Thermal-oxidative induced degradation behaviour of polyoxymethylene (POM) copolymer detected by TGA/MS

Aging processes in dental thermoplastics - Thermoanalytical investigations and effects on Vickers as well as Martens hardness

OBJECTIVE

The influence of various aging protocols, representing and accelerating influences present in the dental context, on possible changes in the microstructure and mechanical properties of thermoplastics was investigated. In order to minimize the complexity of the systems, first pure polymers and then later the equivalent dental polymeric materials were analyzed.

MATERIALS AND METHODS

Pure polymers (Poly(methyl methacrylate) - PMMA, Polyoxymethylene homopolymer - POM-H, Polyether ether ketone - PEEK, Nylon 12 - PA12, Polypropylene - PP) were analyzed before as well as after applying different aging protocols relevant to the oral environment (ethanol, thermocycling, alkaline and acidic setting) by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The thermoanalytical parameters used were glass transition temperature (Tg), melting peak and crystallization peak temperature (Tpm, Tpc) and decomposition behavior. In a second step selected commercially available dental products (Telio CAD - PMMAD, Zirlux Acetal - POMD, Juvora Natural Dental Disc - PEEKD) aged by the protocol that previously showed strong effects were examined and additionally tested for changes in their Vickers and Martens hardness by Mann-Whitney-U test.

RESULTS

The combinations of pure polymers and viable aging protocols analyzed within this study were identified via TGA or DSC as PA12 & thermocycling, POM-H & denture cleanser/lactic acid/ethanol, PP & lactic acid. The dental polymeric materials PMMAD and POMD due to aging in lactic acid showed slight but significantly (p < 0.01) reduced Vickers and partly Martens hardness. PEEK showed the greatest material resistance within this study.

Sunlight and marine weathering of poly(oxymethylene): Evolution of the physico-chemical properties

Plastic pollution is now an environmental problem that affects all environmental compartments. The study of plastic degradation in terrestrial, marine and other freshwater environments is emerging. Research is mainly focused on plastic fragmentation into microplastics. In this contribution, an engineering polymer, poly(oxymethylene) (POM), was studied under different weathering conditions using physico-chemical characterization techniques. A POM homopolymer and a POM copolymer were characterized by electron microscopy, tensile tests, DSC, infrared spectroscopy and rheometry tests after climatic and marine weathering or artificial UV/water spray cycles. Natural climatic conditions were the most favorable for POM degradation, especially under solar UV, as evidenced by the strong fragmentation into microplastics when subjected to artificial UV cycles. The evolution of properties with exposure time was found to be non-linear under natural conditions, in contrast to artificial conditions. Two main stages of degradation were evidenced by the correlation between strain at break and carbonyl indices.

Analysis of Thermal Degradation Kinetics of POM under Inert and Oxidizing Atmospheres and Combustion Characteristics with Flame Retardant Effects

Degradation behavior of combustible fuel is the core factor in determining combustion characteristics. To investigate the effect of ambient atmosphere on the pyrolysis process of polyoxymethylene (POM), the pyrolysis mechanism of POM was studied with thermogravimetric analyzer tests and Fourier transform infrared spectroscopy tests. The activation energy, reaction model, and estimated lifetime of POM pyrolysis under different kinds of ambient gases have been estimated in this paper based on different results of the kinetics. The activation energy values, obtained with different methods, were 151.0-156.6 kJ mol^-1 in nitrogen and 80.9-127.3 kJ mol^-1 in air. Then, based on the Criado analysis, the pyrolysis reaction models of POM in nitrogen were found to be mastered by the "n + m = 2; n = 1.5" model, and by the "A3" model in air. The optimum processing temperature for POM was estimated, with a range from 250 to 300 °C in nitrogen and from 200 to 250 °C in air. IR analysis revealed that the significant difference in POM decomposition between N₂ and O₂ atmospheres is the formation of isocyanate group or carbon dioxide. Combustion parameters of two POMs (with and without flame retardants) obtained using cone calorimetry revealed that flame retardants can effectively improve the ignition time, smoke release rate, and other parameters of POM. The outcomes of this study will contribute to the design, storage, and transportation of polyoxymethylene.

Environmental remediation through various composite membranes moieties: Performances and thermomechanical properties

Pollution harms ecosystems and poses a serious threat to human health around the world through direct or indirect effects on air, water, and land. The importance of remediating effluents is paramount to reducing environmental concerns. CO₂ emissions are removed efficiently and efficaciously with mixed matrix membranes (MMMs), which are viable replacements for less efficient and costly membranes. In the field of membrane technology, MMMs are advancing rapidly due to their good separation properties. The selection of filler to be incorporated in mixed matrix membranes is very considered very important. There has been considerable interest in MOFs, carbon nanotubes (CNTs), ionic liquids (ILs), carbon molecular sieves (CMSs), sulfonated fillers (SFs), and layered silicates (LSs) as inorganic fillers for improving the properties of mixed matrix membranes. These fillers promise superb results and long durability for mixed matrix membranes based on them. The purpose of this review is to review different fillers used in MMMs for improving separation properties, limitations, and thermomechanical properties for environmental control and remediation.

Study on Ring-Opening Copolymerization of Trioxymethylene and Second Monomer Initiated by Phosphotungstic Acid

In this study, a series of polyoxymethylene copolymers are synthesized by bulk cationic ring-opening polymerization by 1,3,5-trioxane (TOX) with 1,3-dioxolane (DOX), octamethylcyclotetrasiloxane (D4), and cyclohexane oxide (CHO) as the second monomer using phosphotungstic acid (PTA) as an initiator. The polymer products were characterized by hydrogen nuclear magnetic resonance (1H-NMR), infrared spectroscopy (IR), thermogravimetry (TG), and differential scanning calorimetry (DSC). And the copolymerization energy barrier was calculated at the b3lyp/6-31g(d) calculation level using density functional theory (DFT) to explore the copolymerization ability of the second monomer with 1,3,5-trioxane. The results showed that CHO as the second monomer more easily participated in the copolymerization reaction, and the copolymers showed better thermal stability.

Photo-chemically induced polycondensation of a pure phenolic resin for additive manufacturing

Additive manufacturing of phenolic resins as a proof-of-concept for the first photo-chemical induced polycondensation by Hot Lithography. Through the dual use of a photoacidgenerator, the first pure 3D printing of Bakelite© is investigated.

Modeling of the Kinetics of Polyoxymethylene Decomposition under Oxidative and Non-Oxidative Conditions

Research on the thermal and thermo-oxidative degradation of polyacetals allows for the development of effective methods of utilization of the waste of these polymers towards the recovery of monomers. For this purpose, in addition to qualitative analysis, it is necessary to understand the mechanisms of chemical reactions accompanying the decomposition process under the influence of temperature. Therefore, in this article, with the experimental results from the thermal analysis of the POM homopolymer of three various stages of life-POM-P-unprocessed sample; POM-R-recycled sample, and POM-O-sample waste-we took steps to determine the basic kinetic parameters using two well-known and commonly used kinetic models: Friedman and Ozawa-Flynn-Wall (OFW). Knowing the values of the course of changes in apparent activation energy as a function of partial mass loss, theoretical curves were fitted to the experimental data. The applied calculation models turned out to be consistent in terms of the nature of the curve changes and similar in terms of E_a in the entire range of mass loss. Both kinetic models showed a very similar course of the E_a curves. The samples that decompose under oxidative conditions obtained the best fit for the reaction of nth order with autocatalysis by product B model and the samples that decompose under inert conditions for the n-dimensional nucleation according to the Avrami-Erofeev model.

POM/EVA Blends with Future Utility in Fused Deposition Modeling

Polyoxymethylene (POM) is one of the most popular thermoplastic polymers used in the industry. Therefore, the interest in its potential applications in rapid prototyping is understandable. Nevertheless, its low dimensional stability causes the warping of 3D prints, limiting its applications. This research aimed to evaluate the effects of POM modification with ethylene-vinyl acetate (EVA) (2.5, 5.0, and 7.5 wt.%) on its processing (by melt flow index), structure (by X-ray microcomputed tomography), and properties (by static tensile tests, surface resistance, contact angle measurements, differential scanning calorimetry, and thermogravimetric analysis), as well as very rarely analyzed emissions of volatile organic compounds (VOCs) (by headspace analysis). Performed modifications decreased stiffness and strength of the material, simultaneously enhancing its ductility, which simultaneously increased the toughness even by more than 50% for 7.5 wt.% EVA loading. Such an effect was related to an improved linear flow rate resulting in a lack of defects inside the samples. The decrease of the melting temperature and the slight increase of thermal stability after the addition of EVA broadened the processing window for 3D printing. The 3D printing trials on two different printers showed that the addition of EVA copolymer increased the possibility of a successful print without defects, giving space for further development.

Thermal Stabilization of Polyoxymethylene by PEG-Functionalized Hydroxyapatite: Examining the Effects of Reduced Formaldehyde Release and Enhanced Bioactivity

Nanostructured hydroxyapatite (HA) functionalized with poly(ethylene glycol) (HA-g-PEG) of different molar mass was used as a thermal stabilizer to prepare polyoxymethylene (POM) composites by a melt processing method. The chemical and crystalline structure of (HA-g-PEG) and POM/HA-g-PEG composites was investigated by means of FTIR and XRD. The thermal properties, degree of crystallinity, and melting behaviour of POM-based composites were analysed with TG, DSC, and TOPEM DSC methods. The tensile strength, Young’s modulus, toughness, and wettability of POM were investigated as well. A preliminary assessment of bioactivity, in vitro chemical stability, and formaldehyde release from POM/HA-g-PEG composites by Schiff’s test was also performed. An SEM/EDX method was used to observe the morphology of POM/HA-g-PEG composites. The results indicate that the addition of 1% HA-g-PEG slightly increases the melting temperature and degree of crystallinity. In small amounts, HA-g-PEG particles probably act as nucleating agents for the POM crystallization process. Incorporation of 5% HA-g-PEG to POM caused a decrease in the crystallinity of the polymer matrix, as a result, some mechanical properties of POM/HA-g-PEG composites also decreased. The thermal stability of POM/HA-g-PEG composites improved significantly from 309°C for unmodified POM to 342°C for POM/10.0% HA-g-PEG 600. The most effective thermal stabilizer was synthesized with the lowest mass-average molar mass PEG. The in vitro bioactivity test confirmed that, as the average molar mass of PEG in HA-g-PEG hybrids increased, POM-based composites indicated higher bioactivity. The in vitro chemical stability analysis results showed that both the POM matrix and the HA-g-PEG additive remain stable during the whole incubation time. Importantly, after seven days of dynamic incubation, no formaldehyde was detected in all filtrates, which is crucial in biomedical applications, among others.

Thermally Triggered Vanishing Bulk Polyoxymethylene for Transient Electronics

Transient materials capable of disappearing rapidly and completely are critical for transient electronics. End-capped polyoxymethylene (POM) has excellent mechanical properties and thermal stability. However, research concerning the inherent thermal instability of POM without end-capping to obtain transient rather than stable materials, has never been reported. Here, POM without end-capping is proposed as a novel thermally triggered transient solid material that can vanish rapidly by undergoing conversion to a volatile gas, and a chemical vapor deposition method is developed to obtain a smooth POM substrate from the synthesized POM powder. Experimental and theoretical analysis was employed to reveal the mechanism whereby the POM substrate formed and vanished. A Cr/Au/SiO2/Cu memristor device, which was successfully deposited on the POM substrate by physical vapor deposition, exhibits bipolar resistive switching, suggesting that the POM substrate is suitable for use in electrical devices. Thermal triggering causes the POM substrate to vanish as the memristor disintegrates, confirming excellent transient performance. The deposited bulk POM material can completely vanish by thermally triggered depolymerization, and is suitable for physically transient substrates and packaging materials, demonstrating great prospects for application in transient electronics for information security.

Investigation on the Crystallization Behaviors of Polyoxymethylene with a Small Amount of Ionic Liquid

Polyoxymethylene (POM) blends with excellent stiffness⁻toughness balance are successfully developed using Tributyl(octyl)phosphonium bis(trifloromethanesulfonyl) imide (TBOP-TFSI), one type of room-temperature ionic liquid, as the nucleating agent. Crystallization behaviors of POM blends have been studied by differential scanning calorimetry (DSC) and polarized light microscopy (PLM). The incorporation of TBOP-TFSI induces the crystal nucleation and fine crystal grain of POM, and also a much shorter hemi-crystalline time with only 0.5 wt% addition. The nucleation effect of ionic liquid leads to considerable improvement in the impact strength of POM blends while not sacrificing its tensile strength. Moreover, antistatic properties with a long-time stable performance are achieved by TBOP-TFSI addition as the electrical resistance reaches 1011 Ω/sq.

Synthesis and characterization of chitosan-coated titanate nanotubes: towards a new safe nanocarrier

Chitosan-coated titanate nanotubes as promising new nanocarriers: two different approaches, two different behaviors.

Processing and adjusting the hydrophilicity of poly(oxymethylene) (co)polymers: nanoparticle preparation and film formation

Handling the insoluble POM: the preparation of organic and aqueous nanoparticle dispersions based on poly(oxymethylene) copolymers and their film formation is described.

In situ formation of benzoxazines in polyoxymethylene: a simple approach for retarding formaldehyde generation and tuning mechanical properties under a semi-interpenetrating network

The thermal stability of polyoxymethylene (POM) significantly increases after blending with bisphenol-A and amine leading to polybenzoxazine formation in POM matrices.

Development of a microbicide-releasing diaphragm as an HIV prevention strategy

Contraceptive diaphragms offer a discreet method of pregnancy protection that women can use when needed with no side effects. Incorporating antiretroviral HIV microbicides into such devices may also provide protection against HIV infection. The paper gives a brief outline of the work being conducted by PATH, CONRAD and QUB on the development of a microbicide-releasing SILCS diaphragm. The design, engineering and manufacturing challenges that have been encountered will be discussed, as well as the potential impact such a device could have in the developing world.