Dibutyltin Mono(2-ethylhexyl) Maleate in food contact grade PVC stabilization systems

Dibutyltin Mono(2-ethylhexyl) Maleate: A Comprehensive Review of its Application in Food Contact Grade PVC Stabilization

Introduction

Polyvinyl chloride (PVC) is a widely used thermoplastic polymer known for its versatility, durability, and cost-effectiveness. Its applications span across various industries, including construction, healthcare, and packaging. However, PVC is inherently unstable at processing temperatures, requiring the addition of stabilizers to prevent degradation, discoloration, and loss of mechanical properties during manufacturing and usage. Tin stabilizers, particularly organotin compounds, have been extensively employed for PVC stabilization due to their exceptional heat stability, processing performance, and compatibility with PVC resins. Among these, dibutyltin mono(2-ethylhexyl) maleate (DBTM) stands out as a prominent choice for food contact grade PVC applications. This article provides a comprehensive overview of DBTM, encompassing its properties, synthesis, mechanism of action, applications in food contact PVC, safety considerations, and future trends.

1. Chemical Properties and Structure

Dibutyltin mono(2-ethylhexyl) maleate, often abbreviated as DBTM, is an organotin compound with the chemical formula C₂₄H₄₄O₄Sn. Its structure consists of a central tin atom bonded to two butyl groups (C₄H₉), one 2-ethylhexyl maleate group, and a chlorine atom.

1.1 Chemical Structure:

      C4H9
       |
C4H9-Sn-OOCCH=CHCOO-CH2CH(C2H5)C4H9
       |
      Cl

1.2 Physical and Chemical Properties:

The following table summarizes the key physical and chemical properties of DBTM:

Property	Value/Description	Reference
Molecular Weight	~511.18 g/mol	[1]
Appearance	Clear, colorless to slightly yellow liquid	[2]
Density	~1.05 g/cm³ at 20°C	[3]
Viscosity	Varies depending on grade, generally low viscosity	[4]
Refractive Index	~1.48	[1]
Solubility	Soluble in organic solvents, slightly soluble in water	[2]
Flash Point	> 150°C	[3]
Boiling Point	Decomposes before boiling	[4]
Tin Content	Typically between 19-21% by weight	[1]
Hydrolytic Stability	Good, under normal conditions	[2]

1.3 Grades and Specifications:

DBTM is available in various grades, with specifications tailored to specific applications. Key parameters include tin content, acid value, color, and clarity. Manufacturers typically provide detailed specifications in their product datasheets.

2. Synthesis and Manufacturing Process

DBTM is typically synthesized through a reaction between dibutyltin oxide (DBTO) and maleic anhydride, followed by esterification with 2-ethylhexanol.

2.1 Reaction Equation (Simplified):

1. (Dibutyltin Oxide + Maleic Anhydride): (C₄H₉)₂SnO + C₄H₂O₃ → (C₄H₉)₂Sn(OOCCH=CHCOOH)
2. (Intermediate + 2-Ethylhexanol): (C₄H₉)₂Sn(OOCCH=CHCOOH) + HOCH₂CH(C₂H₅)C₄H₉ → (C₄H₉)₂Sn(OOCCH=CHCOO-CH₂CH(C₂H₅)C₄H₉) + H₂O

2.2 Manufacturing Steps:

1. Reaction: DBTO and maleic anhydride are reacted in a suitable solvent under controlled temperature and pressure.
2. Esterification: 2-ethylhexanol is added to the reaction mixture to esterify the maleic anhydride moiety, forming DBTM.
3. Neutralization: The reaction mixture is neutralized to remove any residual acidity.
4. Purification: The product is purified through distillation or other separation techniques to remove unreacted reactants and byproducts.
5. Quality Control: The final product undergoes rigorous quality control testing to ensure it meets the required specifications.

3. Mechanism of Action as a PVC Stabilizer

DBTM acts as a PVC stabilizer through several mechanisms:

3.1 HCl Scavenging: DBTM reacts with hydrogen chloride (HCl) released during PVC degradation, preventing autocatalytic degradation.

(C₄H₉)₂Sn(OOCCH=CHCOO-CH₂CH(C₂H₅)C₄H₉) + HCl → (C₄H₉)₂SnCl(OOCCH=CHCOO-CH₂CH(C₂H₅)C₄H₉) + HCl

3.2 Polyene Addition: DBTM can react with polyene sequences formed during PVC degradation, preventing discoloration and embrittlement. The tin atom coordinates with the double bonds in the polyene, disrupting the conjugated system and preventing further propagation.

3.3 Peroxide Decomposition: DBTM can decompose peroxides formed during PVC degradation, reducing oxidative degradation.

3.4 Radical Trapping: DBTM can trap free radicals generated during PVC degradation, preventing chain scission and crosslinking.

3.5 Replacement of Labile Chlorine Atoms: Tin stabilizers can replace labile chlorine atoms in the PVC polymer chain with more stable groups, improving the thermal stability of the polymer.

The efficiency of DBTM as a stabilizer depends on factors such as concentration, processing conditions, and the presence of other additives.

4. Application in Food Contact Grade PVC

DBTM is widely used as a stabilizer in food contact grade PVC applications due to its high efficiency and relatively low toxicity compared to other organotin stabilizers. However, stringent regulations govern its use to ensure consumer safety.

4.1 Food Contact Applications:

• Rigid PVC Films and Sheets: Used for packaging of food products such as meat, cheese, and confectionery.
• Bottles and Containers: Used for beverages, oils, and other liquid food products.
• Flexible PVC Films: Used as cling film for wrapping food items.
• Seals and Gaskets: Used in food processing equipment.
• Pipes and Fittings: Used in food processing plants for conveying potable water and other food-grade liquids.

4.2 Regulatory Compliance:

The use of DBTM in food contact applications is subject to strict regulations imposed by various regulatory bodies worldwide, including:

• European Union (EU): Regulation (EU) No 10/2011 on plastic materials and articles intended to come into contact with food. Specific migration limits (SMLs) are set for tin and specific organotin compounds.
• United States Food and Drug Administration (FDA): 21 CFR 178.2010 – Antioxidants and/or stabilizers for polymers. DBTM is generally recognized as safe (GRAS) for certain applications within specified limitations.
• China: GB 9685-2016 National Food Safety Standard – Standards for Uses of Additives in Food Contact Materials and Articles. Specifies permitted uses and migration limits.

Manufacturers of food contact PVC products must ensure that their products comply with these regulations to ensure consumer safety. Compliance typically involves rigorous testing of the finished product to verify that migration levels of DBTM and its degradation products are below the specified SMLs.

4.3 Formulation Considerations:

When formulating food contact grade PVC, several factors need to be considered:

• DBTM Concentration: The concentration of DBTM should be optimized to achieve the desired level of stabilization while minimizing migration potential.
• Co-Stabilizers: Co-stabilizers, such as epoxy soybean oil, calcium stearate, and zinc stearate, are often used in conjunction with DBTM to enhance its performance and reduce the overall stabilizer loading. These co-stabilizers can provide synergistic effects, improving heat stability, light stability, and processing characteristics.
• Plasticizers: The choice of plasticizer can also influence the stability and migration characteristics of PVC. Phthalate-free plasticizers are increasingly preferred due to concerns about the toxicity of phthalates.
• Processing Conditions: Processing conditions, such as temperature and residence time, can affect the degradation of PVC and the migration of DBTM. Optimizing processing conditions is crucial to minimize degradation and ensure compliance with migration limits.
• Purity of Raw Materials: High-purity raw materials are essential to minimize the presence of impurities that can contribute to degradation and migration.

4.4 Advantages of DBTM in Food Contact PVC:

• High Heat Stability: Provides excellent heat stability during PVC processing.
• Good Clarity: Maintains good clarity in the finished product.
• Low Odor: Has a relatively low odor compared to some other organotin stabilizers.
• Effective HCl Scavenger: Efficiently scavenges HCl released during PVC degradation.
• Compatibility: Good compatibility with PVC resins and other additives.

5. Safety and Toxicology

The safety of DBTM is a critical concern, especially in food contact applications. Extensive toxicological studies have been conducted to assess its potential health effects.

5.1 Acute Toxicity:

DBTM exhibits relatively low acute toxicity. Oral LD50 values in rats are typically in the range of 2000-4000 mg/kg [5]. However, direct contact with skin and eyes can cause irritation.

5.2 Chronic Toxicity:

Long-term exposure to high doses of DBTM can cause adverse health effects, including liver and kidney damage [6]. However, the levels of DBTM that migrate into food from food contact PVC are typically very low and well below the levels that have been shown to cause adverse effects in animal studies.

5.3 Genotoxicity and Carcinogenicity:

DBTM has not been shown to be genotoxic or carcinogenic in standard tests [7].

5.4 Reproductive Toxicity:

Some studies have suggested that high doses of DBTM can have reproductive effects in animals [8]. However, these effects have not been observed at the low levels of exposure that are typical for humans.

5.5 Migration Studies:

Migration studies are essential to assess the potential for DBTM to migrate from food contact PVC into food. These studies typically involve immersing PVC samples in food simulants under controlled conditions and measuring the concentration of DBTM and its degradation products in the simulant over time. The results of these studies are used to determine compliance with regulatory SMLs.

5.6 Handling and Safety Precautions:

When handling DBTM, it is important to follow appropriate safety precautions:

• Wear appropriate personal protective equipment (PPE), such as gloves, goggles, and a lab coat.
• Avoid contact with skin and eyes.
• Work in a well-ventilated area.
• Wash hands thoroughly after handling.
• Dispose of waste materials in accordance with local regulations.

6. Alternatives to DBTM

Due to increasing concerns about the toxicity of organotin compounds, there is growing interest in developing alternative stabilizers for food contact PVC.

6.1 Calcium-Zinc Stabilizers:

Calcium-zinc stabilizers are a popular alternative to organotin stabilizers. They are generally considered to be less toxic and are widely used in food contact applications. However, they typically provide lower heat stability than organotin stabilizers, and require the addition of co-stabilizers to achieve comparable performance.

6.2 Barium-Zinc Stabilizers:

Barium-zinc stabilizers offer improved heat stability compared to calcium-zinc stabilizers, but they are subject to stricter regulations due to the toxicity of barium.

6.3 Rare Earth Stabilizers:

Rare earth stabilizers, such as cerium and lanthanum compounds, are a relatively new class of PVC stabilizers that offer good heat stability and low toxicity. However, they are currently more expensive than traditional organotin stabilizers.

6.4 Organic Stabilizers:

Organic stabilizers, such as β-diketones and polyols, can also be used to stabilize PVC. They are generally less effective than organotin stabilizers, but they can be used in combination with other stabilizers to achieve acceptable performance.

7. Future Trends

The future of DBTM in food contact grade PVC stabilization is likely to be shaped by several trends:

• Stricter Regulations: Regulations governing the use of organotin compounds are likely to become stricter, further limiting their use in food contact applications.
• Development of Alternative Stabilizers: Research and development efforts will continue to focus on developing alternative stabilizers that offer comparable performance to organotin stabilizers with improved safety profiles.
• Increased Use of Recycled PVC: The use of recycled PVC is expected to increase, which may require the development of new stabilization technologies to address the challenges associated with stabilizing recycled PVC.
• Nanotechnology: Nanomaterials are being explored as potential PVC stabilizers, offering the potential for improved performance and reduced stabilizer loading.
• Bio-based Stabilizers: Research is being conducted on bio-based stabilizers derived from renewable resources, such as vegetable oils and lignin.

8. Conclusion

Dibutyltin mono(2-ethylhexyl) maleate (DBTM) has been a valuable stabilizer in food contact grade PVC applications due to its excellent heat stability, clarity, and processing performance. However, growing concerns regarding the potential toxicity of organotin compounds and increasingly stringent regulations are driving the development and adoption of alternative stabilizer systems. While DBTM continues to be used in certain applications, the long-term trend points towards a gradual replacement with safer and more sustainable alternatives. Continued research and innovation are crucial to developing new stabilization technologies that meet the demanding requirements of the food contact PVC industry while ensuring consumer safety and environmental protection.

Literature Sources:

[1] Arkema. (2010). Technical Data Sheet: Mark® 1900 Stabilizer.

[2] Baerlocher GmbH. (2015). Product Information: Baerostab OM 36.

[3] Galata Chemicals. (2018). Product Bulletin: Mark® 17MOH.

[4] Reagens S.p.A. (2020). Technical Data Sheet: Reagens Tin Stabilizer 181.

[5] World Health Organization. (1980). Organotin Compounds: Environmental Health Criteria 15.

[6] European Food Safety Authority (EFSA). (2005). Opinion of the Scientific Panel on Food Additives, Flavourings, Processing Aids and Materials in Contact with Food on a Request from the Commission related to the Safety Assessment of Organotin Compounds for use in Food Contact Materials.

[7] National Toxicology Program (NTP). (1982). Toxicology and Carcinogenesis Studies of Dibutyltin Compounds.

[8] U.S. Environmental Protection Agency (EPA). (1992). Dibutyltin Compounds: Hazard Assessment.

Disclaimer: This article is for informational purposes only and does not constitute professional advice. The information provided should not be used as a substitute for consulting with qualified experts in the field. The author and publisher disclaim any liability for any loss or damage arising from the use of this information.

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