Bergaya F. Handbook of Clay Science

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C. Modiﬁed Polymers

For non-polar species, such as PP, the alkylammonium clay mineral has almos t the

same apolar character as the PP itself. Consequently, such systems are at ‘‘theta’’

conditions: there is no favourable excess enthalpy to promote the formation of the

nanocomposite (Giannelis et al., 1999; Manias et al., 2001). Thus, the challenge with

PP is to design a system wherein the clay–polymer interactions are more favourable

than the surfactant–clay mineral interactions.

Improving the interactions between the polymer and the clay mineral can be

achieved by PP functionalisation, introducing polar groups in the PP polymer. By

far, the most popular method consists in modifying the polymer by grafting maleic

anhydride. This is usually done in a twin-screw extruder and the grafting is initiated

by peroxides.

Other methods to enhance the polarity of PP consist in making hydroxylated PP,

grafting acrylic acid, or forming random and block copolymers using various polar

monomers (Manias et al., 2001).

Another approach to compatibilise clay minerals and non-polar polymers consis ts

of introducing cationic gro ups into the polymer, facilitating polymer intercalation by

ion-exchange (10.3.4.A).

As mentioned before, organoclay–PS nanocomposites can be prepared by react-

ing the melted polymer with organoclays. Alternatively, the introduction of –NH

functional groups which can be protonat ed, facilitates direct intercalation of the PS

into untreated smectites (Hoffmann et al., 2000 ).

10.3.5. CLAY-POLYMER NANOCOMPOSITES: PROPERTIES AND

APPLICATIONS

Clay mineral– and organoclay–polymer nanocomposites are still under develop-

ment. They were successfully marketed because they fulﬁll at least two market needs:

increased stiffness and increased resistance to the permeation of gases, at reasonable

cost. For example organoclay–PP nanocomposites having a high modulus are suit-

able for use in automotive panels, whereas organoclay–nylon nanocomposites with

low gas permeability can be used for packaging.

Toyota was the ﬁrst to develop commer cial clay–nylon nanocomposites. They

patented almost every conceivable combination of clay mineral type and polymer,

including the methods of preparation of nanocomposites.

Other potential improvements in propert ies were identiﬁed. This may drive new

future developments, as shown in the web sites of the major ‘‘nanoclay’’ manufac-

turers: http://www.nanocor.com, http://www.nanoclay.com, http://www.sued-

chemie.com.

There is an extensive literature on the polymer nan ocomposites used for different

applications, but this chapter only considers the properties of nanocomposites based

10.3.5. Clay-Polymer Nanocomposites: Properties and Applications 599

on polymers of current considerable commercial interest. However, some examples

of nanocomposites involving different polymers that were not been commercialised

such as PS, PMM, polysiloxanes or epoxy resins (Pinnavaia and Beall, 2000) will be

mentioned to illustrate potential applications.

A. Mechanical Properties

The design of new thermoplastic parts takes into account the mechani cal properties

of the raw mate rial. Often, the most important mechanical property of plastic ma-

terials is the elastic modulus (measured both in tension and in compression). The

modulus is in principle sufﬁci ent to calculate stresses and strains exerted by external

forces, and to optimise new designs by computer simulations.

In general, the restricted mobility of polymers at the ﬁller interface is the main

cause of the high modulus of ﬁller-loaded polymers (Lipatov, 1995). This also applies

to clay–pol ymer nanocomposites. This mechanical property is very dependent on the

structure of the nanocomposite such as the extent of exfoliation of the clay miner al.

In turn, the structure is dependent on the physical properties of the clay mineral,

such as its CEC, which can vary from one particle to another and from lot to lot of

the clay (Quarmley and Rossi, 2001).

Other mechanical properties such as the modulus at high temperature, creep,

chemical resistance, weathering resistance have to be evaluated. Properties at break

such as impact resistance, tensile strength and elongation at break, indicate product

quality, and resistance to harsh treatment s, shocks, etc., during service life. Some of

these parameters may sometimes be evaluated by specialist software.

Tensile Pr operties

Clay–nylon nanocomposites can be obtained by melt blending organomod iﬁed clay

with nylon. The data in Fig. 10.3.5 show selected mechanical properties of clay–

nylon 6 nanocomposites (Cho and Paul, 2001).

Regardless of preparation method or type of clay, the modulus and strength of

the nanocomposites are remarkably similar, and proportional to the content of clay

mineral. The data suggest that the organoclay is much more efﬁcient than other

mineral ﬁllers in increasing the mechanical properties of the polymer (Goettler and

Recktenwald, 1998).

The extent of organoclay–polymer interaction in nylon nanocomposites, and

therefore the level of modulus, yield strength, and elongation at break is in general a

function of (i) the type and con centration of clay (Kojima et al., 1993c); (ii) the

choice of adequ ate treatment of the clay (Kodgire et al., 2000; Fornes et al., 2002 );

(iii) the molecular weight of the nylon (Fornes et al., 2002); and (iv) the processing

conditions (Davis et al., 2002).

The reinforcing effect of the clay mineral is further clariﬁed (Fig. 10.3. 6 ) by the

evolution of the dynamic elastic mod ulus as a function of the temperature (Gloaguen

and Lefebvre, 2000). This effect is clearly more pronounced above the glass

Chapter 10.3: Clay Mineral– and Organoclay– Polymer Nanocomposite600

Relative StrenghtRelative Modulus

Mineral content (wt.%)

0 5 10 15

Fig. 10.3.5. Relative mechanical properties of various organoclay-nylon 6 nanocomposites as

a function of the clay mineral content: (a) yield strength, (b) tensile modulus. From (Cho and

Paul, 2001).

Fig. 10.3.6. Dynamic elastic modulus of an organoclay-nylon 6 nanocomposite as a function

of temperature. From (Gloaguen and Lefebvre, 2000).

10.3.5. Clay-Polymer Nanocomposites: Properties and Applications 601

temperature, Tg. This indicates restricted mobility in the amorphous phase close to

the clay mineral surface, at temperatures below the melting point. This effect occurs

with many different polymers and types of synthetic or natural clay minerals. Not

unexpectedly, further improvements of the modulus can be obtained by biaxial

stretching ( Worley et al., 2001).

The modulus and tensile strength in Nylon are known to be highly dependent on

water content. In the literature, mechanical properties of organoclay–nylon nano-

composites are often evaluated on DAM (dry as moulded) specimens. Little atten-

tion was paid to evaluate mechanical properties after adequate equilibrium water

absorption. In nanocomposites based on blends of Nylon and grafted PP, the data

show that not only the rate but also the total water absorption is reduced for the

nanocomposites (Liu et al., 2001). It is clear that the reduced water absorption may

have a deleterious effect on the impact properties of the product (see next section

Impact resi stance and ductility).

Nanocomposites based on PP can be made only if both the clay mineral and the

polymer are modiﬁed. Using unmodiﬁed PP and an organoclay (Hasegawa et al.,

1998; Gloaguen and Lefebvre, 2000; Wang et al., 2001), the modulus increase

(15–25%) is almost the same above and below Tg. It is assumed that in this case, PP

interacts very little with the modiﬁed clay mineral (i.e., the polymer is not crawling

into the interlayer space).

Manufacturing processes to make the nanocomposites normally involve prepar-

ing a masterbatch (made of organoclay intercalated by PP-g-MA), and diluting it

into pure PP. The inﬂuence of MW and MA content in PP-g-MA was extensively

studied, and published results are sometimes contradictory.

In general, modulus improvements for PP- based nanocomposites are lower than

for their nylon counterparts. Intercalated nanocomposites containing 5% clay min-

eral show a 50% increase in tensile modulus and 10–15% increase in tensile strength,

compared to pure PP (Wang et al., 2001). These properties are almost unaffected by

the MA content (1.5–5.8%) and the MW (9,100–330,000) of PP-g-MA and its con-

tent (5–15%) in the nanocomposites.

However, the type of organocl ay appears to be more important than the PP

modiﬁcation: Cloisite

20A (dim ethyl di-hydrogenated tallo w ammonium modiﬁed

clay) is claimed to give 20% higher tensile properties than the more heat-stable

Nanomer

I.30 TC (octadecyl ammonium-modiﬁed clay mineral).

Interestingly, the partial substitution of the PP-g-MA for nylon, has a significant

effect on the interlayer space. Nylon crawls into the interlayer space, (increasing

significantly the basal spacing) but does not improve the mechanical properties.

The Toyota laboratory also studied the mechanical properties of PP/PP-g-MA

nanocomposites based on octadecylammonium modiﬁed clay minerals. Tensile

moduli of the nanocomposites are approximately 30% higher than the pure PP.

Fig. 10.3.7 shows the relative dynamic storage modulus as a function of the tem-

perature (Hasegawa et al., 1998 ). As in the case of ny lon the increase in modulus is

higher above than below the glass temperature of the PP phase. The maximum

Chapter 10.3: Clay Mineral– and Organoclay– Polymer Nanocomposite602

reinforcement is obtained at 801C. Above this temperature, the modulus is reduced

due to the lower modulus of the PP-g-MA, compared to PP.

Modiﬁed routes to the melt intercalation to obtain PP nanocomposites were

considered to improve the exfoliation of the clay mineral, such as swelling of the clay

by a solvent that is subsequently volatilised during melt blending (Wolf et al., 1999 ).

Other concepts were evaluated such as the use of random copolymers instead of

PP-g-MA, or speciﬁc surfa ctants for the modiﬁcation of the clay mineral, having less

favourable interaction with the clay mineral than the PP itself (Manias, 2001). The

modulus improvements obtained with these alternative methods are not significantly

different than those reported for common ly used surfactants. However, the ﬂuoro-

organoclay–PP nanocomposites have almost the same elongation at break as pure

PP, hence they have potentially a better impact resistance than other nanocompos-

ites.

Impact Resistance and Ductility

Achieving a high impact resistance (sometimes called toughness) at low temperature

for polymers such as nylon and PP was long considered a technical challenge. At

ambient temperature, PP in particular, displays a relatively ductile behaviour but this

degenerates below Tg. Compounding the polymer with rubber was successful in

alleviating the brittleness of these materials, although stiffness is reduced. Some

nanocomposites composed of nanop articles of vulcanised rubber have however in-

creased impact resistance with a minimal effect on the modulus (Zhang et al., 2002).

Conversely, rigid ﬁllers are commonly used to improve the modulus of the matrix,

usually with the penalty of decreased toughness. Hence, optimal compounding of PP

Fig. 10.3.7. Dynamic storage modulus of the organoclay-PP nanocomposites and clay-PP

composite in relation to PP. From (Hasegawa et al., 1998).

10.3.5. Clay-Polymer Nanocomposites: Properties and Applications 603

is usually the result of a delicate balance of rubber and mineral ﬁllers, to promote

ductility at low temperature and adequate stiffness.

Organoclay–nylon nanocomposites have a slightly lower impact resistance than

the neat polymer. The inﬂuence of the organ oclay is shown in Fig. 10.3.8, where the

Izod-impact strength is plotted against temperature. The data show that the ductile-

brittle transition is simply moved towards higher temperatur es with increasing con-

tent of organoclays, even though Tg does not change (Cho and Paul, 2001). This can

be at least partially attributed to the reduced water absorption of the nanocompos-

ites.

Fracture toughness results show that, in contrast to neat nylon, the nanocom-

posite at equilibrium water uptake (Section 10.3.5.D) does not show an elasto-plastic

(super-tough) behaviour (Bureau et al., 2001). This may be due to the lower water

content of the nanocomposite. Grafting the surfactant (hexamethylene diamine) to

the polymer apparently makes the organoclay–nylon nanocomposite to have the

same level of impact resistance as the neat polymer (Maul, 1999).

Nanocomposites obtained by reacting co-intercalated epoxy resin and quaternary

ammonium into Na

-montmorillonite with nylons, show higher impact strength

than the neat polymer (Liu and Wu, 2002; Liu et al., 2003).

Nanoﬁllers such as nano–CaCO

, are effective nucleating agents an d improve the

impact resistance of PP (Chan et al., 2002). This is due to the nan oparticles intro-

ducing a massive number of stress concentration sites, hence promoting cavitation

during an impact test. The cavities release the plastic constraints and trigger large-

scale plastic deformation of the matrix, which consume high fracture energy. This

Fig. 10.3.8. Notched Izod impact strength for organoclay-nylon 6/nanocomposites as a

function of temperature. From (Cho and Paul, 2001).

Chapter 10.3: Clay Mineral– and Organoclay– Polymer Nanocomposite604

ideal scenario is unfortunately not found with organoclay–PP nanocomposites,

which display systematically a lower impact resistance than the pure polymer.

B. Fire Retardancy and Thermal Properties

Fire Retardancy

The cone calorimeter is one of the most effective bench-scale laboratory equipments

for studying the ﬂammability of materials. In evaluating new materials, the industry

takes cone calorimeter results into consideration, but commercial products are nor-

mally evaluated against industry standards such as the UL94 tests. Although these

tests are not necessarily better, nor more practical than the cone calorimeter, they are

the industrial bench mark against which materials need to be evaluated. The most

severe ﬁre retardancy rating is referred to as UL94 V-0 (similar to EN/IEC 60695-11-

10 Method B, VDE 0471 Part 11-10 Method B, CSA 22.2 Vertical Burning Test).

The V-0 rating at a given thickness means essentially that under the conditions of the

tests, the material must self-extinguish within a very short time, and must not drip.

To achieve the status of UL94 V-0, polymers most often need to be compounded

with ﬁre retardant (FR) additives.

In general, FR additives have an effect either in the condensed phase or in the gas

phase, by respectively creating an isolating char or preventing oxygen from reaching

the burning material. Examples of additives producing char are phosphorous or

melamine compounds; and those working through the gas phase are synergistic

blends of decabromodiphenyl oxide an d antimony sesquioxide.

Nylon can be easily compounded with FR additives (Gilman et al., 2000). Or-

ganoclay–nylon nanocomposites, containing 3.3% melamine and 5% organoclay

produce a UL94 V-0 rating. In this case, some of the melamine is intercalated in a

layered silicic acid salt (Inoue and Hosokawa, 1998). For reference, normally 6 to

10% melamine is needed to provide a V-0 rating to nylon neat polymer.

Polyoleﬁns are the most difﬁcul t to be compounded for reduced ﬂammability. In

the ﬂame, PP decomposes into small v olatile fragments by chain scission resulting in

a high pre-ignition zon e and ﬂame temperature, contributing to the high exothermic

reaction in the ﬂame.

Organoclay–PP nanocomposites based on PP-g-MA show 70 to 80% reduction in

PHRR in the cone calorimeter, slowing down the combustion process significantly

(Gilman et al., 2000). This behaviour is due to the formation of a powerful char

induced by the clay mineral. At the end of the combustion process, almost all

nanocomposites tested leave behind a layered carbonaceous silicate structure, which

is polymer type-dependent.

Some nanocomposi tes with intercalated polymers in clay minerals show excellent

ﬁre retardancy. For other nanocomposites a minimum amount of delaminated clay

particles is needed to provide good ﬁre retardancy. However, an apparently well-

exfoliated ﬂuorohectorite–polymer nanocomposite has no ﬁre retardant properties.

No fundamental explanation was proposed to explain this surprising behaviour.

10.3.5. Clay-Polymer Nanocomposites: Properties and Applications 605

Organoclay–PP nanocomposites do not reach the UL 94 V-0 rating, but in a

traditional V-0 rated PP compound, 25% of the FR additives such as a blend of

decabromo diphenyl oxide and antimony sesquioxide, can be replaced by an or-

ganoclay in a ratio of almost 1:1 (Nanocor, 2003). The resulting nanocomposite has

the same V-0 rating, a slightly lower speciﬁc gravity, and potentially higher me-

chanical pr operties than, the original compound.

ATH and MH are traditional FR additives for pol yoleﬁns. They have very de-

sirable properties (non-toxic and halogen-free) but have many practical disadvan-

tages, such as their low efﬁciency. Additions of up to 65% by weight (40% in

volume) may be required to achieve UL 94 V-0 ratings. At these high ﬁller levels, a

marked deterioration in mechanical properties and significant viscosity increases,

can limit the mechanical performance and their selection for more demanding ap-

plications. Hence, reducing ﬁller level without compromising ﬁre retardancy is

needed. The use of modiﬁed forms of hydroxides, and their combination with al-

ternative additives acting in a synergistic manner are the most promising ways to

achieve this goal. In the very short list of effective additives that can be used to

partially replace ATH, such as the extremely efﬁcient PAN ﬁbres (Myata and Ima-

hashi, 1992), carbon powder, transition metal oxides, and zinc stannates and hy-

drostannates (Hornsby and Ahmadnia, 2002), organoclays should be considered as

valid candidates.

Even though ATH is normally used for PE and EVA, Table 10.3.4 shows the

results of the partial replacement of MH by an organoclay in a UL94 V-0 rated EVA

compound. The data show that the organoclay is much more efﬁcient than the

hydroxide in providing ﬂame retardancy. For the same V-0 rating, as the loading of

magnesium hydroxide is lower, improved processing and mechanical properties are

expected (Nanocor, 2003).

Other valuable but less known FR additives exist alongside organoclays to improve

the ﬁre resistance of polymers. For example, adding 15% lignin gives a 60% reduction

in PHRR in the cone calorimeter; further reductions in PHRR are obtained in the

presence of lignin with traditional char forming additives (De Chirico et al., 2003).

However, organoclays appear to be unique in their ability to provide both increased

modulus and ﬁre resistance, but at the expense of the impact resistance.

Table 10.3.4. EVA compounds and clay mineral-EVA nanocomposites for UL94 V-0 rating

(thickness: 3.2 mm) (from Nanocor)

Traditional EVA nanocomposites Clay mineral-EVA nanocomposites

% Weight % Volume % Weight % Volume

EVA 40 63 47 69

Mg(OH)

60 37 50 29

Clay mineral 0 0 3 2

Chapter 10.3: Clay Mineral– and Organoclay– Polymer Nanocomposite606

Thermal Stability and Expansion

One property of organoclay–PP nanocomposites is the increase (of up to 130 1C) in

the onset of the degradation tempe rature in TGA experiments (Kodgire et al., 2000).

Unfortunately, this is not an indication of an improvement in the resistance to ageing

or oxidation of the nanocomposite, which in fact is reduced compared to that of the

neat polymer (Tidjani and Wilkie, 2000; Chin and Solera, 2002). The increase in the

degradation temperature is generally regarded as a consequence of the low perme-

ability of the nanocomposites preventing oxygen to reach the polymer, and pre-

venting the volatiles generated during the decomposition of the polymer to leave the

nanocomposites (Gilman, 2000). Thus, the TGA experiment would appear to du-

plicate the observations from the cone calorimeter. However, organoclay–nylon and

–polyimide nanocomposites do not have a higher degradation temperature in TGA

experiments, but they nevertheless display interesting ﬁre retardancy properties.

It is clear that each polymer may be a special case, and there is no general

understanding of this clay mineral–polymer nanocomposite behaviour.

As expected the CLTE of nanocomposites based on HMW (high molecular

weight) nylon is lower than that form ed from LMW (low molecular weight) nylon

(Yoon et al., 2002). The better behaviour of the organoclay–HMW nylon nano-

composite was attributed to its higher viscosity, inducing higher shear during

processing and a higher degree of clay mineral exfoliation, eventually a higher ori-

entation.

Organoclay–PP nanocomposites also have a lower CLTE than the neat polymer.

This is a definite advantage in a number of automotive ap plications such as bod-

ywork panels, because the decrease in CLTE allows the designer to reduce the gap

between the body panels; this is of general beneﬁt to the car design and aesthetics.

C. Electrical and Electrochemical Properties

The electrical beh aviour of nanocomposites based on clay minerals and related solids

combined with various conducting polymers was intensively studied in the last dec-

ade (Ruiz-Hitzky, 1993; Ruiz-Hitzky and Aranda, 1997). Clay mineral–PEO nano-

composites based on homoionic montmorillonites and other smectites, are

anisotropic solid electrolyte materials presenting ionic conductivity values several

orders of magnitude higher than that of the parent silicate. The thermal stability of

this class of materials is strongly enhanced in comparison with conventional

salt–PEO complexes. Moreover, a salient feature of these clay mineral–polymer

nanocomposites compared with electrolytes in solution, is that in the former case, the

silicate layer acts as the anion. In theory, therefore, ionic conductivity should ex-

clusively involve a cationic transport mechanism.

Recent results of solid-state polarisation (Ruiz-Hitzky et al., 2000 ; Aranda et al.,

2003) using Na

-montmorillonite–PEO nanocomposites conﬁrm this hypothesis,

giving a transport number (t

) close to 0.99 compared to the theoretical value of 1

for an ideal pure cationic solid electrolyte. This unique behaviour can serve as a

10.3.5. Clay-Polymer Nanocomposites: Properties and Applications 607

model for ionic conductors; these nanocomposites have important potential appli-

cations as components in electrochemical devices, such as rechargeable solid-state

batteries and electrochemical sensors. Intercalation of PEO into clay minerals can

promote co-adsor ption of other polymers, such as PMMA, improving further the

ionic conductivity (Chen et al., 1999).

An electronic conducting polymer is formed during the intercalation of aniline

into a Cu

-smectite giving PANI, by the conversion of the emeraldine base into the

emeraldine salt (Fig. 10.3.9) by exposure to HCl vapours (Mehrota and Giannelis,

1991). Alternatively, if the clay mineral is ion-exchanged by treatment with aniline

hydrochloride followed by oxidation with ammonium peroxodisulphate, the an-

ilinium cations give after polymerisation the conductive emeraldine salt form of the

polymer (Chang et al., 1992)(Fig. 10.3.10 ). This type of material exhibits interesting

electrical properties, because the conducting polymer remains in the interlayer space

of the clay mineral. The electrical conductivity shows, as expected, a highly aniso-

tropic character; the value measured in the direction parallel to the basal layer

silicate plane is in the order of 10

3

S/cm.

Similarly, iodine doping of polypyrrole spontaneously formed from adsorbed pyr-

role on transition-metal exchanged smectites, enhances dramatically the conductivity

HCl

: Cu

, Fe

,..

PANI-clay nanocomposite

Fig. 10.3.9. Reactions of aniline in the formation of clay-PANI-nanocomposites from

smectites exchanged with transition-metal ions. A) emeraldine-base formation; B) emeraldine-

salt conducting polymer.

Chapter 10.3: Clay Mineral– and Organoclay– Polymer Nanocomposite608