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 first 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 II see Bulletin January/February 2001
The chemical reactions involved during polymerisation and crosslinking convert the original chemical species consisting of small molecules from liquid, paste or solid form to a large polymerised macromolecule or crosslinked structure of insoluble network. Considerable time is usually required when using conventional curing techniques. A variety of rapid setting materials and rapid curing techniques [1-3] have been developed in recent years to avoid long curing times and slow manufacturing processes. As the requirement for rapid curing materials continues the potential of faster curing techniques is being investigated. In some emerging technologies such as optoelectronic devices there is an urgent need to use materials with curing speeds of a few seconds rather than minutes or hours. The aim of this article is to review materials and techniques which can be used to achieve rapid curing of polymeric resins.
Radiation-curable materials
Radiation-curable coatings, adhesives or inks consist essentially of low or medium molecular weight resins (oligomers), monofunctional or multifunctional monomers, additives, pigments and, in the case of UV-curable systems, photoinitiators and/or photosensitisers.
[4,5] The main functions of various constituents in a radiation-curable system are:
- The base oligomer imparts most of the fundamental properties to the final cured or crosslinked material
- The monofunctional monomers enable the formulation to be diluted to the required viscosity
- The multifunctional monomers allow chemical linkages to form between segments of the oligomer
- Initiators may be photo or thermally initiated
- Additives are used to modify or impart specific properties.
In addition there has been significant development in water-based and solventless adhesives and coatings which are considered more 'environmentally friendly' than solvent-based systems.
Oligomers
The overall properties of any coating, adhesive or ink polymerised or crosslinked by various radiation techniques are primarily determined by the nature of oligomer used in the formulation. Most of the radiation-curable systems are based on an oligomer containing acrylate functional groups which can be cured by both UV and EB (electron beam). The main resin types used are acrylated epoxies, polyesters, urethanes and silicones. [4,6,7] These oligomers are formed by reacting acrylic acid with epoxies, polyesters and urethanes. Three other systems which also have been used are cationic, thiolene and unsaturated polyester plus styrene or acrylate monomer. Oligomers are low or medium molecular weight compounds which have been specially designed to impart a desired set of properties. Oligomers may vary in molecular weight from about 400 to 7000. While the higher molecular weight oligomers are quite viscous, the lower molecular weight materials are pourable liquids. The resins based on acrylates are the product of acrylation of different chemical structures. The acrylation process imparts the unsaturation of the group to the ends of the oligomer. Therefore, in principle, most oligomers can be considered as difunctional entities. Each type of acrylated oligomer imparts specific properties to the product. Various oligomers are described in the following sections.
Acrylated epoxies
These resins are normally harder than other acrylated systems and are more suitable as coatings or adhesives on metal cans or rigid substrates and as binders in fibre-reinforced composites. The presence of epoxy or hydroxyl groups in the prepolymer chain improves the adhesion and pigment wetting characteristics of the system. The epoxy structure enhances adhesion to non-porous substrates, as well as improving the chemical resistance.
Acrylated polyesters
These materials generally have low viscosities, reducing the need to use reactive diluents. Acrylated polyesters can also provide good weatherability and high adhesion to various substrates. A broad range of acrylated polyesters can be prepared using a wide variety of acids and polyols. Coatings or adhesives of this type have a wide range of properties. Polyesters normally have good curing characteristics and colour stability and are suitable for outprint paper lacquers, adhesives and wood coatings.
Acrylated urethanes
These oligomers are used for abrasion-resistant topcoats, mar resistant coatings on wood surfaces, printing inks and vinyl flooring. Acrylated urethanes usually provide high flexibility, toughness and abrasion resistance. However, they may also have higher viscosity, cure more slowly and be more expensive than other acrylates.
Acrylated silicones
These are novel resins which are finding applications as release coatings on a variety of substrates. However, these materials are being considered for coatings, adhesives or encapsulants for many new applications, including microelectronics due to their low and high temperature properties.
Cationic systems
Cationic initiators can be used to polymerise or crosslink epoxy based oligomers. This type of oligomer is usually more expensive than other systems but can provide better adhesion to various substrates and improved long term durability. These oligomers are polymerised by cationic rather than free radical reactions. The most common type of UV-curable resin is the cycloaliphatic diepoxide cationic resin ( eg the 3,4 - epoxycyclohexylmethyl - 3',4' - epoxycyclohexane carboxylate).
Unsaturated polyesters
One of the first systems to be introduced commercially using UV technologies consisted of unsaturated polyesters dissolved in styrene and containing benzoin ethers as photoinitiators. It is used almost exclusively for wood finishing, either as clear varnish or as a filler for wood and chipboard. The slow rate of cure and inhibiting effect of air on the surface of the coating make it difficult to produce hard and tack-free clear fillers. Unsaturated polyesters are used predominantly as filler coatings. The low cost of this class of radiation-cured products compared with alternative systems should maintain interest in the wood filling sector of the wood finishing industry.
Monomers and reactive diluents
A large number of mono-, di-, and polyfunctional monomers are now available commercially. They are required as diluents to reduce the viscosity of the formulated mixtures and to promote rapid curing. Monofunctional monomers have only one reactive group or an unsaturated structure ( eg a double bond). This enables the monomer to react, when exposed to radiant energy, and to become incorporated into the cured material or finished product. As well as reducing viscosity, monofunctional monomers can also affect the properties of the cured coating, adhesive or ink. The specific chemical nature of these monomers will determine their ability to promote adhesion to non-porous substrates, harden or flexibilize the cured material, or to add other unique properties to the finished product. Some widely used [4] monofunctional group monomers are as shown in Fig.1.
Multifunctional monomers are unable to form the links between oligomer molecules and other monomers within a formulation. The ultimate properties of a coating, adhesive or ink depend on converting formulations, which are usually applied as liquids or viscous pastes, to solids during crosslinking or curing process. The polyfunctional acrylates can provide rapid curing but have a number of disadvantages, [8] if used alone, including:
- Inferior ability to lower viscosity
- Poorer solvent power for oligomers
- Tendency to leave high residual unsaturation in the final product
- Excessive crosslinking resulting in high stiffness and brittleness.
Some widely used multifunctional monomers are as shown in Fig.2.
A multifunctional monomer can provide a much tighter crosslinked network than a difunctional one.
Initiators
An effective free radical initiator is a chemical agent which, when subjected to electromagnetic radiation, heat or chemical reaction, will readily undergo dissociation into radicals of greater reactivity than the monomer radical. These radicals must also be stable for long enough to react with a monomer and create an active centre. The photoinitiators have been widely used in photopolymerisation and curing of coatings by non-ionising radiation ( eg UV), whereas thermal initiators are used for thermally induced curing reactions.
Photoinitiators
Photoinitiators absorb light and generate radicals or cations, which initiate polymerisation or crosslinking reactions. For free radical polymerisation, there are two main types of photoinitiators:
- Initiation which produce radicals by direct cleavage, eg benzoin ether cleaved as shown in Fig.3. A commercially available photoinitiator of this type is 2,2-dimethoxy-2-phenyl acetophenone with the chemical structure shown in Fig.4.
- Initiators which generate radicals by hydrogen-abstraction from a hydrogen donor, eg benzophenone, which is widely used in commercial UV coating formulations. The initiator produces free radicals by the mechanism [4,8] shown in Fig.5.
The radical ( Symbol.1.) produced initiates free radical polymerisation. Tertiary amines are very good hydrogen donors and are usually used in conjunction with benzophenone in coating formulations.
As indicated earlier, UV light is capable of activating crosslinking reactions via an 'ionic' rather than by a 'free radical' mechanism. Cationic photoinitiators are generally acrysulphonium salts which, when subjected to UV light, decompose to produce an acid catalyst.
Thermal initiators
There is a range of thermal initiators available primarily based on peroxides or azo compounds. For instance, when heated, benzoyl peroxide eventually forms two phenyl radicals ( Symbol.2.) with loss of carbon dioxide ( Fig.6).
The phenyl radicals produced are capable of initiating the polymerisation or curing reactions. There are a number of peroxides available commercially which decompose at different temperatures to generate free radicals. Therefore, depending on the heat source and the temperature, a suitable peroxide can be incorporated in a coating or adhesive formulation to initiate the curing reactions.
Additives
Additives are used to modify the performance of coatings and enhance specific properties. For instance, waxes can improve scuff resistance whilst silicone oils and fatty acids enhance slip and antiblocking characteristics. For UV systems, the absorption of light by additives ( eg pigments) has to be minimised in the frequency range required to start the polymerisation reactions. The main types of additives used in coatings, adhesives or inks are:
- Fillers
- Pigments
- Adhesion promoters
- Wetting agents
- Slip aids
- Stabilisers ( eg antioxidants and photostabilisers)
The use of stabilisers is often essential to improve the long-term serviceability of the material. For example, in a clear coating a small amount of photostabiliser (0.5-1%) can improve resistance to photodegradation and discolouration during weathering. In pigmented or filled systems, wetting agents are incorporated to promote the adhesion characteristics of the coating.
Water-based systems
The use of water-based, radiation-curable systems has been under discussion for many years. However, in recent years considerable progress has been achieved in the development of water-borne coatings and adhesives. [9,11] The main interests in their use are for:
- Wood lacquers
- Clear overprint varnishes
- Screen printing inks
- Gravure and flexo printing inks
- Plastic coatings
- Adhesives
Some of the advantages of water-borne systems, compared to 100% solid radiation-curable systems, are: [11]
- Coating systems have low toxicity, odour and skin irritancy since there is no need for reactive diluents (monomer or oligomer)
- The rheology or viscosity of the formulations can be controlled by water content or by thickeners
- Lower shrinkages due to absence of monomers
- Equipment or spillages are easily cleaned with water after use
- Water-based coatings can be prepared to dry physically before radiation curing, enabling early handling without dust pick up.
Some of the disadvantages are:
- Can have a grain raising effect on substrates ( eg wood)
- In many applications an extra processing step is required to remove the water before radiation curing
- There still may be some organic volatiles (amines or solvents) present.
Undoubtedly the trend towards healthier, safer and environmentally more acceptable adhesives, coatings and other polymer based resins will continue and the number of water-borne, radiation-curable raw materials will also be expected to increase in the future.
Water-borne, acrylate urethane based coatings for wood substrates have recently become commercially available. [11]
Oxygen inhibition
In general oxygen can inhibit free radical reactions.
[8] Polymerisation and crosslinking reactions of acrylated resins can be prevented by the presence of oxygen. Oxygen is a strong radical acceptor and reacts with the alkyl radical (
Symbol.1.) generated during the initiation step of the free radical reactions:
Symbol.1. + O 2 →R Symbol.3.2
This reaction prevents successful addition of the alkyl radical to the monomer (M) during polymerisation and crosslinking:
Symbol.1. + M → R Symbol.4.
As a result the coating on a substrate remains uncured and tacky. The effect is particularly noticeable in thin layers and at surfaces, where the oxygen consumed is easily replaced by more diffusing in from the surrounding atmosphere. Oxygen inhibition can interfere with both UV and EB curing mechanisms. In EB curing, the oxygen inhibition effect is usually not significant, since inert atmospheres are needed in order to reduce the ozone formation. Among the early methods recommended to reduce oxygen inhibition was use of dual cure systems. However, in recent years coating materials have been developed which are used satisfactorily in air. In some of these coating systems, benzophenone is used in conjunction with tertiary amines or methyldiethananolamine which can provide satisfactory performance, particularly when exposed to high UV light intensities . [12]
