LDPE–wood composites utilizing degraded LDPE as compatibilizer

https://doi.org/10.1016/j.compositesa.2013.04.005Get rights and content

Abstract

The effect of degraded low-density polyethylene (dLDPE) as compatibilizer on the morphology and properties of low-density polyethylene (LDPE)/wood flour (WF) composites was investigated. The formation of functional groups on the degraded polyethylene chains enables the dLDPE to be used as a compatibilizer. The SEM images show smooth surfaces with fewer voids and fibre pullout for the dLDPE modified composites. The carbonyl index of the dLDPEs increased up to 7 weeks degradation, while the molecular weight decreased significantly. In the dLDPE treated composites a nucleating effect of the fibres gave rise to increased LDPE melting and crystallization enthalpies. There was no significant improvement in the thermal stability of the dLDPE treated composites. The presence of dLDPE observably influenced the viscoelastic properties and mechanical properties of the composites. It was found that the higher carbonyl index dLDPEs are more efficient compatibilizers in LDPE/WF composites, despite their significantly reduced molecular weights.

Introduction

In recent decades, wood–polymer composites (WPCs) have attracted significant attention [1]. WPCs take advantage of the properties of both wood and plastics. To achieve the maximum effect of wood as reinforcing filler, the polymer matrix must be compatible with the wood fibres [2]. The compatibility and the interfacial adhesion can be improved by using compatibilizers or by modifying the wood surface [3]. It is quite clear that a careful selection of the type and level of compatibilizer is needed in order to produce WPCs with acceptable properties and performance [4], [5].

All organic polymers undergo oxidative degradation, and heat and radiation will accelerate this process. Whilst polyethylene (PE) in general is regarded as quite stable, low density polyethylene (LDPE) will undergo thermooxidative degradation (TOD). TOD is often a complex process involving a combination of different mechanisms [6]. The TOD of PE is believed to proceed by a free radical mechanism. This involves, inter alia, the formation of alkyl macro radicals, which in turn react with oxygen to form peroxy radicals. These can further react by abstracting hydrogen from elsewhere on the polymer, forming a hydroperoxide and recreating the macro alkyl radical. The decomposition of the hydroperoxides results in the production of carbonyl and hydroxide groups [7], [8]. Carbonyl groups are often used to monitor the TOD of polyolefins [9], [10]. All of this indicates that the oxidative degradation of PE should create new functional groups that should allow the polymer to be used as a compatibilizer in LDPE–wood composites. The basic function of a compatibilizer is to form an interphase between the wood and the plastic, thus the compatibilizer should have a domain or functionality that is compatible with the wood fibre as well as a domain that is capable of interacting with the plastic matrix [11]. Thus, the idea of degraded LDPE as a compatibilizer for an LDPE–wood composite system is appealing.

Section snippets

Materials

Unstabilized LDPE was supplied in pellet form by Sasol Polymers, Johannesburg, South Africa. It has Mn = 29417 g mol−1, Mw = 142584 g mol−1 and a melting point of 108 °C. The same LDPE was degraded in an oven at 80 °C for 5, 5.5, 7 and 9 weeks to functionalize the LDPE. The formation of functional groups was monitored by FTIR spectroscopy.

Pine wood flour was a cream-white powder supplied by Taurus furniture manufacturers, Phuthaditjhaba, South Africa. It was sieved to <150 μm sizes, and it has a bulk

Results and discussion

Fig. 1 shows the differences between the LDPE spectra before degradation and after 5, 5.5 and 7 weeks degradation. All the spectra show equally intense absorption peaks around 2840 and 2920 cm−1 that correspond to the Csingle bondH symmetric and asymmetric stretching of methylene groups. Additional peaks are seen in the dLDPE spectra confirming the oxidative degradation of LDPE. The peaks around 1714–1780 and 1000–1100 cm−1 are respectively related to the carbonyl and Csingle bondOsingle bondC stretching vibrations. The 5 weeks

Conclusions

The objective of this study was to investigate the effect of degraded LDPEs in LDPE/WF composites. The LDPE was degraded at 80 °C for different periods of time and used as compatibilizers in LDPE/WF composites. Part of the investigation was to see which degradation period gave the best balance between functionalization of the polymer and reduced molecular weight. FTIR spectroscopy showed that thermal degradation of LDPE resulted in the formation of several functional groups on the LDPE chains,

Acknowledgements

The National Research Foundation of South Africa and the University of the Free State are acknowledged for financial support of the Project.

References (29)

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