Compatibilisation Compatibility

Diblock copolymers, especially those containing a block chemically identical to one of the blend components, are more effective than triblocks or graft copolymers. Thermodynamic calculations indicate that efficient compat-ibilisation can be achieved with multiblock copolymers [47], potentially for heterogeneous mixed blends. Miscibility of particular segments of the copolymer in one of the phases of the bend is required. Compatibilisers for blends consisting of mixtures of polyolefins are of major interest for recyclates. Random poly(ethylene-co-propylene) is an effective compatibiliser for LDPE-PP, HDPE-PP or LLDPE-PP blends. The impact performance of PE-PP was improved by the addition of very low density PE or elastomeric poly(styrene-block-(ethylene-co-butylene-l)-block styrene) triblock copolymers (SEBS) [52]. 

Compatibility of PE with PVC is improved by poly(ethylene-graft-vinyl chloride) or partial chlorinated PE. To compatibilise blends of PE with PET, common for the scrap of beverage bottles, EPDM or SEBS are effective additives [56]. 

The ionic aggregates present in an ionomer act as physical crosslinks and drastically change the polymer properties. The blending of two ionomers enhances the compatibility via ion-ion interaction. The compatibilisation of polymer blends by specific ion-dipole and ion-ion interactions has recently received wide attention [93-96]. FT-IR spectroscopy is a powerful technique for investigating such specific interactions [97-99] in an ionic blend made from the acid form of sulfonated polystyrene and poly[(ethyl acrylate - CO (4, vinyl pyridine)]. Datta and co-workers [98] characterised blends of zinc oxide-neutralised maleated EPDM (m-EPDM) and zinc salt of an ethylene-methacrylic acid copolymer (Zn-EMA), wherein Zn-EMA content does not exceed 50% by weight. The blend behaves as an ionic thermoplastic elastomer (ITPE). Blends (Z0, Z5 and Z10) were prepared according to the following formulations [98] ... 

The company claims easy processing results from the high compatibility of the blend components. The formulation consists of more than 10% PLA (purchased from NatureWorks LLC) plus a biodegradable co-polyester and special additives. FKuR says a special combination of compatibilisers permits coupling between the PLA and the co-polyester. The compound is homogeneous, which allows the film to be drawn down to 8 microns. Film up to 110 microns thick is 90% degraded after twelve weeks in composting conditions. 

All the aromatic polyesters based on DEG have poor compatibility with blowing agents (pentanes or fluorocarbons) and to improve this compatibility compatibilising polyols such as ortho-toluene diamine polyols, propoxylated a-methyl glucoside polyols, oxyethylated p-nonylphenol, amine and amide diols, PO-EO block copolymers, borate esters, silicone compounds and so on, are frequently used [27-30]. 

A miscible polymer blend is one for which the miscibihty and homogeneity extend down to the molecular level, so that there is no phase separation. An immiscible blend is one for which phase separation occurs, as described in the next section. An immiscible blend is called compatible if it is a useful blend wherein the inhomogeneity caused by the different phases is on a small enough scale not to be apparent in use. (Blends that are miscible in certain useful ranges of composition and temperature, but immiscible in others, are also sometimes called compatible blends.) Most blends are immiscible and can be made compatible only by a variety of compatibilisation techniques, which are described in section 12.2.4. Such compatibilised blends are sometimes called polymer alloys. 

Compatible blends are two-phase materials with properties controlled by the properties and geometry of each phase and the nature of the connectivity between phases (compatiblilisers modify/improve the interface). In some cases, the addition of small amoimts of an A/B copolymer compatibiliser will result in a compatibilised A-B blend morphology that has improved mechanical properties. The compatibiliser is considered to be located mainly at the interface between the two immiscible polymers, where it induces local miscibility. The compatibliser lowers the interfacial tension and allows the dispersion of the incompatibile homopolymers into small, microscopic domains. 

These samples, simultaneous PU/PS IPNs, were synthesised by a one-short route. The IPN topology appears to restrict phase separation, which results in materials with broad transition regions. By variation of the crosslink level in either or both polymer networks, the controlled introduction of internetwork grafting or the incorporation of compatibilisers into the PS network, the compatibility of the two polymer networks can be increased. For simultaneous IPNs, it has been found [129] that the network which is first formed... 

Addition of p-cresol formaldehyde (PCF) into phenolic/NBR blends resulted in rednction in the domain size of the dispersed phase and improvement in mechanical properties [244]. PCF resin has an intermediate polarity compared with NBR and resole and can react faster with NBR. Therefore, PCF molecules are likely to be concentrated at the phenolic/NBR interface and act as an external compatibilising agents [245]. Thus compatibility and chemical bonding between NBR and phenolic resin is improved, leading to the enhancement in properties. The other materials used as toughening agents of phenolic resin include elastomers such as natural rubber and nitrile rubber [246, 247], reactive liquid polymers [248] and thermoplastics such as polysulfone, polyamide, polyethylene oxide [249, 250]. 

The term compatibiliser refers to an additive used to improve the miscibility and properties of a polymer blend. It is sometimes more specifically used to mean an additive used to promote adhesion between a polymer and an inorganic smface such as a mineral, or glass fibres. In this report, the coupling agents used to promote adhesion between polymer and inorganic additives are discussed imder the heading of Fillers, and this section concentrates almost entirely on the compatibilisers used to promote the compatibility of two organic polymers. 

Whether the copolymer is a block or a graft, one sequence, P, is chosen to be compatible with polymer A, and the other, Q, with polymer B. The sequence P may even be identical with the polymer repeat unit A, or not. If polymer A and polymer B are incompatible and are being mixed, the simplest arrangement would be to use a block copolymer of A and B as compatibiliser, but this is not the only possibility. 

Functionalisation, or compatibilisation by chemical reaction techniques, usually involves mixing one of the polymers, say A, with a small quantity of chemically modified (functionalised) polymer A, or a chemically modified polymer compatible with A. The chemical modification introduces maleic, methacrylate or similar groups, chosen to react chemically with, or at least promote, compatibility with B. 

Early development work on nanocomposites employed polar polymers such as the polyamides and epoxy resins because they are readily compatible with montmorillonite and similar silicate nanoclays. However, polypropylene can be compatibilised by reacting it with maleic anhydride, and nanoclays can be treated to make them compatible with nonpolar polymers. 

If the polymer is not compatible with the impact modifier, a compatibiliser (see earlier in this chapter) may be needed. The choice will depend on both the main constituents. Maleated ethylene-octene and maleated SEBS are often used, since they also contribute to the impact modifying action. When polyamide-6 is mixed with ABS, the compatibiliser can be styrene-maleic anhydride copolymer, poly(methyl methacrylate co-maleic anhydride) or poly(methyl methacrylate co-glycidyl methacrylate). 

Arkema oflers Lotader compatibilisers for recycled PET and PC/ABS mixtures. They can also improve compatibility between polymers and fillers. 

Wood plastics composites have been successfully and rapidly developed in North America to produce a market of 690,000 tonnes, and they are now becoming more popular in Europe and Asia. Compatibilisers improve the water resistance and rot resistance and transform the mechanical properties, particularly the strength. They also improve the heat distortion temperature. This is because of the poor compatibility between polar cellulosic fibres and nonpolar polyolefins. In practice many manufacturers do not yet use compatibilisers because of their high cost. 

The recycling of mixed waste presents a fmther market opportunity for compatibilisers. European Directives on the recychng of end-of-life products encourage the recycling of plastics products, although in practice this does not often involve mixed waste. This is because the lack of compatibility between polymers degrades the mechanical properties and drastically reduces the market for the recyclate. 

Another application of compatibilisers is in intumescent formulations for PP compositions used in vehicles, where better flame retardancy is being sought for various reasons. The intumescent mixtiue sometimes incorporates a polyamide as the carbonisation polymer, together with ammonium polyphosphate (APP) to improve the fire performance. PA and APP have limited compatibility, and EVA can overcome this. Attempts have been made to demonstrate that such mixtmes can count towards the 80% of recyclable vehicle weight demanded by the EU Directive relating to end-of-vehicle life issues. 

The surface area is also a key parameter when trying to increase the compatibility of immiscible phases. Paul and Newman found that the interfacial area covered by compatibilisers is a function of 1/W, the average radius of the dispersed phase and the volume fraction of the dispersed phase [34]. Generally, the average radius of the dispersed phase decreases upon increasing the compatibiliser concentration, while the interfacial surface area covered by the compatibiliser increases, decreases or remains unchanged. 

Generally, octadecyl-trimethoxy silane, dodecyl-trichlorosilane and y-aminopropyl-trimethoxy-silane are used for the compatibilisation of nonmiscible or poorly miscible polymer-polymer systems. These compounds can connect to the apolar plastic constituent by their aliphatic groups via both physical and chemical bonds while the silane groups connect to the other polar polymer constituents. Figure 9.4 summarises the compatibility effect of commonly used siloxane-type compatibilisers. 

The process does have limitations. There is a need for the skin and core materials to be compatible with each other in terms of adhesion and shrinkage. Adhesion of the layers is necessary to prevent the core material becoming detached from the skin especially if the moulding is likely to be exposed to mechanical loads. Therefore materials must be compatible or a suitable compatibiliser used in the core component. The use of compatibilisers in the core component of co-injection moulding was developed and patented by the Rover Group in collaboration with University of Warwick [1]. Researchers from Warwick have also developed and reported methods to mechanically interlock immiscible materials for co-injection moulding but these are currently in the early development stages [2]. 

---