SikaMelt®-678
Polyurethane Hot Melt for assembly operations
SikaMelt®-678 is a reactive polyurethane hot melt adhesive. It cures on exposure to atmospheric humidity. Due to its long open time SikaMelt®-678 is in particular suitable for assembly application.
- Long open time for assembly (bead application)
- Very high green strength also under warm conditions
- Good adhesion on metals and polar plastics
PRODUCT BENEFITS
- Long open time for assembly (bead application)
- Very high green strength also under warm conditions
- Good adhesion on metals and polar plastics
Usage
SikaMelt®-678 is suitable for permanent bonding of polar plastics as well as for wood, foam, textiles, painted and primed steel. Non polar plastics like PP and PE can be bonded after proper physical pretreatment.SikaMelt®-678 can be used for Automotive interior assembly applications, where a high initial temperature resistance is required because of elevated temperature in the working environment. This product is suitable for experienced professional users only. Tests with actual substrates and conditions have to be performed ensuring adhesion and material compatibility.
Packaging
| Bag | 2 kg |
| Pail | 20 kg |
| Bag (cardboard) | 22 kg |
Product Details
| Chemical base | Polyurethane | |
| Colour (CQP001-1) | White – beige, opaque | |
| Cure mechanism | Moisture curing | |
| Density (uncured) | 1.2 kg/l | |
| Viscosity (by Brookfield) | at 130 °C | 9 000 mPa·s |
| Softening temperature (CQP538-5) | 72 °C | |
| Application temperature |
Short term max. 1h | 110 – 140 °C 150 °C A |
| Open time (CQP559-1) | Medium | |
| Curing time (CQP558-1) | 10 h | |
| Green strength (CQP557-1) | 1.2 MPa | |
| Shore D hardness (CQP023-1 / ISO 48-4) | 40 | |
| Shelf life | 9 months |
| CQP = Corporate Quality Procedure | A) Only valid for nozzle |
CURE MECHANISM
SikaMelt®-678 cures by reaction with atmospheric moisture. At low temperatures the water content in the air is lower, which will result in a lower curing speed (see diagram 1). When bonding hydrophobic (e.g. PP) and/or moisture impermeable substrates a significantly longer curing time has to be taken into account.
This applies especially on assembly applications with an adhesive thickness > 100 µm. For lamination applications of hydrophobic and/or moisture impermeable substrates the adhesive layer shall not exceed 100 µm. In such cases project related tests with original substrates and conditions are mandatory.
Diagram 1: Curing time for 500 µm film
CHEMICAL RESISTANCE
SikaMelt®-678 is resistant to aqueous surfactant, weak alkaline/acids solutions and temporarily resistant to fuels, solvents and mineral oils.
The chemical resistance is influenced by several factors such as chemical composition, concentration, period of exposure and temperature. Therefore a project related testing in case of chemical or thermal exposure is required.
Application
Surface Preparation
Surfaces must be clean, dry and free from grease, oil and dust.
Based on the surface and type of material, a physical or chemical pre-treatment might be required. Type of pre-treatment must be determined by preliminary tests.
For metals best results are achieved, if substrates are heated up between 40 °C and 60 °C prior the assembly process.
Application
With adequate processing equipment SikaMelt®-678 can be applied as film, dot, bead or spray application. For automated applications a suitable filter system is required.
To meet the required application properties the adhesive viscosity can be adjusted by adapting the application temperature (see table Typical Product Data).
During breaks SikaMelt®-678 is to be processed as follows:
For breaks ≥ 1 h the heating needs to be lowered to 80 °C and for breaks ≥ 4 h the heating needs to be switched off.
To ensure a constant quality during the whole production process it is mandatory to protect the adhesive in the melting tank with nitrogen, carbon dioxide or dried air (to avoid possible reaction of the product with humidity). At breaks or shut downs dip nozzle in dried oil in order to prevent curing of the adhesive (avoid blockage).
For advice on selecting and setting up suitable processing equipment contact the System Engineering Department of Sika Industry.
Removal
Equipment and application tools can be cleaned with SikaMelt®-009. Cured material can be swelled with SikaMelt®-001 and needs to be removed mechanically (see also cleaning instruction).
Uncured SikaMelt®-678 may be removed from tools and equipment with Sika® Remover-208 or another suitable solvent.
Hands and exposed skin have to be washed immediately using Sika® Cleaner-350H or a suitable industrial hand cleaner and water.
Do not use solvents on skin.
STORAGE CONDITIONS
SikaMelt®-678 has to be stored at temperature below 30 °C in a dry place.
For transportation purposes, the storage temperature can be exceeded for a period of max. 2 weeks up to 60 °C.