Examples of Porous Materials

Sarah
DeGuzman

A porous substance contains pores, voids, or holes on purpose. In this article, you’ll learn examples of porous materials, the benefits of porous materials, and determine how to reduce porosity.

Examples of Porous Materials

First, let’s take a look at examples of porous materials. This includes wood, paper products, sponges, some rocks, and most fabric. In contrast, marble, glass, and some plastics are not porous and contain very few open pockets of air (or pores).

Wood

Wood is a porous material and fibrous structural tissue found in trees and other woody plants’ stems and roots. The “matrix” or “frame” refers to the skeletal portion of the material. It’s a natural composite of cellulose fibers that are strong in tension and set in a lignin matrix that resists compression.

In most cases, wood is porous because trees have pores to get moist and air. In fact, wood porosity varies between 40% and 80%.

Paper Products

Hydrogen bonding holds interwoven cellulosic fibers which produce paper products. This includes cardboard, printing paper, and filters, also an example of porous materials.

Cardboard

Cardboard sheet as porous materials
Courtesy: IndiaMART

Cardboard is a generic name for heavy-duty paper-based products with a high thickness and durability, as well as other mechanical properties that distinguish paper from different materials, such as foldability, rigidity, and impact resistance. Corrugated fiberboard comprises numerous corrugated and flat layers. It can range in thickness from a thick sheet known as paperboard to a corrugated fiberboard of multiple corrugated and flat layers.

Printing Paper

Printing paper provides another example of porous material. With smaller holes than some of the previous examples, fewer fluids can pass through. However, since air and other gases can pass through, paper is still considered porous.

Newspapers, periodicals, catalogs, books, notebooks, commercial printing, business forms, stationery, and various iterations of these items also fall into this category.

Filters

Filters as porous materials
Courtesy: HENGKO

The filter provides a device or a mass of material with tiny openings specifically designed to remove particulates from gas or liquids. For example, a common residential filter removes dust from the air. 

Commercial applications include the protection of compressed air circuits and noise control. Porous materials combine the properties of lightweight, wide absorption frequency range, and high sound absorption ability. Thus they provide great potential in the field of sound absorption.

Sponges

A sponge is an example of a porous material as it has a large number of empty spaces compared to its volume.

Did you know that the scientific name for sponges is Porifera? It means pore bearing. Ostia, more commonly referred to as tiny pores, covers the sponge. These pores lead to canals, large holes, and chambers within the sponge. Organic sponges feed by filtering liquid and capturing small particles due to their porous nature.

Rocks

Rock as a porous material
Courtesy: iStock

The term “porous” refers to rocks that absorb water. In addition, rocks with rounded grains tend to be softer and crumblier than those with interlocking grains. As a result, porous rocks are more delicate than non-porous rocks.

Gas, air, and liquid fill the spaces between the particles. In fact, particles’ shapes and sizes affect how they aggregate, including how tightly they can pack together, which involves a rock’s porosity. This property is the ratio of the volume of a rock’s empty spaces to its total volume.

Fabrics

Example: Fabric
Courtesy: Shutterstock

All textiles are porous media with a hierarchical structure and varied characteristic scales, starting with the nanopores found in each cotton fiber. Between the twisted fibers that form a yarn, larger pores exist in the micron range. Voids between weft and warp strands create the porous area in fabrics. From the fabric’s surface, air passes via the pores.

Benefits of Porous Materials

The performance of porous materials must meet the expected standards of the material application. Sound absorption, heat insulation, and electrical applications are all common uses for these materials.

Porous materials provide excellent resiliency, high physical strength, and are lightweight, providing a rugged, efficiently handled part. These materials may attain up to 99.8% filtration efficiency.

Porous carbon materials are in high demand due to their superior physical and chemical qualities, including thermal conductivity, electrical conductivity, low density, and chemical stability, as well as their widespread availability.

Because of their unique properties, porous materials can be used in energy-saving and energy-conversion modules. These characteristics may improve material efficiency in terms of power and energy density, stability, and lifetime.

Reduction of Porosity in Material

Porosity for the aforementioned items typically presents as a positive characteristic. In welding applications, the absorption of nitrogen, oxygen, and hydrogen in the molten weld pool causes porosity that ends up trapped in the weld metal. And at times, porosity in a material may be undesirable.

Fortunately, there are processing options available to reduce porosity.

Molten Metal Porosity

The porosity of your parts and components might be affected by the pouring temperature at which you pour them. For example, low metal temperatures may lead to increased porosity. Setting up high-pressure die casting process settings helps maintain a constant pouring rate and pressure.

A mold with sharp corners and edges may allow for inconsistent cooling temperatures, thus promoting porosity. You can set processing settings to guarantee that the mold is appropriately built and has the proper hardness.

Sealants, Paints, and Coatings

Often sealants provide a practical means to reduce porosity. Weatherproofing a wooden fence is an easily relatable example of how porosity can be reduced and the service life of a material extended.

Courtesy: Northland Fence

Cleaning

Contaminants on an alloy can lead to more porosity and reduce service effectiveness. Suitable processing steps such as pickling, passivation, and sandblasting ensure a clean alloy stays as clean and removes potential contaminants.