Electron Beam Radiation
20.4.1. Concepts
In electron beam radiation, the polymer material is bombarded with high-energy electrons that have been created in a large machine built specifically for that purpose. Such a machine is pictured in Photo 20.2. The diagram shows a typical layout of an electron beam operation. Note that both of the major types of products (boxes and continuous extruded lengths) are shown being irradiated. The irradiation horn, the output device for the electron beams, is located inside a concrete enclosure for safety purposes.
The high-energy electrons can cause several changes in the polymer. The nature of these changes depends on the nature of the polymer, the energy of the electrons, and other processing variables, such as the time of exposure and the presence of modifying agents within the polymer material. In general, three main reactions can occur: cross linking, scission, and molecular rearrangement. (Scission, or chain scission, is the breaking of the main backbone of the polymer and is a form of polymer degradation.)
Each of these reactions is initiated by the interaction of the polymer with the high-energy electrons. This interaction usually results in the removal (knocking off) of hydrogen atoms from the polymer chain, as shown in Figure 20.1. The removal of the hydrogens can result in the formation of unpaired electrons (free radicals) or charged atoms (ions), depending on whether the hydrogen atoms leave with or without an electron, which, in turn, depends on the energy of the electrons and the nature of the polymer. Whatever the form of the leaving hydrogen atoms, the sites on the polymer formerly occupied by the hydrogens become active sites. These active sites can bond to other active sites on another molecule to create a crosslink, or they can cause a local rearrangement of the bonding patterns in the polymer to cause either scission or molecular rearrangement.
These reactions can occur in both thermosets and thermoplastics. When a thermoset is irradiated by high-energy electrons, the thermoset can be crosslinked (cured), thus providing a method of crosslinking thermosets in addition to the normal chemical crosslinking methods (peroxide-initiated free radical reactions, ring opening, molecular rearrangements, or standard reactions on multiple sites, as the case may be for each type of thermoset material). The thermosets can also experience scission or molecular rearrangement. If the reactions occur in thermoplastics, then thermoplastic materials can be cross linked by this method, thus turning the thermoplastic and thermoset. Thermoplastics can also experience scission or rearrangement.
20.4.2. Usefulness
The ability to use some method other than the thermally activated method to crosslink a thermoset has advantages. For instance, some molecules may be very sensitive to degradation at even modest temperatures; others may require very high energies to crosslink. In either case, the use of electron beams for curing simplifies the processing. Since the temperature increase associated with electron beam reactions is less than that associated with thermal reactions, molds that may be weakened or degraded by thermal cures (such as a plastic mold) can be used with electron beam cures, provided that the material in the mold is not degraded by the electron beam.
The high level of control of the electron beam radiation level and direction is important in the choice of this method for crosslinking of polymers versus chemical crosslinking. The electron beam method allows the amount of crosslinking to be chosen by simply choosing the amount of radiation that is created. The electron beam method also allows much faster throughput because the crosslinking by electron beam radiation is much faster than chemical crosslinking. A further advantage of electron beam radiation cross-linking is avoidance of the environmentally damaging and hazardous peroxides often required in conventional crosslinking.
The ability to use electron beams to crosslink thermoplastic polymers provides many unique processing opportunities. The thermoplastic material can be processed in the normal method, such as by extrusion or injection molding, and then subjected to the electron beam radiation to change the properties to those of a thermoset. In many cases, the basic properties characteristics of a thermoplastic, such as toughness and elongation, are largely retained, and new properties characteristic of a thermoset, such as higher thermal resistance, are added. This irradiation process is used to make thermoplastic wire and cable insulation more resistant to environmental stress cracking, to raise the thermal stability of thermoplastic parts used in aerospace applications, to increase the toughness and resistance to cuts of golf ball covers, to make shrink-wrap and shrink-tubing, and to increase the hardness of rubber materials. All these attributes, and others discussed later in this chapter, reflect the presence of crosslinks made by the action of the electron radiation.
The second of the reaction paths for polymers irradiated by electrons is scission. Normally, the breaking of bonds along the main polymer backbone has a negative effect on polymer properties and therefore is avoided. A few applications, such as the control of molecular weight for lubricants, viscosity modifiers, and intermediates for making synthetic fibers, are exceptions. These applications depend on the ability to carefully control the amount of radiation and, therefore, to control the amount of degradation that occurs within the polymer. Electron beam irradiation can also be used to sterilize medical products by causing scission in the polymers that constitute the bodies of the unwanted microbes. The products are usually irradiated after they have been placed into their final shipping boxes, an application shown in Photo 20.2. The effectiveness of electron radiation in killing microorganisms should be a reminder that this radiation method can be dangerous to humans as well.
The third reaction path, molecular rearrangement, is used to add flexibility and useful life to polymers, especially hard, amorphous (glassy) polymers, that have been in service for an extended period of time and have begun to harden. This process, called deaging, reverses the aging process in these polymers. The electron beam radiation rearranges the molecules and in the process forces the polymer molecules apart, thus increasing the polymer flexibility. Rearranging molecules by electron beam radiation is also used to decrease the number of defects in some crystal structures (such as semiconductors, precious gems, and decorative glassware).
20.4.3. Equipment
The electron beam machine can be compared to a giant TV picture tube. A power source converts normal ac power into rf (radio frequency) power, which is then converted to very- high-energy de power (2.5 to 4.5 million volts) that is used to heat a filament (usually a tungsten wire). The hot filament emits electrons that enter a chamber where they are accelerated in speed by a magnetic field. The acceleration chamber is evacuated so that the electrons will not be deflected by gas molecules. The high-speed (high-energy) electrons then enter a chamber where the electrons are fanned out by a scanning device (somewhat like the way the electron gun in a TV picture tube sprays electrons across the face of the picture tube). The electrons flow through a delivery horn shaped to match the fan shape of the electrons. The horn has a window on the end that is covered by a thin metal sheet (usually titanium). This sheet is necessary to maintain the vacuum throughout the system. The electrons pass through the window and into the plastic part as the part passes under the window. Because of the radiation hazard present when the machine is on, the entire machine (see Photo 20.3 and Figure 20.2) is enclosed in some massive barrier, such as a concrete box.
The radiation energy of the electron beam machine is measured either in grays (Gy) or in rads (rad), where 1 Gy=1 J/kg and 1 rad=10-2 J/kg. Rads were formerly the most frequently used units, but grays are now favored as the official SI (international) unit of measure.
The dose received by a part is usually measured by dosimetry. This is done by placing a piece of radiation-sensitive tape on the polymer part as it goes through the machine. After exposure, the tape is removed and placed in a device (dosimeter) that compares some physical change (usually color} in the tape with a standard having a known dosage. The dosage is then determined as a function of the physical change.
Photo 20.3 Plasma chamber. (Courtesy of BYU)
20.4.4. Process
The principal control for the electron beam irradiation process is the amount of power supplied to the filament. As the power increases, the number of electrons increases and the amount of radiation impacting on the plastic part is increased. The dosage is therefore increased by increasing the voltage, assuming all other variables are constant. Another variable used to control dosage is the speed of travel of the part. If the part is left under the delivery horn for a longer period of time, the dosage increases. These two parameters (power and time) are mutually adjusted to give the desired effect in the polymer.
Time is a critical variable even after the part has passed through the irradiation process. The active sites can persist for some time and, as the polymer chains move and internally rearrange in response to stresses, can continue to react. The most common postirradiation reactions are cracking, embrittlement, and discoloration. These are all associated with scission, which would be favored by the limited mobility of the polymer chains. Postirradiation reactions can be minimized by annealing because the heated polymer can move more freely and therefore react by crosslinking rather than by scission or rearrangement.
Only a small portion of the total amount of energy in the electron stream is absorbed by the polymer to produce the activated sites that create crosslinks, scission, or rearrangement. Some of the excess energy is absorbed by the polymer to give increased movement and vibrations within the polymer. These movements are registered as increased temperature. The typical temperature rise for a polymer after electron beam irradiation is 43° F (24°C) for each 100 kGy. At the dosages typical for polyethylene, the expected temperature rise is from 75° to 248°F (24° to 120°C). In some cases, the polymer is cooled by fans or water to prevent distortion from this increased temperature.
The depth of radiation penetration depends on the power and the nature of the polymer. For most polymers, a penetration of 0.16 inches (4 mm) is normal for most of the currently available electron acceleration machines.
20.4.5. Detection of Results
Dosage rates for crosslinking of polymers vary widely depending on the type of polymer. For polyethylene, one of the most commonly irradiated polymers, the typical dosage is 10 kGy to 50 kGy, with significant degradation occurring above 1,000 kGy.
Even though the dosage can be detected quite accurately, the results of that dosage are somewhat difficult to determine. Since electron beam radiation causes three different reactions (cross linking, scission, rearrangement), all of which can occur simultaneously or not at all, evaluating the extent to which each reaction occurred can be rather complicated. Tests exist that do measure each of the effects, but some distortion of the results from the other reactions can occur.
The amount of crosslinking is usually determined by a gel fraction test (ASTM D 2765). Because of the high molecular weight of the crosslinked material, the crosslinked portion of the polymer does not dissolve in some solvents, whereas the non crosslinked portions of the polymer do dissolve. This allows the material to be separated into high and low molecular weight portions. Therefore, the fraction of crosslinked material can be determined by weighing the sample, dissolving out the noncrosslinked portion, and then filtering, drying, and weighing the undissolved portion.
Scission also causes changes in the molecular weight of the polymer, but since all fractions are usually soluble, the gel fraction test cannot be used. Other tests that measure properties affected by changes in molecular weight include the melt index test and various viscosity tests. When scission alone occurs, these tests are quite reliable. However, when both scission and crosslinking occur simultaneously, the results can be misleading. For instance, the melt index test can be higher because of the existence of the low molecular weight materials created by scission, but it can also be lower because of the crosslinked materials. The same problem occurs with most of the viscosity tests.
Sedimentation and gel chromatography are testing methods that may be able to separate the low molecular weight and high molecular weight fractions. After separation, the fractions are compared with fractions from the polymer before irradiation to determine the relative amounts of newly formed small or large molecular weight fractions. Both sedimentation and gel chromatography depend on the natural tendency of materials of different molecular weights to separate themselves when subjected to stresses. The stress used for sedimentation is gravity; for gel chromatography the stress is absorptivity on a paper medium.
A process called nuclear magnetic resonance (NMR) can also be used to determine the amount of polymer that may have reacted to crosslinking, scission, or rearrangement. The NMR technique uses the inherent differences in electron density in various parts of the polymer to give different energy-absorption frequencies when the polymer is subjected to a varying magnetic field. Since the cross linked molecules have different local electron densities than the noncrosslinked areas, the relative number of crosslinked polymers can be determined. Differences also exist between rearranged molecules and both cross linked and noncrosslinked polymers.
The thermal transitions of a polymer depend on the molecular weight of the polymer and can be used for detecting the amount of crosslinking and scission. Some of the tests that measure these thermal transitions and can be used for monitoring radiation products include differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), and differential thermal analysis (DTA). In each of these tests, the nonirradiated and radiated samples are compared.
Scission may cause a change in color, as is often the case with degradation. This color change can be detected by comparing the color of the nonirradiated material with that of the irradiated material using a detection device such as a reflectometer. These devices compare the light reflected off the surfaces of the two materials and register the differences. The amount of reflected light is affected by the color of the sample.