Introduction to Polymer Science and Technology Introduction

Examples of the replacement of metals with plastics in house-hold appliances and the advantages gained are given by Hagan&Keetan(1994).

1.3 What can be achieved by appropriate selection of polymer-based materials?

Polymeric materials offer high strength- and stiffness-to-weight ratios, corrosion resistance, moulded-in colour, safety and ease of fabrication into complex shapes, which often results in greatly reduced product costs.

1.3.1 Reduction in cost

Judicious usage of even an expensive material such as carbon-carbon composite (at the cheaper end £100-£150 kg¹) can be cost-effective. Carbon-carbon raw material costs vary according to the type and geometries of fibres, the type of matrix, the end use and method of production (Savage 1993, p373). Carbon-carbon composite brakes in place of steel/ cermet brakes offer significant weight savings in military and commercial aircrafts. In Concorde 600 kg was saved, which means extra payload or fuel saving.

1.3.2 Improvement in performance/safety

Most modern-day feats in sports have been possible, not least, due to the introduction of polymeric materials into sports equipment. A 120-mile-an-hour serve in tennis could not exist without polymer-matrix fibre composite rackets. Research in biomechanics has shown that the early rackets were poorly constructed to damp the high vibrational forces, which are generally regarded as the main cause of "tennis elbow". Todays composite constructions improve the rackets strength and durability, as well as damp the high impact forces involved in these sports.

Huge increases in height achieved by leading pole vaulters depend on the use of carbon-fibre/epoxy and glass-fibre/epoxy prepregs in the construction of modern pole vaults.

Recent successes in cycling are strongly associated with high-tech racing bikes of carbon-fibre composite disc wheels with improved aerodynamics, lightness, rigidity and conservation of momentum.

A Formula-1 car is likely to be subjected to a number of different forms of severe impact loading during a race. These events include strikes from track debris, collisions of various types and impact with the track due to a combination of bumps and perturbations with the aerodynamic down force. Since the early 1980s the construction of Formula-1 racing cars has been dominated by the use of carbon fibre reinforced composite materials.

When carbon fibre composite chassis were first introduced by McLaren, in conjunction with Hercules, a number of designers expressed concern as to the suitability of such brittle materials for this purpose. Indeed, some even went so far as to attempt to have them banned on safety grounds! An incident in the 1981 Italian Grand Prix at Monza went a long way to dispelling these fears and removing the doubt as to the safety of carbon fibre structures under impact. John Watson lost control of his McLaren MP4/1, smashing heavily into the Armco barriers. The ferocity of the crash was sufficient to remove both engine and transmission from the chassis. The remains of the monocoque were catapulted several hundred yards along the circuit until finally coming to rest. The Ulster man was able to walk away from the debris completely unscathed. The wrecked chassis clearly demonstrated the ability of the composite structure to absorb and dissipate kinetic

Introduction to Polymer Science and Technology Introduction

energy. The high stiffness of the chassis allowed the impact to be absorbed by the structure as a whole rather than being concentrated at the point of impact. Furthermore, the composite material was able to absorb the energy of impact by a controlled disintegration of the structure. By contrast, the forces generated from the impact of a vehicle constructed from a ductile metal such as aluminium are sufficient to exceed the materials elastic limit. In an aluminium car the monocoque would have remained in one piece, but collapsed until all of the energy had been absorbed. The driver would doubtless have been killed.

In their web publication entitled "The compelling facts about plastics 2007", the organisation of PlasticsEurope (2007) highlights that "plastics protect us from injury in numerous ways, whether we are in the car, working as a fire fighter or skiing. Airbags in a car are made of plastics, the helmet and much of the protective clothing for a motorcycle biker is based on plastics, an astronaut suit must sustain temperatures from -150 to 120 °C and the fire-fighter rely upon plastics clothing which are protecting against high temperature, and are ventilating and flexible to work in. Plastics safeguard our food and drink from external contamination and the spread of microbes. Plastics flooring and furniture are easy to keep clean to help prevent the spread of bacteria in e.g., hospitals. In the medical area plastics are used for blood pouches and tubing, artificial limbs and joints, contact lenses and artificial cornea, stitches that dissolve, splints and screws that heal fractures and many other applications. In the coming years nanopolymers will carry drugs directly to damaged cells and micro-spirals will be used to combat coronary disease. Artificial blood based on plastics is being developed to complement natural blood".

Date: 2015-12-11; view: 766