The report concludes with the statement: ‘The replacement of polyethylene by paper carrier bags makes no sense ecologically. The production of polyethylene carrier bags requires less energy, and in the process results in less burden to the environment. There is no significant difference in the disposal of polyethylene and paper bags at landfill sites or in incineration plants’. 

The Role of Photodegradable Plastics in packaging 

Environmental groups and the general public have often failed to distinguish between two distinct problems relating to solid-waste management of packaging materials. One is the disposal or recycling of waste packages included in municipal garbage stream. The other, which is more difficult, is the problem of litter, i.e. part of solid waste, which escapes collection and contaminates beaches, forests, and other natural regions, which can be hundreds of miles away from the site where the litter was discarded. 

Garbage can be defined as the discarded solid-waste products of household or industry which are collected and disposed of in some central facility such as a dump, landfill, or incinerator. Litter, on the other hand, may be defined as synthetic object in a place where it should not be. For example, a fallen tree in the forest is not litter, but a discarded wooden box made from the same material in the same place is. Paper in a rubbish bin is not litter. The same piece of paper blowing along the side of a road definitely is. Surveys of litter show that by far the greatest proportion consist of containers or packages of various kinds used for food, beverages or tobacco. 

Plastics have one major advantage over glass and metal in packaging applications in that they are inherently organic materials, just like banana skins and coconut shells, and it is therefore possible in principle to make them degrade by natural mechanisms once they have performed their primary function as a temporary container. As has been shown in the previous sections, the proper use of disposable packages can result in savings of both energy and resources.  

Considering these principles, it is possible to draw up a list of the desirable characteristics for a packaging material. 

1. It must be resistant to the material, which it is to contain and not contribute to     the taste, odour, or toxicity, particularly if it is a food product.
2. It must be light in weight and easily formable into an attractive package.
3. It must be cheap and represent a minimal expenditure of natural resources in its     manufacture.
4. It must be resistant to microorganisms, which might otherwise attack the          materials, which it contains.
5. It must be stable and maintain its desirable physical properties for at least the     lifetime of the product, which it contains.
6. It must be disposable or recyclable by conventional rubbish disposal technology.
7. It should degrade by some natural mechanism if it becomes litter.

Plastics, as currently manufactured, fulfill all of these characteristics except the last, and until recently point 7 has been considered to be inconsistent with point 4 since it was felt that if the plastic were biologically degradable, it would no longer afford adequate protection against the attack of microorganisms on the product which the plastic is intended to contain. Now, however, it is clear that these two requirements need not be mutually exclusive. 

It has been found that the resistance of conventional plastics to micro-organisms is primarily due to two factors: (1) the low surface area and relative impermeability of plastic films and moulded objects and (2) the very high molecular weight of the plastic material. Microorganisms tend to attack the ends of large carbon-chain molecules and the number of ends is inversely proportional to the molecular weight. In order to make plastics degradable, it is necessary first to break them down into very small particles with large surface area, and secondly to reduce their molecular weight. 

Although the merits of  ‘degradable plastics’ as a means of solving some of the problems associated with the disposal of packaging materials in the solid-waste stream remain to be demonstrated conclusively, their effective use in litter control is now well established.  

Environmental Considerations For Packaging Materials 

Energy and Resources 

A major concern of the environmental movement worldwide has been the increasing use of disposable packaging (frequently plastics), which is considered to waste non-renewable resources and impose unacceptable burdens on municipal disposal facilities. In concurrence with susceptible politicians, laws and regulations have been promulgated in many jurisdictions, which have increased both the cost to the taxpayer of garbage disposal and the loss of energy resources. 

Energy costs for the production of various packaging materials  (Table 2) and products were first discussed by Guillet and based on the data of Makhijani and Lichtenberg. More detailed calculations have been published recently by Boustead and Hancock and these incorporated some of the improvements in energy recovery in more modern manufacturing plants. 

Table 2 : Energy requirements for the production of material used in packaging applications.  
 

The energy requirements for selected beverage containers are shown in Table 3. It is obvious that the energy cost of plastics per pound is substantially less than that of all its major competitors. The saving of energy is even more obvious when one includes the weight factor, as is done in Table 3. The energy cost of glass bottles and aluminum cans, for example, is of the order of 20 to 30 times that of comparable plastic containers; therefore many returns per container are necessary to compete with disposable plastics. 

Table 3: Energy requirement per beverage container  
 

It is not possible to make a general rule about the savings of energy with different packaging systems since it depends to a large extent on the number of returns that one can expect with a returnable glass bottle and the amount of energy required to return the bottle and wash it, as compared with the disposable system.

What is clear from these tables is that returnable systems do not automatically save either energy or raw materials.

In a society such as the United States in which a large part of the electrical energy is obtained from burning hydrocarbons, it is clear that a considerably larger amount of petroleum or natural gas is consumed in synthesizing a glass bottle than in making a plastic container to hold the same amount of liquid. 

It  is often assumed that the use of reusable containers such as glass bottles saves energy and resources. This is clearly not the case, as is demonstrated by the data on the milk containers in Table 4. 

Table 4:Comparison of energy cost of manufacture of disposable and returnable milk containers.  
 

When the comparison is made between a glass milk bottle distribution system and plastic pouches, there is a clear energy advantage to the use of plastic disposable pouches and probably advantages in sanitation as well. The energy cost of the bottle is one hundred times that of the plastic pouch.A bottle would have to be used more than 100 times before it would be a lower energy cost system of packaging. Furthermore, since the primary raw material for glass is sand, a cheap and abundant raw material, the primary non-renewable resource used is energy.

Fifty times more oil or natural gas would be needed to make a glass bottle than a plastic package for milk.

A further consideration, now that extensive recycling systems are in place, is to re-evaluate the use of aluminium cans which are promoted because they are easy to recycle and provide a large saving of resources when recycled.However even after extensive publicity and much popular support the maximum recovery of aluminum beverage containers in the US and Canada seldom exceeds 50%.

A country like Canada might be expected to use about 10 billion beverage containers per annum. At 50% recovery, the energy wastage of the five billion not recovered represents a total of 15x10⁹ kilowatt hours(kWh) as compared with 0.3x10⁹ kWh for the plastic bottle, a factor of 30 times less. At an energy cost of 10 cents per kWh, the cost of energy lost in the aluminium containers would be about $1.5 billion dollars per annum.

It should be noted that the energy of combustion of raw material is not included in Table 2 and 3. The reason for this is that if it were included for plastics, it would also have to be included for this is that if it were included for plastic, it would also have to be included for paper, aluminium and steel all of which, in theory, could be burned to produce energy. However, even if the energy of the petroleum is included, there would still be substantial energy advantage to using plastics.

Clearly, the proper way to dispose of plastic materials in garbage is to burn them in incineration system equipped to use the heat of combustion either to generate electricity or to provide steam for municipal heating . In this way one can consider that the use of plastic, using it once or twice as a container and then recovering about the same amount of heat from it as if the barrel of petroleum had been burnt in first place. Such applications obviously represent a considerable conservation of energy and raw materials.