High Pressure Water Cleaning Of Degraded Polymers Biology Essay

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Injection molded test bars, 3mm thick, made from a high-density polyethylene, low-density polyethylene and polypropylene co-polymer were subjected to ultraviolet exposure for 2 weeks, 4 weeks and 8 weeks in the UV irradiation laboratory. Crystallinity measurements were made using Differential Scanning Calorimeter (DSC) at different depth in the unexposed state and after UV exposure. Water spraying was applied to all exposed test bars and crystallinity measurements then were carried out again to find any physical or molecular changes and also to determine the extent to which the degraded material can be efficiently removed in this way.

Keywords: Polymer Recycling; Photodegradation; High Pressure Water

1. Introduction

It has been found that over the last 30 years the use of polymeric materials in daily life has continuously increased. In Western Europe for instance, over 17.5 million tonnes of polymer wastes are generated every year, their environmental effect has been a matter of great public concern [1]. The variation in properties and chemical composition between different types of polymer materials hinders the application of an integrated and general approach to handle these polymer wastes. There are several reasons for the continuous increase in the demand for commodity polymers such as their properties which are low density, low thermal and electric conductivities, and easily moulded, high corrosion resistance, highly durable and low cost.

Polymers are composed completely of organic compounds; their main disadvantage is that their decay process takes a very long time and making use of waste polymers is an important element in economics [1]. Since polymers take a long time to decay, recycling is one of good methods to overcome this problem. Feedstock recycling and mechanical recycling have been used to recycle polymers nowadays and a lot of research has been carried out to improve the current system. The preparation for recycling should not be taken lightly as it is obviously contribute to the efficiency of polymer recycling.

There are many factors that can be linked to the crystallization of polymers such as the shape and stereoregularity of the repeat unit, the presence of branches and crosslinks as well as chain distance in the linear segments [2, 3, 5]. When polymer is subjected under ultraviolet irradiation there are several changes to the crystallinity and crystallizability of the polymer [2]. The main concern of this report is to investigate the changes of crystallization and weight removed from degraded polymers. Apart from that the objective is also to determine whether high pressure water can be used to eliminate degraded polymers.

It is noted that photo-oxidation occur when polymer is subjected under ultraviolet irradiation and hence principal changes occur. The principal changes that normally happen are (i) chain scission; (ii) crosslinking; and (iii) the formation of molecular defects, carbonyl group for instance [6,7]. Chain segments which previously entangled are released by chain scission to allow crystallization.

2. Experimental

2.1 General Strategy

This work was divided into two parts. The first one was rig construction to carry out high pressure water spray on degraded polymers and the other one was to determine the changes occurred in crystallinity value in order to investigate whether this new method can efficiently remove the degraded parts.

2.2 Materials and sample preparation

For DSC analysis, samples were cut using a single point cutter with fly cutting action by milling away material from exposed surface. Before milling operation was carried out the bed was cleaned thoroughly and the chippings were collected at the end of each cutting pass. Samples were cut at different depths; from 0 to 0.1mm, 0.1 to 0.2mm and 0.2 to 0.3mm. For each DSC run, the sample was weighed on a Mettler AT261 and the amount used was ~10 mg. The weighing machine (Mettler AT261) balance reading to 0.01mg, yielding accuracy better than ±0.3% [4,7].

Figure1: Single flying cutting action Figure 2: Chipping or swarfs

2.3 UV exposure

Ultraviolet exposures were carried out in a constant temperature room at 30±1oC using fluorescent tubes type UVA-340 as the UV radiation supply. The tubes were chosen because of their output in the UV range at wavelengths below about 360 nm coincides the spectrum of solar radiation at the earth’s surface fairly closely. One side of the bars was exposed for a range of times from 2 to 8 weeks before milling away the exposed surface. Exposures were conducted uninterrupted, 24 hours per day. Studies that have been done previously showed that very significant molecular degradation occurs within the chosen periods [2].

2.4 Water spraying on degraded bars

Before water spraying was applied to the exposed surface, each of the bars was clamped at center as shown below (figure 3). A water blaster literally blasts water at a high pressure has been chosen to complete the task. The maximum pressure of the water is 95bar and the water flow is 360L/H. Further details on specification are shown in table 1.

Below are the high pressure water washer specifications;

Suitable For

Frequent use

Pressure Gauge (Y/N)

No

Model No.

TRY1350PWB

Electric/Petrol

Electric

Material

Plastic &Metal

Size

Small

Category

Pressure Washers &Sprayers

Water Flow (L/H)

360

Max Pressure Rating (BAR Pressure)

95

Category

Frequent Use Pressure Washers

Table 1

Figure 3: High pressure washer

By using the high pressure water washer, exposed bars were sprayed using high pressure of water (figure 3) to remove the degraded polymers and dried for three days before successive layers were removed. Degraded layers were removed by using milling machine with single flying cutting action method. This method was applied because less heat produced during cutting. Heat produced during cutting may influence the degree of crystallinity value on specimen. In order to reduce the impact of heat on crystallinity value during sample preparation, samples were cut using single flying cutting action.

Figure 4: Degraded bar is clamped Figure 5: Water spray is applied to bar

2.5 Characterization

As mentioned earlier, crystallization measurements were made by differential scanning calorimetry (DSC). Differential scanning calorimetry or DSC is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference are measured as a function of temperature. Both the sample and reference are maintained at very nearly the same temperature throughout the experiment. Generally, the temperature program for a DSC analysis is designed such that the sample holder temperature increases linearly as a function of time [7, 8]. The reference sample should have a well-defined heat capacity over the range of temperatures to be scanned. All measurements that have been carried out were made under flowing nitrogen with a Mettler FP F90 controller connected to a FP85 Heat Flux cell. The equipment was calibrated for temperature and calorimetric sensitivities of the cell with indium to ensure precise measurement of crystallinity. Experiments were carried out under nitrogen flow (50ml/min) to avoid thermal degradation during measurements and also to displace atmospheric oxygen to prevent undesired oxidation of the sample.

A heating rate of 13oC/min was used within the range 40oC-210oC for all three types of polymers chosen and thermogram recorded. This heating thermogram characterizes the material in the form that existed at the end of UV exposure. The crystallization endotherms obtained in the heating runs were used to estimate the crystallinity. The peak areas were obtained between chosen limits for each sample. The limits chosen were for HDPE:70oC-160oC; LDPE: 65oC-140oC;PPCO:90oC-200oC. Heat of fusion is an important parameter for crystallinity measurements. The polyethylene crystal phase was taken to be 292.5kJ/kg while for PPCO was taken to be 267kJ/kg.

3. Results

3.1General observations

The thermogram with different depths from unexposed sample is given as reference in figure 6. Dotted line representing 0-0.1mm, dotted line represents 0.1-0.2mm and straight line represents sample from 0.2-0.3 mm.

The crystallinity values climbed slightly higher towards the centre and the highest crystallinity was recorded from 0.2-0.3mm depth. This observation applies for all samples at all conditions whether they are exposed or unexposed to UV radiation.

Figure 6: DSC thermogram obtained from different depths within a HDPE (unexposed)

The thermogram shown in above figure is likely to be seen in all three samples discussed here. The highest crystal melting point in the thermogram for the sample obtained from the exposed surface (0-0.1mm) is likely to be displaced significantly towards lower temperature. In contrast crystal melting peaks for samples obtained from layer deeper into the bar were displaced less.

3.2 High density polyethylene

Depth (mm)

UV Exposure (Weeks)

Crystallinity (%)

Before Water Spray

Crystallinity (%)

After Water Spray

0.0 to 0.1

unexposed

62.0

-

0.1 to 0.2

unexposed

64.3

-

0.2 to 0.3

unexposed

65.0

-

0.0 to 0.1

2

68.5

64.2

0.1 to 0.2

2

68.1

64.5

0.2 to 0.3

2

67.0

68.5

0.0 to 0.1

4

72.0

67.2

0.1 to 0.2

4

70.3

66.4

0.2 to 0.3

4

69.0

64.2

0.0 to 0.1

8

73.6

64.8

0.1 to 0.2

8

73.8

58.4

0.2 to 0.3

8

71.4

64.8

Table 2: Crystallinity values; before and after water spray for HDPE

The unexposed thermograms for high density polyethylene are given in table 2. It can be observed that there were not much different in crystallinity values as shown in the thermograms. Crystallinity value obtained at the surface recorded the lowest crystallinity, this is common observation attributed to the high cooling rate during molding operation which also pointed out by Craig and White [4].

Figure 7: Crystallinity measurements obtained using DSC first heating for HDPE at different depths from unexposed surface

Figure 8: Crystallinity measurements obtained using DSC first heating for HDPE at different depths from exposed surface; 2 weeks, 4 weeks and 8 weeks

After 2 weeks ultraviolet exposure, the crystallinity values were increased at most depths whereas after 8 weeks a big change in crystallinity was recorded at all positions within 0.3mm of the exposed surface. Interesting results were obtained after the water spray was carried out; the crystallinity values fell slightly lower as shown in figure 8. Water spray on bars seems have slightly altered the secondary crystallization produced by photo-oxidation and change the internal structure of the samples.

Water spray on HDPE bars

15 Second Spray

Exposure Time (Weeks)

Weight % Removal

0

0

2

1.681

2

0.03104

2

0.02615

4

0.1194

4

0.01979

4

0.196

8

0.01526

8

0.01187

8

0.006446

Table 3: Percentage of weight removal for HDPE

Figure 9: Weight removed (%) vs. weeks exposed (HDPE)

Figure 9 shows the time of exposure and percentage of weight that had been removed using high pressure of water. Water spray which was applied to the HDPE bars is believed able to remove a small amount of the degraded polymers although the values obtained were not in good agreement. In average, samples of 2 weeks exposure time recorded the highest value of material removal followed by 4 and 8 weeks. Samples with highest crystallinity values (8 weeks UV exposure) were expected to have the greater average of material removal due to photo-oxidation but the results here contradicted the prediction.

3.3 Lower density polyethylene

Depth (mm)

UV Exposure (Weeks)

Crystallinity (%)

Before Water Spray

Crystallinity (%)

After Water Spray

0.0 to 0.1

unexposed

37.8

-

0.1 to 0.2

unexposed

37.2

-

0.2 to 0.3

unexposed

40.2

-

0.0 to 0.1

2

39.2

41.0

0.1 to 0.2

2

39.8

40.8

0.2 to 0.3

2

39.5

40.8

0.0 to 0.1

4

40.6

41.3

0.1 to 0.2

4

39.1

41.5

0.2 to 0.3

4

39.2

42.0

0.0 to 0.1

8

41.1

45.0

0.1 to 0.2

8

40.4

43.6

0.2 to 0.3

8

42.4

42.9

Table 4: Crystallinity values; before and after water spray for LDPE

Figure 10: Crystallinity measurements obtained using DSC first heating for LDPE at different depths from unexposed surface

Figure 11: Crystallinity measurements obtained using DSC first heating of LDPE at different depths from exposed surface; 2 weeks, 4 weeks and 8 weeks

Water spray on LDPE bars

15 Second Spray

Exposure Time (Weeks)

Weight % Removal

0

0

2

0.01551

2

0.03209

2

0.01792

4

0.05567

4

0.6885

4

0.03723

8

0.1749

8

0.005574

8

0.01394

Table 5: Percentage of weight removal of LDPE

Figure 12: Percentage of weight removal for LDPE

The results obtained (figure 13) shows the percentages of materials have been removed for LDPE samples followed by graph of material removal against week exposed (Figure 16). The lowest material been removed was recorded after 2 weeks of ultraviolet exposure and progressively increased with time of exposure. In terms of average material removal, the highest was recorded after 8 weeks of exposure as expected although the variations were quite different from sample discussed earlier (HDPE).

3.4 Polypropylena co-polymer

Depth (mm)

UV Exposure (Weeks)

Crystallinity (%)

Before Water Spray

Crystallinity (%)

After Water Spray

0.0 to 0.1

unexposed

39.7

-

0.1 to 0.2

unexposed

38.1

-

0.2 to 0.3

unexposed

39.2

-

0.0 to 0.1

2

41.5

43.2

0.1 to 0.2

2

42.2

42.9

0.2 to 0.3

2

41.2

43.9

0.0 to 0.1

4

40.7

47.8

0.1 to 0.2

4

42.8

42.1

0.2 to 0.3

4

39.1

45.9

0.0 to 0.1

8

44.9

45.5

0.1 to 0.2

8

43.9

41.1

0.2 to 0.3

8

41.1

48.2

Table 6: Crystallinity values; before and after water spray for PPCO

Figure 13: Crystallinity measurements obtained using DSC first heating for PPCO at different depths from unexposed surface.

Figure 14: Crystallinity measurements obtained using DSC first heating for PPCO at different depths from exposed surface; 2 weeks, 4 weeks and 8 weeks.

Water spray on PPCO bars

15 Second Spray

Exposure Time (Weeks)

Weight % Removal

0

0

2

0.01073

2

0.007928

2

0.006282

4

0.01227

4

0.01127

4

0.01517

8

0.0163

8

0.03039

8

0.01113

Table 7: Percentage of weight removal for PPCO

Figure 15: Weight removed (%) vs. weeks exposed (PPCO)

The results for material being removed after water blast are presented above for PPCO (Figure 21) and the graph (Figure 22) also plotted. It can be deduced that the degraded material being removed after water spray slightly increased. Result obtained after 8 weeks ultraviolet exposure showed the highest degraded polymer that had been removed, similar variations of weight removed that occurred on LDPE samples.

4. Discussion

Evidently, all samples showed a variation in crystallinity before and after ultraviolet exposure and before and after water spray been carried out. For unexposed surface, it was noted that crystallinity value from 0.1 to 0.2mm depth was recorded as the lowest crystallinity compared to other different layers (0-0.1mm and 0.2-0.3mm) but this was obtained from LDPE and PPCO. For HDPE sample, the crystallinity variation was similar to the common observation that pointed out in previous studies [9, 10, 11]. Samples that were exposed under ultraviolet irradiation showed the effect of photo-oxidation. For all samples that were exposed under UV irradiation, it is believed that residual appeared attributed to the temperature gradient, at this stage chemi-crystallization occurred and crystallinity values increased significantly.

Encouraging results were found after bars had been sprayed about 15 second. In terms of physical appearance, LDPE and PPCO had become slightly softer especially after the bars were dried but there was a different impact on HDPE presumably because this material tougher and has high density than others. Apart from that, it can be seen all bars that were sprayed become swollen as a result of water accumulation. During cutting process it can be seen that the chippings were longer than usual for both polymers probably influenced by water absorbed however this was not happened on HDPE bars. High pressure water does not affect this material at all since there was no different before and after water spray applied. Another possibility is that those samples from HDPE material were totally dried before they were cut.

5. Conclusion

From the experiment it has been found that crystallinity slightly increases with distance from the sample surface as a result of the cooling rate during moulding operation. At the surface of each bar, the lowest crystallinity values were recorded. The studies described here were performed as part of investigation water cleaning of degraded polymers in preparation for recycling. It has been observed that high pressure water cleaning was able to remove small amount of material from the bars that had been exposed for certain period of time.

The results obtained here were encouraging although the small amount of degraded polymers successfully removed were not in good agreement or inconsistent. The amount of material being removed also slightly differs although the sample with the same ultraviolet exposure of time had been sprayed. Apart from that, water has a tendency to change the molecular and internal structure of the polymer. It is believed that the high pressure of water exerted to the bars produced secondary crystallization which increased the crystallinity values of the polymers. Water might change the plasticization of the polymer surface and resulted for crystallinity to increase slightly higher compared with the previous state before water blast. As mentioned, residual appeared after samples were exposed under ultraviolet irradiation in laboratory. It is believed that residual also occurred after water spray due to high pressure exerted to bar being exposed. This phenomenon occurred to all of the samples. Moreover water absorption causing swelling and secondary crystallization as observed. It can be deduced that high water was able to change the internal and molecular structure of the polymers tested here including changing the crystallinity value.

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