Long-term Thermo-oxidative Stability Research Articles

Aromatic nitrile resins with improved processability and thermal properties prepared by collaborative design of structure and blending strategy

The contradiction between processability and thermal properties has been limiting the development of high-performance thermoset resins, especially aromatic nitrile resins (AN). A new AN resin blend of 2-(3-(3,4-dicyanophenoxy)phenyl)-1H-imidazole-4,5dicarbonitrile (PNDCI) and 4-(4-aminophenoxy) phthalonitrile (APN) named PAB based on Brønsted acid-base blending strategy and introduction of asymmetric structure was designed and prepared in this paper. A series of characterizations showed that the PAB has not only excellent processability with a diminished melting point (118°C), wider processing window (90°C), and high curing reactivity, but that the cured PAB resins (C-PAB) also possessed high glass transition temperature (477°C), excellent long-term thermo-oxidative stability, thermomechanical properties and dimensional stability. The present design strategy of blending resins provides PAB as a good reference material for achieving a good balance between processability and high-temperature resistance.

Study on aromatic nitrile-based resins containing both phthalonitrile and dicyanoimidazole groups

In order to prepare aromatic nitrile-based resins with improved curing reactivity, thermal and thermomechanical properties, and to clarify the difference in curing activity between dicyanoimidazole and phthalonitrile groups, a novel class of autocatalytic aromatic nitrile-based resins named PNDCI which contains both dicyanoimidazole (DCI) and phthalonitrile (PN) groups were designed and synthesized. A method for evaluating the reactivity of nitrile groups using the absorption wavenumber of FTIR was proposed. PNDCI monomers have demonstrated higher activity for self-promoted curing reactions than benzimidazole-containing phthalonitrile monomers (BIPN). The introduction of DCI can change the curing pathway of PN and inhibit the formation of phthalocyanine ring which is an important cured product of PN resins. The data of TGA, DMA, etc. revealed that the cured products of PNDCIs showed outstanding thermal stability, long-term thermo-oxidative stability at high temperatures (425°C) and thermal mechanical properties, which is favorable for the preparation of high temperature resistant resins. • Aromatic nitrile-based (PNDCI)resins containing both dicyanoimidazole (DCI) and phthalonitrile (PN) were reported. • 2.The difference in curing activity of the nitrile group of PN and DCI was preliminarily investigated. • 3.The cured PNDCI resin exhibits excellent thermal and thermomechanical properties.

Enhancement of cardanol-loaded halloysite for the thermo-oxidative stability and crystallization property of polylactic acid

A natural antioxidant, cardanol (CA), was loaded on halloysite nanotube (HNT) to fabricate a dual-functional green additive cardanol-loaded halloysite (HNT-CA) for the preparation of fully green polylactic acid (PLA) composites with improved thermo-oxidative stability and crystallization property. Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and transmission electron microscopy (TEM) measurements confirmed that cardanol was loaded in the lumen and attached on the outer surface of halloysite with approximately 22.9 wt% loading efficiency. The anti-aging behavior of HNT-CA was evaluated by the change of oxidation onset temperature (OOT) and weight-average molecular weight (Mw) of PLA composites stabilized by this additive after a 12-day aging test at 120 °C. The results showed that the OOT values of PLA composites with CA dropped from 298.2 °C to 279.0 °C, whereas the PLA composites with HNT-CA decreased from 292.2 °C to 281.5 °C, showing a higher retention. Similarly, the PLA stabilized by HNT-CA presented a higher retention of Mw than that stabilized by CA after thermal oxidation aging. For the PLA composites with HNT-CA, the migration of cardanol in PLA was inhibited by the restriction of halloysite to cardanol, thus showing long-term thermo-oxidative stability. Moreover, the analyses of polarized optical microscope (POM) and differential scanning calorimetry analysis (DSC) showed that HNT-CA increased the crystallization rate of PLA compared to HNT further due to the improved dispersion in PLA. These results indicated that HNT-CA exhibited excellent performance in improving the thermo-oxidative stability and crystallization property of PLA.

New genre of antioxidants from renewable natural resources: Synthesis and characterisation of rosemary plant-derived antioxidants and their performance in polyolefins

Several ester derivatives of rosmarinic acid (rosmarinates) were synthesised, characterised (1D and 2D NMR, UV and FTIR spectroscopy) and tested for their potential use as antioxidants derived from a renewable natural resource. The intrinsic free radical scavenging activity of the rosmarinates was assessed, initially using a modified DPPH (2, 2-diphenyl-1-picrylhydrazyl radical) method, and found to be higher than that of commercial synthetic hindered phenol antioxidants Irganox 1076 and Irganox 1010. The thermal stabilising performance of the rosmarinates in polyethylene (PE) and polypropylene (PP) was subsequently examined and compared to that of samples prepared similarly but in the presence of Irganox 1076 (in PE) and Irganox 1010 (in PP) which are typically used for polyolefin stabilisation in industrial practice. The melt stability and the long-term thermo-oxidative stability (LTTS) of processed polymers containing the antioxidants were assessed by measuring the melt flow index (MFI), melt viscosity, oxidation induction time (OIT) and long-term (accelerated) thermal ageing performance. The results show that both the melt and the thermo-oxidative stabilisation afforded by the rosmarinates, and in particular the stearyl derivative, in both PE and PP, are superior to those of Irganox 1076 and Irganox 1010, hence their potential as effective sustainable bio-based antioxidants for polymers.The rosmarinic acid used for the synthesis of the rosmarinates esters in this study was obtained from commercial rosemary extracts (AquaROX80). Furthermore, a large number of different strains of UK-grown rosemary plants (Rosmarinum officinalis) were also extracted and analysed in order to examine their antioxidant content. It was found that the carnosic and the rosmarinic acids, and to a much lesser extent the carnosol, constituted the main antioxidant components of the UK-plants, with the two acids being present at a ratio of 3:1, respectively.

Novel organo-modifier for thermally-stable polymer-layered silicate nanocomposites

A new novel approach for the stabilisation of polymer-clay nanocomposites has been investigated based on reacting chemically an antioxidant function, a hindered phenol moiety, with an organic modifier based on a quaternary ammonium salt. The chemically linked antioxidant-containing organic modifier (AO-OM) was then introduced into natural montmorillonite (MMt) through a cation-exchange reaction resulting in antioxidant-containing organo-modified clay (AO-OM-MMt). The new antioxidant-containing modified clay, along with other organo-modified clays having a similar organo-modifier but without the reacted antioxidant, were characterised by spectroscopic, thermogravimetric and x-ray diffraction techniques and tested for their thermo-oxidative stability.PA11-based clay nanocomposites samples containing the AO-OM-MMt and the other organo-modified clays, both without and with an added (i.e. not chemically reacted) hindered phenol antioxidant (similar to the one used in the AO-OM) were prepared by melt processing and examined for their processing and long-term thermal-oxidative stability at high temperatures. It was shown that although the new organo-modifier, AO-OM, was also susceptible to the Hoffman elimination reaction, the nanocomposites containing this newly modified clay (PA11/AO-OM-MMt) showed higher melt processing and long-term thermo-oxidative stability, along with excellent clay dispersion and exfoliation, compared to the other PA11-nanocomposites examined here (with and without the conventionally added antioxidant). It is suggested here that the excellent overall performance observed for the PA11/AO-OM-MMt nanocomposites is due to an in-situ partial release of low molecular weight antioxidant species having stabilising functionalities that are capable of acting locally at the interface between the inorganic clay platelets and the polymeric matrix which is a critical area for the onset of degradation processes.

Comparative studies on melt processable polyimides derived from 2,3,3′,4′-oxydiphthalic anhydride and 2,3,3′,4′-thioetherdiphthalic anhydride

Two thermoplastic polyimides were controllably synthesized with similar molecular weight from 2,3,3′,4′-oxydiphthalic anhydride (3,4′-ODPA) or 2,3,3′,4′-thioetherdiphthalic anhydride (3,4′-TDPA) with 4,4′-diaminodiphenyl ether (ODA) by a one-step method of m-cresol. The solubility and mechanical properties of these polymers were investigated for comparison. The thermal and melt processable properties of these polymers were examined carefully by differential scanning calorimetry, thermogravimetric analysis (TGA), rheometer and melt index, and so on. It was found that the glass transition temperature ( Tg) of 3,4′-TDPA/ODA was about 15°C lower than that of 3,4′-ODPA/ODA, the melt indices of 3,4′-TDPA/ODA at different temperatures were higher than those of 3,4′-ODPA/ODA, while the minimum melt viscosity of 3,4′-TDPA/ODA was lower than those of 3,4′-ODPA/ODA. Furthermore, TGA results showed that 3,4′-TDPA/ODA had different degradation mechanism in air compared with 3,4′-ODPA/ODA. The long-term thermooxidative stability results showed that 3,4′-TDPA/ODA had better thermal oxidative stability than 3,4′-ODPA/ODA at 410°C for 6 h in air, which could be associated with the results of apparent activation energy ( Ea) of polymers calculated by TGA dynamics. The natural bond orbital charge of ethero atoms (S or O), and energy gap of the corresponding model compounds were also obtained by computational simulation to correlate with polymer properties.

Effect of organo-modified clay on accelerated aging resistance of hydrogenated nitrile rubber nanocomposites and their life time prediction

Organically modified clay – reinforced hydrogenated nitrile rubber vulcanizate was subjected to accelerated heat aging to estimate its long-term thermo-oxidative stability and its useful lifetime was compared with that of the virgin polymer for the first time. Changes in technical properties such as tensile strength, modulus and elongation at break were studied as a function of time and temperature of aging. The infrared spectroscopic analysis of the degraded products revealed that under aerobic hot aging conditions, hydrogenated nitrile rubber (HNBR) compounds undergo cross-linking reactions that lead to embrittlement and ultimately failure. Incorporation of clay filler, however, resulted in significant improvement of the degradation profile of the nanocomposite at elevated temperatures. Loss of ductility during aging of the nanocomposite was also milder, relative to the unfilled polymer, indicating a restricted degradation by the clay filled rubber, thus prolonging the durability. From the scanning electron microscopy and atomic force microscopy studies, it was found that nanofillers protected the elastomer from surface rupture that took place on oxidation. Life prediction of both virgin elastomer and the nanocomposite indicated a three-fold increase in the effective service temperature range of the HNBR using 8 parts organically modified nanoclay.

Influence of accelerated aging on clay-reinforced polyamide 6

Organically modified clay-reinforced polyamide 6 was subjected to accelerated heat aging to estimate its long-term thermo-oxidative stability and useful lifetime compared to the virgin material. Changes in molecular weight, and thermal and mechanical properties were monitored and connected to the polymer modification encountered during aging. The incorporation of the clay filler was found to result in moderate polymer degradation during processing, which became more pronounced with aging time at elevated temperature, imparting discoloration. Post-crystallization was favoured by short periods of heat exposure, leading to an increase of crystalline content. Conversely, extended chain scission occurring after prolonged residence time negatively affected crystallites' size, lowering the degree of crystallinity. The aging-induced transformations of crystal structures correlated well with materials' mechanical performance, yielding initially hard and brittle specimens which gradually adopted a softening behavior. Relative to the unfilled polymer, the loss of ductility during aging was milder in the case of nanocomposite, indicating that the clay filler restricted degradation, prolonging durability.

Properties of phthalonitrile monomer blends and thermosetting phthalonitrile copolymers

Binary blends of biphenyl phthalonitrile and the n=4 oligomeric phthalonitrile were prepared and characterized by differential scanning calorimetry and rheology studies. The blended phthalonitriles also were polymerized from the melt in the presence of bis[4-(4-aminophenoxy)phenyl]sulfone and the dynamic mechanical and thermal properties of the resulting copolymers were investigated. The properties of the phthalonitrile blends and copolymers were compared with those of the neat polymers. The phthalonitrile blends were found to have larger processing windows relative to that observed for the biphenyl phthalonitrile. The size of the processing window depended on the n=4 phthalonitrile content in the blend. Dynamic mechanical measurements and thermogravimetric analysis showed that temperatures up to 425°C were necessary to completely cure the phthalonitrile copolymers. The dynamic mechanical measurements also revealed that the fully-cured phthalonitrile copolymers did not soften or exhibit a glass transition temperature upon heating to 450°C. Thermogravimetric analysis showed that the phthalonitrile copolymers exhibited excellent thermal stability along with long-term thermo-oxidative stability.

Designed poly(ether-imide)s and fluoro-copoly(ether-imide)s: Synthesis, characterization and their film properties

Two series of amorphous fluoro-poly(ether-imide) (6F-PEI) and one series of fluoro-coploy(ether-imide) (6F-CoPEI) polymers based on 2,2′-bis(3,4-dicarboxyphenyl) hexafluropropane dianhydride (6FDA) and di-ether-containing diamines: 1,2′-bis(4-aminophenoxy)benzene ( o-BAPOB) and 4,4′-bis(4-aminophenoxy)diphenyl sulfone ( p-SED) were synthesized. The solution properties, chemical resistance, thermal stability, mechanical properties, thermo-oxidative and hydrolytic stability of selected 6F-PEIs and 6F-CoPEIs were studied. The dielectric constant ( ɛ′) values of 6F-PEI and 6F-CoPEI were estimated by additive group contribution calculation using mathematical equations defined by the Lorentz–Lorenz's theory and the Vogel's theory, and by Vora–Wang equations, respectively. These polymers not only showed excellent electrical properties but also excellent long-term thermo-oxidative stability and reduced water absorption relative to non-fluorinated polyimides. The estimated dielectric constant of these polymers ( ɛ′ < 3.15) were lower than those of commercially available poly(ether-imide) ULTEM ®1000 and polyimides Kapton ®H, respectively at 1 kHz. The analytical results are summarized and discussed.

Benefits of antioxidants in lubricants and greases assessed by pressurised differential scanning calorimetry

The long-term thermo-oxidative stability of lubricants and greases is a prerequisite to meet today's ‘stay in grade’ and extended drain interval requirements. To achieve this goal, there is a need for a better understanding of thermo-oxidative lubricant degradation and of how antioxidants can affect this process. In this paper a two-stage degradation model is discussed. Primary degradation leads to the formation of very reactive species such as radicals and peroxides. These are the precursors for generating secondary degradation products, such as oligomers, sludge, or deposits. By applying the theory of reaction kinetics and the Arrhenius law, quantitative correlations may be derived between oxidation induction time (OIT) and temperature, and between OIT and antioxidant treat level. Pressurised differential scanning calorimetry (PDSC) data confirm these theoretical findings and demonstrate the benefit of the addition of aminic antioxidants in comparison with high-molecular-weight hindered phenolic antioxidants.

Synthesis, Fabrication, Characterization, Properties and Thermo-Oxidative Stability Study of Poly(ether imide)/MMT Clay Nanocomposites

A series of partially fluorinated poly(ether imide) (6F-PEI) and (6F-PEI)/Organosoluble MMT clay nanocomposite were synthesized. XRD data, indirectly confirmed the exfoliation of diamine modified MMT clay at molecular level in the nanocomposite. Thermal properties, and longterm thermo-oxidative stability, thermal degradation kinetics, moisture absorption, chemical resistance of (6F-PEI)s, and nanocomposite films were studied and reported in this paper.

Synthesis and characterization of a silarylene–siloxane–diacetylene polymer and its conversion to a thermosetting plastic

The synthesis and characterization of a linear silarylene–siloxane–diacetylene polymer 1 and its conversion to a highly cross-linked thermoset 4 are discussed. The linear polymer was prepared via polycondensation of 1,4-bis(dimethylaminodimethylsilyl)butadiyne, [(CH 3) 2N–Si(CH 3) 2–CC–CC–(CH 3) 2SiN(CH 3) 2], 2 with 1,4-bis(hydroxydimethylsilyl)benzene 3. Conversion to a thermoset 4 occurs through the diacetylene groups above 300 °C. The thermoset was observed to exhibit long-term thermo-oxidative stability up to 350 °C in air as determined by thermogravimetric analysis.

Synthesis and Properties of Designed Low-k Fluoro-Copolyetherimides. Part 1

Polyetherimides are versatile melt-processable engineering resins. A series of fluoro-polyetherimides compositions based on diether-containing diamines were designed on paper and their dielectric constants (ϵ′) estimated before their synthesis. Experimentally determined ϵ′ values were compared with the estimated values. The amorphous polyetherimides containing bis-trifluoromethyl groups exhibited not only low dielectric constants—lower than those of the commercially available polyetherimide ULTEM 1000 and polyimide Kapton H at 1 kHz—but also excellent long-term thermo-oxidative stability and reduced water absorption relative to non-fluorinated polyimides.

Thermo-Processable Polyimides with High Thermo-Oxidative Stability as Derived from Oxydiphthalic Anhydride and Bisphenol a Type Dianhydride

Two series of homopolyimides based on oxydiphthalic anhydride and bisphenol A bisether-4-diphthalic anhydride were synthesized with 12 kinds of aromatic diamines. Several thermo-processable homopolyimides were the focus of further investigation. The structure–property relationships of these homopolyimides were examined as functions of the glass transition temperature ( Tg), melting point ( Tm), thermal decomposition temperature ( Td), and melt-flowability. The effects of the chemical and higher-order structures on these properties were discussed in this work to obtain an indication for the molecular design of high-performance thermoprocessable polyimides. A series of thermoplastic copolyimides were prepared to achieve higher Tg and Td without sacrificing thermo-processability. The copolyimides investigated exhibited a comparable or lower melt viscosity, higher Tg and Td, and higher long-term thermo-oxidative stability than those of ULTEM type polyimides. One of them exhibited a low melt viscosity (4700 poise at 400 °C), a high Tg (224 °C), and excellent thermo-oxidative stability.

Polyimide Copolymers Containing Various Levels Of The 6F Moiety For High Temperature And Microelectronic Applications

ABSTRACTTrifluoromethyl group-containing polyimides not only show extraordinary electrical properties, but they also exhibit excellent long-term thermo-oxidative stability. Among the most thermomechanically stable structural polyimides are those from 6F dianhydride (6FDA) and 6F diamines. The effects of substituting non-fluorine containing monomers such as BTDA, mPDA and 4,4′-DADPS for the hexafluoroisopropylidene monomers on the dielectric, thermo-oxidative, thermal and mechanical properties of the copolymers were studied.