Does a plastic utensil swallow up taste and excrete it out like metal,or is it like glass.
Also can plastic be kashered if it became treif?
What are the sources that discuss this?
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This question is dealt with by Dose of Halacha:
R’ Yechiel Yaakov Weinberg (Seridei Aish 1:46) writes that the acharonim are lenient regarding plastic and such utensils may be kashered through hagalah (placing in boiling water). R’ Eliezer Waldenberg (Tzitz Eliezer 4:6:3) compares plastic utensils to stone ones and permits kashering through hagalah.
R’ Yitzchak Yaakov Weiss (Minchas Yitzchak 1:86; 3:67) writes that while one shouldn’t specifically buy non-kosher plastic utensils, if one’s plastic utensils became treif, one may kasher them. One should avoid doing this to kasher for Pesach.
R’ Moshe Feinstein (Igros Moshe OC 2:92) wrote that while one may kasher natural rubber, one cannot kasher materials made from synthetic substances (See Igros Moshe EH 4:7 regarding Teflon). R’ Shimon Eider (Halachos of Pesach 13:n5; 10) writes that R’ Moshe told him that this only applied to kashering for Pesach. One may use hagalah to kasher during the year, however.
As to whether it 'swallows up taste' (bolea), is quite a discussion among the sources quoted above. In short, the reason that glass can't be kashered according to the Rema isn't because it is bolea but because it is made from sand like earthenware. (See beginning of article linked above). The general understanding is that plastic is similar to glass - it isn't bolea or maflit ('excrete it out like metal'), though follows the same argument as glass.
Rav Yosef Henkin has this to say in Kisvei HaGria Henkin part two, page 60 siman 46.
Concerning hagaala for rubber utensils and plastic utensils, it seems that it helps. Only with earthenware vessels where we see the abortion which never comes out completely, there hagaala does not work as per Pesachim 30. Also as far as tevila goes it seems they don't need it. According to Torah law only metal items need tevila, and it helps. And glass items also being that they are created with fire and reformed with fire, we tovel. And even though plastic is also formed and reformed with fire, it seems that the chidush of glass would not extend beyond itself (דאין לך אלא חדושו). He ends off with ובפרט שכנראה הוא מין הבא מן המים וכל הבא מן המים וכל שבים טהור וצ׳ע.
I assume his last point is because plastic is made from petroleum, which is liquid.