A Novel Foam Coating Approach to Produce Abrasive Structures on Textiles

The main component of the foam coating formulation was abrasive particles of silicon carbide, SiC with a particle size of 9 μm (Carborex, Washington Mills). An additive filler of fumed silica, SiO₂ nanoparticle dispersion (AERODISP® W 1226), was purchased from Evonik. A water-based UV-resin binder (Ucecoat® 7177) was purchased from Allnex. CNCs, trade name Celluforce NCC^™, were purchased from Celluforce. A photoinitiator for the UV resin (Esacure KTO 46, IGM Resins B.V.) was also used. Weylchem International GmbH provided the foaming agent, Weylclean SAS 30, a secondary alkane sulfonate sodium salt. The substrate used in the foam coating was a polyamide, knitted, and open structure textile, which is used in the production of one current abrasive product, grammage 74 g/m² (see Figure 7a). Scanning electron microscope (SEM) images of SiC abrasive particles and SiO₂ nanoparticles were obtained with a Merlin Field Emission (FE)-SEM (Carl Zeiss NTS GmbH, Oberkochen, Germany) using secondary electron (SE) detector Figure 1. The electron gun voltage was 2 kV and the probe current was 60 pA.

Figure 1

The foam coating slurries were developed and explored first on a laboratory scale. The laboratory-scale study was focused on studying the foaming ability of abrasive particles with water-based UV-resin and CNC binder systems at high solids content. Thereafter, the foams were applied onto a textile substrate using an Erichsen laboratory coater, and the coating quality obtained was visually evaluated for its abrasive function. Homogenous slurries containing the abrasive, filler, and binder were prepared in tap water by mixing with a laboratory dissolver Dispermat^® TU at 2,000 rpm for 30 min. Before addition to the slurry, the CNC powder was dispersed into deionized water by stirring at 3,000 rpm for 3 h to prepare a 10 wt-% gel. The addition level of CNC to the formulations was as dry wt% of the total dry amount of abrasive, filler, and binder. The foaming agent was added at 0.3 wt% of the total dry matter, and the UV-photoinitiator was added as 5 wt% of the dry UV-resin amount. The five formulations with varying levels of CNC are listed in Table 1.

The viscosity of abrasive slurries was measured with an Anton Paar MCR 301 rheometer using cone-plate geometry. The slurry viscosities at 100 rpm are shown in Table 1. The stabilizing effect of CNC on SiC-water slurry stability was studied with a turbidity measurement using a TurbiScan Lab (Formulation SA; L’Union, France) device. The backscattering value of SiC-water slurry as such and with two parts of CNC was recorded every 4 s for 5 min after slurry preparation. The backscattering values were ratioed against the starting point to clarify the comparison between the two compositions. The SiC-slurries had a solids content of 10 wt-% and 1 wt-% in turbidity measurement.

Abrasive foams were produced on a laboratory scale using a Kenwood Chef mixer for 10 min at maximum speed. Foam half-life time was estimated by following visually the separation of the water from the freshly prepared foam in a measuring vessel. Foam density was measured by weighing after foaming. Table 1 shows the foam densities. Bubble size determination based on optical imaging was targeted to use, according to the method developed for wet foams [22, 23]. The foam sample was collected by dipping a cuvette into the freshly generated foam. Unfortunately, because of the highly non-transparent abrasive foams, the optical imaging was not able to detect the details of the bubbles in foams.

A multiple light scattering analysis with an optical analyzer Turbiscan has proven to a versatile tool to characterize emulsions, suspensions, and foams without dilution in a wide range of particle volume fractions (0–90%) and particle size (0.1–1,000 μm) for formulation and quality control purposes [24]. Foam bubble size evolution can be extracted from the average backscattering of the foam [25, 26]. Since, in this study, foams were produced at high concentrations of abrasive particles and binders, it is assumed that the changes in the backscattering value of these abrasive foams indicate the stability of the whole foam structure, and not merely the growth of bubble size. An abrasive foam stability test with turbidity measurement was conducted by recording the backscattering value of the foam every 2 s for a total of 20 min after foam generation. Foam turbidity was measured from the abrasive formulations chosen to pilot trials and foams were produced with a small rotor-stator PICO-MIX foam generator (Hansa Industrie-Mixer GmbH&Co KG) using mixing speed 1,200 rpm, pump speed of 6 l/h, and target density of 100 g/l. The stability of water-surfactant Weylclean SAS 30 (0.3 wt-%) foam was measured similarly, as a reference.

Abrasive foams were applied on the textile substrate with the Erichsen laboratory coater and dried in the oven for 5 min at 105°C; after this, the resin was cured by a LED UV-lamp at a wavelength of 395 nm. The coating quality was evaluated visually and abrasive particles stayed best on the cloth substrate surface at the highest slurry solids content of 50 wt-% (F1_Lab and F3_Lab). In addition, the use of silica filler in the formulation (F3_Lab, F4_Lab, and F5_Lab) seemed to further diminish abrasive migration into cloth pores.

Laboratory foam coatings of abrasives with silica filler, formulations F3_Lab, F4_Lab, and F5_Lab, were chosen for a sanding test. A reference sample of the existing abrasive product (Abralon 3000) was included in the test. Abrasive surface gloss was measured before sanding and after 1 min of sanding on a clear coat. It is emphasized that the gloss values of foam coated abrasive layers are not similar to the reference abrasive product because of differences in abrasive layer structure and abrasive formulation. However, the sanding test is capable to show the gloss change due to the sanding: a minor gloss increase indicates minimal wearing of the abrasive layer. Sanding results indicated that foam coated abrasive layers had the same level of wearing as the reference sample (see Table 2). The gloss increase was similar or even lower in foam coated abrasives than in the reference sample.

Based on the foaming study and laboratory-scale foam coating results, the formulations F1_Lab and F3_Lab were picked for pilot-scale coating. These two formulations had the highest solids content of 50%, which affected the best abrasive particle attachment on the substrate surface. The performance of silica filler as further diminishing the abrasive layer penetration into cloth pores was also verified in the pilot-scale trial. F3_Lab had a small gloss increase, indicating minimal wearing of the abrasive layer. Pilot formulations were prepared by mixing the components at speeds of 2,000 rpm. After adding the foaming agent, a short gentle mixing was performed, and the slurry was transferred to a foam generator. The foam was generated with a Top-Mix 60 rotor-stator foam generator (Hansa Industrie-Mixer GmbH&Co. KG) operating at pressure ~3 bar, mixing speed 500 rpm, pump speed 10 l/h, and target density 100 g/l.

The generated foam was applied with a Magnojet slot-die applicator (Zimmer Maschinenbau GmbH; Austria) onto a moving textile web using VTT's surface treatment concept pilot line, as shown in Figure 2. Formulations of foam coating trials, slurry viscosities, and foam densities are listed in Table 3. The coating speed was 15 m/min. Three IR and five airfoil dryers were used for drying, and a LED UV-curing unit was used with a UV-lamp (wavelength 395 nm) operating at 100% power and 3 cm distance from the web.

Figure 2

Grammage of the textile substrate and foam coated samples were determined by weighing a known sample size, and coat weight was obtained as a difference of grammages of the substrate and their coated counterparts. The thickness of the uncoated substrate and foam coated samples was determined using an L&W Micrometer, and coating thickness was obtained as a difference of thicknesses of the substrate and its coated counterpart. SEM images of abrasive coated surfaces were obtained with a Hitachi TM4000Plus SEM.

Abrasive-binder slurries with 2–5 wt% of CNC had viscosity in a suitable range for foam generation. Comparison of the viscosity curves of abrasive slurries was done between slurries F3_Lab, F4_Lab, and F5_Lab, which had the same abrasive/filler composition of 70 SiC/30 SiO₂, and CNC proportion was varied. Results show that CNC addition into UV-resin-abrasive mixture made the abrasive slurries shear-thinning having steady viscosity decrease with an increase in shear rate, and the slurry viscosity was higher when CNC proportion was higher (see Figure 3). Slurries of the 70 SiC/30 SiO₂ mixture with UV-resin and CNC (F3_Lab–F5_Lab) had a somewhat higher viscosity than SiC abrasive UV-resin slurry without SiO₂ at CNC concentrations 2 wt-% and 3 wt-% (F1_Lab–F2_Lab), as values in Table 1 show.

Figure 3

CNC performed not only as a rheology modifier but also improved the stability of dispersed abrasives. The stabilizing effect of CNC was verified from SiC-slurries by the turbidity measurement. The abrasive slurry with two parts of CNC showed very stable behavior at 10 wt-% solids and had better stability also at 1 wt-% solid compared with abrasive slurry without CNC (see Figure 4).

Figure 4

Smooth foams were obtained from abrasive slurries using an anionic foaming agent. Foams were stable, having foam half-life time of well over 10 min. The combination of SiC particles with SiO₂ nanoparticles produced even smoother foams. The foam stability characterization with the turbidity measurement indicated that abrasive foam structures are very stable. The backscattering value of abrasive foam was almost constant over the measurement period of 20 min after the foam generation. Reference water-surfactant foam is less stable than abrasive foam, as the backscattering value of the plain water-surfactant foam reference was steadily decreasing during the 20 min after foam generation (see Figure 5). In the abrasive foams, it looks obvious that both solid SiC abrasive particles and co-binder CNC as a gelling agent have improved the foam stability. The foam stabilization with unmodified CNC is discussed in a study of Hu et al. [27] who demonstrated that unmodified CNC particles stabilized methylcellulose aqueous foams, forming weak gels that inhibit bubble coalescence and slow foam drainage. The high concentration of the particles in the abrasive suspension and the resulting yield stress can slow down the drainage, thus leading to a stable foam. As described in a review of foam stability and aging, solid particles with attractive interactions can form aggregated structures, which can be blocked inside the foam liquid channels and increase the viscosity of the liquid phase, leading to a slow down or cessation of drainage [9].

Figure 5

In the pilot trials, the target density was set to 100 g/l, i.e., 90% of foam is air, and the resultant foam density was varying from 110 g/l to 140 g/l. The foam generation of the slurry at 50% solids was working as a continuous process. In the formulation F1_Pilot, the retention of abrasive particles on the substrate surface was not good enough, as there seemed to be partial migration deeper into the cloth pores. The second foam coating formulation F3_Pilot having silica filler with SiC abrasives resulted in improved retention of foam on the cloth surface. There is less migration of abrasive particles to the back of the cloth with the combination of abrasive particles and silica filler (see Figure 6).

Figure 6

The coat weight and thickness of the coatings are shown in Table 4. Coat weight and layer thickness values confirm that the formulation F1_Pilot has penetrated partially into cloth pores. A lower layer thickness of 22 μm and a higher coat weight of 27 g/m² were obtained using the formulation F1_ Pilot compared with the layer thickness of 30 μm and the coat weight of 20 g/m² obtained using the formulation F3_Pilot with filler. The abrasive particle deposition on the substrate is visible from the SEM images, shown in Figure 7.

Figure 7

Foam coating surface treatment is typically used to apply dilute chemical solutions forming thin coatings. This work showed that the foam coating technique is working well to apply very thick abrasive layers, >20 μm as a dried layer on a porous substrate. Abrasive-binder slurries were foamed at a high solids content of up to 50% using low foaming agent concentration (0.3 wt-% of total solids). The high solids abrasive slurries had viscosity ranging from 3,300 mPa×s to 6,600 mPa×s at 100 rpm, and the rotor-stator generator was able to foam the slurries without any problem. Foams were stable in the time scale of the foam coating process, which makes it possible to obtain uniform coating layers. According to the results obtained, foam coating offers a new and promising concept to make abrasives.

CNC performed as a co-binder and dispersing agent and made the abrasive slurries shear thinning. CNC also improved abrasive slurry and foam stability. In the foam coating process, the migration of abrasive particles deeper into the pores of the textile substrate was further reduced by combining the abrasives with silica filler.

Foam coating of abrasives produced an even abrasive layer. The foam coating process has fewer parameters to control than spray coating, which makes the foam coating a good option to produce abrasive layers on the porous textile substrate. However, the new abrasive recipes still need further development to ensure that the performance of the new, demonstrated abrasive structure is fully in line with the current commercial products. Abrasive structures made with foam coating were estimated to be suitable mainly for finessing type sanding.