Curing control

For the renovation of sewer pipes, cured-in-place pipe lining (CIPP) has proven itself as a standard and well-established technique.

However, the technology still retains certain challenges.
As the final production step of a cured liner occurs out in the field at the jobsite, quality control is of particular importance.

But with a wide range of general and highly variable conditions experienced in the outside world, including humidity, groundwater, temperature, and storage conditions, every jobsite is unique.

This uncertainty extends to the installation process, with general conditions such as weather being unpredictable and subject to rapid change.

For these reasons, CIPP is unlikely to become a fully standardised product.

Beyond the controllable confines of the laboratory, the environmental parameters out in the field are impossible to fully control or account for.

If problems occur on a jobsite, the conventional quality assurance during the hardening of the CIPP product – temperature control, for example – is not a precise enough indicator to detect what is really happening.

Adding to the challenges, most quality assurance of the physical product occurs after the curing process has finished, usually during the post-installation inspections and after the jobsite is closed.

It is in this context that the notion of in-situ quality control using dielectrical analysis (DEA) comes into play. DEA is a method of measuring the curing behaviour of materials such as adhesives, paints and thermosetting resins.

With every CIPP product consisting of a thermosetting resin and a carrier material (such as glass, fabric or synthetic fibre), DEA can potentially be an effective means to measure the CIPP curing process and act as the main window to observe what is happening.

After all, to understand the reaction is to ensure the quality of the product.

Reactionary analysis

When working with thermosetting resins, a detailed knowledge of the reactivity of every step of the process is fundamental in order to achieve fully cured composite parts within a minimum cycle time.

Analytical methods are therefore used during the entire development process, from resin formulation to composite processing to final composite analysis.

In addition to providing information on a thermoset resin or composite’s mechanical properties, thermal analysis methods also provide a comprehensive insight into the material’s curing behaviour and achievable properties.

As well as DEA, other methods such as differential scanning calorimetry are well-established techniques for investigating cross-linking behaviour.

Other processes such as dynamic mechanical analysis, thermomechanical analysis, thermogravimetric analysis and laser/light flash analysis can also reveal further information about the properties of a composite material.

Inside the DEA method

DEA is an essential technique across a wide range of application areas in the composite industry.

It helps monitor and understand curing behaviour during the research and development of resin formulations as well as process development in lab-scale processes.

Further, DEA technology can be used for on-line quality control in the manufacturing of composite parts with thermosetting matrices.

DEA provides an on-line insight into the state of cross-linking by detecting the change in dielectric properties of a thermosetting resin.

To this end, the uncured resin must be in contact with a dielectric sensor consisting of two electrodes.

A sinusoidal electrical voltage (excitation) is applied to the electrodes and the resulting current and corresponding phase shift is measured.

The response signal correlates with the ion mobility (also known as ion conductivity) of the resin and the alignment of dipoles.

As the curing reaction progresses, the sample material becomes increasingly viscous.

As a consequence, the mobility of the charge carriers decreases, followed by a corresponding attenuation of the amplitude and an increased phase shift.

The dielectric coefficient ëµr describes the resulting signal, which is comprised of a real part ëµr’ (dielectric permittivity) and an imaginary part ëµr” (dielectric loss factor). In addition, ëµr” is proportional to the ion conductivity ìÄ, which is the reciprocal value of the ion viscosity1.

Since ion viscosity, like shear viscosity, changes as a function of the degree of curing, DEA is the ideal tool for in-situ monitoring of the cross-linking progress of any thermosetting resin.

A typical cure monitoring measurement curve at 1 kHz for an epoxy resin is displayed in Fig. 4, where the ion viscosity and sample temperature are shown over time.

The logarithmic ion viscosity exhibits a significant decrease from 107.8 ë©*cm to 106.9 ë©*cm as the sample temperature increases from room temperature to 150å¡C, which enhances the mobility of ions in the uncured resin.

The sharp increase in ion viscosity after 8.7 minutes indicates the beginning of network formation by cross-linking of the epoxy, which hinders the ion mobility.

This dielectric behaviour correlates with the rheological viscosity, which can be analysed by means of a plate-plate rheometer.

Application of DEA on a CIPP installation site

The no-dig nature of CIPP means the thermosetting resin within the material is cured by exposing it to thermal energy or a stimulated emission of UV light.

This process takes place in-situ (on the jobsite itself) to produce a new, long-lasting, load-bearing pipe.

By placing a DEA sensor in the thermosetting resin after the installation of the liner (but before the commencement of the curing process), the cure phase can be measured, recorded and traced.

However, because of the applied electrical voltage, the sensor should be prevented from any water contact to minimise the risk of any short-circuiting faults.

To better illustrate the process, Fig. 5 shows an in-situ curing measurement from a jobsite.

The red line is the temperature, with the exothermic peak occurring after about 38 minutes at 90å¡C.

The green lines provides the ion viscosity of the thermosetting resin at a certain time and at different frequencies.

Because of the proceeding polymerisation, the material gets more and more solid.

At higher curing rates (higher viscosity), the lower frequencies (1-10 Hz) are relevant, because the ion movement in high viscous materials is handicapped.

This effect leads to an increasing ion viscosity up to a certain final value.

If there is no change in viscosity, the curing is at a maximum value and the job is done.

If the reaction is stopped before reaching the maximum viscosity value, the curing is incomplete and the mechanical, chemical, and long-term values are insufficient.

This can then result in pipe damage or breakdown.

DEA is here to stay

By using DEA measurement, it is easy to control the behaviour of viscosity, and therefore the curing rate, during the curing process of thermoset resins for CIPP products. In particular, the independence of the measurement of humidity and cooling down by means of groundwater gives us a reliable value for quality control in the CIPP market.