The Effect of Binder Systems on Dispersion and Mixing Performance

ABSTRACT

The effects that binder systems have on chemical bound dispersion properties were evaluated. Both polymer and liquid binder systems were assessed for their effects on chemical melt point, dispersive properties, incorporation time, cost competitiveness, and activity level. Multi chemical liquid bound dispersions were also evaluated against raw material pre-weighs for improvements in cycle times, cure properties, physical properties, and cost. Liquid binder systems had the most dramatic impact on overall quality in both single and multi ingredient dispersions.

Types of Chemical Dispersion Binders

Chemical dispersions have been used in the rubber industry for many years. Polymer bound dispersions (PBD) are the oldest and still most widely used chemical dispersions. They can be made with a variety of different polymers, but EPR is the most common because it is not cross linked with sulfur or sulfur containing accelerators, allowing for longer shelf life. EPR polymers can also accept higher filler concentrations, allowing for increased activity levels. PBD’s are ideal for powder raw materials that can cause health hazards and housekeeping issues such as lead or ETU. Drawbacks to PBD’s include cost, lower activity levels, increases in raw material melt point, and added heat history that can cause scorch issues in ultra fast accelerators like ZDMC or BDMC.

Liquid bound dispersions (LBD) are the newest innovation in dispersion technology. The name liquid bound dispersions can be misleading because the end product is still a bead or powder. The name is derived from the oil and other additives that are used to reduce the melt point of the powder chemical, increasing its dispersability. Liquid binder systems tend to be cheaper than polymer binder systems because the primary ingredients (oils and waxes) tend to be cheaper than polymers. LBD’s also allow for higher activity levels which reduce the cost of the binder system.

Science behind Liquid Bound Dispersions

The most common types of chemical bound dispersions are rubber curatives and accelerators, because sulfur and most accelerators are insoluble in rubber at cure addition mixing temperatures (82°C to 110°C). Therefore, to achieve proper accelerator dispersion, the individual particle must be encapsulated by the rubber. “Poor dispersion of accelerators can be seen as undispersed streaks or particles of powder. It can also be seen as small convex flaws on the surface of the part, referred to as pebbling. Poor dispersion of accelerators can cause inconsistency in the crosslink density throughout the cured part, leading to the initiation of a failure event; such as inconsistent modulus, tensile rupture, inconsistent tear, or fatigue failure.” 1 So to achieve excellent accelerator dispersion, you have to try to reach the accelerator’s melt point either in your mixing or curing process.

TABLE 1

Examples of Common Accelerator Melt Points

Accelerator DPTT TMTD MBTS DPG DOTG BDMC ZDMC TBBS MBS ETU
Melt Point 135°C 155°C 180°C 97°C 170°C 230°C 253°C 104°C 80°C 195°C

When looking at the accelerators in TABLE 1 you will notice that few of the accelerators listed would reach their melt points in the mixing process, since normal cure addition mixing temperatures range from 82°C to 110°C. There is also a distinct possibility that any accelerator with a melt point over 160°C would not achieve proper accelerator dispersion in the molding process, especially larger molded SBR or NR parts like conveyor belts or tires. Ultra fast accelerators like ZDMC or BDMC would not reach their individual melt points even in a 204°C salt bath extrusion process. One way to improve the likelihood of proper accelerator dispersion is to use accelerators on a liquid binder system because the melt point of the accelerator is reduced.

Graph 1

In GRAPH 1, three different forms of MBTS are evaluated for melt point and the amount of energy required to melt the material. A standard MBTS powder, MBTS 75% PBD, and a MBTS 75% LBD are used. There is a slight increase in the melt point of the MBTS 75% PBD because of the addition of the polymer in the binder system. EPR polymer is considered stable until it reaches temperatures of 300°C. Although the actual change in melt point going from the MBTS powder to the MBTS 75 PBD is minimal, you can see that even small changes in melt point have a greater impact on the amount of energy required to melt the material. There is a significant difference in the drop in melt point from the MBTS powder to the MBTS 75% LBD. This reduction in melt point in the MBTS 75% LBD has a positive impact on the amount of energy required to melt the material.
MBTS Mill Study
Once the difference in melt point and the amount of energy required to melt the different MBTS samples was evaluated, we wanted to see if these differences actually translated into meaningful results in real life. A mill study was set up using four different forms of MBTS. The MBTS powder, MBTS 75% PBD, MBTS 75% LBD, and a MBTS 90% LBD were added to a SBR compound on a lab mill and reviewed for dispersion quality, incorporation time, cost competitiveness and activity level.
The SBR compound was banded on the mill for 30 seconds, creating a bank of material at the nip of the mill, at which point the MBTS samples were added. We allowed one full revolution on the mill once the MBTS samples were added. Then the mill was stopped to take the 0 second pictures. The mill was re-started and all dispersing action happened at the nip of the mill; there were no cuts made with a mill knife to reduce variability. The mill was stopped at different time intervals so pictures of the dispersion could be taken. The time intervals used were 0, 30, 60, 75, 90, 120, and 135 seconds.

MBTS 75 PBD 0 Seconds
MBTS Powder 0 Seconds

MBTS 75 LBD

MBTS 90 LBD
At 0 seconds, the MBTS 75 PBD is dispersing the best. All of the other MBTS samples still have quite a bit of loose material at the nip.

MBTS Powder 30 Seconds

MBTS 75 PBD 30 Seconds

MBTS 90 LBD 30 Seconds

At 30 seconds, the MBTS PBD is still dispersing the best, the 75 and 90 LBD’s look similar to each other and are dispersing better than the MBTS powder. The MBTS powder still has a lot of loose powder in the nip of the mill.

MBTS Powder 60 Seconds

MBTS 75 PBD 60 Seconds

MBTS 75 LBD 60 Seconds

MBTS 90 PBD 60 Seconds

At 60 seconds, the MBTS 75 LBD looks the best. The MBTS 90 LBD still has streaks in it and the MBTS 75 PBD still has undispersed particles. The MBTS powder still has loose material on the nip of the mill.

MBTS Powder 75 Seconds

MBTS 75 PBD 75 Seconds

MBTS 75 LBD 75 Seconds

MBTS 90 LBD 75 Seconds

At 75 seconds, both the MBTS 75% and 90% LBD’s are completely dispersed. The MBTS 75 PBD still has undispersed particles and the MBTS powder still has a lot of undispersed material in the nip of the mill.

MBTS Powder 90 Seconds

MBTS 75 PBD 90 Seconds

At 90 seconds, the MBTS powder no longer has any loose powder in the nip and the MBTS 75 PBD still has undispersed particles.

MBTS Powder 120 Seconds

MBTS 75 PBD 120 Seconds

At 120 seconds, the MBTS powder still has small amounts of streaking and the MBTS 75 PBD still has undispersed particles and has not improved at all in the last 60 seconds.

MBTS Powder 135 Seconds 

MBTS 75 PBD 135 Seconds

At 135 seconds, the MBTS powder is completely dispersed and the MBTS 75 PBD still has undispersed particles.

MBTS Mill Study Discussion and Results

The MBTS mill study showed the same results when comparing the different MBTS samples for melt point and the amount of energy required to melt the material. The MBTS LBD’s dispersed 44% faster than the MBTS powder and MBTS 75 PBD took longer than the MBTS powder. (see Graph 2).

GRAPH 2

The polymer in the MBTS 75 PBD initially wants to disperse faster because the EPR in the binder system is a good processing polymer; however, since the polymer is stable to 300°C it is not going to melt in any rubber process. Also consider where most curatives and accelerators are added within the mixing cycle – after the fillers, oils, antioxidants, and process aids, which reduces the shear of the compound. Traditionally polymers would not be added after all of these other raw materials, but that is essentially what is happening when using a PBD, which makes it very hard to disperse the polymer in the binder system.
Not only do the MBTS LBD’s disperse faster but they are also cheaper than traditional MBTS PBD’s. Polymer tends to be more expensive than the additives used to make liquid bound dispersions. When PBD’s were introduced many types of EPR / EPDM polymers were very cost competitive; unfortunately, with increased volatility in the oil markets over the last 20 years, many companies have shied away from using PBD’s because the cost can fluctuate dramatically. If you look at just the cost associated with binding the MBTS, there is a substantial difference in cost (See Graph 3).

GRAPH 3

There is a 39% increase in the cost of MBTS to buy it in a PBD form, while there is only a 15% increase to buy MBTS in 75% LBD and only an11% increase to buy MBTS in a 90% LBD. Not only are the raw materials used to make LBD’s cheaper, but since they can be made at higher activity levels, there is less binder which also reduces cost.
Multi Chemical Liquid Bound Dispersions
Multi chemical liquid bound dispersions are also called chemical blends. Chemical blends can be comprised of any non-bulk raw material that is not injected directly into the mixer, such as zinc oxide, fatty acids, antioxidants, curatives, accelerators, resins, metal oxides, and process aids. Chemical blends improve batch to batch consistency, reduce variation in cycle times, and reduce fly loss. Pre-weigh chemical blends in low melt bags offer “peace of mind” by reducing scrap and down time by eliminating human error.
Chemical blends also reduce the number of in house raw materials, which has a positive impact on plant cleanliness, warehouse space needed to store raw materials, and vendor headaches. Even though there is added cost in chemical blends if the chemical blender is buying certain raw materials in truckload quantities that most mixing operations would not buy in such large quantities, these off sets can help minimize the cost.

Chemical Blends vs. Pyramid Weighing

Pyramid weigh outs offer many of the same advantages as chemical blends; however, chemical blends exclusively offer increased raw material consistency and reduced melt points. Chemical blends can offer this because the raw materials are blended at high speeds in a high shear mixer with binder systems designed to reduce the overall blend melt point. Pyramid weigh outs use an automated weighing system that eliminates human error and weigh multiple ingredients to specified weights into low melt bags. Hence the name “pyramid” weighing because raw materials are being added in layers to the bag and not being adjusted in any other way. Pyramid weighing does not improve upon the product; it only improves upon the process by eliminating any human error; whereas chemical blending improves the product and the process (see Figure 1).

FIGURE 1

Chemical Blend vs. Pyramid Weigh Out

See FIGURE 1. By looking at the visual differences between a pyramid weigh out and a chemical blend you can see the differences that high speed mixing and a binder system can have on the raw materials, making them more uniform, which reduces hot spots and poor dispersion in the mix.

How Chemical Blends Improve the Product

When you add multiple curatives and accelerators in your mixing process, you are essentially adding materials with different melt points that once inside the mixer can create pockets of materials that will melt and disperse at different times, which can lead to hot spots and poor accelerator dispersion. By blending the curatives and accelerators together with a liquid binder, a more uniform product is added to the mixer with a uniform melt point, eliminating these pockets of hot spots and poor dispersion. (See Graph 4)

GRAPH 4

Cure System Melt Points

Looking at the cure system in Graph 4 you can see that both the sulfur and the Retarder Esen would not hit their respective melt points in the mixing process, which means it has to happen during the curing process and if it does not, this could lead to a failure event. However, the chemical blend of the curatives in Graph 4 with the binder system will reach melt point in the mixing process, ensuring good accelerator dispersion.

Chemical Blend vs. Pyramid Pre-weigh Study

To perform this study we used an EPDM compound and mixed 10 lab batches using chemical blends and 10 batches using pyramid weigh outs. All batches were mixed in a BR 3 liter lab mixer and sheeted off on a two roll mill. The lab batches were evaluated for consistency in cycle times, cure properties, and compound physical properties.

TABLE 2

EPDM FORMULAS

Raw Material

PHR Pyramid Weigh Out

PHR Chemical Blend

EPDM Polymer

100.0

100.0

Silica

102.0

95.88

TiO2

25.0

Paraffinic Oil

34.0

30.88

Zinc Oxide

5.0

Fatty acid

1.0

Sulfur

2.0

Accelerators

3.4

PEG

1.0

PE 617A

5.0

Process Aids

5.0

Chemical Blend 1

32.21

Chemical Blend 2

24.43

Total

283.4

283.4

TABLE 3

MIX SPECS

MIX SPEC

Upside down, all materials

0 Minutes

Scrape

79°C

Scrape

96°C

Drop

107°C

The cycle times for each batch were reviewed for mix consistency. The chemical blend batches were much more consistent and had an 8% improvement in cycle time. (See Graph 5)

GRAPH 5

The batches were also tested for rheological properties on a Monsanto ODR Rheometer. The test was conducted for 6 minutes at 176°C per ASTM D-2084. The chemical blends had more consistent rheological properties than the pyramid weighed material. (See Graph 6 and 7)

GRAPH 6  

Pyramid Weigh Outs

GRAPH 7

Chemical Blends

In regards to physical properties, the chemical blends had higher tensile properties and % Elongation. Physical property testing was done on a Tech PRO Tensitech per ASTM D-412. Not only were the overall physical properties higher on the chemical blend compounds, but they were also more consistent. (See Graph 8 and 9 and Table 3)

TABLE 3

Pyramid

Range

Blend

Range

Tensile

944.0

113

963.2

106

% Elongation

468.3

70

477.6

45

Durometer

68.6

4

70.6

1

When compared for cost, the chemical blend is more expensive than the pyramid weigh; however, when you take into account the 8% savings in cycle time, it actually costs less to use a chemical blend in this particular compound. The improvement in cycle time using the chemical blend actually equated to $0.029 in savings in this compound. Therefore, the customer saved money by switching from a pyramid weigh out to a chemical blend. (See Graph 10 and 11)

GRAPH 10 

Increased Cost / Lb.

GRAPH 11

Actual Cycle Times

Conclusion

LBD’s are superior to PBD’s because they reduce the melt point of the accelerator, increasing the likelihood of proper accelerator dispersion. Proper accelerator dispersion is essential for homogeneous crosslink density and or modulus of a cured rubber part. LBD’s also disperse faster, cost less, and improve productivity over both powder and PBD’s. Multi ingredient LBD’s are called chemical blends and they are superior to pyramid weigh outs because, like their single raw material counter part, they also reduce melt point and improve uniformity. This improves batch to batch uniformity and reduces cycle times. Chemical blends also have a positive impact on tensile and % elongation. In this study, the cost of the chemical blends were lower then the cost of pyramid weigh outs when taking into account the savings in cycle time.

REFERENCES

1). Effective Curative Dispersion for Uniform Mechanical Properties by Fred Ignatz-Hoover and Byron To of Flexsys, Rubber World November 2010.

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