CA1254707A - Method and apparatus for manufacturing porous polytetrafluoroethylene material - Google Patents

Abstract

ABSTRACT

An improved method and apparatus for manufacturing a porous polytetrafluoroethylene (PFTE) material with high strength properties. Such method comprises forming a molded article from a mixture of unsintered PFTE and liquid lubricant by extruding and rolling, and subsequently combining the previously-used separate steps of 1) eliminating liquid lubricants by evaporation or extraction, 2) rendering the molded article porous by stretching, and 3) sintering the porous article to fix the porous structure, into one simultaneous operation carried out at a temperature not lower than 390°C, to obtain a material having a Young's Modulus of more than 10,000 kg/cm2 and a matrix tensile strength of more than 1,100 kg/cm2 . By simultaneously carrying out the above 3 steps a resultant saving in equipment necessary, time, energy, and labour is realized.
The apparatus incorporates a hot air furnace through which the molded article is drawn under continuous tension. Air temperature may be controlled by circulating air through a heater.

Description

ôl BACKGROUND OF TXE INVENTI ON
This invention relates to a method for manu-facturing a porous polytetrafluoroethylene material with high mechanical strength and more particularly to improve-ment in the economization of energy required for steps for removing a liquid lubricant, stretching and sintering the article.
The invention also relates to an apparatus for use in such method.
A porous polytetrafluoroethylene (hereinafter referred to "PTFE" j ma-terial takes advantage of excellen~
heat resistance, chemical resistance, electric insulàtion and repellency of PTFE and is used in various ~ilters, diaphragms and other waterproof gas-permeable materials, insulating materials for cables and sealing materials or the like.
Various methods for manufacturing a porous PTFE
material have been well known. Among others, what is commerciall~ appealing to the market is a stretching process for making porous PTFE whose basic idea is disclosed in -the applicant's Japanese Patent Publication No. 13560/67 which was published ~ugust 1, 1967.
In the first step of the process, powdered PTFE
and a liquid lubricant are mixed and the mi~ture is shaped by a paste ex-trusion and a calender rolling or a combination thereof to obtain an unsintered molded article in the form of a film, tube or a rod and the like. The second and subsequent steps comprises: (1) eliminating the liquid lubricant contained in the molded article by evaporation or extraction, cl ,' /~g

(2) rendering the molded article porous by s~x~t-e~
and (3) sintering the porous article at a temperature higher than the melting point of PTFE to fix the porous structure. The reason why the second and subsequent steps are thus subdivided is as follows.
Stretching of molded PTFE containing a liquid lubricant at a temperature heretofore employed (from room temperature to a temperature lower than the crystalline melting point of PTFE) fails to lead to uniform stretching due to the action of interfacial tension of the liquid lubricant and give rise to a porous structure heterogeneous in pore si~e distribution. The sintering step must be carried out at a temperature higher than the crystalline melting point of PTFE. However, this step has been taken separately from the stretching step since the stretching has heretofore been carried out mostly at a temperature lower than the crystalline melting point of PTFE.
It has been customary to di~ide the process into unit steps to be carried out at different temperatures and different s~eeds, thus requiring much time and energy for processing and wasting much labor.

:

~ZS~7~t7 SU~RY OF THE INVENTION
.
It is, therefore, an object of the present invention to provide a method for manufacturing PTFE, which enables one to carry out eliminating of lubricants, stretching and sintering in one step in a temperature not lower than 430C, thereby eliminating the second and subsequent steps which have heretofore been employed.
Another object of the invention is to provide an apparatus for manufacturing PTFE, thereby greatly reducing processing time and saving energy required for the production of the porous PTFE material.
As a result of extensive researches into ` temperature, speed and other operative conditions in the stretching and sintering steps, it has been found that when an unsintered PTFE molded material containing a li~uid lubricant is stretched at a temperature of not lower than 390C under the conditions under which a temperature distribution is precisely controlled, removal o~ the lubricant by evaporation can proceed simultaneously with stretching and sintering can also be carried out simultaneously with stretching after sintering. Through further researches on the conditions under which the steps are readi~y carried out, it has now been found that the method of the present invention can proceed advanta~eous~
ly by employing an oven circulating hot air as a control system for temperature distribution and providing in the l~S~'7U7 furnace a catalyst which facilitates the combustion of the vaporized l~brican-t.
The present invention is based on the above findings an~ provides a method for manufacturing a porous polytetrafluoroethylene materials, comprising molding a mixture of an unsintered polytetrafluoroethylene and a liquid lubricant lnto an article by extruding and rolling, and then by simultaneously carrying out four steps of a) removing the liquid lubricant by evaporation, b) stretch-ing the article in an unsintered condition, c) sinteringthe article in a stretched condition, and d) stretchin~
the article in a sintered condition at a temperature of more than 390~C to obtain a sintered porous polytetra-fluoroethylene material having a Young's modulus of more than 10,000 kg/cm , and a matrix tensile strenqth of more than 1,100 kg/cm2.
BRIEF DESCTIPTION OF T:HE DRAWINGS
Fig. 1 is a graph schematically representing the manner in which the temperature is raised in the furnace;
Fig. 2 shows a graph of differential thermal spectrum in the proximity of the crystalline melting point;
Fig. 3 is a schematical diagram sho~ing the manner in which stretched tension is measured in the furnacei Fig. 4 is a ~raph showing -the resul-t of measure-ment o~ the tension;

l~S~7~ 7 Fig. 5 is a graph showing a relation between temperature and stretching ratio of PTFE under a certain tension; and Fig. 6 is a schematical cross-sectional view of an apparatus used to carry out the method of the invention.
,DETAI~ED DESCRIPTION OF THE INVENTION
When the unsintered PTFE molded article enters into a temperature not lower than the crystalline melting point o PTFE, the temperature of molded PTFE increases with simultaneous initiation of the evaporation o:r the liquid lubricant,--pro,viding the result t~at PTFE is maintained at a temperature less than the boiling point of the liquid lùbricant due to absorption of heat of vaporization.
Under such condition, tension is applied to moldecl PTFE
- 15 so that its stretching proceeds within a limited range in an unsintered condition and proceeds to increase t:he surface area where the liquid lubricant has been removed.
Increasing the surface area facilitates the evaporation of the liquid lubricant still left in the molded article.
That is, porosification and evaporation of the liquid luhricant are simultaneously achieved. A~er the liquid lubricant has bee~,evaporated thoroughly, the temperatuxe of PTFE gradually increases and sinterin~ starts when it reaches crystalline melting point of PTFE. PTFE maintains higher viscosity at a temperature not lowex than the .. , ~ ~ .
~. ... . .

l~ZS~7U'7 crystalline melting point and develops a fibruous s-tructure when stretched by tension applied from the both ends thereof. The process is schematically shown in Figs. 1 to 5. Fig. 1 shows a temperature profile in the state where the temperature is increased. ~hen the molded article at room temperature reaches an inlet A
of the furnace, it is gradually heated and its temperature is elevated to the initial boiling point B
of the liquid lubricant, so that the rate of the increase in the temperature becomes lower due to heat of vaporization. Further, when the temperature of the molded article reaches a dry distillation point C' of lubricant, the rate of increase in the temperature rises and the temperature reaches the crystalline melting point of PTFE. Since the temperature of the furnace as set is still higher, the article absorbs a certain amount of heat at this crystalline melting point and finally its temperature becomes closer to the temperature of the furnace.

Referring to Fig. 2 showing data of differential thermal analysis of the unsintered PTFE
molded article in the proximity of crystalline melting point, the endothermic curve shows a peak at a temperature of 3~8C and a shoulder at a temperature of 338C and further a small peak in the proximity of 380C. These peaks depend lZS4~ 7 on the crystalline structure formed when PTFE was polymerized. On the other hand, after sintering, the above-described peaks in the endothermic curve diminish and instead a sharp peak appears at a temperature of 327~Co S This means that the crystalline structure is varied before and a~ter sintering step, and it is obvserved that the endothermic peak is shifted to the side of high te~perature when the rate of increase and decrease in the temperature is higherO
10 -- FigO 3 is schematical diagram showing how much tension is applied to the molded article in a steady state in the ~urnace where the liquid lubricant is removed by evaporation, the article is stretched in an unsintered condition, it is sintered, and it is stretched after sintering. In this inst~nce, a tension meter using a strain gauge is coupled to a pulley to continuously record the stress loaded on the opposite ends of PTFE in the furnace.
Fig. 4 is a graph representing a dynamic tension curve when a stretch ratio is varied at a constant furnace temperature while the article is supplied to the furnace at a speed of 3m/min. Measurements are performed at furnace temperatures ranging from 200C to 450C.
Fig. 5 is a graph showing a relation betwen the furnace temperature and stretch ratio under a certain ot7 constant dynamic tension, ob-tained from -the graph in Fig. 4.
It is understood from Fig. 5 that if the PTFE
molded article containing the liquid lubricant enters into the furnace as shown in Fig. 1 under conditions under which, for example, dynamic tension is exerted to load the article a stress of 100 g, little stretchiny occurs between the points A and B in Fig. 1 but stretching of not exceeding about 100% occurs at the point C at about 250C. However, the article is subjected to a higher temperature as high as up to about 350C between the points C and D and then stretched up to 800%. Between the points D and E, if the upper limit of the furnace temperature set is, for example, 400C, stretching of not lower than 1400~ is achieved simultaneously with the sintering.
It is, of course, a normal procedure to prede-termine -the stretch ratio at the top and the bottom of the furnace so that the dynamic tension is varied according to that ratio and the stretch ratios shown at various temperatures are changed accordingly. However, it has been discovered during repeated tests that when the furnace temperature was set, for e~ample, 400C in excess of -the endo-thermic peak shown in Fig. 2 ~380C), the porous PTFE material had greater mechanical properties, particularly, Young's ~" 25 modulus and matriY tensile s~t-th. More specifically, ~2S4~'7 the furnace temperature which was set at 380DC or less may not satisfy either Young's modulus of not lower than 10000 kg/cm2 or matrix tensile strength of not lower than 1100 dg/cm2 . However, the furnace temperature set at 390C or more may satisfy both the requirements, and the self-supported porous PTFE
material may be obtained convenient to handle.

It is also confirmed that the PTFE molded article not containing a liquid lubricant is not qualifie~ to satisfy Young's modulus of not lower than 10000 kg/cm2 and matrix tensile strength of not lower than 110~ kg/cm2 when supplied to the furnace.

Although the physical background of the method accordin~ to the invention is still unclear, it is thought that the phenomenom results from the rate of temperature increase up to the point at which a temperature above the endothermic peak is reached, and is also i.nfluenced by the rate of temperature decrease from such point.

The method of the present invention may be suitably applicable to the various articles in the form of a filnl, tube or rod.

Liquid lubricants which can be mixed with PTFE
include those which are capable of wetting the surface of PTFE and capable of being evaporated at a temperature not higher than the melting temperature of PTFE.
_9_ ..:

47~'7 Preferred lubrican-ts include those having a boiling point not higher than 260C may be suitably used as they are readily evaporated from the molded article. Generally, petroleum hydrocarbons are used as they are easy to handle and resonable to price.

The furnace temperature at which the invention is carried out must be not lower than 390C.. Although the crystalline melting point of PTFE is 347~C, it is required to use a temperature exceeding the crystalline melting point of 380l if it is desired to satisfy higher Young's modulu.; and matrix tensile strength.

One the othe:r hand, the furnace temperature affects the rate of i)lcrease in the temperature of PTFE
molding. More specif.ically, as the furnace temperature is increased to more than 450C or 500C, simultaneous operation of removal of the liquid lubricant, stretching and sinter.Lng of PTFE can be carried out at a higher speed. According to the present invention, it has now been confirmed that the method may be carried out as long as the temperature distribution within the urnace is controlled within the range of not more than 30OCo The higher the furnace temperature, the more accurate the temperature distribution. By accurate temperature control, PTFE can be heated and cooled homogeneously at high speed. With low accuracy, fluc-tuations are observed lZ5~7U'7 in melting of crystals and recrystallization, which tend to cause micro-breakage during the treatment and result in fluctuations in the physical properties of the porous material. According to the present inven-tion, it has been found that the oven which is adapted to circulate hot air at a high speed to homogenize the furnace temperature distribution is well suitable for heat means which controls the temperature with high accuracy in the high temperature atmosphere. Further, it has been also discovered that incorporation into the furnace, of a catalyst which facilitates oxidation of evaporated lubricant lessens a solvent concentration in the furnace atmosphere so as to maintain it below explosion limit, and that thermal energy by combustion is used to the best advantage so that energy 1S cost is greatly reduced. Since the hi~Jher the circulating air velocity is, the more the te~per~ture distrlbutlon accuracy is improved in the furnace. More than 5 m/sec.
air velocity is preferable at a working temperature and 10 - 40 m/sec. wind velocity is more preferable. The velocity may be changed according to the shape of the molded article.
Next, an apparatus will be described hereinafter into which the invention is well embodied.
The basic structure of the apparatus is shown in Fig. 6 and includes heat means, and supply and take-~47U'7 up m~ans for the molded article. Heat means is a circulating hot air system. To this end, a double-cylinder piped furnace3 is employed wherein the molded article passes through an inner cylinder portion 4 and a heater 9 is incorporated into an outer cylindrical portion 5. These two cylindrical portions are in connection with one another, and a hot air in the furnace is circulated at a constant flow rate by a fan 7. The furnace temperature is detected by a thermocouple 8 or the like disposed with the inner cylindrical portion as the center and is fed back by the heater for temperature control. The furnace temperature is influenced by an inflow of the air from the furnace inlet and outlet as well as a supplied speed of molded articles and combustion caloric energy of the lubricant.
A pressure box 2 at the furnace inlet and a nozzle 11 a-t the outlet are arranged to provide an adjust-able mechanism which controls inflow of the air. A port-ion of the air in the furnace is forcibly discharged by an exhaust pipe 10 to maintain oxygen concentra-tion required for combusiton in theJfurnace.
Catalysts 6 ar~ mounted at one or more places where the hot air is circulated. A platinum group ca-talyst is one of the catalysts having excellent oxidizing capacity and is suitably used for the purpose of the invention.
Means for supplying and taking-up the molded ~ 12 -~..

4L7~'7 artlcle will be ~escribed hereinafter. Basically, a take-up portion 13is required to be driven at a speed higher than a speed at which a supplying portion 14 does since a stretching operation is interposed. When the molded article is, for example, a film, means for supplying and taking up the film is preferably in the form of roller means adapted for rolling up or supplying the ~ . When tne molded article is a tube or a rod, means is preferably in the form o~ a pair of capstanslor 12 witha groovetoconform to the outer diameter. In addition a guide roll, a supply stand, take-up machine or various detectors are arranged as the case may be.

A li~uid lubricant naphtha No. 5 (manufactured by Shell Petroleum Co.; boiling range 152-197C) with 25 parts by weight was mixed with a PTFE fine powder F104 ~manufactured by Daikin Kogyo Co., Ltd.) with 100 parts by weight and then extruded to a tube of the outer diameter of 3 mm, the inner diameter of 2 mm by a ram extruder.
Next, the tube was subjected to simultaneous treatments such as removal of the lubricant and stretching, sintering the tube as shown in Table 1 at the temperature ranging from 350 - 520C by the use of the apparatus shown in Fig. 6. As a result, a homoaeneous porous tube was obtained in either case. The lubricant was fully evaporated and removed. An extracted residue (the tube was extracted in - 13 ~

.~ ..

~4~7~)7 acetone to measure weight decreases before and after extraction) was shown less than 0.1 weight % in either case. Table 2 shows properties of porous tubes obtained, wherein strength characteristics of the tube obtained at the furnace temperature of more than 390PC
are matrix strength of 1100 kg/cm and Young's modulus of more than 10000 kg/cm2 . On the other hand the tube obtained using an atmospheric temperature of 360C
produces values for the matrix tensile strength, or Young's modulus where one of these is less than the appropriate value state above.

Table 1 Test Condition . .

Furnace Supply Take-up Stretching Test No. Temperature Speed Speed Ratio (C) (m/min.) (m/min.)(~) 1 360 1.5 6.0 300 2 400 3.0 6.0 100

3 400 3.0 12.0 300

4 420 3.5 38.5 1000 520 7.0 28.0 300 6 360 2.0 22.0 1000 ~Zs47(~7 TabIe 1 Characteristics of Porous Tube Dimensions . Property of Matter Test Outer Inner Poro- Bubbling Ma~ri~ Ten- Younq's No. Dia. Dia. sitv Point sile Strength ModuIus (mm) (mm) (%) (kg/cm~) (kg/cm~) 1 2.2 1.5 65 0.32 825 8000 2.3 1.7 42 1.20 1150 10500 3 2.2 1.6 68 0.43 1340 11000 4 2.3 1.7 82 0.18 1240 14000 2.4 1.9 76 0.22 1200 20000 6 2.1 1.6 86 0.15 1150 7000 * Calculation is made according to the follow-ing formula FM = FR x dR

wherein FM: matrix tensile strengh kg/mm2, FR: actual -tensile strength of porous sample kg/mm2, dR: apparent specific density of porous sample.

. .--_ A __ __ A liquid lubricant naphtha No. 5 with 26 parts by weight was mixed with a PTFE fine powder F104 with 100 parts by weight and then extruded to give a rod of the outer dia-meter of 25 mm by a ram extruder. The rod was ~urther rolled out to a film measured in the width of 650 mm and . ~ 15 -lZS~7~

the thickness of 0.25 mm.. The film was subjected to simultaneous treatments such as removal of the lubricant and stretching, sintering the film under a condition as shown in Table 3 by the use of the apparatus having a supply stand, a guide roll, and a take-up machine suitable for the film. Characteristics of the porous film as obtained are shown in Table 4 from which it is understood that the film is obtained with a higher strength and sufficient permeability.
Table 3 Test Condition - Furnace -Supply Take-up Stretching Test No. Temperature Speed Speed Ratio (C)(m/min.~ (m/min.) (~) 7 350 2.0 12.0 500 8 400 4.0 12.0 200 9 400 4.0 24.0 500 400 4.0 44.0 1000 11 540 10.0 30.0 200 12 540 10.0 60.0 500 13 350 3.0 33.0 1000 l~S4t70'7 Table 4 Characteristics of Porous Film _ ... _ _ . . . . _ _ Matrix Tensile Dimension Strength in Test Film Perme- Longitudinal Youn~'s No. Thickness Porosity ability DirectionModulus (mm~ (~)(Gurley(kg/cm2) (kg/cm2) 7 0.19 71.74.1 1200 7500 8 0.21 46.548.2 1120 11000 9 0.20 72.16.5 1150 10900 0.19 84.72.0 1360 13000 11 0.19 6&.59.4 1270 15000 12 0.18 80.04.5 1460 17000 13 0.17 89.01.5 1200 6500 For production of the PTFE porous materials the three individual steps or processes have been heretofore required, such as removal of the liquid lubricant, and stretching and sintering of the molded article to obtain the porous materials from the paste extruded materials.
According to the present invetnion, these three s-teps can be carried out simultaneously in one unit operation, and the present invention greatly economizes in the amount of equipment, time and energy required for producing porous PTFE materials, thus greatly economizing in the cost, labor, and energy required.

While the invention has been described in detail and with reference to specific embodiments thereof, i-t will be apparent to one skilled in the art that various changes in modifications can be made therein without departing from the spirit and scope thereof.

Claims (12)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for manufacturing a porous polytetrafluoroethylene material comprising extruding or rolling or a process consisting of both extruding and rolling a mixture comprising unsintered polytetrafluoroethylene and a liquid lubricant to form a molded article, and carrying out continuously in a one unit operation the steps of evaporating the liquid lubricant from the article simultaneously with stretching the article in an unsintered condition, and on substantial evaporation of the lubricant, sintering the stretched article simultaneously with further stretching the article in a sintered condition, all at a temperature of more than 390°C to obtain mechanical strength characterized by a Young's modulus of more than 10,000 kg/cm2, and a matrix tensile strength of more than 1,100 kg/cm2.

2. A method of manufacturing a polytetrafluoroethylene porous material as described in Claim 1, characterized in that control of the temperature at which the steps of evaporating the lubricant simultaneously with stretching the molded articles in an unsintered condition and sintering the stretched article simultaneously with further stretching are carried out, is by circulating hot air at an air velocity of more than 5 m/sec., and that the evaporated lubricant is burned by means of an oxidizing catalyst.

3. A method for manufacturing a polytetrafluoroethylene porous material as described in Claim 1, characterized in that the molded article is in the form of a film, tube or rod.

4. A method for manufacturing a polytetrafluoroethylene porous material as described in Claim 1, characterized in that the boiling range of the lubricant is less than 260°C.

5. A method of manufacturing a porous polytetrafluoroethylene material as described in Claim 1, characterized in that the temperature at which the steps of evaporating the lubricant simultaneously with stretching the molded article in an unsintered condition and sintering the stretched article simultaneously with further stretching the same are carried out, is more than 450°C with temperature variation of less than 30°C.

6. A method of manufacturing a porous polytetrafluoroethylene material as described in Claim 1, characterized in that the temperature at which the steps of evaporating the lubricant simultaneously with stretching the molded article in an unsintered condition and sintering the stretched article simultaneously with further stretching are carried out, is more than 500°C with a temperature variation of less than 30°C.

7. An apparatus for manufacturing a porous polytetrafluoroethylene material from an unsintered molded article comprising a mixture of polytetrafluoroethylene and a liquid lubricant which is subjected to the steps, carried out continuously in a one unit operation, of evaporating the lubricant from the article simultaneously with stretching the unsintered molded article and then sintering the stretched article simultaneously with further stretching the same in a sintered condition, said apparatus comprising a furnace including a first passageway through which the molded article passes and a second passageway in which a heater is disposed, said first and second passageways being coupled to one another at the ends thereof, means for circulating hot air through said first and second passageways in series, means for detecting a furnace temperature, means for adjusting an inflow of the air at an inlet and an outlet of the furnace, means for forcibly exhausting a portion of the furnace hot air, a catalyst disposed within said second passageway for facilitating oxidization of the vaporized lubricant, means for supplying the molded article to the furnace, and means for actuating take-up of the molded article at a speed greater than the speed at which the molded article is supplied.

8. An apparatus as claimed in claim 7 wherein said furnace includes an inner cylinder disposed within an outer cylinder with said first passageway defined within the inner cylinder and the second passageway defined about the first passageway between the inner and outer cylinder.

9. A method for manufacturing a porous polytetrafluoroethylene material comprising forming a strand from a mixture comprising unsintered polytetrafluoroethylene and a liquid lubricant, drawing the strand under continuous tension through a hot air circulating furnace at a temperature of more than 390°C in a continuous one unit operation to evaporate the lubricant from the strand simultaneously with stretching the strand and, on substantial evaporation of the lubricant, to sinter the stretched article simultaneously with further stretching, thereby to obtain the resultant strand as said material with mechanical strength characterized by a Young's modulus of more than 10,000 kg/cm2, and a matrix tensile strength of more than 1,100 kg/cm2.

10. A method as claimed in Claim 1 wherein the strand is in the form of a film, tube or rod.

11. A method as claimed in Claim 10 wherein the evaporated lubricant is burned by means of an oxidizing catalyst.

12. A method as claimed in Claim 9, 10 or 11 wherein the variation of the temperature is less than 30°C.