Development of vegetable oil based jute fibre reinforced composites

Jute fibre reinforced composites were prepared from epoxy resin blends consisting of epoxidized soybean oil (ESO) and glycerol-based (GER), pentaerythritol-based (PER) aliphatic EP or bisphenol-A based aromatic EP (DGEBbisphenol-A) cured with DETDbisphenol-A or bisphenol-AR917 hardener. In each case 25, 50 and 75%

ESO was added to the synthetic resin. Composite laminates were prepared by hand lamination followed by hot pressing to achieve high (59-61%) and uniform fibre content. The effect of ESO on storage modulus, glass transition temperature, mechanical properties and morphology was determined [198].

Effect of the alkali treatment on the mechanical properties of the reinforcing fabric

Prior to composite preparation, a systematic study [199,] was carried out to select the optimal fibre treatment conditions resulting in the best fibre mechanical properties, based on the literature on the alkali treatment of elementary fibres [83,84]. The maximal force values per fibre determined from strip tensile tests of differently treated jute fabrics are displayed in Table 4.3.1.

Table 4.3.1 Effect of alkali concentration and treatment time on maximal force values per fibre [N]

determined from strip tensile tests of jute fabrics

maximal force [N]

NaOH concentration [mass%]

0 0.5 1 2 4 8

treatment time [h] 0 567±56 0.5 496±36 523±45 522±50 460±55 395±35 1 506±71 538±63 560±54 518±47 363±30 2 569±24 534±38 544±38 523±62 373±25 4 579±77 574±46 527±29 507±36 399±24 8 554±50 596±17 559±60 520±48 349±43

From the 25 different alkali treatment conditions only 4 resulted in a modest increase in the maximal force values (marked with grey background), in all other cases the tensile strength of the

73 fabric decreased. The highest increase in maximal force was detected when the fabric was immersed into 1% NaOH solution for 8 h, however the increase is only around 5%, which taking into account the standard deviation cannot be considered as a significant amelioration. According to Kabir et al. [200] the non-cellulosic materials (hemicellulose and lignin) can be partially dissolved from natural fibres with the applied alkali treatment. As hemicellulose and lignin act as binding material in the elementary jute fibres, less force is sufficient to move the elementary fibres from each other, resulting in a decrease in the maximal force values.

On the other hand, the removal of hemicellulose and lignin leads to a rougher and bigger fibre surface, consequently better interfacial properties and higher glass transition temperatures [199,201,202]. The ultimate mechanical properties of the composites are therefore influenced both by the decreased tensile strength of the fibres and by the better fibre-matrix adhesion, leading to contrary results in the literature on the effect of the alkali treatment. According to our experience, the composite mechanical properties deteriorated due to the alkali treatment [199].

Additionally, it has to be noted that although the alkali treatment partially removes the components thermally less stable than cellulose, according to Sebestyén et al. due to the residual alkali ions the thermal degradation of the cellulose fraction is shifted to lower temperatures [203], all together leading to fibres with lower thermal stability. Furthermore, it is supposed that above a certain residual alkali ion concentration, the alkalisation process continues inside the fibres in an uncontrollable way leading to swollen and highly porous composites [201].

Based on these results we did not consider the alkali treatment of the jute fabric justified, consequently untreated fabric was used for the preparation of composite specimens.

Storage modulus and glass transition

Storage modulus curves of reference and EP/ESO jute fibre reinforced composites as a function of temperature are shown in Figure 4.3.1. Table 4.3.2 shows the storage modulus values at 0 °C (below Tg) and 150 °C (beyond Tg) and Tg determined by the tan δ peaks.

74 Figure 4.3.1 Storage modulus curves of the jute reinforced EP/ESO composites

In most cases the storage modulus decreased with ESO addition. Three exceptions occurred: in the case of PER with 25 and 50% ESO in glass transition range and in rubbery state, as well in the case of DGEBA with 50% ESO in glassy and rubbery states the storage modulus values were higher than in the case of the neat synthetic systems. The Tg values of the composites decreased by 40 °C compared to the corresponding neat EP systems (see 4.2.1). In the literature this effect is attributed to the reaction of hydroxyl groups of the jute reinforcement and anhydride type curing agent [194]. As for the effect of ESO-addition on the Tg values of the composites, in aliphatic matrices, the Tg rather increases when the amount of ESO is increased, in good correspondence

2000

75 with the behaviour of the neat EP systems. Also in good agreement with the neat resin, in aromatic DGEBA the Tg values rather show a decreasing tendency, yet these values are higher than in the aliphatic systems due to the more rigid aromatic backbone of DGEBA. Also, similarly to the neat resins, the observed peak doubling in tan δ curves of samples containing 75% ESO suggests that at higher ESO content phase separation occurred. In these cases dual Tg values were determined as well (Table 4.3.2).

Table 4.3.2 Storage modulus and Tg values of jute reinforced EP/ESO composites

base resin GER ESO

Tensile and flexural properties of EP/ESO jute reinforced composites are shown in Table 4.3.3.

As the efficiency of the jute fibre reinforcement in woven structure was modest, the mechanical properties of the composites were rather determined by the mechanical properties of the matrices. It has to be emphasized that in the case of natural fibre reinforced composites matrix properties have significantly higher effect on the tensile properties than in the case of high performance fibre reinforced composites, because of the comparable fibre and matrix properties.

In aliphatic matrices the tensile strength showed a decreasing tendency due to ESO-addition, above 50% ESO content the tensile strength values were lower than in neat ESO composite, which could be the result of the phase separated structure [14]. The same tendency was observed in case of tensile modulus, but the values decreased below the level of neat ESO composite only from 75% ESO content. The flexural strength was between the values of the neat aliphatic resin and neat ESO composites, in the case of PER composite with 25% ESO even a slight increase could be detected. From 75% ESO-content, the flexural modulus values decreased below the level of

76 neat ESO composite. In aromatic DGEBA systems all mechanical properties decreased with ESO addition, however they stayed above values of the neat ESO composites in all cases.

Table 4.3.3 Mechanical properties of the jute reinforced EP/ESO composites

base resin GER ESO

The mechanical properties of the 25% ESO-containing composites approach the properties of the reference DGEBA composite in the most values. In those applications where high Tg is not a crucial requirement, the jute fibre reinforced aromatic DGEBA epoxy resin composite can be replaced by 25% ESO-containing hybrid epoxy components. Given that besides the natural jute fibre and ESO, both PER, GER and the anhydride based hardener can be potentially synthesized from bio-based sources as well, this leads to a replacement by a fully bio-based composite without significant compromise in mechanical properties.