Thank you for sharing your experience and observations with us.

Blistering in a composite occurs usually as a result of osmosis. Osmosis is caused by a macroscopic solution in the composite. In this case, the macroscopic solution could consist of water and some other components, like for example monomers of the adhesive that is used. The macroscopic solution ought to be prevented in composites, especially in case of water (becuase of it's small size and capability for hydrolysis)

Macroscopic solutions in this kind of industrial applications are initiated by diffusion from the environment. This can be diffusion from the internal environment where it gives a constant load of hot water (95 degrees Celsius), but also and simultaneously, from the external environment (e.g. 20 degrees Celsius and 70% relative humidity). In this case the internal water is hot, which results in a relative high solubility in - and diffusivity through - the layer of polypropylene. Mind that temperature effects on the permeability behavior may be enormous with this type of polymeric materials: a mass transfer increase with a factor 1000(!) compared to 20 degrees Celsius is not uncommon. This reduces the time lag, which is the time the water requires to obtain a (critical) solubility at the polypropylene - aluminium interface, where the adhesive is.

With the adhesive many things can be wrong that would promote the occurrence of a macroscopic solution. I will mention a few:

(1) an adhesive that is susceptible to water hydrolysis,
(2) un reacted parts or cells in the adhesive due to reaction process conditions that are insufficient,
(3) a polluted surface of the metal (condensed particles / mixtures)
(4) etc....

Another or simultaneous phenomenon is internal stress by temperature. The materials have different degrees of thermal expansion, which can result in stresses with a higher energy than the interfacial stress.

IMPORTANT NOTICE:

A total collapse usually occurs as a result of the above described water diffusion with subsequent chemical attack of the interfacial region in combination with temperature initiated mechanical stress. Our chemical-physical simulation programmes can analyze this situation and provide a material/adhesive and processing suggestion.

Sincerely,
Composite Agency
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