We have a problem at work and I need some help on a theory.
We ship a liquid in plastic bottles using a pump closure. Recently some of the bottles have been arriving to customers having leaked. The pumps are still in the closed position and the boxes are not damaged.
We heat the liquid slightly before filling the bottles (120∘F) but it has mostly cooled when filling takes place (70∘F – 90∘F). Filling is done near sea level. The pumps that we use are a screw on type with a dip tube that reaches the bottom of the bottle. There is a small amount of air in the bottles after filling. Sometimes during filling air bubbles are incorporated briefly into the liquid but float to the surface after 15-30 seconds. This liquid will solidify at around 55∘F.
The liquid, when it does leak, is exiting from the spout of the pump. The vast majority do not leak.
If you squeeze a bottle you can see the liquid flowing up the dip tube.
I had several bottles leak when I drove from our filling facility to Asheville, NC (~2300 ft.)
My theory: When we fill a bottle and then screw on the cap, we are increasing the pressure of the air trapped inside the bottle. The pressure of the air could also be affected by the heat of the liquid when bottled and amount of air still incorporated into the liquid when the cap is applied. When the product is then shipping to or over higher elevations, the air pressure decreases which allows the air inside the bootle to expand which causes pressure on the liquid. The liquid is then pushed up through the dip tube and out the spout of the bottle. This may be exaggerated by the solidification of re-liquification of the liquid as it does contract slightly when it becomes a solid.
Am I completely off my rocker or is this tracking with anyone else? Let me know if photos would help or more explanation is needed.
EDIT: Additional thought. When the leaking occurred for me on the trip to NC, the bottles were stored in the car which was reaching 30∘F at night and probably 90∘F during the day. I know that heat will make the liquid expand as is proven when we overheat a drum and have to clean up the mess. So it could be a combination of lower ambient pressure (altitude) and expansion of the liquid (heat) that is causing the internal pressure to rise. Therefore the pump is acting like a pressure release value.
Best Answer
As I understand it, you have two theories.
I think we can probably rule out both of these, and here's why:
Temperature drop
As we know from the ideal gas law, $$\displaystyle \frac{P_i V_i}{T_i} = \frac{P_f V_f}{T_f}$$ Where $P$ is pressure, $V$ is volume and $T$ is temperature. Subscripts are $i$ for initial and $f$ for final. This suggests that as the liquid cools (assuming constant volume) the pressure would also go down. So that would seem to eliminate the possibility of the temperature change causing a pressure increase and therefore a leak.
Placement of cap
If we suppose that the volume of the cap is $A \text{cm}^3$ and that the cap effectively seals completely when the threads first engage (which is unlikely, practically speaking), and the volume of air within the bottle is originally $B \text{cm}^3$, then the increase in pressure would be $(A+B)/B$. So if the volume of the cap is significant compared to the volume of air within the filled bottle and if the cap seals immediately, then it could be a factor. However, it seems unlikely to me that the screw top seals immediately on engagement of the threads. This is easily checked -- screw on the top half-way and squeeze the bottle. If gas escapes, it's not that tightly sealed and we can probably eliminate this theory.
Reduction in ambient pressure
An altitude change, especially if the product is shipped by air, would have the effect of reducing the external (ambient) pressure outside the bottles which could indeed cause a leak.