STYRENE-BUTADIENE RUBBER LATEX POLYMERS WITH IMPROVED AUTO-ADHESION K. Don Kim, Ph.D., Senior Scientist, OMNOVA Solutions Inc., Akron, OH Tibor Pernecker, Ph.D., Senior Scientist, OMNOVA Solutions Inc., Akron, OH Tim Saddow, Sr. Market Development Manager, OMNOVA Solutions Inc., Chester, SC Abstract Continuously decreasing availability and increasing costs create incentives to identify suitable alternatives to natural rubber. Adhesive formulators have blended natural rubber latex with small amounts of styrene-butadiene latex, albeit with a negative impact on adhesive performance. For some pressure sensitive adhesive applications natural rubber has been hard to replace due to its superior auto-adhesion characteristics, low tack and good compatibility with tackifiers. Newly engineered SBR latexes might enable adhesive formulators to substantially substitute SBR for NR. Such latexes offer improved auto-adhesion through molecular architecture and polymer characteristics. These new SBRs are fully miscible up to 20% with NR, and the homogeneous blends show the same or better autohesion properties as natural rubber in a certain ratio. The autohesion characteristics are gradually changing with increasing SBR concentration higher than 20%. In this paper we discuss the design of new synthetic SBR which can be blended in with NR at high concentrations without sacrificing adhesive performance for cold seal, tamper evidence, and similar applications. The SBR latexes are fully miscible up to 20% with NR and the homogeneous blends show the same or better (90/10 NR/SBR) autohesion properties as natural rubber. Autohesion gradually deteriorates with an increasing SBR concentration over 20%. However, natural rubber’s low tack is preserved over a wide range of SBR. Introduction Natural rubber (NR) was the earliest polymer used in manufacturing pressure sensitive adhesives (PSA). Its low Tg, excellent ability to bond under light pressure and ease of processing made it ideal for PSA applications. In today’s applications it is typically used as a raw material to formulate adhesive products. Relative to other polymers, natural rubber is limited by its high molecular weight, low miscibility with low molecular weight resins, low polarity and low UV and thermo-oxidative stability resulting in discoloration during the lifetime of a PSA product [1, 2]. Over several decades, NR gradually has been replaced by versatile pure acrylics in formulating PSA, but in some applications it appears to be difficult to replace it due to its unique characteristics. One such feature is its autohesion, also known as auto-adhesion or self adhesion. In cold seal or tamper evident envelope adhesives the choice of the adhesive is based on natural rubber’s ability to quickly form a strong autohesion contact. In these applications, after the formation of adhesion contact by two NR layers, fast polymer diffusion commences, the interface between layers begins to disappear over time and a strong cohesive bond forms between the two sides. The NR-NR interface becomes an organic part of a NR film. Upon initiating the separation of the interface, i.e., opening a mailing envelope, a NR adhesive typically delaminates from the carrier paper or film and provides tamper evidence. Diffusive bonding provides the strong interaction at the interface.

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Diffusion theory of polymers describes such a phenomenon with clarity [3]. In order for diffusion bonding to take full effect, high molecular mobility at the interface, good compatibility, and sufficiently high molecular weight must be met, so that fast autohesion can occur [4]. According to the theory of polymer interfaces, a full cohesive strength is reached when the polymer molecules at an interface diffuse a distance comparable to their radius of gyration and form large number of molecular entanglements [5, 6]. For high molecular weight polymers like polybutadiene and polyisoprene (NR), the interdiffusion process is completed within 3 seconds at room temperature [7]. This implies that autohesion of NR is practically instantaneous, making it ideal for selected adhesive applications. The excellent autohesion feature of NR represents a key experimental obstacle, however, for studying its autohesion by traditional “T-peel” or any other mechanical test method. For selected applications such as deep freezer and low temperature adhesive, high performance and packaging tape, NR is not being used exclusively but, instead, it is blended with low molecular weight acrylics and styrene butadiene resin (SBR) latexes for enhanced performance and cost effectiveness. Optimization of blended compositions however is cumbersome using mechanical testing and other methods, like small angle neutron scattering, and these methods are not readily available to formulators. Probe tack experiments are well documented in the PSA industry. Creton and coworkers [8] conducted extensive and detailed investigations of model adhesives using a customized probe tack analyzer combined with in situ optical observation. Their experiments focused on adhesion between a flat steel probe and an adhesive layer. Unfortunately, this method can not be directly applied for studying autohesion of NR/SBR blends because the cohesive strength of the adhesive sample is often greater than the adhesion between a steel probe and the adhesive, resulting in complete delamination of the adhesive from the steel probe. We resolved this problem by using paper saturated with NR and NR/SBR as a test specimen in probe test experiments and, after determining appropriate test conditions, successfully used the probe tack test method for optimization of the composition of NR/SBR blends. In this paper we discuss the value proposition behind NR replacement with a newly developed synthetic SBR, the performance of progressively increasing SBR content blends, as well as the limits of replacement based on the compatibility of the two elastomers. In the process, we describe a unique technique to measure autohesion of NR. Economic Retrospective Natural rubber is an elastic hydrocarbon polymer derived from the sap of plants such as the Para rubber tree, Hevea Brasiliensis. According to the International Rubber Study Group [11], total NR and Synthetic Rubber (SR) production reached 23.6 million tons in the twelve months ending September 2008, around 0.7 million tons higher than at the same point in 2007. SR accounted for 13.5 million tons (57%) and NR accounted for 10.1 million tons (43%). Although the bulk of rubber produced is synthetic, derived from petroleum, NR pricing historically has tracked prevailing global price of West Texas Intermediate crude oil, as shown in Fig. 1.

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Until the early 2000s, natural rubber and NR latex have experienced slow demand growth and declining real dollar prices. However, over the past 3-4 years, rapid industrial growth in China, India, Eastern Europe, and SE Asia has translated into a greater consumption of NR. Greater demand, along with limited reinvestment in plantations, has resulted in dramatic fluctuations in NR and latex pricing. NR latex reached an all time high in August 2008 of $1.80/dry lb, while NR reached an all time high in October 2008 of $1.48/dry lb. NR prices have seen a significant price decline since September/October 2008 reflecting concerns over the impact of the global economic slowdown (lower consumption) and coming off the peak production season during the third quarter (higher output). What Can an Adhesive Formulator Do? Adhesive formulators using NR latex have seen significant price fluctuations in their raw materials in the last few years for the reasons just described. Continued price volatility and uncertainty should be considered when formulating decisions are made. Engineered synthetic SBR latexes enable the water based adhesive formulator to potentially replace a portion of the NR latex with a synthetic product without sacrificing product performance. In addition to the performance evaluation of a blended formula, the economics must be evaluated. Fig. 2 shows the change in raw material costs of NR latex and SBR using 40% styrene and 60% butadiene monomers ratio for a representative SBR in PSA, as well as two different blends of NR latex and SBR [12].

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The following conclusions can be derived from Fig. 2:

! Blending SBR latex with NR latex lowers formulated raw material costs. ! Over the four year period shown, NR latex increased 60%, while the blended latex increased 42

– 52%, saving the adhesive formulator 13 – 26% in raw material costs. Doing the research now on how to formulate with SBR latex will give the adhesive formulator the ability to switch based on economics. Evaluating the price/performance relationship of a blended formulation allows for reacting quickly to future market changes. Experimental SBR Latex Synthesis SBR latex was synthesized with a typical emulsion polymerization method in a pressurized reactor. Styrene, butadiene, and carboxylic acid are the primary monomers, and surfactant and other additives including an initiator are charged to create the desirable molecular weight, particle size and gel content. Both chemistry and process tools were utilized to precisely control the synthetic polymer’s molecular weight, its distribution and polymeric chains architecture. Using experimental designs, we determined the optimum synthetic polymer composition and latex physical properties to emulate NR. Physical properties of the synthetic SBR and NR latexes are shown in Table 1, hence the methodologies used to measure and determine the physical parameters are described below. The NR latex for our study was LA-XT natural rubber from Dynatex and was used without modifications.

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Table 1. Physical Properties of Natural Rubber and SBR

Property Natural Rubber SBR

Glass transition temperature (Tg) ! 64 oC ! 50 oC

Gel content 50 % 68 %

Molecular weight, Mw 10, 943,000 g/mole 69,700 g/mole

Polydispersity, Mw/Mn 66.0 8.6 Critical entanglement molecular weight,

Me 4,100 g/mole 3,500 g/mole

Molecular weight between crosslink points, Mc

5,000 g/mole* 460 g/mole

* From reference [9]

Sample Preparation The face stock was prepared by saturation of the 42 gsm (28-lbs) crepe paper with natural rubber latex using a laboratory roll padder. The saturated sheets were air dried for 2 hours, then placed in an oven at 93°C for 5 minutes. The saturated paper created an anchorage with the latex film in it. The NR latex was mixed with SBR latex providing six samples with NR/SBR ratios of 100/0, 90/10, 80/20, 70/30, 60/40, and 0/100. Free films of each sample were made using Meyer rod on silicone release liner. These films were dried under room temperature for 15 min and subsequently in a 93oC oven. The thicknesses of the dried films were between 25 and 35 µm. Three layers of each film were laminated to a resulting in 75 to 105 µm thick film of each. The sheets were placed in the oven under weight at 50°C overnight to provide the enough annealing time between film and saturated paper.

Autohesion Testing The instrument used for the measurement of autohesion is TA.XT plus Texture Analyzer manufacture by Texture Technologies shown in Fig 3. It has a stainless steel cone type probe connected to the probe carrier, which is moved vertically. During the sample testing, the force vs. distance between the probe tip and the bottom adhesive film were recorded by a computer. Figure 3. Texture Analyzer (Texture Technologies, Model TA.XT)

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The saturated paper laminated with latex film was placed in housing unit under probe as a stationary adhesive layer. For the top layer, the probe tip was wrinkle free wrapped with the piece of same laminated paper. A Peltier plate which was located under the specimen docking area maintained a temperature of 23°C. This set-up allowed an adhesive film to come in contact with the film and the force could be recorded during upward movement of probe. The autohesion test can be divided into two distinctively sequential steps. First is the bonding or compression step, where the probe, covered with saturated paper, comes in to contact with the stationary specimen and keeps it under compression for a predetermined period of time. Debonding follows this step during which, at constant velocity, the probe separates from the stationary saturated paper at a constant velocity. Test conditions were identified by finding the minimum required compression force, contact time, and probe speed for appearance of fibrillation, fibril formation at separation of NR-NR specimens. An example of the force vs. distance is shown in Fig. 4. Four different stages can be distinguished during the debonding process; (a) linear, elastic deformation obeying Hook’s law, (b) stress relief by cavity germination, (c) evolution of fibril structure, and (d) fibril failure. The optimum testing conditions were 10 sec contact time, 10 lbf compression force, and 0.05 mm/sec probe speed.

Probe Tack Force The tack force was measured using a stainless steel probe and it was defined as the value of peak force recorded between the stainless steel probe surface and the stationary latex film samples during vertical motion of the probe.

Glass Transition Temperature (Tg) A Differential Scanning Calorimeter (DSC Q100 from TA Instrument) was used to measure glass transition temperature, Tg, to detect the compatibility between NR and SBR. Samples of latex films were scanned as temperature changed from -90oC to 125oC at a rate of 15oC/min in a nitrogen atmosphere.

Figure 4. Typical graph of force versus debonding distance during separation process

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Critical Entanglement Molecular Weight (Me) The AR-2000 advanced rheometer from TA Instrument was used to measure viscoelastic properties of NR and SBR films. Films were prepared by drying at room temperature for 48 hrs. Dynamic mechanical analysis (DMA) measurements were conducted by heating the samples at 3oC/min, and 10 rad/sec frequency 8 mm diameter parallel plate was used with 2.5 mm sample thickness in the range of temperature from -80 to 250oC. The following equation was used to calculate Me. [9]

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o is determined from the point where tan # shows the minimum value in Fig. 5.

Average Molecular Weight Between Crosslink Points (Mc) Mc was determined with the insoluble material swollen in a solvent. The dried polymer was encapsulated in a sealed PTFE membrane filter and tumbled in a sealed vial of toluene for 24 hrs. After 24 hrs, the vial was removed from the rotator, the sample rinsed with clean toluene, and any surface solvent was quickly blotted with absorbent lint- less towel. After weighing the swollen gel, a sample was placed on an absorbent towel in the hood for 30 min and then dried in the oven at 100oC for an hour. The dry gel was weighed. Flory-Rhener equation describes the equilibrium swelling theory well and useful for calculation of Mc to evaluate the degree of crosslinking related to Me. [9]

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Results and Discussion Autohesion of Natural Rubber and SBR During the debonding stage of autohesion experiments (Fig. 6-B), sample failure occurred at the SBR-SBR interface without fibril formation along the separation surface, while extensive fibrillation was observed with NR-NR samples (Fig. 6-A). After reaching the peak force value, widespread cavity germination took place with NR samples. The cavity walls oriented in the direction of the applied stress end formed elongated fibril structures.

Figure 6. Free films of Natural Rubber/SB blends and their corresponding probe tack curves: A – 100% Natural Rubber (NR), B – 100% SB, C – 90/10 NR/SB, D – 80/20 NR/SB, E – 70/30 NR/SB, F – 60/40 NR/SB, (The line in insert C marks the integration limit for area calculation)

A B C D E F

(b) Styrene Butadiene Rubber

Figure 5. DMA Curves of NR and SBR used for calculation of Me

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Upon further deformation, additional cavities formed within the fibrils and some of the primary fibrils split in to finer ones. This break up process continued, along with thinning of the primary fibrils, until the applied stress overcame the cohesive force of the individual fibrils, resulting in cohesive failure without any sign of the original NR-NR interface. In addition to elastic deformation of the primary fibrils, the stress energy was also relived by continuous cavitations inside the elongated fibrils and their break up. On the molecular level, during the debonding process of two polymer interfaces, the stress energy can be dissipated by the work against secondary forces and by a chain pull-out mechanism, i.e., by disentanglement of diffused polymer chains at the interfacial layer. The cohesive strength of an interfacial layer depends on entanglement density (De). When the De value is high, the strong secondary electrostatic forces delay individual chain disentanglements, and the interface appears stronger. After stress energy reaches a critical value, the entangled chains start to orient and stretch into the direction of the applied stress. Further deformation leads to disentanglement of the individual chains and structural failure. Using the entanglement density of the individual polymers we can estimate failure mechanism of SBR-SBR or NR-NR interfaces. The entanglement density is the ratio of the weight average molecular weight of the soluble fraction (Mw) and the entanglement molecular weight (Me) i.e., De = Mw/Me. Literature suggests that only polymers which have Me > 10,000 g/mole are able to show fibrillation during the debonding process [10]. While this is valid for the debonding of SBR-SBR interface during the autohesion experiments, it is not valid for the NR-NR interface. The Me and the calculated De values for NR and SBR from equation (1) are 4,100 and 3,500 g/mole and 2,656 and 20, respectively. (Table 1) The low peak force and the clean failure at the SBR-SBR interface suggest that the failure occurred by a simple chain pull out mechanism due to lack of sufficient interaction between the polymer chains and low De value of SBR compared to NR. The Me value of natural rubber is similar to that of SBR, but its high soluble fraction (50% compared to 32% for SBR, see Table 1) and large De i.e., average number of entanglements per chain, provide sufficient cohesive energy for fibril formation during the debonding stage. Both elastomers contain a substantial amount of gel (Table 1). The molecular weight between the crosslink points (Mc) was calculated from equation (2) and is not high enough (<1000, see Table I) to form entanglements with the linear polymer chains in the case of SBR. On the other hand, in NR the Mc is slightly higher than Me giving an opportunity for entanglement formation between longer network chains and linear NR chains, therefore contributing to fibril formation during deformation. Autohesion of NR/SBR blends The deformation characteristics for NR-SBR blends are similar to those of NR. A 10% addition of SBR to NR increased the initial peak force (Fig. 7) and had a fibril “reinforcement” effect during the debonding process in NR/SBR autohesion experiments (Fig. 6-C). In these experiments the sample failure was not cohesive i.e., fibril break, but delamination from either the upward moving probe’s surface covered by paper saturated with the NR/SBR (90/10) blend or from the stationary saturated paper. The fibrils appeared to be too strong to break. An increase of SBR to NR ratio led to lower peak force, work of adhesion (area under the curve), initial slope during the debonding process (Figure 7-a), and fibril failure without delamination from either saturated paper surfaces (Figure 6-D, E, F). One possible explanation for the fibril “reinforcement” is the potential plasticization of the polyisoprene molecules by the low molecular weight poly (styrene-butadiene) molecules. The presences of low

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molecular weight poly (styrene-butadiene) chains promote alignment of the stereo-regular polyisoprene, similarly to the well known stress induced crystallization with cis-polyisoprene. In NR/SBR blends with 20% or higher SBR concentration phase separation was observed (see picture in Figure 6). The NR/SBR blend free films became opaque with increasing SBR concentration. The change in visual appearance was substantiated by DSC. As it is shown in Fig. 8, the 90/10 NR/SBR blend has only one Tg which is close to that of NR. At higher than 20% SBR concentration, a second small transition appears and becomes pronounced with increasing SBR concentration (the medium sized endotherm, just after the glass transition temperature of NR, is commonly referred to as enthalpic recovery).

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Figure 7. Effect of NR/SBR blend composition on initial slope, work of adhesion, peak force, and probe tack force

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The DSC data show that the SBR is fully miscible toNR, up to 20%. Above 20%, the SBR/NR interface influences fibril deformation during debonding (Fig. 7). The De value of SBR dominates over that of NR (20 compared to ~2000), gradually deteriorating the autohesion properties of natural rubber (Fig. 6- D, E, F) and its ability to dissipate energy during the debonding process. At high levels of SBR, the presence of gel, which is incapable of forming chain entanglements neither with SBR nor with NR chains, gradually limits a blend’s ability to form fibrils during the debonding process, and subsequently dissipate stress by viscoelastic deformation.

Figure 8. DSC Curves of blended latex samples

Probe Tack In probe tack experiments (Fig. 7-d) the low tack of NR was preserved over a wide range of blended compositions because of the low adhesion of NR/SBR blends to the stainless steel probe. In these experiments the cohesive strength of NR and NR/SBR blends, which was mainly provided by NR, was higher than the blends’ adhesion to stainless steel.

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Conclusions We have shown that engineered synthetic SBR latexes can be blended with NR for adhesives in cold seal, tamper evident and similar applications. These SBR latexes are fully miscible up to 20% with NR, and the homogeneous blends show the same or better (90/10 NR/SBR) autohesion properties as natural rubber. Autohesion gradually deteriorates with an increasing SBR concentration over 20% by weight. However, natural rubber’s low tack is preserved over a wide range of SBR concentrations in NR/SBR blends. Literature Citations 1. Istvan. Benedek, Pressure Sensitive Design and Formulation, Application, vol. II, Martinus Nijhoff Publisher and VSP, 2006 2. Alphonsus V. Pocius, Adhesion and Adhesives Technology, Hanser, 2002 3. S.S. Voyutskii, Autohesion and Adhesion of Polymers, Interscience, New York, 1963 4. U. Giese, R.H. Schuster, KGK Kautchuk Gummi Kunstoffe, 54. Jahrgang, 12, 600 (2001) 5. Y. Liu, J.C. Haley, K. Deng, W. Lau, M. A. Winnik, Macromolecules, 40, 6422 (2007) 6. J.C. Haley, Y. Liu, M.A. Winnik, W. Lau, J. Coat. Tecnol. Res., 5 (2), 157 (2008) 7. C.M. Roland, G.G. Bohm, Macromolecules, 18, 1310 (1985) 8. R. Schach, Y. Tran, A. Menelle, C. Creton, Interdiffusion and Tack at Interfaces Between Immiscible Polymer Melts, Proceedings of the 29th Annual Meeting of The Adhesion Society, Jacksonville, USA, 2006 9. L.H. Sperling, Introduction to Physical Polymer Science, J. Wiley & Sons (1992) 10. A. Zosel, Int. J. Adhesion and Adhesives, 18, 265 (1998) 11. Rubber Industry Report, International Rubber Study Group, Vol. 8, No 4-6, October- December 2008 12. CMAI Price Database, 2009 Acknowledgments The authors would like thank Mr. Ian Everhard to conduct the autohesion tests, and the Analytical Solutions Group at the Akron Technology Center for DSC and Rheometer testing of the samples.

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TECH 32 Technical Seminar Speaker Styrene-Butadiene Rubber Latex Polymers with Improved Auto-Adhesion K. Don Kim, Ph.D., OMNOVA Solutions Inc.

K. Don Kim is a senior scientist in Akron Tech Center of OMNOVA Solutions Inc. Kim received his Ph.D. in polymer science and engineering from Lehigh University in 1993. His specialty is emulsion polymerization, suspension polymerization, colloidal and surface chemistry, and polymer chain interdiffusion. Kim has worked for Hansol Chemicals in South Korea, Reichhold Inc, and OMNOVA Solutions Inc since 2001. In his career in the chemical industry, he has focused on paper coating, floorcare coating and pressure sensitive adhesives. Kim holds nine patents and co-authored some technical papers in polymer science. He can be reached at don.kim@omnova.com.

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