A new polymer system for tire innerliners.

The tire industry has a continuing quest for improved innerliners. These improvements range from the processability of the rubber or its compounds in tire plants to the performance of the liner after vulcanization. Halogenated butyls are the preferred rubbers for tire innerliners. They have the adhesion flex, and heat-resistant properties, and low air permeability required of a high quality innerliner compound.

Low air permeability of the innerliner compound is particularly important, not only in maintaining tire pressure, but also in reducing the build-up of air pressure inside the tire carcass to a minimum. High intra-carcass pressure results in increased degradation of the carcass components, thereby lowering the resistance of the tire to failure through flex fatigue or ply separation.

Air pressure retention of tubeless tires is important for long term performance and durability. The design of high performance tubeless tires usually takes into consideration, also, other factors:

* oxidative degradation;

* adhesion;

* rheometry;

* fatigue life;

* processability in tire plants.

This article describes a new experimental polymer (XBIIR), a modified bromobutyl rubber which contains 15 phr poly-[alpha]-methylstyrene resin, for tire innerliners which is aimed at improving:

* processing (milling, extrusion calendering, tire building operations) in tire plants, and

* performance (i.e. air retention) characteristics of tires.

Specific reference is also made to butyl rubbers (halobutyls) because of their low air and water vapor permeability, compared to other polymers, and compatibility with highly unsaturated rubbers. Compounds were prepared with the new polymer and conventional bromobutyl polymer. They were then tested to evaluate (a) the effects of processing in tire plant conditions, and (b) the performance characteristics, primarily air retention. The physical properties of these compounds are also discussed to assess the utility of the new polymer.

The XBIIR polymer is white in color and has significantly less cold flow than the conventional butyl polymers. This polymer can be used to prepare compounds which do not require oil and/or other plasticizers, without adversely affecting the processing characteristics. In addition, there are some aspects of processing, such as green strength and shrinkage, which are also improved.

Background

Over the years, butyl rubbers have been the established polymers for many commercial uses. Vulcanizates of butyl and halobutyl elastomers possess a variety of inherent properties, such as low permeability to air, high damping of low frequency vibrations and good resistance to aging, heat, acids, bases, ozone and other chemicals. Table 1 shows a comparison of these properties with those of other rubbers (ref. 1).

The differences in air permeability of various polymers are large. Butyl rubber (IIR) is about one-tenth as permeable as styrene-butadiene rubber (SBR) at room temperature and about one-eighteenth as permeable as cis-polybutadiene. With increasing temperature the relative differences diminish (ref. 2).

The low permeabillity to air, moisture, vapors and gases renders butyl polymers suitable for a wide variety of applications, such as tire innerliners, tubes, tire curing bladders and various air bladders.

Inflation pressure

The performance of a tire is influenced by the tire operating pressure. Rolling resistance, force and moment (handling), spring rate (ride) and wear (uneven and rapid) are the performance factors most affected by inflation pressure (ref. 3).

The air pressure loss, and subsequent operation of a tire at low air pressure, may result in structural deterioration:

* loss of adhesion and separation of the carcass and belt compounds;

* bead failures due to increased flexing of the lower sidewall;

* liner and mid-sidewall failure induced by flex cracking;

* pressure build-up within structural components resulting in separations.

Maintenance of the inflation pressure is, therefore, critical in a radial tire to achieve the good performance and maintain its structural durability. Therefore, in designing the tire, the innerliner compound must have low air permeability for good air retention capability.

Permeability

Pressure loss is dependent upon the permeability coefficient (Q) of the innerliner compound. As air permeates through the tire for the air chamber, it causes degradation of those components which are subject to heat and stress. The major factor which affects the level of air permeability of a rubber compound is the type of rubber used. Halobutyl

Table 1 - a comparison of properties of butyl and

other rubbers

                   NR   SBR   BR   CR    NBR   EPDM   Butyl
Impermeability      P    P     P    G     G      P     E
Resilience          E    G     E    G     G      G     P
Resistance to:
 - Heat             P    P     P    G     G      E     E
 - Weather          P    P     P    G     G      E     E
 - Acids/bases      P    P     P    G     P      E     G
 - Oils/fuels       P    P     P    G     E      P     P
  (P = poor, G = good, E = excellent)

Table 2 - test formulation

Innerliner compound        BIR      XBIIR     XBIIR
   (phr)                  (oil)     (oil)   (no oil)
Polymer (Polysar BB2030)   100        -        -
Polymer (XB)                -        115      115
N-660 carbon black         60         60       60
Sunpar 2280                 7         7        -
Stearic acid               1          1        1
Pentalyn 'A'                4         4        4
MBTS                       1.3       1.3      1.3
Zinc oxide                  3         3        3
Sulfur                    0.5        0.5      0.5

Table 3 - test formulation

Innerliner compound           Polysar        XBIIR
   (phr)                   Bromobutyl 2030
Polymer (Polysar BB2030)        100            -
Polymer (XBIIR)                  -            115
N-660 carbon black              60            60
Sunpar 2280                      7             0
Stearic acid                     1            1
Pentalyn 'A'                     4             4
MBTS                            1.3           1.3
Zinc oxide                       3             3
Sulfur                          0.5           0.5

rubbers are particularly suited for innerliner applications because of their low permeability to both air and water vapor. Fillers and processing oils are two types of processing ingredients which have significant effects on the level of permeability. The subject of fillers will not be covered here.

Oil used in rubber compounds improves the processing characteristics. However, this ingredient adversely affects the air permeability of tire innerliners. The permeability to air of a compound containing 15 phr oil can be reduced by at least 25% by having the amount of oil (ref. 4).

Tire fabricators have always expressed their desire for a new innerliner polymer which, when compounded, can provide even lower air permeability than the commercial polymers presently used in innerliner compounds. In addition to the air permeability requirements, improved processing of innerliner compounds in all of the tire operations (i.e. milling through tire building) is also required. All these improvements are expected to be achieved without sacrificing the current satisfactory balance of physical properties.

Experimental

Model innerliner compounds were prepared with XBIIR and BIIR polymers. The study was conducted using two groups of compounds:

* Table 2 shows the formulations used in the first group for the laboratory evaluation of the effect of process oil with the XBIIR polymer.

* Table 3 shows two innerliner compounds, using BIIR and XBIIR polymers, in a second group which were prepared on a large scale, factory environment, to assess the processability parameters. These compounds were used to produce tires and tested for comparable performance, especially air retention.

All tests were carried out in accordance with procedures shown in the appendix.

Results and discussion

Laboratory mixed compounds

The laboratory study of the XBIIR compounds showed that process oil had a major effect on some of the key innerliner properties which are used to predict processing behavior in a tire plant and also, tire performance. Compared to the BIIR polymer, the XBIIR polymer showed less shrinkage, higher adhesion, lower permeability to air, higher tack to itself, and less stickiness to metal in all compounds (figures 1 to 4). The improvements were more pronounced in the no-oil compounds.

Factory mixed compounds

A comparison of the physical properites of each of the factory mixed compounds is shown in table 4. The beneficial features of the XBIIR polymer for tire innerliners will be described in greater detail.

Processability effects - An assessment was made of the processability of the unvulcanized compounds. Green strength development was found to be better with the XBIIR polymer. This higher green strength should be of value for better/easier handling at the tire-building stages and should also be beneficial during the blowup prior to curing to avoid splice openings.

The XBIIR polymer produced a compound with much less cold flow than the BIIR polymer. As a result, the XBIIR compound was easier to handle after storage.

In the processing of innerliner compounds, dimensional stability, low shrinkage and die swell are all important factors. A good extrusion rate would also aid productivity. All of these advantages, shown in figures 5-7, are demonstrated with the new experimental polymer.

Melt viscosity measurements on the compounds were carried out using the capillary rheometer (Rheograph 2001) at 104 [deg] C. Compared to the BIIR compound, the XBIIR compound showed lower die swell, good surface appearance and higher viscosity at high shear rates and slightly lower shear thinning behavior. Figure 8 shows the melt viscosities versus shear rates for the compounds over the shear rate range tested (20-1800 [sec.sup.-1]. The higher melt viscosity and the lower post-extrusion swell of XBIIR, compared to BIIR, indicate that there will be improvements in calendering and extrusion operations.

Benefits in processing were also observed in the calendering of the compounds. Shrinkage data obtained from the calendering process show good gauge uniformity. No sticking of the XBIIR liner to the fabric was observed and this demonstrates an important feature for maintaining dimensional stability of the calendar sheet.

Handling of liner compounds at the tire-building stages can be improved with the XBIIR. The lower tack of this compound to stainless steel is an advantage when removing green tires from the building drum. Figures 9 and 10 show tack comparison of the XBIIR and BIIR compounds to stainless steel and to itself. Higher tack of the XBIIR to itself is important for better splicing and less separation.

Physical properties - The vulcanized properties of the XBIIR and BIIR compounds are shown in figures 11 to 14. In general, the stress-strain properties for these compounds were the same. XBIIR shows less change upon aging than the BIIR.

Although the innerliner in a tire is not a structural component, the adhesion level is important to ensure fewer blisters between the liner and the carcass after curing. The high level of hot cured adhesion of the XBIIR compound sugests that fewer blisters can be expected.

Another outstanding feature of the XBIIR compound is lower permeability to air and water vapor than the standard BIIR compound.

Tire performance - Tires containing the two innerliner compounds were built and cured in the conventional manner. Figure 15 shows the better air retention for the XBIIR compound obtained on duplicate tires. These data support the laboratory findings of the permeability of the respective compounds.

The tires for the air retention test were cut and the innerliner gauges measured from the center/shoulder line. The gauge of the XBIIR compound was lower than the BIIR compound, 0.084 cm to 0.096 cm. Nevertheless, the air retention of XBIIR was better.

Table 4 - physical proprties of innerliner compounds
                                     Bromobutyl   XBIIR
Compound properties                     2030
Compound Mooney
ML 1 + 4' (100 [deg] C)                 63.8       76.5
Scorch at 138 [deg] C                    11         10
5 point rise
Mill shrinkage (%)                      23.3        16
Specific gravity                        1.14       1.15
Monsanto rheometer (30 min. motor)
(3 [deg] arc. 166 [deg] C, 100 cpm)
[M.sub.H] (dNm)                         34.0      34.0
[M.sub.L (dNm)                          11.0       12.0
[M.sub.H]-[M.sub.L] (dNm)               23.0       22.0
T2 (min)                                 2.4        2.6
T25 (min)                                3.8        4.0
T50 (min)                                4.6        5.0
T90 (min)                                8.0       10.2
Green strength at 23 [deg] C
Modulus at 100% elongation (MPa)         .32        .43
Modulus at 200% elongation (MPa)         .26        .34
Modulus at 300% elongation (MPa)         .21        .26
Garvey die extrusion
(#1/2" Royle, 104 [deg] C, 70 RPM)
Rate (cm/min)                           87.2        92.2
Die swell (%)                           66.6       43.4
Appearance                              A/10       A/10
Tel Tak at RT
To self (psi)                            49         58
To stainless steel (psi)                 41         32
Vulcanizate properties
(cure: 30 min. t 166 [deg] C
Hardness, Shore A                        56         66
Modulus at 100% elongation (MPa)         1.3       1.7
Modulus at 300% elongation (MPa)         4.7       4.7
Tensile strength (MPa)                   9.3       8.8
Ultimate elongation (%)
Aged 168 h at 120 [deg] C
Hardness, Shore A                        62         71
Modulus at 100% elongation (MPa)         2.1       2.1
Modulus at 300% elongation (MPa)         6.3       6.1
Tensile strength (MPa)                   8.9       8.7
Ultimate elongation (%)                  580       590
Static peel adhesion at 100 [deg] C
to 100% NR carcass (kN/m)                10.7      19.1
Permeability
to air, (preconditioned 24 h at RT
and 0.35 MPa) at 65 [deg] C (Q x [10.-8])3.4       1.8
to water vapor at RT (Q x10.sup.-12])    19.4      14.8
Low temp. stiffening
Gehman [T.sub.2] ([deg] C)              -26.3     -22.0
Gehman [T.sub.10] ([deg] C)             -46.4     -43.3
Gehman [T.sub.100] ([deg] C)            -60.0     -59.1
Monsanto flex fatigue (cam #24)
Unaged (kc)                              212       128
Aged 168 h at 100 [deg] C (kc)            33       28
Aged 168 h at 120 [deg] C (kc)            25       34
Aged 336 h at 120 [deg] C (kc)            20       20
Aged 504 h at 120 [deg] C (kc)            16       14

It is possible, within practical considerations, to obtain reduced air loss of innerliner compounds based on this study. The rate of air loss is approximated according to the following equation (ref.4): [P.sub.t] = [P.sub.o].[e.sup.-kt] where k = A/V . Q/T (the rate constant of pressure loss in the tire) [P.sub.t] = pressure after time t [P.sub.o] = original pressure A = surface area of tire innerliner V = contained air volume Q = effective permeability constant of the tire construction T = effective average thickness of the tire components

If the objective is to obtain tires with different permeabilities of the innerliner compounds and the same rate of pressure loss (k), then K must be constant.

Or [k.sub.1] = [k.sub.2] where [k.sub.1] = rate of air loss for tire with [Q.sub.1] [k.sub.2] = rate of air loss for tire with [Q.sub.2]

For tires of the same design, size and construction, the A/V ratio will also be the same. The air retention will then be controlled by the innerliner thickness and the air permeability.

Therefore [Q.sub.1]/[T.sub.1] = [Q.sub.2]/[T.sub.2]

where [T.sub.1] and [T.sub.2] are the thickness for the tires with [Q.sub.1] and [Q.sub.2] permeability constants.

Assuming that [Q.sub.2] is significantly smaller than [Q.sub.1], an opportunity exists for reducing the gauge and still maintaining a high level of performance, in terms of air retention. In addition, it is anticipated that this approach to gauge reduction could result in some cost savings.

The tire designer, therefore, can utilize the air permeability reduction of the innerliner compound to maximize a high level of air retention in tires. The XBIIR polymer in an innerliner compound demonstrates a cost effective way of achieving this objective.

It is important to note that the tires in this study were also tested to obtain Department of Transport (DOT) and endurance data. Road testing was done to 48,000 km and no unusual occurrences were observed. There were no splice openings and/or liner separations upon examination of the tires.

Summary

A new experimental polymer, XBIIR, compared in a tire innerliner to the conventional bromobutyl polymer, shows that the XBIIR polymer has the characteristics shown in table 5.

The XBIIR polymer in a no-oil innerliner formulation has been observed to give easier handling, from compounding to the tire-building operations. The better air-retention characteristics of the XBIIR polymer in an innerliner compound confirmed in tires can be expected to improve tire performance (rolling resistance for fuel economy) and durability. An opportunity exists to improve tire performance without additional cost.

References

[1] Polysar Butyl Handbook, pp. 319-324, Ryerson Press, Toronto, 1966.

[2] K.J. Kumbhani, "Specialty applications of butyl rubber," presented at a meeting of the Rubber Division, American Chemical Society, Los Angeles, CA, Spring, 1985.

[3] B.L. Collier and J.T. Warchol, SAE Paper #800087, "The effect of inflation presure on bias-belted and radial tire performance."

[4] C.W. von Hellens, "Innerliners for high performance tires," presented at a meeting of the Rubber Division, American Chemical Society, Indianapolis, IN, May 10, 1984.

[5] U.S. Patent #4,754,793 dated July 5, 1988.

Table 5 - XBIIR characteristics vs. conventional
          bromobutyl in a tire innerliner
Stickiness to metal                  Less
Shrinkage                            Less
Die swell                            Lower
Extrusion rate                       Higher
Compound flow                        Less
Green strength                       Higher
Permeability
 - to air
 - to water vapor                    Lower
Hot cured adhesion to NR carcass     Higher
Fatigue life                         Equal
          Appendix - test procedures
Monsanto rheometer   ASTM D 2084-81
Permeability to air  Polysar test
                     Q = volume of gas (cc at N.T.P.)
                     passing, per second, through a
                     specimen of [1cm.sub.2] area and 1 cm
                     thickness when the pressure
                     difference across the specimen
                     is one atmosphere.
Static peel adhesion Test specimen - 2 cm strips died
                     from a press-cured laminate
                     cured to 100% NR carcass stock.
                     Specimens line top and bottom
                     with rubberized nylon fabric.
                     NR carcass formulation
                     SMR-5CV                  -100.0
                     N550 carbon black        -  30.0
                     Circosol 4240            -   5.0
                     Zink oxide               -   5.0
                     Stearic acid             -   1.5
                     Santocure NS             -   0.5
                     Sulfur                   -   2.4
Monsanto fatigue to  Cam #24, not adjusted to set.
failure              Test results reported as the
                     geometric mean of 12 specimens.