|
How to formulate UV-curing coatings
R.C.W. Zwanenburg Introduction This paper is meant to be a practical guide for formulating UV- and EB-curing systems. As the vast majority of the commercially available UV- and EB-systems is based on free radical curing systems, emphasis will be on this technology. Within these free radical curing systems, coatings based on unsaturated polyesters in styrene are very limited in applications (mainly wood coatings) and also in geographical area where they are produced and used (mainly Italy and Spain/Portugal). By far the most UV-curing-coatings, for a variety of substrates ranging from paper and wood to plastics and glass - is based on acrylate chemistry. The wide formulating latitude of this chemistry mainly causes this: the formulator has a choice among more than fifty different reactive diluents (monomers, oligoether-acrylates etc.) and a wide range of oligomers. Because of this wide range of formulating raw materials, it is not always easy to select the raw materials that will give the desired performance properties. This paper will help you selecting the right oligomers and monomers/reactive diluents. This will be done in three steps:
At the end a few words will be said about formulating UV-systems using a different chemistry: cationic curing.
Oligomers are raw materials that can be compared with the 'resin' in a classical coating. Choice of the oligomer is critical: very often, the oligomer is an or even the most important component in the formulation by weight. Because of this, its choice has a major impact on the final performance of the system. Examples of performance characteristics are:
One of the difficulties for the formulator is the fact that the exact chemical composition of oligomers is not revealed by the raw material suppliers for reasons of confidentiality. Despite this, selection can be made on the basis of several criteria:
In acrylate chemistry, there are several families of oligomers. Each of these has its own advantages and disadvantages. The main oligomer families are:
The name of this family of oligomers may lead to confusion: in most cases, epoxy acrylates do not have any free epoxy groups left. The name epoxy acrylates is derived from the base resin for their synthesis: an epoxy resin. Within the epoxy acrylate oligomers, there are essentially five sub groups
Aromatic difunctional epoxy acrylates have three main drawbacks: their high viscosity, their limited flexibility and the fact that they do yellow to some extent. Because of this, they are less suitable for application on flexible substrates; low viscosity application techniques like spray-, dip-, curtain coating, and applications with high requirements in terms of colour stability over a longer period of time (white and light coloured substrates that have to last long). Acrylated oil epoxy acrylates, in Europe essentially epoxidised soyabean oil acrylate. This oligomer has a good pigment wetting properties and is flexible in combination with a relatively low viscosity and low cost. Main disadvantage is its slow cure speed. This type of oligomers is mainly used in pigmented systems or to reduce cost. Epoxy novolac acrylates are really specialty products, mainly used in the electronic industry for printed circuit boards (solder resist) because of their high reactivity, excellent heat resistance. Disadvantages are: high viscosity, lack of flexibility and relatively high costs. Aliphatic epoxy acrylates are also specialty products. There are various types available to the formulator, varying in functionality, molecular weight and chemical backbone. Properties like flexibility (good for difunctional, poor for trifunctional or higher), reactivity (good for difunctional, excellent for trifunctional) and viscosity (very low for difunctional, medium for trifunctional) and adhesion (good to excellent for difunctional)depend on the functionality. Some difunctional aliphatic epoxy acrylates have the interesting property to be compatible with water, enabling to formulate water dilutable systems with these. Miscellaneous epoxy acrylates. This group comprises essentially specialty oligomers with fatty acid modification, which give good pigment wetting properties and therefore are used in printing inks and pigmented coatings. Because of their higher molecular weight and lower functionality (part of the acrylate groups has been replaced by fatty acid) these oligomers are not quite as reactive as e.g. bisphenol A epoxy acrylates. Urethane Acrylates (1) The chemistry of urethane acrylates is very versatile and therefore there are many different types available to the formulator. Essentially, there are four parameters that can be varied synthesizing urethane acrylates:
Functionality for urethane acrylates varies in practice between one and six. Generally speaking: the lower the functionality, the lower the reactivity, the better the flexibility and the lower the viscosity. Functionality two and three are good compromises for general-purpose oligomers. Type of isocyanate. Essentially, four types of isocyanates are used for urethane acrylate synthesis: monoisocyanates, aliphatic diisocyanates, aromatic diisocyanates and polymeric isocyanates. Monoisocyanates are used for monofunctional urethane acrylates only, and this type of oligomer is described above. Diisocyanates are by far the most widely used in urethane acrylate synthesis. We can divide them in aliphatic and aromatic diisocynates. Aromatic diisocyanates are used for the manufacture of the so-called aromatic urethane acrylates. The incorporation of an aromatic diisocyanate makes the urethane acrylate harder, gives it a better scratch resistance. Aromatic urethane acrylates are also significantly lower cost than aliphatic urethane acrylate. This makes them interesting for those applications, where the performance of a urethane acrylate is needed (e.g. a good flexibility or abrasion resistance) but the formulation has to be relatively low cost. The major drawback of aromatic urethane acrylates is that they tend to yellow and therefore they are less appropriate for long lasting applications on white or light coloured substrates. Aliphatic diisocyanates are used in aliphatic urethane acrylates. Aliphatic urethane acrylates are slightly more flexible than aromatic urethane acrylates with the same functionality, a similar polyol modifier and at similar molecular weight. The main advantage of aliphatic urethane acrylates is the fact that they are virtually non-yellowing and therefore can be used for long lasting applications, on white or light coloured substrates. If the polyol modifier is well adapted as well, it is possible to formulate systems with good weathering resistance with aliphatic urethane acrylates. Examples of uses of aliphatic urethane acrylates are for topcoates for parquet flooring, on plastic substrates, for optical fibers, for screen inks for flexible packaging etc. Polymeric isocyanates are used less for urethane acrylates than diisocyanates. They are essentially used for higher functionality (3 and higher) urethane acrylates.
The polyol modifier is the backbone of the urethane acrylate (if one is used). Polyol modifiers vary in chemical type (essentially either polyether or polyester), functionality (typically ranging from two to four) and molecular weight. Polyether urethane acrylates are typically more flexible than polyester urethane acrylates and often lower cost. In addition, a polyether urethane acrylate will have a slightly lower viscosity that a polyester urethane acrylate with the same functionality and approximately the same molecular weight. The molecular weight. For di- and trifunctional urethane acrylates, the polyol modifier used mostly determines this property. See above for the influence on properties. For higher functionality urethane acrylates this is not always the case and therefore the general guidelines for property/molecular weight correlation may be different.
Like for urethane acrylates, the chemistry of polyester acrylates is a versitile one. Because of this, a variety of polyester acrylates is available to the formulator, varying in functionality, chemical backbone and molecular weight.
There are rather few polyether acrylates available to the formulator. These products typically are low to very low in viscosity (can be as low as a medium viscosity monomer!) and they often have very high flexibility. An interesting property of some of these oligomers is that they are compatible with water and therefore can be used in formulating water thinnable systems.
This is a rather new family of oligomers. They vary in functionality, chemical backbone, type and degree of amine modification and molecular weight. These variations make that these products range from very low viscosity (as low as a medium viscosity monomer) to medium viscosity. They all have in common that they have high reactivity and typically low irritancy. Amine modified polyether acrylates are typically used with very little monomer or reactive diluent (because of their intrinsically low viscosity) and without free amines or acrylated amine synergist. Although the water resistance and chemical resistance of amine modified polyether acrylates is much better than of polyether acrylates; they still lack the flexibility and toughness of urethane acrylates. Main usages are in wood coatings (furniture) and paper coatings.
Like urethane acrylates, the chemistry of acrylic acrylates is very versatile. Variations are possible in functionality, chemical backbone (monomers used) and molecular weight. Today, rather few acrylic acrylates are available to the formulator, often used because of their good adhesion to difficult substrates.
These comprise adhesion promoters, melamine acrylates, silicone acrylates etc. Typically, these are specialty products.
Monomers are used as reactive diluents in formulations. For this, often low cost, multipurpose products are used. However, because sometimes quite high levels of monomers are used in the formulation, especially in low viscosity applications, the influence of the monomer on the performance properties of the system can be significant. Especially for those cases, the choice of monomer becomes critical. In addition to being reactive diluents, monomers are also used to achieve a variety of desired properties: improve adhesion, reactivity, chemical resistance, scratch resistance etc. Some monomers are not used as reactive diluents at all, but only to achieve a desired effect, like the reactivity boosters.
Functionality. The rule of thumb here is: the higher the functionality, the higher the reactivity. This is easy to understand: the higher the functionality, the higher the number of acrylate double bonds that is available for crosslinking.
Type of chemical backbone:
Hydrocarbon type monomers typically have low surface tensions; this is particularly true for the mono- and difunctional types. Low surface tension is important for substrate wetting, which in turn is required for good adhesion. Hydrocarbon type monomers typically have good flexibility (especially for mono- and difunctional types). In addition, they have very low yellowing and good weathering resistance properties. As they are hydrophobic in nature, hydrocarbon type monomers have excellent water resistance properties.
The monomers described are listed in increasing functionality, are grouped by chemical type and within each chemical type are listed in decreasing molecular weight. Consequently, generally viscosity increases and flexibility decreases going from the top to the bottom.
In this part, we will look at how the raw materials described in the first two parts are used by looking at some starting point formulations. We will briefly explain why the acrylate oligomers and monomers that are used in these formulations were selected.
Overprint varnishes (OPV) are one of the main usages for UV in the graphic arts. In this application, full advantage is taken from some of the features of UV-curing:
Typically, this kind of varnishes is applied by roller coating and therefore are low viscosity. One of the problems sometimes encountered is the use of highly porous and therefore absorbant paper. This leads to absorption of the low viscosity varnish into the paper resulting in low gloss. Using special additives can solve this: the bentone gives the varnish some degree of thixotropy, which reduces the absorption into the paper. The transparent zinc oxide has a particle size that matches the size of the pores of the paper. This further reduces the absorption. Transparent zinc oxide has little effect on the gloss and the transparancy of the coating.
As for the acrylates used in this formulation: skin- and eye irritation is often a problem, as industrial hygiene from the printers is not always as it should be and often the workers do not use the protective gloves, they should use when working with UV-varnishes. Although this is undesirable and the varnish manufacturer should stress the use of protective gloves on the work floor, also low irritancy raw can be used. The oligomer used here is a bisphenol A diacrylate oligomer, diluted in propoxylated neopentyl glycol diacrylate. This oligomer is used as is has excellent reactivity and high gloss and has low skin and eye irritancy. The propoxylated neopentyl glycol diacrylate is used as a reactive diluent as it has low skin- and eye irritancy, good diluting properties, low skin- and eye irritation and excellent substrate wetting because of its low surface tension.
Melamine paper, widely used for furniture applications, is a very difficult substrate to get good adhesion to with a UV-coating (3). There are two reasons for this:
the low surface tension of the substrate The formulation above works because the selection of acrylates addresses these two problems. CN704 is an acrylated adhesion promoter for non-porous substrates, which improves adhesion because it greatly reduces polymerisation shrinkage of the coating. To achieve this effect, the amount of this product in the formulation has to be high: reduction of shrinkage only works if the main components by percentage in the formulation are low shrinkage. CN131, a monofunctional epoxy acrylate, has also low shrinkage, but it improves the cure speed of the formulation, as it is faster curing than CN704. The propoxylated neopentyl glycol diacrylate is used while it low shrinkage. It further improves adhesion, as it improves wetting of the substrate, a vital element to get good adhesion to a substrate. The acidic adhesion promoter is used to further improve adhesion to this very difficult substrate.
UV-coatings are widely used on wooden substrates, essentially for furniture and parquet flooring. In this application, cure speeds are not quite as high as in the graphic arts industry. The main advantages of UV in these applications are:
The example above is a starting point formulation for a scratch- and abrasion resistant topcoat for wood, either furniture or parquet. For furniture, the scratch resistance is important. For wood parquet, both scratch- and abrasion resistance are important.
The difficulties to overcome in this formulation for wood furniture are:
The main oligomer used here is an amine modified polyether acrylate: the lowest viscosity oligomer available to the formulator. This type of oligomer gives excellent reactivity, especially surface cure. This makes achieving low gloss levels very difficult, as gloss is a result of micro roughness at the surface. This is achieved by the particles of the matting agent that migrate to the surface during cure. This is more difficult if the surface cure is high. Consequently, more matting agent has to be used, which leads to higher viscosity, which is undesired in this formulation. The solution of this problem is the use of a special matting agent, which can be used at higher concentrations, without resulting in a significantly higher viscosity. The propoxylated neopentyl glycol diacrylate is used in this application because it has low surface tension.
Adhesion to flexible PVC is typically difficult to achieve because of migration of the plasticizer. Best results are obtained if (3):
Low shrinkage is achieved by use of the high molecular weight aliphatic polyester urethane acrylate, which further is non-yellowing, which makes application on light colours and even white PVC possible. Obviously, for a flexible substrate, a flexible coating is needed to avoid cracking. This, again, is obtained by using the aliphatic urethane acrylate. Finally, the aliphatic urethane acrylate is non-irritant.
UV-coatings are also used as topcoats for motorcycle helmets. In addition to the obvious advantages as
no solvent emission As we have seen before, this has to be achieved by combining several approaches (3):
Low shrinkage and low yellowing is obtained by using a high molecular weight aliphatic polyester urethane acrylates oligomer. 1,6-Hexanediol diacrylate and tetrahydrofurfuryl acrylate are both swelling monomers that attack the polyurethane coating to give excellent adhesion. Pentaerythritol tetraacrylate gives excellent scratch resistance and high reactivity. High reactivity and low viscosity without losing the adhesion characteristics are achieved by using THEIC triacrylate, which further improves scratch resistance. Two non-yellowing photoinitiators are used. Combination of photoinitiators often improves adhesion, as it modifies the ration of surface to through cure and therefore the 'curl' of the formulation.
Polystyrene is another difficult substrate for UV-coatings to adhere to (3). Adhesion in this formulation is achieved by the use of two swelling acrylates: an oligoether acrylate ethoxylated trimethylolpropane triacrylate and 1,6-hexanediol diacrylate. Because of this, adhesion is excellent, even at this very low viscosity.
Polyolefines (polyethylene and polypropylene) are the most difficult plastic substrates for UV- (and other) coatings to get adhesion to (3), as they have very low surface tensions and are completely inert, which makes it impossible to find acrylates, which will attack these substrates, forming an IPN. Therefore, polyolefines (typically used for flexible packaging) are typically corona- or flame-treated before coating. Even then, it is extremely difficult to achieve good adhesion. The only parameters that can be used are:
Low shrinkage applies to all components in this formulation. Propoxylated neopentyl glycol diacrylate is used as a reactive diluent with very low surface tension.
Adhesion to metals is also very difficult to achieve with UV-curing coatings (3). The best adhesion is achieved by using acidic adhesion promoters, that attach the metal surface and provide a chemical bond between the metal surface and the coating. Further polymerisation shrinkage has to be limited as much as possible. 2-(2-Ethoxyethylacrylate) is used in this very low viscosity formulation, as it is the highest reactivity and lowest viscosity monofunctional acrylate monomer. Ethoxylated trimethylolpropane triacrylate is used, as it is the lowest viscosity trifunctional acrylate, trifunctional and therefore used to improve the adhesion. The acrylated polyester adhesion promoter reduces shrinkage. The low viscosity aliphatic epoxy acrylate gives improves adhesion and reactivity, but does not increase viscosity.
This is an example of a solvent containing formulation, which can be spray applied. Adhesion is achieved by combining an acidic adhesion promoter with a carboxylic functionalized oligomer. Dipentaerythritol pentaacrylate is an oligoacrylate with free OH-functionality, which also improves adhesion. This product is essentially used to improve reactivity: it is the fastest curing acrylate available. It further improves scratch resistance.
(1) CRAY VALLEY Cationic Chemistry
The products available to the formulator for cationic curing are essentially: As the chemistry is completely different to acrylates, different formulating rules apply. One of the most important things to remember is that polyols are used in combinations with cycloaliphatic acrylates to improve reactivity and flexibility. This is because polyols will lead to chain transfer reaction, which will increase the mobility of the polymerising species, resulting in higher cure speeds and reducing the average molecular weight of the polymerised film, resulting in better flexibility. In formulating cationic systems, one has to respect stoichiometric ratios between the cycloaliphatic epoxies and the polyols.
References | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||