Mechanochemistry

Due to the degradation and immiscibility of the plastic waste, high content of stabilisers, compatibilizers, and fillers have to be used, which is not economically feasible for the recycling industry. Mechanochemistry is a good alternative method, where reactive blending occurs in the mechanical pulverization of polymers without using any additives (Streletskii et al. 2015). The mechanochemistry denotes the chemical and physicochemical transformation of substances during the agglomeration caused by the mechanical energy. The technique includes a variety of reactions, such as fast decomposition and synthesis, graft modification, and polymorphic transformation. The advantages include simple process, ecological safety and possibility of obtaining a product in the metastable state. This technology has thus attracted wide focus (Guo et al. 2010).

Mechanical milling, typically ball milling, refers to the process that utilises high-energy ball milling technology to co-pulverize shredded recycled plastic, resulting in a considerable decrease in the polymer size (Kaupp 2009). The mechanical effects generated in the process, such as impact, compression, fracture, extension and shearing, can induce chain scission and hydrogen abstraction within the material particles, thus producing a large number of free radicals. The free radicals from different molecular chain species react with each other to induce chemical crosslinking and coupling (Liu et al. 2013).

Solid-state shear pulverization (S3P) is a novel, continuous and one-step cryogenic extrusion process to recycle plastic waste (Akchurin and Zakalyukin 2013). The normally incompatible plastic waste is subjected to high shearing forces to shred the carbon-chain backbone of plastic, thus generating a large amount of free radicals, which can form graft copolymers. The pulverised particles are then used to prepare high-quality products through the injection moulding, rotational moulding, powder coatings, or blending with virgin plastics (Guo et al. 2010). Lebovitz et al. (2003) prepared recycled PPs through the S3P process. The mechanical properties can be seen in Table 2.2.

The mechanochemical process only requires simple devices, and is a time-saving, cost-saving and eco-friendly process. The technology will contribute

Table 2.2 Properties of blends of postconsumer plastics (Lebovitz et al. 2003)

Feedstock

S3P

Tensile properties

Izod impact

Flexural properties

Ultimate

(MPa)

Elongation

(%)

(J/m)

Modulus

(MPa)

Strength

(MPa)

Recycled PP

Yes

375

32

1710

64.3

Recycled PP

No

330

37

1900

59.3

Virgin PP

28.2

38

21

1430

48

HDPE-recycled PP 70:30

Yes

19.6

8

11

960

27

LLDPE-recycled PP 70:30

Yes

12.9

8

32

510

19

considerably to economy, industry and environment in the future. But presently, the technology is just in the experimental stage, and still needs investigations for commercialization.

 
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