Water-Based Vs. Solvent-Based Adhesives

SpecialChem | Edward Petrie - Feb 3, 2003

Questions often appear in the Specialchem4adhesives TechDirects regarding the relative merits of solvent and water based adhesives. In this editorial, we will look at the properties and processing characteristics of each and discuss the drivers that cause formulators to consider alternative technologies.


Organic solvents are used as adhesive carrier fluids and diluents, as well as for surface preparation and cleanup. Over the last decade, environmental and workplace safety regulations on solvents have become increasing stringent. Both the formulator and end-user drive most of the efforts towards environmentally friendly systems. Solvents are a factor in formation of ground level ozone as a result of volatile organic compound (VOC) emissions that take place during formulation, application, drying, and curing. VOC emissions are subject to permitting and enforcement in order to reduce urban "smog".

In the past several decades, several alternatives to solvent-based adhesive systems have been developed. The driving factors for this development were (1) environmental and work place safety, as described above, and (2) the oil crisis in the 1970s which threatened to significantly cause a rise in solvent prices. These solvent-alternative adhesives include water-based systems, radiation (UV/EB) cured adhesives, and hot melts. They supplement the 100% solids, liquid systems (e.g., polyurethanes, epoxies, acrylics) in the adhesive end-users arsenal. Water based adhesives have at times been proposed and many specific development efforts initiated to replace solvent based adhesives in certain applications. The comparison between water and solvent-based systems is the prime subject of this editorial.

Although some of these new technologies have provided replacements for solvent-based adhesives, the conversion of non-solvent alternatives has slowed down considerably over the decades. This slowdown can be attributed to somewhat disappointing performance of the replacing products and the comfort zone that the formulators and end-users have around current technologies.

There are many water-based products on the market, and many have gained great success as contact and pressure sensitive adhesives and as latex sealants. However, they generally are not direct replacements for solvent-based adhesives. Water-based adhesives are slower drying than solvent-based adhesives because of the water's higher heat retention capacity. They require about three times more heat to dry and need more time to achieve steady-state performance during production runs. The water vapors in drying tunnels and ovens have also been noticed to cause corrosion and maintenance problems. Converting to water-based adhesive may require oven retrofits and installation of corrosion proof storage, mixing, and piping systems.

With regard to physical properties, the water-based adhesives and sealants generally do not provide the shear or peel strength that solvent-based systems provide. Once cured, the water-based adhesives usually do not have the moisture resistance of solvent-based adhesives. However, water-based adhesives can withstand wide temperature ranges.

Water-based adhesives greatly reduce VOC emissions and may eliminate associated permit and control costs. Explosion risks are eliminated; hazardous waste generation and associated management and disposal costs are reduced. However, there may be increased cost required because of wastewater treatment.

The purchase price of water-based adhesives is generally 15-20% less than solvent based adhesives. Since water-based adhesives generally have the same amount of solids as solvent-based adhesives, comparison based entirely on the price of each type of adhesive is valid. A study has shown that the capital costs for water-based adhesives are about 8% less than conventional solvent-based adhesives systems, even when costs of air emissions controls are excluded.[1] The additional requirement of emission control and explosion-proof equipment systems makes solvent based adhesive systems much more expensive.

Manufacturers considering a switch to water-based adhesives must take into account not only the cost of capital, but also operating costs. The higher operational costs associated with water-based adhesives include production costs (such as maintenance and corrosion protection) and energy costs. However, there also may be a reduction in insurance costs associated with using a water-based adhesive.

With all factors considered including capital equipment costs and environmental and health benefits, significant cost savings may be achieved by implementing water-based adhesive technologies. However, performance requirements (strength, durability, and manufacturing efficiency) must be considered for the specific application. There is seldom a one-for-one replacement.

Should you have any comments or feedback, please contact me.

Edward Petrie.

References:
1. McMinn, B.W., et. al., "Solvent-Based to Water-Based Adhesive Coated Substrate Retrofit, Volume I: Comparative Analysis", EPA-600/R-95-011a, National Risk Management Research Laboratory, Research Triangle Park, NC, April 1996.

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