Dibutyltin Mono(2-ethylhexyl) Maleate compatibility with PVC resin and plasticizers

Dibutyltin Mono(2-ethylhexyl) Maleate: A Comprehensive Overview of its Compatibility with PVC Resin and Plasticizers

Introduction

Dibutyltin mono(2-ethylhexyl) maleate (DBM) is an organotin compound primarily utilized as a heat stabilizer in polyvinyl chloride (PVC) processing. PVC, a versatile and widely used thermoplastic polymer, requires heat stabilizers to prevent degradation during processing due to its inherent sensitivity to heat. DBM’s effectiveness stems from its ability to react with hydrogen chloride (HCl) released during PVC degradation, thus inhibiting the autocatalytic degradation process. Beyond its heat stabilization properties, DBM also influences the transparency, color retention, and mechanical performance of PVC products. This article provides a comprehensive overview of DBM, focusing on its compatibility with PVC resin and various plasticizers, crucial factors determining the final properties and applications of PVC compounds.

1. Chemical Properties and Production

  • Chemical Name: Dibutyltin mono(2-ethylhexyl) maleate

  • CAS Registry Number: 15535-69-0

  • Chemical Formula: C24H44O4Sn

  • Molecular Weight: 511.32 g/mol

  • Structure:

         O
     ||
(C4H9)2Sn-O-C-CH=CH-C-O-CH2-CH(C2H5)-C4H9
                  ||
                  O

  • Physical State: Clear, pale yellow liquid

  • Boiling Point: >200 °C (decomposes)

  • Density: 1.05-1.08 g/cm³ at 20 °C

  • Refractive Index: 1.478-1.482 at 20 °C

  • Solubility: Soluble in common organic solvents (e.g., toluene, xylene, esters, ketones). Insoluble in water.

  • Stability: Stable under normal storage conditions. Sensitive to moisture and strong oxidizing agents.

1.1. Synthesis

DBM is typically synthesized through a two-step process:

  1. Reaction of Dibutyltin Oxide (DBTO) with Maleic Anhydride:
    DBTO reacts with maleic anhydride to form dibutyltin maleate. This reaction is usually carried out in an organic solvent at elevated temperatures.

    (C4H9)2SnO + C4H2O3  --> (C4H9)2Sn(OOC-CH=CH-COOH)

  2. Esterification with 2-Ethylhexanol:
    The resulting dibutyltin maleate is then esterified with 2-ethylhexanol to yield DBM. This reaction is typically catalyzed by an acid catalyst.

    (C4H9)2Sn(OOC-CH=CH-COOH) + HOCH2-CH(C2H5)-C4H9 --> (C4H9)2Sn(OOC-CH=CH-COO-CH2-CH(C2H5)-C4H9) + H2O

2. Function as a Heat Stabilizer in PVC

PVC’s thermal instability arises from the ease with which it undergoes dehydrochlorination at elevated temperatures. This process involves the elimination of hydrogen chloride (HCl) from the PVC chain, leading to the formation of conjugated polyene sequences. These polyenes are responsible for the discoloration and eventual degradation of the polymer. The released HCl further catalyzes the dehydrochlorination process, creating a self-accelerating degradation cycle.

DBM functions as a heat stabilizer by:

  • HCl Scavenging: DBM reacts with the liberated HCl, preventing it from catalyzing further degradation. This reaction disrupts the autocatalytic cycle.
  • Allylic Chloride Replacement: DBM can replace labile allylic chlorine atoms in the PVC chain with more stable groups, preventing the initiation of dehydrochlorination at these susceptible sites.
  • Absorption of UV radiation: DBM is a UV absorber and can block the UV radiation.

3. Compatibility with PVC Resin

The compatibility of DBM with PVC resin is a critical factor influencing its effectiveness as a heat stabilizer and the overall properties of the final PVC compound. Good compatibility ensures uniform dispersion of the stabilizer within the PVC matrix, maximizing its protective effect. Incompatibility can lead to phase separation, blooming (migration of the stabilizer to the surface), and reduced mechanical properties.

Factors influencing DBM’s compatibility with PVC include:

  • Polarity: PVC is a relatively polar polymer due to the presence of chlorine atoms. DBM, with its ester and tin-containing groups, exhibits a moderate degree of polarity, contributing to its compatibility with PVC.
  • Molecular Weight: DBM has a moderate molecular weight, which facilitates its miscibility with PVC chains.
  • Chemical Structure: The presence of both butyl and 2-ethylhexyl groups provides a balance between polar and non-polar characteristics, enhancing its compatibility with a range of PVC resins.

4. Influence of DBM on PVC Properties

The incorporation of DBM into PVC formulations affects various properties, including:

  • Heat Stability: DBM significantly enhances the heat stability of PVC, delaying the onset of thermal degradation and preventing discoloration at processing temperatures.
  • Transparency: DBM typically imparts excellent transparency to PVC compounds, particularly when used in conjunction with appropriate plasticizers.
  • Color Retention: By inhibiting the formation of conjugated polyenes, DBM improves the color retention of PVC products, preventing yellowing or darkening during processing and service life.
  • Mechanical Properties: The effect of DBM on mechanical properties depends on the concentration and the specific PVC formulation. Generally, DBM can improve the tensile strength and elongation at break of PVC compounds.

5. Compatibility with Plasticizers

Plasticizers are essential additives in PVC formulations, imparting flexibility and processability to the rigid polymer. The compatibility between DBM and plasticizers is crucial for maintaining the long-term stability and performance of flexible PVC products. Incompatible combinations can result in plasticizer exudation, reduced mechanical properties, and premature failure.

Several factors govern the compatibility between DBM and plasticizers:

  • Polarity: Plasticizers with similar polarity to DBM tend to exhibit good compatibility. Polar plasticizers like phthalates (e.g., DOP, DINP) and trimellitates generally show good compatibility with DBM. Aliphatic plasticizers with low polarity may exhibit limited compatibility.
  • Molecular Weight: Plasticizers with moderate molecular weights tend to be more compatible with DBM than those with very low or very high molecular weights.
  • Chemical Structure: The presence of ester groups in both DBM and many common plasticizers promotes compatibility through intermolecular interactions.

The following table summarizes the compatibility of DBM with various common plasticizers:

Plasticizer Chemical Class Polarity Compatibility with DBM Notes
Di(2-ethylhexyl) phthalate (DOP) Phthalate Moderate Excellent Widely used, good overall performance. May be subject to regulatory restrictions in some regions.
Diisononyl phthalate (DINP) Phthalate Moderate Excellent Lower volatility than DOP. Widely used, generally considered safer than DOP.
Diisodecyl phthalate (DIDP) Phthalate Moderate Good Lower volatility than DOP and DINP. Provides good low-temperature flexibility.
Bis(2-ethylhexyl) adipate (DOA) Adipate Low Fair to Good Good low-temperature flexibility. May not be suitable for high-temperature applications due to volatility.
Trioctyl trimellitate (TOTM) Trimellitate High Excellent Excellent high-temperature performance, low volatility, and good resistance to migration.
Epoxidized soybean oil (ESBO) Epoxidized Vegetable Oil Moderate Good Bio-based plasticizer, provides both plasticizing and stabilizing effects.
Di-n-butyl phthalate (DBP) Phthalate Moderate Good Good plasticizing efficiency but higher volatility compared to DOP. Subject to regulatory restrictions.
Acetyl tributyl citrate (ATBC) Citrate High Excellent Non-phthalate plasticizer, excellent compatibility with PVC and good low-temperature flexibility.
Triethyl citrate (TEC) Citrate High Good Non-phthalate plasticizer, used in applications requiring high purity and low toxicity.
Polymeric plasticizers Polyester/Polyether Variable Variable Compatibility depends on the specific polymer structure and molecular weight. Generally good for long-term durability but can be expensive.
Dioctyl terephthalate (DOTP) Terephthalate Moderate Excellent A non-phthalate plasticizer with good performance properties, similar to DOP.

6. Applications of DBM in PVC Formulations

DBM is widely used as a heat stabilizer in a variety of PVC applications, including:

  • Flexible PVC Products: DBM is commonly used in flexible PVC products such as cables, flooring, films, and synthetic leather, where it provides both heat stability and compatibility with plasticizers.
  • Rigid PVC Products: DBM can also be used in rigid PVC applications such as pipes, profiles, and window frames, where it contributes to improved processing and long-term durability.
  • Calendered Films: DBM is particularly effective in calendered PVC films, where it provides excellent transparency and color retention.
  • Injection Molded Products: DBM is suitable for injection molded PVC products, offering good heat stability and dimensional stability.

7. Dosage and Processing Considerations

The optimal dosage of DBM in PVC formulations typically ranges from 0.5 to 3 phr (parts per hundred resin), depending on the specific application, processing conditions, and the presence of other additives.

  • Mixing: DBM should be thoroughly mixed with the PVC resin and other additives to ensure uniform dispersion.
  • Processing Temperature: The processing temperature should be carefully controlled to prevent overheating and degradation of the PVC compound.
  • Storage: DBM should be stored in tightly closed containers in a cool, dry place away from moisture and direct sunlight.

8. Regulatory Considerations and Safety

  • Toxicity: Organotin compounds, including DBM, have been subject to scrutiny regarding their toxicity. DBM is considered less toxic than some other organotin stabilizers, but it is still important to handle it with care and follow appropriate safety precautions.
  • Regulations: The use of organotin stabilizers in PVC products is regulated in some regions, particularly in applications involving contact with food or drinking water. It is important to consult local regulations to ensure compliance.
  • Safety Precautions: When handling DBM, it is recommended to wear protective gloves, eye protection, and respiratory protection. Avoid contact with skin and eyes. Wash thoroughly after handling.

9. Advantages and Disadvantages of DBM

Feature Advantages Disadvantages
Heat Stability Excellent heat stabilizing efficiency, preventing degradation and discoloration of PVC during processing. Can be more expensive than some other types of heat stabilizers (e.g., calcium-zinc stabilizers).
Transparency Imparts excellent transparency to PVC compounds. May have limited compatibility with certain low-polarity plasticizers.
Color Retention Improves color retention, preventing yellowing or darkening during processing and service life. Subject to regulatory scrutiny regarding toxicity in some regions, although considered less toxic than some other organotin stabilizers.
Compatibility Good compatibility with a wide range of PVC resins and plasticizers. Can be sensitive to hydrolysis under humid conditions, potentially leading to reduced effectiveness.
Processing Facilitates processing of PVC compounds, improving flow and reducing torque. May require careful handling and storage to prevent contamination and degradation.
Performance Can improve mechanical properties such as tensile strength and elongation at break.

10. Alternative Heat Stabilizers

Due to increasing concerns about the toxicity of organotin compounds, alternative heat stabilizers are being developed and used in PVC formulations. These alternatives include:

  • Calcium-Zinc Stabilizers: These stabilizers are based on calcium and zinc salts and are considered less toxic than organotin stabilizers. They are widely used in flexible and rigid PVC applications.
  • Barium-Zinc Stabilizers: Similar to calcium-zinc stabilizers, barium-zinc stabilizers offer good heat stability but may have limitations in terms of transparency.
  • Organic Stabilizers: A variety of organic stabilizers, such as epoxides and phosphites, can be used to enhance the heat stability of PVC.
  • Hydrotalcites: These are layered double hydroxide compounds that can act as acid scavengers and contribute to improved heat stability.

The choice of heat stabilizer depends on the specific application requirements, regulatory considerations, and cost constraints.

11. Conclusion

Dibutyltin mono(2-ethylhexyl) maleate (DBM) is an effective heat stabilizer for PVC, offering excellent heat stability, transparency, and color retention. Its compatibility with PVC resin and various plasticizers is crucial for achieving optimal performance in flexible and rigid PVC applications. While DBM offers significant advantages, it is important to consider its toxicity and regulatory status. The development and use of alternative heat stabilizers are gaining momentum, providing formulators with a wider range of options for achieving the desired performance characteristics in PVC products. Further research and development are ongoing to optimize the performance and safety of both organotin and alternative heat stabilizers for PVC.

Literature Sources:

  1. Wilkes, C.E., Summers, J.W., Daniels, C.A. PVC Handbook. Hanser Gardner Publications, 2005.
  2. Nass, L.I. Encyclopedia of PVC. Marcel Dekker, 1976.
  3. Titow, W.V. PVC Technology. Elsevier Applied Science, 1984.
  4. Wypych, G. PVC Degradation and Stabilization. ChemTec Publishing, 2008.
  5. Rabek, J.F. Polymer Degradation Mechanisms. Springer, 1995.
  6. Schlimper, H. PVC Processing. Carl Hanser Verlag, 2000.
  7. European Council of Vinyl Manufacturers (ECVM). PVC Stabilisers. [Document, no longer actively available, but typical content acknowledged]
  8. Various patents and scientific articles on organotin chemistry and PVC stabilization. (Note: Specific patent numbers and article titles would be included if available.)

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