Melt Stability Research Articles

An introduction to toughened polyamide‐4,6

AbstractImpact modified polytetramethylene adipamide (polyamide‐4,6) has recently been introduced. The author compares this compound to a toughened polyamide‐6,6 and a toughened polyamide‐6 in selected impact resistance and elevated temperature tests. Based on the results, toughened polyamide‐4,6 promises similar impact strength; and, better retention of stiffness, creep modulus, and toughness at elevated temperatures. Apparent improvements in melt stability make it suitable for extrusion, as well as injection molding applications.

Rheological and chemorheological studies of cyclic aryl ether ketone and aryl ether thioether ketone oligomers containing the 1,2-dibenzoylbenzene moiety

The melt stability, shear rate, and temperature dependence of steady-state shear viscosity of molten cyclic aryl ether ketone and thioether ketone oligomers containing the 1,2-dibenzoylbenzene moiety have been investigated. The isothermal chemorheology of the ring-opening polymerization of cyclic oligomers 4 and 9 in the presence of a nucleophilic initiator was also conducted. The cyclic aryl ether ketone oligomers are thermally stable in the melt, and their melt viscosity is several orders of magnitude lower than their high molecular weight linear counterparts. At a given temperature, the steady-state shear viscosity of the molten cyclics initially undergoes shear thinning as the shear rate increases, and once the shear rate is above 10 s -1 , the molten cyclic oligomers behave like Newtonian fluids. For the amorphous cyclic oligomers studied, the steady-state shear viscosity at 100 s -1 at a given temperature only depends on their glass transition temperature. The cyclic aryl thioether ketone oligomers are thermally unstable in the melt and undergo ring-opening polymerization in the absence of an initiator to form high molecular weight linear polymers with a concomitant rapid increase in viscosity. The rate of change in viscosity increases with temperature and is promoted by the addition of a catalytic amount of elemental sulfur or a disulfide such as 2,2-dithiobis(benzothiazole). It is hypothesized that the ring-opening polymerization is initiated by the in situ generated thiyl radical(s) and proceeds via a free-radical route.

Melt Stability Study of Various Types of Poly(l-lactide)

Poly(l-lactide) (PLLA) prepared by ring-opening melt polymerization using tin−octoate as catalyst undergoes a rapid melt degradation. The melt stability of PLLA was evaluated by using a Haake melt rheometer at various temperatures. The degradation rate constants were calculated from the changes in the melt viscosity for as-polymerized, precipitated, and acid treated PLLA. Furthermore, stability measurements were performed for as-polymerized PLLA modified with various types of peroxides (hydroperoxide, dialkyl peroxide, peroxy ester, and diaroyl peroxide). The energy of activation for the melt degradation of as-polymerized PLLA was calculated to 119.4 kJ/mol at temperatures between 180 and 210 °C. Precipitation of PLLA gave a reduction of the tin catalyst content in the polymer, and this also reduced the extent of the melt degradation. A further reduction of the tin content, and accordingly a reduction in the melt degradation, was achieved after acid treatment of the polymer. Addition of peroxide in open a...

Studies on antioxidants. III. 2[(3,5‐di‐tert‐butyl‐4‐hydroxyphenyl)methylene]hydrazine carbothioamide as a melt stabilizer for unstabilized polypropylene

AbstractPolypropylene's high processing temperatures initiate thermal oxidative degradation and loss of physical properties. Antioxidants protect polymers through the severe conditions of processing and fabrication and also through end use. Small amounts of antioxidants like 2,6‐di‐tert‐butyl‐4‐methylphenol and 2[(3,5‐di‐tert‐butyl‐4‐hydroxyphenyl)methylenel]‐hydrazine carbothioamide are shown to be effective in protecting isotactic polypropylene (IPP) against thermooxidative degradation. 2[3,5‐Di‐tert‐butyl‐4‐hydroxyphenyl)‐methylene]hydrazine carbothioamide was melt compounded with IPP using a twin screw extruder. The melt flow index of the extruded sample was measured. The extruded granules were injection molded to get specimens suitable for testing mechanical properties, such as heat deflection temperature, Izod impact strength, tensile strength, tensile modulus, flexural strength, and flexural modulus. 2[(3,5‐Di‐tert‐butyl‐4‐hydroxyphenyl)methylene]hydrazine carbothioamide was found to be an effective melt stabilizer for unstabilized polypropylene. © 1995 John Wiley & Sons, Inc.

The antioxidant role of α‐tocopherol in polymers. I. The nature of transformation products of α‐tocopherol formed during melt processing of LDPE

AbstractThe antioxidant role of α‐tocopherol (vitamin E) in low‐density polyethylene (LDPE), and the nature of its transformation products formed during extrusion of the polymer are investigated. The melt stabilizing effectiveness of α‐tocopherol was found to be very high, higher than that of the commercial hindered phenol antioxidants, Irganox 1076 and 1010, after single and multiple extrusion. The high antioxidant activity of α‐tocopherol as a melt stabilizer is due, at least in part, to its transformation products. The importance of the processing history and the parent antioxidant concentration on the transformation products is discussed. Transformation products of α‐tocopherol were analyzed after each of the four extrusion passes of the polymer. These were fractionated, analyzed, and characterized by HPLC and spectroscopy, respectively. The main products formed are diastereoisomers of dimers and trimers, as well as aldehydes; the relative concentration of each was shown to depend on processing severity (number of extrusion passes) and the initial concentration of α‐tocopherol. The dihydroxydimer was found to be formed at a high concentration relative to the other products and proportional with the initial concentration of tocopherol. Based on both the identity and distribution of transformation products, a mechanism is proposed for the melt stabilization effect of α‐tocopherol in LDPE. © 1994 John Wiley & Sons, Inc.

Influence of the controlled rheology process (vis‐breaking) on additive effectiveness in polypropylene

AbstractControlled rheology or VIS‐breaking is the process whereby polypropylene, peroxide, and other additives such as stabilizers are extruded to simultaneously shorten polymer chain length by chemical action of the peroxide and to blend in additives. Although the influence of additives on the VIS‐breaking process has been studied, little attention has been paid to the role of additives present during VIS‐breaking on the final resin properties. The effectiveness of a light mineral oil used as a costabilizer for radiation resistance and Irgafos 168 as a melt stabilizer was determined in polypropylene resins prepared with different peroxide levels. It was found that exposure of Irgafos 168 to peroxide during the VIS‐breaking process did not hinder the action of Irgafos 168 as a melt stabilizer during injection molding. At low levels of peroxide, the presence of oil led to an improvement of radiation resistance compared to resins that contained no oil. However, at higher peroxide levels, oil‐containing resins showed less resistance to radiation than did those without oil. These results were interpreted as an interaction of peroxide with the function of oil in the final resins. © 1994 John Wiley & Sons, Inc.

Initiation of shape instabilities of free boundaries in planar Cauchy–Stefan problems

The linearized shape stability of melting and solidifying fronts with surface tension is discussed in this paper by using asymptotic analysis. We show that the melting problem is always linearly stable regardless of the presence of surface tension, and that the solidification problem is linearly unstable without surface tension, but with surface tension it is linearly stable for those modes whose wave numbers lie outside a certain finite interval determined by the parameters of the problem. We also show that if the perturbed initial data is zero in the vicinity of the front, but otherwise quite general, it does not affect the stability. The present results complement those in Chadam & Ortoleva [4] which are only valid asymptotically for large time or equivalently for slow-moving interfaces. The theoretical results are verified numerically.

Magnetic properties and stability of melt spun Nd/sub 5/Fe/sub 17/

Nd/sub 5/Fe/sub 17/ (also referred to as A/sub 2/-phase) is prepared by rapid-quenching and subsequent annealing. Due to the fine, homogeneous distribution of the elements in the melt-spun ribbons, only a short reaction time is required in the annealing treatment. The samples are almost single-phase, as revealed by X-ray and magnetization measurements. Unlike Nd/sub 2/Fe/sub 17/, the A/sub 2/-phase does not interstitially dissolve nitrogen, but decomposes into NdN and alpha -iron. >

The growth, structural and optical characterization of large area, single crystalline thin films of 3-nitroaniline (mNA)

The classical organic non-linear optical material 3-nitroaniline (mNA) has been used as a model compound to study the growth of such materials in thin, single crystalline films. Using the excellent melt stability of mNA, we have grown from the melt phase (under ideal conditions) large area (1 × 6 cm 2 ) films which vary in thickness from 5–20 μm. The use of the Laue X-ray technique has allowed the identification of the in-plane film orientation to be carried out. The high structural perfection of the films was defined using glancing angle X-ray topography. The optical characterization of such films, using a Maker fringe experiment, has allowed a value for the d 31 coefficient to be measured which compares favourably with similar coefficients defined for bulk single crystals of high optical and structural quality.

Absence of isothermal thickening for a blend of linear and branched polyethylene

In this paper we investigate the effect on melting stability of blending linear with branched polyethylene. Linear polyethylene (LPE) was blended in solution with branched polyethylene (BPE) at a concentration of 10% LPE. Both homopolymers and the blend were held at 126°C, for 80 h. At this temperature solid-liquid phase separation takes place in pure LPE and in the blend; pure BPE remains molten. The melting point of the isothermally crystallized blend lamellae was lower than the melting point of pure LPE lamellae crystallized under the same conditions. This depression in melting temperature has previously been explained in two ways: by the inclusion of BPE into the LPE crystals, and because of the different environment of the crystals. We assess these two factors and find them insufficient to explain the decrease in melting temperature observed. We discuss another important factor, the dimension of the crystals in the chain direction. By transmission electron microscopy we show that crystals isothermally grown in the blend are thinner than LPE crystals grown under the same conditions. This difference in crystal thickness is sufficient to explain the observed depression in melting temperature. We believe that the lamellae in the blend fail to thicken during the initial stages of crystallization. We envisage three mechanisms that explain the suppression of thickening present in the blend, and one of these is eliminated.

Aromatic ether–ketone–‘X’ polymers. Part 3.—Effects of structure on the melt stability of some ether–ketone–imide copolymers

A modified Friedel–Crafts synthesis has been used to make a series of aromatic ether–ketone–imide copolymers. Melt viscosity measurements of the polymers were made using capillary rheometry, and the effects of structural variations on the stability of the melt viscosity of the copolymers were evaluated.

The role of radiation in melt stability in zone-melt recrystallization of SOI

Under circumstances in Zone-Melt-Recrystallization (ZMR) of Si-on-Insulator (SOI) structures where radiative heat loss is significant, the ∼50% decrease in emissivity when Si melts destabilizes the Si molten zone. We have demonstrated this both experimentally using a slowly scanned e-beam line source and numerically with a finite-element computational simulation. The resulting instability narrows the process window and tightens requirements on beam control and background heating uniformity, both for e-beam ZMR systems and optically-coupled systems such as a graphite strip heater.

Postcrosslinking of linear polyesters. II. UV‐induced crosslinking agents having carboxyl reactive group

AbstractA previous article reported that polyallyl compounds such as triallyl cyanurate could be applied as an effective melt‐blend‐type ultraviolet (UV)‐induced crosslinking agent for linear polyesters to improve their thermal resistances without sacrificing their melt processabilities. The present article reports the results from further investigation to seek more effective crosslinking agents for the polyesters. Compounds having both allyl and carboxyl reactive groups, such as diallyl glycidyl isocyanurate and 2‐[β‐(3,5‐diallylisocyanuro)ethyl]carbamoyl‐2‐oxazoline have been proved to be more effective UV‐induced crosslinking agents than the simple polyallyl compounds reported previously. The former compounds were coupled with carboxyl terminal groups of the polyesters by reaction in the melt and act as an effective UV‐induced crosslinking agent, which was proved by the observation that the polyester having more carboxyl terminals resulted in higher UV crosslinkabilities. Addition of such compounds to the polyester did not seriously affect the melt stability of the mixtures.

Effect of ionic strength on the chromatin conformational transitions. A differential scanning calorimetry study

The thermal denaturation of hen erythrocyte chromatin at low ionic strength (0.25 m m EDTA, pH 8.0 (A), and 0.25 m m EDTA, pH 8.0, 5 m m NaCl (B)) was studied by high sensitivity differential scanning microcalorimetry and absorbance at 260 nm. The melting profiles recorded by the two methods were decomposed into sums of three (case A) and two (case B) Gauss curves and were found to be of identical shape. In case A, the three individual peaks at 61, 72, and 82°C have enthalpies 4.1, 8.4 and 12.5 kcal/mol base pairs −1 , respectively. The enthalpies of pure DNA melting at these temperatures are 6.2, 6.9 and 7.6 kcal/mol base pairs −1 , respectively. The smaller in comparison to pure DNA enthalpy of the peak at 61°C indicates a decreased thermal stability of the chromatin portion melting during this transition. The disappearance of this transition at increased ionic strength (case B) is accompanied by a transfer of its enthalpy to the peak at 72°C and its hyperchromicity to the peak at 82°C. During this complex transformation the overall melting enthalpy remains unchanged (9.5±0.5 kcal/mol base pairs −1).

Postcrosslinking of linear polyesters. I melt‐blend‐type UV‐induced crosslinking agents

AbstractPostcrosslinking of such linear polyesters as poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT) are known to be attractive means to improve their thermal resistances without sacrificing their melt processibilities. Effective melt‐blend‐type ultraviolet‐induced crosslinking agents were investigated among allyl compounds. Such polyallyl compounds as triallyl cyanurate and triallyl isocyanurate were found to be practically promising agents from the viewpoints of crosslinkability and stability in the polyester melt. The crosslinkability of the ally compounds increased with increasing the number of the allyl groups in the molecule and electron‐withdrawing property of the neighboring group to which the allyl group was attached. The crosslinking mechanism likely includes both the polymerization of the allyl compound to form semi‐interpenetrating polymer networks in the polyester matrix and the reaction between the polyester and the allyl compound moiety. The crosslinked PET and PBT showed good mechanical properties even above the melting points of the ordinary PET and PBT.

Dopant segregation during vertical Bridgman-Stockbarger growth with melt stabilization by strong axial magnetic fields

Ga-doped germanium was grown in a vertical Bridgman-Stockbarger system with melt stabilization by axial magnetic fields of 30 kG. It was found that radial segregation of gallium is negligible and that the initial transient of axial macro-segregation is in apparent compliance with the theory of diffusion controlled plane front solidification.

Mechanisms of antioxidant action: Mechanochemical transformation products of 2,2,6,6–tetramethyl-4–hydroxypiperidinoxyl in polypropylene

The effect of the processing operation on the nature and quantity of transformation products of 2,2,6,6-tetramethyl-4-hydroxypiperidinoxyl (〉NO.) and its corresponding hydroxylamine (〉NOH) was investigated in polypropylene. It was found that both 〉NO. and 〉NOH are actively involved in a regenerative cycle and that this redox couple is responsible for the stability of the polymer whether the initial additive is 〉NO. or 〉NOH. The alkyl hydroxylamine (〉NOR), on the other hand, does not appear to play a significant role in the melt stabilization of the polymer. Quantitative measurements of 〉NOH and 〉NO. concentrations show that, under conditions of both restricted oxygen (CM) and oxygen excess (OM), the concentrations of both species change reciprocally throughout the processing operation. In a closed mixer (CM), the total overall concentration of 〉NO. and 〉NOH remains at a high constant level, whereas in an open mixer (OM), their total concentration decreases continuously to a low level, indicating the irreversible destruction of the redox couple (〉NO. and 〉NOH). The photo-oxidative stability of the polymer was found to depend on the total concentration of the redox couple left in the system after processing in either CM or OM.

Increasing the stability of melting of self shielding flux cored wires

Increasing the stability of melting of self shielding flux cored wires

Mechanisms of antioxidant action: Tellurium dithiocarbamates as antioxidants for polypropylene

AbstractThe tetrakis (dialkyldithiocarbamato)tellurium(IV) complexes are highly effective thermal antioxidants and melt stabilizers for polypropylene and the evidence suggests that they are converted to the bis(dialkyldithiocarbamato)tellurium(II) complexes and the corresponding dithiocarbamoyl disulfides during processing. The Te(IV) complexes are also effective photoantioxidants and their activity increases with increasing processing time. The bischelates are in general less effective when used alone.

Structure and thermal stability of supercooled Li2SO4 melts

Structure and thermal stability of supercooled Li2SO4 melts