Spun Polyester to Polypropylene Thread Converter
Pick the bag closing thread you run today and see its closest equivalent in the other range, matched on breaking strength or on thickness, with what actually changes on the machine if you switch. Works both ways — spun polyester bag closing thread to polypropylene bag closing thread, or back again.
| Specification | Your thread | Equivalent |
|---|
Logic behind the matching
Two bag closing threads can be called equivalent on two different bases, and they do not give the same answer.
Breaking strength is the right basis for most changeovers, because what you are really replacing is the seam's ability to hold the bag. Thickness, matched on resultant Tex, is the basis to use when stitch appearance and needle fit matter more — switching on thickness keeps the seam looking and feeling the same, but may change what it can carry.
Whichever basis you pick, the tool returns the nearest thread in the other range and states the exact percentage between the two. Within 25% it reads as an equivalent. Beyond that it is still the closest thread we hold, and the percentage tells you how far apart they are — our polypropylene bag closing thread range extends both lighter and heavier than our spun polyester bag closing thread range, so at those ends the nearest ticket sits further away.
Every specification behind the comparison — construction, breaking strength, elongation and needle range — comes straight from those two product pages. Resultant Tex is calculated from the published construction: 590.5 ÷ Ne × ply for spun polyester bag closing thread, denier ÷ 9 for polypropylene bag closing thread.
Spun polyester bag closing thread vs polypropylene bag closing thread
| Specification | Spun Polyester | Polypropylene |
|---|---|---|
| Breaking strength range | 5.08 – 9.02 kg | 1.80 – 15.00 kg |
| Resultant Tex range | 120 – 240 | 47 – 560 |
| Elongation | 12 – 24% | 20 – 30% |
| Seam stability under load | Very low elongation and high modulus — seams stay tight under load | Higher elongation — more give in the seam under the same load |
| Moisture and humidity | Standard polyester behaviour; not specified for hydrolysis resistance | Virtually no moisture absorption; resists hydrolysis, mould and mildew |
| Chemical exposure | Not specified for chemical exposure | Chemically inert |
| UV and sunlight | Excellent UV resistance, stain free and colourfast | UV resistant, suited to outdoor bags and bulk packaging |
| Abrasion resistance | Abrasion and friction resistant on woven PP, HDPE and jute without fluffing | Abrasion resistant on rough surfaces |
| Machine performance | Micro-molecular lubrication for long continuous runs | Uniform twist feeds consistently on portable, semi-automatic and high-speed heads |
| Weight | Conventional polyester density | Low density — excellent strength-to-weight ratio |
The number that decides most arguments
Elongation. Spun polyester bag closing thread runs at 12 – 24% across our range, polypropylene bag closing thread at 20 – 30%. Under the same load the polypropylene seam has more give in it. On a light sack that is irrelevant; on a stacked, handled, heavy bag it is the difference between a seam that stays tight and one that works loose. That single line is why spun polyester bag closing thread remains the default for FIBC panels and cement.
Choose spun polyester bag closing thread for
- Heavy bags where the seam must not open under load
- FIBC and jumbo bag panels, loops and reinforcement
- Cement, mineral and other high-weight packaging
- Outdoor and yard storage with prolonged sun exposure
Choose polypropylene bag closing thread for
- Damp, humid or wash-down environments
- Chemical, mineral and bulk agro packaging
- High-speed closing lines
- Weight-sensitive applications where every gram counts
Frequently asked questions
Which is stronger, spun polyester or polypropylene bag closing thread?
Neither, as a blanket statement — it depends on the ticket. What differs is how each fibre gets there. Polypropylene reaches a given breaking strength at a higher Tex, so the polypropylene thread that matches a spun polyester one on strength is the thicker of the two, and usually needs a heavier needle.
When should I choose polypropylene bag closing thread over spun polyester?
When moisture, humidity or chemical exposure is the problem. Polypropylene absorbs virtually no moisture and is chemically inert, which suits damp stores, wash-down areas and chemical or mineral packaging. Its low density also gives a good strength-to-weight ratio where weight matters.
When should I stay with spun polyester bag closing thread?
When the seam must not give under load. Spun polyester runs at 12–24% elongation against 20–30% for polypropylene, and that lower stretch with high modulus is what keeps a heavy bag seam tight. For FIBC panels, cement and mineral bags, and anything stacked or handled hard, it is the safer choice.
Can I swap threads without changing the needle?
Sometimes, but do not assume it. The equivalent finder shows both needle ranges side by side and tells you whether the swap moves you up or down a size. A strength-matched change from spun polyester to polypropylene often means a heavier needle, which the fabric also has to tolerate.
Related calculators
- Sewing Thread Count Converter Ne, Tex, dtex, denier, metres per kg and yards per lb, in both directions and ply-aware.
- Bags Per Cone Calculator for Bag Closing Thread How many bags one cone will close, and the cones a run of 10,000, 1 lakh or 5 lakh bags needs.
- Monthly Bag Closing Thread Requirement Calculator Bags per day into cones per month, with how long a single cone lasts on your line.
- Bag Closing Needle Size Calculator The needle range published for a given bag closing thread, or every bag closing thread that runs on the needle already in your machine.
Thinking about changing over?
Send us your current thread, your bag and your machine, and we will tell you whether a switch is worth making before you trial it on the line. Talk to our technical team.