**2.2 PET degradability**

86 Material Recycling – Trends and Perspectives

5. **Chemical recycling** is related to the recovery of the chemical compounds based on following depolycondensation particular reactions glycolysis, methanolysis, hydrolysis,

In spite of long lasting efforts, because of the low cost and low performance applications of the recycled material, at present, the widely accepted opinion is that the POSTC-PET mechanical recycling without a structural up – gradation is not an efficient procedure.

The chapter presents an overview on the structural up-grading of POSTC-PET by macromolecular chain extension during mechanical recycling (reactive processing), a

The main parameters controlling the POSTC-PET mechanical recycling are: the

POSTC-PET contamination can be of the following three types: *macroscopic and microscopic* 

*Macroscopic physical contamination* of POSTC-PET is easy to clear off as it consists of dust, glass chops, stones, adhesives, product residues, plastics such as PVC and PE, earth

*Microscopic physical* c*ontamination* is more difficult to clear off especially because is partially attached to the bottle because it is about adhesive or other impregnated impurities resulted after abrasion or impact. These impurities break the thread either during granulation in the melt processing or during the spinning in the fibre industry. This leads to decrease the

*Chemical contamination* is the result of *adsorption* of flavouring, oil, pesticides, household chemicals, and fuel if the bottles were re-filled with such products in a secondary utilization. The proportion of POSTC-PET interaction with these compounds depends on the diffusion behaviour of contaminants and the sorption properties of the polymer. The removing of these contaminants implies *undergoing the reverse processes, namely desorption*. The adsorbed chemical impurities into the polymer *settle on the risk potential of* POSTC-PET mechanically recycled especially if the food packages are targeted. The recycling by desorption can not be considered because of its very low productivity, this process being an extremely slow one. For diminishing as much as possible the impurity content, the POSTC-PET melts are filtered during the mechanical recycling at extrusion, before passing throuth the nozzle, using

POSTC - PET requests a severe control of the contamination level especially if it is recycled into food packaging. Currently, the impurity content limits are established and generally accepted for POSTC-PET recycling as food and non-food packaging (EGPMFC, 1999; Franz,

acidolysis, amynolysis,etc. (Carta, 2001; Karayannidis, 2003; Minoru, 2003;);

procedure considered efficient for the enhancement of its properties.

**2. Parameters controlling the POSTC – PET mechanical recycling** 

However the results are not spectacular.

contamination level and the degradation degree.

*physical contamination and chemical contamination.*

impregnation due to improperly storage.

quality and productivity of the recycling.

particular filters (Yang Tang & Menachem, 2008).

**2.1 POSTC – PET contamination** 

mechanically recycled into performing products, it is compounded at melt processing.

During POSTC-PET conditioning and melt processing, the polymer is degraded by mechanical and thermal agents that act in the presence of water and oxygen. If during the first life the POSTC-PET is exposed to UV radiations rather than to thermo-mechanical and hydrolytic degradation, the photo-oxidation must be considered too (Cioffi et al., 2002; Chen et al., 2002; Raki et al., 2004).

The degradation occurs at the weakest thermodynamic links namely at the ester those between the terephthalic acid and diethylene glycol of POSTC-PET macromolecules (Sandi et al., 2005; Vasiliu et al., 2002). In figs. 1 – 5, the main reactions that characterize the PET thermo-hydrolitic degradation are exposed.

By thermal-oxidative degradation (fig.1, - Awaja & Pavel, 2005 ), the macromolecular chains break resulting in the formation of volatile products (i.e. acetaldehyde – fig. 5 Alexandru & Bosica, 1966 ) , 1.8 – 3 % cyclic and linear oligomers (fig.4 - Awaja & Pavel, 2005) and shorter chains with acid carboxylic and vinyl ester end groups. In hydrolytic degradation (fig.2 - (Awaja & Pavel, 2005), the mechanism is similar, with the difference that the end groups of the short macromolecules resulted from degradation are carboxylic acid and hydroxyl ester.

Fig. 1. PET thermal degradation mechanism with the formation of carboxyl acid and vinyl ester end group (Awaja & Pavel, 2005).

Overview on Mechanical Recycling

Pavel, 2005).

by Chain Extension of POSTC-PET Bottles 89

Fig. 4. Cyclic and linear oligomeric compounds resulted from PET degradation (Awaja &

Fig. 2. PET Hydrolysis mechanism resulting in carboxyl acid and hydroxyl ester end group appearance (Awaja & Pavel, 2005).

Fig. 3. The dependence of the carboxyl end group by the number of reprocessing of POSTC-PET (Spinace & De Paoli, 2001).

Fig. 2. PET Hydrolysis mechanism resulting in carboxyl acid and hydroxyl ester end group

Fig. 3. The dependence of the carboxyl end group by the number of reprocessing of POSTC-

appearance (Awaja & Pavel, 2005).

PET (Spinace & De Paoli, 2001).

Fig. 4. Cyclic and linear oligomeric compounds resulted from PET degradation (Awaja & Pavel, 2005).

Overview on Mechanical Recycling

melt processing degradation.

high performance properties.

**3.1 Drying /degassing** 

2000).

by Chain Extension of POSTC-PET Bottles 91

The result of the degradation reactions is a severe drop in the molecular weight which leads to the failing of intrinsic viscosity, melt strength and melt processability and finally, to poor usage properties and a low quality of the products obtained from reprocessed polymers. Because of the severe molecular weight diminishing during POSTC-PET reprocessing, the intrinsic viscosity may decrease from 0.72 dl g-1, the virgin polymer specific value, down to 0,04 - 0,26 dl g-1 (Raki et al., 2004; Zong Zhang et al., 2004;Seo et al., 2006; Cuberes et al.,

Because of the formation of shorter macromolecules as a result of the hydro –thermal degradation, the crystallization capacity of the POSTC-PET increases and its degradability becomes more pronounced. This process known as chemi-crystalization is a complex one because at the beginning it is a chemical one (diminishing the macromolecules length due to degradation) and in the end it is a physical phenomenon (crystallization of the shorter macromolecular chains) (Pralay, 2002;). As a result of an increased crystallinity, the glass transition (Tg), melting temperature (Tm), melting heat and density of the POSTC-PET are greater. Also because of the dependence of the crystallinity on the degradation degree, the colour of POSTC-PET can differ from transparent (un-degraded or poorly degraded), to

The strong degrading tendency during the melt processing is specific for all polycondensation polymers, not only for PET, and is observed in case of primary polymer melt processing too. The higher the molecular weight of the primary polymer the greater the

The structural changes resulted from degradation can be so dramatic that the melt processing of POSTC-PET may become not viable. It is therefore easy to understand why the mechanically recycling of POSTC-PET can consider only applications which do not require

Considering the general opinion according to which POSTC-PEC can be mechanically recycled only into low-property goods, it becomes clear the interest to find new economic solutions for the reprocessing of these materials into products with practical importance. In the last 20 years the researchers have been concerned in the up-gradation of POSTC-PET by increasing the macromolecular weight based on chain extension reactions (Cavalcanti et.al.,

The efficiency of these reactions is controlled by many factors. Their presentation begins with emphasis the importance to eliminate humidity by drying before melt processing and

Before the chain extension, the POSTC-PET is dried to remove the humidity. It was observed that drying before chain extension and degassing and /or operation under vacuum during chain extension are able to decrease the degradation of POSTC-PET during

translucent (small degraded) and opaque (great degraded).

**3. The chain extension up-gradation of POSTC-PET** 

2007, Awaja & Pavel, 2005, Villalobos et.al, 2002, Karaianidis, 1993).

to stabilize the POSTC-PET at melt processing.

Fig. 5. The acetaldehyde formation during PET thermal degradation (Alexandru & Bosica, 1966).

The increasing of the carboxylic end group with the number of the reprocessing of the POSTC-PET is presented in fig. 3 (Spinace & De Paoli, 2001, Silva Spinace, 2001). The end groups content puts on view the POSTC-PET degradation degree, the carboxyl end groups being correlated with the thermal and hydrolytic degradation and the hydroxylic end groups with hydrolytic ones.

CO O CH2 CH2 O CO

CH CH2

CH CH3

+ HO CH2 CH2 OH

O OC

CO O CH2 CH2 OH <sup>+</sup> HOOC

Fig. 5. The acetaldehyde formation during PET thermal degradation (Alexandru & Bosica,

The increasing of the carboxylic end group with the number of the reprocessing of the POSTC-PET is presented in fig. 3 (Spinace & De Paoli, 2001, Silva Spinace, 2001). The end groups content puts on view the POSTC-PET degradation degree, the carboxyl end groups being correlated with the thermal and hydrolytic degradation and the hydroxylic end

1966).

groups with hydrolytic ones.

+ HOOC

CO O OC <sup>+</sup> CH3 CHO

CO O

CO O

The result of the degradation reactions is a severe drop in the molecular weight which leads to the failing of intrinsic viscosity, melt strength and melt processability and finally, to poor usage properties and a low quality of the products obtained from reprocessed polymers. Because of the severe molecular weight diminishing during POSTC-PET reprocessing, the intrinsic viscosity may decrease from 0.72 dl g-1, the virgin polymer specific value, down to 0,04 - 0,26 dl g-1 (Raki et al., 2004; Zong Zhang et al., 2004;Seo et al., 2006; Cuberes et al., 2000).

Because of the formation of shorter macromolecules as a result of the hydro –thermal degradation, the crystallization capacity of the POSTC-PET increases and its degradability becomes more pronounced. This process known as chemi-crystalization is a complex one because at the beginning it is a chemical one (diminishing the macromolecules length due to degradation) and in the end it is a physical phenomenon (crystallization of the shorter macromolecular chains) (Pralay, 2002;). As a result of an increased crystallinity, the glass transition (Tg), melting temperature (Tm), melting heat and density of the POSTC-PET are greater. Also because of the dependence of the crystallinity on the degradation degree, the colour of POSTC-PET can differ from transparent (un-degraded or poorly degraded), to translucent (small degraded) and opaque (great degraded).

The strong degrading tendency during the melt processing is specific for all polycondensation polymers, not only for PET, and is observed in case of primary polymer melt processing too. The higher the molecular weight of the primary polymer the greater the melt processing degradation.

The structural changes resulted from degradation can be so dramatic that the melt processing of POSTC-PET may become not viable. It is therefore easy to understand why the mechanically recycling of POSTC-PET can consider only applications which do not require high performance properties.
