There's a rule of thumb in chemistry that say that increasing the temperature of a reaction by 10 degC doubles the rate at which it proceeds, but the converse is true - if the temperature is reduced by 10 degrees, the reaction takes twice as long. 20 degC will slow it by a factor of four.
As your curing mastic is outside, this rule of thumb suggests that low overnight temperatures could have slowed down the curing reaction enough to give you an extended curing time.
First you need to establish whether the mastic is indeed 'curing' or just drying out, and whether or not the 'curing' process - if it is curing - needs air.
Since it didn't go off in a tube all by itself, it isn't a straight curing process so the equation above does not apply.
The product specs mention a cure time.
The other thing to consider is that there has been a long run of low humidities. This will affect the diffusion rate of water vapour into the material, and low temperatures affect the cure rate. The TDS mentions both of these as affecting the cure time. The rate of cure is given as
2mm per 24hr at 23degC and 55%RH. Temperature and humidity in most of the UK has been somewhat below these figures, for some time.
People have mentioned technical reasons why it may be slow to set. Even in good conditions silicone can sometimes be very slow setting - just give it time.
NT
Thanks very much all for your responses. After a week now it does seem to be setting a bit. Much better than it was but still not fully set. Looks like it will need a bit more time.
Thanks
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