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Citation: Meng, L.; Li, X.; Liu, M.; Li,
C.; Meng, L.; Hou, S. Modified
Ammonium Polyphosphate and Its
Application in Polypropylene Resins.
Coatings 2022,12, 1738. https://
doi.org/10.3390/coatings12111738
Academic Editor: Ivan Jerman
Received: 17 October 2022
Accepted: 10 November 2022
Published: 13 November 2022
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coatings
Article
Modified Ammonium Polyphosphate and Its Application in
Polypropylene Resins
Lingyu Meng 1,2 , Xiangrui Li 1, Mingli Liu 1, *, Chunfeng Li 1, Lipeng Meng 2, * and Sen Hou 1
1School of Materials Science and Engineering, Beihua University, Jilin 132000, China
2
Institute of Forestry Resource Utilization, Jilin Forestry Scientific Research Institute, Changchun 130000, China
*Correspondence: liumingli17@163.com (M.L.); menglipengmlp@126.com (L.M.);
Tel.: +86-186-0449-6062 (M.L.); +86-189-4312-0592 (L.M.)
Abstract:
Herein, a simple and environment-friendly method of coupling agent treatment of APP
(ammonium polyphosphate) is provided and an optimum process of modification via coupling
agent is identified. The effects of coupling agent type, dosage, modification time, and modification
temperature on the modification of ammonium polyphosphate (APP) were investigated using an
orthogonal experimental design. The modified ammonium polyphosphate (KAPP) was characterized
under optimal process conditions using Fourier Transform Infrared (FT-IR), X-ray Diffraction (XRD),
Thermogravimetry (TG), and Scanning Electron Microscope (SEM) analysis. The treatment greatly
improved the water solubility, dispersibility, and thermal stability of KAPP; and the application
of KAPP in polypropylene (PP) was investigated. The flexural properties, thermal stability, and
flame retardancy were studied using mechanical testing, thermogravimetric analysis, oxygen index,
and UL-94 vertical combustion. The results show that the KAPP-added polypropylene composites
have better bending properties when compared with the APP-added PP composites. SEM analysis
suggests that the surface of KAPP became smoother and flat; dispersion was better, compatibility
with the PP matrix was improved, and there were no prominent voids and gaps in the cross-section.
A different degree of improvement in flame retardancy was also observed as per the LOI and vertical
combustion results, wherein the PP composites prepared by adding 20% KAPP achieved the LOI of
27.6% and passed the UL-94 test with V-0 rating.
Keywords: orthogonal design; coupling agent; solubility; flame retardancy
1. Introduction
With the rapid development of polymers and chemicals, plastic products are widely
used in various areas of the daily life of people [
1
–
3
]. However, the flammable plastics
also pose a fire hazard that may be life-threatening and can cause severe property damage.
Polypropylene (PP), one of the world’s four most versatile plastics [
4
], has been widely used
in construction, packaging, and automotive applications [
5
–
7
] because of its low density, ex-
cellent abrasion resistance and strength, electrical insulation, and chemical resistance [
8
–
11
].
However, PP is flammable, easily deformed after burning, continues to burn when exposed
to open flames, and the molten drip phenomenon can lead to secondary combustion, which
greatly limits the applications of PP. Therefore, improving the flame retardancy of PP and
reducing the melt-drop phenomenon have been extensively studied [
12
–
14
]. Currently, the
primary methods to improve the flame-retardant performance of PP are the addition of
flame retardant and coating treatment [
15
,
16
]. The addition of flame retardant has become
the most common method because of its simple process [17,18].
Ammonium polyphosphate (APP), as an inorganic flame retardant with high phos-
phorus and nitrogen content, good thermal stability, low smoke, and non-toxicity, and is
in line with the concept of green development. Owing to these characteristics, APP has
become a research focus for additive flame retardants [
19
–
21
]. The industrially produced
Coatings 2022,12, 1738. https://doi.org/10.3390/coatings12111738 https://www.mdpi.com/journal/coatings
Coatings 2022,12, 1738 2 of 17
APP has been reported to have a relatively low degree of polymerization, is easily soluble
in water, and has poor compatibility with the organic material substrates, which severely
limit the use of flame-retardant plastics in humid environments [
22
–
24
]. To address these
issues, the surface hydrophobic treatment of APP is one of the commonly used methods.
The surface hydrophobic treatment of APP is primarily a microencapsulated, surface modi-
fier and coupling agent treatment [
25
,
26
]. However, there are still some problems in the
application of microencapsulated modified APP, such as microencapsulation of melamine
formaldehyde resin to treat APP, owing to which there is free formaldehyde in the ma-
terial that is harmful to human body [
27
]. Also, the use of isocyanate and melamine to
prepare microencapsulation is damaging. The use of isocyanate and melamine to prepare
microencapsulated APP increases the viscosity [
28
]. Also, surfactants such as fatty acids
and low-valent metal salts to modify APP may have an impact on APP as organic solvents
are used in the subsequent treatment process [
29
–
32
]. Moreover, the process is complex and
costly. Although silane coupling agent treatment is the most common; however, coupling
agent in the treatment process should follow the best dosage rules. Any dosage more than
the best dosage would both increase the cost and cause powder coagulation. Also, the use
of a simple and environment-friendly coupling agent treatment of APP has rarely been
reported.
Herein, different types of silane coupling agents were used to modify the surface
of APP. The effects of conditions such as coupling agent dosage, modification time, and
modification temperature on the solubility of APP were investigated. The dispersion, ther-
mal stability, and surface morphology of KAPP prepared using the optimum modification
process have been studied and the modification mechanism has been discussed in detail.
On this basis, different proportions of AAPP and KAPP were applied to polypropylene
resins to compare their bending properties, oxygen index, and vertical burning properties.
2. Materials and Methods
2.1. Materials
The main materials used in this experiment are shown in Table 1.
Table 1. Experimental materials.
Product Name Manufacturer Remarks
Ammonium polyphosphate
China, Shandong Yusuo Chemical
Technology Co.
Average degree of polymerisation
(n) of 30–50, industrial grad
KH-550
China, Shandong Yusuo Chemical
Technology Co. Analysis pure
KH-560
China, Shandong Yusuo Chemical
Technology Co. Analysis pure
KH-570
China, Shandong Yusuo Chemical
Technology Co. Analysis pure
Liquid paraffin China, Sinopharm Group
Chemical Actual Co. Analysis pure
Polypropylene China, Maoming Shihua
Dongcheng Chemical Co.
Density 0.901 g/cm
3
; melt index 4
g/10 min, industrial grade
Antioxidant China, Dongguan Dinghai Plastic
Chemical Co. B225, industrial grad
Maleic anhydride grafted
polypropylene
China, Dongguan Dinghai Plastic
Chemical Co.
grafting rate 0.6–0.8%,
industrial grad
Internal and external compound
lubricant
China, Dongguan Dinghai Plastic
Chemical Co. industrial grad
Distilled water - Laboratory homemade
2.2. Sample Preparation
(1)
Preparation of modified ammonium polyphosphate
Several factors affect the water solubility of ammonium polyphosphate, primarily the
type of coupling agent, amount of coupling agent, reaction temperature, and reaction time.
Herein, water solubility of ammonium polyphosphate was taken as the response index
Coatings 2022,12, 1738 3 of 17
and, four factors were selected, namely: coupling agent type (A), coupling agent dosage
(B), reaction temperature (C) and reaction time (D); and three levels were selected for each
factor. The experimental factors and levels are listed in Table 2.
Table 2. L9(34) orthogonal test factor level table.
Level
A B C D
Coupling Agent
Types
Amount
Used (%)
Reaction
Temperature (◦C)
Reaction
Time (h)
1 KH-550 3 50 1
2 KH-560 5 60 2
3 KH-570 10 70 3
Liquid phase method: 100 g of APP powder was added to 150 mL of anhydrous
ethanol and stirred to obtain an ethanol dispersion of APP. Then, the dispersion of APP was
transferred to a three-necked flask. The coupling agent was dispersed in anhydrous ethanol
and stirred with a magnet for 20 min to obtain the ethanol solution of the coupling agent.
Finally, this solution was added to the three-mouth flask, stirred for a certain duration in a
constant temperature water bath and then filtered.
(2)
Preparation of flame-retardant polypropylene composites
PP, APP, modified APP, and antioxidant were mixed in proportion and each specimen
was kneaded on a dense refiner at 180
◦
C for 10 min. The resultant sample was placed
in a Forming Die and pressed for 15 min on a 180
◦
C plate vulcanizer at the pressure
of 10 MPa cooled, to obtain a sample of the composite material. The formulation of the
flame-retardant polypropylene composite is shown in Table 3.
Table 3. Flame-retardant polypropylene composite formulations.
Samples Mass Fraction %
APP KAPP PP
PP - - 100
10% APP/PP 10 - 90
20% APP/PP 20 - 80
30% APP/PP 30 - 70
10% APP/PP - 10 90
20% KAPP/PP - 20 80
30% KAPP/PP - 30 70
2.3. Characterization
The solubility is based on the method specified in HG/T 2770-2008 “Industry Stan-
dard for Industrial Ammonium Polyphosphate”. The results obtained are the average of
three tests.
Infrared spectroscopy test: the sample was ground into powder using mortar and
pestle into powder of 110 mesh or more, using the KBr press method; Fourier transform
infrared spectrometer (Spectrum 100, Perkin Elmer Company, Waltham, MA, USA) was
employed for the analysis of infrared absorption of the specimen and scan was performed
in the range of 4000–500 cm−1.
Scanning electron microscopy (Hitachi S-3000N, Tokyo, Japan): SEM was to observe
the microscopic morphology of the fracture surfaces of APP and flame-retardant PP com-
posites before and after modification at, an accelerating voltage of 15–20 kV. The powder
sample was sprayed on the conductive tape, flicked by hand, and blown with ear ball
to make it firmly and evenly adhere to the conductive tape, and finally sprayed with
gold. Flame retardant PP cross section was obtained by breaking the transect under liquid
nitrogen quenching, and then, spraying gold on its cross-section.
Thermogravimetric analysis (TG, NETZSCH, STA-449F3, Selb, Germany): The thermal
weight losses of APP and flame-retardant PP before and after modification were measured
Coatings 2022,12, 1738 4 of 17
at a heating rate of 10
◦
C/min in the temperature range of 60–700
◦
C (60 mL/min under
nitrogen atmosphere).
Vertical combustion test (UL 94) for flame retardant PP was conducted according to
UL 94 ISBN O-7629-0082-2 (Tests for Flammability of Plastic Materials for Parts in Devices
and appliances).
Oxygen index test: The samples (100 mm
×
10 mm
×
5 mm) were tested for the
limiting oxygen index (LOI) according to ASTM D2863-17a (Standard Test Method for
Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of
Plastics). The samples were placed in the temperature range of 23
±
2
◦
C and humidity
of 50
±
5% for more than 40 h before the test. The oxygen index was measured using
X-ray light.
Elemental analysis was performed using X-ray photoelectron spectroscopy (XPS) on
the APP powders before and after modification. The data were obtained from the PHI 5300
spectrometer of Perkin Elmer Company, Waltham, MA, USA
The bending properties were tested according to ASTM D790-17 in a three-point
bending mode using a specimen size of 127 mm
×
12.7 mm
×
5 mm, a bending rate of
2 mm/min, and a span of 80 mm, with no less than five specimens in a set.
3. Results and Discussion
3.1. Analysis of Extreme Difference Results
As per Tables 4and 5, the factors affecting the water solubility of modified ammonium
polyphosphate are the type of coupling agent, dosage, reaction temperature, and reaction
time with R-values of 1.42, 0.85, 0.38 and 0.3, respectively, so the four factors in order of
strength were the type of coupling agent, dosage, reaction temperature, and time. It can
be seen from Figure 1that the solubility of modified APP was minimized at a reaction
temperature of 60
◦
C, a coupling agent of KH-550, a dosage of 10%, and a reaction time of 2
h. FT-IR, XRD, TG, and microscopic morphological characterization were conducted for
KAPP prepared under these optimal conditions and the application in PP was explored.
Table 4. Orthogonal design and results table.
No.
Factors
Solubility g/100 mL ∆/%
A
Coupling Agent
Type
B
Coupling Agent
Dosage/%
C
Reaction
Temperature/◦C
D
Reaction Time
/h
1 KH-550 3 50 1 2.4274 33.8
2 KH-550 5 70 2 1.4602 60.2
3 KH-550 10 60 3 1.1204 69.4
4 KH-560 3 70 3 2.0074 45.2
5 KH-560 5 60 1 1.7274 52.9
6 KH-560 10 50 2 1.2904 65
7 KH-570 3 60 2 3.174 13.4
8 KH-570 5 50 3 3.4454 6
9 KH-570 10 70 1 2.6554 27.6
K1 1.67 2.54 2.39 2.27
K2 1.68 2.21 2.01 1.97
K3 3.09 1.69 2.04 2.19
R 1.42 0.85 0.38 0.3
Order of priority of factors Type of coupling agent > Dosage > Reaction temperature > Time
Table 5. Results of extreme difference analysis.
Performance Coupling Agent
Types
Coupling Agent
Dosage/%
Reaction
Temperature/◦C
Reaction Time
/h
Water soluble
1.67 2.54 2.39 2.27
1.68 2.21 2.01 1.97
3.09 1.69 2.04 2.19
1.42 0.85 0.38 0.3
Coatings 2022,12, 1738 5 of 17
Coatings 2022, 12, x FOR PEER REVIEW 5 of 19
Table 4. Orthogonal design and results table.
No.
Factors
Solubility
g/100 mL Δ/%
A
Coupling Agent
Type
B
Coupling Agent
Dosage/%
C
Reaction Tempera-
ture/°C
D
Reaction Time
/h
1 KH-550 3 50 1 2.4274 33.8
2 KH-550 5 70 2 1.4602 60.2
3 KH-550 10 60 3 1.1204 69.4
4 KH-560 3 70 3 2.0074 45.2
5 KH-560 5 60 1 1.7274 52.9
6 KH-560 10 50 2 1.2904 65
7 KH-570 3 60 2 3.174 13.4
8 KH-570 5 50 3 3.4454 6
9 KH-570 10 70 1 2.6554 27.6
K1 1.67 2.54 2.39 2.27
K2 1.68 2.21 2.01 1.97
K3 3.09 1.69 2.04 2.19
R 1.42 0.85 0.38 0.3
Order of priority of factors Type of coupling agent > Dosage > Reaction temperature > Time
Table 5. Results of extreme difference analysis.
Performance Coupling Agent
Types
Coupling Agent
Dosage/% Reaction Temperature/°C Reaction Time
/h
Water soluble
1.67 2.54 2.39 2.27
1.68 2.21 2.01 1.97
3.09 1.69 2.04 2.19
1.42 0.85 0.38 0.3
Figure 1. Factor water mean trend graph.
Figure 1. Factor water mean trend graph.
3.2. Effect of 3-Aminopropyltriethoxysilane on Properties of Ammonium Polyphosphate
3.2.1. Effect of 3-Aminopropyltriethoxysilane on Dispersibility of Ammonium
Polyphosphate in Solvents
Figure 2presents photographs of APP and KAPP powders placed in different solvents
(distilled water, paraffin) for different durations. Figure 2a,b shows the hydrophilicity of
unmodified APP and hydrophobicity of KAPP. The unmodified APP was dispersed in
water and the distilled water became more turbid after 24 h and the precipitation occurred
quickly after addition. In contrast, KAPP added to distilled water was partially dissolved
and settled completely to the bottom after 24 h. The photographs of the dispersion of
unmodified APP and KAPP in the organic phase (liquid paraffin) are shown in Figure 2c,d,
respectively. The unmodified APP did not appear to disperse, settling in the liquid paraffin
within 10 min, whereas KAPP dispersed better in the liquid paraffin and gradually dis-
solved to from turbid solution after 24 h. These results can indicate that the modified APP
changes from hydrophilic to hydrophobic due to the access of organic groups, the solvent
degree and dispersion in organic solvents improved and the modification effect was better.
Coatings 2022, 12, x FOR PEER REVIEW 6 of 19
3.2. Effect of 3-Aminopropyltriethoxysilane on Properties of Ammonium Polyphosphate
3.2.1. Effect of 3-Aminopropyltriethoxysilane on Dispersibility of Ammonium Polyphos-
phate in Solvents
Figure 2 presents photographs of APP and KAPP powders placed in different sol-
vents (distilled water, paraffin) for different durations. Figure 2a, b shows the hydrophilic-
ity of unmodified APP and hydrophobicity of KAPP. The unmodified APP was dispersed
in water and the distilled water became more turbid after 24 h and the precipitation oc-
curred quickly after addition. In contrast, KAPP added to distilled water was partially
dissolved and settled completely to the bottom after 24 h. The photographs of the disper-
sion of unmodified APP and KAPP in the organic phase (liquid paraffin) are shown in
Figure 2c,d, respectively. The unmodified APP did not appear to disperse, settling in the
liquid paraffin within 10 min, whereas KAPP dispersed better in the liquid paraffin and
gradually dissolved to from turbid solution after 24 h. These results can indicate that the
modified APP changes from hydrophilic to hydrophobic due to the access of organic
groups, the solvent degree and dispersion in organic solvents improved and the modifi-
cation effect was better.
Figure 2. shows photographs of APP and KAPP powders placed in different solvents (distilled
water, paraffin) for different durations. (a) APP with KAPP 10 min in water;(b) APP with KAPP 24
h in water; (c) APP with KAPP 10 min in paraffin; (d) APP with KAPP 24 h in paraffin.
3.2.2. Effect of 3-Aminopropyltriethoxysilane on Thermal Stability of Ammonium Poly-
phosphate
As shown in Figure 3, the thermal decomposition of APP before and after modifica-
tion is divided into two main stages. The first stage from 250 to 400 °C is divided into two
parts, corresponding to the release of NH3 from APP to generate polyphosphoric acid and
further H2O crosslinking to produce metaphosphoric acid, pyrophosphoric acid and P2O5
crosslinking products. This result is based on from the changes in the initial decomposi-
tion temperature (T5%) of APP before and after modification (see Table 6). The initial de-
composition temperature of unmodified APP was 297 °C and the initial decomposition
temperature of modified APP was 276 °C, slightly ahead of schedule. This change was
probably due to the introduction of the coupling agent in the initial stage, which promoted
the early decomposition of APP. The second stage occurs at 450–700 °C and was domi-
nated by the continued decomposition of metaphosphoric acid and P2O5 from the first
stage of dehydration crosslinking. At this stage, the maximum weight loss rate of the un-
modified APP and modified APP were 5.49%·min−1 and 1.61%·min−1, respectively, and the
temperature at the maximum weight loss rate were 568 °C and 601 °C, respectively. The
Figure 2.
Shows photographs of APP and KAPP powders placed in different solvents (distilled water,
paraffin) for different durations. (
a
) APP with KAPP 10 min in water;(
b
) APP with KAPP 24 h in
water; (c) APP with KAPP 10 min in paraffin; (d) APP with KAPP 24 h in paraffin.
Coatings 2022,12, 1738 6 of 17
3.2.2. Effect of 3-Aminopropyltriethoxysilane on Thermal Stability of
Ammonium Polyphosphate
As shown in Figure 3, the thermal decomposition of APP before and after modification
is divided into two main stages. The first stage from 250 to 400
◦
C is divided into two
parts, corresponding to the release of NH
3
from APP to generate polyphosphoric acid and
further H
2
O crosslinking to produce metaphosphoric acid, pyrophosphoric acid and P
2
O
5
crosslinking products. This result is based on from the changes in the initial decompo-
sition temperature (T5%) of APP before and after modification (see Table 6). The initial
decomposition temperature of unmodified APP was 297
◦
C and the initial decomposition
temperature of modified APP was 276
◦
C, slightly ahead of schedule. This change was
probably due to the introduction of the coupling agent in the initial stage, which promoted
the early decomposition of APP. The second stage occurs at 450–700
◦
C and was dominated
by the continued decomposition of metaphosphoric acid and P
2
O
5
from the first stage of
dehydration crosslinking. At this stage, the maximum weight loss rate of the unmodified
APP and modified APP were 5.49%
·
min
−1
and 1.61%
·
min
−1
, respectively, and the tempera-
ture at the maximum weight loss rate were 568
◦
C and 601
◦
C, respectively. The maximum
weight loss rate of the modified APP was significantly lower when compared with that of
the unmodified APP and the residual at 700
◦
C (49.2%) was much higher than that of the
unmodified APP (37.6%). Hence, the use of KH-550 modified APP significantly enhanced
the thermal stability of the product at higher temperatures.
Coatings 2022, 12, x FOR PEER REVIEW 7 of 19
maximum weight loss rate of the modified APP was significantly lower when compared
with that of the unmodified APP and the residual at 700 °C (49.2%) was much higher than
that of the unmodified APP (37.6%). Hence, the use of KH-550 modified APP significantly
enhanced the thermal stability of the product at higher temperatures.
Table 6. TGA data of APP and KAPP.
Samples T5%
/
°C Tmax
/
°C Rmax
/
%·min−1 Residiue
/
°C
APP 296 568 −5.49 37.6
KAPP 276 601 −1.61 49.2
100 200 300 400 500 600 700
20
30
40
50
60
70
80
90
100
Weight/%
Temperature/℃
APP
KAPP
（a）
100 200 300 400 500 600 700
-6
-5
-4
-3
-2
-1
0
DTG/%·min
-1
Temperature（℃）
APP
KAPP
（b）
Figure 3. (a) TG and (b) DTG curves for APP and KAPP.
3.2.3. Effect of 3-Aminopropyltriethoxysilane on Microscopic Morphology of Ammo-
nium Polyphosphate
The Scanning electron microscopy (SEM) images of unmodified APP and modified
KAPP are presented in Figure 4. As per Figure 4a,c, the aggregation of unmodified APP
particles was more prominent. The aggregation of modified KAPP was improved and
dispersion became better. The microscopic morphology of the unmodified APP and the
modified KAPP are shown in Figure 4b,d. The unmodified APP was rough and has many
tiny attachments, which indicates that the unmodified APP is more hygroscopic, and the
powder particles aggregated with each other, resulting in poor dispersion. After KH-550
modification, the surface of KAPP had few particles but is generally smooth and flat. This
result suggests that the morphology of KAPP was significantly changed after the
Figure 3. (a) TG and (b) DTG curves for APP and KAPP.
Coatings 2022,12, 1738 7 of 17
Table 6. TGA data of APP and KAPP.
Samples T5%/◦C Tmax /◦C Rmax/%·min−1Residiue/◦C
APP 296 568 −5.49 37.6
KAPP 276 601 −1.61 49.2
3.2.3. Effect of 3-Aminopropyltriethoxysilane on Microscopic Morphology of
Ammonium Polyphosphate
The Scanning electron microscopy (SEM) images of unmodified APP and modified
KAPP are presented in Figure 4. As per Figure 4a,c, the aggregation of unmodified APP
particles was more prominent. The aggregation of modified KAPP was improved and
dispersion became better. The microscopic morphology of the unmodified APP and the
modified KAPP are shown in Figure 4b,d. The unmodified APP was rough and has many
tiny attachments, which indicates that the unmodified APP is more hygroscopic, and the
powder particles aggregated with each other, resulting in poor dispersion. After KH-550
modification, the surface of KAPP had few particles but is generally smooth and flat.
This result suggests that the morphology of KAPP was significantly changed after the
modification with KH-550. These structures may endow KAPP new properties, which can
further promote the flame-retardant effect.
Coatings 2022, 12, x FOR PEER REVIEW 8 of 19
modification with KH-550. These structures may endow KAPP new properties, which can
further promote the flame-retardant effect.
Figure 4. Scanning electron micrograph of APP and KAPP. (a)APP magnification ×200; (b)APP
magnification ×800; (c)KAPP magnification ×200; (d)KAPP magnification ×800.
3.3. Modification Mechanism Analysish
3.3.1. Infrared Analysis
For comparison and analysis of Figure 5, the peak at 880 cm
−1
was assigned to P = 0
telescopic vibration peak, 1014 cm
−1
to P-O telescopic vibration peak, 1435 cm
−1
to N-H
bending vibration peak, 1700 cm
−1
to C = 0 telescopic vibration peak and 3212 cm
−1
for the
N-H telescopic vibration peak, and the above peaks are consistent with the characteristics
of Type I-APP in Tables 2 and 3. The above peaks coincide with the characteristic peaks
of type I-APP in Tables 2 and 3. The symmetrical stretching vibration of the Si-O bond at
a wavelength of 800 cm
−1
suggests that KH-550 was successfully grafted on the surface of
APP, further demonstrating that although the silane coupling agent did not change the
crystal structure of APP, the functional groups of APP were changed and the surface mod-
ification was achieved.
Figure 4.
Scanning electron micrograph of APP and KAPP. (
a
) APP magnification
×
200; (
b
) APP
magnification ×800; (c) KAPP magnification ×200; (d) KAPP magnification ×800.
3.3. Modification Mechanism Analysish
3.3.1. Infrared Analysis
For comparison and analysis of Figure 5, the peak at 880 cm
−1
was assigned to P = 0
telescopic vibration peak, 1014 cm
−1
to P-O telescopic vibration peak, 1435 cm
−1
to N-H
bending vibration peak, 1700 cm
−1
to C = 0 telescopic vibration peak and 3212 cm
−1
for the
N-H telescopic vibration peak, and the above peaks are consistent with the characteristics
of Type I-APP in Tables 2and 3. The above peaks coincide with the characteristic peaks
of type I-APP in Tables 2and 3. The symmetrical stretching vibration of the Si-O bond at
a wavelength of 800 cm
−1
suggests that KH-550 was successfully grafted on the surface
of APP, further demonstrating that although the silane coupling agent did not change
Coatings 2022,12, 1738 8 of 17
the crystal structure of APP, the functional groups of APP were changed and the surface
modification was achieved.
Coatings 2022, 12, x FOR PEER REVIEW 9 of 19
4000 3500 3000 2500 2000 1500 1000 500
Transmitance%
Wavenumber/cm
-1
APP
KAPP
1700
3212 1435 880
1014
800
Figure 5. Infrared spectra of APP and KAPP.
3.3.2. XRD Analysis
As shown in Figure 6, the positions of diffraction peaks on several crystalline surfaces
of APP were not changed, and no new diffraction peaks have appeared in the modified
MH. This result indicates that the modified MH has the same internal crystal structure as
the unmodified APP, and the coupling agent only acted on the surface of APP. However,
the diffraction peak of the modified APP is higher than that of the pre-modified APP,
which signifies that the crystallinity of the APP was higher, and the regularity was better
after the surface modification. In summary, the coupling agent modified APP, the crystal
structure of APP was not destroyed.
X-ray Photoemission Spectroscopy (XPS) was employed to analyze the elemental
changes on the surface of the powders before and after modification and the binding states
of the elements to investigate the modification mechanism. The full XPS spectra of APP
and KAPP are provided in Figure 7a. The main binding energies in the XPS spectra of APP
are P2P at 135.14 eV, P2S at 192.21 eV, C1S at 284.8 eV, N1S at 401.95 eV and O1S at 532.72 eV.
For KAPP, the presence of Si2P and Si2S binding energy at 103.88 eV and 153.6 eV, respec-
tively, and the detection of Si elements in KAPP are evidence of the successful modifica-
tion of APP surface with KH-550.
Also, as per Figure 7b, the binding energy of P2p in the modified KAPP was lower
than that of P2P in the unmodified APP. This result indicates that a chemical reaction oc-
curred between APP and KH-550 during the modification process and that chemical bind-
ing state of the surface P elements had changed. The inference is that the P-O-N portion
of the powder particle surface changed to P-O-Si, and as Si element has a lower electro-
negativity, the binding energy of P-O-Si is lower than that of P-O-N, This phenomenon
resulted in, a change in the binding energy of P2P in KAPP. To verify this inference, a split-
peak fitting process for the P elements in APP and KAPP was performed, and the results
from the fitting are presented in (Figure 8)
Figure 5. Infrared spectra of APP and KAPP.
3.3.2. XRD Analysis
As shown in Figure 6, the positions of diffraction peaks on several crystalline surfaces
of APP were not changed, and no new diffraction peaks have appeared in the modified
MH. This result indicates that the modified MH has the same internal crystal structure as
the unmodified APP, and the coupling agent only acted on the surface of APP. However,
the diffraction peak of the modified APP is higher than that of the pre-modified APP, which
signifies that the crystallinity of the APP was higher, and the regularity was better after the
surface modification. In summary, the coupling agent modified APP, the crystal structure
of APP was not destroyed.
Coatings 2022, 12, x FOR PEER REVIEW 10 of 19
10 20 30 40 50 60
Intensity
(
a.u.
)
2θ/(°)
APP
KAPP
14.84°
16.25°
23.21°
25.53°
27.55°
39.23°
Figure 6. XRD pattern of APP and KAPP.
Figure 7. XPS profiles of APP and KAPP all elements (a) and P 2p (b).
0 200 400 600 800 1000 1200 1400
Intensity(a.u.)
Binding Energy (eV)
Si
2p
Si
2s
P
2p
P
2s
C
1s
O
1s
N
1s
KAPP
APP
(a)
124 126 128 130 132 134 136 138 140
0
2000
4000
6000
Intensity(a.u.)
Binding Energy (eV)
APP
KAPP
（b）
Figure 6. XRD pattern of APP and KAPP.
X-ray Photoemission Spectroscopy (XPS) was employed to analyze the elemental
changes on the surface of the powders before and after modification and the binding states
of the elements to investigate the modification mechanism. The full XPS spectra of APP and
KAPP are provided in Figure 7a. The main binding energies in the XPS spectra of APP are
Coatings 2022,12, 1738 9 of 17
P
2P
at 135.14 eV, P
2S
at 192.21 eV, C
1S
at 284.8 eV, N
1S
at 401.95 eV and O
1S
at 532.72 eV. For
KAPP, the presence of Si
2P
and Si
2S
binding energy at 103.88 eV and 153.6 eV, respectively,
and the detection of Si elements in KAPP are evidence of the successful modification of
APP surface with KH-550.
Coatings 2022, 12, x FOR PEER REVIEW 10 of 19
10 20 30 40 50 60
Intensity
(
a.u.
)
2θ/(°)
APP
KAPP
14.84°
16.25°
23.21°
25.53°
27.55°
39.23°
Figure 6. XRD pattern of APP and KAPP.
Figure 7. XPS profiles of APP and KAPP all elements (a) and P 2p (b).
0 200 400 600 800 1000 1200 1400
Intensity(a.u.)
Binding Energy (eV)
Si
2p
Si
2s
P
2p
P
2s
C
1s
O
1s
N
1s
KAPP
APP
(a)
124 126 128 130 132 134 136 138 140
0
2000
4000
6000
Intensity(a.u.)
Binding Energy (eV)
APP
KAPP
（b）
Figure 7. XPS profiles of APP and KAPP all elements (a) and P 2p (b).
Also, as per Figure 7b, the binding energy of P
2p
in the modified KAPP was lower than
that of P
2P
in the unmodified APP. This result indicates that a chemical reaction occurred
between APP and KH-550 during the modification process and that chemical binding state
of the surface P elements had changed. The inference is that the P-O-N portion of the
powder particle surface changed to P-O-Si, and as Si element has a lower electronegativity,
the binding energy of P-O-Si is lower than that of P-O-N, This phenomenon resulted in, a
change in the binding energy of P
2P
in KAPP. To verify this inference, a split-peak fitting
process for the P elements in APP and KAPP was performed, and the results from the
fitting are presented in (Figure 8)
Coatings 2022,12, 1738 10 of 17
Coatings 2022, 12, x FOR PEER REVIEW 11 of 19
Figure 8. P
2p
binding energy of APP (a) and KAPP (b).
3.3.3. XPS Analysis
The variation in elemental content on the surface of APP and KAPP is provided in
Table 7. KAPP and APP had elemental content of N and P 8.04% and 10.46%, while P as
7.21% and 10.2%, respectively. This is due to the presence of the cladding layer, which
slightly reduced the elemental content of N and P on the surface of KAPP. Also, the N/P
ratio of KAPP (N/P = 0.98) is lower than that of APP (N/P = 1.16), with a decrease in the
relative content of N elements, This result also proves that the P-O-N part of the particle
surface changed to P-O-Si after the modification treatment.
Based on the results of FTIR and XPS analysis, a relevant modification mechanism is
proposed, As shown in Figure 9.
Figure 8. P2p binding energy of APP (a) and KAPP (b).
3.3.3. XPS Analysis
The variation in elemental content on the surface of APP and KAPP is provided in
Table 7. KAPP and APP had elemental content of N and P 8.04% and 10.46%, while P as
7.21% and 10.2%, respectively. This is due to the presence of the cladding layer, which
slightly reduced the elemental content of N and P on the surface of KAPP. Also, the N/P
ratio of KAPP (N/P = 0.98) is lower than that of APP (N/P = 1.16), with a decrease in the
relative content of N elements, This result also proves that the P-O-N part of the particle
surface changed to P-O-Si after the modification treatment.
Table 7. Comparison of APP and KAPP surface elements.
Projects O (wt%) C (wt%) N (wt%) P (wt%) Si (wt%)
APP 20.29 ±1.5 51.05 ±1.5 10.46 ±0.4 10.2 ±0.2 -
KAPP 21.05 ±1.5 49.72 ±2.0 8.04 ±0.5 8.21 ±0.4 7.98 ±0.1
Based on the results of FTIR and XPS analysis, a relevant modification mechanism is
proposed, As shown in Figure 9.
Coatings 2022,12, 1738 11 of 17
Coatings 2022, 12, x FOR PEER REVIEW 12 of 19
Figure 9. KH550 modified APP reaction mechanism diagram.
Table 7. Comparison of APP and KAPP surface elements.
Projects O (wt%) C (wt%) N (wt%) P (wt%) Si (wt%)
APP 20.29 ± 1.5 51.05 ± 1.5 10.46 ± 0.4 10.2 ± 0.2 -
KAPP 21.05 ± 1.5 49.72 ± 2.0 8.04 ± 0.5 8.21 ± 0.4 7.98 ± 0.1
3.4. Application of 3-Aminopropyltriethoxysilane Modified Ammonium Polyphosphate in Flame
Retardant PP
3.4.1. Analysis of Bending Properties of Flame Retardant PP
In general, the addition of additive flame retardants gradually decreases the mechan-
ical properties of composites The surface modification of coupling agent can be used to
improve the compatibility with the polymer matrix and, in the process, enhance the me-
chanical properties. As per Figure 10, with the rise in flame retardant (APP) addition, the
bending strength of flame-retardant PP composites was significantly decreased. Also, the
elastic modulus displayed the trend of first increasing and then decreasing. After the sur-
face modification of APP by KH-550, the mechanical properties of flame-retardant PP
composites were improved. Specifically, at 30% flame retardant addition, the bending
strength of flame-retardant PP composites increases 48.3 from 46.8 The above results show
that KH-550 effectively enhanced the compatibility between APP and PP matrix along
with an increases in the intermolecular interaction force, which led to the improvement of
mechanical properties of flame-retardant PP composites.
Figure 9. KH550 modified APP reaction mechanism diagram.
3.4. Application of 3-Aminopropyltriethoxysilane Modified Ammonium Polyphosphate in Flame
Retardant PP
3.4.1. Analysis of Bending Properties of Flame Retardant PP
In general, the addition of additive flame retardants gradually decreases the mechan-
ical properties of composites The surface modification of coupling agent can be used to
improve the compatibility with the polymer matrix and, in the process, enhance the me-
chanical properties. As per Figure 10, with the rise in flame retardant (APP) addition,
the bending strength of flame-retardant PP composites was significantly decreased. Also,
the elastic modulus displayed the trend of first increasing and then decreasing. After the
surface modification of APP by KH-550, the mechanical properties of flame-retardant PP
composites were improved. Specifically, at 30% flame retardant addition, the bending
strength of flame-retardant PP composites increases 48.3 from 46.8 The above results show
that KH-550 effectively enhanced the compatibility between APP and PP matrix along
with an increases in the intermolecular interaction force, which led to the improvement of
mechanical properties of flame-retardant PP composites.
Coatings 2022, 12, x FOR PEER REVIEW 13 of 19
Figure 10. Mechanical properties of flame-retardant PP composite.
3.4.2. Microscopic Morphology of Flame Retardant PP
To analyze the mechanism behind the enhancement of bending properties of the
composites after modification, SEM analysis was performed on the bending sections of
the composites. Figure 11 displays the bending section morphology of the three flame re-
tardant PP composites. The powder in 20% APP/PP composites was poorly dispersed and
consisted of multiple prominent holes and gaps of different sizes at the interface. These
can easily fracture when the material is subjected to concentrated stress; thus, causing a
reduction in the mechanical properties of the composites. These results indicate that the
unmodified APP powder is not compatible with the PP matrix, When the material is sub-
jected to stress, the powder particles are dislodged; thus, affecting its dispersion in the PP
matrix. On the other hand, for the modified KAPP added to the PP matrix (20 KAPP/PP),
the powder particles become better dispersed, without prominent holes and gaps, which
also corresponds to the mechanical properties exhibited by the flame-retardant PP.
PP
10%APP/PP
20%APP/PP
30%APP/PP
10%KAPP/PP
20K%APP/PP
30%KAPP/PP
46
48
50
52
54
56
58
60
Tensile strength
Elastic Modulus
Flexural strength/MPa
2500
2750
3000
3250
3500
3750
4000
Flexural Modulus/MPa
Figure 10. Mechanical properties of flame-retardant PP composite.
Coatings 2022,12, 1738 12 of 17
3.4.2. Microscopic Morphology of Flame Retardant PP
To analyze the mechanism behind the enhancement of bending properties of the
composites after modification, SEM analysis was performed on the bending sections of
the composites. Figure 11 displays the bending section morphology of the three flame
retardant PP composites. The powder in 20% APP/PP composites was poorly dispersed
and consisted of multiple prominent holes and gaps of different sizes at the interface.
These can easily fracture when the material is subjected to concentrated stress; thus, caus-
ing a reduction in the mechanical properties of the composites. These results indicate
that the unmodified APP powder is not compatible with the PP matrix, When the ma-
terial is subjected to stress, the powder particles are dislodged; thus, affecting its dis-
persion in the PP matrix. On the other hand, for the modified KAPP added to the PP
matrix (20 KAPP/PP), the powder particles become better dispersed, without prominent
holes and gaps, which also corresponds to the mechanical properties exhibited by the
flame-retardant PP.
Coatings 2022, 12, x FOR PEER REVIEW 14 of 19
Figure 11. SEM images of bending section of flame-retardant PP composite. PP: (a) magnification.
500×; (b) magnification 1000×. 20%APP/PP: (c) magnification 500×; (d) magnification 1000×. 20%
KAPP/PP: (e) magnification 500×; (f) magnification1000×.
3.4.3. Thermal Stability
As can be seen by Figure 12 TGA (a) DTG (b), the effect of ammonium polyphosphate
on the thermal stability of flame-retardant PP before and after modification was investi-
gated. Thermogravimetric analysis of flame-retardant PP prepared from APP and differ-
ent proportions of KAPP was conducted under a nitrogen atmosphere, as listed in Table
8. The overall flame-retardant PP composites had a T5% lower than that of PP at 387 °C.
This is due to the possible catalytic degradation of the main chain of PP by APP and KAPP,
which accelerated the dehydration of PP into carbon and further promoted the flame-re-
tardant effect. The addition of APP and KAPP had little impact on the maximum weight
loss rate. However, the maximum weight loss rate of its flame-retardant PP composites
was significantly reduced, and the R max decreased gradually with the rise in KAPP ratio.
By comparing 10% APP/PP with 10% KAPP/PP, the initial decomposition temperature
decreased from 383 °C to 379 °C and the high temperature carbon residue of 10%
KAPP/PP increased, from 10.69% to 12.69%. This is because, as compared with APP,
KAPP promoted more carbon formation in the cohesive phase of the flame-retardant PP.
As per DSC results presented in Figure 13a, all the samples displayed a heat absorp-
tion peak at around 165 °C, which corresponds to the molten state of PP, indicating that
the addition of APP and KAPP did not affect the melting temperature of PP and can facil-
itate the control of the processing temperature of the product. The thermal degradation
temperature of the flame-retardant PP composite is presented in Figure 13b, which is con-
sistent with the results obtained by DTG. As the glass conversion temperature of the pol-
ypropylene matrix is −10 °C, it is not reflected in the figure.
Figure 11.
SEM images of bending section of flame-retardant PP composite. PP: (
a
) magnification.
500
×
; (
b
) magnification 1000
×
. 20%APP/PP: (
c
) magnification 500
×
; (
d
) magnification 1000
×
. 20%
KAPP/PP: (e) magnification 500×; (f) magnification1000×.
3.4.3. Thermal Stability
As can be seen by Figure 12 TGA (a) DTG (b), the effect of ammonium polyphosphate
on the thermal stability of flame-retardant PP before and after modification was investi-
gated. Thermogravimetric analysis of flame-retardant PP prepared from APP and different
proportions of KAPP was conducted under a nitrogen atmosphere, as listed in Table 8. The
overall flame-retardant PP composites had a T5% lower than that of PP at 387
◦
C. This is
due to the possible catalytic degradation of the main chain of PP by APP and KAPP, which
accelerated the dehydration of PP into carbon and further promoted the flame-retardant
effect. The addition of APP and KAPP had little impact on the maximum weight loss
rate. However, the maximum weight loss rate of its flame-retardant PP composites was
significantly reduced, and the R max decreased gradually with the rise in KAPP ratio.
Coatings 2022,12, 1738 13 of 17
By comparing 10% APP/PP with 10% KAPP/PP, the initial decomposition temperature
decreased from 383
◦
C to 379
◦
C and the high temperature carbon residue of 10% KAPP/PP
increased, from 10.69% to 12.69%. This is because, as compared with APP, KAPP promoted
more carbon formation in the cohesive phase of the flame-retardant PP.
Coatings 2022, 12, x FOR PEER REVIEW 16 of 20
Table 8. Thermal weight loss data for flame retardant PP under nitrogen atmosphere.
Samples
T5%/°C
Tmax/°C
Rmax/%·min−1
Residiue/°C
PP
387
452
−22.1
4.08
10% APP/PP
383
461
−18.66
10.69
10% KAPP/PP
379
463
−18.33
12.69
20% KAPP/PP
412
468
−18.76
19.69
30% KAPP/PP
314
469
−18.21
17.72
100 200 300 400 500 600 700
0
20
40
60
80
100
Weight/%
Temperature/℃
PP
10%APP/PP
10%KAPP/PP
20%KAPP/PP
30%KAPP/PP
（a）
100 200 300 400 500 600 700
-25
-20
-15
-10
-5
0
5
DTG/%·min-1
Temperature/℃
PP
10%APP/PP
10%KAPP/PP
20%KAPP/PP
30%KAPP/PP
（b）
Figure 12. TGA (a) and DTG (b) curves for flame retardant PP.
Figure 12. TGA (a) and DTG (b) curves for flame retardant PP.
Table 8. Thermal weight loss data for flame retardant PP under nitrogen atmosphere.
Samples T5%/◦C Tmax /◦C Rmax/%·min−1Residiue/◦C
PP 387 452 −22.1 4.08
10% APP/PP 383 461 −18.66 10.69
10% KAPP/PP 379 463 −18.33 12.69
20% KAPP/PP 412 468 −18.76 19.69
30% KAPP/PP 314 469 −18.21 17.72
As per DSC results presented in Figure 13a, all the samples displayed a heat absorption
peak at around 165
◦
C, which corresponds to the molten state of PP, indicating that the
addition of APP and KAPP did not affect the melting temperature of PP and can facilitate the
control of the processing temperature of the product. The thermal degradation temperature
of the flame-retardant PP composite is presented in Figure 13b, which is consistent with the
Coatings 2022,12, 1738 14 of 17
results obtained by DTG. As the glass conversion temperature of the polypropylene matrix
is −10 ◦C, it is not reflected in the figure.
Coatings 2022, 12, x FOR PEER REVIEW 16 of 19
100 120 140 160 180 200 220
Heat flow/mW
Exothermic→
Temperature/℃
PP
10%APP/PP
10KAPP/PP
20%KAPP/PP
30%KAPP/PP
166.7℃
164.4
℃
166.5
℃
162.9
℃
165.2
℃
（
a
）
Figure 13. DSC curves for flame retardant PP. (a) DSC temperature range 100–220 °C; (b) DSC
temperature range 350–550 °C.
3.4.4. Flame Retardant Properties
To further test the flame resistance of the samples, the oxygen index (LOL) of the
flame-retardant PP was tested against UL-94 vertical combustion and the relevant results
are listed in Table 9. These results further indicate that KAPP is more efficient in improv-
ing the fire resistance of PP.
350 400 450 500 550
Heat flow/mW
Exothermic→
Temperature/℃
PP
10%APP/PP
10%KAPP/PP
20%KAPP/PP
30%KAPP/PP
452℃
461℃
463℃
468℃
469℃
(b)
Figure 13.
DSC curves for flame retardant PP. (
a
) DSC temperature range 100–220
◦
C; (
b
) DSC
temperature range 350–550 ◦C.
3.4.4. Flame Retardant Properties
To further test the flame resistance of the samples, the oxygen index (LOL) of the
flame-retardant PP was tested against UL-94 vertical combustion and the relevant results
are listed in Table 9. These results further indicate that KAPP is more efficient in improving
the fire resistance of PP.
Coatings 2022,12, 1738 15 of 17
Table 9. Oxygen index and vertical combustion test results for flame retardant PP.
Samples LOL/% UL-94 Dripping Cotton Ignited
PP 18.6 NR Yes Yes
10% APP/PP 20.2 NR Yes Yes
20% APP/PP 22 NR No Yes
30% APP/PP 24.4 V-2 No Yes
10% APP/PP 24.2 V-2 Yes Yes
20% KAPP/PP 27.6 V-0 No No
30% KAPP/PP 29.1 V-0 No No
4. Conclusions
Herein, the optimum modification process of coupling agent (KH-550, 10%, 2 h)
was screened, and the surface modification treatment of APP was conducted using the
optimum modification process. The water solubility, dispersibility and thermal stability of
the modified KAPP were improved, and the maximum weight loss rate of the modified
KAPP was significantly lower compared with that of APP under nitrogen atmosphere, and
the residual amount at 700
◦
C (49.2%) was much higher than that of the unmodified APP
(37.6%). The addition of KAPP significantly improved the uniform distribution of the flame
retardant in the PP matrix and enhanced its bending properties for the same amount of
flame-retardant addition. SEM analysis suggests that the surface of KAPP became smooth
and flat; the dispersion was better and compatibility with the PP matrix was improved,
and there were no obvious voids and gaps in the cross section. The proposed process
may provide a new and facile way to simultaneously improve the flame retardant and
mechanical properties of PP.
Author Contributions:
Conceptualization, L.M. (Lingyu Meng) and M.L.; methodology, L.M. (Lipeng
Meng); software, L.M. (Lingyu Meng); validation, L.M. (Lingyu Meng) and X.L.; formal analysis,
C.L.; investigation, L.M. (Lingyu Meng); resources, L.M. (Lingyu Meng) and M.L.; data curation,
X.L.; writing—original draft preparation, L.M. (Lingyu Meng); writing—review and editing, M.L.;
visualization, C.L. and S.H.; supervision, L.M. (Lipeng Meng); project administration, M.L.; L.M.
(Lipeng Meng) and C.L.; funding acquisition, M.L. All authors have read and agreed to the published
version of the manuscript.
Funding:
This research was funded by the Funding for Capital Construction in the Budget of Jilin
Province (innovation capacity construction) (Granted No. 2022C039-4), the Funding for Capital Con-
struction in the Budget of Jilin Province (innovation capacity construction) (Granted No. 2021C036-8),
Technology Development Innovation Platform (Base) and Talent Project: 20220508119RC, Wood
Material Science and Engineering Key Laboratory of Jilin Province, Beihua University, 132013 P,R
China, Beihua University Postgraduate Innovation Plan Project (Beihua Yanchuanghe Zi [2022] 009).
Jilin Forest Processing Industry Public Technology Research and Development Center.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data is contained within the article.
Acknowledgments:
The authors are grateful to the capital construction funds within the budget of
Jilin Province for providing financial support for the work of this project and to Beihang University
University for providing financial support and experimental equipment for this project.
Conflicts of Interest: The authors declare no conflict of interest.
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