Designation: D2734 − 16

Standard Test Methods for

Void Content of Reinforced Plastics1
This standard is issued under the fixed designation D2734; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.

1. Scope* D2584 Test Method for Ignition Loss of Cured Reinforced

1.1 These test methods cover the void content of reinforced Resins
plastics or “composites.” The test methods are applicable to
3. Summary of Test Methods
composites for which the effects of ignition on the materials are
known. Most plastics, glass, and reinforcements fall into this 3.1 The densities of the resin, the reinforcement, and the
class. These test methods are not applicable to composites for composites are measured separately. Then the resin content is
which the effects of ignition on the plastics, the reinforcement, measured and a theoretical composite density calculated. This
and any fillers are unknown. This class may include silicone is compared to the measured composite density. The difference
resins, which do not burn off completely, reinforcements in densities indicates the void content. A good composite may
consisting of metals, organic materials, or inorganic materials have 1 % voids or less, while a poorly made composite can
which may gain or lose weight, and fillers consisting of oxides, have a much higher void content. Finite values under 1 %
carbonates, etc., which may gain or lose weight. Note that should be recognized as representing a laminate density
separate weight loss tests of individual materials will usually, quality, but true void content level must be established by
but not necessarily, give the same result as when all the complementary tests or background experience, or both.
materials are combined.
4. Significance and Use
NOTE 1—There is no known ISO equivalent to these test methods.
4.1 The void content of a composite may significantly affect
1.2 The values stated in SI units are to be regarded as some of its mechanical properties. Higher void contents
standard. usually mean lower fatigue resistance, greater susceptibility to
1.3 This standard does not purport to address all of the water penetration and weathering, and increased variation or
safety concerns, if any, associated with its use. It is the scatter in strength properties. The knowledge of void content is
responsibility of the user of this standard to establish appro- desirable for estimation of quality of composites.
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use. 5. Interferences
5.1 The density of the resin, in these test methods, is
2. Referenced Documents assumed to be the same in the composite as it is in a large cast
2.1 ASTM Standards:2 mass. Although there is no realistic way to avoid this
D618 Practice for Conditioning Plastics for Testing assumption, it is nevertheless not strictly correct. Differences in
D792 Test Methods for Density and Specific Gravity (Rela- curing, heat and pressure, and molecular forces from the
tive Density) of Plastics by Displacement reinforcement surface all change the composite resin density
D1505 Test Method for Density of Plastics by the Density- from the bulk resin density. The usual change is that bulk
Gradient Technique density is lower, making void content seem lower than it really
is.
5.2 For composites with high void contents, this error will
1
These test methods are under the jurisdiction of ASTM Committee D20 on lower the true value an insignificant amount, from a true 7 %
Plastics and are the direct responsibility of Subcommittee D20.18 on Reinforced down to a calculated 6.7 %, for example. For composites with
Thermosetting Plastics.
Current edition approved Sept. 1, 2016. Published September 2016. Originally low and void contents, the value may be lowered from a true
approved in 1968. Last previous edition approved in 2009 as D2734 – 09. DOI: 0.2 % to a calculated − 0.1 %. This would indicate an obvious
10.1520/D2734-16.
2
error, and illustrates that as the void content gets lower the
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
constant error in resin density gets progressively more impor-
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on tant. Note that these values are for example only, that different
the ASTM website. resin systems can give different errors, and that it is left to the

*A Summary of Changes section appears at the end of this standard

Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States

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 D2734 − 16
individual tester to determine the accuracy of the calculated 7.1.3 Test Method C:
result in his particular measurement. 7.1.3.1 Densities calculated from weight and volume mea-
5.3 For the special case of semi-crystalline plastics, such as surements are acceptable if the specimens are smooth, uniform,
polyphenylene sulfide (PPS) and polyetheretherketone and of such shape that the volume can be calculated accurately
(PEEK), an interference due to the level of crystallinity present from the dimensions.
in the composite can cause significant variation in the mea- 7.1.3.2 Procedure—The volume of each specimen shall not
surement of void content by this test method. The level of be less than 2 cm3 (0.125 in.3). Make dimensional measure-
crystallinity can be affected by a variety of circumstances, ments with a micrometer at all edges (12 in all for a 6-sided
including the molding conditions. For these polymers, the rectangular block). Use the averages for each dimension to
density used in the calculation must be the actual density of the calculate the volume.
resin in the composite. 7.1.3.3 The tolerance on the accuracy of the micrometer
measurements shall be 60.0013 cm (60.0005 in.). With
NOTE 2—The actual degree of crystallinity of the composite can be maximum tolerance buildup on a small sample, this could
measured by techniques such as differential scanning calorimetry (DSC)
or by X-ray diffraction. result in an error in the calculated volume of 0.6 %. For larger
samples, and with some measurements being in error on the
6. Conditioning plus side and some on the minus side, the error in the
6.1 Conditioning—Condition the test specimens at 23 6 calculated volume should not exceed 0.2 %.
2°C (73.4 6 3.6°F) and 50 6 10 % relative humidity for not 7.1.3.4 Calculate the density by dividing the weight by the
less than 40 h prior to test in accordance with Procedure A of volume; express as grams per cubic centimeter.
Practice D618, for those tests where conditioning is required. 7.2 Density of the Glass or Other Reinforcement—Most
In cases of disagreement, the tolerances shall be 1°C (1.8°F) glass reinforcement is E glass, which typically has a density
and 62 % relative humidity. between 2.54 and 2.59 g/cm3 ; S glass density is 2.46 to 2.49
6.2 Test Conditions—Conduct tests in the standard labora- g/cm3. However, if a density determination is necessary, use
tory atmosphere of 23 6 2°C (73.4 6 3.6°F) and 50 6 5 % Test Methods D792. Pay particular attention to Note 11 of that
relative humidity, unless otherwise specified in the test meth- test method, which discusses removal of trapped air by
ods. In cases of disagreement, the tolerances shall be 1°C exposure of the sample to a vacuum. This step can be assumed
(1.8°F) and 65 % relative humidity. to be necessary in every determination of glass density. Use a
vacuum of 3 mm Hg or better. Several cycles of atmospheric
7. Procedure pressure-to-vacuum may be required before the trapped air is
7.1 Density of the Resin and the Composite—Three test completely removed.
methods are presented for these measurements. Measure the NOTE 3—It is suggested that the density of the glass supplied be verified
density on pieces of resin that are bubble-free and that were with the glass fiber producer.
cured under heat, time, and pressure conditions that are as close 7.3 Resin Content of Composite—Determine in accordance
as practicable to the conditions under which the composite was with Test Method D2584. The ignition loss in that test method
cured. Density measurements supplied by the resin manufac- is the resin content of the sample and is to be recorded as the
turer are acceptable if they are certified for each batch. weight percent as indicated.
7.1.1 Test Method A—Measure densities using Test Methods
D792. Paragraph 1.1 of that test method requires the specimen 8. Theoretical Density
to have smooth edges and surfaces. For composites, this
requirement may necessitate hand sanding the specimen with 8.1 Calculation—Using the values determined in 7.1, 7.2,
400-grit emery paper to remove fuzzy edges caused by cutting. and 7.3, calculate theoretical density of a composite as follows:
The specimen should be as free as possible from geometric T d 5 100/ ~ R/D1r/d ! (1)
irregularities which tend to trap air bubbles. Remove any
bubbles by a wire, or other mechanical means. Do not use a where:
vacuum to remove bubbles, because cut surfaces of composites Td = theoretical composite density,
may be porous and exposure to a vacuum will force water into R = resin in composite, weight %,
the pores, causing an error in the density measurement. In some D = density of resin,
cases of extreme porosity, just immersing the composite in r = reinforcement in composite, weight %, and
d = density of reinforcement.
water, without using a vacuum, will allow an unacceptably
large amount of water take-up by the composite. In these cases, 8.2 Examples:
seal the porous surfaces by coating them with a known amount From 7.1:
of sealer of known density. This will require weighing the D 5 1.230 g/cm 3 (2)
specimen, then spraying-on or wiping-on the sealer, then
reweighing. A correction for this added material will then be From 7.2:
required in the calculations. d 5 2.540 g/cm 3 (3)
7.1.2 Test Method B—Measure densities in accordance with
Test Method D1505. The comments in 7.1.1 concerning fuzzy From 7.3:
edges and removal of bubbles apply here also. R 5 28.55 weight %, (4)

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 D2734 − 16
r 5 71.45 weight % (5) where:
T d 5 100/ ~ 28.55/1.230171.45/2.540! 5 1.949 g/cm (6) V = void content, volume %,
Md = measured composite density,
9. Void Content R = resin in composite, weight %,
9.1 Test Method A: r = reinforcement in composite, weight %,
9.1.1 Calculation: D = density of resin, and
d = density of reinforcement.
V 5 100~ T d 2 M d ! /T d (7)
9.2.2 Example—Using the same number as in 8.2 and 9.1.2
where: but to only three significant figures:
V = void content, volume %,
Td = theoretical composite density, and
Md = measured composite density.
V 5 100 2 1.90 S 28.6 71.4
1
1.23 2.54 D 5 2.4 % (12)

Note that dividing two densities gives an unlabeled ratio, 10. Report
which in this case is the fraction of material missing. It is
equally correct to interpret this as a weight fraction or volume 10.1 Report the following information:
fraction, but in expressing voids it is always considered to be 10.1.1 Complete identification of the materials tested,
a volume fraction. 10.1.2 Density of all specimens,
9.1.2 Example: From 8: 10.1.3 Weight fraction of resin and reinforcement, theoreti-
cal density (if calculated), and void content of all composite
T d 5 1.949 g/cm 3 (8)
specimens,
From 7.1: 10.1.4 Method of test, and
M d 5 1.903 g/cm3 (9)
10.1.5 Date of test.
V 5 100 3 ~ 1.949 2 1.903! /1.949 5 2.36 % (10) 11. Precision and Bias
Note that four significant figures have been used in these 11.1 This test method does not yet contain a numerical
calculations. This represents a level of accuracy achieved only precision and bias statement and it shall not be used as a referee
by careful work with optimum samples. For many, or perhaps method in case of dispute. The precision and bias of this test
the majority of, determinations, such accuracy is not attained method are under investigation by a task group of Subcommit-
and only three significant figures are warranted. tee D20.18. Anyone wishing to participate in this may contact
9.2 Test Method B—Users may find this test method more the Chairman, Subcommittee D20.18, ASTM, 100 Barr Harbor
convenient when only the void content value is wanted and the Drive, West Conshohocken, PA 19428.
theoretical density value is of no interest:
9.2.1 Calculation: 12. Keywords

V 5 100 2 M d S R r
1
D d D (11)
12.1 composites; ignition loss; plastics; reinforced plastics;
void content; weight fractions

SUMMARY OF CHANGES

Committee D20 has identified the location of selected changes to this standard since the last issue (D2734 - 09)
that may impact the use of this standard. (September 1, 2016)

(1) Corrected the spelling of “diffraction” in Note 2. (5) Corrected format of variable and units in Eq 9 (9.1.2).
(2) Corrected the spelling of “take-up”in 7.1.1. (6) Modified terms and variables of Eq 11 (9.2.1) to be
(3) Corrected the spelling of “centimeter” in 7.1.3.4. consistent throughout the document.
(4) Modified terms and variables of Eq 1 (8.1) to be consistent
throughout the document.

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 D2734 − 16
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