Editorial Type:
Article Category: Research Article
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Online Publication Date: 17 Nov 2021

CHANGES IN THE MECHANICAL, MICRO-, AND NANO-STRUCTURAL PROPERTIES OF REINFORCED VULCANIZED NATURAL RUBBER COMPOUNDS: THEIR DEPENDENCE ON THE SiO2/CB RATIO

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Page Range: 58 – 81
DOI: 10.5254/rct.21.78991
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ABSTRACT

A systematic study on the influence of fillers on the structural properties at micro and nanoscale of vulcanized compounds based on a natural rubber matrix reinforced with silica and/or carbon black is presented. Several compounds with different SiO2/CB ratios (1.5/1, 2.25/1, and 3/1) and total filler contents (55, 60, and 65 phr) were prepared and vulcanized at 150 °C. The experimental techniques used were rheometric, swelling and dynamic mechanical tests, and positron lifetime spectroscopy. From these techniques, cure reaction parameters, the fraction at the maximum degree of swelling, storage modulus, loss tangent, and fractional free volume were measured. Using a recent model based on a hydrodynamic description and the percolation of aggregates in a rubber matrix, it was found that regardless of the filler combinations, the dynamic storage modulus is well represented as a function of the filler volume fraction. Besides, beyond a critical SiO2/CB ratio (2.25/1) in the formulations of the compounds, the loss tangent does not depend on the SiO2/CB ratio. The results obtained show a direct correlation among mechanical properties, swelling and fractional free volume, and the type and amount of fillers in the reinforced compounds.

Copyright: 2022
Fig. 1.
Fig. 1.

Rheometer torque curves at 150 °C for the unfilled NR compound and compound filled with CB, SiO2, and a mix of both. (a) Ftotal = 55 phr; (b) Ftotal = 60 phr; (c) Ftotal = 65 phr.

Fig. 2.
Fig. 2.

(a) t05 as a function of the silane content. The data labels correspond to the total filler content. Each arrow corresponds to a total filler content of 55, 60, and 65 phr in the compound. (b) t05 as a function of the CB content. The data labels correspond to the total filler content. The arrow is an eye guide. (c) t100 as a function of the CB content. The data labels correspond to the total filler content.

Fig. 3.
Fig. 3.

Contour plot of Δτ as a function of the filler content in compounds.

Fig. 4.
Fig. 4.

Contour plot of the rubber fraction at the maximum degree of swelling ν2m as a function of the filler content.

Fig. 5.
Fig. 5.

Δτ/Λ as a function of the maximum degree of swelling ν2m. The errors associated with Δτ/Λ are within the symbols.

Fig. 6.
Fig. 6.

Storage modulus (E′) and the hysteresis loss (tan δ) as a function of the test temperature. (a) Cured unfilled compound; (b) cured filled compounds with 55 phr; (c) cured filled compounds with 60 phr; (d) cured filled compounds with 65 phr. In the label, each sample is identified according to the SiO2/CB ratio.

Fig. 7.
Fig. 7.

Glass transition temperature as a function of the total filler volume fraction.

Fig. 8.
Fig. 8.

Peak height tan δ at Tg as a function of the storage modulus in the rubbery state E′30°C (see text). Dotted lines are only for an eye guide.

Fig. 9.
Fig. 9.

E′/E′o as a function of the total filler volume fraction. Data points were fitted using the model proposed by Baeza et al.41 (see text). For the sake of clarity, the hydrodynamic contribution in Eq. 5 is represented by a straight dotted black line only for the full-CB samples.

Fig. 10.
Fig. 10.

E′/E′o as a function of the rubber fraction at the maximum degree of swelling. Dotted line is only for an eye guide.

Fig. 11.
Fig. 11.

Loss tangent at 60 °C as a function of the silica content.

Fig. 12.
Fig. 12.

Nanohole free volume as a function of the total filler volume fraction. In this figure, only the data points corresponding to the NR, full-CB, and high-silica compounds are presented. Lines are only eye guides.

Fig. 13.
Fig. 13.

νsol (left y axis) and ν2m (right y axis) as a function of fv for the different SiO2/CB formulations in compounds. Data labels correspond to the total filler content. The error bars associated with fv are within the symbols.

Fig. 14.
Fig. 14.

E′/E′o as a function of fv for the different SiO2/CB formulations in compounds. Data labels correspond to the total filler content. The error bars associated with fv are within the symbols.

Contributor Notes

*Corresponding author. Email: asomoza@exa.unicen.edu.ar
Received: 01 Feb 2021
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