I have read through this thread -- as well as those on other forums that discuss this topic -- and I thought I would give my impressions of PETG tubing in comparison to rigid acrylic. I have used only Primochill brand tubing, so it is only this brand for which I can offer insight.
Several people have contested the claim that rigid acrylic is prone to cracking. If you think that it does not crack, you are simply incorrect. I have had two pieces of bent tube that developed hairline cracks... one from accidentally dropping it and the other from causes unknown. Moreover, I had one piece that cracked in half, but this was due to excessive stress caused by my attempting to make a piece fit that was slightly too long. Now, does this mean that rigid acrylic tubing is weak? I guess that depends on how we define weak, so I would not say that. I can say that it is when compared to PETG tubing.
I cannot begin to adequately extoll the virtues of PETG tubing compared to acrylic:
-It is extremely strong and is far more resistant to cracking and scratches. For those who would respond that they haven't had issues with acrylic cracking or scratching, I noticed other virtues of the material.
-It is much easier to work with. It bends more cleanly, and I was able to achieve much cleaner bends (with no variation in diameter along the bend) with far less effort/caution than with rigid acrylic.
-I also found that PETG is capable of sharper bends , requiring a smaller turn radius. Primochill compression fitting collars are easier to slide over PETG tubing and can easily round bends, which means that bends can be made right up to the point of insert -- without needing as much straight tube to line up the compression collar.
-Although PETG is stronger than acrylic, I found it much easier to cut through cleanly, requiring less effort and chipping less.
-PETG tubing is more malleable than rigid acrylic, so if a tube is slightly mis-measured... or if the tubing run was a little off center, it will bow easily, allowing insertion into the fitting. --Lastly, PETG is cheaper -- way way cheaper -- than rigid acrylic. A pack of 4 24" Primochill rigid acrylic tubes costs about 25$, while I purchased a pack of 12 36" Primochill PETG tubing for about the same price (26$ I think).
This is not to say that there aren't any negatives:
-One thing that is noticeable upon close inspection is that PETG tubing does not have the same glass-like transparency that rigid acrylic does . It is difficult for me to articulate the difference in transparency, but I'll try: the rounded PETG tubing has what looks like small lines running down the tubes, which refract light. It's almost as though the tube is made round by making thousands of bends until it appears round (I know that this is not the case, I'm simply trying to describe the way it looks). This is only noticeable upon very close inspection, though, and I would wager that from a few feet away or in a picture, one could not tell the difference between clear acrylic and PETG.
-B Negative reports water permeation as a drawback, so I did a little research. Syan43806 writes:
""PETG" is just another name for PET or Polyethylene Terephthalate. It's literally the same plastic that is found in your soda bottles.
I work in the polymer industry and we manufacturer PET. Overall, this is very interesting but there is ONE concern I do have with PET vs Acrylic and that's the diffusion rate of water in PET vs Acrylic. I don't know about how water acts in acrylic but water can diffuse through PET. For example, a 1 year old soda bottle will look squished because it's lost a lot of water."
This supports B Negatives concern; however, in the same thread GTUK responds
"The water vapour transmission rate of both PET and PETG are similar, but when the PET is stretched or orientated under the correct conditions, the transmission rate can be reduced by about one third. The tube extrusion process does not introduce any orientation into PET."
So the degree to which water permeation is a factor will depend on the manufacturing quality, but it does allow some water vapor to escape over time. If it loses the same amount or less than a plastic soda bottle, though, I imagine I'll be doing maintenance on my loop before this becomes a factor.
-The last purported drawback I have read about is that, as opposed to acrylic, PETG is not resistant to staining. I use Mayhem's red dye, so this was a red flag until I read the following, also written by GTUK:
"The chemical and stain resistance of PET and PETG are very good and will certainly out-perform acrylic. In fact, acrylic (PMMA) is very susceptible to chemical attack, especially when stressed. This normally causes stress cracking, even if the part is annealed to reduce any residual stress."
So, it seems that contrary to what some have claimed, PETG is actually more resistant to staining and tolerates dyes and other chemicals better than acrylic.
In conclusion, I will just say that which tubing to use is a matter of personal preference I suppose. To me, the benefits of PETG far outweigh the drawbacks, and it is now my tubing of choice. I hope that this post has provided users with good info on which they should choose.