Ask Dr. Dave
Answer: Many traditional applications that used solvent-based adhesives have been converted to water-based alternatives, mainly in so-called low- and medium-performance applications (typically, simple applications involving porous substrates like paper and wood). However, some waterborne systems that pass boil-in-bag and hot-fill applications enable extension into high-performance applications.
You obviously also have the alternative of 100%-solids liquid adhesives (e.g., two-component polyurethanes) or hot melts that do not require drying and can also bond non-porous substrates such as films and foils. The cost of these alternatives tends to be higher, but performance is also very high. However, it is worth bearing in mind that an important trend will be packaging away from plastic and back to the future of non-toxic, degradable and/or recyclable packaging materials such as glass and paper with degradable non-toxic adhesives, where waterborne adhesives will play a major role in the future.
Question: What alternatives to fumed silica are available for thickening adhesives?
Answer: Fumed silicas are used as thixotropes in many adhesive and sealant systems mainly because they introduce thixotropy in the systems. Fumed silicas are available in both hydrophilic (often called “untreated”) and hydrophobic (“treated”) versions. The untreated versions have traditionally been used successfully in many products, but they do have some deficiencies in some systems (particularly in polar systems), including their inability to maintain stable viscosity on aging. The treated versions give much more stable systems, but are considerably more expensive.
Several alternatives to fumed silicas are available, including castor oil derivatives, bentone clays and fibers. Fibers are interesting thixotropes because, rather than relying on chemical interactions, they build viscosity through the physical entanglement of fibers and fibrils. Under shear, these fibers probably align and the material becomes lower in viscosity. Two common fibers that are used as thixotropes are aramid fibers and engineered cellulose fibers. Unfortunately, there is no alternative to extensive testing of these thixotropes. You will need to test them in your product and control the amount and time of shearing during mixing. Also be sure to measure the effects on thixotropy, immediately after both mixing and shelf aging.