Comparing Dibutyltin Mono(2-ethylhexyl) Maleate with other organotin stabilizers

Dibutyltin Mono(2-ethylhexyl) Maleate: Properties, Applications, and Comparison with Other Organotin Stabilizers

Abstract: Dibutyltin mono(2-ethylhexyl) maleate (DBM), an organotin compound, serves as a crucial heat stabilizer for polyvinyl chloride (PVC) polymers. This article provides a comprehensive overview of DBM, including its chemical properties, synthesis methods, stabilization mechanism, and applications. Furthermore, it compares DBM with other common organotin stabilizers, such as dibutyltin dilaurate (DBTL), dibutyltin mercaptide (DBTM), and dioctyltin stabilizers, highlighting their respective advantages and disadvantages in terms of stabilization efficiency, processing characteristics, and regulatory compliance. This article aims to provide a detailed understanding of DBM and its role in PVC stabilization.

Table of Contents:

1. Introduction
2. Chemical Properties of Dibutyltin Mono(2-ethylhexyl) Maleate (DBM)
   2.1 Chemical Structure and Nomenclature
   2.2 Physical and Chemical Properties
3. Synthesis of DBM
4. Stabilization Mechanism of Organotin Stabilizers in PVC
   4.1 HCl Scavenging
   4.2 Substitution of Allylic Chlorine Atoms
   4.3 Prevention of Polyene Formation
5. Applications of DBM in PVC
6. Comparison of DBM with Other Organotin Stabilizers
   6.1 Dibutyltin Dilaurate (DBTL)
   6.2 Dibutyltin Mercaptide (DBTM)
   6.3 Dioctyltin Stabilizers
   6.4 Performance Comparison Table
7. Factors Affecting the Performance of Organotin Stabilizers
   7.1 Concentration
   7.2 Synergistic Additives
   7.3 Processing Temperature and Time
8. Regulatory Considerations and Toxicity
9. Future Trends in Organotin Stabilizer Development
10. Conclusion
11. References

1. Introduction

Polyvinyl chloride (PVC) is a versatile thermoplastic polymer widely used in various applications, including construction materials, packaging, medical devices, and automotive components. However, PVC is inherently unstable at processing temperatures due to the presence of labile chlorine atoms in its polymer chain. Thermal degradation of PVC leads to the release of hydrogen chloride (HCl), which autocatalytically accelerates the degradation process, resulting in discoloration, embrittlement, and loss of mechanical properties. Therefore, heat stabilizers are essential to prevent or retard PVC degradation during processing and service life.

Organotin compounds have been widely used as efficient heat stabilizers for PVC since the 1930s. They offer excellent heat stability, clarity, and weather resistance to PVC products. Among the various organotin stabilizers, dibutyltin mono(2-ethylhexyl) maleate (DBM) stands out as a prominent choice due to its balanced performance characteristics. DBM is effective in preventing PVC degradation and maintaining its desirable properties. This article provides a comprehensive overview of DBM, comparing it with other organotin stabilizers to provide a deeper understanding of its role in PVC stabilization.

2. Chemical Properties of Dibutyltin Mono(2-ethylhexyl) Maleate (DBM)

2.1 Chemical Structure and Nomenclature

DBM is an organotin compound with the following chemical structure:

(C4H9)2Sn(OOCCH=CHCOO(CH2)3CH(C2H5)C4H9)

The IUPAC name for DBM is dibutyltin mono(2-ethylhexyl) maleate. It is also known by several other names, including:

• Dibutyltin monoester of maleic acid and 2-ethylhexanol
• DBM stabilizer
• Dibutyltin 2-ethylhexyl maleate

2.2 Physical and Chemical Properties

DBM is typically a colorless to pale yellow liquid at room temperature. Its physical and chemical properties are summarized in Table 1.

Table 1: Physical and Chemical Properties of Dibutyltin Mono(2-ethylhexyl) Maleate (DBM)

3. Synthesis of DBM

DBM is typically synthesized through a reaction between dibutyltin oxide (DBTO) and maleic anhydride, followed by esterification with 2-ethylhexanol. The reaction can be represented as follows:

1. Reaction of DBTO with Maleic Anhydride:

(C4H9)2SnO + C4H2O3 → (C4H9)2Sn(OOCCH=CHCOOH)

2. Esterification with 2-Ethylhexanol:

(C4H9)2Sn(OOCCH=CHCOOH) + C8H18O → (C4H9)2Sn(OOCCH=CHCOO(CH2)3CH(C2H5)C4H9) + H2O

The reaction is usually carried out in the presence of a catalyst, such as sulfuric acid or p-toluenesulfonic acid, to accelerate the esterification process. The water produced during esterification is removed to drive the reaction to completion. The final product is then purified to remove any unreacted reactants and byproducts.

4. Stabilization Mechanism of Organotin Stabilizers in PVC

Organotin stabilizers, including DBM, protect PVC from thermal degradation through several mechanisms:

4.1 HCl Scavenging

The primary mechanism of PVC degradation involves the dehydrochlorination of the polymer chain, leading to the formation of conjugated polyenes. HCl, a byproduct of this process, acts as an autocatalyst, accelerating further degradation. Organotin stabilizers react with HCl to neutralize it, preventing its autocatalytic effect. This reaction can be represented as follows:

(C4H9)2Sn(OOCCH=CHCOO(CH2)3CH(C2H5)C4H9) + HCl → (C4H9)2SnCl(OOCCH=CHCOO(CH2)3CH(C2H5)C4H9) + HOOCCH=CHCOO(CH2)3CH(C2H5)C4H9

The chlorine atom attached to the tin center is more reactive than the chlorine atoms in the PVC polymer chain, making this reaction thermodynamically favorable.

4.2 Substitution of Allylic Chlorine Atoms

PVC contains labile allylic chlorine atoms, which are more susceptible to dehydrochlorination than other chlorine atoms in the polymer chain. Organotin stabilizers can substitute these allylic chlorine atoms with more stable groups, such as maleate or carboxylate moieties, thereby preventing their decomposition and the subsequent formation of HCl.

PVC-CH=CH-CHCl-CH2-PVC + (C4H9)2Sn(OOCCH=CHCOO(CH2)3CH(C2H5)C4H9) → PVC-CH=CH-CH(OOCCH=CHCOO(CH2)3CH(C2H5)C4H9)-CH2-PVC + (C4H9)2SnCl

4.3 Prevention of Polyene Formation

The formation of conjugated polyenes is responsible for the discoloration and embrittlement of PVC. Organotin stabilizers can react with polyenes to interrupt their conjugation, preventing further extension of the polyene chain and minimizing color development. The exact mechanism of this reaction is complex and not fully understood, but it is believed to involve the addition of the organotin moiety to the polyene chain.

5. Applications of DBM in PVC

DBM is primarily used as a heat stabilizer in rigid and flexible PVC formulations. Its applications include:

• Rigid PVC profiles and pipes: DBM provides excellent heat stability during extrusion and injection molding of rigid PVC products, ensuring good surface finish and dimensional stability.
• Flexible PVC films and sheets: DBM is used in the production of flexible PVC films and sheets for various applications, such as packaging, flooring, and wall coverings. It contributes to the clarity, flexibility, and durability of these products.
• PVC plastisols: DBM can be used as a stabilizer in PVC plastisols, which are used for coating fabrics, automotive underbody coatings, and other specialized applications.
• Medical devices: Certain grades of DBM are approved for use in medical devices, such as blood bags and tubing, due to their low toxicity and good compatibility with PVC.

6. Comparison of DBM with Other Organotin Stabilizers

DBM is just one of many organotin stabilizers used in PVC. Other common organotin stabilizers include dibutyltin dilaurate (DBTL), dibutyltin mercaptide (DBTM), and dioctyltin stabilizers. Each of these stabilizers has its own advantages and disadvantages in terms of stabilization efficiency, processing characteristics, and regulatory compliance.

6.1 Dibutyltin Dilaurate (DBTL)

DBTL is a dialkyltin dicarboxylate stabilizer with the following chemical structure:

(C4H9)2Sn(OCOC11H23)2

DBTL is a highly effective heat stabilizer, providing excellent clarity and color retention to PVC. However, it has a strong odor and is more prone to plate-out (the migration of stabilizer to the surface of the PVC product) than DBM. DBTL is also more susceptible to hydrolysis, which can reduce its effectiveness over time. Furthermore, DBTL is classified as a toxic substance in some regions and its use is becoming increasingly restricted.

6.2 Dibutyltin Mercaptide (DBTM)

DBTM is a dialkyltin mercaptide stabilizer containing sulfur atoms. A common example is dibutyltin bis(isooctyl mercaptoacetate). These stabilizers are known for their exceptional heat stability and early color hold, particularly in highly plasticized PVC formulations. The presence of sulfur atoms enhances their reactivity with HCl and their ability to prevent polyene formation. However, DBTM stabilizers often impart a strong odor to the PVC product and can cause staining if exposed to sulfur-containing environments. They can also negatively affect the welding properties of PVC.

6.3 Dioctyltin Stabilizers

Dioctyltin stabilizers, such as dioctyltin bis(2-ethylhexyl thioglycolate) (DOTG) and dioctyltin maleate (DOTM), are characterized by their lower toxicity compared to dibutyltin stabilizers. This is due to the longer alkyl chains (octyl) attached to the tin atom, which reduces their bioavailability and toxicity. Dioctyltin stabilizers are widely used in food-contact applications and medical devices where low toxicity is a critical requirement. However, dioctyltin stabilizers generally offer lower heat stability than dibutyltin stabilizers, particularly at high processing temperatures. They are also more expensive.

6.4 Performance Comparison Table

Table 2 summarizes the key performance characteristics of DBM and other common organotin stabilizers.

Table 2: Performance Comparison of Organotin Stabilizers

7. Factors Affecting the Performance of Organotin Stabilizers

The performance of organotin stabilizers in PVC is influenced by several factors, including concentration, synergistic additives, and processing temperature and time.

7.1 Concentration

The concentration of the organotin stabilizer is a critical factor in determining its effectiveness. An insufficient concentration may not provide adequate heat stability, while an excessive concentration can lead to plate-out and other undesirable effects. The optimal concentration depends on the specific PVC formulation and processing conditions, but it typically ranges from 0.5 to 3 phr (parts per hundred resin).

7.2 Synergistic Additives

The performance of organotin stabilizers can be enhanced by the addition of synergistic additives, such as epoxy compounds, phosphites, and zeolites.

• Epoxy compounds: Epoxy compounds, such as epoxidized soybean oil (ESBO), can scavenge HCl and act as plasticizers, improving the compatibility of the organotin stabilizer with the PVC matrix. They can also react with polyenes to prevent discoloration.
• Phosphites: Phosphites can act as antioxidants and prevent the oxidation of the organotin stabilizer, thereby extending its effectiveness. They can also decompose peroxides formed during PVC degradation.
• Zeolites: Zeolites can adsorb HCl and other acidic degradation products, preventing their autocatalytic effect. They also act as drying agents, removing moisture from the PVC formulation.

7.3 Processing Temperature and Time

The processing temperature and time significantly impact the effectiveness of organotin stabilizers. Higher processing temperatures and longer processing times accelerate PVC degradation, requiring higher concentrations of stabilizer to provide adequate protection. It is crucial to optimize processing conditions to minimize PVC degradation and maximize the performance of the organotin stabilizer.

8. Regulatory Considerations and Toxicity

Organotin compounds have been subject to increasing regulatory scrutiny due to concerns about their toxicity and environmental impact. Certain organotin compounds, particularly those with short alkyl chains, such as tributyltin (TBT) and triphenyltin (TPT), have been shown to be highly toxic to aquatic organisms and are restricted or banned in many countries.

Dibutyltin compounds, including DBM, are considered to be less toxic than TBT and TPT, but they are still subject to regulatory limits in some applications. The European Union (EU) has restricted the use of dibutyltin compounds in certain consumer products, such as textiles and childcare articles, due to concerns about endocrine disruption.

Dioctyltin compounds are generally considered to be the least toxic of the organotin stabilizers and are approved for use in food-contact applications and medical devices in many countries. However, manufacturers must ensure that the dioctyltin stabilizers used in these applications meet specific purity requirements and do not contain unacceptable levels of dibutyltin impurities.

It is essential for manufacturers to comply with all applicable regulations regarding the use of organotin stabilizers to ensure the safety and environmental sustainability of their PVC products.

9. Future Trends in Organotin Stabilizer Development

The development of organotin stabilizers is driven by the need for more effective, less toxic, and more environmentally friendly products. Future trends in this area include:

• Development of new organotin structures: Researchers are exploring new organotin structures with improved stabilization efficiency and reduced toxicity. This includes the development of sterically hindered organotin compounds, which are less likely to undergo hydrolysis and other degradation reactions.
• Use of synergistic blends: The use of synergistic blends of organotin stabilizers with other additives, such as epoxy compounds, phosphites, and zeolites, is becoming increasingly common. These blends can provide enhanced heat stability, improved color retention, and reduced toxicity compared to single-component organotin stabilizers.
• Development of tin-free stabilizers: Due to the environmental and regulatory concerns surrounding organotin compounds, there is a growing interest in the development of tin-free alternatives. Calcium-zinc stabilizers, magnesium-aluminum hydrotalcites, and organic co-stabilizers are being investigated as potential replacements for organotin stabilizers in certain PVC applications. However, these alternatives often do not provide the same level of performance as organotin stabilizers, particularly in demanding applications requiring high heat stability and clarity.
• Recycling and Circular Economy: Focus on developing stabilizers that are compatible with PVC recycling processes and contribute to a circular economy. This includes stabilizers that do not impede the recycling process and can be effectively reused in recycled PVC materials.

10. Conclusion

Dibutyltin mono(2-ethylhexyl) maleate (DBM) is a widely used and effective heat stabilizer for PVC. It provides good heat stability, clarity, and processability to PVC products. While other organotin stabilizers, such as DBTL and DBTM, may offer superior performance in certain aspects, DBM provides a balanced combination of properties that makes it suitable for a wide range of applications. Dioctyltin stabilizers are preferred for applications requiring low toxicity, such as food-contact applications and medical devices. However, the choice of the optimal organotin stabilizer depends on the specific requirements of the PVC formulation, processing conditions, and regulatory constraints. The future of organotin stabilizer development is focused on creating more effective, less toxic, and more environmentally friendly products, while also exploring tin-free alternatives to meet evolving regulatory requirements and sustainability goals.

11. References

[1] Gächter, R., & Müller, H. (1993). PVC Additives: Performance, Chemistry, Developments, and Trends. Hanser Publishers.

[2] Wilkes, C. E., Summers, J. W., & Daniels, C. A. (2005). PVC Handbook. Hanser Gardner Publications.

[3] Titow, W. V. (1984). PVC Technology. Springer Science & Business Media.

[4] Nass, L. I., & Heiberger, J. B. (1986). PVC: Polymer Properties, Mechanism of Degradation, and Stabilization. Van Nostrand Reinhold.

[5] Schlimper, H. (2000). Stabilization of Polyvinyl Chloride. Elsevier Science.

[6] Owen, E. D. (1984). Degradation and Stabilization of PVC. Elsevier Applied Science.

[7] Becker, H., & Braun, D. (1993). Polymer Degradation. Hanser Publishers.

[8] Anonymous. "Dibutyltin maleate". Registry of Toxic Effects of Chemical Substances (RTECS). National Institute for Occupational Safety and Health.

[9] European Chemicals Agency (ECHA). Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH). Accessed via ECHA website.

[10] Wicks, Z. W., Jones, F. N., & Pappas, S. P. (1999). Organic Coatings: Science and Technology. John Wiley & Sons.