Waterborne acrylic-epoxy coatings

Waterborne two-component acrylic-epoxy coatings are gaining popularity as topcoats in moderate duty industrial and high performance architectural (HIPAC) applications. This increased popularity is due to their attractive handling, application, and performance properties, along with their low solvent content and odor. The objectives of this work were to characterize the cure and property development of these coatings, evaluate performance properties of cured films, and investigate a new epoxy resin dispersion in existing acrylic-epoxy formulations. These evaluations confirmed that existing acrylic-epoxy coatings have long pot life and short dry times while displaying a range of chemical resistance and physical properties. IR spectroscopy and differential scanning calorimetry (DSC) results indicated that the extent of cure at ambient conditions over a 21-day period was minimal; however, dynamic mechanical analysis (DMA) and solvent swell results did illustrate noticeable crosslink density development under these conditions. DSC results demonstrated more complete reaction and cure after heating. Direct substitution of a novel epoxy resin dispersion into these formulas resulted in lower required solvent content, shorter dry time, higher gloss, higher crosslink density, and improved water and scrub resistance. Presented at the 79th Annual Meeting of the Federation of Societies for Coatings Technology, on November 5–7, 2001, in Atlanta GA. 7201 Hamilton Blvd., MailStop R3108, Allentown, PA 18195-1501.     

Structure and properties of dynamically cured EPDM/PP blends

The structure and properties of polyolefin blends of ethylene–propylene–diene terpolymer (EPDM) and polypropylene were studied. Blends were prepared in a laboratory internal mixer where EPDM was cured with PP under shear with dicumyl peroxide (DCP) at different shear conditions (blend–cure). Blends were also prepared for comparison from EPDM which were dynamically cured in the absence of PP and blended later (cure–blend). The effect of DCP concentration, intensity of the shear mixing, and rubber/plastic composition were studied. In blend–cure, the melt viscosity increased with increasing DCP concentration in blends of 75% EPDM and 25% PP, but it decreased with increasing DCP concentration in blends of 75% PP and 25% EPDM. In cure–blend, however, the melt viscosity increased with increasing DCP concentration for all compositions. The melt viscosity decreased with increasing intensity of the shear mixing presumably due to the formation of the smaller segregated microdomain of the crosslinked EPDM gels in both blend–cure and cure–blend materials. The crystallization rate was higher in EPDM/PP blends than in PP homopolymer. The crystallization rates for various blending conditions were also compared.     

A study of the effects of postcure treatments on polyester–melamine coating matrices

The effect of postcure high energy (γ), ultraviolet (UV) and thermal treatment on the properties of polyester–melamine clearcoats of a range of compositions has been investigated. Two initial cure conditions were used, of which one was “optimally” cured and the other under‐cured. It was found that postcure treatments, particularly γ and UV, led to coatings of similar mechanical and thermal properties irrespective of initial cure, although the change in properties on postcure treatment was greater for the under‐cured samples. The results were interpreted in terms of the effect of the treatments on the structure of the crosslinked matrices. The study suggests the possibility of the development of a dual‐cure process for polyester–melamines, whereby cure optimization and property improvement can be achieved. This could also be used to “correct” for small variations in thermal cure levels brought about by adventitious on‐line fluctuations in cure oven conditions. POLYM. ENG. SCI. 46:532–539, 2006. © 2006 Society of Plastics Engineers     

Effects of graphite fiber and epoxy matrix physical properties on the temperature profile in a composite during cure

An investigation was carried out into the effects of graphite fiber and epoxy matrix physical properties on the temperature profile in their laminates during cure. An accurate prediction of the temperature profile within a composite is of paramount importance in determining the processing conditions. In this study, a model for the temperature profile was established first. Then, three physical parameters of composites that change during cure, namely, density, heat capacity, and thermal conductivity, were varied in magnitude, and the ensuing effect on the temperature profile was examined. The range of their variation was based upon the values measured in our laboratory as well as those reported in the literature. Changes in the neat resin thermal conductivity were found to significantly influence the temperature profile.     

Aspects of Latency in Epoxy Film Adhesives

Studies have shown that the cure of epoxy resins with urea hardeners, either alone or in conjunction with dicyandiamide, is influenced by the ability of adventitious agents, products, or by-products to volatilise from the reacting system. Although a generalised mechanism has not been identified, the phenomenological findings are important in evaluation and characterisation of adhesives and cure schedules, particularly by thermal analysis, and need to be considered in any detailed chemical study of these systems. Propenal has been identified as a significant by-product of cure under certain conditions.     

A novel rapid cure epoxy resin with internal mold release

Mold preparation, material layup, and cure times for thermoset-based composites often limit their use in high-volume applications. As such, new rapid cure epoxy resins are being developed to achieve a complete cycle time within 3 min. In this research, calorimetry and rheometry are used to examine and model two novel rapid cure epoxy resin systems with internal mold release. The rapid cure epoxy resins followed an autocatalytic cure kinetic and William–Landel–Ferry diffusion model. The rapid cure epoxy resin was shown to achieve 94% cure in 2 min at 150°C. However, adding an additional 2.5 wt% internal mold release hindered the first step of the reaction, which delayed the second reaction step since the final degrees of cure were similar. Furthermore, the resin viscosity followed a modified William–Landel–Ferry equation and at 120°C could maintain a viscosity below 5 Pa s for 4.1 min. These models provided valuable insight into the range of processing conditions these novel resins could experience during impregnation and molding processes.     

Classification of properties of elastomers using the “optimum property concept.” I. Introduction

An attempt is made to distinguish properties of elastomers by types. “Basic properties of materials” or “network properties” in elastomers are properties which either increase or decrease from the liquid to the solid state of materials or over the range of the “elastomeric plateau” of elastomers. From these are distinguished properties that exhibit characteristic maxima and are therefore “maximum properties” or bivalued properties. Mechanical failure properties show the characteristics of “maximum properties.” The maxima in “maximum properties” generally do not coincide. This noncoincidence of the maxima with a change in a “basic property of a material” has major theoretical and practical implications, for example, it is the cause of the crossovers in the relative performance rating of materials under different test conditions. The implications of this noncoincidence of the failure property maxima on the relevance of correlations between these properties are discussed. A change in the testing conditions is reflected in a shift of the optimum value in a “basic property of a material” with respect to a specific “maximum property.” Data and certain conclusions in the literature are interpreted on the basis of this concept. Examples of the limitations of the validity of mathematical relationships are presented. Also, a definition of the term “state of cure” is proposed and a suggestion for the rating of severities of test equipment and applications of elastomeric materials recommended. The effect of increased degrees of crosslinking for a series of polymers and crosslinking agents is assessed. It is suggested that the “mechanisms” of failure properties will remain elusive if their rationalization is attempted on the basis of other failure properties, e.g., the mechanism of abrasion on that of tear strength or cut growth. The main purpose of this proposal is to provide support for a drastic reduction in laboratory testing by identifying those properties which can lead to different relative ratings in routine evaluations and actual applications. A more empirical approach to materials evaluations is recommended based on the calibration of laboratory instrumentation with respect to specific applications. A de-emphasis of routine evaluations of materials on the basis of their “maximum properties” seems to be justified.     

Resin transfer molding (RTM) process of a high performance epoxy resin. I: Kinetic studies of cure reaction

The cure kinetics of a high performance PR500 epoxy resin in the temperature range of 160–197°C for the resin transfer molding (RTM) process have been investigated. The thermal analysis of the curing kinetics of PR500 resin was carried out by differential scanning calorimetry (DSC), with the ultimate heat of reaction measured in the dynamic mode and the rate of cure reaction and the degree of cure being determined under isothermal conditions. A modified Kamal's kinetic model was adapted to describe the autocatalytic and diffusion‐controlled curing behavior of the resin. A reasonable agreement between the experimental data and the kinetic model has been obtained over the whole processing temperature range, including the mold filling and the final curing stages of the RTM process.     

Microstructural effects and the toughening of thermoplastic modified epoxy resins

We have studied an epoxy resin formulation consisting of the diglycidyl ether of bisphenol-A (DGEBA), modified with phenolic hydroxyl-terminated polysulfone (PSF) and cured with an aromatic amine curing agent, diaminodiphenyl sulfone (DDS). A range of microstructures and fracture properties have been obtained by controlling the formulation cure conditions (cure temperature and cure cycle in an isothermal mode). The chemical conversion of the cured resins has been monitored by near-infrared spectroscopy (NIR). Although only a single material formulation was used, three distinct types of microstructure were identified by scanning electron microscope (SEM) observations on samples prepared at different cure temperatures. Surprisingly, the thermal and fracture properties of the cured samples did not vary noticeably, in spite of the significant microstructure variations. The consistency of these fracture toughness results with cure temperature changes was an unexpected result in the light of our earlier observations of a strong dependence of fracture toughness on cure temperature in neat resin systems. The difference in behavior between neat and modified resins reveals that the fracture toughness of the latter is dependent on a combination of the microstructure and the matrix resin properties. This hypothesis was also supported by an observation of high fracture thoughness in a sample cured in a two-step process, which we believe is due to the optimum microstructure and matrix resin properties, being achieved separately during precure and postcure, respectively. The increase in fracture toughness values caused by the modification (ΔG_IC) was calculated from the fracture toughness values of neat and modified resins, prepared under the same cure conditions, using a proposed theoretical equation. © 1993 John Wiley & Sons, Inc.     

Influence of cure via network structure on mechanical properties of a free-radical polymerizing thermoset

An improved understanding has been achieved regarding the relationships among cure chemistry, network structure, and final physical properties of vinyl ester (VE) resins, a thermoset polymer often used as the matrix of fiber reinforced polymers. Mechanical properties of the polymer are found to depend on both cure schedule and cure formulation. The possibilities of phase separation and micro-gel formation being the cause for these differences in mechanical properties are examined. The VE/styrene (S) system does not phase separate under the conditions studied. Though bulk properties of the resin are unaffected by the details of the cure, the microscopic morphology, in particular the type of cross-link formed (intermolecular bond or intramolecular bond), is sensitive to both cure temperature and initiation mechanism as determined by cure formulation. An analysis of cure kinetics shows that both temperature and initiation mechanism affect the apparent ‘reaction order’ of the VE/S system as determined by the autocatalytic equation. This apparent reaction order is interpreted as being an indication of the degree of heterogeneity in the resin. By controlling cure temperature and cure formulation, it is possible to minimize the apparent reaction order and thereby optimize physical properties. Finally, a theory is adapted from other non-network polymer systems to qualitatively describe how cure temperature and initiation mechanism may alter the heterogeneity in network structure via micro-gel formation and how these changes in structure affect changes in the mechanical properties.     

Synthesis of Alumina-Magnesia Spinel

Results of a technological synthesis of alumina-magnesia spinel from a range of doped magnesia-containing raw materials at different temperatures under laboratory conditions are reported. Synthetic chromite-doped spinel with excess periclase is recommended for manufacture of magnesia-spinelide refractories     

Surface pattern formation in UV-curable coatings

Some UV-curable coatings display matte surfaces after cure if they have undergone a certain period of leveling at a temperature above their glass transition temperature and the melting point of any crystalline co-reagents present in the formation. The matte finish of these coatings is due to the presence of coherent surface wrinkles after cure, which are similar in appearance to those induced by differential thermal contraction, when a metal layer is sputtered onto a rubbery or viscous substrate. However, the wrinkles in the UV-cured coatings appear under isothermal conditions, and it is, therefore, inferred that they are due to the dynamics of internal stresses induced by through-thickness variations in the extent of curing.     

The Effects of Cure Temperature and Time on the Bulk Fracture Properties of a Structural Adhesive

The purpose of this investigation is to determine experimentally the possibility of optimizing the room temperature bulk fracture properties of structural adhesives with respect to cure temperature, time and cool-down conditions. The model adhesive, Metlbond, is a solid film modified nitrile epoxy resin supplied in two forms: Metlbond 1113 (supported with synthetic fabric carrier cloth) and Metlbond 1113-2 (without carrier cloth). The effects of carrier cloth on the bulk fracture properties are investigated as well. The uniaxial tensile strength and rigidity values were determined over a wide range of cure temperatures and times with fast and slow cool-down conditions during a previous investigation by the authors. For the present investigation, the fracture toughness of the model adhesives, subjected to opening mode failure, are experimentally determined, with the use of single-edge-cracked specimens, for different cure and cool-down conditions. It is found that the optimum fracture toughness values are obtained at low temperature-long time cure conditions in the absence of carrier cloth when slow cool-down condition is employed. Using the elastic-plastic material behavior assumption, it is shown that an average crack tip plastic zone radius can be determined using the fracture toughness and tensile strength values obtained corresponding to a given cool-down condition. These average plastic zone radii values are used along with the available tensile rigidity values to evaluate the optimum fracture energies of the model adhesives for a number of cure schedules. It is found that the optimum fracture energy levels are obtained at high temperature-short time cure conditions, using slow cool-down in the absence of carrier cloth.     

Polymerization behavior of silver-filled epoxy resins by resistivity measurements

The cure of heavily silver-loaded epoxy resins with several amines has been investigated by resistivity–time measurements under isothermal conditions. For favorable cases activation energies in the range of 14–17 kcal./mole have been derived from slope analyses of the ρ?t plots. The relationship of the technique to similar measurements on unfilled or lightly loaded resins is discussed. Many aspects of the electrical behavior of silver-based conductive adhesive systems are clarified by the kinetic data.     

Surface Strength and Crack Resistance of Very Hard Ceramic Materials Tested by Microindentation

Load and energy conditions for nucleation and crack growth in the vicinity of a local stress concentrator — the impression of a diamond indenter — are formulated. Crack resistance characteristics are given for a wide range of carbide and boride ceramic materials, and their correlation with the wear resistance of friction surface determined under laboratory and industrial conditions is discussed.     

高速铁路用聚氨酯填充材料快速试验方法的研究

针对采用聚氨酯(PUR)作高速铁路填充材料时,现场施工条件下不可能在PUR固化过程中进行加热及室温PUR固化周期长的缺点,探讨实验室可用的快速试验方法。先用差示扫描量热分析和红外光谱分析确定后处理温度,再根据实际情况确定后处理时间,并与未后处理的试样进行性能对比。结果表明,用该方法研究高速铁路用PUR填充材料可大大缩短试验周期。     

Relationship between isothermal and dynamic cure of thermosets via the isoconversion representation

This study deals with the cure of thermoset resins used in the pultrusion of fiber reinforced composites. The objective was to predict the degree of cure under non‐uniform time‐temperature profiles. A simple procedure using differential scanning calorimetry results was developed for predicting the degree of cure vs. time under isothermal conditions from dynamic DSC tests and vice versa. The principal feature of the procedure is the transformation of the degree of cure vs. time curves obtained under isothermal or dynamic DSC conditions into isoconversion curves as time vs. temperature or time vs. heating rate diagrams. The proposed procedure is validated with isothermal and dynamic DSC results from epoxy and polyester resin formulations used in the pultrusion of fiber reinforced composites. The agreement between predictions and experiments was very good and the extension of the procedure for predicting the cure under non‐uniform temperature profiles as in pultrusion seems to be feasible.     

Accelerated ageing versus realistic ageing in aerospace composite materials. IV. Hot/wet ageing effects in a low temperature cure epoxy composite

Samples of an aerospace grade carbon fiber epoxy composite (Hexcel, M20/IM7) were subject to long term ( ≈ 1 year) hot/wet ageing and thermal spiking under a variety of humidity levels and temperature conditions related to “in service” conditions seen by military aircraft. Changes to the chemical and physicochemical structure of the composite were analyzed by a range of different techniques including gravimetric analysis, FTIR, and DMA to compare the effects of the various ageing conditions. The results indicated that the chemical effects of hot/wet and spiking conditions on this incompletely cured type of composite are very complex because of the variations in moisture levels and cure chemistry from the composite surface inwards as the resin ages under the different external environments. Physicochemical changes (such as T_g) and structural effects (such as microcracking) are similarly complex and dependent on composite thickness. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007