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Comment PDF Solids Handling

Measuring Dust and Fines In Polymer Pellets

By Shrikant Dhodapkar, Remi Trottier, and Billy Smith, The Dow Chemical Co. |

The ability to carry out  measurements of dust and fines in polymer pellets can help operators improve equality control, assess equipment performance and optimize the process

Plastics are most commonly produced and sold in pellet form due to the superior handling characteristics of pellets in downstream applications. From the moment the polymer is pelletized from its molten state, the pellets or granules undergo a series of handling steps as they move from the producer to the consumer. For instance, post-pelletization steps include:

• Hydraulic conveying

• Drying (typically via spin drying)

• In-process pneumatic conveying

• Silo storage

• Packaging (via bulk shipment, or in bags or boxes)

• Unloading (via pneumatic conveying) and silo storage

• Conveying to processing equipment

• Blending and feeding

As a consequence of this repeated handling, the pellets often become contaminated with various unwanted species, namely:

FIGURE 1. Many types of dust and fines form during the handling of bulk plastic pellets. A single pellet is shown here for size comparison

FIGURE 2. Pneumatic conveying systems routinely create dust and fines when moving polyethylene pellets. The size distribution shown here was measured using an image analyzer
 

• Dust or fines that are generated during pelletization, conveying and collection in cyclones (Figure 1). The particle size distribution of dust and fines (Figure 2) can vary depending on type of polymer, process conditions and specific hardware of the handling system

• Chips, undersize pellets, miscuts and broken fragments that are generated during pelletization, conveying and collection in cyclones

• Chopped or smashed pellets, so-called worms or smeared polymer crumbs that are generated by rotary airlocks (feeders)

• Broken tails that result from poor pelletization and subsequent handling

• Streamers, floss, angel hair, ribbons or snake-skins that are generated during pneumatic conveying (Figure 3)

FIGURE 3. The steamers, floss, angel hair and ribbons shown here are examples of the types of impurities that typically form when plastic pellets are pneumatically conveyed
 

Many of the terms noted above are synonymous and thus are used interchangeably, based on the shape and size. Streamers can be up to a couple of meters in length.

The presence of these contaminants in pellets can lead to numerous problems in the downstream processes, including the following:

1. Gel formation in film applications

2. Frequent clogging of filters in pneumatic conveying systems

3. Cross-contamination problems in multi-product plants

4. Defects in the finished product

5. Variability of slip additives or processing aids

6. Color issues (such as black specks or color inconsistencies) resulting from degraded fines dislodging from process equipment

7. Process safety issues (such as increased risk of dust explosion in the fines-collection system)

8. Industrial hygiene concerns associated with respirable dust

9. Plugging problems at various stages of the end user’s process, resulting from streamers, floss, angel hair and ribbons

10. Inconsistent pellet feedrates into the throat of an extruder, resulting in gage variations of profiles and films

To alleviate these problems and deliver the cleanest possible product to the customer, many of today’s plastics producers use dust- and streamer-removal devices in their processes. Further discussion of these devices is beyond the scope of this article.

Efforts to quantify the unwanted species in the plastic pellets have long been a source of confusion, contention and miscommunication among plastic producers, users of the pellets (converters) and vendors of solids-handling equipment. Such problems can be avoided if there is mutual agreement by all stakeholders on an appropriate standard for the measurement of dust, fines and streamers. A reliable, robust and well- documented standard serves a variety of purposes including the following:

• Ensures greater quality control in a manufacturing process

• Provides a basis for product specification

• Serves as a troubleshooting tool to relate pellet quality with process conditions

• Helps to identify opportunities for process optimization

• Enables performance evaluation of pellet-cleaning devices

A varied and creative collection of dust- and fines-measurement techniques have evolved over the years based on experiences derived from a wide spectrum of applications. Historically, these techniques have been based on dry sieving or counter-flow air-classification concepts.

However, for plastics applications, the quality of data gathered using these methods is often compromised, due to the presence of electrostatic charges, the existence of waxy and sticky additives, and the presence of fines at particularly small particle sizes (< 50 µm) and low levels (< 50 ppm).

This article summarizes the existing standards for quantifying unwanted contaminants such as dust, fines and streamers in plastic pellets, discusses the limitations of these approaches, and highlights some of the recent advances.

 

Measurement techniques

In general, unwanted species contaminating a batch of plastic pellets can be broadly classified into three groups, based on their size relative to the pellets or granules (the prime product) themselves:

Group 1 (size < pellets). Fines, dust, chips, fragments, undersize pellets, miscuts, smears, worms and ill- formed pellets

Group 2 (size ~ pellets). Tails, smears, worms and small streamers

Group 3 (size > pellets). Streamers, floss, angel hair, ribbons and snake skins

Group 1  species can typically be separated from the pellets by dry sieving or air classification if there is no tendency for the particles to adhere to the pellets, for instance, due to electrostatics, van der Waals forces, surface tension and other adhesive tendencies. Anti-static additives are used to minimize the electrostatic buildup. However, dry sieving is often unable to separate sticky fines (such as those that are rich in waxy additives).

Using an air-classification approach, the finer fraction is elutriated by the counterflowing air. This method is quick but usually results in relatively poor separation efficiency and high variability. Separation via air classification can also be affected by the relative humidity of the elutriation air.

By contrast, wet classification is the most effective method for separation of Group 1 species from pellets and granules. Dispersing the pellets in a liquid or washing them with a liquid breaks down the attractive forces between the fines and the pellets, thereby allowing them to then be easily removed by mechanical separation. However, this approach adds a drying step to the measurement process.

Demineralized water, ethanol or a mixture of water and ethanol are commonly used for washing. Safe handling characteristics and inertness to the polymer (and its additives) are key considerations when selecting the liquid washing media.

Group 2  species are the most difficult and challenging to separate from the pellets, since most separation techniques rely on differences in particle size. The air-classification approach can be used if the aerodynamic diameters of various fractions are significantly different and electrostatic forces are minimal.

One may also exploit the differences in shape for separation. For instance, a new, innovative approach has been developed by the authors of this article, to effectively separate Group 2 species from plastic pellets. The core idea is to use a rectangular aperture (such as a wedge-wire screen) coupled with a wet washing approach, to separate the pellets from the unwanted species.

FIGURE 4. This illustration shows how the separation of fines, tails and short streamers can be improved when the wet separation process incorporates a screen with elongated apertures
 

The width of the aperture is slightly smaller than the diameter of the pellets, which prevents them from passing through. However, the longer dimension of the aperture allows the tails, polymer smears, agglomerates and small streamers that are longer than the pellet size to pass through, along with the wash liquid (Figure 4).

Group 3  species are most effectively separated by dry screening. The screener can be a vibrating type, or a Trommel (rotating cylinder) type. For a meaningful estimation, the sample size must be large enough to capture a sufficient number of streamers.

Existing standards

Today, there are three primary standards for measuring fines in plastic pellets or granules, and each is discussed below:

• FEM-2482: Test method to determine the content of fines and streamers in plastic pellets (1999)

• ASTM D 1921-06: Standard test method for particle size (sieve analysis) of plastic materials (2006)

• ASTM D 7486-08: Standard test method for measurement of fines and dust particles on plastic pellets by wet analysis (2008)

 

 

FEM-2482

The European Federation of Materials Handling (FEM) put forth a well-documented and comprehensive procedure (FEM-2482) in 1999 to address the ambiguity associated with measuring fines and streamers in plastic pellets. In this standard, the fines are defined as the particle fraction below 500 µm. The lower limit of this fraction depends on the needs of downstream processes. Three classes of fines were proposed, namely:

• Type A: 63 to 500 µm

• Type B: 45 to 500 µm

• Type C: 20 to 500 µm

The process schematic for measurement of fines using FEM-2482 is shown in Figure 5.

FIGURE 5. The elements of the wet process that is defined in FEM-2482 are shown here
 

When it comes to plastic pellets, the typical level of fines in the final product can range from 10 to 2,000 ppm. Pellets with fines content in excess of 500 ppm are usually deemed “dusty.” It should be noted that the fines fraction below 20 µm is not measured by this method, since this ultrafine fraction passes through the retention medium (Column C2). However, the ability to measure this fraction can be important for certain applications (such as the manufacture of optical lenses and digital storage media).

The particle fraction above 500 µm with a form deviating from the usual pellet shape is defined as the streamer content. Streamers are also known by numerous other names — angel hair, floss, snake-skins, ribbons, film or foil. As noted earlier, dry screening is an effective way to separate these species from pellets.

The FEM-2482 method is based on wet process with two possible operating modes.

Mode 1. Fluidization mode is used when the density of washing liquid is lower than the true density of the pellets. A retention sieve (Figure 5) is not used in Column C1 during fluidization mode. The upward velocity of the liquid is set according to the Stokes velocity for the largest particle to be separated. The carryover fraction is further classified in Column C2.

Mode 2. Flotation mode  is used when the density of the washing liquid is higher than the density of the pellets. Since water is the safe choice as the washing liquid for most polyolefins pellets whose density is lower than water, this mode is most commonly used.

Using the FEM-2482 method, the agitation and circulation of pellets are achieved by injecting air bubbles in Column C1. The intensity of circulation can be adjusted by the air-flow throttle valve (Figure 5). The authors of this article have evaluated numerous alternatives for mixing and agitation (such as the use of directional water jets and agitators) and have concluded that the use of air bubbles is the most effective means of agitation and recirculation of pellets in the wash column. The top retention sieve shown in Figure 5 is a 500-µm wire mesh that prevents the pellets from being carried over to Column C2.

The FEM-2482 standard also provides details on various aspects of the measurement process, such as:

• Sizing and configuration of the apparatus

• Selection of classification screens

• Ancillary equipment

• Selection of guidelines for wash liquid

• Test procedure

• Interpretation of results

• Error analysis

Sampling guidelines per FEM- 2482).  A sample size of 1 L or larger is recommended for fines analysis. As a general rule of thumb, the maximum size of a sample is dictated by the size of the washing column (C1). To achieve good dispersion and washing, the volume of a sample should not exceed half the volume of Column C1.

For streamer content analysis conducted using the dry-screening approach, the sample volume should be at least 50 L. Since streamers are not dispersed homogenously within the bulk, larger samples are always preferred. Note that this standard lacks details regarding methods and devices to use for separation of streamers from the pellets.

Special care must be given when handling samples during filling, discharging and transportation. The methodology of sampling depends on product and plant surroundings.

Limitations of FEM-2482.  Despite the comprehensive nature of this standard, it has several shortcomings, including the following:

1. Fines smaller than 20 µm in size are not measured or collected by the finest retention medium (Column C2). The limit on the lower cut is due to physical limitations posed by the screening operation.

2. In this standard, fines are defined as the fraction with particle size smaller than 500 µm. However, particles with size greater than 500 µm can exist in the fines (Figure 2). Dry screening may not be the appropriate method for separating these fines from the pellets, especially when the fines are sticky, waxy and specifically fibrous in nature. Similarly, quantification of broken tails with size varying from a fraction of the pellet size to several pellet diameters is especially challenging (Figure 6). Typical pellet size is between 3 and 5 mm.

FIGURE 6. A typical pellet with a tail formed during pelletization is shown in the inset, while the larger image shows the typical size distribution of broken tails

 

 

The first limitation has been addressed by a recent ASTM standard (D 7486-08, discussed below), which recommends using a filtration disc made of glass microfibers to collect fines as small as 0.7 µm.

The second limitation can be addressed by replacing the 500-µm wire-mesh retention screen (shown in Figure 5) with a wedge-wire screen (see Figure 7). The rectangular aperture of the wedge-wire screen is designed to retain the pellets while allowing the fines, tails and small streamers to efficiently pass through. The tails and small streamers align themselves with the liquid flow and pass through the rectangular aperture (Figure 4).

FIGURE 7. As noted in the in FEM-2482 wet process, a wedge-wire screen with elongated aperatures (right) is a more-effective alternative to the standard 500-μm retention screen (left).
 

Alternatively, a punched plate with elongated openings can also be used. However, the use of square wire-mesh with larger opening inevitably results in occlusion of the openings as the tails and streamers hang up on the mesh.

The wedge-wire retention screen also allows chips, undersize pellets, miscuts and broken pellet fragments to pass through and get carried over to Column C2 (Figure 5). Therefore, it is recommended that the fines classification screen-stack (Figure 5) preferably have a top sieve with 1-mm opening. The remainder of the sieves can be chosen per the FEM-2482 guideline.

 

ASTM D 1921-06

This method covers the measurement of particle-size distribution of plastic materials in various forms — pellets, granules and powders. It is based on the dry-sieving approach, hence the lower limit of measurement is about 38 µm (No. 400 sieve).

The standard proposes two methods — A and B — both of which are discussed below.

Method A. This method uses multiple screens stacked on top of each other. A complete distribution of particle sizes can be obtained, which can then be used to determine the mean particle diameter. The suggested sample size is 50 g.

Method B. This is an abbreviated version of Method A that uses limited screens. It is typically used to get specific cuts (such as percent passing through, or the percent retained) on certain screens. The suggested sample size is 100 g.

The problem of electrostatic charge buildup during dry sieving is addressed by adding an anti-static additive (up to 1 wt.%). However, the problem of sticky fines and agglomerates is not addressed in this standard and thus remains unresolved.

One fundamental problem with separation processes using sieves is the inherent inefficiency of sieve-based separation methods to separate fibrous fines that are longer than the mesh opening size, even if the fiber diameter is much smaller.

ASTM D 7486-08

This is a recent addition to the standards for fines and dust measurement in plastic pellets. It proposes a wet washing technique. The sample (typically 100 g) is placed on a filter funnel assembly, consisting of a 200-mm diameter, wire-cloth sieve with 500-µm opening. The filtration disc (90 mm in dia.) is made of glass microfiber, and the media has a pore size of 0.7 µm to 2.7 µm. The typical lower limit is 1.6 µm due to filtration rate limitations.

The pellets are washed with a strong jet of water (500 mL/min) until they are visibly clean and no particles are observed in the wash liquid. The filter disc is then removed from the assembly, dried in an oven and then weighed to measure the fines content.

This method addresses the deficiency of FEM-2482 in its inability to quantify fines smaller than 20 µm, since fines as small as 0.7 µm can be captured on the filter disc. However, smaller sample size (100 g) and possible operator dependence for effective washing of pellets are the shortcomings of this method.

When streamers (such as a fraction greater than 500 µm) are present, the standard proposes using the ASTM D 1921-06 method.

The methods discussed above are summarized in Table 1. The applicability range for each method based on particle size is shown in Figure 8.

 
       
       
       
       
       
       

 

FIGURE 8. This figure shows the range of applicability for the various measurement standards discussed here, based on particle size
 

Tips for success

To ensure maximum reliability during measurement of fines or dust in plastic pellets, readers are encouraged to do the following:

• Understand the nature and content of typical non-pellet fraction (such as fines, dust, streamers) in the bulk material

• Identify the key requirements for downstream processes and be clear about the purpose of the measurement (for instance, are the measurements being sought to reduce process variability, meet product specification, assist in troubleshooting or optimize equipment performance?)

• Identify the most suitable test method(s) for the purpose, and establish specific testing parameters based on the material at hand

• Understand the accuracy, precision and reproducibility of the chosen test method

• Obtain representative samples and follow good sampling guidelines

Readers should note that obtaining a “representative sample” for fines and dust analysis is a difficult task that presents its own challenges. Specifically, the samples inevitably become biased, due to the attraction of fines towards any sampling device or sampling container. For example, the use of a plastic scoop can result in significant loss of fines from the sample, due to electrostatic attraction between the fines and the scoop itself.

It is a common practice to obtain a large sample from the process and reduce it to the appropriate analytical size using a sample splitting process (such as a riffler). However, one must pay close attention to the loss of fines during this step, otherwise the results will be biased. Purging the sampling line before taking the sample, taking spot samples to create a composite, analyzing the entire sample, rinsing the sample container to recover fines and the use of anti-static sprays will help to reduce sampling errors.

Closing thoughts

One of the key metrics of product quality for polymer pellets is the amount of fines, dust and streamers that are generated during handling. The presence of such unwanted species has a direct bearing on downstream applications, and therefore to the acceptability and value of the final product.

During the past decade, significant progress has been made toward the standardization of measurement methods available to quantify these unwanted contaminants. These standards provide a common basis for evaluating product quality, for assessing the performance for pellet-cleaning systems, and for troubleshooting processes. The success of these standards hinges upon the user’s understanding of the underlying concepts and careful attention to the details. In this article, the authors have also introduced an innovative approach to measure tails and short streamers in the pellets.â– 

Edited by Suzanne Shelley

Authors

Shrikant Dhodapkar is a technical leader in the Dow Elastomers Process R&D Group at The Dow Chemical Co. (Freeport, TX 77541; Phone: 979-238-7940; Email: sdhodapkar@dow.com) and Adjunct Professor of Chemical Engineering at the University of Pittsburgh. He received his B.Tech. in chemical engineering from I.I.T-Delhi (India) and his M.S.Ch.E. and Ph.D. from the University of Pittsburgh. During the past 20 years, he has published numerous papers in particle technology and contributed chapters to several handbooks. He has extensive industrial experience in powder characterization, fluidization, pneumatic conveying, silo design, gas-solid separation, mixing, coating, computer modeling and the design of solids processing plants. He is a member of AIChE and past chair of the Particle Technology Forum.

Remi Trottier is a senior specialist in the Solids Processing Discipline of Engineering & Process Sciences at The Dow Chemical Co. (Phone: 979-238-2908; Email: ratrottier@dow.com). He received his Ph.D. in chemical engineering from Loughborough University of Technology, U.K,, and M.S. and B.S. degrees in Applied Physics at Laurentian University, Sudbury, Ont. He has more than 20 years of experience in particle characterization, aerosol science, air filtration and solids processing technology. He has authored some 20 papers, has been an instructor of the course on Particle Characterization at the International Powder & Bulk Solids Conference/Exhibition for the past 15 years, and has authored an article on particle characterization for the “Kirk-Othmer Encyclopedia of Chemical Technology.”

Billy Smith is a senior research chemical technologist in the Fluid and Mechanics Mixing Group in Engineering and Process Sciences at The Dow Chemical Co. (Phone: 979-238-2097; Email: BGSmith2@dow.com). He received his Bachelors degree in Applied Technology in Industrial Management and an Associate of Science degree in physics and chemistry. For the past 10 years, he has worked on numerous projects related to fluid mechanics and mixing technology. He has extensive laboratory experience in both lab equipment and wet chemistry.

 

 

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