EMULSION POLYMERISATION

Abstract: A copolymer emulsion latex with a solids content of 20% to 70% by weight contains at least 40% by weight of vinyl alkanoate, eg acetate, in the monomer charge. This latex has good rheology and dry pick strength for paper coating and is prepared using an unsaturated surfactant and a surfactant providing sharp nucleation.

Description: This invention relates to a process for the preparation of aqueous emulsions containing polymers in which vinyl alkanoates is the major component. Polymers of the invention are utilised in paper coating compositions and demonstrate particularly good rheology and enhanced dry pick strength. Paper coating compositions usually comprise an aqueous polymer binder emulsion, a pigment and optionally other additives common in the technology. The present invention is specifically directed to polymers comprising a vinyl C1 to C13 alkanoate component i.e. forming at least 40% by weight of the polymer more usually at least 50% by weight. Vinyl acetate is an example of a vinyl ester of a C1 to C13 alkanoic acid to which the invention is generally directed. Coating compositions of the invention will contain a pigment, for example clay, which is compounded with the latex binder and used in coating a cellulosic web for example a paper or paperboard web. The characteristics of the latex binder component are significant in the ease of preparation of the paper coating composition, its application and on the qualities of the coated product. In particular the flow characteristics of the liquid coating composition are crucial in the industrial application to the paper or board and, when dried, the latex particles confer adequate strength to the surface for subsequent processing. The invention provides a process for the preparation of a polymer emulsion latex having a solids content from 20% to 70% by weight, preferably 40% to 60%, wherein vinyl C1 to C13 alkanoate forms at least 40% by weight of the monomer charge in which an effective amount of a stabilising system comprising:- A) at least one surfactant capable of providing sharp nucleation (as herein defined in Test method (iii)) in the polymerisation process and B) at least one surfactant including a C14 to C20 unsaturated alkylene moiety. The product of this process has good rheology and enhanced dry pick strength and these desirable product attributes follow from the stabilising surfactant system. In the emulsions prepared for use in paper coatings the polymerisation system will be substantially free of protective colloids. Colloidal materials may be added subsequently to the paper coating composition. DE-A-1 906 386.6 (Reichhold) includes an emulsifier combination of a sulphosuccinic acid ester and a nonionic surfactant based on the reaction product of p-alkylphenol and ethylene oxide to provide a non-coagulating sprayable vinyl ester polymer emulsion. It is not directed to the problem of improving the rheology and dry pick properties of a vinyl alkanoate polymer emulsion. Preferably the stabilising surfactant system is present in an amount of from 1% to 10% by weight, more preferably from 2% to 8%, relative to the polymerisable monomers. Preferably the surfactants A and B are present in a weight ratio of from 5:1 to 1:5. It is believed the presence of surfactant A provides a reduced particle size for the emulsion particles and hence increases the surface area of the binder latex. The surfactant system is applicable to vinyl acetate homopolymers and vinylacetate/acrylates-copolymers. Although vinyl acetate is the preferred vinyl alkanoate monomer because of its availability, cost and known reactivity, other vinyl esters within the class defined are usable in particular vinyl formate, propionate, butyrate, isobutyrate, valerate, 2-ethyl hexanoate, isooctanoate, nonanoate, decanoate, pivalate and Versatate®. A preferred comonomer is ethylene but other ethylenic hydrocarbons, for example propylene, butylene and isobutene and higher alpha-olefins, are usable. Alkyl acrylates are also preferred comonomers. Optionally the copolymer may contain minor monomer components added to provide specific benefits, examples are sodium vinyl sulphonate, acrylic acid, methacrylic acid, acrylamide, hydroxy functional acrylates, vinyl silanes and vinyl halides. A favoured comonomer is a polyethylencially unsaturated compound selected from triallyl cyanurate, triallyl isocyanurate, diallyl maleate, diallyl fumarate, divinyl benzene and diaklyl phthalate. Methods for preparing the polymer emulsions of the invention are well characterised in the literature. Polymer Synthesis (vols I and III) by Sandler & Karo (Academic Press 1974) and Preparative Methods of Polymer Chemistry (2nd Ed) by Sorenson and Campbell (Interscience 1968) provide preparative information. Methoden der Organischen Chemie (Houben-Weyl) Band XIV published by George Thieme Verlag Stuttgart (1961) also provides preparative descriptions. The specific stabilising system to which the invention is directed comprises two surfactants. Surfactant A is capable of providing sharp nucleation in the polymerisation process. The term "sharp nucleation" is defined in Test Method (iii) below. One example of a suitable surfactant is obtainable from Cyanamid of USA under the trade name Aerosol A102® described as the disodium salt of an ethoxylated alcohol half ester of sulphosuccinic acid. Surfactant B is required to contain a C14 to C20 unsaturated alkylene moiety and examples are an oleyl propanol amide sulphosuccinate obtainable from Witco of USA under the trade name Emcol K8300® and the potassium salt of the sulphonation product of oleic acid obtainable from Lankro Chemical of Manchester England under the trade name Lankropol OPA®. The paper coating composition comprises a pigment, for example clays, hydrated silica clays, and other conventional ingredients. The clays used include Kaolin group clays and hydrated silica clays, specific clays are disclosed in "kaolin Clays and their Industrial Uses" by J. M. Huber Corp. (1949) New York chapters 10 to 16. Other pigments, for example calcium carbonate, titanium dioxide, blanc fixe, lithopone and zinc sulphide may be used in addition to clay. In general the paper coating composition will comprise Component Parts Pigment 100 dispersing agent eg. sodium pyrophosphate 0.1 to 0.5 latex (on solids basis) 3 to 30 Cobinder eg. starch or casein 0 to 25 Defoamer, eg. a hydrocarbon oil 0 to 0.2 Water to provide desired solids level. The coating compositions produced with the aid of the latexes of the invention will be applied to fibrous paper webs using conventional means for example trailing blade coaters, air knife coaters and roll coaters. Test Methods: i) Dry pick strength values (cm/sec) were measured on Reeds Aircoat woodfree paper using an IGT Dynamic Pick Tester (AC2 Model) with 25 Kg load and low viscosity ink. ii) Viscosity measurements used either a Brookfield Viscometer at 25°C or a Carrimed Rheometer under the conditions of temperature 25°C Ascent time 1 min. Hold time None Descent time 1 min. Max. shear stress 1500 dynes/sq. cm (150 Pa) Cone diameter 4 cm. Cone Angle 1.5°C iii) The identification and definition of a sharp nucleation surfactant A is obtained by means of the following test method. A vinyl acetate homopolymer is formed by polymerising at 60°C a composition comprising the components Component Weight (g) Vinyl Acetate 120 Test surfactant 6 Sodium bicarbonate 1.2 Sodium persulphate 0.6 Water 1072.2 Total 1200.0 Samples of the reaction medium were taken during the initial stages of the polymerisation. The non volatiles content of each sample was measured and the time to provide 15% conversion from the onset of nucleation was calculated. A sharp nucleation surfactant is one providing a period of below 5 minutes, preferably below 4 minutes, to achieve 15% conversion. Results of surfactants tested are:- Surfactant Period (mins) Aerosol A102® 3.35 Perlankrol RN75® 3.1 Empicol LXV® 4.2 Perlankrol PA® 4.4 Arylan SC15® 6.6 Lankropol OPA® 13.0 Emcol K8300® above 30.0 Perlankrol P208® 26.3 The compositions and commerical sources of Aerosol A102 and Emcol K8300 have been given previously. Perlankrol PA is ammonium nonylphenol 4EQ sulphate and Perlankrol RN75 sodium nonylphenol 20EO sulphate; they are obtainable from Lankro Chemicals of Manchester. Empicol LXV is sodium dodecyl sulphate obtainable from Albright and Wilson of Whitehaven, England. Arylan SC15 is sodium dodecyl benzene sulphonate and Perlankrol P208 is nonyl phenol 20EO; both are obtainable from Lankro Chemicals. The first four listed surfactants satisfy the definition of a sharp nucleation surfactant. Specific Descritpion of the Invention: Examples of the invention will now be given to illustrate the invention. Example I This example describes the preparation and use of a copolymer latex having the composition by weight of: Weight (Kg) % vinyl acetate 3280 76.9% ethylene 890 20.9% sodium vinyl sulphonate 44.5 1.0% vinyl tris methoxyethoxy silane 44.5 1.0% diallyl maleate 8.9 0.2% A surfactant solution comprising a 37% aq solution of Emcol K8300 (276Kg), 30% aq solution of Aersol A102 (178Kg), 25% aq. solution of sodium vinyl sulphonate (178Kg) dissolved in water (2.5 tonnes) was prepared and added to a 8.5 tonnes pressure vessel including an agitator. The pH of this solution was in the range 5.0 to 6.5. A solution of ammonium persulphate (30Kg) in water (160Kg) was prepared and 45% of this solution pumped to the stirred reactor. The temperature was adjusted to 75°C while 10% of the three liquid monomers was added and the vessel pressurised with ethylene to 62 bar. The temperature was held at 75°C for 45 minutes and ethylene added to maintain the pressure of 62 bar. Water (400 Kg) was added to the remainder of the persulphate solution during this period. The pressure was maintained at 62 bar until the desired quantity of ethylene had been reacted; this was after 3.5 hours. A period which had previosuly been identified to allow the desired quantity of ethylene to react. After the 45 minute period the persulphate solution was added continously over a 7 hour period and the remainder (90%) of the monomer mix added over 6 hours. Both additions were made at a constant rate. After completion of all additions the reaction mass was held at 75°C for 30 minutes and the contents of the vessel then cooled to 40°C and discharged to a degassing vessel where finishing solutions A, B comprising: A Kg t-butyl hydroperoxide 13.3 Synperonic NP30® (70% aq) 31.6 Water 180 B Sodium metabisulphite 13.3 Water 180 were added in separate streams. Ammonia solution was added, if required, to bring the pH of the emulsion latex into the range 8.0 to 9.0 together with a small quantity of preservative. The emulsion had a solids content of 54%. A paper coating composition was formulated containing 12 parts per hundred binder on clay (dry/dry) with a final solids content of 45%; this was subjected to the dry pick test. Rheological measurements with the Carrimed Rheometer were performed with a coating composition at 4 parts per hundred binder on clay (dry/dry) with a final solids content of 64.5%. A Comparison emulsion was also prepared in which EMCOL K8300 was replaced by the Aerosol surfactant on a weight for weight basis and formulated into a Comparison composition. The dry pick test (procedure i) gave a result of 250 cm/sec; this is a commercially attractive value. The rheology of the binder composition of Example I and the Comparison composition which did not contain surfactant B was studied; the results were:- Ascent Time (mins x 10²) Example I Composition Comparison Composition Shear rate sec⁻¹ Viscosity poise Shear rate sec⁻¹ Viscosity poise 10 45.85 3.579 65.19 2.517 19 112.7 2.648 121.8 2.450 28 199.2 2.173 170.2 2.542 37 308.0 1.841 201.8 2.810 46 441.0 1.590 213.6 3.283 55 591.0 1.414 208.7 4.004 64 750.4 1.292 191.6 5.060 73 918.5 1.202 168.7 6.545 82 1077 1.149 146.7 8.444 91 1230 1.116 128.0 10.72 Note: 1 poise = 0.1 pascal seconds From these results it is seen the Example I compositions exhibits a decrease in viscosity with shear rate while the viscosity of the Comparison composition increases onshearing and in consequence as the shear stress rises the shear rate eventually declines. This thickening of the Comparison would give difficulties in application of the composition to paper surfaces whereas the Example I composition could be applied without difficulty to a paper surface. Example II This example describes the preparation and use of a copolymer latex having the composition by weight of: vinyl acetate 76.9% butyl acrylate 22% vinyl tris methoxyethoxysilane 1.1% A surfactant solution comprising a 37% aq solution of Emcol K8300 (13g), 31% aq solution of Aerosol A102 (2.5g) and potassium persulphate (0.6g) dissolved in distilled water (140g) was charged to a 1 litre glass reactor, purged twice with nitrogen and heated to 75°C with stirring. A monomer mix comprising vinyl acetate 175 g butyl acrylate 50 g silane A172 2.5 g had been previously prepared and 10% was added to the reactor when the temperature reached 60° to 65°C. The reactor temperature was held at 75°C for 45 minutes. Then a potassium persulphate solution (persulphate 0.5 g in distilled water 30 g) was added continously over a 5 hours period and the remaining 90% of the monomer mix added over 4.5 hours. After completion of all additions the reaction was held at 75° for 30 minutes and the contents of the reactor cooled to 40°C before adding a mixture of nonylphenol 35EO (5g) with distilled water (20 g). Ammonia solution was added to bring the pH of the solution to the range 6.5 to 7.5. The product had a solids content of 54%. A comparison emulsion was also prepared in which Aerosol A102 was replaced by Arylan SC15; the latter did not satisfy the surfactant A definition. Paper coating compositions containing the Example II emulsion and comparison emulsion, each 12 parts per hundred on clay (dry/dry), were prepared with final solids content of 45%. Example II composition had a dry pick value of 245 cms/sec and the comparison composition a dry pick value of 107 cms/sec. Example III This example describes the preparation and use of a copolymer latex having the composition by weight of: vinyl acetate 99.0% ethyl hydrogen maleate 1.0% A surfactant solution comprising a 37% aq solution of Emcol K8300 (13g), 31% aq solution of Aerosol A102 (4g) and ammonium persulphate (0.8g) dissolved in distilled water (240g) was charged to a 1 litre glass reactor, purged twice with nitrogen and heated to 75°C with stirring. Vinyl acetate (25g) was added to the reactor when the temperature reached 60°C to 65°C. The reaction temperature was held at 75°C for 45 minutes. Then an ammonium persulphate solution (persulphate 0.25g in distilled water 20g) was added continuously over a 5 hours period and a previously prepared monomer mix comprising:- vinyl acetate 225g ethyl hydrogen maleate 2.5g added over 4.5 hours. After completion of all additions the reaction was held at 75° for 30 minutes and the contents of the reactor cooled to 40°C before adding a mixture of nonylphenol 35EO (4.4g) with distilled water (22.5 g). Ammonia solution was added to bring the pH solution to the range 6.5 to 7.5. The product had a solids content of 47%. A comparison emulsion was prepared in the same manner as in Example II and paper coating compositions prepared to the same formula as in Example II. The Example III composition had a dry pick value of 106 cms/sec and the comparison composition had a dry pick value of 63 cms/sec. Thus Examples II and III demonstrate the benefit obtained with the use of surfactant A.

Claims: A process for the preparation of a polymer emulsion latex having a solids content of 20% to 70% by weight, preferably 40% to 60%, wherein vinyl C1 to C13 alkanoate forms at least 40% by weight of the monomer charge in which an effective amount of a stabilising system comprising: a) at least one surfactant capable of providing sharp nucleation in the polymerisation process, and b) at least one surfactant including a C14 to C20 unsaturated alkylene moiety is used to provide good rheology and enhanced dry pick strength. The process according to claim 1 wherein the stabilising surfactant system is present in an amount of from 1% to 10% by weight relative to the copolymerisable monomers. The process according to claim 2 wherein the stabilising system is present in an amount of from 2% to 8%. The process according to claim 1, 2 or 3 wherein the surfactants A and B are present in a weight ratio of from 5:1 to 1:5. The process according to any preceding claim wherein the surfactant containing an unsaturated alkylene moiety is selected from oleyl propanol amide sulphosuccinate and an oleic acid sulphonate. A pigmented paper coating composition comprising: a) at least one pigment and b) an aqueous synthetic polymer emulsion latex binder prepared by the process of any preceding claim.

Inventors: FARMER, DAVID BRETLAND, VAFA, ABDOL MANAF
Applicants: UNILEVER N.V., UNILEVER PLC
Application year: 1988
Main classification: C08F18
IPC codes: C09D5, C08F18, C08F2, D21H19