International Journal of Adhesion & Adhesives 22 (2002) 179–185
Formation of a boundary layer of polyoleﬁns
adhesivelybonded to steel
M. Kalnins*, J. Ozolins
Chemical Technology Department, Institute of Polymer Materials, Riga Technical University, 14/24 Azenes street, Riga LV - 1048, Latvia
Accepted 15 October 2001
The change in peel strength A of adhesive joints of PE or ethylene copolymer with vinylacetate with steel, the thickness of a
residual adhesive layer h on a metal substrate after a cohesive fracture of the adhesive joints, and some structural characteristics of
this layer (content of cross-linked and low-molecular parts of the polymer and molecular weight) are studied as functions of a
contact time t under conditions of catalytic contact oxidation. The AðtÞ curves of both the studied polymers are similar. Fracture of
adhesive joints is of cohesive character, and h grows with contact time. The structure of the boundarylayer signiﬁcantlydiffers from
the bulk adhesive. Competing contact oxidation processesFcross-linking and destructionFproceed simultaneouslyin the adhesive
boundarylayer. The contact time function of an empirical quantityK; which estimates the ratio of contribution of two competing
contact oxidation processes, coincides with the t function of the ratio A
¼ A=h; which is a peel energyper unit volume of a polymer
layer involved in deformation followed by fracture and is determined by its strength-deformation characteristics. r 2002 Elsevier
Science Ltd. All rights reserved.
Keywords: A. Hot melt; B. Metals; C. Boundarylayers; D. Destructive testing
In our earlystudies, we have observed that the
adhesion interaction of a polyoleﬁn melt with steel leads
to the formation of a cohesivelyweak polymer
boundary layer. An adhesive joint always breaks in the
adhesive [1,2] (the fracture under the action of highly
polar liquids, when the interfacial separation takes
place, is an exception ). A fracture front usuallylies
at a distance of at least several tens of nanometers from
the interface. In comparison with the bulk, the structure
of the boundarylayer is less organized . This agrees
well with the conception of a weak boundarylayer
offered long ago byBikerman .
The experimental data surveyed in [6,7] show that the
cohesive characteristics of the boundarylayer are
affected bythe contact thermooxidation reactions of
the polymer catalyzed by iron surface compounds.
Oxygen from several sources can take part in the
contact thermooxidation, namelyoxygen which diffuses
through the polymer layer, oxygen captured at the
interface, and, ﬁnally, oxygen which is absorbed or
chemisorbed bythe metal surface oxide.
The main features of contact thermooxidation (oxy-
gen uptake, carbon dioxide evolution, change in the
carbonyl group content, and change in the layer weight)
studied under conditions of a free access of oxygen
through the polymeric layer are kinetically interrelated
with the change in peel strength A . The dependence
of the rate of change of individual kinetic parameters of
contact thermooxidation as well as of A on the thickness
of the adhesive layer and on the contact temperature can
be described bya simple equation. The dependence of A
on the contact time t can be described byan expression
based on the assumption that the magnitude of A is
controlled bytwo main competing processes, namelythe
accumulation of oxygen-containing groups and the
oxidative cross-linking which increase the A values, on
the one hand, and the reactions of oxidative destruction
which decrease A; on the other hand .
During oxidation in contact with steel, the iron
compounds dissolve and penetrate into the bulk of a
polyethylene layer . The intensity of dissolution
correlates with the rate of contact oxidation. The
proﬁles of the iron compounds content are determined.
*Corresponding author. Tel.: +371-7-616918; fax: +371-7901460.
E-mail address: email@example.com (M. Kalnins).
0143-7496/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0143-7496(01)00053-7