Adhesion modification of polyethylenes for metallization using radiation-induced grafting of vinyl monomers

The present paper demonstrates the performance of the radiation grafting technique as a chemically based surface pretreatment method to adhesion modification of polyethylene surfaces toward the metallic layer. Gamma irradiation over a dose range of 4-10 kGy is used for grafting of vinyl monomers including acrylamide (AAm) and 1-vinyl-2-pyrrolidone (NVP) onto the surface of three different polyethylenes including low density polyethylene (LDPE) and two kinds of high density polyethylenes, namely HDPE-I and HDPE-2. The grafting yield is evaluated based on the weight increase of the samples. It is found that the grafting yield is dominated by the crystallinity of the polyethylene, so that lower crystalline percent of polyethylene leads to higher grafting yield. Additionally, lower irradiation dose rate causes higher grafting efficiency. The polyethylene samples are also hydroxylated by exposing to gamma rays in water at dose level of 100 kGy to make the surfaces hydrophilic. Moreover, for comparison purposes, the surface of the polyethylenes is subjected to the conventional surface treatment methods including chemical etching and mechanical abrasion. Surface characterization is performed by contact angle measurements and scanning electron microscope (SEM) before and after the surface modification. A copper layer with nominal thickness ranging 3-10 mu m is deposited on the surface of treated and untreated substrates using both the electroless plating and magnetron sputtering processes. The adhesion of the metallic layer is investigated by the cross-cut and the pull-off tests. It is shown that radiation grafting of AAm monomer on polyethylene surfaces always leads to an excellent adhesion toward copper layer irrespective of metallization process, probably due to strong chemical complexes between AAm monomers and copper atom. However NVP-grafted surfaces show poor adhesion which may be due to hindrance effect of the nitrogen functionality in NVP. The mechanical roughening shows a slight positive effect on the adhesion, but the maximum bonding strength achieved in this case is 1 MPa in the sputtering process. In addition, maximum bonding strength belongs to LDPE treated by radiation grafting of AAm, so that no failure is observed in the pull-off test. The results also indicate that the bonding strength of the radiation grafting depends on the content of grafted AAm, while it seems that the adhesion strength in chemical etching method is dominated by both the chemical and physical interactions. Eventually, it is shown that the radiation grafting is an efficient surface treatment method for metallizing the polyethylene surfaces. (C) 2007 Elsevier B.V. All rights reserved.