Additives.pdf

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ADDITIVES

Introduction

Plastics additives are typically organic molecules that are added to polymers in

small amounts (typically 0.1–5.0 wt%) during manufacture, processing, or convert-

ing so as to improve the inherent properties of the polymer resin. Additives can be

categorized in three major segments: polymer modiﬁers, performance enhancers,

and processing aids. Pigments and Colorants are not included in this overview.

Antiozonants, accelerators, and vulcanizing agents, used in large volumes in elas-

tomers, are also excluded (see R UBBER C HEMICALS ). The global market for plastics

additives in 1998 has been estimated to be of the order of $15–16 billion in value

and 7–8 million tons in volume (1). Poly(vinyl chloride) (PVC) and polyoleﬁns are

the largest consumers of additives and drive the growth rates of 4–5% on average.

Polymer modiﬁers, accounting for about half of the total value (4.5 million

tons globally in 1999), are added primarily to alter the physical and mechani-

cal properties of the plastic. These include plasticizers, foaming (blowing) agents,

coupling agents, impact modiﬁers, organic peroxides, and nucleating/clarifying

agents. Plasticizers, used primarily in ﬂexible PVC, are the highest volume addi-

tives (see P LASTICIZERS ). Coupling agents are among the fastest growing additives

(6–7%) in this class (see S ILANE C OUPLING A GENTS ).

Performance enhancers are added to plastics to provide functionality not in-

herent to polymer itself. These include ﬂame retardants (FRs), Heat Stabilizers

for PVC, Antioxidants, light stabilizers, biocides, and Antistatic Agents. Perfor-

mance enhancers for 40% of the total global market for additives are led by FRs

with a total value of $2 billion. Among performance enhancers, light stabilizers

show above average growth rates of 6–7% (2) (see UV S TABILIZERS ).

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Processing aids are typically surface-active agents that are added by the plas-

tics converters/transformers to improve throughput and alter the surface proper-

ties of the ﬁnished article. Additives in this class include lubricants, slip agents,

antiblocks, and mold-release agents (see R ELEASE A GENTS ).

A key driver of change and new product development in the plastics additives

markets is a host of environmental concerns. This is seen most dramatically in

PVC where concerns over the use of heavy-metal heat stabilizers based on lead

have led to a widespread conversion to tin-based materials and even to nonmetallic

stabilizers. The widely used phthalate-based Plasticizers for ﬂexible PVC have

come under ﬁre because of concerns over their potential adverse effects on the

human reproductive systems. Concerns over the potential for brominated FRs to

form dioxins are fueling the development of new halogen-free systems.

Plastics additives are used extensively in food packaging and as such are

regulated by the U.S. Food and Drug Administration (FDA) (and related inter-

national agencies) as indirect food additives. Regulation by the FDA of a new

additive requires submission of toxicity, as well as migration data from the poly-

mer in question into a variety of food stimulants so as to calculate estimated daily

intake. The level of migration and anticipated annual usage determines the ex-

tent of toxicity testing that is required. Information on the petition process for

obtaining regulations as well as a directory of all indirect food additives can be

obtained through the FDA website www.fda.gov.

Additives are incorporated into polymer matrixes by a variety of methods

and at various points in the manufacturing process. Polymer producers typically

incorporate additives as single components or as blends of two or more additives

during the pelletization/isolation step. Converters and transformers often intro-

duce additives as a concentrate or masterbatch. Concentrate is a mixture is a

mixture of an additive dissolved in a polymer resin carrier at fairly high (10–30%)

concentrations. A masterbatch is a blend of additives and often pigments in a resin

carrier designed for a speciﬁc end-use application.

Modiﬁers

Plasticizers. Plasticizers, through their use in ﬂexible PVC, are the

largest volume polymer additives used in plastics. Flexible PVC accounts for

nearly 90% of the volume of plasticizers used in plastics. Plasticizers are added

at very high loadings (up to 80%) depending on the degree of ﬂexibility required.

Plasticizers are added to inherently hard thermoplastics to increase the ﬂexibility,

softness, and/or extensibility. In addition, secondary beneﬁts of improved process-

ability, greater impact resistance, and higher ductility can often be achieved. Plas-

ticizers are often used as carriers for pigments and are the liquid vehicle for PVC

plastisols. Plasticizers are predominately esters produced through the reaction of

an acid or anhydride with a linear or branched alcohol (3).

Dialkyl phthalates produced by the reaction of phthalic anhydride [85-44-

9] with alcohols varying in chain length from C 4 to C 11 with 2-ethyl-1-hexanol

[104-76-7] and 1-octanol [111-87-5] typically used, are the most commonly used

plasticizers. While somewhat interchangeable, performance properties such as low

temperature ﬂexibility, volatility, processability, and extractability are governed

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by chain length and degree of branching. For example, in interior automotive

PVC applications, the octyl phthalates have been replaced by isodecyl phthalates

because of their lower volatility and thereby enhanced fogging resistance.

The remaining plasticizers are more specialty in nature. Aliphatics are typ-

ically 2-ethylhexyl esters of dibasic acids, such as glutarates, adipates, or seba-

cates. The primary use of aliphatics is when low temperature ﬂexibility and crack

resistance is required. When very low volatility and low migration is required,

the plasticizers of choice are based on esters of trimellitic anhydride [552-30-7].

A typical application for mellitates is in PVC wire and cable jacketing, which re-

quires excellent long-term heat aging properties and extraction resistance. For

particularly demanding performance applications, dibasic acids are polymerized

with diols to produce low molecular weight polymeric plasticizers. Esters of phos-

phorus oxychloride [10025-87-3], the phosphate esters, are typically used as FRs

and also impart plasticizing properties. A ﬁnal class are the epoxies, with epoxi-

dized soybean oil [8013-07-8] (ESBO) being the most common (epoxies based on

tall oils and linseed oil are also available). While primarily added as secondary

thermal stabilizers in PVC because of their ability to scavenge HCl generated

during processing, as plasticizers they exhibit excellent extraction resistance and

low migration.

Typically the large producers of plasticizers are backward integrated into ei-

ther alcohol and phthalic anhydride or both. The key suppliers of the commodity

plasticizers in North America are ExxonMobil (Jayﬂex), BASF (Pluronic), East-

man (Eastman), and Aristech (PX). About 2.5 billion pounds of plasticizer are

consumed in North America.

Foaming (Blowing) Agents. Chemical blowing agents are inorganic or

organic additives that produce a foamed structure. They are used extensively

in PVC but also in polyethylene (PE), polypropylene (PP), and polystyrene (PS)

to improve properties and appearance (insulation against heat and noise, better

stiffness, removal of sink marks in injection-molded parts, and improved electrical

properties) as well as to reduce weight. Chemical-blowing agents can be classi-

ﬁed as either physical or chemical. They are typically added via a concentrate or

masterbatch. The total market for blowing agents in North America is 6800 tons

(4,5).

Physical blowing agents are volatile liquids or compressed gasses that are

dissolved in the polymer and change state during processing to form a cellular

structure. Chemical blowing agents (CBAs) decompose thermally during process-

ing to liberate gasses that form a foamed product. Organic CBAs typically are

solid hydrazine derivatives that generate nitrogen in an exothermic reaction.

Most common is azodicarbonamide [123-77-3], which in pure or modiﬁed form ac-

counts for up to 80% of all CBAs. It begins to decompose at 390 ◦ F. Other types are

the sulfonyl hydrazides

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and p -toluene semicar-

most common is p -toluenesulfonylsemicarbazide [10396-10-8] which is

used in high temperature applications such as acrylonitrile–butadiene–styrene

(ABS), poly(phenylene oxide) (PPO), nylon, and high impact polystyrene (HIPS)

{

High gas yields and pressures for exothermic CBAs make them useful in applica-

tions such as cross-linked PE and extruded products. Uniroyal Chemical (Celogen,

Expandex) is the major producer of these products in North America. Endothermic

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most common is 4,4 -oxybis(benzenesulfonyl hydrazide)

[80-51-3] which is used for low temperature applications

bazides

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CBAs are based on blends of inorganic carbonates and polycarbonic acids. Proper

combination of these materials allows for operating temperature ranges of 150–

300 ◦ C. A common commercial system is based on citric acid [77-92-9] and sodium

bicarbonate [144-55-8]. Endothermic CBAs generally produce a lower gas yield

providing foams with smaller cell structure than exothermic CBAs do. Clariant

(Hydrocerol) is the leading supplier of endothermic CBAs.

Coupling Agents. Coupling agents promote adhesion between polymers

and inorganic ﬁllers by forming stable chemical bonds between the organic matrix

and the surface of the ﬁller. The highest usage of coupling agents is in the treat-

ment of glass ﬁbers for use in thermosets such as epoxies and polyesters. Other

ﬁllers include clay, silica, mica, wollastonite, calcium carbonate and aluminum

trihydrate (ATH).

The most common type of coupling agent is the organosilanes. Silanes have

the general structure RSi(OR ) 3 , where R is a functionalized organic group that

binds to the polymer matrix (ie, amino, epoxy, acrylate, or vinyl) and R is typically

methyl or ethyl. The methoxy or ethoxy groups hydrolyze to silanols which react

with surface hydroxyl groups on the inorganic ﬁllers to form oxane bonds. The

result is improved mechanical or electrical properties. Amino silanes are typically

used for epoxy and phenolic resins, epoxy silanes for epoxy resins, and methacry-

late silanes with unsaturated polyesters. Fillers are typically pretreated with an

aqueous dispersion of silane at levels of 0.2–0.75%. The treated ﬁllers are then

reacted with the polymer matrix during compounding. The silane improves wet-

ting during the compounding process, thereby reducing the surface tension of

the organic–inorganic interface for better dispersion. The ﬁlled compound has

much improved moisture resistance. Dow Corning, Crompton, and Degussa-Huels

(Sivento) are major suppliers of silanes in North America with an estimated mar-

ket value of $70–100 million.

In addition to silanes, a variety of organometallics (primarily titanates, but

also zirconates, aluminates, and zircoaluminates) are used as coupling agents,

although in signiﬁcantly lower volumes. While mechanistically similar, titanates

are more versatile than silanes because they can react with a broader range of

ﬁllers (ie, calcium carbonate). However, they are more susceptible to hydrolysis.

Titanates are often used as dispersing agents for ﬁllers in polyoleﬁns by reducing

the surface energy of the ﬁller, resulting in better impact strength, lower melt

viscosity, and better aged mechanical properties. DuPont and Kenrich are the

primary suppliers of organometallics.

A specialty class of coupling agent are the maleated polyoleﬁns (6). The pen-

dant maleic anhydride unit reacts with surface hydroxyl groups (or siloxy group

in the case of pretreated ﬁllers) while the polymeric portion cocrystallizes with

the polymer matrix. Their main applications are in glass-ﬁlled PP composites

and in non-halogenated FR wire and cable applications. The addition of 1–2%

maleated PP can improve the tensile strength of a 30% glass-ﬁlled PP by up to

40%. In FR applications, 4% maleated PE in a PE/EVA (polyethylene/ethylene–

vinyl acetate) blend containing 65% ATH gives up to three times improvement in

elongation. The maleic anhydride reacts with the basic inorganic FR, ﬁllers, ATH,

and magnesium hydroxide. Maleated polyoleﬁns are marketed in North America

by DuPont (Fusabond) and Uniroyal (Polybond).

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Organic Peroxides. Organic peroxides are used in the plastics industry

to catalyze polymerization reactions or to modify the properties of polymers (7,8).

On the polymerization side, peroxides are used as initiators for PVC, low density

polyethylene (LDPE), PS, and acrylics. As modiﬁers of existing polymers, perox-

ides are used for the curing of unsaturated polyester resins, as cross-linkers of

PE, silicones, and a variety of ethylene-based elastomers (9) and to break down

the molecular weight of PP in a process known as visbreaking or controlled rhe-

ology (10). Peroxides function through the thermal decomposition of the unstable

peroxide bond to generate two free radicals. The reactivity of the peroxide is mod-

iﬁed by altering the organic substituents attached to the peroxide or through the

use of coadditive promotors. The reactivity of the peroxide is deﬁned by the 10-h

half-life temperature , or the temperature at which one-half of the peroxide decom-

poses in a 10-h period. The lower the 10-h half-life temperature, the more reactive

the peroxide. Organic peroxides fall into seven basic groupings depending on the

organic substituent. These are dialkyl peroxides, diacyl peroxides, hydroperox-

ides, ketone peroxides, peroxydicarbonates, peroxyesters, and peroxyketals. The

following table categorizes organic peroxide by grouping and chemical process:

The choice of organic peroxide used for initiating polymerizations is dictated

by the polymerization temperature used to produce the polymer. The low tem-

peratures used for PVC polymerization, for example, dictate the use of organic

peroxides with low 10-h half-life temperatures. For the higher polymerization

temperatures used for LDPE, an organic peroxide with a higher 10-h half-life

temperature may be used. When used as polymerization catalysts, organic perox-

ides are typically used in the 0.1–0.5% range. Approximately half of all organic

peroxides are used in the various polymerization processes.

Unsaturated polyesters are produced through the cross-linking of low molec-

ular weight polyester resins and comonomers such as styrene in the presence of

1.0–2.5% based on resin weight. Fillers, pigments, and reinforcements such as

glass ﬁber are often added as well. Depending on the cure temperature, promotors

are often used. For room-temperature cures and resin-transfer molding, organic

cobalt and copper promotors are added to the ketone peroxides, which are typically

used. Unsaturated polyester resin production is the largest single application of

organic peroxides, accounting for 30–40% of total consumption.

For the cross-linking of ethylene-based polymers for applications such as

wire and cable jacketing and tubings, peroxides are added during compounding/

processing at levels of 0.2–0.4%. In the visbreaking of controlled-rheology PP,

to reduce the molecular weight and melt viscosity of the polymer, similar levels

are used. Controlled-rheology PP is particulary common for ﬁber and extrusion

grades. When peroxides are used together with other additives, particularly an-

tioxidants, a careful balance of concentrations must be chosen since the radicals

formed during thermal decomposition can react with the other additives, lowering

the effective concentrations of each.

Key suppliers of organic peroxides in North America include Akzo Nobel

(Trigonox, Perkadox), LaPorte, and AtoFina (Luperox).

Impact Modiﬁers. Impact modiﬁers function by absorbing the impact

energy and dissipating it in a nondestructive fashion. Typically elastomers,

they are added to a wide range of thermoplastic materials at levels up to

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