Recycling of HDPE

The economics of recycling PET are more favorable than recycling HDPE. To iacrease the recycling of HDPE, the separation of bottles made of these two plastics could be omitted and a mixture processed. Coarse, light-colored powders of the two polymers have been prepared by an experimental soHd state shear extmsion pulverization process (55). The powder has been successfully injection molded without pelletization. 

Recycling of HDPE. Polyolefins, including HDPE, are the second most widely recycled thermoplastic materials after PET (110). A significant fraction of articles made from HDPE (mostly bottles, containers, and film) are collected from consumers, sorted, cleaned, and reprocessed (110—113). Processing of post-consumer HDPE includes the same operations as those used for virgin resins blow mol ding, injection molding, and extmsion. 

In the United States, recycling of HDPE bottles through curbside collection and dropoff systems is very common. According to the American Plastics Council, more than 20,000 American communities have access to plastics recycling. 

Hope et al. studied the effect of various contaminants (types of household industrial chemicals) on the recycling of HDPE bottles. Detergent, bleach, lubricating oil and white spirit bottles were recycled. They found that the contaminants generally were absorbed into the walls of the container and were not removed by thorough washing. The main contaminants in the recycled pellets were 1-2 wt% of white spirit, 0.7 wt% of hydrocarbon oil, and between 100-500 ppm chlorine from the bleach. Another common contaminant in post-consumer... 

Recently there has been effort focused on the recycling of HDPE containers which contain pesticide residues. This area poses special problems which are difficult to overcome since the products formed by p5T olysis or incineration recycling of these containers have not been adequately characterized to date and may involve the production of highly toxic species. 

Equipment has been developed for the recycling of HDPE containers, waste oil and detergent bottles into saleable flakes at rates of 1000 to 5000 lbs per hour. The most used system is based on hydrocyclone separation, a process... 

Recycling of HDPE bottles makes good business sense as it is a high volnme material readily identifiable in the waste stream and collected cnrbside. For example, plastic bottle recycling in the US generated jnst nnder 1 billion kg of material in 2005 - 27.1% of this was HDPE. 

Chemical recycling of HDPE and LDPE is not easily done because these polymers are both long chains of CH2 groups joined together in a linear fashion. Since there are only C-C bonds and no functional groups in the polymer chain, there are no easy methods to convert the polymers to their monomers. This process is readily accomplished only when the polymer backbone contains hydrolyzable functional groups. 

Blends of PET and HDPE have been suggested to exploit the availabiUty of these clean recycled polymers. The blends could combine the inherent chemical resistance of HDPE with the processiag characteristics of PET. Siace the two polymers are mutually immiscible, about 5% compatihilizer must be added to the molten mixture (41). The properties of polymer blends containing 80—90% PET/20—10% HDPE have been reported (42). Use of 5—15% compatbiLizer produces polymers more suitable for extmsion blow mol ding than pure PET. 

In 1994 in the U.S., recycled PET, HDPE, LDPE, and PS had a 16—46% cost advantage (4). This cost advantage largely disappeared by 1996. Bureau of Labor Statistics data indicate U.S. plastics prices in mid-1997 are seven percent below those of mid-1995 after being more than ten percent less in 1996... 

The current and projected HDPE capacities are shown in Table 3, and producers of resins in Table 4. In most cases, an accurate estimation of the total HDPE volume is compHcated by the fact that a large number of plants also use the same reactors for manufacture of HDPE or LLDPE. UHMWPE is produced in the United States (Himont and American Hoechst), in Japan (Asahi), and in Germany (Hoechst) worldwide capacity is approximately 45,000 tons. The use of post-consumer (recycled) HDPE is gradually increasing in volume. The growth of recycling programs is driven principally by economics (110,114) it has increased from a mere 60,000 tons in 1989 to 350,000 tons in 1994 and is expected to increase to 1.4 million t in the year 2000 (115). 

A significant part of HDPE is coUected from consumers for recycling uncoUected HDPE can be disposed of by landfiU or incineration. In landfiU, HDPE is completely inert, degrades very slowly, does not produce gas, and does not leach any poUutants into groundwater. When incinerated in... 

The quantity of these materials is relatively small compared with the amount of waste high-density poly(ethylene) produced each year. Containers made from HDPE are widely used for detergents, oil, and antifreeze, and enormous amounts of material are used in disposable applications aimually. In principle recycled poly(ethylene) could be used for drain pipes, flower pots, dustbins, and plastic crates. The problem remains, however, that economics do not favour recycling of these polymers and in the absence of Government intervention little or nothing can be done to alter commercial attitudes towards recycling. 

The major category of compounds identified in the virgin and recycled HDPE is comprised of aliphatic hydrocarbons, such as pentadecane, hexade-cane, 1-hexadenene, branched alkanes, branched alkenes and others all oligomers of HDPE. Certain differences between virgin and recycled plastics are, however, obvious, e.g. carboxylic acids such as hexadecanoic and oc-tadecanoic acid were found only in the recycled HDPE. Only two ketones were identified, 6-dodecanone and 2-nodadecanone, in the recycled HDPE the former was also present in the virgin material. 

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