Evaluation of Heartwood Extracts Combined with Linseed Oil as Wood Preservatives in Field Tests in Southern Mississippi, USA

SIMPLE SUMMARY: Wood is a sustainable building material with a wide range of applications. Being of biological origin, wood is subject to degradation by several organisms, including termites and decay fungi. Chemical preservatives are often used to protect the wood from biodegradation but concerns regarding the overall safety and availability of chemical preservatives has stimulated research into alternative treatment methods. One approach is transferring heartwood extracts of durable wood species to non-durable wood due to their intrinsic antimicrobial properties, renewability, and perceived lower impact on the environment. Although they are not free of limitations, their efficacy can potentially be improved with methods such as co-impregnating wood with other plant-based hydrophobic chemicals. In the current study, we used heartwood extracts of four wood species and linseed oil to impregnate two non-durable wood species. Results show that co-impregnation with individual heartwood species extract, and oil increased the resistance of non-durable wood against termites and decay fungi. However, their effectiveness to protect was less than chemical preservatives. Further research is needed to examine single-extract component isolates combined with oils as well as possible synergists or co-biocides to enhance overall performance. ABSTRACT: Heartwood extracts of naturally durable wood species are often evaluated as alternatives to chemical wood preservatives, but field data from long-term performance testing are lacking. The current study evaluated the long-term (five-year) performance of two non-durable wood species treated with heartwood extracts of either Tectona grandis, Dalbergia sissoo, Cedrus deodara, or Pinus roxburghii alone or combined with linseed oil. Stakes (45.7 × 1.9 × 1.9 cm) and blocks (12.5 × 3.75 × 2.5 cm) cut from the sapwood of cottonwood and southern pine were vacuum-pressure impregnated with the individual heartwood species extract, linseed oil, or a mixture of each individual wood extract and linseed oil. For comparison, solid heartwood stakes and blocks of the wood species used to obtain extracts were also included in the tests. All samples were exposed for five years to decay and termites at a test site in southern Mississippi using ground contact (AWPA E7) and ground proximity (AWPA E26) tests. Results showed that extract-oil mixtures imparted higher termite and decay resistance in cottonwood and southern pine than linseed oil only or the individual heartwood species extract in both tests. However, these treatments were as not effective as to commercially used wood preservatives, copper naphthenate (CuN) or disodium octaborate tetrahydrate (DOT) in either test. Moreover, solid heartwood P. roxburghii stakes were completely decayed and attacked by termites after five years in the ground contact test. In contrast, C. deodara stakes were slightly attacked by termites and moderately attacked by decay fungi. However, T. grandis and D. sissoo stakes showed slight to superficial attack by termites and decay fungi in ground contact test. In contrast, T. grandis and D. sissoo blocks showed slight decay fungi attack in above-ground tests. However, termites did not attack T. grandis, D. sissoo, and C. deodara blocks. However, decay fungi moderately attacked C. deodara blocks, and P. roxburghii blocks were severely attacked by decay fungi and termites in the above-ground test.