HDPE, PET, PP: choosing a bottle material

A plastic bottle is never neutral: the material it is made from determines its transparency, its chemical resistance, its ability to hold back a gas or an odour, and the temperature it withstands without deforming. Three materials come up most often in bottle packaging: high-density polyethylene (HDPE), polyethylene terephthalate (PET) and polypropylene (PP). What sets them apart is not marketing, but measurable physical and chemical properties, documented by raw-material and scientific-equipment suppliers.

HDPE is a polyolefin — the same chemical family as PP — made by polymerising ethylene through a process that produces tightly packed molecular chains, which gives it a higher density than other polyethylenes. In its natural state it has a milky-white appearance, translucent rather than transparent. Chemically inert, it resists a wide range of common acids, bases and solvents well; only strong oxidising agents eventually weaken it over time. Its temperature tolerance reaches around 120°C in continuous use, and it keeps its impact resistance even at low temperature. These properties make it a common material for bottles used in industrial, cosmetic or pharmaceutical settings, wherever opacity is not an issue and chemical resistance matters more than seeing the contents.

PET belongs to a different chemical family, the polyesters. Its most visible property is excellent transparency: it lets the contents be seen with an optical clarity close to that of glass, which explains its heavy use in beverage bottling and, more broadly, wherever product presentation matters. It also provides an effective barrier against gas passage — particularly carbon dioxide — and against moisture, limiting exchange between the contents and outside air over time. It is, however, more sensitive to impact at low temperature than HDPE or PP, and its heat tolerance is more limited.

PP, or polypropylene, is chemically close to HDPE — the same polyolefin family — but stands apart with a markedly higher temperature tolerance: its heat deflection temperature reaches around 107°C and it can be used continuously up to around 130°C. This characteristic makes it compatible with steam sterilisation in an autoclave; a common cycle of 121°C at 15 psi for twenty minutes is applied to PP containers, provided proper venting prevents them from collapsing. Like HDPE, it naturally has a milky-white, translucent appearance, which a colorant can render opaque. Its chemical resistance is close to that of HDPE, with a somewhat greater sensitivity to aromatic hydrocarbons and aromatic ketones.

These properties — transparency, gas barrier, temperature tolerance, chemical resistance — are what objectively sets the three materials apart from one another. They say nothing, however, about a given bottle's suitability for one specific product: that compatibility depends on the exact formulation of the contents, and only a container-content compatibility trial can confirm it.