The chemical resistance of polymers to inorganic reagents such as acids and alkalis is very high. However, they are vulnerable to some organic solvents, especially if they have chemical similarity with the structural units that form them. The attack involves softening and swelling, leading to their final dissolution. Crystalline polymers present greater resistance to these compounds than amorphous materials of the same chemical composition, as a consequence of the packing between chains that hinders the penetration of the solvent or other reagents.
The non-polar structure of HDPE allows it to maintain high resistance to attack by chemical agents. In general, this resistance improves with increasing density and molecular weight.
The way to measure this behavior is by considering certain changes in the samples upon contact with the substance under test; these changes are swelling, weight loss or elongation at break.
This plastic withstands strong acids (non-oxidizing) and strong bases very well. At levels above 60 ºC, the material resists many solvents, except aromatic and halogenated hydrocarbons, oils, greases and waxes that induce swelling, which is less with aliphatic solvents.
High density polyethylene is partially or totally soluble in certain extreme cases, for example in benzene or xylene at boiling point. Halogens and highly oxidizing substances attack this plastic, e.g. concentrated inorganic acids such as nitric, sulfuric, perchloric acid, etc.
The change in the qualities of that plastic due to the referred compounds and in general, to any substance depends on several factors: concentration, exposure time, molecular weight, residual stresses of the transformation or mechanically induced, mainly.
Mechanical properties
The mechanical properties of a material refer to its ability to withstand forces, the way it deforms and yields to these forces. Thus, the mechanical properties of HDPE depend basically on its structure, which includes molecular weight distribution, molecular weight and crystallinity. But it also depends on external factors such as temperature, chemical environment and time, the latter being understood as a measure of the speed with which forces are applied, as well as their duration.
The stiffness, hardness and tensile strength of high density polyethylene increase with density, because if this increases it is an indicator that the material is more crystalline, and therefore will be more resistant to the same magnitude of applied force than a specimen of lower density.
Also, as the molecular weight increases up to a certain "point" the mechanical properties improve. Molecular weights below that point are usually not as useful. Beyond this point, mechanical strength usually continues to improve, but more gradually as the molecular weight increases.
However, the manufacturing process is made from the molten polymer, and the viscosity of the melt grows exponentially with the molecular weight, so that very high molecular weights require greater efforts and higher energy consumption in the manufacture of parts.
Therefore, a point of equilibrium must be reached between the feasibility of the process and the desired strength of the material.
This balance point is obtained industrially since it is quite flexible. As an example we can mention ultra high molecular weight polyethylene, which has the highest molecular weight achievable, but its cost is also high.
High density polyethylene is very tough, thus demonstrating high impact resistance even at low temperatures, as it is capable of absorbing part of the energy coming from the impacts through deformations. This is achieved thanks to the amorphous zones of the polymer, since these deformations result in a change in the conformation of the material.
Source: UNIVERSIDAD DE SAN CARLOS DE GUATEMALA FACULTY OF ENGINEERING ERNESTO ROCA GIRÓN SCHOOL OF CHEMICAL ENGINEERING