Coupling agents: coupling agents are reactive molecules which react chemically either with a filler/fiber and/or the polymer matrix and improve thus the adhesion between the components. Also some reactive compatibilizers may act in the same way and there is no clear defined terminology in literature. In general, coupling agents are low molecular weight reactive molecules mainly used in improving rubber filler adhesion and glass fiber polymer adhesion, whereas compatibilizers are polymers and act mainly in polymer blends. Coupling agents are a wide range of chemical compounds, starting from fatty acids and its salts such as calcium stearate, organofunctional silanes widely used for glass fibers, titanates, zirconates and anhydrides (Hohenberger, 2001). For example, silane coupling agents improved the tensile strength, elongation and impact strength of PP/PET mixtures (Oyman and Tincer, 2003). Titanate coupling agents could improve the elongation at break and slightly impact strength of mixed plastics at a concentration of 1% similar to chlorinated polyethylene where 10–20% were used (Fellahi et al., 1991). Furthermore coupling agents are used in fiber composites (see below).

Impact modifiers: impact modifiers are mainly elastomeric compounds based on butadiene such as styrene–butadiene–styrene (SBS), styrene–isoprene–styrene (SIS) or ethylene–propylene–diene copolymers (EPM, EPDM) often related structurally to the above-mentioned compatibilizers. By adding impact modifiers to the recyclate, impact strength and elongation is increased while the modulus is usually reduced. The appropriate choice of impact modifier depends on the specific plastic to be toughened; main applications are in PS, PP and engineering plastics such as PA, polybutylene terephthalate (PBT), PET (Greco, 1998; Cruz, 1998).

Metal deactivators: metal deactivators form complexes with metal ions and reduce thus the negative influence of metals on the polymer properties such as reduced oxidation stability. In LLDPE nanocomposites it could be clearly shown that the addition of UV absorbers of benzotriazole, benzophenone and hydroxyphenyltriazine structures extend decisively the lifetime of the nanocomposite film; however, a metal deactivator (MD-1) alone outperformed in these experiments the UV absorber, indicating that the influence of metal impurities is very crucial. A combination of UV-absorber with metal deactivator showed only a minor additional improvement. When MD-1 is added to the LLDPE nanocomposite elongation at break and tensile strength of the tested film samples are still at a level of 70% of the starting values after 300 hours of QUV irridiation compared to a complete loss of mechanical properties of the unstabilized sample after 70 hours (La Mantia et al., 2006).

Melt flow adjustment: in order to adjust the melt flow of a polymer according to the required transformation process the possibilities are somewhat limited. A decreased melt flow (higher melt viscosity, higher molecular weight) can be achieved in some cases by considering the already described ‘repair’ molecules. An increased melt-flow (lower melt viscosity, lower molecular weight) is accessible, e.g. with polypropylene using radical generators such as peroxides, hydroxylamine esters (Roth et al., 2006, 2008) or azoalkanes (Aubert et al., 2007) or in the field of polycondensation polymers (PET, PA) by hydrolytic cleavage. Processing aids, lubricants, waxes and addition of oligomers may help to improve processing, to lower the melt viscosity and to increase the throughput.

Odor reduction: post-consumer recyclates suffer often from odor problems caused by contaminations or degradation products from the first application. Removing odor is a challenging task as very low quantities of volatile products can be the reason which are difficult to analyze or to trace back. Technically, odor can be reduced by adjusted processing including vacuum venting or vacuum venting in the presence of a carrier such as water (Schrader, 2004, 2008). Another potential way to reduce odor is through additives, e.g. RS-3, zeolites (Gioffre and Marcus, 1989) or selected silicates (Heberer et al., 2002).

Despite the ecological benefits of mechanical recycling and the proven advantages of additives to improve the quality of recyclates, the economic aspect cannot be neglected. The additional cost contribution of additives starts within the cent range per kg of recyclate for restabilization, but can as well achieve a much higher range, if larger concentrations of, for example, a reactive compatibilizer have to be used. Benchmark and attractiveness to use the recyclate will be the cost difference to the virgin material of a given application.