I've just been wrapping up several pallets with 'pallet wrap', and with one roll it was fine until half way through the roll, then it ceased to cling - all one continuous length, but two distinctly different properties. The inner half of the roll wrapped ok but had no tendancy to grip itself, unlike the outer half that gripped like 'you know what on a blanket'.
No, it was literally as wrapping a pallet, half way round one circumnavigation (and getting dizzy ) it stopped sticking, and the roll can be unwrapped without clinging. Other rolls continue to work as they should
The make-up of the film is quite complex, with plasticisers and other additives. The initial "stiction" comes from a static charge created as the film is peeled off the roll; I would imagine if the part of the roll in question got too much of X or not enough of Y it may be that the charge does not form properly, or leaks away too quickly.
I'm sure there is more to it than just static though; that would explain why the film tends to fly towards anything in its vicinity, but it also tends to stick to itself for long after the static charge has dissipated. A quick look on Google suggests they put additional polymers in some types of film to improve the stickiness. Maybe your pallet wrap was not one of these types, or maybe the pallet wrap relies totally on this (not on static) and this additive ran out half way through this roll.
Does the (normal) film attract the hairs on your hand/face as you pull it off? If it does, and the defective roll doesn't, the first explanation is probably the one. If not, it does not rely on static and the second thought is more likely.
To my knowledge (and I know *very* little about glues) it simply is a glue. Some types of glues are considered as being liquids (like the one on the back of the PostIts) with a high viscosity. In the case of that cling film it is cohesion (for example mercury has a high cohesion) that clings the two (same material) parts together.
And it failed because they made something wrong. :-)
It's not actually that complex David, it's a pretty straightforward mix as polymers go. The additives - with the plasticizer usually being the greatest , are as much to aid the initial processing as the end properties. The greater the plasticizer content the softer then end product, but more importantly the greater the 'open' time under heat it will have. This is important as PVC degrades with temperature quite easily, and the extruders generate an enormous amount of shear heat in the material.
The best plasticizers for PVC are DOP's, but there was the huge public scare about phthalates, so these are reasonably fazed out.
The extruders have a length/diameter ratio on the screws of 25 or
30:1, and the decreasing flight depth ensures these get an incredibly thorough mix, and give a very homogenous melt from the end of the nozzle.
The soya bean oil in food-grade cling film replaces the straight mineral oil as a process lubricant, and the stabiliser is again to reduce chemical reactions during processing, but also acts as a tackifier in the finished product.
As to how it sticks to itself? Well the material is a hot, liquid gel when it comes out of the end of the extruder into a annular blown film die, where it gets air gently blown up the middle to expand it into a tube. This is hauled off between pinch rollers which slit it on the way, and then coil it tightly on itself whilst still warm. There is still an amount of post-processing shrinkage going on whilst it's being coiled, and this creates a small vacuum between the rolled layers which help it stick to itself. Of course the large rolls are then re-rolled into smaller lengths for the consumers, but these are often done through heated rollers which will warm and stretch it even more.
That's pretty much it for the PVC cling film. Of course, it could be made from LLDPE (linear low-density polyethylene) which will have slightly less stiction than PVC, in which case it may be a co-extrusion with EVA (ethylene vinyl acetate) to give high strength from LLDPE and a tackifier from the EVA.
Sorry , that's probably everything you didn't want to know about it, but I just couldn't help it. I don't get to be a smartass very often .
Natural polymers of LDPE and PVdC are insufficiently clingy on their own, and they do not adhere to themselves. To achieve the desired clinginess, certain polymers with lower molecular weight have to be added; the most common two are polyisobutene (PIB), and poly[ethylene- vinylacetate] (EVA) copolymer. Their chains readily interact with each other and their lower molecular weight makes them more mobile within the host polymer matrix.
I guess half of your roll missed out on the 'coating'. We used to use non clingy clingfilm to wrap fiberglass boats which had cured sufficiently to come out of the mould, but were still slightly green.