Bonding processes surface treatments

Because of their excellent chemical resistance, polyolefins are impossible to join by solvent cementing. Because of their very low surface energy, polyolefins can only be adhesively bonded after surface treatment processes. The most common way of joining polyolefins is by thermal welding techniques. 

Better adhesion can be obtained if parts are formed, treated, and coated with an adhesive in a continuous operation. The sooner an article can be bonded after surface treatment, the better will be the adhesion. After the part is treated, handhng and exposure to shop environments should be kept to a minimum. Some surface treatments, such as plasma, have a long effective shelf life. However, some treating processes, such as electrical discharge and flame treating, will become less effective with a longer time between surface preparation and bonding. 

For a surface treatment process to be practical, not only does it have to produce a clean, stable surface suitable for chemical or physical bonding to the... 

Glass fibers and most other reinforcements require special surface treatment to ensure the bonding and compatibility of the fibers to the plastic in order to maximize performances. Treatments are also used to protect individual filaments during handling and processing (7,14). 

The brush phases are made from the mono-chlorsilanes or the mono-alkoxysilanes which attach a single organic moiety to each reacted hydroxyl group. For example using octyldimethylchlorsilane, dimethyloctyl chains would be attached to the surface like bristles of a brush, hence the term brush phase. In all bonding processes, some hydroxyl groups remain unreacted which would allow the polar character of the silica play a part in retention and also render the surface liable to dissolution in water and dilute acids. For this reason the material is capped by treatment with either trimethylchlorsilane or hexamethyldisilazane to eliminate any unreacted or stearically... 

Adhesion of mbber is limited because of its inherent nonpolar nature and the presence of additives in formulation (processing oils, moulding agents, antiozonant waxes, vulcanization aids). Although, unvulcanized mbbers are somewhat less difficult to bond, most of the mbbers used in industry are vulcanized mbbers. To improve their adhesion, a surface treatment is always necessary. 

It has also to be remembered that the band model is a theory of the bulk properties of the metal (magnetism, electrical conductivity, specific heat, etc.), whereas chemisorption and catalysis depend upon the formation of bonds between surface metal atoms and the adsorbed species. Hence, modern theories of chemisorption have tended to concentrate on the formation of bonds with localized orbitals on surface metal atoms. Recently, the directional properties of the orbitals emerging at the surface, as discussed by Dowden (102) and Bond (103) on the basis of the Good-enough model, have been used to interpret the chemisorption behavior of different crystal faces (104, 105). A more elaborate theoretical treatment of the chemisorption process by Grimley (106) envisages the formation of a surface compound with localized metal orbitals, and in this case a weak interaction is allowed with the electrons in the metal. 

Control of fiber friction is essential to the processing of fibers, and it is sometimes desirable to modify fiber surfaces for particular end-uses. Most fiber friction modifications are accomplished by coating the fibers with lubricants or finishes. In most cases, these are temporary treatments that are removed in final processing steps before sale of the finished good. In some cases, a more permanent treatment is desired, and chemical reactions are performed to attach different species to the fiber surface, e.g. siliconized slick finishes or rubber adhesion promoters. Polyester s lack of chemical bonding sites can be modified by surface treatments that generate free radicals, such as with corrosive chemicals (e.g. acrylic acid) or by ionic bombardment with plasma treatments. The broken molecular bonds produce more polar sites, thus providing increased surface wettability and reactivity. 

Fig. 2.24 The production process for C fibers. Starting with pitch or polyacrylonitrile the fluids are spun into oriented fibers, thermoset, carbonized, and graphi-tized to carbon fibers. Surface treatment improves bonding to resin matrix.

Deep Bonding Process (Tiefbonder-Verfahren, in Ger) for AP Projectiles. This method, developed in Germany during WWII by the late Dr. V. Duffek collaborators, consists of deep surface treatment of a sintered iron AP projectile with a phosphate. The method is claimed to diminish the wear of gun barrels and to increase the effectiveness of armor penetration by AP projs... 

With time (under increased temperature and humidity) the crack tip continues to a weaker region which for this surface treatment appears to be near the oxide/alloy interface. Figure 11 summarizes the analysis of the bond failure for this particular surface treatment. The important aspect here is that under identical conditions, different surface preparations show different modes of failure. Weak boundary layers are not developed using some treatment/bonding combinations. Processes have been developed in which the locus of failure remains in the adhesive ("a cohesive failure") and it is necessary to use a mechanical test in which even more stress is placed on the interfacial region (19). 

Primers can be used to protect both treated metal and nonmetal substrates after surface treatment. The use of a primer as a shop protectant may increase production costs, but it may also provide enhanced and more consistent adhesive strength. The use of a primer greatly increases production flexibility in bonding operations. Usually primer application can be incorporated as the final step in the surface preparation process. The primer is applied as soon as possible after surface preparation and usually no more than a few hours later. The actual application of the adhesive may then be delayed significantly. 

The surface preparation method must be carefully considered, especially if the completed weldbond is to have long-term durability to hostile environments. The surface preparation should provide an optimal surface for both adhesion and welding. Thus, the choice of surface treatment is crucial, and there can be a conflict of requirements. The spot welding process requires a low electrical surface resistance, and many adhesive surface preparation processes provide a high surface resistance because of oxide layer buildup. When it is impossible to harmonize on a surface treatment, current practice tends to favor treatments that yield good weld nuggets at the expense of the adhesive bond. 

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