Phase-Shift Curemeter

for SOFT-SETTING MATERIALS and LIQUID RESIN CURES

Liquids which gel or set present a challenge to any process monitoring. Such products have widespread applications (adhesives, coatings, sealants, composites, cosmetics, foodstuffs, etc.,) and inadequate monitoring can turn their manufacture into a hit and miss procedure.

Yet the challenges are obvious. The techniques for characterising the liquid state (i.e. materials that flow) are usually quite different from those for solids (i.e. materials which don't flow). Bridging the two can demand specialist knowledge and access to sophisticated instrumentation. This is an area for the research lab, not the shopfloor.

The Phase-Shift Curemeter from Polymatrix brings a new simplicity to tracking the liquid-solid transition. Forget the instruments which deliver a treatise on rheology and leave you wondering what the data means. The Phase-Shift Curemeter allows users to focus on the cure - not the theory. It brings a new immediacy to the monitoring gel formation and curing liquid systems.

Not only does it provide outputs for the technologist, it delivers these in real time. There is no longer a need to wait until the cure is over to establish progress at an intermediate stage. Nor is there uncertainty over the state of cure at the start. All reactive systems have a shelf life. Questions which can now be answered include:

  • was it zero cure at the start?
  • how far has the cure progressed now?

Delivering degree of cure in real time is a true innovation. The Phase-Shift Curemeter achieves this and provides a continuous trace which is a fingerprint of the cure.

With the data in hand, the next question is,

  • how does this cure compare with previous cures?

Comparisons over a wide range of samples are only possible if cures can be recorded on the same scale for all samples - i.e. on an absolute scale of cure. The Phase-Shift Curemeter does just that.

An Absolute Scale of Cure

The full scale of cure from a viscous liquid to an elastic solid is a continuous evolution of properties passing through stages which might be described as follows.

Put simply, cure is the development of elasticity in a material: zero cure is zero elasticity and 100% cure is 100% elasticity. A scale of cure is therefore the percentage elasticity in the sample. On this scale the gel point takes on a special significance. It is the half-way house between a liquid and solid - it is 50% cure.

The only parameter which is uniquely dependent on the level of elasticity in a sample is the phase angle delta between stress and strain for a sample under cyclic deformation, i.e.,

  • for a purely viscous liquid, stress and strain are 90° out of phase, i.e. delta = 90°
  • for an elastic solid, stress and strain are in phase, i.e. delta = 0°
  • at intermediate states, 90 > delta > 0°
  • at the gel point, delta = 45°

For a curing system moving from a liquid to a solid, delta reduces over time. If delta is used to define cure then a universal expression for state of cure emerges, i.e.

% cure = (90 - delta)/0.9

and the gel point is at 50% cure.

The upper and lower limits are fixed to give an absolute scale of cure. If the sample has a finite level of elasticity at the start, it will be recorded as such. How far up the scale the cure starts gives an insight into shelf life. Other parameters, such as work life, set time, etc., all have their place in cure control. An absolute scale of cure provides the template for more systematic characterisation.

The New Instrument

An instrument that works on these principles is both a curemeter and a gel timer. This is the Phase-Shift Curemeter. A sinusoidal vibration is transmitted through a sample and the phase difference between the input and output signals is monitored continuously. No other responses are required and no special constraints are placed on sample geometry. The sample cup can be disposable as can the embedded probe for the output signal.

The Phase-Shift Curemeter offers a very simple arrangement for the sample cell. The sample well is a disposable cup held in place by a magnet. The cup can be steel, as in a beer bottle top, or paper or plastic with a steel washer dropped inside. Sample retention doesn't get simpler!

Sample retention in the Phase-Shift Curemeter

The probe is a disposable (non-magnetic) rod - e.g. made from aluminium, wood or a CFRP composite.

The performance of the instrument can be illustrated with respect to the cure of an epoxy resin. The changes within this sample are huge - from free-flowing liquid to rigid solid - yet the measuring technique can encapsulate this in a clear and simple cure profile.

The trace below shows the percentage cure, determined from phase angle at 0.33 Hz, for a sample (5 ml) of 'quick-set' epoxy at ambient temperature in a beer-bottle top with a disposable aluminium probe.

Here is data that everybody can understand. Moreover, it was data obtained at no risk to the instrument. The sample, its container and the (firmly) embedded section of rod are simply removed and discarded. It is no more than a few seconds' work.

The Phase-Shift Curemeter provides a refreshingly new approach to tracking the liquid-solid transition. Simplicity is the key - contact Polymatrix to find out what the instrument can do for you.

% Cure vs time for 'quick-set' epoxy (gel time marked in red)

 

For a copy of a technical paper on the Phase-Shift Curemeter, click HERE.