Crosslinking chemistry and network structure in organic coatings. I. Cure of melamine formaldehyde/acrylic copolymer films

Crosslinking chemistry and network formation in hydroxy and carboxy functional acrylic copolymer resins cured with representative melamine-formaldehyde crosslinking agents have been studied by infrared spectroscopy. Network formation in these systems is dominated by two reactions, the condensation of the hydroxy (or carboxy) functionality of the acrylic resin with melamine alkoxy groups to form acrylic-melamine crosslinks, and the condensation of melamine hydroxy groups to form melamine-melamine crosslinks. The extents of these reactions have been studied as functions of acrylic resin composition, melamine type and concentration, and cure time and temperature. For melamines with just methoxy functionality, the extent of formation of acrylic-melamine crosslinks increased steadily with cure temperature. For melamines with substantial hydroxy functionality, the extent of formation of acrylic-melamine crosslinks increased rapidly then leveled off with increasing cure temperature. The formation of melamine-melamine crosslinks increased slowly with increasing cure temperature. From these data and a statistical model, effective crosslink densities were calculated. The crosslink densities correlated well with solvent resistance.     

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.     

The role of humidity in the photooxidation of acrylic melamine coatings

Predicting the weatherability of acrylic melamine coatings commonly used as enamel clearcoats requires a detailed understanding of each of the factors that influence photooxidation kinetics. Previous work¹ has shown that the photooxidation rate in coatings can be written as the following function of hydroperoxide concentration: photooxidation rate = K[YOOH] + M. The existence of a measurable photooxidation rate in the absence of hydroperoxide (i.e. a non-zero value of the intercept, M) has been observed only in melamine crosslinked coatings. It has also been observed that the photooxidation rate in acrylic melamine coatings increases with increasing humidity. In contrast, for urethane crosslinked coatings the value of M is zero, and the photooxidation rate is independent of humidity. In this paper, infrared spectroscopic measurements of functional group changes (e.g. carbonyl growth and crosslink scission) are used to measure photooxidation rates in acrylic melamine coatings during UV exposures at different humidities. Comparisons of these rates to measured hydroperoxide concentrations for the same coatings and exposures reveal that the increase in photooxidation rate with humidity is due to the fact that the intercept M increases with increasing humidity. Since the intercept is zero under dry conditions, the chemical reactions responsible for the intercept in melamine crosslinked coatings must involve both UV light and moisture. These results confirm the importance of accurately controlling the humidity during UV exposure for predicting the weatherability of melamine crosslinked coatings.     

Use of chemiluminescence to the study of photostability of automotive coatings

Chemiluminescence (CL) imaging has been used to study the photodegradation of two automotive coatings, a melamine-crosslinked acrylic resin and an acrylic Polyurethane, as a function of artificial weathering time. Characteristics of the CL emission from coatings, the influence of light stabilizers and different types of sample were studied. The results indicate that the method is highly sensitive and can be used to measure photodegradation in unstabilized and stabilized coatings after only 48 h and 100–200 h of exposure, respectively. Measurements can be carried out on free clearcoats or on clearcoats applied on metallic panels. It is also possible to assess the degradation of individual layers in multiple-layer coating systems. Extrapolation to the failure time of coatings by CL is not yet achievable, but results show that the technique is useful for rapidly screening the relative performance of new coating formulations or light stabilizers added to clearcoats.     

Functionalised Microgels for Acrylic Coatings

Summary: Aqueous acrylic dispersions of hydroxy-functionalised copolymer microgel particles crosslinked with allyl methacrylate were synthesized by emulsion polymerization. The microgels were investigated as reactive polymer fillers in mixtures with a water-borne film-forming dispersion. Properties of coatings cast from mixtures of aqueous dispersion of hard microgel particles and film-forming water-borne dispersion were investigated. The swelling behaviour of microgels in selected solvents (aliphatic ketones) as a function of microgel composition is discussed as well. It was found that the swelling ability of microgels decreased with growing degree of crosslinking. Microgels comprising copolymerised butyl methacrylate swelled less in aliphatic ketones than microgels without this comonomer. This work was focused mainly on the influence of microgels incorporated in the commercial solvent-borne acrylic binders on the properties of coatings. It was shown that the application of microgels that were redispersed in acetone did not affect the surface appearance and transparency of coatings. Moreover, the presence of microgel network precursors accelerated film curing at ambient temperature and improved the final hardness of coatings.     

Identification of acrylate copolymers using pyrolysis and gas chromatography

A combination of pyrolysis and gas chromatography were used to investigate thermal degradation products formed from acrylic copolymers containing alkyl acrylate and methacrylate. The method provided an analytical tool for characterizing the chemical composition and structure of the degradation products. Thermal degradation of the synthesized copolymers was analyzed using isothermal (250 °C) pyrolysis–gas chromatography. The degradation process, and the nature and amount of pyrolysis products, provides relevant information about the thermal degradation of acrylic copolymers and the mechanism of pyrolysis. During pyrolysis, the formation of corresponding olefins, alcohols, acrylates and methacrylate was observed.     

Natural and artificial weathering characteristics of stabilized acrylic–urethane paints

Depth-profiling by Fourier transform infrared (FTIR) spectroscopy, dynamic mechanical analysis (DMA), microhardness and scanning electron microscopy (SEM) observations have been used to monitor degradation chemistries in two-package acrylic–urethane coatings when exposed to different exposure conditions. Three artificial and three natural weathering protocols (QUV, ASTM D5894, ISO20340, Pipady (south of France), Bandol (south of France) and Kure Beach (USA)) were selected for this study. The same chemical events were found to occur under all conditions, particularly under natural and artificial exposures. Both loss of the amide II band at 1520 cm⁻¹ and carbonyl growth occurred but at relatively low rate owing to the presence of hindered-amine light stabilizers in the film. A less typical loss of urea biuret linkages also occurs during all exposures and results in a change in the balance between urethane and urea links across the depth of the film during weathering. The chemical degradation of the polymer matrix involves the formation of species that are readily ablated from the surface and results in loss of gloss, increase in hardness and a rougher topology. The dramatic loss of gloss observed after Pipady and Bandol exposures show that loss of gloss should not be systematically correlated to the advance in chemical degradation.     

pH对丙烯酸-丁烯醛共聚合反应的影响

具有功能基的聚合物因其所具备的特殊性能而获得越来越多的应用[1] .带醛基的聚合物可在室温下很容易与酶、抗体、抗原、蛋白质、细胞等含氨基的生物高分子反应 ,生成Ｓｃｈｉｆｆ碱 ,将这些生物高分子共价偶联到聚合物上 ,并能保持它们绝大部分的生物活性 ,因而在免疫分析、生物化学、生物医学等领域有着广泛的应用前景[2～ 4 ] .然而 ,到目前为止 ,带醛基的聚合物的合成研究主要集中在丙烯醛的均聚及共聚反应[5～ 8] ,而对其它不饱和醛的聚合研究报道很少 .但是由于丙烯醛聚合物分子链上相邻的醛基极易形成缩醛或半缩醛而使自由醛基的数量…     

Adsorption of the Cu<Superscript>2+</Superscript> ions from aqueous solutions on the active carbon oxidized with hydrogen peroxide and impregnated with nitrogen-containing compounds

Adsorption of the copper ions from aqueous solutions, benzene, and water on the active carbon obtained by chemical activation from walnut shells was studied. The active carbon was additionally oxidized with hydrogen peroxide and then impregnated with nitrogen-containing compounds. As a result of impregnation with melamine, a secondary porosity is formed within the oxidized active carbon, leading to an increase in the specific surface of the sorbent. A modified carbon surface is heterogeneous, and the carbons themselves exhibit catalytic activity in the oxidation of luminol and decomposition of hydrogen peroxide.     

Mechanochemical synthesis of poly(butadiene-b-acrylic acid)

The preparation of block copolymer with polybutadiene, respectively poly(acrylic acid) sequences, using mechanochemical methods was investigated. The synthesis involves the generating of polybutadiene macroradicals, by ultra-high speed stirring of the polymer in toluene solution. The macroradicals are capable to initiate the polymerization of acrylic acid present in the reaction medium, leading to block copolymer formation. The influence of different parameters, such as temperature, duration, acrylic acid/polybutadiene molar ratio, on the yield and composition of the synthesized copolymer was studied.     

聚丙烯酸接枝辛基酚聚氧乙烯醚的合成、表征和胶束化

通过自由基聚合, 丙烯酸(AA)与辛基酚聚氧乙烯醚丙烯酸酯活性大单体(C8PhEO10Ac)共聚,合成了以聚丙烯酸为主链、C8PhEO10Ac 为支链的水溶性两亲接枝共聚物(PAA-g-C8PhEO10Ac), 用凝胶渗透色谱(GPC)测定其相对数均分子质量为4.37×10⁴, 用FTIR和1H-NMR表征了共聚物的结构和组成, 共聚物分子中丙烯酸单体与活性大单体的摩尔比为9:1, 每个共聚物分子中平均约有32个C8PhEO10侧链. 用表面张力法、荧光探针法和透射电子显微镜对共聚物在水溶液中的自组装行为进行了初步研究, 结果表明, 共聚物分子在第一临界胶束浓度cmc1)和第二临界胶束浓度(cmc2)时都形成了球型胶束. 与cmc1时相比, cmc2时溶液表面张力进一步降低, 胶束内部极性进一步减小, 而且胶束粒径增大、结构紧密. 氯化钠的加入可使共聚物溶液的表面张力和胶束内部极性降低.     

Photodegradation of carbon monoxide–styrene copolymer in benzene and formation of block copolymer with methyl methacrylate

Photodegradation of carbon monoxide–styrene copolymer in benzene was investigated. In relation to the average number of main-chain scissions per macromolecule versus photoirradiation time, the straight line did not pass through the origin. This phenomenon was attributed to the presence of a labile bond from the carbonyl group in the main chain, in accordance with the results of Cameron and Lawrence which were found in the study of photodegradation of thermally polymerized polystyrene. In utilization of photodegradative behavior in carbon monoxide–styrene copolymer, a block copolymer of styrene with methyl methacrylate was prepared and was ascertained by precipitated fractionation, elementary analysis, and turbidimetry.     

Weathering resistance of halogen-free flame retardance in thermoplastics

The influence of weathering on the fire retardancy of polymers is investigated by means of a cone calorimeter test, before and after artificial weathering. The surface degradation was monitored using different techniques (ATR-FTIR, microscopy, colour measurement). Different kinds of polymeric materials were chosen, all as they are used in practice: polycarbonate (PC) blends, polyamide (PA) and polypropylene (PP) flame-retarded with arylphosphate, melamine cyanurate (MC) and intumescent formulation based on ammonium polyphosphate (APP), respectively.All samples show material degradation at the surface due to weathering. No significant weathering influence occurs on the flame retardancy when it is a bulk property, as was observed for aryl phosphates in PC blends and MC in PA. When the fire retardancy is dominated by a surface mechanism, dependence on the duration of weathering is detected: for intumescent formulations based on ammonium APP in PP, a worsening in the formation of the intumescent network was observed.     

Scratch resistance and weatherfastness of uv-curable clearcoats

UV-cuing has found an increasing number of industry allocations over the past decade due to its unique benefits, e.g. solvent free formulations, high cure speed and low temperature processing. In addition to these benefits two additional properties of uv-cured coatings are of today's interest, especially in the automotive industry: scratch resistance and resistance against chemicals. One of the most important requirements for a broad use of uv-curable coatings in the coating industry is that coatings are stable against degradation caused by atmospheric influences since coatings for outdoor use are subject to especially harsh weathering conditions, e.g. uv-light, oxygen, moisture and air pollutants. This weathering leads to a degradation of the polymeric binder. Clearcoats containing photoinitiators based on bis-acylphosphinoxide (“BAPO”) and a combination of hydroxyphenyl-s-triazine uv-absorber and a sterically hindered amine as a light stabilizer package show a very good curing behavior as well as an improved weatherfastness over a long period of time and a good scratch and chemical resistance.     

Deamination and degradation of hydroxyl-containing dipeptides and tripeptides in the radiolysis of their aqueous solutions

The effect of the presence of hydroxyl groups at the side chains of di- and tripeptides on the radiation-initiated deamination and main-chain cleavage was examined. It was found that the presence of hydroxyl groups is favorable to peptide main-chain degradation with the formation of amino acid amides. The deamination was intensified or inhibited depending on the positions of OH groups in the molecules of test compounds. As a rule, both deamination and main-chain degradation processes in oxygenated solutions of di- and tripeptides were inhibited because of oxidation of hydroxyl-containing groups in the starting substances.     

Structure and swelling-release behaviour of poly(vinyl pyrrolidone) (PVP) and acrylic acid (AAc) copolymer hydrogels prepared by gamma irradiation

Copolymer network hydrogels were prepared by gamma irradiation of aqueous solutions of poly(vinyl pyrrolidone) (PVP) and acrylic acid monomer (AAc). The composition of the final hydrogels compared to the composition of the initial preparation solutions of hydrogels was determined. The chemical structure and nature of bonding was characterized by IR spectroscopy analysis, while the thermal durability of the prepared hydrogels was assessed by thermogravimetric analysis (TGA). The kinetic swelling in water and the pH-sensitivity of PVP/AAc copolymer hydrogels was studied. The drug release properties of PVP/AAc hydrogels taking methyl orange indicator as a drug model was investigated. The IR spectra indicate the formation of copolymer networks, whereas the TGA study showed that the PVP/AAc hydrogels possess higher thermal stability than pure PAAc and lower than PVP hydrogels. The kinetic swelling in water showed that all the hydrogels reached equilibrium after 24 h and that the degree of swelling increases with increasing the ratio of AAc in the initial feeding solutions. It was found that the degree of swelling of PVP/AAc hydrogels increases greatly within the pH range 4-7 depending on composition.     

Photopolymerizations initiated by oxime derivatives

A number of ester, urethane, and carbonate derivatives of biacetyl monooxime, dimethylglyoxime, and ketone oxime, were synthesized and their photolyses studied by means of ultraviolet spectroscopy. Most of the oxime derivatives photolyzed easily upon UV irradiation. Among them, however, only the free radicals formed by photolysis from the ester and carbonate of biacetyl monooxime could effectively initiate the polymerization of a vinyl monomer such as methyl methacrylate. Based on the results obtained from the monomeric reactions, syntheses of grafting polymers were made. Graft polymers were obtained by using a copolymer of methyl methacrylate and methacrylic acid–biacetyl monooxime ester (copolymer I) and that of methyl methacrylate and vinyl benzioc acid–biacetyl monooxime ester (copolymer II) in good yield and without the formation of homopolymer. It was also found that when copolymer I was employed as a grafting polymer, a considerable amount of main-chain scission was seen, but no chain degradation was noted in the case of copolymer II. Photocrosslinking was attempted by using these copolymers in the presence of divinyl benzene. It was confirmed that copolymer II was photocrosslinkable, whereas copolymer I underwent photodegradation.     

Sulfonated acrylamide copolymers as pseudo-stationary phases in electrokinetic chromatography

Sulfonated copolymers were synthesized, characterized and used as separation media in electrokinetic chromatography. The polymers used were synthesized from AMPS (2-acrylamido-2-methyl-1-propanesulfonic acid) and LMAm (lauryl methacrylamide) in different mole ratios (from 100:0 to 60:40). Electrophoretic mobilities and methylene selectivities were calculated, which showed the expected correlation with the monomer ratios. The chemical selectivities for the separation of nine solutes by the copolymers were compared with that of sodium lauryl sulfate micelles, showing significant differences. No significant difference in chemical selectivities was observed for copolymers with different monomer ratios. No significant change of hydrophobic microdomain of copolymers was found in background buffers with different ionic strength values, based on the investigation of the retention factors, methylene selectivities and polymer effective mobilities. No change of hydrophobic microdomain of the copolymer solutions was found at copolymer concentrations from 0.17 to 3% (w/v), however, plots of k' versus polymer concentration suggested a different copolymer phase at lower concentrations (from 0 to 0.1%, w/v) from that at higher concentrations (from 0.17 to 3%, w/v). The copolymer with AMPS-LMAm (80:20) could be chosen as optimum copolymer as far as the methylene selectivity, peak symmetry and polymer mobility were concerned.     

Natural and artificial photo-aging of non-stabilized and pigmented, hindered-amine stabilized propylene-ethylene copolymers

This paper describes the chemical evolution of two propylene-ethylene copolymer formulations, non-stabilized and carbon black filled - HALS stabilized, exposed to a range of natural and artificially accelerated weathering techniques, commonly used in the field of material testing. With the non-stabilized formulation, a fairly good representation of weathering in outdoor conditions was demonstrated with any of the accelerated exposures through the recognition of similar chemical changes at the molecular level. In contrast, the additives in the stabilized matrix induced very different chemical evolutions depending on the weathering techniques employed. This variability has been used to critically characterize those techniques on a scientific basis.     

Effect of natural weather on the structure and properties of polylactide/Cloisite 30B nanocomposites

The degradation of polylactide (PLA)/Cloisite 30B nanocomposites under natural weathering was investigated as a function of clay loadings (1, 3 and 5 wt.%) for up to 130 days using Fourier transform infrared (FT-IR) spectroscopy, size exclusion chromatography (SEC), nanoindentation measurements, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). For comparative purposes, the neat PLA was also considered. The FT-IR results showed that the photo-oxidation mechanism of PLA was not modified in the presence of Cloisite 30B, but only the degradation rates were accelerated. Moreover, the photo-oxidative degradation of PLA nanocomposite samples led to the formation of vinyl unsaturation, carbonyls, anhydrides and hydroperoxides groups as a result of the occurrence of several chemical mechanisms simultaneously. The decrease of the weight-average molecular weight, and the number-average molecular weight associated with an enhanced polydispersity of the nanocomposite samples indicated that chain scission was the most prominent phenomenon in natural weathering. The thermal degradation of the PLA was faster in the presence of clay. Modulus and hardness measured by nanoindentation increased slightly with exposure time for both neat PLA and PLA nanocomposite samples; the increase is also a function of the clay content. Finally, the weathering effect on the morphology of exposed samples observed by SEM revealed that the fractured surfaces exhibited many voids and cracks. These defects were much more pronounced for the PLA nanocomposites.