Teresa Corrales

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.

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.

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.

Hybrid materials: Magnetite-Polyethylenimine-Montmorillonite, as magnetic adsorbents for Cr(VI) water treatment.

Hybrid materials formed by the combination of a sodium rich Montmorillonite (MMT), with magnetite nanoparticles (40 nm, Fe(3)O(4) NPs) coated with Polyethylenimine polymer (PEI 800 g/mol or PEI 25000 g/mol) were prepared. The intercalation of the magnetite nanoparticles coated with PEI among MMT platelets was achieved by cationic exchange. The resulting materials presented a high degree of exfoliation of the MMT sheets and a good dispersion of Fe(3)O(4) NPs on both the surface and among the layers of MMT. The presence of amine groups in the PEI structure not only aids the exfoliation of the MMT layers, but also gives to the hybrid material the necessary functionality to interact with heavy metals. These hybrid materials were used as magnetic sorbent for the removal of hexavalent chromium from water. The effect that pH, Cr(VI) concentration, and adsorbent material composition have on the Cr(VI) removal efficiency was studied. A complete characterization of the materials was performed. The hybrid materials showed a slight dependence of the removal efficiency with the pH in a wide range (1-9). A maximum amount of adsorption capacity of 8.8 mg/g was determined by the Langmuir isotherm. Results show that these hybrid materials can be considered as potential magnetic adsorbent for the Cr(VI) removal from water in a wide range of pH.

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.

Photophysics and photoreactivity of substituted thioxanthones

The photophysics and the interaction of the excited states of substituted thioxanthones have been studied in order to be able to predict their behaviour as photoinitiators in photopolymerization systems. Electron-donor groups in position 2 in the thioxanthone ring lead to high fluorescence quantum yields, red-shifts in the emission spectra and long singlet lifetimes, explained by the separation of the nπ* and ππ* excited states induced by the higher electron density of the aromatic system. For some of the derivatives a biexponential decay due to slow relaxation of the solvent has been observed. Rate constants for the quenching of the singlet state by amines are near the diffusional limit and are not modified by substituents. On the other hand, substitution of electron-donor groups on the thioxanthone ring in position 2 decreases the triplet reactivity towards amines and alkenes. The incorporation of double bonds on the side chains of the compounds does not affect the lifetimes and reactivity of the thioxanthones.

Photopolymerization of methyl methacrylate initiated by thioxanthone derivatives: photoinitiation mechanism

The photopolymerization of methyl methacrylate initiated by substituted thioxanthones in the presence of an amine has been investigated. The polymerization rates in acetonitrile were measured at several amine concentrations. The dependence of the polymerization rates with the amine concentration and the photoinitiation efficiency at a given amine concentration are highly dependent on the ketone structure. The rate constants for the quenching of thioxanthones excited states by the monomer and the amine were measured by fluorescence and laser flash photolysis, under the polymerization conditions. These data show that the reactivity of the ketone is also highly dependent on the ketone structure. The values of the quenching rate constants show that the initiation efficiency is controlled by the competition between the quenching of the excited states by the monomer and also the amine. From the latter a mechanism is proposed to simulate the dependence of the polymerization rates with amine concentration.

The influence of the photophysics of 2-substituted thioxanthones on their activity as photoinitiators

The polymerization of methyl methacrylate photoinitiated by thioxanthones (TXs) substituted in the position 2 of the chromophore ring in the presence of 2-(N,N-diethylamino)ethanol has been investigated. The photoinitiation efficiency of these systems is highly dependent on the structure of the 2-substituent. At high amine concentrations, compounds with electron-withdrawing substituents appear as more efficient photoinitiators. The photophysics of the ketones was studied in the polymerization medium. These data show that the singlet excited state of TXs is deactivated by the amine with a rate constant near the diffusional control limit. The triplet rate constants for quenching by the amine and the monomer are highly dependent on the ketone structure. Compounds with electron-withdrawing substituents in the position 2 are more reactive. These studies allow to simulate the dependence of the photoinitiation efficiency with the amine concentration and indicate that the active radicals are produced by the interaction of the ketone triplet with the amine.

A chemiluminescence study of micron and nanoparticle titanium dioxide: effect on the thermal stability of metallocene polyethylene

Abstract The thermal activity of a series of nano and micron particle grade anatase and rutile titanium dioxide pigments, with various densities of surface treatments, particle size and surface area, have been determined in monomodal metallocene polyethylene. Their performance has been undertaken by chemiluminescence (CL), differential scanning calorimetry and hydroperoxide analysis. Chemiluminescence study under nitrogen on polyethylene containing nano and micron titania allows one to determine the thermal activity of pigment during processing. Such results show that for pigmented polymers, a marked suppression of emission is observed, which correlates well with the lower initial determined hydroperoxide content. This effect is enhanced with pigment content and coated grades. For untreated pigment, more oxidation is induced by the greater surface area of the pigment particle, which, in turn influences the dispersion and enhances the pigment–polymer contact. At temperatures below the polyethylene melting point, low chemiluminescence was observed due to the restricted mobility of hydroperoxides to give the disproportionation reaction responsible for the emission. Under oxygen, the high activity of Irganox 1010 as a long-term thermal stabiliser is observed, and the activity of titanium dioxide has been seen to be dependent on the particle size. In general, nanoparticles exhibit a greater thermal sensitising effect, which is reduced by the coating. Increasing the content of pigment, the oxidation induction times (OIT) are greatly reduced and are not detected by DSC. It is concluded that the CL method can be used to determine differences in the oxidation level of pigmented polyethylenes.