PETpla.net Insider 09 / 2016

MATERIAL / RECYCLING 39 PET planet insider Vol. 17 No. 09/16 www.petpla.net Where F is the force applied and A is the cross sectional area of the body. Strain (ϵ) is the amount of exten- sion of the body in the direction of the applied force and is defined by: Where l is the original length of the body and Δl is the extension of the body in the direction of the force. Looking at the definitions of stress and strain it can be seen that there is a link between these and the values measured during a burst test. Stress is defined as a force divided by an area, which also defines a pressure so this can be substituted for the pressure applied during the test. The strain is defined as a change in length divided by the original length which can be substituted by volume expan- sion over original volume, or a per- centage volume expansion. Plotting applied pressure (bar) against volume expansion (%) produces an indicative stress-strain curve, see figure 3. Figure 3 Indicative Stress-Strain curve for a burst bottle The resulting stress-strain curve provides the necessary information to determine key material properties. A generic stress-strain curve is shown in figure 4. Figure 4 Typical stress-strain curve for a ductile material Fracture point The point at which a sample cata- strophically fails, typically resulting in a break in the material. It is also where the measurements cease at the endpoint of the curve in figure 4. Any damage to a sample, or irregularities in the composition of the material may lead to a premature fracture. When applied to finished bottles, the fracture point directly corresponds to the burst point. Any defects during manufacture may cause a premature split in the body or base causing an early burst. Low burst pressures can signify a process problem or an issue with preform design. Inspecting and compar- ing a number of bottles after bursting is an easy way to identify a defected part. If the burst pressure is higher than required this may allow the product to be lightweighted, reducing material costs. Elastic & plastic regions Ductile materials, such as plastics/ polymers, have two distinct regions on their respective stress-strain curves. The elastic region is the initial linear gradient of the curve and represents the area where the material would return to its original size and shape if the stress is removed. The gradient of this elastic region is defined as the Young’s modulus (E) of the material and is calculated by: The latter section of the curve is where permanent deformation takes place and is known as the plastic region. Once plastic deformation occurs, the sample will not return to its original size and shape when the stress is removed. Yield point Defined as the stress at which the deformation of a material becomes irreversible. Beyond this point the material begins to rapidly stretch with continuous addition of stress. For finished bottles, this can be identi- fied on a volume expansion graph as the point at which the gradient of the slope begins to rapidly increase, as shown in figure 2. An indicative yield point can be found by analysing the recorded volume data. This is done by locating the change in gradient of the volume expansion curve and performing a least squares linear regression on both regions of the curve to find the intersection point. This process is shown graphically in figure 5. Figure 5 Finding the intersection points on a volume time graph The yield point indicates a bottle’s resilience and its ability to absorb impact energy if dropped or damaged during transportation. Additionally, two supposedly identical bottles could have the same burst point, but one may yield earlier and have a larger expansion volume. This signifies a potential problem during injection moulding which could have affected the molecular weight. Intrinsic viscosity (IV) IV defines the average molecu- lar chain length of a polymer and is therefore a measure of its molecular weight. This reflects properties such as tensile strength and crystallinity. It is important to realise however that many other aspects influence the mechanical properties; the amount of any copolymer added is one of these factors. The expansion volume between the yield point and burst point is related to the IV of the polymer. A higher IV corresponds to a smaller post-yield expansion, and conversely a lower IV corresponds to a larger post-yield expansion. This is due to higher molecular weight polymers reaching the strain hardening point sooner, therefore the yield point is reached later. Figure 6 (page 40) shows how the IV value can be inter- preted from a graph of volume expan- sion versus time. However, a change in IV value should not be assumed by a change in the yield point, as mate- rial composition and process param- eters will also affect this property.