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Adhesives Ingredients
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Adhesives Ingredients

Replacement of Toxic Solvents in Contact Adhesives Using HSP

SpecialChem – Oct 9, 2018

Through this article, SpecialChem is reporting on the research work done by Catherine P. Barrya, Gregory J. Moroseb, Keith Beginc, Michael Atwaterc, and Christopher J. Hansena on “The identification and screening of lower toxicity solvents for contact adhesives”.

The full paper can be reviewed here: TURI publications

a Department of Mechanical Engineering, University of Massachusetts Lowell, One University Avenue, Lowell, MA 01854, United States
b Toxics Use Reduction Institute, University of Massachusetts Lowell, 126 John Street, Lowell, MA 01852, United States
c ITW Polymer Sealants North America, 56 Air Station Industrial Park, Rockland, MA 02370, United States

Replacement of Toxic Solvents in Contact Adhesives Using HSPContact adhesives are the class of adhesives that bond to themselves upon self-contact. These are typically used to bond items with the large surface area, like plastic laminate countertops in kitchens and bathrooms, or any laminate that is bonded to the wood or particle board…

Solvent-based contact adhesive formulations typically consist of a solvent or solvent blend, in which rubber, resin, and additives such as anti-oxidants, fillers etc. are dissolved. The solvent used for such formulations generally include:

 − Toluene
 − Hexane
 − Methylene chloride and more

These solvents are classified as hazardous air pollutants (HAPs) by the U.S Environmental Protection Agency and long-term exposure to these solvents can:

 − Impair central nervous system
 − Cause cancer
 − Affect the central nervous system

This prompted researchers from UMass Lowell, Toxics Use Reduction Institute (TURI) and ITW Polymer Sealants North America to identify alternatives to toxic conventional solvent systems.

Let’s take a look at this approach in detail:

Use of HSP in Finding Alternatives to Toxic Solvents

With the aim to reduce the use of toxic solvents in contact adhesives, the researchers used the concept of Hansen Solubility parameters to find comparable replacement solvent blends with low toxicity issues.

Hansen solubility parameters consist of 3 intermolecular bonding parameters to characterize the behavior of a solvent with other components in a formulation. These parameters are:

 − The dispersion parameter (∂D),
 − The polarity parameter (∂P), and
 − The hydrogen bonding parameter (∂H)

The distance between HSP points insolubility space, δ, is calculated by Eq.:

D² = 4(δD1-δD2)²+ (δP1-δP2)²+ (δH1-δH2)²

Polymer solubility within a solvent is predicted by the similarity of their respective HSP values. Every polymer has a solubility sphere defined by a center and a radius located in the ∂D, ∂P, ∂H parameter space.

Solvents that reside within the volume of the polymer (solute) solubility sphere are predicted as likely to dissolve the polymer. The closer a solvent is located to the center of a polymer solubility sphere, the more likely it is to dissolve that polymer.

The use of HSP provides predictive power that improves beyond a “guess and check” method, enabling the rational search for new chemical combinations.

Polymer Sphere Calculation

Solubility spheres in HSP space were determined for each of the polychloroprene rubber, resin 1, SIS rubber, and resin 2 components. Each solid component was mixed with twenty-seven different solvents and solvent combinations that covered a broad area of HSP sphere.

A sample of the solid component and the solvent mixture were added to a glass scintillation vial. After a dwell time of 24h at ambient temperature with no mechanical agitation, the dissolution of the solid component was assessed.

Within the HSPiP software, a value of 1 is input for a solvent that dissolved the rubber or resin within 24h, while a value of 0 is input for a solvent if the polymer was insoluble.

The table below shows the summary of solvent blend solubility properties:

Formulation Solvent 1  Solvent 2
Solvent 3
∂P ∂H
Name vol% Name vol% Name vol%
SIS-control Toluene 10–30% Hexane 5–50% Acetone 15–45% 16.3 4 3
SIS-HF1 Methyl Acetate 50% Hexane 35% Methyl Cyclohexane 15% 16.2 4 4.7
SIS-HF2 Methyl Acetate 55% Cyclohexene 45% - - 16.3 4.4 5.1
SIS-HF3 Methyl Acetate 20% Cyclohexane 50% Acetone 30% 16.15 4.6 3.7
SIS-HF-LV Methyl Acetate 63% Cyclohexane 20% PCBTF 17% 16.1 5.2 5.3
CR-control Toluene 10–31% Hexane 23–59% Acetone 17–45% 16.0 3.8 2.6
CR-HF1 Methyl Acetate 50% Cyclohexene 35% Methyl Cyclohexane 15% 16.2 4 4.7
CR-HF2 Methyl Acetate 55% Cyclohexene 45% - - 16.3 4.4 5.1
CR-HF3 Acetone 52% Cyclohexene 48% - - 16.1 5.4 3.7
 Table 1: Summary of Solvent Blend Solubility Properties
(SIS = SIS, CR = polychloroprene, HF = HAP-free, LV = low VOC)

Credit: Toxic Use Reduction Institute (TURI), University of Massachusetts Lowell (UML)

Identifying Safer Solvents

TURI has identified a list of approx. 2000 safer chemicals within the HSPiP database, which helped to narrow the search space to find the safer solvents based on metrics related to reduced toxicity, environmental, or flammability hazards to mere dozens.

The HSP values of the two solutions of toluene, hexane, and acetone were found through the software. The closest positioned solvent combinations were plotted on the polymer solubility spheres to ensure that each solvent solution is likely to dissolve the given rubber or resin.

The figure below shows the sphere distance of samples from polychloroprene and SIS sphere respectively:

HSP Values of HAP-free and Low VOC Formulation
HSP values of HAP-free and low VOC formulations superimposed on 2D projections of 3D HSP
solubility sphere for polymer and resin. (a) Polychloroprene formulation (b) SIS formulations

(Click on the image to see the enlarged version)
Credit: TURI, UML

 » Continue reading to explore the detailed experimental tests & their interpretation in order to find safer solvents for contact adhesives using HSP approach!

1 Comments on "Replacement of Toxic Solvents in Contact Adhesives Using HSP"
Roberto C Oct 11, 2018
very interesting , for formulating CR adhesives since the solvents used are comercial grades that we can buy in local market

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