WO2000052238A1

WO2000052238A1 - A process for the manufacture of filaments, particularly ptfe filaments, and an installation for carrying it out

Info

Publication number: WO2000052238A1
Application number: PCT/BR2000/000016
Authority: WO
Inventors: Luigi Biancheri
Original assignee: Manegro Administração E Participações Ltda.
Priority date: 1999-02-26
Filing date: 2000-02-25
Publication date: 2000-09-08
Also published as: AU2652300A; BR9902001A

Abstract

A process and an installation are described for the manufacture of filaments, particularly PTFE-filaments, from a strip, so as to form filaments having low title, but high tensile strength. The process, according to the invention, includes the following steps: stretching a strip by simultaneously applying heat; cutting the strip so as to produce individual filaments; and stretching the filaments by simultaneously applying heat. In the step of stretching the strip, the strip is exposed to a temperature between 240 °C and 350 °C, and, in the steps of stretching the strip and the filaments, the filaments are exposed to a temperature ranging from 330 °C to 420 °C, and in the steps of stretching the strip and the filaments, the stretching rate ranges from 50 to 30,000 %/m and the stretching rate ranges from 50 to 10,000 %/sec.

Description

Title: "A PROCESS FOR THE MANUFACTURE OF FILAMENTS, PARTICULARLY PTFE FILAMENTS, AND AN INSTALLATION FOR CARRYING IT OUT"

This invention refers to a process and installation for the manufacture of fila- ments, particularly filaments of polytetrafluoroethylene (PTFE), which can be used as dental floss, sewing thread, for weaving, for making filters, as fibers for spinning or other applications.

The manufacturing process for PTFE filaments starts with an expanded PTFE strip. In the manufacture, this material may be PTFE resin, either pure or combined with other fillers, such as graphite, mica, glass or other organic or inorganic components. The filament may also be impregnated with materials that impart lubricating properties to it.

The PTFE strips that serve as a initial material for the filaments are manufactured by the process usually called "slurry-extrusion process". This process involves extruding a preformed billet of PTFE mixed with an appropriate fraction of liquid lubricant that will act as an extrusion aid. The extrudate then passes through the calender rollers, where it will be pressed until the desired thickness, in the form of a strip, is reached. Then the strip is dried to eliminate the extrusion aid and then stretched and sintered.

Description of the Prior Art

Document GB 1287874, LENZING, had previously disclosed a process for the production of fibers, in which a sheet is extracted from a spool by a first triple set of rollers. The sheet may be heated and then divided into a plurality of fibers that are passed through a heated guide that is in contact with the multifilament. A second triple set of rollers, located after the heated guide, operates at a higher speed than that of the first set of rollers, causing a stretching of the fibers in the interval between the two sets. The stretching ratio ranges from 1:2 to 1:10, depending upon the material being stretched. After the second triple set of rollers, the fibers pass through a device for thermosetting by heating, where the fibers are heated at a temperature below the softening temperature and then wound.

Brazilian document PI 9509184-0, WESTONE, national phase of application PCT/GB95/02325, discloses a process for forming an elongated PTFE material. In this document, it states it is already known that there is an association between heating and stretching for the purpose of improving the resistance properties of PTFE, as well as the process of expanding PTFE, and makes reference to documents GB-A-202535, which dis^¬ close the contactless heating of an extruded PTFE article, EP-A-391887, which refers to the sintering of the PTFE prior to the stretching, GB-A-1525980, which discloses the stretching of a curtain of threads, while it passes through a curved heated surface One of the objectives of the Brazilian document PI 9509184-0 lies in avoiding the need for cutting the material longitudinally after elongation, in order to prevent possible defects introduced by the cutting This document proposes a process by which the non-sintered strip of PTFE is caused to slide under tension on a heated plate, so that the strip will be longitudinally stretched The strip may be sintered during the contact with the heated plate or submitted to the subsequent step of heating for this same purpose The sintering temperature is at least 327° C, preferably higher than 346 ° C, and after the sintering the PTFE does not undergo any additional longitudinal stretching The heated plate is at a temperature ranging from 35 to 550° C, preferably from 200 to 500° C The time of contact of the PTFE strip with the plate ranges from 0 5 to 10 seconds The stretching ratio (stretched length/original length) is higher than 10, preferably from 20 to 100 The thickness of the PTFE strip ranges from 5 μm to 1 μmm, preferably from 15 to 150 μm Its tensile strength is higher than 50 MPa, preferably higher than 75 MPa The strip obtained by this process has different properties on the face that comes into contact with the heated plate from those on the opposite face

In the process described in the Brazilian document PI 9509184-0, WESTONE, a sheet of PTFE is unwound, passes under tension through knives that subdivide the sheet into several parallel strips Subsequently, the strips pass through two pairs of rollers that operate with different speeds to produce a stretching of the strip in the interval between the pairs of rollers In this interval, the strip is simultaneously subjected to heating by contact with the heated plate, in the same way presented in the figure of document GB 1 ,287,874 Then, the strips are wound in individual packages

The Brazilian document PI 9001696, LENZING, describes a process for stretching a PTFE-based mass, in which a molded body is heated to a temperature ranging from 327 to 450° C, preferably from 350 to 390° C, sintered and only then stretched The mass molded by this process has resistance values in the longitudinal direction of 500 N/mm² (500 MPa), preferably at least 700 N/mm² (700 MPa) and a density from 1 80 to 2 30 g/cm³

Document US 5474727, AXON CABLE, taught a process for the production of a PTFE strip with density lower or equal to 0 3 g/cm³. by which a PTFE strip is subjected to a WO 00/52238 - 3 - PCT/BR00/00016 _

first stretching step - for instance, with stretching ratio ranging from 5:1 to 20:1 - with simulta^¬ neous heating to reduce its density to a value between 0.70 and 0.30 g/cm³, followed by a step of stabilization of the strip without alteration of the state of the PTFE by passing through an oven at a temperature of, for instance, 150 to 250° C, and for a period of time, for exam- pie, from 5 seconds to 2 minutes, while maintaining the strip under tension, so that its density will increase, followed by a stretching step - for instance, with a stretching ratio of 4: 1 - with simultaneous heating so that the density of the strip can reach a value ranging from 0.20 g/cm³ to 0J2 g/cm³. In this case, the sole objective aimed at is a low density, without concern for the thread's tensile strength level.

Objectives of the invention

Since the final product has a cost that is proportional to its weight, what one seeks with the development of new processes is to provide a thread having a lower title or relative mass (mass/length), but with equal or higher breaking strength. A lighter thread results in economy for the manufacturer of the final product, for instance, a dental floss, which employs the filament, that is, for the same weight, the maker will have a longer length of thread. As another example, in the manufacture of fabrics, if the thread is lighter and has the same tensile strength, it will have the same mechanical properties at a lower cost.

The objective of this invention lies, therefore, in providing a process for the manufacture of a PTFE filament that will result in filaments having a lower title or relative weight, but with high breaking strength.

Brief Description of the Invention

The objective is achieved, according to the invention, by a process that includes the following steps:

- stretching a strip by simultaneously applying heat;

- cutting the strip so as to produce individual filaments; and

- stretching the filaments by simultaneously applying heat.

In the step of stretching the strip, the strip is exposed to a temperature between 280° C and 350°C. In the steps of stretching the filaments, these are exposed to a temperature in the range 300° C to 450° C. In the steps involving stretching of the strips and filaments, the specific stretching lies within the range from 50 to 30,000%/m and the stretching rate ranges from 50 to 10,000%/sec. As preceding steps for carrying out the inventive process, either in a separate way or integrated to the latter, the following optional steps may be carried out:

- mixing the PTFE with a liquid lubricant in a proportion between 17% and 29% of lubricant, and 83% and 71% of PTFE, respectively;

- pressing the material so as to form a billet;

- extruding the billet so as to form an extruded preform;

- calendering the extruded preform in order to obtain a strip with a thickness from 0.05 mm to 0.65 mm; and

- drying the strip at a temperature ranging from 100°C to 250°C.

Tests made by using the process of the invention have shown that the filaments obtained present low title, while maintaining good strength, as shown in tables below.

Description of the Drawing

The invention will now be described in greater detail with reference to an embodiment represented in the drawings. Figure 1 shows, schematically, a PTFE-filament pro- duction line.

First, an initial material is mixed in mixer 1 at a temperature lower than 19° C. By using PTFE as an initial material, in the mixing step, the PTFE is mixed with a liquid lubricant, extrusion aid, in a proportion ranging from 17% to 29% of lubricant and 83% to 71 % of PTFE, respectively. This mixture is processed, preferably for 20 to 30 minutes.

In a following step, the material is pressed in a preform machine 2, forming a billet. The billet is then taken to the step of extruding in an extruding machine 3, where the material is forced through an orifice, forming an extruded preform, this process being responsible for arranging the PTFE particles into fibrils.

The extruded preform is then passed through calender rollers 4 in order to form a strip with a thickness of from 0.05 mm to 0.65 mm.

The strip resulting from the calendering is then sent to a drying oven 5 for removing the liquid lubricant. The drying temperature ranges from 100°C to 250° C. After drying the strip is stretched for the first time, passing through tensioning rollers between the two units of pulling rollers 6 that operate with a stretching ratio - that is, the ratio between the entry speed and the exit speed - from 3 to 5, and a stretching tem^¬ perature ranging from 240° C to 350° C In this first stretching, the stretching rate - that de- fines the relationship between the ratio of the entry speed and the exit speed of the strip, re^¬ spectively, and the time under temperature for the stretching to take place - ranges from 50 to 10,000%/sec, preferably from 300 to 5,000%/sec The stretching is carried out under heating, by means of a heating element 7 that may be an oven, a hot-air, steam or high- boiling-point liquid heater, a heated plate or a heated cylinder

After the stretching, the strip is wound in the winder 8

For the production of individual filaments, the bobbin of strip is led to unwinder 9, which makes the alignment of the stπp, which follows to the cutting step in the cutting unit 10 The cutting unit 10 contains parallel blades on which the strip is passed, whereby the individual PTFE-filaments are cut and separated

Following the cutting step, there is a second stretching, the stretched filament between two units of pulling rollers 1 1 that operate with a stretching ratio from 3 to 25 In this second stretching, the stretching rate ranges from 50 to 10,000 %/sec , preferably from 100 to 3,000 %/sec

Between the units of pulling rollers 11 , a heating element is provided, for in- stance, an oven 12, operating at a temperature between 330° C and 440°C, which results in the sintering of the material at this step The heating element may also be a hot-air, steam, high-boiling-point liquid heater, a heated plate or a heated cylinder

The stretched and sintered individual filaments are wound in the winding unit 13 into their respective final packages

At the end of the process, ideally, the density of the PTFE filament ranges from

0 65 to 1 80 g/cm³, and its dimensions range from 0 7 mm to 150 mm in width, and from 10 to 100 μm in thickness In addition, the final filament should still have the characteristic of tensile strength such that its specific resistance or toughness ranges from 1.6 to 5 cN/dTex

Thread-Stretching Experiments The tables below show three series of results of experiments of thread-stretching, where the values of the first and second stretchings are always expressed in %/second In each series of experiments (A, B), the initial thickness of the strip and the temperature of the oven in the first stretching are constant, the temperature of the oven varying in the second stretching for each set of experiments (A1 , A2, A3, B1 , B2, B3)

Series I: Tables of tensile strength, Mpa

Experiment IA1

Experiment I-A2

Experiment I-A3

Experiment I-B1

Experiment I-B2

Experiment I-B3

Series II: Tables of specific resistance, cN/dTex

Experiment II-A1

Experiment II-A2

Experiment: II-A3

Experiment II-B2

Experiment II-B3

Series III: Tables of density, g/cm³

Experiment III-A1

Experiment III-A2

Experiment III-A3

WO 00/52238 - 11 - PCT/BROO/00016

Experiment III-B1

Experiment III-B2

Experiment III-B3

Claims

WO 00/52238 - 12 - PCT/BROO/00016Claims

1. A process for the manufacture of filaments, particularly PTFE filaments, from a strip, characterized by comprising the following steps:

- stretching a strip by simultaneously applying heat; - cutting the strip so as to produce individual filaments; and

- stretching the filaments by simultaneously applying heat, while in the step of stretching the strip, the strip is exposed to a temperature between 240° C and 350°C and, in the steps of stretching the strip and the filaments, the filaments are exposed to a temperature ranging from 330° C to 420° C, and in the steps of stretching the strip and the filaments, the stretching rate ranges from 50 to 30,000%/m and the stretching rate ranges from 50 to 10,000%/sec.

2. A process according to claim 1 , characterized by the stretching ratio in the step of stretching the strip ranges from 3 to 5.

3. A process according to claim 1 or 2, characterized by the stretching ratio in the step of stretching the filaments ranges from 3 and 25.

4. A process according to claim 1 , 2 or 3, characterized by the fact that, in the step of stretching the strip, the stretching rate ranges from 300 to 5,000%/sec.

5. A process according to any one of claims 1-4, characterized by the fact that, in the step of stretching the filaments, the stretching rate ranges from 100 to 3,000%/sec.

6. A process according to any one of claims 1-5, characterized by the fact that, right after the step of stretching the strip, the strip is wound on a bobbin on a winder and led to an unwinder in order to be unwound and subjected to the other steps of the process.

7. A process according to any one of claims 1-6, characterized by the fact that the strip from which the filaments are produced is obtained by means of the following steps:

- mixing the initial material with a liquid lubricant;

- pressing the mixed initial material, forming a billet; WO 00/52238 _ 13 _ PCT/BROO/00016

- extruding the billet to form an extruded preform;

- calendering the extruded preform to obtain a strip; and

- drying the strip.

8. A process according to claim 7, characterized by the fact that the initial material is PTFE, either pure or with other fillers, and that, in the step of mixing the initial material with a liquid lubricant, the proportion of lubricant ranges from 17% to 29%, while the proportion of PTFE, either pure or with other fillers, ranges from 83% to 71%, respectively.

9. A process according to claim 7 or 8, characterized' by the fact that, in the step of calendering the extruded preform, a heating temperature from 40°C to 60° C is applied, the thickness of the strip obtained ranging from 0.05 to 0.65 mm.

10. A process according to any one of claims 7 to 9, characterized by the fact that, in the step of drying the strip, the strip is exposed to a temperature ranging from 100° C to 250° C.

11. An installation for carrying out the process for the manufacture of filaments according to claiml , characterized by comprising the following sequence:

- two units of tensioning rollers (6) to tension and stretch the strip, between which a heating element (7) is placed;

- a cutting unit (10) for cutting the strip and producing individual filaments;

- two units of pulling rollers (11 ), between which a heating element (12) is located; and

- a winding unit (13) for winding the individual filaments on their respective final packages.

12. An installation according to claim 1 1 , characterized by comprising, sequentially, before the tensioning rollers (6):

- a mixer (1 ) for mixing the filament material with a liquid lubricant;

- a preform machine (2) for pressing the material mixed in the mixer (1 ) so as to form a billet; WO 00/52238 - 14 - PCT/BROO/00016

- an extruder (3) for extruding the billet pressed in the preform machine (2);

- calendering roller (4) for calendering the material extruded in the extruder (3) so as to form a strip; and

- a drying oven (5) for drying the strip.

13. An installation according to claim 11 or 12, characterized by comprising. a winder (8) located after the tensioning roller (6) that tension and stretch the strip, and an unwinder (9) suitable for unwinding and aligning the strip that follows to the cutting unit (10).

14. An installation according to any one of claims 11-13, characterized by the fact that the heating element (12) is an oven.