Influence of cure conditions on properties of resol/layered silicate nanocomposites

Influence of Cure Conditions on Propertiesof Resol/Layered Silicate Nanocomposites

Marta Lopez, Miren Blanco, Maria Martin, Inaki Mondragon‘Materials þ Technologies’ Group, Escuela Politecnica, Department of Chemical and EnvironmentalEngineering, Universidad Paıs Vasco/Euskal Herriko Unibertsitatea. Pza. Europa 1, 20018 Donostia-SanSebastian, Spain

The effects on clay exfoliation of organic modificationof montmorillonite (MMT) and the nature of the catalystused during the synthesis and curing of a MMT modi-fied phenolic resol resin were investigated. The impacton the final properties of other parameters such asreactivity ratio and temperature of condensation werealso analyzed in order to optimize the conditions toprepare a customized organoclay-based nanocompo-site. Nanocomposites were analyzed by means of wideangle X-ray scattering (WAXS), optical microscopy(TOM), and atomic force microscopy (AFM) techniques.The formation of either intercalated or quasi-exfoliatedstructure was assessed in some systems. Thermal andmechanical properties of the cured composites wereevaluated and correlated to their morphologies. Morehomogenous clay dispersion was achieved for compo-sites prepared with aminoacid-modified MMT, triethyl-amine (TEA) as catalyst, formaldehyde/phenol molarratio (F/P) 2.0, and curing at 808C. POLYM. ENG. SCI.,00:000–000, 2011. ª 2011 Society of Plastics Engineers

INTRODUCTION

Polymer-layered nanocomposites using nanosized clay

particles as reinforcing agents have attracted a great inter-

est because of their superior properties such as mechani-

cal strength, heat resistance, gas permeability, and flam-

mability compared with neat polymers, especially when

an exfoliated state is achieved [1–15]. Clays, as montmo-

rillonite (MMT), are inexpensive, chemically and ther-

mally stable, and have good mechanical properties. The

enhanced properties that produce the presence of MMT in

composites are presumably a result of nanometer size,

large aspect ratio, and large surface area of the silicate

layers [7, 8]. To increase the organophility of these natu-

rally hydrophilic phylosilicates, the cations in the galleries

of the clay have to be exchanged by cationic modifiers

(e.g., quaternary ammonium salts) [2, 6, 8]. The modified

clay (or organoclay), whose surface energy is decreased,

tends to be more compatible with polymers. Moreover,

the modified clay can react or interact with the monomer

or the polymer [1, 2, 4, 5, 7–11] thus improving the inter-

facial strength between clay nanolayers and the polymer

matrix [3, 6]. The state of dispersion of clays in a poly-

mer matrix can result in the formation of different kind of

composites. The exfoliated state is the most interesting

for the improvement of properties [4, 9]. In thermoset ma-

trix composites, to enhance the intercalation/exfoliation of

clays, polymer–clay compatibility, shear stress exerted by

resin polymerization and molecular diffusion of polymer

chains into the silicate interlayers are considered as the

key factors [4, 9]. The current use of nanoclays has been

basically dedicated to improve the fire retardant properties

of thermoset resins in general [12–15]. In this article, it is

also probed that MMT can also be used to improve the

mechanical properties of such resins.

Phenolic resins are irreplaceable materials for a wide

range of industrial applications such as adhesives, coat-ings, laminates, and composites [16–22]. Phenolic resins

are synthesized by the reaction of phenol with aldehydes,especially formaldehyde, and are classified as resols and

novolacs depending on phenol/aldehyde ratio. Only a fewstudies have been performed on clay-based nanocompo-sites based on phenolic resins due to their three-dimen-

sional molecular structure even before cure, which mayavoid the exfoliation of the clay [23–33]. Moreover, the

formation of water as a byproduct of crosslinking is alsoanother problem of this type of resins.

In a previous study [34], resol type phenolic resin/lay-

ered silicate nanocomposites were synthesized by the inter-

calation of monomer between silicate layers to overcome

the structural problem of MMT dispersion and exfoliation

into phenolic resin matrix. MMT was modified by using an

aminoacid, L-phenyl alanine, to induce condensation reac-

tions between its carboxyl end group and the hydroxyl

groups of formaldehyde and so, compatibility with the phe-

Correspondence to: I. Mondragon; e-mail: [email protected]

Contract grant sponsor: Ministerio de Educacion y Ciencia; contract

grant number: MAT2006-06331; contract grant sponsor: Basque Country

Governments (in the frame of Grupos Consolidados); contract grant

number: IT-365-07; contract grant sponsor: SAIOTEK; contract grant

number: S-PE07UN39; contract grant sponsor: ETORTEK-inanoGUNE;

contract grant sponsor: Eusko Jaurlaritza/Gobierno Vasco (Programa

Realizacion de Tesis Doctorales en Empresas).

DOI 10.1002/pen.22177

Published online in Wiley Online Library (wileyonlinelibrary.com).

VVC 2011 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—-2011

nolic resin matrix could be increased. In this work, the type

of catalyst for curing, as well as the MMT modifier, have

been analyzed with the aim of achieving an optimum

degree of exfoliation of the layered silicate in the phenolic

matrix. Moreover, other parameters as reactivity ratio and

condensation temperature during prepolymer synthesis

have also been investigated to achieve clay exfoliation.

Morphology, thermal behavior, and stability have been

studied by means of transmission optical microscopy

(TOM), atomic force microscopy (AFM), dynamic mechan-

ical analysis (DMA), wide angle X-ray scattering (WAXS),

and thermogravimetric analysis (TGA). Moreover, mechan-

ical properties have been evaluated and correlated to the

morphology of the obtained nanocomposites.

EXPERIMENTAL

Phenol (P), formaldehyde (F) (35–40% aqueous solu-

tion), triethylamine (TEA), and 50% aqueous solution of

NaOH were purchased from Panreac (Barcelona, Spain)

and used without further purification. Untreated NaþMMT

and Cloisite 30B, a MMT organically treated with methyl

tallow (�65% C18, �30% C16, and �5% C14) bis-2-

hydroxyethyl quaternary ammonium chloride, were ob-

tained from Southern Clay Products (Texas, EEUU). L-phe-

nyl alanine and 6-aminocaproic acid were purchased from

Aldrich (Madrid, Spain) and used for modifying

NaþMMT.

Not very bulky aminoacids were chosen for decreasing

the effect of steric hindrance during the formation of the

prepolymer between MMT layers. L-phenyl alanine mont-

morillonite (PheMMT) and 6-aminocaproic acid modified

montmorillonite (6aaMMT) were prepared through the

ion exchange of NaþMMT with the corresponding amino-

acids in acidic environment according to the protocol

reported in previous work [34]. Different amounts of

PheMMT and 6aaMMT were sonicated in formaldehyde

solution and treated in presence of concentrated sulfuric

acid with the aim of promoting the condensation reaction

between the carboxyl end group of both aminoacids and

the ��OH groups of the formaldehyde in aqueous solu-

tion. Condensation reaction between resol chains and ami-

noacid was demonstrated by FTIR technique in a previous

work [34]. Cloisite 30B and NaþMMT clays were sub-

jected to the same treatment in order to compare all the

composites at the same cure conditions [34]. In the case

of Cloisite 30B with ��OH end groups, as seen by FTIR,

condensation reactions did not occur with the ��OH

groups of formaldehyde solution.

To study the influence of cure conditions on the final

dispersion of the clay in the resol matrix, several poly-

merizations were carried out. In a first stage, prepolymers

were synthesized by mixing the previously modified

clays-formaldehyde solutions with formaldehyde and phe-

nol in order to work with a formaldehyde/phenol molar

ratio 1.4. Then, the pH of formaldehyde/phenol mixture

was adjusted to 8 using different catalysts as TEA or 50%

aqueous solution of NaOH. Condensation was carried

out at 808C under reflux until prepolymers showed around

1/1 g/g solubility in water. Same treatment was used for

formaldehyde/phenol molar ratios 2.0 and 1.0 and when

using different condensation temperatures (55, 80, and

958C). Water extraction was performed under vacuum at

45–488C to a solid content of 75–85 wt%. Samples were

stored at 2208C until they were analyzed. Table 1

resumes the starting conditions and the designations for

each synthesized prepolymer.

WAXS measurements were carried out with a powder

diffractometer Philips, equipped with a graphite monochro-

mator and an automatic divergence slit, using an incident

X-ray of Cu Ka radiation with wavelength of 1.54 A.

Morphologies of the nanocomposites were investigated

by TOM using an Olympus BH-2 optical microscope and

by AFM using a Nanoscope IIIa, MultimodeTM from Dig-

ital Instruments operating in tapping mode. An integrated

silicon tip/cantilever, from the same manufacturer, having

a resonance frequency over 300 kHz, was used. The

specimens were prepared by ultramicrotoming at room

temperature.

Dynamic-mechanical analysis (DMA) was carried out

in a Perkin-Elmer DMA-7 analyzer using a three-point

bending device. DMA measurements were carried out

with 24 3 5 3 1 mm3 specimens maintaining a span of

15 mm and using 110 and 100 mN as static and dynamic

forces, respectively. All measurements were carried out at

a constant frequency of 1 Hz with a heating rate of 58C/min using helium atmosphere.

Static flexural properties were determined in a three-

point bending device using an Instron universal testing

machine, model 4206, equipped with a load cell of 1 kN.

Tests were carried out at room temperature with a relative

humidity of 50 6 5% using a crosshead displacement rate

of 0.43 mm/min. Measurements were carried out with

25 3 10 3 1 mm3 specimens and at least five measure-

ments were performed.

Thermogravimetric analysis was carried out using a

Mettler Toledo TGA/SDTA 851. Samples were scanned

from 25 to 10008C at a scanning rate of 108C/min under

nitrogen atmosphere.

RESULTS AND DISCUSSION

Influence of Clay Modifier

Two aminoacids (L-phenyl alanine and 6-aminocaproic)

were used for surface modification of MMT to study the

influence of the nature of the surfactant on the degree of

exfoliation of the layered silicate in the phenolic matrix.

Furthermore, Cloisite 30B, a commercial clay functional-

ized with methyl tallow bis-2-hydroxyethyl quaternary

ammonium, and untreated NaþMMT were also used for

the synthesis of new composites. The d001 spacings of

the natural NaþMMT and modified clays, as analyzed by

2 POLYMER ENGINEERING AND SCIENCE—-2011 DOI 10.1002/pen

X-ray diffraction, are shown in Table 2. In the case of

Cloisite 30B, the long length of the chains of the surfactant

(�1.90 nm in length) justifies the size of the d001 spacing.

On the other hand, the basal spacings for both PheMMT

and 6aaMMT were lower than for Cloisite 30B due to

the small dimensions of the modifier (0.79 nm [35] and

1.22 nm [1] in length, respectively). Furthermore, as Usuki

et al. [1] suggested, the carboxyl (��COOH) end group of

the a-aminoacid can bond with the oxygen (��O��) group

of the silicate surface through hydrogen bonding.

Figure 1 shows X-ray diffraction patterns of compo-

sites and neat resol catalyzed with TEA and using sonica-

tion. Res-6aa and Phe composites exhibited almost no dif-

fraction peaks when compared with the composites with

the unmodified clay (Res-Na). This suggests that silicate

layers of 6aaMMT and PheMMT were better dispersed in

the phenolic matrix than the other composites. This fact

may be attributed to the induced condensation reaction

between the carboxyl end group of the aminoacid and the

��OH groups of the formaldehyde in aqueous acidic solu-

tion, thus acting like an anchoring point between the

layers and the resin, as previously reported [34]. Further-

more, diffraction peak was hardly noticeable for Res-6aa

composite when compared with Phe composite. Interac-

tions between resol reactive molecules and 6-amino-

caproic modifier could be more easily formed due to the

lack of bulkiness and the higher flexibility of the linear

alkyl chain of 6-aminocaproic acid when compared with

L-phenyl alanine aminoacid. In the case of composite C,

diffraction peaks were also hardly observed, that could

indicate that a homogeneous clay dispersion was obtained.

Taking into account TOM images (Fig. 2d) and by study-

ing in detail the XRD pattern, it was observed that the

basal spacing of the composite C appeared around

1.35 nm, whereas the basal spacing of the Cloisite 30B

was 1.60 nm (Table 2). This significant contraction of

interlayer spacing from 1.60 nm to 1.35 nm could be

caused due to the presence of the bulky modifier of the

clay whose steric hindrance avoids the polymerization

of the resol inside the layered silicates (intragallery).

Thereby, the polymerization could be more favorable

outside them (extragallery) [34]. The alkylammonium

chains of the modifier occupied a large space between the

layers and therefore, not much space remained accessible

for the polymer chains to diffuse between the layers [36].

If no polymerization does occur in the intragalleries, the

layers cannot be further separated and polymerization

takes place in the extragallery region, leading to shrinkage

TABLE 1. Characteristics of neat resols and resol–clay composites.

Sample Designation Modifier Catalyst D.W.a (1 g/g) S.C.b (%)

Neat Resol 808C (1.4) Res — TEA 1.20 76

2 wt% PheMMT 808C (1.4) Phe L-phenyl alanine TEA 0.92 81

2 wt% Cloisite 30B 808C (1.4) C TEA 0.95 80

2 wt% NaMMT 808C (1.4) Res-Na — TEA 1.12 81

2 wt% 6aaMMT 808C (1.4) Res-6aa 6-aminocaproic acid TEA 1.00 84

Neat Resol 808C (1.4) Res-NaOH — NaOH 1.05 83

2 wt% PheMMT 808C (1.4) Phe-NaOH L-phenyl alanine NaOH 1.01 84

Neat Resol T 808C (1.0) Res-1 — TEA 1.05 68

2 wt% PheMMT T 808C (1.0) Phe-1 L-phenyl alanine TEA 1.16 71

Neat Resol T 808C (2.0) Res-2 — TEA 1.07 82

2 wt% PheMMT T 808C (2.0) Phe-2 L-phenyl alanine TEA 1.04 80

Neat Resol T 958C (1.4) Res-T95 — TEA 0.66 82

2 wt% PheMMT T 958C (1.4) Phe-T95 L-phenyl alanine TEA 0.41c 82

Neat Resol T 558C (1.4) Res-T55 — TEA 1.15 80

2 wt% PheMMT T 558C (1.4) Phe-T55 L-phenyl alanine TEA 1.66c 84

a D.W., dilutability in water (1 g/1 g).b S.C., solid content (%).c not clear measurement.

*Methyl tallow (�65% C18, �30% C16, and �5% C14).

TABLE 2. Modifiers and d001 spacing of the modified clays.

Clay NaþMMT PheMMT 6aaMMT Cloisite 30B

Modifier —

L-phenyl alanine

6-aminocaproic acid

T ¼ Tallow (�65% C18,

�30% C16, �5% C14)

d001 spacing (nm) 1.10 1.32 1.26 1.60

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2011 3

of the interlayer spacing [34]. Furthermore, the lack of

interactions between the modifier of the Cloisite 30B and

the resol reactive molecules [27, 34] at the synthesis tem-

perature could also favor this behavior. On the other

hand, for Res-Na composite, a peak appeared at 1.26 nm.

As the interlayer spacing of NaþMMT in the composite

increased from 1.10 (Table 2) to 1.26 nm, part of the re-

active process could occur inside the layers although this

increase was not enough to achieve a complete intercala-

tion [34].

The morphology of the composites was also studied by

TOM and AFM to better analyze the dispersion of the

organoclays in the phenolic matrix. TOM images are

shown in Fig. 2a–e. In Fig. 2a, the morphology of the ho-

mogeneous surface of the neat resol matrix is seen. For

Phe and Res-6aa systems, a uniform dispersion of clay in

the matrix of the composite was observed and no signifi-

FIG. 1. X-ray diffraction patterns of composites with 2 wt% clay load-

ing and neat resol.

FIG. 2. TOM pictures of (a) neat resol, (b) Phe, (c) Res-6aa, (d) C, and (e) Res-Na composites.


cant variations were seen when compared with TOM pic-

ture of the neat resol. In contrast, in composites C and

Res-Na (Fig. 2d–e), aggregates of a broad range of sizes

were observed thus indicating poorer clays dispersions. In

the case of Res-Na composite, the lack of modifier in the

clay could explain this behavior. For C composite, as

above described, the steric hindrance of the bulky surfac-

tant and the lack of reactive groups in the surfactant of

the clay could avoid the polymerization of the phenolic

resin inside the layered silicate. Consequently, big

agglomerates remained in the cured material. As above

shown, the faintness of XRD peaks for C composite could

indicate intercalated or nearly exfoliated clay structures.

This fact was unexpected since Cloisite 30B clay does

not react with resol matrix. However, despite XRD results

indicating some extent of intercalation, TOM pictures

confirmed the existence of layer agglomerates for compo-

sites with untreated (with a size around 3–5 lm) or Cloi-

site 30B (5–15 lm) clays.

AFM phase images are shown in Fig. 3a–c. Lines or

scratches in surfaces, appearing after ultramicrotomy cut-

ting due to the resol fragility, made difficult the observa-

tion of individual layers. In Fig. 3a, the globular structure

of neat matrix can be seen [34]. For Phe and Res-6aa

composites, though homogeneously dispersed individual

layers were observed, layers forming intercalated agglom-

erates with lateral size around 20–100 nm were also seen

for Phe composite. Although in overall, the dispersion of

the modified MMT in Res-6aa composite was very similar

to Phe composite one, individual layers seemed to be

more homogeneously distributed in the matrix (Fig. 3b),

which is consistent with the results of XRD. Resol reac-

tive molecules can diffuse into the inner clay layers when

agglomerates are thin [34, 36]. Indeed, XRD and AFM

results showed a fairly good dispersion of PheMMT and

6aaMMT in the phenolic matrix. When clay stacks are

thicker, reactive molecules could only insert inside the

most superficial layers, remaining some agglomerates as

in the case of C and Res-Na composites. Thus, the clay

dispersion in C composite appeared to be very poor, as

also observed by AFM elsewhere [34]. As a conclusion,

the nature of MMT modifier and its possible interactions

FIG. 3. AFM phase images of (a) neat resol, (b) Res-6aa, and (c and d) Phe composite at different magnifi-

cations. Individual layers are indicated by arrows.


and/or reactions with the polymer matrix result in a key

factor in order to achieve clay exfoliation in phenolic

composites.

Flexural properties of composites and neat resol cata-

lyzed with TEA were analyzed by both dynamic and

static mechanical measurements. As shown in Fig. 4, flex-

ural modulus was higher for all the composites than for

the neat matrix. For Phe and Res-6aa composites, the

modulus achieved higher values than for the other sys-

tems. For flexural strength, the highest value was found

for Res-6aa composite with an increase of 15% with

respect to the neat matrix. The increase of flexural

strength for Phe and Res-6aa composites can be attributed

to more homogeneous dispersion of clays, as well as to

the interactions with the matrix [37]. In the case of Res-

6aa composite, these interactions could be more easily

reached than in the Phe composite due to the flexibility

and the lack of bulkiness of the linear alkyl chain of 6-

aminocaproic inside the clay. On the other hand, for

poorer dispersion of clay (C and Res-Na composites), a

slight increase in modulus was observed, which is usual

for polymeric composites even without remarkable inter-

facial interactions between matrix and inorganic fillers

[5]. A significant decrease of flexural strength was

observed for these composites, especially for Res-Na

composite. As stated above, poor clay dispersions were

obtained for composite C due to the presence of the bulky

modifier of the clay whose steric hindrance avoids the po-

lymerization of the resol in the clay. Therefore, if no po-

lymerization does occur in the intragalleries, the layers

cannot be further separated and polymerization takes

place in the extragallery region. Consequently, larger clay

agglomerates remains without being exfoliated. Further-

more, the lack of interactions between the modifier of the

Cloisite 30B and the resol reactive molecules [27, 34]

could also favor this behavior. As stated by other authors

[37], poorly dispersed clay layered silicates serve as stress

concentration and flaws for crack initiation, which results

in premature failure upon mechanical deformation.

On the other hand, Fig. 5a and b shows the thermal

decomposition behavior of neat resol and clay-filled com-

posites. Different stages of degradation were observed for

the neat resol resin in the TGA thermograms: in the first

stage, from 30 to 3508C, the release of formaldehyde due

to the breakage of ether bridges, and also phenol, water,

FIG. 4. Flexural properties of composites depending on the modifier of

MMT and the used catalyst.

FIG. 5. (a) TGA thermograms and (b) DTG curves of neat resol, Phe,

C, Res-6aa, and Res-Na composites.


and the onset of the degradation of organic modifier of

the clay, took place simultaneously. In the second stage,

in the temperature range of 350–7008C, two zones can be

distinguished: at 400–5508C, the oxidation of the network

and at 550–6508C, the formation of the char structure

[16]. Phenolic composites showed a slightly better ther-

mal stability than the neat resol. Silicate layers would act

as a heat barrier, which enhances the overall thermal sta-

bility of the system [34] especially when the clay layers

are homogeneously dispersed throughout the composite

(Phe and Res-6aa composites).

Influence of Catalyst on Composite Synthesis

Different catalysts can be used for phenol-formalde-

hyde resol synthesis, being the most used NaOH,

Ba(OH)2, or LiOH and rarely, hydroxides of divalent met-

als [16, 38–43]. Carbonates (sodium carbonates) and

oxides (calcium or magnesium oxides) are also employed

[38, 39]. Tertiary amines, in particular triethylamine, are

also used [16, 19–22, 41] being it the selected catalyst

throughout this work. NaOH was also chosen to compare

its influence on the final properties of the composites for

being one of the most worldwide used catalysts in pheno-

lic synthesis and as representative of the hydroxides. As it

was studied elsewhere [41], resol curing in presence of

NaOH is normally faster and can produce a bigger

amount of condensed water than for curing of these resins

with TEA. Figure 1 shows X-ray diffraction patterns of

Phe composite catalyzed by NaOH where the diffraction

peak is hardly observed, possibly indicating that a homog-

enous dispersion was obtained. The possible more uni-

form clay dispersion in the matrix of this composite could

not be verified by TOM or AFM because a suitable sur-

face could not be obtained due to the big amount of water

bubbles formed during the resol condensation in the cur-

ing stage. Flexural properties of Res-NaOH and Phe-

NaOH composites are reported in Fig. 4. The presence of

bubbles in specimens catalyzed by NaOH, slightly

decreased the flexural modulus and strength compared

with TEA catalyzed systems. Furthermore, thermal stabil-

ity is shown in Fig. 6. During polymerization, prepoly-

mers catalyzed with NaOH mainly give methylene-type

bridges [41] while using TEA dimethylene ether brigdes

are formed [41]. Consequently, composites with less oxy-

gen content such as those synthesized with NaOH,

resulted in more thermally stables mixtures.

Influence of Formaldehyde/Phenol (F/P) Molar Ratio

The initial F/P molar ratio is one of the most important

factors on the formation of phenolic resol resins [20, 38,

41]. In the past, many authors reported the influence of

the initial formaldehyde to phenol molar ratio in the syn-

thesis of resol resins catalyzed with alkaline catalysts,

such as sodium hydroxide and barium hydroxide [38, 39,

43]. Our group investigated the influence of F/P ratio in

resols synthesis catalyzed with triethylamine [20, 41] but

no studies do exist on its effect during clay nanocompo-

sites synthesis.

In this study, the range of F/P molar ratio for resol fab-

rication was covered by analyzing three resols synthesized

at 808C, with three initial formaldehyde to phenol molar

ratios (F/P¼ 1.0, 1.4, and 2.0), catalyzed with triethyl-

amine. Every initial formaldehyde/phenol mixture was

adjusted to pH ¼ 8 with a different amount of catalyst,

depending on the initial pH of the mixture. Figure 7

shows X-ray diffraction of L-phenylalanine-modified clay

composites taking into account the F/P ratio (Phe-1 and

Phe-2). Faint diffraction peaks were observed. In the case

of Phe-1 composite, the area of the peak was slightly

higher than for the other composites and shifted to higher

angles. This behavior seems to indicate that at low form-

aldehyde content, part of the reactive process could occur

inside the layers (intragallery) but the content of formal-

dehyde was not enough to overcome the attraction forces

between the clays and fully separate them. This trend has

been also verified below by TOM and AFM techniques

(Fig. 8a and b). Thereby, intragallery reactions were

favored [34, 44] but as the amount of formaldehyde was

quickly finished, reactions were earlier stopped. Figure 8a


and NaOH catalyzed composites.


shows the TOM image of Phe-1 composite where some

MMT aggregates of around 5–10 lm are observed in the

matrix, whereas in Fig. 8b, AFM image, a few individual

clay layers are seen, remaining the most of them in

groups forming small stacked aggregates. Moreover, indi-

vidual layers are hardly separated between them. In the

case of composite Phe-2, the behavior seems to be

slightly different. In this case, during polymerization,

extragallery reactions catalyzed by TEA and intragallery

reactions catalyzed by ��COOH groups of L-phenyl ala-

nine should proceed simultaneously to achieve the exfoli-

ation state. In the presence of a high content of formalde-

hyde, both reactions initially might be parallel processes,

but as the reaction continued, more easily accessible

extragallery reactions would be favored [34, 44], thus

leading to larger stacked agglomerates (Fig. 8c). As a

result, intercalated aggregates and exfoliated sheets are

also observed in Fig. 8d. It seems that the small shear

forces exerted on PheMMT agglomerates during polymer-

ization are able to overcome the attraction forces between

the layers due to the weak forces that stack them together

[9, 45], thus exfoliating the smaller stacks. Thereby, Fig.

8d shows a better dispersion of the clay in the matrix.

On the other hand, flexural properties for Phe-1 and

Phe-2 composites are shown in Fig. 9. In neat resols,

FIG. 7. X-ray diffraction patterns of Phe depending on the reactivity

ratio and the temperature of synthesis.

FIG. 8. TOM micrographs of (a) Phe-1 and (c) Phe-2 composites and AFM phase images at different

amplifications of (b) Phe-1 and (d) Phe-2 composites. Aggregates and individual layers are indicated by

arrows.


when formaldehyde content was increased (Res-2), the

polymerization took place faster [41]. Therefore, the pres-

ence of bubbles increased, thus decreasing flexural modu-

lus and strength. In general, when an uniform clay disper-

sion was achieved, significantly improved both flexural

modulus and strength in all the composites. The differen-

ces between neat and modified-clay resol composites were

more significant for Phe-2 composite where the dispersion

of the exfoliated clay was even better than for Phe com-

posite. Anyway, Phe-1 composite showed slightly higher

flexural properties due to the higher homogeneity of the

resol network.

Similar conclusions can be extracted from thermal

behavior shown in Fig. 10. The presence of the thermally

stable MMT can act as a barrier to hinder heat diffusion

and migration of degraded volatiles, thus delaying the

decomposition rate [34]. At increasing reactivity ratio

(Res-2 and Phe-2 composites), the oxygen content

increased [41, 46] thus resulting in less thermally stable

mixtures compared with composites and matrices with

lower content in formaldehyde. As above shown, this

behavior can be mainly observed in the second stage of

thermal decomposition when the char structure is formed.

Res-1 matrix was the most thermally stable. In the case

of Phe-1, the existence of oxygen groups increased when

compared with the neat matrix owing to the clay modifier

and its interactions with the reactives. This fact seems to

be the responsible for the decrease in thermal stability.

Influence of Temperature of Synthesis

There are different studies concerning the influence of

temperature in the resol prepolymer formation. Some of

them were carried out employing fixed synthesis tempera-

tures [19–21, 38, 40, 41, 43] and others combined steps

during synthesis of the resin [47, 48]. No works about its

influence on clay-based nanocomposites formation do

exist. In this study, triethylamine catalyzed resols with F/

P¼1.4 synthesized at 55, 80, and 958C under reflux were

investigated.

The first observed influence of the condensation tem-

perature on the formation of the composite was related to

the time needed to reach the final value of 1/1 g/g dilut-

ability in water. The higher the condensation temperature,

the shorter the synthesis time was. Phe-T55 composite

FIG. 9. Flexural properties of composites and matrices depending on

reactivity ratio and temperature of synthesis comparing to neat matrix

and composite synthesized at 808C with F/P ¼ 1.4.


and composites synthesized with different reactivity ratios.


showed a very slow evolution and the condensation time

was around 5160 min. Phe-T95 reached the prefixed final

point much faster (75 min), whereas Phe spent �290 min.

Figure 7 shows X-ray diffraction patterns of Phe-modified

composites depending on the temperature of synthesis

(Phe-T55 and Phe-T95 composites). As can be observed,

weak diffraction peaks were observed and the interlayer

spacing for all the composites was very similar (around

1.35 nm), although the area of the peak of Phe-T95 com-

posite was slightly higher. This fact indicates that at

higher temperature the amount of stacked clays increased

[26, 27]. This behavior was confirmed by TOM and AFM

(Fig. 11a and b). While at 558C and 808C, the curing

could be controlled because condensation reactions pro-

ceeded slowly, at 958C, they occurred very fast and they

were difficult to control due to formaldehyde and water

evaporation despite using reflux. Thereby, curing took

place less homogeneously and an increasing amount

of bubbles and thick clay agglomerates were present in

Phe-T95 matrix (Fig. 11a and b). On the other hand, as

can be observed in Fig. 11c and d, the dispersion of the

clay in the matrix for Phe-T55 composite was different.

When the synthesis was carried out at 558C and in pres-

ence of TEA as catalyst, the polymerization seemed to be

favored in the extragallery region. As a consequence, only

superficial layers could be separated and thus the silicate

layers appeared poorly dispersed in the matrix, remaining

big agglomerates, as it is observed in Fig. 11c and d.

Thereby, both composites showed poorer dispersions than

composites synthesized at 808C.As shown in Fig. 9, flexural properties were also

affected by the condensation temperature. For Phe-T95

composite, the high temperature used during resin synthe-

sis led to an increase in the polymerization rate of the

network that generated flaws and the formation of nonho-

mogeneous resol network. Thus, though the modulus

value was fairly constant compared with Res-T95 value

mainly due to the presence of the clay, flexural strength

was significantly decreased compared with this value for

Res-T95 matrix and for Phe composite synthesized at

808C. On the other hand, for resol matrix synthesized at

558C, the low temperature used for the synthesis allowed

FIG. 11. Optical pictures of (a) Phe-T95 and (c) Phe-T55 composites and AFM phase images of (b) Phe-

T95 and (d) Phe-T55 composites. Aggregates are indicated by arrows.


the formation of a homogeneous network that conducted

to better mechanical properties respecting to matrices syn-

thesized at higher temperatures. This increase was more

significant for the flexural strength values. Comparing

Res-T55 matrix with the Phe-T55 composite, the exis-

tence of big clay aggregates due to the absence of poly-

merization in the intragallery region led to a slight

decrease in flexural strength values.

Furthermore, the thermal stability of nanocomposites

was also examined by TGA. Figure 12 indicates that the

Res-T55 matrix was more thermally stable compared with

neat matrices synthesized at higher temperatures (Res and

Res-T95). The presence of less oxygen groups owing to

the lower synthesis temperatures [41, 46] and the homo-

geneity of the resol network could be responsible for this

behavior. During polymerization, prepolymers at 558Ccould form mainly methylene-type bridges. Consequently,

resols synthesized at lower temperatures had less oxygen,

thus resulting in more thermally stable mixtures. On the

other hand, when high synthesis temperatures were used

(958C) and taking into account that clays could lead to

oxidation of the network [44, 48], thermal stability was

decreased.

CONCLUSIONS

Resol-layered silicate composites were synthesized by

intercalative polymerization of phenol and formaldehyde

in the presence of differently modified clays. A few

parameters of the synthesis of these resins and casting of

composites were studied to find the optimum conditions

to improve the intercalation/exfoliation of montmorillon-

ite layers in phenolic resol matrices. On one hand, the

nature of the clay modifier was concluded to be one of

the key factors to obtain exfoliated nanocomposites. The

choice of L-phenyl alanine and 6-aminocaproic acid as

clay modifiers and the reactions between modifiers and

phenolic resin resulted, at low clay concentration, an

adequate method to obtain exfoliated nanostructures, as

verified by different techniques. Thus, the significant

improvement in the mechanical and thermal properties of

these composites can be justified by homogenous clay

dispersion.

Moreover, the catalyst used during the prepolymer

synthesis was also assessed. Concerning mechanical

properties–morphology relationships, NaOH was not a

good suitable catalyst for these systems. In contrast,

prepolymers catalyzed with NaOH resulted in more ther-

mal stable composites compared with TEA catalyzed

ones.

The influence of the reactivity ratio during composite

curing was also investigated. For composites with low

content in formaldehyde (F/P ¼ 1 and T ¼ 808C), reac-tive molecules only could be introduced within the most

superficial layers, thus remaining unreacted stacked

layers. In the case of composites with higher formalde-

hyde content (F/P ¼ 2 and T ¼ 808C), the polymerization

of resol occurred faster. Thus, extragallery reactions could

be favored leading to some clay agglomerates. Anyway, a

homogeneous dispersion of the individual layers for the

whole Phe2 composite was observed being more signifi-

cant the differences in mechanical properties between neat

and composite Phe-2. Nevertheless, Phe-1 composite

showed the best flexural properties due to the higher

homogeneity of the resol network and the presence of

the clay. Thermal properties were also affected, being

composite synthesized with F/P ¼ 1 the most thermally

stable due to its lower oxygen content and the network

homogeneity.

Furthermore, the effect of temperature of the synthesis

during the polymerization of the composite was also

assessed. For the composite synthesized at the lower tem-

perature (F/P ¼ 1.4 and T ¼ 558C), only most superficial

layers were separated from the big agglomerates, thus

negatively affecting both flexural and thermal properties.

For composites synthesized at 958C and F/P ¼ 1.4, the

presence of flaws significantly decreased flexural strength,

whereas the thermal stability was similar to the matrix

synthesized at same conditions.

Thus, different conditions of curing could be chosen

depending on the final application of the composite.

FIG. 12. (a) TGA thermograms and (b) DTG curves of neat resol and

composites synthesized changing the temperature of condensation.


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Influence of cure conditions on properties of resol/layered silicate nanocomposites

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