Quality of the raw materials

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Virgin PET had an intrinsic viscosity of 0.78 dl/g and was crystallized and solid state-polycondensed (upgraded) by the chemical manufacturer. PET reground flakes came from an unknown PET source. The flakes consisted of 100% PET before the reground bottle flakes went through a separation and clean-up process. The flakes were not regranulated since this step involves separate machinery and would add to the cost of the reground polymer material. The intrinsic viscosity was 0.03 to 0.05 dl/g lower compared to that of virgin PET. Such difference is negligible. Only extreme variation in viscosities of melt-layers can lead to processing problems. The size of flakes was in general between 9 and 49 mm2.

Figure 3. Bottle shape and the thickness of layers.

The trial processing

For the trial approach, a NISSEIASB-250 T-Series machine (multilayer type) was used. The processing parameters, as chosen, were within standard set-ups. The bottle shape for these trials and its layer distributionare shown in Figure 3.

Trial results

The manufactured bottles were of a high quality. When supplied with a clean regrind, the transparency was similar to that of comparable monolayer bottles. Physical and mechanical properties of the multilayer bottles were the same as the monolayer types (creep under CO2 pressure, top-load, impact strength). Standard treatment like drop testing and bottle squeezing did not result in delamination ofthe layer structure. In conclusion, all trial results indicated that multilayer bottles had comparable properties to those having monolayer structures.

COST SAVINGS

The cost saving in PET bottle production by co-injection technology, using an inexpensive polymer layer, is about 20% as calculated in Table 1. The calculation includes a higher investment cost of machinery for the co-injection technology as compared to the monolayer bottle production which employs less expensive monolayer machinery.

CONTAMINATION ASPECTS

Several aspects regarding potential contamination of the middle layer made from the post-consumer PET bottle flakes, are discussed below.

Bacteriological contamination

The majority of micro-organisms found on PET bottle surfaces is washed away during the cleaning process of the post-consumer reground PET flakes. The remaining minor amounts do not survive the drying and processing temperature in the injection molding unit, which reaches 180 and 270oC, respectively.

Contamination by foreign substances

There is a possibility that PET bottles are used for storage of substances which can cause danger to human health. Analysis was conducted, using health-endangering substances, to evaluate the effect of PET surfaces exposure. The migration and leaching of these substances were examined. It was found that, even

Table 1

Manufacturing cost comparison of PET/PET-regrind bottles

Table 1

Manufacturing cost comparison of PET/PET-regrind bottles

Container: round bottle, free-standing Material: PET/regrind Content: 1000 ml

Equipment/operation

Output

(g)

Price per bottle (DM)

NISSEI ASB-650 N PET 8 cavities (monolayer)

1,371

Virgin-injection-stretch-blow PET 3.20

35

0.1767

NISSEI ASB-650 NT 8 cavities (multilayer)

1,371

Regrind PET (bottle flakes) 1.30

35

0.1435-0.1501

Machinery type

NISSEI ASB

Unit DM/h

650 N (monolayer M/C)

650 NT (multilayer M/C)

650 NT (multilayer M/C)

1. Equipment cost amortization

48.57 100%

62.17 60%

70% PET-regrind

2. Interest 8% p.a.

PET-regrind

30% virgin PET

3. Energy consumption 0.15 DM/kWh (main equipment)

11.55

12.30 40%

4. Energy consumption 0.15 DM/kWh (auxiliary equipment)

3.75

3.75 virgin PET

5. Rent for space 8 DM/m2 per month: 500 h

0.28

0.35

6. Labour (1/3 operator) 33 DM/h

11.00

11.00

7. Maintenance 20% of machine price

9.71

12.43

8. Entire cost per hour

88.75

106.97

106.97

9. Cost and output per hour (production cost) = DM/PC

0.0647

0.0780

0.0780

10. Total resin cost including scrap = DM/PC

0.112

0.0721

0.0655

with strong toxic substances, like pesticides, the migration into PET surfaces is extremely low and furthermore, the re-migration rate (possible leaching of substances) is so low (a small fraction of the migration rate) that the values were well below the average daily intake specified by FDA.

Furthermore, the PET flakes are exposed to a melting temperature in the injection barrel which would degrade most organic substances in the middle layer. In a new multilayer PET bottle, this middle layer is shielded by layers made of virgin PET. The contents of the bottle (food) are therefore separated by a layer of virgin PET from the middle layer.

In the case of inorganic toxic substances which might have migrated into the PET surfaces in the ppb-range and furthermore survived the processing temperatures, these extremely small fractions will be diluted with the melt cake in the injection barrel and hot runner system.

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