Polypropylene strap: where it fits and where it does not

Polypropylene (PP) strap is the economical end of plastic strapping. It is light, easy to handle and runs on a wide range of arch strapping machines and hand tools. That makes it tempting to use everywhere. The better question is where PP’s behaviour matches the job, and where another material is the safer specification.

This guide sets out the practical boundaries as they stand in March 2017, using properties any buyer can ask a supplier to state.

What PP strap is good at

PP strap stretches more than polyester and far more than steel. The background to a polyester strapping patent application published in January 2011 sums up the trade-off: polypropylene is cheaper but may stretch and loosen under high tension, while polyester is very strong and not easily stretched. At the other extreme, the CTU Code text puts the elongation of steel band at about 0.3% when loaded to its maximum securing load. On the right load, PP’s stretch is an advantage. It absorbs small movements, follows a carton that compresses slightly, and does not cut into soft packaging the way a rigid band can.

Typical good fits include:

• Closing and reinforcing corrugated cartons.
• Bundling newspapers, printed matter, light profiles and similar items.
• Unitising light pallets where the layers are stable and the load is not dense.
• High-volume lines with automatic arch machines that heat-seal the joint.

In those settings the lower material cost and lighter rolls are a real saving, and the strap’s limits are rarely tested.

Where PP is the wrong answer

The weakness shows when the strap must hold a high tension for a long time. Under sustained load, PP tends to relax, so retained tension falls further than it would with PET. On a dense or heavy load that has to survive braking, rough handling or a long sea leg, that loss matters.

Treat these as warning signs that PP needs a careful review, or a different material:

• Heavy, rigid loads such as bricks, blocks, stone, steel or dense timber.
• Loads that will stand strapped for weeks or months before use.
• Hot products, or storage near heat sources. PP softens at a lower temperature than polyester.
• Long periods outdoors without a grade intended for that exposure.
• Sharp edges that bear on the strap under tension.
• Any application where the strap is part of a load-restraint arrangement rather than simple unitisation.

For those cases, polyester or steel strap is usually the starting point, with the joint method chosen to match.

Machine grade and hand grade

PP is sold in grades made for different equipment. Machine-grade strap is produced for automatic and semi-automatic machines, where consistent thickness, width and stiffness matter for feeding and sealing. Hand-grade strap is intended for manual tensioners with clips or buckles, or for battery tools.

A roll that runs well on one machine may jam another, and a strap that seals well with heat may not hold in a metal clip. Record the equipment model alongside the strap reference. If you change either one, treat it as a new combination.

A quick decision table

Question	Points towards PP	Points towards PET or steel
Load mass and density	Light, low-density	Heavy, dense, rigid
Time under tension	Hours to days	Weeks to months
Temperature	Ambient	Hot product or warm storage
Edges	Rounded or protected	Sharp, unprotected
Role of the strap	Holding items together	Restraining a heavy unit

The table is a starting point for discussion, not a rule. A short trial on representative loads settles most questions faster than a debate about data sheets.

Joining PP strap

ASTM D3950-16 covers nonmetallic strapping, the category PP strap belongs to, together with its joining methods, and classifies that strapping into five types by material. Its abstract lists joint strength among the properties addressed where applicable, alongside breaking strength and elongation. The point for buyers is that the strap value and the joint value are separate.

PP can be joined by heat sealing in a machine, by friction welding with suitable tools, with a crimped sheet metal clip, or with a buckle. Signode’s overview of seal joints notes that plastic strap joints use a crimp-type seal or a buckle, with the crimp holding by friction. A clip sized for a nominal width still has to suit the strap’s actual thickness and surface, and the sealer must match the clip.

Running a fair trial

If you are deciding between PP and PET for a load:

1. Strap several representative loads with each option, using the intended tool and joint.
2. Record the applied tension, strap positions and joint method.
3. Store and handle the loads as they would be in service.
4. Check tension, movement and joint condition after a defined interval.
5. Choose the option that holds, then record it in the packing instruction.

Write down the result even if PP wins. The next buyer who is offered a cheaper strap needs to know what was tested and why.

Choosing the clip for PP strap

For PP or PET joints made with clips, see YAKU’s sheet metal clips and PET strap pages. Clip, strap and tool should be checked together; performance values are on request. Send the strap reference, tool and load details through the enquiry page, or use our RFQ checklist to prepare the request.