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.

Natural rubber (NR) latex was prevulcanized by ultraviolet (UV) irradiation in the presence of different diacrylate coagents. Prevulcanization was affected by the aliphatic hydrocarbon chain length of the coagents. The following diacrylate coagents with increasing aliphatic hydrophobic chain lengths were used: 1,4-butanediol diacrylate (BTDA), 1,6-hexanediol diacrylate (HDDA), and 1,9-nonanediol diacrylate (NDDA). Oxygen inhibition of the freeradical reactions of prevulcanization and the effects of the inert CaCO 3 filler (concentration range = 0−40 parts per hundred rubber) on properties of the prevulcanized NR films were investigated. The hydrocarbon chain length of the coagent influenced its hydrophobicity and therefore its affinity toward NR, and this affected the prevulcanization outcomes. HDDA showed the greatest affinity for NR and thus was the most suitable coagent for prevulcanization of NR. Prevulcanization via UV irradiation was insensitive to oxygen and the filler tested. The CaCO 3 filler proved to be satisfactory and could be added either before or after UV irradiation.

Piezoresistive sensors for monitoring human motions are essential for the prevention and treatment of injury. Natural rubber is a material of renewable origin that can be used for the development of soft wearable sensors. In this study, natural rubber was combined with acetylene black to develop a soft piezoresistive sensing composite for monitoring the motion of human joints. An additive manufacturing technique based on stereolithography was used, and it was seen that the sensors produced with the method could detect even small strains (<10%) successfully. With the same sensor composite fabricated by mold casting, it was not possible to detect low strains reliably. TEM microscopy revealed that the distribution of the filler was not homogeneous for the cast samples, suggesting a directionality of the conductive filler network. For the sensors fabricated through the stereolithography-based method, a homogeneous distribution could be achieved. Based on mechano-electrical characterization, it was seen that the samples produced with AM combined the ability to endure large elongations with a monotonic sensor response. Under dynamic conditions, the sensor response of the samples produced by 3D printing showed lower drift and lower signal relaxation. The piezoresistive sensors were examined for monitoring the motion of the human finger joints. By increasing the bending angle of the sensor, it was possible to increase the sensitivity of the response. With the renewable origin of natural rubber and manufacturing method, the featured sensors can expand the applicability of soft flexible electronics in biomedical applications and devices.

Natural rubber was prevulcanized using ultraviolet (UV) light and the resulting prevulcanized natural rubber (PVNR) was composited with multiwalled carbon nanotubes (MWCNT) as a conductive filler, to make films suitable for antistatic gloves.These films were characterized in terms of mechanical properties and electrical resistivity. Effects of type of UV radiation and other specifics of the production process, on mechanical properties of neat PVNR were first investigated. This was followed by an investigation of the effects of surfactant type on the dispersion of MWCNT in water and the PVNR matrix. Four surfactants with different headgroups were used in preparing MWCNT dispersions that were then mixed with the PVNR latex to form the different films. As a consequence of its milk-like turbidity, the natural rubber latex was found to require deeply penetrating UVA light for satisfactory prevulcanization.Sodium dodecyl sulfate (SDS) was the best surfactant for dispersing MWCNT both in water and in PVNR matrix. SDS resulted in MWCNT-PVNR composite films with the least electrical resistivity and good mechanical properties for making antistatic gloves.SDS did not interfere with stress transfer at NR-MWCNT interface. SDS was superior to the other surfactants because its head group had a charge opposite to that on the NR surface and lacked a benzene ring that would have interacted strongly with the hydrophobic surface of MWCNT.

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