C. M. Liauw

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.

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.

Effect of interactions between stabilisers and silica used for anti-blocking applications on UV and thermal stability of polyolefin film 2. Degradation studies

Interactions between stabilisers and silicas, produced by both the gel process and by precipitation, have been studied using flow micro-calorimetry (FMC) and diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS). The silicas had approximately equivalent BET surface areas. By virtue of its greater pore volume and lower water content, the gel silica was shown to have the greatest adsorption activity. For both the silicas, the adsorption activity of a stabiliser was related to its basicity and number of adsorbing groups per molecule which gave rise to multi-point adsorption, hence polymeric hindered amine light stabilisers (HALS) and large hindered phenols showed the strongest adsorption. Competitive adsorption of polymeric HALS versus hindered phenols was also investigated. The data obtained has been very useful for predicting preferential adsorption of the stabilisers on to the silicas from a linear low density polyethylene melt which in turn has been related to photo-oxidative and thermo-oxidative stability. The latter investigation will be reported separately.

Studies of synergism between carbon black and stabilisers in LDPE photodegradation

The interaction between two commercial hindered piperidine compounds, three commercial antioxidants, a secondary antioxidant and two types of furnace carbon black (CB) (with a different surface area and surface chemistry) in the photo-oxidation of LDPE film have been studied using FTIR infrared spectroscopy. The nature of stabiliser-CB interactions has also been analysed through the use of flow microcalorimetry. The stabilisers used are, a commercial polymeric HALS, namely Chimassorb 944®, and Tinuvin 622® LD®, primary and secondary antioxidants, namely Irganox 1010® and Irganox 1076®, Cyanox 1790® and Irgafos 168®. For the fillers, two carbon blacks were chosen with different features as the specific surface particle size and chemistry. During photo-oxidation the interactions are variable, being both antagonistic and synergistic. Generally for the primary antioxidants alone their photostabilising effects on the polymer film are low or negligible and can be related to the stability of the additives. However, in the presence of a hindered piperidine light stabiliser no such correlations between polymer and antioxidant stability exist. In most of the cases, antagonistic interactions were found between HALS and the antioxidants. Differences in the performance of the polymeric type of HALS were evidenced, as well as between the CB grades. Less antagonism was found between primary and secondary antioxidants with CB for two-additive formulations. This effect is overcome by a powerful synergistic interaction in three additive formulations. Antagonism was found between one HALS and CB. It is concluded that the nature of the CB plays an important role in controlling their performances as a stabilising agent alone, as well as their interactions with light stabilisers and antioxidants.

Factors affecting the adsorption of stabilisers on to carbon black (flow micro-calorimetry and FTIR studies). Part I Primary phenolic antioxidants

The surface activity of different types of carbon black with phenolic antioxidants is examined using flow micro-calorimetry (FMC), X-ray Photoelectron Spectroscopy (XPS) and Fourier Transform Infrared spectroscopy (FTIR). Significant differences in both the overall adsorption activity and the levels of probe adsorption are observed. Differences in behaviour between types of carbon black are evident and show that the specific surface area is not the most important factor affecting the adsorption activity, but also the chemical nature of its surface. Essentially, two factors were found to affect the behaviour of phenolic stabilisers: Phenolic hydroxyl and ester groups were found to form the strongest interactions with carbon black. Furthermore, steric hinderance of phenolic hydroxyls by alkyl groups is the main factor which influences adsorption activity. In order to characterise different carbon blacks, FTIR and XPS analysis have been used in an attempt to determine the nature of functional groups present on the surface of the carbon blacks. FTIR analysis also shows that some adsorbed antioxidants on the surface of the carbon black could be successfully detected. This provides valuable information regarding the adsorption mechanisms on to carbon black surfaces. Other techniques included thermogravimetric analysis (TGA), N2 BET adsorption studies and Karl Fisher analysis. The latter were performed in order to determine differences in the volatile and water contents, respectively, of the carbon black samples.

Interactions between carbon black and stabilisers in LDPE thermal oxidation

The interactions between two commercial hindered piperidine compounds, three commercial antioxidants, a secondary antioxidant and two types of furnace carbon blacks (with a different surface area and surface chemistry) in the thermal oxidation of LDPE have been studied using the oxidation induction time test. During thermal degradation the interactions were variable, being both antagonistic and synergistic. Generally, for the primary antioxidants alone their thermal stabilising effects on the polymer were decisive and related to their chemical structure, with the main contribution being their synergism with HALS and a secondary antioxidant. Differences in the performance of the polymeric type of HALS were evidenced, as well as between the CB grades. Minor synergism and antagonism was found between HALS and CB depending on the chemical structure of the HALS. All the detected antagonisms were widely overcome by the synergistic interaction found in three- and four-additive formulations. Though the presence of CB (at the concentrations studied) was not a decisive factor in the LDPE thermal stability, its presence nevertheless showed a beneficial effect. The nature of the CB plays some role in controlling its performance as a stabilising agent alone, as well as its interactions with HALS and antioxidants, via adsorption-desorption processes as well as other chemical interactions as was evidenced by adsorption studies via flow microcalorimetry (FMC).

Entrapment of stabilisers in silica: I. Controlled release of additives during polypropylene degradation

In this study antioxidants (AOs) and hindered amine light stabilisers (HALS) were adsorbed to silicas prior to blending and processing in polypropylene films. In the absence of AOs and HALS, polymer stability was found to be highly dependent on the active metal (Ti, Al, Fe) contents of the silicas (as demonstrated by carbonyl index and embrittlement data). Laser light-scattering has been employed to demonstrate that silica enhances polymer crystallinity (and hence reduces clarity) during ageing by acting as a nucleating agent, but that this is moderated when the stabilisers are bound to the silica. The distinct differences arising in stabilisation performance of AOs and HALS when prebound to silica were attributed to the ability of stabilisers to selectively desorb, depending on environmental conditions. On thermal ageing the majority of HALS exhibited increased activity, whereas during photo-ageing the majority showed reduced activity. All physical mixtures of preadsorbed antioxidants demonstrated reduced activity on photo-ageing, as did many on thermal ageing. The reduction in performance was greater than expected, even when the additive effects of single stabilisers were accounted for.

Factors affecting the adsorption of stabilisers on to carbon black (flow micro-calorimetry studies) Part III Surface activity study using acid/base model probes

The first and second part of this series of papers investigated the interaction between carbon black and stabilisers (phenolic antioxidants and HALS, respectively) and showed that the mechanism was dependent on both the chemical nature of the carbon black surface and the molecular structure of stabilisers. In this third part, the interactions between model compounds, of varying acidity, and the same four carbon blacks, are investigated using flow micro-calorimetry (FMC) and Fourier transform Infrared spectroscopy (FTIR). As with the first and second parts, differences in adsorption behaviour between the four types of carbon black were evident and were principally related to the chemical nature of the surfaces and the adsorbates. In this study further insight in to the nature of the interactions between the carbon black surface functional groups and the acidic and basic probes has been acquired. The main forms of interaction are hydrogen bonding and Lewis and Bronsted acid/base interactions, formation of proton transfer complexes was also considered possible in cases of strong adsorption. The adsorption behaviour of acid and basic aromatic probes, together with octadecanol and stearic acid, was also found to be dependant on the carbon black surface topography. Flat graphene layers containing minimal heteroatoms favoured adsorption of the latter species as flat adsorption and/or structural ordering was permissable.

Factors affecting the adsorption of stabilisers on to carbon black (flow micro-calorimetry studies) Part II Hindered amine light stabilisers (HALS)

The interaction of hindered piperidine light stabilisers (HALS) with carbon black has been examined using flow micro-calorimetry (FMC) and Fourier transform infrared spectroscopy (FTIR). Significant differences in both the overall adsorption activity and molar heats of probe adsorption are observed. Differences in adsorption behaviour between different types of carbon black were clearly evident and, as with a previous paper (see reference 1, in the latter paper, surface chemistry of the carbon blacks investigated is extensively analysed by XPS, FTIR, N2 BET adsorption and Karl–Fischer analysis), were mainly to be due to differences in carbon black surface chemistry. The specific surface area merely physically affected the level and heat of adsorption (per unit mass of carbon black). Variation in the degree of substitution of the piperidine amine is an important factor that is found to influence the adsorption activity of HALS, as well as the number of adsorption active and sterically accessible functional groups per HALS molecule. In some cases the adsorbed HALS could be detected by FTIR; shifts in absorption frequencies associated with both the adsorbate and the substrate yielded significant insight into the mode of adsorption of several of the HALS investigated.

The use of microwave and FTIR spectroscopy for the characterisation of carbon blacks modified with stabilisers

Abstract Previous work concerning the overall adsorption activity of different stabilisers [primary and secondary antioxidants and hindered amine light stabilisers (HALS) [Pena, JM, Allen, NS, Edge, M, Liauw, CM. Factors influencing the adsorption of stabilisers onto carbon black: flow microcalorimetry studies J Vinyl Add Technol, 2000;6:62–68] [1] onto carbon black has allowed an understanding of the concentration of stabilisers adsorbed after adsorption and desorption processes. In order to assess directly the amounts of stabilisers adsorbed and desorbed onto the carbon black after the modification, for plastic formulations purposes, microwave dielectric spectroscopy was examined as a potential analytical method, because it is a fast, simple and non-destructive technique. Several carbon blacks (CBs) were modified by adsorption with HALS, primary and secondary antioxidants. UV spectroscopy was used to estimate the CB stabiliser content during modification by assessing the stabiliser concentrations in solution before and after desorption. In addition, the CBs were characterised by X-ray photoelectron spectroscopy (XPS) and FTIR to determine the nature of functional groups on which the adsorption process relies. FTIR was used to study the way in which the stabilisers are adsorbed. Other techniques like thermogravimetric analyses (TGA) were performed in order to estimate the differences in water, volatile content and the amount of adsorbed additive of the CBs. These data were compared with the amounts of stabiliser calculated by flow microcalorimetry (FMC). The results gave several linear microwave calibration plots of the content of adsorbed stabilisers onto the CBs.