All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.
If the animal fat that is added to the plastic is derived from animals slaughtered according to the Sharia, then there is no doubt that it is pure. If such fat is from animals that were not slaughtered according to the Sharia, but the fat is transformed to another substance because of the manufacturing and processing, then, according to the preponderant view of the scholars, transformation renders it pure.
This is the view that was adopted by Shaykhul-Islam Ibn Taymiyyah and Ibn Al-Qayyim . Ibn Taymiyyah, while addressing this issue and underlining that the preponderant view is the purity of such things, said, "This is the correct view in this regard. These things were not declared unlawful by means of religious texts or meaning, so they are not unlawful or in the meaning of what is unlawful, so there is no reason for deeming them prohibited. Rather, they were underlined in the texts addressing the lawful things. They are declared lawful. They also fall into the category of the things about whose permissibility scholars agreed.”
Ibn Al-Qayyim wrote:
“It is impossible for the ruling on impurity to remain when the name and characteristics of the impure substance have changed. The ruling is connected to the name and characteristics of a substance. The presence of the name and characteristics determine the ruling on the substance and is present or absent depending on whether they are present or absent. The religious texts addressing the prohibition on dead meat, blood, pork and alcohol do not apply to crops, fruits, sand, salt, soil, or vinegar, whether in wording, meaning, text or analogy.” [I‘laam Al-Muwaqqi‘een]
If we doubt the purity of these animal fats, then the basic principle is that they are pure. Accordingly, the plastics to which this animal fat is added are pure as well. Given the case of ‘Umoom Al-Balwa (i.e. permissibility due to unavoidable necessity when a matter becomes too widespread and is difficult to avoid) of such plastics, declaring them pure is the correct view in order to spare people hardship.
Allah knows best.