epoxy-phenolic novolac-montmorillonite hybrid nanocomposites: novel synthesis methods and their...

1170 POLYMER ENGINEERING AND SCIENCE, JUNE 2004, Vol. 44, No. 6

Epoxy-Phenolic Novolac-MontmorilloniteHybrid Nanocomposites:

Novel Synthesis Methods and Their Characteristics

SANG MIN LEE2, TAE RYONG HWANG1,YANG SUN SONG2, and JAE WOOK LEE1*

1Applied Rheology Center, Dept. of Chemical EngineeringSogang University

1 Shinsu-Dong, Mapo-Gu, Seoul, 121-742, Korea

2R&D Center, Kolon Chemical Co. Ltd.294, Gajwa-Dong, Seo-Gu, Incheon City, 404-250, Korea

Epoxy resin-layered silicate nanocomposites (ER-LSN) were successfully synthe-sized using pre-intercalated novolac resin-layered silicate nanocomposites (NR-LSN)as epoxy hardeners to develop novel formulations for copper-clad laminate sheets(CCLS) with enhanced properties. We used melt, melt-ultrasonic, solution, and so-lution-ultrasonic intercalation methods to prepare NR-LSN with different kinds ofnovolac resins including phenolic novolac (PN), bisphenol-A novolac (BN) and phenolicaralkyl novolac, so-called Xylok (XK). We employed different kinds of montmorillonites(MMT) that were organically modified with benzyl dimethyl octadecyl ammonium andbis(2-hydroxy-ethyl)methyl tallow ammonium. It was confirmed that the synthesizedNR-LSN and ER-LSN were intercalated or exfoliated, and exhibited stable structureowing to the effective action of high-intensity ultrasound. These materials showed sig-nificantly improved thermal and mechanical properties suitable for halogen-free CCLSwith highly reliable performance. Polym. Eng. Sci. 44:1170–1177, 2004.© 2004 Society of Plastics Engineers.

INTRODUCTION

It is easy to find electric appliances operated by printedcircuit boards, such as TVs, cellular phones, and

computers. These printed circuit boards are made ofcopper-clad laminate sheets (CCLS), which are com-monly prepared in the form of “glass-epoxy” or “paper-phenolic” composites in view of their cost, productivity,and properties.

There are two main problems to be solved in the“glass-epoxy” CCLS industry. One is related to thehigher reliability of printed circuit boards. As the pitchsizes and dimensions of circuits are getting smaller,highly improved properties such as moisture resistance,thermal resistance, and lower dielectric constant are re-quired. The second problem concerns the regulation ofusing halogenated compounds because halogenated

(brominated) epoxy resins have been commonly used inglass-epoxy CCLS as matrix polymers to meet flame-resistance requirements. Thus, manufacturers of CCLShave faced strong pressure to eliminate brominatedepoxy resins in their formulations without losing de-sired properties. Therefore, a number of studies haveattempted to solve these problems by modifying epoxyresin (1, 2), or by changing hardener types and fillers(3, 4).

Recently, o-cresol novolac epoxy resins (CNE) wereintroduced for high-performance CCLS applicationsbecause of their improved thermal and mechanicalproperties as well as their excellent chemical resist-ance (5, 6).

It has been difficult to enhance the properties ofCCLS based on epoxy formulations comprising dicyan-diamide (DICY) hardener. Through chemical variationof the phenolic hardener, however, it appears possibleto improve the properties in glass-epoxy CCLS. Thusepoxy/phenolic systems instead of epoxy/DICY havebeen investigated because of their capability to improveproperties (7�10). These property enhancements de-rive from changes in cured network structures.

*To whom correspondence should be addressed. E-mail: [email protected]© 2004 Society of Plastics EngineersPublished online in Wiley InterScience (www.interscience.wiley.com).DOI: 10.1002/pen.20110

Epoxy-Phenolic Novolac-Montmorillonite Nanocomposites

POLYMER ENGINEERING AND SCIENCE, JUNE 2004, Vol. 44, No. 6 1171

Since Toyota researchers reported that polymer-lay-ered silicate nanocomposites (PLSN) as nanometer-scale reinforcements of the clays offer unprecedentedmechanical property improvements for nylon (11, 12),many PLSN’s have been synthesized for numerouspolymers (13�25). The conventional epoxy resin-lay-ered silicate nanocomposites (ER-LSN), however, havea disadvantage in storage stability, which limits its usein CCLS applications because ion-exchanged ammo-nium ions on the layered silicates play an undesirablerole of epoxy hardeners. Therefore, the motivation ofthis study is to investigate a new ER-LSN synthesistechnique using pre-intercalated novolac resin-layeredsilicate nanocomposites (NR-LSN), which can resolvethe storage stability problems.

Ultrasound is widely used in chemical synthesis andmany processes, especially in emulsification, catalysis,homogenization, disaggregation, scission, and disper-sion. Propagation of high-intensity ultrasound in liquidsleads to cavitation; that is, the formation and collapse ofmicrobubbles (26�28). Because small gas bubbles dis-organize the structure by weakening the molecularforces within liquids, we introduced high-intensity ultra-sound to prepare well-dispersed, more highly interca-lated nanocomposites.

In this study, we utilized all the performance benefitsof CNE, phenolic resins, PLSN, and high-intensitypower ultrasound to produce CCLS with enhancedproperties. We developed a new technique of EP-LSNsynthesis using various pre-intercalated NR-LSN ashardeners of CNE. It was found that high-intensityultrasound is an effective tool for the synthesis ofPLSN prepared by the melt-ultrasonic or solution-ul-trasonic process. Characteristics of CNE-LSN synthe-sized by our new technique were determined by per-forming X-ray diffraction, differential scanningcalorimetry, thermogravimetric analysis, dynamic me-chanical analysis, stress-controlled rheometry, and auniversal testing machine in three-point bending andtensile modes.

EXPERIMENTAL

Materials

The layered silicates used in this study were commer-cially purified and organically treated MMT; dimethylbenzyl hydrogenated tallow quaternary ammoniummodified MMT (Cloisite 10A, referred to as CLAY-A)with cation exchange capacity (CEC) of 125 meq/100 g,and bis(2-hydroxy-ethyl)methyl tallow quaternary am-monium modified MMT (Cloisite 30B, referred to asCLAY-B) with 90 meq/100 g of CEC, purchased fromSouthern Clay Products Inc.

The epoxy resins were also commercially availableCNE (YDCN-500-4P) from Kukdo Chemical Co. Thecommercial grades of novolac resins such as phenolicnovolac (PN, KPE-F2000), bisphenol-A novolac (BN,KBH-F2121), and phenolic aralkyl novolac, so-calledXylok (XK, KPH-F3065) from Kolon Chemical Company,were used as hardeners for CNE. Triphenylphosphine

(TPP) from Junsei Chemical Company was used at alevel of 0.5 phr to the total binder as a catalyst.

Preparation of NR-LSN

Three types of NR-LSN were prepared by direct meltintercalation, melt-ultrasonic, and solution intercala-tion methods with three different novolac resins of PN,BN and XK.

For the direct melt-intercalation process, novolacresins were mixed with a desired amount of CLAY-A orCLAY-B at 160°C for 3 hours in a kettle with a me-chanical stirrer. The amount of layered silicate wascontrolled at 5 phr (or 2.5 phr) based on the totalbinder. Prepared NR-LSN were degassed in a vacuumoven for 30 minutes at 160°C and poured into a stain-less steel tray. After cooling to room temperature, NR-LSN were crushed and stored in a refrigerator.

To confirm the effect of high-intensity ultrasound ondispersion and intercalation of the layered silicates,NR-LSN were synthesized in an internal mixer (HaakeRheocord 90 with Rheomix 600) with 5 minutes of son-ication under 90% filling conditions at 160°C. Themixer has a specially designed horn to impose high-in-tensity ultrasound of 20 KHz frequency with an ampli-tude of 15 �m. A schematic illustration of the device isgiven in Fig. 1.

Different kinds of NR-LSN were prepared by solutionintercalation method using various solvents such aswater, methyl alcohol, acetone, methyl ethyl ketone,xylene, toluene, glycol ether acetate, and benzyl alco-hol. High-intensity ultrasound was also imposed dur-ing the synthesis of NR-LSN.

Fig. 1. Schematic diagram of internal mixer with speciallydesigned ultrasonic horn.

Preparation of CNE-LSN

CNE and synthesized NR-LSN were melted sepa-rately in a forced convection oven at 120°C for CNE,PN-LSN, and XK-LSN, and at 160°C for BN-LSN for 1hour. The molten CNE and NR-LSN were mixed to-gether in a kettle and then mechanically stirred vigor-ously for 3 minutes for CNE-LSN synthesis. The moltenmixtures were poured into a specially designed mold,and subsequently annealed to eliminate voids andmoisture in a vacuum oven at 120°C for 15 minutes.The evacuation temperature was maintained above theglass transition temperature to ensure enough mobilityof resins, while limiting premature curing. Finally,these uncured mixtures were compression molded andsimultaneously cured on a hot press. Samples weremolded at 180°C for 1 hour under 30 Kgf/cm2 pressureand then post-cured at 150°C for 3 hours. The dimen-sion of the molded ER-LSN sample was 2 mm � 10 mm� 75 mm.

Various samples of NR-LSN and ER-LSN were pre-pared and conveniently named according to the follow-ing notation. For NR-LSN, the kind of NR appears asthe first, the kind of clay as the second, and then thekind of processing method as the third. The processingmethod of “M” stands for melt intercalation and “MS”stands for melt intercalation with high-intensity powerultrasound. For ER-LSN notation, the kind of ER ap-pears as the first followed by the NR-LSN notation. Forexample, “XK-B-MS” is the XK-LSN with CLAY-B,which is melt-ultrasonic synthesized and “CNE-XK-B-MS” is the CNE cured with the above-mentioned XK-B-MS. We also prepared neat samples of CNE cured bydifferent NR without the layered silicates, which wereused as references, and noted “CNE-XK” for CNE curedby XK hardeners, for example.

Morphological Investigation

With an X’PERT MPD X-ray diffractometer fromPhilips Co., changes in the basal spacing of preparedPLSN were measured at room temperature. CuK � irra-diation with � � 0.154056 was used, and diffractionpatterns were obtained from 2� range of 1.5�10° at arate of 2°/min.

Thermal Analysis

Differential scanning calorimeter (DSC) studies forthermal characterization were performed on a MDSC-2910 (TA Instruments) to obtain the glass transitiontemperature (Tg ) and heat of cure, and to confirm thecompleteness of curing. The heating rate was10°C/min. The materials were scanned from 20°C to300°C, and the scans were repeated at least twice toverify reproducibility of the measurements. Thermalstability of crosslinked CNE-LSN was also evaluatedusing a thermogravimetric analyzer (TGA, TA Instru-ments Model 2950) at a heating rate of 20°C/min. Thestorage modulus, loss modulus and damping factor(tan) of PLSN were measured with a Dynamic me-chanical Analyzer Q800 (TA Instruments) from 20°C to

300°C at a scan rate of 4°C/min. The dual cantileverbeam test mode was used at a frequency of 1 Hz with0.1% strain.

Rheological Measurements

A stress-controlled rheometer (CVO, Bohlin) wasused to characterize viscoelastic properties of PLSNspecimens. A plate-plate fixture with 25 mm diameterand 1.0 mm plate gap was used. PLSN specimens weretested at a mode of shear rate sweep at 170°C.

Mechanical Property Measurements

Mechanical properties of crosslinked CNE-LSN weremeasured using a universal testing machine (UTM,Model LR5K plus of Lloyd Instruments). Three-pointbending and tensile mode were performed withcrosshead speed of 50 mm/min. All the reported re-sults are averages of 7 measurements at least.

RESULTS AND DISCUSSION

The test results of thermal properties, tensile strengthand water absorption for neat CNE specimens cured byPN, BN and XK without the clays are summarized inTable 1. The water absorption values indicate the in-creased weight percent of specimens by an immersiontest in water for 7 days. CNE-BN has the lower waterabsorption value than CNE-PN and CNE-XK. It alsoshows the lowest cure exotherm compared with CNE-PN and CNE-XK. CNE-XK has poor storage stabilitycompared with others as judged by the change in heatof cure after 4 months. It was also revealed that CNE-XK exhibits the lowest Tg whereas CNE-PN shows thehighest Tg. Accordingly, we suggest that PN and/or BNare better hardeners than XK for CCLS applications be-cause of their lower water absorption value, higher Tg,tensile strength and storage stability. However, a majordisadvantage was found for PN in CCLS applications: afish-eye defect occurred on the glass-epoxy prepregsurface when it was dried after impregnation. This fish-eye defect is a fatal problem especially in very thinCCLS below 1 mm thickness. Thus we regard BN as thebest hardener for CNE, even though we expect that thefish-eye defect can be eliminated in PN-LSN by intro-ducing layered silicates as leveling additives, like otherinorganic fillers eliminating such defects.

Figure 2 represents the X-ray diffraction patterns ofCLAY-A, CLAY-B, PN-A-M, PN-B-M, BN-A-M and BN-

Sang Min Lee, Tae Ryong Hwang, Yang Sun Song, and Jae Wook Lee


Table 1. Comparison of Properties of CNE-LSN SpecimensCured by PN, BN, and XK Without Any Silicates.

Heat of Heat of Tensile WaterCure*1 Cure*2 Tg Strength Absorption

Type (J/g) (J/g) (°C) (MPa) (%)

CNE-PN 239.7 205.3 149.85 39.87 2.131CNE-BN 165.7 156.9 143.13 74.15 1.559CNE-XK 183.1 147.9 116.18 52.84 2.227

*1 Measured immediately.*2 Measured 4 months later.

B-M. CLAY-A and CLAY-B indicate very strong (001)reflection peaks at 2.26° and 2.42°, corresponding tolayer d-spacings of 1.95 nm and 1.82 nm, respectively.The reflection peaks of PN-A-M and BN-A-M shift tolower angles, 3.39 nm all together, and their peak in-tensities are very weak compared with the case ofCLAY-A. The increase in d-spacing of CLAY-A suggeststhat molecules of PN and BN are intercalated into thelayers of CLAY-A during the melt-intercalation process.Moreover, the reduction in peak intensities indicatesthat the phase structures of PN-A-M and BN-A-M areclose to highly intercalated or nearly exfoliated states.

The reflection peaks of PN-B-M and BN-B-M also shiftto lower angles, 3.39 nm and 4.01 nm, respectively. Asdetermined from the d-spacing difference between PN-B-M and BN-B-M, it appears that BN molecules aremore easily incorporated into the interlayer of CLAY-B.The peak intensities are even weaker than that of CLAY-B, suggesting exfoliated structure in BN-B-M.

The larger difference of d-spacing in PN-B-M (1.57nm) than PN-A-M (1.44 nm) suggests that molecules ofPN are more easily intercalated into CLAY-B thanCLAY-A. It reveals that layered silicates of CLAY-B withhydroxyl functionality are more favorable to PN thanCLAY-A with benzyl group to synthesize the PN-LSN,which is consistent with the results found in the BN-LSN system.

This unexpected result can be compared to Lan andPinnavaia’s work, where it was reported that longeralkyl ammonium chains facilitate the formation ofPLSN in epoxy resins and MMT (29). Even thoughlonger chains are more compatible with polymers thanshorter ones in general, the more important factor isthe chemical affinity between the silicates and poly-mers, although the chain length of CLAY-B is a littleshorter than that of CLAY-A.

To achieve fully exfoliated structures of NR-LSN, we

applied high-intensity ultrasound during the synthesisof NR-LSN. The effects of high-intensity ultrasound onNR-LSN structures are shown in Fig. 3a for CLAY-Aand Fig. 3b for CLAY-B, respectively. In Fig. 3a, PN-A-M shows two reflection peaks, one strong and the otherweak, corresponding to the layer d-spacings of 1.65 nmand 3.25 nm, respectively. Also, high-intensity ultra-sound imposed PN-A-MS shows very similar reflectionpeaks at 1.69 nm and 3.39 nm, respectively. The en-larged results show somewhat more highly intercalatedstructures of PN-A-MS, which have been promoted byhigh-intensity ultrasound. As shown in Fig. 3b, PN-B-Mhas reflection peaks corresponding to 1.62 nm and 3.37nm, respectively, whereas ultrasound imposed PN-B-MS has peaks at 1.72 nm and 3.56 nm, respectively.



Fig. 2. X-ray diffraction patterns of layered silicates and vari-ous NR-LSN. (a) CLAY-A; (b) CLAY-B; (c) PN-A-M; (d) PN-B-M; (e)BN-A-M; (f) BN-B-M.

Fig. 3. X-ray diffraction patterns of PN-LSN with and withoutultrasound. (a) PN-A-M and PN-A-MS; (b) PN-B-M and PN-B-MS.

(a)

(b)

The increments of d-spacing caused by high-intensityultrasound are 0.10 nm and 0.19 nm, respectively.Even though the changes in d-spacing are relativelysmaller than our expectation, we verified the effect ofhigh-intensity power ultrasound on structures of PN-LSN, as already confirmed for thermoplastic polymersin our previous works (27, 28).

The XRD patterns of XK-LSN treated by high-inten-sity ultrasound are shown in Fig. 4. It shows that ultra-sound imposed XK-B-MS is nearly exfoliated, whereasXK-B-M sustains an intercalated structure.

Figure 5 represents the viscosities of PN-LSN. Ultra-sound imposed PN-B-MS shows the highest viscositycompared with the melt-intercalated PN-B-M and neatresin of PN, which is the result of the well-dispersed

and highly intercalated structure of CLAY-B in PN-B-MS, due to high-intensity ultrasound treatment. Al-though viscosity increase is not recommended for mostpolymer processes, the viscosity increase of PN-LSNwill not have a significant effect on the CCLS processbecause CCLS manufacturing is mainly carried out ina low viscous solution state.

Figure 6 represents the XRD patterns of CNE-PN-B-M, CNE-PN-B-MS-uncured, CNE-PN-B-MS-2.5 phr,and CNE-PN-B-MS. The d-spacing of CNE-PN-B-M is3.65 nm, whereas that of PN-B-M is 3.39 nm asshown in Fig. 2. The increment of 0.26 nm in basalspacing between them means that CNE can easilypenetrate into PN-B-M gallery before the cure processis completed. In addition, CNE-PN-B-MS-uncuredshows a smooth peak at 3.42 nm, which disappearedand weakened in CNE-PN-B-MS-cured. The observedresults can be explained in terms of chemical affinitybetween CNE and CLAY-B, between CNE and PN, andbetween CNE and PN-LSN, although it is not clearwhich is the most favored pair among them. The rea-sons for different X-ray diffraction patterns in curedand uncured CNE-PN-B cannot be clarified at thisstage; it seems that a possibility may be associated withthe changes in chemical affinity during the cure pro-cess. However, it is clear that the intercalated and/orexfoliated structures of PN-B are not destroyed when itwas mixed with CNE as a hardener. This is a reasonwhy pre-intercalated NR-LSN can be used for synthesisin ER-LSN as epoxy hardeners.

As demonstrated in Figs. 6a and d, melt-intercalatedCNE-PN-B-M indicates relatively strong (001) reflectionpeak at 1.21°, corresponding to a layer spacing of 3.65nm, whereas the reflection peak of melt-sonicated CNE-PN-B-MS shifts to a lower angle of enlarged gallery dis-tance corresponding to 3.94 nm; also, peak intensity isweaker than that of CNE-PN-B-M. Thus high-intensityultrasound seems to play an important role in promot-ing exfoliation of PLSN.

It is interesting to note in Fig. 6 that CNE-LSN con-taining 2.5 phr of CLAY-B shows exfoliated character-istics as exhibited in Fig. 6c, even though CNE-LSNwith 5.0 phr has less exfoliated structures and a weakintensity peak at 3.94 nm. This difference can be ex-plained by aggregation of the layered silicates, whichhinders the formation of a fully exfoliated structure athigher contents of the clays. If it is correct, we can sug-gest that lower angle peaks having relatively small in-tensity as shown in the previous figures are caused bycollisions of the layered silicate platelets.

The curing reaction rate and intercalation rate in thecourse of formation of crosslinked CNE-LSN are impor-tant parameters for determining the final morphologiesof CNE-LSN. If the intercalation rate is comparable toor faster than cure reaction rate, it is very easy to syn-thesize CNE-LSN by simple mixing with CNE, clays, andPN hardeners. However, the intercalation rate stronglydepends on the temperature concerning mobility of theresins, and also depends on the reactivity between NR-LSN and epoxy resins. Thus, direct intercalation process



Fig. 4. X-ray diffraction patterns of XK-B with and without ultra-sound.

Fig. 5. Shear rate sweep rheograms of PN and PN-LSN withand without ultrasound.

by simple mixing cannot be recommended for all epoxyresins. Furthermore, if the silicates containing alkylammonium ions are mixed with epoxy resins, the alkylammonium ions may act as catalysts and/or curingagents of epoxy resins. Because of the above-men-tioned issues, we suggest that our indirect synthesis ofER-LSN, using pre-intercalated NR-LSN as epoxy hard-eners, is a better way than that of conventional directsynthesis of ER-LSN by simple mixing. Because thealkyl ammonium functionalities on silicates may besurrounded with pre-intercalated phenolic novolacmolecules prior to epoxy molecules, the undesirablecure reaction between ammonium groups and epoxyresins can be prohibited. This masking behavior of phe-nolic novolac molecules makes our ER-LSN, which isindirect synthesized via pre-intercalated NR-LSN, have

longer time to exfoliate before the cure, also have betterstorage stability than direct synthesized ER-LSN.

The effect of high-intensity ultrasound in cure be-havior of CNE-PN-B-M is depicted in Fig. 7. It is ob-served that the cure behavior of melt-sonicated CNE-PN-B-MS is very similar to that of melt-intercalatedCNE-PN-B-M. It is considered that degree of dispersiondoes not influence the cure process, and the ammo-nium groups on silicates do not act as cure acceleratorsin CNE-PLSN. These findings demonstrate that our in-direct synthesis technique improves storage stability ofcommon ER-LSN.

The enhanced mechanical properties in ER-LSN suchas Young’s modulus, elastic strength, and toughnessare illustrated in Table 2. Brown et al. reported that abalance between the rate of intercalation and rate ofcrosslinking must be considered in thermoset PLSNsynthesis, especially for the simultaneous intercalationand curing system such as ours (30). They also sug-gested that the intercalation process would be frozen atthe gel point associated with network formation ofepoxy resins. Thus, it is very difficult to synthesize ER-LSN by the simultaneous processes of intercalationand curing due to gelation. However, all of our CNE-LSN samples demonstrate strikingly improved charac-teristics in their mechanical properties. In the case ofYoung’s modulus, CNE-PN-B-MS has 3692 MPa,whereas the neat sample of CNE-PN has 2163 MPa.CNE-PN-B-MS shows approximately 70% higher valuethan the neat sample. Similarly, elastic strength ofCNE-PN-B-MS is 43.62 MPa, which is approximately180% increase from that of neat system. Moreover, it isnoted that our CNE-LSN shows even higher improve-ments in their toughness than neat sample. The tough-ness is 0.03562 MPa for CNE-PN-B-MS, and 0.03747MPa for CNE-XK-A, respectively, which is about 2500%increase from 0.00140 MPa of neat CNE-PN. These un-precedented improvements in their mechanical proper-ties are caused by intercalated or exfoliated morpholo-gies of them. These improved mechanical properties arevery important for high reliability of CCLS formula-tions, especially for heavy device-mounted CCLS appli-cations.

DMA test results as illustrated in Table 3 also revealthat dramatically improved thermal and mechanicalproperties of our CNE-LSN. Tg’s, which are determinedby peak point of tan, are 108.21°C and 108.91°C forCNE-PN-B-M and CNE-PN-B-MS, respectively, whereasTg of neat CNE-BN without any silicates is 56.81°C. The



Fig. 6. X-ray diffraction patterns of CNE-PN-B systems. (a)CNE-PN-B-M; (b) CNE-PN-B-MS-uncured; (c) CNE-PN-B-MS-2.5phr; (d) CNE-PN-B-MS.

Fig. 7. Viscosity changes due to the cure of CNE-PN-B.

Table 2. Mechanical Properties of CNE-PN-B Measured by UTM.

Young’s Flexural ElasticModulus Rigidity Strength Ductility Toughness

Type (MPa) (Nm) (MPa) (mm) (MPa)

CNE-PN 2163 0.0207 2.429 0.4149 0.00140CNE-PN-B-M 2816 0.0233 35.93 1.887 0.02953CNE-PN-B-MS 3692 0.0305 43.62 1.852 0.03562CNE-XK-A-MS 2974 0.0286 28.61 2.162 0.03747

increases in Tg are somewhat higher than our expecta-tion, which is because of the structures of nano-scalecomposites of CNE-LSN.

TGA thermograms show that CNE-BN-B decom-posed at 432.17°C, whereas neat sample of CNE-BNhas the decomposition temperature of 428.24°C. Thisimproved thermal decomposition behavior is very im-portant in formulating halogen-free formulations ofCCLS; that is our main goal, by use of the advantagesof the barrier properties of PLSN.

Figure 8 shows difficulties in solvent selectivity of thelayered silicates in solution intercalation of PN-LSN.The samples of PN-LSN in Fig. 8 are made of acetone forsample-Q, methyl ethyl ketone for sample-R, water forsample-S and xylene for sample-T, respectively. Themixing ratio is 5 wt% of CLAY-B on solvents. Sample-Sshows perfectly separated, precipitated CLAY-B inwater, and sample-Q and sample-R show paste struc-tures caused by swelling of the layered silicates by sol-vents with a little precipitation. Sample-T of xylene,however, exhibits transparent structure, which is re-garded as exfoliated morphology, which emphasizesthe importance of solvent selection in solution interca-lation process.

The effects of high-intensity ultrasound on storagestability of the layered silicate dispersed solution areshown in Fig. 9. All PLSN samples of PN-B in Fig. 9 are

prepared with the same solvent, PN resins, and compo-sitions. The photographs were taken after 7 days stor-age without any agitation. The samples differ from themethodologies in manufacturing; NC-29 is prepared byonly vigorous mixing of PN, CLAY-B and solvent, NC-5is by solution of PN-B-M in solvent, NC-27 is by solutionof PN-B-MS in same solvent, and NC-36 is by imposinghigh-intensity ultrasound on the previous NC-27. NC-29 shows perfectly separated and precipitated struc-ture, NC-5 and NC-27 exhibit partially precipitatedstates, whereas NC-36 reveals perfectly exfoliated stateand enhanced storage stability due to high-intensity ul-trasound. Figure 9 shows the limitation of solution in-tercalation of pre-intercalated PN-LSN from the resultsof NC-5 and NC-27 because there are new factors re-lated to chemical affinities among the layered silicates,polymers and solvents in solution intercalation of PLSN.However, we suggest that high-intensity ultrasound isalso very effective in solution intercalation as well asmelt intercalation as seen from the comparison betweenNC-29 and NC-5, and that between NC-27 and NC-36.

In our systems, there are four major and distinct fac-tors to be considered for PLSN: intercalation, cure,high-intensity ultrasound, and solvent selectivity. Theintercalation in the layered silicates can be interferedwith curing of epoxy resins. Therefore, it is concludedthat rate of cure of epoxy resins is a very important fac-tor as well as rate of intercalation of the layered silicatesto improve properties. Moreover, to prevent layered sili-cates from aggregation, imposing high-intensity ultra-sound on PLSN is very effective in enhancing their in-tercalated morphology and their storage stabilities.



Table 3. Thermal and Mechanical Properties ofCNE-LSN Measured by DMA.

Tg Storage ModulusType (°C) (MPa, 40°C)

CNE-PN n56.81 981CNE-PN-B-M 108.21 3197CNE-PN-B-MS 108.91 3150CNE-PN-A-MS 148.88 2765

Fig. 8. Photographs of solution intercalated PN-LSN with vari-ous solvents.

Fig. 9. Photographs of solution intercalated PN-LSN with dif-ferent synthesis methods.



CONCLUSIONS

We prepared different kinds of NR-LSN by melt-inter-calation and solution-intercalation methods for highlyreliable CCLS applications. Also various kinds of CNE-LSN having intercalated and/or exfoliated morphologywere synthesized by melt-intercalation methods withpre-intercalated NR-LSN as epoxy hardeners under theconditions with or without high-intensity ultrasound.We also investigated thermal and mechanical proper-ties of CNE-LSN produced from various processes.

From this study, it is suggested that chemical affinitybetween phenolic resins and organically modified MMTis a very important factor in preparing NR-LSN as wellas ER-LSN. The hydroxyl modified MMT named CLAY-B is better than the MMT having a benzene groupsnamed CLAY-A to synthesize exfoliated PLSN of PN,BN, and XK resins.

We demonstrated effects of high-intensity ultra-sound on the synthesis of PLSN via melt-intercalationand solution-intercalation for well-dispersed andhighly intercalated PLSN without any precipitated lay-ered silicates.

All our pre-intercalated NR-LSN are very stable dur-ing curing process with epoxy resins, so their interca-lated/exfoliated structures are not destroyed duringcuring processes. They also exhibit the tendency to beexfoliated during simultaneous intercalation and cur-ing processes for CNE-LSN synthesis.

We confirmed that our new developing methods usingpre-intercalated NR-LSN for ER-LSN are very useful toprevent alkyl ammonium ions on the silicates from un-desired cure acceleration reaction with epoxy resins.

The dramatic enhancements of mechanical proper-ties such as toughness were observed. In addition, im-proved thermal properties including glass transitiontemperatures and thermal decomposition tempera-tures were also confirmed.

All improved properties of ER-LSN are attributed todifferent morphologies of intercalation and exfoliation.Also these property improvements are associated withcuring reaction between epoxy resins and phenolic res-ins, which is affected by the presence of layered sili-cates. Therefore, we propose that a balance betweenrate of intercalation and rate of cure must be consid-ered for PLSN synthesis in thermosetting polymers, es-pecially for simultaneous intercalation and curing sys-tem such as ours.

It is emphasized that one of the most important fac-tors in ER-LSN synthesis is concerned with their cur-ing characteristics. We found the possibility that PLSNwith their improved barrier properties produced by ournew synthesis methods is suitable for halogen-freeCCLS applications and other systems demanding highreliability.

ACKNOWLEDGMENT

Financial support of the Korea Institute of IndustrialTechnology Evaluation and Planning (ITEP), and the

special grants of Sogang University are gratefully ac-knowledged.

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