Effect of temperature on the extent of migration of low molecular weight
moieties to rubber surface
Rafael Torregrosa-Coque, Sonia A
´
lvarez-Garcı
´
a, Jose
´
Miguel Martı
´
n-Martı
´
nez
n
Adhesion and Adhesives Laboratory, University of Alicante, 03080 Alicante, Spain
article info
Article history:
Accepted 21 September 2010
Available online 7 October 2010
Keywords:
Rubbers
Interfaces
Infrared spectra
Contact angle
abstract
Vulcanized rubbers contain different low molecular weight additives in their formulation, including
antiozonants, plasticizers, oils, etc. These moieties – mostly paraffin wax – often migrate to the surface
causing a weak boundary layer of non-rubber contaminants which is deleterious for adhesion of rubber
to adhesives (such as polyurethane and polychloroprene adhesives). One of the key steps in
the manufacturing of rubber/adhesive joints is the reactivation, i.e. sudden heating of the thin adhesive
layers on the substrates to be joined under infrared (IR) radiation to 80–90 1C for a few seconds to allow
diffusion of the polymeric chains under pressure. This reactivation may cause the migration of
low molecular weight additives to the rubber surface causing a lack of adhesion. The main aim of this
study was to .identify the influence of the reactivation temperature (40 to 170 1C) on the surface
properties of sulphur vulcanized styrene–butadiene rubber and determine the extent of the diffusion of
paraffin wax and zinc stearate to the rubber surface. The changes produced on the rubber surface
were measured immediately after reactivation treatment by ethylene glycol contact angle measure-
ments, attenuated total reflectance infrared spectroscopy (ATR-IR) and scanning electron microscopy
(SEM). Additionally, the weight loss of the rubber after reactivation at different temperatures was
recorded.
The reactivation of the rubber at different temperatures produced changes in the morphology
and thickness of the paraffin wax layer on the surface. By heating at temperature close to that of the
paraffin wax melting point, the paraffin wax migration was favoured and at the same time the crystals
of paraffin wax on the rubber surface were melted. As a consequence a thicker and smoother film of
melting paraffin wax was formed. By increasing the reactivation temperature, a partial removal of
paraffin wax was produced and the thickness of the paraffin wax film on the rubber surface was
reduced. For reactivation temperatures below 90 1C, the higher the temperature, the lower the weight
loss of the rubber, because the increase in the surface area of the melted paraffin wax layer
that prevented migration from the rubber bulk. However, for reactivation temperature higher than
90 1C, the weight loss of the rubber increased with the reactivation temperature and this was likely
due to sublimation of the paraffin wax on the rubber surface. Besides, even after reactivation at 170 1C, a
thin film of paraffin wax always remained on the rubber surface as was evidenced by contact angle
measurements. On the other hand, a critical reactivation temperature at 90–100 1C existed at which the
migration of zinc stearate to the paraffin wax layer on the rubber surface was favoured.
& 2010 Elsevier Ltd. All rights reserved.
1. Introduction
Nowadays, the sulphur vulcanized styrene–butadiene (SBR)
rubbers are the most widely used polymers in the World, constituting
about 40–50% of the total synthetic rubber consumed. These rubbers
are commonly used in several applications including sole material in
footwear, adhesives, moulded or extruded goods, and modifiers for
asphalt and thermoplastic resins. Rubbers are visco-elastic hydro-
carbon polymeric materials, whose high elasticity allows them to
undergo considerable deformation under relatively low stress and,
once the stress is released, they recover their original form and
dimensions, in a short period of time and without too much
permanent deformation. It is a fact that rubbers by themselves have
poor properties hence their limited commercial value. However, by
using a variety of fillers such as carbon black and silica and other
additives, it is possible to improve their properties in order to produce
the best achievable products for different applications. The nature and
number of additives in rubber formulations depends on the chemical
nature of the raw rubber polymer and the properties expected in the
finished products [1]. SBR rubber formulation includes generally the
addition of fillers, plasticizers, antioxidants, vulcanizing agent and
activators.
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/ijadhadh
International Journal of Adhesion & Adhesives
0143-7496/$ - see front matter & 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijadhadh.2010.09.002
n
Corresponding author. Tel.: +34 965 90 39 77; fax: +34 965 903454.
E-mail address: jm.martin@ua.es (J.M. Martı
´
n-Martı
´
nez).
International Journal of Adhesion & Adhesives 31 (2011) 20–28