The chemical modification of polymer blends by reactive processing

The primary objective of this research was to examine the concepts of the chemical modification of polymer blends by reactive processing using interlinking agents (multi-functional, activated vinyl compounds; trimethylolpropane triacrylates {TRIS} and divinylbenzene {DVD}) to target in-situ interpolymer formation between immiscible polymers in PS/EPDM blends via peroxide-initiated free radical reactions during melt mixing. From a comprehensive survey of previous studies of compatibility enhancement in polystyrene blends, it was recognised that reactive processing offers opportunities for technological success that have not yet been fully realised; learning from this study is expected to assist in the development and application of this potential. In an experimental-scale operation for the simultaneous melt blending and reactive processing of both polymers, involving manual injection of precise reactive agent/free radical initiator mixtures directly into molten polymer within an internal mixer, torque changes were distinct, quantifiable and rationalised by ongoing physical and chemical effects. EPDM content of PS/EPDM blends was the prime determinant of torque increases on addition of TRIS, itself liable to self-polymerisation at high additions, with little indication of PS reaction in initial reactively processed blends with TRIS, though blend compatibility, from visual assessment of morphology by SEM, was nevertheless improved. Suitable operating windows were defined for the optimisation of reactive blending, for use once routes to encourage PS reaction could be identified. The effectiveness of PS modification by reactive processing with interlinking agents was increased by the selection of process conditions to target specific reaction routes, assessed by spectroscopy (FT-IR and NMR) and thermal analysis (DSC) coupled dichloromethane extraction and fractionation of PS. Initiator concentration was crucial in balancing desired PS modification and interlinking agent self-polymerisation, most particularly with TRIS. Pre-addition of initiator to PS was beneficial in the enhancement of TRIS binding to PS and minimisation of modifier polymerisation; believed to arise from direct formation of polystyryl radicals for addition to active unsaturation in TRIS. DVB was found to be a "compatible" modifier for PS, but its efficacy was not quantified. Application of routes for PS reaction in PS/EPDM blends was successful for in-situ formation of interpolymer (shown by sequential solvent extraction combined with FT-IR and DSC analysis); the predominant outcome depending on the degree of reaction of each component, with optimum "between-phase" interpolymer formed under conditions selected for equalisation of differing component reactivities and avoidance of competitive processes. This was achieved for combined addition of TRIS+DVB at optimum initiator concentrations with initiator pre-addition to PS. Improvements in blend compatibility (by tensiles, SEM and thermal analysis) were shown in all cases with significant interpolymer formation, though physical benefits were not; morphology and other reactive effects were also important factors. Interpolymer from specific "between-phase" reaction of blend components and interlinking agent was vital for the realisation of positive performance on compatibilisation by the chemical modification of polymer blends by reactive processing.

Reactive processing of polyolefins using antioxidant systems

Various monoacrylic compounds containing a hindered phenol function (e.g.3,5-di-tert.-butyl-4-hydroxy benzyl alcohol, DBBA and vinyl-3-[3',5'-di-tert.-butyl-4-hydroxy phenyl] propionate, VDBP), and a benzophenone function (2-hydroxy-4-[beta hydroxy ethoxy] benzophenone, HAEB) were synthesised and used as reactive antioxidants (AO's) for polypropylene (PP). These compounds were reacted with PP melt in the presence of low concentration of a free radical generator such a peroxide (reactive processing) to produce bound-antioxidant concentrates. The binding reaction of these AO's onto PP was found to be low and this was shown to be mainly due to competing reactions such as homopolymerisation of the antioxidant. At high concentrations of peroxide, higher binding efficiency resulted, but, this was accompanied by melt degradation of the polymer. In a special reactive processing procedure, a di- or a trifunctional reactant (referred to as coagent), e.g.tri-methylol propane tri-acrylate, Tris, and Divinyl benzene, DVB, were used with the antioxidant and this has led to an enhanced efficiency of the grating reaction of antioxidants on the polymer in the melt. The evidence suggests that this is due to copolymerisation of the antioxidants with the coagent as well as grafting of the copolymers onto the polymer backbone. Although the 'bound' AO's containing a UV stabilising function showed lower overall stabilisation effect than the unbound analogues before extraction, they were still much more effective when subjected to exhaustive solvent extraction. Furthermore, a very effective synergistic stabilising activity when two reactive AO's containing thermal and UV stabilising functions e.g. DBBA and HAEB, were reactively processed with PP in the presence of a coagent. The stabilising effectiveness of such a synergist was much higher than that of the unbound analogues both before and after extraction. Analysis using the GPC technique of concentrates containing bound-DBBA processed in the presence of Tris coagent showed higher molecular weight (Mn), compared to that of a polymer processed without the coagent, but was still lower than that of the control processed PP with no additives. This indicates that Tris coagent may inhibit further melt degradation of the polymer. Model reactions of DBBA in liquid hydrocarbon (decalin) and analysis of the products using FTIR and NMR spectroscopy showed the formation of grafted DBBA onto decalin molecules as well as homopolymerisation of the AO. In the presence of Tris coagent, copolymerisation of DBBA with the Tris inevitably occured; which was followed by grafting of the copolymer onto the decalin, FTIR and NMR results of the polymer concentrates containing bound-DBBA processed with and without Tris, showed similar behaviour as the above model reactions. This evidence supports the effect of Tris in enhancing the efficiency of the reaction of DBBA in the polymer melt. Reactive procesing of HAEB in polymer melts exhibited crosslinking formation In the early stages of the reaction, however, in the final stage, the crosslinked structure was 'broken down' or rearranged to give an almost gel free polymer with high antioxidant binding efficiency.

Effect of processing on sulphur antioxidants in polyolefins

The effect of processing on the antioxidant activity of sulphur-containing compounds, with particular reference to nickel dialkyldithiophosphates and their corresponding di sulphides, were studied in polyolefins under melt, thermal and photo-oxidative conditions. These compounds were evaluated both at low (normal) and high (concentrates) concentrations. In general, the dithiophosphates were found to be very efficient melt stabilisers at normal concentrtion levels, and compare quite favourably with the best commercially available systems. The nickel dithiophosphates were also found to be very efficient thermal stabilisers for polyolefins, but their activity is highly dependent on the alkyl substituent in the molecule. The corresponding disulphides on the other hand showed very little activity under thermal oxidative conditions, and this was attributed to their inefficiency in scavenging alkyl peroxyl radicals since both compounds possess similar peroxidolytic activity. Furthermore, the nickel dithiophosphates were found to be excellent photo stabilisers for mildly-processed polyolefins while the corresponding disulphides only offer slight protection to the polymer. Oxidative processing of the disulphide, however, results in a dramatic improvement in their photo antioxidant activity. Thionophospho-ric acid, a major oxidation product of dithiophosphates, was also shown to have photo antioxidant activity similar to that of the disulphides. A combination of a U.V. absorber with the nickel complex and/or the disulphide resulted in a synergistic stabiliser system which was further augmented by oxidative processing. Moreover, the dilute analogues of such multicomponent stabiliser concentrates also showed excellent melt, thermal and photo-stabilising activity. The mechanistic studies carried out on the nickel complex and the corresponding disulphide clearly identified the thionophosphoric acid a a major transformation product although various triesters were formed as reaction intermediates. The mechanisms of the antioxidant action of the dithiophosphates, which is believed to involve a cyclical process similar to that shown for simple alkyl sulphides and nitroxyls, are discussed.

Reactive processing of polymers: structural characterization of products by 1H and 13C NMR spectroscopy for glycidyl methacrylate grafting onto EPR in the absence and presence of a reactive comonomer

Grafted GMA on EPR samples were prepared in a Thermo-Haake internal mixer by free radical melt grafting reactions in the absence (conventional system; EPR-g-GMA(CONV)) and presence of the reactive comonomer divinyl benzene, DVB (EPR-g-GMA(DVB)). The GMA-homopolymer (poly-GMA), a major side reaction product in the conventional system, was almost completely absent in the DVB-containing system, the latter also resulted in a much higher level of GMA grafting. A comprehensive microstructure analysis of the formed poly-GMA was performed based on one-dimensional H-1 and C-13 NMR spectroscopy and the complete spectral assignments were supported by two-dimensional NMR techniques based on long range two and three bond order carbon-proton couplings from HMBC (Heteronuclear Multiple Bond Coherence) and that of one bond carbon-proton couplings from HSQC (Heteronuclear Single Quantum Coherence), as well as the use of Distortionless Enhancement by Polarization Transfer (DEPT) NMR spectroscopy. The unambiguous analysis of the stereochemical configuration of poly-GMA was further used to help understand the microstructures of the GMA-grafts obtained in the two different free radical melt grafting reactions, the conventional and comonomer-containing systems. In the grafted GMA, in the conventional system (EPR-g-GMA(CONV)), the methylene protons of the GMA were found to be sensitive to tetrad configurational sequences and the results showed that 56% of the GMA sequence in the graft is in atactic configuration and 42% is in syndiotactic configuration whereas the poly-GMA was predominantly syndiotactic. The differences in the microstructures of the graft in the conventional EPR-g-GMA(CONV) and the DVB-containing (EPR-g-GMA(DVB)) systems is also reported (C) 2009 Elsevier Ltd. All rights reserved.

Controlled synthesis and processing of a poly(L-lactide-co-ε-caprolactone) copolymer for biomedical use as an absorbable monofilament surgical suture

Poly(L-lactide-co-ε-caprolactone) 75:25% mol, P(LL-co-CL), was synthesized via bulk ring-opening polymerisation (ROP) using a novel tin(II)alkoxide initiator, [Sn(Oct)]2DEG, at 130oC for 48 hrs. The effectiveness of this initiator was compared withthe well-known conventional tin(II) octoateinitiator, Sn(Oct)2. The P(LL-co-CL) copolymersobtained were characterized using a combination of analytical technique including: nuclear magnetic resonance spectroscopy (NMR), differential scanning calorimetry (DSC), thermogravimetry (TG) and gel permeation chromatography (GPC). The P(LL-co-CL) was melt-spun into monofilament fibres of uniform diameter and smooth surface appearance. Modification of the matrix morphology was then built into the as-spun fibresvia a series of controlled off-line annealing and hot-drawing steps. © (2014) Trans Tech Publications, Switzerland.

Learning from mother nature:exploiting a biological antioxidant for the melt stabilisation of polymers

An overview of the antioxidant role of the biologically active form of vitamin E, α-tocopherol, in polyolefins is discussed. The effect of the vitamin antioxidant on the melt and colour stability of polyethylene (PE) and polypropylene (PP) is highlighted. It is shown that tocopherol is a highly effective antioxidant that results in superior melt stabilisation of polyolefins particularly when used at much lower concentration than that needed for conventional synthetic hindered phenol processing stabilisers. As with other hindered phenols,α-tocopherol imparts also some colour to the polymer but this is shown to be reduced drastically in the presence of other antioxidants, such as phosphites, or other additives, such as polyhydric alcohols.

Preparation of a poly(L-lactide-co-caprolactone) copolymer using a novel tin(II) alkoxide initiator and its fiber processing for potential use as an absorbable monofilament surgical suture

A poly(L-lactide-co-caprolactone) copolymer, P(LL-co-CL), of composition 75:25 mol% was synthesized via the bulk ring-opening copolymerization of L-lactide and ε-caprolactone using a novel bis[tin(II) monooctoate] diethylene glycol coordination-insertion initiator, OctSn-OCH2CH2OCH2CH2O-SnOct. The P(LL-co-CL) copolymer obtained was characterized by a combination of analytical techniques, namely nuclear magnetic resonance spectroscopy, gel permeation chromatography, dilute-solution viscometry, differential scanning calorimetry, and thermogravimetric analysis. For processing into a monofilament fiber, the copolymer was melt spun with minimal draw to give a largely amorphous and unoriented as-spun fiber. The fiber's oriented semicrystalline morphology, necessary to give the required balance of mechanical properties, was then developed via a sequence of controlled offline hot-drawing and annealing steps. Depending on the final draw ratio, the fibers obtained had tensile strengths in the region of 200–400 MPa.