**3.5 Treatment of glass fiber reinforced plastic composite**

Glass fiber reinforced plastic (GFP) scrap consisted of acrylic plastic (PMMA) with glass fiber reinforcement in polyester resin matrix.

Polymethylmethacrylate (PMMA) sheets were formed in vacuum (shower trays and bathtubs) and reinforced with fiberglass in a polyester resin matrix. At the end of the life cycle, the product was disassembled (cut into pieces). A scrap of this composite was used for experiments (**Figure 17**).

#### **Figure 16.**

*The result of simultaneous grinding-separation of a mixture of ductile (ABS) and brittle (PMMA) plastics: (a) ground mixture of ABS+PMMA; (b) separated ABS; (c) separated PMMA (authors image).*

#### **Figure 17.**

*Initial plastic composite scrap of PMMA+GFP (authors image).*

For the milling of composite scrap, different disintegrator mills were used. To treat composite plastic scrap, the focus was on the size reduction of the acrylic plastic constituent and on the separation of the glass fiber constituent.

Disintegrator milling enables size reduction with simultaneous separation of components of low toughness. Composite plastic strips (PMMA+GFP) with dimensions of 100 100 5 mm were retreated.

The reprocessing technology of composite plastic scrap in disintegrators consisted of two steps:


The results obtained during the preliminary grinding of PMMA + GFP composite plastic in disintegrator mills are shown in **Figures 18** and **20 a, b**. The particle

#### **Figure 18.**

*Dependence of the particle size d50 of the milled composite plastic PMMA+GFP on the specific energy of treatment ES [11].*

**Figure 19.**

*Separation of glass fiber from acrylic plastic [12].*

size at the outlet of the DSA-158 disintegrator was approximately 13–25 mm. The precrushed material is suitable for direct grinding in the DSA-2 disintegrator.

Before the separation, the curve of mixed plastics has two modes (**Figure 19**). After separation the acrylic plastic has two main fractions: 46% of 1.4 mm and 25% of 0.355 mm and glass fiber has one main fraction 0.180 mm, which is more than 85%. The results of separating glass fiber from scrap of composite plastic are shown in **Table 7**.

As follows from **Table 7**, the total amount of separated GFP was 45 wt%. As a result, 55% of acrylic plastic from composite plastic scrap can be reused. GFP can be reused as reinforcement in the production of polymer concrete products.

Plastic powder with a particle size of about 1–2 mm can be obtained by two-stage grinding, and 95% by weight of the glass fiber content can be separated by final selective grinding.

The recovered material can be reused in the same production process in which it was obtained. The crushed PMMA powder is applicable as a filler in the casting technology.

#### **Figure 20.**

*The results of milling-separation of the glass fiber and acrylic plastic: (a) pre-crushed plastic scrap, (b) separated GFP large fraction, (c) finally milled mixture, (d) separated GFP fine fraction (authors image).*


#### **Table 7.**

*Results of GFP separation during selective milling by different disintegrators [13].*

### **3.6 Retreatment of tyres and pure rubber**

Tyres can be utilized by disintegration in two ways (**Figure 21**):


Collars with wires will be removed before cutting-off in both cases. At the first stage, the pieces of tyres sized 50 to 150 mm can be used in the pyrolysis technology for oil production. The pieces sized 10 to 50 mm can be used as additional fuel in furnaces. Fine powder of fraction 1–2 mm can be used for producing asphalt-concrete of road pavement, which is most beneficial.

At the second stage, further milling of rubber assumes separation of pure rubber from textile and wire fiber. The technology of treatment pure rubber to reclaimed rubber, used instead of caoutchouc, is cost-effective. The ultra-fine rubber powder

*Retreatment of Polymer Wastes by Disintegrator Milling DOI: http://dx.doi.org/10.5772/intechopen.99715*

**Figure 21.** *Principal scheme of utilization of tyres at room temperature (authors image, based on [14]).*

**Figure 22.**

*Dependences of particles size on the feed pressure:(a) of tyre strips in the process of milling and post-grinding; (b) of rubber particles at different feed pressures: 1–0.5 MPa; 2–1.0 MPa; 3–1.5 MPa; 4–2.0 MPa (d0 = 5 mm) [9].*

with particles less than 100 μm can be used as carbon black in the production of new rubber. However, this rubber powder is more expensive than carbon black.

Direct milling of whole tyres to the powder of 1–2 mm is more effective. In this case, the disintegrator system consists of two special devices - units for milling and post-grinding.

The granularity of the product depends on the pressure of the tyres against the milling tool. The mean size of the product after milling is shown in **Figure 22a**.

The dependence of particle mean size on the pressure is notable. Post-grinding evens the size of the particles. The granularity of the final product is shown in **Figure 22b**. It can be seen that the distribution function is two-modal.

Comparative grinding of pure rubber at normal and low temperatures was also conducted. The results are shown in **Figures 22** and **23**. The effectiveness of grindability of pure rubber at normal temperature is very low but as it follows from

**Figure 23.**

*Dependence of the granularity (a) and specific surface area (b) of ground rubber particles on the specific energy of treatment ES at normal temperature and cryogenic treatment: 1 – Milling at normal temperature with energy 25 kWh/t; 2 – Cryogenic milling with energy 8.3 kWh/t and 3–27 kWh/t (d0 = 5 mm), (authors image).*

**Figure 23a**, the cryogenic grinding is more effective. Results of normal and low temperature grindings for comparison are grouped together graphically in **Figure 23b**.

As can be seen, the relative mean particle size dm/dm0 and the relative increase in the specific surface area ΔS/ΔS0 of rubber particles depend on the specific energy of treatment *E*S. **Figure 23** shows that cryogenic grinding is more effective, particularly due to the increase in the specific surface area of rubber particles.

Highly alloyed steel wire found in car tyre collars may have its own value. To find out this value, it was necessary to study first the possibility of separating the wires.

Selective collar size reduction was achieved by using cryogenic grinding in disintegrator DS-158, during which the separation of wire, textile and rubber particles will take place (**Figure 24**).

The amount of rubber in the collars is relatively low, so the main value is probably in the metal. In addition, the amount of liquid nitrogen needed for cryogenic grinding has been determined theoretically and experimentally.

However, the economic efficiency of selective grinding has not been determined. It depends on the market value of alloy steel, amount of tyre collars to be treated and methods used to separate materials. The alternative is disposing collars to landfills, which is currently allowed.

**Figure 24.**

*Cryogenic grinding by the disintegrator DS-158: (a) initial pieces of collar; (b) ground and separated rubber; (c) textile; (d) steel wire (authors image).*

Grinding of pure rubber and composites using classical disintegrator mills at normal temperatures is much (by orders of magnitude) less effective than grinding in liquid nitrogen (**Figures 22** and **23**). This is due to high elastic properties of rubber. Under impact, kinetic energy leads to strong elastic deformations of rubber pieces, which preferentially heat the material but do not destroy it.

Therefore, it is rational to use such a grinding method for the following:


Changing in material properties consists:


The degree of grinding is directly related to the energy consumption of the process. Thus, the feasibility of energy costs for the rubber grinding process is associated with the minimum particle size obtained during grinding. **Figure 25** shows the dependence of the average diameter of rubber particles on the specific grinding energy.

**Figure 25.** *Dependence of the mean diameter dm (a) and shape (b, c) of treated at normal temperature pure rubber on the specific energy of disintegrator milling ES: b – Pure rubber particles before and c – After milling [14].*

### **3.7 Treatment of printed circuit boards**

Disintegrator milling as a prospective recycling method to relieve metallic components of current generation of printed circuit boards (PCB) (**Figure 26**).

The PCBs stands out with one of the highest concentrations of the rare and precious metals (RPMs). Therefore, the PCBs can/should be become a sustainable source of the RPMs for future generations of technology.

Traditionally, PCBs are processed using cutting processes (shredder mills) and a combination of low intensity impacts with shear and abrasion (hammer mills).

Both methods have disadvantages. During cutting, the knife must pass through all layers of material, which consist not only of epoxy resins and glass fiber, but also of strong and ductile metals and alloys, as well as ceramics (**Figure 27a, b**). This leads to high wear on the cutting edges. The cutting process also does not effectively

**Figure 26.** *Initial PCB plates (authors image).*

separate into its constituent components. During abrasion and shearing, the largest and most protruding pieces are affected, and these are basically the same metal and ceramic components. As a result, a lot of extra energy is wasted on grinding media wear, unnecessary grinding of metal and ceramic components and unnecessary movement of the entire mass of material.

The high-intensity impact that is used in disintegrator mills generates high stresses in the composite materials, breaking primarily the weakest constituents of them. This should lead to the fact that in the first place the bonds between metallic fraction (MF) and non-metallic (NMF) phase will be destroyed, which will allow further classification to achieve high-quality metal concentrate (**Figure 27c, d**). Thus, due to the selectivity of the impact action, a high level of fragmentation can be achieved – this is the main factor for mechanical enrichment. Such selective disassembly is less energy consuming, relieves the rest of the components of the PCB composite from excessive grinding, and, consequently, transformation into technological emissions.

**Figure 27.** *Structure of the PCBs plate: (a) initial material, (b) after cutting with a shredder and (c, d) ground by the disintegrator DSA-2 (author's image).*

#### **Figure 28.**

*Example of possible PCB disintegration degrees with existing disintegration systems (a) and PCB milling and separation stages (b): raw PCB (A), metal- (B), plastic- (C) and foil- (D) enriched components after disintegration and separation (author's image).*

In this part, the integration of a high-energy impact milling for PCBs recycling comminution state-of-the-art was reviewed from the fragmentation (relieving) process perspective. Modifying PCBs wastes through delamination and fragmentation turning it to MF and NMF mixture and makes it possible to further efficient classification into enriched concentrates (**Figure 28**).

A printed circuit board is a multicomponent metal-plastic multilayer composite material that has a complex structure with brittle and plastic components. The mechanisms for reducing the particle size of plastic and brittle materials are different.

At the pre-crushing stage, large pieces of the composite plates quickly disintegrate into their component parts. The result of milling is a direct fracture. Each separated component is then crushed at its own different speed.

Then each of these components is crushed at a different speed:


## **4. Summary**

Based on the study of impact milling by disintegrators different polymer materials the following conclusions may be drawn:

1. Impact milling an effective way for retreatment of polymeric materials due to the stresses initiated in materials due be ground. For impact milling DS-series of multi-functional disintegrators and milling systems operating at direct, separation and selective modes were developed.

