Comparison of mechanical properties of different groups of packaging materials

The mechanical strength of packaging materials is decisive for certain structural aspects, such as the wall thickness of the product. Strength properties are determined by characteristic quantities, which are determined when materials are tested on standardized specimens (so-called shapes). While the definition of a given magnitude is usually based on a certain physical relationship (e.g., tensile stress is the value of the force, recorded during stretching, divided by the cross-sectional area of the specimen, oriented transversely with respect to the direction of the force), the shape of the test specimens as well as the way in which the characteristic magnitudes are defined can depend on the specifics of the material. Thus, standards for testing the properties of plastics, metals and glass are different.

Strength of materials for packaging – characteristics

The tensile strength of plastics is (for example) 18÷35 MPa (megapascals) for high-density polyethylene (PE-HD), 21÷37 MPa for polypropylene (PP) and 47 MPa for polyethylene terephthalate (PET). Aluminum alloys, used for packaging such as beverage cans and closures (3000 series alloys), have higher tensile strengths than the aforementioned plastics. The strength of these lightweight metals is in the range of 125÷230 MPa (214 MPa for alloy 3004), although it should be noted that the characteristic value for this type of material, called yield strength, of about 170 MPa (>145 MPa) is important. This is the value of the stress from which (in tension) the material deforms plastically, and therefore significantly changes its properties. The tensile strength of glass, which is a brittle material (it has no yield strength), is: 0.4÷1 MPa.

Measures of stiffness of various packaging materials

A measure of a material’s stiffness can be Young’s modulus – the modulus of elasticity, determined during a tensile test of the material. This modulus is noticeably smaller for plastics and for this reason is expressed in megapascals (MPa) for these materials, while for metals and glass it is expressed in gigapascals (GPa). It ranges from 600÷1400 MPa for PE-HD plastic, 1300÷1800 MPa for PP, while for PET: 3100 MPa. Aluminum alloys used for packaging are characterized by values of this modulus in the range of 69÷73 GPa (i.e. 69,000

The compressive strength of plastics is usually several tens of megapascals (20 MPa – PE-HD, 40 MPa – PP, 90 MPa – PET), while glass here shows a value in the 6÷10 MPa range.

The hardness of a material is generally defined as the material’s resistance to plunging under a certain load. Depending on the type of material, the methods of hardness testing, including the shape of the cavity and the load during the test are different, so it is not easy to compare the hardness of materials belonging to different groups. In general, however, it should be said that of the three groups of materials mentioned, glass is by far the hardest. Its hardness is determined by the Mohs scale (a scale from 1 to 10 developed for minerals) and is 5÷7. The hardness of aluminum can be determined by the Brinell method (the indenter is in the form of a ball with a diameter of 10 mm) and is about 50 HB, while the hardness of plastics is determined by several methods, such as the method of pressing a ball with a diameter of 5 mm (ball indentation hardness), obtaining values for PE-HD: 35÷58 MPa, for PP: 45÷106 MPa and for PET: 117÷170 MPa.

Changes in the properties of plastics – what do they depend on?

During the use of products, it is also necessary to pay attention to possible changes in the properties of materials with changes in temperature, which is especially true for plastics, for which the temperature of use of products is limited to a certain upper temperature range, above which the mechanical strength significantly decreases. However, changes in the properties of plastics can also apply to lower temperatures. An example of such a material is ethylene-propylene copolymer (sometimes colloquially referred to as polypropylene CoPo). In addition to a different temperature range of use, copolymers can also differ from polypropylene in other properties, such as exhibiting greater transparency.


1 A. Emblem, H. Emblem: Packaging technology. Basics, materials, manufacturing processes, PWN, Warsaw 2014.

2 T. A. Osswald. G. Menges: Material Science of Polymers for Engineers, 3rd Edition, Hanser Publishers, Munich 2012.

3. W. Nowotny: Glass technology, Ch. I, II. Państwowe Wydawnictwa Szkolnictwa Zawodowego, Katowice 1971.




Date of elaboration: 14.11.2023 Dr. Tomasz Jaruga, Eng.