Photoinitiator 651 : Formula Optimization Case Tackling Surface Tackiness and Coating Yellowing

As a new-type foreign trade service provider driven by both technological transformation and export business, HiSiaddi has built a "1+2+3+4=1" service system and can supply photoinitiator 651 sourced directly from multiple well-known original manufacturers.

Boasting R&D capabilities, HiSiaddi has collaborated with manufacturers on technological transformation and accurately captured market demands, repeatedly proposing formula optimization schemes and application improvement suggestions for photoinitiator 651. Below is a case of HiSiaddi’s formula optimization consulting service for photoinitiator 651.

Please contact HiSiaddi customer service if you require formula optimization consulting services.

I. Client Profile & Production Failure Background (Mid-to-high-end European Optical Raw Material Manufacturer, Non-small-scale Trading Entity)

Purchaser: AUSTRO OPTIC COAT GmbH, located in Vienna, Austria. The company specializes in high-end UV transparent coatings for optical lenses, UV encapsulation adhesives for automotive lenses and light-transmitting protective films for precision electronics, supplying top European optical enterprises and automotive component manufacturers. It purchases 49 tons of original BASF Irgacure 651 annually. To cut raw material costs and shorten lead times, the client selected domestically produced high-purity photoinitiator 651 for bulk procurement. After the first 12-ton shipment arrived and was put into production using the original mature formula for imported materials, four critical technical failures emerged consecutively, forcing full production line suspension:

1. Incomplete powder dissolution forming tiny white particle suspensions in adhesive liquid, resulting in pinpoint bright spot defects after optical coating, rendering all finished products undeliverable.

2. Excessive yellowing of cured coatings after short-term room-temperature aging; transparent lenses showed obvious yellowing after 72 hours, failing optical transmittance standards.

3. Uneven curing with tacky surface layers and under-cured bottom layers: the surface dried thoroughly while the inner layer remained soft and sticky on the same production line, failing adhesion tests.

4. Deteriorated storage stability of adhesive liquid; viscosity rose abnormally after standing at room temperature for 3 days, unable to meet continuous feeding requirements for mass production.

The supplying factory could only provide COA quality inspection certificates guaranteeing purity standards, lacking UV formula application engineers to troubleshoot issues from the perspectives of formula compatibility, production processes and post-treatment of raw materials. The client urgently commissioned HiSiaddi’s technical team to conduct joint technical troubleshooting via remote guidance and on-site support to resolve formula and production operational challenges in one stop.

II. HiSiaddi Identifies Root Causes of Failures, Classified into Raw Material, Formula and Production Process Triggers

Combining the physicochemical properties of 651, the client’s existing resin system and workshop production parameters, HiSiaddi’s UV curing application engineers traced and confirmed the root causes of the four failures:

1. Particle precipitation: Excessively large crystal particle size of domestic 651 plus unreasonable feeding and dissolution processes Conventional domestic 651 features D50 of approximately 7–9μm, while original BASF products undergo ultra-fine pulverization to achieve D50 ≤3.5μm. The client retained the feeding method designed for imported materials, adding solid powder directly to acrylic resin under low-speed stirring at room temperature. Coarse grains of 651 dissolved slowly and precipitated microcrystalline particles under local supersaturation. Meanwhile, scattered natural light in the feeding workshop triggered minor premature photolysis of 651, generating insoluble by-products that worsened white spot suspensions.

2. Accelerated yellowing: Excessive formula dosage, trace colored impurities in raw materials and overexposure curing energy The original formula contained 3.2% 651 by weight. Domestic 651 has slightly higher effective activity than imported grades, so over-dosing left residual decomposition fragments that oxidized to form chromogenic groups under heating. Trace colored by-products in raw materials, combined with excessive power of UV lamps and overexposure curing, synergistically accelerated yellowing.

3. Uneven surface & inner curing: Mismatch between light source wavelength and the absorption peak of 651, unbalanced coating thickness control 651 features a characteristic absorption peak at 337nm, yet the client’s existing UV lamps mainly emit 395nm light with insufficient short-wave UV penetration for thick coatings. The surface layer received strong near-source light and cured rapidly, while the photoinitiator in deep layers failed to activate fully, leading to dry surfaces and tacky inner layers.

4. Viscosity rise during adhesive storage: The formula contains trace acidic monomers that slowly hydrolyze 651 and trigger gradual crosslinking of the system The client’s formula incorporates a small amount of carboxyl-modified acrylic resin. 651 undergoes slow hydrolysis in weakly acidic environments, with trace decomposition products triggering gradual pre-polymerization of the system and continuous viscosity increase during storage, shortening the shelf life of adhesives.

III. Four Major Modules of Formula Optimization & Production Process Rectification Solutions

1. Feeding Process & Pre-treatment Optimization to Eliminate White Spot Precipitation Completely

① Raw material pre-treatment: In a yellow-light safe workshop, pre-dissolve 651 with matched active diluents at low temperature to prepare a 25% masterbatch before adding to the main resin, avoiding incomplete dissolution from direct feeding of solid powder. ② Workshop environment control: Install blackout curtains in the batching workshop and replace lighting with UV-free yellow lamps to ensure full light-shielded feeding and prevent premature photolysis and deterioration of 651. ③ Coordinate upstream manufacturers: Deliver subsequent batches of 651 customized with ultra-fine pulverization specifications of D50 ≤3.8μm to optimize solubility from the raw material end.

2. Fine-tune Formula Proportions to Suppress Yellowing via Precise Dosing

Reduce the original 651 dosage from 3.2% to 2.4%, and compound with 0.3% low-yellowing auxiliary photoinitiator 184 for synergistic efficiency. Residual 651 content is lowered while maintaining curing efficiency. Trace antioxidant additives are incorporated into the formula to block free radical oxidative yellowing post-curing.

3. Adjust Production Line UV Equipment Parameters to Resolve Uneven Curing & Tackiness

Add a 340nm band mercury lamp module to the original LED lamp group to match the UV absorption range of 651. Adjust line speed according to coating thickness: reduce production line speed for thick film sections and increase the proportion of short-wave UV light to ensure uniform curing of the entire coating.

4. Improve Formula Compatibility to Enhance Adhesive Storage Stability

Add trace pH buffering additives to the original carboxyl resin system to neutralize weakly acidic components and isolate the acidic environment that causes hydrolysis of 651. Batch small-scale verification proved viscosity fluctuations controlled within 5% after 15 days of room-temperature standing, meeting warehousing turnover requirements.

IV. Phased Small-batch & Pilot Verification to Resume Full Production

1. Laboratory formula testing: Optimized formula samples produced coatings free of white particle defects, with yellowing index meeting standards after 72-hour accelerated aging, uniform curing throughout layers and qualified adhesion performance.

2. 500kg workshop pilot mass production: Full-process specifications formulated by HiSiaddi (light-shielded dissolving, segmented curing) were strictly implemented for 24-hour uninterrupted continuous production. All optical parameters of finished products passed incoming material inspection by downstream automotive lens manufacturers.

3. Full production of remaining bulk goods: All 12 tons of goods were put into production following the optimized scheme, and finished products were successfully warehoused and shipped.

V. Long-term Cooperation Achievements

1. Cost optimization: Comprehensive raw material costs of domestic 651 decreased by 27% compared to original BASF products, and the full-year 49-ton order was fully executed.

2. Long-term formula custody service: The client entrusted HiSiaddi with permanent technical custody of formula optimization and raw material selection for its full series of optical UV adhesives, with pre-delivery small-batch compatibility testing for every batch of incoming raw materials.

3. Supporting development of new products: When the client developed ultra-thin optical protective films, they commissioned HiSiaddi to develop customized compound formulas of 651.

4. Upstream product upgrade: Based on parameter data from this Austrian client, the manufacturer finalized an ultra-fine low-impurity optical special grade of 651, expanding business to two central European optical enterprises in Switzerland and Czech Republic via HiSiaddi.

VI. Case Summary

1. While domestic 651 meets physical and chemical purity standards, subtle differences exist in particle size specifications and trace impurities compared to original imported BASF products. High-end European optical adhesive formulas are highly matched to the physical properties of imported materials, and direct copying of original formulas easily leads to dissolution defects, yellowing and abnormal curing. Raw material manufacturers only control product purity and lack technical reserves for downstream formula application, unable to assist end clients with process debugging.

2. Core Value of HiSiaddi: Based on both the physicochemical properties of 651 and downstream resin systems, conduct full-chain technical rectification covering raw material selection, formula proportioning, workshop feeding processes and curing equipment parameters, breaking the compatibility barrier between domestic raw materials and high-end overseas formulas.

3. For procurement of high-end optical and automotive UV raw materials, supporting formula optimization and production process guidance constitute the core competitive advantage distinguishing professional foreign trade service providers from general spot intermediate merchants, beyond mere product quality.

Please contact HiSiaddi customer service for more formula optimization consulting services.