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Weak Boundary Layer Theory

According to the weak boundary layer theory,as first discribed by Bikerman,

when bond  failure seems to be at the interface,usually a cohesive rupture of

a weak boundary layer is the real event.The failure occurs cohesively withen

the weakest component of the joint ,similar to the weakest link in a chain.(as

the following fig.)

Fig.show:in an adial joint the substrate should be the weakest link.The adhesive joint:

(a)can be divided into at least six regions that are similar to the link.(b)in achain.

     This theory largely suggests that true interfacial failure (i.e.adhesion failure) seldom

     occurs.Failure may occur so near to the interface that it is apparently at the interface,

      but in most cases it is ductile plastic deformation or cohesive failure of a weak boun-

      dary layer.Weak boundary layers can originated from the adhesive , adherend , the

      environment,or combination of any of  the three.

     weak boundary layers can occur on the adhesive or adherend if an impurity  concentrated

    near the bonding surface and forms the weak attachment . when failure occurs , it is the

   weak boundary layer that fails , although failure may seem to occur at the adhesive adherendinterface .the history of a typical adhesive joint may be divided into three time periods .weak  boundary layers could develop during any one of these periods

   1-before and during application of the adhesive

   2-setting and curing of the adhesive

   3-the time period during which the joint is in service and exposed to stress

   before application of the adhesive , the most important task is to remove weak boundary

   layers. the most common material to be removed is atmospheric air . displacement of air bythe adhesive is a process already described ---- wetting .two other examples of weak boundary layer that are present at the  time of adhesiveapplication are common to polyethylene and metal substrates . polyethylene substratesusually have a weak , low molecular weight constituent evenly   distributed throughoutthe polymer this weak boundary layer is present at the interface and contributes to low failing stress when polyethylene is used as an adhesive or adherend . some metallic oxides , such

   other oxides ,such as those associated copper & copper alloys , may be weak & requireremoval prior to application of the adhesive . weak boundary layers such as low molecularweight polyethylene constituents and copper oxides can be removed or strength ened byvarious surface treatments .in some instances ,  new boundary layer may also from during the setting stage . if theseboundary layers are physically weak or if they interface with wetting , the final jointwill be weak reaction by product may cause weak boundary layers to form during this stagecertain active metal surfaces (e.g titanium) are known to react with some chemical hardenersused in adhesive formulations a clean reactive substrate surface may also becomecontaminated by the components in the adhesive itself . for example the adhesive couldcontain water or low molecular weight constituents that preferentially adsorb  on the substratesurface .weak boundary layers also could occur after the joint is made . the most common isenvironmental   moisture that can diffuse through either the adhesive of adherendplasticizer ,solvent , or other low molecular weight substances also may migrate out ofadherend or adhesive or deposit at the interface  thel bonding of plasticized polyvinylchloride , for example , can be difficult because of plasticizers will migrate to interfacewith time notice the way some plasticized binder covers will stick together  on your book –shelf ? this is due to highly mobile nature of the plasticizer .

    another weak boundary layer that can form during aging occurs by the adherend continuingto cure or chemically react with surroundings . for example ,corrosion of an aluminumadherend could occur at the interface and weaken the joint strength . on exposure tomoderately elevated  temperature under cured  phenolic substrates , for example , continueto liberate moisture as a by – product of the curing reaction , thereby degrading the jointstrengththe weak boundary layer that forms after the joint is made can be one of the most troublingproblems faced by the end – user . the early joint strength as measured in the normalinspection process may be excellent . however , with time and aging in certain environmentsthe strength of the joint could degrade precipitously and unexpectedly.