Replacement of Toxic Solvents in Contact Adhesives Using HSP

TAGS:  Solvent-borne Adhesives      Contact Adhesives    

Through this article, SpecialChem is reporting on the research work done by Catherine P. Barry^a, Gregory J. Morose^b, Keith Begin^c, Michael Atwater^c, and Christopher J. Hansen^a 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

Contact 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.:



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		∂D	∂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:

Experimental Testing

Solubility Testing for Safer Solvents

The solvent blends that were identified as likely to dissolve the rubber and resin were experimentally tested for solubility. The vials were shaken at 90rpm on a platform shaker (New Brunswick Scientific C1). Every 30min the vials were shaken vigorously by hand for 30s to disperse agglomerated components, and then returned to the platform shaker. The time at which each component completely dissolved was noted.

The solvent blends in Table 1 were tested to experimentally verify their ability to dissolve the polymer and resin components. The solvent blend was combined with either rubber or resin and observed for dissolution.

The table below shows the dissolution time for each of the rubbers and resin in the solvent mixtures:

Formulation	Rubber	Resin
	(CR or SIS) (h)	(1 or 2) (h)
SIS-control	1.35	0.35
SIS-HF1	1.18	0.35
SIS-HF2	0.98	0.32
SIS-HF3	1.57	0.72
SIS-HF-LV	1.37	0.70
CR-control	5.07	0.05
CR-HF1	5.07	0.05
CR-HF2	5.07	0.05
CR-HF3	4.37	0.08

Table 2: Solubility Results of Solvent Solutions for Polychloroprene, SIS, and Resins
Credit: TURI, UML

The solubility of a solute in a solvent solution is related to the solvent's location in comparison to the solute sphere in HSP space. The closer the solvent is towards the center of the sphere or the junction point, the better the solubility becomes.

Here, CR-HF3 was found to be the closest of all solvent blends to the optimal junction point.

Evaporation Testing

A rapid evaporation rate is essential to reduce the dry time of the adhesive, after which bonding can occur. Moreover, if the evaporation rate is not rapid and too much solvent remains, the strength of the bond may be lessened. Lifting or bubbling of the substrate can also result from trapped solvents.

The evaporation time was quantified for solvent blends in which the rubber and resin fully dissolved. A balance (model PI-214, Denver Instruments) recorded the sample mass as a function of time. A solvent blend specimen of approximately 0.1g of initial mass was measured until the mass reached a plateau value.

The figure shows both control solutions evaporating within 5 min (300s), which was the target range for the new solutions:

Spray Testing

The viscosity of the adhesive is an important property to ensure that it would not clog the nozzle during spray application.

For the test, Oil Red B4 liquid was added to the adhesive to visualize the adhesive spray pattern. A series of nozzle and pot pressure adjustments were made to generate similar patterns of coverage between the baseline formulations and the new solutions.

The table below shows the viscosity measurements. The viscosity of the two target solutions was tested for each round of experiments, which were performed on different days.

Test day	Formulation	Brookfield Viscosity	Viscosity (Pa-s)
Round 1	SIS-Control	32	0.16
	SIS-HF1	48	0.24
	SIS-HF2	50	0.25
	CR-Control	88	0.175
	CR-HF1	88	0.44
	CR-HF1	97	0.485
Round 2	SIS-Control 2	37	0.185
	SIS-HF3	72	0.36
	SIS-HF-LV	61	0.305
	CR-Control 2	37	0.185
	CR-HF3	37	0.260

Table 3: Viscosity of the SIS Adhesive and the Polychloroprene Adhesive Formulations
Credit: TURI, UML

Round 1

In round 1, formulations SIS-HF1, CR-HF1 and SIS-HF2, CR-HF2 have viscosities that are 50%, 151%, 56%, and 177% higher than the target solutions, respectively. The difference in the viscosities is likely related to the difference in the solids loading fraction.

The commercial adhesive ratio of solvent to rubber, resin and additives is based on weight. Hence, the new solvent mixtures, which are denser than the control composition, result in a lower volume of solvent added to the adhesive to maintain the same weight ratio.

Round 2

In round 2, CR-HF3, SIS-HF3, and SIS-HF-LV were created to match the solids loading fractions of the controls. While these adhesives were prepared on a volume basis to lower the viscosity, the viscosities of the new blends remained greater than that of the control.

The viscosity of CR-HF3, SIS-HF3, and SIS-HF-LV are 40%, 94%, and 65% greater than the baselines, respectively. Despite the observed viscosity increases, the viscosities were sufficiently low for spray gun application of the adhesive.

The spray patterns of each adhesive were assessed by spraying the formulations through a spray gun onto a kraft paper substrate. The spray gun can be adjusted in order to produce the same spray patterns over a range of viscosities.

Figures below shows the spray pattern for SIS- and Polychloroprene– based adhesives respectively.

SIS Adhesive spray Pattern Optimized for Solubility (left) & Optimized for Cost and Low VOC (right)	Polychloroprene Adhesive Spray Pattern Optimized for Solubility (left) & Optimized for Cost (right)

Credit: TURI, UML


As soon as the adhesive leaves the nozzle of the spray gun, the solvent starts evaporating and the rapid movement of the air disperses the solvent into the environment. This rapid solvent loss thickens the adhesive making it difficult for the spray gun to spray a continuous and homogenous pattern.

The result would produce the speckled pattern that is seen in SIS-HF3 instead of the smooth web pattern that is observed in the control and SIS-HF-LV. The CR-HF3 at the right of Polychloroprene adhesive spray testing figure showed the similar spray patterns with little little-to to-no adjustment of the spray nozzle.

Bond Strength Testing

The primary function of a contact adhesive is to provide a secure bond. In order to make sure that the contact adhesive has adequate strength, the bonded laminates are tested by the edge lift test.

After 24 h the specimens that were bonded 10 min after spraying (i.e., at the start of the open time) were tested for strength. The peak load was measured. The results are shown in Fig. below:



Based on typical commercial formulations:

• The SIS adhesive peak load is typically expected to be between 222 and 334 N (50–75 lbf), and
• The polychloroprene adhesive peak load is expected to be between 267 and 445 N (60–100 lbf)

Fig. a and b above show all adhesives falling within the given expectations for round 1 of testing. Most of the solutions, including the controls, tested in round 2 fall below the given expectations (Fig.5a and b).

A possible reason for these lower results is that the pressure applied to the laminates – which is applied by a human operator – was likely decreased in round 2 of testing. While this manual pressure application is known to introduce variation into the testing, it is representative of adhesive bonding in the field and is a standard method used in industry to assess the bond strength.

The open time for each adhesive occurs within ten minutes, matching the quick time of the commercial formulations. The edge lift test is performed for specimens seven days after being bonded to make sure that the strength is maintained throughout the open time.

If the load decreases significantly from the 30 min to 60 min (1800 s–3600 s) specimen, then that indicates that the open time is reduced. Specimens that were bonded after 15 min, 30 min, and 60 min from the time that the specimens were sprayed were tested for strength. The results are shown below:



The SIS adhesive has a peak load range between 222 and 334 N and the polychloroprene adhesive has a peak load range between 267 and 445 N.

The first round of adhesive tested, fall within the given range. Some are 2–21% higher than the upper limit of the range which is not considered a problem for industrial applications. These adhesives were able to maintain strength through the full 60 min of open time.

While the second round of adhesives tested fell below the typical range, the controls were also lower. As previously mentioned, this decrease in strength could be due to the difference in the amount of bonding pressure applied to the laminates, solution viscosities, and the lab environment.

The bond strength of SIS-HF3 is reduced with longer open times. However, this drop is not significant enough to indicate a loss of open time. SIS-HF-LV maintains a constant strength throughout the 60 min. Fig. 6b shows CR-HF3 performing adequately compared to the control and maintaining its strength throughout the time period.

Overall, all of the adhesives tested exhibited adequate strength throughout the open time duration of one hour.

Interpretation of Experimental Data

• Similar dissolution times to controls in all cases
• Higher viscosities, but no problems spraying
• Promising evaporation behavior

Performance Characteristics

• Adhesion strength compared is similar to or better than control systems in all cases
• Two blends (HF1, HF2) work well for both polymers & Multiple HAP-free alternatives demonstrate functional equivalence vs. controls
• Low-VOC formulation (HF-LV) provides significantly better performance than water-based products

Conclusion

1. Five solvent blends that do not contain hazardous air pollutants (HAP) were identified
2. Two of these blends (-HF1, -HF2) were optimized for solubility of the rubber and resin solid components
3. The next two blends (both –HF3) trade optimal solubility in order to achieve desirable low prices
4. The fifth blend (SIS-HF-LV) contains no HAP sand has a VOC content under 250 g/L.
5. All five solvent blends were demonstrated to dissolve the given rubber and resin, evaporated sufficiently quickly to initiate the open time, and strength tests confirmed that all five formulations possess strengths equal to or greater than commercial formulations and retain a 1 h open time.

Using these alternative safer solvents can enable consumers and industrial users to use high-quality contact adhesives without compromising their health and environment.

Credits

• https://www.turi.org/
• https://www.uml.edu/Research/OTC/technologies/
• http://itwsealants.com/

 » Download the Full Journal Paper from Here!