Among the numerous possibilities of material selection in injection molding, blow molding or thermoforming, polypropylene provides a unique balance of benefits such as: low cost, easy processing, high stiffness and low density compared to other polymers or glass. As polypropylene is a semi-crystalline polymer, it is naturally translucent or opaque. The introduction of additives called Clarifiers can make PP highly transparent. The material is then referred to as Clarified PP. Clarified PP appears today as an outstanding alternative to other transparent polymers (generally amorphous) or glass. It allows the packaging and medical industries to develop new concepts with a unique combination of properties: transparency, gas barrier, chemical resistance and cost.
Organoleptics (taste and odor) issues have been a problem with PP clarifiers for many years, mainly because most clarifiers are based on acetal sorbitol compounds. These compounds are known to hydrolyze during processing, releasing free aldehydes, which have pronounced tastes and odors. They can also cause plate-out during extrusion. Over the years, the inability to find more stable PP clarifiers has discouraged PP’s competition with PET and other polymers used in food and beverage packaging. New clarifying and nucleating agents for polyethylene and polypropylene have been developed that offer barrier properties and better organoleptics including taste and odor.
Milliken Chemicals, well known for its clarifying agents has developed newer, efficient clarifying agents for both PP and PE that provide better transparency and improve strength. In addition, they impart very low odour to PP and PE products. Millad NX8000 provides about 50% more haze reduction than previous clarifying agents. The proprietary chemical composition is free of the tastes or odors that many other clarifiers impart to PP-packaged goods. Clear polypropylene clarified with Milliken’s Millad 3988i is a better choice than polyethylene terepthalate (PET) for glass replacement in packaging applications. Not only does clear polypropylene have the potential to be a less-expensive plastics packaging solution than PET, due to a combination of raw material cost and lower density, but it may also deliver excellent clarity plus the added benefits of improved moisture barrier performance and higher hot-fill temperatures. It imparts the best clarity without organoleptic problems, and continues to enable substitution of PP for polystyrene, PET, polycarbonate and glass in packaging applications. In addition to clarity, its benefits for PP include ease of use, reduced cost, increased rate of crystallization during manufacturing, greater product strength and rigidity, and improved chemical resistance.
Additives producer RIKA International’s RIKACLEAR PC1, a non acetal sorbitol clarifying agent can lessen haze without producing unwanted tastes and odors. In recent experiments, the material reduced haze in test plaques of PP at levels as low as 1500 ppm, offering a performance equivalent to standard sorbitol-based clarifiers. At the same time, the additive offered substantial improvements in the flexural modulus of the test plaques. The haze reduction and stiffness improvement extended to all types of PP, including homopolymers, random copolymers, impact copolymers and controlled rheology grades. The new product was also found to be a nucleator for PP impact copolymers, where it was reported to be effective at low concentrations (around 500 ppm).
Ciba Specialty Chemicals’ PP clarifier Irgaclear®XT386 has important side benefits beyond haze reduction. The material is based on different chemistry than the most widely used PP clarifier today, DMDBS (1,2,3,4-di-meta, para-methylbenzylidene sorbitol). Because of its high thermal stability, the new material is less likely than standard formulations to cause organoleptic problems, plate-out and blooming. The resistance to high temperatures means that the clarifier can be subjected to multiple processing cycles without degradation. The usage levels for the product, depending on the polymer system, vary from 130-200 ppm. By contrast, a typical loading of DMDBS would be about 1800 ppm. The new clarifier is targeted at thin-wall injection-molded transparent food containers, injection-molded houseware articles and extruded and injection blow-molded bottles. Conventional clarifiers repeatedly processed between 240 and 280º C show notable degradation. This degradation results in reduced clarity, strong discoloration and distinct malodor. This PP clarifier can be processed several times without any notable degradation or performance loss. Particularly in applications where regrind is used, it can lead to better optical, mechanical and physical properties compared to the conventional clarifier systems tested.
New additives for PE film are helping to boost the clarity of this material, while reducing some of its deficits. The traditional way to increase the clarity of LLDPE and HDPE films has always been to blend them with LDPE. However, LDPE degrades the mechanical properties of these films. One clarifier masterbatch - IP1810, by Ingenia Polymers avoids the need for LDPE blending. Addition of 2% of the masterbatch to a LLDPE film composition, reduces haze from 18% to around 8%. But when 20% LDPE is blended with the LLDPE, the haze reduction is not as pronounced, dropping to just under 10%. The masterbatch, with a proprietary chemical composition, is also said to offer a substantial improvement in vapor barrier properties. A nucleating agent for PE film provides not only haze reduction, but improved physical properties, higher processing speeds and lower moisture vapor transmission, according to its developer Milliken Chemical. The material, a carboxylic acid salt trade named Hyperform HPN-20E, eliminates the need to blend LDPE with LLDPE to obtain haze reduction. So high-clarity and high-gloss PE films can be obtained without the reduction in physical strength caused by LDPE.
With blown films made of butene and hexene comonomer LLDPE grades, haze reduction of 38-55% can be obtained with the use of 1000 ppm of the Hyperform HPN-20E additive. Like all nucleating agents, the new material raises the crystallization temperature of a polymer. The Milliken investigators indicate a 5-20% increase in line speeds for LLDPE blown-film formulations using the HPN-20E additive. By addition of the additive, the moisture vapor transmission rate of LLDPE and HDPE films was reduced by about 40% and 20% compared to control films without the additive. Oxygen transmission rate reductions followed the same pattern. While blown PE films have not generally been used in barrier applications, these markets could potentially open up for films clarified with the new additives