A Deeper Understanding of Filter Paper

Filter papers are essential to supply for the scientists working in the lab. Hawach has quantitative and qualitative filter papers to meet almost all the customer’s requirements.

Quantitative filter paper(Quantitative Medium Flow Filter Paper Grade: BIO-40, Quantitative Laboratory Filter Paper Grade: BIO-42 ) is widely used for filtration after filtration, which refers to the weight method analysis test and the corresponding analysis test in quantitative chemical analysis. We offer ash-free filter papers for quantitative analysis, which are top quality, ash-free (ash content <0.01%), and with pure cotton fiber linter filters. And our ash-free filters are sorted by a variety of thicknesses, with characteristics and flow rates from wide pore and fast to narrow pore, and slow rates too.

Used to identify the properties of substances in qualitative analysis, the wet strength qualitative filter paper (Wet Strengthened 10 Micron Filter Paper Grade: BIO-91, Wet Strengthened Chemistry Filter Paper Grade: BIO-113) contains a small amount of chemically stabilized resin to enhance the wet strength and avoid introducing obvious impurities into the filtrate. Our qualitative filter papers(Qualitative Chromatography Filter Paper Grade: BIO-5) which have low ash content (0,06 – 0,15 %) are available for different needs and applications in multiple diameters, thicknesses, and filtration rates.

Introduction of filter paper technical indicators

Filter paper technical indicators are essential characteristics used to assess its suitability for specific filtration tasks. These include basis weight (g/m²), pore size (µm), porosity (%), ash content (%), wetting time (s), filtration speed (mL/min), retention efficiency (%), flow rate (mL/s), wet strength (g/cm), chemical compatibility, pH compatibility, binder type, and certifications. Understanding these indicators helps in choosing the right filter paper for precise and efficient filtration processes in laboratories.

On the other hand, the technical specifications of the filter paper can be mainly divided into two aspects, filtration characteristics, and physical characteristics. Filtration characteristics include air permeability, maximum pore size, and average pore size. Physical characteristics include quantitation, thickness, stiffness, burst strength, resin content, and the like.

Filtration characteristics

1. Air permeability refers to the amount of air passing through the filter paper under a certain area and a certain degree of vacuum, unit: l/m2·s.
2. Maximum pore size is the diameter of the largest sphere that can pass through the filter paper, unit: μm
3. The average pore size is the pore size derived from the “burst” bubbling pressure, unit: μm.

Physical characteristics

1. Quantitation refers to the quantity of filter paper per square meter, unit: g / m2.
2. Thickness refers to the thickness of the filter paper, excluding the corrugation depth, unit: mm.
3. Stiffness is the ability of the filter paper to deform, unit: mg.
4. Burst strength refers to the maximum pressure that can be applied to the unit area of the filter paper, unit: Kpa.
5. Resin content refers to the percentage of resin in the weight of the filter paper.

Relationship between physical characteristics of filter paper and filter performance

1. Quantitation
Quantitation in the filter is mainly reflected in the weight and price of the filter. Under a certain filtration area, the larger the amount, the heavier the filter paper and the heavier the filter. Since the cost of filter paper accounts for a large proportion of the cost of the filter, the cost is higher.

2. Thickness
For corrugated cylindrical filters, the thickness of the filter paper limits the maximum filtration area of the filter.

3. Stiffness and bursting resistance
Stiffness and bursting resistance indicate the resistance to deformation, high-pressure resistance, high stiffness, and high bursting resistance of the filter paper. It can show that the filter has good durability, but if the stiffness is too high and it is easy to become brittle, and the filter is easily damaged.

4. Resin content
The resin is added to the filter paper in order to increase the stiffness of the filter paper and the shaping in the filter paper processing and is generally between 10% and 30% depending on the usage. If the resin content is too high, the resin will easily block the pore structure of the filter paper.

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Filter paper equivalent to Whatman

5. Air permeability
Air permeability is a comprehensive reflection of filter paper performance and is the primary consideration in filter design. It is closely related to the flow resistance, the filtration efficiency, the original resistance, the ash storage capacity (service life), etc. The greater the permeability of the filter paper, the smaller the flow resistance and the smaller the original resistance, and vice versa.

Air permeability is also an indicator of the moisture resistance of filter paper. The larger the air permeability, the easier it is to absorb moisture. The filter paper becomes longer after moisture absorption. The processing size is unstable, which affects mass production. In severe cases, the quality is degraded.

6. Maximum pore size, average pore size
The maximum pore size and average pore size can be measured by the bubble method, and the maximum is the pore diameter derived from the first bubble point pressure, the average is calculated from the bubble point pressure. According to this principle, the values of the maximum and the average pore size can be referred to.

The bubble point test is used to judge the integrity of the filter. The accuracy of the filter can be approximated based on the average and the maximum pore size. The average pore size is closely related to the filtration efficiency, and the smaller the average pore size, the higher the filtration efficiency.

Conclusion of filter paper

Based on the technical indicators of filter paper, the last three chapters have analyzed the relationship between filter paper index and filter performance, as well as the impact on the design and manufacture of filters.

There are many kinds of filter materials, and filter paper has a large proportion in the filter material industry with excellent performance and a reasonable price. The filter with filter paper as the filter medium is closely related to the performance index of the filter paper. The indicators of the filter paper interact with each other and are related to each other.

They are reflected in the performance parameters of the filter and are not a single corresponding relationship. Instead, several index parameters of the filter paper jointly determine the performance of the filter. For example, the life of the filter is determined by several parameters such as air permeability, thickness, surface density, and average pore size. Therefore, only by comprehensively understanding the relationship between the filter paper index and the filter performance, can we design and manufacture high-quality filters suitable for different occasions to meet different needs.