Soil burial conditions

The two most common natural textile fibers encountered in modern fabrics have contrasting responses to soil burial. Under most soil burial conditions cellulose will degrade rapidly whereas wool will decay at a slower rate. These phenomena are demonstrated by the degradation of textile fibers from the Experimental Earthworks Project (Janaway 1996a). Figures 7.9 and 7.10 compare wool and linen buried in the chalk environments at Overton Down for 32 years. The linen is denatured to the point that there is little surviving morphology, whereas the wool retained some fiber structure. 

Starch and cellulosic materials are frequently used as fillers in degradable materials. The addition of starch to LDPE in combination with a pro-oxidant increases the photooxidation rate and the formation of hydroperoxides and carbonyl groups. Starch alone does not increase the photooxidation rate. The addition of starch to LDPE increases its stability in 80°C water. Slower degradation in water is due to leaching out of the pro-oxidant. The addition of starch causes biodegradation process under soil burial conditions. Further increase in the degradation rate can be achieved by preheating polyethylene filled with starch. ... 

Three different films obtained by a hot pressing procedure of powder mixtures containing plasticized PVA, PVAc with and without bactmal PHB were submitted to the biological degradation of soil microorganisms in a respirometric test aimed at simulating soil burial conditions. The compositions of the blend films are summarized in Table 1. 

Figure 2. GPC chromatograms of the PVA fraction of D blend before and after biodegradation under simulated soil burial conditions.

Figure 4. Biodegradation curves of C and D hot pressed PVA based films under simulated soil burial conditions, compared with the mineralization profile of D blend diminished of the PVAc-derived organic carbon.

Solaro, R., Coiti, A., and Chiellini, ., 1998, A new respiiometric test simulating soil burial conditions for the evaluation of polymer biodegradatioa J. Environ. Pofym. Degr. 6 203-208. 

R. Solaro, A. Corti and E. Chiellini, A new respirometric test simulating soil burial conditions for the evaluation of polymer biodegradation , J Environ Polym Degrad, 1998,6,203-8. 

This European certification establishes the performance requirements for biodegradation of plastic materials and products (including packaging, films, and other products) while under cool soil burial conditions. The certification specifies modifications to EN 13432. The soil biodegradation environment is one that maintains a temperature below 30°C and results in mesophilic conditions. The certification requires the use of EN 14855 (ISO 17088) test method to measure the amount of CO2 that is emitted from the degrading plastic sample. Vin otte soil biodegradation certification requires the product must demonstrate each of the following characteristics ... 

The Vin otte soil biodegradation certification specifies that two types of tests are performed on the plastic samples. The first test procedure for modified EN 13432 standard specifies that a satisfactory rate of biodegradation of the plastic material is under soil burial conditions between 20°C and 30°C for 24 months, that is, more than 90% of the carbon in the original plastic sample is converted into CO2 as measured by a CO2 respirometer or wet chemistry methods. The details of the test procedures are listed in ISO 14855, ISO 14851, ISO 14852, or ISO 17566 test methods. 

Soil burial tests are popular despite the precautions that are needed. It is also important that a sufficient number of specimens are exposed so that statistical treatment of the results may be applied to compensate for some of the inevitable variations in the exposure conditions. Certain precautions originally set out in 1937 are still valid, and are as follows ... 

Soil burial is widely used as the method of testing susceptibility to degradation. It closely mimics the conditions of waste disposal used for plastics but it is often difficult to reproduce results obtained because of absence of control over either the climate at the test site or the variety of micro-organisms involved in the degradation. Soil burial is thus used to provide qualitative indications of biodegradability, with more controlled laboratory work with cultured micro-organisms being used to obtain more quantitative detail. 

While these relatively simple soil burial tests are directly relevant to the underground environment, the conditions of testing have deliberately been optimised to ensure a maximum rate of degradation. The relation between this rate and the predicted rate of attack in service can only be deduced by comparison. 

A large number of archaeological artifacts are found in burial conditions. Water acting as a solvent, as well as a carrier of ionic species coming from the soil, is responsible for the migration of the latter—the acid/alkaline attack on the object material and further lixiviation of materials and ionic species from the object. Thus, determination of physical and chemical properties of the soil (pH, conductivity, chemical composition, etc.) is of great importance. 

The excavation of a clandestine grave had revealed the largely skeletonized remains of a young man who had been buried for 5 years in a biologically active soil. The subsoil was clay, with the grave cut being water-filled at the time of the excavation due to a fractured field drain. It was covered by a stack of horse manure used as agricultural fertilizer that had considerably modified the burial conditions. The body had been buried clothed, and items of textile were recovered with the human remains. These included cloth, metal, and leather that had been subject to considerable differential preservation. 

Soil burial tests are important in the assessment of the degradation of a range of polymers by the corresponding microflora. They are performed either under field or laboratory conditions with the use of specific strains without the presence of any additional carbon source other than that supplied from the polymer (Whitney, 1996). There are certain drawbacks that should be taken into account, namely the lack of reproducibility due to the climatic conditions and the change of the microbial soil flora when in situ experiments are performed. However, in situ soil burial tests are often the most useful when the extent of biodegradability of a material is sought, especially when combined with tensile strength tests and microscopic examinations (Cain, 1992). 

Most methods can be modified to suit applications, but for specific applications and to accoimt for long-term efficiency, near faithful conditions may be afforded by humidity chamber techniques, such as soil burial or vermiculite bed. Often a combination of tests is worthwhile in order to reflect performance imder actual climatic and environmental conditions. 

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