Advances of Hot Melt Adhesive
Conventional hot melt adhesive (HMA) and Hot Melt Pressure Sensitive Adhesive (HMPSA) are 100% solid thermoplastic materials. They do not contain any volatile organic compounds. Therefore, they are safe during production, transportation, storage, and application. Most importantly, they are environmentally friendly and are not harmful to human health. Because of the fast set feature, hot melt adhesive is the best candidate among all types of adhesive for high speed production lines. They have been widely used in various industries, such as packaging, bookbinding, wood working, DIY, tapes and labels, hygiene, automobile, medical, electronic, construction, etc.
Although conventional hot melt adhesive and hot melt pressure sensitive adhesive have several advantages, they can not replace cross-linkable solvent and water borne adhesives in some application markets. Hot melt adhesive and hot melt pressure sensitive adhesive have two major shortcomings. First, they are thermoplastic compounds, which will melt or creep when the surrounded temperature is close to their flow point, typically less than 200℃. Therefore, they are not suitable for high temperature applications.
Secondly, almost all hot melt adhesive and hot melt pressure sensitive adhesive formulations contain portions of low molecular weight tackifying resins, which are organic solvent or plasticizer soluble ingredients. A typical case is the bonding of hot melt adhesive or hot melt pressure sensitive adhesive onto Poly-Vinyl-Chloride (PVC). Almost all hot melt adhesive and hot melt pressure sensitive adhesive fail to bond onto heavily plasticized PVC objects. PVC alone is a rigid material, which is very easy to bond by any adhesive due to its high surface energy. However, when a hot melt adhesive or a hot melt pressure sensitive adhesive is used to bond onto plasticized PVCs, it will be gradually softened by the plasticizer-phthalates. The adhesive composition and adhesion performance are therefore gradually changed the passing of time.
How to improve the heat and plasticizer resistances of hot melt adhesive and hot melt pressure sensitive adhesive? Well, instead of constantly improving the heat and plasticizer resistances for those conventional hot melt adhesive and hot melt pressure sensitive adhesive, many new technologies have been introduced to the adhesive industry in the recent years. Two major successful technologies are radiation and moisture curable hot melt adhesives. Once the adhesives are cured or cross-linked, their molecular weights become infinite. They will therefore offer high temperature and plasticizer resistance.
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