Elsevier

Waste Management

Volume 54, August 2016, Pages 53-61
Waste Management

Solid-state drawing of post-consumer isotactic poly(propylene): Effect of melt filtration and carbon black on structural and mechanical properties

https://doi.org/10.1016/j.wasman.2016.04.029Get rights and content

Highlights

  • Contaminated, post-consumer i-PP can be solid-state drawn to an oriented polymer with enhanced properties.

  • By means of orientation, the stiffness of post-consumer i-PP can be increased by a factor 10 and the tensile strength by a factor 15.

  • The properties of oriented post-consumer i-PP tapes are approx. 70% of oriented virgin i-PP tapes.

Abstract

Post-consumer plastic waste obtained via mechanical recycling is usually applied in thick-walled products, because of the low mechanical strength due to the presence of contaminants. In fact, sorted post-consumer isotactic poly(propylene) (i-PP) can be considered as a blend of 95% i-PP and 5% poly(ethylene), with traces of poly(ethylene terephthalate) (PET). By applying a treatment such as solid-state drawing (SSD) after melt extrusion, the polymer chains can be oriented in one direction, thereby improving the stiffness and tensile strength. In this research, molecular processes such as crystal break-up and chain orientation of these complex blends were monitored as a function of draw ratio. The melt filter mesh size – used to exclude rigid PET particles – and the addition of carbon black (CB) – often added for coloration in the recycling industry – were varied to investigate their influence on the SSD process. This research shows that despite the blend complexity, the molecular processes during SSD compare to virgin i-PP and that similar draw ratios can be obtained (λmax = 20), albeit at reduced stiffness and strength as a result of the foreign polymers present in post-consumer i-PP. It is observed that the process stability improves with decreasing mesh size and that higher draw ratios can be obtained. The addition of carbon black, which resides in the dispersed PE phase, also stabilizes the SSD process. Compared to isotropic post-consumer i-PP, the stiffness can be improved by a factor 10 to over 11 GPa, while the tensile strength can be improved by a factor 15–385 MPa, which is approx. 70% of the maximum tensile strength achieved for virgin i-PP.

Introduction

Plastics are used in a wide variety of applications, including packaging products. After a short service life, the packaging material is often disposed. Recycling of these and other post-consumer plastics has gained a lot of attention in recent years. From the environmental point of view mechanical recycling of waste plastics is considered the preferred option (Patel et al., 2000, Wollny et al., 2002, Arena et al., 2003). Post-consumer plastic packaging waste is sorted with near-infrared technology and is washed and dried afterwards (Jansen et al., 2012). Despite improved sorting efficiencies, some polymeric contamination is always present in post-consumer waste. It was shown that a sorted polymer waste stream typically contains 5–10% contamination of another polymer due to multiple types of plastic of which a packaging product is made from Luijsterburg and Goossens, 2013, Seiler, 1995. Like in virgin polyolefin blends, small fractions of a different polymer, which act as stress concentrators if they are phase separated, reduce the elongation-at-break significantly (Robertson and Paul, 1973). Solid particles such as poly(ethylene terephthalate) (PET) can be filtered out during extrusion using (rotating) melt filters equipped with steel meshes. The mesh size is the diameter of the holes of the filter. The diameter of a mesh varies from 80 to 500 μm and filters can be replaced easily. The mesh size determines the dimension and concomitantly, the fraction of non-molten solid particles that end up in the extrudate. Polymer contamination in recycled plastics severely limits the applicability of plastics from sorted plastic waste.

One of the sorted polymer waste streams is poly(propylene) (PP). PP is a versatile polymer that is suitable for many applications because of its relatively high stiffness, tensile strength and impact strength, with isotactic PP (i-PP) as the most common form. The heat and shear forces which are applied during (re)processing the material contribute to degradation processes of the polymer (Ranby and Rabek, 1975, Moss and Zweifel, 1989, Camacho and Karlsson, 2002). i-PP degrades by chain scission when reprocessing the material which influences the viscoelastic (Hamskog et al., 2004, Elloumi et al., 2010) and the mechanical properties (Schnabel, 1981, Pospı́šil et al., 1999, Vulic et al., 2002, Jansson et al., 2003). It was reported that the viscoelastic properties of virgin-recycled i-PP blends can be estimated after a number of reprocessing cycles (Bernardo et al., 1996). For sorted i-PP waste, the presence of contaminations in combination with degradation reactions during the service time of a product and reprocessing lead to undesired brittle failure (Luijsterburg and Goossens, 2013). In order to have sufficient mechanical strength, it is applied in thick-walled injection-molded products or blended with virgin i-PP to improve the mechanical properties. However, blending does not save valuable resources, since a large amount of virgin i-PP has to be added to obtain the desired properties.

In order to obtain a product with a low carbon footprint and sufficient mechanical properties, improvements can be considered at the three stages of the recycling process:

  • Pre-extrusion: reduction of the amount contaminants by improved sorting efficiencies and better separation at the mechanical recycler (not studied during this study).

  • Extrusion: reduction of contamination by filtration/degassing and/or improved dispersion of contaminants by mixing.

  • Post-extrusion step, such as annealing or drawing.

In the drawing process, the material is plastically deformed and polymer chain segments are molecularly oriented in the drawing direction, thereby providing a high stiffness and strength. The extent of molecular orientation is limited by the presence of entanglements, which act as physical cross-links upon drawing and of which the number rapidly increases above a critical molecular weight (Macosko, 1994). Drawing is sometimes done from the solution or gel state to reduce the entanglement density in order to obtain mechanical properties close to the theoretical E-modulus of the polymer (Smith and Lemstra, 1979, Smith and Lemstra, 1980a, Smith and Lemstra, 1980b, Matsuo et al., 1986, Flood and Nulf, 1990). An example of a commercially available highly drawn polymer from linear poly(ethylene) (PE) is DSM’s Dyneema®. In this process environmentally unfriendly solvents are recovered. Teijin’s Endumax® is another example of drawn PE, which is made via an environmentally friendly, solvent-free process (http://www.teijinendumax.com). In this process, the polymer is drawn from the solid state after crystallization.

When the drawing step is carried out after crystallization of the polymer at temperatures between the glass transition temperature and the melting temperature, it is referred to as solid-state drawing (SSD). The simplest form of SSD is a uniaxial tensile test (Cansfield et al., 1976). The SSD of i-PP can also be done continuously in-line in combination with extrusion (Bilotti et al., 2010, Alcock, 2005). The SSD technique can be considered as a dynamic tensile test where the temperature and strain rate can be varied systematically. From literature, the effect of temperature and strain rate on the yield stress, marking the transition between elastic and plastic deformation, was reported for i-PP (Fig. 1) (Roetling, 1966, van Erp et al., 2012). It was found that a higher drawing temperature and a lower strain rate will reduce the yield stress of the material, which reduces the resistance to deformation during the initial stages of the solid-state deformation process. A reduced strain rate gives polymer chains more time to adjust to the applied stress, resulting in a temporary energetically more favorable state.

Similarly, an elevated temperature allows for a higher chain mobility and shorter relaxation times. Hence, crystals break-up at lower stress levels, resulting in a reduced yield stress. The optimum drawing temperature of i-PP is situated above the α-relaxation temperature Tα, which marks an improved molecular mobility within the i-PP crystals (Aharoni and Sibilia, 1979a, Aharoni and Sibilia, 1979b).

The deformation of isotropic semi-crystalline polymers can be regarded as deformation of two interpenetrated networks of a hard crystalline skeleton and a soft amorphous entangled network (Men and Rieger, 2003, Jiang et al., 2007, Jiang et al., 2010). Upon drawing, spherulitic-like crystals formed during the quenching step break-up and rearrange in the drawing direction. This is schematically shown in Fig. 2. In the elastic region, the hard inter-coupled crystalline phase is deformed (Fig. 2B). The transition from elastic to plastic deformation (yield point), is governed by interlamellar and intralamellar slip processes (Fig. 2C). In interlamellar slip, the friction between the lamellae results in the break-up of these lamellae. In intralamellar slip, the lamellae break up as a result of friction inside the lamellae due to deformation under load (Fig. 2D). After yielding, the lamellae undergo stress-induced melting and recrystallization into smaller lamellae that orient in the drawing direction (Fig. 2E). At higher draw ratios, these smaller lamellae unravel ideally into chain-extended structures possessing the ultimate stiffness and strength (Peterlin, 1971, Kamezawa et al., 1979). At these higher deformations, orientation of the amorphous chain segments occurs. For i-PP, the crystalline phase orients up to a draw ratio (λ) of 9, after which it levels off. The orientation of the amorphous phase increases gradually with draw ratio (Yamada and Kamezawa, 1981), but to a different extent (Huy et al., 2004).

The exact mechanisms of elastic and plastic deformation in semi-crystalline polymers are still not fully understood. A possible explanation is that the force is transferred from the amorphous phase to the crystals through so-called tie molecules, polymer chains that are incorporated in multiple lamellae (De Rosa and Auriemma, 2012). However, tie molecules were reported to be of less importance in the local deformational process (Men and Rieger, 2003). If the force is sufficiently transferred from the amorphous to the crystalline phase, crystal deformation is facilitated which leads to an improved ductility. This is related to the number of tie molecules connected to the crystals, which in turn is related to the molecular weight of the polymer or - stated differently - the number of entanglements per chain. Melt-extruded polymers, such as i-PP, contain entanglements, which hamper the unraveling of polymer crystals into fully chain-extended structures upon drawing. For this reason, the stiffness of α-phase crystallized i-PP never reaches the theoretical modulus of i-PP of 49 GPa (Young, 1979).

The mechanical properties of drawn tapes depend on a number of parameters, such as molecular weight (distribution) (Flood and Nulf, 1990, Capaccio and Ward, 1973), crystallinity, and drawing temperature (Bigg, 1988). Furthermore, the initial crystal structure of i-PP proved to influence the drawing process (Luijsterburg et al., 2014). As a result of less Van der Waals interactions, a lower molecular weight polymer can be drawn further, but the resulting mechanical properties are reduced when compared to a high molecular weight polymer (Capaccio and Ward, 1973). On the one hand, a higher drawing temperature increases the mobility of the polymer chain segments and thereby reduces the resistance to deformation. However, the maximum drawing temperature is limited by the melting temperature of the polymer and a higher drawing temperature decreases the relaxation time of polymer chains.

Another possibility to improve the mechanical properties of polymers is the use of (nano)fillers (Bilotti et al., 2010). An example is carbon black (CB), which is applied widely in the recycling industry for coloring purposes and can have a different dimensionality depending on the state of agglomeration and the concomitant hierarchical structure. When applied in polymer blends, nanoparticles can be selectively localized in a particular polymer phase, or be located at the interphase between two phases (Li et al., 2013, Elias et al., 2007, Feng et al., 2003). This selective localization is governed by the physical interactions between the components as well as by the mixing process (Gubbels et al., 1994). In PE-PP blends, carbon black has a preference for the PE phase (Sumita et al., 1991). In such blends, CB can therefore alter the viscoelastic properties, in particular the relaxation time of the PE phase. The existence of a dispersed phase gives rise to the development of a plateau in G′ at low frequencies (Graebling et al., 1993). The distinction between different relaxation mechanisms in polymer blends is often represented by Cole-Cole plots (Manchado et al., 2001, Li et al., 2006, Utracki, 1991), where a homogeneous system is characterized by one circular arc and, a phase-separated system is characterized by the formation of a tail or a second circular arc because of a second relaxation mechanism. For the 20–80 HDPE-PP blend, the Cole-Cole plots shows a small tail, indicative for the relaxation mechanism of the dispersed HDPE phase (Li et al., 2011). The addition of CB on a PE carrier to recycled i-PP could alter the viscoelastic properties of the PE phase and thereby promote stress transfer during tensile loading, leading to an improved ductility.

It is the purpose of this study to explore the potential of the SSD process as a method to improve the mechanical properties of recycled i-PP. In this research, both post-consumer and post-industrial i-PP grades are compared to virgin i-PP. The effect of melt filtration and carbon black on the crystallization behavior, the maximum drawability at stable process operation and the concomitant mechanical properties of the oriented tapes are discussed.

Section snippets

Materials

A fiber-grade virgin i-PP (H540-03Z, code: VPP) was provided by the Dow Chemical Company (Terneuzen, the Netherlands). Post-industrial (Vision PPC BL 2008, code: PIPP250CB) and post-consumer (Vision PPC SK 3005, code: PCPP250) i-PP were provided by AKG Polymers (Vroomshoop, the Netherlands). Sorted Dutch post-consumer i-PP was granulated at MTM Plastics (Niedergebra, Germany) and filtered at Ettlinger (Königsbrunn, Germany, Codes PCPP80, PCPP180). Carbon black (Plasblak PE 4884, 50% carbon

Molecular characterization

The materials used in this study were characterized by HT-SEC, rheology, DSC and TEM before conducting any SSD experiment. The results for HT-SEC and rheology are given in Table 1. Different mesh sizes during melt filtration were used to filter out the solid particles, such as PET. It can be observed that the recycled materials are of lower molecular weight and, hence, lower viscosity than the virgin i-PP (VPP). The differences in molecular weight for PCPP grades are large and indicate the

Conclusions

In this study the potential of the solid-state drawing process to significantly enhance the properties of recycled i-PP was investigated. Molecular processes such as crystallinity and chain orientation were monitored as a function of draw ratio. Like for virgin i-PP, the crystallinity gradually increases with draw ratio. Furthermore, the orientation of crystalline i-PP chain segments increases up to a draw ratio of 10, after which it levels off. On the other hand, the orientation of amorphous

Acknowledgement

This research was financially supported by the Top Institute for Food and Nutrition, Wageningen, the Netherlands (Project SD-001).

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