Polymer Additives and Reinforcements

Transcription

1 Polymer Additives and Reinforcements

2 Polymer solubility-1 Schematic representation of the dissolution process for polymer molecules a) Polymer molecules in solid state just after being added to a solvent c) Second step: solvated polymer molecules dispersed into solution b) First step: a swollen gel in solvent Solubility depends on; Crystalinity Molecular weight Branching Polarity Crosslinking degree

3 Polymer solubility-2 G m H m T S m Gibbs free energy Entropy change during mixing Enthalpy change during mixing Solubility will occur if the free energy of mixing Gm is negative. The entropy of mixing is believed to be always negative. Therefore, the sign and magnitude of Hm determine the sign of Gm.

4 Polymer solubility-3 Two-dimensional lattice model of solubility for a low molecular weight solute Two-dimensional lattice model of solubility for a polymer solute

5 Water absorption of polymers Polymer Water absorption(%) polyester 0,2 polycarbonate 0,2 polystyrene 0,1 polyarcylonitrile 1,5 nylon 6,6 1,5 nylon 6 1,5 polytetrafluoroethylene 0 polypropylene 0 polyethylene 0 polyethyleneoxide 0,4

6 Polymer combustion combustion products flammable Region gas layer pyrolysis layer Heated layer Inner portion that is not affected from combustion Polymer Combustion rate (mm/min) Sytrene-butadiene rubber 57 LDPE 39 HDPE 23 polypropylene 34 ABS 56

7 Conductive Polymers

8 Electrical Properties-2 Polymer Dielectric constant (1 khz) Dielectric Force (kv/mm) Natural rubber 2,9 50 ABS 2,9 18 Polycarbonate 4,4 33 Nylon 6,6 4,1 16 Polyethylene terephthalate 3,0 15 Poly (vinyl chloride) 3,4 22 Polyethylene 2,5 18

9 Density Material Density (g/cm 3 ) Material Density (g/cm 3 ) Al 2,70 PVAc 1,19 Fe 7,87 PP 0,90-0,92 Cu 8,96 Nylon 11 1,04 Au 19,32 Nylon 12 1,02 LDPE 0,91-0,94 Nylon 6 1,12-1,13 PS 1,05 Nylon 6,6 1,13-1,15 PVC 1,37-1,39 PC 1,2 Natural rubber 0,91 PAN 1,17 PET 1,34-1,39 PFTE 2,27

10 Polymer Additives and Reinforcements Additives are usually required; To impart stability against the degradative effects of various kinds of aging processes Enhance product quality and performance. Thermal and light stabilizers, antioxidants, and flame retardants. (influence essentially the chemical interaction of polymers with the environment) Plasticizers, lubricants, impact modifiers, antistatic agents, pigments, and dyes. (usually employed to reduce costs, improve aesthetic qualities, or modify the processing, mechanical, and physical behavior of a polymer) These additives are normally used in relatively small quantities Nonreinforcing fillers are employed in large quantities to reduce overall formulation costs provided this does not result in significant or undesirable reduction in product quality or performance Alloying and blending

11 Plasticizers-I The principal function of a plasticizer is to reduce the Tg of a polymer so as to enhance its flexibility over expected temperatures of application. Plasticizers are usually high boiling organic liquids or low melting solids. They are also sometimes moderate-molecular-weight polymers. Like ordinary solvents, plasticizers act through a varying degree of solvating action on the polymer. Plasticization is difficult to achieve in nonpolar polymers like polyolefins and highly crystalline polymers. Polymer plasticization can be achieved either through internal or external incorporation of the plasticizer into the polymer. a) Internal plasticization involves copolymerization of the monomers of the desired polymer and that of the plasticizer so that the plasticizer is an integral part of the polymer chain. Tg. The most widely used internal plasticizer monomers are vinyl acetate and vinylidene chloride. b) External plasticizers are those incorporated into the resin as an external additive. Typical low-molecular-weight external plasticizers for PVC are esters formed from the reaction of acids or acid anhydrides with alcohols.

12 Plasticizers-II Monomeric plasticizers ( phthalate, terephthalate, adipate, phosphate esters) Polymeric and permanent plasticizers (Linear polyesters obtained from the reaction of dibasic acids such as adipic, sebacic, and azelaic acids with a polyol ) Polymeric stabilizers have higher molecular weights than the monomeric plasticizers, and less volatile when exposed to high temperatures either during processing or in the end-use situations, less susceptible to migration and less extractible. Epoxy Plasticizers (derived from vegetable oil, epoxidized soybean is an example) confer heat stability and light stability on PVC products have relatively low-temperature properties The ideal plasticizer must satisfy three requirements; Compatibility Permanence requires low volatility, extractability, nonmigration, and heat and light stability. Lack of permanence involves long-term diffusion into the environment. Efficiency Also it should be odorless, tasteless, nontoxic, nonflammable and heat stable

13 Fillers and Reinforcements (Composites)-1 Different types of fillers are employed in resin formulations. -Added to improve tensile strength & abrasion resistance, toughness & decrease cost The most common are; calcium carbonate,talc, silica, wollastonite, clay, calcium sulfate, mica, glass structures, and alumina trihydrate.

14 Fillers and Reinforcements (Composites)-2 Particulate-filled composites are generally isotropic (they are invariant with direction provided there is a good dispersion of the fillers) Fiber-filled composites are typically anisotropic. Fibers are usually oriented either uniaxially or randomly in a plane. In this case, the composite has maximum modulus and strength values in the direction of fiber orientation. For uniaxially oriented fibers, Young s modulus, measured in the orientation direction (longitudinal Modulus, E L ) Where E f is the tensile modulus of the fiber, E m is the modulus of the matrix resin and is the volume fraction of the filler. f

15 Alloys and Blends-I An alternative to the development of new polymers is the development of alloys and blends that are a physical combination of two or more polymers to form a new material. To combine the best properties of each component in a single functional material that consequently has properties beyond those available with the individual resin components and that is tailored to meet specific requirements. To optimize cost/performance index and improve processability of a high-temperature or heat-sensitive polymer. The composition dependence of a given property, P, of a two-component polymer system may be described by; where P 1 and P 2 are the values of the property for the isolated components and C 1, C 2 are, respectively, the concentrations of the components of the system. I is the interaction parameter that measures the magnitude of synergism resulting from combining the two components

16 Alloys and Blends-II

17 Alloys and Blends-III Examples of the most significant commercial engineering alloys are polystyrene (PS)-modified poly(phenylene oxide) (PPO) and polystyrene (PS)- modified poly(phenylene ether) (PPE). We can combine amorphous polymer with a crystalline polymers to exploit the strengths of each component while deemphasizing their weakness. Example: Nylon, PET, PBT- crystalline polymers and offer excellent resistance, processing ease and stiffness PC and polysulfone (amorphous polymers) outstanding impact strength and dimensional stability. PC/PET blends (replacement for metal, including automotive, lawn and garden appliances and electrical/electronic,consumer, industrial/mechanical, sporting and recreation, and military equipment.) Nylon/PPO blends (fenders and rocker panels of some automobiles, applications demanding chemical resistant performance under high impact and high heat.

18 Antioxidants and Thermal Stabilizers Antioxidants Free radical scavengers (Primary antioxidants, radical or chain terminators)-inhibit oxidation through reaction with chain-propagating radicals Ex: hindered phenols and aromatic amines Peroxide decomposers (secondary antioxidants or synergists)- break down peroxides into nonradical and stable products. Ex:Organic phosphites and thioesters that serve to suppress homolytic breakdown. Thermal Stabilizers; Thermal stabilizers may be based on one or a combination of the following classes of compounds; Barium/cadmium (Ba-Cd), calcium/zinc (Ca-Zn), organotin, organo-antimony, phosphite chelates, and epoxy plasticizers. Ba/Cd stabilizer systems, which represent the largest share of the PVC stabilizer market, are available as liquids or powders.

19 UV stabilizers UV radiation in the range 290 to 400 nm has potentially degradative effects on polymers since most polymers contain chemical groups that absorb this radiation and undergo chain scission, forming free radicals that initiate the degradative reactions. UV stabilizers are employed to impede or eliminate the process of degradation and, as such, ensure the long-term stability of polymers, particularly during outdoor exposure. Light stabilizers are typically UV absorbers or quenchers. The former preferentially absorbs UV radiation more readily than the polymer, converting the energy into a harmless form. Quenchers exchange energy with the excited polymer molecules by means of an energy transfer mechanism. Other UV stabilizers deactivate the harmful free radicals and hydroperoxides as soon as they are formed

20 Flame Retardants The function of flame retardants in a resin formulation is ideally the outright inhibition of ignition where possible. Where this is impossible, a flame retardant should slow down ignition significantly and/or inhibit flame propagation as well as reduce smoke evolution and its effects. The presence of flame retardants also tends to cause substantial changes in the processing and ultimate behavior of commercial resins. The burning characteristics of polymers are modified by certain compounds; aluminatrihydrates; bromine compounds; chlorinated paraffins and cycloaliphatics; phosphorus compounds,notably phosphate esters; and antimony oxides, which are used basically as synergists with bromine and chlorine compounds Flame retardants can be classified as; (based on the method of incorporation in the resin formulation or mode of action) Additives Reactives Intumescents Nonflame retardant systems

21 Colorants-I The marketability of a polymer product quite frequently depends on its color; therefore the purpose of adding a colorant to a resin is to overcome or mask its undesirable color characteristics and enhance its aesthetic value without seriously compromising its properties and performance. Colorants are available as; (they can be natural or synthetic) Organic pigments -generally transparent, good brightness, variable heat stability, light and migration fastness Dyes-stronger, brighter and more transparent than pigments - have poor migration fastness Inorganic Pigments largely mixed with metal oxides, generally good to excellent light and migration fastness but variable chemical resistance Colorants are used in polymers either as raw pigments (and dyes), concentrates or precolored compounds. Colorants are avalaible in a variety or forms, including pellets, cubes, granules, powder, liquid, and paste dispersions

22 Colorants-II Selecting a colorant for particular application; The ability of the colorant to provide the desired color effect Withstand not only process temperature (heat stability) encountered during manufacture but also, for possible prolonged times, the temperature in the end-use requirements Migration fastness is related to the solubility of the colorant in the polymer. Color migration is manifested by bleeding, blooming or plate-out Inadequate light fastness of a colorant is manifested in the form of fading or darkening The colorant must be compatible with the base resin forming a homogeneous mass and also it must not degrade or be degraded by the resin Incompatibility of the colorant can affect mechanical properties, flame retardancy, weatherability, chemical and UV resistance, heat stability of the resin through interaction of the colorant with the resin and its additives.

23 Antistatic agents Antistatic agents are hydroscopic chemicals that can generate layer of water for the removal of static charges generated on the polymer by pulling moisture from the atmosphere. There are essentially two types of antistats that are commonly used in polymers to get rid of static electricity: those that are applied topically and those that are incorporated internally into the polymer. Both improve the conductance of polymer surfaces by absorbing and holding a thin, invisible layer of moisture from the surrounding air onto the polymer surface. Major types of organic antistatic agents include quaternary ammonium compounds, amines and their derivatives, phosphate esters, fatty acid polyglycol esters, and polyhydric alcohol derivatives such as glycerine and sorbitol. Selection of the appropriate antistat depends on its compatibility with the polymer, the end use of the part, and the desired level of antistatic activity. Other factors that need to be considered include the effect of antistatic agent on color, transparency, and finish of the polymer part; its possible toxicity; stability during processing; and degree of interference with physical properties and ultimately cost effectiveness.