Mechanical Properties of Blends

As discussed in section 2.2 of this literature review, uncompatibilised blends of immiscible polymers exhibit a coarse and unstable phase morphology with poor interfacial adhesion. The mechanical properties of these blends are often poorer than those of either component. The poor mechanical properties can be improved with the addition of a small amount of interfacial agent that lowers the interfacial tension in the melt and enhances interfacial adhesion in the solid.

Mechanical testing is frequently employed to evaluate the compatibility of the polymer blends by comparison of the mechanical property profile with and without compatibilisation [29, 38, 40, 182-184]. Two types of mechanical tests have been used: the low rate of deformation (in a tensile, compressive, or bending mode), and the high speed impact tests.

The low speed mechanical properties of polymer blends have been frequently used to discriminate between different formulations or methods of preparation [183]. Of these, the tensile elongation at break is very sensitive to blend component adhesion strength and is routinely used to evaluate the degree of compatibilisation in polymer alloys [3, 185-187]. Figure 2.31 shows the stress-strain curves for polyethylene-PVC blends with and without chlorinated polyethylene (CPE) compatibiliser [185]. The addition of 20% CPE transformed the brittle nature of unmodified HDPE/PVC and LDPE/PVC into very ductile blends.

Figure 2.31 Effect of chlorinated polyethylene (CPE) on stress-strain curves for polyethylene-PVC blends [185]

One of the most important mechanical properties of polymer blends is the achievement of enhanced impact strength [38]. Toughness is defined as the total area under the stress-strain curve, thus abruptly ending curves without a yield point are characteristic of brittle materials. High impact strength and toughness are generally characterised by yielding accompanied by a high elongation to break and a large area under the stress-strain curve. However, a polymer that has a yield point using slow speed testing may fracture in a brittle manner at high speeds. Also, many polymers that are ductile under normal testing conditions may appear to be brittle if the test specimen contains a notch or a crack [184].

Various testing methods have been developed to quantify the toughness of polymers. These usually involve the delivery of a sharp blow by either a hammer or by a projectile propelled at the polymer or dropped on it [188].

Commonly used impact test methods include Izod impact tests, Charpy impact tests, tensile impact tests and falling weight impact tests. A comprehensive survey on these test methods has been conducted by Perkins [188].

Polymeric systems are roughly classified as [189]:

• Type I polymers: Brittle, having low crack initiation as well as low propagation energy in impact. They fail by crazing as the main fracture mechanism. Therefore they exhibit low unnotched and notched impact strengths. Examples: polystyrene, SAN and poly(methyl methacrylate);

• Type II polymers: Ductile, have high crack initiation energy, but low crack propagation energy on impact. They fail by yielding as the main fracture mechanism. Therefore they normally do not fail when unnotched, but show much lower impact strength when notched.

Example: polyamide, poly(ethylene terephthalate) and polycarbonate.

For blends that are brittle under standard notched Izod impact testing, the impact behaviour of both uncompatibilised and compatibilised blends will be brittle with nearly identical impact strength. Therefore unnotched impact strength will be a more appropriate way to differentiate the toughness change of the notch-sensitive blends through compatibilisation as demonstrated by Tsai [70] and Shieh [87] et al.

When used alone or in combination, both slow speed tensile testing and high speed Izod impact testing have been proven to be useful tools in evaluating the efficiency of blend compatibilisation and mechanism of rubber toughening by many researchers.

CHAPTER 3

EXPERIMENTAL

This research was carried out in two separate studies. The preliminary stage involved the evaluation of the effectiveness of five reactive and non-reactive polar compatibilisers namely, EVA, EMA, E-GMA, E-MA-GMA and HDPE-g-MAH, in compatibilising HDPE with four different types of engineering polymers namely, ABS, PC, PBT and PA6. A laboratory co-rotating twin screw extruder was used for the preparation of HDPE/compatibiliser binary blends.

The compounded binary blends were then blended with the matrix polymers and injection moulded into test bars for mechanical testing and characterisation of the resultant ternary blends.

The second stage of work consisted of reactive grafting of a low molecular weight epoxy onto maleic anhydride functionalised HDPE (HDPE-g-MAH), characterisation of these grafted blends, and investigation of the compatibilisation effectiveness of these reactive grafted blends with ABS, PBT and PA6 matrix polymers. An extrusion plastometer was initially utilised for screening the reactive formulations whereby the most promising formulations were selected for scaling-up using a twin screw extruder. These compounded reactive blends were subsequently injection moulded for evaluation of their compatibility with the three matrix polymers.

This chapter covers the details of raw materials used, approaches in processing of the blends, and also discusses the experimental techniques employed in characterisation of the blends.

3.1 Materials

This section of the thesis covers detailed information of raw materials used in the studies and also discusses the reasons for material selection.