Development and performance optimization of polyurethane-based multifunctional coatings using Taguchi method

Overview of Multifunctional Polyurethane-Based Coatings

  • Inducing multifunctionality is a critical requirement for textile products used in diverse environments to expand their scope and potential applications.

  • The research focuses on fabricating polyurethane (PU)-based water repellent, flame retardant, and antibacterial coatings on cotton fabrics.

  • Polyurethane-based polymers are chosen for their superior flexibility, which is a result of their low glass transition temperature (TgT_g).

  • Key advantages of PU coatings in textile applications:

    • Higher resistance to abrasion.

    • Ability to withstand washing and dry cleaning due to the absence of plasticizers.

  • Safety applications: Functionalized cotton textiles are essential in public spaces such as hospitals, cinemas, and seminar rooms, where they must meet specific requirements for liquid repellency, flame retardancy, and antibacterial protection.

  • The coating process sequentially involves deposition via a knife coating machine, followed by drying and curing of the coated textiles.

Experimental Materials and Fabric Specifications

  • The research utilized specific commercial chemical agents from various suppliers:

    • RUCO-COAT PU 1110: An aqueous, aliphatic polyether polyurethane dispersion.

    • RUCO-GUARD AFR: A fluorocarbon resin formulation used for water and oil repellency.

    • RUCO-FLAM SCO: A halogen and antimony-free flame retardant (FRA) containing organic and inorganic salts.

    • RUCO-BAC AGP: A suspension of silver chloride and titanium dioxide (AgCl/TiO2AgCl/TiO_2) used as the antibacterial agent (ABA).

    • Lutexal HIT: An acrylic-based thickener obtained from BASF.

    • PU 200: Used to prepare the polyurethane dispersion.

    • Liquor Ammonia (2 g/l2\,g/l): Added to the dispersion during preparation.

  • Fabric specifications:

    • Type: 100% Cotton fabric, desized and bleached.

    • Warp and Weft Count: 30/1.

    • Ends per inch: 76.

    • Picks per inch: 68.

    • Dimensions for samples: 1300×1600 mm1300 \times 1600\,mm.

Design of Experiment (DoE) and Coating Formulation

  • The research employed a Taguchi design to monitor the interdependency of chemical concentrations and identify the most influencing parameters.

  • Formulation constants: Coatings were prepared by adding 35 g/l35\,g/l PU followed by 2 g/l2\,g/l liquor ammonia.

  • Detailed experimental design (Table 1):

    • Individual Antibacterial Samples:

    • S1: 0.2% RUCO-BAC AGP.

    • S2: 0.4% RUCO-BAC AGP.

    • S3: 0.6% RUCO-BAC AGP.

    • Individual Water Repellent Samples:

    • S4: 2% RUCO-GUARD AFR.

    • S5: 4% RUCO-GUARD AFR.

    • S6: 6% RUCO-GUARD AFR.

    • Individual Flame Retardant Samples:

    • S7: 20% RUCO-FLAM SCO.

    • S8: 30% RUCO-FLAM SCO.

    • S9: 40% RUCO-FLAM SCO.

    • Combined Multifunctional Samples (S10 to S18):

    • These samples involve varying combinations of 0.2%, 0.4%, or 0.6% ABA; 20%, 30%, or 40% FRA; and 2%, 4%, or 6% WRA.

Fabric Coating Application and Testing Methodologies

  • Application Process:

    • Steps: Knife coating $\rightarrow$ Drying $\rightarrow$ Curing.

    • Layers: Three layers were applied to each sample.

    • Drying/Curing Protocol: After the first and second layers, the fabric was dried. After the third layer, it was both dried and cured.

    • Specific Layering: For individual water repellent and antibacterial finishes, only the top layer contained the active agent. For flame retardancy, all three layers contained the FRA.

  • Quantitative Antibacterial Testing (AATCC 100):

    • Test organisms: Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus).

    • Contact period: 24 hours.

  • Water Repellency Testing (AATCC 22):

    • Specimens: Circular, 700 mm700\,mm diameter.

    • Method: Standard spray test using 250 ml250\,ml of distilled water.

  • Flame Retardancy Testing:

    • Instrument: Vertical flammability tester.

    • Specimen size: 300×1200 mm300 \times 1200\,mm.

    • Conditions: Flame length of 1.500 inch1.500\,inch, sustained for 12 s12\,s.

    • Measurements: After-glow time and char length.

  • Mechanical Characterization:

    • Tensile Strength (ISO 13934-1): Samples 600 mm600\,mm long and 200 mm200\,mm wide; tested in warp and weft directions.

    • Thickness (ASTM D1777): Measured using TMI Precision Micrometers.

Antibacterial Performance and Chemical Mechanisms

  • The zone of inhibition technique demonstrated that all treated samples effectively killed bacteria, whereas the untreated bleached fabric showed no inhibition zone.

  • Findings: Samples S10, S11, and S12 (containing only 0.2% ABA) showed efficient activity despite the presence of other finishing agents.

  • Chemical Reaction of Silver (Ag):

    • Release of silver ions (Ag+Ag^+) occurs due to oxidation caused by dissolved oxygen in aqueous solutions:

    • Equation 1: 4Ag+O2→2Ag2O4Ag + O_2 \rightarrow 2Ag_2O

    • Equation 2: Ag2O+2H+→2Ag++H2OAg_2O + 2H^+ \rightarrow 2Ag^+ + H_2O

  • Proposed Antibacterial Mechanisms:

    • Interaction of Ag+Ag^+ ions with thiol groups in enzymes and proteins.

    • Decrease in antioxidant enzyme activity to inhibit bacterial cell proliferation.

    • Inhibition of cell division and damage to the cell envelope.

    • Interaction with DNA to damage cells.

    • Reactive oxygen species (ROS) generation by Ag nanoparticles absorbed through diffusion and endocytosis.

Water Repellency and Surface Energy Characteristics

  • Water repellency is driven by the presence of C-F bonds in the fluorocarbon-based coatings, which possess very low surface energy.

  • Performance Trends:

    • Individual WRA application: ISO ranking increased from 2 to 4 as concentration rose from 0.2% to 0.6%.

    • Maximum contact angle: 124∘124^\circ (Sample S6).

    • Combined effects: S10 (multifunctional) showed lower repellency compared to S4 (individual finish) at the same concentration.

  • Interference: At higher concentrations, the addition of other agents slightly decreased water repellency, attributed to inefficient polymerization of the fluorocarbon-based emulsion.

Flame Retardancy and Char Length Observations

  • Flame retardancy improves as the concentration of FRA increases, indicated by a decrease in char length.

  • Comparative Analysis:

    • Bleached fabric exhibits the highest char length due to the high flammability of cellulose fibers and lack of self-extinguishing behavior.

    • Comparison of concentrations: For 20% and 30% FRA, individual finishes showed better flame retardancy than combined ones.

    • For 40% FRA, the char length was almost identical for both individual and combined finishes, indicating performance compatibility at higher concentrations.

Mechanical Properties and Structural Effects

  • Tensile Strength:

    • Most coated samples showed higher tensile strength than the untreated bleached sample.

    • Maximumwarp strength: 305 N305\,N (Sample S17).

    • Maximum weft strength: 258 N258\,N (Sample S14).

    • Explanation: Mechanical bonding between fabric and coating layers distributes external loads, enhancing strength.

  • Thickness:

    • Generally increases with the concentration of finishing agents.

  • Elongation at Break:

    • Coated fabrics showed higher elongation due to the elasticity of the polyurethane coating.

    • Maximum warp elongation: 18.66% (Sample S17).

Optimization via Taguchi Method and Grey Relational Analysis

  • The Taguchi method uses the signal-to-noise (S/N) ratio to identify control factors that minimize variability caused by noise factors.

  • Steps for Optimization:

    1. Identify control factors that reduce variability using S/N ratios.

    2. Move the mean toward the target using grey relational analysis.

  • The Quality Loss Function ($\zeta$) was calculated on a scale of 0 to 1 for various factors including Load (Warp/Weft), Char Length (Warp/Weft), and Water Repellency.

  • Detailed S/N Ratios for Multifunctional Samples (Table 3 Examples):

    • S10: Max Load Warp S/N = 48.3; Water Repellency S/N = 0.0.

    • S12: Max Load Warp S/N = 48.5; Water Repellency S/N = 6.0.

    • S16: Max Load Warp S/N = 45.9; Water Repellency S/N = 9.5.

    • S17: Max Load Warp S/N = 49.7; Water Repellency S/N = 6.0.

  • Identification of the Most Influential Factor: Delta (the difference between the high and low effect of each factor) was used to determine influence.

Final Conclusions and Optimal Parameter Selection

  • The study successfully developed a multifunctional coating for cotton providing simultaneous water repellency, flame retardancy, and antibacterial activity.

  • Antibacterial performance was found to be independent of the presence of other agents.

  • Water repellency is most sensitive to the concentration of the fluorocarbon resin and can be affected by the other finishes.

  • Optimal Robust Formulation: Sample S16 was identified as the best performing configuration, consisting of:

    • Antibacterial Agent (ABA): 0.2%

    • Water Repelling Agent (WRA): 2%

    • Flame Retarding Agent (FRA): 20%

  • Funding: Research supported by the Higher Education Commission of Pakistan [grant number 20-2922].