A chemical substance is a unique form of matter with constant chemical composition and characteristic properties . Chemical substances may take the form of a single element or chemical compounds . If two or more chemical substances can be combined without reacting , they may form a chemical mixture . If a mixture is separated to isolate one chemical substance to a desired degree, the resulting substance is said to be chemically pure .
136-1134: A polymer is a substance composed of macromolecules. A macromolecule is a molecule of high relative molecular mass, the structure of which essentially comprises the multiple repetition of units derived, actually or conceptually, from molecules of low relative molecular mass. A polymer ( / ˈ p ɒ l ɪ m ər / ) is a substance or material that consists of very large molecules, or macromolecules , that are constituted by many repeating subunits derived from one or more species of monomers . Due to their broad spectrum of properties, both synthetic and natural polymers play essential and ubiquitous roles in everyday life. Polymers range from familiar synthetic plastics such as polystyrene to natural biopolymers such as DNA and proteins that are fundamental to biological structure and function. Polymers, both natural and synthetic, are created via polymerization of many small molecules, known as monomers . Their consequently large molecular mass , relative to small molecule compounds , produces unique physical properties including toughness , high elasticity , viscoelasticity , and
272-459: A catalyst . Laboratory synthesis of biopolymers, especially of proteins , is an area of intensive research. There are three main classes of biopolymers: polysaccharides , polypeptides , and polynucleotides . In living cells, they may be synthesized by enzyme-mediated processes, such as the formation of DNA catalyzed by DNA polymerase . The synthesis of proteins involves multiple enzyme-mediated processes to transcribe genetic information from
408-452: A chemical reaction form a chemical compound . All compounds are substances, but not all substances are compounds. A chemical compound can be either atoms bonded together in molecules or crystals in which atoms, molecules or ions form a crystalline lattice . Compounds based primarily on carbon and hydrogen atoms are called organic compounds , and all others are called inorganic compounds . Compounds containing bonds between carbon and
544-678: A database and is popularly known as the Chemical substances index. Other computer-friendly systems that have been developed for substance information are: SMILES and the International Chemical Identifier or InChI. Often a pure substance needs to be isolated from a mixture , for example from a natural source (where a sample often contains numerous chemical substances) or after a chemical reaction (which often gives mixtures of chemical substances). Stoichiometry ( / ˌ s t ɔɪ k i ˈ ɒ m ɪ t r i / )
680-441: A lower critical solution temperature phase transition (LCST), at which phase separation occurs with heating. In dilute solutions, the properties of the polymer are characterized by the interaction between the solvent and the polymer. In a good solvent, the polymer appears swollen and occupies a large volume. In this scenario, intermolecular forces between the solvent and monomer subunits dominate over intramolecular interactions. In
816-574: A mixture , referencing them in the chemical substances index allows CAS to offer specific guidance on standard naming of alloy compositions. Non-stoichiometric compounds are another special case from inorganic chemistry , which violate the requirement for constant composition. For these substances, it may be difficult to draw the line between a mixture and a compound, as in the case of palladium hydride . Broader definitions of chemicals or chemical substances can be found, for example: "the term 'chemical substance' means any organic or inorganic substance of
952-618: A "ligand". However, the center does not need to be a metal, as exemplified by boron trifluoride etherate BF 3 OEt 2 , where the highly Lewis acidic , but non-metallic boron center takes the role of the "metal". If the ligand bonds to the metal center with multiple atoms, the complex is called a chelate . In organic chemistry, there can be more than one chemical compound with the same composition and molecular weight. Generally, these are called isomers . Isomers usually have substantially different chemical properties, and often may be isolated without spontaneously interconverting. A common example
1088-502: A bad solvent or poor solvent, intramolecular forces dominate and the chain contracts. In the theta solvent , or the state of the polymer solution where the value of the second virial coefficient becomes 0, the intermolecular polymer-solvent repulsion balances exactly the intramolecular monomer-monomer attraction. Under the theta condition (also called the Flory condition), the polymer behaves like an ideal random coil . The transition between
1224-416: A chemical reaction takes place and a new substance is formed, the compound iron(II) sulfide , with chemical formula FeS. The resulting compound has all the properties of a chemical substance and is not a mixture. Iron(II) sulfide has its own distinct properties such as melting point and solubility , and the two elements cannot be separated using normal mechanical processes; a magnet will be unable to recover
1360-408: A degree of crystallinity approaching zero or one will tend to be transparent, while polymers with intermediate degrees of crystallinity will tend to be opaque due to light scattering by crystalline or glassy regions. For many polymers, crystallinity may also be associated with decreased transparency. The space occupied by a polymer molecule is generally expressed in terms of radius of gyration , which
1496-585: A deviation from a simple linear chain. A branched polymer molecule is composed of a main chain with one or more substituent side chains or branches. Types of branched polymers include star polymers , comb polymers , polymer brushes , dendronized polymers , ladder polymers , and dendrimers . There exist also two-dimensional polymers (2DP) which are composed of topologically planar repeat units. A polymer's architecture affects many of its physical properties including solution viscosity, melt viscosity, solubility in various solvents, glass-transition temperature and
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#17328011268311632-464: A flexible quality. Plasticizers are also put in some types of cling film to make the polymer more flexible. The attractive forces between polymer chains play a large part in determining the polymer's properties. Because polymer chains are so long, they have many such interchain interactions per molecule, amplifying the effect of these interactions on the polymer properties in comparison to attractions between conventional molecules. Different side groups on
1768-479: A given application, the properties of a polymer can be tuned or enhanced by combination with other materials, as in composites . Their application allows to save energy (lighter cars and planes, thermally insulated buildings), protect food and drinking water (packaging), save land and lower use of fertilizers (synthetic fibres), preserve other materials (coatings), protect and save lives (hygiene, medical applications). A representative, non-exhaustive list of applications
1904-515: A high chemical resistance. Polypropylene is the second-most widely produced commodity plastic (after polyethylene ). Phillips Petroleum chemists J. Paul Hogan and Robert Banks first demonstrated the polymerization of propylene in 1951. The stereoselective polymerization to the isotactic was discovered by Giulio Natta and Karl Rehn in March 1954. This pioneering discovery led to large-scale commercial production of isotactic polypropylene by
2040-401: A high surface quality and are also highly transparent so that the laser properties are dominated by the laser dye used to dope the polymer matrix. These type of lasers, that also belong to the class of organic lasers , are known to yield very narrow linewidths which is useful for spectroscopy and analytical applications. An important optical parameter in the polymer used in laser applications
2176-638: A melting point of 130 °C (266 °F). Below 0 °C, PP becomes brittle. The thermal expansion of PP is significant, but somewhat less than that of polyethylene. Propylene molecules prefer to join together "head-to-tail", giving a chain with methyl groups on every other carbon, but some randomness occurs. Polypropylene at room temperature is resistant to fats and almost all organic solvents , apart from strong oxidants. Non-oxidizing acids and bases can be stored in containers made of PP. At elevated temperature, PP can be dissolved in nonpolar solvents such as xylene , tetralin and decalin . Due to
2312-436: A metal are called organometallic compounds . Compounds in which components share electrons are known as covalent compounds. Compounds consisting of oppositely charged ions are known as ionic compounds, or salts . Coordination complexes are compounds where a dative bond keeps the substance together without a covalent or ionic bond. Coordination complexes are distinct substances with distinct properties different from
2448-525: A modifier. The catalysts are differentiated depending on the procedure used for fashioning catalyst particles from MgCl 2 and depending on the type of organic modifiers employed during catalyst preparation and use in polymerization reactions. Two most important technological characteristics of all the supported catalysts are high productivity and a high fraction of the crystalline isotactic polymer they produce at 70–80 °C under standard polymerization conditions. Commercial synthesis of isotactic polypropylene
2584-462: A particular molecular identity, including – (i) any combination of such substances occurring in whole or in part as a result of a chemical reaction or occurring in nature". In the field of geology , inorganic solid substances of uniform composition are known as minerals . When two or more minerals are combined to form mixtures (or aggregates ), they are defined as rocks . Many minerals, however, mutually dissolve into solid solutions , such that
2720-401: A polymer behaves as a continuous macroscopic material. They are classified as bulk properties, or intensive properties according to thermodynamics . The bulk properties of a polymer are those most often of end-use interest. These are the properties that dictate how the polymer actually behaves on a macroscopic scale. The tensile strength of a material quantifies how much elongating stress
2856-419: A polymer is its first and most important attribute. Polymer nomenclature is generally based upon the type of monomer residues comprising the polymer. A polymer which contains only a single type of repeat unit is known as a homopolymer , while a polymer containing two or more types of repeat units is known as a copolymer . A terpolymer is a copolymer which contains three types of repeat units. Polystyrene
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#17328011268312992-432: A polymeric material can be described at different length scales, from the sub-nm length scale up to the macroscopic one. There is in fact a hierarchy of structures, in which each stage provides the foundations for the next one. The starting point for the description of the structure of a polymer is the identity of its constituent monomers. Next, the microstructure essentially describes the arrangement of these monomers within
3128-535: A process called reptation in which each chain molecule is constrained by entanglements with neighboring chains to move within a virtual tube. The theory of reptation can explain polymer molecule dynamics and viscoelasticity . Depending on their chemical structures, polymers may be either semi-crystalline or amorphous. Semi-crystalline polymers can undergo crystallization and melting transitions , whereas amorphous polymers do not. In polymers, crystallization and melting do not suggest solid-liquid phase transitions, as in
3264-402: A range, so the melting point is determined by finding the highest temperature of a differential scanning calorimetry chart. Perfectly isotactic PP has a melting point of 171 °C (340 °F). Commercial isotactic PP has a melting point that ranges from 160 to 166 °C (320 to 331 °F), depending on atactic material and crystallinity. Syndiotactic PP with a crystallinity of 30% has
3400-427: A result, they typically have lower melting temperatures than other polymers. When a polymer is dispersed or dissolved in a liquid, such as in commercial products like paints and glues, the chemical properties and molecular interactions influence how the solution flows and can even lead to self-assembly of the polymer into complex structures. When a polymer is applied as a coating, the chemical properties will influence
3536-437: A separate chemical substance. However, the polymer may be defined by a known precursor or reaction(s) and the molar mass distribution . For example, polyethylene is a mixture of very long chains of -CH 2 - repeating units, and is generally sold in several molar mass distributions, LDPE , MDPE , HDPE and UHMWPE . The concept of a "chemical substance" became firmly established in the late eighteenth century after work by
3672-400: A simple mixture. Typically these have a metal, such as a copper ion, in the center and a nonmetals atom, such as the nitrogen in an ammonia molecule or oxygen in water in a water molecule, forms a dative bond to the metal center, e.g. tetraamminecopper(II) sulfate [Cu(NH 3 ) 4 ]SO 4 ·H 2 O. The metal is known as a "metal center" and the substance that coordinates to the center is called
3808-479: A single chemical substance ( allotropes ). For instance, oxygen exists as both diatomic oxygen (O 2 ) and ozone (O 3 ). The majority of elements are classified as metals . These are elements with a characteristic lustre such as iron , copper , and gold . Metals typically conduct electricity and heat well, and they are malleable and ductile . Around 14 to 21 elements, such as carbon , nitrogen , and oxygen , are classified as non-metals . Non-metals lack
3944-430: A single manufacturing process. For example, charcoal is an extremely complex, partially polymeric mixture that can be defined by its manufacturing process. Therefore, although the exact chemical identity is unknown, identification can be made with a sufficient accuracy. The CAS index also includes mixtures. Polymers almost always appear as mixtures of molecules of multiple molar masses, each of which could be considered
4080-726: A single rock is a uniform substance despite being a mixture in stoichiometric terms. Feldspars are a common example: anorthoclase is an alkali aluminum silicate, where the alkali metal is interchangeably either sodium or potassium. In law, "chemical substances" may include both pure substances and mixtures with a defined composition or manufacturing process. For example, the EU regulation REACH defines "monoconstituent substances", "multiconstituent substances" and "substances of unknown or variable composition". The latter two consist of multiple chemical substances; however, their identity can be established either by direct chemical analysis or reference to
4216-452: A statistical distribution of chain lengths, the molecular weight is expressed in terms of weighted averages. The number-average molecular weight ( M n ) and weight-average molecular weight ( M w ) are most commonly reported. The ratio of these two values ( M w / M n ) is the dispersity ( Đ ), which is commonly used to express the width of the molecular weight distribution. The physical properties of polymer strongly depend on
Polymer - Misplaced Pages Continue
4352-444: A stiffer material that is more resistant to creep than both atactic polypropylene and polyethylene. The density of PP is between 0.895 and 0.93 g/cm . Therefore, PP is the commodity plastic with the lowest density. With lower density, moldings parts with lower weight and more parts of a certain mass of plastic can be produced. Unlike polyethylene, crystalline and amorphous regions differ only slightly in their density. However,
4488-499: A technical specification instead of particular chemical substances. For example, gasoline is not a single chemical compound or even a particular mixture: different gasolines can have very different chemical compositions, as "gasoline" is primarily defined through source, properties and octane rating . Every chemical substance has one or more systematic names , usually named according to the IUPAC rules for naming . An alternative system
4624-537: A tendency to form amorphous and semicrystalline structures rather than crystals . Polymers are studied in the fields of polymer science (which includes polymer chemistry and polymer physics ), biophysics and materials science and engineering . Historically, products arising from the linkage of repeating units by covalent chemical bonds have been the primary focus of polymer science. An emerging important area now focuses on supramolecular polymers formed by non-covalent links. Polyisoprene of latex rubber
4760-432: A variety of different but structurally related monomer residues; for example, polynucleotides such as DNA are composed of four types of nucleotide subunits. A polymer containing ionizable subunits (e.g., pendant carboxylic groups ) is known as a polyelectrolyte or ionomer , when the fraction of ionizable units is large or small respectively. The microstructure of a polymer (sometimes called configuration) relates to
4896-490: A wide range of useful products, such as face masks, filters, diapers and wipes. The most common shaping technique is injection molding , which is used for parts such as cups, cutlery, vials, caps, containers, housewares, and automotive parts such as batteries. The related techniques of blow molding and injection-stretch blow molding are also used, which involve both extrusion and molding. The large number of end-use applications for polypropylene are often possible because of
5032-405: A wide-meshed cross-linking between the "main chains". Close-meshed crosslinking, on the other hand, leads to thermosets . Cross-links and branches are shown as red dots in the figures. Highly branched polymers are amorphous and the molecules in the solid interact randomly. An important microstructural feature of a polymer is its architecture and shape, which relates to the way branch points lead to
5168-442: A yellow-grey mixture. No chemical process occurs, and the material can be identified as a mixture by the fact that the sulfur and the iron can be separated by a mechanical process, such as using a magnet to attract the iron away from the sulfur. In contrast, if iron and sulfur are heated together in a certain ratio (1 atom of iron for each atom of sulfur, or by weight, 56 grams (1 mol ) of iron to 32 grams (1 mol) of sulfur),
5304-436: Is glucose vs. fructose . The former is an aldehyde , the latter is a ketone . Their interconversion requires either enzymatic or acid-base catalysis . However, tautomers are an exception: the isomerization occurs spontaneously in ordinary conditions, such that a pure substance cannot be isolated into its tautomers, even if these can be identified spectroscopically or even isolated in special conditions. A common example
5440-704: Is glucose , which has open-chain and ring forms. One cannot manufacture pure open-chain glucose because glucose spontaneously cyclizes to the hemiacetal form. All matter consists of various elements and chemical compounds, but these are often intimately mixed together. Mixtures contain more than one chemical substance, and they do not have a fixed composition. Butter , soil and wood are common examples of mixtures. Sometimes, mixtures can be separated into their component substances by mechanical processes, such as chromatography , distillation , or evaporation . Grey iron metal and yellow sulfur are both chemical elements, and they can be mixed together in any ratio to form
5576-402: Is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer propylene . Polypropylene belongs to the group of polyolefins and is partially crystalline and non-polar . Its properties are similar to polyethylene , but it is slightly harder and more heat-resistant. It is a white, mechanically rugged material and has
Polymer - Misplaced Pages Continue
5712-592: Is a chemical substance made up of a particular kind of atom and hence cannot be broken down or transformed by a chemical reaction into a different element, though it can be transmuted into another element through a nuclear reaction . This is because all of the atoms in a sample of an element have the same number of protons , though they may be different isotopes , with differing numbers of neutrons . As of 2019, there are 118 known elements, about 80 of which are stable – that is, they do not change by radioactive decay into other elements. Some elements can occur as more than
5848-501: Is a colorless, cloudy material and can be used between −15 and +120 °C. Atactic polypropylene is used as a sealant, as an insulating material for automobiles and as an additive to bitumen . Polypropylene copolymers are in use as well. A particularly important one is polypropylene random copolymer ( PPR or PP-R ), a random copolymer with polyethylene used for plastic pipework . Polypropylene random crystallinity temperature ( PP-RCT ), also used for plastic pipework ,
5984-478: Is a crucial physical parameter for polymer manufacturing, processing, and use. Below T g , molecular motions are frozen and polymers are brittle and glassy. Above T g , molecular motions are activated and polymers are rubbery and viscous. The glass-transition temperature may be engineered by altering the degree of branching or crosslinking in the polymer or by the addition of plasticizers . Whereas crystallization and melting are first-order phase transitions ,
6120-410: Is a long-chain n -alkane. There are also branched macromolecules with a main chain and side chains, in the case of polyethylene the side chains would be alkyl groups . In particular unbranched macromolecules can be in the solid state semi-crystalline, crystalline chain sections highlighted red in the figure below. While branched and unbranched polymers are usually thermoplastics, many elastomers have
6256-492: Is a measure of molecular weight of polypropylene. The measure helps to determine how easily the molten raw material will flow during processing. Polypropylene with higher MFR will fill the plastic mold more easily during the injection or blow-molding production process. As the melt flow increases, however, some physical properties, like impact strength, will decrease. There are three general types of polypropylene: homopolymer , random copolymer, and block copolymer . The comonomer
6392-410: Is a new form of this plastic. It achieves higher strength at high temperature by β- crystallization . Polypropylene is liable to chain degradation from exposure to temperatures above 100 °C. Oxidation usually occurs at the tertiary carbon centers leading to chain breaking via reaction with oxygen . In external applications, degradation is evidenced by cracks and crazing . It may be protected by
6528-400: Is also commonly present in polymer backbones, such as those of polyethylene glycol , polysaccharides (in glycosidic bonds ), and DNA (in phosphodiester bonds ). Polymerization is the process of combining many small molecules known as monomers into a covalently bonded chain or network. During the polymerization process, some chemical groups may be lost from each monomer. This happens in
6664-511: Is an average distance from the center of mass of the chain to the chain itself. Alternatively, it may be expressed in terms of pervaded volume , which is the volume spanned by the polymer chain and scales with the cube of the radius of gyration. The simplest theoretical models for polymers in the molten, amorphous state are ideal chains . Polymer properties depend of their structure and they are divided into classes according to their physical bases. Many physical and chemical properties describe how
6800-402: Is an example of complete combustion . Stoichiometry measures these quantitative relationships, and is used to determine the amount of products and reactants that are produced or needed in a given reaction. Describing the quantitative relationships among substances as they participate in chemical reactions is known as reaction stoichiometry . In the example above, reaction stoichiometry measures
6936-417: Is an example of a chemical substance, with a constant composition of two hydrogen atoms bonded to a single oxygen atom (i.e. H 2 O). The atomic ratio of hydrogen to oxygen is always 2:1 in every molecule of water. Pure water will tend to boil near 100 °C (212 °F), an example of one of the characteristic properties that define it. Other notable chemical substances include diamond (a form of
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#17328011268317072-481: Is an example of a natural polymer, and the polystyrene of styrofoam is an example of a synthetic polymer. In biological contexts, essentially all biological macromolecules —i.e., proteins (polyamides), nucleic acids (polynucleotides), and polysaccharides —are purely polymeric, or are composed in large part of polymeric components. The term "polymer" derives from Greek πολύς (polus) 'many, much' and μέρος (meros) 'part'. The term
7208-407: Is as biaxially oriented polypropylene (BOPP). These BOPP sheets are used to make a wide variety of materials including clear bags . When polypropylene is biaxially oriented, it becomes crystal clear and serves as an excellent packaging material for artistic and retail products. Polypropylene, highly colorfast, is widely used in manufacturing carpets, rugs and mats to be used at home. Polypropylene
7344-428: Is composed only of styrene -based repeat units, and is classified as a homopolymer. Polyethylene terephthalate , even though produced from two different monomers ( ethylene glycol and terephthalic acid ), is usually regarded as a homopolymer because only one type of repeat unit is formed. Ethylene-vinyl acetate contains more than one variety of repeat unit and is a copolymer. Some biological polymers are composed of
7480-409: Is defined, for small strains , as the ratio of rate of change of stress to strain. Like tensile strength, this is highly relevant in polymer applications involving the physical properties of polymers, such as rubber bands. The modulus is strongly dependent on temperature. Viscoelasticity describes a complex time-dependent elastic response, which will exhibit hysteresis in the stress-strain curve when
7616-413: Is easy to coat, print and laminate to give the required appearance and properties for use as a packaging material. This process is normally called converting . It is normally produced in large rolls which are slit on slitting machines into smaller rolls for use on packaging machines. BOPP is also used for stickers and labels in addition to OPP. It is non-reactive, which makes BOPP suitable for safe use in
7752-456: Is extruded and stretched in both the machine direction and across machine direction it is called biaxially oriented polypropylene . Two methods are widely used for producing BOPP films, namely, a bi-directional stenter process or a double-bubble blown film extrusion process. Biaxial orientation increases strength and clarity. BOPP is widely used as a packaging material for packaging products such as snack foods, fresh produce and confectionery. It
7888-423: Is given as 185 to 220 °C, the density as 0.936 to 0.946 g·cm . The β-modification is in comparison somewhat less ordered, as a result of which it forms faster and has a lower melting point of 170 to 200 °C. The formation of the β-modification can be promoted by nucleating agents, suitable temperatures and shear stress. The γ-modification is hardly formed under the conditions used in industry and
8024-522: Is given below. Chemical substance Chemical substances can exist in several different physical states or phases (e.g. solids , liquids , gases , or plasma ) without changing their chemical composition. Substances transition between these phases of matter in response to changes in temperature or pressure . Some chemical substances can be combined or converted into new substances by means of chemical reactions . Chemicals that do not possess this ability are said to be inert . Pure water
8160-447: Is made with two types of Ziegler-Natta catalysts. The first group of the catalysts encompasses solid (mostly supported) catalysts and certain types of soluble metallocene catalysts. Such isotactic macromolecules coil into a helical shape; these helices then line up next to one another to form the crystals that give commercial isotactic polypropylene many of its desirable properties. Modern supported Ziegler-Natta catalysts developed for
8296-427: Is manufactured using melt processing with either chemical or physical blowing agents. Expansion of PP in solid state, due to its highly crystalline structure, has not been successful. In this regard, two novel strategies were developed for expansion of PP. It was observed that PP can be expanded to make EPP through controlling its crystalline structure or through blending with other polymers. When polypropylene film
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#17328011268318432-457: Is poorly understood. The mesomorphic modification, however, occurs often in industrial processing, since the plastic is usually cooled quickly. The degree of order of the mesomorphic phase ranges between the crystalline and the amorphous phase, its density is with 0.916 g·cm comparatively. The mesomorphic phase is considered as cause for the transparency in rapidly cooled films (due to low order and small crystallites). Syndiotactic polypropylene
8568-432: Is the change in refractive index with temperature also known as dn/dT. For the polymers mentioned here the (dn/dT) ~ −1.4 × 10 in units of K in the 297 ≤ T ≤ 337 K range. Most conventional polymers such as polyethylene are electrical insulators , but the development of polymers containing π-conjugated bonds has led to a wealth of polymer-based semiconductors , such as polythiophenes . This has led to many applications in
8704-559: Is the main constituent of wood and paper. Hemoglycin (previously termed hemolithin ) is a space polymer that is the first polymer of amino acids found in meteorites . The list of synthetic polymers , roughly in order of worldwide demand, includes polyethylene , polypropylene , polystyrene , polyvinyl chloride , synthetic rubber , phenol formaldehyde resin (or Bakelite ), neoprene , nylon , polyacrylonitrile , PVB , silicone , and many more. More than 330 million tons of these polymers are made every year (2015). Most commonly,
8840-411: Is the relationships among the weights of reactants and products before, during, and following chemical reactions . Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equals the total mass of the products, leading to the insight that the relations among quantities of reactants and products typically form a ratio of positive integers. This means that if
8976-518: Is typically used with ethylene . Ethylene-propylene rubber or EPDM added to polypropylene homopolymer increases its low temperature impact strength. Randomly polymerized ethylene monomer added to polypropylene homopolymer decreases the polymer crystallinity, lowers the melting point and makes the polymer more transparent. [REDACTED] Polypropylene can be categorized as atactic polypropylene (aPP), syndiotactic polypropylene (sPP) and isotactic polypropylene (iPP). In case of atactic polypropylene,
9112-497: Is used by the Chemical Abstracts Service (CAS). Many compounds are also known by their more common, simpler names, many of which predate the systematic name. For example, the long-known sugar glucose is now systematically named 6-(hydroxymethyl)oxane-2,3,4,5-tetrol. Natural products and pharmaceuticals are also given simpler names, for example the mild pain-killer Naproxen is the more common name for
9248-416: Is usually carried out either in the medium of liquid propylene or in gas-phase reactors. Commercial synthesis of syndiotactic polypropylene is carried out with the use of a special class of metallocene catalysts. They employ bridged bis-metallocene complexes of the type bridge-(Cp 1 )(Cp 2 )ZrCl 2 where the first Cp ligand is the cyclopentadienyl group, the second Cp ligand is the fluorenyl group, and
9384-545: The DNA to RNA and subsequently translate that information to synthesize the specified protein from amino acids . The protein may be modified further following translation in order to provide appropriate structure and functioning. There are other biopolymers such as rubber , suberin , melanin , and lignin . Naturally occurring polymers such as cotton , starch , and rubber were familiar materials for years before synthetic polymers such as polyethene and perspex appeared on
9520-548: The Italian firm Montecatini from 1957 onwards. Syndiotactic polypropylene was also first synthesized by Natta. Interest in polypropylene development is ongoing to the present. For example, making polypropylene from bio-based resources is a topic of interest in the 21st century. Polypropylene is in little aspects similar to polyethylene , especially in solution behavior and electrical properties. The methyl group improves mechanical properties and thermal resistance, although
9656-430: The aPP obtained this way is not completely amorphous but can still contain 15% crystalline parts. Atactic polypropylene can also be produced selectively using metallocene catalysts, atactic polypropylene produced this way has a considerably higher molecular weight. Atactic polypropylene has lower density, melting point and softening temperature than the crystalline types and is tacky and rubber-like at room temperature. It
9792-509: The ability to tailor grades with specific molecular properties and additives during its manufacture. For example, antistatic additives can be added to help polypropylene surfaces resist dust and dirt. Many physical finishing techniques can also be used on polypropylene, such as machining . Surface treatments can be applied to polypropylene parts in order to promote adhesion of printing ink and paints. Expanded Polypropylene (EPP) has been produced through both solid and melt state processing. EPP
9928-441: The adhesion of the coating and how it interacts with external materials, such as superhydrophobic polymer coatings leading to water resistance. Overall the chemical properties of a polymer are important elements for designing new polymeric material products. Polymers such as PMMA and HEMA:MMA are used as matrices in the gain medium of solid-state dye lasers , also known as solid-state dye-doped polymer lasers. These polymers have
10064-532: The amounts of the separate reactants are known, then the amount of the product can be calculated. Conversely, if one reactant has a known quantity and the quantity of the products can be empirically determined, then the amount of the other reactants can also be calculated. This is illustrated in the image here, where the balanced equation is: Here, one molecule of methane reacts with two molecules of oxygen gas to yield one molecule of carbon dioxide and two molecules of water . This particular chemical equation
10200-501: The application, but higher tolerance of impurities is usually expected in the production of bulk chemicals. Thus, the user of the chemical in the US might choose between the bulk or "technical grade" with higher amounts of impurities or a much purer "pharmaceutical grade" (labeled "USP", United States Pharmacopeia ). "Chemicals" in the commercial and legal sense may also include mixtures of highly variable composition, as they are products made to
10336-433: The backbone in a variety of ways. A copolymer containing a controlled arrangement of monomers is called a sequence-controlled polymer . Alternating, periodic and block copolymers are simple examples of sequence-controlled polymers . Tacticity describes the relative stereochemistry of chiral centers in neighboring structural units within a macromolecule. There are three types of tacticity: isotactic (all substituents on
10472-638: The bridge between the two Cp ligands is -CH 2 -CH 2 -, >SiMe 2 , or >SiPh 2 . These complexes are converted to polymerization catalysts by activating them with a special organoaluminium co-catalyst, methylaluminoxane (MAO). Atactic polypropylene is an amorphous rubbery material. It can be produced commercially either with a special type of supported Ziegler-Natta catalyst or with some metallocene catalysts. Melting process of polypropylene can be achieved via extrusion and molding . Common extrusion methods include production of melt-blown and spun-bond fibers to form long rolls for future conversion into
10608-421: The case of water or other molecular fluids. Instead, crystallization and melting refer to the phase transitions between two solid states ( i.e. , semi-crystalline and amorphous). Crystallization occurs above the glass-transition temperature ( T g ) and below the melting temperature ( T m ). All polymers (amorphous or semi-crystalline) go through glass transitions . The glass-transition temperature ( T g )
10744-408: The chemical combination of the known chemical elements. As of Feb 2021, about "177 million organic and inorganic substances" (including 68 million defined-sequence biopolymers) are in the scientific literature and registered in public databases. The names of many of these compounds are often nontrivial and hence not very easy to remember or cite accurately. Also, it is difficult to keep track of them in
10880-451: The chemical compound (S)-6-methoxy-α-methyl-2-naphthaleneacetic acid. Chemists frequently refer to chemical compounds using chemical formulae or molecular structure of the compound. There has been a phenomenal growth in the number of chemical compounds being synthesized (or isolated), and then reported in the scientific literature by professional chemists around the world. An enormous number of chemical compounds are possible through
11016-460: The chemical industry, manufactured "chemicals" are chemical substances, which can be classified by production volume into bulk chemicals, fine chemicals and chemicals found in research only: The cause of the difference in production volume is the complexity of the molecular structure of the chemical. Bulk chemicals are usually much less complex. While fine chemicals may be more complex, many of them are simple enough to be sold as "building blocks" in
11152-401: The chemical resistance decreases. The properties of polypropylene depend on the molecular weight and molecular weight distribution, crystallinity, type and proportion of comonomer (if used) and the iso tacticity . In isotactic polypropylene, for example, the methyl groups are oriented on one side of the carbon backbone. This arrangement creates a greater degree of crystallinity and results in
11288-402: The chemical substances of which fruits and vegetables, for example, are naturally composed even when growing wild are not called "chemicals" in general usage. In countries that require a list of ingredients in products, the "chemicals" listed are industrially produced "chemical substances". The word "chemical" is also often used to refer to addictive, narcotic, or mind-altering drugs. Within
11424-419: The chemist Joseph Proust on the composition of some pure chemical compounds such as basic copper carbonate . He deduced that, "All samples of a compound have the same composition; that is, all samples have the same proportions, by mass, of the elements present in the compound." This is now known as the law of constant composition . Later with the advancement of methods for chemical synthesis particularly in
11560-476: The continuously linked backbone of a polymer used for the preparation of plastics consists mainly of carbon atoms. A simple example is polyethylene ('polythene' in British English), whose repeat unit or monomer is ethylene . Many other structures do exist; for example, elements such as silicon form familiar materials such as silicones, examples being Silly Putty and waterproof plumbing sealant. Oxygen
11696-534: The degree of crystallinity may be expressed in terms of a weight fraction or volume fraction of crystalline material. Few synthetic polymers are entirely crystalline. The crystallinity of polymers is characterized by their degree of crystallinity, ranging from zero for a completely non-crystalline polymer to one for a theoretical completely crystalline polymer. Polymers with microcrystalline regions are generally tougher (can be bent more without breaking) and more impact-resistant than totally amorphous polymers. Polymers with
11832-512: The density of polyethylene can significantly change with fillers. The Young's modulus of PP is between 1300 and 1800 N/mm². Polypropylene is normally tough and flexible, especially when copolymerized with ethylene . This allows polypropylene to be used as an engineering plastic , competing with materials such as acrylonitrile butadiene styrene (ABS). Polypropylene is reasonably economical. Polypropylene has good resistance to fatigue . The melting point of polypropylene occurs in
11968-400: The driving force for mixing is usually entropy , not interaction energy. In other words, miscible materials usually form a solution not because their interaction with each other is more favorable than their self-interaction, but because of an increase in entropy and hence free energy associated with increasing the amount of volume available to each component. This increase in entropy scales with
12104-531: The element carbon ), table salt (NaCl; an ionic compound ), and refined sugar (C 12 H 22 O 11 ; an organic compound ). In addition to the generic definition offered above, there are several niche fields where the term "chemical substance" may take alternate usages that are widely accepted, some of which are outlined in the sections below. Chemical Abstracts Service (CAS) lists several alloys of uncertain composition within their chemical substance index. While an alloy could be more closely defined as
12240-417: The field of organic electronics . Nowadays, synthetic polymers are used in almost all walks of life. Modern society would look very different without them. The spreading of polymer use is connected to their unique properties: low density, low cost, good thermal/electrical insulation properties, high resistance to corrosion, low-energy demanding polymer manufacture and facile processing into final products. For
12376-527: The free energy of mixing for polymer solutions and thereby making solvation less favorable, and thereby making the availability of concentrated solutions of polymers far rarer than those of small molecules. Furthermore, the phase behavior of polymer solutions and mixtures is more complex than that of small molecule mixtures. Whereas most small molecule solutions exhibit only an upper critical solution temperature phase transition (UCST), at which phase separation occurs with cooling, polymer mixtures commonly exhibit
12512-400: The glass transition is not. The glass transition shares features of second-order phase transitions (such as discontinuity in the heat capacity, as shown in the figure), but it is generally not considered a thermodynamic transition between equilibrium states. In general, polymeric mixtures are far less miscible than mixtures of small molecule materials. This effect results from the fact that
12648-414: The heat in an autoclave . Its heat resistance also enables it to be used as the manufacturing material of consumer-grade kettles . Food containers made from it will not melt in the dishwasher, and do not melt during industrial hot filling processes. For this reason, most plastic tubs for dairy products are polypropylene sealed with aluminum foil (both heat-resistant materials). After the product has cooled,
12784-660: The hinge to maximise strength. Polypropylene is used in the manufacturing of piping systems, both ones concerned with high purity and ones designed for strength and rigidity (e.g., those intended for use in potable plumbing, hydronic heating and cooling, and reclaimed water ). This material is often chosen for its resistance to corrosion and chemical leaching, its resilience against most forms of physical damage, including impact and freezing, its environmental benefits, and its ability to be joined by heat fusion rather than gluing. Many plastic items for medical or laboratory use can be made from polypropylene because it can withstand
12920-691: The human body as implantable mesh devices. The degraded material forms a tree bark-like layer at the surface of mesh fibers. PP can be made translucent when uncolored but is not as readily made transparent as polystyrene , acrylic , or certain other plastics. It is often opaque or colored using pigments. Polypropylene is produced by the chain-growth polymerization of propene : [REDACTED] The industrial production processes can be grouped into gas phase polymerization, bulk polymerization and slurry polymerization. All state-of-the-art processes use either gas-phase or bulk reactor systems. The properties of PP are strongly affected by its tacticity ,
13056-398: The idea of stereoisomerism – that atoms have rigid three-dimensional structure and can thus form isomers that differ only in their three-dimensional arrangement – was another crucial step in understanding the concept of distinct chemical substances. For example, tartaric acid has three distinct isomers, a pair of diastereomers with one diastereomer forming two enantiomers . An element
13192-415: The individual chains more strongly in position and resist deformations and matrix breakup, both at higher stresses and higher temperatures. Copolymers are classified either as statistical copolymers, alternating copolymers, block copolymers, graft copolymers or gradient copolymers. In the schematic figure below, Ⓐ and Ⓑ symbolize the two repeat units . Monomers within a copolymer may be organized along
13328-414: The iron, since there is no metallic iron present in the compound. While the term chemical substance is a precise technical term that is synonymous with chemical for chemists, the word chemical is used in general usage to refer to both (pure) chemical substances and mixtures (often called compounds ), and especially when produced or purified in a laboratory or an industrial process. In other words,
13464-425: The large number of chemical substances reported in chemistry literature need to be indexed. Isomerism caused much consternation to early researchers, since isomers have exactly the same composition, but differ in configuration (arrangement) of the atoms. For example, there was much speculation about the chemical identity of benzene , until the correct structure was described by Friedrich August Kekulé . Likewise,
13600-446: The latter case, increasing the polymer chain length 10-fold would increase the viscosity over 1000 times. Increasing chain length furthermore tends to decrease chain mobility, increase strength and toughness, and increase the glass-transition temperature (T g ). This is a result of the increase in chain interactions such as van der Waals attractions and entanglements that come with increased chain length. These interactions tend to fix
13736-434: The length (or equivalently, the molecular weight) of the polymer chain. One important example of the physical consequences of the molecular weight is the scaling of the viscosity (resistance to flow) in the melt. The influence of the weight-average molecular weight ( M w {\displaystyle M_{w}} ) on the melt viscosity ( η {\displaystyle \eta } ) depends on whether
13872-651: The lids are often made of somewhat more flexible LDPE so they can snap onto the container to close it. Polypropylene can also be made into disposable bottles to contain liquid, powdered, or similar consumer products, although HDPE and polyethylene terephthalate are commonly also used to make bottles. Plastic pails, car batteries, wastebaskets, pharmacy prescription bottles, cooler containers, dishes and pitchers are often made of polypropylene or HDPE, both of which commonly have rather similar appearance, feel, and properties at ambient temperature. An abundance of medical devices are made from PP. A common application for polypropylene
14008-409: The literature. Several international organizations like IUPAC and CAS have initiated steps to make such tasks easier. CAS provides the abstracting services of the chemical literature, and provides a numerical identifier, known as CAS registry number to each chemical substance that has been reported in the chemical literature (such as chemistry journals and patents ). This information is compiled as
14144-417: The load is removed. Dynamic mechanical analysis or DMA measures this complex modulus by oscillating the load and measuring the resulting strain as a function of time. Transport properties such as diffusivity describe how rapidly molecules move through the polymer matrix. These are very important in many applications of polymers for films and membranes. The movement of individual macromolecules occurs by
14280-436: The market. Many commercially important polymers are synthesized by chemical modification of naturally occurring polymers. Prominent examples include the reaction of nitric acid and cellulose to form nitrocellulose and the formation of vulcanized rubber by heating natural rubber in the presence of sulfur . Ways in which polymers can be modified include oxidation , cross-linking , and end-capping . The structure of
14416-411: The material will endure before failure. This is very important in applications that rely upon a polymer's physical strength or durability. For example, a rubber band with a higher tensile strength will hold a greater weight before snapping. In general, tensile strength increases with polymer chain length and crosslinking of polymer chains. Young's modulus quantifies the elasticity of the polymer. It
14552-411: The metallic properties described above, they also have a high electronegativity and a tendency to form negative ions . Certain elements such as silicon sometimes resemble metals and sometimes resemble non-metals, and are known as metalloids . A chemical compound is a chemical substance that is composed of a particular set of atoms or ions . Two or more elements combined into one substance through
14688-407: The methyl group (-CH 3 ) is randomly aligned, alternating (alternating) for syndiotactic polypropylene and evenly for isotactic polypropylene. This has an impact on the crystallinity (amorphous or semi-crystalline) and the thermal properties (expressed as glass transition point T g and melting point T m ). The term tacticity describes for polypropylene how the methyl group is oriented in
14824-424: The molecular arrangement of the polymer chains. The crystalline modifications are categorized into the α-, β- and γ-modification as well as mesomorphic (smectic) forms. The α-modifications is predominant in iPP. Such crystals are built from lamellae in the form of folded chains. A characteristic anomaly is that the lame are arranged in the so-called "cross-hatched" structure. The melting point of α-crystalline regions
14960-444: The number of particles (or moles) being mixed. Since polymeric molecules are much larger and hence generally have much higher specific volumes than small molecules, the number of molecules involved in a polymeric mixture is far smaller than the number in a small molecule mixture of equal volume. The energetics of mixing, on the other hand, is comparable on a per volume basis for polymeric and small molecule mixtures. This tends to increase
15096-458: The orientation of the methyl groups ( CH 3 ) relative to the methyl groups in neighboring monomer units. A Ziegler–Natta catalyst is able to restrict linking of monomer molecules to a specific orientation, either isotactic, when all methyl groups are positioned at the same side with respect to the backbone of the polymer chain, or syndiotactic, when the positions of the methyl groups alternate. Commercially available isotactic polypropylene
15232-400: The other hand, lacks any regularity, which prevents it from crystallization, thereby creating an amorphous material. Isotactic polypropylene has a high degree of crystallinity , in industrial products 30–60%. Syndiotactic polypropylene is slightly less crystalline, atactic PP is amorphous (not crystalline). Is polypropylene can exist in various crystalline modifications which differ by
15368-719: The partially negatively charged oxygen atoms in C=O groups on another. These strong hydrogen bonds, for example, result in the high tensile strength and melting point of polymers containing urethane or urea linkages. Polyesters have dipole-dipole bonding between the oxygen atoms in C=O groups and the hydrogen atoms in H-C groups. Dipole bonding is not as strong as hydrogen bonding, so a polyester's melting point and strength are lower than Kevlar 's ( Twaron ), but polyesters have greater flexibility. Polymers with non-polar units such as polyethylene interact only through weak Van der Waals forces . As
15504-409: The pharmaceutical and food industry. It is one of the most important commercial polyolefin films. BOPP films are available in different thicknesses and widths. They are transparent and flexible. As polypropylene is resistant to fatigue, most plastic living hinges , such as those on flip-top bottles, are made from this material. However, it is important to ensure that chain molecules are oriented across
15640-421: The physical arrangement of monomer residues along the backbone of the chain. These are the elements of polymer structure that require the breaking of a covalent bond in order to change. Various polymer structures can be produced depending on the monomers and reaction conditions: A polymer may consist of linear macromolecules containing each only one unbranched chain. In the case of unbranched polyethylene, this chain
15776-399: The polymer and create gaps between polymer chains for greater mobility and fewer interchain interactions. A good example of the action of plasticizers is related to polyvinylchlorides or PVCs. A uPVC, or unplasticized polyvinylchloride, is used for things such as pipes. A pipe has no plasticizers in it, because it needs to remain strong and heat-resistant. Plasticized PVC is used in clothing for
15912-425: The polymer at the scale of a single chain. The microstructure determines the possibility for the polymer to form phases with different arrangements, for example through crystallization , the glass transition or microphase separation . These features play a major role in determining the physical and chemical properties of a polymer. The identity of the repeat units (monomer residues, also known as "mers") comprising
16048-543: The polymer can lend the polymer to ionic bonding or hydrogen bonding between its own chains. These stronger forces typically result in higher tensile strength and higher crystalline melting points. The intermolecular forces in polymers can be affected by dipoles in the monomer units. Polymers containing amide or carbonyl groups can form hydrogen bonds between adjacent chains; the partially positively charged hydrogen atoms in N-H groups of one chain are strongly attracted to
16184-463: The polymer chain. Commercial polypropylene is usually isotactic. This article therefore always refers to isotactic polypropylene, unless stated otherwise. The tacticity is usually indicated in percent, using the isotactic index (according to DIN 16774). The index is measured by determining the fraction of the polymer insoluble in boiling heptane . Commercially available polypropylenes usually have an isotactic index between 85 and 95%. The tacticity effects
16320-402: The polymer is above or below the onset of entanglements . Below the entanglement molecular weight, η ∼ M w 1 {\displaystyle \eta \sim {M_{w}}^{1}} , whereas above the entanglement molecular weight, η ∼ M w 3.4 {\displaystyle \eta \sim {M_{w}}^{3.4}} . In
16456-582: The polymerization of PET polyester . The monomers are terephthalic acid (HOOC—C 6 H 4 —COOH) and ethylene glycol (HO—CH 2 —CH 2 —OH) but the repeating unit is —OC—C 6 H 4 —COO—CH 2 —CH 2 —O—, which corresponds to the combination of the two monomers with the loss of two water molecules. The distinct piece of each monomer that is incorporated into the polymer is known as a repeat unit or monomer residue. Synthetic methods are generally divided into two categories, step-growth polymerization and chain polymerization . The essential difference between
16592-412: The polymerization of propylene and other 1-alkenes to isotactic polymers usually use TiCl 4 as an active ingredient and MgCl 2 as a support. The catalysts also contain organic modifiers, either aromatic acid esters and diesters or ethers. These catalysts are activated with special co-catalysts containing an organoaluminium compound such as Al(C 2 H 5 ) 3 and the second type of
16728-443: The polymers physical properties . As the methyl group is in isotactic propylene consistently located at the same side, it forces the macromolecule in a helical shape , as also found in starch . An isotactic structure leads to a semi-crystalline polymer . The higher the isotonicity(the isotactic fraction), the greater the crystallinity, and thus also the softening point, rigidity, e-modulus and hardness. Atactic polypropylene, on
16864-410: The realm of organic chemistry ; the discovery of many more chemical elements and new techniques in the realm of analytical chemistry used for isolation and purification of elements and compounds from chemicals that led to the establishment of modern chemistry , the concept was defined as is found in most chemistry textbooks. However, there are some controversies regarding this definition mainly because
17000-444: The relationship between the quantities of methane and oxygen that react to form carbon dioxide and water. Because of the well known relationship of moles to atomic weights , the ratios that are arrived at by stoichiometry can be used to determine quantities by weight in a reaction described by a balanced equation. This is called composition stoichiometry . Polypropylene Polypropylene ( PP ), also known as polypropene ,
17136-437: The same side), atactic (random placement of substituents), and syndiotactic (alternating placement of substituents). Polymer morphology generally describes the arrangement and microscale ordering of polymer chains in space. The macroscopic physical properties of a polymer are related to the interactions between the polymer chains. When applied to polymers, the term crystalline has a somewhat ambiguous usage. In some cases,
17272-510: The size of individual polymer coils in solution. A variety of techniques may be employed for the synthesis of a polymeric material with a range of architectures, for example living polymerization . A common means of expressing the length of a chain is the degree of polymerization , which quantifies the number of monomers incorporated into the chain. As with other molecules, a polymer's size may also be expressed in terms of molecular weight . Since synthetic polymerization techniques typically yield
17408-447: The states is known as a coil–globule transition . Inclusion of plasticizers tends to lower T g and increase polymer flexibility. Addition of the plasticizer will also modify dependence of the glass-transition temperature T g on the cooling rate. The mobility of the chain can further change if the molecules of plasticizer give rise to hydrogen bonding formation. Plasticizers are generally small molecules that are chemically similar to
17544-454: The synthesis of more complex molecules targeted for single use, as named above. The production of a chemical includes not only its synthesis but also its purification to eliminate by-products and impurities involved in the synthesis. The last step in production should be the analysis of batch lots of chemicals in order to identify and quantify the percentages of impurities for the buyer of the chemicals. The required purity and analysis depends on
17680-686: The term crystalline finds identical usage to that used in conventional crystallography . For example, the structure of a crystalline protein or polynucleotide, such as a sample prepared for x-ray crystallography , may be defined in terms of a conventional unit cell composed of one or more polymer molecules with cell dimensions of hundreds of angstroms or more. A synthetic polymer may be loosely described as crystalline if it contains regions of three-dimensional ordering on atomic (rather than macromolecular) length scales, usually arising from intramolecular folding or stacking of adjacent chains. Synthetic polymers may consist of both crystalline and amorphous regions;
17816-527: The tertiary carbon atom, PP is chemically less resistant than PE (see Markovnikov rule ). Most commercial polypropylene is isotactic and has an intermediate level of crystallinity between that of low-density polyethylene (LDPE) and high-density polyethylene (HDPE). Isotactic & atactic polypropylene is soluble in p -xylene at 140 °C. Isotactic precipitates when the solution is cooled to 25 °C and atactic portion remains soluble in p -xylene. The melt flow rate (MFR) or melt flow index (MFI)
17952-520: The tubs are often given lids made of a less heat-resistant material, such as LDPE or polystyrene. Such containers provide a good hands-on example of the difference in modulus, since the rubbery (softer, more flexible) feeling of LDPE with respect to polypropylene of the same thickness is readily apparent. Rugged, translucent, reusable plastic containers made in a wide variety of shapes and sizes for consumers from various companies such as Rubbermaid and Sterilite are commonly made of polypropylene, although
18088-553: The two is that in chain polymerization, monomers are added to the chain one at a time only, such as in polystyrene , whereas in step-growth polymerization chains of monomers may combine with one another directly, such as in polyester . Step-growth polymerization can be divided into polycondensation , in which low-molar-mass by-product is formed in every reaction step, and polyaddition . Newer methods, such as plasma polymerization do not fit neatly into either category. Synthetic polymerization reactions may be carried out with or without
18224-401: The use of various polymer stabilizers , including UV-absorbing additives and anti-oxidants such as phosphites (e.g. tris(2,4-di-tert-butylphenyl)phosphite ) and hindered phenols, which prevent polymer degradation . Microbial communities isolated from soil samples mixed with starch have been shown to be capable of degrading polypropylene. Polypropylene has been reported to degrade while in
18360-601: Was coined in 1833 by Jöns Jacob Berzelius , though with a definition distinct from the modern IUPAC definition. The modern concept of polymers as covalently bonded macromolecular structures was proposed in 1920 by Hermann Staudinger , who spent the next decade finding experimental evidence for this hypothesis. Polymers are of two types: naturally occurring and synthetic or man made . Natural polymeric materials such as hemp , shellac , amber , wool , silk , and natural rubber have been used for centuries. A variety of other natural polymers exist, such as cellulose , which
18496-474: Was discovered much later than isotactic PP and could only be prepared by using metallocene catalysts . Syndiotactic PP has a lower melting point, with 161 to 186 °C, depending on the degree of tacticity. Atactic polypropylene is amorphous and has therefore no crystal structure. Due to its lack of crystallinity, it is readily soluble even at moderate temperatures, which allows to separate it as by-product from isotactic polypropylene by extraction . However,
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