Effects of stenter and interdigitation stretching process on breathability improvement in polyethylene films filled with surface treated calcium carbonate

Calcium carbonate has been used as filler in plastics since it is abundant in nature being very cheap. Calcium carbonate is traditionally used in plastics as extender to decrease the cost of the expensive polymers [1,2]. It is difficult to distribute calcium carbonate homogeneously in plastic materials since it has a hydrophilic and a lyophilic character. Therefore, calcium carbonate to be used in plastics industry is coated generally with stearic acid and its salts. Coating process may be chemical dissolving (dry coating) or spraying. It may also be carried out in emulsion, or in the form of aqueous solution application [3,5]. In recent years, a growing application area of calcium carbonate in the plastics industry has been breathable films possessing a microporous structure. Breathable films are produced with very high levels of the particulate mineral (calcium carbonate) to create an interconnecting pathway for the transport of water vapor through the film, while preventing permeation or leakage of liquid water out of the film [6]. The scope of this work consists in studying production of linear low density polyethylene (LLDPE) films containing calcium carbonate (CaCO3) particles surface treated with different chemicals (C14-C18 saturated fatty acids) by using different surface treatment methods (dry or wet) and investigating the effects of different stretching methods (stenter and interdigitation) on breathability performance. Firstly, two types of uncoated calcium carbonate having two different particle sizes (d50: 1.08 µm vs. 1.8 µm and d97: 3.41 µm vs. 7.8 µm) were provided commercially and they were wet and dry coated with saturated fatty acids using a high shear mixer (Cyclomix 5, The Netherlands). Particle size and particle size distribution, specific surface area (BET), thermal analyses (DSC, TGA), and zeta potential measurements of the calcium carbonate before and after coating were carried out. After characterization of the coated calcium carbonates, they were incorporated into linear low density polyethylene (Sabic 218W LLDPE, Tm: 122oC, MFI190oC,2.16 kg: 2 g/10 min) in changing amounts (45 and 55wt.%) using a twin screw extruder (PTLE 2744, Polimer Technics Bursa Turkey, Ø: 27 mm, L/D ratio: 44). The effects of coating chemicals, coating conditions and filler amount on the prepared compounds were studied by evaluating the thermal (DSC and TGA) and morphology (SEM) analyses. In the final stage, blown and cast films were generated and then they were stretched by using stenter and interdigitation stretching methods in different conditions. Afterwards, the produced films were compared in terms of thermal behavior, morphological properties and breathability (water vapor transmission (WVTR)). The obtained results showed that moisture was dropped to 0.1% especially in the case of coating with fatty acids bearing longer carbon number, while moisture values were around 0.3% before coating. Before calcium carbonate coating, contact drying gave the best moisture results (% 0-0.1). Using high shear mixer (Cyclomix) showed better coating quality than conventional dry coating and wet coating. Zeta potentials were -5 mV before coating but they raised to 10-15 mV after coating independent upon the coating chemical and coating conditions. In other words, the tendency to agglomeration was reduced after coating. Particle sizes increased with increasing amount of the filler when considered the SEM images of the compounds. Crystallization temperatures of the compounds decreased with increasing amount of the filler according to the thermal analyses. Furthermore, it was revealed that contact drying before and after coating is very important to reduce moisture of calcium carbonate and prevent agglomeration in LLDPE/CaCO3 compounds and films. According to the WVTR results, stenter stretching method showed better breathability property but interdigitation stretching method delivered more homogeneously stretched films.