Properties of uncured adhesives

Being composite materials, prepared by loading an organic matrix with very large amounts of inorganic fillers, typically from 70 to 80% by weight, the appearance and 

Percent by weight of polyimide and filler in the adhesive composition. 

Adhesive pastes are non-Newtonian fluids whose viscosity depends upon temperature, time, and shear rate. They are applied to the substrates by means of either stamping (pin transfer), screen printing, or dispensing. The performance of the last technique depends on a key factor the paste rheology. There exists at least six semi-empirical models to describe the rheological response of non-Newtonian fluids [4]. For shear-thinning fluids (thixotropic materials), it has been reported 

Carreau s model, which can be written as -q = -qoll + where -qo is 

The incorporation of fillers into organic resins results in increased viscosity with the maximum loading of filler being restricted by the permissible working viscosity, which in turn depends on the dispensing equipment and method 

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The properties of epoxy adhesives in their uncured condition will determine primarily how easily the adhesive can be processed, applied, and cured. They will also determine, to some extent, the performance characteristics of the cured joint. The properties of the individual components as well as that for the mixed formulation are important. 

Properties often used by epoxy resin manufacturers to specify particular grades of resin include the epoxy content, viscosity or softening point, and color. In addition, properties such as density, vapor pressure, flash point, refractive index, solubility characteristics, and hydroxyl content are often reported. The important properties of uncured epoxy formulations with regard to most adhesive applications are... 

Although the primary purpose of this chapter is to discuss mechanical testing and strength of adhesive joints, the reader should be aware that ASTM covers a wide variety of tests to measure other properties. ASTM, for example, includes standard tests to measure the viscosity of uncured adhesives, density of liquid adhesive components, nonvolatile content of adhesives, filler content, extent of water absorption, stress cracking of plastics by liquid adhesives, odor, heat stability of hot-melt adhesives, ash content, and similar properties or features of adhesives. 

Cyanoacrylates are generally specified in relation to their intended end use (see section on Uses). For adhesives this will usually include details of the physical properties of uncured formulations (eg, viscosity, density, flash point), curing... 

Silicone adhesives are generally applied in a liquid and uncured state. It is therefore the physical and chemical properties of the polymers, or more precisely of the polymer formulation, that guide the various processes leading to the formation of the cured silicone network. The choice of the cure system can be guided by a variety of parameters that includes cure time and temperature, rheological properties in relation with the application process, substrates, the environment the adhesive joints will be subjected to and its subsequent durability, and of course, cost. 

In this reaction, no by-products are evolved, and the crosslinking reactions occur within the bulk of the material. A typical hydrosilylation crosslinking system is depicted in Scheme 11, where n and y can vary depending on the required viscosity of the uncured formulation and the final targeted properties of the cured adhesive. 

When formulating a silicone adhesive, sealant, or coating, based on hydrosilylation addition cure, one must consider the following properties of the uncured product pot life, dispensing technique, rheology, extrusion rate, cure performance. These characteristics directly affect the processing properties of the polymer base or crosslinker parts. The degree of cure conversion at the temperature of interest is determined by properties such as tack free time, cure profile and cure time. Once... 

Proper formulation of epoxy adhesives requires knowledge of the chemical reactions that lead to polymerization as well as the chemical and physical properties of both the uncured mixture and the cured material. This chapter reviews the general principles of epoxy resin chemistry including synthesis of the epoxy monomer itself and its possible polymerization reactions. 

The general properties of cured and uncured epoxy resins are reviewed in Chap. 3. The chemical structures of the resin and curing agent will determine these physical properties. They will also determine, to a great extent, the surface chemistry and adhesion properties of the final product. 

These properties have a profound effect on the processing properties of the uncured adhesive and on the end properties of the fully cured product. The properties determined by physical chemistry affect both the cohesive strength of the adhesive film as well as the degree of adhesion to the substrate. They also affect the permanence and durability of the adhesive bond once it is placed into service. 

Extended exposure of the uncured material to temperatures or conditions outside those recommended by the manufacturer will cause change in physical properties of the uncured material and will likely reduce its resulting cohesive and adhesive strength. The reactions that occur due to ambient storage conditions are described in Chap. 3. Figure 17.5 shows the effect of aging conditions on the tensile shear strength of an epoxy film adhesive. 

The most commonly used tests for properties of the materials used in epoxy adhesive formulation and uncured mixed adhesives are viscosity, shelf life, percent of solids, and moisture content. These are generally applied to the resinous components. 

Since epoxy adhesive formulation represents a surprisingly broad area of technology, a road map to the use of this book may be valuable. Chapters 1 through 3 discuss the synthesis of raw materials, epoxy chemistry in general, and the physical and chemical properties that are important for an epoxy adhesive. These properties are important during the three primary phases or conditions of an adhesive (1) uncured, (2) during cure, and (3) fully cured. 

Physical-property tests are used to measure the properties of adhesives in the liquid or gelled states prior to curing and in the solid state after curing. Tests for the uncured state such as viscosity, visual examination, and surface energy or contact angle assure that fillers, if used, have not settled out, that the material has not exceeded its pot life or shelf life, and that the supplier has not changed the formulation. Visual examination and density after cure are performed to verify that voids are not present or, if present, meet specification requirements. Finally, light transmission and index of refraction measurements are important for adhesives used in optoelectronic applications. 

In addition to the benefits of low odor and redueed fogging, these adhesives form stronger bonds to low-energy substrates sueh as EPDM rubber, natural rubber, and other difficult-to-bond plastics. This property seems to be a funetion of the solvent action of the uncured adhesive, so care must be taken to avoid stress eraeking when the adhesive is used on sensitive substrates sueh as polyearbonates and polyaerylates. 

There are three main polymers presently used for structural adhesive bonding and they are phenolics, epoxies and urethanes. We can also include in our classification, the initial physical state of the uncured adhesive since it governs the type of application and curing conditions. Structural adhesives are manufactured in the form of films or pastes. The films are one-part adhesives, i.e., they contain a latent catalyst which requires heat for activation. Structural adhesive films are made in a number of different thicknesses and can either be supported (containing a scrim) or unsupported. In general, film adhesives require the application of pressure during cure in order to obtain ultimate properties. Paste adhesives are either one- or two-part materials. That is, pastes can either contain a latent heat activatable catalyst, or they can be a system which is separated into two parts, one of which contains the cur a t i ve/ca talys t. Two-part pastes cure at room temperature. In this section the chemistry of one-... 

One of the unique properties of ZnO is its ability to retain the tack of uncured rubber compounds over many months of shelf-storage for adhesive tapes. 

Modifiers can inclnde dyes, thickeners, gelling agents, fillers and lubricity controllers and the like as agents that influence the properties of the uncured composition. Plasticizers, reinforcing and/or toughening agents, adhesion promoters and thermally polymerizable prepolymers may be added as modifiers of the cured adhesives. 

Perhaps, the earlier materials found to have a useful capacity for adhesive bonding underwater depended upon the use of a stoichiometric excess of water-scavenging polyamide hardener in an epoxide-based adhesive. This approach can lead to the production of effective joints in the short term, but formulations of this type, which are hydrophilic in the uncured state, are also likely to absorb significant amounts of water in the cured condition. It is a widely accepted view that the extent of joint weakening in susceptible joints, quite apart from the consequences of plasticization, is a function of the water-uptake characteristics of the adhesive (see Glass transition temperature). The consequence is therefore likely to be that such joints will show poor durability in the presence of water, when rapid uptake of water may lead to equally rapid degradation of both cohesive and interfacial properties (see Durability fundamentals). 

Dynamic mechanical analysis provides a useful technique to study the cure kinetics and high temperature mechanical properties of phenolic resins. The volatile components of the resin do not affect the scan or limit the temperature range of the experiment. However, uncured samples must be supported by a braid, a scrim, or paper. This does not influence the kinetic results and can be corrected in the calculations of dynamic mechanical properties (qv). Recent DMA work on phenolic resins has been used to optimize the performance of structural adhesives for engineered wood products and determine the effect of moisture in wood product on cure behavior and bond strength (75-77). 

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