while HDPE shaped bottles is generally resistant to many chemicals, there are specific substances it should avoid contact with, as they can degrade the material or cause it to weaken.
HDPE is a semi-crystalline polymer, which gives it excellent resistance to a variety of chemicals, but strong oxidizing acids are an exception. Concentrated sulfuric acid (H₂SO₄) and nitric acid (HNO₃) are highly reactive and can attack the polymer chains by initiating oxidative degradation. This process involves the breaking of C-H bonds in the polyethylene backbone, leading to the formation of carbonyl groups. The introduction of these polar groups disrupts the material's crystalline structure, leading to embrittlement and a significant loss in mechanical properties, such as tensile strength and impact resistance. This degradation is exothermic, meaning it can generate heat, potentially accelerating the breakdown of the polymer if not properly managed. Over time, the material can become prone to stress cracking, particularly if it is under mechanical load.
Aromatic hydrocarbons, like benzene, toluene, and xylene, are known for their solvent properties, which can be problematic for HDPE. These compounds are non-polar and can interact with the non-polar HDPE chains through van der Waals forces, causing the polymer to swell. This swelling disrupts the ordered crystalline regions of the polymer, leading to a reduction in density and a corresponding drop in mechanical properties such as stiffness and strength. The swelling can also lead to dimensional instability, where the bottle may no longer maintain its shape, particularly if the swelling is uneven. In extreme cases, prolonged exposure can result in partial dissolution of the polymer, making the bottle unusable. The impact of aromatic hydrocarbons is temperature-dependent, with higher temperatures exacerbating the swelling and dissolution effects.
Halogenated hydrocarbons, such as chloroform, carbon tetrachloride, and dichloromethane, are particularly aggressive solvents when it comes to HDPE. These solvents are characterized by their ability to interact with the polymer on a molecular level, leading to a reduction in the material's crystallinity. The halogen atoms in these compounds can create dipole-induced dipole interactions with the polymer chains, effectively disrupting the orderly arrangement of the molecules in the crystalline regions. This disruption leads to a softening of the material, reducing its load-bearing capacity and making it more susceptible to deformation under stress. Prolonged exposure can cause the polymer to absorb the solvent, leading to swelling and a further decrease in mechanical properties. In some cases, the polymer can even become sticky or tacky, particularly in high humidity environments, which further compromises its utility.
HDPE is generally resistant to a wide range of organic solvents, but specific solvents like acetone, ether, and ketones can present challenges. These solvents are capable of penetrating the amorphous regions of the polymer, where the polymer chains are less tightly packed. The interaction between these solvents and the polymer can lead to a phenomenon known as plasticization, where the material becomes softer and more flexible. This effect can be beneficial in some applications, but in the case of HDPE bottles, it leads to a loss of rigidity, which is critical for maintaining the shape and integrity of the container. Prolonged exposure can lead to solvent-induced stress cracking, where small cracks form on the surface of the bottle due to the combination of mechanical stress and solvent attack. These cracks can propagate over time, leading to leakage or catastrophic failure of the container.