The Big Stretch: Testing ‘’K Value’’ for ‘’Creep Behaviour’’

Striking the right balance between molecular weight  - or K value - and performance has long been a conundrum for PVC developers. The 2011 Brighton Conference on PVC shed new light on the latest lab results which show oriented PVC as the toughest contender in the pack.

O-PVC pipes or plates - modified by molecular orientation - show strongly improved mechanical properties compared with standard U-PVC.  In fact, their impact resistance is so high they are almost impossible to break. But what’s the optimum K value?

“We’ve known for years that compositions based on low molecular weight resins (for example those used for injection moulding) did not bring a significant improvement after molecular orientation,” says Joel Fumire, Technical Marketing at Solvin. “The range of useful molecular weights for biorientation was not precisely described”.

So it was back to the drawing board. After hundreds of hours stretching to the breaking point to quantify the relationship between the K value and the mechanical properties, the conclusion on O-PVC was available.

The study used mainly bioriented sheets. It covered the traditional range of commercial PVC resins from a molecular weight as low as Kw=50 up to Kw=69.  These plates were put in a laboratory stretching machine in order to realise the biorientation process. 

The examination of mechanical properties of these plates focused on two important properties: impact resistance and long-term creep behaviour which indicates the long term deformation of a plate under stress.  

The results show that contrary to a generally accepted idea, it is not necessary to have a very high molecular weight. The mechanical properties of the plates are at a high level for all K-values except for very low ones (£ 57). It was also observed that for the higher K-values, the gelation (or fusion) of the plates was reduced compared to medium molecular weights. This finding had a significant influence on the final properties of the item.  

Finally, two industrial pipe extrusions, with a high and a medium molecular weight were produced.  Under a pressure of 32 bar, the pipe manufactured with a K value of 67 resisted during 200 hours while the pipe with a K value of 64 lasted for more than 600 hours. The mechanical test confirmed that the medium K value provided the best result.

And that yes, you can prevent creep behaviour, at least in PVC. If only human behavioural change were as easy!