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Laser irradiation causes structural surface changes to PET materials. As such, the process induces modification to textile fibres. It is known that this reaction enables increased dye uptake capability compared untreated/dyed polyester, as explored by Shamey and Shim (2011).

Researchers have studied laser modified polyester using a UV excimer laser approach able to deliver energy via pulsed beam i.e. the number of repeated pulses applied at different wavelengths. The work of Bahners and Schollmeyer (1989) and Kesting et al. (1990) represent early studies in this field regarding laser modified PET textile fibres, rather than films or other polymeric materials commonly explored at the time.

In UV excimer laser processing, ‘fluence’ describes the amount of energy distributed over an area (e.g. mJ/cm² or J/cm²) known as ‘Energy density’. Bahners et al. (1993, p.13) explain ‘It is well known, that the existence of a threshold fluence is a main characteristic of UV-laser induced ablation of polymer surfaces’. Wong et al. (2003, p.114) further explain, ‘…changes to the surface morphology of PET fibres were found in relation to the laser energy applied.

2.5.1.1 Laser modified PET fibres: Structural surface changes

The surface morphology of laser irradiated PET textile fibres has been widely researched.

Some of these studies include: Bahners and Schollmeyer (1989), Kesting et al. (1990), Bahners et al. (1993), Lau et al. (1997), Wong et al. (2003), Yip et al. (2002), Kan (2008b), Nourbakhsh and Ebrahimi (2012), and Kamel et al. (2012), for example.

As a highly absorbing polymer, both the physical and chemical properties of polyester are affected during fibre-laser interaction, as documented by Kan (2008b). Yip et al. (2002, p.151) explain, ‘…laser treatment on certain polymer materials can be categorised into two groups’ – above ablation threshold or ‘high fluence’ and below ablation threshold or ‘low fluence’.

The morphological configuration of the modified surface has been commonly referred to as periodic ‘roll-like’ ripple structures, as documented by Kesting et al. (1990), for example. Kan (2008, pp.115-116) reported high fluence irradiation caused the polyester surface to become

’sufficiently’ rough (Figure 14) compared to the untreated material (Figure 15). As such, this method encouraged trapped air between the solid and liquid interface preventing water penetration (Figure 16). Lau et al. (1997, p.526) discussed such effect saying, ‘According to surface physics, the unwettability of a hydrophobic material is enhanced by surface roughening’. Wettability decreased with high fluence irradiation causing an increase in wetting time. Bahners et al. (1993, p.12) also documented a higher number of laser pulses applied to the fibre increased the depth of structure yielding coarser modification. Bahners and Schollmeyer (1989, p.1884) further described this effect as ‘a strong thermal contribution to laser-material interaction’.

Figure 14: (Left) Surface structure of laser treated polyester under high fluence (Kan 2008b, p.115) Figure 15: (Right) Surface structure of untreated polyester (Kan 2008b, p.115)

Figure 16: The effect of surface roughness on wetting (Kan 2008b p.118)

Low fluence formed sub-micron structures without deposited surface debris. Nanometric (tiny) sized ripples indicated structural surface modification was reduced (Figure 17). Nourbakhsh and Ebrahimi (2012, p.3), Kamel et al. (2012, p.2) and (Kan 2008b, p.116) reported such finding. The effect of a low fluence approach is therefore of ‘greater practical importance’ (Kan 2008, p.116) in terms of functionality and applicability of the laser process for textile goods, for example. According to Kan (2008b, pp.115-116), air could not be trapped between ripples using a low fluence approach due to ripple size constraints denoting micrometre structures on the fibre surface. Both wetting and dyeing time reduced based on minimised structural change to fibres. These results suggest opportunities for dyeing polyester regarding uptake capability in terms of reduced dyeing cycles for polyester coloration, relevant to this laser-dye research.

Figure 17: Surface structure of laser treated polyester under low fluence (Kan 2008b, p.116)

In terms of the impact of laser treatment on the properties of the fabric, Kesting et al. (1990, p.326) indicated an ability to influence fibre characteristics by controlled surface modification by using a UV excimer laser. These include wetting ad adsorption abilities, for example. Lau et al. (1997, pp.14-15), Nourbakhsh and Ebrahimi (2012, p.1) and Kesting et al. (1990, p.326) discussed the potential to design/alter the optical appearance of polyester fibres due to a change in lustre or reflective ‘glossiness’. Similarly, Bahners and Schollmeyer (1989, p.1884) described a loss of ‘birefringence’ (or optical characteristics) following modification.

Kamel et al. (2012) explored the process as a method for improving properties of woven polyester fabrics, particularly dyeability (further discussed in section 2.5.1.2 of this chapter).

Tensile strength and colourfast properties were also indicated in the study based on laser modification. However, unlike this laser-dye research (as presented and discussed in Chapter 5), detailed investigation regarding textile performance was not evident and therefore limited regarding these aspects of the study. Kan (2008b) documented a more extensive investigation in this area compared to Kamel et al. (2012) by using PET fibres, yarns and fabrics, rather than one type of substrate. Discussion and results by Kan (ibid) included fibre weight and diameter, tensile strength and elongation, yarn abrasion, bending, surface lustre, air permeability and crystallinity.

2.5.1.2 Dyeability studies

Dye uptake capability of PET fibres increases when modified by laser energy, as discussed previously. Lau et al. (1997) and Kamel et al. (2012) have carried out dyeability studies with UV excimer laser treated woven polyester fabrics. Samples were dyed at 130°C for 60 minutes, as studied by Lau et al. (1997) and at different temperatures (70°C -100°C), between 5-90 minute intervals, as Kamel et al. (2012) explored, and therefore below glass transition of 130°C Tg, typically required for polyester coloration according to industry standards. In both studies, conventional disperse dyes were used. Exhaust dyeing methods were carried out in order to understand the rate of dye adsorption against laser modification, compared to untreated fabrics. Adsorption levels were quantified using colour reflectance methods via spectrophotometer as documented by Lau et al. (1997) and K/S measurements to explain colour yield, described by Kamel et al. (2012).

According to Lau et al. (1997, p.527), laser treatment quickened the dyeing process and produced ‘deeper dyeing’ in a shorter time than the untreated fabric (Figure 18). It was argued that the laser etching process caused large molecular chains of the PET fibres to break and form smaller molecules during fibre-laser interaction (ibid). It was therefore concluded that dyeability was improved by laser treatment due to the increase of the overall surface area as a result of UV excimer irradiation. These findings suggest a more permeable substrate for he dyes as a result of laser induced morphology, further supported by statistical/visual reflectance colour data (Figure 19). Laser treated samples yielded lower values attributed to darker shade depths and therefore lower reflective characteristics, which can be understood.

Figure 18 (Left): Exhaustion rate of laser treated and untreated dyed polyester samples (Lau et al. 1997, p. 526) Figure 19 (Right): Colour reflectance of laser treated and untreated dyed polyester samples (ibid)

Kamel et al. 2012 suggested increased dye uptake was proportional to beam exposure time and dyeing time. Uptake levels were defined by plotting ‘colour intensity’ against ‘dyeing time’

in relation to fibre/laser interactions over different time durations. Results showed that the dyeing behaviour of laser treated PET was enhanced compared to untreated samples.

Notably, longer beam exposure and dyeing times produced an increased depth of shade, as Kamel et al. (ibid, pp.3-4) explained. Therefore, K/S colour yield was higher

for laser irradiated samples. However, treated samples with a longer beam exposure time such as 10 minutes, produced lower K/S than 1 or 5 minutes for instance, as illustrated in Figure 20. These results can be attributed to a darkening (rather than a brightening) of colour caused by greater/longer fibre/laser interaction.

Figure 20: Dyeability of CW laser treated PET fabric at different exposure times (Kamel et al. 2012, p. 4)

The study (Kamel et al. 2012) therefore suggests dye-to-fibre penetration leading to increased uptake was proportional to both beam exposure time and dyeing time. It was concluded that pre-treatment with laser irradiation improved the dyeability of PET fibres.

The aforementioned studies offer some insight regarding the molecular change of PET textile fibres induced solely by excimer laser irradiation. Dyeability research provided further knowledge of dye uptake capability post laser treatment. However, the scope for combining laser processing and textile coloration methods is limited for polyester compared to polyamide (PA) synthetic materials (further discussed in section 2.5.2). Therefore, this digital laser-dye research has addressed some of gaps in existing knowledge. An ability to understand, calculate, determine and repeat energy density parameters in relation to colour density based on the dyeability of laser modified polyester has been achieved in this study. In doing so, a controllable laser processing approach to tonally engineer dye on to polyester textile surfaces with digital patterns was found.