Molecular weight of resin

Dusek (1), Shy (2 and Bauer (3) give examples of modelling the structure-property relation of several networks (Tg, gel point, etc.). Examples are described of models on solvent evaporation, calculation of functionalities, molecular weight of resins (4), etc. 

As indicated previously, a significant feature of these resins is the very low content (<0.1%) of the monomers, phenol and formaldehyde. The molecular weights of resins used in plywood manufacture are higher, and the viscosity is in the range of 700 (wet process) or 450 mPa s (dry process) (9). In the manufacture of particle boards, which consist of —95% by weight of wood chips bonded together by the adhesive, the viscosity of the resin used is much lower, about 130 mPa s (9). In both cases, the resin has a dry solids content of 40-50%. 

The molecular weight of the polymer formed is a function of the reaction temperature used during the polymerization. Lower temperatures favor increased molecular weights. The relationship between polymerization temperature and molecular weight of resin (as expressed by relative viscosity) is shown in Figure 6. In commercial practice, the molecular weight is controlled by reaction temperature, and the reaction rate is controlled by the selection of the initiator and its concentration. 

Copolymer resin (p-CAF) was synthesized by the condensation of p-cresol and adipamide with formaldehyde in the presence of hydrochloric acid as catalyst and using varied molar ratios of reacting monomers. A composition of the copolymers has been determined on the basis of their elemental analysis. The number average molecular weight of resins was determined by conductometric titration in non-aqueous medium. The copolymer resins were characterized by viscometric measurements in dimethylsulphoxide (DMSO), UV-visible absorption spectra in non-aqueous medium, infrared (IR) spectra, and nuclear magnetic resonance (NMR) spectra. The morphology of the copolymers was studied by scanning electron microscopy (SEM). 

Although solution viscosity of PLA in solvent is not directly relevant to the processing of molten PLA polymers, this property is often evaluated to determine the molecular weight of resins and processed parts for quality control purposes. The relationship between viscosity and the molecular weight of PLA dissolved in a dilute solution is commonly modeled using the Mark—Houwink equation ... 

More recently, Class and Chu" extended the use of dynamic mechanical measurements to a systematic study of resin-elastomer blends which revealed the relationship between the structure, concentration and molecular weight of resins and their effect on the viscoelastic properties of elastomers. Dynamic mechanical data typical of that obtained from an elastomer or elastomer-resin blend is shown in Fig. 4. G is the elastic or storage modulus, G" is the viscous or loss modulus, and the ratio of G jG gives the tan 6 curve. The temperature at which the tan 6 curve shows a maximum corresponds to a dynamic glass transition temperature. Class and Chu showed that with these types of measurements, the effect of modifying resins on the viscoelastic properties of elastomers can be readily determined. Resins which are compatible with an elastomer will cause a decrease in the elastic modulus G at room temperature and an increase in the tan delta peak or glass transition temperature. Resins which are incompatible with an elastomer will cause an increase in the elastic modulus G at room temperature and will show two distinct maxima in the tan delta curve. 

M c = true molecular weight of effective chains M = number-average molecular weight of resin prior to cross-... 

The number-average molecular weight of most commercially available acetal resins is between 20,000 and 90,000. Weight-average molecular weight may be estimated from solution viscosities. 

Formaldehyde—Alcohol Solutions. These solutions are blends of concentrated aqueous formaldehyde, the alcohol, and the hemiacetal. Methanol decreases the average molecular weight of formaldehyde oligomers by formation of lower molecular weight hemiacetals. These solutions are used to produce urea and melamine resins the alcohol can act as the resin solvent and as a reactant. The low water content can improve reactivity and reduce waste disposal and losses. Typical specifications for commercially available products are shown in Table 7 (117). 

With the improvement of refining and purification techniques, many pure olefinic monomers are available for polymerization. Under Lewis acid polymerization, such as with boron trifluoride, very light colored resins are routinely produced. These resins are based on monomers such as styrene, a-methylstryene, and vinyltoluene (mixed meta- and i ra-methylstyrene). More recently, purified i ra-methylstyrene has become commercially available and is used in resin synthesis. Low molecular weight thermoplastic resins produced from pure styrene have been available since the mid-1940s resins obtained from substituted styrenes are more recent. 

Blends of piperylenes and amylenes (mixed 2-methyl-1-butene and 2-methyl-2-butene) or UOP propylene dimers can be adjusted to produce softening points of 0—100°C and weight average molecular weights of <1200 (32,33). Careful control of the diolefin/branched olefin ratio is the key to consistent resin properties (34). 

Styrenic block copolymers (SBCs) are also widely used in HMA and PSA appHcations. Most hot melt appHed pressure sensitive adhesives are based on triblock copolymers consisting of SIS or SBS combinations (S = styrene, I = isoprene B = butadiene). Pressure sensitive adhesives typically employ low styrene, high molecular weight SIS polymers while hot melt adhesives usually use higher styrene, lower molecular weight SBCs. Resins compatible with the mid-block of an SBC improves tack properties those compatible with the end blocks control melt viscosity and temperature performance. 

Automotive and architectural laminates of PVB develop maximum impact strength near 20°C, as shown in Figure 2. This balance is obtained by the plasticizer-to-resin ratio and the molecular weight of the resins. It has been adjusted to this optimum temperature based on environmental conditions and automobile population at various ambient temperatures. The frequency and severity of vehicle occupant injuries vs temperature ranges at the accident location have been studied (5), and the results confirm the selection of the maximum performance temperature and decreasing penetration resistance at temperature extremes. 

Chain Length. The total number of monomer units (which approximately equals x - - y - - z. in the above formula) in PE chains is called the degree of polymerization. It can vary from small (about 10—20 in PE waxes) to very large (over 100,000 for PE of ultrahigh molecular weight (UHMW)). Consequentiy, the molecular weights of PE resins can range from several hundred to several million. 

As with nearly all other polymers, HDPE resin is a collection of polymer chains of different lengths, varying from short, with molecular weights of 500—1000, to very long, with molecular weights of over 10 million. Relative contents of chains with different lengths (ie, the shape and width of MWD) depend mostly on production technology and on the type of catalyst used for polymerization. The MWD width of HDPE resins can be tailored to specific apphcations. 

Molecular Weight. The range of molecular weights of commercial LLDPE resias is relatively narrow, usually from 50,000 to 200,000. One accepted parameter that relates to the resin molecular weight is the melt index, a rheological parameter which, broadly defined, is inversely proportional to molecular weight. A typical melt index range for LLDPE resias is from 0.1 to 5.0, but can reach over 30 for some appHcations. 

Strong-Acid Catalysts, Novolak Resins. PhenoHc novolaks are thermoplastic resins having a molecular weight of 500—5000 and a glass-transition temperature, T, of 45—70°C. The phenol—formaldehyde reactions are carried to their energetic completion, allowing isolation of the resin ... 

A typical resin has an initial molecular weight of 150 to perhaps 1500. For systems of unsubstituted phenols, the final cross-link density is 150—300 atomic mass units (amu) per cross-link. In other words, 25—75% of the ring-joining reactions occur during the cure phase. 

Fig. 9. Melt flow index as a function of temperature for varying molecular weights of poly(ethylene oxide). WSR = Polyox water-soluble resins.

Molecular Weight. Measurement of intrinsic viscosity in water is the most commonly used method to determine the molecular weight of poly(ethylene oxide) resins. However, there are several problems associated with these measurements (86,87). The dissolved polymer is susceptible to oxidative and shear degradation, which is accelerated by filtration or dialysis. If the solution is purified by centrifiigation, precipitation of the highest molecular weight polymers can occur and the presence of residual catalyst by-products, which remain as dispersed, insoluble soHds, further compHcates purification. 

One of the important attributes of alkyds is their good compatibiUty with a wide variety of other coating polymers. This good compatibiUty comes from the relatively low molecular weight of the alkyds, and the fact that the resin stmcture contains, on the one hand, a relatively polar and aromatic backbone, and, on the other hand, many aUphatic side chains with low polarity. An alkyd resin in a blend with another coating polymer may serve as a modifier for the other film-former, or it may be the principal film-former and the other polymer may serve as the modifier for the alkyd to enhance certain properties. Examples of compatible blends foUow. 

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