Polymers get the laser treatment - surface modification for better adhesion

TWI Bulletin, September/October 2000

Mehdi Tavakoli gained a BSc in Chemistry and then his MSc and PhD in Polymer Science and Technology at Aston University. Since joining TWI in 1989, Dr Tavakoli has been working on new joining techniques with particular interest in surface modification of materials to enhance adhesion and durability. He has over 20 years research, industrial problem solving and product development experience on polymeric materials and has published more than 40 papers and patents. He is currently a technology manager at TWI.

The use of lasers in processing polymers has increased significantly in recent years. Infrared and excimer lasers have been used for welding , ^[1] cutting ^[2] and surface modification of polymers ^[3-6] as well as curing of polymer-based resins. ^[7,8] As Mehdi Tavakoli reports the ability of lasers to generate the exact amount of energy can be used to induce specific chemical reactions in polymers.

The characteristics of laser beams offer new opportunities to achieve photochemical or thermally induced reactions, leading to new applications and products. This publication reviews the use of lasers in the processing of polymers. The main aim of this article is to describe TWI's work on laser surface modification of polymers to enhance adhesion. The potential uses of lasers and electron beams for processing of polymers, first proposed in the early 1990s, are shown in Fig.1.

Types of laser

There are many types of lasers which are used in the surface modification of polymers and curing of polymer based resins. They fall into two main categories - UV and IR.

Ultraviolet lasers

Ultraviolet lasers can emit energy at a number of specified wavelengths depending on the mixture of chemicals used in the laser source. The most common UV lasers are called excimer lasers. Excimer is short for excited dimer, and the laser is commonly generated from the rare gas halide molecules such as: argon fluoride (ArF, wavelength = 193nm), krypton fluoride (KrF, wavelength = 248nm), xenon fluoride (XeF, wavelength = 351/353nm) and xenon chloride (XeCl, wavelength = 308nm). All practical excimer laser devices are pulsed and produce intense bursts of UV laser light, each lasting only 10-30nsec, at repetition rates as high as several thousand times a second. Excimer laser pulses contain energies and peak powers of up to 2J and 50MW respectively, in the deep ultraviolet wavelength range (157-353nm).

Each excimer laser photon carries energy in the range of 3.5-8 electron volts (eV), which is sufficient on its own to break apart many of the molecular based materials found in nature, as well as those artificially synthesised. This is significant, because to remove material with the light from longer wavelength lasers such as Nd:Yag ( λ= 1.06µm, h ν~1.2eV) and CO ₂ ( λ = 10.6µm, h ν~0.12eV) requires greater photon absorption before constituent surfaces can be broken down. This causes high temperatures to be generated, which can lead to undesirable effects, such as thermal-oxidation or photo-oxidation, combustion, charring, melting, flow or boiling of surrounding unirradiated material.

Infrared lasers

The two main types of industrial infrared lasers used for surface modification, which operate in the IR region, are CO ₂ and Nd:YAG lasers.

CO ₂ lasers

The CO ₂ laser is a versatile gas laser, able to operate in either pulsed or continuous mode and produces the highest continuous power output of any laser. Unlike argon, He-Ne, and metal-vapour lasers, the CO ₂ laser operates on a set of vibrational-rotational transitions. This puts its output at much longer wavelengths (9-11µm), in the infrared region. There are several important types of CO ₂ lasers, but all work on the same transitions. Carbon dioxide lasers are usually used only for surface pretreatments of metals ( eg hardening of steel) rather than polymers.

Nd:YAG laser

The Nd:YAG laser is one of the most important classes of solid-state lasers in which the light-emitting species is the rare earth neodymium, in a glass or crystalline matrix. An Nd:YAG has the advantage of being able to be delivered through a fibre optic beam delivery system. The optical and thermal properties of the Nd:YAG laser allows it to be pulsed either continuously with an arc lamp or by a series of flash lamp pulses. Maximum average power from a Nd:YAG laser can reach hundreds of watts, although most operate at much lower powers. The peak power can reach tens or hundreds of kilowatts in a millisecond long pulse generated by a flash lamp, or over 100MW in a Q-switched pulse of 10-20nsec. The pulse power density available from a Q-switched Nd:YAG laser makes it suitable for rapid surface modification of materials.

Diode laser

High power diode lasers (>100W) have been available since early 1997. They are now available up to 4kW and are competitively priced compared to CO ₂ and Nd:YAG lasers. The production of the diode laser light is a far more energy efficient process (30%) than CO ₂ (10%), Nd:YAG (3%) or excimer (<1%) lasers. The interaction with plastics is very similar to that of the Nd:YAG lasers, and applications overlap. The beam from a diode laser is generally rectangular in shape, which, while being preferential for some applications, limits the minimum spot size and maximum power density available. Diode lasers are being considered for surface treatment of metals and polymers as well as curing of polymeric resins.

Laser surface modification of polymers

The use of excimer lasers to modify polymer surfaces to improve adhesion has been demonstrated at TWI in recent years. ^[5] The photoenergy generated by excimer lasers can be used effectively to clean surfaces, to remove materials selectively or to interact with upper surface layers to alter surface chemistry and surface reactivity. The work carried out at TWI using lasers to modify surfaces of polymers to improve adhesion and biocompatibility is described in the following sections. ^[5]

Laser surface modification to enhance adhesion

The use of lasers to modify polymer surfaces to promote adhesion started at TWI in the early 1990s. Successful application of adhesives, paints or coatings normally requires a suitable surface treatment of the substrate prior to bonding or coating. Selection and application of an appropriate surface treatment is one of the major factors in achieving good wettability and improving long-term durability of bonded or coated components. On the other hand, inadequate or unsuitable surface treatment is one of the most common causes of premature degradation and failure. The functions of surface treatment include the removal of contaminants or weak boundary layers and alterations of surface chemistry, topography and morphology in order to enhance adhesion and durability. Surfaces of some polymers, particularly moulded components, are often contaminated with fluorinated or silicone containing release agents which are used during their manufacture. Filled polymers may also have a tendency to reject foreign bodies and additives with which they are incompatible and migration of these species to the surface of a polymer can interfere with the process of adhesion. Some polymers may also have low surface energy ( eg polyolefins) as well as surface releasing properties ( eg fluoropolymers) which can lead to lack of adhesion. Adhesion to these surfaces is also very difficult without a proper surface treatment.

Work carried out at TWI demonstrated that excimer lasers can be used effectively to modify polymer surface to enhance adhesion. ^[4,6] The effects of various types of excimer lasers ( ie XeCl, ArF and KrF) on twelve adherends ( eg polyolefins, polyimides, liquid crystalline polymers and polyetheretherketone (PEEK)) and seven adhesives ( ie cyanoacrylates, epoxies and polyurethanes etc) were investigated. An extensive range of laser processing conditions ( eg laser pulse frequencies) have been used in order to establish an optimum surface modification condition. Single lap shear joints have been prepared and tested to assess the effectiveness of laser treatment and extent of improvement in adhesion. Characterisation of a number of laser-modified surfaces was carried out using a surface texture measuring system, FTIR-ATR, XPS and SEM. The effects of storage after treatment and prior to bonding have been studied. Several adherend/adhesive combinations were subjected to durability testing and exposure to over 2000 hours in a hot/wet environment.

The following conclusions were reached as a result of this study:

• In general, pretreatment with all of the excimer lasers (general order of effectiveness ArF > KrF > XeCl) was effective in increasing adhesion to various substrates under optimised conditions.
• Characterisation of polyolefins, particularly high density polyethylene (HDPE), showed that at low laser pulse frequencies there was little or no physical effect on the surface. Analysis of surfaces of low density polyethylene(LDPE) revealed that surface contamination had been removed by laser treatment. There was also evidence of an increase in the concentration of oxygenated surface functional groups with increase in the pulse frequency of the lasertreatment.
• Most specimens could be stored for up to 15 days after laser treatment and retained their surface reactivity without loss of performance after bonding.
• The failure mode of many joints, prepared under optimised conditions were primarily within the parent material.
• Laser treated PEEK and APC-2 joints exposed to a hot/wet (50°C/96%RH) environment for several weeks showed excellent resistance to ageing - PEEK joints exposed to 1750 hours and APC-2 joints to 2060 hours under theseconditions showed no loss in joint strength.

The effects of laser on joint strengths of a range of polyolefins adherends are shown in Table 1.

Table 1: The effects of excimer laser treatment on joint strength and storage stability of a range of polyolefins bonded with a cyanoacrylate adhesive

The graphs showing the surface roughness characteristics of abraded and laser treated (XeCl laser at 10Hz and 25Hz) surfaces are shown in Fig.2a-2c. The abraded surface ( Fig.2a) showed a sharp but regularly distributed roughened surface. This could be attributed to pores giving troughs below the baseline and debris or loosely attached surface layers providing peaks above the baseline. The specimens treated at 10Hz ( Fig.2b) did not show any irregularities or physical changes on the surface. In contrast, HDPE treated at 25Hz ( Fig.2c) showed both peaks and troughs located along the bond line area. These were wider, covering larger areas compared to those found with the abraded surface.

a) Abraded
b) and c) XeCl laser treated;
b) HDPE treated at 10Hz;
c) HDPE treated at 25Hz

The effects of ageing on joint strengths of PEEK and APC-2 bonded with a structural epoxy film adhesive is shown in Table 2. The typical modes of failure of PEEK and APC-2 are shown in Fig.3 and 4.

Table 2: Effects of ageing at 50°C and 96%RH on joint strengths of PEEK and APC-2 bonded with epoxy adhesive

Laser surface modification to enhance biocompatibility

When a biomaterial contacts tissue, fluid or blood, the surface of the material that comes into contact with the physiological environment must be carefully controlled. Many implantable biomaterials are constructed using polymeric materials and the biocompatibility of these materials is critical to their satisfactory performance. The first physiological process that occurs within the initial stages of exposure is the adsorption of biomolecules onto the surface and this is usually followed by cellular interactions. Both the surface topography and the surface chemistry can significantly affect the type and intensity of these interactions. The possibilities of using lasers to modify polymer surfaces to enhance biocompatibility have been investigated jointly by TWI and the Department of Clinical Engineering at the Royal Liverpool University Hospital. ^[5]

The effects of changing surface topography on neutrophil chemokinesis and fibroblast adhesion was examined using two polymeric substrates; polycarbonate and polyetherimide, modified by laser treatment to produce pillars of varying dimensions on the surfaces of these materials. The range of dimensions for the pillars were 7, 25, 50µm square, and 0.5, 1.5 or 2.5µm deep. Human neutrophils were isolated by centrifugation on ficoll from heparinisal whole blood obtained from healthy volunteers. Isolated neutrophils were exposed to the surfaces for 20 minutes and tracked using image processing and analysis techniques. The mean speed for each cell on each surface was calculated and this data statistically analysed using multivariate analysis of variance to determine any significant effect on speed of movement due to surface topography.

SEM and confocal micrographs from some of the laser treated surfaces with fibroblast cultures are shown in Fig.5-9. Figure 5 shows that the 50µm grid with five pulses allows the cells to spread. The 7µm grid with three pulses ( Fig.6 and 7) cause the cells to spread and elongate. Figures 8 and 9 show typical confocal micrographs. The results showed that there were clearly surfaces that had more effect on cell movement than other surfaces, and were stimulating cells to move faster than on the same untextured surface. Surface topography can stimulate neutrophils to move at different speeds across a surface. It was calculated that further texturing and edge effects may lead to an increase in stimulation of the neutrophils by surface treatments. The results obtained from fibroblasts demonstrated that the textured polymer surfaces showed good cytocompatibility. Further work is planned to produce surfaces which provide better stimulation of neutrophils and higher cellular interactions.

Concluding remarks

TWI has been working on the development of power beam techniques for surface treatments of inorganic and organic adherends and curing of adhesives for many years. The unique characteristics of lasers to induce photochemically and thermally activated reactions on the surface or in the bulk of polymeric materials can effectively be used to provide specific properties leading to many new applications and products. Fundamental knowledge of polymer chemistry and polymer science and structure - property relationship will be required to ensure that chemical and physical changes within the polymers are achieved without inducing undesirable ( eg photo, thermal and thermal oxidative) degradations which may affect product performance and reliability. The emergence of a new generation of low power lasers with unique properties has considerably increased the possibility of their use in many new exciting applications including welding of plastics and in medicine and non-invasive microsurgery (cornea corrections, kidney stones removal etc). Work originated at TWI on laser surface modification of polymers to enhance biocompatibility for medical applications and will continue through a PTP (postgraduate training programme) scheme with the University of Cambridge. Further progress in this area is expected to provide significant contributions to the development of a new generation of medical and implantable devices.

References