Patent Application
20150174962
The invention relates to a pneumatic truck tire, particularly a bus tire, with a tread of a cap/base configuration comprised of a circumferential outer tread cap rubber layer and an underlying tread base rubber layer. The outer tread cap rubber layer is configured with lugs, particularly circumferential ribs with circumferential grooves between the ribs. At least a portion of the tread grooves, including the bottom of a groove, is contained in a rubber encasement which is an extension of the tread base rubber layer.
This invention relates to a truck tire, particularly a bus tire, intended to be used to support and carry relatively large vehicular loads over roads, which may include irregular road surfaces, rather than off-the-road service. The lugs of the tire tread may be in a form of circumferential rubs. Rib treaded tires have been referred to, for example, and not intended to be limiting, in U.S. Pat. Nos. 5,718,782, 5,772,807 and 5,843,249.
It is desired that the outer tread cap rubber layer of the tread, which contains the tread's running surface, is a rubber composition intended to promote durability of the tread, particularly for use on irregular road surfaces.
In one embodiment, the tread cap rubber composition is a natural rubber-rich rubber composition where a major portion of its elastomer is comprised of natural cis 1,4-polyisoprene rubber and a minor portion comprised of at least one of polybutadiene (e.g. cis 1,4-polybutadiene) and styrene/butadiene elastomers.
In another embodiment, the tread cap rubber composition is a polybutadiene rubber-rich (e.g. cis 1,4-polybutadiene-rich) rubber composition with a major portion of its elastomer comprised of the polybutadiene rubber and a minor portion comprised of at least one of cis 1,4-polyisoprene and styrene/butadiene elastomers.
Where lower rolling resistance is desired for such truck, or bus, tire to promote vehicular fuel economy, the outer tread cap rubber layer may contain reinforcing filler composed of a combination of rubber reinforcing carbon black and precipitated silica where a significant portion of the reinforcing filler is precipitated silica. If the precipitated silica exceeds the rubber reinforcing carbon black in the outer tread rubber composition, the rubber composition might be referred to as being a silica-rich rubber.
During service of the tire, particularly over irregular road surfaces, the tread lugs, particularly circumferential tread ribs, may experience significant physical stress which may sometimes promote a degree of small surface crack formation on the tread groove surfaces between the tread lugs, or circumferential tread ribs, particularly in the bottom portion of the tread grooves.
Historically, surfaces of tread grooves have sometimes been protected, when appropriate and if desired, by providing an external thin protective rubber layer over the surface, or surfaces, of the grooves. Further, in one aspect, tread grooves have sometimes been reinforced with stiffer rubber than the tread itself to promote handling of the tire. For an example, which is not intended to be limiting, see U.S. Pat. Nos. 6,213,181 and 5,176,765 and U.S. Patent Application Publication No. 2010/0154948.
However, as a departure from such past practice, it is proposed to provide a rubber encasement, or rubber block, within the tire tread for containing at least a portion of at least one of the tread grooves instead of being a simple rubber coating on the groove surface where the rubber encasement is a part of and extends from (is an extension of) the tread's underlying base rubber layer into the tread's outer cap rubber layer to thereby encompass at least a portion of a tread groove. In this manner, then, for a circumferential groove, the encasement itself would also be circumferential in a sense of extending circumferentially around the tire within the tire tread. The encasement, or rubber block, would thereby become at least a portion, or part, of the surface of the tread groove itself.
In one embodiment, the rubber encasement, namely the rubber block forming the encasement, extends from the tread base layer (is an extension of the tread base layer) through the outer tread cap rubber layer to and including a portion of the outer running surface of the tire tread. Therefore the encasement, or rubber block, is not simply a thin rubber coating over a surface of the tread groove, but instead is a part of the tread groove itself.
In another embodiment, the rubber encasement, namely the rubber block forming the encasement, extends from the tread base layer (an extension of the tread base layer) into the outer tread cap rubber layer without extending to the tread running surface.
As indicated, the rubber encasement is a part of at least a portion of a tread groove and desirably includes the bottom of the tread groove.
The rubber composition of the rubber encasement beneficially promotes at least one physical property such as increased tear resistance, increased cut growth resistance and increased elongation at break to the surface of the tread groove embedded in the rubber encasement compared to the adjoining outer tread cap rubber composition in which the rubber encasement resides. The rubber composition of the tread encasement may also beneficially promote increased fatigue resistance of the surface of the embedded tread groove as compared to the adjoining outer tread cap rubber composition.
In one embodiment, rubber composition of the tread cap may contain silica-rich reinforcing filler comprised of a combination of rubber reinforcing carbon black and precipitated silica of which the precipitated is the majority of the rubber reinforcing filler. The precipitated silica is used together with silica coupling agent In one embodiment, the rubber reinforcing carbon black content of the tread cap rubber composition may be less than 30 parts by weight per 100 parts of elastomer (phr) which, in turn, can promote a reduction of its electrical conductivity (can promote an increase its electrical resistivity).
In one embodiment, the tread base rubber composition contains carbon black-rich filler reinforcement of which the rubber reinforcing carbon black is the majority of the rubber reinforcing filler. Therefore the rubber reinforcing carbon black is the majority of the reinforcing filler, usually in a quantity of at least 50 phr thereof. Where the rubber reinforcing carbon black content in the rubber composition is at least 40 phr is expected to promote electrical conductivity (promote a reduction in electrical resistivity).
The rubber composition of the rubber encasement is an extension of, and thereby of the same rubber composition, of the tread base rubber layer. The tread encasement containing a tread groove thereby is a part of and extends from the tread base rubber layer into the outer tread cap rubber layer. When the tread encasement extends to, and includes a portion of, the running surface of the tread cap rubber layer, a path of least electrical resistance (path of electrical conductivity) can thereby be provided from a rubber reinforcing carbon black rich (with a majority, if not all, of the reinforcing filler being the rubber reinforcing carbon black which may be used in an amount of at least 40, alternatively at 50, phr), tread base rubber layer to the tread running surface of an outer, electrically resistive, tread cap rubber layer which may contain less than 35, and alternatively less than 30, phr of rubber reinforcing carbon black reinforcing filler.
In the description of this invention, terms such as “compounded rubber”, “rubber compound” and “compound”, if used herein, refer to rubber compositions containing of at least one elastomer blended with various ingredients, including curatives such as sulfur and cure accelerators. The terms “elastomer” and “rubber” may be used herein interchangeably unless otherwise indicated. It is believed that such terms are well known to those having skill in such art.
In accordance with this invention, a pneumatic rubber tire is provided having a circumferential tread of a cap/base construction comprised of an outer tread cap rubber layer containing a plurality of lugs in a form of circumferential ribs with intervening grooves between the ribs together with an underlying a tread base rubber layer which underlies the outer tread cap rubber layer;
wherein said tire tread further includes at least one circumferential tread rubber encasement in a form of a rubber block which contains at least a portion of at least one of said tread grooves including the bottom of at least one of said groove(s);
wherein said tread groove-containing rubber block (encasement) is an extension of said tread base rubber layer (e.g. is a unitary part of and therefore of the same composition of the tread base rubber layer) which extends radially outward from the tread base rubber layer into the outer tread cap rubber layer, and contains, and is thereby a portion of, at least a bottom and at least a portion of the walls of at least one of said grooves.
In one embodiment, said groove-containing rubber block (encasement) is a portion of the surface of at one of said tread grooves for which it provides an encased portion.
In one embodiment, a tread groove extends through the outer tread cap rubber layer and into the underlying tread base rubber layer.
In one embodiment, the tread groove in the outer tread cap rubber layer does not extend to the underlying tread base rubber layer.
In selective embodiments, the said groove-containing rubber block (rubber encasement):
(A) extends from said tread base rubber layer to and within said outer tread cap rubber to include a portion of the outer tread running surface of at least one rib of said tread cap rubber layer, or
(B) extends from said tread base rubber layer to and within said outer tread cap rubber without extending to a running surface of the tire (whereas, in such embodiment, the rubber encasement may include the bottom of said groove up to about 50 percent of the height of said groove from its bottom).
The rubber compositions of the tread cap rubber layer, underlying tread base rubber layer and groove-containing rubber block (rubber encasement) are comprised of sulfur cured diene elastomer-containing rubber compositions.
In one embodiment, said tread cap rubber layer is comprised of a precipitated silica rich rubber composition which contains less than 35, alternately less than 30, phr of rubber reinforcing carbon black reinforcing filler and said tread base rubber layer (and thereby said encasement rubber block extending from said tread rubber base as a part of the tread base rubber layer) is a reinforcing carbon black rich rubber composition which contains at least about 40, alternately at least about 50 phr of rubber reinforcing carbon black reinforcing filler.
In one embodiment, the rubber composition of the groove-containing rubber block (rubber encasement) has a cut growth rate property of at least 24 millimeters/minute less than the cut growth rate property of the rubber composition of the tread cap rubber layer according to ASTM test D813 at 23° C.
In one embodiment the rubber composition of said tread cap rubber layer is comprised of, based on parts by weight per 100 parts by weight rubber (phr):
(A) about 51 to about 90, alternatively about 70 to about 90, phr of diene-based elastomers comprised of cis 1,4-polyisoprene rubber (desirably natural cis 1,4-polyisoprene rubber), and about 10 to about 49, alternately from about 10 to about 30, phr of at least one of cis 1,4-polybutadiene rubber and styrene/butadiene rubber, or
(B) from 51 to about 90, alternatively about 70 to about 90, phr of cis 1,4-polybutadiene rubber, and about 10 to about 49, alternately from about 10 to about 30, phr of cis 1,4-polyisoprene rubber (desirably natural cis 1,4-polyisprene rubber),
where the rubber composition of said tread cap rubber layer contains about 40 to about 120 phr of rubber reinforcing filler comprised of:
(C) about 51 to about 90, alternatively about 70 to about 90, phr of diene-based elastomers comprised of cis 1,4-polyisoprene rubber (desirably natural cis 1,4-polyisoprene rubber), and about 10 to about 49, alternately from about 10 to about 30, phr of at least one of cis 1,4-polybutadiene rubber and styrene/butadiene rubber, or
(D) from 51 to about 90, alternatively about 70 to about 90, phr of cis 1,4-polybutadiene rubber, and about 10 to about 49, alternately from about 10 to about 30, phr of cis 1,4-polyisoprene rubber (desirably natural cis 1,4-polyisprene rubber), or
(E) from about 40 to about 60 phr of cis 1,4-polyisoprene rubber and, correspondingly from about 60 to about 40 phr of at least one of cis 1,4-polybutadiene rubber and styrene/butadiene rubber,
where the rubber composition of said tread base rubber layer and said tread groove encasement contain about 40 to about 100 phr of rubber reinforcing filler comprised of:
In one embodiment, the cis 1,4-polybutadiene rubber is:
(A) a first cis 1,4-polybutadiene rubber having a microstructure comprised of from about 90 to about 99 percent cis 1,4-isomeric units, a number average molecular weight (Mn) in a range of from about 120,000 to about 300,000 and a heterogeneity index (Mw/Mn) in a range of from about 2.1/1 to about 4.5/1 (a relatively high heterogeneity index range illustrating a significant disparity between its number average and weight average molecular weights), or
(B) a second cis 1,4-polybutadiene rubber having a microstructure comprised of from about 96 to about 99 percent cis 1,4-isomeric units, a number average molecular weight (Mn) in a range of from about 150,000 to about 300,000 and a heterogeneity index (Mw/Mn) in a range of from about 1.5/1 to about 2/1 (a relatively moderate heterogeneity index range illustrating a moderate disparity between its number average and weight average molecular weights).
Said first cis 1,4-polybutadiene rubber may be the product of a nickel catalyst promoted polymerization of 1,3-butadiene monomer in an organic solvent solution such as, for example polymerization of 1,3-polybutadiene monomer in an organic solvent solution in the presence of a catalyst system as described in U.S. Pat. No. 5,451,646 which is based on polymerization of 1,3-butadiene monomer with a catalyst system comprised of, for example, a combination of an organonickel compound (e.g. nickel salt of a carboxylic acid), organoaluminum compound (e.g. trialkylaluminum) and fluoride containing compound (e.g. hydrogen fluoride or complex thereof).
Said second cis 1,4-polybutadiene rubber may be the product of a neodymium catalyst promoted polymerization of 1,3-butadiene monomer in an organic solvent such as, for example, polymerization of 1,3-butadiene monomer in an organic solvent solution in the presence of a catalyst system comprised of, for example, organoaluminum compound, organometallic compound such as for example neodymium, and labile (e.g. vinyl) halide described in, for example and not intended to be limiting, U.S. Pat. No. 4,663,405.
The silica coupler has a moiety reactive with hydroxyl groups (e.g. silanol groups) on said precipitated silica and another different moiety interactive with said conjugated diene elastomers.
In one embodiment, the silica coupling agent (silica coupler) is comprised of a bis(3-trialkoxysilylalkyl)polysulfide containing an average in a range of from about 2 to about 3.8, alternately from about 2 to about 2.6 and alternately from about 3 to about 3.8, connecting sulfur atoms in its polysulfidic bridge or an alkoxyorganomercaptosilane.
In one embodiment, said silica coupling agent is comprised of bis(3-triethoxysilylpropyl)polysulfide.
In one embodiment bis(3-triethoxypropyl)polysulfide contains an average in a range of from about 2 to about 2.6 connecting sulfur atoms in its polysulfidic bridge.
An important aspect of this invention is use of tire tread groove-containing rubber blocks to promote resistance to tread groove cracking, particularly tear resistance and cut growth resistance.
Drawings as
The precipitated silica for the reinforcing filler is a synthetic amorphous silica (e.g. precipitated silica) such as, for example, those obtained by the acidification of a soluble silicate (e.g., sodium silicate or a co-precipitation of a silicate and an aluminate). Such precipitated silicas are, in general, well known to those having skill in such art.
The BET surface area of the synthetic silica (precipitated silica), as measured using nitrogen gas, may, for example, be in a range of about 50 to about 300, alternatively about 120 to about 200, square meters per gram.
The silica may also have a dibutylphthalate (DBP) absorption value in a range of, for example, about 100 to about 400, and usually about 150 to about 300 cc/g.
Various commercially available synthetic silicas, particularly precipitated silicas, may be considered for use in this invention such as, for example, only and without limitation, silicas commercially available from PPG Industries under the Hi-Sil trademark with designations 210, 243, etc; silicas available from Rhodia, with designations of Zeosil 1165MP and Zeosil 165GR and silicas available from Degussa AG with designations VN2 and VN3, 3770GR, and from Huber as Zeopol 8745.
The following Table A presents various desirable physical properties for the rubber composition of the tread rubber encasement component of the tread (positioned within the tread and containing a tread groove) compared to an example of as tread cap layer which contains the tread groove-containing tread encasement component) as well as a desirable difference in the indicated physical properties of the tread block and example of tread cap rubber layer with a view toward resisting cracking (e.g. surface cracking) of a tread groove, particularly at a bottom of as tread groove as the tread is flexed over time.
The values for the tread cap are presented as being exemplary properties.
The values for the basic tread encasement are presented as being the desirable properties.
The values for the alternate tread encasement are presented as being alternate properties.
The desirable differences between the tread encasement and tread cap are presented as being desirable property differences.
Tear resistance may be obtained according to a tear strength (peal adhesion) test to determine interfacial adhesion between two samples of a rubber composition. In particular, such interfacial adhesion is determined by pulling one rubber composition away from the other at a right angle to the untorn test specimen with the two ends of the rubber compositions being pulled apart at a 180° angle to each other using an Instron instrument at 95° C. and reported as Newtons force. The greater the tear resistance value, the beneficially greater the resistance to surface cracking of the tread grove surface.
The energy (in joules) can be measured by an ATS (Automated Test System instrument by Instron Company) which is a measure of energy to achieve elongation at break. It is understood that the greater the energy value (higher value in joules of energy), represents a beneficially greater tread groove surface resistance to cracking.
DeMattia cut growth rate according to ASTM D813. It is understood that the less the cut growth rate, the beneficially greater the resistance of the tread groove to surface crack growth.
For the Aged Energy value, the cured rubber sample is aged for seven days at 70° C.
For the Aged Tear Resistance value, the cured rubber Sample is aged for seven days at 70° C.
For the Aged Elongation value, the cured rubber Sample is aged for seven days at 70° C.
From Table A, it is readily seen that it is desired for the tread block rubber composition (the cured rubber) as compared to the tread cap rubber composition to have a combination of at least two of the following:
(A) increased tear resistance,
(B) increased DeMattia cut growth resistance
(C) increased elongation at break, and
(D) increased energy to break
It is also seen that the internal tread encasement is not provided with a purpose of adding stiffness (e.g. Shore A hardness) to the tread (particularly the tread cap rubber layer) because it is normally desired to have a similar Shore A hardness as the tread cap rubber layer.
It is readily understood by those having skill in the art that the rubber compositions of the tread components would be compounded with conventional compounding ingredients including the aforesaid reinforcing fillers such as carbon black and precipitated silica, as hereinbefore defined, in combination with a silica coupling agent, as well as antidegradant(s), processing oil as hereinbefore defined, fatty acid comprised of, for example, stearic, oleic, palmitic, and possibly linolenic, acids, zinc oxide, sulfur-contributing material(s) and vulcanization accelerator(s) as hereinbefore mentioned.
Processing aids may be used, for example, waxes such as microcrystalline and paraffinic waxes, in a range, for example, of about 1 to 5 phr or about 1 to about 3 phr; and resins, usually as tackifiers, such as, for example, synthetic hydrocarbon and natural resins in a range of, for example, about 1 to 5 phr or about 1 to about 3 phr. A curative might be classified as sulfur together with one or more sulfur cure accelerator(s). In a sulfur and accelerator(s) curative, the amount of sulfur used may be, for example, from about 0.5 to about 5 phr, more usually in a range of about 0.5 to about 3 phr; and the accelerator(s), often of the sulfenamide type, is (are) used in a range of about 0.5 to about 5 phr, often in a range of about 1 to about 2 phr. The ingredients, including the elastomers but exclusive of sulfur and accelerator curatives, are normally first mixed together in a series of at least two sequential mixing stages, although sometimes one mixing stage might be used, to a temperature in a range of, for example, about 145° C. to about 185° C., and such mixing stages are typically referred to as non-productive mixing stages. Thereafter, the sulfur and accelerators, and possibly one or more retarders and possibly one or more antidegradants, are mixed therewith to a temperature of, for example, about 90° C. to about 120° C. and is typically referred as a productive mix stage. Such mixing procedure is well known to those having skill in such art.
After mixing, the compounded rubber can be fabricated such as, for example, by extrusion through a suitable die to form a tire tread. The tire tread is then typically built onto a sulfur curable tire carcass and the assembly thereof cured in a suitable mold under conditions of elevated temperature and pressure by methods well known to those having skill in such art.
The following Example is provided to further illustrate the invention. The parts and percentages are by weight unless otherwise indicated.
Rubber compositions were prepared to evaluate and compare rubber compositions for physical properties.
Rubber compositions are referred in this Example as rubber Samples A and B.
Rubber Sample A contains a combination of 20 phr of natural (cis 1,4-polyisoprene) rubber and 80 phr of cis 1,4-polybutadiene rubber with 20 phr of the natural rubber.
Rubber Sample B contains a combination of 50 phr of natural (cis 1,4-polyisoprene) rubber and 50 phr of cis 1,4-polybutadiene rubber with 50 phr of the natural rubber.
The basic rubber composition formulation is shown in Table 1 and the ingredients are expressed in parts by weight per 100 parts rubber (phr) unless otherwise indicated.
The rubber compositions may be prepared by mixing the elastomers(s) without sulfur and sulfur cure accelerators in a first non-productive mixing stage (NP-1) in an internal rubber mixer for about 4 minutes to a temperature of about 160° C. If desired, the rubber mixture may then mixed in a second non-productive mixing stage (NP-2) in an internal rubber mixer for about 4 minutes to a temperature of about 160° C. without adding additional ingredients. The resulting rubber mixture may then mixed in a productive mixing stage (PR) in an internal rubber mixer with sulfur and sulfur cure accelerator(s) for about 2 minutes to a temperature of about 110° C. The rubber composition may then sheeted out and cooled to below 50° C. between each of the non-productive mixing steps and prior to the productive mixing step. Such rubber mixing procedure is well known to those having skill in such art.
In the following Table 1, exemplary rubber compositions for a tread rubber cap layer (containing tread grooves) and tread groove encasement candidate (which includes a tread base rubber layer) are shown and reported as rubber Samples A and B.
1Natural rubber SMR-20 or SMR-5
2Cis 1,4-polybutadiene elastomer as Bud ™ 1208 from The Goodyear Tire & Rubber Company
3Cis 1,4-polybutadiene elastomer as Bud ™ 4001 from The Goodyear Tire & Rubber Company
4Carbon black N121(ASTM designation)
5Carbon black N550 (ASTM designation) carbon black having an Iodine number of about 43 with a DBP value of about 121.
6Precipitated silica as HI-SIL ™ 210 KS300 from PPG Industries which is synthetic hydrated, amorphous, precipitated silica
7Precipitated silica as Zeosil 1165 from Rhodia
8Liquid coupling agent comprised of bis(3-triethoxysilylpropyl) polysulfide having an average of from about 3.4 to about 3.8 connecting sulfur atoms in its polysulfidic bridge as Si69 ™ from Evonic
9Coupling agent composite of carbon black (N330) and bis-(3-triethoxysilypropyl) polysulfide having an average of from about 2.1 to about 2.6 connecting sulfur atoms in its polysulfidic bridge as Si266 ™ from Evonic in a 50-50 weight ratio
10Rubber processing oil
11A mixture of microcrystalline and paraffin waxes
12Amine based antioxidants
13Non-staining, unreactive phenol formaldehyde resin
14Sulfenamide based sulfur cure accelerator(s)
The prepared rubber compositions were cured at a temperature of about 170° C. for about 12 minutes for the tread block (tread groove encasement) rubber composition and 150° C. for 32 minutes for the tread cap rubber composition and the resulting cured rubber samples evaluated for various physical properties which are reported in Table 2.
1Automated Testing System (ATS) instrument by the Instron Corporation which incorporates a plurality of tests in one system. Such instrument may determine properties such as ultimate tensile, ultimate elongation, modulii and energy to break data reported in the Table is generated by running the ring tensile test station which is an Instron 4201 load frame based on ASTM D412.
2ASTM D2240
3ASTM D1054
4Data obtained according to a peel strength adhesion (tear strength) test to determine interfacial adhesion between two samples of a rubber composition. In particular, such interfacial adhesion is determined by pulling one rubber composition away from the other at a right angle to the untorn test specimen with the two ends of the rubber compositions being pulled apart at a 180° angle to each other using an Instron instrument.
5ASTM D813
6Data obtained according to DIN 53516 abrasion resistance test procedure using a Zwick drum abrasion unit, Model 6102 with 2.5 Newtons force. DIN standards are German test standards. The DIN abrasion results are reported as relative values to a control rubber composition used by the laboratory.
From Table 2 it can be seen that the energy to break increased from 90 to 170 joules for the tread groove encasement candidate as compared to the exemplary tread cap value which is indicative of a significant increase in the rubber composition's durability. This agrees with the indicated desirable physical properties and associated differences in physical properties found in Table A.
From Table 2 it can also be seen that the tear strength (resistance to tear) at (95° C.) increased from 141 to 238 for the tread groove encasement candidate as compared to the exemplary tread cap value which is also indicative of a significant increase in the rubber composition's durability. This agrees with the indicated desirable physical properties and associated differences in physical properties found in Table A.
In one sense, while the mechanism might not be entirely clear, it appears that contributing to the beneficial increases in predictive durability might be promoted, at least in part, by a significant increase in natural cis 1,4-polyisoprene content together with an optimized reinforcing filler and type, as well as an adjustment of the sulfur cure package in a sense of an increase in sulfur content and sulfur/accelerator ratio, to promote an increase in tear and cut growth resistance.
While certain representative embodiments and details have been shown for the purpose of illustrating the invention, it will be apparent to those skilled in this art that various changes and modifications may be made therein without departing from the spirit or scope of the invention.
