Question from Willis: We are opening a new distribution center in a very hot climate, and some have expressed concern over how that will affect load containment. What should we expect when temperatures vary significantly?
Hi Willis,
Thanks for sending in your question. Temperature is something we rarely think about until seasonal changes expose its impact. I can give you some insight with the help of a little science!
To understand how temperature affects stretch film, we need to begin with how stretch film works. Stretch film is similar to a rubber band in that a force is required to stretch it, and once that force is removed, the rubber band retracts to its original length (or close to it). Stretch film is designed to be stretched as it is applied to the load, then once that stretching force is removed, the film will retract by some percentage, though less than a rubber band. As the film recovers, or retracts, it creates a unitizing force, drawing the contents of the load tightly together to prevent independent component movement during transportation.
The force required to stretch the film increases as the percentage of stretch increases, right up to the point where the film breaks. This is where the force exceeds the strength of the film. Controlling the elasticity of the film is critical to optimizing load containment. The goal is to remove as much residual elasticity as possible, so the film resists further stretching during shipment. However, this balance is delicate. As the percentage of stretch increases, elasticity decreases and brittleness also increases. At higher stretch levels, the film becomes more susceptible to punctures and web breaks, which disrupt production.
There are two primary ways to control elasticity: one is during the film manufacturing process and the other is when the film is applied to the load.
Stretch film is typically made from Linear Low-Density Polyethylene (LLDPE) resin, combined with additives that influence properties such as cling, puncture resistance, and elasticity. After blending, the resin mixture is heated and extruded into a film. How quickly the resin is cooled from its melted or “plastic” state to become a solid also affects its elastic properties. Stretch film manufacturers tightly control this process to ensure a consistent force-to-stretch curve. The upper end of this curve represents the film’s Ultimate Strength, or the maximum resistance to stretching before the film breaks. Keep in mind, the closer the stretch comes to the film’s ultimate strength, the more brittle and susceptible to breaking it becomes.
If the film stretches again after wrapping your load, due to transportation forces, it loses unitizing force. This secondary stretch allows load components to move independently, increasing the likelihood of a load failure. Managing secondary stretch involves a combination of pre-stretching the film in the machine before it is applied and then stretching it again via controlled tension as the load is being wrapped. However, the maximum practical percentage you can stretch the film without a web break becomes the limiting factor for load containment.
Now that we understand the elastic nature of stretch wrap, let’s introduce temperature.
Temperature extremes change the film’s elastic properties. In very cold conditions, film becomes stiffer and more difficult to stretch. In hot conditions, it stretches more easily. Another thing to consider is that while your new distribution center may operate at temperatures a little higher than in your other operations, your product will be transported in a solar powered convection oven, called a trailer. Trailers can reach temperatures as much as 30 degrees above their ambient conditions.
It is generally impractical to remove all elasticity in the stretch film while wrapping pallets, because doing so would increase brittleness and web breaks will follow, especially when sharp pallet corners, box edges or tier sheets are involved. To avoid punctures and web breaks some elasticity will remain in the film. At elevated temperatures, that residual elasticity becomes more active and increases the chances of load failure as the film continues to stretch during transit, which reduces containment force and allows for independent movement of the individual load components.
We study the science of load containment, combining physics and material science to address these challenges. We’ve developed technology that can be installed on your existing equipment, enabling loads to be wrapped with virtually no elasticity left in the film and can do that without web breaks during wrapping. By incorporating reinforcement filaments into the stretch film web, we can achieve much higher levels of stretch while also preventing the web from breaking, even if it is punctured. With minimal residual stretch remaining in the film after your load is wrapped, temperature has far less impact on load stability. The net result is improved load containment, reduced film usage, lower annual film spend, and increased machine throughput.
Thanks for asking!
