Short Notes # 11
Cross-linking with Peroxides
BY
Sin Siew Weng, FIM, FPRIM, AMIC, ANCRT, FIManf, F.Prof
BTM.
Sin Siew Mun, BSc, ANCRT, APRIM
Sin Yoong Cheong, MEng
Sin Yoong Leong, BSc
Lee Aik Kwong, PhD, FPRIM, FMIC, ANCRT
Abstract
The chemistry of peroxides and co-agents are reviewed. Some hints are given to the selection of peroxides with or without co-agents based on techno-economic reasons. The availability of Sin RubtechTM polymer bound predispersed grades are revealed.
First Published 18-Jan-2000
1st Revision: 24-Jan-2000
2nd Revision: 8-Aug-2001
List of Abbreviations
|
BP |
Dibenzoyl peroxide |
|
BCUP |
Tert.butyl cumyl peroxide |
|
BDV |
N-butyl-4,4-bis-(tert.butyl peroxy)-valerate |
|
BDMA |
Butylene glycol dimethacrylate |
|
CO |
Epichlorohydrin homopolymer |
|
CSM |
Chlorosulphonated polyethylene |
|
DAP |
Diallyl phthalate |
|
DYBP |
2,5-Bis-(tert.butylperoxy)-2,5-dimethyl-3-hexyne |
|
DIPP |
a,a’-di-(tert.butyl peroxy) diisopropylbenzene |
|
DTBP |
Ditert.butyl peroxide |
|
DCLBP |
Bis-(2,4-dichlorobenzoyl) peroxide |
|
DHBP |
2,5 – dimethyl-2,5-bis(tert-butyl-peroxy)hexane |
|
DCP |
Dicumyl peroxide |
|
ECO |
Epichlorohydrin-ethylene oxide copolymer |
|
EDMA |
Ethylene glycol methacrylate |
|
EPDM |
Ethylene propylene diene terpolymer |
|
EPM |
Ethylene propylene copolymer |
|
EVM |
Ethylene vinylacetate copolymer |
|
HVA-2 |
m-phenylene bismaleimide |
|
IIR |
Isobutylene isoprene copolymer |
|
NR |
Natural Rubber |
|
PIB |
Polyisobutylene |
|
PP |
Polypropylene |
|
PVC |
Polyvinyl chloride |
|
PBD/s |
1,2 vinyl polybutadiene
resin |
|
PBD/ MA |
PBD/s adducted with maleic anhydride |
|
TMCH |
1,1-bis(tert.butylperoxy)-3,3,5
trimethylcyclohexane |
|
TBPD |
Tert.butyl peroxybenzoate |
|
TAC |
Triallyl cyanurate |
|
TIAC |
Triallyl isocyanurate |
|
TAM |
Triallyltrimellitate |
|
TAP |
Triallyl phosphate |
|
TMPTMA |
Trimethylolpropane trimetharcylate |
|
ZDMA |
Zinc dimethacrylate |
1.0 Introduction
Peroxide cross-linking of polymer was introduced in the 1950’s with the commercial availability of dicumyl peroxide (DCP). The main interest then was the long scorch time provided by DCP which is only activated at > 110OC. Peroxides are now used in many applications for better heat resistant, lower set, translucent/ transparent water clear vulcanisates not possible with sulphur cures, in slow curing polymers like EPDM’s, in saturated EPM/ EVM etc, pharmaceutical products, in products requiring high gloss i.e. bloom-free and/ or bright colours and others. However peroxide cross-linking is still not of widespread use and still not fully appreciated especially with respect to their chemistry, choice and use of co-agents.
2.0 Chemistry
The fundamentals of cross-linking with peroxides were first reviewed by Class, J.B1 in 1995 and lately by the same author in 19992. It is not easy to fully appreciate the multifarious reactions of free radicals.
There are 3 main reactions in peroxide cross-linking:-
Step 1: Peroxide undergoes homolytic cleavage to form 2 free radicals.
Step 2: Free radical abstracts H from polymer chain.
Step 3: Two polymeric free radicals couple to form a carbon-carbon bond.
An example of DCP cross-linking of NR illustrates:
Step 1.
Step 2.
Step 3.
2.1 Mechanisms
2.1.1. Step 1 Peroxide homolytic
cleavage
The reaction is a first-order reaction proportional only to the concentration of the peroxide at any one time. The rate of formation of free radicals depends on the reaction temperature.
The cure temperature selected and time of cure depends on the half-life of the peroxide. The half-life is defined as the time required for half of the peroxide dosage to decompose at the reaction temperature i.e. curing temperature. As a general rule of thumb, the half-life is reduced to 1/3rd of its value every 10OC increase in curing temperature.
Efficiency of formation of free radicals is reduced in the presence
of acidic materials such as clay, stearic acid, salicylic acid etc. as the acid
causes ionic decomposition of the peroxide viz:
2.1.2 Formation of Polymer Chain
Radicals
This is a bimolecular reaction involving an alkoxy free radical and a polymer chain. The reaction is also a first-order reaction. Since there is abundant supply of hydrogen atoms on the polymer chain and the alkoxy free radicals once formed reacts very rapidly, the rate-determining step is Step 1 i.e. homolytic cleavage of the peroxide.
The alkoxy free radical can abstract a hydrogen atom from the polymer chain or from any compounding ingredients containing abstractable hydrogen atoms. The ease of abstraction of hydrogen atoms by alkoxy radicals are in the order:
Phenolic > benzylic > allylic > tertiary
> secondary > primary
Therefore use of aromatic oils which contain benzylic and allylic hydrogen atoms and phenolic antioxidants, will affect the efficiency of polymer chain radicals formation and hence cross-linking efficiency.
There are some compounders who try to minimise the odour of DCP by using a blend of DCP with say 2,5-bis-(-tert-butyl-peroxy-)-2,5-dimethylhexane. It should be pointed out that with mixed peroxides, two radicals of unequal half-life or reactivity are formed and at medium curing temperatures 150+10OC, the more active free radical acts as cross-linkers while the other remains inactive and wasted. Mixed peroxides are best cured at higher temperatures 180+10OC where the reactivities of the 2 peroxides are more or less equalised.
Polymer chain radicals can also undergo undesirable b-scission. (See later Eqn. 9)
2.1.3 Coupling of Polymer Chain
Radicals
Final reaction is the coupling of two radicals on adjacent polymer chains to form a carbon-carbon cross-link without any peroxide fragments.
However in the presence of oxygen, the radical on the polymer chain can couple with O2 to form a hydroperoxy radical leading to polymeric degradation viz:- example;
This is clearly evident in moulded flashes of peroxide cured products. For this reason, peroxide curing cannot be undertaken in hot air systems.
Certain polymers like IIR, PP, PVC, XIIR, PIB, CO & ECO cannot be cross-linked with peroxides as their polymer chain radicals can undergo severe b-cleavage along the chain which of course degrades the polymer example:
In general polymers with tertiary carbon atoms as illustrated above are more prone to b-scission.
3. Role of Peroxide Co-Agents
Whilst peroxide curing has been around some 46 years, co-agents are relatively new making their appearance only in the 1980’s.
Most rubber chemists still accept, for example, that peroxides cure rate can only be increased by temperature rise but not by co-agents/ additives. They are half correct.
Co-agents are polyfunctional multi-unsaturated organic compounds which readily form free more stable radicals when exposed to heat. Elemental sulphur can also be a co-agent but it has not achieved commercial acceptance because of the resultant bad odour. When co-agents are added to peroxide cure systems, they can:
1. Improve cure efficiency
2. Some co-agents can also increase cure rate
3. Minimise b-scission
4. Some co-agents can eliminate steric hindrance for polymer chain radicals coupling.
Co-agents can be classified under 2
general classes.
Type I:- Can improve cross-link efficiency as well as cure rate.
Type I includes: acrylics
methacrylates
bismaleimides
vinyl esters
The chemistry of co-agents is less well studied as compared to peroxides.
These co-agents themselves can undergo hydrogen abstraction of polymer chains producing polymer chain radicals which can lead to chain cross-linking and hence co-agents used with peroxides can increase modulus/ hardness.
Co-agents can also undergo free radical addition resulting in
homopolymersation, eg. Trimethylol propane trimethacrylate can undergo free
radical addition to form a low molecular weight homopolymer which can
eventually be grafted onto the main polymer chain.
Co-agents can also eliminate steric hindrance of polymer chain radical eg.
which leads to improved cross-linking efficiency.
These reactions result in higher cross-link densities and higher cure rate.
Some modern Type I co-agents are available with built-in proprietary scorch retardants.
Some common Type I co-agents are given in Table 1.
Table 1 – Type I Co-agents
|
|
Abbreviation |
|
Trimethylolpropane Trimethylacrylate |
TMPTMA |
|
Ethylene glycol dimethacrylate |
EDMA |
|
m-phenylene bismaleimide |
HVA2 |
|
Zinc dimethacrylate |
ZDMA |
|
Butylene glycol dimethacrylate |
BDMA |
Whilst Type I is generally preferred, these co-agents give polymer chain radicals which are more prone to b-scission.
New work has shown that metallic co-agent like zinc dimethacrylate can also function as adhesion promoters to create strong bonds between untreated metal and a variety of rubber formulations.
Type II – Co-agents that improve cross-link efficiency only.
Type II co-agents given in Table 2 include allylic compounds and low molecular weight vinyl polymers.
Table 2 – Type II Co-agents
|
|
Abbreviations |
|
Triallyl cyanurate |
TAC |
|
Triallyl isocyanurate anhydride |
TIAC |
|
1,2 Vinyl polybutadiene resin |
PBD/S |
|
PBD/S adducted with maleic anhydride |
PBD/MA |
|
Triallyl
trimellitate |
TAM |
|
Triallyl phosphate |
TAP |
|
Diallyl phthalate |
DAP |
4.0 Choice of Peroxides
Like any other compounding ingredients, choice of peroxide is based on techno-economic reasons. The most common peroxide used is DCP. It is the cheapest and conveniently cures at similar S cure systems temperatures ~ 150-160OC. However it has a slight odour which can be masked by Sin RubtechTM Deocide S but otherwise objectionable to some.
Choice is also dictated by technical reasons such as the peroxide reactivity, polarity, transparency, smell and cure temperature required.
Peroxides without carboxy groups
|
Characteristics |
|
|
Less sensitive to acids. Less sensitive to O2 such as
absorbed on carbon blacks. Higher cure temperatures required. |
|
a,a’- di-(tert.butylperoxy)diisopropylbenzene (DIPP) |
|
|
2,5 –
dimethyl-2,5-bis(tert-butyl-peroxy)hexane (DHBP) |
|
|
Dicumyl peroxide (DCP) |
|
|
|
Peroxides with carboxy groups
|
Characteristics |
|
|
More sensitive to acid. Cure problems with O2 as in
carbon black. Scorchy but can cure at lower
temperatures. |
|
Dibenzoyl peroxide (BP) |
|
|
2,4 – dichlorobenzoyl peroxide (DCLBP) |
For translucent & transparent compounding it is common sense to choose a liquid water clear undiluted peroxide e.g. 1,1 – bis(t.butylperoxy)-3,3,5-trimethyl cyclohexane. (TMCH) or 2,5-Bis-(tert.butylperoxy)-2,5-dimethyl-3-hexyne (DYBP).
In selecting a peroxide to suit a desired curing temperature one can apply a general rule of thumb that peroxide reactivity from 1800C to 50oC is in the order:-
Dialkyl > alkyl-aralkyl
> diaralkyl > alkyl-ketal > diaroyl.
Hence the Bis(2,4 dichlorobenzoyl) peroxide can cure even at ~45OC. However the Di-(tert-butyl) peroxide is too volatile at 180OC for commercial use and hence replacing 2 methyl groups with phenyl groups gives Dicumyl peroxide which is not so volatile and cures at ~160OC.
Table 3 Illustrates
Table 3
Choice of Peroxides based on cure temperature and scorch safety
|
Active
Chemical |
Abbrev. |
Safe
Temp. (OC) |
Curing
Temp (OC) |
Form |
|
|
1 |
2,5-Bis-(tert.butylperoxy)- 2,5-dimethyl-3-hexyne |
(DYBP) |
150 |
190 |
Clear Liquid |
|
2 |
a,a’-di-(tert.butyl peroxy)
diisopropylbenzene |
(DIPP) |
140 |
180 |
Low MP,
Semi Crystalline Solid |
|
3 |
Ditert.butyl peroxide |
(DTBP) |
140 |
180 |
Liquid (high volality@180OC) |
|
4 |
Tert.butyl cumyl peroxide |
(BCUP) |
130 |
180 |
Liquid |
|
5 |
Dicumyl peroxide |
(DCP) |
130 |
170 |
Low MP, Semi-crystalline solid |
|
6 |
N-butyl-4,4-bis-(tert.butyl peroxy)-valerate |
(BDV) |
120 |
160 |
Usually predispersed solid or paste |
|
7 |
1,1-bis(tert.butylperoxy)-3,3,5
trimethylcyclohexane |
(TMCH) |
110 |
140 |
Usually predispersed solid or paste |
|
8 |
Tert.butyl peroxybenzoate |
(TBPB) |
100 |
140 |
Usually predispersed solid or paste |
|
9 |
Dibenzoyl peroxide |
(BP) |
80 |
110 |
Usually predispersed solid or paste |
|
10 |
Bis-(2,4-dichlorobenzoyl) peroxide |
(DCLBP) |
70 |
90 |
Usually predispersed solid or paste |
Conclusion
The main applications of peroxides and co-agents are in the saturated polymers such as EPM, EVM & CPE for the wire and cable industry.
In the unsaturated polymer sector, the use of peroxides and co-agents is not expected to make much in-roads against the conventional sulphur cure systems.
Nevertheless Sin RubtechTM has encountered users requesting for these materials. Peroxides and co-agents do have niche markets and also in areas where only peroxide cured vulcanisate can give e.g. the low set, heat resistance etc. required, or the water clear transparency as in BR play balls and speciality solings.
There is today a very wide selection of commercial peroxides and co-agents. Users should refer to these for more specific selection aided by an understanding of their chemistry as given in this Short Note. Material Safety Data Sheet are important as health and safety of storage and handling of peroxides/ co-agents should not be overlooked.
Finally most peroxides/ co-agents in their pure form are liquid or low melting point solids. Dicumylperoxide has a MP ~40OC. One is not surprised to find in this part of the world, a bag of solid DCP on a cold day and a bag of liquid on a hot day.
Sin RubtechTM has made available polymer bound predispersed peroxides and co-agents to our users. (See Technical Bulletin #11).
References
|
1. Class, J.B. (1995) |
Fundamentals of cross-linking with peroxides. Rubb. and Plast. News, Oct 9 |
|
2. Class, J.B. (1999) |
A review of the fundamentals of cross-linking with peroxides. Rubb. World, Aug. 1999 pp 35-39 |
|
3.Costin, R. & Nagel, W. (1995) |
Metallic co-agents for rubber to metal adhesion. Rubb. World. Sept. pp 18-24 |