Advances in Non-Toxic Silicone Biofouling Release Coatings

Chapter 11

Advances in Non-Toxic Silicone Biofouling Release Coatings 1

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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 24, 2017 | http://pubs.acs.org Publication Date: May 4, 2000 | doi: 10.1021/bk-2000-0729.ch011

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Tim Burnell , John Carpenter , Kathryn Truby , Judy Serth-Guzzo , Judith Stein , and Deborah Wiebe 1

GE Corporate Research and Development, Building K 1 , Room 4A49, One Research Circle, Niskayuna, N Y 12309 Bridger Scientific, Inc., Sandwich, MA 02532 2

In this paper, we report two methods to control o i l depletion from silicone foul release coatings: ablative networks and tethered incompatible oils. The synthesis o f ablative and tethered diphenyldimethylsiloxane oils, the incorporation of such oils into the silicone room temperature vulcanized (RTV) network and the foul release properties o fRTVcoatings containing the ablative and tethered oils are discussed. The residence time o f radiolabeled diphenyldimethylsiloxane oils i n silicone RTV topcoats is also addressed. Synthesis o f the radiolabeled diphenyldimethylsiloxane oil and incorporation of the radiolabeled oil into the silicone network are discussed. In addition, the environmental partitioning o f the radiolabeled oils i n both freshwater and marine systems is presented with the material balance.

Marine biofouling is a significant problem for ships and other structures submerged i n a marine environment (I). Both calcareous and non-calcareous fouling types present problems. Calcareous organisms, or "hard" foulers, are found i n both marine and freshwater environments. Those found i n marine water include barnacles, blue mussels, and encrusting bryazoans; those found i n freshwater include zebra muscles. Examples o f non-calcareous organisms, or "soft" foulers, are algae, slime, hydroids and tunicates. Barnacles are the biofouling organism o f interest for this paper. For ship owners, the fouling o f the ship hull has many detrimental effects. B o t h soft and hard fouling leads to increased drag on the ship which decreases both the speed and fuel efficiency o f the vessel, and consequently leads to an increase i n operating expenses (1). It has been reported that for oceangoing freighters, a 20% increase i n fuel costs, or ~ $1 M M per year could be anticipated (2). 3

Corresponding author.

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© 2000 American Chemical Society

Clarson et al.; Silicones and Silicone-Modified Materials ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

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Traditional antifouling coatings, such as paints containing heavy metals, organic biocides and tin or copper ablative coatings, are highly effective at preventing biofouling. Such coatings release toxic substances into the water adjacent to the coating surface, thereby killing the biofouling before strong attachment can occur (3). Consequently, antifouling coatings provide an environmental risk to marine organisms since they release toxins into the environment. Not surprisingly, the use o f cupric oxide i n paints is expected to be limited i n the near future due to environmental concerns and triorganotin species, also very effective, are prohibited for use by the U . S . Navy. For these reasons, much interest has evolved i n nontoxic foul-release coatings, such as silicones (4). These coatings inhibit the strong attachment o f marine organisms v i a an "easy release" mechanism. Several desirable properties o f silicone coatings minimize the adhesive strength o f biofouling attachment and once fouling does occur, it can be removed easily by physical processes such as water pressure washing or gentle scrubbing. The easy release properties o f silicones have been demonstrated empirically to be related to the glass transition temperature (Tg) and surface energy o f the silicone coatings. The low T g (-127°C) is attributed to the flexible siloxane backbone, which gives it its very high molecular mobility, even as a high molecular weight elastomer. Thus, coatings based on polymers with high T g ' s tend to have poor fouling release, even at very low surface energy values. F o r example, Teflon (DuPont), - ( C F C F ) - or poly(tetrafluouroethylene), has a T g o f 130°C due to its rigid backbone and exhibits poor foul release properties. Silicones also have a critical surface tension that coincides with the m i n i m u m o f a plot o f relative attachment versus surface free energy (5). G E ' s foul release coatings, described below, exhibit both low T g and low surface free energy. G E foul release coatings are comprised o f a silicone topcoat and a silicone o i l additive, typically at 10 or 20 weight percent. The silicone topcoat, R T V 11 ( G E Silicones) is a room temperature condensation moisture cure system, which contains a silanol terminated polydimethylsiloxane ( P D M S ) , C a C 0 filler, tetraethoxyorthosilicate ( T E O S ) crosslinker and dibutyltin dilaurate, a Sn(IV) catalyst. The chemistry o f this system is shown below i n Figure 1. Barnacle adhesion testing is a technique used to measure the foul release performance o f foul release coatings and is performed at static exposure sites such as the Indian River Lagoon i n Melbourne, Florida. Barnacle adhesion testing is performed using a force gauge according to A S T M D5618-94. This technique measures the shear force required to remove barnacles adhered to the surface o f a coating. Using barnacle adhesion data, it has been shown empirically that improvements i n foul release are observed i n coatings containing oils. First demonstrated by International Paint i n the 1970's (6), the N a v y and then G E began incorporating oils into their foul release topcoats in the late 1980's and early 1990's, respectively. A s shown below i n Figure 2, R T V 1 1 exhibits superior barnacle adhesion relative to an epoxy control; however, R T V 11 containing 10% free diphenyldimethyl siloxane o i l performs superior to both the epoxy control and R T V 1 1 . Since free oils i n the silicone coating demonstrate improved foul release and since it is desirable to maximize the lifetime o f the o i l i n the topcoat for maximum foul release performance, the following questions need to be addressed. Does o i l need to be at the surface? Can o i l diffusion from the matrix be controlled by attaching either ablative or tethered oils into the silicone topcoat? 2

2

n

3


182

OEt 4

HO—Si/wvSi—OH

Si

+ (/

R T V 11

ÔEt


Sn (IV)

vsAA/Si—0 I

O—Si W W

V

\

/

I. / wwSi—0

\

I

+ 4 EtOH

I. 0 — S i \^v"v/v

w w

=

(-0—Si-^-

Figure 1. RTV 11 Chemistry

Figure 2. Barnacle Adhesion Data for Various

Coatings


183


Mechanisms for O i l Retention in RTV11 Topcoat: Ablative Networks and Tethered Incompatible Oils The incorporation o f ablative and tethered oils into the silicone topcoat o f fouling release coatings is a desirable mechanism for slow, controlled release o f the silicone oil from the R T V topcoat. Once incorporated into the silicone network, the hydrolytically unstable S i - O - C bond i n the ablative o i l (Figure 3) should slowly degrade i n water. Conversely, the tethered o i l is chemically bonded into the silicone network and one end (the non-miscible portion) should phase separate to the surface o f the P D M S . Both ablative and tethered oils contain diphenyldimethylsiloxane functionality, based on previous studies o f the free o i l . The approach was to synthesize both ablative and tethered diphenyldimethylsiloxane copolymers, incorporate the copolymers into the R T V topcoat and then measure the foul release performance o f the coatings. Both oils are shown below i n Figure 3.

Phv (EtO) Si^^\ ~Si-|-0— SijÔ-SrÔ 3

1

0

h

^ S i j - O —Si-j-O-

/r\ -Si

\ Ph

OH

1.75 /3.7

Figure 3. Summary of Ablative (1) and Tethered (2) Diphenyldimethylsiloxane

Oils

The synthesis o f the ablative diphenyldimethylsiloxane o i l involved three steps, shown below in Figure 4. A silanol terminated diphenyldimethylsiloxane copolymer 3 containing 16 mole % diphenylsiloxane was reacted with dimethyldichorosilane at 5°C in the presence o f triethylamine to give the bis-chlorosilane terminated derivative 4. The chlorosilane derivative was subsequently reacted with allyl alcohol and triethylamine to yield the bis-allyl terminated diphenylmethylsiloxane 5. Hydrosilyation with triethoxysilane and Karstedt's catalyst gave the product, bistriethoxy terminated diphenyldimethylsiloxane 1. Note some chain extension was observed in 4, where the molecular weight doubled from approximately 3,300 to 7,000. The tethered diphenyldimethylsiloxane o i l was prepared by a kinetically controlled anionic ring opening polymerization o f hexamethylcyclotrisiloxane ( D ) and hexaphenylcyclotrisiloxane ( D ) i n the presence o f n - B u L i (Figure 5) (7). Once the lithium salt o f D and D 6 was formed in a two step process, it was then quenched with water to give a silanol terminated diphenyldimethylsiloxane product (2). 3

ph

3

p h

3

3


184

HO—Si—/-0—Si-i/o—Si-\-0-~Si—OH

+

2Me Cl Si + 2NEt 2

2

3

3

Ph'


-5*C, toluene

\

I

/

I

\ I

h

T\

I

I

C l — S i— O - S i - ^ O — S i | ^ 0 — S i - J J - O — S i — O — S i — C I ;

2

^ \ ^ O H

+

2HCFNEt

+ 2NEt +

4

3

Et 0

3

2

5C Ph

v

O-Si-f o—Siiio-SH-O

+ 2HCl'NEt

—Si-O

Pt catalyst, 70-75'C + HSi(OEt)

5

3

3

1 Figure 4. Synthesis of Ablative Diphenyldimethylsiloxane

Oil

/ f

THF 1.5 D + 1.25 D 3

p h 3

+ BuLi

i4-0—Si-j-Otoluene ' /4.5

\

4

-Si—j-OTi " 6

\ ph

/3.75

HC1/H 0 2

i-f-O—Si-4-oV

Ph I -Si-

14.5 \ ph

-OH /3.75

Figure 5. Synthesis of Tethered Diphenyldimethylsiloxane

Oil


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Incorporation of Ablative and Tethered Oils into Silicone Network The ablative diphenyldimethylsiloxane copolymer was incorporated at 10 wt % into the P D M S network (Figure 6).

The bis-triethoxysiloxane end groups o f 1 condense

with the silanol terminated P D M S i n the presence o f Sn(IV) to give a crosslinked network containing the hydrolytically unstable S i - O - C moiety.

1


5

SIÔHJ + ( E t O ) 3 S i ^ S / \

0

Si(EtO)3

s

-Sf|o--SiJ^^ Sn(IV) Phv

O - Si-f O — S i f f o - s i - 4 - o - S i - 0

Si-hO— S i w w

+ 6 EtOH Figure 6. Incorporation

of Ablative Diphenyldimethylsiloxane

Oil into RTV

Network

Likewise, the tethered diphenyldimethylsiloxane o i l was also incorporated at 10 wt % into the P D M S network (Figure 7). The silanol endgroups o f 2 condense with T E O S in

the presence o f Sn(IV) to give the triethoxy-terminated

copolymer, w h i c h

subsequently condense with the silanol-termmated R T V 11 to form a crosslinked network.

The incompatible butyl-terminated diphenyldimethyl fragment

should

phase separate to the surface o f the P D M S network.

*Si(-0— S M - o - h S i - 4 - O H

+

S

Advances in Non-Toxic Silicone Biofouling Release Coatings

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