Optical Brightener OB-1 for Recycled PET: Whiteness Control, Processing Trials, Quality Testing, and Supplier Qualification

Optical brightener OB-1 for recycled PET is used when recyclers and converters need to manage yellow undertones and improve the apparent brightness of selected clear, translucent, white, or lightly colored products. The additive can be effective at low concentration, but it is not a substitute for feedstock sorting, washing, drying, filtration, and controlled melt processing. A successful program connects fluorescence chemistry with the real recycled stream, final article, viewing conditions, regulatory duties, and supplier quality system.

This technical guide gives compounders, sheet producers, fiber manufacturers, molders, quality teams, and procurement specialists a practical qualification framework. It emphasizes measurable trials rather than universal dosage claims. Every final formulation should be validated with representative feedstock and commercial equipment because recycled PET variability can dominate the optical response.

Why Recycled PET Color Is Difficult to Control

Recycled PET can combine bottles, trays, fibers, labels, adhesives, pigments, barrier layers, and multiple thermal histories. Even well-sorted clear feedstock may show yellow, gray, green, or blue undertones after washing, drying, extrusion, filtration, and pelletizing. OB-1 can modify fluorescent appearance, but it cannot replace sorting, decontamination, controlled drying, and prevention of new thermal degradation.

Turn this topic into a written trial with controlled materials and acceptance criteria. Record recycled-feedstock lot and blend, OB-1 lot and dose, other additives, drying, feeder calibration, extrusion temperatures, melt temperature, pressure, throughput, residence time, filtration, pelletizing, specimen preparation, and measurement settings. Compare the candidate with the current standard under identical conditions and repeat the study across realistic variability.

Supplier documentation should support, not replace, plant evidence. Request an agreed specification, certificate of analysis, safety data sheet, technical data sheet, regulatory statements, storage and shelf-life guidance, traceability, and change-notification commitment. When a result affects compliance, consumer safety, or customer performance, verify it independently on the finished recycled PET article.

How Optical Brightener OB-1 Works

OB-1 absorbs part of the ultraviolet spectrum and emits visible blue-violet light, which can reduce the visual impression of yellowing. The apparent effect depends on light source, UV content, polymer transmission, concentration, sample thickness, pigments, and background. Therefore, qualification must use instrumental data and representative molded or extruded specimens rather than powder color or a supplier photograph.

Setting the Correct Color Target

A request for maximum whiteness is too vague for production control. Define target L-star, a-star, b-star, yellowness index, fluorescence response, haze, and visual appearance using an approved reference under controlled illumination. The correct target should reflect the final market because sheet, fiber, strapping, injection parts, and opaque compounds have different color and transparency expectations.

Feedstock Classification and Blending

Post-consumer and post-industrial PET streams differ in intrinsic viscosity, color, contamination, copolymer content, and stabilizer history. A single brightener dose will not deliver consistent results if the incoming blend changes without control. Classify bales or flakes, monitor blend ratios, use representative composite samples, and establish correction rules only after sufficient historical data.

Sorting and Contaminant Removal

PVC, polyolefins, polyamide, paper, adhesives, metals, colored PET, and multilayer structures can create color, black specks, gels, odor, or degradation. OB-1 cannot hide serious contamination and may make certain defects more visible under ultraviolet light. Improve optical sorting, label removal, washing, density separation, metal control, and manual quality checks before increasing brightener dosage.

Washing Chemistry and Residues

Alkaline washing, surfactants, antifoams, rinse quality, and carryover affect appearance and processing. Residual caustic, detergent, glue, or organic contamination can contribute to yellowing, hydrolysis, deposits, and odor during extrusion. Define rinse conductivity, pH, cleanliness, and drying criteria and investigate process residues when color shifts unexpectedly.

Drying and Hydrolytic Protection

PET must be dried to an application-appropriate moisture level before melt processing because water promotes hydrolytic chain scission. Excessive drying temperature or time can also create color and energy penalties, especially in oxygen-rich equipment. Measure moisture with a suitable method, verify dryer dew point and airflow, prevent reabsorption, and correlate dryness with intrinsic viscosity and color.

Thermal Stability During Extrusion

OB-1 must tolerate the melt temperatures and residence times used for recycled PET without unacceptable decomposition, color shift, or volatility. Actual melt temperature can exceed barrel settings because of shear, restricted screens, and worn or poorly designed screws. Map the process window, inspect pressure and residence time, and challenge the candidate at normal and worst credible production conditions.

Dosage Window and Overdosing

A small dose may improve apparent whiteness, but the response is not unlimited or perfectly linear. Overdosing can produce an artificial blue cast, reduced transparency, surface effects, poor economics, or no additional benefit due to concentration quenching and optical limits. Run a structured dose ladder in the complete formulation and select the lowest robust level meeting all color and performance criteria.

Dispersion and Addition Method

OB-1 may be supplied as powder, pre-dispersion, compacted form, or masterbatch. Low addition rates make feeder accuracy, dust, segregation, carrier compatibility, and distribution critical. Compare direct dosing with a properly designed masterbatch and verify actual uniformity across time, hopper refills, startup, steady operation, and low throughput.

Masterbatch Carrier Selection

A masterbatch carrier must melt, disperse, and remain compatible with the recycled PET application. Incompatible or low-melting carriers can affect clarity, deposits, odor, migration, mechanical properties, or downstream drying. Require disclosure appropriate to the application and test the masterbatch at full let-down under the final extrusion, molding, spinning, or sheet conditions.

Instrumental Color Measurement

Color values are meaningful only when specimen preparation, thickness, surface, background, illuminant, observer angle, aperture, UV setting, and calibration are controlled. Optical brighteners respond strongly to the instrument’s ultraviolet content, so two devices can report different results on the same sample. Document the complete method, use control standards, verify repeatability, and maintain correlation with customer viewing conditions.

UV-Included and UV-Excluded Measurements

Comparing measurements with ultraviolet energy included and excluded can help separate base polymer color from fluorescent contribution. This distinction is useful when an attractive initial result hides a strongly yellow underlying recyclate. Track both signals during qualification and aging so the plant understands whether improvement comes from feedstock cleanliness, reduced degradation, or fluorescence.

Transparency, Haze, and Clarity

A brightener may change perceived color without improving optical clarity. Fine contamination, crystallization, incompatible polymers, gels, moisture damage, and excessive additive can increase haze or reduce transmission. Measure haze and transmission on representative thicknesses and inspect black specks, gels, streaks, and fluorescence uniformity alongside color.

Intrinsic Viscosity and Molecular Integrity

Intrinsic viscosity reflects molecular weight and is important for many PET applications. OB-1 does not protect PET from hydrolysis or restore chains already broken during repeated processing. Control moisture and temperature, consider approved chain-management strategies when needed, and ensure color improvement is not purchased at the cost of unacceptable molecular degradation.

Acetaldehyde and Volatile Control

Thermal history and residence time can generate acetaldehyde and other volatiles that matter particularly in sensitive packaging applications. Optical brightener is not a volatile scavenger and should not be used to mask evidence of excessive processing. Evaluate relevant volatiles, odor, taste requirements, dryer performance, extrusion conditions, and regulatory obligations for the final article.

Migration and Extraction Testing

Additive mobility depends on molecular structure, concentration, polymer morphology, temperature, time, contact medium, and article geometry. OB-1 suitability must be established for the intended application rather than inferred from general plastics use. Where required, conduct migration or extraction studies on the finished material using applicable conditions, analytical methods, and current regulatory criteria.

Food-Contact and Regulatory Responsibility

Food-contact eligibility depends on jurisdiction, recycled-feedstock authorization, additive listing, use level, functional barrier, migration, and good manufacturing practice. A generic supplier declaration cannot approve the finished recycled PET package. The responsible producer must evaluate current rules, customer requirements, traceability, decontamination validation, and test evidence for every destination market.

Fiber and Textile Applications

Recycled PET fiber processing introduces demands related to spinning stability, filter pressure, denier, dyeing, heat setting, fluorescence, and shade consistency. Brightener response can change with orientation, crystallinity, delustrants, pigments, and fabric construction. Run spinning and textile trials using the actual polymer blend and downstream heat history rather than relying only on injection-molded plaques.

Sheet, Film, and Thermoforming

Sheet thickness, chill-roll conditions, crystallinity, orientation, reheat, and thermoforming alter color and transparency. A dose qualified in pellets may not give the same appearance after sheet extrusion and forming. Evaluate across web width, startup and steady state, trim recycling, multiple passes, and final package geometry under realistic retail lighting.

Strapping and Injection-Molded Parts

Strapping requires stable extrusion, tensile behavior, weld performance, and consistent color, while molded parts introduce gate, residence, and cooling effects. Brightener must not cause unacceptable deposits, plate-out, streaks, or mechanical changes. Test the final process, specimen thickness, pigment package, recycled content, and customer inspection method.

Multiple-Pass Processing Trials

Trim and off-spec material may return to the process, adding another heat history. Repeated extrusion trials can compare color drift, fluorescence retention, intrinsic viscosity, haze, deposits, and black-speck formation. Use identical material and conditions, include an untreated control, and do not claim lifetime equivalence from accelerated passes without correlation.

Light and Heat Aging

Fluorescent appearance may change during sunlight, warehouse lighting, heat exposure, or long storage. The base polymer, contaminants, pigments, UV stabilizers, and OB-1 can age at different rates. Measure color, fluorescence, haze, mechanical properties, and surface condition before and after application-relevant exposure using defined pass criteria.

Incoming Quality Control for OB-1

Each lot should conform to agreed identity, appearance, assay, melting behavior, volatiles, ash, particle characteristics, and color-related controls. Similar commercial names do not guarantee identical purity, isomer profile, physical form, or performance. Confirm supplier test methods, retain samples, trend results, and perform a controlled polymer check when risk or variability justifies it.

Supplier Qualification and Change Control

A capable supplier controls raw materials, synthesis, purification, particle form, testing, packaging, contamination, and traceability. Audit deviations, complaints, subcontracting, capacity, continuity, and technical support as well as the certificate of analysis. Require advance notification for manufacturing-site, process, raw-material, specification, method, physical-form, packaging, and regulatory changes.

Total Cost and Procurement Decision

Low price per kilogram can be offset by higher dosage, poor feeder behavior, dust, color variation, deposits, rejects, customer complaints, or extra correction pigments. Compare cost per tonne of conforming recycled PET across representative feedstock lots and process conditions. Approve only after multiple lots demonstrate a stable dosage window, dependable documentation, acceptable logistics, and repeatable finished-product performance.

Frequently Asked Questions

What dosage of OB-1 should be used in recycled PET?

There is no universal dose. Build a dose-response curve using representative feedstock and the final process. Select the lowest robust level that meets color, fluorescence, haze, clarity, migration, deposit, aging, and economic requirements.

Can OB-1 remove the yellow color of badly degraded PET?

No. Fluorescence can offset part of the visual yellow impression, but it cannot remove chemical degradation, contamination, haze, black specks, odor, or molecular-weight loss. Correct the feedstock and process causes first.

Is OB-1 suitable for food-contact recycled PET?

Suitability depends on current jurisdictional rules, feedstock authorization, additive identity and level, decontamination validation, migration, article use, and customer requirements. Obtain supplier data, but have the responsible manufacturer complete the final compliance assessment.

Should powder or masterbatch be selected?

Choose by feeder capability, dose accuracy, dispersion, dust control, carrier compatibility, drying, deposits, regulatory needs, and total cost. Test both forms on commercial equipment when the decision is important.

Final Qualification Checklist

Approve optical brightener OB-1 for recycled PET only after defining the color target and UV measurement method; controlling feedstock, washing, moisture, and processing; establishing a robust dose window; verifying dispersion, haze, intrinsic viscosity, deposits, volatiles, aging, and application performance; resolving regulatory responsibilities; and qualifying supplier consistency, traceability, change control, continuity, and technical support.

The best result is not the bluest laboratory plaque. It is repeatable commercial material that meets customer appearance and functional requirements without concealing weak recycling control. OB-1 creates value when its fluorescence is measured correctly, its dose is disciplined, and supplier claims are confirmed through representative production evidence.