Advances in TPE polymer blends.

One of the first commercially successful thermoplastic elastomers, TPE, is based on a blend of two polymers; polyvinyl chloride and butadiene-acrylonitrile copolymer, NBR. PVC is semi-crystalline. Despite having asymmetric carbon atoms in the main chain, the free-radical polymerization of vinyl chloride is directed enough to provide random stereo-block sequences that melt at over 100 [degrees] C. Because NBR and PVC form a soluble blend, the Tg of the blend phase in the noncrystalline regions is suppressed enough to give an elastomeric phase. PVC plasticized with typical low molecular weight ester plasticizers has an analogous morphology, as do stereoblock polypropylene and ethylene [Alpha]-olefin copolymers of high ethylene content. These TPEs have a two phase morphology, usually depicted as the fringed micelle type, at use temperature. In the melt they are single phase.

In contrast, TPEs based on insoluble blends have a very different morphology. Most of these materials are based on blends of a semi-crystalline polymer and a rubbery elastomer that are compatible in the melt but are not soluble. They are two phase systems at both use and processing temperature. These blends consist of either two co-continuous phases or a semi-crystalline polymer phase containing a dispersed, crosslinked elastomer phase.

Co-continuous phase blend TPEs

Polyolefin blend thermoplastic elastomers

Polyolefin blend thermoplastic elastomers, based primarily on EPM or EPDM and iPP have been used extensively for a number of years and are an important family of engineering materials (ref. 1). These were the first polyolefin based TPEs and are currently used extensively in applications where a relatively high stiffness and hardness is required, particularly automotive applications.

Olefin polymerization catalysts do not easily lend themselves to A-B-A block polymers synthesis because of the necessity of having a living polymerization. A living polymerization must be free of chain transfer and have a single type of active center that exhibits a rapid initiation rate, slow propagation rate and the absence of chain transfer. While these constraints can be achieved at low temperatures with selected coordination catalyses for olefins, none have proven practical for the commercial production of tri-block copolymers. The metallocene catalysts have single active centers but typically do not possess all of the other necessary attributes for living polymerization.

Blends (ref. 1) of the proper morphology exhibit elastomeric properties and surprisingly good recovery after extension. When compounded and plasticized, they afford a wide range of useful properties, excellent processing, and can be produced at moderate cost. Rubbery blends based on iPP and EPM with an EP phase that is either highly long chain branched or partially cured can give property improvements, apparently due to altered morphology or a somewhat more elastic rubber phase.

iPP and several other crystalline polymers are transformable into elastic materials by changing their crystal structure (ref. 2). With isotactic PP it is possible to obtain 97% recovery from 100% extension by applying high stress during crystallization of melt spun fibers. These fibers have a non-rubberlike stress-temperature response and their elastic nature depends on their morphology. Electron microscopy shows close-packed lamellae with normals mainly parallel to the fiber axis. On extension, these tilt and split apart, creating voids. This nonentropic elastic nature of isotactic polypropylene may play a role in blend technology and perhaps explains the recovery characteristics after strain.

EPM and EPDM are more elastic at use temperature if the ethylene sequences are long enough to crystallize. Here the rubbery phase is elastic in nature. However, when these semi-crystalline elastomers are strained, more crystallinity develops and recovery from large extensions is not high. The concept of using blends of high ethylene content EPM or EPDM with polyethylene and iPP to give thermoplastic elastomers was introduced in early blend work (ref. 3).

EPM/iPP is a very useful blend system and considerable development work has been carried out to produce optimum processing and physical properties for many diverse applications. The use of plasticizers, fillers, stabilizer packages and other additives advance the successful uses of the blends.

Gessler and Kresge (ref. 4) found that there were considerable processing advantages to incorporating oil in blends of EPM and iPP. In the melt, the oil partitions between the phases, greatly lowering the viscosity. This gives very good injection molding and extrusion characteristics. On crystallization, the oil is believed to be adsorbed by the amorphous rubber phase as well as the amorphous regions in the iPP phase and is not rejected to the material's surface. The mechanical properties at 25 [degrees] C of these blends are similar to those without oil at the same volume fraction of polypropylene. Low temperature properties are altered due to the plasticization of the non-crystalline polypropylene phase.

Blends of EPM and, iPP containing carbon black were described by Straub (ref. 5). These materials are injection moldable and are electrostatically printable. Mineral fillers lead to property improvements in the polyolefin blends as well.

Blends of EPM and iPP were studied by Danesi and Porter (ref. 6) in order to establish relationships between morphology and physical properties, as well as to determine the mechanisms of morphology development. Bicontinuous morphology, as well as a dispersed phase of the minor phase in the major phase, were observed by optical microscopy. Jevanoff and co-workers (ref. 7) showed that a polypropylene "skin" can develop on extrusion of EPM/polypropylene blends. In this case it seems that the lower viscosity polypropylene phase migrated to the high shear rate region at the wall, while the internal regions of the extrudates retained a morphology where both phases were continuous.

Blends of semi-crystalline EPDM with polyethylene were examined by Lindsay (ref. 8). Here the polyethylene appeared to nucleate the crystallization of the EPDM phase, which has a lower melting point due to shorter methylene segments. Tensile strengths were higher for the blends than for either polymer alone. Polyethylene blends, however, have lower heat distortion temperature then iPP blends. This has limited their use in many applications.

Blends of S-ES-S with iPP (ref. 9) consist of a thermoplastic elastomer co-continuous phase and a co-continuous crystalline plastic phase. The solubility parameters of the center EB block and the iPP are similar enough to give this morphology over a wide compositional and viscosity ratio range. The blends have a higher modulus than S-EB-S and exhibit significantly higher softening points.

Some of the most current advances in iPP blends (refs. 10-12) have been the use of the recently commercially available ethylene a-olefin copolymers based on metallocene catalysts. These copolymers are typically higher in ethylene content than the ethylene copolymers produced for rubber applications. The high ethylene copolymers are TPEs by virtue of the ethylene crystallinity which acts as physical crosslinks and the low Tg of the amorphous phase. Many of the blends designed for automotive uses are relatively high in stiffness with 30 wt. %, or less copolymer.

Morphology of polyolefin blend thermoplastic elastomers

To provide a low modulus thermoplastic elastomer from non-thermoplastic elastomer materials by blending, it is critical to control the morphology of the system. This has been accomplished by choice of the mixing method, mixing conditions, rheological properties of the blend components, controlling the surface energy (polymer choice and/or polymeric compatibilizers), and chemical reactions during mixing.

In the simplest blends with polyolefins, such as high-molecular-weight EPM and iPP, intensive mixing results in two continuous phases. By adjusting the viscosity ratios and the composition of the copolymer, both phases can be kept continuous over a considerable range of volume fractions in this blend (e.g. 80/20 to 20/80).

At the typical copolymer monomer ratios and molecular weights of EPM copolymers used in blends, the elastomers are insoluble in PP when held quiescently in the melt. This has been inferred from studies of the glass transition temperature of EPM/atactic PP blends by DSC. These blends have a broad transition rather than the narrow change in heat capacity found in soluble polymer systems (ref. 1). Scattering studies (ref. 13) on deuterium-labeled EPM blended with iPP indicate a two-phase melt. Shear could influence solubility during mixing of simple blends, but the nodular nature of the phases and phase size is more consistent with a shear dispersion mechanism than a spinodal decomposition or crystallization on cooling from a thermodynamically soluble system. The iPP is non-spherulitic in nature with a well-defined x-ray diffraction pattern of monoclinic PP.

For simple blends, the iPP phase is continuous and exhibits elastic properties due to the open fiber-like microstructure. The EPM phase will be somewhat more elastic if it is semi-crystalline or if it is highly branched. If this phase is continuous, it will also provide some elastic response, particularly at short time scales. When the ethylene copolymer phase is dispersed as particles within a continuous iPP matrix, the blends are much higher in modulus and recover only poorly from high extensions.

Several hypotheses for the formation of co-continuous phases have been formulated. Avgeropulos et al. (ref. 14) showed that for EPDM/polybutadiene blends the volume fraction and torque ratio of the components determined the phase morphology. Paul and Barlow (ref. 15) found that phase inversion occurred when: f1/f2 = h1/h2 where f1 and f2 are the volume fractions of polymer 1 and 2 and h1 and h2 are the viscosities. Lyngaae-Jorgensen and Utracki (ref. 16) found that the critical volume fraction for a continuous phase, fcr = 0.156, is predicted by percolation theory for monodisperse spherical domains. This is in agreement with experiments on EPDM and iPP.

Mechanical properties of polyolefin blends

The primary mechanical property that distinguishes elastomers from other materials and the characteristic that dictates their use in many applications is a low-modulus, high elongation stress-strain curve. The distinction of thermoplastic as rubbery and nonrubbery is quite arbitrary, but in general, materials are considered rubbery if they can be extended over 100% without failure and return to nearly their original dimensions in a short period of time. Thermoplastic polyolefin blends are produced with a spectrum of stress-strain properties via specific backbone polymer selection, morphology control and compounding with filler and plasticizers.

The stress-strain properties of unfilled blends of a highly elastic amorphous EPDM (ref. 17) and various polyolefin resins are shown in table 1. Blends containing high amounts of EPDM are quite rubbery in nature and have surprisingly low sets at break (20-35%). In contrast, when pure high molecular weight polyolefin resins are extended, they undergo a yield at low elongation followed by a typical drawing mechanism, and there is very little recovery after drawing. Table 2 lists the mechanical properties of blends similar to those above, except that a semicrystalline EPDM was used. This gives a significant improvement in properties, especially elongation to break. Not only is crystallinity in the unstrained state important, but the increase in crystallinity during deformation no doubt has a large effect on the stress-strain properties of the blend.

Table 1 -- properties of EPDM/polyolefin blends

Blend(a)
EPDM(b), parts
         60                            80     70     60
Polypropylene(c), parts                20     30     40
Low-density polyethylene(d), parts            --     --
High-density polyethylene(e), parts           --     --
         20       40

Physical properties
Tensile strength, MPa                 8.3   10.5   13.9
         10.2
Elongation at break, %                220    150     80
         130
Elongation set at break, %                    28     30
         25       33

Blend(a)
EPDM(b), parts
         60                            80     60     80
Polypropylene(c), parts                --     --     --
Low-density polyethylene(d), parts     --     20     40
High-density polyethylene(e), parts    --     --     --
         20       40

Physical properties
Tensile strength, MPa                 5.8    8.0    8.5
         10.2
Elongation at break, %                290    190    210
         130
Elongation set at break, %             30     35     30
         25       33

(a) Internal mixer, about 7 min., max. temperature about 200 [degrees] C

(b) Amorhpous, high molecular-weight ethylene-propylene-dicy clopentadiene (-5 wt. %) terpolymer

(c) r = 0.093 gm/[cm.sup.3] melt index = 4.0 gm/per 10 min. at 230 [degrees] C

(d) r = 0.919 gm/[cm.sup.3] melt index = 2.0 gm/per 10 min. at 190 [degrees] C

(e) r = 0.956 gm/[cm.sup.3] melt index = 0.3 gm/per 10 min. at 190 [degrees] C

Table 2 -- properties of semicrystalline
EPDM/polyolefin blends

Blend(a)
EPDM, parts
       80                               80    80     80
EPDM crystallinity, wt. %             12.9   2.7   12.9
       2.7
High-density polyethylene(b), parts     20    20     --
Low-density polyethylene(c), parts      --    --     20
       20
Physical properties
Tensile strength, MPa                 15.0   5.4   14.5
         7.6
Elongation at break,  %                730   940    720
         880

(a) Mill mixed at 150 [degrees] C; (b) r = 0.95 g/[cm.sup.3]; (c) r = 0.92 gm/[cm.sup.3]

Blends of polyolefins with other polymers

Both EPM and iPP have been extensively studied as one of the phases in blend thermoplastic elastomers. iPP was blended with propylene/hexene-1 random copolymer to give a thermoplastic elastomer. The blends had a single [T.sub.g] (single tan d peak) as well as a single melting point. The stress-strain properties show no yield point and the higher [T.sub.m] induced by the iPP gives significantly improved mechanical properties at elevated temperatures. Random stereoblock polypropylene also responds well in blends with iPP. Stereoblock PP can be produced with various homogeneous catalysts (ref. 18) including metallocene catalysts that are able to control chain stereoregularity (ref. 19). Stereoblock PP and iPP blends appear to co-crystallize and show a single DSC melting point. Stress-strain properties of the blends depend on composition with 15 to 25 wt.% iPP giving good elastic characteristics at 25 [degrees] C. Low temperature properties are not attractive, however, with the Tg of ca. 0 [degrees] C.

TPEs are also produced from blends of iPP with EVA (ref. 20). The polymers are immiscible and form co-continuous phases from 50 to 70 wt. % EVA. Dynamic mechanical spectroscopy of both uncrosslinked and dynamically cross-linked blends displayed two [T.sub.g] corresponding to EVA and iPP phases.

Applications for polyolefin blend TPEs

Blends of iPP and EP copolymers that are designed for specific uses are fully compounded with fillers, reinforcing agents, antioxidants, colorants, plasticizers, etc., and are called TPO (for thermoplastic polyolefin). There are three major market areas: automotive, wire and cable, and mechanical goods.

Automotive parts are the largest single market for TPO compounds (auto. conf. ref.). Excellent weatherability, low density, processing flexibility and relatively low cost make them very common exterior and interior automotive parts, e.g., air dams, bumper covers, fender extensions, grills, rub strips, conduit, grommets and interior trim.

TPOs have replaced PVC and vulcanized elastomers in many electrical applications, such as flexible cords, booster cables, appliance wire and low-voltage jacketing. TPO compounds are available that offer low smoke generation and flame resistance. Excellent electrical properties, water resistance and ozone resistance are the main attributes in these applications.

TPOs are sold as pelletized compounds developed to meet specific applications and processing requirements. They can be processed by most common thermoplastic techniques: e.g. injection molding, extrusion, injection blow-molding, vacuum forming and as blown film. General recommendations on TPO usage and processing have been reviewed (refs. 21 and 22). Specific recommendations should be obtained from suppliers of TPOs.

TPEs based on crosslinked elastomer phase

TPEs produced by blending a plastic phase and an elastomer where the elastomer phase is crosslinked are typically called thermoplastic vulcanizates, TPVs. The TPVs are reviewed in a number of comprehensive articles (refs. 23-25).

Rather than the simple intensive mixing process employed for blends, TPVs are produced by dynamic vulcanization. In this case, the rubber is crosslinked into cured particles during intensive mixing, while the continuous plastic phase undergoes no reactions and remains a viscous fluid.

By far the most widely used of these remarkable thermoplastic elastomers are those based on blends of EPDM and iPP. Other combinations of polymers are very useful and properties can be altered to meet many demands for special physical properties, heat resistance and fluid contact. Depending on the elastomer and plastic phase surface energies, the proper size of the dispersed elastomer phase can only be accomplished by the use of compatibilizers. The compatiblizers can be pre-made block or graft copolymers (ref. 26) or made in-situ by any number of coupling reactions between polymers.

In general, the TPVs exhibit more elastic recovery than the uncrosslinked blends, better set properties and increased fluid resistance. In addition, the fixed nature of the morphology results in less property variability due to morphology changes during processing for typical blends.

Morphology of TPVs

The most studied of the TPVs are those prepared from EPDM and iPP. The morphology of the blends before crosslinking has been well established as being co-continuous over a wide range of blend compositions and viscosity ratios. It has been shown that the blends go from co-continuous during the initial stages of mixing to having a dispersed elastomer phase after crosslinking (ref. 27). In this study, EPDMs having 50 wt. % ethylene at two different viscosities were Brabender mixed with iPP having a somewhat lower viscosity. Blend ratios of 40/60 to 60/40 EPDM/iPP were co-continuous after Brabender mixing with no curative added. On addition of an activated phenolic resin curative to the mix, a dispersed-phase morphology was obtained. This occurred over a wide range of curative levels.

The dispersed-phase morphology is independent of the type of cure system. Peroxide, sulfur and phenolic cures all give the same phase arrangement, although the properties of the TPVs can be quite different due to reactions on the iPP phase and crosslink rearrangement. Peroxide will effectively cure EPDM but results in molecular weight degradation in iPP. If the iPP is too low in molecular weight it has been shown to be brittle rather than tough; apparently due to the lack of proper tie molecules between crystallites. Sabet and Patel (ref. 27) also have shown that sulfur cures change morphology when held at elevated temperatures in the melt. The sulfur crosslinked particles of EPDM appear to coalesce into large domains due to the exchange of the sulfur bonds at elevated temperature. Crosslink bond exchange and subsequent particle growth also result in poor processing characteristics for sulfur cured TPVS.

The size and shape of the crosslinked rubber particles in TPVs depend on the polymers used, the use of or generation on compatibilizers and the mixing conditions. This is a critical area because the physical and rheological properties of the blends are highly dependent on morphological details. Experiments by Coran (ref. 28) showed that the stress-strain properties of EPDM-iPP TPV depended on particle size. Large particles resulted in low elongation to break and as the particles were reduced in size the material followed the same stress-strain curve but had much higher elongations. Because of this situation, most practical TPVs have elastomer particles in the range of one micron or smaller.

There are two important facets of particle shape: aspect ratio and surface smoothness. High aspect ratio particles and particles that have regular faces have a higher maximum packing fraction than spherical particles, [[Phi].sub.m] (ref. 27). [[Phi].sub.m] for close packed spheres is 0.74, while close packing of high aspect ratio cylinders is 0.9. Particles with regular faces can have [[Phi].sub.m] approaching unity. Irregular faces on particles can also lead to high maximum packing fractions. This is important since to produce a soft TPV it is necessary to have a large volume fraction of crosslinked rubber while minimizing particle interaction that restricts flow.

TPVs show a range of particle morphologies. Crosslinked rubber particles range from nearly spherical to highly elongated. In addition, particles can also have very irregular shapes and appear to have fractured surfaces. This range of particle shapes appears to result from different preparation conditions. Smooth spherical particles in typical polymer blending without dynamic vulcanization result from an equilibrium viscous break-up mechanism. These same types of particles might be expected if the morphology of the TPV is formed by the same mechanism followed by subsequently crosslinking the rubber. Once the rubber is crosslinked, the morphology is fixed and the particles will not grow in the melt.

Smooth elongated particles could result from a similar mechanism. The primary morphology is established prior to crosslinking. Crosslinks are then produced in the rubber particles elongated by a shear field fixing this morphology.

The irregular shaped particles are likely not the result of an equilibrium viscous break-up mechanism, but elastic fracture. At the high shear stress available in intensive mixers, larger crosslinked rubber particles can easily be fractured into some smaller size. The size depends on mixing intensity and temperature. Crosslinked elastomers at high temperatures encountered in the mixing process have low stress and elongation to rupture. Moreover, the high volume fraction of dispersed phase results in considerable particle interaction as shown by having a yield value in shear flow (ref. 29).

An interesting aspect of morphology is that of foamed TPV (ref. 30). In EPDM-iPP TPV, foaming occurs only within the iPP phase. As the volume fraction of rubber particles increases, the bubbles change from spherical to highly elongated channels aligned in the extrusion direction. Wide angle x-ray pole figure analysis of the TPVs indicates a low level of crystalline orientation in the flow direction for the extrudates. This preferential orientation is highest on the surface and decreases toward the core.

Crosslink density

The crosslink density of the rubber phase of TPVs plays an important role in physical and rheological properties as well as maintaining a fixed morphology. Increases in crosslink density are accompanied by lower tension set and tensile strength (refs. 27 and 28). Since the presence of a continuous crystalline inhibits determination of crosslink density by equilibrium modulus or direct swelling measurements on a TPV, other methods have been used. The crosslink densities in most studies are inferred by crosslinking the elastomer independently from the plastic phase under conditions that approach dynamic vulcanization.

Swollen-state NMR spectroscopy has been used to estimate the crosslink density of NR based TPVs (ref. 31) by determining the amount of solvent that is swollen into the rubber phase. Another technique (ref. 31) is to swell the rubber phase with a polymerizable monomer, e.g., styrene, polymerize the monomer, and examine the swollen phase with electron microscopy to estimate the degree of initial volume swell from the morphology of the resulting polystyrene-rubber composite (ref. 32). In these techniques, the iPP phase, which could restrict swelling, is removed by hot solvent extraction. The crosslink densities in the rubber phase found for TPVs based on EPDM and iPP are typical of fully cured EPDM (ref. 33).

Rheology

The rheological properties of blends of polyolefins have been reported (refs. 7, 29 and 34). The uncrosslinked blends have some unique characteristics because of the two phase nature of the system and, usually, large differences in the viscosities of the starting polymers. In most cases, the EPDM has a significantly higher viscosity than the iPP. iPP exhibits a steady state shear viscosity with a lower-Newtonian region followed by considerable shear thinning. Most EPDMs are much higher in viscosity and shear thin even at shear rates of less than [10.sup.-3] [s.sup.-1]. Generally, iPP blends with EPDM show intermediate rheological properties. Examination of extrudates (ref. 7) shows that the lower viscosity iPP migrates to the higher shear rate region at the wall of the die. Hydrocarbon plasticizer oils are very effective at lowering the shear viscosity of the blends (ref. 35). Most data indicate that EPM or EPDM blends with iPP containing oil remain as two phase systems in the melt.

Han and White carried out rheological studies on TPVs and compared the properties with iPP and a blend of EP and iPP. One of the largest differences in the TPV is the transient stress at the start-up of flow. iPP, which is relatively low in molecular weight, shows no stress overshoot at low shear rates. At higher shear rates (0.5 [s.sup.-1]), the stress overshoots the steady rate. For blends, which contain a high molecular weight EPM phase, the stress overshoots at all shear rates examined. The stress overshoot for the TPV, which contains the crosslinked EPDM phase in a iPP phase that contains oil plasticizer, is very pronounced over the entire shear rate region.

The steady state shear viscosities of TPV show pronounced shear thinning over the entire shear rate ([10.sup.-5] to [10.sup.3] [s.sup.-1]). This type of rheological behavior is common for block copolymers that are in the ordered state in the melt, e.g., thermoplastic polyurethanes and S-B-S. Conversely the iPP, which is a single phase in the melt, exhibits a lower Newtonian viscosity. The EP/iPP blend has a higher viscosity than iPP and may also have a zero shear viscosity at low rates. While the EP/iPP blend is a two phase system, the chains in either phase do not form an elastic network. The rheological properties of TPV are consistent with a material that has a rest state structure. The crosslinked elastomer particles appear to interact and deform under flow.

Properties of TPVs

The properties of TPV based on EPDM and iPP are considerably improved over those of typical polymer blends. Tensile strengths are much improved. Compression and tension set, in particular, are more in line with crosslinked elastomers than are the blends. Many grades of TPVs have been developed to meet specific application needs. Hardness ranges from around 30A to 50D.

The compression and tension set behaviors of TPV, blends and block copolymers are due to plastic deformation of a crystalline or glassy phase. In contrast, set behavior in chemically crosslinked elastomers appears to be dominated by filler interactions (Mullen's effect) and chemical rearrangement of weak crosslinks.

The outstanding flex fatigue of TPVs appears to be the result of the two-phase nature of the material and the inherent high flex life of iPP above [T.sub.g].

Other properties of TPVs based on EPDM and iPP are inherent in the backbone polymers. The polyolefins have low specific gravity and many compounds are around 0.95 g/cc. The [T.sub.g] of EPDM is about -55 [degrees] C and the low temperature properties of TPV reflect this. Chemical resistance is good for polar materials, as are the electrical properties.

Blends of EPDM and poly(butylene terephthalate) (PBT), have been studied recently as thermoplastic elastomers (ref. 36), both with and without dynamic vulcanization. Due to the high surface energy between EPDM and PBT it was necessary to use a compatibilizer to achieve elastomeric properties. This was accomplished by grafting the EPDM in an extruder with glycidyl methacrylate (GMA) using a peroxide initiator. Tensile properties for the blends are listed in table 3.

Table 3 -- EPDM blends with poly (butylene
terephthalate)

Blend                      Tensile   % elong.   Tensile
                         strength,                  set
                               map

1/1 EPDM/PBT                  15.2         55        --
1/1 EPDM/PBT plus             20.8        285        40
 EPDM-g-GMA(a)
1/1 EPDM/PBT plus             22.8        285        15
 EPDM-g-GMA
(Dynamically cured)(b)

(a) EPDM grafted with 3 wt. % glycidyl methacrylate

(b) Crosslinked during extrusion with 0.5% to 2,5-dimenthyl-2, 5-di(t-butylperoxy)-3-hexyne.

Properties of the blends depend on compatibilization and, as observed for EPDM/iPP blends, dynamic vulcanization enhances tensile set, apparently by making the rubber phase more elastic. These data are consistent with the effects of compatabilizers and crosslinking the elastomer phase as shown for polyolefin blends.

Summary

Blends offer an economical route to thermoplastic elastomers. Proper selection of the polymer pairs and other compounding ingredients allows tailoring of many desirable attributes in the final product. Two phase blends with and without co-continuous phases are typically the easiest to produce, particularly if the polymers are compatible without the necessity of a compatibilizer.

Compatibilizers can be made during processing if needed. The blends are easy to process in typical thermoplastic processing equipment and are often employed where the recovery and set properties are not demanding.

Blends that are crosslinked dynamically into TPVs give major property improvements over the simple blends. Compatibilizers can be used to produce TPVs with polymers of very different solubility parameters. This allows wider latitude in synthesis for production of advanced materials to meet demanding specifications that are processible as thermoplastics.

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