Arbejdsrapport fra Miljøstyrelsen, 4/2005 – Forbehandling af hærdeplastbaserede kompositmaterialer til genanvendelse

Pre-treatment of Composit Waste for Recycling

Summary and conclusions

In this project the efficiency of various methods to pre-treat waste from worn out products made from composite materials has been investigated. The pre-treatment is necessary, if the composite waste is to be recycled. The amount of composite waste is expected to increase considerably in the future, and this waste constitutes a potential resource for the society, which it would be expedient to exploit - depending on an overall evaluation of the environmental, technical and social conditions. In the project the main focus has been on the technical conditions regarding pre-treatment of composite waste, while the economical and environmental conditions have only been evaluated to a limited extent.

The pre-treatment methods have been examined on the basis of the degree to which the valuable properties of the materials are maintained during the pre-treatment and until final recycling takes place. The greater the degree to which the material properties are maintained the greater the value of the material during the final recycling. It is the primary aim that the pre-treated material will contribute substantially when recycling the material and manufacturing new products.

Two key methods for pre-treatment of composite waste in the form of reinforced fibreglass have been investigated:

Thermal pre-treatment, where combustion of the plastic material is combined with energy recovery, and where the properties of the remaining inorganic material, primarily glass fibres, have been examined. The combustion can be carried out by means of several methods, and in this project the main focus has been on a pyrolysis-like combustion which takes place in two steps
Mechanical pre-treatment, where the composite waste is granulated, and where the properties of the granulated material has been examined

Furthermore, test specimens have been produced from the thermally pre-treated material for determination of the properties at final recycling of the material. The practical experiments have been carried out on waste from the windmill industry in the form of cut-outs of composite parts of two different types of plastic material, polyester and epoxy resin.

On the basis of the evaluations regarding the thermal pre-treatment it can be concluded that the tensile strength of the glass fibres during the thermal pre-treatment is reduced by more than 50%. However these reduced properties have shown to be sufficient so that the pre-treated glass fibres when being impregnated with unsaturated polyester or epoxy resin can be used for the manufacture of products, on which moderate demands to mechanical properties are made. Known production methods like hand lay-up, vacuum injection and compression moulding can be used when manufacturing the products.

In order to achieve an easier handling of the thermally pre-treated glass fibres better methods for binding the glass fibres should be developed, making it possible to use them in processes known for the new glass fibre mats and roving fabric. Furthermore, for environmental (irritant dust particles) and process reasons, it will be advisable to develop efficient methods for the removal of inorganic particles and dust such as pigments and fire resisting additives, from the surface of the glass fibres.

Experiments have been carried out applying the pre-treated glass fibres with a size in order to improve the flexural strength of the test plates, which have been produced from the pre-treated glass fibres.

However, the effect was modest, as we only succeeded in improving the flexural strength by 11% for one test plate made of polyester laminate, while the flexural strength for the remaining 5 test plates remained unchanged. However, the results showed a considerably lower deviation of the flexural strength for the resized versus the non-resized test plates, i.e. resizing the pre-treated glass fibres makes it possible to achieve a more uniform quality of the composite materials.

From the thermally pre-treated glass fibres three different types of demonstration specimens have been manufactured by using the vacuum-injection method. The specimens differ in dimension and height, and they show that it is possible to mould simple specimens without major problems. Binding of the pre-treated glass fibres with some kind of glue will make it easier to drape the glass fibres in high moulds, and at the same time prevent the bundles of fibres from sliding away from each other.

Based on the examinations regarding the mechanical pre-treatment it has been demonstrated that it is possible to granulate composite waste into uniform fractions by repeated granulating and sieving the material. Semi-flexible “mats” have been manufactured from the granulated composite fibres, consisting of a mixture of glass and plastic material. These mats have been bound together with solvents of PVA-glue and epoxy resin respectively. These mats can be shaped and partly adjusted to a component, as is the case with new glass fibre mats. The possibilities of modelling the granulated composite materials compared to new glass mats or glass fibre after thermal pre-treatment are, however, limited, as these materials in the form of fibres and bundles of fibres in the “mats” are more stiff.

Test plates of semi-flexible “mats” have been manufactured, however they have low mechanical properties. The reasons for the reduction in mechanical properties are entrapped air and insufficient interaction between granulated composite material and the matrix material. After further development it should be possible to use the semi-flexible “mats” for manufacture of products where moderate demands are made regarding mechanical strength properties. In general, the semi-flexible “mats” of granulated composite material can be used for moulding by vacuum injection, hand lay-up and compression moulding.

Based on the evaluated methods for pre-treatment of composite materials it is not possible to conclude whether thermal pre-treatment of composite materials is more advantageous than mechanical pre-treatment or vice versa. Such an evaluation would be dependent on further criteria such as the composition of the actual composite waste, application possibilities and markets for the pre-treated material, possibilities of collecting and sorting the composite material, and where in the world the composite material is located at the time of removal.

The thermal pre-treatment has the key advantage that the many different types of composite materials based on different plastic materials after the pre-treatment will consist of a relatively uniform glass fraction, which will be easier to gather in sufficient quantities. At the thermal pre-treatment - dependent on the size of the specific waste incinerator - it will in many cases only be necessary with a limited size reduction of the bigger waste parts prior to pre-treatment. The thermal pre-treatment on the other hand requires major investments in the form of expensive ovens or the like.

Often, the mechanical pre-treatment will not necessarily require such advanced and heavy investments of downsizing equipment. However, a high degree of wear will occur on the equipment during the granulation. The resulting granulated composite material will typically consist of a complex mixture of different plastic materials, which will be difficult to produce in a uniform quality and find recycling possibilities, unless a pre-sorting is carried out. On the other hand, if there is a specific market demand for the granulated composite material, the mechanical pre-treatment may be advantageous.

In summary, it can be concluded that in the long term there will probably be a need for plants for both thermal and mechanical pre-treatment, where large amounts of different types of worn out composite material are to be pre-treated and finally recycled in Denmark. The possibilities of final recycling of the pre-treated materials must be seen in the light of the limited strength properties combined with the price. In this connection it should be mentioned that one of the principal reasons for choosing to manufacture products of glass fibre reinforced plastic is the high mechanical strength properties of composite products.

When comparing the recycled material with virgin glass fibres and the properties and price, it is obvious that the challenge in developing economically and environmentally advantageous recycling solutions for composite waste is to procure high value applications for the recycled material.

Good possibilities exist to improve and optimise both examined pre-treatment methods and the resulting recycled material, which should be adjusted to special applications. It is, of course, decisive that the recycled material can be produced to a uniform standard and at competitive prices.

In this project, the emphasis has been on to the technical conditions for the selected pre-treatment methods; however, the present results must be adapted to a specific industrial production, and the economic implications must be examined. Furthermore, a more detailed environmental- and socio-economic evaluation of the various pre-treatment- and subsequent final recycling methods for composite materials should be carried out.