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.     

Crosslinking chemistry and network structure in organic coatings. II. Effect of catalysts on cure of melamine formaldehyde/acrylic copolymer films

In a previous paper, the extents of reaction of various functional groups in different acrylic copolymer melamine-formaldehyde crosslinked formulations were measured by infrared spectroscopy. From these data and a statistical model, various network structure parameters could be calculated. One of these, effective crosslink density, was found to correlate well with film solvent resistance. In this paper the effects of the addition of strong acid catalysts on cure chemistry and network structure are determined. Both solvent-based and water-based formulations have been studied. In the solvent-based formulations studied, the addition of acid catalysts greatly enhanced the crosslinking reactions at low temperatures and dramatically lowered the cure temperature. Acid catalysts also lowered the cure temperature of a water-based formulation based on a high (180°C) cure temperature melamine. In contrast, the addition of acid catalysts had a negligible effect on the cure chemistry of a water-based formulation crosslinked with a low (130°C) cure temperature melamine. For this case the rate limiting process was found to be the rate of removal of neutralizing amine from the film. Cure temperatures in this system could only be lowered by replacing the amine used with a more volatile amine.     

Stress relaxation of natural rubber during irradiation

The site of chain scission and crosslinking in vulcanized natural rubber irradiated with 4 MeV electrons has been determined by analysis of stress relaxation data. Sulfur and peroxide vulcanizates of different crosslink densities were prepared and the crosslink densities determined from stress-strain measurements. Stress relaxation was measured during irradiation using modified commercial relaxometers. The specimens were maintained in an atmosphere of nitrogen to minimize oxidative side effects. Scission is deduced to take place in the vicinity of crosslinks, since the rate of continuous stress relaxation is independent of crosslink density. Scission may be associated both with crosslinks initially present and with those subsequently introduced by irradiation. Crosslinking by radiation is largely a random process. However, there is a crosslinking reaction dependent to a slight extent on crosslink density as well as a small contribution from random scission reactions. G values for the random reactions are given.     

New approaches to reversible crosslinking in epoxy resins

The objective of the research is to investigate epoxy resins where crosslinks formed in curing reactions can be cleaved and reformed without significant sacrifice in resin properties. Experimental disulfide-containing crosslinking agents have been selected for the study, including dihydrazides of dithiodicarboxylic acids (aliphatic and aromatic) and dithioaromatic diamines. Conditions for curing reactions and for subsequent cleavage of crosslinks by reduction have been determined in model systems and confirmed for a prototype epoxy resin. A comparative evaluation of cure kinetics, crosslink density, and thermal and mechanical properties has been carried out for resin cured with aromatic curing agents (dithiodianiline and methylenedianiline) and with aliphatic compounds (3, 3′-dithiopropionic acid dihydrazide and hexamethylenediamine). It has been shown that in resin fully cured with disulfide-containing crosslinking agents, crosslinks are ruptured by reduction, and re-established by oxidation under mild conditions, or by reaction of thiol groups with bifunctional alkylating agents.     

The sulfur reversion process in natural rubber in terms of crosslink density and crosslink density distribution

Unfilled natural rubber compounds composed of conventional (CV), semi-efficient (SEV), efficient (EV) and sulfur donor (SD) vulcanization systems were heat aged to promote sulfur reversion. Rheometry, hardness, strain-strain characteristics including Mooney-Rivlin analysis, equilibrium solvent swell and Double Quantum (DQ) Nuclear Magnetic Resonance (NMR) were used to monitor crosslink density changes. A loss of crosslink density was observed by rheometry, C₁, equilibrium swelling and by DQ NMR as a function of cure extent. No chain scission reactions were operating in the time/temperature conditions used. All crosslink distributions were unimodal and the network homogeneity followed the order of EV > SD > SEV > CV. The crosslink distribution narrowed during the curing process for the CV and SEV systems. Non-oxidative maturation reactions were advantageous in promoting a more random distribution of crosslinks in the polymer matrix.     

Effect of high-temperature curing on the crosslink structures and dynamic mechanical properties of gum and N330-filled natural rubber vulcanizates

The effects of high-temperature curing and overcuring on the cure characteristics, crosslink structure, physical properties and dynamic mechanical properties (DMPs) of gum and carbon black (N330) filled natural rubber (NR) vulcanizates cured with conventional (CV), semi-efficient (SEV) and efficient (EV) cure systems, which have about the same total crosslink densities under a moderate curing temperature of 150°C, were investigated. The gum NR vulcanizates cured with CV, SEV and EV curing systems have about the same glass transition temperature (T_g) and tan δ values below the temperature of about 0°C, but showed some apparent differences in the tan δ values increasing in the order CVG′ and tan δ values above T_g higher than those of the gum NR vulcanizates.

High-temperature curing and overcuring cause decreases to various extents in the cure plateau torque, Shore A hardness, 300% modulus and tensile strength, and lead to apparent changes in the DMPs. Typically, there is an increase in T_g of all three kinds of gum and N330-filled NR vulcanizates because of changes in the total crosslink densities and crosslink types. The CV vulcanizates show the most significant change in cure characteristics, physical properties and DMPs since the highest content of polysulfidic crosslinks appears in the CV vulcanizate, causing the highest level of reversion and having a dominant effect on the properties.     

Radiation crosslinking of elastomers. I. Polybutadienes

The chemical and physical crosslink densities of irradiated polybutadiene were determined using a precise and consistent method. This method included the use of a statistical theory of crosslinking modified to include chain reactions and the use of Langley's theory for calculating the number of trapped chain entanglements. The number of crosslinks formed per unit radiation dose, G(X), was measured for a series of polybutadienes containing various isomeric forms of unsaturation. The physical crosslink density and the molecular weight between entanglements were determined from measurements of the compression modulus of swollen samples.     

Zur Analytik von Harnstoff-Melamin-Formaldehyd-Harzen

Summary A method is described for the distinction between urea-formaldehyde and melamine-formaldehyde resins by precipitating the melamine derivates with picric acid. The analysis of this precipitation by urease-decomposition gives information whether urea and melamine are condensed with formaldehyde together or whether the investigated resin is only a mixture of an urea-formaldehyde and a melamine-formaldehyde resin.Moreover, it is possible to prove the formation of cocondensates of urea, melamine, and formaldehyde during the three-component condensation by combination of these two methods.     

Curing characteristics of carboxyl functionalized glucose resin and epoxy resin

The curing characteristics of carboxylic functionalized glucose resin (glucose maleic acid ester vinyl resin: GMAEV) and epoxy resin have been studied using DSC and FTIR methods. Exothermic reactions attributed to esterification and etherification reactions of the hydroxyl and carboxyl functionalities of GMAEV with the epoxy groups were identified. Exothermic reactions showed very different patterns according to the degree of carboxyl group substituent of GMAEV. The results showed that esterification reaction occurs in the early stage of cure and then etherification followed after completion of the esterification. A cured matrix containing epoxy resin and 50 wt.% of GMAEV was prepared and characterized. The cured matrix showed thermal stability up to 300 °C. The average glass transition temperature and storage modulus of the matrix were as high as 95 °C and 2700 MPa, respectively. The cured matrix of epoxy resin and GMAEV with higher degree of carboxyl group was found to have a lower density due to the formation of bulky groups in the crosslinks.     

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.     

A generalized theory for the glass transition temperature of crosslinked and uncrosslinked polymers

A generalized theory for the glass transition temperature of crosslinked and uncrosslinked polymers has been developed, which takes into account the influences of end groups, branching, and crosslinking, and their functionality distribution. DiBenedetto's theory was found to correctly characterize the influence of crosslinks on the glass temperature. Normalized to constant crosslink functionality, the crosslink constant is a universal parameter suggesting that the entropic theory of glasses is applicable to crosslinked systems. Data on linear polymers and networks from the crosslinking of polymer chains, vinyl/divinyl-copolymers and step-growth polymers, such as polyurethanes, amine-cured epoxies, or inorganic glasses, are presented.     

Functionalized acrylic polyhydroxy urethanes as molecular tool box for photocurable thermosets and 3D printing

Liquid urethane (meth)acrylates represent attractive components of photocurable thermosets for applications ranging from coatings and adhesives to 3D printing. Herein we tailor liquid polyfunctional urethane methacrylates (UMA) derived from acrylic polyhydroxy urethanes. Cyclic carbonate methacrylates react with diamines to form dihydroxy-functional urethane dimethacrylates. In an “one-pot” process the hydroxy groups are functionalized either by reaction with 2-isocyanatoethyl methacrylate (IEMA) or by esterification with methacrylic anhydride (MAA) and acetic anhydride (AA). The hydroxy group esterification substantially lowers the resin viscosity (26–156 Pa•s). Hydroxy functionalization with IEMA and MAA affords tetrafunctional methacrylates. The corresponding photo-cured thermosets exhibit higher crosslinking density and improved stiffness as reflected by increasing the Young's modulus from 2900 to 3700 MPa combined with increasing the glass temperature from 135 to 204°C. Hence, this facile molecular UMA design enables to control functionality and thermoset properties over a wide range and meets the demands of 3D printing applications.     

Bimodal particle distribution for emulsions: The effect of interstitial functional particles

The location of hydroxyl functionality was investigated for thermosetting acrylic latexes. Large and small latex particles with and without hydroxyl functionality were synthesized and characterized. Such large and small latex particles with and without hydroxyl functionality were blended together and a melamine-formaldehyde (M-F) resin was added to crosslink the hydroxyl groups. Mechanical and thermo-mechanical properties were evaluated as function of blend ratio and functionality location. The packing of the latexes was dependent on small to large particle ratio. It was shown that the mechanical properties were also dependent on the location of the functionality. The higher concentration of hydroxyl groups in the small latex particles contributed more to the tensile properties of the latexes than the functionality of the large particles. The location of the M-F resin in the small latex particles resulted in a lowering of the volume fraction needed for a continuous network.     

Characterization of networks from the peroxide cure of polybutadiene. II. Kinetics and sol–gel

The kinetics of cure of polybutadiene with molecular weights varying from 12,500 to 92,000 g/mole and containing 23–93% of 1,2 microstructure were investigated at two levels of dicumyl peroxide and two temperatures. The relative rates of termination were determined as a function of vinyl content and of reactive additives. Sol-gel measurements showed that molecular weights between crosslinks of less than 100 g/mole could be obtained. Network formation was found to be a polymerization process in which each free radical formed started only one crosslink chain. However, at high initiator levels chain scission did occur during cure.     

Hydrogen bond directed self-assembly of core-substituted naphthalene bisimides with melamines in solution and at the graphite interface

A series of red and blue highly fluorescent core-substituted naphthalene bisimide dyes has been synthesized and they have been investigated as supramolecular building blocks. NMR and UV-Vis titration experiments of these dyes with complementary melamines revealed the formation of triple hydrogen bonds (DAD-ADA arrays) in solution. At stoichiometric ratios, ditopic melamine receptors could dissolve otherwise insoluble bisimides by means of hydrogen bonding, even in aliphatic solvents. At the solution/graphite interface, one-dimensional chains of hydrogen bonded naphthalene bisimides and two-dimensional adlayers of ditopic melamines are formed for the pure compounds but little evidence for heterocomplexes between the two complementary building blocks could be obtained.     

Calorimetric investigation of polymerization reactions. IV. Curing reaction of polyester fumarate with styrene

The curing reaction of polyester fumarate with styrene was investigated with a differential scanning calorimeter (DSC) operated isothermally. The change in rate of cure was followed over the whole range of conversion. The rate of cure is accelerated by the gel effect to about ten to fifty times the rate of model copolymerization of diethyl fumarate with styrene. This autoacceleration is much enhanced for systems with higher crosslinking densities and at lower temperatures. The results confirm that both termination and propagation steps of the curing reaction are controlled by diffusion of polymeric segments and monomer molecules over almost the whole range of conversion. The final extent of conversion is short of completion for isothermal cure and even for postcure of polyester fumarate with styrene because of crosslink formation. The final conversion of isothermal cure decreases with increasing crosslinking density and shows a maximum with increasing reaction temperature. This temperature dependency of the final conversion is caused by the difference in the activation energies for two propagation rate constants k_pf and k_ps, which were evaluated to be 7–10 and 5–8 kcal/mole, respectively, for the intermediate stage of the curing reaction.     

Pyrolysis and THM reactions of melamine and its resins

Melamine resins and a melamine polyester copolymer have been investigated with pyrolysis-GC/MS and thermally assisted hydrolysis and methylation (THM). Conventional pyrolysis yields characteristic volatile products only at temperatures above 500 °C and hexamethylenetetramine is the main product. A series of methylated melamines and secondary pyrolysis products were also found. THM yields high amounts of methylated melamines and therefore provides a sensitive method for the detection of melamine in resins and copolymers. Deduced from this results a preparative approach for the synthesis of methylated melamines with aqueous tetramethylammonium hydroxide solution was successfully developed.     

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.     

橡胶活性官能团对增韧环氧树脂动态力学性能的影响

用动态扭摆法测试聚丙烯酸丁酯橡胶增韧环氧树脂的动态力学行为,研究在环氧树脂低固化度和高固化度时,橡胶活性官能团种类(环氧基官能团与羧基官能团)和数量(官能度)对其影响。研究体系中橡胶玻璃化转变温度(Tg)的移动大小,与橡胶和基体树脂健合程度之间的关系。     

Merocyanine dyes containing imide functional groups: synthesis and studies on hydrogen bonding to melamine receptors

The condensation of the CH acidic heterocycles 4-alkyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carbonitrile (5a and b) and barbituric acid (15) with electron-rich thiophene aldehydes and benzaldehyde derivatives affords the respective monomethine dyes 10-13 and 17-19. The formylation of 5a,b and 15 with N,N'-diphenylformamidine or dibutylformamide in acetic anhydride and further reaction with 4-picolinium salts 9a,b provide the dimethine dyes 14 and 20a,b. Triple hydrogen bonding of the imide groups of merocyanine dyes 10-14 has been investigated by NMR titration experiments with melamine 21. Despite rather pronounced variations of the charge-transfer properties within the given series of dyes, minor changes of their binding constants have been observed. These results could be rationalized by semiempirical calculations that reveal small changes in the charge density at the oxygen functionalities involved in hydrogen bonding upon variation of the electron-donating carbocyclic or heterocyclic groups at the terminal double bond. Although the binding constants for triple hydrogen bonding between imides and melamines are rather weak in chloroform, they proved to be strong enough to facilitate dissolution of some of these dyes in aliphatic solvents by coordination to amphiphilic melamines and dipolar aggregation. UV-vis spectral changes observed in methylcyclohexane vs chloroform suggest the formation of colloidal assemblies through noncovalent polymerization.