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Advanced composites and thermosets have revolutionized wind turbine, aircraft, aerospace vehicle, and automotive development. But companies using these materials often don’t understand how the materials cure during manufacturing. Only dielectric cure monitoring, also known as dielectric analysis (DEA), offers critical insights into composite and thermoset processing in real time. This assures users of the integrity of their processes and their materials—and of the reliability of their finished products. Read on for answers to frequently asked questions about dielectric cure monitoring. 


Frequently asked questions

What is dielectric cure monitoring (DEA)?

• Dielectric cure monitoring is a thermal analysis technique for determining cure state.

• Dielectric cure monitoring tracks the cure state of a material by measuring the electrical properties of permittivity and resistivity.

• Permittivity (ε’) is related to energy storage in a material.

• Resistivity (ρ) is related to energy loss in a material. Resistivity is also called ion viscosity.

• Ion viscosity (resisitivity) is proportional to mechanical viscosity for significant portions of cure. See figure 1. on viscosity tracks cure state throughout cure, even after ion viscosity deviates from mechanical viscosity. See Figure 1.

FAQ Figure 1 Comparison of viscosity and ion viscosity during cure.png

Figure 1: Comparison of resistivity (ion viscosity) and mechanical viscosity during cure.

The dielectric cure curve is characterized by four critical points:

• CP(1)–A user defined level of ion viscosity that is typically used to identify the onset of material flow at the beginning of cure.

• CP(2)–Ion viscosity minimum, which typically also corresponds to the physical viscosity minimum. This Critical Point indicates the time when the crosslinking reaction and resulting increasing viscosity begins to dominate the decreasing viscosity due to melting.

• CP(3)–Inflection point, which identifies the time when the crosslinking reaction begins to slow. CP(3) is often used as a signpost that can be associated with gelation.

• CP(4)–A user defined slope that can define the end of cure. The decreasing slope corresponds to the decreasing reaction rate. Note that dielectric cure monitoring continues to reveal changes in the evolving material past the point when mechanical measurement of viscosity is not possible.


What are the benefits of dielectric cure monitoring?

• Dielectric cure monitoring provides insight into cure state.

• DEA can determine the effects of chemistry and formulation.

• DEA can determine the effects of time, temperature and other process parameters.

• Dielectric cure monitoring saves time, effort and expense.

• Electrical measurements are simple.

• Instruments and software are easy to set up and use.

• Sample preparation is simple.

• Sensors are rugged and can be used in presses, molds or ovens.

• Samples can be applied to sensors in any form.

• Materials can be tested with production process configurations.

• Materials can be tested under production process conditions.

• The same measurement can be used in R&D, QA/QC and manufacturing.

• Data from Research and Development will be the same as data from Quality Assurance/Quality Control and manufacturing.

 

What types of companies can benefit from dielectric cure monitoring?

• Raw resin/materials manufacturers

• Suppliers of monomers, resins and catalysts

• Suppliers of adhesives, paints and coatings

• Suppliers of pre-impregnated (pre-preg) composites

• Bulk Molding Compound (BMC)/Sheet Molding Compound (SMC)

• Epoxy-fiber/Polyester-fiber/Polystyrene-fiber thread, sheet or laminates

• Manufacturers of composite end products

• Aircraft

• Automobile

• Electronic components

• Consumer products

• Government agencies with R&D

 

Which applications can use dielectric cure monitoring?

• Formulation, reaction rate, cure and process development and monitoring

• Diffusion

• Water and solvent diffusion

• UV curing

• Dental adhesives

• Optical adhesives

• Photoresist

• Nondestructive materials testing

• Rheology

• Research & Development

• Quality Assurance/Quality Control

• Manufacturing

 

What materials can be studied with dielectric cure monitoring?

• Thermosets

• Epoxies

• Acrylics

• Silicones

• Polyesters/polyurethanes/polystyrenes/polyimides/polyamides

• Composites and laminates

• Bulk Molding Compounds (BMC)

• Sheet Molding Compounds (SMC)

• Paints, coatings and adhesives

• Oils

• Pharmaceuticals

 
What are thermosets? What are thermoplastics?

• Thermosets are materials which solidify (cure) with an irreversible reaction.

• Monomers link into a network and form a polymer.

• Catalysts often facilitate the curing reaction.

• The reaction rate increases as temperature increases.

• Thermosets cannot melt and be reformed.

• Ion viscosity is a measure of the state of cure and network formation.

• Dielectric cure monitoring provides valuable information about thermosets.

• Thermoplastics are materials which melt and can be reformed multiple times.

• Thermoplastics do not cure and ion viscosity is proportional to temperature.

• Dielectric cure monitoring usually does not provide useful information about thermoplastics.

 

What processing environments can use dielectric cure monitoring?

• Ovens

• Presses and molds

• Autoclaves

• Pultruders and extruders

• Batch reaction vessels

• Injection molding

 

How does dielectric cure monitoring work?

The dielectric properties of conductivity σ, and permittivity ε, arise from ionic current and dipole rotation in the bulk material. For polymers, mobile ions are typically due to impurities and additives, while dipoles result from the separation of charge in the monomers making up the material. When analyzing dielectric properties, it is possible to separate the influence of ions from dipoles, as shown in Figure 2, in order to consider their individual effects.

FAQ Figure 2 ion mobility and dipole rotation in curing thermosets.gif

The flow of ions under the influence of an electric field is responsible for conductive current, and therefore for conductivity σ and its inverse, resistivity ρ. Consequently, the effect of mobile ions can be modeled as a conductance, as shown in Figure 3. This conductance may be frequency dependent, and will change as the bulk material changes. The mobility of ions highly depends on the nature of the medium--ions flow more easily through a material with low viscosity and with greater difficulty as the viscosity increases.

FAQ Figure 3 conductance and capacitance in curing thermosets.gif

For dielectric cure monitoring, it is convenient to observe the ion viscosity, which is simply the electrical resistivity ρ—i.e the inverse of conductivity. As the physical viscosity of a curing polymer increases, the ion viscosity presented to ion current also increases. This relationship is the principle behind the usefulness of dielectric cure monitoring, and makes possible the observation of cure state.

 

How are parallel plate sensors different from interdigitated sensors?

Researchers studying dielectric properties often use parallel plate electrodes, for which plate separation sometimes cannot be accurately controlled. The distance between the plates may change upon the application of pressure, or as the material between them expands or contracts. For such situations tan(δ) is used to characterize dielectric properties because ε"/ε' does not vary with plate spacing. However, tan(δ) alone cannot provide information about either permittivity or loss factor, and therefore is limited in usefulness–especially because permittivity and loss factor are themselves complicated functions of several factors.

Interdigitated electrodes on a substrate can be used instead of parallel plate electrodes, as shown in Figure 4. The planar structure of interdigitated electrodes has a geometry which does not change with pressure or expansion or contraction of the material under test, and therefore has the ability to accurately measure both permittivity and loss factor.

FAQ Figure 4 Interdigitated electrodes and parallel plates.gif

 

The raw measurements from dielectric instruments at a given frequency f are typically:

G = conductance (ohms-1)

C = capacitance (farads)

With the known quantities of:

ω = 2πf

ε0 = 8.86 x 10 -14 F/cm

A/D = ratio of area to distance for parallel plate electrodes or geometric constant for interdigitated electrodes

Then it is possible to calculate the resistance between the electrodes:

R = 1/G (resistance)

and the dielectric material properties:

ρ = R *A/D (resistivity or ion viscosity)

σ' = G / (ε0* A/D) (relative conductivity)

ε' = C/ (ε0* A/D) (relative permittivity)

ε" = σ' / ω (loss factor)


Who is Lambient Technologies LLC?

Lambient Technologies LLC, based in Cambridge, Massachusetts, develops instruments, sensors and software for monitoring the dielectric properties of curing polymers. These properties provide insight into the chemistry, formulation, reaction rate, viscosity and cure state of epoxies, polystyrenes,polyurethanes, silicones, SMC, BMC and other thermoset materials.

Dielectric cure monitoring has wide application in research and development, quality assurance/quality control, and manufacturing. Products from Lambient Technologies are designed for flexibility and ease of use – together they form an integrated system for studying polymers and optimizing manufacturing processes.

Lambient Technologies was founded in 2008 by members of the team that developed products commercialized by Micromet Instruments; a Massachussets Institute of Technology spinoff that pioneered the technology of dielectric cure monitoring in the 1980s. Since then, members of our team have improved upon the product line at GeoCenters, Metrissa/Holometrix, and Netzsch Instruments, giving us over 30 years experience in the field.