by Drs. Bob Ruckle and Tom Seung-Tong Cheungm, Siltech Corporation
A wide variety of polymers are used as resins in coatings systems. Often defined by their reactive groups, the myriad of resins offer a wide-range of properties. In the coatings industry, acrylate, epoxy, urethane and polyester coatings are all quite common and have associated typical properties and expectations. Resin manufacturers have spent countless hours and money fine-tuning and expanding the properties of their core chemistry to vary their utility.1-6
One way to alter the fundamental properties of a polymer is to react it with a different polymer generating an AB-type co-polymer or hybrid. With low surface energy, ultra-low Tg and strong slip, release and flow properties, polydimethylsiloxane (aka PDMS or silicone) can bring profound property changes to these hybrids.
PDMS itself has no reactive groups although the polymer can be broken under strong base or acid catalysis and reacted with nucleophilic resin systems. The reactive sites are made from the same raw materials as the native resin polymers. The reaction is complicated by the inherent insolubility of silicone in organic resins. The reaction with silicones is often slower and requires stringent mixing methods.
In this paper, we modify a few coatings systems with reactive silicones and examine the effect on their liquid and cured film properties. We have chosen UV-cured acrylate and cycloaliphatic epoxy systems as examples, but this concept is valid in heat-cured systems as well.
Experimental and methodology:
The overall design is to use two radiation-cured systems, one acrylate and the other cycloaliphatic epoxy-cured. The systems were cured in a UV box with a hand lamp, using the following UV lamps and cure conditions, depending on the nature of study:
- 15Watt Bench UV lamp with 10 mW/cm2 of UV Full (230nm-410nm); exposure time from 30 min to 1 hour for heat sensitive Leneta panels.
- Rheometer LED UV Lamp with 132 mW/cm2 of UV Full; exposure time from 30 sec to 5 min for rheological measurements.
- PC 100S Spot Lamp with 140 mW/cm2 of UV Full; exposure time from 30 sec to 5 min for hardness measurements of small button samples.
- High Pressure Mercury Vapor Lamp with 0.98W/cm2 of UV Full; exposure time from 1 sec to 5 sec for metal panels.
A nitrogen blanket is used for curing acrylate coating that contains free radical photoinitiator. (See Image 1 – System I: UV-cured acrylate (ACR))
System I UV-cured acrylate (ACR):
In Series A, we use one well-studied reactive silicone product, Silmer ACR D208. On average, this is a di-functional silicone acrylate ester with polyethyleneoxide chains for increased solubility. This product is commonly used for screening at Siltech because it has good solubility and good reactivity. Here we examine the silicone at 30 percent and 60 percent of the system so we can see an effect and confirm reactivity.
In Series B, we extend this success with six silicone acrylate esters that vary by linear vs. pendant polymer architecture; presence or absence of a polyalkyleneneoxide chain; and silicone and polyether chain lengths. All of these are run at 10 percent use level.
Finally in Series C, we run a similar selection of six acrylate ester functional silicones focusing mainly on the linear vs. pendant polymer architecture in a different organic resin system at a 22 percent use level.
System II UV-cured cycloaliphatic epoxy (EPC):
Coatings system: The following UV-curable EPC blends are used for the current study:
In this different cure system, we evaluate a series (A) of relatively insoluble epoxy silicones that vary by linear vs. pendant polymer architecture, number of reactive sites and polymer chain length. Each of these is screened at 1 percent and 20 percent use levels.
In Series B, we examine polyalkyleneoxide modified epoxy silicones that vary by linear vs. pendant polymer architecture; type and chain length of polyalkyleneoxide; number of reactive sites and silicone and polyether chain lengths. All of these are run at 1 percent and 20 percent levels.
Test panel preparation
All tested panels are prepared by drawing down approximately 1 ml of the EPC blends formulation on a 4×6.5″ white Leneta paper with wire-wound rod #10. The wet film is cured under a UV bench lamp for one hour with nitrogen blanket.
Coefficient of friction (CoF/Slip)
Slip is measured with ChemInstruments Coefficient of Friction -500. (Test speed: 15 cm/min; travel length: 15 cm; sled weight: 200 grams; and sled surface, which is covered with ASTM-specified rubber). Static coefficient of friction is directly obtained from the equipment, representing the ratio of the horizontal component of the force – required to overcome the initial friction – to the vertical component of the object weight (200 grams). Kinetic coefficient of friction also is obtained directly from the equipment, representing the ratio of the horizontal component of the force (required to cause the object to slide at a constant velocity) to the vertical component of the object weight (200 grams). The greater the value, the higher the friction is for the substrate. The slip rating is determined by averaging percent change of CoF with weighting factors against the control in the same series and normalizing to 10 with all the test samples. The best is 10, and the worst is 0.
Gloss is measured with BYK-Gardner 60° micro-glossmeter. The value is recorded directly from the micro-glossmeter. The lowest is 0, the highest is 100.
Peel force measurements
A 12″ long length of 1.89″ wide Intertape 6100 clear packing tape is used. Half the length of the tape is applied on the coated panel at a 45° angle with a wooden applicator. Care is taken to ensure good contact between the tape and the substrate. One end of a stainless steel string is attached to the transducer and the other end is fastened onto the remaining half of the tape with a 2″ length of standard cellophane tape. Peel force is measured by peeling the tape with ChemInstruments 500 at an angle of 180° and peel rate of 60 cm/min. Record and report an average of 10 tests as the peel force in grams/cm2.
Mar resistance is measured using a Sutherland 2000 Ink Rub Tester – Dry Rub method with the following settings: 500 rubs, 84 rpm stroke speed for all sample sets. Rubbings are done using a 4 lb test block that is attached with a 2×4″ nylon scrubbing pad. Gloss is measured immediately after completion of rubbing for each panel. The mar resistance rating is determined by visual inspection of surface defects and by the percentage change in gloss reading before and after the rubbing test. Record percentage loss of gloss and a subjective rating from 0 to 10, where 10 is the best and indicates no visible effect.
Stains are applied on the panel using 1-5 drops/mark each on separate locations near the center portion of the panel. The following stains were used: red lipstick, green permanent marker, black permanent marker, brown crayon, purple crayon, pencil, red ballpoint pen and yellow highlighter. All of the stained panels were conditioned at room temperature for 1 hour before testing. All the treated panels were then rinsed with tap water for 1 minute and wiped with an IPA-saturated cotton swab. The subjective ratings are obtained by visual comparison of stains remaining on the panels for each series and rating them from 1 to 10, where 10 is best and indicates no remaining stain.
The panel to be tested is placed coated side down on the top of a protective paper that sits on a flat steel plate with rubber pad on the bottom. A steel rod with a 1 cm diameter round steel ball attached at the end of the rod is placed on the back side of the coating surface. A 700 gram weight with a 1.5 cm hole through the middle fitted onto the steel rod drops down freely and vertically along the rod from a distance of 23 cm above the coating surface. The impact resistance is estimated by visual inspection of the size and pattern of the damage. The subjective ratings are obtained by visual comparison of impact damage on the panels for each series from 1 to 10, where 10 is best and indicates no cracking or breaking of the film.
System 1 Series A: Formulations of UV-curable coatings were prepared with epoxy acrylate resin and modified with varying amounts of Silmer® ACR D208 di-functional acrylate ester silicone with a polyethyleneoxide chain for increased solubility. (See Image 3 – Table IA Film).
Conclusions System I Series A: This screening study shows this low molecular weight polyethyleneoxide, acrylate ester silicone material is highly compatible with this acrylate resin and reacts into the film completely. The film properties of the acrylate UV coating can be modified by incorporation of more than 60 percent silicone to give very strong release properties. The peel force and impact resistance data show better release and flexibility as more silicone is used as one expects. However, the CoF data shows an increase with silicone content. The coating becomes rubbery and very flexible to the hand at these high levels of silicone. We believe this is why the CoF data is skewed.
System I Series B: Formulations of UV-curable coatings were prepared with epoxy acrylate resin and modified with varying amounts of silicones, all but one of which are modified by polyalkyleneoxides for solubility, and which differ primarily by silicone and polyether chain lengths. All of these are run at 10 percent use level. (See Images 4, 5 and 6 – System I Series B, Table IB Liquid and Table IB Film).
Conclusions System I Series B: The LINX400, which has no polyether modification, does not cure completely and leaves unreacted silicone oil on the surface. This means the film property tests may not be measuring an inherent change in the film, but rather the surface oil.
All of the test samples modified with 10 percent reactive silicones show better spreading than the control. LINEL10 and LINES15 reactive silicones give somewhat better flow than the control. All other samples show poorer flow than the control. The strongest correlation is between flow and viscosity of the coating rather than surface tension reduction due to the reactive silicone. This is perhaps due to the already low surface tension of the control.
The peel force, slip and CoF for each of the test samples prepared with 10 percent reactive silicone are significantly improved over the control. The best results of the completely cured systems belong to the medium chain length silicones.
There is no dramatic improvement in mar resistance with 10 percent silicone actives in this series. This may be due to the fact that the coating becomes softer and more rubbery as indicated by storage modulus. Alternatively, since using a small amount of silicone (<1% as a slip additive in a UV coating typically improves mar resistance significantly, that effect already may be seen in the control film from the 0.5% Silmer ACR D2 used for foam control.
Most of the samples give lower storage and loss modulus than the control. Only LINEL10 gives higher values, a surprising result.
The uncured material using LINX400 gives the best stain resistance perhaps due to uncured silicone. In general, there is some improvement in stain resistance, with the exception of LINEL25 and LINPS20. We have no explanation for those exceptions.
System I Series C: In this study, formulations of UV curable acrylate coatings are varied with a selection of acrylate ester functional silicones, focusing mainly on absence or presence of a polyalkyleneoxide chain, chain length of silicone and polyether and the linear vs. pendant polymer architecture in a different organic resin systems. All of these are run at 22 percent use level. (See Images 7, 8 and 9 – System I Series C, Table IC Liquid and Table IC Film).
Conclusions System I Series C: The products with no polyether modification do not cure completely and leave unreacted silicone oil on the surface. This means the film property tests may not be measuring an inherent change in the film, but rather the surface oil.
All of the test samples modified with 22 percent reactive silicones show better spreading than the control. This is perhaps due to the fact that adding a reactive silicone to these systems reduces the viscosity of the resin significantly.
All samples prepared with 22 percent reactive silicones give much lower peel force and CoF than the control. The peel force for the samples prepared with linear acrylate ester silicones is significantly lower than those prepared with pendant materials. Higher MW also seems to help this effect.
Most of the samples give lower storage and loss modulus than the control. The most cross-linked systems, PENES30 and PENX8, give the least reduction in the moduli. Again a consistent, albeit moderate, improvement in stain resistance is seen.
System II Series A: Formulations of UV-curable coatings with cycloaliphatic epoxy resin were prepared with 1 percent and 20 percent of a series of relatively insoluble epoxy silicones that vary by linear vs. pendant polymer architecture, number of reactive sites and polymer chain length. (See Image 10 – System II Series A and Image 11 – Table IIA Film).
Conclusions System II Series A: As all of these samples show surface defects and a slightly greasy surface, the materials are not completely compatible and may not have completely cured into the film. It is possible that the film properties being measured are due to the surface oil rather than incorporation of silicone into the backbone.
Slip, CoF and peel force of all of the epoxy silicone modified epoxy coatings is significantly lower than the control. Multi-functional (pendant) epoxy silicones with higher functionality give lower peel force than those with lower functionality. The highest molecular weight linear di-functional epoxy silicone gives lower peel force than the lower molecular weight ones.
Stain and mar resistance improve significantly when the cycloaliphatic epoxy coating is modified with either linear di-functional or multifunctional epoxy silicones. Both stain resistance and mar resistance seem to increase as the molecular weight of either polymer architecture is increased. The mar resistance of the 1 percent series is slightly better than the 20 percent series, whereas the latter is better than the former with respect to stain resistance.
The impact resistance of epoxy coatings modified with 20 percent epoxy silicone improves significantly, whereas there is no improvement in impact resistance for epoxy coatings modified with the 1 percent epoxy silicones. Multifunctional epoxy silicones seem to give better impact resistance than the linear di-functional silicones. The highest MW linear di-functional is better than lower ones.
It is surprising to see that the shear modulus increases as the molecular weight of the linear di-functional epoxy silicone increases in the 20 percent series. The shear modulus of the high MW linear di-functional epoxy silicone is higher than that of the low molecular weight ones. The sample prepared with multifunctional epoxy silicone gives lower shear modulus than those with the linear di-functional ones.
System II Series B: In this series we prepared formulations of UV-curable cycloaliphatic epoxy coating resins modified with 1 percent and 20 percent of polyalkyleneoxide modified epoxy silicones that vary by linear vs. pendant polymer architecture; type and chain length of polyalkyleneoxide; number of epoxy groups and silicone and polyether chain lengths. (See Image 11 – System II Series B and Image 12 – Table II B Film).
Conclusions System II Series B: These epoxy silicone polyethers give smooth coating surfaces indicating they are totally compatible with the epoxy resin system used. This is confirmed by gloss reading, where there is no significant change in initial gloss for 1 percent silicone polyether series. The initial gloss of the 20 percent series is slightly reduced relative to the control.
The epoxy coating modified with 20 percent epoxy polyether silicones gives better impact resistance than the control. The impact resistance increases as the number of epoxy groups in the silicones increases, but only to the level of the linear, di-functional materials. There is no change in impact resistance and pencil hardness for all 1 percent epoxy silicone polyether samples.
Stain resistance and mar resistance of silicone polyether modified epoxies is much better than the control for both 1 percent and 20 percent series. The 1 percent epoxy silicone polyether sample gives better mar resistance than the corresponding sample in the 20 percent series. This may be due to the fact that the latter is softer than the former or that there is enough slip at 1 percent. The reverse is true for stain resistance.
The slip of epoxy coating improves significantly with linear materials. The improvement is greater in the 20 percent series. There is only a minor improvement in slip for these multifunctional materials.
The peel release force of epoxy coating is significantly improved, particularly for 20 percent high MW linear di-functional epoxy silicone polyether materials.
Epoxy coatings modified with 20 percent epoxy silicone polyether material is more flexible than the control, as indicated by tan delta and impact resistance measurements. The sample with greater number of epoxy groups per silicone polyether molecule results in lower damping factor or lower tan delta. This is attributed to a higher level of cross-linking and is expected.
The curing rate of linear di-functional silicone polyether is faster than multifunctional polyether. The curing rate of silicone polyether samples increases as the number of epoxy groups in the silicone polyether increases. In general, the curing rate, hardness and shear modulus of all silicone polyether modified epoxy coatings is lower than the control.
- In general, reactive silicones improve release, slip and CoF at 1 percent incorporation. In most cases, these properties continue to improve with more silicone. Linear di-functional materials are often better than pendant for these properties.
- Mar resistance also is seen at 1 percent and is not improved at higher levels. In fact, the mar resistance properties are often lost at higher loadings, which is believed to be an artifact of the softer films.
- Stain resistance is seen with most reactive silicones across multiple stains and is increased at higher use levels such as 20 percent over 1 percent. High molecular weight and di-functional architecture give the best stain resistance. Having some uncured silicone in the film seems to increase stain resistance.
- Impact resistance and moduli indicate the increased flexibility of the systems with silicone reacted into the film. Higher use levels are needed here for significant changes, with 1 percent showing little or no effect.
- In both systems, the incorporation of polyalkyleneoxide into the reactive silicones increases compatibility and degree of curing. More than 60 percent can be used in one case. This is the recommended path to incorporate silicones into the film. Without exception, materials without polyether showed signs of incomplete cure or incomplete incorporation of the silicone into the matrix. Several properties, such as stain resistance and CoF, were significantly impacted by this free silicone.
- Curing rates are slowed with reactive silicones as compared to the controls.
- The high levels of silicone incorporation tend to make the coating softer and more rubbery.
- Flow and spreading of the coating is improved significantly with reactive silicones, apparently mainly due to reduction in viscosity.
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Dr. Bob Ruckle is global marketing and sales manager at Siltech Corporation. He attained his Ph.D. at the University of Delaware, and then joined Union Carbide’s silicone business in 1989 as a research chemist. During the next 25 years, Ruckle worked in research and development, designing silicone surfactants primarily for the coatings additives market. He joined Siltech in 2009. Contact Bob Ruckle at email@example.com.
Dr. Tom Seung-Tong Cheung is an application lab manager at Siltech Corporation. Following his Ph.D. in chemistry at the University of Western Ontario, Cheung joined the University of Toronto as a research postdoctoral fellow and lecturer, providing a course on “Polymers for Engineering Applications” for several years. He has worked in research and development in various chemical industries for 30 years, specializing in applications of silicone materials in coating, adhesive, sealant and plastics/elastomers industries – covering a wide range of cross-linking chemistries. Contact Tom Seung-Tong Cheung at firstname.lastname@example.org.