application note

DSC as Problem Solving Tool: Measurement of Percent Crystallinity of Thermoplastics W.J. Sichina, International Marketing Manager

Introduction Figure 1. DSC results on ‘as-received’ PET plastic food container. One of the most important properties of semi-crystalline thermoplastics (including PET, nylon, polyethylene, polypropylene, fluoro polymers, PES, PEEK) is the polymer’s percent crystallinity. This refers to the overall level of crystalline component in relationship to its amorphous component. The percent crystallinity is directly related to many of the key properties exhibited by a semi-crystalline polymer including:

· Brittleness · Toughness · Stiffness or modulus · Optical clarity · Creep or cold flow · Barrier resistance (ability to scanning calorimetry (DSC) which · Highly stable performance for prevent gas transfer in or out) measures the heat flow into or from consistent and reproducible · Long term stability a sample as it is either heated, results · Pyris software (Windows NT- Applications where knowing the cooled or under isothermally. based) for ease of use value of the percent crystallinity are The Pyris 6 DSC from critical include: · Pyris Player for single button, PerkinElmer provides an ideal start-to-finsh operation means of measuring the percent · Injection molded parts and crystallinity of thermoplastic PerkinElmer also has available components materials. The instrument offers the special software, Temperature · Plastic food and beverage following important features: Dependent Crystallinity software, containers which provides an enhanced and more · Fibers · High performance in terms of convenient means of measuring · Films sensitivity and resolution percent crystallinities of · Automotive components · Low cost thermoplastics. · Outstanding, reliable The assessment of a polymer’s autosampler for automated percent crystallinity can be most loading of up to 45 samples for easily performed using differential unattended operation Measurement of Percent Percent Crystallinity Results from Purge gas: Approximately 10 Crystallinity PET Container mg As an example of the practical Sample pan: Crimped aluminum Polyethylene terephthalate (PET) is importance of knowing the percent pan widely utilized for the production of food crystallinity of polymers, a major and beverage containers. PET bottle resins provide good impact resistance, food company recently had The heats of melting, DHm, and cold crystallization, DHc, are good optical clarity and barrier resistance, problems with their plastic ketchup provided that the resin achieves a proper bottles. Some consumers noted that determined by integrating the areas percent crystallinity during production. the ketchup bottles contained (J/g) under the peaks. Depending PET containers are manufactured using a significantly less ketchup than upon the sample’s given thermal blow molding process where the resin is specified on the label. The state history, a cold crystallization melted, injected into a mold were the resin authorities were notified and they exothermic peak may or may not be assumes the temporary shape of a tube. verified that the ketchup was not up observed during the DSC Pressurized air is then introduced into the to specified levels. The ketchup experiment. tube, which then expands the shape of the container to its final form. As this occurs, bottles from this particular food The percent crystallinity is then the resin acquires a certain level of company were removed from the determined using the following crystallinity which then affects the final store shelves and the company was equation: properties of the container including fined. It was later found that the optical clarity, gas permeability, impact plastic bottles were defective (due to % Crystallinity = [DHm – DHc] / DHm° resistance, stiffness, resistance to creep, a greater concentration of recyclates · 100% etc. in the plastic) and were less In this equation, the heats of Displayed in Figure 1 are the DSC crystalline that they should have melting and cold crystallization are results, generated on the PerkinElmer been. The lower crystallinity of the in terms of J/g. The term DHm° is a Pyris 6 DSC instrument, for an ‘as- bottle permitted the water in the reference value and represents the received’ PET container. ketchup to seep through to the heat of melting if the polymer were outside (poor barrier resistance) and 100% crystalline. This reference The as-received bottle resin shows this led to the decrease in the weight heat of melting has been established three distinct transitions, all of which of the ketchup inside the bottle. for each of the commonly used are key to the properties exhibited by Testing of the plastic bottles with polymers and some of these are the container: DSC would have shown that the listed below bottles were defective with lower Tg or glass transition at 75 C where the resin undergoes softening than desired percent crystallinity. Polymer DHm° (J/g) Cold crystallization at 101 C (DHc To perform this important Nylon 6 230.1 percent crystallinity measurement, = 1.65 J/g) where the polymer approximately 10 mg of the sample Nylon 6,6 255.8 undergoes some small amount of is cut from the test specimen and crystallization while heating in the PET 140.1 accurately weighed. The following DSC test conditions are used to measure Polypropylene 207.1 Melting at 247 C (DHm = 46.5 J/g) the sample’s crystallinity: where the existing crystalline Polyethylene 293.6 component is destroyed Initial Room temperature temperature: The PerkinElmer Temperature The percent crystallinity of this Dependent Crystallinity software Final 30 C above melting particular PET food container can be temperature: temperature of directly provides these values to assessed based on these DSC results polymer make the assessment of the percent and based on the theoretical heat of crystallinity more convenient and melting for 100% crystalline PET: Heating rate: 10 C/min easier.

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PerkinElmer Instruments 761 Main Avenue Norwalk, CT 06859-0010 USA Tel: 80O-762-4000 or PETech-40 (1+) 203-762-4000 ã2000 PerkinElmer, Inc. Printed in U.S.A. Thermal Analysis Fax: (1+) 203-762-4228 its impact and optical clarity Figure 2. DSC results from PET container resin after being annealed at 180 C properties. for 3 minutes. In order to enhance the properties of the PET resin for certain packaging applications, the blow molded bottle may be held at an elevated temperature between Tg and Tm to stabilize the properties of the resin and to produce an increase in the container’s Tg. This will allow the container to be used for more critical barrier resistance uses, such as storage of orange juice, milk and beer. The PET container resin was annealed for 3 minutes at 180 C in order to produce changes in the structure or morphology of the resin and the DSC results for the annealed resin are displayed in Figure 2. The Tg or softening temperature increased by about 20 C to 93 C, Figure 3. DSC results on quench cooled PET container resin. which is important for better storage of gas sensitive liquids such as orange juice or beer The cold crystallization peak disappears entirely A heat set, pre-melting endothermic peak is observed at 188 C which reflects the annealing processing step The overall level of crystallinity of the PET container resin is significantly increased The percent crystallinity of the annealed container resin is calculated as: % crystallinity = 55.5 / 140.1 · 100% % crystallinity = 39.6%

% crystallinity = (46.5 – 1.65) / too low, the barrier resistance will These results demonstrate that the 140.1 · 100% be low and its creep propensity will brief annealing step produces a much % crystallinity = 32.0% be high. Similarly, if the percent higher Tg (75 versus 93 C) and a crystallinity is too high, the higher crystallinity (32.0% versus Knowing this value is critical for container may suffer with regards to 39.6%) for the PET food container. a food container as, if the value is

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PerkinElmer Instruments 761 Main Avenue Norwalk, CT 06859-0010 USA Tel: 80O-762-4000 or PETech-40 (1+) 203-762-4000 ã2000 PerkinElmer, Inc. Printed in U.S.A. Thermal Analysis Fax: (1+) 203-762-4228 The great sensitivity to PET resin % crystallinity = [44.9 – 42.9] / permeability, and long term stability. to its given processing conditions 140.1 · 100% The PerkinElmer Pyris 6 DSC is may also be demonstrated by quench % crystallinity = 1.4% or essentially ideally suited for the measurement of cooling the resin from the melt, as amorphous (0% crystallinity) the percent crystallinity as the shown by the DSC results in Figure instrument provides high performance 3. Summary (both sensitivity and resolution), The quench cooled PET resin has The measurement of the percent outstanding autosampler, low cost and an extremely low crystalline content crystallinity by DSC is a ease of use with the Pyris software and is nearly entirely amorphous. straightforward and easy-to-perform and Pyris Player. The calculation of By cooling rapidly from the melt, test, which provides extremely the percent crystallinity of polymers is the PET resin does not have time to valuable information on the end use made more convenient with the develop a significant crystalline properties of semi-crystalline Temperature Dependent Crystallinity component. The DSC measured polymers. The DSC percent DSC software offered by heat of melting is 44.9 J/g and the crystallinity is directly related to PerkinElmer. heat of cold crystallinity is 42.9 J/g. essential properties including: From this data, the percent optical clarity, toughness, stiffness, crystallinity of the quench cooled creep, barrier resistance, gas PET container is given as:

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PerkinElmer Instruments 761 Main Avenue Norwalk, CT 06859-0010 USA Tel: 80O-762-4000 or PETech-40 (1+) 203-762-4000 ã2000 PerkinElmer, Inc. Printed in U.S.A. Thermal Analysis Fax: (1+) 203-762-4228