Meadowfoam oil factice and its performance in natural rubber mixes.

Meadowfoam oil factice and its performance in natural rubber mixes

Vulcanized vegetable oil, also known as factice, has been used for more than a 100 years in rubber compounding to improve product quality and reduce manufacturing costs. Factices give dimensional stability to extruded articles, reduce mold fill time and cure cycle time, improve ozone resistance of the rubber compound, give a smooth velvety feel to rubber articles, reduce migration of oils and plasticizers to the surface of low durometer stocks, absorb a large amount of mineral oil and liquid plasticizers and have the ability to flow or promote flow under mechanical pressure (ref. 1).

The main factor in the choice of raw material for factice production has been cost. Unfortunately, most of the oils that are used for economical reasons, such as soybean oil, take a longer time to gel than more expensive oils. These less expensive oils also produce factice that is stickier and often more difficult to handle. Formulations with these oils sometimes require additives or higher amounts of sulfur to improve the physical characteristics of their factices. There is a steady worldwide increase in the demand for rubber products and better quality rubber additives (ref. 2). With the favorable change in the market, the research continues for an oil that can give a factice with better physical characteristics at a reasonable price.

Meadowfoam (Limnanthes alba) oil is a promising oil for factice production. It has over 95% unsaturated fatty acids (60% cis-5-eicosenoic acid, 10-20% cis-5-docosenoic acid, and cis-13-docosenoic acid, 15-20% cis-5, cis-13-docosenoic acid) (ref. 3). The double bonds can be crosslinked easily with sulfur and, therefore, meadowfoam oil is a good candidate to consider for factice production. The unique fatty acids of meadowfoam oil have 90% of their unsaturation at the fifth carbon. As these bonds are close to the carboxyl and glycerol moities, they may align easier and increase the chance of intramolecular bonding, thus producing factice more rapidly (refs. 4 and 5).

To pursue these ideas, we determined color, acetone extract, free sulfur amounts, hardness and factice formation times for meadowfoam oil and compared them to other industrial and potential vegetable oils. It has become customary to judge the quality of a factice by its physical properties. Free sulfur consists of unreacted or lightly bound sulfur. During the vulcanization of rubber, alongside the calculated amount of sulfur, free sulfur coming from factice will also participate in the crosslinking of the double bonds. This can lead to overcuring of rubber. Therefore, industrially acceptable levels of free sulfur in factice do not exceed 2%. Acetone extract values show the amount of unsaponifiable oils, unreacted sulfur and partially sulfurized glyceride oils that are present in the factice. Acetone extract values not greater than 20% are considered first grade, greater than 20% of the weight of factice but less than 35%, medium grade, and greater than 35% commercial grade. However, a high acetone extract value does not necessarily imply that the factice is inferior for a particular application. In fact, factice is specifiable only by its actual performance in tests in rubber, and not by simple chemical tests alone (ref. 1). Therefore, the performance of meadowfoam oil factice, in three natural rubber mixes, was compared to high erucic acid rape-seed oil and soybean oil factices that had similar physical properties. For these studies, brown factices were used in activated white, medium abrasion and engine mounting rubber formulations.

Experimental

Materials and methods

The following commercial materials were used without further purification: Sulfur (sublimed), Fisher Scientific, Fairlawn, NJ; zinc oxide, calcium carbonate, stearic acid, all reagent grade, were from Aldrich Chemical, Milwaukee, WI. Accelerators, n-cyclohexyl-2-benzothiazole sulfenamide (CBS) and benzothiazyl disulfide (MBTS), and the antioxidant, polymerized trimethyl dihydroquinoline, were from R.T. Vanderbilt, Norwalk, CT. SRF carbon black was from Cabot Corp., Boston, MA. Processing oil, Sunthene (0-120), was from Sun Oil, Hillside, CA.

The methods for the preparation of white and brown factices are described in a previous work (ref. 6).

The factices used in rubber experiments had 1.3 to 2.0% free sulfur, 28% to 30% acetone extractable material and 25 to 30 shore A hardness. To achieve these values, vulcanizations were conducted with 20 parts of sulfur per hundred parts of oil (pho), for meadowfoam oil, 20 parts of sulfur pho for rapeseed oil and 35 parts of sulfur pho for soybean oil.

Rubber mixes were compounded on a 15.24 cm x 33.02 cm Farrell laboratory mill, model V522 DC. The front roller had a rotation of 23 rpm and the back roller, 35 rpm. Both rollers were cooled with water. Curing times for samples were determined on a Monsanto oscillating disc rheometer, Model MIV, with die temperatures of 148.9 [degrees] C. Samples were cured, under conditions determined by the rheometer, on a Wabash hydraulic press Model 75-16 STM that had a compression stroke of 30.48 cm. Samples were cured at 148-9 [degrees] C under 6.0 MPa on a 25.4 cm diameter ram. Dumbbell-shaped specimens were cut with a NAEF manual punch press, Model B-36 (ASTM D-412068), type C. Their tensile strengths, stress at 300% elongation and ultimate elongations were determined on an Instron tensile testing machine, Model 4201. Hardness values were measured on a Wallace dead load hardness tester (ASTM D 1415). These tests were conducted at 22.8 [degrees] C and 50% humidity.

Results and discussion

The fatty acid compositions of the selected vegetable oils, determined by gas chromatography, are shown in table I. High erucic acid rapeseed oil is used in better-quality factices because it provides a harder product in a shorter reaction time (ref. 7). Crambe oil, presently in commercial development, has similar fatty acids to rapeseed oil and was compared alongside it. Lesquerella oil, an experimental oil that contains [C.sub.20] hydroxy fatty acids, was compared with castor oil because both have similar oxygenated fatty acids. Jojoba oil contains long chain wax-esters instead of triglycerides and is an excellent oil for manufacturing lubricants (refs. 8, 9). Because factice formation has been reported (ref. 10) it was included in this study with hopes of obtaining a factice with unusual properties. However, with 20 pho of sulfur, jojoba oil did not form factice. [Tabular Data Omitted]

Table 2 compares the physical properties of the factices produced from these oils. Under low sulfur conditions, 10 or 12 pho, meadowfoam oil gels much faster than the other oils tested, giving a firm and nonsticky product. Rapeseed oil under these conditions made a sticky product and was difficult to remove from the reactor. Soybean oil does not gel under these conditions. [Tabular Data Omitted]

When factice is ground in a mill or in a mortar, it will either form a coarse powder or it will take on a fluffy form. If the factice is sticky, particles tend to form a mass that resembles masticated rubber. It does not break down to powder. While meadowfoam oil factice prepared with 10 parts of sulfur pho produced the non-sticky coarse powder factice, rapeseed and crambe seed oils gave the coarse powder only after more than 20 pho of sulfur was used. The ease in formation of powder in factice has proven very valuable in sample preparation, especially in cleaning factice from reactors and stirrers, and in handling and transportation.

The vulcanizations reported in table 3 were run at 160 [degrees] C. The decrease in temperature increases the reaction time and helps to amplify any differences between the oils. The acetone extract and free sulfur values of meadowfoam, rapeseed and crambe factices were in the same range. However, under these conditions meadowfoam oil again gave a harder factice, which was also much lighter in color. The color difference was especially noticeable in powder form. Rapeseed, crambe, castor, lesquerella and soybean oils all gave factices ranging from black-brown to black-green while meadowfoam oil factices were different shades of yellow. [Tabular Data Omitted]

The performance of meadowfoam, rapeseed and soybean oil factices were compared in three rubber mixes, activated white, medium abrasion and engine mounting; results are shown in table 4.

Table : Table 4 - rubber formulations(a)

                             Activated     Medium     Engine
                                 white   abrasion   mounting
Natural rubber                   100.0      100.0      100.0
Zinc oxide                         5.0        5.0        5.0
Stearic acid                       1.0        2.5        3.0
CBS accelerator                   0.75        0.5        0.6

Polymerized

  dihydroquinoline                   -       0.75        1.5
Sulfur                             2.5        2.5        2.5
Activated whiting                 80.0       12.0          -
SRF black                            -       48.0       35.0
Processing oil                       -        4.0        2.5
Factice                          or 75     or 7.5     or 7.5

(a) Quantities are listed in parts by weight.

During rubber compounding, the presence of factice indeed eased the addition of powders and processing oil. However, factice behaved like an oil, contributing to loss of strength and tears in the rubber sheets. To minimize this problem, factice was added in small increments. At this stage there were no detectable differences between meadowfoam oil, rapeseed oil and soybean oil factices.

The color of the factice does not make any difference in formulations that contain carbon black. However, the lighter colored meadowfoam oil factice in activated white mixes gave a noticeably lighter colored product. This color may further be enhanced by additives in factice or rubber formulations.

As the amount of factice increases in formulations, there is a decrease in tensile strength. In activated white formulations, where these levels reached 75 phr (table 5), this decrease reached significant levels. However, with lower amounts of factice, up to 75 phr, as seen in medium abrasion (table 6) and engine mounting (table 7), the decrease in tensile strength was within acceptable limits. [Tabular Datas Omitted]

Activated white formulations containing meadowfoam oil factice had a higher tensile stress than formulations containing rapeseed oil and soybean oil factices. Ultimate elongation was lower in soybean oil factice formulations, but did not differ from the blank sample the meadowfoam and rapeseed oil factices, in spite of the large amounts that were used. The set-at-break values, however, were higher for meadowfoam oil factice containing formulations. This effect may be due to the repositioning of sulfur linkages between the factice and rubber molecules after being stretched. Meadowfoam oil fatty acids have longer chain lengths with higher unsaturation. Therefore, they will have a longer reach and a higher amount of sulfur on its chains that can attach to stretched and aligned rubber molecules more efficiently.

Medium abrasion formulations containing meadowfoam oil factice and rapeseed oil factice have similar tensile stress, force at 300% elongation and hardness values. Tensile stress and hardness values were slightly lower for soybean oil factice-containing formulations. Ultimate elongation and set-at-break values did not deviate significantly from the values of the blank sample.

In engine mounting formulations the different structures of factices do not seem to affect the tensile stress and hardness values. As the amount of factice increases in each formulation, there is a slight drop in the tensile stress and an increase in the elongation at break point. The increase is most noticeable in the formulations containing soybean oil factice. The formulations containing 2.5 phr meadowfoam oil factice had a slightly lower set-at-break value.

Conclusions

From both the physical properties of factices and their performance in rubber mixes, it can be concluded that meadowfoam oil factice has properties equivalent to or better than high erucic acid rapeseed oil, used commercially for the highest quality factice. It is definitely superior in its performance when compared to factices made for other vegetable oils.

References

1. E.S. Lower, Polymer Paint Journal, 174, 4125 (1984). 2. B.F. Greek, Chem. Eng. News, 67, 25 (1989). 3. S.P. Chang and J.A. Rothfus, J. Am. Oil Chem. Soc. 54, 549 (1977). 4. J.B. Harrison, Trans. Inst. Rubber Ind. 28, 117 (1952). 5. C.F. Flint, Proc. Inst. Rubber Ind. 2, 151 (1955). 6. S.M. Erhan, and R. Kleiman, J. Am. Oil. Chem. Soc. (In press). 7. A.H. Clark et al., Trans. Inst. Rubber Ind. 37, 193 (1961). 8. L.H. Princen and J.A. Rothfus, J. Am. Oil. Chem. Soc. 61,281 (1984). 9. K. Kammann and A.I. Philips, Ibid. 62,917 (1985). 10. J. Wisniak "Chemical transformation of the oil" in "The chemistry and technology of jojoba oil," J. Wisniak, Ed., Am. Oil Chemists' Society, Champaign, IL, 1987, ch. 2.