How to define and verify the electrostatic properties of plastics
    components in the medical industry

     by Reinhold Rutks
     

Static electricity, often referred to as the world's oldest electricity, is usually considered a nuisance and something of minor importance. This is often very true and looking further into the subject is regarded as beeing unnecessary or possibly of academic interest. In most companies or organizations there is very little knowledge that covers even basic aspects of electrostatics.

Let us just very briefly jog our minds with some absolutely basic facts;

• static electricity is an imbalance in the charge of a material, which means that there is a larger number of positive and/or negative charges than at equilibrium
• a negatively charged surface has an excess of electrons and a positive surface is lacking electrons
• a charged surface will produce an electrostatic field which is directed perpendicularly from any point of the surface
• static electricity is static – it does not flow across a surface or rather it moves very slowly which means that there will be a charge buildup with time when running a process.

Electric field - The strength of an electric field is defined in terms of the force that an imaginary test charge would experience if it was placed in the field. The magnitude, E, of an electric field at a particular location in an electrostatic system is the force acting upon a unit positive charge placed at that point. Thus, the force experienced by a point charge, q, in a field E is;

F =qE

Units will be 1 newton/C = 1 Nm/As = 1 V/m

In practical work kV/m is used rather than V/m.

Field lines - A practical way of representing the electrostatic field is by use of field lines. At every point on a field line the direction of the field line is determined by the tangent to the field line at that point and the density of the field lines is a measure of the field strength.

Triboelectricity – The electrostatic charge that most of us are familiar with is generated when two material surfaces in close contact are rubbed against each other or are separated from each other. When taking off a synthetic garment, for instance pulling a sweater overhead, one experience sparks due to having generated an electrostatic charge.
The charge buildup is influenced by factors such as speed and contact pressure and charge buildup is higher for surfaces with smaller surface roughness.

Over the years many so called triboelectric series have been published. In such a series the samples are grouped according to their resulting charge with the positively charged samples at the top and the more negatively charged at the bottom of the tables.

The triboelectric series have very little practicle value since materials are getting more and more complex with additives, pigments, fillers etc.
One may be led to believe that two sheets of the same material would not be charged when rubbing. In the real world they do charge, which is a very serious problem in the handling of paper or plastics sheets in the graphics industry or in powder handling in the pharmaceutical and plastics industries.

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Volume and surface resistivity – Many suppliers are giving data for surface and volume resistivity in the specifications for their materials (data given as Ohms/m or Ohms/square). Although it is generally true that an insulating material with a very high resistivity will give a higher charge buildup than a material with a much lower resistivity the correlations are very poor when looking at the practical use in a process.

A very common way of handling static problems is to use antistatic equipment which by using ionization can discharge the material in critical stages of the process in order to give high enough throughput, precision and quality and in some cases even to avoid ignition in flammable atmosphere. If we want to change the properties of the materials used the measurement of electrostatic properties is necessary and the decay time
an excellent way of monitoring the development and later to control the quality of production.
Many smart applications are making active use of electrostatic charge for pinning materials onto eachother in laminating processes, for electrostatic paint and spray processes etc. These applications also call for close monitoring of the main factor – the decay time.

Chargeability and decay time – When studying a process where static buildup may be a problem one can first establish the process parameters such as physical size, speed, temperature, relative humidity, pressure etc and then see what happens when the material is introduced into the process.
Many processes are run at a lower speed due to the otherwise hindering static buildup. With a flow of charge being fast enough to dissipate at a lower process speed but not at a high speed the decay time of the material becomes a very important factor. Thus, being able to change and control the decay time of a material may be the way to achieve higher production throughput and maintain a good quality.

In the JCI 155 Charge Decay Test Unit a high voltage corona discharge is used to deposit a patch of charge on the surface of the material to be tested. A fast response electrostatic fieldmeter measures the voltage generated by this charge. It also measures how quickly this voltage falls as the charge migrates away [1]. Corona charging has been shown [2] to be a simple way to simulate practical charging events. It allows control of initial surface voltage and charge polarity and is applicable for all types of surfaces - whether uniform or with localised conducting features. It provides consistent, reproducible results that are not affected by corona exposure.
Operation of a JCI 155 in conjunction with measurement of the quantity of corona charge transferred to the test surface (for example using a JCI 176 Charge Measuring Sample Support) allows calculation of the ‘capacitance loading’ experienced by charge on the surface. This ‘capacitance loading’ gives guidance to the maximum surface voltage likely to be created by certain quantities of charge on a surface [4]. A high capacitance loading means that only low surface voltage arise per unit of charge.

Test Area - The JCI 155 has a 45x54mm test aperture in the instrument baseplate. This can rest directly on the test surface. Contact with the surface around the test aperture provides a return route for outwardly migrating charge and high local capacitance to trap such charge. With short duration corona charging (e.g.20ms) the presence and position of the outer earth boundary is not important. Measurements can be made on surfaces smaller than the test aperture area.

Charging - The surface is charged by a high voltage corona discharge (3-10kV) from the tips of a small conical cluster of fine wires mounted on the underside of a light moveable plate.
This plate is moved between the fieldmeter sensing aperture and the material surface exposed through the instrument baseplate. Corona is usually generated as a brief pulse (20ms) immediately before the plate is moved away. The plate moves fully away within 20ms.
The moving plate and instrument construction shield the fieldmeter from high voltage connections so reliable measurements can be made down to even quite low surface voltages. An ‘air dam’ is included on the trailing edge of the moving plate. This sweeps away residual air ionisation at the end of the corona period so there is little influence on surface voltage measurements (no more than about 10V).

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Fieldmeter - A fast response ’field mill’ electrostatic fieldmeter [5,6] gives fast, sensitive and stable measurement of surface potential. The response time is below 10ms and charge decay times can be measured from below 50ms to many days.
Instrument software provides for automatic zero setting of the fieldmeter just before each test. In very long charge decay studies arrangements are made to check the fieldmeter zero from time to time. It is not easy to measure decay times with signals where noise is significant in comparison to the signals or the signal differences to be measured. This may apply for materials that dissipate charge either so quickly that the initial peak voltage is very low, only 10-50V, or with materials that dissipate charge very slowly, so that small differences in signal levels need to be measured to get results within modest periods of observation. It is therefore important to identify average values of a noisy signal without slowing down time response.

Sample support - Measurements are normally made both with the material freely supported with an open backing and also resting against an earthed backing surface. These two arrangements represent the extremes of constraints of practical application. The longer of the two decay times is used for assessment of the suitability of the material. A sample support provides a simple arrangement for such measurements.
Simultaneous measurement of the quantity of charge received by the sample surface and the initial peak surface voltage achieved enables calculation of the ‘capacitance loading’ experienced by charge on the sample surface [3,4,8]. The JCI 176 Charge Measuring Sample support provides the ability to measure the corona charge received by the sample. Measurements using the JCI 176 can be with both open and earthed backing.

Powder samples may be presented using a powder sample support with the instrument stood off a few mm to reduce risk of powder dispersal into the air by action of the air dam and consequently risking clogging of the field mill. Powder, and liquid, samples can alternatively be presented for testing using a powder sample support plate mounted between the plates of a standard sample support.

Test conditions - Charge decay characteristics are usually susceptible to absorption of surface moisture from the atmosphere, so measurements are very likely to depend on humidity.
It is hence desirable to carry out testing under defined, or at least known, conditions of temperature and humidity. This can be achieved by carrying out measurements in a controlled environment with adequate time allowed for acclimatisation. The instrument has builtin sensors to measure both temperature and humidity within the test region of the instrument. These measurements are stored along with all the other information on test conditions. Both the instrument and the samples to be measured should be kept long enough in the desired climate.

JCI-Graph - All test conditions and observations are stored to a memory card. This data can be transferred to a PC using a Windows software.
This provides opportunity to display up to 4 graphs at a time of observations and to transfer these together with associated numerical test and result information into wordprocessed documents. A summary table of test and result information is also created.

Test criterion -Experience is that the form of charge decay curves is usually not an exponential. The form of the curve however does not usually depend on the level of the initial peak voltage or, hence, on the level of charging. Thus a ‘decay time’ measured as a set percentage of the initial peak voltage is an appropriate basis by which to rate materials. A point immediately after the initial peak voltage is used as a starting point for timing as timing from this point includes as much information as possible about possibly fast initial voltage drops. The point is chosen to be immediately after completion of competition between opening of the moving plate and the decay of surface potential.
It is also useful to record how the rate of charge decay varies during decay to see whether significant levels of charge may be retained for long times. It is observed that charge decay curves may ‘plateau out’ after an initial perhaps fairly rapid fall of surface voltage. In this situation it may be argued that a better acceptance test criterion would be the time to 10% of the initial peak voltage – as this would better ensure that residual surface voltages were low.
Measurements to both 1/e and to 10% are recommended when practicable.
In special applications it may be better to measure the time until a specified voltage (e.g 200V, 50V or some other value) is reached.
Measurement of the quantity of charge transferred to the sample and the initial peak voltage enables materials to also be assessed in terms of ‘capacitance loading’.

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Decay timing - Decay times are measured using a proprietary technique called ‘stutter timing’. This approach very effectively overcomes problems with signal noise at low signal levels. It is helpful a) with very slow charge decay rates, and b) with very low initial peak voltages (for example 10-50V). Stutter timing works by running and stopping the timing clock according to whether the instantaneous fieldmeter signal is above or below the voltage level of interest. This approach is used both in the algorithm for finding an initial peak surface voltage, from which timing will start, and for determining the end of timing. It is also used in calculation of local charge decay time constants during the progress of charge decay.

Calibration - Instrument performance can be formally calibrated to British Standard BS 7506: Part 2: 1996 [7] using measurements whose accuracy is traceable to National Standards. The JCI 255 Calibrator Unit provides a convenient basis for such formal calibration.

Active use of static charge – In many processes there is a need, in an intermediate stage, to nonpermanently pin two surfaces together without using a chemical or mechanical bond. With suitable static properties this can be achieved and controlled.
One interesting application that illustrates the use of decay measurements and active and controlled use of static electricity is in so called Inmould Labelling where a preprinted label is fed into the mould of an injection moulding machine and there pinned into position and held by static charge. Once safely in position the mould is closed around the label and the plastic melt is injected. The label is thereby made an integral part of the detail/product.

The ”label” to be used must first be picked up from some sort of storage and then safely and accurately positioned into the mould. With a static charge generated in the label stack there is the common risk of accumulating static charge and picking multiple labels. If an antistatic agent is added there is the immediate risk of ”shortcircuiting” the label and not being able to statically pin it into the mould. By use of decay measurements one can develop a suitable material to fulfill the criterions needed and secure quality.

In more complex processes the same approach will help choosing the best size and shapes of particles in for instance dry mixing of pharmaceuticals or how to produce a homogeneous masterbatch for plastics production.

References

[1] J. N. Chubb "Instrumentation and standards for testing static control materials" IEEE Trans. Ind. Appl. (6) Nov/Dec 1990, p 1182.

[2] J. N. Chubb “The assessment of materials by tribo and corona charging and charge decay measurements” Proc Inst Phys Confr ‘Electrostatics 1999’, Univ Cambridge, Mar 1999. Inst Phys Confr Series 163 p329

[3] J. N. Chubb “Measurement of tribo and corona charging features of materials for assessment of risks from static electricity” Trans IEEE Ind Appl 36 (6) Nov/Dec 2000 p1515

[4] J. N. Chubb “New approaches for electrostatic testing of materials” J. Electrostatics 54 (3/4) March p233 (ESA 2000 meeting, Brock University, Niagara Falls, June 2000).

[5] J. N. Chubb “Two new designs of 'field mill' type fieldmeters not requiring earthing of rotating chopper.” IEEE Trans. Ind. Appl. 26 (6) Nov/Dec 1990, p 1178.

[6] J. N. Chubb “Experience with electrostatic fieldmeter instruments with no earthing of the rotating hopper” Proc Inst Phys Confr Electrostatics 1999’ Univ Cambridge, Mar 1999. Inst Phys Confr Series 163 p443

[7] British Standard ’Methods for measurements in electrostatics’ BS 7506: Part 2: 1996.

[8] “Test method to determine the limitation of surface potential created by electrostatic charge retained on.

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