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.
Polymeric-based resins used as adhesives, paints or other coatings, inks, encapsulants or as matrix in polymer composites have to experience changes known as curing or drying to convert from the initial reactive materials to finished products. As Mehdi Tavakoli reports in the second episode of this two part feature, the curing often involves chemical processes known as polymerisation and crosslinking and sometimes only includes evaporation of a carrier ( eg solvents, water) or a drying process.
For Part I see Bulletin November/December 2000
The development of high quality coating systems requires the use of special energy-related chemical reaction mechanisms to enable the formation of complex network structures. These network formation reactions take place between high molecular weight polymer chains containing reactive functional groups or between polymer chains and crosslinking oligomers. In understanding how the various energy sources and curing methods [8,13] work, it should be noted that energies of the order of a few electron volts are often sufficient to break a chemical bond or raise a molecule to an electronically excited state. Therefore, in general, UV, EB, IR (infrared), microwave, and radio frequency radiation sources can generate reactive chemical species similar to those produced by heating an adhesive or a coating in a conventional oven.
Compared to conventional ovens used to drive off solvents or water and to apply heat to cure resins, radiation energy interacts directly with specially formulated materials which are usually applied as 100% solid systems, containing no solvents. This direct action initiates a chemical reaction in a fraction of a second. The space required for a radiation curing source is significantly smaller than for a convection oven, particularly when used in high speed coating and laminating applications. With radiation curing no emission control system or solvent recovery is generally required.
A description of various curing methods available for adhesives and coatings is given below.
UV curing
The UV processing of coatings began with the use of low-pressure mercury lamps for curing styrenated polyesters. These lamps soon gave way to medium and high pressure mercury-arc lamps. The medium pressure lamps were the most widely used. The lamps are normally positioned directly above the feed line and may vary in size from a few centimetres to about two metres in length. The power output may also vary but a typical value is 8W/mm. Mercury vapour lamps, doped with materials such as volatile metal halides, are becoming more important for curing pigmented coatings due to increased emission towards the visible part of the spectrum. Electrodeless lamps with a power output of up to 24W/mm are an alternative, possessing the added advantages of reduced warm up times and better long term performance.
Temperature control around a mercury lamp is necessary. An obvious method is to cool with air. Because oxygen can inhibit free radical polymerisations, air curing is not appropriate to many end uses. However, much progress has been made in this area and many of the current UV lamps can operate in an air atmosphere. The coating can also contain additives or chemical agents which retard or inhibit oxygen absorption.
Visible light curing
An alternative radiation curing technique is the use of visible light for curing of coatings, inks, adhesives and polymeric encapsulants. Some materials have been developed [14] which can be cured with blue light (wavelength = 470nm). These are used as dental restoratives or for bonding fibre optic components or other microelectronic applications. These materials are based upon an oligomeric aromatic methacrylate combined with a photoinitiator which can generate free radicals when exposed to the light. The free radicals produced can initiate polymerisation reactions of the resin which proceed until the viscosity of the system prevents further reaction. The health hazards of using visible light are less than the UV system. However, visible light curing resins are more expensive and oxygen inhibition may be a problem when using porous substrates.
Infrared curing
IR sources all supply a mixture of wavelengths whose general range depends on filament temperatures. Long wave IR (4-1000µm) is readily absorbed by air and difficult to focus. It may also lack the ability to penetrate into the coating and only surface curing may be achieved. Even this may require longer times than medium wave IR (2-4µm). Medium wave IR is important in the curing of coatings
[8,17] of inks because many of the organic materials used in formulations absorb in this range, although for thin films the radiation may penetrate into the substrate. Short wave IR (0.7-2µm) is only slightly scattered by air and can penetrate thick films and substrates. Matching the output spectrum of the lamp to the absorption frequencies of the ink or coating can sometimes increase the efficiency of curing.
The mechanism of IR curing depends on the type of coatings or adhesives used. Evaporation and lowering of viscosity or breaking of a gel to facilitate penetration into the substrate are both possible, as is acceleration of crosslinking.
Since use of IR depends upon absorption, it becomes essentially a line of sight process. Of course, a highly conductive substrate ( eg metal) will distribute the thermal energy out of the line of sight. Conversely, an insulating substrate ( eg wood) will do little to distribute the heat, and hot spots can develop if exposure is non-uniform.
Laser curing
Lasers produce a monochromatic beam of extremely coherent light which can be focused onto a very small area or cover a large area in a short period of time. The temperature coherence of the laser emission which occurs at a well-defined wave-length reduces the extent of undesirable, secondary reactions induced by polychromatic radiation. At the same time, it enables precise control of the penetration profile and may increase curing depths. Several laser types are available which may be suitable for rapid curing of coatings. 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 excimer lasers which can emit energy at some of the following wavelengths:
ArF - 193nm
KrF - 249nm
XeCI - 308nm
XeF - 350nm
In addition, other lasers such as argon ion (363nm) or nitrogen (337nm) can be used.
Laser-induced photopolymerisation of acrylate resin has been reported [18] in recent years. The curing of the resin takes place quasi-instantly by UV laser irradiation. The main advantage of any radiation is to initiate the polymerisation of acrylic monomers. It normally takes a fraction of a second to achieve extensive through-cure of the photosensitive resin. Higher cure speeds, which are sometimes required, can be obtained by replacing the conventional mercury lamp by powerful lasers which currently appear to be the ultimate light sources for achieving instant polymerisation. The capability of UV lasers to induce some ultra-fast curing reactions in acrylic coatings was first demonstrated by Decker [19-22] for both pulsed and continuous irradiation.
One of the most important advantages of laser emission is the large power output concentrated in a narrow beam. It allows curing to be attained extremely quickly, once the initial induction period (due to oxygen inhibition) is over. The laser scanning speed required may vary between 6-50 cm/sec, depending on formulation.
Infrared lasers
The two main types of industrial laser which operate in the IR region are CO
2 (10.6µm wavelength) and Nd:YAG lasers (1.06µm wavelength). A Nd:YAG laser has the advantage of being able to be delivered through a fibre optic beam delivery system.
Both laser types could be used to initiate polymerisation thermally but, to date, little work has been performed to study material systems which may be suitable to IR initiation. If shown to be feasible, high processing speeds due to the fast interaction times are likely.
Although some of the most promising uses of laser curing have been for electronic applications ( eg for direct writing of complex relief patterns) and in the coating industry for the instant curing of optical fibre coatings, many new areas have yet to be investigated. These include the use of lasers for curing of coatings, encapsulants and also curing of adhesives for many different applications. The CO 2 laser has successfully been used at TWI to cure a structural epoxy adhesive using coated steel as adherend. The curing of the adhesive was reduced from about 45 minutes to a few seconds
Electron beam curing
To achieve chemical reactions, the energy of fast electrons must be transferred to the molecules of the material irradiated. The main difference between EB
[4,5,8] and UV curing is that the former does not require any photoinitiators. The electrons, produced as a beam or more recently a curtain, can induce polymerisation reactions to a greater depth than is possible with UV curing. In a self-shielded EB process, electron energies up to 400kV are used as high energy radiation sources to initiate polymerisation.
[14,15] When oligomers and/or monomers are subjected to energetic electrons from a low energy EB machine, both radical and ionic chemical species are formed. Electron beams could be used to cure thicker and more heavily filled coatings or adhesives using less energy. However, one of the main difficulties encountered in the acceptance of EB curing by the adhesives and coating industry is the relatively high initial cost of the equipment compared with UV systems.
Use of EB curing for wood and other building materials has been reported. [16] Varnishing of doors, multilayer pigmented coatings on chipboard panels or lamination and varnishing of decor paper on panels can be achieved with short curing zones and at speeds only limited by the handling equipment (up to 50 m/min). One of the recent applications is for weatherable coatings on wood composite panels.
Glow discharge polymerisation
Glow discharge polymerisation (GDP) [23] usually refers to the formation of polymers by means of an electric discharge. Plasmas are very similar to other ionising radiation, such as y-radiation, UV radiation and high-energy electron beams. Plasmas can initiate polymerisation of certain monomers and also generate free radicals leading to crosslinking of the polymer. However, plasma radiation effects are usually limited to the surface and the depth of the layer affected is much smaller (1-10µm) than by other penetrating radiation.
Microwave/radio-frequency curing
Microwave and radio-frequency curing can be used to cure or dry adhesives or coatings by thermal activation
[13] . The main mechanism of activation involves rotation of polar molecules to align their dipoles in an electric field. The rate at which electrical energy can be dispatched in a dielectric material is proportional to the frequency of the energy and to the square of the electric field strength.
[24] However, few developments in this area have been published. Variable frequency microwave (VFM) processing has been developed as an alternative to convection curing. This method may be used for rapid curing of adhesives, encapsulants, underfills and glob tops. One of the main advantages of VFM curing is that it can eliminate arcing problems experienced in a microwave oven when a metal or a semiconducting material is exposed to it. In this technique the frequency of the electric fields is electronically swept which prevents microwave energy from remaining focused at any fixed location for more than a fraction of a second.
Radio-frequency dielectric heating could be used to cure thermoset based adhesives or coatings, reducing the curing times from minutes to seconds.
Induction heating
Many adhesives contain metallic fillers or are used for bonding metallic adherends. Induction curing can be achieved by causing resistive heating in metal substrates or metallic filler incorporated into the adhesive systems. The temperature of metallic particles or adherend can be increased by the transfer of electrical energy from a high frequency conductor. The curing reaction is initiated as a result of high frequency energy activating the heat curing catalyst ( eg dicyandiamide)
Dual cure systems
UV-curing systems have proven their potential for many applications in the printing, adhesives and coating industries. A limitation, however, is shrinkage which may occur during polymerisation
[24] leading to a lack of adhesion on certain non-porous substrates. The high crosslinking density of these systems often tends to form a hard but rather brittle coating which is very difficult to bend. Dual cure systems
[25,26] can solve a number of these problems by combining a fast UV cure step with a slower thermal, moisture or air curing step. The dual cure character of the coatings can be achieved by incorporating both types of functionalities (
eg radiation-curable and thermally-curable) on one chemical structure or by mixing two products with different types of functionalities together. Dual cure systems can provide adhesives and coatings with improved properties and superior resistance to environmental attack. These systems can consist of acrylated oligomers and traditional two component polyurethanes. Furthermore, thermally activated cure or bake cycles can be used to crosslink some materials not reactive to radiation energy. This secondary curing or post curing process can improve the impact resistance of some cationic cured coatings used on metals.
There are a number of commercially available dual cure adhesive systems or coatings based on combinations of UV or visible light, anaerobic, moisture, catalyst and particularly heat.
Concluding remarks
A wide range of rapid curing polymeric materials as well as rapid curing techniques has been developed in recent years. Selection of rapid and reactive resins with suitable curing agents and design of new formulations for rapid curing have been the subject of numerous papers and patents. Environmental concern, which is discouraging the industrial use of solvents is one of the main factors driving adoption of radiation curing. Organisations which use coatings, adhesives and inks are particularly susceptible to the environmental lobby against the use of solvents to cure polymer based resins. Apart from avoiding solvents, the other main advantages of using radiation curing are saving on floor space in the plant, lower energy requirements and increased production speeds. In terms of coating materials, acrylate resins are the main leaders for use in radiation or rapid curing applications with epoxy acrylates dominating the market. The dominant position of acrylic systems is due to their high reactivity, fast cure and versatility which enables the creation of a wide range of adhesives, coatings etc, with a broad spectrum of properties. Polyester systems (
eg for wood coatings) are cheaper than acrylated resins, since they are based on commodity unsaturated polyester resins and styrene but have the disadvantage of having a slow rate of cure. Coating systems based on aliphatic epoxy resins cured by a cationic mechanism have been introduced to the market in recent years. These resins can provide superior properties but are more expensive.
Radiation curing techniques based on new technologies ( eg lasers, electron beams, induction heating) are being accepted or considered for many new industrial applications. Demands to use solvent free resins and to reduce curing times and manufacturing costs have been increased in recent years. This in turn will lead to selection of rapid or radiation curing techniques for new applications ( eg in optoelectronics). New rapid curing materials and methods have also been developed for critical and non-industrial application such as in dentistry (visible light curing filling) and in medicine and surgery ( eg tissue bonding and bone cements).
References
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