The raw material studied is oil produced by industrial processing of the residues of the bottom of rubber tree latex cups. It is produced by Bayan Industry Company (BIC) (Abidjan-Côte d’Ivoire). This fluid was obtained by collecting the residue, drying it and grinding it.

The 98% purity Wijs reagent was purchased from Acros Organics. Soluble starch (99%), glacial acetic acid (100%) and phenolphthalein (99.5%) were purchased from Merck. Chloroform (99.2%) was purchased from Prolabo, ethanol (96%) from Analar, sodium thiosulphate (99%) from Sds, potassium hydroxide (85%) from Chem-Lab, carbon tetrachloride (99%), hydrochloric acid (37%) from Riedel-de Haën, hexane (99.17%) from Panreac, potassium iodide (99%) and methanol (99%) from Carlo Erba.

The density and specific gravity of the oil was determined in accordance with NF EN ISO 6883 [26] . Volumes of 5 mL of dry oil and water were weighed. The masses determined for the test plugs and distilled water respectively were used to determine these parameters, at the ambient temperature of the laboratory (29˚C).

Relation (1) was used to calculate the density ρ_h (kg∙m⁻³) of the cup-bottom oil.

${\rho }_h=\frac{m}{V}$(1)

where, m, the mass of the oil sample (kg) and V, the volume of the oil sample (m³).

Viscosity is one of the oil parameters that directly influences the correct operation of diesel engines. It was determined using a falling ball viscometer (Thermo Scientific) fitted with a thermostatic bath (Lauda) [27] . The measurement variable used is the time taken for the ball to move t (measured by an electronic chronometer) over a given distance. The measurements were taken at 38˚C. The cylindrical tube of the viscometer was filled with oil, then the ball was dropped into the tube. The relationship (2) was used to calculate the dynamic viscosity η of the oil:

$\eta =k\left[\rho -{\rho }_h\right]t$(2)

With

k: the ball constant (mPa∙s∙cm³/g∙s)

t: the falling time of the ball (s)

ρ and ρ_h the respective densities of the ball and the oil (g∙cm⁻³)

The kinematic viscosity of a fluid is the ratio of its dynamic viscosity η to its density ρ (relationship 3) and is expressed in m²/s or cSt.

$v=\frac{\eta }{{\rho }_h}$(3)

The cloud point was determined in accordance with ASTM D 2500 [28] . After preliminary heating in a water bath, the sample was immersed in a freezing bath (ice + sodium chloride crystals) and then cooled at a specified rate. The temperature at which the first crystals appear corresponds to the cloud point.

Moisture content is an important parameter for guaranteeing sufficient quality of vegetable oil for use as biofuel. It was determined in accordance with international standard ISO 662 [29] . The method involves measuring mass loss by weighing the sample after oven drying at 103˚C ± 2˚C for 24 hours. 10 g of the oil sample was weighed into a ceramic capsule and dried in an oven at 103˚C ± 2˚C for 24 hours. After cooling in a desiccator, the sample was weighed again. The water content (T) is given by relationship (4).

$T=\frac{m_1-m_2}{m_1-m_0}\ast 100$(4)

where, m₀, the mass (in g) of the empty capsule; m₁ and m₂ the masses (in g) of the capsule + sample before and after drying respectively.

The iodine value was determined in accordance with standard NF ISO 3961 [30] . The method consists of dissolving the test sample in solvent and adding Wijs reagent. After a given time, potassium iodide and water are added, followed by titration of the liberated iodine with a sodium thiosulphate solution. 10 mL of carbon tetrachloride and 10 mL of Wijs’ reagent are added to a 250 mL flask containing 2 g of test sample. The mixture was kept in the dark for 1 hour. 100 mL of distilled water was then added to the mixture, which was determined iodometrically using a 0.1 M sodium thiosulphate solution, with stirring. A blank test was carried out under the same operating conditions. The iodine value (I_I in g of I₂/100g) is calculated from relationship (5).

$I_I=\frac{12.6\times C\times \left(V_1-V_2\right)}{m}$(5)

With, C, the concentration of the sodium thiosulphate solution (mol/L); V₁ and V₂, the volumes (mL) of the sodium thiosulphate solution used for the blank test and for the oil respectively.

m is the mass of the test sample (g) and 12.69, the number of grams of iodine corresponding to 1 mL of thiosulphate.

The peroxide value is an index of rancidity and therefore provides information on the quality and stability of the oil. The peroxide value was determined in accordance with the international standard ISO 3960 [31] . The principle is based on the iodometric determination of the oil sample dissolved in a mixture of glacial acetic acid/chloroform. To a 250 mL conical flask containing 2 g of the test sample, 30 mL of a glacial acetic acid/chloroform mixture (3:2 v/v) is added. The mixture is stirred until the sample is dissolved. 0.5 mL of a saturated potassium iodide solution and 30 mL of distilled water were added. The mixture was titrated with a 0.1 M sodium thiosulphate solution. A blank test was carried out under the same operating conditions. The peroxide value (I_P (meq O₂/kg)) is calculated from relationship (6).

$I_P=\frac{\left[V_1-V_2\right]\times 10}{m}$(6)

V₁ and V₂, the volumes of sodium thiosulphate solution used for the sample and blank respectively (mL) and m, the mass of the test sample (g).

Acid values give an indication of the age and quality of the oil or fat. The acid value has been determined in accordance with the international standard ISO 660 [32] . The principle is to determine the level of free fatty acids present in the oil by titrimetry using a solution of alcoholic potash. 2g of oil is dissolved in 10 mL of a mixture of diethyl ether and ethanol (1:1 v/v). After neutralization with a potassium hydroxide solution, the mixture was titrated with a 0.5N solution of ethanolic potash in the presence of phenolphthalein. A blank test was carried out simultaneously under the same operating conditions. The acid value (I_A (mg KOH/g)) is determined from relationship (7).

$I_A=\frac{\left[V_1-V_2\right]\times N\times 56.1}{m}$(7)

where V₁ and V₂ are the volumes of ethanolic potash solution used for the sample and blank respectively (mL); m is the mass of the test sample (g); N is the normality of the 0.5 N ethanolic potash solution and 56.1 is the molar mass of KOH (g/mol).

This index was determined using the international standard ISO 3657 [33] . The principle consists of neutralizing the free fatty acids in the oil while hot with an excess of alcoholic potash solution, then titrating this excess with a hydrochloric acid solution. 25 mL of the alcoholic potash solution is added to a conical flask containing 2 g of the test sample. The mixture was refluxed for 1 hour. The soapy solution obtained was titrated while hot with a hydrochloric acid solution, in the presence of phenolphthalein. A blank test was carried out under the same operating conditions. The saponification number (I_S mg KOH /g) is calculated from relationship (8).

$I_S=\frac{\left[V_1-V_2\right]\times N\times 56.1}{m}$(8)

With: V₁ and V₂, the volumes of HCl solution used for the sample and blank respectively (mL); m, the mass of the test sample (g); N, the normality of the HCl solution and 56.1 the molar mass of KOH (g/mol).

Table 1 shows the physical parameters of rubber latex bottom-of-cup oil.

The oil density value of 951 kg/m³ clearly shows that the sample is of vegetable origin. This value is between 900 - 960 kg/m³, and this interval is the one in which the density of the majority of vegetable oils falls [34] . Furthermore, the density of the oil studied is very close to that of castor oil (ρ = 952.5 kg/m³) [35] . It should be noted that this type of oil is used as varnishes, paints and biolubricants in heavy machinery working at very high temperatures [36] . The kinematic viscosity of the oil studied (48.57 cSt) is close to that of jatropha oil (49.9 cSt) [37] . As this oil is used to produce biofuels, the cup bottom oil could be used. Viscosity is one of the most important characteristics of biodiesel. It affects fuel drop size, jet penetration, atomization quality, spray characteristics and combustion quality [38] . Very high or very low fuel viscosity affects the engine [39] . For example, if the viscosity is very low, there will be insufficient lubrication, which will increase wear and leakage. A more viscous fuel will form larger droplets during injection, affecting combustion quality and leading to increased exhaust emissions [40] . To remedy this, preheating is sufficient [41] . Also, the presence of water in a fuel is detrimental as it promotes microbial growth, corrosion of tanks and filters, disturbs ignition and reduces the efficiency of stationary engines [34] . A high value would require heating before use, to enable this oil to meet fuel specifications. The value determined for cup bottom oil is 0.0845%. As this value is low, cup bottom oil could be used directly as a biofuel without drying.

The cloud point found in this work is −1.5˚C. This value is of the same order of that of sweet almond oil (Prunus amygdalus) (−2˚C) and jatropha oil (−2˚C) [35] [40] [42] . The oil studied in this work could be used in the formulation of biolubricants and biofuels.

Table 2 gives a summary of chemical characteristics of rubber latex cup bottom oil.

The cup bottom oil analyzed in this study has a peroxide value of 9.95 meq O₂/kg. This value indicates that the oil has deteriorated and is unfit for consumption [22] . The I_P of a rancid oil is between 10 - 20 meq O₂/kg [43] . It has an acid value of 2.805 mg KOH/g. This value is lower than the limit value of 3 mg KOH/g recommended by the West African pre-standard [34] . Indeed, high acidity can cause severe corrosion of the fuel system of a combustion engine [44] . Many researchers have reported that free fatty acids (FFAs) above 3% in oil require pre-treatment for optimal conversion to biodiesel, as high FFAs result in the oil being lost as soap rather than biodiesel [45] . This index is close to those of sweet almond (Prunus amygdalus) (2.811 mg KOH/g) and African star apple (Chrysophyllum albidum) (2.88 mg KOH/g) oils [42] . These oils could be used for industrial applications in the production of biofuels and biolubricants. The iodine value gives an idea of the unsaturation of an oil. It represents the degree of unsaturation [44] . From a fuel quality point of view, the more saturated the oil, the better it is. Or the lower its index, the better it is. The value of the iodine index (92.42 g I₂/100g) between 50 and 100 shows that this oil is monounsaturated, semi-drying and of the oleic type [11] [34] . This semi-drying oil could therefore be used in the production of alkyd resins (binders for paints and varnishes) and shoe polishes [22] . It could also be used as a biofuel and lubricant [34] [39] [46] . According to standard EN14214 (European Committee for Standardization) [47] , as the iodine value is less than 120 g I₂/100g, this oil could be used as a raw material for biodiesel production [42] . The saponification value (191.72 mg KOH/g of oil), which is higher or lower, shows that this oil could be used to make soaps. This is the case for oils from buriti palm and macauba pulp, which have saponification numbers of 190 and 192 mg KOH/g respectively [46] .