From Science & Research
From Science & Research Talc filled PLA Micronized talc: a functional filler for PLA nucleation The limited service temperature of standard PLA is narrowing the application opportunities in many disposable items (i.e. hot beverages cups) as well as in durable applications where service temperature is a relevant property. In general, for semi-crystalline polymers, by increasing the degree of crystallinity it is possible to improve the service temperature. Because of the limited crystallization kinetic of PLA, such polymer is not able to crystallise during standard shaping processes (such as in injection moulding). The usage of nucleating agent improves the crystallization speed, allowing PLA to enhance its properties. Highly micronized talc is a common nucleator for many semi-crystalline polymers (the most common one is polypropylene) and some properties of micronized talc as nucleator for PLA were investigated. Talc is a natural mineral and it can be identified as an hydrated magnesium sheet silicate. Talc is ranked as the softest mineral (Mohs scale) and it is hydrophobic and chemically inert. Thanks to its platy structure, talc is able to improve mechanical performances of polymers, offering quite high specific surface to better interact with the polymer. Because of its affinity with polymers, talc surface is a perfect substrate for crystal growth. Experimental Concerning PLA, the ability of different talc grades to enhance crystallization in such polymer was measured. The basic evaluation performed on PLA was related to differential scanning calorimeter (DSC) experiments. DSC is an easy method to evaluate crystallization, recording the exothermic peak, typically observed periment for most of semi-crystalline polymers. But when the cess is very slow, polymer chain structure re-organization can urther melting experiment. to neat PLA, once the polymer is in molten state and the history completely erased, if cooled under controlled conditions rystallization doesn’t take place. By melting the sample still olled conditions, it is possible to record an exothermic peak at °C, showing the PLA crystallization (Fig. 1). ntal evaluation, three different talc grades were considered: talc icronized talc), talc HTPultra5c (ultrafine talc) and talc NTT05 nce talc). By modifying PLA with minor amounts of micronized sible to improve the crystallization behaviour, allowing modified hieve crystallization under cooling conditions. Two different talc rates were evaluated: 1% and 5%, by weight. Modification was rmed by dispersing talc in PLA via a 25mm twin screw extruder, ding talc upstream together with resin; also neat PLA was xtruded, as a reference for the process conditions. Table 1: half crystallization time for PLA modified with talc at different isothermal holding temperatures t 1/2 @ 90°C [s] t 1/2 @ 100°C [s] t @ 110°C [s] 1/2 Neat PLA 596 222 268 PLA + 1% HTP1c 107 59 63 PLA + 5% HTP1c
From Science & Research 50 — Heat FlowEndo Up (mW) 45 — Fig. 1: DSC curves of neat PLA In Fig. 2 it is possible to see the different DSC patterns for talc modified PLA at 1% talc HTP1c loading. In general all the three samples of talc gave same results in terms of crystallization temperature. By increasing the talc loading (5%), a higher crystallization temperature is recorded with no specific distinctions between the three talc samples. Talc loading plays a major role in PLA nucleation rather than the talc fineness. A relevant experiment, in order to better understand the crystallization conditions of talc modified PLA, is related to isothermal crystallization. Only talc HTP1c was considered as PLA modifier in this experiment. In DSC, the samples were heated up to 200°C at 10°C/min, held 5 min at 200°C and cooled rapidly (at 100°C/min) down to the testing temperature, holding the specimen at testing temperature for a certain time, until crystallization takes place. Time was recorded and it quantifies the crystallization kinetic. Crystallization occurs at a temperature higher than glass transition temperature (Tg) because below Tg, molecular mobility is virtually zero, with no possibility of chain folding. PLA Tg is in the range of 60- 70°C and experiments were performed from 90 to 110°C as testing (hold) temperature for the isothermal crystallization on PLA modified with talc HTP1c at both 1% and 5% loading. In this experiment, the presence of talc significantly reduces the time to crystallization (generally expressed as time to achieve 50% of crystallization, t 1/2 ) allowing nucleated PLA to achieve crystallinity in a more reasonable time for practical process purposes. In Fig. 3, the behaviour of PLA modification with talc HTP1c at both 1% and 5% loading is shown. For each type of modification, three different temperatures were investigated. In table 1, the t 1/2 values are summarized. The behaviour of the other two talc grades is basically similar to HTP1c. Talc loading plays a relevant role in shortening t 1/2 . Based on such experiments, it appears that moulded PLA items must be kept at relatively high temperature for a certain time to develop the expected degree of crystallinity. Such process can be performed either from the melt of from quenched state, with a visible impact on production costs. The presence of a talc (as nucleator) in the resin helps to shorten such time improving the productivity. The reduction of crystallization time is also driven by the talc concentration. The minimum crystallization time is recorded at 100°C. 40 — 35 — 30 — 25 — 20 — 15 — 10 — 5 — 0 — Crystallization Melting -50 -20 0 20 40 60 80 100 120 140 160 180 °C Fig. 2: DSC crystallization curves of talc modified PLA 60 — Heat FlowEndo Up (mW) 55 — 50 — 45 — 40 — 35 — 30 — 25 — 20 — 15 — 10 — 5 — 0 — 140 — Heat FlowEndo Up (mW) 120 — 100 — 80 — 60 — 40 — 20 — 0 — neat PLA PLA + 1% HTP1c PLA + 5% HTP1c 40 60 80 100 120 140 160 180 °C Fig. 3: Isothermal crystallyzation curves of talc modified PLA at different crystallization temperatures 100°C 110°C neat PLA PLA + 1% HTP1c PLA + 5% HTP1c 0.2 1 2 3 4 5 6 7 8 9 min bioplastics MAGAZINE [03/14] Vol. 9 25
