Subiman Ghosh

Thermal degradation and ageing of segmented polyamides

The degradation behaviour and kinetics of degradation of segmented polyamides with varying block lengths have been studied by non-isothermal thermogravimetry in air and nitrogen and by infrared spectroscopy. In air, all the polymers show two stage decomposition, whereas in nitrogen, the decomposition occurs in a single stage. In both atmospheres the degradation, however, follows first order kinetics. The infrared spectroscopic analysis of the degraded products reveals that the decomposition occurs in the polyether linkage followed by polyamide hard block. The mechanism of degradation is of course a complex one. In the case of thermooxidative degradation, a decrease in hard block molecular weight has a great influence on the activation energy values. The effect of ageing on the mechanical properties of these polymers has also been studied. On ageing, it is observed that both tensile strength and elongation at break drop sharply in the initial stage. After this initial drop, both properties show a marginal change with time and temperature of ageing. Retention of physical properties is better with high hard block molecular weight polymers.

Phase modification of SEBS block copolymer by different additives and its effect on morphology, mechanical and dynamic mechanical properties

The objective of this study is to examine the phase modification of styrene-ethylene butylene-styrene (SEBS) block copolymer by different additives and its influence on morphology and mechanical, and dynamic mechanical properties. The additives chosen are the coumarone-indene (CI), phenol-formaldehyde (PF), paraffin hydrocarbon (PAHY) resins, as well as aromatic oil (AO), polystyrene (PS), polypropylene (PP), ethylene vinyl acetate (EVA) (VA 28 and 45%), and ethylene propylene diene monomer ( EPDM ) rubber. It is interesting to note that of all the additives, PP has the most prominent effect. The mechanical properties of SEBS polymer are enhanced to a large extent by PP. The value of tan δ maximum of SEBS at both the low and the high temperature transitions is decreased. All the resins and PS increase the storage modulus and the tensile modulus of the SEBS polymer. CI resin and AO modify the hard and soft phases of SEBS polymer. AO, EPDM rubber, and EVA lower the mechanical strength of the SEBS polymer. The results are explained on the basis of morphology studied with the help of scanning electron microscopy.

Studies on adhesion between rubber and fabric and rubber and rubber in heat resistant conveyor belt

Abstract Adhesion strength between topping rubber and fabric (nylon 66 and polyester-nylon 66) and cover rubber and topping rubber of heat resistant conveyor belt based on SBR and chlorobutyl rubbers have been measured over a range of peel rates and temperatures and after ageing of the joints at various temperatures and durations. The adhesion strength between SBR topping compound and nylon 66 was greatest. TMTM-based SBR cover compound registered highest adhesion strength with the SBR topping compound. Decreasing the rate or increasing the temperature of testing reduced the peel strength of the joints. Rubber to fabric adhesion decreased with ageing time and temperature, while rubber to rubber joint strength initially decreased and then increased or remained constant.

Melt rheology of segmented polyamides: Effect of block molecular weight

Segmented polyamides, also known as polyether-ester-amides, are composed of polyether and polyamide structural units. The rheological behavior of segmented polyamides with respect to the variations in the molecular weight of hard and soft blocks has been studied using a Monsanto Processability Tester. These systems exhibit pseudoplastic flow behavior. The shear viscosity of the segmented polyamides decreases with a decrease in hard block molecular weight up to 1500. However, at low shear rates, the shear viscosity shows marginal change with an increase in soft segment molecular weight. The equilibrium die swell increases with an increase in shear rate, but decreases with increasing temperature. The stress relaxation study of the segmented polyamides reveals that the stress developed during extrusion relaxes exponentially for all the systems. The equilibrium die swell at a fixed temperature and shear rate, the time required to relax a fixed amount of stress and the stress developed after a certain time interval decrease with a decrease in hard block molecular weight up to 1500, but increase with an increase in soft segment molecular weight. The activation energy of the melt flow process increases with the rate of shear in most of the cases.

Oxidative degradation of polyethylene in presence of phase transfer catalyst: Part II—Thermal studies

Abstract Low density polyethylene (LDPE) has been modified by oxidative degradation with phase transferred permanganate and both the treated and untreated materials have been examined by TGA and DSC. In TGA, LDPE shows three degradation steps whereas the modified LDPEs show only two, owing to elimination of short branches during oxidation. From DSC studies it is evident that all melting endotherms contain two peaks which can be explained as resulting from the two structure forms of LDPE. There is a small difference in melting peak temperature as well as in percentage crystallinity of LDPE and modified LDPEs but a greater difference is observed in degradation pattern. While LDPE has one degradation step, modified LDPEs follow multistep degradation.

Atomic force microscopy studies of molded thin films of segmented polyamides

Block copolymers comprise two or more blocks of chemically different monomers. If the blocks are sufficiently large and the interactions between them sufficiently adverse, the blocks will segregate [1, 2]. Since the blocks are inter connected, they can only segregate on a microscopic scale. For example, an AB diblock copolymer with A and B blocks of equal molecular weight will form a mesophase structure of A and B lamellae. The thickness of the lamellae is commensurate with the size of the polymer coil. A particularly interesting type of block copolymer is the ABA triblock copolymer where A constitutes a thermoplastic material (e.g., polystyrene) and B an elastomer (e.g., polybutadiene). Another interesting class of block copolymers is the (AB)n type of copolymers where “A” represents the soft segment and “B” corresponds to the hard block. Segmented polyamides composed of polyether and polyamide structural units is a typical example of (AB)n type block copolymers. Since the structural units are linked via an ester group, such systems are also referred to as polyether – ester – amides [3, 4] having the chain structure as shown below.

Influence of block molecular weight on the adhesion properties of segmented polyamides

The effects of molecular weight variations in the hard and soft segments on the adhesion strength of segmented polyamides against aluminium, copper, and steel were investigated using 180° peel strength measurements. It was found that the adhesion strength of the segmented polyamides was largely influenced by block molecular weight variations. The nature of the substrate, the rate of peeling, cooling in different environments, and thermal ageing, etc. had significant effects on the adhesion strength of the joints, whereas variation in the moulding conditions used in these experiments did not have much impact on the strength of the joints. The joint strength increased with a decrease in hard block molecular weight at a constant soft block molecular weight of 1000, or with an increase in soft block molecular weight at a constant hard block molecular weight of 1100.

PROMOTION OF ADHESION OF LOW-DENSITY POLYETHYLENE BY POLYMER BLENDING

LDPE blends of variable composition are prepared with several polymer additives, which include chromic acid-etched LDPE, polyhydroxyetherimide (PHEI), butylated silica and LDPE oxidized with phase-transferred permanganate. The strength of adhesion measured as the force required to peel off the laminated aluminium foil by 180 ° is found to be a function of time of etching, blend composition and the effective amount of polar groups on polymer-metal interface. LDPE-based additives, e.g. chromic acid oxidised LDPE and LDPE oxidised under phase transfer catalysis in benzene, promote practical adhesion by factors of 8 and 16, respectively. In the case of the silane additive there is virtually no enhancement of the peel load despite a large number of polar groups present in the blend. The results can be interpreted in terms of cohesive failure.