Chapter 2

Effects of Excimer Laser Treatment on Self-Adhesion Strength of Some Commodity (PS, PP) and Engineering (ABS) Plastics

Erol Sancaktar*, Hui Lu and Nongnard Sunthonpagasit

Department of Polymer Engineering, University of Akron, Akron, Ohio, USA

*Corresponding author: erol@uakron.edu

Abstract

This Chapter presents the effects of KrF excimer laser irradiation on the self-adhesion (weld) strength of commodity (polypropylene (PP), polystyrene (PS)), and engineering (acrylonitrile butadiene styrene (ABS)) thermoplastics. after laser irradiation, the polymer samples were welded using the ultrasonic welding method. The tensile stress-strain behaviors of the welded samples were obtained subsequently. It was found that laser irradiation increased the weld strength of ultrasonically welded samples. Increases in the weld strength values, in comparison to the untreated weld samples, were as high as 810% for PS. For ABS, the maximum increase in weld strength for increases in pulse frequency was 350%. The maximum increases in weld strength corresponding to increases in pulse energy and pulse number were 400% and 460%, respectively. For PS, the maximum increase in weld strength for increases in pulse frequency was 690%. The maximum increases in weld strength corresponding to increases in pulse energy and the pulse number were 690% and 810%, respectively. For PP, the increase in weld strength was as high as 191% due to laser treatment.

Keywords: Excimer laser surface treatment, ultrasonic welding, weld strength, polypropylene, polystyrene, acrylonitrile butadiene styrene

2.1 Introduction

Due to their high speed and precision, excimer lasers find applications in semiconductor processing, optical communications and medical fields, in which polymers have extensive applications. Furthermore, it has been shown that excimer lasers can be used to enhance adhesion strength [1], as well as in evaluating effects of process conditions such as injection molding [2] and nanoclay exfoliation [3]. In addition to desirable electrical and optical properties, polymers also have high strength to weight ratio, good corrosion resistance, and low processing cost. As an important joining method for plastics, ultrasonic welding plays an important role in processing of thermoplastics because it is easily automated, and it is the most rapid way to weld thermoplastics with low costs. In this Chapter, we present results utilizing ultrasonic welding to assess any improvements we may have in self-adhesion behavior of PS, PP and ABS polymers as induced by excimer laser treatment of their surfaces to be welded.

Successful welding often requires a suitable surface treatment of material prior to bonding. Selection and application of appropriate surface treatments are major factors in good weldability and durability. There is a wide range of surface treatments available for removing contaminants and weak boundary layers from polymer surfaces. These methods include mechanical methods, such as abrasion, grit and shot blasting, as well as chemical methods such as solvent degreasing, acid etching, and compatibilization by adhesion promoters in the form of primers. The disadvantages of these methods include environmental hazards due to undesirable emission, and poor controllability of surface finish.

A relatively recent technique for altering the surface properties of polymers is the use of excimer lasers [4]. Therefore, the relationships between weld strength and the parameters of excimer laser used to treat the welding surfaces are especially important.

In this Chapter, the influences of pulsed UV laser parameters on the self-adhesion (weld) properties (weld strength and break strain) of commodity polymers polystyrene (PS) and polypropylene (PP), as well as an engineering polymer, ABS (acrylonitrile butadiene styrene), are described. The laser parameters studied include pulse number, frequency, and energy.

2.2 Background and Literature Survey

2.2.1 Excimer Laser Surface Treatment

2.2.1.1 Overview of Excimer Laser Processing

The term laser is an acronym for “Light Amplification by the Stimulated Emission of Radiation”. Helium or neon buffer gas containing a halogen atom and rare gas binary complex produces the lasing plasma under high voltage in an excimer laser [5]. Several complexes which produce characteristic emission wavelengths [6] are commonly used. Recombination of electrons and ionized rare-gas ions in the plasma yields electronically excited rare gas atoms. These atoms react with halogen atoms and produce excited molecules, which relax to their ground states by emitting a ultraviolet (UV) photon.

The pulse durations for excimer lasers are in the nano-to pico-second range, thus providing the capacity to deliver high peak power output at several UV wavelengths. This makes them attractive for a wide range of applications such as micro-machining, surface modification including surface treatment for adhesion [1], corneal sculpting and marking, and process evaluation [2, 3, 7].

By subjecting the polymer surface to UV-laser light in atmospheric environment, some part of its chemical structure in its hydrocarbon group (CHx) chain can be altered to form intensely polar groups, such as carbonyl (-C=O) and hydroxyl- (-OH). The presence of these polar groups on the surface can enhance adhesion. High energy flux can also cause instant fragmentation of polymeric chains without any oxidation [8–11]. The increased surface roughness produced in this manner (ablation) can serve to enhance adhesion [8, 12–15]. Enhancement of polymer surface conductivity has also been reported as a result of laser irradiation [8, 16–18].

Thermal-oxidation and photo-oxidation generally cause main chain scission and crosslinking. The presence of oxygen typically propagates thermal oxidation [19]. Both thermal and/or photo-oxidation can also be initiated in the presence of free radicals (R•) formed by the thermolysis and/or photolysis of impurities, additives or photoinitiators. Photon absorption can initiate stepwise degradation of macromolecules. Typically, photo-oxidation and thermal-oxidation involve reactions between the polymer, polymer alkyl radical (P•), polymer oxy radical (polymer alkoxy radical), polymer peroxy radical (polymer alkylperoxy radical), polymer hydroperoxide (POOH), and the hydroxy radical. Hydroxy (OH) and hydroperoxy (OOH) groups are formed in reactions between polymer oxy radicals (PO•) and polymer peroxy radicals (POO•) with the same and/or neighboring polymer molecule (PH), respectively. Both groups can be formed along the polymer chain, or its ends.

Surface modification by laser irradiation can be carried out in variety of ways depending on the purpose of the surface modification (etching, ablation, deposition, evaporation, surface functionalization, etc.), the type of the laser used, the ambient conditions, and the materials to be treated. The desired level of surface modification can be achieved by the choice of an appropriate type of laser, by considering the optical and thermal properties of the material to be treated, matched with the wavelength, pulse energy, and pulse frequency of the laser to be used.

2.2.1.2 Mechanism of Thermal-oxidation by Laser Irradiation

The theory originally developed by Bolland and Hughes [20] to explain the thermal oxidation of olefins and rubber can also be applied to explain thermal oxidation of other polymers.

For ABS and PS, hydroperoxide and acetophenone groups have been identified on the polymer chain as a result of thermal oxidation. The volatile products are phenol, benzaldehyde and acetophenone. The major physical change due to thermal oxidation of PS is chain scission. Sequences of neighboring hydroperoxide groups are formed through intermolecular hydrogen abstraction. The main chain scissions observed on thermal oxidation of PS are generally attributed to the decomposition of tertiary alkoxy radicals [21]. The chemical structure of excimer laser irradiated PP shows that carbonyl (-C=O) or hydroxyl (-OH) groups are formed. It is known that the presence of these strongly polar groups on a polymer surface can improve the adhesion properties [13].

2.2.1.3 Mechanism of Photo-oxidation by Laser Irradiation

Incident light will either be reflected from the surface or scattered or absorbed in the bulk of the polymer. The absorption of light by polymers is related to their structure. For example, saturated hydrocarbons do not absorb above 250 nm, but if double bonds (chromophores) are present, longer wavelength laser light can be absorbed [22].

Laser light above 290 nm wavelength may also degrade polymers such as polyolefins, which do not contain chromophores in their repeat units. This is due to structural irregularities of polymers or is caused by traces of impurities left over from manufacturing e.g. catalyst residues or oxidation products. The first may absorb in the UV range, leading to photochemical transformation. Moreover, in semicrystalline polymers, scattering of light by the crystallites likely increases its path in comparison to amorphous materials, causing semicrystalline polymers to absorb rather high quantities of energy at low concentrations of chromophoric groups.

Subsequent to photo-absorption, the chromophores are raised to excited states at higher energy levels. This excitation energy is dissipated by several processes such as fluorescence, phosphorescence, and radiation decay. Energy can also transfer from the excited state to a suitable acceptor molecule.

In the presence of oxygen, polymers which simultaneously...