Formulating Adhesives and Sealants with VOC Restrictions

TAGS:  Solvent borne Adhesives      Science-based Formulation    

Growing concern over volatile organic compounds (VOC) is motivating adhesive and sealant formulators to:

• Capture and recycle solvents
• Reduce solvent use, or
• Switch to solvents with better environmental profiles.

Even in “water-based” adhesives and sealants, solvents are sometimes used in conjunction with water to dissolve or suspend components or to provide a more efficient film-forming function. For example, many water reducible coatings contain small amounts of a “co-solvent” such as a glycol ether or alcohol to aid in coating and handling.

Volatile organic compounds or VOCs are organic chemicals that have a high vapor pressure at ordinary room temperature conditions. The definition of VOC from a regulatory standpoint depends somewhat on the region and application.

• In the US, VOCs are legally defined in the various laws and codes under which they are regulated, and different regulations exist in each state. Definitions now include hazardous air pollutants (HAPs) and VOC-exempt solvents.
• In the EU, VOC is any organic compound having an initial boiling point less than or equal to 250°C and that can damage human senses.

VOCs are necessary components in many types of adhesive systems primarily used to adjust viscosity and provide good coating properties.

Regulations limiting VOCs are getting stricter. These are primarily enforced through:

• The Environmental Protection Agency (EPA) in the US.
• The Registration, Evaluation and Authorization of Chemicals (REACH) in the EU.

These regulations are designed to limit the use of organic chemical compounds that are harmful to human health and contribute significantly to carbon emission and atmospheric ozone depletion.

The following steps are currently being taken towards the reduction of VOC emissions in the development of new formulations and the reformulation of existing adhesives and sealants.

1. Minimizing conventional solvent content
2. Use of alternative solvents
3. Use of an alternative technology

Let’s review the solvents used in the formulation process of adhesives and sealants and the actions that the formulator must take to implement the above strategies in detail. Some solvents used in cleaning, substrate preparation, and other auxiliary processes have been discussed here.^{[1, 2]}

Solvents and the Need for Reducing VOCs

In the adhesives and sealants industry industrial solvents are vital. They are used as:

• Component ingredients in formulated products, or
• Processing aids in manufacturing (e.g., cleaning fluids).

Different applications require specific solvating or other properties, and different solvents can be often blended to achieve the specific properties required for an application.

The classical definition of a solvent is that it is a liquid with the ability to dissolve, suspend, or extract other materials without causing chemical change to the material or to the solvent. Solvents operate on the principle of “like dissolves like”. Therefore, for a solvent to work, it needs to have similar chemical characteristics to the substance that it is trying to dissolve.

Other factors that are important in selecting a solvent or solvent blend are listed below.

Performance Properties	Non-performance Properties
Dissolving (solvating) power Viscosity Evaporation rate Color Odor Toxicity Flammability	Raw material cost Disposal cost and cost to reduce emission levels Conservation of resources and availability Hazardous emissions (types and levels) Health and safety factors

Important Factors in Selecting a Solvent

There are many kinds of solvents with different physical and chemical properties. Organic solvents can be classified by chemical structure. There are three main classification as shown in the table below.

Oxygenated and hydrocarbon solvents represent most of the total organic solvents used. Halogenated solvents have been more consigned to niche applications because of their cost and notoriety as a “green-house gas”.

Type	Examples
Oxygenated solvents	Alcohols, glycol ethers, ketones, esters, and glycol ether esters
Hydrocarbon solvents	Aliphatic and aromatic hydrocarbons
Halogenated solvents	Chlorinated, fluorinated, and brominated hydrocarbons

Classification of Common Industrial Organic Solvents

The primary applications for solvents in the adhesives industry are for cleaning and formulating. As one might expect, the evolution of environmentally safe solvents has been faster in the cleaning area than in the formulating area.

The emission of any volatile organic compound to the atmosphere is undesirable for several reasons.

1. They contribute to the formation of photochemical smog. 
2.  Many are implicated in aggravation of lung diseases, such as asthma.
3. They absorb infra-red radiation and, therefore, act as greenhouse gases.

For these reasons, there is an increasing move away from using organic solvents, even at very low concentrations in both adhesive formulations and in prebonding treatments, such as substrate cleaning, surface preparation, and primers.

Solvent	Pounds of Ozone Produced per   Pound of Solvent Emitted
Methyl acetate	0.1
p-chlorobenzotrifluoride (PCBTF)	0.1
Tertiary butyl acetate	0.2
Acetone	0.4
Methanol	0.7
Isopropyl alcohol	0.7
n-butyl acetate	1.1
Methyl ethyl ketone	1.5
Glycol ether PPM	2.6
Ethyl acetate	2.7
Glycol ether EB	2.9
Toluene	4.0
Methyl isobutyl ketone	4.3
Xylene	7.5

Ozone Producing Effect of Various Solvents³

Minimizing Conventional Solvent Usage by Maximizing Solvent Efficiency

#1 First Approach

One of the most straightforward methods of minimizing solvent in adhesive formulations is to assure that the efficiency of the conventional solvent system is optimal. To obtain low viscosity, or alternatively high solids content at a given viscosity, solvents with good solvency power must be employed.

The solvent selection for high solids formulations is not simple. However, the wide range of solvents available enables formulators to design solvent systems that will produce the required properties (e.g., spray or brushability, flow, leveling, drying time).

Ketones, such as methyl isobutyl ketone (MIBK) and methyl ethyl ketone (MEK), constitute a class of solvents that combine good solvency and effectiveness in reducing viscosity. Moreover, the relative low density of these solvents permits the most volume possible per unit weight of solvent – an important factor in complying with VOC regulations, generally expressed as grams / liter.

High solids formulations can only be achieved with the proper choice of solvent for the specific resin in question. Fortunately, several tools have been developed to make this task easier.

• The solubility parameter approach is based on the Hildebrand solubility parameter, fractional polarity, and hydrogen-bonding index. This is widely used to construct solubility maps, which can be used to select optimal solvent systems for a specific resin base.
  
  
  
  Solubility Map of a Typical Alkyd Resin⁴



• Another solvent selection tool has been produced by the American Chemical Society.⁵ Although not specifically designed for polymeric systems, it has been found to be generally useful in all industries exploring solvent alternatives. This tool allows one to interactively select solvents based upon the Principal Component Analysis (PCA) of the solvent's physical properties.
  
  Solvents which are close to each other in the map have similar physical and chemical properties, whereas distance solvents are significantly different. In addition to the PCA scores other data including the physical properties, functional groups and environmental data have been included to aid in the rational selection of solvents.

#2 Second Approach

Another approach to minimizing solvent content is to use more effective dispersing agents in highly filled adhesive and sealant formulations. These dispersing agents are capable of effectively reducing the viscosity of the system without added solvent. In fact, the required solvent to meet a specified viscosity may be reduced by the addition of coalescing agents, such as organosilanes or organotitanates.


Silanes are versatile products that can be used in several different ways to benefit adhesive and sealant applications. In addition to being useful as an additive for improved compounding and filler dispersion, silanes can be employed as:

• An internal additive to improve adhesion, cohesion, and physical properties in general.
• A primer to the substrate surface before application of the adhesive or sealant.
• As a crosslinking agent in several reactive formulations.

Organometallic coupling agents typically can also provide a dual function of improving processing and improving adhesion. Titanates have been used predominantly to modify the viscosity of filled thermoset and thermoplastic systems. It has been shown that a small percentage of titanate in a heavily filled resin system can reduce the viscosity significantly.

Low VOC or VOC Exempt Solvents Having a Better Environmental Profile

VOC exempt solvents are organic compounds that are exempt from restrictions placed on most volatile organic compounds in the United States. This class currently includes:

• Acetone
• Dimethyl carbonate
• Methyl acetate parachlorobenzotrifluoride (PCBTF)
• Tert-butyl acetate, and
• Propylene carbonate.

Under EPA regulations, a chemical is VOC-exempt, if it:

• Has vapor pressure of less than 0.1 millimeters of mercury (at 20°C); or, if the vapor pressure is unknown.
• Consists of more than 12 carbon atoms, or
• Has a melting point higher than 20°C, and does not sublime (i.e., does not change directly from a solid into a gas without melting).

The European Union uses the boiling point in its definition of VOCs. It defines VOC as any organic compound having an initial boiling point less than or equal to 250°C measured at a standard atmospheric pressure of 101.3 kPa.

VOCs are sometimes categorized by the ease they will be emitted. For example, the World Health Organization (WHO) categorizes indoor organic pollutants as:

• Very volatile organic compounds (VVOCs) – boiling points of less than 100°C
• Volatile organic compounds (VOCs) – boiling points of 100° to 250°C
• Semi-volatile organic compounds (SVOCs) – boiling points of 250° to 400°C

VOC exempt solvent are not counted as VOCs for the purposes of regulations as shown below.


Thus, the use of a VOC exempt solvent in a solvent blend can give a formulator flexibility to use other conventional solvents and still meet regulatory requirements. Most recently, changes have been proposed based on the recognition that not all VOCs are equal in their contribution to ozone production.

However, a VOC exempt solvent generally cannot be used as a “drop-in” replacement for a VOC solvent. In addition to cost, there are several factors described below that must be considered and balanced in order to develop solvent blends for specific formulations having reduced VOCs.

Property	Concerns
Evaporation rate	Can affect drying time, application methods, and solvent balance.
Resin solubility	Blends must include enough strong solvent to dissolve the resin.
Solvent activity	Stronger solvent blends have higher solvent activity and reduce resin solution viscosity more efficiently.
Solvent balance	Solvent balance in a blend will affect evaporation rate and solvency. A rule of thumb is that the slowest evaporating solvent should be an active solvent and not a diluent.
Flash point	The flash point of the solvent blend must be managed to address concerns about user safety and transportation.
Weight to volume ratio	The lower the solvent density the more volume may be included in the formulation of a given weight.

Considerations Necessary in Choosing a Low VOC Solvent Blend⁶

Barry, et. al. provides an excellent example of the processes to use in identification and screening of lower toxicity solvents for application in contact adhesive formulations.⁷ Contact adhesives, which bond to themselves upon contact, frequently contain volatile organic compounds (VOCs). Commercial formulations containing various elastomers and resins impose demanding requirements.

A database of solvents selected from Hansen Solubility Parameters was used to identify solvent blends based on solubility, evaporation rate, and density. Promising new solvent blend candidates are experimentally tested, and their performance compared to commercial formulations.

Alternative Technology to Reduce VOC Emissions

Use of an alternative technology towards the reduction of VOC emissions in the development of adhesives and sealants formulations. These technologies include:

• Solvent free
• High-solids
• Waterborne
• Hot-melt, and
• Radiation polymerizing

» Click Here To Learn More About These Technologies in Detail!