Plastic forms the containers, films, and durable goods that shape modern life, leading many to question its physical structure. The answer to whether plastic is porous or nonporous is nuanced and rooted in materials science. Most consumer plastics are considered nonporous based on their physical characteristics, yet they still allow gases and liquids to pass through them on a molecular scale. Understanding the distinction between porosity and permeability is necessary to grasp why plastic behaves this way.
Defining Porosity and Nonporosity
The term porosity describes the physical presence of voids or empty spaces within a solid material. For a material to be truly porous, these internal spaces must be interconnected and accessible from the surface. This structure allows a fluid like water or air to flow completely through the material. This interconnected space is often referred to as effective porosity.
Nonporous materials, by contrast, are those that visibly lack these interconnected channels or holes that would permit bulk fluid flow. Examples of nonporous substances include materials like glass or polished metals, where the structure is dense and continuous. Understanding this fundamental difference is the basis for classifying most common plastics, which do not permit water to flow through them in the way a sponge or a piece of sandstone would.
The General Answer: Why Plastic is Considered Nonporous
On a macroscopic level, the vast majority of plastics encountered in daily use are nonporous. This characteristic stems directly from their internal molecular architecture, which is built from long chains of repeating molecular units called polymers. These macromolecules are densely packed together during the manufacturing process, resulting in a solid material that acts as a barrier to bulk liquids and gases.
Plastics can be categorized by how these chains are arranged. Amorphous polymers have chains that are randomly intertwined, while semi-crystalline plastics contain regions where the chains are tightly folded into highly ordered, dense crystal structures. Even in the looser amorphous regions, the chains are so closely packed that they prevent the formation of the large, accessible channels required to qualify the material as porous. The dense, continuous nature of both amorphous and semi-crystalline structures makes plastic an effective solid barrier in everyday contexts.
Permeability Versus Porosity
The common misconception that plastic must be porous for gases or odors to pass through it confuses porosity with permeability. Permeability describes the rate at which a substance can dissolve into, move through, and exit a solid material on a molecular level. This passage occurs not through physical holes, but through a three-step process: the molecule first dissolves into the surface, then diffuses through the space between the polymer chains, and finally releases from the opposite side.
The rate of permeability varies significantly depending on the specific plastic and the substance attempting to pass through it. Polyethylene terephthalate (PET), used for carbonated beverage bottles, is valued for its low permeability, offering a good barrier against oxygen and carbon dioxide to keep soda fresh. Conversely, Low-Density Polyethylene (LDPE), often used for plastic films, has a higher permeability, allowing gases like oxygen to pass through more readily.
Practical Implications of Plastic’s Structure
The nonporous structure of plastic has direct benefits in applications requiring sanitation. Since the material lacks interconnected voids, it is easy to clean and does not provide hiding places for bacteria and moisture, which is a major advantage for reusable food containers. However, the same continuous structure is still subject to chemical breakdown, and heating or freezing plastic containers can cause them to shed microplastic particles that raise concerns about food contamination.
The varying permeability of different plastic types is a determining factor in material selection for industrial uses. High-Density Polyethylene (HDPE) is a common choice for water storage tanks and piping because it is resistant to water vapor and chemical permeation, ensuring the contained substance remains uncontaminated. Other specialized plastics, such as certain nylons and fluoropolymers like PVDF, are selected for use in fuel tanks or chemical piping because they exhibit ultra-low permeability to aggressive substances like organic solvents and fuel components.