Gutta percha, natural rubber and balata-chemical characterization of polyisoprenes in the context of cultural heritage

Abstract

Gutta percha, natural rubber, and balata are latexes produced by tropical trees that since the mid-nineteenth century have been extensively used commercially and in the creation of artworks and daily use objects. All three latexes are stereoisomers of polyisoprene that differ in their molecular structure. In gutta percha and balata, the molecular functional group is in the trans- position; in natural rubber, it is in the cis- position. This difference in their molecular structure affects the physical properties of these materials and their aging, turning natural rubber tacky and gutta percha brittle. Commercially focused analysis of natural polyisoprenes has typically used chemically modified materials. The present study characterizes natural, chemically unadulterated latex harvested from trees, aiming to differentiate the cis- and trans-forms. The analysed materials comprised fresh latexes harvested in Malaysia and Indonesia, naturally aged botanical samples, some over 100 years old, and minimally processed 10-year-old commercial stock; a total of 50 samples were examined, out of which 23 are discussed in detail. The harvesting practices of latexes were recorded during fieldwork in Southeast Asia in 2015–2019, in Cipetir (Tjipetir), on Lingga Island in Indonesia, and in Malay rubber plantations. The harvesting methods are the first indicators as to the type of latex harvested based on the pattern of incisions and time of coagulation upon exposure to air. The interviews of tappers, especially of gutta percha and balata, serve as a record of a disappearing craft. Therefore, this fieldwork report contributing to a general knowledge of working methods with natural latexes is of particular value considering the scarcity of published records. The laboratory analytical study of trans- and cis- forms relied on nuclear magnetic resonance (NMR) spectroscopy, which is the most accurate method to unambiguously determine the stereo chemistry of each polyisoprene. The characteristic spectroscopic features of cis- and trans-positions were clearly shown in all samples, in fresh latex, in over 100-year-old material, and in early twentieth century artifacts. This result indicated that the aging of the polyisoprenes does not prevent the identification of their stereochemistry. This analytical work is the first thorough examination of gutta percha, natural rubber and balata in the context of cultural heritage to guide preservation strategies for objects made of these materials.