Recycling of Plastic Materials Part 10 docx

Since all finished fuel tanks are in fact fluorinated, the mechanical, rheological and thermal properties of the recovered material were compared only with the fluorinated samples.. Tors

the values determined for injection-moulded test specimens with those exhib-ited by the original tank material and those specified by the quality order since the test specimens were only 3 mm thick, compared with the average thickness

of a tank, which is between 5 and 8 mm

Degree of crystallinity

The degree of crystallinity of both the fluorinated and unfluorinated materials, both after one regrinding and one remoulding, lay within the provisions of the quality order

Melting temperature

The remelting temperature of fluorinated and unfluorinated materials, both after regrinding and after remoulding, lay below the temperature range stipu-lated by the quality order

Flow index

No difference could be detected between fluorinated and unfluorinated sam-ples evaluated by a spiral flow mould test

The recycling of material from used fuel tanks

Parallel investigations were carried out to establish how much fuel the tank material absorbs, and the time involved for the fuel to migrate from the material when no longer exposed to fuel

At a room temperature, samples punched from the original (fluorinated) tank achieved constant weight, with weight increases in the range 6.6-7.3% Since material recycling cannot take place without elimination of the fuel, drying tests were carried out under a range of conditions It was established that tempera-tures exceeding 100oC were necessary to eliminate all fuel

The material used for the investigation was obtained from Volkswagen Golf and Passat vehicles, which have employed plastic fuel tanks for some 10 years The time required for tank removal, cleaning, and elimination of tank compo-nents, made either of metal or a plastic other than HDPE, averaged some 5 min

30 sec At a labour cost of DM 45/h this implies some DM 4.2 for recovery of a fuel tank yielding 3.8 kg of HDPE with, at a price of about DM 1.8/kg for secondary HDPE, a nominal value of DM 6.9

Ballistic test

All test samples showed a ductile fracture It is, however, difficult to compare

perature of 230-260oC Subsequent material reprocessing caused no problems other than the unpleasant odour, which was absent from samples made from virgin material

Since all finished fuel tanks are in fact fluorinated, the mechanical, rheological and thermal properties of the recovered material were compared only with the fluorinated samples

Torsion test

The recovered material met the requirements of the quality order in respect of torsional properties

Tensile test

The moduli of elasticity of samples made from recovered material were much higher than those made from unused tanks Though perhaps partially attribut-able to cross-linking, it is more likely to reflect a change of specification of the Lupolen 4261A, whose modulus of elasticity was in 1978 given as 1200 N/mm2 and in 1989 as 850 N/mm2 The higher value of the modulus of elasticity does not necessarily constitute a poorer quality of material so far as the requirements of plastic fuel tanks are concerned

Charpy impact test

As before, none of the test specimens broke, so satisfying the criteria of the quality order It was noted that the impact properties did not deteriorate on re-cycling, as usually occurs

Ballistic testing

Since all specimens exhibited ductile fracture it was possible to compare used and unused polymers It was noted that, although the materials had been in ser-vice for ten years before recycling, the low-temperature impact properties had not deteriorated As before, it is difficult to compare the values determined for injection-moulded test specimens with those exhibited by the original tank ma-terial and those specified by the quality order since the test specimens were only

3 mm thick, compared with the average thickness of a tank, which is between 5 and 8 mm

M E Henstock and K Seidl 149 Before the tank was reground and reprocessed the fuel was extracted for 24 h

at 120oC, to avoid the risk that reprocessing might reach the fuel ignition

tem-Degree of crystallinity

The results fell within the±10% tolerance band of the quality order

Melting temperature

The melting temperature, evaluated by differential scanning calorimetry, was lower than that specified by the quality order

SUMMARY AND CONCLUSIONS

The monetary value of recoverable materials in road vehicles has fallen as plastics have replaced steel Increases have occurred in the cost of the labour for scrapyard dismantling and in the amount of unsaleable residue and, therefore,

in the associated disposal costs Improvements in scrapyard economics may pos-sibly be achieved by the prior removal from vehicles of large polymeric compo-nents and their recycling as well-characterised plastics fractions Some trials with material recovered from used plastic fuel tanks show promising results for the manufacture of new tanks

REFERENCES

1. M E Henstock, Design for Recyclability, London, The Institute of Metals, pp 3-6

(1988).

2 K C Dean and J W Sterner, Dismantling a Typical Junk Automobile to Produce Quality Scrap, United States Bureau of Mines, RI 7350, Washington, (1969).

3. M E Henstock, Conservation and Recycling, 2 (1), 69 (1988).

4. R Franklin, Recycling Cars, Materials Reclamation Weekly, 8 December, 21 (1990).

5. K D Marshall, The Economics of Automotive Weight Reduction, Soc of Automot.

Engrs, Paper No 700 174, Automot Engng Congr., Detroit, 12 January, 1970.

6. M C Flemings, K B Higbie, and D J McPherson, Report of Conf.: Energy

Conservation and Recycling in the Aluminum Industry, Massachusetts Institute of

Technology, (co-sponsored by the Center for Materials Science, M.I.T and the U.S Bureau of Mines, with the cooperation of the aluminium Ass.), 18-20 June, 1974.

7. Anon, The Energy Content of Plastics Articles, Association of Plastics Manufacturers in

Europe, Distributed in the United Kingdom by the British Plastics Federation,

Publication No.309/1, (April, 1986).

8 K Seidl, Development of a Total Energy Balance for the Manufacture of Fuel Tanks in Steel and Plastics, Unpublished calculations (1991).

9 K Muller, The Increasing Use of Plastics and Its Impacts on the Recyclability of

Automobiles and on Waste Disposal in West Germany, The United States and Japan,

Vortrag zur Recyclingplas II, Washington, D.C., 18-19 June, 1987.

Automobile Recycling, Univ of Wisconsin, 16 October, 1975.

11 The calculations are intended to illustrate only the absolute changes in quantities of recoverable metal with change in model year and to facilitate comparison by application

of metal prices at arbitrary times, i.e April, 1976 and December, 1986 No attempt has been made to determine the revenues obtainable by scrapping each model year after a predetermined period and then deflating prices to 1988 levels.

12 M E Henstock, Design for Recyclability, The Institute of Metals, London,

p 74 (1988).

13 K E Boeger and N R Braton, Resources and Conservation, 14, 133 (1987).

14 G R Daborn and M Webb, Treatment of Fragmentizer Waste by Starved Air

Incineration - a Brief Feasibility Study, Department of Industry, Warren Spring

Laboratory, LR 465 (MR) M, October, 1983.

M E Henstock and K Seidl 151

10 R W Roig, M Narkus-Kramer, and A L Watson, Impacts of Materials Substitution

in Automobile Manufacture on Resource Recovery, Symp., The Technology of

Ground Rubber Tire-Polymer Composites

K Oliphant, P Rajalingam, and W E Baker

Department of Chemistry, Queen’s University, Kingston, Ontario,

Canada K7L 3N6

INTRODUCTION

Discarded tires represent a significant component of the overall plastics recy-cling challenge They are an easily segregated, large volume part of the waste stream and present their own, somewhat unique, waste utilization problems The whole issue is complicated by many alternative proposals, varying govern-ment legislation and preferences, incomplete technical information, and eco-nomic uncertainties.1

A number of reviews have already discussed this overall disposal problem and examined many of the proposed approaches.1-5The major problem lies in finding approaches that are both economically and environmentally sound Some of the methods of utilizing scrap tires that have been investigated are: burning, pyrol-ysis, use in cleaning up oil spills, road surfaces, roofing materials, and play-ground surfaces (for details see above mentioned reviews) While some of these approaches have been put into practice, the scrap tire disposal problem is still clearly a case where supply far outstrips available uses, and new methods of uti-lization (or technological advances to extend existing ones) are clearly needed One area that has the potential to utilize large volumes of discarded tires is their use as a filler in polymer composites It is the problems associated with this approach, and the technological advances made in overcoming these problems, that are the focus of this review Unfortunately, much of the work in this area has been undertaken by industry and is not available in the literature The liter-ature which is available, however, is presented, along with an in-depth look at the work carried out in our laboratories

GROUND RUBBER TIRE COMPOSITE BEHAVIOR

Although the use of ground rubber tire (GRT) as a filler in polymer blends is a potentially attractive approach, it is fraught with a number of difficulties Gen-erally, when the large GRT particles are added to either thermoplastic or thermoset matrices there is a large drop in mechanical properties, even at rela-tively low filler loadings.6,7 Given that the approach here is to use the GRT as a low cost additive, and that there are a number of other materials competing in this regard, overcoming this large drop in properties has to be accomplished with little added cost (both in terms of additives and additional processing) This has proven to be quite a challenging task In the following sections the major fac-tors influencing thermoplastic GRT composite properties are discussed along with approaches to improving these properties

Tire Grinding

In order to be used as a filler in polymer composites, tires are first ground into a fine powder on the order of 100-400µm This is accomplished typically through either cryogenic or ambient grinding General reviews of the size-reduction pro-cess have been published.8-10A typical process4generally involves tire splitters

to cut the tire initially, followed by a two-roll grooved-rubber mill or hammer

154 Ground Rubber Tire-Polymer Composites

Figure 1 Ground rubber tire particles (left) cryogenically ground, (right) ambiently ground.

mill The bead wire is removed by hand or with magnets and fiber is removed at intermediate operations with hammer mills, reel beaters, and air tables that blow a steady stream of air across the rubber, separating the fiber

Between ambient and cryogenic grinding there is a noticeable difference in the nature of the ground rubber tire (GRT) particles As shown in Figure 1, the sur-face of the cryogenically ground rubber is smooth and regular (because the parti-cles are cooled below their glass transition temperatures before fracture) compared to the rough irregular surfaces of ambiently ground material There has been no complete study (to our knowledge) on the advantages (or disadvan-tages) of cryogenically versus ambiently ground GRT particles in polymer com-posites Such a study is complicated because of the influences of particle size, particle size distribution and contaminant levels on GRT-polymer composites, and the fact that these all vary from supplier to supplier It has been found, how-ever, that significant differences in composite properties are found for GRT of similar mesh (particle) size sourced from different suppliers and further study of particle characteristics is currently underway in our laboratories

Characteristics of Tire Particles

It is the complex nature of the GRT particle that complicates its use as a filler

in polymer composites The most pertinent feature of GRT particles is that they are still highly cross-linked This has two major consequences:

• there is little breakdown of the particles under normal melt compounding conditions (see section on particle size)

• there is a sharp interface resulting in poor adhesion between the GRT par-ticles and the matrix (see section on adhesion)

GRT particles are also compositionally quite complex Tires contain a number

of different rubbers (SBR, butyl rubber, natural rubber, polybutadiene rubber etc.), carbon black filler, antioxidants, and additional additives, the exact com-position depending on the type of tire and the part of the tire (e.g tread vs side-wall, vs liner) Elementally, a typical tire is comprised of carbon 83%, hydrogen 7%, ash 6%, oxygen 2.5%, sulfur 1.2%, and nitrogen 0.3%.1 There is approxi-mately 45-55% rubber hydrocarbon, 10-15% acetone extractables, 20-30% car-bon black, and 6% ash.11

Polymer Matrix

As this paper focuses on the use of GRT as a filler in thermoplastic systems the literature pertaining to thermoset systems will only be briefly reviewed In gen-eral, addition of GRT to rubber vulcanates reduces all physical properties, the extent of deterioration depending upon the amount and particle size of the GRT added.12-14The use of coupling agents15has been reported to improve the proper-ties of these systems GRT particles are also found to decrease the tensile, flex-ural and storage shear modulus in composites with an unsaturated polyester resin.16Very poor properties are again reported for a GRT-phenolic compound.15 Similar poor behavior for GRT-thermoplastic composites is often reported Deanin and Hashemielya17report on GRT composites made with six different polymers (HIPS, PP, HDPE, LLDPE, LDPE, and ABS) and five different elasto-mers (SBS, SEBS, SIS, butyl, and EDPM) The addition of the GRT reduced the tensile strength noticeably, and is not reported to provide for any increase in im-pact strength Poorer properties are reported for more brittle matrices The GRT

in this study was, however, broken down to some extent in a prior mastication step, reportedly enabling it to form into a “thin thermoplastic sheet” Its exact nature, compared to the still highly cross-linked large GRT particles typically employed in GRT composites was not reported however Tuchman and Rosen15 examined composites of GRT and PP, ABS, PS, LDPE, and HDPE Addition of GRT was reported to reduce all the mechanical properties of ABS, LDPE, and HDPE The Izod impact strengths of PP and PS, however, are reportedly in-creased (by up to three times for PP at 40 wt% GRT) upon the addition of GRT Phadke and De,18however, report that there is a decrease in the impact strength

of PP when GRT is added

Duhaime and Baker6report on the properties of LLDPE-GRT composites The impact strength is seen to drop by 35% even at 10 wt% filler loading The impact strength continues to drop until at 30 wt% filler it is 50% lower than that of the pure LLDPE It remains approximately constant, however, from 30-60 wt% GRT The impact behavior was characterized using a Rheometrics drop-weight instrumented impact tester, in which a piezoelectric load cell is tip-mounted to a high velocity dart permitting the recording of the load-displacement curve for the entire impact event For pure LLDPE, impact failure is seen to be a ductile yielding process in which the dart draws the material out as it passes through.19

For the GRT-LLDPE composites the failure is similar except that ‘yielding‘

oc-curs at much lower forces and elongations, and the material is not drawn out to

156 Ground Rubber Tire-Polymer Composites

as high elongations because of premature failure caused by the large, poorly bonded rubber particles In tensile tests, the tensile strength and ultimate elon-gation are seen to decrease steadily with increasing GRT content At 40 wt% GRT the tensile strength has decreased by 48% and the tensile elongation by 80%

Oliphant and Baker7report on blends of GRT with LLDPE and HDPE The ad-dition of GRT to LLDPE results in composites with properties similar to those described by Duhaime.6The deleterious effects of the GRT particles on HDPE is, however, more pronounced than for blends of the GRT with LLDPE (70% de-crease in impact strength for HDPE compared to 50% for LLDPE at 40 wt% GRT), although the observed trends are similar In contrast to the failure of LLDPE described previously, the failure of pure HDPE, although it involves some plastic deformation, is observed to occur through catastrophic propagation

of a crack through the impact zone This type of failure is also observed in the GRT-HDPE composites (compared to the ductile tearing process observed for GRT-LLDPE composites) It is this difference in impact failure which is sug-gested to be responsible for the poorer properties of GRT-HDPE composites It is postulated that in HDPE-GRT composites the failure remains semi-brittle be-cause the particles are too large to induce a brittle to ductile transition (or a shift

in the brittle to ductile transition temperature to below the test temperature) Failure then occurs largely through crack propagation and the large particles

Table 1

Influence of melt flow index on the impact energy of LLDPE/GRT composites

Pure PE 40 wt% GRT

*LDPE

act as serious flaws, providing an easy path for the crack to follow The addition

of GRT to a semi-brittle matrix is therefore believed to require much higher lev-els of adhesion (to retard crack growth at the particle/matrix interface), or much lower particle sizes (to lower the brittle-ductile transition temperature) This is borne out to some extent experimentally (see reference 7 and section on adhe-sion)

Rajalingam and Baker20have studied GRT composites with a number of differ-ent LLDPE’s and a LDPE The impact properties of the pure matrices and their corresponding 40 wt% GRT composites are given in Table 1 Although there is some variation in the percent drop in impact strength with MFI, the addition of GRT is shown in all cases to have a similar influence on mechanical properties There are two exceptions of note: the higher molecular weight 1 MFI LLDPE ap-pears to produce composites with lower material property drop, and the reduc-tion in impact strength for the LDPE is slightly higher than for the LLDPE composites Deanin and Hashemiolya17have also reported that LDPE produced poorer GRT composites than LLDPE It is also interesting to note that the LLDPE of MFI = 1.0 produces a 40 wt% GRT composite with slightly higher im-pact strength than for pure LLDPE’s with MFIs of 12 and 20 dg/min Thus, if the higher viscosities of the composite can be tolerated in processing, these materi-als may prove to be useful composites

In general then, it is seen that simple addition of GRT to most polymers by melt blending results in a significant deterioration in mechanical properties

Particle Size

The large rubber particle size used in GRT composites is reported to be one of the two major factors (the other being adhesion) contributing to the poor me-chanical properties generally observed for GRT-polymer composites The impor-tance of particle size (and particle size distribution and shape) on mechanical properties of composites in general is well known in the literature.21For rubber toughening applications it is generally reported that there is an optimum parti-cle size (typically in the 0.1-5µm range) for toughening brittle polymers,21and a minimum particle size (or inter-particle distance) for toughening semi-ductile polymers (typically less than 1 µm).22 For hard particulate fillers, impact strength is generally observed to increase with a decrease in particle size In general, for optimum composite properties, a low particle size is desired In GRT-polymer composites, however, the particle size is (relatively) quite large

158 Ground Rubber Tire-Polymer Composites