The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials, from a hard and brittle "glassy" state into a viscous or rubbery state as the temperature is increased. An amorphous solid that exhibits a glass transition is called a glass; the reverse transition, achieved by supercooling a viscous liquid into the glass state, is called vitrification. The glass-transition temperature Tg of a material characterizes the range of temperatures over which this glass transition occurs, it is always lower than the melting temperature, Tm, of the crystalline state of the material, if one exists. Hard plastics like polystyrene and poly are used well below their glass transition temperatures, i.e. when they are in their glassy state. Their Tg values are well above room temperature, both at around 100 °C. Rubber elastomers like polyisoprene and polyisobutylene are used above their Tg, that is, in the rubbery state, where they are soft and flexible. Despite the change in the physical properties of a material through its glass transition, the transition is not considered a phase transition.
Such conventions include a constant cooling rate and a viscosity threshold of 1012 Pa·s, among others. Upon cooling or heating through this glass-transition range, the material exhibits a smooth step in the thermal-expansion coefficient and in the specific heat, with the location of these effects again being dependent on the history of the material; the question of whether some phase transition underlies the glass transition is a matter of continuing research. The glass transition of a liquid to a solid-like state may occur with either compression; the transition comprises a smooth increase in the viscosity of a material by as much as 17 orders of magnitude within a temperature range of 500 K without any pronounced change in material structure. The consequence of this dramatic increase is a glass exhibiting solid-like mechanical properties on the timescale of practical observation; this transition is in contrast to the freezing or crystallization transition, a first-order phase transition in the Ehrenfest classification and involves discontinuities in thermodynamic and dynamic properties such as volume and viscosity.
In many materials that undergo a freezing transition, rapid cooling will avoid this phase transition and instead result in a glass transition at some lower temperature. Other materials, such as many polymers, lack a well defined crystalline state and form glasses upon slow cooling or compression; the tendency for a material to form a glass while quenched is called glass forming ability. This ability can be predicted by the rigidity theory. Below the transition temperature range, the glassy structure does not relax in accordance with the cooling rate used; the expansion coefficient for the glassy state is equivalent to that of the crystalline solid. If slower cooling rates are used, the increased time for structural relaxation to occur may result in a higher density glass product. By annealing the glass structure in time approaches an equilibrium density corresponding to the supercooled liquid at this same temperature. Tg is located at the intersection between the cooling curve for the glassy state and the supercooled liquid.
The configuration of the glass in this temperature range changes with time towards the equilibrium structure. The principle of the minimization of the Gibbs free energy provides the thermodynamic driving force necessary for the eventual change, it should be noted here that at somewhat higher temperatures than Tg, the structure corresponding to equilibrium at any temperature is achieved quite rapidly. In contrast, at lower temperatures, the configuration of the glass remains sensibly stable over extended periods of time. Thus, the liquid-glass transition is not a transition between states of thermodynamic equilibrium, it is believed that the true equilibrium state is always crystalline. Glass is believed to exist in a kinetically locked state, its entropy, so on, depend on the thermal history. Therefore, the glass transition is a dynamic phenomenon. Time and temperature are interchangeable quantities when dealing with glasses, a fact expressed in the time–temperature superposition principle. On cooling a liquid, internal degrees of freedom successively fall out of equilibrium.
However, there is a longstanding debate whether there is an underlying second-order phase transition in the hypothetical limit of infinitely long relaxation times. Refer to the figure on the upper right plotting the heat capacity as a function of temperature. In this context, Tg is the temperature corresponding to point A on the curve; the linear sections below and above Tg are colored green. Tg is the temperature at the intersection of the red regression lines. Different operational definitions of the glass transition temperature Tg are in use, several of them are endorsed as accepted scientific standards. All definitions are arbitrary, all yield different numeric results: at best, values of Tg for a given substance agree within a few kelvins. One definition refers to the viscosity; as evidenced experimentally, this value is close to the annealing point of many glasses. In contrast to viscosity, the thermal expansion, heat capaci
Poly or polylactic acid or polylactide is a biodegradable and bioactive thermoplastic aliphatic polyester derived from renewable biomass from fermented plant starch such as from corn, sugarcane or sugar beet pulp. In 2010, PLA had the second highest consumption volume of any bioplastic of the world; the name "polylactic acid" does not comply with IUPAC standard nomenclature, is ambiguous or confusing, because PLA is not a polyacid, but rather a polyester. Producers have several industrial routes to usable PLA. Two main monomers are used: lactic acid, the cyclic di-ester, lactide; the most common route to PLA is the ring-opening polymerization of lactide with various metal catalysts in solution, in the melt, or as a suspension. The metal-catalyzed reaction tends to cause racemization of the PLA, reducing its stereoregularity compared to the starting material. Another route to PLA is the direct condensation of lactic acid monomers; this process needs to be carried out at less than 200 °C. This reaction generates one equivalent of water for every condensation step.
The condensation reaction is reversible and subject to equilibrium, so removal of water is required to generate high molecular weight species. Water removal by application of a vacuum or by azeotropic distillation is required to drive the reaction toward polycondensation. Molecular weights of 130 kDa can be obtained this way. Higher molecular weights can be attained by crystallizing the crude polymer from the melt. Carboxylic acid and alcohol end groups are thus concentrated in the amorphous region of the solid polymer, so they can react. Molecular weights of 128–152 kDa are obtainable thus. Polymerization of a racemic mixture of L- and D-lactides leads to the synthesis of poly-DL-lactide, amorphous. Use of stereospecific catalysts can lead to heterotactic PLA, found to show crystallinity; the degree of crystallinity, hence many important properties, is controlled by the ratio of D to L enantiomers used, to a lesser extent on the type of catalyst used. Apart from lactic acid and lactide, lactic acid O-carboxyanhydride, a five-membered cyclic compound has been used academically as well.
This compound is more reactive than lactide, because its polymerization is driven by the loss of one equivalent of carbon dioxide per equivalent of lactic acid. Water is not a co-product; the direct biosynthesis of PLA similar to the polys has been reported as well. Another method devised is by contacting lactic acid with a zeolite; this condensation reaction is a one-step process, runs about 100 °C lower in temperature. Due to the chiral nature of lactic acid, several distinct forms of polylactide exist: poly-L-lactide is the product resulting from polymerization of L,L-lactide. PLA is soluble in solvents, hot benzene and dioxane. PLA polymers range from amorphous glassy polymer to semi-crystalline and crystalline polymer with a glass transition of 60 °C and melting points of 130-180 °C. PLA has a glass transition temperature 60–65 °C, a melting temperature 173–178 °C and a tensile modulus 2.7–16 GPa. Heat-resistant PLA can withstand temperatures of 110 °C; the basic mechanical properties of PLA are between those of polystyrene and PET.
The melting temperature of PLLA can be increased by 40–50 °C and its heat deflection temperature can be increased from 60 °C to up to 190 °C by physically blending the polymer with PDLA. PDLA and PLLA form a regular stereocomplex with increased crystallinity; the temperature stability is maximised when a 1:1 blend is used, but at lower concentrations of 3–10% of PDLA, there is still a substantial improvement. In the case, PDLA acts as a nucleating agent, thereby increasing the crystallization rate. Biodegradation of PDLA is slower than for PLA due to the higher crystallinity of PDLA; the flexural modulus of PLA is higher than PLA has good heat sealability. Several technologies such as annealing, adding nucleating agents, forming composites with fibers or nano-particles, chain extending and introducing crosslink structures have been used to enhance the mechanical properties of PLA polymers. Polylactic acid can be processed like most thermoplastics into film. PLA has similar mechanical properties to PETE polymer, but has a lower maximum continuous use temperature.
With high surface energy, PLA has easy printability which makes it used in 3-D printing. The tensile strength for 3-D printed PLA was determined. There is poly – used as PLDLLA/TCP scaffolds for bone engineering. PLA can be solvent. PLA is used as a feedstock material in desktop fused filament fabrication 3D printers. PLA printed solids can be encased in plaster-like moulding materials burned out in a furnace, so that the resulting void can be filled with molten metal; this is known as "lost PLA casting". Being able to degrade into innocuous lactic acid, PLA is used as medical implants in the form of anchors, plates, rods, as a mesh. Depending on the exact type used, it breaks down inside the body within 6 months to 2 years; this gradual degradation is desirable for a support structure, because it transfers the load to the body as that area heals. The strength characteristics of PLA and PLLA implants are well documented. PLA can also
Plasticizers or dispersants are additives that increase the plasticity or decrease the viscosity of a material. These are the substances; these solids. They decrease the attraction between polymer chains to make them more flexible. Over the last 60 years more than 30,000 different substances have been evaluated for their plasticizing properties. Of these, only a small number – 50 – are today in commercial use; the dominant applications are for plastics polyvinyl chloride. The properties of other materials may be modified when blended with plasticizers including concrete and related products. According to 2014 data, the total global market for plasticizers was 8.4 million metric tonnes including 1.3 million metric tonnes in Europe. Plasticizers for plastics are additives, most phthalate esters in PVC applications. 90% of plasticizers are used in PVC, giving this material improved flexibility and durability. The majority is used in cables, it was thought that plasticizers work by embedding themselves between the chains of polymers, spacing them apart, or swelling them and thus lowering the glass transition temperature for the plastic and making it softer.
For plastics such as PVC, the more plasticizer added, the lower their cold flex temperature will be. Plastic items containing plasticizers can exhibit improved durability. Plasticizers can become available for exposure due to migration and abrasion of the plastic since they are not bound to the polymer matrix; the "new car smell" is attributed to plasticizers or their degradation products. However, multiple studies on the makeup of the smell do not find phthalates in appreciable amounts due to their low volatility and vapor pressure. Plasticizers make it possible to achieve improved compound processing characteristics, while providing flexibility in the end-use product. Ester plasticizers are selected based upon cost-performance evaluation; the rubber compounder must evaluate ester plasticizers for compatibility, processibility and other performance properties. The wide variety of ester chemistries that are in production include sebacates, terephthalates, gluterates, phthalates and other specialty blends.
This broad product line provides an array of performance benefits required for the many elastomer applications such as tubing and hose products, wall-coverings and gaskets, belts and cable, print rolls. Low to high polarity esters provide utility in a wide range of elastomers including nitrile, polychloroprene, EPDM, chlorinated polyethylene, epichlorohydrin. Plasticizer-elastomer interaction is governed by many factors such as solubility parameter, molecular weight, chemical structure. Compatibility and performance attributes are key factors in developing a rubber formulation for a particular application. Plasticizers function as softeners and lubricants, play a significant role in rubber manufacturing. Antiplasticizers exhibit effects that are similar, but sometimes opposite, to those of plasticizers on polymer systems; the effect of plasticizers on elastic modulus is dependent on both temperature and plasticizer concentration. Below a certain concentration, referred to as the crossover concentration, a plasticizer can increase the modulus of a material.
The material's glass transition temperature will decrease however, at all concentrations. In addition to a crossover concentration a crossover temperature exists. Below the crossover temperature the plasticizer will increase the modulus. Antiplasticizers are any small molecule or oligomer additive which increases the modulus while decreasing the glass transition temperature. Plasticizers used in PVC and other plastics are based on esters of polycarboxylic acids with linear or branched aliphatic alcohols of moderate chain length; these compounds are selected on the basis of many critieria including low toxicity, compatibility with the host material and expense. Phthalate esters of straight-chain and branched-chain alkyl alcohols meet these specifications and are common plasticizers. Ortho-phthalate esters have traditionally been the most dominant plasticizers, but regulatory concerns have led to the move away from classified substances to non-classified which includes high molecular weight ortho-phthalates and other plasticisers in Europe.
Plasticizers or water reducers, superplasticizers or high range water reducers, are chemical admixtures that can be added to concrete mixtures to improve workability. Unless the mix is "starved" of water, the strength of concrete is inversely proportional to the amount of water added or water-cement ratio. In order to produce stronger concrete, less water is added, which makes the concrete mixture less workable and difficult to mix, necessitating the use of plasticizers, water reducers, superplasticizers, or dispersants. Plasticizers are often used when pozzolanic ash is added to concrete to improve strength; this method of mix proportioning is popular when producing high-strength concrete and fiber-reinforced concrete. Adding 1-2% plasticizer per unit weight of cement is sufficient. Adding an excessive amount of plasticizer will result in excessive segregation of concrete and is not advisable. Depending on the particular chemical used, use of too much plasticizer may result in a retarding effect.
Plasticizers are manufactured from lignosulf
Polycarbonates are a group of thermoplastic polymers containing carbonate groups in their chemical structures. Polycarbonates used in engineering are strong, tough materials, some grades are optically transparent, they are worked and thermoformed. Because of these properties, polycarbonates find many applications. Polycarbonates do not have a unique resin identification code and are identified as "Other", 7 on the RIC list. Polycarbonates received their name. A balance of useful features, including temperature resistance, impact resistance and optical properties, positions polycarbonates between commodity plastics and engineering plastics; the main polycarbonate material is produced by the reaction of bisphenol A and phosgene COCl2. The overall reaction can be written as follows: The first step of the synthesis involves treatment of bisphenol A with sodium hydroxide, which deprotonates the hydroxyl groups of the bisphenol A. 2CMe2 + 2 NaOH → Na22CMe2 + 2 H2OThe diphenoxide reacts with phosgene to give a chloroformate, which subsequently is attacked by another phenoxide.
The net reaction from the diphenoxide is: Na22CMe2 + COCl2 → 1/n n + 2 NaClIn this way one billion kilograms of polycarbonate is produced annually. Many other diols have been tested in place of bisphenol A; the cyclohexane is used as a comonomer to suppress crystallisation tendency of the BPA-derived product. Tetrabromobisphenol A is used to enhance fire resistance. Tetramethylcyclobutanediol has been developed as a replacement for BPA. An alternative route to polycarbonates entails transesterification from BPA and diphenyl carbonate: 2CMe2 + 2CO → 1/n n + 2 C6H5OHThe diphenyl carbonate was derived in part from carbon monoxide, this route being greener than the phosgene method; the ring-opening polymerization of cyclic carbonates has been investigated. Polycarbonate is a durable material. Although it has high impact-resistance, it has low scratch-resistance. Therefore, a hard coating is applied to polycarbonate eyewear lenses and polycarbonate exterior automotive components; the characteristics of polycarbonate compare to those of polymethyl methacrylate, but polycarbonate is stronger and will hold up longer to extreme temperature.
Polycarbonate is transparent to visible light, with better light transmission than many kinds of glass. Polycarbonate has a glass transition temperature of about 147 °C, so it softens above this point and flows above about 155 °C. Tools must be held at high temperatures above 80 °C to make strain-free and stress-free products. Low molecular mass grades are easier to mold than higher grades, but their strength is lower as a result; the toughest grades are much more difficult to process. Unlike most thermoplastics, polycarbonate can undergo large plastic deformations without cracking or breaking; as a result, it can be processed and formed at room temperature using sheet metal techniques, such as bending on a brake. For sharp angle bends with a tight radius, heating may not be necessary; this makes it valuable in prototyping applications where transparent or electrically non-conductive parts are needed, which cannot be made from sheet metal. PMMA/Acrylic, similar in appearance to polycarbonate, is brittle and cannot be bent at room temperature.
Main transformation techniques for polycarbonate resins: extrusion into tubes and other profiles including multiwall extrusion with cylinders into sheets and films, which can be used directly or manufactured into other shapes using thermoforming or secondary fabrication techniques, such as bending, drilling, or routing. Due to its chemical properties it is not conducive to laser-cutting. Injection molding into ready articlesPolycarbonate may become brittle when exposed to ionizing radiation above 25 kGy. Polycarbonate is used for electronic applications that capitalize on its collective safety features. Being a good electrical insulator and having heat-resistant and flame-retardant properties, it is used in various products associated with electrical and telecommunications hardware, it can serve as a dielectric in high-stability capacitors. However, commercial manufacture of polycarbonate capacitors stopped after sole manufacturer Bayer AG stopped making capacitor-grade polycarbonate film at the end of year 2000.
The second largest consumer of polycarbonates is the construction industry, e.g. for domelights, flat or curved glazing, sound walls, which all use extruded flat solid or multiwall sheet, or corrugated sheet. A major application of polycarbonate is the production of Compact Discs, DVDs, Blu-ray Discs; these discs are produced by injection molding polycarbonate into a mold cavity that has on one side a metal stamper containing a negative image of the disc data, while the other mold side is a mirrored surface. In the automotive industry, injection-molded polycarbonate can produce smooth surfaces that make it well-suited for sputter deposition or evaporation deposition of aluminium without the need for a base-coat. Decorative bezels and optical reflectors are made of polycarbonate. Due to its low weight and high impact resistance, polycarbonate is the dominant material for making automotive headlamp lenses. However, automotive headlamps require outer surface coatings because of its low scratch resistance and susceptibility to ultraviolet degradation (yellowin
Poly known as acrylic, acrylic glass, or plexiglass as well as by the trade names Crylux, Acrylite and Perspex among several others, is a transparent thermoplastic used in sheet form as a lightweight or shatter-resistant alternative to glass. The same material can be used as a casting resin, in inks and coatings, has many other uses. Although not a type of familiar silica-based glass, the substance, like many thermoplastics, is technically classified as a type of glass hence its occasional historical designation as acrylic glass. Chemically, it is the synthetic polymer of methyl methacrylate; the material was developed in 1928 in several different laboratories by many chemists, such as William Chalmers, Otto Röhm, Walter Bauer, was first brought to market in 1933 by German Röhm & Haas AG and its partner and former U. S. affiliate Rohm and Haas Company under the trademark Plexiglas. PMMA is an economical alternative to polycarbonate when tensile strength, flexural strength, polishability, UV tolerance are more important than impact strength, chemical resistance and heat resistance.
Additionally, PMMA does not contain the harmful bisphenol-A subunits found in polycarbonate. It is preferred because of its moderate properties, easy handling and processing, low cost. Non-modified PMMA behaves in a brittle manner when under load under an impact force, is more prone to scratching than conventional inorganic glass, but modified PMMA is sometimes able to achieve high scratch and impact resistance; the first acrylic acid was created in 1843. Methacrylic acid, derived from acrylic acid, was formulated in 1865; the reaction between methacrylic acid and methanol results in the ester methyl methacrylate. Polymethyl methacrylate was discovered in the early 1930s by British chemists Rowland Hill and John Crawford at Imperial Chemical Industries in England. ICI registered the product under the trademark Perspex. About the same time and industrialist Otto Röhm of Rohm and Haas AG in Germany attempted to produce safety glass by polymerizing methyl methacrylate between two layers of glass.
The polymer separated from the glass as a clear plastic sheet, which Röhm gave the trademarked name Plexiglas in 1933. Both Perspex and Plexiglas were commercialized in the late 1930s. In the United States, E. I. du Pont de Nemours & Company subsequently introduced its own product under the trademark Lucite. In 1936 ICI Acrylics began the first commercially viable production of acrylic safety glass. During World War II both Allied and Axis forces used acrylic glass for submarine periscopes and aircraft windshields and gun turrets. Airplane pilots whose eyes were damaged by flying shards of PMMA fared much better than those injured by standard glass, demonstrating better compatibility between human tissue and PMMA than glass. Civilian applications followed after the war. Common orthographic stylings include polymethyl polymethylmethacrylate; the full IUPAC chemical name is poly. Although PMMA is called "acrylic", acrylic can refer to other polymers or copolymers containing polyacrylonitrile. Notable trade names include Acrylite, Lucite, R-Cast, Optix, Oroglas, Altuglas and Sumipex.
PMMA is produced by emulsion polymerization, solution polymerization, bulk polymerization. Radical initiation is used, but anionic polymerization of PMMA can be performed. To produce 1 kg of PMMA, about 2 kg of petroleum is needed. PMMA produced by radical polymerization is atactic and amorphous; the glass transition temperature of atactic PMMA is 105 °C. The Tg values of commercial grades of PMMA range from 85 to 165 °C. PMMA is thus an organic glass at room temperature; the forming temperature goes up from there. All common molding processes may be used, including injection molding, compression molding, extrusion; the highest quality PMMA sheets are produced by cell casting, but in this case, the polymerization and molding steps occur concurrently. The strength of the material is higher than molding grades owing to its high molecular mass. Rubber toughening has been used to increase the toughness of PMMA to overcome its brittle behavior in response to applied loads. PMMA can be joined using cyanoacrylate cement, with heat, or by using chlorinated solvents such as dichloromethane or trichloromethane to dissolve the plastic at the joint, which fuses and sets, forming an invisible weld.
Scratches may be removed by polishing or by heating the surface of the material. Laser cutting may be used to form intricate designs from PMMA sheets. PMMA vaporizes to gaseous compounds upon laser cutting, so a clean cut is made, cutting is performed easily. However, the pulsed lasercutting introduces high internal stresses along the cut edge, which on exposure to solvents produce undesirable "stress-crazing" at the cut edge and several millimetres deep. Ammonium-based glass-cleaner and everything short of soap-and-water produces similar undesirable crazing, sometimes over the entire surface of th
A polyamide is a macromolecule with repeating units linked by amide bonds. Polyamides occur both and artificially. Examples of occurring polyamides are proteins, such as wool and silk. Artificially made polyamides can be made through step-growth polymerization or solid-phase synthesis yielding materials such as nylons and sodium poly. Synthetic polyamides are used in textiles, automotive applications and sportswear due to their high durability and strength; the transportation manufacturing industry is the major consumer, accounting for 35% of polyamide consumption. Polymers of amino acids are known as proteins. According to the composition of their main chain, synthetic polyamides are classified as follows: All polyamides are made by the formation of an amide function to link two molecules of monomer together; the monomers can be amides themselves, α,ω-amino acids or a stoichiometric mixture of a diamine and a diacid. Both these kinds of precursors give a homopolymer. Polyamides are copolymerized, thus many mixtures of monomers are possible which can in turn lead to many copolymers.
Additionally many nylon polymers are miscible with one another allowing the creation of blends. Production of polymers requires the repeated joining of two groups to form an amide linkage. In this case this involves amide bonds, the two groups involved are an amine group, a terminal carbonyl component of a functional group; these react to produce a carbon-nitrogen bond. This process involves the elimination of other atoms part of the functional groups; the carbonyl-component may be part of either a carboxylic acid group or the more reactive acyl halide derivative. The amine group and the carboxylic acid group can be on the same monomer, or the polymer can be constituted of two different bifunctional monomers, one with two amine groups, the other with two carboxylic acid or acid chloride groups; the condensation reaction is used to synthetically produce nylon polymers in industry. Nylons must include a straight chain monomer; the amide link is produced from an amine group, a carboxylic acid group.
The hydroxyl from the carboxylic acid combines with a hydrogen from the amine, gives rise to water, the elimination byproduct, the namesake of the reaction. As an example of condensation reactions, consider that in living organisms, Amino acids are condensed with one another by an enzyme to form amide linkages; the resulting polyamides are known as polypeptides. In the diagram below, consider the amino-acids as single aliphatic monomers reacting with identical molecules to form a polyamide, focusing on the amine and acid groups. Ignore the substituent R groups – under the assumption the difference between the R groups are negligible: For aromatic polyamides or'aramids' e.g. Kevlar, the more reactive acyl chloride is used as a monomer; the polymerization reaction with the amine group eliminates hydrogen chloride. The acid chloride route can be used as a laboratory synthesis to avoid heating and obtain an instantaneous reaction; the aromatic moiety itself does not participate in elimination reaction, but it does increase the rigidity and strength of the resulting material which leads to Kevlar's renowned strength.
In the diagram below, Aramid is made from two different monomers which continuously alternate to form the polymer. Aramid is an aromatic polyamide: Polyamides can be synthesized from dinitriles using acid catalysis via an application of the Ritter reaction; this method is applicable for preparation of nylon 1,6 from adiponitrile and water. Additionally, polyamides can be synthesized from dinitriles using this method as well. Polyamide-imide Kohan, Melvin I.. Nylon Plastics Handbook. Hanser/Gardner Publications. ISBN 9781569901892