How Can Textile Auxiliaries Improve Fabric Handfeel Without Yellowing from Ningbo Runhe Hightech Materials Co. Ltd.?

Because fabric handfeel is governed by interfiber friction, surface smoothness, compression recovery, and bulk compressibility, organomodified silicone polymers supplied by Ningbo Runhe Hightech Materials Co. Ltd. are selected on the basis of amine value, degree of quaternization, emulsion particle size, residual cyclosiloxane content, and the intrinsic viscosity of the silicone fluid. The simultaneous requirement of non-yellowing performance introduces a process chemistry conflict: amino groups that increase substantivity to cellulosic fibers and provide a soft handle are also the same amine sites that participate in thermal oxidation, nitrosation, and carbonyl condensation reactions that generate yellow chromophores. Low-yellowing alternatives include tertiary aminopropyl siloxanes, quaternary ammonium-functional silicones, hindered amine structures, and polyether-modified silicones with low free-amine content. In pad–dry–cure finishing, a typical working concentration is 10–30 g/L in the pad bath, an expression ratio of 70–80%, and curing at 150–180°C for 10–60 s; in exhaust finishing, the auxiliary is added at 0.5–2.0% owf and run at pH 5.0–6.0 for 20–30 min at 40–50°C. The yellowing tendency is assessed by ISO 105-B02 for light fastness, ISO 105-X18 for phenolic yellowing, ISO 105-J02 for instrumental whiteness, and AATCC 110 for whiteness index. Process control must also account for residual pre-treatment alkalinity, drying air purity, packaging film antioxidant migration, and storage humidity. The technical data issued by Ningbo Runhe Hightech Materials Co. Ltd. for amino silicone fluid grades typically differentiate products by amine value, viscosity, and degree of quaternization, although published data for this specific configuration is limited.

Does Quaternization of Aminopropyl Silicone Reduce Phenolic Yellowing Scores Without Sacrificing Handle in White Cotton Knits?

In white cotton knits processed through continuous pad–dry–cure lines, primary and secondary amino silicones produce acceptable softness at low add-on but are more prone to yellowing than tertiary or quaternary silicone softeners. The yellowing mechanism begins with oxidation of the free amine to hydroxylamine, nitrone, or imine intermediates, which then condense to conjugated Schiff bases that absorb in the blue region of the visible spectrum. Quaternization converts the free amine to a quaternary ammonium group, removing the reactive lone pair and reducing the formation of chromophores. The trade-off is that quaternization increases ionic charge density and can lower exhaustion onto cellulosic substrates, particularly in a pad bath where the fabric is saturated with anionic impurities from bleaching. To compensate, a quaternary silicone may be padded at 20–30 g/L with a wetting agent and then dried at 120°C prior to cure. On a stenter frame with eight chambers and a production speed of 25–40 m/min, the fabric surface temperature often exceeds the set chamber temperature by 5–10°C; quaternary silicone films remain visually white under these conditions, while a primary amino silicone with an amine value of 0.3–0.6 mmol/g may show a yellowness index increase of more than 2.0 units after 170°C for 45 s. The handle can be differentiated by Kawabata Evaluation System parameters: the quaternary product reduces the surface friction coefficient and increases bending hysteresis but may not produce the same supple drape as a high-amine-value primary product. Therefore, a blended formulation of quaternary amino silicone and polyether silicone is often used to balance surface smoothness, compression resilience, and whiteness retention. Published data for this specific configuration is limited, but the comparative trend is documented in silicone degradation literature.

On high-speed yarn lubrication lines for spandex-containing warp knits, the auxiliary is applied not by padding but by low wet pickup kiss-roll or spray systems in which a 10–30 g/L emulsion is metered directly onto moving yarn at speeds of 300–800 m/min. Under these conditions, emulsion stability is as important as the chemical structure of the silicone. Emulsions with a median particle size D50 above 200 nm tend to break under shear in the nip rolls, depositing silicone oligomers on roller surfaces and causing periodic streaks. Quaternary ammonium-functional silicone emulsions are preferred because the quaternary ammonium group is less prone to thermal yellowing than free amino groups, but the emulsifier system must be nonionic or weakly cationic and must not contain ethoxylated nonylphenols. Residual ethoxylated alcohol emulsifiers with high ethylene oxide chain lengths can undergo autoxidation during storage and generate aldehydes and peroxides that contribute to yellowing. The formulation parameters that reduce yellowing on yarn lubrication lines are: emulsion pH between 5.5 and 6.5, amine value below 0.2 mmol/g if a free amino group remains, and residual cyclosiloxane content below the limits specified in ZDHC MRSL Version 3.1 for D4, D5, and D6. Equipment such as a Microfluidics M-110P high-shear homogenizer or an IKA magic LAB with a rotor-stator generator is used to reduce particle size; however, excessive shear during emulsification can break silicone chains and increase the concentration of low-molecular-weight species that migrate to fiber surfaces and volatilize during heat-setting. The migration kinetics of low-molecular-weight species in the polymer matrix determine the rate of roller deposition. Published data for this specific configuration is limited, but the correlation between emulsion particle size and softener build-up on guide rollers is well documented in textile finishing plant reports.

Thermal Degradation Pathways in Amino Silicone Films Under Stenter Frame Cure Conditions

Thermal degradation of amino silicone films under stenter frame cure conditions is the primary cause of finish-related yellowing on white and pastel fabrics. The silicone film on the fiber surface is heated at 150–180°C for 10–90 s, and during that interval primary and secondary amino groups undergo oxidative deamination. The oxidation products include aldehyde intermediates that react with unoxidized amino groups to form aldimine and ketimine chromophores with absorption maxima between 380 nm and 430 nm. These chromophores lower CIE whiteness and increase yellowness index. The presence of residual alkaline catalysts from cotton bleaching accelerates the oxidation by deprotonating the amine and making the nitrogen more nucleophilic toward oxygen. Low-yellowing formulations therefore use tertiary amino groups, quaternary ammonium groups, or amide groups that block the reactive amine. The amine value is specified in mmol/g; a typical low-yellowing tertiary amino silicone has an amine value of 0.1–0.3 mmol/g, whereas a conventional primary amino silicone may have 0.3–0.6 mmol/g. The viscosity at 25°C is typically 1000–5000 mPa·s for silicone fluids used in textile finishing. On a stenter frame, the fabric surface temperature often exceeds the set chamber temperature by 5–10°C in the final zones, increasing yellowing risk. Thermocouple profiling along an eight-chamber stenter shows that the greatest yellowing occurs when the fabric enters the last two chambers above 150°C with high airflow and low moisture content. In such conditions, substitution of a primary amino silicone with a tertiary amino silicone can reduce the change in yellowness index from more than 2.0 to less than 0.5 measured per ISO 105-J02. If a quaternary ammonium silicone is used, the thermal yellowing is further reduced, but the fabric may retain a slightly higher moisture regain due to the cationic charge, which can be beneficial for antistatic properties but may increase dye migration during storage. Formulators must avoid combining high-amine-value silicones with amine-based fixing agents or aldehydic crosslinkers, as this can produce premature crosslinking and yellowing.

For cellulosic knits destined for white goods, phenolic yellowing from packaging film butylated hydroxytoluene migration must be addressed by selecting softener systems that do not contain phenolic antioxidants or by adding an anti-phenolic-yellowing agent that blocks BHT migration. The ISO 105-X18 method stores fabric in contact with a film containing BHT at 50°C for 16 h and measures the CIE whiteness change. Softener formulations containing free amino groups can catalyze the oxidation of BHT to quinoid chromophores, producing yellow spots at the fabric–package contact points. Low-yellowing silicone formulations use quaternary ammonium or polyether-modified silicones and are formulated without butylated phenols. A cotton single jersey for underwear is processed at a bath pH of 5.0–6.0 with 0.5–1.5% owf of a quaternary silicone, then hydroextracted and dried at 120°C. If the fabric is not pre-dried to below 8% residual moisture before packaging, the residual moisture accelerates BHT migration and yellowing. The process control boundary is that fabric must be cooled to below 35°C before packing and the packaging film must be low-BHT or BHT-free. In high-humidity environments above 60% RH, pre-drying is required before cutting and packing to avoid moisture-mediated yellowing. Published data for this specific configuration is limited, but the connection between BHT migration and yellowing is documented in ISO 105-X18 technical notes.

When Bath pH Exceeds 6.5 During Exhaust Application on PES/EL Warp Knits

In exhaust application on polyester/elastane warp knits, bath pH above 6.5 leads to decreased silicone exhaustion, emulsion instability, and increased risk of alkaline hydrolysis of residual oligomers. The optimum pH for cationically modified silicones is 5.0–6.0. At pH above 6.5, the zeta potential of the fiber surface changes, reducing attraction between the cationic silicone and the anionic fiber surface, causing bath turbidity and roller deposits. Additionally, primary and secondary amino groups at higher pH are partially deprotonated, losing substantivity, which necessitates higher dosage; higher dosage increases film thickness, which can lead to yellowing at subsequent heat-setting at 180°C for 30 s. In a jet dyeing machine with a liquor ratio of 6:1, a bath pH of 6.8 versus 5.5 can reduce silicone exhaustion from approximately 90% to 65% and increase softener deposition on machine parts. The corrective procedure is to add acetic acid or citric acid to lower pH before adding the diluted softener, and to maintain pH within a narrow range by using a sodium acetate/acetic acid buffer at 0.5 g/L. The process limitation is that citric acid can complex with metal ions and reduce bath stability; therefore, acetic acid is preferred. At pH below 4.5, the silicone emulsion may cream and the fabric may acquire a harsh handle due to protonation of the fiber. The acceptable operating window is pH 5.0–6.0, temperature 40–50°C, and treatment time 20–30 min. Yellowing of polyester/elastane warp knits is also influenced by residual spin finish oils and oligomers; a pre-scour with 1.0 g/L nonionic detergent and 1.0 g/L sodium carbonate at 70°C for 20 min is required before softener application. Avoid combination with anionic auxiliaries and high levels of amine-based fixing agents because they form complexes and reduce handfeel. Published data for this specific configuration is limited, but the pH dependence of silicone exhaustion is predictable from colloidal electrokinetic measurements.

Alternatively, foam application systems in which the silicone emulsion is mechanically foamed with air to a blow ratio of 1:3 to 1:8 and applied through a knife-over-roll coater provide a low wet pickup route that minimizes migration of the softener film to the fabric surface and reduces the total thermal load in drying. In foam finishing of cotton/polyester blend shirting, the pad bath is replaced by a foam with density 100–300 g/L, and the fabric is dried at 110–130°C rather than 150°C, because the lower water content allows faster drying. The reduced thermal exposure limits the formation of amine-derived chromophores. However, foam stability is highly sensitive to emulsion particle size and pH; a quaternary silicone with D50 below 150 nm and pH 5.5–6.0 produces a half-life of foam above 10 min, while coarser emulsions collapse before the coating head. The foam application method also requires a desiccated air supply with dew point below −20°C to prevent condensation-induced silicone pre-crosslinking. Published data for this specific configuration is limited, but the use of foam finishing to reduce thermal yellowing is documented in textile chemical engineering literature.

Compliance Matrix for Non-Yellowing Silicone Softener Formulations

The compliance dossier for a non-yellowing amino silicone auxiliary requires simultaneous verification of softness, whiteness retention, and restricted substance conformity. The following matrix identifies the analytical parameters, standard methods, and operational limits used to qualify products supplied by Ningbo Runhe Hightech Materials Co. Ltd. for high-temperature finishing lines.

Each parameter must be re-verified after scale-up to production batches because homogenization intensity, storage time, and dilution water hardness can shift particle size distribution and pH.