In this article we will discuss about the waste generated from rubber industry and its disposal.
Sources of Waste from Rubber Industry:
Industrial scrap is usually able to be classified and segregated. Consumer scrap is rarely fully identifiable. Value is very much dependent upon classification cleanliness and non-contamination, hence whilst industrial scrap can find a market with relative ease; this is not so with consumer scrap.
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1. Industrial Waste:
Contractors exist who will collect from, reprocess and return thermoplastics to the processor. They will also make additions to make up for processing losses of additives. This type of operation is particularly useful for materials such as P.V.C. which are more susceptible to degradation during processing.
2. Used Products:
Waste used products of both plastics and rubber have in recent years become potentially emotive topics for discussion. Often unnecessarily and unwisely. Usually the factor which determines the value as a recyclable material has little to do with the material. It is determined by the cost of collection, segregation and decontamination.
Generally, plastic containers are light and have a high volume/weight ratio. They can be baled but still they are expensive to transport. Segregation is visually possible into containers of similar types and often this also provides a separation of different polymers but this is made more difficult where closures made from dissimilar materials are fitted. Labelling and the adhesive used can also provide a very significant contaminant where very thin walled containers are concerned.
Tyres are also in the problem area; not because they cannot be recycled but because they are reused and are collected for this purpose. The most simple way is by applying a new layer of tread rubber to the tyre’s periphery.
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A retreaded tyre’s has virtually a new tyre life and hence, is a very economical technique. In preparing the tyre to receive the new tread rubber, the periphery is “buffed”, removing some of the old tread. The powdered rubber is termed crumb and it is sold as a processing ingredient for use in small quantities in a fresh compound.
Another use for old tyres is found by manufacturing a material, known as ‘reclaim’. To make ‘reclaim’ the whole tyre is shredded and the rubber compound part is separated from the fabric and steel reinforcements. The rubber powder is then treated by a combination of heat, mechanical work and chemicals.
The resulting softened rubber crumb is finally refined by passing it through a very closely set 2 roll mill and refined into sheets approximately .02 mm thick. These filmy layers are wound on a drum until enough layers are there to cut off as a sheet approximately ¾” thick. Again this material is used to impact particular properties to fresh compound as well as to make cheaper compounds.
Quality and Nature of Waste from Rubber Industry:
From the above it will be concluded that polymers are compatible with each other in varying amounts. Some are compatible with others; most have a deleterious effect whilst others are almost completely incompatible. It is good practice to mix ‘like with like’ but save for industrial waste, this is almost impossible. Compromise is therefore necessary.
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1. Rubbers:
Whilst it is advantageous to separate the polymers, many products are designed using blends of two or more synthetic rubbers with material. Most tyres are blended in this way-truck tyres contain more natural rubber than passenger tyres.
The best that can be achieved with re-using tyre waste is to use truck scrap in truck tyres and passenger waste in passenger tyres. However, addition of vulcanised waste above 10-15% reduces the product quality below the acceptable level.
With specialist rubber polymers, such as silicone and viton, recovery is more simple but the product is more critical so that careful tests should be applied.
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2. Thermoset Plastics:
Thermoset Plastics waste is usually cured or part cured. It is in lumps and flash. Hairing little use it is usually discounted as rubbish. Some processors grind it to a powder and incorporate a small percentage as a filler.
3. Thermoplastics:
Thermoplastics industrial waste is usually in the form of sprues and runners or misformed products-granulating machinery is available to deal with this form of waste. Additionally waste is caused when colours or polymers are changed i.e., when producing the same product in a colour range or when changing from one product to another.
It is then necessary to ‘purge’ the injection barrel of the machine. The usual result is a lump of polymer which is both difficult to granulate and of varying colour/polymer and of very little value. It is possible to purge into a water tank but few if any factories use the technique.
Consumer waste can vary from highly contaminated to clean and it is this aspect which most affects its re-use. Some contaminants may be driven off during recovery processing but others may prove dangerous not only to the processing plant but in the future use of the product.
1. Rubbers:
From what has preceded it may be rightly concluded that to recycle rubbers is not easy. Examining this list, the inevitable conclusion must be that, whilst there are opportunities for use, few, if any, depend entirely on waste rubber and the use of waste rubber will not result in exceptional economies.
Nevertheless it would be entirely wrong to convey the impression that marginal savings only are possible. There are a number of companies which make good business based on recycling rubber. What is true is that at the head of each company is a person who regards the additional profit as important and has the desire to use waste material.
2. Tyres:
Brief mention has already been made of retreading. To the layman conservationist, it is a process that is full of opportunity but when the conservationist is also a motorist many pitfalls become apparent and he purchases new tyres.
A large section is concerned with the pros and cons of retreading and in general may be summarised by saying that with normal careful use, a tyre casing should outlast two treads.
But, it may only require one unfortunate occasion with a brick or sharp kerb to create the beginning of a casing weakness which, with continued normal use, will get worse and possibly render the tyre unfit for retreading.
It is this hidden factor which causes caution in many road users. It must be accepted that some slightly weakened cases cannot be identified visually from the exterior or interior of the tyre.
By the same token, if a tyre is driven over or against an object early in its first ‘life’ and does not immediately puncture, it will have a potential weakness. Yet there are few who would in those circumstances remove the tyre for examination. There is in all of us a degree of illogicality!
The retread industry is very aware of the problems of casing inspection and discussions are in progress towards instrumenting a more positive method of examination for ply failure and ply separation.
It has always been a target for researchers to manufacture a better form of recyclable rubber. Recent years have presented a number of new patents which are being carefully examined but as yet no breakthrough has been found at a commercial level. Such a breakthrough would help the rubber industry but would be unlikely to affect the national problems of tyre disposal or re-use.
3. General Rubber Goods:
There is currently very little use made of any used product of the general rubber goods sector. Diversity of compound, costs of collection and segregation are technically reasons for lack of industrial interest but practically, it must also be acknowledged that there is lack of a necessity. In its formative years, the main products were shoe soling and cycle tyres. They were collected as today are tyres. If necessary, they could doubtless be collected, and re-used.
4. Thermoset:
A similar result is conceivable from thermoset plastics as is obtained with rubber. However, the market is very much smaller and the need is negligible. Thermoset polymers are usually compounded with 40-60% filler and the fillers are relatively cheap materials.
Furthermore, more than one type of filler is used. It is easy to see that variety, low percentage of polymer and a small market do not encourage much interest in recycling.
It has long been known that, by re-grinding waste thermoset material to a powder, a percentage could be introduced in place of a new filler. The cost and operation of grinding often made it unattractive and there are few companies who bother with the process today.
It is reasonable to suppose that some form of pyrolysis might also be applied to recover a base resin. Here again, the cost and usefulness angles are probably the major factors affecting its adoption.
5. Thermoplastic Polymers:
Opportunities for re-use must also be considered separately for factory waste and consumer waste. The criteria of contamination and identity being paramount importance, particularly where thin sections are involved, or where the recovered product is in contact with special environments such as medical or food applications.
Factory Waste:
Most factories have considered recycling their own waste and adopted one of a number of courses of action. For critical products virgin material is used. For non-critical products sprues and runners are re-introduced after simple re-granulation.
Additionally, sprue and runners from critical applications may be used in down-graded products. Careful segregation of polymers, grades of polymers, and colours, is practised. Degraded scrap (e.g. PVC) and contaminated mixtures such as occur when colour or material is changed on a machine are discarded as rubbish.
Usually colour is found to deteriorate before physical properties are grossly affected and this is the guide which determines whether material is discarded. A particular example of this is shown in the photographs of low-density polystyrene. The material was beginning to show signs of brittleness after the sixth pass but it is apparent that some loss of clarity had occurred much earlier.
It is, however, important to appreciate that every unit of work which is applied to a plastic causes a change in physical properties which, although not grossly affecting the product quality, will require some slight alteration in processing conditions.
Table 23.1 shows the effects on a material which has simply been re-ground after one extrusion. Not only has the characteristic quality of the material been changed but it has been found necessary to change the output rate of the machine.
Decisions whether to recycle should, therefore, be both technical and economic ones. Plastics generally involve a low labour content; hence material cost and production rate are the main ways of economising.
It is not sufficient to say that because a material is capable of recycling it should be recycled within that particular factory nor indeed within any factory.
Consumer Waste:
Conversion of consumer waste incurs the additional hazards of:
(i) Product and general contamination.
(ii) Identity.
Losses and changes, caused by the actual recycling of the material, are multiplied out of all proportion when the product and dirt have to be considered. To promote the re-use of consumer waste it is necessary to revise many of the concepts which are at the foundation of plastics processing.
The most important of these are:
(i) Thin wall section.
(ii) Minimum material content.
At the extreme, a lump of grit can easily be larger than the thickness of the plastic sheet on which it resides. Re-extrusion must inevitably give rise to a faulty product with holes or grit protruding from both faces.
Material cost is relatively high for virgin polymers. Consumer scrap should be much less although it will attract costs of collection, separation and washing. Hence, more material can be afforded.
This fact creates problems elsewhere because more energy will be required to heat and cool the material, most of which are good heat insulators, and processing time will be increased.
A number of companies in Europe, Japan and America have attacked this problem, some successfully, but it is largely a market development exercise, not a problem of plastics technology.
Ultimate Disposal of Waste from Rubber Industry:
Limitations on re-use currently make a need for a satisfactory method of disposal, but even with good techniques and markets for recycled polymers, there will come a time when no further re-use is possible and disposal is the only course of action.
Traditional methods of disposal have received criticism from many quarters but cheaper alternatives are very elusive and, until such time as new techniques are evolved and adopted, landfill is likely to remain the most frequently used technique.
The first task is, therefore, to make the best use of existing methods. Having ensured this, it will become important to attack the problem of resource utilisation. This is the second, but infinitely more important task.
Landfill:
The Department of the Environment report “Refuse Disposal” quoted the figures given in Table 23.2.
Direct incineration and pulverising are being adopted increasingly but direct tipping will remain the major method for a number of years.
In landfill, plastics and rubbers represent relatively inert materials which take many years to decay, longer than metals but less than ceramics. Large articles can thus form pockets in a tip into which concentrations of other materials may collect. Otherwise, however, they will not, by themselves cause pollution by leaching.
Degradation, whether natural or accelerated, can be a different matter, and acceleration may be caused by spontaneous combustion. Hence it is important that plastics and rubbers, like many other materials, should be carefully deposited in land fill.
It is generally accepted that pulverisation of the refuse will greatly assist in the creation of a homogeneous tip, unlikely to cause trouble to water-courses or to those who wish to make use of the reclaimed lands.
Reversion to Base Materials:
Ultimate disposal may come close to being a complete recycling process, if the polymer is reverted to its base ingredients i.e., oils and gases. Polymers are hydrocarbons and hence may be treated in this way. With most materials the route is not easy and many small fractions result which are not readily reverted into the polymer from which they originated.
A notable exception is methyl methacrylate from which acrylic sheet is made, and back into which acrylic sheet may be turned. For the remainder, the best that can be done is to use the distillate as a feed stock in companies who make a wide range of chemical hydrocarbons, solvents, gases, etc.
Conversion to Heat:
It is but another step to convert the hydrocarbon to heat energy. The polymers from which plastics are manufactured are currently derivatives from fuel oils, generally destined to be burned.
Manufacture into plastics may be regarded as an interlude which makes an additional use of the oil before its eventual incineration. Calorific values (save for PVC) are high, sulphur content is low and, save for fire retardant grades, plastics burn easily. (Table 23.3.)
Furthermore, incineration results in the maximum percentage volume reduction. Tyres have been reduced to around 5% of the original weight and volume.
To sum up, recycled and carefully discarded rubbers and plastics can make their contribution so that the advantages which they have given to society in their prime uses may be converted into still more advantages when they are discarded.