ELECTRON BEAM TECHNOLOGY EQUIPMENT
STATUS AND APPLICATIONS UPDATE
by: Joe Lovin
Electron beam equipment and technology have matured from
a little known technology in the mid 1950's to a mature, reliable process
system of choice for many processes today. Not only is the process’
chemistry better understood, E- beam initiated chemistry or sterilization
offers a more economical and controllable processing tool. Also, the
equipment penetration range and throughput can more readily be tailored
to individual applications needs. The equipment is also much improved
in reliability and maintainability as a result of maturity in the base
equipment design itself as well as the utilization of computer based
control systems to manage the system to provide rapid self diagnostics
for trouble shooting of system problems even while the system is operational.
Historical Perspective
Commercial exploitation of the radiation for the purpose
of sterilization of medical products began in the late 40's and early
50's; whereas, the use of radiation for cross-linking of commercial
products was not seriously investigated and exploited until the mid
50's.
Johnson & Johnson is generally recognized as the pioneers
of the use of machine generated radiation on a routine basis for sterilization
of sutures. In or about the mid 50's both the Cryovac Division of W.
R. Grace
and The Raychem Corporation began serious efforts to exploit
machine generated radiation for the purpose of cross-linking plastics
to obtain improved properties.
These early efforts utilized primitive Cockcroft-Walton,
van der Graff generators or resonant transformer type devices. Needless
to say control problems, poor machine reliability as well as the lack
of machine predictability caused a great deal of grief in the beginning.
This resulted in the medical industry’s turning to the more reliable
isotope radiation as a source for sterilization, but isotope throughput
capability was too slow to be commercially feasible for cross linking
so that industry elected to push for improved machine reliability and
maintainability to achieve their goals.
Fortunately for the industry the efforts of these early
pioneers and others that came later were successful not only in realizing
dramatic equipment improvements, but also the understanding and breadth
of radiation chemistry has been enhanced.
Equipment Today
Penetration and machine production capacity for a given
application tends to be very industry specific. Terminal energy available
and type of equipment is generally segregated as follows:
150 kev to 300 kev, single gap, non scanned beams. These
units are available in widths from less than one meter to more than
three meters. Beam currents can vary from a few milliamperes to more
2000 milliamperes.
Typical applications for this type of equipment are: curing
of coatings on such substrates as wood panels, floor coverings, magnetic
media, printing inks, etc... This equipment has also been extensively
applied to cross- linking small single strand wire as well as to cross-link
relatively thin plastic sheeting or plastic laminates or co-extrusions.
This equipment can be procured with oil filled or gas
filled power supplies.
All of the systems today are supplied with a PLC based
control system with the specific type or manufacturer of the PLC specified
by the buyer.
450 kev to 750 kev scanned beam systems are the next general
decrement of equipment in use today. This type of equipment is available
in widths from approximately 0.5 meters up to approximately 1.8 meters
in width. Beam currents are generally available from 25 milliamperes
to approximately 250 milliamperes. Typical applications include curing
of coating, cross-linking of plastic sheeting and tubing, and polymerization
of liquid and semi-liquid emulsions. This range of equipment has been
extensively applied in the tire and rubber and plastics industry.
This equipment is typically supplied with gas filled power
supplies but has also been supplied with oil-filled power supplies.
These units are more often supplied with PLC based controls but have
in some recent instances been supplied with P. C. Based control in combination
with PLC's.
1 Mev to 4.4 Mev, scanned beam systems are the next general
decrement of equipment in use today. This type of equipment is available
in widths from 0.5 meters to 1.8 meters in width. These units are characterized
by beam power rather than beam current available. Beam powers of these
units run from as little as 25 kw to 150 kw. These units have a broad
range of application from cross-linking of thicker cross sections of
materials to polymer rheology modification and sterilization of medical
products in the higher penetration ranges.
The equipment is always supplied with gas filled power
supplies and all of the more modern equipment is supplied with PLC based
control systems. These systems can be difficult to bring back on line
when a power supply has failed or a vacuum failure has occurred. This
is due to conditioning time required to reestablish the high voltage
and the vacuum system. This is particularly true above 3 Mev. Another
recently emerging issue is the cost of gas with sulfur hexafloride gas
filled power supplies, especially for the larger power supply vessels.
The price of this gas has more than doubled in the last four years and
is seeing more and more pressure from state and federal regulatory agencies
in a similar manner as Freon 12.
- 5 Mev to 10 Mev scanned beam linacs offer the highest level of
penetration in the machine produced business. Most of these systems
are offered in scan widths from 0.5 meters to 1.8 meters and most
are pulsed output devices. Even though linacs have been around for
a number of years, there have been significant improvements in power
output and reliability in approximately the last 5 years. Beam power
levels are available from 25 Kw up to 350 Kw, and continuous wave
machines are available that operate at significantly lower frequency
than previously available S and L band microwave sourced machines.
This design feature offers greater power density capability, reduced
susceptibly to temperature drift, and in addition the newer design
eliminates the need for SF6 gas insulation. More over, the R. F.
components are cheaper and more readily available than their microwave
counterparts.
This range of penetration is commonly used for medical
product
sterilization, cross-linking of thick section products
, food disinfestation,
waste water remediation, Polymer rheology modification,
gem stone
enhancement, and shelf life extension for food and fruits.
Successful Proven Applications
Cross-linking is by far the most successful application
of electron beam technology, due in part to the early intense pioneering
effort in this applications arena, plus the obvious economic benefits
to be achieved by the use of this technology.
In addition, cross-linking applications tend to be in
line operations or high volume batch operations that demand high reliability,
controllability, and predictability of all system components.
The usual high volume cross-linking operation is a physically
large, multistep, multi-machine operation that is usually manned by
not more than one operator and in some cases an assistant. Therefore,
all line components must be capable of producing high volumes of product
with demonstrable consistent properties 24 hours a day, seven days per
week, year round with minimal attention from operating personnel and
maintenance personnel.
All these requirements play to the advantage of the modern
electron beam, and in particular to the more recently engineered systems
that have had intense scrutiny of reliability and are more user friendly.
More sophisticated industrial operations require that
production lines be statistically evaluated on start up to demonstrate
first, statistical stability and second, that they are in demonstrable
statistical control. It is generally the E-Beam component of these types
of operations that is the first line component to be qualified as stable
and in control. This is a result of many years of intense scrutiny of
the system reliability, and more recently, the application of high speed
computers to monitor the operation of E-Beam systems.
Cross-linking as a broad category covers the entire range
of penetration with beam current requirements (beam power) varying from
a few mA to as much as 2000mA. The product categories range from printing
inks to floor coverings to thick wire insulation and plastic pipe and
tire applications.
The second most successful application of E-Beam technology
as measured by volume of use is medical sterilization. E-Beam sterilization
of medical products has gained a solid foothold in medical product sterilization
as a high throughput, environmentally friendly alternative to gamma
and ETO sterilization. It should be emphasized, however, that all of
these sterilization technologies have an appropriate niche depending
on end use needs and regulatory pressures.
The sleeping giants in the application of radiation technology
are food irradiation and pasteurization, decontamination of waste water
and sewage sludge. The treatment of contaminated effluent from industrial
stack emissions, such as sulfur and nitrogen compounds as well as hydrocarbon
contaminated effluent offer tremendous opportunity for remediation through
the use of E-Beam technology.
Applications Information
Critical information needed to develop the process specifications
for a new application are the process target speed (throughput), dose,
depth of penetration into the product required, and finally the dose
uniformity needed to insure that the desired product or process parameters
are achieved.
Figure 1 and Figure 2 are examples of the energy distribution
for a range of different terminal energies. The penetration range in
unit density material varies from less than one millimeter at 300kV
to slightly over 5 cm at 10 Mev.
Low penetration is generally sufficient for the curing
of coatings, printing inks, and the cross-linking of thin plastic sheet
or thin wire insulation. The dose uniformity needed to cure coating
and inks through the thickness of the product is easily achieved with
very low penetration equipment. The dose uniformity across the width
of the product will generally vary as much as plus or minus 5% with
a typical low energy single gap accelerator. This kind of dose uniformity
is very adequate for the end use, however.
As the material thickness and/ or density or both increase,
higher penetration equipment is needed to accomplish the desired end
results. Radiating the product from both sides may also be necessary
to achieve the desire dose uniformity. Figure 3 shows an example of
this type of dose distribution when a product has been irradiated from
both sides
with a 10 Mev beam. Typically two sided irradiation is
used when cross-linking relative thick products that need a reasonably
high degree of dose uniformity. It is fairly common to use two sided
irradiation for boxed, bulk products that are processed in the medical
sterilization business. More than 50 cm of product of a density of 0.15g/cc
can be radiated to reasonably uniform dose levels using a 10 Mev beam
and double sided irradiation. (see Fig 3)
To achieve the maximum uniformity utilizing two sided
irradiation, the product density thickness would need to equal to the
density thickness at the 50% point on the depth-dose curve. For single
sided irradiation the most uniform dose would be achieved when the product
density thickness results in the dose at the beam exit side of the product
is equal to the surface dose at the beam entry point.
Dose is proportional to current density and exposure time.
Stated in mathematical form Dose = K x (I/A) x T:
Dose(kGy) = K(kGy-kg/mA-min x I(mA)/kg x T(min). K factor
is empirically derived by performing dose tests with the particular beam of interest.
The K factor is a figure of merit for the performance of the electron
system. A more specific form of the dose equation can be written as
follows: Dose = K x mA / 1
kg/min Beam
Width
The equation above predicts bulk or mass throughput which
is defined as the dose one can expect through the entire bulk of a product
being passed under the beam under the conditions stipulated in the equation.
If one wishes to predict surface throughput, you simply have to substitute
meters per minute for kilograms per minute in the above equation to
do so.
Some useful numbers to remember when dealing with dose
predictions are: 1 KW of Beam power will process 360 Mega-Rad- kilograms
of material or 800 Mega-Rad pounds of material per hour, assuming 100%
efficiency.
Figure 4 shows a plot of surface dose K factor or figure
of merit for low penetration equipment. Similar curves can be developed
for higher penetration equipment, but would typically be written for
predicting bulk throughput rather than surface dose.
Recent developments in electron beam technology, particularly
in high penetration equipment like the Rhodotron developed by IBA can
provide beam power capabilities in demonstrated terminal energies of
5 Mev and
10 Mev in beam power levels to 350 kw. Terminal energies
as low as 500 Kev are possible with this type machine but IBA has elected
to concentrate on high end equipment to date.
Other advantages to the IBA design are excellent temperature
stability of the machine, the elimination of SF-6 gas, and a very robust
system design that has resulted in a very high system reliability.
The unique capabilities of radiation processing can be
summarized as follows:
- Contrary to conventional wisdom, radiation treatment is a very
environmentally friendly process. Radiation processing provides
a unique tool for the remediation of many forms of environmental
contamination. Food treated with radiation for shelf life extension
or disinfestation of insects has a lower chemical burden than the
same products treated by chemical means.
- Radiation processing, such as cross-linking, can provide unique
product properties not achievable by other means. Where there are
alternatives to radiation, radiation almost always provides higher
processing speeds with superior properties. Electron beam technology,
in particular, is capable of high processing speeds.
The costs of switching to radiation technology are: retooling
cost and the learning curve for the new technology (training, etc.)
Help in accomplishing these hurdles are abundantly available from both
equipment suppliers, suppliers of radiation chemistry, and engineering
firms that specialize in radiation technology. Users that might want
to utilize radiation technology but who prefer not to invest in the
purchase and installation of a radiation system can work with companies
that specialize in radiation contracting such as Steris, E-Beam Services,
and others.
The future of the Industry
The future of the electron beam processing industry seems
to be very bright for both commercial applications as well as medical
sterilization applications.
Even though cross-linking has been the most successful
commercial application to date, it is reasonable to assume that there
are many untapped cross-linking application yet to be exploited. Cross-linking
has proven to be a path to creating new niche markets as well providing
a tool for the development of new products.
In the food industry, the potential has barely been tapped.
This has been largely due to misinformation and smear tactics used by
anti-radiation activists, even in the face of overwhelming scientific
evidence to the contrary. Fortunately the adverse publicity and smear
tactics have been largely discredited today. The future of pasteurization
and shelf life extension of foods will be developed on a controlled
basis and the potential volume is huge. The pay off is a significant
improvement in wholeness of food as well as a large reduction in the
loss of available food worldwide.
Even though this sterilization of medical products is
a proven, accepted use of radiation , the application of electron beams
has been more limited than the application of Cobalt due to the limited
penetration of e-beam and in many cases power limitations. New higher
power beams controlled by modern high speed computers will open new
applications for electron beam sterilization.
Another arena that has been proven technically, but not
exploited commercially to any significant degree, is the decontamination
of waste streams and the remediation of contaminated ground water. There
are competing technologies in some of these arenas, but they are quite
often more expensive to implement than electron beam technology and
seldom as fast as electon beam treatment.
Rheology modification of plastics is another proven successful
application of electron beam technology that is not widely known with
the possible exception of the degradation of scrap PTFE.
In summary, the future of radiation processing by means
of electron – beam technology is very bright . Modern technological
improvements in the electron beam systems, as well as greatly expanded
understanding of radiation technology offers vast horizons of opportunity
for the industry in the near term as well as the long term.
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