crosslinking and crosslink density. - cross linked polymer
I was asked to compare the curing to the cross-linking and cross-linking density.
Curing is the means by which the coating meets the performance required by the user's needs.
This may occur in environmental conditions and may also involve baking, UV radiation, or exposure to other forms of energy.
Curing is usually evaluated by measuring physical properties and solvent resistance according to customer specifications.
Cross-linking is the process of forming chemical links between polymer chains.
This may start to extend and/or branch chains for low molecular weight polymers, but eventually form a network.
The cross-linked polymer expands in a good solvent to form a gel, but does not dissolve to form a solution.
Treatment may or may not involve links.
The loss of solvent and/or water may be all that is needed, and that is the case with the lacquer and most latex paint.
Dissolvedor dispersed polymers have a high enough molecular weight to give the final film the same performance.
The advantage of cross-linking is that it allows the use of low viscosity and low MW polymers in high solid coatings, but at the same time provides control of the coating structure, as well as improvements in performance such as film hardness, toughness and solvent resistance. [
Cross-linking density n is an indicator of the amount or degree of cross-linking.
It is defined as the molar number of effective red chains per mass or volume of the polymer, which is the reciprocal of the average molecular weight or mass between cross-linking points ,[M. sub. c]
Expansion analysis is a useful technique for measuring cross-linking density.
In one version of the measurement, remove the coated small disk from the part or panel with a pin and place it in the hole of the concave microscope slide.
Similar methods include placing a square film sample on a microscope slide and covering it with glass.
In each case, the size of the sample is measured with an optical microscope/Camera/Image analysis combination.
Suitable solvent (
It's usually Asian blue)
Then introduce and measure the size of the expansion.
The expansion is calculated from the initial size and swollendimensions and then converted to volumetric expansion ,[DELTA]
V/V, assuming the sample is expanded in an equal tropical zone.
The volumetric expansion value is replaced with an equation of molecular weight between cross-links, E. G [M. sub. c]= [([Q. sup. 1/3]-Q/2)/([Q. sup. 2]/2+ [Q. sup. 3]/3)]where Q = 1/[([DELTA]v/v)x 100]; finally n =1/[M. sub. c]
Dynamic Mechanical Analysis (DMA)
In addition, free film is used to measure the cross-linking density of the coating.
Rubber plateau modulus [
See JCT coating, 5 (10), 44 (2008)]
The equation derived from the elastic theory of rubber is substituted: n = e'/3 RT, where n = the number of moles of the elastic effective network chain for each cc, e' = dynes /[cm. sup. 2]
R is gasconstant (8. 314 x 107 erg/(mol K)
T is the temperature in K.
Figure 1 shows a DMA diagram of the storage modulus (rigidity)and tan[delta](damping)
Temperature of acrylic resin
Baked melamine coating (
Actually overbaked)at 160[degrees]
Rubber platform modulus at 140 [degrees]
C was found to be 3. 396 x [10. sup. 9]dynes/[cm. sup. 2]
, Which means that the cross-link density is 3. 30 x[10. sup. -2]mol/cc.
There are some problems with what temperature should be chosen for cross-link density calculation.
There is a school of thought that the "minimum temperature" is correct.
However, curves running differently may show a change in the "minimum" temperature at the same time as the "minimum" itself.
For basically the same material, different cross-linking density values are given using "Minimum Temperature.
It seems unreasonable.
So my lab tends to choose a specific temperature (
It's usually standard baked. 140[degrees]
C in the case of the film described in figure 1. [
Figure 1 slightly]
Although this method does not measure cross-linking density, one indicator of the degree of cross-linking is the remaining gel fraction of the free film of the coating after several hours of solvent reflux in the asoxextractor extractor in order to extract soluble substances.
In a program, a small square of film is placed on the screen that is clearly weighed, and the screen is folded around the film.
Determine the weight of the coating by weighing and subtracting the screen weight.
Put the folded sample into the Soxhletextractor and return it for 5 hours with 50: 50 methanol: acetone.
Then remove, dry and re-weigh the stone door.
The gel content is the remaining weight after subtracting the screen weight from the final sample weight.
In my experience, not many customers need cross-link density data.
Some car companies
Japanese manufacturers in particular)like to see[T. sub. g]
Function values, tensile strength, number of break elongation, elastic modulus value, and cross-linking density of Baketemperatures (
Baked, regular, and roasted).
Individual or absolute values are not as important as data patterns, which enable engineers to look for weaknesses in low baking and possible degradation. By clifford K.