Pharmaceutical Manufacturing Handbook: Production and

TABLE 2 Properties of Different Grades of PEG
Property
200
Physical Liquid
state
Average 190 – 210
molecular
weight
Melting —
( ° C)
Density
(g/cm 3 1.11 – 1.14
)
Kinematic
viscositya 3.9 – 4.8
(cS)
a
At 98.9 ° C.
400
Liquid
380 – 420
—
1.11 – 1.14
6.8 – 8.0
600
Liquid
570 – 613
—
1.11 – 1.14
9.9 – 11.3
By Grade
1000 2000
Solid
950 – 1050
37 – 40
1.15 – 1.21
16.0 – 19.0
OINTMENTS AND CREAMS 273
number of oxyethylene groups present, their molecular weights vary from few hundreds
to several thousands. Usually the number that follows PEG represents their
average molecular weight. They are available as liquids or solids based on molecular
weight. PEGs 600 or less are liquids, whereas PEGs above 1000 are solids. PEG
liquids are usually clear or pale yellow in color. Their viscosity increases with
increase in molecular weight. Solid PEGs are usually white in color and available
as pastes, waxy fl akes, or free - fl owing solids based on their molecular weight. Table
2 shows the physicochemical properties of some PEGs.
PEGs are hydrophilic materials and are extensively used in the preparation of
hydrophilic ointments and creams. They are nonirritants and are easily washed
from skin surfaces. Products with varying consistency are prepared by mixing different
grades of PEGs. Excessive heating is avoided while melting PEGs. This will
prevent oxidation and discoloration of products. In addition, use of purifi ed grades
that are free from peroxide impurities, inclusion of suitable antioxidants, and
heating under nitrogen atmosphere can minimize the oxidation. PEGs are prone
to etherifi cation or esterfi cation reactions due to the presence of two terminal
hydroxyl groups. They are incompatible with some antibiotics, antimicrobial preservatives,
iron, tannic acid, and salicylic acid and also interact with plastic containers
made of polyvinyl chloride and polyethylene. PEG - containing products are
usually packed in aluminum, glass, or stainless steel containers to avoid such interactions.
Although low - molecular - weight PEGs are hygroscopic, they do not promote
microbial growth. PEG - containing products are generally stored in well - closed
containers in a cool, dry place. These products can cause stinging sensation on
mucus and some hypersensitivity reactions, especially when applied onto open
wounds [7, 8] .
Stearic Acid Stearic acid is obtained by hydrolysis of fat or hydrogenation of
vegetable oils. Compendial stearic acid contains a mixture of stearic acid and palmitic
acids. It is available as powder or crystalline solid which is white to yellowish
white in color and possesses a characteristic odor. Although stearic acid is insoluble
Solid
1800 – 2200
45 – 50
1.15 – 1.21
38 – 49
3000
Solid
2700 – 3300
48 – 54
1.15 – 1.21
67 – 93
4000
Solid
3000 – 4800
50 – 58
1.15 – 1.21
110 – 158
8000
Solid
7000 – 9000
60 – 63
1.15 – 1.21
470 – 900