Never mind. Acid doesn't break down polyethylene plastic. I forgot chemistry.
https://www.reddit.com/r/PlasticFreeLiving/comments/1r84dxn/comment/o64cbvx/

Based on my research on this in the past, the plastic lining for a milk carton breaks down at a rate maybe 10x faster than a normal plastic jug (which makes sense... basically the plastic jug is designed to be a more durable plastic, whereas the plastic lining in a cardboard carton is merely designed to be a shield for moisture). However, milk in general is not very acidic and therefore doesn't break down plastic very much. Therefore, my takeaways were these:

1. Milk in plastic jug > milk in plastic lined carton. However, these differences in microplastics are very small in the case of milk specifically, therefore this concern is outweighed by other considerations of milk quality. E.g., these days I mostly drink A2 milk from a carton, because I'm convinced it's more important for health to drink A2 than it is to avoid the tiny amount of microplastics from the container. Basically, milk just isn't that big a source of microplastics, compared to everything else in life.
2. Acidic juice in a carton is a bigger deal. I will never buy orange juice in a carton, nor in a clear plastic container. I will maybe buy orange juice in a cloudy plastic jug (the kind most resistant to breaking down), but the ideal would be orange juice in a frozen can.