CA1335678C

CA1335678C - Composition and process for making porous articles from ultra high molecular weight polyethylene

Info

Publication number: CA1335678C
Application number: CA000593956A
Authority: CA
Inventors: Harvey L. Stein
Original assignee: Hoechst Celanese Corp
Current assignee: CNA Holdings LLC
Priority date: 1988-03-28
Filing date: 1989-03-16
Publication date: 1995-05-23
Anticipated expiration: 2012-05-23
Also published as: ATE115607T1; DE68919883T2; US4880843A; EP0335613A2; EP0335613B1; EP0335613A3; SG72654A1; DE68919883D1

Abstract

Porous shaped articles made from a molded composite of ultra high molecular weight polyethylene and polyethylene wax are disclosed. These articles are produced by free-sintering a non intensively mixed blend of UHMW-PE particles and particles of a polyethylene wax. The process involves mixing the UHMW-PE and wax both in powdered form until a heterogeneous mixture is formed, but under conditions which do not lead to any substantial fracturing of the UHMW-PE particles or melting of either component. The heterogeneous mix is then transferred to a press mold to form a shape and pressure is applied sufficient only to maintain the enclosed volume of the shape. The mold is heated to a temperature above the melting point of the UHMW-PE for a period of time to allow the particles to soften, expand, and contact one another at their surfaces. The mixture is then quickly cooled.

Porous articles so produced may exhibit stiffness values in excess of about 2000 psi while at the same time showing excellent porosity values of less than about 25 inches water pressure drop.

Description

~`
, J~ 1335678 CQMPOSITION ANP PROCESS FOR
MAKING POROUS ARTIC~ES FRo~ ULTRA HIGH
MOIECULAR WEIG~T PO~ HYLENE

- 5 ~ BACKGROUND OF THE INVENTION

The present invention relates to an ultra high molecular weight polyethylene composition particularly suited for making molded plastic articles of high strength and high porosity, to articles produced therefrom, and to a process for their production.

Ultra high molecular weight polyethylene (UHMW-PE) is known in the art to possess properties markedly superior to similar polyethylenes of lower molecular weight. Such properties include toughness, impact strength, abrasion resistance, anti-friction properties and resistance to corrosion and chemical attack. But because of its extremely high molecular weight (at least 106) and the high viscosity of UHMW-PE melts, it is extremely difficult to process the material by conventional techn;ques such as injection molding, blow molding or screw extrusion. Such processing also may give rise to a degradation of the polymer chains with a conseguential diminution of physical properties.

Porous sheets and articles made from polyethylene are known in th.e art. For example, US Patent 3,024,208 discloses a process for the production of porous polyethylene articles ~ade by sintering (heating) of particles of polyethylene having a molecular weight of ' r ~ ~_ ~` 133S678 about 10,000 to 1,000,000 under heat and pressure without melting of the particles. US Patent 3,954,927 discloses a method for preparing porous articles comprising first forming a heterogeneous mixture of UHMW-PE particles with 50 to 95% by weight of a hydrocarbon such as mineral oil or a paraffin wax, heating the mixture to a temperature above the melting point of the hydrocarbon, forming the mixture into a shape, heating the shape to a second temperature above the melting point of the polyethylene until the polyethylene particles are completely fused, cooling the shape and then extracting the hydrocarbon to form the porous article. This extraction process leads to the formation of voids in the fused mass which creates porosity.

While these and similar processes offer the opportunity to minimize degradation of the polyethylene caused by conventional processing techniques as referred to - 20 above, it has been found that such porous articles are often brittle and easily fracturable, particularly when manufactured to give articles of higher porosity.
Although low porosity articles may exhibit reasonably good stiffness, such low porosity articles are not suitable for many applications. Thus, there is a compromise of one property vs. the other. Accordingly, there is a continuing need to develop a process for producing porous articles made from UHMW-PE which offers better control of porosity while at the same time maintaining or improving the ~trength and flexibility of the article.

i33~678 SUMMARY OF THE I~VENTION

It has now been discovered that strong, flexible porous articles can be produced by free-sintering a non intensively mixed blend of UHMW-PE particles and particles of a polyethylene wax. The process involves mixing the UHMW-PE and wax both in powdered form until a uniform heterogeneous mixture is formed, but under conditions which do not lead to any su~stantial fracturing of the UHMW-PE particles or melting of either component. The uniform heterogeneous mix is then transferred to a press mold to form a shape and pressure is applied sufficient only to maintain the enclosed volume of the shape. The mold is heated to a temperature above the melting point of the UHMW-PE but below its degradation temperature for a period of time to allow the particles to soften, expand, and contact one another at their surfaces. The mixture is then quickly cooled.
Porous articles so produced may exhibit flexural stiffness values in excess of about 2,000 psi while at the same time showing excellent porosity values of less than about 25 inches water pressure drop, as defined hereinafter.

DETAILED ~ESCRIPTION OF THE INVENTION

The present invention provides a composition and process for producing molded porous articles of UHMW-PE
having high porosity and good flexural strength. Such articles have utility as filter funnels, immersion filters, filter crucibles, porous sheets, pen tips, marker nibs, aerators, diffusers and light weight molded parts.

The UHMW-PE used in the present invention generally exhibits a molecular weight of at least 1 x lo6 , up to about 6 x 106 as determined by the intrinsic viscosity of a 0.05 weight percent solution in decahydronaphthalene at 135 C in accordance with ASTM
D-4020-81. Such polyethylenes may be produced by solution polymerization of ethylene using the well known coordination catalysts such as developed by Karl Ziegler.

Polyethylene waxes preferred for the purposes of this invention are commercially available materials exhibiting a melting point of at least about 100C, preferably within the range of about llO to 150C, a density within the range of about 0.92 to about o.ss g/cm3 at 20C, and a molecular weight in the range of a~out 1,000 to about 20,000, more preferably from about 2,000 to about lO,000. Most preferred are non-polar, non-oxidized waxes.

A molding composition is formed by dry blending particles of the UHMW-PE and particles of polyethylene wax in any suitable non-intensive mixing device for a period of time sufficient to form a uniform heterogeneous blend. This blending is conducted at a temperature well below the melting point of either component, preferably room temperature, and for a period of time sufficient to form a uniform blend of non-melted and non-softened particles. Suitable mixers include ribbon blenders, double cone mixers, drum tumblers, cement mlxers, twln shell ~Vec) mlxers or slmllar devlces. It ls lmportant that the mlxing be non-lntenslve and be limlted to a tlme requlred to form a unlform blend ln order to avold excesslve heat generatlon and to avold excesslve fracturing of the particles.
Blend ratios of the components may range from about 40 to 95% by weight of the UHMW-PE and correspondingly about 5 to about 60% by weight of the polyethylene wax. Wax levels above about 60% by weight tend to glve rise to molded artlcles of poor or nonexlstent porosity, whlle at levels below about 5% by welght there is little increase or even a diminutlon of the flexural strength of the article produced from the blend. Pre-ferred levels are at least about 10~ wax up to about 50% wax, more preferably from about 15 to about 45% wax.
Molded articles may be formed by a free sintering process which involves introducing the UHMW-PE/Wax mixture into either a partially or totally confined space and sub~ecting the mixture to heat sufficient to melt the wax and cause the UHMW-PE
particles to soften, expand and contact one another. Suitable processes include compression molding and casting. The mold ls heated wlth a heated hydraulic press or infrared heaters to a temperature above the melting point of the UHMW-PE, generally in the range of about 175C to about 205OC; more preferably in the range of about 185C to about 195C. Heating time depends upon the mass of the mold, and lles generally within the range of about 5 to about 15 minutes. Subsequently, the mold is cooled and the porous artlcle removed.

.~
41~, 1 3 3 S ~ 7 8 73831-43 During the heatlng process, the wax component melts and tends to form agglomerates with the UHMW-PE particles. This permits the wax to fill particle interstitial spaces and surface irregularities. The UHMW-PE, on the other hand, softens and undergoes surface sintering and bonding with nelghboring parti-cles. Upon coollng, and as shrinkage occurs, larger vold volumes are created ln the UHMW-PE/Wax composite by changes in the wax morphology. The wax is further believed to act as a particle bonding agent giving rise to artlcles havlng greater flexiblllty and strength as compared wlth articles produced from UHMW-PE alone. Mlcro photographs of the cooled composlte show that lt ls not a homogeneous blend of the polymer and wax, but rather comprlses numerous agglomerates of UHMW-PE partlcles whlch are both surface-fused together and surround dlscrete particles or reglons of the wax.
It has been found that the partlcle slze distributlon of the UHMW-PE particles used in the mixture is extremely influ-ential upon the degree of poroslty of the finished artlcle. An excess of flne partlcles tends to flll the lnterstlces or volds resulting in an article of lower porosity. An excess of larger partlcles tends to provide insufficient surface area for particle-to-particle contact during the sintering process.
Preferably the UHMW-PE particles have a loose bulk density within the range of about 350 to 500 grams per llter as measured by ASTM D-1895 and a partlcle slze dlstrlbution of at least about 95% through a 0.5 .. ..~, ~ ~ , 7 1335678 mm screen and not greater than about 15% through a 0.063 mm screen as measured by ASTM D-1921. The particle size distribution of the polyethylene wax is not necessarily critical, but the formation of a uniform heterogeneous mixture of the wax particles and the UHMW-PE particles is facilitated if the particle size distribution of each material approximate one another. The loose bulk density of the wax particles is preferably within the range of about 400 to 500 grams per liter.

In the following examples, various compositions are formulated and the porosity and stiffness values are reported.
Test samples are prepared by forming a 2-1/2 inch diameter disc, one quarter inch thick, in a suitable mold. The mold is filled with the appropriate polymer and the sides are tapped to settle the powder for uniformity and to improve packing. The top of the mold is leveled, the mold is covered and inserted into a hydraulic press. The press is closed sufficiently to contact the mold with just enough pressure to maintain an enclosed volume. A temperature of about 190C is maintained on the press for a period of 12 minutes.
The mold is then removed from the press and cooled guickly. The sample is removed from the mold and allowed to air cool for 24 hours.

The percent or degree of porosity is determined by placing the disc prepared as described above in an air chamber. The air chamber consists of a two section aluminum chamber six inches long and three and one 8 133~678 7383~-43 quarter inches outslde dlameter. Internal dlmenslons are approxlmately four inches ln length and two lnches ln diameter.
The dlsc is lnserted ln a recessed area between the two cham-bers, and the chamber ls then closed and sealed. Each sectlon of the chamber is equipped with one quarter lnch lnlet and outlet taps for dlfferentlal pressure (D.P.) measurement ln lnches of water.
An alr flow of 25 SCFH ls then establlshed and the differentlal pressure is determlned wlth a manometer. Thls measurement of dlfferential pressure is indlcative of the degree of poroslty of the speclmen tested; the lower the dlfferential pressure, the higher the porosity. From experience wlth thls method, dlfferential pressures of less than 25 lnches water pressure drop are lndlcative of artlcles wlth excellent poro-slty.
Tlnlus-Olsen stlffness values of the varlous samples are measured ln accordance wlth ASTM-D 747-84a.
Thls test method determines the apparent bendlng modulus of plastic materlals by measurlng force and angle of bend of a centilever beam. A free sintered speclmen is clamped ln a vlse and a controlled load ls applled to the free end. The vlse ls connected through gear tralns to a motor and ls capable of unlform clockwise rotation. It ls provlded with a polnter for lndlcatlon of angular deflectlon. Rotatlon of the welghlng system about the same polnt as the vlse results due to movement of the blending plate. The 9 1 33 ~ 6 78 71173-76 magnitude of the movement is indicated with a pointer moving over a load scale.
The relationship of percent deflection vs load is determined on three (3) specimens, which are prepared in a mold equipped for eight (8) 1.5" x 0.25" x 0.125" specimens. Sintering procedure is identical to that described for the 2.5" diameter disk. Load readings are taken at 3, 6, 9, 12, 15, 20, 25 and 30~
deflection and the slope is calculated. Stiffness is determined by use of the equation:

E s 4S3 X M (load scale reading) wd 100 ~
where E = stiffness or apparent bending modulus S - 0.25", the distance between specimen mounting plate and bending surface w ~ 0.25", specimen width d = 0.125", specimen thickness M = 1.0 lbf-in, the total bending moment value on the pendulum system ~ = reading or angular deflection scale in radians The following examples are illustrative of the invention.

Various blends were formed of UHMW-PE powder having a molecular weight of about 3 x 106 and polyethylene wax powder (Hostalen ~ GUR UHMW-PE and Hoechst O Wax PE 190, both available from Hoechst lo 1 3 3 5 6 7 8 73831-43 Celanese Corporatlon) at various levels shown ln Table 1. Both components had a partlcle slze dlstribution of greater than g5%
by welght through a 0.5 mm screen and less than 15% by welght through a 0.063 mm screen. The components were mixed for about 30 mlnutes at room temperature ln a non-lntenslve blender. Test discs were prepared from each composltlon by the moldlng method descrlbed above. A control sample conslsting of 100% of the UHMW-PE was also evaluated and deslgnated as control A.
Poroslty and T/0 stlffness values were then obtalned for each sample by the methods descrlbed above. ~esults are reported ln Table 1.

TA~LE 1 % UHMW-PE % WAX T/0 Stlffness, Poroslty, lnches ~sl H20 D.P.

Control A ~100%) - 1886 18.1 EX. 1 90% A 10 1591 15.4 EX. 2 80% A 20 1842 9.3 EX. 3 70% A 30 2704 6.0 EX. 4 60% A 40 2978 5.4 EX. 5 50% A 50 3078 10.9 EX. 6 40% A 60 3006 24.5 EX. 7 30% A 70 3259 No Alr Flow EX. 8 20% A 80 16,014 No Alr Flow EX. 9 10% A 90 24,065 No Alr Flow As can be seen from the data ln Table 1, a distlnctlve lmprovement in porosity was achleved at levels of polyethylene wax ranglng from about 10 to up to about 60%, wlth an exhlbitlon of a concomitant improvement of stiffness values of this -particular batch of UHMW-PE beglnnlng at wax levels between about 20 and 30% by weight.

Various blends were formed and processed as described in Examples 1 - 9 above using a different lot of UHMW-PE desig-nated as Control B. Particle slze dlstribution of the components in the blend were as described above. Porosity and stiffness values are as reported in Table 2.

% UHMW-PE % WAX T/0 Stlffness, Porosity, lnches PSi H20 D.P.

~ontrol B (100%) - 1488 43.8 EX. 10 95~ B 5 1480 38.2 EX. 11 90% B 10 1969 37.2 EX. 12 85% B 15 2050 29.5 EX. 13 80% B 20 2145 25.9 EX. 14 75% B 25 2960 23.0 As demonstrated in Table 2, an improvement in stiff-ness value for this particular lot of UHMW-PE was demonstrated at a wax level somewhere between 5 and 10% with a consistent enhancement of porosity as the wax level was increased.

The UHMW-PE used ln Examples 10 - 14 above (Control B) was blended with 25% by weight of Hoechst Wax PE-130 (Example . ~.

~ 133~i678 15) and 25% by weight of Hoechst Wax PE-520 ~Example 16).
Blending and moldlng was carrled out as ln Examples 1-9 and the partlcle slze distrlbutlon of the materlals was as speclfled in Examples 1-9. Poroslty and stlffness values are as reported ln Table 3.

% UHMW-PE % WAX T/0 Stlffness, Poroslty, lnches Psl H20 D.P.

Control A (100%)- 1488 43.8 EX. 15 75% A25% 3664 16.5 10EX. 16 75% A25% 2308 11.5 Test results demonstrate a marked lmprovement ln both stlffness values and poroslty at a preferred 25% level of addl-tion of the various waxes wlth thls partlcular batch of UHMW-PE
polymer.

Thls example illustrates the lmportance of the parti-cle slze dlstribution of the UHMW-PE polymer used in the manu-facture of the porous articles of thls inventlon. An UHMW-PE
polymer havlng a bulk denslty of 399 g/l and havlng a partlcle flnes fractlon of 35.1% passlng through a 0.063 mm screen was employed as Control C. Thls materlal was blended wlth 25% by welght of Hoechst PE 190 wax and processed as ln Examples 1 - 9.
Stlffness and porosity results are shown ln Table 4.

.

% UHMW-PE% WAX T/O Stiffness, PorositY~ inches Psl H20 D.P.

Control C (100~) - 1197 39.2 EX. 17 75%25 2306 35.1 As demonstrated from the data of Table 4, there is an increase in stiffness of the UHMW-PE polymer at the 75/25 blend ratio, but very little increase in porosity which can be attributed to the high fines content of the UHMW-PE used ln thls test.

Claims

1. A molding powder composition comprising a uniform mixture of from about 40 to about 95% by weight of particles of a polyethylene polymer having a molecular weight within the range of from about 1 x 106 to about 6 x 106 and from about 5 to about 60%
by weight of particles of polyethylene wax having a molecular weight within the range of from about 1,000 to about 20,000, the particle size distribution of the particles of polyethylene polymer being within the range of at least about 95% by weight smaller than 0.5 millimeters and not greater than about 15% by weight smaller than 0.063 millimeters.

2. The composition of Claim 1 containing from about 50 to about 90% by weight of the polyethylene polymer and from about 10 to 50% by weight of the polyethylene wax.

3. The composition of Claim 1 wherein the polyethylene wax has a molecular weight within the range of about 2,000 to about 10,000, and a melting point within the range of about 100°C to about 150°C.

4. The composition of Claim 3 wherein the polyethylene wax has a particle size distribution in the range of at least about 95% by weight of particles smaller than 0.5 millimeters and not greater than about 15% by weight smaller than 0.063 millimeters.

5. The composition of Claim 1 containing from about 55 to 85% by weight of the polyethylene polymer and from about 15 to about 45% by weight of the polyethylene wax.

6. The composition of Claim 5 containing about 75% by weight of the polyethylene polymer and about 25% by weight of the polyethylene wax.

7. A process for forming a porous article comprising:
(A) providing a molding powder comprising the mixture of polyethylene polymer particles and polyethylene wax particles as defined in any one of Claims 1 through 6, under conditions which do not lead to any substantial fracturing of the polyethylene polymer particles or melting of either component of the molding powder;
(B) forming the molding powder into a desired shape;
(C) heating the shape to a temperature within the range of from about 175°C to about 205°C while maintaining the shape under pressure just sufficient to maintain the enclosed volume of the shape and for a period of time sufficient to melt the polyethylene wax particles to permit the polyethylene polymer particles to expand and soften and to allow the polyethylene wax particles to contact the polyethylene polymer particles at their surfaces; and (D) thereafter cooling the shape.

8. The process of Claim 7 wherein the mixture is prepared by subjecting the particles of polyethylene polymer and polyethylene wax to non-intensive mixing insufficient to cause substantial fracturing of the polyethylene polymer particles and at a temperature below the melting point of the polyethylene wax.

9. A porous shaped article produced by the process of Claim 7.

10. The article of Claim 9 having a porosity of less than about 25 inches of water as measured by differential pressure and a Tinius-Olsen stiffness value of at least about 2000 psi.