Technology · Impregnation
Nonwoven impregnation: oil loading and uniformity
Loading a nonwoven is not the same as saturating it. A usable impregnated cloth carries a defined amount of liquid, distributed predictably enough that the cleaning interface behaves the same at the start and end of a roll.
Key takeaways
- Loading must be defined by mass per area or mass per mass, with a tolerance and a method.
- Substrate pore structure and liquid rheology determine pickup and retention.
- Average loading and distribution uniformity are separate acceptance criteria.
- Winding, storage and time can change distribution after the loading line.
- Every loading claim should be traceable to a measurement on the supplied lot.
1. Start with the substrate, not the liquid
A nonwoven is a network of fibres with a pore structure that varies at a scale smaller than the cloth. Liquid enters that network through capillary action, wetting and, in some processes, applied pressure. The same liquid will load differently into a spunlace web, a needlepunched web and a thermally bonded web, even when basis weight is identical.
The parameters that matter most are fibre chemistry, fibre diameter, web density, pore size distribution, thickness and the bonding method. A denser web may hold less liquid by volume but release it more slowly. An open web may pick up quickly but drain or migrate during winding. Neither is automatically better; the cleaning method determines which behaviour is wanted.
2. Define loading precisely
Two definitions are common, and they produce different numbers:
Loading per area: LA = (mwet − mdry) / A, expressed as g/m².
Loading per substrate mass: LM = (mwet − mdry) / mdry × 100, expressed as %.
If a specification says "20% loading" without stating the basis, two suppliers can deliver materially different liquid doses. The specification should state the definition, the target, the tolerance, the sample size and the conditioning before weighing.
Weighing is simple but sensitive to evaporation. The time between removal from the sealed pack and weighing, the ambient temperature and the balance resolution all affect the result. For volatile liquids, a sealed weighing method or a controlled enclosure is preferable.
3. Application methods and their behaviour
| Method | How it works | Strengths | Watch-outs |
|---|---|---|---|
| Dip and squeeze | Web passes through a bath, then through nip rolls | High throughput, full wetting | Pickup depends on nip pressure and web speed; bath contamination and evaporation |
| Kiss / gravure transfer | A metered film transfers from a roller to the web | Controlled add-on, suitable for higher viscosity | Web tension and nip uniformity affect transfer; pattern may not penetrate thickness |
| Spray | Liquid is atomised onto the web | Good for low add-on and wide webs | Overspray, drift, edge build-up and nozzle wear |
| Metered slot or coating | A pump delivers a defined volume across the width | Repeatable add-on, closed system | Requires compatible rheology; cleaning and changeover are more complex |
4. Uniformity across width and along length
Uniformity is not the same as a correct average. A roll can average exactly to target while being wetter in the centre and drier at the edges, or wetter at the outside of the wound roll due to migration. Both conditions change the cleaning result.
A practical sampling plan takes strips across the web width at intervals along the roll, measures each sample separately, and reports mean, range and standard deviation or coefficient of variation. Acceptance limits are set against the consequence of variation: a wide cleaning roll with a tight downstream residue target needs tighter limits than a general-purpose wipe.
Edges deserve separate attention. Slitting exposes new fibre ends, edge density differs from the centre, and drying at the edges can be faster. Edge loading can be higher or lower than the body of the web depending on the method.
5. Drying and equilibration
If the process uses a carried solvent or water, drying removes part of the liquid and concentrates the functional component. The drying profile must be controlled so that the web surface does not dry faster than the core, which can cause migration of the functional component to the surface. Over-drying can degrade the substrate or the liquid.
After drying, the roll should be equilibrated before sealing. Winding a warm roll and sealing it immediately can produce condensation, a changed distribution or a pressure-driven migration over time. The correct equilibration temperature and time are process-specific and should be established during development.
6. Migration after production
Impregnated rolls are not static. Liquid can move within the web under gravity, winding tension and temperature gradients. The visible symptom is a loading difference between the outside and inside of the roll, or between the top and bottom of a stored roll. Packaging, storage orientation and time all influence the result.
Control measures include a compatible release liner, controlled winding tension, a defined storage orientation, temperature limits and a shelf life supported by stability data. Where migration is a known risk, the specification should include a re-check at the end of shelf life, not only at release.
7. Measurement and release
- Gravimetric loading on conditioned samples, using the defined basis.
- Distribution sampling across width and along roll, with acceptance limits.
- Liquid identity confirmed against the approved grade.
- Substrate identity and basis weight checked against the specification.
- Visual and packaging inspection for contamination, sealing and labelling.
- Lot records retained so that a delivered roll can be traced to its production data.
A note on claims
Loading and uniformity values are meaningful only when tied to a substrate, a liquid and a measurement method. A generic statement such as "uniformly impregnated" is not a specification. Ask for the sampling plan and the acceptance limits.
Related guidance
See factors affecting oil loading and uniformity for a production-oriented checklist, and material compatibility for the interaction between the liquid, the substrate and the roll.
Frequently asked questions
What does "oil loading" mean?
Oil loading is the mass of liquid carried per unit area of substrate, or per unit mass of substrate. The two definitions are not interchangeable, so the specification must state which one applies and include a tolerance and a measurement method.
Why is uniform loading difficult?
Nonwoven webs vary in density and pore structure. Liquid moves preferentially into lower-density regions, and edges can dry or wick differently from the centre. Winding tension and storage can also redistribute liquid after production.
Can loading be measured non-destructively?
Gravimetric measurement is destructive but direct. Non-destructive methods such as moisture or oil sensors can be used for trending, but they require calibration against gravimetric results for the specific substrate and liquid.
Does a heavier cloth hold more liquid?
Not necessarily. Basis weight influences capacity, but pore structure, fibre surface energy, liquid viscosity and the impregnation method determine how much liquid is retained and how it is distributed.
How is uniformity specified?
By a sampling plan across the web width and along the roll, with acceptance limits. A single average value does not demonstrate uniformity, and a single sample cannot represent a roll.
Next step
Define loading and uniformity with our process team
Send the substrate, liquid, target loading and cleaning method. We will confirm the measurement approach and the trial plan.
Typical response time: one business day. Your details stay confidential.