A study of moisture diffusion in polymeric packaging materials especially at high temperatures

Master'sMASTER OF ENGINEERIN

Non-Fickian Absorption Characteristics of Adhesive Joints: Capillary Effects and Residual Properties

Mechanical performance of polymer-based adhesive joints is generally susceptible to moisture absorption. This study quantifies the effects of moisture content on the strength, stiffness, and energy properties of adhesive bonded joints. For this purpose, moisture absorption characteristics of structural adhesive joints (Araldite 2015) with different thicknesses (0.5, 1.0, and 1.5 mm) were firstly established under accelerated aging condition (deionized water at 60 oC). A thickness-dependent non-Fickian moisture absorption model was then used to characterize the moisture absorption of the adhesive joints. Results suggested that the moisture absorption of the adhesive joints was governed by the capillary action. Subsequently, adhesive joints with aluminum 6061 adherent and 0.5 mm-thick Araldite 2015 adhesive compound were subjected to dry, 0.1, 0.15, 0.18, and 0.2 pct of moisture content. The specimens were tested in shear and tension loadings at 1 mm/min. The resulting variations in the mechanical properties were fitted using a residual property model. It was noticed that all properties degraded upon moisture attack. For strength and energy properties, the degradation was more severe in tension. As for the stiffness, the decrease in the property was similar in both tensile and shear. The results from this study showed that moisture attack is an important aspect to be considered when designing for the service lifetime of adhesive bonded structures

Modelling of dimensional stability of fiber reinforced composite materials

Various methods of predicting the expansion and diffusion properties of composite laminates are reviewed. The prediction equations for continuous fiber composites can be applied to SMC composites as the effective fiber aspect ratio in the latter is large enough. The effect of hygrothermal expansion on the dimensional stability of composite laminates was demonstrated through the warping of unsymmetric graphite/epoxy laminates. The warping is very sensitive to the size of the panel, and to the moisture content which is in turn sensitive to the relative humidity in the environment. Thus, any long term creep test must be carried out in a humidity-controlled environment. Environmental effects in SMC composites and bulk polyester were studied under seven different environments. The SMC composites chosen are SMC-R25, SMC-R40, and SMC-R65

Hygrothermal Effects on Viscoelastic Properties of Amine-Hardened Epoxy

In this work the behaviour of a DGEBA/TETA epoxy resin system, a candidate resin for electrically conductive nanocomposites, has been characterized after exposure to hygrothermal conditions for up to 2400 hours. The objectives of the study were to determine the diffusion coefficient, the viscoelastic and glass transition temperature changes as result of long-term aging. Due to the large number of samples required, inherent material properties and physical constraints of the measurement instruments, significant efforts were made to develop an appropriate molding technique to fabricate consistently high quality samples. Over 200 thin samples were fabricated for gravimetric measurements and dynamic mechanical analysis (DMA) testing at iso-strain and temperature-sweep. As far as we know, this is the first known comprehensive report of the variation of storage modulus as a function of time, temperature and water uptake for an epoxy resin system. When room temperature DMA tests were conducted on progressively aged samples, we found material behaviour to be highly dependent on the hygrothermal aging conditions, such as temperature, relative humidity and exposure time. In the case of storage modulus, it was found that higher dry aging temperature can be detrimental on the modulus as exposure time increased. This effect, however, was attenuated when coupled with relative humidity conditions where the modulus values increased. This behaviour can be associated with reported hydrogen bonding mechanisms occurring in the material. When tests were conducted after at least 80 days of aging using temperature ramp-up in the DMA, the reverse is found. The effects of structural relaxation of physical aging including volumetric contraction became evident during ramp-up. For dry aging condition samples, the storage modulus increased while relative humidity conditions reduced it, consistent with plasticization. It would appear then that the temperature ramp-up method is able to delineate physical aging and plasticization effects prior to the onset of glass transition. Finally, the glass transition temperature is greatly influenced by hygrothermal conditions. The results for the different conditions in this study can be interpreted using various theoretical concepts published in the open literature. They include the formation of hydrogen bonds between the water molecules and the OH groups of the epoxy. Loss modulus and tan δ as measured by DMA indicated that the former tends to be more sensitive than the latter, better discerning the effects of relative humidity

A fully coupled diffusion-reaction scheme for moisture sorptionedesorption in an anhydride-cured epoxy resin

Thermoset materials frequently display non-classical moisture sorption behaviors. In this paper, we investigated this issue from an experimental point of view as well as in terms of modeling the water transport. We used the gravimetric technique to monitor water uptake by epoxy samples, with several thicknesses exposed to different levels of humidity during absorption and desorption tests. Our results revealed that the polymer displays a two-stage behavior with a residual amount of water that is desorbed progressively. We proposed a phenomenological reaction-diffusion scheme to describe this behavior. The model describes water transport as a competition between diffusion and the reaction, during which the local diffusivity and solubility depend on the local advancement of the reaction. We then implemented our model using COMSOL Multiphysics and identified it using a MATLAB-COMSOL optimization tool and the experimental data. We discussed the relation between the hydrophilicity of the product of the reaction and the diffusion behavior. We examined the reaction-induced modification of the water concentration field. It is worth noting that part of the phenomenology can be explained by the presence of hydrolyzable groups

Moisture and Interfacial Adhesion in Microelectronic Assemblies

In this research, a systematic and multi-disciplinary study was conducted to understand the fundamental science of moisture-induced degradation of interfacial adhesion. The research is comprised of both experimental and modeling components of analysis and consists of four primary components. First, the moisture transport behavior within underfill adhesives is experimentally characterized and incorporated into a finite element model to depict the moisture ingress and interfacial moisture concentration for each respective level of moisture preconditioning. Second, the effect of moisture on the variation of the underfill elastic modulus is demonstrated and the physical mechanisms for the change identified. Third, the aggregate effect of moisture on the interfacial fracture toughness of underfill to both copper and FR-4 board substrates is determined. This includes the primary effect of moisture being physically present at the interface and the secondary effect of moisture changing the elastic modulus of the adhesive when absorbed. Last, the recovery of both the elastic modulus and interfacial fracture toughness from moisture preconditioning is assessed with reversible and irreversible components identified. Using adsorption theory in conjunction with fracture mechanics, an analytical model is developed that predicts the loss in interfacial fracture toughness as a function of moisture content. The model incorporates key parameters relevant to the problem of moisture in epoxy joints identified from the experimental portion of this research, including the interfacial hydrophobicity, epoxy nanopore density, saturation concentration, and density of water. This research results in a comprehensive understanding of the primary mechanisms responsible for the interfacial degradation due to the presence of moisture. The experimental results obtained through this research provide definitive data for the electronics industry to use in their product design, failure analysis, and reliability modeling. The predictive model developed in this research provides a useful tool for developing new adhesives, innovative surface treatment methods, and effective protection methodologies for enhancing interfacial adhesion.Ph.D.Committee Chair: Jianmin Qu; Committee Member: C. P. Wong; Committee Member: S. Mostafa Ghiaasiaan; Committee Member: Suresh K. Sitaraman; Committee Member: W. Steven Johnso

Moisture absorption behavior and diffusion characteristics of continuous carbon fiber reinforced epoxy composites: a review

Throughout their service life, carbon fiber reinforced epoxy composites are exposed to a range of environmental conditions. The long-term durability of these composites is an ongoing concern because exposure to moisture can degrade the polymeric matrix and deteriorate the mechanical properties of the composite. Carbon fiber reinforced epoxy composites are sensitive to their environments via humidity, elevated temperatures, and liquid exposure. This paper reviews the moisture uptake behavior and diffusion characteristics of continuous carbon fiber epoxy composites which will serve as a good source for understanding the moisture diffusion behavior of these composites in hot/wet environments

Humid Ageing of Organic Matrix Composites

In this chapter, several aspects of the ageing phenomena induced by water in organic matrix composites are examined, essentially from the physico-chemical point of view. It is first important to recognize that there are two main categories of humid ageing. First there are physical processes, mainly linked to the stress state induced by matrix swelling and sometimes matrix plasticization. This kind of ageing can occur in matrices of relatively high hydrophilicity (affinity with water). Highly crosslinked amine cured epoxies are typical examples of this behavior. The second category of humid ageing involves a chemical reaction (hydrolysis) between the material and water. Unsaturated polyesters are typical examples of this category. They display a low to moderate hydrophilicity, swelling and plasticization have minor effects, but hydrolysis induces a deep polymer embrittlement and, eventually, osmotic cracking. Whatever the ageing mechanism, it needs the water to penetrate into the material and depends on the water concentration and its distribution in the sample thickness. This is the reason why the first and second sections are respectively dedicated to water solubility and diffusivity in matrices, interphases and composites. In each case, the elementary processes are distinguished, to examine the effects of temperature and stress state and to establish structure–property relationships. It is shown that, in most of these aspects, research remains largely open. The last section is devoted to hydrolysis, its kinetic modeling, including the case of diffusion controlled hydrolysis, and its consequences on polymer properties. Structure reactivity relationships are briefly presented. The very important case of osmotic cracking, which can be considered as a consequence of hydrolysis, is also examined