Lodge Stressmeter®

 Dielectric
    Analyzers

 Viscometers

 Rheometers

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Accurately measures first normal stress difference and viscosity
On-line and lab
Wide shear rate range

Non-linear better than linear

An International Union of Pure and Applied Chemistry Working Party showed that two low-density polyethylene resins with significantly different film-blowing properties gave the same values for the dynamic modulus G' (a linear elastic property) but measurably different values for the first normal stress difference N₁ (a non-linear elastic property). The resins could not be differentiated by means of intrinsic viscosity or GPC measurements.

The Lodge Stressmeter® is a new slit-die rheometer designed to measure N₁ and the viscosity h of liquids in steady shear flow, in both on-line and sample modes, up to shear stress and shear rate values higher than those accessible with any available commercial rheometer.

Sensitive on-line sensor

N₁ is extremely sensitive to small changes in the high-molecular-weight end of polymer molecular weight distributions for concentrated polymer solutions (fig. 1) and molten polymers.

Figure 1: Adding 1 part in 10,000 of high molecular weight polyisobutylene to a stirred concentrated solution of medium molecular weight polyisobutylene doubles N₁ but has negligible effect on h

Robust Rheometer

The Stressmeter has no moving parts (other than a gear pump); the pressure transducers, designed and constructed in house are very stable. As a result, the Stressmeter is robust. By way of demonstration, the Stressmeter which was used to record the fig. 1 data was once struck with a hammer during measurement; there was no noticeable effect on the data or subsequent performance.

Molten Polymer Elasticity

One version of the Stressmeter has been designed for making measurements on molten polymers and other high viscosity liquids. Fig. 2 shows agreement between the values of N₁, over a common range of values of shear stress, obtained for a sample of degassed molten polystyrene from a cone-and-plate rheometer and a Melt Stressmeter.

Figure 2: For molten degassed polystyrene, Stressmeter N₁ data agrees with cone-and-plate data (kindly provided by Prof. J. Meissner)

Oils at high shear rates

Another model of the Stressmeter has been used for measurements (in sample mode only) of N₁ and h for oils and other low-viscosity liquids at sheear rates of up to 10⁶ and 10⁷ sec^-1 respectively at temperatures up to 200°C.