Melt Blending

Melt blending is one of the most frequently-used ways of mechanically recycling plastic waste. It refers to blending recycled plastics with similar type of virgin plastics or different types of recycled plastics in the melt process. Blending recycled PP with virgin PP not only can reduce cost, but the new blended PP can maintain equal performance with the virgin plastic products (Yin et al. 2013). However, plastic waste collected from kerbside is a mixture of various types and qualities of polymers, thus extensive research has been carried out on reprocessing commingled plastic waste.

Blending recycled plastics with virgin plastics of similar components is often used for mechanically recycling industrial plastic waste, which is industrial plastic scrap off-cuts and off-specification items obtained from processing operations. The mechanical recycling of industrial plastic waste has been widely adopted due to ease of separation of different types of plastics, low level of impurities present, and their availability in large quantities. Meran et al. (2008) mixed recycled PP with its virgin material. As shown in Fig. 2.1, tensile strength of recycled PP can be effectively improved by mixing its virgin plastic. They believed that recycled PPs have the capacity of being as good as any engineering grade under optimised mixing rates and reprocessing conditions.

Tensile strength of recycled PP blended with its virgin material (Meran et al. 2008)

Fig. 2.1 Tensile strength of recycled PP blended with its virgin material (Meran et al. 2008)

Domestic plastic waste, consisting mostly of packaging materials from kerbside recycling collections, contains various types of polymers. A segregation of the plastic waste before recycling is time- and cost-consuming, and never fully separated. Therefore, the study on reprocessing the commingled plastic waste from roughly sorted waste is very practical. Table 2.1 shows mechanical properties when recycled PP is blended with recycled low-density polyethylene (LDPE) and HDPE (Hasanah et al. 2014; Strapasson et al. 2005). The mechanical properties show similar trends when the recycled PP is mixed with recycled LDPE and recycled HDPE. With the decrease of PP rate, the yield strength and Young’s modulus of the

Table 2.1 Mechanical properties of the recycled PP-LDPE and PP-HDPE composites

Blending

rate

Yield

strength (MPa)

Young’s modulus (MPa)

Elongation at break (%)

PP-LDPE composites (Strapasson et al. 2005)

100:0

25.1

1304

600-700

75:25

22.8

1149

100

50:50

13.8

845

3.7

25:75

11.4

435

400-529

0:100

7.6

157

111

PP-HDPE composites (Hasanah et al. 2014)

100:0

45

2340

14

80:20

34

1690

8

60:40

29

1290

7

50:50

29

1250

6

40:60

27

1200

15

20:80

26

920

25

0:100

25

900

25

materials are decreasing, thus the materials are becoming more ductile. Elongation at break dramatically decreases when 50% of PP is mixed with 50% HDPE or LDPE, due to incompatibility of the different blends.

 
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