TWI Industrial Member Report Summary 974/2010
By A Brun, A Bahrami and R Shepherd
Due to their small physical dimensions combined with other, specific properties (high electrical conductivity, high heat conductivity, high mechanical strength), carbon nanotubes (CNTs) have attracted a lot of interest in various industry sectors.
The aim of this project was to explore the use of carbon nanotube material as possible reinforcing filler for a range of engineering applications and as an absorber for laser welding of thermoplastics.
In the rubber industry, carbon black is already used as filler for reinforcement of the material. However, the unique properties of carbon nanotubes (CNTs), such as their high surface area, make them an interesting choice to improve rubber properties. It is not unusual for rubber compounds for engineering applications to be specifically designed for their end use by means of adjusting the 'recipe' or formulation. Since there is a wide range of polymer types available to the rubber compounder (which form the base ingredient for the specific formulation and confer the basic material properties required, such as chemical resistance) then the possibility for variation in the final recipe is essentially infinite given the range of other ingredients that can be added to enhance or extend performance.
Given this potential choice it was necessary to focus on an elastomer for a specific application. Rubber materials have been used in the Oil and Gas industry for flexible bonded hoses for 40 years. Being able to enhance their properties in increasingly harsh environments offers significant benefits for long term performance and durability. The work here, therefore, has taken an existing compound from an oilfield application, and used this as a basis for the investigation of the effect of the addition of carbon nanotubes on the mechanical performances and permeation characteristics of the rubber; the development of a new formulation was outside the scope of this project.
In this report, the term rubber has been used interchangeably with the term elastomer. The term elastomer here is meant as thermoset elastomer. Thermoplastic Elastomers (TPE) are not the subject of this report. Also, for the purpose of this report, elastomer containing carbon black but no carbon nanotubes will be called unreinforced elastomer, although strictly speaking elastomers containing carbon black are reinforced. The elastomers containing both carbon black and carbon nanotubes will be called reinforced elastomers.
Transmission laser welding of polymers using carbon black as the absorber is used in the automotive industry for components such as registration number plates, door handles, displays, electronic keys, water pumps, pneumatic valves and light clusters. In 1998, TWI also invented and patented the Clearweld process, which uses an absorber that does not degrade into any coloured by-products. The transmission laser welding process using carbon black (CB) or Clearweld is fast and produces a strong weld. However, because of their strong absorption and good mechanical properties, CNTs could be a good laser absorber, as well as reinforcement for the weld material thus increasing the strength of the joint. Work was thus carried out to compare the performances of CB, CNT and Clearweld as infra-red absorber for laser welding of polymers.
To provide comparison data between reinforced and unreinforced elastomers on:
- Mechanical performances.
- Permeability of CH4 and CO2.
- To provide comparative data on laser welding of HDPE filled with CNTs, CB and Clearweld.