Jakob Lange

Residual stress build-up in thermoset films cured above their ultimate glass transition temperature

The stress build-up during isothermal cure below the ultimate glass transition temperature of epoxy and acrylate films is investigated in detail. Four systems are studied; two acrylates and two epoxies, with different crosslink densities. Relaxation modulus and film shrinkage are measured simultaneously during cure. The stress build-up is measured independently using a bi-layer beam bending technique. A model for the build-up of cure stresses is proposed, in which stresses are generated by the cure shrinkage and decay by viscoelastic relaxation. The relaxation is described by a simple, modified Maxwell model. Owing to the absence of memory in the Maxwell model, the resulting equation is simple and numerical stress computation straightforward. The stress build-up over time is thus simulated for the four model systems based on the relaxation and shrinkage data, and the simulations compared with the experimentally observed stress build-up. The model successfully predicts the cure stresses where more standard elastic methods fail. It is found that the amount of stress build-up during cure varies greatly between the different systems. In general, a higher crosslink density results in higher stress build-up. The stress on cure ranged from less than 1% of the total stress on cure and cool-down in a lightly crosslinked epoxy to more than 30% of the total stress in densely crosslinked epoxies and acrylates. Finally simple approximations for estimating the stress levels after cure and cool-down from basic material properties, e.g. modulus and cure shrinkage, are proposed.

Barrier coatings for flexible packaging based on hyperbranched resins

Hyperbranched resins have been evaluated for use as barrier coatings. Different types of modified hydroxy-functional hyperbranched polyester resins were prepared and partially end-capped with acryl ...

Build-up of structure and viscoelastic properties in epoxy and acrylate resins cured below their ultimate glass transition temperature

Abstract The build-up of structure and viscoelastic properties with conversion during cure below the ultimate glass transition temperature of epoxy and acrylate resins has been investigated. Using a torsional dynamic mechanical analyser, dynamic shear modulus and change in sample thickness was monitored simultaneously, thus giving information on both the physical properties (stiffness) and the progress of the reaction (shrinbdkage) in one experiment. Two step-wise curing epoxy systems and two chain-wise curing acrylate systems with different crosslink densities were studied and compared. The results showed that in the epoxies vitrification was a distinct event, occurring separately from gelation and ending with the end of the cure reaction. In the acrylates vitrification began immediately after gelation, the two events being indistinguishable, and lasted until the end of the reaction, leaving the sample in its transition zone. Scaling of modulus—cure time data obtained at different frequencies showed that the data for each system followed one single curve, independent of frequency over five decades. This made it possible to estimate the modulus development at low frequencies early in the reaction, which is difficult to measure directly. From the shrinkage and storage moduli approximate values of the relaxation modulus as a function of chemical conversion were calculated. The relaxation modulus curves at different conversions were then shifted along the time axis to provide a relaxation master curve. The data and understanding gained in this work provide the basis for analysing the time-dependent mechanical behaviour during cure, e.g. build-up and relaxation of residual stresses.

Influence of crosslink density, glass transition temperature and addition of pigment and wax on the scratch resistance of an epoxy coating

The scratch resistance of a series of cationically cured epoxy coatings with varying crosslink densities has been investigated using five different scratch tests. The tests simulate a variety of scratch modes, ranging from single scratches to repeated surface wear. The relative influences of crosslink density and glass transition temperature (Tg) on scratch resistance, as well as the effects of adding pigment and wax to the coatings, were examined. The results show the scratch resistance to increase both with increasing crosslink density and increasing Tg in all tests. It is also shown that adding pigment or wax either increased or decreased the scratch resistance of the coating, depending on which type of scratching was performed.

Development of scratch tests for pre-painted metal sheet and the influence of paint properties on the scratch resistance

Abstract At present, the high scratch sensitivity of coatings used for pre-painted metal sheet is a major limitation to the use of these materials. This paper is concerned with the development of scratch tests, and the simulation of the forming (contact situation in a sheet forming die) and handling (single and repeated stylus-type impact, and surface abrasion) of pre-painted metal sheet. Using the tests, the scratch resistance of a series of polyester coatings has been investigated. The results show the scratch resistance to increase with increasing ductility of the coatings. The same trend was detected in both forming and handling tests, irrespective of if the coating contained pigment or not, or if the coating surface was lubricated. However, pigment addition and lubrication of the surface were found to increase the overall scratch resistance in some tests but to decrease it in others. The tests exhibited varying sensitivities, both to changes in coating properties and to the presence of pigment and lubricants, but no general difference between the forming and handling tests was found.

Gelation behaviour during chainwise crosslinking polymerisation of methacrylate resins

The gelation process in chainwise crosslinking polymerisation of methacrylate resins with average functionalities of 2.1-50 (1.05-25 methacrylate groups/molecule) was investigated using dynamic mechanical analysis. For all systems a crossover in tan delta (tan delta independent of frequency) was observed at gelation. The gel time was found to decrease with increasing functionality of the system. Within the measured frequency ranges, power law behaviour for the dynamic modulus was observed at the gel point in all systems. A value of the power law exponent n of 0.4 +/- 0.2 to 0.6 +/- 0.1 (increasing with increasing functionality of the system) was determined for gelation during reactions at T greater than or equal to T-g infinity. This trend suggests that the differences in screening between the systems dominate over the difference in fractal dimension. For reaction temperatures below T-g infinity a value of n = 0.3 +/- 01 was obtained, which was attributed to the influence of micro-vitrification

Electrochemical impedance spectroscopy as a tool for investigating the quality and performance of coated food cans

In the present work, electrochemical impedance spectroscopy (EIS) is explored as a tool for evaluating the quality of coatings on food cans. The properties of the coating-metal interface are evaluated using an equivalent electrical circuit representing intact and defect coating areas. The element corresponding to the charge transfer capacitance under pores, proportional to the wetted metal surface, is found to be the most useful element for quality evaluation. It is shown that with EIS, differences can be detected between new and filled cans from different suppliers, as well as between different product formulations. Three measurement techniques, i.e., initial state characterization, rapid dc aging, and long-term electrolyte aging, are applied to a series of petfood and beverage cans, and the results compared with the performance of the same cans after filling and storage. It is shown that all three measurement techniques give basic guidance as to the long-term performance of the filled cans. However, the electrolyte-aging test was found to give the best precision in the performance prediction.

Influence of structure and chemical composition on oxygen permeability of crosslinked epoxy -amine coatings

The oxygen barrier properties of a series of coatings based on diglycidyl ethers of bisphenol and butanediol reacted with a wide range of amines in different stoichiometric ratios have been investigated. The oxygen permeability was analysed with respect to the molecular structure in general and the concentration of polar functional groups in particular. The results showed that formulations based on aliphatic amines gave good barrier performance, that coatings prepared from aromatic and cyclo-aliphatic amines yielded intermediate barrier properties, and that polyether amine formulations produced very poor barriers. It was also observed that pendant methyl groups had a strong detrimental effect on barrier properties and that using an excess of amine monomer significantly improved the barrier performance. Attempts at predicting the oxygen permeability from physical and chemical characteristics of the coatings showed that the glass transition temperature could not be correlated with the permeability. However, it was found that a polarity index, calculated as the sum of the concentrations of hydroxyl and amine moieties weighted by their individual cohesive energy density, was a good predictor of oxygen permeability for all types of formulations.

The effects of silica fillers on the gelation and vitrification of highly filled epoxy-amine thermosets

Highly filled thermosets are used in applications such as integrated circuit (IC) packaging. However, a detailed understanding of the effects of the fillers on the macroscopic cure properties is limited by the complex cure of such systems. This work systematically quantifies the effects of filler content on the kinetics, gelation and vitrification of a model silica-filled epoxy/amine system in order to begin to understand the role of the filler in IC packaging cure. At high cure temperatures (100°C and above) there appears to be no effect of fillers on cure kinetics and gelation and vitrification times. However, a decrease in the gelation and vitrification times and increase the reaction rate is seen with increasing filler content at low cure temperatures (60-90°C). An explanation for these results is given in terms of catalysation of the epoxy amine reaction by hydrogen donor species present on the silica surface and interfacial effects.

Understanding Puncture Resistance and Perforation Behavior of Packaging Laminates

This paper presents the development of a method to measure damage to packaging laminates by penetration. A hemispherical probe was shown to provide the most reproducible results and the rate of loading was found to be a small factor. The puncture mechanism for different types of laminates was examined. It was shown that the presence of paper in the structure has a decisive effect on puncture resistance. For paper-based laminates, the paper layer limits the strength by rupturing first and then producing premature rupture of the other layers; the result is a 4-10-fold improvement in strength by eliminating paper as an outer layer. For laminates without paper, all layers rupture together so changing the composition of the non-paper layers has a direct influence on the puncture resistance. The direction of the penetration through the laminate usually has an effect on the energy to puncture.