Blending of commercial plastic materials is a tra-ditional and economical way to produce new sys-tems having desirable properties. Ultimate prop-erties of end products are heavily dependent onthe chemical structure and physical properties ofbasic constituents and on processing conditions.All these characteristics, in fact, dictate the fea-tures of the achieved morphology.1,2 However, inmany cases, the applicability of these systems canbe strongly limited by the usual mutual incom-patibility between different polymers. Thus, the
drawback is generally overcome considering blendsbased on interactive polymers3–5 or blends preparedby using opportune additives6,7 (graft or block co-polymers acting as compatibilizers) or by reactivemixing (the compatibilizer is formed in situ).8,9
Compatible blends show properties different fromthose of starting homopolymers and the occurrenceof typical synergism plays a fundamental role forpractical applications in many industrial fields.
In the list of compatible blends, formulationsbased on nylon 6 (Ny6) and ethylene-vinyl alcohol(EVOH) copolymers can be properly included. Theinherent chemical structure of these two poly-meric materials assures the compatibility of finalblends since the occurrence of hydrogen bonding-type interactions between amide groups of nylonand hydroxyl groups of EVOH copolymers, as al-ready demonstrated elsewhere.10
The technological interest toward Ny6/EVOHformulations having the EVOH as the dispersedphase can be justified especially for packagingapplications. In fact, polyamides are engineeringsemicrystalline thermoplastics with attractivemechanical properties, low permeability with re-spect to CO2, while EVOH copolymers are semi-crystalline materials showing excellent oxygenbarrier properties.
Moreover, previous studies have clearly shownthat more entangled morphologies are obtained inthe Ny6-rich region of the phase diagram.10
In this paper, attention was paid to study howthe composition affects the properties of the Ny6/EVOH system and the results were interpreted interms of specific interactions between the compo-nents. Rheological, morphological, thermal, andmechanical analysis and gas permeability testshave been performed. Peculiar results have beenobtained for Ny6-rich formulations and especiallyfor the 75/25 w/w Ny6/EVOH composition.
EXPERIMENTAL
Materials
The polymers used were a film grade polyamidenylon 6 supplied from SNIA Tecnopolimeri underthe code Ny6 F34L (hrel 5 3.4 at 20°C in sulfuricacid) and an ethylene-co-vinyl alcohol (EVOH)copolymer produced by NIPPON GOHSEY andcontaining about 29% in moles of ethylene. Thebulk density of the materials was 1.19 and 1.14g/cc for EVOH and Ny6, respectively. Due to theirhygroscopic nature, both polymers were driedovernight in a vacuum oven at 80°C prior to use.
Blends Preparation and Film Blowing Process
A dry blending of Ny6 and EVOH pellets, havingapproximately the same particle size, was ef-fected by hand mixing the polymers in a box inthe opportune weight ratio. The melt mixingstage, to obtain blends containing 10, 25, 50, 60,75% b.w. of EVOH copolymer, was realized usinga BRABENDER extruder (L/D 5 400/20) linkedto a head with a rectangular 20-mm-wide flat dieoperating at a screw speed of 20 RPM. A take-upspeed of 0.65 m/min and a temperature profile of230–240–240°C from the hopper to the head werefixed. For comparison, the same procedure wasalso applied on pure constituents.
The extruded strips of material were groundwith a blade mill (RETSCH, Muhle, Germany)and vacuum dried for the film-blowing processand rheological measurements. Films were pro-duced by using the same extruder employed inthe mixing step, fitted with a 20-mm annularblown film die and a HAAKE take-up system. Theextrusion was carried out at 20 rpm using thethermal profiles indicated in Table I. The choice ofthese operative conditions was dictated by thebubble stability. The blow-up ratio and the drawratio were fixed at values of 3 and 10, respec-tively.
The morphological analysis and the reproduc-ibility of the mechanical results obtained on sam-ples randomly cut along the film indicate that agood blending of the components was obtained.
Characterization Techniques
The presence of amide and OOH groups on themolecular structure of Ny6 and EVOH, respec-tively, make these polymers and their relativeblends very hygroscopic with substantial effectson their properties. In this paper, if not otherwisespecified, film data were collected on samples en-vironmentally conditioned for at least 2 monthsafter their production in order to monitor the realproperties of materials during their use.
Melt rheological measurements were carriedout using a capillary viscometer (CEAST Mod.Rheoscope 1000) at T 5 250°. A flat entry capil-lary with D 5 1 mm and a length/diameter ratio(L/D) equal to 40 was used and the Rabinowitschcorrection on the shear rate was performed. Carewas taken to minimize the effects of degradationand oxidation by quickly charging the dried ma-terials in the barrel under a nitrogen flow.
FTIR analysis was performed on thin filmsmounted on an infrared film holder with a PerkinElmer spectrometer mod. Paragon 500, at a reso-lution of 2 cm21. The films were prepared from
Table I. Composition and Thermal Profiles in FilmBlowing Extrusion for NY6/EVOH System
diluted solutions of blends (about 10% by weight)in DMAc containing 10% by weight of LiCl, bycasting on Teflon sheets. Sheets were put in avacuum oven at 80°C to allow the solvent to evap-orate. The recovered thin films were immersed inwater and repeatedly washed to remove the lith-ium chloride and finally dried in vacuum at 80°Cfor 24 h.
Scanning Electron Microscopy (SEM, Philipsmod. XL20) has been performed on films surfacesafter etching by immersion in a concentrated so-lution (40% b.w. in water) of tetrabutylammo-nium hydroxide (Aldrich reagent) at T 5 100°Cfor 5 h. Samples were coated with AuPd alloy.
Calorimetric measurements were carried outusing a DSC Mettler TC11, on the temperaturerange from 120 to 260°C and with a scanningrate of 20°C/min under nitrogen flux.
Dynamic mechanical measurements at 1 Hzwere made in tensile mode with a zero startingdeformation using a Polymer Laboratories dy-namic mechanical thermal analyzer MKIII appa-ratus. Tests were carried out at 4°/min in drynitrogen with a strain amplitude equal to 16 mm.Rectangular specimens 6 mm wide and 8 mm longwere used.
Mechanical tensile tests were performed by anInstron 4301 equipped with a 100 N cell. Thefilms were cut in the flow direction in rectangularstrips 10 mm wide and at least 120 mm long andtested with a cross-head speed of 50 mm/min anda grip distance of 100 mm. Tensile tests were alsorealized on specimens dried in a vacuum oven atroom temperature for about 2 h. At least tenspecimens of each film were tested to verify thereproducibility of the results. In this work, theaveraged values of Young Modulus (E) are re-ported. The standard deviation was approxi-mately equal to 20 for all the samples analyzed.
Permeability tests were carried out at 30°C bymeans of a Lyssy GPM 200 quasi-isostatic appa-ratus on films of Ny6, EVOH, and blends at 10,25, and 50% by weight of EVOH. The film perme-ability was measured with respect to CO2 and O2gases under steady-state conditions. All sampleswere conditioned for at least 12 h in the appara-tus, with dry He at 30°C.
RESULTS AND DISCUSSION
Rheological Analysis
The effect of the EVOH content on the rheologicalbehavior of the Ny6/EVOH system was analyzedby a capillary rheometer.
The flow curves, viscosity (h) vs. shear rate (g),for Ny6, EVOH, and their investigated blends areshown in Figure 1 on the range of shear rate goingfrom 10 to 1000 s21 at 250°C. All over the consid-ered range of g the viscosity of Ny6 is alwayshigher than that of EVOH [i.e., the ratio (K) be-tween the viscosity of the dispersed phase(EVOH) and the viscosity of the matrix (Ny6) isless than 1 in the whole shear rate range inves-tigated]. The viscosity of the blend with an EVOHcontent less than 50% by weight is higher thanthat of pure constituent polymers, while the vis-cosity values for the 40/60 and 25/75 w/w Ny6/EVOH blends lie between that of pure materials.
The viscosity as a function of composition atfour different shear rates is reported in Figure 2.
It is seen that at a certain blend ratio the bulkviscosity goes through a maximum. This maxi-mum corresponds to a blend having an EVOHcontent close to 25% by weight. At this purpose itis worth noting that, for our system, a simplestoichiometric calculation on the molar ratio be-tween the hydroxyl groups of EVOH and amidicgroup of Ny6 have shown that an almost stoichi-ometric ratio corresponds approximately to thecontent of 25% by weight of EVOH copolymer.Such observation, strictly valid only for com-pletely miscible systems, where molecular con-tacts exist between blend components, is an indi-cation that our system, at least in the amorphous,could be miscible, as already suggested by otherauthors.10 In the melt, this will give, in turn, adependence of properties from the stoichiometryof the interacting groups.
The existence of a maximum in the viscosity-composition dependence is generally verified in
Figure 1. Capillary melt viscosity vs. shear rate forthe pure polymers and the blends at T 5 250°C.
PROPERTIES OF NYLON 6/EVOH BLENDS 2447
presence of interfacial interactions or when thecomponent polymers are intimately mixed.
In Figure 2, experimental viscosity results dataobtained at g 5 26 s21 were compared with thosepredicted with the simplest method of determin-ing the blend viscosity at temperatures well aboveTg. This model is represented by the followingequation:
log h0 5 w1log h01 1 w2log h02 (1)
where h0, h01 and h02 are the zero shear rate, orlow shear rate, viscosities of the blend, Ny6 andEVOH, respectively, and w1 and w2 are theweight fractions of the pure constituents. The vis-cosity data calculated by means eq. (1) are repre-sented in Figure 2 by the straight line.
Comparing the experimental data with thosecalculated with eq. (1), it is shown that a positivedeviation is verified for all the blends investigatedat g 5 26 s21. Moreover, at a fixed Ny6/EVOHcomposition, the entity of the positive deviationbecomes greater as the shear rate decreases, ow-ing to the major effect manifested by the interfa-cial hydrogen bonding interactions, at low shearrates.
It is important to point out that the inclusion of10 and 25% by weight of EVOH modifies also theshape of the flow curve compared to those of purepolymers. In fact, while Ny6 and EVOH materialsshow a Newtonian behavior at low shear rates,
the 90/10 and in particular the 75/25 w/w blendsare highly shear thinning in the whole shear raterange examined. Increasing the EVOH content to60% by weight, the shape of the flow curve ap-proaches that of pure materials and the viscosityvalues of 40/60 and 25/75 w/w Ny6/EVOH blendslie between those of the pure constituents. Sincethe rheology of blends depends on their morphol-ogy, these results indicate that in the neighbor-hood of the 50% of the EVOH content a concen-tration-dependent change in morphology occurs.The reported rheological behaviors show the crit-ical role of the EVOH concentration in determin-ing the level of mutual interaction and the mor-phology of the blends investigated. They indicatethat in blends having Ny6, as the matrix there isa higher tendency to hydrogen bonding interac-tions than in EVOH matrix formulations. On theother hand, in the first case, a fine phase disper-sion can be obtained also due to the fact that theNy6 with a higher viscosity can transfer the shearforce more easily onto the softer EVOH phase.Further morphological features regarding Ny6-rich formulation will be discussed later in thiswork.
FTIR Analysis
Keeping in mind the rheological results, FTIRspectroscopy was carried out with the aim of iden-tifying a dependence of specific interactions on
Figure 2. Melt viscosity vs. blend composition for Ny6/EVOH system at differentshear rates. (The additivity rule is depicted as a straight line at a shear rate of 26 s21.)
2448 INCARNATO ET AL
blend composition. Thin films for infrared analy-sis were prepared by casting from diluted solu-tions obtained by dissolving melt prepared blendsin DMAc/LiCl. The casting procedure is the onlyone that permits obtaining a very thin film, suit-able for a quantitative FTIR analysis. In fact,films obtained by melt-pressing are too thick. Thefollowing considerations on solvent cast filmshould hold also on films prepared from melt ifone takes into account the high compatibility ofNy6 and EVOH in the melt.10 Venkatesh et al.13
reported a similar procedure using formic acid asa solvent and found a partial esterification ofOOH groups of EVOH to format groups.14 As thenonlinear behavior of blends properties suggestedthat the stoichiometric ratio between vinyl alco-hol and amidic groups is of fundamental impor-tance, we preferred to avoid formic acid and chosean organic polar solvent, DMAc, containing 10%by weight of lithium chloride dissolved in it,which is able to solubilize both Ny6 and EVOH attemperatures $ 100°C, without any chemical in-teraction with the polymers. Particular care wastaken to completely remove all solvent and saltfrom the films. No indication of ester formationwas found in the spectra, as a possible result ofchemical interaction betweenOOH of EVOH andOCOOH end groups of Ny6. This event was al-ready excluded in a previous work15 where it wasshown how it is possible to recover selectively andquantitatively the EVOH portion of the blend bysolvent means.
FTIR analysis (see Fig. 3 and Table II) shows a
Figure 3. FTIR transmission spectra in the 1800–1500 cm21 wavenumber range ofNy6/EVOH blends: (a) 90/10; (b) 75/25; and (c) 50/50.
Table II. Half-Height Width ( A) and FrequencyValue (F) of the Amide I Band fromInfrared Spectra of the Blends
Blend Composition (w/w) A (cm) F (cm21)
90/10 3.4 1638.475/25 2.9 1639.050/50 3.2 1637.8
PROPERTIES OF NYLON 6/EVOH BLENDS 2449
peculiar behavior of the amide I band (1639cm21): while there is a minimum shift (1–2 cm21)in the frequency value, a more pronounced
change in the width at half-height is observed,whose trend is nonlinear with the composition,but shows a minimum for the composition 75/25.
Figure 4. Surfaces after SEM micrographs (52003) of film etching: (A) Ny6/EVOH90/10; (B) Ny6/EVOH 75/25; and (C) Ny6/EVOH 50/50.
2450 INCARNATO ET AL
According to the theory of vibrational spectros-copy,16 the sharpness of a stretching vibrationcan be related to the number of allowed vibra-tions. Hence, it looks like that at 75/25 composi-tion a minimum in molecular motion is achieved,as a consequence of a maximum interaction of thepolar groups (carboxyl on Ny6 and hydroxyl onEVOH).
These observations prompted us to attempt asimple stoichiometric calculation on the molar ra-tio between the hydroxyl groups of EVOH andamidic groups onto Ny6. As already reported inthe present paper, this evaluation showed that atthe weight composition 75/25 an almost stoichio-metric ratio between the repetitive groups corre-sponds. Consequently, it can be reasonably as-sumed that we are in the presence of a specificinteraction that reaches its maximum at equimo-lar concentrations.
Morphological Analysis
Scanning electron microscopy has been effectedon the surface of films as obtained by blow-extru-sion. Surfaces have been immersed for a pro-longed time (5 h) in hot concentrated aqueoustetrabutylammonium hydroxide (about 100°C).The use of such a strong etching agent was found
to be necessary due to the highly interpenetratedsystem and to selectivity problems (preliminarytests have shown that such organic base onlydissolves the vinyl polymer in the blend).
Due to their small thickness, it was impossibleto analyze a cross-section of the films, whichwould have been more revealing of the internalstructure according to previously reported litera-ture.17 Nevertheless, clear differences amongfilms of different composition were found, as re-ported in Figure 4(A), (B), and (C). The featuresthat need to be emphasized are: at very lowEVOH content (90/10), no evidence of dispersedEVOH domains were found; at the 75/25 compo-sition, the etching reveals circular domains hav-ing an average diameter of 0.2–0.6 mm; at highEVOH content (50/50) still circular domains areobserved, with 1–3 mm dimensions. A differenceof about one order of magnitude for the two blendcompositions is once more a clear indication thatfor the 75/25 composition strong compatibility, oreven amorphous miscibility, does occur.
The above observations seem to indicate thatan EVOH content of 10% is not sufficient to causea clear phase separation. Increasing the EVOHcontent circular domains of copolymer are evi-dent. Clearly, the size of these domains dependsupon the concentration of EVOH, the viscosity
Figure 4. (Continued from the previous page)
PROPERTIES OF NYLON 6/EVOH BLENDS 2451
ratio of the two phases, the level of compatibility,and the method of processing. Moreover, theblend composition plays a key role varying thespecific interfacial area between the two poly-meric constituents and the deformability of thedispersed phase during the processing condition,thus affecting final morphology and, conse-quently, the eventual synergism between poly-mers mixed together. The increase of the EVOHcontent produces an increase in the size of dis-persed domains. For this system, in fact, both theincreasing of the concentration of the dispersedphase and the concomitant reduction of the mu-tual specific interactions are responsible for thelarger domain observed in the Ny6/EVOH 50/50w/w blend.
Moreover, moving from the Ny6-rich regions tothe EVOH-rich regions, the viscosity ratio be-tween the dispersed phase and the matrix goesfrom a value less than 1, as required for dropletdeformation, to a value greater than 1.
Thermal and Mechanical Characterization
The thermal behavior of the Ny6/EVOH systemhas been already analyzed in a previous paper18
in which particular attention was paid to the crys-tallinity behavior as a function of the blend com-position. The obtained results have indicated thatthe crystallinity of both components is onlyslightly modified under the applied blending con-ditions.
Analyzing in detail DSC thermograms of thewet samples reported in Figure 5, it is evidentthat all the materials investigated show a broadendothermic signal centered at about 90°C, indic-ative of loss of water. This broad endotherm isshifted to higher temperatures increasing theEVOH content, and it seems that the adsorbedwater is particularly restrained in the 75/25 Ny6/EVOH blend.
Since the water absorption preferentially inter-ests the amorphous regions19 owing to their loworder and poor packing density with respect to thecrystalline phases, the thermal behavior alreadyconsidered for the Ny6/EVOH 75/25 w/w blendcould be attributed to the occurrence of a tightamorphous structure. This structure can be at-tributed to the highest degree of intermolecularhydrogen bonding with respect to the other com-positions as also revealed by rheological and FTIRresults.
On the other hand, from DMTA experiments,we have found that the Tg values vs. the EVOH
content show a nonmonotonic trend with a posi-tive deviation from the mixing rule for Ny6-basedsystems and a negative deviation for systems con-taining an excess of EVOH copolymer. In partic-ular, from Figure 6, it appears that at 25% byweight of EVOH, a maximum in the glass-transi-tion value is verified. In light of FTIR data, thisbehavior can be attributed to stronger hydrogen-bonded interactions within the amorphous phaseat this composition.
Tensile mechanical tests were conducted onwet and dried samples. In Figure 7, Young Mod-ulus (E) values are reported as a function of com-position. It clearly appears that wet samples showlower E values and a negative deviation behaviorfrom the ideal log additive rule. As expected,19 theadsorbed water restrained in the amorphous re-gion tends to plasticize it and to reduce the me-chanical performance of the samples. Such aneffect assumes the highest extent for the 75/25Ny6/EVOH blend. In fact, as was previously ob-served, analyzing the thermal behavior of wet sam-ples, the adsorbed water is particularly trapped inthis blend. Figure 7 also shows the data of driedsamples. In this case, it clearly appears that E val-ues positively deviate from the ideal additive rule.Since the crystallinity content of the samples is not
Figure 5. DSC curves relative to the wet samples ofthe pure Ny6 (sample A), the blends with EVOH at 10%(sample B), 25% (sample C), 50% (sample D), and thepure EVOH (sample E).
2452 INCARNATO ET AL
significantly different, the observed trend can beattributed to the strong interactions among thecomponents in the amorphous phase.
In light of the above results, there seems to be acontradiction between the increase of Tg as recordedby DMTA measurements and the decrease of thetensile modulus reported for wet samples in Figure7. Actually, while the tensile tests were carried outin normal environmental conditions, the DMTA
tests have been performed in a flux of dry nitrogenand, during the measurement, this would partiallyallow a drying of thin samples analyzed. However,DMTA results are more consistent with the tensileresults obtained on dry samples.
Barrier Properties
The study of transport phenomena in polymerblends is motivated not only by the need to use
Figure 6. Glass-transition temperature vs. blend composition for wet films. (Theadditivity rule is depicted as a straight line.)
Figure 7. Tensile modulus vs. blend composition for dry (F) and wet (E) film samples.
PROPERTIES OF NYLON 6/EVOH BLENDS 2453
blend structure as barrier resins but also the needto characterize the polymeric solid state.
Permeability measurements with respect toCO2 and O2 were performed at T 5 30°C. Thepermeability (P) and the permselectivity (PCO2
/PO2
) are reported in Table III. As can be observed,the permeability of Ny6/EVOH system decreasesas the EVOH weight fraction is increased. However,the addition of 10% of EVOH is not sufficient toincrease significantly the barrier properties of Ny6.On the other hand, the increase of EVOH content to25% produces a decrease in the permeability valueat about 65% with respect to the matrix.
It is well known20 that gas permeation in poly-mers takes place only in the amorphous part,while crystalline regions act as excluded volumesfor the sorption process and as an impermeablebarrier for diffusion. Taking into account that for
the Ny6 rich blends thermal studies have shown18
that the addition of EVOH does not modify signif-icantly the crystallinity in the Ny6 matrix, thepresence of a physical crosslinking within theamorphous phase is presumably responsible forthe remarkable gas barrier properties measured.In fact, as previously discussed, the Ny6/EVOHsystem shows specific interactions in the amor-phous phase which reach a maximum at 75/25composition reducing the polymer segmental mobil-ity. Moreover, analyzing the permselectivity datareveals that while the permselectivity of 90/10 and75/25 blends is unchanged with respect to the ma-trix, the blend 50/50 shows a higher value.
Oxygen permeability vs. EVOH weight fractionis depicted in Figure 8. The dependence of gas per-meability on blend composition and morphology cangive significant information regarding the level ofcompatibility. For a miscible blend, the logarithm ofthe composite permeability frequently correlateslinearly with the volume fraction of either of com-ponents of the blend. The additivity law is:
ln P 5 f1ln P1 1 f2ln P2 (2)
Where P describes the permeability of the systemand fi and Pi are the volume fraction and perme-ability coefficient of components, respectively. Fig-ure 8 illustrates the deviations of the Ny6/EVOHexperimental results from this additivity rule.
Table III. Permeability z 1015 (cm3 cm/cm2 sec Pa)with Respect to O2 and CO2 and Permselectivity(PCO2/PO2) of Ny6/EVOH System
Figure 8. Oxygen permeability vs. blend composition for NY6/EVOH system at T5 30°C. (The additivity rule is depicted as a straight line.)
2454 INCARNATO ET AL
Based on this plot, it comes out that the mixing rulecorrelates quite well the data of the system analyzed.
CONCLUSIONS
In this work, Ny6/EVOH blends were investigatedin depth highlighting the strong dependence be-tween composition and final properties of this system.
Rheological results have shown the existence of amaximum in the viscosity-composition dependencefor the 75/25 w/w blend and a positive deviation forall the considered g values. It was found that theshape of the flow curves of the Ny6/EVOH blends,compared to those of pure polymers, depends uponthe EVOH content, confirming the occurrence ofmorphological changes with composition.
Moreover, FTIR analysis has pointed out theexistence of specific interactions in the amor-phous phase for Ny6/EVOH blends that reach amaximum at the 75/25 composition. Such an ob-servation, in agreement with rheological results,indicated the occurrence of an interconnectedamorphous phase with a reduced segmental mo-bility. On the other hand, at this composition, amaximum in the glass transition was relieved.Finally, a remarkable decrease in the gas perme-ability values with respect to the Ny6 matrix canbe obtained adding 25% b.w. of EVOH.
From the obtained results, we can conclude thatthe specific interactions betweenOCOOH of Ny6 andOOH of EVOH generate a highly compatible sys-tem with synergistic effects for certain composi-tions.
This work was partially supported by “Progetto Finaliz-zato: Materiali Speciali per Tecnologie Avanzate II,” CNR.
REFERENCES AND NOTES
1. Elemans, P. H. M.; van Gisbergen, J. G. M.; Meijer,H. E. M. In Integration of Fundamental Polymer
Science and Technology; P. J. Lemstra, L. A. Kleintjens,Eds.; Elsevier Appl. Sci.: London, 1988; Volume 2.
2. Meijer, H. E. M.; Lemstra, P. J.; Elemans, P. H. M.Makromol Chem, Macromol Symp 1988, 16, 113.
