In earlier years, customers did not have any odor requirements for hot melt glue. However, in recent years, customers have been demanding less odor from the adhesive, with increasingly stricter requirements, even to the point of being harsh. This article clarifies the nature of odor, explains methods for evaluating odor, and expresses views on odors for reference by those in the hot melt glue industry and users of hot melt glue.
Subject to influences from raw materials, production processes, and distribution channels. hot melt glues are made of components including essential resins, tackifying resins, diluents or softening oils, and additives. The inherent odor of the raw materials is an important source of the odor of hot melt glue. For example, most EVA has a faint acetate odor, and synthetic rubber has its own odor, which varies only by intensity. Tackifying resins containing rosin have a pine odor, while C9C5 resins have their own distinctive odors. Even fully hydrogenated resins have their own odors, although they may not be detectable by the human nose. Other raw materials also have odors, which vary in intensity and strength. During production processes, hot melt glue may also acquire new sources of odor, but odor reduction can be achieved by controlling the production process. During logistics and transport, hot melt glue may also be susceptible to being tainted by new odors.
The evaluation of hot melt glue odor is done using instrumental testing methods and olfactory testing methods. Instrumental testing is purely physical, measuring the amount of volatile organic compounds, or VOCs, for example. However, it has some limitations. VOC contents may meet standards, but the odor may not meet customer requirements, and there is no necessary relationship between volatile gases and flavors. This article focuses on olfactory testing methods. Currently, there is no national standard for the odor of hot melt glue, but there is a standard for the measurement of adhesive odors, which is suitable for solvent adhesives but not hot melt glues because customers require a higher level of odor control for the latter. Different hot melt glue companies and users have different standards for evaluating adhesive odors.
Whether using a six-level or four-level standard, odor evaluation requires at least three members on the evaluation panel, each of them having received professional training in odors. Different companies have different odor testing standards. The best approach is to simulate the actual application of hot melt glue in terms of odor requirements, for example, by placing the same amount of the same type of hot melt glue in different sealed containers of the same size, heating them to a certain temperature for a certain time, then allowing them to cool to room temperature, opening the lids, and allowing the evaluation team to sniff and evaluate, recording their individual evaluation values. This process is repeated three to five times to obtain the average value, which is the final evaluation result.
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