Background of the Invention Polyoxymethylene (POM) is a thermoplastic polymer possessing good physical properties that may be processed into a variety of articles. Although POM is moderately tough, it is often desirable to increase the impact resistance of POM resins by adding toughening agents.

The following disclosures may be relevant to various aspects of the present invention and may be briefly summarized as follows : US patent 3,749, 755 discloses the use of an elastomeric graft copolymer to toughen POM resins. The elastomeric graft copolymer is made from the emulsion polymerization of an acrylic ester, a monomer containing two double bonds, and an optional additional monomer in the presence of sulfonate emulsifiers. However, tougheners made from sulfonate emulsifiers can lead to thermal degradation of the POM.

US patent 4,639, 488 discloses the use of elastomeric graft polymers with a base made from the emulsion polymerization of dienes, styrenes, and/or (meth) acrylonitrile and a shell made from the polymerization of styrene and/or methyl methacrylate. POM compositions containing elastomeric graft polymers made using potassium fatty acid salts as emulsifiers are shown to undergo less thermal degradation of the POM than those containing elastomeric graft polymers made using sodium hexadecyl sulfonate as an emulsifier.

US patent 5,290, 858 discloses POM resin compositions with improved weatherability that are toughened with core-shell polymers that have no detectable anions present. The core-shell polymers are made using a nonionic surfactant.

However, surfactants with lower ionic character, such as nonionic surfactants, are less efficient and thus either more surfactant is needed or longer polymerization times are required for the preparation of the toughener when such surfactants are used. Thus, it is desirable to have a method of obtaining thermally stable, toughened POM resins that permits the use of ionic surfactants in the preparation of the toughener.

Summary of the Invention Briefly stated, and in accordance with one aspect of the present invention, there is provided a process for making a core shell toughener, comprising: a polymerizing in an aqueous solution vinyl monomers in the presence of an initiator and an ionic surfactant having no metal ions; adding methyl methacrylate and further polymerizing the mixture to form a latex ; coagulating the latex in the presence of a salt to form a coagulum ; washing the coagulum with water, and drying the washed coagulum.

Pursuant to another aspect of the present invention there is provided a process for making a core shell toughened polyoxymethylene, comprising: a polymerizing in an aqueous solution vinyl monomers in the presence of an initiator and an ionic surfactant having no metal ions; adding methyl methacrylate and further polymerizing the mixture to form a latex ; coagulating the latex in the presence of a salt to form a coagulum ; washing the coagulum with water; drying the washed coagulum ; and melt blending the coagulum with polyoxymethylene.

While the present invention will be described in connection with a preferred embodiment thereof, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

Detailed Description of the Invention The present invention relates to a process of preparing a core-shell toughener (e. g. solid core shell resin), preparing a core shell toughener for POM resins and preparing thermally stable, toughened POM resins. In the process of the present invention the core-shell toughener is prepared by polymerizing an aqueous solution of core monomers in the presence of an ionic surfactant and an initiator in a reactor. The monomers, surfactant, and initiator may be added to the reactor continuously or in portions. The contents of the reactor will preferably be stirred to ensure a homogenous mixture. The reactor may be heated. When polymerization of the core monomers to generate a core layer is complete, shell monomers are added to the reactor to generate a shell layer on the core layer. Further initiator may be added when the shell monomers are added. When polymerization of the shell monomer is complete, the resulting polymer is in the form of latex, and the contents of the reactor are cooled to room temperature, if necessary. The latex is coagulated with a coagulating agent, to form solid core-shell toughener coagulum particles that can be in the form of flakes or other forms known to those skilled in the art. The solid particles are then washed with water sufficiently thoroughly to ensure that when the resulting core-shell toughener is melt- blended with POM and the resulting POM resin composition has good thermal stability. Optionally, other additives listed herein can be combined with the POM and melt-blended with the resulting core shell toughener described above resulting in good thermal stability. The thermal stability of the POM resin composition is determined by using the thermally evolved formaldehyde (TEF) test procedure described in US patent 5,011, 890, which is hereby incorporated by reference, from column 21, line 61 to column 22, line 47. The resulting POM resin is deemed to have good thermal stability when no more than about two (2) weight percent of formaldehyde is evolved after 30 minutes of heating. The POM resin composition obtained by blending the core-shell toughener made by the process of the present invention has good toughness and thermal stability as measured by the TEF test described herein.

The core monomers used in the process of the present invention are vinyl monomer. Examples of vinyl monomers include C2-8 alkyl acrylates such as ethyl acrylate, propyl acrylate, butyl acrylate, cyclohexyl acrylate, 2- ethylhexyl acrylate, etc. n-Butyl acrylate is preferred. Conjugated dienes such as butadiene or isoprene are other examples of vinyl monomers.

Butadiene is a preferred conjugated diene. Two or more of any of the foregoing monomers may also be used. One or more comonomers such as aromatic vinyl or vinylidene compounds, vinyl or vinylidene nitriles, or alkyl methacrylates may also be used. Examples of vinyl or vinylidene compounds include styrene, C-methylstyrene, and vinyl toluene. Examples of vinyl or vinylidene nitriles include acrylonitrile and methacrylonitrile. Examples of alkyl methacrylates include methyl methacrylate and butyl methacrylate.

One or more cross-linking and grafting monomers may also be used in the generation of the core layer. Examples of cross-linking monomers include aromatic divinyl monomers, such as divinylbenzene and alkane polyol polyacrylates such as ethylene glycol diacrylate, ethylene glycol dimethacrylate, 1, 4-butanediol diacrylate, hexandiol diacrylate, hexandiol dimethacrylate, trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, etc. , where 1, 4-butanediol diacrylate is preferable. Examples of grafting monomers are allyl esters of unsaturated carboxylic acids such as allyl acrylate, allyl methacrylate, diallyl maleate, dially fumarate, daillyl itaconate, etc. , where allyl methacrylate is preferable.

The initiator used may be a radical precursor suitable for initiating free radical polymerization reactions. Examples are azo initiators, such as 2,2'- azobis (isobutyronitrile) (AIBN) and 4, 4'-azobis (4-cyanovaleric acid), and peroxide initiators such as cumene hydroperoxide, diisopropylbenzene hydroperoxide, and hydrogen peroxide.

The surfactant used is at least one ionic surfactant that contains no metal ions. The surfactant will preferably be an ammonium phosphate or sulfate. For example, a suitable surfactant is, but not limited to, ammonium trideceth-6-phosphate.

The shell monomer is methyl methacrylate. Optionally, other vinyl comonomers can be combined with the methyl methacrylate such as C2-8 alkyl acrylates such as ethyl acrylate, propyl acrylate, butyl acrylate, cyclohexyl

acrylate, 2-ethylhexyl acrylate, etc; aromatic vinyl or vinylidene compounds such as styrene, a-methylstyrene, and vinyl toluene ; vinyl or vinylidene nitriles such as acrylonitrile and methacrylonitrile ; or alkyl methacrylates in the preparation of the shell. A preferred vinyl comonomer is ethyl acrylate.

The coagulating agent used is at least one salt of an alkaline earth metal, such as calcium, magnesium, or barium; or a transition metal such as zinc, copper, palladium, iron, titanium, or nickel. The salt is preferably a bivalent metal salt. Acetate salts are preferred. A preferred coagulating agent is calcium acetate.

Core monomers will preferably comprise about 50 to about 90 weight percent of the monomers used to make the core-shell toughener and shell monomers will preferably comprise about 10 to about 50 weight percent of the monomers used to make the core-shell toughener. When present, grafting and cross-linking monomers will preferably be used in about 0.01 to about 5 mole percent, based on the total amount of monomers used.

The POM used in the present invention can be either one or more homopolymers, copolymers, or a mixture thereof. Homopolymers are prepared by polymerizing formaldehyde or formaldehyde equivalents, such as cyclic oligomers of formaldehyde. Copolymers can contain one or more comonomers generally used in preparing polyoxymethylene compositions.

Commonly used comonomers include alkylen oxides of 2-12 carbon atoms.

If a copolymer is selected, the quantity of comonomer will not be more than 20 weight percent, preferably not more than 15 weight percent, and most preferably about two weight percent. Preferable comonomers are ethylene oxide and butylen oxide, and preferable polyoxymethylene copolymers are copolymers of formaldehyde and ethylene oxide or butylen oxide where the quantity of ethylene oxide or butylen oxide is about two (2) weight percent. It is also preferred that the homo-and copolymers are: 1) those whose terminal hydroxy groups are end-capped by a chemical reaction to form ester or ether groups; or, 2) copolymers that are not completely end-capped, but that have some free hydroxy ends from the comonomer unit. Preferred end groups, in either case, are acetate and methoxy.

The polyoxymethylenes used in the compositions of the present invention can be branched or linear and will generally have a number average molecular weight in the range of 10,000 to 100,000, preferably 20,000-90, 000, and more preferably 25,000-70, 000. The molecular weight can be conveniently measured by gel permeation chromatography in m-cresol at 160 °C using a DuPont PSM bimodal column kit with nominal pore size of 60 and 1000 A. The molecular weight can also be measured by determining the melt flow using ASTM D1238 or ISO 1133. The melt flow will be in the range of 0.1 to 100 g/min, preferably from 0.5 to 60 g/min, or more preferably from 0.8 to 40 g/min. for injection molding purposes. Other structures and processes such as films, fibers, and blow molding may prefer other melt viscosity ranges.

The core-shell toughener and POM may be melt-mixed with additional additives such as reinforcing agents, plasticizers, heat and light stabilizers, chemical stabilizers, lubricants, mold-release agents, minerals, fillers, nucleating agents, and other additives known to those skilled in the art.

Examples of chemical stabilizers include but are not limited to calcium carbonate; calcium stearate; magnesium carbonate; or an amine or an amine derivative, such as tris (hydroxymethyl) methylamine, or hydantoin, allantoin, or their derivatives.

The core-shell toughener and POM and optional additives used in the present process may be melt-blended using any melt-blended method known in the art. For example: 1) the component materials may be mixed to homogeneity using a melt-mixer such as a single or twin-screw extruder, blender, kneader, Banbury mixer, roll mixer, etc. to give a resin composition ; or 2) a portion of the component materials can be mixed in a melt-mixer, and the rest of the component materials subsequently added and further melt- mixed until homogeneous.

The core-shell toughener will preferably be present in the POM resin composition about 1 to about 30 weight percent, based on the total weight of the core-shell toughener and POM.

The resulting POM resin compositions have good thermal stability as measured by the TEF test described above relative to US patent 5,011, 890.

The toughened, thermally stable POM resins produced by the process of the present invention may be further molded into articles using any suitable

melt-processing technique. Commonly used melt-molding methods known in the art such as injection molding, extrusion molding, blow molding, and injection blow molding are preferred. The compositions of the present invention may be formed into films and sheets by extrusion to prepare both cast and blown films. These sheets may be further thermoformed into articles and structures that may be oriented from the melt or at a later stage in the processing of the composition.

Examples General preparation of the core-shell polymer A five-liter reactor was equipped with a reflux condenser, an N2 inlet, and a mechanical stirrer. Demineralized water (1200 g) that had been flushed with N2 overnight and ammonium trideceth-6-phosphate (sold as Polystepe P-12A by Stepan Co., Northfield IL) (0. 844 g) were placed in the reactor. A solution (referred to as"reaction mixture A") of butyl acrylate (448.5 g), allyl methacrylate (1.0 g), 1, 4-butanediol diacrylate (0.5 g), and ammonium trideceth-6-phosphate (18.0 g) in water (255 g) was prepared. A solution (referred to as"reaction mixture B") of methyl methacrylate (142.5 g) and ethyl acrylate (7.5 g) was also prepared. A solution (referred to as"initiator solution") of 4, 4'-azobis (4-cyanovaleric acid) (sold as Vazo (D 69WSP from E. I. du Pont de Nemours, Inc., Wilmington, DE) (2.5 g) in water (500 mL) was also prepared and kept at about 40 °C. A portion of reaction mixture A (75g) was added to the reactor and stirred at 83 °C for 10 minutes. Initiator solution (50 mL) was then added to the reactor, causing the contents of the reactor to turn milky. Fifteen (15) minutes after the initiator solution was added, the continuous addition of the remainder of reaction mixture A commenced and continued for 121 minutes. Initiator solution was added to the reactor in 20 mL portions at 25,55, 85, and 115 minutes after the initial addition of the initiator solution. An additional 20 mL of initiator solution was added 24 minutes after the completion of the addition of reaction mixture A. Thirty-four (34) minutes after the completion of the addition of reaction mixture A, the addition of reaction mixture B to the reactor commenced and lasted for 62

minutes. Throughout the polymerization, the reaction mixture was maintained at 83 to 85 °C. The reaction was cooled 39 minutes after the completion of the addition of reaction mixture B. The cooled latex was filtered through a filter cloth and collected. The mean diameter particle size of the latex was 192 nm as measured by photon correlation spectroscopy using a Beckman Coulter N4 Plus instrument. The latex was further processed as described below.

Core-shell polymer A A portion of the latex was dried in a vacuum oven at 50 °C and the dried polymer was ground to particles with an average diameter of about 2 mm in size.

Core-shell polymer B A 500 mL solution of the latex was slowly added to a vigorously stirred solution held at about 75 °C of calcium acetate monohydrate (10 g) in water (1000 mL) to generate a coagulum. The coagulum was washed in a column with water and dried in a vacuum oven at 45 °C overnight.

Core-shell polymer C A 500 mL solution of the latex was slowly added to vigorously stirred solution held at about 75 °C of calcium acetate monohydrate (10 g) in water (1000 mL) to generate a coagulum. The coagulum was washed once with water and dried in a vacuum oven at 45 °C overnight.

Core-shell polymer D A 500 mL portion of the latex was slowly added to vigorously stirred solution held at about 75 °C of calcium acetate monohydrate (10 g) in water (1000 mL) to generate a coagulum. The coagulum was not washed prior to drying in a vacuum oven at 45 °C overnight.

Melt-blending of core-shell polymers with POM to determine thermal stability POM homopolymer of about 45,000 number average molecular weight (2000 g), Irganoxt) 245 hindered phenol antioxidant (available from Ciba

Specialty Chemicals, Basel, Switzerland) (2.0 g), polyacrylamide stabilizer (described in US patent 5,011, 890) (10 g), and ethylene/vinyl alcohol copolymer derived from 29 weight percent ethylene and 71 weight percent vinyl alcohol (2.0 g) were dry-blended at room temperature to prepare blend A.

Blend A was melt-blended with the core-shell polymers prepared as described above and, in some cases, calcium carbonate (with about 0.7 Dm diameter) in a 18 mm Leitstriz co-rotating twin screw extruder equipped with an atmospheric vent and running at 350 rpm with temperature settings of 200- 210 °C. The compositions of the resulting Examples 1-3 and Comparative Examples 1-4 are given in Table 1.

Table 1

30 g core-shell polymer 3.0 g calcium Example 1 300 g blend A B carbonate 30 g core-shell polymer Example 2 270 g blend A B 30 g core-shell polymer Example 3 270 g blend A C Comparative 270 g blend A Ex. 1 Comparative 30 g core-shell polymer 3.0 g calcium 270 g blend A Ex. 2 A carbonate Comparative 30 g core-shell polymer 270gb) endA Ex. 3 A Comparative 30 g core-shell polymer 270 9 blend A Ex. 4 D The thermal stability of each of the compositions of Examples 1-3 and Comparative Examples 1-4 was determined using a thermally evolved formaldehyde (TEF) test procedure and the results are given in Table 2. A weighed sample of the composition was placed in a tube and the tube was fitted with a cap for the introduction of nitrogen to the test sample to facilitate the removal of any evolved gases from the apparatus while maintaining the sample in an oxygen-free environment. The tube that contained the sample was heated at 254 °C in a silicon oil bath. The nitrogen and any evolved gases transported thereby were bubbled through 75 mL of a 40-g/liter aqueous sodium sulfite solution. Any evolved formaldehyde reacts with the sodium sulfite to generate sodium hydroxide. Generated sodium hydroxide was continuously neutralized with standard 0.1 N HCI. The results were

obtained as a chart of mL of titer versus test time. The percent evolved formaldehyde was calculated by the formula V*N* (0. 03*100/SW) where V is the volume of titer in milliliters, N is the normality of the titer, and SW is the sample weight in grams. The factor"0. 03" is the milliequivalent weight of formaldehyde in g/milliequivalent. Thermally evolved formaldehyde results are conveniently reported after fifteen minutes and thirty minutes of heating. Samples evolving less than 0.30 weight percent of formaldehyde at 15 minutes and less than 2.0 weight percent of formaldehyde at 30 minutes are considered to have acceptable stability.

Table 2 Weight percent formaldehyde generated After 15 min. After 30 min. Example 1 0. 12 0. 45 Example 2 0. 18 0. 68 Example 3 0. 12 0. 91 Comparative Ex. 1 0. 03 0. 26 Comparative Ex. 2 1. 37 2. 26 Comparative Ex. 3 4. 67 8. 62 Comparative Ex. 4 0. 62 2. 22 Comparative Example 3, prepared from core-shell polymer that was not coagulated, has unacceptable stability. In Comparative Example 2, small- particle size calcium carbonate was added to the melt-blend, but the thermal stability was still unacceptable. The coagulum of Comparative Example 4 was not washed and the resin has unacceptable thermal stability.

Melt-blendinq of core-shell polymers with POM to determine impact resistance Core-shell toughener made as generally described above and coagulated with calcium acetate, washed, and dried was melt-blended at about 200-220 °C using a Leistriz co-rotating twin screw extruder with POM homopolymer (DelrinE 500 fluff, available from E. I. du Pont de Nemours, Inc., Wilmington, DE), polyacrylamide stabilizer (described in US patent 5,011, 890), Irganox0 245 hindered phenol antioxidant (available from Ciba Specialty Chemicals, Basel, Switzerland), and ethylene/vinyl alcohol copolymer derived from 29 weight percent ethylene and 71 weight percent vinyl alcohol in the proportions given in Table 3 to prepare the compositions of Examples 4 and 5. The same components except for the core-shell toughener were melt-blended to prepare the composition of Comparative Example 5. The compositions were molded into ASTM T-bars and notched Izod impact resistance and flexural modulus were determined using ASTM D- 256, and ASTM D-790, respectively and are shown in Table 3.

Table 3 Example 4 Example 5 Comparative Example 5 DelrinE 500 89.4 84. 4 99. 4 Core-shell 10 15-- toughener Polyacrylamid 0. 45 0. 42 0. 45 e stabilizer Ethylene/vinyl alcohol 0.09 0.08 0.1 copolymer IrganoxE 245 0.09 0. 08 0. 1 Notched Izod impact 112 109 85 resistance jim Flexural modulus 2068 2013 2772 (MPa)

All ingredient amounts are given in weight percent based on the total weight of the composition.

It is therefore, apparent that there has been provided in accordance with the present invention, a process for making a core-shell toughener and toughening polyoxymethylene resins that fully satisfies the aims and advantages hereinbefore set forth. While this invention has been described in conjunction with a specific embodiment thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.