The influence of carbon blacks on the extrusion operation for hose production.

The extrusion process in hose production is the link between mixing and a curable preform that will ultimately become a highly engineered conveyor of material. Since a number of finished performance properties, e.g., dimensional quality and structural integrity, depend significantly on the extrusion operation, this process is essential to final and long-term performance of any hose product. Finally, the extrusion operation itself can be one of the bottlenecks that limits the throughput and speed of hose production.

If we analyze a typical hose compound, we find that the polymer is chosen primarily for its final performance properties and secondarily for its manufacturing flexibility. The next most critical material in the hose compound is the reinforcement system, which, in most cases, means carbon black. In the extrusion operation, carbon black influences viscosity, smoothness, wall-gauge control, green strength, cleanliness and cost. Therefore, proper carbon black selection is strategic in making the extrusion operation as efficient as possible, and significantly influences the final product's performance. Improper carbon black selection can cause poor dispersion, high contamination, surface roughness, low green strength and poor wall-gauge control. These problems can result in high scrap, poor efficiency and premature product failures.

Basic characteristics of a good extrusion

The attributes of a good extrusion begin with proper wall-gauge control, since the dimensions of the extrudate will become the dimensions of the final hose. Given the fact that all rubber compounds exhibit swelling coming out of the die, and this die swell is directly proportional to extrusion shrinkage in the length direction, control of this property is critical to the final dimensions of the hose. Normally, the extrusion shrinkage/die swell should be kept to a minimum since lower shrinkage values tend to produce products that have lower levels of dimensional variation.

The next characteristic of a good extrusion is viscosity, a property that also must be kept under control. If viscosity is too low, then the extrudate will sag and collapse; if it is too high, then production efficiency can be affected, along with performance properties (e.g., adhesion of various extrusion layers).

Another key property of an extrusion is green strength. This characteristic is very difficult to measure, but it certainly influences the collapse resistance and strength of the extruded tube. Many times green strength also influences the proper application of textile and wire reinforcements to the hose.

The heat history of the compound through the extrusion process is also a critical parameter during processing. Significant care must be taken to keep energy and temperature as low as possible to maintain the cure reaction below its threshold energy and temperature limits.

Cleanliness and smoothness of the extrudate are two other important elements of a good extrusion - properties that are often overlooked unless they become the cause of excessive in-process scrap. Cleanliness refers to the inherent cleanliness of the raw materials and the proper dispersion of these materials during the mixing process. The raw materials must also be free of contamination, which can cause defects (e.g., die marks on the extrudate) and will slow the efficiency of the operation (via screen clogging). The extrusion smoothness refers directly to the properties of the reinforcing raw materials (e.g., carbon black) and their ability to maintain a sleek surface on both the inside and the outside of the extrudate. This is critical on both the inside diameter (ID) as well as the outside diameter (OD) of the extrudate. Flaws on the inside of the extrudate can cause premature failures and may be targets for chemical attacks such as electro-chemical degradation.

Finally, cost is certainly one of the most important attributes of a good extrusion. Many times a less expensive raw material is selected to reduce compound cost at the expense of the quality and throughput in the extrusion operation without properly weighing the resulting scrap and inefficiency against the total systems cost. This often occurs when a strategy of selecting high loadings of poor extruding carbon blacks or white fillers is used, resulting in compromises to the manufacturing operation.

Influence of the mixing process

Since the mixing process precedes the extrusion process, it has a tremendous influence on the final extrudate characteristics. The three main areas of influence are dispersion, mastication and heat history.

The proper dispersion of ingredients is critical not only to the performance of the extrudate, but the final product as well. Proper dispersion will eliminate the problem of scrap due to undispersed materials, produce a surface that is as smooth as possible and ensure an extrusion process that runs efficiently. In contrast, improper dispersion can lead to high scrap, surface imperfections and shutdowns due to screen-pack plugging and die marks.

Mastication of the polymer is difficult to measure and its effect on the extrusion process is very subtle. The most common problem usually results from too much mastication during the mixing process, which can cause variability in green strength and wall-gauge control - the latter usually from stretching rather than from die swell. The phenomenon is caused by a breakdown of the polymer's molecular weight during the mixing operation. Although somewhat rare in occurrence, it can still occur, especially when multiple mixes are required to disperse the raw material ingredients completely.

Finally, the heat history of the compound begins accumulating with the mixing process, so any excess stress on the curative reaction will carry over to the other operations that follow. Such occurrences are described as fast- or short-scorch compounds and, in severe cases, the compound will actually cure during the extrusion process, causing significant scrap and downtime. Since carbon black has one of the greatest influences on the proper dispersion of ingredients during the mixing operation, proper selection is absolutely critical to the efficiency and proper performance of both the mixing and extrusion operations.

Particle size and surface area, structure and cleanliness

Before we can discuss the influence of carbon black on the extrusion process, we must first have a basic understanding of the characteristics of carbon black. The morphological characteristics of carbon black that most directly affect the extrusion process - and the mixing process that precedes it - include particle size, structure and cleanliness.

Carbon black is actually a group of aggregates consisting of particles of similar size, whereby the diameter of the individual particles in the aggregate is considered to be the particle size, and the number of panicles in the aggregate is the structure. The best analogy is that of a bunch of grapes on a stem. The size of the individual grape represents the particle size, while the number of grapes on the stem represents the structure. Since surface area usually inversely relates to particle size, this attribute is used to describe a carbon black's particle size. Therefore, the higher the surface area, the smaller the particle size generally is.

The structure of the carbon black is the number of particles in an average aggregate. This attribute is measured by the amount of oil (DBP) absorbed into the aggregate. High DBP absorption equals high structure, while low DBP equals low structure.

Finally, the manufacturing process to make carbon black can form undispersible contaminants called grit. This contamination can cause extrudate defects and block screen packs, as previously discussed. Therefore, it is extremely important that the cleanliness properties of a carbon black be considered during the selection process to prevent this contamination.

How carbon black morphological characteristics meet the needs of a good extrusion

Effect of carbon black on the mixing process

We have already seen that the mixing process is critical to the quality of the extrusion process, so we must first evaluate the influence of carbon black on mixing-process properties before we can fully evaluate its influence on the extrusion process. For the purpose of this article, we will only look at those properties of the mixing process that have already been defined as being significant to the extrusion process:

* Loading capacity for cost;

* mixing temperature and predicted mixing time for heat history;

* dispersibility for proper incorporation of ingredients; and

* viscosity for green strength.

Table 1 shows the influence of carbon black characteristics on mixing-performance properties. The effect of panicle size shows that a low-surface-area (large-particle-size) carbon black is the best choice for efficient mixing and low compound cost. Therefore, when given a choice of carbon blacks that are capable of meeting the performance properties, the largest particle size carbon black should be chosen. The effects of structure are not as easy to determine. Usually, a high-structure black is considered the best choice for good mixing characteristics. In fact, this is especially tree with high-surface-area carbon blacks, since high-structure variations are the only means to achieve reasonable dispersion under normal mixing conditions in commercial operations. However, for some applications using low surface-area blacks, a slightly lower structure black can strike the best balance between cost and ease of processing. Generally, the higher the surface area and the lower the structure, the poorer the resulting mixing performance or the longer the mixing cycle required to achieve equal dispersion.

Table 1 - general effects of changes in surface area and structure on mixing properties

Mixing property         Increasing surface      Increasing
                              area                structure

Loading capacity            Decreases            Decreases
Predicted mixing time       Increases            Decreases
Mixing temperature          Increases            Increases
Dispersibility              Decreases            Increases
Viscosity                   Increases            Increases

Effect of carbon black on the extrusion process

Carbon black affects all aspects of the extrusion process. The morphological selection of carbon black is key to the viscosity, shrinkage, wall-gauge control, smoothness, temperature, throughput, green strength and scorch potential for almost all extruded hose compounds. This fact, combined with the tremendous influence that carbon black has on the mixing process, makes the proper selection of carbon black the second most critical part of the compound's development. (Selection of the base polymer is still the most critical part of a hose compound's design.)

In table 2, we see that both surface area and structure play an important role in determining the extrusion performance of a given compound. However, since many hose compounds only require the reinforcing ability of a low-surface-area carbon black, structure will tend to dominate the effect of any given carbon black on the hose extrusion process. As with mixing, the general rule is the higher the surface area - and especially the lower the structure - the worse the resulting extrudate becomes.

Table 2 - general effects of changes in surface area and structure on extrusion properties

Extrusion property      Increasing surface       Increasing
                               area               structure

Viscosity                   Increases            Increases
Wall gauge control        Little effect          Increases
Extrusion shrinkage         Decreases            Decreases
Extrusion smoothness        Decreases            Increases
Extrusion temperature       Increases            Increases
Extrusion rate              Decreases             Variable
Scorch time               Little effect          Decreases
                                                 Increases

Effects of carbon black on specific extrusion attributes

Die swell, extrusion shrinkage and wall-gauge control Three extrusion attributes - die swell, extrusion shrinkage and wall-gauge control - are all interrelated. As previously mentioned, die swell is directly proportional to extrusion shrinkage because as the rubber compound swells coming out of the die, it also shrinks in length to compensate for the change in diameter. The greater the die swell, the greater the extrusion shrinkage. This phenomenon can be measured very accurately and can be used to evaluate the effect of various carbon blacks on the control of the resulting wall gauge (ref. 1). Wall gauge control is related to die swell in an indirect way. The lower the die swell (and extrusion shrinkage), the lower potential variation there will be in the wall gauge. The reasoning behind this is that the lower the die swell, the lower the amount of total variation (batch-to-batch and day-to-day) there is in an extrusion operation. The lower the variation in swell, the lower the variation in the wall-gauge dimension created by the die. Therefore, the fewer the adjustments that are necessary to maintain the correct wall gauge from batch-to-batch and day-to-day.

The main carbon black attribute that controls die swell is the structure. The higher the structure of the carbon black, the lower the die swell is. We will see later that a slight upward adjustment in the structure of a carbon black yields lower die swell, even though the carbon black has slightly lower surface area (larger particle size). See table 4 for more details.

Table 4 - extrusion performance and select physical properties of NBR compounds with various types of carbon black

Carbon black                       N650       N660       N774

                                      Extrusion performance

Loading                              80         80         95
Loading rating                        5          5          2
Mooney viscosity                     92         74         73
ML 1+4 @ 100 [degrees] C
Viscosity rating                      1          3          4
Extrusion shrinkage, %               21         30         34
Shrinkage rating                      1          3          4
Extrusion smoothness              16/16       9/16      11/16
Garvy rating
Smoothness rating                     1          5          3
Green strength, MPa                0.83       0.74       0.79
Green strength rating                 2          5          4
Dispersion - Phillip's              8.3        7.8        8.0
  rating
Dispersion rating                     3          5          4
Total extrusion rating               13         26         21
Best carbon black for               2nd        6th        4th
  NBR hose extrusion

                                       Physical properties

Hardness, Shore A                    76         74         74
Tensile, MPa                      18.39      18.25      16.60
Elongation                          340        348        363
100% modulus                       5.78       4.60       4.32
Die C tear-kN/m                   48.34      49.91      48.16
Compression set                    72.5       71.3       70.5

Carbon black                       N762      N990     IRX1046

                                       Extrusion performance

Loading                              95        140         95
Loading rating                        2          1          2
Mooney viscosity                     70         60         90
 ML 1+4 @ 100 [degrees] C
Viscosity rating                      5          6          2
Extrusion shrinkage, %               35         50         27
Shrinkage rating                      5          6          2
Extrusion smoothness              11/16       7/16      16/16
 Garvy rating
Smoothness rating                     3          6          1
Green strength, MPa                0.81       0.62       1.02
Green strength rating                 3          6          1
Dispersion - Phillip's              7.8        9.1        9.1
  rating
Dispersion rating                     5          1          1
Total extrusion rating               23         20          9
Best carbon black for               5th        3rd        1st
  NBR hose extrusion

                                       Physical properties

Hardness, Shore A                    73         74         75
Tensile, MPa                      16.39      13.50      16.73
Elongation                          393        503        360
100% modulus                       4.16       2.93       5.18
Die C tear-kN/m                   50.79      48.98      50.01
Compression set                    71.8       68.3       70.7

Note that this relationship should not be confused with the stretching and snapback that can occur when the takeaway system of the extruder runs faster than the actual extruder speed. Such a problem causes stretch in the extrudate and the elasticity of the compound will try to compensate for the stretch by snapping back to its reference position. Since this stretch can cause extreme variation in the resulting extrudate, excessive stretch must be avoided and corrected if found.

Green strength and collapse resistance

Two other extrusion attributes are also related, since the collapse resistance of a given hose extrudate is directly related to its green strength. The higher the green strength, the better the collapse resistance. Green strength is also a critical attribute when a reinforcing textile or wire is added during the extrusion process. Again, the higher the green strength, the better the resistance to pull-through or cutting of the extrudate by the textile or wire reinforcement.

The morphology of carbon black affects green strength in terms of both loading and structure. The higher the loading, the better the compound is able to deflect the force of a textile or wire cutting through its surface. Also, the higher the loading, the better the collapse resistance of the resulting tube - especially for low viscosity compounds.

Structure also affects the green strength of the extrudate. The higher the structure, the higher the resulting viscosity, and the better the deflection potential of the resulting extrudate against collapse and cut-through by reinforcing textiles or wire. Therefore, the best carbon black for good extrudate green strength is a highly loadable, high-structure carbon black. Since the lower the surface area, the larger the particle size and the higher the loading, therefore, a low-surface-area, high-structure carbon black will be the best choice for green strength.

Smoothness and cleanliness

As previously mentioned, smoothness and cleanliness are two properties that are often overlooked when designing a good extrusion compound. Only when they cause excessive scrap or low extrusion performance do compounders tend to design to improve these attributes. However, both are extremely important to any hose extrusion operation.

A rough extrusion can cause poor adhesion to textile, wire or other rubber layers. Such roughness can also cause unnecessary variations in wall gauge and could ultimately lead to such defects as spider marks or thin spots. As with other carbon black attributes, a low-surface-area, high-structure carbon black will provide the smoothest extrusion available. Lack of cleanliness of undispersed raw materials or contamination by foreign materials will obviously cause defects and potential failure points in the extrudate. These are usually dealt with as they are seen, but many times the defects are not visible. In such cases, the result can then be an increase in the pressure behind the screen pack, causing either an unusually high temperature or frequent screen-pack changes. Hence, it is desirable to select a clean, easily dispersed carbon black to make sure that there is no contamination from either undispersed material or from foreign material in the carbon black.

Selection criteria of CBs for hose extrusion applications

Usually, the normal carbon black selection criteria for hose extrusion applications focus on the loadability of the carbon black into the polymer. This selection procedure is especially common when expensive polymers are being used for the application. Here it is typical to take a large-particle-size black with low structure to capitalize on its high loading capability as the main selection criterion. However, many times these high-loading carbon blacks are not of the proper design for hose extrusion applications because of their low structure. Carbon blacks that fall into this category are N990, N762 and N660. An alternative is to use a high-structure, large-particle-size carbon black (e.g., N650 or N774). These grades yield reasonably good extrusions, but their low-loading capability can make the compound's cost too high. Taking an NBR hose compound as an example, let us examine these carbon blacks against a new product specially designed by Cabot specifically for hose applications: IRX1046. IRX1046 demonstrates a unique combination of large particle size (low surface area) and high structure.

Carbon black selection for hose extrusion applications

For the purpose of this article, we will examine a basic NBR rubber, one of the more frequently used polymers in hose compounds. Therefore, the data shown can be extrapolated across a wide variety of applications and can also be related to other polymers such as CPE, CSM and CR. EPDM was not chosen for this article because its normally high oil loadings detract from the influence of the carbon black on extrusion characteristics. The compound shown in table 3 will be the basis for our evaluation, with carbon black loadings being varied to match equal hardness.

Table 4 shows the results of laboratory testing based on the compound shown in table 3. The results highlight those tests that relate to extrusion performance, plus some basic physical properties to show the relative performance capabilities of the carbon blacks tested. The extrusion performance tests evaluated in this trial included: Loading (for cost); viscosity; extrusion shrinkage (for wall-gauge control); smoothness using the Garvy rating; green strength; and dispersion.

Table 3 - typical NBR hose formulation 

Formulation                        phr

NBR (34 ACN/50 Mooney)             100
Zinc oxide                           5
Stearic acid                         1
Antioxidant/antiozidant            1.5
DOP                                 10
Sulfur                             1.5
Accelerator                        1.5
Carbon black                  Variable

Finally, each carbon black was rated for its ranking in each category and the combined ratings were used to judge the final recommendations for extrudability in NBR hose compounds. The carbon blacks evaluated - N660, N762, N650, N774, N990 and the new IRX1046 grade - were selected for their normal use in NBR-type applications. The results show that IRX1046 is by far the best choice for extrudability in NBR hose compounds. This carbon black ranked first or second in every performance test and had the best overall rating of any of the blacks tested. N650 also proved to be an acceptable black for NBR hose extrusion applications, although it lacked the loadability, green strength and dispersibility of IRX1046.

The carbon black grades that were found to be the worst for NBR hose extrudability were N660 and N762, yet these products are often specified simply because of their loadability and physical properties. The results shown in table 4 clearly demonstrate that neither of these carbon blacks should be used for NBR hose applications, because their low extrusion ratings will cause more problems than their loading advantages can overcome.

Cleanliness

As started previously, commercial carbon black grades can carry contamination from their manufacturing process. Extreme efforts must be taken to eliminate these contaminants from the carbon black before it is used for critical applications such as hose extrusion. One of the best tests to determine the effect of contamination is to measure the pressure rise while a compound is being passed through a narrow-meshed screen pack. This test, called the Delta P test, not only measures the contamination of a carbon black, but also its inherent dispersibility as well.

In Table 5, we see the results of several carbon blacks tested through a 325-mesh screen. The pressure is measured both before the compound begins extruding and after the extrusion is almost complete. For the purpose of this test, 120 phr of each carbon black were mixed with 175 phr of an oil-extended EPDM. This polymer was selected for its inherent oil loading, viscosity and general cleanliness. The carbon blacks chosen for this trial were N650, a grade of N650 designed for cleanliness, N550 and the new IRX1046 grade. These blacks were chosen because of their good dispersion characteristics, since poor dispersing carbon blacks would give unusually high results in this type of test.

Table 5 - delta P test of EPDM compounds indicating the relative cleanliness of various carbon black grades

Carbon black       Blank    N550     N650      N650    IRX1046
                                              clean

Loading                0     120      120       120        120
Pressure before    1,715   1,650    1,734     1,664      1,685
Pressure after     1,821   2,385    3,246     1,920      1,891
Delta P              106     735    1,512       256        206
Rating                NA       3        4         2          1

The results show that IRX1046 had the lowest pressure rise of the grades tested, followed by the N650 designed for cleanliness. The normal N650 and the normal N550 grades both exhibited a much higher pressure rise, indicating that material was plugging the screen, requiring additional pressure to flow the material through the screen at a consistent speed. The blank was an unfilled control designed to show the base level of pressure rise in an unfilled compound.

Conclusion

Based on the data from the NBR hose compound evaluation and the Delta P testing, a new grade of carbon black specifically designed for hose extrusions - IRX1046 - showed the best combination of extrusion properties for NBR and similar polymers used in hose applications. This carbon black demonstrated equal loadability to both N774 and N762, but provided significant improvements in viscosity, shrinkage, smoothness, green strength and dispersion. Therefore, IRX1046 offers the best all-around combination of high loading, excellent extrudability, good green strength and cleanliness to make it the preferred carbon black for many extruded hose applications.