Norman S. Allen

Free radical macrophotoinitiators: an overview on recent advances

This paper provides an overview of the recent advances on polymeric photoinitiators for UV curing. During the last decade, significant developments have been achieved in the synthesis of macrophotoinitiators, due to the advantages derived of their macromolecular nature, in comparison with their corresponding low molecular weight analogues. In particular, a variety of macromolecules containing the two main types of free radical photoinitiators: hydrogen-abstracting (thioxanthone, benzil, anthraquinone, camphorquinone) and photofragmenting chromophores (benzoin ether, acylphosphine oxides) are described. For hydrogen-abstracting photoinitiator, the photoinitiation activity have been examined in terms of volume and nature of substituent in the polymeric coil, and their influence to prevent the recombination of radicals favouring their reaction with the monomer. Also, copolymers bearing chromophore and amine groups with potential synergistic effects of activity are reported. It has been found that the approach of the tertiary amine to the chromophore to produce the corresponding exciplexes is dependent on both the monomeric or polymeric nature of chromophore and the tertiary amine. Type II polymeric photoinitiators, such as benzoin ether derivatives having the benzoin methyl ether moieties connected to the main chain through the benzyl aromatic are reported. And a fragmentation mechanism involving the formation of an stable quinoid structure and aliphatic acyl radical is proposed for the above copolymers, which would justified their lower initiating efficiency than the corresponding low molecular weight model. In addition, polymers bearing phosphine oxide moieties are described. The efficiency in the polymerisation of all photoinitiators was found to be similar and irrespective of the presence of flexible spacer in their structure. However, it was found that the flexible oligomethylene spacer enhanced the compatibility of the new polymeric photoinitiators in acrylic adhesive formulations. Finally, polysilanes as photoinitiators are reported. Under UV irradiation, polysilanes undergo main-chain scission leading to free silyl radicals capable of reacting with olefinic monomers. The silyl radicals generated by photolysis can be oxidised by appropriate onium salts to yield cationic initiating species (photoinitiated radical promoted cationic polymerisation). The photoinitiation efficiency of polysilanes having different aliphatic and aromatic side groups has been investigated and compared with commercial low molecular weight photoinitiators as benzoin.

Photoinitiators for UV and visible curing of coatings : Mechanisms and properties

Abstract The radiation curing industry is one of the most rapidly developing fields in the entire coatings industry. The low toxicity, cheapness, speed, control and ease of formulation and operation are some of the main advantages of this growing technology. UV and/or visible light radiation is used to induce photochemical polymerization or crosslinking of a monomer, oligomer or prepolymer formulation containing a certain type of unsaturation, such as an acrylic group, and an appropriate initiator. The latter is used to absorb the light energy and transform it into active species, such as radicals or ions, capable of inducing such reactions. Applications extend to general coatings for paper, board, wood, tapes, compact discs and holograms, inks, photoresists for imaging processes and adhesives for welding and sealing in electronic circuit boards. The photoinitiator is the key to the control of these processes and, in recent years, has seen many new developments. These include the need for water-soluble, co-reactive and polymeric structures with low migration rates, as well as cheaper UV/ visible sensitizers with enhanced speed. New and effective cationic systems are also on the scene and, although expensive, are attracting significant academic and commercial interest.

Behaviour of nanoparticle (ultrafine) titanium dioxide pigments and stabilisers on the photooxidative stability of water based acrylic and isocyanate based acrylic coatings

A series of nano-particle grade anatase and rutile titanium dioxide pigments have been prepared with various densities of surface treatments, particle size and surface area. Their photochemical activites have been determined and compared in water based acrylic and isocyanate acrylic coatings with typical benzophenone and hindered piperidine light stabilisers. Their performance on wood and aluminium substrates is assessed by FTIR, colour and gloss change and mass loss following artifical weathering. UV absorption analysis of benzophenone and benzotriazole chromophore based UV absorbers shows that they absorb more strongly in the near UV below 350 nm. However, nanoparticle rutile absorbs more strongly above this range and therefore, operates as a strong opacifier. Anatase also possesses opacifying behaviour, but to a lesser degree than rutile above 380 nm. In water based acrylics the absorbers undergo decomposition during irradiation whereas the nanoparticles, by virtue of their inorganic nature are inherently stable. Mass loss experiments indicated that anatase is a photosensitiser, though the intensity of the effect was found to be dependent upon the nature of the coating. Rutile was found to be an effective stabiliser with performance greater than or equal to the organic absorbers and HALS. Combinations of anatase HALS were found to be antagonistic. Little or no synergy was observed between rutile and HALS. The outstanding performance of both anatase and rutile nanoparticles is visibly and colourimetrically evident on clear acrylic wood coatings with rutile being the more effective. The 70 nm particles are more effective than the 90 nm particles. However, this data for anatase is in marked contrast to the chemical changes from FTIR analysis. Here the difference may be due to the colour bleaching effect on the lignin products through the photosensitising activity of the anatase. From a commercial point-of-view coated nanoparticles offer a significant opportunity for cost-effective benefits over conventional organic absorbers and HALS for the photoprotection of acrylic coating systems.

Photocatalytic Coatings for Environmental Applications

Abstract A series of nano- and micronparticle-grade anatase and rutile titanium dioxide pigments have been prepared with various densities of surface treatments, particle size and surface area. Their photocatalytic activites have been determined in a series of paint films by FTIR, chalking, color, gloss change and weight loss after artifical weathering. The pigments have also been examined by rapid assessment methodologies using photodielectric microwave spectroscopy, 2-propanol oxidation and hydroxyl analysis. The microwave response under light and dark cycles provides an extended timescale probe of charge-carrier dynamics in the pigments. Pigment particle size, surface area and properties clearly play an important role in dispersion and any polymer–pigment interactions. Photooxidation studies on several types of paint films show a clear demarcation between nanoparticle- and pigmentary-grade titanium dioxide, with the former being more active because of their greater degree of catalytic surface activity. The photosensitivity of titanium dioxide is considered to arise from localized sites on the crystal surface (i.e. acidic OH), and occupation of these sites by surface treatments inhibits photoreduction of the pigment by ultraviolet radiation; hence, the destructive oxidation of the binder is inhibited. Coatings containing 2–5% by weight alumina or alumina and silica are satisfactory for general-purpose paints. If greater resistance to weathering is desired, the pigments are coated more heavily to about 7–10% weight. The coating can consist of a combination of several materials, e.g. alumina, silica, zirconia, aluminum phosphates of other metals. For example, the presence of hydrous alumina particles lowers van der Waals forces between pigments particles by several orders of magnitude, decreasing particle–particle attractions. Hydrous aluminum oxide phases appear to improve dispersibility more effectively than most of the other hydroxides and oxides. Coated nanoparticles are shown to exhibit effective light stabilization in various water- and oil-based paint media in comparison with conventional organic stabilizers. Hindered piperidine stabilizers are shown to provide no additional benefits in this regard, often exhibiting strong antagonism. The use of photocatalytic titania nanoparticles in the development of self-cleaning paints and microbiological surfaces is also demonstrated in this study. In the former case, surface erosion is shown to be controlled by varying the ratio of admixture of durable pigmentary-grade rutile (heavily coated) and a catalytic-grade anatase nanoparticle. For environmental applications in the development of coatings for destroying atmospheric pollutants such as nitrogen oxide gases (NOX), stable substrates are developed with photocatalytic nanoparticle-grade anatase. In this study, porosity of the coatings through calcium carbonate doping is shown to be crucial in the control of the effective destruction of atmospheric NOX gases. For the development of microbiological substrates for the destruction of harmful bacteria, effective nanoparticle anatase titania is shown to be important, with hydrated high surface area particles giving the greatest activity.

Aspects of the thermal oxidation of ethylene vinyl acetate copolymer

Abstract The thermal oxidation of ethylene-vinyl acetate copolymer [EVA-17 and 28% w/w VA (vinyl acetate) units] has been examined by thermo-gravimetric and hydroperoxide analysis, FTIR (Fourier transform infra-red) fluorescence spectroscopy and yellowness index. Thermal analysis indicates the initial loss of acetic acid followed by oxidation and breakdown of the main chain. The degradation rate is greater in an oxygen atmosphere as is the formation of coloured products. FTIR spectroscopic analysis of the oxidised EVA shows evidence for de-acetylation followed by the concurrent formation of hydroxyl/hydroperoxide species, ketone groups, α,β-unsaturated carbonyl groups, conjugated dienes, lactones and various substituted vinyl types. Hydroperoxide evolution follows typical autooxidation kinetics forming ketonic species. In severely oxidised EVA evidence is given for the subsequent formation of anhydride groups. The initial fluorescence excitation and emission spectra of EVA is not unlike that reported for polyolefins confirming the presence of low levels of unsaturated carbonyl species. There are however, significant differences in a long wavelength component in the fluorescence emission indicating the presence of other active chromophores. These long wavelength emitting components grow in intensity and shift to longer wavelengths with ageing time. However, unlike studies on PVC these emission spectra are limited due to the vinyl polyconjugation lengths and tend to be consistent with the formation of specific degraded units, possibly polyunsaturated carbonyl species of a limited length confined to the EVA blocks. During oxidation of EVA the original unsaturated carbonyl species remain as distinct emitting chromophores. This suggests that the growth and decay of these chromphores is virtually constant indicating that they could be an integral part of the EVA polymer that are responsible for inducing degradation. Degradation is limited to the vinyl acetate moieties where hydroperoxides can lead to the formation of polyconjugated carbonyl groups. The EVA degradation is therefore, different from that of PVC where in the latter case polyconjugated vinyl groups are evident through conjugated absorption bands in the UV spectrum. In the case of degraded EVA no such bands are observed. Also, degraded coloured EVA is not bleached by treatment with bromine, maleic anhydride or peracetic acid. Primary phenolic antioxidants exhibit variable activity in inhibiting the yellowing of EVA while combinations with phosphites generally display powerful synergism.

Design of eutectic photoinitiator blends for UV/visible curable acrylated printing inks and coatings

Abstract The optimisation of photoinitiator systems used in UV printing inks and coatings has been carried out by means of Design of Experiments (Mixture designs). Mixture designs have been used to improve the reactivity of photoinitiator blends in order to design cost effective, synergistic, near eutectic photoinitiator blends for curing UV printing inks. Both the reactivity and bulk stability of photoinitiator blends have been studied. The photoinitiator blends consisted of three commercial photoinitiators, benzophenone (BP), isopropyl thioxanthone (ITX), which are Type II photoinitiators and benzil dimethyl ketal (BDK) a Type I photoinitiator. Ethyldimethylamino benzoate (EPD), a commercial aromatic tertiary amine, which is an efficient electron/proton donor was used to enhance the reactivity of the Type II photoinitiators. The results highlighted that the EPD content plays a crucial role in the final reactivity of the photoinitiator blends. Relatively high proportions of EPD with respect to the overall centroid (25% each component) were required to design photoinitiator blends of enhanced reactivity. These results agreed with mechanistic studies published in the literature where it was observed that with Type II photoinitiators, such as BP and ITX, the tertiary amine synergist acted as the main initiating radicals of the polymerisation process. On the other hand, from the stability point of view, high proportions of EPD decreased the stability (eutectic properties) of the photoinitiator blends. BP showed good solvating properties in the photoinitiator blends and had the highest influence on stability of the photoinitiators studied. Relatively high proportions of BP enhanced the stability of photoinitiator blends. Multiple response optimisation for stability and reactivity resulted in photoinitiator blends that exhibited both enhanced reactivity and stability.

Photooxidative and thermal degradation of polyethylenes: interrelationship by chemiluminescence, thermal gravimetric analysis and FTIR data

Abstract The thermal and photooxidative behaviour of polyethylenes of different manufacturing histories (linear low, metallocene and high density) have been investigated by FTIR spectroscopy and the data related to the chemiluminescence (CL) and thermal analysis of the polymers. The CL intensity and activation energies for thermal decomposition were found to be dependent on the rates of oxidation of the polymers under light and heat. On thermal oxidation, the activation energies were found to decrease with time and followed the order HDPE>m-PE>LLDPE, which correlated with the results obtained by means of CL analysis, where a higher intensity of CL at low temperature was found for HDPE. On photooxidation, the order followed their instability, i.e. m-PE>HDPE>LLDPE. The rates of thermal oxidation in each case were found to be the same irrespective of the nature of the oxidation product, i.e. hydroperoxide, vinyl and carbonyl groups. The influence of branching in the polymer appears to be important, showing a decrease of thermal stability as branching in polyethylene increases.

Ageing and stabilisation of filled polymers: an overview

The durability of filled polymers is a scientific and technological problem involving a complex interplay of phenomena and mechanisms many of which are little understood. There are a number of issues that require addressing in this regard. The first relates to the stability of the filler itself, especially if it is a coloured pigment. In this regard organic pigments may fade or change colour to an unacceptable shade. The second involves complex filler-polymer interactions and these, in turn, are further complicated by the nature of the environmental ageing conditions such as temperature, atmosphere, humidity and light source. The third problem relates to filler-stabiliser interactions where chemical and physical adsorption processes or sensitisation reactions can control the nature of both the oxidation and stabilisation processes. In this regard fillers with high surface areas and pore volumes can markedly influence such mechanisms, especially hindered phenolic antioxidants and hindered piperidine light stabilisers (HALS). This paper presents an overview of our current understanding of the complexities of filler-polymer and additive interactions with a major emphasis on carbon black, silica and titanium dioxide pigments.

Aspects of the thermal oxidation, yellowing and stabilisation of ethylene vinyl acetate copolymer

The thermal oxidation of ethylene-vinyl acetate copolymer [EVA-17 and 28% w/w VA (vinyl acetate) units] has been examined by thermo-gravimetric and hydroperoxide analysis, FTIR (Fourier transform infra-red), fluorescence spectroscopy and yellowness index. Thermal analysis indicates the initial loss of acetic acid followed by oxidation and breakdown of the main chain. The degradation rate is greater in an oxygen atmosphere as is the formation of coloured products. FTIR spectroscopic analysis of the oxidised EVA shows evidence for de-acetylation followed by the concurrent formation of hydroxyl/hydroperoxide species, ketone groups, α, β-unsaturated carbonyl groups, conjugated dienes, lactones and various substituted vinyl types. Hydroperoxide evolution follows typical auto-oxidation kinetics forming ketonic species. In severely oxidised EVA, evidence is given for the subsequent formation of anhydride groups. The initial fluorescence excitation and emission spectra of EVA is not unlike that reported for polyolefins confirming the presence of low levels of unsaturated carbonyl species. There are, however, significant differences in a long wavelength component in the fluorescence emission indicating the presence of other active chromophores. These long wavelength emitting components grow in intensity and shift to longer wavelengths with ageing time. However, unlike studies on PVC these emission spectra are limited due to the vinyl polyconjugation lengths and tend to be consistent with the formation of specific degraded units, possibly polyunsaturated carbonyl species of a limited length confined to the EVA blocks. During oxidation of EVA the original unsaturated carbonyl species remain as distinct emitting chromophores. This suggests that the growth and decay of these chromphores is virtually constant indicating that they could be an integral part of the EVA polymer that are responsible for inducing degradation. Degradation is limited to the vinyl acetate moieties where hydroperoxides can lead to the formation of polyconjugated carbonyl groups. The EVA degradation is therefore, different from that of PVC where in the latter case polyconjugated vinyl groups are evident through conjugated absorption bands in the UV spectrum. In the case of degraded EVA no such bands are observed. Also, degraded coloured EVA is not bleached by treatment with bromine, maleic anhydride or peracetic acid. Primary phenolic antioxidants exhibit variable activity in inhibiting the yellowing of EVA while combinations with phosphites and hindered piperidine stabilisers display powerful synergism confirming the importance of hydroperoxides as precursors. Thermal oxidation was also was displayed through the inhibition of lactone, carboxylic acid and alkene groups illustrating the fact that oxidation and yellowing are synonymous reactions.

Photochemical study and photoinitiation activity of macroinitiators based on thioxanthone

In this work we studied the photopolymerization of methyl methacrylate using polymeric initiators based on thioxanthone chromophore. As co-initiator were used low molecular weight tertiary amines and bound to a polymer chain. The photoinitiation efficiency of these systems was compared with that of the corresponding low molecular weight analogue. A higher efficiency was obtained with the system comprising the thioxanthone bound to a polymer chain and the free amine. The results carried out with low molecular weight TX and the polymeric co-initiator showed that the efficiency is independent of the amount of amine units in the polymer chain. The photophysics of the different systems show that the polymer chain does not affect the emission characteristics of the thioxanthone chromophore. Bimolecular rate constants for the singlet and triplet quenching of thioxanthone by the amine were determined using fluorescence and laser flash photolysis techniques. The dependence of the photoinitiation efficiency with the amine concentration was simulated from the measured quenching rate constants, and shows that the active radicals are produced from the interaction of the ketone triplet with the amine. The ketyl radical yield was slowly higher for the thioxanthone bound to the polymer. The difference between the photoinitiation activity of the polymeric system and the low molecular weight models is explained in terms of the effect of the polymer chain on the reactivity of the amino radicals.