Effect of Chemicals on Plastics
Sina Ebnesajjad
Series Editor, Plastics Design Library
2.1 Introduction
Plastics often come in contact with various chemicals in applications in chemical processing, semiconductor, automotive,
aerospace, consumer, and other applications. Even in mundane applications plastics have to contend with the ever-present ambient
oxygen and moisture. Many applications rely on the interactions (or lack thereof) of polymers with chemicals in industries, such as
microlithography, membrane technologies, medical device, pharmaceutical, plastics recycling, and drug delivery.
This chapter describes the general effects of chemicals on plastics, which will be referred to as polymers for brevity
purpose. Metals are also not immune from attack by chemicals. Water corrodes iron while acids dissolve many metals.
The perception is metals are by and large not attacked by a large number of chemicals, starting with organic solvents.
This perception is basically correct but it does not extend to plastics. Solvents, acids, bases, and other chemicals affect
overwhelming majority of polymers.
There are several examples of different types of interactions between plastics and chemicals. Water does not wet
polyethylene or polypropylene or affect them in a perceptible manner. Acetone and other ketones swell polyvinyl chloride
(PVC). Polyvinyl alcohol is completely dissolved in water. Cellulosic polymers react with acids such as concentrated sulfuric
acid as manifested by the color of the polymer turning black. In this reaction the acid removes the hydrogen and oxygen
molecules from the cellulosic in the form of water polymer leaving behind the carbon backbone that is naturally black.
An ever-present issue when working with plastics is that chemical exposure affects nearly every one of them. A chemical
environment often poses one of the most demanding tests of a polymer’s durability. There are two basic ways that a chemical
can affect a polymer: chemical and physical. The extent of the impact of the chemicals varies based on a variety of factors.
Foremost among these factors are the chemical structure and composition of the polymers and the properties of the impacting
chemical. Increasing the temperature of the environment and stress compound the effects. There are other factors that
influence the severity of the effect of chemicals on plastics are described in this chapter.
2.2 Effect of Chemicals on Plastics
Chemical environments decrease the integrity of polymers by two mechanisms—physical and chemical means. The primary
effect is physical or solvent effect while chemical effect or degradation occurs in minority of the cases. Physical effects are
mainly a function of the polymer and solvent structures. A number of predictive tools have been developed which are helpful
though less than perfect. They include a number of solubility parameters such as Hildebrand and Hanson systems. Solubility
parameter is discussed later in this chapter. Other parameters such as polarity (or lack thereof) of the polymer and solvent can
be used as rough estimators of interaction between these materials.
Highest losses in mechanical properties of a polymer take place when the solubility parameters of a polymer and solvent
match. Same statement can be made when polarities of a solvent and a primary polymer bond match. A close match of the
solubility parameters or polarities results in the incompatibility of the polymer and the solvent. To be clear, incompatible here
means the solvent attacks the polymer. When electing a polymer for a given chemical environment, materials must be chosen
that have the largest solubility or polarity differences with the chemical environment. For example, nylon 6/6 resists cleaning
solvents such as carbon tetrachloride, and polystyrene and ethylene glycol are incompatible. Nylon 6/6 has polar amid bonds
while carbon tetrachloride is a nonpolar solvent. In comparison water is a polar liquid and is absorbed by nylon 6/6. Similarly
polystyrene and ethylene glycol are both polar thus interact.
2.2.1 Water—A Potent Solvent
Resistance of polymers to chemicals often receives a great deal of attention while the effect of water, the most commonly
encountered liquid, in our environment is ignored. Polymers and composites used to make parts for various applications are
nominally resistant to moisture; otherwise, their use as engineering materials would be precluded. For example, water-soluble
ethyl vinyl acetate polymers would not be a suitable candidate material for a majority of applications.
xxviii
Effect of Chemicals on Plastics
Water transmission through polymeric parts occurs in two ways—sorption and diffusion. Sorption is the entrance of
water molecules into the resin; diffusion is the distribution, by random molecular motion, throughout the polymer. If the
water in the form of vapor, the equilibrium water absorption is a function of the relative humidity (partial pressure of vapor).
At low partial pressures, there is a linear relationship between water absorption (and consequent dimensional change),
accordance to Henry’s Law—the concentration of water within a thermoplastic equals a constant times the partial pressure [1].
Variations from the ideal case, that is a uniform distribution of water molecules, are caused by molecular clusters of water
that form at high concentrations, and by “site effects” around a molecular bond. Site effects, which occur in nylons,
polyesters, polyurethanes, and polycarbonates, account for the dramatic changes in physical properties when dry, molded
material is moisture-conditioned.
When a thermoplastic is immersed in liquid water, effects of the water are more rapid than those from a vapor environment.
Attainment of equilibrium is controlled to a greater degree by sorption. Sorption becomes a direct function of water contact or
wetting. No thermoplastic is wetted out completely by water, since the surface tension of water (72.5 dynes/cm) is too high.
For wetting to take place surface energy of polymer must exceed 72.5 dynes/cm, which can only be achieved by physical
surface treatment or chemical modification [2].
2.3 Chemical Reaction or Degradation Mechanism
Some chemicals actually degrade the polymer structure. They act by breaking down the chains into smaller ones thus
reducing molecular weight (molecular degradation), react with the chemical bonds of functional groups or a combination of
both mechanisms. Oxygen, water, alkalis, and acids are examples of chemicals that can react with some plastics. A number of
the properties of polymers, including tensile strength, elongation, impact strength, and fatigue, are determined by the size of
its molecules. If a chemical environment results in a reduction in molecular weight by chemical reaction, then this will affect
especially the tough and resilient properties of the material.
A feature of chemical mechanism is the irreversibility of the impact on polymers. After molecular weight of a polymer has
been reduced, say in a fabricated part, there are no practical ways to restore the original molecular weight. In contrast a
physical effect such as swelling may be reversible in some cases. For example, the plastic part can be removed from the
offending environment and heated to force the chemical out of the part. The restoration is unlikely to work completely for a
high boiling point solvent.
There are many examples of molecular degradation of a polymer by chemicals. Polycarbonates are esters of bisphenol A
and carbonic acid. Their molecules will slowly break down into its constitutive compounds by hydrolysis by water at elevated
temperatures. Bases are strong catalysts during the hydrolysis. Acids are weak catalysts while alcohols and carboxylic acids
bring about molecular degradation via ester interchange. Amines can cause molecular damage in some cases through
transamination. Ammonia and low molecular, aliphatic, primary and secondary amines are quite aggressive in the presence of
traces of water. High-molecular and slightly basic amines are less effective in degrading polycarbonates [3].
Nylons contain amid groups (aCOaNHa). They can react with water and split the polymer chain. An amide group
consists of a nitrogen atom and carbonyl (CQO), generally speaking the chemical formula can be written as
RaNHaCOaRʹ. The degradation stems from the fact that polymerization reactions that form nylons are actually equilibrium
reactions as seen in Eqs. (2.1) and (2.2). Eq. (2.1) shows the reaction scheme for nylons such as nylon 6 while Eq. (2.2)
shows the reaction for nylons such as nylon 6/6.
ð2:1Þ
ð2:2Þ
Since they are condensation reactions, Le Chatelier’s Principle predicts that the addition of water to nylon will push the
reactions back toward the left. This would break the polymer chains down into monomers again by hydrolysis. Hydrolysis
reduces the strength of nylon and causes it to become brittle as well. Fortunately, polymerization is also an exothermic
process, so at room temperature the forward direction of the polymerization reaction is heavily favored. Nylon 66 soaked in
water becomes brittle after about 2 months at 66°C.
Thermoplastic polyesters have similar physical properties to nylon 66 but have much lower moisture absorption. They are
attacked by ethylene dichloride and are susceptible to hydrolysis upon prolonged contact with hot water.
Oxygen has a prominent role in degrading polymers such as PVC, polyolefins, polyvinyl fluoride, and others. PVC degrades
by a thermo-oxidative mechanism in the presence of oxygen at elevated temperatures. Degradation usually begins by oxygen
attack at an unsaturation point followed by a loss of hydrochloric acid and decrease in unsaturation of the chain. Consequently,
the polymer molecule is destabilized resulting in chain scission or splitting of the chain into two fragments [4].
Effect of Chemicals on Plastics
xxix
2.4 Physical Mechanism
There are several ways chemicals interact with polymers physically, that is, without any reaction or change in the chemical
structure of the polymer. The important interactions include absorption and swelling, plasticization and dissolution.
Permeation is another physical phenomenon that is closely related to the absorption and swelling of polymers.
2.4.1 Absorption and Swelling
The phenomena of absorption and swelling are covered together because swelling is an extension of absorption of solvents
and chemicals by plastics. Interactions of chemicals with polymeric materials take place according to van der Waals forces
that govern the intermolecular interactions. The components of these forces have been further identified by other researchers
[5]12]. They have classified the van der Waals intermolecular forces into four components:
1.
2.
3.
4.
Dispersion (or nonpolar) force
Dipole-dipole force
Dipole-induced-dipole (induction) force
Hydrogen bonding
van der Waals interactions can take place between any pair of molecules. When one or more of these interactions take
place between the molecules of a chemical and the molecules of a polymer, it absorbs the chemical. Specific volume of the
amorphous region in polymers is larger than that of crystalline regions. Accumulation of a chemical in amorphous regions of
a polymer results in swelling which is a consequence of an increase in the volume of the polymer relative to its original state
prior to interaction with that chemical. The types of van der Waal’s interaction between polymer and chemicals depend on
their respective chemical structures.
For example, water can be absorbed into nylon 6/6 by forming hydrogen bonds with its amide groups. At elevated
temperatures nylon swells by absorbing water. Low-density polyethylene (LDPE) becomes increasingly susceptible to attack
by aromatic, chlorinated, and aliphatic hydrocarbons as temperature increases. Attack of LDPE by aliphatic hydrocarbons is
an example of dispersion (nonpolar) forces.
Another important consideration in absorption or swelling is the size of the solvent molecules. While smaller liquid
hydrocarbon molecules such as heptane or hexane can swell LDPE, larger wax molecules with similar chemical structures do
not have similar effects.
Swelling can be treated thermodynamically as the phenomenon involving two processes of mixing and expansion.
Diffusion of solvent into the polymer matrix is a type of mixing phenomenon whereas the expansion due to swelling is
similar to an elastic deformation. For systems exhibiting limited swelling, degree of swelling is defined as the mass of
absorbed liquid by unit mass or unit volume of the polymer [13].
Some linear polymers can dissolve in certain solvents. For example, polymer comprised of styrene]ethylene]butylene]
styrene dissolves in heptanes and tertiary butyl acetate to form a uniform solution of 20% solids [5]. If chemical (covalent
bond) cross-links are introduced to tie the chains in a network, the polymer cannot dissolve in solvents. Instead the solvent is
absorbed into the polymer network thus giving rise to swelling.
2.4.2 Plasticization
A plasticizer is defined as an additive that is incorporated in a plastic to impart softness and flexibility in order to facilitate
the manufacturing process. When added to a plastic a plasticizer increases its workability and flexibility. Plasticizers tend to
lower the melt viscosity, the glass transition temperature, and/or the elastic modulus [16].
Plasticizers are low molecular or oligometric additives that are compatible with rigid thermoplastic polymers, rendering
them semirigid or leathery/rubbery in behavior. They can be either non-polymeric materials or polymer impact modifiers.
Some forms of copolymerization can also produce a degree of internal plasticization. Certain plasticizers can also perform
other functions, assisting in viscosity control, in the dispersion of particulate additives such as fillers and pigments, and
general lubrication of the compound [17].
Some plastics may be mixed with high boiling temperature (low-volatility) liquids to give products of lower Tg. An
important example is PVC, which is often mixed with liquids such as di-iso-octyl phthalate, tritolyl phosphate, or other
diesters to reduce the Tg below room temperature. Some solvents, including aromatic and chlorinated hydrocarbons, ketones
and ethers, will soften PVC by acting as additional plasticizers
Liquid plasticizers work by increasing the free volume in the materials thus facilitating the segmental motions that
constitute the glass transition process. If their volatility is a problem in a given case, it may be possible to use either a
xxx
Effect of Chemicals on Plastics
polymeric plasticizer or a chemically bound plasticizer. The resultant plasticized PVC is flexible and to some degree quite
rubbery. Other commonly plasticized materials are cellulose acetate and cellulose nitrate. It is important to note that such
plasticizers may be able to modify the chemical properties of the plastic material since the plasticizer may be readily
extracted by certain chemicals and chemically attacked by others while the base polymer remains unaffected [18]20].
2.4.3 Dissolution or Polymer Solubility (Adapted from Ref. [18])
The solution properties of polymers have been subjected to intensive study and to complex mathematical treatment [21]23].
This section, however, confines the discussion to a qualitative and practical level [21].
One chemical will be a solvent for another if the molecules are able to coexist on a molecular scale, that is, the molecules
show no tendency to separate. In these circumstances, the two species are said to be compatible. This definition concerns
equilibrium properties and gives no indication of the rate of solution, which will depend on other factors such as temperature,
the molecular size of the solvent, and the size of voids in the solute.
Molecules of two different species will be able to coexist if the force of attraction between different molecules is not less
than the forces of attraction between two like molecules of either species. This is shown more clearly by reference to Fig. 2.1,
which shows two types of molecules A and B. The average forces between the like molecules are FAA and FBB, and the
average forces between dissimilar molecules are FAB. If FAA was the largest of these three forces, then the A molecules would
tend to congregate or cohere, rejecting the B molecules. A similar phase separation would occur if FBB was the greatest.
It is, therefore, seen that only when FAB$FAA and FAB$FBB will coexistence or compatibility be possible. Obviously, if it
is possible to obtain some measure of these forces, it should be possible to make predictions about polymer solubility. What
then is a suitable measure of the forces holding like molecules together? One would expect the latent heat of vaporization, L,
to exceed that cohesion energy by an amount corresponding to the work done by evaporation, an amount approximating to
RT, where R is the gas constant and T the absolute temperature. Such a diagram of (L—RT) might be a sufficient measure if
all of the molecules were of about the same size.
However, it is reasonable to suppose that compatibility should not be greatly affected by molecular size and that the
shorter polymer molecules in Fig. 2.2(a) should be just as compatible as the longer ones in Fig. 2.2(b), although their
FAA
FBB
FAB
Figure 2.1 Two different molecular species will be compatible if FAB$FAA and FAB$FBB. In other circumstances the molecules will tend to separate if they have sufficient energy for molecular movement.
B
B
B
AA
B
AAB
AA
AA
B
AA
A
AA
AA
AA
B
B
AAA B
(a)
Figure 2.2
Polymer molecules: (a) short and (b) long.
B
B
AAA B
AA
A
AA
A A A AA
AAA
AA
AA
B
A
B
B
B
A
AA
AAA AAA
AA
AA
A
B
AA B AAAA
AA
B AAA
B
A
(b)
B
B
A A AA
B
Effect of Chemicals on Plastics
xxxi
theoretical latent heats of vaporization will be greatly different. In such circumstances, a reduced diagram of (L—RT)/M will
give a measure of intermolecular energy per unit weight.
Similarly, a measure of the intermolecular or cohesion energy per unit volume will be given by the following expression,
where D is the density.
L 2 RT
M=D
Eq. (2.3) is known as the cohesive energy density [21,24] with units of megapascal. The square root of this expression is
more commonly encountered in quantitative studies and is known as the solubility parameter and given the symbol δ.
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
L 2 RT
MPa1=2
δ5
M=D
ð2:3Þ
The solubility parameter is, thus, an experimentally determinable property, at least for low-molecular-weight materials. In
the case of polymers that cannot be vaporized without decomposition, a method from a knowledge of structural formula has
been devised by Small and others [24,25]. It is now possible to provide an estimate of FAA and FBB, but the magnitude of
FAB has to be considered separately for each different system.
2.4.3.1 Amorphous Nonpolar Polymers and Amorphous Nonpolar Solvents
It is generally assumed in these circumstances, by analogy with gravitational and electrostatic attractions, that FAB will be
equal to the geometric mean of FAA and FBB. Thus, if by definition FAA.FBB, then, FAA.FAB.FBB. Considering these
conditions, it can be seen that compatibility will occur between amorphous nonpolar polymers and solvents only when
FAA5FAB5FBB; that is, when polymer and solvent have similar solubility parameters (in practice to within about 2 MPa1/2).
Reference to the values of δ in Tables 2.1 and 2.2 provides examples of this. Cellulose diacetate (δ523.2) is soluble in
acetone (δ520.4), but not in methanol (δ529.6) or toluene (δ518.2). It should be noted that apart from the problem of
achieving a molecular level dispersion, it is not necessary for the solvent to be liquid; it could be an amorphous solid. Such
tables are of greatest use with nonpolar materials with values of δ do not exceed 19.4 MPa1/2 and when the polymers are
amorphous.
Table 2.1 Solubility Parameters (δ) of Polymers
Polymer
δ (MPa1/2)
Polymer
δ (MPa1/2)
Polytetrafluorethylene
12.6
Polymethyl methacrylate
18.8
Polychlorotrifluoroethylene
14.7
Polyvinyl acetate
19.2
Polydimethyl siloxane
14.9
Polyvinyl chloride
19.4
Polyethylene
16.3
Bisphenol A polycarbonate
19.4
Polypropylene
16.3
Polyvinylidene chloride
20.0]24.9
Poly-t-butyl methacrylate
16.9
Ethylcellulose
17.3]21.0
Poly-n-butyl methacrylate
17.7
Cellulose dinitrate
21.5
Poly-n-hexyl methacrylate
17.5
Polyethylene terephthalate
21.8
Polybutyl acrylate
18.0
Acetal resins
22.6
Polyethyl methacylate
18.4
Cellulose diacetate
23.1
Polymethylphenyl siloxane
18.4
Nylon 66
27.7
Polyethyl acrylate
18.8
Polymethyl α-cyanoacrylate
28.8
Polystyrene
18.8
xxxii
Effect of Chemicals on Plastics
Table 2.2 Solubility Parameters (δ) and Partial Polarities (P) of Some Common Solvents
Solvent
δ (MPa1/2)
P
Solvent
δ (MPa1/2)
P
Dimethylpropane
12.9
0
Chloromethane
19.8
—
2-Methylpropene
13.7
0
Dichloromethane
19.8
—
Hexane
14.9
0
1,2-Dichloroethane
20.0
0
Ethoxyethane
15.1
0.03
Cyclohexane
20.2
—
Octane
15.5
0
Carbon disulfide
20.4
0
Methylcyclohexane
15.9
0
Acetone
20.4
0.69
2-Methylpropanoate
16.1
—
Octanol
21.0
0.04
2,4-Dimethylpentan-3-one
16.3
0.3
Butanenitrile
21.4
0.72
2-Methyl butyl acetate
16.3
—
Hexanol
21.8
0.06
Cyclohexane
16.7
0
2-Butanol
22.0
0.11
2,2-Dichloropropane
16.7
—
Pyridine
22.2
0.17
3-Methyl-1-butyl acetate
16.9
—
Nitroethane
22.6
0.71
Pentylacetate
17.3
0.07
Butanol
23.3
0.10
Tetrachloromethane
17.5
0
Cyclohexanol
23.3
0.08
Hexan-2-one
17.7
0.4
2-propanol
23.4
—
Piperidine
17.7
—
Propanol
24.3
0.15
Xylene
18.0
0
Dimethyl formamide
24.7
0.77
Methoxymethane
18.0
—
Hydrogen cyanide
24.7
—
Toluene
18.2
0
Acetic acid
25.7
0.30
1,2-Dichloropropane
18.4
—
Ethyanol
25.9
0.27
Ethyl acetate
18.6
0.17
Formic acid
27.5
—
Benzene
18.8
0
Methanol
29.6
0.39
4,4-Hydroxymethylpentan-2-one
18.8
—
Phenol
29.6
0.06
Trichloromethane
19.0
0.02
Glycerol
33.7
0.47
1,1,2-Trichloroethene
19.0
0
Water
47.7
0.82
Tetrachlorethane
19.2
0.01
2-Hydroxyethoxyethan-2-ol
19.6
—
2.4.3.2 Crystalline Nonpolar Polymers and Amorphous Solvents
Most polymers with regular structure will crystallize if cooled below the melting point, Tm. This is in accordance with the
thermodynamic law that a process will occur only if there is a decrease in Gibbs free energy (]ΔG) in going from one state to
another. Such a decrease occurs upon crystallization as the molecules pack in an orderly fashion. Since a process occurs only when
it is accompanied by a decrease in free energy, there is no reason why a crystalline nonpolar polymer should dissolve in a solvent
at temperatures well below the melting point. However, as the melting point is approached, the TΔS term in Eq. (2.5) increases.
ΔG 5 ΔH 2 TΔS
ð2:4Þ
Here, T is the absolute temperature, ΔS the entropy change, and ΔH the enthalpy change. With increasing temperatures,
ΔG can turn negative and dissolution can, therefore, occur.
Hence, at room temperature, there are no solvents for polyethylene, polypropylene, poly-4-methylpentene-1, polyacetal, or
polytetrafluoroethylene, but at temperatures of about 30°C below their melting points solvents with similar solubility
parameters are effective. It should also be noted that at room temperature swelling may occur in the amorphous zones of a
polymer in the presence of solvents of similar solubility parameter.
Effect of Chemicals on Plastics
xxxiii
2.4.3.3 Amorphous Nonpolar Polymers and Crystalline Solvents
This situation is identical to the previous one and occurs, for example, when paraffin wax is mixed into rubber at above its
melting point. On cooling, the paraffin wax tends to crystallize, some of it on the surface of the rubber. Such a bloom is one
way of protecting a diene rubber from ozone attack.
2.4.3.4 Amorphous Polar Polymers and Solvents
Molecules are held together by one, or more, of the four types of forces: dispersion, dipole, induction and hydrogen bonding.
In the case of aliphatic hydrocarbons dispersion forces predominate. However, many covalent bonds contain dipoles, with one
end being partially positively charged and the other partially negatively charged. Such dipoles may interact with dipoles on
other molecules and lead to enhancement of the total intermolecular attraction.
Molecules that possess dipoles and interact in this way are said to be polar. Many well-known solvents (eg, water)
and polymers (eg, PVC) are polar and it is generally accepted for interaction both the solubility parameter and their degrees
of polarity should match. This is usually expressed in terms of partial polarity [24], which expresses the fraction of total
forces due to dipole bonds. Some figures for partial polarities (P) of solvents are given in Table 2.2, but there is a lack of
quantitative data on the partial polarities of polymers. A comparison of polarities has to be made by common sense rather
than a quantitative approach. For example, hydrocarbon polymers are expected to have a negligible polarity and are more
likely to dissolve in toluene rather than in diethyl ketone, although both have similar solubility parameters.
2.4.3.5 Crystalline Polar Polymers and Solvents
It has already been pointed out that at temperatures well below their melting point crystalline nonpolar polymers will not
interact with solvents, and similar considerations can apply to a large number of polar crystalline polymers. It has, however,
been possible to find solvents for some polar, crystalline polymers, such as the nylons, PVC, and the polycarbonates. This is
because of the specific interactions between polymer and solvent that may often occur by, say, hydrogen bonding.
For example, nylon-6,6 will dissolve in formic acid and glacial acetic acid and phenol, all solvents that not only have
similar solubility parameters but also are capable of acting as proton donors while the carbonyl groups in the nylon molecules
act as proton acceptors.
More interesting examples are given with PVC and the polycarbonate of bis-phenol A—both are slightly crystalline
polymers. It is noticed here that while dichloromethane is a good solvent and tetrahydrofuran a poor solvent for the
polycarbonate, the reverse is true for PVC, yet all four materials have similar solubility parameters. A likely explanation is
that a form of hydrogen bonding occurs between the polycarbonate and methylene chloride and between PVC and
tetrahydrofuran. In other words, there is a specific interaction between each solvent pair:
R
CH2
CH2
Cl
C
CH2
H
CH2
C
O
CH2
CH2
H
O
O
O
H
C
Cl
Cl
Many studies have been made to assess the propensity to hydrogen bonding of chemical structures [24]. As a result, the
following broad generalizations may be made:
1. Proton donors include highly halogenated compounds such as chloroform and pentachlorethane; less halogenated materials
are weaker donors.
2. Polar acceptors include, in roughly descending order of strength, amines, ethers, ketones, aldehydes, and esters, with
aromatic materials usually being more powerful than aliphatic ones.
3. Some materials such as water, alcohols, carboxylic acids, and primary and secondary amines may be able to act
simultaneously as proton donors and acceptors. Cellulose and polyvinyl alcohol are two polymers that also function in
that way.
4. A number of solvents such as the hydrocarbons, carbon disulfide, and carbon tetrachloride are quite incapable of forming
hydrogen bonds.
2.4.3.6 Thermosetting Plastics
Covalently cross-linked plastics cannot dissolve without chemical change. They will, however, swell in solvents of similar
solubility parameter, the degree of swelling decreasing with increasing cross-link density.
xxxiv
Effect of Chemicals on Plastics
2.4.4 Environmental Stress Cracking
A weakness of many polymers is their tendency to fail at fairly low stress levels when exposed to certain hostile chemicals.
Many rigid plastics are unaffected when exposed to chemicals in the absence of stress. They may, however, crack under
stresses well below the normal yield stress in the same chemical environments.
Another example is stress cracking of polyolefins such as high-density polyethylene in the presence of surfactants. When
polyethylene is held under stress in the presence of some detergents, its behavior changes from short-time ductile failure at
high stresses to brittle fracture at low stresses after longer times with very small break elongations [26].
The mechanism for this stress-cracking phenomenon is not entirely understood and, indeed, it is likely that different
mechanisms govern different circumstances. There do, however, appear to be four main types [18]:
1.
2.
3.
4.
Solvent cracking of amorphous polymers
Solvent cracking of crystalline polymers
Environmental stress cracking (ESC)
Thermal cracking
Different molecular mechanisms for ESC have been proposed over the years [27,28]. Interlamellar failure has been
postulated as the controlling mechanism of ESC, with the concentration of the tie molecules as a factor in ESC
resistance. Brown [29] concluded that the mechanism of slow crack growth involves the disentanglement of the tie
molecules from the crystals. The number of tie molecules and the strength of the crystals that anchor them are
considered the controlling factors.
Three examples of ESC of amorphous polymers include polystyrene with white spirit, polycarbonate with methanol, and
polysulfone with ethyl acetate. Susceptibility to ESC is not predictable thus requiring end-use testing of a polymer prior to
finalizing the part design.
In the case of crystalline polymers cracking is probably caused by the action of the chemical environment in the
amorphous regions of the more complex morphologies. Benzene and toluene impact on polyethylene are two such examples.
The more troubling issue with polyethylene, however, is ESC by exposure to a broad range of common chemicals such as
soap, alcohols, surfactants, and silicone oils [30]. Most are highly polar materials that do not cause swelling, but are simply
absorbed either into, or on, the polymer. This seems to weaken the surface and allows cracks to propagate from preexisting
minute flaws, some degree of which is inevitable in molding polymers.
Cracking caused by heat (thermal cracking) appears to act in a similar manner, but in this case oxygen is the hostile
environment, activated at 70]80°C with some polyethylene grades.
2.4.4.1 Factors Influencing the ESC Behavior
ESC behavior of a polymer is highly dependent on the concentration of the stress-cracking agent, exposure temperature,
exposure time, and most importantly and the level of strain of the polymer.
Polymer transition to brittle behavior is quickened to shorter times by increase in temperature, cyclic loading, stress
resulting in micro-yields, and stress concentrations. The effect of temperature is complex. Physical aging is a manifestation of
small-scale relaxation processes that take place in the amorphous regions of a glassy polymer, causing volume contraction
and densification of the sample. The polymer structure remains unchanged but the local packing of the chain alters. This
leads to dimensional changes and alteration of physical properties such as brittleness, tensile strength, and the glass transition
temperature. As the extent of physical aging increases there are corresponding decreases in the enthalpy, the specific volume,
and the fracture toughness, while increases in glass transition temperature, the yield stress, and tensile modulus of the
material may also be observed.
Localized concentration of the stress due to local geometrical features as notches, voids, and inclusions will increase the
stress and modify the nature of the stress field. Craze initiation is accelerated by stress fields with high dilational stress and
retarded under hydrostatic pressure.
There are critical polymer properties and variables that affect ESC. The higher the molar mass the longer the polymer
chains, which results in more tie molecules and increased ESC resistance. ESC resistance decreases with increasing the
degree of crystallinity. Higher comonomer content and longer comonomer short chain branches (higher α]olefins) provide
better ESC resistance of linear LDPE due to a decrease in the degree of crystallinity. Increased pigment content usually
decreases the ESC resistance. The thermal history of the material and the processing conditions are also important factors for
the ESC resistance behavior of the polymers.
Effect of Chemicals on Plastics
xxxv
2.4.4.2 Characteristics of ESC Failures
More than 25% of plastic part failures have been found to be due to ESC. Environmental stress crack failures share several
typical characteristics [31]:
Brittle fracture: ESC failures are caused by brittle fracture, even in materials that would normally be expected to produce a
ductile yielding mechanism. The crack initiation sites for ESC failures are always on the surface. They normally
correspond to localized areas of high stress, such as microscopic defects or points of stress concentration. The initiation
location is generally related to direct contact with an active chemical agent, either liquid or gas.
Multiple cracks: Multiple individual cracks are initiated, and these subsequently coalesce into a unified fracture. Numerous
crack origins and the corresponding unions are illustrative of an ESC failure mechanism.
Smooth morphology: The crack origin areas usually exhibit a relatively smooth morphology, indicative of slow crack
growth. However, aggressive chemical agents can produce rapid initiation and extension, characterized by more coarse
surface features.
Craze remnants: The presence of opened craze remnants, either within the crack origin regions or in adjacent areas, is
further indication of ESC. In many cases, the final fracture will develop via ductile overload after the crack length has
reached a critical size.
Stretched fibrils: The final fracture zone can include stretched fibrils and other features indicative of ductile cracking.
It is important to note that ESC is not a chemical attack mechanism; therefore, features that are normally associated with
chemically induced molecular degradation will not normally be present.
Alternating bands: Recent experimentation has shown that ESC commonly develops by a progressive crack-extension
mechanism. Examination of fracture surfaces created under laboratory conditions reveals a series of alternating bands
corresponding to crack extension cycles. The observed bands are thought to represent repeated cycles of crazing, followed
by crack extension via brittle fracture, consistent with the steps involved in creep and ESC failure mechanisms [31].
2.4.4.3 Prevention and Coping with ESC
ESC must be considered in designing parts from polymers. For some material such fluorinated polymers ESC is not considered
an extensive problem. Permeation variables have a strong influence on stress cracking which should be considered in part design
and material selection. Different polymers differ in their propensity to ESC, primarily based on their degree of crystallinity.
Lowering the crystalline phase content of the part tends to increase resistance to stress cracking due to the increasing break
elongation. High crystallinity can be mitigated by the use of copolymers, whenever possible. Adding a comonomer almost
always decreases the crystalline phase content of polymers.
Resin processing can affect crystallinity. Reducing the processing temperature and time and rapid cooling (or quenching)
at the end of the fabrication process reduce crystalline content thus increase amorphous content. If the cooling is too fast,
parts will contain residual stress that could reduce ESC resistance.
Increasing the molecular weight of the polymer reduces its crystallinity and enhances its stress crack resistance. Longer
chains have higher tensile strength (ie, load-bearing ability).
Chemicals with structures similar to the polymer tend to permeate and plasticize, thus, reducing its mechanical
strength. ESC effect of chemicals on polymers can be measured by exposing the polymer to the chemical under the
desired conditions. Tensile properties of the exposed sample can then be measured. Any loss of elongation and tensile
strength would indicate ESC.
2.5 Permeation of Chemicals Through Plastics
Permeation can be defined as the passage of gases and liquids through a second material such as a solid. It is a significant
consideration in the selection of plastic material for the construction of chemical processing equipment because process fluids
may travel across the thickness of the polymer by permeation. Permeated species in sufficient quantities could cause
corrosion, contamination, or unacceptable environmental emission, singly or in combination.
In its simplest form, permeation can be expressed as a product of the solubility multiplied by diffusion coefficient of
the permeant in the polymer. Permeation of a gas can be calculated from Eq. (2.5). This equation is derived from Fick’s first
law of mass transfer. Permeation concerns the movement of a species through the molecules of another species (eg, a gas
through a polymer). It does not take into account transport of material through cracks, voids, and in general physical flaws in
the structure of the second species such as the polymer. To be sure, both phenomenon result in the migration of chemicals
xxxvi
Effect of Chemicals on Plastics
through the structure. This means that after an appropriate plastic material has been selected to meet the permeation
requirements of a process, the equipment must be fabricated carefully to avoid flaws in the polymer structure.
P5D S
ð2:5Þ
P (cm3/s cm atm) is the permeability of the gas, D is the diffusion coefficient (cm3/s), and S (cm3/cm3 atm) is the
solubility coefficient.
No permeation would occur, if either diffusion or solubility coefficients are zero. The lowest diffusion rates occur with
crystalline polymers below the Tg, since there is very little space through which diffusing molecules may pass [18].
Amorphous polymers below the Tg have a somewhat higher permeability, but diffusion is still difficult. For amorphous
polymers above the Tg, in the flexible and rubbery states, there is more space (free volume) available through which diffusing
molecules may pass, and so these materials show comparatively high diffusion rates with low-molecular-weight diffusing
fluids. For crystalline polymers between Tg and Tm, the diffusion rate is very dependent on the degree of crystallization.
Several factors affect the permeation rate of the polymer. Temperature increase raises the permeation rate for two
reasons. First, solubility of the permeant increases in the polymer at higher temperatures. Second, polymer chain movements
are more abundant which allow easier diffusion of the permeant. The permeation rate of gases increases at higher partial
pressures. For liquids, permeation rates rise with an increase in the concentration of the permeant. Unless the permeant
species are highly soluble in the polymer, the permeation rate increases linearly with pressure, concentration, and the area of
permeation.
The effect of thickness is usually nonlinear. The permeation rate is very high at a low thickness and rapidly decreases with
an increase in the thickness. After a critical thickness is reached, the effect of thickness is diminished and the permeation
rate reaches a plateau. At lower thicknesses, the effect of surface structure begins to play a significant role in the permeation.
A more oriented (ordered) surface will inhibit permeation.
Chemical and physical characteristics of the polymer have powerful impacts on the rate of permeation, as much as four
orders of magnitude [32]. Chemical affinity for the permeant, intermolecular forces such as van der Waals and hydrogen
bonding forces, degree of crystallinity, and degree of cross-linking are the influential variables.
A similarity of chemical functional structures of the polymer and the permeant will promote solubility and permeation
rate. Higher intermolecular forces of the polymer result in less permeation because of the resistance that they present to the
development of space between adjacent molecules required for the passage of the permeant. Crystallinity is an important
factor, which can be controlled during the processing of the polymer. The crystalline phase can be considered impermeable
by most species because of its orderly structure (packing), which usually minimizes its specific volume. The amorphous
phase has the opposite construction and is disordered with interchain space available for permeation. Cross-linking acts
somewhat similar to crystallinity, though less effective, to limit the space for permeation. Cross-linking is size-dependent and
smaller species may permeate.
The molecular size of the permeant, its chemical structure, and its condensation characteristics affect permeation.
Diffusion of the permeant increases as its molecular size decreases, thus contributing to an increase in permeation. Molecular
structure is important. A polar chemical will normally have a lower permeation rate in a nonpolar polymer than a nonpolar
species and vice versa. This is due to the ability of chemicals with similar structures to the polymer to swell the polymer, that
is, to create space between the chains for permeation. A more easily condensed chemical will also be more effective in
swelling the polymer, resulting in higher rates of permeation.
2.6 Methods for Determination of Chemical Resistance of Plastics
An important point about chemical resistance testing is the nonuniformity of the available data. There are a few standard test
methods but many people conduct their own tests and do not follow the standard methods. Only a few of the currently
applied chemical resistance tests have been standardized. One reason for the absence of widespread acceptance and use of
standardized chemical resistance test methods is the magnitude of the number of applications and conditions which too
numerous to capture in standardized tests.
Most companies conduct their own test methods. Determination of the level of attack is usually done by:
1. appearance of the specimen compared before and after testing,
2. weight change of the specimen due to exposure,
3. performing mechanical tests (tensile, impact) after the chemical resistance tests.
Effect of Chemicals on Plastics
xxxvii
Any data obtained from outside sources should be considered with a thorough knowledge of the method and criteria used
in the determination of chemical resistance of the polymer.
The most common method for testing the chemical resistance of a part is by immersing it in the solvent, acid, base, or
other chemical. Immersion of an unloaded part in water at elevated temperatures (with or without detergent/disinfectant) can
also be used to test the hydrolytic stability of a product. A small amount of a detergent or surfactant is sometimes added to
water to reduce its surface tension to wet the polymer surface.
Room temperature immersion testing of polymer coupons is a good starting point. And it may be sufficient testing if the
application happens to be at the room temperature free of load on the plastic part. If the end-use conditions deviate from the
ambient conditions it will be necessary to test the part using methods and conditions that approach the actual application of
the part.
Immersion testing can be set up to test chemical resistance at elevated temperatures by use of a heated bath or in reflux
mode. Elevated temperature testing can also be used as an accelerated technique as a proxy for aging tests. Data obtained
from accelerated testing should be carefully analyzed because time and temperature are not always interchangeable. Testing
against gases or at elevated pressures requires more complex equipment. Tests can be carried out on either stressed or
unstressed parts. The worst-case scenario is to test the part under stress as previously discussed in Section 2.4.4,
Environmental Stress Cracking. This mode is, however, realistic and required if the part is expected to perform under load
(under stress) in the application.
Another cautionary note has to do with the processing of parts that can influence the behavior of the product when it
comes into contact with an aggressive chemical environment. For example, a polymer coupon may resist a solvent while the
actual part may be impacted. One simple way this can happen is because of molecular weight decrease of parts as a result of
processing steps during part production. The actual parts might contain smaller molecular weight species that could dissolve
in a solvent while larger molecular weight molecules would not.
There are several ASTM and ISO test methods for determining chemical resistance of polymers. Brief descriptions
provided by the respective standard organizations have been given for some of the chemical resistance methods in the
following sections.
It important to review the following statement issued by ASTM, applicable to each “standard” (test method) it has issued:
“. . .the full text of the standard itself must be referred to for its use and application. ASTM does not give any warranty
express or implied or make any representation that the contents of this abstract are accurate, complete or up to date.”
2.6.1 ASTM D543 Method Specification for Evaluating the Resistance of Plastics to Chemical Reagents
The following description has been provided by American Society for Testing Materials (refer www.ASTM.org) for the D543
test method.
Significance and Use
The choice of types and concentrations of reagents, duration of immersion or stress, or both, temperature of the test, and
properties to be reported is necessarily arbitrary. The specification of these conditions provides a basis for standardization
and serves as a guide to investigators wishing to compare the relative resistance of various plastics to typical chemical
reagents.
Correlation of test results with the actual performance or serviceability of plastics is necessarily dependent upon the
similarity between the testing and the end-use conditions. For applications involving continuous immersion, the data obtained
in short-time tests are of interest only in eliminating the most unsuitable materials or indicating a probable relative order of
resistance to chemical reagents.
Evaluation of plastics for special applications involving corrosive conditions should be based upon the particular reagents
and concentrations to be encountered. The selection of test conditions should take into account the manner and duration of
contact with reagents, the temperature of the system, applied stress, and other performance factors involved in the particular
application.
Scope
1. This practice covers the evaluation of all plastic materials including cast, hot-molded, cold-molded, laminated resinous
products, and sheet materials for resistance to chemical reagents. This practice includes provisions for reporting changes
in weight, dimensions, appearance, and strength properties. Standard reagents are specified to establish results on a
xxxviii
Effect of Chemicals on Plastics
comparable basis. Provisions are made for various exposure times, stress conditions, and exposure to reagents at
elevated temperatures. The type of conditioning (immersion or wet patch) depends upon the end-use of the material. If
used as a container or transfer line, specimens should be immersed. If the material will only see short exposures or will
be used in close proximity and reagent may splash or spill on the material, the wet patch method of applying reagent
should be used.
2. The effect of chemical reagents on other properties shall be determined by making measurements on standard specimens
for such tests before and after immersion or stress, or both, if so tested.
2.6.2 ASTM D1239 Method Specification for Resistance of Plastic Films to Extraction by Chemicals
The following description has been provided by American Society for Testing Materials (refer www.ASTM.org) for the
D1239-07 test method.
Significance and Use
This test method is intended to be a rapid empirical test to determine the loss of the plasticizer or other
extractable components from the plastic film when immersed in liquids commonly used in households.
Scope
This test method for resistance of plastic films to chemicals covers the measurement of the weight loss of film after
immersion in chemicals.
1. There is no known ISO equivalent to this test method.
2. Film is defined as sheeting having nominal thickness not greater than 0.25 mm in accordance with Terminology D 883.
2.6.3 ASTM D3681 Method Specification for Chemical Resistance of “Fiberglass” (Glass-Fiber-Reinforced
Thermosetting Resin) Pipe in a Deflected Condition
The following description has been provided by American Society for Testing Materials (refer www.ASTM.org) for the
D3681-06 test method.
Significance and Use
This test method evaluates the effect of a chemical environment on pipe when in a deflected condition. It has been found that
effects of chemical environments can be accelerated by strain induced by deflection. This information is useful and necessary
for the design and application of buried fiberglass pipe.
Pipes of the same diameter but of different wall thicknesses will develop different strains with the same deflection. Also,
pipes having the same wall thickness but different constructions making up the wall may develop different strains with the
same deflection.
Scope
1. This test method covers the procedure for determining the chemical-resistant properties of fiberglass pipe in a deflected
condition for diameters 102 mm and larger. Both glass-fiber-reinforced thermosetting resin pipe and glass-fiber-reinforced
polymer mortar pipe are fiberglass pipes.
2. For the purposes for this standard, polymer does not include natural polymers.
2.6.4 ASTM D4398 Method Specification for Determining the Chemical Resistance of Fiberglass-Reinforced
Thermosetting Resins by One-Side Panel Exposure
The following description has been provided by American Society for Testing Materials (refer www.ASTM.org) for the
D4398-07 test method.
Significance and Use
The results obtained by this test method may serve as a guide in, but not as the sole basis for, predicting the possible
performance of the particular glass-fiber-reinforced thermosetting resin laminate in the one-side exposure to the specific
environment under evaluation. No attempt has been made to incorporate into the test method all of the factors that may enter
into the serviceability of a glass-fiber-reinforced resin structure when subjected to chemical environments.
This test method provides for the determination of changes in the physical properties of the test panel and test media
during and after the one-side exposure in the test media. Determination of changes includes: Barcol hardness, appearance of
Effect of Chemicals on Plastics
xxxix
panel, appearance of test media, flexural properties, and thickness. (Barcol hardness test characterizes the indentation
hardness of materials through the depth of penetration of an indentation device, loaded on a material sample and compared to
the penetration in a reference material.)
Scope
This test method is intended for use in the evaluation of the chemical resistance of fiberglass-reinforced thermosetting resins
that are subjected to one-side panel exposure to specific environments. It takes into consideration the cold wall effects and
radiation losses of heat transfer through the laminate wall.
2.6.5 ASTM C868 Method Specification for Chemical Resistance of Protective Linings
The following description has been provided by American Society for Testing Materials (refer www.ASTM.org) for the
C868-02 (2008) test method.
Significance and Use
The results obtained by this test method should serve as a guide in, but not as the sole basis for, selection of a lining material
for particular application. Simple chemical-resistant evaluations of the lining materials may be performed more conveniently
by other pertinent methods as a prescreening test for this procedure in accordance with Test Methods C 267 and D 471.
Scope
1. This test method covers a procedure for evaluating the chemical resistance of a polymer-based protective lining in
immersion service. The method closely approximates the service conditions, including the temperature differential between
the external and internal surfaces of the equipment, which may accelerate permeation of the lining by a corrosive media.
2. This test may be used to simulate actual field use conditions insofar as a qualitative evaluation of the lining system after a
predetermined period of exposure.
2.6.6 ISO 4600 Method Specification for Determination of ESC—Ball or Pin Impression Method
(Refer www.ISO.org)
Abstract
The test is applicable to finished products and to test specimens, prepared by molding and/or machining, and can be used for
the assessment of both ESC of a plastic product or material exposed to different environments, and for the determination of
ESC of different plastics materials exposed to a specific environment.
2.6.7 ISO 4599 Method Specification for Plastics—Determination of Resistance to ESC—Bent Strip Method
(Refer www.ISO.org)
2.6.7.1 ISO 6252 Method Specification for Plastics—Determination of ESC—Constant Tensile Stress Method (Refer www.ISO.org)
Abstract
The test is applicable to test specimens, prepared by molding and/or machining, and can be used for the assessment of both
ESC of a plastic product or material exposed to different environments, and for the determination of ESC of different plastics
materials exposed to a specific environment.
2.7 PDL Chemical Resistance Rating
The PDL Resistance Rating is determined using a weighted value scale developed by PDL and reviewed by experts. Each of
the ratings is calculated from test results provided for a material after exposure to a specific environment. It gives a general
indication of a material’s resistance to a specific environment. In addition, it allows users to search for materials most likely
to be resistant to a specific exposure medium.
After assigning the weighted value to each field for which information is available, the PDL Resistance Rating is
determined by adding together all weighted values and dividing this number by the number of values added together. All
numbers to the right of the decimal are truncated to give the final result. If the result is equal to 10, a resistance rating of 9 is
assigned. Each reported field is given equal importance in assigning the resistance rating since, depending on the end-use,
different factors play a role in the suitability for use of material in a specific environment. Statistically, it is necessary to
consider all available information in assigning the rating. Supplier resistance ratings are also figured into the calculation of
the PDL Resistance Rating. Weighted values assigned depend on the scale used by the supplier.
xl
Effect of Chemicals on Plastics
Table 2.3 PDL Chemical Resistance Rating
Weighted Weight
value
change
Diameter
length
change
Thickness
change
Volumea
change
Mechanicalb Visualc observed change
property
retained
BTTd (min)
Permeation Hardness
change
rate (µg/
(units)
cm2/min)
.0 to 0.1
0 to 0.25
0 to 25
.97
#51
#0.9
10
0 to 0.25
9
.0.25 to 0.5 .0.1 to 0.2 .0.25 to 0.5 .2.5 to
5.0
94 to ,97
.1 to #2
8
.0.5 to 0.75 .0.2 to 0.3 .0.5 to 0.75 .5.0 to
10.0
90 to ,94
.2 to #5
7
.0.75 to 1.0 .0.3 to 0.4 .0.75 to 1.0 .10.0 to 85 to ,90
20.0
Slightly discolored,
slightly bleached
.5 to #10
6
.1.0 to 1.5
.0.4 to 0.5 .1.0 to 1.5
.20.0 to 80 to ,85
30.0
Discolored yellows,
slightly flexible
.10 to
#30
5
.1.5 to 2.0
.0.5 to
0.75
.1.5 to 2.0
.30.0 to 75 to ,80
40.0
Possible stress crack
agent, flexible, possible
oxidizing agent, slightly
crazed
.30 to
#120
4
.2.0 to 3.0
.0.75 to
1.0
.2.0 to 3.0
.40.0 to 70 to ,75
50.0
.120 to
Distorted, warped,
softened, slight swelling, #240
blistered, known stress
crack agent
3
.3.0 to 4.0
.1.0 to 1.5 .3.0 to 4.0
.50.0 to 60 to ,70
70.0
Cracking, crazing, brittle, .240 to
#480
plasticizer, oxidizer,
softened swelling,
surface hardness
2
.4.0 to 6.0
.1.5 to 2.0 .4.0 to 6.0
.60.9 to 50 to ,60
90.0
Severe distortion,
oxidizer and plasticizer
deteriorated
.480 to
#960
1
.6.0
.2.0
.90.0
.0 to ,50
Decomposed
.960
0
Solvent dissolved,
disintegrated
.6.0
No change
0 to 2
.2 to 4
.0.9 to 9
.4 to 6
.6 to 9
.9 to 90
.9 to
12
.12 to
15
.90 to
900
.15 to
18
.18 to
21
.900 to
9000
.21 to
25
.25
.9000
a
All values are given as percent change from original.
Percent mechanical properties retained include tensile strength, elongation, modulus, flexural strength, and impact strength. If the % retention is greater than 100%, a value of
200 minus the % property retained is used in the calculation.
c
Due to the variety of information of this type reported, this table can be used only as a guideline.
d
Breakthrough time: time from initial chemical contact to detection.
b
Table 2.3 gives the values and guidelines used in assigning the PDL Resistance Rating. The guidelines—especially in the
case of visual observations—are sometimes subject to an educated judgment. An effort is made to maintain consistency and
accuracy.
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