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copolymers and nano-TiO2 as environmentally safe. Kubacka et al.[25] state that TiO2 nanoparticles are non-toxic and that nano-TiO2/ethylene-vinyl alcohol copolymer composites are environ-mentally friendly.

During the lifecycle, nanoparticles may be released. Such

composites show that friction leads to the gradual loss of SiO2nanoparticles [27]. In the case that SiO2 nanoparticles are applied intires, one may expect them to be released by wear. It has beenshown that many of the particles released by the interactionbetween tires and road pavement are

on aseem proper to consider the impact of nanoparticlate TiO2 andamorphous silica after release.

2. Titania and amorphous silica nanoparticles can behazardous

There is now substantial evidence that inhaled TiO2 nano-particles are hazardous to humans [1,4858]. Inhaled TiO2 nano-particles can increase the risk of pulmonary and cardiovasculardisease and will increase such risk in contexts where there isalready a large environmental exposure to non-manufactured smallparticles, as is common in urban areas [1,48,49,5153,58]. There isfurthermore evidence that TiO2 nanoparticles can be translocatedfrom the nasal area to the central nervous system via the olfactorynerve and bulb, thus posing a hazard to the central nervous system[5456]. Ingested titania nanoparticles may also be hazardous [52].Ingestionmay lead of inammation of the intestines and perhaps ofother organs [52].

There is evidence that TiO2 nanoparticles may exhibit ecotox-icity in water, negatively affecting sh and unicellular organisms including algae, and TiO2 nanoparticles are possibly ecotoxic insoils [48,49,51,52,59,60]. TiO2 may also enhance the bio-accumulation of Cd in sh [61].

There is evidence that amorphous SiO2 nanoparticles may behazardous to humans [6264] and may exhibit ecotoxicity [65]. Amain molecular mechanism of cytotoxicity in case of both amor-phous SiO2 and TiO2 nanoparticles in the absence of light appears tobe oxidative damage linked to reactive oxygen species, whereasTiO2 particles exposed to light and/or UV radiation may alsodamage cells due to photocatalytically enhanced oxidation [58,6674]. Changes of the nanoparticulate surface, which may beintroduced to achieve a better performance of nanocompositese.g. [8,16,75,76], may in turn affect hazard [1,48,49,51,67,77].

All in all, amorphous SiO2 and TiO2 nanoparticles can behazardous, with actual hazard to a considerable extent dependenton surface characteristics and in case of TiO2 also on crystal struc-ture [1,4953,58,77]. Claims that nanocomposites are environ-mentally safe [3,24], environment(ally)-friendly [11,24,25] oreco-friendly [12] and that TiO2 nanoparticles are non-toxic [6,25]do not seem to have a rm foundation in empirical data. Moreover,traditional methods of particulate control such as wastewatertreatment plants and lters are often not well suited to efcientlycatching TiO2 and SiO2 nanoparticles [48,49,78].

3. Options for hazard reduction

Safety by design extending to the full life cycle of the nano-composite [48,49], would seem a matter to consider for nano-composites. In situ formation of nanoparticles in thenanocomposite is conducive to hazard reduction in the part of thelife cycle up to, and including, the production stage [48,49]. This isnot possible in case of TiO2 nanoparticles which have a coating orcontain dopants, but may be an option for nanocomposites withSiO2 nanoparticles e.g. [18].

When design is for stability and durability, the full life cycle mayin principle include more than one use cycle, and thus recycling. Insuch a case, from a safety perspective, nanoparticle behaviourduring recycling should also be considered and from the point ofview of performance, the characteristics of the recycled materialmerit consideration.

There is only very limited research into the performance ofrecycled nanocomposites which contain organic polymers. Onestudy has considered recycling of layered silicate-thermoplasticolen elastomer nanocomposites, focussing on mechanical

L. Reijnders / Polymer Degradati874performance [79]. In this study it was found that thoughdegradation of the nanocomposite during recycling occurred,mechanical properties remained signicantly better than those ofthe neat polymer. More in general one might expect that theoxidative properties of titania and silica nanoparticles are condu-cive to polymer degradation [80] and will increase thermaldegradation over the level occurring in neat polymer when recy-cling involves heating [7,10]. If nanocomposite design aims atimproving mechanical performance and if degradation would bea substantial problem for the application of the recycled material,the alternative of self-reinforced mono-materials may be worthconsidering [81]. Such self-reinforced mono-materials are nowavailable for the polymers polyethylene, polypropylene and poly(ethylene terephthalate) [81]. An alternative option would seem tobe a persistent suppression of oxidative damage, which in turn cancontribute to nanocomposite stability [82].

More generally, in designs aiming at reduction of nanoparticlerelease more than safety may be at stake, because loss of nano-particles may lead to a deterioration of characteristics whichnanoparticles should enhance, such as (di)electric behaviour, ameretardant properties, protection against corrosion and gloss e.g.[26,19,24,34,35,37,39]. In designing nanocomposites for stability,better xation of nanoparticles leading to reduced release wouldimprove performance and might reduce hazard.

On the other hand, there may also be design characteristicswhich are linked to the release of nanoparticles. The rst examplethereof concerns tribomechanical performance. In this respectnanoparticle lled polymer composites have been shown toperform well. The apparent reason therefore is that due to inden-tion and scratch of such nanocomposites, nanoparticles are grad-ually removed [27]. A second example is design of thenanocomposite for biocidal activity using photocatalytically activeTiO2 particles. Due to the photocatalytic activity, the polymer islikely to be degraded near the TiO2 particles, and this may beexpected to further the release of such nanoparticles [51].

When design is for nanocomposite stability and durability,hazard reduction by changing the surface of nanoparticles seemsworthwhile considering [19,48,49,52]. It has been found thatsurface modication of SiO2 nanoparticles by a variety of silanesconducive to proper nanocomposite formation with polyethylenereduces the generation of reactive oxygen species [80]. On the otherhand, it has been found this reduction in the generation of reactiveoxygen species may be largely lost on incorporation of the modiedSiO2 nanoparticles in the nanocomposite [80]. In such a case,persistent suppression of oxidative damage to polymers by usingsuitable antioxidants may reduce nanocomposite hazard. Reduc-tion of hazard may also come from coating of TiO2 particles withsilicates to strongly reduce photocatalytic activity, which maycontribute to the reduction of environmental hazard of suchparticles [48,49]. Such coating may also be conducive to the dis-persability of the nanoparticles [75] and to the stability of thenanocomposite, thus limiting the release of TiO2 nanoparticles[48,51]. Changes in structure or composition may also be helpful inhazard reduction. For instance, anatase-structured TiO2 is morecytotoxic than rutile-structured TiO2 [52] and dopant ions mayreduce the photocatalytic activity of TiO2, though one should beselective in this respect because a variety of dopants have beenshown to increase chemical interactions with cells which may behazardous [48,51].

Also, within the context of hazard reduction, design of nano-composites might focus on being conducive to types of release inwhich titania or silica nanoparticles remain included in muchlarger sized pieces of nanocomposites, as (largely) occurs in therelease of platinum-group nanoparticles from catalytic convertersused in motorcars [48]. This would probably limit hazard because

nd Stability 94 (2009) 873876relatively large particles tend to be less hazardous than smaller

coatings. Asia-Pacic Coatings Journal 2007;20:223.[5] Aglan A, Allie A, Ludwick A, Koons L. Formulation and evaluation of nano-

thermal catalytic activity of micro- and nano-particulate titanium dioxide in

on aoxidizing condensed mediums. Dyes and Pigments 2007;75:31527.[11] Zan L, Tian L, Liu Z, Peng Z. A new polystyreneTiO2 nanocomposite lm and

its photocatalytic degradation. Applied Catalysis A General 2004;264:23742.[12] Nkayama N, Hayashi T. Preparation and Characterization of poly (L-lactic acid)/

TiO2 nanoparticle nanocomposite lms with high transparency and efcientphotodegradability. Polymer Degradation and Stability 2007;92:125564.

[13] Hsu L, Chein H. Evaluation of nanoparticle emission from TiO2 nanopowdercoating materials. Journal of Nanoparticle Research 2007;9:15763.

[14] Kaegi R, Ulrich A, Sinnet B, Vonbank R, Wichser A, Zuleeg S, et al. SyntheticSiO2 nanoparticle emission from exterior facades into the aquatic environ-ment. Environmental Pollution 2008;156:2339.

[15] Bandyopadhyay A, Maiti M, Bhowmick AK. Synthesis, characterisation andproperties of clay and silica based rubber composites. Material Science andTechnology 2006;22:81828.

[16] Takamura M, Yamauchi T, Tsubokawa N. Grafting and crosslinking reaction ofcarboxyl-terminated liquid rubber with silica nanoparticles and carbon blackin the presence of Sc(OTf)3. Reactive and Functional Polymers 2008;68:11138.

[17] Yan J, Zhou S, Gu G, Wu L. Effect of the particle size of nanosilica on thestructured polymeric coatings for corrosion protection. Surface and CoatingsTechnology 2007;202:3708.

[6] Allen NS, Edge M, Verran J, Stratton J, Maltby J, Bygott C. Photocatalytic titaniabased surfaces: environmental benets. Polymer Degradation and Stability2008;93:163246.

[7] Pandey JK, Reddy KR, Kumar AP, Singh RP. An overview on the degradability ofpolymer nanocomposites. Polymer Degradation and Stability 2005;898:23450.

[8] Lin OH, Akil HM, Ishak ZAM. Characterization and properties of activatednanosilica/polypropylene composites with coupling agents. PolymerComposites 2008; doi:10.1002/pc.20744.

[9] Chen XD, Wang Z, Liao ZF, Mai YL, Zhang MQ. Role of anatase and rutile TiO2nanoparticles in photooxidation of polyurethane. Polymer Testing2007;26:2028.

[10] Zeynalov EB, Allen NS, Calvet NL, Stratton J. Impact of stabilizers on theparticles [48]. On the other hand, such a design characteristic maybe at variance with required tribomechanical performance [27].

Some nanocomposites would seem to be associated witha substantial irreducible hazard. A biocidal coating with photo-catalytically active TiO2 in an organic polymer matrix would seeman example thereof, because a signicant release of relativelyhazardous TiO2 nanoparticles is a likely characteristic. Anotherexample seems to be the use of TiO2 nanoparticles in biodegradablenanocomposites [11,12,25] because the degradation of nano-composites may be expected to further the release of nanoparticlesand because the photocatalytic activity of TiO2 nanoparticles whichis exploited for the purpose of biodegradation is a major contrib-utor to environmental hazard [1,48,49,51].

4. Conclusions

As there is evidence that amorphous SiO2 and TiO2 nano-particles can be hazardous, in the design of nanocomposites withsuch nanoparticles, hazard reduction extending to their full lifecycle would seem a matter to consider. Options for hazard reduc-tion include: better xation of nanoparticles in nanocomposites,including persistent suppression of oxidative damage to polymerby nanoparticles, changes of nanoparticle surface, structure orcomposition, and design changes leading to the release of relativelylarge particles.

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The release of TiO2 and SiO2 nanoparticles from nanocompositesIntroductionTitania and amorphous silica nanoparticles can be hazardousOptions for hazard reductionConclusionsReferences