The work of this thesis was structured into 2 main activities: (1) to devise a surface functionalization technique for commercial carbon fibres (CFs) based on lignin precursor developed under the European Union FP7 Project “Functionalized Innovative Carbon Fibres Developed from Novel Precursors with Cost Efficiency and Tailored Properties” (FIBRALSPEC), grant agreement No. 604248. (2) To perform the exploration of low cost carbon fillers for application to polymer composites.
1st chapter presents a general introduction of the different carbon materials and their applications. 2nd chapter is concerned with the existing commercial carbon fibre manufacturing techniques and precursors. A small portion on the surface modification techniques is also added in this chapter. The 3rd chapter deals with the production techniques and limitations of Carbon nano tubes as a filler for the application to composites. A brief introduction on biochar materials has been added as well. The 4th chapter deals with functionalization study of commercial carbon fibres and lignin based carbon fibres. Commercial carbon fibres T700 were purchased from Toray, Japan to study the surface modification through low pressure oxygen plasma at Polito. Treatment parameters in an oxygen environment such as holding time (1~10 minutes), plasma power (100~200 W), flow rate (250 Standard Cubic Centimeters) and plasma chamber pressure (53 Pa) were set. Morphology of the carbon fibres before and after plasma treatment was studied through Field Emission Scanning Electron Microscopy (FESEM). Chemical nature of the functional groups formed on the carbon fibres surface after the treatment was studied through Fourier Transform Infrared (FTIR) spectroscopy and atomic percent was quantified through X-Ray Photoelectron Spectroscopy (XPS). Raman spectroscopy was carried out to study the structural changes in the carbon fibres. Wettability test was carried out to study the interaction of the surface functional groups with epoxy matrix. Tensile strength of the CFs was determined after the plasma treatment to ensure optimum mechanical performances of the treated fibres in the subsequent composites. In order to ensure the effectiveness of the plasma treatment the same samples were studied after six months of storage in ambient conditions. On the basis of the obtained results from the activities above, optimum plasma treatment parameters such as treatment time, plasma power, oxygen flow rate, plasma chamber pressure were singled out and applied on the lignin based carbon fibres. The lignin based CFs were plasma treated for 5 minutes at 100 W and 200W at a flow rate of 250 SCCM and 53 Pa plasma chamber pressure. Surface morphology was studied through FESEM. Plasma treated fibres showed canals and pits on the surface. The fibre started to damage at a plasma power of 200W. Also the oxygen pickup reduced at this treatment power as depicted by the XPS analysis. The 200 W, 5 minutes treatment was identified as an upper limit for the treatment parameter. The treated fibres were shredded finely and dispersed in epoxy resin using an overhead mixer to produce composites. Mechanical and tribological analysis was carried out and compared with the neat epoxy and untreated CFs composites. The plasma treated carbon fibre composites outperformed their counterparts. Based on the observations we recommended low pressure oxygen plasma treatment for the surface modification of the lignin based carbon fibres intended for commercial use.
To further support our recommendations we produced carbon fibers from waste cotton clothes in Polito and applied the same treatment to them. The temperature profile for the thermal treatment was deduced from thermogravimetric analysis of cotton fibres in argon environment. XPS and FTIR analysis was carried out to ensure the absence of any impurity in the cotton fabrics. Carbonization process was carried out in a Carbolite furnace (TZF12/65/550) at the temperatures of 400o C, 600o C and 800o C for one hour in nitrogen environment at a ramp rate of 15o C/minute. The sample prepared at 800o C was selected to study the plasma treatment due to its more ordered structure and high carbon content as depicted by the Raman and XPS analysis respectively. The carbon fibres were treated with oxygen plasma at 100 W and 200 W for 5 minutes. Surface morphology and structure of the treated CFs were studied via FESEM and Raman spectroscopy. Surface of the treated fibres showed pits and canals confirming the action of the plasma elements while a degradation of the ID/IG ratio in the Raman spectra evidenced the effects of the plasma elements on the structure of the CFs. The functional groups on the surface of the plasma treated CFs were studied through X-Ray Photoelectron Spectroscopy and Fourier Transform Infrared spectroscopy. Chemical groups like alcohols, carboxyl and carbonyl were found on the surface of the treated CFs. BET analysis showed that surface area of the fibers increased after treatment. The plasma treated CFs retained higher amount of the epoxy resin in the wettability test. The plasma treated fibres were applied in composites. Epoxy based composites were fabricated with the pristine and treated CFs in 1% and 3% by weight. Mechanical and tribological analysis was carried out on all composites. The composites of the plasma treated fibres showed superior mechanical and tribological properties when compared to their untreated CFs counterparts. Morphology of the mechanical and tribological specimen were studied with FESEM to investigate the interaction of the filler with the matrix. Above results supported our earlier argument and low pressure oxygen plasma was recommended as a suitable treatment for the modification of the carbon fibres.
The 2nd part of the thesis emphasizes the application of cheap precursor based carbon materials for the tailoring of composites properties. In recent years, low-cost carbons derived from recycled materials have gained a lot of attention for their potentials as filler in composites and in other applications. The electrical, frictional and mechanical properties of polymer composites can be tailored using different percentages of these fillers. In the Carbon lab at Polito we synthesized carbon nano materials (CNMs) from waste polyethylene bags in two different morphologies namely carbon nano beads (CNBs) (P1) and a mix of carbon nano tubes (CNTs) and carbon nano beads (P2) using chemical vapour deposition (CVD) technique by varying the carrier gas pressure. Morphology of the CNMs were studied through FESEM and their purity through Thermogravimetric Analysis (TGA) and Raman spectroscopy. Epoxy based composites were fabricated using these CNMs as filler in 1% and 3 % by weight. Mechanical properties and tribological properties were compared with the epoxy composites of commercial Multi Walled Carbon Nano Tubes (MWCNT). It is observed that the in house generated CNMs composites show overall better mechanical and tribology properties compared to the neat epoxy and the commercial MWCNTs based composites. Morphology of the composites was analysed through FESEM to study the interaction of the filler with the matrix that lead to improved performances. A model on the fracture behaviour was proposed on basis of FESEM analysis. Chapter 6 is concerned with this activity.
In chapter 7, the maple based biochar has been explored as a cheap alternative filler to enhance the polymer properties. In this regard, the mechanical, tribological and electrical behavior of composites with two types of biochar based on maple wood namely biochar and biochar HT were investigated and compared with those of a composite containing multiwall carbon nanotubes. HT is heat treated at 900o C in nitrogen at 1 hour. Superior mechanical properties (ultimate tensile strength, Young modulus and tensile toughness) were noticed at low biochar concentrations 2~4 wt. %). Biochar based composites showed equivalent tribology properties to the composites fabricated with MWCNTs. Furthermore, dielectric properties in the microwave range comparable to low carbon nanotubes loadings can be achieved by employing larger but manageable amounts of biochar (20 wt. %) rending the production of composites for structural and functional application cost-effective.
Conclusive remarks and future plans are compiled in chapter 8
