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and Composite Performance

Surface modification of carbon fibres is necessary to improve interfacial adhesion between carbon fibre and matrix. Plasma treatment of carbon fibres has attracted much interest as a means to tailor the adhesion of the constituents in high performance composites. Through plasma treatment, the surface energy of carbon fibres can be altered, which determines the wettability between the plasma treated fibres and polymer matrices. This chapter reviews current progress on the surface treatment of carbon fibres in low pressure and atmospheric pressure plasmas. In this chapter the effect of both plasma treatments on the surface and bulk properties of the fibres; namely surface morphology, surface composition, fibre wettability and surface free energies, carbon graphitic order and the fibre tensile properties are discussed. The micro- and macro- mechanical performance of composites containing plasma treated carbon fibres is also presented. Comparisons of both surface and bulk characterisations of the fibres are drawn between low and atmospheric pressure plasma treatments. This chapter also highlights the suitability of continuous atmospheric plasma treatment of carbon fibres to tailor the fibre-matrix interface during composite manufacturing.

54 3.1 Introduction

Advanced fibre reinforced polymer composite materials are often used in load-bearing and structural applications nowadays due to their high strength, high modulus and low density (1.55 g cm-3). These composites consist of a continuous matrix phase, a reinforcing phase and an interface/interphase between the two constituents. The matrix protects the fibres, holds them in place (in the desired alignment) and determines the chemical and thermal resistance of the composite while the reinforcing phase, i.e. the fibres, carries the majority of load acting in the fibre direction [13, 96]. The overall performance of composite materials does not only depend intrinsically on the properties, alignment and orientation of the fibres and properties of the matrix but also on the adhesion at the fibre/matrix interface. Adequate adhesion allows for sufficient stress to be transferred from the matrix through the interface to the reinforcing fibres when load is applied. The easiest way to quantify adhesion is to determine the thermodynamic work of adhesion WA (Eq. 2.1) by measuring the contact angle θ of a (pre-) polymer wetting the fibres

and knowing (or measuring) the liquid surface tension γla [96].

(3.1)

Sufficient adhesion between fibres and matrix can be achieved when hydrogen and covalent bonds form between the fibre surface functionalities and the matrix and/or through mechanical interlocking [97-99]. Untreated carbon fibres are known to be microporous and generally have imperfect graphite crystalline structure along the fibre axis [100, 101]. Untreated carbon fibres also are chemically inert, therefore, difficult to bond to the matrix [98, 101, 102]. In order to enhance the fibre-matrix interfacial bonding, carbon fibres that are available on the market are always surface treated and often also sized. To guarantee an optimum adhesion between carbon fibres and thermosets, commercially available carbon fibres are modified by electrochemical surface oxidation (using NH4HCO3 or K2CO3/KOH or KNO3 as electrolytes) [103-106] and sized [101, 107-111].

The majority of the market is dominated by thermosetting matrices because of the low viscosity of thermoset systems, making them easy to process and form into any shape. The processing

55 advantages as well as the market’s demand require commercially available carbon fibres generally engineered to be compatible with thermosetting resins. Nevertheless, the application of thermoplastic as matrix should not be ignored in order to exploit the full advantages that these matrices can offer for the composite industry. Thermoplastic materials are tough and offer enhanced impact and abrasion properties, enhanced moisture and corrosion resistance, unlimited shelf life, clean processing with no exothermic reactions, they do not emit volatile organic vapours, are recyclable and can be processed using one-step moulding as compared to thermosetting composites. However, the problem associated with thermoplastic matrix systems is often the poor adhesion between carbon fibres and thermoplastics. Therefore, various surface treatments of carbon fibres are employed to improve the interfacial adhesion between fibres and matrix. Further oxidation of carbon fibres in dry environment (using O2 or O3) [100, 112], wet (HNO3) [100] or electrochemical (using HNO3, KMnO4, H2SO4, NaOH) oxidation [100] and other post-surface treatments such as electrocoating (PMMA, MPD) [113, 114], plasma (air, O2, N2, He, Ar, CF4, CHClF2) [22, 29, 30, 40, 115-118], plasma polymerization (C3H5NH2, C4H8O2, C8H10, C3H4O2, C6H18OSi2, C4H2O3, C7H6O2) [100, 119-123] and plasma enhanced chemical vapour deposition (CH4, C3H8, C6H6) [100, 124], deposition of active form of pyrolytic carbon or carbon nanotubes [125, 126] and polymer grafting on the fibre surface [127], have been explored to improve the interaction between thermoplastic matrices and the reinforcing fibres. However, compared to most carbon fibre surface modifications, plasma treatments of carbon fibres offer many advantages [97-99]; they are:

 dry, clean and environmental friendly compared to wet oxidation process.

 efficient in altering the chemistry of materials without affecting the bulk properties of the material and, therefore, allow tailoring of the surface properties of materials.

 versatile because many feed gases can be used to introduce various active functional groups or plasma polymer layers that are chemically bonded onto the fibre surface.

Although low pressure plasma (LPP) treatments are the more established route for surface modification of carbon fibres, they have a low-productivity due to the use of sealed vacuum chambers allowing often only for batch treatments [29, 128]. Atmospheric pressure plasma (APP) treatment, however, is more adaptable for continuous in-line modification because it can

56 be operated without the need for complex vacuum systems [29, 128]. In addition to the ability for continuous processing, APP also allows sustainable plasma glow discharge using various feed gases which is important for any continuous treatment [29, 129]. Bismarck et al. [130], Park et al. [131], Erden et al. [132] and Ho et al. [29] have demonstrated that the surface free energy of carbon fibres can be increased using both LPP and APP treatments. Such increased in fibre surface energy led to enhanced interface between the plasma treated fibres and polymer matrices, which results in better composites performance [40, 97, 131]. In this chapter, the focus is on surface treatment of carbon fibres using plasma treatment to enhance the compatibility between the fibres and polymer matrices. The impact of both APP and LPP on surface and bulk fibre properties is discussed.