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ADVANCES IN NANOMER® ADDITIVES FOR CLAY/POLYMER NANOCOMPOSITESbyTie Lan, Ying Liang, Gary W. Beall and Karl Kamena

Nanocor IncorporatedCorporate Technical CenterArlington Heights, Illinois USA

The fundamental bases of clay/polymer nanocomposites are the clay utilized and thesurface modification, which renders that clay compatible with the polymer of interest.Nanocor's surface modified clays are sold under the trade name, Nanomer®. Theselection of appropriate clay involves a number of variables including, among others,morphology, cation exchange capacity, cationic form, charge density, aspect ratio,color, and degree of purity. The most critical of these are discussed in some detail asthey relate to surface modification and ultimately the formation of nanocomposites.There are a number of surface modification options, including onium ion exchange, ion-dipole treatment and coupling agents. These chemistries are also briefly discussed.Performance properties for three clay/polymer nanocomposites, epoxy, nylon-6 andnylon MXD6 are presented.

Additives '99, San Francisco, CA, March 22-24, 1999

INTRODUCTIONPioneering work conducted at Toyota Central Research Laboratories Inc. on nylon-6nanocomposites1 has generated several commercial products and stimulated a greatdeal of interest in expanding the technology to other resin systems. The majority ofresearch conducted to date has focused on two broad areas of application. The first isengineering polymers, where increases in modulus, tensile strength, heat distortiontemperature and retention of impact have made nanocomposites candidates for manyproducts that have not been traditionally plastic. The second is packaging, wherenanocomposites increase barrier to gases (i.e. oxygen, carbon dioxide, water vapor),while retaining clarity, and where they expand the hot fill window.

Ube Industries Ltd. offered the first commercial nanocomposite products, a timing beltcover for Toyota Motors2 and nylon-6 film for packaging3. In October 1998 Bayer AGannounced its intention to offer a line of nylon-6 nanocomposites for food packagingfilm4. More recently, Eastman Chemical Company announced it is moving ahead withthe development of thermoplastic nanocomposites for the polyethylene terephthalate(PET) packaging market5.

A number of factors combine to fuel interest in nanocomposite technology using clayminerals:

Low loading levelsTransparencyIncorporation flexibilitySafetySynergies with other additivesLow cost .

Nanocomposites typically contain 2-10% loadings on a weight basis, yet propertyimprovements can equal and sometimes exceed traditional composites containing 20-35% mineral or glass. Machine wear is reduced and processability is better.

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Because mineral and glass fillers have densities twice that of polymers, weight sensitiveapplications, such as auto parts, offer attractive opportunities.Nanoclay particles have a dimension below the visible light wavelength. ProperlyorientedThe particles are tough. They can withstand solvents, polymerization temperatures andcompounding shear. Consequently, they can be processed without concern aboutdegradation. The implications are obvious for initial fabrication and for recycle.Clays are generally innocuous materials and have been used safely in consumer productsfor decades. Timelines for regulatory approval are shortened.

A wealth of experience demonstrates they act synergistically with other minerals.Recent research shows they are also synergistic with glass fiber6, extending flexibility tocreate customized systems with unique sets of properties.

Lastly, clays are cost-effective. Research has progressed to the point where claynanocomposites can be formulated at cost premiums of 10-30%. Often lower costresins can be nanocomposited to perform like higher cost cousins with no premiumwhatsoever.

The predominant nanoclay in commercial products, and in on-going research, isnaturally-occurring montmorillonite, the most abundant member of the smectite family ofclays. All smectite clays are impure as mined. They require extensive purification for usein polymers such as plastic. Smectite clays are hydrophilic and therefore not compatiblewith most polymers. They must be surface modified to render them suitable for morehydrophobic systems. This paper focuses on fundamental mineralogical characteristicsof montmorillonite and how surface modification affects three basic categories ofnanocomposite properties.

BACKGROUNDAs stated previously, the two critical factors affecting performance pertain to keymineralogical characteristics and the types of surface modification used to rendernanoclay compatible with various polymers.

Clay Characteristics

Purity level, cation exchange capacity and aspect ratio are the clay characteristics ofgreatest importance.

Purity is critical to achieving maximum increases in mechanical properties for engineeringapplications and optimum clarity in packaging. Montmorillonite, as mined containsbetween 5 and 35% impurities. These include both crystalline (e.g. quartz, feldspar,calcite, etc�) and amorphous materials (i.e. amorphous silica), which must besubstantially removed prior to surface modification.

In engineering applications impurities act as stress concentrators, resulting in poorimpact resistance. In films and packaging they contribute to haze. It is, therefore,important to choose deposits that have the lowest amount of impurities or that areamenable to beneficiation. A study7 comparing elongation-at-break of PVOHnanocomposites containing clays of different purity levels, demonstrated thatmontmorillonite should be at least 97.5% pure in order to ameliorate adverse effects.

Another important factor is cation exchange capacity (CEC). Montmorillonite's CECarises mainly from isomorphous substitution of divalent cations (ie. Mg+2, Fe+2) fortrivalent aluminum in the octahedral or innermost layer. Montmorillonites haveconsiderable substitution, 10-30 times greater than kaolin clay for example, causing acharge imbalance in the structure that is compensated by exchangeable cations on the

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surface. CEC provides montmorillonite with the surface activity necessary foracceptance of modifiers.

A final, important factor is the aspect ratio of the clay particle. Montmorillonites areunique in having a platey structure and a unit thickness of less than one nanometer,while the other two dimensions are in the micron range. Theoretical aspect ratios rangefrom 300 to 1500. The average surface area for a montmorillonite in its totally dispersed(exfoliated) state is in the range of 700 sq. meters/gm. With such lofty aspect ratios andsurface areas, exfoliated montmorillonites should be highly effective as reinforcers andbarrier materials at low addition levels. This is indeed the case.

Surface ModificationMontmorillonite's surface is hydrophilic and requires modification to make it compatiblewith most polymer systems. Surface modification can be accomplished through twoprimary mechanisms, ion exchange and ion-dipole interaction. The edges of the plateletscan also be treated with traditional silane coupling agents. However, edges account forless than 1 % of the total surface area. As a result, silane-coupling is normally used asan adjunct to ion exchange or ion dipole treatment.

Ion exchange modification has been practiced since 19498. The original applications forthese materials were as thixotropes for solvent-based drilling muds, paints and greases.Ion exchange involves the substitution of onium ions, which contain aminefunctionalities, for the exchangeable cations on the platelet surface. Examples of thistype of Nanomer are grades I.24T and I.30TC.

Ion dipole treatment is a relatively new method7. It consists of attaching organicmolecules that contain groups such as alcohols, carbonyls or ethers to the exchangeablecations on the surface of the clay. These partially-negative charged species interact withpartial positive charges existing on the exchangeable cations. In this process watermolecules are displaced from coordination to the cations and the surface becomeshydrophobic. Examples of this art are the pyrrolidone-based Nanomers, I.35K and I.46D.

EXPERIMENTALFormation of Nanomer powders, epoxy, nylon-6 and nylon MXD6 nanocomposites

Onium ion modified montmorillonites (Nanomers I.24T, I.28E, I.30E, I.30TC and I.28MC)are manufactured for both epoxy and nylon systems using ion exchange. The process isa variation of that used traditionally for the manufacture of organophilic clay thixotropes.The final product form is a flowable powder.

Epoxy nanocomposites are prepared by exfoliating directly in the resin and curing. In thispaper we are reporting results for Dow DER® 331 epoxy, cured with two HuntsmanJeffamine® agents, D230 and D400, as well as ECA® 100 from Dixie ChemicalCompany. Jeffamines® are amine-based. ECA® 100 is an anhydride. Chemical resistancedata are also reported for Epon® 828 cured with Epi-Cure® 3164, a mixed-chemistrycuring agent. Both products are available from Shell Chemical Company.

Nylon nanocomposites are prepared using two incorporation routes, one involvingNanomer addition to nylon monomer such as e-caprolactam with subsequentpolymerization (Toyota technology)* and the other utilizing a twin screw compounderand high polymer. The results reported for compounding were obtained onnanocomposites made through a 27mm co-rotating Leistritz D36 compounder.Nanomers can be added straight or they can be first made into nylon concentrates andletdown to achieve the desired loading.

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High barrier polymers have also been used to produce nanocomposites. One suchpolymer is nylon MXD6, a product of Mitsubishi Gas Chemical Ltd. MGC's grade 6007 isused to extrude nanocomposite films through the compounding route and the Leistritzcited above.

Formation of Nanomer concentrates and epoxy nanocompositesConcentrates such as Nanomer C.40EBA are also designed for epoxy systems and havebeen prepared using a number of approaches. Concentrates are useful because they canbe added to neat epoxy with relative ease. In concentrate form a Nanomer is partiallyexfoliated, requiring less shear to achieve a uniform mix prior to curing. Nanocorformulates concentrates with the assistance of neutral (non-ionic) surface modifiers. Thedesired clay to epoxy resin (liquid resin) weight ratio varies from 1:75 to 1:1. Modifiersand epoxy resin co-intercalate into the clay galleries by ion dipole interaction, separatingthe clay platelets sufficient to fully exfoliate during letdown. The compositions arecompounded into uniform mixtures and the final product form is also a flowable powder.

* Nanocor is a licensee of Toyota Central Research and Development Laboratories, Inc.

RESULTS AND DISCUSSIONThe above-described Nanomers and concentrates can all be utilized to make commercialnanocomposites. Application possibilities are quite varied, but at present they generallyfall into the categories of those requiring high temperature stability or gas barrier.Performance characteristics are given for these categories. In addition chemicalresistance data is presented for amine-cured epoxy nanocomposites

All of the data reported in this paper has been generated at Nanocor's Technical Center.The facility includes laboratory capabilities which mimic full-scale processes at thecompany's Mississippi production facility. The Center is also equipped to makenanocomposite test specimens using both polymerization and compounding routes.

Figures 1 and 2 ( Hard copy ) are Dynamic Mechanical Analysis (DMA) printouts for theepoxy nanocomposites. Figure 1 covers the amine-cured specimens. An additive loadingof 7% substantially improves modulus, especially in the glass transition (Tg) region.Improvements below the Tg are in the range of 30-50%, but above the Tg they areorders of magnitude greater. Perhaps more significant is an upward shift in the Tg of 10-20°C, depending on the Nanomer.

For the higher temperature anhydride-cured nanocomposites at 10% loadings (Figure 2)modulus improves to a lesser extent, but Tg increases are somewhat greater, 12-23°C.Notice that C.40EBA outperforms I.28E in modulus below the Tg. The situation reversesabove it. This points to the need to match Nanomer surface chemistry to the property-driven application, rather than simply the polymer itself.

Chemical resistance information is given in Table 1. Nanomer I.30E was exfoliated ineach specimen at 6% loading. Using relative uptake as the comparison basis, the threenanocomposites show good-to-excellent performance, except in the case of DERÒ 331-230 exposed to MEK. Resistance to chemical attack is beneficial for many traditionalepoxy applications, most notably coatings.

TABLE 1RELATIVE UPTAKE (ASTM D543-87) NOTE: Unfilled resin = 1.0

Nanocomposite H2O230C

H2O500C

H2SO4230C

H2SO4500C

Toluene MEK

DER 331-230 0.68 0.72 0.75 0.89 0.85 -DER 331-400 0.76 0.68 0.50 0.75 0.34 0.31EPON® 828-3164 0.21 0.15 0.32 0.11 0.85 0.68

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Table 2 ( hard copy ) lists property data for nylon-6 nanocomposites made by both thepolymerization and compounding routes. The one using polymerization contains aNanomer in which the surface treatment reacts with the nylon matrix to form aconnected or tethered system. As the data demonstrates, tethering improves mechanicalproperties versus an untethered system (I.30TC), but this is not the case for oxygentransmission. OTR is higher than the untethered system, meaning that for barrierapplications an unthethered system is preferred. It is another example of the need tomatch nanoclay surface chemistry to intended application.

An anomaly occurs in impact resistance. Conventional theory would hold that a tetheredsystem is more stable and consequently impact should improve. This is not the case,however. Applications needing higher impact require the use of modifiers, even fortethered systems. Further research is ongoing to determine why this occurs and how tomaintain impact without using modifiers.

Considering the nylon MXD6 nanocomposite, Graph A shows barrier improvement foroxygen across the entire relative humidity range using just 5% Nanomer. Moresignificantly, the magnitude of improvement is greatest in the region of highest utility (rh50-85%). For this nylon MXD6 grade, Nanomer addition shifts the curve down and tothe right. Under carefully controlled laboratory conditions, it is possible to achieve near-glasslike permeability in the 55-65% rh interval. Lastly, a noticeable improvement inhaze is seen in films exposed to very high relative humidity (90+%). P>

Our experience with nylon MXD6 nanocomposites has led us to re-think the mechanismof barrier enhancement. Prior theory held that enhancement is due to creation of atortuous path to gas diffusion. Platelet orientation work confirms this is a majorcontributor, but it does not account for the 100 times permeation reduction that weobserve in MXD under tightly controlled conditions. We have concluded that other, asyet unconfirmed, factors are at work as well. The issue is under intense study. Ifadditional mechanisms can be elucidated, applying the knowledge to more commonlyused plastics such as polyolefins, will be of great benefit.

CONCLUSIONOne question frequently asked is "how broadly can this technology be applied in thelong run?" It is impossible to give a definitive answer at this state of knowledge. We areat present developing products for a half dozen additional polymers, and this is by nomeans the limit for polymer candidates. Nanocor believes clay nanocomposites can domuch more than enhancing classic engineering properties and barrier. Range-findingwork provides evidence for improvement in electrical performance, uv stabilization, fireretandancy and control of polymer crystallization. Research in Japan is keying on the useof partially dispersed systems as three-dimensional "platforms" for additive fixation. Asmore industry R&D resources are committed to the technology's potential, furtherdevelopment in these areas will undoubtedly expand its possibilities.

It is clear that several treatment chemistries can be used to compatibilizemontmorillonite for polymer nanocomposites. The main commercial chemistries areonium ion and ion dipole treatments. An expanding universe of treatment agents isamenable to these chemistries.

Montmorillonite-based nanocomposites can be produced commercially, using bothpolymerization and compounding routes. The compounding route includes directNanomer incorporation and concentrate letdown options.

Performance properties improve significantly when proper conditions are met with regardto mineralogy, surface compatibilization and incorporation route.

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Because polymers are complex materials and their real world applications highperformance, care must be taken to match clay purity, morphology and surfacemodification to each system's intended purpose. Cost-effective montmorillonite additivesare entering the market now. As more is learned about how and why nanoscale clayadditives interact with polymers, the potential for high-performance, low-costnanocomposites will be fully realized.

ACKNOWLEDGEMENTSOur appreciation for valuable input is given to Dr. Okane Okada and Dr. Arimitsu Usukiof Toyota Central Research and Development Laboratories, Inc. We also wish to thankJames Ancmon for his work in preparing and testing specimens and Elaine K. McNearfor manuscript preparation.

REFERENCESOkada, A. and Usuki, A., Materials Science and Engineering, C3. (1995) 109-115Deguchi, R., Saegusa, S., Ube Industries, Ltd., Nishi, T., Noruma, T., Toyota MotorCorp., Okada A., Kurauchi, T. Toyota Central R & D Labs, Inc., "Nylon 6-Clay Hybrid-Synthesis, Properties and Application to Automotive Timing Belt Cover", Proceedings ofthe International Congress and Exposition for SAE International, Detroit, MI, February 25- March 1, 1991.Fugimoto, T., Ube Industries, Ltd., "Nylon Clay Hybrid for Packaging Applications",Proceedings of Future Pak �96, Chicago, IL, November 1996.Scherer, C. "PA for Cast and Blown Film Applications" Proceedings of K98 in Dusseldorf,Germany, October , 1998Eastman Chemical Press Release at Nova Pack Americas �99, February 12, 1999,Wyndham Palace Resort Spa, Orlando, FloridaGoettler, L.A. and Rechtenwald, D.W., Solutia Inc., "Nylon Nanocomposites:Performance Attributes and Potential Applications", Proceedings of Additives �98,Orlando, FL, February 1998Beall, G. W. and Tsipursky, S. J., "Nanocomposites Produced Utilizing a Novel ClaySurface Modification" Proceedings of Additives 98, Orlando, Florida February 16-18,1998Jordan, J.W., "Organophilic Bentonites I", J. Phys. Chem., 59, 294-305 (1949)

Nanomer is a registered trademark of Nanocor, Inc.

Copyright © 1999, NanocorNanocor Inc. is a wholly owned subsidiary of AMCOL International Corporation.