Search

In this paper, the chemorheological and dynamic mechanical behavior of melamine-formaldehyde (MF) resins of four different formaldehyde/melamine (F/M) molar ratios (1.25, 1.5 1.75 and 2.00) are Investigated. ME resins polymerize via a polycondensation reaction involving formation of up to 10 wt% of [H.sub.2]O on cure. This typically results in rapid and extensive foaming of the resin when it is cured under atmospheric pressure. Experimental adaptation for the foaming behavior of ME resins is used to gather rheological Information concerning the curing kinetics and the mechanical response of neat ME resins of different molar ratios. Likewise, the procedures developed allow curing of the resins under atmospheric pressure, hence allowing volatile evacuation as occurs during venting procedures (commonly used during compression molding of ME molding compounds) or as a result of absorption by hydrophilic fillers or substrates. The results show that increased moisture content in the B-stage leads to faster reaction rates and greater foaming. Gelation and vitrification times are identified for each molar ratio, and are found to increase with decreasing molar ratio. The dynamic mechanical behavior of carefully molded neat ME samples of different molar ratios is studied using DMTA. [T.sub.g] is found to be 200°C for the resin with the lowest formaldehyde content (F/M = 1.25), and around 230°C for the other resins. The storage shear modulus above [T.sub.g] is studied, and the results show that the crosslink density increases with increasing molar ratio., INTRODUCTION Polymers based on melamine (1,3,5-triamino-2,4,6-triazine) and formaldehyde have been used commercially for over 60 years. Originally used as wood adhesives, they are now mainly found in flooring and decorative [...]

The development of recyclable crosslinked thermosetting fibres is a challenging research topic. In the present work, we have designed and synthesized polyurethane fibres from fructose-derived 5-chloromethylfurfural (CMF) and lignin-derived monomeric phenols. The greenhouse gas emissions associated with the production of CMF showed comparable results to that of 5-hydroxymethylfurfural (HMF), a high potential sugar-based platform molecule. The wet-spun biobased polyurethane fibres produced could be conveniently crosslinked using Diels-Alder chemistry to effectively enhance the glass transition temperature and mechanical properties. At a mildly elevated temperature (140 °C), the chemically crosslinked fibres could be effectively de-crosslinked, which enabled complete separation from a mixture with poly(ethylene terephthalate) (PET) and cotton fibres. These results outline a potential strategy to design and fabricate new biobased fibres with reversible crosslinking, which may enable fibre-to-fibre recycling., (© 2024 Wiley‐VCH GmbH.)

Failures to replicate evidence of new discoveries have forced scientists to ask whether this unreliability is due to suboptimal implementation of methods or whether presumptively optimal methods are not, in fact, optimal. This paper reports an investigation by four coordinated laboratories of the prospective replicability of 16 novel experimental findings using rigour-enhancing practices: confirmatory tests, large sample sizes, preregistration and methodological transparency. In contrast to past systematic replication efforts that reported replication rates averaging 50%, replication attempts here produced the expected effects with significance testing (P < 0.05) in 86% of attempts, slightly exceeding the maximum expected replicability based on observed effect sizes and sample sizes. When one lab attempted to replicate an effect discovered by another lab, the effect size in the replications was 97% that in the original study. This high replication rate justifies confidence in rigour-enhancing methods to increase the replicability of new discoveries., (© 2023. The Author(s).)

Several concurrent developments are shaping the future of plastics. A transition to a sustainable plastics system requires not only a shift to fossil-free feedstock and energy to produce the carbon-neutral building blocks for polymers used in plastics, but also a rational design of the polymers with both desired material properties for functionality and features facilitating their recyclability. Biotechnology has an important role in producing polymer building blocks from renewable feedstocks, and also shows potential for recycling of polymers. Here, we present strategies for improving the performance and recyclability of the polymers, for enhancing degradability to monomers, and for improving chemical recyclability by designing polymers with different chemical functionalities., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Significantly increased production of biobased polymers is a prerequisite to replace petroleum-based materials towards reaching a circular bioeconomy. However, many renewable building blocks from wood and other plant material are not directly amenable for polymerization, due to their inert backbones and/or lack of functional group compatibility with the desired polymerization type. Based on a retro-biosynthetic analysis of polyesters, a chemoenzymatic route from (-)-α-pinene towards a verbanone-based lactone, which is further used in ring-opening polymerization, is presented. Generated pinene-derived polyesters showed elevated degradation and glass transition temperatures, compared with poly(ϵ-decalactone), which lacks a ring structure in its backbone. Semirational enzyme engineering of the cyclohexanone monooxygenase from Acinetobacter calcoaceticus enabled the biosynthesis of the key lactone intermediate for the targeted polyester. As a proof of principle, one enzyme variant identified from screening in a microtiter plate was used in biocatalytic upscaling, which afforded the bicyclic lactone in 39 % conversion in shake flask scale reactions., (© 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

Survey institutes recently have changed their measurement of generalized trust from the standard dichotomous scale to an 11-point scale. Additionally, numerous survey institutes use different question wordings: where most rely on the standard, fully balanced question (asking if "most people can be trusted or that you need to be very careful in dealing with people"), some use minimally balanced questions, asking only if it is "possible to trust people." By using two survey-embedded experiments, one with 12,009 self-selected respondents and the other with a probability sample of 2,947 respondents, this study evaluates the generalized trust question in terms of question wording and number of scale points used. Results show that, contrary to the more commonly used standard question format (used, for example, by the American National Election Studies and the General Social Survey), generalized trust is best measured with a minimally balanced question wording accompanied with either a seven- or an 11-point scale.

An economically sustainable process was developed for propionic acid production by fermentation of glycerol using Propionibacterium acidipropionici and potato juice, a by-product of starch processing, as a nitrogen/vitamin source. The fermentation was done as high-cell-density sequential batches with cell recycle. Propionic acid production and glycerol consumption rates were dependent on initial biomass concentration, and reached a maximum of 1.42 and 2.30 g L(-1) h(-1), respectively, from 50 g L(-1) glycerol at initial cell density of 23.7 gCDW L(-1). Halving the concentration of nitrogen/vitamin source resulted in reduction of acetic and succinic acids yields by ~39% each. At glycerol concentrations of 85 and 120 g L(-1), respectively, 43.8 and 50.8 g L(-1) propionic acid were obtained at a rate of 0.88 and 0.29 g L(-1) h(-1) and yield of 84 and 78 mol%. Succinic acid was 13 g% of propionic acid and could represent a potential co-product covering the cost of nitrogen/vitamin source., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Six-membered cyclic carbonates are potential monomers for phosgene and/or isocyanate free polycarbonates and polyurethanes via ring-opening polymerization. A two-step process for their synthesis comprising lipase-catalyzed transesterification of a polyol, trimethylolpropane (TMP) with dimethylcarbonate (DMC) in a solvent-free system followed by thermal cyclization was optimized to improve process efficiency and selectivity. Using full factorial designed experiments and partial least squares (PLS) modeling for the reaction catalyzed by Novozym®435 (N435; immobilized Candida antarctica lipase B), the optimum conditions for obtaining either high proportion of monocarbonated TMP and TMP-cyclic-carbonate (3 and 4), or dicarbonated TMP and monocarbonated TMP-cyclic-carbonate (5 and 6) were found. The PLS model predicted that the reactions using 15%-20% (w/w) N435 at DMC:TMP molar ratio of 10-30 can reach about 65% total yield of 3 and 4 within 10 h, and 65%-70% total yield of 5 and 6 within 32-37 h, respectively. High consistency between the predicted results and empirical data was shown with 66.1% yield of 3 and 4 at 7 h and 67.4% yield of 5 and 6 at 35 h, using 18% (w/w) biocatalyst and DMC:TMP molar ratio of 20. Thermal cyclization of the product from 7 h reaction, at 110°C in the presence of acetonitrile increased the overall yield of cyclic carbonate 4 from about 2% to more than 75% within 24 h. N435 was reused for five consecutive batches, 10 h each, to give 3+4 with a yield of about 65% in each run., (Copyright © 2012 American Institute of Chemical Engineers (AIChE).)