Evaluation of Maleated Polyethylene in Natural Rubber and Teline monspessulana Flour Composites

The effect of the addition of maleated polyethylene (MAPE) to compounds of natural rubber (NR) and Telinne monspessulana flour (TMF) previously mercerized was investigated. Two factors were analyzed: A. concentration of MAPE with five levels 2; 4; 6; 8; 10 phr (parts per hundred rubber), B. concentration of TMF with two levels 25 and 40 phr. The effect of MAPE on compatibility between NR and HTM was evaluated by tensile testing the compounds. The mixing was performed in a laboratory scale mill. The test tubes were obtained by cutting or die-cutting crosslinked peroxide sheets, these were obtained during the compression molding process. Analysis indicate that the MAPE coupling agent improved the compatibility between HTM and NR, this effect was evidenced by the values of tensile strength and elongation at break. However, the gel content determination indicates that the addition of 10 phr of MAPE crosslinking decreases due to competition with coupling reaction MAPE HTM. Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 17 February 2020 doi:10.20944/preprints202002.0236.v1 © 2020 by the author(s). Distributed under a Creative Commons CC BY license.

mixing NR with paper sludge, Ismail, Rusli, & Azura and Ismail, Rusli, & Rashid, claim that the addition of MANR increases the tensile strength and fatigue of the compounds, whose effect is attributed to the improvement of the interfacial adhesion between the filling and the rubber in the presence of the compatibilizing agent. Another coupling agent that has been investigated is the maleated polyethylene (MAPE). Sameni, Ahmad, & Zakaria, used as a compatibilizer in wood flour compounds and natural rubber, finding that there is increase in the modulus of elasticity and tensile strength as the MAPE content increases.
In this study, the effect of the maleated polyethylene (MAPE) was investigated as a coupling agent in natural rubber compounds and Telinne Monspessulana flour (TMF), whose species is invasive of the Colombian Andean Zone. The incorporation of MAPE was carried out during the mixing stage. The compound was reticulated using dicumil peroxide (DCP). The effect of the MAPE content on fiber-rubber compatibility was analyzed by evaluating the tensile properties. EXPERIMENTAL PART.

Methods.
Experimental design. A completely random bifactorial 2x6 design was performed. Factor A: TMF. with two levels 25 and 40 phr. Factor B: MAPE with six levels 0; 2; 4; 6; 8 and 10 phr; for a total of twelve formulations. The sum NR +MAPE was taken to 100 parts (phr) and was taken as reference to quantify the ingredients, as seen in Table 1. Antioxidant, process aid and DCP remained constant in all formulas. The output variable was assigned to each of the mechanical properties of the rubber compound. 5 repeats per variable were performed.  confidence level. The mean comparison test was performed using Tukey HSD.

Preparation of compounds
Mercerization. The flour TM was treated with NaOH 8% w/v, for 4 hours at 35 °C, according to the procedure described by Buitrago et al [19] .
Mixing. It was made in a laboratory roller mill, capacity 600 cm 3 . The order of addition was the following, first the natural rubber, then the MAPE, then the mercerized TMF, at the end of the incorporation stage the process aid and the antioxidant was added. The DCP was added three (3) minutes before the end of the stage. The temperature of the mixture was 120±5°C, the cycle 12 minutes.
Pressing. The sheets were obtained by compression moulding. A hydraulic press with heating system was used by means of electrical resistance. The operating parameters were: Mould temperature 160±1 °C, specific pressure 5.24 MPa, pressing time 6 minutes.

Methods of analysis.
Gel content. The degree of cross-linking was determined by the Soxhlet technique according to ASTM D 2765-01, method A [20] .

Content of the gel
This technique is used to quantify the percentage of polymer chains that are cross-linked.
The MAPE factor affects the gel percentage differently at level 25 phr and 40 phr of the TMF factor as seen in Figure 1.  The decrease in gel content may be evidence of interference of the coupling reaction (MAPE TMF) with the reticulation reaction (NR-NR via peroxide). This effect found in this paper is consistent with other investigations in which it is stated that the coupling reaction between the MAPE functional groups with the hydroxyl groups of flour negatively affect the crosslinking density of polymers with DCP [23][24][25] . If it is required to formulate a compound with MAPE level at 10 phr, it will be necessary to increase the content of DCP above 1 phr to maintain cross-linking density.

Mechanical properties.
Young's modulus The Young's modulus of the rubber compound is formed by the contribution of the NR, TMF, MAPE modules, the influence of the mixing conditions (shear, temperature and time) and the cross-linking density. The mixing process decreases the viscosity of the NR compound and therefore, has a direct consequence in the reduction of its rigidity. Theoretically the plant fiber module has values between 500-2500 Mpa. The coupling agent module was measured in the laboratory and its value was 425 Mpa. The highest value of the module was obtained with 40 phr of fiber and 8 phr of MAPE, being 130% higher with respect to the compound without coupling agent. The decrease in stiffness in the compound that has 40 phr of TMF and 10 phr of MAPE may be related to the decrease in cross-linking density (see Figures 1 and 2).

Tensile strength.
When analyzing the effect of the coupling agent on the compounds made with 25 phr of TMF, it was found that the tensile strength is significantly higher from 8 phr. Clearly the addition of the coupling agent improved stress transfer in the rubber compound. The compound data with 4 phr of coupling agent were very scattered.  [29 ] and Lu et al [30 ] who indicated that tensile strength improved due to the incorporation of maleate-type coupling agents in compounds made from natural fibers.

Elongation at Break.
The behavior of this property is affected by TMF factors, MAPE and gel percentage (see  The effect of the incorporation of the coupling agent in compounds having 25 phr of TMF indicates that despite the variation seen in Figure 4, it is not possible to conclude that the changes were significant due to the high dispersion of data in the compound without MAPE. Compounds that have MAPE with 40 phr of fiber showed increased elongation compared to test zero. Carefully analyzing the range of compounds with MAPE (2-10 phr ) it is observed that: the compound with 8 phr of MAPE has less elongation in comparison to MAPE tests 2 and 4 whose difference may also be due to the restriction of movement due to the coupling, since in the 8 th the maximum traction value was produced.
The increase of the elongation at break of the compounds with 10 phr of MAPE, regarding to the formulations with 6 and 8 phr, is possibly related to the decrease of the degree of crosslinking of the mixtures (see Figures 1 and 4). In theory, by decreasing the degree of crosslinking of the polymer chains, the elongation at break of the compound increases.
Energy to Rupture.

Hardness
The hardness behaviour of NR compounds was affected by TMF and MAPE factors, as seen in Figure 6. Regarding to the reinforcing load content, although the higher amount of flour, the more stiffness of the material increase its Young's modulus, this condition restricts the mobility of chains in the polymer, decreasing both the material elongation before a certain tension and the amount of energy it absorbs before its break.