Coatings, sealing and bonding is known to man since a long period of time. From the natural resins, oils and fats used in the
‘mummification’ of human bodies to the most recent times, sealants, coating and bonding is an interesting developing field of sciences. As the development of synthetic resins started around the 20th century, the production of coatings and adhesives vastly improved. A German industrial chemist, Otto Bayer discovered poly-addition process for the synthesis of polyurethanes from polyisocyanate and polyol. Polyurethane is a polymer composed of organic components joined by carbamate (urethane) links. This versatile material can be found in liquid coatings and paints, tough elastomers such as roller blade wheels, rigid insulation, soft flexible foam, elastic fibre or as an integral skin. Polyurethane raw materials are used in the manufacture of foams, industrial rigid foams for insulation, construction industry, energy sector as sealants, energy absorbing components in automobile interiors, packing industry and some application also reach out to the health sector [21]. The major use of polyurethane is in the automobile industry as an adhesive and sealant. The broad range of applications for polyurethanes also include wood finishing, corrosion resistance and construction, textile coatings and many more.
Brockmann defined polyurethanes as the polymers produced by addition reactions between polyisocyanates (di-functional or higher) and hydroxyl-rich-ethers [1]. As of today, polyurethane adhesives are available in solvent based moisture adhesives, thermoplastic hot melts, thermo-setting systems and emulsions.
As there is a wide diversity of mechanical properties of the polyurethane adhesives, they have a broad spectrum of applications.
4.1 Chemistry and structure
Polyurethanes are formed by reacting a polyol (alcohol with hydroxyl groups) with diisocyanates or polymeric isocyanates in the presence of suitable catalysts and additives. Polyurethane adhesives have a distinctive feature of polymer structure with hard and soft segments. Soft segments are formed by long-chain polyester polyols, which has a low glass transition temperature, whereas the hard segments are created by the cross-linking of
diisocyanates with short-chain diols or diamines. These hard segments of the adhesive have higher glass transition temperature values (see figure 6). Due to the different glass transition temperature ranges, polyurethane adhesives can be generated in two different ranges [1]. Generally, the glass transition temperature of polyurethanes is higher than that of silicones, but lower than that of highly cross-linked, structural epoxy resin adhesives, hence the mechanical properties can be compared to those of the epoxy resin adhesive systems.
Polyurethanes are in the class of compounds called as the
‘reaction polymers’, which include epoxies, unsaturated polyesters and phenolics. The raw materials for polyurethane adhesives are the isocyanates, both aromatic and aliphatic isocyanates are used for the synthesis of polyurethanes. Aromatic isocyanates such as toluene diisocyanates (TDI), methyl-di-phenyl-isocyanates (MDI) are used for the synthesis of polyurethane adhesives. Commonly
Figure 6 Soft and hard segments of polyurethane adhesive [1]
used aliphatic isocyanates are hexa-methylene diisocyanate, isophorone diisocyanate, tri-methyl hexa-methylene diisocyanate [1] (see figure 7 for types of isocyanates).
Figure 7 Different configurations of isocyanates [1]
The properties of polyurethane adhesives are highly influenced by the type of isocyanate and polyol used for the synthesis.
Isocyanates react with other substances by addition or poly-condensation and this reaction is also based on the electrophilic character of the carbon atom in the double bond system. Another group of polyisocyanates commonly used in the generation of polyurethane adhesives are blocked isocyanates. This is an isocyanate which has been reacted with mono-functional alcohols or amines, to prevent its reaction at room temperature [22]. The commonly used means of generating polyurethanes is the reaction of diisocyanates or polyisocyanates with primary mono-alcohols, di-alcohols and poly-alcohols. Polyols of ether and ester origins are widely employed in the synthesis. Polyether polyols are produced
they are resistant to alkaline hydrolysis. Polyester polyols have a higher tensile strength and a higher resistance to heat [7].
In the presence of moisture, a two-stage reaction takes place during the synthesis, firstly an unstable carbamic acid is formed, in the second step this carbamic acid splits off carbon dioxide to give an amine. This newly formed amine reacts further with isocyanate groups to give a urea group. If the reaction takes place in room temperature, the urea group formed in turn reacts with free isocyanates, cross-linking the polymer chains to form biurets [7].
4.2 Classification of Polyurethane adhesives
Classification of polyurethane adhesives can be done based on different parameters and characteristics of the adhesives. Each of these systems are mainly based on the type of curing or the components used in the adhesives. But keeping the work of this thesis at view, polyurethane adhesives will be classified based on the components used in the adhesives and the mechanism of curing.
4.2.1 One Component Polyurethane adhesives
A group of polyurethanes called as the ‘Isocyanate-terminated polyurethanes’, which are the pre-polymers when polyol reacts with an excess of polyisocyanates, cure in atmospheric pressure and ambient moisture to form biurets or poly-urea by cross-linking [1], [22], [23]. One component reactive system uses two constituents which will form a polymeric system on the substrate [24]. Hydroxyl polyurethanes are a group of thermoplastic polyurethanes with a hydroxyl group content of 0.5-1.0%, which are produced by the reaction of MDI (diisocyanate) with polyester diols. The crystallinity of the individual hydroxyl polyurethanes determines the curing time of the adhesive [22]. Moisture-curable thermoplastic polyurethane hot-melts are formed by the mixture of hydroxyl polyurethanes with polyisocyanates. At room
temperature, due to their cross linking, the heat resistivity of the polyisocyanates or pre-polymers (compared to those of one component polyurethanes) cured with low-molecular-weight polyols or poly-amines [1]. They are used in the automobile industry in bonding metals to plastics, textile foam coatings and in the wood furniture industry.
4.2.3 Moisture Curable Polyurethane Adhesives
In this thesis, the used adhesive is a moisture curable one component polyurethane adhesive. The curing principle is described by the reaction of isocyanate groups with water.
Polyisocyanate and polyol combinations with an excess of isocyanate groups crosslink at atmospheric moisture to give an insoluble higher molecular weight polyurethane/poly-urea [22]. The properties of moisture-curable one component system are governed by the type of
polyisocyanate used for the synthesis. The drying time depends on the temperature and the amount of atmospheric moisture present during the process of curing (see figure 8 for chemical reaction).
Figure 8 Formation of moisture curable one component polyurethane adhesive
Comparison of One-component and Two-component polyurethane adhesives [22] (see table 1).
Table 1 Comparison of one component and two component polyurethane adhesive
One-component Two-component
Chemistry is limited to room temperatures
Chemistry can be defined even at higher temperatures
Very long open times Limited open time, variable from seconds to hours No mixing needed Must be meter mixed Simple dispensing equipment
4.3 Processing of One component adhesive systems
Processing any polyurethane adhesive material requires equipment to store, meter, heat, mix and dispense the chemicals such as the polyol or polyether or polyesters and diisocyanates or polyisocyanates [24]. All adhesives undergo a transition from the state of liquid to solid, during the creation of an adhesive joint with the exception of pressure-sensitive adhesives (see chapter 3.1.1 Pressure sensitive adhesives). The chemical components are mixed in the mixing head, which results in the formation of polyurethane. Depending on the application of the adhesive, the mixing head can vary from a high-pressure or low-pressure mix head to apply the adhesive [24] (see figure 9). Viscosity is a
rheological parameter that has an obligation to bring the adherents in close proximity, and to allow the adhesive forces to originate [1].
Particularly, thermoplastic moisture curable one component polyurethane adhesives are widely used in the automotive and construction industry. For bonding of smaller component in medical and electronic application, UV curing is used for one component adhesives. The specific interaction between the photo initiators and high energy UV radiation allows a very rapid curing process [1]. Use of robots for the application of adhesives and curing using UV radiations are used in industries to increase productivity (see figure 10).
Figure 9 (Left) High pressure mixing head; (Right) Low pressure mixing head for application of adhesive
Figure 10 (Left) Flame treatment using robots; (Right) application of adhesive using robots
4.4 Field of Application
As the market today is competitive, it is preferred to use complex structures made of different layered materials to achieve the desired combination of properties for respective applications. It is found that one effective way to make strong and durable lamination is through the process of adhesive bonding. Adhesive bonding has its benefits when compared to other processes of joining such as welding or soldering; when these other processes are used for the joining process, the surfaces are permanently changed by thermal stresses whereas, using adhesives the strains are dissipated over the whole surface of the substrate without creating concentrated stress point. Also by using mechanical interlocking systems such as riveting, nailing, sewing or screwing methods, the surfaces are weakened which might affect the application of these materials [22]. Choosing the best adhesive based on the application, one of the important factor to be considered is the type of loads (static, cyclic, impact or dynamic loads) applied later on the joint.
Applications of polyurethane adhesives are very vast and these adhesives are used to bond many different material types including ceramics, metals, glass, plastics and composites. For this thesis, usage of moisture curable one component polyurethane adhesive in the automobile industry is in view and in this chapter, topics related to application of polyurethane adhesive in the automotive industry are described.
Direct Glazing is a process followed in the automotive industry; it constitutes of fitting fixed glass panes with the use of polyurethane adhesives. One component moisture curing products are used as they create an elastic joint between a painted body and the glass;
also, they perform the important function of coping with vibrations and movement. One side of the glass is pre-heated with an adhesion promoting glass primer (discussed in chapter 2.6) to enhance the process of adhesion. These glass primers contain
silanes and are efficient when applied as a thin coat [1]. The curing time of the moisture cured one component polyurethane adhesive is determined by the diffusion of water vapour, if the humidity is low during the process, it can delay the process of curing. The cured adhesive in this process will have a glass transition temperature lower than -40ᵒC, so the mechanical properties remain unchanged within the whole range of operating temperatures.