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Theory of Antistatic Agent Migration

Frank Lochel and Michael Watson – Chemax Polymer Additives

Abstract

A theory of antistatic agent migration is proposed based upon specific additive and polymer properties. Two examples are discussed. The first looks at the properties of a variety of phosphate esters in low-density polyethylene. Their predicted and actual performance is discussed. The second example explores coco amine POE 2 incorporated into a range of polymers looking at predicted versus actual results.

Polymer Properties

To begin developing our theory of chemical migration in polymeric materials, we begin by first considering the gross structure of the polymer itself. Polymers can be amorphous, crystalline or a blend of both. This difference is analogous to a crystal of quartz versus an amorphous quartz called an opal. One would never expect a quartz crystal to be permeable to any chemical while opal is easily permeable. The difference is in the molecular structure of these materials. The crystal has very high intermolecular bonding strength, the opal is composed of spheroids of SiO2 that are packed in an ordered array but with large voids between the spheroids – just like a box filled with uniformly sized balls. Recognizing these differences, we theorized that migration in polymers is most likely to occur in amorphous regions rather than crystalline regions. Given this theory there are three polymer properties that will impact migration rate: Specific gravity, glass transition temperature and polarity. Specific gravity

Specific Gravity is the most easily measured indicator of relative crystallinity within a chemical group. If one compares the specific gravity of LDPE versus HDPE one would expect that migration through LDPE would be faster than through HDPE. This effect is even more pronounced

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when comparing more diverse polymers such as LDPE versus Polystyrene.

LDPE HDPE PP PS ABS

Specific Gravity

0.91-0.93

0.94 – 0.96

0.902 – 0.906

1.04 – 1.10

1.01 – 1.07

Glass Transition Temperature Tg

Tg is a good indicator of the intermolecular bonding strength in a polymer. It is expected that the lower the Tg the more permeable the polymer If a very fast migrating additive is present during processing of the polymer, it will have maximum mobility while at a temperature > Tg.

Polarity

It is not the polarity of the molecules in the polymer that determine additive migration speed, but the differences in polarity between the polymer and the additive that is important. One would expect a very polar additive to migrate quickly through LDPE, itself non polar. We would expect a non-polar additive to migrate quickly through very polar polymer such as PVC.

PP LDPE HDPE PS ABS Relative Polarity Rank in order of increasing polarity

1 2 2 4 5

Additive Properties There are a variety of additive properties that can impact migration. The significance of some of these is in direct relationship to the polymer, such as polarity. Others are independent of the polymer, such as molecular weight. Outlined below are the suspected impact on migration for molecular weight, solubility, viscosity, pH, HLB, thermal stability and terminal group chemistry. We believe that these properties taken as a whole provide a predictive model for migration.

LDPE HDPE PP PS ABS Glass Transition Temperature

-110 °C -90°C 0° 100°C 110°C

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Molecular Weight

Some additives can be formulated to meet polarity and functional group expectations as relatively small molecules others are only possible as large molecules. Due to the interstitial spaces in the polymer, the smaller the molecule the faster the migration.

Solubility If the additive is relatively insoluble in the polymer then the

polymer will be stressed by the presence of the additive and in its effort to seek thermo dynamic stability will endeavor to exclude the additive from its structure – this accelerates migration. If additive solubility is extremely poor, the polymer may suffer structural weakness. An example is if excessive moisture is present in a polymer additive the polymer, when processed, will lack integrity.

Conversely if the additive is very soluble in the polymer,

migration will be slow but dispersion of the additive in the interstitial matrix will be greater.

pH

We have found that pH has a significant influence on antistat performance. Additives having extreme pH’s have been found to have excellent surface activity and fast migration speeds.

HLB

The Hydrophilic-Lipophilic Balance (HLB) indicates whether an additive is water or oil soluble. Due to the oil soluble nature of most polymers one would expect that low HLB (oil soluble) additives would be most useful in polymers. The that HLB is a good representative for additive polarity. The greater the HLB the more polar the additive. Surfactant additives are ranked on a 1 to 20 scale where 1 to 6 are oil soluble, 7-9 water dispersible and 10 and above water-soluble.

Thermal Stability

The additive must be thermally stable at the processing temperature of the polymer. While this is secondary to the other properties discussed, not considering this important parameter can be fatal to any model.

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Terminal Group Chemistry

For migrating antistats terminal group chemistry provides the ability to hold moisture on the polymer’s surface. The holding force is hydrogen bonding between the water molecules and the antistat’s polar terminal groups.

Method

Summary: We used surface resistivity measurements over time to demonstrate migration speed. We conducted two studies. First we evaluated a diverse range of phosphate ester chemistries (not typically seen as antistatic agents in polymers) to determine if we could predict their performance. We based the evaluation upon the properties mentioned by looking at their performance in one polymer, LPDE (Low Density Polyethylene). Second we tested one known antistatic agent, coco amine POE2, commonly know as CAM2, to determine if we could predict its performance in a range of polymers. For different additive types various properties will carry greater weight in predicting performance. Major influences of antistat migration are: additive solubility in the polymer, relative polarity of the additive versus that of the polymer, molecular weight and functional group concentration at the surface.

Procedure

Phosphate Esters The phosphate esters synthesized for this worked have one of two forms, either Mono-Ester or Di-Ester. O O

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R-O-P-OH R-O-P-OR

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OH OH Mono-Ester Di-Ester

Phosphate esters can further be classified based on their R-groups. For this study we used three main groups and modified them with-in the group by adding various levels of polyoxyethylene (POE) to vary solubility, molecular weight and HLB. These groups are: Linear Chain: These should have the least resistance to migration based on spatial configuration. The following materials were selected:

• C4:(Butyl) mono-ester with 2 moles ethylene oxide (C4 POE2)

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• C10: (Decyl) di-ester with 4 moles ethylene oxide (C10 POE4)

• C10-12:(Decyl-Lauryl) mono-ester with 6 moles ethylene oxide (C10-12 POE6)

Branched Chain:

These should be slower migrating versus the linear materials.

• C8: (2 Ethyl Hexyl) di-ester with 2 moles ethylene oxide (C8 POE2)

• TDA-3:Tridecyl Alcohol di-ester with 3 moles ethylene oxide (TDA POE3)

• TDA-6: Tridecyl Alcohol di-ester 6 moles ethylene oxide (TDA POE6)

• TDA-9: Tridecyl Alcohol di-ester 9 moles ethylene oxide (TDA POE9)

Aromatic:

These should be slowest migrating based on their larger spatial configuration than either the linear or branched R-groups. Their aromatic nature may make them more soluble in some polymers such as PET or Styrene.

• Phenol di-ester with 6 moles ethylene oxide (Phenol POE6)

• Octyl Phenol monoester with 7 moles ethylene oxide (OP POE7)

• Nonyl Phenol diester with 9 moles ethylene oxide (NP POE9)

• Dinonyl Phenol diester with 8 moles ethylene oxide (DNP POE8)

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Properties of Various Phosphate Esters

On a relative scale with other polymers we would expect additives to migrate faster through LDPE due to its low specific gravity and low Tg. Given that LDPE is a non-polar polymer we would expect the additives with the greatest polarity to migrate fastest. Within additives with similar polarity we would expect those with lower molecular weight to migrate fastest. The fastest migrating additive may not make the best antistatic agent. If it migrates too fast it may be left on the barrel of the extruder, oil out on the surface of the polymer or loose its effect over a short period of time. We were able to keep the pH and thermal stability relatively constant over the range of phosphate esters. The terminal groups were the same within the mono-ester and di-ester subgroups. Therefore the predictive factors are polarity (as described by HLB) and molecular weight. Based on molecular weight we would expect the C4 POE(2) mono-ester to be the fastest and the dinonyl phenol POE(8) to be the slowest. Looking at polarity we would expect the phenol POE(6) di-ester to be the fastest and the tridecyl alcohol POE(3) di-ester to be the slowest. The relative predictive effect of polarity versus molecular weight will be seen by observing two sets of three phosphate esters: In the first set they all have a similar molecular weight (near 730).

• C10: (Decyl) Di Ester with 4 moles ethylene oxide

• TDA-3:Tridecyl Alcohol diester with 3 moles ethylene oxide

• Phenol diester with 6 moles ethylene oxide

Linear Phosphate Esters Aromatic Phosphate Esters Branched Chain Phosphate Esters

Property C4 C10 C10-12 Phenol Octyl Phenol

Nonyl Phenol

Dinonyl Tridecyl Alcohol

Tridecyl Alcohol

Tridecyl Alcohol

C8

Moles EO 2 4 6 6 7 9 8 3 6 9 2

Mono or Di Ester

Mono Di Mono Di Mono Di Di Di Di Di Di

Molecular Wt

242 730 516 778 594 1294 1458 726 990 1252 498

pH <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3

Thermal Stability °C

>350 >350 >350 >350 >350 >350 >350 >350 >350 >350 >350

HLB 13.9 12.2 13.9 16 13.6 13.7 10.9 9.9 12.6 14.1 10.9

Terminal Group

(OH)2 (OH) (OH)2 (OH) (OH)2 (OH) (OH) (OH) (OH) (OH) (OH)

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In the second set they all have a similar HLB (polarity) value (10 –11):

• C8: (2 Ethyl Hexyl) Di Ester with 2 moles ethylene oxide

• TDA-3:Tridecyl Alcohol diester with 3 moles ethylene oxide

• Phenol diester with 6 moles ethylene oxide

Predicted Antistatic Performance of Various Phosphate Esters in LDPE

Actual Antistatic Performance of Various Phosphate Esters in LDPE

Conclusion: First we will consider the impact of molecular weight. Since we expected differences between the mono-ester (OH)2 and di-esters (OH), we will address each separately. Below are the results sorted by terminal group and by molecular weight.

Performance Sorted by Terminal Group and Molecular Weight

Within the mono-esters the lowest molecular weight material acted as expected. The C4 POE2 migrated fast but lost performance overtime. The midsize mono-ester also reacted as predicted. C10-12 POE6 migrating slower with improved

Property C4

C8 C10 C10-12

Phenol

Octyl Phenol

Nonyl Phenol

Dinonyl phenol

Tridecyl Alcohol

Tridecyl Alcohol

Tridecyl Alcohol

Moles EO 2 2 4 6 6 7 9 8 3 6 9 Mono or Di Mono Di Di Mono Di Mono Di Di Di Di Di Natural LDPE 13 13 13 13 13 13 13 13 13 13 13 Immediate SR 11 12 12 12 12 12 12 12 12 12 12 30 DaysSR 12 12 <11 11 <11 12 <11 <11 <11 <11 <11

Property C4

C8 C10 C10-12

Phenol

Octyl Phenol

Nonyl Phenol

Dinonyl phenol

Tridecyl Alcohol

Tridecyl Alcohol

Tridecyl Alcohol

Moles EO 2 2 4 6 6 7 9 8 3 6 9 Mono or Di Mono Di Di Mono Di Mono Di Di Di Di Di Natural LDPE 13 13 13 13 13 13 13 13 13 13 13 Immediate SR 11 13 11 12 12 13 12 12 11 12 12 10 Days SR 12 13 11 11 11 13 11 11 11 10 11 30 Days SR 12 13 11 11 12 13 11 10 11 11 -

Property C8 Tridecyl Alcohol

C10 Phenol Tridecyl Alcohol

Tridecyl Alcohol

Nonyl Phenol

Dinonyl C4 C10-12 Octyl Phenol

Moles EO 2 3 4 6 6 9 9 8 2 6 7

Molecular Wt

498 726 730 778 990 1252 1294 1458 242 516 594

HLB 10.9 9.9 12.2 16 12.6 14.1 13.7 10.9 13.9 13.9 13.6

Terminal Group

(OH) (OH) (OH) (OH) (OH) (OH) (OH) (OH) (OH)2 (OH)2 (OH)2

Initial SR 13 11 11 12 11 12 12 12 11 12 13

10 Day SR 13 11 11 11 10 11 11 11 12 11 13

30 day SR 13 11 11 12 11 11 11 10 12 11 13

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performance over time. Only the aromatic compound failed to meet the predicted performance. This could be due to the aromatic nature of the molecule. Within the di-esters, we observed either loss in extrusion or no effect, for the lowest molecular weight material (C8 POE2). There is a general trend towards slower initial migration as the molecular weight rises. It can also be seen that the larger molecular weight items improve overtime. Of those materials with very similar molecular weights but different HLB values there were some interesting observations. The linear (C10) and the branched (TDA POE3) had identical properties even though there was almost 2.5 points difference in their HLB. Given this performance the aromatic material (Nonyl Phenol POE9) would be expected to migrate faster due to the significantly different HLB value (3.8 units higher than TDA POE3). Instead it migrates slowly initially and looses performance quickly.

Performance Sorted by Terminal Group and HLB Property Tridecyl

Alcohol C8 Dinonyl C10 Tridecyl

Alcohol Nonyl Phenol

Tridecyl Alcohol

Phenol Octyl Phenol

C4 C10-12

Moles EO 3 2 8 4 6 9 9 6 7 2 6

Molecular Wt

726 498 1458 730 990 1294 1252 778 594 242 516

HLB 9.9 10.9 10.9 12.2 12.6 13.7 14.1 16 13.6 13.9 13.9

Terminal Group

(OH) (OH) (OH) (OH) (OH) (OH) (OH) (OH) (OH)2 (OH)2 (OH)2

Initial SR 11 13 12 11 11 12 12 12 13 11 12

10 Day SR

11 13 11 11 10 11 11 11 13 12 11

30 Day SR

11 13 10 11 11 11 11 12 13 12 11

Secondly we will consider the importance of HLB. Within the mono-esters there is very little difference in the HLB values, although significant performance differences resulted. This would imply that molecular weight and R-group spatial configuration play a more significant role in their migration. Within the di-esters, some interesting observations can be seen. We had expected that the higher HLB (more polar material) would migrate fastest. The ones with the highest HLB: Phenol POE6, TDA POE9 and Nonyl Phenol POE9 actually had slow initial migration with improvement overtime. Within the materials with similar HLB values molecular weight was very predictive. C8POE2 didn’t function at all, TDA POE3 was fast and steady and Dinonyl Phenol POE8 improved overtime.

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Coco amine POE2 in Various Polymers CH2CH2—OH C12H25—N CH2CH2—OH

Properties of Cocoamine POE 2 Molecular Weight

Viscosity PH Thermal Stability

HLB Terminal Group

275 150mPa/s 10 >500°F 6.1 (OH)2 Coco amine POE2 has a relatively low molecular weight implying fast migration in any polymer, especially an amphorous polymers. Its linear R-Group also indicates fast migration is expected. The relatively low HLB indicates it would move slower through non-polar polymers versus polar polymers. The terminal group indicates sufficient hydrogen bonding to form a water layer at the surface of the polymer.

Properties of Various Polymers

Polymer Property LDPE HDPE PP PS ABS

Specific Gravity 0.91-0.93

0.94 – 0.96

0.902 – 0.906

1.04 – 1.10

1.01 – 1.07

Glass Transition Temperature

-110°C -90°C 0°C 100°C 110°C

Relative Polarity Ranked in order of increasing polarity

2 2

1 4 5

All of these polymers are processed above their Tg’s so we would not expect this to have an impact on initial antistatic performance. The specific gravity would indicate that PP is the least crystalline polymer and presents the lowest resistance to migration. The polarity of the polymers would indicate that coco amine POE2 would move faster through the PE polymers than through PP, PS or ABS.

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Predicted Performance Polymer Property LDPE HDPE PP PS ABS Natural Polymer SR 13 13 13 12 13

Initial SR 11 12 12 11 12 60 Day SR 10 10 10 9 11

Actual Performance

Polymer Property LDPE HDPE PP PS ABS Natural Polymer SR 13 13 13 12 13

Initial SR 10 12 11 11 11

3 Day SR 10 11 10 11 10 21 Day SR 10 11 10 10 10

60 Day SR 10 10 10 9 10 Use concentration 0.4% 0.4% 0.8% 2% 0.9%

Conclusion: The expectation that coco amine POE2 would migrate faster in LDPE versus HDPE was accurate given that HDPE is more crystalline than LDPE. HDPE did show improvement overtime as would be expected with an additive that is slower migrating. While PS, with the highest specific gravity, did show the slowest migration there was no differentiation between PP and ABS even though there is 0.10 unit difference in their specific gravities. This would need to be studied further to determine other causal factors. Final Conclusion: Given these results it can be seen that additive properties can predict performance in different polymers. The various properties carry different weight in predicting performance. In this study they can be ranked as molecular weight first, functional group second and HLB third. While HLB is a good objective measurement of additive polarity more work needs to be done to better understand each polymers relative HLB. For example, coco amine POE2 has an HLB of 6.1, which is the least polar of all additives in this study. It still migrated quickly through LDPE. Possibly LDPE’s relative HLB is well below 6 allowing materials like coco amine POE2 to migrate quickly. Further Work: In addition to further determining the relative HLB of the base polymers there are a number of other additive properties we would like to explore to determine their predictive effect:

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Refractive Index (RI)

Clarity is a major concern in many polymers and a number of additives tend to haze polymers. Matching the RI of the additive to that of the polymer is expected to have a predictive effect on final clarity. .

Functional Group Orientation

The molecular structure of an effective antistat must be designed so that the lipophyllic portion of the molecule is soluble in the polymer while its hydrophilic portion must orient toward the polymer surface. Additionally the spacing of the functional groups at the polymer surface must be of the appropriate distance to maximize hydrogen bonding with water to form a continuous water layer. While we did see there was a difference in performance between mono-esters with two hydroxyl groups and di-esters with one hydroxyl group the result was actually the inverse of what would be expected. The two-hydroxyl groups of the mono-ester should increase hydrogen bonding, but this did not take place. It is not a function of just having two hydroxyl groups present because coco amine POE2, with the same groups, has excellent performance. The way these groups orient themselves on the surface may be the answer.

pH

While we briefly addressed pH we need a more systematic study of its effect on migration using a broader range of pH’s.

Viscosity

Viscosity is related to both molecular size and polarity. At post extrusion storage temperature viscosity becomes an important factor in additive migration speed. The lower viscosity materials will be expected to migrate faster.

Acknowledgements: We would like to acknowledge the hard work and dedication of Mr. Sam Habib and Mr. Chris Sculthorpe, researchers at Chemax Polymer Additives, for their contributions to this paper.