Properties of adhesives

Rubber-grade resins are mostly in the softening point range 70-100°C R B. A deviation of 5-10°C in softening point may cause problems. The softening point of a resin affects the properties of adhesives. Hence, for pressure-sensitive rubber adhesives the decrease in the softening point of the resin produces a more tacky adhesive with less cohesive strength. 

The degree of agglomeration of fillers affects the dispersion and the rheological properties of adhesives [41], and disagglomeration of fillers during adhesives manufacturing should be produced to obtain acceptable properties. 

Like other adhesives, PMDI is a specific mixture of different molecules. The greater part of the functional groups of polyisocyanates are of low reactivity because of the different positions. The molecular weight distribution influences the reactivity, but also the typical properties of adhesives like viscosity, wetting and fluidity. In the monomer form (MDI) the functionality is 2 and the NCO-content is 33.5%, while PMDI has an average funetionality of 2.7 with a NCO-content of approximately 30.5%. The HCl-content is usually below 200 ppm. PMDI is used whenever the color of the finished adhesive is not of concern [138]. 

A surface is that part of an object which is in direct contact with its environment and hence, is most affected by it. The surface properties of solid organic polymers have a strong impact on many, if not most, of their apphcations. The properties and structure of these surfaces are, therefore, of utmost importance. The chemical stmcture and thermodynamic state of polymer surfaces are important factors that determine many of their practical characteristics. Examples of properties affected by polymer surface stmcture include adhesion, wettability, friction, coatability, permeability, dyeabil-ity, gloss, corrosion, surface electrostatic charging, cellular recognition, and biocompatibility. Interfacial characteristics of polymer systems control the domain size and the stability of polymer-polymer dispersions, adhesive strength of laminates and composites, cohesive strength of polymer blends, mechanical properties of adhesive joints, etc. 

The polyelectrolyte cements are modern materials that have adhesive properties and are formed by the cement-forming reaction between a poly(alkenoic acid), typically poly(acrylic acid), PAA, in concentrated aqueous solution, and a cation-releasing base. The base may be a metal oxide, in particular zinc oxide, a silicate mineral or an aluminosilicate glass. The presence of a polyacid in these cements gives them the valuable property of adhesion. The structures of some poly(alkenoic acid)s are shown in Figure 5.1. 

No reaction Contaminant layer determines bond strength Properties of adhesive-contaminant mixture determine bond strength ... 

Many properties of adhesive bonds are influenced by the Tg or mobility of the molecular chain structure, as shown in Fig. 3.16. When chain segments can move easily, such as when the temperature exceeds the Tg, they can deform under impact or assume new alignments under mechanical or thermal expansion stresses. This movement spreads the applied energy over a greater number of atoms and thus gives the bond a better chance to resist stress. Brittleness is, therefore, reduced and flexibility is increased. 

Diethylenetriamine and Triethylenetetramine. Diethylenetriamine (DETA) and trieth-ylenetetramine (TETA) are very reactive, low-viscosity liquids that are widely used with DGEBA epoxy resins. The application characteristics and cured properties of adhesive formulations prepared with these two curing agents are very similar. The lower vapor pressure of TETA generally favors its use. 

Strength Properties of Adhesive in Shear by Tension Loading in the Temperature Range of -450 to -57°F, Test for D 2557... 

Strength Properties of Adhesives in Shear by Tension Loading at Elevated Temperatures, Test for D 2295... 

Many applications for adhesives and sealants require high strength and durability at low temperatures. Many of these same applications also require resistance to thermal cycling between high and low operating temperatures. Unfortunately, the properties of adhesives and sealants at low temperatures are not as well studied or documented as they are at high temperatures. 

There are two properties of adhesive joints that protect them from exposure to chemical or solvent environments high degree of crosslinking and low exposure area. 

Mathews, D. H., and Silver, I., Methods for Testing the Strength Properties of Adhesives and Test Data, NEVOID Report 3923, 1962. 

Test Method for Strength Properties of Adhesive Bonds in Shear by Compression Loading... 

Test Method for Impact Strength of Adhesive Bonds Test Method for Tensile Properties of Adhesive Bonds Test Methods for Pressure Sensitive Adhesive Coated Tapes Used for Electrical and Electronic Applications... 

D 1344 Method of Testing Cross-Lap Specimens for Tensile Properties of Adhesives... 

American Society for Testing and Materials, Standard test method for strength properties of adhesives in two-ply wood construction in shear by loading, ASTM D 2339-85, 1985, Philadelphia, PA. 

On the nanoscale, the surface chemistry of the scaffold must recreate the important cell-ECM properties of adhesion and control. Biocompatibility of the scaffold surface with cells is key for allowing adhesion and migration of cells. The amino acid sequence of arginine-glycine-aspartic acid (RGD) has been identified on fibronectin and other ECM glycoproteins as a key adhesion domain, and the design of synthetic scaffolds incorporating the peptide has been successful... 

Thermal conductivity and expansion are important properties of adhesives used in electronics. Both properties influence the performance of computer chips. Generally, the chip has a protective cover which is attached by an adhesive. The adhesive bond must be maintained during thermally induced movement in the chip. The chip is bonded to its base with an adhesive which must also take thermal movement and, in addition, transfer heat from the chip. Two epoxy adhesives were used in the study silica filled epoxy (65 and 75 wt% SiO2 epoxy) and epoxy containing 70 wt% Ag. Figure 15.6 shows their thermal conductivities. The behavior of both adhesives is completely different. The silver filled adhesive had a maximum conductivity at about 6()"C whereas the maximum for SiOz filled adhesive was 120"C. The Tg of both adhesives was 50 and 160 C, respectively. Below its Tg, the thermal conductivity of the adhesive increases at the expense of increased segmental motions in the chain molecules. Above the Tg the velocity of photons rapidly decreases with increasing temperature and the thermal conductivity also decreases rapidly. 

Special considerations sepiolite treatment below 550°C does not alter the properties which modify the rheological properties of adhesives filler presence may affect adliesion (see Figure 8.54) rubber particles are hygroscopic and can absorb 1.4% moisture with the rate of 0.01% per minute (similar to carbon black) " ... 

In this section, the most important properties of adhesives and the resulting adhesive layers, which have to be taken into consideration, are again summarized in a systematic scheme. For supplementary information, the reader is referred to the respective sections. 

Table III. T-Peel and Lap-Shear Properties of Adhesive Joints of Adhesive A Formulations at Various Levels of ME Ac Toughener...

Table IV. Bulk Properties of Adhesive B Formulations Containing 5 phr of Toughener...

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