Plasma Treatment of Crosslinked Polyethylene Tubes for Improved Adhesion of Water-based Paints

Materials Research. 2019; 22(1): e20171046 DOI: http://dx.doi.org/10.1590/1980-5373-MR-2017-1046 Plasma Treatment of Crosslinked Polyethylene Tubes for Improved Adhesion of Waterbased Paints Gabriel Ferreira Fernandesa, Milena Kowalczuk Manosso Amorima, Rafael Gustavo Turria, Paulo Silas Oliveiraa,c, Ismael Leandro Graffb, Elidiane Cipriano Rangela , Steven Frederick Durranta* a Laboratório de Plasmas Tecnológicos, Instituto de Ciência e Tecnologia de Sorocaba, Universidade Estadual Paulista - UNESP, Avenida Três de Marco, 511, Alto da Boa Vista, 18087-180, Sorocaba, SP, Brasil b Departamento de Física, Universidade Federal de Paraná, Curitiba, PR, Brasil c Instituto Federal de Educação, Ciência e Tecnologia de São Paulo, Campus Registro, SP, Brasil Received: November 28, 2017; Revised: August 21, 2018; Accepted: October 25, 2018 Cold plasmas fed trichloromethane-argon mixtures were used to treat cross-linked polyethylene (PE) to improve the adhesion of water-based paint. The effects of the plasma treatment undertaken at different percentages of CHCl3 in the plasma feed, CCl, were investigated using Infrared Spectroscopy in Diffuse Reflection (DR) mode, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS), goniometry for surface contact angle measurements, profilometry for roughness measurements, and paint adhesion tests. All the treatments modify the surfaces by introducing chlorine. Oxygen is present in the bulk and on the surfaces of the treated and untreated material. The treatments do not alter the bulk, but tend to increase the surface roughness and contact angle. When CHCl3 is present in the plasma subsequent paint adhesion is improved from a very poor classification of 4 to an excellent classification of 0 (assessed according to the appropriate Brazilian standards (BS EN ISO 2409:2007 and BS 3900-E6:2007A)). Surface roughness is not increased at low CCl but improved paint adhesion occurs for all CCl > 0. Although the causes of the improved paint adhesion with the plasma treatment are unclear, they may be related to the plasma activation of unsaturated carbon and oxygen functionalities. Keywords: paint, roughness, cold plasma, trichloromethane, adhesion. 1. Introduction Plasma treatment is well established for the surface modification of diverse materials 1. For example, air cold plasmas have been used to improve the adhesion of acrylic paint to 2024 aluminum alloy 2, which is used in aircraft manufacture and therefore has to resist harsh conditions, such as the presence of moisture, organic materials, and wide temperature fluctuations. Such treatments allow the elimination of an otherwise essential surface-cleaning step and a reduction in the amount of primer needed, potentially reducing the overall painting time and the paint weight, the latter being an important consideration for aircraft. Plasma-deposited films have also been used as promoters for the adhesion of alkyd paint to titanium alloy (Ti-6Al-4V) substrates 3. Several silanes were used as monomers, improving the adhesion of the paint under monomer-deficient conditions. Improved adhesion in the cases mentioned here are attributable to increases in surface roughness and decreases in surface contact angle induced by the treatments, which lead to increased covalent, van der Waal’s or hydrogen bonding 4. *e-mail: Wolkenhauer et al 5 examined the adhesion of paints (and adhesives) on wood-plastic composites following atmospheric dielectric barrier discharge plasma treatment in air. Contact angle measurements, atomic force microscopy and tensile bond strength testing were applied. These revealed that the polar component of the surface energy and the roughness increased under treatment. The treatments also improved adhesion of water-borne, solvent-borne and oil-based paints. The efficacy of improving urethane paint adhesion to a polypropylene car bumper using oxygen, water, and acetylene plasmas instead of a primer has been reported 6. Each treatment increased the adhesion strengths, as assessed by lap-shear tensile strength values, in dry conditions but the treatment in acetylene was more robust when subsequently exposed to wet conditions. The improved adhesion in the presence of acetylene is attributed to the existence of reactive unsaturated hydrocarbons and oxidized hydrocarbons in the deposited film. Irradiation of poly(ethylene glycol-co-1,3/1,4 cyclohexanedimethanol terephthalate (PETG) using H2 and CF4 plasmas typically increased surface roughness and decreased surface contact angles as measured by profilometry and goniometry, respectively 4. Paint adhesion was strongest 2 Fernandes et al. when wettability and work of adhesion were increased. (Work of adhesion is defined as the reversible thermodynamic work that is needed to separate the interface from the equilibrium state of two phases to a separation distance of infinity). As summarized elsewhere 7, chlorine-containing plasmas have been employed to modify the surfaces of polymers. For example, when polypropylene is treated in CCl4 plasmas or CHCl3 plasmas, chlorine and often also oxygen is introduced into the treated surface, changing the surface roughness and surface contact angle. Such treatments are simple, one-step, rapid, dry, almost pollution free, and may readily be automated and scaled-up as required. In the present study, the use of cold CHCl3 - Ar plasmas to improve the adhesion of water-based paint to crosslinked polyethylene material used in fuel gas installations is investigated. The motivation for this is to be able to use these tubes for external use. Obtaining good paint adhesion is difficult but necessary for such outside use, which is a common requirement in Brazil. Infrared Spectroscopy in Diffuse Reflectance (DR) mode, Energy Dispersive X-ray Spectroscopy (EDS) and X-ray Photoelectron Spectroscopy (XPS), were used to investigate the chemical structure and composition of the treated polymer. Scanning Electron Microscopy (SEM) was used to confirm surface morphologies. Goniometry, profilometry, and a Brazilian standard paintadhesion test were employed to reveal surface contact angles, surface roughness, and quality of adhesion, respectively. 2. Experimental 2.1 Plasma treatment Plates of rough dimensions 12 mm x 7 mm x 1mm were produced by cutting, opening out, and flattening pieces of high density cross-linked polyethylene tube, PE-Xc (HENCO, Doorn, Netherlands). The substrates were cleaned by immersion in an ultrasonic bath containing 50% water and 50% detergent (DET LIMP S32) for 8 minutes, then rinsed in running water, immersed in an ultrasonic bath of distilled deionized water for 8 min., rinsed again in running water, and finally treated in an ultrasonic bath of isopropyl alcohol for 8 min. and dried in a hot air current. Treatments were undertaken in a Radiofrequency PECVD system consisting of a cylindrical stainless-steel chamber equipped with i (...truncated)