Compatibilization of CR/EPM blends for power transmission belt applications.

Compatibilization of dissimilar elastomer blends is an area of active interest both from technological and scientific points of view. The synthetic or natural elastomers are often blended with one another to generate a given property profile not achievable by either of the blend components. Neoprene or polychloroprene rubber (CR) is the material of choice in most power transmission belts. However, the emerging needs of the automotive industry, namely higher use temperature and longer warranty periods require significant improvements in the heat/ozone/cut growth resistance of neoprene. Such requirements can be satisfied by blending CR with EPM rubbers that are known for better heat, ozone and cut growth resistance. As such, the CR/EPM blends are incompatible, resulting in poor mechanical properties.

The two objectives of this work were:

* Develop compatibilized CR/EPM blends for PTBs application with improved heat, ozone and cut growth resistance.

* Provide guidelines and fundamental understanding of compatibilization and morphological behavior of CR/EPM blends.

Power transmission belts

Description

Power transmission belts represent a broad category which includes all belts which transmit power. They are used extensively in automotive and industrial applications and include a wide range of belts: v-belts, poly-rib belts, micro-v-belts, timing belts, etc.

PTB material

In most belts neoprene (or chloroprene rubber, CR) has been the material of choice due to its unique property profile: ozone resistance, oil resistance, toughness, dynamic flex life, good adhesion to other materials and heat resistance up to 100 [degrees] C. Until recently, CR belts had served the needs of the automotive industry, but new materials are in demand to meet the emerging needs of that industry. First of all, neoprene belts are encountering greater heat duress in service due to increasing underhood temperatures (up to 150 [degrees] C). Secondly, to meet the automotive industry's target 100,000 mile warranty periods, the PTBs must have a lower failure rate with high mean life and at higher temperatures. Improved neoprene type polymers and other materials with significant improvements in belt life are being introduced. A cross sectional view of the PTB is shown in figure 1.

[Figure 1 ILLUSTRATION OMITTED]

PTBs needs/requirements

The goal of the PTB market is to develop a 100,000 mile belt. The three major limitations of current PTBs are undercord cracking, noise and short service life. Undercord cracking is a deterioration due to a combination of heat, ozone and stress. Although a "cracked" belt may still be operational, it is less efficient and more prone to breakage. Noise resistance is related to the coefficient of friction and hence, abrasion resistance becomes an important factor. Beyond laboratory scale testing, PTB manufacturers perform parts testing for frequency of the "squeaks" generated. Service life is affected by a variety of factors such as heat/ozone resistance and fabric breakage.

Specific laboratory tests used for screening PTB (undercord and overcord) compounds are as follows: Heat resistance, 48 hours at 125 [degrees] C; ozone resistance (static and dynamic); De Mattia cut growth; wear resistance (Pico, Taber, weight loss after running belts); tensile; elongation; flexural modulus; Mooney viscosity/scorch; adhesion; oil swell, ASTM #3 oil and low temperature flexibility (Gehman brittleness, run belt at low temperature).

Approaches

We addressed the following issues to formulate a CR/EPM blend with improved heat, ozone and cut growth resistance:

* Effect of EPDM (ethylene, diene content, Mooney, molecular weight distribution) on blend properties.

* Compatibility of "modifiers" with CR and EPM separately.

* Selection and evaluation of compatibilizers for CR/EPM blends.

* Effect of various curing agents.

* Effect of carbon black on CR/EPM blends.

In general, our compatibilization approaches involved:

* Designing a compatibilizer in such a way that it had structural similarity with one phase and specific interaction with the other phase, e.g. E-MA-AA, "ethylene" segment had structural similarity with EPM and "MA-AA" segment could interact specifically with neoprene.

* Miscibility in one phase (EPM) and specific interaction of the other phase (CR) with the compatibilizer (EPDM-g-PMMA).

* Chemical modification of either phase (EPDM EPDM-g-PMMA).

* Good melt viscosity match between blend components (CR, EPDM, EPDM-g-PMMA or E-MA-AA), which could increase blending efficiency.

These compatibilization approaches are also applicable to other elastomer blends.

Experimental

Materials

The Neoprene GRT (Du Pont Chemical), EPDM (Vistalon 7000, Exxon Chemical) and E/MA/AA (Escor acid terpolymers, Exxon Chemical) were commercial polymers. We synthesized the EPDM-g-PMMA in our laboratory as described previously (ref. 3).

Methods

Blend samples were compounded in a small scale (45 cc) Brabender mixer and were examined in the optical microscope as thin sections (100-200 nm) using phase contrast technique (neoprene phase [right arrow] dark grey, V-7000 or compatibilizer of V-7000 + compatibilizer phase [right arrow] "white"). The scanning electron microscopy (SEM) data were obtained for those blend samples which did not produce satisfactory phase contrast. Dispersion of carbon black in the above blends was analyzed using transmission electron microscopy (TEM). Due to the proximity of both Tg and tan [Delta] values of neoprene and EPM rubbers, we could not use DSC and DMTA data to study the compatibilities of the above blends.

Results and discussion

Neoprene GRT/EPDM/escor acid terpolymer alloys

Escor acid terpolymers (ethylene/methacrylate/acrylic acid terpolymer, E/MA/AA) was selected as a potential compatibilizer for 70/30 CRIV-7000 blends. These terpolymers are used as high performance adhesive polymers. They are designed for bonding polar to non-polar substrates and have a unique adhesion profile offering excellent adhesion to a wider variety of substrates than other ethylene based polymers. These new bonding resins adhere to diverse materials such as glass, metals, nylon, PET, PP, PE and cured rubber. Additionally, they provide excellent mechanical compatibility with other polymers and are used to modify the adhesion properties of many plastics and elastomers. The property profiles of various Escor acid terpolymers grades are listed in table 1.

Table 1 - properties of Escor acid terpolymers

                                         ASTM method       ATX 325

Melt index, g/10 min                     D 1238 (E)            20
Acid number(1)                           Exxon method          45
Density, g/cc                            D 792              0.942
Tensile strength                         D638(2)            1,200
  psi (MPa)
Elongation, %                            D638(2)            1,800
Flexural modulus                         D 790              1,300
  psi (MPa)                                                   (9)
Tensile Impact,                          D 1822(S)
  ft.-lbs./in.(2) (kJ/[m.sup.2])@
  73 [degrees] F (23 [degrees] C)                          350
(735)
  -40 [degrees] F (-5 [degrees] )                          255
(535)
Hardness, Shore D                        D 2240                21
DSC melting point,                       D 3417            149 (65)
  [degrees] F ([degrees] C)
Vicat softening point,                   D 1525 (rate B)   140 (60)
  [degrees] F ([degrees] C), 200g load

                                         ATX 350

Melt index, g/10 min                           6
Acid number(1)                                15
Density, g/cc                              0.951
Tensile strength                           1,400
  psi (MPa)
Elongation, %                              2,000
Flexural modulus                           2,700
  psi (MPa)                                 (20)
Tensile Impact,
  ft.-lbs./in.(2) (kJ/[m.sup.2])@
  73 [degrees] F (23 [degrees] C)        340 (715)
  -40 [degrees] F (-5 [degrees] )        235 (495)
Hardness, Shore D                             28
DSC melting point,                       167 (75)
  [degrees] F ([degrees] C)
Vicat softening point,                   146 (63)
  [degrees] F ([degrees] C), 200g load

(1) Milligrams KOH/gm polymer.

(2) Compression molded type IV specimens crosshead speed 2 in./min. (5.1) cm/min.).

This E/MA/AA compatibilizer has some structural similarity to EPDM while it could interact with neoprene either through hydrogen bonding or covalent linkage as shown:

[ILLUSTRATION OMITTED]

We compounded two different grades of Escor acid terpolymers with neoprene and with V 7000 separately, and also evaluated them as a compatibilizer for 70/30 CR/V-7000 blends. The blend samples (table 2) were analyzed using light microscopy. Analysis of the light microscope pictures indicated that the degree of dispersion and the particle-size of the dispersed phase (i.e. the terpolymer phase of the binary blends, figures 2 and 3, and the V-7000 + terpolymer phase of the ternary blends, figures 4 and 5), depended on the structure of the terpolymer. In both the binary and ternary blends, the effect of the compatibilizer (i.e. the so called "compatibility effect" which was a measure of the degree of dispersion and reduction in particle size of the dispersed phase) improved in the order: ATX 350 [is greater than] ATX 325

[Figure 2-5 ILLUSTRATION OMITTED]

Table 2 - properties of Escor acid terpolymers

                   Components

Blend #   CR   V-7000   ATX 350   ATX 325

1         70     30        --        --
2         70     --        10        --
3         70     --        --        10
4         70     20        10        --
5         70     20        --        10

Interestingly, the molecular structure of this compatibilizer seemed to control its efficiency. An Escor acid terpolymer having low melt index ([is less than] 4), low % AA (1-2%) and high % MA (35 ~ 45%) was found to be a better compatibilizer.

Viscosity study

The phase morphology of the dispersed phase (size, shape and dispersity) of a blend depended on key factors such as interfacial adhesion and viscosity ratio of the blend components. To understand this phenomenon in CR/V-7000/Escor acid terpolymer blends, we collected the viscosity vs. shear rate data (figure 6) of the blend components mentioned above. Based on these data, droplet formation of dispersed phase in the neoprene matrix was expected with some differences in particle size among various grades of Escor acid terpolymer. Since the ATX 325 did not have good viscosity match with both the blend components, V-7000 and CR, the compatibility of CR/V-7000/ATX 325 blend was expected to be poor. The observed compatibility differences of the CR/V7000/ATX 350 or 325 blends might also be due to terpolymer composition such as % MA, polarity (i.e. % AA), etc.

[Figure 6 ILLUSTRATION OMITTED]

CP/V-7000/Escor acid ATX 350 alloys for PTBs application

We selected the best compatibilizer from our initial screening study, i.e. Escor acid ATX 350 and evaluated the CR/V7000/ATX 350, 70/30/10 blend in a standard PTB formulation. The physical properties of the blends relative to CR are listed in table 3. The beneficial effect of the compatibilization of CR/V-7000/ATX 350 (70/30/10) blends was evident in terms of significant improvements in heat aging (both tensile and elongation change), ozone resistance, cut growth resistance and other physical properties.

Table 3 - EPDM/neoprene blends for PTB

                                         Control      A      B

Neoprene GRT                                 100    70      70
Vistalon 7000                                --     30      30
Escor ATX 350                                --     --      10

Mooney viscosity
(ML1 + 8 @ 100 [degrees] C)                  61     82      76

Physical properties
Cure 20 min @ 160 [degrees] C
100% modulus, MPa                            7.5    6.9     8.2
200% modulus, MPa                            15.6   13.4    14.8
Tensile, MPa                                 19.0   15.4    16.5
Elongation, %                                256    237     245
Hardness, Shore A                            80     82      83

Air oven age, 48 hr. @
  140 [degrees] C
Tensile, MPa                                 16.2   14.7    16.5
Elongation, %                                115    149     172
Hardness, Shore A                            90     90      90

Static ozone resistance
100 ppm [0.sub.3], 37.8 [degrees] C,
  bent loop
Hours to 2x crack                             8    >500      >500
Hours to visable crack                       184   >500      >500
Dynamic ozone resistance
100 ppm [0.sub.3], 37.8 [degrees] C
0-25% extension, 30 cycle/min.
Hours to 2x crack                             24    112    160
Hours to break                               112    297    440

De Mattia cut growth
Average crack rate (inch/megacycles)
Room T, 120 [degrees] C angle
  (0.50 in)                                  586    531     25
100 [degrees] C, 90 [degrees] angle
  (0.75 in.)                                 938    891     15

Carbon black interaction

For blends of dissimilar elastomers, achieving optimum carbon black distribution in all phases of the final product is still a problem. In blends of elastomers that differed significantly in terms of unsaturation or viscosity, carbon black resided preferentially in the higher unsaturation, lower viscosity phase. Among various elastomers, the relative carbon black response (distribution) in different 50/50 preblends was ranked from highest to lowest as: butadiene rubber (BR), styrene-butadiene rubber (SBR), neoprene (CR), nitrile rubber (NBR), EPDM, halobutyl (C1 or BR-IR) and butyl (IIR) rubber (ref. 4). Polarity was also a factor controlling carbon black migration in dissimilar elastomer blends (ref. 5). In other rubber blends, the interaction of isobutylene polymers with reinforcing carbon black was a central issue in the tread formulation to improve the abrasion resistance of isobutylene containing tread compounds and thereby treadwear resistance. Again, here also, migration of carbon black from isobutylene to SBR phase was a problem, causing the poor abrasion resistance of isobutylene polymers.

Neoprene and V-7000 did not differ significantly in the melt viscosity (figure 5). However, the polarity difference between CR and V-7000 was appreciable enough to cause carbon black migration from V-7000 to CR phase. We used transmission electron microscope (TEM) technique to identify the carbon black distribution in CR/V-7000 blends. Since large differences in unsaturation existed between CR and V-7000, we used osmium tetroxide staining in our TEM studies. This staining technique rendered the high unsaturation polymer, i.e. CR phase more opaque to provide contrast for TEM analysis. As shown in figure 7, the carbon black localized selectively in CR phase of the control - CR/V-700O, 70/30 blend. A good phase boundary existed between CR phase which was "opaque" and V 7000 phase (which was "light"). Since the entire carbon black aggregates localized in CR phase, the CR phase size increased and the "opaque/light" area ratio appeared larger than the corresponding blend components ratio, CR/V-7000, 70/30. In contrast, as shown in figure 8, the phase morphology of the compatilized blend was significantly different from that of the above control blend. The carbon black dispersed in both CR (opaque) and V-7000 (light) phases of this compatilized blend. The CR phase size was not as big as in the control. Still good phase boundary existed between the phases.

[Figure 7-8 ILLUSTRATION OMITTED]

One possible explanation for delocalized dispersion of carbon black in the compatibilized blend was due to some kind of ionomeric interaction between the compatibilizer (which contains [CO.sub.2] H groups) and carbon black particles (which has e po]ar surface ) in presence of the curing agent, i.e. ZnO.

Neoprene GRF/EPDM/EPDM-g-PMMA alloys

The EPDM-g-PMMA was prepared as described previously (ref. 3). The compatibilizer, EPDM-g-PMMA was compounded with neoprene using a small scale Brabender (volume = 45 cc) as before. The blend samples were examined in the light microscope (LM) as thin sections (100-200 nm) using the phase contrast technique. The blends were also examined in the scanning electron microscope (SEM), by staining with [OSO.sub.4]. Analysis of both LM and SEM data indicated that the CR/EPDM-g-PMMA, 70/10 blend had the smallest zone and particle sizes of the dispersed phase, [is less than or equal to] 1 micrometer, and the CR/EDPM/EPDM-g-PMMA, 70/30/10 blend had particle sizes = 1-3 micrometers. Both the above blends had better phase dispersion than the CR/EPDM, 70/30 blend which had its particle size = 5-10 micrometers. These data indicated that EPDM-g-PMMA had better interfacial adhesion with CR resulting in reduced particle size of the dispersed phase in the above blends. Both the CR/EPDM-g-PMMA, 70/30 and the CR/EDPM/EPDM-g-PMMA, 70/20/10 blends had better compatibility than the CR/EPDM, 70/30 blend.

Large quantities of the above copolymer were synthesized as before and were purified by solvent extraction using acetone and heptane in sequence. The chemical identity of each fraction was confirmed by analyzing the FTIR spectral data. The acetone soluble fraction (35 wt.%) was recovered and was identified as the homopolymer PMMA. Analysis of the FTIR spectral data of both heptane soluble and insoluble reactions (25 and 40 wt. % respectively) indicated the presence of strong absorption peaks at 1,725 cm-l which was due to the "C=O" group in the PMMA chains. Thus, both fractions contained the PMMA grafts linked to the EPDM backbone.

The compatibility effect of both heptane soluble and insoluble fractions with neoprene/EPDM 70/30 blends were also investigated using the above test method. Selected blend samples were examined in the optical microscope as before. The poor dispersion and larger domain sizes ([is greater than or equal to] 12-25 micrometer) of the dispersed phase (V 7000 or heptane insoluble fraction of the graft copolymer) indicated the incompatibility of the CR/EPDM (70/30) blend and CR/EPDM-g-PMMA (heptane insoluble) blend. On the contrary, significant improvement in the phase morphology, better dispersion and smaller domain size ([is less than] 1 micrometer) clearly indicated the improved compatibility of CR/EPDM-g-PMMA (heptane soluble), 70/30 blend.

To demonstrate the beneficial effect of the above compatibilizer in improving other mechanical properties of neoprene/EPDM blends, we developed a simple test procedure using the same ratio of curing agents and antioxidants but excluding the carbon black and oil. The blends were compounded and their mechanical properties were measured as described before (ref. 3). The compatibility effect was evident in terms of the significant improvement in tensile properties, heat aging and dynamic ozone resistance of those blends containing EPDM-g-PMMA (heptane soluble) as shown in table 4. The above results also indicate that only small amount (10 wt.%) of the EPDM-g-PMMA (heptane soluble) was required to reveal the beneficial effects of compatibilization in the CR/EPDM/EPDM-g-PMMA (heptane soluble), 70/20/10 blend.

Table 4 - properties of CR/EPDM/EPDM-G-PMMA
blends
Components/                 Blends #
properties           1     2     3      4       5

Neoprene            100    70    70     70      70
EPDM                 --    30    20     10      --
EPDM-g-PMMA          --    --    10     20      30
Tensile MPa        13.3   7.7    10   12.1    12.8
Elongation %        726   396   385    489     556
(*) Tensile MPa     5.0   7.0   8.0   11.9     12.1
(*) Elongation %    275   396   385    489     556
Dynamic ozone       144   168   336   >480     >480

(*) Heat aged at 140 [degrees] C for 48 hours

Conclusion

The dissimilar elastomer blends such as neoprene/EPM blends required the compatibilizers to improve their properties. Two such compatibilizers, E/MA/AA and EPDM-g-PMMA were identified. Two different triblends, CR/EPM/EMA-AA, 70/30/10 and CR/EPM/EPDM-g-PMMA, 70/20/10 with significant improvement of their key properties, i.e. heat, ozone and cut growth resistance in PTB application were developed. The distribution of the carbon black in both phases of the CR/EPM/E-MA-AA, 70/30/10 blend was better than that of the control CR/EPM, 70/30 blend.

Our compatibilization approaches involved: Miscibility of one phase (EPM) and specific interaction of second phase (CR) with the compatibilizers; designing a compatibilizer in such a way that it had structural similarity with one phase and specific interaction with the other phase; chemical modification of one phase such as: e.g. EPDM -- [is greater than] EPDM-g-PMMA; and a preformed block or graft copolymer of EPDM and CR was not required to compatibilize CR/EPM blends.

References

(1.) P. Arjunan and R.B. Kusznir, U.S. Patent 5,281,651 (1994).

(2.) P. Arjunan, U.S. Patent. 5,397,833 (1995).

(3.) P. Arjunan, U.S. Patent: 5,352, 739 (1994).

(4.) J.E. Callan, Rubber Chem. Technol., 44 (3); 814 (1971).

(5.) D. Craig and R.B. Fowler, Rubber World, 146 (6), 79 (1962).

(6.) D. Craig and R.B. Fowler, Rubber World, 91 (6), 966 (1962).