Abstract
Modifications
in polymeric structure of plastic materials can be brought about either by
conventional chemical means, usually involving silanes or peroxides, or by
exposure to ionizing radiation from either radioactive sources, or highly
accelerated electrons. Chemical crosslinking typically involves the generation
of noxious fumes and sensitizing by-products of peroxide degradation.
Increased utilization of
electron-beams (e-beams) for modification and enhancement of polymer properties
has been well documented over the past forty years. Of specific interest to the
plastics industry has been the use of e-beam processing (EBP) to improve
thermal, chemical, barrier, impact, wear, and other properties of inexpensive
commodity thermoplastics, extending their utility to demanding applications
typically dominated by higher-cost engineered materials. EBP of cross-linkable
plastics has yielded materials with improved dimensional stability, reduced
stress cracking, higher service temperatures, reduced solvent and water
permeability, and significant improvements in other thermomechanical
properties.
The purpose of this paper is to review the basic effects
EBP on polymers, as well as to highlight several specific recent cases of its
utilization to improve key properties of selected plastic products.
Introduction
to Irradiation Processing
For over sixty years the physical and chemical changes
induced by absorption of radiation sufficiently high in
energy to produce ionization have been the subject of both
university and industrial research. Early work dealing with
chemical effects of ionizing radiation utilized the natural
radioisotopes radium and radon as radiation sources (1).
At this time, the most common commercial sources of ionizing
radiation are 60Co and 137Cs for gamma
irradiation, and electron accelerators for e-beam (beta)
irradiation (2). When the electron-beam generated by an
accelerator is directed at a target consisting of a high-atomic-number
metal, such as tungsten or gold, X-rays with a broad spectrum
of energies can also be produced. The amount of energy absorbed,
also known as the dose, is measured in units of kiloGrays
(kGy), where 1 kGy is equal to 1,000 Joules per kilogram,
or MegaRads (MR or Mrad), where 1 MR is equal to 100,000,000
ergs per gram (3).
Industrially,
EBP is performed using commercially-available accelerators, which are equipped
with a variety of material handling systems, and are capable of significant
throughput. The accelerators are typically described in terms of their energy
and power (4). Low-energy accelerators range from 150 keV to 2.0 MeV. Medium-energy
accelerators have energies between 2.5 and 8.0 MeV. High-energy accelerators
have beam energies above 9.0 MeV. The beam energy required depends directly on
the application for it is to be used. For example, for coatings curing and
crosslinking of food wrap film accelerators with energies of 150-500 keV are
typically used. On the other hand, crosslinking of wire and cable jacketing
can require electron energies of 1.25-5.0 MeV, depending on wire diameter. (3)
Accelerator power is a product
of electron energy and beam current. Available beam powers range from 5 to
about 300 kW (5). For example a 5.0 MeV accelerator at 30 mA will have the
power of 150 kW (3).
Accelerators can generally be
classified according to exactly how they generate accelerated electrons. The
five main types of accelerators are: electrostatic direct-current (DC),
electrodynamic DC, radiofrequency (RF) linear accelerators (LINACS),
magnetic-induction LINACs, and continuous-wave (CW) machines. (3)
In general, DC accelerators are
characterized by high power output and high efficiency, while LINAC systems are
typically much more compact and can generate higher beam energies. However,
they are also considerably less efficient. Similarly, CW machines can be fairly
compact, and can achieve high beam energies. Regardless of the exact nature of
the accelerator, in all EBP facilities, the target materials are passed under
the accelerator’s scan-horn using conveyors, carts, reel-to-reel equipment, or
other specialized handling means (4). Worldwide, there are approximately
700-800 electron beam accelerators in industrial use today (6).
All
forms of ionizing radiation interact with matter by transferring energy to the
electrons orbiting the atomic nuclei of target materials. These electrons may then
be either released from the atoms, yielding positively charged ions and free
electrons, or moved to a higher-energy atomic orbital, yielding and excited
atom or molecule (free radical). These ions, electrons, and excited species are
the precursors of any chemical changes observed in irradiated material (5).
Thus, by using ionizing radiation, it is possible to synthesize, modify,
cross-link, and degrade polymers. Likewise, ionizing radiation has the ability
to break the chains of DNA in living organisms, such as bacteria, resulting in
microbial death and rendering the space they inhabit sterile (3). Table 1
provides examples of established current industrial applications of irradiation
processing.
Effects
of Ionizing Radiation on Polymers
The effects of ionizing
radiation on polymeric materials can be manifested in one of three ways. The polymer may undergo one or
both of the two possible reactions: those that are molecular-weight increasing in nature, or molecular-weight
reducing in nature. Or, in the case of
radiation-resistant polymers, no significant change in molecular weight will be
observed. The conventional term for irradiation-induced increase in molecular
weight is crosslinking. The corresponding term for irradiation-induced
decrease in molecular weight is chain scissioning (or degradation). (7)
Each of the two types of
reactions are currently being harnessed in an economically beneficial manner to
add value to a wide variety of thermoplastics, elastomers, and other materials.
For example, the beneficial changes
observed in crosslinked polyethylene (XLPE) include increased modulus, tensile
and impact strength, hardness, deflection and service temperature, stress-crack
resistance, abrasion resistance, creep and fatigue resistance, and barrier
properties (8). On the other hand, the chain scissioning effects observed in
polytetrafluoroethylene (PTFE) have been commercially exploited as an effective
means to produce fine micropowders from scrap or off-spec materials (9).
With respect to processing
economics, EBP typically requires lower energy expenditure than conventional
thermochemical processes to produce the same net effects. For example,
radiation vulcanization of rubber requires an absorbed dose of 80 kGy, or 80
J/g, while thermochemical vulcanization at 150 oC producing end
material of the same cross-link density requires energy expenditure of 281 J/g.
It has been observed that radiation vulcanization is 3-6 times more energy
efficient than thermochemical vulcanization. (7)
While radiation responses of various polymers to the
three types of radiation mentioned earlier are to a great extent (and with
notable exceptions) similar, due to its high throughput efficiency and lack of
a nuclear source requirement, EBP is currently the method of choice for
irradiation processing of polymers.
Predicting
Irradiation Response of Polymers
In order to predict the behavior
of carbon-chain polymers exposed to ionizing radiation, an empirical rule can
be used. According to this rule,
polymers containing a hydrogen atom at each carbon atom, predominantly undergo
crosslinking, whereas those polymers containing quaternary carbon atoms and
polymers of the -CX2-CX2- type (where X is a halogen), chain scissioning
predominates (7). Aromatics, like polystyrene (PS) and polycarbonate (PC) are
relatively resistant to EBP and are thus well suited to serve as packaging
materials for medical disposables which are slated to be radiation sterilized
(10).
During irradiation, chain
scissioning occurs simultaneously and competitively with crosslinking, the end
result being determined by the ratio of the yields of the two reactions. For some polymers, such as polyvinyl
chloride (PVC), polypropylene (PP), and polyethylene terephthalate (PET), both
directions of transformation are possible, and certain conditions exist for the
predominance of each one.
The ratio of crosslinking to
scissioning depends on factors including total irradiation dose, dose rate, the
presence of oxygen, stabilizers, and radical scavengers, and steric hindrances
derived from structural or crystalline forces (7).
Overall property effects of crosslinking can be complex
and contrary, especially in copolymers and blends. For example, after EBP, highly crystalline polymers like
high-density polyethylene (HDPE) may not show significant changes in tensile
strength, a property derived from the crystalline structure, but a significant
improvement in properties associated primarily with behavior on the amorphous
regions, like impact and stress-crack resistance (11).
Selected
Applications of EBP
Controlled Rheology of PE Copolymer
One
of the most valuable applications of EBP involves the use of relatively low
irradiation doses to induce beneficial changes in the melt rheology of bulk
resin pellets. For crosslinking polymers, this will result in a reduction of
the melt-flow rate (MFR), while with degrading polymers, the effect will be an
increase in MFR. Additional benefits of CR resins include narrower molecular
weight distribution (MWD), changes in strain hardening, and increased melt
strength. As a practical example, in extrusion coating, EBP-induced strain
hardening allows up to 300% higher line speeds.
While there are several
thermochemical options for manufacturing CR pellets, EBP has been demonstrated
as being a reliable, high-throughput, environmentally favorable alternative.
Organic peroxides employed by the conventional methods typically decompose in
processing to yield noxious fumes and sensitizing by-products (12). Likewise,
the use of additives in thermochemical processing necessitates tight control
over a large number of key variables, including temperature profiles, peroxide
concentration, residence time, quenching, screw design, etc. Conversely,
production of CR resins by EBP does not involve any additives, and the success
of the process depends on a single easily controlled variable – dose.
In a recent experiment, several
grades of commercially-available ethylene-vinyl acetate copolymer (EVA) with a
nominal melt flow index (MFI) of 30 g/10 min were irradiated by EBP to doses
ranging from 5 to 20 kGy using a 4.5 MeV DC accelerator. Figure 1 graphically
summarizes the observed relationship between irradiation dose and MFI as per
ASTM D 1238. The results confirm that EBP is, in fact, a suitable alternative
method for generating CR grades of EVA.
Impact Strength Increase in Rotomolded Drums
Rotomolded HDPE drums are
frequently used in transportation of hazardous chemical wastes, including
specifically to encase leaking or fragile metal drums. Therefore, properties
such as environmental stress crack resistance (ESCR) and impact strength are
key in their performance and utility. Themochemical crosslinking to improve
these properties is somewhat unfavorable due to the possibility of reactive
peroxide residues that could potentially react with the hazardous materials in
the drum.
In seeking to demonstrate the
suitability of EBP to such a demanding and yet at the same time sensitive
application, an experiment was conducted in which 100-liter rotomolded HDPE
drums were irradiated to doses ranging from 75 to 300 kGy using a 10.0 MeV RF
LINAC. ASTM Type III specimens for tensile, impact, and percent insolubles
(gel) testing were die-cut from the walls of the irradiated drums. Testing of
the samples was performed as per ASTM D 256 (Izod impact), ASTM D 638 (tensile
properties), and ASTM D 2765 (gel content).
The results
indicate that Izod impact of drums processed by EBP increased with dose up to
300% of the control (non-irradiated) value at maximum dose (300 kGy).
Similarly, the gel content was observed to increase with dose from 0% (control)
to over 88% at maximum dose. Table 2 presents the full data set from this
experiment.
Effect of EBP on Two Injection Molding Nylons
Commercially, polyamides
(Nylons) are used in a variety of
markets in a number of demanding applications. Due specifically to their
high dimensional stability, excellent chemical resistance, and superior
electrical properties, Nylon 11 and 12 based formulations have been used in
injection molding of various under-the-hood and other heavy-duty automotive
parts, including timing sprockets, cooling fans, wire connectors, brake-fluid
reservoirs, and door latches. Other industrial injection molded Nylon 11 and 12
products include underwater bearings, seals, gears, sliding bearings,
insulating components, and various parts for textile machinery and household
appliances. (10)
As expected, aromatic Nylons are
considerably less responsive to ionizing radiation that linear aliphatic
Nylons. However, in theory it should be
possible to improve the tensile properties of Nylon blends, where one of the
components is a linear aliphatic Nylon. In fact, such blends are currently
being marketed for exterior weathering applications, where the injection molded
part is to be continuously exposed to solar radiation. It is therefore of
interest to evaluate whether the tensile strength of such a blend could be improved by EBP.
In a recent experiment, sets of standard tensile bars
made from two commercially-available Nylons were supplied by the resin producer
and irradiated by EBP to a doses of 25, 50, and 75 kGy using a 4.5 MeV DC
accelerator. The first formulation (TR-55) was an aromatic cycloaliphatic
totally amorphous Nylon homopolymer manufactured by the condensation of o-laurolactam, isophthalic acid, and
bis(4-amino-3-methyl-cyclohexyl)methane. The second formulation (TR-55LX) was a
proprietary blend of the first material with a linear aliphatic Nylon. The
second formulation also incorporated an additives package, which included free
radical scavengers. After irradiation the samples were tested for tensile
strength and elongation as per ASTM D 638.
The results, summarized in Table 3, show that while the
tensile strength of the aromatic Nylon did not improve, in the case of the
aromatic-linear aliphatic Nylon blend, a 66% increase in tensile strength was
observed, along with a substantial decrease in elongation. This is especially
remarkable, since the Nylon blend included free radical scavengers, which act
to retard radiation-induced crosslinking. It can be hypothesized that a
scavenger-free blend of the same
composition would show an even greater increase in tensile strength following
EBP.
Conclusion
EBP is a unique and powerful
means of bringing about controlled, beneficial changes in polymers,
particularly since the changes are brought about in solid-state, as opposed to
alternative chemical and thermal reactions carried out in hot, melted polymer.
The influence of ionizing radiation on properties and performance differs
depending on whether the target polymer degrades or cross-links, and this in
turn depends on specific sensitivities or susceptibilities inherent in the
polymer backbone.
Organic peroxides commonly used
for thermochemical crosslinking of plastics are relatively unstable chemicals
which pose hazards in handling due to their heat sensitivity, flammability, and
tendency to violently decompose upon contamination. Likewise, their use
typically involves the generation of noxious fumes and sensitizing degradation
by-products.
In each of the three highlighted applications, it was
possible to use EBP to favorably modify a specific target property without the
use of organic peroxides or other crosslinking additives, thus demonstrating
the utility of EBP as an environmentally responsible alternative to
conventional thermochemical processing techniques.
Acknowledgements
The author would like to recognize Dante Ferrari of AT
Plastics and A.J. Vezendy of EMS-Chemie for providing materials and testing expertise
for portions of the work described herein.
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