Chapter 4 Density Test PST472 PDF

PST472 POLYMER PHYSICAL TESTING & ANALYSIS DENSITY TEST 1 LESSON OUTCOMES Upon the completion of this chapter, the students should be able to:  Define density, specific gravity and bulk density.  Explain the relationship of density to the structure and...

PST472 POLYMER PHYSICAL TESTING & ANALYSIS DENSITY TEST 1 LESSON OUTCOMES Upon the completion of this chapter, the students should be able to:  Define density, specific gravity and bulk density.  Explain the relationship of density to the structure and polymer’s chemical composition.  Describe the basic principle and the significance of density measurement.  State and describe the types of density measurement methods.  Calculate the density and specific gravity for a materials. 2 INTRODUCTION  Density is an important physical property of plastics for several reasons:  As a design considerations when weight is a factor  The identification of unknown materials  The assessment of the structure (crystalline, cellular, etc.) of the material in a product  It is also commonly important to consider the density in establishing the actual cost of a resin. 3 INTRODUCTION  Density is a measure of how heavy an object is for a given size.  For example:  A rock is obviously more dense than a crumpled piece of paper of the same size.  A styrofoam cup is less dense than a ceramic cup.  One of the most critical is that the density of a substance will determine if it will float on another.  Less dense substances will float on (or rise through) more dense substances. 4 INTRODUCTION  Definition: Mass per unit volume of a substance Density, ρ = Mass, m (g)___ Volume, V(cm3)  Density is expressed in grams per cubic centimeter (g/cm3), pounds per cubic foot (lb/ft3), or kg/m3. The SI unit:5 kg/m3 INTRODUCTION  Density is also defined as weight per unit volume (specific weight).  Although the terms mass and weight are used almost interchangeably, there is a difference between them.  Mass is a measure of the quantity of matter, which is constant all over the universe.  Weight is proportional to mass but depends on location in the universe.  Weight is the force exerted on a body by gravitational attraction (usually by the earth). 6 INTRODUCTION  ASTM D792 describes a test method to determine the density of plastics based on weight and volume.  Density is a direct function of the polymer’s chemical composition.  The specific gravity of carbon/hydrogen polymers is ≈ 1.0 (similar density to water).  Most other atoms produce higher density, such as the fluoropolymers. 7 INTRODUCTION  Polyethylene is denser than polypropylene because of its closer packing.  Polystyrene is somewhat denser because the ring is denser than the linear -C- C - system and has fewer hydrogen atoms per carbon.  Oxygen, chlorine, fluorine, and bromine are dense atoms, which increase the density of polymers that 8 have such atoms in their molecular configuration. INTRODUCTION  The density for different types of materials are shown below: 9 THEORY  The density determination is performed by means of Archimedes' buoyancy principle.  Archimedes' principle states that a body immersed in a fluid apparently loses weight by an amount equal to the weight of the fluid it displaces.  This method allows determination of the density of solids. 10 BASIC PRINCIPLE  Measuring the amount of water displaced from a full container when the object is completely immersed. 11 DENSITY MEASUREMENT METHOD  There are three types of density measurement: 1) Buoyancy method – suitable for relatively large and solid test specimens. 2) Pycnometer method – suitable for small test specimen. Ex: sand. 3) Density column method – widely used for polymers to assess very small differences in density (ASTM D1505). 12 DENSITY MEASUREMENT METHOD 1) Buoyancy method 13 DENSITY MEASUREMENT METHOD 2) Pycnometer method:  It consists of a flask and a cap with a thin overflow tube.  Flask and cap is weighed when filled with liquid alone and when containing the test specimen immersed in the liquid. 14 Density measurement method 3) Density column method:  The basic principle is shown below.  It is a special method based on buoyancy.  It involves the preparation of a density gradient column.  Two miscible liquids of different density (high and low) are placed in connected identical containers; the downstream container is fitted with a stirrer.  The liquids are slowly fed to the bottom of the cylindrical column through a control valve and a thin tube. 15 cont:  It leads to a continuous density gradient in the column.  The first fed – almost pure light liquid ends up at the top.  The last fed – almost pure heavy liquid is at the bottom  Calibrated glass floats can be used to establish a density scale,  A small test specimens, gently introduced at the top, sink to the level corresponding to their density.  Alcohol-water columns are widely used for PE.  Ethanol-carbon tetrachloride can be used for resin identification.. 16 SPECIFIC GRAVITY  For practical purposes, density may be regarded as numerically equal to relative density (sometimes called specific gravity).  The specific gravity (SG) or relative density is a dimensionless unit defined as the ratio of density of the material to the density of water at a specified temperature.  Specific Gravity can be expressed as: SG = ρ / ρH2O Where, ρ = density of fluid or substance (kg/m3) ρH2O = density of water (kg/m3) = 1000 kg/m3 17 SPECIFIC GRAVITY  SG for common polymers: Polyethylene: Others: Polyamide: LDPE 0.90-0.93 ABS 1.04-1.07 PA-12 1.02 VLDPE 0.79-0.85 PS 1.05 PA-11 1.04 LLDPE 0.91-0.94 PVAc 1.19 PA-6 1.12- HDPE 0.96-0.97 PC 1.2 1.13 PSU 1.24 PA-66 1.13- PET 1.34-1.39 Rubber: 1.15 PVC 1.37-1.39 Natural rubber 0.91 POM 1.41-1.43 Butyl rubber 0.92 PTFE 2.27 Styrene-butadiene 0.93 18 CALCULATION  Example: If the density of iron is 7850 kg/m3, the specific gravity of iron is: SG = (7850 kg/m3) / (1000 kg/m3) = 7.85 19 Bulk density Bulk density is the mass per unit volume, including voids inherent in the material, of the raw plastic material as it is purchased from the material supplier. It is the density of a processable material such as a molding compound in loose form (granular, pellet, etc.). Bulk density is useful as an incoming quality check on material. 20 Apparent density Apparent density is a measure of the fluffiness of the material. Apparent density is the same as bulk density. It is the mass per unit volume of a void-containing material. The term is usually applied to particulate and loose material (e.g., powders or pellets), and the voids are interparticle spaces. A method of determining the apparent density is described in ASTM D1182. 21 For unvulcanized rubber:  Density is simple but effective quality control check on batches of compounded rubber against gross errors.  It is also widely used in the rubber industry to check the filler/polymer ratios of mixed compounds (especially masterbatches) and for calculating blank weight for mouldings.  By measuring the density of a compound, the weighing errors of mixing loss can also be determined.  For the most purposes the density of a rubber is quoted to 0.01 Mg/m3 and the commonest method of determination is by weighing the sample in air and then in water.  The standard procedure is given in ISO 2781. 22 Thank you 23