Blown Film Formulation and Process Notes 26 — Film Haze

Gloss refers to the ability of a plastic film surface to specularly reflect incident light; when the surface scatters light, gloss decreases;

Transparency is a measure of the scattering or absorption loss of incident light by a plastic film; high transparency means little scattering or absorption loss of incident light;

Haze is the percentage of transmitted light intensity that deviates from the incident light by more than 2.5° in the total transmitted light intensity; greater haze means that film gloss and transparency, especially image clarity, decrease

Since the advent of plastic films, optical properties such as surface gloss and transparency have been an important aspect of their performance. Especially in the packaging field, where plastic films are most widely used, high surface gloss and high transparency are important indicators. Therefore, how to reduce or eliminate the haze of plastic films has long received widespread attention (). However, as people's aesthetic concepts have changed, people have presented diverse requirements for the surface characteristics of packaging materials. Matte (frosted) surface packaging (with high haze and low gloss) has also begun to become popular. In addition, with the saturation of production capacity for high-gloss, high-transparency packaging products such as BOPP (biaxially oriented polypropylene) and intensifying market competition, the development of matte films has become a new profit growth point and investment hotspot for such film manufacturers. Therefore, the influencing factors and mechanisms of plastic film haze have received further attention.

Factors affecting the haze of plastic films mainly fall into two major aspects: first, raw materials, such as resin type, resin parameters, and additives; second, processing conditions, such as different processing processes and different processing conditions under the same process.

1 Effect of raw materials on haze

Resin factors include resin type, melt index (MI), relative molecular mass (Mt) of the resin and its distribution (MWD), and main-chain regularity and branching, all of which ultimately affect film haze by affecting the crystallinity or melt elasticity of the resin.

When the MI of LDPE is 1–5 g per 10 min, gloss is best, while at 0.2–0.5, the matte (haze) effect is at its

excellent. For HDPE, LLDPE, and LDPE, under the same processing conditions and when the resin MI is all around 1.0, HDPE has the highest haze. MWD also affects film haze performance; a broad MWD will cause the resin film

gloss to decrease, because the irregularity of the material surface increases. The more regular the main chain and the fewer or shorter the side chains, the higher the crystallinity and the greater the haze.

2 Effect of additives

The addition of the anti-blocking agent SiO2 increases film haze because small-particle-size SiO2 is uniformly dispersed in the resin, improving its ability to scatter visible light.

Nucleating agents: nucleating agents are easily and uniformly dispersed in the resin, and the crystallization process fully exerts heterogeneous crystallization, effectively increasing the density of crystal nuclei and promoting uniform distribution of crystal nuclei, leading to grain refinement and smaller crystal size, which reduces light scattering and light reflection on the film surface. This leads to lower haze and significantly improved transparency.

Previous articles have discussed the effects of processing processes and processing conditions on the haze of PE films.

Specific articles: Blown Film Formulation and Process Notes 12, Blown Film Formulation and Process Notes 21, Blown Film Formulation and Process Notes 22, Blown Film Formulation and Process Notes 23, Blown Film Formulation and Process Notes 24