Truck tire technology for developing countries.

While the trend worldwide is to switch from bias to radial truck tires, the rate of change in continental Asia and Latin America is much slower with projections of actual units produced continuing to increase for bias tires for at least die next five years in these regions. As may be seen in figures I and 2, the North American bias market continues a rapid decline, but severe service conditions, along with the very high capital investment needed for radial truck tire production (especially all steel), favors the continued popularity of bias tires for certain regions around the world. The vast majority of truck tires for developing countries are tube type; virtually 100% in India, Pakistan, Africa and most of Latin America. Mexico has been driven by original equipment manufacturing in recent years to tubeless radials following U.S. trends.

Everyone agrees with the advantages of radial tires in both treadwear and retreadability, but due to the continued popularity of bias truck tires in developing countries and ongoing developments in this field, we will concentrate on this area in this article.

General criteria

When dealing with the design and construction/compound features for overseas we would like to not only take a helicopter view, but a satellite view, if you will. What we learn from one region in large part can be applied to another, since virtually all truck tires are tube type and overloaded, with road conditions inferior to North America and Europe.

There are many internal and external factors (figure 3) affecting both the manufacturing process and the usage of tires. It needs to be emphasized that the design process must focus on "robustness" to counteract all the difficult conditions faced at the particular overseas location, from cradle to grave - meaning from the raw materials used for manufacturing of the tires and miscellaneous products through the retread life of the tire.

Standards and regulations

Sometime in the future we may have identical ISO (International Organization for Standardization) (ref. 1) regulations or norms for tire design (dimensions, rim use, load, pressure). However, at the present in any given country, they may follow ETRTO (ref. 2) (Europe), T&RA (ref. 3) (USA), JATMA (Japan), their own (India, Mexico, etc.) (ref. 4) or a combination of these. For instance, 12.00-20 tires in Tanzania must be designed narrow per ETRTO standards or tires in dual positions will have "kissing" problems if designed to Tire & Rim Association (T&RA) standards. This may be seen in table 1, where the new bias design section width between the two standards only varies 3mm, but maximum growth for duals is 343 versus 324 or a difference of 19 mm; the 324 basically requires the design of a new tire well below the new tire target of 312 mm. Rim spacers required to be used on some tires when others do not have the kissing problem, is not popular or practical.

                           TRA              ETRTO
Section width          Bias    Radial   Bias     Radial
 Design new tire, mm   315     315      312      313
 Maximum grown
 overall width, mm     343     340      337(*)   319(**)
* 324 mm recommended for dual fitments
** For fitment in dual formation. For specialized tires designed
for fitment in single formation only, allowed 322 mm for normal
road service and 338 mm for special service

In Pakistan, the 8.25-20 tire is used on a rim that is one-half inch too narrow by U.S. norms, and this not only reduces the air volume (and thus load carrying capacity) but also changes the tire's deflection and flange digging. One cannot expect the whole country to change their rims, so the supplier works with original equipment where possible to eventually correct the specifications, and otherwise adapts to the market - i.e., making it "robust." The change of 1" less rim width results in approximately 6% less load capacity per T&RA formulae as follows (ref. 5):

Load = Kx.425x[p.sup.58]5 x ([S.sub.625]).sup.1.39] x (Dr + [S.sub.625]) [S.sub.625] = S x 180 [degrees] - [SIN.sup.1] (Rm/S) 141.3[degrees] Rm = Design rim width (inches) S = Tire section width on rim having design rim width

Table 2 shows differences in selected popular size bias and radial tube type tires for use around the world. One tubeless tire size is shown only to illustrate they are not exempt from differences in non-standardization. Also note (in table 2) the footnote [+ or -] 15%" allowed by ETRTO on pressure, which further emphasizes differences. Loads permitted on axles and vehicles vary throughout the world as the examples in table 3 illustrate (refs. 7 and 8).

U.S. bridge formula and    80,000 lbs. gross vehicle wt.
Federal axle load limits   34,000 lbs. max per tandem
                           axle
                           20,000 lbs. max. on single
                           axle
Middle East                58,000 lbs. max. tandem
                           rear axle
Scandinavia                40,000 lbs. max. on single
                           drive axle

Pakistan/India             No enforced limits
Typical Pakistan survey    14,000 lbs. on front axle
                           30-35,000 lbs. on single
                           drive axle
Typical Northern India
overload market survey     40-60,000 lbs. GVW

The gross vehicle weight (GVW) loads in the U.S. are further restricted by the number and spacing of a truck's axles (ref. 9). The Pakistan and India data are actually misleading the other way, in that the small tire sizes used (8.25-20 and 9.00-20 for Pakistan front/rear and 9.00-20 and 10.00-20 for India front/rear) will actually have the tires under 50-100% overload situations. The need for a "robust" tire becomes imperative. Some specific construction and design features for overseas overload and poor condition road markets are:

* Heavier ply gauges (or squeegee gum inserts) to protect against higher shear stresses. (See further explanation of ply gauge versus rubber between cord optimization below).

* Extra chafers or thicker rubber skim on top (sometimes harder and more abrasion resistant compound) to help for abrasion resistance and protect against rim digging due to overloads and improperly matched lock rings on tubetype rims.

* Designing tire larger for increased air volume for load carrying capability (this may be constrained by wheel space or competitive/cost reasons). Remember, the main difference between a 9.00-20 and 10.00-20 is the increased air volume inside to carry the load. (Working with O.E. vehicle manufacturers to upsize is potentially ideal, but not always practical.)

* Thicker sidewalls for better protection against sidewall cut/impact damage.

* Use of twin bead construction in place of single bead.

* More "robust" shoulder ribs and rugged tread elements or dual radii shoulder lugs to withstand sharp drop offs at pavement edges. The roads are oftentimes too narrow for two vehicles to pass at the same time and still have both stay on the road. Tie bars also are used to help solidify ribs or lugs to counteract these road conditions. Tire manufacturers use a curb shoulder tear simulation test which predicts how adequately a given design or tread compound (or combination) is to resistance to shoulder tearing.

Increased ply gauge background support

There is a theory for optimizing cord spacing for durability which dates back to 1963 (ref. 6). The ratio of the distance between adjacent cords ("X") to the gauge between plies ("y") should be .7 for bias tires to equalize cordwise and interply shear figure 4). While interply shear strain is reduced by adding gauge between plies, it is also important to increase intercord distance, by reducing end count, to reduce cordwise shear strain. Of course, reducing the ply end count beyond a certain point will require more plies in order to insure adequate load carrying capability of the carcass.

Beyond the design/construction criteria for international truck tire development are the material features. Consideration of these issues calls for an examination of:

* Philosophies required for creating a raw material approval system;

* compounding approaches for diverse markets; and

* structural reinforcement candidates for the casing.

International sourcing of raw materials for truck tires

Developing a system to address the raw material requirements for truck tires in the international market requires the installation of three key procedures:

* Development of "world specifications";

* standard operating procedure for screening sources;

* method of manufacturing qualification of sources.

The philosophy of a "world spec" is put into practice with management's commitment to rigid raw material property standards which are not relaxed for special local problems, such as high import duties or local content laws. Although these special situations continue to exist in some countries, the flexibility to circumvent the problems that they incur must be achieved via creation of a broad range of approved world sources. These sources must further be assured of supplying materials which ensure the health and safety of customers, the driving forces for which come from both the (tire) industry as well as from governmental regulatory agencies in different world areas (ref 10).

Having established the world specification framework, the next step is to develop the screening method, which is based upon an iterative submission - test - report/recommend procedure. Specimen submission requires samples not only projected to conform with requirements, but also be representative of typical production. When the sample is submitted to a U.S. laboratory, it must be accompanied by appropriate MSDS information for compliance to Occupational Safety & Health Administration regulation (ref 11). Testing at this stage is by no means limited to the standards listed on the specification. It will encompass virtually all measurable aspects considered to have a potential impact upon final compound rheological or physical properties. Examples of the extensive lists of characteristics evaluated at this stage are listed in table 4. It should be noted that test result comparisons at this stage are also not restricted to specified limits, since a standard control sample is also routinely tested, and its properties available for additional comparisons.

[TABULAR DATA OMITTED]

The final step in the international raw material qualification requires production mixing and processing trials of the candidate source's product in an overseas manufacturing environment. The objectives at this stage are to ascertain such characteristics as propriety of packaging and labeling, lot-to-lot uniformity, and "runability" in general. The impact of increased raw material aging must also be assessed at this stage due to the frequent requirement for transoceanic shipment plus potentially long delays at customs.

Compounding of components for international truck tires

Tire body components

Two of the most significant performance demands upon casing life in international markets are:

* High load carrying capacity (to the point of 50-100% overloads in some working environments); and

* frequent high ambient temperature service.

Fortunately, many of the formulation philosophies employed to individually achieve these special requirements are consistent with one another. For example, low compound heat generation in ply and belt/breaker skim coats is critical for overload service, and this same feature is desirable for high ambient operating temperatures. These coexisting requirements commonly lead to high natural rubber-content formulations in international tire body skim coats. Likewise, chemical antidegradant properties beneficial to high temperature service are also useful for maximizing resistance to compound fatigue in overload service. These latter needs typically lead to the use of the paraphenylenediamine class of antioxidants/antiozonants.

Truck tire sidewalls for international markets must perform with long flex lives under frequently overloaded and/or underinflated conditions. This kind of service places special emphasis upon flex fatigue and cut/growth resistances; regardless of whether the component is being used in a bias or radial application. Formulation tools for achieving these characteristics include:

* The use of a relatively high polybutadiene content to provide a polymer blend with appropriate matrix flex resistance; and

* a relatively low ratio of accelerator/sulfur to maximize cross-link chain flexibility.

Medium and heavy truck tire chafers for the bias tire international market are reinforced with nylon and have skims compounded with the objectives of maximizing abrasion resistance while operating under extreme compression. Two of the tools used to achieve these goals are:

* Use of styrene/butadiene rubber in the polymer blend; and

* introduction of a high loading of an abrasion-resistant, tread-grade carbon black.

* As previously mentioned, most truck tires in international markets are tubetype. However, the tubeless tires that are being manufactured tend to employ innerliners characterized by high levels of resistance to air permeation. This property is critical for the following reasons:

* Service in hot environments demands high leak-down resistance, since tests have indicated that the rate of pressure loss can increase 150%+ over a 28-day period with a 30 [degrees] F rise in temperature (ref. 12).

* truck tire inflation levels may be maintained at very high levels to maximize load carrying capabilities.

The key compounding tool to achieve the desired permeation resistance is the use of halobutyl rubber at relatively high levels (60 phr or higher).

Treads

Tread compounding for international truck tires, particularly bias, represents the key area in which the "satellite view" must be abandoned, and formulations for one area may indeed significantly differ from another. Service environment and design application (rib/lug/semi-lug) are the major parameters which guide the international truck tread chemist.

Service environment requirements demand the consideration of finding an optimum level of heat, treadwear and cut/chip resistances. Every ingredient in the recipe must be optimized to achieve the balance, and yet this balance changes significantly by geographic market. Lug tire treads for India and the Middle East, for example, require particular consideration for high ambient temperature service and the resultant need for low heat generation recipes. Alternatively, lug treads for mountainous areas of South America require significant attention to cut/chip resistance for winding roads with rough surfaces.

Rib tire tread formulations are somewhat more straightforward to formulate, since the component thicknesses are lower than like gauges in lug and semi-lug treads. The thinner component permits the compounder to target higher levels of absolute wear resistance without incurring excessive heat generation in the finished product.

Textile reinforcements for international truck tires

Bias

For the bias construction, tire casing reinforcement fabric is virtually 100% nylon. Nylon's domination of this application, including both body plies and breaker/belts, is attributable to its unique combination of strength and durability, particularly with respect to fatigue resistance. This latter characteristic is benefited by the flexibility of its polymer chain, which unlike rayon, polyester or aramid, contains no restrictive ring structures (ref. 13).

Depending upon supply availability and the decisions of design engineers, nylon-reinforced international truck tires may be built with either nylon 6 or nylon 6.6. These two polymer versions differ slightly as noted in figure 5 . The criteria for selection of type 6 vs. 6.6 is largely limited to user's cost, since the physical property differences are negligible in impact upon tire performance.

Cord constructions for nylon truck tires are based upon multiples of the standard yam weights (measured in denier: g/9000m): 840, 1260, 1680 and 1890 deniers.

Preparing nylon for bonding to rubber is a process that is common for domestic and international tire manufacturing; an adhesive dip based upon a resin (commonly resorcinol/formaldehyde) and latex (frequently a terpolymer blend of styrene/butadiene/vinyl pyridine) are blended. The finished dip is then applied to the fabric via a heat treating operation in which the dip is dried and the dimensional stability of the fabric is improved. Design of the reinforcements for the chafer component for international bias truck tires offers flexibility in that they may be cord structures, similar to those used in the plies and breaker/belts, or they may be square woven.

Radial

Reinforcements for international medium and heavy truck radial belts are primarily based upon steel cord. Belt packages for these tires typically consist of three or four plies strengthened with high carbon steel filaments stranded into a variety of cord constructions.

The top belt ply (closest to the tread) may be reinforced with a high elongation cord construction which has been achieved by more extensive twisting of the filaments and cable. Depending upon the manufacturer's recommendation, this belt may be removed and replaced at the end of the first tread service life for improved retreadability.

Radial truck tire body ply reinforcements include a wider range of candidate materials: steel, nylon aramid or polyester; nevertheless, steel cord again predominates this application. Cord constructions are designed to maximize flexibility and fatigue resistance. The steel radial casing is always monoply in construction, likewise for aramid ply tires produced in Morocco.

Nylon, and to a lesser extent polyester, are used in international radial body plies where a multiply construction is required, or where suitable equipment to handle steel ply is not available.

Concluding remarks

It is worth emphasizing that by no means should one believe that a tire produced by a foreign developing nation will not be commensurate in quality to a U.S. or European tire. If properly designed for their own overload conditions the local product may very well outperform an imported product not designed for those conditions. This has been seen many times on both bias products and radial products. Radial truck tires will oftentimes be removed, due to anomalies in the bead region if not properly designed for overload. Likewise, tread patterns in truck and passenger tires may pick up/retain stones and cause tread chunk outs or stone drilling leading to carcass failures, or render the casing unacceptable for retreading, unless percent void and sipe depths are modified for the "secondary type" roads they will be used on. Companies have truly become global, and the interaction of original equipment (OE) vehicle manufacturers and their suppliers (i.e. tires for us) cannot be overemphasized in the future. ISO 9000 will have a further influence on this standardizing of product and exporting across borders.

In conclusion, it should be emphasized that the multinationals in the tire business are developing and manufacturing tires worldwide to meet the demands of very severe operating conditions (loads, poor roads, etc. already described), and we have attempted to show how this process is influenced by different criteria in the international arena.

References

[1.] 1988 International Standard ISO 4209/1, 3rd Edition, Truck and Bus Tyres and Rims (Metric Series), International Organization for Standardization, 1988. [2.] Standards Manual 1993, Brussels, Belgium: The European Tyre and Rim Technical Organization, 1993. [3.] 1993 Yearbook, Akron, OH: The Tire and Rim Association, Inc., 1993. [4.] Data Manual 1993, New Delhi, India: Indian Tyre Technical Advisory Committee, 1993. [5.] Engineering Design Information, Akron, OH: The Tire and Rim Association, Inc., 2-67E, 1993. [6.] Mathematics Underlying the Design of Pneumatic Tires - John F. Purdy, Chief Mathematician Goodyear Tire & Rubber Co., Edwards Brothers, Inc., Ann Arbor, MI 1963 pp 62-64. [7.] "Heavy trucks: Vocational uses & customer requirements," Ed Saxman, product planning manager, Volvo-GM, Conference on Highway Commercial Vehicle Interface MVMA Medium & Heavy Truck Orientation Program. [8.] "Constraints on heavy truck design engineers," Garrick Hu, Director of Engineering, Quality & Service, Navistar Int'l Transportation Corp. Conference on Highway Commercial Vehicle Interface MVMA Medium & Heavy Truck Orientation Program. [9.] Walt Moore, Senior Editor, Construction Equipment, Oct. 1991 "Are heavier loads on the horizon?" [10.] W. Curt Woods, "Rubber chemicals for the rubber industry in the 1990s " Jan. 28, 1993 meeting of the Akron Rubber Group, P. 5. [11.] OSHA Hazard Communication Rule: 29CFR [12.] General Tire Service Bulletin D-19, 1/17/90, P. I. [13.] B. D. Lambillotte, Fabric reinforcements for rubber," Journal of Coated Fabrics, Vol. 18 Jan. 1989) PP 162-179