Melt Pressing

Dense Symmetrical Membranes. These membranes are used on a large scale ia packagiag appHcations (see Eilms and sheeting Packaging materials). They are also used widely ia the laboratory to characterize membrane separation properties. However, it is difficult to make mechanically strong and defect-free symmetrical membranes thinner than 20 p.m, so the flux is low, and these membranes are rarely used in separation processes. Eor laboratory work, the membranes are prepared by solution casting or by melt pressing. 

Many polymers, including polyethylene, polypropylene, and nylons, do not dissolve in suitable casting solvents. In the laboratory, membranes can be made from such polymers by melt pressing, in which the polymer is sandwiched at high pressure between two heated plates. A pressure of 13.8—34.5 MPa (2000—5000 psi) is appHed for 0.5 to 5 minutes, at a plate temperature just above the melting point of the polymer. Melt forming is commonly used to make dense films for packaging appHcations, either by extmsion as a sheet from a die or as blown film. 

High molecular weight, rather high melting polymers were obtained and could be melt pressed or cast into films or spun into fibers. These polymers were soluble... 

Nondestructive radiation techniques can be used, whereby the sample is probed as it is being produced or delivered. However, the sample material is not always the appropriate shape or size, and therefore has to be cut, melted, pressed or milled. These handling procedures introduce similar problems to those mentioned before, including that of sample homogeneity. This problem arises from the fact that, in practice, only small portions of the material can be irradiated. Typical nondestructive analytical techniques are XRF, NAA and PIXE microdestructive methods are arc and spark source techniques, glow discharge and various laser ablation/desorption-based methods. On the other hand, direct solid sampling techniques are also not without problems. Most suffer from matrix effects. There are several methods in use to correct for or overcome matrix effects ... 

Polyethylene (PE) was a commercial LD type (without additives) with a density of 0.92 and polypropylene (PP) was also a commercial material with a density of 0.91. The polvolefin samples were melt pressed to 1 mm thick sheets (plates) which were wiped clean with acetone and used directly for the grafting experiments with the vapor-phase process. 

Ester interchange between the additive and polymer would be expected if the mixture were held for an excessive time in the melt. In order to avoid the potential for interchange, the blendings were conducted in solution and heated only for a time necessary to melt press films for flammability testing. An alternative method of film preparation by solvent casting of films was discarded due to difficulties in preparing uniform, sufficiently thick, solvent free films for evaluation. 

Ir- and F-NMR spectra were determined for all soluble polymers. Most of the HFB polymers (8a-e) showed good solubility in common solvents (e.g., THF, CHCI3) whereas, most polymeric derivatives of the bishaloaromatics (9a-f), especially those with PFB as comonomer, were insoluble. Inherent viscosities ranging from 0.1 to 0.7 were obtained for soluble polymers. Coherent films could be either solution cast or melt pressed for a number of polymers. 

For higher molecular weight polymers, films were cast from solution for soluble polymers and melt pressed ca. 300 for insoluble polymers. 

Polymer Characterization. Melt-pressed films of the nylon samples were examined using a Nicolett 5DX FTIR. The samples were pressed at 240° C - 260° C at 4,000 to 20,000 psi. Samples in 88% formic acid were precipitated into methanol and also examined as KBR pellets. All samples were scanned a minimum of 100 times. 

Tensile testing was performed on an Instron A1020C at elongation rates of 50%/min and 100%/min. A minimum of 7 samples were tested per material type. Both melt-pressed films and in situ disks were examined. Some saunples were also conditioned in a hxunidity chamber before testing to insure that the samples contained the same amount of water which acts as plasticizer. 

The thin films of the nylon samples that were melt-pressed for FTIR examination gave similar results. Linear samples pressed out easily to make thin films that were excellent for IR evaluation. Star branched materials would not press thin enough to give good spectra even at maximum press pressure. It was originally hoped that star and linear nylons would have different crystal structure forms (JL4) unfortunately, FTIR has shown little differences other than those that occur from differences in sample preparation (Figure 5). 

Softening range data can serve as guides to proper temperatures for melt fabrication, such as melt pressing, melt extruding, and molding. They also are related to the product s thermal stability. 

The fusion temperature of these polymers is low enough to allow the spinning of fibres and melt pressing of films 263). They can also be blended with normal thermoplastics such as polystyrene or polyethylene oxide)2711. The conductivity shows a percolation threshold of about 16% which is expected for a random distribution of conducting spheres. 

A du Pont model 940 thermomechanical analyzer was employed for the Tg measurements on the PEMA/PVdF polyblends by the TMA technique. The sample specimens were cut from 30-mil, melt pressed sheets of the respective blends and examined by heating from —60° to 100°C at a rate of 5°C/minute. Sharp, unambiguous transitions were observed. 

Figure 3.3 A typical laboratory press used to form melt-pressed membranes. (Courtesy of Carver, Inc., Wabash, IN)...

The all-sulfonate system is a rigid, brittle material which crystallizes either in the solid state or from solution to become completely insoluble in common solvents. Only melt-pressed films can be made, and these exhibit the properties given in Table I. 

Melt spinning Fiber increased melt-pressing increased ... 

The samples used in this study are listed in Table I with selected characterization data. Also given are our notations for the samples. S and B represent polystyrene and polybutadiene, respectively. The block polymers are denoted S/Bi, S/B2, etc. The letters identify the polymer components with the first letter indicating the center segment. The polymers were isolated from the polymerization solution by precipitation into cold (0°C), well-stirred methanol. After careful drying in a vacuum oven (25°C), film samples were prepared by melt pressing at temperatures ranging from 25 °C for homopolybutadiene to 150 °C for homopolystyrene. 

Amorphous films of PEN, obtained by melt pressing at about 290 °C and quenching in ice water, were stored for different periods of time at room temperature. The physical ageing of these PEN samples was enhanced by means of annealing at different temperatures Tg below Tg = 123 °C, for selected annealing times tg. The samples were annealed in an ambient atmosphere under N2 flow using a hot-stage device at Tg — 80, 90, 105 and 115°C. In this case one has to assume that water vapour is not completely removed. 

Blends of these starting materials were obtained by coprecipitation from solution in hexafluoroisopropanol. Amorphous films were then obtained from the precipited powder by melt pressing in vacuo for different times varying from 0.2 min to 45 min followed by quenching in ice-water. In this way PET/PEN blends with weight compositions 90/10,70/30,60/40,44/56,30/70 and 10/90 were prepared. 

The PET and PEN blends are completely amorphous after quenching from the melt as revealed by the DSC experiments (Zachmann et al, 1994). Figure 5.8 shows in detail the variation of the microhardness with melt-pressing time tm for the PET/PEN composition 44/56. For all compositions, H shows first a rapid initial increase with exhibiting a maximum just before = 10 min and, then, for longer times, a gradual decrease down to values which can be even lower than the starting ones. 

Figure 5.8. Microhardness H variation of a PET/PEN (44/56 by mol) blend vi melt-pressing time tm- (From Baltd Calleja et al, 1997b.)...

Let us recall that when the melt-pressing time is about 0.2-0.5 min two TgS are observed, indicating that there are two phases present. In case of tm >2... 

In conclusion, in order to obtain the optimum mechanical properties of these blends, one should use melt-pressing times in the range 5-19 min. Otherwise, the mechanical properties represented by microhardness may be reduced by 10-15%. 

Samples of PET/PEN copolymers with 10, 20, 30, 50, 80 and 100 mol% PEN have been synthesized. Amorphous films of the samples were obtained by melt pressing above the melting point and quenching in ice-water. The samples were then crystallized by annealing the glassy materials at various temperatures. The degree of crystallinity was calculated from the amorphous density measured on quenched samples and from the crystal density derived from the crystal unit cell. 

Fig. 24 X-ray diffraction patterns of as-synthesized (a) and coalesced (b) PCL-b-PLLA melt-pressed films observed following various enzymatic degradation times [20]...

---