Controlling the Microstructure of Polymers.
Designing new chemistry has always been key to the development of polymers, going back to the initial efforts leading to the introduction of polyethylene and polyamides in the 1930s up to the oxidative polymerization methods developed in the 1970s at GE Plastics leading to brand new high performance polyethers. Improving performance through chemistry remains central, as demonstrated by new developments in the last decade: much publicized new metallocene catalysis leading to improved control over polyolefin molecular architecture, and new inherently conductive polymers. However, in virtually all applications, achieving the targeted performance still depends to a large extent on obtaining the right microstructure.
Phase Morphology in Blends
Properties of polymer blends depend on the composition, the intrinsic properties of the ingredients and the microstructure. Thus, the key to optimizing the performance of the material is control of morphology. Distinct types of morphology are required to optimize the impact strength, the stiffness/ductility balance or the barrier performance. The optimized microstructure results from strict control over processing conditions and equipment, and from methods of generating and stabilizing the structure, such as reactive compatibilization.
Processing must be carefully examined since the forming stage has a dramatic influence on the final morphology. During processing, the material undergoes complex deformation which will dictate the final microstructure. For example, a strong elongational flow component is present during convergent flow through a die. Depending on the rate of cooling, extrudates may show uniaxial orientation of elongated drops or enhanced dispersion.
Dispersed, lamellar and co-continuous are the three main classes of morphology found in polymer blends. These different blend structures are required for different applications. For example, a lamellar morphology is effective for reduction of permeability, while impact properties can be improved by engendering either a co-continuous structure (useful for large impact energy absorption) or small drops (useful for dissipation of crazes and cracks).
Toughness is probably the most examined property for polymer blends. Impact resistance depends strongly on morphological features. Depending on conditions (i.e. temperature, velocity), three fracture mechanisms can be observed in polymers. Brittle fracture with pure elastic deformation occurs at low temperature or high impact velocity. The two other mechanisms involve plastic deformation of some form: either shear yielding or cavitation. At higher temperature or lower velocity, plastic deformation occurs, causing extensive crazing, accompanied by volume increase. The required fracture energy in this mode depends on the number and size of crazes and is higher than for brittle fracture. At still higher temperature or lower velocity, some polymers exhibit larger deformation between extremely small flaws through shear, either diffuse shear yielding or localized shear bands. The fracture energy is still higher for this type of mechanism. In general, crazing seems to be the principal toughening mechanism but shear yielding also contributes. The shear contribution is particularly important for more ductile plastics. Crazes and shear bands are both initiated at rubber particles. In fact, the two mechanisms could be synergistic as shear bands act as obstacles to the crazes and keep the latter's size smaller. The optimum particle size differs from system to system but is usually in the range 0.5 to 2 pm.
In commercial blends, the morphology is normally stabilized by the addition of a compatibilizer or by inducing its formation during reactive processing. Particles grafted to the matrix are more efficient to diffuse cracks and absorb energy, and less likely to agglomerate in a shear field than when no grafting is present. This is the case for toughened engineering polymers that contain relatively small concentration (10 per cent or less) of a dispersed phase, which exists in the form of small droplets.
A different type of morphological structure, co-continuity of the phases, can prove beneficial by drawing on properties of both constituents: stiffness from a rigid polymer and high strain at break from a ductile one. For example, co-continuous morphology was observed to improve toughness: formation of a network, or honeycomb structure in which a modifier actually becomes a continuous phase. Such a structure was observed when chlorinated PE or EVA was added to rigid PVC. It was shown that for certain extrusion conditions, honeycomb structures were formed, in which the modifier phase surrounds the PVC primary particles and, at a composition of only 10 per cent, formed the continuous phase. The impact strength of the compound extruded at the optimum temperature was almost 5 times that of the same composition extruded outside the optimum processing window.
Under certain processing conditions and with the right rheology, the dispersed phase in a blend can form lamellae. A lamellar morphology is effective for reduction of permeability. In some cases, a permeability level similar to that obtained by co-extrusion was observed. Figure 1 shows hydrocarbon permeability for HDPE containers. The lamellar morphology shows significant improvement in permeability over the pure HDPE or the blend with droplet dispersion. The platelet-type dispersion, responsible for the high efficiency of the barrier phase, is obtained under specific processing conditions, in particular under low shear flow. Intensive melt compounding and dispersive mixing destroy the platelets and reduce the effectiveness of the PA.
Molecular orientation of polymer chains considerably affects the final properties of plastic products, such as stiffness, creep and impact resistance, barrier properties and transparency. Orientation of the macromolecular chains takes place in most forming operations, albeit to very different extents. In the forming processes where the deformation takes place in the molten state, most of the orientation is lost through relaxation of the chains before the structure can be frozen in place. For example, injection molding induces relatively low degrees of orientation in articles, causing some anisotropy in the physical properties. By contrast, if the material is in the solid state, relaxation is severely limited and the applied deformation will result in high levels of orientation, thereby inducing dramatic changes in the physical properties.
The manufacture of plastic films represents a very large segment of the plastics industry and constitutes the major application area taking advantage of molecular orientation, Biaxial orientation is particularly important in films, where it allows one to produce thinner films having superior mechanical, optical, and barrier properties, and, if required, the ability to shrink when reheated. The most widely used biaxial orientation processes are tubular film blowing and cast film biaxial orientation (or tentering). The specific properties required for the various applications (stiffness, shrink, optical properties, barrier, etc.) will depend to a great extent on the structure imparted through proper processing conditions. Processing allows one to obtain anisotropy in the properties by controlling the thermomechanical history of the material: levels of stiffness, shrink or tear resistance. Differences in the machine and transverse directions can be obtained by controlling the extent of deformation in these two d irections, allowing one to tailor the performance for the specific end use.
Solid State Forming
The first developments in highly oriented polymers were performed on fibres in the 1970s, in particular the polyaramid fibres ([Kevlar.sup.TM]) and later on the polyethylene ([Spectra.sup.TM] or [Dyneemaa.sup.TM]) fibres having tensile modulus and strength in the vicinity of 200 GPa and 3 GPa, respectively. These gel spun fibres are used in numerous applications: reinforcements in composites, antiballistic protection, and protective gloves and clothing.
Extension of these principles to articles of large dimensions can yield engineering materials of great potential. Different orientation processes have been investigated in order to do so: die-drawing for oriented tubes and shapes, roll-drawing for flat and simple geometry profiles, and uniaxial and biaxial solid state extrusion. These processes have been used successfully to form different materials, from polyolefins to engineering and specialty resins, and mechanical properties comparable to fiber reinforced plastics have been achieved. Figure 2 provides an example of the dramatic improvements that can be obtained by these processes: the tensile modulus can be increased by an order of magnitude or more by solid state roll-drawing. However, the development efforts have met with limited success in the market, because of the complexity of the processes, low throughput, and the difficulties of designing with these highly anisotropic materials. So far, successful applications include pallett strapping and orient ed piping. The significant development efforts still underway warrant that new end uses will emerge and provide more tools to material users.
Michel Dumoulin is director of materials and processes at the Industrial Materials Institute of the National Research Council, in Boucherville, QC. Abdellak Ajji is research officer, also at the Industrial Materials Institute. They are involved in development of science and technology in polymer processing.