What is ‘Creep’?
19 Aug 2014
The term “creep” in any material describes a material’s deformation over time under a load. If a material cannot withstand the force of that load from any direction, it begins to deform. Such stresses within the Earth’s crust can be great enough to cause solid rocks to flow like plastics, fold over and eventually break.
Similar types of deformation occur in man-made plastics and have had a major influence on the design of plastic pallets. Plastic pallets have had a great advantage over their cheaper wooden counterparts because they are exempt from international inspection standards for bio safety. In addition, they can resist weathering, chemicals, rot, mildew and corrosion and can be easily sanitised. These durable pallets can be used over and over again for more than 100 trips.
Their major problem has been that they can collapse over time from plastic creep if they are stacked on top of each other to store a very heavy load for a long period.
Plastic pallets are made from high-density polyethylene (HDPE), which is the most common material for plastic pallets, or polypropylene (PP), polyvinyl chloride (PVC) or recycled polyethylene terephthalate (PET), originating usually from plastic drinks bottles.
Thermoplastics such as HDPE, PVC, PP and PET deform with increased temperature and pressure. They can do so even at room temperature. When these pallets are stacked over a long period, the highest load is exerted on the bottom pallet. The plastic creep usually begins as a buckling at the corners of the pallets that develops into a stiffening and brittleness in the material.
If left unattended, this process eventually breaks the pallet frame. In the meantime, the stacked pallets can fuse together, while the goods they contain are either squashed or squeezed out from the sides.
Damaged plastic pallets are not easy to repair and can cost up to ten times as much as hardwood pallets. In the case of HDPE, which is usually considered a lightweight though very strong thermoplastic with an excellent resistance to chemical attack, continuous loading will cause it to elongate. In contrast, polypropylene, a thermoplastic but a more crystalline material, becomes stiffer under stress and more resistant to plastic flow than HDPE.
The strongest thermoset plastic material for pallets is polyvinyl chloride (PVC). It can sustain two and half times the load that HDPE can over the same period of time. PVC is over seven times stronger than low-density polyethylene (LDPE), the material that is usually recycled into PET and is the weakest of the thermoplastics.
The answer to the dilemma is to use a thermoset material for the plastic pallets. In practice, this is the combination of a thermoplastic resin with a thermosetting reinforcement to make a composite thermosetting polymer. The reinforcements can be carbon fibres for a super-strength pallet carrying high-value loads or glass and mineral fibres or even wood.
The resulting pallets are stronger and lighter than the standard thermoset pallets. Their disadvantage is that they are also more brittle and can break on impact. These pallets are also difficult to recycle or remanufacture.