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.
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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