Technical guide · Impregnation
Factors affecting oil loading and uniformity in impregnated nonwovens
Loading is a result of substrate, liquid and process acting together. Changing one of them changes the others, which is why an impregnated product must be specified by method as well as by target value.
Key takeaways
- Substrate pore structure sets the capacity; liquid rheology sets how fast and how evenly it fills.
- Higher line speed and shorter dwell time reduce pickup and can increase variation.
- Drying redistributes the functional component unless the profile is controlled.
- Winding and storage can change distribution after the loading line.
- Uniformity needs a sampling plan across width and along the roll.
1. Substrate structure
A nonwoven is not a uniform sponge. Fibre diameter, web density, pore size distribution, thickness and bonding method all vary across the web and between lots. Liquid fills the smaller pores first at low pressure and the larger pores when pressure or dwell time increases, so the same nominal substrate can produce different pickup under different conditions.
Basis weight is a useful starting point but not a predictor of loading. A denser web may hold less liquid per unit volume yet release it more slowly, while an open web may pick up quickly and then drain or migrate during winding. Both behaviours can be appropriate depending on the cleaning method.
2. Liquid properties
- Viscosity: higher viscosity slows capillary penetration and requires longer dwell or higher pressure.
- Surface tension: affects wetting of hydrophobic fibres and the risk of unwetted patches.
- Volatility: fast-evaporating liquids change loading during the run and after opening the package.
- Temperature: changes viscosity and evaporation rate at the applicator.
- Compatibility: interaction with the fibre and any binder can alter strength or release behaviour.
These properties should be documented for the approved grade. A substitution of "equivalent" liquid without revalidation is a common cause of loading drift.
3. Application method
| Method | Strength | Main control point |
|---|---|---|
| Dip and squeeze | Simple, high throughput | Nip pressure and liquid level |
| Roll coating | Controlled film thickness | Roll speed ratio and gap |
| Spray | Localised or patterned loading | Flow rate, overlap and atomisation |
| Knife / slot | Consistent film on flat webs | Gap, web tension and viscosity |
Line speed interacts with every method. A process that reaches target loading at low speed may fall short at production speed because dwell time is shorter and the liquid has less time to penetrate.
4. Drying and migration
If the liquid is applied with a carrier, drying removes the carrier and concentrates the functional component. Fast surface drying can pull the component toward the surface, leaving the core different from the face. Over-drying can degrade the substrate or the liquid and change how the cloth releases contamination.
After drying, the roll should be equilibrated before sealing. Winding a warm roll and sealing it immediately can cause condensation and redistribution. Winding tension also drives liquid movement: a tightly wound roll can show a wetter outside diameter after storage.
5. Measurement and acceptance
Gravimetric measurement remains the reference method. Weigh conditioned samples before and after loading, or extract the liquid and weigh it, using a defined basis such as grams per square metre or percent of substrate mass. Report the method, because the two bases are not interchangeable.
Uniformity requires multiple samples across the web width and along the roll. Report mean, range and a dispersion measure, and state acceptance limits. A single sample cannot represent a roll, and an average can hide a dry edge or a wet centre. See nonwoven impregnation for the underlying process.
Specify the method, not only the number
"20 g/m²" is not a complete specification. It needs the measurement basis, tolerance, sampling plan, liquid grade and application method. Without those, two suppliers can both meet the number and deliver different products.
Frequently asked questions
Why does the same product grade give different loading on different lots?
Substrate basis weight, web density and pore structure vary within manufacturing tolerances, and the liquid pickup responds to those variations. A loading specification must therefore include a tolerance and a sampling plan, not only a target value.
Does higher loading always mean better cleaning?
No. Excess liquid can increase transfer, extend drying time and make the cloth harder to handle. Loading should be set against the cleaning function and the residue limit, not maximised.
What causes edge-to-centre variation?
The web edges differ in density, the slitting process exposes new fibre ends, and drying or wicking can be faster at the edges. Application method and web tension also distribute liquid unevenly.
How should uniformity be reported?
Report the sampling positions, the number of samples, the mean, the range and a dispersion measure such as standard deviation or coefficient of variation. A single average value does not demonstrate uniformity.
Next step
Set a loading specification that can be checked
Send the substrate, liquid grade, target loading and measurement basis. We will confirm the sampling plan and the process window.
Typical response time: one business day. Your details stay confidential.