Ask Dr. Dave

Ask Dr. Dave

September 1, 2009
/ Print / Reprints /
ShareMore
/ Text Size+
Adhesives and sealants troubleshooting tips from Dr. Dave Dunn.

Question: What are the best thixotropes for adhesives and sealants, and why are they used?

Answer: Thixotropes are widely used in adhesive, sealant, and coating systems. Thixotropy is a rheological term for time-dependent shear thinning, which in practical terms means that the products can be sheared using pumps or other dispensers to lower their viscosity for application; however, they rapidly restructure to products that will not sag or slump on vertical surfaces. Fumed silicas have become the benchmark against which any alternatives are measured, and they are essential components in some products, particularly silicone sealants, where they act as the primary reinforcement. Fumed silicas are available in both hydrophilic (often called “untreated”) and hydrophobic (“treated”) versions. The untreated versions have been successfully used in a number of products for many years but demonstrate deficiencies in some systems, notably in their inability to maintain stable viscosity on aging. The treated versions provide much more stable systems but are somewhat more expensive.

Alternatives to fumed silicas include castor oil derivatives, clays and fibers. Asbestos is a formerly popular fiber-type thixotrope that is no longer used for health and safety reasons.  Fibers are interesting thixotropes because they build viscosity through the physical entanglement of fibers and fibrils rather than relying on chemical interactions. Under shear, these fibers align and the material becomes lower in viscosity. Aramid fibers and engineered cellulose fibers are two examples of modern fibers used as thixotropes. 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. You’ll also need to ensure that you measure the effects on thixotropy, both immediately after mixing and after shelf aging. Be careful when measuring viscosity as a function of shelf life; I have seen instances where a product appears to be in specification when packaged in a tube but cannot be dispersed several months later due to structuring within the tube.

Links

Did you enjoy this article? Click here to subscribe to Adhesives & Sealants Industry Magazine.

Recent Articles by Dave Dunn

You must login or register in order to post a comment.

Multimedia

Videos

Image Galleries

ASI April 2014 Photo Gallery

Our April 2014 issue is now available!

Podcasts

ExxonMobil Tackifier Expansion

Dwight Tozer, vice president of ExxonMobil’s Adhesion Industry business, discusses the company’s latest tackifier expansion project with Editor-in-Chief Susan Sutton.

More Podcasts

Adhesives & Sealants Industry Magazine

ASI August 2014 cover

2014 August

Our August issue includes what is traditionally our most popular article of the year: the ASI Top 25. You won't want to miss it!

Table Of Contents Subscribe

THE ADHESIVES STORE

handbook-sealant-tech.gif
Handbook of Sealant Technology

The Handbook of Sealant Technology provides an in-depth examination of sealants, reviewing their historical developments and fundamentals, adhesion theories and properties, and today’s wide range of applications.

More Products

ASI 2014 Buyers GuideASI's Buyers' Guide

Annual purchasing resource for equipment used in the manufacture/formulation of adhesives, sealants, pressure sensitives, tapes and labels and for application of finished adhesives. 

Clear Seas Research

With access to over one million professionals and more than 60 industry-specific publications, Clear Seas Research offers relevant insights from those who know your industry best. Let us customize a market research solution that exceeds your marketing goals.

STAY CONNECTED

facebook_40px twitter_40  youtube_40pxlinkedin_40 google+ icon ASI 30px