GB1575010A - Calenderable propylene polymer compositions - Google Patents

Description

(54) CALENDERABLE PROPYLENE POLYMER COMPOSITIONS (71) We, IMPERIAL CHEMICAL INDUSTRIES LIMITED, Imperial Chemical House, Millbank, London SW1P 3JF, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:: This invention relates to crystalline polypropylene compositions of low melt flow index which can be calendered into foils having a melt flow index of below 5 g/l0mins. The invention further relates to a process for making such compositions and which is able to employ powdered polypropylenes (for example powders 80% by weight of which are capable of passing through a 410 ,a, preferably a 250 , sieve as described in ASTM Sieve Specification El 1-61). The foils may be embossed and/or thermoformed into articles.

Calendering techniques are very well known and have been used successfully on polyvinyl chloride for many years. Attempts were made to calender polypropylene as soon as it became available in about 1960. Calendering offers the possibility of faster outputs. It also affords the possibility of obtaining foils from powdered polypropylenes as starting materials whereas most commercial extrusion techniques require the powder to be first converted into granules usually having dimensions of 1 to 3 mm and this is an expensive extra step. Calendering also offers the possibility of making thinner foils than can be made by extrusion.

During the past 15 years, the attempts made to calender polypropylene commercially have failed and it was reported in "Plastiques Modernes et Elastomeres" of May 1970, page 245 that the calendering of polyolefins was not customary. This was probably because the polypropylene underwent rapid degradation while in the molten states associated with the various stages in a calendering process. Degradation causes the polypropylene to stick to the calender rolls making calendering commercially unattractive, especially when attempting to make foils having smooth surfaces. It also produces unacceptable odour and colour changes and causes the compositions when converted into foils to have melt flow index of over 5 g/ 10 mins whereupon the foils become technically unacceptable for their main potential uses.

It has now been discovered that provided a soap and a particulate inorganic material are used together with highly effective stabilisers, then low melt index propylene polymers and copolymers become adequately degradation resistant while in the molten states associated with a calendering process. This is surprising because many fillers such as talc which contain transition metal ions generally promote the degradation of polypropylene.

Accordingly, this invention provides a composition calenderable into foils and which comprises: a) a crystalline homopolymer of propylene and/or a crystalline copolymer of propylene with up to 25% (preferably from over 5% up to 17%) by weight of ethylene, b) from 0.1% to 1.5% (preferably 0.15% to 0.3%) by weight of a "highly effective stabiliser" as hereinafter defined, c) from 0.1% to 2.0% (preferably 0.15% to 0.8%) by weight of a soap as hereinafter defined, and d) 10% to 60% (preferably 15% to 45%) by weight of a solid particulate inorganic material.

The percentages of ethylene quoted are based on the weight of the copolymer present in the composition and the percentages quoted in b, c and d are based on the weight of the composition.

For the purposes of making foils which can be embossed and/or thermoformed it is desirable that the composition from which the foil has been made should have a melt flow index of not more than 2 g/ 10 mins. (Melt flow index is measured by a modification of British Standard 2782 Part 1/105C/1970 using a 2.16 kg load in which the test is operated using a temperature of 230"C instead of 1900C). However, as the presence of many fillers increases the viscosity of polypropylenes, the unfilled polypropylene may have a melt flow index as high as 15 g/10 mins.

It is preferred that foils which are to be thermoformed into containers should have thicknesses of less than 0.5 mm and the compositions containing inorganic materials of this invention may be calendered into foils as thin as 0.1 mm whereas corresponding extruded foils are believed to have a minimum thickness in excess of 0.2 mm. Accordingly, this invention provides a novel polypropylene foil characterised by a thickness of less than 0.2 mm. Optimisation of the workshop skills involved in calendering the composition of this invention indicates the probability of manufacturing foils as thin as 0.05 mm or even less.

It is preferred that foils which are to be embossed should have a thickness of from 0.5 mm up to 2.0 mm. Accordingly, this invention provides novel embossed polypropylene foils preferably having a thickness of from 0.5 mm to 2.0 mm (especially a thickness of the order of 1.0 mm, e.g. 0.85 to 1.15 mm) which has been calendered from a polypropylene composition according to this invention.

The polypropylenes of this invention may comprise propylene homopolymers, but better impact resistances and flexibilities can be achieved using copolymers comprising 8%to 17% by weight of copolymerised ethylene. The preferred compolymers are the so-called sequential copolymers. These are made by carrying out a homopolymerisation of propylene to a point where many of the polymer chains have ceased to grow and then introducing ethylene into the polymerisation zone. This causes both propylene and ethylene units to be introduced into those chains which are still growing and the proportion of ethylene introduced increases with time. Sequential copolymers of this type are described by T G Heggs in "Chemistry and Industry", 7 June 1969 on page 744.

By blending with from 5% to 15% by weight of rubber, propylene homopolymers can be given impact resistances and flexibilities within the range of impact resistances and flexibilities possessed by the sequential copolymers. The percentage weight of rubber is based on the weight of homopolymer. Examples of rubbers which may be used are the polyisobutylenes, the various butyl rubbers and ethylene-propylene rubbers such as those described on pages 255 to 258 of "Chemistry and Industry" of 16 March 1974. Ethylenepropylene diene modified rubbers are especially suitable. The precise quantities of rubber needed to confer the required impact resistance and flexibility on the polypropylene will vary from rubber to rubber and can be determined by routine impact and flexibility tests.

The compositions used in the performance of this invention comprise a "highly effective stabiliser" especially in amounts not exceeding 0.7%by weight of the composition. Whether or not a stabiliser is a "highly effective stabiliser" is determined as follows: A homopolymer of propylene containing the stabiliser under test, at least 0.2%buy Ojo by weight of calcium stearate but no inorganic material, and having a melt flow index of 2 g/ 10 mins is charged to a Banbury mixer and heated to 170"C at which temperature it is molten.The molten charge is quickly transferred to a two-roll mill operating at a temperature in the range 1800C to 185"C. Polymer from the two-roll mill is then gradually fed to a four-roll calender over a period of an hour and calendered into foil 0.3 mm thick. The rolls of the calender are arranged in an inverted 'L' formation as illustrated in Figure 1 of the drawings. The top two rolls of the calender are maintained at a temperature of 185"C, the middle roll at 180"C and the bottom roll at 175"C. The melt flow index of polymer taken from foil which has been made during the first two minutes of calendering is measured as is the melt flow index of polymer taken from foil made in the last two minutes of calendering.These two melt flow indexes must differ by less than 15% if the stabiliser is to be regarded as highly effective and the term "highly effective stabiliser" is defined accordingly.

Not all stabilising compounds are equally efficient, and so although some stabilisers are efficient enough to allow the compositions to contain as little as 0.1% by weight of the stabiliser, other less efficient stabilisers may require to be used in higher concentrations in order to be highly effective.

Preferred highly effective stabilisers are pentaerythritol tetra-P-3,5-ditertiary butyl-4hydroxyphenyl propionate and tris(3,5-ditertiary butyl-4-hydroxybenzyl) isocyanurate but stabilisers such as 1,1,3-tris(2'- methyl-5'-tertiary butyl-4-hydroxy phenyl) butane are also effective. Good stabilising properties are also possessed by compounds with structures analogous to the structures of the three stabilisers mentioned above and in particular additional lower alkyl groups may be introduced onto the benzene ring or alkyl groups already present may be varied. A lower alkyl group is defined as containing 1 to 6 carbon atoms.

The soaps used in the performance of this invention are defined as substances which act as lubricants between the composition and the calender rolls. Typical soaps are the metal salts of carboxylic acids, preferably 12 to 18 carbon monocarboxylic acids such as lauric, palmitic, oleic and stearic acids. The metal ions should be derived from the metals of groups 1,2 and 3 of the periodic table according to Mendeleef if it is required to make durable thermoformed articles or they should be derived from the transition metals if it is desired to make photodegradable articles. Specific examples include sodium, calcium, magnesium, zinc oleates or stearates for durable articles or ferric stearate (preferably mixed with free stearic acid) for degradable articles.Other useful soaps are described in the second edition of the Encyclopedia of Chemical Technology edited by R E Kirk, and D F Othmer, Volume 7, page 284.

The particulate inorganic material preferably has a hardness of less than 6.8 on the Mohs scale. Examples of inorganic materials are slate flour, dolomite (Mohs 3.5 to 4.5), barytes (2.5 to 3.5) calcium carbonates (chalk 3.0), kaolin (2.0 to 2.5), gypsum (1.5 to 2.0) or talc (1.0). The most suitable inorganic materials have a Mohs hardness of 3.0 or below. A soft inorganic material is believed to be essential to facilitate thermoforming and it also appears to have a desirable effect on the feel of the thermoformed foil. It is preferred that the inorganic material be free from any fibrous components such as asbestos fibres since these may have a deterious effect on the ability of the foil to stretch during thermoforming.It is also preferred that the inorganic material has a lamellar (or platy) structure presumably because the lamellae have the ability to slip relative to each other and facilitate internal movements within the foil during thermoforming. The preferred lamellar or platy material is talc, especially talc containing less than 1 % by weight of asbestos fibres. Our best results have been obtained with the Chinese talc known as Haichen talc.

The particles of the inorganic materials should preferably be capable of passing through ASTM Sieve 140 (105 ) and preferably 97%by weight of the particles should be capable of passing through ASTM Sieve 325 (44 it) The preferred materials should comprise at least 30% by weight of particles having a largest dimension of between 10 and 18 microns. The presence of very coarse particles adversely affects the feel of the sheet material and inhibits thermoforming. It has also been discovered that foils containing inorganic material, particularly talc, are less prone to show visible defects such as irregular lines, crepes or patterns which do not significantly affect the physical properties of the foil, but which look unsightly.

It is also preferred that the compositions of this invention should also comprise from 1% to 8% by weight of a pigment having a hardness of less than 6.8 on the Mohs scale and many conventional pigments for polypropylene fulful this condition. The presence of the pigment is particularly desirable in order to achieve a uniform background on which to print. For printing the preferred pigment is titanium dioxide. Anatase titanium dioxide is preferred because it has a hardness of only 5.5 to 6 on the Mohs scale and so interferes less with the thermoforming or creasing of the foil. However, rutile titanium dioxide which has a hardness of 6 to 6.5 causes less long term degradation of the foil and may be preferable if the sheet material is to be used in containers which are expected to have a long life.Both the rutile and anatase titanium dioxide pigments should preferably be coated with up to 5% by weight of alumina and up to 2% by weight of silica (percentages based on the total weight of the pigment). The percentages are based on the weight of the titanium dioxide. The pigment may be used in combination with up to by weight of an optical brightener such as ultramarine.

The surface of articles thermoformed from foils made from compositions containing inorganic material generally have good ability to receive printing inks. However, this ability can be further enhanced by subjecting the surface to an oxidation treatment of the type described in the 2nd edition of the book "Polythene" edited by Renfrew and Morgan and published by Iliffe, see pages 542 and 543. The most convenient of these treatments is the one which uses a corona discharge. Oxidation treatment is desirable where the containers are themoformed from foils made from compositions containing no organic material.

This invention also provides a process for making calendered foils from the compositions of the invention, the process comprising: a) charging the composition in powder form to a shearing mixer and causing the shearing mixer to melt the polymer or copolymer component of the composition, b) transferring the composition from the shearing mixer to the calender rolls while maintaining the polymer or copolymer in molten condition, c) calendering the composition, and d) cooling the calendered composition to cause it to solidify into a foil.

The compositions may be calendered onto coated or uncoated textiles fabrics to form articles comprising an integral textile fabric.

Shearing mixers are mixers which generate heat by inflicting shear on the polymer or copolymer and which make use of this heat to raise the temperature of the polymer from at least just below the point at which it is solid to just above the point at which it is molten.

Suitable shearing mixers are described in Chapter 15 of the book "Polythene" (ibid).

Preferably, the temperature of the polymer or copolymer is raised to above 1700C in an internal mixer. The preferred-internal shearing mixers are a Banbury mixer or a combination of an extruder and two-roll mill.

The molten charge from the shearing mixer is preferably transferred (i.e. fed) to the calender or may not form part of the shearing mixer. The two-roll mill or extruder permits a gradual feed of the charge to the calender. It is preferred that the two-roll mill be operated at temperatures above 175"C or alternatively that material fed to the calender from an extruder leave the extruder at a temperature above 175"C. It is also preferred that the hottest roll of the calender be at 1800C or more.

When shear mixing using a two-roll mill and an extruder feed to the calender, the two-roll mill should be operated at a temperature of 1800C to 2000C and the extruder should cause the polymer or copolymer to reach a maximum temperature of at least 2000C and preferably up to 220"C.

The foils preferably of thickness from 0.05 mm to 0.5 mm produced according to this invention can be thermoformed into containers such as tubs for edible fats, trays for chocolates, and boxes for horticultural purposes. Thicker foils (for example 0.5 mm to 2.0 mm) may be thermoformed (for example by vacuum forming) into articles including motor vehicle components such as fascias, door panels, floor and head-linings, wheel archers, car seat trims and parcel shelves; cooling tower packigns; and effluent treatment packings. For certain products it might be necessary to fill the shaped article with a suitable cellular foam material, for example polyurethane foam, or other support structure. Use may also be made of the vacuum-covering technique.

This invention also provides a process for producing an embossed plastics foil comprising providing a calendered foil derived from compositions according to this invention, heating the foil until at least the surface of the foil is rendered plastic (e.g. to a temperature of 180"C to 220"C), impressing a design on the heated foil by means of an embossing tool, preferably a pressure roller, carrying the embossing design, and cooling the embossed foil. Such embossed foils may then be thermoformed into articles of the general type indicated above.

COMPARATIVE EXAMPLES A TO E AND EXAMPLES 1 TO 5 Two copolymers of propylene containing 8% and 15% by weight of copolymerised ethylene as specified in Table 1 were obtained in the form of a powder having an average particle size of 250 it. The copolymers were mixed with various combinations of powdered additives as specified in Table 1 by charging the copolymer and additives to a Henschel rotary arm fluidising mixer.

Each powder mixture obtained was in turn transferred to a Banbury mixer and heated to 1700C which is just above the melting point of the copolymer. The heated charge from the Banbury was transferred to a two-roll mill operating at a temperature of from 175"C to 1800C. The charge was fed from the mill gradually onto a bank of four steam heated calender rolls arranged in an inverted 'L' configuration as illustrated in Figure 1. The top two rolls were heated to 1850C, the middle roll was heated to 1800C and the bottom roll was heated to 175"C. Where possible, the charge was fed to the calender over a period of one hour during which time it was being steadily calendered into foils 0.3 mm thick.

The melt flow index of the compositions being fed to the calender was monitored to provide an indication of the amount of degradation which was occurring. The change in colour of the composition was also noted. These results are set out in Table 2. Attempts to calender polypropylene compositions containing no soap result in degradation which causes the composition to stick to the calender rolls preventing any useful calendering operation.

The foils made according to Examples 1 to 5 were suitable for thermoforming into tubs for margarine.

The calenders-which may be used in the performance of this invention are illustrated by the drawing.

The drawing is a diagram of an inverted 'L' formation of calender rolls.

The drawing shows calender rolls 1, 2, 3 and 4 arranged in an inverted 'L' formation.

Molten composition 5 is gradually fed to the gap between rollers 1 and 2 from a two-roll mill which is not shown. The composition 5 is calendered by the rolls to produce a foil 6. TABLE 1 EXAMPLE % BY WEIGHT MELT *% BY WT OF *% BY WT OF *% BY WT OF *% BY WT OF *% BY WT OF *% BY WT OF *% BY WT OF *% BY WT OF ETHYLENE IN FLOW INDEX STABILISER STABILISER STABILISER STABILISER DILAURYL (L) CALCTUM **TALC ***TiO2 COPOLYMER OF COPOLYMER 2 3 4 OR DISTEARYL (S) STEARATE g/ 10 min (HIGHLY EFFECTIVE THIODIPROPIONATE STABILISER) A 8 0.2 0.25 - - - - 0.5 - B 8 0.2 0.018 - - 0.063 0.153 S 0.180 - C 8 0.2 - - - 0.5 - 0.5 - D 8 0.2 - 0.1 0.1 - - 0.5 - E 8 0.2 0.2 - - - 0.50L 0.20 - 1 8 0.2 0.25 - - - - 0.5 40 2 8 0.2 0.25 - - - - 0.5 20 3 8 0.2 0.20 - - - 0.50 L 0.20 40 4 8 0.2 0.25 - - - - 0.5 40 5 1.5 1.0 0.25 - - - - 0.5 40 Stabiliser 1 is pentaerythritol tetra-ss-3.5-ditertiary butyl-4-hydroxyphenyl propionate Stabiliser 2 is 2,2'-methylene bis (4-methyl-6-tertiary butyl phenol) Stabiliser 3 is distearyl penta@rythritol diphosphite Stabiliser 4 is 2.6-ditertiary butyl-4-methyl phenol *% by weight based on the weight of the composition **The talc used was a particulate Haichen talc 97% of whose particles passed through ASTM Sieve 32.5 and at least 29% of the particles had a maximum diameter in the range 10 to 20 microns ***The TiO2 used was an anatase coated with 1.5% by weight of alumina and 0.7% of silica (the percentages being based on the weight of the TiO2) TABLE 2 Melt Flow Index (g/10 min) Changes with Time Erampel Colour Change 0 15 mins 30 mins 45 mins 60 mins A 0.34 0.46 - 0.48 0.74 Acceptable colour change B 0.45 1.37 20.60 48.40 Abandoned Little colour change C 2.64 3.24 Abandoned - - Acceptable colour change D 1.40 1.60 1.55 Abandoned - Bad colour change E 0.70 0.79 0.92 0.96 1.00 Bad colour change 1 0.15 0.13 0.16 0.15 0.15 Acceptable colour change 2 0.16 0.17 0.20 0.21 0.21 Acceptable colour change 3 0.12 0.12 0.13 0.14 0.13 Acceptable colour change 4 0.19 0.15 0.22 0.21 0.29 No discernable colour change 5 0.83 0.90 0.92 0.95 0.97 Acceptable colour change

Claims (53)

WHAT WE CLAIM IS:

1. A crystalline polypropylene composition calenderable into foils and which comprises: a) a crystalline homopolymer of propylene and/or a crystalline copolymer of propylene with up to 25% by weight of ethylene, b) from 0.1 %to 1.5%by weight of a "highly effective stabiliser" as hereinbefore defined, c) from 0.1% to 2.0% by weight of a soap as hereinbefore defined, and d) 10% to 60% by weight of a solid particulate inorganic material.

2. A composition according to claim 1 comprising 0.15 % to 0.3 4bby weight of the "highly effective stabiliser".

3. A composition according to claim 1 comprising not more than 0.7% by weight of the "highly effective stabiliser".

4. A composition according to any of the preceding claims wherein the "highly effective stabiliser" comprises pentaerythritol tetra-ss-3,5-ditertiary butyl- 4-hydroxyphenyl propionate.

5. A composition according to any one of claims 1 to 3 wherein the "highly effective stabiliser" comprises tris(3,5-ditertiary butyl-4-hydroxybenyl) isocyanurate.

6. A composition according to any one of the preceding claims comprising 0.15%to 0.8% by weight of a soap.

7. A composition according to claim 6 comprising 0.2% to 0.5% by weight of a soap.

8. A composition according to any one of the preceding claims wherein the soap is a metal salt of a carboxylic acid containing 12 to 18 carbon atoms.

9. A composition according to any one of the preceding claims wherein the soap is a metal salt of lauric, palmitic, oleic or stearic acids.

10. A composition according to any one of the preceding claims wherein the metal ions of the soap are derived from metals in group 1, 2 or 3 of the periodic table according to Mendeleef.

11. A composition according to claim 10 wherein the metal ions are derived from calcium.

12. A composition according to claim 10 wherein the metal ions are derived from calcium.

13. A composition according to claim 12 wherein the soap is calcium stearate or calcium oleate.

14. A composition according to claim 10 wherein the metal ions are derived from zinc.

15. A photodegradable composition according to any one of claims 1 to 9 wherein the metal ions of the soap are derived from iron.

16. A composition according to claim 15 wherein the soap comprises ferric stearate.

17. A composition according to claim 16 wherein the ferric stearate is mixed with free stearic acid.

18. A composition according to any one of the preceding claims wherein the particulate inorganic material has a hardness of less than 6.8 on the Mohs scale.

19. A composition according to claim 18 wherein the inorganic material is a calcium carbonate.

20. A composition according to any one of the preceding claims wherein the inorganic material has a hardness of 3.0 or below on the Mohs scale.

21. A composition according to claim 20 wherein the inorganic material is chalk.

22. A composition according to any one of claims 1 to 18 wherein the inorganic material has a lamellar structure.

23. A composition according to claim 22 wherein the inorganic material is talc or kaolin.

24. A composition according to any one of the preceding claims wherein the particles of inorganic material are small enough to pass through a 44,u C sieve (ASTM Sieve Number 325).

25. A composition according to claim 24 wherein at least 29% of the particles have a maximum diameter in the range 10 to 20 microns.

26. A composition according to any one of the preceding claims which contains from 1% to 8% by weight of a pigment having a hardness of less than 6.8 on the Mohs scale.

27. A photodegradable composition according to claim 26 wherein the soap comprises ferric stearate.

28. A composition according to claim 27 wherein the pigment is rutile.

29. A composition according to any one of the preceding claims wherein the polypropylene has a melt flow index of 15 g/ 10 min or below.

30. A composition according to claim 29 wherein the polypropylene has a melt flow index of 2 g/10 min or below.

31. A composition according to any one of the preceding claims wherein the polypropylene is a sequential copolymer of propylene with 8% to 17% by weight of ethylene.

32. A composition according to any one of the preceding claims wherein the composition comprises from 5 to 15 by weight (based on the weight of the polypropylene) of a rubber.

33. A composition substantially as hereinbefore described and illustrated by any one of Examples 1 to 5.

34. A process for making calendered foil from a composition as claimed in any one of claims 1 to 33 wherein the process comprises: a) charging the composition in powder form to a shearing mixer and causing the shearing mixer to melt the polymer or copolymer component of the composition, b) transferring the composition from the shearing mixer to the calender rolls while maintaining the polymer or copolymer in molten condition, c) calendering the composition, and d) cooling the calendered composition to cause it to solidify into a foil.

35. A process according to claim 34 wherein the composition is transferred to the calender rolls from a two-roll mill.

36. A process according to claim 35 wherein the two-roll mill is operated at a temperature above 175uC.

37. A process according to claim 34 wherein the composition is transferred to the calender from an extruder.

38. A process according to claim 36 wherein the temperature of the composition as it leaves the extruder is above 175"C.

39. A process for making calendered foil substantially as hereinbefore described and illustrated by any one of Examples. 1 to 5.

40. A calendered foil made from a composition as claimed in any one of claims 1 to 33.

41. A calendered foil as claimed in claim 40 when made by a process as claimed in any one of claims 34 to 39.

42. A calendered foil as claimed in claim 40 or 41 having a thickness of from 0.05 mm to 0.2 mm.

43. A calendered foil as claimed in claim 40 or 41 having a thickness of from 0.5 mm to 2.0 mm.

44. A calendered foil as claimed in any one of claims 40 to 43 when embossed.

45. A process for producing an embossed foil as claimed in claim 44 which comprises: a) heating the foil until at least the surface of the foil is plastic, b) impressing a design on the heated foil by means of an embossing tool, and c) cooling the impressed foil.

46. A process according to claim 45 wherein the foil is heated to a temperature of 1800C to 220"C.

47. A process according to claim 45 or 46 wherein the embossing tool is a pressure roller.

48. An embossed foil made according to any one of claims 45 to 47.

49. A process for making articles in which a foil according to any one of claims 40 to 44 or 47 is thermoformed.

50. A component for a motor vehicle which has been made by thermoforming a foil as claimed in any one of claims 40 to 44 or 48.

51. A container which has ben made by thermoforming a foil as claimed in any one of claims 40 to 44 or 48.

52. Packing for a cooling tower or effluent treatment process which has been made by thermoforming a foil as claimed in any one of claims 40 to 44 or 48.

53. An article comprising an integral textile fabric formed by calendering a composition according to any one of claims 1 to 33 onto a coated or uncoated textile fabric.