Organometallic Polymers: Development of Controlled Release

20 Organometallic Polymers: Development of Controlled Release Antifoulants VINCENT J. CASTELLI and WILLIAM L. YEAGER

Downloaded by UNIV OF PITTSBURGH on May 5, 2017 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0033.ch020

David W. Taylor Naval Ship R&D Center, Annapolis, Md. 21402

The savings associated with the use of long-lived antifoulant coatings are quite r e a l , especially for the Navy. Estimates of the rates of fouling for ships vary, as do the effects of these fouling rates on hull drag and the attendant increases in fuel consumption. For Navy ships, it i s estimated that the increase in fuel consumption averages 8 - 10% per year from day one out of drydock. These increases are cumulative, and there i s evidence that the rate increases even faster following the period of incipient antifouling coating failure. Since the current drydocking schedule for most Navy ships i s five years, i t i s our estimate that on the average 20 - 25% of the fuel consumption per year i s attributable exclusively to the fouling of the hull. Since the Navy's annual fuel bill at post energy c r i s i s costs exceeds $700 million, this wasted fuel i s costing the Navy about $150 million per year. Savings as low as 1% of these huge costs would permit extensive return-on-investment on the cost of the development of long life antifouling coatings. The need to control the a c t i v i t i e s of plant and animal pests is well accepted. However, environmental concerns have necessitated that prudence be exercised especially when the more toxic pesticides are used. Many of these toxicants have been shown to be resistant to degradation and to possess a c t i v i t y toward plants and animals other than the original target specie(s). It was this concern that alerted the Navy to the problems present with i t s current antifouling paint systems. Antifouling paints, by their very nature, are designed to leach out a pesticide to prevent the attachment and growth of fouling organisms. The i n i t i a l leaching i s far in excess of the amounts required to control fouling, but since the rate of solution of the pesticide decays exponentially, large amounts of chemical are wasted in the i n i t i a l phases, only to leave the coating depleted of toxin in a short period of time. Therefore, the coating i s susceptable to fouling. This waste i s further compounded because once these toxic substances leave the coating film, they are free to interfere with the l i f e processes of a l l susceptible organisms. 239 Paul and Harris; Controlled Release Polymeric Formulations ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

240

CONTROLLED RELEASE POLYMERIC

FORMULATIONS

THE "WHYS" OF CONTROLLED RELEASE 1. D E C R E A S E

PESTICIDE


2. L I M I T S P H E R E O F

DOSE

ACTION

3. S A V E M O N E Y A N D ENVIRONMENT Figure 1

CONTROLLED RELEASE BY CHEMICAL ATTACHMENT

Figure 2

Paul and Harris; Controlled Release Polymeric Formulations ACS Symposium Series; American Chemical Society: Washington, DC, 1976.


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The idea therefore, at that time, was to produce a "nontoxic" toxicant whose effects would only be apparent on those surfaces treated with i t . The quandry this posed was resolved with the idea of producing a surface which provided toxic dosages "on demand," at the instant a fouling organism started to settle. This postulated material was termed a "contact toxin," and was our f i r s t exposure to the concept of controlled release. The rationale for the controlled release approach i s summarized in Figure 1. The basic controlled release idea using chemical bonding i s indicated conceptually in Figure 2. One practical rendition i s also described in the same figure. This particular approach was u t i l i z e d by Dyckman, et a l (1972, 1974, 1975) to formulate polymeric resins whose pesticide portion was primarily organometallic toxicants, and were name organometallic polymers (OMPs). As may have been predicted, the performance of any compound of this nature w i l l be influenced primarily by the factors described i n Figure 3. These three factors a l l have profound effects on the a b i l i t y of the system to perform effectively, but the relative contribution of each subsystem has yet to be determined. However, the characteristics of the individual pieces of the system have been well studied. Perhaps the most intensely studied subsystem has been the pesticides, since i t i s ultimately their task to provide the required control. Some of the more common marine pesticides are given in Figure 4. These include the metallic materials such as copper, one of man's f i r s t antifoulants, and copper alloys, such as the various copper-nickel alloys so favored for submerged marine applications. Also included are the inorganic compounds, such as copper oxide, which presently handles over 75% of current demand for antifoulant coatings, arsenic salts, which are no longer used to any extent in marine antifouling coatings due to gross occupational hazards associated with their manufacture and application and mercuric salts, which though they have displayed fine fouling resistance suffer from the same problems as do the arsenic salts. More recent additions to the f i e l d of marine pesticides are the organic antifoulants particularly the polychlorinated phenols, which provide excellent control of marine plants, but possess negligible effect on most fouling animals, and the aryl and alkyl n i t r i l e s , whose benefits and drawbacks are just beginning to be explored. The newest class of antifoulants to be investigated are the organometallic pesticides, such as the t r i a l k y l and t r i a r y l t i n compounds, which have demonstrated relatively wide spectrum activity against most species of fouling plants and animals, t r i a r y l lead compounds, whose effectiveness has been shown to extend over a wide range of fouling organisms, and various organomercurials and organoarsenicals whose effectiveness i s l i t t l e disputed but which suffer from even more troublesome manufacture and application hazards than their inorganic cousins.



FORMULATIONS

FACTORS INFLUENCING EFFECTIVE ACTION


®

PESTICIDE

2) P O L Y M E R T Y P E

3) L I N K A G E B E T W E E N P E S T I C I D E AND POLYMER Figure 3

COMMON MARINE PESTICIDES METALLIC -

Cu Cu ALLOYS

INORGANIC -

ORGANIC -

ORGANOMETALLIC -

CuO ARSENICAL SALTS MERCURIAL SALTS POLYCHLOROPHENOLS ARYL AND ALKYL NITRILES TRIALKYL AND TRIARYL TINS TRIARYL LEADS VARIOUS ORGANOMERCURIALS VARIOUS ORGANOARSENICALS Figure 4



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In choosing a pesticide from the above l i s t suitable for a controlled release formulation, certain factors need to be considered. One factor i s that antifouling control should be obtained at relatively low dosage level to permit slow degradation of the controlled release formulation with attendant long service l i f e . The Navy, for example, is presently seeking hull coatings capable of remaining fouling free for five years. Another consideration i s the a b i l i t y of the pesticide to provide suitable binding sites for chemical attachment. In the case of metallic and inorganic compounds, these are obviously not suitable when bonded ionicly, because of problems i n maintaining effective counter-ion concentration at the surface in a medium such as seawater. If these compounds were bonded covalently however, the effect would be the production of organometallic compounds, which might be effective agents. But by far, organic and organometallic antifoulants provide the greatest opportunity when attached through convalent chemical bonds. This is especially true for toxic compounds possessing certain types of pendants such as hydroxyl, amino and carboxylic groups, just to name a few. The second subsystem affecting the performance of the controlled release pesticide i s the "inert" polymer backbone. It is the carrier for the pesticide, and i t s properties determine the possible applications for the materials. Examples of polymer types, properties, and potential areas of usage are provided i n Figure 5. Most present applications of plastics in the marine environment can be accomodated by the polymers l i s t e d , and for these not covered suitable modifications can be developed. These modifications w i l l be determined by the last element of our system, the type of chemical bond. Though, i n theory, any chemical bond which can provide an adequate joint between the polymer and the pesticide i s suitable, certain bond types are preferred. This preference i s generally related to the ease of synthesis of the required monomers, and therefore have been limited. Those linkages which have been investigated at this Laboratory appear i n Figure 6. The most encouraging results to date has been in cases where the bonding has been through an ester linkage. Field results in which the polymers have been various vinyl monomers with carboxylic groups esterfied to organometallic t i n species have shown four years of 100% resistance to a l l forms of fouling i n continuing tests. Field results of polymer systems in which the bonding has been through ether linkages is less encouraging with test samples lasting less than eighteen months. The third system in which the linkage has been through a carbon-carbon bond, has been the most disappointing with test panels not even lasting six months. With the above empirical data as a guide, emphasis at our Laboratory has been placed on refining the polymer-pesticide systems which use the ester linkage. Also work i s underway investigating high performance polymeric systems - epoxies for the formulation of fouling resistant glass reinforced laminates and polyurethanes for high


244



POLYMER TYPES A N D

FORMULATIONS

APPLICATION

TYPICAL POLYMERS

DESIREABLE PROPERTIES

USAGE

ACRYLICS

CLARITY

VIEWPORTS

VINYL URETHANES

FORMS FILM

COATINGS

EPOXIES, POLYESTERS

COMPATIBILITY WITH REINFORCEMENT, STRENGTH

STRUCTURES

POLYBUTADIENES

FLEXIBILITY, WATER RESISTANCE

CABLE INSULATION, PLIANT MEMBRANES

Figure 5

INTERCONNECTING LINKAGES TYPES-

^ φ

Ο Η R - C - O R '

2)

R -

Ο -

3)

R — R'

R'

R= INERT" CARBON BACKBONE R - BIOCIDE Figure 6



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performance coating applications. With the empirical data available to date on the types of controlled release polymers described, i t i s beneficial to examine the possible theories which might describe the actual mode of action. The more plausible theories are outlined in Figure 7. Realistically, the true mechanism is probably a combination of some that are given. The f i r s t postulates that the molecular weight of the majori t y of the polymer i s low enough to permit solution of the intact toxic substituted polymer. This toxic polymer would then be free to diffuse away from the surface and then might be ingested by some potential fouling organism in which the toxic groups would be cleaved from the backbone and, disrupt the normal a c t i v i t i e s of the organism. Considering the observed long antifouling service l i f e of some of the polymer systems of relatively high molecular weight (over 50,000 in some cases), this explanation i s not entirely feasible. The next pathway assumes that the controlled release system functions in the manner originally envisioned. Specifically, this postulates that the basic mode of bond cleavage occurs only when an organism makes intimate contact with the surface, as i n attachment. It i s then thought that the body fluids of the animal in some way catalyze the hydrolysis of the ester linkage and produce a local high concentration of the antifoulant which then i s able to eliminate the fouling organism. After this has occurred innumerable times on the same polymer chain, the hydrophilicity of

POSSIBLE MODES OF ACTION 1. INTACT TOXIC POLYMER SOLVOLYSIS 2. SURFACE BIOTIC BOND CLEAVAGE (CONTACT TOXIN) 3. SURFACE ABIOTIC BOND CLEAVAGE 4. BULK ABIOTIC BOND CLEAVAGE 5. PESTICIDE DETOXIFICATION AND SUBSEQUENT BOND CLEAVAGE Figure 7


246


FORMULATIONS

the chain i s increased to the point where solution of the stripped polymer chain i s possible, and in so doing, a fresh, completely toxic surface is exposed. In this model, the increase in the hydrophilicity of the stripped polymer chain i s assumed to be sufficient to permit solvolysis, and precludes the existence of large numbers of hydrophobic chain members, as well as limits the chain to essentially two-dimensional structure devoid of cross-linking. There is no evidence which discounts this theory of the "contact toxin, i n fact, bouyed by the long resistance to fouling exhibited by many of the test panels, may indeed be wholly or p a r t i a l l y responsible for their apparent success. The third pathway assumes that the fouling resistance of these polymers i s the result of a normal degree of weathering to which a l l materials exposed in the marine environment are subjected. The chemical forces which are responsible for this degradation are also responsible for the slow hydrolysis of the linkage anchoring the pesticide. This would provide a slow but nearly constant outflow of pesticide from the surface which would be i n excess of the lethal concentration for the fouling organisms. This model also assumes that after a sufficient number of hydrolyses of the toxic pendant groups occurs the hydrophilicity is sufficient to permit solvolysis. Also assumed is an absence of large numbers of hydrophobic chain members as well as a limit of the chain to essentially two-dimensional structure, devoid of cross-linking. The available evidence does not discount this model, and may in fact be used to support i t . Since the phenomenon of weathering i s known to be essentially a slow process, i t i s possible for a material to display the observed antifouling action, and may be p a r t i a l l y or wholly responsible for the performance indicated. Another model postulates that these polymeric materials can be envisioned as tight sponges in which the "holes" are f i l l e d by chemically bound toxicant. The toxicant bond to the polymer is cleaved through some hydrolysis mechanism and slowly diffuses from the boundary layer in contact with the surface into the seawater environment. This exposes new pendant toxicants which are then cleaved, and then diffused, etc. A l l during this time period, the stripped polymer chains remain intact on the surface. After sufficient time has elapsed, the stripped polymer network appears as a sponge of sorts in which the necessary agents which affect hydrolysis are diffusing into the matrix, and the hydrolyzed pesticide is diffusing out of the network. This model differs i n one important aspect from the currently recognized model of conventional antifouling paints in that the rate of toxicant outflow from the controlled release polymer i s governed by the chemical hydrolysis of the polymer-pesticide bond, not simply the dissolution of the toxicant. This theory also is supported by evidence from the f i e l d which indicates that even the most highly cross-lined organometallic polymer resins are effective antifoulants after four years of marine exposure i n a


11



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Polymers

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continuing series of tests. The last model proposed i s more "blue sky" than any of the preceding in that i t postulates that i t i s the toxic surface i t self which prevents fouling by giving the fouling organisms a "bad taste" when they attempt to settle on the surface, and effectively preventing a l l fouling. The degradation of the surface would be concentrated primarily on the pesticide and would detoxify the agent before i t ever l e f t the surface. In effect, this would be an antifoulant which would only be toxic on the surface to which i t was applied, and have only the most minor of environmental impacts, since the leachate from the surface would be of low inherent toxicity. This model cannot be eliminated from the data available at present, but should possibly be taken as a goal to strive for, rather than celebrating i t as one achieved. Literature Cited 1. Dyckman, E.J., J.A. Montemarano and D.E. Gilbert, "Biologically Active Polymeric Coating Materials," NSRDC Rept #4526, Apr 1975 2. Dyckman, E.J. and J.A. Montemarano, "Antifouling Organometallic Polymers: Environmentally Compatible Materials," NSRDC Rept $4186, Feb 1974 3. Dyckman, E.J., J.A. Montemarano, D.M. Andersen and A.M. Miller, "Non-Polluting Antifouling Organometallic Polymers," NSRDC Rept #3581, Nov 1972 4. Montemarano, J.A. and E.J. Dyckman, "Antislime Organometallic Polymer of Optical Quality," NSRDC Rept #3597, Sep 1972


Organometallic Polymers: Development of Controlled Release

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