Greenhouse screen

1. A greenhouse screen comprising strips of film material that are interconnected by a yarn system by means of knitting, warp-knitting or weaving process to form a continuous product, wherein at least some of the strips comprise a film material in the form of a multilayer polyester film having a thickness less than 60 μm and comprising at least two layers, wherein at least one layer is white and at least one layer is black, said at least one white layer comprises polyester and a white pigment in an amount between 5 and 50 weight-% based on the total weight of said white layer, and said at least one black layer comprises polyester and a black opacifying agent.

2. A greenhouse screen as claimed in claim 1 , wherein said multilayer polyester film comprises at least three layers: at least two white layers and at least one black layer.

3. A greenhouse screen as claimed in claim 2 , wherein the film comprises white layers as outer cover layers and a black layer as a middle layer.

4. A greenhouse screen as claimed in claim 2 , wherein the film comprises a first white layer and a black layer as outer cover layers and a second white layer as a middle layer.

5. A greenhouse screen as claimed in claim 2 , wherein the outer cover layers each have thickness at least 4 μm and not more than 15 μm.

6. A greenhouse screen as claimed in claim 2 , wherein a middle layer has a thickness that amounts to at least 50% of the total thickness of the film.

7. A greenhouse screen as claimed in claim 1 , wherein said multilayer polyester film is a two-layer film comprising one white layer having a thickness of at least 4 μm and a black layer.

8. A greenhouse screen as claimed in claim 1 , wherein the white layer(s) comprise(s) at least 15 weight-% white pigment based on the total weight of said white layer.

9. A greenhouse screen as claimed in claim 1 , wherein said white pigment is TiO 2 .

10. A greenhouse screen as claimed in claim 1 , wherein the black layer contains between 0.2 and 15 weight-% black opacifying agent based on the total weight of said black layer.

11. A greenhouse screen as claimed in claim 10 , wherein the black opacifying agent is carbon black.

12. A greenhouse screen as claimed in claim 1 , wherein the thickness of said multilayer polyester film is equal to or less than 45 μm.

13. A greenhouse screen as claimed in claim 1 , wherein the multilayer polyester film comprises a flame retardant agent.

14. A greenhouse screen as claimed in claim 13 , wherein the flame retardant agent is a phosphorous-containing flame retardant and that the multilayer polyester film comprises between 1500 and 3500 ppm phosphorous.

15. A greenhouse screen as claimed in claim 13 wherein the flame retardant is a copolymerisable phosphorus-containing flame retardant compound selected from compounds of formula (I):

wherein:

R 1 is an ester-forming group selected from —COOR 4 , —OR 5 and —OCOR 6 ;

R 2 and R 3 are independently selected from halogen atoms, hydrocarbon groups having 1-10 carbon atoms and R 1 ;

R 4 is a hydrogen atom, a carbonyl group or a hydrocarbon group having 1-10 carbon atoms,

R 5 is a hydrogen atom or a hydrocarbon group having 1-10 carbon atoms;

R 6 is a hydrocarbon group having 1-10 carbon atoms;

A is a divalent or trivalent hydrocarbon group having 1-8 carbon atoms;

n1 is 1 or 2;

and n2 and n3 are each 0, 1, 2, 3 or 4.

16. A greenhouse screen as claimed in claim 15 , wherein said compound contains two ester-forming functional groups.

17. A greenhouse screen as claimed in claim 15 , wherein R 4 is a hydrocarbon group having 1-10 carbon atoms, said hydrocarbon group containing a hydroxyl group or a carboxyl group.

18. A greenhouse screen as claimed in claim 15 , wherein R 5 is a hydrocarbon group having 1-10 carbon atoms, said hydrocarbon group containing a hydroxyl group or a carboxyl group.

19. A greenhouse screen as claimed in claim 15 , wherein R 6 is a hydrocarbon group having 1-10 carbon atoms, said hydrocarbon group containing a hydroxyl group or a carboxyl group.

20. A greenhouse screen as claimed in claim 1 , wherein the black layer comprises between 1 and 25 weight-% white pigment based on the total weight of said black layer.

21. A greenhouse screen as claimed in claim 1 , wherein the white layer comprises between 0.05 and 5 weight-% of a UV-stabilizer.

22. A greenhouse screen as claimed in claim 1 , wherein the multilayer polyester film is uniaxially drawn.

23. A greenhouse screen as claimed in claim 1 , wherein said strips of said multilayer polyester film form at least 20% of the surface area of the greenhouse screen.

24. A greenhouse screen as claimed in claim 1 , wherein all strips in said screen are of said multilayer polyester film and are arranged closely edge to edge, so that they form a substantially continuous surface.

25. A greenhouse screen as claimed in claim 1 , wherein the screen comprises open areas that are free from strips of film material.

26. A greenhouse screen as claimed in claim 1 , wherein the strips extend in a longitudinal direction of the screen and are held together by a yarn framework comprising longitudinally extending warp threads and transverse connection threads extending across the strips and connecting to the warp threads, at least said longitudinally extending warp threads are of black yarn.

27. A greenhouse screen as claimed in claim 26 , wherein said transverse connection threads extending over a white surface of the strips are of transparent or white yarn and the transverse connection threads extending over a black surface of the strips are of black yarn.

28. A greenhouse screen as claimed in claim 1 , wherein the multilayer polyester film is biaxially drawn.