Electron-Beam Processing of Plastics: An Alternative to Chemical Additives

E-BEAM Services, Inc.

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 1,000,000 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 MeVDC 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 MeVDC 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.

References

(1)Glocker, G. and S.C. Lind; The Electrochemistry of Gases and Other Dielectrics. New York: Wiley (1939).

(2)Singh, A. and J. Silverman; Radiation Processing of Polymers.New York: OxfordUniversity Press (1992).

(3)Bly, J.H.; Electron Beam Processing.Yardley, PA: International Information Associates (1988).

(4)Minbiole, P.R.; Radiation Physics and Chemistry, 46:4-6 421-428 (1995).

(5)Cooper, W.J., R.D. Curry, and K.E. O’Shea; Environmental Applications of Ionizing Radiation. New York: Wiley-Interscience (1998).

(6)Machi, S. Radiation Physics and Chemistry, 46:4-6 399-410 (1995).

(7)Ivanov, V.S.; Radiation Chemistry of Polymers.Utrecht, The Netherlands: VSP BV (1992).

(8)Lee, D.W.; Kunststoffe,83:2 127-130 (1993).

(9)Lee, D.W.; Kunststoffe,81:7 609-613 (1991).

(10)Dominghaus, H.; Plastics for Engineers. Munich: Carl Hanser Verlag (1993).

(11)Wilson, J.E.; Radiation Chemistry of Monomers, Polymers, and Plastics.New York: Marcel Dekker (1974).

(12)Anonymous; Organic Peroxides Product Safety Guide. Greenwich, CT: Witco Corporation (1997).

Table 1: Selected Industrial Applications of Irradiation Processing.

Irradiation
Target / Net
Effect / DoseRange (kGy)
Food / Cold pasteurization / 0.3-30
Medical disposable items / Sterilization / 10-60
Cellulose/Pulp / Depolymerization / 5-50
Coatings / Curing / 30-160
Polyolefin foams / Crosslinking / 40-80
Heat-shrinkable materials / Memory Imparted / 75-250
Rubber / Vulcanization / 80-400
Fluoropolymers / Degradation / 500-1500
Gemstones / Coloration / 10,000+

Table 2: Izod Impact and Gel Fraction in Rotomolded HDPE Drums.

Irradiation Dose (kGy) / Insolubles (Gel) Fraction (%) / Izod Impact (J/m)
0 / 0.0 / 160
75 / 49.0 / 186
150 / 80.3 / 523
225 / 86.2 / 656
300 / 88.4 / 688

Table 3: Tensile Strength and Elongation of EB-processed Nylons.

Tensile Strength (MPa) / Control / 25 kGy / 50 kGy / 75 kGy
TR-55 / 60.1 / 52.4 / 52.7 / 53.2
TR-55LX / 43.5 / 72.2 / 71.0 / 68.3
Elongation at Break (%) / Control / 25 kGy / 50 kGy / 75 kGy
TR-55 / 121 / 108 / 115 / 131
TR-55LX / 132 / 5 / 5 / 4

Figure 1: Melt Flow Index as a Function of Dose in EVA.

Key Words

Electron-beam; Irradiation; crosslinking; Controlled Rheology