SCC in FRP Laminate-Sodium Hypo 1

[George77577]

We have an ongoing issue with cracking around nozzle attachments in over the road filament wound tankers. Only see this in sodium hypochlorite service. Appears to be predominate in the high stress areas such as nozzles and to some extent head attachment.

1. Road tankers have very high dynamic loads but since we don't see this in HCL have to assume its related to Hypo.

2. Change to higher elongation resin...onset of problems is sooner. So assume resin is less resistant and effect is corrosion related but not obvious in the resin matrix.

3. Surface veils may play a role and increase rigidity = more susceptiple to cracking in high stress areas.

4. Hypo is unstable and possible corrosion effect is related to the E-glass fibers being attacked once veil layers are breached.

5. Degree of cracking and types vary from small micro to larger but only in Hypo service.

Anyone have any ideas on the root cause? We have onboard samples and they provide some direction but not conclusive. Its been a battle for some years still trying to uncover.

 

[Compositepro]

The cracks seem more frequent in resin rich areas and are oriented in directions where the part geometry restrains shrinkage. The stresses causing the cracks appears to be due to resin shrinkage. the delams and blistering probably occurred after the cracking. The hypochlorite may be causing some shinkage but initial cure shrinkage is probably causing most of the stress. Addition of external loads probably initiated the cracks.

 

[George77577]

Interesting observations. The laminate thickness is .170-.200". Would you say possible solution are:

1. Reduce the laminate thickness to say around .125" (Industy standard)

2. Control exotherm to reduce shrinkage?

Although we have this same laminate in HCL and many other services and don't see any cracking. Could be the Hypo is creating stress corrosion situation......possibly by decompostion. Chlorinated brine is pretty aggressive to vinyl esters.

Here is an interesting observation you might consider: Bis-A-vinyl esters.....brominated versions even better typically perform better in hypo service not as well in acid and oxidizers. Novolac VE not typically great performer in hypo but excell in acid and oxidizing environments. We have on board samples not coupons but built into the molded surface and Novolac seems to do better. Not enough data to support this yet. However, the resin used most often is a modified VE that bridges the peformance gap of the two.

See the coupons of a brominated VE and a novolac can see there is a big difference in peformance and these are not in a high stress area.

 

[Compositepro]

I'm not sure what to make of the last picture. Most of my experience is in aerospace where the resins used are epoxy,
bismaleimides, polyimides and cyanates. These all have a volumetric cure shrinkage of about 2% and shrinkage causes all kinds of problems. Polyester and vinyl ester resins have cure shrinkage of 5 to 10%. Aerospace cure cycles are typically 2 to 8 hours so that exotherm is restricted to less than 5 to 10F. Exotherm during cure creates tremendous stresses to be stored in parts because some areas gel before others. This results in directional shrinkage rather than uniform shrinkage. Total shrinkage will always be the same, so if it is not uniform some areas will see higher than average shrinkage. I know that it is common practice with polyesters to use the exotherm to get a greater degree of cure but I believe it must have a detrimental effect on strength compared to a slow postcure at elevated temperature.