Head cut, known as “head fraction” Terpineol

Terpineol is a product obtained by the hydration of pinene or turpentine, appearing as a colourless to pale yellow, viscous and transparent liquid, exuding a pleasant lilac-like fragrance with hints of pine and citrus. Commercially available terpineol currently comes in specifications such as 85%, 96%, MU grade, electronic grade and perfumery grade. These products are largely from the heart cut of the distillation process, that is, the middle fraction of the distillate. This fraction has the lowest impurity content and the highest concentration of α-, β- and γ-terpineol isomers, as well as small amounts of 4-terpineol, linalool and other substances.

The head cut is the initial by-product fraction obtained during the distillation of terpineol, generally accounting for around 5%–10% of the total distillate output (varying according to feedstock and processing conditions). This fraction has a lower boiling range and is primarily composed of monoterpene hydrocarbons such as α-pinene, camphene, limonene and α-terpinene, along with small amounts of terpineol isomers.

At present, the head-cut fraction of terpineol is often used specifically as a bio-based solvent, with wide applications in printing ink thinners, UV-curable coating co-solvents, and non-aromatic electronic component cleaners. Meanwhile, the demand for bio-based propylene in the plastics market remains strong. Whether produced via the petrochemical cumene process or the terpene process, the outcome is equivalently the co-production of acetone. Against the backdrop of citrus greening disease (Huanglongbing) causing significant citrus yield losses in certain regions, the industry is also exploring the feasibility of using terpineol as a substitute for limonene.

Another promising area of research is the use of terpineol as a natural insecticidal or repellent adjuvant to enhance the penetration of formulations. Improving the penetration of crop protection products often requires adjusting the solubility and spreading properties of active ingredients. From a pharmacokinetic perspective, changes in a compound’s solubility and spreading behaviour on the target insect’s wax layer are key to its penetrative performance. The insect cuticle is made up of lipids and alkane polymers and is covered with a hydrophobic wax layer. When a mixture exhibits better spreading on the surface, surface tension decreases, contact angle is reduced, and the active compound can dissolve and penetrate the waxy barrier more effectively. For example, studies have shown that mixing camphor — the main component of rosemary oil — with 1,8-cineole can increase camphor’s penetration through the cuticle of cabbage looper larvae by as much as nineteen times.

The main components of head-cut terpineol — such as α-pinene, camphene, and limonene — are monoterpene hydrocarbons whose physicochemical characteristics can significantly alter solubility and spreading properties. Firstly, as pure hydrocarbon compounds, monoterpenes are highly hydrophobic or lipophilic. The waxy surface layer of an insect cuticle is likewise composed of hydrophobic lipids, acting as a barrier to external substances; the lipophilic nature of monoterpenes enables them to interact well with and dissolve into these wax components.

Secondly, monoterpenes typically have low surface tension, which helps them to spread more effectively over the insect surface. Research indicates that hydrocarbons often exhibit a pronounced “enhancement effect” when used in synergistic formulations. For example, in a study on the synergistic effects of thymol and p-cymene against larvae and adults of the housefly (Musca domestica), p-cymene significantly increased the penetration of thymol. Similarly, in research on the synergistic effects of natural pyrethrins in controlling Aedes aegypti mosquitoes, a variety of plant essential oils were found to markedly enhance the insecticidal activity of pyrethrins even at low dosages.

Why not simply purchase the pure monomers?

Many may wonder why α-pinene, camphene or other monomers are not procured directly for crop protection purposes. The reason is that the head cut is a direct by-product of terpineol distillation, and its recovery offers clear advantages in terms of cost. This is particularly attractive for crop protection companies that value formulation synergy while controlling expenses. Moreover, in many countries and regions, the regulatory registration requirements for mixtures and pure monomers differ; often, registration is only necessary when certain components exceed a specified concentration threshold. In the United States, the head cut can be registered under EPA List 4B inert ingredients, entering the North American supply chain as a natural inert adjuvant — a flexibility that holds strong appeal.

What about simply buying commercial pine oil?

Here, “pine oil” refers specifically to the by-product from producing terpineol from turpentine, which is also a mixture. Whether it can be used directly is the key question at the heart of promoting head-cut terpineol.

Commercial pine oil is generally classified by purity into grades of 50%, 65% and 85%, with an atmospheric boiling range usually between 168°C and 230°C. The terpenol content increases with purity: for example, 85% pine oil can contain over 85% terpenols. The high terpenol content is the primary source of its commercial value, with such alcohols including terpineol, guaiol, citronellol, menthol and others.

By contrast, the distilled head cut has a lower boiling point and a higher proportion of terpene hydrocarbons, being derived entirely as a direct distillation by-product — which gives it its commercial advantage. Its terpenic hydrocarbon content includes α-pinene, limonene, camphene, myrcene and others.

Representative target insects	Terpene Alcohols Family	Terpenes Family
Flea		Limonene
Tick		Limonene
Mosquito	α-Terpineol, Citronellol, Linalool	Limonene, α-pinene, Camphene
Aphid	Menthol, α-Terpineol, Linalool	Limonene
Japanese pine sawyer beetle		α-Pinene
Mite	α-Terpineol, Citronellol, Linalool, Menthol	α-Pinene, Camphene
Termite		Camphene
Fly	Citronellol	Limonene
Moth	Linalool
Fruit fly	Linalool
Whitefly	Menthol
Rice weevil	Menthol
Table 1. Representative target insects corresponding to terpenes

Both terpenols and terpene hydrocarbons belong to the terpene family and have significant effects in crop protection. Comparatively, hydrocarbon compounds have stronger wax-dissolving ability, serving as adjuvants to enhance the penetration of formulations into insect surfaces; terpenols, by contrast, contain a hydroxyl group (–OH), giving them superior amphiphilicity — enabling formulations to maintain wetting and spreading, and reducing loss. Furthermore, terpene hydrocarbons have higher volatility, making them suitable for rapid repellence and fumigation applications, while terpenols are better suited to longer-lasting repellents and insecticides.