Modification of Polymer Substrates Using Electron Beam-Induced Graft Copolymerization

by Dr. Stephen C. Lapin, PCT Engineered Systems LLC

Introduction

Low energy electron beam (EB) systems with accelerating potentials up to 300 kV have been used in industrial processes for more than 30 years. Most of the systems feature a self-shielded design, meaning the systems include all shielding needed to prevent emission of secondary X-rays as the substrate is transported in and out of processing zone.¹ The most common substrates are flexible webs; however, systems designed to transport flat and even 3-dimensional objects are now known.²

Electron beams are a form of ionizing radiation, meaning that the accelerated electrons have enough energy to break chemical bonds in organic materials including polymers. The most common result of the breaking of chemical bonds is the formation of free radicals. EB applications take advantage of processes resulting from the formation of these radicals. EB processes can be classified by the effects resulting from the formation of free radicals that include: (A) curing, (B) cross-linking, (C) scissioning and (D) grafting.³ These processes are illustrated in Figure 1.

EB curing occurs when the radicals that are formed initiate the polymerization of monomers and oligomers. Acrylate functional materials are used most commonly for their high reactivity. Curing usually is associated with the rapid conversion of liquid ink, coating or adhesive to a solid cross-linked polymer layer. EB curing is used in a variety of printing, packaging and industrial applications.

EB cross-linking occurs when the radicals that are formed recombine with each other. Cross-linking usually starts with a polymer material and results in the joining of adjacent polymer chains to form a three-dimensional network. A relatively small number of cross-links often can have a large impact on the thermal and mechanical properties of a polymer. Common applications for low energy EB cross-linking include: (1) processing of polyethylene films to provide heat shrink properties for packaging applications⁴ and (2) processing of pressure sensitive adhesives to improve heat resistance and shear properties.⁵

EB scissioning occurs when the radicals that are formed fail to recombine and are terminated by reactions with oxygen and/or hydrogen abstraction. The net result of EB scissioning of a polymer is a reduction in the molecular weight. Industrial processes for EB scissioning are less common than curing or cross-linking. An example of scissioning is the processing of polytetrafluoroethylene (PTFE) to make low molecular fragments for use in waxes and lubricants.⁷ Most polymers undergo cross-linking and scissioning and the process that predominates depends on chemical structure and morphology of the polymer. Scissioning can be applied to biopolymers and is used to kill bacteria and other pathogens in EB sterilization processes.

EB-induced graft copolymerization (EIGC) occurs when radicals formed in and on a polymer substrate become a site for initiation of monomer polymerization. The net result is that two dissimilar polymers are joined covalently to form a new copolymer material. EB grafting is less well known than curing or cross-linking but is an important process for creation of new functional materials. The EB grafting process and applications are the subject of the discussion in this paper.

EB Grafting process

EIGC has several advantages over other grafting methods. They include:

1. Ability to ionize polymers that have limited reactivity in chemical processes
2. Clean, non-chemical method to generate polymer radicals
3. Consistent, controlled process
4. Low energy usage
5. Scalable from slow to fast process speeds
6. Scalable from narrow to wide webs
7. Easy integrated into complete process lines

Many common low cost polymer substrates, such as polyethylene and polypropylene, are very unreactive and lack functional groups that can be used for chemical grafting. EB can ionize these polymers easily, creating radical sights for grafting. Because these polymers are normally unreactive, they can also benefit greatly in performance and value as a result of EIGC. The number of radical sites created is proportional to the EB dose applied to the polymer substrate. Once the optimum dose is determined, it can be maintained at a very constant level as the EB dose is controlled automatically with increases or decreases in line speed. The output of the equipment itself is very consistent also, with little variation over very long periods of operation. The process can be scaled using EB systems available from under 0.4 to over 3.0 meters wide (Figure 2). The EB systems also are compact – typically occupying only 2 to 4 meters of space in the web direction. This facilitates integration into a process line.

The depth of EB energy deposition into materials is controlled by the accelerating potential of the equipment and the elemental composition and density of the material being irradiated. This can be very accurately predicted by Monte Carlo simulations.⁷ Most organic polymers show very similar energy deposition characteristics. The net result is that energy deposition is predicted by the density of the polymer alone. A plot of the relative dose versus the basis weight (weight per unit area) of the polymer essentially factors out the density and provides a very useful tool to determine energy deposition as a function of the material basis weight.

Figure 3 shows the energy deposition for systems operating from 100 to 300 kV. A material with a basis weight of 20 g/m2 will receive a very uniform dose from the front to back surface using an accelerating potential of 150 kV or more. Materials up to 400 g/m2 will get a relatively uniform (+ less than 20 percent) dose from the front to back using a potential 300 kV. Another way to look at this is to consider a thick material – for example, 1.0 mm with a density 1.0 = 1000 g/m2. EB energy deposition can be controlled in this case from less than 20 microns to more than 400 microns into the material. This is very useful for controlling the location of the radicals that are formed and the resulting grafting that occurs. An interesting aspect of this is that low density materials, such as microporous membranes and fabrics – woven or non-woven – can be functionalized on internal surfaces since the air voids within these materials have very little electron stopping power. The same depth/dose curves can be used to predict energy deposition in low density materials as long as the materials have a uniform density on a microscopic scale – ie, the average basis weight is about the same for any given spot on the material. Note that although the energy deposition in materials can be predicted accurately, the actual yield of radicals that are formed is highly dependent of the type of polymer being used.⁸

EIGC may be performed by two main methods: (1) simultaneous irradiation or (2) pre-irradiation methods. These are illustrated in Figure 4.

In the simultaneous irradiation method, the polymer substrate is coated or saturated with neat monomer or a monomer solution. The substrate/monomer combination is then irradiated to initiate polymerization of the monomer. This may be followed by a washing process that removers uncured monomer or polymer, which is not grafted to the substrate. An optional thermal drying step may be used to evaporate residual wash solvent from the graft copolymer substrate.

A disadvantage of the simultaneous irradiation method is the formation of homopolymer, which is not grafted to the substrate. This can be minimized by using relatively dilute monomer solutions, including inhibitors in the solutions, or minimizing the EB dose levels that are used. Some degree of homopolymerization may not be an issue as long as it is removed upon washing or is anchored well enough to the substrate to provide the desired functionality.

In the pre-irradiation method, the polymer substrate is irradiated to generate radicals. As long as the substrate is maintained in a vacuum or inert atmosphere, the radials have a relatively long lifetime and can initiate polymerization of monomer, which is subsequently brought into contact with the irradiated substrate. A washing process may be used to remove unreacted monomer. A thermal oven may be used to evaporate the wash solvent. Homopolymerization is less of an issue with pre-irradiation compared to simultaneous irradiation methods.

In an alternate version of the pre-irradiation method, the polymer substrate may be irradiated in air forming either peroxy or hydroperoxy groups. Grafting is then initiated by decomposition of the peroxides into radicals at an elevated temperature in the presence of a monomer.

With all methods the main process factors used to control EIGC are (1) EB voltage, (2) EB dose, (3) type of solvent used to dilute the monomer, (4) monomer concentration, (5) temperature and (6) dwell time for grafting reaction.⁹

Polymer substrates

A wide variety of polymer substrates may be used for EIGC. The majority are synthetic polymers, such as polyethylene (PE), polypropylene (PP), polyamides (PA), polyether sulfone (PES), poly(vinylidenefluoride) (PVDF), poly(tetrafluoroethylene) (PTFE) and poly(ethylene-co-tetrafluoroethylene) (ETFE). Additional graft copolymers originate from modified natural backbone polymers, such as cellulose, starch, alginate and chitosan. From a morphological point of view, the polymer substrates may be in form of beads, gels, fibers, fabrics, films and membranes.¹⁰

Graft copolymers

There are a wide variety of graft copolymers that can be formed by EIGC. A broad classification would be neutral and ionic copolymers. Ionic copolymers may be subdivided into anionic, cationic and bipolar types. The monomer used for grafting determines the type of copolymer that is formed (Figure 1D). A list of sample monomers that have been used in EIGC are shown in Table 1. A monomer, such as a perfluoroacrylate, will produce highly fluorinated graft side chains that result in a very hydrophobic copolymer. Acrylamide monomer, by contrast, gives a polar graft side chain resulting in hydrophilic copolymer. Acrylic acid produces graft side chains containing carboxy groups. The acid groups may be neutralized to form the corresponding metal salt, which then may be medium for exchange with other metal cations.

The monomer itself may provide the desired properties when copolymerized on the polymer substrate. Another option is to subject the copolymer to a post-grafting reaction where the side chain is chemically converted to give the desired functionality. An example is the use of gycidyl (meth)acrylate, which produces epoxy functional side chain groups. The epoxy groups can be reacted with other materials, including phenols, amines, phosphoric acid and amino acids (Table 1) to give the desired properties.¹⁰ In most cases, the goal is to start with a very inert polymer – such as PE, PP, PVDF or PTFE – and produce active or contrasting properties in the grafted copolymer.

Applications

There are many applications for copolymers produced by EB grafting. Examples include:

1. Specialty fabrics – woven or non-woven – with modified properties such as water repellence or water absorption¹¹
2. Reinforcing fiber for composites, where enhanced bonding properties between the fiber and matrix resin results in higher performance properties¹²
3. Plastic films with enhanced adhesion properties such as print receptivity or enhanced bonding of multilayer structures¹¹
4. Production of media used for separation and purification purposes

The use of radiation-induced grafting for the production of separation media is an active area of research and development and it was the subject of a recent review article.¹⁰ The separating media may be in the form of beads, gels, fibers, fabrics and membranes. For commercial purposes the media may be packaged in many different configurations, including tanks, columns, modules and cartridges.

There are a wide variety of industrial separation and purification applications that include:

1. Water treatment
2. Environmental
3. Chemical industry processing
4. Food processing
5. Battery and fuel cell separators
6. Biotechnology and biomechanical application

Sample applications from published literature are listed in Table 2. The variety of applications are very broad and include the recovery of toxic and high value metals from waste water^18-21 and a biomedical application for the purification of a racemic mixture to give the enantiomer the desired pharmaceutical activity.²²

Conclusions

EIGC is a very versatile method for the production of specialty graft copolymers. EB provides control of depth and concentration of radials in and on polymer substrates. The selection of monomers and post-grafting chemical transformations allows the production of copolymers with desired functionality. EIGC allows the production of copolymers tailored for specific end-use application. Low energy electron beam equipment is well suited to integration in commercial EIGC production lines.

References

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   21. Das S, Pandey AK, Athawale A, Kumar V, Bhardwaj YK, Sabharwal S, Manchanda VK, Chemical Aspects of Uranium Recovery from Seawater by Amidoximated Electron Beam Grafted Polypropylene Membranes, Desalination, 232, 2008, pp. 243–253.
   22. Nakamura M, Kiyohara S, Saito K, Sugita K, Sugo T, Chiral Separation of dl-Tryptophan Using Porous Membranes Containing Multilayered Bovine Serum Albumin Cross-linked with Glutaraldehyde, Journal of Chromatography A, 822, 1998, pp. 53–58.

Dr. Stephen C. Lapin is the BroadBeam applications specialist for PCT Engineered Systems LLC. He has a Ph.D. in organic photochemistry and 30 years of industrial experience, which includes 22 US patents and being published in more than 40 publications. He has been a RadTech member since 1988. Contact Stephen Lapin at [email protected].