Core points
- Melt blown PP cartridges are self-bonding depth media: polypropylene micro-fibres are thermally entangled without any chemical binder, so nothing can migrate into the filtrate.
- The gradient-density structure (open outside, dense inside) is what gives high dirt-holding capacity and low pressure drop at the same time.
- Nominal ratings commonly run 1-100 um; absolute-rated melt blown grades reach 99.9% efficiency at ratings down to about 0.2 um in specialised versions [source: Filter Cartridge Tech absolute-rating datasheet].
- Pure PP gives broad chemical compatibility - strong acids, alkalis and most solvents.
- Surface finishes (smooth, orange-peel, embossed, dotted, slit, matte) are available to tune flow and release behaviour.
How the gradient structure is made
Melt blown production melts polypropylene granules, spins the polymer into micro-fibres, and deposits them onto a forming mandrel. Because fibres self-bond as they cool - no adhesive is involved - the cartridge is a single continuous PP structure. Density is controlled during deposition: the outside is laid open, the inside progressively tighter. Contaminant capture then happens through the whole wall, classified by density, instead of building a blocking cake on one surface.
Nominal vs absolute ratings - why it matters commercially
Most melt blown cartridges are sold with nominal ratings: useful for comparing media, but not a tested guarantee. Where a process needs a guaranteed cut-off - final filtration before a membrane, for example - absolute-rated grades are tested to a defined efficiency (commonly 99.9% at the stated micron size) [source: Filter Cartridge Tech absolute-rating datasheet]. BROLERN publishes nominal ratings for its melt blown line; absolute-rated validation is available per project.
Typical parameters (BROLERN production range)
| Media | 100% polypropylene micro-fibre, thermally self-bonded |
|---|---|
| Filtration rating | 1-150 um nominal |
| Lengths | 5-60 inch (127-1524 mm) |
| OD / ID | 60 / 62 / 114 mm OD; 28-110 mm ID |
| Surfaces | Smooth, orange-peel, embossed, dotted, slit, matte |
| Binders | None - no wetting agents, no antistatic additives |
FAQ
Why does a gradient-density cartridge last longer?
Density rises from surface to core, so the whole depth shares the dirt load instead of a surface skin; there are no blind spots restricting flow.
Nominal or absolute?
Use nominal for general clarification; specify absolute-rated grades when downstream equipment needs a guaranteed cut-off.
Chemical compatibility?
Pure PP covers most acids, alkalis and organic solvents - confirm edge cases against a resistance chart for your liquid.
Conclusion
For economical depth filtration of liquids, gradient-density melt blown PP remains the default. Specify rating by duty, stage coarse-to-fine for dirty feeds, and reserve absolute-rated or membrane grades for final polishing. BROLERN supplies the full range from 1 to 150 um with surface and length options for OEM lines.