{"id":13420,"date":"2026-09-11T10:08:43","date_gmt":"2026-09-11T08:08:43","guid":{"rendered":"https:\/\/www.gremco.de\/?p=13420"},"modified":"2026-09-11T10:08:43","modified_gmt":"2026-09-11T08:08:43","slug":"pet-support-braids-for-hollow-fiber-membranes","status":"publish","type":"post","link":"https:\/\/www.gremco.de\/en\/magazin\/pet-support-braids-for-hollow-fiber-membranes\/","title":{"rendered":"PET Support Braids for Hollow Fiber Membranes"},"content":{"rendered":"
Clean drinking water and the efficient treatment of municipal and industrial wastewater are among the most pressing technical challenges of our time. A key technology for both areas is the PVDF hollow fiber membrane<\/strong> \u2014 an ultra-fine, hollow polymer fiber used as a filtration element in water and wastewater plants worldwide. What many outside the industry don’t realise: these high-performance membranes need a mechanical backbone. Inside every fiber sits a braided or knitted PET support braid<\/strong> that makes the membrane operationally robust in the first place. This article explains how a hollow fiber membrane is built, where it is used, how it is manufactured from the PET support braid and a PVDF coating \u2014 and what really matters when specifying the substrate.<\/p>\n A hollow fiber membrane<\/strong> is essentially a very thin, hollow polymer fiber \u2014 comparable to a micro-spaghetti whose wall has pores so fine that only water passes through, while bacteria, viruses, suspended solids and sludge flocs are retained. A single membrane module contains several thousand of these fibers bundled in parallel, delivering 50 to 500 m\u00b2 of filter area in a fraction of the footprint of alternative filtration technologies.<\/p>\n Functionally, a PVDF hollow fiber membrane consists of three layers:<\/p>\n Figure 1: Cross-section of a PVDF hollow fiber membrane. Three layers \u2014 PVDF separation layer on the outside, PET support braid in the middle, hollow lumen inside for the filtered permeate.<\/em><\/p>\n The filtration flow runs from outside to inside: the raw water surrounds the fiber, and a slight vacuum (in submerged operation, typical for MBR) or an applied pressure (in pressurised operation, typical for UF) drives the water through the PVDF layer and the PET braid into the lumen.<\/p>\n Without a mechanical substrate, a PVDF layer only a few micrometres thick would be unusable in service. The real-world loads are substantial \u2014 and they don’t come primarily from filtration itself:<\/p>\n Rather than loading all of this onto the thin separation layer alone, the PET support braid<\/strong> takes the mechanical function \u2014 the PVDF layer only has to separate, not carry. This extends the service life of a typical MBR module from a few months to more than ten years.<\/p>\n By far the largest application for PVDF hollow fiber membranes is the membrane bioreactor<\/strong> in municipal and industrial wastewater treatment. The MBR is the central growth technology in the water market \u2014 driven by the new EU Urban Wastewater Directive, the push for water reuse in dry regions, and rising requirements for effluent quality.<\/p>\n The difference from a conventional plant is fundamental: in an MBR, the hollow fiber membrane replaces the secondary clarifier and usually the tertiary filtration as well. What used to be passive sedimentation in two large tanks becomes active filtration in a single tank \u2014 directly within the biological process.<\/p>\n Figure 2: Comparison of conventional treatment plant (five stages) versus membrane bioreactor (three stages). The hollow fiber membrane replaces the secondary clarifier and usually the tertiary stage.<\/em><\/p>\n Concretely:<\/p>\n Beyond MBR, PVDF hollow fiber membranes serve a range of further applications in which a PET-supported substrate is mechanically advantageous:<\/p>\n PET support braids for hollow fiber membranes are a classic B2B precursor: no end consumer ever sees them, but the downstream industry can’t function without them. The value chain runs through five stages:<\/p>\n Figure 3: Value chain of the PVDF hollow fiber membrane. GREMCO supplies stage 2 (support braid) to membrane module manufacturers in stages 3 and 4.<\/em><\/p>\n The direct customer for the PET support braid is the membrane module manufacturer<\/strong> at stage 3 \u2014 not the plant builder, the engineering consultancy, or the treatment plant operator. These players buy finished modules, not raw materials.<\/p>\n How does a braided PET tube and a dense PVDF coating turn into a permeable membrane with sub-micrometre pores? A coating is normally a barrier layer \u2014 but for a membrane it has to be controllably porous. The answer lies in a chemical-physical process called phase inversion<\/strong> (NIPS \u2014 Non-solvent Induced Phase Separation<\/em>).<\/p>\n Figure 4: Continuous PVDF hollow fiber membrane production line. Seven stations from bare PET support braid to finished porous membrane.<\/em><\/p>\n On a continuous line, the support braid runs through seven stations:<\/p>\n Before the bath, the coating is a homogeneous solution<\/strong>: PVDF molecules evenly distributed in the solvent, no pores. If this layer simply dried, it would form a massive, impermeable polymer film.<\/p>\n In the water bath, a double exchange<\/strong> then occurs: water diffuses into the layer while solvent diffuses outward. Water and the solvent (e.g. NMP) are miscible and exchange with each other. PVDF, however, is not soluble in water \u2014 once enough water has entered the layer, the chemical equilibrium tips and the polymer precipitates.<\/p>\n Because the solvent continuously diffuses outward, the PVDF doesn’t form a dense gel but a branched, three-dimensional network. The places that used to hold solvent become micropores \u2014 typically in the range 0.01 to 0.1 \u00b5m.<\/p>\n Figure 5: Phase inversion over time. The double exchange water <> solvent transforms the homogeneous PVDF solution into a porous polymer network with micropores of 0.01\u20130.1 \u00b5m.<\/em><\/p>\n The quality of the PET support braid directly impacts the finished module. Coating quality, pin-hole rate, TMP values and ultimately module lifetime depend measurably on the specifications of the substrate. The following parameters are typical for high-quality PET support braids in the membrane industry:<\/p>\n <\/p>\n Each of these specifications relates directly to the phase inversion process or to module operation:<\/p>\n GREMCO GmbH<\/strong> is an owner-managed manufacturer of technical protective sleeves based in Augsburg, Germany, with more than 40 years of experience in the automotive and aerospace industries. Our core business is braided and knitted textile sleeves under the FITCOFLEX\u00ae<\/strong> brand \u2014 used in wiring harnesses, cable management systems and high-temperature applications. Volumes are correspondingly large: several tonnes of raw material per year, millions of metres of textile sleeving.<\/p>\n Out of this weaving and braiding expertise, we have developed in recent years a dedicated product line of PET support braids for hollow fiber membranes<\/strong>. Outer diameters between 1.30 and 2.15 mm, tensile strength > 400 N, roundness < 0.05 mm, controlled surface profile with no protruding fibres. Quality management at IATF 16949<\/strong> and ISO 9001:2015<\/strong> level \u2014 the standard we bring with us from the automotive business.<\/p>\n What this heritage is worth to the membrane industry: process control is tighter in automotive, FAI and PPAP documentation is daily business, statistical process monitoring is standard. We bring exactly this discipline to the membrane market \u2014 where many suppliers historically come from chemistry or water and still have to establish automotive-grade quality.<\/p>\n An ultra-thin, hollow polymer fiber with an outer separation layer of polyvinylidene fluoride (PVDF) featuring micropores of 0.01 to 0.1 \u00b5m. It is bundled into membrane modules and deployed in water and wastewater treatment \u2014 most commonly in membrane bioreactors (MBR).<\/p>\n MBR stands for membrane bioreactor. A wastewater treatment process in which the biological stage (aeration tank) is combined with membrane filtration, eliminating the secondary clarifier. Benefits: 30 to 50 % smaller footprint, higher MLSS, and reuse-ready effluent quality.<\/p>\n It provides the mechanical strength of the fiber. While the PVDF layer handles filtration, the PET support braid ensures the fiber survives pressure cycles, backpulse cleaning and chemical cleaning regimes. Tensile strengths > 400 N are achievable with a high-quality support substrate.<\/p>\n Through phase inversion (NIPS). A PVDF polymer solution is applied to the braid, which then runs through a water bath. In the bath, water and solvent exchange, the polymer precipitates and forms a porous, three-dimensional network with micropores in the 0.01 to 0.1 \u00b5m range.<\/p>\n Membrane module manufacturers who apply the PVDF coating themselves and assemble the result into finished filtration modules. Plant builders, engineering consultancies or treatment plant operators do not buy raw materials \u2014 they procure finished modules.<\/p>\n Outer diameter (typically 1.30 to 2.15 mm), roundness (< 0.05 mm), tensile strength (typically > 400 N), sizing content (< 0.62 %), number of protruding fibres (< 5 per metre) and wall thickness (< 0.325 mm). Each of these parameters has a direct effect on coating quality, pin-hole rate and the lifetime of the finished module.<\/p>\n Besides PET, polyamide (PA), fibreglass and aramid are options depending on the requirement. PET is the standard for most hollow fiber applications while aramid and fibreglass are used in specialty cases with higher temperature or chemical loads.<\/p>\n Are you developing a new hollow fiber membrane or qualifying a second-source supplier for PET support braids? Our technical team can advise on selection, geometry and specification \u2014 and provide samples for coating trials. We supply membrane module manufacturers worldwide across the UF, MF and MBR segments.<\/p>\n <\/p>\n GREMCO GmbH \u00b7 Sterzinger Str. 6 \u00b7 86165 Augsburg \u00b7 Germany<\/p>\n Phone: +49 (0)821 27 26 3-0<\/p>\n Email: info@gremco.de<\/p>\n Web: www.gremco.de<\/p>\n <\/p>\nWhat is a PVDF hollow fiber membrane and how does it work?<\/h2>\n
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Why does a hollow fiber membrane need a PET support braid?<\/h2>\n
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Applications of PVDF hollow fiber membranes<\/h2>\n
Membrane bioreactors (MBR) \u2014 the most important application<\/h3>\n
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Further applications of PVDF hollow fiber technology<\/h3>\n
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The value chain \u2014 where the PET support braid sits<\/h2>\n
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From support braid to membrane: the phase inversion process<\/h2>\n
The production line step by step<\/h3>\n
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What happens chemically in the water bath<\/h3>\n
<\/em><\/p>\nSpecification requirements for PET support braids<\/h2>\n
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\n Parameter<\/strong><\/td>\n Value range<\/strong><\/td>\n Impact on the process<\/strong><\/td>\n<\/tr>\n<\/thead>\n\n \n Outer diameter (OD)<\/strong><\/td>\n 1.30 \u2013 2.15 mm<\/td>\n Module geometry, packing density<\/td>\n<\/tr>\n \n Inner diameter (ID)<\/strong><\/td>\n > 0.65 \u2013 > 1.25 mm<\/td>\n Permeate flow, pressure drop<\/td>\n<\/tr>\n \n Wall thickness<\/strong><\/td>\n < 0.325 mm<\/td>\n TMP, energy consumption<\/td>\n<\/tr>\n \n Roundness<\/strong><\/td>\n < 0.05 mm<\/td>\n Uniform PVDF coating thickness<\/td>\n<\/tr>\n \n Tensile strength<\/strong><\/td>\n > 400 N<\/td>\n Module lifetime, backpulse resistance<\/td>\n<\/tr>\n \n Linear shrinkage<\/strong><\/td>\n 0.09 \u2013 1.00 %<\/td>\n Form and dimensional stability<\/td>\n<\/tr>\n \n Sizing content<\/strong><\/td>\n < 0.62 %<\/td>\n PVDF coating adhesion<\/td>\n<\/tr>\n \n Protruding fibres<\/strong><\/td>\n < 5 \/ m, ? 2 mm<\/td>\n Pin-hole rate<\/td>\n<\/tr>\n \n Density (picks per inch)<\/strong><\/td>\n 23 \u2013 49 PPI<\/td>\n Braiding geometry, substrate stability<\/td>\n<\/tr>\n \n Standard colour<\/strong><\/td>\n White<\/td>\n Visual defect detection<\/td>\n<\/tr>\n \n Material alternatives<\/strong><\/td>\n Polyamide, fibreglass, aramid<\/td>\n Specialty applications<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n \n
GREMCO PET Support Braids: Automotive Precision for the Membrane Industry<\/h2>\n
Frequently Asked Questions (FAQ)<\/h2>\n
What is a PVDF hollow fiber membrane?<\/h3>\n
What does MBR stand for?<\/h3>\n
What function does the PET support braid serve in a hollow fiber membrane?<\/h3>\n
How is the PVDF coating applied to the PET support braid?<\/h3>\n
Who buys PET support braids for hollow fiber membranes?<\/h3>\n
Which specifications matter most for a PET support braid?<\/h3>\n
Which materials besides PET are possible for support braids?<\/h3>\n
Request technical consultation & samples<\/a><\/h2>\n
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\n Request technical consultation now<\/strong><\/p>\n