SELF-ADESIVE MULTILAYERED MONOLITHS MADE FROM DIFFERENTS NATURAL RUBBER-CLAY

NANOCOMPOSITES FILMS

Fábio do Carmo Bragança, Sergio Augusto V. Jannuzzi, Fernando Galembeck,1*

1* Universidade Estadual de Campinas - Unicamp, Campinas - SP – fernagal@iqm.unicamp.br

Filmes nanocompósitos de borracha natural e argila, preparados por casting, foram unidos sem ajuda de adesivos para a obtenção de diferentes monolitos laminados de três camadas. Os filmes sobrepostos para formar os laminados possuem valores de módulo elástico significativamente diferentes, mas são altamente compatíveis, pois possuem a mesma matriz polimérica. As duas faces dos filmes utilizados apresentaram características distintas, em função de terem secado em contato com o ar e com o molde de polietileno, mas a adesão nas interfaces, após a montagem dos laminados, foi intensa o suficiente para que esses se rompessem no interior dos filmes, deixando as interfaces intactas mesmo após aplicação de solicitação mecânica. Os laminados foram cortados com sucesso por um feixe de íons focalizado montado dentro de microscópio eletrônico de varredura, revelando lamelas de argila esfoliada e tactoides no interior dos filmes. Os resultados desse trabalho mostram que filmes nanocompósitos preparados com latexes, mesmo apresentando diferentes propriedades mecânicas podem ser auto-aderentes, pois possuem a mesma matriz polimérica. Palavras-chave: laminados, nanocompósitos, borracha natural, argila, auto-adesão.

Multilayered materials made from natural rubber-clay nanocomposites films

Natural rubber clay nanocomposite films prepared by casting were transformed into multilayer thick slabs by successively layering films on top of each other. Adhesion between layers does not require any adhesive, whenever the nanocomposite base polymer is the same in adjacent layers. The self-adhesive joints resist exposure to peeling mechanical action. Trilayer laminate slabs formed by high- and low-modulus films have mechanical properties intermediate to the component films. The two surfaces of each cast film show different properties due to the formation in contact with air and the casting substrate, but this does not have a major effect on the film-film interfaces. Laminate multilayers were successfully cut by a focused ion beam mounted within a scanning electron microscope, revealing exfoliated clay particles and tactoids. These results show an interesting property of clay nanocomposite formation using latex: materials with different mechanical properties can be self-adherent due to the use of the same base polymer, particularly non-vulcanized rubber, opening new possibilities for designing polymeric materials. Keywords: multilayer films, nanocomposite, natural rubber, self-adhesion. Introduction Natural rubber-clay nanocomposites have been studied by many researchers1-6 that used different

preparation techniques, as coagulation or casting, with and without crosslinking. The rubber

reinforcement degree depends of the route chosen.7 Previous work from the author’s group8,9

showed interesting mechanical properties of clay nanocomposites prepared by casting with natural

and synthetic latexes. The strong interaction between clay and polymer particles receives an

important contribution from electrostatic adhesion, where the counterions play a decisive role

forming ionic bridges uniting the two immiscible phases that both contain excess negative charges.

In this work, natural rubber nanocomposite films prepared by casting with different clay contents (5

and 30% wt) are shown to be self-adherent so that they can be bonded without adhesive agents thus

forming complex monoliths. Since the Young modulus of the 30% wt nanocomposite is much

Anais do 10o Congresso Brasileiro de Polímeros – Foz do Iguaçu, PR – Outubro/2009

higher than that with low clay content, this is an unusual example of combination of hard and soft

materials with excellent adhesion, since there is no major barrier for polymer diffusion across the

interfaces.

Experimental Material Centrifuged natural rubber latex with 58.3% wt dry rubber (stabilized with 0.69% ammonia) was

supplied by Orbys, São Paulo. Sodium montmorillonite (Na-Cloisite, herefrom named Na-MMT)

was acquired from Southern Clay (cation exchange capacity = 102 mequiv./100 g of clay).

Nanocomposite films preparation The films were prepared according a previous work of this group described in reference 9.

Films assembly into three-layer solid The films were layered on top of each other and joined using a metal roll with 2 cm diameter, as

shown in Figure 1. Two types of monoliths were prepared, as shown in Figure 2. Type A laminate

has one film with 30% wt clay sandwiched between two films with 5% wt clay and Type B has one

film with 5% wt clay in between two films with 30% wt clay.

Figure 1. Illustrative cartoon of the laminate assembly using a metal cylinder. During film assembly, special care was taken in the identification of upper and lower film surfaces.

Figure 2. Dimensions and assembly of the two monolith types.

After the films were joined, the slabs were submitted to different treatment conditions. To perform

the experiment in triplicate, one set with three monoliths of each type was heated in an oven at

120oC for 2 hours. These were named A-H120 and B-H120. Another set was heated at 60oC for the

Anais do 10o Congresso Brasileiro de Polímeros – Foz do Iguaçu, PR – Outubro/2009

same time (A-H60 and B- H60). A third set was pressed under 245kPa for 60 minutes (A-P60 and

B-P60). The last set was pressed under 245kPa for 10 minutes (A-P10 and B-P10).

Instruments and measuring procedures Both sides of the cast films were examined using a wide angle X-ray diffractometer, Shimadzu

XRD-7000 in the reflection mode with an incident Cu Kα radiation, 15.4nm wavelength under

1.0°/min scan rate.

In the multilayered solids, tensile tests were performed according to DIN 52504 standard using

specimens cut from the samples. These were kept at 24oC under 50% relative humidity for 24 h

before the tensile measurements that were performed using an EMIC DL2000 universal testing

machine at a 200 mm/min strain rate. At least five specimens were tested for each sample and the

reported values are the calculated averages. Young moduli were obtained from the angular

coefficient in the initial linear portion (ε < 0.5%) of the stress–strain plots.

The samples were also observed using a FEI focused ion dual beam model NanoLab 200. The ion

beam was tilted 52° relative to the electron beam and was only used for etching, not for imaging.

Samples were etched with a Ga+ ion beam current of 7 nA under 30 keV acceleration voltage. The

etched surface was then polished with a low beam current (3 nA) prior to imaging with SEM.

Results and Discussion Nanocomposite Films for the nanocomposite films used in the monoliths assembly, compared to clay. This presents an

intense first order diffraction peak (d001) at 2Θ = 7.5o corresponding to an 1.17nm interlayer space.

Figure 3 shows a series of X-Ray diffractograms from 5% and 30% wt films. The lower films face

show a d001 peak shift for a low angle region corresponding to 1.0nm increase in the interlayer space

compared to the pure clay in 5% film, and less than that in 30% one. The diffractogram from the

upper films faces of the 5% does not show the characteristic (d001) diffraction peak indicating the

absence or full exfoliation of clay and in 30% film, the d001 peak appears less intense and lightly

shifted.

Figure 3. WAXRD for natural rubber-clay nanocomposite films prepared by casting. (a) film with 5% wt clay; (b) film with 30% wt clay. Upper face: dried in contact with air and lower face: dried in contact with plastic mold surface.

Anais do 10o Congresso Brasileiro de Polímeros – Foz do Iguaçu, PR – Outubro/2009

To observe films morphology, the films bulk and interfacial regions, a Ga+ focused ion beam was

used to etch the films revealing the clay distribution. Figure 4 shows the cavities drilled by the ion

beam in the bulk films where the clay particles can be observed. Figure 4 (e,f) show the flawless

interface between films delimited by a white line, undamaged by the ion beam. This evidences the

usefulness of this technique for the examination of bulk soft polymers.

Figure 4. Images from the interior of the laminates, obtained using a SEM/FIB. (a) 30% bulk film etched by the gallium beam, (b) 30% bulk film marked region observed with high magnification, (c) 5% bulk film etched by the gallium beam, (d) 5% bulk film marked region observed under higher magnification, (e) interface between 5 and 30% clay films delimited by a with line and (f) interface region under higher magnification.

Mechanical properties as maximum stress, elongation at break, elastic modulus and the energy

values obtained from the mechanical measurements are show in Table 1. For type A monoliths

maximum stress and elongation at break presented equivalent values to each other, except for A-

H120. Elastic modulus for all type A monoliths were similar independent of the heating or pressing

conditions and were 14 times higher than F5%.

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Type B monoliths performance depend on preparations conditions. B-P10 and B-H60 maximum

stress, elongation at break and elastic modulus were similar to each other and different from B-P60

and B-H120 properties, which in turn were similar.

The rupture energy for the samples was obtained using stress-strain curves integration. Type A

monoliths presented values lower than F5%. For type B monoliths the tenacity values change with

the experimental condition, but in general they are superior to the reference films.

Table 1. Mechanical properties of the layered monoliths prepared under various experimental conditions and the reference films.

Sample Max stress (MPa) Elongation at break (%) Elastic modulus (MPa) Energy (J)

Natural rubber 2.6 ± 0.2 845 ± 54 0.7 ± 0.1 3.5 ± 0.3

F5% 3.9 ± 0.9 556 ± 54 2.9 ± 0.4 4.7 ± 0.6

F30% 7.7 ± 0.9 247 ± 21 205 ± 59 6.6 ± 0.6

A-P60 4.4 ± 0.4 305 ± 6 41 ± 9 3.8 ± 0.1

A-P10 4.1 ± 0.8 300 ± 17 41 ± 10 3.4 ± 0.5

A-H60 4.2 ± 0.5 303 ± 19 47 ± 10 3.5 ± 0.4

A-H120 3.7 ± 0.1 252 ± 3 45 ± 11 2.9 ± 0.1

B-P60 7.2 ± 0.6 289 ± 23 138 ± 15 4.7 ± 0.8

B-P10 8.1 ± 0.5 361 ± 31 81 ± 14 8.0 ± 1.0

B-H120 6.8 ± 0.9 301 ± 26 110 ± 33 5.2 ± 0.8

B-H60 8.0 ± 0.4 342 ± 17 97 ± 14 6.0 ± 0.6

Fracture patterns in the specimens used in the mechanical tests were observed by SEM. Figure 5

shows that the layer adhesion persisted even under stresses required for tensile fracture but the 5%

wt films underwent greater extension prior to fracture, since they appear forming depressions in the

fracture surfaces.

Figure 5. SEM images of the rupture region in three-layer specimen used in the mechanical test. (a) A-H60 central region under higher magnification.(b) B-H60 central region under higher magnification.

Anais do 10o Congresso Brasileiro de Polímeros – Foz do Iguaçu, PR – Outubro/2009

With these results is possible to assume that clay particles are largely exfoliated and very well

dispersed in the rubber matrix.

Despite clay accumulation in the lower film region, microscopy techniques used to observe the

monolith bulk morphology show individual clay lamellae and also some aggregates throughout the

matrix.

As expected, modulus increases when a higher amount of 30% wt clay nanocomposite is used and

elongation at break increases when using higher amounts of 5% wt clay film.

Conclusions Rubber-clay nanocomposite films can be assembled in multilayer slabs or monoliths with interfaces

strongly resistant to mechanical efforts and to swelling pressure. This can be used to produce

materials combining layers of soft and hard films with excellent compatibility. Film multilayering

can also be used to overcome limitations to the making of rubber-clay nanocomposites from

aqueous dispersions, due to energy and time consumption for water evaporation.

Acknowledgment The authors thank Carlos A. P. Leite and the Center for Semiconductor Components of Unicamp

for the SEM/FIB images. This is a contribution from the Millenium Institute for Complex Materials,

PADCT/CNPq.

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