Polymer powder composition and its method of preparation

1. A polymer composition in form of polymeric particles comprising

at least one stage forming a layer (B) comprising a polymer (B1) having a glass transition temperature between 80° C. and 150° C., and

at least one stage forming a layer (A) comprising a polymer (A1) having a glass transition temperature between −40° C. and 30° C.,

obtained by a multistage process wherein the weight ratio r of the layer (A) in the composition comprising the two layers is at least 1 wt %, and at most 30 wt %, wherein the layer (A) comprising a polymer (A1) having a glass transition temperature between −40° C. and 30° C. is a stage forming the outer layer of the polymeric particles and is present only in external layer (A),

wherein the layer (B) comprising polymer (B1) having a glass transition temperature of between 80° C. and 150° C. is a stage forming an inner layer of the polymeric particles,

wherein the mass average molecular weight Mw of polymer (A1) is less than 1,000,000 g/mol and the mass average molecular weight of polymer (B1) is at least 300,000 g/mol,

and wherein the weight ratio r in wt % of the layer (A) comprising polymer (A1) in a complete polymer particle is at least as defined by following formula

r

=

be

aT

+

1

+

1

363

-

T

wherein the variable T is the glass transition temperature Tg expressed in Kelvin of the polymer (A1) of the layer (A) and the exponent a is at least 0.024 and factor b is at least 0.0053 and the exponent a is at most 0.028 and the factor b is at most 0.007.

2. The polymer composition according to claim 1 wherein the polymer (A1) is a (meth) acrylic polymer.

3. The polymer composition according to claim 1 wherein the polymer (A1) comprises at least 70 wt % monomers chosen from the group consisting of C1 to C12 alkyl (meth)acrylates.

4. The polymer composition according to claim 1 wherein the polymer (A1) comprises acrylic or methacrylic monomers that are chosen from the group consisting of methyl acrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate, butyl acrylate, tert-butyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate and mixtures thereof.

5. The polymer composition according to claim 1 wherein the polymer (B1) is a (meth) acrylic polymer.

6. The polymer composition according to claim 1 wherein the polymer (B1) comprises at least 70 wt % monomers chosen from the group consisting of C1 to C12 alkyl (meth)acrylates.

7. The polymer composition according to claim 1 wherein the polymer (B1) comprises acrylic or methacrylic monomers that are chosen from the group consisting of methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate and mixtures thereof.

8. A method for manufacturing the polymer composition comprising the steps of:

a) polymerizing by emulsion polymerization of a monomer or monomer mixture (B m ) to obtain layer (B) comprising a polymer (B1) having a glass transition temperature of between 80° C. and 150° C.

b) polymerizing by emulsion polymerization of a monomer or monomer mixture (A m ) to obtain one layer (A) comprising polymer (A1) having a glass transition temperature of between −40° C. and 30° C., and

c) drying by spray drying to obtain polymer powder composition,

wherein the weight ratio r of the obtained layer (A) in the composition comprising the two layers is at least 1 wt % and is at most 30 wt %, wherein the layer (A) comprising polymer A(1) having a glass transition temperature of between −40° C. and 30° C. is a stage forming the outer layer of the polymeric particles,

wherein the layer (B) comprising polymer (B1) having a glass transition temperature of between 80° C. and 150° C. is a stage forming an inner layer of the polymeric particles,

wherein the mass average molecular weight Mw of polymer (A1) is less than 1,000,000 g/mol and the mass average molecular weight of polymer (B1) is at least 300,000 g/mol, wherein the weight ratio r in wt % of the layer (A) comprising polymer (A1) in the complete polymer particle is at least as defined by following formula

r

=

be

aT

+

1

+

1

363

-

T

wherein the variable T is the glass transition temperature Tg expressed in Kelvin of the polymer (A1) of the layer (A) and the exponent a is at least 0.024 and factor b is at least 0.0053,

and wherein step a) is made before step b).

9. The method according to claim 8 wherein step b) is performed in presence of polymer (B) obtained in step a).