Polyethylene (PE) is one of the most widely manufactured and utilized plastics globally, serving countless applications from consumer packaging and food containers to industrial pipes and chemical storage tanks. PE is popular due to its durability, low cost, and generally high resistance to chemical attack. Understanding how this material interacts with various substances is necessary for ensuring safety and preventing material failure. The chemical resistance of polyethylene is not a simple yes or no answer, but depends entirely on the material’s inherent structure and the specific chemical it encounters.
How Polyethylene Structure Determines Chemical Resistance
The chemical resistance of polyethylene begins with its fundamental molecular nature as a long chain of simple carbon and hydrogen atoms. This structure is entirely non-polar, meaning the material lacks the positive and negative charge centers that attract and react with polar molecules. This non-polarity gives PE a natural shield against highly polar substances, such as water, strong inorganic acids, and strong bases, following the principle of “like dissolves like.”
Chemical resistance is directly linked to the density of the polyethylene, which is determined by its internal packing efficiency. Polymers with higher density exhibit greater crystallinity, meaning their long molecular chains are more tightly aligned and ordered.
This tight, crystalline structure creates a dense, impenetrable barrier that makes it difficult for chemical molecules to physically penetrate the polymer matrix. Conversely, areas of lower density are known as amorphous regions, where the chains are more disordered and loosely packed. Chemicals primarily interact with and attempt to penetrate the material through these amorphous sections. Therefore, a higher degree of crystallinity translates to a more robust chemical barrier, offering superior protection against permeation and swelling.
Chemical Families Polyethylene Resists Effectively
Polyethylene demonstrates high stability against a large category of chemicals due to the polarity mismatch between the plastic and the chemical agent. The non-polar structure of PE makes it especially suited for handling highly polar substances. This includes common, strong inorganic acids such as hydrochloric acid, sulfuric acid, and phosphoric acid in typical working concentrations.
The material also provides excellent resistance to strong alkaline solutions, including sodium hydroxide (lye) and potassium hydroxide. The highly charged, polar molecules of the acid or base do not chemically react with or physically penetrate the neutral, non-polar PE chains.
Resistance also extends to most aqueous solutions and neutral salt solutions, such as sodium chloride or calcium chloride. These solutions are highly polar and water-based, meaning they are effectively repelled by the non-polar PE structure.
For compatible substances, PE containers can maintain mechanical strength and integrity over long periods of exposure. This compatibility makes PE the material of choice for laboratory bottles, industrial storage tanks, and municipal water piping.
Chemicals That Cause Polyethylene Degradation
While polyethylene resists many substances, its non-polar nature makes it vulnerable to attack by other non-polar chemical families, such as organic solvents. Non-polar solvents, including aromatic hydrocarbons like benzene and toluene, or petroleum products like gasoline, can penetrate the amorphous regions of the polymer. This penetration causes the plastic to swell and soften, a process known as plasticization, which reduces the material’s mechanical strength and rigidity.
Another major threat comes from powerful oxidizing agents, which cause true chemical degradation rather than just physical swelling. Strong oxidizing acids, such as concentrated nitric acid or fuming sulfuric acid, can chemically attack the carbon-hydrogen bonds in the PE chain. This attack leads to oxidation and chain scission, permanently breaking the polymer into shorter segments and resulting in embrittlement and failure.
Halogens, such as elemental chlorine or bromine, also act as strong oxidizers. These elements cause degradation similar to that seen with strong oxidizing acids.
A different failure mechanism, known as environmental stress cracking (ESC), can be triggered by surface-active agents like certain industrial detergents and surfactants. ESC causes premature brittle failure when the polyethylene is simultaneously exposed to a chemical agent and mechanical stress. Elevated temperatures drastically accelerate all these forms of degradation, lowering the material’s resistance to solvents, oxidizing agents, and ESC agents.