Polyurethanes in the Automotive Sector

Polyurethanes in the Automotive Sector

1. A Historical Perspective on Polyurethanes

In the early 1900s, most coatings and paints were produced using animal fats, milk, or natural oils. Between the First and Second World Wars, many companies turned to the polymer industry. Polymers offered significant advantages over their predecessors for surface protection. In the early 1930s, the first epoxy resin used in the coatings sector introduced the concept of "pot life," and dual-container packaging, where resins were stored in one container and "curing agents" in the other, began to be used for the first time. Epoxies did not conduct electricity and this property provided serious usage advantages for cathodic protection. However, the inability of epoxy resins to maintain chemical stability under UV light (chalking) led to new developments in this sector. In 1937, German Otto Bayer and his team produced polyurethane resins and obtained a patent. This polyurethane technology was not ready for use until the early 1950s. Initial trials were conducted during the Second World War with the external fuselage painting of a combat aircraft. These resins demonstrated excellent resistance to UV light compared to epoxy and other oil-based coatings used until that time. This resistance enabled rapid adoption of polyurethanes on surfaces exposed to sunlight. Following these trials, polyurethane resins, which were first advancing in Germany, quickly industrialized and began to be used worldwide.

2. Polyurethanes from a Chemical Perspective

Organic polymers (macromolecular materials) with covalent bond structures are generally classified into three main groups. These are fully synthetic or chemically modified plastics. • Thermoplastics • Thermosets • Elastomers Polymers are formed by the combination of units called monomers and complete this formation through reactions termed "polyreaction." If these monomers contain at least two functional groups, they form through polyaddition or polycondensation; if a double bond, they form through polymerization reaction.  

2.1. Chemical Formation Mechanism of Polyurethane Resins

Polyurethane resins used at the sectoral level are formed through polyaddition. An example presentation of copolymer formation is given in Figure 5.

2.2. Types of Polyurethanes

Polyurethanes can be grouped into three main categories based on their properties, applications, and curing mechanisms.

2.2.1. 2C Polyurethanes

These polyurethanes generally consist of low molecular weight polyol resins and their hardeners are isocyanate-terminated prepolymers. After curing, they form thermoset or elastomer plastics. Since they are two-component (2C), they can be cured at room temperature; curing can be accelerated with heat. An example curing mechanism is given in Figure 6.

2.2.2. 1C Moisture-Cured Polyurethanes

These polyurethanes generally consist of low molecular weight polyol resins and in their hardening mechanism, moisture converts some of the isocyanate groups to amino groups. Carbon dioxide is released as a byproduct. The amino groups become reactive components for the remaining isocyanates. The released carbon dioxide can cause foaming problems in thick applications. After curing, they form elastomer plastics. Curing occurs through an addition reaction. The curing is anisotropic. An example of an anisotropic curing curve is given in Figure 7. Since it cures with moisture, a minimum of 40% relative humidity is required for curing to be complete. An example curing mechanism is given in Figure 8. A comparison of these different types of polyurethanes is given in Table 1.

2.3. Use of Polyurethanes in the Automotive Sector

2.3.1. Plastic Parts

Polyurethanes are used quite frequently in the automotive sector. Polyurethanes have many advantages such as comfort, processability, and energy conservation. Polyurethane foams are used in seats, armrests, side mirror arms, and other parts requiring high fatigue resistance. They are frequently used for their high strength, light weight, heat and sound insulation properties. These types of polymers are generally paintable and are heat-cured polyurethanes produced in injection molds with high strength, known as PURRIM.

2.3.2. Paints

In many exterior coating systems, polyurethanes consisting of hydroxyl groups for acrylic resin and isocyanate hardeners are used. Polyurethane resins combined with acrylic resin are frequently used in automotive paints due to their high UV resistance and durability under weathering conditions. Paints containing these types of resins are also used in clear coat systems on vehicle exteriors because they provide high gloss and image clarity for aesthetic purposes. High resistance to moisture and contamination contributes positively to this application. These types of polyurethanes, which provide excellent adhesion to many surfaces due to surface tension, are recommended to have epoxy resin-based primer coatings for pre-treatment on metal surfaces before application for enhanced corrosion resistance. These paints form the uppermost layer of the paint system and; • In addition to aesthetic properties, provide protective characteristics against external factors, • Applicability and adhesion over primer or intermediate coats, • Color, gloss, hardness, flexibility, scratch resistance, resistance to sunlight are other important properties. These paints are generally 2C Polyurethane systems as described in Section 2.

2.3.3. Adhesives

Polyurethanes form an important part of adhesives widely used in the automotive sector. They are preferred for their high strength, elasticity, and adhesive properties on different surfaces (rubber, glass, wood, ABS, electronic components, etc.). These adhesive systems are generally 1C moisture-cured or 2C polyurethane adhesives. The UV resistance properties of these adhesives can be modified with carbon fillers. Additionally, polyurethanes are useful in the production of new applications developed from discarded and recycled materials. For example, due to polyurethane's adhesive properties, end-of-life vehicle tires can be transformed into surfaces for children's playgrounds, sports courses, or sports stadiums. These types of recycling developments help protect the natural resources of the world. The steel industry uses diisocyanates as the basis for binders in making molds for casting. From the adhesive properties of polyurethane, benefits are also gained in the production of high-performance composite products in the automotive sector. Composite wood products made from sustainable forest resources are a genuine alternative to panel products produced from large mature trees, which take years to grow. This application ensures that more trees are planted than are harvested and helps reduce deforestation by using fast-growing young trees that absorb more carbon dioxide than mature trees.

2.3.4. Corrosion Protection Applications

In the automotive sector, polyurethanes are applied in corrosion protection applications in sealing operations called "mastic" in the market to block air contact on open surfaces after welding. This process is frequently used in welding areas that are not fully welded, examples of which are given below. These sealing products are adhesives and are generally 1C moisture-cured polyurethanes. • Seam welds, • Spot welds, • Butt welds. These types of polyurethanes are also preferred because they provide high sound and heat insulation. The sound and heat transmission coefficients of these materials are at approximately the attached values. • Heat transmission value λ for 2 mm thickness: 0.4 W / (m*K) • Sound transmission 40 dB (Engine Compartment) – 5 mm application Polyurethanes used in these applications are also paintable and can be applied as stone strike protection on painted cover edges. When applied in suitable thickness for underbody protection, these systems provide 10 years of corrosion resistance to the vehicle. Hande Kılıç Senior Paints and Corrosion Expert MAN Türkiye A.Ş.