed with aromatic 
amines such as, for example, aniline, chloroaniline, naphthylamine, 
4-(p-aminophenoxy)diphenyl sulfone, and/or, on the other hand, with 
aliphatic amines having 4 to 20 carbon atoms. In these mixtures, 
carboxylic acid and amine are preferably used in equimolar amounts (see 
DE-A-3,804,401). 
Finally, it is also possible to regulate the molecular weight by using the 
component A in excess (see German Patent Application P 39 35 467.9). 
The catalysts used are preferably compounds of the following formulae: 
##STR8## 
In these formulae, R, R' and R", independently of one another, are phenyl, 
1- or 2-naphthyl, 2-, 3- or 4-chlorophenyl, 2-, 3- or 4-tolyl, C.sub.1 - 
to C.sub.20 -alkyl or -cycloalkyl; and 
X is Cl or Br. 
The polycondensation reaction is preferably carried out in the presence of 
a catalyst and additionally in the presence of a cocatalyst selected from 
the group of pyridines having at least one nucelophilic substituent, 
tin(II) compounds, and mixtures thereof. The catalyst and cocatalyst 
contents each range from 0.01 to 2.0, preferably 0.02 to 1.0 mol %, 
relative to the total of components A and B. 
The relative proportions of catalyst and cocatalyst can be selected as 
desired within the above-mentioned limits. Preferably, the phosphorus 
compound and the substituted pyridine are used in the molar ratio of 1:1, 
the phosphorus compound and the tin(lI) compound in the molar ratio of 1:1 
to 2:1, and the phosphorus compound, the substituted pyridine and the 
tin(II) compound in the molar ratio of 1:1:1 to 1:1:0.5. 
The substituted pyridines are preferably compounds of the formula: 
##STR9## 
In this formula, 
##STR10## 
where R and R' are C.sub.1 - to C.sub.20 -alkyl or -cycloalkyl. 
The tin(II) compounds are preferably salts of organic mono- or 
di-carboxylic acids having 2 to 16 carbon atoms in the carbon skeleton 
such as, for example, tin dioctanoate, tin dilaurate, tin dodecanedioate 
and tin oxalate, and also tin alcoholates such as, for example, tin 
glycolate. 
Surprisingly, it has been found that the catalysts and the 
catalyst/cocatalyst combinations employed in the present process are 
clearly superior to conventional catalysts with regard to activity. The 
polyamides which have been prepared by the process of the invention have a 
significantly improved heat stability so that they can be processed at 
temperatures above 340.degree. C. without damage, i.e. without darkening 
and without reduction in the molecular weight. During this processing, the 
solution viscosity, which is a measure of the molecular weight, does not 
diminish. Injection-molded test specimens from a polyamide which has been 
prepared according to the process disclosed in DE-A-3,609,011 developed an 
intense brown coloration after storage for only 24 h at 200.degree. C. in 
air. In contrast, a test specimen from a comparable polyamide according to 
the present invention remained unchanged under corresponding test 
conditions.

Having generally described this invention, a further understanding can be 
obtained by reference to certain specific examples which are provided 
herein for purposes of illustration only and are not intended to be 
limiting unless otherwise specified. 
The comparative examples, not according to the invention, are indicated by 
capital letters. 
The viscosity numbers (J) were determined using 0.5 % by weight solutions 
of the polyamides in a phenol/o-dichlorobenzene mixture (ratio by weight 
i:1) at 25.degree. C. in accordance with DIN 53 728. 
EXAMPLE 1 
A 21.62 g (0.05 mol) amount of 4,4'-bis(p-aminophenoxy)diphenyl sulfone and 
8.64 g (0.052 mol) of isophthalic acid were melted at 250.degree. C. 
together with 48 mg (0.0005 mol) of methylphosphonic acid (Kl) in a 
polycondensation reactor fitted with a stirrer, a nitrogen feed and a 
distillation arm. After 20 minutes, the temperature was increased to 
300.degree. C. Meanwhile, the viscosity of the melt steadily increased and 
the water liberated during the reaction was removed by distillation. After 
30 minutes at 300.degree. C., the reaction was terminated. 
The viscosity number (J) was 28 cm.sup.3 /g. Solid phase postcondensation 
at 250.degree. C. and 0.5 mbar gave, after 24 h, a polyamide having J = 71 
cm.sup.3 /g. 
EXAMPLES 2 TO 15 AND COMATIVE EXAMPLES A AND B 
Examples 2 to 15 and the Comparative Examples A and B were carried out 
similarly to Example 1, but the catalysts were varied as shown in the 
Table below. The catalysts (K) and cocatalysts (CP and CZ) used correspond 
to the formulae as follows: 
______________________________________ 
##STR11## 
##STR12## 
##STR13## 
##STR14## 
##STR15## 
##STR16## 
##STR17## 
##STR18## 
##STR19## 
##STR20## 
##STR21## 
Ex- 
am- (Co) catalysts 
Mol % J J* 
ple K CP CZ K CP CZ [cm.sup.3 /g] 
[cm.sup.3 /g] 
______________________________________ 
1 K1 -- -- 1.0 -- -- 28 71 
2 K2 -- -- 0.6 -- -- 34 86 
3 K4 -- -- 1.0 -- -- 27 68 
4 K7 -- -- 1.0 -- -- 30 78 
5 K3 -- -- 0.5 -- -- 38 81 
6 K5 -- -- 0.5 -- -- 26 69 
7 K6 -- -- 0.5 -- -- 31 75 
8 K1 CP1 -- 0.5 0.5 -- 30 104 
9 K1 CP1 CZ 0.5 0.5 0.5 30 88 
10 K2 CP1 -- 0.2 0.2 -- 30 78 
11 K2 -- CZ 0.5 -- 0.25 
26 104 
12 K4 CP2 -- 0.5 0.5 -- 36 84 
13 K7 CP1 CZ 1.0 1.0 1.0 40 96 
14 K3 CP1 -- 0.5 0.5 -- 37 82 
15 K5 -- CZ 0.5 -- 0.5 29 75 
A TPP -- -- 1.0 -- -- 26 58 
B H.sub.3 PO.sub.3 
-- -- 1.0 -- -- 24 49 
______________________________________ 
*after solid phase postcondensation. 
The polyamides prepared in Examples 1 to 15 and in Comparative Examples A 
and B were pressed at 310.degree. C. and a pressure of 100 bar to give 
sheets of 1-mm thickness and these were stored in a circulating-air oven 
having a fresh air supply of about 10%, for 24 h at 200.degree. C. The 
polyamides which had been prepared for comparative purposes were 
discolored dark brown while the polyamides which had been prepared 
according to Examples 1 to 15 were virtually unchanged. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.