Editor: Otto Tromm | Test date: 3 Jun
Introduction
What happens when a low-cost polypropylene rope that has been stored at the back of a warehouse for years is put through a breaking strength test?
This orange 4 mm polypropylene rope had been in storage for 7.5 years, and the test results showed something notable.
The average breaking strength of this 7.5-year-old 4 mm polypropylene rope was 2.90 kN (296 kg), measured across 5 tests.
After 7.5 years, this rope still meets the ISO standard, which sets a minimum of 285 kg (2.80 kN).
This is consistent with an earlier test on 8 mm polypropylene rope, which also met the standard after 10 years. Read that test here.
What is 4 mm polypropylene rope?
Polypropylene (PP) is the lightest synthetic rope material and the only one that floats on water by nature. This rope has a classic 3-strand twisted construction: three fibre bundles twisted around each other to form a round, stable structure.
That construction makes the rope easy to splice and compatible with basic knots, although knotted strength does reduce breaking strength.
Polypropylene is the most cost-effective synthetic fibre used in rope applications. The trade-off is lower UV resistance compared to polyester or nylon, and a lower breaking strength per diameter. PP is generally not recommended for safety-critical applications.
How was the breaking strength of 4 mm polypropylene rope measured?
The tests were carried out on a universal testing machine fitted with rope-specific clamps suitable for thin twisted ropes.
The test speed was 20 mm/s, in line with the standard method for ropes under 12 mm diameter.
5 individual tests were carried out on the same rope, which was 7.5 years old at the time of testing.
The rope was tested without pre-tension: the free length between the clamps was brought to a taut position gradually, but no measurable pre-load was applied.
This is consistent with the practical test setup used at Prorope. Results were recorded as peak load at the point of failure.
Important: the original breaking strength of this specific rope was not known. A direct comparison between new and aged condition is therefore not possible.
The test results are compared against the ISO standard for new 4 mm polypropylene rope, which sets a minimum of 285 kg (2.80 kN).
Testing was conducted in accordance with ISO 2307:2019.
Breaking strength of 4 mm polypropylene rope
The 5 tests produced the following results:
- Average breaking strength: 2.90 kN (296 kg)
- Highest measured value: 3.09 kN
- Lowest measured value: 2.74 kN
- Number of tests: 5
The spread between the highest (3.09 kN) and lowest (2.74 kN) measurement is 0.35 kN. That is a relatively narrow spread for a 7.5-year-old rope, indicating consistent material behaviour despite its age.
The break point pattern is notable: in 4 of the 5 tests, 2 strands broke first, with the third strand failing last. In the 5th test, all 3 strands broke near-simultaneously.
This is consistent with the twisted construction: the strands do not always share the load equally, and minor irregularities in the rope or the clamping determine which strand fails first.
The most significant result: this 7.5-year-old rope, with an average of 2.90 kN (296 kg), still meets the ISO standard, which sets a minimum of 285 kg (2.80 kN) for new 4 mm polypropylene rope.
Even the lowest measured value of 2.74 kN (280 kg) falls only marginally below this standard. The fact that this rope was stored without direct UV exposure likely explains why the material retained most of its tensile strength.
4 mm polypropylene rope compared to other ropes
To put the performance of this rope in context, the test results are compared with other 4 mm ropes tested by Prorope:
- HMPE 12-strand braided: 17.93 kN — 6.2× stronger than this PP rope
- HMPE with sheath: 6.70 kN — 2.3× stronger
- HMPE: 5.59 kN — 1.9× stronger
- Polypropylene 4 mm (this rope, 7.5 years old): 2.90 kN (296 kg)
The difference compared to HMPE is substantial. HMPE has the highest strength-to-weight ratio of all synthetic fibres: a 12-strand braided HMPE at 4 mm reaches 17.93 kN — more than 6× the breaking strength of this polypropylene rope at the same diameter. Where absolute strength in 4 mm is required, HMPE is the appropriate choice. Where a low-cost, lightweight, floating cord is needed for temporary use, polypropylene is fit for purpose.
When to use 4 mm polypropylene rope
This rope is best suited to applications where low cost, light weight, and buoyancy are the determining factors — not maximum strength or long-term outdoor service life.
- Temporary demarcation and marking: construction sites, fields, event grounds. Low cost to deploy, easy to replace.
- Packaging and bundling: light loads indoors or in sheltered environments where UV is not a factor.
- Float lines and water applications: polypropylene floats by nature; well-suited as a marker line on water.
- Storage in sheltered conditions: as demonstrated by this rope, PP rope stored without direct UV exposure retains a large proportion of its tensile strength over many years.
- Agricultural use: temporary fastening, supplementing fencing, plant guides — applications where regular replacement is acceptable.
The working load limit (WLL) of this rope depends on the required safety factor. At a safety factor of 5:1 (standard for general lifting applications), the WLL based on the average tested breaking strength of 2.90 kN is approximately 0.58 kN (59 kg).
Applications involving personnel or critical loads require a higher safety factor; this rope is not suitable for those uses.
When is polypropylene rope not suitable?
Polypropylene has limitations that are relevant to any professional application:
- Prolonged UV exposure: PP degrades faster than polyester or nylon under direct sunlight. Rope left outdoors without protection can lose significant strength within one to two seasons. This rope performed well because it was stored in a sheltered environment.
- Safety-critical applications: with an average breaking strength of only 2.90 kN and relatively low abrasion resistance, this rope is not suitable for lifting, lifelines, climbing applications, or any situation where failure poses a risk to persons.
- High tensile load: compared to HMPE (17.93 kN at the same diameter) or even polyester, PP delivers the lowest breaking strength per diameter of the common synthetic materials.
- Sustained static load at elevated temperatures: polypropylene exhibits creep (permanent elongation) under sustained load at higher temperatures. Do not use for applications where the rope remains under tension in direct sunlight for extended periods.
- Shock loading: the low elongation of PP (lower than nylon or polyester) makes it less suitable for applications involving dynamic or shock loading, such as tow lines or anchor lines.
Alternatives to 4 mm polypropylene rope
For applications where polypropylene falls short, two straightforward alternatives are available:
- Braided polyester (4 mm, per metre, White) — polyester offers significantly better UV resistance and higher abrasion resistance. For outdoor applications with prolonged exposure, polyester is the more suitable choice. Breaking strength is generally higher than PP at the same diameter, and the elongation (10–15%) provides moderate shock absorption.
- Braided nylon (polyamide) (14 mm, price per metre, White) — nylon has the highest elongation of all synthetic fibres (20–35% at break) and absorbs shock loading effectively. Well-suited for tow and anchor lines. Note: nylon loses 10–15% of its strength when wet — a relevant factor for load calculations.
Conclusion
This 7.5-year-old 4 mm polypropylene rope is best suited to temporary, lightly loaded applications in sheltered environments — such as marking, bundling, and float lines — where low cost and light weight take priority over maximum strength or long outdoor service life.
The fact that after all those years the rope still meets the ISO standard of 285 kg with an average of 296 kg confirms that polypropylene stored in sheltered conditions retains its value — but it remains unsuitable for safety-critical or long-term outdoor applications.
This test was carried out by Otto Tromm
The test data were collected by Prorope. This text was generated with AI on the basis of those data and checked for factual accuracy. Read how we test and publish →