Recycling compatibilisers

Recycling of glass fibre-reinforced plastics is reviewed, with special emphasis on remelting of thermoplastic composites, mechanical recycling of thermoset composites, depolymerisation and dissolution of thermosets and thermoplastics, closed loop recycling of glass, and the use of glass as a mechanical compatibiliser. 32 refs. 

J. Pospfsil, I. Forteln, D. Micheilkovei, Z. Kruli , and M. Slouf, Mechanism of reactive compatibilisation of a blend of recycled LDPE/HIPS using an EPDM/SB/aromatic diamine co-additive system, Polym. Degrad. Stab., 90(2) 244-249, November 2005. 

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]. 

Styrene-butadiene or styrene-(ethylene-co-propylene) block copolymers are common compatibilisers for the commingled recyclate from PCW [27]. Improvement of the mechanical properties of heterogeneous PCR (33% PE+39% PVC+28% PET) or (44% PE+1% PP+28% PET+9% PS+2% PVC+16% other plastics) was performed with EPDM or hydrogenated SBS triblocks [52]. 

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

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. 

Attempts have been made to use compatibiUsers in reclaiming three-component plastics waste consisting of PE, PP and PS. The PET (bottles) were first separated, and the remaining three polymers were successfiilly compatibilised with a mixture of EPDM rubber and styrene-butadiene block copolymer. Provided that a substituted diamine stabiliser was also added, the recycled mix achieved impact strength values comparable with those of virgin polyolefins. 

DSM has produced a carbonyl-bis-caprolactam compatibiliser called Allinco that has been used by IKV to compatibilise PET and polyamides so that PET bottles with a polyamide barrier layer can be recycled and made into strong tape. A different approach to compatibilising the same pair of polymers is to use a specially modified PET functionalised with suUb-isophthalic units. 

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. 

Compatibilisers are certain to benefit from the recycling trend, but the reuse of polymers for plasties (as opposed to their use as fuel or their conversion to feedstock for chemical intermediates) may be aeeompanied by the reuse of at least a proportion of the additives. Researeh is being earned out to make possible the systematic recovery of additives from used artieles. Several observers believe that energy recovery and feedstock recycling will often be more eeonomie than eonversion to second-life plastics products. 

The mechanical compatibilisation of polymer blends is conceptually an attractive route, leading to unique property combinations and the recycling of mixed polymer scrap. There is considerable impetus behind discovering polymers that exhibit such compatibilising effects and to understand the mechanisms by which they function. Blends would result if certain types of plastic soft-drink bottles were granulated and processed. A bottle type of interest consists of a PET container with an HOPE pedestal, which is needed because the blown bottle has a rounded bottom. It is quite obvious that PET and any PO would be grossly incompatible and that a very effective compatibiliser would be needed to give the blend adequate mechanical properties for any subsequent application [76]. 

The reluctance of most polymers to mix with one another has important ramifications in the plastics recycling technologies. Sometimes compatibilisers are necessary. While some believe it is simply an industrial conspiracy that prevents recycling of plastics, it is actually quite difficult to achieve [253]. 

Although thermoplastics and thermoplastic composites are potentially easy and economical to recycle, in practice there are some impediments to the implementation of widespread recycling. The main one is that the used materials must be collected, separated and cleaned economically. This is feasible in some instances but often it is not. In general, polymers are immiscible with one another, and, if melt processed as a mixture, the result is phase separation to give domains of one polymer in the other. This morphology leads to rather poor mechanical properties. Therefore, there are efforts to find better separation techniques in order to avoid the problem or to use compatibilisers [152] that lower the interfacial tension, improve the adhesion of the two phases, and encourage smaller domains of the disperse phase. 

La Mantia, F.P. (ed.) (1993) Recycling of Plastic Materials, ChemTech, Ontario. Bonner, J.G. and Hope, P.S. (1993) Compatibilisation and reactive blending, in Polymer Blends and Alloys (eds M.J. Folkes and P.S. Hope), Blackie Academic and Professional, Glasgow, pp. 46-74. 

Cengiz [53] studied a recycled polypropylene (rPP)/organoclay nanocomposite containing ethylene-methyl acrylate-glycidyl methacrylate (E-MA-GMA) and polypropylene-grafted-maleic anhydride (PP-g-MA) compatibilisers. The rPP/ Cloisite 15A nanocomposite provided the highest improvement in mechanical properties. PP-g-MA was indicated as a better compatibiliser and furthermore, the improvement of both tensile and impact properties were better at a compatibiliser to clay ratio of 3. In addition, a significant enhancement of dispersion level as well... 

In one study, rPP and recycled polyethylene (rPE) were blended with two compatibilisers, polyethylene-grafted-maleic anhydride (PE-g-MA) and ethylene propylene diene monomer (EPDM) copolymer, and OMMT [56]. Scanning electron microscopy (SEM) micrographs showed that the blend with EPDM exhibited a better compatibilisation than PE-g-MA. The presence of OMMT caused an increase of the storage modulus and loss modulus additionally, OMMT improved the thermal stability. 

Mural and co-workers [57] also optimised the mechanical properties of an rPP and recycled high impact polystyrene (rHIPS) blend at a composition of 70/30 wt%. Consequently, this composition was mixed with a styrene-ethylene-butylene-styrene (SEES) block copolymer triblock copolymer and Cloisite 20A OMMT. Using X-ray diffraction, the samples containing 3 wt% of nanoclay were found to lack the characteristic nanoclay peak, which indicated the mixed intercalated and exfoliated clay layers where the intercalated layers were further pushed toward the interphase [76]. The incorporation of a compatibiliser and nanoclay also improved the thermal stability of the PP/HIPS blend. SEES and nanoclay performed as an interfacial compatibiliser, which led to the reduction in particle size of rHIPS and the promotion of interfacial adhesion. 

Lei and co-workers [58] used both PE-g-MA and titanate for the compatibilisation of recycled high-density polyethylene (rHDPE) with nanoclay. Both compatibilisers... 

Mahanta and co-workers [71] prepared a blend of rPC, recycled acrylonitrile-butadiene-styrene (rABS) and nanoclay, Cloisite SOB or Cloisite 15A. The blend was compatibilised with both PP-g-MA and solid epoxy resin. The mechanical properties of the rABS/rPC blend without a compatibiliser decreased in comparison to neat matrices. However, upon the addition of 5 wt% of a PP-g-MA compatibiliser, the mechanical properties improved. Similarly, further addition of the epoxy led to a synergistic behaviour in mechanical performance, particularly the modulus and tensile strength. Also, a greater improvement of the modulus was achieved in the rPC-rich blend by simultaneously adding two nanoclays. The thermal stability of the blends compatibilised with PP-g-MA and epoxy exhibited positive thermal properties. 

With the objective of a successful and economical recycling process in which the recycled polymer has largely acceptable properties, considerable effort must be made to encompass all the aspects of recycling in future studies to enhance the competitiveness of these systems. The first step could be the improvement of interfacial adhesion in prepared nanocomposites to achieve better physical and mechanical properties from recycled polymer wastes. Many procedures such as compatibilisation, functionalisation and surface modification could be developed in the future. Furthermore, the addition of effective nanofillers including available nanofillers or a combination of nanofillers will provide further progress and new opportunities in these systems. In addition, the development of fabrication techniques and also, the optimisation of available methods such as melt mixing should be performed, due to its important role in the final properties of recycled products. 

Therefore, in many cases, the main research objective is how to improve the properties of one - usually widely available and cheap - plastic material by the addition of another plastic. The most common examples of these are PE-PA, PP-PA, PE-PET, PE-ABS and so on, e.g., recycled PET and HDPE composites were prepared with different proportions of raw materials in the work of Avila and co-workers. It was found that changes in the ratio of raw materials significantly influenced the characteristics of the composites. The properties of the final products could also be improved by the application of compatibilisers however, the tensile strength of the composites was typically below 30 MPa [24]. 

Bertin and Robin [40] used both virgin and recycled LDPE/PP blends with a compatibiliser on the basis of the following summary ... 

Any recyclate derived from multi-layer mouldings will consist of a combination of the materials used to make up the individual layers. This does not usually present a problem as the layers are all of suitable properties for blow moulding and compatibilised with each other. 

Compatibilisers are commercially available for combining a range of materials and this technology can also be applied to recyclate blends. The advantages of this are the removal of the requirement for costly and/or difficult separation processes, and the production of commercially valuable materials. The compatibilisers can be added during reprocessing usually at levels of around 2-5%. The cost of the compatibiliser must be taken into account, but this can... 

Murphy and co-workers [3] have looked into the effect on physical properties of blending ultrafine rubber particles into thermoplastics. A large range of blends were produced using a munber of recycled thermoplastics and recycled rubbers in a batch process. The influence that the particle size of the rubber, the total amount of recycled rubber, and the degree of compatibilisation had on the physical properties was determined and reported. 

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