Fibers aramid. surface treatment

This chapter is concerned primarily with the surface treatments of high performance fibers, including glass, carbon (or graphite), aramid, polyethylene... 

Aramid fibers are widely used as reinforcing fibers in high performance composites. One disadvantage is the poor adhesion to the matrix materials. This arises from the lack of functional groups in the polymer. To overcome the lack of adhesion, the fibers are treated by so-called finish formulations, which is essentially a surface treatment. 

It should be noted, however, that the thermal and mechanical properties of vegetable fiber reinforced polymer composites are notoriously lower than those of similar composites reinforced with synthetic fibers (e.g., carbon, glass, aramid) [1, 2,12]. The above-mentioned techniques, i.e., fiber drying and surface treatment or the addition of a compatibilizer, are mostly not enough to adjust the properties of vegetable fiber reinforced polymers to the desired level. Moreover, even though these treatments enhance adhesion, there is some controversy in the literature about their effect on the mechanical properties of the fiber itself and even when a more pronoxmced gain is noticed after treatment, the improvement for the composite is often within the scatter of the results. In addition, the cost and environmental impact of some of these treatments, especially of those more elaborated, often prevent their industrial scale applications. 

A significant number of studies on p-aramid/epoxy systems are related with surface treatments of the reinforcing fibers. The purpose of these treatments is threefold (i) roughening of the fiber surface in order to enlarge the physical interface with the matrix resin allowing also for mechanical anchoring, (ii) chemical activation of the fiber surface, and (iii) creating chemical bonds across the fiber/matrix interface. 

Aramid fibers were subjeeted to a variety of surfaee treatments to improve the interfaeial stress transfer between a thermoplastie matrix and the treated fiber eomposite. Analytical techniques to characterize the effect of surface treatment included DSC, Optical Microscopy, AFM and micromechanical analysis using Raman spectroscopy. Correlations between the different analysis methods were identified. It was found that plasma modified and chloride grafted fibers had the largest degrees of transcrystallinity, highest nucleation rates and greatest interfaeial shear strength between fiber and matrix. 

Figure 2 Graph shows the sizes of the transcrystalline zones formed around aramid fibers embedded in a Pebax matrix. The fibers have varying surface treatments...

Figure 6 Average Max. ISS v. Average Fragment Length for aramid fiber with various surface treatments with a Pebax 7033 matrix...

Fig. 5.21. Surface amine concenlralion (O) of aramid fiber and ILSS ( ) of epoxy matrix composites as a function of ammonia plasma treatment time. After Brown et al. (1991).

Y. Ren, C. Wang, and Y. Qiu. Influence of aramid fiber moisture regain during atmospheric plasma treatment on aging of treatment effects on surface wettability and bonding strength to epoxy. Appl. Surf. Sci., 253(231 9283-9289, September 2007. 

In this study, in order to improve the adhesion properties of aramid fibers to rubber as matrix, nylon thin films were securely formed on the surfaces of the aramid fibers by a radio frequency ion-plating (RFIP) method which represents an application of low temperature plasma treatment. These fibers were coated with a RFL (resorcinol-formaldehyde-latcx) adhesive which has high affinity to both nylon and rubber The adhesion properties of the fibers to rubber were evaluated, and the effects of the RFIP method were confirmed by a comparison of the RFIP treated fibers with those subjected to the low temperature plasma treatment. The usefulness of such surface modification methods will be discussed. 

It is shown that the adhesion properties of rubbers can be enhanced by low temperature plasma treatment of the aramid fibers with inactive surfaces, and coating with RFL adhesive immediately after the treatment. 

Sydenstricker et al. [38] evaluated the IFSS of sisal fibers, both untreated and subjected to different surface chemical treatments, by SFP tests. The sisal fibers were inserted at a constant 3 mm depth in polyester blocks. The authors found [38] that the values of IFSS that are given in Table 9.1 showing treated sisal fibers, up to a certain level of surface chemical treatment, present better IFSS with polyester as possible composite matrix. It can also be inferred from Table 9.1 that the value for both the untreated and treated fibers are markedly lower when compared to synthetic fibers with polymeric matrix. For instance, Tanaka et al. [40] reported an IFSS of 20 MPa for aramid fiber embedded in epoxy matrix. 

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