Solution Properties and Environmental Fracture Resistance Laboratory

Molecular weight determination of polymers: Gel Permeation Chromatography (GPC)

A constant flow of solvent is used to elute a polymer solution through a series of thermostated columns at high temperature so that the polymeric chains are separated on the basis of their size. A subsequent analysis using refractive index and viscosity detectors allows the quantification of the polymer fractions which are eluted at each retention time. A previous calibration (universal calibration) relates the retention times with the molecular weight of the polymers, so that the molecular weight distribution can be obtained and fundamental magnitudes of the polymers like their mean molecular weight or the polydispersity index can be calculated. LATEP has a high-temperature GPC equipment (Polymer Char GPC IR-6).

Short Chain Branching (SCB) distribution on polyolefins by GPC-IR

The use of an infrared detector with multiple filters allows the simultaneous detection of several wavelengths. The combination of an infrared detector with a GPC system is ideal for polyolefin analysis enabling the determination of the average short chain branching (SCB) for each molecular weight, as well as the molecular weight distribution. The introduction of an online viscometer gives the possibility of analyzing the long chain branching, providing complete information about polymer composition. LATEP owns a Polymer Char GPC-IR5 equipment for the synchronized detection of molecular weight and chemical composition of the polymer.

Analytical fractionation by Temperature Rising Elution Fractionation (TREF)

In semicrystalline polymers, the Chemical Composition Distribution (CCD) along with the molecular weight distribution determines the microstructure of the polymer. In polyolefins, the presence, content and distribution of side chains determine their properties, so a previous structural characterization is essential. The TREF technique is based on two steps, being the first one a cooling stage in which a solution of polymer is cooled onto an inert support, fractionating the polymer on the basis of its crystallization capacity. In a second part, the elution stage, the temperature is increased while a solvent flows through the fractionation column, so that the different fractions of polymer elute at different times on the basis of their crystallization capacity.

Analytical fractionation by Crystallization Analysis Fractionation (CRYSTAF)

The CRYSTAF technique is intended to rapidly determine the CCD of a polymer. Unlike to the TREF technique, the analysis in CRYSTAF is conducted in one step, where the crystallization of the polymer chains from a diluted solution is accomplished in a continuous mode. The analysis is carried out measuring the concentration of polymer in the solution during the crystallization process while the temperature is decreasing.

LATEP has an analytic Polymer Char TREF-CRYSTAF equipment (model 200+) for the characterization of the CCD of polymers.

Preparative fractionation of polymers (composition or molar mass)

The preparative fractionation of polymers allows the physical separation of the families which constitute the chemical composition distribution of a polymer due to differences on their crystallization capacity or molar mass. After the fractionation, each of these families can be characterized but other techniques such as GPC, TREF, CRYSTAF, NMR, DSC, TGA, etc.

LATEP has a Polymer Char PREPmc2 equipment which is fully automated to fractionate polymers on the basis of their composition or their molecular weight.

Determination of the hydroxyl value

The hydroxyl value is defined as the number of miligrams of potassium hydroxide equivalent to the hydroxyl content of one gram of sample. Quantifying the content of hydroxyl group is of great importance for the production of intermediate products such as polyols.

LATEP has an automatic Metrohm 848 Titrino Plus titrator to perform this type of tests following the procedure described in ASTM D4274.

Environmental Stress-Cracking (ESCR)

The environmental stress-cracking (ESCR) is a specific test for polyethylenes which evaluates their resistance to cracking when the material is subjected to environmental stress. Polyethylene resins are extremely resistant to most chemicals and solvents in the absence of stress. However, many polyethylenes show cracking processes when exposed to the same chemical environment under stress. The ESCR test, described in ASTM D1693 standard, is based on notching a polyethylene plaque, bending it to increase the tension in the region of the notch, and subjecting it to a tensioactive agent at certain temperature, generally 50 °C. The test consists on registering the time that a crack in the specimen needs to grow.

PENT test: Slow Crack Growth resistance of polyethylene

The PENT test (Pennsylvania Notch Tensile test) is exclusive for polyethylenes which are intended to be used in the fabrication of pipes for water or gas conduction. The test, described in ASTM F1473 and ISO 16241 standards, evaluates the slow crack growth resistance of the material. The specimen has two types of notches, the main notch and two side notches. It is important to make the side grooves coplanar with the main notch. Specimens shall be notched at room temperature. Then the prism-shaped specimen is subjected to axial stress (perpendicular to the notches plane) at a temperature of 80 °C. After a period of time, a crack is formed in the material and it starts growing. The PENT test determines the time at which the final failure of the specimen occurs.

LATEP has several equipments for PENT tests, some of which have been developed in the own laboratory for observing the process of formation and progression of the crack.

Full Notch Creep Test (FNCT)

FNCT is a test specially design for evaluating the Slow Crack Growth (SCG) resistance of polyethylene used of pipe application. The test is carried out according to the international ISO 16770 standard, using a square bar of material previously notched on all faces in coplanar way. A constant stress is applied on the specimen at 80 °C (could be even higher in high resistance materials) using also a tensioactive agent for accelerating the SCG process. LATEP has a FNCT equipment to carry out conventional and accelerated FNCT test at 90ºC and using a solution of Lauramine oxide (Dehyton).

Resistance to ozone cracking

Resistance to ozone cracking test evaluates the resistance to ozone cracking in rubber samples. Generally, ozone attacks rubber oxidizing and breaking its double bonds by oxidative cleavage reactions causing the degradation of the polymer.

To evaluate the resistance, a concentration of ozone of 50 or 200 ppcm is generated within an ozone chamber at 40 °C. Generally, the rubber specimens are previously deformed in order to favor the ozone attack to the sample. The evolution of the material with time and the possible formation of cracks are evaluated visually. The test procedure is described in ISO 1431 standard.

LATEP has a HAMPDEM 2000AM ozone chamber which generates a controlled concentration of ozone of up to 200 ppcm.