Study on Cellular Structure and Mechanical Property of Foaming/Cross-linking Polyethylene System

The cellular structure and mechanical property of a sequential foaming/cross-linking polyethylene system were studied in this work. By adjusting the components, foaming starts before the cross-linking reaction initiated and the melt strength increases during the foaming process. Rubber Process Analyzer (RPA) was used for the in situ monitoring and measuring of the foaming and cross-linking process. The cellular structure and expansion ratio of polyethylene foam can be modulated by controlling the type and ratio of foaming agent and cross-linking agent, as well as the foaming/cross-linking conditions. The mechanical strength was tested by universal mechanical testing machine, the melt strength were also characterized and analyzed. Experimental results demonstrated that the cross-linking controlled the cellular size and improved mechanical strength.

In this paper, a sequential foaming/cross-linking system was designed, in which foaming reaction was initiated before the cross-linking reaction started. The cellular structure and mechanical strength were In this experiment, high density polyethylene (8007) was used as the XPE matrix resin, DBPH was used as the cross-linking agent. The XPE samples preparation process was as follows: firstly, take out some DBPH and 8007, make the quality ration was 1:20, then mix polyethylene resin and cross-linking agent, finally, put the obtained mixture into twin screw extruder, the temperature was set at 140℃.

Samples preparation
XPE with different dilution ratio and adding 2% OBSH to dilution XPE were investigated, for comparing, the control samples were also prepared.
The sample compositions are shown in Table 1

Expansion ratio test
The densities of foamed samples, ρ, were determined by deducing the Archimedes law involving weighing polymer foam in water with a sinker using an electronic analytical balance (HANG-PING FA2104), calculated using equation 1.
where a, b, and c are the weights of the sample in air without sinker, the totally immersed sinker, and the specimen immersed in water with sinker, respectively, and ρ w is the density of water. The volume expansion ratio of the cross-linking polyethylene foaming material, Rv, was the ratio of the bulk density of initial mixture density (ρ 0 ) to that of the cross-linking polyethylene foaming material (ρ), calculated as follows:

Gel content test
The gel content was determined by using xylene as the solvent according to ASTM-D2765, oil bath temperature was 170ºC. A polyethylene sample of 0.400 ± 0.015 grams by weight was cut into small pieces and placed in a 100-mesh stainless steel wire cloth. The sample was dropped into xylene, boiled and reflux condensation for 8 hours. After the extraction cycle, the sample was dried in a vacuum oven at 160°C for 2 hours. The gel content (%) was calculated as follows: where m 1 is the weight of stainless steel wire cloth, m 2 is the weight of initial sample and stainless steel wire cloth, m 3 is the total weight of sample and stainless steel wire cloth after extraction.

Scanning electron microscope (SEM) test
Those foam samples were immersed in liquid nitrogen for 10min, and then fractured; the surface was vacuum coated with gold examined using a NOVA Nano SEM450 (oxford instruments). The size and size distribution of the cells were measured using Nano-measure software. Length and width of each test sample is 80mm×10mm.

Flexural modulus test
Flexural test helps in evaluation of attributes of sample when it is subjected to a loading condition similar to simple beam loading [13]. 3 Results and discussion

Interaction between cross-linking and foaming reactions
MFI is one of the most common parameters of polyethylene, it could affect the cross-linking reaction and the degree of cross-linking of polyethylene [14].
Similarly, it also has great influence on the dispersion of the foaming agent in the polyethylene, the gas diffusion speed, the interfacial tension between gas and liquid, and melt strength properties [15]. In this paper, 7042 was used as the dilution resin of XPE.

Effect of XPE on cellular structure
Gel content of polyethylene material means the  Figure 2 and Figure 3. in the middle and OX8 presented the slowest pressure growth rate. This phenomenon was mainly caused by different MFI, the MFI of OX4 was 3.8, and it was higher than the MFI of OX8 (2.7). The higher MFI of polyethylene, the lower viscosity, and resulted in the higher gas diffusion rate [15]. After 70 seconds, cross-linking reaction started and the pressure rate slowed down, the pressure rate of OX4 slower than OX6, which was opposite with the initial pressure rate discipline. This appearance was mainly due to the cross-linking reaction, the more the dilution ratio, the less the cross-linking agent, the lower the viscosity, and the higher the pressure growth rate.
Cellular structure was the important property of polyethylene foam, which determined the properties of polyethylene foam. In this experiment, the cellular structure was tested by SEM. Figure 6a, Figure 6b and For comparison, the foam structures of the samples without cross-linking agent were also studied. Figure   7a, Figure 7b and Figure 7c were the SEM pictures of OB4, OB6 and OB8, respectively, their corresponding diameters were 518.8μm, 478.8μm and 466.1μm and these cellular sizes are much larger than OX samples.
Although the cell were all closed structure, but their shape was not regular circular structure, some cellular size was large enough and some cellular size was small, their standard deviation of errors (σ) were 256.8μm, 194.4μm and 233.5μm, respectively, the cell size distribution was scattered without cross-linking, which is possibly due to the lack of three-dimensional network structure restricting the random bubble growth during the foaming process. More interesting is that the higher the content of 7042, the smaller the size of cellular, which is consistent with the trend of MFI, and similar to that shown in Figure 5b.This comparing experiment demonstrated that the cross-linking agent not only control cellular size, but also control the cellular shape.

Effect of XPE on mechanical property
Due to closely relationship between cellular structure and mechanical property, the cross-linking polyethylene foaming materials' mechanical property will be transformed with the cellular size and structure changed which resulted by the different XPE dilution ratio [7,25]. In this experiment, flexural strength was taken as the typical mechanical property to test. Figure   8 showed  OB6 and OB8 were also measured. As shown in Figure 8, the elasticity modulus values of OB polymer mixtures were lower than OX polymers. The main reason for this phenomenon was that OB mixtures didn't contain cross-linking agent, OB polyethylene mixture molecular chains were independent with each other, and they didn't formed the effective and stability connection structure, finally resulted in the weak mechanical property [26]. In addition to this, there was another phenomenon: the higher content of 7042, the lower elasticity modulus value. The appearance mainly caused by low elastic modulus of linear density polyethylene 7042 [27].

Conclusions
In this paper, the sequential foaming/cross-linking system was developed, the cellular structure and mechanical property were researched. Test results showed that foaming reaction and cross-linking reaction interfered with each other. Additionally, the cross-linking reaction could control cellular structure during foaming process and finally improved mechanical strength.