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Chapter 5
Conclusions
In this work a general investigation on a novel laminate characterized by a nano-clay filled polypropylene matrix reinforced by a glass fibre mat was carried out. Although strictly interconnected and interdependent, the research on the nanocomposite and on the final laminate was conducted separately.In both cases, however, as a first step, a feasible and, possibly, low cost manufacturing strategy was investigated.
Regarding the matrix preparation a melt compounding procedure in a twin-screw extruder was chosen as NCP production route and it was shown that, in order to achieve better nano-filler dispersion, to reduce the clay tactoids size and to reduce the presence of voids in the resulting hybrid it is preferable:
• to adopt powder form PP;
• to keep the compounding temperature between 160° and 170° along the extruding barrel and the screw speed at 500 rpm;
• to dry out the NC before compounding.
DSC analysis performed on the hybrid demonstrated the nano-particles act as nucleating agent promoting polymer crystallization. Indeed when 5% of NC is added to PP, the Tc onset is raised from 118.2°C to 121.2°C and the increasing of the crystalline phase in the resulting hybrid is up to 8% in the 30’ cooled samples and up to 14.6% in the 16h cooled samples.
Although FIB is not a technique suitable to ‘see’ the single NC platelets, it could demonstrate the twin screw extrusion actually managed at least to stretch out the NC stacks.
Through XRD patterns study it was shown that the melt compounding technique always increases the initial NC interlayer distance up to 38.0 Å in a sample with 5% of C15A (being the pristine distance 31.3 Å) and up to 33.6 Å in a sample with 5% of I.30P (being the pristine distance 25.9 Å).
The use of the compatibilizer seemed not to significantly affect the dispersion of the Nanocor® filler, whereas a clear connection was found between the assessed dispersion level of the Southern Clay® product and the ratio PPgMA/NC in the nanocomposite.
Besides, it was shown that when a lower NC loading level is adopted, a higher dispersion level is achieved; to be precise 40.4 Å can be reached instead of
Chapter 5-Conclusions 95
37.7 Å if only 2% of I.30P is loaded instead of 5%, PPgMA/NC weight ratio being equal.
It was also shown that annealing the material does not cause collapsing of the intercalated layers, on the contrary it can slightly increase the interlayer distance (namely from 31.2 Å in a 19’ cooled sample to 32.8 Å in a 16h cooled one).
DMTA was used to evaluate possible variations in the glass transition temperature but no noticeable change could be detected.
Tensile testing showed that nano-filling technique improves the PP Young’s modulus up to a 17.70% higher value when 5% of NC is added. Although this improvement strictly depends on NC and PPgMA loading, it was shown it is not worth exceeding 10-15% of compatibilizer loading.
A very strict correlation between the crystalline phase amount of the manufactured NCP specimens and their elastic modulus was found (see Figure below), leading to the assumption that the improved mechanical nanocomposite performance is probably more related to the nucleating effect of nano-clays on PP than to their level of dispersion.
R2 = 0,9054 2,5 3 3,5 4 4,5 5 25,00% 30,00% 35,00% 40,00% 45,00% 50,00% Crystallinity Y oung' s M odul us ( G P a)
Figure 5.1: Dependence of elastic modulus on crystallinity
Such result indicates that the main issue, in NCP manufacturing, is not the achieved dispersion level but if and how this is connected with the NC nucleating effect and, consequently, with the mechanical behaviour of the final material. For this reason, it is clear that the nucleation mechanism of polymer crystals on organo-clay needs to be further investigated.
The laminate was prepared by pressure moulding two NCP plaques with a GF mat in between them and it was shown that it is preferable:
• not to exceed 200°C in the hot pressing stage;
• for the ‘sandwich’ thickness before laminating to be as close as possible to the desired final laminate thickness.
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Different GF mat grades were tested and it was shown that the best resistance to delamination, the highest tensile modulus and the most enhanced fibre-matrix adhesion were yielded, in order, by the pre-preg laminates followed by the emulsion coated and the epoxy functionalized ones. The worst performance was provided by the heat cleaned GF mat which demonstrated to be unsuitable for our purposes. The most significant results are summarized in the following table (the percentage below the tensile modulus reports its improvement in comparison with a pure polypropylene specimen).
Table 5.1: Significant results of tensile tests perfomrmed on laminates NC loading
and grade PPgMA wt%
GF mat grade E (GPa) Yield Stress (MPa) Elongation at break
NC free PPgMA free (+10.24)3.78 23.29 3.37%
2 (+24.61%)4.27 26.52 2.00% 5% of I.30P 5 (+56.94%)5.38 40.67 1.10% 2 (+18.74%)4.07 25.88 0.95% 5% of C15A 5 Epoxy f unc tionalised 5.74 (+67.27%) 38.22 0.95%
NC free PPgMA free (+47.10%)5.05 26.95 0.70%
2 (+40.55%)4.82 38.38 1.26% 5% of C15A 5 Em ulsion coat ed 5.48 (+59.72%) 41.09 1.33%
NC free PPgMA free (+215.35%)10.82 111.65 1.27%
2 (+187.27%)9.85 106.87 1.29% 5% of C15A 5 Pr e-pr eg 10.28 (+199.80%) 105.75 1.32%
SEM micrographs helped to qualitatively evaluate the level of glass cloth-matrix adhesion achieved and to foresee the tensile behaviour of the different laminates manufactured. Actually a better tensile behaviour was always confirmed to be related with a better wetting out of the fibre surface by the poliolefinic matrix estimated by this technique.
