Polyurethane technologies for concrete infrastructure rehabilitation

Transcription

1 N. Knight, A. Colson, H. Shah, A. Stephenson, J. C. Medina Polyurethane technologies for concrete infrastructure rehabilitation The rehabilitation of transportation infrastructure has become increasingly important as aging roads, bridges, and highways see record traffic levels. Public and private infrastructure owners are faced with the daunting task of maintaining existing assets while balancing limited budgets and stakeholder expectations. The Dow Polyurethanes business has developed products designed to provide maintenance providers with economical polyurethane-based solutions to concrete infrastructure rehabilitation projects. In this paper, a two-component polyurethane system designed for use as an aggregate binder in concrete rehabilitation applications such as spall repair, patching, and nosing replacement will be discussed. Best practices for selecting mineral aggregates and in-field processing will also be presented. In addition, polyurethane sealants designed specifically for use in concrete construction and rehabilitation will be highlighted. 1 Polymeric infrastructure binders 1.1 Introduction Concrete is, by definition, a composite material composed of an aggregate or aggregate blend dispersed in a binding matrix. Most often, the aggregate is composed of natural mineral materials extracted through mining operations. The chemical composition of the binding matrix may vary, but the most common binding matrices used in infrastructure construction are Portland cement and asphalt. Portland cement exhibits high compressive strength and long-term durability, but is also brittle and prone to spalling or impact-induced flaking. Portland cement concrete can also require several weeks of curing before becoming load-bearing, limiting its use in rapid repair or patching projects. polymer concrete system to meet the needs of specific applications. The selection of a polymeric binder matrix is based on a variety of factors including material cost, ease of installation, return-to-service time, relative polymer rigidity or flexibility, resilience, resistance to chemical attack, and even aesthetic appeal. Ultimately, however, the binder must be selected based on its ability to address performance gaps that cannot be addressed using cementitious or asphalt concrete. Many polymer concrete types have been described in the academic and trade literature, and a complete survey of the various chemistries and their associated merits is beyond the scope of this paper. Instead, the reader is encouraged to consult a comprehensive review on the topic and explore the references provided therein [1]. This paper will focus on the advantages of using polyurethane polymers as a binding matrix in concrete systems and discuss the key factors that must be considered when designing a polymer concrete system based on an elastomeric polyurethane polymer. 1.2 Experimental details Typical procedure for preparing polymer plaques for mechanical analysis Nicole Knight knight2@dow.com Adam Colson, Amber Stephenson Polyurethanes/Application Development & Tech Services Harshad Shah Polyurethanes/R&D Juan Carlos Medina Polyurethanes/Marketing The Dow Chemical Company, Freeport, TX, USA Paper, 215 Polyurethanes Technical Conference, 5 7 October 215, Orlando, FL, USA Published with kind permission of CPI, Center for the Polyurethanes Industry, Washington, DC, USA The class of materials known as polymer concrete has emerged to address the performance gaps associated with traditional Portland cement concrete. As its name implies, polymer concrete employs a polymeric material as the binding matrix for the mineral aggregate. Although typically not as cost-effective as standard Portland cement concrete, polymer concrete can provide a number of performance benefits and advantages. For example, polymeric systems can often be designed to cure faster than Portland cement, and some polymer systems can achieve substantial curing within a few hours and attain full physical properties within just a few days. Additionally, the breadth of mechanical properties offered by the various types of commercially available polymer systems enables the concrete formulator or design engineer to customize a Polyol and isocyanate components were combined in 3 ml disposable FlackTek SpeedMixer cup for 6 s at 2,2 min -1. The blended material was poured into an 3.5 cm x 3.5 cm x.3 cm (12 inch x 12 inch x 1/8 inch) open mold on top of a.3 cm (1/8 inch) thick steel sheet and allowed to cure at 25 C for seven days Typical procedure for preparing polymer concrete samples The polyol component of the polyurethane elastomer system and the mineral aggregate were placed in a 7.6 l (2 gal) plastic bucket and blended with a jiffy-type mixer for 6 s, after which the isocyanate component was added to the bucket. The contents of the bucket were mixed for another 6 s, and the slurry was poured or troweled into a form for curing. 112 PU MAGAZINE VOL. 13, NO. 2 APRIL/MAY 216

2 1.2.3 Method for determining gel time of neat polymer systems Gel time measurements were carried out by hand-mixing 1 g samples of the polyurethane polymer system at 25 C. The gel time was defined as the point at which discrete, persistent strings of polymer could be drawn from the bulk material by slowly withdrawing a glass stir rod from the curing polymer Tensile strength and elongation measurements The tensile strength and elongation at break of neat polymer systems were determined according to ASTM D412. Specimens were cut from plaques using a die to ensure uniformity Compressive strength determination The compressive strengths of composite samples were determined according to method B of ASTM C579. Composite cubes of 5.1 cm (2 inch) edge length were formed using a brass mold and allowed to cure for seven days at room temperature prior to testing. Samples were compressed at a rate of 4 mm/min until a global maximum in compressive strength was achieved Method for measuring compressive stress and resilience Composite samples were prepared according to method B of ASTM C579 and allowed to cure for 24 h at room temperature prior to analysis. The sample thicknesses were Fig. 1: Polymer concrete in an expansion joint system Header (polymer concrete) Concrete or asphalt measured within.3 mm; then the sample cube was placed on the testing platform and a 45.4 kg (1 lb) load was applied. The specimen was further compressed at a rate of 4 mm/min until a deflection of 3 mm was reached. The stress at 3 mm of deflection was recorded, and the sample was removed from the testing device. The sample thickness was re-measured after 5 min, and the resilience was calculated according to the equation 1. (3 mm + final thickness initial thickness) Resilience = (1) (3 mm) Method for measuring dry and wet adhesion of polymer concrete composites to Portland cement concrete The adhesion strength of the polymer concrete composite to Portland cement concrete was measured using a modified version of ASTM C37. Specimens were prepared by pouring a polymer concrete slurry into a brass mold containing a half-briquette of cementitious concrete and allowing the composite to cure for at least five days at 25 C, resulting in a hybrid briquette containing a polymer concrete/cementitious concrete interface of 6.5 cm 2 (1 inch 2 ). The briquettes were loaded onto an electro-mechanical frame, and a load was applied until bond failure or substrate failure occurs. For wet testing, the briquettes were immersed in water for 24 h prior to testing. 1.3 Polyurethane polymer concrete Polyurethane systems are attractive candidates as binder materials in polymer concrete owing to the potential for customization of mechanical properties and cure schedules. At one extreme end of the performance spectrum, polyurethanes can be formulated to produce rigid materials that can successfully compete with alternative polymer types such as epoxies, unsaturated polyesters, or polymethacrylates. At the opposite end of the spectrum, dynamic elastomeric materials with high resilience and fatigue resistance are possible. Although other thermosetting polymers such as epoxies and polymethacrylates can be formulated to provide flexibility, many of these systems rely on exotic flexibilizing agents or plasticizers. Polyurethanes, on the other hand, can be formulated to provide unmatched elastomeric performance without supplemental additives. There are several applications in the infrastructure rehabilitation sector that can benefit from the use of polymer concrete materials prepared using an elastomeric polymer binder. One such application for which elastomeric polyurethane binders are especially well suited is in the preparation of polymer concrete headers in expansion joint systems. Expansion joints are installed between adjacent slabs of Portland cement concrete or asphalt concrete on bridges and highways where thermally-induced expansion or contraction is expected to occur. This dynamic joint serves as a buffer that prevents expanding concrete slabs from damaging one another during expansion and provides a flexible seal that prevents ingress of water or debris when the adjacent slabs contract. In the simplest form, the expansion joint consists of the native cementitious concrete or asphalt slabs, a sealant that bridges the two slabs, and polymer concrete segments known as headers or nosings that have been installed at the interfaces of the cementitious concrete slabs (fig. 1). The polymer concrete performs several critical functions in the expansion joint system. First, the polymeric binder is designed to improve the durability of the joint by reducing the occurrence of spalling, which is defined as the chipping or flaking of the native cementitious concrete or asphalt slab over time. The polymer concrete header is positioned in the joint so as to be directly exposed to impact from traffic and maintenance equipment, absorbing most of the impact at the joint. The polymer concrete header also provides a surface at which to bond the sealant, effectively acting as a primer for the native slab material. The sealant, and to a lesser degree the polymer concrete headers, can be considered consumable components and are replaced with new materials at regular intervals after being worn down by traffic, equipment, and environmental exposure. PU MAGAZINE VOL. 13, NO. 2 APRIL/MAY

3 1.3.1 Selection and development of an elastomeric polyurethane binder system The Dow Chemical Company has developed a polyurethane system suitable for use in elastomeric polymer concrete applications. Key properties of the liquid polyol and isocyanate components are shown in table 1; relevant mechanical properties of the neat cured polymer in table 2. The individual polyol and isocyanate components were formulated to maintain sufficiently low viscosities to facilitate mixing and wetting with mineral aggregates. A roughly 2:1 (wt : wt) mixing ratio was targeted to allow for suitable wetting of aggregates with polyol prior to initiating the curing reaction by addition of isocyanate. As demonstrated in the tensile stress versus strain curve (fig. 2), the neat cured polyurethane polymer exhibits classical elastomeric behavior. By contrast, figure 2 also contains a stress versus strain plot obtained from a plasticized epoxy material. Unlike the polyurethane binder, the plasticized epoxy exhibits a distinct yield point at approximately 5 % elongation, indicative of Tab. 2: Key physical and mechanical properties of the cured elastomeric polyurethane polymer Polymer properties Gel time (1 g at 25 C) in min 5 6 Hardness in Shore A Tensile strength in MPa / psi 9.65 / 1,4 Elongation at break in % 2 plastic deformation. Such plastic deformation would not be desirable in applications requiring long-term dimensional stability, as is the case in expansion joint headers. The elastomeric polyurethane binder was also designed to provide relatively fast return-to-service times and enable end users to carry out concrete rehabilitation projects in a timely manner. The gel time of a 1 g sample of the neat polymer system was observed to be about 5 6 min at 25 C, and the gel time effectively doubled when 15 2 parts of polymer were combined Tab. 1: Key physical properties of the elastomeric polyurethane components Fig. 2: Stress vs. strain curves for neat elastomeric polyurethane and plasticized epoxy binder materials (1, psi = MPa) with 8 85 parts of mineral aggregate. In the latter case, the aggregate slowed the reaction rate by absorbing the heat generated from the reaction between the polyol blend and the isocyanate. The data presented in figure 3 demonstrate how the neat polymer system develops tensile strength over time. The data were collected by casting a plaque onto a steel sheet (see chapter 2.1) and removing specimens for testing at regular intervals. The steel sheet was used to draw heat away from the curing polymer, similar to how mineral ag- Component properties Polyol blend Isocyanate blend Appearance White liquid Amber liquid Viscosity at 25 C in MPa s 8 1,2 1 2 Specific gravity in g/cm Parts by weight 1 52 Tensile stress in psi 3,5 3, 2,5 2, 1,5 1, Elongation in % Dow elastomeric PU binder Plasticized epoxy binder Fig. 3: Development of ultimate tensile strength over time for the polyurethane elastomer Fig. 4: Compressive stress curves for polymer concrete composites prepared with an elastomeric polyurethane binder and three different aggregate types (1, psi = MPa) Development of tensile strength in % Cure time at 25 C in h Compressive stress in psi 3, 2,5 2, 1,5 1, 5 1 cm (3/8 inch) pea gravel Generic fine aggregate (ASTM C33) Black Lab Blend (engineered) Compression in % 114 PU MAGAZINE VOL. 13, NO. 2 APRIL/MAY 216

4 gregates draw heat from the polymer during the polymer concrete curing process. The polymer achieved 5 % of its ultimate tensile strength within 8 h of casting and achieved about 9 % of its ultimate strength within 24 h. The plaque developed 1 % of its ultimate tensile strength within 48 h of being cast. Compared to Portland cement, the polyurethane binder provides rapid curing and the potential to return damaged assets to service within a relatively short period of time Aggregate selection and effect on composite properties The preceding discussion focused mainly on the selection of an elastomeric binder for polyurethane polymer concrete applications. However, the selection of an appropriate aggregate blend is also a critical factor in determining the ultimate performance of composite systems. Empirical and theoretical evidence suggests that a strong correlation exists between the size distribution and morphology of the aggregate and key performance properties such as compressive strength and modulus [1 3]. Examples of the effect of aggregate type on the compressive strength of polymer concrete composites prepared using the previously described polyurethane elastomer are shown in figure 4. Composite samples prepared using 1 cm (3/8 inch) pea gravel or fine aggregate (as defined by ASTM C33) were found to have peak compressive strengths of MPa (1,1 1,3 psi). By contrast, composites prepared using an engineered aggregate blend were shown to have a peak compressive strength of approximately 17.9 MPa (2,6 psi). This engineered aggregate, produced by Black Lab Corp as Black Lab Blend, contains a proprietary blend of course and fine aggregates designed to provide efficient particle packing within the polymeric binding matrix. The data (fig. 4) demonstrate how judicious selection of aggregates can significantly impact performance in polymer concrete composites. Table 3 contains a summary of additional performance properties obtained from the composite composed of the Dow elastomeric polyurethane binder and Black Lab Blend engineered aggregate. Although the selection of an appropriate aggregate blend is a critical factor in achieving the desired performance of polymer concrete systems, it is equally important to define a suitable polymer loading level in the composite. Polymer concrete composites generally contain between ten and 2 parts of polymeric binder by weight, but the optimal polymer loading is ultimately determined by the size and shape distribution of the aggregate blend. If too little polymeric binder is incorporated, the composite can become friable as a result of poor aggregate encapsulation and exhibit poor adhesion to the surrounding cementitious concrete due to incomplete wetting at the concrete interface. Conversely, the use of excess polymeric binder adds unnecessary cost to the composite. Figure 5 provides examples of polymer concrete specimens prepared using 1 cm (3/8 inch) pea gravel and elastomeric binder at loadings of 1, 15, and 2 wt%. As shown, the 1 cm (3/8 inch) pea gravel aggregate was not sufficiently bound in the composite until a polymeric binder loading of 2 parts by weight was used. The amount of polymeric binder required to effectively encapsulate all aggregate particles can be reduced significantly by selecting an aggregate containing blends of coarse and fine particles, and an example of such a composite containing a polymeric binder loading of 16 parts by weight is shown in figure 6. Another critically important factor to be considered when selecting an aggregate blend for use in polyurethane polymer concrete systems is the moisture content of the aggregate. The isocyanate component of polyurethane systems has the potential to react with residual moisture in the aggregate blend, resulting in the generation of CO 2 gas. The reaction between the isocyanate component and water has several major deleterious effects on the polymer concrete composite. First, the generation of CO 2 gas results in swelling of the composite during cure. This swelling causes the composite to expand beyond the desired confines of the form and results in unpredictable composite shapes. 1 % polymer 15 % polymer 2 % polymer Fig. 5: Polymer concrete specimens prepared with 1 cm (3/8 inch) pea gravel and various weight percentages of elastomeric polyurethane binder Fig. 6: Polymer concrete specimen prepared with a blend of coarse and fine aggregates containing 16 parts of polymer concrete by weight Property Average value Polymeric binder content 18 wt% Compressive strength 17.9 MPa / 2,6 psi Compressive stress at 5 % deflection 9.7 MPa / 1,4 psi Resilience at 5 % deflection 95 % Dry adhesion to Portland cement concrete* 2.7 MPa / 4 psi Wet adhesion to Portland cement concrete* 1.9 MPa / 275 psi * Portland cement concrete surface was primed with an epoxy primer Tab. 3: Summary of critical performance properties of the polymer concrete composite prepared using the Dow elastomeric polyurethane binder and Black Lab Blend engineered aggregate. 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5 Swelling also leads to the generation of voids and defects that can significantly reduce compressive strength and durability. Figure 7 shows examples of polymer concrete materials prepared using dry and wet aggregate blends. The composite prepared using dry aggregate maintained its intended shape and density during cure, while the composite prepared using wet aggregate swelled beyond the confines of the form and produced an anisotropic shape with lower density and significant voids and defects. Moisture-induced swelling of the polymer concrete composite is best avoided by selecting an aggregate blend that has been kiln-dried and packaged in a manner that prevents the ingress of bulk water or transmission of water vapor. An example of such an aggregate blend is produced by Black Lab Corp as Black Lab Blend Installation and processing considerations for polyurethane polymer concrete Due to the relatively rapid cure rates of polyurethane systems, the installation process for polyurethane polymer concrete is fundamentally different from the installation of cementitious concrete, and a brief description of preferred process operations and best practices for installation is warranted. Upon verification of suitable surface preparation, the polyol component and the dry aggregate should be mixed together using a mortar or Jiffy-type mixer. Mixing should be sustained for at least 9 s but can be extended for as long as necessary to achieve complete wetting of the aggregate particles. Next, the entire volume of isocyanate should be added at one time, and the slurry should be mixed aggressively for a period of 6 9 s. Prolonged mixing beyond the specified mixing time can result in heat generation and should be avoided to prevent premature setting of the polymer system. The slurry should be poured or troweled into the mold or service cavity immediately upon completion of the mixing cycle. No more than 3 4 min should elapse from the beginning of the isocyanate mixing cycle until the slurry is dispensed into the mold or cavity. A mason s trowel may be used to level the surface and remove excess composite material. If construction forms or molds are employed, the composite should be allowed to cure for at least 2 h before removing the forms. Longer cure times should be anticipated if installation is carried out at temperatures below 21 C (7 F). Conversely, installers should plan for shorter working times at temperatures above 24 C (75 F). 1.4 Conclusions This paper has described the critical factors that must be considered when selecting polymeric binders and aggregate blends to be used in the preparation of polyurethane polymer concrete composites. It is anticipated that this information will prove valuable to individuals and organizations interested in developing infrastructure rehabilitation applications based on polyurethane technology. 2 Polymeric infrastructure sealants 2.1 Introduction Joints are used throughout the concrete construction industry to allow for the movement of cementitious or asphaltic concrete during initial cure as well as throughout daily and seasonal temperature fluctuations. Concrete is highly susceptible to cracking without proper jointing at engineer specified locations. For example, summer heat causes expansion of concrete slabs which can break apart if a slab is too large or without proper control jointing throughout. Two types of joints are commonly used: expansion and control joints. Control joints are also described as contraction joints in the industry. All application surfaces and service cavities must be adequately prepared before beginning any installation operations. The International Concrete Repair Institute has published specific technical guidelines for preparing concrete surfaces prior to installation of polymeric overlays, and readers are strongly encouraged to consult these guidelines to determine acceptable surface preparation methods [4]. All surfaces must be carefully inspected immediately prior to installation to ensure the absence of any bulk water, and it is highly recommended that any concrete or cementitious surfaces be dried with a hand torch or a high-pressure stream of desiccated air prior to installation. In applications requiring exceptional wet adhesion, a twocomponent primer may be applied to the dried and sandblasted surface prior to application of the polymer concrete system. Fig. 7: Polymer concrete samples prepared using a predried aggregate blend (a) and an aggregate blend containing substantial residual moisture (b) Fig. 8: Expansion joint Joint sealant Backer rod Optional: polymer concrete header material a b Fig. 9: Contraction joint Joint sealant Sawed joint Backer rod Induced cracking directed by saw cut 116 PU MAGAZINE VOL. 13, NO. 2 APRIL/MAY 216

6 Expansion and control joints are often incorrectly referenced but serve the same purpose, which is to allow the concrete to remain intact on the surface upon inherent movement. The major difference between an expansion and contraction joint is in the concrete itself. In the simplest terms, an expansion joint is the interface between two separate substrates (fig. 8). A polymer concrete header described above and depicted in figure 8 would require an expansion joint sealant. A control joint is when the joint is prepared by sawing partially into a single substrate (fig. 9) such as when a road is poured as a solid concrete slab and cut into smaller segments. This sawed joint provides a weak plane in the concrete which can initiate cracking in a controlled manner upon movement of the concrete. The in-plane crack is underneath the sealant and not apparent on the roadway surface. All joints must also be properly sealed with a joint sealant to prevent water ingress, which can freeze during the winter and contribute to concrete cracking, and to keep debris out. Joints and joint sealants are used in a number of concrete construction applications, including but not limited to roadways, bridges, parking garages, sidewalks, warehouses, and stadiums. A depiction of roadway failure due to insufficient jointing is shown in figure 1 and a representative roadway with proper jointing is depicted in figure 11. Joint sealant requirements are specified by engineers depending on the concrete substrates involved and the environment. One important parameter is the modulus of the material. The modulus, or stress at a specified maximum extension, must be lower than Tab. 4: List of materials employed in sealant study the tensile strength of the concrete to ensure cracking does not occur due to the strength of the sealant. While cementitious concrete has high compression strength, it has a relatively low tensile strength of MPa (3 7 psi) [5 6]. Asphaltic concrete has a substantially lower tensile strength of MPa ( psi) [7]. The ultimate tensile strength of the joint sealant is generally not as important, as extensions beyond a certain point would pose more severe problems, such as complete roadway failure or building collapse. The modulus of a joint sealant is dictated by both the chemical backbone of the polymer as well as the formulation components of the sealant system. Major components of a sealant system include the polymer itself (commonly polyurethane, silicone, polysulfide, or latex), fillers, and plasticizers. Additives to improve durability can be used as well and can be application specific. Thus, this paper will focus on the fundamental design parameters of a polyurethane sealant system. 2.2 Experimental details Table 4 shows the components used for preparing the sealants for this study Sample preparation Isocyanate prepolymer synthesis Sealant prepolymers were prepared by reacting Voranol polyol(s) with Isonate 125M to produce isocyanate prepolymers with low hard segment contents. The polyol components were added to a dry glass jar under Product name Description Functionality Voranol polyols Polyether polyols 2 3 Isonate 125M Methylene diphenyl diisocyanate (MDI) 2 Dibutyltin dilaurate Catalyst Benzoyl chloride Stabilizer 2,2 -Dimorpholinodiethylether (DMDEE) Moisture cure catalyst Diisononyl phthalate (DINP) Plasticizer Alkylsulphonic acid esters of phenol (ASE) Plasticizer 2,2,4-Trimethyl-1,3-pentanediyl diisobutyrate (TPD) Plasticizer Stearate coated ground calcium carbonate Filler nitrogen, capped, heated to 5 C, and mixed to homogeneity by swirling. One drop of benzoyl chloride and one drop of dibutyltin dilaurate catalyst were then added. The mixture was stirred using a glass stir rod. MDI was added, stirred to homogeneity, the jar was capped, then placed in a preheated oven at 8 C. After ca. 3 min, the prepolymer was swirled again to melt and disperse any solids that had crystallized. The prepolymer reaction was completed in a programmable oven at 8 C for 3 6 h, after which, the oven automatically turned off, and the prepolymers remained in the oven and cooled slowly to ambient conditions until removed. Typically several prepolymers were prepared at once. Therefore, the heating program started after the final prepolymer was prepared, and the earlier prepared prepolymers were reacted the longest. Reaction completion was measured by NCO titration Sealant film preparation Prepolymer, followed by DINP and CaCO 3, if used, were weighed into a FlackTek Speed- Mixer cup. The components were mixed in a speed mixer at 8 min -1 for 3 s and then at 2,35 min -1 for 1 min. Although no bubbles were apparent, the mixture was degassed at room temperature under vacuum to remove dissolved gasses. The DMDEE (.5.1 wt%) catalyst was weighed, the headspace filled with nitrogen, and the components were mixed again at 8 min -1 for 3 s and then at 2,35 min -1 for 1 min. A film was cast on polyethylene sheets using a draw down bar at 1.27 mm (5 mil) thickness. The film was moisture cured in ambient conditions for at least four days. The residue was allowed to cure in the mix cup to afford a sample suitable to obtain hardness measurements Sample testing The tensile properties of the thin films were obtained on micro-tensile bar samples (ASTM Standard D178) that were prepared from cured plaques using a manual punch press in a dogbone shape with a width of.47 cm (.187 inch) and a length of 2.1 cm (.827 inch). Sample thicknesses varied and PU MAGAZINE VOL. 13, NO. 2 APRIL/MAY

7 were measured using a linear gauge which displayed thickness to the nearest 1 µm (.5 inch). Tensile properties were measured using an MTS electromechanical test frame. Extension rate was set at 5 inch/min. Pneumatically actuated grips were used. 2.3 Results and discussion Dow Polyurethanes has developed a platform of 1K low and high modulus polyurethane sealant materials. The purpose of this study is to highlight the latitude one has in polymer backbone and formulation design, specifically targeting low modulus sealants for use in concrete infrastructure construction and rehabilitation. We confined our working parameters to focus mainly on polyurethane prepolymers with low hard segment contents to allow for sufficient reactivity without excessive bubbling due to the formation of CO 2 upon moisture curing. A stress at 1 % elongation (otherwise described as 1 % modulus) of ca..14 MPa (2 psi) was identified as a key target as to differentiate from commercial sealants on the market today. To achieve this target, an average polyol functionality of ~2 was selected to provide low modulus sealants with acceptable levels of crosslinking. With regards to processing, a minimally engineered self leveling system was preferred for our screening as to more easily identify relationships between raw materials and formulation components whilst not confounding the results with intricate rheological behavior. Initial screening looked at the impact of covalent crosslink density when utilizing a set Voranol diol and Isonate 125M as the isocyanate to prepare Prepolymers 1 3 for use in a formulated system including plasticizer and filler. Specific formulation components are detailed in table 5. Figure 12 shows the effect of covalent crosslink density on the stress at 1 % elongation (1 % modulus). Decreasing the crosslink density resulted in a decrease in 1 % modulus as expected. Ultimate elongation (%) also tracks with crosslink density as shown in figure 13. The catalytic route to produce the polyols can also affect ultimate sealant properties. For example, the use of KOH catalyzed polyols resulted in lower modulus and higher elongation sealants vs. DMC catalyzed polyols of similar average molecular weight. This is likely due to the reactive plasticization of the residual monols in higher MW KOH polyols. An example comparing the two polyol manufacturing processes for a similar MW polyol in a formulated system is shown in figure 14. Fig. 1: Cracked roadway As more in depth screening of polyol and isocyanate components are available through Dow s literature archives [8], the remainder of this paper will focus on interactions of basic formulation components, namely the polymer, plasticizer, and filler. The first formulation screen utilized Prepolymer 3 based on the observation that a lower crosslink density imparts lower modulus values. A mixture design of experiments was initiated with a polymer component range of 25 5 parts, plasticizer range of 1 3 parts, and a filler range of 3 55 parts. We sought to achieve a low level of tackiness to minimize dirt pickup in a sealant system. We have previously observed that levels higher than 3 parts plasticizer can result in a tacky surface for these higher EW based sealants. Another key performance target was to minimize the 1 % secant modulus or stress at 1 % strain. The formulations screened and the properties of the resulting sealants are shown in table 6. A plot of the data (not shown in this paper) illustrates the correlations between the formulation components Fig. 11: Concrete roadway with construction joints Fig. 12: Stress at 1 % vs. covalent crosslink density (1 psi = kpa) 8 Fig. 13: Elongation vs. covalent crosslink density 2, Stress at 1 % in psi Prepolymer 1 Prepolymer 2 Prepolymer 3 Ultimate elongation in % 1,75 1,5 1,25 1, 75 5 Prepolymer 2 Prepolymer 3 25 Prepolymer 1 Lower Covalent crosslink density Higher Lower Covalent crosslink density Higher 118 PU MAGAZINE VOL. 13, NO. 2 APRIL/MAY 216

8 and the performance measurements. The strongest correlation exists between the plasticizer level and the 1 % modulus, where high levels of plasticizer afford low modulus sealants. High levels of plasticizer also led to higher tack sealants, and most of the sealants possessed some level of tack. A subjective tackiness value of 1 4 was assigned: 1 being the most desired having a typical sealant-like feel; 2 being slightly tacky; 3 having moderate tackiness; and 4 being tacky. The polymer and filler levels have minimal relation to the 1 % modulus. However, increasing filler content reduces elongation, and should be taken into consideration if elongation is an important performance target. In the construction Tab. 5: Formulation and mechanical property details for sealant systems throughout (plasticizer: diisononyl phthalate; filler: stearate coated ground CaCO 3 ) Tab. 6: Formulation design and film properties of formulations using Prepolymer 3 industry, joint sealants typically require elongation of several hundred percent and are often overperforming in this property, as movement beyond the requirement may be detrimental to the structure itself, be it a concrete building or roadway. The observations discussed above, namely observation of tackiness in most systems and a lack of correlation of modulus to filler levels, prompted us to explore a new polymer backbone, Prepolymer 2. The same mixture design was evaluated and is outlined below in table 7. The sealants prepared with Prepolymer 2 were less tacky compared to those prepared with the Prepolymer 3 and are more representative of a typical sealant. Prepolymer Reference Fig Fig Fig Fig. 14 Fig. 14 Formulation in parts by weight Prepolymer Plasticizer Filler Mechanical properties Modulus (1 %) in MPa (psi).48 (7).3 (44).19 (28).11 (16).13 (19) Ultimate tensile strength in MPa (psi).93 (135) 1.85 (269) 2.52 (366) 1.54 (223) 2.68 (389) Ultimate elongation in % 337 1,18 1,867 1,918 1,641 DOE experiment Composition in pbw (prepolymer/dinp/caco 3 ) Modulus (1 %) in MPa (psi) Ultimate tensile strength in MPa (psi) Ultimate elongation in % A 4 / 3 / (18.5) 1.45 (21) 2,66 4 B 25 / 2 / (27.6) 1.33 (193) 1,74 2 C 35 / 1 / (59.4) 1.85 (268) 1,369 1 D 5 / 1 / (55.7) 1.8 (261) 1,348 1 E 25 / 3 / (12.6).68 (98) 1,733 4 F 37.5 / 2 / (32.3) 1.79 (26) 1,798 3 G 5 / 2 / (27.2) 1.88 (272) 1,911 3 Tack level Since the Prepolymer 2 formulations exhibit less tack vs. Prepolymer 3 formulations, the formulator may utilize higher plasticizer levels to drive a lower modulus. Overall, the materials with Prepolymer 2 had higher 1 % modulus and tensile strengths and lower elongations because of the lower EW polyol used to prepare the prepolymer. However, low modulus sealants are typically defined as those with <.4 MPa (58 psi) tensile strength at 1 % extension [9], and several formulations afforded low modulus properties. Lower modulus systems of <.2 MPa (3 psi) are typically required for roadway applications, and this was achieved in DOE experiment L. The correlations (not shown in this paper) again demonstrate that the 1 % modulus has a strong dependence on the amount of plasticizer used. Additionally, the elongation does not show a correlation to filler content as was seen when using Prepolymer 3, but does correlate to the amount of prepolymer used. Given the dependence of the 1 % modulus on plasticizer levels, a brief screen of alternative plasticizer components was performed. Prepolymer 2 was used at a 4/3/3 ratio of prepolymer/plasticizer/ CaCO 3 with either diisononyl phthalate (DINP) or non-phthalate plasticizers comprised of alkylsulphonic acid esters of phenol (ASE) or 2,2,4-trimethyl-1,3-pentanediyl diisobutyrate (TPD). The mechanical properties and subjective observations are described in table 8 and the mechanical properties are shown graphically in fig- Fig. 14: Comparison of diol source (1 psi = kpa) 18 2,5 Tab. 7: Formulation design and film properties of formulations utilizing Prepolymer 2 DOE experiment Composition in pbw (prepolymer/dinp/caco 3 ) Modulus (1 %) in MPa (psi) Ultimate tensile strength in MPa (psi) Ultimate elongation in % H 4 / 3 / (37) 3.2 (438) 1,486 I 25 / 2 / (52) 2.45 (356) 1,411 J 35 / 1 / (83) 2.58 (374) 1,154 K 5 / 1 / (96) 1.89 (274) 793 L 25 / 3 / 45.2 (29) 2.33 (338) 1,67 M 37.5 / 2 / (59) 2.36 (343) 1,266 N 5 / 2 / (64) 1.32 (191) 757 Tensile strength at 1 % in psi Polyol process A: KOH catalyzed Diol source Polyol process B: DMC catalyzed 2, 1,5 1, 5 Elongation in % PU MAGAZINE VOL. 13, NO. 2 APRIL/MAY

9 A brief overview of polymer backbone and formulation design of low modulus sealants for use in concrete infrastructure construction/rehabilitation was presented. Trends on the effect of formulation on sealant properties led to defined polymers with low hard segment content to enable the preparation of low modulus sealants. Stress at 1 % and ultimate tensile strengths were reduced with decreasing crosslink density of the poly ol components while increasing elongation. Most notably, we identified that the plasticizer levels are key for formulating low modulus systems. Observing the correlation between crosslink density and performance enabled access to ultra low modulus sealants, which are differentiated from many commercial polyurethane sealants on the market today.! n o i t n e t t A The authors wish to thank Mike Malone for his support in evaluation of polymeric concrete specimen, Haris Shafi for his contributions during his 214 summer internship, Ike Latham for guidance in designing and analyzing the design of experiments for the sealant work, and Michelle ThomasTipps and Ann Chase for analytical support. 4 References [1] Bedi, R.; Chandra, R.; Singh, S. P.; Journal of Composites, 213, 213, 12 [2] Muthukumar, M.; Mohan, D.; European Polymer Journal, 24, 4, 2167 [3] Shigang, A.; Liqun, T.; Yiqi, M.; Yongmao, P.; Yiping, L.; Daining, F.; Computational Materials Science, 213, 67, 133 [4] I.C.R.I.; International Concrete Repair Institute, 213 [5] Guidelines for Concrete Mixtures Containing Supplementary Cementitious Materials to Enhance Durability of Bridge Decks Report 54 ; National Cooperative Highway Research Program, 25 [6] Heuts, M.; Kelly, D. G.; Wolf, D. W.; C8J9228 C8G731 C8G73 C8J9 ed. US, 28 [7] Huber, G. A.; Decker, D. S.; Engineering Properties of Asphalt Mixtures and the Relationship to Their Performance ; ASTM International: Philadelphia, PA, 1995 [8] Company, T. D. C.; Reactivity and Mechanical Performance of Polyurethane Films Made with CASE Dow Polyols and Isocyanates ; [Online Early Access]; litorder.asp?filepath=/polyurethane/ pdfs/noreg/ pdf (accessed 2 June 215) [9] Mittal, K. L.; Pizzi, A.; Handbook of Sealants Technology ; CRC Press: Boca Raton, 29 Fig. 15: Plasticizer screen of a 4/3/3 Prepolymer 2/plasticizer/CaCO3 formulation (1 psi = kpa) DINP ASE Plasticizer type TPD 2, 1,8 1,6 1,4 1,2 1, Elongation in % 2.4 Conclusions 3 Acknowledgments Stress at 1 % in psi ure 15. The plasticizer type did not affect the mechanical property performance. However, the viscosities of the formulations were impacted, where DINP afforded the most viscous formulation and TPD afforded the least viscous formulation. The lower viscosity formulations using ASE and TPD did impart a higher tack level on the final sealant properties, but had minimal issues with bubble formation that can plague a polyurethane sealant system (bubble formation was observed in the residue remaining in the mixing cup. TPD did not show any signs of bubble formation. The films used for mechanical testing did not show signs of bubble formation). Additionally, a lower viscosity base formulation can give the formulator more freedom in filler and additive usage. Tab. 8: Plasticizer screen of a 4/3/3 Prepolymer 2/plasticizer/CaCO3 formulation Relative observations (1 = lowest, 3 = highest) Modulus (1 %) in MPa (psi) Ultimate tensile strength in MPa (psi) Ultimate elongation in % DINP.214 (31) 2.19 (318) 1, ASE.2 (29) 2.26 (328) 1, TPD.193 (28) 2.5 (297) 1, Plasticizer Formulation viscosity Bubble formation Tack level E-MAGAZINE Did you know already that you can read our magazines online? In case you re interested please contact our subscription department by mail or phone: Noemi Jäger Tel.: service@gupta-verlag.de 12 PU MAGAZINE VOL. 13, NO. 2 APRIL/MAY 216