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Benzoate plasticizers have been recognized as the plasticizers of choice for latex adhesive applications and have a proven track record that dates back to the 1950s. By nature, benzoates are non-phthalates and, as such, have gained attention as phthalate replacements. A value-added benzoate plasticizer product, K-Flex® 850S, has been developed to meet the global need for phthalate replacements and reflects the current plasticizer needs of the latex adhesive market.
An evaluation was recently conducted to demonstrate the performance of the new K-Flex 850S product. The study consisted of testing the new benzoate blend in both polyvinyl acetate homopolymers and polyvinyl acetate/ethylene copolymers at three levels vs. diisobutyl phthalate, di-2-ethylhexyl terephthalate, and current dibenzoate controls. Several key tests, including viscosity response and glass transition (Tg) suppression, indicated that K-Flex 850S is an excellent plasticizer for latex applications and compares very favorably with phthalates and other competitive benzoates. It also offers latex applications formulators wide latitude in compatibility with other polar adhesive polymers.
BackgroundBenzoate esters are established plasticizers for the adhesive market. As a family, benzoates have been available commercially since the 1930s.1 Diethylene glycol dibenzoate, dipropylene glycol dibenzoate and a 50/50 blend of these two plasticizers form the basis of the product line for use in adhesives.2-4 The principal reason why benzoates were established as general purpose plasticizers for latex applications was their polar nature (compared to general purpose vinyl plasticizers, such as diisononyl phthalate), which allowed their compatibility with commonly used polymers in the adhesives or coatings industries.
General purpose phthalates are not utilized as often in the adhesives industry as they are in the vinyl industry. Polar phthalates, such as butyl benzyl phthalate (BBP), di-n-butyl phthalate (DBP) and diisobutyl phthalate (DIBP) are more commonly associated with use in adhesives than the longer chain dialkyl phthalates.
In terms of regulatory pressure, di-2-ethylhexyl phthalate (DOP or DEHP) was initially suspect, but the lower phthalates also became increasingly regulated as time passed, leading producers of plasticized compounds to seek non-phthalate alternates.5 The pressure to move toward using non-phthalates-particularly in Europe-has increased in the 21st century. Benzoate esters have been effectively used as non-phthalate alternatives since before the need arose for such a technology and are the natural choice for adhesives.6
Generic grades of benzoates have been used for years for the diverse high solvator applications, but benzoate manufacturers are keeping pace with the needs of the industry with new technologies and products.7-10 Although other plasticizers can and have been used in the adhesive industry, benzoates have proven to offer the best balance of performance and value.6
Adhesive EvaluationK-Flex 850S is a proprietary blend of dibenzoates that keys off of the excellent performance of diethylene glycol dibenzoate, which is one of the blend’s components. As this product is based on existing dibenzoate technology, it:
- Is listed on most of the chemical inventories in the world
- Has 21CFR175.105, 21CFR176.170 and 21CFR176.180 U.S. Food and Drug Administration (FDA) approvals
- Is free of both health and environmental hazard labels at the levels used in adhesives in Europe
Three basic performance characteristics are considered to be critical in the evaluation of any plasticizer or plasticizer blend for use in a latex adhesive. Undoubtedly, the first is compatibility of the plasticizer with the polymer used in the adhesive. Without at least partial compatibility, the plasticizer cannot provide good performance in an adhesive.
The next parameter is viscosity response (normally a viscosity increase) of the base emulsion to the plasticizer. It is most desirable to have as good a viscosity response as possible to allow the adhesive compounder to add water back to adjust the adhesive to the required viscosity for the application.
The last characteristic is Tg suppression of the polymer by the plasticizer. In the case of polyvinyl acetate homopolymer, Tg suppression is desired, as the Tg of the homopolymer is about 35-40°C. Tg suppression is considered to be an efficiency parameter, so a greater Tg suppression is usually desired. However, in some instances, less depression is required to allow for solids addition, to optimize formulation cost, or to control other performance characteristics. In copolymers, the Tg is often low enough by itself, so a plasticizer is added to enhance other performance characteristics.
The addition of a plasticizer affects other adhesive performance characteristics as well. Neat plasticizer volatility will affect the volatile organic content of the adhesive. If a plasticizer is compatible, it can also affect the adhesive’s set and open times, adhesion, strength, and bond water resistance, among other characteristics.
Evaluation ProtocolAs dibenzoates and dibenzoate blends are well known to the latex adhesive industry, a basic adhesive evaluation was conducted to illustrate the performance and advantages of using K-Flex 850S. The following protocol was designed to demonstrate the performance of K-Flex 850S in latex adhesives:
Two emulsions selected for formulation
- a polyvinyl acetate homopolymer, polyvinyl alcohol protected
- a polyvinyl acetate/ethylene copolymer, polyvinyl alcohol protected 0°C Tg
- K-Flex 500: a classic blend of diethylene glycol dibenzoate and dipropylene glycol dibenzoate
- A competitive blend of two dibenzoates
- A competitive blend of three dibenzoates
- Diisobutyl phthalate (DIBP): still used in adhesives and therefore used here as a primary phthalate control
- Di-2-ethylhexyl terephthalate (DOTP): considered a non-phthalate by some and a general purpose plasticizer replacement in vinyl; GP vinyl plasticizers are generally not polar enough for many adhesive applications, so DOTP was added here as a negative control
Results and DiscussionThe evaluation presented here is abstracted from the data presented at the 2010 Fall Convention of The Adhesive and Sealant Council, Inc. meeting (October 3-5, 2010) in Covington, KY.11 As mentioned earlier, the plasticizer’s volatile organic compound (VOC) content will have an effect on the total VOC of the adhesive. The lower the VOC of the plasticizer, the better. In general, benzoates are quite low in VOC. A modified ASTM D 2369 VOC test (0.5 g sample with 1 ml toluene) was used to determine VOC. Figure 1 illustrates how K-Flex 850S is similar in neat VOC to the control benzoates and significantly lower than DIBP. DOTP is very low in VOC, as would be expected for a higher molecular weight dialkyl phthalate.
Compatibility is the key parameter in any plasticized formulation. As expected, it was experimentally proven that all of the benzoates and DIBP were compatible at the level of test. However, DOTP was completely incompatible with polyvinyl acetate and only partially compatible with the polyvinyl acetate/ethylene copolymer.
Figure 2 shows photos of some of the dried adhesive films used to judge compatibility in the polyvinyl acetate homopolymer. Note that, in the case of DIBP and K-Flex 850S, the films are clearer than the film without plasticizer and free of any exudation. This indicates excellent film formation and compatibility of these plasticizers with the polymer. In the case of the DOTP film, the photo does not show significant exudation but does show a very hazy film, which is indicative of incompatibility. The unplasticized emulsion will form a film at room temperature, as water is a fair plasticizer in a homopolymer emulsion stabilized with polyvinyl alcohol.
Figure 3 illustrates the data on the effect of select plasticizers on the Tg of the polymers tested. In the case of the polyvinyl acetate homopolymer, the benzoates are similar in Tg suppression while DIBP is slightly better in suppression. DOTP does not affect Tg at any level of incorporation in the homopolymer, further indicating its complete lack of compatibility with the homopolymer. In the copolymer, DOTP displays partial compatibility. At 5% loading, DOTP does depress Tg well, but the response levels off at the 10% and 15% loading. This is not the case for the benzoates, which are all similar in their ability to increasingly suppress Tg as the plasticizer concentration increases.
Figures 4 and 5 show the viscosity response data. While there are some slight variations in the viscosity response of the compatible plasticizers of this evaluation, they all provide excellent response and are all similar in this aspect of performance. As expected, DOTP does not provide adequate viscosity response.
In polyvinyl alcohol-protected emulsion adhesives, the polyvinyl alcohol provides reasonable wet tack and set times before any contribution provided by a plasticizer. Figure 6 illustrates the set times of the 5% copolymer and the 10% homopolymer adhesives (these concentrations were selected as representative of average plasticizer loadings in these types of adhesives). In each case, the benzoates shortened set times in the adhesives as compared to the unplasticized base polyvinyl alcohol-protected emulsion.
Compatible plasticizers can lengthen the open time of an adhesive; long open times are also indicative of good wet tack. Figure 7 illustrates the effect of the evaluated plasticizers on open times. Both the benzoates and DIBP provided excellent open times.
ConclusionWhile it is certainly important today for our industry to identify ways to become greener and more committed to using safe materials, the overriding fact remains that the materials we use must function as required while providing value. The results of this evaluation indicate that K-Flex 850S, which has been tailored to provide the value demanded as a non-phthalate alternative, provides both the value and the performance characteristics required for the adhesive market. In addition, it is in line with today’s stricter environmental and health standards.
For more information, contact the authors by e-mail at firstname.lastname@example.org or email@example.com.
Editor’s note: K-Flex® is a registered trademark of Emerald Kalama Chemical, LLC.
References1. Farben, I.G., “Improvements Relating to the Production of Artificial Materials from Highly Polymeric Materials,” GB 496, 574, 12/1/1938.
2. Reeves, Howard, et. al., “Polyalkylene Glycol Dibenzoate, Process of Making Same and Resinous Compositions Plasticized Therewith,” U.S. 2,956,978, October 18, 1960.
3. Arendt, W.D., “Utilization of Glycol Benzoates in Polyvinyl Acetate Emulsion adhesives and Acrylic latex Caulks,” Proc. Adhesive & Sealant Council Seminar, Cambridge, MA, 186-210.
4. Arendt, William D., “Plasticization of Polyvinyl Acetate Adhesive Emulsions,” Tech Pap Soc Manuf Eng., 1985, AD85-782, p. 23.
5. Gravel, Shamsi P., McBride, Emily L., “Dibenzoate Plasticizers Offer a Safer, Viable Solution to Phthalates,” Adhesives & Sealants Industry, February 2010, pp. 15-18.
6. Skeist, Irving, Edit., Handbook of Adhesives, Third Edition, Chapter 21, Harold Jaffe and Franklin Rosenblum, Van Norstrand, Reinhold NY, NY, 1990, p. 395.
7. Arendt, William D., “Increasing Plasticizer to Eliminate Solvent from Latex Adhesives,” Adhesives Age, vol. 36, August 1993, pp. 17-19.
8. Arendt, W.D., Streeter, B.C., Holt, M.S., “New Benzoate Plasticizers for Latex Caulks and Adhesives,” J. Adhes. Sealant Counc., October 1997, pp. 85-103.
9. Arendt, W.D., Streeter, B.C., Holt, M.S., “New Benzoate Plasticizers for Latex Adhesives,” Adhesives Age, vol. 41, March 1998, pp. 36-39.
10. Arendt, William D., Strepka, Arron M., Stanhope, Bruce, “New Cost-Effective Benzoate Plasticizer Blend Enhances Latex Adhesive Systems,” Adhesives Age, vol. 45, June 2002.
11. McBride, Emily L., and Arendt, William D., “Introduction of a New Benzoate Plasticizer Product for Latex Applications,” presentation at the The Adhesive and Sealant Council, Inc. Fall 2010 Convention, Cincinnati, OH, October 5, 2010.
SIDEBAR: Test MethodsPreparation
The adhesive emulsions were prepared by mixing the plasticizer into the polymer for 10 minutes at 750 RPMs using a Caframo outfitted with a Jiffy blade. The plasticizer was added within the first minute of mixing.
Plasticizer samples of about 0.5 g in 1 ml of toluene were held at 110°C for one hour in a type 2A oven.
Emulsions were drawn down 10 mils wet on glass and allowed to dry overnight. Samples were evaluated for clarity and exudation.
The viscosity response of the emulsions to the plasticizers was determined at low shear by a 30-second reading at 20 RPMs on a Brookfield DVII, RV viscometer (22°C, ± 2°C).
Set time was determined by laminating two strips of 50-lb unbleached Kraft paper and measuring the time it took before 80% fiber tear occurred when peeling the strips apart. The top and bottom strips of paper were 1 x 14 in. and 1.5 x 14 in., respectively. Approximately 1 g of emulsion was metered onto the bottom strip using a #20 wire wound rod while the top strip was laminated to the bottom strip with a second rod.
Open time was determined using a method akin to the set time method. The top and bottom strips of 50-lb unbleached Kraft paper were 1 x 14 in. and 1.5 x 14 in., respectively. Approximately 1 g of emulsion was metered onto the bottom strip using a #20 wire wound rod; the top strip was laminated to the bottom strip with a second rod after the determined time interval (a multiple of 5 seconds) had elapsed. Strips were peeled apart immediately following lamination and were considered set once greater than 50% fiber tear occurred.
The glass transition data was gathered on 10 mg samples of 10 mil wet film that had dried at least overnight. Samples were run by jumping from room temperature to -90°C and then ramped at 10°C per minute on a TA Instruments Q2000 DSC.<