Sulfur-Free Prevulcanization of Natural Rubber Latex by Ultraviolet Irradiation

Abstract

The possibility of prevulcanizing natural rubber latex via ultraviolet (UV) irradiation was investigated using 2-hydroxy-2-methyl-1-phenylpropanone as the photoinitiator and 1,9-bis­(acryloyloxy)­nonane as the coagent. Effects of the following process variables were assessed on the tensile strength and the cross-link density of thin films prepared from the prevulcanized natural rubber (PVNR) latex: the duration of UV irradiation, the distance between the light source and the latex being irradiated, the mixing time of the latex with the coagent and the phtoinitiator, the depth of the latex pool, and the mass ratio of the photoinitiator to the coagent used. The PVNR films produced under optimal conditions had nearly 4-fold the tensile strength of the film produced without UV irradiation. Fourier transform infrared spectroscopy and 13C nuclear magnetic resonance spectroscopy were used to assess the UV reactions, and a mechanism was proposed for UV-induced prevulcanization with the above identified photoinitiator and coagent.

Compositing prevulcanized natural rubber with multiwalled carbon nanotubes to make antistatic films

“…The mass ratio of the coagent and photoinitiator was always 1:1 and 4 g of this mixture was used per 100 g rubber. The mixture was prevulcanized by UV irradiation in petri dishes (diameter = 100 mm; depth = 15 mm) in a dark room at 28°C, under previously identified optimal conditions: an irradiation time of 1 h; an irradiation distance (the distance between the UV lamps and the top surface of the latex pool) of 45 mm; and a latex pool depth of 10 mm 12 . Ultraviolet A (UVA; 315–400 nm; Actinic BL TL 8 W/10 1FM/10X25CC lamp; Philips Lighting) and ultraviolet B (UVB; 280–315 nm; 8 W G5 lamp; Lampton Lighting, Thailand) lamps were used for irradiation in different experiments.…”

“…Prevulcanization by UV light is based on generation of free radicals that initiate the formation of crosslinks between the coagent (diacrylate in this case) molecules and the NR chains. 12 UV light absorbed by a photoinitiator results in its homolytic cleavage to generate two radicals. These radicals begin the prevulcanization process by attacking the C═C bonds in polyisoprene chains of NR and in the coagent molecules.…”

“…Although thiol coagents are effective for prevulcanizing NR latex via UV irradiation, they are difficult to use because of their unpleasant odor and poor storage stability 11 . The thiols‐associated problems can be avoided by using a diacrylate coagent, although the resulting films have a low tensile strength (~12 MPa) 4,12 . Therefore, UV‐based prevulcanization needs to be optimized using a diacrylate coagent to establish conditions for forming films with the necessary strength.…”

“…These radicals attack the NR chains to produce a network of NR chains crosslinked by the coagent. Based on this mechanism, 12 crosslinking for improving the mechanical properties of NR films can be manipulated by the amounts and types of photoinitiators and coagents used, the irradiation time, and the wavelength of UV light. Effects of these factors, except the wavelength of UV light, were previously reported by us in the context of prevulcanization of NR 4,12 .…”

Design of a Prototype Photoreactor UV-LEDs for Radiation Vulcanization of Natural Rubber Latex

“…Design of a Prototype Photoreactor UV-LEDs for Radiation Vulcanization of Natural Rubber Latex (Hansupalak et al, 2016).…”