The advantages of energy curing technology are clear. It increases efficiency, is highly economical, enables wide range of applications, saves energy costs, most environmental friendly technology.
The application of ultraviolet light (UV) and electron beam (EB) curable systems are not anymore restricted to graphic arts and wood coatings. Although, these applications still represent the largest volumes for UV/EB products. During the past years, the radcure technology has successfully entered a number of new application areas:
* Printing inks and varnishes.
* Adhesives.
* Electronical media.
* Optical fiber coatings.
* Coatings for wood, paper/board and plastics.
* Dental composites.
* Release coatings (ie labels).
* Printed circuit boards.
This list is by far not complete. The quality and productivity of radiation curable systems are essential in the manufacturing of many products. Considering today's share for radcure application, this enviromental friendly technology has still a large potential for new applications.
UV/EB curable inks are commercially available for most printing methods like screen, letterpress, flexo and litho-graphic printing. The print quality is often improved compared to conventional inks. Most common substrate can be printed with radiation curable inks: paper/board, labels, glass and plastic bottels, display and packaging materials, plastic foils, CDs, optical fibres, metals.
UV/EB varnishes are available for most application methods. The varnish protects the print and substrate, the gloss level can be matt or very high.
Laminating adhesives require good adhesion onto the printing ink, the substrate and the laminating foil combined with low viscosity and good reactivity. Carefully formulated products, based on oligomers and monomers with low skrinkage are available for sheet and web applications.
UV curable systems for wood coatings are used for many applications and products. Common application methods are by rollercoater, spraying, casting and vacuum-coater. Filler, basecoat, clear and pigmented topcoat are used for kitchen or office furniture, chairs, parkettfloor, decorative parts in cars, panels etc.
Like wood, coatings for plastics can be applied by most common methods. The coatings will protect the plastic from scratch, stain, abrasion etc. Examples of applications are vinylfloors, helmets, lenses, headlights for cars, ski's and snowboards, tennis rackets, golf balls, packagings,
In the manufacturing of printed circuit boards UV curable systems are essential. Solder masks and etch resists are basic products. There are many other, more specific applications used in this field like marking inks, topcoats, ad-hesives, casting insulation etc.
Coatings are normally clear transparent films and will protect the substrate. The surface look can be controlled from matt to high gloss. Inks and paints have a similar composition but contain additionally a color pigment and fillers. Paints are opaque and will cover the substrate.
The solvents are volatile organic compounds (VOC) and mainly used in order to apply the coating or paint by the re-quired application method. Liquid inks or coatings with low viscosity for i.e. spray application contain more solvents then a paste ink.
The used solvents are alcohol's or distilled gasoline products. Besides the viscosity reduction, solvents are used to control the time for flow out, gloss and time for the physical drying process.
The solvents evaporate after the coating is applied onto the substrate and leave a thin homogenous film of the solid resin binder and evt. pigments.
An ink or a coating may contain up to 70 per cent volatile organic solvents, these will evaporate during the drying process and not be present in the final coating. Increased awareness of the environmental impact of organic volatile com-pound has raised the interest to reduce the solvent content in these coatings and inks. Other concerns with solvents are flammability and risk to health by inhalation.
Waterdilutable inks and coatings are widely used in order to overcome the environmental draw back and handling of solvent based inks and coatings. The advantage is obvious, water instead of flammable organic solvents is used to reduce the viscosity of the ink or coating. Although with the same drying process as for solvent based coatings, the evaporation rate of water slows down the output and increases the energy consumption. Most water dilutable inks and coatings are not totally free of VOCs, they often contain small amount of solvents to improve stability and compatibility.
A solventfree fine powder, a mixture of resin, pigment and hardener, is applied onto the heated substrate. The pow-der melts due to the high temperature and forms a film with good resistance to solvents and impact. Due to the ex-cellent properties, powder coatings are used in high performance applications.
The required high temperature (>100°C) limits the application of powder coatings mainly to metals.
Semisolid or liquid resins with chemical unsaturation or reactive groups ie unsaturated polyester, epoxy, or polyure-thane resins are the basic resins to formulate chemical curing inks and coatings. A second component, the hardener is mixed to the coating just before the application. The polymerisation reaction starts and the liquid coating is trans-formed to a solid film. The chemical reaction provides a polymer film with excellent properties and resistance to chemicals and other effects. Based on the application and other requirements, these inks and coatings may contain volatile organic solvents or water in order to adjust the viscosity.
Basically radiation curable inks and coatings are also chemically cured systems. The hardener is a photoinitiator in the case of UV curable systems. Under the exclusion of light, especially in the uv-range, the coating remains liquid and stable. After the coating is applied to the substrate, it will be exposed to uv-light, this will initiate the polymerisa-tion.
The main resins used to produce radiation curable inks, coatings and adhesives contain chemically reactive groups. These, mostly liquid or semisolid resins will polymerize upon the exposure to high energy radiation or due to the reac-tion with the breakdown products of the photoactive compound (photoinitiator). This initiates a chain reaction, which will transform the liquid resin into a solid plastic. This process, called polymerisation offers a solventfree application of a liquid and low viscosity coating or ink and curing in fraction of seconds. The cured film shows excellent gloss and resistance to chemicals, scratch or abrasion.
The chemistry of radiation curable inks, coatings or adhesives can be divided in radically and cationic curable sys-tems. Despite the unsaturated polyester/styrene systems, acrylate terminated resins (oligomers) and diluents (monomers) are the mainly used for the formulation of UV or EB curable inks and coatings. Resins with methacry-late, vinylether or maleate functionality are used to a lesser extent in more specific applications.
The UV-curing process :
* The radiation in the range of 200-400 nm is emitted by a UV-source, normally a medium to high pressure mer-cury bulb. The photoinitiator absorbs the emitted light and transfers the energy into radicals, highly reactive chemical energy.
* The free radicals crack the doublebonds of the oligomer and monomer mixture and initiate a chain reaction.
* The chain reaction builds up a network with the polymerized oligomers and monomers. The flexibility of the grow-ing molecules decreases with time and will finally stop the chain reaction. Now the coating turned from liquid to solid.
* Post (exposure) curing (about 24 hours) is not visible but may have an impact on the final properties, such as adhesion or flexibility.
As the reaction takes place in fractions of a second, the production speed increases and the energy consumption is normally reduced compared to solvent or water borne systems.
A typical composition of a UV-curable ink or coating:
* Resins (Oligomers) 30-60 per cent
* Reactive diluents (Monomers) 5-50 per cent
* Pigments, fillers, matting agents 5-40 per cent
* Photoinitiators, synergists 2-15 per cent
* Additives 1-5 per cent
Electron beam curing (EBC)
The main steps in the polymerisation reaction are:
* Low voltage electron beams are produced with a cathode and accelerated to ultra high speed. The electrons are bombarded onto the carbon doublebonds, crack them and create free radicals.
* The free radicals will react with other doublebonds and the chain reaction is initiated.
* The chain reaction build up a network with the polymerized oligomers and monomers. The flexibility of the growing molecules decreases with time and will finally stop the chain reaction. Now the coating turns from liquid to solid.
As the electron beam is much more intensive, the radiation will penetrate also thick layers and highly pigmented coatings. Pigments, fillers and other light absorbing materials will not affect the curing, also the ink or coating ab-sorbed by the substrate will polymerize with electron beam curing. Except the missing photoinitiator, the composition of a EB-curable ink or coating is similar to a UV curable system.
As oxygen inhibits the polymerisation reaction, the electron beam curing process operates under inert atmosphere using nitrogen. The high investment and the cost to run a electron beam curing (EBC) operation limits the technology to high volume production or those applications where the EBC features are essential.
Cationic curable systems
The composition of a cationically curable system is comparable to radical systems. The photoinitiator, ie a triallyl-sulphonium salt, breaks down to free acid compounds, which will initiate the polymerization reaction. The binders used are mainly based on cycloaliphatic epoxy resins, polyols and other epoxy or vinylether functionalised, mostly low viscous resins. For specific applications, these systems may show better adhesion or barrier properties. Contrary to radical curable systems, the cure of such inks and coatings are not inhibited by oxygen, but high humidity may af-fect the reaction. The acid produced under the radiation can be readily neutralized by alkalis used for cleaning or pre-treatment of the substrate. The post-curing effect of cationic curable inks and coatings is significant and will accel-erate at elevated temperature. Cationic systems could be used in most applications, following the above criteria.
Hybrid curable systems are mixtures of radical and cationic curable raw materials (acrylate/vinylether; ep-oxy/acrylate; maleate/vinylether, etc.). For specific applications and requirements the hybrid curable system's com-bined features may offer a solution.
The final properties of a cured ink or a coating are dominated by the monomers and oligomers used in the formula-tion. The character of the oligomer or oligomer mixture define the flexibility, hardness and resistance to chemicals or weathering. Monomers are used in order to adjust the viscosity required by the application. Additionally, related to the monomer's functionality, they will increase the hardness and the crosslinking density of the cured film. For inks and coatings, acrylate terminated resins and diluents are widely used due to their higher reactivity. In adhesives, composites and similar applications, methacrylates are preferred due to lower shrinkage and in general lower irrita-tion.
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