Trends in tire design and the impact on reinforcing fillers in radial car tires.

New tire designs often significantly affect the usage of reinforcing fillers. Depending on the tire performance target, rubber compound changes can be required for a number of the tire components. Modifications may be required in filler loading or the filler type. Additionally, some tire design changes selectively impact individual tire components in terms of weight or distribution. Thus, determining the overall impact of a design change on the reinforcing fillers is not always straightforward.

This article will focus on reinforcing filler usage in passenger tires. Filler loading levels and filler types will be discussed for the individual components. Each component's contribution to the tire's total weight will be reviewed. The effort will be to characterize the filler systems at a macro level and not to delineate properties such as particle size, surface area or surface treatment. Run flat tires, green tires and tire size migration are all significant trends in the tire industry. A detailed examination of each will be made relative to the potential impact on reinforcing fillers.

Background

Each year, Smithers Scientific Services analyzes hundreds of tires from markets around the world. Detailed reconstructive engineering is performed on many of these tires. The tires are dimensionally measured, dissected, and the components are analyzed for construction, mechanical properties and chemical composition.

Although tire constructions and compound formulations vary significantly, there is enough commonality within a given design category that on a macro level it is possible to develop an "average" tire. Thus for radial passenger tires, a composite tire representing the group can be used as a reference by which to judge the impact of design changes as they relate to reinforcing fillers. It is important to note the comparisons made in this article are based only on tires analyzed by Smithers, and should not be considered representative of the entire market.

Radial passenger tires--components

A radial passenger tire will often have more than twenty individual components. The weight percent of the individual component for an average tire is shown in figure 1. From a weight perspective, the rubber components contribute the most, with the tread alone accounting for roughly 30% of the total tire weight.

Any significant tire design change can alter the component weight percentages. Some tires have subtreads, or a unique tread base material, and some do not. The majority of tires have no white sidewall, but for those that do, it is a significant component in terms of weight. Tires with higher speed ratings tend to have cap plies, while the lower speed rated versions may not. The load rating of a tire may dictate usage of one or two body or carcass plies. It is important to keep these design variables in mind when using this composite tire as a reference.

Rubber accounts for approximately 85% of the total radial passenger tire's weight as shown in figure 2. Stripping out the steel and textile materials, Figure 3 charts the rubber components by percent contribution to the total rubber weight in the tire.

The tread is more than double the weight of the next heaviest rubber component. The tread is also one component that is likely to be modified whenever a tire design change is made. Also, in terms of the volume of reinforcing filler used, the tread is one of the more highly loaded compounds.

Figure 4 provides the typical loading level found in six selected components. The loading levels are based on the reinforcing materials as a combined value. Thus, carbon black and silica are both included in the value. Levels vary by tire component, but for most compounds the reinforcing filler weight is equivalent to approximately one third of the total rubber weight. The high hardness and stiffness required of bead fillers demand a high loading level. Tread rubber, with abrasion requirements, also typically has more reinforcement. Subtreads are often added to lower the tire's rolling resistance and, as expected, the loading level is lower.

Carbon black and silica, especially silica used with a coupling agent, have similar polymer network reinforcement capabilities. The total level of reinforcing filler does not change dramatically when silica is used as a partial replacement for carbon black. An analysis of selected tread compounds with and without silica shows that the total filler level on a percentage basis increases by 2% in tread compounds with partial carbon black replacements (table 1).

Replacing carbon black with semi-reinforcing or non-reinforcing fillers requires higher loading levels to negate the impact on abrasion resistance. In the opposite direction, it is possible that newer technology fillers and nanoparticles can allow for a decrease in the overall filler level.

Combining the component weight and the loading level allows for the calculation of reinforcing material weight. Figure 5 provides a breakdown of each component in terms of the weight percentage that each contributes to the total weight of reinforcing material in an average passenger tire.

In terms of volume of reinforcing material consumed, the rubber components are ordered as follows: 1) Tread; 2) other (all other stocks including carcass skim and cushions); 3) belt skim; 4) black sidewall; 5) liner; 6) bead filler; 7) subtread.

Tire design trends

Passenger car tires are certainly global products, but in terms of the marketplace, tires are often developed to meet regional performance goals. As a way to keep abreast of new technology, Smithers will target tires for analysis that are developed for a specific attribute. Compared to the average or composite tire, designs targeted specifically for tread-life, wet traction, snow traction or ride comfort may have significantly different constructions or compound formulations. Tailoring a tire to meet a performance goal, however, is not a new concept. Also, relative to a global impact on reinforcing filler usage, these niche applications usually have a small impact, since by their nature the tires are limited in market share.

There are trends and new technologies that have potential to grow beyond a market niche or region. Automated tire manufacturing has become more prevalent with the Michelin C3M, Pirelli MIRS, Bridgestone BIRD and Goodyear IMPACT systems. There has also been a move toward higher speed ratings on tires, as S and T tires are increasingly joined by H, V and even W rated tires. However, the three tire design trends that will be reviewed in more detail in this article are run fiat tires, green tires and tire size migration.

Run fiat tires are growing in OE applications. In Europe, BMW in particular is being very aggressive. In the United States, there has been less push from vehicle manufacturers, and the market is not demanding this feature. The U.S., as a result of the TREAD Act, now has legislation that requires tire pressure monitoring systems on new vehicles, and thus, one of the significant system hurdles has been removed.

Green tires have many facets. Most of the emphasis has been on low tire rolling resistance, but on the other end there have been development efforts put into petroleum-free tires. Many tire companies now market eco-friendly products. The silica industry has benefited from increased usage in tread compounds on most of these tires. The normal trade-off of rolling resistance and wet traction can be diminished by using silica and a coupling agent to replace a portion of the carbon black.

Tire size migration is a trend primarily driven by the U.S., but as with many larger trends, there is spill over into other regions. Some of the trend is driven by the popularity of large vehicles and SUVs, but separately there is a trend of putting larger tires on the same size vehicle. As new vehicles need replacement tires the average size will continue to grow.

According to data published by the RMA on the most popular replacement tire sizes in the U.S.:

* In 2003: No 16" tires in the ten most popular replacement tire sizes;

* In 2004: One 16" in the top ten; and

* In 2005: Three 16" tires in the top ten.

According to data published by the RMA on the most popular original equipment tire sizes in the U.S.:

* In 2005: Eight 16" and larger tires in the top ten;

* In 2005: Four 17" tires in the top ten.

Impact of tire design on fillers

Run flat tires

Run flat tire technology is divided into two types, including insert and self-supporting. The performance goal for each is very much the same, to carry the vehicle a limited distance if there is a loss of inflation pressure, but the designs are significantly different.

Ideally, a run fiat tire will have ride, handling and fuel economy identical to a standard construction tire. Practically speaking, this is a challenge. With either type of run flat, material is being added. In the case of an insert tire, this material is in the form of a "ring" inside the tire. Different concepts, including a tire-in-a-tire, and materials, including polyurethane, have been used for the "ring." The self-supporting run flat (SSR) tires typically rely on adding an inner sidewall component that dramatically stiffens that region.

Structurally, insert run flat tires are more similar to standard tires, and thus require fewer design modifications to move them towards typical tire performance standards. SSR tires have a usage advantage in that no special wheels or mounting are typically required.

[FIGURE 6 OMITTED]

Aspect ratio can play a part in the selection of the run flat tire type. Lower aspect ratio tires, with shorter sidewalls, are an easier application for self-supporting tires. Fortunately, insert tires more easily address the higher aspect ratios, as the added interior height provides the space required for the insert.

The two run flat tire types are similar in that they need to have low heat build-up properties. They are also similar in an ongoing effort to develop new materials and constructions for both the "ring" in the insert run fiat and the inner sidewall in the SSR.

Self-supporting run flat tires (SSR)

Figures 6 and 7 are cross-sectional diagrams of a standard tire and a self-supporting run flat tire, respectively. The increased sidewall gauge for the SSR tire is evident. The addition of an inner sidewall in the SSR accounts for the most significant change.

Both the tire weight and material volume are increased for the SSR tire. A selected comparison of two sizes analyzed by Smithers is provided in table 2.

Minimizing the weight and volume change is a focus for tire manufacturers. New materials with high stiffness and flex fatigue will likely emerge for the inner sidewall.

[FIGURE 7 OMITTED]

Currently, many of the inner sidewalls, from a filler perspective, are similar to black sidewall compounds. Carbon black predominates, with loading levels around 30%. Some manufacturers use two inner sidewalls, with one sidewall containing silica as a partial replacement for the carbon black. Assuming 10% of the SSR's increase in tire weight is the result of an inner sidewall(s), the total reinforcing filler usage in the tire would increase by approximately 10%.

Tread rubber is the other component in SSR tires in which Smithers notes differences. Increased tire weight, combined with the need for low tire heat build-up, strongly encourage the development of a low hysteresis tread compound. High usage of silica has been found in many of the SSR tires analyzed (table 3).

Insert run flat tires

It is difficult to group insert run fiat tires into one category. The different approaches to the "ring," to the materials for an insert, and to retaining the tire on the wheel after loss of inflation pressure, can create more differences than similarities. Sometimes comparing these tires is difficult, simply because conventional tire size nomenclature is not used. However, because the tire structure does not carry the load when un-inflated, the insert/ring does, the insert tire tends to be similar to standard tires. A significant design consideration for the entire assembly is weight, and trying to compensate for the addition of the insert. Figure 8 is a cross-sectional diagram of one type of insert run flat tire.

[FIGURE 8 OMITTED]

Table 4 provides a rough comparison of a standard tire versus a self-supporting run fiat and an insert tire.

Similar to the SSR tires, the insert run flat tires also have the need for low tire heat build-up and low tread rubber hysteresis. The tread compounds in the tires analyzed by Smithers have silica as the primary reinforcing filler (table 5).

Green tires

From a design perspective, what is a "green" tire? Certainly, one performance goal that is connected with green tires is better fuel economy. For a tire designer, that means low rolling resistance, and for a rubber compounder, that means low hysteresis.

A number of design features can be altered to enhance rolling resistance, although sometimes at the cost of performance trade-offs in other areas. Many manufacturers have publicized, and even used as a marketing tool, their efforts to optimize the tire profile or body shape in order to achieve the maximum energy return after loaded deflection. Gauge and distribution control of components, likewise, has been a focus.

The tread, because of its mass and its location, is often the most critical component in achieving a performance goal. Manufacturers have gone as far as creating multi-zone treads to target specific performance characteristics across the tire footprint. The low rolling resistance, sought in green tires, relies heavily on a low hysteresis tread compound.

The hysteretic properties of tread rubber can be lowered dramatically with no change in the reinforcing filler system, but typically the trade-offs are not acceptable. Modifications to the filler system, both in filler type and loading level, can result in lower hysteresis with minimal impact on other properties. Silica, in combination with coupling agents, can be effective when partially replacing the carbon black. It should be noted there is a cost penalty, primarily associated with the cost of the coupling agents. New developments in carbon black surface treatment and structure, including long chain carbon, are likewise aimed at improving the filler to polymer interaction, and thus creating a more energy efficient material.

For many tires, fuel economy is a consideration, but it is interesting to look specifically at the tires that are marketed as energy efficient or green. Table 6 gives a comparison of the reinforcing filler system in selected tires analyzed by Smithers.

The green tires have extensive use of silica. It is also interesting to note the increase in total filler content. In many applications, silica-containing treads show a total filler content increase of approximately 2%. In the case of the green tires, the total filler increases only 0.5%.

Size migration

Although not exactly a design trend, tire sizes change over time. Change can be driven by vehicle developments and/or by consumer preferences. In terms of volumes of rubber and of rubber raw materials, tire size trends can be enormously important. The U.S. has led the upsizing trend in tires. The direction over the last ten years has been towards increasing rim diameter and increasing tire width. Aspect ratio is more complicated. In high performance applications, the aspect ratio has definitely decreased. However, sport utility vehicles, which tend to have higher aspect ratios, have become more popular.

An analysis by Smithers of a series of tire sizes from a manufacturer's product line provides a good weight comparison. Since the design and construction are consistent, the impact of weight can be attributed primarily to size alone. Table 7 shows the sizes in the study.

In comparing the weights of tires when only one size parameter is changed, the average weight change for a size change can be calculated. The average result is shown in table 8.

Small increases in rim diameter and section width can have a big impact on tire weight. Small decreases in aspect ratio typically have a minor impact on tire weight. Although the average weight changes above cannot be applied across all tires, it is still interesting to look at the trend.

One way to examine tire size migration driven by new vehicles is to look at the most popular original equipment sizes and replacement sizes. The OE sizes will eventually move into the replacement market, and because the replacement tire market is more than three times larger, the OE market is a good early indicator of how the total tire market will eventually shift.

In 2005, according to figures published by the RMA, the most popular sizes were as shown in table 9, along with a limited sampling of tire weight data.

The average tire weight of the top five most popular OE sizes is 24% higher than the weight of the replacement tires. There is a direct correlation between tire weight and the volume of rubber and reinforcing filler. A more detailed look is possible by comparing two of the tires above, one from each segment. The comparison in table l0 shows the change if the replacement tire size was migrated to the larger OE tire size.

Clearly, tire size changes may have the single biggest impact on reinforcing fillers as a category, in terms of the volume of material consumed.

Summary

Three current trends in tire design, including: run flat tires, green tires and tire size migration, all have a potential impact on reinforcing fillers.

Self-supporting run flat tires must have stiffer sidewalls, and rigidity is typically accomplished, in part, by the addition of an inner sidewall(s). The material for the inner sidewall may change in the future, but currently, many of the formulas use reinforcing filler loadings equivalent to 30% of the component weight. Insert run flat tires do not have the requirement for stiffer sidewalls, and while the ring insert may represent an opportunity for reinforcing fillers, the tire by itself does not use more filler. A strong incentive is present, with both types of run fiat tires, to have very low heat build-up and low rolling resistance. In these applications, utilization of silica in the tread compounds is prevalent. Tread rubber, from a limited sampling of tires, showed the ratio of carbon black to silica to be roughly 1:2 for SSR tires, and 1:6 for insert run fiat tires.

Green tires, which are specifically marketed as fuel efficient, typically require low hysteresis tread compounds. The reinforcing filler systems in many of these compounds have silica replacing a significant portion of the carbon black. Great variation exists, but the average ratio of carbon black to silica was 1:2.

Tire size migration, while not truly a design change, can certainly be a large driver in the total usage of reinforcing fillers. As tires grow in size, the volumes of rubber and reinforcing filler follow. In the U.S., if the five most popular original equipment sizes were also the five most popular replacement sizes, reinforcing filler usage could potentially grow by 20% for those tires.

Future tires will likely combine many design features. A green run fiat tire is certainly in the future. Large tires run somewhat counter to the green concept, as more mass is not good for energy efficiency. The green run fiat tire of the future may have a large rim diameter, but a relatively narrow width and a low aspect ratio. The tire could also have a multi-segmented tread with zones targeted to specific performance goals. Fortunately for the reinforcing filler industry, complex tire performance requirements create a need for higher technology materials.

This article is based on a paper presented at High Performance Fillers 2007, a Rapra Technology conference, www. rapra.net/conferences.

by David L. Schwarz, Smithers Scientific Services

([e-mail]/www.smithersscientific.com)

Table 1--tread compound filler

Tread filler system          Carbon black   Silica   Total filler

Carbon black - average              33.5%     0.0%          33.5%
Carbon black + silica-avg.          17.4%    18.1%          35.5%

Table 2--self-supporting run flat tires compared to standard tires

Standard vs. SSR   Tire weight   Material volume

205/45R17               +11.4%            +13.9%
225/45R17               +22.2%            +18.5%

Table 3--tread compound filler of SSR tires

Tread filler system      Carbon black   Silica   Total filler

Standard tire--average          23.4%    11.2%          34.6%
SSR tire--average               11.7%    24.5%          36.2%

Table 4--run flat tires compared

Standard                 Tire      Material     Outside   Section
P205/60R16              weight      volume     diameter    width
as reference
Vs. self-supporting      8.9%         2.6%       -4.1%     -3.8%
run flat
195/55R16
Vs. insert run flat    -21.2%    -26.5% w/o       -4.8%     -2.9%
                                     insert
195/620R420A
(195/50R16.5)                       (+13.9%
                                   w/insert)

Note: Tires shown are not the same size

Table 5--tread compound filler of insert run
flat tires

Tread filler system          Carbon black   Silica   Total filler

Standard tire--average              23.4%    11.2%          34.6%
Insert run flat tire--avg.           5.4%    31.2%          36.6%

Table 6--tread compound filler of "green" tires

Tread filler system      Carbon black   Silica   Total filler

Standard tire--average          23.4%    11.2%          34.6%
Green tire--average             11.0%    24.1%          35.1%

Table 7--tire sizes studied

Section      13" rim      14" rim      15" rim      16" rim   Aspect
width       diameter     diameter     diameter     diameter    ratio

225                                              P225/60R16       60
215                                              P215/60R16       60
215                                 P215/65R15                    65
215                    P215/70R14   P215/70R15                    70
215                    P215/75R14   P215/75R15                    75
205                                                               65
205                     205/70R14   P205/70R15                    70
205                    P205/75R14   P205/75R15                    75
195                                 P195/65R15                    65
195                     195/70R14                                 70
195                    P195/75R14                                 75
185                     185/65R14                                 65
185        185/70R13    185/70R14                                 70
185                    P185/75R14                                 75
185       P185/80R13                                              80
175                     175/65R14                                 65
175       P175/70R13   P175/70R14                                 70
175                                                               75
175       P175/80R13                                              80

Table 8--tire size impact on weight

Size parameter       Change   Average weight change

Rim diameter (in.)       +1                     +6%
Section width (mm)      +10                     +8%
Aspect ratio             -5                     -2%

Table 9--2005 most popular sizes

Original          Tire weight    Replacement   Tire weight
equipment tires          (KG)          tires          (KG)

P215/60R16               11.0     P235/75R15          14.3
P265/70R17               17.7     P225/60R16          11.8
P205/65R15               10.0     P205/65R15          10.0
P245/65R17               16.2     P215/70R15          10.6
P235/60R16               14.7     P205/70R15           9.2
Average of 5             13.9   Average of 5          11.2

Table 10--effect of increasing tire size

Change IN =>     Tire   Material   Rubber     Reinforcing
               weight     volume   weight   filler weight
P225/60R16
vs.              +45%       +48%     +47%            +31%
P265/70R17

Note: Reinforcing filler weight comparison based only on six components

Figure 1--tire components--weight %

Bead wire          3%
Tread             30%
BSW               11%
WSW                1%
Liner              6%
Belt skim         11%
Subtread           4%
Bead filler        5%
Cushion            1%
Shldr. wedge       1%
Lower SW         0.3%
Body skim         10%
Rubber chafer      4%
Other rubber       1%
Cap ply            1%
Chafer/flipper   0.1%
Body plies         3%
Belt plies         8%

Note: Table made from pie chart.

Figure 2--rubber vs. reinforcements--weight %

Reinforcement   15%
Rubber          85%

Note: Table made from pie chart.

Figure 3--rubber components--weight %

Other rubber      1%
Tread            36%
BSW              13%
WSW               1%
Liner             7%
Belt skim        13%
Subtread          5%
Bead filler       5%
Cushion           1%
Shldr. wedge      1%
Lower SW        0.3%
Body skim        12%
Rubber chafer     5%

Note: Table made from pie chart.

Figure 4--reinforcing filler--loading level (%)

Reinforcing filler %

Tread         34.6
BSW           29.3
Liner         33.2
Belt skim     32.9
Subtread      30.5
Bead filler   36.1

Note: Table made from bar graph.

Figure 5--reinforcing filler--weight %

Tread         34%
BSW           13%
Liner          8%
Belt skim     16%
Subtread       5%
Bead filler    7%
Other         17%

Note: Table made from pie chart.