Application of PBDMA for enhancement of EPR loaded with chopped fibers.

Grafting of maleic anhydride (MA) to many polymers, particularly to polyethylene and polypropylene, is one of the most effective synthetic methods of cardinal modification of polymer properties for increased compatibility, paintability and adhesion. These polymers are widely used in many important applications. However, their potential utilization is limited due to a low concentration of grafted maleic anhydride, usually 1% by weight or less, and a spontaneous hydrolysis of anhydride with formation of the less reactive toward many substrates carboxyl groups. Low reactivity, poor compatibility and high temperature of softening of the saturated hydrocarbon polymer chain of polyethylene and polypropylene may also cause processability problems and restrict some applications.

Low molecular weight maleated polybutadienes (PBDMA) can serve as an alternative to MA-grafted polyolefins to overcome these limitations. PBDMAs are long existing polymers useful as vulcanization coagents, nonextractable plasticizers and adhesion promoters (ref. 1). In contrast to maleated polyolefins, the concentration of the highly reactive grafted MA of PBDMAs is high and variable from 5% to 20% by weight. PBDMA is available in liquid or powder form to eliminate many processability problems. Double bonds of its unsaturated backbone provide active sites for sulfur or peroxide curing. The elastomeric nature of PBDMA and a great value of solubility parameter (table 1) make this polymer useful in a wide range of applications in many areas of fiber, plastic and rubber technologies. We studied one of the most promising PBDMA usages as a reactive compatibilizer.

Table 1 - properties of EPR loaded with fiber pulp and PBDMA

                   Fiber    Ricobond                Max.
                  loading     1756       Pull      torque
No.   Fiber         phr       phr      direction   lb.(*)in.

 1    Cotton         5        None         L        23.0
                                           T
 2    Cotton         5          5          L        30.0
                                           T
 3    Nylon-6        5        None         L        25.5
                                           T
 4    Nylon-6        5          5          L        42.5
                                           T
 5    Polyester      5        None         L        28.0
                                           T
 6    Polyester      5          5          L        31.5
                                           T
 7    Polyester     10        None         L        20.5
                                           T
 8    Polyester     10         10          L        40.5
                                           T
 9    Control 1    None       None         --       21.0
10    Control 2    None        10          --       27.0

                               Modulus
                             lb./sq. in
                                              Tensile
                  Shore    @     @     @      strength
No.   Fiber        A/D    25%   50%   100%   lb./sq. in.

 1    Cotton      66/19   110   150    250      1,200
                          100   130    170      1,100
 2    Cotton      71/25   225   325    450      1,150
                          110   175    326      1,050
 3    Nylon-6     68/21   200   250    275      1,050
                          140   175    275      1,100
 4    Nylon-6     72/25   225   375    525      1,200
                          160   250    325      1,150
 5    Polyester   68/20   120   150    240      1,150
                          110   140    230      1,150
 6    Polyester   71/22   180   240    400      1,225
                          130   180    280      1,200
 7    Polyester   70/19   180   210    240        950
                          150   190    210        850
 8    Polyester   76/26   230   360    650      1,150
                          180   260    460      1,100
 9    Control 1   61/16    80   110    160      1,250
10    Control 2   69/19   140   170    240      1,300

                  Elong.   Rebound    Tear
No.   Fiber         %         %      lb./sq.

 1    Cotton       500       43        165
                   520                 175
 2    Cotton       320       43        195
                   320                 205
 3    Nylon-6      480       43        195
                   530                 190
 4    Nylon-6      340       44        190
                   380                 210
 5    Polyester    540       43        160
                   560                 170
 6    Polyester    400       41        185
                   460                 196
 7    Polyester    420       43        140
                   380                 140
 8    Polyester    280       43        190
                   350                 200
 9    Control 1    510       45        150
10    Control 2    480       40        145

No.   Fiber       [KO.sub.25%]   [KRM.sub.50%]

 1    Cotton          1.15             --
 2    Cotton          2               2.1
 3    Nylon-6         1.4              --
 4    Nylon-6         1.4             1.5
 5    Polyester       1.1              --
 6    Polyester       1.4             1.6
 7    Polyester       1.1              --
 8    Polyester       1.3             1.7
 9    Control 1        --              --
10    Control 2        --              --

It is known that many nucleophilic reagents, including amines and alcohols both monomeric and polymeric, water, etc., can readily open the MA ring (figure 1). This type of reaction occurs, e.g., between polyamides and MA-grafted EPDM (Royaltuf 465A, CK Witco) in production of impact resistant nylon. We explored PBDMAs in interaction with aramid fiber and EPDM rubber matrix using not only the reaction of the anhydride ring (step 1), but also the reactivity of the polymer backbone unsaturation (step 2). The PBDMA was grafted to aramid during rubber compounding on the roll-mill at 45-50 [degrees] C (step 1) and then introduced into the rubber matrix through sulfur or peroxide vulcanization at 160-175 [degrees] C (step 2). As a result, aramid fiber was chemically bound to the rubber matrix.

[Figure 1 ILLUSTRATION OMITTED]

We expanded application of PBDMAs as reactive compatibilizers and vulcanization coagents in reinforcement of ethylene-propylene rubber (EPR) loaded with chopped nylon, polyester and cotton. These functional fibers contain end amino (nylon), hydroxyl (polyester) or pendant (cotton) hydroxyl groups. This article illustrates the universal character of PBDMA enhancing fiber reinforcement of rubber compounds.

Experimental

EP rubber masterbatches were modified with PBDMA and fibers on a 7-inch two-roll lab mill. We kept track of mill direction in order to evaluate orientation of the fibers and tested properties in two directions: the longitudinal (L) direction, which is parallel to milling direction, and transverse (T) direction, which is perpendicular to milling direction. Milling was performed per instructions in ASTM D-3184.

We used two types of chopped fiber: Cut with the ratio of the length to the diameter of about 500, and pulp with this ratio less than 20.

The rubber formulations were evaluated for cure characteristics on an ODR with 1 arc (ASTM D-2084). Parts were prepared for physical testing (ASTM D-3182 and D-3183) and tested for tensile and modulus strength, elongation (ASTM D-412) and tear strength (ASTM D-624 Die C) in the L- and T- directions. Shore hardness (ASTM D-2240, Shore A and D) and rebound resilience (ASTM D-2632) were tested as well. We studied only static properties at a rate of deformation of 20 inches/minute.

Samples of pulps have been received from Akrochem as nylon-6 Akroflock NW-31, polyester Akroflock PW-31 and cotton Akroflock CWV-1. Three grades of aramid cut were used: Kevlar 1F178 of DuPont, Twaron 1080 of Acordis and Technora T320 of Teijin.

Fibers were applied without special drying.

Two types of saturated EPR (Nordel of DuPont) rubber formulations were used (table 2).

Table 2 - rubber formulations

Formulation 1       Phr

Nordel IP 340       100
Carbon black N550    70
Sunpar 2280, oil     20
ZnO                   5
Vul-Cup 40 KE         5

Formulation 2       Phr

Nordel IP 3430      100
Carbon black N550    20
Sunpar 2280, oil      7
ZnO                   5
Vul-Cup 40 KE         6
Hi-Sil 233           30

As PBDMA, we used Ricobond 1756 having a high content of 1,2-units (65-75%), which is beneficial for peroxide curing. Curing was performed at 175 [degrees] C for 29 minutes.

Results and discussion

Improvement of properties of EPR compositions

Figure 1 illustrates the reaction of cyclic anhydride (after grafting maleic anhydride is converted into succinic anhydride) with end amino or hydroxyl groups of fiber polymers. Indeed, we found that during mixing on a roll mill at 40-50 [degrees] C, nylon pulp reacts with PBDMA forming gelled material within the rubber matrix (ref. 2).

A special study showed that PBDMAs react with monomeric amines, even with low nucleophilic secondary aromatic amines, in solution at room temperature. The less reactive aliphatic alcohols complete the reaction in 20-30 minutes at 100 [degrees] C or a higher temperature. For our case, it means that reactions between PBDMA and hydroxyl containing polymers require a higher temperature. This temperature was achieved in process of vulcanization. Data of table 1, and figures 2 and 3 confirm a significant improvement of the properties of vulcanized EPR loaded with all types of tested chopped fibers, amino or hydroxyl terminated, in the presence of PBDMA.

[Figures 2-3 ILLUSTRATION OMITTED]

Addition of PBDMA to the fiber pulp loaded compositions increased the modulus at low elongation (25-50%) in longitudinal (L) direction by 10 and up to 220% with average of 160%. Thus, a coefficient of enhancement, [KRM.sub.50%], calculated as the ratio of modulus at 50% elongation in the L direction of the rubber formulation containing chopped fiber and PBDMA to that of the rubber formulation with chopped fiber alone, is in the range of 1.5-2.2.

A coefficient of orientation, [KO.sub.25%], calculated as the ratio of the modulus at 25% elongation in the L direction to that in the transverse direction, has not increased in most cases, except for in cotton filled formulations during which this coefficient increased two times.

It should be noted that cotton pulp became an active ingredient of rubber formulation only in the presence of PBDMA.

Conclusions on the comparison of the efficacy of different chopped fiber in EPR reinforcement should be done with some cautions. First, the differences depend not only on chemical structure of the polymers, but also on the geometry of fibers, surface area, etc. As we mentioned, the chemical interaction between fiber and PBDMA depends on the temperature of mixing of components of the rubber formulation (temperature of treatment in internal mixer, Brabender or mill) and some other aspects of technology, which have a definite effect on the cured rubber performance.

Figure 2 demonstrates actual curves of deformation of several samples filled with fiber pulp, as presented in table 1. We can see that the shapes of the curves are typical for elastomers and differences between samples are in the value of the modulus depending on the type of reinforcement. For comparison, we placed at the bottom of figure 2 the curve of deformation of the same EPR formulation, but filled with Kevlar cut with length of 6 mm. This type of deformation combined plastic like deformation at low elongation with practically constant Young modulus and rubbery-like deformation at higher elongation. This is the principal difference between reinforcement with fiber pulp and cut. The second notable difference is obviously a more heterogeneous character and a higher level of orientation and anisotropy of rubber compounds reinforced with cut fiber than those with pulp.

A rubbery-like shape of the deformation curve of fiber pulp reinforced EPR and relatively low level of anisotropy allow us to consider, from a practical point of view, the reinforcement by reactive combination of PBDMA and functional fiber as a mostly uniform increase of rubber matrix crosslink density.

This reactive combination of reinforcing agents adds a new concept to the explanation of the nature of fiber reinforcement, not only through mechanical interaction (ref. 3), but through potential chemical interactions as well.

The improvement of properties of EPR formulations in strain-stress terms does not exhaust the full potential of PBDMA. Chopped fiber loading alone increased rubber tear (Die C) up to 15-25%. But with the addition of PBDMA, rubber tear increases up to 40%. Figure 3 also illustrates that samples with the lower value of rubber tear have a larger elongation. Consideration of the tear strength mechanism and related matters could be found in books (refs. 4 and 5).

From the theory of composite mechanics, we can expect the highest tear increase in T-direction, and we observed this trend, for instance, in sample 4 (table 1). Nevertheless, a limited amount of the composition variations makes this observation preliminary.

Our research was performed as a study of fiber reinforced compositions in the presence of PBDMA in static test conditions. It was found (ref. 5) that generally, fiber incorporation into rubber matrix increases the dynamic modulus and broadens the loss peak with the simultaneous decline in magnitude of the peak.

There is one more remarkable advantage of the addition of PBDMA to the blends of EPR and chopped fiber. It is known that compatibility of such different polymers as EPR and nylon is very poor because of great differences in solubility parameters (table 3), and blending of these two materials is a difficult, mostly unsolved problem. Addition of PBDMA makes the preparation of the blend significantly better from two points - economy (less time, less energy consumption, efficient equipment utilization) and performance (improved fiber wetting, uniform distribution, cured rubber properties enhancement). A perfect fiber incorporation into rubber matrix has a critical importance for the quality of the rubber goods, especially in dynamic applications preventing fiber separation during exploitation. Improvement of the processability was discussed in detail previously (ref. 2).

Table 3 - solubility parameters of solvents and polymers (Hildebrand's)

Solvents

Water                                   23.4
Formamide                               19.2
Glycerol                                16.5
Succinic anhydride                      15.4
Ethylene carbonate                      14.5
Methanol                                14.5
Dimethyl sulfoxide                      13.4
Caprolactam                             12.7
Pyridine                                10.7
Acrylonitrile                           10.5
Acetone                                 10.0
THF                                      9.9
Toluene                                  8.9
Hexane                                   7.3
Tolylene-diisocyanate                   11.6
Epichlorohydrin                         11.0

Polymers

Polyethylene                         7.9-8.1
Ethylene-propylene rubber                7.9
Butyl rubber                             7.9
Polyisoprene                             8.1
Polybutadiene                            8.4
Polystyrene                          8.6-9.1
Nitrile rubbers(A)                  9.4-10.3
Polychloroprene                      8.8-9.2
Poly (vinylpyridine-butadiene)(B)        9.5
Maleated polybutadiene(C)           9.6-10.6
Poly(ethylene-terephthalate)            10.7
Cellulosic                           10-11.5
Nylon 6,6(D)                            13.6
Poly(vinyl chloride)                     9.6
Polyurethanes                       9.5-10.0
Amino, epoxy, phenoxy resins        8.5-14.0

(A) - The solubility parameter for HNBR will be slightly less than NBR's

(B) - 75/25 butadiene/vinylpyridine copolymer

(C) - Calculated for PBDMA with content of grafted maleic/succinic anhydride 15-20%

(D) - The solubility parameter for aramid will be slightly higher than that for nylon 6,6

The improvement of fiber incorporation was observed for all studied fibers.

It is important to note that PBDMA is a reactive compatibilizer toward functional fibers and at the same time, PBDMA provides reactive sites for sulfur and peroxide curing. PBDMA as a compatibilizer might be used for production of new types of composites or for improvement of many properties of polymer blends, such as adhesion, processability, uniformity, etc.

Improvement of properties of EPR compositions with aramid cut; PBDMAs as reactive compatibilizers

Earlier (ref. 2), studying the enhancement of properties of EPR loaded with 6-mm aramid cut, we showed that addition of PBDMA improves fiber orientation, the network density of the rubber matrix, the value of the modulus at low elongation and adhesion between the fiber and the rubber matrix. This study was performed for four types of the most widely used elastomers - unsaturated, saturated, polar and nonpolar - encompassing about 90% of total world rubber consumption.

In that paper, we studied EPR formulation #1 loaded with Kevlar cut oriented in L- and T- directions. Now, applying EPR formulation #2, which is essentially harder, and testing non-oriented materials, we achieved a very high increase in rubber tear (Die C) (table 4). Furthermore, improvement of rubber tear by addition of Kevlar fiber together with PBDMA was effective at high temperatures up to 150-170 [degrees] C. Table 5 illustrates this improvement of hot tear in comparison with one of the best industrial EPR formulations.

Table 4 - properties of EPR loaded with Kevlar cut and PBDMA

                                  Control +
Property              Control   Kevlar, 5 phr

Max. torque, lb.(*)       41           49
inch                   1,800        1,700
Tensile, psi             175          610
Modulus at 50%, psi      195          300
Tear, ppi                565          490
Elongation, %          68/21        86/36
Shore A/D                 47           46

                                       Control +
                       Control +     Kelvar, 5 phr
                        Ricobond       Ricobond
Property              1756, 10 phr   1756, 10 phr

Max. torque, lb.(*)          51             59
inch                      1,950          1,320
Tensile, psi                340          1,170
Modulus at 50%, psi         185            275
Tear, ppi                   300            125
Elongation, %             77/29          90/39
Shore A/D                    42             49
Table 5 - tear and hot tear of EPR loaded with Kevlar cut and PBDMA

                             23            150           177
Formulation              [degrees] C   [degrees] C   [degrees] C

Control                      196            84            87
Control + 5 phr Kevlar
 +10 phr Ricobond 1756       270           150           135
Industrial formulation       320           107            98

Table 6 presents data on the comparison of three types of commercially available chopped aramid fibers - Kevlar, Twaron and Technora. All tested fibers have close properties in reinforcement of EPR in the presence of PBDMA.

Table 6 - comparison of different brands of aramid chopped fiber reinforcement with and without Ricobond (EPR formulation #1)

                    Ricobond     Max
          Fiber      1756      torque        Pull      Shore
No.       2.5 phr   2.5 phr    lb.(*)in.   direction    A/D

1A                   None        19          L         73/23
          Kelvar                             T
1B        1F178      2.5         26          L         75/25
                                             T
2A                   None        23          L         76/26
          Technor                            T
2B        a T320     2.5         26          L         77/25
                                             T
3A                   None        23          L         71/21
          Twaron                             T
3B        1080       2.5         26          L         74/22
                                             T
Control   None       None        20          --        56/13

                      Modulus      Tensile
          Fiber     lb./sq. in.    strength   Elongation
No.       2.5 phr   @25%   @100%   lb./sq.        %

1A                   460    440        970       520
          Kelvar     150    250        900       570
1B        1F178      610    725      1,150       420
                     180    300      1,050       490
2A                   360    370        920       500
          Technor    150    270        880       520
2B        a T320     570    650      1,150       460
                     220    400      1,150       500
3A                   350    360        950       520
          Twaron     120    200        800       620
3B        1080       550    680      1,150       480
                     140    300      1,150       510
Control   None       100    220      1,200       560

          Fiber     Rebound
No.       2.5 phr      %      [KO.sub.25%]   [KRM.sub.25%]

1A                    43          3.1              --
          Kelvar
1B        1F178       43          3.4             1.3
2A                    43          2.4              --
          Technor
2B        a T320      43          2.6             1.6
3A                    43          2.9              --
          Twaron
3B        1080        43          3.9             1.6
Control   None        43           --              --

L - longitudinal;

T - transverse direction;

length of chopped fiber - 6 mm

Conclusion

This article describes enhancement of several important properties of EPR with the application of combination of chopped fiber and maleated polybutadiene (PBDMA), like:

* Modulus at low (25-50%) elongation;

* robber tear and hot tear; and

* processibility.

Fibers tested were both synthetic (nylon and polyester pulp and aramid cut) and natural ones (cotton pulp).

Based on previous studies and on the common chemical mechanism of interaction of functional fibers with PBDMA, it is possible to make a deduction that results obtained for EPR are applicable to many other rubbers including unsaturated polybutadiene, SBR, NBR and polychloroprene, and saturated HNBR. PBDMA should be a strong enhancer of reinforcement with amino, hydroxyl or other functional fibers.

A combination of fibers and PBDMA can be useful (ref. 5) for the production of belts (power transmission, v-belt, conveyor, timing), hoses, seals, friction products, etc.

[Figure 4 ILLUSTRATION OMITTED]

References

(1.) Ronald E. Drake and John M. Labriola. "Adhesion promoting using maleated polybutadiene resins in rubber compounds, "presented at a meeting of the Rubber Division of the American Chemical Society, Toronto, May, 1991.

(2.) Arkady S. Estrin. "Enhancement of the properties and dispersion of chopped aramid fiber loaded rubber compositions with application of maleated low molecular weight polybutadienes," Rubber and Plastic News, February, 2000.

(3.) Martin A. Doherty. "Para aramid chopped fibers and pulp masterbatches: Alternate consideration for the rubber compounder," presented at a meeting of the Rubber Division of the ACS, Orlando, September, 1999.

(4.) A.N. Gent, "Strength of elastomers," Chapter 10 in "Science and Technology of Rubber," 2nd Edition, Eds. James E. Mark, Burak Erman, Frederic R. Eirich, Academic Press, San Diego - New York, etc., 1994.

(5.) K. De and J. K. White, Eds., "Short fiber polymer composites," Woodhead Publishing Limited, Cambridge, England, 1996.