Sometimes manufacturers request a particular hot melt color or clarity. Where does the color come from? Why are some adhesives more transparent than others? Does color and clarity mean quality? Why do customers like light colored and transparent adhesives?
All materials in this universe, when their electrons are excited by certain energy, may emit a wavelength showing different colors. These colors are observed by human eyes or detected by instruments. Most polymeric materials containing more polar functional groups and /or double or triple bonds, tend to generate colorful bodies. As a result, when a hot melt formulation contains more polar ingredients, such as non-hydrogenated hydrocarbons resins and natural rosin and its derivatives, they are normally darker than those hydrogenated ingredients.
Disregarding color, if the ingredients used in a hot melt formulation are very compatible, having similar solubility parameters, the formulated product will be clear or transparent. On the other hand, for a less compatible and incompatible formulated adhesive, the appearance is cloudy or opaque.
Clarity may not be a big concern for many applications. However, the heat stability for those cloudy and opaque formulations can be relatively poor when compared to some clear formulations. Incompatible ingredients in the formulation tend to separate away from each other upon heating at elevated temperatures due to their widely different solubility parameters. Note that the above mentioned cloudy and opaque appearance does not apply to those materials with crystalline such as wax and ethylene segment of Ethylene Vinyl Acetates (EVA).
Many users, particularly in the Asian countries, often ask for water white and clear HMPSAs for most applications simply due to esthetics reason. Consequently, water white, hydrogenated tackifying resins are needed for those formulations. All selected ingredients must be compatible as well. In the principle of adhesion, a more polar adhesive normally generates better adhesion to substrates due to its higher surface energy. In other words, the selection of more expensive water white ingredients for esthetic purpose may adversely hurt adhesive performance. To compensate the loss of portion intrinsic adhesion resulted from lower surface energy; adhesive’s rheology is then becoming an important factor affecting adhesion performances. The rheology or mechanical property of adhesives will be discussed on a separated article.
Indeed, besides the esthetic reason, clear adhesives exhibit much better heat stability and pot life when they are heated at elevated temperatures for a long period of time. This is very important particularly for the process using an open coating system, such as a roll coating line.