Melt rheology of polyolefins

Abstract

Rheology has a key position in polymer research, being an important link in the so-called ‘chain of knowledge’ reaching from the production of polymers to their end-use properties. A review of the melt rheology of polyolefins, which are the most widely used group of thermoplastic polymers today, is given in this paper both in terms of application and characterisation aspects. The materials are discussed according to their phase structures (single- and multi-phase polymers) and their chain structures (linear and branched). Aspects of the molar mass distribution, the chain structure and topology are discussed both from an experimental and theoretical point for the single-phase systems. For the technically more important types of multi-phase polymers like compounds and blends, the importance of rheological properties in the development of the phase structure is outlined as well as the possibility to use rheometry for structure investigations. In any case, the importance of considering the stress or strain history of a material sample in a rheological investigation is discussed. Finally, an outlook on the present and future developments in the field of polyolefins is given.

Introduction

For thermoplastic polymers, the knowledge and moreover the design of flow behaviour is essential for all forms of production and processing, as big parts of these occur in the molten state [1]. Additionally, rheology has attained a key position in polymer research, being an important link in the correlation chain (see Fig. 1) from the catalyst over polymerisation and chain structure to processing behaviour and final properties [2]. Thereby it forms an important knot in the so-called ‘chain of knowledge’ reaching from the production of polymers to their end-use properties, which has become increasingly important in view of the increasing speed of material development as a result of quickly changing customer requirements.

Polyolefins, which are normally defined as polymers based on alkene-1 monomers or α-olefins, are the most widely used group of thermoplastic polymers today. Based on their monomeric units and their chain structures, they can be divided into the following subgroups:

• Ethylene-based materials — polyethylenes (PEs) — produced under low pressure conditions with transition metal catalysts of various types and showing a predominantly linear chain structure. This subgroup includes high density PE (HDPE), medium density PE (MDPE), linear-low density PE (LLDPE) and other varieties, which are distinguished through the regulation of density and subsequently mechanical properties through the incorporation of higher α-olefins (mostly butene, hexene and octene) as comonomers. The linear nature of their polymeric chains can be disturbed twofold: by longer comonomers like butene, hexene or octene acting as short side chains [3] and by catalysts forming polymerisationally active oligomers being incorporated further as long chain branches (LCBs). Examples for the latter case including implications for the rheology of such systems are given by Yan et al. [4].

• Ethylene-based polymers (PEs) produced in a radical polymerisation under high pressures with oxygen or peroxides as chain initiators and showing a predominantly branched chain structure. According to their reduced crystallinities and densities, these materials are termed low density polyethylenes (LDPEs). A variation of the degree of branching is possible by various measures like temperature control, peroxide feed, residence time etc., which strongly affects the material's rheological and terminal properties [5].

• Propylene-based polymers produced with transition-metal catalysts — polypropylene (PP) and its copolymers — showing a linear chain structure with stereospecific arrangement of the propylene units. Mostly, the isotactic species — iPP — is used today, but also syndiotactic — sPP — or special stereoblock structures have become technically relevant, as they are available from single-site catalysts. A wide variation of material properties can be achieved with the incorporation of ethylene and/or higher α-olefins in various fashions; single-phase and multiphase materials are possible [6].

• Polymers based predominantly or exclusively on higher α-olefins (e.g. poly-butene-1), produced with transition-metal catalysts and having a linear and stereospecific chain structure.

• Olefinic elastomers based on transition metal or single-site catalysts, with or without the incorporation of dienes, which make these materials partially crosslinkable. These polymers are normally based on ethylene and propylene, mostly amorphous with high molar masses and rarely homogeneous in their phase structures. Nowadays, such elastomers are sometimes substituted with metallocene-based ultra-low density PEs (ULDPEs) termed plastomers [7], which have a more homogeneous structure and can be varied in their properties more easily.

Also of importance are an inhomogeneous group of materials that are based inhomogeneous group on blends of different polyolefins. The terminology is not completely ‘sharp’ here; extruder-based mixtures with solid substances (fillers and reinforcements) and elastomers of any kind as well as other polyolefins are normally called ‘compounds’, while similarly produced mixtures with non-PO polymers (and also elastomers) as well as reactively produced mixtures incorporating grafting steps are called ‘blends’. All of these materials have attained importance in areas where the physical limits of polyolefins need to be exceeded or basic characteristics of their chemical nature (e.g. hydrophobicity and apolarity) need to be changed.

Some limiting cases will not be treated within this review. These include wax-like atactic PP (used e.g. in glue systems and as asphalt modifiers) as well as olefin-oligomers produced as constituents for lubrification systems. Common to these materials is their low molar masses, which goes along with Newtonian behaviour in the molten state or even a liquid nature at room temperature.

Before going into details, the scope of this paper should be clarified. It will cover the melt rheology of polyolefins, including the limiting case of solidification — which is, in case of practically all technically relevant polyolefins, crystallisation. From a theoretical point of view, these properties can be split into:

• Linear viscoelastic properties, the range of material behaviour where a linear relationship between stress and strain exists. These are theoretically the simplest properties, having a direct relation to the molecular structure or superstructure in case of multiphase systems and being mainly used for investigation of the same. Normally, the determination of storage and loss moduli (G′, G″(ω)) combining low material and time demand with high precision [8] is used for this purpose. However, creep and relaxation measurements — J(t) and G(t) — have an important position here [9], giving improved access to the long-time (low frequency/rate) behaviour of viscoelastic materials.

• Non-linear viscoelasticity, indicating the stress (or strain) sensitivity of material behaviour. Technically most relevant is the classical determination of the flow curve, forming the base for all simple types of flow modelling. The most simple case of steady-state viscosity measurements is the determination of the melt flow rate (MFR) as outlined e.g. by Bremner et al. [10], but for meaningful values a good definition of the state of deformation during and even before the measurement is necessary. Problems in reproducibility and comparability frequently have their origin in a neglect of these influence factors. Also, a differentiation between stress- and strain-controlled measurements must be made [11], [12]. This has lead to two different philosophies in instrument design with different strengths and weaknesses, depending on the nature of the investigated systems.

Other non-linear properties include normal stresses (or normal stress coefficients) and measures for elasticity. These are mostly relevant in free-surface moulding processes like extrusion, blow moulding or foaming, and their determination is normally less straightforward. This also puts a practical limitation to viscoelastic modelling of such processes through the limited availability of the necessary data.

Although capillary measurements have lost some importance in recent years, these instruments are still very valuable for steady-shear investigations at very high shear rates for the determination of melt fracture phenomena [13] and related effects (sharkskin structure, oscillations and pumping, spurt effect, etc.). Mostly, purely optical detection is used for quantifying these effects, which are effectively limiting the output rates in all extrusion-type conversion processes. No uniform classification of melt fracture phenomena can be found for different types of polymers, also because of the significant influence of the polymer chain structure.

Extensional properties still have a special position in the whole field of melt rheology. Partially, the problem can be defined with ‘what should we measure — what is relevant’; and not even for the normal case of uniaxial extension a standard measuring procedure is available [14], [15]. Depending on whether somebody wants to determine ‘pure’ rheological properties or rather processing behaviour in technologies with a strong elongational flow component (e.g. film blowing, foaming, coating, etc.) the choice of an ‘appropriate’ measuring technique will have to be different. Details will be discussed in Section 2.2.2.

Generally, the appropriate range of deformation (strain) or load (stress) will have to be used in obtaining the relevant parameters for a given process (see Fig. 2). Also, time dependence and history effects have to be considered.

Polyolefins are nowadays used in practically all application areas of thermoplastics and are processed with all standard conversion techniques. Despite the fact that they are considered to be ‘well established’ and ‘well known’ materials, a multitude of new grades continuously appearing on the market and having partially new structural features are demanding special attention [16], [17], [18].

Moreover, the development of polymeric materials can be seen as a ‘push–pull-approach’. Both the input of new knowledge and results from the scientific side of the development — new catalysts, new polymers or basic facts on flow or solidification processes — and the steady change of the market situation — through customer demands, general trends or the overall socio-economic situation — are contributing to the speed of evolution.

The main types of processing, for which rheological requirements need to be considered, are:

• Extrusion — flat (cast) film, blown film including biaxially oriented film (BOPP), pipe and profile [19], [20]

• Extrusion coating and foaming [21]

• Injection moulding [22], [23]

• Extrusion blow moulding, injection-stretch blow moulding (ISBM) and thermorming [24]

• Fibre spinning [25]

• Special processes like rotomoulding or powder-slush moulding [26], [27]

Processing simulations today are still mostly limited to the viscous part of the behaviour of polymer melts and frequently make use of generalised newtonian models for description (as shortly discussed in Section 2.1). This is certainly not sufficient in case of free-surface or elongational-flow dominated conversion processes. But even if the full viscoelastic nature of a material is taken into account, the solidification as an integral part of the forming process still remains open. In case of polyolefins, solidification means crystallisation, which will strongly interact with the flow processes as such. This will be discussed to some extent in Section 3.3.