Barrier properties in paints and coatings

Barrier properties in paints refer to their ability to act as a protective layer. This prevents the passage of substances like moisture, chemicals, or gases. This is crucial in various applications, from protecting industrial equipment to preserving the aesthetic appeal of buildings.

 

Key barrier properties

The key properties of barrier coatings include:
 

• Moisture barrier: Prevents the penetration of moisture. Thus, protecting materials from corrosion, mold, and mildew.
• Gas barrier: Controls the diffusion of gases like oxygen, nitrogen, and carbon dioxide. This is essential for preserving product quality and preventing degradation.
• Thermal barrier: Provides insulation against extreme temperatures. Thus, protecting materials from heat damage or cold embrittlement.
• Chemical barrier: Resists the penetration of chemicals, protecting materials from corrosion and degradation.
• UV barrier: Blocks harmful ultraviolet radiation, preventing discoloration and degradation of materials.

Figure 1: Key Properties of Barrier Coatings on a Substrate¹

Key characteristics of barrier coatings

The main characteristics possessed by barrier coatings include:
 

• Permeability: The ability of a material to allow the passage of substances. A good barrier material should have low permeability to the substances it is intended to block.
• Adhesion: The ability of a coating to adhere to the substrate surface. Good adhesion is essential for the durability and performance of a barrier coating.
• Chemical resistance: The ability of a material to withstand exposure to chemicals without degradation.
• Thermal resistance: The ability to withstand extreme temperatures without losing its barrier properties.
• Mechanical strength: The ability to resist wear, abrasion, and other mechanical stresses.

Factors affecting barrier properties of paints & coatings

• Paint formulation: The specific ingredients and their proportions determine the paint's barrier performance.
• Application technique: Proper application, including surface preparation and coverage, is crucial for optimal barrier formation.
• Environmental conditions: Temperature, humidity, and other factors can influence curing and performance.
• Substrate preparation: The condition of the surface being painted affects adhesion and barrier properties.

Different types of paints have different barrier properties. Some of the common barrier paints and coatings are explained below:
 

1. Epoxy coatings: Known for their excellent moisture and chemical resistance. They are widely used in industrial and marine applications.
2. Polyurethane coatings: Offer good durability, abrasion resistance, and chemical resistance, making them suitable for various applications.
3. Metal coatings: Coatings like aluminum, zinc, and chromium provide corrosion resistance. They are often used in conjunction with other barrier coatings.
4. Aluminum paints: Made with aluminum particles and oil varnish. This paint is resistant to corrosion, electricity, and weather. It can be used on metals and wood.

5. Anti-corrosive paints: Used to resist corrosion, this paint is often used on metal surfaces like pipes and external structures. Two-component epoxy coatings offer optimum protection against mechanical erosion and electrochemical attack. These compounds can be applied to metal surfaces and can operate under continuous immersion. Products feature:
    

   • exceptional wear/friction resistance,
   • guard against flow-accelerated corrosion,
   • particle impingement, and
   • cavitation

    

   They also exhibit low shrinkage, outstanding electrical insulation, superior compressive strength, and fill gaps. Selected grades are engineered to improve the efficiency of fluid-handling equipment such as valves, pumps, and pipes.

6. Barrier coatings: A coat that seals a surface to make it impenetrable to water. It is often applied before bottom paint on a hull to prevent blistering.
7. Bituminous paints: Made from dissolved asphalt or tar, this paint is waterproof and alkali resistant. However, it will deteriorate if exposed to sunlight.
8. Asbestos paints: Made by mixing distemper or oil paint with finely ground asbestos. This paint is heat- and sound-insulating, and highly resistant to fire. It can also shield metals from corrosion and keep off rodents.
9. Acrylic latex paints: A water-based paint that adheres well to concrete and is easy to apply. It is a good choice for concrete bollards.
10. Cellulose paints: A plant-based coating that can be applied in multiple layers. Cellulose paints are temporary and can be easily removed for another coating.
11. Lamellar pigments, such as glass flakes and micaceous iron oxides enhance the coating's barrier properties. The flakes provide a 'tortuous path' for any molecule that may be detrimental to the substrate, as shown schematically in Figure 2.
     

Figure 2: Water Barrier Properties Difference in the Paints with Different Pigment Shapes²

Thermal barrier coating (TBC) materials are designed to provide insulation and protection against high temperatures. They are used in applications like gas turbine engines, jet engines, and industrial furnaces. TBC thickness typically ranges from 0.5 to 1.5 mm. The optimal thickness depends on the specific application and the desired level of protection.

 

Structure of TBC materials

The effectiveness of a TBC lies in its composition and multi-layered structure. Most TBCs consist of four distinct layers as shown in Figure 3.

 

Bond coat

This metallic layer is usually composed of MCrAlY alloys (where M can be nickel, cobalt, or iron). It serves as an adhesive between the substrate and the ceramic topcoat. It provides oxidation and corrosion resistance, enhancing the durability of the coating.

 

Thermally grown oxide (TGO) layer
 

It is formed between the bond coat and the topcoat which is primarily aluminum oxide (Al₂O₃). It acts as a barrier against further oxidation of the bond coat and helps maintain the integrity of the coating system.

 

Ceramic topcoat
 

Typically made of yttria-stabilized zirconia (YSZ), the ceramic topcoat is the main thermal barrier. Its low thermal conductivity and high melting point (around 2700°C) make it an excellent insulator against high temperatures. Rare earth zirconates, such as gadolinium zirconate (Gd₂Zr₂O₇), have emerged as promising alternatives to YSZ.
 

• These materials offer lower thermal conductivity and higher phase stability at elevated temperatures.
• Additionally, their resistance to sintering and erosion makes them suitable for long-term applications in harsh environments.

Recent advancements include multilayer and functionally graded coatings, where different materials are layered or graded to optimize performance. These coatings can tailor thermal expansion properties and improve the durability of the TBC system. For example, a multilayer TBC with alternating layers of YSZ and rare earth zirconate can combine the benefits of both materials, offering superior thermal protection and longevity.

 

Substrate

The base material is often a superalloy. It provides the structural strength required for the component to function under high-stress conditions. 
 

Figure 3: Thermal Barrier Coating on Turbine Blade³

Properties of TBC materials
 

• High-temperature resistance: The ability to withstand extreme temperatures without significant degradation or failure.
• Low thermal conductivity: Low thermal conductivity helps minimize heat transfer, reducing the temperature of the underlying material.
• Porosity: A porous structure allows the expansion and contraction of the coating without cracking.
• Adhesion: The ability to adhere strongly to the underlying substrate is essential for the coating's durability and effectiveness.
• Oxidation and corrosion resistance: The bond coat and TGO layer protect against oxidation and corrosion, which are common at high temperatures.
• Wear resistance: The ability to withstand mechanical abrasion and erosion.
• Thermal shock resistance: The ability to withstand rapid temperature changes without cracking or spalling.

Benefits, challenges, and future advancements
 

TBCs offer numerous benefits, including:
 

• extended component lifespan,
• improved efficiency, and
• cost savings

By protecting components from thermal degradation, TBCs enable higher operating temperatures, leading to better fuel economy and reduced emissions. While initial application costs can be high, the long-term benefits outweigh these expenses.

However, challenges such as durability and environmental resistance remain. Ongoing research focuses on improving bond coat strength, developing new materials, and exploring additive manufacturing techniques. Future advancements may also include smart coatings with self-healing capabilities. Overall, TBCs represent a significant advancement in material science, providing essential protection and enhancing the performance of high-temperature components.

 

Food packaging
 

Barrier coatings are essential for food packaging to prevent contamination, maintain freshness, and extend shelf life. They can be applied to various packaging materials, including:
 

• Rigid packaging: Plastic bottles, cans, and cartons can be coated to provide a barrier against moisture, oxygen, and grease.
• Flexible packaging: Films, laminates, and pouches can be coated to protect food from environmental factors and maintain product integrity.

Earlier barrier coatings for food packaging were made from synthetic polymeric chemicals known as per- and polyfluoroalkyl substances (PFAS). PFAS are a diverse class of compounds characterized by having a hydrophobic (water-hating), fluorine-saturated carbon chain joined to a hydrophilic (water-loving) functional group. This unique structure gives PFAS the ability to repel both water and fat readily.

Unfortunately, PFAS can't be separated from the paper easily, which means the paper can't be recycled or repulped. The whole family of compounds has also been shown to have harmful effects on human health.


Common barrier properties for food packaging

→ Moisture barrier: Prevents moisture vapor transmission (MVTR), which can lead to product spoilage and contamination. Moisture vapor transmission is the transfer of water vapor from one side of a package to the other. The water vapor transmission in a specified time range is measured by the MVTR, also known as the water vapor transmission rate. The units are expressed as either grams/100 in²/day or as grams/m²/day. The lower the MVTR, the better the moisture barrier properties. A properly formulated barrier coating for paperboard can achieve an MVTR measurement of 2/day or 2/day. Figure 4 shows the different moisture and gas barrier requirements for different food packaging applications.

→ Oxygen barrier: Controls oxygen transmission rate (OTR), preventing oxidation and preserving product freshness. Oxygen permeability is measured in units such as cm³m²/sPa or ccmil/dayatm. Various methods are used to measure this property. When reporting results, it is essential to include units, temperature, humidity, and thickness for accurate interpretation and comparison.

 

 Figure 4: Oxygen Transmission Rate and Water Vapor Transmission Rate Requirements for the Barrier of Various Food Products⁴

→ Grease barrier: Resists grease and oil penetration, maintaining product appearance and preventing contamination. Oil and grease resistance in a barrier coating was traditionally measured by the Kit test. This test involves applying varying mixtures of castor oil, toluene, heptane, and turpentine to a product for 15 seconds. Each mixture is scored a number on a scale of 0 to 12, from least aggressive to most aggressive. The highest-numbered mixture that does not stain the surface is reported as the 'kit rating.' Hot oils and greases are also used for testing. They better reflect how well a synthetic polymer-based barrier coating resists actual greases that food packaging typically contacts.

→ Flavor barrier: Helps prevent flavor transfer between different products. Sensory evaluation, instrumental analysis, and permeation tests are common methods to measure this. Flavor barrier performance is influenced by:
 

• packaging material type,
• processing conditions, and
• flavor compound properties

By combining these methods, an accurate assessment of flavor barrier properties can ensure product flavor integrity.

 

Paper packaging
 

Paper packaging, such as cartons and boxes, can be coated to provide barrier properties and protect against water leakage, vapor, solvents, oils, fatty acids, and other substances. Depending on the product, barrier coatings are required to show certain valuable characteristics. The important properties encompassed by barrier coatings are given below:
 

• oil & grease resistant
• liquid water resistant
• low in odor permeability
• visual appearance (glossy, satin, matt)
• heat seal ability
• non-blocking
• high wet block resistance
• high scuff resistance
• heat resistant
• recyclability
• consistent coefficient of friction (COF)
• direct food contact FDA-compliant
  
   

Common barrier coatings for paper packaging
 

• Wax coatings: Provide moisture and grease resistance. However, they have limited oxygen barrier properties.
• Lacquer coatings: Offer better moisture and oxygen barrier properties than wax coatings.
• Heat sealable barrier coatings: Many paper or paperboard packages (for example, paper cups for food or drink services) require the paper or paperboard to be heat sealable. This makes it possible to form cups on a cup machine. Polyethylene (PE) extrusion-coated paperboard dominates in such applications by providing both required barrier and heat seal properties. However, as discussed above there are increasing demands for replacing PE with a barrier coating which not only provides excellent water and oil resistance but also can seal the paper material on application of heat.

Bio-based materials for barrier coatings on paper packaging

There is growing interest in using bio-based materials for barrier coatings on paper packaging to improve sustainability. Generally being water-soluble like starch, these polymers have excellent film-forming and barrier properties against nonpolar gases such as oxygen.

Prominent examples are polyvinyl alcohols (PVOHs) and ethyl vinyl alcohol copolymers (EVOH). Neither of these polymers harms the degradation properties of paper. This is because they are fully biodegradable. Although the raw materials do not have entirely renewable origins (a pure cellulose/starch combination is 100% renewably sourced) the barrier paper will still be made of more than 95% renewably sourced raw materials, which is a very good figure.
 

• Cellulose acetate butyrate: A biodegradable polymer that can provide good moisture and oxygen barrier properties.
• Polylactic acid: A biodegradable polymer that can be used for barrier coatings, especially in combination with other materials.
• Green PE coating: PE Green is a fully renewable option to traditional PE and provides excellent humidity protection. PE Green is made of renewable, plant-based raw material to get barrier packaging that is 100% renewable and recyclable. In converting, it performs the same way as PE and is therefore easy to introduce to production by customers.

Flexible packaging
 

Flexible packaging materials, such as films and laminates, are widely used in food packaging. Barrier coatings are essential for protecting these materials from moisture, oxygen, and other contaminants. Common barrier coatings for flexible packaging include:
 

• Aluminum foil: Provides excellent moisture and oxygen barrier properties. However, it can be challenging to laminate with other materials.
• Ethylene vinyl alcohol: A high-barrier polymer that offers excellent moisture and oxygen resistance.
• Nylon: Provides good moisture and oxygen barrier properties, especially when used in combination with other materials.
• PE coating: PE is the most commonly used barrier coating. Polyolefin barriers, such as LDPE and HDPE polymers, provide excellent humidity protection.
• PET coating: PET provides a barrier and performs other functions. Black or white PET coatings that provide heat resistance act as an excellent grease barrier and possess solid WVTR (water vapor transmission rate) properties.
• PP coating: PP coating offers heat resistance for microwave ovens and is also suitable for deep freezing. Good sealing properties secure performance in use.
• High-barrier coating: High-barrier coating consists of a multilayer EVOH and PE polymer structure. It provides excellent oxygen, humidity, and aroma protection. The high barrier-coated boards and papers are also greaseproof.

Barrier coatings for flexible packaging are used in various applications, including:
 

• Food packaging: Packaging for snacks, meats, dairy products, and other food items.
• Beverage packaging: Packaging for juice, coffee, tea, and other beverages.
• Pharmaceutical packaging: Packaging for drugs, medical devices, and other healthcare products.
• Consumer goods packaging: Packaging for personal care products, household goods, and other consumer products.