Lamination of High-Density Polyethylene by Bulk Photografting and the Mechanism of Adhesion Huiliang Wang, 1,2 Hugh R. Brown 1 1 Steel Institute, University of Wollongong, NSW 2522, Australia 2 Department of Chemistry, Beijing Normal University, Beijing 100875, China Received 8 April 2004; accepted 18 October 2004 DOI 10.1002/app.21724 Published online in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: The photolamination of high-density poly- ethylene (HDPE) by bulk photografting is described, along with a discussion of the adhesion mechanism. HDPE can be photolaminated very easily with a thin poly(acrylic acid) layer, photopolymerized from acrylic acid, with very strong adhesion obtained after a short time of UV irradiation; the adhesion failure mode is polyethylene breakage. Thicker HDPE sheets require longer irradiation times for strong adhesion. Methacrylic acid or hydroxyethyl methacrylate provides no adhesion of HDPE at all after irradiation. When glycidyl acrylate is used alone between HDPE sheets, the peel strength of the photolaminated polyethylene is only approximately 320 N/m, but when glycidyl acrylate or hy- droxyethyl methacrylate is grafted with acrylic acid, very good adhesion can be obtained. It is proposed that stronger adhesion is produced by a less branched grafted chain struc- ture, which permits much more chain entanglement. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1097–1106, 2005 Key words: adhesion; polyethylene (PE); graft copolymers; hydrophilic polymers INTRODUCTION Yang and Rånby 1–9 developed a method for laminat- ing polymer films with a bulk (no solvent) surface photografting process, in which a thin layer of an acrylic monomer containing a dissolved photoinitiator is sandwiched between two thin films and then pho- topolymerized. Most importantly, photolamination occurs simultaneously during the photografting pro- cess and results in good adhesion of the two films. Kang and coworkers 10,11 reported the photolamina- tion of ozone-pretreated low-density polyethylene (LDPE) films via a novel technique of UV-induced graft copolymerization with acrylamide or acrylic acid (AA) under atmospheric conditions and in the com- plete absence of an added initiator or oxygen scaven- ger. This photolamination technique can be applied to a wide variety of plastic films and produces laminates of high mechanical strength and high and selective bar- rier properties toward different gases and vapors. However, this method has an unavoidable drawback, in that it can only be used for thin and UV-transparent films. Most of the work on photolamination by bulk pho- tografting has been done with LDPE films, for which enough adhesion can be obtained that sample strips are broken in a peel test rather than the interface failing. In this situation, the reported apparent peel strength at break for a laminate in which both films were 0.188-mm-thick films of LDPE was 1050 N/m. 8 High-density polyethylene (HDPE) has a surface that is more difficult to graft than that of LDPE because of the linear chain structure of HDPE and its higher degree of crystallinity. The reported apparent peel strength at break for a laminate of two HDPE films 0.04 mm thick was only 290 N/m 3 . In the work de- scribed here, our efforts were focused on the photo- lamination of HDPE, and thicker HDPE sheets were used. In previous work, the photolamination of polymeric films was normally performed with a single monomer. The monomers were usually water-soluble. The most effective monomer was found to be AA, and so it has been most studied. It is likely that the adhesion of polymeric films photolaminated with a water-soluble monomer will become weaker when the films are soaked in water for some time or even when they are held in a humid atmosphere for a long time. There- fore, we studied the use of a mixture of water-soluble and water-insoluble monomers to form less water- soluble grafted copolymers and hence improve the water resistance of photolaminated films. Yang and Rånby 3 suggested that the lamination method involves the formation of hyperbranched graft macromolecules and a crosslinked macromolec- ular network obtained by the addition of multifunc- Correspondence to: H. R. Brown (hugh_brown@uow. edu.au). Journal of Applied Polymer Science, Vol. 97, 1097–1106 (2005) © 2005 Wiley Periodicals, Inc.