U.S. patent application number 15/599597 was filed with the patent office on 2019-08-22 for assemblies containing polyetherketoneketone tie layers.
The applicant listed for this patent is Arkema Inc.. Invention is credited to Christopher A. Bertelo, Gregory S. O'Brien.
Application Number | 20190256707 15/599597 |
Document ID | / |
Family ID | 42542373 |
Filed Date | 2019-08-22 |
![](/patent/app/20190256707/US20190256707A1-20190822-D00000.png)
![](/patent/app/20190256707/US20190256707A1-20190822-D00001.png)
United States Patent
Application |
20190256707 |
Kind Code |
A1 |
Bertelo; Christopher A. ; et
al. |
August 22, 2019 |
ASSEMBLIES CONTAINING POLYETHERKETONEKETONE TIE LAYERS
Abstract
Tie layers comprised of amorphous polyetherketoneketone are used
to join substrates to form laminates and other assemblies.
Inventors: |
Bertelo; Christopher A.;
(Doylestown, PA) ; O'Brien; Gregory S.;
(Downingtown, PA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Arkema Inc. |
King of Prussia |
PA |
US |
|
|
Family ID: |
42542373 |
Appl. No.: |
15/599597 |
Filed: |
May 19, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13148170 |
Aug 16, 2011 |
9683100 |
|
|
PCT/US2010/023131 |
Feb 4, 2010 |
|
|
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15599597 |
|
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61150128 |
Feb 5, 2009 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B32B 27/18 20130101;
B32B 2307/54 20130101; B32B 15/08 20130101; B32B 9/005 20130101;
B32B 15/20 20130101; B32B 2307/702 20130101; B32B 9/045 20130101;
B32B 2419/00 20130101; Y10T 428/31855 20150401; B32B 2439/00
20130101; B32B 17/06 20130101; B32B 2535/00 20130101; B32B 27/36
20130101; B32B 2605/00 20130101; B32B 2307/50 20130101; B32B 27/365
20130101; B32B 27/281 20130101; C08L 71/00 20130101; B32B 27/12
20130101; B32B 27/286 20130101; B32B 27/08 20130101; B32B 27/285
20130101; B32B 27/34 20130101; B32B 27/288 20130101; B32B 5/02
20130101; B32B 2262/101 20130101; B32B 2405/00 20130101; Y10T
428/31645 20150401; B32B 7/14 20130101; B32B 2255/26 20130101; C09J
171/00 20130101; C08G 2650/40 20130101; B32B 2597/00 20130101; Y10T
428/31692 20150401; B32B 2307/704 20130101; Y10T 428/26
20150115 |
International
Class: |
C08L 71/00 20060101
C08L071/00; B32B 27/28 20060101 B32B027/28; B32B 27/36 20060101
B32B027/36; B32B 5/02 20060101 B32B005/02; B32B 7/14 20060101
B32B007/14; B32B 9/00 20060101 B32B009/00; B32B 9/04 20060101
B32B009/04; B32B 15/08 20060101 B32B015/08; B32B 15/20 20060101
B32B015/20; B32B 27/08 20060101 B32B027/08; B32B 27/12 20060101
B32B027/12; B32B 27/18 20060101 B32B027/18; B32B 27/34 20060101
B32B027/34 |
Claims
1. A multilayer assembly comprising a first substrate, a second
substrate and an adhesive, wherein a discontinuous adhesive layer
consisting of amorphous polyetherketoneketone is positioned between
and directly in contact with said first substrate and said second
substrate, and wherein the amorphous polyetherketoneketone consists
of repeating units represented by formulas I and II:
-A-C(.dbd.O)--B--C(.dbd.O)-- I -A-C(.dbd.O)-D-C(.dbd.O)-- II
wherein A is a p,p'-Ph-O-Ph- group, Ph is a phenylene radical, B is
p-phenylene, and D is m-phenylene and the isomer ratio of formula
I:formula II (T:I) is 60:40 so as to provide amorphous
polyetherketoneketone; wherein a contact surface of said first
substrate is the same as or different from a contact surface of
said second substrate and each of the contact surface of said first
substrate and the contact surface of said second substrate is
selected from the group consisting of glass, and prepreg tape.
2. The assembly of claim 1, wherein said discontinuous adhesive
layer is from about 1 to about 100 microns thick.
3. The assembly of claim 1 which selected from the group consisting
of a storage tank, a pipe, and a tube.
4. The assembly of claim 1 which selected from the group consisting
of a sheet and a film.
5. A method of making an assembly, said method comprising joining a
first substrate to a second substrate using a discontinuous
adhesive layer consisting of amorphous polyetherketoneketone said
amorphous polyetherketone ketone consisting of repeating units
represented by formulas I and II: -A-C(.dbd.O)--B--C(.dbd.O)-- I
-A-C(.dbd.O)-D-C(.dbd.O)-- II wherein A is a p,p'-Ph-O-Ph- group,
Ph is a phenylene radical, B is p-phenylene, and D is m-phenylene
and the isomer ratio of formula I:formula II (T:I) is 60:40 so as
to provide amorphous polyetherketoneketone; wherein a contact
surface of said first substrate is the same as or different from a
contact surface of said second substrate and wherein each of the
contact surface of said first substrate and the contact surface of
said second substrate is selected from the group consisting of
glass and prepreg tape, said method further comprising the step of
positioning the discontinuous adhesive layer between said first and
second substrates, whereby said discontinuous adhesive layer is
directly in contact with a surface of said first substrate and
directly in contact with a surface of said second substrate.
6. A multilayered laminated assembly comprising a first substrate,
a second substrate, and a discontinuous adhesive layer, said
discontinuous adhesive layer having first and second surfaces and
positioned between said first substrate and said second substrate,
the laminated assembly being configured such that the first
substrate is directly attached to said first surface of the
discontinuous adhesive layer, and the second substrate is directly
attached to said second surface of the discontinuous adhesive
layer, wherein said discontinuous adhesive layer is from about 1 to
about 100 microns, and consists of amorphous polyetherketoneketone,
wherein the amorphous polyetherketoneketone consists of repeating
units represented by formulas I and II:
-A-C(.dbd.O)--B--C(.dbd.O)-- I -A-C(.dbd.O)-D-C(.dbd.O)-- II
wherein A is a p,p'-Ph-O-Ph- group, Ph is a phenylene radical, B is
p-phenylene, and D is m-phenylene and the isomer ratio of formula
I:formula II (T:I) is 60:40 so as to provide amorphous
polyetherketoneketone; wherein a contact surface of said first
substrate is selected from the group consisting of glass and
prepreg tape, and wherein a contact surface of said second
substrate is selected from the group consisting of glass, prepreg
tape, and metals, wherein said assembly is laminated.
7. The multilayered laminated assembly of claim 6, which is
selected from the group consisting of aircraft parts, automobile
parts, boat parts, machinery parts, heavy equipment parts, storage
tanks, pipes, sports equipment parts, tools, and biomedical device
parts.
8. The multilayered laminated assembly of claim 6, wherein said
assembly is laminated using sufficient heat and/or pressure, such
that the desired degree of adhesion with the surface of the first
and/or second substrate(s) is obtained.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 13/148,170, filed Aug. 16, 2011 which is a national stage
application under 35 U.S.C. .sctn. 371 of PCT/US2010/023131, filed
Feb. 4, 2010, which claims benefit to U.S. Provisional Application
No. 61/150,128, filed on Feb. 5, 2009.
FIELD OF THE INVENTION
[0002] The invention relates to the use of amorphous
polyetherketoneketones as tie layers in assemblies such as
composites, laminates, and the like.
DISCUSSION OF THE RELATED ART
[0003] In recent decades, considerable development effort has been
invested in the design of laminates, composites and other
assemblies comprising multiple layers of different materials. For
many end-use applications, it is not feasible to use a single
material to fabricate a desired component due to particular
multiple performance requirements that cannot be met by any known
material. For example, a part may need to be simultaneously high in
strength and stiffness as well as pressure resistant,
solvent/chemical resistant, and dimensionally stable at high
temperatures. However, it has often proven to be challenging to
achieve satisfactory adhesion or bonding directly between the
dissimilar layers that may need to be utilized in a composite or
laminate. Poor compatibility between composite layers can limit the
properties exhibited by such assemblies. In particular, certain
thermoplastics (particularly crystalline and/or high temperature
thermoplastics) exhibit poor adhesion to other materials, which has
been attributed to the inability of such thermoplastics to
adequately "wet" surfaces of dissimilar substances, leading to
problems with delamination and loss of structural integrity when
the composite is placed in use in a highly demanding environment.
Accordingly, it would be advantageous to develop improved methods
of assembling composites, laminates and the like which avoid such
difficulties by ensuring good bonding between the composite or
laminate layers.
BRIEF SUMMARY OF THE INVENTION
[0004] An assembly is provided by the present invention which is
comprised of a first substrate and a second substrate, wherein a
tie layer comprised of amorphous polyetherketoneketone is
positioned between and in contact with said first substrate and
said second substrate. Such assemblies may be prepared by coating
one substrate with amorphous polyetherketoneketone and then joining
the other substrate with the coated substrate by pressing the
substrates together while heating the polyetherketoneketone tie
layer. Alternatively, a coextrusion process can be utilized. The
invention is especially useful where one substrate is comprised of
a crystalline and/or high temperature thermoplastic such as a
crystalline poly(aryletherketone) that does not exhibit completely
satisfactory adhesion to the surface of the other substrate in the
absence of the tie layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1 Shows micrographs of fibers sized with PEKK and PEEK,
and demonstrates the failure mode in fibers of each sizing.
DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS OF THE INVENTION
[0006] Assemblies in accordance with the present invention are
advantageously manufactured using tie layers comprised of amorphous
polyetherketoneketone. The amorphous polyetherketoneketones
suitable for use in the present invention comprise (and preferably
consist essentially of or consist of) repeating units represented
by the following formulas I and II:
-A-C(.dbd.O)--B--C(.dbd.O)-- I
-A-C(.dbd.O)-D-C(.dbd.O)-- II
where A is a p,p'-Ph-O-Ph- group, Ph is a phenylene radical, B is
p-phenylene, and D is m-phenylene. The Formula I:Formula II (T:I)
isomer ratio in the polyetherketoneketone is varied so as to
provide an amorphous (non-crystalline) polymer. An amorphous
polymer, for purposes of this invention, means a polymer that does
not exhibit a crystalline melting point by differential scanning
calorimetry (DSC).
[0007] Polyetherketoneketones are well-known in the art and can be
prepared using any suitable polymerization technique, including the
methods described in the following patents, each of which is
incorporated herein by reference in its entirety for all purposes:
U.S. Pat. Nos. 3,065,205; 3,441,538; 3,442,857; 3,516,966;
4,704,448; 4,816,556; and 6,177,518. Mixtures of
polyetherketoneketones may be employed.
[0008] In particular, the Formula I:Formula II ratio (sometimes
referred to in the art as the T/I ratio) can be adjusted as desired
by varying the relative amounts of the different monomers used to
prepare the polyetherketoneketone. For example, a
polyetherketoneketone may be synthesizing by reacting a mixture of
terephthaloyl chloride and isophthaloyl chloride with diphenyl
ether. Increasing the amount of terephthaloyl chloride relative to
the amount of isophthaloyl chloride will increase the Formula
I:Formula II (T/I) ratio. Generally speaking, a
polyetherketoneketone having a relatively high Formula I:Formula II
ratio will be more crystalline than a polyetherketoneketone having
a lower Formula I:Formula II ratio. An amorphous
polyetherketoneketone having a T/I ratio of from about 55:45 to
about 65:35 is particularly suitable for use in the present
invention.
[0009] Suitable amorphous polyetherketoneketones are available from
commercial sources, such as, for example, certain of the
polyetherketoneketones sold under the brand name OXPEKK by Oxford
Performance Materials, Enfield, Conn., including OXPEKK-SP
polyetherketoneketone.
[0010] The polymeric composition used to form the tie layer may
additionally be comprised of components other than the amorphous
polyetherketoneketone, such as stabilizers, pigments, processing
aids, fillers, and the like. In certain embodiments of the
invention, the polymeric composition consists essentially of or
consists of amorphous polyetherketoneketone. For example, the
polymeric composition may be free or essentially free of any type
of polymer other than amorphous polyetherketoneketone.
[0011] The present invention is useful for multilayer structures,
such as, for example, films, sheets, pipes and hollow bodies, such
as storage tanks, wherein a tie layer comprised of amorphous
polyetherketoneketone is used to adhere a first substrate layer to
a second substrate layer. The multilayer structure thus may
comprise a tie layer (T), a first substrate layer (S1) directly
attached to a first face (surface) of tie layer (T), and a second
substrate layer (S2) directly attached to a second face (surface)
of tie layer (T), such that the tie layer (T) is sandwiched between
first substrate layer (S1) and second substrate layer (S2). Of
course, additional layers and components may be present in the
assembly, including one or more further tie layers positioned
between other substrate layers.
[0012] One or both of the substrate layers joined by the tie layer
can, for example, be a sheet or a thin film. The substrates may be
comprised of any suitable material such as, for example, metal,
plastic (thermoplastic or thermoset), ceramic, or glass. The
substrate may be a composite material such as, for example, glass
fiber-reinforced plastic. The thickness of the thin film or of the
sheet can he suitably chosen and can be, for example, from
approximately 0.01 to approximately 10 mm.
[0013] In one embodiment, at least one of either the first
substrate or the second substrate is comprised of a crystalline
(including semi-crystalline) and/or high temperature thermoplastic
as these materials often exhibit interfacial adhesion to various
types of substrate surfaces that is not completely satisfactory. A
tie layer of amorphous polyetherketoneketone in accordance with the
present invention helps to improve such adhesion, thereby enhancing
the mechanical and other properties of the resulting assembly.
Suitable crystalline and/or high temperature thermoplastics
include, but are not limited to, polyaryletherketones (such as
crystalline polyetherketone (PEK), crystalline polyetheretherketone
(PEEK), crystalline polyetherketoneketone (PEKK),
polyetheretheretherketone (PEEEK), polyetheretherketoneketone
(PEEKK), polyetherketoneetheretherketone (PEKEKK), and
polyetherketoneketoneketone (PEKKK)), polyamides, polyetherimides,
polyamideimides, polysulfones, polyethersulfones, polyarylethers,
polycarbonates, liquid crystal polymers, polyphenylene sulfides,
polyarylenes (polyphenylenes), polyamides, polyphthalamides,
polyaromatic esters and the like.
[0014] In another embodiment, at least one of either the first
substrate or the second substrate is metallic, e.g., a metal sheet,
foil, or the like. The substrate may be comprised of any suitable
metal or metal alloy such as steel, aluminum, aluminum alloy,
copper, gold, silver or the like.
[0015] The tie layer typically is relatively thin, e.g., from about
1 to about 100 microns thick. In one embodiment of the invention,
the interface between the first and second substrate surfaces is
completely filled or covered by the tie layer, although in other
embodiments the tie layer may be discontinuous.
[0016] In one embodiment, the assemblies of the invention are
manufactured by adapting known coextrusion processes, particularly
where both of the substrate layers joined by the tie layer are
based on thermoplastic polymers. For example, the apparatus used
for making such assemblies can be any conventional or standard
extruder, dies or stream distributors generally employed for the
coextrusion of thermoplastic polymers. The thickness of each of the
polymeric layers will depend on the regulation of the flow rate of
each of the extruders. Generally, the die and extruder temperatures
should be selected based on the characteristics and properties of
the polymers to be used in the tie layer and the substrate layers
so that the materials are rendered capable of being extruded.
[0017] The assemblies of the invention can be extruded into any
conventional form, including film, plate, sheeting, tubing or any
other shape conventionally obtainable by coextrusion.
[0018] Compression molding, intermittent matched die consolidation,
double belt press consolidation, composite roll forming, transfer
molding, as well as other such techniques can also be utilized in
connection with the present invention. For example, the assembly
may be prepared by placing a sheet or film corresponding in
composition to the desired tie layer between a first substrate and
a second substrate and heating the resulting "sandwich" at a
temperature effective to soften at least one of the layers
sufficiently to enable it to flow and come into intimate contact
with an adjacent layer, thereby forming an adhesive bond when the
assembly is cooled. Typically, it will be desirable to apply
pressure on the "sandwich" so as to enhance the degree of adhesion
achieved between the tie layer and the substrate layers.
Thermoforming of the assembly can be carried out so as to attain a
particular desired shape or contour.
[0019] In yet another embodiment, the assembly may be formed by
first adhering a tie layer in accordance with the present invention
to a first substrate. This may, for example, be accomplished by
coextrusion of the tie layer and the first substrate or by pressing
a sheet of the tie layer and a sheet of the first substrate
together while heating or by extruding the tie layer onto the first
substrate. The resulting tie layer/first substrate subassembly can
then be joined to a second substrate by bringing the second
substrate into contact with the other side of the tie layer, with
sufficient heat and pressure being applied so as to create the
desired degree of adhesion between the tie layer and the surface of
the second substrate. Overmolding or extrusion molding techniques
could also be utilized. For example, the tie layer/first substrate
subassembly can be positioned in a mold and a heated polymeric
composition (corresponding to the composition of the desired second
substrate) introduced into the mold such that it comes into contact
with at least a portion of the available surface of the tie layer.
If desired, the overmolding conditions may be selected such that
the tie layer/first substrate subassembly undergoes a change in
shape during such overmolding (e.g., the subassembly may be
thermoformed). Alternatively, the heated polymeric composition may
be extrusion molded as a coating onto the tie layer surface to form
the second substrate layer.
[0020] In yet another embodiment, a melt of the tie layer may be
extruded between preformed sheets of the first and second substrate
layers. This could be done just prior to lamination of the two
sheets.
[0021] The tie layer may also be formed by applying a solution of
the amorphous polyetherketoneketone to a substrate surface and then
removing the solvent. Suitable solvents for amorphous
polyetherketoneketones are known in the art and include, for
example, halogenated hydrocarbons (particularly chlorinated
hydrocarbons such as o-dichlorobenzene, 1,2,4-trichlorobenzene,
methylene chloride and tetrachloroethylene), nitrobenzene, and
aqueous mineral acids (e.g., sulfuric acid and/or nitric acid). The
solvent may be removed from the coated substrate by any suitable
method such as heating and/or application of vacuum.
[0022] Assemblies prepared in accordance with the present invention
may be utilized in any of the end use applications where such
laminates or composites conventionally are employed or have been
proposed to be employed. Representative applications include
composites and laminates (including two- and three dimensional
panels and sheets) for aerospace/aircraft, automobiles and other
vehicles, boats, machinery, heavy equipment, storage tanks, pipes,
sports equipment, tools, biomedical devices (including devices to
be implanted into the human body), building components, wind blades
and the like. Benefits of the invention described herein include
higher tensile strength, higher compressive strength, higher peel
strength, enhanced solvent, chemical and water resistance, and
improved resistance to delamination, as compared to assemblies
prepared without tie layers based on amorphous
polyetherketoneketone,
EXAMPLES
[0023] Example 1, Production of PEKK film--A thin film of amorphous
polyetherketoneketone A-PEKK (OXPEKK SP from Oxford Performance
materials, T/I ratio of 60/40) was produced by melt processing
pellets of A-PEKK on 1 inch Davis Standard extruder fitted with a
12 inch film die. The polymer was processed with a relatively low
screw speed (20-80 RPM) and at 315-325.degree. C. The extruder was
fitted with a standard cast film take off stack from Davis standard
operating at 150.degree. C.
[0024] Films of crystalline grades, with T/I ratios of 70/30 or
higher, are also possible but more difficult to produce. The rapid
cooling and crystallization of these films requires higher extruder
temperatures and melt temperatures (365-375.degree. C.), and higher
temperatures for the take off equipment (220-250.degree. C. or
higher).
[0025] Example 2, Production and testing of laminated materials
using PEKK as the tie layer--Test samples consisted of 1'' by 5''
metal or glass strips (see table 1) and 1'' by 1'' by 0.07'' sheets
of PEKK formed by the process described in Example 1. The surfaces
of the test samples were cleaned with acetone to remove any grease
or oil prior to 2.5 molding. The samples were compression molded
using a Carver press and two 12-inch by 12-inch aluminum blocks
with small 1'' by 5'' indentations to hold the test samples. The
press mold was preheated to 220-230.degree. C. (for A-PEKK sheets)
or 260-290.degree. C. for crystalline PEKK. After preheating the
mold was removed from the press and the test sheets and
1''.times.1'' PEKK sheets carefully assembled into a 3 layer
structure so as to not contaminate the bonding surfaces. The
construction consisted of one PEKK sheet between two sheets of the
appropriate tests materials. The mold was then reassembled and
placed back in the press and reheated to the test temperature for
c.a. 1 minute. The assemblies were then pressed at 2000 psi for 5
minutes and then removed and allowed to remain at room temperature
for at least 24 hrs at 50% relative humidity prior to testing the
strength of the bonding.
[0026] The strength of the bond was evaluated using a Zwick-Roell
2050 tensile testing in a instrument using a method similar to ASTM
D3528-96 (2008) Standard Test Method For Strength Properties Of
Double Lap Shear Adhesive Joints By Tension Loading with a cross
head speed of 1.27 mm/min. The results are presented in Table 1 and
Table 2 below.
TABLE-US-00001 TABLE 1 Lap shear tests of similar substrates using
PEKK as an adhesive Max Stress Max Stress Break Stress Break Stress
Failure Substrate Avg (psi) Std Dev. Avg (psi) Std Dev Mode
Aluminum 1491 97.17 1380 99.1 cohesive Steel 2589 427.1 2590 427
cohesive Stainless Steel 1779 319.4 1730 365 cohesive Galvanized
Steel 1046 628.4 1010 650 cohesive Anodized 1521 229.4 1520 229
cohesive Aluminum Glass N/A N/A N/A N/A substrate Alclad 1684 280.8
1540 191 cohesive Aluminum Zinc Dichromate Samples broke during
assembly, zinc coating delaminated
TABLE-US-00002 TABLE 2 Lap shear tests of using PEKK as an adhesive
for similar and dissimilar substrates Max Stress Max Stress Break
Stress Break Stress Failure Substrate Avg (psi) Std Dev. Avg (psi)
Std Dev Mode Aluminum- 1417 123.7 1320 143 cohesive Aluminum
AramidFilm 146.9 12.64 142 16.5 cohesive/ Nomex- substrate Aluminum
AramidFilm 108.8 10.84 114 4.79 cohesive Nomex- AramidFilm Nomex-
AramidFilm 140.5 8.9259 139 11.3 cohesive Nomex - Steel AramidFilm
144.2 15.92 138 12 cohesive/ Nomex - Grit substrate Blasted Steel
Grit Blasted 1817 349.3 1820 349 cohesive Steel- Aluminum Grit
Blasted 2107 273.4 2110 273 cohesive Steel - Grit Blasted Steel
Prepreg Tape- 1116 191.1 498 190 substrate Grit Blasted Steel Grit
Blasted 2217 579 2220 579 cohesive Steel - Steel Aluminum- 234
192.4 186 151 adhesive/ Prepreg Tape substrate Prepreg Tape- 1256
217.3 859 284 substrate/ Prepreg Tape adhesive Prepreg Tape- 333.5
66.96 136 99.6 substrate Steel Steel- 1250 226.4 1250 226 cohesive/
Aluminum adhesive Steel - Steel 2811 223.4 2790 230 cohesive
Illustration of the Improved Adhesion that PEKK Provides vs Other
Similar High Temperature Polymers
[0027] Polyether-ether-ketone is a polymer with a similar structure
and use temperatures to PEKK, however it is produced with a pure
para structure with no isophthalate (i.e. T/I=100). The
microphotographs of the Figure illustrate that PEEK has much poorer
adhesion to glass fibers as the fibers seem to pull away from the
matrix when cryrofractured, while the microphotograph of the
samples made with PEKK show that the fibers do not pull out and
that PEKK is still attached to the fibers. As the Figure
illustrates, the failure in the PEKK composite is cohesive as the
failure is in the PEKK matrix. The fracture surface in this case
shows significant ductility produced as a consequence of the
effective load transfer between the fiber and the PEKK matrix. In
contrast, the failure in the PEEK system is adhesive as the fibers
are cleanly pulled from the matrix. The fractured surface shows a
clean brittle rupture process, indicating that in this case the
interaction of the fiber with the matrix is not as effective. Load
transfer is not expected to be efficient in this system. The energy
of breaking a piece by cohesive failure is dependant on the
strength of the matrix itself and is generally regarded as higher
than an adhesive failure.
* * * * *