PET-Support Braids

PET Support Braids for Hollow Fiber Membranes

PET Support Braids for PVDF Hollow Fiber Membranes: Structure, Function, and Manufacturing

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 — 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 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 — and what really matters when specifying the substrate.

What is a PVDF hollow fiber membrane and how does it work?

A hollow fiber membrane is essentially a very thin, hollow polymer fiber — 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 m² of filter area in a fraction of the footprint of alternative filtration technologies.

Functionally, a PVDF hollow fiber membrane consists of three layers:

Figure 1: Cross-section of a PVDF hollow fiber membrane. Three layers — PVDF separation layer on the outside, PET support braid in the middle, hollow lumen inside for the filtered permeate.

• Outside — the PVDF separation layer. A few micrometres thin coating of polyvinylidene fluoride (PVDF), with micropores of 0.01 to 0.1 µm. This is where the actual filtration happens — the pore size corresponds to typical ultrafiltration and microfiltration values (UF/MF) and reliably retains even viruses.
• Middle — the PET support braid. A braided or knitted tube of polyethylene terephthalate monofilaments with an outer diameter of typically 1.30 to 2.15 mm. It gives the fiber the mechanical strength it would otherwise lack under operating pressure or during backpulse cleaning. An unsupported PVDF fiber rarely exceeds 80 to 100 N of tensile strength, while a high-quality support braid enables > 400 N.
• Inside — the hollow lumen. This is where the filtered water — the so-called permeate — collects and is drawn out centrally from the module.

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.

Why does a hollow fiber membrane need a PET support braid?

Without a mechanical substrate, a PVDF layer only a few micrometres thick would be unusable in service. The real-world loads are substantial — and they don’t come primarily from filtration itself:

• Pressure-cycling. In MBR operation, the membrane constantly alternates between filtration (vacuum from the inside) and backpulse (pressure pulse from inside to outside to dislodge deposits). These load cycles would destroy an unsupported fiber within weeks.
• Hydraulic shear forces. The aeration typically used in the bioreactor (to mix the sludge and clean the membrane) exposes the fibers to intense flow.
• Chemical cleaning. CIP and CEB cycles using hypochlorite or acid attack the mechanical structure. A high-quality support substrate maintains its geometry over thousands of cleaning cycles.
• Module handling. Installation, maintenance and module replacement subject the fiber bundles to mechanical stress.

Rather than loading all of this onto the thin separation layer alone, the PET support braid takes the mechanical function — 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.

Applications of PVDF hollow fiber membranes

Membrane bioreactors (MBR) — the most important application

By far the largest application for PVDF hollow fiber membranes is the membrane bioreactor in municipal and industrial wastewater treatment. The MBR is the central growth technology in the water market — driven by the new EU Urban Wastewater Directive, the push for water reuse in dry regions, and rising requirements for effluent quality.

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 — directly within the biological process.

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.

Concretely:

• 30 to 50 % smaller footprint than a conventional plant of the same capacity.
• Better effluent quality. The pore size of 0.01 to 0.1 µm reliably retains bacteria and viruses in addition to suspended solids.
• Higher MLSS in the bioreactor (typically 8 to 15 g/L versus 3 to 5 g/L in conventional plants) — boosting biomass performance.
• Reuse-ready effluent. Water quality is good enough for irrigation, industrial processes, or pretreatment for reverse osmosis.

Further applications of PVDF hollow fiber technology

Beyond MBR, PVDF hollow fiber membranes serve a range of further applications in which a PET-supported substrate is mechanically advantageous:

• Tertiary filtration. Polishing stage after a conventional treatment plant, before discharge into sensitive waters or before reuse applications.
• Anaerobic MBR (AnMBR). For high-strength industrial wastewater from food and beverage, pharmaceutical and brewery operations — with biogas recovery as a side benefit.
• Drinking water ultrafiltration. UF of surface water or groundwater for potable supply, especially under highly variable raw water quality.
• RO pretreatment. Protecting the expensive RO membrane from fouling, particularly in seawater desalination.
• Water reuse. Treating wastewater to industrial, agricultural or indirect potable quality. A growth topic in the Mediterranean, Middle East, and parts of Asia.

The value chain — where the PET support braid sits

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:

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.

1. Raw yarn. PET monofilaments, typically from specialised yarn spinners.
2. Support braid. Weaving and braiding operations (such as GREMCO) braid or knit the precise, thin tube from these filaments.
3. PVDF coating. The membrane module manufacturer coats the braid with PVDF using phase inversion.
4. Module build. Several thousand hollow fibers are bundled into a module, potted with epoxy at the ends, and fitted into a housing.
5. MBR or UF plant. The finished module is installed by the plant builder in a water or wastewater facility.

The direct customer for the PET support braid is the membrane module manufacturer at stage 3 — not the plant builder, the engineering consultancy, or the treatment plant operator. These players buy finished modules, not raw materials.

From support braid to membrane: the phase inversion process

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 — but for a membrane it has to be controllably porous. The answer lies in a chemical-physical process called phase inversion (NIPS — Non-solvent Induced Phase Separation).

The production line step by step

Figure 4: Continuous PVDF hollow fiber membrane production line. Seven stations from bare PET support braid to finished porous membrane.

On a continuous line, the support braid runs through seven stations:

1. PET reel. The bare support braid runs continuously off the supply reel.
2. Coating die. A viscous polymer solution — the “dope” — is applied uniformly to the braid. The dope is PVDF dissolved in a polar aprotic solvent (typically NMP, DMAc or DMF), often with pore formers such as PVP or PEG.
3. Air gap. A short stretch in air during which a small amount of solvent evaporates and humidity begins to initiate phase inversion at the surface. This significantly shapes the final skin structure of the membrane.
4. Coagulation bath. The key step. The dope-coated braid is submerged in a water bath — details below.
5. Multiple rinse baths remove residual solvent and additives.
6. Often with a glycerine treatment that keeps the pores open during drying.
7. Take-up. The finished hollow fiber is wound onto large spools and later assembled into modules.

What happens chemically in the water bath

Before the bath, the coating is a homogeneous solution: PVDF molecules evenly distributed in the solvent, no pores. If this layer simply dried, it would form a massive, impermeable polymer film.

In the water bath, a double exchange 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 — once enough water has entered the layer, the chemical equilibrium tips and the polymer precipitates.

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 — typically in the range 0.01 to 0.1 µm.

Figure 5: Phase inversion over time. The double exchange water <> solvent transforms the homogeneous PVDF solution into a porous polymer network with micropores of 0.01–0.1 µm.

Specification requirements for PET support braids

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:

Each of these specifications relates directly to the phase inversion process or to module operation:

• High roundness ensures a uniform dope coating thickness. Uneven coating = uneven phase inversion = inconsistent pore structure and local weak spots.
• Low sizing content is the prerequisite for good adhesion of the PVDF layer. Coating delamination in the bath is one of the worst quality issues in membrane production — direct scrap.
• No protruding fibres prevents “spikes” where the coating ruptures when entering the coagulation bath. A direct lever on the pin-hole rate.
• High tensile strength secures long module lifetime. Membranes more often fail mechanically (backpulse, cleaning cycles, handling) than through filtration failure.
• Low wall thickness reduces TMP (transmembrane pressure) in the finished module — and with it energy consumption per m³ of permeate.

GREMCO PET Support Braids: Automotive Precision for the Membrane Industry

GREMCO GmbH 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® brand — 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.

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. 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 and ISO 9001:2015 level — the standard we bring with us from the automotive business.

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 — where many suppliers historically come from chemistry or water and still have to establish automotive-grade quality.

Frequently Asked Questions (FAQ)

What is a PVDF hollow fiber membrane?

An ultra-thin, hollow polymer fiber with an outer separation layer of polyvinylidene fluoride (PVDF) featuring micropores of 0.01 to 0.1 µm. It is bundled into membrane modules and deployed in water and wastewater treatment — most commonly in membrane bioreactors (MBR).

What does MBR stand for?

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.

What function does the PET support braid serve in a hollow fiber membrane?

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.

How is the PVDF coating applied to the PET support braid?

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 µm range.

Who buys PET support braids for hollow fiber membranes?

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 — they procure finished modules.

Which specifications matter most for a PET support braid?

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.

Which materials besides PET are possible for support braids?

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.

Request technical consultation & samples

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 — and provide samples for coating trials. We supply membrane module manufacturers worldwide across the UF, MF and MBR segments.