1. Unimodal vs bimodal HDPE — and why bimodal is now preferred

Traditional unimodal HDPE is produced with a relatively narrow molecular weight distribution (MWD). It crystallises quickly after melting, develops strength rapidly, and behaves predictably under short-term mechanical testing. For decades, most geomembrane welding standards—including ASTM D6392—were written around this material behaviour.

Bimodal HDPE, by contrast, deliberately combines two distinct molecular populations:

• Short, mobile chains that enable processing and early nucleation

• Long, high-molecular-weight chains that deliver superior long-term performance

The practical purpose of bimodal HDPE is not faster strength gain, but better durability. Compared with unimodal resins, bimodal HDPE is:

• More ductile and forgiving during installation
• Significantly higher in stress crack resistance (SCR)
• Better suited to applications where tensile strain is unavoidable
• More resistant to slow crack growth, surfactants, and long-term degradation mechanisms

These advantages are why bimodal HDPE now underpins high-performance geomembranes and why it is explicitly recognised in the latest GM42 HDPE specification. However, its different crystallisation kinetics mean it does not always behave the same way as unimodal HDPE immediately after welding.

That mismatch is where the current testing standard falls short.

2. The problem everyone sees, but the spec doesn’t

On many projects, welds in unimodal HDPE pass shear testing immediately at the standard ASTM D6392 crosshead speed of 50 mm/min. Bimodal HDPE welds, however, often under-read in shear during the first hour or two, particularly under elevated liner temperatures, before comfortably meeting specification after 24 hours.

The industry response is familiar:

• Re-welding otherwise sound seams
• Excessive cooling or handling delays
• Waiting periods that stall production

None of these improve weld quality. They simply compensate for a test method that does not reflect the material’s early-age behaviour. Recent site trials comparing bimodal and unimodal HDPE under sun-heated conditions showed this behaviour to be repeatable and systematic, not workmanship-related.

3. Why bimodal HDPE behaves differently

Bimodal polyethylene crystallises differently after melting. Short chains nucleate early, while long chains, which are responsible for toughness and crack resistance, diffuse, entangle, and crystallise more slowly. Until that structure relaxes and locks in, the weld zone exhibits time-dependent viscoelastic behaviour.

At the molecular level, wider MWD systems (bi- and trimodal resins) show:

• Slower crystal growth when long-chain diffusion dominates
• Temporary impedance of ordering as early nuclei form
• Delayed stabilisation of load-bearing morphology

In practical terms, this means strength exists, but it is still forming. A slow shear test conducted too early measures creep and compliance as much as bond strength.

4. What ASTM D6392 currently says—and doesn’t

ASTM D6392 fixes seam shear testing speeds as follows:

• HDPE and LMDPE: 50 mm/min

• LLDPE, VLDPE, fPP, PVC: 500 mm/min

A note permits the use of a manufacturer-recommended speed when required, but in the absence of explicit guidance, Sections 7.1 and 7.2 default to the standard values. Bimodal HDPE is not recognised as a separate material class, despite its known differences in crystallisation kinetics and early-age mechanical response. As a result, the standard unintentionally penalises higher-performance resins during the very period when QA decisions are most time-critical.

5. Field evidence — key findings

Recent site trials comparing bimodal and unimodal HDPE welds under realistic thermal conditions showed:

• Test speed matters
  Increasing shear test speed from 50 mm/min to ~150–200 mm/min produced higher and more representative shear values for bimodal HDPE within the first hour after welding.

• Temperature matters, but speed matters more
  Lower coupon temperatures improved results, but cooling alone did not reliably resolve low early-age shear readings at 50 mm/min. Test speed remained the dominant factor.
  

• Unimodal HDPE is less sensitive
  Unimodal HDPE welds showed comparatively stable shear results across test speeds and temperatures.
  

• Handling and grip compliance influence outcomes
  Reduced free length and consistent grip placement improved measurement consistency but did not change the underlying trend.

The implication is clear: early-age bimodal welds may be under-estimated without deeper knowledge.

6. Interpretation

Immediately after welding, bimodal HDPE weld zones are still re-ordering. A slow crosshead speed allows viscoelastic deformation to dominate the measurement, producing artificially low shear values.
Increasing the test speed to approximately 200 mm/min reduces time-dependent creep during the test itself and captures the true strength of the fused interface. The result aligns with the material’s delayed crystallisation kinetics and removes false negatives without changing the welding process.

7. Practical guidance for sites using bimodal HDPE

Until the standard is updated, a pragmatic QA approach for bimodal HDPE includes:

• Record coupon temperature and time-since-weld at testing.
• If early-age shear at 50 mm/min is borderline, re-test a sibling coupon at ~200 mm/min, clearly reporting the speed used.
• Use light water cooling to stabilise coupons where appropriate, but treat test speed as the primary control variable.
• Maintain consistent grip placement close to the weld line to minimise compliance artefacts.

8. Proposed change to ASTM D6392

Add a bimodal-specific provision to Sections 7.1 / 7.2:

For HDPE geomembranes manufactured with a multimodal (e.g. bimodal) molecular weight distribution, seam shear testing performed within 0–24 hours after welding shall be conducted at a constant crosshead speed of 200 mm/min. For testing conducted ≥24 hours after welding, either 50 mm/min or 200 mm/min may be used, provided the speed is reported. Coupon temperature at time of testing shall be recorded

Rationale:
Bimodal HDPE exhibits slower post-melt crystallisation driven by long-chain diffusion. Early-age shear testing at 50 mm/min under-represents weld strength compared with 150–200 mm/min on the same seams. Aligning test speed with material kinetics yields accurate QA results without delaying construction, while remaining consistent with the existing D6392 framework.

9. Summary

• Bimodal ≠ unimodal, especially in the first hour.

• Raise the test speed to read the weld, not the creep.

• Codify the change so QA keeps pace with modern HDPE technology.