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Substitution of plasticizers in NBR vulcanizate with cashew nut shell oil.

The most common plasticizers used in NBR vulcanizates are phthalates, mainly di-octyl phthalate (DOP) and di-butyl phthalate (DBP). Since the introduction of Registration, Enforcement and Restriction of Chemicals (REACH) legislation, European countries have disallowed the use of these phthalate plasticizers in imported products, and a search is going on for a cost-effective substitution of these plasticizers. One such material is cashew nut shell oil (CNSL), a by-product of the cashew industry. It is acceptable under REACH legislation (CAS No. 8007-24-7). Commercial CNSL is supplied after heat treatment of extracted oil either during extraction or at a later stage. The refined CNSL mostly contains: (I) 90% anacardic acid which is de-carboxilated on heating to Cardanol (m-pentadecadienyl phenol) and (II) 10% Cardol (refs. 1-4).

As early as 1931, Harvey (ref. 5) had reported that when natural rubber was mixed with CNSL, the insolubility of vulcanizates in petroleum solvent is increased. It also improved incorporation of fillers and other ingredients, i.e., CNSL behaved as a good plasticizer. In 1943, Harvey (ref. 6) reported that various kinds of rubbery articles can be prepared by mixing polymerized chloroprene with thickened CNSL. He also observed that thickened CNSL (ref. 7) acted as an excellent plasticizer for neoprene rubber reducing durometer hardness and improving elongation and aging characteristics of cured rubber. Ferrin (ref. 8) reported improvement in low temperature properties of SBR when plasticized with Cardiolite-625, the ethyl ether of the mono phenolic component of CNSL. Sanghi, Bhattacharya and Banerjee (ref. 9) reported that Cardanol had plasticizing properties in nitrile rubber compounds with improved tear, solvent resistance and aging properties. Ghatge, et al. (refs. 10 and 11) and Rajapkse, et al. (ref. 12) derived antioxidants from CNSL and studied their effect on natural rubber gum and channel black compounds. In 2010, Thachil, et al., studied plasticization behavior of CNSL in NBR vulcanizates (ref. 13).

In this background, it seemed fit to study the effects of CNSL on NBR vulcanizates with special attention on oil and fuel aged properties. We planned to study three industrial recipes already in our use and substituted the existing plasticizers with CNSL, both partially and fully.

Experimental

Our experiment was based on three separate formulations meeting three different specifications, as follows:

* Recipe A: A general purpose, highly loaded, NBR-PVC formulation needing oil resistance, low hardness (~55SH) and ozone resistance. Plasticizer type, quantity and results are presented in table 1.

* Recipe B: These vulcanizates were to meet the international standard SAE J-30/R7 for hoses. It is an NBR recipe meeting 125[degrees]C heat aging and fuel resistance (table 2).

* Recipe C: These vulcanizates needed to have Fuel C and both methanol and ethanol blended Fuel C resistance. Results are presented in table 3.

Mixing was done as per formulations (not disclosed here) in a kneader without any oil. The masterbatch was divided in required portions. Oil was added in the second stage on a mixing mill followed by accelerators. Slabs and buttons were produced on a 100 ton hydraulic press at 160[degrees]C for 15 minutes. All tests were done per standard ASTM procedures and results are presented in the three tables.

Results and discussion

Recipe A--ozone/oil resistant NBR-PVC formulation

In recipe A, the polymer used was an NBR-PVC grade with 30% PVC with a hardness range of 50-55[degrees]durometer including 32 phr plasticizer and 50% and 100% substitution of DOP by CNSL.

The original solo plasticizer DOP was used in recipe A1 and replaced partly and fully in recipes A2 and A3. TP 95 was used in recipe A4 and 15% extra accelerator over the CNSL compound was used in recipe A5. From table 1 and figures 1 and 2, we observed that tensile strength decreased and elongation increased as DOP was substituted with CNSL in recipes A2 and A3. Hardness was almost equal in all five compounds. The lowest tensile strength of CNSL improved in recipe A5, where extra accelerator was added to improve the state of cure. Interestingly, the TP95 vulcanizate had the highest tensile strength. The oil aged tensile strength of CNSL with extra accelerator were almost equal to the DOP/ TP95 results (figure l).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Figure 2 shows that initial elongation and ASTM oil #1 and #3 aged elongations were highest with CNSL followed by the TP 95 and DOP mixed batches. S'(max.)--S'(min.), i.e., crosslink density, was reduced when DOP was replaced with CNSL. The lowest value observed with total CNSL was recipe A3. Crosslink density increased when extra accelerator was added in the CNSL vulcanizate (A5). This was reflected in compression set values also (figure 3). It was highest with CNSL and lowest with CNSL having extra accelerator. Chloroform swell also followed the same trend as compression set.

The heat aging results at 100[degrees]C, 70 hours (figure 4) show that the aged elongations of both CNSL and TP 95 were equally good. It was followed by mixed plasticizers. All elongations at 120[degrees]C had reduced to a very low value, indicating complete deterioration.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

From recipe A, we concluded:

* Tensile strength of 32 phr CNSL was lower than solo DOP vulcanizates; but the extra accelerator improved tensile strength.

* Elongation of CNSL was highest and maintained during heat (100[degrees]C) and ASTM #1 and #3 oil aging.

* S'(max.)--S'(min.) was lowest with CNSL, resulting in the highest compression set. However, both had improved when extra accelerator was added.

* Hardness varied in a small band.

Recipe B--NBR formulation meeting SAE J-30/R7 specification for hose

This formulation needed 120[degrees]C heat resistance, ASTM #3, Fuel C, CE20 (Fuel C + 20% EtOH) and cold resistance (-40[degrees]C) before and after ASTM oil #3 aging. We used an all NBR recipe which met international fuel hose standard, SAE J-30/R7. Here, TP95, the solo original plasticizer (B1), was gradually replaced by CNSL in B2 and B3. Recipe B4 had 20 phr DOP. All results are presented in table 2.

At 20 phr plasticizer level, the gradual replacement of TP 95 by CNSL (recipes B2 and B3) resulted in marginal lowering of tensile strength and an increase in elongation values; but hardness remained constant. Oil and fuel aged tensile strengths of CNSL were equal or better than solo TP 95, DOP or mixed compounds (figure 5). Also, original and aged elongations of CNSL were found highest (figure 6). Original and oil aged hardness were almost equal in all recipes (figure 7). But, the fuel aged hardness of CNSL was definitely lower than in other solo/mixed plasticizers.

Recipe B conclusions include:

* A positive volume swell after ASTM #3 oil aging indicated lower leaching of CNSL in oil. Fuel swells were higher.

* During 120[degrees]C heat aging, tensile strength, elongation and hardness had increased over original values.

* All aged tensile strengths and elongations of CNSL were equal/better than both TP 95 and DOP.

* Hardness of B3 (CNSL) after oil aging was equal; but after fuel aging it was lower than B1 (TP 95).

* Mh values of CNSL were lower than TP 95.

* Cold resistance of all four vulcanizates was acceptable both before and after ASTM oil aging.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

Recipe C NBR-PVC/Fuel hose

This formulation needed ASTM #1 and #3 oils, Fuel C, CE 5, and CE 20 aging.Here, higher ACN content NBR/PVC with 30% PVC content was used as polymer. Replacement of both DOP and TP 95 with CNSL were studied.

In this experiment, we wanted to test in a fuel resistant recipe meeting both 48 hours and 120 hours aging in Fuel C, CE20 (20% ethanol blended Fuel C) and CM5 (5% methanol blended Fuel C). The results are presented in table 3.

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

From figure 8 we observed that the lower initial tensile strength of CNSL had not improved after heat, oil or fuel aging. Fifty percent substitution of TP 95 (recipe C4) and DOP (recipe C5) resulted in intermediate heat, oil and fuel aged tensile strength. Both initial and aged elongations of CNSL compound were higher than both TP 95/DOP compounds (figure 9).

All initial and oil aged hardnesses were equal; but fuel aged hardness of CNSL was lowest due to higher swelling of CNSL vulcanizates in fuels. This property seemed to be a typical characteristic of CNSL vulcanizates and was possibly due to less extraction of CNSL by fuels. From figure 10, we observe that the highest compression set of CNSL (recipe C3) was due to the lowest Mh-Ml, i.e., crosslink density. Results of mixed (recipes C4 and C5) batches were very similar to the CNSL compound. The addition of 15% extra accelerator in the CNSL (recipe C6) compound resulted in a much improved crosslink density and compression set.

[FIGURE 10 OMITTED]

Lastly we studied the combined effects of all three recipes by averaging the results (figure 11). Original tensile strength of CNSL was a bit lower than both TP95 and DOP, but fuel aged tensile was almost equal, i.e., average retention of tensile strength during fuel aging was almost equal in all recipes. Both original and aged elongations of the CNSL vulcanizate were higher than both plasticizers (TP 95 and DOP). The fuel aged combined hardness of CNSL was lower. Results improved when more accelerators were added, including:

* Average initial tensile strength of (A + B + C) varied between 124 (TP 95) and ~115 Kg/[cm.sup.2] (CNSL). DOP and mixed had intermediate results.

* Average fuel aged tensile strengths varied between 66 (CNSL) and ~69 Kg/ [cm.sup.2.]

Figure 12 shows the effect of various plasticizers on original and fuel aged tensile strength. CNSL with 15% extra accelerator showed a remarkable improvement. Elongation results were excellent with CNSL (figure 13) and CNSL with added accelerator.

So we found that CNSL could be a good plasticizer for use in fuel resistant NBR formulations. Some drawbacks were encountered in some recipes. Performance generally improved with increased accelerator level. The fuel aged tensile appeared to be formulation specific. The fuel aged elongation was definitely highest with CNSL. The volume swell of CNSL vulcanizates in fuels was slightly higher and consequently the aged hardness observed lower. In ASTM oil swelling, CNSL was less extracted by oils. Though the low temperature property before and after ASTM No. 3 swell was observed to be acceptable at -40[degrees]C, the oil freezes between -35 to -40[degrees]C. So further studies are needed if low temperature properties are critical.

Summary

* Tensile strength of CNSL was 2-10% lower, depending on the recipe.

* Oil, heat and fuel aged tensile strengths were equal or lower (-5%) than both TP 95 and DOP.

* Both initial and oil/fuel aged elongations of CNSL were highest.

* 15-20% extra accelerator improved crosslink density (Mh-Ml) and also compression set.

* All CNSL recipes had characteristically lower fuel aged hardness, lower Mh and slightly higher volume swell in oils and fuels.

* Volume changes in ASTM #1 and #3 oils are marginally positive in the solo CNSL recipe.

* Average fuel aged tensile strength varied between 66 (CNSL)--69 Kg/[cm.sup.2].

Cashew nut shell oil (CNSL) was used partly and fully in place of currently used plasticizers, DOP and TP 95, in three different formulations. The first recipe was an NBR-PVC based vulcanizate needing ASTM #1 and #3 oil resistance and ozone resistance. It was observed that though initial tensile strength was reduced by ~10% when CNSL replaced DOP, the elongation was higher and oil resistance properties were either equal or improved. Addition of 15% extra accelerator increased tensile strength, improved compression set and overall results. The second formulation needed heat (120[degrees]C), oil and fuel resistance as per the SAE J30 R7 specification. Here, when we replaced TP 95 by CNSL, we observed that apart from lower fuel aged hardness, all other properties were almost comparable. The third recipe needed excellent fuel resistance. When CNSL replaced TP 95 or DOP, initial tensile strength was reduced. But all other oil aged and fuel aged properties remained equal or improved. Lastly, properties of five frequently used plasticizers were compared with the CNSL in a general purpose fuel resistant formulation. Here, lower initial and fuel aged tensile strength, higher volume swell in fuels and higher compression set were observed with CNSL loaded compound. However, blends of plasticizers or 15% more accelerator in the CNSL formulation overcame most of these problems. In summary, a cost-effective substitution of phthalate and ester plasticizers was possible by CNSL. Some small changes would be necessary.

[FIGURE 11 OMITTED]

[FIGURE 12 OMITTED]

[FIGURE 13 OMITTED]

References

(1.) Points, S.P. and Aggarwal, J.S., J. Col. Soc: 13(1), pp. 10-14, (1974).

(2.) Aggarwal, J.S., Paint India, 17(1), Annual pp. 103-112, (1967).

(3.) Attanasi, Orazio, Serra-zanetti, France, Perloni, France, Scagliarini, Alessandro, Chim. Ind. (Milan), 61(10), pp. 718-726 (1979), ca 92-6,074 w (1980).

(4.) Harvey M.T and Caplon, S., Lnd. Eng. Chem. 32, p. 1,306 (1940).

(5.) Harvey, M.T., U.S. Patent 1819416 (1931).

(6.) Harvey M.T., U.S. Patent 2323-30 (1943).

(7.) Harvey M.T., U.S. Patent 2409277 (1946).

(8.) Ferrin, J.P. (General Tire & Rubber) U.S. Patent 2776693 (1957).

(9.) Sanghi, L.K., Bhattacharya, A.S. and Banerjee, S.J., Inst. Rubber Ind., pp. 188-191, 196 (1974).

(10.) Ghatge, N.D. and Gokhale, R.G,, Rubber Age (New York) 101 (2), pp. 52-57 (1971).

(11.) Ghatge, N.D. and Gokhale, R. G., Ind. J. Technol., 9(10), pp. 391-395 (1971).

(12.) Rajapkse, R.A., et al, Polymer 19 (2) pp. 205-211 (1978).

(13.) JSR Technical Data; Bulletin No. NBR-B_No. 004.

Ratan Singh and P.S. Bhattacharya, Bony Polymers ps_bhattacharya@yahoo.com
Table 1--substitution of DOP and TP 95 with CNSL
in NBR-PVC recipe

Recipe Al A2

Plasticizer (phr) CNSL/DOP/TP95 0/32/0 16/16/0

Tensile strength Kg/[cm.sup.2] 109 103

Elongation % 525 625

Hardness Durometer 56 54

ASTM No. 1 oil Change in hardness +5 +5
aging 100[degrees]C,
70 hrs. Change in +2 +10
 tensile strength

 Change in -11 -9
 elongation at break

 Change in volume -10.5 -9.7

ASTM No. 3 oil Change in hardness -1 -4
aging at 100[degrees]C,
72 hrs. Change in -12 -6
 tensile strength

 Change in -13 -17
 elongation at break

 Change in volume -0.6 +1.9

Cold test -40[degrees]C, 24 hrs. NC NC

Compression set % 53 57
100[degrees]C, 70 hrs.

Chloroform Change in vol. 402 470

MDR result S' max.-S' min. 8.5 7.2

 T10%/T90% 1.6/3.5 1.4/3.2

Recipe A3 A4

Plasticizer (phr) CNSL/DOP/TP95 32/0/0 0/0/32

Tensile strength Kg/[cm.sup.2] 95 114

Elongation % 670 660

Hardness Durometer 55 56

ASTM No. 1 oil Change in hardness +3 +4
aging 100[degrees]C,
70 hrs. Change in +15 -5
 tensile strength

 Change in -2 -14
 elongation at break

 Change in volume -8.8 -11.1

ASTM No. 3 oil Change in hardness -2 +1
aging at 100[degrees]C,
72 hrs. Change in -8 -8
 tensile strength

 Change in -9 -12
 elongation at break

 Change in volume +1.2 -1.3

Cold test -40[degrees]C, 24 hrs. NC NC

Compression set % 62 60
100[degrees]C, 70 hrs.

Chloroform Change in vol. 553 380

MDR result S' max.-S' min. 5.4 8.2

 T10%/T90% 1.5/3.2 1.5/2.9

Recipe A5 *

Plasticizer (phr) CNSL/DOP/TP95 32/0/0

Tensile strength Kg/[cm.sup.2] 105

Elongation % 580

Hardness Durometer 56

ASTM No. 1 oil Change in hardness +5
aging 100[degrees]C,
70 hrs. Change in +5
 tensile strength

 Change in -9
 elongation at break

 Change in volume -8.6

ASTM No. 3 oil Change in hardness -3
aging at 100[degrees]C,
72 hrs. Change in -10
 tensile strength

 Change in -13
 elongation at break

 Change in volume +1.6

Cold test -40[degrees]C, 24 hrs. NC

Compression set % 49
100[degrees]C, 70 hrs.

Chloroform Change in vol. 410

MDR result S' max.-S' min. 8.1

 T10%/T90% 1.3/2.8

* 15% extra accelerator was added to A3 formulation.

Table 2--CNSL in NBR formulation needing
heat (120[degrees]C[degrees], fuel and cold resistance

Recipe B1 B2 B3 B4

CNSL 0 12 20 0

TP 95/DOP 20/0 8/0 0/0 0/20

Tensile strength Kg/[cm.sup.2] 135 128 130 115

Elongation % 375 400 460 410

Hardness Durometer 68 67 68 66

Heat aging Change in hardness 11 13 11 11
120[degrees]C,
70 hrs. Change in +3 +9 +13 +11
 tensile strength

 Change in -33 -37 -40 -37
 elongation at break

ASTM No. 3 Change in hardness 1 5 2 10
aging,
100[degrees]C, Change in 3 8 12 16
72 hrs. tensile strength

 Change in -35 -32 -35 -40
 elongation at break

 Change in volume -1.2 -0.8 +0.9 -4.5

Fuel C Change in hardness -17 -21 -27 -23
aging,
40[degrees]C, Change in -40 -40 -37 -38
48 hrs. tensile strength

 Change in -41 -37 -36 -31
 elongation at break

 Change in volume 21.7 24 27 22

CE 20 (20% Change in hardness -21 -23 -27 -26
EtOH),
40[degrees]C, Change in -45 -43 -43 -40
48 hrs. tensile strength

 Change in -45 -45 -40 -38

 elongation at break 29.2 30.5 32 26.5

 Change in volume

Cold -40[degrees]C, 48 hrs. NC NC NC NC
resistance
 do after NC NC NC NC
 ASTM#3 aging

ODR prop. Mh-MI 17.5 17.3 16.7 21.1

Table 3--results of CNSL substitution in recipe C

Properties Plasticizer C1 C2 C3

 TP 95 17 -- --

 CNSL -- -- 17

 DOP -- 17 --

Tensile strength Kg/[cm.sup.2] 118 119 102

Elongation % 360 370 420

Hardness Durometer 72 71 69

Heat aging Change in -2 -4 +2
100[degrees]C, 70 hrs. tensile strength

 Change in -21 -16 -12
 elongation at break

ASTM No. 1 Change in hardness 11 11 12
100[degrees]C, 70 hrs.
 Change in volume -11.2 -12 -9.1

ASTM No. 3 Change i n +1 +3 +1
100[degrees]C, 70 hrs. tensile strength
aging
 Change in -11 -5 -4
 elongation at break

 Change in volume -6.4 -6.3 -4.7

Fuel C Change in hardness -12 -12 -18
40[degrees]C, 48 hrs.
aging Change in -41 -46 -40
 tensile strength

 Change in -28 -35 -14
 elongation at break

CM 5 Change in hardness -15 -15 -22
40[degrees]C, 48 hrs.
aging Change in -49 -45 -45
 tensile strength

 Change in -34 -47 -13
 elongation at break

CE 20 Change in hardness -16 -17 -22
40[degrees]C, 48 hrs.
aging Change in -49 -49 -42
 tensile strength

 Change in -36 -48 -13
 elongation at break

Fuel C Change in hardness -12 -12 -19
40[degrees]C, 120 hrs.
 Change in -46 -43 -35
 tensile strength

 Change in -36 -34 -3
 elongation at break

CM 5 Change in hardness -13 -15 -21
40[degrees]C, 120 hrs.
 Change in -47 -47 -42
 tensile strength

 Change in -36 -33 -10
 elongation at break

CE 20 Change in hardness -15 -16 -24
40[degrees]C, 120 hrs.
 Change in -45 -49 -41
 tensile strength

 Change in -32 -40 -11
 elongation at break

MDR result S' (max.)-S' (min.) 32.9 34.7 27.7

Properties Plasticizer C4 C5

 TP 95 7 --

 CNSL 10 10

 DOP -- 7

Tensile strength Kg/[cm.sup.2] 106 107

Elongation % 405 380

Hardness Durometer 69 70

Heat aging Change in -1 +1
100[degrees]C, 70 hrs. tensile strength

 Change in -11 -11
 elongation at break

ASTM No. 1 Change in hardness 10 11
100[degrees]C, 70 hrs.
 Change in volume -9.0 -8.9

ASTM No. 3 Change in +3 -3
100[degrees]C, 70 hrs. tensile strength
aging
 Change in -4 2
 elongation at break

 Change in volume -4.7 -4.8

Fuel C Change in hardness -17 -16
40[degrees]C, 48 hrs.
aging Change in -38 -38
 tensile strength

 Change in -12 -13
 elongation at break

CM 5 Change in hardness -21 -21
40[degrees]C, 48 hrs.
aging Change in -47 -48
 tensile strength

 Change in -25 -24
 elongation at break

CE 20 Change in hardness -23 -22
40[degrees]C, 48 hrs.
aging Change in -45 -47
 tensile strength

 Change in -25 -27
 elongation at break

Fuel C Change in hardness -17 -16
40[degrees]C, 120 hrs.
 Change in -38 -36
 tensile strength

 Change in -11 -9
 elongation at break

CM 5 Change in hardness -19 -19
40[degrees]C, 120 hrs.
 Change in -44 -39
 tensile strength

 Change in -24 -4
 elongation at break

CE 20 Change in hardness -21 -21
40[degrees]C, 120 hrs.
 Change in -44 -44
 tensile strength

 Change in -24 -24
 elongation at break

MDR result S' (max.)-S' (min.) 29.3 28.4
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Author:Singh, Ratan; Bhattacharya, P.S.
Publication:Rubber World
Date:Mar 1, 2011
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