Overcoming technological issues associated with color additives in rotational molding via SSSP.
Rotational molding is an established manufacturing process used to make large, usually hollowed plastic components via a low-shear melting technique. Unlike injection molding, rotomolding utilizes powdered polymeric materials that are placed in a mold and slowly rotated and heated for ~20 min under low shear to produce these products. Rotationally molded parts are often used outdoors as garbage bins, chemical tanks, and playground equipment. This can be problematic since plastics are sensitive to ultraviolet (UV) radiation, especially polyethylene (PE), which makes up 80% of all rotational molded products . If not protected, outdoor exposure can lead to significant degradation and product failure, particularly at high temperatures and higher elevations where UV radiation and/or acid rain are particularly intense. To increase the overall lifetime of the final product, additives such as antioxidants, UV and thermal stabilizers, and other processing aids are mixed or compounded into the plastic prior to molding. Additionally, other additives, such as color pigments, are often added to the powders in order to give the final product an appealing appearance.
In rotational molding, the blending of additives is performed using one of three techniques: 1) dry-mixing, 2) turbo-blending, or 3) a four-step (i) dry-mixing, (ii) compounding, (iii) pelletizing, and (iv) solid-state grinding operation. Dry-mixing is a low-shear process that blends polymeric powders with UV stabilizers, antioxidants, and other processing aids in a paddle mixer or blender at room temperature. Although dry-mixing is the most utilized and cost-effective method, limitations include inferior dispersion and agglomeration of additives, which leads to decreased lifetimes, poor color consistency, intensity, swirling, and opacity, and decreased mechanical performance in the final products [1,2]. Additionally, dry-blended powders can cause a "plate-out" effect of pigment that is deposited on the interior of the mold surfaces after processing. This is because the pigment does not become well encapsulated in the polymer matrix. Ultimately, plate-out leads to increased processing costs. To overcome some of the dispersion issues, turbo-blending, a process in which additives are bonded to the surface of the plastic powder by rotating both together at high speeds and at elevated temperatures, has been employed. Although the dispersion of additives improves, many of the same issues with the final product remain as for products made from dry-blended powders. These issues include returns and reruns as well as dramatically reduced product lifetimes, ultimately resulting in wasted energy and higher costs.
To overcome many of the shortcomings associated with dry-mixing and turbo-blending, a four-step process consisting of dry-mixing, compounding, pelletizing and solid-state grinding was developed. This operation is becoming more widespread within industry to produce high-value polymeric powders; it has been shown to yield products with greater life-spans. Crawford and Throne.  found that powders turbo-blended with the UV-absorber carbon black (CB) produced a rotomolded product with a useful lifetime of two years. Those powders with added CB produced via the four-step operation yielded products that had ten years of outdoor use, five times that of turbo-blended materials . It has also been noted that the four-step method is the best approach to use when color concentration is greater than 0.2 wt% .
It is well known that the potential property improvement of a composite material depends greatly on the degree of dispersion [3-6]. This explains why in the study by Crawford and Throne , the overall life-span of the materials subjected to the four-step process is significantly enhanced. During melt compounding, CB is subjected to high shear forces, which effectively break up and disperse the filler in the polymer matrix. In dry-mixing and turbo-blending, the additive is subjected to only very low shear forces and thus remains heavily agglomerated and poorly dispersed. Although the four-step technique yields superior products, the multiple operations are energy intensive and costly. Furthermore, the high processing temperatures can lead to degradation of additives and decrease their useful lifetimes, especially organic pigments. For some rotomolding powders (e.g., polypropylene [PP] and nylon 6), cryogenic milling is required to grind down the extruded pellets [1,7]. This process adds significantly to the costs and overall energy required to make rotomolding-grade powders. Thus, an opportunity exists to develop a cost-effective and energy-efficient process method for producing high-value powders containing well-dispersed additives.
Solid-state shear pulverization has previously been shown to achieve excellent dispersion of nanofillers, including as-received graphite and carbon nanotubes, that cannot be well dispersed in polymers by conventional melt processing [4-6]. Additionally, SSSP has been shown to yield in-situ compatibilization of phase-separated polymer blends even when the polymers do not contain reactive functional groups [8,9]. Here, we describe the first study in which SSSP is used as a one-step solid-state compounding process that intimately mixes and uniformly disperses colorant in a polymer matrix . This is done while pulverizing the polymer into a fine powder useful for rotomolding. Figure 1 compares flow charts for the three commercial processing techniques to make rotomolding powder containing colorants to the SSSP flow chart. As we show below, compared to the four-step compounding/grinding operation, SSSP can achieve excellent product performance while eliminating several costly processing steps.
The continuous, industrially scalable process known as SSSP was used to disperse colorant materials in a polymer matrix. This process employs a modified twin-screw extruder (screw diameter = 25 mm, length/diameter = 26.5). [See refs. 4-6 and 8-11 for further details.] A key difference between the SSSP apparatus used in this study and conventional twin-screw melt extruders is that the SSSP apparatus is maintained at a temperature below the glass and/or melt transition temperature of the polymer through a circulating cooling system set at -7[degrees]C around the pulverizer barrels. This allows for repeated fragmentation and fusion steps in the solid state without the limitations of thermodynamics, viscosity, and degradation often encountered in melt processing of polymers.
The SSSP apparatus (Figure 2) consists of three zones: mixing, conveying, and pulverization. The material enters as pellets but exits the pulverizer in the solid state as powder or particulate, depending on processing conditions.
Following SSSP, the powder output was melt-mixed at 200[degrees]C for 2 min using a low-shear, batch melt-mixer. This was done in order to mimic a rotational molding operation. The same procedure was used for neat PP pellets with hand-mixed organic colorant. The product from the batch mixing was then compression-molded into films using a PHI (model No. 0230CX1) hot press at 200[degrees]C and 5-ton ram force. Samples were then immediately cooled in a cold press to room temperature. Finally, some of the SSSP powder output as well as neat polymer pellets and colorant were melt processed with a single-screw melt extruder and subsequently air-cooled to room temperature. The barrel temperature was held at 190[degrees]C-205[degrees]C. The resulting output was cut into ~2-mm pellets.
Materials and Testing
Polypropylene (PP) (MFI = 10 g/10 min) pellets and organic purple powder were generously supplied by Aptar-Mukwonago. Organic green powders were generously donated by Accurate Color.
The Young's modulus (E), tensile strength ([sigma]y), and elongation at break ([epsilon]B) were measured via ASTM D1708 standard method using an MTS Sintech 20/G tensile tester, in conjunction with TestWorks 4 software. Tensile specimens with a gage length of 17 mm, width of 5 mm, and thickness of ~ 0.4 mm were cut using an ASTM standard dogbone die from Dewes Gumbs Co. Multiple samples were tested at a crosshead speed of 50 mm/min and the results were averaged. The color opacity, lightness, a* (red-green axis) and b* (blue-yellow axis) were measured on PP/colorant compression-molded disks using a LabScan XE spectrophotometer system with CIELAB color scale.
Results and Discussion
Colorant Dispersion in SSSP Powders The high shear and compressive forces that are applied to the material during SSSP result in repeated fragmentation and fusion steps in the solid state. Furthermore, SSSP can simultaneously break up and disperse additives, leading to a uniform dispersion in the polymer matrix [4-6]. Figure 3 shows the powdery outputs that resulted from SSSP processing of PP/organic green and PP/purple colorant. A uniformly colored output was achieved with no visual evidence of colorant agglomerates, indicating excellent breakup and dispersion of colorant particles throughout the PP matrix. The pigments were also well encapsulated in the polymer matrix, which was evidenced by the fact that no pigment was deposited when powder was rubbed between a thumb and finger. Conventional colorant dispersion in a polymeric powder results in color rub-off.
In order to simulate a rotational molding operation, a low-shear, batch melt-mixer was used. Figure 4 shows compression-molded bars of PP/organic green and PP/organic purple colorants prepared via only MM, respectively. For both organic green and organic purple pigments, there is poor dispersion of colorant following low-shear MM, which is evident from the color streaks in the final product. This is a common occurrence with organic pigments and often requires one to utilize the four-step method described in Figure 1 in order to overcome this issue.
SSSP prior to low-shear MM yields a different result. Shown in Figure 5 are compression-molded bars of PP/organic green and PP/organic purple materials subjected to SSSP. In contrast to the results in Figure 4, it is evident from Figure 5 that the pigments are well dispersed in the PP matrix. This is because the color streaks are eliminated in the final product, a result that could also be achieved with the four-step process. This also suggests that SSSP can be used as a cost-effective replacement of this four-step method since SSSP requires only a single-step solid-state compounding operation to yield a high-value polymeric powder. It is also worth noting that costs can even be further reduced if one uses SSSP to produce non-PE powders (e.g., PP and nylon). This is because SSSP can eliminate the use of cryogenic milling, a more expensive process that is currently employed to produce non-PE powders [1,7].
It is well known that poor dispersion of colorant can lead to major reduction in mechanical performance. For example, Crawford et al.  showed that in general, dry-tumble blending in the mold and turbo-mixing produced moldings with similar impact strength (for 0.3 wt% pigment loading). However, compounding followed by solid-state grinding resulted in a material that had impact strength almost twice that of turbo-blended samples , a result they suggested was caused by better dispersion and consistency of the pigment particles in the polymer.
Table 1 gives the mechanical properties of neat PP and PP/organic green and PP/organic purple color resulting from our study. It is evident that samples prepared via SSSP prior to low-shear, batch MM show slightly greater mechanical performance and have less variability between samples as compared with those materials just strictly melt-mixed. This is because the pigment particles are well dispersed in the polymer matrix after SSSP followed by MM.
Reduction of Colorant Amount
The cost of colorant resin is significantly higher than the cost of neat polymer resin, as much as 10 to 100 times more, depending on pigment and polymer. Thus, any reduction in the use of colorants needed to make a plastic part while maintaining the desired color can lead to significant savings. Shown in Figure 6A are lightness and opacity and Figure 6B are a* and b* values measured for compression-molded bars made from PP/organic purple colorant subjected to MM only (open symbols) or SSSP prior to MM (closed symbols). By extrapolating the open symbols to the trend lines, it is evident from Figure 6 that SSSP leads to a 10%-15% reduction in colorant amount needed to match the color characteristics of materials made from MM powders. We can achieve similar color characteristics using less colorant in our SSSP/MM samples than in the MM samples because of the more effective breakup and dispersion of colorant particles during SSSP.
An important outcome from SSSP processing of colorant materials is the ability to develop secondary and tertiary colors by blending primary and secondary colors together. Figure 7 shows the formation of a tertiary tan color by blending a combination of lightgreen, dark purple, and white. These final outcomes are expected based on the color wheel .
In order to determine whether conventional melt processing would be able to create similar outcomes, we ran the same colorant blend formulations in a conventional melt extruder. Shown in Figure 8 are the results for tan color. It is evident that color shifting occurs in these blends over time. In contrast, the SSSP-processed materials exhibited no color shift. The large shift in color with melt processing is due to the inability to intimately mix and evenly distribute the colorant particles during melt processing. Another reason is the major viscosity mismatches between PP homopolymer and colorant pellets. Because SSSP can efficiently break up and evenly disperse the colorant particles prior to melt processing, color deviations are no longer prevalent. These results illustrate that SSSP can be used as an effective tool for overcoming a major industrial issue of color shifting.
The significance of these results is twofold. During melt processing (e.g., injection/blow molding), a significant amount of material (from defective parts or color shifting) is thrown away. SSSP can be a cost-effective solution for recycling these waste colorant materials to make new materials with unique colors. For example, if a processor wants to make a tan part but has only light green, dark purple, and white waste materials, then instead of processing neat PP and adding tan colorant, the processor can use SSSP to combine the waste materials to make the tan color. This would save on polymer and colorant costs and increase recyclability.
In addition, these findings indicate that SSSP can, in conjunction with primary and secondary colorants, be used to prepare polymer with a wide array of novel secondary or tertiary colors with uniform color distribution. This is because SSSP, employed with any number of ranges of colorant concentrations and combinations and melt viscosities, can yield excellent dispersion and mixing of the colorants without regard to viscosity mismatch issues that limit the utility of melt processing. (See, for example, Figure 8, which demonstrates the much poorer color distribution achieved via melt processing relative to SSSP.) Thus, SSSP can serve as a novel and relatively facile and inexpensive method by which polymer processors can obtain a wide range of uniformly distributed colors in their polymer products.
A novel, solid-state compounding approach known as SSSP was used to disperse colorant in PP prior to low-shear MM. We found that SSSP can overcome several major limitations often encountered in rotomolding-grade dry-mixed or turbo-blended powders with added pigments, including color streakiness, poor mechanical properties, and pigment rub-off. Since SSSP is a single-step process, it has the potential to be more cost-effective for producing high-value powders. Furthermore, SSSP can be used to create a variety of colors by either recycling waste materials or combining colors in the proper concentrations and reduce colorant amount needed to color-match a sample that was MM only.
The ability of SSSP to achieve excellent dispersion of colorant additives has implications for other applications besides rotomolding, including injection and blow molding. With the latter operations, polymeric powder would first be melted and pelletized prior to melt processing. The SSSP method can be equally effective in dispersing many other additives important to the plastics industry besides colorants, including those not well dispersed by conventional melt processing. The innovative SSSP technique has demonstrated potential for making polymeric materials that cannot be made by conventional methods [4-6]. Additionally, in some cases it has cost and property performance advantages because SSSP can achieve equal or better dispersion in a single-step operation  than can be achieved by a multistep process.
The authors acknowledge contributions of materials and equipment by Aptar-Mukwonago and Accurate Color as well as support from Initiative for Sustainability and Energy (ISEN) and the NSF-MRSEC program at Northwestern University.
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Note: The authors presented an earlier version of this paper at ANTEC[R] 2012.
Philip J. Brunner (1) and John M. Torkelson (1,2) (1) Northwestern University Evanston, Illinois, USA (2) NuGen Polymers, LLC
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|Title Annotation:||solid-state shear pulverization|
|Author:||Brunner, Philip J.; Torkelson, John M.|
|Date:||Oct 1, 2012|
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