The clear PC window turn cloudy over period of time – obscuring signage on a school bus in Florida. Thin crack along the right side of cover opposite to metal plate fastening clear window to cover
Tests:
The window of used unit had a thin powdery (dandruff like flakes) dirty yellow (sand color) coating on the inner surface but not on the outer surface. The FTIR scan of white powder indicated it to be Silicon polymer.
The conformal coating on PC board of the used unit showed very poor adhesion to PC board and was very easy to peel. This indicated conformal coating not to have cured properly. ATR on the small section of conformal coating indicated it to be PDMS. FTIR of white potting material was also found to be Silicon elastomeric similar to conformal coating.
TGA test on new conformal coating showed weight loss about 0.6% between 23 C to 100 C (operating range of LED lights). The coating began to decompose at 371 C. The residue left was 52%. TGA on conformal coating from the used part showed weight loss of 0.3 % (half that of the new unit) between 23 C and 100 C and additional 3.6% between 100 C and 330 C. The coating began to decompose at 389 C (higher than the new coating). The residue left was 30% (much lower than the new unit).
The TGA on black inner NBR gasket indicates further loss of 0.8% below 100 C. This could be due to plasticizers used in making gasket.
Two conclusions are drawn:
(1) The white powdery coating is silicon very similar to one used in conformal coating.
(2) There is high degree of weight loss from conformal coating during use.
Explanation of failure mechanism:
The one part moisture cured PDMS conformal coating is not fully cured when PC board is installed. The LED light source and PC board is completely secured by two rubber gaskets making the enclosure air tight with no route for vapor to escape. When unit is powered the inside gets hotter. Higher temperature and stagnant air facilitates further curing giving off water and methanol . Methanol and water begins to permeate coating and reaches to interface between conformal coating and epoxy PC board destroying adhesion between coating and epoxy board. Methanol will also carry some of the uncured silicon oligomers and deposit on the surface.. Over period of time due to water and heat, silicon oligomers deposited on the inside of the window surface begins to cure. With increase in cure, molecular weight increases and forms powdery layer on PC.
It is also advisable to consult with gasket mfg. to make sure that no phthalate type esters are used as plasticizers. Phthalates are incompatible with polycarbonate.
SUGGESTIONS
- Optimize moisture curing process and make sure that coating has cured before board is being installed.
- Since moisture is detrimental to polycarbonate, use two-part silicon which cures faster and has better adhesion to PC board, making it less permeable to water.
- Consult Dow Corning for right type of conformal coating to be used.
Later development: A cheese plate was placed on window of the used unit with coated surface of window in contact with cheese plate. Much of the white powdery material transferred from wondow to cheese plate. This shows how easy it is silicon to transfer on PC substrate.
II. Crack formation in Lexan cover
Failure mode: A visible crack is observed on the right side of cover unit after being in use for several years. The crack was either next to the inner NBR gasket in one unit and on the surface opposite to fasteners holding window to cover by a painted metal restrainer in the second unit.
Close observation indicated this to happen only on the lower right side of cover but not in tub.
Cracking of part in service occurs due to following reasons:
(1) Accidental abuse of part
(2) Uneven stresses in part caused by either part being installed before being fully relieved of residual thermal stressed from molding process or from mechanical fastening process.
(3) An environmental stress crack due to some stress crack agent – either from solvent used in paint, grease from metal parts used or some chemical being released from the other parts of unit during use.
(4) Thermal and hydrolytic degradation of Polycarbonate.
Tests:
(1) MFR test was conducted on Top/ Bottom section of cover plate from unused and used units at 260 C under 2160gm weight. The average MFR for new unit was 3.4 g/10’ as compared to 3.2 for used unit. No significant increase in the MFR (which is inversely proportional to molecular weight of resin), indicated that Lexan has not undergone hydrolytic or thermal degradation.
(2) The TGA on section of used and new cover plate showed decomposition onset of 490 .8 C for the used and 477.8 C for unused unit. Both left 6.2% residue indicating that Lexan has 6.2% of CGF and not 10% as specified. The lower decomposition temperatures for the unused unit indicate migration of some chemical into it.
(3) The drop impact strength of used and unused units were in range of 230 to 275 lb-inch and hence degradation seems not to be excessive.
(4) The DSC test showed Tg of Lexan cover to be 150 C for both used and new unit. No degradation.
(5) The FTIR of cover of used and new unit showed no sign of it being PC/ABS blend. Interestingly the peak at 1772 is split for used unit. This indicates that the agent is attacking C=O-O units.
(6) The weight gain of gaskets when immersed in ASTM 903 oil at 100C for 16 hour was less than 1%. So both the red and black gaskets are made from NBR and not SBR as suspected. The FTIR scans showed significant diminishing of butadiene phase (peaks at 966 and 798 1/cm) and diminishing peak at 1259 1/cm. It appears that NBR is degrading under heat and oxygen.
(7) The diameter of inner black gasket was measured along the entire length. The mean diameter from as gasket from unused unit was 134 mil while the diameter of right side of gasket (where crack is found) in used unit ranged from 120 to 130 mil. The smaller diameter than mean indicates permenant set of gasket. The density of new gasket was 1.25 as opposed to 1.28 for the used part indicating loss of plasticizer of crosslinking.
CONCLUSION
- Both cover and tub are made of PC and not PC/ABS
- Lexan has lower amount of CGF than specified
- The carbonate unit of unused unit has altered after installation and storage – possibility of some low MW volatile permeating in PC.
- The NBR gasket has degraded and has lost compression recovery.
- The right lower side of cap has experienced higher stress.
FAILURE MECHANISM:
The inner NBR gasket holding window to cover has degraded. The lower right side of this gasket has permanent set caused by prolonged exposure to excessive stress. The change in density and loss of butadiene phase in FTIR indicates continuous cross linking of butadiene. This could explain the permanent set.
SUGEGSTIONS
(1) Consult with NBR gasket producer and discuss permanent set issue.
(2) Anneal both closure and tub to make sure all residual stresses are relieved prior to mounting and fastening.
(3) Degrease fasteners or asked for degreased fasteners.
(4) Make sure the paint on metal restrained is completely cured prior to installation. Paint thinners are the worst chemical stress crack agents.