WO2020170272A1

WO2020170272A1 - Core-shell latex with non-yellowing properties

Info

Publication number: WO2020170272A1
Application number: PCT/IN2020/050160
Authority: WO
Inventors: Dr. Jatindranath MAITI; Dr. Swapan Kumar GHOSH
Original assignee: Asian Paints Ltd.
Priority date: 2019-02-23
Filing date: 2020-02-20
Publication date: 2020-08-27

Abstract

A core-shell latex/binder comprising styrene acrylic emulsion/paint formulation including core shell latex particles, is provided, involving styrene rich core and acrylate shell based polymeric particles wherein said core has at least 40 wt.% styrene with respect to total monomer. Also, is provided a two-stage process of synthesizing the same wherein while the core comprises high amount of styrene of at least 40% by wt, said emulsion has at least 50 wt.% solid content to meet high specific productivity. Said core shell styrene-acrylate latex paint showed very high UV resistance properties when compared to formulations having non-core-shell styrene acrylate latex.

Description

TITLE: CORE-SHELL LATEX WITH NON-YELLOW I NG PROPERTIES

Field of I nvention

The present invention provides for core-shell latex/binder comprising styrene acrylic emulsion/paint formulation including core-shell latex particles involving styrene rich core and acrylate shell based polymeric particles wherein said core has at least 40 wt.% styrene with respect to total monomer. Also, is provided a two-stage process of synthesizing styrene core acrylate shell polymeric particles preferably comprising a styrene hard core and soft shell wherein while the core comprises high amount of styrene of at least 40% by wt, said emulsion has at least 50 wt.% solid content to meet high specific productivity. Said core-shell styrene-acrylate latex paint showed very high UV resistance properties when compared to formulations having non-core-shell styrene acrylate latex.

Background Art

Exposure to ultraviolet (UV) radiation may cause significant degradation of many materials. UV radiation causes photo-oxidative degradation which results in breaking of the polymer chains, produces radicals and reduces the molecular weight, causing deterioration of mechanical properties and leading to useless materials, after an unpredictable time.

Polystyrene (PS), one of the most important materials from the modern plastic industry, has been used all over the world, due to its excellent physical properties and low-cost. When polystyrene is subjected to UV irradiation in the presence of air, it undergoes a rapid yellowing and a gradual embrittlement (Yousif, E. and Haddad, R. Photodegradation and photo-stabilization of polymers, especially polystyrene: review Springer plus. 2013; 2:

398. doi: 10.1186/2193-1801-2-398)

The photo-stabilization of polymers may be achieved by various ways. Stabilizing systems have been developed which depend on the action of stabilizer: (1) light screeners, (2) UV absorbers, (3) excited-state quenchers, (4) peroxide decomposers, and (5) radical scavengers; among these, it is generally believed that excited-state quenchers, peroxide decomposers, and radical scavengers are the most effective (Yousif et al.2013).

For certain expositions the formation of yellow color is believed to be the result of an interaction between antioxidant and hindered amine UV stabilizer in the polymer composition. US4464496A (Non-yellowing polymer composition) provides non-yellowing combinations of antioxidant and hindered amine UV stabilizer. Although the advancement is particularly useful in "AES (Acrylonitrile Ethylene Styrene)" polymers, may also be applied to other polymers normally subject to similar yellowing. This non-yellowing antioxidant-UV stabilizer of this prior art comprise(A) at least one antioxidant selected from the group consisting of: bis- ( 3 , 5- d ialky I- 4-hydroxy) hydro cinnamic acid antioxidants likethiodialkylene bis-(3,5-dialkyl-4-hydroxy) hydrocinnamate, N,N’-alkylenebis- (3, 5- dialky I- 4- hydroxy) hydrocinnamam ide, 3, 5- dialky I- 4- hydroxy benzyl antioxidants such as 0,0-dialkyl-3,5-dialkyl-4-hydroxybenzyl phosphonate; oxamidobis alkyl (3,5)-dialkyl-4-hydroxyphenyl) propionate; and[3-(3,5-dialkyl- 4-hydroxyphenyl) -propionam ido] alkyl stearate with (B) a UV stabilizer package comprising:(i) at least one UV stabilizer of the hindered amine type, with or without(ii) at least one UV absorber.

US5624982A (Stabilizer system for non yellowing polymer composition) discloses another stabilizer system for the suppression of yellowing in styrenic polymers, consisting essentially of a benzotriazole such as 2-(2’-hydroxy-3’,5’-di- tert-amylphenyl)benzotriazole, a hydroxy- benzocyanurate such as t ris- (3,5-di- tert-butyl-4-hydroxybenzyl)-isocyanurate and a hindered amine light stabilizer selected from the group consisting of (i) bis (2,2,6,6,-tetramethyl-4- piperidinyl)sebacate; and (ii) b is- ( 1 ,2,2,6, 6-pentamethyl-4-piperdinyl)ester of (3,5- di-tert- butyl- 4- hydroxy benzyl)- but ylmalonate. Additionally a dialkylthiodipropionate and a zinc dialkyldithiocarbamate may be included in the stabilizer system.

An interesting aspect for latex formulations exhibiting yellowing upon exposure to ultraviolet light is reduced by formulations in which the outer layer of a polymeric particle comprises incorporated alkyl acrylate monomer, alkyl methacrylate monomer, or a combination of these monomers with little or no styrene. Thus, yellowing is reduced when styrene incorporated into the outer layer of a polymeric particle in a latex formulation is replaced with an alkyl methacrylate such as methyl methacrylate.

A core-shell particle comprises two or more polymerizable layers comprising a polymeric core, and a polymer composition of which each layer is different. Depending on the composition (ratio of core and the shell), particle size, thickness of the shell, constituent monomers, distribution of monomers in the core shell particle, method for manufacture - the mechanical, physical chemical properties of the core-shell particles change significantly.

Diverse processes are available for the production of polymer emulsions such as batch process, semi-continuous process, continuous process, additional emulsifier process and seed emulsion polymerization process etc. For the same monomer, if the production process is different, the resulting product quality, production efficiency and cost are different. For instance, when the styrene-acrylic emulsion is made of semi-continuous process, there is a tendency that the coating film is soft and easy to stick back and easy to stain as the ambient temperature rises during use; low scrub resistance etc. whereas the said properties are improved by emulsion polymerization.

Core/shell latexes composed of polystyrene (PST) and poly (Bu acrylate) (PBA) made by a two-stage emulsion or microemulsion polymerization process were reported by Perez-Carrillo, L. A. et al. (Polymer (2007), 48(5), 1212-1218). Low- seed content (LSC) latexes were made by batch polymerization in microemulsions stabilized with DTAB in the presence of an organic salt (di-Bu phosphite). High-seed content (HSC) latexes were produced by microemulsion or emulsion polymerization in semi-continuous process. These latexes were subsequently used to form core/shell particles of PST/PBA or PBA/PST and their mechanical properties were examined and compared. The results indicate that core/shell particle size and the location of the polymers have important effects on the mech. properties.

While studying effects of methacrylic acid amount on latex properties Zhang, et al (Xiangzheng Jinan Daxue Xuebao, Ziran Kexueban (Jinan, China) 2011, 25(2), 123-127) prepared seed latex with narrow particle size distribution via emulsion polymerization using styrene and Bu acrylate as monomers and varying the quantities of emulsifiers and initiators and was then used to achieve stable core/shell latex separately with batch and semi-continuous emulsion polymerization of monomers consisting of methyl methacrylate, Et acrylate and methacrylic acid (MAA). Monodisperse core-shell latex particles were obtained when the mass ratio of core/shell was kept at 5:2, and MAA mass content was 3- 5% in the shell formation monomers. The particle size distribution of the latexes prepared by semi-continuous technique were narrower than those prepared by batch process. With increase of the amount of MAA in polymerization, the stability of latex deteriorates, and particle size and size distribution increase with more strawberry latex particles.

According to Mendizabal, E. et al (Annual Technical Conference-Society of Plastics Engineers (2002), 60th (Vol. 3), 3864-3867) latexes with high solid content polymer latex of core-shell styrene-Bu acrylate or Bu acrylate-styrene can be prepared by microemulsion polymerization. The soft core/hard shell and hard core/soft shell structured polymers were obtained by a two-stage emulsion polymerization process using semi-continuous addition of the monomers. It was found that as the amount of rigid polymer increases the material become stiffer and present a lower elongation at break.

In another report Fernando et al. (Polimeros, 27(3), 225-229, 2017) disclosed synthesis of Core-shell particles on seeded suspension polymerization by using polystyrene (PS) as polymer core, or seed, and methyl methacrylate (MMA) as the shell forming monomer. Two synthesis routes were evaluated by varying the PS seed conversion before MMA addition. 1H NMR spectroscopy showed that the use of PS seeds with lower conversion led to the formation of higher amount of poly(styrene-co-MMA). The copolymer acted as a compatibilizer, decreasing the interfacial energy between both homopolymers. As a consequence, more PMMA clusters densely grouped at the surface were formed, as was revealed by TEM measurements. Samples in this system showed enhanced resistance to cyclohexane attack compared with pure PS, with a PS extraction of only 37% after 54 hours test. Core-shell multi-stage polymer particles were provided (KR20120067314A) to improve impact strength, yellowing and hydrolysis, and melting stability of an impact-modified thermo plastic resin. The core-shell particle comprised of two or more polymerizable layers and a polymeric core, polymer composition of each layer was different. The one or more polymerizable layers comprised a gradient polymer and the glass transition temperature of the polymeric core was 0°C or lower. A manufacturing method of the core-shell particle comprised: a step of synthesizing the core shell copolymer by emulsion polymerization; a step of controlling pH value of the core shell polymer; and a step of cohering the core shell polymers under the pH of 4-8 by the addition of electrolyte water aqueous solution.

The polymeric core layer comprised a butadiene and styrene and the polymeric shell layer was methyl methacrylate wherein core characterized in that it comprises a rate-shell copolymer impact modifier particle.

US4683269A relates to an opaque binder system containing a core-shell polymer and polymeric binder. The system is prepared and blended in-situ and demonstrates improvement in contrast ratio and scrub properties over corresponding compositions. Synthetic method for opaque polymeric binder system consists essentially of: (a) emulsion polymerizing about 5 to 95% by weight of at least one monomer not having an amine moiety, selected from the group consisting of vinyl, acrylic, methacrylic, and mixtures thereof, to produce homogeneous film forming polymeric particles having a T_g of less than about 45° C.;(b) continuing the polymerization of step (a) in the presence of previously formed from at least one monomer not having an amine moiety non-film forming hard, solid polymeric core particles (formed from styrene)with T_g greater than about 70° C., and gradually added to the polymerization of the homogenous film forming particles, and wherein the ratio of homogeneous film forming polymeric material to the polymeric material used to produce the core particles varies from about 1:9-9: 1, respectively; further said method of step (a) includes monomers selected from the group consisting of: methyl acrylate, butyl acrylate, ethylhexylacrylate, hydroxy ethyl methacrylate, methacrylic acid, acrylic acid, styrene, vinyl acetate and mixtures thereof.

The above prior art relates to preparation of polymeric opacifier by core shell polymer method and its application is different as the core particle was prepared separately, and during opaque binder preparation, core was co-feeded with different shell monomer composition and vinyl acetate was used in shell.

U SO 7646447 discloses core/shell polymer preparation capable of increasing a composition’s whiteness of opacity. The shell polymer has either a crystalline structure of a T_g of at least about 50° C. The shell polymer comprises (i) at least one nonionic monomer selected from the group consisting of soft alkyl acrylate monomers, soft methacrylate ester monomers, and mixtures thereof, and (ii) at least one additional nonionic monomer. The core has a different composition than the composition of the shell and a T_g of at least about 50° C. The core comprises alkenyl aromatic monomers. The core and shell polymers are homopolymers or copolymers and wherein both the core and the shell contained styrene monomer.

Although acrylate latex has many outstanding advantages, such as excellent light resistance, weather resistance, acid and alkali resistance and corrosion resistance but possess disadvantages such as poor water resistance, easy brittleness at a low temperature, and easy viscosity at a high temperature etc. One approach is to modify the acrylate latex by a silicone functional monomer having high temperature resistance, low surface energy and good water resistance. The silicone modified acrylate coating may improve the hardness, tensile strength, gas permeability and wear resistance, UV light resistance etc. of the coating film. Core shell latex obtained of styrene butadiene monomers with an organic silicon modification by emulsion polymerization process using deionized water as a dispersion medium showing an improvement in yellowing performance is reported by CN104311739B. The latex particle size of 120-150 nm, tablets narrow diameter distribution was monodisperse and exhibited good flowability, appropriate viscosity, good mechanical and chemical stability, alkali, water resistance, improved yellowing performance in compared to acetate emulsion used in the fiberglass mesh, textile coatings currently on the market.

CN103193924A relates to a preparation method of styrene-acrylate copolymer painting emulsion for elastic painting of inner and outer walls of a building. The preparation of styrene-acrylate copolymer painting emulsion comprises the preparation method of styrene-acrylate seed emulsion and the preparation method of painting emulsion and the styrene-acrylate copolymer painting emulsion mainly comprises styrene, acrylate, silane coupling agent, emulgator, initiator, defoamer, pH adjusting agent, molecular weight regulator and water.

This prior art does not relate to produce styrene core and acrylate shell latex particle and have demonstrated that combining the seed emulsion aggregation method and the phase-control dispersion film formation technology of styrene- acrylate latex particle, the compactness, mechanical property and the outdoor aging resistance problems of a paint film can be solved.

CN101092467A relates to core-shell polymer of styrene-acrylate copolymer emulsion, wherein: said monomer is a soft monomer, hard monomer, and having an epoxy group and a group silane functional monomer 45 to 55%; solids content of the emulsion at 48 to 52%, wherein: a glass core Tg humidity at 25 deg.] C ~ 35 °C, glass shell humidity Tg-50 5 °C. For said core-shell styrene- acrylate copolymer emulsion, soft monomer is selected from butyl acrylate, ethyl acrylate, and iso-octyl acrylate; the hard-single selection body styrene and methyl methacrylate and functional monomer is selected acrylic acid, glycidyl methacrylate monomers and silane. In this prior art epoxy and silane containing monomers were added in the core and both core and shell contained styrene monomers directed to high wet adhesiveness and high washing resistance of the coating film.

This prior art uses pre-emulsified seed polymerization and semi-continuous, and to increase the conversion of the reaction by a redox system, and by controlling the polymerization temperature, suppressing the hydrolysis of the silane, condensation crosslinking, reducing the residual monomer em ulsion was prepared phase translucent blue appearance, odorless, soft shell structure to form a hard core, suitable for different am bient tem peratures, the present advancem ent does not use film-form ing aid in the form ulation, reducing the amount of high boiling point solvent, thereby reducing the volatile organic content of the product (VOC) .

CN 1 251 843 A relates to the organosilicon modified acrylate latex with nucleocapsid structure and its preparing process. The vinylalkoxyl silane monom er is used as modifier of acrylate latex, which is copolymerized with alkyl methylacrylate, alkyl acrylate, hydroxyalkyl acrylate and vinyl arom atic com pound monom ers in seed em ulsion polym erization mode. The product features high stability ( more than one year of storage period) and can be used for external wail paint, water-proof paint and glass-decorat ive paint to obviously im prove t he hardness, tensile strength, water resistance, adhesion and washing resistance of coated film . I n this prior art, silicon modified styrene acrylate particle was prepared by single stage seeded em ulsion polym erization process using redox initiator wherein the shell also contained styrene.

Application of styrene-acrylic em ulsion using the modified silane coupling agent, although im proves the performance of part of the styrene-acrylic em ulsion, but still has the problem of weather resistance, stain resistance and coating undesirable effect of m em brane defects, that lead to the following docum ented under CN12491 1 1 C below.

CN 1 2491 1 1 C discloses a method for producing a modified acrylic em ulsion and wherein styrene, butyl acrylate, acrylic acid are used as polym erizable monom er, an organic fluorine and silicone-modified monomer, an organic fluoro after pretreatm ent with a silane coupling agent, dodecylbenzene sulfonic acid as catalyst, using a modified core-shell acrylic em ulsion polym erization process, wherein the organic silicone- modified monom er is a m ixture of octam ethylcyclotetrasiloxane and vinyl cyclotetrasiloxane, fluorine- modified organic monom er is trifluoroacetic acid, propylene carbonate, trifluoroacetyl acrylam ide, acetyl acrylam ide isoflurane.

Hence while several core shell latexes are known in literature, it is the need in the art to provide for styrene rich core shell latex particles which in being styrene rich would overcom e the yellowness properties of paint when incorporated in paint form ulations, would have better exterior durability and would also offer accelerated weather stability, and further on the other hand would also offer the possibilities of MMA reduction and cost advantage in paint form ulations when com pared to the products com mercially available.

Obj ects of t he i nve nt ion

It is the primary object of the present invention to provide for a core-shell latex/binder com prising styrene acrylic em ulsion form ulation including core shell latex particles com prising styrene core and acrylate shell based polymeric particles said particles in having high amount of styrene as core and acrylate shell would still favour exterior application that would yield non-yellowing core shell latex properties when exposed to UV exposure even when poor UV stability of styrene is known.

It is another object of the present invention to provide for high amount of styrene use in exterior em ulsion/paint form ulation without com prom ising the yellowness and durability properties of the paint thereby demonstrating high UV resistance properties.

It is yet another object of the present invention to provide for said binder and a process of manufacture thereof that would also offer MMA reduction and consequent cost reduction advantage in paint form ulations when com pared to the products available together with accelerated weather stability, im proved exterior durability. Summary of the invention

Thus, according to the basic aspect of the present invention there is provided a core-shell latex/binder comprising styrene acrylic emulsion formulation including core shell latex particles involving styrene rich core and acrylate shell based polymeric particles wherein said core has at least 40 wt.% styrene with respect to total monomer.

While there are several references available on core-shell particles having styrene core and acrylate shell, however it is the surprising finding of the present invention that even having a styrene rich core-shell particle an emulsion/ paint thereof, the same can overcome yellowness properties of paint wherein such styrene rich acrylic systems can be used in exterior as well as interior coating applications free of any non-yellowing upon outdoor exposure together providing for a cost advantage when compared with existing products.

Preferably in said core-shell latex/binder wherein said core shell latex particles with styrene hard core and acrylate soft shell comprises ethylene glycol dimethacrylate (EGDMA) reacted with styrene for immobilizing the core wherein amount of said EGDMA is at least 0.15 wt% with respect to total composition.

More preferably said core-shell latex/binder comprising said styrene acrylic emulsion formulation as styrene acrylate latex includes said core shell particles with styrene core and acrylate shell has at least 50 wt%-70 wt% solid content to meet the high specific productivity.

According to a preferred aspect of the preset invention there is provided said core-shell latex/binder wherein said core shell latex particles has polarity difference between the core and shell comprising more polar/hydrophilic polymers in the shell than core said shell being free of styrene.

More preferably in said core-shell latex/binder said shell of said core shell latex particles is crosslinked by crosslinkers including silane-based monomer cross- linker, diacrylate based cross-linker preferably 1,4 Butanediol Dimethacrylate (BDDMA), Tricyclodecane Dimethanol Diacrylate (TDD), wherein said silane- based monomer preferably vinyl trimethoxysilane (VTMO) is at least 0.15 wt% with respect to total composition.

According to another preferred aspect of the present invention there is provided a core-shell latex/binder suitable for paint formulation favouring reduction of at least 40 wt % Methyl methacrylate MMA monomer with respect to total monomer and obtaining cost advantage in paint formulations.

Preferably in said core-shell latex/binder said shell of said core shell latex particles involves polar monomers with polarity difference as compared to the core and having low Tg: -25 - + 15 °C favours good film formation capability coupled with alkali & water resistance, mechanical, freeze-thaw and accelerated stability required for coating application, and wherein said styrene rich core with high Tg: 10 - 100 °C controls the physical and mechanical properties of latex particle.

According to another aspect of the present invention there is provided a process for the preparation of said core-shell latex/binder comprising the steps of a two- stage process

Providing core shell latex particles with styrene immobilized core obtained of initial and complete polymerization of all styrene-based monomers, with said core having at least 40 wt.% styrene with respect to total monomer content, followed by delayed addition of acrylate monomers and obtaining therefrom said core shell latex particles involving styrene rich core and acrylate shell based polymeric particles.

Preferably in said process as said two stage process involving thermal initiators wherein in the first stage (a) all styrene-based monomers are polymerized including reacting low amounts 0.05 - 1 wt% of Ethylene glycol dimethacrylate (EGDMA) with respect to total composition with styrene to immobilize the core,

in the second stage (b) delayed addition of the acrylate monomers takes place involving lower monomer addition time of up to 4 hours generating core-shell morphology of said styrene acrylate latex particles involving styrene rich core and acrylate shell.

More preferably in said process said shell is free of styrene and is crosslinked by low amounts 0.05 to 1 wt.% (with respect to total composition) of silane based cross linker preferably VTMO (Vinyl trimethoxysilane).

According to another preferred aspect of the present invention there is provided a process wherein in said two stage process involving thermal initiators comprises providing a mixture of water, Dowfax 2A1 , Atpol 5731/70N and sodium bicarbonate (SBC) in a glass kettle and heating the mixture gradually heated to about 80° C; providing two sets of solutions as solution (1): an initiator solution of potassium persulfate (PPS), and water, as solution (2): a pre-emulsion 1 (PE 1) comprising monomer, surfactant, and water; as the first stage, adding 5-7.5% of the above pre-emulsion (PE 1 as solution 2) to the kettle above at 80°C, followed by addition of above initiator solution 1, and allowing the mixture a time of 15 minutes to generate latex seed particles followed by which remaining (PE 1 as solution 2) was fed into the reaction kettle over a period of about 105 minutes; adding EGDMA monomer after about 60 minutes of completion of PE1 feeding (PE 1 as solution 2), and thereafter about 10 mins PPS and SBC solution were added; as the second stage, providing a second feed pre-emulsion (PE2) comprising monomer, surfactant and water was fed into the reaction kettle over a period of about 120 minutes; preferably, adding VTMO monomer after about 60 minutes post completion of second feed of pre emulsion (PE2) followed by preferably adding into the kettle a solution of tert-Butyl hydroperoxide (TBHP) and sodium formaldehyde sulfoxylate (SFS) separately and holding the reaction mixture for about 45 minutes followed by cooling to room temperature and adding ammonia solution and filtering to obtain therefrom said styrene acrylate latex particles with core-shell morphology involving styrene rich core and acrylate shell.

More preferably said PE1 includes Styrene, Butyl acrylate, Methacrylic acid and Ethylene glycol dimethacrylate; and wherein said PE2 includes Methyl methacrylate (MMA), Butyl acrylate (BA), Methacrylic acid (MAA), Cyclic trimethylpropane formal acrylate (CTFA), Tricyclodecane DimethanolDiacrylate (TDD), 4-hydroxy butyl acrylate (HBA), Vinyl trimethoxysilane (VTMO).

According to another preferred aspect of the process said process is free of any separate step of preparation of core particle and favours Methyl methacrylate (MMA) reduction in the levels of 40 to 100 wt% with respect to total monomer providing for cost advantage in paint formulations.

Preferably in said process wherein said shell monomers include single or plurality of monomers selected from Butyl acrylate (BA), Methacrylic acid (MAA), Cyclic trimethylpropane formal acrylate (CTFA), Tricyclodecane Dimethanol Diacrylate (TDD), 4-hydroxy butyl acrylate (HBA), Vinyl trimethoxysilane (VTMO) including Methyl methacrylate (MMA).

Detailed description of the invention

As discussed hereinbefore, the present invention solves the technical problem of yellowing of styrene containing pigments in presence of light and provides for improved core-shell latex/binder comprising styrene acrylic emulsion formulation including core shell latex particles involving styrene rich core and acrylate shell based polymer and process thereof, by preferably including suitable silicone grafting monomer on the shell comprising acrylates and having therein a polystyrene core. This preparation of high solid containing high performance styrene-acrylic emulsion improves weatherability, stain resistance, light resistance, and adapts to different environments of use.

According to an embodiment of the present invention styrene rich core and acrylate shell based polymeric particles are provided wherein said core has at least 40 wt.% styrene with respect to total monomer.

According to a preferred embodiment of the present invention a small amount of dimethacrylate (EGDMA) is reacted with styrene to immobilize the core and the shell is preferably crosslinked by a silane-based monomer as cross linker preferably VTMO cross linker. The core-shell latex particles consist of a hard core and a soft shell. The core-shell, non-core-shell styrene-acrylate and pure acrylate-based paints were exposed to UVB (313nm) for accelerated weather testing. Non(core-shell) styrene acrylate-based paints degraded after 500 hours exposure while core- shell latex exhibited same level of yellowness when compared with pure acrylate system (1000 hours of UV exposure).

Thermal initiator was used in the two-stage process of the present invention wherein the amount of core particle is high compared to the prior art systems and the shell of the particle was free of styrene and the silicon containing monomer in the process of the present invention is low.

The Core-shell latex/binder of the present invention is based on the polarity difference between the core and shell latex particle and contains more polar polymer in the shell. For comparative purposes styrene acrylate latex particle and an emulsion thereof was also prepared by single stage seeded emulsion polymerization process which shows poor UV stability.

The solution proposed by the present invention is different from other available core-shell particles of the prior art systems having styrene core and acrylate shell in the following respects:

1. Multiple monomers are employed in the process of the present invention;

2. 50wt% solid content to meet the high specific productivity;

3. 40 wt% styrene content (with respect to total monomer);

4. No separate preparation of core particle;

5. Lower monomer addition time (4 hours compared to 7-8 hours);

6. Cross-linking the shell forming polymer by silicon-based monomer preferably (VTMO) to obtain VTMO based shell to have better exterior durability;

7. Offers accelerated weather stability;

8. Moreover, developed emulsion of the present invention by a two-stage process offer the possibilities of MMA reduction and cost advantage in paint formulations when compared to the available and existing products due to styrene price advantage.

The present invention also provides for: a. A method of synthesizing styrene core acrylate shell i.e. the core - shell latex particles consisting of a hard core and a soft-shell polymeric particles wherein multiple monomers are employed in this process, which is a two-stage process; b. In said two stage process first all styrene monomers are polymerized, followed by delayed addition of the acrylate monomers, generating core-shell morphology; c. Preferably a small amount of dimethacrylate (EGDMA) is reacted with styrene to immobilize the core;

d. The shell has been crosslinked by using VTMO cross linker which is a silane based cross linker. The present invention thus relates to a method of synthesizing styrene core acrylate shell polymeric particles by a two-stage seeded emulsion polymerization process. First, all styrene monomers are polymerized, followed by delayed addition of the acrylate monomers, generating core-shell morphology. A small amount of ethylene glycol dimethacrylate (EGDMA) is reacted with styrene to immobilize the core. The shell has been preferably crosslinked by using VTMO cross linker. The core - shell latex particles consist of a hard core and a soft shell. Other cross- linkers suitable for the present invention other than VTMO includes Diacrylate based cross-linkers such as 1,4 Butanediol Dimethacrylate (BDDMA), and Tricyclodecane DimethanolDiacrylate (TDD).

Example I: Process of preparing core shell latex particles with styrene (hard) core and acrylate (soft) shell

Example 1 Core shell styrene acrylate system: A 1 liter glass kettle equipped with a mechanical stirrer, reflux condenser, thermometer and inlet tube for pre emulsion feeding is placed in a water batch. The demineralized water, Dowfax 2A1 , Atpol 5731/70N and sodium bicarbonate (SBC) were added to the glass kettle. The mixture was then gradually heated to 80° C under stirring at 200 rpm. Separately two solutions were prepared in the flask: (1) an initiator Solution of potassium persulfate (PPS), and water, (2) a pre-emulsion 1 (PE 1) of monomer, surfactant, and water. 7.5% of the above pre-emulsion was added to the kettle at 80°C, followed by addition of above initiator solution. The mixture was allowed for 15 minutes to produce latex seed particles. After 15 minutes, remaining PE1 was fed into the reaction kettle over a period of 105 minutes. After 60 minutes of PE1 feeding, EGDMA monomer (ethylene glycol dimethacrylate) was added to the PE 1 flask. Ten minutes after completing PE 1 feeding, PPS and SBC solution were charged to the reactor. Ten minutes after charging the PPS and SBC solution, a second feed pre-emulsion (PE2) composed of monomer, surfactant and water was fed into the reaction kettle over a period of 120 minutes. After 60 minutes of PE2 feeding, VTMO monomer was added to the PE 2 flask. After completing the last feed PE2, a solution of TBHP and SFS then were added separately into the kettle. The reaction was held for 45 minutes. The reaction was cooled down to room temperature and ammonia solution was added to the reaction kettle. The resulting emulsion was filtered through a 80 nylon-mesh.

Example 2 Non-core shell styrene acrylate system:

A 1 liter glass kettle equipped with a mechanical stirrer, reflux condenser, thermometer and inlet tube for pre-emulsion feeding is placed in a water batch. The demineralized water, Dowfax 2A1 , Atpol 5731/70N and sodium bicarbonate (SBC) were added to the glass kettle. The mixture was then gradually heated to 80° C under stirring at 200 rpm. Separately two solutions were prepared in the flask: (1) an initiator Solution of potassium persulfate (PPS), and water, (2) a pre- emulsion of monomer, surfactant, and water.

5% of the above pre-emulsion was added to the kettle at 80°C, followed by addition of above initiator solution. The mixture was allowed for 15 minutes to produce latex seed particles. After 15 minutes, remaining PE was fed into the reaction kettle over a period of 240 minutes. After 60 minutes of PE feeding, VTMO monomer was added to the PE flask. After completing the feed PE, a solution of TBHP and SFS then were added separately into the kettle. The reaction was held for 45 minutes. The reaction was cooled down to room temperature and ammonia solution was added to the reaction kettle. The resulting emulsion was filtered through a 80 nylon-mesh.

Example 3 Pure acrylate system:

Same procedure as example 2

Abbreviation: Methyl methacrylate (MMA), Butyl acrylate (BA), Methacrylic acid (MAA), Cyclic trimethylpropane formal acrylate (CTFA), TricyclodecaneDimethanolDiacrylate (TDD), 4 -hydroxy butyl acrylate (HBA), Vinyl trim ethoxysilane( VTMO) Ethylene glycol dimethacrylate (EGDMA), Sodium bicarbonate (SBC), potassium persulfate (PPS), and water (DMW).

It was found by way of the present invention that Ethylene glycol dimethacrylate (EGDMA) is an essential ingredient in said two stage process wherein arresting of styrene in the core is completed, and otherwise it is not. EGDMA is selectively employed in the core particle preparation stage after 60 minutes of PE 1 feeding. It was also found that preferably when VTMO is employed UV stability of the particles could be enhanced by 30-50%. For the said purpose VTMO is selectively added in the shell particle preparation stage after 60 minutes of PE 2 feeding.

Example II : Propertieswith regard to the follow ing :

Shell having low Tg favors good film formation capability coupled with alkali & water resistance, mechanical, freeze-thaw and accelerated stability required for coating application, and wherein said styrene rich core with high Tg controls the physical and mechanical properties of latex particle as displayed below.

Abbreviation: Non-volatile material (NVM); Minimum film forming temperature (MFFT). Exam pie 111 : Com paratives

The core-shell, non-core-shell styrene-acrylate and pure acrylate-based paints were exposed to UVB (313nm) for accelerated weather testing.

It was found that non-core-shell styrene-based paints degraded after 500 hours of UV exposure while core- shell latex based as well as pure acrylic based systems do not degrade. Thus, both samples (core-shell as well as pure acrylic) were tested until 1000 hours of UV exposure. It was found that core-shell based system showed same level of yellowness when compared with pure acrylic based system even though it contains high amounts (up to 40 wt % with respect to monomer) of styrene.

The additional advantage by crosslinking with VTMO is that enhanced scrub resistance properties could be attained.

Also, styrene acrylate latex particle and an emulsion thereof was prepared by single stage seeded emulsion polymerization process for comparative purposes which shows poor UV stability, stability reduced by 30-50%.

This result indicates that the core-shell styrene-acrylate latex paint of the present invention showed very high UV resistance properties when compared with a formulation having non-core-shell styrene acrylate latex and also with a formulation having the particles prepared in a single stage.

Other significant features of the present advancement are the following: i. High solid content of styrene acrylate latex; ii. 50wt% solid content to meet the high specific productivity; iii.40 wt% styrene content (with respect to total monom er) ; iv. No separate preparation of core particle; v. Lower monom er addition tim e (4 hours compared to 7-8 hours) ; vi. Non-yellowing coating film appearance upon outdoor exposure; vii. Reduction of MMA used and cost advantage in paint form ulations when com pared to the available products with respect to styrene price advantage; viii. Cross-linked (VTMO) shell to have better exterior durability; ix. Offers accelerated weather stability.

It is thus possible for the present advancem ent to provide for core-shell latexes with high styrene content in the core which does not im part yellowness properties in paint upon extended outside ( ultraviolet) exposure, said core having polarity difference based on the monom er content with that of the shell and said core having distinct and high Tg is also different as com pared to the low Tg of the shell enabling film formation, which high styrene content core to provide such non-yellowing was hitherto before unknown though several core-shell latexes are well known in literature.

Such dual attribute arising from the same core-shell latex particles wherein the shell on one hand enabling low Tg favouring good film formation capability coupled with alkali & water resistance, m echanical, freeze-thaw and accelerated stability required for coating application, and on the other hand styrene rich core with high Tg controls the physical and m echanical properties of latex particle is indeed a significant finding. Preferably, crosslinking the shell with VTMO offers the additional benefits to the said particle with regard to the non yellowing/exterior/ durability properties even though the styrene content of said particles is very high.

Claims

We Claim :

1. Core-shell latex/binder comprising styrene acrylic emulsion formulation including core shell latex particles involving styrene rich core and acrylate shell based polymeric particles wherein said core has at least 40 wt.% styrene with respect to total monomer.

2. Core-shell latex/binder as claimed in claim 1 wherein said core shell latex particles with styrene hard core and acrylate soft shell comprises ethylene glycol dimethacrylate (EGDMA) reacted with styrene for immobilizing the core wherein amount of said EGDMA is at least 0.15 wt% with respect to total composition.

3. Core-shell latex/binder as claimed in anyone of claims 1 or 2 wherein said styrene acrylic emulsion formulation as styrene acrylate latex including said core shell particles with styrene core and acrylate shell has at least 50wt%-70 wt% solid content to meet the high specific productivity.

4. Core-shell latex/binder as claimed in anyone of claims 1-3 wherein said core shell latex particles has polarity difference between the core and shell comprising more polar/hydrophilic polymers in the shell than core said shell being free of styrene.

5. Core-shell latex/binder as claimed in anyone of claims 1-4 wherein said shell of said core-shell latex particles is crosslinked by crosslinkers including silane based monomer cross-linker, diacrylate based cross-linker preferably 1,4 Butanediol Dimethacrylate (BDDMA), Tricyclodecane Dimethanol Diacrylate (TDD), wherein said silane based monomer preferably vinyl trimethoxysilane ( VTMO) is at least 0.15 wt% with respect to total composition.

6. Core-shell latex/binder as claimed in anyone of claims 1-5 suitable for paint formulation favouring reduction of at least 40 wt % Methyl methacrylate MMA monomer with respect to total monomer and obtaining cost advantage in paint form ulations.

7. Core-shell latex/binder as claimed in anyone of claims 1-6 wherein said shell of said core shell latex particles involving polar monomers with polarity difference as compared to the core and having low Tg: -25 - + 15 °C favours good film formation capability coupled with alkali & water resistance, mechanical, freeze- thaw and accelerated stability required for coating application, and wherein said styrene rich core with high Tg: 10 - 100 °C controls the physical and mechanical properties of latex particle.

8. A process for the preparation of core-shell latex/binder as claimed in anyone of claims 1-7 comprising the steps of a two-stage process

9. A process as claimed in claim 8 as said two stage process involving thermal initiators wherein in the first stage (a) all styrene-based monomers are polymerized including reacting low amounts 0.05-1 wt% of Ethylene glycol dimethacrylate (EGDMA) with respect to total composition with styrene to immobilize the core,

in the second stage (b) delayed addition of the acrylate monomers takes place involving lower monomer addition time of upto 4 hrs generating core-shell morphology of said styrene acrylate latex particles involving styrene rich core and acrylate shell.

10. A process as claimed in anyone of claim 8 or 9 wherein said shell is free of styrene and is crosslinked by low amounts 0.05 t o 1 wt.% (with respect to total composition) of silane based cross linker preferably VTMO (Vinyl trimethoxysilane) .

11. A process as claimed in claim 8-10 wherein in said two stage process involving thermal initiators comprises providing a mixture of water, Dowfax 2A1 , Atpol 5731/70N and sodium bicarbonate (SBC) in a glass kettle and heating the mixture gradually heated to about 80° C; providing two sets of solutions as solution (1): an initiator solution of potassium persulfate (PPS), and water, as solution (2): a pre-emulsion 1 (PE 1) comprising monomer, surfactant, and water; as the first stage, adding 5-7.5% of the above pre-emulsion (PE 1 as solution 2) to the kettle above at 80°C, followed by addition of above initiator solution 1, and allowing the mixture a time of 15 minutes to generate latex seed particles followed by which remaining (PE 1 as solution 2) was fed into the reaction kettle over a period of about 105 minutes; adding EGDMA monomer after about 60 minutes of completion of PE1 feeding (PE 1 as solution 2), and thereafter about 10 mins PPS and SBC solution were added; as the second stage, providing a second feed pre-emulsion (PE2) comprising monomer, surfactant and water was fed into the reaction kettle over a period of about 120 minutes; preferably, adding VTMO monomer after about 60 minutes post completion of second feed of pre emulsion (PE2) followed by preferably adding into the kettle a solution of tert-Butyl hydroperoxide (TBHP) and sodium formaldehyde sulfoxylate (SFS) separately and holding the reaction mixture for about 45 minutes followed by cooling to room temperature and adding ammonia solution and filtering to obtain therefrom said styrene acrylate latex particles with core-shell morphology involving styrene rich core and acrylate shell.

12. A process as claimed in claim 8-11 wherein said PE1 includes Styrene, Butyl acrylate, Methacrylic acid and Ethylene glycol dimethacrylate; and wherein said PE2 includes Methyl methacrylate (MMA), Butyl acrylate (BA), Methacrylic acid (MAA), Cyclic trimethylpropane formal acrylate (CTFA), Tricyclodecane DimethanolDiacrylate (TDD), 4-hydroxy butyl acrylate (HBA), Vinyl trimethoxysilane (VTMO).

13. A process as claimed in anyone of claim 8-12 wherein said process is free of any separate step of preparation of core particle and favours Methyl methacrylate (MMA) reduction in the levels of 40 to 100 wt% with respect to total monomer providing for cost advantage in paint formulations.

14. A process as claimed in anyone of claim 8-13 wherein said shell monomers include single or plurality of monomers selected from Butyl acrylate (BA), Methacrylic acid (MAA), Cyclic trimethylpropane formal acrylate (CTFA), Tricyclodecane Dimethanol Diacrylate (TDD), 4-hydroxy butyl acrylate (HBA), Vinyl trim ethoxysilane (VTMO) including Methyl methacrylate (MMA).