Identifying Organic Contaminants in Sub Tier Castings Using FTIR Fingerprinting
Attenuated total reflectance FTIR identifies organic parting agents on raw castings before expensive coating processes fail on customer lines.
Siloxane Residues
Meaning
Trace amounts of organosilicon compounds remaining on a surface after contact with silicone-based oils, rubbers, sealants, or greases. In high precision manufacturing, siloxane residues are problematic because they spread rapidly and are difficult to remove with standard cleaners. These molecules migrate across surfaces through evaporation and redeposition, creating a thin, low energy film that interferes with bonding and coating processes.
Coating Interference
Surface tension is significantly reduced by the presence of these residues, causing liquid coatings to bead up rather than spread. This results in defects known as fish eyes, where the paint pulls away from the contaminated spot to form a circular void. Even a few parts per billion can cause a total failure of an expensive paint line.
This issue is particularly common in facilities that mix molding and painting operations in the same building.
Electronic Reliability
Electrical contacts can fail when silicones migrate onto the metal surfaces and decompose into silicon dioxide. This insulating layer increases electrical resistance and can lead to the failure of switches or relays. In semiconductor cleanrooms, strict controls are placed on the use of silicone products to prevent this contamination.
The use of low outgassing materials is the standard way to mitigate this risk.
Detection Method
Verification of surface cleanliness involves the use of infrared spectroscopy or contact angle goniometry. A high contact angle with water indicates the presence of a hydrophobic contaminant like siloxane. Special solvent blends or plasma cleaning are often required to completely strip the residue.