12 Compound Development and Applications George Burrowes The Goodyear Tire & Rubber Company, Lincoln, Nebraska, U.S.A. Brendan Rodgers The Goodyear Tire & Rubber Company, Akron, Ohio, U.S.A. I. INTRODUCTION The rubber industry represents a critical link in a diverse range of associated manufacturing and service industries. Products find varied applications such as in automobiles, medical devices, mining, and many manufacturing sys- tems. The automotive industry in particular owes much of its success to the quality of tires and associated industrial products such as hoses and belts. Tires are essential to the efficient operation of a nation’s transportation and logistics infrastructure. It is therefore appropriate to discuss compound development techniques and to view selected applications of elastomers and other compounding ingredients in important rubber products such as auto- motive hoses and belts, conveyor belting, and tires. II. COMPOUND DEVELOPMENT A. Sources of Compound Development Compound formulation development and reformulation provides a means to rapidly meet new regulatory requirements, respond to competitive concerns, improve existing products, and facilitate new product development. 4871-9_Rodgers_Ch12_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 569 Copyright © 2004 by Taylor & Francis The modern Compound Development department must be able to respond rapidly to internal company needs, changes in the marketplace, and external requirements such as environmental and economic constraints to the availability of a raw materials supply. The sources of information for new compound development include raw materials suppliers, scientific publica- tions, universities and research institutes, and internal company development teams. The techniques available to the compound development scientist rely on several tools that can be divided into two groups: 1. Information Technology. Information technology (IT) systems centered on the deployment of knowledge management systems and tools for experimental designs are basic to the efficient operation of a Compound Development team. The functions provided include a. Information such as approved formulations. b. Vendor-supplied data. c. Knowledge records, i.e., reports. d. Experimental data storage and easy retrieval. Data include formulations and associated compound properties such as vulcanization kinetics and rheological properties, classical mechanical properties, and dynamic and hysteretic properties. 2. New Compound Development. Formulation development to meet a new performance requirement can be conducted at various levels. a. The most elementary is screening of a series of formulations based on the experience of the scientist. This may involve incremental changes in one or more selected components in a formula. Alternatively it may involve substitution of one material for another. b. More sophisticated tools using ‘‘designed experiments’’ can be employed. These essentially fall into two categories: simple factorial designs where two or more components in a for- mulation are varied in an incremental manner, and full multiple regressions where three or more components in a formulation are changed in defined increments, data are collected, multiple regression equations are computed, graph- ical representation of data are computed, and optimized formulations are calculated for the desired mechanical properties. c. Computational techniques based on neural networks and genetic algorithms are now being used. This enables boundaries to be established within which a designed experi- ment may be developed to fine-tune a specific formulation. 4871-9_Rodgers_Ch12_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 570 Copyright © 2004 by Taylor & Francis Such techniques when developed enable many more compo- nents in a formulation to be considered without the experi- menter being overcome with excessive amounts of data. d. Predictive Modeling. Many proprietary models have been developed that enable an estimation of how a formulation will perform in a product such as a tire. A number of elementary relationships are available to the researcher, such as the effect of tangent delta on tire traction and the influence of compound rebound on rolling resistance. Basic computational tools can be readily assembled to calculate the effect of changing the hysteretic properties of several compounds in a tire simul- taneously and estimate the resulting rolling resistance. On completion of the laboratory development phase, adequate testing is essential to verify that the product will meet performance expectations and the predicted performance parameters. B. Examples of Formulations Formulations are available in several industry publications such as the Natural Rubber Formulary and Property Index published by the Malaysian Rubber Producers Research Association (1). Typical examples of compound formulations cited frequently in the technical literature are tabulated for general reference purposes (Tables 1–3). Further optimization can be con- ducted on these formulations should a specific set of mechanical properties be required to meet the product mission profile, product manufacturing envi- Table 1 Examples of Roofing and Automotive Hose Cover Compounds Roof sheeting (phr) Radiator hose (phr) EPDM 100.00 EPDM 100.00 N347 120.00 N660 130.00 Talc 30.00 N762 95.00 Paraffinic oil 95.00 CaCO 3 45.00 ZnO 5.00 Paraffinic oil 130.00 Stearic acid 2.00 ZnO 3.00 MBTS 2.20 Stearic acid 1.00 TMTD 0.65 DTDM 2.00 TETD 0.65 ZDBDC 2.00 S 0.75 ZDMDC 2.00 S 0.50 4871-9_Rodgers_Ch12_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 571 Copyright © 2004 by Taylor & Francis ronment, or compliance with regulatory constraints. For a brief discussion on compound mixing, reference should be made to Barbin and Rodgers (2). A further point to be noted in the context of this discussion is the importance of defining optimum compound mixing temperatures, internal mixer compound dwell time, and required final compound viscosity. Compound viscosity is important to ensuring quality component extrusions, which are a function of throughput, extrudate temperature, adherence to contour or gauge control, and appearance, which may be adversely affected by bloom of any compound constituents. Table 2 Model Tread Compounds Model truck tire tread compound, Example 1 Natural rubber 50.00 Polybutabiene 25.00 SBR 25.00 Carbon black (N220) 65.00 Peptizer 0.25 Paraffin wax 1.00 Microcrystalline wax 2.00 Paraffinic oil 10.00 Polymerized dihydrotrimethylquinoline (TMQ) 1.00 7PPD 2.50 Stearic acid 2.00 Zinc oxide 5.00 TBBS 1.25 Sulfur 1.00 DPG 0.30 Retarder (if required) 0.25 Model truck tire tread compound, Example 2 Natural rubber 100.00 Carbon black (N220) 50.00 Peptizer 0.25 Paraffin wax 1.00 Microcrystalline wax 2.00 Paraffinic oil 3.00 Polymerized dihydrotrimethylquinoline (TMQ) 1.00 Stearic acid 2.00 Zinc oxide 5.00 TBBS 1.00 Sulfur 1.00 DPG 0.25 Retarder (if required) 0.20 4871-9_Rodgers_Ch12_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 572 Copyright © 2004 by Taylor & Francis III. INDUSTRIAL PRODUCTS The term ‘‘industrial rubber products’’ represents a very broad product array ranging from all-rubber single-component articles such as roofing membranes or automotive weatherstripping through to sophisticated composites such as timing belts and multilayer hoses. Industrial products utilize the full spectrum of elastomeric material, textile, and metal reinforcement. Generalizations about product materials, performance, and so on are therefore impossible. It is more appropriate to choose a few products that must operate in Table 3 Tire Sidewall and Casing Compounds (phr) Model tire sidewall compound Natural rubber 60.00 Polybutabiene 40.00 Carbon black (N330) 48.00 Peptizer 0.15 Paraffin wax 1.00 Microcrystalline wax 2.00 Paraffinic oil 3.00 Polymerized dihydrotrimethylquinoline (TMQ) 1.50 7PPD 3.50 Stearic acid 2.00 Zinc oxide 3.00 TBBS 0.95 Sulfur 1.25 Retarder (if required) 0.15 Model tire casing ply compound (phr) Natural rubber 65.00 Polybutabiene 35.00 Carbon black (N660) 65.00 Peptizer 0.25 Paraffin wax 1.00 Microcrystalline wax 1.00 Paraffinic oil 8.00 Polymerized dihydrotrimethylquinoline (TMQ) 1.00 7PPD 2.50 Stearic acid 2.00 Zinc oxide 3.00 DCBS 0.90 Sulfur 4.50 Retarder (if required) 0.25 4871-9_Rodgers_Ch12_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 573 Copyright © 2004 by Taylor & Francis increasingly demanding environments, represented in the main by the auto- motive industry, and examine the evolution of these products in order to satisfy the ever-rising performance expectations of recent years. For this reason, the discussion that follows will focus on two types of hoses and three types of belts that have recently undergone considerable modifications in construction and material components to continue to meet rapidly upgrading performance expectations in their particular areas of operation. A. Coolant Hose Radiator hoses (Fig. 1) are designed to provide a flexible connection permitting coolant fluid transfer between the engine block and the radiator. These hoses have an inner tube resistant to the coolant fluid (usually an ethylene glycol–water mixture) at the operating temperature and hydrolysis- resistant textile reinforcement and are covered by a heat- and ozone-resistant material. A discussion of radiator hoses also applies in principle to heater hoses (internal diameter normally 19 mm or below), because ethylene glycol–water mixtures are the heating medium for the vehicle interior. However, unlike radiator hoses, heater hoses are generally not exposed to continuous move- ment while the vehicle is in motion. The term ‘‘coolant hoses’’ will be used in this text for information that is pertinent to both radiator and heater hoses. Automotive bodies and engines are becoming increasingly compact because of aerodynamic styling. At the same time engines are operating at Figure 1 Radiator hose. 4871-9_Rodgers_Ch12_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 574 Copyright © 2004 by Taylor & Francis higher temperatures for improved fuel efficiency; there is an increasing desire for turbocharging, emission control, and power assist devices. Therefore, under-the-hood temperatures, including those to which the coolant hoses are exposed, have continued to increase in recent years. The automotive manu- facturers’ expectation is that the coolant hoses on their engines will perform well over the lifetime of the vehicle. In 1988, a radiator hose life goal of 100,000 miles was quoted (3). Nowadays, the life goal for these hoses has been extended to 10 years or 150,000 miles (4). 1. Manufacturing Process In the traditional manufacturing process for coolant hoses, a rubber inner tube material is first extruded, then passed through a textile knitter, braider, or spiraling equipment to apply one or more reinforcing layers of continuous filament yarn. A rubber cover material is extruded over the reinforced carcass, and the unvulcanized hose is cut into predetermined lengths. With the aid of a glycol-based lubricant, the individual hose pieces are placed over shaped mandrels that hold them in position during vulcanization with high pressure steam. After that, pieces of vulcanized hose are stripped off the mandrel and trimmed to the required length. Some small internal diameter heater hoses are made on flexible mandrels and vulcanized by continuous processes. 2. Classification of Hoses and Materials For the automotive industry, the most common performance standard for coolant system hoses is SAE J20, which classifies them according to type of service. For example, SAE 20R3 and SAE 20R4 are normal service heater and radiator hoses, respectively. In addition to outlining a series of other re- quirements, this standard also defines the physical properties of each ‘‘ class’’ of the elastomeric materials to be used in the various hose types (5). It is common practice in the industry to use compound performance in accelerated aging tests as a predictor of the serviceability of hose in a vehicle. Some limited data exist to back this up (6). Table 4 shows the physical property requirements for the three most common classes of hose material. Class D-1 material requirements are based on oven aging for 70 hr at 125jC, with a 125jC compression set; they are usually met by sulfur-vulcanized EPDM. Class D-3 material requirements are based on more stringent oven aging, 168 hr at 150jC; the same 125jC compression set requirement applies. This material class is usually peroxide-vulcanized EPDM. 4871-9_Rodgers_Ch12_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 575 Copyright © 2004 by Taylor & Francis In this context, the compression set test is performed under constant strain conditions for 70 hr at the stated temperature (7). It is a measure of recoverability of the rubber material after aging under 25% compression; low compression set contributes to good coupling retention for a given rubber material. For hose materials of classes D-1 and D-3, with compression set measured at 125jC, the stability of the cross-link is the controlling factor, so a sulfur donor or peroxide cure system is necessary. Table 4 Material Physical Properties of Main Coolant Hose Types—SAE 20R3 Heater Hose for Normal Service and SAE 20R4 Radiator Hose for Normal Service SAE J20 a Class D-1 Class D-3 Class A GM6250M b Original properties Durometer, Shore A 55–75 55–75 55–75 60–75 Tensile strength, min, MPa 7.0 7.0 5.5 7.6 Elongation, min, % 300 300 200 250 Oven age 70 hr/125jC 168 hr/150jC 70 hr/175jC 168 hr/165jC Durometer, Shore A +15 +15 +10 0–15 Tensile strength change, max, % À20 À35 À15 À30 Elongation change, max, % À50 À65 À40 À55 Compression set (ASTM D395 Method B) 125jC 125jC 125jC 150jC 70 hr, max, % 75 75 40 60 Coolant immersion (tube only) Hours at boiling point 70 168 70 168 Volume change, % À5 to+20 À5 to +20 0 to +40 +20 Durometer, points Shore A À10 to+10 À10 to +10 À10 to +10 À15 to +15 Tensile strength change, max, % À20 À20 À30 À15 Elongation change, max, % À50 À25 À25 À15 Elastomer EPDM EPDM Silicone EPDM Vulcanization system Sulfur Peroxide Peroxide a Property requirements extracted from SAE Standard SAE J20 (Oct 1997). b Property requirements extracted from General Motors Engineering Standards GM6250M (June 1997). 4871-9_Rodgers_Ch12_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 576 Copyright © 2004 by Taylor & Francis Class A materials have the most stringent requirements on aging, compression set and coolant immersion. Silicone elastomers are usually required for this class. 3. Coolant Hose Materials Rayon, suitable for 120jC service, has long been used as a cost-effective reinforcing yarn for coolant hoses. However, with increasing under-the-hood temperatures, the more heat-resistant aramids, capable of operating up to 230jC, are used in preference to rayon for the more demanding coolant hose applications (8). Though meta-aramid is significantly more expensive than para-aramid, the former is often used for its greater abrasion resistance, essential when yarns contact each other in hoses subjected to high levels of vibration, as well as for its greater resistance to hydrolysis and heat. 4. Ethylene Propylene Elastomer–Based Coolant Hoses Before the 1960s, natural rubber and styrene butadiene rubber (SBR) were the base elastomers for the tubes of automotive coolant hoses, with polychlo- roprene being used whenever an ozone-resistant cover was required. How- ever, with the advent of ethylene propylene diene (EPDM) technology, ethylene propylene elastomer compounds rapidly gained widespread accept- ance for coolant hoses because of their outstanding resistance to hot coolant fluid and to the dry heat of vehicle engine compartments. Though other elastomers, most notably silicones, find some limited use, EPDM-based coolant hoses are used almost universally by the modern automotive industry. For this reason, most of the discussion that follows will be devoted to EPDM and its associated compounding issues. 5. Elastomer Characteristics The following generalizations can be made on the required characteristics of EPDM elastomers for coolant hose compounds (9,10): Molecular Weight. The highest molecular weight grades are com- monly used because they increase hot green strength and improve tensile strength properties, compression set resistance, and collapse resistance of inner tubes during application of reinforcement textile. They also improve the capability for filler and oil loading so as to enable cost optimization. Ethylene Content. Higher ethylene content improves ambient tem- perature green strength, tensile strength, extrusion rate, and man- drel loading capability. High ethylene content can, however, be detrimental to flexibility and set properties at low temperature and 4871-9_Rodgers_Ch12_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 577 Copyright © 2004 by Taylor & Francis may result in nervy extrudates. In practice, coolant hose compounds often contain a blend of high and low ethylene EPDM elastomers. Diene Content (Unsaturation Level). With sulfur cure systems, in- creasing levels of termonomer in the EPDM elastomer increase cure rate, tensile modulus, and compression set resistance, but reduce scorch safety and in some cases may compromise heat resistance. Ethylidene norbornene (ENB), which gives the fastest cross-linking, is the preferred termonomer for coolant hose EPDM elastomers compared with dicyclopentadiene (DCPD) or 1,4-hexadiene (1,4HD). For peroxide curing there is, in principle, no need for diene to be included in the elastomer. However, diene content will improve cure rate and cross-link density. Molecular Weight Distribution (MWD). A broad distribution will improve overall processing characteristics, including extrusion smoothness. However, physical properties, especially compression set, may be compromised. The breadth of the molecular weight distribution can influence cure state and cure rate, broader MWD grades curing to a lower cure state and slower than narrow grades (6). A recent development in catalyst technology has resulted in the production of EPDM elastomers with narrow molecular weight distributions intended to provide good physical properties, along with a high level of chain branching to improve polymer processing (11). 6. Sulfur Vulcanization Both sulfur and peroxide cure systems find application in coolant hoses. Because the cure system is the most important factor influencing the heat and compression set resistance of a hose, aspects pertinent to coolant hoses will be discussed in detail below. Several review articles cover the basics of sulfur curing of EPDM elastomers (12–14). Sulfur-based vulcanizing systems produce excellent stress/strain properties and tear strength in EPDM coolant hoses, as well as being very cost-effective. Low sulfur/sulfur donor systems are preferred for coolant hose compounds because they give a near optimum balance of cure rate, heat resistance, compression set, and mechanical properties. Such cure systems have been reported in the literature (15). Because EPDM elastomers have far fewer cure sites than diene rubbers, they require higher levels of accelerator to achieve practically useful cure rates. The heat resistance of a sulfur-cured EPDM compound is improved by the addition of the synergistic combination of zinc salt of mercaptobenz- 4871-9_Rodgers_Ch12_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 578 Copyright © 2004 by Taylor & Francis [...]... test fuelsa Fuel permeation [(g/m2)/day] 30R6 Low-pressure hose, synthetic rubber tube, and cover 100jC 48 hr, RT, Fuel C 70 hr, RT, Fuel G 600 Fuel C, RTb 30R7 Low-pressure hose, synthetic rubber tube, and cover 125jC 48 hr, RT, Fuel C 70 hr, RT, Fuel G 14 days, 40jC, sour gas No 1 550 Fuel C, RTb 30R8 Low-pressure hose, synthetic rubber tube, and cover 135 jC Intermittent 150jC 48 hr, RT, Fuel C 70 hr,... in belt sidewalls modify the frictional behavior and wear resistance of raw edge belts More important, fiber-loaded rubbers help support the tensile member cords and also enable the belt to withstand the high sidewall pressures resulting from its wedging in and out of each pulley during rotation In V-belts or V-ribbed belts, the fibers in the below-cord rubber are deliberately oriented so that they lie... ethers, e.g., methyl-t-butyl ether (MTBE), and other additives present in fuels to compensate for loss in octane number caused by the removal of lead from gasoline Corporate Average Fuel Economy (CAFE) standards adopted in the 1970s resulted in reductions in vehicle size and weight and more compact engine compartments with reduced air flow Coupled with the addition of more under-the-hood heat sources,... industrial, and automotive drives (most notably for snowmobiles) where high impact loads must be withstood and high loads must be transmitted Banded V-belts (Fig 8) in which belts are held together side by side in a single unit by being vulcanized to a tie band (of rubberized fabric) Figure 8 Banded V-belts Copyright © 2004 by Taylor & Francis laid across the top Instead of a series of individual V-belts... banded belt may be used to distribute power more evenly, reduce vibration, and prevent belt turnover Used on agricultural, textile, and heavy industrial equipment Double-V (hexagonal) back-to-back V-belts with a central cord line (Fig 9) They can transfer power from either side in drives where the belt passes in a zigzag pattern around a number of pulleys Used in agricultural and some industrial applications. .. combinations of antioxidants, such as acetone-diphenylamine reaction product (ADPA) with the relatively nonextractable a-methylstyrenated diphenylamine (a-MSDPA), are now recommended for NBR-based fuel hose tubes to be used in air-aspirated engines with carburetors (44) Hydrogenated nitrile butadiene rubber (HNBR) and NBR are compatible, and their peroxide-cured blends (50/50) were found to have better... machinery, which must withstand high shock loads and in variable-speed belts that transmit high loads To obtain adequate adhesion between para-aramid and most rubbers, treatment with RFL alone is not sufficient A two -part dip system must be used in which an isocyanate or epoxy type predip is added ahead of an RFL dip Isocyanates in toluene will penetrate the cord, improving cuttability and fray resistance but... Fiberglass and aramid cords may be used in certain specialized applications Low modulus, high twist polyamide cords that impart ‘‘elastic’’ char- Copyright © 2004 by Taylor & Francis 2 3 4 acter to belts are used in some washing machine and dryer applications to eliminate the need for tensioning mechanisms A rubber layer that encapsulates and adheres to the tensile member Rib material, usually rubber reinforced... ethylenepropylene-diene terpolymers (EPDM), are increasingly being used as the base for automotive V-ribbed belt materials Because this type of elastomer is more cost-effective and offers broader operating temperature ranges than polychloroprene or its other alternatives, EPDM-based V-ribbed belts have been the subject of intense research and development activity in the industry Products based on sulfur-cured and. .. World War II, V-belts from the mid-1940s on were produced from materials based on synthetic rubber, mainly SBR Today, general-purpose diene elastomers like SBR are used Figure 9 Hexagonal belt Copyright © 2004 by Taylor & Francis in cost-sensitive, lower performance belt applications However, polychloroprene, mainly the sulfur-modified grades, is now the main base elastomer for most V-belt materials . an ethylene glycol–water mixture) at the operating temperature and hydrolysis- resistant textile reinforcement and are covered by a heat- and ozone-resistant material. A discussion of radiator hoses also. under-the-hood temperatures, the more heat-resistant aramids, capable of operating up to 230jC, are used in preference to rayon for the more demanding coolant hose applications (8). Though meta-aramid. (12). Peroxide-vulcanized hoses tend to have better resistance to heat and com- pression set than those with sulfur-based systems (6 ,13) . Dicumyl peroxide and bis(t-butylperoxyisopropyl)benzene, on polymer or inert