polyurethane adhesives and sealants based on … ©2015 american chemistry council polyurethane...

©2015 American Chemistry Council

Polyurethane Adhesives and Sealants Based on Hydrophobic Polyols

Adam Colson

Amber Stephenson

Nita Xu

Wenwen Li

Bindu Krishnan The Dow Chemical Company 2301 N. Brazosport Blvd Freeport, TX 77541 ABSTRACT Thermosetting polyurethane polymers are staples of adhesive and sealant formulations used in a variety of industrial applications. Polyurethane materials generally provide reliable service under most application conditions, but the long-term performance and durability of some polyurethanes can be compromised by direct and prolonged contact with bulk water or high ambient humidity. The Dow Chemical Company has recently developed a class of hydrophobic polyols designed to impart hydrophobicity to polyurethane formulations while retaining the traditional performance and processing benefits associated with polyether polyols, including low viscosity, tunable molecular weight, and fidelity of polyol functionality. This paper describes the rational design of adhesive and sealant materials using the VORAPEL™ family of hydrophobic polyols. An experimental design was employed to explore the correlation between polyol functionality and equivalent weight and the associated thermal and mechanical properties of cured polyurethane materials. The hydrophobic nature of polymers prepared using VORAPEL™ polyols was demonstrated through a series of wet aging studies which provided evidence of improved resistance to water uptake and plasticization compared with polymers prepared using non-hydrophobic polyols of comparable molecular weight and functionality. Furthermore, it has been demonstrated that the use of the VORAPEL™ hydrophobic polyols in adhesive and sealant formulations does not adversely impact key performance properties when compared with analogous formulations prepared with non-hydrophobic polyols. The information provided in this paper is intended to serve as a design template to enable formulators and application development specialists to effectively incorporate VORAPEL™ polyols into their own product designs. INTRODUCTION Thermosetting polyurethane polymers are commonly employed as adhesives and sealants in a variety of industrial applications due to the broad range of physical and mechanical properties that can be achieved through judicious selection of formulation components. While polyurethane materials are well-known for providing reliable service in demanding applications, the long-term performance and durability of some polyurethanes can be adversely affected by exposure to high ambient humidity or by direct or prolonged contact with water. Not only does water have the potential to participate in hydrolysis reactions that can degrade the polymer backbone, but it can also have a plasticizing effect on the polymer network, resulting in an apparent reduction in critical performance properties such as tensile strength or modulus. The Dow Chemical Company has developed a novel class of hydrophobic polyether polyols that can impart hydrophobic character to polyurethane formulations while simultaneously offering the benefits of low viscosity, tunable molecular weight, and fidelity of polyol functionality. At present, two of these hydrophobic polyols are commercially available: VORAPEL™ D3201 and VORAPEL™ T5001. The former is a hydrophobic diol with a hydroxyl equivalent weight of approximately 1000, while the latter has a functionality of 3 with a hydroxyl equivalent weight of approximately 200. Both polyols are clear, colorless liquids exhibiting very low viscosities (~ 300-500 cPs). Papers presented at the 2013 and 2014 Polyurethane Technical Conferences described the formulation and testing of polyurethane elastomers and coatings prepared using the VORAPEL™ D3201 and VORAPEL™ T5001 polyols.1,2 These materials exhibited physical and mechanical properties typical of materials prepared using polyether-based polyols but were


also shown to possess higher resistance to water ingress and plasticization when compared to materials prepared using non-hydrophobic polyether polyols of comparable equivalent weight and functionality. The objective of this work is to expand upon previous studies using the hydrophobic VORAPEL™ polyols and demonstrate the utility of these polyols in adhesive and sealant formulations. The rational design of adhesive and sealant formulations described herein should provide a foundation for formulation scientists and application development specialists to incorporate this class of polyols into polyurethane formulations and capitalize on the unique benefits afforded by the VORAPEL™ polyols. Basic formulation science using conventional polyether polyols is also provided in this paper. EXPERIMENTAL DETAILS Sample Preparation and Testing Plaques used for thermal and mechanical testing of adhesive formulations were prepared by pre-blending the polyols, catalyst, chain extender, and drying agent in a disposable FlackTek SpeedMixer cup for 40 seconds at 2100 rpm. The isocyanate component was then added, and the mixtures were mixed again for 40 seconds at 2100 rpm. The blends were poured into flat circular molds which were left open to the atmosphere. The plaques were allowed to cure for approximately four hours at room temperature, followed by a two-hour post-cure at 80 °C. Samples for lap shear adhesion testing were prepared and tested according to ASTM D1002 for metal substrates and D3163 for plastics. Aluminum 6061T6 and cold-rolled steel adherends were obtained from ACT Test Panels and were wiped with 2-butanone prior to application of adhesives. Plastic adherends were lightly abraded with medium grit sandpaper and wiped with methanol prior to use. Tensile strength and elongation at break were determined using ASTM D1708, which was also employed to estimate values for Young’s modulus. The glass transition temperature was determined using dynamic mechanical analysis in shear mode and was assigned as the temperature at which the tan delta peak reached a maximum. VORAPEL™-based prepolymers suitable for use in one-component, moisture-cured sealant formulations were prepared by reacting four mass equivalents of VORAPEL™ D3201 with one mass equivalent of ISONATE™ 143L at 80 oC for 6.5 hours in a three-neck round-bottomed flask under a blanket of inert gas. Cured films approximately 50 mils in thickness were prepared by mixing 50 g of prepolymer with one drop of 2,2’-dimorpholinodiethylether catalyst, degassing the mixture under vacuum, and casting a film onto a sheet of polypropylene. The film was allowed to cure at approximately 25 oC for 7 days prior to testing. RESULTS AND DISCUSSION The purpose of this paper is to present a case study on the rational design of materials having specific performance requirements using the VORAPEL™ family of polyols. Prior to beginning experimental work, minimum performance targets were established to provide context for the exploratory formulation work and guide the development of VORAPEL™-based adhesive systems. These performance targets were defined as follows:

I) Minimum tensile strength of 1500 psi (filled and un-filled systems) II) Minimum elongation at break of 150% in the un-filled system and 100 % in the filled system III) Glass transition temperature (Tg) of 25 °C or greater IV) Lap shear adhesion of 1000 psi (filled or unfilled systems on aluminum) V) Preferred inorganic filler loading of 30-40 weight percent

It is important to note that the performance targets listed above were based on specific ongoing application development efforts at Dow and should not necessarily be construed as performance specifications for general purpose adhesive formulations. Three series of experiments were designed to establish the useful range of key thermal and mechanical properties that can be accessed using the VORAPEL™ D3201 and T5001 polyols and identify basic polymer formulations that could potentially meet the performance criteria listed above. In the first experimental series (Experiments A-E in Figure 1), carbodiimide-modified MDI (ISONATE™ 143L) was reacted with blends of VORAPEL™ D3201 and T5001 in which the weight fraction of VORAPEL™ T5001 in the triol/diol blends was systematically increased over the range of 0.15 to 0.75. In the second experimental series (Experiments F-J in Figure 1), 0.25 mole equivalents of the polyol blend were replaced


with a chain extender (dipropylene glycol) to increase the amount of hard segment in the polymers. In the third experimental series (Experiments K-O in Figure 1), the mole fraction of chain extender was increased from 0.25 to 0.50. Figure 1 contains a graphical summary of the experimental design used in this work, and Table A1 in the Appendix contains the detailed formulations and tabulated thermal and physical properties for the screening experiments. The experimental design described in Figure 1 enabled the direct observation of the effect of increasing polyol functionality and hard segment on the thermal and mechanical properties of polymer systems containing the VORAPEL™ D3201 and T5001 polyols. Figure 2 contains data plots showing the effect of increasing polyol functionality and hard segment content on the tensile strength, Young’s modulus, glass transition temperature (Tg), and elongation at break for the entire series of screening formulations. Figure 3 contains data plots of shear storage modulus (G’) and tan � observed over a broad range of temperatures using dynamic mechanical analysis. The data presented in Figures 2 and 3 demonstrate the great diversity of thermal and mechanical properties that can be accessed via judicious selection of polyol functionality and chain extender content.

Table 1. List of materials employed in the current study

Component Description Eq. Wt. Functionality

VORAPEL™ D3201 Hydrophobic polyol 1000 2 VORAPEL™ T5001 Hydrophobic polyol 200 3

VORANOL™ 220-056N Polyether polyol 1000 2 VORANOL™ CP 450 Polyether Polyol 150 3

VORANOL™ 220-530 Amine-initiated polyol chain extender 106 2

Dipropylene Glycol (DPG) Chain extender 67 2 ISONATE™ 143L Carbodiimide-modified MDI 144 2.1 ISONATE™ 181 MDI prepolymer 182 2

PAPI™ 27 Polymeric MDI 134 2.7 VORATRON™ EG 711 Moisture scavenger paste

Bismuth/Zinc neodecanoate mix Catalyst

Organotin complex Catalyst (3-glycidoxypropyl)

trimethoxysilane Adhesion promoter

70 nm precipitated calcium carbonate (stearic acid treated) Inorganic filler

0.7 µm precipitated calcium carbonate (stearic acid treated) Inorganic filler

Calcined kaolin clay Inorganic filler Treated fumed silica Rheology modifier Amphiphilic acrylate

copolymer Processing additive/emulsifier

2,2’-dimorpholinodiethylether Catalyst for moisture-curable sealants

High molecular weight benzyl phthalate

High molecular weight benzyl phthalate


Figure 1. Summary of experimental design parameters and experimental designations for polymers containing VORAPEL™ T5001 and

D3201

Figure 2. Selected performance properties observed from initial screening studies (Formulations A through O).


Figure 3. Shear storage modulus (G’) and tan � plots obtained from Formulations A through O.

Based upon the performance targets described earlier, Formulations L and M appeared to be the most promising candidates for further development and testing. A model created using the data obtained from the original fifteen screening experiments predicted that a polyol blend containing a 0.22 weight fraction of VORAPEL™ T5001 in a triol/diol blend and 0.5 mole equivalents of chain extender would produce a material with a Tg of 25 °C and a tensile strength greater than 1500 psi. This formulation, designated as Formulation P in Table A1, was found to have a tensile strength of 2262 psi, an elongation at break of 279%, and a Tg of 28 °C. The effect of inorganic filler on the mechanical properties of formulations L, M, and P was studied by incorporating calcined kaolin clay into the formulations at loading levels of 20 and 40 weight percent, as shown in Figure 4. The tensile strengths of Formulations L, M, and P all dropped slightly at the 20 weight percent filler loading level, but showed an overall increase at the 40 weight percent loading level. The estimated Young’s moduli of Formulations L and P increased at both the 20 and 40 weight percent filler loading levels, while the modulus of Formulation M experienced a net increase only at the 40 weight percent loading level. As might be expected, addition of filler resulted in a decrease in elongation at break for all three formulations tested. Based on the previously imposed performance criteria that the filled formulations should have an elongation at break of at least 100% with a filler loading of at least 30-40 weight percent, Formulation M was excluded from additional testing.


Formulations L and P containing 40 weight percent calcined kaolin clay and 1 weight percent (3-glycidoxypropyl)trimethoxysilane were applied to aluminum 6061T6 substrates and lap shear testing was carried out according to ASTM method D1002. The filled version of Formulation L produced a bond with lap shear strength of 1100 psi, while the filled version of Formulation P gave a lap shear strength of 940 psi, both of which were very near to the targeted lap shear strength requirement of 1000 psi (Figure 5). Based on the results of the screening experiments described above, a more sophisticated adhesive formulation was developed, and lap shear adhesion to a variety of substrates was evaluated. An analogous formulation was also prepared to provide a comparison between the adhesive properties of the hydrophobic VORAPEL™-containing system and an adhesive containing non-hydrophobic polyols of comparable molecular weight and functionality. Weight percentages of the individual polyol components were adjusted to maintain the same average functionality in the two systems. The composition of the VORAPEL™-based system (Formulation Q) and the non-hydrophobic system (Formulation R) are provided in Table 2, and the results of lap shear testing are shown in Figure 6. Interestingly, the hydrophobic system exhibited nearly identical adhesive performance as the non-hydrophobic system, suggesting that the use of VORAPEL™ polyols does not adversely impact adhesive properties when compared to analogous systems containing non-hydrophobic polyols, such as polypropylene oxide-based polyether polyols.

Figure 4. Effect of inorganic filler on key mechanical properties of selected VORAPEL™-containing formulations

Figure 5. Lap shear strength of filled Formulations L and P on aluminum 6061T6 substrates


Table 2. Composition of hydrophobic adhesive System Q and non-hydrophobic adhesive System R

Formulation Q R Parts

ISONATE™ 143L 5.00 5.00 ISONATE™ 181 5.00 5.00

VORAPEL™ T5001 3.78 0 VORAPEL™ D3201 8.34 0

VORANOL™ 220-056N 0 8.34 VORANOL™ CP 450 0 2.87 VORANOL™ 220-530 3.16 3.16 VORATRON™ EG 711 1.14 1.14

Bismuth/Zinc neodecanoate mix 0.01 0.01 (3-glycidoxypropyl)trimethoxysilane 0.79 0.77 70 nm precipitated calcium carbonate

(stearic acid treated) 16.33 15.77

Treated fumed silica 0.34 0.33 Amphiphilic acrylate copolymer 0.15 0.14

Figure 6. Lap shear adhesion of hydrophobic adhesive System Q and non-hydrophobic System R on a variety of substrates

The performance benefits of employing VORAPEL™ hydrophobic polyols in wet environments can be demonstrated by carrying out wet aging studies on cured polymer systems. Bulk samples of Systems Q and R were prepared without inorganic fillers and were immersed in water for periods of 15, 30, and 45 days at 25 °C. Changes in mass, tensile strength, elongation at break, and estimated Young’s modulus after wet aging were recorded and the data are summarized in Figure 7. The polymer sample prepared using the VORAPEL™ hydrophobic polyols (Q) absorbed nearly half as much water as the sample prepared from non-hydrophobic polyols of similar equivalent weights and functionality (R). Sample Q retained approximately 20% more of its original tensile strength and about 13% more of its original tensile modulus compared with the non-hydrophobic analogue R. It is believed that the steeper declines in mechanical properties observed for the non-hydrophobic system can be attributed to plasticization caused by higher water uptake in the sample. The significant increase in elongation at break observed in the non-hydrophobic formulation also suggests a higher degree of plasticization caused by ingress of water.


Figure 7. Summary of performance analyses conducted on hydrophobic System Q and non-hydrophobic System R after various intervals

of water aging at 25 °C

Thus far, the developmental methodology and data presented in this paper have focused primarily on the formulation of polyurethane systems for adhesive applications. However, VORAPEL™ D3201 is also suitable for use in sealant formulations and can impart resistance to water uptake and subsequent declines in mechanical properties. As illustrative examples, two 2.5% NCO prepolymer resins were prepared by reacting ISONATE™ 143L with VORAPEL™ D3201 and VORANOL™ 220-056N, respectively. The prepolymers were cast onto polypropylene sheets at a thickness of approximately 50 mils and allowed to cure for 7 days at 25 oC. After curing, the tensile strength, elongation at break, and stress at 100% elongation were measured for each polymer according to ASTM D1708. The polymers were then immersed in water for 7 days at approximately 25 oC, and changes in the mass, tensile strength, elongation at break, and stress at 100% elongation were recorded. Figure 8 contains a summary of the mechanical performance of the cured resins before and after water aging. Compared to the non-hydrophobic resin, the cured sealant resin prepared using hydrophobic VORAPEL™ D3201 was found to have retained a greater percentage of its original properties after water aging. As with the adhesive formulations described earlier, it is important to demonstrate that fully-formulated sealant systems containing VORAPEL™ D3201 exhibit performance comparable to that of analogous formulations containing non-hydrophobic polyols of similar equivalent weight and functionality. Table 3 contains a summary of the composition of the fully formulated hydrophobic and non-hydrophobic sealant formulations as well as key performance properties. The data presented in Table 3 confirm that the sealant formulation containing VORAPEL™ D3201 performed comparably to the system containing the non-hydrophobic VORANOL™ 220-056N analogue.


Figure 8. Summary of performance analyses conducted on cured hydrophobic and non-hydrophobic sealant resins before and after water

aging for 7 days at 25 °C

Table 3. Composition and associated mechanical properties of fully formulated sealants systems based on VORAPEL™ D3201 and

VORANOL™ 220-056N

Hydrophobic

Sealant

Non-Hydrophobic

Sealant Component Parts Parts

VORAPEL™ D3201 Prepolymer (2.5% NCO w/ ISONATE™ 143L) 46 --

VORANOL™ 220-056N Prepolymer (2.5% NCO w/ ISONATE™ 143L) -- 46

High molecular weight benzyl phthalate 20 20 0.7 µm precipitated calcium carbonate

(stearic acid treated) 21 21

70 nm precipitated calcium carbonate (stearic acid treated) 9.9 9.9

2,2’-dimorpholinodiethylether 0.5 0.5 PAPI™ 27 1 1 Properties Tensile Strength (psi) 155 145

Elongation at Break (%) 374 434 Stress @ 100% Elongation 72 63


CONCLUSIONS This work has described the rational design of adhesive and sealant materials using the VORAPEL™ family of hydrophobic polyols. A simple experimental design was employed to explore the relationship between polyol structure and functionality and the associated thermal and mechanical properties. Using this structure/property map, basic formulations were selected to meet specific thermal and mechanical performance targets. The hydrophobic nature of polymers prepared using VORAPEL™ polyols was demonstrated through a series of water aging studies which revealed improved resistance to water uptake and plasticization compared with polymers prepared using non-hydrophobic polyols of comparable molecular weight and functionality. Furthermore, it has been demonstrated that the use of the VORAPEL™ hydrophobic polyols in adhesive and sealant formulations does not adversely impact key performance properties when compared with analogous formulations prepared with non-hydrophobic polyols. The formulation development process described herein is intended to serve as a design template to enable formulators and application development specialists to effectively incorporate VORAPEL™ polyols into their own product designs. ™ Trademark of The Dow Chemical Company (“Dow”) or an affiliated company of Dow. ACKNOWLEDGEMENTS The authors wish to thank Avery Watkins and William Heaner for fruitful technical discussions. REFERENCES

1) Watkins et al. “Hydrophobic Polyols for Improved Moisture Resistance in CASE Applications” Polyurethanes Technical Conference 2013.

2) Watkins et al. “Spray Urethane Coatings with VORAPEL™ Hydrophobic Polyols” Polyurethanes Technical Conference 2014.

Biographies Adam Colson is an Associate Research Scientist specializing in polyurethane adhesives technology and application development at The Dow Chemical Company. Dr. Colson holds a Ph.D. in Chemistry from Rice University and a B.S. in Chemistry from Idaho State University. Amber Stephenson (Ph.D., Chemical Engineering, University of California – Berkeley, Certification in Management of Technology, Haas School of Business) is the Global R&D Manager for Polyurethanes Adhesives & Sealants at The Dow Chemical Company and has worked in Process R&D, Product R&D, Application Development and Intellectual Property Management across Dow businesses. Nita Xu is currently a Technologist in the Polyurethanes product development group at The Dow Chemical Company. Nita has 25 years of experience in teaching chemistry and conducting research in colleges and industrial companies. Her work has focused on the synthesis, characterization, and applications of polymers. She has 1 granted US patent and 11 publications in peer reviewed journals. Mrs. Xu received a B.S. in chemistry in 1987 in Shandong University in China. Wenwen Li is a Senior chemist at The Dow Chemical Company working on polyurethane product developments. Dr. Li holds a Ph.D. in chemistry from Carnegie Mellon University as well as a M.S. in Materials Science and a B.S. in chemical engineering from Shanghai Jiao Tong University in China. Bindu Krishnan is a Research Scientist for the Automotive Business in The Dow Chemical Company. Bindu holds a Ph.D in Organic Polymer Chemistry from the University of Bordeaux, France. She has also completed a postdoctoral fellowship at Goodyear Polymer center (University of Akron) before joining Dow. Dr. Krishnan has 11 peer reviewed journal articles, 3 granted US patents, and 10 patent applications


APPENDIX Table A1. Formulations and tabulated thermal and physical properties for screening Experiments A through P.

This paper may contain copyrighted material, the use of which has not always been specifically authorized by the copyright owner. In accordance with Title 17 U.S.C. Section 107, the material in this paper is being used for nonprofit educational purposes and will not be made available for distribution. ACC believes this constitutes a ‘fair use’ of any such copyrighted material as provided for in section 107 of the US Copyright Law. For more information, go to:http://www.copyright.gov/title17/92chap1.html#107. If copyrighted material from this paper is further used for purposes that go beyond “fair use,” permission from the copyright owner must be obtained.

polyurethane adhesives and sealants based on … ©2015 american chemistry council polyurethane...

Documents