Chapter 1
Polyethylene-based Biocomposites and Bionanocomposites: State-of-the-Art, New Challenges and Opportunities

Sigrid Lüftl1* and Visakh. P. M.2

1IWILL E.U., Vienna, Austria

2Department of Ecology and Basic Safety, Tomsk Polytechnic University, Tomsk, Russia

*Corresponding author: slueftl-consultant@outlook.com

Abstract

Biodegradable polymers have experienced a growing interest in recent years for applications in packaging, agriculture, automotive, medicine, and other areas. One of the drivers for this development is the great quantity of synthetic plastic discarded improperly in the environment. R&D in the industry and in academic research centers searches for materials that are reprocessable and biodegradable. On the one hand, tailor-made composites of synthetic polymers and natural and biodegradable materials have gained much interest as they show mostly improved properties like tensile strength, modulus, heat distortion temperature, flame retardancy, or abrasion resistance. On the other hand, traditional polymers have the advantage of the availability of cost-efficient production techniques and, in most cases, a high level of recyclability with a well-established recycling infrastructure that can eventually be affected by the introduction of systems containing natural polymers that already exist. In polyethylene (PE)-based biocomposites and bionanocomposites both the PE matrix and the filler can be either of natural or synthetic origin; at least one of the components has to be biodegradable or come from a renewable source. The main source for biobased PE is currently ethanol obtained by fermentation of primary renewable material. Although biobased PE has the same properties as PE derived from fossile fuels, the challenges in PE biocomposites are the homogeneous distribution of the filler within the matrix and the coupling between the hydrophobic matrix and the mostly hydrophilic filler. To enhance the interfacial bonding between the matrix and the filler, typically coupling agents like maleic anhydride grafted PE are used and the fillers surface is mostly chemically modified.

Keywords: Biocomposite, nanofiller, biobased polymer, biodegradable polymer, renewable material source, polyethylene, exfoliation, intercalation, coupling agent, interfacial bonding

1.1 Introduction

Recently, the company ABB presented a 525 kV cable system that is designed for both subsea and underground applications. The extruded high voltage direct current (HVDC) cable system transmits power at significantly higher voltages than was previously possible in cables using extruded insulation. The new insulation material consists of crosslinked polyethylene (XLPE) developed by Borealis [1]. The BorlinkT Superclean compound is specifically designed to meet the requirements in cleanliness for high voltage applications, as contaminants lead to damages of the cable system and eventually to breakdown failures. Tests on the influence of contaminants showed that the larger the size of metallic contaminants at a specific concentration the lower is the electrical breakdown strength [2]. This example typically shows that the incorporation of small particles into polymers can alter the properties of a product drastically.

For more than 100 years several materials, e.g., mineral fillers, metals and fibers of synthetic and natural origin, have been added to polymers to form composites. Composites consist of at least two different phases, a continuous one called matrix and a dispersed one that is commonly the filler. In comparison to neat resins, these composites show mostly improved properties like tensile strength, modulus, heat distortion temperature, flame retardancy, or abrasion resistance. As a result, composites have become very popular for structural applications and they are sold in even larger volumes than neat resins. Furthermore, apart from improving properties, some fillers offer the advantage to decrease material costs [3].

Advances in synthetic techniques over the past 25 years have led to an increasing number of publications and applications dealing with polymer nanocomposites (PMNC) [4]. For example, scientists have reported about superior mechanical properties [5-13], flame retardancy [9-20], thermal stability [13, 14, 17, 20-23], optical [22-29], electrical [17, 24, 26-35], and gas barrier properties [12, 15, 36, 37], as well as the reinforcement effect as a result of the incorporation of a relatively low concentration of nanofillers into a polymer matrix [3, 38, 39]. However, the matrix material is not only limited to polymers, as there exist metal matrix nanocomposites (MMNC) and ceramic matrix nanocomposites (CMNC) too [40]. Typically, filler materials are called nanofillers if at least one dimension of the dispersed particles is in the nanometer range [5]. A variety of nanofillers has been used up to date in combination with commodity and engineering polymers. The range covers natural fillers, clays and layered silicates, graphene, carbon nanotubes, fibers, among others. In 2011 the European Commission recommended that nanomaterials (NM) be defined as a natural, incidental or manufactured material where more than 50% of the particles had one or more dimensions in the size range of 1-100 nm [41].

The properties of the nanocomposites depend on several parameters such as the chemistry of polymer matrices, the nature of fillers, the modification of filler and the preparation methods. In every case, to obtain an enhancement in the mechanical and physical characteristics of the compounded nanocomposite a uniform dispersion of the nanofillers in the polymer has to be achieved first. Specifically, the dispersion of nanofillers is still a challange that limits the actual reach of their potential or theoretical properties. With some nanopaticles the matrix polymer must first be intercalated between the layers of layered nanofillers (Figure 1.1). This requires the aid of polar additives or compatibilizer. Then, these disrupted sheet-like nanoplatelets must be exfoliated chemically or physically to separate and distribute them as much as possible as individual particle (Figure 1.1), since the full surface area of each separated nanosheet or platelet is what creates optimum properties. Thus, one easy way to determine the degree of exfoliation is the melt flow index (MFI), as it drops with increasing distribution of the filler within the matrix. In order to facilitate the processing some companies offer masterbatches composed of a polyolefin matrix and a nanofiller loading fraction of 0.4 to 0.6 in pellet form [42].

Figure 1.1 Dispersion of layered nanofiller (grey structure) in a polymer matrix (black lines).

Polyethylen (PE)-based nanocomposites can be obtained by different techniques that are in use for the preparation of other polymeric nanocomposites like melt processing, solution casting, and in-situ processing techniques. However, to prepare composites with enhanced properties in comparison to the net resin, attention has to be drawn to the successful coupling of the filler material with the polyethylene matrix. Further, to take full advantage of the reinforcement by nanoplatelets to obtain enhanced thermal, mechanical or barrier properties one has to ensure that they are oriented in the appropriate direction and that curling or curving is avoided [43]. To assess the quality of dispersion in nanocomposites, optical and electron microscopy techniques are most commonly used for a qualitative characterization.

Ecological and environmental concerns about the depletion of fossil fuels and greenhouse gas (GHG) emissions have favored the development of materials derived from renewable natural sources. Hence, worldwide efforts have been made to obtain sustainable polymeric materials with a low impact on the environment for various end-use applications [44]. Further, in light of improving manufacturing techniques and enhancing both mechanical and thermal properties of new innovative materials, different natural sources for monomers and fillers have gained considerable research interest.

Today, biocomposites form a specific class among the composites materials. Their matrix can be of conventional plastic material that is derived from renewable raw materials. In this case the polymer is classified as biobased (Figure 1.2). However, the matrix can also be a biodegradable polymer, which means that it decomposes under specific conditions by the action of microorganisms, UV-light and/or water. The organic or inorganic filler in biocomposites consists of fibers or coherent or loose particles. If the filler belongs to the class of nanofillers then the composite is called nanobiocomposite.

Figure 1.2 Classification of polymers in dependence of their raw material origin and biodegradability.

1.2 History of the Synthesis of Polyethylene: From Fossil Fuels to Renewable Chemicals

The story of plastic resin polyethylene dates back to the end of the 19th century. In 1898, while working with diazomethane in ether, the German chemist Hans von Pechmann accidentally synthesized a white product having a waxy texture, but the amount of the unknown substance was not enough for a subsequent characterization [45]. In 1900, Eugen Bamberger and Friedrich Tschirner repeated the experiment of Pechmann and could analyze the white, waxy resin. As the two chemists identified long -CH2- chains, they decided to name it polymethylene [46]. However, at...