adsorption theory
the adsorption theory states that adhesion results from molecular contact between two materials and the surface forces that develop. a bond develops from the adsorption of adhesive molecules on the substrate and the resulting attractive forces, usually designated as secondary or van der waals forces. for these forces to develop, the respective surfaces must not be separated more than five angstroms in distance. therefore, the adhesive must make intimate, molecular contact with the substrate surface. the process of establishing continuous contact between an adhesive and the adherend is known as "wetting." fig. 2.7 illustrates good and poor wetting of an adhesive spreading over a surface. good wetting results when the adhesive flows into the valleys and crevices on the substrate surface poor wetting results when the adhesive bridges over the surface perturbations. obtaining intimate contact of the adhesive with the surface essentially ensures that the interfacial flaws are minimized or eliminated .at minimum .poor wetting
causes (1) less actual area of contact between the adhesive and adherend and (2) stress risers at the small air pockets along the interface. this results in lower overall joint strength. wetting can be determined by contact angle measurements. it is governed by the young equation, which relates the equilibrium contact angle, e, made by the wetting component on the substrate to the appropriate interfacial tensions:
12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75">lv cos
12خ¸" style="width: 10.5pt height: 25.5pt" type="#_x0000_t75"> =
12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75"> sv -
12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75"> sl
the term
12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75"> sv is the interfacial tension of the solid material in equilibrium with a fluid vapor,
12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75">lv is the surface tension of the fluid material in equilibrium with its vapor, and
12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75">sl is the interfacial tension between the solid and liquid materials. complete, spontaneous wetting occurs when
12خ¸" style="width: 10.5pt height: 25.5pt" type="#_x0000_t75"> = 0°, or the material spreads uniformly over a substrate to form a thin sheet. a contact angle of 0° occurs with a pure water dropinglet on a clean, glass slide. therefore, for complete spontaneous wetting, cosine
12خ¸" style="width: 10.5pt height: 25.5pt" type="#_x0000_t75"> > 1.0 or when:
12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75">sv >
12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75"> sl +
12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75">lv
wetting is favored when the substrate surface tension,
12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75">sv, or its critical surface energy,
12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75">c ,is high, and the surface tension of the wetting liquid,
12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75">lv, is low (i.e.,
12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75">c substrate >
12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75"> adhesive) low energy polymers, therefore easily wet high energy substrates such as metals. conversely, polymeric coatings and polymeric substrates having low surface energies will not be readily wet by other materials and are useful for applications requiring nonstick, passive surfaces.
for good wetting:
12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75"> adhesive «
12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75"> e substrate
for poor wetting:
12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75"> adhesive »
12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75"> c substrate
thus, as can be seen from the surface energy data in table 2.2, most common adhesive liquids readily wet clean metal surfaces, ceramic surfaces, and many high energy polymeric surfaces. however, common adhesives do not wet low energy surfaces such as polyethylene and fluorocarbon. the fact that good wetting required the adhesive to have a lower
wetting occurs when ? = 0 ,or the material spreads uniformly over a substrate to form a thin sheet .a contact angle of 0 occurs with a pure water dropinglet on a clean glass slide.
therefore,for.complete,spontaneous wetting, cosine ? > 1.0 or when :
? sv > ? sl + ? l v
wetting is favored when the substrate surface tension, ? sv ,or its critical surface energy,
?c,is high ,and the surface tension of the wetting liquid,? l v ,is low{i.e.,?c substrate > ?adhesive}.
low energy polymers,therefore easily wet high energy substrates such as metals.conve-
-rsely,polymeric substrates having low surface energies will not be readily wet by other materials and are useful for applications requiring nonstick ,passive surfaces.
for good wetting: ? adhesive< < ? c substrate
for poor wetting ? adhesive> > ? c substrate:
thus,as can be seen from the surface energy data in table2.2,most common adhesive liquids readily wet clean metal surfaces ,ceramic surfaces ,and high energy polymeric surfaces.howeve r,common adhesive do not wet low energy surfaces such as polyethylene and fluorocarbons.the fact that good wetting requires the adhesive to have a lower
introduction to adhesives and sealants
surface tension than the substrate explains why organic adhesives .such as epoxies .have excellent adhesion to metals . but offer week adhesion on many untreated polymeric sub-
strates ,such as polypropylene ,and the fluorocarbons .
the lower fig provides a simple view of the relationship of wetting and adhesion .here the contact angle of droping of an epoxy resin is about 42 dynes\cm .the expected bond strengths
would increase as the contact angle decreases . therefore the bond strength of the epoxy
adhesive on the epoxy substrate would be expected to be greatest ,followed by polyvinyl
chloride ,polyethylene ,and polytetrafluoroethylene , in that order.
(contact angle of an uncured epoxy adhesive on four surfaces of varying critical surface
tension ,note that as the critical surface tension of the surfaces decrease the contact angle increases indicating less wetting of the surface of the epoxy .)
some important concepts develop out of the principle that for good wetting to occur
? adhesive must be less then ? c substrate you would , therefore expect polyethylene and
fluorocarbon, if used as adhesives ,would provide excellent adhesion to a variety of
surfaces including both polymers and metals .
in fact they do provide excellent adhesion .however ,commercial polyethylene generally
has many lower molecular weight constituents that create a week boundary layer thus
preventing practical adhesion .polyethylene make an excellent base fore hot melt adhesives
once the weak low molecular weight constituents are removed.
flourocarbons cannot be easily melted or put into solution.thus,they are difficult to get
into a fluid state to wet the surface and then solidify without significant internal stresses.
investigators are attempting to develop epoxy resins with fluorinated chains so that they
can easily wet most surfaces.with this process the surface tension of the epoxy adhesive
can be reduced from 45 to 33 dynes/cm,and good bond strengths are achieved on untreated
low energy substrates.
it is easy to see why silicone and fluorocarbon surfaces provide good release character-
-istics and are applied as coatings to injection molds,extrusion screws,and cooking ware.
most resins will not easily wet these surfaces.higher surface energy oils are weak boundary
layers.if a very thin layer of such oil is on the substrate,the adhesive will" bond "to the oil and the failure mode would be cohesive within the oil film.
it is also interesting to note that by making a coating (or adhesive)more likely to wet a substrate (by lowering its surface tension) , you may be inadvertently making it more
difficult for any coating or adhesive to wet this new material once it is cured.graffiti-
resistant paints work in this manner.
to optimize joint strength,it is often necessary to reduce the surface tension of the adhes-
-ive or increase the surface tension of the substrate.reducction in surface tension of the
adhesive is generally achieved by substituting or adding a lower surface energy base poly-
-mer or to add a"wetting agent"or"adhesion promoter".
to increase the surface energy of substrate a surface treatment generally must be employed.
prebond surface treatment are employed to either (1) remove weak boundary layer .
(2)increase the surface energy of the substrate . or (3) provide a consistent surface for bonding .there are three main categories of surface treatments : mechanical ,chemical,
& energetic .within each categories there are several processes as shown in lower table.
once intimate contact is achieved between the adhesive and substrate through wetting
it is believed that permanent adhesion results primarily through forces of molecular
attraction . other types of forces can also prevail due to the close proximity of the adhe-
sive & substrate .these forces provide the basis of the other common theories of adhesion .
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .