Injection moulding
Injection moulding Injection molding of PLA cutlery By: Shilpa Manjure and Michael Annan Natur-Tec - A Division of Northern Technologies International Corp. (NTIC) Circle Pines, Minnesota, USA Background Disposable plastic items are typically made out of two types of plastics: polypropylene (PP) and polystyrene (PS). Plastic utensils, in particular, are highly regarded for the affordability and convenience. However, once these utensils are contaminated with food, recycling them becomes challenging. On the other hand, food service-ware and packaging made from compostable plastics, such as Ingeo Poly(lactic) acid (PLA), allow for easy disposal in composting, and thereby provide a viable alternative to recycling of conventional plastic-based materials. There is no need to clean the item as you would for high-quality conventional recycling. All compostable plastic products go into one bin together with the food waste, thereby making it simpler to facilitate diversion of food waste from landfill to composting. However, for manufacturers, PLA is a thermoplastic material that comes with its own unique challenges. This article examines from a manufacturer’s perspective, how the injection molding of PLA-based compounds compares with molding of PS and PP, and in particular, how the performance of cutlery made from Natur-Tec’s modified Ingeo PLA compares with cutlery made from PS or PP. Comparison of thermal properties In order to understand molding behavior and performance of a material, it is important to first understand its thermal properties. Table 1, summarizes the thermal properties of PLA, PS and PP. Commercial grade, atactic PS is an amorphous material, i.e. has 0 % crystallinity, and as such it does not have a melting point. The glass transition temperature (T g ) of this PS is 100 °C (89 to 102 °C depending on the molecular weight). The glass transition temperature is an important thermal property of any polymer, and is the temperature region where the (amorphous region of) polymer transitions from a hard, glassy material to a soft, rubbery material as temperature increases. Hard plastics, such as PS, are used well below their T g or in their glassy state. The T g of PS is well above room temperature, and as such PS can be used with hot foods up to 90 °C without softening. PP, on the other hand, has a T g of 0 °C and is a more flexible polymer as compared to PS at room temperature. This is a common way to distinguish PP cutlery from PS cutlery in the market. PP cutlery tends to be bendable or pliable, whereas PS cutlery tends to be stiff and hard. PP and PLA are both semi-crystalline polymers with a melting point in the range of 160 °C. Despite having a similar melting point, PLA is different from PP. PLA has a high melting point similar to that of PP, and a T g above room temperature similar to that of PS. This makes PLA cutlery, rigid or glassy at room temperature. However, above its T g of 55 °C, PLA cutlery starts to soften and is difficult to use in high temperature applications. Although it is a semi-crystalline polymer, PLA has a much slower crystallization rate as compared to PP. Therefore, PLA parts made with a cold mold are essentially amorphous. PP food service ware is usable in hot food applications inspite of its much lower T g because of its “crystallinity” and faster rate of crystallization – achieve a crystallinity of 30 – 70 % in 5 – 10 seconds [1]. When a PP part is above its T g , the amorphous regions soften, but the crystals which contribute to the morphological structure help the part in maintaining form until its melting point is reached. This same principle can be applied to PLA. Figure 1, clearly demonstrates these differences among the three materials by measuring storage modulus (stiffness) as a function of temperature. PS (orange curve) maintains its stiffness until 100 °C, above which it deforms. Amorphous Ingeo 2003D PLA (green curve) follows the same trend until it reaches its T g around 55 °C, after which it deforms. As discussed earlier, PP is a semicrystalline material and slowly decreases in stiffness (brown curve) until it reaches its melting temperature of 140 °C. Crystallized PLA (Ingeo 3100HP – blue curve) is rigid at room temperature, similar to PS, and decreases in stiffness at approximately 60 °C. However, the crystalline domains of PLA hold the structure together and prevent the product from deformation till its melting point of 155 °C is reached. This is very similar to PP behavior as can be seen from the brown (PP) and blue (Ingeo 3100HP PLA) curves. Thus, developing crystallinity in PLA helps increase resistance to heat in compostable foodservice ware applications. There are, of course, other ways to improve heat resistance in durable, non-compostable PLA applications. Molding and crystallization of PLA From the above discussions, it is clear that crystallization is an efficacious way to improve high-heat performance in compostable food service ware products. There are two methods in which one can develop crystallinity in a compostable part as summarized below: a) One-Step Process or In-mold annealing: Crystallization of a part by changing the mold temperature to improve performance of the molded part has been practiced and studied for traditional plastics [3]. The same can be applied to PLA, where crystallization is carried out in the mold itself by heating the mold to the crystallization temperature of the specific PLA grade, typically in the range of 100 – 130 °C. Crystallization rate is affected by the D-content present in the PLA. Lower the D-content, faster 16 bioplastics MAGAZINE [03/16] Vol. 11
Injection moulding molding is the crystallization rate [4]. This is particularly important for a molding process as it directly affects the cycle times in the mold. Cycle times for an Ingeo 3100HP PLA based cutlery is on the order 30 – 45 seconds depending on the mold design, runner system and heating channels. Therefore, this method is, currently a more expensive way of crystallizing a PLA part, as the cycle times to crystallize in the mold are much higher than those for PP or PS that are only 5 – 10 seconds. The main advantage of an in-mold annealing process is that one can utilize the full capacity of the molding equipment, and the process set-up is straightforward. Additionally, the warpage of the part is minimal as compared to a post-annealing process described in the next section. b) Two-Step Process or Post-annealing: This is currently the most popular way of crystallizing PLA, especially for cutlery. The cutlery is molded in step one in a cold mold, followed by step two in which the cutlery is annealed in a convection oven set at the PLA crystallization temperature [5]. The advantage is one can get benefit from the faster cycle times of the cold mold to make almost amorphous parts in step one and keep the molding cost much lower. The disadvantages of the post-annealing method are (i) molding capacity can only be fully utilized with an upfront investment in suitable ovens or automation (ii) it can be labor intensive if not automated, and (iii) part warpage is an issue depending upon the geometry of the cutlery, as the material relaxes when reheated above its T g . Performance of cutlery made with Natur-Tec’s modified Ingeo PLA compound Natur-Tec has launched a 2-part resin solution, BF3002HT, consisting of a highly-filled, impact-modified Ingeo PLA based masterbatch, that can be blended with virgin Ingeo PLA at the time of injection molding. Competitive filled-PLA compounds that are currently available in the market do not use the masterbatch approach and typically use 100 % of the compounded resin for molding cutlery. A key advantage of the Natur- Tec 2-part solution is that only 50 % of the resin used for molding goes through two heat histories, which in turn, helps in maintaining the molecular weight, and therefore provide improved mechanical strength for the final part, as compared to a part manufactured with the 100 % fullycompounded resin. Performance Test Methods: There is no standardized quantitative test method to compare various cutleries, other than a military specification describing a method that is at best semi-quantitative [6]. As a result, to quantify the stiffness/flexibility of a cutlery and performance in hot water, Natur-Tec developed two in-house tests with standard Instron equipment used for tensile/compressive testing 1. Rigidity Test: In the rigidity test, the handle of a cutlery piece was clamped to the upper jaw of the Instron and pushed down vertically until it was bent or broken. 2. Hot Water Test: In the hot water test, which simulates performance in hot fluids, the cutlery was immersed in hot water at controlled temperature between 80 and 90 °C for 20 seconds before it was compressed in the vertical direction. Glass transition temperature, T g , °C Melting temperature, T m , °C % Crystallinity Crystallization rate PS 100 NA 0 NA PP 0 140 – 170 30 – 70 Fast PLA 55 160 30 – 50 Slow Table 1: Typical thermal properties of PLA, PS and PP Figure 1: Change in storage modulus (stiffness) as a function of temperature for Ingeo 2003D PLA, polystyrene, polypropylene and crystallized Ingeo 3100HP PLA [2] Storage modulus, MPa 10,000 1,000 100 10 20 Amorphous 2003D Crystalline 3100HP PS PP Good range 60 100 140 180 Temperature, °C bioplastics MAGAZINE [03/16] Vol. 11 17
