Influence of gamma irradiation on the thermal stability of blends of PP with previously treated sisal fiber

In this work, the thermal degradation of PP blends with previously treated sisal fiber (10mm), and gamma irradiated in air at different doses (10, 25, 50, 60, 70 kGy), is studied. The treatment to which sisal fibers were prev iously submitted were: (a) the addition of a coupling agent (Silane A-172), 1% by weight, (b) a chemical treatment at different times (1/2 and 1 h) wi th NaOH, and (c) the addition of a functionalized polypropylene known as Po lybond 3150 and 3200, 5% by weight. After the treatment, mixtures of PP wit h 20% of sisal fibers were prepared. Their thermograms were analyzed using different integral methods in order to evaluate the activation energy. For the blend of PP with sisal fibers, previously treated for 1 h with NaOH, th e largest value amounted to 60 kJ/mol (approximately) for irradiation doses under 40 kGy. At higher irradiation doses a decay on the activation energy was observed. When the fibers were treated with NaOH for 30 min, the activ ation energy decreased from 55 kJ/mol to values under 45 kJ/mol, when subje cted to high irradiation doses (> 50 kGy). This can be explained by the fac t that, when sisal fibers are treated with NaOH solutions over a longer tim e, impurities and lignin are better removed resulting in a higher surface r oughness; as a consequence a higher polymer to fiber interaction is possibl e, which, in turn, makes the blend more stable at low irradiation doses. In stead, when the sisal fibers were treated with silane, the activation energ y for thermal degradation decreased at higher irradiation doses until an in version of the tendency was observed for the 50 kGy dose; this may imply th at this irradiation induces the production of PP-filler bridges. With compo sites containing compatibilizers, a similar behavior as with silane was obs erved but with the inversion on the tendency at 70 kGy. From these results, it can be concluded that a l-h treatment with NaOH produces a higher therm al stability than the others. Moreover, this treatment has the best profit/ cost ratio, which makes it commercially profitable when compared with other s. The mathematical analysis shows that the blend of PP with NaOH-treated s isal fiber at the two treatment times (1 h and 1/2 h), showed competing deg radation and single decomposition processes for the irradiation intervals b etween 0-50 kGy and 50-70 kGy, respectively. Regarding the blends of PP wit h silane-treated sisal fiber or Polybond addition showed bimolecular decomp osition behavior for both irradiation intervals. (C) 2001 Elsevier Science Ltd. All rights reserved.