A Practical Guide to Polymeric Compatibilizers for Polymer Blends, Composites and Laminates.

Jozef Bicerano, Ph.D.

Introduction Fundamental Considerations Overveiw of Available Compatibilization TechnologiesRepresentative Examples of Vendors and their TechnologiesTechnology 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.

AdhesionHolding together of two bodies by interfacial forces or mechanical

interlocking on a scale of micrometers or less.

Adhesion promoter See Coupling agent.

Chemical adhesionAdhesion 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 blendImmiscible 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 compatibilityMeasure 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 bondingBonding 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.

LaminateMaterial 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.

MiscibilityCapability of a mixture to form a single phase over certain ranges of

temperature, pressure and composition.

Miscible polymer blend Polymer blend that exhibits miscibility.

MorphologyShape, 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.

NanocompositeComposite in which at least one of the phases has at least one

dimension of the order of nanometers.

Phase domainRegion 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 blendMacroscopically 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 2 presents the "practical fundamentals" of compatibilization. The five key factors that every compatibilization additive developer must consider in order to improve the likelihood of achieving technical and commercial success simultaneously are identified and discussed. These five factors are (1) performance versus price, (2) the thermodynamic equilibrium phase diagram, (3) metastable morphologies often induced by processing conditions, (4) practical implications of kinetic barriers to equilibration and (5) morphology-property-connections.

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.

Phase separation occurs if AB has a sufficiently large positive value to overcome the entropic effect. The entropic effect decreases rapidly in relative importance with increasing effective degree of polymerization n, so that miscibility decreases with increasing n. The product AB? quantifies the combined effects of degree of polymerization and intermolecular interactions on miscibility. Equation 3, where d0, d1, d2 and d3 are fitting parameters, can produce all of the observed types of binary amorphous polymer blend phase diagrams shown in Figure 3. This

equation can be used either correlatively by fitting the theory to experimental data on phasic behavior or predictively by fitting to the interaction energies predicted by atomistic simulations.

Figure 3:

Schematic

illustration of

possible

types of

polymer blend

phase

diagrams, for

binary blends

where

additional

complications

that can be

introduced by

competing

processes

(such as the

crystallization

of a

component)

are absent.3

The

coefficients

d1 and d2

refer to a

general

functional

form (see

Equation 3)

for the binary

interaction

parameter

AB.

While most commercially successful compatibilizers are random copolymers, block copolymers consisting of dissimilar blocks (most commonly, blocks differing greatly in chain rigidity) have always been viewed as obvious candidates for use as compatibilizers. Each type of block interacts more favorably with a different polymeric component in the blend. Since the blocks are connected to each other by covalent bonds, they cannot "get away" from each other. Consequently, their favorable interactions with and penetration into the phase domains of dissimilar polymers force these polymers to become more intimately mixed. Compatibilization is considered to have occurred if the phase domains of the immiscible polymers in the blend become small enough that the blend can be considered to manifest "microphase" instead of "macrophase" separation. It is even better if the componenta can be mixed at the nanoscale.4 The design of nanostructured blends creates opportunities to develop novel materials whose property profiles can be tailored more precisely for specific applications. The use of block copolymers as compatibilizers provides the ability to achieve such nanoscale self-assembly.

The thermodynamics of blend compatibilization by block copolymers have been investigated extensively by Leibler5 and by Balazs et al. 6,7 These researchers formulated models for predicting the molecular architecture and composition of effective compatibilizers for any given binary polymer blend. While Leibler's model can be applied equally to premade and reactive compatibilizers, the latter have more complexity due to the intriguing interfacial reaction kinetics. The role of such reaction kinetics in blend compatibilization has been studied both theoretically (Fredrickson and Milner,8,9 O'Shaughnessy et al.10,11 ) and experimentally (Macosko et al.12 ) in recent years, but much remains to be done before robust models that can routinely be used to guide reactive blend design become available. Preliminary data on the compatibilizing influence of fillers in polymer blends have been reported by Rafailovich et al.13 (for organoclays) and by Lipatov et al. , 14,15,16 (for silica). This is also an area where much further work is needed to develop robust models that can truly

guide polymer blend as well as polymer composite design.

In addressing a specific set of problems via modeling, one can usually readily decide which method is most appropriate. Once a choice is made, a particular experimental and/or modeling capability to screen additives and processing conditions can generally be found. The ability to predict the thermodynamic equilibrium mixing behavior in a blend, mixture, or composite with reasonable reliability helps target experimental work towards the most promising directions. This statement is valid regardless of the intended application of the blend, mixture, or composite material. A recent review article on industrial applications of polymer modeling 17 includes some examples of applications of thermodynamic equilibrium mixing considerations.

The three major classes of compatibilizers can be distinguished from each other in terms of the primary mechanism by which they reduce the interfacial tension between incompatible polymers and thus favor finer dispersion with more regular and stable equilibrium morphologies:

Figure 4: Use

of a block

copolymer for

compatibilizati

on. The block

copolymer will

prefer to

migrate to the

interface to

reduce the

interfacial

tension. Red

blocks are

compatible

with Polymer

A (matrix).

Blue blocks

are

compatible

with Polymer

B (dispersed

phase). The

consequence

will be lower

interfacial

tension,

better

interfacial

adhesion and

better

dispersion.

Block or graft copolymers (Figure 4).

Figure 5: Use

of an

nonreactive

polymer

containing

polar groups

for

compatibilizati

on by the

creation of

nonbonded

interactions

[in order of

increasing

strength,

dispersive,

polar

cohesive and

hydrogen

bonding

(strongest

type of polar

cohesive)]. If

all else is kept

equal, the

stronger and

more

"specific" the

nonbonded

interactions,

the higher is

the

compatibilizati

on

effectiveness.

In general,

the

compatibilizer

must be

compatible

with one

phase

(generally

with the

nonpolar

phase) and

must create

specific

interactions

with the other

phase.

Nonreactive polymers containing polar groups (Figure 5).

Figure 6: Use

of a reactive

functional

polymer for

compatibilizati

on. Reaction

at the

interface

between

functional

groups on the

different

polymers

creates, "in-

situ", a

grafted block

copolymer.

The

functionalized

copolymer is

miscible with

the matrix

and can react

with

functional

groups of the

dispersed

phase.

Reactive functional polymers (Figure 6). Many compatibilizers of this class also contain nonreactive polar groups in addition to reactive groups. Maleic anhydride (MAH, see Figure 7 for an example of how it works) is the most commonly used type of reactive group in such polymers. The second most commonly used type of reactive group is glycidyl methacrylate (GMA, see Figure 8 for an example of how it works) which introduces epoxy functionalities.

Figure 7:

Compatibilizat

ion by MAH-

grafted

reactive

functional

polymers.

Maleated

polymers can

be prepared

directly by

polymerizatio

n or by

modification

during

compounding

via the

reactive

extrusion

process.

Their

anhydride

groups can

react with

amine, epoxy

and alcohol

groups. In this

example, the

reaction

between a

maleated

polymer and

the -NH2 end

groups of

Polyamide

6,6 (Nylon

6,6)

compatibilizes

a

polyamide/pol

yolefin blend.

Figure 8:

Compatibilizat

ion by GMA-

grafted

(epoxidized)

reactive

functional

polymers.

They react

with amine,

anhydride,

acid and

alcohol

groups,

making them

effective in

compatibilizin

g polar

polymers with

nonpolar

polymers

according to

the

mechanism

shown above.

Some of these types of polymers (especially those containing polar functional groups and/or reactive groups) are often also effective as coupling agents between polymers and inorganic fillers in composites (Figure 9) and/or as adhesion promoters between incompatible polymers in a laminate or between polymers and a substrate such as glass or a metal. In all cases, they owe their effectiveness to the same fundamental underlying cause; namely, their favorable effect in modifying the thermodynamic equilibrium state towards which the morphology of the system will evolve unless its evolution is hampered by kinetic barriers as will be discussed next.

Figure 9: A

polymeric

coupling

agent

attaches an

inorganic filler

to the

polymer

matrix and

thus

compatibilizes

the filler with

the polymer

by

nonbonded

(physical)

interactions

and/or

chemical

bonds. It must

be compatible

with the

polymer

(ideally, it

should have

the same

chemistry as

the polymer),

as well as

being able to

interact with,

react with, or

even better

"glue" to the

filler.

Metastable Morphologies Induced by Processing Conditions

The latest edition of a book by Bicerano3 was used as the main resource for this subsection.

The morphology of a polymer blend or composite is often not at thermodynamic equilibrium but instead at a metastable state that the morphology is "frozen into" as a result of the processing conditions used in fabrication. Metastability refers to the ability of a system to exist indefinitely in a state separated by an energy barrier from a thermodynamically more stable state. The "classic" example is that people often say that "diamonds are forever" although graphite is thermodynamically more stable than diamond. A diamond will, in fact, become transformed into graphite if it is heated for a sufficiently long time at a sufficiently high temperature. In polymer blends and composites, factors that can cause and influence deviations from thermodynamic equilibrium include the relative viscosities of polymeric components during the blending process, details of mixing equipment and conditions and post-fabrication physical aging by annealing.

High shear may produce morphologies that deviate strongly from thermodynamic equilibrium; broadening greatly the volume fraction range over which phase co-continuity may occur in a polymer blend. Such morphologies may be "frozen in" by kinetic barriers when the specimen is cooled. A dramatic example is how the use of optimal melt processing conditions along with appropriately chosen compatibilizers has led to lamellar co-continuous morphologies, thereby producing blends whose solvent and gas barrier properties differed drastically from those of ordinary blends of the same composition.18 In this example, kinetic barriers were used to help design metastable morphologies with desirable properties. It is also possible to use high shear to help disperse fillers in polymers and to create morphologies where stiff anisotropic fillers (such as fibers and platelets) have a preferred orientation.

Annealing tends to coarsen the blend morphology, by reducing the total interfacial area per unit volume so that the interfacial components of the Gibbs free energy G can be minimized.

Economic value can be gained by the development of combinations of blend or composite

formulations and processing conditions that enable the components to mix well at lower shear rates. Less sophisticated (and hence less expensive) mixing equipment can then be used to attain the desired morphology, reducing equipment costs. Energy costs can sometimes also be reduced, provided that the ability to process at a lower shear rate can be attained without requiring a substantial increase in the processing temperature. It is also valuable to design processing conditions that can shorten cycle times and/or enable thin or complex-shaped objects to be manufactured faster and with better quality. Metastable morphologies induced by the processing conditions are important in making any of these process improvements.

It is crucial, for promising blend and composite formulations, to explore how the phase structure depends on the processing conditions. Physical phenomena in polymers take place over a vast range of length and time scales. Atomistic simulations describe physical processes whose trea™ent requires the explicit consideration of the atoms. Simulations at the continuum level describe the behavior of the bulk material. Mesoscale simulation methods (such as dissipative particle dynamics and dynamic density functional theory) bridge between these two scales. They describe phenomena taking place at length and time scales that are larger than atomistic but smaller than macroscopic, such as the collective behavior of chain segments consisting of several repeat units lumped together into "beads" connected to adjacent "beads" by "springs". They provide valuable insights on morphology evolution over time in heterophasic polymer systems. There is, therefore, intense ongoing research to improve their abilities to predict the dynamic pathway along which the morphology evolves from an initial state towards thermodynamic equilibrium. Nonetheless, much additional work is needed to develop reliable rules for predicting (even at a merely qualitative level) kinetic effects on the phase structure. An empirical "statistical design-of-experiments" approach is, hence, currently (and possibly for the foreseeable future) most often the best approach for optimizing such effects.

Practical Implications of Kinetic Barriers to Equilibration

The compatibilization of immiscible polymers is one of the most important, widespread and difficult problems in contemporary applied polymer science. In investigating various methods of compatibilizing immiscible blends, one can roughly distinguish two broad types of approaches:

1. Modification of Processing Conditions. These methods could include:(a) Increasing the processing temperature.(b) Increasing the motor speed and/or improving the mixing by some other means.2. Modification of Polymer Formulation. The additives could include:(a) "Standard" (premade) compatibilizers.(b) Reactive compatibilizers.(c) Other substances (such as silica, carbon, or clay nanoparticles) that may manifest a compatibilizing effect under some conditions.

Some techniques [such as 1(a), 2(a) and 2(b) and perhaps in many cases also 2(c)] rely on thermodynamics to "break up" macrodomains and ensure "true" homogeneity of the system. Other methods [1(b) and 2(c)] rely on kinetics to "break up" domains constantly and force the system to remain "approximately" homogeneous in metastable morphologies with domain sizes not exceeding ~1 micron. Several of these techniques are often combined in practice. For example, it is quite common to increase both the temperature and the shear rate during processing, while also including both a compatibilizer and other substances in the formulation.

It is difficult to prescribe a priori which method should be used for any particular problem. Each method has its own advantages and disadvantages. For example:

If it were practically feasible, increasing the processing temperature to the point where two polymers become miscible would certainly solve thermodynamic incompatibility problems. However, this solution is impractical for many realistic systems in which the transition from a two-phase system to a one-phase system occurs far above the decomposition temperature of one or both components.

Improving mixing can be relatively easy and straightforward, but the mixture can quickly phase separate into large droplets once shear (a kinetic factor) is removed.

Compatibilizers (such as short chains of block copolymers or random copolymers) can reduce the interfacial tension to near-zero levels and promote mixing on the nanoscale. However, this effect is limited by the migration knietics of compatibilizer molecules towards interfaces and can thus be very slow,.

Reactive compatibilizers rely on chemical reactions that take place during processing to attach themselves to the polymers that are being blended and thus compatibilize immiscible polymers with each other. In practice, they can be either more effective or less effective than standard compatibilizers, depending on the choices of reactive groups and catalysts.

The addition of lower molecular weight molecules (compatibilizers) sometimes leads to a dramatic worsening of various properties (such as stiffness, toughness, or flame retardancy) even if these additives improve the compatibility of the polymers in the blend.

The addition of nanoparticles may be a useful and interesting method of compatibilization, but its mechanism is not well-understood and so far there have been only a few studies describing this effect which is at the frontiers of compatibilization science and technology.

Morphology-Property Connections

The latest edition of a book by Bicerano3 was used as the main resource for this subsection.

The qualitative connections between polymer blend or composite morphology and mechanical properties, as well as the mechanisms by which an additive can improve the mechanical properties, are known. Many additives can often perform multiple roles and sometimes do so

simultaneously in a given polymeric system. Here is a summary of the most commonly found multiple roles. These roles will be illustrated with many examples in later pages of this report.

A "blend compatibilizer" often also functions as an "impact modifier". The morphological changes resulting from enhanced compatibility can increase the impact strength at ambient temperature and also help retain acceptable impact strength at lower temperatures than is possible in the absence of the additive. These morphological changes typically are the development of much smaller (in some instances, interpenetrating) phase domains that are better connected to each other, enabling improved load transfer across phase boundaries.

If a polymer (or blend) contains reinforcing fillers (such as inorganic fibers), an additive that can compatibilize the polymers in a blend may also act as a "coupling agent" between the polymer(s) and inorganic fillers, helping disperse the fillers and bond them to the polymer(s) and thus increase the stiffness (modulus), strength and impact toughness of the composite.

A compatibilizer may often also act as an "adhesion promoter" between a polymer (or blend) and a substrate, or between adjacent layers consisting of dissimilar polymers in a multilayer structure. Better interlayer adhesion results in better mechanical properties.

Both analytical (micromechanical) and numerical simulation (most commonly, finite element) methods for the semi-quantitative prediction of such effects are still under development. For multilayer systems with good interlayer adhesion and known layer properties, the equations of lamination theory or numerical simulations can often be used to predict some key properties quantitatively as a function of the properties and the arrangement of the layers in the laminate. More generally, the ability to make reliable quantitative predictions remains further in the future.

In a practical blend or composite design project, it will generally be useful to use the qualitative and semi-quantitative insights that can be gained from theory and simulations to provide some guidance to experimental work intended to link the formulations of products of interest to their final mechanical properties. It will, however, be essential both to verify the qualitative validity of anticipated connections between morphology and mechanical properties and to quantify these connections as a part of product design and optimization, by means of careful experiments.

In relation to the mechanical and other properties, it is important to keep in mind when blends and composites can provide the most value and thus offer the greatest profit potential. It is when their properties are not simple weighted averages of the properties of their components, with all of the compromises and tradeoffs inherent in such an average. The best blends and composites offer far more than just a compromise between the properties of their components. Instead, they offer synergies whereby the product can provide combinations of performance characteristics that are unattainable by using any single polymer, at a

reasonable price. If an additive supplier is able to provide compatibilizers that enables certain polymers to blend better or certain fillers to be incorporated more effectively into polymers and thus provide such synergistic combinations of properties, it will be rewarded by the market.

Here is an example of what is meant by a synergistic combination of properties. Polymers (just like other materials) become embrittled as the temperature is lowered. It is highly desirable for exterior body panels in cars to have high impact strength at very low temperatures. A car producer would want to be able to sell the same car in Alaska, with comparable safety and quality attributes, as it is able to sell in Texas. On the other hand, plastic parts used in exterior body panels are normally painted by the "e-coat" electrostatic painting process where they are subjected to elevated temperatures for a prolonged period in a baking oven. One needs to avoid warpage and/or other dimensional changes of a panel during this manufacturing step so that the polymer must be able to maintain its high stiffness ("modulus") up to very high temperatures and thus avoid "creep". In other words, the polymer needs to have a very high "heat distortion temperature". An empirical trend (with fundamental underlying physical causes) is that the low-temperature fracture toughness (resistance to brittle fracture under impact) of a polymer decreases with increasing high-temperature stiffness (elastic modulus). One reason why General Electric's NORYL™ GTX blends have been successful in this application is that they are able to provide a desirable combination of adequate low-temperature toughness and high-temperature stiffness, while still remaining at a reasonable price.

Another interesting example of a synergistic combination of properties comes from the frontiers of composite materials development, in nanocomposites where the "exfoliation" and dispersion of highly anisotropic clay platelets with a thickness of ~1 nanometer in polypropylene is improved by using MAH-grafted polyropylene. For low clay loadings (up to 2.5% by weight), it is observed that the tensile strength, modulus and fracture toughness all increase substantially. 19

It should be clear by now that any polymeric compatibilizer can potentially also serve as an impact modifier, if incorporated in the right amount, into an appropriate polymeric system, by using a suitable processing technique. It is important to emphasize, next, that while all polymeric compatibilizers thus have the potential to serve as impact modifiers, all polymeric impact modifiers are not necessarily compatibilizers. It is possible for some polymeric additives to serve as highly effective impact modifiers in certain polymeric systems without also playing the role of a compatibilizer. In order to understand this subtle but important distinction, we must delve deeper into the mechanisms of toughening a polymer by incorporating another phase in it.

Rubber particle incorporation is a common toughening method. However, voids and even rigid particles are sometimes used as tougheners. Toughening occurs by imparting either the ability to craze (in brittle matrix polymers such as polystyrene) or the ability to undergo shear

yielding (in pseudoductile matrix polymers such as Polyamide 6,6) more effectively. It has also been shown, in work on rubber-toughened polypropylene, that energy dissipation due to viscoelastic relaxation may sometimes be an additional toughening mechanism. The main initial role of the inclusion (whether it is a rubber particle, a void, or a rigid particle such as CaCO3) is to act as a stress concentrator in its vicinity because of the difference between its stiffness and the stiffness of the surrounding matrix material. The local initiation and then the propagation of many crazes or shear bands (or both, in polymers which exhibit mixed failure modes) increases the energy dissipation required to cause failure so that the polymer becomes "tougher". The optimum rubber phase morphology correlates with the nature of the matrix phase. The extent to which a polymer can be toughened at a given rubber volume fraction depends on its intrinsic toughness:

For brittle (crazing) thermoplastic matrix polymers, the controlling parameter is the optimum rubber particle size. This parameter decreases with increasing matrix ductility, so that if the matrix polymer is less brittle then smaller rubber particles may be able toughen it.

For pseudoductile (shear yielding) thermoplastic matrices, the controlling parameter is the critical average distance between the surfaces of two neighboring rubber particles. This parameter increases with increasing matrix ductility, so that if the matrix polymer is more ductile then rubber particles that are further apart from each other may be able to toughen it.

Much work has been reported on the quantification of these trends in terms of intrinsic characteristics of polymers (such as characteristic ratio and entanglement density), the morphologies of polymers (such as the effects of crystallinity), and characteristics of the particles of the second phase (volume fraction, size distribution and spatial distribution).

It has also been found that rubber-toughenable thermosets with high glass transition temperature (Tg) are more readily obtained if the high Tg is attained by enhancing the chain stiffness than if it is attained by increasing the crosslink density.

It should be clear from the paragraph above that many entities can act as impact modifiers without serving as compatibilizers. These entities include rubber particles (which are polymers), and in some instances voids or even rigid particulate fillers. Such entities can "toughen" a polymer without playing any role in compatibilizing immiscible polymers, in coupling polymers to fillers, or in helping enable the adhesion of dissimilar materials in laminates. The focus of this report is on compatibilization technologies. Consequently, while many examples of impact modification by compatibilizers will be highlighted to provide a complete perspective of their versatility as additives, we will not discuss impact modifiers which are not also compatibilizers.

Overveiw of Available Compatibilization Technologies

The information provided in this section was assembled through extensive searches on the worldwide web which has become the best available source of product information. Most companies provide detailed information online regarding their products, often including case studies describing the use of their products and/or citations to relevant articles in the open literature. There are also many online databases [such as SpecialChem (which contains a very extensive additives database), Omnexus, MatWeb, CAMPUS and IDES Prospector] of commercial polymers, blends and additives. These databases all provide free access to their compilations, but some require the payment of fees to gain access to their "premium content".

The author considered whether to list the URLs of the many worldwide web pages from which information was extracted and decided not to list them. Unlike a book or a journal article, URLs are quite ephemeral. They can change and/or be removed at any time, potentially resulting in considerable frustration and waste of time for a person looking for them a year or two after they were cited. Readers interested in more detailed information about the products discussed in this section are recommended, instead, to visit the most current websites of the online databases named above and of the companies named below.

Companies sometimes change identity because of events such as mergers and acquisitions. Furthermore, product lines are sometimes sold from one company to other. Trademarks generally outlive such events. Consequently, searching the worldwide web by using the tradename of a product as a keyword may also be a good strategy to find the most recent information about a product line a few years after the completion of this report.

Company Product Tradename

MODIFIED POLYOLEFINS

DuPont

Ethylene-VAc-CO (CO denotes carbon monoxide), ethylene-BA-

CO and ethylene-BA-GMA terpolymers; ethylene-MA, ethylene-EA

and ethylene-BA copolymers.

Use of CO as a comonomer results in the incorporation

of -C(O)- (ketone) groups along the chain backbone.

Elvaloy

DuPont A very broad range of MAH-grafted polyolefins. Fusabond

DuPont

Ethylene-methacrylic acid (MAA) ionomers. Zn2+ or Na+ is used as

the counterion in the different product grades.

MAA repeat unit: -CH2-C(CH3)(COOH)-.

Anionic MAA repeat unit: -CH2-C(CH3)(COO-)-.

Surlyn

DuPont Poly(vinyl alcohol), repeat unit: -CH2-CH(OH)-. Elvanol

STYRENIC BLOCK COPOLYMERS

BASF Styrene-butadiene (SB) diblock copolymers.

B repeat unit: -CH2-CH=CH-CH2-. Styrolux

BASF Styrene-butadiene-styrene (SBS) triblock copolymers. Styroflex

Dexco Polymers

Styrene-butadiene-styrene (SBS) and styrene-isoprene-styrene

(SIS) triblock copolymers.

I repeat unit: -CH2-CH=C(CH3)-CH2-.

VECTOR

Kraton Polymers

SBS and SIS triblock copolymers, their hydrogenated midblock

versions and their hydrogenated midblock versions grafted with

functional groups such as MAH. .

KRATON

Kuraray SBS and SIS triblock copolymers (hydrogenated B or I block).

See Figure 22 for the chemical structures.SEPTON

OTHER TYPES OF COMPATIBILIZERS

Degussa Methacylate-based polymeric compatibilizers. DEGALAN

Dow Chemical

Polycaprolactone (PCL) polyesters, PCL polyester /

poly(tetramethylene glycol) (PTMEG) block polyols.

PCL repeat unit: -(CH2)5-COO-.

PTMEG repeat unit: -(CH2)4-O-.

TONE

Polymer

Chemistry

Innovations

Methacrylate-terminated reactive polystyrene.

See Figure 27 for the chemical structure. Methacromer

Struktol Mixture of aliphatic resins with a molecular weight below 2000

g/mole, blend of medium molecular weight resins.STRUKTOL

Table 2: A representative (but not comprehensive) selection of polymeric compatibilizer suppliers and

their products, some acronyms used in this report, and trade names for the products. The products

listed below will be discussed further in Section 4.

Table 2 lists the companies and products that will be discussed further in providing examples of the use of polymeric additive technologies. The information provided in Table 2 is intended to constitute a representative sampling of the types of additive technologies and is not (nor was it intended to be) a comprehensive listing. The suppliers of polymeric compatibilizers cited in Table 2 will be discussed in the next section, in alphabetical order. It is hoped that sufficient detail will have been provided in this broad survey to give the reader a good idea of the type of additive product that may be most appropriate for his/her needs and thus focus further effort.

The largest number of polymeric compatibilizers, by far, are the modified polyolefins. Polymeric additives manufactured by DuPont are used in this review to provide illustrative examples of such additives and their utility. Most types of modified polyolefins contain polar groups that enhance their compatibility with polar polymers, and their abilities to couple to (and disperse) inorganic fillers more effectively and to adhere to substrates. In some modified polyolefins, some or all polar functional groups are reactive. Reactive functionalities may further strengthen the effectiveness of an additive by creating chemical bonds to a polar polymer, filler, or substrate. The abundance of competing modified polyolefin additive

technologies from many vendors reflects the tremendous commercial importance of the polyolefins as inexpensive commodity polymers that can be used for a wide range of applications. The importance of polyolefins has been growing in recent years. This trend is driven both by advances in catalyst technology that have made it possible to "tailor" polyolefins more precisely than was possible in the past and by the desire to expand the use of polyolefins in applications where the incumbent materials are much more expensive engineering thermoplastics.

Styrenic block copolymers constitute the second largest general 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. The growth of this technology (as illustrated here in the context of products from BASF, Dexco Polymers, Kraton Polymers and Kuraray) is a result of the synergistic superposition of three key factors that encourage intense research and development activity towards its continued development:

1. These types of block copolymers have many important applications on their own right, in addition to their use as additives.2. Polystyrene is a relatively inexpensive commodity polymer that has a very broad range of applications. Consequently, new additives that improve its properties and/or allow it to be blended with a broader range of polymers will be valuable. 3. Advances in anionic polymerization technology, as well as in the ability to predict the effects of molecular architecture on the properties of a block copolymer, have resulted in the ability to "tailor" styrenic block copolymers increasingly more precisely for targeted applications.

Other types of commercially available polymeric compatibilizers include methacrylate-based polymers (Degussa), polycaprolactone polyesters and polycaprolactone / poly(tetramethylene glycol) block polyols (Dow Chemical), methacrylate-terminated reactive polystyrene (Polymer Chemistry Innovations), and mixtures of aliphatic resins of low or medium molecular weight (Struktol).

Automotive and electrical/electronic applications provide the broadest range of opportunities for new polymeric compatibilizers; as blend compatibilizers, coupling agents, adhesion promoters and/or impact modifiers. Significant opportunities also exist in the packaging, major

appliance, sports/recreation equipment and medical device industries; and in the continued development of plastics recycling technologies.

Representative Examples of Vendors and Their Technologies

BASF

Figure 10:

Characteristic

s and

applications

of BASF's

Styroflex SBS

triblock

copolymers.

BASF makes the Styrolux™ styrene-butadiene (SB) diblock and Styroflex™ styrene-butadiene-styrene (SBS) triblock copolymers. These polymers have many important applications on their own right, in addition to being useful as polymer blend compatibilizers and as impact modifiers in polymers (especially polystyrene) and blends. See Figure 10 for the characteristics and applications of Styroflex. Such versatility is also shared by the styrenic block copolymers (SBCs) of other manufacturers (discussed later) and enhances the growth of SBC technology.

Degussa

The DEGALAN™ products of Degussa are specially-designed thermoplastic methacylate-based polymeric compatibilizers for polymer blends. Acrylic polymers typically manifest excellent resistance to UV light and saponification, colorfastness and durable gloss and good chemical resistance. The selection of suitable methacrylic comonomers makes it possible to obtain coating systems with excellent resistance, especially to outdoor exposure. Coatings manufactured according to standard formulations do not yellow even after prolonged weathering and show no change in color. They are also remarkable for their durable high gloss and very low tendency to chalking. Applications include heat-seal lacquers, PVC finishes, concrete coatings, marine and container paints, low-odor interior paints, metal

coatings, printing inks, exterior paints, ceramic transfer lacquers and halogen-free plastisols.

Dexco Polymers

Dexco Polymers is a joint venture between Dow Chemical Company and ExxonMobil Chemical Company. It makes VECTOR™ styrene-butadiene-styrene (SBS) and styrene-isoprene-styrene (SIS) triblock copolymers, which are thermoplastic elastomers, via anionic polymerization.

Different VECTOR polymer grades differ in their relative amounts of rigid (polystyrene) and soft (polybutadiene or polyisoprene) blocks, molecular weights, molecular architecture (whether the arrangement of the blocks is linear or radial), whether any residual diblock copolymer is present, whether any other component is present and/or the physical form in which the product is supplied (pellet or powder). These differences cause variations in properties and processing characteristics. For example, increasing molecular weight generally improves mechanical properties but reduces the ease of melt processing. Increasing the relative amount of the rigid blocks results in a stiffer (higher modulus) polymer. Any change in the composition or molecular architecture can also alter the thermodynamics and kinetics of mixing with other polymers and thus affect the action of these polymers as blend compatibilizers and/or impact modifiers.

VECTOR block copolymers are used by producers and compounders of olefinic and styrenic thermoplastics, engineering resins, thermosets, blends and alloys, to enhance the toughness and impact strength of such materials at ambient and low temperature. When used in blends, they enhance the compatibility between appropriate types of dissimilar polymers (such as styrenic polymers and olefinic polymers). Diblock-free grades also extend the high-temperature performance range of the modified base resin compared to conventionally polymerized styrenic block copolymers containing diblock residues. Some grades can be used as base feedstocks for the manufacture of more advanced engineering resins. Others are tailored to overcome the deleterious effects of additives such as flame retardants. The superior heat resistance of halide-free VECTOR grades manifests itself in in the improved color stability of the base resin and is especially evident after multiple-heat exposures of in-plant recycle. Some VECTOR grades may be qualified for certain food contact and/or medical applications. The recycling of plastics (where compatibilization of dissimilar polymers is of crucial importance) is another focus of product development activities. For homogeneous recovered plastics, VECTOR block copolymers can renew the properties, resulting in near-virgin product performance.

The VECTOR grades available as of the date of this report are 2411, 2411P, 2518, 2518P, 4461, 6241, 6507, 7400 and 8508 (SBS); and 4111A, 4113A, 4114A, 4211A, 4215A, 4230 and 4411A (SIS). The product grades containing the letter "P" (2411P and 2518P) are provided as powders while the other grades are provided as pellets. The following grades include a diblock copolymer component: 2411, 2411P, 4113A, 4114A, 4215A and 4230. In

VECTOR 7400, a linear, pure SBS triblock copolymer is extended with 33% mineral oil. The molecular architecture is radial in VECTOR 2411, 2411P and 4230; and linear in the other grades.

Dow Chemical Company

The TONE™ polycaprolactones are truly biodegradable when composted and thus of special interest when biodegradability is desired. TONE P-767 and P-787 are linear polycaprolactone polyesters with high crystallinity and a low melting temperature, used in various thermoplastic blend applications. They have broad miscibility or mechanical compatibility with many polymers (see Table 3), resins and pigments. Applications include use as dispersants, compatibilizers and reactive modifiers for other polymers such as polyesters and nylon fibers. TONE P-767 can be injection molded, extruded, slot-casted into films, or blended with other polymers. It is available in pellet or powder form. TONE P-787 can be extruded or blended with other polymers. It was specially formulated for use in high melt strength thermoplastic applications.

Miscible

Poly(vinyl chloride) (PVC), poly(styrene-co-acrylonitrile) (SAN, 24 % to 29 %),

poly(acrylonitrile-co-butadiene-co-styrene) (ABS), polydroxyether of bisphenol-A,

phenoxy resin, polycarbonate, nitrocellulose, cellulose butyrate, cellulose propionate,

chlorinated polyether, polyepichlorohydrin, poly(vinylidene chloride), styrene/allyl

alcohol copolymers.

Mechanically

Compatible

Polypropylene, poly(1-butene), polyethylene, natural rubber, styrene/butadiene

elastomers, styrene/butadiene block copolymers, unsaturated polyesters, epoxies,

phenolics, poly(vinyl acetate), poly(vinyl butyral), polybutadiene, ethylene/propylene

rubber, polyisobutylene, polyoxymethylene, polyoxyethylene.

Table 3: Miscibility and compatibility of polymer blends containing poly( -caprolactone).

The TONE polyol-based urethane product family consists of grades which are either liquids at room temperature (25°C) or have melting temperatures not too far above it. They can be formulated for adhesion to various substrates at ambient and at elevated temperatures. The applications of TONE 7241, a linear polycaprolactone polyester / poly(tetramethylene glycol) (P™EG) block polyol designed for use in elastomers and microcellular systems with enhanced flex-fatigue performance and hydrolytic stability, include polyol blend compatibilization.

DuPont

DuPont makes four product lines of functionalized polyolefins. The many applications of these materials include polymer blend compatibilization, coupling of polymers to fillers, promotion of adhesion of polymers to substrates as well as to dissimilar polymers in multilayer structures and impact modification of polymers. Different grades of each product line are optimum

choices for use in different applications. Many of these polymers meet the requirements of the Food and Drug Administration of the USA for use in a number of regulated applications.

Elvaloy™ ethylene-VAc-CO (VAc: vinyl acetate, CO: carbon monoxide), ethylene-BA-CO and ethylene-BA-GMA terpolymers; and ethylene-MA, ethylene-EA and ethylene-BA copolymers, can toughen (impact modify) and flexibilize (plasticize) other polymers. Because of their high molecular weights, unlike conventional plasticizers, they do not migrate to the surface and hence are not lost through evaporation or extraction. They can flexibilize and toughen many polymers; such as PVC, ABS, polypropylene, PET, PBT and polyamides. They also serve as compatibilizers in polymer blends and coupling agents between polymers and fillers.

Fusabond™ MAH-grafted polyolefins include modified conventional as well as metallocene polyethylenes, ethylene propylene rubbers, polypropylenes, ethylene-BA-CO terpolymers and ethylene-VAc copolymers. They are used as coupling agents between polymers and fillers and as high-performance impact modifiers for engineering polymers. Each grade offers its own specific interpolymer adhesion characteristics. Their functionalization makes them effective in helping bond together polymers used in toughened, filled and blended compounds. For example, MAH-grafted polyolefins can compatibilize and thus help blend, polyamides with polyolefins. Polyamide-polypropylene blends that can be made by using such compatibilizers can be used in applications such as parts for automotive cooling systems. Such applications require the high-temperature properties of the polyamide. However, since moisture absorption can degrade the polyamide, polypropylene is also needed to reduce moisture absorption. The Fusabond coupling agents can also provide new levels of functionality in polymer-wood composites and in other wood alternatives.

Surlyn™ ethylene-methacrylic acid ionomers (with Zn2+ or Na+ used as the counterion in the different product grades) provide impact toughness, abrasion resistance and chemical resistance various consumer and industrial products. They can either be used by themselves or blended with other polymers. They can be injection-molded, extruded, foamed, thermoformed, or used as a powder-coatings or resin modifiers. The resulting applications range from tough, cut-resistant golf ball and bowling pin covers, to footwear components, glass coatings, abrasion resistant surfaces and buoys. Their high resistance to chemicals and oils enables them to provide unique packaging options for perfumes and cosmetics.

Polymer Blend Compatibilizer DuPont's Recommendations

PA6/PE PE-g-MAH, E-MAA (Zn) Fusabond E, Surlyn 1652

PA6/PP PP-g-MAH Fusabond P

PBT/PP Ethylene-BA-GMA Elvaloy PTW

PBT/PA Ethylene-BA-GMA Elvaloy PTW

PET/Polyolefin Ethylene-BA-GMA Elvaloy PTW

PC/ABS Ethylene-Acrylate Elvaloy AC, Elvaloy PTW

PC/PBT Ethylene-Acrylate Elvaloy AC, Elvaloy PTW

Table 4: Some important types of polymer blends and both the best generic compatibilizer

chemistries and the compatibilizers recommended by DuPont for each of them. PA6 denotes

Polyamide 6 (Nylon 6). PC denotes polycarbonate.

Figure 11:

Example

showing the

finer

dispersion

and more

regular and

stable

morphologies

that can result

from

compatibilizati

on. Both

micrographs

show the

morphology

of a blend of

30%

Polyamide 6

with 70%

linear low-

density

polyethylene.

A grade of

Fusabond

has been

used at a

level of 10%

as a

polymeric

compatibilizer

in one of the

two samples.

Table 4 lists some important types of polymer blends and provides both the best generic compatibilizer chemistries and the compatibilizers recommended by DuPont for each of them. Compatibilization reduces the interfacial energy between two polymers and thus increases the adhesion between them. Compatibilizers also generally provide finer dispersion, more regular and stable phase morphology, better mechanical properties, improved surface characteristics and enhanced recyclability. Figure 11 shows a dramatic example of the finer dispersion and more regular morphologies that can result from the addition of a suitable compatibilizer.

Figure 12:

Effects of

using a small

amount of

Elvaloy as an

impact

modifier in

polymers. (a)

PC(50)/PBT(5

0)/Additive(10

) blend

compared

with

PC(50)/PBT(5

0). Effect the

choice of

impact

modifier on

notched Izod

impact

strength at

room

temperature

(23 °C) and at

0 °C. (b)

Great

increase in

impact

strength of

PVC, with

negligible

reduction in

heat distortion

temperature.

Figure 12 shows the effects of using a small amount of Elvaloy as an impact modifier. Figure 12(a) illustrates how an additive can often perform more than one role in a blend. Various grades of Elvaloy, which compatibilize polycarbonate (PC) with poly(butylene terephthalate) (PBT), also serve as impact modifiers in PC/PBT blends. It can also be seen that, while the use of any of these additives improves the impact strength compared with the uncompatibilized blend, various grades differ drastically in the magnitude of their effectiveness. This example thus also illustrates the need to select the specific product grade within a given additive product line very carefully to obtain the desired level of properties at the lowest possible cost. Figure 12(b) shows that a small amount of suitable grade of Elvaloy can improve the impact strength of poly(vinyl chloride) (PVC) drastically with very small reduction in the heat distortion temperature.

Figure 13:

General

structure of a

multilayer film

(laminate).

Multilayer structures ("laminates", see Figure 13) are used in many packaging applications. The combination of layers generally provides a mix of the individual performances of the polymers involved (such as barrier, sealability, moisture or chemical resistance and stiffness)

that is usually impossible to achieve with a single polymer. The recyclability of the resulting multilayer material is also desired. The interlayer compatibilization of multilayer.polymeric materials (such as Polyamide/PE, Polyamide/EVOH/PE, PE/EVOH/PP, PE/EVOH/PE and PET/PE) is, hence, crucial. Functionalized polyolefins are very useful in such "adhesion promoter" applications.

Elvanol™ 71-30 is poly(vinyl alcohol). It is prepared in aqueous solutions. Transparent films with high tensile strength, tear resistance and barrier properties are formed upon evaporation of water. Elvanol 71-30 provides excellent adhesion to porous, water-absorbent surfaces. It also provides a combination of excellent film forming and binder characteristics. Its applications are in adhesives, paper and paperboard sizing and coatings, textiles, films and building products.

Kraton Polymers

KRATON Polymers makes both clear and oil-extended grades of its styrenic block copolymers, which are thermoplastic elastomers.

KRATON D polymers are elastic and flexible. The choice of soft block influences the properties. For example, styrene-butadiene-styrene (SBS) is especially suitable for footwear and for the modification of bitumen/asphalt, while styrene-isoprene-styrene (SIS) is preferred for the production of pressure-sensitive adhesives.

The middle blocks of SBS and SIS can be hydrogenated to make KRATON G block copolymers. These polymers include styrene-ethylene/butene-styrene (SEBS) and styrene-ethylene/propylene-styrene (SEPS). KRATON G block copolymers have the added benefits of enhanced oxidation and weather resistance, higher service temperatures and increased stability during processing by common thermoplastic processing technology. Their applications include use as sealants and high performance adhesives.

KRATON FG polymers are KRATON G polymers that have been grafted with functional groups such as maleic anhydride. KRATON FG polymers can manifest improved adhesion to polar substrates such as metals and polyamides. They can be used as impact modifiers for polar polymers such as polyesters, polyamides and epoxies. They can also help compatibilize polyamides and thermoplastic polyesters with polyolefins.

Kuraray

Figure 14:

Four types of

SEPTON

block

copolymers:

(Top left)

Hydrogenated

poly(styrene-

b-isoprene)

[polystyrene-

b-

poly(ethylene/

propylene)

(SEP)]. (Top

right)

Hydrogenated

poly(styrene-

b-isoprene-b-

styrene)

[polystyrene-

b-

poly(ethylene/

propylene)-b-

polystyrene

(SEPS)].

(Bottom left)

Hydrogenated

poly(styrene-

b-butadiene-

b-styrene)

[polystyrene-

b-

poly(ethylene/

butylene)-b-

polystyrene

(SEBS)].

(Bottom right)

Hydrogenated

poly(styrene-

b-isoprene/bu

tadiene-b-

styrene)

[polystyrene-

b-

poly(ethylene-

ethylene/prop

ylene)-b-

polystyrene

(SEEPS)]4 .

Each type of

polymers has

its own

unique set of

properties.

Kuraray uses its isoprene technology to make the SEPTON™ hydrogenated styrenic block copolymers (Figure 14), which are thermoplastic elastomers.

Figure 15:

Main

structural and

morphological

features of

the SEPTON

hydrogenated

styrenic block

copolymers

made by

Kuraray. The

styrenic block

copolymers

made by

other

companies

(such as

BASF, Dexco

Polymers and

Kraton

Polymers)

also possess

similar

general

features.

Prior to processing, the polystyrene end blocks are associated in rigid domains. In the presence of heat and shear (such as the shear imposed during processing), the polystyrene domains soften and permit flow. After cooling, the polystyrene domains reform and harden, locking the rubber network in place. This physical phenomenon provides SEPTON its high tensile strength and its elasticity. These general features are illustrated in Figure 15.

Figure 16:

Scanning

Electron

Micrographs

(×1000),

illustrating

compatibilizati

on by

SEPTON.

When blended with polyolefins, SEPTON improves various properties, including the impact strength. It can also compatibilize polyolefins with polystyrenes. In the Kuraray product literature, examples are given of the use of SEPTON as a polypropylene impact modifier and as a compatibilizer in blends of polypropylene with ABS. The much better mutual dispersion of ABS and polypropylene in the blends using a SEPTON compatibilizer can be seen from the micrographs shown in Figure 16.

Property ABS(70)/PP(30) ABS(70)/PP(30)/SEPTON(5)

Notched Izod (J/m) 49 88

Unnotched Izod (J/m) 167 549

Flexural Modulus (MPa) 2040 1980

Table 5: Data from Kuraray, showing how its SEPTON 2104 compatibilizer, when added at a level of

5% by weight, improves the impact strength of a 70/30 blend of ABS and polypropylene (PP) at room

temperature (25 °C) drastically while causing only negligible loss in stiffness.

The data listed in Table 5 show that the notched and unnotched Izod impact strength both increase drastically as a result of the improved morphology resulting from compatibilization, while the loss in stiffness (as measured by the flexural modulus) is negligible. This example, therefore, also illustrates how an additive can perform multiple roles. SEPTON clearly serves both as a compatibilizer (Figure 16) and as an impact modifier (Table 5) in this particular blend.

Polypropylene 100 80 80 80

SEPTON 2004 0 20 0 0

SEPTON 2007 0 0 20 0

Ethylene-Propylene Rubber 0 0 0 20

Izod Impact Strength (J/m, at 25 °C) 117 614 547 164

Izod Impact Strength (J/m, at -20 °C) 38.5 141 122 90

Flexural Modulus (MPa) 752 572 671 656

Flexural Strength (MPa) 23.3 18.3 19.3 18

Table 6: Data from Kuraray, showing tremendous improvements in the Izod impact strength of

polypropylene at both ambient and low temperatures with the use of SEPTON 2004 or SEPTON 2007

as an impact modifier. Formulations are indicated in terms of the percentages of their ingredients by

weight. There are only small reductions in flexural modulus and strength. Note that SEPTON is far

more effective than ethylene-propylene rubber as an impact modifier.

Table 6 shows tremendous improvements in the Izod impact strength of polypropylene at both ambient and low temperatures, with only small reductions in flexural modulus and strength.

Polymer Chemistry Innovations Inc.

Figure 17:

Chemical

structure of

Methacromer

™ PS12

reactive

polystyrene.

More than

85% of the

polymer

chains are

terminated

with a

methacrylate

group.

Polymer Chemistry Innovations Inc. makes the Methacromer™ PS12 methacrylate-terminated reactive polystyrene. The chemical structure of this polymer is shown in Figure 17. Its physical properties resemble those of polystyrene woth a low molecular weight. It allows formulators to modify polymers with a high degree of control. It is especially attractive to adhesive manufacturers since it can be used to increase the shear strength with only minor effects on the peel strength. It is available in a standard molecular weight range of 11,000 to

15.000 g/mole, with 12,000 g/mole as the target molecular weight. The molecular weight can be modified to meet individual specifications. The polydispersity is low: [(Mw/Mn)<1.1]. More than 85% of the polymer chains are terminated with a methacrylate group. It reacts readily with the acrylates and acrylamides, imparting toughness while keeping the polymer thermoplastic.

Struktol

Struktol's product line of STRUKTOL™ polymer additives includes the STRUKTOL TR product grades (among which TR 060 and TR 065 can be considered primarily as compatibilizers), as well as the more recently developed STRUKTOL TPW product grades.

TR 060 and TR 065 are normally incorporated at low levels (0.5% to 1 %) into a formulation. They both meet the requirements of the Food and Drug Administration of the USA for use in a number of regulated applications.

TR 060 is a mixture of light-colored aliphatic resins with a molecular weight below 2000 g/mole. It has good solubility in aliphatic, aromatic and chlorinated hydrocarbons. It is a compatibilizer and blending aid that reduces splay in colored and/or filled polyolefins. It is very compatible with the polyolefins. It can be used to increase extrusion output rates. It has a natural tackiness at process temperatures. This "adhesive" nature enables it to act as an effective binder. This is especially important in polymers where high filler levels require the most uniform blending in order to maintain or improve the physical properties. In addition, its low molecular weight provides some viscosity reduction during processing, improving the flow characteristics. TR 060 has been shown to improve the blending of thermoplastic olefin (TPO) compounds, flame retardant formulations and filled polymer systems. In general, it is recommended for use with polyolefins, ABS, styrene-acrylonitrile (SAN) copolymers, general-purpose polystyrene, high-impact polystyrene, rigid poly(vinyl chloride) (PVC) and polyesters such as poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT).

TR 065 is a blend of medium molecular weight resins designed as a high temperature process aid and blending agent. It works up to process temperatures of 370°C. Its compatibilizing action is useful in blending polymers, processing recycle materials and incorporating impact modifiers. It is effective in binding filler materials to the polymer system by virtue of its adhesive nature at process temperatures. In many cases, especially when high filler levels are involved, this more homogenous blend results in better physical properties and fewer processing problems. Its components make it compatible with many polymers, whether polar or nonpolar in nature. In general, it is recommended for use with polyesters such as PET and PBT, polyamides, nitriles, PVC, ABS, SAN and general-purpose or high-impact polystyrene.

The new STRUKTOL TPW product grades are polymeric additives that have been developed specifically for the improved processing of wood-filled thermoplastics. While they can be

viewed primarily as lubricants, their roles also include compatibilization. For example, the general purpose grade (TPW 101), which consists of a mixture of zinc stearate and waxes, can improve the processing characteristics of highly filled polyolefin compounds, as well as improving filler dispersion and providing metal release for both molding and extrusion operations. TPW 113, which is a blend of complex modified fatty acid esters, can provide superior filler wetting and dispersion characteristics in a wide range of polymer systems.

Technology Outlook

The development of polymer blends, composites and laminates is of great economic importance for the plastics industry and for other industries where the use of such products is becoming increasingly common. Advanced polymer modification techniques have grown in importance during the last two decades as the "point of diminishing returns" has been approached in improving the performance/price balance by altering just the chemical structures of polymers.

The most important polymer modification techniques are blending dissimilar polymers, preparing composites where a matrix polymer is modified by fillers, and creating multilayer (laminate) structures. The objective is to seek synergies between the components so that one can attain better performance without increasing cost or maintain acceptable performance at lower cost.

Polymeric compatibilizers are polymers that can be used (normally in small percentages) as additives to help assemble dissimilar components into polymer blends, composites and laminates with improved properties. These more attractive properties generally result from phase separation on a finer scale (microscale or even better nanoscale, instead of macroscale) along with stronger interconnections between phase domains. Impact modification (toughening) is one major benefit that can often be attained by using polymeric compatibilizers. It can be inferred from the anticipated continued growth of markets for polymer-based heterophasic products that polymeric compatibilization technologies will also contine to grow in importance.

The five key factors that every compatibilization additive developer must consider in order to improve the likelihood of achieving technical and commercial success simultaneously are (1) performance versus price, (2) thermodynamic equilibrium phase diagram, (3) metastable morphologies often induced by processing conditions, (4) practical implications of kinetic barriers to equilibration and (5) morphology-property-connections. Progress in the development of predictive methods based on theory and simulation was summarized. Methods for the prediction of the thermodynamic behavior of nonreactive systems are quite well-established. Significant further progress is needed for the development of more robust models for the thermodynanmic equilibrium state of reactive systems, for the dynamic

behavior of both nonreactive and reactive systems, and for the relationships between morphology and mechanical properties under large deformation. While major progress can be anticipated in all of these "frontier" areas of materials science over the next decade, a semi-empirical approach will be most useful in the practical development of new technologies for the foreseeable future.

The largest number of polymeric compatibilizers, by far, consist of modified polyolefins, most of which contain polar groups and some of which also contain reactive groups. Styrenic block copolymers, which are thermoplastic elastomers, constitute the second largest class of polymeric compatibilizers. Other commercial 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.

Significant progress can be anticipated over the next decade in the development of more refined grades (tailored for specific applications) of both modified polyolefin and styrenic

block copolymer (and perhaps also selected non-styrenic block copolymer) technologies. A major guiding principle for such work will be the desire to attain control over the resulting morphology at an increasingly finer scale. The development of compatibilizers for biodegradable polymer-based systems (a relatively minor area at this time) may also grow if the environmental and regulatory driving forces towards biodegradable polymer technology development gain strength.

Many companies are in the polymeric compatibilizer market with products falling into the same two major classes (modified polyolefins, styrenic block copolymers) competing for similar types of applications so that competition is fierce. On the other hand, these are currently also the two most versatile polymeric compatibilizer families. The markets for other types of polymeric compatibilizers (where the competitive landscape is less crowded) are more limited.

Customized additive compunding services are provided by many companies. Organizations that provide such services range from the technical service depar™ents of giant multinational corporations to small specialty compounding shops. Customized compounders can provide complete technology solutions and hence great value to their customers. It is anticipated that such specialized services (the detailed discussion of which fell outside of the scope of this review of polymeric compatibilizer products) will also continue to grow over the next decade.

Automotive and electrical/electronic applications provide the broadest range of opportunities for new polymeric compatibilizers; as blend compatibilizers, coupling agents, adhesion promoters and/or impact modifiers. Significant opportunities also exist in the packaging, major appliance, sports/recreation equipment and medical device industries. The continued development of plastics recycling technologies may also stimulate the growth of compatibilization technologies if it becomes driven by stronger environmental and regulatory

forces in the future.

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