Method for making propylene monofilaments, propylene monofilaments and their use

Abstract

In a process for manufacturing monofilaments of a polypropylene having a melt flow index (MFI) at 230° C./2.16 kg of 2 to 16 g/10 min and possessing a diameter of greater than 0.050 mm and an improved abrasion resistance, from 20 to 0.1% by weight of an additive is added to the polypropylene upstream of the extruder, the melt is spun into a water bath, drawn into monofilaments and the monofilaments are wound up. The monofilament of a polypropylene having a melt flow index (MFI) at 230° C./2.16 kg of 2 to 16 g/10 min and possessing a diameter of greater than 0.050 mm and an improved abrasion resistance has a strength of at least 50 cN/tex for an elongation corresponding to the maximum tensile stress (elongation at break) of less than 30%. It also has an abrasion of less than 0.05% and a relative fracture energy>100% after treatment for 24 hours at 120° C.

Description

The invention relates to a process for manufacturing monofilaments of a polypropylene having a melt flow index (MFI) at 230° C./2.16 kg of 2 to 16 g/10 min and possessing a diameter of greater than 0.050 mm and an improved abrasion resistance; it also relates to monofilaments of this polypropylene and to their use.

Two-dimensional polypropylene textile articles are of great interest as chemically and mechanically resistant filtration means for filtering in the chemical, pharmaceutical and food industries. In this field, relatively coarse monofilaments, possessing a diameter of greater than 0.050 mm, are most particularly required.

Pure polypropylene monofilaments have the drawback of forming a great deal of dust because of the low abrasion resistance of this substance during the weaving process. The problem of abrasion is also known in the case of other thermoplastics. Thus, EP-A2-0 784 107 cites melt-spun monofilaments, of polyamide, polyester or polypropylene, that are intended for the forming fabrics of paper machines and for the wires of edge cutters. According to that patent, abrasion-resistant monofilaments are obtained using 70 to 99% by weight of a fibre-forming polymer and 1 to 30% by weight of a maleic-anhydride-modified polyethylene/polypropylene rubber, and other additives. However, the examples are limited, as regards the fibre-forming polymer, to the polyamide nylon-6 and to polyethylene terephthalate, as well as a PA-6,6/PA-6 copolyamide. The spinning rates are not specified.

Also known, from EP-A-1 059 370, is a process for manufacturing polypropylene multifilaments intended for textile applications. A metallocene-catalysed isotactic polypropylene is used as starting material, the melt flow index of the polypropylene having to be greater than 19 g per 10 minutes if it is desired to achieve the desired shrinkage characteristics. This is because, to obtain a low shrinkage, high MFI values are necessary. The above patent describes fully oriented yarns (FOYs) of 10 dpf (denier per filament) [0.03953 mm] and partially oriented yarns (POY) of 2 dpf (denier per filament) [0.01768 mm]. As regards the yarns manufactured, only general indications are given. There is no description of monofilaments.

The problem of the invention consists in providing an economic process for manufacturing coarse abrasion-resistant polypropylene monofilaments. Another problem of the invention consists in manufacturing coarse polypropylene monofilaments that exhibit improved abrasion resistance during weaving.

Yet another problem consists in providing the use of coarse monofilaments, possessing good abrasion resistance, for manufacturing two-dimensional high-performance articles, especially those intended for filtration.

The problems are solved according to the invention by the fact that 20 to 0.01% by weight of an additive is added to the polypropylene upstream of the extruder, the melt is spun into a water bath, drawn into monofilaments and the monofilaments are wound up.

As polypropylene, it is preferred to use a homopolymer possessing a melt flow index of more than 2 g/10 min, preferably from 6 to 13 g/10 min, in particular from 12 to 13 g/10 min. An MFI of more than 16 g/10 min has the drawback of giving insufficient mechanical properties and poor abrasion resistance.

It is appropriate to use an additive consisting of a combination of lubricant, filler and heat stabilizer. As combination of lubricant, filler and heat stabilizer, 0.5 to 1.2% by weight of polyethylene waxes, calcium carbonate and sterically hindered phenols have proved to be particularly suitable.

In another embodiment, it is appropriate to use as additive 0.05 to 1.0% by weight, in particular 0.3 to 1.0% by weight, of a lubricant. Particularly suitable, as lubricant, are metal salts of carboxylic acids, linear or branched hydrocarbons, fluoroelastomers and polydimethylsiloxanes.

In yet another embodiment, it is appropriate to use fillers as additive. As fillers, 0.01 to 0.1% by weight of aerosils and 0.1 to 1.0% by weight of calcium carbonate have proved to be particularly suitable.

In yet another embodiment, it is appropriate to use as additive 0.1 to 0.8% by weight of a heat stabilizer. Sterically hindered phenols, phosphites and phosphonites have proved to be particularly suitable.

In yet another embodiment, it is appropriate to use as additive 1 to 20% by weight of a polypropylene/polyethylene copolymer possessing a melting point≧140° C.

It is appropriate that the monofilaments possess a strength of at least 50 cN/tex for an elongation corresponding to the maximum tensile stress (elongation at break) of less than 35%, preferably less than 30%. This is because a strength of less than 50 cN/tex has the drawback of increasing the number of yarn breakages during the weaving process.

It is also important for the monofilaments to exhibit less than 0.05% abrasion. This is because when the abrasion is greater than 0.05%, irregularities occur during weaving, these being due to the reeds becoming fouled too quickly. This means that the interval between cleaning operations must be shortened, thereby reducing the productivity of the loom.

It is appropriate for the monofilaments to be characterized by a relative fracture energy>100% after treatment for 24 hours at 120° C., in particular after oven ageing. This has the advantage of increasing the lifetime of the filters, when they are subjected to a relatively high thermal stress and to aggressive chemicals.

The monofilaments according to the invention are particularly suitable for the manufacture of two-dimensional articles intended for filtration in the chemical, pharmaceutical and food industries.

The invention will now be described in greater detail with the aid of examples.

Polymers

As fibre-forming polymers, five different commercially available polypropylenes were used in the tests, these having a melt flow index (MFI) at 230° C./2.16 kg varying from 6.0 to 13.0 g/10 min. Each time, 50 kg of polypropylene granules were blended, using 100 kg drums and an eccentric mixer. The blending was carried out using two different processes, depending on the additive. The various processes will be explained in the examples. The granules/additive blend was fed directly into the extruder and melted.

Spinning Conditions:

• Extruder:
  • diameter: 40 mm; barrel length: L/D=25
  • extruder pressure: 80 bar
  • output: 19.76 kg/h
  • 5 heating zones;
• Spinning pump:
  • swept volume: 10 cm³/revolution;
• Spinning unit:
  • electrical heating;
• Metering pump: 23.19 rpm;
• Dies:
  • diameter: 0.7 mm
  • length of the capillary: 3×D;
• Water bath:
  • die/water bath distance: 45 mm
  • temperature: 30° C.
    Drawing Stages and Heating Channels
• Drawing stage 1:
  • 7 rolls; roll diameter: 230 mm; 1 heating channel
• Drawing stage 2:
  • 7 rolls: roll diameter: 230 mm; 1 heating channel
• Drawing stage 3:
  • 7 rolls; roll diameter: 230 mm; 2 heating channels
• Drawing stage 4: 4 rolls; roll diameter: 230 mm.
• Spinning preparation:
  • 5% aqueous solution.
    Preparation of the specimens:

EXAMPLES 2, 5, 6 and 7

In the case of the pulverulent additives, such as the fillers, lubricant, heat stabilizer, etc., the process starts by rolling the granules for half an hour in an adhesive, such as BAYSILON M 100® (brand name of Bayer AG), then the rest of the additives are added and blended for a further 1.5 hours.

EXAMPLE 4

In the case of modified polyolefins, the blend of granules, consisting of polypropylene and PP/PE modified polyolefin, having a melting point>140° C., were blended for one hour.

The examples are given in Table 1.

TABLE 1
								Relative
					Specific			fracture
					fracture		Linear	energy in the
				Elongation	energy	Mechanical	density	case of oven
		Diameter	Strength	at break	[cN.cm/	constant	uniformity	ageing for	Abrasion
Example	Additives	[mm]	[cN/tex]	[%]	dtex]	[cN/tex]	[U%]	24 h/120° C.	[%]
1	0	0.159	55.6	18.4	0.349	238.5	2.43	77.3	0.1717
2	0.15/0, 3/0.35	0.160	57.7	19.3	0.373	253.5	1.66	183.6	0.0156
3	0	0.160	54.7	19.1	0.354	239.1	1.80	76.2	0.5543
4	10	0.159	51.1	19.1	0.329	223.3	1.82	180.2	0.0254
5	0.5	0.160	54.8	17.9	0.334	231.8	2.60	69.0	0.017
6	0.15/0.15	0.159	55.3	18.2	0.344	235.9	2.16	68.3	0.018
7	0.15/0.3, 0.35	0.159	55.6	18.6	0.356	239.8	2.31	71.0
8	0	0.160	54.3	19.1	0.35	237.1	1.94	139.8	0.0128
9	0	0.160	56.3	19.4	0.37	248.0	1.67	103.3	0.0386
*Fracture energy as a percentage of the initial value;
Example 1: (control example 1): polypropylene of 13.0 g/10 min MFI;
Example 2: polypropylene of 6.0 g/10 min MFI, with a combination as additive;
Example 3: (control example 2): polypropylene of 12.0 g/10 min MFI;
Example 4: polypropylene of 12.0 g/10 min MFI, with PP/PE of melting point >140° C. as additve;
Example 5: polypropylene of 13.0 g/10 min MFI, with a lubricant as additive;
Example 6: polypropylene of 13.0 g/10 min MFI, with a combination based on heat stabilizers as additve;
Example 7: polypropylene of 13.0 g/10 min MFI, with a combination as additive;
Example 8: polypropylene of 13.0 g/10 min MFI, with anti-gas fading stabiliztion;
Example 9: polypropylene of 9.0 g/10 min MFI, with anti-gas fading stabilization.

To allow more detailed explanations, the results have been shown graphically and photographically.

These show:

FIG. 1, a histogram indicating the specific fracture energy, the oven ageing and the abrasion with and without the addition of an additive according to Example 2;

FIG. 2, the specific fracture energy, oven ageing and abrasion behaviour with and without the addition of an additive according to Example 4;

FIG. 3a, the abrasion behaviour without the addition of an additive as a function of the yardage manufactured, according to Example 1 (control example); and

FIG. 3b, the abrasion behaviour with the addition of an additive and as a function of the yardage manufactured, according to Example 8 (embodiment example).

In FIG. 1, the pair of columns on the left represent the specific fracture energy, that in the middle the oven ageing and that on the right the abrasion, with and without the addition of an additive according to Example 2. The columns on the left reflect the prior art, those on the right represent the results obtained with the monofilaments according to the invention. This figure shows, as regards the abrasion, an improvement of more than 100%. Now, 100% less abrasion means that the loom can operate for at least twice as long before it has to be cleaned. Similar results are obtained for the relative fracture energy. In this case, the right-hand column of the pair of columns in the middle also shows an improvement, by more than 50%. The specific fracture energy, indicated by the right-hand column of the pair of columns on the left, also shows an improvement over the prior art.

FIG. 2 differs from FIG. 1 by the fact that it shows the abrasion with and without the addition of an additive according to Example 4.

FIG. 3a shows photographs indicating the state of the reeds of a loom after manufacturing 100 m, 200 m and 300 m of fabric from pure polypropylene monofilament [control trial (Example 1)]. The fouling by polypropylene fluff after 300 m is so great that the loom had to be stopped.

FIG. 3b shows photographs indicating the state of the reeds of a loom after manufacturing 100 m, 200 m and 300 m of fabric from the monofilament according to the invention [illustrative example (Example 8)]. Even after manufacturing 300 m of fabric, the amount of fluff produced remains less than that obtained for 100 m in the control example.

Measurement Methods:

• • Melt flow index according to ASTM D1238;
  • Linear density determined according to SN 197 012 and SN 197 015, performance supplemented by DIN 53830;
  • Calculation of the mechanical constant MC from the following formula:
    MC={square root}{square root over (D)}·F [cN/tex
  • where D denotes the elongation in % and F denotes the strength in cN/tex.
    Description of the Abrasion Tests:

Manufacture of the Sectional Beams.

The sectional beams, each of 1000 m, were manufactured using monofilaments from 80 bobbins of the various embodiments.

Weaving Trials:

The weaving trials were carried out on a ribbon loom.

• Maximum possible production: 4000 rpm;
• The shed was formed by cams;
• Working mode: without weft reentry;
• Warp yarn density: 22.80 yarns/cm;
• Reed:
  • opening, 0.175 mm
  • dent thickness: 0.264 mm
  • dent width: 7.0 mm;
• Rotation speed of the loom: 1000 rpm;
• Weaving speed: 10 m/h;
• Weave: L1/1 sheet cloth.
  Evaluation of the Abrasion Behaviour:
  • visual assessment of the reeds;
  • gravimetric determination of the amount of fluff produced.

For the visual examination, the reeds were photographed after the operating time for 100 m, 200 m and preferably 300 m, and they were assigned a rating.

Evaluation of the abrasion behaviour using the gravimetric method is described below. To do this, all of the fluff formed is collected after the operating time for 300 m, it is weighed and related to the weight of the warp yarns using the following formula: $\mathrm{amount}\mathrm{deposited}\mathrm{in}\%=\frac{\mathrm{mass}\mathrm{of}\mathrm{fluff}\mathrm{deposited}\times 100}{\mathrm{number}\mathrm{of}\mathrm{warp}\mathrm{yarns}\times \frac{\mathrm{length}\mathrm{of}\mathrm{warp}\mathrm{yarns}\times \mathrm{linear}\mathrm{density}}{10,000}}$

The monofilaments according to the invention, possessing a diameter≧0.050 mm, are suitable for manufacturing, without abrasion, fabrics intended for filtration.

By virtue of the process according to the invention and the monofilament according to the invention, it has become possible for the first time to weave, practically without any abrasion, a polypropylene monofilament and to considerably increase the operating time of the loom. This monofilament is particularly suitable for the manufacture of fabrics used for filtration in the chemical, pharmaceutical and food industries.

Claims

1-10. (canceled)

11. A process for manufacturing monofilaments of a polypropylene having a melt flow index (MFI) at 230° C./2.16 kg of 2 to 16 g/10 min and possessing a diameter of greater than 0.050 mm and an improved abrasion resistance, said process comprising the steps of:

a) adding from 20 to 0.01% by weight of an additive to the polypropylene upstream of the extruder,

b) spinning the melt into a water bath,

c) drawing into said monofilaments,

d) and winding up the monofilaments.

12. The process according to claim 11, wherein the additive is 0.5 to 1.2% by weight of a combination of lubricant, filler and heat stabilizer.

13. The process according to claim 11, wherein the additive is 0.05 to 1% by weight of a lubricant.

14. The process according to claim 11, wherein the additive is 0.01 to 1.0% by weight of fillers.

15. The process according to claim 11, wherein the additive is 0.1 to 0.8% by weight of a heat stabilizer.

16. The process according to claim 11, wherein the additive is 1 to 20% by weight of a polypropylene/polyethylene copolymer possessing a melting point≧140° C.

17. Monofilaments of a polypropylene having a melt flow index (MFI) at 230° C./2.16 kg of 2 to 16 g/10 min and possessing a diameter of greater than 0.050 mm and an improved abrasion resistance, a strength of at least 50 cN/tex for an elongation corresponding to the maximum tensile stress (elongation at break) of less than 35%.

18. Monofilaments according to claim 17, wherein the maximum tensile stress is less than 30%.

19. Monofilaments according to claim 17, further having an abrasion of less than 0.05%.

20. Monofilaments according to claim 17, further having a relative fracture energy>100% after treatment for 24 hours at 120° C.

21. A two-dimensional article for filtration in the chemical, pharmaceutical and food industries made from monofilaments as defined in claim 7.