Properties of uniaxially stretched polyolefin films: effect of processing conditions

Abstract

Orientation of plastic films greatly improves many of the mechanical and optical properties. Often the orientation is done in two directions, machine and transverse. Main processes for producing oriented films favours orientation in transverse direction. This leads to better properties in transverse direction which is not favourable for film used for making roll-fed labels where the machine direction properties are more desirable. The key properties for self-adhesive roll-fed labels are bending resistance, elastic modulus and optical clarity. The need for film that performs better in machine direction has led to development of films that are only oriented in machine direction using high draw ratios. The properties of the machine direction oriented film can be adjusted by changing the film recipe as well as the orientation conditions.

This Master of Science thesis focuses on machine direction orientation of polyolefin films and especially on the effect of orientation conditions to final film properties. Film samples were made using pilot scale cast film line with machine direction orientation unit. Various process parameters were altered in the machine direction orientation unit and the effect of changing these parameters on the key properties of the film was measured. Altered process conditions were draw ratio, draw temperature, draw gap, annealing temperature and annealing tension. The key mechanical properties measured were: elastic modulus, bending resistance and ultimate tensile strength. The optical properties that were measured were: clarity, haze and gloss. Differential scanning calorimetry was used to evaluate morphological changes occurring due to different process conditions. Two film recipes were used, one for label facestock and one for release liner. The cast film production conditions were kept the same as the purpose was to only focus on the effect of orientation process conditions.

The results indicate that all of the measured properties change with draw ratio. Most of the properties are also affected by the draw temperature and draw gap. Increasing draw ratio increases elastic modulus, bending stiffness and ultimate tensile strength. Decrease in optical properties was noticed with increasing draw ratio, and also when using a medium draw temperature. Increasing preheat temperature mostly caused decrease in mechanical and optical properties, but exceptions exist. Morphological changes occur when the film is oriented using different operating parameters. Formation of fibrillar crystalline structure was observed. It was discovered that the development of the optical properties contradicted the literature, but the mechanical properties developed similarly to those in literature. Also the formation of fibrillar crystalline structure has been reported by other researchers.