Adhesive, selection viscosity

Doyle, Daryl J. (1990). Criteria for Proper Adhesive Selection From Application to Viscosity. Society of Manufacturing Engineers Technical Paper AD90-450, Adhesives 90, October 1-4, 1990, Schaumburg, Illinois. 

This section starts with the chapter Quality Control of Raw Materials by Wakabayashi. The main components of adhesives are first described and then quality control procedures are discussed such as chemistry, impurity content, molecular weight, viscosity, density, and quality control of raw materials in storage. Next, in the chapter Processing Quality Control by Haraga, the processing quality control of adhesively bonded joints in actual production lines is discussed. Topics such as control of environmental conditions and materials, inspection of surface treatment of adherends, adhesive selection for easy process control, and... 

The viscosity of the adhesive can also be a significant factor in adhesive selection. Low-viscosity adhesives (<50 mPa-s) should be used where gaps are small and the adhesive is required to flow or wick into parts that have been pre-assembled. A medium-viscosity adhesive (up to 1000 mPa-s) may be more appropriate for the bonding of, say, cylindrical parts where the diametral gap is of the order of 0.1 mm. High-viscosity adhesives (>10,000 mPa-s) would be specified for larger gaps or where the adhesive is required not to flow away from the joint prior to assembly and cure. 

Additives. Because of their versatility, imparted via chemical modification, the appHcations of ethyleneimine encompass the entire additive sector. The addition of PEI to PVC plastisols increases the adhesion of the coatings by selective adsorption at the substrate surface (410). PEI derivatives are also used as adhesion promoters in paper coating (411). The adducts formed from fatty alcohol epoxides and PEI are used as dispersants and emulsifiers (412). They are able to control the viscosity of dispersions, and thus faciHtate transport in pipe systems (413). Eatty acid derivatives of PEI are even able to control the viscosity of pigment dispersions (414). The high nitrogen content of PEIs has a flame-retardant effect. This property is used, in combination with phosphoms compounds, for providing wood panels (415), ceUulose (416), or polymer blends (417,418) with a flame-retardant finish. 

Casting. Because the sol is a low viscosity Hquid, it can be cast into a mold (step B, Fig. 1). The mold must be selected to avoid adhesion of the gel. The sol can be appHed as a coating on a substrate or drawn into fibers or emulsified. 

Plasticizers and Processing Aids. Petroleum-based oils are commonly used as plasticizers. Compound viscosity is reduced, and mixing, processing, and low temperature properties are improved. Air permeabihty is increased by adding extender oils. Plasticizers are selected for their compatibihty and low temperature properties. Butyl mbber has a solubihty parameter of ca 15.3 (f /cm ) [7.5 (cal/cm ) ], similar to paraffinic and naphthenic oils. Polybutenes, paraffin waxes, and low mol wt polyethylene can also be used as plasticizers (qv). Alkyl adipates and sebacates reduce the glass-transition temperature and improve low temperature properties. Process aids, eg, mineral mbber and Stmktol 40 ms, improve filler dispersion and cured adhesion to high unsaturated mbber substrates. 

Solvents. Solvents affect adhesive viscosity, bond strength development, open time, cost, and ultimate strength. Blends of three solvents (aromatic, aliphatic, oxygenates, e.g. ketones, esters) are generally added, and in their selection environmental and safety regulations must be considered. A graphical method has been proposed to predict the most adequate solvent blends for solvent-borne CR... 

Desirable fixative properties superior to PVP homopolymer can be specified by judicious selection of the amount of vinyl acetate. Hair sprays are limited in the molecular weight of the resin because if they are too hLgh the resulting viscosity of the formulation will result in a poor (coarse) spray pattern. Increasing the VP/VA ratio causes properties to increase in the direction shown by the arrows. Other applications for VP/VA copolymers are uses as water-soluble or remoistenable hot melt adhesives, pharmaceutical tablet coatings, binders, and controlled-release substrates. 

Fillers are relatively nonadhesive substances added to the adhesive formulation to improve its working properties, strength, permanence, or other qualities. The improvements resulting from the use of fillers are listed in Table 1.8. Fillers are also used to reduce material cost. By selective use of fillers, the properties of an adhesive can be changed significantly. Thermal expansion, electrical and thermal conduction, shrinkage, viscosity, and thermal resistance are only a few properties that can be modified by the use of fillers. Common fillers are wood flour, silica, alumina, titanium oxide, metal powders, china clay and earth, slate dust, and glass fibers. Some fillers may act as extenders. 

The primary reactive diluents are monoepoxy low-molecular-weight epoxy resins. These may be used at rather high concentration with little effect on cured properties. Often they are employed to make selective improvements on certain properties such as adhesion, thermal cycling resistance, and impact strength. Some lower-viscosity commercial epoxy resins are already reduced with these diluents. 

Fillers generally represent one of the major components by weight in an adhesive formulation. However, their concentration is quite often limited by viscosity constraints, cost, and negative effects on certain properties. The degree of improvement provided by a filler in an epoxy formulation will heavily depend on the type of filler and its properties (particle size, shape, size distribution, and concentration), surface chemistry, dispersion characteristics, dryness, and compatibility with the other components in the formulation. Table 9.3 summarizes the properties of selected fillers. 

Selecting a flame retardant for an adhesive system has many ramifications, depending on the formulation being modified, the end use, how it will be processed, and the cost/performance ratio. When one is choosing a flame retardant, characteristics such as water extraction, particle size, viscosity, toxicity, dusting, uniformity, as well as economics must be considered. The materials chosen to perform the function of flame retardation must not interfere with the final product s performance. The major problem with incorporating flame retardants in adhesives is that very often a significant amount is required, and they interfere with the other properties of the adhesive and contribute to the cost. This is why bromo bisphenol epoxy resins are often employed in flame-retardant epoxy adhesives. 

Silk screen application is often used when the adhesive has to be applied to specific controlled areas. The liquid adhesive is forced through pores in a cloth or screen. It is possible to coat only selected areas by masking parts of the screen so that adhesive does not pass through in the unwanted areas. Adhesives generally must be specifically formulated for silk screen processing. Very low-viscosity adhesives, with flow characteristics similar to those of coatings, are best for silk screening operations. 

Other properties that are heavily influenced by the choice of monomer include cure speed (in general higher functional monomers cure more rapidly), viscosity, and durability of the film. Table 1 lists some monomers, their viscosities, and the properties that they enhance (reprinted with permission from Sartomer). it is important to note several trends on the chart. Cure speed increases with an increase in functionality (all of the recommended monomers in that column are at least trifunctional and several are tetra- or penta-functional). Viscosity also increases as the functionality of the monomer is raised (all of the low viscosity diluents are diacrylates). The adhesion promoting monomers are all di- or mono-functional. Most formulas contain several different monomers and sometimes also oligomers as there is often a balancing act that must be performed when selecting materials that will provide the required performance properties while still maintaining the correct viscosity and surface tension. 

Selected reactive diluents are employed to reduce the viscosity of Uvimer M formulations and to Impart a greater flexibility to the cured product. The choice of a monomer or reactive diluent for a Uvimer ig based on a number of factors, including compatibility with the base resin, odor, volatility, toxicity, and its effect on specific properties such as adhesion. Changing the diluent monomer does not drastically alter the basic properties of the resin so that it is possible to supply a given UvimerTM with a choice of diluents in order to comply with particular user requirements. 

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