Polyoxymethylene compositions and method of manufacturing the same

A polyoxymethylene composition comprises an antioxidant of 0.05 to 5% by weight and a basic compound containing at least one tertiary nitrogen atom and a metallic salt of carboxylic acid in the same one molecule of 0.01 to 2% by weight based on polyoxymethylene.

The present invention relates to a polyoxymethylene composition exhibiting 
improved heat-resistance, and discoloration properties and reduced 
adhesion to metallic molds during molding. 
PRIOR ART AND PROBLEMS TO BE SOLVED 
Polyoxymethylene resins are polymerized from formaldehyde or cyclic 
oligomers thereof, that is, trioxane, or copolymerized from trioxane and 
comonomers, such as cyclic ethers and cyclic formals, and their ends are 
subjected to a stabilizing treatment. In addition, antioxidants and other 
heat stabilizers are added to polyoxymethylene so as to prevent it from 
decomposing. 
Stereo-hindered phenol compounds or stereo-hindered amine compounds have 
been proposed as antioxidants added to the polyoxymethylene resins while 
polyamides, urea derivatives, hydrazine compounds, amidine compounds, 
hydroxides of alkaline metals or alkaline earth metals, salts of organic 
or inorganic acids and the like have been proposed as other heat 
stabilizers. However, a polyoxymethylene resin composite with these 
compounds incorporated therein is disadvantageous in that it changes color 
to a yellowish brown under the influence of heat and oxygen within the 
cylinder of a molding machine. Moreover fine particular substances and 
tarry substances (usually termed "mold deposits") are transferred to 
surfaces of the metallic mold over the course of a large number of molding 
cycles, resulting in a reduced surface luster of a subsequently molded 
products. Accordingly, various kinds of devices and proposals have been 
suggested but without achieving the desired satisfactory results. 
MEASURES FOR SOLVING THE PROBLEMS 
The present invention is based on the discovery that the use of the 
following group of compounds, together with antioxidants, exhibits a 
remarkable effect in terms of heat-resistance, discoloration and reduction 
of mold deposits (MD). 
Thus the present invention relates to a polyoxymethylene composite 
comprising an antioxidant in an amount between 0.05 to 5% by weight and a 
basic compound containing a tertiary nitrogen atom and a metallic salt of 
carboxylic acid in the same molecule in an amount between 0.01 to 2% by 
weight. The present invention also relates to a method of manufacturing 
the polyoxymethylene composite, comprising adding the above components to 
molten polyoxymethylene. 
The basic compound used in the present invention is a compound containing 
at least one tertiary nitrogen atom and a metallic salt of carboxylic acid 
in the same one molecule and represented by the following formula (1). It 
is preferable that all nitrogen atoms are a tertiary nitrogen atom. 
##STR1## 
wherein at least one of R.sub.1, R.sub.2, R.sub.3, R.sub.4 is a carboxylic 
group represented by the following formula (2) and the rest are an alkyl 
group having 1 to 7 carbon atoms, R.sub.5 is an alkylene group having 2 to 
10 carbon atoms or an alkylene group having 1 to 3 ether bonds midway, M 
is at least one metal selected from alkaline metals or alkaline earth 
metals, L is an integer of 0 to 6, m is 1 or 2, n is an integer making the 
total valences of metal M equal to the total number of carboxylic groups 
in said basic compound. 
##STR2## 
wherein R.sub.6 is an alkylene group having 1 to 6 carbon atoms and 
directly connected to a nitrogen atom in the formula (1). 
This basic compound is for example a salt formed of at least one compound 
selected from the group consisting of N,N-dimethyl-glycine, 
N-methylimino-diacetic acid, nitrilo-triacetic acid, 
nitrilo-propionicacid, ethylene diamine tetraacetic acid, ethylene diamine 
tetrapropionic acid, diethylene triamine pentaacetic acid, triethylene 
tetramine hexaacetic acid, triethylene tetramine hexapropionic acid, 
cyclohexylene dinitrilo-tetraacetic acid, cyclohexylene 
dinitrilo-tetrapropionic acid and ethylene dioxy-bis 
(ethylamine)-N,N,N',N'-tetraacetic acid and at least one metal selected 
from the group consisting of lithium, sodium, potassium, rubdium, cesium, 
beryllium, magnesium, calcium, strontium and barium at a stoichiometric 
ratio. Salts formed of carboxylic acids, such as N-methylimino-diacetic 
acid, nitrilo-triacetic acid and ethylene diamine tetrapropionic acid, and 
an alkaline metal, such as lithium and sodium, or an alkaline earth metal, 
such as magnesium and calcium, are preferably used. In particular, salts 
formed of a carboxylic acid represented by the formula (1), wherein each 
of R.sub.1, R.sub.2, R.sub.3, R.sub.4 is a carboxylic group, and an 
alkaline metal are specifically effective. Particularly, ethylene diamine 
tetraacetic acid tetrasodium represented by (the following formula (3)) is 
most superior. 
##STR3## 
These basic stabilizers do not lead to the formation of colored products 
since the control together with the antioxidants the thermal decomposition 
of polyoxymethylene as a product. Accordingly, decompositon gas can be 
prevented from generating during the molding process while the basic 
stabilizers have the minimal sublimating and decomposing properties by 
themselves, thereby reducing the formation of mold deposits. 
Antioxidants used in the present invention include hidered phenols and 
hindered amines. The preferred hindered phenols include 
2,2'-methylene-bis(4-methyl-6-t-butylphenol), hexamethylene 
glycol-bis(3,5-di-t-butyl-4-hydroxyhydrocinnamate), 
tetrakis[methylene(3,5-di-t-butyl-4-hydroxyhydrocinnamate)]methane, 
triethylene glycol-bis-3-(3-t-butyl-4-hydroxy-5-methylphenyl)propionate, 
1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxy-benzyl)benzene, 
n-octadecyl-3-(4-hydroxy-3,5-di-t-butylphenol)propionate, 
4,4'-methylene-bis(2,6-di-t-butylphenol), 
4,4'-butylidene-bis-(6-t-butyl-3-methyl-phenol), 
2,2'-thiodiethyl-bis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], 
di-stearyl-3,5-di-t-butyl-4-hydroxybenzylphosphonate and 
2-t-butyl-6-(3-t-butyl-5-methyl-2-hydroxybenzyl)-4-methylphenylacrylate 
used singly or in combination. Of them, hexamethylene 
glycol-bis(3,5-di-t-butyl-4-hydroxyhydrocinnamate), 
tetrakis[methylene(3,5-di-t-butyl-4-hydroxyhydrocinnamate)]methane and 
triethylene glycol-bis-3-(3-t-butyl-4-hydroxy-5-methylphenyl)propionate 
are particularly preferred. 
The hindered preferred amines include piperidine derivatives having a 
stereo-hindering group which include 
4-acetoxy-2,2,6,6-tetramethylpiperidine, 
4-stearoyloxy-2,2,6,6-tetramethylpiperidine, 
4-acryloyloxy-2,2,6,6-tetramethylpiperidine, 
4-methoxy-2,2,6,6-tetramethylpiperidine, 
4-benzoyloxy-2,2,6,6-tetramethylpiperidine, 
4-cyclohexyloxy-2,2,6,6-tetramethylpiperidine, 
4-phenoxy-2,2,6,6-tetramethylpiperidine, 
4-benzyloxy-2,2,6,6-tetramethylpiperidine, 
4-(phenylcarbamoyloxy)-2,2,6,6-tetramethylpiperidine, 
bis(2,2,6,6-tetramethyl-4-piperidyl)oxalate, 
bis(2,2,6,6-tetramethyl-4-piperzyl)malonate, 
bis(2,2,6,6-tetramethyl-4-piperizyl)adipate, 
bis(2,2,6,6-tetramethyl-4-piperidyl)sebacate, 
bis(1,2,2,6,6-pentamethyl-piperidyl)cebacate, 
bis(2,2,6,6-tetramethyl-4-piperizyl)terephthalate, 
1,2-bis(2,2,6,6-tetramethyl-4-pyperidyloxy)ethane, 
bis(2,2,6,6-tetramethyl-4-piperizyl)hexamethylene-1,6-dicabamate, 
bis(1-methyl-2,2,6,6-tetramethyl-4-piperizyl)adipate, 
tris(2,2,6,6-tetramethyl-4-piperidyl)benzene-1,3,5-tricarboxylate and the 
like. 
Also, high molecular piperidine derivatives are preferably used. 
The antioxidant used in the present invention is added to polyoxymethylene 
in an amount between about 0.05 to 5% by weight, preferably 0.1 to 3% by 
weight, and the basic compound is added to polyoxymethylene in an amount 
between 0.01 to 2% by weight, preferably 0.03 to 1% by weight. In the case 
where these components are added below the minimum weight percentages 
noted above, insufficient effects are obtained. In the case where they are 
added in excess of the maximum weight percentages noted above, the 
beneficial effects, such as heat-resistance, are not improved in spite of 
their presence in large amounts. Instead, a discoloration tendency occurs, 
which is, of course, undesirable. 
Any suitable technique to add and blend the compounds may be employed, e.g. 
melt blending. That is to say, the antioxidant and the above-described 
basic compound is added to polymethylene, (after subjecting its terminal 
end groups to a in an amount stabilizing treatment), of between 0.05 to 5% 
by weight and 0.01 to 2% by weight, respectively. The resulting mixture is 
melted and blended to obtain the composition. Furthermore, the above 
described basic compound and an antioxidant is added to crude 
polyoxymethylene. That is, the basic compound is added to polyoxymethylene 
(after stopping the polymerization) in which the end groups thereof have 
not been subjected to stabilizing treatment. The resulting mixture may 
thus be melted and blended in a uniaxial or biaxial extruder or similar 
apparatus (e.g. as disclosed in Japanese Patent Publication No. Sho 
50-21514) provided with a vent hole within the temperature between 
180.degree. to 250.degree. C. and a mean residence time of 1 minute or 
more. 
The basic compound according to the present invention is added in the form 
of powder, but it may be added in the form of a 0.1 to 10%-aqueous 
solution. 
According to the present invention, the basic compound serves not only as a 
stabilizer but also as a modifier for releasing unstabilized end portions 
from unstable crude polyoxymethylene and thereby transform the unstable 
polymer to a stabilized polymer. 
The polyoxymethylene composition obtained by these methods can be used as a 
molding material as they are but known substances usually added to usual 
thermoplastic resins and thermo-setting resins, that is to say, 
plasticizers, ultraviolet ray absorbants, antistatic agents, surfactants, 
flame-retardants, coloring agents, such as dyestuffs and pigments, agents 
unguents for improving the fluidity and releasability, lubricants, 
crystallization promotors (nucleating agents) and the like can be suitably 
used according to particular end-use requirements. In addition, fibrous, 
plate-like and granular inorganic fillers, such as glass fibers, can be 
added to the composition of the present invention. It goes without saying 
also that other resins or high molecular substances can also be added to 
the composition of the present invention.

cl PREFERRED EMBODIMENTS 
The present invention is below described with reference to the Examples and 
Comparative Examples. 
E (%) designates a quantity of unstable end portions, which are decomposed 
by alkalies, and is represented by the following equation: 
EQU E=[(W.sub.0 -W)/W.sub.0 ].times.100(%) 
In the above described equation, W.sub.0 designates a weight (about 5 g) of 
dry polyoxymethylene used in the measurement and W designates a weight of 
polyoxmethylene obtained by heating said dry polyoxymethylene in a 1 
weight %-aqueous solution of ammonia of 500 ml for 30 minutes at 
180.degree. C. and then cooled followed by washing and drying the 
deposited polyoxymethylene. 
In addition, the color tone of the composite was evaluated by maintaining a 
sample of about 10 g within a melt-indexer for 60 minutes at 200.degree. 
C. and then measuring the color tone of the sample when molded in the form 
of a disc by means of a colorimeter. The evaluation is classified as shown 
in the following Table. A L-value represents the whiteness, wherein the 
larger value of L is preferable. In addition, a b-value represents the 
yellowness wherein the smaller value of b is preferable. 
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Rank 
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A L-value: 86 or more; b-value: less than 1.0 
B L-value: 85 or more; b-value: less than 3.0 
C L-value: 82 or more; b-value: less than 6.0 
D L-value: less than 82; b-value: 6.0 or more 
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The heat-stability was evaluated by subjecting the sample to the heat 
treatment in the same manner as in the measurement of color tone and 
measuring the melt index (MI) value before and after the treatment. That 
is to say, the lower MI value after the residence time means that the 
thermal decomposition is reduced and the heat-resistance is superior. 
Adhesion to a metallic mold during the molding cycle was evaluated by 
visually observing the stain of the metallic mold after the continuous 
molding of the polyoxymethylene composite sample under appointed 
conditions in an injection molding machine. The evaluation is represented 
by A, B, C which are defined as follows: 
A--Not stained 
B--Slightly stained 
C--Stained 
EXAMPLES 1, 2 
An antioxidant [triethylene 
glycol-bis-3-(3-t-butyl-4-hydroxy-5-methylphenyl)propionate] of 0.5 parts 
by weight and a basic compound shown in Table 1 of 0.1 parts by weight 
were added to polyoxymethylene copolymer, which has MI-value of 9.0 and E 
of 1.0% and of which ends were subjected to the stabilizing treatment, of 
100 parts by weight and the resulting mixture was molten and blended at 
210.degree. C. in the extruder to obtain pellets. The test results of the 
resulting compositions are shown in Table 1. 
COMATIVE EXAMPLES 1, 2 
Pellets were obtained in the same manner as in Examples 1, 2 excepting that 
the substances shown in Table 1 were added in place of the basic compounds 
shown in Examples 1, 2 at the ratio of 0.1% by weight. The test results of 
the resulting composite are shown in Table 1 together with those in 
Examples 1, 2. 
EXAMPLES 3, 4 
Antioxidants used in Examples 1, 2 and basic compounds shown in Table 2 in 
the form of a 5 weight %-aqueous solution were added to polyoxymethylene 
copolymer, which has MI-value of 8.6 and E of 2.5% and of which ends were 
not subjected to the stabilizing treatment, at a ratio of 0.5% by weight 
and 0.15% by weight and the resulting mixture was molten and blended at 
210.degree. C. and a pressure of a vent hole of 200 torr in a biaxial 
extruder provided with a vent hole to obtain pellets. The test results of 
the resulting compositions are shown in Table 2. 
COMATIVE EXAMPLES 3, 4 
The treatment was carried out in the same manner as in Examples 3, 4 except 
that the basic compounds added in the Examples were not added and the 
substances shown in Table 2 were added at a ratio of 0.15% by weight in 
the form of a 5% aqueous solution to obtain pellets. The test results of 
the resulting compositions are shown in Table 2 together with those in 
Examples 3, 4. 
EXAMPLES 5 to 8 
Antioxidants added in Examples 1, 2 and basic compounds shown in Table 3 
were added to polyoxymethylene copolymer, which has MI-value of 8.6 and E 
of 2.5% and of which ends are not subjected to the stabilizing treatment, 
at a ratio of 0.7% by weight and 0.1% by weight, respectively, and the 
resulting mixture was molten and stirred for 15 minutes in a 
Laboplastmil.RTM. mixer while maintaining a suction pressure of 50 torr. 
The test results of the resulting composition are shown in Table 3. 
COMATIVE EXAMPLES 5 to 8 
The treatment was carried out in the same manner as in Examples 5 to 8 
except that the substances shown in Table 3 was added at a ratio of 0.1% 
by weight in place of the basic stabilizers added in Examples 5 to 8. The 
test results of the resulting compositions are shown in Table 3 together 
with those in Examples 5 to 8. 
TABLE 1 
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Additive 
(basic compound) 
Color tone 
MI MD 
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Example 1 Ethylenediamine tetra- 
A 10.4 A 
acetic 
acid tetrasodium 
2 Nitrilo-triacetic acid 12.7 A 
trisodium A 
Compara- 
1 Calcium starate 
D 16.0 C 
tive 2 Polyamide* C 12.8 B 
Example 
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*Nylon 6, 66, 610 copolymer 
TABLE 2 
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Additive Color E 
(basic compound) 
tone MI MD (%) 
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Example 3 Ethylene diamine 
A 10.4 A 0.8 
tetraacetic acid 
tetrasodium 
4 Triethylene tetra- 
A 10.7 A 0.3 
mine 
hexaacetic acid 
hexasodium 
Compara- 
3 Triethylamine C 16.1 C 0.7 
tive 4 -- A 16.6 C 1.6 
Example 
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TABLE 3 
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Additive Color E 
(basic compound) 
tone MI MD (%) 
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Example 5 Ethylene diamine 
A 10.3 A 1.0 
tetraacetic acid 
tetrasodium 
6 Nitrilo-triacetic 
A 12.8 A 0.9 
acid trisodium 
7 N--methylimino- 
A 11.8 B 1.0 
diacetic acid 
disodium 
8 Triethylene tetra- 
B 10.9 A 0.3 
mine 
hexaacetic acid 
hexasodium 
Compara- 
5 Imino-diacetic acid 
C 10.8 A 0.3 
tive disodium 
Example 6 Magnesium hy- B 13.8 C 1.1 
droxide 
7 Sodium carbonate 
D 10.3 C 0.3 
8 -- A 16.6 C 2.1 
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As is evident from the above description and Examples, according to the 
present invention, polyoxymethylen can significantly be improved in terms 
of heat-stability, reduction of metallic mold staining after long-term 
continuous molding, significantly reducing the discoloration even after 
high-temperature treatment during molding, and maintaining the superior 
color tone. Therefore, according to the invention, polyoxymethylene resin 
compositions superior in quality are obtained.