CA1046685A - Stabilization of acetal polymers - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A stabilized composition comprising an acetal polymer and from 0.02 to 12 parts by weight to 100 parts by weight of acetal polymer of a block copolymer of the general structure A-B-R, wherein:
A is a polylactamic block of recurring units:

wherein PM is a linear polymethylene chain having from 3 to 13 carbon atoms, B is a polyoxymethylene block of recurring units

Description

~o46685 The present inVention concerns a composition comprls- -ing an acetal polymer and a stabilizer belonging to ~ new class.
In the course of the present specification, by "poly-mers" or by "acetal polymers" will be designated those products having a molecular weight greater than 10,000, which are obtain-ed in the art by polymerization of an aldehyde, or by copoly- ~`-merization of a plurality of different aldehydes, or by copoly-merization of one or more different aldehydes with other non-aldehyde monomers, wherein the terminal hydroxyl groups of the marcomolecules have been transformed into other groups endowed with greater thermal stability.
Various polymers of aldehydes or compris~ng aldehyde monomer moieties have been prepared in the art~ such as:
- the homopolymers or aidehydeg or of.their cyclic oligomers such as : formaldehyde, acetaldehyde, trioxane:and tetraoxane;
- the copolymers o$ at least two dif~erent aldehydes;
- the copolymers.containing in the macromoleculax chains, aldehyde unitt and recurring units, which can be defined by the general formula ', Rl .
.
. ' . .:
tOr~CH2r~ (,1 :tL~ -L
~ 2 - : ~ whe~-ln n i9 an int~ger ~rom zero to 5 and Rl and R2 are inert subst~tuents which are free o~ interfering functional groups.
;: Copolymers o~ th~s type are disclosed ~or example s by Canadlan Patent No. 773~159 (J.F. Megee~., issued Dec. 5, 1967.
: Th~se eopolymers can ~te obtained copolymerizing an aldehyde ~such:as formaldéh~ e). or its cyslic oligomer tsuch 30. ~ as trioxanet~ with:d~f~erent monomers, suc~ as for instance:
l , `~ t~t cycl~ic et~ers, such as ethylene oxide, 1-3 dioxolane , and eplchlorohydrin~

cb~
.t ~t 104tj685 - unsaturated vin~l compounds, such as st~rene, vinyl methyl ketone, acrolein, vinyl ether, and vinyl ~cetate;
- ketenes, such as ketene and dimethylketene.
It is known that the acetal polymers contain for each macromolecule at least one termlnal hydroxyl group which renders the polymers unstable at the manufacturing andFrocess~ng tem-peratures.
, Therefore, in the art, these polymers are treated by a proper reagent for the purpose of transforming the terminal , , , hydroxyl groups into others endowed with greater stability.
For this purpose, the terminal hydroxyl groups of the macromolecules are transformed into ester groups by re~ction with anhydrides or carboxylic acids or with ketenes, or into ester groups of the carbamic- or thiocarbamic acid by reaction with isocyanates or, respectively, isothiocyanates.
Further processes are known for the transesterifi-cation o~ the terminal hydroxyl groups of the polymers, or for the etherification of the same, by reaction, for example, with orthoesters.

2~ As known, notwithstandi,ng the transformation of the terminal groups, the acetal pol~mers are practically unusable a~ plastic ~aterial, because of their strong sensitiyity towards ox~gen~ heat, ultraviolet xadiations a,nd the traces of impurities alwa~s present in a technical product.
It follows that decompos~tion of the polymer occurs, espeCially when heated, such as during the moldin~ treatments in the molten state.
To ob~iate these undesirable phenomena the use of ~t~bilizing substances which are incorporated into the acetal ' ' ~ol~mer is expedient in the techn~que.
Normall~,antioxidants are used to inhibit the deterio-xation due to,the action o~ oxygen and of heat, said antl-cb~ 2 -~ 046685 oxidants being normally chosen among substituted phenol or substituted bisphenols.
However in the practice, it has been ascertained that this stabilization with an antioxidant is not sufficient, and during the processing of the acetal polymer in the molten state, substances originated b~ the de~radation of the polymer are ~lways evolved.
Said degradation products, for instance, consist of formaldehyde, in the case of polymers or of copolymers of said monomer, or of transformation products of formaldehyde such as formic acid.
These degradation products of the acteal polymers are blocked by incorporating in the polymer substances having a basic character whether with a high or with a low molecular weight.
In thè latter case, the sub~tances gene~ally utilized are pol~amides.
A problem Which arises in the stabilization of acetal polymers consists in the degree of mutual compatibility of the stabilizer and poly.mer, taking into account also the high c~stall~n~ty.degree of the acetal polymers.
A problem relat~ve to the stab~ltz~tion,concexns the homogenlzation, at molecular level, amon~ the stabilizers and the acetal poly~er, To th~s ~ntent are opposed at least t~o obstacles o~ a technical and of a physical-chemlcal nature, ;res~?ect~vel~ .
As kno~n, the add~t~n ~f stablllze~s to the polymer occurs commonly dur~n~ ex~rusion and a thox.ough ho~ogenization ~s usually reached with machines ~extruders2. which exert also 3Q a xe~axkable cutting stress on the macromolecul~r ch~ins of the acetal pol~mer.
. This action leads to phenomena of mechanical-chemical ~,.

cb/ _ 3 _ ~ ' , ~ ' ' ' ' ,' '', ' ' ' '. ' ' , . ' ' : , ~ .' . ' 104~685 degradation of the material; a ~act which brin~s forth a dec-rease of the molecular wei~ht due to scission phenomena and conclusively, given the particular chemical structure of the polyacetals, to a thermal instability.
In any event, the scission of the macromolecular chain, originates two shorter chains, each carrying at one end a thermically unstable group. In the case of homopolymers of formaldehyde and trioxane the two chains resulting from the scission, deteriorate by thermal effect, with liberation of formaldehyde, even unzippering themselves completely.
In the case of copolymers the degradation continues up to the first non-aldehydic unit contained in the chain, ~here the sequence is interrupted by bonds of other type. In any event, a moxe or less important decrease of the molecular welght takes place. `
The abave mentioned difficulties are overcome or at lea~t strongly reduced, according to the present invention, by the use of a composition comprising a new stabilizer for the acetal polymer.
Thus, the invention provides a stabilized composition comprising an acetal polymer and from 0.02 to 12 parts by weight to 100 parts by weight of acetal polymer of a block copolymer Qf the general structure ArB~R ~ wherein:
A is a polylactamic block consisting of recurring un~ts .
--r c tPM~ N~

O EI , "
obtainable from one or more monomeric lactams of the formula I_lPM)~
29 ~CO--NEI~
: ' .,' ' ` ' , ` ' cb/ ~ 4 ~

,, , ~ , ., , . ., . ... - - ~ . -.

10~68S
wherein PM is a linear polymethylen~ chain having ~rom 3 to 13 carbon atoms, non substituted or having at least one hydrogen atom replaced by a radical selected from the group consisting of the alkyl, aryl, cycloalkyl and arylalkyl radicals.
B is a polyoxymethylene ~lock consisting of recurring units:
~ CH2 f R is a terminal group selected in the class consisting of the ester, ether and urethan groups.
The polylactam block may consist of different lactams.
The preferred lactams are: E-caprolactam, ~-lauryllactam, ~-piperidone, ~-enantiolactam and ~-pyrrolidone.
The A-B-R copolymer preferred for the purposes of the invention ~as a molecular welght from 1,000 up to 120,000 and a proportion of blocks B of from 5 to 90~ by weight.
A-B block copolymers have been dèscribed in our co-pending Canadian application Serial No. 241,9~, filed December 17, 1975, as new materials useful in the field of yarns and of molded products or as technopolymers for special applications as a substitute for metals.
2Q According to said app~ication, A-B-R block copolymers are obtained by preparing ~rst a polymer A consisting of a polylactam unit and then puriying the polymer thus obtained and bringing it in a particulate form suitable for the sub-~equent reaction with formaldehyde.
Then the copolymer A-B i9 prepared b~ reacting formaldehyde monomer with the thus treated polymer A.
Finally copolymer A-B is stabilized in order to transform the terminal unstable hydroxyl groups of the polyoxy-methylene block B into stable groups.
~ According to the present invention, the block copolymer A~B R acts as new and ef~icient stabilizer for acetal polymers.
, ~ ' .

, Cb/ - 5 -In particular the said stabilizer aYoids those draw-backs relative to the mutual incompatibilities which occur with the use of known stabilizers such as the polyamides.
The polyoxymethylene block present in the A-s-R block copolymer improves the mutual compatibility of the different heterochains and allows a thorough homogenization between the acetal polymer and the stabilizer.
In this connection, it should be noted that the pro-portion of the polyoxymethylene block in the block copolymer is ~n important factor. The best results are obtained by maintain-lng said proportion within the specified range of values.
In any case, the homogenization between the stabilizer and the acetal polymer occurs under mild conditions, for example in extruders of common type and having such a mixing action that it does not permit tho~e scission and deterioration phenomena which bring forth a decrease of the molecular weight of the ; acetal polymer in addition to a 105s of useful product.
In other words, it is possible to obtain a perfect homogenization between the acetal polymer and the stabilizer, while avoiding that cuttIng action which induces the undesirable effects already described.
On the other hand, the perect homogenization permits ~ bettex utilization of the stabilizer, and conclusively the use o~ lesser amounts of stabilizer, as compared to those used when operating with conventional stabilizers.
The) physical-mechanical properties, mainly the mechan-~cal-dynamic ones of the acetal polymers are affected, sometimes xemarkably, by the presence of extraneous components, since ` these substances constitute a disturbing element within the

3~ cr~stalline structure, for ~nstance in the resulting molded ~rticles, Therefore the necessity arises o~ using the stabilizers . .

; cb! ~ 6 ~

iO46~;85 in amounts as low ~s possible -and this necessity is satisfied with the present stabilizer, Furthexmore, the A-B~R block copolymer, besides having a greater stabilizing activ~ty, has the characteristic of not being extractable ~with aqueous solutions or with organic sol-vents) from the acetal polymer or from the relative articles.
Thus, the composition according to the present inven-tion, can be used in the manufacture of mechanical components and of containers in which pharmaceutical products and food-stuffs can be packed.
The A-B-R block copolymer is generally obt~ined in the form of a powder having a granulometry of from 50 to 600 microns and an apparent density of the order of 0..1-0.8 g/ml, depending upon the method of preparation.
In continuous stabilization the acetal polymer can be mixed and homogenized with the stabilizer according to the conventional methods industrially known, for example by straight dry-blending or by mixing a main stream of acetal polymer with a master-batch containing from 15 to 40% by weight of A-B-R
2~ copolymer, followed by homogenization under hot conditions in a suitable equipment (extruder etc.).
. Obviously the feedings are controlled in such a way that the concentration of the stabilizer in the acetal polymer ~alls in the interval of values hereinbefore indicated.
An antioxidant of the type previously mentioned may be added in the stabilization of the acetal polymer, Said anti-oxidant is usually.chosen among the substituted phenols and substituted bisphenols.
~ Examples of such compounds are: 4,4'-thiobis)6-tert- .
3~ butyl.meta-cresol~; 4,4'-but~lideneb;s~6~tert~butyl-meta-cresol);
pentaerythritylrtetrabeta~4-hydroxy,3,5,di-tert~butyl-phenyl) ~ropionate; n-octadecylbetat.4,hydroxy-3,5-di-tert-butyl-phenyl) propionate7 2,2'-methyleneb~s(.4-methyl-6-tort-butyl-phenol).

c~ ~ 7 ~

The said antioxidant is ~enerally incorpor~ted in a proportion of from 0,02 to 2 parts b~ wei~ht~ preferqbly from 0,1 to 0.6 parts for each 100 p~rts of acetal polymer.
In the follo~ing experi~ental Examples, the parts and the percent~ges are intended b~ weight unless otherwise speci-,$ied.
Example 1 Stabilizer preparation O.208 parts of metalllc lithium are added to 85 parts of pure alpha-pyrrolidone maintained at 55C under stirring and under extremely inert conditions. ~t the end of the re-action of formation of the metal-lactam, 90 parts of anhydrous '"
nitrobenzene and 3.81 parts of N-acetylpyrrolidone are added to the mas's, bringing the temperature to 20C.
' The system ~n reaction is maintained under these condi-tions for 45 hours.
At the end of the operation, the su~pension formed is diluted and washed thoroughly with toluene so as to el'iminate all soluble polymerization residue.
The analysis of polymer A thus o~tained indicates:
conyersion percentage o~ the monomer charge : 86.8 ~'reduced viscosity (liters , g 1~ : ,l.Q7 In the Examples, the A polymer viscosity is measured ~, at 35C from a solution of m-cresol containing 0.5 wt.% of '~
polymex and expressed aa the ratio ~ reduced = c~Oncpenctratcio-n , in liters, g-~,granulometry > 88 micron~ : ~.9 88~44 microns ~ 56 < 44 microns : 42.4~
200 parts o~ polymer A thus pxepared are introduced into a polymerization reactor containing 1000 parts o~ toluene.
. . .

Cb/ ' - 8 -, ' ' ~04~;68~ , The reactor is supplied wlth a yi~orous stirrex, with a proper system to ensure inert conditions by mean~ o~ a nitro~en flo~
and with a thermostating jacket.
Pure gaseous monomer ~ormaldehyde is introduced at a rate of 2.5 parts per minute for a period of 15 minutes.
The temperature is maintained at about 5C by brine circulation in the reactor jacket.
When the feeding of formaldehyde ls completed, the stirring is maintained for 10 minutes longer, then the mixture is filtered. After washing and drying in a vaccum oven at 60C, the resulting A-B copolymer is then esterified.
For this purpose, a mixture comprising 1.5 parts of pure acetic anhydryde and 3.5 parts of a mixture of ClO-Cl4 n~paraffins for each part o~ A-B copolymer is reacted for 15 ,minutes at 140C. On completion of the reaction, the result-ing suspension is filtered, washed thoroughly with hexamethyl- , phosphoric triamlde and then with acetone.
~ he analysis o~ the resulting A-B-R copolymer yields the following data:
20 ~ oyerall yield (%1 : 8~.8 - A block '~%l : 90.1 ~,nitrogen ~%) : 14.84 , intrinsic viscosity : O.91 In the Examples, the viscosity of the A-B-R copolymex and i of the acetal polymer is measured at 60C from a solution of p~chlorophenol with 2% c~-pinene containing O.5 wt.% o~
polymer and expressed as the ratio ~ intrinsic = c~Onrceenattravteion in liters. g~l.
K220 ; 0.005 30 This latter datum is o~tained in a thermaldegradation test and expressed as the we~ght loss percentage per mtnute during 1, ' the first 30 minutes, measured at 220C by a thermoscale in a ¦ ob/ _ g _ 1046t;85 nitrogen atmosphere. The measuring instrument ~llows the con-t~nuous discharge of the ~aseous products b~ means of a nitrogen flow.
Stabilization of the acetal polymer 22.3 parts of A-B-R copolymer obtained as previously described are mixed with 12 parts of pentaerythrityl-tetrabeta-r~4-hydroxy-3,5-di-tert-butylphenyl) propionate and with 65.7 parts of acetylated polyox~methylene having an intrinsic Yiscosity of 1.40. This formaldehyde homopolymer has been obtained by polymerization of pure monomer formaldehyde in toluene and in the presence of an anionic initiator, and sub-sequently it has been esterified with acetic anhydride for the purpose of block~ng the terminal hydroxyl groups.
20 parts of the product rich in stabilizer, prepared as hereinbefore described are added to and mixed with 980 parts o~ an acetylated polyoxymethylene having an intrinsic viscosity of 1,82. In this way the resulting composition contains 0.40%
of A-B-R block copolymer and 0.24% of phenolic antioxldant.
After careful homogenization, this composition is melted, extruded and transformed into 2x2 mm granules by a screw extruder operating at 180-22QC and with an auto~atic cutting blade, The following tests have been run on the granules:
220 : as previously defined;
D22~ : thermal degradation test in air at 220C; expressed as the weight loss percentage of the polymer after 10 minutes and 20 minutes of heating. The degrada-tion products are continuously discharged by flush-ing with a stream of air.
The results are reported ~POM~l~ in Table 1.
Correspond~ngly~ the same tests are performed on a aample o~ the same acetylated polymer stabilized with 0.24% of cb~ - 10 - -.

10~6685 pentaerythrityl-tetrabeta (4-hydroxy-3,5-di-text-butylphenyl) propionate and tr~nsformed into granules as pre~iously described, The ~esults (POM-2~ are summarized in Table 1.
Table 1 colour Melt-index at 195 C K220 1OlD220 POM-l white 2.10 0.03 0.5 1.2 POM-2 white 4.20 0.13 6.2 11.8 Example 2 Stabilizer re aration P P
0.208 parts of metallic lithium are added to 113 parts of pure ~-caprolactam at the temperature of 110C operating under stirring in an inert atmosphere.
After 10 minutes the reaction of formation of the metal-lactam is practically completed.
Then, 150 parts of pure and anhydrous toluene and 4.65 partg of acetyl-caprolactam are added.
The reaction system is maintained at boiling point for

4 hours.
At the end of the operation the resulting suspension of polymer A i9 cooled and washed thoroughly with toluene so as to eliminate all soluble residue.
The analysis of a sample of polymex A shows the follow-lng res~lts:
~ conversion : 86,8%
- reduced viscosity : 0.95 - Granulometry:
88 microns = 1.3%
88-44 m~crons - 65.3%
44 microns S 33.4%
Operating ~n ~ manner identical to that of Example l, the polymer A is introduced in an amount equal to 200 partq into a Xeactor contalning 1000parts o~ n~heptaneO
! ~
ob/

104~i685 Pure gaseous monomer formaldehyde is introduced into the reactor at a rate o~ 2.5 parts per minute for 10 minutes.
The A-B copolymer thus obtained is esterified with ~cetic anhydride and purified with hexamethylphosphoric triamide in a way analogous to that indicated in Ex~mple 1.
On the final A-B-R copolymer the following determinations h~ve been performed:
- Overall yield ~%~ - 85.2 - A block ~%~ = 92,4 10 - Nitrogen t%) = 11.45 r intrinsic viscosity = 0.70 K220 - 0.005 St b'lization of the acetal ol er a 1 p ym The stabilized A~B copolymer thus prepared and 1,1,3 tris, ~-methyl-4-hydroxy- 5-tert-butyl-phenyl~butane ~Topanol C~ are added in an amount of 0.55% and 0.35~ respectively, to an acetylated polyoxymethylene in powder prepared in a way analogous to that of Example 1.
The intxinsic viscosity of the formaldehyde homopolymer i9 of 1.35.
The mixture is carefully homogenized and then melted in the cell ~thermoplastic type~ of a Plasti Corder PLV 151 R
~Brabender~.
The cell i9 thermostated with oil at 225C and the number of revolutions of the rotors is of 120 per minute.
Two tests are pexformed with different xesidence times.
The percent loss b~ wei~ht durin~ the plastifiCation measured and the~determinations of the ~elt-Index and of the thexmal stabilit~ a~e pe~formed on the product. The results ~POM-3~ are reported in Table 2.

cb~ - 12 - ;

~4~;685 Exam ~ 3 (comparison) A sample of poly-caprolactam in powder, finely sub-divided ~having a reduced viscosity of 0.94 as measured in m-cresol~ is added together with 1,1,3-tris-~2-methyl-4-hydroxy-

5-tert-butyl-phenyl)butane ~Topanol CA) in an amount of 0.55%
and 35~, respectively, to an acetylated polyoxymethylene in powder identical to that of Example 2.
On the mixture, after careful homogenization, the tests and determinations indicated in Example 2 are performed., ~, 10The results are reported (POM-4~ in Table 2.
Table 2 Time POM-3 PO~-4 p K Melt-Index at ~ p K2 Melt-Index at 220 lgsoC ~g/10~) 20 195C ~g/10') 10~ 0,65 0.05 13.8 0.8 0.07 14.3 ' 20' 0.95 0.05 14.8 1.2 0.06 15.8 ~p ~ weight loss percentage of polymer during the plastification.
Example 4 and comparison Example 5 The stabilized compositions obta~ned in Example 2 and 3 are melted in the cell of the Plasti Corder thermostated wi,th heating oil at 230C while the number of reYolutions of the xotors is of 20 per minute.
Tests with dif~erent residence times are performed.
In Table 3 are reported the results relati~e to the acetylated polyoxymethylene stabilized with the A-B-R copolymer and the antioxidant ~POM-5) and to the acetylated polyoxymethylene ~tabilized with polycaprolactam and the antioxidant ~POM-6).
Table 3 , Time ~O~-5 POM-6 ~ P K220Melt~Index ~ P K22~ Melt-~ndex

6~ 0,45 0,0913.~ 0.6 0.12 13.3 . . . -.
10~ 0.60 0.06,13.3 0.8 0.08 13.8 20~ 0,75 0,C513.6 1,0 0.06 14.2 ~:
cb~ ' - 13 -.

~046685 Exam~le 6 . ~
''Stabilizer preparation 1.2 parts o~ metallic potassium are added at 180C
under stirring in an lnert atmosphere to 197 parts of ~-lauryl-lactam and after 20 minutes 7.17 parts of N-acetyllauryllactam ~re'added. The mixture is brought to the temperature of 200 and left under these conditions for 60 minutes. Subsequently 800 parts of preheated dimethylsulfoxide are added to the result-ing polymer A melted and cooled at 190C.
The resulting solution gives by cooling a suspension of polymer A in a very dispersed form. The suspension is washed thoroughly with benzene, so as to eliminate all soluble residue.
The analysis shows the following results:
conversion : 90.1%
reduced ~iscosity : 0.90 Granulometry:
88 microns = 1.2%
88-44 microns = 37,7 < 44 microns - 61.1%
B~ operating in a manner identical to that of Example 1 the polymer A thus obtained is fed in an amount equal to 200 paXts into a reactor containing 1000 parts of ~enzene. Puxe and gaseous monomer formaldehyde is introduced into the reactor at a ~ate of 2.5 parts per minute for 15 minutes.
The copolymer A-B thus obtained ls wa~hed thoroughly With hexamethylphosphoric triamide and with acetone. At the end of the operation it is dried at 50~C in a vacuum oyen.
Then the A-B copolymer ~s esterified with acetic an-hydride in the way indicated in Example 1. The following determinations are performed on the stabilizer A-B copolymer.

Cb~ ' - 14 -,, . - : : .

466~5 , Oyerall ~iela (%) = 83.6 - ~ block ~%1 a 8 7 ~
~ Nitrogen ~%) = 6,18 - intrinsic viscosity = 0.70 K220 -- 0 . 008 Stabilization of the acet~l polymer 3.4 parts of stabilized A-B copolymer and 3 parts of ~ bis-~2-methyl-4-hydxoxy-5-tert-butyl-phenyl~butane are added to 993.6 parts of acetylated polyoxymethylene having an 10 intrinsic viscosity of 1.52. :
The powder after careful homogenization under cold conditions, is melted and txansformed into granules in the way described in Example 1.
Then the granules are submitted to the thermal degrada-tion te~ts and to a particular thermal treatment (Test CR) by use of an apparatus fox the determination o~ the Melt-Index.
In particular the granules are introduced into the apparatus and the Melt-Index ~n grams~ i~ determined for various residence times with a charge of 2160 g and at 230C.
In this way it is po~sible to follow the variation of the fIuid-~ty ~and ther~fore of the molecular weight~ and of the color of the extruded product as a function of the residence time.
The result~ ~OM-~) are summarized in Table 4 and 5.
Example 7 ~.comparison).
A sample of polyamide prepared by copolymerization of hexamethylenediamine adipate, hexamethylenediamine sebacate and ~-caprolactam in a 4:4:3 weight ratio and l,l-bis ~-methyl-4-hydroxy-5-tert-butylphenyl)butane are added in an amount of 0.34~ and 0.3%, respec~vely to an acetylated polyoxymethylene i~ powdex identical to that utilized in Example 6.
The products are in the form of ~ fine powder and the blend after careful homogenization is melted and pelletized Cb/ . - .15 1{)46685 as described in Example 1. The granules are submitted to the thermal degradation tests and to the CR test of Example 6, The results axe reported in Tables 4 and 5 under POM-8.
Table 4 Color 220 220 10~ 20~
PO~-~ white 0.04 0.4 0.9 POM-8 white 0.05- 0.~ 1.3 Table 5 ~CR test at 230C~
_ _ _ Residence POM-~ POM-8 time Melt-index Color Melt-index Color ~gramsl ~grams) 5' 10.4 ~hite 10.5 white 10 ~ 10 . 2 white 11.0 white 15' 10.5 white 11.9 white 20' 10.6 white 14.2 yellowish-white 30' 15.7 yellowish 30.4 brown white 40~ ~ 23.2 yellowish- 50 brown-white black i~ Exam~le 8 ; One part o~ non-stabilized A-B copolymer prepared 3 as described in Example 6, is etherified w:lth a reactive system consisting of 0.4 parts of triethylorthoformate, 0.8 paxts of e-caprolactone and 2.0 parts of n-dodecane using as catalyst 0.006 parts of diethylsulfate. The operation i9 carxied out at 125C ~or 15 minutes. The polymex, aftex being waQhed thor-oughly with toluene containing 1% triethanolamine and with ace-tone, i9 dried in an oven at 60C. 99,1% o~ the charged product 30 ~- recovered.
3.2 paxts o~ the thus stab~lized ~-B copolymer and 3.5 parts of 1,3,5~tximethyl,2,4,6,tris(3,5-di~tert-butyl-4-~hyd~oxybenzyllbenzene ~ONOX 330G~I are added to 993.3 paxtQ
' cb/ ~ 16 - .
:

10466~S
of etherified polyoxymethylene ~polyoxymethylene diethyl ether) having an intrinsic viscosity o~ 1.39.
The resulting blend, containing 0.32% of A-B copolymer ~nd 0.35% of antioxidant, after careful homogenization is melted ~nd transformed into granules as in Example 1.
Thermal degradation tests are performed on the granules and the results are reported in Table 6 under POM-9.
Example 9 3.5 parts of 2,2'-methylenebis(4-methyl-6-tert-butyl-phenol) and 3.2 parts of stabilized A-B copolymer prepared as in Example 8 are added to 993.3 parts of an acetal copolymer haying an intrinsic viscosity of 1.42, obtained by polymeri-zation of trioxane with 2% of ethylene oxide.
After careful homogenization, the mixture is melted and transformed into granules as described in Example 1. The granules, are submitted to thermal degradation tests whose results are reported in Table 6 under POM-10.
Table 6 Color K220 D220 10' 20~
~OM-9 white 0.02 0.4 1.0 POM-10 white 0.02 0.5 1.1 Example 10 4.0 parts of stabilized A-B copolymer prepared as in Example 8 and 5.0 parts of a polymeric product obtained ~rom caprola~tam and beta U-hydroxy-3,$-di-tert-butylphenyl) propionic acid ~in a 75:25 weight ratio~, are added to 991 parts of an acetal copolymer having an intrinsic viscosity of 1.42, obtained by polymerization of trioxane with 2% of ethylene oxide.
~he acetal copolymer contains 0.40% of A-B copolymer and 0.5~ of the po~ymeric product (antioxtdant).
,:, ' ' ~- ' c~ 7 -~ .

~o~668S
After homogenization, the blend is melted and trans-formed into granules as described in Example 1.
Thermal degradation tests are performed on the granules.
sesides the product is submitted to a treatment with boiling water at 100C for a period of 500 hours. At the end of the operation, after drying in a vacuum oven, the granules are again submitted to the thermal degradation tests.
The results ~POM-ll and POM-ll treated) are summariz-ed in Table 7.

Example 11 (comparison) 3.5 parts of dicyandiamide and 5.0 parts of l,l-bis--~2-methyl-4-hydroxy-5-tert-butylphenyl)butane are added to 991.5 parts of an acetal copolymer identical to that of Example 10.
After careful homogenization, the blend is melted and transformed into granules as described in Example 1.
The granules are submitted to the thermal tests and the treatment indicated in Example 10.
The results (POM-12 and POM-12 treated) are summar-ized in Table 7.

. Table 7 10' 20' POM-ll 0.02 0.4 0.9 POM-ll treated0.02 0.4 1.0 POM-12 0.02 0.5 1.3 28 POM-12 treated 0.04 1.0 3.6 : .

Cb/ - 18 .~

Claims (5)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A composition stabilized against oxidation and thermal degradation comprising (a) an acetal polymer having a molecular weight greater than 10,000 and (b) from 0.02 to 12 parts by weight to 100 parts by weight of said acetal polymer of a block copolymer of the general structure A-B-R having a pro-portion of blocks B of from 5 to 90% by weight and wherein:
A is a polylactamic block consisting of recurring units:

obtainable from one or more monomeric lactams of the formula:

wherein PM is a linear polymethylene chain having from 3 to 13 carbon atoms, non-substituted or having at least one hydrogen atom replaced by a radical selected from the group consisting of alkyl, aryl, cycloalkyl and arylalkyl radicals, B is a polyoxymethylene block consisting of recurring units:
R is a terminal group selected from the class consisting of ester, ether and urethane groups.

2. A composition according to claim 1, wherein said lactams are selected from the group consisting of .epsilon.-caprolactam, .delta.-pyrrolidone, .omega.-lauryllactam, .delta.-piperidone and .omega.-enan-tiolactam.

3. A composition according to claim 1, wherein said block copolymer has a molecular weight of from 1,000 to 120,000.

4. A composition according to claim 1, which comprises from 0.02 to 2 parts by weight to 100 parts by weight of said acetal polymer of an antioxidant selected from the group con-sisting of substituted phenols and substituted bisphenols.

5. A composition according to claim 4, wherein said antioxidant is present in an amount of from 0.1 to 0.6 part by weight to 100 parts by weight of acetal polymer.