The influence of aging on surface free energy of corona treated packaging films

In packaging, plastic films are very often applied as overprinting materials. The printing properties of plastic films depend on the value of the surface free energy. Usually, during storage but before printing, the surface free energy is decreasing as a result of ageing. The aim of this study was to analyse the influence of elevated temperature and UV radiation on ageing properties and variation of the free surface energy for three commercially available plastic films: polyethylene, polypropylene and polyethylene terephthalate. The investigation was done experimentally, and the surface free energy was calculated using two approaches, Owens-Wendt and van Oss-Chaudhury-Good. The time change of polar fractions was also analysed. The calculation results were compared and it was concluded that UV radiation causes more changes in surface free energy than elevated temperature. In some cases, surface free energy values calculated with the applied methods show similar trends.     

Radiochemical ageing of ethylene-propylene-diene elastomers. 4. Evaluation of some anti-oxidants

The preceding papers of this series were devoted to the identification and quantification of the main chemical changes resulting from the radiochemical ageing of EPDM (77.9% ethylene, 21.4% propylene, 0.7% diene) and EPR (76.6% ethylene, 23.4% propylene) films irradiated under oxygen atmosphere using ⁶⁰Co γ-rays. It was shown that two processes are involved in the EPDM radio-oxidation. The random γ-radiolysis of the polymer provides a constant source of macro-alkyl radicals that are likely to initiate a selective oxidation of the polymer through free-radicals reactions involving the abstraction of labile hydrogen atoms. In the present paper, infrared spectroscopy has been used to study the γ-degradation of EPDM cross-linked with dicumyl peroxide and/or stabilised with two types of anti-oxidants (hindered phenol or amine-type). The results show that the anti-oxidants are not efficient in preventing oxidation. To understand the lack of efficiency of the stabilisers, the impacts of the various formulations on the rate of degradation of EPDM against chain oxidation involved in thermal and UV ageing were also studied.     

New high-tensile thermally stable copolymers of poly(p-phenylene-1,3,4-oxadiazole)

A one-stage method is developed for the synthesis of new irregular copolymers of poly(p-phenylene-1,3,4-oxadiazole), whose polymer chain contains a 5,5-dioxodibenzothiophenylene fragment. The polymer synthesis is performed through direct copolymerization of terephthalic acid, 5,5-dioxodibenzothiophen-3,7-dicarboxylic acid, and hydrazine sulfate in oleum at an elevated temperature. The introduction of 13.5–17% structural fragments of 5,5-dioxodibenzothiophene into a copolymer macromolecule makes it possible to improve the thermal stability and fire-resistance of fibers and films produced on their basis and to achieve a 30% increase in breaking strength and a fivefold greater breaking extension relative to those values for a homopolymer based on terephthalic acid.     

Effect of Aliphatic Chain Length in the Ligand on Photophysical Properties and Thin Films Morphology of the Europium Complexes

Chelating ligands based on polydentate diamides of 2,2'-bipyridyl-6,6'-dicarboxylic acid with a high affinity for lanthanide ions have been synthesized. The effect of the size of lipophilic aliphatic substituents in the ligand on the photophysical characteristics of europium complexes in acetonitrile solutions and in the solid state, as well as on the morphology of thin films obtained by the spin-coating method, was studied. The external and internal luminescence quantum yields have been measured, the luminescence lifetimes at 300 and 77 K were determined, and the sensitization efficiency values for europium complexes were calculated. The phosphorescence of gadolinium(III) complexes was used to determine the energy difference between the triplet level of the ligand and the resonance level of europium.     

Different types of water in the film formation process of latex dispersions as detected by solid-state nuclear magnetic resonance spectroscopy

 The properties of polymer films prepared from latex dispersions are influenced by the drying or film formation process. In order to investigate this process, various systems of aqueous latex dispersions were dried until a specific solid content was reached. The samples investigated were based on vinyl acetate, vinyl acetate/ethylene and pure acrylics employing different surfactants and polyelectrolytes as stabilisers of the dispersions. The role of water in these partially dried films was investigated using ¹H and ²H solid-state NMR spectroscopy. Different types of water could be distinguished in the spectra. The drying latex films were found to contain interfacial external water, water at ionic and nonionic groups at surfactants in the polymer/water interface and also water inside the swollen polymer. These different types of water were examined separately using various NMR techniques. Received: 22 October 1999/Accepted in revised form: 19 November 1999     

Effect of oxygen plasma treatment on the surface properties of tin-doped indium oxide substrates for polymer LEDs

The effect of oxygen plasma treatment on the surface properties of tin-doped indium oxide (ITO) substrates and the changes in surface properties of treated ITO substrates with ageing time were investigated by X-ray photoelectron spectroscopy (XPS), contact angle and surface free energy measurements. Experimental results show that oxygen plasma treatment increases the oxygen concentration, decreases the carbon concentration, and enhances the surface free energy and polarity, and thereby improves the surface properties of ITO substrates. However, the improved ITO surface properties tended to decay and the surface free energy decreased, with ageing time. In addition, the ageing effect of treated ITO substrates on the performance of polymer light-emitting diodes (LEDs) was studied with respect to the driving voltage, electroluminescent luminance and efficiency. We observe that the ITO substrates aged for various times result in significant differences in optical and electrical characteristics which become worse as the ageing time increases. The optical and electrical performance of polymer LEDs is closely related to the surface properties of ITO substrate and the interface characteristics of ITO/polymer.     

Oligomeric bishydroxybutyl terephthalates from terephthalic acid

A process that uses terephthalic acid and excess 1,4-butanediol for the production of polybutylene terephthalate was investigated and the reaction conditions were maximized for fast reaction and minimal loss of butanediol by dehydration to tetrahydrofuran. For best results a mixed catalyst was selected: butylstannoic acid and titanium tetrabutoxide (0.05 and 0.025 mole %, respectively, versus terephthalic acid) with a starting material ratio of butanediol: terephthalic acid of 1.7 at 210°C. The mixture cleared at 2.5 hr with a loss of 6% of butanediol. HPLC analysis showed that the clearing point corresponds to the disappearance of terephthalic acid and monohydroxybutyl terephthalate from the reaction mixture. Thus the polycondensation conditions can be applied before the clearing point with the result that the reaction is faster and tetrahydrofuran formation is minimized. Tetrahydrofuran is formed from butanediol in the reaction mixture by acid catalysis. Excess butanediol results in the formation of larger amounts. End group cyclization of terephthalate esters accounts for the slow tetrahydrofuran formation in the absence of excess butanediol.     

Stability of a salicylate-based poly(anhydride-ester) to electron beam and gamma radiation

The effect of electron beam and gamma radiation on the physicochemical properties of a salicylate-based poly(anhydride-ester) was studied by exposing polymers to 0 (control), 25 and 50 kGy. After radiation exposure, salicylic acid release in vitro was monitored to assess any changes in drug release profiles. Molecular weight, glass transition temperature and decomposition temperature were evaluated for polymer chain scission and/or crosslinking as well as changes in thermal properties. Proton nuclear magnetic resonance and infrared spectroscopies were also used to determine polymer degradation and/or chain scission. In vitro cell studies were performed to identify cytocompatibility following radiation exposure. These studies demonstrate that the physicochemical properties of the polymer are not substantially affected by exposure to electron beam and gamma radiation.     

Ageing,weathering and stabilisation of polymers

The common chemical basis of thermal ageing and weathering are outlined, and the mechanisms of antioxidant action which follow from thermal and photo-oxidation are discussed. The physical behaviour of antioxidants and stabilisers in polymers is assuming increasing importance as the polymer environment becomes more aggressive, and the rationale for the development of polymer-bound antioxidants is discussed.     

Ordered structures in proton conducting membranes from supramolecular liquid crystal polymers

Highly sulfonated forms of poly(p-phenylene terephthalamide) (PPTA) have been prepared in three different molecular configurations; sulfonated diamine form (S-PPTA), sulfonated terephthalic acid form (S-invert-PPTA), and the bi-sulfonated form (S2-PPTA). All three polymers are water soluble to a certain degree and films were cast from solution for S-PPTA and S-invert-PPTA. S-PPTA films absorb less water than S-invert-PPTA (under controlled humidity conditions) and consequently, the conductivity for this polymer is also slightly lower. Although the conductivities are comparable to Nafion (of the order of 10(-2) to 10(-1) Scm(-1)), proton mobility is more restricted. X-ray diffraction showed that the rigid molecules are aligned in opposite directions for the two polymer films, being homeotropic in S-PPTA films and planar for S-invert-PPTA. SEM analysis demonstrated layering in the same direction as the alignment of the polymer chains. The variation in the polymer alignment is most likely the result of the differences in the solution properties and the film forming process. It is possible, however, that this alignment could be exploited to enhance proton transport and thus these films are of interest for fuel cell membranes.     

Melt stabilisation of Phillips type polyethylene, Part I: The role of phenolic and phosphorous antioxidants

The role of a phenolic and three phosphorous (phosphite, phosphonite and phosphine) antioxidants in the melt stabilisation of polyethylene was studied in a Phillips type polyethylene by multiple extrusions. The polyethylene was stabilised with a single antioxidant at 700 ppm and with phenolic/phosphorous antioxidant combinations containing 700 ppm of each component. The functional groups (methyl, vinyl, vinylidene, trans-vinylene and carbonyl) of polyethylene and the residual amount of phosphorous antioxidants were analysed quantitatively by FT-IR methods developed in our laboratory. The rheological characteristics, the colour and the residual thermo-oxidative stability of the polymer were determined and compared. Blown films were prepared and their mechanical strength measured by the Elmendorf and Dart-drop tests. The comparison of the different characteristics revealed that the chemical reactions taking place during the first processing of the nascent polymer powder, as well as the chemical composition of the antioxidants determine the reactions taking place in further processing operations. The changes in the characteristics of stabilised polyethylene during processing are controlled by the phosphorous stabiliser. The effect and final result depend on the chemical structure of the given antioxidant. The phenolic antioxidant itself does not hinder the formation of long chain branches. It reduces the rate of oxidation of the various phosphorous stabilisers, but does not modify the mechanism of stabilisation of the phosphonite and the phosphine. The reactions of the phosphite are significantly modified by the presence of a phenolic antioxidant.     

Thermal ageing of rocket propellant formulations containing ε-HNIW (ε-CL20) investigated by heat generation rate and mass loss

New types of rocket propellant batches have been formulated with the objective of achieving higher burning rates. The main ingredients are (1) the energetic plasticizers glycidyl azide polymer-α,ω-diazide (GAP)-A (short chain GAP with azide end groups), trimethylolethane trinitrate (TMETN) and 1,2,4-butanetriol trinitrate (BTTN), (2) the energetic substances ammonium perchlorate (AP) and ε-CL20 (ε-HNIW, hexanitrohexazaisowurtzitane, crystallised in ε-phase). The binder is GAP (glycidyl azide polymer, diol component) cured with the polyisocyanate Desmodur™ N100. From the point of view of stability and ageing, the interesting fact is that the formulations contain none of the typical stabilisers for the nitric acid ester components TMETN and BTTN, although their contents range up to 21 mass%. One reason for doing so is to increase the content of the high energy ingredients. Seven formulations were examined in more detail. To assess basic stability the autoignition temperature test, Dutch mass loss test and vacuum stability test were used. To investigate ageing, two measurement quantities are applied: heat generation rate (heat flow) as function of time at 70, 80 and 89 °C and mass loss as function of time at the temperatures of 70, 80 and 90 °C. The evaluation of the measurements was done with reaction kinetic models. One batch (#189) containing BTTN shows significantly lower activation energy and pre-exponential factor. From mass loss one gets as activation energy for #189 of 101 kJ mol⁻¹ in comparison to the range of 126-135 kJ mol⁻¹ for the six other batches. But, based on the ageing caused by chemical decomposition reactions, all seven batches showed a good ageing behaviour. A use time period of up to 20 years of use seems realistic.     

A comparison of crystalline- and graft polymer-based chemosensors

Herein, we report a highly sensitive luminescent thin film chemosensor constructed out of a small-molecule donor/acceptor system. Two types of films were compared: one using a small-molecule crystalline donor/acceptor pair and the other using a donor-graft polymer/small-molecule acceptor pair. The acceptor selected for this proof of concept responds to acid, causing its absorption and emission bands to red-shift, which increases spectral overlap with the donor. This increase in overlap greatly enhances energy transfer from the acceptor to the donor. Signal amplification was ascertained by measuring the ratio of acceptor fluorescence when the donor was excited versus direct excitation of the acceptor. Both types of films exhibited large amplification. For the polymeric system, the mechanism of energy migration was investigated by the use of steady-state fluorescence spectroscopy. The mechanism was determined to be dominated by an exciton-hopping process.     

Processing and characterization of waxy maize starch films plasticized by sorbitol and reinforced with starch nanocrystals

Nanocomposites films have been processed from a filler and a matrix having the same nature, i.e. waxy maize starch. The filler consists of nanoplatelet-like starch particles obtained as an aqueous suspension by acid hydrolysis of starch granules and the matrix was prepared by plasticization and disruption of starch granules with water and sorbitol. Nanocomposite films were obtained by casting and evaporating the mixture of the aqueous suspension of starch nanocrystals with the gelatinized starch. The resulting films were conditioned before testing and the effect of accelerated ageing in moist atmosphere was investigated. The thermal properties of the nanocomposite films were determined from DSC measurements and the mechanical characterization was performed in both the linear and nonlinear range.     

Inhibited degradation of nylon 66 in presence of nitrogen dioxide,ozone, air,and near—ultraviolet radiation

Degradation of nylon 66 films of different morphologies was studied in the presence of nitrogen dioxide, ozone, oxygen, and near-ultraviolet radiation (λ > 2900 Å). Films cast from formic acid solution showed normal random degradation, whereas films cast from benzyl alcohol solutions and dried at elevated temperatures under nitrogen showed very strongly inhibited random degradation. This inhibition may be due to protection of peptide groups by hydrogen bonding with benzaldehyde or benzoic acid or even to their chemical reactions at elevated temperatures. Oxygen was not rigorously excluded during preparation of the films. Degradation of nylon 66 films cast from formic acid solutions at room temperature containing benzaldehyde or benzoic acid, respectively, is also inhibited. The energy of activation for inhibited degradation in presence of nitrogen dioxide is relatively small, indicating that the process is either controlled by diffusion of polymer radicals from medium cages or by diffusion of gases into the polymer. The degradation kinetics can be expressed by “weak”-link random degradation. The weak links are in the present case unprotected peptide groups. The functional relationship between chain scission rate constants and NO₂ pressure is linear.     

Preparation of polyamides via the phosphorylation reaction. Poly-4,4′-benzanilides

4,4′-Diaminobenzanilide (DAB) was reacted with diacids via the Yamazaki phosphorylation reaction to yield simple polyamides and random copolyamides. The use of DAB was found to promote polycondensation because DAB contains a preformed amide linkage which minimizes the amounts of by-products formed. It also maximizes polymer solubility since the monomer is unsymmetrical. The order of the inherent viscosity values of the polymers obtained from DAB by reaction with different diacids, isophthalic > aliphatic > terephthalic, was found to parallel polymer solubility in the reaction medium. The inherent viscosity of the polyamide based on terephthalic acid, DAB-T, was increased greatly by copolymerization, isophthalic acid being more effective than an aliphatic diacid in this regard. None of the polymers formed a thermotropic nematic phase, but the copolymer having an equimolar ratio of terephthalic and adipic acid formed anisotropic solutions in 100% sulfuric acid at polymer concentrations exceeding 40%. Strong films were cast from such solutions.     

Mechanism of antioxidant interaction on polymer oxidation by thermal and radiation ageing

The mechanism of polymer oxidation by radiation and thermal ageing was investigated for the life evaluation of cables installed in radiation environments. The antioxidant as a stabilizer was very effective for thermal oxidation with a small content in polymers, but was not effective for radiation oxidation. The ionizing radiation induced the oxidation to result in chain scission even at low temperature, because the free radicals were produced and the antioxidant could not stop the oxidation of radicals with the chain scission. A new mechanism of antioxidant effect for polymer oxidation was proposed. The effect of antioxidant was not the termination of free radicals in polymer chains such as peroxy radicals, but was the depression of initial radical formation in polymer chains by thermal activation. The antioxidant molecule was assumed to delocalize the activated energy in polymer chains by the Boltzmann statics (distribution) to result in decrease in the probability of radical formation at a given temperature. The interaction distance (delocalization volume) by one antioxidant molecule was estimated to be 5–10 nm by the radius of sphere in polymer matrix, though the value would depend on the chemical structure of antioxidant.     

Thin-film polymerization and characterization of Sumitomo's Sumikasuper®-type liquid crystalline polymers

Using the thin film polymerization approach, we have studied the texture evolution when synthesizing a novel liquid crystalline polymer (LCP) system based on Sumikasuper^® LCPs. The main monomers used were p-acetoxybenzoic acid (ABA), 4,4'-biphenol (BP), isophthalic acid (IA), phthalic acid (PA) and terephthalic acid (TA). Polarizing optical microscopy (POM), FTIR, X-ray diffraction (XRD) and atomic force microscopy (AFM) were employed to study the thin film polymerization process and characterize the products. The generation and evolution of liquid crystal phases were monitored; the results revealed that there exists a composition range for the monomers to react and form liquid crystal materials. The critical temperature for LC formation in ABA/BP/IA system decreased with increasing ABA content. FTIR results confirmed the formation of polymers. AFM investigation suggested a similar process of morphological change to that observed using POM. An increasing surface roughness of the thin films with the progress of polymerization was also obtained from AFM analysis. A nematic LC texture of the polymer system was suggested by XRD examination. Results obtained by replacing BP with acetylated BP, and by conducting polymerization using two-monomer systems, suggest that BP units are included in polymers obtained by the thin film polymerization method.     

Characterising in situ activation and degradation of hindered amine light stabilisers using liquid extraction surface analysis-mass spectrometry

Changes in the molecular structure of polymer antioxidants such as hindered amine light stabilisers (HALS) is central to their efficacy in retarding polymer degradation and therefore requires careful monitoring during their in-service lifetime. The HALS, bis-(1-octyloxy-2,2,6,6-tetramethyl-4-piperidinyl) sebacate (TIN123) and bis-(1,2,2,6,6-pentamethyl-4-piperidinyl) sebacate (TIN292), were formulated in different polymer systems and then exposed to various curing and ageing treatments to simulate in-service use. Samples of these coatings were then analysed directly using liquid extraction surface analysis (LESA) coupled with a triple quadrupole mass spectrometer. Analysis of TIN123 formulated in a cross-linked polyester revealed that the polymer matrix protected TIN123 from undergoing extensive thermal degradation that would normally occur at 292 °C, specifically, changes at the 1- and 4-positions of the piperidine groups. The effect of thermal versus photo-oxidative degradation was also compared for TIN292 formulated in polyacrylate films by monitoring the in situ conversion of N-CH₃ substituted piperidines to N-H. The analysis confirmed that UV light was required for the conversion of N-CH₃ moieties to N-H – a major pathway in the antioxidant protection of polymers – whereas this conversion was not observed with thermal degradation. The use of tandem mass spectrometric techniques, including precursor-ion scanning, is shown to be highly sensitive and specific for detecting molecular-level changes in HALS compounds and, when coupled with LESA, able to monitor these changes in situ with speed and reproducibility.     

PIXE Spectroscopy for Determination of Volatile Corrosion Inhibitor Concentration in Anticorrosion Polymer Films

Summary. Anti-corrosion polymer films used for 0 to 6 years were investigated to determine the type and content of volatile corrosion inhibitors (VCI) and how long the films emit it to protect the metal surface encased by the film. By FTIR and DSC low density polyethylene (LDPE) was identified as the polymer carrier. Particle induced X-ray emission (PIXE) spectroscopy of anti-corrosion polymer films revealed significant concentration of molybdenum implying that corrosion inhibitor was molybdate based. The films were divided into three groups according to different rate of the VCI concentration decrease with increasing age. The division of films into groups and diffusion of VCI was confirmed by significant decrease of Mo content determined by PIXE spectroscopy after accelerated ageing in vacuum at 50°C. It was concluded that films with initial concentration of Mo above 6000 ppm emit VCI for at least 6 years while the films with less than 4000 ppm of molybdenum do not emit the VCI in the ambient conditions.