Francesco P La Mantia Pvc and Mixed Plastics

Francesco P La Mantia

Tue, 09 Aug 2011 14:50:26 | Pvc and Mixed Plastics

Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universita di Palermo, Viale delle Scienze, 90128 Palermo, Italy

Mechanical properties of secondary plastic materials obtained by recycling of post-consumer plastic containers for liquids are very poor. This is due to the incompatibility between component polymers (PET, PVC, and PE) and degradation of components during the heterogeneous reprocessing. The use of different classes of additives, such as stabilizers, inert fillers, elastomeric modifiers, and compatibilizers can improve the processability, enhancing the thermomechanical resistance of the polymers, and the mechanical properties.

Inert fillers reduce the cost of the recycled plastics but the mechanical properties, although enhanced, remain insufficient for secondary applications. Elastomers can remarkably improve some mechanical properties. Some compatibilizing action is contributed by the functionalized polyethylene and sty-rene-butadiene-styrene rubber and CaO coated with organo-titanates. Finally, good results are obtained for blends produced from this mixture and recycled polyethylene.

INTRODUCTION

The collection of plastic post-consumer containers for liquid is relatively simple to organize and can be an inexpensive source for recycling of large amounts of plastic materials. Most plastic containers are made out of three polymers, namely, high density polyethylene, HDPE, polyvinylchloride, PVC, and polyethyleneterephthalate, PET.

The presence ofthese three polymers in the collection ofplastic containers creates difficulty in recycling of such mixed plastic materials. The present technologies are based on separation of polymers which are then recycled as single polymers. In general, these recycling plants include different stages: separation of HDPE by flotation, identification of PVC and PET containers, automatic sorting, washing, and micronization.1

The difficulties arising in recycling of mixture of HDPE, PVC, and PET are due to the incompatibility of these polymers, their different melting points, ranging from 130 to 260oC, and thus different processing temperatures, and degradability of PVC at temperatures above 200oC.

Possible industrial recycling processes of mixed plastics wastes are based on fast extrusions of these mixtures, having PE as continuous matrix, in ad hoc designed equipment to produce finished products in a single step.2

A new process3 is based on the homomicronisation of the polymer mixtures to obtain a homogeneous new thermoplastic material.

When small amounts of PET are present in the mixture, the processing can be carried out at temperature below the melting point of this polymer which acts as a solid filler.4'5 No significant degradation of PVC is observed under these conditions.

In previous papers,6-8 it has been demonstrated that by adding polyethylene (or recycled polyethylene) to a plastic mixture coming from collection of containers for liquids, these mixtures with PE content larger than 50% can be extruded in conventional machineries at high temperatures. The degradation of PVC is controlled, at last in part, by adding stabilizing agents and by using suitable processing conditions.

The mechanical properties strongly depend on composition, and for blends with PE content larger than 65% are similar to those of the recycled polyethylene apart for the elongation at break.6 7 Mechanical properties, and in particular elongation at break and impact strength, can be further improved by adding suitable modifying agents.8

Aim of this work is to study the processing in a conventional single screw extruder of mixed plastics wastes coming from a collection of containers for liquids without any blending with polyethylene and to characterize the resulting secondary materials. The influence of the processing conditions and the effect of stabilizing agents, fillers, modifiers, and compatibilizers are also considered.

EXPERIMENTAL

MATERIALS

The post-consumer plastics containers for liquids, PCL, used in this work have the following composition: PET « 45%, PE « 35%, and PVC - 20%. This composition is representative of the average composition of the plastic fraction obtained in 1992 by obligatory separate collection of containers for liquids in Italy. This composition is different from that obtained in previous years and used in previous papers.6-8 The difference reflects a lower consumption of PVC for mineral water bottles replaced by PET, whose concentration in this separate collection is continuously increasing. The PE content, mainly HDPE, especially used for detergents and cosmetics bottles, is almost constant.

The plastics waste was reduced to small flakes in rotating knives mill.

The thermal stabilizing agents used in the work are reported in Table 1.

Table 1: Thermal stabilizers
Stabilizer	Supplier	Concentration (%)
Irgafox B603	Ciba-Geigy	0.1
Tribasic lead sulfate	-	0.5

Irgafox B603 is a blend of three antioxidants, namely Irganox 1010, Irganox 1076 and Irgafox 163, suitable for polyethylenes and polyesters. Tribasic lead sulfate is a typical stabilizer for PVC.

To improve the processing behavior of the mixtures and to reduce the mechanical stress in the melt, which can enhance the degradation of the polymers, two lubricants, see Table 2, were used throughout.

Table 2: Lubricants
Lubricant	Supplier	Concentration (%)
P520	Hoechst	0.15
Calcium stearate	Ciba-Geigy	0.15

Calcium stearate is an external lubricant, while P520 is an apolar low molecular weight polyethylene wax and then an internal lubricant.

Table 3: Modifiers
Modifier	Code	Supplier
Kraton G1650	SEBS	Shell
Vistalon 3708	EPDM	ESSO
Paraloid EXL 3647	MBS	Rohm & Haas
FC 45	EVA	Enichem
Table 4: Compatibilizers
Compatibilizer and its code		Supplier
OCT/AB		Micromin
OCT/CH		Micromin
SEBS-g-MA		Shell
PE-g-MA		Enichem

The inert fillers added to the plastics waste were calcium carbonate, sawdust and glass fibers. CaCO3 is a powder of about 30 ^m in diameter. Sawdust was obtained by milling of wood in a blade mill through a 20 mesh grid. The glass fibers had D = 10 ^m and length-to-diameter ratio of about 400.

Whereas the fillers have been used especially to reduce the cost of the secondary materials, the modifiers reported in Table 3 have been used to improve some mechanical properties.

Kraton G is a SEBS thermoplastic elastomer with a middle block of hydro-genated polybutadiene and molecular weight of about 64,000. Vistalon is an EPDM rubber with a molecular weight of about 278,000. Paraloid is a methylmethacrylate-butadiene-styrene, MBS, elastomer with a core-shell structure.

FC 45 is a EVA sample with about 14% of EVAc.

Finally, four compounds, see Table 4, have been used in the attempt to enhance the adhesion between different phases of the mixtures.

The two OCT samples are small particle size (less than 10 ^m) CaO coated with organo-titanates. In particular AB is coated with neopentyl(diallyl)oxy, tri(dioctyl)phosphato titanate, whereas CH is coated with neopentyl(diallyl)oxy, tri(dioctyl)pyrophosphato titanate.

PE-g-AM is a linear polyethylene functionalized with maleic anhydride. Melt index is 2.6 and the MA content is about 1.8%.

SEBS-g-MA is a SEBS elastomer functionalized with maleic anhydride commercially known as Kraton FG 1901.

PROCESSING

All extrusion runs were performed with a laboratory single screw extruder (D=19 mm, L/D = 25) fitted with a venting port. The blends were mixed by hand and fed to the extruder. Most of the extrusion runs were done with a temperature profile of200-230-240-260oC and a screw speed of 80 rpm. Other runs were carried at a screw speed of 20 rpm.

The extruded materials were cooled in a water bath, granulated and extruded a second time to achieve a good homogenization. As already shown in previous works,6-8 this procedure gives rise to the best balance between degradation and homogenization.

MECHANICAL PROPERTIES

The sheets for the stress-strain curves were obtained by compression molding in a laboratory press at 260oC.

The bars for the impact tests were obtained by injection molding in a laboratory molder (Mini Max molder CS 183, Custom Scientific, USA). In both cases the temperature was 260oC.

Stress-strain curves were obtained with an Instron model 1122. Impact tests were carried out in Izod mode with a CEAST Fractoscope. The results were averages of at least seven measurements.

PVC DETERMINATION

In order to determine the undegraded PVC content in the recycled material, about 0.2 g of the extruded mixtures were put in a glass tube, heated up to 280oC at a heating rate of about 5oC/min and held at this temperature for about 15 min under a slight flow of nitrogen. The evolved gases were absorbed in water and the resulting HCl titrated with NaOH.

The amount of HCl is directly related to the content of undegraded PVC in the extruded material.

: Figure 1. PVC degraded during processing of PLC and PLC with different additives. The dashed area represents the scatter of the experimental results.

RESULTS AND DISCUSSION

EFFECT OF STABILIZERS AND LUBRICANTS

Figure 1 reports the amount of PVC degraded during two extrusion steps for PLC and its mixtures with all stabilizing and lubricating additives. When no stabilizing agent is used, the degradation of PVC is quite extensive: only slightly more than a half of the initial theoretical PVC content in the raw material is found in the recycled material. The lubricants do not improve significantly the stability of PVC which is only slightly enhanced by the antioxidants. This is somehow surprising since these antioxidants are typical stabilizers for polyolefines, PE, and not for PVC. In explanation of this result, one must take into account the possible interactions occurring during degradation of blends.9-11 The degradation of PVC can be enhanced by the formation of polyethylene radicals, produced by thermomechanical stress but stabilized by these antioxidants.

The slight stabilizing action of the lubricants can be attributed to a lower mechanical stress acting on the melt during processing. The mechanical stress can act, indeed, as a "catalyst" of the thermal degradation increasing the level of the degradation.

An efficient stabilizing action is contributed by the lead compound (which is a typical stabilizer of PVC) and a mixture of antioxidants and lubricants with lead salt. With these stabilizing systems, about 80% of the initial undegraded PVC has been measured in the recycled material.

: Figure 2. Elastic modulus of the samples shown in Figure 1.

Figure 3. Tensile strength of the samples shown in Figure 1.

During processing, both PE and PET can also undergo severe degradation processes. In particular, PE can be degraded by thermomechanical stress and PET by hydrolytic chain scission since the material is not dried before process ing. It is not possible, however, to examine these processes of degradation, and only the degradation of PVC, the most severe and dangerous, was considered.

: PLC Antio* tub Pb Antiox+Pb Lub+Pb Antiox+Lub+Pb

Figure 4. Elongation at break of the samples shown in Figure 1.

PLC Antiox Lub Pb AntioxtPb Lub+Pb Antiox+Lub+Pb

Figure 5. Impact strength of the samples shown in Figure 1.

The above discussion refers to the extrusion steps performed at a die temperature of260oC and screw speed of 80 rpm. By changing them, for example, increasing temperature and decreasing screw speed, the degradation of PVC

becomes more rapid. After extrusion carried out at a die temperature of 260oC and 20 rpm, small amounts of PVC were detected in the material. Decreasing rotation speed from 80 rpm to 20 rpm, the residence time in the extruder increases from about 3 min to about 10 min.

The mechanical properties of these materials reflect, at least in part, the amount of PVC degraded. In Figures 2-5 the elastic modulus, E, the impact strength, IS, the tensile strength, TS, and the elongation at break, EB, are reported for all samples shown in Figure 1.

Due to a strong incompatibility and severe degradation of the components, the mechanical properties of PLC are very poor and much lower than those expected on the basis of an additivity rule for the mixture components.

Table 5: Mechanical properties of the PLC mixture and individual components
Sample	E, GPa	TS, MPa	EB, %	IS, J/m
HDPE	5-6	22-24	600-700	750-800
PVC	13-16	6-8	30-40	60-90
PET	12-14	33-36	350-450	40-50
PLC	5-6	4-4.4	1.3-1.6	20-22

In Table 5, the mechanical properties of the individual components (bottle grade materials) are reported together with those of the recycled mixture. All mechanical data of PCL are below those evaluated according to the additivity rule, but elongation at break and impact strength are exceptionally low, indicating a lack of adhesion between the phases.

Regarding the mechanical properties of the stabilized mixtures, modulus, impact strength, and elongation at break show improvements up to 25%, but their values remain low, in particular elongation at break and impact strength.

The above results suggest that an appropriate stabilizing system should be used to improve properties of the recycled material and in particular to avoid the degradation of PVC and HCl evolution. HCl can be entrapped in the material, reducing its resistance and contributing to the pollution of environment.

The next extrusion runs were carried out using the following stabilizing system: lead stabilizer = 0.5 wt%, B603 = 0.15 wt%.

PLC PLC+CaC03 PLC+Sawdust PLC+GF

Figure 6. Elastic modulus of PLC and PLC with 10% inert fillers.

PLC PLC+CaC03 PLC+Sawdust PLC+GF

Figure 6. Elastic modulus of PLC and PLC with 10% inert fillers.

: 12 il

PLC PLC+CaC03 PLC+Sawdust PLC+GF

Figure 7. Tensile strength of PLC and PLC with 10% inert fillers.

PLC PLC+CaC03 PLC+Sawdust PLC+GF

Figure 7. Tensile strength of PLC and PLC with 10% inert fillers.

FILLERS

Inert fillers are used in the thermoplasts to improve some properties ofthe polymers but, in particular, to reduce the cost of finished products. To estimate both effects three inert fillers have been tested. Figures 6-9 report the mechanical properties of the mixture filled with 10% calcium carbonate, glass fibers, and sawdust. All these fillers give rise to a small increase of modulus and different effects on the other mechanical characteristics. Glass fibers are very effective in improvement of all properties, in particular impact and tensile strength are remarkably enhanced. Significant improvements of the tensile strength are also observed by adding CaCO3 and sawdust.

: Figure 8. Elongation at break of PLC and PLC with 10% inert fillers.

: Figure 9. Impact strength of PLC and PLC with 10% inert fillers.

Figure 10. Elastic modulus as a function of fillers content. 14 12 10 8 6 4' 2 0

Figure 11. Tensile strength as a function of fillers content.

The effect of the concentration on the mechanical properties can be observed from Figures 10-13. All fillers show qualitatively similar results, namely, the mechanical properties increase up to a content of about 20% then remain constant or decrease. Concentrations of CaCO3 and sawdust as high as 20% are economical considering a very low cost of these materials, whereas more expensive glass fibers can be used at a level of 10% to remarkably improve the mechanical properties at low cost.

The positive effect on the mechanical properties and on the cost ofthese fillers is counter-balanced, at least in part, by an increase in the viscosity of the mixture and then by the increase of the extrusion power or by a reduction of the

Figure 11. Tensile strength as a function of fillers content.

output flow rate. The presence of 10% fillers reduces the flow rate of by 25-30% for all investigated fillers, Figure 14. Increasing the filler content a further reduction is observed, Figure 15, but this reduction is very small compared with that observed at a concentration of 5%. This result suggests that at shear rates experienced in the extruder the viscosities of the filled melts are very similar, remarkably larger than that of the unfilled molten mixture and almost independent of the type and content of filler.

	n Glass Fibers
	« Sawdusl
■ fii A"	• CaCCG
M f	• CaCCG
r Filler, %

Figure 12. Elongation at break as a function of fillers content.

- s -
. co /
/	0 Glass Fibers * Sawdust
	■ CaC03
	Filler, %

Figure 13. Impact strength as a function of fillers content.

MODIFYING AGENTS

Better results have been observed by adding 10% of the above described modifier agents, Figures 16-19.

: Figure 14. Output flow rate for PLC and PLC with 10% inert fillers.

Q, g/min
Q, g/min	» CaCOS
	* Sawdust
	■ Glass Fibers
« i

0 10 20 30 40 50 Figure 15. Output flow rate as a function of the filler content.

The elastic modulus, Figure 16, decreases and this result was expected from a low modulus of these rubbery compounds. Because of the same reason, significant enhancement of the other properties is observed. SEBS and EPDM remarkably improve tensile strength, elongation at break, and impact strength, Figures 17 to 19, respectively. Elongation at break, in particular, is low and all the samples show a fragile behavior.

These modifying agents do not act as compatibilizing agents. The improvement of the elongation at break and impact strength of the mixture is mainly due to the inherent properties of the elastomers. The SEM micrograph of the system with EPDM, Figure 20a, does not show a better adhesion between the phases than that observed in non-modified PLC, Figure 20b.

: Figure 16. Elastic modulus of PLC and PLC with 10% modifiers.

Figure 17. Tensile strength of PLC and PLC with 10% modifiers.

COMPATIBILIZERS

Some improvement of the mechanical properties is also observed on addition of compatibilizers, Figures 21-24. The grafted polymers have been used successfully as compatibilizers for blends of polyolefines and polar polymers such as PET and PA-6, and the coated CaO samples gave interesting results in the recycling of PET/HDPE blends.12

: Figure 18. Elongation at break of PLC and PLC with 10% modifiers.

Figure 19. Impact strength of PLC and PLC with 10% modifiers.

Two CaO samples induce a significant improvement of modulus and tensile strength, but only a modest improvement of impact strength. The two grafted samples, and in particular the SEBS rubber, are particularly effective for improving tensile and impact strength. The mechanical properties are enhanced by adding these materials and these improvements can be, in this case, attributed to a better adhesion between the three phases. SEM micrographs of PLC with OCT/CH and grafted PE, Figures 25 a and b, show, indeed, a more homogeneous morphology compared with that of the non-compatibilized PLC, Figure 20b. In particular, a minor number of voids is observed, and, moreover, the dimensions of the dispersed particles are slightly lower.

: Figure 20. SEM micrographs: (a) PLC with 10% EPDM, (b) PLC.

COMPARISON BETWEEN DIFFERENT SYSTEMS

In this paragraph, the results of the above discussed systems are compared with those for a blend obtained by adding 50% of recycled polyethylene, RPE, to the PLC mixture. RPE is mainly composed of low density polyethylene coming from recycling of greenhouse films. This new system has been investigated considering the results already obtained for similar compositions.6'7