Adhesive formulations joints

Tetrahydrofurfuryl alcohol is used in elastomer production. As a solvent for the polymerization initiator, it finds appHcation in the manufacture of chlorohydrin mbber. Additionally, tetrahydrofurfuryl alcohol is used as a catalyst solvent-activator and reactive diluent in epoxy formulations for a variety of apphcations. Where exceptional moisture resistance is needed, as for outdoor appHcations, furfuryl alcohol is used jointly with tetrahydrofurfuryl alcohol in epoxy adhesive formulations. 

Some rubber base adhesives need vulcanization to produce adequate ultimate strength. The adhesion is mainly due to chemical interactions at the interface. Other rubber base adhesives (contact adhesives) do not necessarily need vulcanization but rather adequate formulation to produce adhesive joints, mainly with porous substrates. In this case, the mechanism of diffusion dominates their adhesion properties. Consequently, the properties of the elastomeric adhesives depend on both the variety of intrinsic properties in natural and synthetic elastomers, and the modifying additives which may be incorporated into the adhesive formulation (tackifiers, reinforcing resins, fillers, plasticizers, curing agents, etc.). 

St. Clair et. al. investigated a series of maleimide and nadimide terminated polyimides and developed LARC-13 [8,9]. Changing the terminal group from maleimide to nadimide, the value of the lap shear strength of a titanium lap shear joint increased from 7 to 19 MPa [9]. They also added an elastomeric component to the adhesive formulation. The introduction of 15 wt% of a rubbery component, ATBN (amine terminated butadiene nitrile polymer) and ADMS (aniline terminated polydimethyl siloxane) enhanced the adhesive properties as follows 19 MPa to 25 MPa (ATBN) titanium T-peel strength 0.2 kN/m to 1.4... 

Polar solvents are required with polar resins nonpolar solvents with nonpolar resins. When solvents are used in the adhesive formulation, they must be completely evaporated from the bond line prior to cure, or otherwise bubbles could form in the bond line, causing a weak joint. The substrate must also be tested so that the solvents that are used in the adhesive formulation do not attack or degrade it. 

Controlling flow or viscosity is an important part of the adhesive formulation process. If the adhesive has a propensity to flow easily before and during cure, then one risks the possibility of a final joint that is starved of adhesive material. If the adhesive flows only with the application of a great amount of external pressure, then one risks the possibility of entrapping air at the interface and getting too thick of a bond line. These factors could result in localized high-stress areas within the joint and reduction of the ultimate joint strength. 

When used in epoxy adhesive systems, solvents are generally employed for reducing the viscosity for formulation or application purposes. Once the adhesive is applied to the substrate, the solvent must evaporate prior to cure. Otherwise, bubbles or vapor pockets could form in the bond line, causing a physically weak joint with poor adhesion. The solvent in the adhesive formulation must not adversely affect the substrate to which it is applied. Plastics, elastomers, and polymeric foams are especially sensitive to certain solvents used in epoxy adhesives. 

Fillers and extenders are used in epoxy adhesive formulations to improve properties and to lower cost. Properties that can be selectively improved include both the processing properties of the adhesive as well as its performance properties in a cured joint. However, the use of fillers can also impair certain properties. Typically, the formulator has to balance the improvements against property decline. The advantages and disadvantages of filler addition in epoxy formulations are listed in Table 9.1. Common fillers used in epoxy formulations and the properties that they are used to modify are shown in Table 9.2. 

Extenders may also provide one or several secondary functions in addition to cost reduction. Depending on the type and concentration of the extender, it could reduce shrinkage that occurs on polymerization or during aging, lower the coefficient of thermal expansion of the adhesive formulation, provide a higher viscosity to avoid adhesive starvation in the joint, or perform a number of other property improvements. 

Depending on the substrate, the curing temperatures, and the service temperatures that are expected, the adhesive formulator may want to adjust the coefficient of thermal expansion of the adhesive system. This will lessen internal stresses that occur due to differences in thermal expansion between the substrate and the adhesive. These stresses act to degrade the joint strength. 

The effect of simultaneous exposure to both mechanical stress and a chemical environment is often more severe than the sum of each factor taken separately. Mechanical stress, elevated temperatures, and high relative humidity can be a fatal combination for certain adhesive formulations if all occur at the same time. Add to this the possible cyclic effects of each factor, and one can easily see why it is important to understand the effects of environment on the joint. 

Optimal joints can generally be fabricated by the correct combination of epoxy adhesive formulation (i.e., type of resin, curing agent, fillers, modifiers) and surface treatment process. 

Specific substrates are described under each classification. Of course, adhesive applications are not limited to joints having only one type of substrate. Metal-to-plastic, aluminum-to-steel, metal-to-wood, glass-to-metal, and an infinite variety of other joint configurations are all possible. In these applications the nature of each substrate needs to be understood and considered in the overall selection of an adhesive formulation and bonding process. 

However, with the proper combination of surface treatment and adhesive, these differences in durability to aggressive environments can be minimized, as shown in Table 16.3. The initial shear strength and permanence depend on the type of alloy and the pretreatment used. Note that the data presented here show only the relative differences in joint strength for one specific epoxy adhesive and are not representative of other adhesive formulations. There was no attempt to maximize any of these values through choice of the adhesive. 

Plasticizer migration from the vinyl part into the adhesive bond line can degrade the strength of the joint. Adhesives must be tested for their ability to resist the plasticizer. PVC can be made with a variety of plasticizers. An adhesive suitable for a certain flexible PVC formulation may not be compatible with a PVC from another supplier. Nitrile rubber adhesives have been found to be very resistant to plasticizers and are often the preferred adhesive for flexible PVC films. However, certain epoxy adhesive formulations have also been found to provide excellent adhesion to flexible PVC substrates. Several such starting formulations are presented in Table 16.7. A comparison of the performance of several classes of adhesive when bonding PVC to itself and to various other materials is given in Table 16.15. 

It should be apparent by now that numerous parameters can affect the performance of a joint, and many combinations of those parameters are possible. Therefore, a prime rule in any adhesive or sealant testing program is to standardize and document test variables as thoroughly as possible. The adhesive formulator, supplier, and end user should all utilize the exact same procedures and specimen construction. One should make every effort to ensure that similar tests that are performed at different locations in the company and by different personnel are identical duplicates of one another. 

Although preliminary studies suggest that face-laminations can be obtained using adhesives made from condensed tannins that pass the standards of the American Institute of Timber Construction (< ), less is known about how to formulate adhesives using these materials for end-jointing of wood. This study, funded by the USDA Small Business Innovation Research Program, was undertaken to develop adhesive formulations containing 50% of sulfite extracts obtained from the bark of southern pine trees and demonstrate their use in... 

In adhesives formulated in the F- and M-Series (Table I), approximately 5 to 10 minutes at ambient temperature was required to reach an end-joint tensile strength of 500 psi, a value deemed adequate to permit gentle handling of the end-jointed members. Between 15 and 20 minutes at ambient temperature was required to reach a tensile strength of 1,500 psi (11). 

Table II. Gluebond Quality of Lodgepole Pine End-joint and Douglas-fir Parallel Laminates Obtained with the F-Series Adhesive Formulations...

Wattle tannin resins are also used to manufacture other resins, such as foams comparable to phenolics, as waterproofing additives, and binders for corrugated cardboard or charcoal briquettes. This discussion, however, deals only with particleboard, plywood, glulam, and finger-jointing exterior-grade wood adhesives. Formulations of the adhesives will be mentioned ad hoc, if at all necessary, as they have already been extensively discussed in articles and reviews in the relevant literature.(7)... 

On that basis, the book intends to bridge current issues, aspects and interests from fundamental research to technical apphcations. In seven chapters, the reader will find an arrangement of latest results on fundamental aspects of adhesion, on adhesion in biology, on chemistry for adhesive formulation, on surface chemistry and pretreatment of adherends, on mechanical issues, non-destructive testing and durability of adhesive joints, and on advanced technical applications of adhesive joints. Prominent scientists review the current state of knowledge about the role of chemical bonds in adhesion, about new resins and nanocomposites for adhesives, and about the role of macromolecular architecture for the properties of hot melt and pressure sensitive adhesives. Thus, insight into detailed results and broader overviews as well can be gained from the book. 

One of the tests is maintaining strength through a gap. This can be affected by air inhibition, as well as by the diffusion of accelerators. In another test, air was beaten into several adhesive formulations. The aerobic adhesives were only marginally affected by air inclusion when cured between surfaces. Like anaerobics, fillets or adhesive squeezed outside of a bonded joint will remain uncured unless exposed to UV light. 

Numerous geometrical factors of the specific interconnection will also influence anisotropic adhesive formulation and processing, including lead planarity, IC pad metallization, and IC test patterns. The planarity of the leads on the substrate and/or device and the compliance of the conductive particles will determine if anisotropically conductive adhesives can be used in a particular application. For systems with large disparities between lead height, no electrical interconnection will be formed, as shown in Fig. 5. Fine-pitch IC packages for surface-mounted applications, such as the plastic quad flat pack (PQFP), often use gullwing leads that olfer much compliance to the joint. Even if the... 

The fracture stress of adhesive-bonded joints (adhesion strength) is a consequence of processes that occur in the course of their formation, but all attempts to formulate a fundamental relationship between formation and failure of adhesive-bonded joints have so far been unsuccessful. This is mainly due to the lack of methods for meastuing adhesion that would permit determination of the failure equilibrium work. Accordingly, the relationship between the experimentally determined value of the adhesion strength and the thermodynamic characteristics can be one of correlation only. 

It is believed that the above formulations are applicable to both cases - either using self-healing material as a thin film in an adhesively bonded joint or using self-healing material as both the adherends and the adhesive layer. 

In order to evaluate the healing efficiency for the Mode II fracture test, end-notched flexure (ENF) specimens can be used (see a schematic in Figure 8.11). Recently, Ouyang and Li [57] derived a formulation of a Mode II energy release rate for the ENF joints bonded with dissimilar materials. When the adhesively bonded joint becomes a standard ENF specimen, that is, the two adherends have identical thickness and are made of identical material, we have [57]... 

Nevertheless, the adhesives formulated with PVAc-based polymer dispersions suffer from some drawbacks. Due to the high poly(vinyl acetate) thermoplasticity the adhesive joints obtained with PVAc-based formulations are sensitive to high temperatures and have a poor resistance to creep under static load. Moreover, the presence of poly (vinyl alcohol) (PVA), used as protective colloid in the emulsion polymerization phase, makes them also easily affected by both moisture and water. 

---