About the PCA

When you receive your Photocalorimeter Accessory (PCA), there will be some assembly required, as well as installation and maintenance.

For detailed information not covered in this topic, please see your PCA Q Series Getting Started Guide, which can be accessed with the other manuals that are either downloaded with the software or present on the software CD.

Select from the following topics for more information:

Introducing the PCA

The Photocalorimeter Accessory (PCA) is used with the TA Instruments DSC Q2000, Q200, Q1000 and Q100 Differential Scanning Calorimeters.  This accessory allows samples placed inside the DSC cell to be irradiated with ultraviolet or visible light.  When the samples (usually photopolymers) react to the light, heat is released (i.e., an exothermic reaction occurs).  This heat is measured and used to study the relative reactivity and/or kinetics of reaction.  The reactions studied are typically rapid and results are obtained in less than 15 minutes.

The TA Instruments PCA accessory is based on a filter photometer, which contains a high pressure mercury lamp that delivers light over the spectral range 250 to 650 nanometers (nm).  A broadband filter (320 to 500 nm) is provided standard with the instrument.  Light is transmitted from the instrument to the DSC cell via a one-meter long, 3-mm diameter, dual extended range (250 to 700 nm), quartz light guide.  The light guide attaches to the cell itself using a special adapter. The figure here shows those system components.

 

PCA with DSC

 

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Experiment Considerations

There are several fundamental instrument characteristics that affect the quality of PCA results.  Those characteristics are:  wavelength range, light intensity at the DSC cell sample and reference positions, baseline noise, exposure time, and temperature. The following paragraphs describe aspects for each which should be considered when planning an experiment using the PCA.

NOTE:  The patented TzeroTM design of the DSC cells provide important advantages when performing PCA experiments, including the ability to directly measure and balance the light intensities at the sample and reference platforms inside the cell and the ability to perform dual sample experiments. These advantages are obtained only when running in T4 or T4P heat flow mode. Therefore, it is required that PCA experiments be run in T4 or T4P heat flow mode.

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Wavelength Range

The PCA is a filter photometer instrument.  A broadband filter supplied with the instrument covers 320 to 500 nm and is suitable for most UV and Visible PCA studies.  A broadband filter covering the UV (250 to 450 nm) region is available as an option.  (Visible longpass filters with cutoffs at 390 nm and 490 nm are also available as options.  These latter filters mount at the DSC end of the light guide.)

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Light Intensity

The PCA is based on a high intensity, high pressure mercury lamp capable of producing a total light intensity of about 25 W/cm2.  This intensity is far in excess of that required for most photocalorimetry experiments.  (Most experiments are performed with intensities between 20 to 100 mW/cm2 at the sample.)  Therefore the intensity reaching the DSC cell is adjusted using a combination of aperture control at the PCA unit and neutral density and/or cutoff filters at the end of the light guide.

The light intensities at the end of each "arm" of the dual light guide must not only be regulated to the proper level, they must also be balanced to produce minimum baseline offset.  The TA Instruments Q Series DSC's are based on a unique, patented design that allows the heat flows at the sample and reference platforms in the DSC cell to be independently measured. In the absence of a sample or reference material and pans (i.e., an empty cell), the heat flows observed are directly related to the light intensities at the platforms.  Therefore the actual intensity experienced by the sample can be determined.  In addition, the intensities at the sample and reference platforms can be balanced by a simple adjustment on the cell light guide adapter.

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Baseline Noise

Ideally, the light intensity delivered to the DSC cell through the light guide should be constant.  However, the high pressure mercury lamp used in the PCA has some inherent fluctuations in its output.  The DSC cell is sensitive enough to detect those small fluctuations in light intensity as noise in the heat flow baseline.  There is no system adjustment that can be made to eliminate this noise.  Fortunately, the noise is typically less than + 100 W/cm2, which has no effect on the heat flows associated with the photoinitiated events being studied since these heat flows are several orders of magnitude larger.

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Exposure Time

Photoinitiated curing reactions are fast thermal events.  Complete cure is achieved in several seconds to several minutes.  This can make differentiating the curing behavior of similar materials difficult even if low light intensities are used.  The ability to vary exposure time, therefore, provides an additional experimental variable that can help improve differentiation of behavior and/or provide conditions that better mimic those found in real-world processes (e.g., photocuring of a film coating as the film rapidly passes under a light source).  The PCA connects to the DSC cell via an event cable that opens and closes a shutter at the light source.  Exposure times as short as 0.6 seconds can be selected when setting up a PCA method.

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Temperature

Most PCA experiments are run isothermally.  The PCA is compatible with the TA Instruments FACS and RCS coolers.  PCA experiments can be performed in the temperature range 50 to 250C with the dual quartz light guides supplied.  Furthermore, once the PCA experiment is complete, a standard DSC experiment can be run on the fully/partially cured sample material over the broader temperature range covered by the selected cooler by making only a few minor changes to the system.

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Using the PCA Safely

CAUTION:  The operator of this instrument is advised that if the equipment is used in a manner not specified in this manual, the protection provided by the equipment may be impaired.

There are several major areas of concern pertaining to personal safety when using the Photocalorimeter Accessory (PCA).  For all detailed information regarding safety, please refer to the filter photometer manual.

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Electrical Safety

You must unplug the instrument before doing any maintenance or repair work; voltages as high as 120/240 volts AC are present in this system.

WARNING:  High voltages are present in this instrument.  Maintenance and repair of internal parts must be performed only by TA Instruments or other qualified service personnel.

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Radiation Danger

WARNING: NEVER look into the beam from the light guide, or a reflection of the beam.  The high degree of ultraviolet radiation can permanently damage the retina of your eye and result in blindness.

WARNING: You should NEVER expose your skin to UV beams.  Exposure of skin surface will result in severe burns.  ALWAYS wear LATEX gloves when working with ultraviolet beams.

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Additional Safety Warnings

WARNING:  For all detailed information regarding safety please read the filter photometer manual

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Warranty Information

WARNING:  Please take care when using your PCA to protect it from misuse or mishandling.  TA Instruments offers no warranty after the initial installation of the Photocalorimeter Accessory Dual Light Guide or the 200 W Mercury Lamp.

WARNING:  The life of the standard Extended Range Liquid Dual Light Guide is reduced dramatically if it is exposed for extended periods to temperatures above 35C.  The recommended (default) upper temperature for the PCA experiments is 80C. This upper limit is based on the fact that the end of the ligh guide, mounted in the adaptor over the cell, "steady states" at 35C when the cell is held isothermally at 80C.

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