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Ind. Eng. Chem. Res. 2008, 47, 7524–7532
Current Research and Development Status and Prospect of Hot-Melt Adhesives: A Review Wen Li, Laziz Bouzidi, and Suresh S. Narine* Alberta Lipid Utilization Program, Department of Agricultural Food and Nutritional Science, 4-10 Agriculture/Forestry Centre, UniVersity of Alberta, Edmonton, Alberta T6G 2P5, Canada
Fundamental characteristics, general physical and mechanical behavior, and the recent developments in the knowledge of hot-melt adhesives (HMAs) are introduced. The current research and development status for HMAs is reviewed, with an emphasis on the development of new types/generations of HMAs. In particular, some crucial issues and challenges on the technological improvements and the materials development are discussed. On the basis of the current predicted shortage of energy resources and environmental concerns, prospective research on the development of green HMAs is suggested. 1. Introduction Hot-melt adhesives (HMAs) have been in use since the 1950s.1-5 HMAs are solvent-free thermoplastic solid materials which are characteristically solid at low temperatures (generally below 82 °C), are low-viscosity fluids at high temperatures (generally above 82 °C), and rapidly set upon cooling, as illustrated in Figure 1.4 Note here that the open-time is the maximum amount of time after applying the adhesive that one can wait before applying the second substrate and still make an acceptable bond. Today, HMAs are used in a variety of manufacturing processes including packaging, bookbinding, product assembly, and box and carton heat-sealing as well as other pressure-sensitive applications such as disposable products, stamps, and envelopes.6-11 A typical HMA is formulated with four main components: polymer (about 33%), tackifier/resin (about 33%), wax (about 32%), and antioxidant (about 1%). The main physical properties and function of the four components are summarized in Table 1.1,4,7 In comparison with other adhesives, e.g., the conventional solvent-based adhesives, HMAs have shown a variety of excellent mechanical properties and physical functionalities. They form a strong bond quickly, simply by cooling, are compatible with most materials, and are clean and easy to handle. In general, HMAs are not sensitive to water. In particular, their bonding strength can hardly be affected by water, moisture, or humidity, although if HMAs are applied to a damp or wet surface, their bonding performance may become poor with time. HMAs can be formulated to increase their water sensitivity, as when used for stamps, envelopes, and paper products that are to be recycled.1,3 Furthermore, the HMA technology during processing and use also exhibits numerous unique advantages compared to other adhesives. The primary advantages include the reduced cost of the adhesives, removal of volatile organic compounds (VOCs) emissions from the process, elimination of explosion risk of solvent-based adhesives, the removal of dryers in the production line, and simple operation for various substrates or situations.1,4-8 Since HMAs are used to adhere the same or two different substrates, the most important attention should be paid to the adhesive or bonding property at interfacial regions where adhesive coatings or films locate. The adhesion performance of * Author to whom correspondence should be addressed. Tel.: +1 780 492 9081. Fax: +1 780 492 8855. E-mail: Suresh.narine@ ualberta.ca.
HMAs has been intensively studied, but few studies have focused on the strength issue under the various conditions to which HMAs are subjected during production, transportation, and service. Tora et al.12 recently evaluated the shear, tensile, and bending strength of different joint types for different HMAs. With the use of statistical selection models, Neter et al.13 showed that the optimal combination of factors for the bonding strength is joint thickness, adhesive type (e.g., ethylene vinyl acetatebased), and temperature conditions (e.g., ambient temperature). Beside adhesive property or bonding strength, other parameters related to the adhering systems are also important for the evaluation of the comprehensive performance of an HMA. For the same HMA, the adhesion performance of an adhering system depends strongly on the materials property of substrates as
Figure 1. Illustration of temperature vs time curve of a typical HMA (ref4). Table 1. Primary Ingredients of a Typical HMAa ingredient
physical propertiesb
polymer
high MW (>10 000) Tg usually < RT physically cross-links on cooling strong low MW ( RT
resin
diluent
wax
a
amorphous low MW (
