The coating chemistry promotes constant renewal of the surface layer. The 

 TBT moiety is chemically bonded to a polymer backbone (e.g., TBT 

 methacrylate copolymer) . This bond is designed to be hydrolytically 

 unstable under slightly alkaline conditions. Therefore, the biocide is 

 released only by chemical hydrolysis of the tributyltin itself. 



This controlled release process has two major advantages: (1) release is 

 governed by hydrolysis of the TBT group rather than dissolution of paint 

 particles, and (2) the release rate is more effectively controlled (slowed 

 down) by altering the polymer's water absorption characteristics. 



Compared to Type 1 or 2 paints, these polymeric, film-forming resin 

 coatings are also characterized by an initial higher release rate (with 

 free association higher than copolymer) during the "conditioning" period 

 (approximately the first month after the freshly-painted hull is placed in 

 the water) , followed by a constant low release rate of antifouling 

 toxicant (Figure 1.6) . A portion of this high initial release rate is due 

 to unbound (free) TBT in the paint. Paint manufactures have indicated 

 that these levels should drop for the copolymer paints as better quality 

 control practices are implemented. The controlled biocide release also 

 gives the antifoulant paint a controlled life span (the thickness of the 

 initial application of the paint determines the life span) . Current data 

 suggest that 5 to 7 years is the average life span of an application of 

 copolymer TBT paint. Also, copolymer paints can be applied directly to 

 the ship's hull surface without having to sandblast previous copolymer 

 layers away; thus, reducing shipyard costs of removing old paint, and 

 subsequently less TBT is released into the environment as spent paint 

 waste. 



1.1.3 Organotin Compounds 



Qrganotin compounds are now one of the most studied groups of 

 organametallic chemicals, in terms of industrial and agricultural uses and 

 applications. The first applied use was as a mothproofing agent in 1925. 

 In 1932, organotin compounds were used for stabilizing chlorinated 

 benzenes and the diphenyls used in transformers and capacitors. This was 

 followed by dibutyltin dilaurate and other dibutlytin salts in 1936 being 

 used to stabilize polyvinyl chloride (PVC) . The biocidal properties of 

 diverse organotin molecules were first discovered in the 1950 's by a group 

 headed by G.J.M. van der Rerk at the Institute for Organic Chemistry, 

 T.N.O. , Utrecht, Holland, under the sponsorship of the International Tin 

 Research Institute in Greenford, Middlesex, England. 



The largest use of organotin compounds today is in stabilizing PVC 

 polymers, employing diorganotin moieties. PVC is used extensively in the 



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