Mold corrosion inhibition

The tendency of poly(arylene sulfide) resin to cause mold corrosion upon molding is inhibited by admixing with said poly(arylene sulfide) resin prior to the molding thereof a suitable amount of at least one metal salt of metaboric acid, tetraboric acid, hypophosphorous acid, hypophosphoric acid, metaphosphoric acid, orthophosphoric acid, pyrophosphoric acid or an aliphatic polycarboxylic acid containing 4 to 10 carbon atoms wherein the metal is selected from Groups IA or IIA of the Periodic Table of the Elements.

This invention relates to poly(arylene sulfide) resin compositions. In 
another aspect this invention relates to a method for inhibiting the 
tendency of molten poly(arylene sulfide) resin to cause metal in contact 
therewith to corrode. In a further aspect, this invention relates to a 
method for at least substantially eliminating the tendency of poly(arylene 
sulfide) resin to cause corrosion of the molds employed in molding 
processes. 
Today poly(arylene sulfide) resin engineering thermoplasics having 
outstanding ability to withstand high temperatures and chemical attack are 
commercially available. It has been observed that occasionally during the 
injection molding of certain poly(arylene sulfide) resins mold corrosion 
occurs. It is presently believed that this corrosion is due at least in 
part to the evolution of some sulfur dioxide by the molten poly(arylene 
sulfide) resin. The sulfur dioxide when combined with moisture could 
adversely affect many metal molds. The amount of corrosion has varied with 
different lots of poly(arylene sulfide), with molds of different 
composition, and with molding conditions. The corrosion, when observed, 
has varied from only a slight attack of the mold after extensive molding 
to very extensive damage after molding for only a short time. Molds of 
beryllium copper and Vega tool steel are very susceptible to such mold 
corrosion while molds of stainless steel, such as 303 Stainless, 304 
Stainless, and 316 Stainless, and molds with chrome or nickel plating are 
most resistant to such corrosion. The use of stainless steel molds and 
chrome or nickel-plated molds can pose an economic burden upon plastic 
fabrications that do not already have such molds. Thus, other means of 
combatting the corrosion problem would be beneficial for the development 
of the poly(arylene sulfide) resin technology. 
Accordingly, an object of the present invention is to provide a method for 
reducing the possibility of mold corrosion during the molding of 
poly(arylene sulfide) resins. 
A further object is to provide a novel poly(arylene sulfide) resin molding 
composition. 
Other objects, advantages, and features of this invention will be apparent 
to those skilled in the art upon reading the following description, 
examples, and appended claims. 
In accordance with this invention the tendency of poly(arylene sulfide) 
resin to cause mold corrosion under molding conditions is inhibited by 
incorporating into said poly(arylene sulfide) resin prior to the molding 
thereof a corrosion inhibiting amount of at least one metal salt of 
metaboric acid, tetraboric acid, hypophosphorous acid, hypophosphoric 
acid, metaphosphoric acid, orthophosphoric acid, pyrophosphoric acid, or 
an aliphatic polycarboxylic acid containing 4 to 10 carbon atoms wherein 
the metal is selected from Groups IA and IIA of the Periodic Table of the 
Elements. 
The Periodic Table of the Elements referred to above is given in the 
Handbook of Chemistry and Physics, Chemical Rubber Company, 45th Edition 
(1964), page B-2. 
The present invention can be applied to any normally solid poly(arylene 
sulfide) resins, whether linear, branched, or lightly crosslinked. The 
invention can be used, for example, with poly(arylene sulfide) resins 
prepared as described in U.S. Pat. No. 2,513,188 wherein polyhalo aromatic 
compounds are reacted with sulfur and metal sulfide at the fusion 
temperature. It can also be used with resins manufactured by the method 
described in British Patent No. 962,941 wherein metal salts of 
halothiophenols are heated at a polymerizing temperature. The invention is 
particularly useful with polymers prepared by the solution reaction of 
polyhalo compounds with alkali metal sulfides as described in U.S. Pat. 
No. 3,354,129. If it is desired to employ poly(arylene sulfide) resins of 
lower melt flow than those obtained through the process of the just 
previously mentioned patent, the polymers obtained in that process can be 
modified, e.g., according to the method disclosed in U.S. Pat. No. 
3,699,087 or that disclosed in U.S. Pat. No. 3,717,620. The present 
invention can also be used upon p-phenylene sulfide polymers prepared as 
described in U.S. Pat. No. 3,919,177, wherein p-phenylene sulfide polymers 
are produced by reacting at least one p-dihalobenzene with a mixture in 
which at least one suitable source of sulfur, at least one alkali metal 
carboxylate, and at least one organic amide are contacted. Since the 
techniques of producing poly(arylene sulfide) resins disclosed in the 
above-mentioned patents are known to those skilled in the art, further 
description of those processes will not be set forth herein. For more 
detail one may refer to the specific patents, which are incorporated 
herein by reference. 
The present invention is particularly useful for molding grade poly(arylene 
sulfide) resins. Generally such poly(arylene sulfide) resins have melting 
points in the range of about 280.degree. C. to about 400.degree. C. The 
melt flow of such poly(arylene sulfide) resin, determined by the method of 
ASTM D 1238-70, modified to a temperature of 316.degree. C. using a 5-kg 
weight, generally will be within the range of about 0.5 to about 250, 
preferably about 20 to about 50, g/10 min. 
The present invention is particularly useful for the poly(arylene sulfide) 
resins which are linear, branched, or lightly crosslinked poly-(phenylene 
sulfide) resins. Molding grade poly(phenylene sulfide) resins can be 
molded into a variety of useful articles by molding techniques which are 
known in the art. Molding should be carried out generally above the 
melting point or softening point but below the decomposition point of the 
particular polymer being molded. Suitable molding techniques include 
injection molding, compression molding, vacuum molding, extrusion and the 
like. While the present invention is particularly suitable for preventing 
corrosion that occurs when poly(phenylene sulfide) resins are injection 
molded, it is considered that the invention will substantially eliminate 
corrosion that occurs as a result of any technique involving contacting of 
metal with molten poly(arylene sulfide) resins. 
Specific examples of suitable metal salts include calcium metaborate, 
calcium, tetraborate, lithium metaborate, lithium tetraborate, magnesium 
metaborate, potassium tetraborate, sodium metaborate, sodium tetraborate 
and strontium tetraborate, barium monoorthophosphate, barium 
pyrophosphate, barium hypophosphite, calcium metaphosphate, calcium 
triorthophosphate, lithium dihydrogenorthophosphate, lithium 
triorthophosphate, magnesium hypophosphite, magnesium pyrophosphate, 
magnesium triorthophosphate, potassium monohydrogenorthophosphate, 
potassium triorthophosphate, potassium tetrametaphosphate, potassium 
hexametaphosphate, tetrasodium pyrophosphate, sodium triorthophosphate, 
sodium hypophosphite, strontium diorthophosphate, and cesium 
triorthophosphate. 
Suitable salts of said aliphatic polycarboxylic acids containing 4 to 10 
carbon atoms per molecule include those derived from saturated or 
unsaturated di- and tricarboxylic acids and from saturated, di- and 
tricarboxylic acids additionally substituted with 1 to 4 hydroxyl groups 
per molecule. 
Specific examples of salts derived from the aliphatic polycarboxylic acid 
include sodium succinate, potassium succinate, sodium adipate, sodium 
suberate, potassium maleate, sodium malate, cesium tartrate, sodium 
potassium tartrate, calcium tartrate, dipotassium hydrogen citrate, 
trisodium citrate, trilithium citrate, calcium citrate, disodium 
saccharate and the like and mixtures. Mixtures of the salts of the 
polycarboxylic acids and of the salts of the mineral acids can be employed 
also. 
The amount of salt combined with the poly(arylene sulfide) resin is any 
amount which is sufficient to reduce the tendency of the resin to cause 
mold corrosion. Generally, the salt is employed in an amount in the range 
of about 0.05 to about 5 parts by weight per 100 parts by weight of resin 
(phr) more preferably from about 0.1 to about 1 phr. 
The salt can be incorporated in the poly(arylene sulfide) resin by any 
suitable technique which results in a composition comprising poly(arylene 
sulfide) resin and a suitable amount of said metal salt. Preferably the 
metal salt and the resin are admixed in such a manner as to obtain a 
substantially uniform distribution of the salt in the resin. The mixing 
temperature can range from about room temperature, e.g. 20.degree. C., to 
about 50.degree. C. above the melting point of the resin. 
Like other poly(arylene sulfide) resin compositions, the resin compositions 
of this invention have utility in uses where high melting point and high 
temperature stability are desirable. The poly(arylene sulfide) resins of 
this invention can also include other additives such as fillers, pigments, 
stabilizers, softeners, extenders, and other polymers. In injection 
molding, for example, it is quite common to prepare poly(arylene sulfide) 
resin composition containing about 20 to about 50 weight percent of 
conventional glass fiber filler, based on the weight of the poly(arylene 
sulfide) resin and the glass fiber filler. Generally glass fibers about 
1/4 inch to about 2 inches in length are employed. Also, as known in the 
art, such glass-filled compositions can be prepared by feeding continuous 
glass roving through an extruder along with the thermoplastic. Of course, 
it would be counterproductive to employ an additive which increases the 
corrosion tendency of the present inventive composition. 
The present invention and its advantages will now be determined. 
It has been found difficult to find lab-scale tests which correlate 
consistently well with mold corrosion experienced during injection molding 
of poly(phenylene sulfide), abbreviated PPS for convenience, a presently 
preferred arylene sulfide resin for molding. This suggests several 
mechanisms causing corrosion may be present. However, three lab tests have 
been developed which can be usefully employed to determine whether or not 
a given lot of PPS will probably cause mold corrosion. The tests are 
described as follows: (1) acidity test: The PPS sample in powder form is 
charged to a glass tube 20 cm long and 10 min. O.D. which is closed off at 
the bottom, open at the top and which has a restriction about 7.5 cm from 
the tube bottom narrowing the O.D. in that area to about 1/2 that of the 
remainder of the tube. Sufficient PPS is charged to give a depth of about 
4 cm. The charged tube is stoppered with a cork stopper, placed in a 
heated block maintained at a nominal temperature of 350.degree. C. and 
allowed to equilibrate for 5 minutes. At that time a moist piece of 
alkacid paper (pH paper) about 1 cm long is dropped into the opened tube 
allowing it to fall to the restriction and the tube is restoppered. Color 
changes that occur in the test paper are noted after 30 seconds and at 
intervals thereafter until terminating the test when 30 minutes has 
elapsed. Acidic readings at any time of the test indicate the presence of 
gases that are potential sources of mold corrosion. (2) Copper mirror 
test: The glass tube, method of heating, and sample charging, are as 
described in the acidity test except that the test temperature employed is 
200.degree. C. After charging the tube a commercially obtained copper 
mirror about 2.5 cm long and just wide enough to slide into the glass tube 
is inserted and allowed to fall down to just above the restricted portion. 
The copper mirror consists of a vacuum-deposited film of copper metal 
(having a thickness equivalent to 10.+-.5% transmission of normal incident 
light of 5000 angstroms) on a sheet of clear, transparent, polished glass. 
The mirrors can be obtained from Evaporated Metal Films Corp., Ithaca, 
N.Y. After insertion of the mirror, the tube is stoppered, placed in the 
heating block and remains there for the duration of the test, e.g. up to 
about 24 hours or longer, if necessary. The mirror is periodically removed 
and visually examined by placing it against a white background using a 
standard light source as per ASTM D 1729. A failure time in hours is noted 
when complete removal of the copper film in any part of the mirror occurs 
as shown by the white background showing through. (3) Sulfur dioxide test: 
A 50 mg sample of the PPS powder is inserted into a glass tube similar to 
that previously described having a sealed end. The other end can be 
connected to a gas chromatography device by a valving arrangement. The 
tube is heated to 350.degree. C., held for 15 minutes and the gases are 
passed to the analyzer. The results are expressed in terms of grams 
SO.sub.2 evolved per gram PPS. The greater the SO.sub.2 evolution the 
greater the propensity for mold corrosion.

EXAMPLE 
Blends of polyphenylene sulfide (PPS) with the specified salt inhibitors 
are prepared by individually mixing 25 g portions of PPS (Ryton.sup.R P4) 
with a specified amount of the salt in a Wiring Blender at room 
temperature. The resulting blends, and controls, after recovery were 
individually tested according to the methods previously described. The 
results are presented in the following table. 
TABLE 
__________________________________________________________________________ 
PPS - SALT BLENDS 
Inhibitor Salt Tests 
Run Level 
SO.sub.2 
Copper Mirror 
Acidity Rating 
No. 
Name (phr).sup.(a) 
Wt. % 
hours (after 30 min.) 
Remarks 
__________________________________________________________________________ 
1 calcium hypophosphite 
0.2 0.344 
4-5 moderately strong 
Invention 
1A calcium hypophosphate 
1.0 .312 6-22.sup.(c) 
strong Invention 
2 magnesium sulfate 
0.2 .279 2-3 neutral Control 
2A magnesium sulfate 
1.0 .296 2-3 strong Control 
3 sodium tetraborate 
0.2 .154 6-7 strong Invention 
3A sodium tetraborate 
1.0 .132 4-19 strong Invention 
4 trisodium citrate 
0.2 .149 5-6 basic Invention 
4A trisodium citrate 
1.0 &lt;.01 5-6 basic Invention 
5 trisodium phosphate 
0.2 .184 4-19.sup.(c) 
moderately strong 
Invention 
5A trisodium phosphate 
1.0 .05.sup.(b) 
4-19.sup.(c) 
neutral Invention 
6 tetrasodium pyrophosphate 
0.2 .176 3-4 strong Invention 
6A tetrasodium pyrophsophate 
1.0 .094 3-4 strong Invention 
7 none 0 .241 2-3 strong Control 
__________________________________________________________________________ 
.sup.(a) phr is parts by weight inhibitor per 100 parts by weight resin. 
.sup.(b) average of 6 runs, ranging from 0.0001 to 0.114 
.sup.(c) range indicates the sample failed sometime in this time span. 
That is failure occurred sometime between quitting time for the day and 
resumption of work the following morning. 
The results given in control runs 7 (no inhibitor) and, 2 and 2A (a salt 
out of the scope of the invention) indicate the baseline for comparison 
with the remaining invention runs. Thus, the SO.sub.2 evolved from the 
control compositions is about 0.24 wt. % or higher, the copper mirror is 
degraded in 2-3 hours and the acidity rating after 30 minutes is described 
as strong. Invention run samples generally perform better than the control 
run samples in at least two of the three tests or substantially better in 
at least one test. For example, in invention run 1A, although the SO.sub.2 
evolution exceeds that of control run 7, the value for the copper mirror 
test is at least double that of the control. 
The foregoing description and example have been provided to enable those 
skilled in the art to understand the present invention and its preferred 
embodiments. Obvious variations of the invention claimed below are 
considered to be within the scope of the claimed invention.