Michele Edge

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.

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.

Physicochemical aspects of the environmental degradation of poly(ethylene terephthalate)

The degradation of amorphous poly(ethylene terephthalate) bottle and amorphous sheet materials are investigated under different environmental conditions (wet soil, 100 and 45% relative humidity and UV irradiation) by measuring the rate of chain scission, using viscometric analysis, end-group analysis by FTIR and crystallinity via density measurements at different temperatures. Using the Arrhenius expression the lifetime of the biaxially oriented polyester bottle material and activation energy for degradation are found to be very dependent upon the environmental conditions, with hydrolysis being a dominant process. Negligible degradation is observed at temperatures below the glass transition (c. 80°C) in dry conditions. From density measurements at 45 and 100% relative humidity the crystallinity exhibits an initial facile increase only at temperatures about 70°C, due to plasticisation by the moisture and annealing, followed by an inflection which increases from 32 to 35% with the severity of the degradation conditions. This inflection is consistent under all degradation conditions at 0·5 of a chain scission and is then followed by a much slower rate due to combined hydrolytic degradation/oxidation of the polymer chains. The initial rapid increase is found to be faster at lower rates of chain scission due to an annealing/plasticisation effect by the moisture. This is confirmed by the fact that, under both dry conditions and UV irradiation, where significant rates of chain scission are observed, especially at high temperatures and prolonged times (>500 days), the crystallinity shows no significant increase. Hydrolytic degradation on thermal ageing is confirmed using end-group analysis, which shows that hydroxyl and carboxyl rates are synonymous and increase significantly only under high humidities and temperatures above the Tg. Upon UV exposure, however, chain scission is accelerated in the presence of wet soil with the rate of carboxyl formation exceeding that of the hydroxyl group. The latter is associated with the importance of a Norrish Type II intramolecular hydrogen atom abstraction and unzipping mechanism. Metal ion contents in the polyester film material are found to vary significantly with the ageing condition. Thus, whilst antimony appears to be extracted the copper content is enhanced significantly, which may contribute to the accelerated degradation of the polyester. The implications of these results in terms of the ageing of bottle material are discussed.

The enhanced performance of biocidal additives in paints and coatings

The addition of film biocides to coatings is necessary to prevent microbial spoilage. The biocides must be mobile so that they can migrate to the coating interface and across the cell membrane to destroy microbes. Unfortunately, concurrent losses of biocide by aqueous extraction require the addition of relatively high initial levels. This presents problems since biocides are fundamentally toxic and at such increased levels they pose a risk to the surrounding ecosystem. Legislative directives currently in place aim to reduce the amount of biocide released to the environment. This study has shown that typical coating biocides can be encapsulated within modified silica frameworks. These porous frameworks offer a means to inhibit the aqueous extraction of the biocide. In such combinations the biocides retain their anti-microbial properties, while controlled delivery facilitates a dynamic equilibrium to maintain a minimum inhibitory concentration at the coating interface, over an extended time period. There is evidence that biocide housed in such frameworks has a longer effective activity for a given initial concentration, since it is to some extent protected from the usual environmental degradation processes.

Stabiliser interactions in the thermal and photooxidation of titanium dioxide pigmented polypropylene films

The thermoxidative (oven ageing) and photooxidative degradation of polypropylene films (200 μm thick) containing a range of antioxidants and light stabilisers, together with anatase and rutile titanium dioxide pigments are studied by Fourier Transform infra-red (FTIR) spectroscopy. Rates of thermal and photooxidative degradation are determined by measuring the formation of non-volatile carbonyl and hydroperoxide oxidation products which absorb in the infra-red region of the spectrum with maxima at 1710 and 3410cm−1, respectively. During photooxidation the rutile pigment is synergistic in stabilisation with phenolic antioxidants and hindered piperidine stabilisers (HAS) but antagonistic with benzotriazole and benzophenone absorbers. In the case of anatase marked autocatalytic oxidation is observed giving strong antagonistic effects. With mixed antioxidant/stabiliser combinations, synergistic stabilisation is not significantly influenced by rutile but markedly antagonised by the presence of the anatase pigment. The most effective light stabiliser in retarding the catalytic oxidative effect of anatase is the polymeric HAS, Chimassorb 944. During thermal oxidation the polymeric HAS exhibit strong thermal antioxidant activity compared with the non-polymeric HAS. Both antioxidants and the benzophenone absorber are strongly antagonistic with rutile while the HAS and benzotriazole stabilisers display weak synergism. The anatase strongly accelerated and catalysed the thermal stabilisation effects with all the stabilisers and antioxidants. For the stabiliser combinations, the HAS are more effective at inhibiting oxidative degradation than the absorbers in the presence of both pigment types. The thermal catalytic effects of anatase are strongly suppressed by antioxidant/stabiliser combinations.

Aspects of poly(ethylene terephthalate) degradation for archival life and environmental degradation

Abstract The degradation of poly(ethylene terephthalate) (PET) (as substrate for audio-visual materials, amorphous sheet and bottles) has been studied by both accelerated thermal and photo-ageing methods. The degradation has been considered from the opposing viewpoints of environmental acceleration of degradation and prolongment of archival lifetime. In the former case, samples of both polyester sheet and bottles have been aged in contact with dry and wet soils in dark and light conditions at different temperatures to emulate environmental breakdown. In the latter case, non-processed 35 mm cinematographic film has been aged at various relative humidities and temperatures in contact with film containers (metal can) to emulate archival storage conditions. Results of both accelerated ageing studies indicate that breakdown of PET motion picture film is negligible at 60°C and relatively unaffected by variations in humidity of the surrounding environment, over the time period studied (300 days), due to its high crystallinity (55%). At 70 and 80°C the motion picture film exhibits signs of crosslinking rather than degradation due to the high crystallinity and emulsion inhibiting the diffusion of oxygen into the polymer. The presence of iron from the container has an accelerating effect on the degradation rate of motion picture film material but only at temperatures above 90°C. In contrast, normal amorphous polyester sheet and orientated bottles degrade due to their much lower crystallinity (1 and 30%, respectively) and at higher temperatures (70–90°C) breakdown, as characterized by viscosity and chain scission measurements, is indicative of significant polymer deterioration. Breakdown is enhanced by increasing temperature, increasing relative humidity and UV irradiation. The polyester bottles are more stable than sheet due to a greater degree of orientation in the former case and hence higher degree of crystallinity. Both soil (in the case of amorphous PET sheet and bottles) and metal storage can (in the case of cinematograph film) have a significant effect on stability. At temperatures above the glass transition, i.e. 80°C, differences in rates of degradation up to 45% relative humidity are not significant. Videotapes of various archival histories have also been studied using high resolution light microscopy. White crystalline deposits and surface conglomerates are observed with increasing age and these appear to be consistent with artificial ageing experiments.