Filter papers Flashcards
Which categories of filter paper do you know? Please give at least three examples and name a few products!
Industrial applications
- automotive filters
-air conditioning
Food applications
- Tea bags
-coffee filters
Laboratory filters
What fibres can be used to produce filter papers (3-4 examples)?
Organic fibre material:
- Coarse long fibre. Sometimes specially treated for high porosity
- Fine short fibres
- Special long non wood fibres such as abaca or hemp
Other fibres:
- Synthetic fibres
- Glass fibres
- Carbon fibres
Flash dried pulp
Please give a brief explanation of the pulp flash drying process! What are advantages and disadvanges compared to sheet dring?
No paper web is formed. The pulp is thickened in a press device and then shredded and flufrfed. Fluffed pulpblown into a stream of hot gas and a series of drying towers. After drying it is cooled.
Advantages + Investment cost of system much lower than sheet drying. + able to dry pulp which would not be strong enough for sheet drying process.
Disadvantages – More energy and power, lower max capacity.
What are important quality parameters for filter papers?
Porosity
Filtration efficiency and dust holding
Pore size and distribution
Please explain two different measurements to measure the pore size and the pore size distribution of a filter paper!
Pore size is measured using the bubble test. The sample is fixed to a bowl and covered with a fluid with known surface tension, air is pressed through the bottom, air pressure at the first bubble indicated the biggest pore.
Pore size distribution is measured using mercury porosimetry. Here mercury is forced into the pores – the pressure required to do so is inversely proportional to the pore size.
Which filtration mechanisms do you know and what does this mean for the filter paper structure?
In depth/volume filtration
- Dust enters the structure of the paper and is trapped between the fibres. The filter is gradually filled up with dust which decreases the permeability meaning the filter needs to be changed
- Entry side of the filter should be more porous than the exit side to allow dust to move into the structure
Surface/cake filtration
- Dust is stopped at the surface where it builds up in a layer. This acts as a filter itself. Resistance to flow increases as cake increases, so does filtration efficiency.
- Pore size should be small so dust is prevented from entering the structure which would limit the life time.
What is the difference between filtration and thickening? Which effect normally dominates on a Fourdrinier (long-wire) paper machine?
Filtration effect dominates – this is why we see two sidedness, fillers and fines are often washed out at the wire side of the paper.
Please give a brief explanation about tea bag paper and coffee filter production! What are typical basis weights and which raw materials can be used?
There are 2 categories, heat sealable and non-heat sealable.
Low basis weight sometimes below 20GSM for coffee filters often 60-100GSM.
Need to be high strength - especially wet strength considering the application, for this abaca fibres are used. In addition wood pulp and cellulose rayon fibres often make up the composition – in heat sealable papers the sealed layer may contain polyethylene or polypropylene.
