Selection of Thickeners for Waterborne Adhesives

Introduction

The flow behavior of a waterborne adhesive is an important processing parameter in many applications. Adhesives that must be applied as coatings (e.g., pressure sensitive adhesives, laminating adhesives) or adhesives that must not flow or sag before cure (e.g., auto adhesives, sealants applied to vertical surfaces) are applications that use thickeners in the formulation.

Thickeners are additives that are often used to adjust the rheology of most waterborne adhesives. Aside from viscosity modification, thickeners provide multiple functions including suspension of formulation ingredients, improved coating characteristics, improved stability, moisture retention, etc. Thickeners and plasticizers together account for the largest dollar value of additives that go into adhesives. They represent 55% of the total additive market.

There are many different types of thickeners and their use will depend on the function to be preformed, the base polymer, other additives in the system, and many other factors. Thickeners come from both natural and synthetic sources. And rheology modification occurs with a combination of differing mechanisms.

This article will review how various thickeners work and their value to the industry. The different types of thickeners that are available for waterborne adhesives will be identified, and the selection process is discussed to assist the formulator in choosing the correct thickener for his or her formulation.

Types of Thickeners

Natural thickeners include cellulosic materials and naturally occurring polymer comprised of polysaccharide or amino acid building blocks. Examples include starch, cellulose, alginate, agar, collagen, gelatin, guar gum, pectin and xanthan gum. The most common is modified cellulose, usually hydroxy ethyl cellulose (HEC). These naturally occurring thickeners are generally water soluble. Table 1 provides a summary of common natural thickeners.

Synthetic thickeners are polymers that can be grouped into one of three classes:

• ASEs - Alkali-swellable or soluble acrylic emulsions,
• HASEs - hydrophobically modified alkali-swellable acrylic emulsions, and
• HEURs - hydrophobically modified, ethoxylated urethane resins.

Several companies produce these viscosity thickeners including Elementis (Rheolate series) Ashland (Jaypol series) and R.T. Vanderbilt.

Cellulosic Thickeners

Cellulosic thickeners can be used to thicken and stabilize waterborne adhesives. With a range of molecular weights and thickening efficiencies, hydroxyl ethyl cellulose (HEC) polymers help control flow during formulation and coating. They resist sagging once the adhesive is applied. In addition, the products’ non-ionic nature provides wide formulation latitude and offers good heat and shelf-life stability. When choosing one of the HEC products, the formulator generally must consider initial viscosity (as these products come in grades that vary from 2 to 50,000 cps at several percent concentration in water) solubility time, and thickening efficiency. The higher viscosity products generally find use in caulk and sealant formulations.

Hydroxy ethyl cellulose is a nonionic polymer made by swelling cellulose with NaOH and reacting with ethylene oxide. HEC is soluble in hot or cold water and gives clear, colorles