Laser modified PA fibres: Structural surface changes

Laser modified polyamide has been studied by Yip et al. (2002), Shahidi et al. (2013), Bahtiyari (2011), Bahners (1993) and Nourbakhsh and Ebrahimi (2012). Plain woven fabrics and knitted structures were explored in these studies. The effects on dyeing were also investigated (Yip et al. 2002; Shahidi et al. 2013; Bahtiyari 2011), further discussed in section 2.5.2.2.

Yip et al. (2002, p.152) reported on the morphological and chemical modifications of laser treated fibres commenting, ‘Today, pulsed UV laser is one of the most commonly used non-contact treatment techniques in modifying surface properties of polymers physically as well as chemically’ (Yip et al. 2002, p.73). Sub-micrometre sized structures formed on the fibre surface with low fluence methods had significant effects on wetting and dyeing properties of woven PA fabrics. Therefore, hydrophilic properties were improved with a low fluence, rather than high fluence approach, similarly described by Shahidi et al. (2013, p.42). This method was considered most appropriate in terms of practicality. As with laser modified PET fibres, ripple like structures were formed with PA textiles. Bahners et al. (1993, p.12) argued, ‘The resulting surface properties may have an important impact on processing, as they can affect technical properties of synthetic and even natural fibres…’ e.g. wetting. They further explained that surface changes to UV excimer laser irradiated PA (nylon 6,6) and PET fibres were dependent on parameters such as the range of fluences and material constants i.e. the type of substrate, for example.

Yip et al. (2002) further explored the effects on dyeing properties with PA (nylon 6) plain woven fabrics using a UV excimer laser. As with PET fibres treated by UV excimer laser, they also report ripple-like structures regarding PA materials describing them as ‘…strictly perpendicular to the stress direction of the fibre’ (ibid. p.73). These results are similarly documented by Shahidi et al. (2013, p.42) who have studied surface modification of polyamide to improve PA fibre properties for dyeing using UV excimer laser. They comment (ibid.), ‘…the change in coloration closely corresponds with the ripple-like structures and the changes in chemical properties induced by laser treatment’. Therefore, fibre modification is possible by the deliberate change in composition or structure, as described (ibid. p.33). Bahtiyari (2011, p.115) further discussed ‘the change of macromolecular structure’ to improve dyeability using polyamide knitted fabrics and a CO2 laser system. A decrease in the crystallinity (polymeric molecular order) of PA fibres led to a greater affinity for dye uptake in disperse dyeing (ibid.

p.116). Bahtiyari (2011, p.17) argued that this was due to an amorphous (formless) morphological change induced by higher laser intensities, as reported in the study.

2.5.2.2 Dyeability studies

With conventional coloration methods, synthetic textile polymers are typically difficult to dye as they require high temperatures and more resources in the dyeing process such as energy usage, chemicals and auxiliaries. Therefore, laser methods that improve dye uptake capability are beneficial to this field. Nourbakhsh and Ebrahimi (2012, p.1) explained PA (and PET) fibres are characteristically hydrophobic without laser treatment. In textile processing however, dyeing, printing and finishing are largely based on wet treatments (ibid). With a laser approach, surface modification improved fibre characteristics such as adhesion, wetting and the penetration of dyes into fibres. This interaction counteracted the hydrophobic tendencies of synthetic fibres.

Shahidi et al. (2013, p.42) studied the surface modification of plain knitted polyamide via UV excimer laser as a method for improving dyeabilty; Yip et al. (2002) explored the effects on dyeing with laser modified knitted PA fabrics, also with a UV excimer system; Bahtiyari (2011) studied knitted PA using a CO2 laser. In the study (Bahtiyari 2011, pp.115-116), improved dyeability was further attributed to ‘the change of macromolecular structure’ - a decrease in the crystallinity (polymeric molecular order) of PA fibres leading to a greater affinity for dye uptake in disperse dyeing.

Commercially available acid, reactive and disperse dyes were used in each of the studies (Yip et al. 2002; Shahidi et al. 2013; Bahtiyari 2011). Yip et al. (2002) carried out dye bath exhaustion and spectral reflectance data methods in order to observe dye uptake. Dye exhaustion (%) was measured against time (mins) for all dye baths and spectral values (R %) against wavelength (nm) for dyed samples. Shahidi et al. (2013) also examined dye bath exhaustion to compare untreated and laser treated samples. Bahtiyari (2011) studied the effect of laser exposure on the colour yields (K/S) using a laboratory dyeing machine and spectroscopy measurements, also carried out in this digital laser-dye research, further discussed in Chapter 5 of this thesis.

In acid dyeing experiments, Yip et al. (2002, p.77) and Shahidi et al. (2013, p.43) reported a faster rate of dye exhaustion with laser treated samples compared to untreated (Figure 22).

Figure 22: Exhaustion rate in acid bath – laser treated and control PA fabric samples (Shahidi et al. 2013, p.43)

Both studies documented equilibrium (sameness) was reached between treated and untreated sample over a shorter dyeing time in contrast to other dye baths containing different dye types. A change in molecular structure/bonds to laser modified fibres accelerated acid dye adsorption consequently decreasing the equilibrium time reached.

Bahtiyari (2011, p.15) investigated three types of acid dye – fast acid, milling and levelling dyes (as well as disperse and reactive dyes). A notable increase was recorded with milling type acid dyes with controlled parameters using CO2 laser in pulsed mode. Consequently, K/S colour yield values were significantly higher, indicating deeper dyeing (Figure 23). Shade changes were also described in terms of CIE L* a* b* colour measurements (L*: lightness, a*:

redness and greenness, b*: yellowness and blueness values). Acid milling dyes produced lower K/S % yield at higher intensities. This can be attributed to significant deeper dyeing caused by notably darker, rather than brighter shades, as previously discussed. With fast acid and levelling dyes a more linear/steady increase in colour yield was reported in relation to results higher laser intensities against colour yield. This can also be said for reactive dyeing experiments. Samples dyed with disperse dyes report the lowest K/S colour yield indicating the fabric was only stained and not dyed, therefore producing paler shades.

Figure 23: %KS of treated and untreated fabrics (Bahtiyari 2011, p.115)

Steeper exhaustion curves were documented by Shahidi et al. (2013, p.43) and Yip et al.

(2002, p.77) for disperse dyeing. This result was determined by fibre/dye capability. In both studies, ‘No significant change’ (ibid) was recorded between laser treated and untreated samples in the first 40 minutes of dyeing, also reported by Bahtiyari (2011, p.115). After this time, Yip et al. (op. cit) documented increased dye absorption with treated fabrics. However, Bahtiyari (2011, p.115) did not report this. Instead, Bahtiyari (ibid.) explained, ‘It is well known that disperse dyeing is unrelated with the macromolecular structure of fibres and that generally the change in fibre crystallinity does not affect the disperse dyeing of polyamide’. Yip et al.

(2002, p.78) suggested that laser irradiation increases overall dye uptake of PA fibres in disperse dyeing due to an increased surface area. However, based on these studies (Yip et al. 2002; Bahtiyari 2011; Shahidi et al. 2013), it is thought that the rate of exhaustion is not significantly altered by laser treatment in acid dyeing.

In contrast to acid and disperse dyes, both the rate of exhaustion and the total amount of dye absorption increased throughout dyeing with reactive dyes. These results are documented by Yip et al. (2002, pp.77-78), Bahtiyari (2011, p. 115) and Shahidi et al. (2013, pp.42-43).

Referring to laser treated fabrics, Shahidi et al. (ibid.) commented, ‘…darker shades are obtainable using only the usual amounts of dyestuffs’. Yip et al. (op cit.) reported that these results are due to ‘an increase in the number of end groups…’ This effect of the treated fibre therefore led to accelerated dyeing and greater overall dye uptake.

In general, regarding exhaustion, results in each of the aforementioned dyeability studies revealed laser treated samples absorb more dye overall or at the point of equilibrium (where applicable). Therefore, deeper shade depths were achieved compared to the untreated control samples. Yip et al. (2002, p.78) explained, ‘The shade improvement by laser treatment can be attributed to scattered reflection from the roughed fibre surface’. Findings were further supported by lower spectral reflectance results for all laser modified/dyed samples. For example, Yip et al. (2002, p.77) reported higher spectral values for untreated samples. On the principle of colour physics as described by Itten (1970, pp.15-16), this result can be expected due to lighter/paler shade depths typically attributed to untreated fabrics compared to laser modified samples. Paler shade depths exhibit higher reflectance compared to darker depths as with treated samples. Modified fabrics have higher absorbance and lower reflectance values due to a ‘deeper dyeing’ effect.

This literature review discussion acknowledges greater experimentation carried out with PA compared to PET synthetic fibres, in this research field. Therefore, the digital laser-dye investigation presented extends current knowledge regarding laser modified polyester in terms

of the approach and the development of processes. The work has been carried out in a way that is also relevant to other polymer fibres including polyamide as well as natural and multi-fibre fabrics. Based on a rigorous experimental framework explored, applicable to textiles, methods and techniques can be repeated and adjusted in relation to a specific fibre/fabric type. Consequently, experimental laser parameters were assessed in terms of performance and functional properties using ISO procedures in order to further understand the effects of laser treatment. This approach supported industrial applicability of processes regarding commercial relevance of new knowledge generated and consideration for the potential development of aspects studied beyond this doctoral project.