The Preparation and Properties of Thermo-reversibly Cross-linked Rubber Via Diels-Alder Chemistry

The overall goal of this procedure is to prepare recyclable rubbers via Diels-Alder chemistry. This method can help answer key questions in the field of rubber recycling, such as how disposals can be done in a creative fashion. The main advantage of this technique is that the methodology is universal, and can be applied in a number of polymers and rubbers.

Prepare a 10 weight percent rubber solution by dissolving 45 grams of maleated ethylene-propylyne rubber, or EPM, in 500 milliliters of tetrahydrofuran at 23 degrees Celsius in a closed beaker under vigorous stirring. Add 2.8 grams of freshly distilled furfurylamine to the 10 percent rubber solution. Stir the reaction mixture in a closed system at 23 degrees Celsius for at least one hour.

Precipitate the reaction mixture by pouring it slowly into five liters of acetone under mechanical stirring. This yields the polymer product as white threads that are easily fished out of the beaker using large tweezers. Dry the collected furan-modified product to constant weight in a vacuum oven at 35 degrees Celsius.

Cut the resulting plaque of rubber into small pieces with scissors, and wash them thoroughly by immersing them in acetone to remove any unreacted furfurylamine. Compression mold the resulting slightly-yellowish product for 10 minutes at 175 degrees Celsius and 100 bar to convert the intermediate amyte acid product into the amide product. Measure the Shore A Hardness of the compression molded samples at least 10 times by pressing a durometer onto a rubber sample, covering the entire cylindrical surface of the durometer.

Measure the Young's Modulus, ultimate tensile strength, and elongation at break by performing tensile tests on samples of approximately one millimeter thick and five millimeters wide. Use a clamp length of 15 millimeters and a strain rate of 500 plus or minus 50 millimeters per minute. Determine the compression set at 23 degrees Celsius by compressing cylindrical samples between two metal plates to 3/4 of their initial thickness for 70 hours.

Let them relax at 50 degrees Celsius for one half hour and measure the thickness. Weigh 40 grams of furan-functionalized EPM and 04 grams of phenolic antioxidant, and dissolve them in a large closed beaker with 500 milliliters of THF at 23 degrees Celsius. Add 1.48 grams of the aliphatic bismaleimide to the beaker with the 10%rubber solution.

Stir the reaction mixture for at least one hour at 50 degrees Celsius in the closed beaker. Then, remove the cap to open the system to evaporate the solvent. Dry the collected product to a constant weight in a vacuum oven at 35 degrees Celsius.

Then compression mold the product for 30 minutes at 175 degrees Celsius and 100 bar. Anneal the resulting product by storing it in an oven at 50 degrees Celsius for at least three days. Determine the transmission infrared spectrum, Shore A hardness, Young's modulus, elongation at break, ultimate tensile strength, and compression set at 23 degrees Celsius, as described in the text protocol.

It is important to heat the material to above the Diels-Alder decrossing temperature for five minutes prior to compression molding the cross-link rubber. Reprocess the samples after testing by cutting them into small pieces using scissors and compression molding these under the same conditions into new homogeneous samples with the same dimensions. Cut a piece of compression-molded cross-linked rubber of approximately 50 milligrams.

Determine the initial weight of the rubber sample precisely by weighing it in a 20 milliliter glass vial. Immerse the weighed rubber in 15 milliliters of Declan. Let the rubber swell in the Declan for approximately three days, until its weight does not increase any more, indicating the equilibrium swelling is reached before cautiously taking the swollen sample out of the vial.

Carefully dab the surface of the swollen sample with a paper tissue to remove any solvent from the surface without squeezing it. Weigh the swollen rubber sample in a new sample vial. Dry the swollen sample in a vacuum oven at 80 degrees Celsius until a constant weight is reached and determine the sample's dry weight.

Characteristic furan peaks can be seen to disappear upon cross-linking, and reappear upon de-cross-linking of the furan-modified rubber. Reprocessing of the thermal reversibly cross-linked rubber by compression molding yields homogenous sample spars contrary to conventionally cross-linked rubbers. Upon Diels-Alder cross-linking, and reworking these rubbers, the hardness increases and the compression set decreases to approach values obtained for commercially cross-linked reference samples.

Similarly, upon Diels-Alder cross-linking and reworking these rubbers, the Young's modulus increases, the tensile strength increases, and the elongation decreases, to values obtained for commercially cross-linked reference samples. Following this procedure for the modification of rubbers, other methods, like the recycling of rubbers can be performed in a much more efficient way in devulcanizing conventionally cross-linked rubbers.