The invention relates to a method for the manufacture of styrene maleic anhydride copolymers by emulsion polymerization for surface sizing and hollow particle pigments.

Styrene maleic anhydride resins are low molecular weight styrene maleic anhydride copolymers and derivatives in partial ester form. Styrene maleic anhydride copolymers are the most common polymeric materials currently used for surface sizing applications because of their chemical diversity, and they are key additives used in many different fields, especially in water-based formulations, such as dispersions, coatings, polishes and finishes. Styrene maleic anhydride resins can be used to improve properties of formulations, such as gloss aspect, hardness, colour development, adhesion, stability and water resistance. Styrene maleic anhydride resins have usually a styrene to maleic anhydride ratio ranging from 1: 1 to 4: 1. In surface sizing the styrenic units in the polymers probably impart hydrophobicity to the paper surface while maleic units help anchor the polymers onto the paper substrates. One can easily control both the hydrophobic and the hydrophilic nature of the material by varying the styrene to maleic anhydride ratio. Styrene maleic anhydride copolymers are manufactured by polymerization of styrene with maleic anhydride, optionally followed by esterification in the presence of an alcohol. The obtained copolymer is subjected to hydrolysis in the presence of an aqueous base and the hydrolysis is usually per- formed on-site before use. Styrene maleic anhydride resins are commonly available in powder or flake form which are dissolved in water by reaction with an alkaline agent, such as ammonium hydroxide, sodium or potassium hydroxide or with an amine such as monoethanol amine, at a temperature around 90°C. The preparation of the water solution requires elaborate mixing in a mixing tank with

temperature controlling equipment. Great care is needed in order to manufacture aqueous solutions with desired quality and properties. Generally the solid content in the solution is 10-20 wt%. The aqueous styrene maleic anhydride copolymer solution is mixed with large amounts of aqueous starch dispersions before application to the paper. Based on the above it can be noted that there exists a need for an improved method to manufacture of styrene maleic anhydride copolymers, which are ready to use as such without dissolving the copolymer in water.

An object of the invention is a method for the manufacture of styrene maleic anhydride copolymers by emulsion polymerization which yields an emulsion with high solid content and which can be readily used as such in applications like surface sizing.

A further object of the invention is a styrene maleic anhydride copolymer emulsion.

A further object is a method for the manufacture of hollow polymer particles.

Characteristics of the method for the manufacture of styrene maleic anhydride copolymers by emulsion polymerization, of the styrene maleic anhydride copolymer emulsion and of the method for the manufacture of hollow polymer particles are stated in the claims.

The above described objectives are achieved and the disadvantages of the methods and styrene maleic anhydride copolymers of the state of the art are avoided or significantly decreased by the method and the styrene maleic anhydride copolymer according to the invention. It has been noted that styrene maleic anhydride copolymers with properties especially suitable for surface sizing can be manufactured by an emulsion polymerization method. The emulsion polymerization may be carried out in one step or in two steps. In the two step method first a seed copolymer is produced from styrene and maleic anhydride with an emulsion polymerization method using a suitable initiator. The solid content of

the obtained seed copolymer is 1-40 wt% and particle size 60-100 nm. Then preemulsion comprising styrene, maleic anhydride, water, an emulgator and an initiator are added into the reactor containing the seed copolymer, water, an emulgator and an initiator. The reaction temperature is kept at a temperature of 20-100 °C, preferably 50-55 °C. A suitable emulgator is any suitable emulgator known in the art for emulsion polymerization or a mixture of emulgators and preferably sodium dodecylbenzene sulphonate, sodium C12-Cls-alkyl ether sulphate, sodium C12-Cls-alkyl sulphate, sodium C12-Cis-alkyl phenol ether sulphate or sodium dodecyl sulphate. A suitable initiator is any suitable initiator known in the art for emulsion polymerization and preferably ammonium persulphate, potassium persulphate or sodium persulphate and sodium metabisulphite are used as a redox initiator.

In the one step method no seed latex is manufactured. The process is started by charging to a reactor earlier described styrene maleic anhydride preemulsion. The polymerization is performed at a temperature of 20-100 °C, preferably 50-55 °C, in order to prevent the isomerization of maleic acid to fumaric acid. As an inhibitor any suitable initiator for emulsion polymerization and preferably a redox type initiator as described above is used.

The method according to the invention for the manufacture of styrene maleic anhydride copolymers comprises following steps: a) manufacturing a preemulsion from styrene, maleic anhydride, water, an emulgator or a mixture of emulgators, an initiator and optionally from a seed copolymer, b) polymerizing the preemulsion from a) in the presence of additional water, an emulgator and an initiator at a temperature of 20-100 °C, preferably 50-55 °C.

A milky latex is obtained as a product and the solid content is typically 5-50 wt %.

The amount of maleic acid in the copolymer is 1-30 mol-% and the amount of styrene is 70-99 mol-%. The particle size distribution of the obtained latex may be adjusted between 50-500 nm by varying the amount of monomers or the amount of the emulgator. The glass transition temperature of the obtained latex is typically 110 °C, and it varies between 90-115 °C.

Compared to the traditional methods for the manufacture of styrene maleic anhydride copolymers, such as high pressure liquid polymerization, it can be seen that the method according to the invention exhibits several advantages. The method is very simple because the emulsion polymerization is performed in water resulting in a product which is ready for use as such. The control of the particle size and particle size distribution is easier and very accurate particle size distribution can be obtained. Also the solid contents up to 50 wt % can be obtained, which is much higher when compared to the solid content of the dispersions according to prior art. The maleic anhydride content may be adjusted between 1-30 wt % depending on the desired polymer.

The obtained styrene maleic anhydride copolymers can be used in several applica- tions, such as surface treatment agents, modifiers for liquid coating, polymeric surfactants, dispersing and stabilizing agents and chemical intermediates. Especial- ly suitable they are as surface sizing agents for paper manufacture, in which case it is a clear advantage that the copolymer is ready-for-use in a water phase and ammonia treatment can be performed easily.

The styrene maleic anhydride copolymer according to the invention may also be swelled by a basic agent or by an inert hydrocarbon such as pentane in order to prepare hollow particle pigments for several applications such as coatings.

The method according to the invention for the manufacture of hollow polymer particles comprises the following steps:

a) manufacturing a preemulsion from styrene, maleic anhydride, water, emulgator or a mixture of emulgators, an initiator and optionally from seed copolymer, b) polymerizing the preemulsion from a) in the presence of additional water, an emulgator and an initiator at a temperature of 20-100 °C, preferably 50-55 °C, c) swelling by a base, preferably ammonia, the particles manufactured in b), d) manufacturing an outer shell on the swollen particles from c).

Thus hollow particles are made from cheap starting materials such as styrene and maleic anhydride. The carboxylated core is manufactured by emulsion polymerization as described earlier from styrene and maleic acid, the obtained polymer particles are swelled by a base such as ammonia and an outer shell is manufactured on the swollen particles according to any suitable method at the state of the art, as is described in FI-970936 suitably using styrene, methyl methacrylate, divinyl benzene and/or acrylic acid. Hollow polymer particle latex thus obtained exhibits a narrow particle size distribution.

The invention is illustrated in more detail in the following examples which however are not meant to limit the scope of the invention.

Example 1 Manufacture of styrene maleic anhydride copolymer (SMA) by emulsion polymerization Manufacture of preemulsion: 14.40 g of maleic anhydride (2 % of monomers), 1.70 g of ammonium

persulphate and 3.09 g of sodium dodecylbenzene sulphonate are dissolved in 504.0 g of water, 705.63 g of styrene (98 % of monomers) is added to the solution and the mixture is agitated to obtain an emulsion.

Polymerization 61.45 g of the earlier prepared preemulsion, 1.410 g of ammonium persulphate, 0.431 g of sodium dodecylbenzene sulphonate and 408.96 g of water are charged to a reactor. The reaction is performed under a nitrogen atmosphere and the heating of the reaction mixture to 50 °C is started. 962.5 g of the earlier prepared preemulsion is evenly charged below the liquid surface and 60.2 g of sodium metabisulphate solution (3.17 g in 101.22 g water) is added to solution. The charging is performed during 8 hours, then the temperature is kept at 50 °C for an additional hour. The obtained product is a milky latex with solid content of 38 % and pH of 1.7. The particle size distribution is narrow and the average particle size is approx. 170 nm (by transmission electron microscope). The glass transition temperature of the polymer is 102 °C. Surface sizing tests in paper manufacturing were performed with the product without pH adjustment and with pH adjustment.

Results obtained revealed that the products are highly suitable for surface sizing.

Example 2 Manufacture of SMA Manufacture of preemulsion 90.05 g of maleic anhydride (25 % of monomers), 1.54 g of ammonium persulphate and 1.54 g of sodium dodecylbenzene sulphonate are dissolved in 252.1 g of water, to the solution 270.12 g of styrene (75 % of monomers) is added and the mixture is agitated to obtain an emulsion.

0.436 g of sodium dodecylbenzene sulphonate dissolved in 950.7 g of water is charged to a reactor. The reaction is performed under nitrogen atmosphere and heating to 50 °C is started. When the temperature is settled 12.72 g of sodium metabisulphite solution (2.10 g in 59.8 g water) and 6.86 g of ammonium persulphate solution (0.707 g in 14.17 g water) are added. 532.5 g of the pre- emulsion and 46.3 g of sodium metabisulphite solution are charged below the liquid surface during 4 hours. Then 6.25 g of ammonium persulphate solution is added in order to enhance the post-polymerization after which the temperature is kept at 50 °C for an additional two hours'time. The obtained product is milky latex with solid content of 21 % and pH of 1.2. The particle size distribution is narrow and the average particle size is approx. 80 nm (by transmission electron microscope). The glass transition temperature of the polymer is 109 °C. Surface sizing tests in paper manufacturing were performed with the polymer with good results.

Example 3 Manufacture of hollow styrene maleic anhydride particles via seed polymerisation 8.81 g of SMA copolymer prepared in Example 1 is used as seed latex.

A preemulsion of 400 g water, 153 g styrene, 17.8 g of maleic anhydride, 0.5 g ammonium persulphate and 0.7 g sodium dodecyl benzene sulphonate is prepared as described in Example 1. Into the reactor are added 352 g of water, the seed latex, 0.5 g of ammonium persulphate and 0.7 g of sodium dodecyl benzene sulphonate.

The reaction is performed under nitrogen atmosphere and at a temperature of 50 °C. The preemulsion and 0.66 g sodium methabisulphate in 20 g water are charged during 6 hours, and then additionally kept at 50 °C for 1 hour. This gives

a milky latex with a solid content of 13 % and a particle size of 320 nm. Results obtained reveal that the product is highly suitable for surface sizing.

To the white latex 190 g of 4 % ammonia is added at 80 °C during 45 minutes. The latex is further mixed for 50 minutes, which gives a latex with 11 % solid content and a particle size of 360 nm, and a hollow void size of 200 nm.