Classical Applications

CHAPTER 7

Tackling the Drawbacks of UV Systems

The most important intrinsic drawbacks of the radically induced radiation curing technol-ogy together with proposed and practiced solutions to overcome these disadvantages aresummarized in this chapter. Two of these drawbacks are related to the radical intermediatesitself, which firstly hamper the curing reaction at the surface (oxygen inhibition) and sec-ondly contribute to yellowing (initial yellowing), until they are completely decayed. Thethird main drawback is associated with the lack of radical formation in areas to which thelight does not penetrate (shadow areas) and therefore no curing occurs.

7.1 OXYGEN INHIBITION

The reaction of oxygen with the different species formed during the photopolymerisationand the effects on the network formation throughout the film thickness of a coating are

FIG. 7.1. Scheme of the oxygen inhibition reactions.

179

180 UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

sketched in Figure 7.1. The termination of the reaction by hydroperoxides, the incompletenetwork formation at the surface and the remaining acrylate double bonds are commonlycalled the “problem of oxygen inhibition”.

In the photoinduced radical polymerization of multifunctional monomers, the efficiencyof crosslinking is due to the efficiency of the propagation step. In the presence of air, theoxygen diradical reacts much faster with the photoinitiator or propagating radical to forma relatively stable peroxy-radical,1 which does not initiate the acrylate polymerization, butrather acts as an inhibitor. This inhibition results in an induction period of the polymeriza-tion until all oxygen is consumed.2 Therefore, in thin films the complete polymerizationis retarded and in thicker films the acrylate conversions at the air–coatings interface arevery low, resulting in tacky surfaces. This detrimental action of oxygen inhibition is shownschematically in Figure 7.1, depicting the lower acrylate consumption especially at the in-terface to air. The extension of the inhibited layer thickness is dependent on the oxygendiffusion into the coating. The penetration or diffusivity of oxygen in the coating layer it-self is dependent on several factors, for instance the type and polarity of the materials used,as well as the viscosity, which is a major influencing factor. This oxygen penetration intothe film can be derived from an approximate solution of Fick’s diffusion equation (7.1.1).

d = [6D(�t)

]1/2, (7.1.1)

with d = distance (cm), D = oxygen diffusivity (cm2/s) and �t = exposure time.The oxygen diffusivity in water-like liquids (viscosity 1 mPa s) is on the order of

10−5 cm2/s, and in typical UV resins with increasing viscosity from 10−6 to 10−8 cm2/s.Thus in typical UV polymerizations times ranging from 0.5 to 5 s, the oxygen moleculescan penetrate distances of 0.1–10 µm. This theoretical estimate3 can be confirmed, asshown in Figures 2.14 and 7.2, where a polyether acrylate has been photopolymerizedand the remaining concentration of double bonds has been determined by confocal Ra-man microscopy4 as a function of layer thickness. During curing of the unmodified resin,

FIG. 7.2. Double bond conversion as a function of depth into the films with a neat polyether acrylate resin, withamine modified resin (oxygen consumption) and with wax additive (oxygen barrier).

TACKLING THE DRAWBACKS OF UV SYSTEMS 181

a polyether acrylate, up to a layer depth of about 10 µm the double bond conversion re-mains very low, until in deeper layers the conversion increases significantly. By modifyingthe resin with amines or by applying a wax additive, the conversion is already high at thesurface and almost as high as in the bulk regions. It has been shown,5,6 that the detrimentaleffects of oxygen inhibition are less pronounced when the diffusion of atmospheric oxygeninto the liquid coating film is decreased by:

• decreased oxygen concentration in the atmosphere,• increased formulation viscosity or• decreased sample temperature;

and the cure speed increased by

• high photoinitiator concentrations and/or efficient photoinitiators,• highly reactive formulations or• high light irradiance.

Several studies have shown that this inhibitory effect of oxygen in the photopolymerisa-tion of acrylate based formulations depend on the type and concentration of the photoini-tiators selected, the formulation reactivity and the irradiance.7,8

The Bowman group has presented a detailed study into the impact of oxygen onphotopolymerisation kinetics recently.9 They found that the inhibition rate constant is∼106 times greater than the propagation rate constant. They also investigated the effectof dissolved oxygen on the mechanical properties of the film and found, that it has a negli-gible effect on glass transition and modulus, since the concentration of oxygen terminatedshort-chain species is very low compared to the crosslinked polymer chains.

There are several methods known to reduce the effect of oxygen inhibition.

7.1.1 Physical Methods

1. High irradiance and/or high energy density2. Inerting the exposure atmosphere3. Physical barriers, like wax or protective films

7.1.1.1 High irradiance and high energy density The most applied method for over-coming oxygen inhibition is the use of high irradiance and high energy density in order toproduce a high concentration of radicals, which quench the oxygen effectively and finallyresult in a high curing speed and a tack-free surface. The disadvantage associated withthis method is the multiple overexposure of the coating compared with the energy densityneeded to cure the bulk of the coating.

The effect of high irradiance on the double bond conversion of a urethane acrylate for-mulation is shown in Figure 7.3. The influence of the irradiance on the conversion of aurethane acrylate formulation under air shows, that the total conversion improves signif-icantly when the irradiance is increased from 15 to 90 mW/cm2. A comparison of theeffect of irradiance of curing under inert conditions (carbon dioxide) with air is given inFigure 9.41.


FIG. 7.3. Conversion as a function of irradiance.

7.1.1.2 Inerting the exposure atmosphere The detrimental effects of atmospheric oxy-gen can be successfully overcome by inerting the exposure atmosphere.10 It does not playany significant role, which type of inerting medium will be applied, either nitrogen, argon,carbon dioxide or other inert gases. The comparison of nitrogen and carbon dioxide, as wellas the different influence factors, like oxygen content, sample temperature, monomer vis-cosity, film thickness, type and concentration of photoinitiator, monomer reactivity, lightirradiance have been evaluated, with special emphasis on carbon dioxide as the inert at-mosphere are described in more detail in Chapter 9.2.1 (see refs. 5,6).

Carbon dioxide has the advantages over nitrogen that it is:

• easily available and cheaper than nitrogen,• heavier than air and can therefore be maintained in a container without much loss.

In recent years, the use of inert gas was extended to several applications, especially infoil coating and in the printing sector.11 The motivation behind this was to achieve fastercuring, to reduce the photoinitiator content, to reduce the number of lamps and to improvethe quality. The cost for producing the inert atmosphere in belt systems is offset by savingsin the plants and for the photoinitiators, so that this scenario may be worthwhile evenindependently of the improvement in quality.12

The influencing factors on the coating performance by working under carbon dioxideatmosphere and application examples are discussed in Chapter 9.

7.1.1.3 Physical barriers Simple but not widely usable possibilities for preventing theadmission of oxygen include the use of floating waxes, as used in the 1960s for UV curablestyrene/UP resin systems in combination with relatively low-power 30 W/cm lamps.13 Theeffect is demonstrated in Figure 7.2.14 Furthermore, inerting can be achieved by coveringthe coated substrate with a transparent protective foil,15 an evident example is realized inthe UV curing of adhesive through a transparent foil.


7.1.2 Chemical Methods

1. Amine synergists2. High photoinitiator (PI) concentrations and PI type3. Acrylate monomer structure4. High reactivity5. Formulation viscosity6. Oxygen scavenging by a dye sensitizer7. Other additives

Hoyle has recently published an overview covering several of these methods to reducethe detrimental effects of oxygen inhibition on photocuring.16 He also demonstrated theeffect of various additives on the polymerization exotherms by polymerizing acrylates to-gether with additives in a photo-DSC (differential scanning calorimetry) in air with andwithout the additive in comparison to nitrogen atmosphere.

7.1.2.1 Amine synergists A long and well-known method for overcoming the oxygeninhibition is the addition of amines to the formulation, either as additives or chemicallybound to acrylates via Michael addition.17 The proposed mechanism of the amine-co-synergist has been discussed in Chapter 4 (Figure 4.23).

The main effect is the oxygen scavenging reaction of the amine, based on the good hy-drogen atom donor properties of the C–H-group adjacent to the nitrogen. The once formedC-centered radical, produced by hydrogen abstraction of a photoinitiator or propagatingacrylate radical, either can scavenge an oxygen molecule to form a peroxy radical, whichitself can further abstract a hydrogen from another amine, or initiate the polymerizationdirectly. Despite the excellent oxygen scavenging properties of the amines, they exhibitsome distinct disadvantages, like yellowing of the coating, poor weatherability, plasticiz-ing effect, which limit their general usage.

7.1.2.2 High photoinitiator concentrations Since oxygen reacts readily with photoini-tiator or propagating radicals, a high concentration of radicals in the system consumesoxygen and prevents diffusion into deeper layers. The effect of the photoinitiator concen-tration on the polymerization kinetics is given in Figure 9.41 as a comparison of the effectof curing under air and inert atmosphere. Furthermore, of course, the quantum yield of theradical formation is dependent on the photoinitiator type (see Chapter 4.1.4).

7.1.2.3 Acrylate monomer structure and viscosity Effective monomer structures in or-der to reduce the effect of oxygen inhibition are based on considerations to provide eitherlabile hydrogen atoms as discussed in the case of amines, or to increase viscosity.

As discussed in Chapter 2, the type and structure of the resins and diluents have tobe chosen by the application requirements, however, if the requirements allow the use ofethylene or propylene glycol or their thioether analogues to be used, the oxygen inhibitioneffect is significantly reduced.18 The proposed mechanism is similar to the mechanismdemonstrated in Figure 4.23, however with the substitution of the –N–CH– group by the–O–CH– counterpart.


7.1.2.4 High reactivity The reactivity of the monomers or oligomers governs the curespeed and therefore the reaction time during which oxygen can penetrate into the sam-ple. However, photopolymerization will not occur if the diffusion of oxygen is very high(e.g., low viscosity), even if the reactivity of the resin is very high, until the concentra-tion of the oxygen dissolved in the sample has dropped by two orders of magnitude,19 andthe monomer can compete successfully with the oxygen for the addition to the initiatingradicals.

7.1.2.5 Formulation viscosity The rate of oxygen diffusion into the un-cured liquid filmis significantly determined by the viscosity of the UV-curable formulation.20 The effect ofthe viscosity has been demonstrated by comparing the polymerization rates of films of anurethane resin (Laromer LR 8987) as a function of temperature, which changes dramati-cally the viscosity. The film thickness of 5 µm has been chosen in a range where the oxygeninhibition is most pronounced. An increase in the temperature causes a decrease in formu-lation viscosity. At −19 ◦C, where the viscosity is rather high the polymerization rate (per[M]) is on the order of 1, and drops continuously to 0 by increasing the curing temper-ature over 6, 25, 50 ◦C to 80 ◦C. This behaviour is plotted in Figure 9.42 in comparisonto the curing under carbon dioxide. The behaviour under air is due to decreased viscosityand therefore enhanced oxygen diffusion, leading to reduced polymerization rates. Similarresults were obtained by photo-DSC evaluations.21

7.1.2.6 Oxygen scavenging by a dye The conversion of dissolved oxygen into singletoxygen in the presence of a dye sensitizer and the scavenging of the singlet oxygen by1,3-diphenylisobenzo-furan to generate 1,2-dibenzoyl-benzene, which can work as a pho-toinitiator has been described.22 However, associated with the use of this dye, the coatingis slightly coloured, and therefore this approach has not been used extensively, probablydue to the limited application range for slightly coloured coatings.

Recently, a novel system of singlet oxygen generator (zinc 2,9,16,23-tetra-tert-butyl-29H ,31H -phthalocyanine (Zn-ttp)) and singlet oxygen scavenger (dimethylanthracene(DMA)), has been published.23 The combination of Zn-ttp/DMA and pre-illumination caneffectively consume the molecular oxygen dissolved in the system. As a result, the inhibi-tion period was significantly reduced and the rate of polymerization increased dramatically.

7.1.3 Conclusions

As discussed in this chapter, there are several possibilities to overcome the detrimentaleffects of oxygen inhibition, especially at the surface of an UV-cured coating. The mostconvenient way for working in an air atmosphere is to compensate the oxygen inhibitionreaction by high irradiance and energy density, by selection of efficient photoinitiators anduse of high photoinitiator concentrations, by selection of highly reactive formulations orby incorporation of additives, like amines or waxes. Each of these methods has its ownadvantages and disadvantages.

The very simple, yet, more expensive way is the inerting of the curing atmosphere.There are exposure units available where nitrogen is cycled around the exposure set-up.


FIG. 7.4. Possibilities to cure in shadow areas.

Well known, however, only recently further developed is the inerting with carbon dioxide.The influencing factors on the curing conversion in the presence of carbon dioxide as theinerting medium are discussed in Chapter 9. Since carbon dioxide is heavier than air, a newprocess called Larolux® has been introduced, where the exposure can be performed in acarbon dioxide pool with very low-irradiance lamps. This process, as well as the curingunder vacuum conditions (UV plasma curing), which can favourably be used for three-dimensional curing, will also be described in more detail in the Chapter 9.

7.2 SHADOW AREAS

“The brighter the light, the deeper the shadow” and “every cloud has a silver lining”. Theseworldly wisdoms also hold true for UV curing. The curing of relatively complex three-dimensional objects is non-trivial. Such objects may appear for instance in car bodies orcar doors, exhibiting hollow regions, the inside of which can hardly be reached by UVradiation and will therefore remain uncured. To resolve this issue, several approaches havebeen published:

• Light tunnels with lamps illuminating the object from all sides• Placing lamps on robots

This approach has been pursued in several evaluations (see Figure 2.17).

• Curing under inert conditions using reflective walls

The curing under inert conditions using reflective (aluminium) walls will be describedin Chapter 9 (Larolux® process).


• Dual cure systems:• UV and thermal,• UV and oxidative drying.

Dual cure systems combining UV-radiation with thermal curing have recently beendeveloped.24–26 They contain two types of functional groups: usually UV curable acry-late double bonds, and thermally curable groups, for example, polyol and isocyanate ormelamine-amino functionalities. The basic principle of these chemistries is depicted inFigure 7.4. There are several possibilities to design such systems. The simplest approach ofjust combining the individual molecules of multifunctional acrylates, polyols and polyiso-cyanates for example, results in the formation of interpenetrating networks, which are ofteninhomogeneous. This is due to a phase separation of the independently formed relativelyunpolar polyacrylate network and the relatively polar polyurethane network. Thereforemolecules have been designed which combine the different functionalities in one mole-cule, like acrylate double bonds and isocyanates and on the other hand hydroxyl groupsand acrylate double bonds. The newly developed chemistries are discussed in more detailin Chapter 9.

The dual curing approach can also be transferred to water-based systems.27 Furthermore,dual curing can also be obtained with one type of chemistry, but different initiator types,namely a photoinduced radical initiator and a thermal activatable peroxide. With theseinitiators pure acrylate based coating systems can be used for dual curing.

• UV plasma curing

As shown in Fig. 7.4, the UV plasma curing is as well as the Larolux® process a novelinnovative concept of “inert” curing, which is based on the UV curing of (preferably three-dimensional) parts in an evacuated plasma chamber. The process is described in more detailin Chapter 9.

7.3 INITIAL PHOTOYELLOWING28

Certain effects that are unique to radiation curing hamper the development of new UVcoating applications. One such effect is initial photoyellowing that develops in the coatingdirectly after exposure to ultraviolet (UV) or electron beam (EB) radiation (Figure 7.5),which has to be distinguished from the yellowing occurring (with almost all coatings) dur-ing aging and weathering. This initial photoyellowing is at least in part reversible and will,therefore bleach to some extent in the first few hours after the exposure. This variabilityin the colour of the coating makes exact colour matching very difficult until several hoursafter the radiation curing step, which, in turn, makes an inline quality control difficult.To expand the use of radiation curing into new applications this effect needs to be betterunderstood.

There are several influencing factors, which have been evaluated (ref. 28). The initial yel-lowing resulting from the curing reaction diminishes in the hours after exposure, the ratedepending on the storage conditions (Figure 7.6). The bleaching rate is faster the higherthe storage temperature. The yellowing development is not exclusively dependent on thepresence of a photoinitiator, but rather due to the retarded decay of the formed radicals.


FIG. 7.5. Different stages of photoyellowing (due to photocuring and photoaging, respectively).

FIG. 7.6. Initial photoyellowness decay as a function of storage conditions.

The effect of the photoinitiator on initial photoyellowing is shown in Figure 7.7, where thesame resin is exposed to e-beam radiation in the absence of a photoinitiator, and to UVin the presence of an α-hydroxy alkyl acetophenone photoinitiator. The absolute values ofyellowing (b∗) are a function of the e-beam dose (7 mrad, 6.8; 3 mrad, 4.0; 15 mrad, 11)or the photoinitiator concentration. Attempts at measuring the initial photoyellowing withvarious concentrations of photoinitiator were realized. As expected, the higher the concen-tration, the more coloured the film becomes up to a certain limit, after which the initialyellowing plateaus or even starts to decline again. A similar dependence of the photoini-tiator concentration on the reactivity of UV-printing inks has also been observed.29 Thiseffect is not well understood, however, it is possible that at high photoinitiator concentra-tions and therefore high radical concentrations termination reactions (radical combination)could remove radicals from the system or shorten the average chain length of the polymer.This effect in turn would yield less long-lived radicals in the polymer matrix. Therefore,


FIG. 7.7. Comparing initial photoyellowness and yellowness decay of e-beam exposed with UV exposed resin.

FIG. 7.8. Photoproducts of the decomposition of irgacure® 184 photoinitiator.

it is not the presence of photoinitiators per se which is responsible for the initial yellowing,but rather, the initial photoyellowing seems to be an inherent effect of the radical concen-tration.

The GC-MS spectrum of a cured solution of Irgacure® 184 in methanol showed the pres-ence of a variety of scission products (Figure 7.8): benzene, cyclohexanol, cyclohexanone,benzaldehyde, 2-hydroxycyclohexanone, 1,1-dimethoxy-cyclohexane, methyl benzoate,2-hydroxy-1-phenylethanone, Irgacure® 184, 2-diphenylethanone, benzoic acid, benzil,


FIG. 7.9. Initial photoyellowing due to photoinitiator type in UA resin laromer® LR8987 under nitrogen.

o-benzoylbenzoin. It is conceivable that many of these by-products could lead to yellowingupon undergoing secondary reactions such as oxidation or condensation. However, noneof the fragments found can directly account for the initial photoyellowing phenomenon.Similar experiments were performed for other α-hydroxy alkyl acetophenones: Darocur®

1173 and Irgacure® 2959 were found to behave very similarly to Irgacure® 184. Con-sequently, all the α-hydroxy alkyl acetophenone photoinitiators, which have been inves-tigated, also showed similar initial photoyellowing values. Acylphosphine oxides wereintroduced in radiation curable systems more than a decade ago by BASF and morerecently by Ciba. 2,4,6-Trimethylbenzoyldiphenylphosphine oxide (Lucirin® TPO) and2,4,6-trimethylbenzoylethoxyphenylphosphine oxide (Lucirin® TPO-L) are commonlyused for pigmented systems or systems with high film thickness. Despite being yellow,these photoinitiators display photobleaching upon irradiation due to the destruction of thevisible-light chromophore group of TPO. As the absorption of the photoinitiator bleaches,UV or visible light may penetrate deeper and deeper into the film. The initial photoyel-lowing of these photoinitiators is similar to the initial photoyellowing of α-hydroxy alkylacetophenones (see Figure 7.9). However, the initial discoloration presented a slightly redcoloration instead of the typical yellow coloration. Furthermore, combinations of Lucirin®

TPO with other Photoinitiators may display synergistic effects and therefore show less ini-tial photoyellowing than would be expected by simply adding the contributions from eachphotoinitiator. In order to avoid the formation of the yellow-coloured benzil, two otherclasses of photoinitiators have been tested. These are photoinitiators that undergo a pri-mary process of hydrogen atom abstraction from the environment (or an intramolecularH-abstraction) and onium salt photoinitiators, which are primarily used for cationic poly-merisation, but may also be used to initiate radical reactions. Benzophenone presents thedisadvantage of developing significant discoloration when exposed to sunlight. Moreover,the yellowing during ageing becomes even worse if amines are used as co-initiator. For-tunately, some resins may also function as H-donors, so that the use of amines may beavoided.

Despite the poor long-term performance of benzophenone yellowing, it shows onlya very slight initial photoyellowing (see Figure 7.9). No benzil was found in GC-MS


analysis of a cured solution of benzophenone in propan-2-ol, which further confirmedthat no α-cleavages took place. It has been reported30 that the initial photoyellowingof isopropyl-thioxanthone (ITX) with N-methyldiethanolamine as co-initiator is due tophotoproducts from ITX. Unfortunately, ITX already exhibits a yellow coloration andshows an even stronger initial photoyellowing. For these reasons thioxanthones are usu-ally only used in pigmented systems such as screen inks.31 Similar to benzil dimethylketal (BDK), the initial photoyellowing for ITX decreases in the presence of amines whenN,N-dimethylethanolamine is added as co-initiator. Phenylglyoxylates have, unfortunately,only been the subject of a limited number of studies probably because of its non-trivialphotochemistry. It has been claimed32 that these molecules undergo an intramolecular H-abstraction followed by a fragmentation similar to a Norrish type II. It may, however, befeasible that this structure could yield a Norrish type I reaction. Initial photoyellowingresults were excellent with this initiator. However, the initial photoyellowing of this pho-toinitiator depends on the conditions under which the curing takes place. The initial pho-toyellowing was very slight with a urethane acrylate under air. However, the initial pho-toyellowing was significant under inert conditions. Furthermore, the real time-IR spectrashowed that the polymerisation realized under inert conditions achieved a high conversionof 71%, whereas, the reaction under air only reached a conversion of 44% (in thin films(film thickness ca. 10 µm)). The presence of benzaldehyde found in the GC-MS spectrumof a photolyzed solution of methyl phenylglyoxylate (Nuvopol® PI 3000) suggests that(apart from H-abstraction) a reaction type Norrish I may also take place.

Segurola et al.33 drew similar conclusions by evaluating the photoyellowing and discol-oration of the photoinitiator types, summarizing that 1-hydroxy-cyclohexyl-phenyl ketone(Irgacure 184) exhibited very little discoloration and gave the best results of all photoinitia-tors studied and from the near UV absorbing photoinitiators Lucirin TPO (acyl phosphineoxides) caused lower yellowing compared to α-aminoalkyl ketone photoinitiators due tophotobleaching.

Amines are usually used to improve the reactivity of a system or as hydrogen donors.However, as systems containing amines present a strong discoloration when exposed tohigh temperatures it is advantageous to avoid their use in formulations. However, aminescan also have a positive influence on the initial discoloration; as mentioned above, BDKshows a lower initial photoyellowing in the presence of amines, as does Nuvopol® PI 3000.It has been claimed34 that the reduction of the initial photoyellowing in the presence ofamines with BDK could be explained by the photoreduction of the carbonyl chromophoreto carbinol compounds through intermolecular hydrogen abstraction. However, in the caseof most Norrish Type I photoinitiators the presence of amines leads to significantly strongerinitial photoyellowing.

Triarylsulfonium salts are commonly used as cationic photoinitiators. Cyracure® UVI-6990 is a mixture of sulfonium salts. In the unexposed form it already exhibits a slightred colour. The acid obtained with this photoinitiator after curing leads to a very violentreaction with some vinylethers. However, it is suitable for initiating a polymerisation withepoxides. As in the case of iodonium salts urethane acrylates could be polymerised rad-ically too. After UV curing a film of the aliphatic epoxide Basoset® 162 and a film ofLaromer® LR 8987 with Cyracure UVI-6990 as photoinitiator, the films showed no in-creased initial photoyellowness. However, a slight red colour was observed. This initial


discoloration may be a result of (i) coloured by-products which are formed from radi-cals or (ii) side reactions of the super acid HPF6 with the radiation curable resin (e.g.,condensation reactions or rearrangements). Dektar and co-workers have already studiedthe photoproducts resulting from the photolysis of triphenylsulfonium salts by GC-MS.35

Cyracure UVI-6990 is expected to yield additional photolysis products as a result of beinga mixture and because of its more complex structure.

The initial photoyellowing may depend on the quantity of photoinitiator which is pho-tolyzed and thus on the quantity of light received by the sample. The more the sample isirradiated, the more yellow the film is. This observation is true for low irradiation doses,however, at higher doses this simple correlation no longer holds – in fact it reverses. It isfeasible that at very high irradiation doses, the coloured photoproducts, which are them-selves sensitive to UV-light, are further photolyzed. Also at higher radical concentrations,radical combination reactions become more favoured, thereby removing reactive speciesfrom the system. This is also reflected in the dependence of the photoyellowing on the pho-toinitiator content (Figure 7.10). The photoyellowing goes through a maximum, indicatingthat with higher photoinitiator contents more radicals are produced, however, under theseconditions more radical termination reactions also become more likely.

The binder seems to play an important role in the initial photoyellowing. Four classesof radiation curable resins have been tested: (i) urethane acrylates, UAs (aliphaticpolyurethane acrylates are the most suitable for industrial outdoor applications), (ii) oli-goether acrylates, POs, (and amine modified oligoether acrylates, POAs), (iii) epoxy acry-lates, EAs, and (iv) polyester acrylates, PEs. Using Irgacure® 184 as photoinitiator, differ-ent behaviours were observed between the different binders, which suggest that the initialphotoyellowing depends on both the photoinitiator and the matrix it is in. The best initialphotoyellowing result was obtained with the aliphatic urethane acrylate Laromer® UA19T(see Figure 7.11). Tripropylene glycol diacrylate (TPGDA) and diethylene glycol diacry-late (DEGDA) gave better results, however, TPGDA and DEGDA are very rarely used assole binders in a formulation.

FIG. 7.10. Influence of the content of the photoinitiator on the initial photoyellowing.


FIG. 7.11. Initial photoyellowing of different (Laromer®) acrylate resins types exposed under air and nitrogenatmosphere.

In most resin classes a slight trend is detectable favouring either air or inert conditionsfor curing, e.g., aromatic epoxy acrylates (EAs) show less initial photoyellowing undernitrogen, while UV monomers show less initial yellowing under air. These differences aregenerally considered to be too small to be significant for normal applications. Furthermore,the data also show that amine modified polyether acrylates (POAs) exhibit a stronger initialdiscoloration than the polyether acrylates (POs). UAs and POs yielded the best results.Investigations employing Lucirin TPO as the photoinitiator gave very similar results to theresults obtained with Irgacure® 184.

Further experiments investigating the reactivity, absorption and viscosity of different ra-diation curable resins showed that there was no direct correlation between these propertiesand the observed initial photoyellowing. It is worth noting that in many cases the extent ofinitial photoyellowing does not correlate with long-term yellowing from artificial weather-ing or ageing experiments. On the contrary, the systems known for their low yellowing inageing experiments (urethane acrylates) seem to be particularly prone to initial photoyel-lowing.

7.3.1 Conclusions

Thus, both the photoinitiator and the binder play an important role for the initial photoyel-lowing of radiation cured coatings. Except for BDK, photoinitiators undergoing photoscis-sions exhibit very similar initial photoyellowing results – qualitatively and quantitatively– presumably because they all have the benzoyl radical in common. With the exceptionof ITX, which exhibits a very strong discoloration after UV exposure, photoinitiators thatundergo H-abstraction present low levels of initial photoyellowing. However, it is thesephotoinitiators that present yellowing upon ageing, especially, benzophenone and its deriv-atives. Cationic onium photoinitiators show only a relatively slight initial photoyellowing;


FIG. 7.12. Scheme for explanation of initial photoyellowness in radical initiated coatings.

their low solubility as well as the reddish colouring of the cured films make these cationicphotoinitiators suitable for only a small number of applications. The complete absence ofphotoinitiators in the case of electron beam curing does not solve the problem of the initialphotoyellowing effect either. On the contrary, electron beam curing has led to relativelystrong initial photo-yellowing effects.

Binders show varying trends in their initial photoyellowing depending on their chemicalcomposition or resin class. Stabilisers, amines and the atmosphere during curing influ-ence the strength of the initial photoyellowing to some extent and may allow incrementalmodifications of the initial photoyellowing. However, these differences in initial photoyel-lowing are minor in comparison with the contribution from the combination of resin andphotoinitiator used in a formulation. The chemical nature of the polymer network as wellas it’s density seems to be strong influencing factors regarding initial photoyellowing. Bothelectron beam and thermal curing also show strong initial yellowing which suggests thatthe yellowing is inherent to these highly crosslinked polymers. It is conceivable that thecoloration is due to radicals or highly reactive species that are trapped in the polymer ma-trix and therefore have a significant lifetime of several hours to weeks36 (Figure 7.12).Any modification of the conditions that increases the mobility (i.e., delay vitrification) ofthese reactive centres (temperature, solvents) will reduce initial photoyellowing. Further-more, reducing the number of radical species also reduces the observed photoyellowing(photoinitiator, energy density, stabilizers). Unfortunately, the phenomenon of initial pho-toyellowing is very complex and there is no universal solution. Therefore, curing condi-tions and raw materials need to be carefully examined for each application to reduce initialphotoyellowing to a minimum.

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