Polyurethane adhesives
Moisture-curing, two-part, and reactive hot-melt polyurethanes; assembly windows, moisture control, joint movement, conditioned strength, and handling.
Tags: consumables, adhesives, polyurethane, assembly
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For Sikaflex-252, Sika reports 35 minutes open time and 40 minutes skin time at 23°C and 50% RH. Assemble within open time and before skin formation. Verify cure through the actual joint thickness and moisture conditions. A surface skin or an open-time value leaves the assembly’s load-bearing properties unverified. Sikaflex-252 PDS, pp. 1–2
Select the polyurethane process by its exact product, substrate, preparation, moisture, temperature, bondline, fixturing and service exposure.
Three processing families
Polyurethane chemistry commonly involves reaction of polyol and isocyanate components. Henkel describes the resulting hard and soft segments and notes that excess moisture can produce carbon dioxide and bubbles. Formulation controls how that chemistry is packaged and used. Henkel, Engineering Adhesives for Repeated Sterilization, polyurethane section.
| Family | What drives setting/cure | Selection consequence |
|---|---|---|
| One-component moisture-curing adhesive | Reaction with available moisture | Cure depth and exposure conditions matter; a surface skin does not establish cure throughout the joint. |
| Two-component reactive polyurethane | Reaction after the specified components are mixed | Metering and mixing replace dependence on atmospheric moisture as the principal cure mechanism. |
| Polyurethane reactive hot melt, often PUR | Applied warm; initial setting followed by moisture reaction | Handling strength and chemical cure are separate events; equipment and moisture access matter. |
Sikaflex-252 exemplifies the first family, 3M DP604NS the second, and 3M TS230 White the third. These named products are examples, not interchangeable grades. Sikaflex-252 US PDS; 3M DP604NS product description; 3M TS230 White TDS.
Assembly time is not full cure
Set separate limits for mixed working life, joint closure, fixture removal and full cure.
The Sikaflex-252 US PDS version 04.01, April 2025 plots cure depth against time under different temperature and humidity conditions. Verify the actual section depth and exposure. Sikaflex-252 PDS, pp. 1–2
For TS230 White, September 2024 TDS, the stated 4-minute open time concerns a 3.2 mm molten bead on a nonmetallic surface, while 150 seconds to handling strength is measured from bond creation to supporting 34 kPa tensile load. Neither means full moisture cure. The sheet says cure depends on humidity, temperature, substrate, and bondline thickness, and prohibits bonding metal/glass to themselves or each other because moisture transmission is insufficient. TS230 White TDS, pp. 1–2.
Surfaces, moisture, and foaming
Identify the actual bonding face: bare metal, coating, plastic grade, composite skin, or porous core. Cleanliness alone does not establish compatibility. Sikaflex-252 requires clean, dry, contaminant-free surfaces and substrate-specific pretreatment validated on original materials; stress-cracking-prone plastics require particular evaluation. Sikaflex-252 preparation guidance.
Moisture cure does not mean that indiscriminately wetting parts improves bonding. Henkel warns that excess water in a component or on a part can generate unwanted bubbles. Conversely, the SikaForce-101 CT45 Indian PDS, version 02.01, November 2025, deliberately describes a foaming panel adhesive and specifies a water-mist application. That process also requires a way for carbon dioxide to escape when neither substrate is porous. Follow that product's instructions rather than transferring its water addition to a different polyurethane. Henkel moisture discussion; SikaForce-101 CT45 PDS.
Expanded adhesive occupying a gap is not evidence of a qualified load-bearing joint. CT45 requires bonding pressure determined for the actual core, below its compressive strength, maintained through the press time. CT45 pressing instructions.
Joint movement and a bounded strength example
Prefer geometry that distributes load and limits peel/cleavage at edges. Set bondline thickness and movement requirements deliberately; bulk elongation at break is not an allowable repeated joint strain. A flexible adhesive can accommodate displacement while still requiring adequate bonded area and load-path design. 3M Joint Design for Structural Adhesives, November 2017.
The 3M DP604NS Black TDS marked “Dezember 2023” provides a concrete two-part example:
| Property | Reported value or boundary |
|---|---|
| B:A ratio | 1:1 by volume and by weight |
| Handling strength | 20 minutes to 50 psi overlap shear |
| Aluminum overlap shear at 23 °C | 650 psi, approximately 4.48 MPa |
| Aluminum overlap shear at 82 °C | 340 psi after 15 minutes in the test oven |
The aluminum strength is an ASTM D1002 result, with MEK/abrasion/MEK preparation, 2024-T3 clad aluminum about 0.063 in thick, 1 in specimen width, 0.5 in overlap, approximately 0.005 in bondline, and 0.1 in/min jaw speed. The sheet describes seven-day room-temperature curing at about 50% RH, using 23–24 °C in its cure descriptions. Values are typical, not design allowables. The 15-minute oven dwell is a test-conditioning detail, not a continuous hot-service qualification. DP604NS TDS, pp. 2–4.
Environment, equipment, and storage
Separate application limits, short heat excursions, and sustained service. Sikaflex-252's chemical guidance distinguishes general water resistance from temporary fuel/oil resistance and excludes several aggressive chemical classes. “Water resistant” cannot establish every hot-fluid, immersion, or aging condition. Verify the finished assembly under its intended exposure. Sikaflex-252 chemical-resistance section.
Use the specified cartridge, mixer, and ratio for two-part material; a package that fits an applicator does not establish correct proportioning. For PUR, use compatible heated dispensing equipment. TS230 White's sheet limits use to 135 °C or below and requires discarding adhesive held at application temperature for more than 16 hours. Such residence-time limits matter independently of shelf life. DP604NS handling instructions; TS230 White equipment instructions.
Protect moisture-sensitive stock after opening. CT45 specifically requires dry storage and protection of opened contents from humidity. Retain the lot, expiry, storage history, component ratio, preparation, assembly time, cure conditions, and bondline specification with the manufacturing record. CT45 storage instructions.
Handling and isocyanates
Read the current local SDS for the exact product and each component. The retrieved Netherlands Sikaflex-252 SDS, revision 14 May 2026, version 18.3, identifies isocyanates and skin/respiratory sensitization hazards. Follow its mist/vapor avoidance, ventilation, glove, and respiratory-protection provisions; do not infer safety from a cured polymer's properties. Regional documents and formulations need their own review. Sikaflex-252 NL SDS, sections 2 and 8.