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Advances in Polymer-to-Metal Bonding for Underwater Environments

by Robert E. Lindberg
Consultant
REL Enterprises
BIW Connector Systems, Inc.

Bonding 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 partially intact.

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 appropriate.

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.

Adhesion Failure

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 and metals

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 the bond.

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 increase in the pH moves the equilibrium constants so far in favor of debonding that virtually no organic/mineral interface can withstand the chemical attack. Any mechanical stress at that "loose" edge initiates complete debonding of the polymer from the mineral substrate. Blistering illustrates the same phenomenon, commonly seen in paint on metal substrate. As the moisture penetrates through the polymer to the hydrophilic metal surface, microscopic irregularities and contaminates offer sites on the metal surface to form oxidation-reduction reactions. That liberates OH- ions, the continuation of which increases the pH level to very 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, further exposing metal that can react, ultimately delaminating the polymer completely.

BONDiT™ System Description
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, such as polyurethanes.

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 stress.

BONDiT™ Technical 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.

Robert Lindberg 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 reltek@reltekllc.com.

Advances Bonding