Prevention of preformed polymer is the greatest concern when storing Deltech methylstyrenes (para Methylstyrene/Vinyl Toluene) in bulk quantities. In addition, oxidation can cause degradation of product quality by color formation or free particle formation during long-term storage.
The formation of polymer in stored methylstyrene (MS) will generally degrade the product quality and prevent use in most applications. Practice of proper inventory management is required to insure the product quality is maintained until product is used. Deltech MS can be safely stored for extended periods if the proper conditions are maintained. Understanding the polymerization characteristics, maintenance of storage equipment and proper polymerization inhibitor use for MS combine to support good inventory management.
Heat initiated free radical polymerization can occur in MS and other reactive monomers such as styrene monomer. When heated, the monomer molecule can become unstable and form a free radical which is a reactive species. The free radical reacts with another MS molecule and starts a polymer chain. The chain grows in length until some type of chain terminating event occurs or until all molecules are reacted. The polymerization reaction is exothermic (energy is produced by the reaction) which causes the temperature of the bulk liquid to increase and accelerate the formation of more free radicals and more polymerization. MS polymerization is self promoting and autoaccelerating. Once started the energy generated by the reaction can rapidly increase the polymerization rate and amount of polymer formation. In a bulk storage, the polymerization can proceed at a rate that causes the temperature of the bulk liquid to increase to a level where vigorous vapor venting can occur which creates a dangerous fire hazard.
Polymerization inhibitors are added to reactive monomers to prevent undesired polymerization during transport or storage. An inhibitor reacts with the free radical to prevent reaction with MS molecules and polymer formation. Use of inhibitors to control polymer formation in bulk liquid is discussed below.
Storage of reactive monomers in bulk tanks can also result in another type of polymer contamination due to polymerization of uninhibited monomer on tank surfaces. During storage of bulk liquid MS, some of the MS is vaporized and the vapors condense into droplets on the roof and sides of the storage tank. These monomer droplets do not contain polymerization inhibitor and may begin to polymerize if allowed to adhere to the roof or sides of the tank. Once polymer begins to form, additional droplets are trapped and the polymerization can increase to form sheets of polymer on tank sides or large accumulations from the roof (stalagtites). If the polymer accumulation becomes large enough, the weight causes detachment from the tank sides or roof into the bulk liquid. The polymer is soluble in MS monomer and dissolves to create preformed polymer in the product which degrades product quality and prevents use in many applications. Use of a properly designed storage tank is required to reduce the potential for polymer accumulation on tank surfaces.
Methylstyrenes are combustible liquids and the accepted equipment design and material handling techniques for this hazard class must be practiced to prevent conditions that can cause fire or explosion. The storage tank used should meet all applicable code requirements including adequate grounding connections on tank and pump. MS storage tanks should be installed within a suitable bermed enclosure to contain any unintended leaks or spills. Consult the Deltech MSDS for recommendations for recycle or disposal of off-specification MS.
Additional storage tank design requirements not related to safe handling practices are also necessary to allow MS storage without product degradation. A vertical storage tank with self-supporting dome roof is recommended for MS storage. The tank should be designed to meet applicable code standards for atmospheric tanks.
TANK ARRANGEMENT
A simplified storage tank arrangement is shown:
The tank should be equipped with:
| Roof mounted vacuum-pressure relief device (Protectoseal No. 830) |
| Roof mounted manhole/emergency relief device (Protectoseal No. 51300) |
| Roof mounted gauging/sampling hatch (Protectoseal No. 4980) |
| Side mounted 20" manhole for inspection/cleaning entry |
| Internal sump with drain valve |
| Temperature indicator |
TANK LINING
Tank roof and sides must be smooth to prevent polymer accumulation. Carbon steel tank construction with the proper tank lining material will provide the desired smooth surface. Use of the dome roof without internal supports permits simple lining installation and allows monomer vapors that condense on the roof surface to quickly drain back to the liquid. Internal weld connections and weld joints should be ground smooth before lining application.
Condensed monomer vapors do not contain polymerization inhibitor. Uninhibited monomer can form polymer deposits if allowed to collect on tank surfaces. Non-pourous, non-wettable smooth tank linings prevent retention and attachment of uninhibited monomer droplets. As a result monomer drains quickly back into the bulk of the inhibited liquid monomer before polymerization can occur. Baked phenolic, modified epoxy and catalyzed epoxy linings all perform well to prevent entrainment and polymer accumulation above the liquid level. Tank linings must be applied according to manufacturer's instructions.
| Recommended tank linings: | Interzinc 22 or Interzinc 91 | |
| Matcoat Protective Coatings | ||
| PO Box 10762 | ||
| Houston, TX 77292 | ||
| 713 682-1711 |
CAUTION: Inorganic zinc coatings are not electrically conductive. The bottom 30 centimeters of the tank should be coated with an inorganic silicate lining to provide adequate electrical grounding.
TANK INSULATION
Maintaining MS product storage temperature below 85 oF is important to maximize product quality for extended storage periods. If atmospheric conditions require, the tank should be insulated with suitable insulation to prevent direct sunlight impingement on the tank. For storage in very warm climates a package refrigeration system should be also be installed to allow controlling storage temperature within the desired range.
PUMP REQUIREMENTS
Self-priming centrifugal pumps with a suitable mechanical seal should be used for MS service. Continual flow through the pump must be verified when the pump operates to prevent deadheading the pump which can cause heat generation within the pump. Heating the MS can cause thermally induced free radical polymerization as described above.
Methylstyrenes are treated with tertiary Butyl Catechol (TBC) as an inhibitor to prevent premature free radical polymerization. Products are shipped with a minimum of 12 parts per million (ppm) of TBC and may contain up to 50 ppm TBC upon customer request. TBC acts a proton donor to stabilize a free radical and prevent reaction of the free radical with a monomer molecule to start a polymer chain. The chemistry of free radical inhibition can be found below.
An inhibitor level of 10 ppm is normally effective in preventing polymerization for 90 days at maximum temperaures of 85 oF. An inhibitor level of 40 ppm is normally effective in preventing polymerization for 180 days at maximum temperaures of 60 oF. TBC is an effective inhibitor only in the presence of oxygen. MS products shipped by Deltech are saturated with oxygen allowing full utilization of the TBC inhibitor. Use of nitrogen or inert gas blanketing on MS storage tanks is not recommended.
INHIBITOR DEPLETION
The most significant cause of TBC depletion is related to monomer storage temperature. Temperature of stored monomer should be maintained at less than 85 oF to avoid excessive depletion of TBC. As shown below the TBC concentration will deplete over time under ideal storage conditions which will leave the monomer unprotected against free radical polymerization.
Due to the solubility of TBC in water, the presence of free water in the MS storage can cause the TBC content of the monomer to be rapidly depleted. MS storage tanks should be routinely checked to insure that water is not present.
INHIBITOR MAINTENANCE
The TBC concentration in MS monomer is determined using ASTM Test Method D4590. Deltech will provide assistance in specifying the equipment required for TBC analysis at the storage location. Deltech will also perform this analysis as a service for MS customers. Contact Deltech for sample shipping instructions and the laboratory contact for sample receipt.
If TBC concentration in MS monomer is depleted below 10 ppm, additional TBC should be added to the storage to increase the concentration to the desired range. Contact Deltech for assistance in obtaining the TBC solution suitable for addition to MS monomer.
MAINTAINING OXYGEN CONTENT
A minimum oxygen concentration of 15 ppm in MS monomer is required to ensure the TBC inhibiting capablity is effective. Deltech ships MS monomer saturated with oxygen (30-40 ppm). Saturation ensures that minimum oxygen concentration is maintained. Storage conditions and TBC depletion in storage can cause oxygen concentration to decrease over time. In a bulk storage which is vented to atmosphere, intermittent or continuous agitation of the monomer usually assures that minimum oxygen concentration will be maintained. Circulating the tank using the transfer pump is a convenient method of tank agitation.
In bulk storage, use of nitrogen or inert gas padding or blanketing is not recommended. If inert gas blanketing is necessary due to regulatory requirements, a rigorous program of oxygen addition must be enforced to maintain the minimum oxygen concentraion in MS monomer. The monomer should be sparged with clean dry air on a scheduled basis that will add the required oxygen content to the monomer. An oxygen monitor should be used to test the monomer to verify oxygen content is adequate and determine the proper air sparging schedule.
POLYMER TESTING
The presence of preformed polymer in MS monomer can be determined using the simple test method ASTM D2121. Performing the test is required because MS polymer is soluble in the monomer and can be present in quantities that degrade product quality although not visually apparent.
1. Obtain a representative sample of MS monomer from storage.
2. Add 5 milliliters (ml) of sample to a clean, clear test tube.
3. Add 25 ml of dry reagent grade methanol to the test tube.
4. Mix thoroughly.
Solution will remain clear if no polymer is present.
Solution will be hazy at low (less than 20 ppm) polymer concentration.
Solution will be cloudy/milky at higher (greater than 20 ppm) polymer concentration.
If quality of MS monomer is not known, product should be tested for polymer concentration before use.
INVENTORY MANAGEMENT AND TESTING
Deltech recommends that MS stored in bulk for periods exceeding 90 days be routinely monitored to ensure product quality is maintained. Storage temperature should be checked daily and product analysis performed as shown below:
| Test | Frequency | Test Method | ||
| TBC | Once per week | ASTM D4590 | ||
| Polymer | Once per week | ASTM D2121 | ||
| Color | Once per week | ASTM D1209 |
Maximum storage life for MS products (no observable degradation of quality due to polymerization or oxidation products) in bulk storage is about 90 days if recommended storage conditions are not followed. Storage life will be reduced at temperatures above 85 oF are sustained. Storage life can be extended indefinitely if the tank configuration and lining meet the recommended requirements, inhibitor concentration is maintained, monomer is saturated with oxygen and bulk liquid storage temperature is maintained at 60 oF. Proper precautions must also be observed to assure that localized heat sources do not increase the surface temperature of the tank or associated piping.
Storage temperature is the single most critical factor in the storage life of MS products. Use of an insulated tank and refrigeration through a close loop circulation system are recommended when the temperature of the stored product may exceed 85 oF without refrigeration. For longest term MS storage, a temperature of 60 oF is recommended.
Deltech technical product representatives are available to answer questions concerning MS storage and handling. Recommendations may be case specific depending upon use and storage conditions. Deltech also offers assistance in analysis for TBC and polymer content, setting up customer testing capability and recovering downgraded product (depending upon MS condition).
For assistance contact:
| Gene Fluharty | Telephone: | 225 358-3170 | |
| Deltech Corporation | Email: | efluharty@deltechcorp.com | |
| 11911 Scenic Highway | |||
| Baton Rouge, LA 70807 | |||
This information is provided as a service to Deltech customers. No warranty, express or implied, is given nor is freedom from any patent to be inferred. Inasmuch as any assistance furnished by Deltech with reference to the proper use and disposal of Deltech products is provided without charge, Deltech Corporation assumes no obligation or liability thereof.