Applications of Clay-Polymer Nanocomposites

Chapter 4.4

Applications of Clay–Polymer Nanocomposites M. Galimberti, V.R. Cipolletti and M. Coombs Politecnico di Milano, Dipartimento di Chimica, Materiali, Ingegneria Chimica G. Natta, Via Mancinelli 7, 20131 Milano, Italy

Chapter Outline 4.4.1. Improvements of Polymer Properties by Clay Minerals 541 4.4.1.1. Structure of Clay Mineral Polymer Nanocomposites (CPN) 541 4.4.1.2. Clay Minerals for Better Mechanical Reinforce ment 541 4.4.1.3. Clay Minerals for Improved Barrier Properties 548 4.4.1.4. Clay Minerals for Improved Thermal Properties, Heat Resistance, and Dimensional Stability 549 4.4.1.5. Clay Minerals for Improved Resistance to Degradation Properties 550

4.4.1.6. Clay Minerals for Improved Flame Resistance 550 4.4.1.7. Clay Minerals for Improved Chemical Resistance 551 4.4.1.8. Clay Mineral Additions for Rheological Applications 552 4.4.1.9. Clay Minerals as Blend Compatibilizer 553 4.4.1.10. Clay Minerals to Increase the Glass Transition Temperature (Tg) 553 4.4.1.11. Clay Minerals and Ionic Conductivity of Thermoplastic Polymers 553 4.4.1.12. Clay Minerals in Elastomers for

Developments in Clay Science, Vol. 5B. http://dx.doi.org/10.1016/B978-0-08-098259-5.00020-2 © 2013 Elsevier Ltd. All rights reserved.

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Faster Curing and Higher Cross-linking Density 554 4.4.2. Master Batches and Concentrates in Thermoplastic Pellets 554 4.4.2.1. Nanocomposites Based on Polyolefins 554 4.4.2.2. Polyamide Nanocompo sites 554 4.4.3. Application of CPN in the Automotive Field 555 4.4.3.1. CPN in Timing Belt Cover 557 4.4.3.2. CPN in Engine Cover 557 4.4.3.3. CPN in Stepassists, Doors, Centre Bridges, Sail Panels, and Box-rail Protectors 557 4.4.3.4. CPN in Seat Backs 558 4.4.3.5. CPN for Rear Floor 558 4.4.3.6. CPN in Tyres, Tyre Treads, Tyre

Innerliners, and Tyre Curing Bladders 558 4.4.3.7. CPN in Base Compounds 564 4.4.3.8. CPN in Rubber Automotive Compounds other than Tyres 564 4.4.4. Applications of CPN in Sporting Goods 564 4.4.4.1. Barrier Coating Technology 565 4.4.4.2. Tennis Balls 565 4.4.4.3. Other Inflated Sport Balls and Rubber Tubes 566 4.4.4.4. Applications in Footwear 566 4.4.5. Applications of CPN in Packaging 566 4.4.6. Applications of CPN in Coatings 568 4.4.7. Application of CPN for Wire and Cables 569 4.4.8. Application of CPN for Biomedical Applications 569 4.4.9. Application of CPN for Fuel Cells 569 Appendix. Table of Polymers 570 References 578

This Chapter aims to give the reader the tools for answering three questions: 1. Why use a clay mineral as filler for a polymer for given applications? (Section 4.4.1) 2. Are there commercial products based on clay polymer nanocomposites (CPN)? (Sections 4.4.2–4.4.9). 3. What are the limits preventing larger scale applications of CPN? (answer along the paper) General information on CPN is reported in different reviews and books (see references in chapter 13 in Volume A). Additional references are reported

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on thermoplastics (Mittal, 2010), thermosets (Kotsilkova, 2007), and rubbers (Bandyopadhyay et al., 2006; Sengupta et al., 2007; Maiti et al., 2008; Sabu and Ranimol, 2010; Galimberti, 2011; Mittal et al., 2011).

4.4.1 IMPROVEMENTS OF POLYMER PROPERTIES BY CLAY MINERALS In all types of polymer matrices, clay minerals bring about technical advantages such as better (i) mechanical properties, (ii) barrier properties, (iii) thermal properties, (iv) degradation properties, (v) flame resistance, and (vi) rheological properties. A clay mineral can increase the glass transition temperature (Tg) of a polymer, thus positively affecting its heat distortion temperature.

4.4.1.1

Structure of Clay Mineral Polymer Nanocomposites (CPN)

Three successive levels of organization can be defined for a clay mineral, in consideration of size, aggregates, particles, and layers (see Chapter 1 and Bergaya et al., 2011a). At the upper level of clay mineral organization, items such as distribution and dispersion or aggregation of pristine or organomodified clay minerals have to be examined. A clay mineral, due to its large specific surface area (SSA), is a filler with enormous potentialities for a polymer, provided that the particles are evenly dispersed in the matrix. To achieve good dispersion in a hydrocarbon polymer matrix, the inorganic clay mineral is compatibilized with coupling agents such as silanes or by substituting the exchanging cations with organo-cations, in particular alkylammonium ions (Bergaya et al., 2011b), thus forming an organo-modified clay mineral, usually indicated as organoclay (OC). When considering the lowest level of clay mineral organization, both intercalated and exfoliated CPN structures can be formed (see Fig. 13.1 in chapter 13 in Volume A). Most literature refers to the intercalated structure as the most effective one for remarkably improving some polymer properties. However, some authors dispute the validity of the general assumption that hydrocarbon polymer chains can easily intercalate in clay mineral particles and suggest that data available in the literature, showing variation of the basal spacing, could also be caused by intercalation of small molecular mass substances and with different tilting angles of the alkyl chains (Galimberti et al., 2007, 2009a,b, 2010).

4.4.1.2

Clay Minerals for Better Mechanical Reinforcement

The static and dynamic-mechanical properties of a polymer matrix, both thermoplastic and elastomeric, were clearly improved by highly dispersed clay mineral particles.

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4.4.1.2.1 The Origin of Reinforcement Reinforcement is supposed to arise from the immobilization of polymer chains on the clay mineral surface, though it is not possible at present to determine the level of adhesion. The most important source of reinforcement seems to be the intercalation of polymer chains in the clay mineral particles (for elastomeric matrices see, e.g. Joly et al., 2002), even though this interpretation is debated by Galimberti et al. (2007, 2009a, 2010). In the case of a thermoplastic polymer matrix, methods were not developed to establish chemical bonds between polymer chains and clay minerals. In the case of elastomers, a sulphur-containing silane is used to promote the formation of a chemical bond between the clay mineral and the unsaturated elastomer by condensation of the SiOR groups with the clay mineral hydroxyl groups and, in a further step, through the vulcanization reaction. The ammonium ion is an accelerator of cross-linking reactions and thus promotes the formation of areas of high cross-linking density surrounding the clay mineral particles. However, the most important source of reinforcement, for both thermoplastics and elastomers, is the intercalation of polymer chains (provided that this event really takes place). The dispute on the intercalation of polymer chains, which might appear as purely theoretical, is actually on a very important aspect of the reinforcement of polymer melts and elastomers. In fact, it is well known that a complex modulus arises from the contribution of several factors (Medalia, 1978) (Fig. 4.4.1): the structure of the polymer network due to entanglements and cross-links between the chains, the hydrodynamic effects due to the volume fraction of the rigid filler, the type of filler, and its so-called structure. A filler with a ‘high structure’, which means a considerable amount of voids and thus the ability to accommodate many polymer chains transforming them into a rigid filler, would highly contribute to the reinforcement. A clay mineral could in principle accommodate the polymer chains in the space between two opposite layers. However, this aspect has to be verified. A filler network is formed when filler particles aggregate either by direct contact or via layers of polymer shells. The formation of a filler network is thus favoured by a high filler SSA. This should be the case of most clay minerals used for the preparation of elastomer nanocomposites. With reference to Fig. 4.4.1, the SSA of a filler influences the modulus at low strain, whereas the filler structure is responsible for the modulus at large strain (Medalia, 1978). 4.4.1.2.2 Filler Networks at Low Clay Mineral Contents A clay mineral is able to form a network at low concentration, much lower with respect to traditional fillers such as carbon black (CB) and silica. At least about 25–30 mass% of a nanostructured filler is required to reinforce an

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G* Shear modulus

Filler network

In-rubber structure Hydrodynamic effects Polymer network 0.1%

100% Log(shear deformation)

FIGURE 4.4.1 Complex modulus G* versus shear deformation: contributions to the modulus.

elastomeric matrix (Medalia, 1978), whereas a filler network is formed at clay mineral contents 40 phr. The Goodyear Patent (Zanzing et al., 2005) reported the use of a blend of E-SBR with unmodified Mt (10 mass% in the blend), prepared from the E-SBR latex, in a tyre tread based on a prevailing amount of BR. The final compound contained 3 phr. The use of Mt allowed the reduction in the amount of silica from 70 to 60 phr, obtaining a compound with lower abrasion and with hysteresis at low temperature analogous to that of silica-based compounds. Michelin (Simonot et al., 2006) patented a tyre tread rubber composition containing a synthetic aluminosilicate MxSiAlyOa(OH)b, (H2O)c, where M ¼ Kþ, Naþ, Caþþ and a mixture of these cations; x > 0; y > 0; a 0; b 0; c 0 and a þ b > 0. This aluminosilicate was used with a sulphurcontaining silane as a coupling agent, in the absence of any ammonium cation, to completely replace silica. When the curing rate was increased, equivalent mechanical properties and lower hysteresis were obtained. Pirellli Tyre (Romani and De Cancellis, 2006) reported the use of OC in a studded tyre suitable for driving on ice and snow. The modulus was increased with 20 phr of Mt modified with dimethyl ditalloyl ammonium ions instead of aramide fibres and silica. Tyre innerliners are based on BR due to the low free volume of this family of rubbers. In particular, BR is predominantly used to reach a higher reactivity of the cross-linking reactions. This leads to better liner adhesion with adjacent tyre components. The orientation of the clay mineral particles, of the outmost importance in a tyre innerliner, is affected by a number of factors such as processing energy, obstructions by filler particles and degradation of the alkylammonium ions of the OC at high temperatures (Rodgers et al., 2011). Exxonmobil published data on the effect of OC on the impermeability of compounds based on a BIMS rubber at different temperatures without revealing further information on the nature of the CPN (Rodgers et al., 2009). OC was used in place of a small amount of CB and traditional ingredients were adopted. The increase in Delta Torque, a higher 300% modulus and a higher hardness suggested a higher cross-linking density, probably due to effect of the alkylammonium ions. The usual increase of the permeation coefficient by an exponential correlation was much lower in the case of the CPN. Thus, the oxygen permeation was reduced into the tyre casing, which would lead to increased tyre intra carcass pressure (Rodgers et al., 2011). Analogous results were reported using BIMS-based CPN in truck tyres (Rodgers et al., 2005). In the light of the increasing numbers of truck tyres on the market and taking into consideration that these tyres should have a very

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long service life, an increase of air retention due to the use of OC in inner liners in these types of tyres appears particularly relevant. The best clay mineral dispersion was achieved by the two following approaches: (i) the use of clay mineral particles with high aspect ratio and (ii) control of the exfoliation degree of the clay mineral particles in the innerliner compound. The technical solutions proposed in the patent literature are summarized in Table 4.4.2.

TABLE 4.4.2 Technical Solutions from Patent Literature for Achieving the Best Clay Mineral Dispersion in a Rubber Matrix for Tyre Inner Liner Clay mineral

Rubber matrix

Technical advantage

Refs.

Clay mineral with a high aspect ratio Kaolinite POLYFIL DL from Huber Co

XIIR (X ¼ Cl)

Air permeation resistance at 60 C improved up to seven times

Wada et al. (2006)

Mica: aspect ratio of at least 50, average particle diameter from 40 to 100 mm. From 10 to 50 phr

XIIR/NR/BR

High air retention with good processability, low hysteresis and high crack growth resistance

Miyazaki (2006)

‘Packets’ with average thickness: 10–140 nm. 40 phr

BR

Impermeability increased by seven times

Elspass et al. (1998)

OC with dialkylammonium: aggregates of at least three layers. 9 phr

High molecular mass BIMS and a non-ionic compatible polymer having a molecular mass of less than a half

Oxygen transmission reduced to about onefourth

Elspass and Peiffer (2000)

OC with distearyl dimethyl ammonium. 40 phr

SBR/IR

Impermeability increased by 40%. Improved durability at low T

Ishida and Fujiki (2003)

Clay mineral stacks

Continued

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TABLE 4.4.2 Technical Solutions from Patent Literature for Achieving the Best Clay Mineral Dispersion in a Rubber Matrix for Tyre Inner Liner—Cont’d Clay mineral

Rubber matrix

Technical advantage

Refs.

Exfoliated clay minerals Smectites dispersed in water, a surfactant (a quaternary ammonium), polymerizable monomers (isoprene and styrene), a polymerization initiator

(i) Polymers obtained therefrom. (ii) Trimethyl ammoniumfunctionalized paramethylstyrene-coisobutylene-co-isoprene

Reduction of oxygen transmission even to one third

Elspass et al. (1999)

Improved impermeability

Parker et al. (2003)

Exfoliated clay minerals Na clay mineral. Exchange in water with cetyl trimethyl ammonium bromide Clay mineral þ a tertiary amine such as talloyl dimethylamine. 3 phr

BIMS

Permeability reduced by 20%

Dias et al. (2005b)

Delaminated talc with a surface area of 10–40 m2/g, þCB with a low surface area (20 m2/g)

XIIR

Lower permeability, good fatigue

Krueger (2008)

Clay mineral exfoliation with reactive rubbers Silicate/reactive rubber ¼ 25/75

E-SBR þ amine terminated butadiene-acrylonitrile oligomer

Oxygen transmission reduced to less than one half

Kresge and Lohse (1997)

Clay mineral with an aqueous dispersion of a functionalized diene-based elastomer (2:1)

Diene-based elastomer with functional groups: acid, anhydride, protonated amine-modified epoxide group

Clay dispersion

Ajbani et al. (2003)

Clay mineral þ exfoliating agents: alkyl amines

Poly(isobutylene-co-palkylstyrene) and poly (isobutylene-co-isoprene)

Permeability was reduced to one-half

Gong et al. (2004)

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TABLE 4.4.2 Technical Solutions from Patent Literature for Achieving the Best Clay Mineral Dispersion in a Rubber Matrix for Tyre Inner Liner—Cont’d Technical advantage

Clay mineral

Rubber matrix

Refs.

and silanes. 10 phr OC

elastomers functionalized via Friedel–Craft reaction with a Lewis acid and an anhydride

Single layers of OC, oriented perpendicular to oxygen diffusion. 5 phr

XIMS with a polar group. Curing performed by exposing inner liner to water

Air retention improved up to 30

Grah (2004)

Mt modified with a benzalkonium chloride

Reactive PI, containing N,Ndimethylvinylbenzylamine, succinimide amine

Impermeability improved up to 70%

Wang et al. (2004)

Clay mineral exfoliation with reactive rubbers Clay mineral ionically bonded with and an ammonium group

Rubber based on isobutene þ a liquid polymer reactive with an amino group: liquid maleic anhydride-modified BR

Impermeability improved by 70% (8 phr OC, 2 phr liquid MAH-IIR in 100 BIIR)

Maruyama et al. (2005)

Clay þ ionic liquid surfactant such as an imidazolinium salt

Ionic liquid surfactant such as an imidazolinium salt

BIIR, NR

Wang et al. (2005c)

Clay minerals

Polyisobutylene ammonium chloride, from amine þ HCl

Clay dispersion

Gong et al. (2006)

Exxonmobil (Dias et al., 2002) reported a 40–50% better impermeability when naftenic oil was replaced by polybutene oil (MM 400–10,000) when an OC was used in halogenated butyl rubber. The same company (Tsou and Dias, 2002) reported a blend between a copolymer of isobutene, for example, parahalo-methylstyrene, and an engineering resin, such as Nylon. The OC is premixed with the copolymer and, optionally, with the resin. An improvement of impermeability of higher than 100% was detected for BIMS/Nylon compared with BIMS. Also reported was a barrier performance improvement of 50% using the CPN of an elastomer combined with a high barrier thermoplastic resin (Dias et al., 2005a). Michelin (Bergman, 2006) reported the use of a ditalloyl dimethylammoniumMt, at a level of 4 phr, and a plasticizer such as a terpene resin with a Tg > 50 C

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in a matrix made of a brominated copolymer of isobutylene and p-methyl-styrene. The terpene hydrocarbon was added to improve processability but also impermeability, whose increase was at a maximum of about 13% OC. Michelin (Feeney and Balzer, 1998) reported the coating of rubber articles with liquid mixtures containing (i) an elastomer, preferably BR; (ii) exfoliated vermiculite with an aspect ratio >25 (up to 100); and (iii) a surfactant such as a polyether/dimethyl polysiloxane copolymer. The mixture had a solid content up to 30%, and the vermiculite/polymer ratio varied from 1:20 to 1:1. The permeability was reduced up to 250%. This technology has found successful application in sporting goods such as tennis balls. Tyre curing bladders, usually made with BR, experience severe service conditions (heat and pressure) and undergo multiple cycles of expansion and contraction, thus causing tears and ruptures. The primary technical objective is to improve the strength of a bladder compound. Various types of OC were added to a resin-cured bladder formulation and the elastic modulus was increased by 20–25% compared to the base compound, in particular by modifying Mt with benzyl hydrogenated tallow dimethylammonium ions (Samadi and Kashani, 2010).

4.4.3.7 CPN in Base Compounds OC were reported in the patent literature to be effective ingredients for achieving an outstanding improvement of dynamic-mechanical properties in base compounds (Galimberti et al., 2002, 2003; Giannini et al., 2003, 2004). Pirelli Tyre announced in 2007 the use of a base compound containing an OC in P Zero tyres designed for high and ultra-high performances. Higher stiffness, better handling/comfort trade-off, no decay, and dynamic modulus stability with temperature were claimed. Compared to aramide reinforced fibres, OC gave better ultimate properties and a much more isotropic behaviour, that is, equal performance in longitudinal and lateral directions.

4.4.3.8 CPN in Rubber Automotive Compounds other than Tyres Many automotive parts are made with rubber compounds: fuel systems, under-hood, hood bump stops, engine bay rubbers, power train, chassis and underbody, front and rear bumper bars, front and rear mudguards, front and rear screen seals, door seals, and trims. Few data have been published on OC applications in these automotive parts.

4.4.4 APPLICATIONS OF CPN IN SPORTING GOODS CPN were first applied and commercialized for sports equipment, in particular for sports balls and other pneumatic applications, leading to commercial application in the case of tennis balls. This type of CPN is based on elastomers.

Chapter

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565

Barrier Coating Technology

The application of CPN in the field of sports balls was aimed at improving the pressure retention without impairing other properties, namely, the bounce, the feel, and the reproducibility of performances. The core of the technology (Feeney et al., 2006a,b) was the application of a barrier coating containing highly exfoliated clay mineral particles. This technology is based on three main aspects: (i) the use of highly exfoliated clay mineral particles with a large aspect ratio. Vermiculite particles with a thickness of about 1–3 nm and lateral dimensions of 1–30 mm were selected. The coating was made with a mixture of the aqueous vermiculite dispersion with an aqueous dispersion of butyl rubber. A large number of exfoliated particles was found in the relatively thin (about 10–30 mm) coating layer; (ii) the control of the clay mineral–polymer interface, obtained by modifying the clay mineral surface and by the addition of neutral siloxane-based surfactants; and (iii) a good dispersion of the clay mineral particles in the polymer matrix after drying the coating. Transmission electron microscopy revealed well-dispersed particles of 1–2 nm thick and 200–400 nm diameter with wavy or curved morphology, as well as micrometre-sized particles. An aspect ratio of about 100–400 was estimated. The diffusion coefficients were reduced by two orders of magnitude, as determined by applying a butyl rubber latex (rubber particles of about 1 mm in diameter) containing 20–30 mass% vermiculite on PPO-coated Anapore ceramic disc (Takahashi et al., 2006). Air D-Fense products (inmat.com, 2012) were commercialized by InMat company by applying coating layers on elastomeric substrates via spray or dip coating processes. The product used for sporting applications was Air D-Fense 2000, having a solid content of 12.7–13.3 mass%, pH 6.5–7.5, shelf life of 48 months (manufacture data), permeability of 2.5–3.5 cm3 mm/m2 day atm. at 23 C (on a PP substrate), and a strain to first damage at 25 C >25%. The reduction of gas permeability with respect to the bare butyl rubber was up to 300 times. A product with a lower permeability, 1–1.5 cm3 mm/ m2 day atm., is also commercialized.

4.4.4.2

Tennis Balls

The first commercial application of a CPN for barrier coating was in tennis balls, the Double CoreTM Tennis Ball commercialized by Wilson Sporting Goods in 2001. The most important requirements for a tennis ball are the pressure retention and a relatively high bounce. They require the use of natural rubber. BR would give a better pressure retention but would cause a worse bounce. Other requirements are limited mass, a nice sound during the play, and a very high reproducibility of performances. A tennis ball typically loses its air pressure in a few days upon opening the can and become unplayable even before two weeks. Two main approaches were developed over the years

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to achieve better air retention without affecting the other ball properties: a different pressurizing gas and the use of a barrier coating inside the ball. Attempts to use SF6 as the gas, in mixture with air, failed because of the variation of sound during the play and of poor reproducibility. Traditional barrier materials were not suitable for the technology applied for the preparation of tennis balls, which implies curing temperatures >100 C and 5% change in volume when the ball is pressurized. The barrier coating provided by CPN was seen as the ideal solution to the relatively poor pressure retention of natural rubber. The inner core of the tennis ball was coated with a 20–30-mm layer of a CPN based on vermiculite and BR (Air D-fenseTM product by InMatTM LLC). The barrier was equivalent to that provided by 10 mm of natural rubber. The coating added no more than 0.5 g to the ball mass, that is,