I’m going on a trip and I don’t have a proper netilas yadayim cup. Can I use a disposable cup?(
The halacha requires that one use a keli (vessel) for netilas yadayim. Poskim differ as to whether a disposable cup (such as a paper cup, or plastic bottle) is acceptable. Igros Moshe (OC 4:39) ruled that one should not use a disposable cup for kiddush because kiddush requires that the cup have chashivus, or stature. Some draw the conclusion from this teshuva that a disposable cup is called akeli; although it is not acceptable for kiddush it would be acceptable for netilas yadayim, which does not require chashivus. Minchas Yitzchak (10:23) rules that one should avoid using a disposable cup.
Rav Schachter, shlit”a, ruled that since even a disposable cup is often used multiple times in the course of a meal, it clearly qualifies as a keli. Many of the lenient poskim, however, agree that if one can avoid the question by thinking ahead and packing a proper (that is, durable) cup, one should do so. (see Piskei Teshuvos 159:7).