Both papers are made of polymer carrier material (different chemistry) that transfers the toner on to the substrate. The difference is
Imageclip is a two step process, self weeding transfer paper, that transfers only the toner on the substrate while
Duracotton is a one step process but not self weeding although zero hand and zero polymer background is achievable per manufacturer's and end user's statement. Another thing is
Imageclip works very well on light color substrate besides white and ash. Hence, there is no polymer box/window.
The disadvantage when using
Imageclip is due to it's two step nature it will involve two temp and pressure settings. The first stage requires low pressure and low temp setting to transfer the polymer from the polymer carrier paper on the image paper, this setting is used to weed out unwanted polymer in open space area. The second stage requires high pressure and high temp, this setting is used when the image is transferred on the substrate. The paper does not work with very light color image. However, I have used it for photo and images with varying gradient color with very good result.
The good thing is the two steps for a pair of transfer papers do not have to be done in succession. In the first step the batch is done in low temp and low pressure to transfer the polymer and weed out the unwanted polymer in each of the image paper then raise the temp and pressure in the second step to press the batch on the substrates.
The first step is critical. If it is done right
Imageclip will yield a truly no polymer box/window transferred image. Screen printed look and feel.
Here is the link for factory overview of
Imageclip : Technical Products solutions - Photo-Trans® ImageClip™ - Neenah Paper Inc.
Sublimation like image is achievable with
Duracotton and it does not have the limitation that
Imageclip has when it comes to very light color image. I suggest that you try both papers especially on light color and different brands substrate. Wash them and make a side by comparison. I have red that Duracotton may exhibit white polymer box/window on some brand of light color garment after the first wash. Make your decision first hand then go from there. Some members use both paper and take advantage of each paper's property. Depending on the substrate color. More selection of light color substrate versus white and ash only.
Both
Duracotton and
Imageclip are very good products. So, it really is up to the user's choice, after making a side by side comparison, as to what will best fit the requirement.
Sorry, I tend to get carried away. I've been called Mr. TMI

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Hope this helps.