MECHANISM OF ADHESION JOINT FORMATION

To understand the properties of the adhesion joints one should consider the mechanism of their formation. The latter consists of several stages:

• spreading of the adhesive on the surface of a solid body and its wetting

• equilibrium establishment (which is not always possible) of the adhesion contact, depending on the viscosity of the adhesive and on the processes of diffusion and adsorption

• formation of the chemical or physical structure of the adhesive on curing, accompanied by the emergence of a surface layer distinguishable in its properties from the bulk. This stage also includes the setting of the adhe-sive, possible crystallization, evolution of new phases, etc.

The emergence of forces of molecular interactions responsible for adhesion is possible only under conditions under which the molecular contact occurs at the adhesive-substrate interface. This process is dependent on the physical re-lief of the surface. The process of the formation of adhesive joints also depends on the kinetics of spreading, which, in turn, depends on the rheological proper-ties of adhesive. As we have already noted, the state of true thermodynamic equilibrium in many cases cannot be achieved.

The spreading of liquid adhesives on a surface is determined by the general regularities for the impregnation of porous and disperse solids.⁴⁸Roughness of the surface affects wetting and the contact angle on the real surfaceΘ′is deter-mined by the Ventsel-Deryagin relationship cosΘ′= KcosΘ, whereΘis an equi-librium contact angle and K is the roughness coefficient, i.e., the ratio of the true surface area to the apparent surface. If K > 1, spreading can take place only at Θ >0. One can show that:

∆Θ= (KcosγL Θ−1) [2.56]

i.e., the increase in the roughness of the surface, K, contributes to the spreading of liquid having a contact angle close to, but always less than, 90^o. It is also known that the equilibrium value of the contact angle of wetting is attainable over a very long period of time, similar to the establishment of the equilibrium value of the surface tension of polymer system during their curing.³⁶

There is a theory describing the kinetics of surface wetting due to the action of capillary forces in relation to the surface tension predicted by a Laplace equa-tion.⁴⁸One can calculate the rate of liquid flow into a narrow or wedge-like slot, or a round hole. The theory takes into consideration the rheological properties of

liquids. In particular, it can be applied to viscoelastic liquids, which include polymer adhesives. In this case, the wetting depends on the polymer structure.

With an increase in temperature of melt or polymer solution, the intermolecular interactions become weaker and molecules assume the most probable conforma-tions. The rate of change of the molecular arrangement and interaction with a surface determines the kinetics of adhesion development due to chain flexibility and intermolecular interactions in the polymer system and also the energy of the interaction with the surface. With the penetration of the adhesive due to pos-sible orientation of molecules during flow, the viscosity may increase. The rela-tionship between the wetting rate and the relaxation time affects the structure of the adhesion joint formed, which cannot be predicted by the thermodynamic theory because it does not account for the non-equilibrium properties of a sys-tem. If the adhesive viscosity is high or it is increased rapidly during the process of wetting, defects are likely to occur on account of incomplete wetting. Limiting strengthening of the adhesion joint can be accomplished by maximum filling of microdefects in the substrate surface. High viscosity of the adhesive, features of the surface topography, an insufficient duration of the plastic state, and other causes, result in pores and cavities found in the adhesive layer. As a result, the area of actual contact with the adhesive decreases and potential sources of adhe-sion bond failure appear.

The molecular description of the formation of an adhesive layer does not take into account the polymeric structure of adhesives either. It is based on de-termination of the number of adhesion or molecular bonds, n, per unitary area of a true contact surface and the energy of a single bond between two surfaces.⁴⁹ During the formation of such bonds the adhesion strength, A, can be expressed by a specific work spent on the destruction of the adhesive joint:

A = S n U_tΣ _{i i} [2.57]

where n_iis the number of adhesion bonds of the i-th order per unitary area of the true contact surface, St, and Ui is the energy of these bonds. The true surface area may be calculated as follows:

( )

where n1is the number of groves on the surface having a depth, hi, and half of the angle at the apex of the triangle formed by the section of furrow,α, d is the pore diameter, n₂is the number of pores per cm², t is time of contact,ηis the melt vis-cosity, and P is the pressure under which the melt flows into microdefects. Be-cause work of adhesion, irrespective of the nature of the bonds, is proportional to the true contact area, we can derive:

( )

where x1is the average energy of a single bond multiplied by the number of ad-hesive joints per cm²of the true contact area.

The appearance of the maximum number of bonds, determined by the sur-face relief and adhesive rheology, is one of the most essential conditions for the formation of contacts and increase in adhesion strength. Microrheological ef-fects include adsorption interactions with the surface and are determined by the flexibility of chain and molecular mobility in the adsorption layer.

A valid molecular-kinetic treatment of the formation of adhesive bonds has been given by Voronin and Lavrentyev.^50,51When discussing the mechanism of formation and failure of adhesion joints, it is assumed that for segments of the polymeric chain in the boundary layer there are two possible states:

• segments bound with the surface active centers with the number of bonds n2

• segments bound with similar ones with the number of bonds, n1.

In this instance, n1+ n2= n0= const. The rate of change of the number of bonds is:

dn

dt² = n− ν_{2 2}+ n_{1 1}ν [2.60]

where ν₁ and ν₂ are the frequencies of the formation and destructions of the bonds of kinetic units of the polymer with the active centers of the substrate. In a general form:

ν₁=ν_i0exp -U^α kT







 [2.61]

where U is activation energy of the formation or destruction of adhesive joints.

Under conditions of equilibrium:

dn

dt = 0; n = n 1+

2 2 o

2 1

ν ν/ [2.62]

Then, adhesion strength in a simplified form can be expressed as:

A = n f f =n n f

2 av; av 2 N2i i

2i

∑ ∑

whereΣn_2i= n2, n2iis the number of bonds between polymer segments and active centers on the surface, fiis the energy of binding. When loads are applied to the adhesion joint, some bonds, prior to failure, will pass with average frequency from state 1 into state 2, i.e., the number of active bonds will be less than n₂. Therefore, the experimentally determined value of A is A = n3fav, where n3=n2−n2να, andα is a coefficient depending on the rate of loading. In a gen-eral form:

A = n f

(1+ )(1 )

o av

2 1

ν ν/ −αν [2.64]

From a molecular-kinetic concept, it follows that n₁and f_avhave an impor-tant effect on the strength of the adhesion joint. Depending on n1, the density of the polymer boundary layer, directly adjacent to the solid surface, varies.

All these approaches enable us to get some relationships to establish the durability of adhesion joints, accounting not only for adhesive but also for cohe-sive failure of an adhesion joint.