the melt state the concept of melting temperature for thermoplastic po
The Melt State The concept of melting temperature for thermoplastic polymers can be confusing. Depending on the source (textbook website, etc.) melting temperature typically refers to either (1) the temperature at which a semi-crystalline polymer becomes amorphous upon heating, or (2) the temperature at which any thermoplastic becomes fluid enough (low enough viscosity) to enter the “melt state" and thus be able to be processed by various molding processes .Tm transformation from semi-crystalline to amorphous (thermodynamic melting)Semi-crystalline thermoplastic polymers do show a thermodynamic melting temperature, Tm, which corresponds to the temperature (or range of temperatures) at which the crystalline phase becomes amorphous upon heating. When semi-crystalline polymers reach Tm, they often show a dramatic change in optical properties, going from translucent to transparent. You will often be able to observe this when you "cook" the unknown polymers - IF the polymer is of the semi-crystalline variety, AND if, in the as-received condition, the level of crystallinity is high enough (the level of crystallinity will depend on how the polymer was processed).If you think the above sounds complicated, well, it is! Observing the polymers' behavior in lab should help, though, as will the DSC results you will be given. Can you explain why semi-crystalline polymers undergo the optical change described above at their thermodynamic Tm? The "melt state" (physical melting)Polymers do not typically "melt" in the same way that small-molecule or atomic materials (like water or metals) do. Polymers rarely flow like small-molecule liquids (such as water, alcohol, or even liquid metals) – at Tm or any other temperature. Polymer scientists and engineers often speak about polymers that are hot enough to process as being in "the melt state." The melt state refers to thermoplastics(amorphous or semi-crystalline) that are hot enough that their viscosity has been lowered enough for the processes. Processes like injection molding, blow molding, etc, all take place in the melt state. In the melt state, most thermoplastics are quite viscous (even more viscous than heavy molasses). As a consequence, these molding processes typically require very high pressures - thousands of psi. Thus, even though non-crystalline materials like plate glass or amorphous polymers like polystyrene, do not, technically speaking, "melt" (they don't show a true thermodynamic Tm) they do achieve the "melt state" – physical melting if you will .In the table at the end of this section of the candidate "unknown" polymers, you will find data on bothT m, (thermodynamic melting T) for the semi-crystalline thermoplastics and also a typical processing temperature for each of the thermoplastics listed. Molecular Weight Among the most important characteristics of any polymer sample is its molecular weight. All polymers consist of "long" chain molecules. However, the actual average chain length (molecular weight) and the distribution of chain lengths (molecular weight distribution) have a tremendous influence on the properties of a polymer .The actual quantitative measurement of molecular weight and its distribution requires sophisticated equipment and is well beyond the scope of this lab. However, just about every polymer property, including strength and toughness, is influenced by molecular weight - as are the ways in which a polymer can be processed. One of the ways in which we can sometimes qualitatively observe molecular weight is by attempting to draw fibers from a polymer that is in the melt state. Be very careful about this "fiber-forming" behavior, however. Most of the polymers we will test are quite capable of being drawn into extremely fine fibers - as thin as a spider's web - but only within a very narrow temperature range - perhaps just a few °C. Your ability to draw fibers, then, will depend on your lab technique and on your patience .Polymer fibers are an extremely important class of materials. Polymers like nylon, polyethylene, polypropylene, Kevlar, and polyesters (such as PET) have countless applications in fiber form. In order for a polymer to be able to be drawn into a fiber, it needs to have a relatively high molecular weight -and it needs to be at just the right temperature! Thus, if you can manually (and very carefully) draw long fibers out of a polymer sample (with a pair of tweezers, for example - as noted above, the fibers might look like a strand from a spider's web) that is good qualitative evidence of relatively high molecular weight. Be warned that this is difficult. The fiber-drawing ability of a polymer is very temperature sensitive. It will take patience on your part to either (a) draw fibers successfully, or (b) convince yourself that it cannot be done for a given polymer. The ability of a polymer to resist flames is very important in many applications. For example, polymer sused in the interiors of airplanes must meet stringent flame-retarding requirements. Polymer fibers used in clothing and fabrics, particularly bedding and pajamas, must conform to similar requirements. Unfortunately, synthetic polymers are usually derived from petroleum products and many of these polymers burn quite freely. In addition, some also give off toxic combustion products. Some polymer shave an inherent resistance to flame. Other polymers can be treated in various ways to increase their flame-retarding ability. In this lab, we will be working with polymers that have not been treated - thus we will be observing their inherent flame resistance (or lack thereof).We will use observations of the flame characteristics of polymers in two main ways in this lab. First, we will see which polymers resist burning - thus showing superior performance to those that burn freely. Second, through careful observation of flame characteristics, we will gather more information to aid in identifying the unknown polymers. For example, the flame colors and odors of burning polymers are frequently characteristics of the elements present in the polymers. Thermal Degradation Any polymer, if heated hot enough, will undergo thermal degradation. This is the temperature (or temperature range) at which the strong covalent bonds within the polymer chains finally break down and the material is irreversibly degraded. For some polymers (PVC is one well-known example) the thermal degradation temperature, Td, can be quite close to the processing temperature. You should be able to observe thermal degradation (color changes to yellow or brown, charring, smoke formation) inmost of the polymers tested in this lab. The ability of a polymer to resist flames is very important in many applications. For example, polymers used in the interiors of airplanes must meet stringent flame-retarding requirements. Polymer fibers used in clothing and fabrics, particularly bedding and pajamas, must conform to similar requirements. Unfortunately, synthetic polymers are usually derived from petroleum products and many of these polymers burn quite freely. In addition, some also give off toxic combustion products. Some polymer shave an inherent resistance to flame. Other polymers can be treated in various ways to increase their flame-retarding ability. In this lab, we will be working with polymers that have not been treated - thus we will be observing their inherent flame resistance (or lack thereof).We will use observations of the flame characteristics of polymers in two main ways in this lab. First, we will see which polymers resist burning - thus showing superior performance to those that burn freely. Second, through careful observation of flame characteristics, we will gather more information to aid in identifying the unknown polymers. For example, the flame colors and odors of burning polymers are frequently characteristics of the elements present in the polymers Experimental Procedure Safety issues This lab involves a number of potentially hazardous procedures. The use of the fume hood will be necessary in the flame characteristics or "burning" portion of the lab, as noted below. During the thermal properties or "cooking" part of the lab the polymers are likely to give off some odors that some students may find unpleasant. We will try to minimize the effects of this. The hot plate you use will get very hot – up to 300°C or above - towards the end of the "cooking" sessions. Be very careful not to burn yourself! Lab samples Samples of four or five polymers, identified only by a number or letter, will be distributed in the form of either small, thin disks or pellets. Two other known polymer samples will also be tested. Testing the known polymers should help you determine the identities of the unknown polymers .An important goal (but not the only goal!) of this lab is to try and identify the unknown polymers from a list of 15 or so possibilities tabulated in the end of this section (See also the Report section below.)• Visual Examination Examine each sample visually and make notes as to the size, shape, appearance, and texture of the materials. Note carefully the color and optical clarity of the materials. Make relative observations. For example, .number 3 is optically clearer than number 2, but not as clear as number 1." Optical clarity will also vary with the thickness of a sample.• Thermal Characteristics ("Cooking with plastics")This is the most time-consuming and difficult part of the lab. In this section, relatively simple(but not easy!) tests will be used to qualitatively and semi-quantitatively determine some of the thermal properties of the polymer samples. The properties and characteristics we will investigate are described below. During this portion of the lab, you will gather data using your senses, particularly your eyes. It is critical that you very carefully observe what the polymers are doing during this experiment, as well as what temperatures they are doing it at! Make careful and detailed notes about what you see. Also, your ability to obtain accurate results depends on your willingness to be patient as you SLOWLY heat up the polymer samples. The faster you heat, the more difficult (read: "impossible") it will be to get good data. If you heat the sample sat about 5°C per minute, it will take you about ONE HOUR to go from room temperature to 300-325°C (which is about as hot as most of our heating setups will get). Safety note: The heating plates will get more than hot enough to give you a very nasty burn. Please be careful !Each of these properties/characteristics will be investigated using the cooking plates we have prepared for this lab. These will allow you to heat small samples of the polymers relatively slowly, on a layer of aluminum foil, while monitoring the temperature using a digital thermocouple and recording visual and other observations. A schematic of the setup is shown in Figure 1.100thermometersheet of Al foil thermocouple-polymer samples hot plateFigure 1- Schematic of experimental setup for thermal characteristics portion of lab One experimental difficulty concerns the accurate measurement of temperature. In order to identify the unknown polymers, you will (among other things) have to observe and record thermal properties such as Tg and Tm, and then compare your results with tabulated values(such as those in Table 1 below). It may be necessary to make temperature corrections to your results - this will be discussed in lab. You will also have at your disposal the two known polymers in order to help you calibrate your heating setup.- Thermo plasticity. Heat a small sample of polymer on a glass slide on the hot plate .Thermoplasticity may be determined by observing the reversible change from the solid to the melt state upon heating of thermoplastics (as opposed to thermosets). Care must be taken not to heat the sample above its degradation temperature (see below). You will likely observe some differences in thermoplasticity among the samples.- Softening temperature (range). Probe a small sample with tweezers or the like as it is being heated relatively slowly. The range of softening temperatures is thus qualitatively determined. It is also possible to characterize the melt state as highly viscous, highly elastic, or intermediate. processed, you may observe a discrete temperature at which the polymer very suddenly changes shape. This temperature may be related to Tg. Be sure to record and describe any such behavior! Crystalline melting point (thermodynamic melting). This may be qualitatively explored by heating small samples past the softening range and carefully observing any (often relatively sudden) increases in optical transparency and the temperature range at which this occurs. Part 2203 The Properties of Materials – 2020• Does the polymer stop burning when removed from the flame? In other words, is it" self-extinguishing?" (BE CAREFUL – this is frequently more difficult to determine than you might imagine, especially for pellet samples) “Self-extinguishing" seems to exist on a continuum, as opposed to being an either/or phenomenon.• Rate of burning (fast or slow, compared to other samples)• Color(s) of flame• Color and consistency of smoke, if any• Is there soot in the smoke?• Does the polymer drip while burning? (this one is TRICKY! especially for small samples)• What does the burnt polymer smell like (after you've extinguished it? (please be safe -don't inhale too deeply!)You will probably want to test several samples of each polymer to confirm their behavior. Good lab practice, and keen observation skills, are needed! Be especially wary, as noted above, of both the "self-extinguishing" and "dripping" aspects of the burning tests. Results All observations, qualitative and quantitative, should be recorded in your lab notebook and also should be available to be shared with the other lab teams. You should be able to make tables of your results for reporting purposes .Try to be as quantitative as you can in your observations. When you can't be quantitative, try to make our qualitative observations in the form of relative comparisons (that is, compare the unknown samples to one another in terms of their fluidity, ability to be drawn into fibers, etc.). Melt state temperature (physical melting). This is the temperature at which the polymer will flow. How to identify it? The temperature at which you can draw fibers from the polymer is a pretty good indicator of melt state .You may also wish to allow the polymer samples to cool slowly from the melt state (but below the degradation temperature) until they recrystallize. The temperature at which polymers crystallize while cooling is Tc, and is usually not the same temperature as Tm. (Can you think why the two temperatures would be different?). Changes in appearance upon crystallization may be observed and all observations should be recorded. Note: There is likely to be at least one unknown polymer whose identity will be very difficult to determine unless it is both heated from room T to the melt state and then cooled from the melt state to room T, all the while being carefully observed.- Fiber-forming ability. This refers to the ability to form very thin, spider web-like fibers. Take a pair of tweezers and try to draw fibers out of the melt-state polymers. Record your observations. As noted earlier, it is likely that if you can do this at all you will only be able to do it in a very narrow temperature range for any given polymer. That temperature range should be somewhere near the "typical" processing temperature, shown in Table 1, for the polymer.- Thermal stability/degradation temperature. As the polymer is heated past the melting point and melt-flow range, it will eventually begin to thermally degrade. This may be observed as a color change (often, to yellow and then brown), gas evolution, embrittlement, or irreversible increases or decreases in fluidity. Record all your observations during this process and try to estimate the degradation temperature of each polymer (although these are not listed in the textbook).• Flammability and flame characteristics Flame resistance is an important consideration for many polymers. In addition, the flame characteristics of a polymer can provide excellent clues to the identity of an unknown polymer .Before trying to burn any of the polymer samples, refer to "The Three Step Plastics Identification Method" at the end of this section to see what type of information is catalogued there. This should help you once you begin the burning tests. Pick up a sample of polymer with a clean pair of tweezers, and then, under the fume hood, attempt to ignite the sample using the flame source provided. Carefully observe the behavior of the polymer during this procedure. Note whether the polymer will burn, whether it continues to burn when the lamp flame is removed, what color(s) the polymer flames show, the color and consistency of any smoke generated, the presence or absence of soot in the smoke, and whether the burning polymer drips. Very carefully (sniff gently!) observe the smell of the polymer - this should be done just after the sample has been extinguished. Note all your observations in your lab notebook .The main items to look for are noted below:• Does the polymer burn at all?
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