Crosslink Density

1-Crosslink density Crosslinking refers to the bonds between the chains of a polymer. Usually cross linking is done in order to achieve better mechanical properties. By reducing the level of unsaturation, the polymer can be saved from degradation. A polymer is immersed in a solvent at a specified temperature and change in mass or volume is measured correspondingly.

1.1- Procedure A 1” X 1” of the rubber was cut and weighed. An empty beaker was taken and weighed. This rubber was then placed in beaker and weighed again. It was then filled with toluene. The beaker was then covered. The rubber was allowed to swell as it absorbs the toluene. After every 24 hours, the rubber sample was taken out, carefully dried with a dry cloth and weighed. This was done until a constant weight was achieved. The sample was the removed from toluene and dried at room temperature for 24 hours. After that the sample was placed in a hot air oven at 80 0 C for about 7 hours. Then the sample was again dried at room temperature for 24 hours.

1.2- Calculations By using the Flory-Rehner equation, crosslink density is measured:

Where Ve = effective number of chains in a real network per unit volume (cross-link density) Vr = volume fraction of polymer in a swollen network in equilibrium with pure solvent (0.19) x1 = polymer-solvent interaction parameter V1 = molecular volume of solvent (1.069 x 10-4 m3/mol)

The results are recorded in the Table 1. Day 0 1 4 5 6 7 8 9

Weight (g) 1 1.94 2.30 2.35 2.31 0.98 0.96 0.95

Table 1 weight and volume of rubber with time

volume 0.62 1.205 1.426 1.460 1.438 0.609 0.596 0.590

ρ of rubber = 1610 kg/m3 X = 0.668 (for natural rubber and SBR blend) Ve = 194 mol/m3 (cros-link density)

1.3- Conclusion The cross-link density greatly effects the mechanical properties of the rubber. Low cross-link densities decrease the viscosities of polymer melts. Intermediate crosslink densities transform gummy polymers into materials that have elastomeric properties and potentially high strengths. Very high cross-link densities can cause materials to become very rigid or glassy. The polymer is firstly observed to gain weight and swell as it absorbs toluene, but as the number of cross-links increased, the absorption rate decreased. During the swelling stage after the equilibrium was reached, a decreased in weight of the swell is observed. This may be due to the dissolution of rubber contents partly into the solvent.

2.1- Swelling index When a cross-linked polymer is immersed in a good solvent, it will absorb a portion of solvent and swell. The swollen gel can be characterized as a solution. The extent of swelling depends on the forced between the polymer and solution. The free energy of mixing will cause the solvent to penetrate into the polymer and dilute the polymer solution. As the polymer chains in the cross-linked polymer network begin to elongate under the swelling action of the solvent they generate an elastic reactive force in opposite to this formation. The volumetric swelling reaches to the steady state when two forces balance each other. [1,2]

2.1- Procedure A weighed sample of rubber was immersed in a toluene and the container was sealed. This was kept at room temperature. After every 24 hour, the sample was taken out, dried and weighed. The change in the weight was recorded and plotted against the number of days.

2.2- Calculation Weight of sample before dipping in toluene = W 1 = 1.00 g Weight of sample when it becomes constant = W2 = 2.30 g Swell index = (W2 - W1 / W1) * 100 Swelling index = 130 % From the reference to the Table 1, graph is plotted as shown:

Weight (g) 2.5 2 1.5

Weight (g)

1 0.5 0

0

1

2

3

4

5

6

7

8

9

10

Time (in days)

Figure 1, Weight of rubber with time

3-Bound rubber content Rubber composites are reinforced by fillers such as carbon black and silica by formation of bound rubber. Bound rubber is formed by the interactions between rubber chains and filler particles. The filler-polymer interactions for bound rubber involve physical adsorption and mechanical interactions.

3.2- Calculations Rb % = Wfs – W[mf / (mf +mv)] / Wt [ mr / ( mf + mr) ] *100 Wfs is the weight of filer and gel Wt is the weight of sample mf is the fraction of filler in compound mr is the fraction of rubber in compound mf = 08.40 / 29.97 = 0.280 mr = 11.21 / 29.97 = 0.374 W = 1.00 g

Wfs = 0.95 g Rb % = 91.2 %

Ingredient Natural rubber SBR Silica Sulfur TMTD Zinc oxide Stearic Acid

phr 40 30 30 2 2 2 1

Mass in grams 11.21 8.40 8.40 0.56 0.56 0.56 0.28

Table 2, Formulation of natural rubber and SBR blend

By the value of Rb % of 91.2 % calculated above reveals that the filler-rubber interlinking is good.

References [1]. http://www.academia.edu/11566466/Swelling_Index_Analysis_of_Therm oset_Elastomer (last accessed on 22/4/2015, 10:00 AM) [2]. ASTM 2765 “Standard Test Methods for Determination of Gel Content and Swell Ratio of Crosslinked Ethylene Plastics” [3]. ASTM D6814-02 ”Standard Test Method for Determination of Percent Devulcanization of Crumb Rubber Based on Crosslink Density1” [4]. Sung-Seen Choi, Eunah Ko “Novel test method to estimate bound rubber formation of silica-filled solution styrene-butadiene rubber compounds” 2014. [5]. Cambridge Polymer Group “ Swelling measurements of crosslinked polymers”