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WATT 91 > 7 N/mm²

The term WATT 91 > 7 N/mm² refers to a critical performance benchmark for wood adhesives, particularly those used in demanding applications such as window and door manufacturing. It is based on the DIN EN 14257 standard, which was originally known as WATT ’91 (Wood Adhesives Temperature Test).

1. Test Description: WATT ’91 (EN 14257)

The WATT ’91 test is a standardized procedure used to determine the thermal resistance of a wood adhesive joint, primarily for PVAc (white glues).

Test Procedure

  1. Preparation: Test specimens are prepared as lap joints (two pieces of wood glued with a small overlap) according to the specifications of EN 205. The wood is typically conditioned to a specified moisture content.
  2. Conditioning: The bonded specimens are cured for a standard period (e.g., 7 days) at room temperature and standard humidity.
  3. Heat Exposure: The cured test specimens are then placed in an air-circulating oven and conditioned for 1 hour at an elevated temperature of 80°C (176°F).
  4. Testing: Immediately after heat exposure, the specimens undergo a tensile shear test (pulled apart) while still maintained at the elevated temperature of 80°C.
  5. Measurement: The test measures the maximum load the joint can withstand before breaking. The result is expressed as the tensile shear strength at failure in Newtons per square millimeter (N/mm²).

2. Significance of WATT 91 > 7 N/mm²

The requirement for a failure load greater than 7 N/mm² is a minimum performance criterion often required by industry bodies (such as the ift Rosenheim in Germany for window technology) to certify an adhesive for exterior and thermally stressed applications.

  • Thermal Resistance: The primary function of this threshold is to ensure that the adhesive joint retains sufficient strength when exposed to high temperatures. This is critical because thermoplastic adhesives, such as PVAc, can soften under heat.
  • Real-World Application: In real-world use, wooden elements like window frames, doors, and conservatory structures can reach high temperatures, especially when exposed to direct sunlight. If the adhesive joint softens excessively, structural integrity may fail or creep (permanent deformation) may occur.
  • Window and Door Industry: For adhesives used in bonding the corner joints of windows and doors, as well as other structural elements, achieving this minimum value is essential to prove long-term performance and durability. Adhesives certified for the highest water-resistance category (D4) and applications where joints are exposed to weather conditions frequently meet or exceed this 7 N/mm² minimum.

In summary: WATT 91 > 7 N/mm² means that the adhesive joint is strong enough to resist a tensile shear force greater than 7 Newtons for every square millimeter of the bonded area while being subjected to a continuous temperature of 80°C.