Thermoplastics revisited - second time around for auto parts

TWI Bulletin, September - October 1996

Kalpana Mistry joined TWI in November 1995 from the British Plastics Federation, where she gained valuable insight into the plastics industry. She is currently working on welding research programmes for the automotive industry

This industry review aims to assess the current state of thinking in Europe with regard to recycling of plastics in the automotive industry. Kalpana Mistry highlights the influence of welding and joining on plastics recycling issues.

Worldwide production of plastics has been increasing constantly since the advent of new synthetic materials in the 1930s. From 100 000 metric tons in 1930 to figures now in excess of 80 million metric tons. [1] Meeting the demand for tougher, stronger and lighter engineering materials has led to the development of new blends and reinforced grades of almost every generic polymer type. As a major user of engineering thermoplastics, the automotive industry has enjoyed the technical and commercial benefits offered by plastics. Demands for lighter weight vehicles to aid fuel economy requirements, and also increased occupant safety from minor collisions, has led to the predominance of protective plastic front and rear bumper systems.

The true relationship between plastics and cars began as a result of the 1973 oil crisis. The escalating cost of oil forced designers to re-examine the factors influencing car manufacture. The problem was overcome by replacing metal trim used for external applications with plastic parts, thereby reducing vehicle weight and improving fuel economy. As designers gained confidence in identifying plastics alternatives for components made with traditional materials, more plastics-based items were introduced and finally established into the metal dominated material loop.

Increased use of plastics led to new polymer processing methods to manufacture items such as injection moulded bumpers, Fig.1, and blow moulded fuel tanks. Novel fabrication techniques in welding and joining followed suit. An early example of a welded structure includes the two part plastic bumper joined by the resistive implant method, Fig.2.

The environmental impact of using plastics material was not immediately apparent. Designers are now facing the consequences of material selection decisions made at least ten years ago, and questions concerning disposal options for end of life vehicles (ELVs) have arisen. Dismantling ELVs into constituent materials raises many technical and economic issues. Metal content recovery has been long established, and accounts for 77% of a vehicle by weight. [2] Disposal of dismantled plastics components including welded parts, which comprises 7% of the total vehicle weight, presents a different picture for every option considered. There are five basic options available.

• reuse
• landfill
• incineration
• pyrolysis
• recycling

Reusing components is limited by quality and condition of parts. Landfill is becoming increasingly scarce and expensive. Incineration and pyrolysis both require heavy capital investment and thorough environmental consideration. Recycling is complicated and expensive; paying a premium at each stage of collection, separation and identification.

Disposal

Both the automotive and plastics industries are aware of their shared responsibility to dispose of plastics waste from ELVs in a safe but economic way. They also face pressure from government legislation and public concerns over natural resource conservation and preservation of the environment. Many pilot projects have been commissioned to address the problems associated with disposal and every investigation reveals limitations to the methods considered. As yet, there is no easy answer.

Reuse

Undamaged automotive parts from scrapped vehicles which can be resold as spare parts are removed. Charlton Recycled Autoparts in Cambridge specialises in carrying out this component recovery process on Ford vehicles, Fig.3. Parts are cleaned and logged into a computer database. Details such as car model, year and serial number are recorded. Up to 19% of the ELV weight is currently recovered by this route. [2]

Landfill

At present, landfill sites are the most common waste disposal method, with incineration coming a close second. [3] Initially a cheap option, landfill sites are becoming progressively less available and correspondingly more expensive. Governments are introducing compulsory payments in the form of tax to raise revenue justified by the demand for space. However, landfill space will always be required to dispose of inert waste from which no further value can be recovered.

Incineration

Incineration is a popular method of waste disposal in Japan and Switzerland where the terrain is unsuitable for landfilling. Two main criticisms of the process claim it is wasteful of materials and releases gaseous emissions, [3] it can be justified where other methods would use more energy resources than energy recovered. The problem of dioxin emissions was solved by redesigning the incinerator plant and using carbon-bed filters.

Modern incinerator plants are designed to recover energy from plastic waste. Plastics have a high calorific value making them energetically equivalent to brown coal. [4] The main disadvantage of incineration is the expense of building the plant.

Pyrolysis

Pyrolysis is the thermal decomposition in the absence of air or oxygen. It is carried out in a fluidised bed at temperatures ranging from 400-800 deg.C. Under these conditions, plastics are not burned but reduced to simple petrochemical raw materials. No waste materials are created in pyrolysis. Useful products can be generated from a mixture of polyethylene (PE), polypropylene (PP) and polystyrene (PS) in the ratio 3:1:1. Typical output consists of 40-60% gas (methane, ethane and propane) and up to 50% liquid in the form of light paraffins and tar.

Studies show that pyrolysis products are more valuable than the energy obtained from burning plastics in incinerators. Another process similar to pyrolysis is hydrolysis. The process uses high pressure steam to hydrolyse certain plastics back to their original constituent materials. Both pyrolysis and hydrolysis require considerable capital investment and need the reassurance of a reliable market for their products. [5]

Recycling

Recycling is essentially a two stage process. The first step is reclamation of collected material, then conversion of reclaimed material into a form or product to be used again. [6]

Reclaimed plastics need to be stored by material type and compacted before they are supplied to a recycling company. Sorting and compaction takes place at Material Reclamation Facilities (MRFs). Most recycling companies will only handle materials which consist essentially of clean single polymer type.

The plastic is then converted into granulate. This process ensures the end material has undergone sieving and magnetic extraction of any ferrous material. The plastic may also undergo a recompounding process - a melt extrusion process which allows the incorporation of additives such as pigments and stabilisers. The melt is filtered through a fine metal screen before being pelletised and cooled. Sophisticated processing plant exists for materials which are not of clean single polymer type. Such plants are able to separate some plastics from others by using flotation techniques and hydrocyclones. UK plants of this type currently recycle polypropylene from automotive battery cases and recycle polyethylene film from agricultural applications. [6]

Many polymer suppliers now offer both virgin and recycled grades of plastics. The recycled version comes with information sheets stating mechanical properties and differences in processing characteristics. Recycled plastics material retains many of the original characteristics of virgin plastics. Most studies have revealed that recycling increases the melt flow properties and causes some reduction of key mechanical properties. [2] A bag of recycled material may contain scrap from many sources; from a polymer producer, a customer's production process and post-consumer material. In addition, this combination can be modified with some quantity of virgin polymer. [7] These recycled plastics are prohibited from food contact and safety critical applications.

Recycling in the automotive industry

History of plastics in cars

Plastics and plastics based composite materials play a key role in the construction and operation of modern motor vehicles. Plastics are used because they exhibit many desirable engineering properties:

• high strength to weight ratio
• good corrosion resistance
• excellent insulation properties
• low processing costs

These properties have led to an increase in the amount of plastics used per vehicle from less than 2% by volume before 1950 to an average above 20% today, representing over 100kg weight in a present day car. Increased use of plastics followed the 1973 oil crisis. Fuel prices quadrupled, targets for the reduction of fuel consumption were set by CAFE (Corporate Average Fuel Economy) regulations in the USA. The automotive industry responded by replacing dense metal parts with lighter weight plastics components. [8]

Further requirements of the FMVSS (Federal Motor Vehicle Safety Standard) part 581 of safety from minor collisions at 8 km/hr frontal impact and 4.8 km/hr at 30 deg. corner impact resulting in no damage, led to the development of front and rear plastics bumpers. Plastics began to replace many traditional materials like metal, leather and paper in key areas of application, noticeably for interior trim, Fig.4, exterior body parts and interior engine under bonnet parts. The amount of joining required for possibly larger or more complex plastics shapes, is less than with metal fabrication. Examples of welded structures in automotive applications are shown in the Table and Fig. 5.

Welding plays an important role in fabricating of major automotive components. Most of the processes highlighted create permanent joints without using a consumable, producing a homogeneous weld between the two parts. At the end of the vehicle's useful life, the dismantling process takes these factors into account. The plastics and automotive industries both realise that proper treatment of ELVs can only be achieved by creating practical systems for collection, sorting and processing of plastics.

Collection, separation and identification

Collection

Collection of plastics from ELVs is subject to the basics of recycling. Scrap must be clean and from a single polymer type. This implies that sorting of different plastics obtained from ELVs needs to be carried out prior to collection. The collection of scrap should be from authorised disposal locations only. The last owner of an ELV faces the responsibility of disposing the vehicle at specified disposal centres.

Separation

The ease of dismantling and separating ELV scrap into constituent materials is directly proportional to the cost of the process. In economic terms, the car is not worth dismantling past the 30 minute barrier, [2] that is, if the car takes longer than 30 minutes to dismantle the cost is prohibitive ( Fig.6).

Ease of dismantling increases with use of push-fit assemblies ie with no adhesives or welds. However, this ignores the obvious advantages offered by modern welding and joining techniques and their importance in automated assembly. Ford Motor Company are targeting future car designs with the motto 'Design for Disassembly'. Ford believe that reducing the dismantling time will reduce the cost of the recycling operation, which in turn will lower the cost of recycled material. [3] In the case of welded parts, those components joined without the aid of a consumable are in effect single polymer types, and hence can be collected as such.

Exceptions to this rule are welded parts consisting of two or more different polymer types ie the front and rear light units which consist of a PMMA or PC lens welded without a consumable by hot-plate or ultrasonics to an ABS body. The joint is permanent and the part would be considered too small to separate. The most economic solution points to landfill. However, recent research work in Germany has successfully granulated the whole unit and reprocessed the material to manufacture a new light body from blended material, to perform the role previously filled by ABS (this is possible because ABS and PMMA are compatible plastics).

Another exception is welded parts containing a consumable in the component structure. For example, the use of resistive implant braid in bumpers introduces a metallic component into an otherwise plastics part. Linear vibration welding the two parts together effectively produces a bumper of a homogeneous polymer composition.

With regard to weldability of recycled plastics, recent studies by Lucas Automotive and Cookson Group established that using recycled PP to make new car battery cases had little or no effect on the weldability of the plastics (using hot plate welding). [4] TWI research also shows that ultrasonic welding of recycled parts has little or no effect on weldability.

Before dismantling the entire ELV to separate the materials, decisions need to be made with regard to the length of the dismantling process, and whether it is economically viable. Identification plays a crucial part in these decisions.

Identification

Motor manufacturers are assisting the dismantling process by stamping or making plastics parts with acronyms for identification. Identification of polymers is now a key issue and dismantler manuals have been developed to help the process. [2]

Existing UK recycling schemes

ACORD (The Automotive Consortium on Recycling and Disposal) involves major UK manufacturers and vehicle scrapping processors. ACORD was established with the objective of developing new and improved technologies for vehicle recycling and disposal. In effect, ACORD is the political arm of the UK recycling effort. It was formed by the SMMT (Society of Motor Manufacturers and Traders). CARE (Consortium for Automotive Recycling) is a group of car manufacturers and dismantlers working together to improve environmental standards.

The overall management and co-ordination of CARE activities is based at the Rover Group.

German activities in automotive plastics recycling

The German government has passed legislation with prohibitive tariffs aimed at the prevention of waste, by re-using and recycling. In the case of plastics, there is still uncertainty as to which particular recycling process (material, chemical or thermal) is the most environment friendly and cost effective option.

In August 1992, a draft regulation relating to the disposal of ELVs was issued by the Environment Ministry. In essence, the draft requires vehicle manufacturers and retailers to reclaim ELVs for recycling. PRAVDA (Project gruppe Autoverwertung der Deutschen Automobilindustrie) involves Germany's leading car manufacturers, plastics raw material suppliers, dismantlers and recyclers at six sites across Germany. The PRAVDA scheme is a pilot project which explores the logistical and technical feasibility of dismantling, sorting and potential usage of separated plastics parts. A major objective of PRAVDA is to establish secondary uses for material recycled from scrap components within the automotive sector.

EU proposal for a directive on ELVs

The directive [9] proposes to improve the existing situation of material waste, putting the main responsibility with the car producer. The latest version (September 1996) of the draft which is awaiting comment by the member states, details tougher targets. It includes a proposal to eliminate PVC materials in all vehicles from 1 January 2002.

The proposal highlights key elements concerning ELV disposal including:

• collection
• treatment
• recovery
• reporting

Collection

The EU encourages setting up systems for collecting ELVs by car producers, treatment facilities (collection/dismantling centres) and public authorities.

The draft suggests that collection of an ELV from the last holder would be validated by a responsible dismantler. Authorised treatment facilities would present the last holder with a deregistration certificate to confirm collection.

It also proposes that the last owner of an ELV should discard the vehicle free of charge; costs induced could be transferred to the new vehicle on the market. However, these two statements present problems with controlling collection operations.

Treatment

The EU draft would like all ELVs to be made environmentally sound by standardised procedures at the authorised dismantling centres. These centres will be subject to the proposed manufacturers guidelines for standardisation and information as described in the directive. These dismantling guidelines can be interpreted as:

• Technical requirements for siting of ELV depollution stages. This means designated sites for dismantling and storage areas for removed fluids such as brake fluid, fuel and engine oil.
• Recommended procedures for treatment of ELV at authorised dismantling centres only.
• Provision of dismantler manuals informing treatment facility of hazardous substance location on ELV and also the materials used for constituent vehicle parts.
• An identification system outlined to set up and maintain computer database systems, for keeping records of vehicle type and component details.
• Common material coding standards to identify marked parts.

Recovery

The draft recommends that member states aim to achieve the following targets.

• Reuse and recovery for all ELVs, increased to 85% per vehicle, from 2002, ie maximum disposal of 15% (landfill and incineration without energy recovery).
• From 2015, reuse and recovery increased to 95% per vehicle, ie maximum disposal of 5%.
• New vehicles launched from 2002 have to be re-usable and recoverable to 90% per vehicle based on new vehicle weight.

Reporting

With the agreement of member states, the directive requests a report on the implementation of actions relating to ELV disposal. The first report will cover the period 1998 to 2000.

The proposal concludes with the option for EU member states to implement by way of agreements, not legislation. The commission has just released the new directive to clarify allocation of duties and responsibilities for all concerned.

CONCLUSIONS

• Plastics play an important role in the manufacture of automotive components.
• Reduced weight and improved fuel efficiency would have been impossible without plastics.
• The problems associated with recycling of ELV plastics scrap has been given priority over other more serious issues of fuel conservation and exhaust emissions. Realistically, only a certain proportion of plastics recovered from dismantling of ELVs can be recycled.
• The 30 minute barrier has been established and any endeavour to remove every last piece of plastic from an ELV is simply not economical. The remains from dismantling should ideally be incinerated, and its energy recovered.
• Full scale recycling is not economically feasible, instead the theory of part recycle/part incinerate is a better proposition for the industry.

References

Acknowledgments

Thanks are due to Derek Wilkins of CARE/Rover Group, John Amner of Ford Motor Company, Bill Harris and Geraint Williams of Jaguar Cars and Nicki Taylor of TWI.