Powder testing: Low cohesion index, catastrophic discharge failure
7 minute read
Why one number is never enough - and what bird sand teaches us about hopper reliability.
The most dangerous assumption in powder testing
There is a classification system widely used in powder technology that assigns flow categories to Carr's Index and Hausner Ratio values: excellent, good, fair, passable, poor, very poor, extremely poor. It is simple, reproducible, and has been in use for decades.
It is also dangerously incomplete. And the Powder Flow Analyser's Cohesion Index, if read in isolation, can lead you to the same wrong conclusion.
The assumption is this: low resistance to flow means reliable powder handling. If a powder scores well on a single-point flowability measurement, it should be safe in hoppers, feeders, and bins.
Bird sand has a Cohesion Index of 7.9. On a standard flowability scale, that is essentially free-flowing. Bird sand also has a Bridging Factor of 2,601 - and causes severe, persistent hopper discharge failures.
Two completely different ways a powder can fail
Powder flow failures come from two distinct mechanisms, and a single measurement cannot distinguish between them.
Failure mode 1: Cohesive bonding (high Cohesion Index)
The powder particles stick together and to equipment surfaces. Resistance is pervasive and continuous - the powder is always difficult to move. Typical symptoms: build-up on hopper walls, poor flow initiation, smearing, inconsistent restart, sensitivity to humidity.
This is what a high Cohesion Index measures. A high CI powder is genuinely cohesive - particles are attracted to each other and resist separation. The flow problem is driven by stickiness.
Failure mode 2: Structural arching (high Bridging Factor)
The powder particles interlock, form force chains, and create stable arches that span hopper outlets. Resistance is sudden and intermittent - the powder may flow normally for a period, then stop completely as a stable arch forms. Typical symptoms: sudden stoppages in hoppers, ratholing, collapse events, strong sensitivity to outlet size and hopper geometry.
This is what the Bridging Factor measures. A high Bridging Factor powder is not necessarily sticky - it may feel free-flowing in your hand - but it forms stable structures under the stress conditions inside a hopper. The flow problem is driven by geometry and particle mechanics, not by surface chemistry.
|
The critical distinction Cohesion Index measures how hard it is to separate particles. Bridging Factor measures how likely the resistance is to fail in sudden, irregular events - arches, bridges, ratholes. These are different failure modes requiring different interventions. Both can cause catastrophic discharge failure. Only one is captured by CI. |
Three real examples
Bird sand - low CI, extreme Bridging Factor
Bird sand is a granular, relatively coarse material. By hand, it feels free-flowing. Its Cohesion Index of 7.9 confirms this - low resistance, particles separate easily. You would classify it as a well-behaved material that should discharge reliably from any reasonable hopper geometry.
Its Bridging Factor is 2,601. The powder forms stable arches repeatedly during testing, producing an extremely irregular, jagged force trace rather than the smooth curve of a genuinely free-flowing material. In a hopper with a marginal outlet size, bird sand will arch, stop flowing, and require manual intervention to restart - despite a CI that says it should be fine.
The Cohesion Index is not wrong. The Bridging Factor is simply measuring something different - structural instability rather than stickiness. Both pieces of information are necessary to predict real behaviour.
Wallpaper adhesive - the extreme case
Wallpaper adhesive powder has a Cohesion Index of 7.4. Again, low - similar to bird sand, and you might classify it as easy to handle. Its Bridging Factor is 1,999. But the story doesn't end there.
Its Compaction Coefficient at low speed is the highest of any material in our sample library. Its Mean Cake Strength is 548 g. Its Cake Height Ratio is 0.93 - meaning 93% of the powder bed has formed a coherent cake after compaction cycling.
Wallpaper adhesive powder is the definitive illustration of why single-parameter classification fails. CI alone classifies it as free-flowing. The full parameter set reveals a material with extreme structural resistance to initial movement, catastrophic bridging tendency, and severe storage set-up. It requires substantial mechanical intervention to restart after any storage period.
Plain flour - the contrast
Plain flour has a Cohesion Index of approximately 14 - moderate, but not dramatically high. Its Bridging Factor is in the 300s - moderate. Both glass and aluminium vessel results are similar, indicating its behaviour is bulk-controlled rather than wall-dependent.
Flour's flow problems are real, but they are driven by a completely different mechanism: consolidation and compressibility. It packs readily under load (high compressibility, 24%), rearranges over time (very high relaxation, 86.8%), and changes bulk density significantly during storage and transport. The dominant risk for flour is not bridging - it is density drift and post-storage set-up.
The correct intervention for flour is completely different from the correct intervention for bird sand or wallpaper adhesive. You only know which intervention is needed if you have read the right parameters.
Bird sand - Test videos, data and behaviour summary
Wallpaper adhesive - Test videos, data and behaviour summary
White flour - Test videos, data and behaviour summary
Cohesion scatter: Separates 'sticky cohesive' powders (high CI) from ‘bridging/arching’ powders (high Bridging Factor) - both can cause flow problems, but in different ways.
How to use CI and Bridging Factor together
Plot them on two axes and you get a clear picture of the dominant failure mode:
| CI | Bridging Factor | Failure mode | Typical intervention |
| High | Low-Moderate | Cohesive fine - smooth, pervasive resistance | Flow aids, surface treatment, humidity control, feeder design |
| Low | High | Structural arching - sudden, geometry-driven stoppage | Hopper geometry, outlet sizing, wall material, vibration |
| High | High | Combined - highest risk | Multiple interventions; this combination is rare but severe |
| Low | Low | Generally free-flowing | Check geometry and equipment design if problems occur |
Neither axis alone tells the story. A powder in the bottom-right quadrant (low CI, high Bridging Factor) will pass every single-point flowability test and cause persistent hopper problems in production.
What this means for testing practice
When you run a Cohesion test on the PFA, always report both parameters. Never draw a conclusion from CI alone. A powder with low CI and high Bridging Factor is not a well-behaved material - it is a material with a different failure mode that a stickiness measurement cannot detect.
The Bridging Factor is particularly sensitive to vessel material - glass vs aluminium - because structural failure mechanisms depend heavily on wall interaction. Testing in both vessels, when behaviour is unclear, reveals whether the failure is intrinsic to the powder or dependent on the surface it contacts. This distinction is directly relevant to hopper liner selection and wall finish specification.
If your Bridging Factor is high, the next steps are not formulation changes - they are geometry and equipment design: outlet size, hopper half-angle, wall material, and whether vibration is needed. If your CI is high, the next steps are surface chemistry: flow aids, surface treatment, humidity control, and particle engineering.
Different numbers. Different root causes. Different solutions.
|
Key takeaway The Cohesion Index and Bridging Factor are not two measures of the same thing. They measure two different failure mechanisms. Both are always worth reading. A low CI powder can cause catastrophic discharge failure if its Bridging Factor is high. Always interpret them together. |
