Latex Processing Research Articles

High-value application of kaolin by wet mixing method in low heat generation and high wear-resistant natural rubber composites

In this study, a silane coupling agent and kaolin, a natural mineral resource, were introduced into the wet mixing process of natural rubber latex, the high wear-resistant and low-generation heat natural rubber composites were prepared. The sputtering, collision, and deposition of coupling agent/fillers/natural rubber latex mixed emulsion onto the roller effectively could break up the filler aggregates, enable fillers to be better infiltrated, distributed, and dispersed in the rubber. The effects of dry/wet mixing processes and the ratio of kaolin to silica on the crosslinking density, filler dispersion, extrusion rheological properties, gas barrier properties, dynamic mechanical properties and thermal stability of NR composites were investigated, and the silanization mechanism was summarized by FTIR and XRD. The experimental results showed that kaolin improved the crosslinking density and failure temperature, reduced the Payne effect, extrusion swell, and rolling resistance. Compared with 60 phr silica-filled natural rubber, the gas barrier property and wear resistance of 60 phr kaolin-filled natural rubber were improved by 64 % and 27 %, respectively, and the rolling resistance was reduced by 57 %. When the ratios of silica to kaolin were both 20:40, the crosslinking density, gas barrier property, and wear resistance of wet mixing were improved by 24 %, 45 %, and 9 %, respectively, compared with dry mixing. This study provided a new method for the high-value application of kaolin in wet mixing and green tires.

A Study on the Preparation of a Vulcanizing Mixture and Its Application in Natural Rubber Latex.

The traditional preparation process of natural rubber latex requires tedious treatment of a variety of rubber additives. In this paper, a new process of wet mixed grinding was used to prepare a reinforced vulcanization mixture and a rapid vulcanization effect. The effect of different amounts of vulcanization mixtures on the mechanical properties of natural latex film was studied, and the pre-vulcanization process of latex and the vulcanization process of film were optimized. The results showed that with the increase in the amount of vulcanization mixture, the tensile strength increased from 5.96 MPa to 29.28 MPa, and the tear strength increased from 7.59 kN/m to 52.81 kN/m. When the vulcanization temperature of the latex film is heated from 80 °C to 100 °C, the vulcanization time is shortened by 5~6 times. The new vulcanization mixture prepared in this work has the characteristics of simple production and fast vulcanization speed, which provides a new solution for the development of the latex product industry.

Processing rubber latex (Hevea brasiliensis) in agroforestry in Menggala Mas Village, Tulang Bawang Tengah District, Tulang Bawang Barat Regency

Local knowledge of rubber latex processing is important for village communities that depend on rubber plants for their livelihood. This research aims to determine the processing of rubber latex in agroforestry carried out by the community in Menggala Mas Village, Lampung Province, Indonesia. This research approach is qualitative. Data was obtained by observation, in-depth interviews, and documentation studies. Then, the data was processed by making data transcripts, coding, categorizing, drawing temporary conclusions, triangulating, and drawing conclusions so that the results obtained could be analyzed to determine the data processing. Local communities carry out rubber in agroforestry. The research results showed that three applications of agroforestry planting patterns were found, namely alley cropping (alley pattern), Trees along border (fence pattern), and random mixture (random pattern). The dominant combination in alley cropping is rubber-pulai, and rubber-mentru-afrika wood. The dominant combinations in Trees along the border are rubber-acacia, rubber-jackfruit-cempedak, rubber-petai-teak. The dominant combinations in the random mixture are rubber-jengkol, rubber-petai-jengkol-acacia, rubber-jackfruit-cempedak, rubber-pulai-noni-jengkol, rubber-banana-gadung-sungkai. The rubber latex processing process in agroforestry is carried out traditionally, starting from the tapping process, sap acidification, sap harvesting (napeu), sap separation, and sales and harvesting of agroforestry crops. The community's local knowledge has been passed down from their parents to reduce the costs of processing rubber latex from expensive materials so that the community can gain more profits. This has been proven if local communities can manage their land well, which can become a long-term source of livelihood. The government needs to assist with strategies for processing rubber latex as well as managing rubber agroforestry land to increase farmers' knowledge and harvest yields

Cellulose nanocrystals from oil palm trunk biomass as a bio‐reinforcing filler for improved mechanical properties of modified natural rubber composites

AbstractCellulose nanocrystals (CNC) obtained from biomass are increasingly demanding as a candidate for rubber composite reinforcement due to their sustainability and environmentally friendly characteristics. This work explored CNC extracted from oil palm trunks, an agricultural waste, for its reinforcing efficacy for natural rubber (NR) composite. To achieve good compatibility of the hydrophilic CNC in non‐polar NR, the NR was modified in a simple and green epoxidation condition via its latex stage. The epoxidized natural rubbers (ENR) containing 15% and 25% epoxide content, symbolized as 15E and 25E, respectively, were prepared. The ENR/CNC nanocomposites were fabricated by mixing CNC aqueous suspension and ENR latex, followed by co‐coagulation by methanol. A low amount of CNC (0.5–5 phr) was incorporated into the rubber and it was found that the ENR/CNC nanocomposites showed improvement in mechanical properties compared to the NR/CNC nanocomposite. The 25E/CNC nanocomposite with 1 phr of CNC achieved a 22.6% increase in tensile strength while the 25E/CNC nanocomposite with 2 phr of CNC resulted in a 27.1% increase in tear strength. The enhanced performance of the fully bio‐based rubber nanocomposites is attributed to the CNC–rubber interaction and good CNC dispersion in the ENR matrices, evidenced by the results of SEM and crosslink density.Highlights Cellulose nanocrystal (CNC) was successfully obtained from the oil palm trunk. NR was modified into epoxidized natural rubber (ENR) for increased polarity. ENR/CNC mixing was done in a simple and environmentally friendly latex process. Only 2 phr of CNC resulted in a 27.1% increased tear strength of rubber composite. SEM clearly evidenced improvement of CNC–ENR interaction.

Toxicity reduction of natural rubber gloves using protein crosslinking technique

AbstractFor natural rubber (NR) gloves, residual latex processing chemicals are the important substances that cause adverse reactions to users who are sensitive to them. Many attempts have therefore been made to reduce or eliminate these leachable chemicals from the gloves such as natural compound addition and polymer coating. The current study proposed a novel alternative method to reduce chemical allergens from NR gloves by protein crosslinking through the Maillard reaction that is possible to adapt to the existing glove production. Commercial NR gloves were initially treated with various concentrations of glutaraldehyde, a crosslinking agent, ranging from 0.00% to 1.00%w/w prior to being tested for cytotoxicity, contact angle, water absorption, and mechanical properties. Artificial sweat and culture medium with serum (complete medium) were used as vehicles for in vitro cytotoxicity tests. Results from the test revealed that, regardless of the vehicle type, increasing glutaraldehyde concentration reduced the toxicity of NR gloves, indicating the reduction of leachable toxic substances after the treatment. Contact angle and water absorption resistance were found to increase with increasing glutaraldehyde concentration, reflecting the enhanced hydrophobicity of the treatment. The mechanical properties of the gloves were not significantly different after such treatment. The results confirm the potential use of this protein crosslinking method to reduce the allergy problem of NR gloves.

Self‐flocculation and its mechanism of natural rubber latex by accelerant‐modified silica

AbstractIn this paper, an accelerant with vulcanization‐promoting and silanization‐promoting effects, 1,3‐diphenylguanidine (DPG), is loaded on green filler silica by high‐energy ball milling technology. Meanwhile, DPG breaks the ionization equilibrium of latex and realizes the self‐flocculation. The DPG‐induced self‐flocculation technology promotes the dispersion of silica and the silanization reaction, forming more complete filler‐rubber network and three‐dimensional crosslinking network. X‐ray diffraction, Fourier‐transform infrared spectroscopy, X‐ray photoelectron spectroscopy, and thermogravimetric analysis results show that the ball milling technology makes DPG strongly adsorbed on the surface of silica, and the loading rate is 14.4%. Compared with dry mixing, the ball milling‐self‐flocculation technology increase the silanization index, tensile strength and aging coefficient by 8%, 31%, and 19%, respectively. The curing rate is improved, and the rolling resistance is the lowest. This research provides a green flocculation technology of the raw materials in formula without other flocculants and acidic wastewater discharge, avoiding the serious dust pollution and high energy consumption in the dry mixing process, as well as the poor aging resistance of rubber in the acid flocculation process of latex.

A thermal hazard risk evaluation of emulsion polymerisation and vinyl acetate monomers in the latex manufacturing process

Latex is extensively used in industrial products. However, completing some processes at scale leads to unacceptable levels of risk that need to be quantified and mitigated. Systemic risks must be eliminated wherever possible, and safety takes priority over efficiency and quality. To assess the process risks accurately, four raw materials were examined in this study: polyvinyl acetate (PVA), latex process-initiator-ammonium persulfate (APS) and hydrogen peroxide (H2O2), and vinyl acetate monomer (VAM). The physicochemical composition of the PVA latex process was determined via calorimeters, including differential scanning calorimetry (DSC) and vent sizing package 2 (VSP2). The calorimetry results showed that the protective colloid was a critical component in the polymerisation reaction. In addition, when adding initiators to the system, it is vital to observe the normal ratio of materials and keep the stirring system operating. The scenario system also simulated the effects of shutting down various inhibitory programs, including the build-up of free radicals that could result in a runaway reaction when the initiator was added in excess. On the other hand, the result of the risk matrix displayed as a medium level, indicating that although the probability of an accident is low, the resulting severity is at disaster level. As a result, this study provides process safety engineers with a reliable frame of reference for assessing the potential dangers in the PVA latex manufacturing process.

The coagulant dipping process of nitrile latex: investigations of former motion effects and coagulant loss into the dipping compound.

Coagulant dipping, the process used in thin glove manufacture, involves electrolyte ions diffusing from the surface of a hand-shaped former into latex compound, causing a deposit (wet gel) to accumulate on the former. In this work, two aspects of the process were examined, both experimentally and theoretically. For the experimental work, a commercial nitrile latex was used. The motion of formers through a latex dipping tank is intuitively expected to affect the electrolyte diffusion and hence the wet gel growth. This was investigated at laboratory scale with small glass formers moving in a metre-long dip tank. Former velocities ranged from zero to almost 0.2 m s-1. No effect of former lateral movement on wet gel thickness was observed. One obvious explanation is that most of the coagulant diffusion occurs within the wet gel deposit, which provides protection to the diffusive flux. However, the critical zone is just ahead of the coagulating front, where coagulant is present in the liquid compound at concentrations below the level needed for coagulation. A fluid mechanical model was constructed that assumed a uniform fluid flow along the side face of a rectangular former. The model confirmed that for calcium nitrate, the most commonly used coagulant, the effect of movement is very small. In the second investigation, coagulant leakage into the host latex compound during the dwell time was investigated by taking samples during repeat static dips. This experiment was modelled using diffusion theory, focusing on the critical zone just outside the wet gel at the point of former withdrawal. The model and experiment agreed well, both showing a small but definite coagulant leakage that tended towards a plateau concentration. Coagulant leakage from a moving former was also considered, from a theoretical perspective. In this case, the mechanism is advective movement of coagulant from the critical zone into the host compound. In the worst case, where all of this coagulant is swept away, the model suggested that the plateau coagulant concentration could reach an amount that would cause coagulation. Reduced flow in the critical zone (boundary layer) and former shape are factors that would reduce leakage.

A cross-sectional study of poisoning deaths by rubber latex processing acid in Tripura, India

BackgroundRubber latex processing acid poisoning is a frequently encountered phenomenon in Tripura. Formic acid is the preferred choice for coagulating rubber latex in rubber sheet manufacturing units. The objective of this study aimed to assess the epidemiological profile of poisoning deaths by rubber processing acid and to record their autopsy findings. MethodsThis was a 2-year prospective cross-sectional study of deaths by rubber processing acid ingestion, which were brought for postmortem examination in two health care institutes of Tripura. Profiles of the cases were noted from hospital records, family members, Inquest and Challan papers from police and autopsy findings. All data were entered in SPSS software version 23.0, and Pearson's correlation was used to correlate the corrosion of stomach and survival time. ResultsOf 42 cases, 30.90% were between the ages of 41 and 50 years, and the majority were rubber tappers (69.01%). Familial disharmony (52.38%) was the most common cause of acid consumption. Close proximity (within 1 km) of the rubber tapping area to the consumption spots was observed (59.52%). Overall, 97.62% of victims had corrosion of the stomach and congestion of internal organs, and the majority (66.67%) died within 24 h of consumption. ConclusionThis study emphasises the mortality brought on by rubber latex processing formic acid in areas surrounding rubber plantation. The poisoning prevention and control can be strategised in pre-exposure and exposure levels among vulnerable population based upon Information, Education and Communication model.

Desulfurization of H2S‐rich biogas using water scrubbing: Performance in pilot scale scrubber and scale‐up estimation for the concentrated latex factory

AbstractA high about 3% H2S concentration in the biogas produced from concentrated latex process wastewater needs to be removed before further utilization. Water scrubbing is appropriate for the latex factory having plenty of recycled water available. Effects of liquid to gas (L/G) ratios at 0.5–4.0 on H2S removal were tested at 0.06 m3/h biogas flow rate using a pilot scale scrubber. The experimental results were then simulated to validate the scale‐up for the factory, using the process simulation software Aspen HYSYS (HYSYS, Hyprotech and Systems). The highest 99.71% H2S removal was obtained by using L/G ratio 4.0. Interestingly, using L/G ratio 1.0 provided 96.01% H2S removal, which is slightly lower than on using L/G ratio 4.0, but is more practically appropriate for the real volumes of recycled water and biogas generated from the factory. The experimental data agree with the simulations well. The packed column diameter and height were estimated as 0.5 and 3 m for 40 m3/h biogas. Key characteristics of the scrubber effluent were analyzed for further discharge management. The scrubber effluent qualified for direct discharge to the post‐treatment ponds. This work shows that using recycled water for desulfurization of the H2S‐rich biogas is feasible at atmospheric pressure operation.

Browning mechanism in stems of fresh‑cut lettuce

Cellular fluids and latex released from cut cells in the stems of lettuce when it is harvested are the major source of stem browning. In the present study, the browning process of cellular fluids and latex released from romaine and iceberg lettuce stems was investigated. An increase in LOX enzyme and total phenolics also occurs during lettuce stem browning. A decrease in ascorbic acid content, an increase in phenylalanine ammonia lyase and lipoxygenase activity were important factors responsible for the browning. Metabolomic and proteomic analyses indicated that changes in the level of glutathione metabolism, indole alkaloid biosynthesis, oxidative phosphorylation, galactose metabolism were also associated with stem browning. The main causes of stem browning in fresh cut lettuce were comprehensively investigated in the present study and the resulting data provided information that can be used in the study of browning in lettuce and other vegetables during harvest or fresh-cut processing.

ANALISA PRODUKTIVITAS TANAMAN KARET (Havea brasiliensis Muell. Arg) KLON PB 260 DENGAN MENGGUNAKAN BERBAGAI SISTEM SADAP DI AFDELING I KEBUN BANGUN PT. PERKEBUNAN NUSANTARA III

The productivity of rubber plantations is determined by the type of clone, the age of the plant, the level of land suitability, and the exploitation system applied. The exploitation system applied determines the productivity of the rubber plantation because it is related to the use of the skin and the physiological process of latex. Implementing the right exploitation system can ensure high and sustainable productivity. On the other hand, the implementation of the wrong exploitation system can result in low productivity and shorten the economic life of the plant. This study aims to determine the productivity of rubber plants in clone PB 260 with various tapping systems. This research was conducted at Afdeling I Kebun Bangun PT. Perkebunan Nusantara III using a descriptive analysis method by collecting secondary data on the productivity of the PB 260 clone rubber plant from 2016 – 2020. The parameters observed were the production data of the PB 260 clone rubber plant with various tapping systems. The results of this study indicate that the analysis of rubber plant productivity data in 2016 – 2020 had the highest production in 2017 with a planting year of 2009 and the lowest in 2020 with a planting year of 2000. Meanwhile, overall, the highest production was in 2018 and the lowest was in 2016. The factors that influence the high and low productivity are caused by the tapping system that is not permanent or changing, the lack of application of fertilization, the reduction in the number of trees in the field caused by being attacked by JAP (White Root Fungus), KAS (Dry Alur Sadap), Moldy Road, fallen trees due to strong winds, and the age of the plants. The relationship between production and cost of goods/kg is that the greater the production obtained, the lower the cost of goods/kg.

ANALISA PRODUKTIVITAS TANAMAN KARET (Havea brasiliensis Muell. Arg) KLON PB 260 DENGAN MENGGUNAKAN BERBAGAI SISTEM SADAP DI AFDELING I KEBUN BANGUN PT. PERKEBUNAN NUSANTARA III

The productivity of rubber plantations is determined by the type of clone, the age of the plant, the level of land suitability, and the exploitation system applied. The exploitation system applied determines the productivity of the rubber plantation because it is related to the use of the skin and the physiological process of latex. Implementing the right exploitation system can ensure high and sustainable productivity. On the other hand, the implementation of the wrong exploitation system can result in low productivity and shorten the economic life of the plant. This study aims to determine the productivity of rubber plants in clone PB 260 with various tapping systems. This research was conducted at Afdeling I Kebun Bangun PT. Perkebunan Nusantara III using a descriptive analysis method by collecting secondary data on the productivity of the PB 260 clone rubber plant from 2016 – 2020. The parameters observed were the production data of the PB 260 clone rubber plant with various tapping systems. The results of this study indicate that the analysis of rubber plant productivity data in 2016 – 2020 had the highest production in 2017 with a planting year of 2009 and the lowest in 2020 with a planting year of 2000. Meanwhile, overall, the highest production was in 2018 and the lowest was in 2016. The factors that influence the high and low productivity are caused by the tapping system that is not permanent or changing, the lack of application of fertilization, the reduction in the number of trees in the field caused by being attacked by JAP (White Root Fungus), KAS (Dry Alur Sadap), Moldy Road, fallen trees due to strong winds, and the age of the plants. The relationship between production and cost of goods/kg is that the greater the production obtained, the lower the cost of goods/kg.

EFFECT OF AMMONIA AND COD TO SULFATE RATIO ONTREATMENT PERFORMANCE OF CONCENTRATED LATEXPROCESSING WASTEWATER

The objective of this study is to investigate the effects of ammonia nitrogen (NH3-N) and COD to sulfate ratio on the treatment performance of concentrated latex processing wastewater (CLPW). In a batch experiment, the CLPW influent COD, % granular sludge and pH were controlled at about1,200 mg/l, 10% and 7, respectively. The influent NH3-N concentrations varied at 100, 500, 1000 and3000 mg/l, whereas COD/sulfate ratio varied at 0.3, 0.6, 1 and 1.3. Treatment performance was determined by methane generation rate and COD removal efficiency at different NH3-N and COD to sulfate ratio. The NH3-N concentration at 100 and 500 mg/l (which were presented in the real CLPW), did not result in inhibition on methane producing bacteria (MPB), since methane generation was detected. Inhibition on MPB was observed at 1,000 mg/l NH3-N. Strong inhibition occurred at 3,000 mg/l NH3-N, since % COD removal was relatively reduced to 7.98 % and nomethane generation. The COD/sulfate ratio at lower than 0.6 resulted in inhibition of MPB, but in the real CLPW the ratio is normally higher than 0.6. Ammonia concentration higher than 1000 mg/l and COD to Sulfate at lower than 0.6 resulted in inhibition of methane producing bacteria in the treatment of CLPW.

Mechanism of the foaming agent‐assisted microwave drying process on the construction of natural raw rubber network and cross‐linking network

AbstractThe drying process of natural rubber latex significantly affects the structure of the raw rubber network and vulcanizate crosslinking network, resulting in various anti‐aging performances. In the present study, a microwave generator was used as an efficient source of clean energy; potassium oleate was introduced as a foaming agent to increase the porosity and water loss channel of the latex system. Aiming at dehydrating and drying natural rubber latex efficiently, an aging resistant rubber composite was prepared. Meanwhile, the mechanism of the foaming agent‐assisted microwave drying process on the raw rubber network and the cross‐linking network was studied. The experimental results show that the prepared rubber using by this process has higher plastic retention and fluidity. Moreover, it contains more non‐rubber components (e.g. protein and acetone extract) and better network structure of raw rubber and vulcanized rubber. It is found that applying this process increases the tensile product by 13.5% and the retention rate of the tensile product after aging by 15.3 times. This process is important for the development of the rubber industry in the direction of green environmental protection, energy conservation, and high efficiency.

Ultrafine fully vulcanized natural rubber modified by graft-copolymerization with syrene and acrylonitrile monomers

Natural rubber (NR) is known as one of the most important renewable agricultural resources in Thailand. To enhance the NR's value, development and utilization of NR in form of commercial product are necessary. Commercial product based on ultrafine fully vulcanized powdered rubber (UFPR) was widely used in numerous potential applications such as toughening filler or friction modifier in brake pads. However, to meet the required thermal stability of friction modifier in brake pads for replacing the synthetic rubber powder, the modification of NR by graft-copolymerization with styrene (ST) and acrylonitrile (AN) monomer and production into the powder form are promised related to a sustainable economic system. From the beginning, the grafting process of NR latex was prepared by emulsion graft-copolymerization with ST and AN monomers onto deproteinized natural rubber (DPNR). The results revealed that the grafting with ST/AN monomer content of 80/20 weight ratio possessed the monomer conversion and grafting efficiency as high as 71% and 63%, respectively. After that, the obtained DPNR-g-(PS-co-PAN) was radiated by an electron beam, followed by a spray drying process to produce ultrafine fully vulcanized powdered natural rubber (UFPNR-g-(PS-co-PAN)). The SEM micrographs showed that UFPNR particles are relatively spherical with a particle size of approximately 3.56 μm. Furthermore, thermal stability i.e., degradation temperature at 5 % weight loss (Td5) of the modified UFPNR was substantially improved from 334 to 350°C. The benefits of these advanced technologies give an obvious conclusion proved that the modified UFPNR is a good candidate for replacing a commercial synthetic powdered rubber and suitable for using as toughening filler in high performance polymer composite applications.

Design Strategy for Vulcanization Accelerator of Diphenylguanidine/Cyclodextrin Inclusion Complex for Natural Rubber Latex Foam with Enhancing Performance.

Vulcanization is an essential process to obtain high-performance rubber products. Diphenylguanidine (DPG) is often used as the secondary accelerator in the vulcanization process of natural rubber (NR) latex. However, DPG would make NR latex emulsion exhibit gelation, resulting in the negative vulcanization efficiency. In addition, exposure to DPG might lead to some physiological diseases during the production process of DPG doped NR latex. Hydroxypropyl-β-cyclodextrin (HP-β-CD) with the hydrophobic interior and hydrophilic exterior has the advantages of good water solubility, high bioavailability, reliable stability, and low toxicity. In this study, the inclusion complex of diphenylguanidine-hydroxypropyl-β-cyclodextrin (DPG-HP-β-CD) is prepared by ball milling with a host-guest molar ratio of 1 : 1, which has also been applied to the foaming process of NR latex. The mechanical properties of DPG-HP-β-CD inclusion complex/natural rubber latex foam (DPG-HP-β-CD/NRLF) have been significantly improved, including the tensile strength, elongation at break, hardness, compression set, resilience, and antiaging performance. Further, the usage of DPG has been reduced, leading to the reduction of toxicity and environmental hazards.

Treatment of Rubber Latex Processing Wastewater using Activated Carbon Derived from Palm Kernel Shell

Treatment of Rubber Latex Processing Wastewater using Activated Carbon Derived from Palm Kernel Shell

Morphological characterization of gram-positive and gram-negative bacteria from treated latex processing wastewater

A preliminary morphological screening and isolation of bacterial colony from latex industrial wastewater was carried out. Bacteria colonies from latex processing wastewater were isolated from a local latex processing industry. It was found that 17 bacterial isolates had been purified grown on nutrient agar under 35˚C. The colonies were then purified and morphologically indicated via Gram staining and motility test. After morphological observation, it was identified that out of 17 isolates, 9 isolates were Gram positive and 8 isolates were Gram negative. There are 11 out of 17 colonies were rod-shaped bacterial colonies, while the other 6 colonies were cocci-shaped bacteria. There were 11 colonies of gliding bacteria, three colonies were non-motile bacteria and the other three colonies were flagellated bacteria. This study is only limited to morphological observation as the main aim of this study was to investigate the potential occurrence of viable growth in treated latex processing wastewater. The bacterial colonies were classified base on their morphological properties shown. This study has classified several genera such as Staphylococcus, Escherichia, Thiobacillus, Arthrobacter and other Genus. The growth curve of 17 isolates studied and the chemical oxygen demand were determined.

Retraction Note to: Secondary treated wastewater of latex processing: reusing for irrigation or treatment by membrane filtration

Retraction Note to: Secondary treated wastewater of latex processing: reusing for irrigation or treatment by membrane filtration