Filter element with improved testability after dry steaming

Abstract

The present invention relates to a melt-joined filter element with improved testability after dry steaming or alternatively after sterilization by irradiation, to a method for producing the filter element according to the invention, and to the use of the filter element for filtering solutions.

Claims

1. A melt-joined filter element with improved testability after dry steaming or after sterilization by irradiation, the filter element comprising a porous, permanently hydrophilized polymer membrane that has been permanently hydrophilized by a hydrophilizing agent so that a surface tension of the membrane at 20° C. is greater than 72 mN/m, the hydrophilizing agent having a specified degradation temperature; and a housing formed from polypropylene having a melting point that is at least 125° C. lower than the degradation temperature of the hydrophilizing agent with which the polymer membrane is permanently hydrophilized, wherein the polymer membrane is bonded to the housing by melt joining by a melt joining process without a degradation of the hydrophilizing agent in a melt joining region to the housing due to the difference between the melting point of the polypropylene of the housing and the degradation temperature of the hydrophilizing agent, and wherein strips of plastics film are not introduced in the melt joining region, and wherein the hydrophilizing agent is poly-2-ethyloxazoline and the polymer membrane comprises polyethersulfone.

2. The filter element of claim 1, wherein the permanent hydrophilization of the polymer membrane is effected by chemical or physical modification of an unmodified polymer membrane with the hydrophilizing agent.

3. The filter element of claim 2, wherein the melting point of the polypropylene is at least 140° C. lower than the degradation temperature of the hydrophilizing agent.

4. The filter element of claim 2, wherein the hydrophilizing agent has a degradation temperature of at least 270° C.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 shows a thermogram of Aquazol® 5 (poly-2-ethyl-oxazoline, MW: 5,000; CAS No. 25805-17-8).

DETAILED DESCRIPTION

(2) Determination of the melting point of a thermoplastic plastics material or plastics material mixture within the meaning of the present invention by means of DSC:

(3) Dynamic differential scanning calorimetry (DSC) is used to determine the melting point of a thermoplastic plastics material. The determination is carried out on the basis of DIN EN ISO 11357-1:2009.

(4) In the case of crystalline or partially crystalline plastics materials, the melting point is defined as the peak melting temperature Tpm, as described under point 10 of DIN EN ISO 11357-3:2011(E). The determination of Tpm is likewise carried out in accordance with that DIN standard. If the DSC curve of the plastics material has more than one Tpm, the Tpm with the highest value is chosen as the measure of the melting point of the plastics material.

(5) In the case of amorphous or semi-crystalline plastics materials, the melting point is defined as the midpoint temperature Tmg, as described under point 3 of DIN EN ISO 11357-2:1999(E). The determination of Tmg is likewise carried out in accordance with that DIN standard. If the DSC curve of the plastics material has more than one Tmg, the Tmg with the highest value is chosen as the measure of the melting point of the plastics material.

(6) Production of polymers for TGA analysis from monomers which are used for the hydrophilization:

(7) The monomers are used in the same concentration and in the same solvent as in the hydrophilization of the membrane (see Implementation Example 1). The polymerization is then carried out under the same conditions as in the process for hydrophilizing the membrane, but in the absence of the membrane: In particular, the temperature, the oxygen content, the layer thickness, optionally the irradiation intensity or dose and/or the concentration and type of initiator are set or used analogously to the process for producing the membrane.

(8) After the polymerization, the resulting polymeric hydrophilizing agent is washed with the solvent of the impregnating solution and dried at a temperature of 110° C. until a constant weight is reached. The resulting polymer is pulverized and a sample is removed for TGA.

(9) Determination of the degradation temperature of a hydrophilizing agent within the meaning of the present invention by means of TGA:

(10) Dynamic thermogravimetric analysis (TGA) is used to determine the degradation temperature of the hydrophilizing agent. The determination is carried out in accordance with DIN EN ISO 11358:1997. The change in the mass of a sample of the hydrophilizing agent under a flowing air atmosphere in dependence on the temperature increasing at a constant rate of 10 K/min is recorded. Before the TGA measurement, the hydrophilizing agent is pre-dried at 110° C. until a constant mass is reached. As described in the DIN standard, ms is defined as the mass of the sample before heating. The degradation temperature defined according to the invention is the temperature at which the sample mass has lost 2% of its weight, that is to say only 98% of the mass before heating are still present, that is to say sample mass=0.98×ms.

(11) Dry Steaming:

(12) A filter element, for example a 10″ filter cartridge, is inserted dry into a stainless steel housing, and the housing is connected to a steam supply. The steam feed is opened, steam flows through the filter element in the housing, and a steam pressure of 2 bar, corresponding to a steam temperature of 134° C., is set by means of a control valve arranged downstream of the housing. A differential pressure between the inlet and outlet of the filter housing of from 0.05 to 0.1 bar is set with the aid of a control system of the steam feed. After 30 minutes, the steam feed is closed and the housing is cooled with ambient air at room temperature until temperatures<30° C. are reached.

(13) Wet Steaming:

(14) Wet steaming corresponds to the steaming procedure of dry steaming but, in a departure, the 10″ filter element is wetted with water for 90 seconds before being inserted in the filter housing and is used wet. Because steam is unable to flow through the water-wetted membrane when the steam supply is opened, the steam feed is opened only minimally and the steam is discharged at the filter housing through air outlet valves until the housing and the filter element reach a temperature>100° C. and steam is able to pass through the filter element. At no time does the steam pressure exceed a differential pressure of 0.5 bar between the inlet and the outlet of the housing. The differential pressure when a temperature>100° C. has been reached and the steam thus passes through the filter element is from 0.05 to 0.1 bar.

(15) Integrity Test, Bubblepoint and Diffusion Test:

(16) The integrity test, consisting of a bubblepoint and diffusion test, is performed with a Sartocheck® 3 and Sartocheck® 4 integrity tester (Sartorius Stedim Biotech GmbH). Setting parameters are 3 minutes stabilization time, 3 minutes test time for the diffusion test, and the diffusion test pressure specified by the manufacturer for the corresponding filter element (2.5 bar for filter elements according to the invention described herein).

(17) Porometer:

(18) Measuring instrument PMI, Porous Materials Inc., USA, Capillary Flow Porometer

(19) Commercial Comparative Filter Elements:

(20) The following commercial filter elements were tested for comparison:

(21) Pall, type: Supor EKV 0.2 μm, prod. code: AB1EKV7PH4, Ch. IR7867, (2-layer PES membrane);

(22) Pall, type: Fluorodyne EX EDF 0.2 μm, prod. code: AB1UEDF7PH4, Ch. IR8838, (1 layer each of PES and PVDF membrane);

(23) Pall, type: Fluorodyne II 0.2 μm, Prod. code: AB1DFL7PH4, Ch. IR8255, (2 layers of PVDF membrane);

(24) Millipore: type Durapore 0.22 μm, Prod. code: CVGL7TP3, Ch. F1KA97385, (1 layer of PVDF membrane);

(25) Millipore, type: Express SHC, Prod. code: CHGE01TS3, Ch. C1EA82133, (2 layers of PES membrane);

(26) 3M Cuno, type: Cuno 3M Life Assure, PNA020F01BA, Ch. M01812-002, (1 layer of PES membrane).

IMPLEMENTATION EXAMPLES

(27) 1. Permanent Hydrophilization of Membranes:

(28) A starting membrane of type 15407 (Sartorius Stedim Biotech GmbH), a sterile filter membrane of polyethersulfone with a mean pore size of 0.2 μm, was wetted with an impregnating solution of the hydrophilizing agent used for permanent hydrophilization (see Table 2 below). In the described examples, the hydrophilizing agent is a polymer. The concentration of hydrophilizing agent in the impregnating solution is in the range of from 0.5 to 6.0 percent by weight, wherein water is used as the solvent. After contacting the membrane with the impregnating solution, the membrane was placed between two polyethylene (PE) films and excess impregnating solution was displaced from the membrane with a roller or was squeezed out between two rubberized rollers. The impregnated membrane was then exposed to electron beam radiation (EC-Lab 400, Crosslinking AB, Sweden), wherein an acceleration voltage of 190 kV and doses of from 25 to 75 kGy were used. The irradiation zone was rendered inert with nitrogen, that is to say oxygen present was displaced by nitrogen. After the irradiation, the membrane was extracted with suitable solvents in order to detach hydrophilizing agent that was not permanently bonded and show the permanency of the hydrophilization. The modified membrane was then dried in a drying cabinet and tested for its properties.

(29) TABLE-US-00002 TABLE 2 Polymer Name, concentration CAS No., in the Molecular weight impregnating Irradiation Abbreviation MW solution dose PVP Polyvinyl- 1.3% by weight 50 kGy pyrrolidone, 9003-39-8, 50,000 POZ Poly-2-ethyl- 0.5% by weight 75 kGy oxazoline, 25805-17-8, 5,000 PEG Polyethylene 0.7% by weight 70 kGy glycol, 25322-68-3, 4,000 PVP-VA Polyvinylpyrrolidone- 1.0% by weight 50 kGy (VA: vinyl co-vinyl acetate) acetate S630 630S 25086-89-9, 51,000

(30) 2. Thermoanalytical Tests on Polymers (TGA):

(31) The tests were carried out by means of the simultaneous thermoanalyser STA 449 F3 “Jupiter” (Netzsch-Gerätebau GmbH) under a flowing air atmosphere with a heating rate of 10 K/min. The polymers were pre-dried at 110° C. The analysed temperature range was from 10 to at least 500° C. The analysis was carried out and the results were evaluated in accordance with DIN EN ISO 11358. In order to compare different polymers, the degradation temperature was defined as the temperature at which the mass loss is equal to 2%. The data of the TGA for Aquazol® 5 (poly-2-ethyloxazoline, MW 5,000; CAS No. 25805-17-8) are illustrated by way of example (FIG. 1).

(32) TABLE-US-00003 TABLE 3 Degradation temperatures of the hydrophilizing agents used in the examples Name, CAS No., Degradation Abbreviation Molecular weight MW temperature POZ Poly-2-ethyloxazoline, 328° C. 25805-17-8, 5,000 PVP Polyvinylpyrrolidone, 300° C. 9003-39-8, 50,000 PEG Polyethylene glycol, 275° C. 25322-68-3, 4,000 PVP-VA Polyvinylpyrrolidone-co-vinyl 250° C. 630S acetate S630, 25086-89-9, 51,000

(33) 3. Construction of Filter Elements by the Melt Joining Process

(34) The construction of filter elements is carried out by pleating a membrane permanently hydrophilized according to Implementation Example 1 with POZ, PVP, PEG or PVP-VA. A polypropylene spunbonded fabric is thereby arranged above and below the permanently hydrophilized membrane on a knife pleating machine. The pleated membrane composite is arranged in a 10″ polypropylene outer and inner housing (Sartorius Stedim Biotech GmbH). Polypropylene end caps having a melting point Tpm, measured by means of DSC (see “General methods”), of 166° C. are melted and fitted. To that end, a metal block is heated to a temperature which is above the melting point of the housing components used for the melt joining, and positioned 8 to 12 mm in front of the end caps for 60 seconds in order to produce the melt. The membrane composite with the inner and outer housing is then inserted into the polymer melt. The protecting cap and connection adapter are then joined by melting on both sides and joining the end caps and the connection components of polypropylene. The finished cartridges undergo a rinsing step with water of 90 seconds, autoclaving at 134° C. for 30 minutes, rinsing (again) in the same manner, an integrity test, and drying for 12 hours at 80° C. in a convection drying cabinet.

(35) 4. Integrity Test on Filter Elements after Dry and Wet Steaming

(36) Table 4 compares the results of an integrity test on commercial filter elements with filter elements according to the invention after dry steaming. Filter elements are known which do not pass the integrity test after dry steaming (134° C., 30 minutes) with subsequent minimal wetting on account of insufficient wetting.

(37) TABLE-US-00004 TABLE 4 Integrity test after dry steaming Product, Diffusion/Limit Bubblepoint/ Integrity Prod. code, value Limit value test Lot. No [ml/min] [bar] passed commercial filter elements Millipore Express SHC,  18.4/<28.2 5.44/>4.0 yes CHGE71TS3, Ch. C1EA82133 Millipore Durapore ®,   12/<13.3  3.36/>3.45 no CVGL71TP3, Ch. F1KA97385 Pall Supor ® EKV,  168/<17 3.29/>3.2 no AB1EKV7PH4, Ch. IR7867 Pall Fluorodyne ® EX 42.5/<30 3.44/>3.2 no EDF, AB1UEDF7PH4, Ch. IR8838 Pall Fluorodyne ® II, 16.2/<12 3.23/>3.2 no AB1DFL7PH4, Ch. IR8255 Cuno 3M Life Assure, n.d./<51 n.d./>3.2 no PNA020F01BA, Ch. M01812-002 filter elements according to the invention Element according to 16.8/<25 4.03/>3.5 yes the invention PES/POZ, 10″, Ch. 11022083 (membrane according to Table 2) Element according to 12.5/<13 4.15/>3.5 yes the invention PES/PVP, 10″, Ch. 120008583 (membrane according to Table 2) comparative examples of filter elements not according to the invention Comparative example: 16.1/<13 3.81/>3.5 no Element PES/PEG, 10″, Ch. 09005883 (membrane according to Table 2) Comparative example: 17.4/<13 3.90/>3.5 no Element PES/PVP-VA, 10″, Ch. 09011983 (membrane according to Table 2)

(38) By comparison, filter elements successfully pass the integrity test after wet steaming (134° C., 30 minutes) with subsequent minimal wetting (Table 5):

(39) TABLE-US-00005 TABLE 5 Integrity test after wet steaming Product, Diffusion/Limit Bubblepoint/ Integrity Prod. code, value Limit value test Lot. No [ml/min] [bar] passed commercial filter elements Millipore Express SHC,   17.5/<28.2  5.5/>4.0 yes CHGE71TS3, Ch. C1EA82133 Millipore Durapore ®,   10.8/<13.3  3.29/>3.45 no CVGL71TP3, Ch. F1KA97385 Pall Supor ® EKV, 13.5/<17 3.45/>3.2 yes AB1EKV7PH4, Ch. IR7867 Pall Fluorodyne ® EX 23.3/<30 4.38/>3.2 yes EDF, AB1UEDF7PH4, Ch. IR8838 Pall Fluorodyne ® II, 10.3/<12 3.88/>3.2 yes AB1DFL7PH4, Ch. IR8255 Cuno 3M Life Assure, 43.3/<51 4.54/>3.2 yes PNA020F01BA, Ch. M01812-002 filter elements according to the invention Element according to 18.1/<25 3.83/>3.5 yes the invention PES/POZ, 10″, Ch. 11022083 (membrane according to Table 2) Element according to 12.7/<17 3.95/>3.5 yes the invention PES/PVP, 10″, Ch. 120008583 (membrane according to Table 2) comparative examples of filter elements not according to the invention Comparative example: 11.1/<13 4.47/>3.5 yes Element PES/PEG, 10″, Ch. 09005883 (membrane according to Table 2) Comparative example: 10.4/<13 4.05/>3.5 yes Element PES/PVP-VA, 10″, Ch. 09011983 (membrane according to Table 2)

(40) Millipore-Express-SHC filter elements are manufactured with film strips as auxiliary agent in the cap sealing region and do not meet the above requirements. Millipore-Durapore® filter elements exhibit in principle, that is to say even in the case of wet steaming or in the case of non-minimal wetting, bubblepoint values which are too low and therefore do not meet the requirements. Pall-Fluorodyne®-II- and -EX filter elements exhibit degradation under the applied thermal loads and do not meet the requirements. The microfiltration membranes contained in those products discolor/oxidize as a result of the steaming (see Implementation Example 6).

(41) The diffusion results of the integrity test can be shown in dependence on the degradation temperature. The lower the degradation temperature of the hydrophilizing agent, the greater the damage to the hydrophilization upon thermal joining during production of the filter element. There are accordingly formed larger, non-wettable membrane regions and consequently greater diffusion currents, the greater the damage, that is to say the lower the degradation temperature. There is thus obtained a sequence of falling degradation temperatures relative to increasing diffusion values, normalized to the nominal filter area:

(42) TABLE-US-00006 Temperature: 328 300 275 250° C. Diffusion value: 16.8 20.8 26.8 29 ml/min/m.sup.2,

(43) which is shown in Table 6 with assignment to the further data.

(44) TABLE-US-00007 TABLE 6 Integrity test after dry steaming and diffusion result relative to degradation temperature of the hydrophilizing agent Product, Diffusion Prod. code, [ml/min/m.sup.2] Integrity test Degradation Lot. No mean per charge passed temperature filter elements according to the invention Element 16.8 yes 328° C. according to the invention PES/POZ, 10″, Ch. 11022083 (membrane according to Table 2) Element 20.8 yes 300° C. according to the invention PES/PVP, 10″, Ch. 120008583 (membrane according to Table 2) comparative examples of filter elements not according to the invention Comparative 26.8 no 275° C. example: Element PES/PEG, 10″, Ch. 09005883 (membrane according to Table 2) Comparative 29 no 250° C. example: Element PES/PVP- VA, 10″, Ch. 09011983 (membrane according to Table 2)

(45) 5. Degradation/Discoloration of the Membranes of Filter Elements after Heat Treatment

(46) For the product Durapore® 0.22 μm, degradation, visible by the brownish discoloration of the polymer membrane, is noted. For the product Fluorodyne® II and Fluorodyne® EX EDF, degradation, visible by the brownish discoloration of the polymer membrane, is likewise noted.

(47) Measurement of the discoloration is carried out using a D186 reflection densitometer, Greteg, Althardstr. 70, CH-8105 Regensdorf.

(48) The membrane material is removed from a filter element in the delivery state and from a filter element which has been subjected to dry steaming. The D186 densitometer is calibrated with the calibration color chart provided. All the membranes were wetted and lie flat on the same PP base plate, so that the measuring instrument can be placed flat on the membrane and measured. Calibration for the color yellow is 1.48 units. The results of the measurement of the yellow component are given in the following:

(49) Sample 1: Fluorodyne® II, 10″ cartridge, main filter membrane, material as stated

(50) TABLE-US-00008 Starting membrane after dry steaming 0.42 0.63 0.42 0.62 0.44 0.62

(51) After heat treatment, the membrane material exhibits a discoloration into the yellow/brown region, indicating thermal oxidative degradation of the membrane material.

(52) Sample 2: Fluorodyne® EX EDF, 10″ main filter membrane, material as stated

(53) TABLE-US-00009 Starting membrane after dry steaming 0.42 0.61 0.44 0.62

(54) After heat treatment, the membrane material exhibits a discoloration into the yellow/brown region, indicating thermal oxidative degradation of the membrane material.

(55) Sample 3: Durapore® 0.22, 10″ membrane, material as stated

(56) TABLE-US-00010 Starting membrane after dry steaming 0.24 0.56 0.24 0.59

(57) After heat treatment, the membrane material exhibits a discoloration into the yellow/brown region, indicating thermal oxidative degradation of the membrane material.

(58) Sample 4: Filter element according to the invention PES-POZ, 10″ main filter membrane

(59) TABLE-US-00011 Starting membrane after dry steaming 0 0 0 0 0 0

(60) After heat treatment, the membrane material does not exhibit discoloration into the yellow/brown region. There is no indication of thermal oxidative degradation of the membrane material.

(61) Sample 5: Filter element according to the invention PES-PVP, 10″ main filter membrane

(62) TABLE-US-00012 Starting membrane after dry steaming 0 0 0 0 0 0

(63) After heat treatment, the membrane material does not exhibit discoloration into the yellow/brown region. There is no indication of thermal oxidative degradation of the membrane material.