Effect of chain structure on the properties of Glass fibre/polyethylene composites

Abstract

Three types of polyethylenes (low density: LDPE, medium density: MDPE, and high density: HDPE) were used to investigate the effect of chain branching on the dispersion and adhesion in Glass fibre reinforced polymer composites. The interaction between the polyethylene matrix and the Glass fibres was investigated in terms of differences in mechanical behaviour, morphological characteristics, rheological and thermal properties between the three polymer composites systems. Addition of Glass fibres enhanced the mechanical properties for all systems. The degree of enhancement, however, depended on the branching and crystallinity of each polymer. The long chain branching (LCB) in LDPE resulted in higher increases both in the Elastic (Young’s) modulus in the solid state and in the Storage modulus in the melt. The higher crystallinity of HDPE was responsible for higher increase in tensile strength and less fibre pull-out upon addition of Glass fibres. Rheological results also confirm the same observation for LCB. The addition of Glass fibres also resulted in improved thermal stability of the various polyethylene samples.

Introduction

Polymer composites are playing an increasingly significant role in aerospace [1], [2], marine [3], automotive [4], construction [5] and other fields due to their unique properties such as high strength to weight ratio and corrosion resistance [6]. In particular, thermoplastic based materials are being used increasingly in many applications in modern life because of their ease of processing and recyclability [7], [8]. Polyethylene (PE) is one of the most versatile and widely used thermoplastics in the world because of its toughness, near zero moisture absorption, excellent chemical inertness, low coefficient of friction, ease of processing and unusual electrical properties [9]. Polyethylene is classified into several different categories based mostly on its density. The mechanical properties of PE depend significantly on variables such as the extent and type of branching, the crystal structure and the molecular weight. The main forms of polyethylenes are High Density Polyethylene (HDPE), High-molecular weight HDPE (HMW-HDPE), Ultra High Molecular Weight HDPE (UHMW-HDPE), Low density Polyethylene (LDPE), Linear Low Density Polyethylene (LLDPE), Very low density Polyethylene (VLDPE). These are divided based on density and branching. Commercially, the most important polyethylene grades are HDPE, LDPE, LLDPE and MDPE [10].

LDPE is made through a high pressure polymerisation process and is generally characterised by the long chain branching (LCB) structure. This prevents the molecules form packing as closely together during crystallization which results in low crystallinity. LDPE is more flexible than HDPE, and has lower tensile and compressive strength than HDPE due to its lower crystallinity. Generally LDPE is used in food packaging materials and plastic film applications such as plastic bags and film wraps [10], [11]. MDPE is made by the same low pressure process as HDPE but it contains a small amount of short chain branches (SCBs). It is softer than HDPE and never sleek as LDPE together with the common thickness. MDPE is typically used in gas pipes and fittings, sacks, shrink film, packaging film, carrier bags and screw closures [10], [11]. HDPE is made of long chains without major branching. It contains less than 1 side chain per 200 carbon atoms in the main chain resulting in long linear chains with high crystallinity and more rigidity. HDPE is used in applications such as milk jugs, detergent bottles, margarine tubs, garbage containers and water pipes [10], [11].

These thermoplastics are usually used as matrix materials for making composites. The fibres are used as reinforcements (or fillers) to improve properties such as strength, rigidity, durability and hardness [12] and reduce the cost of material. Many studies are available on polyethylene composites based on both natural [13], [14] and synthetic fibres [15], [16]. Glass fibres are most used as reinforcements to plastics due to their low cost and fairly good mechanical properties compared to synthetic fibres. Glass fibre reinforced polymers have been widely used in the automotive and aerospace industries for their superior properties like high strength and low weight [17], [18], [19].

In the present work, three types of polyethylene based resins (LDPE, MDPE and HDPE) were used to prepare the Glass fibre reinforced composites. The effects of addition of GF (Glass fibre) on mechanical, thermal and rheological properties were evaluated. The main aim of this study is to investigate the effect of the chain structure (especially branching) on the interaction between the polymer matrix and GF. Also, the effect of the branching on mechanical, rheological and thermal properties of these composites will be described.