Biodegradation of Silicones (Organosiloxanes)

539 24 Biodegradation of Silicones (Organosiloxanes) Prof. Dr. Habil. Jerzy Lukasiak 1, Dr. Agnieszka Dorosz 2, Dr. Magdalena Prokopowicz 3, Doc. Eng. Pawel Rosciszewski 4, Dr. Bogdan Falkiewicz 5 1 Tel.: 48-58-3493150; Fax: 48-58-3493152; E-mail: jluka@amg.gda.pl 2 Tel.: 48-58-3493151; Fax: 48-58-3493152; E-mail: dorosz@amg.gda.pl 3 Tel.: 48-58-3493151; Fax: 48-58-3493152; E-mail: mprokop@biology.pl 4 Department of Binders and Film Forming Polymers, Professor Ignacy Moscicki's Industrial Chemistry Research Institute, ul. Rydygiera 8, 01-793 Warsaw, Poland; Tel. 48-22-8393852 5 Department of Biotechnology, Intercollegiate Faculty of Biotechnology, University of Gdansk and Medical University of Gdansk, ul. Kladki 24, 80-822 Gdansk, Poland; Tel. 48-58-3012807; Fax: 48-58-3012807; E-mail: bogdan.falkiewicz@wp.pl 1 Introduction... 540 2 Historical Outline... 541 3 Chemical Structures... 541 4 Occurrence and Functions... 541 5 Properties, Degradation and Biodegradation... 543 5.1 Degradation and Biodegradation In vitro... 543 5.1.1 Environmental Fate and Degradation of Silicones... 543 5.1.2 In vitro Biodegradation of Silicones... 545 5.2 Biodegradation In vivo... 554 5.2.1 Silicone Implants and Their Components... 554 5.2.2 Other Silicones... 556

540 24 Biodegradation of Silicones (Organosiloxanes) 6 Silicone Industry... 556 6.1 Production of Silicones... 557 6.2 Main Commercial Silicone Producers... 557 7 Patents... 558 8 Outlook and Perspectives... 558 9 References... 560 cpdms cst D 4 D 5 D 6 DMSD MST OMCTS PDMS PEMS TMS VMS polydimethylcyclosiloxane centistokes octamethylcyclotetrasiloxane decamethylcyclopentasiloxane dodecamethylcyclohexasiloxane dimethylsilanediol methylsilanetriol octamethylcyclotetrasiloxane polydimethylsiloxane polyethermethylsiloxane trimethylsilanol volatile methylsiloxane 1 Introduction The overwhelming majority of synthetic polymers are characteristic of strong resistance to action of biological factors, and upon their application in practice, they become an extremely inconvenient burden to the environment. This results not so much from their specific durability, but rather from the fact that they originated relatively recently in comparison with the period of formation of evolutionary processes in nature. There are no natural, specific mechanisms of decomposition for synthetic polymers in the environment, unless they happen to fit into the degradation routes that exist in the natural world. There are differences between chemical and biological decomposition (Lenz, 1993; Huang and Huang, 1994). Biodegradation is a term for all transformations to which a polymer (a chemical compound) is subjected under the influence of biotic factors, e.g., biotransformation by microorganisms or metabolisms in higher organisms. Biodegradation takes place mainly under the influence of enzymes. Active microorganisms in a biodegradation process are mainly bacteria, fungi, algae, and yeasts (Lenz, 1993), which are capable of secreting active agents to the natural environment, e.g., enzymes, organic acids. In the first stage of the interaction, microorganisms are subjected to adhesion on the polymer's surface, and subsequently, the entire process of reaction of bacteria cell metabolism products with substrate is started. Research into the unification of the criteria of assessment of the biodegradation processes has been in progress; however, so far there are no international unequivocal

4 Occurrence and Functions 541 methods or procedures of testing biological degradation of polymers. The applied methods must be test-material specific (Stevens and Annelin, 1997). 2 Historical Outline The degradation and biodegradation of polydimethylsiloxane (PDMS) and other siloxanes in natural and anthropogenic environments have been poorly examined (Lukasiak and Falkiewicz, 1996, 1998a). Initially, they were treated as non-degradable in the environment, and for many years, they were considered to be inert with respect to living organisms. Then, at the turn of the 1960s and 1970s, the possibility of, respectively, their chemical (environmental) degradation and biodegradation was proven. Until recently, no more than 100 papers focusing on the biodegradation of various siloxanes and their products could be found. Nevertheless, this lack of studies did not affect the general assessment that polysiloxanes are a group of polymers that is hard to biodegrade. 3 Chemical Structures In a broader sense, all organosilicon compounds are called silicones (Smith, 1991). However, for many years now the use of the name silicones has been limited to oligo- and polymeric organosiloxanes that contain alternating silicon and oxygen atoms in the chain (Si-O-Si), which are the subject matter of this report. From the point of view of environmental fate and transport characteristics, three major classes of organosiloxane compounds may be singled out (Allen et al., 1997):. volatile methylsiloxanes ( VMS) (Hobson et al., 1997);. PDMS and its derivatives, excluding polyethermethylsiloxanes (Fendinger et al., 1997a);. polyethermethylsiloxanes (PEMS) (Powell and Carpenter, 1997). There has been very little research concerning biodegradation of organosiloxanes; until recently, only some data on degradation, and its products in the case of a few structures containing siloxanes, have been published (Table 1). They are presented in this review. 4 Occurrence and Functions Polyorganosiloxanes, both in their liquid form and in solid plastics, are broadly utilized in different household and industrial applications (Tomanek, 1991; ECETOC, 1994; Chandra et al., 1997). Because silicones are generally very stable polymers, they are used in various adverse environments, such as those with corrosive chemicals, high humidity, high temperature, electric fields, etc. (Tomanek, 1991). They are applied in such areas as protective encapsulating materials for semiconductors in computers, bases for coolants in transformers, foam-control agents in laundry detergents, lubricants for bearings, coatings to protect facades and historical monuments, rubber-like sealants for windows and bathrooms, basis of gaskets used for engine parts, adhesives for fixtures and fittings, and protectors of electric circuits (Tomanek, 1991). They are also used in several kinds of domestic products, such as detergents, shampoos, conditioners, deodorants, creams, gels, textiles, water repellents, and other products that after use are directly

542 24 Biodegradation of Silicones (Organosiloxanes) Tab. 1 Some organosiloxanes studied in the context of their biodegradation, covered by the chapter Chemical name Formula Abbreviation Polydimethylsiloxane (CH 3 ) 3 SiO[(CH 3 ) 2 SiO] n Si(CH 3 ) 3 PDMS Cross-linked polydimethylsiloxane - Cross-linked PDMS Copolymer of dimethylsiloxane and vinylmethylsiloxane (-[(CH 3 ) 2 SiO] x [(CH 3 )(CH 2 ˆ CH)SiO] y -) n ± (Dialkyl or alkylaryl)cyclotetrasiloxanes (CH 3 RSiO) 4 ± R ˆ CH 3 or (CH 2 ) 3 CH 3, CH(C 2 H 5 )CH 3, C 6 H 5,(CH 2 ) 3 Cl Linear poly(dialkyl or alkylaryl)siloxanes XO(CH 3 RSiO) n X n > 15; Xˆ H or SiCH 3,RˆCH 3 or ± (CH 2 ) 3 CH 3, CH(C 2 H 5 )CH 3,C 6 H 5, (CH 2 ) 3 Cl, (CH 2 ) 2 CF 3,C 6 H 2 Cl 3 Branched or cross-linked poly(dialkyl or HO(R 2 SiO) a (RSiO 1.5 ) b ± alkylaryl)siloxanes a b > 10 and a : b ˆ 0.5 0.7 R ˆ CH 3 or (CH 2 ) 3 CH 3, CH(C 2 H 5 )CH 3 Polydimethylcyclosiloxane [(CH 3 ) 2 SiO] n cpdms Octamethylcyclotetrasiloxane [(CH 3 ) 2 SiO] 4 D 4, OMCTS Decamethylcyclopentasiloxane [(CH 3 ) 2 SiO] 5 D 5 Dodecamethylcyclohexasiloxane [(CH 3 ) 2 SiO] 6 D 6 Trimethylsilanol (CH 3 ) 3 SiOH TMS Dimethylsilanediol (CH 3 ) 2 Si(OH) 2 DMSD Methylsilanetriol CH 3 Si(OH) 3 MST 4-[(3-Methoxyphenyl)methyl]-2,2,6,6-tetramethyl-1-oxa-4-aza-2,6-disilacyclohexane hydrochloride MPSC evaporated into the air (only the low-molecular-weight ones) or released into the environment through sewage and wastewater treatment plants (Chandra et al., 1997). Silicones can be found in all environments: soil, air, and water (Pellenbarg, 1979a,b, 1982, 1988; Watanabe et al., 1984a, 1988; Pellenbarg and Tevault, 1986; Pellenbarg and Carhart, 1990, 1991; Lukasiak et al., 1993a,d, 1998; Pellenbarg et al., 1997; Fendinger et al., 1997b; Gr mping et al., 1998; Gr mping and Hirner, 1999; Wang et al., 2001). Because of the high-scale production and wide range of applications of silicones, human exposure to silicones and their health effects are under strict control in the European Union, the United States, and Japan ( Wischer and Stevens, 1997; Hatcher and Slater, 1997; Miyakawa, 1997). Octamethylcyclotetrasiloxane (D 4,OMCTS) and PDMS, as well as the majority of other organosilicon products, do not meet the requirements to be regarded as substances hazardous to the environment, according to the criteria of the European Community (ECETOC, 1994; Stevens, 1998a). Non-toxicity of selected siloxanes toward many species of organisms, including man, has

5 Properties, Degradation and Biodegradation 543 been verified in numerous tests (Hobbs et al., 1975; Siddiqui and Hobbs, 1982, 1983, 1984; Bruggemenn et al., 1984; Aubert et al., 1985; Guillemaut-Drai et al., 1988; Frye, 1988; Isquith et al., 1988a; Kolesar et al., 1989; Craig and Caunter, 1990; Siddiqui and York, 1993; ECETOC, 1994, and references cited therein; Kent et al., 1994; Siddiqui et al., 1994a,b; Sousa et al., 1995; Hobson and Silberhorn, 1995; Tolle et al., 1995; Fackler et al., 1995; Spivack et al., 1997, and references cited therein; Stevens, 1998a and references cited therein; Burns-Naas et al., 1998a,b; Utell et al., 1998; Looney et al., 1998; Klykken et al., 1999; Padros et al., 1999), but some of these substances were clastogenic in vitro (Isquith et al., 1988b). Nonetheless, it is worth noting that early tests of the behavior of PDMS with respect to higher organisms indicated a possibility of bioconcentration of PDMS by animals ( Watanabe et al., 1984b; Opperhuizen et al., 1987). On the other hand, these results are disputable and are not proven in other studies (Hobbs et al., 1975; Bruggemenn et al., 1984; Guillemaut-Drai et al., 1988; Annelin and Frye, 1989; ECETOC, 1994). 5 Properties, Degradation and Biodegradation The knowledge of the physicochemical properties of an individual siloxane is crucial for understanding and prediction of its possible fate in the environment or even under in vivo conditions. The properties vary significantly, and therefore the fates of various silicone materials may vary from almost complete inertness to easy degradability in the environment. In order to predict their environmental fate, all silicone materials manufactured in significant amounts were divided into the three classes mentioned above (or, more precisely, into the following nine classes: organochlorosilanes; organoalkoxysilanes; hexamethyldisilazanes; volatile methylsiloxanes; polydimethylsiloxanes; modified polydimethylsiloxanes, excluding polyethermethylsiloxanes; polyethermethylsiloxanes; silicone resins; and silicone elastomers [ Allen et al., 1997]) based on their physicochemical properties rather than their chemical structures. Some of them are normally used under both in vitro and in vivo conditions, some only in vitro. Unfortunately, only a few were tested for their susceptibility to being biodegraded. 5.1 Degradation and Biodegradation In vitro A majority of studies dealing with the processes of biodegradation of silicone materials were performed under in vitro conditions. Their results are strongly connected with the environmental fate, analysis, and environmental degradation of silicones; hence, the review should be commenced with these issues. 5.1.1 Environmental Fate and Degradation of Silicones PDMSs are characterized by their very low water solubility (usually < 1 part per million [ppm]). In the case of high-molecular-weight PDMSs, the water solubility is approximately 1 ppt (parts per trillion) or even lower (Mazzoni et al., 1997; Varaprath et al., 1996; Stevens, 1998a). Low solubility is also characteristic of low-molecular-weight siloxanes, e.g., D 4 (approximately 0.05 mg/l [Varaprath et al., 1996] or 0.07 mg/l [Hobson and Silberhorn, 1995]). Hence, they are simply hard to test with respect to their biodegradation, as even recommended tests applicable for sparingly soluble substances require a concentration of the examined substance in a water solution of approximately 10 to