Development of Adhesion
dhesion Promoters for Water-based Coatings for Untreated
PP/EPDM
Helen Brannon a, Phil Coxa, Richard Greenwooda, Jon Preecea, Bjoern Heinzb, Audrée
Audr Andersenc
a.
School of Chemical Engineering, University of Birmingham, Edgbaston B15 2TT
b.
BASF SE, 67056 Ludwigshafen, Germany
c.
BASF
SF Coatings GmbH,
GmbH 48165 Muenster, Germany
Delamination of coatings from polyolefin surfaces presents a major challenge
challe
to the
automotive industry. It occurs because of insufficient binding between the coating and
polyolefin surfaces.. Polyolefins are very hydrophobic and have a low surface energy.
energy
Therefore they are difficult to coat with polar coatings. In the automotive industry, car
bumpers are often made from injection moulded polyolefins e.g. polypropylene (PP)
(PP which
are mixed with rubber components
component e.g. the copolymer EPDM (ethylene
ylene propylene diene
monomers). Currently a flaming technique is used to increase the surface energy of the PP,
therefore increasing the wettability
ttability of the surfaces, thus, improving the adhesion properties.
The flaming is the current state of the art technique. However, the goal of the project is to
eliminate this flaming step in the coating process for increased efficiency and efficacy. [1-5].
A halogen free adhesion promoter system that
that allows for good adhesion between plastic
substrates
ubstrates and water based coatings is to be developed. This adhesion promoter must be
amphiphilic and compatible with water based systems. It has to be directly incorporated into
the basecoat.
One method reported to improve adhesion to a polyolefin surface is swelling of the polymers
with solvents, although currently the
the mechanism is not well understood. However, solvents
are known to swell polymers and improve the mobility of polymer chains and their ability
abil to
physically entangle at an interface.
interface. It is thought that entanglements might be induced to occur
with other polymer chains from a waterborne basecoat thus facilitating physical adhesion via
molecular chain entanglement [6-8] (Fig 3).
Figure 1 - Surface Swelling Results: Solvent uptake () and loss () on pure PP
substrate () and PP + 2%EPDM ()
The first route investigated was the examination of the swelling characteristics of
commercially relevant PP/EPDM substrates. Three
T
solvents were selected,
selected based on their
Hansen solubility parameters,, to test with the PP/EPDM substrates and positive
ositive results have
been collected with an increase in substrate mass when exposed to the solvents
solven (Fig 1).
The rate of solvent loss was
as also investigated (Fig 1). Assuming any remaining
rema
increase in
mass is due to swelling from the solvent in the PP/EPDM, it can be seen that appreciable
swelling remains in the polymer matrix after 100 minutes drying and provides a suitable
timescale for the adhesion promoter to work from any applied basecoat.
Candidate adhesion promoters have been collected based on the hydrophilic head/
hydrophobic tail concept and are being tested on untreated and flamed substrates as a
standard. The candidate adhesion
adhesion promoters are to be tested on the swollen substrates (Fig
(
2).
Figure 2 - Solvent Swelling Concept
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