Polymers exhibit very high chemical resistance to inorganic reagents such as acids and alkalis. However, they are vulnerable to certain organic solvents, especially if those solvents are chemically similar to the structural units that make up the polymers. Such attack causes softening and swelling, eventually leading to complete dissolution. Crystalline polymers exhibit greater resistance to these compounds than amorphous materials of the same chemical composition, due to the packing between chains that hinders the penetration of solvents or other reagents.
The nonpolar structure of high-density polyethylene allows it to maintain high resistance to chemical attack. In general, this resistance improves with increasing density and molecular weight.
This behavior is measured by observing certain changes in the samples upon contact with the test substance; these changes include swelling, weight loss, or elongation at break.
This plastic is highly resistant to strong acids (non-oxidizing) and strong bases. At temperatures above 60 °C, the material resists many solvents, except for aromatic and halogenated hydrocarbons, oils, greases, and waxes, which cause swelling—though this swelling is less pronounced with aliphatic solvents.
High-density polyethylene is partially or completely soluble in certain extreme cases, such as in benzene or xylene at their boiling points. Halogens and highly oxidizing substances attack this plastic; examples include concentrated inorganic acids such as nitric, sulfuric, and perchloric acids, etc.
The change in the properties of that plastic due to the compounds mentioned—and, in general, due to any substance—depends on several factors: primarily, concentration, exposure time, molecular weight, and residual stresses from processing or mechanically induced stresses.
's Mechanical Properties
The mechanical properties of a material refer to its ability to withstand forces and the way it deforms and yields under those forces. Thus, the mechanical properties of high-density polyethylene depend primarily on its structure, which includes molecular weight distribution, molecular weight, and crystallinity. However, they also depend on external factors such as temperature, the chemical environment, and time—the latter understood as a measure of how quickly forces are applied, as well as the duration of those forces.
The stiffness, hardness, and tensile strength of high-density polyethylene increase with density, since a higher density indicates that the material is more crystalline and will therefore be more resistant to the same magnitude of applied force than a specimen of lower density.
Similarly, as the molecular weight increases up to a certain “point,” the mechanical properties improve. Molecular weights below this point are generally not as useful. Beyond this point, mechanical strength typically continues to improve, but more gradually, as the molecular weight increases.
For this reason, it is generally assumed that increasing the molecular weight is the best way to improve mechanical properties; however, the manufacturing process involves working with molten polymer, and the viscosity of the melt increases exponentially with molecular weight. Consequently, very high molecular weights require greater effort and higher energy consumption in the manufacture of parts.
Therefore, a balance must be struck between the feasibility of the process and the desired strength of the material.
This balance is achieved industrially because the material is quite flexible. One example is ultra-high-molecular-weight polyethylene, which has the highest achievable molecular weight, but is also quite expensive.
High-density polyethylene is very tough, which means it exhibits high impact resistance even at low temperatures, as it is capable of absorbing some of the energy from impacts through deformation. It achieves this thanks to the polymer’s amorphous regions, since these deformations result in a change in the material’s structure.
Source: UNIVERSITY OF SAN CARLOS DE GUATEMALA, FACULTY OF ENGINEERING, SCHOOL OF CHEMICAL ENGINEERING—ERNESTO ROCA GIRÓN