Simulasi pengaruh penambahan serat ijuk dan kenaf terhadap indeks avrami, kinetika kristalisasi nakamura, dan waktu paruh kristalisasi polipropilena kopolimer impak = Simulation of effect on the addition of 'ijuk' and 'kenaf' fiber to avrami index, nakamura crystallization kinetics, and half-time of impact copolymer polypropylene