AIP Conference Proceedings [Author(s) DISRUPTIVE INNOVATION IN MECHANICAL ENGINEERING FOR INDUSTRY COMPETITIVENESS: Proceedings of the 3rd International Conference on Mechanical Engineering (ICOME 2017) - Surabaya, Indonesia (5–6 October 2017)] - Purified natural rubber latex: Investigating effect of single anionic surfactant and centrifugation process to latex properties and addition of bio-filler towards latex film performance