Magic-angle 13C-NMR of cured and degraded acrylic copolymer/melamine formaldehyde coatings

Magic-angle carbon-13 NMR has been used to probe the structure and dynamics of acrylic copolymer melamine formaldehyde crosslinked coatings. Changes in chemical composition that occur in conventional accelerated weathering tests were found to be dominated by hydrolysis of acrylic-melamine crosslinks and subsequent formation of melamine-melamine crosslinks. Evidence for photo-oxidation was also observed. These results are in substantial agreement with infrared measurements also made on these coatings. Gated high-power decoupling experiments were used to determine relative mobilities of the different carbon resonances as a function of acrylic copolymer composition and as a function of degradation. It was found that motion of the side-chain carbons on the acrylic copolymer was not sensitive either to the chemical composition of the coating or to the extent of degradation. Mobilities of the main-chain carbons of the acrylic copolymer decrease with increasing glass transition temperature of the acrylic copolymer. For acrylic copolymers containing styrene, it is found that the main-chain carbon mobilities decrease with weathering. The melamine triazine ring becomes very rigid after degradation consistent with the formation of short melamine-melamine crosslinks.     

Nitroxide kinetics during photodegradation of acrylic/melamine coatings

Photooxidation of bis (2,2,6,6-tetramethylpiperidinyl-4) sebacate to nitroxide has been followed by electron paramagnetic resonance in acrylic melamine coatings under a number of ultraviolet light exposure conditions. The strong relationships observed between nitroxide kinetic behaviour, coating composition and exposure condition suggest that nitroxide kinetics analysis could be useful in probing coating photodegradation. This approach, together with infrared spectroscopic measurements of chemical changes on degradation, has suggested a novel mechanism, viz. photo-assisted hydrolysis, to explain the observation that photodegradation in acrylic melamine coatings proceeds more rapidly under more humid conditions. Photo-assisted hydrolysis of melamine ether crosslinks yields formaldehyde which can promote free radical oxidation leading to a more rapid photodegradation of these coatings under humid conditions than under dry conditions.     

丙烯酸类单体与倍半铝的络合常数及其与共轭双烯烃的交替共聚

用紫外光谱法测定了丙烯腈、丙烯酸甲酯、甲基丙烯酸甲脂和甲基丙烯酸丁酯与倍半铝 的络合常数(K),其值分别为123,82.6,50和17.51/mol,它们和单体的e值有关:K=2.57 exp(1.39e)。 丙烯酸类单体与共轭双烯在倍半铝存在下的交替共聚反应速度,随络合常数值及两种单体e值之差增大而增大,说明单体极性效应在这种给电子—受电子单体共聚中起重要作用。     

Biomimetic synthesis of arteannuin h and the 3,2-rearrangement of allylic hydroperoxides

The acyl endoperoxide arteannuin H, recently reported as a novel natural product from Artemtsia annua, has been obtained in two steps from the photooxidation of dihydroartemisinic acid, thereby confirming biogenetic speculation regarding its derivation from a secondary allylic hydroperoxide. The little studied 3,2-rearrangement reaction of such allylic hydroperoxides is also discussed.     

High Solid Acrylic–Melamine Latexes with Tunable Crosslinking Capability

Miniemulsion polymerization is employed to produce high solid content (50%) acrylic/melamine latexes with varied crosslinking capability, for their potential application as waterborne crosslinkable coatings. This synthesis strategy allows the efficient incorporation of a hydrophobic crosslinker, iso‐butylated melamine (iBMF), into polymer particles, and to obtain latexes with different iBMF concentrations and acrylic polymer with varied content of hydroxylic groups (OH–). The involved crosslinking mechanisms and the physical transformations during film thermosetting of acrylic/iBMF nanocomposite are exhaustively investigated by combining thermal, spectroscopic, and thermo‐mechanical analyses. The influence of reactive groups concentration (iBMF and OH– content in the acrylic polymer) on the rate of curing, the crosslinking degree, and consequently onto the sensitive properties of cured films are discussed here.     

Detection of traces of oxidized and reduced sulfur compounds in small samples by combination of different high-performance liquid chromatography methods

Depending on the sulfur species, picomoles of different inorganic sulfur compounds can be detected and separated by HPLC in one arrangement in a sample volume less than 50 μl. The combination of fluorescence labelling of reduced inorganic sulfur compounds such as sulfide (S²⁻), sulfite(SO₃²⁻ and thiosulfate (S₂O₃²⁻) with monobromobimane followed by an extraction of elemental sulfur (S°) by chloroform treatment enables the detection of all mentioned sulfur compounds as well as sulfate (remaining aqueous phase) in the same sample. While the derivatized sulfur compounds could be detected by their fluorescence emission at 480 nm, elemental sulfur is identified by its UV absorption at 263 nm. Sulfate in the remaining aqueous phase is detected by HPLC with indirect UV detection at 254 nm. Detection ranges for the different sulfur compounds examined are as follows: sulfide (5 μM to 1.5 mM), sulfite (5 μM to 1.0 mM), thiosulfate (1 μM to 1.5 mM), elemental sulfur (2 μM to 32 mM) and sulfate (5 μM to >1 mM).     

PVC/MBS共混物的紫外光交联及光氧化反应

PVC及其共混物应用广泛,MBS作为PVC制品的透明抗冲改性剂,在提高PVC抗冲性能的同时,必然影响PVC在紫外光照下发生的光化学反应过程,我们用紫外可见光研究了MBS对PVC紫外光解脱HCl动力学,发现MBS对PVC光解脱HCl有抑制作用,本文利用FTIR光谱研究PVC／MBS共混体系中,MBS对PVC光氧化、光交联及光降解过程的影响。     

Influence of molecular weight on the thermal decomposition of hydroxyl terminated polybutadiene

Thermogravimetric analysis of hydroxyl terminated polybutadiene (HTPB) and its fractions of different molecular weights separated by preparative GPC shows two major stages of weight loss of different nature in a nitrogen atmosphere. The first stage is primarily depolymerisation, cyclisation and crosslinking of molecules and the second stage is mainly the decomposition of the residue from the first stage. The kinetic parameters, viz. activation energy E and pre-exponential factor A using four different non-isothermal integral equations show a systematic increase with increase in molecular weight for the first stage, whereas for the second stage, the effect of molecular weight on E and A values is not prominent. The increase in E and A values for the first stage is attributed to the formation of greater number of cyclised and crosslinked products from molecules of higher dimensions. Quantitative correlations between the kinetic constants and the molecular weight parameters were derived for the first stage as a quadratic curve following the equation: E or ln A = K₁ − K₂/M (where K₁ and K₂ are empirical constants whose values are different for the different molecular weight averages, viz. M_n, M_w and M_z and for the different equations).     

The photooxidative degradation and stabilisation of water-borne acrylic coating systems

The influence of manufacturing and processing parameters on the photooxidative stability of aqueous acrylic based latices is examined by FTIR and hydroperoxide analysis. These are based on emulsion polymerised formulations of methyl methacrylate and butyl acrylate. Here acrylic latices based on different formulations are chosen as homo-polymers and copolymers. The nature of impurities and oxidation products generated during their manufacture are characterised and inter-related to their influence on subsequent photooxidative degradation. This involves the use of reflectance FT-IR spectroscopy to show functional group changes together with colourmetric U.V. analysis to determine the photochemical generation of hydroperoxides. The early chemical changes and their subsequent influence on the physical and chemical properties of the latices during the later stages of photooxidation are found to exhibit a close inter-relationship. To date the nature of the surfactant and the composition of the latex in terms of end group modification are important parameters. De-esterification and hydroperoxide formation are found to be important processes during latex photooxidation while the addition of low levels of co-monomer to the emulsion give latices with improved photostability. While co-reactive hindered piperidine stabilisers are found to be effective the incorporation of simple terminal dialkyl acrylamide/methacrylamide groups are equally as effective. Oxygen scavenging via the formation of alkylamino radicals is disussed.     

Influence of molecular mass on the liquid crystal alignment of photosensitive fluorinated polyester films

Photosensitive fluorinated polyesters (polymer-n) of varying molecular mass M _n (number-average molecular mass) were synthesized. The thin films formed from polymer-n samples could induce liquid crystal (LC) alignment after irradiation by linearly polarized ultraviolet light. The LC alignment direction on the irradiated films was investigated in detail by linearly polarized infrared spectroscopy and polarizing optical microscopy. It was found that LC alignment behaviour changed with change in the molecular mass of polymer-n: irradiated films with lower or higher M _n induced homeotropic or homogenous alignment, respectively. There was no clear morphological anisotropy in these aligned films, as observed by atomic force microscopy. The surface energies of the irradiated films were also measured using the indirect contact angle method, where both surface energy and its polar component increased with increasing M _n. The variation in M _n could be considered as a main reason for varying alignment behaviour.     

Sensitizing effect of polyfunctional monomers on radiation crosslinking of polychloroprene

Radiation-induced crosslinking of polychloroprene containing nine different polyfunctional monomers (PFM) has been investigated. The PFMs were kneaded into polychloroprene on a Labo Plastomill at 80°C, and then the mixtures were irradiated using an electron beam accelerator, to initiate the crosslinking reaction. The results showed that the crosslinking density (1/M_c) of the product is linearly proportional to the absorbed dose (D) and the initial PFM content (C₀), i.e. 1/M_c=(A+KC₀)D, where A and K are constants. The values of K increased quickly with the specific unsaturation (moles of double bonds per 100 g of the monomer) of the PFMs. The tensile strength of crosslinked polychloroprene with 4G (Polyethyleneglycol 200 dimethylacrylate) or UA101H (Urethane prepolymer) was found to be the best.     

Ultraviolet weathering of HDPE/wood-flour composites coextruded with a clear HDPE cap layer

This study examined the effect coextruding a clear HDPE cap layer onto HDPE/wood-flour composites has on the discoloration of coextruded composites exposed to accelerated UV tests. Chroma meter, FTIR-ATR, XPS, SEM, and UV vis measurements accounted for the analysis of discoloration, functional groups, and degree of oxidation of both uncapped (control) and coextruded composites before and after UV exposures. Two separate discoloration characteristics occurred in the discoloration of composites. For uncapped WPCs (control), chemical changes due to photooxidation resulted in darkening followed by physical changes, including loss of colored wood components from the surface, as well as increased roughness on the surface, which led to lightening of WPCs. By contrast, because a hydrophobic cap layer prevented the loss of colored components from the surface, coextruding a clear hydrophobic HDPE cap layer over WPCs significantly decreased the discoloration during the weathering process. Photooxidation of wood components at the interface accounted for the discoloration of coextruded WPCs before the failure of cap layer. Moreover, as the cap layer absorbed a specified amount of UV light and reduced oxygen available to interface, it decreased the photooxidation rate at the interface compared to that at the WPCs surface.     

Photocatalysis of ethanol oxidation by tetrachloroferrate(III) supported on Dowex 2‐X8

FeCl₄^? heterogenized on a Dowex 2‐X8 anion exchange resin catalyzes the photooxidation of ethanol to acetaldehyde under visible and near‐UV irradiation (>345 nm). The rate of reaction is proportional to the oxygen partial pressure up to 1 atm. Oxidation is suggested to occur through the formation of 1‐hydroxyethylhydroperoxide, initiated by the photodissociation of a chlorine atom. The hydroperoxide re‐oxidizes the iron(II) species, both the oxidation and reduction steps producing acetaldehyde. This mechanism is consistent with the increases in yield with ethanol concentration in ethanol–toluene mixtures towards an asymptotic limit. Copyright © 2014 John Wiley & Sons, Ltd.     

Radiation preparation and swelling behavior of sodium carboxymethyl cellulose hydrogels

Sodium carboxymethyl cellulose (CMC) is a kind of degraded polymer under γ-irradiation. However, in this work, it has been found that CMC crosslinks partially to form hydrogel by radiation technique at more than 20% CMC aqueous solution. The gel fraction increases with the dose. The crosslinking reaction of CMC is promoted in the presence of N₂ or N₂O due to the increase of free radicals on CMC backbone, but gel fraction of CMC hydrogel is not high (<40%). Some important values related to this kind of new CMC hydrogel synthesized under different conditions, such as radiation yield of crosslinking G(x), gelation dose R_g, number average molecular weight of network M_c were calculated according to the Charlesby–Pinner equation. The results indicated that although crosslinked CMC hydrogel could be prepared by radiation method, the rate of radiation degradation of CMC was faster than that of radiation crosslinking due to the character of CMC itself. Swelling dynamics of CMC hydrogel and its swelling behavior at different conditions, such as acidic, basic, inorganic salt as well as temperature were also investigated. Strong acidity, strong basicity, small amount of inorganic salts and lower temperature can reduce swelling ratio.     

四丁基碘化铵催化甲基丙烯酸甲酯的可逆-休眠自由基溶液聚合

以四丁基碘化铵(BNI) 为有机催化剂, 碘单质(I₂) 与偶氮二异庚腈(ABVN) 原位生成的碘代异庚腈为引发剂, 进行甲基丙烯酸甲酯(MMA) 的溶液聚合. 以甲苯为溶剂, MMA:I₂:ABVN的摩尔比为200:1:1.7, 考察了催化剂用量对聚合的影响. 结果表明, 加入催化剂可以缩短诱导期, 当I₂:BNI摩尔比为1:1时聚合反应的诱导期最短(1.7 h); 当BNI:I₂摩尔比为0.25:1~2:1之间时, 聚合物实测分子量与理论值十分接近, 分子量分布较窄, 分子量分布指数(M_w/M_n) 多在1.2以下. 考察了在N,N'-二甲基甲酰胺(DMF)、 四氢呋喃(THF)、 苯甲醚、 苯和甲苯5种溶剂中的聚合反应, 发现在苯和甲苯中聚合可控性最佳, M_w/M_n多在1.2以下; 苯甲醚和THF中聚合速率较快, 聚合物分子量分布较苯中的略宽. 以DMF为溶剂时所得聚合物分子量分布很宽, 聚合可控性差. 核磁共振分析聚合物为碘封端结构, 碘原子封端的聚合物链所占比为91.6%.     

Application of a pyrolysis-GC/MS system incorporating with micro-UV irradiation to rapid evaluation of the weatherability of acrylic coating paints for house exterior walls

A pyrolysis-GC/MS system incorporating with micro-UV irradiator (Xe-Hg lamp) was applied to the evaluation of the weatherability of the acrylic coating paints for house exterior walls. The deterioration of the materials during UV irradiation under thermal, and oxidative atmosphere was evaluated within a relatively short period of time with evolved gas analysis-mass spectrometry (EGA-MS) using a sub-milligram polymer sample. The EGA-MS thermograms indicated that both micro-UV irradiator with Xe-Hg lamp and weather meter with metal halide lamp caused similar photo, thermal, and oxidative degradation to the coating paint samples. Strong correlations between the results obtained by the UV/Py-GC/MS and that obtained by conventional weathering test using metal halide lamp was observed. It was suggested that UV/Py-GC/MS method could be used for compensating the conventional method by reducing the test period.     

Determination of fluoride in feed mixtures by capillary isotachophoresis

An isotachophoretic (ITP) method for the determination of fluoride in feed mixtures was developed. A sample of feed mixture, after extraction with 1 M HCl, was analysed using a ZKI 02 column-coupling isotachopherograph. Leading electrolytes for presentation and analytical capillaries consisted of 0.008 M HCl-0.022 M -aminocaproic acid (EACA)-0.001 M CaCl₂-0.05% hydroxypropylmethyl cellulose (HPMC) and 0.002 M HCl-0.005 M EACA-0.05% HPMC, respectively. The terminating electrolyte was 0.01 M tartaric acid. The fluoride released from samples by microdiffusion in 25% perchloric acid was determined using an Ionosep 900.1 single capillary isotachopherograph with 0.002 M HCl-0.005 M EACA-0.05% HPMC as the leading electrolyte and 0.002 M tartaric acid as the terminating electrolyte. The detection limit, depending on the sample treatment, was as low as 4 μg/g as fluoride. A comparison of the developed ITP method with ion- selective electrode method was carried out.     

Distribution of melamine in polyester–melamine surface coatings cured under nonisothermal conditions

The influence of experimental cure parameters on the diffusion of reactive species in polyester–melamine thermoset coatings during curing has been investigated with X‐ray photoelectron spectroscopy and attenuated total reflectance Fourier transform infrared. The diffusion of melamine plays a vital role in the curing process and, therefore, in the ultimate properties of coatings. At a low (<20%) hexamethoxymethylmelamine (HMMM) crosslinker concentration, the matrix composition is uniform, but at high HMMM concentrations, excess HMMM rapidly segregates to the air–coating interface. The rate of migration is governed by the difference in the surface free energies of polyester and HMMM and the concentration gradient of HMMM between the bulk and the surface. An increased rate of energy absorption also increases the rate of migration of HMMM to the surface. A physical model has been proposed to explain this surface segregation phenomenon in terms of cocondensation and self‐condensation reactions. It suggests that an appropriate amount of melamine can be segregated on the surface and allowed to self‐condense to form a desired thickness of a melamine topcoat through the control of the binder composition and cure conditions. This technique can be implemented to apply a melamine topcoat during cure. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 83–91, 2004     

三氯化铁对聚丙烯光氧化降解的作用

用红外及紫外光谱法研究了三氯化铁(FeCl₃)对聚丙烯(PP)光氧化降解的影响。实验结果表明,FeCl₃对聚丙烯光氧化有显著的促进作用,但促进作用与FcCl₃含量之间不成线性关系,存在一促进作用的极大值(在本实验条件下,FeCl₃含量为0.5%时,促进作用最大)。实验还表明,FeCl₃的作用可能包含了通过光解产生自由基引发PP光氧化反应、催化分解氢过氧化物以及光屏蔽等几种作用。根据FeCl₃的作用机理,解释了FeCl₃对PP光氧化降解的促进作用与其含量的依赖关系中存在的极位现象。     

Exchange interaction energy between two one-active electron atoms at large distances

Developed formulas for the exchange interaction energy between two neutral one-active electron atoms interacting at large distances R are presented in terms of known basic integrals obtained from an asymptotic method for the alkali dimers M₂ dissociating to M(ns) + M(ns), M(n′l) + M(n′l), l = 0, …, 3 and M (np) + M(np). Detailed illustrative numerical results are displayed for the molecular states of Na₂ dissociating to the limit Na(3s) + Na(3p). Comparisons with very accurate ab initio results as well as with some available experimental data show that accurate potential energy curves for these excited states may be obtained in a very large range of internuclear distances by connecting ab initio curves for small and intermediate values of R with long-range curves obtained as the sum of usual multipolar Coulombic energy and asymptotic exchange energy.