How to Make a Rosin Ester

The glycerol ester of wood rosin is often used as a emulsifier in citrus sodas.

Rosin esters are a class of chemical compounds that have a number of practical applications. They are used as adhesives, thickeners and emulsifiers. They are derived from a class of plant products called rosins, which consist of eight similar three-ringed hydrocarbon molecules and are derived from sap. The formation of a rosin ester involves the reaction of the carboxcylic acid (COOH) moiety of the rosin molecule with an alcohol. The result is an esterified rosin molecule, which has unique chemical properties that are useful in a number of industries.

Things You'll Need

• Pine sap from the longleaf pine (Pinus palustris)
• Filter paper
• Vacuum funnel
• Vacuum nozzle
• Silicone tubing
• 500 ml Erlenmeyer flask with stopper and tube adapter
• 500 ml Erlenmeyer flask
• Protective gloves
• Hot plate with temperature dial
• Thermometer that goes up to 200 degrees Centigrade
• Glycerol/glycerine
• Acetone
• Nitrile gloves

Instructions

Obtain Purified Rosin

1. 

   Yellow-grade rosin is obtained by purifying sticky tree resin.

   Obtain 200 grams of tree resin from a rosin-producing pine tree. In the United States, this is often Pinus palustris, the longleaf pine. This resin contains various rosin molecules and turpentine oils. The presence of the oil impurities is what keeps the resin sticky and liquid. Put the resin in a 500 ml Erlenmeyer flask with a tube adapter.

2. Place the flask on a hot plate. Lightly stopper the flask such that air flow isn't completely blocked. Attach one end of the silicone tubing to the tube adapter and the other end to a vacuum nozzle. Turn the vacuum suction on.

3. Turn the hot plate on to 180 degrees Centigrade. Carefully observe the fluidity of the tree resin. Once the resin becomes liquid, measure the temperature with a thermometer every 5 to 10 minutes. If the temperature exceeds 150 degrees, turn the hot plate down to 150.

   Sticky resin may accumulate on the thermometer. You can dissolve this resin by wiping it down with a cloth or tissue moistened with acetone. Always wear protective gloves when handling acetone.

4. Heat the mixture for 90 minutes to ensure that all terpentine oils have evaporated. Stir the mixture periodically. If you have access to a stirring hot plate and magnetic stir bars, you can keep the solution under medium stirring once it has become heated and lost some of its viscosity.

5. Place an Erlenmeyer flask on a hot plate. Turn the heat up to 150 degrees. Attach the vacuum funnel to the top of the flask. Attach one end of the silicone tubing to the vacuum funnel and the other end to the vacuum suction. Place a piece of filter paper at the top of the funnel and turn the vacuum suction on.

   Carefully pour the heated liquid rosin into the funnel. Always wear protective gloves when handling heated glassware and liquids. If the funnel filtration slows down, pour 2 to 5 ml of acetone into the funnel to dissolve any solidified rosin. If you use acetone to clear the funnel, leave the rosin product on the hot plate for at least 30 minutes to ensure that the acetone evaporates.

6. Turn the vacuum filtration off and remove the purified rosin product from the hot plate. Rosin is stable, so you can complete the later steps of the reaction at a later time by storing the rosin in a safe location and heating it up to liquid form to resume the reaction. The rosin product should appear clear and will be brown to pale yellow in color, depending on how many impurities remain. The purification steps can be repeated to obtain slightly purer rosin.

   Rosin is a hard but brittle material that chips when strong blows are applied. However, when heated to its softening point (83 to 90 degrees Centigrade for glycerol rosin esters), the rosin becomes a thick, syrup-like substance that can be applied to surfaces or dissolved into solution.

Create a Rosin Ester

1. Heat the purified rosin to 150 degrees in a 500 ml Erlenmeyer flask under light vacuum suction. If the rosin has solidified, heat it on the hot plate with gentle agitation or stirring until it becomes a sticky liquid.

2. Add glycerol (glycerine) to the solution. The ratio of glycerol to rosin should be approximately 1:8. Thus, if you have 150 ml of rosin product from the first reaction, add 18.75 ml of glycerol. The glycerol will mix in and homogenize with the resin solution.

   Depending on the finished product that you wish to create, you can use alcohols other than glycerol. These will result in products with different physical properties and may require different amounts of time or degrees of heating to successfully complete the process.

3. Turn the heat plate up to 300 degrees. This heat will be sufficient to carry out the esterification process, in which the alcohol groups of the glycerol molecule bind to the carboxyl (COOH) groups of the rosin molecules. Gently stir the solution periodically while it is heating. If you have access to a stirring hot plate and a magnetic stir bar, you can continually stir the reacting solution at medium speed.

4. Heat the solution for 60 minutes. The heated reaction is near the boiling point of glycerol, so most of the glycerol will be gone by the end of the stirring process. After the reaction is complete, remove the vacuum apparatus and stopper from the flask and pour the contents into the container that you want to store the rosin ester in. Do not put the rosin ester in a plastic container if you plan on dissolving the solidified product in acetone.

5. 

   The final product is a clear yellow solid that can be used as a thickener or emulsifier.

   Store the finished product in a cool dry place. The rosin ester will be a clear yellowish or brownish solid at room temperature. It can be stored for up to 12 months without significant alteration or degradation.

Tips & Warnings

• Never work with rosins or their byproducts around an open flame. Rosins are highly flammable.

• Perform all chemical procedures under a chemical hood or in a well-ventilated area. Gaseous by-products of this reaction are irritating to the lungs and may cause asthma after prolonged exposure.

• Do not attempt to use any finished products of this reaction in food products. Without additional purification procedures, rosin esters are not fit for human consumption.

References

• Eastman: Rosin Esters
• FAO: Glycerol Ester of Gum Rosin
• Springer Protocols: Synthesis of a Light-Colored Rosin Ester