Not resistant to:

Polyethylenes are not resistant to fuming nitric acid or fuming sulfuric acid.

Halogens and chlorinating agents such as chlorosulfonic acid and phosgene slowly attack them. LDPE is sensitive to environmental stress cracking (ESC), which occurs when a material is subject to strain, or an internal stress, in the presence of polar liquids or the vapors of such liquids. Such ESC is also asso-ciated with detergents or silicone fluids, although there are many other envi-ronmental stress-cracking agents, such as chloroform, xylene and paraffin.

The ESC effect is minimized by reducing the residual stresses in the molding, careful component design, and by using the lowest MFR (the highest molecu-lar weight) grade. At any particumolecu-lar density level, LLDPE has better ESC tance than LDPE and the higher molecular weight grades have the best resis-tance.

In many cases the resistance of PO materials to light is satisfacto-ry, but, if it is not, the cheapest way of improving the light resistance is by the addition of carbon black. Ultraviolet (UV) absorbers and screening agents are also used to give protection. Unless so protected, appearance will initially suf-fer, and then the material will seriously degrade. The most commonly used non-black light stabilizers are those based on ultraviolet absorbers, such as 2-hydroxy-4-alkoxybenzophenones and 2-(2'hydroxyphenyl) benzotriazoles), nickel (11) chelates, and polymeric hindered amine light stabilizers (HALS).

Mixed stabilizer systems are often much more efficient than a single stabilizer.

As PO materials have comparatively limited resistance to oxygen at elevated temperatures, antioxidants are also used for their protection. Unless they are protected, the electrical properties will suffer. LDPE has limited resis-tance to oxygen at elevated temperatures, but not as susceptible to oxidation at high temperatures (for example, those experienced during melt processing) as polypropylene (PP). The antioxidants used are similar to those used for PP, but the level of use is lower (~0.05 to 0.1%) because of compatibility prob-lems. 1,3,5-tris-(5-t-butyl-4-hydroxy-2-methylphenyl-butane or bis- (2-hydroxy-5-methyl-3-(1-methylcyclohexyl)-phenyl)-methane are used for prod-uct protection, whereas for protection during processing, and for non-toxic applications 2,6 di-t-butyl-4-methylphenol (BHT) is used. Metal deactivators such as N,N’dibenzaloxalyldihydrazide or, N,N'-bis-(3-(3',5’di-t-butyl-4'-hydroxy-phenyl)-propionyl)-hydrazine are also used.

10. Material Detection or Identification

With a density of approximately 0.92 g/cm³LDPE (solid, non-filled material) will float in both water and in saturated magnesium chloride. There is no sol-186

vent for the polymer at room temperature, but at higher temperatures (approx-imately 55˚C/131˚F), LDPE is soluble in hydrocarbons and chlorinated hydro-carbons, such as xylene and trichloroethylene. Below approximately 60˚C/

140˚F, PE is insoluble in all organic solvents, but it does swell in aliphatic, aro-matic and chlorinated hydrocarbons. The lower the density the more it swells.

The natural color of the material is a milky white and so a wide color range is possible. When this material is heated in a flame it ignites easily and burns with a yellow-tipped, blue flame, giving off only a little smoke. It forms burning drops and, when the flame is extinguished, gives a smell like candle wax.

The easiest way of differentiating between the different PO materi-als is by density and melting point. LDPE has a melting point of 110˚C to 125˚C (230˚F to 257˚F), LLDPE is 115˚C to 128˚C (239˚F to 262˚F), HDPE is 130˚C to 135˚C (266˚F to 275˚F) and PP is 165˚C to 175˚C (329˚F to 347˚F).

When LDPE is heated in the absence of a flame the material will soften and melt to give a clear liquid as the crystal structures are destroyed. The melt is stable, in the absence of air, up to approximately 300˚C/572˚F when it decom-poses to give low molecular weight hydrocarbons. Cross-linked PE will not melt but will become rubbery at approximately 115˚C/239˚F. LDPE can be cut or scratched easily with a knife. The shrinkage of LDPE is of the order of 0.02 to 0.05 in/in (2% to 5%) when the density is 0.910 to 0.925 g/cm³. It is in the range of 0.015 to O.04 in/in (1.5% to 4%) when the density is 0.926 to 0.94 g/cm³.

11. Coloring

As the natural color of the material is off-white, a wide color range is possible.

This does not include transparent colors in thick sections. It is sold in both compounded colors and as natural material for coloring on the processing machine, by techniques such as dry coloring, masterbatching and liquid color-ing. When coloring LDPE, organic dyes should not be used, due to the prob-lem of color leeching or bleeding (often called blush). For most coloring pur-poses organic and inorganic pigments are preferred. When dry coloring LDPE, colorant levels of up to 1% are generally used. Wetting agents are used with dry colorants, primarily for cleanliness in the processing shop, rather than for aiding the dispersion of the colorant. Universal type master batches are often used with LDPE at concentration levels of 1%. However, to obtain a more uni-form color on a molded component, the addition level may need to be increased to 3% to 5%. When color uniformity is particularly important, LDPE-based master batches are often preferred. If opacity of color is required, then inorganic pigments are generally used.

There are no limitations to using liquid colorants with LDPE, how-ever, levels have to be kept to a minimum (< 3%), otherwise screw slip may occur.

When coloring LDPE, its softness and very easy flow characteris-tics must be remembered. To improve the dispersion of the pigments within the melt, it is necessary to create an adequate mixing/grinding action within the barrel of the machine. In order to achieve this, it is often necessary to reduce the machine temperatures to the lowest values that will still enable components of the required quality to be produced.

12. Materials Handling

LDPE will absorb less than 0.02% water in 24 hrs at room temperature. This means that drying is not normally necessary. If drying is necessary, the resin should be dried in a hot air oven for 3 hrs at 65˚C/149˚F, or in a desiccant dryer for 1 – 1.5 hrs at 80˚C/176˚F.

LDPE is normally sold in the form of pellets. This is the easiest form of the feedstock for use in extrusion. The simplest hopper design is

nor-187

se ction 8: guides for the foll o w ing ma terials

mally satisfactory. Sometimes LDPE may be obtained as a powder, or regran-ulated scrap may be added to the feedstock at fairly high concentrations. In these cases, it may be necessary to use a crammer-feeder on the hopper.

13. Screw and Barrel Design

LDPE is extruded using a standard screw. A typical design is shown below.

Total Length

24 D

Length of Feed Section 6 D

Length of Compression Section 10 D Length of Metering Section 8 D Channel Depth in Feed Section 0.1 - 0.15 D

Compression Ratio 3:1 to 4:1

Flight Width 0.1 D

Flight Pitch P = D (1 D) that is, helix angle 17.7E

Screws designed to the above specification are shown in the following table:

the length of each zone is shown in brackets, (i.e. 6 D).

Screw Diameter 2.5" (62 mm) 3.5" (87 mm) 4.5" (112 mm) Feed Zone Depth (6 D) 0.35" (8.89 mm) 0.50" (12.7 mm) 0.56" (14.2 mm) Compression (10 D)

Metering Zone Depth (8 D) 0.09" (2.29 mm) 0.13" (3.30 mm) 0.15" (3.81 mm) It is quite common for polyethylene screws to have a mixing head or section after the metering section. This zone is 2 to 3 D in length and con-sists of a number of studs arranged in a pattern around the root of the screw.

As the melt passes through the mixing head, it is chopped and sheared by the rotating studs. This considerably increases the mixing and, hence, the unifor-mity of the melt (Note: The motor power required will be increased on adding a mixing head).

Barrier screws are frequently used with LDPE, as they produce a more even melt temperature. For higher outputs with a given barrel diameter, the barrel is fitted with a grooved feed section. The grooves run parallel to the axis of the extruder and force the solid pellets along the screw. The output from grooved barrel designs depends on the temperature of the grooved zone.

For maximum output, the zone is cooled to prevent melting in the grooves.

When a grooved feed is employed, then a mixing zone, usually fitted before the metering zone, is also required. Grooved extruders produce higher out-puts, but the motor power requirements are higher than those for a compara-bly sized conventional design. Barrel wear can also be a problem with the grooved feed system.

14. Barrel and Die Temperatures

It must be remembered that the barrel and die temperature interact with the screw design, screw speed and material rheology to determine the melt tem-perature. Target melt temperatures, together with guides for the barrel and die temperatures, are shown, for three applications, in the table below.

Blown Film Cable Coating Extrusion Coating Melt Temperature (˚C) 170 - 190 190 - 200 320 - 330

(˚F) 335 - 375 375 - 395 610 - 630 Zone Temperatures (˚C/˚F)

Zone 1 170/340 175/350 240/460

Zone 2 180/355 190/375 320/610

Zone 3 185/365 195/385 330/625

Die 185/365 195/385 325/620

15. Die Design and Construction

For blown film production, the spiral mandrel design has largely replaced the 188

side fed die type. This die has at least four ports and more usually about eight.

A typical gap width at the die exit is 0.04 in (1 mm).

Damage to the die lips will cause die lines on the product. These are especially noticeable in thin blown film extrudates. Soft metal scrapers, of copper or possibly brass, must, therefore, be used to remove any blockage of the die lips. A specially made scraper, which fits inside the die lips, is useful to ensure they are clean at start up.

LDPE is not a very damaging or easily damaged material. Provided dies are correctly designed, and do not leak, they should run for lengthy peri-ods without the need for stripping and cleaning. Molten LDPE should be removed when the die is hot.