Lambient



What is Dielectric Cure Monitoring (also known as 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.

 

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

Ion Viscosity Graph


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.


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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.


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What types of companies can use 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


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What 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


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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


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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.


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What processing environments can use Dielectric Cure Monitoring?

• Ovens

• Presses and molds

• Autoclaves

• Pultruders and extruders

• Batch reaction vessels

• Injection molding


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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.

Ion Dipole Seperated Influence

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.

Effect of Mobile Ions

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.

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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.


Interdigitated Electrode Usage

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)


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Who is Lambient Technologies?

Lambient Technologies LLC, based in Boston, Massachusetts USA, 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 ‘spin-off ’ 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.

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What instruments does Lambient Technology manufacture?

LT-451 Dielectric Cure Monitor–Optimized for Research & Development

LT451 Dielectric Cure Monitor

• Wide excitation frequency range–0.001 Hz to 100 KHz

• Available with one, two or four channels.

• Each channel has one dielectric and one thermocouple input

• Two digital start/trigger inputs, two digital outputs

• Four analog inputs (±10 VDC range)

• RS-232 communications, or USB with converter


LTF-631 High Speed Dielectric Cure Monitor–Optimized for QA/QC and studies of rapidly curing materials

LTF 631 High Speed Dielectric Cure Monitor

• Excitation frequencies–10 Hz, 100 Hz, 1.0 KHz, 10 KHz

• Available with one, two or four channels.

• Each channel has one dielectric and one thermocouple input

• Max sampling rate is 55 ms/channel or faster

• Two digital start/trigger inputs, two digital outputs

• Four analog inputs (±10 VDC range)

• RS-232 communications, or USB with converter


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What software does Lambient Technologies provide?

CureView software for Windows XP/Vista/Windows 7

Cureview Software

• Communicates with LT-451/LTF-631 via RS-232, or USB with converter

• Complete package for instrument control, experiment set up, data acquisition, data analysis and reporting

• Controls LTP-350 MicroPress

• Determines Critical Points of ion viscosity curves

• Provided with LT-451 or LTF-631 instruments


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What reusable sensors does Lambient Technologies manufacture?

  • Ceramicomb-1" ceramic sensor with interdigitated electrodes

    Ceramicomb-1 ceramic sensor

    • For use in presses, molds or harsh environments

    • May be mounted so the electrodes are flush with a platen or mold surface

    • Constructed with silver-palladium electrodes embedded in an alumina substrate, and thermocouple positioned below ceramic surface

    • Protected by a stainless steel sheath with a nominal 1.0" (2.54 cm) diameter

    • Maximum operating temperature is 250 °C.


    Single electrode sensors for use where space is limited

    Single Electrode Dielectric Sensor

    • For use in presses, molds or harsh environments

    • Opposite face of platen or mold acts as second electrode of a parallel plate configuration

    • May be mounted so the electrodes are flush with a platen or mold surface

    • Stainless steel electrodes embedded in a polyimide insulator

    • Unsheathed (8 mm dia.) or sheathed (0.5" dia.) design

    • Maximum operating temperature is 200 °C for unsheathed and 250 °C for sheathed design


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    What disposable sensors does Lambient Technologies manufacture?

    Varicon sensors for use where a thin, flexible sensor is required

    Disposable Varicon Sensor

    • Patterned on a polyimide substrate

    • Electrode array allows choice of three sensitivities, selected by cutting off portions of the array at designated lines

    • 15" (38 cm) long and only 0.004" (100 um) thick

    • Sensitive to the dielectric/conductive properties of materials within approximately 0.004" (100 um) of the electrode surface

    • Maximum operating temperature is approximately 350 °C.


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    Mini-Varicon sensors for use where an inexpensive sensor is required

    Mini-Varicon Sensor

    • Patterned on a polyimide substrate

    • Electrode array allows choice of two sensitivities, selected by cutting off portions of the array at designated lines

    • 1.5" (3.0 cm) long and only 0.004" (100 um) thick

    • Available with or without leads

    • Sensitive to the dielectric/conductive properties of materials within approximately 0.004" (100 um) of the electrode surface

    • Maximum operating temperature is approximately 350 °C.


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    What accessories does Lambient Technologies manufacture?

    LTF-350 MicroPress for reproducing time-temperature processing

    LTP-350 MicroPress

    • Independent temperature control of upper and lower platens

    • Temperature range is ambient to 350 °C

    • Maximum applied force is 2000 pounds on 3" x 3" platens


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    How do dielectric cure monitoring instruments work?

    Dielectric Cure Monitoring Instruments

    • The LT-451 and LTF-631 generate a sinusoidal excitation signal at a selected frequency

    • The excitation signal drives one electrode of a dielectric sensor

    • The response signal on the second electrode depends on the dielectric properties of the Material Under Test

    • The response is measured with either digital or analog techniques

    • The response is attenuated in amplitude and shifted in phase relative to the excitation

    • The attenuation is expressed logarithmically as gain in decibels

    • The phase is expressed as degrees


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    How does the LT-451 work?

  • Operation of the LT-451 is shown in the block diagram of Figure 5.a.

    LT 451 Operation

    Figure 5.a. Block diagram of LT-451 data acquisition system


    • A sampled data acquisition system measures the sensor response using an analog-to-digital converter (A/D converter)

    • Digital signal processing (DSP) calculates gain and phase

    • DSP can extract the gain and phase from a very noisy signal

    • Gain and phase are used to calculate the resistance and capacitance between the electrodes of the sensor

    • Sensor parameters are used to calculate ion viscosity (resistivity) and permittivity from resistance and capacitance


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    How does the LTF-631 work?

    Operation of the LTF-631 is shown in the block diagram of Figure 5.b.

    LT 631 Operation

    Figure 5.b. Block diagram of LTF-631 data acquisition system


    • The LTF-631 measures the gain and phase of the response using analog computation techniques

    • Less expensive and faster than the sampled data acquisition/digital signal processing used in the LT-451

    • Slightly more noise sensitive and less accurate than the LT-451

    • Analog computation can perform well when rapid data acquisition is a desired tradeoff for precision.


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  • How do the LT-451 and LTF-631 compare to each other?

    Specification

    LT-451

    LTF-631



    Dimensions / weight

    17" W x 16" L x 5.25" H

    (43.2 cm W x 40.6 cm L x 13.3 cm H)

    15 lbs (6.8 Kg)

    17" W x 16" L x 3.25" H

    (43.2 cm W x 40.6 cm L x 8.3 cm H)

    15 lbs (6.8 Kg)


    Number of channels

    1, 2 or 4

    (mux for additional 4 channels available)

    1, 2 or 4

    Sensor interfaces/channel

    Mid-conductivity and/or high conductivity

    Mid-conductivity

    Other inputs

    2 start/trigger

    4 analog voltage (±10 VDC)

    2 start/trigger

    2 analog voltage (0 — 5 VDC)

    Other outputs

    2 digital TTL level (0 — 5 VDC)

    2 digital TTL level (0 — 5 VDC)

    Thermocouple/channel

    Type J

    Type J

    Frequency range

    0.001 Hz to 100 KHz

    10 Hz to 10 KHz

    Available frequencies

    1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9

    x 10n Hz in each decade

    10, 100, 1.0 KHz, 10 KHz

    Measurement method

    Digital sampled data acquisition with digital signal processing

    Analog computation

    Measurement speed/chan

    0.001 Hz – 9 Hz: freq dependent

    10 Hz – 100 KHz: ~3 sec

    All frequencies: 55 ms or faster

    Communication interface

    RS-232C / USB with adapter

    RS-232C / USB with adapter

    Software

    CureView

    CureView

    Cost

    Greater

    Lesser


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