Thiourethane thermoset coatings from bio‐based thiols

Three bio‐based thiols were synthesized via the thermal thiol‐ene reactions between sucrose soya ester (SSE) and multifunctional thiols; then, thiourethane coatings were produced from these thiols and their coating properties were studied. A series of high bio‐renewable content thiol oligomers were synthesized according to the previously reported thermal thiol‐ene reaction. Fourier transform infrared spectra (FTIR) confirmed the complete consumption of the double bonds in SSE, and gel permeation chromatography confirmed the formation of high‐molecular‐weight oligomers. The viscosity of these oligomers remained low due to their compact and branched structures. Thermoset thiourethane coatings were prepared from these thiol oligomers and polyisocyanate trimer resins with dibutyltin diacetate as the catalyst. FTIR spectra confirmed the formation of the thiourethane group. However, coatings based on isophorone diisocyanate (IPDI) polyisocyanate resin showed a lower degree of cure because of the decreased resin mobility due to the rigid cyclohexane ring. Generally, all the coatings showed good adhesion to aluminum panels, and had high gloss. However, they exhibited low tensile strength, modulus and chemical resistance due to the flexibility of the fatty acid chain. Coatings based on more rigid IPDI‐based polyisocyanate showed higher T_g, hardness and direct impact resistance compared with the hexamethylene‐diisocyanate (HDI) based polyisocyanate counterparts. Thermogravimetric analysis results showed that coatings based on mercaptanized soybean oil have better thermal stability than those from di‐pentene dimercaptan or glycol di‐3‐mercaptopropionate. Two T_g values were found by both differential scanning calorimetry and dynamic mechanical thermal analysis of thiourethanes from HDI‐based polyisocyanate and di‐pentene dimercaptan or glycol di‐3‐mercaptopropionate based oligomers due to phase separation resulting from the poor compatibility between HDI‐based polyisocyanate and the respective oligomers. Copyright © 2011 Society of Chemical Industry     

Structural and mechanistic aspects of the thermal degradation of poly(vinyl chloride)

A critical review of the title subject supports the following major conclusions. Thermal dehydrochlorination of poly(vinyl chloride) (PVC) begins with internal allylic chloride and tertiary chloride structural defects formed during polymerization. The tertiary chloride is associated with 2,4-dichloro-n-butyl, 1,3-di(2-chloroethyl), and chlorinated long branches. Mechanisms for the formation of all of the labile defects are well established. ‘Carbonylallyl’ structures and certain isotactic conformers of ordinary monomer units are unimportant as initiators of thermal dehydrochlorination. Both the initiation and the subsequent formation of conjugated polyene sequences occur via carbenium chloride ion pairs or by a closely related concerted four-center quasi-ionic route. Six-center concerted processes, pathways involving free radicals, and other mechanistic schemes suggested recently are not involved in polyene elongation. However, during thermal degradation, ordinary monomer units are converted into internal allylic chloride defects by a mechanism that may include the abstraction of hydrogen by triplet cation diradicals derived from polyene intermediates. Cyclization reactions seem likely to contribute to the termination of polyene growth. When PVC is thermolyzed in blends with other polymers, unusual kinetic phenomena are detected that remain to be fully explained.     

Kinetics and mechanism of the thermal degradation of poly(vinyl chloride)

The kinetics of the thermal degradation of solid powdered poly(vinyl chloride) (PVC) under nitrogen was studied by thermogravimetry, rate of hydrogen chloride evolution, and rate of polyene sequence formation. These results are accommodated by a chain mechanism involving initiation by random dehydrochlorination at normal monomer residues of PVC, and a series of intermediates, each leaking to a stable conjugated polyene sequence. Structural irregularities such as allylic and tertiary chlorine are responsible for a fast initiation process at the very beginning of the degradation. Mean rate constants and activation parameters for random initiation, propagation, and termination reactions of the PVC degradation chain were calculated by simulation. Activation enthalpy/entropy correlations for the experimental data available for dehydrochlorination of chloroalkanes and chloroalkenes in the gas and in the liquid phase or nonpolar solvents and elementary reactions of PVC degradation show that initiation is an HCl elimination through a transition state of four centers requiring a synperiplanar conformation of the >CH–CCl< group, whereas propagation is a dehydrochlorination through a transition state of six centers requiring a cis configuration of the double bond.     

Novel thermal curing of cycloaliphatic resins by thiol–epoxy click process with several multifunctional thiols

Novel thermosets were prepared by the base‐catalysed reaction between a cycloaliphatic resin (ECC) and various thiol crosslinkers. 4‐(N,N‐Dimethylaminopyridine) (DMAP) was used as base catalyst for the thiol–epoxy reaction. A commercial tetrathiol (PETMP) and three different thiols synthesized by us, 6SH‐SQ, 3SH‐EU and 3SH‐ISO, were tested. 6SH‐SQ and 3SH‐EU were prepared from vinyl or allyl compounds from renewable resources such as squalene and eugenol, respectively. Thiol 3SH‐ISO was prepared starting from commercially available triallyl isocyanurate. A kinetic study of the mixtures was performed using differential scanning calorimetry. Stoichiometric ECC/thiol/DMAP formulations were cured at 120 °C for 1 h, at 150 °C for 1 h and post‐cured for 30 min at 200 °C. The materials were characterized using Fourier transform infrared spectroscopy, thermogravimetric analysis and dynamic mechanical thermal analysis. The results revealed that the materials obtained from the synthesized thiols had higher thermal stability and glass transition temperatures than those obtained from the commercial PETMP. In addition, all the materials obtained exhibited very good transparency. This study proves the ability of multifunctional thiols to crosslink cycloaliphatic epoxy resins, leading to more flexible materials than those obtained by cationic homopolymerization of ECC or base‐catalysed ECC–anhydride copolymerization. © 2017 Society of Chemical Industry     

Benzenethiols as inhibitors for the corrosion of copper

The effect of benzenethiol (BT) and some substituted benzenethiols — {o-methyl benzenethiol (o-MBT), o-amino benzenethiol (o-ABT), p-amino benzenethiol (p-ABT) and m-methyl benzenethiol (m-MBT)} — on the corrosion of copper in H₂SO₄ and NaOH solutions has been investigated by means of spectrophotometric measurements, potentiostatic polarization studies, cyclic voltammetry and potential stepping experiments. In acid solutions, under ordinary conditions, the thiols are adsorbed onto the metal surface in their molecular forms through their thiol(-SH) moiety and when the potential is made more noble oxidative dimerization of the thiols takes place, and the dimer replaces the thiol. Both thiols and their dimers are not effective as inhibitors. In alkaline solutionsin situ polymerization of thiol takes place to yield polythiol which is a very effective inhibitor both in acidic and alkaline media.     

Hybrid dual-curable cyanate ester/boron phosphate composites via sequential thiol-ene photopolymerization and thermal polymerization

The aim of this study was to improve hybrid dual-curable cyanate ester/boron phosphate composites via sequential thiol-ene photopolymerization and thermal polymerization for high performance applications such as aerospace and electronic devices. A novel 2,2′-diallylbisphenol A dicyanate ester (DA-BADCy) which is the allyl group containing cyanate ester was synthesized and characterized. DA-BADCy, silicon containing monofunctional thiol compound, trifunctional thiol compound and boron phosphate were cured using both ultraviolet (UV) and thermal methods. Using thiol-ene system, cyanate ester formulations, which are normally prepared at high temperatures, were prepared at room temperature. This study maintains ease of application for cyanate esters. Thermal stability, flammability and thermal conductivity of the samples were evaluated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), limiting oxygen index (LOI) and laser flash method, respectively. The samples were characterized with the following analysis; gel content, water absorption capacity and stress–strain test. Hydrophobicity of the samples was determined by the contact angle measurements. Moreover, the surface morphology of the samples was investigated by a scanning electron microscopy (SEM–EDS). The obtained results prove that the composites have good thermal and mechanical properties and with the help of easier preparation techniques, they can be used in many applications such as aerospace, electronic devices, materials engineering.     

Effect of thermal decomposition on dielectric relaxation process of poly(vinyl chloride)

Dielectric absorption caused by the molecular relaxation of thermally decomposed poly(vinyl chloride) in air was studied on samples in the form of thin films. It was found that, with the progress of thermal decomposition, the magnitude of α dielectric absorption changes in three stages: (1) initial decrease in magnitude corresponding to the process of the formation of polyene sequences caused by dehydrochlorination; (2) ensuing increase corresponding to the deformation of polyene sequences attributed to oxidation; (3) final decrease corresponding to the formation of crosslinks.     

Non-innocent ligands in copper complexes catalyzed oxidation of thiols

Copper(II) chloride complexes with nitrogen-containing ligands (amines, amides, heterocycles) were studied as catalysts of selective oxidation of thiols to disulfides. This process has an important biochemical analogue – the formation of disulfide bridge in proteins [Wang C, Wesener SR, Zhang H, Cheng Y-Q. An FAD-dependent pyridine nucleotide-disulfide oxidoreductase is involved in disulfide bond formation in FK228 anticancer depsipeptide. Chem and Biol. 2009;16:585–593]; which affects their activity [Zhang L, Chou CP, Moo-Young M. Disulfide bond formation and its impact on the biological activity and stability of recombinant therapeutic proteins produced by Escherichia coli expression system. Biotech Adv. 2011;29:923–929]. Also, thiol oxidation is an important process of oil sweetening. The search for effective and stable catalysts for this reaction is still topical [Ganguly SK, Das G, Kumar S, Sain B, Garg MO. Mechanistic kinetics of catalytic oxidation of 1-butanethiol in light oil sweetening. Catal Tod. 2012;198:246–251]. This article describes a mechanistic study of thiol oxidation and non-innocent ligands transformations during this reaction. The studied ligands are capable of oxidizing thiols, the reaction being followed by reoxidation of the reduced forms of the ligands with air oxygen. The oxidative activity of the ligand correlates with catalyst activity for thiol oxidation.     

Acceleration and Inhibition Effect of Tertiary Amines on Thermal Degradation of Poly(Vinyl Chloride)

Poly(vinyl chloride) (PVC) is occasionally discolored yellow or red by the formation of polyene chains in the polymer backbone. It has been noted that the formation of such polyene structures is caused by dehydrochlorination of the PVC, accelerated by tertiary amines. Thus, in the present study, we investigated the influence of amines on the formation of polyene structures in PVC, using resonance Raman spectroscopy. The amount of polyene produced by thermal treatment with amine vapor exposure was determined based on the resonance Raman intensity ratio of the polyene band to the PVC band. The results showed that the discoloration of PVC, indicating the formation of polyene structures, was most prominently caused by 1,4‐diazabicyclo[2.2.2.]octane (DABCO), bis(2‐dimethylaminoethyl)ether (BDMEE), or N,N,N′,N′‐tetramethyl‐1,6‐diaminohexane (TMDAH), because of their high basicity and nucleophilicity. In addition, the formation of polyene structures was inhibited by the co‐presence of amine and protic solvent (water, ethanol, and 2‐propanol) or additives containing hydroxyl groups (glycerin, poly[vinyl alcohol]), suggesting that amine nucleophilicity toward PVC is reduced by the protonation of amine to lone pairs of tertiary amines. J. VINYL ADDIT. TECHNOL., 26:253–258, 2020. © 2019 Society of Plastics Engineers     

Thermal stabilization of PVC by “plasticizer thiols”

Various new carboxylate esters containing one or more sulfhydryl groups are shown to be remarkably effective as both thermal stabilizers and plasticizers for poly(vinyl chloride) (PVC). These “plasticizer thiols” function well as primary stabilizers in the absence of metal‐containing additives when they are introduced at either a typical plasticizer level or a conventional stabilizer level. Their syntheses are straightforward and, in some cases, require only an acid‐catalyzed esterification performed with commercially available starting materials. Unlike typical thiols, the purified plasticizer thiols do not have offensive odors when their molecular weights are relatively high.     

On the thermal degradation of poly(vinyl chloride). III. Structural changes during degradation in nitrogen

The structural changes in poly(vinyl chloride) during thermal degradation in nitrogen at 190°C have been investigated. From gel permeation chromatography analyses no chain scission, but only crosslinking reactions were observed. An increase in the molecular weight was measured even at 0.3% conversion. For longer polyene sequences and at higher conversions, a crosslinking reaction competed with the “zipper” propagation. The secondary reactions, were more extensive at longer polyene sequence lengths. The growing polyene sequences can be terminated not only by branching reactions but also at existing pendent chloromethylene groups. A decrease in the amount of short chain branching with conversion also indicated other types of secondary reactions. Such a decrease was also observed during thermomechanical degradation in a Brabender Plastograph. The average polyene sequence length was calculated to be around 10, depending somewhat on the type of analysis used. Although allylic chlorine atoms seem to be the main points of initiation, other sites cannot be excluded as the number of initiation points increases appreciably during the early stages of the degradation. Such an increase is, of course, also consistent with a radical mechanism.     

Stabilization of poly(vinyl chloride). VIII. Synergisms between epoxy compounds and metal soaps

Effects of bisphenol A type epoxy compounds involving various average molecular weights on the zinc stearate/calcium stearate and the cadmium stearate/barium stearate synergetic soaps induced thermal stabilization of poly(vinyl chloride) (PVC) were investigated by colorimetry. The remarkable stabilization effects of epoxides could not be observed on the PVC films without synergetic soaps, while the stabilization of PVC was markedly enhanced by combined use of epoxides and synergetic soaps. The appearance of excessive coloration of cool color producing metal chloride–polyene complexes which were an origin of abrupt discoloration of stabilized PVC was retarded by using epoxides together with synergetic soaps. Moreover, as for PVC with or without synergetic soaps, the epoxy compounds did not inhibit the formation of longer polyene chains which were a chromophore for yellow orange of aged PVC. Further colorimetries and IR or X-ray photoelectron spectroscopies on the various PVC containing epoxy compounds and zinc chloride indicated that the epoxy groups caught the zinc chloride. The synergetic effect between epoxy compounds and synergetic metal soaps is ascribed to the action that the epoxides serve as an acceptor for the excessive cool color producing metal chloride produced from zinc stearate and cadmium stearate to retard the abrupt discoloration of stabilized PVC.     

高稳定性巯基-丙烯酸酯光固化体系的制备及性能研究

传统巯基-丙烯酸酯光固化配方由于使用一级硫醇,贮存稳定性差,气味大,应用范围受限。文章以气味较低的二级硫醇-丙烯酸酯体系为对象,系统研究了二级硫醇含量对配方的稳定性、涂层的热性能、机械性能和基本涂膜性能的影响。结果表明:二级硫醇-丙烯酸酯体系具有较高的稳定性,在室温下放置25 d仍保持良好的流动性,而一级硫醇-丙烯酸酯体系放置1 d即发生凝胶。相比于纯丙烯酸酯体系,加入10%(巯基摩尔含量)的二级硫醇即可显著抑制体系的表面氧阻聚,同时双键转化率由47%上升到78%;随着二级硫醇含量的继续增加,涂层的玻璃化转变温度、储能模量和硬度逐渐降低,固化膜的柔韧性、耐冲击性和聚合网络的均一性逐渐增加。因此,通过改变巯基含量可有效调控固化膜的性能,扩大光固化技术的应用领域。     

Photocrosslinked norbornene–thiol copolymers: Synthesis,mechanical properties,and cure studies

A number of norbornene-functionalized resins were prepared and their photoinitiated crosslinking reaction with a multifunctional thiol was studied. Access to these novel resins can be gained by several synthetic routes. The most general of these routes is the [4π + 2π] cycloaddition reaction (the Diels–Alder reaction) of the corresponding multifunctional acrylate ester with cyclopentadiene (CPD) monomer. The photocrosslinking reaction of these resins with multifunctional thiols such as pentaerythritol tetramercaptopropionate (PETMP) is quite rapid and sensitive to low UV dose as well as dose rate. FT-IR cure studies also indicated that the cure rate slows down dramatically as the conversion approaches the calculated gel points for the systems under study. Mechanical properties of cured thin films of these materials exhibited a wide range of tensile and dynamic mechanical properties, which depended on the structure of starting materials used in the preparation of the norbornene resin and the crosslinking thiol.     

Mucoadhesive interaction of cysteine grafted poly(2-hydroxyethyl aspartamide) with pig mucin layer of surface plasmon resonance biosensor

A new type of thiolated polyaspartamide was synthesized and characterized with a surface plasmon resonance (SPR) biosensor. l-Cysteine grafted poly(2-hydroxyethyl aspartamide) (PHEA)s was obtained by grafting S-(Trityl)-l-cysteine to poly(succinimide) and subsequently deprotecting Trityl groups. The degree of substitution was confirmed by ¹H NMR, and elemental analysis. The amount of free thiol groups immobilized on the PHEA was determined by colorimetric reaction of thiols with Ellman's reagent. The stability of thiol groups at different pH levels was also reported. Mucoadhesive interaction was determined by pig mucin immobilized SPR biosensor. It is concluded that cysteine grafted PHEA enhances the interaction with mucin but that the amount of free thiol groups among the grafted thiols determines the mucoadhesive properties.     

An investigation of mechanisms of synergistic interactions in PVC stabilization

Synergism in poly(vinyl chloride) stabilization has been studied by measuring rates of hydrogen chloride evolution from samples of polymer in the presence of stabilizers in di-(2-ethylhexyl) phthalate solution in an inert atmosphere. Barium, cadmium, calcium, and zinc laurates, when used alone, allow escape of hydrogen chloride well before stoichiometric uptake is achieved, whereas synergistic mixtures of calcium–zinc and barium–cadmium laurates absorb almost the theoretical quantity of hydrogen chloride. Cadmium and zinc laurates replace labile chlorine atoms in the polymer backbone by ester groups, reducing formation of long polyene sequences: barium and calcium laurates delay the formation of cadmium and zinc chlorides, apparently by reconverting them into their respective laurates. Polyols function by forming complexes with the prodegradant cadmium and zinc chlorides, but contrary to popular belief phosphites possess little activity in this respect. Instead, they slow down the rate of polymer degradation by removal of labile chlorine atoms, by reaction with hydrogen chloride, and by peroxide decomposition.     

Coupling reaction with thiols as the efficient method of functionalization of ‘‘clickable” polylactide

The effectiveness of coupling reactions with thiols proceeding under different conditions is compared as the method of polylactide functionalization. Metal-free syntheses of polylactides (PLAs) fitted at one chain end with unsaturated (allyl, propargyl or acrylate) groups were performed by applying acid catalyzed activated monomer polymerization of lactide using allyl and propargyl alcohols or hydroxyethyl acrylate as initiators. Functional PLAs were used for thiol-ene and thiol-yne reactions proceeding according to radical as well as nucleophilic addition. The coupling efficiency for thiols containing carboxyl groups (in comparison with model benzyl thiol) was investigated by MALDI TOF method.     

Polymer reagents: preparation of polymer esters of thiocarboxylic acids based on crosslinked polystyrene

Summary The process of acylation of polymer thiols based on microporous poly (styrene-co-divinylbenzene) (crosslinking agent content 1–2%) with carboxylic acids was investigated depending of the degree of functionalization, on the crosslinking of the polymer carrier and on the structure of acids. In the presence of dicyclohexylcarbodiimide and 4-dimethylaminopyridine the acylation reactions can be conducted to high degrees of the transformation of SH groups already at room temperature. The extent of such refunctionalization reactions depends neither on the degree of functionalization of the starting polymer thiol nor on the structure of the acids. Reactions of polymer esters of thiocarboxylic acids with primary, secondary and tertiary alcohols activated with mercury (II) trifluoroacetate lead to low ester yields.     

Thermal degradation of poly(vinyl chloride) blends

Poly(vinyl chloride) was mixed with various poly(methacrylate)s and polycarbonates by combined precipitation from common solutions. The thermal stability of the samples was measured at 180°C under nitrogen, the HCl evolved was detected by conductometry. UV-Vis-spectra of degraded samples were measured to investigate the influence of the poly(methacrylate)s on the lengths of polyenes formed during the degradation of poly(vinyl chloride). The experiments show that the nature of the ester group is the dominating factor for the thermal stability of poly(vinyl chloride) in these blends. Poly(n-butylmethacrylate) exhibits the best stabilization for poly(vinyl chloride) in this series. Polycarbonates with a higher glass transition temperature than the temperature of degradation destabilize poly(vinyl chloride). Stabilization experiments with dibutyltin-bis(isooctylthioglycolate) show a costabilizing effect of the poly(methacrylate)s and polycarbonates.     

Photochemical reactions of polyene sequences produced during the degradation of poly(vinyl chloride)

Photosensitized reactions of tetrahydrofuran (THF) and dichloromethane (DCM) solutions of polyene sequences introduced into poly(vinyl chloride) molecules by chemical degradation have been investigated. The distribution of polyenes produced following further thermal degradation in DCM were shifted toward longer sequences than were observed for the same reaction in THF. Benzophenone-sensitized bleaching of the polyene absorption in aerated THF solutions was characterized by induction periods, but the presence of oxygen had little effect on the same reaction in DCM. The bleaching process seems to involve reaction of the polyenes with the tetrahydrofuranyl radical formed by abstraction of the α-hydrogen from THF by triplet benzophenone. No induction periods were observed for benzoin-sensitized reactions, but the rate of reaction was faster in DCM than in THF. The differences are discussed in terms of the difference in reactivity of the alkyl and alkoxy radicals.