Daniel Graiver

A Review of the Fate and Effects of Silicones in the Environment

Silicones are well-known useful materials varying in structure, reactivity, and chemical and physical properties, but they all contain a covalent bond between the silicon atom and an organic group. Most common of these polymers are those based on polydimethylsiloxane (PDMS) having a siloxane (Si–O–Si) repeat unit and two methyl groups on each silicon atom. All these polymers are manmade, and the organosilicon linkage is not found in nature. It was therefore erroneously assumed that these polymers do not degrade naturally in the environment. It is the purpose of this review to refute this myth and to describe the degradation processes of PDMS in the environment and any potential ecological impact on the terrestrial, aquatic, and atmospheric compartments. Although it was found that minor degradation takes place by hydrolysis of PDMS to dimethylsilandiol followed by oxidation of the methyl group to aldehyde and ultimately to CO2 by Arthobacter and Fusarium oxysporium schlechtendahl, the major degradation processes are abiotic. High molecular weight PDMS are initially depolymerized by soil hydrolysis of the siloxane bonds to yield organosilanol terminated oligomers. These organosilanols and low molecular weight linear PDMS and cyclics are evaporated into the atmosphere and are oxidized there by hydroxyl radicals to benign silica, water, and CO2.

Synthesis and characterization of nanoscale zinc oxide particles: I. laser vaporization / condensation technique

Abstract A method which combines laser vaporization of metal targets with controlled condensation in a diffusion cloud chamber is used to synthesize nanoscale metal oxide and metal carbide particles (10 – 20 nm). In this work we present the results for the synthesis and characterization of ZnO as an example of metal oxide nanoparticles. In part II (this issue), we present the results for silicon carbide nanoparticles synthesized using the same method described here.

Hydrolysis and Condensation of Hydrophilic Alkoxysilanes Under Acidic Conditions

A hydrophilic silane was obtained from the reaction of ethylene carbonate and 3-aminopropyldiethoxymethylsilane. This silane undergoes rearrangement to yield an AB2-type hyperbranched polymer under anhydrous conditions but hydrolyzes and condenses to produce linear siloxanes under acid hydrolysis. The hydrolysis and condensation reactions as a function of time, HCl concentration and water content were studied by 29Si NMR. The compositions of the silanol containing hydrolysis intermediates and the siloxanes condensation products were identified under different conditions. The instantaneous composition was found to depend on the specific combination of the acid and the water. Under certain conditions the intermediate silane-diols were stable and did not condense even under mild acidic conditions.

Silylation of Non-Terminal Double Bonds of Natural Oils

Silylation of non-terminal double bonds of unsaturated fatty acids with reactive organosilicons was successfully accomplished using the ‘Ene’ reaction route. The ‘Ene’ reaction, which is a subset of the famous Diels Alder reaction, enables grafting of vinyl silanes onto unsaturated organic molecules irrespective of the position of the double bond. This procedure was used to graft vinyltrialkoxysilanes onto unsaturated fatty acids of various triglyceride oils. Methyl oleate and methyl stearate were used as model compounds to investigate this reaction. 1H NMR and TGA were used to characterize the structure of the silylation products and to determine the extent of grafting as a function of reaction conditions. It was found that this grafting reaction follows a second order kinetics. Under extreme conditions a silyl-ester redistribution reaction can also occur where an alkoxy group attached to the silicon atom is exchanged with an alkoxy group of the organic ester. Grafting vinyltrimethoxysilane onto natural oils such as soybean, canola and Abyssinian oils leads to a convenient one-component, moisture activated cure system of these natural oils.

Graft and block copolymers with polysiloxane and vinyl polymer segments

A convenient new process to make silicone/organic block and graft copolymers has been recently demonstrated. This dual copolymerization process combines conventional condensation polymerization of the siloxane segments with free radical polymerization of the organic vinyl polymer segments. The copolymerization process is relatively simple and economical compared with other copolymerization techniques as it uses commonly available starting materials and available process equipment. Silicone segments containing alkene side chains or end-groups are prepared in the usual way by polycondensation using an acid or base catalyst. The double bonds of the alkene groups are oxidized to carbonyls which are then used to initiate vinyl monomer polymerization and link the siloxane with the vinyl segments. This initiation step is based on a redox system of copper(II) salts which generates free radicals on the alpha carbons next to the carbonyl groups. This copolymerization process is relatively fast and proceeds at high yields.

Polysiloxanes: Direction of Applications and Perspectives

The business for silicon-based materials continues to grow rapidly worldwide and by the end of the decade will likely exceed 10 billion US dollars. Some of this growth will be based on new families of these materials described in the other Chapters of this book, but if current trends continue, the bulk of this expansion will be based on polysiloxanes with polydimethylsiloxane (PDMS) leading as the workhorse polymer for this rapidly expanding field.

Water Soluble Polysiloxanes

Water soluble polysiloxanes were prepared by condensation of 3-aminopropyl diethoxymethylsilane followed by the reaction of the amines with ethylene carbonate to yield carbamate side chains with terminal hydroxyl groups attached to each silicon atom in the polymer chain. The molecular weight of these carbinol functional polysiloxanes was controlled by adding known concentrations of hexamethyldisiloxane during the polycondensation reaction. The structure of these polymers was confirmed by 1H NMR and FTIR. The resulting polysiloxanes are water soluble and their solubility is independent of the pH or the molecular weight of the polymer. Furthermore, no additives are needed in the water to enhance the solubility of these polymers. The mechanical properties of films that were cast from water solutions were studied and indicated the presence of strong hydrogen bonds that provide for elastomeric films with no need for additional crosslink agents. The hydrophilic nature of the polymer was further confirmed by contact angle measurements and thermal properties data clearly indicate the presence of bound water associated with the large number of hydroxyl groups in the polymer. SEM images of paper coatings prepared from these polysiloxanes revealed a smooth surface with no apparent structural defects that can be used as an anti-graffiti coating.

The Use of Glycerol Carbonate in the Preparation of Highly Branched Siloxy Polymers

Glycerol is an important biomass feedstock readily available as a by-product from the production of fatty acid methyl esters (FAME). Glycerol can be converted to glycerol carbonate, which readily reacts with many nucleophilic reagents including amines. Here, we demonstrate the reaction of glycerol carbonate and 3-aminopropylalkoxylsilane to yield hydroxyl alkyl terminated silanes. This reaction follows a second order rate constant irrespective of the type of aminosilane that was used. These hydroxyl alkyl terminated silanes were further polymerized to yield highly branched siloxy polymers via self-polycondensation through exchange of the ethoxy groups attached to the silicon atom with the terminal hydroxyl groups. The biobased content of these products was determined by ASTM 6866 to be as high as 69 %. The synthesis of the silane monomers as well as the structure and key properties of the hydroxyl alkyl terminated siloxy polymers were determined by DSC, 1H NMR, FTIR, TGA and GPC.

Synthesis of dipotassium dimethylsilanediolate monohydrate

Dipotassium dimethylsilanediolate monohydrate has been synthesised for the first time. The compound has been extensively characterised. The utilization of this new compound as an initiator or monomer in the interfacial polymerisation of silicones is discussed.

Network Development in Films Cast from Emulsions of Polydimethylsiloxane

A current trend in the coatings industry is to shift from solvent based systems to emulsions. In elastomeric coatings cross-linking must occur at some stage during the drying of the latex to obtain useful films. It is also particularly desirable for cross-linking to occur at room temperature. This paper describes such a system based on polydimethylsiloxane (PDMS). It consists of an anionically stabilized emulsion of high molecular weight PDMS terminated with silanol (1), colloidal silica and dialkytin-dicarboxyl-ate curing agent (2). Network formation begins at room temperature before the films are dried and rubbery coherent films form before all the water evaporates from the system. This paper describes the cure process and offers an interpretation of the observed phenomena.