PETpla.net Insider 09 / 2016

MATERIAL / RECYCLING 38 PET planet insider Vol. 17 No. 09/16 www.petpla.net Reaching bursting point by Laurence Newman MEng. (Hons) Torus Measurement Systems, Telford, United Kingdom and contributions by Phil Brannon, ColorMatrix This article details the importance of understanding a PET bottle’s mechanical properties, particularly with the contin- ued developments in lightweighting, recycled PET and bar- rier technology. It demonstrates how the data from a stand- ard burst test can be used to explore stress-strain curves, fracture point, elastic & plastic regions, Young’s modulus and intrinsic viscosity to gain a more detailed overview of the finished bottle. Lightweighting, rPET and barrier technology PET is predominantly used in the manufacture of plastic bottles. It is rela- tively strong yet flexible with low perme- ability providing a suitable and cost- effective packaging solution. During production it is important to understand the capabilities and limitations of the finished bottle design to ensure it is fit for purpose, with particular emphasis on the bottle’s mechanical properties. These properties are vital for ensuring the bottle maintains structural integ- rity during its lifecycle. With the rise in different manufacturing requirements, such as lightweighting, recycled PET and barrier technology it has never been more important to physically verify the bottle’s performance. Lightweighting is always a key area to save material costs, and due to recent advances in technology and design, there has been a heightened focus in this area. Inevita- bly as a bottle is lightweighted the wall thickness decreases and its strength is reduced. In the environmentally conscious society we live in, the use of recycled PET as a sustainable mate- rial is encouraged and as such is often used as a copolymer with virgin PET. Barrier technology within PET bottles has been another area of recent intense interest, with the addition of extra mate- rial at the injection moulding stage to prevent infrared, UV, oxygen and water ingress/emittance. Each of the manu- facturing techniques detailed can have unknown effects on mechanical per- formance, with bottle integrity requiring reverification. While tensile and com- pressive testing can be carried out on preforms and thin PET sheets, these do not replicate the final bottle in its applica- tion and ignore any changes or defects that may occur during the blow mould- ing stage. Burst testing provides a real and physical representation of how the bottle performs under stress and what its limitations are. Furthermore, advanced analysis of the resultant data can provide a more detailed overview of the bottle’s mechanical performance and material properties. Basic burst testing An industry standard test to deter- mine a bottle’s bursting point. The bottle is filled and primed before the pressure is increased at a controlled ramp rate, resulting in a correspond- ing volume increase, until the bottle bursts, as depicted in figure 1. Figure 1 Bottle burst sequence of Fill, Pressurise and Burst Typically, pressure and volume are both plotted against time to graphi- cally represent the bottle expansion and burst, see figure 2. Measured features include burst pressure and volume expansion. Figure 2 Typical results graph from a standard burst test Advanced burst testing A number of important features can be identified by analysing the data obtained from the controlled burst test, to give an indication of the mechanical properties of the finished bottle. Stress-strain curve Stress (σ) is used to express the force acting upon a body and is defined by: