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Evaluation of silicone reclaim for HCR compounds.

Given the current market conditions for silicone polymer, users are looking for new technologies to reduce their demand and curve the cost implications. Partial polymer offsets and extenders are being widely evaluated.

Silicone reclaim technology has been around for decades. Until recently, there has not been a large justification for using it, nor much effort put into perfecting its uses. This study shows the capabilities for using silicone reclaim in HCR compounds.

Introduction

The reclaim evaluated in this study is cured HCR silicone compound that has been devulcanized using a unique and innovative process. The reclaim in this study is not ground or dry powder forms of vulcanized silicone compounds.

This study looks at the physical property differences at various loadings of silicone reclaim. Peroxide technologies are evaluated to determine the best approach to linking the reclaimed material back into the HCR compound. Additionally, it reviews the performance differences when using reclaim from a higher quality compound. Lastly, we evaluate options for modifying the reclaim to better interact with the compound chemistry. Variables are evaluated for rheology, physical properties, heat aging, processability and dispersion.

All processing and testing were performed at Ace Products & Consulting's independent laboratory in Ravenna, OH. Mixing of all formulations was done on a 1.7 liter tilt style laboratory mixer. Testing was performed under the following conditions:

* Slab curing: 6' at 177[degrees]C (350[degrees]F)

* Pellet curing: 10' at 177[degrees]C (350[degrees]F)

* Compression set: 22 hours at 177[degrees]C (350[degrees]F)

* Heat age: 70 hours at 232[degrees]C (450[degrees]F)

* Tear method: Die B

* Compression set method: Pellet

* Rebound resilience method: Pellet

* Sample conditioning: 72[degrees]F/22% RH

Reclaim material

The reclaim for this study was obtained from a molding operation. Two different grades of HCR molding compounds were used in this study. The initial material was flash and fabricated components that did not meet the dimensional specification of the end customer. This scrap material is ground into 1/4" chunks. The chunked material is then loaded into small proprietary hoppers that utilize special venting to uniformly process the material. The chunked material is then placed in an autoclave. Utilizing a combination of temperature, pressure and moisture, the material is devulcanized. The times and temperatures are modified according to the type of compound being reclaimed.

When the material is removed from the autoclave, it is often still in its original form. While looking unchanged, it can be plied into another form with one's fingers. The moisture levels are high due to the steam used during the devulcanization. This moisture must be mostly removed before the reclaim can be used in HCR compounds. The smaller 1/4" chunks allow the material to be reclaimed more consistently and aid the refining process. Larger reclaimed products can cause contamination lumps due to the difficulty to refine. Elevated ranges of heat are used during the refining process to flash off high levels of the moisture. The number of refining steps is determined by the dispersion and consistency required for the end application. Refining the material into a paste form improves the dispersion and breakdown of the reclaim into the HCR compound (figure 1).

Part level evaluation

The purpose of the part level evaluation was to determine at what level of substitution (reclaim for polymer) the physical properties start to fall off (table 1).

Reclaim was evaluated as a 10 phr, 20 phr, 30 phr and 50 phr replacement for a 40 durometer silicone base. A 70 durometer general purpose molding formulation was used for the experiment. A single vinyl specific peroxide, DBPH, was used as the cure system.

For the part level evaluation, raw reclaim silicone was used. The raw reclaim is tacky and requires some additional handling. As a result, on the batches with reclaim material present, the mixer rotors were coated with some of the precipitated silica before starting the mix (table 2).

The 30 phr and 50 phr batches had more material hang up on the mixer. The 10 phr and 20 phr batches mixed and milled identical to the control compound.

Rheology data showed a reduction in cure rate time that trended in line with the amount of reclaim. The MH value dropped off on the 50 phr compound. This is a good indicator of a lower crosslink density. ML values trended higher as the amount of reclaim used increased. This is due to the higher viscosity of the reclaim in comparison to the silicone base (table 3).

The physical property testing corresponded with the rheology results. The 30 phr and 50 phr compounds had considerable property drops. The reclaim batches had higher durometer versus the control batch (table 4).

The 10 phr and 20 phr batches had improved tensile results when compared to the control. The tensile started a sharp dropoff at 30 phr. Elongation and tear had a downward trend in correlation with reclaim loadings. Compression set of the 10 phr and 20 phr did not differ much from the control, while there was a sharp increase on the 30 phr and 50 phr. The rebound did not drop off until the 50 phr compound (table 4).

Heat age results as a percentage remained similar on most properties. There was a noticeably lower percentage of drop-off on the 50 phr for tensile; however, that is likely due to the low initial value (table 5).

It was determined that the optimum substitution level for reclaim in this formulation is between 10 phr and 20 phr.

Cure system evaluation

The initial test formulation uses DBPH as the curative peroxide. DBPH is vinyl specific; therefore, an evaluation was done adding a non-vinyl specific peroxide to the formulation. Most of the vinyl groups on the reclaim are gone, so it would not link in as well with a vinyl specific peroxide (table 6).

The same mixing procedure was used as the part level evaluation. There were no processing differences observed with the peroxide addition.

Both the control compound and 20 phr reclaim compound had improved physical properties with the addition of the Vulcup peroxide (table 7).

The addition of the non-vinyl specific Vulcup peroxide did show some minor improvements on physical properties. Like the part level experiment, there was an increase in durometer when reclaim was added to the formulation. Tensile improved slightly on the reclaim batch with the addition of the Vulcup; however, all other properties remained similar or slightly reduced (table 8).

Reclaimed compound quality evaluation

The reclaim used in the initial studies was a general purpose 50 durometer molding compound. It did not have very high specification. An experiment was done to determine how much effect the quality of the reclaimed compound has on the end compound using the reclaim. A 55 durometer molding compound with a 5% higher polymer content was used in comparison. The compound being reclaimed has higher tear and heat age properties. In this evaluation, we ran a control, a batch with 20 phr of the original GP reclaim, and a batch with 20 phr of the more polymer-rich reclaim (HS).

Both the 20 phr GP (original reclaim) and 20 phr HS processed equally. The durometer increased even more with the HS reclaim than the GP. Basic physical properties did not change much between the GP compound and the HS compound, other than the durometer. Tensile, elongation, tear, compression set and rebound were almost identical (table 9). There was a noticeable improvement in heat age property loss for the HS reclaim (table 10).

The quality of reclaim evaluation was repeated with the addition of the Vulcup peroxide. In the previous study, the nonvinyl Vulcup showed small improvement on the physical properties. The same mixing procedure and formulation was used for this study. A control with VCP, 20 phr GP reclaim with VCP and 20 phr HS reclaim with VCP was mixed for comparison. There were no differences observed in processing.

With the addition of the VCP, the 20 phr HS reclaim showed improvements in unaged physicals over the GP reclaim (table 11), but little difference in aged physicals (table 12).

Modified reclaim evaluation

In this evaluation, the raw reclaim was modified to improve compatibility with the chemistry of the HCR compound. A modified reclaim was made using a 50/50 blend of raw reclaim and 40 durometer silicone base. A proprietary blend of oligomers was added with a small amount of structural filler. This was done to try and provide additional site on the reclaim for it to cure into the system. This was done with a hot mix process. The modified reclaim was then compared against a control compound with no reclaim. The modified reclaim was added as a 50 phr replacement. The modified reclaim has 50% raw reclaim in it; therefore, the total substitution of base to reclaim is 25% (table 13). The 20 phr GP with VCP reclaim information was added to this evaluation for comparison.

The modified reclaim showed superior results in basic physical properties over the 20 phrGP/VCP reclaim. It did have an adverse effect on compression set and heat age properties, however (tables 14 and 15).

An additional modified reclaim was made using a 90/10 blend of GP reclaim with a 40 durometer silicone base. A blend of proprietary oligomers and precipitated silica was used to help treat the material in a heated mix cycle. This material was evaluated as a 25 phr offset for the silicone base, which equates to around 22 phr of GP reclaim (table 16).

The 90/10 reclaim had similar improvements as the 50/50 over the GP reclaim at 20 phr (table 17). Like the 50/50, it also had more swing in heat age testing (table 18). The compression set was better on the 90/10 than the 50/50; however, both were a higher percentage than the 20 phr GP (tables 14 and 17).

Conclusions

Reclaim can be used as a partial substitution for silicone, depending on the specification of the final compound. The quality of the material reclaimed does have an influence on the end compound. A maximum of 20 phr substitution is recommended based on the phr evaluation. The addition of a non-vinyl specific Vulcup peroxide provides additional improvement to physical properties and crosslink density. Treated and modified reclaim improves physical properties over standard GP reclaim.

Types of compound used for reclaim and additional treatments should be explored. Depending upon the end application, the reclaim source or treatment might vary. Reclaim processing parameters could be evaluated, as well.

This article is based on a paper presented at the 193rd Technical Meeting of the Rubber Division, ACS, May 2018.

by Erick Sharp, Ace Products and Consulting

Caption: Figure 1--reclaim silicone in paste form
Table 1--part level
evaluation formulation

Material                        phr

RBB-2000-35         100/90/80/70/50
Silicone reclaim      0/10/20/30/50
Min-U-Sil 10                     50
HiSil 233                        15
Black 1206                        1
ACE-SD-DBPH                     1.2

Table 2--part level evaluation mix
procedure

Addition                                     Time

Base, reclaim, pigment and DBPH-50     30 seconds
Precipitated silica                    90 seconds
Ground quartz                          90 seconds

Table 3--part level evaluation rheology data

              MH       ML    [Tc.sub.90]    [T.sub.s1]

Control    88.47     8.54           1.33          0.57
10 phr     89.14     9.48           1.26          0.54
20 phr     89.06    10.38           1.26          0.54
30 phr     83.39    10.62           1.19          0.51
50 phr     73.31    15.53           1.19          0.53

Table 4--part level evaluation basic physicals

Test              Control       10       20       30       50
                               phr      phr      phr      phr

Durometer            72.8     75.1     74.8      77A     77.8
Tensile             676.3    725.9    705.3      662    559.3
Elongation          126.3    112.3      100     86.3     61.7
Tear die B           27.8     25.8     25.4     22.8       19
Compression set     22.22    25.37    26.92    36.57    51.88
Rebound             49.60    47.70    48.20    48.20       38

Table 5--part level evaluation heat aged physicals

Test           Control         10         20         30        50
                              phr        phr        phr       phr

Durometer           79         81         82       81.6      88.5
Durometer %      8.52%      7.86%      9.63%      5.43%    13.75%
Tensile          593.5      620.6      618.4      576.8     537.5
Tensile %      -12.25%    -14.51%    -12.32%    -12.86%    -3.89%
Elongation        84.1      76.47       67.8       61.2      41.8
Elongation %    -33.4%     -31.9%     -32.1%       -29%    -32.2%

Table 6--peroxide
evaluation formulation

Material                        phr

40 durometer silicone base   100/80
Silicone reclaim               0/20
Ground quartz filler             50
Precipitated silica              15
Pigment                           1
DBPH-50                         1.2
VCP-40                        0/1.2

Table 7--peroxide evaluation basic physicals

Test              Control    Control    20 phr    20 phr
                                 VCP                 VCP

Durometer            72.8         74      74.8        77
Tensile             676.3      701.1     705.3     726.8
Elongation          126.3      116.1       100      69.8
Tear die B          27.79      27.22     25.43     25.33
Compression set     22.22      26.12     26.92      31.3
Rebound              49.6       51.9      48.2        49

Table 8--peroxide evaluation heat aged
physicals
Test           Control    Control     20 phr    20 phr
                              VCP                  VCP

Durometer           79         77         82        80
Durometer %      8.52%      4.05%      9.63%     3.90%
Tensile          593.5        646      618.4     617.4
Tensile %      -12.25%     -7.86%    -12.32%      -15%
Elongation        84.1       68.3       61.1      54.7
Elongation %    -33.4%     -41.2%     -38.8%    -21.6%

Table 9--quality of reclaim evaluation basic
physicals

Test              Control    20 phr GP    20 phr HS

Durometer            72.8         74.8           78
Tensile             676.3        705.3        705.6
Elongation          126.3          100        100.4
Tear die B           27.8         25.4         26.3
Compression set     22.22        26.92         26.7
Rebound              49.6         48.2         47.6

Table 10--quality of reclaim evaluation
heat aged physicals

Test            Control    20 phr GP    20 phr HS

Durometer            79           82           77
Durometer %       8.52%        9.63%       -1.82%
Tensile           593.5        618.4          646
Tensile %       -12.25%      -12.32%       -8.45%
Elongation         84.1         61.2         68.3
Elongation %     -33.4%       -38.8%      -31.91%

Table 11--quality of reclaim with VCP
evaluation basic physicals

Test              Control    20 phr GP    20 phr HS

Durometer              74           77           79
Tensile             701.1        726.8        749.1
Elongation          116.1         69.8         93.9
Tear die B           27.2         25.3         25.9
Compression set     26.12        31.30        27.48
Rebound              51.9           49         48.7

Table 12--quality of reclaim with VCP
evaluation heat aged physicals

Test           Control    20 phr GP    20 phr HS

Durometer           77           80           82
Durometer %      4.05%        3.90%        3.80%
Tensile            646        617.4          627
Tensile %       -7.86%      -15.05%       -16.4%
Elongation        68.4         54.7         52.3
Elongation %   -41.15%       -21.6%       -44.3%

Table 13--modified
reclaim 50/50 evaluation
formulation

Material                        phr

40 durometer silicone base   100/50
Silicone reclaim 50/50         0/50
Ground quartz filler             50
Precipitated silica              15
Pigment                           1
DBPH-50                         1.2
VCP-40                          1.2

Table 14--modified reclaim 50/50
evaluation basic physicals

Test              Control    20 phr       50/50
                              GPVCP    modified

Durometer              74        77          80
Tensile             701.1     726.8       779.4
Elongation          116.1      69.8        88.0
Tear die B           27.2      25.3        23.5
Compression set     26.12     31.30       48.12
Rebound              51.9        49        46.3

Table 15--modified reclaim 50/50
evaluation heat aged physicals

Test           Control     20 phr       50/50
                            GPVCP    modified

Durometer           77         80          85
Durometer %      4.05%      3.90%       6.25%
Tensile            646      617.4       620.0
Tensile %       -7.86%    -15.05%     -20.45%
Elongation        68.4       54.7        51.0
Elongation %   -41.15%     -21.6%     -42.05%

Table 16--modified
reclaim 90/10 evaluation
formulation

Material                        phr

40 durometer silicone base   100/75
Silicone reclaim 90/10         0/25
Ground Quartz Filler             50
Precipitated Silica              15
Pigment                           1
DBPH-50                         1.2
VCP-40                          1.2

Table 17--modified reclaim 90/10
evaluation basic physicals

Test              Control    20 phr       90/10
                              GPVCP    modified

Durometer              74        77          79
Tensile             701.1     726.8         738
Elongation          116.1      69.8        88.5
Tear die B           27.2      25.3       24.95
Compression set     26.12     31.30        35.8
Rebound              51.9        49        45.6

Table 18--modified reclaim 90/10
evaluation heat aged physicals

Test            Control     20 phr       90/10
                             GPVCP    modified

Durometer            77         80          84
Durometer %       4.05%      3.90%       6.33%
Tensile             646      617.4       626.6
Tensile %        -7.86%    -15.05%     -15.09%
Elongation         68.4       54.7        48.2
Elongation %    -41.15%     -21.6%     -45.58%
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Author:Sharp, Erick
Publication:Rubber World
Date:Jun 1, 2018
Words:2748
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