So I happen to have some pretty extensive first hand knowledge of Grabber warmer products.
TLDR: Their performance starts degrading immediately after manufacture and degrades faster as they hit their expiration date. After the expiration, performance is unsatisfactory.
My uncle, was one of the people who first introduced air activated chemical warmers to the US market. He personally travelled to several different manufacturers of these products before choosing a company in Japan to import from.
The company he eventually became the operations manager of, was the company that imported and distributed Grabber Warmers. Grabber Performance Group was the specific distributor under the holding company John Wagner Enterprises when he finally retired. I personally worked in distribution facility for a year while I was between jobs (recession). It helps to be family with the corporate big wigs and I had some technical expertise they needed at the time.
We got Grabber warmer products for free for my entire life until my uncle retired. I was using Grabber products since before I can remember which is the mid eighties and I was born in 1982. I am originally from MI and that is where the distribution of Grabber products also was. MI gets cold in hunting season and just in general. We used the warmers all the time sledding, snowmobiling, hunting, ice fishing, and at outdoor sporting events. We used them when we didn’t even need them. I especially remember using the toe warmers for pond hockey as those skates are not insulated.
The first run of toe warmers were just a half moon shaped hand warmer you stuck over your toes and ideally put a wool sock over it. There was a bunch of fumbling around to get it in place inside your boot. The next run we’re thinner and had adhesive built onto it. You would open the pack and peel off the adhesive cover like a band-aid and stick it on the top of your toes and it stayed in place.
Grabber Warmers are not being made anymore in that guise. They sold to the company who sells an identical product we know as Hot Hands. This was many years ago now. Maybe 8 or 10. If you are using Grabber labeled products, they are old and well past their expiration. The expiration should also be clearly labeled but that label does wear off over time.
In theory, yeah, they should not degrade in the sealed package. They do though. Maybe because the packaging is very finely porous. Like a milk jug.
When the product is fresh, it is at is best for heat output and longevity of warmth production. I have gotten real burns before when using them on body areas with thin skin and inadequate layers between my skin and the warmer. At that point, linear degradation seems to take affect and the warmer does not get as hot or last as long. Eventually, and usually after the expiration has passed, product performance really degrades and gets much worse after that.
I have opened warmers 10 years past expiration and still got heat out of them but it is pretty minimal and fizzles out fast. Maybe the degradation is more like a half-life type. I had family members who tried to revitalize them by putting them in the oven or microwave. The microwave is not recommended. The fact is, a controlled chemical reaction is taking place and eventually the substances that are reacting with each other are used up and a waste product is formed.
I remember old ones having chunks that had formed inside the packet and would still work a little bit. Sometimes you would get ones that never worked or worked poorly within the expiration time but that was rare.
We used the case boxes for all kinds of storage solutions and for moving boxes. We had all kinds of Grabber swag like pens, coffee mugs, shirts, and such. I have Grabber clothing products designed for the warmers with fitted pockets to put them in. Some of them were prototypes that never made it to market. Some of this stuff were truly high end products. I had a pair of winter gloves that would stay waterproof even when you immersed your hand in water for over a minute but were still breathable. Probably would cost $200 bucks today. My hand muff has a pocket specifically designed for the Mega Warmer referred to in the OP. Most of the winter clothing never made it to market though and they cut back that R&D in the mid 90s.
