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New non-dusting, free-flowing dry concentrate hexamethoxymethylmelamine (HMMM)

For several years, the tire industry has been using various hard-to-handle liquids that are blended onto high surface area carriers, such as precipitated silicas or diatomaceous earth calcium silicate, to form an easier-handling, dry-liquid concentrate. These liquid/powder blends typically range between 50-70% active. It has been well documented that silica imparts many desirable properties to a rubber compound, enhancing certain physical properties including cut-growth resistance, hardness, tear resistance, resilience and adhesion (refs. (1) and (2)). The high surface area of PPG's Hi-Sil 233 precipitated silica makes it an ideal carrier for Cytec's Cyrez 963 liquid resin. This product, called Cyrez 964 RPC, has been successfully used by the tire and rubber-product manufacturers. Recently, a new generation of silicas has been introduced that, in addition to enhancing rubber compound physical properties, has many additional advantages heretofore unobtainable in silicas. These advantages include higher liquid carrying capacity, higher bulk density, lower dusting, better flowability and high rubber reinforcement.

This article focuses on CRA100 RPC, a new product which uses Hi-Sil SC72 (ref. (3)) silica, as a carrier for Cyrez liquid resin. The results show that the new silica allows the compounder to achieve all of the compound physical properties previously obtained with Hi-Sil 233 silica, and also to achieve enhanced rubber physical properties over other common carriers of liquids such as the diatomaceous earth calcium silicate products. CRA100 RPC is a less dusty, highly flowable product that is easily dispersable and much more adaptable to bulk forms of packaging such as one-ton flexible intermediate bulk containers (FIBC).
Table 1 - physical property measurements of carrier materials

Test Procedure Properties analyzed

BET [N.sub.2] adsorption ASTM D 5604-95 Surface area
pH ASTM D1512-90 Acidity
Transmission ASTM D3849-89 Aggregate morphology
 electron microscopy
DBP absorption ASTM D2414-92 Agglomerate void volume
Angle of repose Flowability
Resin capacity TGA % resin, final product
X-ray fluorescence ASTM C575-86 Salt content


[Part 1 of 2]

 Table 2 - model wire coat compound formulation

Ingredient Compound 1 2 3
 phr

Natural rubber 100 100 100
N326 Carbon black 55 40 40
 Hi-Sil 233 0 15 0
 Treated
 diatomaceous earth 0 0 15
Cobalt napthenate
 (10.5% cobalt) 1.5 1.5 1.5
Stearic acid 1.2 1.2 1.2
Polymerized
 trimethylquinoline 3.5 3.5 3.5
Aryl paraphenylenediamine 0.3 0.3 0.3
Penacolite B19S 3.0 3.0 3.0
Zinc oxide 8.0 8.0 8.0

Rubbermakers sulfur 3.8 3.8 3.8
Cyrez 963 (neat) 3.0 3.0 3.0
 Cyrez 963 RPC - - -
 Cyrez 964 RPC - - -
 CRA100 RPC - - -
OBTS 0.7 0.7 0.7
N-Cyclohexylthiophthalimide 0.2 0 0

[Part 2 of 2]

 Table 2 - model wire coat compound formulation

Ingredient 4 5 6
 phr

Natural rubber 100 100 100
N326 Carbon black 55 55 55
 Hi-Sil 233 - - -
 Treated
 diatomaceous earth - - -
Cobalt napthenate
 (10.5% cobalt) 1.5 1.5 1.5
Stearic acid 1.2 1.2 1.2
Polymerized
 trimethylquinoline 3.5 3.5 3.5
Aryl paraphenylenediamine 0.3 0.3 0.3
Penacolite B19S 3.0 3.0 3.0
Zinc oxide 8.0 8.0 8.0

Rubbermakers sulfur 3.8 3.8 3.8
Cyrez 963 (neat)
 Cyrez 963 RPC 4.2 0 0
 Cyrez 964 RPC 0 4.6 0
 CRA100 RPC 0 0 4.2
OBTS 0.7 0.7 0.7
N-Cyclohexylthiophthalimide 0.2 0.2 0.2





History of resin use in wire coat compounds

Early work with organic wire adhesion promoters led to the development of the "HRH" system which refers to its three components (ref. (4)) hexamethylenetetramine (HEXA), resorcinol and precipitated silica. HEXA is often called a formaldehyde donor since it can react with water to form formaldehyde and ammonia. An evolution of this original HRH system has seen the HEXA component being replaced by hexamethoxymethylmelamine (HMMM). HMMM has a distinct advantage in that methylene linkages are formed without formaldehyde serving as an intermediate and hence, HMMM could be properly classified as a methylene donor (ref. (5)). Over the years, HMMM usage has continued to grow until today it has replaced most of the HEXA, particularly in cases where the latter may be undesirable for any reason, including dispersion problems, toxicity, and importantly, the release of an amine during vulcanization (ref. (6)).

Experimental results have shown that bonding systems containing HMMM provide superior, i.e. higher pull-out adhesion and rubber coverages under aging conditions including heat-aging, humidity-aging and steam-aging when compared to HEXA (ref. (7)). Today's modern tire adhesion bonding system typically contains HMMM, a resorcinol or a prereacted resorcinol-formaldehyde component, and a precipitated silica. The optimized system might contain the following proportions:

* HMMM - 3-6 phr;

* Resorcinol - 2-3 phr or R/F resin 3-4 phr;

* Silica - 10-15 phr

Cyrez/resorcinol systems function primarily because they are polar and migrate to the high-energy wire interface and form a resin-rich layer on the brass of the wire surface. Thus, the resin system protects the steel cord from attack by moisture and oxygen, reduces dezincification, stabilizes the [Cu.sub.(.sub.2.sub.-.sub.x.sub.)]S interfacial linkage, and imparts both original and humidity-aged adhesion. Finally, resins increase rubber tear strength and pull-out adhesion (ref. (8)).

Experimental

Properties of carrier materials were characterized by the procedures shown in table 1. The model wire coat recipe studied is shown in table 2. Compounds were mixed according to ASTM D3182-89 using a two-stage mix in an internal mixer. Specimens were cured at 150[degrees]C to a time corresponding to [T.sub.9.sub.0] + appropriate mold lag and tested according to the procedures listed in table 3. Adhesion was determined to brass-coated (64% copper) steel wire using the tire cord adhesion test (TCAT) (refs. (9), (10), (11), (12), (13)). Wire composite specimens were tested as cured, and after aging:

* for five days in a circulating air oven at 90[degrees]C;

* for five days in controlled humidity of 90% RH at 90[degrees]C, and;

* for two days in 5% salt-fog at 32[degrees]C.

Results and discussion

Non-black fillers in tire wire coat compounds

Silica has been shown to improve the wire-to-rubber adhesion of compounds containing resorcinol/formaldehyde donor resins (refs. (4), (14), (15), (16), (17)). Tate (ref. (16)) found significant improvements in steam-aged and humidity-aged adhesion upon using silica in compounds containing an organocobalt adhesion promoter. Evans, Waddell and coworkers (ref. (17)) reported that the increases in interfacial adhesion due to silica use was not simply a result of increased tear strength of the rubber compound, but that silica use had an effect on the relative concentrations of the inorganic compounds formed in the interfacial bonding layer on the brass-coated wire during rubber compound cure (ref. (18)). As a replacement for carbon black, silica improved the tear strength, cut-growth resistance and adhesion to adjoining rubber compounds (refs. (2) and (17)).
 Table 3 - rubber physical test methods

Test Procedure Properties analyzed

Cure ASTM D 2084-92 Minimum torque
 Maximum torque
 [TS.sub.2] scorch time
 [T.sub.9.sub.0] cure time
Stress/strain ASTM D 412-87 Elongation at break
 Break strength
 Modulus
Tear ASTM D2262-83 Molded groove tear
 (modified)
Zwick resiliometer Hardness
 Rebound
Fatigue ASTM D813-87 Cut growth
 TCAT(1), (2),
Wire adhesion (3), (4), (5) Original adhesion
 Humid-aged adhesion
 Oven-aged adhesion
 Salt-aged adhesion

(1) . D.W. Nicholson, D.I. Livingston, G.S. Fielding-Russell and
A.N. Gent, Tire Sci. Technol., 6, 71 (1978).

(2) . D.W. Nicholson, D.I. Livingston and G.S. Fielding-Russell,
Tire Sci. Technol., 6, 114 (1978).

(3) . G.S. Fielding-Russell, D.W. Nicholson and D.I. Livingston,
Tire reinforcement tire performance, ASTM STP 694, 153 (1979).

(4) . G.S. Fielding-Russell, D.W. Nicholson and D.I. Livingston,
Rubber Chem. Technol., 53, 950 (1980).

(5) . R.A. Ridha, J.F. Roach, D.E. Erickson and T.F. Reed, Rubber
Chem Technol., 54, 835 (1981).


[Part 1 of 2]

Table 4 - property comparisons - black vs. silica vs. diatomaceous
earth

Property 55 phr 40 phr N326 /
 N326 15 phr Hi-Sil 233

[TS.sub.2], min 4.4 5.2
[T.sub.9.sub.0], min 25.0 31.2
Minimum torque, dNm 3.1 3.3
Maximum torque, dNm 36.8 31.5
Break strength, MPa 24.3 23.6
Elong. @ break, % 506 532
Modulus @ 300%, MPa 10.9 10.0
Hardness @ 23[degrees]C 76 75
Rebound @ 100[degrees]C, % 53.6 54.4
Tear strength, N/mm 9.6 12.2
Cut-growth resistance, mm @ 11.5 8.8
100,000 cycles
TCAT adhesion, [alpha] ( )=% Rubber coverage
Original 3.7(90) 5.8(90)
Humid, 120 hr, 90[degrees]C, 90%RH 2.2(90) 3.8(90)
Oven, 120 hr, 90[degrees]C 2.5(90) 2.8(95)
Salt, 48 hr, 32[degrees]C 3.9(80) 8.6(95)

[Part 2 of 2]

Table 4 - property comparisons - black vs. silica vs. diatomaceous
earth

Property 40 phr N326 /
 15 phr Microcel E

[TS.sub.2], min 4.8
[T.sub.9.sub.0], min 29.9
Minimum torque, dNm 3.1
Maximum torque, dNm 30.3
Break strength, MPa 21.1
Elong. @ break, % 445
Modulus @ 300%, MPa 10.9
Hardness @ 23[degrees]C 76
Rebound @ 100[degrees]C, % 52.9
Tear strength, N/mm 7.0
Cut-growth resistance, mm @ failed
100,000 cycles
TCAT adhesion, [alpha]
Original 3.5(90)
Humid, 120 hr, 90[degrees]C, 90%RH 2.0(80)
Oven, 120 hr, 90[degrees]C 2.4(90)
Salt, 48 hr, 32[degrees]C 4.0(90)





Table 4 is a comparison of the properties of the model wire coat compound shown in table 2 when 15 phr of N326 carbon black is replaced by 15 phr of silica (compound #2) and diatomaceous earth (compound #3), respectively. The substitution of silica for carbon black beneficially increased the compound scorch time, tear strength, cutgrowth resistance and wire-to-rubber adhesion, but also increased the compound cure time and decreased the rheometer maximum torque and compound modulus. The replacement of carbon black with diatomaceous earth was detrimental to compound tear strength, cut-growth resistance, break strength and elongation at break. This is consistent with previous reports indicating that diatomaceous earth does not disperse in rubber to the same degree as typical precipitated silicas (ref. (19)). Thus, precipitated silica is a superior reinforcement of rubber when compared to diatomaceous earth.

Physical properties of HMMM carrier materials

The physical properties of the commonly used carriers for HMMM resin are shown in table 5. Precipitated silica and diatomaceous earth calcium silicate have high dust levels ([less than]200 mesh screen fractions) and high angle of repose values indicating poor flowability. An additional problem with diatomaceous earth is that it may also contain detectable levels of the suspected carcinogen [alpha]-quartz, depending on its source and purification procedures. Amorphous precipitated silicas contain no detectable [alpha]-quartz based on x-ray diffraction due to the lack of longrange structure in the [(-O-Si-O-).sub.n] units (ref. (20)).

One limitation of precipitated silica is that it has a maximum capacity for HMMM of 65% in order to maintain the flowability of this product. SC72 silica has less than 1% dust, a low angle of repose indicating good flowability, contains no detectable [alpha]-quartz and is capable of carrying 72% by-weight HMMM. Finely-divided materials often exhibit bridging behavior in gravity-flow systems. SC72 silica, in contrast, has excellent free-flow properties even with 72% by weight HMMM. Table 6 shows the physical properties of the resin powder concentrate materials. Of particular interest is the very low angle of repose (29[degrees]) for the CRA100 RPC when compared to the other two commercial powdered products, and the higher bulk density of the CRA100 (44.2 [lb/ft.sup.3]). The improved non-dusting and flowability for CRA100 RPC is due to the uniformly sized spherical particles of silica carrier which maintain their integrity, even after mixing in factory-scale ribbon blenders, as shown in figure 1 (right) versus the 964 RPC (left).

Rubber properties of RPC forms of HMMM

Samples of RPC materials were compounded in the model wire-coat formulation shown in table 2. In order to maintain an equal loading of HMMM at 3 phr the following levels of concentrate were used: (1) 963 RPC, 4.2 phr, (2) 964 RPC, 4.6 phr and (3) CRA100, 4.2 phr. Rubber cure and physical properties and wire-adhesion performance are shown in table 7. Compounds #5 and #6, which contained the HMMM on silica, showed significantly higher tear strength and adhesion pull-out force than did compound #4, which contained diatomaceous earth calcium silicate. Table 8 is a summary of the rubber property comparisons at the 95% confidence level for: (1) replacing 15 phr of N326 carbon black with 15 phr of Hi-Sil 243LD silica, (2) using 4.2 phr of 963 RPC in place of 3.0 phr of liquid 963, (3) using 4.6 phr of 964 RPC in place of 3.0 phr of Cyrez 963, and (4) using 4.2 phr of CRA100 RPC in place of 3.0 phr of liquid 963. In all cases use of a minus sign indicates that the value of the rubber property is detrimental, a zero indicates no statistical difference and a plus sign indicates the test value for the property is beneficial. The use of silica in place of carbon black benefited the scorch safety, tear strength, cut-growth resistance and adhesion of the wire composite with detrimental effects on cure time and compound modulus and hardness. The use of 963 RPC increased cure time and reduced rebound values for the rubber composite. The use of 964 RPC increases cure time and scorch safety and as expected, provided increased tear strength and cut-growth resistance, and reduced rebound values for the rubber composite. The use of CRA100RPC increased cure time and reduced rebound values for the rubber composite. The benefits of using 964 and CRA100 may be attributed to the positive effects of using silica, even at the very low levels present in the wire coat compound, since they are also the most significant properties changed when silica is used as a replacement for carbon black.
 Table 5 - properties of carrier materials

Property Hi-Sil SC72 Hi-Sil 223 Diatomaceous
 earth

BET, [m.sup.2]/g 150 150 133
DBP, ml/100g 250 200 269
pH 7 7 8.8
Bulk density,
[lb/ft.sup.3] 12.1 10 8
[greater than]100 mesh, % 84.93 0 2.1
[less than]200 mesh, % 0.51 100 94.1
Angle of repose 29[degrees] 60[degrees] 60[degrees]
Resin capacity, % 72 65 72

 Table 6 - properties of resin powder concentrates

Property CRA100 Cyrez 964 Cyrez 963
 RPC RPC RPC

% resin 73.66 65.21 72.76
Bulk density,
[lb/ft.sup.3] 44.2 36.6 24.5
Angle of repose 29[degrees] 62[degrees] 59[degrees]
Screen fraction
[greater than]100 mesh 94.8 46.6 46.4
[less than]150 mesh 0.8 14.2 32.3
[less than]200 mesh 0.1 0.3 9.6





Summary

The benefits of using SC72 silica as a high-capacity carrier for 72% HMMM resin over the current powdered carriers are shown. The 29[degrees] angle of repose of the CRA100 RPC containing HMMM on SC72, versus the 60[degrees] angles obtained for the current commercial materials, indicates significant advantages in the flowability for this new product. Coupled with the improved bulk density of this new product, this will provide packaging and handling advantages and the ability for use in bulk-handling and weighing systems. The dust levels of less than 1% will provide housekeeping and environmental advantages. Along with the absence of potential [alpha]-quartz, these are important health considerations. Finally, the rubber compound physical properties, particularly tear strength and cut-growth resistance, of the wirecoat compound are enhanced by the use of precipitated silica in the compound formulation and the original and humid-aged wire adhesion performance of the wire composite are improved by the use of CRA1 00 RPC.
Table 7 - physical properties - model wire-coat compound containing
RPC materials

 Compound
Property 4 5 6
 Cyrez 963 Cyrez 964 CRA100
 RPC RPC RPC

[TS.sub.2], min 5.6 5.2 5.5
[T.sub.9.sub.0], min 22.8 24.2 25.0
Minimum torque, dNm 3.6 3.5 3.6
Maximum torque, dNm 35.6 38.7 37.9
Break strength, MPa 25.5 23.3 24.9
Elong. at break, % 523 488 526
Modulus @ 100%, MPa 2.5 2.5 2.3
Hardness @ 23[degrees]C 75 77 77
Rebound @ 100[degrees]C, % 56.4 55.4 55.0
Tear strength, N/mm 11.8 19.5 14.2
Cut-growth resistance, 13.3 13.5 11.4
mm @ 100,000 cycles
TCAT pull-out force, N ( ) = % Rubber coverage
-Original 523(75) 656(87) 738(91)
-Humid/120hr, 90[degrees]C, 607(72) 682(87) 672(84)
90%RH
-Oven/120hr, 90[degrees]C 636(82) 765(83) 721(87)


[Part 1 of 2]

Table 8 - comparison of rubber properties for compound formulations

Property 15 phr silica Cyrez 963 RPC
 ipo ipo
 N326 black Cyrez liquid

Cure time (1) (1)
Scorch safety (1) (2)
Viscosity (1) (2)
Maximum torque (3) (2)
Break strength (2) (2)
Elong. @ break (1) (2)
Modulus (3) (2)
Hardness (3) (2)
Rebound (1) (3)
Tear strength (1) (2)
Cut-growth (1) (2)
resistance
Adhesion, original (1) (2)
Adhesion, humid- (1) (2)
aged

[Part 2 of 2]

Table 8 - comparison of rubber properties for compound formulations

Property Cyrez 964 RPC CRA100 RPC
 ipo ipo
 Cyrez liquid Cyrez liquid

Cure time (1) (1)
Scorch safety (1) (1)
Viscosity (2) (2)
Maximum torque (2) (2)
Break strength (2) (2)
Elong. @ break (2) (2)
Modulus (2) (2)
Hardness (2) (2)
Rebound (2) (2)
Tear strength (1) (1)
Cut-growth (1) (1)
resistance
Adhesion, original (2) (2)
Adhesion, humid- (1) (1)
aged

(1) - denotes a beneficial compound change;

(2) - denotes no statistical difference

(3) - denotes a detrimental compound change





Acknowledgements

"New non-dusting free-flowing dry concentrate hexamethoxymethylmelamine (HMMM)" is based on a paper given at the October, 1995 Rubber Divison meeting.

"Tetraisobutylthiuram monosulfide (TiBTM) - a unique retarder/kicker in one molecule" is based on a paper given at the October, 1995 Rubber Divison meeting.

"The effects of HTS on cure kinetics of accelerated sulfur vulcanization" is based on a paper given at the October, 1995 Rubber Divison meeting.

References

(1) . M.P. Wagner, Rubber Chem. Technol., 49, 703 (1976).

(2) . W.H. Waddell, L.R. Evans and T.A. Okel, Tire Technol. Int. `94, 22 (1994).

(3) . "High-capacity precipitated silica carriers for the rubber industry," D.L. Scott, J.T. Dew, L.R. Evans and W.H. Waddell, presented at the 148th Rubber Division, ACS meeting, Cleveland, OH (1995).

(4) . J.R. Creasey and M.P. Wagner, Rubber Age, 100, 72 (1968).

(5) . M.J. Nichols and R.F. Ohm, Adhesives Age, 19, 31 (1976); 19, 25 (1976).

(6) . "Introduction to Cyrez adhesion promoter systems," American Cyanamid Co., Bound Brook, NJ.

(7) . "Resorcinol bonding systems for steel cord adhesion," A. Peterson and M. Dietrick, Koppers Co., Monroeville, PA, 1984.

(8) . A. Peterson and M. Dietrick, Rubber World, 190, 24 (1984).

(9) . D.W. Nicholson, D.I. Livingston, G.S. Fielding-Russell and A.N. Gent, Tire Sci. Technol., 6, 71 (1978).

(10) . D.W. Nicholson, D.I. Livingston and G.S. Fielding-Russell, Tire Sci. Technol., 6, 114 (1978).

(11) . G.S. Fielding-Russell, D.W. Nicholson and D.I. Livingston, Tire reinforcement tire performance, ASTM STP 694, 153 (1979).

(12) . G.S. Fielding-Russell, D.W. Nicholson and D.I. Livingston, Rubber Chem. Technol., 53, 950 (1980).

(13) . R.A. Ridha, J.F. Roach, D.E. Erickson and T.F. Reed, Rubber Chem Technol., 54, 835 (1981).

(14) . J.R. Creasey, B.D. Russell and M.P. Wagner, Rubber Chem. Technol. 41, 1300 (1968).

(15) . M.P. Wagner, Rubber Chem. Technol., 50, 356 (1977).

(16) . P.E.R. Tate, Rubber World, 192, 37 (1985).

(17) . "Use of precipitated silica to improve brass-coated wire-to-rubber adhesion," L.R. Evans, J.C. Hope, T.A. Okel and W.H. Waddell, Rubber World, April, 1996.

(18) . "Mechanism by which silica improves brass-coated wire-to-natural rubber Adhesion," W.H. Waddell, L.R. Evans, E.G. Goralski and L.J. Snodgrass, presented at the 148th Rubber Division, ACS meeting, Cleveland, OH (1995).

(19) . "Improved rubber compounding with Hi-Sil ABS precipitated silica," PPG Industries, Inc. Pittsburgh, PA.

(20) . E. Gorlich, Ceramic Int., 8, 3 (1982).
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Author:Waddell, Walter H.
Publication:Rubber World
Date:Aug 1, 1996
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