Radiation Crosslinking of Polymers
Prepared by Lewis A. Parks, Ph.D.
February 2, 2010
Crosslinking (or cross-linking, cross linking) is a process where the long chains of polymers are linked together increasing the molecular mass of the polymer as a result.
Properties of polymer products that can be improved by crosslinking include:
· Mechanical properties, such as tensile strength
· Scratch resistance
· Performance at higher temperatures, often with an increase in the melting temperature
· Resistance to chemicals because of lowered solubility in organic solvent
· Gas permeation reduction
· Shape memory retention. Elastomers may be crosslinked to a slight degree to give them memory they will return to their original shape after being expanded.
Depending upon the polymer, different techniques may be used to cause crosslinking. In all cases, the chemical structure of the polymer is altered through the crosslinking process. This can be done by adding different chemicals in conjunction with heating and, sometimes, pressure. One of the earliest examples of crosslinking is the vulcanization of natural rubber by adding sulfur under heat, which creates the links between the latex molecules. Vulcanization gives the rubber its strength properties over temperature ranges in which non-vulcanized rubber could not perform.
Alternatively, the polymer may be crosslinked using high-energy ionizing radiation, ie electron beam (or e-beam, e beam), gamma, or x-ray. Gamma irradiation is usually most economical at lower doses (~80 kGy and below) and for large, high density parts. Electron beam is commonly used for small parts, particularly low density parts, and linear product processed reel to reel (eg, wire, cable, tubing).
Irradiation creates free radicals which will often chemically react in various ways, sometimes at slow reaction rates. The free radicals can recombine forming the crosslinks. The degree of crosslinking depends upon the polymer and radiation dose. One of the benefits of using irradiation for crosslinking is that the degree of crosslinking can be easily controlled by the amount of dose. Other subtle influences include the additives in the base polymer and the type of radiation used (related to the dose rate). Another influence which may not be as subtle is oxidation during irradiation. This effect will be more predominant when using gamma irradiation as compared to the much faster electron beam irradiation process. Furthermore, oxidation can continue after irradiation cuasing changes in properties with time. This oxidation process may reduced by antioxidants added to the polymer resin.
A competing process, called scissioning, occurs when polymers are irradiated. In this case, the polymer chains are broken and molecular mass decreases. Scissioning and crosslinking occur at the same time where one may predominate over the other, depending upon the polymer and the dose. The rate of occurrence for these two processes is the G value, defined as the number of events that occurs with the absorption of 100 eV of energy. These can be converted to units of μmol J-1 by multiplying G by 0.1036 (Ref 1, p 351).
Another byproduct which can be created, depending upon the polymer, is gas emission. For example, polyethylene will release hydrogen during irradiation, usually at relatively insignificant amounts. When evaluating a polymer to irradiate, interaction of the polymer or the gas with ozone created during the irradiation of the neighboring air may need to be considered.
Table 1 lists polymers which are radiation crosslinkable. The table lists their G values for crosslinking (GX) and scissioning (GS), overlooking the subtle influences. These data are from several sources, and listed ranges can arise from the different brands of polymers studied within a polymer family. Although some polymers may crosslink at a greater rate than scission, a low GX value would indicate a need for higher doses to achieve property improvements.
Table 1 Selected radiation crosslinkable polymers
(1) Ref 1, p 351, 355 and references therein
(2) Ref 2, p 563, 574
(3) Ref 3
GX of the polymers listed in Table 1 can generally be enhanced through the addition of crosslinking promoters, called prorads. This can reduce the dose needed for crosslinking, which can further result in reducing potential adverse effects, such as oxidation, free radical generation, and gas evolution. The use of prorads provides opportunities to create radiation crosslinked products from base polymer materials which could not otherwise be created. Although not as common, antirads can be added to reduce the degree of radiation crosslinking.
Scission and crosslinking both occur in nearly the same amounts when polypropylene is irradiated, with purely amorphous polymer (atactic) crosslinking at higher rates than semi-crystalline polymer (atactic / isotactic, atactic / syndiotactic) (Ref 2, p 564). This enhanced crosslinking behavior is also seen for amorphous polyethylene and some other polymers compared to their semi-crystalline species Polypropylene has a high rate of oxidation during and after irradiation, with this effect much more pronounced using gamma irradiation compared to electron beam (Ref 4). For this reason, antioxidants are added to make polypropylene parts radiation sterilizable. The primary gases released during irradiation of polypropylene under vacuum are hydrogen and methane. When irradiated in air, carbon dioxide and carbon monoxide are also produced. Because of scissioning and, more importantly, oxidation that occur, radiation crosslinking of polypropylene is an unlikely candidate for property improvements.
Notable products that are made using radiation crosslinking are listed in Table 2. There are many other niche products that are manufactured using crosslinking. This table, along with Table 1, should be used as a catalyst to realize other potential applications using radiation crosslinkable materials, such as those listed in Table 1.
Table 2. Notable products made using radiation crosslinking.
* Suppliers of radiation crosslinkable polyamide (enhanced with prorads) are:
· Frisetta Polymer, www.frisetta-polymer.de.
· PTS Plastic Technology Service, http://www.pts-marketing.de/. Also suppliers of radiation crosslinkable polyester elastomer and other elastomers.
Two common chemical crosslinking techniques are silane and peroxide crosslinking. Their benefits and disadvantages are listed in Tables 4 and 5 and can be contrasted to the benefits and disadvantages of using radiation crosslinking, listed in Table 3.
Table 3. Benefits and disadvantages of radiation crosslinking
Table 4. Benefits and disadvantages of silane crosslinking (Ref 5)
Table 5. Benefits and disadvantages of peroxide crosslinking (Ref 5)
One of the advantages of radiation crosslinking is that the amount of crosslinking can be controlled by the amount of dose used. In fact, resins of materials listed in Table 1 may be irradiated at doses lower than those used for crosslinking to create long chain branching, a precursor to crosslinking. This influences the melt flow properties of the materials by decreasing their melt flow index and enhancing melt strength during processing. Examples are irradiated polyethylenes (Ref 6, 7) used in products such as films (Ref 8) and foams (Ref 9) which can benefit from enhanced melt strength during manufacture, and ethylene vinyl acetate (Ref 3, p 564).
In summary, radiation crosslinking, although not as widely used as chemical crosslinking, offers several advantages over chemical crosslinking. Because of the high capital cost of radiation processing equipment, radiation crosslinking is usually performed in service centers located around the world.
Reference Data: crosslinking (aka cross-linking and cross linking), electron beam (aka e-beam and e beam), gamma ray, irradiation, peroxide crosslinking, polymers, prorad, radiation crosslinking, silane crosslinking, vulcanization, x ray
1 Robert J. Woods and Alexei K. Pikaev, Applied Radiation Chemistry: Radiation Processing, (John Wiley & Sons, Inc., New York 1994).
2 K. Dawes and L. C. Glover, Physical Properties of Polymers Handbook, ed. James E. Mark (American Institute of Physics Press, Woodbury, NY, 1996).
3 K. Makaucchi, M. Asano, and T. Abe, Nippon Kagaku Kaishi, 686 (1976).
4 I. Ishigaki and F. Yoshii, Radiat. Phys. Chem. 39, 527 (1992).
5 Ron Goethals, presentation at Crosslinking Polyolefins 2005, Dϋsseldorf.
6 Song Cheng, Edward Phillips, and Lewis Parks, Radiat. Phys. Chem. 78, 563 (2009).
7 Radiation Treated Ethylene Polymers and Articles Made From Said Polymers, United States Patent US 7,094,472 B2.
8 Low Level Irradiated Linear Low Density Ethylene / Alpha-Olefin Copolymers and Film Extruded Therefrom, United States Patent US 4,525,257 (1985).
9 Compitable Linear and Branched Ethylenic Polymers and Foam Therefrom, United States Patent US 6,593,386 B1.