Robert E. Lindberg
BIW Connector Systems, Inc.
of organic polymers to inorganic surfaces is hardly new technology.
The Sumerians and Babylonians used asphalt
and pitch for floor covering and paving. Naturally occurring polymers
were used by the ancient Egyptians to varnish their sarcophagi.
After thousands of years, these organic coatings are still at least
That may be true for the dry desert, but when it comes to polymer to mineral
coatings submerged in water survivability may be measured in hours. Few agents
are as destructive to organic bonds as bionic water. (Water naturally has two
ionic forms: HO-, H3O+, as well as being a polar molecule.)
The 1940 evolution of composite plastics filled with glass fiber for structural
material replacement of aluminum and steel brought on the discovery of this troubling
phenomenon. Prolonged exposure to moisture severely degraded the composite's
strength-to-weight ratio. The loss of strength demonstrated the power of the
intruding water to debond resin from hydrophilic glass.
Figure 1 Completely delaminated polyurethane boot
The same phenomenon has been confronted in many other circumstances, particularly
in under-sea applications where organic polymer moldings and coatings debond
from metals. Paint coatings have received considerable study for underwater
applications due to the hydrolyzation problem. The past fifteen years of
naval experience reveal susceptibility of rubber to metal bonded materials
outboard ship where electrical connectors fail because of overmold polymers
debonding. Connector/cable assemblies rated for fifteen years of continuous
ocean submersion may fail in as little as a few months before a new ship
has even left the dock. (See 1.)
Figure 2 Partially deleminated neoprene boot
Studies attributed cathodic action between the connector shell in the electrolytic
ocean water and ship hull sacrificial zinc plates as a major contributor
to the failure. Credit for recent significant contributions goes to NWCS,
NSSC, TRI, Electric Boat, NRL, and BIW Connector Systems. Researchers are
in agreement that water with high alkalinity, resulting from the cathodic
action, attacks the polymer/metal bond interface.
The cathodic action begins at the exposed edge of the polymer to metal interface.
The delamination appears to creep along the interface until the overmolded
boot of the connector (or any other polymer to metal bond) completely delaminates.
However, the rate of delamination is dependent on a number of variables with
the nature of the polymer being very significant; polyurethane delaminates
many times faster than neoprene. On the other hand, polyethylene has not yet
revealed a failure mode, although currently, inadaquate test data is available.
1 illustrates a completely delaminated polyurethane boot resulting from cathodic
delamination tests, while 1 illustrates a partially delaminated neoprene sample,
tested concurrently with the sample of 1.
Cathodic action occurs naturally in the marine environment in the presence
of dissimilar metal structures. Since, typical connectors of underwater assemblies
are made of relatively noble metals, such as monel, stainless steel, and titanium
compared to the sacrificial zinc plates on ship hulls, a battery current results
with the ocean water acting as an electrolyte. The connector and the zinc form
the electrodes of a battery. Electrons flow from the zinc (anode,) through
the steel ship hull to the connector shell (cathode.) The electrons leaving
the zinc produce a positive zinc ion that dissolves in the sea water. Positive
and negative ions diffuse through the seawater between the electrical connector
shell and the zinc plate to complete the electrical circuit. This is more precisely
defined as Electrolytic Conductance by anions and cations.
The electrolytic reactions produce a problem: the caustic OH- ion is released
at the connector shell. Under conditions that allow the buildup of the OH-
the pH level can rise from the normal sea level of 8.2, to perhaps 11 to 12
pH. Organic polymers typically used in manufacture of underwater connectors
begin to break down at that high pH level, especially, the adhesive bond between
the overmolded sealing boot and the connector shell. Once the seal fails, the
connector is flooded and electrical failure is probable.
Since the connector shell is the cathode in this reaction that seems to lead
to delaminatioin of the connector boot, the name Cathodic Delamination seems
Further research indicates cathodic action is an incomplete description of
the noted debonding process, and, after fifty years of intensive study, debate
continues on the mechanism of bonding and debonding between polymers and metals.
Recent studies not only confirm that polymer/metal bonds will hydrolyze in
the presence of moisture, but, lead to the understanding that consequential
debonding cannot be stopped. High pH environments, a consequence of cathodic
action in the environs of ship hulls, especially aggravate the condition. In
addition, thermal shock is found to be a major factor in debonding.
The debonding mechanism, particularly cathodic debonding recently received
significant attention. NAVSEA through NWSC at Crane, Indiana, and NRL, Florida,
funded studies at Texas Research Institute. Various connector configurations
were tested. The focus of the effort was to stimulate industry suppliers to
develop solutions to the problem and produce a reliable long term polymer to
metal bond--with a fifteen year service life as the target. The test report
is available from NWSC.
The principle product derived from that research, BONDiT?, is a reliable metal
to polymer bonding system able to withstand long service in continuously submerged
ocean environments, subjected to caustic chemical attack by cathodic action
and thermal shocks and be compatible with existing field service practices.
The developer is Bob Lindberg, consultant to BIW Connector Systems, Inc. of
Santa Rosa, California.
Additional research at BIW has lead to a better understanding of the debonding
mechanism. Assuming a good bond initially, there are two primary contributors
to adhesion failure: polymer/substrate interface stresses, and water desorption.
In practice these work together to reduce a good cohesive bond to a failure.
It is well established that moisture will travel through a polymer to reach
the interface, and proceed by diffusion along the interface beginning at any
exposed polymer to metal interface. The diffusion along the interface is as
much as 450 times faster than through the polymer. However, depending on geometry
and material, moisture may saturate the polymer and fill the interfacial region
faster than the moisture flow along the interface. Typical polymer saturation
ranges widely from 0.015% to 24% of mass, while the interfacial region saturation
remains relatively constant at 3.0% to 3.5% of surface area regardless of the
polymer, the hydrophilic nature of the mineral surface being the biggest determinant.
Figure 3 Chemical equation for equilibrium state polymer to metal
adhesion in moisture
There is little doubt a saturated interface hydrolyzes the polymer/metal bonds.
However, the reaction is reversible, and hence, if the interface is dried,
the bonds will return to their former state. (See 3.) When in equilibrium in
the moisture environment a percentage of each side of the equation exists,
with individual metal/polymer bonds having some probability of being in either
state. Equilibrium constants determine the probability, and correspondingly
which side of the equation is favored. In any case, some percentage of debonding
will exist--its unavoidable.
Variables such as activation energies, bond temperature, bond chemical structure,
and pH of the water have significant effects on the probability.
Figure 4 Comparison of thermal expansion coeeficients for polymers
In and of itself, the equilibrium bond conditions may be satisfactory for the
application. Bond failure occurs only when the interfacial region separates
beyond the range that dispersion forces can act to induce chemisorption. Thermal
stress constitutes the most significant of the mechanical stresses in a typical
application causing separation. Polymer coefficients of thermal expansion range
from two times to twenty times that of common metals. (See 4.) For polymers
having adequate bond density this does not pose a problem, baring actual fracture
of the bulk material. However, with a lowered bond density resulting from hydrolysis
of the interfacial region the thermal expansion of the polymer causes the polymer
to physically break away from the metal interface, thereby, permanently breaking
Another factor causing stress is swelling due to moisture absorption by the
polymer. The degree of swelling is dependent on the pH level of the bond interface
and nature of the polymer: polyurethane having very hydrophilic qualities,
and polyethylene very low, that is, hydrophobic. Since the metallic substrate
will not swell the adhesive-substrate interface is the plane of discontinuity
and resulting stress build-up. When this accompanies desorption by water (hydrolyzation
of the adhesive bond) and thermal shock in the desorbed adhesive, then failure
in adhesion inevitabley results.
A fourth factor is
also clearly evident. The cathodic environment increases the available
OH- ions at the exposed edges of the interface. That moves
the equilibrium point of the reaction in favor of debonding. Further
the pH moves the equilibrium constants so far in favor of debonding that
virtually no organic/mineral interface can withstand the chemical attack.
stress at that "loose" edge initiates complete debonding of
the polymer from the mineral substrate. Blistering illustrates the same
seen in paint on metal substrate. As the moisture penetrates through
the polymer to the hydrophilic metal surface, microscopic irregularities
offer sites on the metal surface to form oxidation-reduction reactions.
That liberates OH- ions, the continuation of which increases the pH level
high levels, measusred as high as pH 14. That in turn increases the rate
of hydrolyzation of the polymer/metal interface and the blister spreads,
exposing metal that can react, ultimately delaminating the polymer completely.
BONDiT™ is a multicomponent system, to provide high moisture resistant metal
to polymer bonds. It provides a moisture and thermal shock resistant substrate
for a wide range of polymers, as a tie coating to metal substrate. It is
designed with constants that greatly favor the bonding side of the chemical
equation (See 3.) The constants of the individual polymer/metal bond give
it a probability 104 greater than those commonly in use today. Designed to
also reduce the moisture concentration significantly at the interface, it
moves the equilibrium constants in favor of bonding.
The BONDiT™ system is designed to "manage" thermal stresses
and provide a balanced transition from rigid metal to high expansion polymers,
Accelerated Life Tests (ALT) conducted at BIW demonstrate bond survival rate
improvements of 3,000% over common systems in use today. In all cases the bulk
adherend failed under adhesion pull tests, before and after accelerated aging,
with such overmold materials as polyurethane, neoprene, and LDPE. ALT run at
Texas Research Institute under the direction of NWSC show no degradation of
the BONDiT™ system.
The entire system works together to provide a reliable bond between the metal
substrate and the overmolded polymer. The BONDiT™ system is proven to withstand
environment of moisture, cathodically active conditions, and thermal and mechanical
Specifications and Applications
Normal operating temperatures for BONDiT™ are
-40F to 150F. BONDiT™ will
maintain adhesion to metal substrate up to temperatures of 400F in super
heated water or steam, and certain oils, albeit with decreasing bond strength
proportional to increasing temperature above 200F. Peel strength exceeds
180 lb. per inch width at 75?F, and impact of 52 in-lbs. Thermal shock tests
employed include -40F to 150F water of 180 cycles, 60?F to 200?F water
of 30 cycles, and 75F to 400F water and oil of 14 cycles. Accelerated moisture
soak tests run at 150?F and 195?F in excess of 400 hours continuous submersion
produced no bond failures. BONDiT™ sustained soak tests in 11.5 pH, 195?F
salt water in excess of 250 hours before evidencing bond weakening.
Figure 5 NWSC Accellerated Life Test profile
ALT conducted by NWSC and NRL at Texas Research on a submarine
mission profile including elevated temperatures of 66C, thermal
shocks to 6?C, in cathodically
active sea water, demonstrated no failure modes in excess of ten equivalent
year cycles. A report has been published, Accellerated Life Test of TR333/BQN-25
Hydrophone and TR143/BQN-3 Transducer Cables, Serial No. 7523/U91170 dated
April 29, 1991, and is available through NWSC, Crane Indiana. (See 5 for
illustration of the equivalent year of service life test cycle.)
Figure 6 Examples
of BONDiT™ coated connector shells
Applications include virtually any configuration of metal. Tested
metals range from 316 stainless steel and 4130 steel to Monel 400,
titanium, and aluminum.
Commercial and military marine applications include sonar cable assemblies
and towed arrays and coating of ROV masts requiring overmolding of acoustical
rubber. 6 illustrates connector shells coated with BONDiT™.
BONDiT™is a new technology developed to solve adhesion problems in sea water for
polymer to metal bonding. Further development is required to fully realize
its full potential, but, already it has proved a superior engineering solution
to existing marine adhesion systems.
is the inventor of the BONDiT™ system, and
at the time of this article was a consultant to BIW Connector
Systems. Another of his recently developed underwater
products is the polyethylene cable splice kit which allows cable
repair without the use of a thermoplastic press: it permits field
repairs with the use of a standard heat gun. He holds a B.S.E.E
and B.S. Physics from California Polytechnic University, and an
M.A. in business marketing and finance. For further information
he may be contacted at the RELTEK LLC in Santa Rosa, CA (707) 284-8808
or at firstname.lastname@example.org.