Neusol Dyes

Neusol Dyes constitute a group of solvent-soluble colourants based on metal-complex, azo, and anthraquinone chromophores, supplied as low-dusting granules or free-flowing powders. They are intended for non-aqueous thermoplastics, solvent-borne inks, transparent coatings, and wood stains where molecular dissolution is required in place of particulate dispersion. The range is differentiated by Colour Index classification, hue, solubility window, and thermal stability class. Commercial model designations encode these variables; representative designations include Neusol Yellow 2GH, Neusol Red 335, and Neusol Blue 2B, while the suffix defines shade strength or solvent polarity. Because the colorants are dissolved rather than dispersed, films cast at 20 µm dry film thickness show no particle-derived haze when measured according to ASTM D1003. Batch-release testing typically includes spectrophotometric dye content, moisture content by ISO 787-2:1981, sulfated ash by ISO 787-3:2000, and heavy-metal extractables by EN 71-3 where the grade is intended for toy applications. Certificates of analysis should be consulted for batch-specific values, as published data for this specific configuration is limited.

Solubility Behaviour Across Ketone, Ester, and Aromatic Carriers

Methyl ethyl ketone, ethyl acetate, and cyclohexanone provide complete molecular dissolution for standard yellow and red grades at 20–25 °C under low-shear agitation. Aromatic hydrocarbon carriers require a polar co-solvent to maintain clarity during storage at temperatures below 10 °C; precipitation in mixed-solvent systems is avoided by adding the dye as a pre-dissolved concentrate rather than as dry powder to the finished ink. Dissolution is not shear-driven in the same manner as pigment deagglomeration. A baffled vessel with a low-shear agitator operating at 500 rpm for 10 min generally suffices for powders, while dense granules may require 1200 rpm in a high-speed disperser to accelerate wetting. Before press-ready dilution, the concentrate is strained through 1 µm absolute filter bags or cartridge filters to remove undissolved agglomerates; filter pressure increase above 0.3 bar indicates incomplete dissolution or moisture ingress. Strongly acidic or proton-donating modifiers should be avoided with anthraquinone blue and black grades because protonation can shift λmax and reduce tinctorial strength.

In nitrocellulose/polyurethane wood-coating concentrates, the dye is pre-dissolved in methyl ethyl ketone at 20–30 wt% before let-down into a non-ionic stabilizer-free base. The resulting tinting concentrate remains stable at 5 °C storage for 30 days if sealed against moisture; hazy sediment at the bottom of the vessel is an early sign of solvent-water imbalance. In polyvinyl butyral-based glass interlayers, plasticiser migration of the dye into triethylene glycol bis(2-ethylhexanoate) must be assessed because mobile low-molecular-weight yellow grades can shift in hue during autoclave lamination at 140 °C and 10 bar autoclave pressure. Production-scale experience shows that colour drift is minimised by selecting metal-complex structures and by adding the dye to the plasticiser phase rather than to the PVB resin dry blend.

In thermoplastic masterbatch production, Neusol Dyes are processed in co-rotating twin-screw extruders with L/D 40:1 and gravimetric feeding, either as dry granulate or as side-stream solvent-free liquid concentrate after the melting zone. Because low-molecular-weight solvent dyes can alter melt viscosity through plasticisation, torque and melt-pressure instrumentation is used to detect feeding irregularities. Thermal grade selection is screened by thermogravimetric analysis under nitrogen at a heating rate of 10 K/min; a 5 % mass-loss temperature no lower than 280 °C is typically specified for ABS, and 320 °C for polycarbonate. These thresholds reduce decomposition-related shade dulling and die-lip plate-out. Pre-drying at 40 °C for 4 h is recommended when moisture exceeds 0.5 wt%, particularly for metal-complex blue and black grades that can undergo hydrolytic degradation during compounding. Screen packs of 60/100/60 mesh are used to trap undispersed dye agglomerates, and pressure rise across the screen pack above 20 bar is treated as a process alarm. Combination with aliphatic amine-based additives is discouraged for certain anthraquinone blue grades because reductive fading and unwanted crosslinking side reactions may occur in amine-cured epoxy systems.

Liquid side-stream injection is used in extrusion lines with 35 mm to 75 mm screw diameters and throughputs from 50 kg/h to 400 kg/h. The injection point is placed after the first kneading block to avoid excessive volatilisation of carrier solvent; gear-pump metering at 0.5–2.0 wt% dye solids by polymer mass is typical for transparent PETG or ABS. Torque fluctuations greater than ±5 % indicate feed instability or initial solubility mismatch. Melt filtration downstream with 25 µm candle filters or screen packs removes residual agglomerates; candle-filter differential pressure above 30 bar triggers a filter-change protocol in continuous runs.

What Limits Lightfastness and Migration Resistance in Neusol Dyes?

Photochemical stability is grade-dependent and is evaluated by xenon arc exposure according to ISO 4892-2 using daylight filters and 0.35 W/m² irradiance at 340 nm. Metal-complex solvent dyes generally fade more slowly than uncomplexed azo and anthraquinone structures, but they do not equal high-performance organic pigments in exterior applications. For indoor graphics and wood coatings, end-use specifications frequently require a blue wool rating of 4 or higher after 200 h xenon arc exposure; lower ratings restrict use to short-term or interior exposure. Migration is assessed by contact bleeding against white plasticised PVC under load at 60 °C for 72 h using DIN 53405 methodology, with visual evaluation of stain transfer. Low-molecular-weight azo grades migrate more readily than higher-molecular-weight metal-complex structures in flexible PVC and acrylics; grades intended for ABS and PMMA are therefore selected by molecular weight and polarity rather than by colour strength alone. Solubility in plasticisers and processing oils must be accounted for, because oil-soluble dyes can transfer to adjacent substrates during thermoforming or lamination.

For exterior signage and automotive interior laminates, weathering is conducted with controlled irradiance, black-standard thermometer, and wetting cycles. Long-term behaviour is not reliably estimated from short-duration xenon tests because dye photofading may display non-linear reciprocity with irradiance and humidity. In acrylic face films, blistering and adhesion loss are confounded with dye photodegradation; cross-cut adhesion per ISO 2409 and colour change per ISO 7724 are recorded before and after exposure. For polycarbonate headlamp bezels, heat ageing at 120 °C for 500 h is used to detect thermal yellowing independent of light exposure. Published data for this specific configuration is limited; therefore, production qualification trials are necessary for each polymer-additive-dye combination.

Neusol Dyes Versus Pigment Dispersions in Transparent Coatings

Pigment dispersions require wetting, deagglomeration, and stabilisation; even finely divided pigments introduce scattering centres and reduce transparency in thin films. Neusol Dyes dissolve at the molecular level and introduce no particulate phase above the base polymer, which is confirmed by haze measurement according to ASTM D1003 on 20 µm cast films. Ink and coating viscosity is measured by flow cup according to ISO 2431:2019 or by rotational rheometer with coaxial cylinder geometry at 25 °C; the absence of particulate pigment phase eliminates low-shear thixotropy and reduces shear-dependent viscosity. Gloss retention and chroma development on black opacity charts are documented by spectrophotometric reflectance rather than visual grading. The trade-off is lower exterior durability and higher migration potential relative to selected high-performance pigment dispersions.

In high-speed flexographic printing, dye-based inks tolerate rapid solvent evaporation without pigment re-agglomeration on the deck or anilox roller. This reduces the need for heavy solvent blends and allows higher press speeds. However, dye-based inks do not provide the same opacity as pigmented white or opaque systems; therefore they are used in coloured transparency, metallised film, and topcoat applications where clarity or metallic flop is required. Cost-in-use comparisons should account for lower dispersion energy, reduced filtration waste, and reduced press downtime from doctor-blade wear, but not for weathering exposure where pigments are more durable.

For solvent-borne wood stains applied by gravity-fed spray equipment, a 1.2 mm nozzle and 2 bar atomising air pressure are common; wet film thickness is held between 50 µm and 70 µm to prevent solvent entrapment and colour flotation. The stain is dried at 23 °C and 50 % RH for 24 h before topcoat application. Adhesion of the topcoat to stained wood is evaluated by cross-cut test according to ISO 2409. Wood-stain concentrates are filtered through 1 µm cartridges before final reduction because undissolved dye particles can appear as dark specks in low-angle raking light on veneer.

When Low Migration Thresholds Exclude Conventional Solvent Dyes

Food-contact plastic packaging requires positive-list verification before any solvent dye is specified. Neusol Dyes are not presumed suitable for direct food-contact surfaces; compliance must be demonstrated under EN 1186-1 migration testing and EN 13130-1 analytical procedures. Where the overall migration limit of 10 mg/dm² applies under EU 10/2011, migration test results are provided for the finished laminate or coated substrate, not for the colorant alone. In toy applications, soluble heavy-metal release is measured by EN 71-3, and cobalt- or chromium-containing metal-complex grades require additional declarations under REACH Annex XVII. When specific migration limits below 10 µg/dm² are targeted for low-migration printing inks, conventional solvent dyes are usually excluded, and high-molecular-weight insoluble pigments or reactive chemistries are required. Pharmaceutical packaging may also require extractables screening under USP 661.1, where the colourant must be identified among volatile and non-volatile extracted substances. For these regulated applications, published data for this specific configuration is limited; suitability is confirmed by migration laboratory studies on the exact substrate-adhesive-ink stack.

In a high-speed rotogravure press running at 250 m/min on corona-treated polyester, Neusol Dye-based inks are diluted with an ethyl acetate and n-propanol blend to press-ready viscosity measured at 18–25 s by ISO 2431:2019 flow cup. The absence of particulate pigment reduces doctor-blade wear and maintains print smoothness on fine highlight screens. Ink color strength is checked by drawdown on polyester and measurement of optical density before the production run; optical density values are influenced by dye concentration, film thickness, and anilox engraving volume. Solvent-retention failures in high-speed drying are controlled by monitoring residual solvent by gas chromatography, with ASTM D5403 or equivalent oven method. The process window is narrow at maximum press speed; addition of more than 1 wt% of a poorly soluble dye grade to the press-ready ink can cause precipitation and register variation. This application boundary is managed by pre-dissolving the dye in ketone concentrate and filtering at 1 µm before solvent reduction.