An investigation into faulty plastic sleeves

Introduction

One Eighty solves problems involving all engineering materials, not only metals. The non-metallic materials that we work with include polymers, composites, ceramic, glass and organic and inorganic chemicals.

One Eighty has a well-established offering solving problems with non-metallic materials. This month’s newsletter showcases a case study on polymeric materials used for plastic sleeve display files.

One Eighty’s Non-Metallic Materials Vertical is headed up by engineer Sandy Mariaye (Pr.Sci.Nat). Sandy has over four years’ experience solving a range of engineering challenges with the full spectrum of non-metallic engineering materials

Investigation

The Case Study Background

A manufacturer of plastic display sleeves approached One Eighty to solve a manufacturing defect which reduced the value and efficacy of the file. The manufacturer had made several attempts to eliminate the defect, but had not been able to solve the problem. The edges of the sleeves in the area of the seal were severally wrinkling which had an unsightly appearance and which also disabled the proper use of the file. This problem needed to be fixed and One Eighty was tasked with establishing the cause and finding the solution to fix it.

What We Found

Results

The Work Done

One Eighty used high-level technological testing methods, firstly to identify the polymer that was used for the sheets making up the file. The chemical chain type and molecular weight had to be determined, together with the additives incorporated within the polymer. This was achieved by a combination of a melting point test, a burn test and a density test. In addition, thermogravimetric analysis (TGA) and melt flow index (MFI) were used to gain the further detail around the polymers characteristics.
A sample from a file that did not show the wrinkling problem was tested against a sample that did show the wrinkling problem. In the case of the defected polymer sheet, the polymer was found to be a 70:30 linear low-density polyethylene (LLDPE) to linear low-density polyethylene (LDPE) blend. The polymer sheets that were not defected were found to be of a different ratio of the same polymer blend 70:30. Both polymers had the same levels of stearamide and oleamide additives.
While it can be argued that a small difference of 10% in the polymer ratio should not considerably affect the sealing properties and appearance of the file sleeves, the tests and research conducted by One Eighty emphasises the importance and relevance in understanding and identifying the required chemical balance within a polymer blend to achieve the desired heat properties. From a molecular perspective, the interactions or bonds formed are considerably different in a 70:30 LLDPE:LDPE polymer blend when compared to an 80:20 LLDPE-LDPE blend. The LDPE chains are longer and more tangled as compared to LLDPE. LDPE forms additional intramolecular and intermolecular bonds in a LDPE: LLDPE polymer blend which increases the overall bonding strength of the blend.
The bonding strength depends highly on the number and type of bonds formed between LLDPE and LDPE polymers which was found to be directly proportional to the levels of LDPE in the polymer blend. Consequently, a polymer blend containing a higher level of LDPE will require more energy (heat) for sealing. If the sealing processing parameters are not altered (commercially not viable from a cost and time perspective), the sealing process used for an 80:20 LLDPE-LDPE polymer blend will not provide sufficient energy for the 70:30 blend. This will result in the polymer chains not expanding uniformly which is in turn observed as wrinkles on the plastic sleeves.

Conclusion

Conclusion

From the tests designed and conducted, it was clear that a small shift the ratio of the LLDPE: LDPE blend was the cause for the wrinkling of the plastic sleeves upon sealing. While a 70:30 LLDPE to LDPE polymer blend was a more cost-effective option, its behaviour when subjected to heat differs significantly due to a change in bonding strength. Visible and undesirable changes in the plastic sleeves will result in the final products being regarded as defective and aesthetically unfavourable in the market.

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