Process for the production of multilayer blow-molded hollow articles

1. A multilayer blow-molded hollow article comprising

at least an outside layer and an inside layer comprising thermoplastic polymers,

optionally one or more layers located between the outside layer and the inside layer,

wherein both the outside layer and the inside layer have only one of their two principal surfaces directly adhered to another layer of the article,

wherein any middle layer has both of its principal surfaces directly adhered to another layer of the article,

wherein at least one of the layers is a uniform continuous layer with a uniform degree of thickness in both a longitudinal and circumference direction,

wherein at least one of the layers is a non-uniform continuous layer with varying degrees of thickness that includes variations in both circumferential thickness and longitudinal thickness, resulting in a visual surface pattern effect of areas with elevations, cavities, grooves, ridges, streaks or swirls, and/or a visual color effect of streaks, waves and/or swirls caused by varying color or translucency, and

wherein a ratio of dynamic viscosity of the thermoplastic polymer forming the non-uniform layer to the thermoplastic polymer forming the uniform layer is in the range of from 9.0 to 4.0.

2. The article of claim 1 , wherein the uniform continuous layer has a thickness of between 0.1 to 5 mm.

3. The article of claim 1 , wherein the thermoplastic polymers are selected from the group consisting of polyolefins, polyolefin copolymers, polystyrenes and combinations thereof.

4. The article of claim 1 , wherein the thermoplastic polymers are selected from the group consisting of high density polyethylene, medium density polyethylene, linear low density polyethylene, polypropylene homopolymer, polypropylene random copolymer and combinations thereof.

5. The article of claim 1 , wherein any layer contains one or more colorants.

6. The article of claim 1 , wherein the article is a two-layer article and wherein the outside layer is the uniform continuous layer and the inside layer is the non-uniform continuous layer.

7. The article of claim 1 , wherein the article is a two-layer article and wherein the outside layer is the non-uniform continuous layer and the inside layer is the uniform continuous layer.

8. The article of claim 1 , further comprising a middle layer, wherein the outside layer is the uniform continuous layer and the middle layer and the inside layer are non-uniform continuous layers.

9. The article of claim 1 , further comprising a middle layer, wherein the outside layer and the inside layer are non-uniform continuous layers and the middle layer is the uninform continuous layer.

10. The article of claim 1 , wherein non-uniform layer thickness is either continuous and varies along the circumference and along the longitudinal direction of the non-uniform layer of the article, or the non-uniform layer thickness is discontinuous and becomes zero at one or more parts along the circumference and along the longitudinal direction of the non-uniform layer of the article and is interrupted by any of the other layers of the article.

11. A multilayer blow-molded hollow article comprising

at least an outside layer and an inside layer comprising thermoplastic polymers,

optionally one or more layers located between the outside layer and the inside layer,

wherein both the outside layer and the inside layer have only one of their two principal surfaces directly adhered to another layer of the article,

wherein any middle layer has both of its principal surfaces directly adhered to another layer of the article,

wherein at least one of the layers is a uniform continuous layer with a uniform degree of thickness in both a longitudinal and circumference direction,

wherein at least one of the layers is a non-uniform continuous layer with varying degrees of thickness that includes variations in both circumferential thickness and longitudinal thickness, resulting in a visual surface pattern effect of areas with streaks or swirls, and/or a visual color effect of streaks, waves and/or swirls caused by varying color or translucency, and

wherein the article is prepared by coextrusion blow-molding in two consecutive steps, where in the first step an at least two-layer tubular parison is formed by coextrusion of at least two different thermoplastic polymers from a multilayer coextrusion die head whereby (i) the polymer forming a continuous layer with a uniform degree of thickness has a lower dynamic viscosity when passing through the coextrusion die head than the polymer forming a discontinuous or a nonuniform continuous layer; and (ii) the polymer forming a continuous layer with a uniform degree of thickness is extruded at a higher extrusion rate than the polymer forming a discontinuous or a non-uniform continuous layer, and in the second step, said parison is blow-molded to form the multilayer polymeric hollow article.

12. The article as claimed in claim 11 , wherein the ratio of dynamic viscosity of the polymer forming the non-uniform continuous layer to the dynamic viscosity of the polymer forming the uniform continuous layer, when passing through the die head, is of from 9.0 to 4.0.

13. The article as claimed in claim 12 , wherein the ratio of dynamic viscosity is of from 7.7 to 4.6.

14. The article as claimed in claim 11 , wherein the rate of extrusion of the polymer forming the uniform continuous layer to the rate of extrusion of the polymer forming the non-uniform continuous layer is of from 30.0 to 2.5.

15. The article as claimed in claim 14 , wherein the rate of extrusion is of from 8.9 to 4.2.

16. The article as claimed in claim 11 , wherein the thermoplastic polymers are selected from the group consisting of polyolefins, polyolefin copolymers and polystyrenes.

17. The article of claim 1 , wherein the non-uniform continuous layer with varying degrees of thickness results in a visual surface pattern effect of areas with streaks or swirls, and/or a visual color effect of streaks, waves and/or swirls caused by varying color or translucency.