Now one instrument can test high- & low-shear viscosity.

Consistent melt viscosity is an important criterion for achieving product

uniformity. Unfortunately, rheological testing can be time-consuming and labor-intensive, and one instrument is often not sufficient to test the full range of shear rates desired. A new measurement device is said to solve this long-standing problem by making it faster and easier to check resin viscosity over a wide range of shear rates.

For routine QC work, capillary rheometers are widely used to assess resin processability. They are also used for R&D to qualify which material best fits a process or application. However, a conventional single-bore/single-die capillary rheometer often requires making multiple measurements that can take as long as half a day. A new triple-bore capillary rheometer from Goettfert of Rock Hill, S.C., reportedly provides a variety of flow measurements in a single test within half an hour. This instrument makes it easy to conduct normally tedious tests like Bagley-corrections, which correlate with elongational viscosity or melt strength, or Mooney viscosity measurements to determine slip velocity.

Stretching the speed range

Goettfert's new Rheograph 6000 uses microprocessor-controlled servo hydraulics to push capillary rheometry into new territory. Traditionally, the linear flow behavior of polymer melts at low shear rates has been characterized with rotational rheometers, while the nonlinear shear-thinning region at high shear rates is measured with capillary rheometers, which typically have electromechanical drives. According to Goettfert sources, there has been a need to broaden the speed range of capillary rheometers to cover both high and low shear rates. The new instrument accomplishes that with a servo-hydraulic drive system that combines a high-pressure cylinder with a low-pressure cycle and high-precision control.

Rheograph 6000 uses a microprocessor-controlled stepper motor to actuate the high-load hydraulic system. The servo-hydraulic drive offers a dynamic speed range of 1:400,000 as compared with 1:20,000 when using a single-bore capillary rheometer. Minimum piston speeds of 0.0001 mm/sec can be obtained for very low shear. For ultra-high shear rates, the maximum ram speed is 40 mm/sec, twice that of the older Rheograph 2003 model.

The new instrument also offers a maximum force of 13,500 lb, three times higher than that of its predecessor.

Why three bores are better

The Rheograph 6000 utilizes a new measuring cell concept, which consists of a triple-bore, triple-die system enclosed in a single barrel. According to Goettfert, this development expands the range even farther. Parallel operation of three dies of 0.5 to 3 mm diam. gives a shear-rate ratio larger than 1:500. As a result, a total shear range of eight decades can be achieved, compared with three decades typically achievable with a single-bore unit.

According to Goettfert, the triple-bore/triple-die configuration offers two possibilities. Within a single experiment, the shear rate range can be extended by using different die-orifice diameters, or the formerly time-consuming triple-point Bagley-correction can be performed in one experiment using a set of capillaries with the same orifice diameter but different lengths. Also, the extrudate swell as a function of shear rate can be determined to provide a measure of the melt's viscoelastic behavior.

Elastic parameters such as the first normal stress difference can be estimated by the unit's software. Optional die-swell and melt tensile tests are possible with the aid of the company's Rheotens elongational tester. The Rheograph 6000 sells for around $100,00-150,000.