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POLYMER MATERIALS IN THE FOOD INDUSTRY
POLIMERNI MATERIJALI U PREHRAMBENOJ INDUSTRIJI
Katana, Branko; Buric, Mladen
Abstract: Owing to the continuous development of the food industry, machines with
increased production efficiency are constantly being designed, whereby strict hygiene
standards have to be respected. During the process of designing machines for the
food industry, there is an ever increasing need for the use of new materials exhibiting
very good mechanical and tribological properties, such as polymer materials. The
subject of this paper is to examine the working life of a wheel assembly made of
polyethylene terephthalate (PET) material and to compare it with other materials
used for this application, such as polyamide 6 (PA 6) and polyamide 6C (PA 6C).
Key words: Tribology, wear, polyamide, polyethylene terephthalate
Sažetak: Neprestanim razvojem prehrambene industrije kontinuirano se razvijaju
strojevi sve veće proizvodne efikasnosti pri strogim higijenskim standardima.
Tijekom procesa konstruiranja strojeva za prehrambenu industriju pojavljuje se sve
veća potreba za korištenjem novih materijal sa vrlo dobrim mehaničko tribološkim
svojstvima kao što su polimerni materijali. Predmet ovog rada je ispitivanje radnog
vijeka sklopa kotača od polietilen tereftalat (PET) materijala u odnosu na često
korištene materijale za tu primjenu kao što su poliamid 6 (PA 6), poliamid 6C (PA
6C).
Ključne riječi:
Tribologija, trošenje, poliamid, polietilen tereftalat
Authors' data:
Branko, KATANA, PhD Student, University of Zagreb, Faculty of Mechanical
Engineering and Naval Architecture, branko.katana2@gmail.com
Mladen, BURIC, PhD Student, University of Zagreb, Faculty of Mechanical
Engineering and Naval Architecture, mladen.buric@yahoo.com
1. Introduction
New machines used in the food industry have been developed with the aim to
improve production efficiency with the least possible losses, abiding by the strict
hygiene standards, and to increase the life cycle of machines with long service
intervals, including the least possible production interruption caused by various
failures that can result in major financial losses in the production process.
Polymer materials used in the food industry must be covered by adequate regulations
in place such as EU Regulation No. 10/2011 and FDA (Food and Drug
Administration) regulations to avoid contamination of the product or manufactured
article that comes in contact with the polymer. Polymer materials in the food industry
do not contain additives that can contaminate the product; hence they don't pose a
threat to human health when consumed.
Stainless steels type AISI 304 and AISI 316 usually come in contact with polymer
materials. Disadvantage of steel materials is the abrasive effect on polymer materials
which leads to excessive polymer wear. This problem has been tackled and solved by
the adding glass fibers to polymer material which consequently improves the
tribological and mechanical properties of polymers. However, polymer with glass
fibers can have an abrasive effect to stainless steel as it can be seen in Figure 1.
Figure1.
Abrasive effect of the slide bearing made of PA6 + 25% GF to the sleeve
made of AISI 316
To optimize the wear of the polymer - stainless steel system, whether it is a linear,
circular or spiral movement, there is a need to improve the mechanical and
tribological properties of polymer materials.
Another problem regarding the use of machines in the food industry is their frequent
cleaning with hot water or steam containing various acids and alkalies. The most
common methods of cleaning comprise CIP (Cleaning in Place) and SIP (Sterilisation
in Place) which can cause damage to polymer materials in terms of swelling,
cracking, dimensional changes etc.
The subject of this research paper is to analyse the typical polymer damage and
measures for their prevention. Polyethylene terephthalate (PET) is a very good
material exhibiting a variety of applications in the food industry due to its fine
tribological properties, good thermal stability, small changes in mechanical properties
and resistance to various chemicals. All this makes PET a very good choice for
manufacturing slide bearings, wheels and other parts of machines used in the food
industry.
2. Review of previous research papers
Polymer materials such as PET fall under a group of nonlinear elastic materials ie.
load-unload displacement curves in the stress – strain diagram is not a polynomial of
the first degree, which in other words means that the property of materials depends on
the time and temperature; hence empirical relation stating that the allowable
maximum load of the slide bearing equals one third of the maximum pressure loading
cannot be applied [1] [2] [3] [4]. To obtain accurate values of the predicted service
life of polymer bearing materials, it is necessary to carry out an experimental research
in order to compare the results obtained by experimental measurement with the
values obtained by calculation or FEM analysis [3] [5] [6] [7].
Polymer materials used for slide bearings should exhibit some of the following
features [6] [8] [9] [10]:
1) Good sliding properties of materials, low friction and material wear
2) High chemical resistance
3) Good thermal and dimensional stability
4) Small changes in mechanical strength depending on the working temperature
5) Little absorption of the working medium or other medium that comes in
contact with the slide bearing
Wear in polymer materials occurs due to the following reasons:
1) Given that the material is not compatible with the working fluid, a change
occurs in the structure of the polymer material which consequently can lead to
changes in mechanical properties
2) Polymer material can absorb the working fluid causing it to swell up which can
cause bearing damage
3) As a result of contact between the sleeve and bearing, heat might develop in
the area of contact which can lead to bearing damage
4) Inadequate surface roughness of the sleeve might lead to excessive wear of the
polymer slide bearing
3. Theoretical principles and test results
In order to test if we can use a polymer material for a machine, it is necessary to
calculate the pressure between the surface and the wheel i.e. inside the slide bearing.
Equations (1) – (5) have been adapted to calculate the wheel contact pressure
according to the Hertz theory of contact as can be seen in Figure 2 [1] [11] [12].
Figure 2. Pressure between the surface and wheel
3
1
3
e
e
a S F r E
= ⋅ ⋅ ⋅ ⋅
¸ (1)
3
1
3
e
e
b I F r E
= ⋅ ⋅ ⋅ ⋅
(2)
11 12
11 12
e
r r
rr r
⋅
=+
(3)
1 2
1 2
1 1
1 1
2
e
E E E
ν ν
− −
= −
(4)
3
2
F
pa b
π
⋅
=⋅ ⋅ ⋅
(5)
s
F
pB D
=⋅
(6)
In relation to the formula (1) and (2), correction factors are S = 1.486 and I=0.717 and
equivalent wheel radius is r
e
= 16.875mm, depending on the wheel radius r
11
=22.5
mm and roundness of the wheel r
12
=67.5 mm [11] [12]. Permissible pressure must be
reduced by at least 15% of the p
dop
allowable surface pressure for a given material.
Calculation of the pressure between the surface made of AISI 316 (ν=0.25, E=193
MPa) and the wheel can be seen in Table 1.
Material
Poisson
coefficient,
ν
Elasticity
modulus,
E /MPa
Equivalent
elasticity
modulus,
E
e
/MPa
Contact
half
width
a /mm
Contact
half
width
b /mm
Hertzian
surface
pressure
p/MPa
Allowable
surface
pressure
p
dop
/MPa
PET 0.37 3400 7730.96 1.4786 0.7124 67.99 79
PA 6 0.39 3300 7199.42 1.5126 0.7288 64.96 96
PA 6C 0.41 3200 7552.29 1.4886 0.7182 66.98 83
Table 1. Calculation of the Hertzian contact pressure between the surface made of
AISI 316 and polymer wheel at a load of F
r
=150 N for different types of materials
Contact slide pressure p
s
between pin and wheel is calculated with formula (6), width
B of wheel is 21 mm and pin diameter D is 11mm. The result of p
s
is 0.64 MPa that
is less than p
dop
for all three polymer materials.
Testing of the dimension change of PET material was carried out in real operating
conditions using the Procomac bottle rinsing machine. The results of the tests with
polymer wheels can be seen in Figure 3
.
Figure3. Wear of polymer wheels on the inner and outer diameter, with the initial
inner diameter 11.3 mm and outer diameter 45 mm
In the course of testing the wheels of Procomac rinser, the following operating
parameters were used:
- Peripheral speed at the wheel's outer diameter of 45mm is v
t
= 0.6 m/s
- Lubrication by water
- Force exerted on the wheel F = 150N
- Pin used to securely attach the wheel is made of AISI 316
- Roughness of Pin is Ra 0.6 μm
- Roughness of the inside wheel is Ra 0.4 μm
- Roughness of the outside wheel is Ra 0.4 μm
- Testing circle is 200 hours
- Surface on which the outer edge of the wheel rests upon is a main strip made of
AISI 316 with a roughness of Ra 0.6 μm
We can see that external dimensions of the wheel made of PA6C haven’t changed,
whereas the internal dimension with the pin inside changed from 11.5mm to
13.45mm and the inside roughness changed from Ra 0.4 μm to Ra 0.26 μm.
Internal measure of the wheel made of PA6 is slightly modified, but due to the
swelling of the material, the wheels are forced to brake on the pin which leads to
excessive wear on the outer part of the wheel, while roughness from inside
changed from Ra 0.4 μm to Ra 0.15 μm.
As for the internal measure of the wheel made of PET, the measure of the wheel
changed from 11.3mm to 11.5 mm which is negligible compared to the initial state
the roughness from inside changed from Ra 0.4 μm to Ra 0.25 μm .
Dimensions of the pin made of AISI 316 remained unchanged i.e. 11.05 mm, as
well as the roughness.
4. CONCLUSION
On the basis of obtained test results, it can be concluded that polymer material PET.
Tested in the function of wheels of a filling machine, exhibited very good mechanical
tribological properties in comparison to other test materials, such as PA6 and PA6C
in the given operating conditions. The machine whose wheels (made of PET) were
subjected to testing, indicated a significant shift in extending the service intervals
from six to ten working months so that the bottle-filling station could operate
smoothly in terms of production throughout the entire filling season. PET is poorly
used in industry as a structural material for construction; however, it is expected that
further testing and development would provide for broader use of this material.
5. REFERENCES
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978-1-78242-117-1.
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[3] Domitran, Z., Žeželj, D., & Katana, B. (2015) Influence of contact pressure and
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