Electron Beam vs. Peroxide Cross-Linking

Radiation processing can be defined as the technology of improving the desired properties of materials and products by their treatment with ionizing radiation - that is radiation with quantum energy higher than the ionization potentials of the subject materials. All forms of ionizing radiation interact with matter by transferring energy to the electrons present in the atoms 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 an excited atom or molecule. These ions, electrons, and excited species are the precursors of any chemical changes observed in irradiated materials. Thus, by using ionizing radiation, it is possible to synthesize, modify, cross-link, and degrade polymers.

Electron beam technology utilizes highly energetic electrons generated by powerful beam accelerators to bring about beneficial and commercially valuable changes to thermoplastic materials. Of specific interest to the plastics industry has been the use of electron beam technology to enhance thermal, chemical, barrier, impact, wear, and other key properties of inexpensive commodity thermoplastics, thereby extending their utility to demanding applications typically dominated by higher-cost engineered materials.

Industrial and commercial products made of PE have been cross-linked to modify their various performance properties for over fifty years. A sampling of these diverse products would include wire and cable insulation, foam sheeting, food packaging films, various gaskets and seals, hot water tanks, piping and tubing, and medical devices. Cross-linking enhances several key properties of PE, including:

• Impact Strength - At both at low and ambient temperatures, Dart- and Izod-type impact strength can be increased roughly five-fold.
• Barrier Properties - For many solvents and liquids, barrier properties can be increased up to 50%. The U.S. Coast Guard requires that built-in PE fuel storage tanks on boats be cross-linked.
• ESCR - With respect to traditional ESCR tests, cross-linking essentially eliminates stress cracking in rotomolded PE parts.
• Heat Resistance - Cross-linking effectively raises continuous service temperature of PE parts from 122^oF to near 200^oF.

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 ESCR and impact strength are key in their performance and utility. Themochemical cross-linking 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.

Recently, a study was conducted in which 100-liter rotomolded HDPE drums were irradiated to electron beam doses ranging from 75 to 300 kGy. As illustrated in the table below, 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.