Chapter 14
Acrylic-Acid-Grafted Polyethylene by ElectronBeam Preirradiation Method Some Effects of Reaction Variables 1
2
2
Junji Harada , Rey T. Chern , and Vivian T. Stannett
Downloaded by CORNELL UNIV on August 10, 2012 | http://pubs.acs.org Publication Date: November 12, 1991 | doi: 10.1021/bk-1991-0475.ch014
1
Tsukuba Research Laboratories, Mitsubishi Paper Mills, Ltd., 46 Wadai, Tsukuba, Ibaraki, Japan Department of Chemical Engineering, North Carolina State University, Raleigh, NC 27695-7905 2
High density polyethylene (HDPE) and low density polyethylene (LDPE) samples preirradiated with an accelerated electron beam (accelerated for 1-64 Mrad) were grafted with acrylic acid under various conditions. The effects of irradiation dose, reaction time, monomer concentration and inhibitor concentration on the grafting were studied. According to the DSC analysis on dry grafted samples, the graft reaction occurs both in the amorphous region and at the boundary of crystallites. The asymptotic grafting increased exponentially with the preirradiation dose. The grafting rate and asymptotic grafting value of HDPE are much higher than those of LDPE. The grafting of the 64 Mrad irradiated HDPE reached 26,000% within 2 hours of reaction.
It is well known that graft polymerization can be initiated by many methods, for example, high energy (gamma ray, electron beam) irradiation, plasma treatment, ultraviolet light radiation, decomposition of chemical initiators and oxidation of the polymers. Among these methods, electron beam irradiation may be the most convenient and most effective for industrial usage because of its high irradiation dose rate, ease in generating active sites in many kinds of polymers, effective penetration into the polymer and moderate initiation reaction conditions (room pressure and temperature). Electron beam irradiation induced graft copolymerization can be used to modify either the surface or the bulk of the polymer. Many factors affect the modification reaction, they include the radiation chemistry of the substrate polymer (i.e. crosslinking or degradation type), extent of swelling of the grafted polymer by the monomer solution, the electron beam acceleration voltage, the grafting reaction time and temperature, and many others. By graft modification of the polymer surface, one can change the polymer's wettability (1,2), adhesion (3,4), printability, metalization, anti-fog properties (5), anti-statics properties (6-8), and biocompatability (9-13). Bulk graft modification of the polymer has been used to improve water absorbency (1416), fire retardancy (17), and to produce battery separators (18-26), ion exchange 0097-6156/91/0475-0238$06.00/0 © 1991 American Chemical Society
In Radiation Effects on Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
Downloaded by CORNELL UNIV on August 10, 2012 | http://pubs.acs.org Publication Date: November 12, 1991 | doi: 10.1021/bk-1991-0475.ch014
14. HARADA ET AL.
239
Acrylic-Acid-G rafted Polyethylene
and ion trap materials (8,27-30), separation membranes (29,31,32), deodorant materials, immobilized enzymes (33,34) and so on. Two standard methods of radiation grafting were developed early in the field and are still the main techniques available. These are the mutual (direct, simultaneous) and preirradiation (indirect, consecutive)methods. In this study, we used only the preirradiation method. This method can achieve effective grafting reactions, especially for a highly reactive monomer, such as acrylic acid, because the grafting reactions are carried out with little homopolymerization which is one of the main disadvantages of the mutual grafting method. This paper will consider the acrylic acid grafting reaction to polyethylene film by the electron beam preirradiation method. Acrylic acid is one of the most reactive monomers and grafted poly (acrylic acid) is expected to impart ion exchange and ion trap capabilities, wettability, water absorbent properties, anti static properties, permselectivities and so on. Polyethylene is a typical crosslinkable polymer by irradiation and is not swollen by water or acrylic acid. Several papers have been published about radiation induced grafting of acrylic acid onto polyethylene (18-25). Ishigaki et al., studied the preirradiation grafting using Mohr's salt as an inhibitor, and produced comparatively low grafting materials which are suitable as battery separators (23-25). The present paper deals with comparatively high temperature and redox reagent free grafting reactions, especially concerning the effects of irradiation dose, reaction time, monomer concentration and other reaction conditions. Experimental 3
Materials High density polyethylene (HDPE) film (d=0.963g/cm, 95μπι thick, crystallinitv=70%) and low density polyethylene (LDPE) film (d = 0.926g/cm3, 95μπι thick, crystallinity=46%) were used. Polytetrafluoroethylene (PTFE) film (ΙΟΟμπι thick) was used. Acrylic acid (Aldrich, including 200ppm hydroquinone monomethyl ether as a stabilizer) was used without any purification. Vacuum distilled acrylic acid was used only for the experiment on the effects of the Mohr's salt. Mohr's salt (ammonium iron (II) sulfate hexahydrate) was ACS reagent grade (Aldrich). All acrylic acid aqueous solution concentrations were expressed in volume percentage. Irradiation An electron curtain type EB accelerator (Energy Sciences Inc., CB 175/15/180) was used. The irradiation was carried out at 175 KV accelerated voltage, and under a nitrogen atmosphere (oxygen concentration level was 50-100 ppm). The irradiation dose rate range was 3.4 to 13.5 Mrad/sec. Actual irradiation dosage to thefilmwas measured by absorbance change of the cellulose triacetate film (Fujifilm,FTR-125) with the CTAfilmdose reader (Narumi Co. FDR-01). According to the dose-depth profile at 175KV electron energy level of this system (35), the irradiation dosage was expected to be almost constant (>95%) throughout the sample thickness (
