WO2019126913A1

WO2019126913A1 - A method for preparing aqueous copolymer dispersions

Info

Publication number: WO2019126913A1
Application number: PCT/CN2017/118186
Authority: WO
Inventors: Feng WANG (Walter); Honggang CHEN (Henry); Lingyun HUANG (Roy); Xiaohui JIA (Jesse); Liliang ZHU (Eric)
Original assignee: Wacker Chemie Ag
Priority date: 2017-12-25
Filing date: 2017-12-25
Publication date: 2019-07-04

Abstract

The invention relates to a method for preparing an aqueous dispersion of a copolymer comprising polymerized comonomers vinyl ester and ethylene, characterized in that ethylene and the vinyl ester are charged in multiple steps, where the initial charge ratio of ethylene to the vinyl ester is 0.2 to 0.6:1; and the vinyl ester is copolymerized with ethylene in the presence of a) anionic surfactant 1, which has the following chemical formula: C _nH _2n+1O-EO _x-PO_y-Q (I), where n is an integer between 6 and 44, inclusive; x is an integer between 0 and 11, inclusive; y is an integer between 0 and 11, inclusive; at least one of x and y is non-zero; and Q is any one selected from among sulphate, sulfonate, carboxylate and phosphate salts.

Description

A Method for Preparing Aqueous Copolymer Dispersions

Field of the Invention

The present invention relates to a method for preparing aqueous copolymer dispersions, in particular vinyl acetate-ethylene (VAE) copolymer dispersions, which remarkably increases the conversion rate of ethylene in the copolymerization reaction.

Background of the Invention

Vinyl ester-based polymer dispersions are used in a variety of applications, including paper and packaging, wood glues, paints, coatings and inks, as well as leather treatment and textile bonding. Amongst them, dispersions based on vinyl ester-ethylene copolymers, such as VAE copolymer dispersions, are widely used in the field of coatings due to their low glass transition temperatures and low minimum film-forming temperatures resulted from the introduction of ethylene.

Unlike many other dispersions, VAE copolymer dispersions are produced under high pressure where ethylene is polymerized with vinyl acetate in micelles and the resulting latex particles. However, in the current polymerization process of VAE dispersions, the utilization of ethylene is relatively low and the unreacted ethylene is usually burnt and emitted to the atmosphere, causing environmental pollution and a significant increase in the production cost of VAE copolymer dispersions. Therefore, it will bring about huge economic and environmental benefits to improve the conversion rate of ethylene in VAE copolymer dispersions. However, there are few literatures in this regard.

US6673854B discloses a VAE copolymer dispersion containing a phosphate-functionalized surfactant and a hydroxyethyl cellulose, wherein ethylene, in an amount of from 5 to 25 wt% (based on the total weight of the comonomers) , is charged all prior to the initiation of the polymerization, and the copolymer dispersion can be used to prepare coatings having a high pigment volume concentration (PVC ≥ 65%) at low costs. US6673854B also investigates the effect of different surfactants on the scrub resistance of high-PVC coatings, and evaluates and compares ether phosphates containing from 4 to 10 ethylene oxide (EO) units and sodium laureth sulfates containing from 2 to 3 EO units. However, US6673854B does not deal with any aspect regarding the conversion rate of ethylene in the polymerization of VAE dispersions.

US6824635B discloses an aqueous polymeric binder free of alkylphenol ethoxylates, obtained by polymerizing 50 to 90 wt%vinyl acetate, 5 to 49 wt%ethylene and 1 to 10%N-methylol acrylamide in the presence of a combination of an anionic surfactant and a nonionic surfactant, wherein ethylene is charged all prior to the initiation of the polymerization and the anionic surfactant is a sodium laureth sulfate containing from 1 to 12 EO units and the nonionic surfactant is a secondary alcohol ethoxylate containing from 7 to 30 EO units or a branched primary alcohol ethoxylate having from 3 to 30 EO units. However, US6824635B only studies the effect of the ratio of anionic surfactants to nonionic surfactants on the viscosity, accelerated precipitation experiment, peel value and wet tensile strength of polymeric binders, and does not discuss anything about the conversion rate of ethylene in copolymerization.

US 4287329 discloses a method for preparing VAE copolymer elastomers having a high Mooney viscosity (≥30 ML (1 + 4) ) and a low gel content (≤ 2%) , wherein the copolymer is obtained by polymerizing 40 to 70 wt%vinyl acetate and 30 to 60 wt%ethylene in the presence of at least one surfactant, having a hydrophilic lipophilic balance (HLB) of at least 22, and a protective colloid. In Example 2 therein, a composition of a polyoxyethylene nonionic surfactant (HLB = 29) and sodium laureth sulfate was used as the surfactant, and a polyvinyl alcohol as the protective colloid. However, US 4287329 does not describe the degree to which ethylene is converted by the method therein.

Summary of the Invention

The present invention relates to an improvement of the method for preparing vinyl ester-ethylene copolymer dispersions, in particular VAE copolymer dispersions, which can significantly increase the conversion rate of ethylene in the copolymerization to higher than 88%, or even higher than 98%, greatly reducing the cost of disposal of unreacted ethylene as well as the environmental load. Meanwhile, the increased conversion rate of ethylene during the polymerization speeds up the reaction, shortens the polymerization time and reduces equipment occupancy, which further brings down the production cost of vinyl ester-ethylene copolymer dispersions. Moreover, the dramatically decreased pressure in the reactor at the end of the reaction due to the conversion of ethylene greatly improves the safety of industrial production. In addition, the preparation method is easy, and the resulting dispersions have a small particle size and is stable.

As used herein, the term “anionic surfactant 1” means an anionic surfactant containing polyoxyethylene units, which generally has a general formula C _nH _2n+1O-EO _x-PO _y-Q, where Q is a sulfate salt, a sulfonate salt, a carboxylate salt or a phosphate salt.

As used herein, the term “anionic surfactant 2” means an anionic surfactant containing no polyoxyethylene units, which generally has a general formulaC _nH _2n+1-Q, where Q is a sulfate salt, a sulfonate salt, a carboxylate salt or a phosphate salt.

As used herein, the term “multiple-step charge” means including at least one charge prior to the initiation of polymerization and one charge during polymerization.

As used herein, the term “initial charge of XX monomer” means the amount of the monomer fed prior to the initiation of polymerization.

As used herein, the term “polymerization time” means the time window from the initiation to the end of polymerization. The end of polymerization means that the copolymerization of ethylene and vinyl ester is completed substantially with not more than 2 wt%, or even not more than 1 wt%of the total vinyl ester remaining.

The inventors of the present invention have surprisingly found that the multiple-step charge of ethylene and a vinyl ester in the presence of anionic surfactant 1 during the copolymerization of the vinyl ester and ethylene can significantly increase the conversion rate of ethylene.

The first aspect of the present invention provides a method for preparing an aqueous dispersion of a copolymer comprising polymerized comonomers vinyl ester and ethylene,

wherein ethylene and the vinyl ester are charged in multiple steps with an initial charge ratio of ethylene to vinyl ester of 0.2 to 0.6: 1;

and the vinyl ester is copolymerized with ethylene in the presence of a) anionic surfactant 1,

which has the following chemical formula:

C _nH _2n+1O-EO _x-PO _y-Q (I)

where

n is an integer between 6 and 44, inclusive;

x is an integer between 0 and 11, inclusive;

y is an integer between 0 and 11, inclusive;

at least one of x and y is non-zero; and

Q is any one selected from among sulphate, sulfonate, carboxylate and phosphate salts.

According to the method of the invention, the polymerization is carried out in a semi-continuous process comprising the stages: (1) prior to the initiation of polymerization, the initial materials are prepared; and (2) during polymerization, the remaining materials are charged to the initial materials in one or more steps.

According to the method of the invention, the process that ethylene and the vinyl ester are charged in multiple steps means each of them is charged in multiple steps. According to one embodiment of the invention, the multiple-step charge of ethylene and the vinyl ester comprises: the first charge prior to the initiation of the polymerization, and the second charge during the polymerization.

According to the method of the invention, in order to increase the conversion rate of ethylene, the initial charge ratio of ethylene to the vinyl ester should suitably be in the range of 0.2 to 0.6: 1, for example, 0.2: 1, 0.3: 1, 0.4: 1, 0.5: 1 or 0.6: 1, preferably 0.4 to 0.6: 1.

According to the method of the invention, the initial charge of ethylene is suitably in the range of from 40 to 95 wt%, for example, 40 wt%, 50 wt%, 60 wt%, 70 wt%, 80 wt%, 90 wt%or 95 wt%, preferably from 50 to 90 wt%, based on the total charge thereof. The initial charge of the vinyl ester is suitably in the range of from 10 to 60 wt%, for example, 10 wt%, 20 wt%, 30 wt%, 40 wt%, 50 wt%or 60 wt%, preferably from 10 to 40 wt%, based on the total charge thereof.

According to the method of the invention, the amount of the vinyl ester in copolymerizations is usually from 70 to 96 wt%, for example, from 75 to 90 wt%, and the proportion of ethylene is generally from 4 to 30 wt%, for example, from 10 to 25 wt%, based on the total weight of the comonomers.

According to the method of the invention, in chemical formula (I) of said anionic surfactant 1: n can be 6, 8, 10, 12, 15, 18, 20, 22, 25, 28, 30, 35, 38, 40, 42 or 44, preferably an integer between 8 and 36, more preferably an integer between 10 and 20; x can be 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or 11, preferably an integer between 2 and 10, more preferably an integer between 2 and 6; y can be 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or 11, preferably an integer between 0 and 5; and Q is preferably a sulfate or sulfonate salt.

According to one embodiment of the invention, in chemical formula (I) of said anionic surfactant 1: n is an integer between 6 and 44 inclusive, preferably an integer between 8 and 36, more preferably an integer between 10 and 20; x is an integer between 1 and 11 inclusive, preferably an integer between 2 and 10, more preferably an integer between 2 and 6; y is 0; and Q is any one selected from among sulfate, sulfonate, carboxylate and phosphate salts.

According to another embodiment of the invention, in chemical formula (I) of said anionic surfactant 1: n is an integer between 6 and 44 inclusive, preferably an integer between 8 and 36, more preferably an integer between 10 and 20; x is an integer between 1 and 11 inclusive, preferably an integer between 2 and 10, more preferably an integer between 2 and 6; y is 0; and Q is a sulfate or sulfonate salt.

According to the method of the invention, said anionic surfactant 1 can be a surfactant of formula (I) or a combination of several surfactants of formula (I) , for example, one or more ether sulfates, such as one, or a combination, of ethoxysulfates, propoxylates and ethoxypropoxy sulfates. Examples of said anionic surfactant 1 include, but are not limited to C _12-15-O-EO ₃ sulfate, C _12-15-O-EO ₁₁ sulfate, C ₁₂-O-EO ₄ sulfate, C ₃₂-O-PO ₇-EO ₆ sulfate, C _16-18-O-PO ₇-EO ₅ sulfate, C ₂₀-O-PO ₇-EO ₁₀ sulfate, C _16-17-O-PO ₇ sulfate, C ₁₃-O-PO ₇ sulfate, or any combination thereof, preferably C _12-15-O-EO ₃ sulfate and C ₁₂-O-EO ₄ sulfate.

According to the method of the invention, said anionic surfactant 1 can be used in a suitable amount of from 0.1 to 1.5 wt%, for example 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt %, 0.7 wt%, 0.8wt%, 0.9 wt%, 1 wt%or 1.5 wt%, preferably from 0.35 to 1 wt%, based on the total weight of the vinyl ester and ethylene monomers.

According to the method of the invention, at least part, for example at least 25 wt%, at least 40 wt%, at least 55 wt%, at least 70 wt%, or at least 85 wt%, preferably 100 wt%, of said anionic surfactant 1 is fed prior to the initiation of the polymerization, based on the total weight thereof.

According to the method of the invention, one or more surfactants as follows can be present in the system during the copolymerization of the vinyl ester and ethylene:

b) anionic surfactant 2;

c) a nonionic surfactant; and

d) a polymerizable surfactant.

According to the method of the invention, said anionic surfactant 2 can be any exemplary anionic surfactants known in the art, including sulfates, sulfonates, carboxylates and phosphates, preferably sulfates and/or sulfonates. The sulfates typically refer to alcohol sulfates, such as sodium dodecyl sulfate and sodium hexadecyl sulfate, but are not limited thereto; and the sulfonates typically refer to alkylaryl sulfonates, such as sodium dodecyl benzene sulfonate, sodium cetyl benzene sulfonate and sodium octadecyl benzene sulfonate, but are not limited thereto.

According to the method of the invention, said anionic surfactant 2 can be used in a suitable amount of from 0.005 to 0.05 wt%, for example, 0.005 wt%, 0.01 wt%, 0.02 wt%, 0.03 wt%, 0.04 wt%or 0.05 wt%, based on the total weight of the vinyl ester and ethylene monomers.

According to the method of the invention, at least part, for example at least 25 wt%, at least 40 wt%, at least 55 wt%, at least 70 wt%, or at least 85 wt%, preferably 100 wt%, of said anionic surfactant 2 is fed prior to the initiation of the polymerization, based on the total weight thereof.

According to the method of the invention, suitable nonionic surfactants can be alkyl alcohol ethoxylates and/or ethylene oxide-propylene oxide (EO-PO) copolymers, preferably alkyl alcohol ethoxylates.

The alkyl alcohol ethoxylate surfactants include linear alkyl alcohol ethoxylates and branched alkyl alcohol ethoxylate surfactants, wherein the alkyl group has 4 to 40 carbon atoms, preferably 8 to 18 carbon atoms, and is ethoxylated by 2 to 60, preferably 4 to 40, EO units. Suitable alkyl alcohol ethoxylate surfactants include C ₁₂-C ₁₄ fatty alcohols containing 3 to 40 EO units, C ₁₃-C ₁₅ fatty alcohols containing 3 to 40 EO units, C ₁₆-C ₁₈ fatty alcohols containing 11 to 60 EO units, C ₁₀ or C ₁₃ fatty alcohols containing 3 to 40 EO units or polyoxyethylene sorbitan monooleate containing 20 EO units, preferably oleyl alcohols, stearyl alcohols or isotridecyl alcohols containing 2 to 60 EO units, more preferably oleyl alcohols, stearyl alcohols or isotridecyl alcohols containing 4 to 40 EO units, for example, isotridecyl alcohols containing 15 EO units.

The EO-PO copolymer surfactants can be selected from among EO-PO copolymers having an EO content of from 10 to 40 wt%and a molar mass of from 500 to 3,000, and particularly preferably having an EO content of from 10 to 30 wt%and a molar weight of from 1,000 to 3,000, based on the total weight of the copolymer.

According to the method of the invention, the nonionic surfactants can be used in an amount of less than 3 wt%, suitably from 0.1 to 3 wt%, for example, 0.1 wt%, 0.3 wt%, 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%or 3 wt%, based on the total weight of the vinyl ester and ethylene monomers.

According to the method of the invention, at least part, for example at least 25 wt%, at least 40 wt%, at least 55 wt%, at least 70 wt%, or at least 85 wt%, preferably 100 wt%, of the nonionic surfactants is fed prior to the initiation of the polymerization, based on the total weight thereof.

According to the method of the invention, the polymerizable surfactants refer to those containing alkenyl groups or alkynyl groups, wherein, their molecules are covalently bonded to and protect the latex particles during the polymerization. The polymerizable surfactants are typically polymerizable anionic surfactants. Examples of suitable polymerizable anionic surfactants include, but are not limited to, sodium vinyl sulfonate, sodium styrene sulfonate, sodium-2-ethanesulfonate, preferably sodium vinyl sulfonate.

According to the method of the invention, the polymerizable surfactants can be used in an amount of less than 0.5 wt%, suitably from 0.05 to 0.5 wt%, for example, 0.05 wt%, 0.1 wt%, 0.15 wt%, 0.2 wt%, 0.25 wt%, 0.3 wt%, 0.35 wt%, 0.4 wt%, 0.45 wt%or 0.5 wt%, preferably from 0.05 to 0.4 wt%, based on the total weight of the vinyl ester and ethylene monomers.

According to the method of the invention, at least part, for example at least 25 wt%, at least 40 wt%, at least 55 wt%, at least 70 wt%, or at least 85 wt%, preferably 100 wt%, of the polymerizable surfactants is fed prior to the initiation of the polymerization, based on the total weight thereof.

According to the method of the invention, the surfactants present in the system are preferably selected from among the following combinations during the copolymerization of the vinyl ester and ethylene:

Combination I: anionic surfactant 1 and anionic surfactant 2;

Combination II: anionic surfactant 1 and a polymerizable surfactant;

Combination III: anionic surfactant 1, anionic surfactant 2 and a nonionic surfactant;

Combination IV: anionic surfactant 1, a nonionic surfactant and a polymerizable surfactant; or

Combination V: anionic surfactant 1, anionic surfactant 2, a nonionic surfactant and a polymerizable surfactant.

In Combination I, the mass ratio of anionic surfactant 1 to anionic surfactant 2 is suitably (20-50) : 1, preferably (25-40) : 1.

In Combination II, the mass ratio of anionic surfactant 1 to the polymerizable surfactant is suitably (20-50) : (8-20) , preferably (25-40) : (10-16) .

In Combination III, the mass ratio of anionic surfactant 1 to anionic surfactant 2 to the nonionic surfactant is suitably (20-50) : 1: (12-30) , preferably (25-40) : 1 : (18-27) .

In Combination IV, the mass ratio of anionic surfactant 1 to the nonionic surfactant to the polymerizable surfactant is suitably (20-50) : (12-30) : (8-20) , preferably (25-40) : (18-27) : (10-16) .

In Combination V, the mass ratio of anionic surfactant 1 to anionic surfactant 2 to the nonionic surfactant to the polymerizable surfactant is suitably (20-50) : 1 : (12-30) : (8-20) , preferably (25-40) : 1 : (18-27) : (10-16) .

According to the method of the invention, e) a protective colloid can also be present in the system to stabilize the latex particles formed during the copolymerization of the vinyl ester and ethylene.

The protective colloid can be selected from among water-soluble fiber derivatives, polyvinyl alcohols and mixtures thereof, preferably water-soluble fiber derivatives, more preferably hydroxyethylcellulose. The hydroxyethylcellulose is typically used in the form of an aqueous solution having a dynamic viscosity at 25℃ of from 100 to 5,000 mPa·s, preferably from 150 to 1,000 mPa·s. The polyvinyl alcohols can be partially or fully hydrolyzed and have an average alcoholysis degree of from 85 to 99.9 mol%. Examples of suitable polyvinyl alcohols include, but are not limited to, PVOH 25/88, PVOH 04/88 and PVOH 05/88.

According to the method of the invention, the protective colloid can be used in an amount of less than 3 wt%, suitably from 0.05 to 3 wt%, for example, 0.05 wt%, 1 wt%, 0.5 wt%, 1 wt%, 1.5 wt %, 2 wt%, 2.5 wt%or 3 wt%, preferably from 0.8 to 2 wt%, based on the total weight of the vinyl ester and ethylene monomers.

According to the method of the invention, at least part, for example at least 20 wt%, at least 40 wt%, at least 60 wt%, or at least 80 wt%, preferably 100 wt%, of the protective colloid is fed prior to the initiation of the polymerization, based on the total weight thereof.

According to the method of the invention, the copolymerization of the vinyl ester and ethylene is initiated by a redox initiator. Suitable oxidizing agents can be one or more members selected from a group consisting of sodium persulfate, potassium persulfate, ammonium persulfate, hydrogen peroxide, t-butyl peroxides (e.g. t-butyl hydroperoxide) , potassium peroxodisulfate, t-butyl peroxypivalate, cumyl hydroperoxide and azobisisobutyronitrile, preferably one, or a combination, of sodium persulfate, potassium persulfate, ammonium persulfate and hydrogen peroxide. The oxidizing agent is typically used in an amount of from 0.05 to 3.0 wt%, based on the total weight of the vinyl ester and ethylene monomers.

Suitable reducing agents are one or more members selected from a group consisting of alkali metal or ammonium sulfites, bisulfites (e.g. sodium sulfite) , derivatives of sulfoxylates (e.g. formaldehyde-zinc sulfoxylate or formaldehyde-sodium sulfoxylate) , sulfinic acid or salts thereof (e.g. 2-hydroxy-2-sulfinatoacetate, disodium 2-hydroxy-2-sulfinatoacetate, zinc 2-hydroxy-2-sulfinatoacetate, or disodium 2-hydroxy-2-sulfinopropionate) , ascorbic acid or salts thereof (e.g. sodium ascorbate) , isoascorbic acid or salts thereof (e.g. sodium erythorbate) and tartaric acid, preferably one, or a combination, of sulfinic acid or salts thereof, ascorbic acid or salts thereof, isoascorbic acid or salts thereof and tartaric acid. The reducing agent is typically used in an amount of from 0.05 to 3 wt%, based on the total weight of the vinyl ester and ethylene monomers.

According to the method of the invention, the redox initiator is preferably a combination of sodium persulfate and ascorbic acid or isoascorbic acid in order to better initiate the copolymerization of ethylene and the vinyl ester and thus increase the conversion rate of ethylene. In this combination, sodium persulfate is used in an amount of from 0.06 to 0.4 wt%, preferably from 0.1 to 0.3 wt%, based on the total weight of the vinyl ester and ethylene monomers; and the mass ratio of sodium persulfate to ascorbic acid or isoascorbic acid is 1 to 5: 1, preferably 2 to 4: 1. According to the method of the invention, the redox initiator is preferably metered and fed during the polymerization.

According to the method of the invention, the vinyl ester can be, for example, vinyl acetate, vinyl butyrate, vinyl propionate, vinyl pivalate, vinyl 2-ethylhexanoate, 1-methyl vinyl acetate, vinyl laurate or any combination of the vinyl alkyl esters, but are not limited thereto. Preference is given to vinyl acetate.

In order to extend the polymer’s properties, in addition to the vinyl ester and ethylene monomers, there can be present during the polymerization other comonomers, for example: halogenated ethers such as vinyl chloride; olefins such as propylene; ethylenically unsaturated carboxylic acids and their derivatives such as fumaric acid, maleic anhydride, maleic anhydride, acrylamide and acrylonitrile; pre-crosslinked comonomers or post-crosslinked comonomers such as divinyl adipate, diallyl maleate, allyl methacrylate, triallyl cyanurate, acrylamide glycolic acid (AGA) , methyl methacrylamidate (MAGME) , N-methylol acrylamide (NMA) , N-methylol methacrylamide (NMMA) , N-methylol allyl carbamate, isobutoxy ethers or esters of N-methylol acrylamide, isobutoxyl ethers or esters of N-methylol methacrylamide, isobutoxy ethers or esters of N-methylol allyl carbamate; epoxy functional comonomers such as glycidyl methacrylate (GMA) and glycidyl acrylate; and silicon-functional comonomers such as vinyltrialkoxysilane and vinylmethyldialkoxysilane.

The other comonomers herein can be used in an amount of less than 10 wt%, for example, less than 8 wt%, less than 5 wt%, less than 2 wt%or less than 1 wt%, based on the total weight of the vinyl ester and ethylene monomers, and are preferably metered and fed during the polymerization.

In the present invention, the comonomers are preferably vinyl esters and ethylene without any other comonomers included, more preferably vinyl acetate and ethylene.

According to the method of the invention, it is necessary, prior to the initiation of the polymerization, to adjust the pH value of the initial materials once prepared and charged into the reactor to a level of less than 6, preferably less than 5, more preferably from 3 to 4. An organic or inorganic acid, preferably phosphoric or formic acid, is commonly used to adjust the pH. A buffer substance can also be added to the initial materials in the reactor, but the method herein preferably does not include any buffer substance. A catalyst, such as ferrous ammonium sulfate, can also be usually added to the initial materials in the reactor to initiate and catalyze the polymerization reaction.

According to one embodiment of the invention, the initial materials prepared prior to the initiation of the polymerization comprises from 10 to 40 wt%of the vinyl ester based on the total amount thereof, from 50 to 90 wt%of ethylene based on the total amount thereof, 100 wt%of the protective colloid based on the total amount thereof and 100 wt%of the surfactants based on the total amount thereof.

According to the method of the invention, the pressure in the reactor is usually stable during the polymerization which is achieved by controlling the charge of ethylene, and can be set different to meet the requirements of formulations on the pressure. It is usually between 20 and 100 bar, preferably between 45 and 85 bar. When the charge of ethylene is completed, the pressure keeps dropping with the consumption of ethylene, while, by the end of the polymerization, it basically remains stable again, for ethylene in the reactor no longer reduces.

According to the method of the invention, the initial materials are usually heated to a temperature 10 to 40℃ lower than the desired temperature during the polymerization, and the reactor is further heated to the reaction temperature by the heat released from the polymerization reaction before the remaining monomers and other materials, if any, are metered and fed while the redox initiator is kept feeding. When the polymerization is completed, the contents in the reactor can be transferred to the downstream degassing tank when the pressure in the reactor stops decreasing.

According to the method of the invention, the feeding of ethylene can be completed together with that of the vinyl ester during the polymerization, and is preferably completed before the feeding of the vinyl ester comes to an end. When the feeding of the vinyl ester is completed, the redox initiator continues to be fed, optionally with an increased feed rate until the exothermic reaction slows down and/or the amount of the unreacted vinyl ester is reduced to less than 2 wt%, preferably less than 1 wt% (based on the total weight thereof) . When the polymerization is completed, the redox initiator can continue to be fed for post-polymerization to remove the residual vinyl ester and achieve nearly 100%vinyl ester conversion.

According to the method of the invention, since the multiple-step charge of ethylene and the vinyl ester in the presence of anionic surfactant 1 can remarkably increase the conversion rate of ethylene during the polymerization and accelerate the reaction progress, it takes less time for the vinyl ester and ethylene to polymerize in this case than in conventional conditions, which can reduce the production cost of the vinyl ester-ethylene copolymer dispersion. The polymerization time herein is suitably less than 4 hours, particularly less than 3.5 hours, and more particularly less than 3 hours.

According to the method of the invention, at the end of the copolymerization of the vinyl ester and ethylene, a defoamer can be fed to the system with excessive foam for the subsequent use of the dispersion. The defoamer is preferably fed at the end of the polymerization so as to prevent the defoamer from destroying the latex particles and affecting the progress of the polymerization; and more preferably, after the completion of the polymerization reaction when the contents in the reactor are transferred to the downstream degassing tank so as to streamline the feeding operation and cost.

Suitable defoamers can be one, or a combination, of mineral oil-based defoamers, higher aliphatic alcohol-based defoamers and polyether-based defoamers, preferably mineral oil-based defoamers, wherein the mineral oil-based defoamer is a defoamer with a mineral oil (such as white oil, diesel or kerosene) as a carrier and a hydrophobic substance (such as fatty acid/fatty acid metal soap, fatty acid amide and higher aliphatic alcohol) as an active defoaming ingredient, the higher aliphatic alcohol-based deformer can be an dispersion of C7-9 alcohols or C12-22 alcohols, and the polyethered defoamer includes GP-based defoamers, GPE-based defoamers and GPES-based defoamers. The defoamer can be used in an amount of less than 2 wt%, for example, less than 1 wt%, less than 0.5 wt%, less than 0.1 wt%, less than 0.05 wt%or 0.01 wt%, based on the total weight of the vinyl ester and ethylene monomers.

The second aspect of the present invention provides an aqueous copolymer dispersion comprising:

a) a copolymer obtained by polymerizing a vinyl ester, ethylene and, optionally a polymerizable surfactant and/or other comonomers according to the first aspect of the invention;

b) an anionic surfactant 1 according to the first aspect of the invention;

c) water; and

optionally, one or more components as follows:

d) a protective colloid according to the first aspect of the invention;

e) an anionic surfactant 2 according to the first aspect of the invention;

f) a nonionic surfactant according to the first aspect of the invention; and

g) a deformer according to the first aspect of the invention.

The aqueous copolymer dispersion according to the second aspect of the invention is preferably obtained by the preparation method described in the first aspect of the invention.

In the dispersion herein, the glass transition temperature (Tg) of the polymer is between 0 and 20℃, preferably between 0 and 8℃, and the average particle diameter D _n of the dispersion is between 100 and 350 nm, preferably between 100 and 200 nm. For the preparation of copolymer dispersions having a low Tg in particular, the method herein can improve ethylene conversion and create greater economic and environmental benefits.

The copolymer dispersions herein are suitable for use as or in coatings, including interior and exterior wall coatings, especially interior wall coatings, and for use as or in binders for adhesion to various substrates, preferably paper, cardboard, wood, fibrous materials such as cotton fabric, and plastics such as polymeric films, e.g. polyethylene, polyvinyl chloride, polyamide, polyester and polystyrene films or acrylonitrile-butadiene-styrene substrates.

Detailed Description of the Preferred Embodiments

The advantages of the preparation method herein are evaluated by the following test methods.

1. Evaluation of ethylene conversion rate

Ethylene conversion rate = actual consumption of ethylene /total charge of ethylene

The actual consumption of ethylene refers to the amount of ethylene polymerized in the final polymer, which can be calculated based on the glass transition temperature of the polymer under test.

As described in Fox T.G., Bull. Am. Physics Soc. 1, page 123 (1956) , the calculation formula is:

1/Tg = x ₁/Tg ₁ + x ₂/Tg ₂+ …+ x _n/Tg _n

where x _n is the weight fraction of monomer n (based on the total weight of the monomers to be polymerized) ;

Tg _n is the glass transition temperature (absolute temperature) of the homopolymer of monomer n, and the Tg of the homopolymer is detailed in Polymer Handbook 2nd Edition, J. Wiley &Sons, New York (1975) ; and

Tg is the glass transition temperature of the polymer, which is determined by the method described below.

2. Determination of glass transition temperature of the polymer

The glass transition temperature (Tg) of the polymer is determined according to ASTM E1356, by differential scanning calorimetry (DSC) , using a Mettler DSC820 with a liquid N ₂ cooling system. The test range is -80℃～130℃ at a heating rate of 10℃/min. The Tg value herein is the mid-point of the Tg measured.

3. Determination of solid content in the dispersion

The solid content of the dispersion herein refers to the ratio of the weight of the dispersion after drying to the weight of the same before drying. In the specific test method, a suitable amount (e.g. 1 to 2 g) of the dispersion is dried at 105℃ for several hours (e.g. 1 to 2 h) and then the result is obtained by calculating the ratio of the weight of the dried product to the weight of dispersion before drying.

4. Evaluation of particle size of the dispersion

The particle size of the dispersion herein is characterized using a Beckman

LS 13320 laser particle size analyzer with PVAC. RF780D as an optical model. Prior to testing, the dispersion sample is diluted to a suitable concentration (e.g. 1 g/mL) . The particle size of the copolymer dispersion is evaluated using the number average particle diameter D _n.

Information on ingredients referred to in the Examples and Comparative Examples:

LA/40-S, active substance being a sodium laureth sulfate containing 12 carbon atoms and 4 EO units, active content approx. 31 wt%, available from Rhodia Solvay Corporation, used as anionic surfactant 1.

LA/120-S, active substance being a sodium laureth sulfate containing 12 carbon atoms and 12 EO units, active content approx. 30 wt%, available from Rhodia Solvay Corporation, used as anionic surfactant 1.

OSS 50 KS, active substance being a oleic acid sulfonate containing 18 carbon atoms, active content 49-53 wt%, available from BASF, used as anionic surfactant 2.

1879, active substance being a tridecyl alcohol ethoxylate containing 13 carbon atoms and 15 EO units, active content approx. 40 wt%, available from Clariant, used as the nonionic surfactant.

A315, active substance being a sodium C10-C13 alkyl benzene sulfonate, active content approx. 15 wt%, available from Sasol Performance Chemicals, used as anionic surfactant 2.

H95, active substance being a mixture of sodium secondary alkyl sulfonates with an average chain length of C15, active content approx. 95 wt%, available from LANXESS, Germany, used as the nonionic surfactant.

Sodium vinyl sulfonate, a polymerizable surfactant, available from Alfa Chemistry.

Natrosol ^TM 250, a hydroxyethyl cellulose, available from Ashland.

PVOH 04/88 aqueous solution, a 20 wt%aqueous solution of polyvinyl alcohol having an alcoholysis degree of 88 mol%and a viscosity of 4 mPa·s in 4 wt%aqueous solution (20℃, DIN 53015) .

PVOH 25/88 aqueous solution, a 10.3 wt%aqueous solution of polyvinyl alcohol having an alcoholysis degree of 88 mol%and a viscosity of 25 mPa·s in 4 wt%aqueous solution (20℃, DIN 53015) .

223, a mineral oil-based defoamer, available from BASF.

XL10, a vinyltrimethoxysilane, available from Wacker Chemicals.

Unless otherwise specified, the amounts in the following Examples and Comparative Examples are in parts by weight.

Examples 1-6 and Comparative Examples 1-3

The aqueous copolymer dispersion was prepared in the following steps:

Prior to the initiation of the polymerization:

Preparation of the initial reactor material: A mixture of water, anionic surfactant 1, anionic surfactant 2 (if any) , nonionic surfactants (if any) , sodium vinyl sulfonate, 1 wt%aqueous solution of ammonium ferrous sulfate and hydroxyethyl was prepared, and adjusted to pH 3.8 to 4.0 with formic acid; then the adjusted mixture was sucked together with from 10 to 40 wt%of vinyl acetate (based on the total amount thereof) into an evacuated reactor while stirring, into which from 50 to 90 wt%of ethylene (based on the total amount thereof) was passed in until the pressure in the reactor was 32 bar. The initial charge ratio of ethylene to vinyl acetate was controlled in the range of 0.2 to 0.6: 1. The temperature in the reactor was set to 45℃.

During polymerization:

Starting reaction, metering and charging: When the temperature rose to the set point or so, the redox agent for polymerization was metered and charged. When an increase of temperature by about 2℃ in the reactor or a decrease of the jacket temperature by about 2℃ was detected, the remaining vinyl acetate was metered and charged typically at a constant rate through 120 to 180 minutes. 15 minutes after vinyl acetate started to be fed, the remaining ethylene was metered and kept feeding until about 15 minutes before the feeding of vinyl acetate was completed.

Completion of polymerization: When the feeding of vinyl acetate was completed, the redox initiator, previously fed at a constant rate, continued to be metered and kept feeding for at least 60 minutes. In case there was still a significant reaction at the end of the aforementioned timeline, the redox initiator was continuously fed at a constant rate until there was no further reaction.

Cooling /standing /adjusting:

Upon completion of the polymerization, the reactor pressure dropped below 20 bar and the contents in the reactor could be transferred to the downstream degassing tank. In order to eliminate the residual vinyl acetate, a post-polymerization reducing agent was fed to the reactor, and the final level of the remaining vinyl acetate was far below 5,000 ppm, in which case vinyl acetate was considered to be completely reacted.

When the product was cooled to about 30℃, a mineral oil-based defoamer was fed and stirred for 10 minutes, and then an aqueous solution of sodium hydroxide at a concentration of 10 wt%was added to adjust PH to about from 4.0 to 6.0.

Table 1 shows the ingredients and their amounts in the aqueous copolymer dispersions of Examples 1-6 and Comparative Examples 1-3, where the amounts in each case are based pro rata on the total charge, as 100 parts by weight, of the vinyl acetate and ethylene monomers. Table 2 shows the solid content, Tg, average particle size D _n of the resulting dispersion and the calculated conversion rate of ethylene.

By comparing Comparative Examples 1-6 and Comparative Examples 1-2, it was found that the conversion rate of ethylene was increased remarkably when the copolymerization of the vinyl ester and ethylene was carried out in the presence of said anionic surfactant 1. An increase of the conversion rate of ethylene by 1%can bring about huge economic and environmental benefits. Among them, the ethylene conversion rates of Examples 1-4 and Example 6 were higher than 92%, while the ethylene conversion rate of Example 5 was slightly lower than that of the Examples, but was significantly higher than that of Comparative Examples 1 and 2, mainly due to the less amount of anionic surfactant 1. By comparing Comparative Examples 1-6 and Comparative Example 2, it was found that the length of EO chain segments of anionic surfactant 1 had a significant effect on the conversion rate of ethylene. By comparing Comparative Examples 1-6 and Comparative Example 3, it was found that the initial charge ratio of ethylene to the vinyl ester in the range of 0.2 to 0.6: 1 was also a key factor in increasing the ethylene conversion rate.

Example 7 and Comparative Example 4

The aqueous copolymer dispersion was prepared in reference to Examples 1-6 and Comparative Examples 1-3, except that a polyvinyl alcohol was used as the protective colloid prior to the polymerization, that from 50 to 90 wt%of ethylene (based on the total amount thereof) was passed in until the pressure in the reactor was 25 bar, and that the remaining vinyl acetate and the comonomers vinyltrimethoxysilane and glycidyl methacrylate were metered and fed at the same time during the polymerization.

Table 3 shows the ingredients and their amounts in the aqueous copolymer dispersions of Example 7 and Comparative Example 4, where the amounts in each case are based pro rata on the total charge, as 100 parts by weight, of the vinyl acetate and ethylene monomers. Table 4 shows the solid content, Tg, average particle size D _n of the resulting dispersion and the calculated conversion rate of ethylene.

The ingredients and the polymerization process of Example 7 and Comparative Example 4 were identical except that different surfactants were used: anionic surfactant 1 in Example 7 while anionic surfactant 2 in Comparative Example 4. The result was that the Tg of the dispersion obtained in Example 7 was significantly lower than that of Comparative Example 4. According to the aforementioned Fox formula : 1/Tg = x ₁/Tg ₁ + x ₂/Tg ₂ + …+ x _n/Tg _n, it was considered that comonomers vinyltrimethoxysilane and glycidyl methacrylate had the same effect on the Tg of the polymers in Example 7 and Comparative Example 4 under the same polymerization conditions, and because the residual vinyl acetate in the system after the completion of the polymerization was far below 5,000 ppm, the actual consumptions of vinyl acetate could be considered almost the same in both cases, i.e, they was equal to the total charge of vinyl acetate (≈95.28 parts by weight) . Therefore, the Tg of the final dispersion in Example 7 was remarkably lower than in Comparative Example 4, and it was inferred that the conversion of ethylene monomer in Example 7 was significantly higher than in Comparative Example 4.

Claims

A method for preparing an aqueous dispersion of a copolymer comprising polymerized comonomers vinyl ester and ethylene, characterized in that ethylene and the vinyl ester are charged in multiple steps, where the initial charge ratio of ethylene to the vinyl ester is 0.2 to 0.6: 1;

and the vinyl ester is copolymerized with ethylene in the presence of a) anionic surfactant 1, which has the following chemical formula:

C _nH _2n+1O-EO _x-PO _y-Q (I)

where

n is an integer between 6 and 44, inclusive;

x is an integer between 0 and 11, inclusive;

y is an integer between 0 and 11, inclusive;

at least one of x and y is non-zero; and

Q is any one selected from among sulphate, sulfonate, carboxylate and phosphate salts.
The method of Claim 1, characterized in that from 40 to 95 wt%of the total ethylene is initially charged,

and from 10 to 60 wt%of the total vinyl ester is initially charged.
The method of Claim 1-2, characterized in that in the chemical formula (I) of said anionic surfactant 1: y is 0; and/or, Q is a sulfate or sulfonate salt.
The method of any one of Claims 1-3, characterized in that anionic surfactant 1 is used in an amount of from 0.1 to 1.5 wt%, based on the total weight of the vinyl ester and ethylene monomers.
The method of any one of Claims 1-4, characterized in that one or more surfactants as follows are present during the copolymerization:

b) anionic surfactant 2;

c) a nonionic surfactant; and

d) a polymerizable surfactant.
The method of Claim 5, characterized in that anionic surfactant 2 is used in an amount of from 0.005 to 0.05 wt%;

and/or the nonionic surfactant is used in an amount of from 0.1 to 3 wt%;

and/or the polymerizable surfactant is used in an amount of from 0.05 to 0.5 wt%;

based on the total weight of the vinyl ester and ethylene monomers.
The method of any one of Claims 1-6, characterized in that e) a protective colloid is present during the copolymerization.
The method of any one of Claims 1-7, characterized in that the redox agent for initiating the copolymerization is a combination of sodium persulfate and ascorbic acid or isoascorbic acid.
The method of Claim 8, characterized in that the mass ratio of sodium persulfate to ascorbic acid or isoascorbic acid is 1 to 5: 1.
The method of any one of Claims 1-9, characterized in that the duration of the copolymerization is less than 4 hours.