A Practical Guide to Polymeric Compatibilizers for Polymer Blends, Composites and Laminates.
Jozef Bicerano, Ph.D.Introduction
Fundamental Considerations
Overveiw of Available Compatibilization Technologies
Representative Examples of Vendors and their Technologies
Technology Outlook
Introduction
The development of polymer blends, composites and laminates is a very active area of science and technology; of great economic importance not only for the plastics industry but also for many other industries where the use of such products is becoming increasingly more common.
Most pairs of polymers are immiscible with each other. Even worse is the fact that they also have less compatibility than would be required in order to obtain the desired level of properties and performance from their blends. Compatibilizers are often used as additives to improve the compatibility of immiscible polymers and thus improve the morphology and resulting properties of the blend. Similarly, it is often challenging to disperse fillers effectively in the matrix polymer of a composite, or to adhere layers of polymers to each other or to other substrates (such as glass or metals) in laminates. Continued progress in the development of compatibilization technologies is, hence, crucial in enabling the polymer industry to reap the full benefits of such approaches to obtaining materials with optimum performance and cost characteristics.
Term / Definition
Additive / Substance added to a polymer.
Adhesion / Holding together of two bodies by interfacial forces or mechanical interlocking on a scale of micrometers or less.
Adhesion promoter / See Coupling agent.
Chemical adhesion / Adhesion in which two bodies are held together at an interface by ionic or covalent bonding between molecules on either side of the interface.
Compatibility / Capability of the individual component substances in either an immiscible polymer blend or a polymer composite to exhibit interfacial adhesion.
Compatibilization / Process of modification of the interfacial properties in an immiscible polymer blend that results in formation of the interphases and stabilization of the morphology, leading to the creation of a polymer alloy.
Compatibilizer / Polymer or copolymer that, when added to an immiscible polymer blend, modifies its interfacial character and stabilizes its morphology.
Compatible polymer blend / Immiscible polymer blend that exhibits macroscopically uniform physical properties throughout its whole volume.
Composite / Multicomponent material comprising multiple different (nongaseous) phase domains in which at least one type of phase domain is a continuous phase.
Co-continuous phase domains / Topological condition, in a phase-separated, two-component mixture, in which a continuous path through either phase domain may be drawn to all phase domain boundaries without crossing any phase domain boundary
Continuous phase domain / Phase domain consisting of a single phase in a heterogeneous mixture through which a continuous path to all phase domain boundaries may be drawn without crossing a phase domain boundary.
Coupling agent / Interfacial agent comprised of molecules possessing two or more functional groups, each of which exhibits preferential interactions with the various types of phase domains in a composite.
Degree of compatibility / Measure of the strength of the interfacial bonding between the component substances of a composite or immiscible polymer blend.
Discontinuous or discrete or dispersed phase domain / Phase domain in a phase-separated mixture that is surrounded by a continuous phase but isolated from all other similar phase domains within the mixture.
Extender / Substance, especially a diluent or modifier, added to a polymer to increase its volume without substantially altering the desirable properties of the polymer.
Filler / Solid extender.
Hard segment phase domain / Phase domain of microscopic or smaller size, usually in a block, graft, or segmented copolymer, comprising essentially those segments of the polymer that are rigid and capable of forming strong intermolecular interactions.
Immiscibility / Inability of a mixture to form a single phase.
Immiscible polymer blend / Polymer blend that exhibits immiscibility.
Interfacial adhesion / Adhesion in which interfaces between phases or components are maintained by intermolecular forces, chain entanglements, or both, across the interfaces.
Interfacial bonding / Bonding in which the surfaces of two bodies in contact with one another are held together by intermolecular forces.
Interfacial region / Region between phase domains in an immiscible polymer blend in which a gradient in composition exists.
Laminate / Material consisting of more than one layer, the layers being distinct in composition, composition profile, or anisotropy of properties.
Matrix phase domain / See Continuous phase domain.
Miscibility / Capability of a mixture to form a single phase over certain ranges of temperature, pressure and composition.
Miscible polymer blend / Polymer blend that exhibits miscibility.
Morphology / Shape, optical appearance, or form of phase domains in substances, such as high polymers, polymer blends, composites and crystals.
Multiphase copolymer / Copolymer comprising phase-separated domains.
Nanocomposite / Composite in which at least one of the phases has at least one dimension of the order of nanometers.
Phase domain / Region of a material that is uniform in chemical composition and physical state.
Polymer allloy / Polymeric material, exhibiting macroscopically uniform physical properties throughout its whole volume, that comprises a compatible polymer blend, a miscible polymer blend, or a multiphase copolymer.
Polymer blend / Macroscopically homogeneous mixture of two or more different species of polymer.
Polymer composite / Composite in which at least one component is a polymer.
Soft segment phase domain / Phase domain of microscopic or smaller size, usually in a block, graft, or segmented copolymer, comprising essentially those segments of the polymer that have glass transition temperatures lower than the temperature of use.
Thermoplastic elastomer / Melt-processable polymer blend or copolymer in which a continuous elastomeric phase domain is reinforced by dispersed hard (glassy or crystalline) phase domains that act as junction points over a limited range of temperature.
Table 1: IUPAC-recommended definitions1 of key terms.
Before proceeding any further, it is important to summarize the definitions of some key terms, as recommended by the International Union of Pure and Applied Chemistry (IUPAC), in order to avoid any confusion. These IUPAC definitions are listed in Table 1.
This report provides a practical guide to the science and technology of polymeric compatibilizers for polymer blends, composites and laminates. This definition of its scope has several important implications:
- The report does not include any quantitative information regarding current or projected market sizes and market segmentation by product type and geographical region.
- The focus of the report is on additives that are used as compatibilizers, rather than being on polymer blends, composites, or laminates themselves. Consequently, while many blends, composites and laminates are discussed as examples of the optimum selection, use and effects of compatibilizers, we do not catalog and review the vast range of existing and developmental polymer blends, composites, laminates and their applications. It suffices to state that automotive and electrical/electronic applications provide the broadest range of opportunities for new compatibilizers. Significant opportunities also exist in the packaging, major appliance, sports/recreation equipment and medical device industries; as well as in the continued development of plastics recycling technologies.
- Since our focus is mainly on "polymeric compatibilizers" (additives that are polymers) used in blends, composites and laminates, many types of compatibilization additives (surfactants, most liquid or powder additives of low molecular weight, silane and titanate coupling agents; and silane, phenolic, titanate and zirconate adhesion promoters) are not discussed.
- Our focus is on providing a "practical guide" consisting entirely of information that specialty chemical and polymer producers and compounders can use. Consequently, a lengthy review of the vast and rapidly growing academic literature on compatibilization is avoided. We also avoid a lengthy review of the rapidly growing patent literature, much of which consists of patents on technologies which (while they may have significant merit) will never become commercially significant. The author believes that these deliberate omissions are essential in order to help focus the reader's attention on the information that will be most useful in practice by avoiding lengthy digressions from the practical focus.
Section 3 provides a brief overview of the commercially available polymeric compatibilizers. The largest number of compatibilizers, by far, are modified polyolefins, most of which contain polar groups enhancing the compatibility of polyolefins with polar polymers, their ability to couple with (and thus disperse) inorganic fillers more effectively, and their ability to adhere to substrates. Some modified polyolefins contain reactive groups that may further enhance their effectiveness. Styrenic block copolymers constitute the second largest class of compatibilizers. These thermoplastic elastomers have hard blocks that segregate into a glassy glassy hard phase and soft blocks that segregate into a rubbery soft phase. Other polymeric compatibilizers include methacrylate-based polymers, polycaprolactone polyesters, polycaprolactone polyester / poly(tetramethylene glycol) block polyols, methacrylate-terminated reactive polystyrene, and mixtures of aliphatic resins of low or medium molecular weight.
Section 4 discusses selected products of specific vendors as representative examples. The multiple roles that the same additive can perform (especially blend compatibilizer, filler coupling agent, adhesion promoter and impact modifier) are highlighted with many examples
Section 5 provides an outlook on compatibilization technologies.
Fundamental Considerations
Performance Versus Price
As an empirical rule2 shown in Equation 1, if a polymeric product remains a commodity material competing for use in commodity-type applications, the price that the average customer is willing to pay will only increase proportionally to the logarithm of the improvement in its performance:
In this equation, Price2>Price1, Performance2>Performance1 are the corresponding performance levels, "c" is a positive proportionality constant and "ln" is the natural logarithm. See Figure 1 for a schematic illustration. This equation can be generalized readily to more complex cases where the overall "desirability" for a particular application depends on several performance criteria that have different levels of relative importance.
Figure 1: Schematic illustration of the "commodity trap"; namely, the empirical rule2 that, if a polymeric product remains a commodity material competing for use in commodity-type applications, then the price that the average customer is willing to pay for this material will only increase proportionally to the logarithm of the improvement in its performance
The main implication of this equation is that whatever is done to improve the performance of a polymer (blending, incorporation of fillers, lamination, processing in a different way, etc.) must not be allowed to increase by much the sales price required to make a profit if its improved performance remains in the commodity product range. We will refer to this fundamental limitation on the price that the market will be willing to pay for a commodity polymer as the "commodity trap". It is only if the performance can be increased sufficiently to make the material competitive for higher-valued specialty applications (thus escaping the "commodity trap") that a significant price increase can be allowed. A few examples will be provided below.
Car manufacturers are usually reluctant to pay a large price premium (sometimes any price premium at all) for the improved performance of parts fabricated from engineering plastics unless they are producing extremely expensive (and prestigious) vehicles such as Rolls Royce or Ferrari. More generally, automotive consumers are often willing to pay for features that are noticeable by their five senses (such as more attractive fascia, more comfortable controls, high-intensity discharge headlights, advanced sound systems and a quiet interior), as well as for major enhancements in vehicle quality and safety. On the other hand, if the effects of a new feature or component of a vehicle cannot be "sensed" by the consumer and if it also has no implications in terms of significantly enhanced real or perceived quality and safety, consumers will not be willing to pay any price premium for it and cost will be the overriding consideration.
If an inexpensive polymer (such as a polyolefin) can be modified so that its properties become competitive with those of an expensive engineering plastic, it can escape the "commodity trap" since new potential applications become possible for it. It can then command a significant price premium over the "ordinary" (commodity) grades of the polymer. It must, however, still remain cheaper than the engineering plastic which it displaces in a higher-valued application. See Figure 2 for a schematic illustration.
Figure 2: Schematic illustration of two situations where blending and/or compounding are especially attractive from a commercial viewpoint. The thick vertical brown line represents the minimum acceptable performance required to qualify a material for a certain application. The ellipses represent regions on the "price-performance plane". EP1 is an expensive engineering polymer that far exceeds the performance requirements of the application. EP2 is a cheaper blend or composite of EP1 with less expensive ingredients, still exceeding the minimum performance requirements. CP1 is a commodity polymer that does not meet the performance requirements of the application. CP2 is a blend or composite of CP1 that exceeds the minimum performance requirements and can thus be sold at a substantially higher price.
Most people agree about the desirability of recycling but are unwilling to pay any price premium at all for plastic parts with enhanced recyclability. As a result, the growth rate of post-consumer recycling enabled by the use of compatibilization additives has been considerably slower than it would have been if its environmental benefits really outweighed economic factors in most people's minds. This is clearly an area where new or improved compatibilization technologies can make a significant impact.
The effects of market forces summarized above are sometimes modified (on some occasions drastically) by governmental regulations. Such regulations are most often related to safety or to environmental benefits. Regulations can involve international, national, or local governing bodies. They can differ significantly between different regions of the world, such as the United States and the European Union. They can modify the technologies and products that are available, as well as the relative costs of the available choices. Examples include governmental demands for increasing fuel economy and reducing tailpipe emissions in vehicles and for increasing the amount of plastic recycling. When such changes are mandated by governments, the cost-effectiveness of useful polymer compatibilization technologies can change drastically.
Thermodynamic Equilibrium Phase Diagram
The latest edition of a book by Bicerano3 and illustrations of compatibilizer structure and action posted on the website of SpecialChem were used as the main resources for this subsection.
The rapid screening of possible compatibilizers by predicting how their molecular architectures, chemical structures and concentrations affect the thermodynamic equilibrium phase diagram is a challenging but useful starting point. ("Molecular architecture" refers to the overall pattern of construction of a molecule. For example, a molecule that contains five subunits of chemical structure A and five subunits of chemical structure B could have its A and B subunits arranged randomly, or in an alternating fashion as in ABABABABAB, or in "blocks" of A and B subunit as in AAAAABBBBB, etc.) At present, such relatively routine predictive screening is only feasible for formulations without reactive components since the techniques for dealing with complexities introduced by chemical reactions in reactive compatibilization are less developed.
The fundamentals of compatibilization have been studied for many years, especially for the equilibrium (thermodynamic) properties. Methods for predicting the phasic behavior of nonreactive mixtures have advanced tremendously in sophistication and accuracy (and hence in reliabilty and practical utility) in recent years. It has been shown that, with the proper selection of the material parameters describing the system components and their mutual interactions, the same fundamental physical theory can give all observed types of phase diagrams. Different simulation methods differ mainly in the details the calculation of how the enthalpy (H) and the entropy (S) change upon mixing. Thermodynamic equilibrium is determined by the drive towards minimum Gibbs free energy, G=H-TS, where T is the absolute temperature.
The simplest example involves the calculation of the phase diagrams of binary amorphous polymer blends. These phase diagrams can be predicted (or can at least be correlated) quite easily as functions of the chemical structures and molecular weights of the component polymers by using the Flory-Huggins solution theory. According to this theory, the enthalpy of mixing ( Hmix) between mixture components A and B (and thus the deviation from ideal mixing at thermodynamic equilibrium) is proportional to the "binary interaction parameter" AB. The case of AB=0 indicates ideal mixing where Hmix=0. The very rare case of AB<0 indicates an enthalpic driving force towards mixing ( Hmix<0). For the vast majority of mixtures, AB>0 (and hence Hmix>0), indicating that the components enthalpically prefer to be surrounded by other molecules of their own kind. Larger positive AB indicates stronger enthalpic driving force towards phase separation. Entropy always favors mixing. The total free energy of mixing, Gmix, is the sum of enthalpic and entropic terms. For a binary blend of polymers A and B, it is given by Equation 2, where R is the gas constant, Vtot is the total volume of the two polymers, Vref is a reference volume (in practice, Vref=100 cm3/mole is often used), A and B are the component volume fractions and n A and n B are their degrees of polymerization in terms of Vref.