Discolored specks

Gels are generally defined as small, more or less round defects in extruded products, especially film or thin walled tubing. Some people define gels as any particle in an extruder plastic product that has visual properties different from the rest of the product. This includes discolored specks, contamination, crosslinked polymer... 

We will define gels as small spherical droplets or specks with a distinct boundary that can be observed by simple visual inspection. The material making up the gel particle is basically the same as the polymer of the surrounding film. Therefore, a gel particle is different from contamination. In many cases the gel particle has no discoloration see for instance Fig. 11.38. Small droplets with strong discoloration are generally referred to as discolored specks this topic is discussed in Section 11.3.7.4. 

Section 11.3.7.4 also shows expressions that can be used to relate the frequency of discolored specks in the raw material to the frequency of discolored specks in the extruded product. These expressions can also be used to relate the frequency of gels in the raw material to the frequency of gels in the extruded product. 

Discolored specks are a common problem in extrusion and molding. This problem is similar to another common defect, which is the problem of gels. Like gels, discolored specks are formed not only in extrusion and molding at the processor but also in polymerization at the resin producer. As a result, in order to get a handle on the problem, we need to know how many specks are in the incoming raw material. We will distinguish between a speck formed in polymerization and located inside the pellet (P-speck) and a speck formed in extrusion (E-speck). 

Today commercial instruments are available that allow analysis of millions of pellets. Optical Control Systems (OCS) GmbH produces systems for defect detection. OCS makes instruments for defect detection in pellets as well as in extruded sheet and film. Pellet defect detection systems by OCS are used by many resin producers. Therefore, most resin companies know how many defects occur in the pellets they produce. Figure 11.45 shows an example of pellets with discolored specks. 

Figure 11.45 Pellet with black and discolored specks (courtesy OCS)...

If we have one pellet with a discolored speck for every N pellets, we can determine the average incidence of specks in the extruded product. If the cross-sectional area of the extruded product is and the volume of the pellet or powder particle is Vp, the average length over which a P-speck will occur in the extruded product is ... 

Pellets can be scanned and sorted not only for discolored specks but also for irregular pellets, for instance, pellets with tails. Figure 11.46 shows a picture of pellets with tails such pellets can cause problems in extrusion and are preferably eliminated from the feed stream. 

The causes listed in Fig. 11.47 by no means present a complete listing. Figure 11.47 shows some of the more common causes of discolored specks however, other causes can certainly play a role. A detailed discussion of all the causes listed in Fig. 11.47 is beyond the scope of this book, but we will discuss one cause in detail screw wear. 

By careful process monitoring we can often detect the effects of wear. The process parameters that are affected by wear are meit temperature, output, and motor ioad. As wear progresses, the quality of the extruded product tends to deteriorate. This may manifest itself as discoloration, streaks, discolored specks, holes, etc. ft is very important to monitor the specific energy consumption (SEC) and specific extruder throughput (SET) because changes in these parameters often correiate with wear. SEC is the ratio of motor power divided by the throughput, ft is the mechanical power consumed per unit mass of plastic. The SEC tends to correiate with the melt temperature. The SEC is normally expressed in kWh/kg. A typicai vaiue of the SEC is 0.25 kWh/hr for extrusion of polyoiefins. 

Discolored specks are ubiquitous in the plastic extrusion industry. Controlling specks starts with quantifying P-specks in the incoming raw material. Once we can control P-specks to an acceptable level, we can address E-specks. Important factors that affect E-specks have been identified with one factor, screw wear, discussed in detail. 

Specks are a common problem in extruded products, particularly in thin or transparent products. The specks can be black, brown, yellow, or almost any other color different from the matrix material. Specks are usually caused by contamination, degradation, or wear. Degradation can manifest itself as discoloration, specks, pinholes, loss of volatiles (smoking), or loss of physical properties in the extruded product. 

In Chapter 11, the section on gels in extruded products has been expanded as this continues to be a problem experienced by many extrusion companies. There is also a new section on discolored specks in extruded products. In this section expressions are included that allow prediction of the incidence and frequency of specks or gels based on their frequency in the incoming raw material. Included is a discussion on new instruments that are now available to detect defects in pellets produced at the resin supplier with the ability to remove pellets with defects from the pellet stream. 

The origin of defects causing discoloration in polyolefin pellets can be identified using light and electron microscopy. For example, PE pellets from an in-plant recycle re-pelletlzing process contained pellets that were off color and had black specks, as shown In Fig. 11.6(a). One of these defects was isolated using the cross sectioning technique, as shown in Fig. 11.6(b). The cross section revealed an intense reddish particle that caused the discoloration of the pellet. 

The volume of the pellet is 20 mm We will consider a plastic raw material that contains one pellet with a discolored P-speck for every 10,000 pellets this corresponds to 100 ppm. With this input data, we find the average length over which a P-speck will occur is 5806 mm or 5.8 m. If we generate specks in the extrusion process, the average length over which a speck occurs in the extruded product will be less than 5.8 meter. 

Discoloration of the plastic inside the extruder can be caused by degradation, contamination, and several other causes. Figure 11.47 shows a listing of possible causes of specks generated in the extrusion process. 

Polymer degradation can result in discoloration and other associated visual defects (such as gels and black specks), inconsistent flow characteristics and a general deterioration in physical properties. Molecular weight can increase as a result of crosslinking reactions or decrease as a result of chain scission reactions. 

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