Degradation melt flow index

Too little has been published about the flow properties of PET as a criterion for processing. The results of melt flow index (MFI) testing conditions do not correlate with the processing behavior in the case of PET. This may be caused by the discrepancy between the shear rates in testing and processing. MFI is defined as the amount of polymer melt (in g) extruded within 10 min through an orifice of specified diameter at a standard load and temperature. In the case of PET, this method was not very popular until recently due to the sensitivity of this material to hydrolytic degradation. 

The processing of polymers should occur with dry materials and with control of the atmosphere so that oxidative reactions may be either avoided, to maintain the polymer s molar mass, or exploited to maximize scission events (in order to raise the melt-flow index). The previous sections have considered the oxidative degradation of polymers and its control in some detail. What has not been considered are reactions during processing that do not involve oxidation but may lead to scission of the polymer chain. Examples include the thermal scission of aliphatic esters by an intramolecular abstraction (Scheme 1.51) (Billingham et al., 1987) and acid- or base- catalysed hydrolysis of polymers such as polyesters and polyamides (Scheirs, 2000). If a polymer is not dry, the evolution of steam at the processing temperature can lead to physical defects such as voids. However, there can also be chemical changes such as hydrolysis that can occur under these conditions. 

Metal oxide semiconductor chemical sensors in combination with MDA have been shown to be useful to estimate the oxidative stability of polypropylene during processing instead of traditional melt flow index analysis (50). An array of sensors was used to receive a detailed analysis of volatiles. At quality measurements of different poly(butylene adipate)s the use of indicator products has been proven better than analyses of the decrease in molecular weight or mass loss for early degradation detection. Adipic acid, quantified using gas chromatography, was then used as the indicator product [51]. 

Nevertheless, the melt flow indexer is a very useful instrument besides measuring MFI. It can be used for studying plastic thermal degradation, by consecutive running of the same material or by running hot melt slow and recording MFI over time. By doing so, various antioxidants and their various amounts can be tested... 

Thermoplastic elastomers (TPEs) with blocks of polydiene rubber are subject to degradation at the carbon-carbon double-bond sites and require proper stabilization. In SIS block copolymers, chain scission is the predominant degradation mechanism. In an SIS block copolymer, the addition of a more effective stabilizer, AO-3, alone or blended with a secondary antioxidant, PS-1, can provide a significantly superior performance over AO-1 alone or with PS-1. Resistance to discoloration after static oven aging at 80°C (176°F) is improved dramatically (Fig. 5). Viscosity stabilization (melt flow index stability) (Fig. 6) is also improved drastically using AO-3/PS-1. 

Melt flow index (MFI) analysis of composites are shown in Table 2. MFI values of composites without peroxide showed small differences by increasing amount of CaCOs. Contrariwise, MFI values of composites with peroxide are proportional to the increasing amount of CaCOs. MFI values of composites without DAP are between 9 and 10 g/10 min, however addition of 0.06 %wt. DAP to the composites resulted MFI values to vary between 22 and 26 g/10 min. This increase in MFI values is a result of degradation of PP by the DAP. 

The thermal degradation reactions were carried out at a barrel temperature of 230°C, whereas peroxide-induced degradation reactions were catalyzed by injecting 2,5-dimethyl-2,5-fc i-(t-butylperoxy) hexane at rates of 0.05, 0.1, and 0.2 phr (per hundred parts resin) into the feed hopper of the extruder. The polypropylene resin was supplied by Himont and had a melt flow index of 0.14 g/10 min. 

An example of grafting maleic anhydride onto polypropylene in a twin-screw extruder was described by Wong (14). Polypropylene was coated with 1.1 wt% maleic anhydride and fed to a 53-mm twin-screw extruder at 30 kg/hr. Four temperature control zones were used, and styrene monomer at 1.4 wt% was injected into the second zone. They obtained 0.8 wt% bound maleic anhydride on the product, and the melt flow index was 17 dg/min. When 500 ppm Lupersol 130 catalyst was added and no styrene was injected, the melt flow index increased to 258 dg/min, indicating signiflcant degradation. No grafting was obtained when an antioxidant was added to the feed. 

Keywords peroxide, molar weight distribution (MWD), rheology, crystallization, extrusion, melt flow index (MFI), controlled rheology (CR), peroxide-degradation, residence time distribution (RTD), halflifetime of peroxides, melt elasticity, die swell, viscosity curve, shear rate, elongational viscosity, melt fracture, heterophasic PR... 

This property is measured by a melt flow index extrusion plastomer, which measures the weight of polymer extruded through a standard orifice in 10 minutes (see Table 16.1). Alternatively, a multi-functional extrusion plastomer that measures melt flow rate resistance to thermal degradation is available. 

The degradation curves determined for the melt flow indexes of all the polyolefins studied are presented in Fig. 4. 

Shenoy, A. V. and Saini, D. R., Estimation of melt elastidty of degraded polymer from melt flow index, Pofym. DegnuL StaUL 11, TSn-TXn (198S). 

It was however, found that the data for temperatures lower than 280°C lie on a single curve, whereas the data for temperatures 280°C and higher did not. This was due to the degradation of the polymer. Nevertheless, a master curve incorporating even the degraded polymer ta could be obtained by using a mod ed shift factor a, defined as the ratio of the temperature shift factor Or and the melt flow index MFI as shown in Fig. 9.34. The dependence of the melt index on the temperature and the residence time te as shown in Figs. 9.35 and 9.36 was determined in the form... 

Controlled degradation for the improvement of melt flow index, MWD, and production of waxes. 

FIG. 11 Dependence of melt flow index (MFf) and of PP degradation on peroxide amount. Degradation at 190°C, 5 min, 2,5-dimethylhexane-2,5-di-tert-butylperoxide as initiator. (Adapted from Ref. 191.)... 

---