ADDITIVES FOR GEOMEMBRANES

Additives are critical for the proper performance of geomembranes and geosynthetics in general.

PIGMENTS

Pigments are used in geomembranes to provide the colour to control the surface temperature of the geomembrane but more importantly they act as UV stabilizers. Most geomembranes are either black, beige or shades of grey. Carbon black and titanium dioxide are the main two pigments used in geomembranes. Carbon black is an excellent UV screen, absorbing most of the UV radiation that strikes the geomembrane and converting it to heat. Titanium dioxide, on the other hand, reflects almost all UV radiation. Both these pigments offer excellent UV protection. Geomembranes can be exposed for years with minimal UV degradation when using these pigments together with special stabilizer additives.

CARBON BLACK

Carbon black not only acts as a UV screening agent (absorbing damaging UV light) but also performs a radical trapping function by binding up damaging free radicals. Carbon black only reaches its maximum effectiveness when a certain fine particle size is achieved (e.g. 20 nm) and the particle-to-particle distance is minimized in order to provide dense coverage (this is a function of proper distribution and dispersion of the carbon black aggregates). The carbon black used in HDPE geomembranes is generally required to conform to category Group 3 and/or lower as defined in ASTM D-1765.

Levels of carbon black only up to 2.5–3.0% can be used in HDPE since larger amounts can detract from the mechanical properties of HDPE simply because the high crystallinity of HDPE leaves little free volume (i.e. space) to accommodate additives.

Carbon black pigments come in different particle sizes. The more expensive carbon blacks have a smaller average particle size (i.e. 18–22 nm) while the cheaper carbon blacks are coarser (e.g. 60–150 nm). The smaller particle size carbon blacks are more efficient at blocking UV light and preventing ultraviolet degradation of polymeric geomembranes (see Figure 2.21). The level of carbon black in the geomembrane referred to as the carbon black content (CBC) is determined according to ASTM D-1603, ASTM D-4218 or ISO 6964. Not only are the amount and particle size of the carbon black important but also is its uniform dispersion in the geomembrane. The test method for determining the extent of dispersion of carbon black is ASTM D-5596 or ISO 11 420.

To check that the right amount of carbon black is added and that it is dispersed thoroughly within the geomembrane sheet, both carbon black content and carbon black dispersion tests are performed. Carbon black content tests measure the percentage of carbon black added to the geomembrane by heating the geomembrane until only the carbon black remains. For HDPE geomembranes, typically 2 to 3% carbon black content is within acceptable limits.

Carbon black dispersion tests estimate dispersion of carbon black within a geomembrane sheet by observing a thin film of the geomembrane under a microscope. The dispersion observed is qualitatively compared to a chart containing several degrees of carbon black dispersion. Most geomembrane specifications allow a dispersion rated as A-1, A-2 or B-1. For HDPE and LLDPE, the carbon black agglomerates need to fall mainly (i.e. 9 out of 10 views) into Category 1 or Category 2 and only 1 is allowed in Category 3. No carbon black agglomerates in Category 4 or Category 5 are acceptable. Poor carbon black dispersion may result in reduced UV and environmental stress crack resistance and may reduce tensile properties.

Note that the LLDPE carrier resin from black masterbatches can preferentially reside in the centre of the GM and can cause delamination problems. This is often identified as black streaks when performing carbon black dispersion tests.

STABILIZER PACKAGE

Antioxidants and stabilizers are added to polymeric materials to inhibit oxidation and extend the induction period to onset of degradation. Because geosynthetics are manufactured at high temperatures (200 to 220 ?C), antioxidants are needed that function at the high temperatures associated with manufacturing as well as the lower temperatures associated with in-service applications. Consequently, manufacturers generally use a combination of two or more types of antioxidants and stabilizers to provide overall stability. Following pic shows the autooxidation cycle for the oxidiation of polyolefins. Cycle-breaking antioxidants and stabilizers function at steps (a), (b) and (c) to scavenge the damaging free radicals.

The additive package and specifically the stabilizer package, needs to be based on best practice principles which means that it should comprise a high performance stabilizer package formulated for potentially aggressive and exposed environments. It is quite difficult to write a specification for polyolefin geomembranes around a particular additive, or group of additives, because they are generally proprietary. Furthermore, there is ongoing research and development in the stabilization area and thus additives are subject to changes over time. If additives are included in a specification, the description must be very general as to the type and amount.

A best practice stabilization package for polyolefin geomembranes can comprise at least three antioxidant/stabilizer additives:

? Hindered Phenolic Antioxidant (HPA).

? Hindered Phosphite Processing Stabilizer (HPPS).

? Hindered Amine Stabilizer (of high molecular weight, i.e. oligomeric or polymeric)

(non-migratory) (HALS-HMW).

? Hindered Amine Stabilizer (of low molecular weight) (migratory) (HALS-LMW).

Antioxidants

Typical antioxidants used in polyolefin geomembranes are shown in following Table.

Hindered Amine Stabilizers

Hindered amine light stabilizers (HALS) are widely used in polymeric geomembranes to impart both UV and thermooxidative (i.e. heat) protection. HALS are the stabilizer product of choice for polyolefins because they exhibit multi-functional stabilization activity. HALS is multifunctional in its capacity by: trapping free radicals, decomposing alkyl hydroperoxides (which are unstable intermediates in the oxidation cycle), quenching excited states and regenerating its active species. Commercially available HALS contain the tetramethyl piperidinyl moiety which is the precursor to the ‘active site’ for stabilization activity. Once activated, the nitroxyl active site undergoes many cycles until it is ultimately deactivated.

Although nitroxyl deactivation is a major pathway by which basic HALS lose effectiveness, there are also other causes such as chemical side reactions with acidic compounds.

HALS are the stabilizers of choice for polyolefin geomembranes because they exhibit multi-functional stabilization action and can protect the geomembranes against both heat and UV light. HALS are capable of decomposing radical precursors, trapping free radicals and also can potentially quench excited states, all of this at the same time as regenerating the active species.

The tetramethyl piperidinyl group of HALS stabilizers is the precursor for the active site for stabilization activity and once activated, the resultant nitroxyl active site undergoes many regeneration cycles until it is ultimately deactivated. Typical HALS used in polyolefin geomembranes are shown in following Table.

HALS stabilizers can lose their effectiveness because of the following reasons:

? nitroxyl deactivation (which is the major pathway);

? acid attack (protonation of the active nitroxyl site);

? extraction and loss;

? chemical side reactions.

The total concentration of the HALS additives should be equal to or greater than 5000 ppm (i.e. 0.5%) for 15–20 year exposed lifetimes in HDPE and fPP.

Initially only low-molecular weight HALS additives were developed such as Tinuvin 770. Whilst they were effective light stabilizers for polyolefins they had shortcomings such as a high migration rate and only moderate resistance to extraction. To overcome these disadvantages high-molecular weight (or polymeric) HALS were developed. These additives show remarkable effectiveness for stabilizing polyolefins against not only UV degradation but also thermal oxidation.

Tinuvin 770 and Tinuvin 622 are HALS additives based on a polyester structure. Unfortunately the polyester backbone is prone to hydrolytic- and photolytic-cleavage reactions which can lead to additive loss. The hydrolysis reactions can be accelerated by the presence of acids and typical formulation components such as stearates. Chimassorb 944 is a polymeric hindered amine stabilizer which has stable triazine backbones in its structure. Due to the absence of ester groups, Chimassorb 944 is not prone to hydrolytic breakdown and furthermore the triazine rings in the backbones impart superior thermal and photochemical stability as well as low volatility.

Note that combinations of the various antioxidants and stabilizers listed in following Table are often used to provide optimum stabilization; for instance low and high molecular weight HALS are often used in combination, such as Tinuvin 770/Chimassorb 944 or Uvinul 4050/Uvinul 5050.

Following Figure shows the effective temperature ranges of different antioxidant and stabilizer types used in polymeric geomembranes. Note: High molecular weight HALS are also commonly referred to as oligomeric or polymeric stabilizers and they are difficult to extract from the geomembrane polymer matrix.

Commercial examples

5000 ppm Tinuvin 783 (Ciba) comprises Chimassorb 944 (high molecular weight hindered amine) and Tinuvin 622 (a low molecular weight hindered amine stabilizer) in a 1:1 ratio.

5000 ppm of a 1:1 mixture of a high molecular weight hindered amine (Cyasorb UV-3529 from Cytec) in combination with a low molecular weight hindered amine stabilizer (Cyasorb UV-3853 from Cytec) which is highly mobile, to protect the surface of the geomembrane but also has high solubility in the polymer to resist extraction.

Typical Stabilization Formulations

Polyolefin stabilization packages have as a rule of thumb at least 5000 ppm (i.e. 0.5 wt%) of HALS stabilization to give greater than 20 years UV stability in combination with2.5% wt. carbon black.

Typical polyolefin geomembrane additive packages are shown below as examples. For polyolefin geomembranes in potable water contact:

? 3000 ppm Irganox 1010 (hindered phenolic);

? 1500 ppm Irgafos 168 (hindered phosphite);

? 5000 ppm Chimassorb 944 (hindered amine).

For non-potable applications:

? 3000 ppm Irganox 1010 (hindered phenolic);

? 1500 ppm Irgafos 168 (hindered phosphite);

? 3000 ppm Chimassorb 944 (hindered amine);

? 2000 ppm Tinuvin 770 (hindered amine).

Note for non-potable applications, Tinuvin 770 is used to replace part of the Chimassorb944 for the purpose of reducing cost, since Tinuvin 770 is not approved for contact with potable water.

For improved antioxidant leaching resistance in water contact applications, the following mixture has been used:

? 3500 ppm of Irganox 1330 (hindered phenolic);

? 5000 ppm of Chimassorb 944 (hindered amine);

? 2000 ppm of Tinuvin 770 (hindered amine).

The last formulation uses Irganox 1330 in place of Irganox 1010. Irganox 1330 is a trifunctional hindered-phenolic antioxidant whereas Irganox 1010 is tetrafunctional. The molecular weight of Irganox 1330 is actually lower than that of Irganox 1010 (cf. 775 versus 1178). The extraction and leaching resistance of Irganox 1330 however, is purported (by Ciba) to be higher due to the molecule being more rigid and planar (due to its central phenyl core), while the Irganox 1010 molecule is more pliant and mobile.

Irganox 1330 is also preferred over Irganox 1010 in terms of its resistance to alkaline hydrolysis and its low adsorption potential onto carbon black. Irgastab FS811 at levels of 1 wt% (10 000 ppm) has been used for the stabilization of fPP geomembranes as well as fPP roofing products. This additive package at 1 wt% loading is expected to give the 0.75 mm FPP product a 10 year lifetime. Note Irgastab FS 811 is a 5:3:2 mixture of Chimassorb 944, Tinuvin 770 and Irgastab FS 042, and so

the formulation is as follows:

? 5000 ppm of Chimassorb 944 (a high molecular weight hindered amine);

? 3000 ppm of Tinuvin 770 (a low molecular weight hindered amine);

? 2000 ppm of Irgastab FS 042 (a hydroxylamine stabilizer).

The hydroxyl amine (Irgastab FS 042) has good extraction resistance and is often used

in applications for instance, where there is a high surface area to volume ratio such as in

fibres.

Non-Reactive HALS

In recent years it has been recognized that strong acids can interact with basic hindered amine stabilizers and significantly reduce their effectiveness. The reason is that standard HALS stabilizers are basic in nature (i.e. alkaline) and therefore acidic compounds can deactivate them by a neutralization reaction to form a non-active salt. Acidic species (e.g. in some mining leach solutions) can deactivate hindered amines not having the N-OR group, and this interferes with the activity of ‘normal’ hindered amine stabilizers.

A solution to this problem has been the development of methylated HALS. The low basicity of methylated HALS is of particular value in the stabilization of polyolefins where the activity of the more basic hindered amine stabilizes is significantly reduced because of interaction with the acid species.

BLOOMING OF ADDITIVES

Note if the total stabilizer package exceeds or approaches 0.5–0.6% (i.e. 5000–6000 ppm), then waxy ‘blooms’ can form on the liner surface due to exudation of the additives by migration/diffusion processes to the liner–air interface. These additives can interfere with weldability of the HDPE as they act as weak boundary layers. Before wedge welding, this waxy bloom needs to be removed with a polar solvent such as acetone or limonene.

INTERACTION OF ADDITIVES

The typical pigment used for black geomembranes is N110 carbon black. If the carbon black loading exceeds 3 wt% and in particular falls in the range 3–5 wt%, then this can reduce the stability of the overall formulation since antioxidants can become absorbed and immobilized on the surface of the carbon black. Carbon black has a very high surface area and it can adsorb and immobilize antioxidants and reduce their effectiveness.

Small particle size blacks such as the 17 nm N110 type often possess high surface areas and therefore exhibit greater degrees of jetness (or ‘blackness’) than larger particle size blacks. Small particle size blacks are highly effective UV light absorbers but they are difficult to disperse at high loadings and often result in a high viscosity concentrate.

Another commonly used carbon black is Vulcan P (from Cabot) (17 nm particle size) at levels of 2.75–3.25 wt%. This is approaching the loading range where antioxidant adsorption onto carbon black may become an issue to consider.

The degree of steric hindrance of phenolic hydroxyl groups by alkyl groups in orthopositions has been found to be the major factor affecting the adsorption activity of phenolic antioxidants onto carbon black. Irganox 1330 shows negligible adsorption. Therefore it may be concluded that the 2,6-ditert-butyl phenyl group has insignificant interaction with the carbon black due to the steric hindrance (Pe?na et al ., 2002).

It is known that different grades of carbon black adsorb Irganox 1010 to different extents. The ester groups of Irganox 1010 act as sites for adsorption on the carbon black. Therefore owing to the presence of the same phenolic component and ester linkage, Irganox 1010 and Irganox 1076 show broadly similar responses. Despite having a smaller number of functional groups per molecule, Irganox 1076 has a greater adsorption/ desorption energy onto carbon blacks due to sterically more exposed ester groups.

Pe?na et al . (2002) concluded that stabilizer adsorption activity is mainly governed by the number of active functional groups per molecule, together with the steric accessibility of the functional groups to the carbon black surface. Steric hindrance of the adsorption active functional groups (H–N< and Ar–O–H) by alkyl groups has been shown to significantly reduce adsorption activity. Carbonyl groups have also been shown to contribute to additive adsorption activity. Therefore Irganox 1010 is more adsorbed than Irganox 1330 due to the four ester groups per molecule in the case of Irganox 1010. The nature of the carbon black substrate also plays an important part in adsorption. It has been shown that highly oxidized carbon blacks, bearing plentiful carboxylic and phenolic functionality, are more effective at adsorbing stabilizers.

The use of Irganox 1010 in combination with the wrong type of carbon black can therefore result in a lower degree of stabilizer performance due to the high surface area per unit mass of the carbon black coupled with some surface porosity. This finding supports the move away from Irganox 1010 to Irganox 1330.