**3.4 Silicones and silica as flame retardant additives**

Silicone-based compounds (silicones, silsesquioxanes, silicas, and silicates) usually are used not only as fillers, incorporated in the polymer matrix but also as the flame retardant additives. They are endowed with excellent thermal stability and high heat resistance, with very limited release of toxic gases during thermal decomposition. Several types of silicone polymers were applied as flame retardants in polycarbonates (Iji & Serizawa, 1998). For the PC materials modified with branched methyl- and phenylsiloxanes and end-capped by methyl groups, limiting oxygen index (LOI) increased over 35 %, whereas LOI was about 27 % for pure PC. The effect of content and block size of PDMS on LOI values was investigated for PC-*b*-PDMS copolymers. The PDMS block size influenced the dispersibility of the PDMS in the PC and the moderate PDMS dispersion (i.e. 50 nm mean inclusion size) resulted in high flame retardancy (Nodera & Kanai, 2006). Silica particles "*in situ*" formed by thermal degradation of PDMS mostly remained within the char layer. This highly oxidized char layer has the structure which prevented volatile fuel production and served as an additional thermal insulator.

The effect of silica gel structure on the flammability properties of thermoplastics has been investigated by many workers. The effect of pore volume, particle size and surface silanol concentration of silica gel in polypropylene (PP) was studied (Gilman et al., 1999). The performances of various types of silica, silica gel, fumed silica and fused silica as flame retardants in PP and polyethylene oxide were investigated. The fumed silica and silica gel accumulating near the surface acted as a thermal insulation layer and reduced the polymer concentration near the surface in contact with flame. The fused silica did not accumulate near the surface and it is mainly present in the polymer melt layer. The accumulation of silica on the surface of the burned polymer has been also observed in PMMA composites (Kashiwagi et al., 2003).

Nanometric particles in polymer matrices are known to enhance the fire resistance. POSS with general formula (RSiO1.5)8 is an inorganic silica-like nanocage. Eight organic groups located at the corners give possibility for compatibility POSS compounds with organic polymers. These inorganic nanocages are referred to as preceramic compounds. On combustion of such polymer composite, POSS acts as a precursor forming thermally stable ceramic materials at high temperature. The incorporation of POSS in polymers modifies both the viscosity and the mechanical properties of the molten polymer. It also affects the thermal stability and fire performances by reducing the quantity of heat released upon

Modification of Thermoplastics with Reactive Silanes and Siloxanes 181

The CFS have also found applications for production of contact lenses. This type of membranes must to pass oxygen and this ability is determined on a base of so called "*equivalent oxygen percentage"* (EPO). A minimal value of EPO equals 5-7 %. For the soft lenses made of PDMS, with thickeness 0.2 mm, EPO equals ~20 %. Hydrophobic properties of this material can lead to a damage of a cornea, and lenses made of pure PDMS are not used. They are produced of silicone-methacrylic copolymers or the carbofunctional silanes, i.e. (methacryloxypropyl)trimethylsiloxysilane or (methacryloxypropyl)pentamethyl-

Hydrogels have been extremely useful in biomedical and pharmaceutical applications for a long time. Poly(2-hydroxyethyl methacrylate) PHEMA, a biocompatible hydrogel, can absorb a large amount of water and is used to make ophthalmic prostheses (contact or intraocular lenses), vascular prostheses, drug delivery and soft-tissue replacement. Bioactive PHEMA-silica hybrids can be produced either by addition of silica nanoparticles to HEMA monomer-PHEMA solution, or using HEMA solution and tetraethoxysilane (TEOS) (as a silica precursor) through *in situ* sol-gel processes. A hybrid monomer was prepared of HEMA and 3-aminopropyltriethoxysilane (APTS) and its polymers, and blend materials were synthesized through basic catalyzed hydrolysis and condensation of TEOS, followed by *in situ* radical polymerization of HEMA. Obtained hybrid materials were used to make

Composites prepared from a silicone rubber and a hydrogel are often used as drug release materials. A silicone rubber-polyacrylamide (PAAm) composite hydrogel exhibits pHindependent swelling degree, high specific surface area and non-ionogenic character

Polylactides, due to their biocompatibility and biodegradability, have also been used for biomedical applications for decades. Poly(lactide-*b*-methylvinylsiloxane-*b*-lactide) triblock copolymers, in which the vinyl groups were functionalized with carboxylic acids were synthesized. At neutral pH, the carboxylate functional polysiloxane central block binds to the surface of magnetic nanoparticles, while the polylactides serve as tail blocks to provide dispersibility in polylactide solvents through interparticle steric repulsive forces. Potential application for these magnetic materials include magnetic field-directed drug delivery, magnetic cell separations and magnetic hypertherma therapy for a treatment of tumors

The permanent fungicidal and bactericidal properties of polysiloxanes bearing quaternary ammonium salt (QAS) groups have been known for five decades. Novel low surface energy antimicrobial coating, based on hybrid siloxane epoxy coatings containing QAS moieties, was capable of self-contaminating in a variety of environments. These coatings have demonstrated the ability to eliminate up to 99.9 % of pathogenic bacteria on the surface (Pant et al., 2008).

The attachment of -helical polypeptides to the colloidal silica is interesting as the core-shell composite particles call to mind naturally existing, protein-caged materials like viruses. These hydrophobic, polypeptide-functionalized particles could be used in investigation designed to

Polyurethane elastomers since 1970s have found a wide range of biomedical applications due to a combination of good biocompatibility, good hydrolytic and oxidative biostability,

model enzyme activation or the properties of hydrophobic proteins in cell membranes.

disiloxane. Lenses made from these materials reach EPO value ~10 % (Arkles, 1983).

bioactive scaffolds for bone engineering (Luciani et al., 2008).

(Mashak, 2008).

(Ragheb & Riffle, 2008).

combustion. Metal-bearing POSS nanoparticles at tiny concentrations (ca. 1 wt. %) can markedly enhance the char yield in PP (Fina et al., 2005).

Halogen free siloxane modifiers can significantly reduce heat release characteristics of thermoplastic polyurethanes (TPUs). Segmented PUs derived from PTMO, MDI, 1,4-butanodiol were modified with secondary aminoalkyl functional PDMS via solution polymerization (Wang et al., 2000). Addition of 15 % of the PDMS allowed to reduce the cone calorimetry heat release rate by a factor of about 2/3 and hence improved fire resistance, while maintaining mechanical behaviour. It was suggested that the low surface energy characteristics of PDMS promoted migration to the air-polymer interface to form a predominately PDMS enriched surface, which was oxidized at elevated temperatures in air to a silicate-like material and this served as a protective layer, which further reduced burning of the underlaying polyurethane.

Water repellent of silicone-based materials, forming in the fire protective coatings, appear to be attractive flame-retardants, which have many advantages such as low smokes, low toxicity and halogen-free. It is known that intumescent flame retardant (IFR) system is composed of an acid source, a carbonization compound and a blowing agent. The main disadvantages of IFR system are its moisture sensitivity and poor compatibility with polymer matrix. Flammable properties of polyolefins (e.g. LOI for polypropylene is only 17 %) restrict their application in electric and electrical devices, wire and cable, public chairs, packaging films and transportations. One of the most effective methods for obtaining the halogen-free flame-retardant polyolefin is use of IFR. The effect of polydimethylsiloxane (PDMS) in IFR polypropylene containing melamine phosphate (MP) was studied (Lu et al., 2009). It was found that the values of LOI increase gradually with the increase of siloxane content in PP/MP/PDMS composites. Studies of the water resistance of these materials showed improved water leaching of MP. A polysiloxane was found to be a very effective additive for the flame retardancy and water resistance of the IFR-PP materials. An improved thermal stability for PP/MP/PDMS composites was also observed. IFR system containing polysiloxane and silane-modified SiO2, ammonium polyphosphate (APP) in PP matrix was investigated as well (Gao et al., 2010). It was showed that both polysiloxane and silica effectively enhance the flame retardancy of the IFR-PP, and LOI values exceeded 36 %.

Silane-functionalized PP and maleic anhydride modified PP were efficient as a coupling agents for PP-aluminium hydroxide (AH) composites, in order improve the interaction between the phases. However, PP modified with vinyltriethoxysilane showed better effect on the mechanical properties (Bohrz et al., 2006).

#### **3.5 Applications of silane-modified thermoplastics in medicine**

Synthetic polymers with pendant sugar moieties are of great interest not only as simplified model of biopolymers bearing oligosaccharides, but also artificial glycoconjugates in biochemistry and medicine. A new polysiloxane copolymer with pendant glucosylthioureylene groups was prepared in reaction of amino-functionalized PDMS with glucosyl isothiocyanate. They kept the optical activity and could be used not only as biomedical and biotechnological materials, but also as a model of amphiphilic polymers. Another examples of these materials are saccharide-based polysiloxanes, i.e. PDMS grafted amylase or PDMS grafted saccharose (Zhou et al., 2004).

combustion. Metal-bearing POSS nanoparticles at tiny concentrations (ca. 1 wt. %) can

Halogen free siloxane modifiers can significantly reduce heat release characteristics of thermoplastic polyurethanes (TPUs). Segmented PUs derived from PTMO, MDI, 1,4-butanodiol were modified with secondary aminoalkyl functional PDMS via solution polymerization (Wang et al., 2000). Addition of 15 % of the PDMS allowed to reduce the cone calorimetry heat release rate by a factor of about 2/3 and hence improved fire resistance, while maintaining mechanical behaviour. It was suggested that the low surface energy characteristics of PDMS promoted migration to the air-polymer interface to form a predominately PDMS enriched surface, which was oxidized at elevated temperatures in air to a silicate-like material and this served as a protective layer, which further reduced

Water repellent of silicone-based materials, forming in the fire protective coatings, appear to be attractive flame-retardants, which have many advantages such as low smokes, low toxicity and halogen-free. It is known that intumescent flame retardant (IFR) system is composed of an acid source, a carbonization compound and a blowing agent. The main disadvantages of IFR system are its moisture sensitivity and poor compatibility with polymer matrix. Flammable properties of polyolefins (e.g. LOI for polypropylene is only 17 %) restrict their application in electric and electrical devices, wire and cable, public chairs, packaging films and transportations. One of the most effective methods for obtaining the halogen-free flame-retardant polyolefin is use of IFR. The effect of polydimethylsiloxane (PDMS) in IFR polypropylene containing melamine phosphate (MP) was studied (Lu et al., 2009). It was found that the values of LOI increase gradually with the increase of siloxane content in PP/MP/PDMS composites. Studies of the water resistance of these materials showed improved water leaching of MP. A polysiloxane was found to be a very effective additive for the flame retardancy and water resistance of the IFR-PP materials. An improved thermal stability for PP/MP/PDMS composites was also observed. IFR system containing polysiloxane and silane-modified SiO2, ammonium polyphosphate (APP) in PP matrix was investigated as well (Gao et al., 2010). It was showed that both polysiloxane and silica effectively enhance the flame retardancy of the IFR-PP, and LOI values exceeded 36 %.

Silane-functionalized PP and maleic anhydride modified PP were efficient as a coupling agents for PP-aluminium hydroxide (AH) composites, in order improve the interaction between the phases. However, PP modified with vinyltriethoxysilane showed better effect

Synthetic polymers with pendant sugar moieties are of great interest not only as simplified model of biopolymers bearing oligosaccharides, but also artificial glycoconjugates in biochemistry and medicine. A new polysiloxane copolymer with pendant glucosylthioureylene groups was prepared in reaction of amino-functionalized PDMS with glucosyl isothiocyanate. They kept the optical activity and could be used not only as biomedical and biotechnological materials, but also as a model of amphiphilic polymers. Another examples of these materials are saccharide-based polysiloxanes, i.e. PDMS grafted amylase or PDMS

markedly enhance the char yield in PP (Fina et al., 2005).

burning of the underlaying polyurethane.

on the mechanical properties (Bohrz et al., 2006).

grafted saccharose (Zhou et al., 2004).

**3.5 Applications of silane-modified thermoplastics in medicine** 

The CFS have also found applications for production of contact lenses. This type of membranes must to pass oxygen and this ability is determined on a base of so called "*equivalent oxygen percentage"* (EPO). A minimal value of EPO equals 5-7 %. For the soft lenses made of PDMS, with thickeness 0.2 mm, EPO equals ~20 %. Hydrophobic properties of this material can lead to a damage of a cornea, and lenses made of pure PDMS are not used. They are produced of silicone-methacrylic copolymers or the carbofunctional silanes, i.e. (methacryloxypropyl)trimethylsiloxysilane or (methacryloxypropyl)pentamethyldisiloxane. Lenses made from these materials reach EPO value ~10 % (Arkles, 1983).

Hydrogels have been extremely useful in biomedical and pharmaceutical applications for a long time. Poly(2-hydroxyethyl methacrylate) PHEMA, a biocompatible hydrogel, can absorb a large amount of water and is used to make ophthalmic prostheses (contact or intraocular lenses), vascular prostheses, drug delivery and soft-tissue replacement. Bioactive PHEMA-silica hybrids can be produced either by addition of silica nanoparticles to HEMA monomer-PHEMA solution, or using HEMA solution and tetraethoxysilane (TEOS) (as a silica precursor) through *in situ* sol-gel processes. A hybrid monomer was prepared of HEMA and 3-aminopropyltriethoxysilane (APTS) and its polymers, and blend materials were synthesized through basic catalyzed hydrolysis and condensation of TEOS, followed by *in situ* radical polymerization of HEMA. Obtained hybrid materials were used to make bioactive scaffolds for bone engineering (Luciani et al., 2008).

Composites prepared from a silicone rubber and a hydrogel are often used as drug release materials. A silicone rubber-polyacrylamide (PAAm) composite hydrogel exhibits pHindependent swelling degree, high specific surface area and non-ionogenic character (Mashak, 2008).

Polylactides, due to their biocompatibility and biodegradability, have also been used for biomedical applications for decades. Poly(lactide-*b*-methylvinylsiloxane-*b*-lactide) triblock copolymers, in which the vinyl groups were functionalized with carboxylic acids were synthesized. At neutral pH, the carboxylate functional polysiloxane central block binds to the surface of magnetic nanoparticles, while the polylactides serve as tail blocks to provide dispersibility in polylactide solvents through interparticle steric repulsive forces. Potential application for these magnetic materials include magnetic field-directed drug delivery, magnetic cell separations and magnetic hypertherma therapy for a treatment of tumors (Ragheb & Riffle, 2008).

The permanent fungicidal and bactericidal properties of polysiloxanes bearing quaternary ammonium salt (QAS) groups have been known for five decades. Novel low surface energy antimicrobial coating, based on hybrid siloxane epoxy coatings containing QAS moieties, was capable of self-contaminating in a variety of environments. These coatings have demonstrated the ability to eliminate up to 99.9 % of pathogenic bacteria on the surface (Pant et al., 2008).

The attachment of -helical polypeptides to the colloidal silica is interesting as the core-shell composite particles call to mind naturally existing, protein-caged materials like viruses. These hydrophobic, polypeptide-functionalized particles could be used in investigation designed to model enzyme activation or the properties of hydrophobic proteins in cell membranes.

Polyurethane elastomers since 1970s have found a wide range of biomedical applications due to a combination of good biocompatibility, good hydrolytic and oxidative biostability,

Modification of Thermoplastics with Reactive Silanes and Siloxanes 183

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excellent mechanical properties and good processability. Commercial functionalized copolymers contain ca. 10 wt. % PDMS and are either segmented polyurethanes or polyureas that utilize polycarbonate or polyether as soft segments. They can be utilized in the manufacturing of intra-aortic balloons The anti-thrombogenic property of PDMS based polyurethanes is also utilized in artificial heart grafts, vascular grafts, implants and other such medical applications (Sheth et al., 2004).
