The thermomechanical environment and the microstructure of an injection moulded polypropylene copolymer
Introduction
In the injection moulding process, a hot polymer melt is forced under pressure to flow between cold mould walls. The high viscous melt has a complex behaviour (e.g. shear thinning and viscoelastic nature) and a low thermal diffusivity. As a consequence, the process is characterised by an unsteady and non-isothermal flow, a high and local viscous dissipation and a spatial variation of the viscosity. Furthermore, the molten polymer can also experience the effect of significant normal stresses, especially at the flow front and in zones of pronounced geometry variation. As a result of these effects the obtained moulding has a microstructure gradient through the thickness, varying also along the respective flow path. This microstructure determines, to some extent, the mechanical behaviour of the mouldings.
The microstructure developed by semicrystalline thermoplastics when cooled down under specific thermomechanical conditions, namely shear fields and important thermal gradients, is widely reported in the literature [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. This structure features a hierarchy of macromolecular arrangements and a through-the-thickness morphological gradient, which in a simple description is composed of:
- (i)
two external highly oriented skin layers (in the case of polyolefins constituted by a peculiar shish–kebab crystalline structure [14], [15], [16]);
- (ii)
a central spherulitic core.
Between these two layers a transition zone is commonly found presenting deformed spherulite structures typically associated to the crystallisation under high shear and temperature gradients. The local thermomechanical environment imposed during processing dictates the relative dimensions and the morphology of these layers.
This work explores the development of the microstructure of an injection moulding propylene copolymer under the local thermomechanical environment imposed during processing, in terms of microstructural features associated to the skin and core layers.
Section snippets
Skin layers
The skin layers start forming during the filling phase due to the rapid cooling of the hot and very oriented melt against to the cold mould walls (development of the frozen layer). When observed by polarised light microscopy (PLM), the skin has a homogeneous appearance with no discernible morphological features. However, it has a peculiar microstructure, characteristic of a crystallisation under high stress fields and cooling rates, normally referred to as shish–kebab structures [14], [15], [16]
Thermomechanical environment in injection moulding
In injection moulding, the thermomechanical environment imposed to the polymer is associated with the combined effect of a particular set of processing variables (machine setting), mould geometry (feeding system, flow type and part geometry) and thermorheological properties of the material. As a result, specific fields of pressure, temperatures, shear rates and stresses are developed. These can be estimated by computer simulations of the injection moulding process or by adequate mould
Material and moulding programme
Axi-symmetrical dumbbell-like specimens of 1.5 mm of diameter were injection moulded using a propylene-ethylene sequential copolymer (APPRYL 3120 MR5). This commercial polymer has a narrow molecular weight distribution, and incorporates about 8% of ethylene, being typically used in automotive applications.
The moulding programme was defined accordingly to a central composite experimental design, including variations in three levels of the melt temperature (Tinj=200, 230, 270 °C), the mould
Thermomechanical environment quantification
The thermomechanical variables were calculated in a commercial mould filling simulation package, C-MOLD, considering only the filling stage. The results at the end of filling were considered. A complete finite element mesh of the moulding was used (cavities plus feeding system). The rheological behaviour of the melt was experimentally assessed by capillar rheometry [28]. In the simulations the material was assumed as a temperature dependent shear thinning fluid, with the viscosity, η, given by:
Results
In Table 2 are listed the results of the microstructure characterization and of the thermomechanical environment quantification. They are also resumed in Fig. 3, Fig. 5, Fig. 10, showing their variations with Yc and τw.
Level of orientation of the skin.
As observed in Fig. 5, the level of crystalline phase orientation of the skin, Ωs, is higher for the higher values of τw, as reflected by the Debye patterns. The variations of Ωs with the thermal and shear stress levels are shown in Fig. 6.
Ωs increases markedly with the shear level, and it slightly decreases for high Yc values. Ωs is maximised for the thermomechanical environment leading to the highest τw and lowest Yc. The variations are more accentuated for the lowest shear stresses, tending
Degree of crystallinity of the skin.
In Fig. 7 are presented the variations of degree of crystallinity of the skin layer, χs.
The trends reflect the effect of shearing on the crystallization kinetics. In fact, χs increases both with Yc and τw. Shearing was found to strongly increase the nucleation density and the growth rate during crystallization [37].
Double texture index of the skin.
The bimodal orientation of the crystalline phase (assessed by Cα) is, on the whole, higher for the highest Yc values (Fig. 5). A higher Cα reflects a stronger orientation of the a∗-axis component relatively to the c-axis. Fig. 8 presents the variations of Cα with Yc and τw.
The shear level basically determines the values of Cα. They decrease with τw, principally for the lowest shear stress levels. This means that the shear level increases the c-axis orientation component relatively to the a∗
β300-form contents.
The amount of β-phase in the skin is very small (index β300), but nevertheless, dependent on the processing conditions. The skin crystallises essentially in the α-form, although a small amount of β-form can also be found. Nevertheless, it seems interesting to analyse the variations of the β300 index with processing. β300 index is higher for both the extreme limits of the thermomechanical environment (Fig. 5). The variations of this index with the thermal and shear stress levels are not very
Skin thickness
The skin is the result of crystallisation under high stress fields and cooling rates. The molecular orientation induced by the flow cannot completely relax before the temperature reaches Tc, constraining the development of the crystalline structure. In this case, highly oriented structures are formed (shish–kebab structure); otherwise spherulites are developed. Demiray et al. stated that the skin thickness is controlled by the material relaxation time and the cooling rate [11]. A minimum shear
Conclusions
For this moulding, the thermal and shear stress levels are strongly coupled: the former increases as the latter decreases. The effect of the thermomechanical environment on the development of the microstructural features of the skin and core is resumed in Table 3.
For the mouldings and processing window used, the following relationships were established:
- •
The skin ratio, Sa, increases with the stress level, τw and decreases with the thermal one, Yc.
- •
The level of orientation of the skin, Ωs,
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