Elastomeric compounds utilizing polymer blends for improved wire adhesion.

The importance of good adhesion between the elastomeric and wire components of wire reinforced rubber articles cannot be over emphasized. Not only is this tree for tires, but for hoses and belting as well. Failure at the wire to rubber interface can lead to catastrophic results. In the case of wire reinforced hose products, delamination can lead to serious injury or even death. For this reason, the development of compounds that meet not only the chemical and physical requirements of their intended application, but also have optimum adhesion characteristics is necessary.

The intention of this article is to give examples of how, through the proper blending of different polymers, it is possible to gain improved heat, fluid and ozone resistance, optimum physical properties and achieve good adhesion at the rubber/wire interface. The emphasis here will be placed on compounding for the purpose of making wire reinforced hose. Some discussion will also be given to material costs and how they are affected by the blending of polymers. Although the emphasis of this work is on the use of blending polymers to improve the performance of wire reinforced hose, the technologies presented here could be used in compounding efforts for conveyor or power transmission belts.

Before discussion of compounding for use in hose applications can begin, some explanation should be given of the required performance characteristics for wire reinforced hose. Some of the most important properties that a wire reinforced hose can have are heat and oil resistance and adequate adhesion to the wire substrate. The need for heat resistance in a hose is easy to understand when consideration is given to the installation site for many applications. Engine compartments, machinery housings and other confined areas often reach temperatures that would cause rubber compounds with poor heat resistance to fail prematurely. The lack of oil resistance in a hose tube compound would severely limit the number of fluids that it can transport at any temperature.

The need for adhesion between the rubber and wire components in a wire-reinforced hose is worthy of some explanation. In a hose composed of two layers of wire braid, it is critical to have the wire locked into place in order to avoid movement of the layers in an independent, lateral motion. The two wire plies must be moving radially and in unison in order to get the benefit of the strength of both plies. If one ply, upper or lower, is slipping on the other, friction will begin to build and lead to deterioration of the elastomer. This in mm leads to a weak spot in the hose and the potential for failure due to burst is increased greatly. A common test for evaluating the "locked" status of wire is to check for length change of the hose under pressurization. If the length change is minimal, it can be assumed that the hose dimensions will vary little during its useful life. A common practice in hose manufacturing is to pressurize the finished product briefly before packaging in order to lock the wire into place. This is sometimes referred to as proofing a length of hose.

A second reason for needing adequate adhesion between the rubber and wire plies in a hose is for coupling retention. If the wire plies are moving in the vicinity of the hose coupling, the dimensions of the portion of the hose inside the coupling will vary. If these dimensions, inside and outside diameter, are not static, it is only a matter of time before the coupling will work its way loose from the hose.

A third reason for needing adhesion between the rubber and wire is to avoid the condition known as wire flare. Flaring occurs when the end of a hose has an outside diameter greater than is found along the body of the hose. The end of the hose mushrooms out when the wire does not adhere well to the rubber tube or cover. It is difficult, if not impossible, to attach a coupling to a hose that is experiencing wire flare. The wire in the braided state is trying to return to the relaxed unbraided configuration. If the energy of the wire is greater than the adhesive force, flaring will occur.

Experimental

Mixing for the test compounds occurred in a laboratory "B" internal mixer. Masterbatches were prepared and the cure systems were added on a two-roll mill. All mixes were allowed to stand at room temperature prior to beginning any evaluations. Rheometric testing was done per ASTM D5289. Conditions for testing are given in table 2. Testing of original physical properties was done per ASTM D412 with the results in table 2. All test specimens were prepared using ASTM D 3182. Ozone aging tests were run per ASTM D1149. Fluid aging tests were performed per ASTM D471. Heat resistance testing was performed per ASTM D573. Adhesion testing was performed per ASTM D2229 with samples cured 30 minutes at 330 [degrees] F.

Table 2 - polymer comparison

Type         Oil        Heat          Ozone           Cost
          resistance   resistance   resistance
NBR          Good         Fair         Fair      $0.85-1.00/lb.
PVC/NBR      Good         Fair         Good      $0.95-1.20/lb.
CR           Good         Good         Good      $1.60+/lb.
SBR          Poor         Poor         Poor      $0.60-0.85/lb.

Materials

The formulations used for this study are given in table 1. All raw materials used here are commercially available and can be considered typical for use in a variety of rubber goods. This is especially tree for fluid transfer hose. The polymers chosen for this study are known for their inherent ease of extrusion. Noticeably absent from this study is the utilization of an adhesion promoter in any of the formulations. This is by design since the purpose of this work is to show the effect of the polymer systems alone on rubber to wire adhesion. Further explanation of why no adhesion promoter was used is given in the discussion section of this article. All filler, oil, activator and cure systems are the same for each of the cover and tube compound systems. In a production situation, a cover or tube compound would likely include the use of a process aid to help with extrusion. No process aids were included here since they are known to hinder adhesion.

Table 1 - formulations

Component        1     2     3     4     5     6
NBR            100    85    70    --    --    --
NBR/PVC         --    --    --   100    85    70
CR              --    15    30    --    --    --
SBR             --    --    --    --    15    30
N762 black      75    75    75    75    75    75
DOA             20    20    20    20    20    20
Stearic acid     1     1     1     1     1     1
Zinc oxide       5     5     5     5     5     5
BBTS           1.5   1.5   1.5   1.5   1.5   1.5
Sulfur         2.8   2.8   2.8   2.8   2.8   2.8

A comparison of the polymers used in this evaluation is given in table 2. The polymers were chosen with the intention of utilizing a variety of compositional and structural properties in the blends. All of the compounds studied here can be characterized as having low or high unsaturation levels and also as chlorinated or nonchlorinated. The goal was also to have a variety of chemistries involved in the polymer systems for the purpose of gaining characteristic advantages found with different elastomers. Combining chlorinated with nonchlorinated materials can often give the compounder performance and process characteristics that are hard to match with any one of the polymers alone. By varying the level (and source) of unsaturation in a blend, it is possible to gain properties such as improved low temperature or oil swell.

The acrylonitrile butadiene (NBR) polymer used in this study is typical for use in hose compounding for either tube or cover. This particular polymer's Mooney viscosity (50), yields very nice extrusion characteristics when properly compounded. Although the inherent oil resistance found with NBR makes it useful for a number of applications, it has its limitations. As the ACN percentage is increased, giving improved resistance to non-polar fluids, a significant loss h, low temperature flexibility is seen. The medium ACN grade used here has a good oil resistance and low temperature flexibility combination. The cost factor from the use of NBR would be considered midrange among the polymers included in this study.

The styrene-butadiene (SBR) polymer used in this study is also typical for use in hose compounding. Given the low polarity of SBR it is not up for consideration for tube compounds that may be exposed to non-polar fluids. SBR is more often used in hose tube applications such as sandblast or solids transfer hoses. When properly compounded, SBR can give excellent abrasion resistance. Inclusion of SBR in a polymer blend compound generally leads to lower material costs.

The chloroprene (CR) used in this study can be found in many molded goods and hose applications. The high polarity of chloroprene makes it a good choice for situations where oil resistance is of benefit. It can be generally said that chloroprene based compounds have the best heat resistance of all polymers included in this study. Other benefits of chloroprene include added ozone resistance and increased flame resistance, all resulting from the presence of the halogen atom on the monomeric unit. The drawback to chloroprene is the expense.

The reinforcement system for all test compounds was typical for extrusion formulations. A single black system of N762 was used. The level of black used in this evaluation was typical for a high-grade extrusion compound. In a real world setting, a higher loading would probably be considered in efforts to reduce material costs.

The oil systems used in these evaluations had to be varied in order to ensure compatibility with the base polymer. The polar NBR based systems could easily utilize the aliphatic ester DOA.

In all formulations, the level of zinc oxide was held constant. The ZnO used was in powder form and typical for use in most any rubber compound.

The cure system used in these formulations was based upon what the author has seen give an adequate rate of cure for hose formulation. Experience has shown that cure systems of this type (sulfur with a TBBS accelerator) have enough speed to be practical at traditional autoclave temperatures, but not so fast as to give too short a dwell time to give an adequate flow before the onset of crosslinking. The wire used for adhesion testing was a typical brass-coated steel system. The gauge was 0.012". The composition of the brass was 65% copper and 35% zinc. This type wire is used frequently in a production setting for wire reinforced hose. Coating deposition is 4g/Kg.

Results

Original properties

For a rubber compound to be considered for use in wire reinforced hose, some basic requirements must be met. These include adequate oil, heat and ozone resistance, along with certain original physical properties. The test compounds used in this study all began with properties that would be considered adequate for use in a number of hose tube (compounds 1-3) and cover (compounds 4-6) applications. In wire reinforced hose products, the most sought after include high tensile strength (1,200 to 3,000 psi) with high modulus at low elongations and high durometer. The failure to have high modulus at low elongation has been found to be critical for coupling retention during impulse testing. Another original that has been found to influence the performance of hose is durometer. Compounds having hardness values below 75 (Shore A) often perform poorly in test or field situations. The original properties for the compounds in this study are given in table 3.

Table 3 - original physical properties

Press cured                          1       2       3
 10 min. @ 330 [degrees] F
Hardness, pts Shore A               72      71      68
Elongation, %                      210     238     285
Tensile, psi                     2,110   1,985   1,823
Modulus, psi
  5%                               190     162     124
  25%                              410     377     318
  100%                             890     776     723
Rheometer, MDR
350 [degrees] F, 1 deg. arc
  ML                               1.9     2.4     2.8
  MH                              34.9    31.8    29.6
  TS2(min.)                       1:06    1:21    1:28
Compression set,
  ASTM D395
  22 hrs. @ 212 [degrees] F(%)    22.6    26.8    29.4

Press cured                          4       5       6
 10 min. @ 330 [degrees] F
Hardness, pts Shore A               74      75      75
Elongation, %                      190     226     215
Tensile, psi                     1,860   1,928   2,050
Modulus, psi
  5%                               130     110     120
  25%                              280     223     245
  100%                             678     726     838
Rheometer, MDR
350 [degrees] F, 1 deg. arc
  ML                               1.3     2.0     2.3
  MH                              30.7    33.6    35.9
  TS2(min.)                       0:48    0:43    0:32
Compression set,
  ASTM D395
  22 hrs. @ 212 [degrees] F(%)    42.9    41.0    36.8

Adhesion

Values were measured for adhesion using ASTM D2229 and are given in table 4. The adhesion values given here can only be used for the purpose of comparing compounds and not for predicting what an actual piece of production hose would yield. Many new variables are brought into the picture when a hose is manufactured. These test results give information to show relative differences only. The improvement in the adhesion that was seen as the NBR base compounds had CR added is quite pronounced. The relationship of CR content to increased adhesion is practically linear. The same held true for the PVC/NBR compounds as they received increased levels of SBR. The heat aged tests showed an overall decline in adhesive strengths, but the effect of the blended polymers can still be considered a plus.

Table 4 - adhesion characteristics

Original adhesion                       1      2      3
Cured 30 min. @                      37.8   48.6   55.1
 330 [degrees] F(lb./in.)
Wire coverage, %                       60     70     80
Heat aged adhesion
70 hrs. @212 [degrees] F (lb./in.)   36.1   44.6   52.8
Wire coverage, %                       60     70     70

Original adhesion                       4      5      6
Cured 30 min. @                      31.5   46.7   51.2
 330 [degrees] F(lb./in.)
Wire coverage, %                       50     70     70
Heat aged adhesion
70 hrs. @212 [degrees] F (lb./in.)   25.9   39.8   43.8
Wire coverage, %                       50     70     70

Heat aging

The values for heat aged testing of the test compounds are given in table 5. The effects of adding SBR to the NBR/PVC are plainly a detriment to the heat stability of these type compounds. The loss of elongation, marked increase in hardness and loss of tensile suggest that a hose cover compound having too high a level of SBR will likely suffer from loss of flexibility with time. The NBR based compounds did not suffer this same problem as the level of CR was increased. No significant loss of flexibility would be expected with NBR based compounds containing CR.

Table 5 - heat, oil and ozone aging data

Heat aging                     1      2      3      4      5      6
70 hrs. @ 212 [degrees] F
 Hardness chg.; pts.          +4     +5     +5     +7    +13    +18
 Elongation chg.; %          -18    -16    -18    -28    -38    -49
 Tensile chg.; %              -5     -5     -4    -14    -21    -30
Oil aging
70 hrs. @ 212 [degrees] F
 Hardness chg.; pts.           0     -2     -4     -7    -17    -22
 Elongation chg.; %           -3     +8    +17     -6    -24    -36
 Tensile chg.; %              -8    -12    -23    -19    -31    -54
 Volume chg.; %               -5     +4    +12     +4    +18    +31
Ozone aging
70 hrs. @ 100 pphm          pass   pass   pass   pass   fail   fail

Oil aging

Since the NBR/PVC based compounds (4-6) are intended for use as cover only, no testing was included with regard to oil resistance. The series of test formulations (1-3) that are designed for tube compound use exhibited good retention of original properties when exposed to IRM 903. The development of a positive oil swell with the addition of CR is a plus since a negative volume change would only lead to coupling retention problems when the hose is in a pressurized use situation. This negative swell with no CR present could be addressed by lowering the ACN content, but heat aging characteristics would be sacrificed. The benefit of improved adhesion and positive volume change is a good combination. The results of oil aging tests are given in table 5.

Ozone aging

Only the compounds considered for hose cover use were subjected to ozone exposure. The concentration used was 100 pphm for an exposure time of 70 hrs. The rating used is based on a comparison of the samples aged and unaged. It is plain that as the SBR level is increased, the ozone resistance is diminished. The increased level of unsaturation of SBR is not very tolerant of ozone exposure. A compound of this type in the field would require the addition of an antiozonant agent. The results of ozone testing are given in table 5.

Discussion

There are many reasons for attempting to improve the adhesion of rubber to wire through the use of compounds based on polymer blends. The obvious reasons are shown with the examples used in this study. By combining two different polymers, it is often possible to get the beneficial characteristics of them both. Improving the adhesion of NBR/PVC based recipe while lowering the cost, with the inclusion of SBR, is an example of practical compounding. Although not covered in this work, this same compound prepared with the proper SBR may also show improved abrasion resistance. Given the correct filler and process aid system, this compound could also be made to extrude more easily than if it had remained based solely on NBR/PVC.

Considering the compounds (1-3) that were based on a traditional medium ACN NBR, the improvement in adhesion was accompanied by a positive oil swell. The negative oil swell that was present in the control would likely lead to coupling retention problems in a pressurized hose. Another benefit of including some level of CR in a NBR compound that was not covered in this work is the improvement in processability. The crystallization that occurs in CR, that is not present with NBR, is an asset during the extrusion and braiding process. Often during a braiding operation, the tube is frozen prior to application of the braid. This is done in an effort to make the hose dimensionally stable while the wire is being applied. The fast crystallization of CR allows this dimensional stability to be attained more easily.

The question could be asked, "Why not just add an adhesion promoter to the compound?" An explanation of the hose making process is in order to answer this more easily. The first step in the hose making process is to extrude the tube onto a mandrel. If an adhesion promoter works to adhere the tube compound to anything it comes in contact with, as would normally be the case, the ease with which the mandrel can be removed is reduced. The same relationship holds tree for the cover compound and the wrap material (typically nylon fabric).

Some mention should be given at this point to the chemistry causing the improved adhesion of these compounds. A proposed mechanism for rubber to copper adhesion is given in figure 1. The brass plating in the steel wire contains enough Cu (ca. 68%) to cause this relationship to exist. The mandrel material is typically nylon or rubber for most hydraulic hose products. The lack of Cu present in either of these mandrel materials gives more support to the proposed mechanism and how the blended polymer would be preferable to a system that included the use of adhesion promoters.

Figure 1 - proposed rubber to brass adhesion mechanism

Several mechanisms have been suggested for the adhesion of rubber to brass. All begin with the formulation of cuprous sulfide as given by:

    2Cu +S---[Cu.sub.2]S

This may be followed by one of three reactions:
(1) Reacting with additional sulfur to form cupric sulfide:
    [Cu.sub.2]S+S---2CuS
(2) Reacting with an active rubber sulfur species:
    [Cu.sub.2]S+R---S*---[Cu.sub.2]---S---S---R
(3) Reacting directly with the rubber, in this case adding
directly to the double bond:
    [Cu.sub.2]S+R---CH=CH---R---R---CH---CH---R
                                S    H
                                Cu

Considering the improvement in adhesion that was found when the level of unsaturation was increased, it is worthwhile to consider the third scenario given above. Comparing the degree of unsaturation with the adhesion values reveals a very linear relationship. This could also explain the difficulty found with wire adhesion for compounds based on EPDM or other saturated polymer systems. Even when an ample amount of cuprous sulfide is available, there needs to be a sufficient number of sites of unsaturation present to gain adhesion between the wire and rubber.

Other reasons to not include the use of an adhesion promoter include improved hot mill handling, improved bin stability and the negative effect that adhesion promoters can have on cure systems.

Conclusion

The formulations given for review here are to be considered as starting points for any compound development work. The goal was to show the effect of blending polymers for the purpose of gaining a variety of chemical and physical characteristics along with improving the adhesion at the rubber to brass coated wire interface. The data shown here were gathered through common laboratory testing methods. From these data some assumptions can be made:

* Combining small amounts of SBR or CR into an NBR based elastomer can be accomplished without making significant sacrifices on the original physical properties.

* Significant changes in oil, heat and ozone aging characteristics can be made through the inclusion of SBR and CR in an NBR based elastomer.

* The brass-coated wire adhesion of NBR based compounds can be improved through the addition of either SBR or CR to the formulation.

* Wire reinforced hose formulations can be developed without the need to include adhesion promoters that may interfere with the manufacturing process. Some simple roles should be followed, however, for this type of compounding to be successful. General guidelines include;

* Polymers to be blended should have some basic similarities such as comparable viscosities and polarities. Some latitude can be taken on these properties, but the risk of phase separation exists if the difference is too great.

* Oil selection should be done with consideration given to all polymers involved. Using an oil having a solubility parameter similar to only one part of a multi-phase system could lead to an incompatibility and subsequent loss of heat age stability.

* The cure system should be considered with regard to the blend components. Although sulfur type cures may work for all polymer phases of a compound, the source of sulfur must be efficient for all components.

* Filler systems must be universally appropriate for the polymer phases in a blend. This is normally accomplished with carbon black, but in a non-black compound it could provide an obstacle. This is particularly true if the filler alters the pH of a compound significantly.

Although generic in nature, the polymer blend based formulations presented here can serve as possible starting places for hose compounds that will be in contact with brass coated wire reinforcement. With some further refinement to include process aids and customization of the cure system, compounds of this type could be of benefit to the hose industry.

Acknowledgements

"Enhanced viscosity EVM elastomers for GP molded and extruded applications" is based on a paper given at the September, 1998 meeting of the Rubber Division. "Elastomeric compounds utilizing polymer blends for improved wire adhesion" is based on a paper given at the September, 1998 meeting of the Rubber Division.

[Figures 2-6 ILLUSTRATION OMITTED]

References

(1.) Y. Ishikawa, International Polymer Science and Technology, 19, No. 2, 1992.

(2.) W.J. van Ooij, Rubber Chem. Technology, 52, 605 (1979); 57, 421(1984).

(3.) G.T. Carpenter, Rubber Chem. Technology, 51, 788, 1978.

(4.) G. Haemers, Rubber World, 182, Sept., 1980.

(5.) W.J. van Ooij, "Effect of rubber to brass adhesion- effect of rubber composition on the adhesion," paper 19, 112th Meeting of the Rubber Division, October, 1977.

(6.) W.E. Weening, "Rubber to steel bonding," IRI European Conference, April, 1975.

(7.) M.P. Wagner, "Compounding for wire adhesion," paper presented at Society of Automotive Engineers Meeting, Detroit, Michigan, May, 14-18, 1973.3