CA2051966A1

CA2051966A1 - Core-shell polymer and it's use

Info

Publication number: CA2051966A1
Application number: CA 2051966
Authority: CA
Inventors: Ichiro Sasaki; Takao Teraoka; Junji Oshima
Original assignee: Individual
Current assignee: Takeda Pharmaceutical Co Ltd
Priority date: 1990-09-21
Filing date: 1991-09-20
Publication date: 1992-03-22
Also published as: JPH04135155A

Abstract

ABSTRACT OF THE DISCLOSURE

The present invention relates to a core-shell polymer which is useful for improving the impact strength of polyoxy-methylene resin, to a polyoxymethylene resin composition containing the core-shell polymer and to a molded article made of the polyoxymethylene resin. The core-shell polymer is produced by an emulsion polymerization reaction using an oligomeric surfactant and a neutral radicals-liberating polymer-ization initiator. This core-shell polymer improves the impact strength, elongation at the weld line, weatherability, thermal stability, etc. of the molded article and the molded articles are used as various products.

Description

2~a~

Core-Shell Pol~mer and Its IJse The pres&nt invenl:ion relates to a core shell polymer and a resin composition insuring high impact s-trength and improved weld characteristics as produced by melt-blending said core-shell polymer.
[Background of the invention]
Polyoxymethylene (PO~) resin has been employed as a molding material in the manufacture of various parts such as gears, reels, cord clips, etc. but because these moldings are not good enough in impact strength, many attempts have been made to improve PO~ resin in this quality parameter.
However, because of the very structure of POM
resin, no blending resin is available that is sufficiently compatible ~ith POM resin.
Furthermore, because of the high crystallinity of POM resin, any improvement in its physical properties that may be ob-tained by alloying with other resins compromises its weld strength and elongation.

Moreover, because o~ its inadequate thermal stabi-lity, POM resin is not suited for high-temperature blending.
Heretofore a number o:E core-shell polymers have been proposed for melt-blending for the purpose of improving the impact strength oE matrix resins. ~ny core-shell polymer consisting of a rubbery elastomer core and a glassy polymer shell, in particular, has the advantage of greater reproducibility of dispersion uniformity because the state of its dispersion in a matrix resin is less susceptible to the influence of melt-blending conditions.
Such core-shell polymers have heretofore been used as impact modifier for a variety of matrix resi.ns such as polycarbonate, poly(butylene terephthalate), 2 ~

polyamide, poly(phenylene oxicle), etc. as well as various allo-~s thereo~.
However, the core-shel:l polymers heretofore avai-lable contain ingredients that enco~lrage thermal degradation of POM resin. q~herefore, these known core-shell polymers can hardly be even blended with POM
resin. If they could be blerlded, the resulting compositions would be inadequate in -thermal stability.
A POM resin composition with improved impact strength is dlsclosed in U.S.P. ~,~04,716, for instance. This is a POM resin composition forming thermoplastic IPN tinterpene-trated polymer networks) with a polyure-thane elas~omer but has many dis-advantages. Thus, in order to ob-~ain a su~ficiently high impact strength, it is necessary to use the polyurethane elastomer in a fairly large proportion so that the modulus of elasticity is markedly sacri~iced.
Moreover, it is impossible to obtain a composition having satisfactory thermal stability, weatherability, fluidity and weld strength and elongation characteristics.
European Patent Laid-open Publication No. 115,373 discloses a POM resin composition containing a rubbery elastomer prepared by emulsion-polymerization of Cl 8 alkyl acrylates. However, -the production of this com-position requires special blending conditions and if the ordinary blending conditions are used, a sufficiently stable POM resin composition cannot be obtained. Moreover, no ingenuity has been exercised in regard to therrnal stab;lity in the emulsion polymerization stage.
U.S.P. ~,713,~1~ discloses a POM resin composition containing a core-shell polymer and a reactive titanate. However, even with this core-shell polymer, the POM resin composition is unstable, undergoing decomposition.

Particularly the core-shell polymer used in the examples described in ll.S.P. ~,713,~1~ and EP~A-115,373 is deficient in thermal stability (Comparative Example 1 of this specifica-tion).
Disclosed in IJ.S.P. 4,639,~ is a POM resin composition containing a rubbery elastomer obtained by emulsion polymerization of butadiene but here is no exercise of ingenuity in the emulsion polymer, either, and the thermal stability of thls composition is poor.
U.S.P. 3,749,755 discloses a POM resin composition containing a rubbery elastomer but its thermal stability is unsatisfactory.
Japanese Patent Examined No. 15331/19~4 discloses a method for producing a thermoplastic resin like acrylonitrile-acrylate-styrene (AAS resin) using emulsion polymerization technique improved on impact strength. This is, however, copolymer not a blend mix-ture.
It is generally acknowledged that a polymer blend composed of crystalline polymers is insufficient in the strength and elongation of welds. For example, a POM
resin composition containing a poly urethane elastomer as a blending resin for improved impact strength is markedly compromised in weld strength and elongation.
Moreover, among engineering plastics, POM resin does not necessarily rank high in weatherability. When blended with a poly urethane elastomer for improved impact strength, POM resin provides only a composi-tion markedly compromised in weatherability.
In the above state of the art, therefore, devel-opment of an impact modifier which, in a POM resin composition, provides sufficient impact strength and insures sufficient weld strength and elongation as well as improved thermal stability has been keenly demanded.
Moreover, POM resin is particularly poor in weatherability among various engineering plastics.

2 ~
-- 4 ~

This parameter has not been overtly improved by the prior art mentioned above and, therefore, development of a P~M resin composition improved not only in impact strength but also in weatherability has been demanded.
The inventors of the present invention explored this field of art attempting to develop a core-shell polymer capable of providing an improved POM resin composition and found that the surfactant and the polymerization initiator used in the preparation of the core-shell polymer had adverse effects on the thermal stability of POM resin. Based on this finding, attempts were made to improve the weld strength and elongation and weatherability of POM resin, and it ~as ultimately discovered that the above-mentioned problems could be solved all at once by melt-blending a core-shell polymer of the construction described hereinafter. The present invention is based on the above findings.
[Detailed description of the invention]
The present invention is therefore directed to a core-shell polymer comprising a rubbery polymer core and a glassy polymer shell as produced by emulsion polymerization in the presence of an oligomeric surfactant and a neutral radicals-liberating polymerization initiator, to a process for producing the core-shell polymer, to a polyoxymethylene resin composition containing the core-shell polymer, and a resin product molded from the composition.
In accordance with the present invention, an emulsion polymerization is carried out using the following surfactant and initiator.

~ r~ 9 ~
- 4a -The surfactant -to be used in the present inventio~ is an oligomeric surfactan-t such as those which have been used in emulsion polymerization reactions for certain special purposes.
For example, oligomeric surfactants of the following formula can be employed.

2 ~ 5 R, R2 ~ ~ R3 Rl ~ r ~2l-1 R2n - S(O)z ~ c- c - _ - c - c ~ c _ c _ ~ 11 X~ ~1 I~ 2 JY2 ~ Xn In the above formula, R means an alkyl group of 5 to 20 carbon atoms, preferably 6 to 12 carbon atoms; Z
is equal to 0, 1 or 2; preferably 0 or l, and more preferably 0; n is a positive in-tegral number;
R2n~ respectively means -H~ -C~3~ -C2Hs or -COO~I;
R2n respectively means -H~ -CH3~ -C2Hs~ -COOH or -CH2COOH; Xn means -COOH, -CONH2~ -OCH3~ -OC2~s~ -CH20H~
~ , -CONH2~ -COOC2~140H~ -COOC3H60H~ -CONHCH20H~
~J
-CO~HCH3~ -CON~C2Hs~ -CONHC3H7~ -COOCH3~ -COOC2Hs~ -CN~
-OCOCH3~ -OCOC2Hs~ or -COOCH2-~ H2-The molecular wei.ght of the oligomeric surfactant to be used in accordance wi.-th the invention is about 200 to 5000, preferabl.y about 1500 to 3000, with the degree of polymerization (~lYa) ranging from about 6 to 50.
The oligomeric surfactant as such may be water-soluble. If not, it is converted to a water-soluble salt by reacting with an oxide, hydroxide or alcohol.
The water-soluble salt mentioned just above in-cludes, among others, alkali metal salts, alkaline earth metal salts, Group III heavy me-tal salts, ammonium salt, substituted ammonium ~alts, e-tc.~ and most preferably the ammonium salt.
These oligomeric surfactants can be synthesized, for example as described in Japanese Patent Publication No. 47-34832, by addition-polymerizing relevant monomers in an anhydrous sol.vent in the presence of an 2 ~ j ! t alkyl mercaptan or further oxid:izing the oligomer with hydrogen peroxide or ozone -to the corresponding sulfoxide or su:lfone.
The alky:L mercapl:an merltiorled above incl~ldes, among others, n-octyl mercaptan, n-dodecyl mercap-tan, t-dodecylmercaptan, n-decyl mercaptan and 90 on.
The monomers mentioned above include ~,~-ethylen-ically unsaturated monomers having at least one polar group, such as (meth)acrylic acid, ~-ethyl acrylate, 10 ~-methyl acrylate, ~,~-dimethyl acrylate, caproic acid, itaconic acid, fumaric acid, maleic acid, (meth)acryl-amide, vinyl ethyl ether, vinyl methyl ether, allyl alcohol, vinylpyrrolidone, (meth)acrylonitrile, ethyl-acrylonitrile, methyl (meth)acrylate, ethyl acrylate, 15 hydroxyethyl ~meth)acrylate, hydroxypropyl (meth)acrylate, vinyl acetate, vinyl propionate, N-isoproylacrylamide, N--ethylacrylamide, N-methylacrylamide, glycidyl (meth) acrylate, N-me-thylol-acrylamide and so on.
The solvent used for the above-mentioned addition polymerization is preferably a lower alkanol such as methanol, ethanol, isopropyl alcohol and so on.
The above addition polymerization is generally carried out in the temperature range of about 20 to 25 100C.
The proportion of said oligomeric surfactant in the prac~ice of the present invention is selected with reference to the particle stabilizing power of the surfactant.
In the present invention oligomeric anionic surfactant is used preferably.
The neutral radicals-liberating polymerization initiator includes initiators of the azo typel such as azobis (isobutyronitrile), dimethyl 2l2'-azobis (iso-35 butyrate), 2,2'-azobis (2-amidinopropane) dihydrochloride, etc. and peroxides such as cumene ~ 3 hydroperoxide, diisopropylbenzene hydroperoxide, hydrogen peroxide and so on. These ini~iators can be used independently or in combina-tion.
The emulsion polymerization in a reaction system containing said oligomeric surfactant and ini-tiator gives rise to a core-shell polymer which is sub-stantially free of sulfur oxide compounds or lean in sulfur oxide compounds.
The low sulfur oxide compound (e.g. sulfate, persulfate, etc.) content means tha-t the result of an ordinary qualitative test for sulfate ions is negative.

R typical test is as follows. Five grams of a sample (core-shell polymer) is weighed into a 50 ml conical ~lask, 20 ml of deioni~ed water is added and the mixture is stirred wi-th a magnetic stirrer for 3 hours at room temperature.
The mixture is then ~ilterecl through a No. 5 C
filter paper and the filtrate is divided into halves.
To one of the halves is added 0.5 ml of 1% barium chloride aqueous solution and the relative turbidity of the halves is evaluated (qualitative test for sulfate ion).
The impact streng-th of a POM resin composition containing such a core-shell polymer, particularly one free of sulfur oxide compounds, is very excellent.

The core-shell polymer according to the present invention can be produced by the so-called seeded emulsion polymerization method, which is a serial multi-stage emulsion polymerization method in which a polymer formed in the preceding stage is covered with a polymer formed in the following stage.
It is preferable that in the seed particle-forming stage, the monomer, surfactant and water be fed to the reactor and, then, the initia-tor be added so as to initiate the emulsion polymerization reaction.
The first-s-tage polymerization i.5 the reaction forming a rubbery polymer.
The monomer for cons-tituting such rubber polymer includes, among others, conjugated dienes and alkyl acrylates contai~ing 2 to 8 carbon atoms in the alkyl moiety, as well as mixtures thereof.
Such a monomer or monomers is polymerized to give a rubbery polymer with a glass transition -temperature of not higher than -30C.
Among said conjugated dienes can be reckoned buta-diene, isoprene, chloroprene ancl so on, although butadiene is particularly pre~erred.
Among said alkyl acrylates whose alkyl moieties contain 2 to ~ carbon a-toms each are ethyl acrylate, propyl acryla~e, bu-tyl acrylate, cyclohexyl acryla-te, 2-ethylhexyl acrylate and so on, although bu-tyl acrylate is particularly desirable.
In this -First stage polymerization reaction, mono-mers copolymerizable with said conjugated dienes and/oralkyl acrylates can be copolymerized. ~nong such monomers can be reckoned various aromatic vinyl or vinylidene compounds such as styrene, vinyltoluene, ~-methylstyrene, etc., vinyl or vinylidene cyanide compounds such as acrylonitrile, methacrylonitrile, etc., and alkyl methacrylates such as methyl methacrylate, butyl methacrylate and so on.

When the first-stage polymerization system does not contain a conjugated diene or, if i-t does, in a proportion of not more than 20 weight % o~ the total monomer ~or the first-stage reaction, an improved impact strength can be implemented by incorporating a crosslinking monomer and a grafting monomer in small proportions. The crosslinking monomer mentioned above includes, among others, aromatic divinyl monomers such as divinylbenzerle etc., and alkane polyol polyacrylates or polymethacrylates such as ethylene glycol diacrylate, ethylene gl~col dimethacrylate, but~lene glycol diacrylate, hexanediol diacr~late, haxanediol dimethacrylate, oligoet:hylene glycol diacrylate, olio-ethylene glycol dimethacr~late, trimethylolpropane di-acrylate, trlmethylolpropane dimethacrylate, trimethylolpropane -triacrylate, trimethylolpropane trimethacrylate and so on. Parti.cularly preferred are butylene glycol diacrylate and hexanediol diacrylate.
The grafting monomer includes, among others, allyl esters of unsaturated carboxylic acids, such as allyl acrylate, allyl methacrylate, diallyl maleate, diallyl fumarate, diallyl i-taconate and so on, although allyl methacrylate is particularly preferred.
The above crosslinking monomer and grafting monomer are used in a proportion o~ 0.01 to 5 weigh-t %
each, preferably 0.1 to 2 weight % each, based on the total monomer for the first-stage polymeri~ation reaction.
The rubbery polymer core preferably accounts for 50 to 90 weight % of the total core-shell polymer. If the proportion of the core is either below or above the above-mentioned range, the resin composition prepared by melt-blending the core-shell polymer may not be improved well in impact strength.
Moreover, the low-temperature impact strength may not be adequately improved if the glass transition tem-perature of the core is higher than -30C.
The outer phase of the core shell polymer is con stituted by a glassy polymer.
As examples of the monomer constituting the glassy polymer, there may be mentioned methyl methacrylate and various monomers copolymerizable with methyl methacrylate.
This monomer is either methyl methacrylate as such 9 ~

or a mixture of methyl methacrylate and one or more other monomers copolymerizable ~:ith me-thyl methacrylate, and forms a cJlassy polymer with a glass transition temperature of not lower than 60C.
The monomers copolymerizable with methyl meth-acrylate include various vinyl polymerizable monomers, e.g. alkyl acrylates such as ethyl acrylate, butyl acrylate, etc., alkyl methacrylates such as ethyl meth-acrylate, butyl methacrylate, etcO, aromatic vinyl or vinylidene compounds such as s-tyrene, vinyltoluene, ~-methylstyrene, etc., and ~inyl or vinylidene cyanides such as acrylonitrile, methacrylonitrlle and so on.
Particularly preferred are ethyl acrylato, styrene and acrylonitrile.
This outer shell phase preferably accounts for 10 to 50 weight % of the total core-shell polymer. If the proportion of the shell phase is below or above the above-mentioned range, the resin composition prepared by melt-blending the core-shell polymer may not be improved sufficiently in impact strength.
An intermediate phase may be interposed between the first-stage polymer phase and the final-stage polymer phase. Such an intermediate phase can be provided by subjecting a polymerizable monomer having functional groups, such as glycidyl methacrylate, unsaturated carboxylic acids, etc., a polymerizable monomer forming a glassy polymer such as methyl methacrylate, or a polymerizable monomer forming a rubbery polymer such as butyl acrylate.
A variety of intermediate phases can be selected according to the desired properties of the core-shell polymer.

The polymerizing proportions may be appropriately chosen according to the monomers used. For example, when a glassy polymer is to be used as the intermediate 1 1 - 2 ~

phase, its polymerizing ratio can be calculated assuming this phase as a part of the shell and when -the intermediate phase is a rubbery pol~mer, i-~s ratio can be calculated as a part of the core.
The structure of a core-shell polymer having such an intermediate phase may, for example, be a multi-layer system including an additional layer between a core and a shell or a salami-like sys-tem in which an intermediate layer is dispersed as small particles in the core. In a core-shell polymer of the salami type, the intermediate phase which i~ usually dispersed may form a new core in the center of the core polymen.
Such a core-shell polymer is sometimes formed when styrene or the like is used as the monomer for constituting the intermediate phase.
The use of such a core-shell polymer having an intermediate phase results not only in improvements in impact strength but also improved flexural modulus, increased heat distortion temperature and improved appearance (molding delamination and pearlescence, variation of color due to change in refractive index).
The core-shell polymer of the present invention can be made available in the form of granules, flakes or powders, for example by the following procedures.
(1) A latex is produced by the per se known seeded emulsion polymerization method in the presence of said surfactant and initiator.
(2) This latex is then subjected to the freqze-thaw cycle to separate the polymer.

(3) Then, the polymer is dehydrated centrifugally and dried.
By the above recovery procedure, the solvent and surfactant used in the emulsion polymerization can be largely removed.
Alternatively, at step (2) above, the latex as it is may be dried and used.

2~3~ ~b The spray-drying method using a spray drier can also be utilized for recovery oi -~he core-shell polymer from the latex.
The core-shell polymer thus isolated may be pro-cessed into pellets by means of an extruder or pelletizer or be directly melt-blended with matrix resin for achieving improved impact strength.
The POM resin composition of the present invention contains 5 to 100 weight parts, pre-ferably 10 to 80 weight parts, of said core-shell polymer based on 100 weight parts of POM resin.
If the proportion of the core-shell polymer is less than 5 weight parts, no improvement may be realized in impact strength, while the use o~ the core-shell polymer in excess of 100 weight parts may xesult in marked decreases in the rigidity and thermal properties of the product resin.
The POM resin which can be used in the present invention may be a homopolymer of formaldehyde or a co-polymer of formaldehyde or a cyclic oligomer thereof with an alkylene oxide containing at least 2 geminal carbon atoms in the backbone chain and any of such polyoxymethylene homopolymer resins and polyoxyrnethylene copolymer resins can be employed.
In the production of a POM resin composition according to the present invention, the melt-blending method is employed.
Melt-blending is generally performed in an appro-priate temperature range between 1~0C and 240C, where the resins melt and the viscosity of the composition will not be too low.
The melt-blendinc3 operation can be performed using a calender, Banbury mixer or a s:inc31e-screw or multi-screw extruder.
The resin composition of the present invention may further contain various additives and other resins in ~J ~ ?j ~

appropriate proportions.
Among the additives rnentioned above are flame Letardants, mold releases, wea-ther resistance agents, antioxidants, antistatic agents, heat resistanc~
agents, colorants, reinforcements, surfactants, inorganic fillers, lubricants and so on.
The resin compositions of -the invention may be molded into ar-ticles of desired shapes, by ordinary molding techniques such as injec-tion molding, extrusion molding, compression molding and so on, at a temperature of 200-300C.
The core-shell polymer of the present invention, when melt-blended with POM resin, imparts an excellent impact strength.
Moreo~er, the resin composi-tion containing the core-shell polymer of the invention is more thermally stable than the corresponding resin composition containing any of the known core-shell polymers and displays better fluidity, thermal stability, appearance, weatherability and weld strength and elongation than the resin composition containing a polyurethane elastomer.
[Examples]
The following working examples and reference exam-ple are intended to illustrate the present invention infurther detail and should by no means be construed as limiting the metes and bounds of the invention. It should be understood that, in the working and reference examples, all parts are by weight. The following abbreviations are used in the examples.
Styrene St Acrylonitrile AN
Ethyl acrylate EA
Methyl methacrylate M~A
2-Ethylhexyl acrylate 2EHA
Butadiene Bd Butyl acrylate BA
1,4-Butylene glycol diacrylate BGA
Allyl methacrylate AQMA
Methacrylamide MAM
Methacrylic acid MAA
2,~' Azobis(isobutyronitrile~ AIBN
Deionized water DIW
2,2'-Azobis(2-amidinopropane) dihydrochloride V50 (Wako Pure Chemicals, V50) Hydrogen peroxide H2O~
Vitamin C (ascorbic acid) VC
Sodium persulfate SPS
Sodium octylsulfosuccinate NP
(Neocol P, Dai-ichi Kogyo Seiyaku Co. Ltd.) OS soap (potassium oleate, Kao Corporation) OS
Tetrasodium ethylenediaminetetraacetate EDTA
Dodecyl mercaptan DMP
Oligomeric surfactant Surfactant A
This surfactant was synthesized as in Example 13 20 described in Japanese Kokai Patent Application No. 53-10682, adjusted to pH 7.5 with aqueous ammonia and diluted with purified water to make a solid content of 10%.

~ J3 ~ 3 n-dodecyl- S - ¦-C-C ~ C-C
Il COVC~13 a }I COOI~ b (wherein a:b = 7:3, a -~ b = 13.6 [Composition]
MAA 155 g MMA 360 ~
n-DMP 109 g AIBN 4.4 Isoprop~l alcohol 314 g Molecular weic3ht 1310 Oligomeric surfactant Surfac-tant B
This surfactant was synthesized as follows;
A 7-liter pol~meriza-tion reactor equipped wi-th a reflux condenser was charged wi-th 1550 g of isopropyl alcohol, 231 g of MMA, 546 g of MAA, 137 g of hydroxyethyl acrylate and 170 g of t-DMP, and the charge was heated to 60C with stirring in a nitrogen stream. Then, 21 g of AIBN was added to initiate a polymerization, and the internal temperature was increased to 75C. The reaction mix-ture was cooled to not more than 40C, then 2000 g of DIW was added thereto, and adjusted -to p~l 7.5 with aqueous ammonia.
Isopropyl alcohol was distilled off under reducted pressure, and it was dilut~d with DIW to make a solid content of 10~.
r~ l~3 ~ r' I ~ r~Cl~3~
n-dodecyl-S- -C-C - ~I-C-C - - ¦-C-C - - H

~ COOCH3 ~H COOC2~0 lH COO~ C
[wherein a:b:c: 47:24:129, a ~ b -~ c = 39.3, Molecular weight 3500~4000] ) Example 1 Production of core-shell polymer A
A 7-liter autoclave was charged with 975 g of DIW, 1.47 g of 25~ aqueous am~lonia, 10.5 g of surfactant A, and 0.525 g of MAM and, after nitrogen purging, the internal temperature was increased to 70. A seed monomer mixture of the following composition was then added and dispersed over l0 minutes, after which 10.5 g of a 10~ aqueous solution of V50 was added for the formation of seed particles.
Seed monomer mix-ture EA 51.608 g AQMA 0.263 g BGA 0.105 g Then, 1168.8 g of DIW, 21 g of surfactant A, 4.2 g 3 ~ ~ 3 of 25% aqueous ammonia, 10.5 g of a 10% aqueous solution of EDTA, 0.525 g of t-DMP and 3.497 g of MAM
were added and the temperature was increased to 70C.
Then, 10O92 g of an initiator solution of the following composition was added to lnitiate the core polymerization.
Initiator solution 10% V50 105.0 g 25% Aqueous ammonia 4.2 g Then, the following core monomer mixture and sur-factant solution were continuously fed over a period of 240 minutes. The balance of the initiator solution was fed over 480 minutes. After completion of feed, the mixture was stirred for 12 hours to give a core latex.
Core monomer mixture Bd 420.00 g 2EHA 376.~5 g MMA 195.30 g Surfactant solution Surfac-tant A 105.00 g 5~ Aqueous MAM 35.07 g Shell polymerization was initiated by adding 14.5 g of the following initiator solution.
Initiator solution 10% V50 13.5 ~
25% Aqueous ammonia 0.9 g Thereafter, the following shell monomer emulsion was continuously fed over 120 minutes for further seeded polymerization.
Shell monomer emulsion MMA 404.1 g EA ~5 0 BGA 0.9 g Surfactant A 27.0 g DIW 630.0 g 25% Aqueous ammonia 0.54 g 3 ~

The temperature was increased to 90C and the reaction mixture was kept ~or 1 hour. Af-ter cooling, the mixture was filtered -through a 300-mesh stainless steel screen to give a core-shell pol~ner latex.
This latex was frozen, filtered through a glass filter and dried.in an air current at 40C for 24 hours to give core-shell pol~er A.
Example 2 Production of core-shell polymer B
A 5-li-ter polymerization reactor equipped with a reflux condenser was charged with 1.200 g of DIW, 1.68 g of 25% aqueous ammonia, 7 g of surfactant A and 0.14 g of MAM and the charge was heated to 70C with stirring in a nitrogen stream. Then, 27.86 g of a seed monomer mixture of the following composition was added and dispersed over 10 minutes, followed by addi-tion of 21 g of a 10% aqueous solution of V50 to initiate a seed polymerization.
Seed monomer mixture EA 27.664 g AlMA 0.14 g BGA 0.056 g After 7 g of MAM was added, a monomer emulsion prepared by adding 210 g of surfactant A, 900 g o DIW
and 2.80 g of 25% aqueous ammonia to 1400 g of a core monomer mixture of the following composition and a mixture of 21.0 g of a 10% aqueous solution of V50 and 0.63 g of 1% aqueous ammonia were continuously fed over 180 minutes for further seeded polymerization Core monomer mixture BA 1215.2 g MMA 140.0 g BGA 2.8 g AlMA 7.0 g The reaction temperature was increased to 80C for keeping for 1 hour and, then, cooled to 70C.
After 9 g of a 10% aqueous solution of ~50 and 0.27 g oE 1% aqueous amlnonia were added, the following shell monomer emulsion, 12 g of a 10% aqueous solution of V50 and 0.36 g of 1% aqueous ammonia w~re continuously fed over 60 minutes for further seeded polymeriza~ion.
Shell monomer emulsion MMA 540.0 g EA 60.0 g Surfactant A 30.0 g DIW 500.0 g 25% Aqueous ammonia 0.92 g The temperature was increased to 80C, where the mixture was kept for 1 hour. After cooling, the reaction mixture was filtered through a 300-mesh stainless steel screen to give a core-shell polymer latex.
This latex was frozen at -15C, filtered through a glass filter and dried in an air current at 60C for 2 hburs to give core-shell polymer B.
Example 3 Production of core-shell polymer C
According to the method of Example 1, core-shell polymer C was produced using surfactant B instead of surfactant A.
Example 4 Production of core-shell polymer D
A 2-liter polymerization vessel equipped with a reflux condensex was charged with 600 g of DIW and 20 g of surfactant B and the mixture was stirred under a nitrogen stream and heated to 35C. 35 g of EA was added to the above mixture and dispersed for 10 minutes. 12 g of a 3% aqueous solution of H2O2 and 12 g of a 2~ aqueous solution of VC were added for polymerization of seed latex.
665 g of a core monomer mixture of the under-mentioned composition was mixed with 135 g of surfactan-t B and 95 g of DIW. Then, the mixture was ~ed to the reaction mixture over a period of 2~0 minutes, followed ~y 72.5 ~ 3 g of a 3% a~ueous solution of H~O2 and 72.5 g of a 2%
aqueous solution of VC were continuously fed over a period of 300 minutes for seeded polymerization. While the monomer solution was fed, the reaction -temperature was kept at the range from 35C to 40C.
Core monomer mixture BA 697.20 g AlMA 1.40 g BGA 1.40 g The reaction mixture was kept for one hour at the same temperature after finish of feeding monomers, and was subject to the shell pol~merization.
32.9 g of a 3~ aqueous solution of H2O2 and 32.~ g of VC was fed to the reaction mixture over a period of 15 150 minutes, and 431 g of a shell monomer emulsion of the under-mentioned composition was continuousl~ fed over a period of 90 minutes for seeded pol~nerization.
While the monomer solution was fed, the reaction temperature was kept at the range from 35C to 40C.
Shell monomer emulsion St 240 g AN 60 g Surfactant B ~7.0 g DIW 102.0 g The reaction mixture was kept for one hour at the same temperature, then, cooled and filtered through a 300-mesh stainless steel sieve to give a core-shell polymer latex.
This latex was frozen at -15C and filtered through a glass filter. The solid was then dried in a current of air at 60C overnight to give core-shell polymer D.
The compositions of core-shell polymers A to D are shown in Table 1.
Example 5 Production of POM resin composition (1) Seventy parts of Tenac C4510, a POM copolymer 2~ 3 .resin o Asahi Chemical [ndustry Co., Ltd., and 30 parts of core-shell polymer A prepared ln Example 1 were dried to a moisture con-tent of not rnore than 0.3%
and using a twin-screw extruder (PCM-30; Ikegai Corporation), the mixture was melt-blended a-t a cylinder temperature o~ 200C and a die head temperature o~ 200C to give pellets of POM resin composition (1).
Examples 6 to 12 Production of POM resin compositions (2) to (8) In the same manner as Example 5, pellets o~ POM
resi.n compositions (2) to (8) were produced according to the formulas shown in Table 2.
Comparative Examples 1 and 2 Production of core-shell polymers E and F
In the same manner as Example 4, core-shell polymers E and F were produced according to the compositions shown in Table 1.
Comparative Examples 3 to 7 Production of POM resin compositions (9) to (13) In the same manner as Example 5, pellets of POM
compositions (9) to (13) were produced according to the formulas shown in Table 2.
Impact strenqth testinq of resin prodllcts Resin compositions (1) through (13) were dried at 110C for 1 hour and using an injection molding machine (TS-100, Nissei Plastics Co.), each composition was molded at a cylinder temperature of 200C and a nozzle temperature of 200C.
Notched Izod testpieces, 3.2 mm -thick, were prepared in accordance with JIS K7110. The impact strength of these testpieces were measured at 23C in accordance with ~IS K7110.
Incidentally, melt-blending could not be made with POM resin compositions (10) and (11) (Comparative Examples 4 and 5). The results of blending are shown ~S,)~3~"~
- 2] -below in the table 2.
Determination of welcl e~ ion reten-tion rates of resin products Using testpieces conforming to JIS k7113 , the ratio of the elonga-tion at break of a testpiece with two point g~tes a-t both ends to that of a testpiece with a one-poin~ gate at one end was determined by the tensile test method according to JIS
k7113. The results are set forth in the table 2.
[Weatherability Test]
The color difference between non-exposed and exposed injection molded specimen obtained from POM
resin compositions (5) and (13) by the Sunshine Super Long-Life Weather Meter~ (Suga Test Instruments) were measured using ~80 Color Measuring System~ (Nippon Denshoku Kogyo).
The results are shown in Table 3.
[Thermal Stability Test]
The color difference between non-kept and kept injection molded specimen obtained from POM resin compositions (5) and (13) in the drier setting at 150C
for 50 hours were measured using ~80 Color Measuring System~.
The results are shown in Table 3.
[Thermal s-tability Test (]cept melting)]
The color difference between non-kept and kept injection molded specimen obtained from POM resin compositions (5) and (13) in the in~ection molding system setting the cylinder temperature of 230C before molding, were measured using **80 Color Measuring System~.
The results are shown in Table 3.
[Qualitative test for sulfate ion]
The sulfate ions in core-shell polymers A ~o E, KM-330 were determined.
Thus, 5 g of each sample was weighed into a 50 ml conical flask, 20 ml of deionized water was added and the mixture was stirred with a magneti.c stirrer for 3 hours.
The mixture was filtered through a No. 5 C filter paper and the filtrate was divided into halves. Then, 0.5 ml of a 1~ aqueous solution of barium chloride was added to one of the halves and the relative turbidity of the two halves was examined.
In this qualitative test, no sulfate ion was detected in core-shell polymers A to D but sul-fate ions were detected in core-shell polymers E and KM~330.

Table 1 Compositions of Core-Shell Polymers _~_ _ Ex. No. 1 2 3 4 Comp. Comp.
Ex. 1 ~x. 2 l I _ I
Impact Modifier A B C D E F
Core , 11 BA 60.76 _ 66.234 79.68 69.51 BGA 0.0070.1428 0.007 0.133 0.16 0.14 AQMA 0.0180.357 0.018 0.133 0.16 0.35 Bd 28.1 _ 28.1 _ _ _ _ MMA 13.05 7.0 13.05 _ _ _ I - _ ._ EA 3.451.3832 3.45 3.5 _ _ I -- _ 2EHA 25.2 _ 25.2 _ _ _ l _ I
MAM O.12 O.357 O.12 _ . _ Core/MID//Sh01170//30 70//3070//3070//30 80//2070//30 Shell _ I
MMA 27.0 27.027.0 _ 18.0 27.0 I _ EA 3.0 3.0 3.0 _ 2.0 3.0 _ BGA 0.06 _ _0.06 _ AN _ _ _ 6.0 _ _ I
St _ _ _ 24.0 _ _ I .
Surfactant A A B B NP OS
¦ Polymeri~ation V50 V50 V50 H202/VC SPS V50 Initiator , _ _ ~ J
- 2~ -Table 2 . _ ¦ Examples 5 6 7 8 9 10 11 12 Resin (1) (2) (3)(4) (5) (6) (7) (8) composition _ Impact (A) (B) (D)(C) (C) (C) (C) (C) Modifier I
Ratio of POM
to Impact Modifier POM-l 70 70 100100 100 100 Impact Modifer30 30 40 40 20 60 40 40 l _ I
Izod impact 31.218.0 14.8 33.9 22.1 38.2 41.6 35.1 (kgf-cmlcm) l . _ I
Elongation(%) 70/200 601170 (with weldIwithout weld) . l - I

POM-1: Tenac C4510 (Asahi Chemical Industry Co., Ltd.; POM copolymer) POM-2: Tenac C3510 (Asahi Chemical Indus-try Co., Ltd.; POM copolymer) POM-3: Tenac 4010 (Asahi Chemical Industry Co., Ltd.; POM homopolymer) 2 ~ ~5 J JA ~

Table 2 (Continued) Examples Co~p. 6x, Comp. Ex. 4 Comp, Ex. 5 Col~p. ~. 6 ~ 7 Resin (9) (10) (11) (12) (13) composition _ Impact (E) (F) KM-330 TPU TPU
¦Modifler Ratlo oflOO/40 100/40 100140 100/40 100/20 POM-l to Impact ¦ Modifier Izod impact _* _* _* 13.6 (kgf cm/cm) _ Elongation _* _* _* 2.8/400 weld/without KM-330 : impact modifier (Rohm & Haas Co.) TPU ; polyurethane elastomer; Elastollan ET-680-10 (Takeda Badische Urethane Industries, LTD.) * The resin compositions (9) and (11) (Comparative Example~ 3 and 5) foamed copiously owing to decomposition of POM during blending and could not be molded.
The resin composition (10) (Comparative Example 4) made smoke and had discolored during blending.
Table 3 Resin compositions (5) (13) Weatherability (~E) 0.90 11.0 Thermal stability(~E) 3.5 12.1 Thermal stability 1.2 5.8 (kePt melting~ (~E) ~ ,

Claims

1) A core-shell polymer comprising a rubbery polymer core and a glassy polymer shell which is produced by an emulsion polymerization reaction using an oligomeric surfactant and a neutral radicals-liberating polymerization initiator.

2) The core-shell polymer as claimed in claim 1, wherein the oligomeric surfactant is an oligomeric anionic surfactant.

3) The core-shell polymer as claimed in claim 1, wherein the neutral radicals-liberating polymerizing initiator is an initiator of azo type.

4) The core-shell polymer as claimed in claim 3, wherein the initiator of azo type is azobis(isobutyronitrile), dimethyl 2,2'-azobis(isobutyrate) or 2,2'-azobis(2-amidinopropane)-dihydrochloride.

5) Tha core-shell polymer as claimed in claim 1, wherein the neutral radicals-liberating polymerization initiator is an initiator of a peroxide type.

6) The core-shell polymer as claimed in claim 1, wherein a transition temperature of the rubbery polymer is not over -30°C.

7) The core-shell polymer as claimed in claim 1, wherein a transition temperature of the glassy polymer is not less than 60°C.

8) The core-shell polymer as claimed in claim 1, wherein an amount of the rubbery polymer core is from 50 to 90 weight% per the whole core-shell polymer.

9) The core-shell polymer as claimed in claim 1, wherein an amount of the glassy polymer shell is from

10 to 50 weight% per the whole core shell polymer.
10) The core-shell polymer as claimed in claim 1, which has substantially no detectable sulfur oxide compounds.

11) The core-shell polymer as claimed in claim 1, wherein a sulfur oxide contained in the core-shell polymer is not detected by the method which comprises stirring 5 g of the core-shell polymer in 20 m? of deionized water for 3 hours at room temperature, filtering the mixture through a No. 5 C filter paper dividing the filtrate into halves, adding 0.5 m? of 1%
barium chloride aqueous solution to one of the halves, and observing the relative turbidity of the two halves.

12) A method of producing a core-shell polymer which comprises an emulsion polymerization reaction using an oligomeric surfactant and a neutral radicals-liberating polymerization initiator.

13) A core-shell polymer which is produced by the method as claimed in claim 12.

14) A polyoxymethylene resin composition comprising the core-shell polymer as claimed in claim 1.

15) The polyoxymethylene resin composition as claimed in claim 14, wherein an amount of the core-shell polymer is 5 to 100 weight parts based on 100 weight parts of the polyoxymethylene resin.
15) A polyoxymethylene resin composition comprising the core-shell polymer as claimed in claim 13.

16) A molded article made of the resin composition as claimed in claim 14.

17) A molded article made of the resin composition as claimed in claim 15.

18. A process for producing a core-shell polymer comprising 50 to 90% by weight (based on the core-shell polymer) of a rubbery polymer core having a glass transition temperature of not higher than -30°C and 10 to 50% by weight (based on the core-shell polymer) of a glassy polymer shell having a glass transition temperature of not less than 60°C, the said core-shell polymer containing substantially no detectable sulfate ions, which process comprises:
(1) a first stage polymerization by polymerizing at least one monomer that includes at least one member selected from the group consisting of conjugated dienes and alkyl acrylates containing 2 to 8 carbon atoms in the alkyl moiety, thereby forming the rubbery polymer core, and (2) a second stage polymerization by polymerizing a monomer that is methyl methacrylate alone or in admixture with one or more other monomers which are copolymerizable and give the resulting polymer a glass transition temperature of not less than 60°C, using the rubbery polymer core as a seed particle, wherein both of the first and second stage polymeriza-tions are conducted in an emulsion using a neutral radicals-liberating polymerization initiator and an oligomeric surfactant having a molecular weight of 200 to 5,000 and the formula:

(wherein:

R is an alkyl group of 5 to 20 carbon atoms;

z is 0, 1 or 2;
n is a positive integer corresponding to the molecular weight;
R1, R3,....R2n-1 are each H, CH3, C2H5 or COOH;
R2, R4,....R2n are each H, CH3, C2H5, COOH or CH2COOH;
and X1, X2,.... Xn are each COOH, CONH2, OCH3, OC2H5, CH2OH, , COOC2H4OH, COOC3H6OH, CONHCH2OH, CONHCH3, CONHC2H5, CONHC3H7, COOC2H5, CN, OCOCH3, OCOC2H5 or COOCH2 or a water soluble salt thereof.

19. A process according to claim 18, wherein the oligomeric surfactant has a molecular weight of from about 500 to 5,000 and is in a water soluble anionic salt form.

20. A process according to claim 19, wherein the monomer of the first stage polymerization also includes one or more comonomers that are copolymerizable with the conjugated dienes and the alkyl acrylates and are selected from the group consisting of aromatic vinyl or vinylidene compounds, vinyl or vinylidene cyanide compounds and lower alkyl methacrylates.

21. A process according to claim 19, wherein the monomer of the first stage polymerization contains 0 to 20% by weight (based on the total amount of the monomer) of a conjugated diene and also contains a crosslinking monomer and a grafting monomer each in an amount of 0.01 to 5% by weight based on the total amount of the monomer.

22. An article molded of a polyoxymethylene resin composi-tion which comprises 5 to 100 parts by weight (per 100 parts by weight of the polyoxymethylene resin) of the core-shell polymer produced by any one of claims 18 - 21, the said article having a thermal stability that is not substantially lower than that of the polyoxymethylene resin only.