I recall this being covered here before. Early one-piece bottles were prone to stress cracks until the manufacturing process improved and the chemicals used in the process were refined along with the manufacturing.
(Making the bottom as it is today would have resulting in too much risk of cracking)
It was injection-molded polyester that was initially used to create the plastic bottles. The methods were relatively new. Over time, the methods were improved, and we now have stretch blow moulding: One way to get more reliable performance combined with shapes (such as the bottom…a petaloid).
They undoubtedly did it to save costs – a one-piece bottle of PET plastic only requires one step of blow-molding over a two-piece bottle made of blow-molded PET plus a base of polyethylene that then has to be glued in place.
The old two-piece bottle had a rounded end inside the polyethylene base that was, I suspect, intended to resist the pressure within the bottle. It wasn’t obvious how to make a stable base that didn’t also concentrate the forces at sharp corners (which would be weak points), making them vulnerable to fracturing.
It could be as simple as that no one had figured out how to make that 5-point molded bottle well enough at first, rather than any technical development in plastic molding. It does take time yto come up with ideas, after all.
But the present design requires fewer parts, takes fewer steps, and is lighter. There’s no doubt in my mind that that’s why they changed over – it’s cheaper for all those reasons.
PET = polyethylene terephthalate is the plastic these bottles are made of.
I could have sworn it had something to do with the non-recyclability of those black plastic bases. At least, I seem to remember from my childhood that when they were phased out, some environmental group took credit for a campaign to get rid of them. Is that ringing a bell for anyone?
I don’t recall it, and it doesn’t seem right. Polyethylene, which is what those bases were made of, is the most common plastic, and very recyclable.
Replacing the 2-piece bottles with 1-piece bottles meant that you didn’t have to worry about separating the PET bottle from the PE base, but I think that’s secondary in the minds of soft drink bottlers to the smaller expense and lighter weight of the one-piece bottles in the first place.
Yeah, but flat doesn’t work well for the one-piece design. The “petaloid” shape is the best available compromise so far on a platform bottom and the curves to avoid stress concentration points. Trying to make the bottle the shape of the old bottle, with a single piece looking like the two piece, would bow out the bottom and make it round instead of flat. Then it wouldn’t stand at all.
Why won’t the new bottles do these things?
Originally, recycling places would only take clear, non-colored plastic. Soda bottles or milk jugs. It may have been some economics of how plastic was returned to industry for reuse, and non-colored being only stock they had economical supply lines. So for every soda bottle, someone would have to separate the black bottom from the clear bottle before it could be used.
Thinking about it, smaller drink bottles, like Gatorade bottles, have a flattened bottom that still stands up better than the petaloid design. They also have a complex shape to provide reinforcement and prevent bulging, but I think something like that on a 2-L bottle would be more stable.
This is not an authoritative source, but a local Pepsi rep told me the change was all about cost. The two piece bottle tested better because the bottle was more stable standing up, but the added costs of acquiring and assembling 2 pieces wasn’t worth it. I think it was inevitable once the manufacturing process was perfected.
BTW: Look at the difference in price between soda sold in cans and bottles. Those bottles are expensive.
While the two different plastics may indeed be recyclable, your recycling center may not take all plastics. Until recently, we couldn’t recycle large-mouth plastic tubs (margarine) for this very reason. Their expanded their operations so now we can recycle all plastics.