By Evan E. Koslow, PhD, Rasmina Musovic and Esko Musovic
In our first article on catalytic carbons, we reviewed some of the fundamentals of these materials, including how to test for catalytic properties, the impact of particle size on performance and the enormous variation in the as-received performance of commercial catalytic carbons from different manufacturers. We used a variation in the method originally developed by Calgon Carbon for the measurement of catalytic performance using hydrogen peroxide reduction. We demonstrated that carbon particle size is one of the most important variables that impacts performance. There were, however, enormous variations in the as-received performance of catalytic carbons provided by different manufacturers. Basically, some products had extremely high performance and others displayed a completely different and lower level of performance. One might conclude that there are two different technologies being used to produce these materials and that they result in products with two distinctly different performance signatures. In this article, we begin to examine the severe problems of converting catalytic activated carbons into carbon block.
Conversion to carbon block is the primary means available to use small-mesh activated carbon and this small particle size will boost the performance of the catalytic material enormously. Conversion of powdered carbon into a carbon block is not a simple proposition because in many cases, the original catalytic performance of a carbon will be nearly entirely destroyed during conversion to a carbon block. We have previously demonstrated that it is not the result of applying heat during the molding or extrusion of carbon block. Instead, it appears to relate to the amount, type and heating of the carbon in the presence of a thermoplastic binder. We will also examine the fouling of catalytic carbons within carbon blocks and determine that only a single combination of carbon, binder and process conditions results in the retention of high performance. Every other combination of carbon, binder and processing results in significant or even disastrous reductions in performance.
Testing of blends containing carbon and poly(ethylene-vinyl acetate) (pEVA) binder
In our first test (see Figure 1), we blend 14 percent by weight of a standard pEVA (Microthene® FE-532) binder with high-performance Carbon B and then carry out a measurement of catalytic activity for a 1.000-gram sample of this mixture without any heat treatment. This sample contains the same amount of carbon (0.860 grams) as a pure carbon sample we use to form our baseline estimate of performance. In this test, we observed a 30-percent loss of peroxide reduction capacity for a mixture of carbon and binder (that hasn’t even been heated to form a block) compared to a sample having the same weight of the original carbon. Clearly, the presence of the binder alone causes an immediate and large loss of performance.
We next repeated this experiment using this blend of pEVA binder and Carbon B that is heated to different temperatures for 15 minutes. Both sample weight (1.000 gram) and binder content (14 percent) are constant for all of the tests. We obtained the results shown in Table 1. Not only did the presence of FE-532 binder (even when not heated) cause a serious loss in performance, but when heated, this impact became extremely severe with 79- to 89-percent reductions in catalytic performance of the carbon (this means a four- to 10-fold reduction in performance).
Although Carbon B is a high-performance material, this high performance can be entirely destroyed using this binder, even at relatively low concentrations and under almost any applied heating regimen. The as-received performance of the carbon is not really important. Instead, finding a means to preserve this high performance is the critical step to designing a high-performance carbon filter.
We repeated this test for the other catalytic carbons as shown in Table 2. These results are a bit surprising. Carbon C appears to significantly resist fouling by this binder, but it started as a relatively low-performance material. Carbon J is also a low-performance material and this binder does further serious damage. Carbon J emerges with almost no residual catalytic behavior. The two high-performance carbons (Carbons B and H) are both seriously damaged when processed with this binder. One might as well have used the low-performance Carbon C, which resisted the potentially profound fouling by this binder.
Testing of blends containing carbon and low density poly(ethylene) (LDPE) binder
We examined the impact of Microthene low-density poly(ethylene) (LDPE) grade FN-510 upon the performance of Carbon B 80 x 325-mesh catalytic carbon. The first series of trials examined the impact of processing temperature at a fixed blend of 14-percent LDPE and 86-percent carbon. This concentration of binder was close to the lower limit used for this class of binder. (Table 3 shows these data.) All samples were oven-cured for 15 minutes at the processing temperatures shown (centigrade).
Processing carbon with LDPE at higher temperatures results in severe losses of catalytic activity. We are uncertain about the anomalously large loss of catalytic activity at 145°C or what may be an anomalously low amount of fouling at 160°C, but we replicated the results at 145°C, so it appears to be a reliable outcome. The overall trend is less severe damage at low processing temperatures.
Unfortunately, during both molding and extrusion, it is necessary to operate the process at higher temperatures to drive the required heat transfer and bring core temperature to the fusion point. This implies that the outer portion of a carbon block closer to the heat source, which is where the bulk of the carbon is to be found, will be severely damaged, while the core of the cylindrical block might be impacted to a lesser degree.
All processing temperatures produced serious negative impacts for both pEVA and low-density poly(ethylene), but it is clear that the FN-510 binder would be preferred in most cases. (Even so, it is likely that well over 50 percent of the catalytic activity will be lost with LDPE, even under the best circumstances and perhaps as much as 80 to 90 percent in the case of pEVA binders.) We blended the other catalytic carbons with the FN-510 binder and processed this mixture at 190°C for 15 minutes to simulate traditional molding operations. These results are shown in Table 4.
The results shown in Table 4 for pLDPE are very similar to those shown in Table 2 for pEVA. The low-performance Carbon C significantly resists damage by the binder compared to the other carbons. In the end, the high-performance Carbon H and the low-performance Carbon C have nearly identical performance within the simulated molded block. Carbon B and Carbon J have both collapsed in the face of this binder.
Testing of blends containing carbon and UHMW poly(ethylene) binder
GUR® 2126 ultra-high molecular weight poly(ethylene) (pUHMWPE) is an ultra-low melt-flow index binder usually used in relatively high percentages (20 to 40 percent being the typical commercial range) for the production of carbon blocks by molding. We produced blends of 20 and 30 percent by weight GUR 2126 UHMWPE with 80 x 325-mesh Carbon B and heated this mixture to 190°C for 15 minutes. Without correcting for the high weight of binder, the ΔT10/Tmax for this polymer was measured several times (see Figure 5).
The 30-percent concentration had a roughly 41- to 44-percent reduction in catalytic activity, while dropping binder content to 20 percent (a practical minimum) caused a 28-percent reduction. These results demonstrate that the use of an ultra-low melt-flow index polymer alone is insufficient to protect the catalytic carbon. Because GUR 2126 binder must be used in high weight percentages, the loss in activity is partially caused by the displacement of carbon by the binder. The remainder of the damage appears to be related to some form of fouling, but not surface-area fouling, which is generally not possible with this binder. Clearly, binders impact catalysis through a more complex mechanism and in a more profound manner than they effect adsorption capacity. We repeated this test with the other activated carbons and obtained the results shown in Table 6.
Again, we observe that Carbon C has a strong resistance to binder fouling. In general, the impact of the pUHMWPE binder is more restrained on all of the carbons than the medium melt-flow pEVA and pLDPE binders. Note that there remains significant loss of performance when using pUHMWPE, but that can likely be tolerated in many cases. Here, Carbon H has an advantage.
Testing of blends containing carbon and KYBLOCK® poly(vinylene fluoride) (pPVDF) binder
We produced a series of mixtures of KYBLOCK FG-42 with catalytic Carbon B. We studied the typical commercial range of this binder’s loading rate from five to 14 percent by weight. In all cases, we heat-treated the samples for 15 minutes at 190°C to simulate a typical molding process. Table 7 summarizes these results for Carbon B.
KYBLOCK FG-42 binder has three unusual properties. First, it has a high absolute density of approximately 1.75 g/cm3, which is nearly double that of the polyolefins. This high density makes the volume of binder within a given blend extremely small, so that displacement of carbon is minimized. Second, KYBLOCK FG-42 is composed of very small (roughly four-micron) particles (in comparison to GUR 2126, for example, that has a particle size approaching 60 microns) that generally do not compete with the larger carbon particles for space, but instead occupy the smallest particle-packing position within a carbon block. Finally, this binder has a very low melt index similar to that of the pUHMWPE. In conclusion, KYBLOCK FG-42 has a realistic capacity to maximize the loading of carbon into a given volume of carbon block and sustain the catalytic performance of the various carbons.
The results shown in Table 7 demonstrate that the KYBLOCK FG-42 binder would have minimal or modest impact on the performance of a carbon block up to loading rates of about 12 percent by weight. A 14-percent loading rate is generally beyond the commercial and economic limits of this binder and probably defines a practical upper limit. It appears that the five- to 10-percent weight range provides excellent protection to the carbon with a loss in activity of only 2.9 to 6.5 percent, which is the best we have recorded in all of our studies. We have repeated the test at 14 percent by weight of KYBLOCK FG-42 to examine the typical variance in this catalytic activity assay. Although two tests is not a statistically useful study of variance, these measurements do demonstrate that the test is reasonably reproducible to a few percent.
Notice that the loss of performance observed with GUR 2126 UHMWPE binder (23.5 percent) is similar to that measured for 12 to 14 percent of KYBLOCK FG-42 (22.7 to 26.8 percent). The FG-42 binder, however, is a four-micron powder, while the GUR 2126 is a much larger 40- to 60-micron powder. This means that while the FG-42 can be effectively applied at very low concentrations (five to 10 percent), this is not possible with the GUR 2126 material. At low concentrations, the FG-42 produces a very small displacement of carbon and appears to not seriously impact catalytic properties. We believe that the FG-42 advantage is the ability to function at low application rates and that the relatively lower performance of the GUR 2126 relates to its inability to be used at such low concentrations.
Table 8 shows the results obtained with the other catalytic carbons when blended with eight percent by weight pPVDF binder and processed at 190°C for 15 minutes. Here, we obtain a really shocking outcome. After all of our expectations that the pPVDF binder should perform very well and avoid fouling of these catalytic carbons, we find that this simply is not true. Two carbons absolutely collapse when mixed with this binder and two carbons retain essentially all of their original performance. Carbons B and C are essentially not impacted by the presence of this binder, while Carbons H and J collapse, with Carbon J nearly completely destroyed. Because Carbon B is a high-performance material and Carbon C is a lower-performance product, Carbon B comes on top here with the highest overall score (>40°C at 10 minutes in our peroxide test) for any blend of binder and carbon we have studied.
Selecting a catalytic carbon based upon an as-received assay of catalytic performance is not recommended when converting such carbons into a carbon-block product. The ability to preserve the catalytic performance within the final product is critical and requires detailed testing. We have found that certain catalytic carbons with low performance (Carbon C here) can significantly resist binder fouling, but the best approach is to identify a high-performance catalytic carbon (such as Carbon B here) that can be processed with the right binder (for example, pPVDF) to obtain really superior results. There is no question that producing a product with catalytic carbons is not a simple proposition and will require a complex series of considerations.
About the authors
Evan Koslow, PhD, President of KT Corporation is the original inventor of the carbon block extrusion process, with roughly 80 patents. He was the founder of KX Industries, L.P. Dr. Koslow has received the WQA Award of Merit and both the American Filtration Society Senior Scientist Award and New Product Award. He can be reached via email at firstname.lastname@example.org. Rasmina and Esko Musovic are a husband-and-wife team who have worked at both KX Industries and KT Corporation for over 20 years. They have extensive experience in carrying out research and testing of activated carbon materials and products.